Micro Electro Mechanical Systems

Unit-I Overview Of MEMS & Micro System

• Reference Books 1. Nadim Maluf, Kirt Williams : An Introduction to Microelectromechanical Systems Engineering, ARTECH HOUSE, INC. 685 Canton Street Norwood, MA 020622.Tai Ran Hsu : MEMS AND MICROSYSTEMS: DESIGN AND MANUFACTURE, TMH, 20023. Julian W. Gardner, Vijay K. Varadan, and Osama O. Awadelkarim, “Microsensors MEMS and Smart Devices,” John Wiley & Sons, 2001

• WHAT IS MEMS?- Any engineering system that performs electrical and mechanical functions with components in micrometers is a MEMS.

• HOW SMALL ARE MEMS DEVICES?- They can be of the size of a rice grain or even smaller!Ex : 1. Inertia sensors for air bag deployment systems in automobiles2. Microcars

• Courtesy of Denso Research Laboratories, Denso Corporation, Aichi, Japan)

• Available MEMS products1. Micro sensors - acoustic wave, biomedical, chemical, inertia, optical, pressure, radiation, thermal etc.2. Micro actuators - valves, pumps and microfluidics; electrical and optical relays and switches; grippers, tweezers and tongs; linear and rotary motors, etc.Read/write heads in computer storage systems.Inkjet printer heads.

3. Micro device components - palm-top reconnaissance aircrafts, mini robots and toys, micro surgical and mobile telecom equipment, etc.

• Microsystems = sensors + actuators + signal transduction

MINIATURIAZATION

• The Principal Driving Force for the 21st Century Industrial Technology

• There has been increasing strong market demand for: “Intelligent,” “Robust,” “Multi-functional,” and “Low-cost” industrial products.

• Miniaturization is the only viable solution to satisfy such market demand

Miniaturization Makes Engineering Sense!!!

• Small systems tend to move or stop more quickly due to low mechanical inertia.

• It is thus ideal for precision movements and for rapid actuation.

• Miniaturized systems encounter less thermal distortion and mechanical vibration due to low mass.

• Miniaturized devices are particularly suited for biomedical and aerospace applications due to their minute sizes and weight.

• Small systems have higher dimensional stability at high temperature due to low thermal expansion.

• Smaller size of the systems means less space requirements.This allows the packaging of more functional components in a single device.

• Less material requirements mean low cost of production and transportation.

• Ready for mass production in batches.

MEMS as a Microsensor:• Sense the existence and intensity of certain

physical, chemical or biological quantities – temperature, pressure, sound, light, radiation, magnetic flux, chemical composition etc.

• Advantage :- sensitive and accurate with minimum amount of sample.- mass produced in batches.

• Broad classification : Bio, Biomedical, Chemical, Optical, Thermal & Pressure sensors

• Transduction unit converts the parameter available from the microsensor to an electrical voltage or current.

MEMS as a Microactuator- motor:

• The electrical signal applied is converted to motion.

• Most popular actuation method involves the electrostatic forces generated by charged parallel conducting plates or electrodes.

Components of a Microsystem

• A microsystem is an Engineering system that contains MEMS components that are designed to perform specific Engineering functions.

• A typical airbag deployment system employs a micro-inertia sensor built on the principle of a micro-accelerometer.

• Two micro-accelerometers are used to measure deceleration in horizontal and vertical directions.

• They are mounted on a single chip (3 x 2 mm) with signal transduction and processing unit.

Intelligent Microsystems - Micromechatronics systems

Unique Features of MEMS and Microsystems

• Components are in micrometers with complex geometry using silicon, si- compounds and polymersEx :A micro gear-train bySandia National Laboratories

• A simple capillary tubular network with cross-sectional area of 20x30 μm Work on the principle of driving capillary fluid flow by applying electric voltages at the terminals at the reservoirs.

• Typical MEMS and Microsystem products> Micro-gears> Micro-Motors> MICRO-turbines> Micro-Optical components

• ICs have made possible for miniaturization of many devices and engineering systems in the last 50 years.

Microsystems & Microelectronics• These two technologies share many common

fabrication technologies.

