SMART MATERIALS and MEMS (15ME745)

SMART MATERIALS

Prepared by-Amaresh Kumar, Asst. Prof., MED SRI VENKATESHWARA COLLEGE OF ENGINEERING, BANGALORE Page 1

2015 SCHEME VTU CBCS SYLLABUS SMART MATERIALS and MEMS

(15ME745)

PREPARED BY:

MR. AMARESH KUMAR DHADANGE ASSISTANT PROFESSOR

DEPARTMENT OF MECHANICAL ENGINEERING,

SRI VENKATESHWARA COLLEGE OF ENGINEERING,

VIDYANAGAR CROSS, YALAHANKA AIRFORCE STATION ROAD,

BENGALURU-562157

CONTACT:+91- 974236647

SMART MATERIALS


2015 SCHEME VTU CBCS SYLLABUS

SMART MATERIALS and MEMS

Syllabus

Course Code Credits L-T-P Assessment Exam

SEE CIA Duration

Smart Materials and MEMS 15ME745 03 3-0-0 60 40 3Hrs

Course Objective:

This course provides a detailed overview to smart materials, piezoelectric

materials structures and its characteristics.

The study of Smart structures and modelling helps in Vibration control using

smart materials in various applications.

Helps to understand the principles and concepts of using MEMS, ER & MR

Fluids for various applications.

MODULE 1: Introduction, Memory Alloys

(Refer Text book: Smart structures, A V Srinivasan)Chapter No.:1, 2, 3

Unit1: Introduction: Closed loop and Open loop Smart Structures.

Applications of Smart structures, piezoelectric properties. Inchworm Linear

motor, Shape memory alloys, Shape memory effect-Application, Processing and

characteristics. 5hrs

Unit 2: Shape Memory Alloys: Shape Memory Alloys: Introduction,

Phenomenology, and Influence of stress on characteristic temperatures,

modelling of shape memory effect. Vibration control through shape memory

alloys. Design considerations, multiplexing embedded NiTiNOL actuators. –

5hrs

MODULE -2: Electro rheological and Magneto rheological Fluids, Fibre

Optics

(Refer Text book: Smart structures, A V Srinivasan) Chapter No.:4, 7

Unit-3 Electro rheological and Magneto rheological Fluids: Mechanisms

and Properties, Characteristics, Fluid composition and behaviour, Discovery

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and Early developments, Summary of material properties. Applications of ER

and MR fluids (Clutches, Dampers, others). – 5hrs

Unit-4 Fibre Optics: Introduction, Physical Phenomenon, Characteristics,

Fibre optic strain sensors, Twisted and Braided Fibre Optic sensors, Optical

fibres as load bearing elements, Crack detection applications, Integration of

Fibre optic sensors and shape memory elements. – 5hrs

MODULE-3: Vibration Absorbers, Biomimetics

(Refer Text book: Smart structures, A V Srinivasan) Chapter No.:5, 8, 9

Unit 5: Vibration Absorbers: Introduction, Parallel Damped Vibration

Absorber, Analysis, Gyroscopic Vibration absorbers, analysis & experimental

set up and observations, Active Vibration absorbers. Control of Structures:

Introduction, Structures as control plants, Modelling structures for control,

Control strategies and Limitations. – 6hrs

Unit 6: Biomimetics: Characteristics of Natural structures. Fibre reinforced:

organic matrix natural composites, Natural creamers, Mollusks. Biomimetic

sensing, Challenges and oppurtunities. – 5hrs

MODULE -4: MEMS, Piezoelectric Sensing and Actuation

Unit7: MEMS: History of MEMS, Intrinsic Characteristics, Devices: Sensors

and Actuators. Microfabrication: Photolithography, Thermal oxidation, thin

film deposition, etching types, Doping, Dicing, Bonding. Microelectronics

fabrication process flow, Silicon based Process selection and design. – 5hrs

Unit 8: Piezoelectric Sensing and Actuation: Introduction, Cantilever

Piezoelectric actuator model, Properties of Piezoelectric materials, Applications.

Magnetic Actuation: Concepts and Principles, Magnetization and

Nomenclatures, Fabrication and case studies, Comparison of major sensing

and actuation methods. – 5hrs

MODULE-5: Polymer MEMS & Micro fluidics, Case Studies

Unit 9: Polymer MEMS & Micro fluidics: Introduction, Polymers in MEMS

(Polyimide, SU-8, LCP, PDMS, PMMA, Parylene, Others) Applications

(Acceleration, Pressure, Flow, Tactile sensors). Motivation for micro fluidics,

Biological Concepts, Design and Fabrication of Selective components. Channels

and Valves. – 6hrs

Unit 10: Case Studies: MEMS Magnetic actuators, BP sensors, Microphone,

Acceleration sensors, Gyro, MEMS Product development: Performance,

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Accuracy, Repeatability, Reliability, Managing cost, Market uncertainties,

Investment and competition. – 5hrs

TEXT BOOKS:

1. “Smart Structures –Analysis and Design”, A. V. Srinivasan, Cambridge

University Press, New York, 2001, (ISBN: 0521650267).

2. “Smart Materials and Structures”, M. V. Gandhi and B.S.Thompson

Chapmen & Hall, London, 1992 (ISBN:0412370107)

3. “Foundation of MEMS, by Chang Liu. Pearson Education.

(ISBN:9788131764756)

COURSE OUTCOMES:

CO 1: Describe the methods of controlling vibration using smart systems and

fabrication methods of MEMS.

CO 2: Explain the principle concepts of Smart materials, structures, Fibre

optics, ER & MR Fluids.

CO 3: Explain the principle concepts of vibration controller, Biomimetics

CO 4: Analyze the properties of smart structures, MEMS, with the applications

and select suitable procedure for fabrication.

CO 5: Summarize the methods and uses of Micro fabrications, Biomimetics,

types of polymers used in MEMS, Fibre optics, piezoelectric sensing and

actuation.

Kx Blooms Knowledge Level (K1, K2, K3, K4, K5, K6)

Knowledge, Comprehension, Application, Analysis, Synthesis, Evaluation

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MODULE 1:

Introduction, Memory Alloys


Introduction: Closed loop and Open loop Smart Structures. Applications of

Smart structures, piezoelectric properties. Inchworm Linear motor, Shape

memory alloys, Shape memory effect- Application, Processing and

characteristics. – 5hrs

Shape Memory Alloys: Introduction, Phenomenology, and Influence of stress

on characteristic temperatures, modelling of shape memory effect. Vibration

control through shape memory alloys. Design considerations, multiplexing

embedded NiTiNOL actuators. – 5hrs

PREFACE:

Smart structures or smart materials systems are those which incorporate

actuators and sensors that highly integrate into the structures and have

structural functionality, as well as highly integrated control logic, signal

conditioning, and signal power amplification electronics. Such actuating,

sensing and controlling are incorporated into a structure for the purpose of

influencing its states or characteristics, be they mechanical, thermal, optical,

chemical, electrical, or magnetic. For example, a mechanically smart structure

is capable of altering either its mechanical states (its position or velocity) or its

mechanical characteristics (its stiffness or damping). Optically smart

structures could, for example, change colour to match its background.

In the following decades, it is expected that there will be widespread application

of the technology under development, in its current and evolutionary forms.

The breath of application of this technology is expected not only towards high-

tech but also towards civilian fields.

THE NEEDS

The demand for new generations of industrial, military, commercial, medical,

automotive and aerospace products has fuelled research and development

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activities focused on advanced materials and smart structures. This situation

has been further stimulated by the intellectual curiosity of humankind in

synthesising new classes of bio-mimetic materials. And, of course, global

competition among the principal industrial nations has also been a parameter

in the equation governing the rate of technological progress. A fundamental

axiom of this field of advanced materials is that the ultimate materials are the

biological materials which replicate such characteristics and properties in

synthetic materials and which can be employed in diverse scientific and

technological applications. Thus, by integrating the knowledge bases

associated with the mega-technologies of advanced materials, information

technology and biotechnology, the creation of a new generation of Biomimetics

materials and structures can be facilitated, with inherent brains, nervous

systems and actuation systems –this is at present a mere skeleton compared

with the anatomy perceived in the not-too-distant future. This quantum jump

in materials technology will revolutionise the future in ways far more dramatic

than the way the electronic chip has impacted on our lifestyles.

These new materials are termed Smart Materials or Intelligent Materials and

they will typically feature fibrous polymeric composite materials, embedded

with powerful computer chips of gallium arsenic which will be interfaced with

both embedded sensors and embedded actuators by networks of embedded

optical-fibre wave-guides, through which large volumes of data will be

transmitted at high speeds.

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Today‟s material revolution is the cornerstone of the triumvirate of mega

technologies, which comprise the essential, integrates of this embryonic field.

These technologies will have a mutually symbiotic relationship and will

significantly impact on one another resulting in synergistic technological

advances which cannot be foreseen today. However, a natural consequence of

advancing on these technological disciplines will be the impending revolution in

smart materials and structures.

The classes of smart materials and intelligent structures are diverse and the

applications of them are largely unknown. However, what is known is that this

new generation of materials will certainly revolutionise our quality of life as

dramatically as the state-of-art materials did in the past, with stone

implements triggering the Stone Age, alloys of copper and tin triggering the

Bronze Age, and the smelting of iron ore triggering the Iron Age. The time-line

of humankind is located at the dawn of a new age, The Smart Materials Age.

History of Smart Materials

The first recorded observation of smart material transformation was

made in 1932 on Gold-Cadmium. In addition, in 1938 the Phase

Transformation was observed in Brass (Copper Zinc). It was not until 1962,

however, that Beehler and co-workers found the transformation and attendant

shape memory effect in Nickel-Titanium at the Naval Ordinance Laboratory.

They named this family of alloy NiTiNOL after their lab. A few years after the

discovery of NiTiNOL, a number of other alloy systems with the shape memory

effect were found. A number of companies began to provide Ni-Ti materials and

components, and an increasing number of products, especially medical

products, were developed to market.

Smart materials, similar to living beings, have the ability to perform both

sensing and actuating functions and are capable of adapting to changes in the

environment. In other words, smart materials can change themselves in

response to an outside stimulus (e.g. Stress, Pressure, Temperature Change,

Magnetic Field, etc.) or respond to the stimulus by producing a signal of some

kind. Hence, smart materials can be used as “sensors”, “actuators” or in some

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cases as “self-sensing actuators” or “senoric actuators” in general. Smart or

intelligent materials form a group of new and state of the art materials now

being developed that will have a significant influence on many of our

technologies.

The adjective “Smart implies that these materials are able to sense

changes in their environments and then respond to these changes in

predetermined manners--behavior that are also found in living organisms. In

addition, the concept of smart materials is being extended to rather

sophisticated systems that consist of both smart and traditional materials. The

field of smart materials attempts to combine the sensor (that detects an input

signal), actuator (that performs a responsive and adaptive function) and the

control circuit on as one integrated unit. Actuators may be called upon to

Change Shape, Position, Natural Frequency, or Mechanical Characteristics in

response to changes in Temperature, Electric Fields, and/or Magnetic fields.

Fig: Integrated Sensor-Actuator systems with controller are analogous to

biological systems.

Overview of Smart Materials

Smart Materials are materials capable of controllable response to their

environment. Smart Materials can be used as 'Actuators," "Sensors", or in some

cases as Self-Sensing Actuators. These materials present new and exciting

design opportunities leading to innovations and improved performance.

TYPES OF SMART MATERIAL

There are a number of types of smart material, some of which are already

common. Some examples are as following:

a) Piezoelectric Materials

b) Electrostrictive Materials

c) Magnetostrictive Materials

d) Magneto Electric Materials

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e) Magneto Rheological

f) Fluids Electro Rheological Fluids

g) Shape Memory Materials

h) Fiber-Optic Sensors

a) PIEZOELECTRIC MATERIALS: These materials that produce a voltage

when stress is applied. Since this effect also applies in the reverse manner, a

voltage across the sample will produce stress within the sample. Suitably

designed structures made from these materials can therefore be made that

bend, expand or contract when a voltage is applied.

b) SHAPE MEMORY ALLOYS: Shape memory polymers are Thermo responsive

materials where deformation can be induced and recovered through

temperature changes.

c) MAGNETIC SHAPE MEMORY ALLOYS: are materials that change their

shape in response to a significant change in the magnetic field.

d) PH-SENSITIVE POLYMERS: These materials which swell/collapse when the

pH of the surrounding media changes.

e) TEMPERATURE-RESPONSIVE POLYMERS: These materials which undergo

changes upon temperature.

f) HALOCHROMIC MATERIALS: commonly materials that change their colour

as a result of changing acidity. One suggested application is for paints that

can change colour to indicate corrosion in the metal underneath them.

g) CHROMOGENIC SYSTEMS: Change colour in response to electrical, optical

or thermal changes. These include electrochromic materials, which change

their colour or opacity on the application of a voltage (e.g. liquid crystal

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displays), thermochromic materials change in colour depending on their

temperature, and photo chromic materials, which change colour in response

to light - for example, light sensitive sunglasses that darken when exposed to

bright sunlight.

h) NON-NEWTONIAN FLUID: It a liquid which changes its viscosity in response

to an applied shear rate. In other words the liquid will change its viscosity in

response to some sort of force or pressure.

SMART STRUCTURES

A smart structure has the capability to respond to a changing external

environment such as loads and shape change as well as to a changing internal

environment (such as damage or failure). Smart structure consists of a

structure provided with a set of actuators and sensors coupled by a controller;

if the bandwidth of the controller includes some vibration modes of the

structure, its dynamic response must be considered. If the set of actuators and

sensors are located at discrete points of the structure, they can be treated

separately.

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The distinctive feature of smart structures is that the actuators and

sensors are often distributed and have a high degree of integration inside the

structure, which makes a separate modeling impossible.

Smart structures are a new emerging materials system which combines

contemporary materials science with information science. The smart system is

composed of sensing, processing, actuating, feedback, self-diagnosing and self-

recovering subsystems Moreover, in some applications like vibroacoustics, the

behavior of the structure itself is highly coupled with the surrounding medium;

this also requires a coupled modeling. A Smart Structure is that which has the

ability to respond adaptively in a pre-designed useful and efficient manner to

changes in environmental conditions and changes in its own condition.

TYPES OF SMART STRUCTURES

Smart Structures Can Distinguished by two types:

1. Closed Loop Smart Structures

2. Open Loop Smart Structures

CLOSED LOOP SMART STRUCTURES

A closed-loop smart structure senses the changes to diagnose the nature of

the problem, takes action to mitigate the problem, and also stores the data of

the episode for future reference.

OPEN LOOP SMART STRUCTURES

An Open Loop smart structure means that the design is such that

structural integrity is enhanced only when needed, and the structure relapses

to its normal state when there is no need for enhanced integrity

Smart structures are on an evolutionary path, in the following sequence:

Actively smart structures

Very smart structures

Intelligent structures Wise structures (moral and ethical decisions).

Awareness in smart structures.

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Collective consciousness

Man-machine integration.

POTENTIAL FEASIBILITY (Possibility) OF SMART STRUCTURES

Potential Feasibility in Smart structures has features intermediate between

those of smart materials and smart systems. Actively smart structures not only

have a sensor feature and an actuator feature, but, unlike passively smart

materials, they also involve external biasing or feedback

One can make a distinction between smart structures and intelligent

structures, reserving the latter term only for applications incorporating

cognitive capabilities and adaptive learning in the design, generally entailing

the use of fast, real-time, information processing with neural networks.

POTENTIAL FEASIBLITTY IN SMART STRUCTURES

a) Improve vibration

b) Reduce acoustic noise

c) Monitor their own condition and environment

d) Automatically perform precision alignments

e) Change their shape or mechanical properties on command

f) Develop a base technology to build smart rotorcraft to actively control

External/internal noise

Rotor-induced vibration

Transmission-induced noise & vibration

Aeromechanical stability

Performance and flight stability

g) Refine key technology elements of smart structures for rotorcraft

environment, actuators, sensors (MEMS, fiber optics), controllers and

power conditioning

The design and implementation of smart structures necessitate the

integration

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a) Smart sensors and actuators,

b) Microelectronics

c) Structural modelling and design

d) Intelligent control techniques

e) Reflectivity and Emissivity of Light, Sound, and Heat

f) Control Material Properties

g) Stiffness, Shape, Damping, Thermal Properties

h) Reduce Fatigue & Extended Lifetime

i) Monitor and Adapt to operating Condition

KEY ELEMENTS OF SMART STRUCTURES

The Main Key Elements in Smart Structures is

a) Fiber optic sensor based system to detect and control stress in smart

structures.

b) Active vibration and noise control.

c) Piezoelectric sensors and actuators.

d) Modification of the parametric stability signature.

e) Sensors

f) Actuators

g) Control Systems

SENSORS

A sensor is an essential requirement for a smart structure. In biological

systems, the sensor output can be of various types, but in man-made

structures the most convenient sensor output is an electrical signal. Thus a

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sensor is usually a transducer involving a specific transduction principle for

transforming a particular form of energy input into an electrical signal.

Smart Sensors: Accelerometers; Force Sensors; Load Cells; Torque Sensors;

Pressure Sensors; Microphones; Impact Hammers; MEMS Sensors; Sensor

Arrays

ACTUATORS

Like sensors, actuators are also an essential component of most of the

conceivable smart structures. An actuator creates controllable mechanical

motion from other forms of energy. Micro actuators offer special advantages,

but they should be, by and large, compatible with the materials and processing

technologies of silicon microelectronics. They should be capable of being

powered and controlled electrically, thus allowing full utilisation of integration

with on-chip electronics. There are two types of micro actuators: „mechanisms‟

which provide displacement through rigid-body motion; and „deformable

microstructures‟ which provide displacement by mechanical deformation or

straining.

Smart Actuators: Displacement Actuators; Force Actuators; Power Actuators;

Vibration Dampers; Shakers; Fluidic Pumps; Motors

SMART TRANSDUCERS:

Ultrasonic Transducers; Sonic Transducers; Air Transducers Measurement,

Signal Processing, Drive and Control Techniques

Quasi-Static and Dynamic Measurement Methods; Signal-Conditioning

Devices; Constant Voltage, Constant Current and Pulse Drive Methods;

Calibration Methods; Structural Dynamics and Identification Techniques;

Passive, Semi-Active and Active Control; Feedback and Feed forward Control

Strategies.

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APPLICATIONS OF SMART STRUCTURES

Smart structures Mainly Applied in the Field of

a) Electrical Engineering

b) Mechanical Engineering

c) Aerospace Engineering

d) Civil Engineering

e) Engineering Mechanics

Some of the other Applications are

a) Fast response valves

b) High-power-density hydraulic pumps

c) Active bearings for reduction of machinery noise

d) Footwear

e) Sports equipment

f) Precision machining

g) Vibration and acoustic sensors

h) Dampers

i) Carbon Fibre Reinforced Concrete

j) Smart Concrete

PIEZOELECTRIC MATERIALS

The piezoelectric effect was discovered in 1880 by the Jacques and Pierre

Curie brothers. They found out that when a mechanical stress was applied on

crystals such as tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane

sugar, electrical charges appeared, and this voltage was proportional to the

stress. First applications were piezoelectric ultrasonic transducers and soon

swinging quartz for standards of frequency (quartz clocks). An everyday life

application example is your car's airbag sensor. The material detects the

intensity of the shock and sends an electrical signal which triggers the airbag.

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The Piezoelectric Effect

In physics, the piezoelectric effect can be described as the link between

electrostatics and mechanics. The piezoelectric effect describes the relation

between a mechanical stress and an electrical voltage in solids. It is reversible:

an applied mechanical stress will generate a voltage and an applied voltage will

change the shape of the solid by a small amount (up to a 4% change in

volume). Piezoelectricity is the ability of some materials (notably crystals and

certain ceramics) to generate an electric charge in response to applied

mechanical stress.

If the material is not short-circuited, the applied charge induces a voltage

across the material. The piezoelectric effect is reversible, that is, the

piezomaterials exhibit. The direct piezoelectric effect – the production of

electricity when stress is applied.

The converse piezoelectric effect – the production of stress and/or strain when

an electric field is applied. (For example, lead zirconate titanate crystals will

exhibit a maximum shape change of about 0.1% of the original dimension.)

Simple molecular model for explaining the piezoelectric effect

Figure shows Simple molecular model for explaining the piezoelectric effect:

an unperturbed molecule [left], the molecule subjected to an external force

[middle], a polarizing effect on the material surfaces [right].

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Before subjecting the material to some external stress: the centres of the

negative and positive charges of each molecule coincide, the external

effects of the charges are reciprocally cancelled, as a result, an

electrically neutral molecule appears.

After exerting some pressure on the material: the internal structure is

deformed, that causes the separation of the positive and negative centers

of the molecules, as a result, little dipoles are generated.

Eventually: the facing poles inside the material are mutually cancelled, a

distribution of a linked charge appears in the material‟s surfaces and the

material is polarized, the polarization generates an electric field and can

be used to transform the mechanical energy of the material‟s

deformation into electrical energy.

Piezoelectric Materials

The piezoelectric effect occurs only in non conductive materials.

Piezoelectric materials can be divided in 2 main groups: crystals and ceramics.

The most well-known piezoelectric material is quartz (SiO2).

Piezoelectric materials strain when an electric field is applied across them.

Also, they produce voltage under strain. The first property makes them suitable

as actuators to control structural response, where as second property makes

them suitable as sensors. Thus, Piezoelectrics have the property to transform

mechanical energy to electrical energy and vice-versa.

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Typically, the electrical dynamics of the piezoelectric are ignored when they

are used as sensors. Piezoelectric materials can be crystals and ceramics and

they are brittle. Piezoelectric sensors are generally made of polymers such as

poly vinylidene fluoride, PVDF. It can be easily formed in to very thin sheets

(films) and adhered to any surface.

More Piezoelectric Materials

Here is a list of other piezoelectric materials:

Lithium tantalate

Polyvinylidene fluoride

Lanthanum gallium silicate

Potassium sodium tartrate

Piezoelectric constitutive relations

The piezoelectric material is assumed to be linear. The actuation strain is

modelled like thermal strain. Piezoceramics can be assumed as orthotropic

material like composite unidirectional laminate. The constitutive relations are

based on the assumption that the total strain in the actuator is the sum of the

mechanical strain induced by the stress, the thermal strain due to temperature

and the controllable actuation strain due to the electric voltage. The X3 axis is

assigned to the direction of the initial polarization of the piezoceramics, and X1

and X2 axes lie in plane perpendicular to X3 axis.

Coupled electromechanical constitutive relations are:

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Where, lb/in2

σ = stress vector, N/mm2 or lb/in2

є = strain vector

e= vector of piezoelectric coefficients, N/V-mm

d= vector of piezoelectric strain coefficients, mm/V

C= vector of stiffness coefficients, N/mm2

S= vector of compliance coefficients, mm2/N

A= vector of thermo elastic coefficients, N/mm2K

α = vector of thermal coefficients of expansion, l/K

ΔT= temperature change, K

In a plane perpendicular to the piezo-polarization, it has isotropic

properties, i.e., transversely isotropic material in the plane 1-2. For orthotropic

material, there is no temperature shear strain. However, there is a shear strain

induced due to electric field E1 and E2. It is possible to introduce modified

coefficients to combine thermal and induced strain.

DEPOLING AND COERCIVE FIELD

Depoling and coercive field (a measure of a ferromagnetic or ferroelectric

material to withstand an external magnetic or electric field)

During the manufacture of a piezoceramics, a large (greater than

1KV/mm) field is applied across the ceramic to create polarization. This is

called coercive field. During subsequent testing, if a field greater than the

coercive field, Ec, is applied opposite to the polarization direction, the ceramic

will lose its piezoelectric properties- called Depoling. Again, Depoling is

possible. If applied field is aligned with the initial polarization direction, there is

no Depoling. A sufficiently high voltage can cause arching or a brittle fracture.

Depoling is also possible if the temperature exceeds Curie temperature or if a

large stress is applied.

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FIELD-STRAIN RELATION

A linear model fits well only for a small field. For large strain, a nonlinear

variation occurs and a cubic variation fits better.

Fig: Deformation of a cube of PZT subjected to a uniform electric field

During poling the material is subjected to a small electric field, the

dipoles in this process respond collectively to produce a macroscopic expansion

along the poling axis and contraction perpendicular to it.

The geometry and deformation of a simple cube of PZT, which has been

poled in the 3-direction and is subjected to and electric field in this direction, is

shown in above figure. The relationship between applied field strength and

resulting strain is quantified by the piezoelectric moduli dij, where i is the

direction of the electric field and j the direction of the resulting normal strain.

Therefore, for the above example

33 = d33 𝑉

𝑡……………(1)

11 = d31 𝑉

𝑡…………….(2)

Where V is the voltage applied in the 3-direction and t the thickness of

the specimen, as shown in figure.

Once depoled the piezoelectric material loses the property of dimensional

response to an electric field.

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Hysteresis

Many physical systems naturally exhibit hysteresis. A piece of iron that

is brought into a magnetic field retains some magnetization, even after the

external magnetic field is removed. Once magnetized, the iron will stay

magnetized indefinitely. To demagnetize the iron, it would be necessary to

apply a magnetic field in the opposite direction. This effect is exploited

commercially; for example, it provides the element of memory in a hard disk

drive. Hysteresis phenomena occur in magnetic and ferromagnetic materials,

as well as in the elastic, electric, and magnetic behavior of materials, in which

a lag occurs between the application and the removal of a force or field and its

subsequent effect.

Electric hysteresis occurs when applying a varying electric field and

elastic hysteresis occurs in response to a varying force. The term "hysteresis" is

sometimes used in other fields, such as economics or biology; where it

describes a memory, or lagging effect, in which the order of previous events can

influence the order of subsequent events.

If the displacement of a system with hysteresis is plotted on a graph

against the applied force, the resulting curve is in the form of a loop. In

contrast, the curve for a system without hysteresis is a single, not necessarily

straight, line. Although the hysteresis loop depends on the material's physical

properties, there is no complete theoretical description that explains the

phenomenon. The family of hysteresis loops, from the results of different

applied varying voltages or forces, form a closed space in three dimensions,

called the hysteroid.

http://en.wikipedia.org/wiki/Physics

http://en.wikipedia.org/wiki/Iron

http://en.wikipedia.org/wiki/Hard_disk_drive



http://en.wikipedia.org/wiki/Voltage

http://en.wikipedia.org/w/index.php?title=Hysteroid&action=edit&redlink=1

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CREEP AND STRAIN RATE EFFECTS

Creep only occurs with open loop PZTs. Like hysteresis, creep is related

to the effect of the applied voltage on the remains polarization of the piezo

ceramics. Creep decreases logarithmically with time. If the operating voltage of

a (open loop) Piezo is increased (decreased), the remnant polarization (piezo

gain) continues to increase (decrease), apparently itself in a slow creep (positive

or negative) after the voltage change is complete. The following equation

describes the effect:

L(t) L(1+ lg (t/0.1)) …(1)

Creep of Piezo motion as a function of time.

Where,

L = displacement 0.1 seconds after the voltage change is complete [m].

= creep factor which is dependent on the properties of the actuator (on the

order of 0.01 to 0.02).

Creep and the strain rate dependence of d*31 (piezoelectric strain

coefficient) are small but measurable which become more significant for

larger strains and lower frequencies. At low frequencies, the tendency of

piezoceramics to creep under prolonged application of electric fields. For

high frequencies, creep can be ignored. For low frequencies or static

applications creep must be accounted for. The degree of creep is reduced

when piezoceramics are elastically constrained.

piezoelectric strain coefficient VS frequency for piezo ceramics

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STRAIN RATE

Strain rate in piezoelectric materials is defined as the strain per unit time. The

time required to accumulate a given strain is expressed as the elongation per

time.

= /t= (L-Lo)/(Lot)

=change in length/ (original length) (time)

Where, L = final length

Lo =original length

T = time

Example:

Case 1: Loading a material fast (reducing load time) we get a stress versus

strain diagram.

Case 2: Loading the same material slow (increasing load time) and we get

another stress versus strain diagram.

If the stress strain diagrams are different for case 1 and case 2 then the

material is strain rate sensitive.

If the stress strain diagrams are the same for case 1 and case 2 then

the material is not strain rate sensitive. At room temperature steel is

regarded as not strain rate sensitive. Viscoelastic materials are strain

rate sensitive.

Strain rate sensitive means a material's stress versus strain

characteristics are dependent on the rate of loading.

INCHWORM LINEAR MOTOR

The inchworm motor is a device that uses piezoelectric actuators to move a

shaft with nanometer precision. In its simplest form, the inchworm motor uses

three piezo-actuators (2 and 3, as in Figure) mounted inside a tube (1) and

electrified in sequence to grip a shaft (4) which is then moved in a linear

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direction. Motion of the shaft is due to the extension of the lateral piezo (2)

pushing on two clutching piezos (3).

Inchworm motor

Operation

The actuation process of the inchworm motor is a six step cyclical process

after the initial relaxation and initialization phase. Initially, all three piezo are

relaxed and unexpended. To initialize the inchworm motor the clutching piezo

closest to the direction of desired motion (which then becomes the forward

clutch piezo) is electrified first then the six step cycle begins as follows (see

Figure 2 ):

Step 1. Extension of the lateral piezo.

