MEMS :- An Overview

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SHRI RAM MURTI SMARAK WOMEN COLLEGE OF

ENGINEERING & TECHNOLOGY, BAREILLY

Submitted to Department of EC &AEI in partial fulfillment of

B.Tech Degree

Seminar report on

Micro-electro-mechanical Systems (MEMS)

Submitted by

Hina Bhatia

Electronics & Communication

Batch:-2008

Roll no:-0845010028

April, 2011


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Certificate

This is to certify that the seminar report entitled Micro-electro-mechanical Systems

submitted by Ms.Hina Bhatia, in partial fulfillment of the requirement for the award of the

degree of Bachelor of Technology in Electronics & Communication to SRMS Womens

College of Engineering & Technology, is record of candidates own work carried out by

her under my supervision and guidance. I am satisfied with her performance and her

presently carried out work.

Mr.Anuj Agarwal

Head of the Department

Department of EC & AEI

SRMSWCET, Bareilly


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Acknowledgement

It was a great pleasure to be a part of this seminar. At this stage after the completion of the

report, I would like to take this opportunity to thank each and everyone without their

support, encouragement and guidance; it would not have been possible to complete it

successfully.

First and foremost I would like to thank the Almighty, without whose blessings and

support, I would not have been able to complete the task assigned to me.

Then I would like to thank Mr. Anuj Agarwal Sir, H.O.D EC & AEI Department,

SRMSWCET, Bareilly, whose advice and guidance enabled me to complete this report.

I would also like to thank Mr. Sanjeev Kumar Sir, Dean SRMSWCET, Bareilly whose

continuous support and encouragement acted as a moral support in the completion of this

report.

I would also like to express my gratitude towards faculty of our department Mr. Kuldeep

Kumar Sir, Miss. Nazia Praveen Mam, Miss. Shruti Parashar Mam, Miss. Taru Agarwal

Mam and others, whose guidance and critical examination helped a lot in gaining the

required knowledge and improving my skills.

Lastly, I would like to thank my friends and family members as they acted as a backbone

and gave continuous moral support and encouragement to complete the report.

Hina Bhatia


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Abstract

Imagine a machine so small that it is imperceptible to the human eye. Imagine working

machines no bigger than a grain of pollen. Imagine thousands of these machines batch

fabricated on a single piece of silicon, for just a few pennies each. Imagine a world where

gravity and inertia are no longer important, but atomic forces and surface science

dominates. Imagine a silicon chip with thousands of microscopic mirrors working in

unison, enabling the all optical network and removing the bottlenecks from the global

telecommunications infrastructure. You are now entering the micro-domain, a world

occupied by an explosive technology known as MEMS. A world of challenge and

opportunity, where traditional engineering concepts are turned upside down, and the realm

of the "possible" is totally redefined.

Micro-electro-mechanical systems (MEMS) is the technology of very small mechanical

devices driven by electricity. They range in size from the sub micrometer (or sub micron)

level to the millimeter level, with a batch of millions of machines fabricated on same

silicon substrate. In this report, we will deal with about MEMS are, their fabrication

techniques, merits and de-merits and their application in daily life.
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Contents

Title Pg No.

1. Introduction 62. History 83. Fundamental of mechanics 104. Mechanical to electrical transduction 135. Materials used for MEMS manufacturing 156. MEMS Basic Processes 167. MEMS Manufacturing Technologies 228. Components of MEMS 389. Advantages of MEMS 4010.Current limitations of MEMS 4211.

Applications of MEMS 4312.Future aspects 45

13.Conclusion 4614.References 47


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Introduction

Micro-Electro-Mechanical Systems, or MEMS, is a technology that in its most general

form can be defined as miniaturized mechanical and electro-mechanical elements (i.e.,

devices and structures) that are made using the techniques of micro-fabrication. It is theintegration of mechanical elements, sensors, actuators, and electronics on a common silicon

substrate through micro fabrication technology. The critical physical dimensions of MEMS

devices can vary from well below one micron on the lower end of the dimensional

spectrum, all the way to several millimeters. They usually consist of a central unit that

processes data, the microprocessor and several components that interact with the outside

such as micro-sensors.

Examples of MEMS device applications include inkjet-printer cartridges, accelerometers,

miniature robots, micro-engines, locks, inertial sensors, micro-transmissions, micro-

mirrors, micro actuators, optical scanners, fluid pumps, transducers, and chemical, pressureand flow sensors. New applications are emerging as the existing technology is applied to

the miniaturization and integration of conventional devices.

These systems can sense, control, and activate mechanical processes on the micro scale,

and function individually or in arrays to generate effects on the macro scale. The micro

fabrication technology enables fabrication of large arrays of devices, which individually

perform simple tasks, but in combination can accomplish complicated functions.

MEMS are not about any one application or device, nor are they defined by a single

fabrication process or limited to a few materials. They are a fabrication approach thatconveys the advantages ofminiaturization, multiple components, and microelectronics

to the design and construction of integrated electromechanical systems. MEMS are not only

about miniaturization of mechanical systems; they are also a new paradigm for designing

mechanical devices and systems.

While the electronics are fabricated using integrated circuit (IC) process sequences (e.g.,

CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated

using compatible "micromachining" processes that selectively etch away parts of the

silicon wafer or add new structural layers to form the mechanical and electromechanical

devices.

MEMS promises to revolutionize nearly every product category by bringing together

silicon-based microelectronics with micromachining technology, making possible the

realization of complete systems-on-a-chip. MEMS is an enabling technology allowing the

development of smart products, augmenting the computational ability of microelectronics
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with the perception and control capabilities of microsensors and microactuators and

expanding the space of possible designs and applications.

Microelectronic integrated circuits can be thought of as the "brains" of a system and

MEMS augments this decision-making capability with "eyes" and "arms", to allowmicrosystems to sense and control the environment. Sensors gather information from the

environment through measuring mechanical, thermal, biological, chemical, optical, and

magnetic phenomena. The electronics then process the information derived from the

sensors and through some decision making capability direct the actuators to respond by

moving, positioning, regulating, pumping, and filtering, thereby controlling the

environment for some desired outcome or purpose. Because MEMS devices are

manufactured using batch fabrication techniques similar to those used for integrated

circuits, unprecedented levels of functionality, reliability, and sophistication can be placed

on a small silicon chip at a relatively low cost.


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History

The invention of the transistor at Bell Telephone Laboratories in 1947 sparked a fast-

growing microelectronic technology. Jack Kilby of Texas Instruments built the first

integrated circuit (IC) in 1958 using germanium (Ge) devices. It consisted of one transistor,

three resistors, and one capacitor. The IC was implemented on a sliver of Ge that was glued

on a glass slide. Later that same year Robert Noyce of Fairchild Semiconductor announced

the development of a planar double-diffused Si IC. The complete transition from the

original Ge transistors with grown and alloyed junctions to silicon (Si) planar double-

diffused devices took about ten years. The success of Si as an electronic material was due

partly to its wide availability from silicon dioxide (SiO2) (sand), resulting in potentially

lower material costs relative to other semiconductors.

Since 1970, the complexity of ICs has doubled every two to three years. The minimum

dimension of manufactured devices and ICs has decreased from 20 microns to the submicron levels of today. Current ultra-large-scale-integration (ULSI) technology enables the

fabrication of more than 10 million transistors and capacitors on a typical chip.