Microelectronics Microsystems (silicon based)

Primarily 2-dimensional structures

Complex 3-dimensional structure

Stationary structures May involve moving components

Transmit electricity for specific electrical functions

Perform specific biological, chemical,Electro-mechanical and optical functions

IC die is protected from contacting media

Delicate components are interfaced with working media

Microelectronics Microsystems (silicon based)

Use single crystal silicon dies, silicon compounds,ceramics and plastic materials

Use single crystal silicon dies and other materials - GaAs, quartz, polymers, ceramics and metals

Fewer components to be assembled

Many more components to be assembled

Mature IC design methodologies

Lack of Engineering design methodology & standards

Complex patterns with high density of electricalcircuitry over substrates

Simpler patterns over substrates with simpler electrical circuitry

Microelectronics Microsystems (silicon based)

Large number of electrical feed-through and leads

Fewer electrical feed-through and leads

Industrial standards available

No industrial standard to follow in design, material selections, fabrication processes and packaging

Mass production Batch production, or on customer-need basis

Fabrication techniques are proven and well documented

Many microfabrication techniques are used for production

Microelectronics Microsystems (silicon based)

Manufacturing techniques are proven and well documented

Distinct manufacturing techniques

Packaging technology is relatively well established

Packaging technology is at the infant stage

Primarily involves electrical and chemical engineering

Involves all disciplines of science and engineering

• Microsystems use more materials than microelectronics – Silicon, Quartz, GaAs, Polymers & Metallic materials.Glass, Plastic and Metals are used for packaging.

• Microsystems perform a great variety of functions.

• Microsystems involve moving parts – Microvalves, pumps and gears.Many have liquid flow through the system – biosensors, analytic systems.

Micro-optical systems have light beam passing through it.

• Microsystems are complex 3 dimensional structures.

• The sensing elements and many core elements are in contact with the working media – technical problems in design and packaging.

• Manufacturing & Packaging technologies lack maturity.

The Multi-disciplinary Nature of Microsystems Engineering

• Electrochemistry is widely used in electrolysis to ionize substances in micromanufacturing and chemical sensors.

• Electrohydrodynamics is used in driving mechanisms in fluid flow in microchannels and conduits.

• Molecular Biology is involved in the design and manufacture of Biosensors and biomedical equipment.

• Plasma physics is used in the production and supply of ionized gases required for etching and deposition in microfabrication.

• Scaling laws provide the rules for scaling down of physical qualtities involved in the design of microdevices.

• Quantum physics is used to modeling physical behavior of materials and substances in microscale.

• Molecular physics provides models in the description of the materials at microscale.

• Mechanical engineering principles are used in the design of microsystem structure and packaging.

• Electrical engineering involves electrical power supply, functional control and signal processing circuit design.

• Chemical engineering involves processes with chemical reactions in micromanufacturing.

• Materials engineering offers selection of materials for the design and microfabrication.

• Industrial engineering relates to production and assembly of microsystems.

Major commercial success:• Pressure sensors and inertia sensors

(accelerometers) with worldwide market of:- Airbag inertia sensors at 2 billion units per year.- Manifold absolute pressure sensors at 40 million units per year.- Disposable blood pressure sensors at 20 million units per year.

Emerging trends

Old MEMS New MEMS• Pressure sensors BioMEMS• Accelerometers IT MEMS for

Telecommunication• Other MEMS (OptoMEMS and RF

MEMS)

Application in Automotive Industry

• More than 65 million vehicle will be produced in a year.

• Areas of application are - Safety- Engine and power train- Comfort and convenience- Vehicle diagnostics and health monitoring- Telematics like GPS, Route map etc.

(1) Manifold or Temperature manifold absolute pressure sensor

(2) Exhaust gas differential pressure sensor(3) Fuel rail pressure sensor(4) Barometric absolute pressure sensor(5) Combustion sensor(6) Gasoline direct injection pressure sensor(7) Fuel tank evaporative fuel pressure sensor(8) Engine oil sensor(9) Transmission sensor(10) Tire pressure sensor

Application in Aerospace Industry• Cockpit instrumentation. • Sensors and actuators for safety - e.g. seat

ejection• Wind tunnel instrumentation • Sensors for fuel efficiency and safety• Microsattellites• Command and control systems with

MEMtronics

• Inertial guidance systems with microgyroscopes, accelerometers and fiber optic gyroscope.