Step 2. Extension of the shaft clutch piezo.

Step 3. Relaxation of the forward clutch piezo.

Step 4. Relaxation of the lateral piezo.

Step 5. Extension of the forward clutch piezo.

Step 6. Relaxation of the shaft clutch piezo.

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six steps working of a Inchworm linear motor

Uses of Inchworm motor

The inchworm motor is commonly used in scanning tunnelling

microscopes (STM's). An STM requires nanometer scale control of its scanning

tip near the material it is observing. This control can be accomplished by

connecting the scanning tip to the shaft of the inchworm motor. The inchworm

motor, in turn, allows control in a direction normal to the plane of the observed

material's surface.

The inchworm motor can be used in the patch clamping of cells. This

technique is most often performed with an optical microscope and requires

micro-manipulation of a glass pipette.

SHAPE MEMORY ALLOYS:

A BRIEF HISTORY

The discovery of martensite in steels in the 1890s by Adolf Martens was a

major step toward the eventual discovery of shape memory alloys. The

martensitic transformation was perhaps the most widely studied metallurgical

phenomenon during the early 1900s. The martensitic transformation, as

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observed in the Fe-C system, was established as an irreversible process. The

concept of thermo elastic

Martensitic transformation, which explained the reversible transformation of

martensite, was introduced in 1949 by Kurdjumov and Khandros, based on

experimental observations of the thermally reversible martensitic structure in

CuZn and CuAl alloys. By 1953, the occurrence of thermo elastic martensitic

transformation was demonstrated in other alloys such as InTl and CuZn. The

reversible martensitic transformation and the alloys that exhibited them

remained unutilized until 1963. The breakthrough for engineering applications

occurred with the discovery of NiTi by Buehler and co-workers while

investigating materials useful for heat shielding. It was noticed that in addition

to its good mechanical properties, comparable to some common engineering

metals, the material also possessed a shape recovery capability. Following this

observation, the term “NiTiNOL” was coined for this NiTi material in honour of

its discovery at the Naval Ordnance Laboratory (NOL). The term Shape Memory

Effect (SME) was given to the associated shape recovery behavior.

Definition of Shape Memory Alloy:

Shape Memory Alloys (SMAs) refer to a group of materials which have the

ability to return to a predetermined shape when heated. The shape memory

effect is caused by a temperature dependent crystal structure.

SHAPE MEMORY EFFECT

The shape memory effect is caused by a temperature dependent crystal

structure.

• There are two common shape memory effects - One Way and Two Way effects.

• In the case of One Way effect, the material always remembers the shape at

Parent State (Austenite Phase)

• In the case of Two Way effect, the material is trained to remember two

shapes, one at the Parent Austenite phase and the other at the Martensite

Phase.

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Where,

Ms= Martensite Start Temperature As= Austenite Start Temperature

Mf= Martensite Finish Temperature Af=Austenite Finish Temperature

Figure. Schematic representation of shape memory effect.

At point (1) the alloy is in the twinned martensitic phase; when load is applied

(1–2) the crystal configuration changes from twinned to detwinned, and the

deformation remains also when the load is removed (2–3). Increasing the

temperature results in a change into the austenitic phase, with a consequent

recovery of the original shape (3–4), this is maintained also when the alloy cools

down.

In austenite phase, the structure of the material is symmetrical; each “grain” of

material is a cube with right angles (a). When the alloy cools, it forms the

martensite phase and collapses to a structure with different shape (b). If an

external stress is applied, the alloy will yield and deform to an alternate state

(c). Now, if the alloy is heated again above the transformation temperature, the

austenite phase will be formed and the structure of the material returns to the

original “cubic” form (a), generating force/stress. The change in the SMA

crystalline structure is not thermodynamically reversible process due to

internal frictions and creation of structural defects. When heated, SMA follows

the upper curve, As is the temperature, where austenite phase starts to form

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and in Af the material is 100 % austenite. When the alloy cools, it follows the

lower curve: Ms is the temperature, where martensite starts to form and in Mf

the alloy is 100 % martensite.

SHAPE MEMORY EFFECT

1. One-Way Shape-Memory Effect:

The ability of SMA to recover a specific shape upon heating and does not

remember the alternate shape when cooled (below the transformation

temperature) is known as One-way shape memory effect

A shape memory element can be deformed in its martensitic state to almost

any “cold shape.” The basic restriction is that the deformations may not exceed

a certain limit, typically 8%. These apparent plastic deformations can be

recovered completely during heating when the reverse transformation occurs

and results in the original “hot shape.” This strain and shape recovery during

heating is called the one-way shape-memory effect because only the hot shape

is memorized (Fig.).

2. Two-Way Memory Effect (TWME):

The ability of SMA to recover a specific shape upon heating and then return to

an alternate shape when cooled (below the transformation temperature) is

known as two-way shape memory effect.

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The two-way memory effect involves memorization of two shapes. Figure 2

shows that a cold shape is obtained spontaneously during cooling. Different

from the one-way memory effect, no external forces are required to obtain the

“memorized” cold shape. During subsequent heating, the original hot shape is

restored. The maximum strains are in general substantially smaller than those

of the one way memory effect. A strain limit of about 2% has been mentioned,

although higher TWME strains have been found in specific cases. In 1972, Tas

proposed the term “two-way memory effect” (abbreviated to TWME) to refer to

this spontaneous, reversible shape change between a “hot” shape linked to the

parent phase and an acquired “cold” shape linked to the martensitic phase.

This spontaneous shape change was observed only after particular thermo

mechanical procedures.

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The Shape memory effect can be

Thermal induced SME

The SMA is cooled from high temp Austenite phase to low temp Martensite

phase and it is deformed below Mf followed by heating above As to cause

shape recovery to occur this type of SME is called Thermoelastic SME

Mechanical induced SME

The martensite phase transformation can also be induced by applying stress

at temperature above Ms. The material so formed is called stress induced

martensite(SIM) Driving forced is mechanical not thermal .It has been

experimentally established above the Ms The stress required to produce SIM

increases with increasing temperature.

PHASE TRANSFORMATION

Fig: Shows the Mechanism of Shape memory effect

Original parent crystal,

Self accommodated Martensite,

(c-d) Deformation in Martensite proceeds by the growth of one variant at

the expense of other (i.e. Twinning or Detwinning)

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(e) Upon heating to a temperature above Af,

Each variant reverts to the parent phase in the original orientation by reverse

transformation Crystallography, the martensite transformation occurs in two

steps (1) Bain Strain (2) Lattice invariant shear. The crystallography changes

that occur are shown in fig. The austenite structure is illustrated in (a) and

progression to fully martensite structure is illustrated in (b) through (d). It can

be noted that as the martensite interface progress from one atomic layer to

another, each atom is required to move by only a very small amount (less than

1inter atomic distance).The end results of all these small coordinated

movements in new martensite structure, and the movement needed to produce

the new structure called “Bain strain”. The second part of martensite

transformation is the “lattice invariant shear “. In this step, the shape and

volume change resulting from the Bain strain is accommodated to reduce the

strain and make martensite phase stable

The martensite transformation in steel involves both volume change and

shape change, whereas SMA, s undergoes only a Shape change. During

transformation the overall shape of the new phase or the surrounding

austenite must be altered to accommodate new structure. There are two

general mechanisms by which the accommodation takes place (1) Slip and (2)

Twinning. In both the cases each individual cell or parallelogram has the new

martensite structure, but overall shape is that of original austenite. Slip is a

permanent process and is common accommodation mechanisms in many

martensites. Twinning can accommodate shape changes in a recoverable way

but cannot accommodate volume changes. For Shape memory to occur the

shape accommodation should be fully reversible or in other ways Twinning

should be the dominant accommodation process.

TANAKA’S CONSTITUTIVE MODEL

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Consider an SMA wire subjected to an isothermal mechanical loading and

unloading. Assume the wire is in 100% Austenitic state and at a temperature

between Ms and As.

The stress state in a SMA component is function of 3 primary state variables.

They are ξ, the fraction of martensite. „T‟ the temperature at which the

component is operative. „&‟ the strain at which the component is functioning.

Therefore

,,T

T

T

Integrating w.r.t time from the initial condition o oT o

We can write a unified constitutive relation

][][][ oooo TTD

Where D= young‟s modulus, θ = Thermoelastic tensor, = Transformation

tensor

An isothermal condition implies.

oTT lin o (Not fully austenite)

Assume zero initial strain.

o =0 at t=0 0o 0TT o

Under this condition if we load and unload we are going to be in the linear

region in which D

linlin D

lin = The subscript linear represents the limit of linearity for the stress strain

relationship above which the stress strain relation will be non-linear because of

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non-zero martensite.

When 0 (i.e., the applied stress exceed lin ) the excess stress induced

martensite and the constitutive relation becomes

][][ ooo D

With lin 0 lino 0

linlin DD Because linlin D

Thus the governing equation for the constitutive modeling of the Shape Memory

effect is when EQN lin

D

TESTING OF SMA WIRES

Stress–Strain Characteristics of SMA

Fig Stress strain curve of SMA

●Austenite

■ Martensite

The initial flat region is termed as “Martensite plateau”. Here the

martensite structure deforms by movement of Twin boundary which are quite

mobile .Hence the yield strength of martensite is extremely low compared to

austenite which deforms by slip. Only certain amount of martensite

deformation can be accommodated by detwinning and once this exceeds the

material again deforms elastically and eventually yield a second time by

irreversible process of dislocation movement (slip).

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The stress strain curve can be divided into three well defined regions in

martensite phase

The initial low plateau is due to stress induced growth of martensite

variant at the expense of adjacent unfavorably oriented one (This process

is known as detwinning)

At higher stresses there is second region which is linear but not

necessarily purely elastic The deformation mechanism in the stage is a

mixture of elastic deformation of detwinned martensite together with

formation of new orientation of martensite which intersect with that

which is already present and provide additional recoverable strain

The third region is where plastic deformation as in case of yielding of all

conventional metal

APPLICATIONS OF SMART MATERIALS

Smart materials find a wide range of applications due to their varied response

to external stimuli. The different areas of application can be in our day to day

life, Aerospace, Civil Engineering applications and mechatronics to name a few.

a) Aircraft and spacecraft: variable area fan nozzle (VAFN), vibration

dampers for launch vehicles and commercial jet engines.

b) Automotive: control low pressure pneumatic bladders in a car seat that

adjust the contour of the lumbar support / bolsters.

c) Robotics: it possible to create very lightweight robots.

d) Civil Structures: application is Intelligent Reinforced Concrete (IRC),

which incorporates SMA wires embedded within the concrete. These

wires can sense cracks and contract to heal macro-sized cracks. Another

application is active tuning of structural natural frequency using SMA

wires to dampen vibrations.

e) Piping: oil line pipes for industrial applications, water pipes and similar

types of piping for consumer/commercial applications.

f) Telecommunication: autofocus (AF) actuator for a smart phone.

g) Medicine: fixation devices for osteotomies in orthopedic surgery,

https://en.wikipedia.org/wiki/Pneumatic

https://en.wikipedia.org/wiki/Car_seat

https://en.wikipedia.org/wiki/Autofocus

https://en.wikipedia.org/wiki/Smart_phone

https://en.wikipedia.org/wiki/Osteotomy

https://en.wikipedia.org/wiki/Orthopaedic_surgery



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in dental braces to exert constant tooth-moving forces on the teeth.

h) Optometry: Eyeglass frames made from titanium-containing SMAs

i) Dentistry: The prevalence of dental braces using SMA technology to

exert constant tooth-moving forces on the teeth.

j) Engines: Experimental solid state heat engines, operating from the

relatively small temperature differences in cold and hot water reservoirs,

have been developed.

k) Crafts: Sold in small round lengths for use in affixment-free bracelets.

a) Characteristics OR properties of SMA

1. SMA is having ability to return to their original or present shape and

size on a simple temperature change.

2. They posses high damping capacity.

They have pseodoelasticity property.

3. They posses high tensile strength.

4. They are resistant to corrosion.

6. They posses re-orientation of temperature phase transformation.

7. They posses high ductility.

8. They posses good thermal stability.

SHAPE ALLOY TYPES

Since the discovery of Ni-Ti, at least fifteen different binary, ternary and

quaternary alloy types have been discovered that exhibit shape changes and

unusual elastic properties consequent to deformation. Some of these alloy

types and variants are shown in table

a) Titanium-palladium-nickel

b) Nickel-titanium-copper

c) Gold-cadmium

d) Iron-zinc-copper-aluminum

e) Titanium-niobium-aluminum

https://en.wikipedia.org/wiki/Dental_braces

https://en.wikipedia.org/wiki/Glasses

https://en.wikipedia.org/wiki/Dental_braces

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f) Uranium-niobium

g) Hafnium-titanium-nickel

h) Iron-manganese-silicon

i) Nickel-titanium

j) Nickel-iron-zinc-aluminum

k) Copper-aluminum-iron

l) Titanium-niobium

m) Zirconium-copper-zinc

n) Nickel-zirconium-titanium

Phenomenology of Phase Transformation in Shape Memory Alloys (SMAs):

Shape Memory Alloys (SMAs), have two phases, each with a different

crystal structure and therefore different properties. One is the high

temperature phase called austenite (A) and the other is the low temperature

phase called martensite (M). Austenite (generally cubic) has a different crystal

structure from martensite (tetragonal, orthorhombic or monoclinic). The

transformation from one structure to the other does not occur by diffusion of

atoms, but rather by shear lattice distortion. Such a transformation is known

as martensitic transformation. Each martensitic crystal formed can have a

different orientation direction, called a variant. The assembly of martensitic

variants can exist in two forms: twinned martensite (Mt), which is formed by a

combination of “self-accommodated” martensitic variants, and detwinned or

reoriented martensite in which a specific variant is dominant (Md). The

reversible phase transformation from austenite (parent phase) to martensite

(product phase) and vice versa forms the basis for the unique behaviour of

SMAs.

Upon cooling in the absence of an applied load, the crystal structure

changes from austenite to martensite. The phase transition from austenite to

martensite is termed the forward transformation. The transformation results in

the formation of several martensitic variants, up to 24 for NiTi. The

arrangement of variants occurs such that the average macroscopic shape

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change is negligible, resulting in twinned martensite. When the material is

heated from the martensitic phase, the crystal structure transforms back to

austenite, and this transition is called reverse transformation, during which

there is no associated shape change

Fig: Temperature-induced phase transformation of an SMA without

mechanical loading.

A schematic of the crystal structures of twinned martensite and austenite for

an SMA and the transformation between them is shown in above Fig. There are

four characteristic temperatures associated with the phase transformation.

During the forward transformation, austenite, less than zero loads, begins to

transform to twinned martensite at the martensitic start temperature (Ms) and

completes transformation to martensite at the martensitic finish temperature

(Mf). At this stage, the transformation is complete and the material is fully in

the twinned martensitic phase. Similarly, during heating, the reverse

transformation initiates at the austenitic start temperature (As) and the

transformation is completed at the austenitic finish temperature (Af).

If a mechanical load is applied to the material in the twinned martensitic phase

(at low temperature), it is possible to detwin the martensite by reorienting a

certain number of variants (see Fig. below). The detwinning process results in a

macroscopic shape change, where the deformed configuration is retained when

the load is released.

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Fig: Schematic of the shape memory effect of an SMA showing the

detwinning of the material with an applied stress.

A subsequent heating of the SMA to a temperature above Af will result in a

reverse phase transformation (from detwinned martensite to austenite) and will

lead to complete shape recovery (see Fig. 1.5). Cooling back to a temperature

below Mf (forward transformation) leads to the formation of twinned martensite

again with no associated shape change observed.

The process described above is referred to as the Shape Memory Effect (SME).

The load applied must be sufficiently large to start the detwinning process. The

minimum stress required for detwinning initiation is termed the detwinning

start stress (σs). Sufficiently high load levels will result in complete detwinning

of martensite where the corresponding stress level is called the detwinning

finish stress (σf ).

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MODELLING OF SHAPE MEMORY EFFECT

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MODULE -2:

Electro rheological and Magneto rheological Fluids, Fibre Optics

(Refer Text book: Smart structures, A V Srinivasan) Chapter No.:4, 7

Electro rheological and Magneto rheological Fluids: Mechanisms and

Properties, Characteristics, Fluid composition and behaviour, Discovery and

Early developments, Summary of material properties. Applications of ER and

MR fluids (Clutches, Dampers, others). – 5hrs

Fibre Optics: Introduction, Physical Phenomenon, Characteristics, Fibre optic

strain sensors, Twisted and Braided Fibre Optic sensors, Optical fibres as load

bearing elements, Crack detection applications, Integration of Fibre optic

sensors and shape memory elements. – 5hrs

INTRODUCTION

In this topic we consider fluids whose properties change in response to an

applied electric or magnetic field. These are known as Electrorheological [ER]

and Magnetorheological [MR] fluids, respectively, because the most remarkable

filed-induced change is a tremendous increase in their ability to support shear

stress. Most engineering applications of ER and MR fluids exploit their

controllable yield stress to vary the coupling or load transfer between moving

parts, for example in dampers and clutches.

MAGNETORHEOLOGICAL (MR) FLUID

A Magnetorheological fluid (MR fluid) is a type of smart fluid in a carrier fluid,

usually a type of oil. When subjected to a magnetic field, the fluid greatly

increases its apparent viscosity, to the point of becoming a viscoelastic solid.

Importantly, the yield stress of the fluid when in its active ("on") state can be

controlled very accurately by varying the magnetic field intensity. The upshot of

which is that the fluid's ability to transmit force can be controlled with an

electromagnet, which gives rise to its many possible control-based applications.

WORKING PRINCIPLE OF MR FLUIDS

The magnetic particles, which are typically micrometer or nanometer scale

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spheres or ellipsoids, are suspended within the carrier oil are distributed

randomly and in suspension under normal circumstances, as below.

When a magnetic field is applied, however, the microscopic particles (usually in

the 0.1–10 μm range) align themselves along the lines of magnetic flux, see

below. When the fluid is contained between two poles (typically of separation

0.5–2 mm in the majority of devices), the resulting chains of particles restrict

the movement of the fluid, perpendicular to the direction of flux, effectively

increasing its viscosity. Importantly, mechanical properties of the fluid in its

“on” state are anisotropic. Thus in designing a Magnetorheological (or MR)

device, it is crucial to ensure that the lines of flux are perpendicular to the

direction of the motion to be restricted.

MECHANISMS AND PROPERTIES

The ER and MR effects are the result of the formation of structures within a

fluid in response to an electric or magnetic field. These structures, actually

aggregations of solid particles, dominate the flow of the fluid, and can prevent

flow entirely at lower stresses. In this section, we discuss these microscopic

phenomena and present basic models of the corresponding macroscale fluid

mechanics.

FLUID COMPOSITION AND BEHAVIOUR

Both ER and MR fluids are suspensions of particles in inert carrier liquids. The

particles, typically of the order of 1 to 10 µm in size, are added to fluids, such

as mineral oils or silicone oils, in weight fractions as large as 50%, with

fractions of around 30wt% being common. Most ER and MR fluids also contain

small amounts of additives that affect the polarization of the particles or

stabilize the structure of the suspension against settling, but for many

engineering purposes these may be neglected in modeling the fluids‟

mechanical response.

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In the absence of an external electric or magnetic field, and ER or MR fluid may

be characterized as Newtonian, i.e. as resisting shear strain γ with the shear

stress τ proportional to the product of the strain rate γ| and viscosity η :

τ = η γ| ------------------------ (Eqn. 1)

This response is represented by the line passing through the origin in Fig as

shown below, which shows shear stress as a function of strain rate.

This is widely acknowledged as an approximation – most ER and MR fluids are

non-Newtonian even when no field is applied because of their heavy loading of

solid particles and, to some extent, because of the additives they contain.

However in most applications the filed - induced component of the shear stress

is much larger than the η γ| term and equation 2.1 is an adequate model of

the rate-dependent part of the total shear stress.

The effect of an electric field E on an ER fluid, of a magnetic field H on an MR

fluid, is to cause the particles to form chains, or fibrils, in the direction of the

field. This process of fibration occurs in a few milliseconds after application of

the field .When there is no motion of the fluid or of the walls of its container,

the fibrils are static structures and span the gap between the walls if the

particle fraction is large enough.(This is one reason fluids with low particle

fractions exhibit weak ER and MR effects).In an MR fluid, the formation of

particle chains occurs when the magnetically polarizable particles move into

alignment with the applied filed and are then drawn together like magnets

whose opposite poles attract the adjacent particles in the chain. The formation

of fibrils in an ER fluid in response to an external electric field happens in a

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similar way, but the chemistry underlying the electrical polarization of the

particles has been the subject of much research attention.

It now appears that ER fluids are divisible into two classes depending on the

mechanism by which particle polarization and interaction occurs. One type of

ER fluid requires the presence of some amount of water in order to manifest an

Electrorheological response; other, anhydrous ER fluids contain no water, and

particle chain formation is thought to occur in them by a different mechanism

(Weiss, Carlson, and Coulter).For our purposes, it is unimportant to

distinguish which electrical polarization mechanism is at work in a given ER

fluid, but we note in passing that this contributes to the sensitivity of ER fluids

to water contamination

MR fluids differ from conventional “magnetic fluids”, which contain particles

of much smaller size, typically of the order of 10nm.The effect of Brownian

motion is greater at this scale, and prevents the particles from forming fibrils in

the presence of a magnetic field. The magnetic fluid instead experiences a body

force proportional to the magnetic field gradient, and may flow in response to

this force. This behavior is exploited, for example, in sealing applications;

however, here we are concerned only with the MR and ER effects exhibited by

fluids containing larger particles.

The ER and MR shear stress increases with increasing field strength, and is

typically proportional to the field strength raised to the power between 1 and

2.The upper limit on the induced shear stress occurs when an MR fluid

reaches magnetic saturation or when an ER fluid breaks down electrically,

typically at field strengths of around 250 A/mm or 4kV/mm, respectively.

Discovery and Early developments,

Currently, there is no universally accepted theory for the causes of the MR

effect. However, it is generally agreed that the change in properties is due to

the realignment of particles in the fluid to form fibrils, or long strands of

suspended particles, that resist shear (Fig. 1). It is believed that the observed

alignment of particles is related to the displacements and torque produced in

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the medium by the field and the translational motion and relocation of particles

to positions that have local minimum potential energy. This results in an

increase in viscosity and an increase in the shear strength of the material. A

Bingham plastic model is often considered sufficient to model MR devices:

τtotal = τy (H) + ηp ẙ , (1)

Where τ total is the total shear stress of the material, τ y is the yield stress as a

function of the magnetic field, H is the magnetic field strength, ηp is the plastic

viscosity or post yield viscosity, and ˙ γ is the shear strain rate in the fluid.

Apparent viscosity ηa is defined as the total shear stress divided by the shear

rate:

ηa = τtotal / ẙ,

Figure 1. Suspended magnetisable particles align in a magnetic field. The MR

fluid will then resist a certain amount of shear before the chains begin to

break. This yield point increases as magnetic field strength increases until the

magnetic saturation point is reached. These results in the solid feel of MR

materials.

THE BINGHAM PLASTIC AND RELATED MODELS

One might expect that the formation of fibrils with in an ER or MR fluid

would increase the fluid viscosity, but infect the slope of the shear stress

versus shear strain rate curve and thus the viscosity, changes little if at all.

The effect of fibrils is instead to produce a shear stress that is largely

independent of the strain rate; this is commonly referred to as the yield stress

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and denoted τy. Adding this term to the Newtonian model results in the

Bingham plastic model which has the stress-strain rate relation

τ= τy (F) + η γ| ------------------------ (Eqn.2)

Where in a given application F is the strength of the applied electric or

magnetic field (i.e. E or H).The response predicted by this model is plotted in

fig.2 which depicts the strong dependence of the yield stress on the field

strength.

Fig: Shear stress versus shear strain rate.

This model, or the extensions of it that predict similar overall response, is by

far the most popular for use in the design of devices that depend on the post

yield shear resistance of an ER or an MR fluid.

In practice, the dynamic viscosity (the lope of τ vs. γ| curve) is determined by a

linear regression fit of a line to experimental data, and the intersection of this

line with the shear stress axis is taken as the value of the yield tress τy.

Although this is a good approximation at higher strain rates and is entirely

adequate for most dynamic response calculations, the data measures at small

train rates can depart from this idealization. Initiating motion or flow requires

overcoming a static yield stress τy.s which is often larger than the dynamic

yield stress τy.d, but the measured stress quickly falls into its dynamic value

as shown by the dashed path in the fig 3 below. Once τ has reached its

dynamic value, it tends to flow the fitted st.line towards τy and γ| decreases.

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More detailed models than the Bingham plastic of equation 2 can of course be

devised and fitted to experimental data. It may be necessary, for instance, to

capture the dynamics associated with the momentum of the fluid, or to

represent the finite compliance of the container. An example of this extension

of the Bingham model in the case of an MR fluid damper is presented below.

PRE-YIELD RESPONSE

According to the Bingham Plastic model, stress less than the yield stress τy

produces no flow of the ER or MR fluid; but in reality the fluid naturally

responds to stress in this range, and for many purposes it may be regarded as

viscoelastic solid. The figure4 below shows typical stress-strain characteristics

for an ER or MR fluid loaded upto and beyond yield. Note that yield occurs at

approximately the same strain γy regardless of field strength, while τy increases

with F as discussed above. For clarity we have shown the yield strain as

corresponding to the peak stress on each curve, but in practice the correct

definition of yield for these materials is not so clear.

The shear stiffness of viscoelastic solids, including unyielded ER and MR fluids,

is often represented by the complex shear modulus G*---------

G* = G' + jG''

The real part G' is called the storage modulus and measures the materials

ability to elastically store strain energy, while the imaginary part G'' is termed

the loss modulus and is associated with the dissipation of energy during

deformation.

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The Loss factor is then defined as the ratio of the loss modulus to the storage

modulus i.e.

Loss Factor = G''/G'

The loss factor can be determined by measuring the phase difference between a

strain wave input to a material and the resulting stress wave. A predominantly

Elastic material will exhibit a small phase difference and a very small Loss

factor, typically less than 0.1

In a viscous material the phase difference will approach 90o and the

corresponding Loss Factor will be quite large.

POST-YIELD FLOW AND DEVICE GEOMETRY

In many devices where ER or MR fluids are employed, at any time a small

portion of the fluid is subjected to the applied electric or magnetic field, while

the remainder is free to flow as a conventional, low-viscosity fluid. Ordinarily

the field is created across a small gap whose surfaces serve as both

electrodes(in case of an ER fluid) or pole pieces (in case of an MR fluid)and as

the walls of a channel confining the fluid. For purposes of analysis, these walls

are often modeled as parallel, flat plates; this is the only configuration we shall

consider in detail here. The resulting equations are frequently applied to

annular or other non-flat geometries with acceptable results, but more realistic

models are available, e.g., for cylindrical dampers.

Restricting consideration to a gap formed by two parallel flat plates, we

must still take account of how shear is created in the fluid. The two

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possibilities are illustrated in the figure below(Fig 5), where it is shown that

shear may be produced in the fluid either by forcing it through the gap under

pressure(fixed-plate configuration) or by moving one plate with respect to the

other (sliding-plate configuration).In either case, and for either type of fluid ,the

gap is small in the direction of the field ,often well under 1mm,and of the order

of millimetres in length in the direction of flow or motion. This close spacing of

the plates is required in order to produce a field strong enough to activate the

fluid ;the ER and MR effects, that is, the formation of fibrils in the direction of

the applied field, will occur over a great distance within the fluid if an adequate

field can be generated.

MATERIAL BEHAVIOR/PROPERTIES

To understand and predict the behavior of the MR fluid it is necessary to model

the fluid mathematically, a task slightly complicated by the varying material

properties (such as yield stress).

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As mentioned above, smart fluids are such that they have a low viscosity in the

absence of an applied magnetic field, but become quasi-solid with the

application of such a field. In the case of MR fluids (and ER), the fluid actually

assumes properties comparable to a solid when in the activated ("on") state, up

until a point of yield (the shear stress above which shearing occurs). This yield

stress (commonly referred to as apparent yield stress) is dependent on the

magnetic field applied to the fluid, but will reach a maximum point after which

increases in magnetic flux density have no further effect, as the fluid is then

magnetically saturated.

SHEAR STRENGTH

Low shear strength has been the primary reason for limited range of

applications. In the absence of external pressure the maximum shear strength

is about 100 KPa. If the fluid is compressed in the magnetic field direction and

the compressive stress is 2 MPa, the shear strength is raised to 1100 KPa. If

the standard magnetic particles are replaced with elongated magnetic particles,

the shear strength is also improved.