IC fabrication is dependent upon sensors to provide input from the surrounding

environment, just as control systems need actuators (also referred to as transducers) in

order to carry out their desired functions. Due to the availability of sand as a material,

much effort was put into developing Si processing and characterization tools. These tools

are now being used to advance transducer technology. Today's IC technology far outstrips

the original sensors and actuators in performance, size, and cost.

Attention in this area was first focused on micro-sensor (i.e., micro-fabricated sensor)

development. The first micro-sensor, which has also been the most successful, was the Si

pressure sensor. In 1954 it was discovered that the piezoresistive effect in Ge and Si had

the potential to produce Ge and Si strain gauges with a gauge factor (i.e., instrument

sensitivity) 10 to 20 times greater than those based on metal films. As a result, Si strain

gauges began to be developed commercially in 1958. The first high-volume pressure sensor

was marketed by National Semiconductor in 1974. This sensor included a temperature

controller for constant-temperature operation. Improvements in this technology since then

have included the utilization of ion implantation for improved control of the piezoresistor

fabrication. Si pressure sensors are now a billion-dollar industry.

Around 1982, the term micromachining came into use to designate the fabrication of

micromechanical parts (such as pressure-sensor diaphragms or accelerometer suspension

beams) for Si micro-sensors. The micromechanical parts were fabricated by selectively

etching areas of the Si substrate away in order to leave behind the desired geometries.

Isotropic etching of Si was developed in the early 1960s for transistor fabrication.
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Anisotropic etching of Si then came about in 1967. Various etch-stop techniques were

subsequently developed to provide further process flexibility.

These techniques also form the basis of the bulk micromachining processing techniques.

Bulk micromachining designates the point at which the bulk of the Si substrate is etched

away to leave behind the desired micromechanical elements. Bulk micromachining has

remained a powerful technique for the fabrication of micromechanical elements. However,

the need for flexibility in device design and performance improvement has motivated the

development of new concepts and techniques for micromachining.

Among these is the sacrificial layer technique, first demonstrated in 1965 by Nathanson

and Wickstrom, in which a layer of material is deposited between structural layers for

mechanical separation and isolation. This layer is removed during the release etch to free

the structural layers and to allow mechanical devices to move relative to the substrate. A

layer is releasable when a sacrificial layer separates it from the substrate. The application of

the sacrificial layer technique to micromachining in 1985 gave rise to surface

micromachining, in which the Si substrate is primarily used as a mechanical support upon

which the micromechanical elements are fabricated.

Prior to 1987, these micromechanical structures were limited in motion. During 1987-1988,

a turning point was reached in micromachining when, for the first time, techniques for

integrated fabrication of mechanisms (i.e. rigid bodies connected by joints for transmitting,

controlling, or constraining relative movement) on Si were demonstrated. During a series

of three separate workshops on micro-dynamics held in 1987, the term MEMS was coined.

Equivalent terms for MEMS are micro-systems (preferred in Europe) and micro-machines(preferred in Japan).
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Fundamental of mechanics

To understand the design and operation of MEMS devices, basic knowledge of mechanics

of materials is necessary. These concepts include stress, strain, Hookes law, Possions

ratio and film stress.

When a solid body of homogenous material is subjected to force, the body will respond to

force by changing shape. For non-solid materials the change is dramatic but in case of solid

materials the change is very small, often too small to be noticed through eyes except in the

case of large force that may cause irreversible change.

Figure1.Rod elongation due to tensile force F

Consider a solid rod of length Lo (shown in Fig.1) and diameter D subjected to force F

uniformly applied to the ends of the rod. The tensile force will cause the rod to lengthen by

amount L. this lengthening of rod is described by axial strain as

o

Strain is defined as the change in length of material per length of the material and is adimensionless quantity.

Stress is defined as the force applied to the material per unit area. Its unit is Pascal (Pa).

Tensile stresses are considered as negative, while compressive stresses are taken as

positive.

()

2


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Here the force is termed as axial force as it acts perpendicular to the axis. Shear force acts

parallel to the surface of the body and generate shear stress and strain.

Materials are also characterized on the basis of their stress-strain curve. Using a tensile test

machine, a bar of material under the test is subjected to uniform axial loading force.

Sensitive measuring devices determine the change in length as a controlled tensile force isapplied to the bar. By measuring the change in length the stress-strain curve is plotted.

Curve for both ductile and brittle material is plotted. Linear region ends at stresses

corresponding to proportional limit. Stresses beyond the proportional limit leads to

permanent deformation of the bar. For ductile material, beyond proportional limit are

regions of plastic deformation and reach maximum stress value before fracture. But for

brittle material there is no plastic deformation and it fractures soon after reaching

proportional limit.

Figure2. Stress-Strain Curve For Brittle Metals And Silicon

Hookes law describes linear relation between stress and strain,

where, E is the slope of linear region of stress-strain curve and is called as Youngs

modulus. MEMS devices are generally designed to operate at stresses in the linear region,

so is an important parameter in MEMS design. For a single silicon crystal, E=190GPa.

Another important mechanical effect is change in lateral dimension due to an axial force F.The change in width is characterized by a lateral strain l, which is related to axial strain a

by poissonsratio by


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For silicon poissons ratio is taken as 0.28 and for other material it ranges from 0.2 to 0.5,

with most metals near 0.3.

Stresses in thin films:- control of strain levels in deposited thin films is critical for

many MEMS devices. MEMS mechanical material, require tensile strain less than -.001 or

the membrane will break. Strain in thin films is difficult to measure directly and ismeasured by finding the center deflection i.e. the difference in height between centre and

edge assuming uniform bow across the substrate. The stress level is calculated using

Stoney equation,

But the limitation of this technique is that an average value for whole wafer is given, with

no indication of local stress variation. Other technique for measuring thin film stress

involves fabrication of MEMS structures such as cantilever beam and ring beam structures.By using cantilever beam supported at both ends compressive stress can be determined by

identifying largest unbroken beam in series of beam of varying dimension. Depending on

compressive stress level and young modulus of beam, tensile stress is determined.

Stress gradients are serious concern in MEMS devices and results in non-uniformities in

thin film deposition process, which cause atomic structure variations that create uneven

strain through film. Factors leading to stress are intrinsic or extrinsic stresses. Intrinsic

stresses result from non-equilibrium nature of thin film deposition process. Extrinsic stress

is caused due to external factors of the film structure. The most common source of extrinsic

stress is difference in thermal coefficients of expansion between film deposited and thesubstrate.


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Mechanical to electrical transduction

The measurement of physical quantities such as pressure, acceleration and mass change is

based on sensing mechanism that converts change in these quantities to electrically

measurable parameters such as resistance, capacitance and changes in characteristic

frequencies. MEMS sensors have structural elements that move in response to the physical

quantity being measured. The common transduction methods are:-

Piezoelectric effect:- It is the phenomena whereby force applied to certaincrystalline material causes an electrical charge to be generated on the surface of the

crystal, with the amount of the change directly related to the applied force. This

happens because force creates stress in crystal leading to strain. On atomic level strain

implies slight change in positions of atom in the crystal. In piezoelectric materials, the

unit cell of material contains positively and negatively charged ions that are not

symmetrically oriented with respect to each other. The application of stress leads to

creation of electric dipoles. These dipoles induce surface charge on the crystal that in

turn creates the electric field and cancels the stress induced electric field from dipoles.