• Altitude determination and control systems with micro sun and Earth sensors.

• Power systems with MEMtronic switches for active solar cell array reconfiguration, and electric generators

• Propulsion systems with micro pressure sensors, chemical sensors for leak detection, arrays of single-shot thrustors, continuous microthrusters and pulsed microthrousters

• Thermal control systems with micro heat pipes, radiators and thermal switches

• Communications and radar systems with very high bandwidth, low-resistance radio-frequency switches, micromirrors and optics for laser communications, and micro variable capacitors, inductors and oscillators.

Application in Biomedical Industry

• Disposable blood pressure transducers: Lifetime 24 to 72 hours

• Catheter tip pressure sensors• Sphygmomanometers • Respirators• Lung capacity meters• Barometric correction instrumentation

• Medical process monitoring• Kidney dialysis equipment• Micro bio-analytic systems: bio-chips, capillary

electrophoresis, etc.

Application in Consumer Products• Scuba diving watches and computers• Bicycle computers• Sensors for fitness gears• Washers with water level controls• Sport shoes with automatic cushioning control• Digital tire pressure gages• Vacuum cleaning with automatic adjustment

of brush beaters• Smart toys

Application in Telecommunication Industry

• Optical switching and fiber optic couplings• RF relays and switches• Tunable resonators

Micro Optical Switch - 2-Dimensional

Micro Optical Switch - 3-Dimensional

Working principle of Microsystems

• Microsensors or transducers are the most widely used MEM devices at present.

• A sensor is a device which converts one form of energy to another form.

• It provides the user with an energy output in response to a specific measurable input.

• A smart sensor unit would include automatic calibration, interference reduction, compensation for parasitic effects, offset correction & self test.

MicrosensorsAcoustic Wave sensors

• Used to measure chemical compositions in a gas.

• They generate acoustic waves by converting mechanical energy to electrical.

• Acoustic devices are also used to actuate fluid flow in microfluidic systems.

• Activation energy is provided by two methods – piezoelectric and magnetostrictive.

Biomedical sensors and Biosensors• Biomedical industry will be a major player in

MEM devices.• BioMEMS includes Biosensors, Bioinstruments

& surgery tools and Biotesting & analysis equipments.

• Design and manufacture of this type of sensor and instrument require the knowledge and experience in Molecular biology as well as physical chemistry in addition to engineering.

• Major technical issues involved are - Functionality for biomedical operations- Adaptability to existing instruments and equipments.- Compatibility with biological system of the patients.- Controllabilty, mobility and easy navigation for operations such as laproscopic surgery.- MEMS structures with high aspect ratio – ratio of dimension in depth of the structure to that of the surface.

• Two classes of sensors – Biomedical & Bio.• Biomedical Sensors• Biomedical instruments that are used to

measure biological substances as well as for medical diagnostics.

• They typically require a minute amount of sample and can perform analysis much faster.

• Electrochemical sensors - certain biological substances like glucose in human blood, can release certain elements by chemical reaction.

• These elements can alter the electricity flow pattern in the sensor which can be detected.

• A small sample of blood is introduced to a sensor with a polyvinyl alcohol solution.

• The sensor is made up of two electrodes – one is a platinum film and the other is Ag/AgCl film

• A chemical reaction takes place between the glucose in Blood and the oxygen in polyvinyl alcohol.Glucose + O2 -> gluconolactone + H2O2

• The H2O2 produced is electrolyzed by applying a –ve potential to the platinum electrode.

• This produces positive Hydrogen ions which flow towards this electrode.

• The amount of glucose concentration is proportional to the current flow between the electrodes.

• Biosensors• They work on the principle of the interaction

of the analytes that need to be detected with biologically derived biomolecules such as enzymes, antibodies & other form of protein.

• These biomolecules when attached to the sensing elements can alter the output signal when they interact with the analyte.

• Proper selection of biomolecules and sensing element is essential for a specific analyte.

• Biotesting & analytical systems• Operation of these systems involve the

passage of minute samples of the order of nanoliters in capillary tubes or microchannels.