PARTICLE SEDIMENTATION

Ferro-particles settle out of the suspension over time due to the inherent

density difference between the particles and their carrier fluid. The rate and

degree to which this occurs is one of the primary attributes considered in

industry when implementing or designing an MR device. Surfactants are

typically used to offset this effect, but at a cost of the fluid's magnetic

saturation, and thus the maximum yield stress

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MR FLUID SURFACTANTS

MR fluids often contain surfactants including, but not limited to

a) Oleic acid

b) Tetra methyl ammonium hydroxide

c) Citric acid

d) Soy lecithin

These surfactants serve to decrease the rate of ferroparticles settling, of which

a high rate is an unfavourable characteristic of MR fluids. The ideal MR fluid

would never settle, but developing this ideal fluid is as highly improbable as

developing a perpetual motion machine according to our current

understanding of the laws of physics. Surfactant-aided prolonged settling is

typically achieved in one of two ways: by addition of surfactants, and by

addition of spherical ferromagnetic nanoparticles. Addition of the nanoparticles

results in the larger particles staying suspended longer since to the non-

settling nanoparticles interfere with the settling of the larger micrometre-scale

particles due to Brownian motion. Addition of a surfactant allows micelles to

form around the ferroparticles. A surfactant has a polar head and non-polar

tail (or vice versa), one of which adsorbs to a nanoparticles, while the non-polar

tail (or polar head) sticks out into the carrier medium, forming an inverse or

regular micelle, respectively, around the particle. This increases the effective

particle diameter. Steric repulsion then prevents heavy agglomeration of the

particles in their settled state, which makes fluid remixing (particle

redispersion) occur far faster and with less effort. For example,

Magnetorheological dampers will remix within one cycle with a surfactant

additive, but are nearly impossible to remix without them.

While surfactants are useful in prolonging the settling rate in MR fluids, they

also prove detrimental to the fluid's magnetic properties (specifically, the

magnetic saturation), which is commonly a parameter which users wish to

maximize in order to increase the maximum apparent yield stress. Whether the

anti-settling additive is nanospheres based or surfactant-based, their addition

decreases the packing density of the ferroparticles while in its activated state,

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thus decreasing the fluids on-state/activated viscosity, resulting in a "softer"

activated fluid with a lower maximum apparent yield stress. While the on-state

viscosity (the "hardness" of the activated fluid) is also a primary concern for

many MR fluid applications, it is a primary fluid property for the majority of

their commercial and industrial applications and therefore a compromise must

be met when considering on-state viscosity, maximum apparent yields stress,

and settling rate of an MR fluid.

MODES OF OPERATION AND APPLICATION

An MR fluid is used in one of three main modes of operation, these being flow

mode, shear mode and squeeze-flow mode. These modes involve, respectively,

fluid flowing as a result of pressure gradient between two stationary plates;

fluid between two plates moving relative to one another; and fluid between two

plates moving in the direction perpendicular to their planes. In all cases the

magnetic field is perpendicular to the planes of the plates, so as to restrict fluid

in the direction parallel to the plates.

Flow mode

Shear Mode

Squeeze-Flow Mode

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The applications of these various models are numerous. Flow mode can be

used in dampers and shock absorbers, by using the movement to be controlled

to force the fluid through channels, across

Which a magnetic field is applied. Shear mode is particularly useful in clutches

and brakes in places where rotational motion must be controlled. Squeeze-flow

mode, on the other hand, is most suitable for applications controlling small,

millimetre-order movements but involving large forces. This particular flow

mode has seen the least investigation so far. Overall, between these three

modes of operation, MR fluids can be applied successfully to a wide range of

applications. However, some limitations exist which are necessary to mention

here.

LIMITATIONS

Although smart fluids are rightly seen as having many potential applications,

they are limited in commercial feasibility for the following reasons:

a) High density, due to presence of iron, makes them heavy. However,

operating volumes are small, so while this is a problem, it is not

insurmountable.

b) High-quality fluids are expensive.

c) Fluids are subject to thickening after prolonged use and need replacing.

d) Settling of Ferro-particles can be a problem for some applications.

Commercial applications do exist, as mentioned, but will continue to be few

until these problems (particularly cost) are overcome.

APPLICATION OF MR AND ER FLUIDS

Because the state of MR materials can be controlled by the strength of an

applied magnetic field, it is useful in applications where variable performance

is desired. Microprocessors, sensor technologies and increasing electronic

content and processing speeds have created real-time control possibilities of

smart systems used MR devices. Beginning of the commercialization of MR

technology was year 1995 and use of rotary brakes in aerobic exercise

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equipment. From this moment application of Magnetorheological material

technology in real-world systems has grown steadily. During the past few years

a number of commercially available products (or near commercialization) have

been developed, e.g.

a) Linear MR dampers for real-time active vibration control systems in heavy

duty trucks.

b) Linear and rotary brakes for low-cost, accurate, positional and velocity

control of pneumatic actuator systems.

c) Rotary brakes to provide tactile force-feedback in steer-by wire systems.

d) Linear dampers for real-time gait control in advanced prosthetic devices.

e) Adjustable real-time controlled shock absorbers for automobiles.

f) MR sponge dampers for washing machines.

g) Magnetorheological fluid polishing tools.

h) Very large MR fluid dampers for seismic damage mitigation in civil

engineering structures.

i) Large MR fluid dampers to control wind-induced vibrations in cable-stayed

bridges.

OR

IN CLUTCH

MR clutch similar to MR brake operates in a direct-shear mode and transfers

torque between input and output shaft. There are two main types

constructions of MR clutch: cylindrical and frontal. In the cylindrical model MR

fluid works between two cylindrical surfaces and in frontal MR fluid fills gap

between two discs. During work magnetic field produced by coils increases

viscosity of fluid and causes transfer of torque form input to output shaft.

Useful torque is available after 2-3 milliseconds from stimulation.

IN DAMPERS

Magnetorheological dampers of various applications have been and continue to

be developed. These dampers are mainly used in heavy industry with

applications such as heavy motor damping, operator seat/cab damping in

construction vehicles, and more. As of 2006, materials scientists and

mechanical engineers are collaborating to develop standalone seismic dampers

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which, when positioned anywhere within a building, will operate within the

building's resonance frequency, absorbing detrimental shock waves and

oscillations within the structure, giving these dampers the ability to make any

building earthquake-proof, or at least earthquake-resistant.

There are several ways of using smart materials in an active system that can

sense and respond to ground vibrations. Perhaps unsurprisingly, vibration-

damping systems for buildings have been explored most extensively in Japan.

This has a vibration-reduction system that also reduces wind induced sway,

which can be pronounced in Osaka from time to time. Without a system of that

sort, the occupants of the building might find themselves experiencing feelings

of sea-sickness during high winds. The building uses a vibration control

system called DUOX, which is able to quickly damp out oscillations several

centimetres in amplitude at the top floor.

Vibration-control systems show that for this kind of sophisticated and adaptive

control of a structure, we need more than just a „smart‟ response to a

stimulus. In general, we need some kind of feedback so that the response can

be continuously adjusted to the stimulus. And often the sensing of the

stimulus and the production of the response might be carried out by separate

entities – perhaps by two different smart materials. For example, to control

vibrations we need a system that senses movement, coupled to a system that

adjusts its mechanical properties to counteract that movement.

MR dampers are semi-active devices that contain Magnetorheological fluids.

After application of a magnetic field the fluid changes from liquid to semi-solid

state in few milliseconds, so the result is an infinitely variable, controllable

damper capable of large damping forces. MR dampers offer an attractive

solution to energy absorption in mechanical systems and structures and can

be considered as “fail-safe” device.

IN BRAKE

The MR brake operates in a direct-shear mode, shearing the MR fluid filling the

gap between the two surfaces (housing and rotor) moving with respect to one

another. Rotor is fixed to the shaft, which is placed in bearings and can rotate

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in relation to housing. Resistance torque in the MR brake depends on viscosity

of the MR fluid that can be changed by magnetic field.

MR brake allows for continuous control of torque. When there is no magnetic

field the torque is caused by viscosity of carrier liquid, bearings and seals. MR

brake is especially well suited for a variety of applications including pneumatic

actuator control, precision tension control and Haptic force feedback in

applications such as steer-by-wire.

AUTOMOTIVE AND AEROSPACE

If the shock absorbers of a vehicle's suspension are filled with MR fluid instead

of plain oil, and the whole device surrounded with an electromagnet, the

viscosity of the fluid (and hence the amount of damping provided by the shock

absorber) can be varied depending on driver preference or the weight being

carried by the vehicle - or it may be dynamically varied in order to provide

stability control. This is in effect a Magnetorheological damper. The MagneRide

magnetic ride control (a kind of active suspension) is one such system which

permits the damping factor to be adjusted once every millisecond in response

to conditions. GM is the origin company of this technology as applied to

automobiles. Many other companies have paid for the use of it in their own

vehicles. As of 2007, BMW manufactures cars using their own proprietary

version of this device, while Audi and Ferrari offer the MagneRide on various

models. All Corvettes made since 2005 have also employed a dynamic MR

suspension system.

General Motors and other automotive companies are seeking to develop a MR

fluid based clutch system for push-button four wheel drive systems. This

clutch system would use electromagnets to solidify the fluid which would lock

the driveshaft into the drive train.

Magnetorheological dampers for use in military and commercial helicopter

cockpit seats, as safety devices in the event of a crash, are under development.

This decreases the shock delivered to each passenger's spinal column thereby

decreasing the rate of permanent injury during a crash.

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Porsche has introduced Magnetorheological engine mounts in the 2010

Porsche GT3 and GT2. At high engine revolutions, the Magnetorheological

engine mounts get stiffer to provide a more precise gearbox shifter feel by

reducing the relative motion between the power train and chassis/body.

HUMAN PROSTHESIS

Magnetorheological dampers are utilized in semi-active human prosthetic legs.

Much like those used in military and commercial helicopters, a damper in the

prosthetic leg decreases the shock delivered to the patients‟ leg when jumping,

for example. This results in an increased mobility and agility for the patient.

MILITARY AND DEFENSE

The U.S. Army Research Office is currently funding research into using MR

fluid to enhance body armour. In 2003, researchers stated they were five to ten

years away from making the fluid bullet resistant. In addition, Humvees,

certain helicopters, and various other all-terrain vehicles employ dynamic MR

shock absorbers and/or dampers.

OPTICS

Magnetorheological Finishing, a Magnetorheological fluid-based optical

polishing method, has proven to be highly precise. It was used in the

construction of the Hubble Space Telescope's corrective lens.

MECHANISM FOR NOISE REDUCTION

For example, in a car or an aircraft. The “smart structure” that does this task

might involve a smart material that senses changes in air pressure, due to

sound waves, and converts that stimulus to an electrical signal that can be

used to drive a loudspeaker to broadcast “antinoise” sound waves that cancel

out the noise. Both parts of the system can use piezoelectric materials, which

are smart materials that interconvert mechanical and electrical energy.

WORKING PRINCIPLE OF MR FLUID TECHNOLOGY

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The MR fluid is a smart fluid whose properties can be controlled in the

presence of magnetic field. In the absence of magnetic field, the rheological

properties of the MR fluid are similar to that of base fluid except that it is

slightly thicker due to the presence of metal particles.

Figure1 Principle of MRF Technology

In the absence of magnetic field, these metal particles align themselves along

the direction of flow (figure 2(a)) however when a magnetic field is applied each

metal particles becomes a dipole aligning itself along the direction of magnetic

field (Figure 2(a) and (b)). Thus a chain like structure is formed along the line of

magnetic flux which offers mechanical resistance to the flow resulting in an

increase in the viscosity of fluid .This mechanical resistance created due to the

chain column imparts yield strength to the fluid, making it stiff like a semi-

solid. This stiffness and hence the yield strength depends on the strength of

the magnetic field and also the quality and quantity of metal particles. The MR

effect is reversible. When the magnetic field is removed the fluid returns to its

original condition. The MR fluids with their controllable properties are found to

be useful in the implementation of smart fluid concept. Where the fluid motion

is controlled by varying its viscosity with the help of magnetization .The

suppleness of MR fluid technology, the controllability and the quick response of

the rheological properties makes it a smart fluid with application areas where

fluid motion is controlled by varying the viscosity.

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Figure 2: Working of MRF Technology

DESIGN OF MAGNETIC CIRCUITS

Fundamental Equations

Circuit laws similar to those of electric circuits apply in magnetic circuits as

well.

That is, a magnetic circuit can be replaced by an equivalent electric circuit for

Ohm‟s Law to be applied. If the magneto motive force of a magnet is F and the

total magnetic flux is Φt, and assuming the magnetic resistance (reluctance) of

the circuit is R, then the following equation is valid.

Assuming the vacant length of the circuit as ℓg and the vacant cross-sectional

area as ag, the magnetic resistance is then given by the following equation.

μ is the magnetic permeability of the magnetic path and is equivalent to the

magnetic permeability

μ0 of a vacuum in the case of air. (μ0=4πX10-7 [H/m])

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Although the current in an electric circuit rarely leaks outside the circuit, as

the deference in the magnetic permeability between the conductor yoke and

insulated area in a magnetic circuit is not very large, leakage of the magnetic

flux also becomes large in reality. The amount of the magnetic flux leakage is

expressed by the leakage factor σ, which is the ratio of the total magnetic flux

Φt generated in the magnetic circuit to the effective magnetic flux Φg of the

vacant space.

In addition, the loss in the magnetic flux due to the joints in the magnetic

circuit must also be taken into consideration. This is represented by the

reluctance factor f. Since the leakage factor σ is equivalent to the increase in

the vacant space area, and the reluctance factor f refers to the correction

coefficient of the vacant space length, the corrected magnetic resistance

becomes as follows.

The inverse of this magnetic resistance is known as permeance (P) and

generally, this permeance is used in the calculations. Substituting this in

Equation (1), P then becomes as follows.

If you assume the cross-sectional area of the magnet as am, the length as ℓm,

the demagnetized field in the magnet as Hd, the magnetic flux density as Bd,

and the magnetic flux density in the magnet to be uniform, then F and Φt are

expressed as follows.

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Based on Equation (6), the permeance coefficient is determined by the following

equation.

Substituting Equation (5) into this equation, the permeance coefficient becomes

as follows.

Therefore, the external permeance as seen from the magnet can also be said to

be the permeance coefficient of the magnet when converted to a per unit

volume figure. The above equation shall serve as the basic equation for

determining the permeance. Although the reluctance factor f is approximately

1.1 ‒ 1.3 and no big error will result if a normal value of 1.2 is assumed, the

leakage factor σ has to be determined based on calculation since it will

fluctuate to a certain extent. Based on Equation (3), the leakage factor σ is

determined as follows.

Since Ft/Fg is equivalent to the reluctance factor here, σ then becomes

Since Pt is the sum of the vacant space permeance and the leakage permeance,

it therefore becomes

Although Pg can be computed easily as Pg=μag/ℓg, as the leakage permeance is

quite complex, the respective terms are generally simplified and computed as

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shown in Figure 8. The respective permeance is determined in this manner to

compute σ.

Vibration Dampers

Damping – is the ability of a vibrating system or structure to dissipate energy.

Mostly – mechanical energy is converted to heat energy

When the dissipation is by internal friction or hysteresis characteristics due to

its molecular structure – material damping

When it is generated by the friction, snapping, rubbing, slapping or impacting

at the joints and interfaces of structural assemblies, - it is structural damping

Damping reduces dynamic load

CONVENTIONAL DAMPER

In conventional dampers, fluid flows from the inner cylinder to outer

cylinder through the foot valves. In outer cylinder there is air and fluid. As

piston moves down, the fluid level in outer cylinder increases and free air

behaves as a compressed medium and produces damping effect .When the

piston expands, direction of flow gets reverse.

MRF DAMPER

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MR dampers are slightly different from the conventional dampers .They

don‟t have valves in the piston as piston of conventional dampers.MR damper

shown in the figure is used in the suspension of highway vehicles. In this type

of damper there is an annular orifice passage, through which MR fluid can be

transmitted from one chamber to another. In non magnetized condition, fluid

can move through the orifice but when suspension is required, the coil is

energized and current starts to flow through the coils, and develops a magnetic

field. Due to the effect of this magnetic field, fluid in the orifice behaves like a

semi solid and offers resistance to the fluid flow. Thus the fluid in the chamber

starts to behave like a shock absorbing medium. These types of dampers are

used for seat vibration control in vehicles. This technology is quicker, effective

and requires less maintenance.

These MRF applications bring additional functionality at the same time

as keeping the simplicity. Other possible MRF applications using this mode are

dampers for knee prosthesis, vibration dampers, seismic dampers for civil

industry, active engine mounts and prop shaft mounts.

Using this simple mechanical principle the damping arrangement

becomes controllable and the vibration transmission and excitation frequency

for a suspended seat can be adjusted accordingly. Proper choice of MRF

parameters extended to seat suspension could eliminate any resonance

problems and allow the system to be isolated from high frequency problems.

SMART MATERIALS


ER CLUTCH

WORKING: These materials in the fluid responds to applied magnetic

fields and is thus referred to as magneto rheological materials. Such materials

can be utilized in devices or can be incorporated in traditional composites to

form advanced intelligent composite structures, whose continuum magneto-

rheological response can be actively controlled in real time. Applications that

can be benefit from materials whose rheology can be continuously, rapidly and

reversibly varied are numerous.

The most common MR materials are of liquid state. The controllable

rheological response of such fluids results from the polarization induced in the

suspended particles by application of an external magnetic field. The

interaction between the resulting induced dipoles causes the particles to form

columnar structures, parallel to the applied field.

These chains like structure restrict the flow of the fluid, thereby

increasing the viscous characteristics of the suspension. The mechanical

energy needed to yield these chains like structures increase as the applied

magnetic field increase resulting in a field dependent yield stress. In the

absence of an applied field, the controllable fluids exhibit Newtonian like

behaviour. MR fluid is filled in the casing and the fluid is there between the

disc and the aligned electro magnets.

The electro magnets are arranged so that the field of the magnet is

directly focussed towards the fluid area. The electro magnet in the assembly is

worked by using a 12 volt battery.

MR fluid is composed of particles suspended in a carrier fluid such as oil.

When a magnetic field is applied the magnetic particles will align and form

SMART MATERIALS


chains and solid like structures within the fluid. During engaged phase MR

fluid is excited by passing current through the electromagnetic from a battery.

Magnetic field will be generated in the region of MR fluid. This causes

suspended magnetically polarisable particles in MR fluid to become magnetic

dipoles and attract each other and align according to the magnetic field. This

results in an immediate stiffening of the MR material and helps in transferring

torque.

OPTICS AND ELECTROMAGNETIC INTRODUCTION:

Everywhere on this planet hair-thin optical fibers carry vast quantities of

information from place to place. There are many desirable properties of optical

fibers for carrying this information. They have enormous information-carrying

capacity, are low cost, and have protection from the many disturbances that

can afflict electrical wires and wireless communication links. The superiority of

optical fibers for carrying information from place to place is leading to their

rapidly replacing older technologies. Optical fibers have played a key role in

making possible the extraordinary growth in world-wide communications that

has occurred in the last 25 years, and are vital in enabling the proliferating use

of the Internet.

Of key importance in the course of these developments in information

technology has been a few basic, but vitally significant, events. Principal among

these are the invention and development of the laser, the growing appreciation

that this might make optical communications practically useful, the production

of very pure glass, which was sufficiently transparent that long distance

transmission of light through glass fibers became practical, and the digital

revolution. We will examine the role that each of these has played in creating

the "Information Age."

All early light wave communication systems had two major problems

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1. The transmission path needed an unobstructed line of sight between

transmitters and receivers. This is because light cannot go around corners

by itself.

2. The transmission path was affected by weather conditions.

These problems were overcome by developing waveguides. The early

designs were expensive and could not compete with electrons travelling

through copper wire

OPTICAL FIBER SENSOR

A sensor that measures a physical quantity based on its modulation on the

intensity, spectrum, phase, or polarization of light travelling through an optical

fiber. An optical sensor is a device that converts light rays into electronic

signals. Similar to a photo resistor, it measures the physical quantity of light

and translates it into a form read by the instrument. Optical sensors have a

variety of uses. They can be found in everything from computers to motion

detectors.

For example, when the door to a completely darkened area such as the

inside of a copy machine is opened, light impacts the sensor, causing an

increase in electrical productivity. This will trigger an electric response and

stop the machine for safety.

Fiber optic sensors are also resistant to electromagnetic interference, and

do not conduct electricity so they can be used in places where there is high

voltage electricity or flammable material such as jet fuel. Fiber optic sensors

can be designed to withstand high temperatures as well.

Fiber optic sensors can also be configured to measure the internal chemical

states in structures, such as the penetration of corrosion-causing de-icing salts

in bridge decks.

https://en.wikipedia.org/wiki/Electromagnetic_interference

https://en.wikipedia.org/wiki/High_voltage



https://en.wikipedia.org/wiki/Jet_fuel

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CLASSIFICATION OF OPTICAL FIBER SENSOR

TYPES OF OPTICAL FIBER SENSOR

Distinction is often made in the case of fiber sensors as to whether measurand

act externally or internally to the fiber.

A. Intrinsic

In an intrinsic sensor, the fiber itself is the sensing element (the fiber is

directly affected by the measurand). Where the sensors are embedded in or are

part of the fiber and for this type there is often some modification to the fiber

itself. The sensors are termed internal or intrinsic sensors.

Optical fibers can be used as sensors to measure strain,

temperature, pressure and other quantities by modifying a fiber so that the

quantity to be measured modulates the wavelength or transit time of light in

the fiber. Sensors that vary the intensity of light are the simplest, since only a

simple source and detector are required. A particularly useful feature of

intrinsic fiber optic sensors is that they can, if required, provide distributed

sensing over very large distances.

https://en.wikipedia.org/wiki/Strain_(materials_science)

https://en.wikipedia.org/wiki/Temperature

https://en.wikipedia.org/wiki/Pressure

https://en.wikipedia.org/wiki/Wavelength

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Special fibers like long-period fiber grating (LPG) optical fibers can be used for

direction recognition. Photonics Research Group of Aston University in UK has

some publications on vectorially bend sensor applications.

Optical fibers are used as hydrophones for seismic and sonar applications.

A fiber optic microphone and fiber-optic based headphone are useful in areas

with strong electrical or magnetic fields, such as communication amongst the

team of people working on a patient inside a magnetic resonance imaging (MRI)

machine during MRI-guided surgery.

https://en.wikipedia.org/wiki/Long-period_fiber_grating

https://en.wikipedia.org/wiki/Aston_University

https://en.wikipedia.org/wiki/Hydrophone

https://en.wikipedia.org/wiki/Sonar

https://en.wikipedia.org/wiki/Microphone#Fiber_optic_microphone

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Properties of intensity based sensors

• Versatile

• Simple design and easy signal interpretation

• Usually suffer from intensity fluctuations and low sensitivity

B. Extrinsic

Where the transducers are external to the fiber and the fiber merely

registers and transmits the sensed quantity, the sensors are termed extrinsic

sensors.

In an extrinsic sensor, the fiber simply transports light to or from the sensing

element. In this, the fiber carries the light from the source and to the detector,

but the modulation occurs outside the fiber transducer acts as fiber.

Extrinsic fiber optic sensors use an optical fiber cable, normally

a multimode one, to transmit modulated light from either a non-fiber optical

sensor, or an electronic sensor connected to an optical transmitter.

A major benefit of extrinsic sensors is their ability to reach places which are

otherwise inaccessible. An example is the measurement of temperature

inside aircraft jet engines by using a fiber to transmit radiation into a

radiation pyrometer located outside the engine. Extrinsic sensors can also be

used in the same way to measure the internal temperature of electrical

transformers, where the extreme electromagnetic fields present make other

measurement techniques impossible.

Extrinsic fiber optic sensors provide excellent protection of measurement

signals against noise corruption. Extrinsic sensors are used to measure

vibration, rotation, displacement, velocity, acceleration, torque, and

temperature.

https://en.wikipedia.org/wiki/Optical_fiber_cable

https://en.wikipedia.org/wiki/Multi-mode_optical_fiber

https://en.wikipedia.org/wiki/Modulation

https://en.wikipedia.org/wiki/Aircraft

https://en.wikipedia.org/wiki/Jet_engine

https://en.wikipedia.org/wiki/Radiation

https://en.wikipedia.org/wiki/Pyrometer

https://en.wikipedia.org/wiki/Electrical_transformer



https://en.wikipedia.org/wiki/Electromagnetic_field

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COMPARISON OF EXTRINSIC & INTRINSIC SENSOR

A. Extrinsic

• Applications- temperature, pressure,

liquid level and flow.

• Less sensitive

• Easily multiplexed

• Ingress/ egress connection problems

• Easier to use

• Less expensive

B. Intrinsic

• Applications-rotation, acceleration,

strain, acoustic pressure and

vibration.

• More sensitive

• Tougher to multiplex

• Reduces connection problems

• More elaborate signal demodulation

• More expensive

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THE PHYSICAL PHENOMENA

Total Internal Reflection

The basic principle governing the operation of fiber optics in applications is

total internal reflection, which can occur when light waves travelling in one

medium encounter another medium. An example is light travelling in air

encountering a surface of water. Because the refractive indices for water and

air are different, the light waves behave differently at the interface.

Fig. Total internal reflection

Consider medium 1 with a refractive index 1n . Light waves entering medium 1

encounter another medium of refractive index 12 nn , as shown in Fig. Note how

a ray of light with an incidence angle 1 experiences refraction with an angle of

refraction 2 . As the angle 1 continues to increase, the angle 2 also increases

until 902 for c 1 where c is defined as critical angle of incidence.

For c 1 the ray of light is reflected back entirely into medium 1, which

represents the condition for total internal reflection with no loss in

transmission. The ray numbered 5 in the figure shows this condition and

behaves as if it has struck a perfectly reflecting surface. Such rays obey the law

of reflection so that the angles on incidence and reflection are equal. They also

obey the law of refraction or Snell‟s law, which states that a ray undergoes

refraction when it propagates from one medium to another such that the ratio

of the sine of the incident angle and the sine of the refracted angle is equal to

the ratio of the reciprocal of refractive indices of the media, that is, 1

2

2

1sin

sinn

n .

Thus, the critical incidence angle c may be calculated from 21 sin nn c and,

therefore, 1

2sin nn

c for 12 nn ,which suggests that total internal reflection can

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occur only when light waves traveling in a medium with a certain index attempt

to move into another medium with a lower refractive index.

As stated earlier, total internal reflection occurs when c 1 . Clearly, rays of

light propagating from a dense medium through successive layers of less dense

media will continue to bend and experience total internal reflection. The

principle of total internal reflection can be used to “pipe” light from one location

to another. In a glass rod or fiber, light entering the glass medium at one

location is trapped as it were, as shown in Fig. and with successive reflections

continues to travel through the fiber entirely confined.

Fig. Light rays “trapped” inside a fiber

This physical phenomenon serves as the foundation for wide variety of

engineering uses of fiber optics. An optical fiber can be viewed as l light guide

obeying Snell‟s law. The fiber has a core through which light waves can

propagate by total internal reflection from a cladding of lower refractive index.

FIBER CHARACTERISTICS

Optical fibers are usually made of Glass2SiO (Sapphire, Fluoride Glasses

and Neodymium doped Silica are utilized for specialized applications.), a high-

index material, mixed with various Dopants to control the refractive index. The

core is surrounded by cladding material, which is also glass of slightly lower

refractive index. The difference can be as small as 0.001 or 0.002. Fibers in

which refractive index is uniform across the core thickness, as shown in Fig,

are known as step- index fibers. In step-index fibers, rays travelling close to the

longitudinal axis of fibers traverse a shorter distance than those at an angle to

the axis. Consequently, the travel times for these rays are different, leading to

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light dispersion. However, if the core is made of glass with refractive indices

varying smoothly across the core diameter as shown in Fig. then the travel

distances can be controlled in such a way that the light dispersion is

minimized.

Although the discussion above introduces the concepts of light transmission

through optical fibers, it is somewhat incomplete because it does not include

oscillations along the other principal directions. In fact, propagation of light

along the optic fibers can be looked upon as electromagnetic waves with a wide

variety of modes. Thus it should be noted that even with an optimum grading

of refractive index through the thickness, fibers that are truly multimode

exhibit inevitable dispersion.

However, with drastic reduction in core thickness, one can reduce the

number of modes that can be physically supported by the fiber. For example, a

core with a 10µm diameter is only about four wavelengths across and therefore

cannot allow any more than a single mode, thus avoiding the dispersion

problem.

Fig. Index profiles for step-index and graded-index fibers.

The Cores of Optical fibers are made of extremely low-impurity silica and the

cladding is typically fluorine-doped glass 125µm in diameter. In view of the

lower specific gravity of silica compared to that of copper(2.2 vs.8.9), the

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diameter of an optical fiber can be less than tenth of diameter of a

corresponding copper wire to convey the same information. The consequent

weight reduction, for example, in a large aircraft communication system can be

a saving of 1000lb (see Lacy, 1992). In addition, fiber optics possesses an

enormous capacity to carry information because of a vast bandwidth, which is

an incredible 50THz

Typical single mode fibers have core diameter of about 5µm, which compares

favourably with 1 to 3µm diameter for structural fibers. However, multimode

fibers are much large in size and may be 100µm to 200µm in diameter.