Piezoelectric material also shows the reverse effect i.e. when an electric field is applied

it causes mechanical deformation. This is an important phenomenon in MEMS

actuators. The disadvantage of the thin film piezoelectric is the complication they bring

to the device processing. Issues include potential contamination of the IC and non-

standard film deposition.

Figure 3.Ion position in piezoelectric material with and without applied pressure.

Capacitance measurements:-Here a pressure sensor is made with flexiblemembrane that moves in response to the changes in applied pressure, by putting one


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electrode of the membrane and fixing the second membrane, a capacitor is created with

capacitance as function of the applied pressure. As the pressure increase the membrane

moves closer to fixed membrane and capacitance increases and vive versa. The

advantage of this technology is the limited temperature dependence the measured

capacitance and its simplicity. The disadvantage is the non-linearity of the dependence

of the capacitance on the pressure. Accelometers are developed using this technique.

Figure 4.Schematic drawing of pressure sensor using capacitance measurement

technique

Signal transduction using magnetic effect such as Halls effect generates a voltageproportional to the applied magnetic field. These sensors are used in the detection of the

electric current, proximity detection and position rotation shafts.

Piezoresistivity:- It is the property of the material that described the change in theresistance of the material as the function of the mechanical stresses applied to it.Piezoresitivty depends on the silicon doping concentration, temperature, direction of

current flow and direction and the type of force relative to the orientation of the crystal.

Figure 5. Phenomena of current flowing in silicon bar and piezoresistivity.


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Materials used for MEMS manufacturing

Silicon:- It is the material used to create most integrated circuits used in consumerelectronics in the modern world. The economies of scale, ready availability of

cheap high-quality materials and ability to incorporate electronic functionality makesilicon attractive for a wide variety of MEMS applications. Silicon also has

significant advantages engendered through its material properties. In single crystal

form, silicon is an almost perfect Hookean material, meaning that when it is flexed

there is virtually no hysteresis and hence almost no energy dissipation. As well as

making for highly repeatable motion, this also makes silicon very reliable as it

suffers very little fatigue and can have service lifetimes in the range ofbillions to

trillions of cycles without breaking. The basic techniques for producing all silicon

based MEMS devices are deposition of material layers, patterning of these layers by

photolithography and then etching to produce the required shapes.

Polymers:-Even though the electronics industry provides an economy of scale forthe silicon industry, crystalline silicon is still a complex and relatively expensive

material to produce. Polymers on the other hand can be produced in huge volumes,

with a great variety of material characteristics. MEMS devices can be made from

polymers by processes such as injection molding, embossing or stereo-lithography

and are especially well suited to micro-fluidic applications such as disposable blood

testing cartridges.

Metals:-Metals can also be used to create MEMS elements. While metals do nothave some of the advantages displayed by silicon in terms of mechanical properties,

when used within their limitations, metals can exhibit very high degrees of

reliability. Metals can be deposited by electroplating, evaporation, and sputtering

processes.

Commonly used metals include gold, nickel, aluminum, copper, chromium,

titanium, tungsten, platinum, and silver.
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MEMS Basic Processes

MEMS technology is based on a number of tools and methodologies, which are used to form

small structures with dimensions in the micrometer scale (one millionth of a meter). Significant

parts of the technology have been adopted from integrated circuit (IC) technology. For

instance, almost all devices are built on wafers of silicon, like ICs. The structures are realizedin thin films of materials, like ICs. They are patterned using photolithographic methods, like

ICs. There are however several processes that are not derived from IC technology, and as the

technology continues to grow the gap with IC technology also grows. There are three basic

building blocks in MEMS technology, which are the ability to deposit thin films of material on

a substrate, to apply a patterned mask on top of the films by photolithographic imaging, and to

etch the films selectively to the mask. A MEMS process is usually a structured sequence of

these operations to form actual devices.

Thus these operations are deposition, lithography and etching.

1.Deposition processes:-One of the basic building blocks in MEMS processing is theability to deposit thin films of material with a thickness anywhere between a few

nanometers to about 100 micrometers.

Physical deposition:-There are two types of physical deposition processes.

Physical vapor deposition (PVD):-It consists of a process in which a material isremoved from a target, and deposited on a surface. Techniques to do this include

the process of sputtering, in which an ion beam liberates atoms from a target,

allowing them to move through the intervening space and deposit on the desiredsubstrate.

Evaporation (deposition), in which a material is evaporated from a target usingeither heat (thermal evaporation) or an electron beam (e-beam evaporation) in a

vacuum system.

Chemical deposition:-It techniques include chemical vapor deposition ("CVD"), in

which a stream of source gas reacts on the substrate to grow the material desired.

This can be further divided into categories depending on the details of the

technique, for example, LPCVD (Low Pressure chemical vapor deposition) and

PECVD (Plasma Enhanced chemical vapor deposition).

Oxide films can also be grown by the technique ofthermal oxidation, in which the

(typically silicon) wafer is exposed to oxygen and/or steam, to grow a thin surface

layer ofsilicon dioxide.
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2.Patterning:-Patterning in MEMS is the transfer of a pattern into a material.

3.Lithography:-Lithography in MEMS context is typically the transfer of a pattern into

a photosensitive material by selective exposure to a radiation source such as light. A

photosensitive material is a material that experiences a change in its physical propertieswhen exposed to a radiation source. If a photosensitive material is selectively exposed to

radiation (e.g. by masking some of the radiation) the pattern of the radiation on the material

is transferred to the material exposed, as the properties of the exposed and unexposed

regions differs this exposed region can then be removed or treated providing a mask for the

underlying substrate. Photolithography is typically used with metal or other thin film

deposition, wet and dry etching.

Photolithography:- It is a process used in micro-fabrication to selectively removeparts of a thin film or the bulk of a substrate. It uses light to transfer a geometricpattern from a photo mask to a light-sensitive chemical "photoresist", or simply

"resist," on the substrate. A series of chemical treatments then either engraves the

exposure pattern into, or enables deposition of a new material in the desired pattern

upon, the material underneath the photo resist.

Electron beam lithography:-It is the practice of scanning a beam ofelectrons in apatterned fashion across a surface covered with a film called the resist, exposing the

resist and of selectively removing either exposed or non-exposed regions of the

resist developing. The purpose, is to create very small structures in the resist that

can subsequently be transferred to the substrate material, often by etching. It was

developed for manufacturing integrated circuits, and is also used for creating

nanotechnology architectures.

The primary advantage of electron beam lithography is that it is one of the ways to

beat the diffraction limit of light and make features in the nanometer regime. This

form ofmaskless lithography has found wide usage in photomask-making used in

photolithography, low-volume production of semiconductor components, and

research & development.

The key limitation of electron beam lithography is throughput, i.e., the very long

time it takes to expose an entire silicon wafer or glass substrate. A long exposure

time leaves the user vulnerable to beam drift or instability which may occur during

the exposure. Also, the turn-around time for reworking or re-design is lengthened

unnecessarily if the pattern is not being changed the second time.
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Ion beam lithography:-It is the practice of scanning a focused beam of ions in apatterned fashion across a surface in order to create very small structures such as

integrated circuits or other nanostructures. Ion beam lithography has been found to

be useful for transferring high-fidelity patterns on three-dimensional surfaces.

X-ray lithography:-It is a process used in electronic industry to selectively removeparts of a thin film. It uses X-rays to transfer a geometric pattern from a mask to a

light-sensitive chemical photoresist, or simply "resist," on the substrate. A series of

chemical treatments then engraves the produced pattern into the material

underneath the photoresist.