• They are pumped by electrohydrodynamic means – electro-osmosis or electrophoresis.

• Electrohydrodynamics involves controlling the flow of an ionized fluid by the application of electrical field.

• Analysis is carried out by separating the various species in the biological samples.

• Analytes include various biological substances and human genomes.

• Different species have different electro-osmotic mobility.

• Optical means are used to identify the species after separation.

• These systems are built with microelectronic circuits for signal transduction, conditioning and processing.

• Hundreds of such capillary tubes can be constructed on a single chip for parallel testing

• A simple system consist of two capillary tubes or microchannels of diameter 30 μm.

• Shorter channel is connected to the sample injection reservoir A and analyte waste reservoir A’.

• The longer channel is connected to the buffer solvent reservoir B & B’.

• The biological sample consists of species S1, S2, S3 with distinct electro-osmotic mobility.

• Application of an electric field between A & A’ initiates the flow of injected sample from A to A’ and the sample gets collected near the intersection due to higher resistance to flow.

• A high voltage electric field is applied between B & B’ which drives the sample with buffer solvent to flow from B to B’.

• The species get separated because of their inherent difference in electro-osmotic mobility.

Chemical sensors• These are used to sense various chemical

compounds such as exahaust gas.• Many materials are sensitive to chemical

attack – most metals are vulnerable to oxidation.

• Significant oxide layer over the surface of the metal can alter the its properties.

• The presence of Oxygen can be detected by change in resistance of a metal.

• In a practical application, the detection of Oxygen has to be much more rapid than wait for oxidation to begin.

• Material’s sensitivity to specific chemicals is used as the basic principle for many chemical sensors.1. Chemiresistor sensors – Organic polymers are used with embedded metal inserts. These polymers can cause changes in electrical conductivity of the metal when it is exposed to certain gases.

Ex : A polymer pathalocyanine is used with copper to sense ammonia (NH3) and Nitrogen Oxide (NO2) gases.

2. Chemicapacitor sensors – Some polymers can be used as the dielectric material in a capacitor.Exposure of these polymers to certain gases can alter the dielectric constant of the material which in turn changes the capacitance between the metal electrodes .Ex : Polyphenyl acetylene (PPA) is used to sense CO, CO2, N2 and CH4.

3. Chemimechanical sensors – Certain polymers change shape when exposed to chemicals including moisture.Presence of the chemical is identified by measuring variation in dimensions.Ex : Moisture sensor using pyraline PI-2722.4. Metal oxide gas sensors – This type of sensor works on the principle similar to chemiresistor sensors.Several semiconducting metals, such as SnO2, change their resistance after absorbing some gases.

The reactivity between the measured gas and the semiconducting metal can be insreased either by heating or by metallic catalysts.Catalysts are deposited on the surface of the sensor and can speed us the reactions , increase the sensitivity of the sensor.

Optical sensors• Devices that convert optical signal to

electronic output have been developed and used many consumer electronic products.

• Micro-optical sensors have been developed to sense the intensity of light.

• Solid state materials that provide strong electron – photon interactions are used as the sensing materials.

• Four main types of sensors – Photovoltaic, Photoconductive, Photodiode and Phototransistor.

• Photovoltaic junction can produce an electric potential when the transparent substrate of semiconductor is exposed to incident photon energy.

• Photoconductors change their electrical resistance when exposed to light waves.

• Photodiode and Phototransistor starts conducting with incident photons on the junction or base.

• They have extremely short response time in generating the electrical signals.

• Selection materials for optical sensors is based on quantum efficiency – materials ability to generate electron-hole pairs from input photons.

• Semiconducting materials like Si and GaAs are common photoconducting materials – GaAs has higher quantum efficiency but expensive.

• Alkali metals like Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb) and Cesium (Cs) are also used for optical sensors – last one is more commonly used.

Pressure sensors• Most of these sensors work on the principle of

mechanical deformation and stress on the diaphragm induced by the measured pressure.

• This distortion and stress is then converted to electrical signal through transducers.

• Two types – Absolute and Gage.• Absolute sensors have an evacuated cavity on

one side of the diaphragm – the measured pressure is the absolute value with vacuum as the reference.