Ultimate tensile strengths of the order for 106 psi have been measured for short

test sections of freshly drawn glass fibers. A slight degradation may occur in

the manufacturing process leading to a compromised value of about of 800000

psi. With a Young‟s modulus of 10X106 psi, which is close to that of aluminum,

and the ability to survive strains of the order 8%, the applicability of optical

fibers for strain-sensing becomes evident. With appropriate adhesives, fiber-

optic strain sensors can have long life-much longer than that of conventional

strain gages.

With proper selection of adhesives and coatings, optical fibers are reported

to have sustained one million cycles in a simulated graphite-reinforced

helicopter tail subjected to delamination tests. Polyamides that have a high

temperature/high modulus characteristic have been found to be ideal in

providing a thin, hard coating and the increase in fiber diameter because of the

coating can be mere 10µm .Polyamides also known to mechanical degradation

(Dunphy, Meltz and Morey, 1995).

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BASIC FIBER OPTIC COMMUNICATION SYSTEM

Figure. Basic fiber optic communication system

Fiber optics is a medium for carrying information from one point to another in

the form of light. Unlike the copper form of transmission, fiber optics is not

electrical in nature. A basic fiber optic system consists of a transmitting device

that converts an electrical signal into a light signal, an optical fiber cable that

carries the light, and a receiver that accepts the light signal and converts it

back into an electrical signal. The complexity of a fiber optic system can range

from very simple (i.e., local area network) to extremely sophisticated and

expensive (i.e., long distance telephone or cable television trunking). For

example, the system shown in Figure 8-1 could be built very inexpensively

using a visible LED, plastic fiber, a silicon photo detector, and some simple

electronic circuitry. The overall cost could be less than $20. On the other hand,

a typical system used for long-distance, high-bandwidth telecommunication

that employs wavelength-division multiplexing, erbium-doped fiber amplifiers,

external modulation using DFB lasers with temperature compensation, fiber

Bragg gratings, and high-speed infrared photo detectors could cost tens or even

hundreds of thousands of dollars. The basic question is “how much

information is to be sent and how far does it have to go?” With this in mind we

will examine the various components that make up a fiber optic

communication system and the considerations that must be taken into account

in the design of such systems.

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ADVANTAGES OF OPTICAL FIBER SENSORS

a) All-passive dielectric characteristic: elimination of conductive paths in high-

voltage environments, it tells about Inability to conduct electric current.

b) Freedom from electromagnetic interference (EMI) and radio frequency

interference (RFI)

c) Inherent safety and suitability for extreme vibration and explosive

environments

d) Tolerant of high temperatures (>1450 C) and corrosive environments,

Greater Sensitivity

e) Light weight, and compact size

f) Multifunctional

g) Multiplexing capabilities

h) Resistant to harsh environment (Environmental Ruggedness)

i) Wide Dynamic Range

j) Remote sensing capability

k) Multifunctional sensing capabilities such as rotation, acceleration, electric

and magnetic field measurement, temperature, pressure, acoustics,

vibration, linear and angular position, strain, humidity, viscosity, chemical

measurements

l) Large Bandwidth

MAJOR DISADVANTAGES

Following are the disadvantages of using optical fiber sensors:

a) Detection systems may be complex and expensive

b) High cost

c) Unfamiliarity to the end user

d) Requirement for precise installation procedures

e) Development of usable measuring systems is complex

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APPLICATIONS OF FIBER OPTICS SENSORS

Fiber optic sensors are used in several areas. Specifically:

a) Mechanical Measurement such as rotation, acceleration, electric and

magnetic field measurement, temperature, pressure, acoustics, vibration,

linear and angular position, strain, humidity, viscosity, chemical

measurements

b) Electrical & Magnetic Measurements

c) Chemical & Biological Sensing

d) Monitoring the physical health of structures in real time.

e) Buildings and Bridges: Concrete monitoring during setting, crack (length,

propagation speed) monitoring, spatial displacement measurement, neutral

axis evolution, long-term deformation (creep and shrinkage) monitoring,

concrete-steel interaction and post-seismic damage evaluation.

f) Tunnels: Multipoint optical extensometers, convergence monitoring,

concrete / prefabricated vaults evaluation, and joints monitoring damage

detection.

g) Dams: Foundation monitoring, joint expansion monitoring, spatial

displacement measurement, leakage monitoring, and distributed

temperature monitoring.

h) Heritage structures: Displacement monitoring, crack opening analysis,

post-seismic damage evaluation, restoration monitoring, and old-new

interaction.

i) Detection of Leakage

FIBER-OPTIC STRAIN SENSORS

STRAIN MEASUREMENT

The basic design philosophy of engineering structures has changed

drastically from a conservative “safe life” approach to a more scientific “damage

tolerant” approach “safe life” was set on the basis of fatigue test data, the

extent of whose scatter determined setting the safe limit below the lowest

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values. With this approach, components that may have many additional years

of useful service had to be retired. Advances in the science and technology of

fracture mechanics provided a more rational basis to view structural failure on

the basis of size propagation characteristics of cracks. Thus, it was argued that

the mere presence of cracks in a structure could not be the basis to declare

that the product has outlived its usefulness; rather, cracks lengths and

propagation characteristics must determine component life. The success of the

latter approach is dependent on the accuracy with which structural

components with defects are modeled and tested in order to generate a basis in

regard to critical crack lengths. It therefore follows that the ability to monitor

cracks and their dynamics will be mandatory to ensure safe operation of

structural systems. Embedded fiber optic sensors are invaluable in such

monitoring and diagnostic tasks.

In a broad classification, sensors are described either as extrinsic or intrinsic.

In extrinsic fiber sensors the parameter under study (strain, temperature,

pressure, vibration, chemical concentration or other phenomena) at some

location along the fiber and exits the sensor either by re-entering the input

fiber or by entering another fiber to reach a detection device. In intrinsic

sensors, on the hand, changes in one or more optical parameters (intensity,

phase, polarization, wavelength, frequency, timing, or modal content) are

observed as light, propagating through the fiber, experiences change due to the

influence of the measurand.

Only the basic principles that govern the unique ideas implicit in the

arrangements of several sensors are described below. Of these, the most

promising are perhaps the Bragg grating and the white light interferometer

(Huston, 1999). Additional details on these and other sensing mechanisms are

available in the list references included here.

EXTRINSIC FABRY-PEROT SENSORS

The basic principle that governs the operation of Fabry perot sensors can

be understood with reference to Fig. The instrument is constructed by

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providing an air gap (typically 4mm in length and measured to an accuracy of

m5 ) between a single mode fiber and reflection from a multiple mode fiber.

The sensor is attached to a structural component through adhesives that

faithfully transmit any deformation to the sensor leading to a change in the

length of the gap. This, in turn, causes a phase change between the light of the

reference signal (reflected at the glass-air interface at the left end of the gap in

the figure) and the light from the sensing signal (reflected at the glass-air

interface at the right end of the gap) because of interference between the two

reflections. This phase change is a measure of motion at the gap location and

serves as the basis for an accurate measurement of strain.

Fabry-Perot sensor

OTHER FIBER-OPTIC MEASUREMENT TECHNIQUES

BRAGG GRATING SENSOR

The Bragg Grating fibre optic sensor is a relatively new type of fibre optic

sensor. It is generally classed as an interferometer.

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In this technique a stable Bragg grating is permanently impressed (photo

etched) into the core of a germanosilicate fibre by exposure to a two-beam

ultraviolet interference pattern. The region of periodic variation in the index of

refraction of the fibre core then acts as a very narrowband reflection filter.

The reflection signal is therefore a narrow spine with a centre wavelength that

is linearly dependent on the back reflected Bragg wavelength and the mean

index of refraction of the core. Figure illustrates the working principles of a

fibre optic Bragg grating. Consequently, any external parameters which act to

alter the grating characteristics results in a shift in the reflected Bragg

wavelength and constitutes the mode of measurement.

A strain resolution of 0.1μe and temperature resolution of 0.1oc is readily

achievable. The Bragg sensor has temperature compensation is possible by

overlaying two gratings. All other interferometers have a sinusoidal response

characteristic, requiring recalibration on initialization (i.e. following power

interruption).

The Bragg sensor, on the other hand, is initially calibrated (determined from

grating characteristics) and any deviations from the Bragg wavelength are

proportionately related to an exact parameter (i.e. Strain). Bragg fibre optic

sensors could be particularly useful when the Bragg gratings are arranged

along the fibre length such that the gratings are written into the core of the

optical fibre at various Bragg wavelengths. Each of the reflected wavelength

signals from the corresponding gratings could be monitored by the use of a

coupler, detector and tunable optical filter, thus achieving single-fibre

multiplexing of the sensors.

WHITE LIGHT INTERFEROMETRY

In this technique, a strained (active) fiber and an unstrained (reference)

fiber are spliced together and broadband (white) light is sent into both. The

light reflected from the ends of the fibers will interfere in a manner that

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depends on their relative lengths, and so varies as the active fiber is strained.

The change in the interference pattern measured as an indicator of strain.

These fibers have proven to be stable over time under static loads. They are

used in civil and geotechnical engineering applications that require strain

measurements with long-term stability (Huston, 1999).

CRACK DETECTION

When a fiber-optic strain sensor experiences strain along with the

structure to which it is attached, neither maintain nor damage in the way of

normal operation. On the other hand, an optical fiber may be used to detect

localized damage to its supporting structure. One possible configuration is

where a crack in a structural member to a crack in the embedded fiber at the

same location. The development of cracks can be detected by monitoring the

intensity of light transmitted through the embedded fiber. Light will be lost

from a crack in the fiber, and this loss will increase as the crack opens. This

reduction in transmitted light will reveal the presence of a crack, but not its

location within the structure.

Both the existence and the location of a crack can be determined by utilizing a

more sophisticated interrogation method. In optical time domain reflectometry

(OTDR), a very brief pulse of light is sent into the embedded fiber, where it

propagates normally until it reaches the crack. Part of the incident pulse is

reflected pulse is reflected from the crack and travels back toward the input

end of the fiber. When such a reflected pulse is detected, a crack is known to

exist. Furthermore, because the speed of light in the fiber is known, the time

between the generation of the input pulse and the arrival of the reflection can

be used to compute the location of the crack.

INTEGRATION OF FIBER-OPTIC SENSORS AND SHAPE MEMORY

ELEMENTS

The sensitivity of optical fibers to sense strains in a structure and the

ability of shape memory alloy wires to actuate offer an unusual opportunity to

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combine the two in structural applications. A grid pattern of optical fibers can

be visualized from which not only the strain level but also the location of

undesirable strain may be established. With this information, actuation of the

structure can be initiated by energizing the corresponding shape memory alloy

wires grid resulting in a smart structure.

An embedded optical fiber will be damaged along with the surrounding

material.

Aircraft wings, bridge decks, and buildings are some of the examples in which

a judicious combination of these two technologies may contribute to a greater

level of structural integrity than otherwise possible. Although this approach is

conceptually attractive, the technology needs to be developed through research

efforts. The issues pertain to the use of both fibers (shape memory, optical) in

addition to structural fibers in a composite structure. Structural weight and

stress concentration are to be balanced against cost questions. Feasibility

studies are underway, and if successful, the impact on the design of smart

structures is evident. An extremely revolutionary development may include the

potential use of optical fibers as sensors and load-bearing elements. Clearly, a

success in such an attempt will have enormous implications on the design and

development of structures in the future.

MACH ZEHNDER INTERFEROMETER

The development of fiber optic sensors based on white light

interferometery has been attractive in recent years. The use of such techniques

for distributed strain or temperature sensing in advanced composite or other

structural materials.

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Due to its protection to Electromagnetic Interference (EMI), especially in

noisy environments, fiber optic strain sensors have been attractive in the past

few years. Common electrical strain gauges have a low electrical output level

and desensitizing these gauges to EMI is a difficult task.

Mach–Zehnder interferometer is a device used to determine the

relative phase shift variations between two collimated beams derived by

splitting light from a single source. The interferometer has been used, among

other things, to measure phase shifts between the two beams caused by a

sample or a change in length of one of the paths. In strain sensor

configuration, the applied strain causes the variation of light intensity in two

ways.

First, the strained fiber of the main branch of a Mach-Zehnder

interferometer exhibits length variation, so the superposed light intensity at the

output of the Mach-Zehnder interferometer is proportional to the strain. This

variation of intensity due to the phase shift results from the elongation of the

fiber. Second, the length variation in fiber reduces the distance between the

head of the fiber and the fixed mirror which in turn leads to intensity variation

of the light. We also consider the changes of the refraction index due to the

strain caused by length variation.

Principle of Fiber Optic

A Mach-Zehnder interferometer is formed from two couplers connected

by two arms of unequal optical lengths. The light is split in two arms of the

input coupler of the interferometer and they are later recombined at the output

coupler.

In this sensor structure, the length difference of the two arms is due to

the applied strain on the main arm. The unequal length of the arms causes the

phase difference between the split beam which is a function of wavelength and

arm length difference.

https://en.wikipedia.org/wiki/Phase_(waves)

https://en.wikipedia.org/wiki/Collimated

https://en.wikipedia.org/wiki/Interferometer

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To improve the resolution we have to increase the sensitivity of the

intensity to the applied strain. Therefore, we use a Mach-Zehnder

interferometer followed by a displacement sensing system. In difference to the

ordinary displacement sensors, in our proposed system, the reflecting mirror is

fixed so that we can measure the displacement of the head of fiber as

illustrated in the zoomed part of Figure. It is clear that the gathered beam

intensity is a function of the fiber elongation.

The reflected beam passes through the MachZehnder and has been

affected again by the strain. The Mach-Zehnder affects the reflected light

similar to the earlier light by changing the phase of the light field. In addition,

the applied strain alters the refractive index of the optical fiber which leads to a

change in the optical path.

So the improvement of the precision is due to the four strain effects:

a) The input light is affected by the elongation of the main arm of the Mach-

Zehnder

b) The elongation of the fiber modifies the intensity of the reflected beam

c) Mach-Zehnder does have an effect on the reflected light as well as on the

input light

d) The optical path varies because of the changes in refractive index

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MODULE-3

Vibration Absorbers, Biomimetics


Vibration Absorbers: Introduction, Parallel Damped Vibration Absorber,

Analysis, Gyroscopic Vibration absorbers, analysis & experimental setup and

observations, Active Vibration absorbers.

Control of Structures: Introduction, Structures as control plants, modeling

structures for control, Control strategies and Limitations.

Biomimetics: Characteristics of Natural structures. Fiber reinforced: organic

matrix natural composites, Natural creamers, Mollusks. Biomimetics sensing,

Challenges and opportunities.

INTRODUCTION

It is possible to forget that in the majority of cases vibration is only

considered when there is a problem. It is therefore of great practical importance

to be able to reduce vibration amplitudes. We will now consider vibration

absorbers. These go under various names, undamped vibration absorbers are

also called detuners. Damped vibration absorbers are sometimes just called

vibration absorbers.

Vibration absorbers are devices attached to flexible structures in order to

minimize the vibration amplitudes at a specified set of points. Design of

vibration absorbers has a long history. First vibration absorber proposed by

Frahm in 1909 [Den Hartog, 1956] consists of a second mass-spring device

attached to the main device, also modeled as a mass-spring system, which

prevents it from vibrating at the frequency of the sinusoidal forcing acting on

the main device. If the absorber is tuned so that its natural frequency coincides

with the frequency of the external forcing, the steady state vibration amplitude

of the main device becomes zero. From a control perspective, the absorber acts

like a controller that has an internal model of the disturbance, which therefore

cancels the effect of the disturbance.

http://www.mech.uwa.edu.au/bjs/Vibration/TwoDOF/Absorbers/Undamped/default.html

http://www.mech.uwa.edu.au/bjs/Vibration/TwoDOF/Absorbers/Damped/default.html

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When there is a sinusoidal excitation with a fixed frequency then in theory it

possible to stop vibration. Since the top mass is not moving there can be no

resultant force on it. The lower spring and mass are vibrating so that the

exciting force is balanced by the force applied by the lower spring. As the lower

spring/mass system is vibrating freely without any force applied to it and the

mass to which is attached is at rest, then the lower spring/mass system is

vibrating at its undamped natural frequency.

We shall first consider the undamped vibration absorber.

VIBRATION ABSORBERS: UNDAMPED

The steady state solution when a sinusoidal force is the excitation is given by,

However when the lower spring/mass is undamped we have c2=0 and the

equation becomes,

It is clear that

Thus the detuned frequency is the undamped natural frequency of the lower

spring/mass system.

http://www.mech.uwa.edu.au/bjs/Vibration/OneDOF/Transient/Undamped/default.html#Wn


http://www.mech.uwa.edu.au/bjs/Vibration/TwoDOF/Externalforce/SS/default.html

http://www.mech.uwa.edu.au/bjs/Vibration/OneDOF/Transient/Undamped/default.html#Wn

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VIBRATION ABSORBERS: DAMPED

An undamped vibration absorber (detuner) is effective at or very close to a

single frequency. In practice the excitation frequency may not be constant and

therefore a detuner is not the appropriate way to improve the response.

A damped vibration absorber is effective over a greater frequency range.

Consider the response of a single degree-of-freedom system and how it is

modified by both undamped and damped absorbers.

The undamped absorber stops the vibration at one frequency, the detuned

frequency. However, there are two resonances nearly as large as the original

resonance. When an optimized damped absorber is added the two resonance

peaks are greatly reduced (a factor of 10) but there is no detuned frequency

with a zero response.

PARALLEL DAMPED VIBRATION ABSORBER


http://www.mech.uwa.edu.au/bjs/Vibration/TwoDOF/Absorbers/Optimization/default.html

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The analysis of the parallel absorber shows that it is possible to obtain an

undamped ant resonance in a dynamic absorber system, which exhibits a well-

damped resonance. Although the bandwidth of frequencies between the damped

peaks is not significantly increased, the amplitudes of the mass are considerably

smaller within the operation range of the absorber. The damped absorber mass and

the main mass attain null simultaneously so that the vibratory force is transmitted

entirely to the undamped absorber.

A comparison of the results with those of the conventional absorber indicates that the

parallel damped dynamic vibration absorber has definite advantages over the

conventional damped vibration absorber.

ANALYSIS

The analysis that follows consists mainly of

The derivation of the governing equation of motion and

Derivation of the condition for the amplitude the main mass to be independent

of the damping ratio.

The latter condition provides the frequencies at which the amplitude of the main mass

are independent of the damping ratio C/Cc. in addition for the particular case of

practical interest so called favourable tuning has been determined. Favourable tuning

refers to the frequency at which the absolute value of the amplitude is independent of

damping. Under this favorable tuning the mass ration required to provide the greatest

spread between the frequencies is determines also the equations that provide the

optimum ratio of damping has been derived and represents a condition at which the

slope of the response curve is zero.

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GYROSCOPIC VIBRATION ABSORBERS

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ANALYSIS

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EXPERIMENTAL SETUP AND OBSERVATIONS

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ACTIVE VIBRATION ABSORBERS

We conclude this chapter with a brief discussion of the active vibration absorber, also

known as a proof-mass actuator and by various related names. The physical

arrangement of these devices components is very similar to that of the passive

absorber, with the addition of a controllable force element in parallel with the absorber

spring and dashpot, as shown in the fig. The existence of this force between the

primary structure and the absorber mass alters the dynamics of the combined system

(e.g. its natural frequencies and the magnitude of the response of the main mass to a

harmonic disturbing force).

In many applications of the active control force fa is derived by feeding back a

combination of the relative displacement and velocity between the primary and

secondary masses. This results in a system that is mathematically equivalent to a

passive absorber in which the spring and dashpot are adjustable, allowing the

absorber to be tuned during operation. Although such a system may perform better in

the harmonic steady state than a conventional, passive device, with the introduction of

active passive structures are generally immune.

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In addition, the generation of the force fa for example by a hydraulic actuator,

consumes power and the associated hardware increases the complexity of what is

basically a simple device, these drawbacks must be weighed against the benefits that

may follow from flexibility in specifying fa in a given application.

CONTROL OF STRUCTURES

INTRODUCTION

In categorizing smart structures as open-loop or closed-loop, we have implied that

some are capable of responding actively to change in their state or environment, for

example by altering their effective mechanical properties .much of this book is devoted

to analyzing means for sensing or inducing mechanical phenomena ,such as force or

displacement ,often through the use of electrical signals .in this chapter we shall

consider the device or subsystems that generate the signals to which the actuators

respond and the effects they produce in some common configuration.

In closed-loop structure, sensor outputs are processed by the controller to generate

actuator commands. Open-loop structures may employ neither sensors nor actuators;

on the other hand, nothing prevents the use of actuators without feedback. Such an

open-loop smart structure still needs a controller to generate the signals applied to its

actuators. Thus, a closed-loop smart structure requires a controller, while an open-

loop smart structure may not incorporate one. We will be concerned here primarily

with feedback systems, that is, closed-loop structures.

It will come as no surprise that the introduction of feedback can radically alter the

dynamics of a structural system, affecting its natural frequencies and modes, its

transient response, and even its stability. Fortunately, in studying smart structures

we can take advantage of the literature in the field of structural control, which has

manufactured greatly over the last two decades.

Much of this work has deal with conventional structures modified by the addition of

discrete sensors or actuators, as opposed to the more fully integrated systems implied

by the term smart structure, but it has gone far toward connecting the sometimes

disparate fields of structural dynamics and control theory.

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MODELING STRUCTURES FOR CONTROL

In discussing control theory in the abstract, it is common to assume the

availability of a model of the plant to be controlled, typically in the form of differential

equation are system of differential equations i.e. an input-output relation or state-

space model. For many purposes, the number of inputs and outputs are the number

of states can be regarded as a parameter that is defined on a case- by- case basis, and

theoretical developments can proceed under the assumption that this number is

“small” in the sense that the sheer size of the matrices to be manipulated will present

no numerical difficulties.

For Ex. this approach may work well for mechanical systems with a few degrees of

freedom, even bearing in the mind that each degree of freedom gives rise to two states

of variables one displacement and one velocity.

However, it is common to perform static and dynamic analyses on structures with

many degrees of freedom. When a structural dynamic model results from the finite

element dicretization of a large or complex structure, it may have tens or thousands of

degree of freedom. Although numerical methods to complete the natural frequencies

and natural modes of such a system are available, the computational tools of modern

control cannot all be expected to be so robust. This is due to good numerical

properties of most structures‟ coefficient of matrices as well as to the types of

problems historically of the greatest interest to the controls and structures

communities.

A consequence of this disparity is that the straight forward reduction of even a

moderate structural model to state-variable form can produce a state-space model of

ungainly dimensions. This has motivated interest in reducing the size of structural

models while retaining acceptable accuracy, and in exploiting the structure of the

equations governing mechanical systems.

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The idea behind model reduction or reduced order modeling is to identify those

degrees of freedom or state variables least relevant to the goals of analysis and

systematically eliminate the corresponding equations from the model. This may mean

isolating those degrees of freedom that contribute least to the forced response or those

states that have little effect on the controlled law. In some cases it is possible to

determine by inspection which equation may best be dispensed with, for example,

those corresponding to in-plane motion when out-of-plane bending is the deformation

mode of interest. More generally, though, a mathematical criterion must be devised to

weigh the relative importance of the degrees of freedom or system states. Once it is

determined which equations are to be eliminated, the remaining equations are

modified to include an estimate of the effect of those that are removed.

CONTROL STRATEGIES AND LIMITATIONS

Assuming a model of reasonable size (dimension) and acceptable accuracy is

available to represent a structure; we can turn to the calculation of a control law. In

the simplest cases, it may be possible to proceed directly by using the tools. More

often, though, it will be expedient to consider the physics of the problem

simultaneously with the mathematics. For example, negative feedback of position or

rate is in many ways similar to adding stiffness or damping to a mechanical system.

Part of stiffness or damping in an active vibration absorber may be achieved through

feedback gains.

However, if the sensed motion of one mass is used in computing the control force

to be applied to another mass to which the first is not directly (mechanically)

connected the coupling in the closed-loop system will be qualitatively different from

that in the passive, open-loop structure.

This is not necessarily undesirable, but it serves to illustrate that the dynamics of

the active system can quickly become much more complex than those of the structure

alone. This complexity extends to such fundamental matters as stability; further, it

can introduce a very large number of parameters with less obvious physical meanings

than “stiffness” and “damping”, for example, if every mass in a MDOF system is

subject to control forces depending on the motion of every other mass. When a

structure consists of lumped masses and springs, it is possible at least in principle to

dedicate a sensor and an actuator to each degree of freedom. Possible controller

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structures then range from the strictly local, where the signal driving each actuator is

derived from the output of only the corresponding sensor, to the configuration in

which each actuator signal is computed using information from every sensor. The

latter introduces new coupling into the equations of motion, and may affect the

symmetry of the coefficient matrices as well. These effects have been investigated

extensively, and some (very stringent) conditions under which they are minimized are

known.

Many of the same phenomena are observed when spatially discrete sensors and

actuators are used to control a continuous structure. In addition, it is more likely that

the sensors and actuators will be at different locations on the structure (although the

configuration in which they are collocated is an important special case ad permits

some simplifications). Because the plant has infinitely many degrees of freedom and

thus infinitely many states, the effects of unmodeled dynamics must be addressed, for

example, by ensuring that all significantly contributing modes are included in any

finite-dimensional model.

It is also often necessary to represent the control forces (actuators outputs) in the

modal or state coordinates, and then to construct physical control forces or actuator

commands from the result of modal or state-space control law computations.

ACTIVE STRUCTURES IN PRACTICE

In this section, we shall consider some examples of smart structures that illustrate

the potential of integrating sensing, actuation and control into a structural system.

Note that even in such proof-of-concept work a multidisciplinary approach is generally

necessary, and how the initial problem statement has ramifications throughout the

design, analysis, and operation of even a simple smart structure. Additional

representative applications may be found in the references given at the end of this

chapter, and in Chopra (1996). The reader interested in aerospace technology is

encouraged to refer to Wie (1988) and Frank et al. (1994), who present broad,

accessible treatments of the development of active smart structure for space flight.

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BIOMIMETICS

Characteristics of Natural structures. Fiber reinforced: organic matrix natural

composites, Natural creamers, Mollusks. Biomimetics sensing, Challenges and

opportunities.

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MODULE -4

MEMS, PIEZOELECTRIC SENSING AND ACTUATION

MEMS: History of MEMS, Intrinsic Characteristics, And Devices: Sensors and

Actuators. Micro fabrication: Photolithography, Thermal oxidation, thin film

deposition, etching types, Doping, Dicing, Bonding. Microelectronics fabrication

process flow, Silicon based Process selection and design.

Piezoelectric Sensing and Actuation: Introduction, Cantilever Piezoelectric

actuator model, Properties of Piezoelectric materials, Applications. Magnetic

Actuation: Concepts and Principles, Magnetization and Nomenclatures,

Fabrication and case studies, Comparison of major sensing and actuation

methods.

Micro-electromechanical Systems (MEMS)

What is MEMS?

A micro-electromechanical system (MEMS) is a process technology used to

create tiny integrated devices or systems that combine mechanical and

electrical components. They are fabricated using integrated circuit (IC) batch

processing techniques and can range in size from a few micrometers to

millimeters. These devices (or systems) have the ability to sense, control and

actuate on the micro scale, and generate effects on the macro scale.

HISTORY OF MEMS

MEMS (micro-electro-mechanical systems) are tiny electo mechanical devices

made by some of the same methods as integrated circuits. The results are some

of the smallest machines ever made, capable of being built on a silicon wafer

alongside the circuits that control them. Most MEMS devices are still

experimental, but they are already being used in cars to deploy airbags and

actuate antilock brakes, in integrated optical switches to handle Internet

traffic, and in many other areas.

MEMS were first proposed in the 1960s, but not commercialized until the

1980s. Engineers and scientists wanted to use integrated circuit fabrication

techniques to make tiny mechanical systems, which could, if necessary, be

https://ethw.org/Integrated_Circuits

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connected to electronic circuits on the same chip. One of the first commercial

applications of MEMS was the tiny nozzle assembly used in the cartridges of

inkjet printers. Each of the nozzles in an inkjet printers print head consists of a

hollow chamber. Inside, ink flows in, is heated with tiny electric heating

elements, and is then expelled through a port. The chamber and all its features

are created using the same photolithography techniques as an integrated

circuit.

In 1982, automotive airbag systems (which had been proposed in the 1950s)

were introduced using MEMS sensors to detect a crash. The Analog Devices

Corporation elaborated this idea, producing an “accelerometer” for airbag

systems in 1991, where the mechanical and electronic portions were integrated

on the same chip. The accelerometer chip detects the sudden increase or

decrease in speed that occurs during a crash. The same company later

introduced a gyroscope-on-a-chip, capable of working with an automobile‟s

global positioning system to create more accurate maps and directions for

drivers.