4.Etching processes:-There are two basic categories of etching processes:

Wet etching:-It consists in selective removal of material by dipping a substrateinto a solution that dissolves it. The chemical nature of this etching process

provides a good selectivity, which means the etching rate of the target material is

considerably higher than the mask material if selected carefully. Various type of

wet etching techniques are:

Isotropic etching:- Here, etching progresses at the same speed in alldirections. Long and narrow holes in a mask will produce v-shaped grooves

in the silicon. The surface of these grooves can be atomically smooth if the

etch is carried out correctly, with dimensions and angles being extremely

accurate.

Anisotropic etching;-Some single crystal materials, such as silicon, willhave different etching rates depending on the crystallographic orientation of

the substrate. This is known as anisotropic etching and one of the most

common examples is the etching of silicon in KOH (potassium hydroxide),

where Si planes etch approximately 100 times slower than other

planes (crystallographic orientations). Therefore, etching a rectangular hole

in a (100)-Si wafer results in a pyramid shaped etch pit with 54.7 walls,instead of a hole with curved sidewalls as with isotropic etching.

HF etching:-Hydrofluoric acid is commonly used as an aqueous etchant forsilicon dioxide, usually in 49% concentrated form.They were first used in

medieval times for glass etching. It was used in IC fabrication for patterning

the gate oxide until the process step was replaced by RIE.
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Electrochemical etching (ECE) for dopant-selective removal of silicon is acommon method to automate and to selectively control etching. An active p-

n diode junction is required, and either type of dopant can be the etch-

resistant (etch-stop) material. Boron is the most common etch-stop dopant.

In combination with wet anisotropic etching as described above, ECE has

been used successfully for controlling silicon diaphragm thickness in

commercial piezoresistive silicon pressure sensors. Selectively doped

regions can be created either by implantation, diffusion, or epitaxial

deposition of silicon.

Dry etching:-It refers to the removal of material, typically a masked pattern ofsemiconductor material, by exposing the material to a bombardment of ions

(usually a plasma of reactive gases such as fluorocarbons, oxygen, chlorine, borontrichloride; sometimes with addition ofnitrogen, argon, helium and other gases) that

dislodge portions of the material from the exposed surface. The dry etching process

typically etches directionally or anisotropically.various dry etching techniques are:-

Xenon difluoride etching:-XeF2 is a dry vapor phase isotropic etch forsilicon originally applied for MEMS in 1995 at University of California,

Los Angeles. Primarily used for releasing metal and dielectric structures by

undercutting silicon, XeF2 has the advantage of a stiction-free release unlike

wet etchants. Its etch selectivity to silicon is very high, allowing it to work

with photoresist, SiO2, silicon nitride, and various metals for masking. Its

reaction to silicon is "plasmaless", is purely chemical and spontaneous and

is often operated in pulsed mode. Models of the etching action are available,

and university laboratories and various commercial tools offer solutions

using this approach.

Plasma etching:- It is a form of plasma processing used to fabricateintegrated circuits. It involves a high-speed stream of glow discharge

(plasma) of an appropriate gas mixture being shot (in pulses) at a sample.

The plasma source, known as etch species, can be either charged (ions) or

neutral (atoms and radicals). During the process, the plasma will generate

volatile etch products at room temperature from the chemical reactions

between the elements of the material etched and the reactive species

generated by the plasma. Eventually the atoms of the shot element embed
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themselves at or just below the surface of the target, thus modifying the

physical properties of the target.[1]

Sputtering:- It is a process whereby atoms are ejected from a solid targetmaterial due to bombardment of the target by energetic particles.[1] It is

commonly used for thin-film deposition, etching and analytical techniques.

Reactive ion etching (RIE):-In reactive ion etching (RIE), the substrate isplaced inside a reactor, and several gases are introduced. A plasma is struck

in the gas mixture using an RF power source, which breaks the gas

molecules into ions. The ions accelerate towards, and react with, the surface

of the material being etched, forming another gaseous material. This is

known as the chemical part of reactive ion etching. There is also a physical

part, which is similar to the sputtering deposition process. If the ions have

high enough energy, they can knock atoms out of the material to be etched

without a chemical reaction. It is a very complex task to develop dry etch

processes that balance chemical and physical etching, since there are many

parameters to adjust. By changing the balance it is possible to influence the

anisotropy of the etching, since the chemical part is isotropic and the

physical part highly anisotropic the combination can form sidewalls that

have shapes from rounded to vertical. RIE can be deep (Deep RIE or deep

reactive ion etching (DRIE)).

Deep reactive ion etching:-Deep RIE (DRIE) is a special subclass of RIEthat is growing in popularity. In this process, etch depths of hundreds of

micrometres are achieved with almost vertical sidewalls. The primary

technology is based on the so-called "Bosch process",named after the

German company Robert Bosch, which filed the original patent, where two

different gas compositions alternate in the reactor. Currently there are two

variations of the DRIE. The first variation consists of three distinct steps

(the Bosch Process as used in the Plasma-Therm tool) while the second

variation only consists of two steps (ASE used in the STS tool). In the 1st

Variation, the etch cycle is as follows: (i) SF6 isotropic etch; (ii) C4F8

passivation; (iii) SF6 anisoptropic etch for floor cleaning. In the second

variation, steps (i) and (iii) are combined. Both variations operate similarly.

The C4F8 creates a polymer on the surface of the substrate, and the second

gas composition (SF6 and O2) etches the substrate. The polymer is

immediately sputtered away by the physical part of the etching, but only on
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the horizontal surfaces and not the sidewalls. Since the polymer only

dissolves very slowly in the chemical part of the etching, it builds up on the

sidewalls and protects them from etching. As a result, etching aspect ratios

of 50 to 1 can be achieved. The process can easily be used to etch

completely through a silicon substrate, and etch rates are 36 times higher

than wet etching.

5.Dicing: The finished wafer is sawed or machined into small squares, or dice, from

which electronic components can be made.

6.Packaging: The individual sections are then packaged, a process that involves

physically locating, connecting, and protecting a device or component. MEMS design is

strongly coupled to the packaging requirements, which in turn are dictated by the

application environment.


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MEMS Manufacturing Technologies

Various manufacturing technologies of MEMS are discussed below:-

Bulk MicromachiningThe oldest micromachining technology is bulk micromachining. This technique involves

the selective removal of the substrate material in order to realize miniaturized mechanical

components. Bulk micromachining can be accomplished using chemical or physical means,

with chemical means being far more widely used in the MEMS industry.

A widely used bulk micromachining technique is chemical wet etching, which involves the

immersion of a substrate into a solution of reactive chemical that will etch exposed regions

of the substrate at measurable rates. Chemical wet etching is popular in MEMS because it

can provide a very high etch rate and selectivity. Furthermore, the etch rates and selectivity

can be modified by: altering the chemical composition of the etch solution; adjusting the

etch solution temperature; modifying the dopant concentration of the substrate; and

modifying which crystallographic planes of the substrate are exposed to the etchant

solution.