• In the gage type no evacuation is necessary.• Two ways to apply pressure to the diaphragm.

With back side pressurization, there is no interference with signal transducer which is normally mounted on the top surface of the diaphragm.The other method, front side pressurization is used under special circumstances due to the interference of the pressure medium with the signal transducer.

• The sensing element is normally made up of thin silicon die varying in size from a few micrometers to few millimeters square.

• A cavity is created on one side of the die by microfabrication which creates a diaphragm that deforms under pressure.

• The thickness of the silicon diaphragm is in micrometers.

• A constraint base made up of metal or ceramic (Pyrex glass) supports the silicon die.

• The entire assembly is then packaged into a robust casing made of metal, ceramic or plastic with proper passivation of the die.

• 4 piezoresistors are implanted on the surface of the silicon diaphragm forming a Wheatstone bridge circuit.

• The resistors R1 & R3 experience elongation while R2 and R4 get compressed when pressure is applied.

• This causes increase in resistance of R1 & R3 while decrease in resistance of R2 and R4.

• This converts the stress on the diaphragm to change in electrical resistance and in turn to voltage.

• These changes are reflected in the dynamic deflection mode operation of the Wheatstone bridge as (1)

32

3

41

1

RR

R

RR

RVV ino

where Vo and Vin are the measured and supplied voltage to Wheatstone bridge.

• Thin wire bonds are used to transmit the voltage change through two metal pads.

• Micropressure sensors with piezoresistors have high gain and exhibit a good linear characteristics – in plane stress to change in output.

• The main drawback is that it is temperature sensitive.

• Another method of transduction is capacitance variation on applied pressure.

• Two electrodes of thin metal film are placed at the bottom of top cover and on the surface of the diaphragm.

• Any deformation of the diaphragm due to applied pressure will decrease the space between the electrodes and hence vary the capacitance.

• This method is independent of temperature.• The capacitance of the parallel plate capacitor

is given by (2)d

AC or

where εr is the relative permittivity of dielectric medium, εo is the permittivity of free space (vacuum) = 8.85 pF/m

• Capacitors are common transducers as well as actuators in microsystems.

• Capacitance variation can be measured by bridge circuits similar to Wheatstone bridge.

• The variable capacitance can be measured by measuring the output voltage Vo.

(3)

where ∆C is the capacitance change and C is the capacitance of other capacitors in the bridge.

• Sensitivity of the capacitance micropressure sensors are low compared to piezoresistors.

• They are also nonlinear.

ino VCC

CV

22

• Ex : a) Determine the capacitance of a parallel plate capacitor with a plate dimension of L=W=1000 μm with an air gap of 2 μm.b) Determine the voltage output of the capacitance bridge with above capacitor used as the variable capacitor when the air gap is reduced in steps of 0.25 μm.

a) 4.425 pF

b) Airgap μm cap pF ∆C pF Vo/Vin

----------------------------------------------------------------------------------------

2.00 4.425 0 0

1.75 5.063 0.633 0.0331.50 5.910 1.477 0.0711.25 7.080 2.650 0.1151.00 8.860 4.430 0.1670.75 11.813 7.383 0.2270.50 17.720 13.290 0.300

• A manifold absolute pressure (MAP) sensor for an Automobile is shown.

• It uses a capacitor as signal transducer.

• Third method is to use a vibrating beam for pressure measurement.

• A thin n-type silicon beam is placed across a shallow cavity on a silicon die.

• A p-type electrode is diffused at the surface of the cavity under the beam.

• The beam is made to vibrate at its resonant frequency by applying an ac signal to the diffused electrode.

• When external pressure is applied to the diaphragm, the stress is transferred to beam.

• This induced stress along the beam causes a shift of the resonant frequency of the beam which is a measure of the pressure applied.

• This kind of signal transduction is immune to temperature and has linear characteristics but are expensive to fabricate.

Thermal Sensors• Thermocouples are the most common

transducer used to sense heat.• An electromotive force is produced at the

open ends of two dissimilar metallic wire junction exposed to heat.

• This emf can be correlated to the temperature at the junction.

• These wires and the junction can be very small in size.

• By introducing an additional junction in the thermocouple circuit and exposing it to a different temperature, a gradient can be introduced in the circuit.