INTRINSIC CHARACTERISTICS

There is no doubt that MEMS will continue to find major new applications in

the future. The reason for technology development and commercialization may

vary by case. Nevertheless, there are three generic and distinct merits for

MEMS devices and micro fabrication technologies: Miniaturization,

Microelectronics Integration and Parallel fabrication with high precision. MEMS

products will compete in the market place on the grounds of functional

richness, small sizes, unique performance characteristics (e.g., fast speed),

and/or low cost.

a. Miniaturization

The length scale of typical MEMS devices generally ranges from 1 mm to 1 cm.

Small dimensions give rise to many operational advantages, such as soft

springs, high resonance frequency, greater sensitivity, and low thermal mass.

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Small size allows MEMS devices to be less intrusive in biomedical applications

(e.g., neuron probes).

However, all things do not work better when miniaturized. Some physical

phenomena do not scale favorably when the dimensions are reduced, while

certain physical phenomena that work poorly at the macro scale suddenly

becomes very practical and attractive at the micro scale.

Being small also means that MEMS devices can be integrated

nonintrusive in crucial systems, such as portable electronics, medical

instruments, and implants (e.g., capsule endoscopes). From a practical point of

view, smaller device footprint leads to more devices per wafer and greater

economy of scale. Hence the cost of MEMS devices generally scales favorably

with miniaturization.

b. Microelectronics Integration

Circuits are used to process sensor signals, provide power and control, improve

the signal qualities, or interface with control/computer electronics. MEMS

products today are increasingly being embedded with computing, networking,

and decision-making capabilities. By integrating micromechanical devices with

electronic circuitry and offering the combined system as a product, significant

advantages can be produced in a competitive market place.

The ability to seamlessly integrate mechanical sensors and actuators with

electronics processors and controllers at the single wafer level is one of the

most unique characteristics of MEMS. This process paradigm is referred to as

monolithic integration fabrication of various components on a single

substrate in an unbroken, wafer-level process flow.

c. Parallel Fabrication with Precision

MEMS technology can realize two or three-dimensional features with small

dimensions and precision that cannot be reproducibly, efficiently, or profitably

made with traditional machining tools. Combined with photolithography,

MEMS technology can be used to realize unique three-dimensional features

such as inverted pyramid cavities, high aspect ratio trenches, through-wafer

holes, cantilevers, and membranes. To make these features using traditional

machining or manufacturing methods is prohibitively difficult and inefficient.

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MEMS and Microelectronics are also different from traditional machining, in

that multiple copies of identical dies are made on a same wafer. This practice

can contribute to lowering the cost of individual units. Modern lithography

systems and techniques provide not only finely defined features, but also

uniformity across wafers and batches.

DEVICES: SENSORS AND ACTUATORS.

a. TRANSDUCERS

MEMS technology enables revolutionary sensors and actuators. In general

terms, sensors are devices that detect and monitor physical or chemical

phenomenon, whereas actuators are ones that produce mechanical motion,

force, or torque. Sensing can be broadly defined as energy transduction

processes that result in perception, whereas actuation is energy transduction

processes that produce actions.

Sensors and actuators are collectively referred to as transducers, which serve

the function of transforming signals or power from one energy domain to

another. There are six major energy domains of interests: (1) electrical domain

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(denoted E); (2) mechanical domain (Mec); (3) chemical domain (C); (4) radiative

domain (R); (5) magnetic domain (Mag); and (6) thermal domain (T). These

energy domains and commonly encountered parameters within them are

summarized in Figure The total energy within a system can coexist in several

domains and can shift among various domains under right circumstances.

b. SENSORS

Sensors fall into two categories, physical sensors and chemical/biological

sensors. Physical sensors are used to measure physical variables such as force,

acceleration, pressure, temperature, flow rate, acoustic vibration, and magnetic

field strength. Chemical sensors are used to detect chemical and biological

variables including concentrations of chemicals, pH, binding strength of

biological molecules, protein–protein interactions, and so forth.

The most important sensor characteristics of concern are summarized in the

following:

i. Sensitivity. The sensitivity is defined as the ratio between the magnitude of

output signal and that of the input stimulus. Note that the sensitivity values

may be a function of the input amplitude and frequency, temperature,

biasing level, and other variables. When electronics signal amplification is

used, it is meaningful to distinguish values of sensitivity before and after

amplification.

ii. Linearity. If the output signal changes proportionally with respect to the

input signal, the response is said to be linear. Linear response of a sensor

alleviates the complexity of signal processing.

iii. Accuracy. The ability of a sensor to provide results close to the true value.

iv. Precision. The ability of a sensor to give the same reading when repeatedly

measuring the same quantity under the same conditions. Repeatability is the

precision of a device over a short term, whereas reproducibility is the

precision of a device over a long term.

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v. Responsively, or resolution. It is also known as the detection limit or

minimal detectable signal (MDS). This term signifies the smallest signal a

sensor can detect with confidence. It is generally limited by noise associated

with the transduction elements and circuits.

vi. Noise. Noise can be applied to anything that obscures a desired signal. Noise

can itself be another signal (“interference”); most often, however, we use the

term to describe “random” noise of a physical (often thermal) origin. While

interference noise can be corrected or eliminated, such as by careful

electrical shielding, random noises are ubiquitous and have much more

fundamental origins. Noise can also arrive from the circuits.

vii. Dynamic range. The dynamic range is the ratio between the highest and the

lowest detectable signal levels. In many applications, a wide dynamic range

is desired.

viii. Bandwidth. The bandwidth characterizes sensor ability to measure fast-

changing signals. Sensors behave differently to constant or time-varying

signals. Oftentimes, sensors may cease to respond to signals of extremely

high frequencies. The effective frequency range is called the bandwidth.

ix. Drift. Drift may occur because electrical and mechanical properties of

materials vary over time. Sensors with large drift cannot be used

successfully to detect slow changing signals, such as monitoring stress

building up in a civil structure over time.

x. Sensor reliability. Sensor performance may change over time and when

placed under harsh conditions. Reliability and trustworthiness of sensors in

a wide temperature range (5°C to 105°C) is demanded of such sensors. Many

industries have established guidelines and standards involving sensor use.

xi. Crosstalk or interference. A sensor intended for measuring one variable

may be sensitive to another physical variable as well. An acceleration sensor

with sensitivity in one particular axis may respond to acceleration in other

orthogonal axes. Sensor cross talks should be minimized in practical

applications.

xii. Development costs and time. It is always desirable that the sensor

development process be inexpensive and fast. Fast time-to-market is

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important for commercial sensors that are built with custom specifications.

The reduction of cost and development time, to the level currently enjoyed by

the application specific integrated circuit (ASIC) industry, would be very

appealing.

c. ACTUATORS

Actuators generally transform energy in non-mechanical energy domains into

the mechanical domain. For a particular actuation task, there could be several

energy transduction mechanisms. For example, one can generate a mechanical

movement by using electrostatic forces, magnetic forces, piezoelectricity, or

thermal expansion.

The following are general criteria when considering actuators designs and

selections:

i.Torque and force output capacity. The actuator must provide sufficient

force or torque for the task at hand. For example, micro optical mirrors are

used to deflect photons. In some cases, micro actuators are used for

interacting with a fluid (air or water) to actively control the fluid. Such

actuators must provide greater force and power to pro-duce appreciable

effects.

ii.Range of motion. The amount of translation or angular movement that the

actuator can produce under reasonable conditions and power consumption

is an important concern.

iii.Dynamic response speed and bandwidth. The actuator must be able to

produce sufficiently fast response. From the point of view of actuator control,

the intrinsic resonant frequency of an actuator device should be greater than

the maximum oscillation frequency.

iv.Ease of fabrication and availability of materials. To reduce the potential

costs of MEMS actuators, there are two important strategies. One is to

reduce the costs of materials and processing time. Another is to increase the

process yield for a given process in order to produce more functional units in

each batch.

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v.Power consumption and energy efficiency. Many micro actuators are

envisioned for use in small and mobile systems platforms. The total available

power for such systems is generally limited. In this and many other MEMS

applications, low-power actuators are preferred to increase the duration of

operation.

vi.Linearity of displacement as a function of driving bias. If the

displacement varies with input power or voltage in a linear fashion, the

control strategy would be simplified.

vii.Cross-sensitivity and environmental stability. The actuator must be

stable over the long term, against temperature variation, humidity

absorption, and mechanical creep. Long-term stability of such actuators is

extremely important for ensuring commercial competitiveness and success.

viii.Footprint. The footprint of an actuator is the total chip area it occupies. In

cases of dense actuator arrays, the footprint of each actuator becomes a

primary point of consideration.

OR

a) TRANSDUCER

A transducer is a device that transforms one form of signal or energy into

another form. The term transducer can therefore be used to include both

sensors and actuators and is the most generic and widely used term in MEMS.

b) SENSOR

A sensor is a device that measures information from a surrounding

environment and provides an electrical output signal in response to the

parameter it measured. Over the years, this information (or phenomenon) has

been categorized in terms of the type of energy domains but MEMS devices

generally overlap several domains or do not even belong in any one category.

These energy domains include:

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Mechanical Force, Pressure, Velocity, Acceleration, Position

Thermal Temperature, Entropy, Heat, Heat Flow

Chemical Concentration, Composition, Reaction Rate

Radiant Electromagnetic Wave Intensity, Phase, Wavelength,

Polarization Reflectance, Refractive Index, Transmittance

Magnetic Field Intensity, Flux Density, Magnetic Moment, Permeability

Electrical Voltage, Current, Charge, Resistance, Capacitance, Polarization

c) ACTUATOR

An actuator is a device that converts an electrical signal into an action. It can

create a force to manipulate itself, other mechanical devices, or the

surrounding environment to perform some useful function.

MICRO FABRICATION:

Conventional macroscale manufacturing techniques e.g. injection moulding,

turning, drilling etc, are good for producing three dimensional (3D) shapes and

objects, but can be limited in terms of low complexity for small size

applications. MEMS fabrication, by comparison, uses high volume IC style

batch processing that involves the addition or subtraction of two dimensional

layers on a substrate (usually silicon) based on photolithography and chemical

etching. As a result, the 3D aspect of MEMS devices is due to patterning and

interaction of the 2D layers. Additional layers can be added using a variety of

thin-film and bonding techniques as well as by etching through sacrificial

„spacer layers‟.

a. Photolithography

A most common lithography process involves depositing photo-sensitive

chemicals on a silicon wafer, exposing it with light through a mask, and

removing (develop) photo resist material that has been modified by light. The

starting point of a lithography process is to coat a wafer with photo resist

through spin coating. A wafer is held on a rotating stage. Photoresist is applied

to the center of the wafer at rest position. The wafer is then spun at high speed,

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causing the photo resist to move towards the edge of the wafer under

centrifugal forces. After the wafer spinning is stopped, a uniform thin layer of

photo resist is coated on the front surface of a wafer. Process variables include

the wafer spinning speed, the viscosity of the resist, and the types of resists

(e.g., target wavelength, sensitivity). Typical thickness of photo resist is

generally 1–10 mm.

Figure: Process steps of photo resist spin coating. Figure: Process flow for

patterning photo resist with a photo mask.

A lithography patterning procedure involves multiple steps.

Step a: A wafer is first covered with a uniform thin layer of resist

Step b: A mask, consisting of a transparent substrate (e.g., glass or quartz)

with opaque features, is brought close to the resist-coated wafer.

Step c: High energy, collimated light rays strikes the mask-wafer assembly.

Resist regions that are not covered by opaque features are exposed, changing

the chemical composition of the resist. For positive resist, the exposure by light

causes the resist to be more soluble in a wet chemical developer.

Step d: This allows the opaque features on the mask to be faithfully

transferred to the wafer.

A pattern in photo resist can be further transferred to an underlying layer,

using the photo resist as a mask layer. The process is shown in Figure.

It starts with a wafer with a thin film coating (step a).

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The wafer is covered with a spin-coated layer of photo resist (step b)

The wafer is then immersed in a chemical solution that preferably etch the thin

film but not the photo resist (step c).

(Alternatively, the thin film may be etched with dry etch methods.) With proper

timing control, the thin film covered by the photo resist would stay intact;

whereas the thin film in areas not covered by the photo resist would be

removed (step d).

The photo resist is then preferentially removed, leaving the thin film behind as

patterned (step e).

b. Thin Film Deposition

Functional materials, conductors and insulators can be incorporated on a

wafer through additive deposition process. One such deposition process is

direct transfer of the material from a source to the wafer surface in an atom-

by-atom, layer-by-layer fashion. Examples include metal evaporation and metal

sputtering. The process is generally conducted in a low-pressure environment

so that atoms may travel from the source to the wafer surface without

interruptions caused by air molecules. One such system, an evaporator, is

diagrammed in Figure.

Figure: Patterning a thin film using the photo resist as a mask.

Figure: A process of evaporating a thin film.

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A wafer and a metal source are both placed inside a vacuum jar. The metal can

be transferred either by heating it (evaporation) or by bombarding it with high-

energy ions (sputtering). The achieved thickness is proportional to the power

and time. In practice, the routine thickness of metal thin films ranges from 1

nm to 2 mm.

A second method for placing thin film materials on a wafer surface is chemical

vapor deposition. Two or more active species arrive at the vicinity of the wafer

surface (step a). They react under favorable conditions (with energy provided by

heating or plasma). The reaction of these species produces a solid phase, which

is absorbed onto the nearby wafer surface (step b). The byproducts of the

reaction (if any) may be removed by the surrounding media. Continuous

reaction causes a layer of material to be built on the wafer surface (step c).

Typically the average thickness of thin film deposited by CVD, evaporation or

sputtering is below 1 mm. To deposit films of greater thickness is typically too

time consuming or impractical.

Fig: A schematic diagram of the metal evaporation equipment.

Fig: A process of chemical reactive deposition (e.g., chemical vapor deposition).

c. Thermal Oxidation of Silicon

Silicon dioxide is an important insulating layer for microelectronics and MEMS.

One prominent method of forming a high-quality silicon-dioxide layer is by

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reacting silicon wafers with oxygen atoms at high temperatures (e.g., 900°C

and above).

Figure: A schematic diagram of the oxidation equipment and process.

Figure: Process of thermal oxidation explained at the atomic level.

(a) Oxygen atoms arrive at the surface of bare silicon.

(b) Reaction between oxygen and silicon turns part of the surface into silicon

dioxide.

(c) Gradually, a continuous layer of silicon dioxide is formed, separating the

oxygen atmosphere with the silicon substrate.

(d) Newly arrived oxygen species must diffuse across the oxide layer in order to

react with silicon atoms on the other side of the oxide.

The reaction rate is limited by the diffusion process.

Wafers are often placed inside a heated quartz tube. On the surface of the

wafer, a layer of oxide is formed and separates the interior silicon from the

oxygen atoms. The atoms on the outside must diffuse through the oxide layer

and react with the fresh silicon on the inner interfaces of silicon and oxide. One

can imagine that, as the thickness of oxide grows, the rate of oxidation growth

decreases. The deposition rate and the ultimate thickness are dependent on the

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temperature. For most applications, the thermal oxide thickness is below 1.5

mm.

d. ETCHING TYPES

Bulk micromachining involves the removal of part of the bulk substrate. It is a

subtractive process that uses wet anisotropic etching or a dry etching method

such as reactive ion etching (RIE), to create large pits, grooves and channels.

Materials typically used for wet etching include silicon and quartz, while dry

etching is typically used with silicon, metals, plastics and ceramics.

Wet Etching

Wet etching describes the removal of material through the immersion of a

material (typically a silicon wafer) in a liquid bath of a chemical etchant. These

etchants can be isotropic or anisotropic.

Isotropic etchants etch the material at the same rate in all directions, and

consequently remove material under the etch masks at the same rate as they

etch through the material; this is known as undercutting. The most common

form of isotropic silicon etch is HNA, which comprises a mixture of hydrofluoric

acid (HF), nitric acid (HNO3) and acetic acid (CH3 COOH). Isotropic etchants are

limited by the geometry of the structure to be etched. Etch rates can slow down

and in some cases (for example, in deep and narrow channels) they can stop

due to diffusion limiting factors. However, this effect can be minimized by

agitation of the etchant, resulting in structures with near perfect and rounded

surfaces.

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Figure. Isotropic etching with (a) and without (b) agitation, and anisotropic wet

etching of and silicon (c and d respectively).

Anisotropic etchants etch faster in a preferred direction. Potassium hydroxide

(KOH) is the most common anisotropic etchant as it is relatively safe to use.

Structures formed in the substrate are dependent on the crystal orientation of

the substrate or wafer. Most such anisotropic etchants progress rapidly in the

crystal direction perpendicular to the plane and less rapidly in the direction

perpendicular to the plane. The direction perpendicular to the plane etches

very slowly if at all. Figures c and d shows examples of anisotropic etching in

and silicon. Silicon wafers, originally cut from a large ingot of silicon grown

from single seed silicon, are cut according to the crystallographic plane. They

can be supplied in terms of the orientation of the surface plane. Dopant levels

within the substrate can affect the etch rate by KOH, and if levels are high

enough, can effectively stop it. Boron is one such Dopant and is implanted into

the silicon by a diffusion process. This can be used to selectively etch regions

in the silicon leaving doped areas unaffected.

Dry Etching

Dry etching relies on vapour phase or plasma-based methods of etching using

suitably reactive gases or vapors usually at high temperatures. The most

common form for MEMS is reactive ion etching (RIE) which utilizes additional

energy in the form of radio frequency (RF) power to drive the chemical reaction.

Energetic ions are accelerated towards the material to be etched within a

plasma phase supplying the additional energy needed for the reaction; as a

result the etching can occur at much lower temperatures (typically 150º-250ºC,

sometimes room temperature) than those usually needed (above 1000ºC). RIE

is not limited by the crystal planes in the silicon, and as a result, deep trenches

and pits, or arbitrary shapes with vertical walls can be etched.

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Deep Reactive Ion Etching (DRIE) is a much higher-aspect-ratio etching

method that involves an alternating process of high-density plasma etching (as

in RIE) and protective polymer deposition to achieve greater aspect ratios.

Figure. Deep Reactive Ion Etching (DRIE)

Etch rates depend on time, concentration, temperature and material to be

etched. To date there are no universally accepted master equations to predict

etched performance and behavior.

e. Doping

Figure: Two step diffusion doping.

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Doping is the process of planting Dopant atoms into the host semiconductor

lattice in order to change the electrical characteristics of the host material. The

initial source dopants can be placed on the surface of the wafer or precisely

injected into the silicon lattice using the ion implantation method. The Dopant

atoms can further diffuse from high-concentration to low-concentration regions

under thermal activation.

A schematic diagram of a representative process used for doping selective

regions of silicon with Dopant atoms is shown in Figure. The desired shape of

the resistor is shown in the top-most figure. The resistor feature should be

moderately doped. The two ends of the resistor should have higher doping

concentration in order to form ohmic contacts with metal leads.

A mask shield is first deposited in step a and patterned to form windows. The

wafer is ex-posed to a source of Dopant, which cannot penetrate the mask

shield layer but can enter the sill-icon via the open windows. The arms of the

resistor are then patterned (in step b) for performing a lower dose doping.

Finally, metal leads are deposited and patterned to connect with the resistor

(step c).

Dopant atoms perform random walk (Brownian motion) in a semiconductor

lattice under elevated wafer temperature. Though the movement of individual

Dopant atoms is random, the overall population of Dopant atoms moves from

high-concentration regions to low-concentration ones. This process is called

thermal diffusion.

It is important to notice that

(1) existing doping procedure can only be performed on top surfaces of wafers

(2) high temperature encountered by a wafer during a process, even in steps

after the doping process, can cause Dopant redistribution and changes of

electrical characteristics.

f. Wafer Dicing

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A wafer consists of multiple dies; each must be broken into separate pieces

before being pack-aged individually. The traditional process for separating dies

is through a dicing process. A high-speed rotating dicing saw blade is used to

cut trenches in silicon wafer. The cutting process, being mechanical in natures,

produces particles. Water is sprayed onto the wafer to lubricate and remove

heat. The thinned trenches allow silicon to be broken off easily without

fracturing. Obviously, this step, with the particles, vibration, and water, can

damage almost all freestanding MEMS mechanical components.

Figure: A dicing saw blade cutting trenches in silicon wafer to facilitate

die separation.

Laser ablation has also been used to dice wafers in lieu of dicing saw. Since

2006, new laser stealth dicing (SD) technologies were developed, using laser

permeable to the silicon wafer to create internal modified lines invisible on the

surface. This technology would benefit MEMS device package immensely.

g. Wafer Bonding

Wafer-to-wafer bonding is a versatile technique that allows wafers with

disparate materials, surface profiles, and functional characteristics to be joined

to form unique structures. Wafer bonding involves bringing two wafers close

with proper spatial alignment to form permanent bonding under proper

physical and chemical conditions. Wafer bonding can be per-formed using a

variety of materials and temperature conditions. Wafer bonding can be aided

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through interfacial layers of thin films deposited on the wafer surfaces.

Bonding allows flexibility in processing.

Bonding processes can be categorized according to the temperature of

operation: room temperature bonding, low-temperature bonding (<100°C) and

high-temperature bonding (>100°C). Wafer bonding can be direct, i.e., without

involving any intermediate adhesive layer, or indirect, where an adhesion layer

is used. Bonding can be initiated by mechanical contact force, molecular

attractive force, or electrostatic force.

Wafers can be chemically or mechanically modified after bonding. For example,

they can be thinned to a desired thickness by mechanical polishing or chemical

etching. Wafer to wafer transfer has been demonstrated to achieve surface

planarization and to produce devices such as mirrors and membranes, even

ones with large sizes. Most bonding operations are conducted at the wafer

scale. However, bonding can be performed at die level or device level.

THE MICROELECTRONICS FABRICATION PROCESS FLOW

Figure: Fabrication process for an integrated circuit.

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A basic understanding of the fabrication technology for integrated circuit,

which precedes that of MEMS in history, is necessary to understanding the

micromachining process. A fabrication process for integrated circuits generally

involves many steps of material deposition, material removal, and patterning,

as illustrated in the following example.

A brief description of each step is presented below:

Step 1.0. The figure depicts the cross section of a starting bare silicon wafer.

The wafer thickness is not drawn to scale. Material process steps occurring on

the backside of a wafer are not drawn in future steps for the sake of simplicity.

Step 2.0. A layer of oxide is deposited. The oxide is patterned using the

following subsequent steps (2.1 through 2.3). This oxide layer is here only to

serve a transitional purpose. (This point will become obvious later.)

2.1. A photosensitive resist layer is deposited on top of the oxide by spin

coating.

2.2. The photosensitive resist is lithographically exposed and developed.

2.3. The photo resist is used as a mask for etching the oxide.

Step 3.0. The photo resist is removed using organic solvents. The patterned

oxide is used as a mask against impurity doping performed in step 3.1 through

3.3.

3.1. A layer of material containing Dopant impurities is deposited.

3.2. The wafer is thermally treated, causing the Dopant to diffuse into

silicon in areas not covered by the oxide.

3.3. The Dopant-source layer deposited in step 3.1 is removed.

Step 4.0. The oxide is removed. Note that many steps and layers of materials

are involved to transform a bare wafer (step 1.0) to a wafer with Dopant in

selective places (step 4.0).

Additional processes (steps 4.1 through 4.4) are then performed to produce

another layer of patterned oxide.

4.1. Another layer of silicon oxide is grown.

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4.2. A photosensitive resist is deposited.

4.3. The resist is lithographically patterned.

4.4. Using the resist as a mask, the oxide is etched.

Step 5.0. The resist deposited in step 4.2 is removed. From Step 4.0 to 5.0, the

major difference is oxide cover in undoped regions. An oxide layer is then

deposited and patterned (5.1. through 5.4).

5.1. A very thin oxide is grown. This so-called gate oxide layer must have

very high quality and are free of contaminants and defects.

5.2. A resist layer is again deposited.

5.3. The resist is lithographically patterned.

5.4. The resist serves as a mask for selectively etching the gate oxide.

Step 6.0. The resist deposited in step 5.2 is removed. The active regions not

covered by oxide will provide electrical contact to metal. A gate electrode, made

of polycrystalline silicon, is then deposited and patterned in steps 6.1 through

6.4.

6.1. A layer of polycrystalline (doped) silicon is deposited.

6.2. A layer of photosensitive resist is deposited.

6.3. The resist is patterned lithographically.

6.4. The resist serves as a mask for etching the underlying

polycrystalline silicon selectively.

Step 7.0. The resist is removed by using organic solvents. The difference

between steps 6.0 to 7.0 is the addition of polycrystalline silicon. Each

transistor must be connected with each other and to the outside through low-

resistivity metal wires. The metal wires are made in steps 7.1 through 7.3.

7.1. A layer of metal is deposited.

7.2. And 7.3. The metal is coated with resist and lithographically

patterned.

7.4. The resist serves as a mask for etching the metal.

Step 8.0. The resist is removed, realizing a complete field effectors transistor.

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The process used in the industry follows the basic flow diagramed in Figure but

involves more detailed steps for quality assurance, functional enhancement,

and for increasing the yield and repeatability. Many more steps may incur after

step 8.0 as well.

A complete process run from the start to the finish may take 3 months, and

20–40 mask plates.

SILICON BASED PROCESS SELECTION AND DESIGN

SILICON BASED PROCESS SELECTION

MEMS devices were first developed on silicon wafers because of the easy

availability of mature processing technologies that had been developed within

the microelectronics industry, and the availability of expertise in process

management and quality control.

Silicon actually comes in three general forms: single crystal silicon,

polycrystalline silicon, and amorphous silicon. In single crystal silicon (SCS)

material, the crystal lattice is regularly organized throughout the entire bulk.

Single crystal silicon is often encountered in three cases:

(1) Single crystal silicon wafer grown from a high-temperature

melt/recrystalization process

(2) Epitaxial grown silicon thin films;

(3) Single crystal silicon obtained from recrystalizing polycrystalline or

amorphous silicon by global or local heat treatment.

Figure: Crystal structure of single crystal silicon and polycrystalline silicon.

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A polycrystalline silicon material (so called polysilicon, polySi, or poly) is made

of multiple crystalline domains. Within each individual domain, the crystal

lattice is regularly aligned. However, crystal orientations are different in

neighboring domains. Domain walls, also referred to as grain boundaries, play

important roles in determining electrical conductivity, mechanical stiffness,

and chemical etching characteristics. The polysilicon material can be grown by

low pressure chemical vapor deposition (LPCVD), or by recrystalizing

amorphous silicon through global or local heat treatment.

Amorphous silicon, on the other hand, exhibits no crystalline regularity.

Amorphous silicon films can be deposited by chemical vapor deposition

methods (CVD), at a lower temperature than that required to deposit

polysilicon. Due to low temperature, atoms do not have enough vibration

energy to align themselves after they are incorporated into the solid.

Amorphous silicon can be grown using the LPCVD method. (In a typical,

horizontal, low-pressure reactor, the transition temperature above which

polycrystalline structure forms during deposition is 580°C.) Amorphous silicon

can be formed by plasma enhanced chemical vapor deposition (PECVD) method

as well.

The two most fundamental classes of fabrication technologies are bulk

micromachining and surface micromachining. Bulk micromachining processes

involve selectively removing the bulk (silicon substrate) material in order to

form certain three-dimensional features or mechanical elements, such as

beams and membranes. Bulk micromachining may be combined with wafer

bonding to create even more complex three-dimensional structures.

SILICON BASED PROCESS DESIGN

We use the example of a micro machined pressure sensor to illustrate a

representative MEMS process. The process involves two wafers a bottom wafer

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is etched to form a cavity whereas a top wafer is used to make the membrane.

A description for each step in the diagram follows.

Figure: Process for a micro machined pressure sensor.

Step a. The wafer is cleaned thoroughly to remove any large particles, dirt

particles, and invisible organic residues. A combined mechanical wash

and oxidizing acid bath may be used, followed by a rinse by ultrapure

water.

Step b. The cleaned wafer is placed inside a high-temperature furnace filled

with running oxygen gas or water vapor. Oxygen atoms present in the

air or dissociated from the water molecule will react with silicon to form

a protective silicon dioxide thin film.

Step c. The wafer is removed from the furnace and cooled to room

temperature. It will be very clean. A layer of thin film photoresist is

deposited on the front surface of the wafer. The photo resist is typically

spin coated. Alternatively, photo resist thin film can be deposited by

vapor coating, mist coating, or electroplating.

Step d. The photo resist is exposed through a mask with a high-energy

radiation (such as ultraviolet ray, electron beam, or X-ray).

The entire wafer is then placed inside a developing solution that removes

loosely bound photosensitive polymer. The soft bake process in step c

ensures that the photo resist will not be indiscriminately stripped by the

developer.

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Step e. The photo resists needs to be baked again, this time at a higher

temperature and often for a longer duration than the soft bake. This

second baking step, called “hard bake”, removes remaining solvents and

makes photo resist that remains on the wafer stick to the wafer even

stronger.

The photo resist mask is here used to selectively mask the underlying layer, the

silicon oxide, against a hydrofluoric acid etchant bath.

Step f. The photo resist is removed using an organic solvent etchant such as

acetone. The hard-baked photo resist is chemically resistant to the HF

etchant but not to acetone. The organic solvent does not etch the oxide

and the silicon.