There are two general types of chemical wet etching in bulk micromachining: isotropic wet

etching and anisotropic wet etching. In isotropic wet etching, the etch rate is not dependent

on the crystallographic orientation of the substrate and the etching proceeds in all directions

at equal rates. In theory, lateral etching under the masking layer etches at the same rate as

the etch rate in normal direction. However, in practice lateral etching is usually much

slower without stirring, and consequently isotropic wet etching is almost always performed

with vigorous stirring of the etchant solution. Figure 6 illustrates the profile of the etch

using an isotopic wet etchant with and without stirring of the etchant solution.

Figure 6: Illustration of the etch profile, with and without stirring, using an isotropic

wet chemical etchant.


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Any etching process requires a masking material to be used, with preferably a high

selectivity relative to the substrate material. Common masking materials for isotropic wet

silicon etching include silicon dioxide and silicon nitride. Silicon nitride has a much lower

etch rate compared to silicon dioxide and therefore is more frequently used.

The etch rate of some isotropic wet etchant solution mixtures are dependent on the dopant

concentration of the substrate material. For example: the commonly used mixture of

HC2H3O2:HNO3:HF in the ratio of 8:3:1 will etch highly doped silicon (> 5 x 1018

atoms/cm3) at a rate of 50 to 200 microns/hour, but will etch lightly doped silicon material

at a rate 150 times less. Nevertheless, the etch rate selectivity with respect to dopant

concentration is highly dependent on solution mixture.

The much more widely used wet etchants for silicon micromachining are anisotropic wet

etchants. Anisotropic wet etching involves the immersion of the substrate into a chemicalsolution wherein the etch rate is dependent on crystallographic orientation of the substrate.

The mechanism by which the etching varies according to silicon crystal planes is attributed

to the different bond configurations and atomic density that the different planes exposed to

the etchant solution. Wet anisotropic chemical etching is typically described in terms of

etch rates according to the different normal crystallographic places, usually , ,

and . In general, silicon anisotropic etching etches more slowly along the

planes than all the other planes in the lattice and the difference in etch rate between the

different lattice directions can be as high as 1000 to 1. It is thought that the reason for the

slower etch rate of the planes is that these planes have the highest density of

exposed silicon atoms in the etchant solution, as well as 3 silicon bonds below the plane,

thereby leading to some amount of chemical shielding of the surface.

The ability to delineate the different crystal planes of the silicon lattice in anisotropic wet

chemical etching provides a high-resolution etch capability with reasonably tight

dimensional control. It also provides the ability for two-sided processing to embody self-

isolated structures wherein only one side is exposed to the environment. This assists in

packaging of the device and is very useful for MEMS devices exposed to harsh

environments, such as pressure sensors. Anisotropic etching techniques have been around

for over 25 years and are commonly used in the manufacturing of silicon pressure sensors

as well as bulk micro-machined accelerometers.

Figure 7 below is an illustration of some of the shapes that are possible using anisotropic

wet etching of oriented silicon substrates including an inverted pyramidal and a flat

bottomed trapezoidal etch pit.


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Figure 7: Illustration of shape of the etch profiles of a oriented silicon substrate

after immersion in an anisotropic wet etchant solution.

Figures 8a and 8b are SEM photographs of a silicon substrate after an anisotropic wet

etching. Figure 8a shows a trapezoidal etch pit that has been subsequently diced across the

etch pit and Figure 8b shows the backside of a thin membrane that could be used to make a

pressure sensor. It is important to note that the etch profiles shown in the Figures are only

for a oriented silicon wafer; substrates with other crystallographic orientations willexhibit different shapes. Occasionally, substrates with other orientations are used in MEMS

fabrication, but given the cost, lead times and availability, the vast majority of substrates

used in bulk micromachining have orientation.

Fig:8a Fig:8b

Figures 8a and 8b: SEMS of a oriented silicon substrate after immersion in an

anisotropic wet etchant.


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Useful anisotropic wet etching requires the ability to successfully mask certain areas of the

substrate and consequently an important criterion for selecting an etchant is the availability

of good masking materials. Silicon nitride is a commonly used masking material for

anisotropic wet etchants since it has a very low etch rate in most etchant solutions. Some

care must be exercised in the type of silicon nitride used, since any pin hole defects will

result in the attack of the underlying silicon. Also, some low-stress silicon-rich nitrides can

etch at much higher rates compared to stoichiometric silicon nitride formulations.

Thermally grown SiO2 is frequently used as a masking material, but some care must be

exercised to ensure a sufficiently thick masking layer when using KOH etchants, since the

etch rates of oxide can be high. Photoresists are unusable in any anisotropic etchant. Many

metals including Ta, Au, Cr, Ag, and Cu hold up well in EDP and Al holds up in TMAH

under certain conditions.

In general, the etch rate, etch rate ratios (100)/(111), and etch selectivitys of anisotropic

etchants are strongly dependent on chemical composition and temperature of the etchant

solution.

Frequently, when using bulk micromachining it is desirable to make thin membranes of

silicon or control the etch depths very precisely. As with any chemical process, the

uniformity of the etching can vary across the substrate, making this difficult. Timed etches

whereby the etch depth is determined by multiplying the etch rate by the etch time are

difficult to control and etch depth is very dependent on sample thickness uniformity,

etchant species diffusion effects, loading effects, etchant aging, surface preparation, etc. To

allow a higher level of precision in anisotropic etching the MEMS field has developed

solutions to this problem, namely etch stops. Etch stops are very useful to control the

etching process and provide uniform etch depths across the wafer, from wafer to wafer, and

from wafer lot to wafer lot. There are two basic types of etch stop methods that are used in

micromachining: dopant etch stops and electrochemical etch stops.

Surface MicromachiningSurface micromachining is another very popular technology used for the fabrication of

MEMS devices. There are a very large number of variations of how surface

micromachining is performed, depending on the materials and etchant combinations that

are used. However, the common theme involves a sequence of steps starting with the

deposition of some thin-film material to act as a temporary mechanical layer onto which

the actual device layers are built; followed by the deposition and patterning of the thin-film

device layer of material which is referred to as the structural layer; then followed by the


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removal of the temporary layer to release the mechanical structure layer from the constraint

of the underlying layer, thereby allowing the structural layer to move.

An illustration of a surface micromachining process is shown in Figure 9, wherein an oxide

layer is deposited and patterned. This oxide layer is temporary and is commonly referred to

as the sacrificial layer. Subsequently, a thin film layer of polysilicon is deposited and

patterned and this layer is the structural mechanical layer. Lastly, the temporary sacrificial

layer is removed and the polysilicon layer is now free to move as a cantilever..

Figure 9: Illustration of a surface micromachining process

Some of the reasons surface micromachining is so popular is that it provides for precise

dimensional control in the vertical direction. This is due to the fact that the structural and

sacrificial layer thicknesses are defined by deposited film thicknesses which can be

accurately controlled. Also, surface micromachining provides for precise dimensional

control in the horizontal direction, since the structural layer tolerance is defined by the

fidelity of the photolithography and etch processes used. Other benefits of surface

micromachining are that a large variety of structure, sacrificial and etchant combinations

can be used; some are compatible with microelectronics devices to enable integrated

MEMS devices. Surface micromachining frequently exploits the deposition characteristics

of thin-films such as conformal coverage using LPCVD. Lastly, surface micromachining

uses single-sided wafer processing and is relatively simple. This allows higher integration

density and lower resultant per die cost compared to bulk micromachining.

One of the disadvantages of surface micromachining is that the mechanical properties of

most deposited thin-films are usually unknown and must be measured. Also it is common

for these types of films to have a high state of residual stress, frequently necessitating a

high temperature anneal to reduce residual stress in the structural layer. Also, the

reproducibility of the mechanical properties in these films can be difficult to achieve.