• This arrangement produces the Seebeck effect

• The voltage generated by the thermocouple is given as (4)where β is the Seebeck coefficient (thermo electric power of thermocouple materials) and ∆T is the temperature difference between the hot & cold junctions.

• In practice, the cold junction is maintained at a constant temperature (normally 00C by dipping in ice water).

• β depends on the thermocouple wire materials and the range of temperature measurements.

TV

• One serious drawback of the thermocouple as microthermal transducers is that the output decreases as the size of the wires and beads (junction) is reduced.

• A microthermopile is ideal micro heat sensor.

• A thermopile operates with both hot & cold junctions but thermocouples are arranged in parallel while the voltage output in series.

• Materials for the thermopile wires are same as that used for thermocouples.

• The voltage output from a thermopile can be expressed as (5)where N is the number of pairs in the thermopile.

• An example thermopile is shown. 32 polysilicon-gold thermocouples were constructed on a silicon die (3.6 mm x 3.6 mm x 20 μm).

• Typical output signal of 100 mV is available from a 500 K blackbody radiation source of Qin = 0.29 mW/cm2 with 50 ms response time.

TNV

• A capacitance type pressure sensor can also be used to measure temperature.

• In this method, the deformation of the diaphragm occurs when exposed to thermal sources and the variation in capacitance gives the measure of temperature.

• The sensitivity of such devices is low compared to thermopiles.

Microactuation• Actuation means generation of mechanical

movement for controlling some activity.• Actuator is an important part of microsystem

that involves motion.• 4 methods for actuation –

1) Thermal forces 2)Shape memory alloys 3) Piezoelectric crystals 4) Electrostatic forces

• Electromagnetic actuation is popular in macro scales but are rarely used in microdevices due to unfavorable scaling rules.

• The driving power for the actuators is dependent on the specific application

Thermal forces• Bimetallic strips are used as actuators based

on thermal force.• These strips are made by bonding together

two materials with different expansion coefficients.

• The strip will bend when heated or cooled from initial reference temperature due to incompatible thermal expansion.

• The strip will return to its initial shape when the thermal force is removed.

• Microclamps and Microvalves use this principle of microactuation.

• In these cases, one of the strips is used as a resistance heater and the other could be made from silicon or polysilicon.

• α1 & α2 are the thermal expansion coefficients of the constituent materials with α1 > α2.

Shape memory alloys• A more accurate and effective method of

microactuation is by using shape memory alloys (SMA) such as Nitinolor or TiNi alloys.

• These alloys tend to return to their original shape at a preset temperature.

• The SMA strip is originally in a bent shape at a designated preset temperature T attached toa silicon cantilever beam.

• The beam is set straight at room temperature.

• When the strip is heated to temperature T, it will return to the bent shape causing the attached silicon beam to bend with the strip.

• Used in micro rotary actuators, micro joints, robots and microsprings.

Piezoelectric crystals• Certain crystals like quartz, deform with the

application of an electric voltage.• Also an electric voltage gets generated when

the crystal deforms with an applied force.

• Such a crystal can be attached to a flexible silicon cantilever beam which gets bent when a voltage is applied to the crystal.

• Piezoelectric actuation is used in microclamps and micropositioning mechanism.

Electrostatic forces• Accurate assessment of electrostatic forces is

an essential part of the design of many actuators and micromotors.

• The induced electrostatic field between two charged particles with charge q & q’ separated by a distance r is given by

Newtons (6)

• The force is repulsive if both charges are +ve or –ve and attractive if they are opposite.

2

'

4

1

r

qqF

• If we consider two charged plates instead of a single charge then the capacitance between the plates is given by equation (2).

• The energy associated with the electric potential between the plates is

(7)Negative sign indicates a loss of potential energy with increasing applied voltage.

202

22

1V

d

ACVU r

• The associated electrostatic force normal to the plates (in the d direction) is

(8)• Ex : Determine the Electrostatic force on the

plates of a parallel plate capacitor with a plate dimension of L=W=1000 μm with an air gap of 2 μm.

• 11 mN force is generated with100 V

22

0

2V

d

A

d

UF rd

NVVFd262

26

612

10106.1)102(2

101085.8

• The electrostatic force in the width (W) and Length (L) directions can be derived from (8).