Step g. The silicon wafer is immersed in a wet silicon etchant, which does not

attack the silicon oxide. Only the silicon in the open oxide window is

etched, resulting in a cavity with sidewalls defined by crystallographic

planes. The cavity may reach the other side of the wafer if the open

window is large enough for the given wafer thickness.

Step h. The wafer at the end of stage (g) is tilted to provide a clear view of the

through-wafer cavity.

Step i. A wafers are processed be very clean, because tiny particles adhering to

the bonding surfaces of either wafer will prevent good bond strength

from being reached.

Step j. The bonded top wafer is thinned by using mechanical polishing or

chemical etching. The remaining thickness of the top wafer determines

the thickness of the membrane. Thin membranes are desired to have

high sensitivity.

Step k. Strain sensors are then made on the prepared membrane. A thin film

layer (e.g., oxide) is deposited and patterned. It serves as a barrier layer

to ion implantation. Areas on the silicon wafer hit directly by energetic

Dopant ions will become doped and form a piezoresistors, which

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changes its resistance upon applied stress due to membrane bending

under pressure difference.

Figure: Pressure sensor fabrication process.

The second class of micro fabrication process for MEMS is surface

micromachining. Free-standing mechanical elements can be created by

removing an underlying place-holding thin film layer, instead of the substrate

underneath. This spacer layer, called the sacrificial layer, constitutes the

primary characteristic of a surface micromachining process. The general

concept of this process was first envisioned by physicist Richard Feynman.

Figure illustrates a typical surface micromachining process involving one

structural and one sacrificial layer. A sacrificial layer is first deposited and

patterned. This is followed by the deposition of a structural layer on top of the

sacrificial layer material. Following the fabrication of layered structures, the

sacrificial material is selectively removed to free the structure layer on top. For

example, cantilevers residing on the surface of a substrate can be made using

oxide as a sacrificial layer and polycrystalline thin film as a structural layer. In

fact, surface micromachining is so named because micro mechanical devices

reside within a thin boundary on the front surface of the wafer.

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Figure: Sacrificial surface micromachining.

These two classes of processes bulk and surface micromachining are not used

to the exclusion of one another, nor are they the only types of processes

available for MEMS. Increasingly, bulk and surface micromachining processes

are combined to create more complex structures with desired functionalities

that cannot be realized using bulk or surface micromachining alone. Further,

these two major classes of processes can also be combined with other classes

of processes such as wafer bonding, laser machining, micro molding, and three

dimensional assemblies, to incorporate a variety of bulk and thin film

materials.

PIEZOELECTRICITY

INTRODUCTION:

The phenomenon of piezoelectricity was discovered in the late nineteenth

century. It was observed that certain materials generate an electric charge (or

voltage) when it is under a mechanical stress. This is known as the direct

effect of piezoelectricity. Alternately, the same material would be able to

produce a mechanical deformation (or force) when an electric field is applied to

it. This is called the inverse effect of piezoelectricity.

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Many important properties of piezoelectric materials stem from its crystalline

structures. Piezoelectric crystals can be considered as a mass of minute

crystallites (domains). The macroscopic behavior of the crystal differs from that

of individual crystallites, due to orientation of such crystallites. The direction of

polarization between neighboring crystal domains can differ by 90 or 180.

The properties of piezoelectric elements are time dependent. The stability as a

function of time is of particular interest. Material characteristics may be

degraded through aging effects due to the intrinsic process of spontaneous

energy reduction. The speed of aging can be con-trolled through the addition of

composite elements or through accelerated aging.

CANTILEVER PIEZOELECTRIC ACTUATOR MODEL

Piezoelectric actuators are often used in conjunction with cantilevers or

membranes for sensing and actuation purposes. General models for such

piezoelectric actuators are rather complex. Accurate analysis often involves

finite element modeling. For limited cases, such as cantilever actuator with two

layers, analytical models have been successfully developed.

The deflection of a two-layer piezoelectric structure can be described by

compact formula. Consider a cantilever with two layers, one elastic and one

piezoelectric, joined along one side. These two layers share the same length. A

compact model for calculating the curvature of bending has been made under

the following assumptions:

The induced stress and strain are along axis-1, or the longitudinal axis of

the cantilever;

Cross sections of the beam originally plane and perpendicular to the beam

axis remain plane and perpendicular to the resulting curved axis;

The beam maintains a constant curvature throughout the beam;

Shear effects are negligible;

Beam curvature due to intrinsic stress may be ignored;

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The beam thickness is much less than the piezoelectric-induced curvature;

Second order effects such as the influence of d33 and electrostriction are

ignored;

Poisson‟s ratio is isotropic for all films.

Figure: Bending of a piezoelectric bimorph.

The beam bends into an arc when the piezoelectric layer is subjected to a

longitudinal strain, slong. The radius of curvature can be found by

Where, Ap and Ae are the cross-sectional areas of the piezoelectric and the

elastic layer, Ep and Ee are the Young‟s modulus of the piezoelectric and the

elastic layer, and tp and te are the thickness of the piezoelectric and the elastic

layer.

Once the radius of curvature is known, the vertical displacement at any

location (x) of the cantilever can be estimated.

The amount of force achievable at the free end of a piezoelectric bimorph

actuator equals to the force required to restore the tip of the actuator to its

initial unreformed state. Since the displacement is linearly related to force

according to

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Piezoelectric sensors and actuators with more than two layers are commonly

encountered.

Several general techniques can be found in, both under simple or arbitrary

loading.

PROPERTIES OF PIEZOELECTRIC MATERIALS

Since semiconductor materials are often used in making circuits and MEMS, it

is of interest to discuss piezoelectricity of important semiconductor materials.

a. Quartz

The most familiar use of quartz crystal, a natural piezoelectric material, is

resonator in watches. In a quartz-crystal oscillator, a small plate of quartz is

provided with metal electrodes on its faces. Quartz-crystal oscillators are able

to produce output frequencies from about 10 kilohertz to more than 200

megahertz and, in carefully controlled environments, can have a precision of

one part in 100 billion, though one part in 10 million is more common.

b. PZT

The lead zirconate titanate (PZT) system is widely used in polycrystalline

(ceramic) form with very high piezoelectric coupling. One of the most widely

used methods to prepare thin film PZT material for MEMS is sol-gel deposition.

Using this method, relatively large thickness can be reached easily using single

or multiple layer deposition.

Using a processing technique called screen printing, even thicker PZT films can

be reached in a single pass with the highest piezoelectric coupling coefficient

being 50 pC/N, significantly lower than what is achievable in bulk PZT. The

screen printing ink consists of sub-micron PZT powders obtained commercially,

and lithium carbonate and bismuth oxide as bonding agent. After screen

printing, the deposited materials are dried and then fired at high temperature

for densification. The sol-gel deposition process is constantly being advanced.

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c. PVDF

The polyvinylidenfluoride (PVDF) is a synthetic fluoropolymer with monomer

chains of (CH2-CF2-)n. It exhibits piezoelectric, pyroelectric, and ferroelectric

properties, excellent stability to chemicals, mechanical flexibility, and

biocompatibility. The piezoelectric effect of PVDF has been investigated and

modeled.

Thin-stretched PVDF films are flexible and easy to handle as ultrasonic

transducers. The material is carbon based, usually deposited as a spin cast

film from a dilute solution in which PVDF powder has been dissolved. As for

most piezoelectric materials, process steps after deposition greatly affect the

behavior of the film. For example, heating and stretching can increase or

decrease the piezoelectric effect. PVDF and most other piezoelectric films

require a polarizing after deposition.

d. ZnO

ZnO material can be grown using a number of methods, including rf or dc

sputtering, ion plating, and chemical vapor deposition. In the MEMS field, ZnO

is most commonly deposited by magnetron sputtering on various materials,

with the Z axis close to the normal of a substrate.

A popular electrode material on top of the ZnO thin film is aluminum, which

can be etched using a solution of KOH, K3Fe(CN)6, and water (1 g:10 g:100 ml).

ZnO itself can be etched using wet etchants such as CH3COOH:H3PO4: water (1

ml: 1 ml: 80 ml) at fast rate. Techniques have been developed to avoid excessive

undercutting and produce fine features.

APPLICATIONS

Piezoelectric materials can be used in many micro sensors and actuators. We

will focus on the discussion of four types of sensors: inertia sensors, pressure

sensors, tactile sensors, and flow sensors. Meanwhile, two examples of

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piezoelectric actuators will be reviewed. The case studies collectively will reveal

design, materials, and fabrication issues specifically related to piezoelectric

MEMS devices.

a. Inertia Sensors

Commercial MEMS accelerometers are primarily based on electrostatic or

Piezoresistive sensing. Piezoelectric sensors require more complex materials

and fabrication processes. Nonetheless, piezoelectric acceleration sensors have

been made in the past. Integrating piezoelectric material in MEMS is not

straightforward. First, controlling the microstructure of piezoelectric thin films

requires careful calibration and, often, dedicated equipment. Secondly, many

piezoelectric thin films are not chemically inert. Care must be exercised to

prevent damages to piezoelectric thin films during processing.

b. Acoustic Sensors

There is growing interest in using micromachining technology to create

microphones. MEMS based microphones offer good dimensional control,

miniaturization, and direct integration with on chip electronics, arrayed format,

and potentially low cost due to batch processing. Piezoelectric microphones

using diaphragms made of silicon nitride, silicon, and even organic thin film

has been made

c. Tactile Sensors

The thrust of the tactile sensor research is to quantitatively measure contact

forces (or pres-sure), mimicking human-like spatial resolution and sensitivity,

large bandwidth, and wide dynamic range.

d. Flow Sensors

Flow sensors can be built using piezoelectric principles in similar fashion as

Piezoresistive flow sensors, although the material deposition and optimization

will require more efforts in general. For example, floating-element shear-stress

sensors have been made using piezoelectric bi-morph sensors.

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e. Surface Elastic Waves

Piezoelectric materials, under proper electrical bias, can launch elastic waves

in bulk or thin films. Two most commonly encountered elastic waves are the

surface acoustic wave (SAW) and flexural plate wave (or Lamb wave). The SAW

occurs on samples of appreciable depth, whereas Lamb waves occur in thin

plates of materials.

MAGNETIC ACTUATION ESSENTIAL CONCEPTS AND PRINCIPLES

Magnetization and Nomenclatures

A magnetic field may cause internal magnetic polarization of a piece of

magnetic material within the field. This phenomenon is called magnetization.

A piece of magnetic material is made of magnetic domains. Each magnetic

domain is said to consist a magnetic dipole. The strength of internal

magnetization of the bulk magnetic material depends on the extent of ordering

of these domains. These domains contribute to a net internal magnetic field

within the magnetic material itself, if they are somewhat aligned.

Magnetic field intensity (Symbol H) represents the driving magnetic influence

external to a magnetic material. Its convenient SI unit is A/m. The

conventional unit in CGS unit system is oersted (1 A/m = 4p>103Oe).

Another term, called magnetic field density (Symbol B), represents the

induced total magnetic field inside a piece of magnetic material. The total

magnetic field accounts for the influence of the induction field and the internal

magnetization. The term B is often referred to as magnetic induction or

magnetic flux density as well. The magnitude of B can be expressed in units

within the SI unit system: Tesla, or Wb/m2, or within the CGS unit system:

Gauss (1T = 104 Gauss). The convenient SI unit for Weber is V·s. The

convenient SI unit for Weber is V·s. The convenient SI unit for Tesla is

therefore V·s/m2.

The magnetic field densities of commonly encountered magnetic objects are:

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• Common refrigerator magnet: 100–1000 Gauss;

• Rare earth magnet used in Magnetic Resonance Imaging: 1–2 T;

• Magnetic storage media: 10 mt, or 100 Gauss;

• Earth magnetic field (near equator):1 Gauss.

The relationship between B and H can be described using the following

equation:

Relative permeability of the magnetic material, and M the internal

magnetization. The magnetic susceptibility, x, is defined as mr-1. A magnetic

material with a weak and positive x is called paramagnetic; one with a weak

and negative x is diamagnetic. For paramagnetic and diamagnetic materials,

the relative permeability is very close to 1.

For ferromagnetic materials (e.g., iron, nickel, cobalt, and some rare earths),

the values for relative permeability are very large. A ferromagnetic material is

so named because iron is the most common example of this group.

Ferromagnetic materials are often used in MEMS actuation applications. The

linear relationship between B and H is only valid within a certain range of H.

The full magnetization curve for a ferromagnetic material is illustrated in

Figure. There are a number of important features to note:

- After the external induction field reaches a certain level, the magnetization

will reach a saturation point, called saturation magnetization. The saturation

represents a situation when all available domains within a piece of magnetic

material have been aligned to one another.

- A ferromagnetic material will lose a portion of its magnetization upon the

removal of the external magnetic field. The fraction of the saturation

magnetization which is retained after H is removed is called the remnance of

the material. The coercively is a measure of the reverse field needed to drive the

magnetization to zero after having reached saturation at least once. The area

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enclosed by the hysteresis curve indicates the amount of magnetic energy

stored in a magnetic material.

Their differences are easily explained using the B-H hysteresis curves. The

curve to the left in Figure shows the hysteresis curve of a hard magnetic

material, such as a permanent magnet.

Figure: Magnetization hysteresis curve

Permanent magnetic materials not only exhibit large remnant field. They also

require larger reverse field and energy input in order to switch or destroy the

built-in magnetic field. The curve to the right shows the hysteresis curve of a

soft magnetic material. The core of a transformer, for example, is desired to be

made of a soft magnet. The small stored energy allows high efficiency, low

power consumption, and rapid transition.

Figure: Representative hysteresis curves for hard and soft magnets

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FABRICATION OF MICRO MAGNETIC COMPONENTS

Magnetic actuators involve unique materials and unique structures (e.g.,

solenoids). The material preparation and fabrication techniques for

representative components of a micro magnetic system are discussed in the

following.

a. Deposition of Magnetic Materials

Although it is possible to attach small pieces of magnetic materials to micro

mechanical structures for realizing sensors and actuators, this process is

generally very inefficient. Monolithic integration of magnetic materials is more

accurate and widely practiced.

Figure: A process for electroplating of magnetic materials.

The most common technique for depositing ferromagnetic materials for micro

devices applications is electroplating. A chemical solution consisting of

constituent ions of the desired magnetic material is used as a wafer bath. The

work piece for metal deposition (wafer) is biased negatively with respect to a

counter electrode, which is placed in the bath during the electroplating session.

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In many cases, the wafer is not conductive on its own. Under these

circumstances, the surface of a wafer is first coated with a thin film metal layer

for providing negative electric biasing. This thin film layer is called the seed

layer. Common seed layer materials are copper, aluminum, or gold. Thin metal

layers of Cr or Ti are often used to enhance adhesion between the seed layer

metal and the substrate.

A typical electroplating process flow using a seed-layer is shown in Figure. A

substrate covered with a metal seed layer is prepared. In order to produce

patterned ferromagnetic thin film, a mold electroplating method is often used

(Figure. a). The mold, made of a thin film insulating layer (e.g., patterned

photoresist), is deposited and patterned (Figure b). The wafer is immersed in an

electroplating solution (Figure c). Electroplated metal grows in the open

windows, where the seed layer is exposed to the electroplating bath (Figure e).

The electro-plating mold is then selectively removed. The electroplating process

may result in thickness smaller than the height of mold (Figure d), or greater

(Figure e), depending on the duration of the plating step. When the thickness of

electroplated metal reaches beyond the height of the mold, it tends to grow

laterally.

Figure: Mushroom shaped electroplated metal, revealed after the

electroplating mold is removed.

The constitutions of electroplating bath and pertinent processing parameters

for two representative materials including NiFe and CoNiMnP are summarized

in Table.

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Besides electroplating, other fabrication processes for realizing thick magnetic

materials are also available. One such process relies on commercial polyimide

mixed with ferrite magnetic powers at different concentration levels. The

magnetic polymer composite, consisting of particles of magnetic materials

suspended in nonmagnetic media, offers the ability to incorporate magnetic

materials of arbitrary characteristics available at the bulk scale to

micromachining applications. Patterned polymer magnetic film can be

produced using either screen printing technique or, if the polymer matrix is

photo definable, spin coating followed by photolithography.

Design and Fabrication of Magnetic Coil

On chip integrated solenoids are of great interest. They can be used for

electromagnetic source, coil actuator, as well as inductors, telemetry coils, and

transformers for integrated circuits. Solenoids by conventional machining

involve wounding conducting wires around a ferromagnetic core (Figure). In

micro fabricated devices, however, this practice is prohibitively difficult due to

the small scales and lack of automated tools. Instead, the most prevalent and

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manufacturable form of electromagnet is a single layer, planar coil with air core

(Figure a). Such a coil is not capable of generating strong magnetic flux density

because of the lack of a magnetic core and the lateral spreading of wires away

from the center of the coil.

More efficient electromagnet coils have been built with integrated core and

wrap-around coils. Such coils can be classified into two categories according to

the orientation of the magnetic core those with the magnetic flux normal to the

substrate plane or those with the flux lying parallel to the substrate plane.

Figure: Magnetic field line distribution of a solenoid magnet.

Figure: A planar magnetic inductor

A number of techniques for realizing micro solenoids with integrated

ferromagnetic core are shown in Figure.

The simple scheme of electroplating a high-permeability magnetic material

with a planar coil improves upon the performance of a single layer magnetic

coil (Figure b). However, the improvement is not significant as the issue of

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strong dispersion of magnetic field persists. To contain and even concentrate

the magnetic field lines, innovative structures and fabrication processes are

involved. For example, through-wafer magnetic cores can be built by taking

advantage of the sloped surfaces created using anisotropic etching (Figure c).

The equivalent of horse-shoe magnet can be made by processing magnetic core

materials on both sides of a wafer. The magnetic flux density can be increased

by reducing the cross-sectional area of the core (Figure d).

Figure: Process flow for multiple layer planar coils.

All solenoids in Figure involve single-layered conducting wires. Coils with

multiple turns can be made by stacking (Figure). The process involves

depositing a metal coil layer, covering it with a dielectric material, planarizing

the dielectric material, and repeating the cycle. In theory, this process can be

repeated for an infinite number of times. In reality, there is practical limit to

how many layers can be stacked due to finite process time and potential

degradation of registration quality and surface roughness as the number of

layers increases.

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Three-dimensional coils have been fabricated on various substrate surfaces,

including nonplanar surfaces, using techniques ranging from micro contact

printing on a cylinder, three-dimensional assembly, laser direct lithography,

fluid self-assembly, and even conductive loops formed by bonding wires.

Magnetic coils with in-plane magnetization can be built as well. The process

flow for a typical multiturn coil is shown in Figure. The basic process consists

of three major steps: deposit and pattern a bottom conductive layer (Figure a),

electroplate the magnetic core as well as vertical conductive posts (Figure b and

c), and deposit and pattern a top conductive layer (Figure d).

Figure: Fabrication of three dimensional micro coils

CASE STUDIES OF MEMS MAGNETIC ACTUATORS

Micro magnetic actuators can be categorized according to the types of magnetic

sources and of the microstructures involved. The source of the magnetic field

can be a permanent magnet, an integrated electromagnetic coil (with or without

the core), or an external solenoid. Multiple types of sources may be used in a

hybrid manner. Force-generating microstructure, located on a chip, can be of

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one of the following kinds: permanent magnet (hard ferromagnets), soft

ferromagnets, or integrated electromagnetic coil (with or without a core).

Case 1: Magnetic Motor

Figure: A variable-reluctance magnetic micro motor.

The first example is a planar variable-reluctance magnetic micro motor with

fully integrated stator and coils in Figure. The stators are made of integrated

electromagnets, whereas the rotor is made of a soft magnetic material. The

motor has two sets of salient poles, one set in the stator (which usually has

excitation coils wrapped around the magnetic poles) and another set on the

rotor.

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When a phase coil is excited, the rotor poles located closest to the excited

stator poles are attracted to the stator pole (Figure a, b). Due to the rotation of

the rotor, the said rotor poles will align with the stator poles. The excited phase

coil is turned off, and the next phase is excited for continuous motion. In this

design, the wound poles of all phases are arranged in pairs of opposite polarity

to achieve adjacent pole paths of short lengths. The stator coils arranged in one

or more sets and phases are excited in sequence to produce continuous rotor

rotation.

A toroidal-meander type integrated inductive component is used in the motor

for flux generation. Multilevel magnetic cores are “wrapped” around planar

meander conductors (Figure c). This configuration can be thought of as the

result of interchanging the roles of the conductor wire and magnetic core in a

conventional inductor (Figure d).

Figure: Fabrication process of toroidal-meander type magnetic coil.

Step a. The fabrication process begins with an oxidized silicon wafer.

Step b. A 200-mm-thick titanium thin film is deposited as the

electroplating seed layer.

Step c. Polyimide was spin coated on the wafer to build electroplating

molds for the bottom layer of the magnetic core.

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Step d. This polyimide is coated with an aluminum metal thin film and

again with a photoresist layer, which is photo lithographically patterned.

Step e. Electroplating of Nickel-iron Perm alloy is grown to fill the openings

in the polyimide

Step f. Another layer of polyimide is spin coated to insulate the bottom

magnetic core.

Step g. A 7-mm-thick metal film (either aluminum or copper) was

deposited and patterned on top of the polyimide insulator

Step h. More polyimide is spin coated on the patterned metal to

planarizing the wafer and insulate the meander conductor

Step i. The polyimide is patterned using the same procedure as was done

previously

Step j. holes are opened all the way to the bottom magnetic core, and an

electroplating process is conducted to pro-duce the top magnetic core

Case 2: Magnetic Beam Actuation

One of the most unique performance characteristics of magnetic actuation

over electrostatic, thermal, and piezoelectric actuators is the ability to

generate torque and achieve large angular displacement.

Figure: Flap magnetic actuator

One actuator has been developed for dynamic aerodynamic control. The

actuator consists of a rigid flap supported on one side by two fixed-free

cantilevers. Each flap is made of polycrystalline silicon (as the support

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structure) and electroplated ferromagnetic materials. The cross-sectional side

view is shown in Figure a. When an external magnetic field is present, an

internal magnetization is developed in the ferromagnetic piece. The situation

is similar to the one experienced by piece #4, case c of Figure. Experimental

characterization of a single flap has been conducted to show that up to 65

angular displacements can be achieved.

The magnitude of the internal magnetization equals the saturation

magnetization, being approximately 1.5 T for the prepared material. The

direction of the internal magnetization vector lies within the plane of the flap

according to shape anisotropy (Figure b).

In a no uniform magnetic field, a torque and a force are developed on the

micro flap. (The force element is ignored in analysis.) The magnetic torque and

the angular displacement.

The process is carried out on a silicon wafer because of the high temperature

associated with the structural layer (LPCVD polycrystalline silicon) and

sacrificial layer (LPCVD oxide).

Figure: Fabrication process.

SMART MATERIALS


Step a. LPCVD oxide is deposited and patterned, followed by deposition

and patterning of polycrystalline silicon.

Step b. A metal seed layer is deposited, followed by spin coating and

patterning of photoresist as the electroplating mold.

Step c. Electroplating occurs in regions not covered by the photoresist, to

a height decided by the thickness of the spin-coated photoresist.

Step d. The photoresist is removed.

Step e. Subsequently, the sacrificial layer (oxide) is removed in HF acid

bath.

Case 3: Plate Torsion with Lorentz Force Actuation

A moving-coil electromagnetic optical scanning mirror capable of one-axis

rotation is discussed in Figure. A mirror plate is supported by torsional hinge

structures consisting of multilayered polyimide films with aluminum lead

wires in between. The mirror consists of a planar micro plate, with a smooth

side for optical reflection and the opposite side hosting a planar coil. Two

permanent magnets are placed on the side of the mirror, such that the

magnetic field lines are parallel to the plane of the mirror and orthogonal to

the torsional hinges. When current passes through the coil, Lorentz forces will

develop and cause rotational torque on the mirror. The direction of the torque

depends on the direction of the current input.

Figure: Diagram of a micro mirror.

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Case 4: Multi axis Plate Torsion Using On-Chip Inductor

Another micro mirror capable of rotation along two axes is discussed. A mirror

is suspended in a rotational gimbal that provides rotation degrees-of-freedom

in two axes, Figure a. Four planar electroplated electromagnetic coils are

located at the backside of the mirror, one occupying each quadrant. Current

passing through each coil will generate a magnetic dipole. A strong permanent

magnet based on rare-earth materials is located underneath the mirror plate.

Based on the polarity of the dipole with the external field, an attractive or

repulsive force will be acted on each coil under the interaction of induced

magnetic dipole and the permanent magnet.

Figure: Optical micro mirror capable of two-axis rotation.

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A rotational torque can be generated if two neighboring coils (joined by at least

one side) are biased in the same direction. There are four distinct possibilities

for generating torques (Figure b-e). The selective activation of these coils will

induce coupled angular displacement along two axes, in a highly selective

manner. The motions are controlled electrically under a strong biasing

electromagnetic field provided by a stationary permanent. The permanent

magnet eliminates any power consumption for biasing, and can operate over

relatively large distance and large displacement angles. The large magnetic field

H compensates for the fact that the electromagnetic field created by each

planar coil is relatively weak.

Case 5: Bidirectional Magnetic Beam Actuator

Figure: A bi-directional magnetic actuator.

A bidirectional cantilever-type magnetic actuator is discussed in Figure. At the

tip of a silicon cantilever beam, permanent magnet arrays are electroplated so

as to achieve a vertical magnetic actuator by taking advantage of the vertical

magnetic anisotropy of the magnetic arrays. The adoption of array shapes in

design of permanent magnets (multiple vertical posts as opposed to a large

sheet) allows suppression of the residual stress between the electroplated

CoNiMnP films and the establishment of preferred internal magnetization,

normal to the plane of the cantilever. A commercial inductor is used as the

electromagnet to drive the bidirectional actuator. The magnetic force acting on

the cantilever is

SMART MATERIALS


Where V is the volume of the magnetic piece, Ms the magnetization of the

magnet. To generate large force, it is advantageous to use spatially disperse

magnetic field to increase the gradient.

Case 6: Hybrid Magnetic Actuator with Position Holding

Magnetic actuation with hybrid magnetic source has been used to achieve a

latchable, bistable electrical switch. MEMS technology has been used to realize

switches and relays. A variety of techniques have been investigated.

The structure and principle of a bistable magnetic switch is illustrated in

Figure. A cantilever is elevated above the substrate surface by a torsion

support. The cantilever consists of soft ferromagnetic material (Perm alloy in

this case) on top and a layer of high conductivity metal (gold) on the bottom

(for electrical contact purposes). The biasing magnetic field is contributed by

two sources a planar coil (on-chip) and a permanent magnet (off-chip). A

planar coil is embedded beneath the cantilever. A permanent magnet located

on the backside of the silicon provides a constant background magnetic field

H0.

Figure: A bistable magnetic switch.

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The length of the cantilever is much greater than its width and thickness.

When it is magnetized inside an external magnetic field, the internal

magnetization (labeled M) is always in the longitudinal direction due to shape

anisotropy. The interaction between the internal magnetization (M) and the

external magnetic field creates a torque. However, the internal magnetization

has two stable directions, due to the initial alignment between the cantilever

and the external magnetic field. Depending on the direction of M, the torque

can be either clockwise or counterclockwise. Both angular positions,

corresponding to OFF and ON states, are stable.

The unique design of this switch is the fact that the bi-directional

magnetization can be momentarily reversed by using a second magnetic field.

This allows the torque and the position of the switch to be switched by

supplying a small current. Towards this end, a planar coil situated between

the cantilever and the external magnet is used to generate a magnetic field to

compensate the field created by the external magnet. The permanent magnet

holds the cantilever in that position under the next switching event is applied.

COMPARISON OF MAJOR SENSING AND ACTUATION METHODS

Relative advantages and disadvantages of electrostatic sensing, thermal

sensing, Piezoresistive sensing and piezoelectric sensing are summarized in

Table. Often, the choice of a sensing principle is not just based on sensitivity.

Table: Comparison of various sensing methods.

Advantages Disadvantages

Electrostatic sensing

Simplicity of materials;

Low voltage, low current operation;

Low noise;

Rapid response.

Large footprint of device necessary to provide sufficient

capacitance;

Sensitive to particles and

humidity

Thermal sensing

Simplicity of materials; Relatively large power consumptions;

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Elimination of moving parts.

Generally slower response than electrostatic sensing.

Piezoresistive sensing

High sensitivity achievable;

Requires doping of silicon to achieve high- performance

piezoresistors;

Simplicity of materials

(metal strain gauge).

Only allow doping front-facing

surfaces;

Sensitive to environmental

temperature changes.

Piezoelectric

sensing

Self-generating no power

necessary.

Complex material growth and

process flow;

Relative poor DC response due

to electric leakage across the material;

Piezoelectric material cannot

sustain high-temperature operations.

Table: Comparison of various actuation methods.

Advantages Disadvantages

Electrostatic

actuation

Simplicity of materials; Trade-off between magnitude of

force and displacement;

Fast actuation response.

Susceptible to pull-in

limitation.