Additionally, the release of the structural layer can be difficult due to a stiction effect

whereby the structural layer is pulled down and stuck to the underlying substrate due to


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capillary forces during release. Stiction can also occur in use and an anti-stiction coating

material may be needed.

The most commonly used surface micromachining process and material combination is a

PSG sacrificial layer, a doped polysilicon structural layer, and the use of Hydrofluoric acid

as the etchant to remove the PSG sacrificial layer and release the device. This type of

surface micromachining process is used to fabricate the Analog Devices integrated MEMS

accelerometer device used for crash airbag deployment. Figure 10 and Figure 11are SEMs

of two surface micro-machined polysilicon MEMS devices.

Figure 10: Polysilicon micro-motor fabricated using a surface micromachining

process.

Figure 11: Polysilicon resonator structure fabricated using a surface micromachining

process.


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Another variation of the surface micromachining process is to use a metal structural layer, a

polymer layer as the sacrificial layer, and an O2 plasma as the etchant. The advantage of

this process is that the temperature of the sacrificial and structural layer depositions are

sufficiently low so as not to degrade any microelectronics in the underlying silicon

substrate, thereby integrating MEMS with electronics. Also, since the sacrificial layer is

removed without immersion in a liquid, problems associated with stiction during release

are avoided. A process similar to this is used to produce the Texas Instruments Digital

Light Processor (DLP) device used in projection systems.

Wafer BondingWafer bonding is a micromachining method that is analogous to welding in the macro-scale

world and involves the joining of two (or more) wafers together to create a multi-wafer

stack. There are three basic types of wafer bonding including:

o Direct Or Fusion Bondingo Field-Assisted Or Anodic Bondingo Bonding Using An Intermediate Layer.

In general, all bonding methods require substrates that are very flat, smooth, and clean, in

order for the wafer bonding to be successful and free of voids.

Direct or fusion bonding is typically used to mate two silicon wafers together or

alternatively to mate one silicon wafer to another silicon wafer that has been oxidized.

Direct wafer bonding can be performed on other combinations, such as bare silicon to asilicon wafer with a thin-film of silicon nitride on the surface as well.

As mentioned, wafer bonding is analogous to welding in the macro-scale world. Wafer

bonding is used to attach a thick layer of single crystal silicon onto another wafer. This can

be extremely useful when it is desired to have a thick layer of material for applications

requiring appreciable mass or in applications where the material properties of single crystal

silicon are advantageous over those of thin-film LPCVD materials. Direct wafer bonding is

also used to fabricate Silicon-On-Insulator (SOI) wafers having device layers several

microns or more in thickness.

Another popular wafer bonding technique is anodic bonding. In anodic bonding a silicon

wafer is bonded to a Pyrex 7740 wafer using an electric field and elevated temperature. The

two wafers can be pre-processed prior to bonding and can be aligned during the bonding

procedure. The mechanism by which anodic bonding works is based on the fact that Pyrex


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7740 has a high concentration of Na+ ions; a positive voltage applied to the silicon wafer

drives the Na+ ions from the Pyrex glass surface, thereby creating a negative charge at

glass surface. The elevated temperature during the bonding process allows the Na+ ions to

migrate in the glass with relative ease. When the Na+ ions reach the interface, a high field

results between silicon and glass, and this combined with the elevated temperatures fuses

the two wafers together. As with direct wafer bonding, it is imperative that the wafers are

flat, smooth, and clean and that the anodic bonding process is performed in a very clean

environment. An advantage of this process is that Pyrex 7740 has a thermal expansion

coefficient nearly equal to silicon and therefore there is a low value of residual stress in the

layers. Anodic bonding is a widely used technique for MEMS packaging.

In addition to direct and anodic bonding there are other wafer bonding techniques that are

used in MEMS fabrication. One method is eutectic bonding and involves the bonding of a

silicon substrate to another silicon substrate at an elevated temperature using an

intermediate layer of gold on the surface of one of the wafers. Eutectic bonding works

because the diffusion of gold into silicon is extremely rapid at elevated temperatures. In

fact this is a preferred method of wafer bonding at relatively low temperatures.

Another wafer bonding technique used in MEMS is glass frit bonding. In this process a

glass is spun or screen-printed onto a substrate surface. Subsequently this wafer is

physically contacted to another wafer and the composite is annealed to flow the glass

intermediate layer and bond the two wafers.

Lastly, various polymers can be used as intermediate layers to bond wafers including epoxy

resins, photoresists, polyimides, silicones, etc. This technique is commonly used during

various fabrication steps in MEMS such as when the device wafer becomes too fragile to

handle without mechanical support.

High-Aspect Ratio MEMS Fabrication Technologies

Both bulk and surface silicon micromachining are used in the industrial production ofsensors, ink-jet nozzles, and other devices. But in many cases the distinction between these

two has diminished. A new etching technology, deep reactive-ion etching, has made it

possible to combine good performance typical of bulk micromachining with comb

structures and in-plane operation typical ofsurface micromachining. While it is common in

surface micromachining to have structural layer thickness in the range of 2 m, in HAR

silicon micromachining the thickness can be from 10 to 100 m. The materials commonly
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used in HAR silicon micromachining are thick polycrystalline silicon, known as epi-poly,

and bonded silicon-on-insulator (SOI) wafers although processes for bulk silicon wafer also

have been created (SCREAM). Bonding a second wafer by glass frit bonding, anodic

bonding or alloy bonding is used to protect the MEMS structures. Integrated circuits are

typically not combined with HAR silicon micromachining. The consensus of the industry at

the moment seems to be that the flexibility and reduced process complexity obtained by

having the two functions separated far outweighs the small penalty in packaging.

Deep Reactive Ion Etching of Silicon:-

Deep reactive ion etching or DRIE is a relatively new fabrication technology that has

been widely adopted by the MEMS community. This technology enables very high

aspect ratio etches to be performed into silicon substrates. The sidewalls of the etched

holes are nearly vertical and the depth of the etch can be hundreds or even thousands ofmicrons into the silicon substrate.

Figure 12 illustrates how deep reactive ion etching is accomplished. The etch is a dry

plasma etch and uses a high density plasma to alternately etch the silicon and deposit an

etch resistant polymer layer on the sidewalls. The etching of the silicon is performed

using a SF6 chemistry whereas the deposition of the etch resistant polymer layer on the

sidewalls uses a C4F8 chemistry. Mass flow controllers alternate back and forth between

these two chemistries during the etch. The protective polymer layer is deposited on the

sidewalls as well as on the bottom of the etch pit, but the anisotropy of the etch removesthe polymer at the bottom of the etch pit faster than the polymer is removed from the

sidewalls. The sidewalls are not perfected or optically smooth and if the sidewall is

magnified under SEM inspection, a characteristic washboard or scalloping pattern is seen

in the sidewalls. The etch rates on most commercial DRIE systems varies from 1 to 4

microns per minute. DRIE systems are single wafer tools. Photoresist can be used as a

masking layer for DRIE etching. The selectivity with photoresist and oxide is about 75 to

1 and 150 to 1, respectively. For a through wafer etch a relatively thick photoresist

masking layer will be required. The aspect ratio of the etch can be as high as 30 to 1, but

in practice tends to be 15 to 1. The process recipe depends on the amount of exposed

silicon due to loading effects in the system, with larger exposed areas etching at a muchfaster rate compared to smaller exposed areas. Consequently, the etch must frequently be

characterized for the exact mask feature and depth to obtain desirable results.