• These forces are induced with partial alignment of the plates in the respective directions.

• In general (9)can be used to derive these forces where i is the direction in which the misalignment occurs.

(10)(11)

ii x

UF

20

2V

d

WF rL

20

2V

d

LF rw

• The force in W direction is independent of width while the force in L direction is independent of length.

• Electrostatic forces are the prime driving forces of micromotors.

• The drawback of electrostatic actuation is that the force that is generated by this method is low in magnitude.

• Its application is primarily limited to acuators for optical switches, microgrippers and tweezers.

MEMS with microactuators

Microgrippers• The electrostatic forces generated by parallel

charged plates can be used as the driving force for gripping objects.

• Either normal force (parallel plates) or in-plane force (pairs of misaligned plates) can be used to provide the gripping force.

• Former one is simple in practice but consume more space for electrodes.

• Later arrangement has multiple pairs of misaligned plates and is referred to as the comb drive.

• The electrostatic force generated by these pairs of misaligned plates tends to align them.

• This action bends the arm and closes the extension arms for gripping.

• These microgrippers can be used in micromanufacturing processes and microsurgery.

• EX : For the comb drive operating in air as shown, determine the voltage required to pull the moving electrode 10 μm from the unstretched position of the spring. The spring constant k is 0.05 N/m. The gap between the electrodes is 2 μm and width is 5 μm.

• The required travelling distance δ = 10 μm.Equivalent spring force F = kδ = 0.5 μN.There are two sets of electrodes and each set has to generate 0.25 μN.By equation (11), we get

which yields V = 150.33 V

26

6126

1022

1051085.81025.0 V

Micromotors• Two types – linear and rotary.• They work based on elctrostatic forces.• The sliding force generated by a pair of

electrically energised misaligned plates produces the motion in a linear motor.

• Each of the two sets of base plates contain many electrodes made of conducting plates of length W.

• The fixed plate has a pitch of W whereas the moving plate has slightly higher pitch say W+W/3.

• On energising a pair of electrodes A & A’ the moving plate moves towards left until both A & A’ are aligned.

• At this point energise the pair B & B’ to move the top plate a further distance of W/3.

• This process can be repeated for other sets of electrodes to induce additional motion.

• The smaller the preset misalignment of the electrode plates, the smoother the motion becomes.

• By redesigning the structure of the plates, rotary motors can be made to work on the same principle.

• The major problem in micromotor design is the bearings for the rotors – electric levitation principles are used for this purpose.

• Electrodes are installed on the outer surface of the rotor poles and on the inner surface of the stator poles.

• The pitches of the electrodes are slightly mismatched for the electrostatic driving force to develop when misaligned pairs of electrodes are energised.

• The air gap between the rotor and stator plates can be as small as 2 μm.

• The outside diameter of the stator poles are about 100 μm while the length of the rotor poles is about 25 μm.

• One major design problem encountered is the wear and lubrication of the moving parts.

• Typically these motors rotate at about 10,000 RPM.

• At such high speeds, the bearings quickly wear off and the rotor starts wobbling.

• Microtribology which deals with friction, wear and lubrication is a critical research area in Microsystem design.

Microvalves• Microvalves are primarily used in industrial

systems that require precision control of flow - gas flow for manufacturing process or blood flow in biomedical systems.

• The growing market is in the pharmaceutical industry in microfluidic systems for precision analysis and separation of constituents.

• In the basic design of microvalves, a cantilever silicon diaphragm moves to close the inlet on heating the resistor rings mounted on it.

• Removal of heat from the diaphragm opens the valve again allowing the flow of fluid.

• The heating rings are made of aluminum 5 μm thick.

• The valve has a flow capacity of 300 cm3/min at a pressure upto 100 psi.

• 1.5 W of power is required to close the valve at 25 psi.

• In another type of construction, a liquid is heated using electrodes which in turn bends the silicon membrane causing the flow to stop.

• Even the flow rate can be controlled by controlling the bending of the diaphram.

Micropumps• A simple micropump can be constructed by

using electrostatic actuation of a diaphragm.