Thermal actuation

Capable of achieving

large displacement (angular or linear);

Relatively large power

consumption;

Moderately fast actuation response.

Sensitivity to environmental temperature changes.

Piezoelectric

actuation

Fast response possible; Requires complex material

preparation;

Capable of achieving

moderately large displacement.

Degraded performance at low

frequencies.

Magnetic actuation

Capable of generating large angular displacement;

Moderately complex processes;

The possibility of using very strong magnetic as

bias.

Difficulty to form on-chip, high-efficiency solenoids.

SMART MATERIALS


MODULE-5

POLYMER MEMS & MICRO FLUIDICS, CASE STUDIES

Polymer MEMS& Micro fluidics: Introduction, Polymers in MEMS (Polyimide,

SU-8, LCP, PDMS, PMMA, Parylene, Others) Applications (Acceleration,

Pressure, Flow, Tactile sensors).Motivation for micro fluidics, Biological

Concepts, Design and Fabrication of Selective components, Channels and

Valves.

Case Studies: MEMS Magnetic actuators, BP sensors, Microphone,

Acceleration sensors, Gyro, MEMS Product development: Performance,

Accuracy, Repeatability, Reliability, Managing cost, Market uncertainties,

Investment and competition.

POLYMER MEMS

INTRODUCTION

Polymers are large, usually chainlike molecules that are built from small

molecules. Long chain polymers are composed of structural entities called mer

units, which are successively repeated along the chain. A bulk polymer is made

of many polymer chains. The physical characteristics of a polymer material

depend not only on its molecular weight and make up of polymer chains, but

also on the ways the chains are arranged.

Polymers can be classified into three major classes: fibers, plastics, and

elastomers (rubbers). Major discerning characteristics of these three groups are

summarized in Table. The largest number of different polymeric materials

comes under the plastics classification. Polyethylene, polypropylene, polyvinyl

chloride (PVC), polystyrene, fluorocarbons, epoxies, phenolics, and polyesters

are all classified as plastics. Many plastic materials are manufactured by

different vendors and carry different trade (common) names. For example,

acrylics (Polymethyl methacrylate, PMMA) are known as Acrylite, Diakon,

Lucite, and Plexiglas in trade. Vendors may incorporate additive substances

SMART MATERIALS


into polymers to adjust their physical, chemical, electrical, and thermal

characteristics and to change their appearances.

According to their origins, polymers can be categorized into two groups:

naturally occurring polymers and synthetic polymers. Naturally occurring

polymers those derived from plants and animals, include wood, rubber, cotton,

wool, leather, and silk. Synthetic polymers are derived from petroleum

products.

Polymers can be classified by its response to temperature. Thermal plastic

polymers (thermalplasts) can be remelted and reshaped repeatedly whereas

thermal setting polymers (thermalsets) take on a permanent shape after being

melt-processed once.

The melting of a polymer crystal corresponds to the transformation of a solid

material, from an ordered structure of aligned molecular chains, to a viscous

liquid in which the structure is highly random.

The mechanical properties of polymers differ from those of metals and

semiconductors in several major aspects. An excellent review of mechanical

properties of polymers:

Polymer materials cover a wide range of Young‟s modulus.

The modulus of elasticity may be as low as several MPa for highly elastic

polymeric materials, but may run as high as 4 GPa for some of the very

stiff polymers.

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Maximum tensile strengths for polymers are on the order of 100 MPa,

much lower than that of metal and semiconductor materials.

Many polymers exhibit viscoelastic behaviour.

- The mechanical properties are influenced by temperature, molecular weight,

additives (many proprietary), degree of crystallinity, and heat treatment history.

Mechanical properties of certain polymers can change dramatically over narrow

temperature range.

- Many organic polymers are dielectric insulators. However, certain polymers

exhibit interesting conducting behaviours. In recent years, conducting polymer

materials are being actively pursued for making transistors, organic thin film

displays, and memory. Such conductive polymers include polypyrrole,

polyaniline, and polyphenylene sulphide, to name a few.

- Polymers can be processed using a large number of techniques, including

injection molding, extrusion, thermoforming, blow molding, machining, casting,

compression molding, rotational molding, powder metallurgy, sintering,

dispersion coating, fluidized-bed coating, electrostatic coating, calendaring, hot

forming, cold forming, vacuum forming, and vapour deposition. Many

techniques can be combined with Micro fabrication.

-

POLYMERS IN MEMS

Micromachining technology for MEMS was derived from integrated circuit

fabrication. Naturally, silicon has been the predominant material choice. In

recent years, polymers have emerged as an important new class of materials for

use in MEMS applications. There are a number of unique merits associated

with polymer materials.

The cost of the material is much lower than that of single-crystal silicon.

Many polymer materials allow unique low-cost, batch-style fabrication

and packaging techniques such as thermal micromolding, thermal embossing

and injection molding. Instead of processing on one wafer at a time, polymer

SMART MATERIALS


substrates can potentially be processed in a high-throughput, roll-to-roll

fashion.

Certain polymers offer unique electrical, physical and chemical

properties that are not available in silicon and silicon-derived materials.

Examples of such properties include mechanical shock tolerance,

biocompatibility, and biodegradability.

There are barriers for using polymers in MEMS. The viscoelastic behaviour of

polymers is undesirable in certain applications. Many polymer materials have

lower glass-transition and melting temperatures. The low thermal stability

limits fabrication methods and application potentials.

The list below includes polymers that have been used successfully and widely

for MEMS applications. Some items in the list represent a family of polymers

while others represent a specific product:

a) Polyimide

b) SU-8

c) Liquid crystal polymer

d) Poly dimethyl Siloxane

e) Polymethyl methacrylate (also known as acrylics. Plexiglas, or PMMA)

f) Parylene (poly paraxylylene)

g) Poly tetra fluoro ethylene (Teflon) and Cytop.

SMART MATERIALS


Polyimide

Polyimides represent a family of polymers that exhibit outstanding mechanical,

chemical, and thermal properties as a result of their cyclic chain-bonding

structure. Bulk processed polyimide parts are used widely, from cars (struts

and chassis in some cars) to microwave cookware. It is widely used in

microelectronics industry as an insulating material as well. Polyamides are

formed from the dehydrocyclization of polyamic precursors into cyclic polymers

by incorporating aromatic groups R and R¿. These aromatic groups are chosen

to affect the properties of the final polyimide. For example, by chemically

altering the polyamic acid precursor to include R–groups sensitive to UV,

SMART MATERIALS


photo-patternable precursors can be made to crosslink where exposed to UV

light.

Cured polyimide films exhibit intrinsic stress on the order of 4*106 Pa to 4*107

Pa, as measured using suspended micro fabricated polyimide strings. Further,

mechanical and electrical properties of polyimide may exhibit direction-

dependent behaviour. Many properties such as index of refraction, dielectric

constant, Young‟s modulus, thermal expansion coefficient, and thermal

conductivity vary with processing conditions.

Polyimide is commercially available as cured sheets, semi-cured sheets, or

viscosity solutions for spin-coating. The structure of a typical commercial

polyimide HN-type Kapton. In MEMS, polyimide is used as insulating films,

substrates, mechanical elements (membranes and cantilevers), flexible joints

and links, adhesive films, sensors, scanning probes, and stress-relief layers.

Polyimide materials offer many favourable characteristics in these roles,

including

a. Chemical stability;

b. Thermal stability up to around 400 c;

c. Superior dielectric properties;

d. Mechanical robustness and durability; and

e. Low cost of materials and processing equipment.

Polyimide can be used as structural elements for sensors and actuators.

Unfortunately, polyimide is neither conductive nor strain sensitive. Functional

materials such as conductors or strain gauges need to be integrated externally.

Thin film metal strain gauges have been integrated with polyimide, exhibiting

an effective gauge factor on the order of 2 to 6. An alternative is to modify the

polyimide material for sensing purposes.

SMART MATERIALS


SU-8

The SU-8 is a negative tone, near UV photo-resist first invented by IBM in the

late 1980s, with the main purpose of allowing high aspect ratio features to be

made in thick photosensitive polymers. The photoresist consists of EPON®

Resin SU-8 (from Shell Chemical) as a main component. The EPON resin is

dissolved in an organic solvent (GBL, gamma-butyrolacton), with the quantity

of solvent determining the viscosity and the range of achievable thickness.

Processed layers as thick as 100 mm can be achieved, offering tremendous new

capabilities for masking, molding, and building high aspect ratio structures at

low cost. The cost of SU-8 lithography is considerably lower than that of other

techniques for realizing high aspect ratio microstructures, notably LIGA

process and the deep reactive ion etching. SU-8 has been integrated in a

number of micro devices, including micro fluid devices, SPM probes, and micro

needles. It can also serve as a thick sacrificial layer for surface

micromachining.

Figure: Polyimide chemistry.

a) Generic polyamic acid precursor, with thermally stable R and R‟ groups chosen for specific final proper ties in the

b) Resultant general polyimide structure after imidization (dehydrocyclization) reaction.

Liquid Crystal Polymer (LCP)

The liquid crystal polymer is a thermoplastic with unique structural and

physical properties. This characteristic differentiate LCP from most

thermoplastic polymers (e.g., Kapton), whose molecule chains are randomly

oriented in the solid state.

SMART MATERIALS


Owning to its unique structure, LCP offers a combination of Electrical,

Thermal, Mechanical and Chemical properties unmatched by other engineering

polymers. One of the earliest LCP films used in MEMS is a Vectra A-950

aromatic liquid crystal polymer. The reported melting temperature of Vectra A-

950 is 280 C. The specific gravity ranges from 1.37 to 1.42 kg/m3, and the

molecular weight is greater than 20,000 g/mol. Extensive tests showed that

LCP was not attacked or dissolved by at least the following chemicals common

in Micro fabrication:

a. Organic solvents including acetone and alcohol,

b. Metal etchants for Al, Au and Cr,

c. Oxide etchants (49% Hf and buffered Hf), and

d. Developers for common photoresist and su-8 resist.

Characteristics of Liquid Crystal Polymer (LCP)

LCP is virtually unaffected by most acids, bases and solvents for a

consider-ably long time and over a broad temperature range.

LCP films have excellent stability.

It has very low moisture absorption („0.02%) and low moisture

permeability,

The LCP film also shows excellent chemical resistance.

Compared with Kapton, LCP has a lower cost (50%–80% lower than that

of Kapton) and is melt processable at lower temperatures.

For LCP film with uniaxial molecule orientation, its mechanical

properties are anisotropic and dependent on the polymer orientation.

The thickness of LCP film could vary from several microns to several

millimetres.

PDMS

Elastomers are materials that can sustain large degree of deformation and

recover their shape after a deforming force. Poly (dimethylsiloxane) (PDMS),

SMART MATERIALS


an elastomer material belonging to the room temperature vulcanized (RTV)

silicone elastomer family, offers many advantages for general MEMS

applications.

Optically Transparent

Electrically Insulating

Mechanically Elastic

Gas Permeable

Biocompatible

The biological and medical compatibility of the material is reviewed in. PDMS is

widely used in micro fluidics. The primary processing method is molding,

which is straightforward and allows fast, low cost prototyping.

A number of unique process characteristics of PDMS are worth noting:

The volume of PDMS shrinks during the curing step. Compensation of

dimensions at the design level should be incorporated to yield desired

dimensions.

Due to volume shrinking and flexibility, deposited metal thin films on cured

PDMS tends to develop cracks, affecting the electrical conductivity.

The surface chemical properties (such as adhesion energy) can be varied by

altering the mixing ratio and through surface chemical or electrical

treatment.

PDMS is commercially supplied as a viscous liquid it can be cast or spin coated

on substrates. Unfortunately, the PDMS material is not photo definable. It

therefore cannot be simply spin coated and patterned like photosensitive

resists. Though UV curable PDMS is being developed, the technology is not yet

mature. It is possible to use plasma etching to pattern PDMS thin films.

However, the etch rate is rather slow. The measured etch rate is approximately

7nm/min at 800 W power and 100 V bias. Etching of PDMS with O2 plasma

leaves the surface and line edges rough.

SMART MATERIALS


PMMA

PMMA is supplied in many different forms, including bulk, sheets, and

solutions for spin-coating. PMMA bulk, most commonly known by its trade

name acrylics, has been used in making micro fluidic devices. The photo

definable PMMA thin film is a widely used e-beam and X-ray lithography resist.

Spin coated PMMA has been used as a sacrificial layer as well. Deep reactive

ion etching processes for PMMA thin films has been demonstrated.

PARYLENE

Parylene is a thermalsets polymer. It is the only plastic material that is

deposited using chemical vapour deposition (CVD) process. The deposition

process is conducted under room temperature. A Parylene deposition system

consists of a source chamber connected to a vacuum deposition chamber. A

dimer (dipara-xylene) is heated inside the source chamber to approximately

150C. It sublimates into a gaseous monomer, which then enters the vacuum

chamber and coats on objects within. Three parylene dimer variations are

available from commercial vendors, including Parylene C (widely used),

Parylene N (for better dielectric strength and penetration), and Parylene D (for

extended temperature performance). The Parylene film offers very useful

properties for MEMS applications, including

Very Low Intrinsic Stress,

Room Temperature Deposition,

Conformal Coating,

Chemical Inertness,

Etch Selectivity.

Parylene coating is deal for electrical isolation, chemical isolation, preservation,

and sealing. Parylene has been used for microfluidics channels, valves, retina

prosthesis, sensors (acceleration sensors, pressure sensors, microphones, and

SMART MATERIALS


shear stress sensors). The thickness of Parylene coating is generally controlled

by the amount of dimer loaded. Thickness monitors and end-of-point detectors

for in-situ Parylene thickness monitoring have been developed, for example

based on thermal transfer principles.

FLUOROCARBON

Fluoropolymers such as Teflon and Cytop provide excellent chemical inertness,

thermal stability, and nonflammability due to the strong C-F bond. They can be

used as a surface coating, insulation, antireflection coating, or as adhesion

agent. Cytop is a trademarked material (by Asashi Glass Company of Japan). It

exhibits many good properties as Teflon but offers high optical transparency

and good solubility in specific fluorinated solvents. Fluoropolymers films can be

spin coated or deposited by PECVD method. In MEMS, Teflon and Cytop films

have been used for electrical insulation, adhesive bonding, and friction

reduction.

OTHER POLYMERS

In addition to the seven polymers mentioned above, a number of emerging

polymer materials are pursued for use as functional structural layers, unique

sacrificial layers, adhesive layers, chemical sensors, and mechanical actuators.

These include biodegradable polymers, wax (paraffin), and polycarbonate.

These three classes of polymers are briefly reviewed below.

Biodegradable polymer materials have been developed and investigated for

implantable medical applications, drug delivery vehicles, and tissue

engineering matrixes. Biodegradable polymers such as polycaprolactone,

polyglycolide, polylactide, and poly lactide-co-glycolide have been demonstrated

in MEMS use. Biodegradable polymers are thermoplasts. Microstructures have

been formed by micro molding, for applications such as microfluid channels,

reservoirs, and needles.

SMART MATERIALS


Paraffin provides many interesting properties not found in other materials. For

example, Paraffin has low melting temperature (40 - 70) and high volumetric

expansion (14–16%). The melt temperature of Paraffin can be controlled by

mixing several types of Paraffin‟s with different melting temperatures together.

It can be selectively etched by certain organic solvents (such as acetone) very

quickly at room temperature, and offers good chemical stability against many

strong acid solutions such as HF.

The use of Paraffin can lead to many interesting transduction mechanisms and

Micro fabrication techniques. At large scale, Paraffin has been used as linear

actuators for dexterous endoscope. At small scales, paraffin-based actuators

have been used for microfluid valuing and pumping by encapsulating Paraffin

patches inside a volume with integrated heaters. Wax can be used as a mold

for fabricating complex micro structures.

Paraffin can be deposited using thermal evaporation, and patterned using

plasma generated with an oxygen and Freon 14 gas mixture. Since the melting

temperature is low (75 C for Logitech 0CON-195 or n-Hexatriacotane), all steps

following Paraffin deposition must use low temperature processes or engage

active substrate cooling.

Polycarbonate is a tough, dimensionally stable, transparent thermoplastic that

can be used in many applications that demand good performance

characteristics over a wide range of temperatures. Commercial polycarbonates

are supplied in three grades: machine grade, window grade, and glass-

reinforced grade. Un-notched polycarbonate has very high-impact strength,

excellent dielectric strength, and electrical resistivity. Polycarbonate can be

processed with injection molding, extrusion, vacuum forming, and blow

molding. Polycarbonate parts can be bonded easily and welded. In MEMS,

polycarbonate has been used for Micro fabrication of micro channels using

either sacrificial etching or molding. Polycarbonate sheets with ion track etched

SMART MATERIALS


holes have found use for filters with unique ionic filtering capabilities due to

the nanometre-sized diameters and uniformity of these holes.

Despite the progress in recent years, a large number of polymer materials that

are widely used at the macroscale are not yet exploited for MEMS applications.

Many polymer materials can potentially find applications in MEMS in the

future. These candidates include conductive polymers, electro active polymers

such as polypyrrole, photopatternable gelatin, polyurethanes, shrinkable

polystyrene film, shape memory polymers, and piezoelectric polymers such as

Polyvinylidene fluoride (PVDF).

Further, there are seemingly endless ways to modify polymer materials. For

example, it has been discovered that the functional, electrical and mechanical

properties of many polymers can be altered by additives such as nanoparticles,

carbon nanotubes, and nanowires.

REPRESENTATIVE APPLICATIONS

Many unique materials properties and fabrication techniques of polymer

materials can best be understood by examining applications that involve them.

We shall review four types of sensor devices, made using thin film polymers or

polymer bulk substrates.

Acceleration Sensors

Acceleration sensors can be made entirely or partially out of polymer materials

using a variety of transduction principles. These generally involve depositing

functional thin films on polymer substrates or microstructures.

Pressure Sensors

The surface micromachining process and the use of metal as strain gauges

completely eliminate the need to use thin film silicon or substrates, thus

reducing the cost of development and the cost of final devices.

Flow Sensors

SMART MATERIALS


Most existing micro machined sensors have been developed using single crystal

silicon substrates. An important reason for making sensors out of silicon lies in

the fact those piezoresistive elements can be realized in silicon by selective

doping. However, silicon devices are relatively expensive and brittle when

compared to polymer and metal-based devices. A silicon beam may fracture

easily in the presence of shock or contact. Flow sensors with polymer elements

are re-ported.

Tactile Sensors

Among the various types of sensors discussed in this book pressure,

acceleration, flow, and tactile sensors the tactile sensors has the most stringent

requirement of sensor robustness. They must be able to withstand direct

contact and over loading. It is advantageous to incorporate polymers in tactile

sensors in increase the level of robustness.

MICRO FLUIDICS APPLICATIONS

PREVIEW

Microfluidics represents a new and interdisciplinary research area. This

chapter serves as an introduction of this exciting research area to interested

readers. Because materials for microfluidics channels, reactors, sensors, and

actuators must be compatible with biochemical fluids and particles, this

subject area challenges MEMS developers to incorporate new materials and

develop practical, effective, and low cost solutions to sensing and actuation.

MOTIVATION FOR MICROFLUIDICS

Sophisticated chemical and biological analytical procedures, for applications

such as medical diagnosis and environmental monitoring, have traditionally

been conducted in dedicated laboratories by highly trained personnel. These

protocols are performed on bench tops and in test tubes and beakers. This

bench-top norm has limited accessibility, long turn-around time, complex

logistics (e.g., sample transportation and storage), and high costs.

SMART MATERIALS


A microfluidics system for chemical and biological diagnosis is known as a

“laboratory-on-a-chip” or a “micro total analysis system” (mTAS). The naming

of the field reveals its inspiration and motivation. The last three letters of the

word “microfluidics” “ics” are identical to the last three letters of the world

“microelectronics”.

Much like microelectronics circuits revolutionized the signal processing and

communication, fluid reactions carried on in integrated, miniaturized

microfluid channels and reactors promise major changes in practices of

medical diagnosis and intervention, drug discovery, environmental monitoring,

cell culture and manipulation of bio particles, gas handling and analysis (e.g.,

component separation or heat transfer), heat exchange, chemical reactors (for

power and force production) , and bioterrorism defence.

Some major benefits of using microfluid platforms to replace bench-top

chemistry are:

a. A microfluidics system reduces dead volumes associated with a chemical

assay system with large-scale chambers and connectors.

b. A microfluidics system reduces the amount of chemical assays and solutions

required and thus can potentially reduce cost by saving the amount of

expensive chemicals and biological samples used for a given analysis.

c. A microelectronics-style bulk fabrication will reduce the cost of sophisticated

systems. Lithography and parallel fabrication reduces the difficulty of

building sophisticated fluid piping and reaction networks.

d. Microfluidics can achieve high level of multiplicity and parallel operations to

increase the efficiency of chemical and biological discovery.

e. Micro scale fluid elements also find broader use beyond biological and

chemical analysis. Many applications are being sought in areas such as

optical communication, tactile display (e.g., refreshable Braille display), IC

chip cooling, and fluid logics. Microfluid has also been used for performing

novel Micro fabrication and nanofabrication.

SMART MATERIALS


ESSENTIAL BIOLOGY CONCEPTS

Microfluidics systems are used to handle and interact with biological and

chemical particles and substances, including cells and polymers (e.g., DNA and

proteins). In this section, a concise overview of major characteristics of

essential biological and chemistry elements that pertain to the design,

fabrication and functions of microfluidics systems are presented.

A MEMS developer should be at least conversant about key terminologies and

concepts of biology and chemistry. A few of these are reviewed below. Interested

readers should refer to textbooks in the area of biology and chemistry for more

details.

Cells are basic functional units of life. The function of a cell is determined by

the genetic sequence it carries. A basic human cell stores genetic codes,

reproduces such codes upon cell division, and manufactures protein molecules

based on such codes. Cells can develop a rich variety of functional

differentiation based on the genetic codes. Cells communicate with their

outside environment through a highly sophisticated, compliant cell wall. The

cell wall is made of a lipid bilayer lined with ion channels, tiny channels that

allow ions (such as potassium and sodium) to pass selectively, in two ways.

Bacteria and viruses are special forms of cells. Bacteria, for example, do not

contain cell nucleus. A virus, on the other hand, does not have the ability to

divide and reproduce. It may only do so after infecting a host cell and taking

over the reproduction mechanism.

DNA Life is only possible because each cell, upon division, transmit to the next

generation the vital information about how it works. The substance that carries

the information is a polymer called deoxyribonucleic acid (DNA), a large

molecule with a molecular weight as high as several billion. The monomers that

comprise the nucleic acids, called nucleotides, are composed of three distinct

parts a five-carbon sugar, a nitrogen-containing organic base, and a

phosphoric acid molecule (H3PO4). Four nitrogen-containing bases are found

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cytosine (C), thymine (T), adenine (A), and guanine (G). The cell of humans

carries a total of 3 billion base pairs of nucleotide molecules. Segments of the

DNA chain, called genes, regulate the production of proteins based on the

specific sequence of nucleotide arrangement of the gene. The code transmits

the intended primary structure of the protein to the construction “machine” of

the cell.

Protein If DNA is the basic code of life, protein is the agent that carries out the

intent of the code. Protein is a natural polymer. It makes up about 15% of our

bodies and has molecular weighs that range from approximately 6000 to over

1,000,000 grams per mole. A protein molecule is made of a chain of a-amino

acids. 20 basic types of amino acids are found in life. The order of amino acids

in the protein is called the primary structure, conveniently indicated by using

three letter codes for the amino acids.

Lock-and-Key Biological Binding Chemistry and biology is filled with

examples of lock-and-key protocols highly selective, self-regulated assembly of

two or more entities with recognition deriving from chemical bond forces

and/or folded shapes of proteins. Many biological binding events are very

specific and strong, allowing chemical recognition and mechanical construction

of molecular conjugates. There are no engineering equivalent of such selective

and automated selection processes given its tailor-ability, accuracy of

selectivity, and prevalent use. Some of the most commonly exploited biological

binding protocols include:

- Binding between antibody and antigens

- Binding between biotin and streptavidin molecules

- DNA complementary binding

Molecular and Cellular Tags Certain cells, chemical and biological molecules,

and ions, when present in a fluid environment, are too small and scattered to

be detected easily. To report the location, species, binding characteristics, and

the environment conditions (pH, temperature) of a biological cell or molecule,

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special tags (or beacons) are frequently used. Tags are designed to bind

specifically to cells or molecules of interest, and allow visualization,

identification, selection, and capturing of such cells or molecules. Tags vary in

size and operational principles. Frequently used tags include fluorescent

particles and molecules, and surface-functionalized beads and particles made

of magnetic, metallic, or dielectric materials.

DESIGN AND FABRICATION OF SELECTIVE COMPONENTS

A microfluid chip is made of many categories of components. We discuss two

most important ones in this section, namely channels and valves. Other

component categories include heaters, mixers, fluid reactors, and reservoirs.

The design and fabrication methods for these elements must be compatible

with those of channels and valves.

1. Channels

Microfluid channels are the most important components in a microfluid

system, despite its relatively simple form and function compared with others

(such as pumps, valves). The selection of the channel material is the starting

point for any development efforts of microfluidics systems. There are several

important aspects that must be taken into consideration when selecting

channel materials and subsequent fabrication methods. These include:

a. Hydrophobicity of the channel wall. Liquid moves freely in channels with

hydrophilic walls by capillary action, simplifying sample loading and

priming.

b. Biocompatibility and chemical compatibility. Ideally, the channel wall should

not react with the fluid, particles or gases within.

c. Permeability of channel material to air and liquid. High permeability will

cause excessive loss of fluid or, in the case multiple channels are placed

close to each other, cross-contamination.

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FIGURE: Representative fabrication processes of channels on silicon

substrates.

d. Retention of chemicals on walls. Walls that retain chemicals may cause

cross-contamination during repeated use.

e. Optical transparency. Optically transparent walls facilitate observation and

quantitative assay analysis.

f. Temperature of the processing. Low temperature processing is always

desirable. High-temperature processes would narrow the choice of structural

and surface coating materials.

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g. Functional complexity and development cost. The materials for channels

should be amendable to integration of active components such as pumps

and valves.

The barrier to prototyping and manufacturing should be low.

The materials often order the fabrication method and performance

specifications. Material selection plays an important role of determining the

channel geometries achievable. Materials and technologies for other

components, such as pumps and valves, must be compatible with the channel

wall material and fabrication process.

Research work in microfluidics started in two distinct research communities:

the MEMS community and the analytical chemistry community. These two

communities used different sets of materials.

In the early days of microfluid systems development and applications, the

channels were often made of inorganic materials commonly found in MEMS

studies, such as silicon, silicon dioxide, silicon nitride, polycrystalline silicon,

or metal. The fabrication processes include bulk etching (wet or dry etching),

sacrificial etching, wafer-to-wafer bonding, or any combination of these steps. A

few representative fabrication methods are illustrated Figure for making

channels of various cross-sectional shapes.

In the analytical chemistry community, researchers developed channel

fabrication processes based on familiar materials (glass) and simply fabrication

(wafer bonding). There are many problems associated with silicon-based

microfluid devices despite its ability to generate sophisticated channel cross

sections. Silicon, for example, is not an optically transparent material. Special

fluid imaging and tracking methods have to be developed. The silicon micro

fabrication is not accessible by chemists and biologists and often proves to be

very expensive and inaccessible for rapid prototyping.

In Table, I compare the relative merits of several representative material

systems for microfluid channels according to the criteria presented earlier:

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Valves

Valves are important elements in a micro fluid system. They provide complex

system-level functionalities to a laboratory-on-a-chip system. The following

factors are generally considered when selecting or developing a micro machined

valve:

- The reliability of valve operation. Ideally, a valve should be leak free during

off state and open during ON state.

- The repeatability of valve operation.

- The ability to withstand large pressure.

- The simplicity of valve construction.

- The simplicity of valve operation and control.

- Biocompatibility with the fluid and biological particles.

Valves can be classified according to the mode of operations into several

categories:

- Cyclic valves can be operated multiple times. They can be constant on,

meaning the valve holds its open position without active input of power, or

constant off, meaning the valve maintains sealed position without active power.

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- One-time valves are operated only once during the life of operation.

Constant-on valves will seal off a channel permanently when activated.

Constant-off valves will open once it is activated, for applications such as

collection of environmental samples.

Since the valve is critical for the performance of a microfluid system and for

enabling miniaturization, many valve designs have been developed in the past.

Generally, valve structures fall into the following categories:

- Hard-membrane valves

- Soft-membrane valves

- Plug valves

- Threshold valves

Hard membrane valves use membranes of one of the following materials:

single-crystal silicon, polycrystalline silicon, LPCVD silicon nitride, piezoelectric

thin films, metal thin films, or non-elastomeric organic polymers (such as

Parylene, polycarbonate). Hard membrane valves can be operated by a variety

of principles, the most common ones being based on piezoelectric, electrostatic,

electromagnetic, thermal bimetallic, pneumatic and thermal pneumatic,

actuators. Valves can be based on hybrid combination of principles. For

example, a pneumatic valve may use electrostatic force for holding closed-gap

positions. Hard membranes generally cannot provide good seal in the off state,

especially for regulating valves.