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Figure 12: Illustration of how deep reactive ion etching works.

Figure 13 is a SEM of a MEMS component fabricated using DRIE and wafer bonding. This

device was made using an SOI wafer wherein a backside etch was performed through the

handle wafer, stopping on the buried oxide layer, and a frontside DRIE was performed on

the SOI device layer. Then the buried oxide was removed to release the microstructure,

allowing it to freely move.

Figure 13: SEM of a MEMS device fabricated using two sided DRIE etching

technology on an SOI wafer.


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Figure 14 is a cross section SEM of a silicon microstructure fabricated using DRIE

technology. As can be seen, the etch is very deep into the silicon substrate and the

sidewalls are nearly vertical.

Figure 14: SEM of the cross section of a silicon wafer demonstrating high-aspect ratio

and deep trenches that can be fabricated using DRIE technology.

Deep Reactive Ion Etching of Glass:-

Glass substrates can also be etched deep into the material with high aspect ratios and this

technology has been gaining in popularity in MEMS fabrication. Figure 15 shows a

structure fabricated into glass using this technology. The typical etch rates for high aspectratio glass etching range between 250 and 500 nm per minute. Depending on the depth of

the photoresist, metal or a polysilicon can be used as a mask.

Figure 15: SEM of high aspect ratio structures etched into a glass substrate.


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LIGA

Another popular high aspect ratio micromachining technology is called LIGA, which is a

German acronym for LIthographie Galvanoformung Adformung. This is primarily a non-

silicon based technology and requires the use of synchrotron generated x-ray radiation. Thebasic process is outlined in Figure 16 and starts with the cast of an x-ray radiation sensitive

PMMA onto a suitable substrate. A special x-ray mask is used for the selective exposure of

the PMMA layer using x-rays. The PMMA is then developed and will be defined with

extremely smooth and nearly perfectly vertical sidewalls. Also, the penetration depth of the

x-ray radiation into the PMMA layer is quite deep and allows exposure through very thick

PMMA layers, up to and exceeding 1 mm. After the development, the patterned PMMA

acts as a polymer mold and is placed into an electroplating bath and Nickel is plated into

the open areas of the PMMA. The PMMA is then removed, thereby leaving the metallic

microstructure (Figure 17).

Figure 16: An illustration of the steps involved in the LIGA process to fabricate high

aspect ratio MEMS devices.


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Figure 17: A tall, high aspect ratio gear made using LIGA technology.

Because LIGA requires a special mask and a synchrotron (X-ray) radiation source for the

exposure, the cost of this process is relatively expensive. A variation of the process which

reduces the cost of the micro-machined parts made with this process is to reuse the

fabricated metal part (step 5) as a tool insert to imprint the shape of the tool into a polymer

layer (step 3), followed by electroplating of metal into the polymer mold (step 4) and

removal of the polymer mold (step 5). Obviously this sequence of steps eliminates the need

for a synchrotron radiation source each time a part is made and thereby significantly lowers

the cost of the process. The dimensional control of this process is quite good and the tool

insert can be used many times before it is worn out.

Hot Embossing

A process to replicate deep high-aspect ratio structures in polymer materials is to fabricatethe metal tool insert using LIGA or a comparable technology and then to emboss the tool

insert pattern into a polymer substrate, which is then used as the part. Figure 18 is a

diagram illustrating the hot embossing process. A mold insert is made using an appropriate

fabrication method having the inverse pattern made into it. The mold insert is placed into a

hot embossing system that includes a chamber in which a vacuum can be drawn. The

substrate and polymer are heated to above the glass transition temperature, Tg, of the

polymer material and the mold insert is pressed into the polymer substrate. The vacuum is

critical for the polymer to faithfully replicate the features in the mold insert since otherwise

air would be trapped between the two surfaces resulting in distorted features. Subsequently,

the substrates are cooled to below the glass transition temperature of the polymer materialand force is applied to de-emboss the substrates.

As shown in Figure 20, hot embossing can successfully replicate complicated, deep and

high-aspect ratio features. This process can make imprints into a polymer hundreds of

microns deep with very good dimensional control. The advantage of this process is that the

cost of the individual polymer parts can be very low compared to the same structures made


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Figure 20: SEM of a variety of small test structures made in a plastic substrate using

hot embossing technology. The height of the plastic microstructures is nearly 300 um

and the smallest features have a diameter of about 25 um.

Other Micromachining TechnologiesIn addition to bulk micromachining, surface micromachining, wafer bonding, and high

aspect ratio micromachining technologies, there are a number of other techniques used to

fabricate MEMS devices.

Electro-Discharge Micromachining

Electro-discharge micromachining or micro-EDM is a process used to machine a

conductive material using electrical breakdown discharges to remove material. A working

electrode is made from a metal material onto which high voltage pulses are applied. The

working electrode is brought into close proximity to the material to be machined, which is

immersed in a dielectric fluid. The minimum sized features that can be made with micro-

EDM are dependent on the size of the working electrode and how it is fixture, but holes as

small as tens of microns have been made using this method. One issue with micro-EDM is

that it is a slow serial process.

Laser Micromachining

Lasers can generate an intense amount of energy in very short pulses of light and direct that

energy onto a selected region of material for micromachining. Among the many types of

lasers now in use for micromachining include: CO2, YAG, excimer, etc. Each has its own

unique properties and capabilities suited to particular applications. Factors that determine

the type of laser to use for a particular application include laser wavelength, energy, power,

and temporal and spatial modes; material type; feature sizes and tolerances; processing

speed; and cost. The action of CO2 and Nd:YAG lasers is essentially a thermal process,

whereby focusing optics are used to direct a predetermined energy/power density to a well-

defined location on the work piece to melt or vaporize the material. Another mechanism,

which is non-thermal and referred to as photoablation, occurs when organic materials are


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exposed to ultraviolet radiation generated from excimer, harmonic YAG, or other UV

source. Similar to microEDM, laser micromachining can produce features on the order of

tens of microns, but it is a serial process and therefore slower.

MEMS Process IntegrationProcess integration is defined as understanding, characterizing and optimizing to the extent

possible, the interrelationship of the individual processing steps in a process sequence.

Given the customization of MEMS process sequences, it should not be surprising that

process integration is of critical importance. Also, MEMS developers must have good data

on the electrical materials properties as well as data on the mechanical material properties.

Coupled with the enormous diversity of materials and processing techniques used in

MEMS fabrication, means that process integration can be a major part of a product

development effort.


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Components of MEMS

There are basically four major components of MEMS. These are:-

Microactuator Microstructure Microelectronics

Microactuator

A microactuator is a microscopic servomechanism that supplies and transmits a measured

amount of energy for the operation of another mechanism or system. As a general actuator,

following standards have to be met.

Large travel High precision Fast switching Low power consumption Power free force sustainability

Various types of actuator areElectrostatic,Electromagnetic,Piezoelectric,Fluid, Thermal.

Figure 21 shows micro-actuator used in hard drives.
http://en.wikipedia.org/wiki/Microscopichttp://en.wikipedia.org/wiki/Servomechanismhttp://en.wikipedia.org/wiki/Actuatorhttp://en.wikipedia.org/wiki/Electrostatichttp://en.wikipedia.org/wiki/Electrostatichttp://en.wikipedia.org/wiki/Electrostatichttp://en.wikipedia.org/wiki/Electromagnetichttp://en.wikipedia.org/wiki/Electromagnetichttp://en.wikipedia.org/wiki/Electromagnetichttp://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Thermalhttp://en.wikipedia.org/wiki/Thermalhttp://en.wikipedia.org/wiki/Thermalhttp://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Electromagnetichttp://en.wikipedia.org/wiki/Electrostatichttp://en.wikipedia.org/wiki/Actuatorhttp://en.wikipedia.org/wiki/Servomechanismhttp://en.wikipedia.org/wiki/Microscopic


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Advantages of MEMS

1. MEMS devices are made using integrated circuit-like processes, which enables theability to integrate multiple functionalities onto a single microchip. The ability

to integrate miniaturized sensors, miniaturized actuators and miniaturized

structures along with microelectronics has far-reaching implications in countless

products and applications.

2. MEMS borrow many of the production techniques of batch fabrication from theintegrated circuit industry and therefore, the per-unit device or microchip cost of

complex miniaturized electromechanical systems can be radically reduced - similar

to the per die cost reductions we have experienced in the IC industry. Although the

cost of the production equipment and each wafer can be relatively high, the fact that

this cost can be spread over many die in batch fabrication production can drastically

lower the per part cost.

3. Integrated circuit fabrication techniques coupled with the tremendous advantages ofsilicon and many other thin-film materials in mechanical applications allows the

reliability of miniaturized electromechanical systems to be radically improved. It is

well known that the most expensive and most unreliable components of a

conventional macro-scale control system are the sensors and actuators. It is

expected that as these miniaturized sensors and actuators are integrated onto a

single microchip with electronics, we will see similar improvements in system

reliability such as we have experienced in the transition from discrete electronics on

a printed circuit board to integrated circuits. The lower cost of these miniaturized

electromechanical systems also allows them to be easily and massively deployed

and more easily maintained and replaced as needed.

4. Miniaturization of micro-systems enables many benefits including: increasedportability, lower power consumption, and the ability to place radically more

functionality in a smaller amount of space and without any increase in weight.

5. The ability to make the signal paths smaller and place radically more functionalityin a small amount of space allows the overall performance of electromechanical

systems to be enormously improved.


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6. In short, MEMS translates into products that have lower cost, higher functionality,improved reliability and increased performance.

Current challenges with MEMS

MEMS technology is currently used in low- or medium-volume applications. Some of the

obstacles preventing its wider adoption are:

1. Limited OptionsMost companies who wish to explore the potential of MEMS have very limited options for

prototyping or manufacturing devices, and have no capability or expertise in micro-

fabrication technology. Few companies will build their own fabrication facilities because of

the high cost. A mechanism giving smaller organizations responsive and affordable access

to MEMS and Nano- fabrication is essential.

2. PackagingThe packaging of MEMS devices and systems needs to improve considerably from its

current primitive state. MEMS packaging is more challenging than IC packaging due to the

diversity of MEMS devices and the requirement that many of these devices be in contact

with their environment. Currently almost all MEMS development efforts must develop a

new and specialized package for each new device. Most companies find that packaging is

the single most expensive and time consuming task in their overall product developmentprogram. As for the components themselves, numerical modeling and simulation tools for

MEMS packaging are virtually non-existent. Approaches which allow designers to select

from a catalog of existing standardized packages for a new MEMS device without

compromising performance would be beneficial.

3. Fabrication Knowledge RequiredCurrently the designer of a MEMS device requires a high level of fabrication knowledge in

order to create a successful design. Often the development of even the most mundane

MEMS device requires a dedicated research effort to find a suitable process sequence for

fabricating it. MEMS device design needs to be separated from the complexities of the

process sequence.


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Applications

There are numerous possible applications for MEMS and Nanotechnology. As a

breakthrough technology, allowing unparalleled synergy between previously unrelated

fields such as biology and microelectronics, many new MEMS and Nanotechnology

applications will emerge, expanding beyond that which is currently identified or known.

Here are a few applications of current interest:

1.Biotechnology:-MEMS is enabling new discoveries in science and engineering such

as the Polymerase Chain Reaction (PCR) micro-systems for DNA amplification and

identification, enzyme linked immunosorbent assay (ELISA), capillary electrophoresis,

electroporation, micro-machined Scanning Tunneling Microscopes (STMs), biochips for

detection of hazardous chemical and biological agents, and micro-systems for high-

throughput drug screening and selection.

2.Medicine:-There are a wide variety of applications for MEMS in medicine.

The first and by far the most successful application of MEMS in medicine (at

least in terms of number of devices and market size) are MEMS pressure sensors,

which have been in use for several decades. The market for these pressure sensors

is extremely diverse and highly fragmented, with a few high-volume markets and

many lower volume ones. Some of the applications of MEMS pressure sensors in

medicine include:

The largest market for MEMS pressure sensors in the medical sector is thedisposable sensor used to monitor blood pressure in IV lines of patients in

intensive care.

MEMS pressure sensors are used to measure intrauterine pressure duringbirth. The device is housed in a catheter that is placed between the baby's

head and the uterine wall. During delivery, the baby's blood pressure is

monitored for problems during the mother's contractions.

MEMS pressure sensors are used in hospitals and ambulances as monitorsof a patients vital signs, specifically the patients blood pressure and

respiration. The MEMS pressure sensors in respiratory monitoring are used inventilators to monitor the patients breathing.

MEMS pressure sensors are used for eye surgery to measure and controlthe vacuum level used to remove fluid from the eye, which is cleaned of

debris and replaced back into the eye during surgery


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Special hospital beds for burn victims that employ inflatable mattressesuse MEMS pressure sensors to regulate the pressure inside a series of

individual inflatable chambers in the mattress. Sections of the mattress can

be inflated as needed to reduce pain as well as improve patient healing.

Physicians office and hospital blood analyzers employ MEMS pressuresensors as barometric pressure correction for the analysis of concentrations

of O2, CO2, calcium, potassium, and glucose in a patient's blood.

MEMS pressure sensors are used in inhalers to monitor the patientsbreathing cycle and release the medication at the proper time in the

breathing cycle for optimal effect.

MEMS pressure sensors are used in kidney dialysis to monitor the inletand outlet pressures of blood and the dialysis solution and to regulate the

flow rates during the procedure.

MEMS pressure sensors are used in drug infusion pumps of many types

to monitor the flow rate and detect for obstructions and blockages that

indicate that the drug is not being properly delivered to the patient.

Another application are MEMS inertial sensors, specifically accelerometers and

rate sensors which are being used as activity sensors. Perhaps the foremost

application of inertial sensors in medicine is in cardiac pacemakers wherein they are

used to help determine the optimum pacing rate for the patient based on their activity

level.

MEMS devices are also starting to be employed in drug delivery devices, for both

ambulatory and implantable applications. MEMS electrodes are also being used in

neuro-signal detection and neuro-stimulation applications.

A variety of biological and chemical MEMS sensors for invasive and non-

invasive uses are beginning to be marketed. Lab-on-a-chip and miniaturized

biochemical analytical instruments are being marketed as well.


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3. Communication:-High frequency circuits are benefiting considerably from the

advent of RF-MEMS technology. Electrical components such as induc

Documents

MEMS :- An Overview