Soft membrane valves uses valves made of elastomer such as PDMS [80]. The

operation principles of elastomeric membranes are limited compared with those

of hard membranes. Since the membrane is soft, it is difficult to integrate

elements such as electrodes. However, soft membrane seal very well and is the

material of choice for conventional valves.

Plug valves can be based on a variety of principles. For example, valves can be

developed by exploiting the large swelling and shrinking capability of ionic

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hydro gels in response to chemical concentrations, pH, temperature, and

electric field or by congregating magnetic or chemically modified particles.

Threshold valves changes its on/off state depending on the pressure or flow

rate. Threshold valves often leverages surface tension principles. Burst valve is

a special kind of threshold valve its state changes from closed to open when the

pressure at its head reaches a certain level.

CASE STUDIES

Case 1: Parallel-Plate Capacitive Accelerometer

Capacitive acceleration sensors are surface micro machined on a wafer with

integrated MOS detection circuitry. The sensor consists of a metal-coated oxide

cantilever with electroplated gold patch at its distal end serving as a proof

mass. The counter electrode is made of heavily doped p type silicon. The

capacitor gap is defined by an epitaxy silicon layer grown on a silicon surface.

Step a. A surface micromachining process was developed, using thermal oxide

as the cantilever structural material and epitaxially grown silicon as the

sacrificial layer.

Step b. The process starts with an n-type, silicon wafer. A heavily boron doped

region is made using an oxide layer as the doping barrier.

Step c. Between steps a and b, the oxide growth, deposition and patterning of

photoresist, and the subsequent oxide etch and photoresist removal are

skipped.

Step d. Another layer of oxide is deposited and patterned,

Step e. Serving as a mask for etching via hole.

Step f. A barrier for doping to form drain, source, and electrical conduction

paths on the slopes via hole.

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Step g. The doping is conducted using ion implantation. During the hole etch,

the heavily doped region will not be attacked because the etchant

reduces it‟s etch rate on heavily doped silicon.

FIGURE: Capacitive accelerometer.

Step h. The oxide barrier is removed.

Step i. This step is followed by the growth of another layer of thick oxide, which

serves as the dielectric insulator, the cantilever, and etching barrier in

regions other than the gate.

Step j. A layer of metal is deposited and patterned. It provides electrical

interconnects to the bottom p+ electrode, electrode on top of the oxide

cantilever, and gate of the field effect transistor.

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Step k. The metal layer consists of Cr followed by thick gold, with the Cr used

to satisfy a critical requirement of enhancing the adhesion between the

gold and the substrate. Finally, a wet silicon etch is performed to

undercut epitaxial silicon beneath the oxide cantilever.

The released cantilever is naturally bent due to intrinsic stress present in

the metal and oxide thin films. The upward bending is approximately 1.5 at the

end of the cantilever. Under the influence of applied acceleration, the beam will

further deform from the stationary profile.

Case 2: Torsional Parallel-Plate Capacitive Accelerometer

FIGURE: Schematic diagram of a surface micro machined parallel-plate

capacitor serving as an accelerometer.

Exposure of a silicon wafer with circuitry to high temperature under

prolonged duration may cause dopants in active regions of the circuits (e.g.,

source and drain) to diffuse out. This will irreversibly change electrical

characteristics or introduce device failure in extreme cases. The top-level

surface micro machined structures need to be deposited and processed under

relatively low temperatures. The oxide structural layer used in Parallel-Plate

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Capacitive Accelerometer requires high temperature to be deposited and is

therefore not suited here.

The new device consists of a flat nickel-top plate supported by torsional

bars. Counter electrodes are located on the substrate surface. Since the plate

weight is asymmetrically distributed with respect to the rotational axis,

acceleration along normal axis to the substrate will cause the top plate to rock

in one direction or another.

Step a. The process starts with a silicon wafer that has already gone through

the complete cycle of IC fabrication. Conducting electrode patches on

the IC wafer serve as the bottom electrodes.

Step b. First, a conductive layer, metal 1, is deposited over the substrate

surface.

Step c. This serves as a seed layer for subsequent electroplating. A second layer

of conductive metal (metal 2) is deposited and patterned, forming

bottom electrode patterns.

Step d. The combined thickness of these two metal layers is 5 mm. Next, a

photoresist layer is deposited and patterned, opening windows to reach

the metal 1 layer.

Step e. Electro-plating of nickel takes places in the open window to define the

movable plate.

Step f. The thickness of the electroplated nickel determines the thickness of the

movable plate and that of the torsional bar. The photoresist is removed,

following by the etching of the sacrificial metal. The bottom conductive

layer (seed layer) is etched as well. It is important to make sure that the

metal 1 layer underneath the anchor is not removed.

All steps, including deposition and etching, take place under room

temperature.

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The sensor‟s response is calibrated within a broad temperature range (- 55

to 125 C) as dictated by military and automotive industry specifications. Here,

capacitive sensing is advantageous over other modes of sensing (e.g.,

piezoresistive) as the temperature sensitivity is relatively low.

Case 3: Membrane Parallel-Plate Pressure Sensor

A membrane pressure sensor can detect pressure differential across the

membrane. Two pressure ports are typically required. To simplify the pressure

sensor design and use, absolute pressure sensors are often desirable. In such

sensors, the reference pressure at one side of the membrane is integrated. One

popular choice is to provide a zero pressure reference (vacuum) by hermetic

sealing.

FIGURE: Fabrication process of pressure sensor with sealed cavity.

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A pressure sensor with batch processed, hermetically sealed vacuum

chamber. The use of vacuum avoids expansion of trapped air and increases the

bandwidth by eliminating air damping inside the cavity. The device must retain

resolution over a temperature range from - 25 C to 85 C. However, the need for

vacuum packaging and integration with integrated circuits presents a critical

challenge.

The sensor cross section is shown in the last step of Figure. A membrane made

of doped silicon serves as the pressure sensing element and one electrode. The

counter electrode consists of patterned metal thin film on the bottom substrate

(made of glass in this case). A fabrication process was developed where the

micro machined silicon membrane was transferred to a glass substrate.

Step a. The process begins with a silicon wafer.

Step b. An oxide mask is deposited and patterned.

Step c. Serving as a chemical barrier during a wet anisotropic etching of

silicon

Step d. 9-mm-deep recessed region is created with the slopes being surfaces.

Step e. An-other layer of oxide is grown, this time as a conformal coating.

Step f. The oxide is then photo lithographically patterned.

The depositing of photoresist film over a wafer with recessed cavities poses a

challenge. The uniformity of the spin-coated resist layer will be negatively

impacted by the presence of surface topology. Further, photo exposure action

on the bottom of the cavity, which is 9-mm away from the ideal focus plane of

the photolithography exposure tool, reduces line width resolution. During

process, caution should be exercised whenever photolithography is performed

on wafers with significant topographic features.

Step g. A boron diffusion step at 1175 C is conducted.

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Step h. To form doped regions as thick as 15 mm. This is followed by the

stripping of the oxide and growth and patterning of yet another oxide

layer.

Step i. The second oxide layer is used in a subsequent boron dif-fusion

step to define a doped region (depth = 3 mm), which becomes the

thickness of the membrane diaphragm.

Step j. A layer of silicon oxide is deposited and patterned, to form a

dielectric insulation.

Step k. The researchers patterned via holes in the oxide, which allow a

subsequently deposited polysilicon to contact the boron doped region and

provide electrical contact with the membrane later.

A short diffusion session (at 950 C) is performed to dope the polysilicon; this is

followed by a chemical mechanical polishing (CMP) step to increase the top

surface smoothness. The polishing step enhances the yield of the sealing step

later.

Step l. A layer of metal (consisting of Cr and Au) is deposited and

patterned, with the gold facing the front side of the wafer.

Step m. An oxide layer is deposited and patterned to re-side on the bottom

of the cavity to provide electrical isolation in case the top membrane

touches the bottom electrode. The researchers flip bonded the wafer onto

a glass wafer, which is coated with a composite Ti-Pt-Au layer.

Step n. The backside of the silicon wafer is etched in an anisotropic silicon

etchant to dissolve the silicon other than the heavily doped, raised

membrane.

Alternatively, hermetically sealed cavities can be formed by chemical vapour

deposition under vacuum to encapsulate strategically placed etch holes.

Capacitive pressure sensors with or without hermetic sealing can also be made

using surface micromachining processes.

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Case 4. Membrane Capacitive Condenser Microphone

A condenser microphone, which is a pressure sensor for measuring acoustic

pressure fronts created when sound waves travel through air or liquid. Sound

waves are oscillating pressure waves.

A condenser microphone consists of a parallel-plate capacitor; with one

solid plate (called a diaphragm) that moves under incoming acoustic waves

while another one being perforated (called a backplate). The perforation

reduces the amount of plate deformation. A capacitor formed between these

two electrodes would therefore change its value in response to incoming sound

waves. The monolithic integration of the capacitor with integrated circuit is key

to realizing high resolution and miniaturization.

FIGURE 4.11: Fabrication process of condenser microphone.

The condenser microphone does not involve wafer bonding. The schematic

diagram of the microphone is shown in the Figure. Mechanically, the device

consists of a perforated plate made of polyimide thin film, and a solid plate

made of the same material. Metal conducting thin films are integrated with

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both plates. The capacitor is electrically connected to the on-chip integrated

circuitry.

The fabrication process of this device combines surface and bulk

micromachining steps. It starts with a silicon wafer that contains fully

processed integrated circuit elements.

Step a. The wafer is covered with a passivation oxide dielectric.

Step b. A composite metal thin film (consisting of chromium, platinum,

and chromium) is deposited and photo lithographically patterned. A layer

of photo patternable polyimide is deposited and patterned, overlapping

with the metal thin film below. The researchers used a Cr layer to

increase adhesion between the platinum layers to the surrounding

structural layers.

Step c. A layer of aluminium is deposited on top of the polyimide, with its

thickness defining the gap of the future parallel-plate capacitor.

Step d. On top of the aluminium, a composite metal thin film (Cr/Pt/Cr) is

deposited and patterned to form a conducting plate with perforation

holes.

Step e. Another layer of polyimide is deposited and patterned, with proper

registration to the perforated electrode plate below.

Step f. Next, a layer of chromium is deposited on the backside of the wafer

and patterned. The chromium layer provides sufficient selectivity during

the subsequent deep reactive ion etching, which etches through the

backside of the wafer, exposing the backside of the first Cr/Pt/Cr

composite layer.

Step g. The aluminium sacrificial layer is then etched away, resulting in a

finished device.

In the finished device, the diaphragm exhibits a tensile intrinsic stress of 20

MPa, which is ideal for keeping the diaphragm flat. The thickness of the

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diaphragm is 1.1 mm, whereas the thickness of the backplate is 15 mm. The

gap of the capacitor is 3.6 mm. The size of the membrane is 2.2 mm by 2.2

mm, with the size of acoustic holes and the spacing between them being 30 *

30 mm2 and 80 mm, respectively.

Case 5. Capacitive Boundary-Layer Shear Stress Sensor

Fluid flowing past a solid surface introduces a boundary layer, inside which the

flow velocity is reduced. Inside the boundary layer, the velocity varies with the

distance to the wall surface (y). The shear stress is defined as the velocity

gradient at the boundary multiplied by the viscosity of the fluid:

Shear stress sensors reveal critical fluid flow conditions at the bottom of the

boundary flow, which are difficult to measure conventionally. The area integral

of shear stress produces drag force. The shear stress information can be used

for active control of turbulent flow field, for actively monitoring fluid drag, and

for achieving drag reduction.

Techniques for measuring fluid shear stress fall into two categories: direct

measurement and indirect measurement. Two popular techniques are the hot-

wire/hot-film anemometer (indirect measurement) and the floating-element

technique (direct measurement).

Floating-element shear stress sensor was the first MEMS shear stress

sensor developed. The floating element shear stress sensor determines the

magnitude of local shear stress directly by measuring the drag force it

experiences. As shown in Figure, a suspended floating element is flush

mounted on the surface of a wall. The displacement of the floating element due

to the shear force (drag force) acting on the plate is transduced into plate

displacement, which can be measured by a variety of techniques, including

electrostatics, piezoresistivity, piezoelectricity and optical sensing.

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FIGURE: A floating element shear stress sensor.

The electronics detection uses a differential capacitance readout scheme.

Three passivated electrodes are located on the surface of the wafer underneath

the element and a thin conductor is embedded in the polyimide. The coupled

capacitances between the drive electrode in the center and the two

symmetrically placed sense electrodes are modified by motion of the floating

element. This change in capacitance is transduced by connecting the sense

electrodes to a pair of matched depletion-mode MOSFET‟s on chip.

The fabrication process for the device begins from a silicon wafer with MOS

circuits already defined. The entire wafer is passivated with 750-nm-thick

atmospheric chemical vapour deposition of silicon dioxide and a 1-mm

polyimide layer (Dupont 2545). The polyimide is added between the passivated

electrodes and the sacrificial layer to eliminate stress cracking in the silicon

dioxide layer. A 3-mm-thick aluminium layer is evaporated as the sacrificial

layer. It is patterned photo lithographically. A 1-mm-thick polyimide layer is

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coated again and cured. This is followed by the evaporation of a 30-nm thick

chromium layer, which serves as the floating electrode. A 30-mm-thick

polyimide layer is applied in seven coating steps. A layer of aluminium is

deposited on the top layer and serves as a mask for etching the polyimide to

define the plate and the cantilever. .

Case 6: Multiaxis Capacitive Tactile Sensor

In this case, a compact sensor capable of measuring normal contact and shear

contact in two axes was made. A parallel-plate capacitor is formed by bonding

a silicon wafer with a glass one. One piece consists of a cone-shaped silicon

mesa suspended by a circular silicon membrane with a thickness of t and a

radius of a. The glass piece consists of a recessed region in which electrodes

are patterned. Four electrodes, each with an area of L2, are arranged in a quad

configuration. Four capacitors are formed, between the four electrodes and the

suspended plate. These are denoted C1 through C4. If a normal force is applied

perpendicular to the substrate, the distance between the movable mass and

the bottom electrodes is reduced uniformly for all four capacitors.

FIGURE: Schematic diagram of a bulk-micro machined parallel-plate capacitor

serving as a differential mode tactile sensor.

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However, if a shear force is applied to induce rotational movement of the silicon

mass, the changes of capacitance for the four capacitors will be different. Two

capacitances increase while the other two decrease, with almost the same

degree of change.

The processing of the silicon part began with a standard p-type silicon wafer,

which is polished on both sides (Figure a). All standard IC processes were

performed on the front side. First, a buried n-type layer (3.5 mm deep) was

formed by doping. Then a 6-mm-thick n-type epitaxial silicon layer was grown

(Figure b). The buried n type layer and the epitaxial layer constitute the

thickness of a flexible membrane. A deep p-type diffusion doping is performed

to electrically isolate each capacitor electrodes (not shown). A composite layer

of silicon oxide followed by silicon nitride is grown on both sides. The silicon

nitride and oxide on the backside is patterned to serve as an etch mask for wet

anisotropic etching (Figure d). After the etching, a contact pad on top of a

membrane is formed by anisotropic silicon wet etch (Figure e). The silicon

nitride and oxide layers are then re-moved using wet chemical etchants. The

silicon wafer is then bonded to a glass wafer, which consists of a recessed

region (3 mm deep) with patterned electrodes on the bottom. Anodic bonding is

achieved at 400 C with a voltage bias of 1000–1200 V.

FIGURE 4.14: Fabrication process of tactile sensor.

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The sensor output with respect to applied calibration forces has been

characterized. In the range of 0 to 1 gram, normal forces cause a capacitance

change of 0.13 pF, whereas shear forces causes a differential capacitance of

0.32 pF. The capacitance change is linearly proportional to the calibration force

within this range.

Case 7: Gas Chromatography Channels

Chromatography involves a sample (or sample extract) being dissolved in a

mobile phase (which may be a gas, a liquid or a supercritical fluid). The mobile

phase is then forced through an immobile, immiscible stationary phase. The

phases are chosen such that components of the sample have differing solubility

in each phase. A component which is quite soluble in the stationary phase will

take longer to travel through it than a component which is not very soluble in

the stationary phase but very soluble in the mobile phase. As a result of these

differences in mobility‟s, sample components will become separated from each

other as they travel through the stationary phase.

The time between sample injection and an analyte peak reaching a detector at

the end of the column is termed the retention time (tR). Each analyte in a

sample will have a different retention time. The time taken for the mobile phase

itself to pass through the column is called tM.

In 1975, a group of researchers at Stanford University unveiled an integrated

gas chromatographer (GC) made from glass wafers shown in Figure. The gas

chromatographer device separate components within a gas mixture and

analyze relative concentrations of gas species. Gas separation channels have

semicircular cross sections, 200 mm across and 40 mm deep On a 4 diameter

(100 mm) wafer, channels as long as 1.5 m was realized. The channel walls

were made of glass.

The system is used for chemical trace analysis of pollutants and toxic elements

in the environment. Instead of packing a channel with solid phase materials,

the wall of the channel serves as the absorption element. Under a same

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pressure gradient applied between the inlet and the outlet, difference gas

molecules exhibit different Gas species that have started at the same location

and time would arrive at the exit at different time. With the advancement of

micro fabrication technology, the GC chip can be further miniaturized. Long

GC columns can be packed with higher efficiency using three-dimensional

channels fabricated in silicon, glass, or polymer materials. tR.

Figure: Schematic diagram of an integrated gas chromatography system.

Case 8: Electrophoresis in Micro channels

Electrophoresis separation of multiple species in a liquid sample is a powerful

technique for assaying or purification. According to our earlier discussions

about electrophoresis, different species travel at different speed under a given

electric field. In order to maximize the efficiency of electrophoresis separation,

those species should begin within close vicinity of each other, rather than being

spread out over a long sample plug.

Figure: Schematic diagram of a glass micro electro separation chip.

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EP separation with precision defined plug sizes can be achieved using double-T

type injectors. A glass microchip for electro separation of biological molecules

has been developed.

First, a buffer solution is injected between the buffer and waste ports. An

analyte is then injected between the analyte and analyte waste ports. An

analyte plug with precision volume is formed between the two T junctions. An

EP potential is applied between the buffer and the waste ports, causing the

analyte plug to move along the EP column towards the waste port. Different

species within the plug of analyte are detected at end of the separation column.

Figure: Operational principle of electro separation chip.

There are many designs and fabrication methods to implement this EP

separation system. One of the simplest, and earliest demonstration, is

discussed below. It begins with a glass wafer, a material that is no different

from conventional EP separation columns. A Cr thin film is deposited over the

glass, and photo lithographically patterned (step b). The patterns in the Cr are

used to define the position and size of channels. Glass is etched in regions not

covered by Cr using a solution containing HF and NH4F to a desired depth

(step c). The Cr mask is removed (step d). Another glass chip is positioned on

top of the glass substrate and permanently bonded. The channel is therefore

made of glass entirely. Metal electrodes are inserted into ports externally.

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Case 9: Neuron Probes with Channels

Neural physiology is one of the grandest scientific and medical quests of the

mankind. Understanding of neurological physiology will lead to prevention and

curing of many debilitating illnesses that severely affect the quality of life, such

as Alzheimer‟s‟ disease and Parkinson‟s disease. In order to study neurological

signal processing, which is conducted in complex three-dimensional tissues

with myriads of connections, advanced engineering tools for imaging,

recording, and affecting neurological behaviours are needed. One of such tools

is micro machined neurological recording probe, which can be made with small

size and high density for minimizing unintended damages and extracting rich

data.

Here we discuss a representative work by Wise‟s group that incorporates

microfluid channels in micro machined neural probes. Such probes are used

for injecting solutions. The probe must be sufficiently stiff to penetrate neural

tissues. In this case, it is made of single crystal silicon. We focus on the

fabrication process of the silicon channel. Because the channel is long, it is

impractical to use embedded sacrificial layers and later remove it. The lateral

undercut of the channel would take a very long time. Also due to the length of

the channel, the cross section of the channel need to be relatively large in order

to produce sufficient flow of chemical solutions under moderate-to-low

pressure differences (to avoid harming biological tissues). It is difficult to use

deposited sacrificial-layer material to realize large cross section because the

deposition process would take a long time.

The fabrication process requires only one mask. The front surface is doped to

form a 3-mm-thick region with high concentration (part b). The concentration

is sufficiently high to effectively reduce the etch rate in anisotropic silicon

etching solutions such as EDP. The front surface of silicon wafer is etched

using reactive ion etching, which does not discriminate against silicon material

of different doping concentrations (part c). The intended channel region is

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opened through this layer in the form of a chevron pattern, as shown in Figure

d. Anisotropic silicon etching is performed to undercut materials underneath

the doped regions. The etching profile underneath the Chevron-shaped mask is

shown in Figure e. significant undercut can be achieved to produce channels

with relatively large cross-sectional areas.

Figure: Micro fabrication technique.

Deep boron diffusion is performed to define the probe shank. The entire inner

surface of the channel reaches a concentration necessary to produce etches

stop effects. The channel is sealed using thermal oxidation and LPCVD

deposited dielectrics. After the depositing and patterning of electrodes and

dielectric shields, the silicon wafer is then dissolved in anisotropic etching

solutions, which selectively remove bulk silicon with only background

concentrations, leaving silicon shanks freestanding

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Figure: Fabrication process of embedded micro channels

Case 10: PDMS Microfluid Channels

Channels made of Polydimethylsiloxane (PDMS) are very popular because of

the easy accessibility of material, rapid fabrication, and many desirable

performance aspects of the material. The PDMS material can be obtained in

viscous liquid precursor form from many vendors under various trade names,

such as Sylgard Silicone Elastomer from Dow Corning and RTV silicone from

GE Silicones. The most commonly used PDMS materials are Sylgard 184 (Dow

Corning) and RTV 615 (GE Silicones).

The precursor materials consist of two parts, the base and curing agent. The

two parts are mixed and then cured under room temperature, in vacuum, or

under elevated temperatures (rapid cure), with recommended mixing ratio,

resulting in a Thermoset, transparent elastomeric solid. The Sylgard 184

SMART MATERIALS


silicone elastomer, for example, may be cured fewer than one of the following

recommended conditions - 24 hours at 23 C, 4 hours at 65 C, 1 hour at 100 C

or 15 minutes at 150 C.

FIGURE: PDMS molding.

To realize precision three-dimensional features, the uncured precursor can be

poured over surfaces with three-dimensional patterned features (Figure, made

by a variety of means (including bulking etching, photoresist patterning, etc)

(steps a and b). Once the elastomeric material is removed, the surface features

translate into recessed or raised regions (step c). The PDMS material can then

be bonded to another piece of substrate to form an enclosed channel (step d).

The matching substrate can be silicon, glass, polyimide sheet, or even another

piece of PDMS.

The PDMS material exhibits volume shrinking in all directions after it is

removed from the mold. The dimensional change resulting from shrinking is

influenced by the material, by the amount of materials poured, and by the

curing method. This must be carefully calibrated for each use.

Case 11: Parylene Surface Micro machined Micro Channels

Surface micro machined channels have been made using photoresist as the

sacrificial layer and chemical vapour deposited Parylene thin film as the

structural layer. The use of the Paralene-photoresist systems replaces the high

temperature LPCVD polysilicon/oxide system.

SMART MATERIALS


A representative fabrication process for realizing Parylene channels,

monolithically connected to fluid inlet/outlet ports in silicon substrate is

shown in Figure.

FIGURE: Fabrication process for a Parylene channel.

The process starts with a silicon wafer (step a), which is coated with a thin

layer of silicon dioxide

The oxide on the backside is photo lithographically patterned, and used as a

mask for anisotropic silicon etching (step b).

A layer of photoresist is spin coated and patterned (step c).

The front side of the wafer is then coated with a layer of Parylene thin film (step

d).

A layer of polyimide is spin coated and patterned to mechanically enhance the

stiffness of the channel to prevent collapsing (step e).

The remaining silicon in the etched backside holes are removed until the

silicon dioxide on the front side is reached (step f).

This can be accomplished through anisotropic wet etching or plasma etching.

The oxide at the bottom of the cavity is then removed in a hydrofluoric acid

bath (step g).

The photoresist sacrificial material is removed using acetone to create open

channels through the now-open inlet and outlet ports (step h).

SMART MATERIALS


Many components have been incorporated in such a system, these include:

- One-time valve

- On-chip thermal pneumatic source

- Electro osmosis pumps

Case 12: PDMS Pneumatic Valves

Soft membrane valves using elastomeric (rubber) polymers are almost

exclusively used in macroscopic valves and pumps. The advantage of soft

membrane valves is good seal against liquid or air in the off state. However, soft

membrane valves are more challenging from the design and fabrication point of

view, as soft membranes and perhaps matching seats must be integrated into a

micro system. PDMS is a commonly used soft membrane material because of

its relatively simple processing and desirable softness. Large deformation (50–

150 mm) has been reached on a membrane (1*1 to 2*2 mm2) under pressure

input of approximately 100 mW for various working fluids, including air.

One representative method for making functional valves using external

pneumatic control is discussed here. The valve involves two layers of PDMS

thin film (Figure). Both the first and second layers follow the PDMS molding

method discussed in the previous case. For the first layer, the thickness of the

PDMS is kept as small as possible; hence, the ceiling directly above a channel

is very thin (Figure). The PDMS precursor is allowed to settle and planarizing

before being cured.

The second layer consists of pneumatic control lines (Figure). The first and

second layers are bonded together with channels crossing each other. Oxygen

plasma treatment can make the two layers bond permanently. The two-piece

PDMS assembly is then bonded to a substrate. The channel formed in the first-

layer PDMS is used to transport liquid, whereas channels in the second-layer

PDMS are used to convey pressure, either by gas or liquid.

SMART MATERIALS


FIGURE: Pneumatic controlled PDMS valve.

FIGURE: Fabrication process of a peristaltic pump in PDMS.

Pressure applied in second-layer channels pushes the PDMS membrane down,

sealing the channel underneath.

This method can be used to construct micro pumps. One possible

configuration is shown in Figure. The channel in the first layer crosses the

pressure lines above it in three interaction areas, forming three well-defined

PDMS membranes. Three pres-sure lines working in a peristaltic fashion will

push liquid continuously, in two possible directions.

SMART MATERIALS


PROBLEMS

Problem 1: Design

For a microfluid channel with a length of 1 mm, and a square cross-sectional

area of 20 mm2, find the volumetric flow and average flow speed if one end of

the channel is subjected to a water column that is 5 m tall. The other end is

connected to atmosphere pressure.

Problem 2: Fabrication

Identify three practical methods of forming the channel with dimensions

discussed in Problem 1, if the height of the channel is 4 mm. Part of the

channel must be transparent in the visible spectrum for optical observation.

Sacrificial etching is generally not practical due to large channel length.

Problem 3: Design

Find the Reynolds number of the flow situation of Problem 1 if the width of the

channel is 5 mm.

Problem 4: Review

Find a method to make an array of fluid channels with the length of 1–10 mm

and the size of the channel cross section being 10 nm exactly. The cross

section of the channel should be a circle or a square. Discuss the method of

patterning. Pay attention to practicality, efficiency, and accuracy.

Problem 5: Design

A segment of a microfluid channel is 10 mm long, with a rectangular cross

section of 30 mm wide and 1 mm tall. What is the required pressure to achieve

a volumetric flow rate of 10nl/min for water at room temperature?

Problem 6: Review

Draw detailed fabrication process for the gas chromatography chip of Case

14.1. Justify the choice of masking layer.


SMART MATERIALS


Draw detailed fabrication process of neuron probes with integrated fluid

transport channels according to. Draw the process at a representative cross

section along the probe. Discuss the etching selectivity in each step.

Problem 8: Design

The PDMS pneumatic valve discussed in Case 14.6 utilizes a thin elastomer

membrane with a certain area defined by the crossing of the fluid and control

lines. It forms reliable seals due to the contact of PDMS surfaces. Discuss at

least three strategies for reducing the threshold voltage necessary to close the

valve. For each strategy, discuss the effect on fabrication process.


Design a complete fabrication process for making a Parylene cantilever probe

with integrated fluid delivery channel. The channel is opened at the free end of

the cantilever. The probe consists of a bulk silicon micro machined handle. The

handle further consists of an etched cavity that fluidically communicates with

the integrated channel. The cavity serves as a fluid reservoir and inlet. Note the

sidewall of the cavity and the handle can be sloped or vertical. The drawing

shows a case with vertical walls. Detailed lithography steps can be ignored in

the drawing. The process drawing should illustrate progression for both the

channel and the handle pieces.

Problem 10: Challenge

Develop a micro valve with a footprint of no more than 1 mm2 that can be

controlled by electricity. The valve should be able to completely stop a liquid

flow with a back pressure of 30 KPa. The valve must be operated with a voltage

of less than 100 V. The smaller the footprint and the electric voltage, the better.

The leak rate of the valve should be zero. No out-of-chip pneumatic sources

should be used. The valve must be able to be repeatedly operated

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SMART MATERIALS and MEMS (15ME745)