• The deformable silicon diaphragm forms one of the electrodes of a capacitor which can be actuated and deformed toward the top electrode by applying a voltage.

• The upward motion of the diaphragm increases the volume in the pumping chamber which decreases the pressure.

• This causes the inlet check valve to open and allow in flow of fluid.

• When the applied voltage is removed, the diaphragm returns to its original position pressurising the pumping chamber.

• This causes the outlet check valve to open and the allow the out flow of the fluid.

• This pump has a diaphragm of 4 mm x 4 mm x 2 μm thick; The gap between the diaphragm and the electrode is 4 μm.

• The pumping frequency is upto 100 Hz.• At 25 Hz, a pumping rate of 70 μL/min is

achieved.• A piezopump has piezoelectric material coated

outside the tube wall which create a wave motion of the wall.

• This wave motion exerts pressure on the fluid inside the tube causing the pumping action.

Microaccelerometer• An accelerometer is an instrument that

measures the acceleration (rate of change of speed) of a moving object.

• Microaccelerometers are used to detect the associated dynamic forces in a mechanical system in motion and are widely used in automotive industry.

• ±2g range accelerometers are used in suspension system and antilock breaking system (g = 9.81m/s2).

• ±50g range accelerometers are used to actuate the airbags in case of collision.

• Most accelerometers are built on the principle of vibration – a mass supported by a spring and a damping device (dashpot).

• In the case of microaccelerometers, a different arrangement is necessary due to limited space

• A minute silicon beam with an attached mass (seismic mass) constitute a spring mass system – and the air surrounding it forms the damping effect.

• A piezoresistor is implanted on the beam to measure the deformation due to attached mass and thus the acceleration of the body which is related to the driving dynamic force that causes the vibration.

• Accurate measurement of the acceleration (vibration) enables us to measure the applied dynamic force.

• So accelerometers of different ranges can provide us information on excessive vibration in a vehicle or state of the suspension system or state of the engine etc.

• Different types of accelerometers available commercially – piezoelectric, piezoresistive, capacitive and resonant membrane.

• The most widely used application of microaccelerometer is for airbag deployment with integrated transduction.

• A beam with an electrode is attached to two tethers at both ends (made up of elastic material and anchored at one end).

• The thin beam acts as the seismic mass and the attached electrode is placed between two fixed electrodes.

• In the event of acceleration, the displacement of the mass will be in the direction opposite to acceleration which is correlated to change in capacitance between the fixed electrodes.

• The arrangement shown will measure the acceleration only in the direction along the length of the beam mass.

• To measure the acceleration in both x & y directions, following arrangement has to be used.

• Another compact arrangement is shown where the beam is replaced by a squareplate that can displace both in x & y directions.

Microfluidics• Microfluidic systems are widely used in

biomedical & pharmaceutical precision manufacturing processes.

• Applications - Chemical analysis, biological & chemical sensing, drug delivery, molecular separation (DNA analysis, amplification, sequencing or synthesis of nucleic acids) and environmental monitoring.

• They are also essential part of precision control system for automotive & aerospace.

• Advantages 1. Ability to work with small samples – significantly smaller and less expensive biological and chemical analysis.2. Better performance with reduced power consumption.3. Biotech systems – it can be combined with traditional electronics on a single piece of silicon.4. They are disposable after use – safety in application, cheaper in cleaning & maintenance.

• A microfluidic system consist of nozzles, pumps, channels, reservoirs, mixers, oscillators and valves.1. Microsensors used measure fluid properties – pressure, temperature & flow.2. Actuators used to alter the state of the fluid – microvalves, micropumps.3. Distribution channels regulating flow in various branches of the system – capillary networks.

They have a cross-sectional area in square micrometers and a fluid flow of a few hundred nanoliters to a few microliters.

Microchannels of noncircular cross-section are produced by chemical etching in open channels – two open channels are bonded to form a closed conduit.Typical length of these microchannels is a mm.4. System integration – integrating the microsensors, valves and pumps through microchannels.Involves the required electrical system to provide electrohydrodynamic forces, transduction and control of fluid flow.

• Microfluidic systems are built with quartz, glass, plastic, polymer, ceramic, semiconductor and metal.

• Electrohydrodynamic pumping is an effective way of moving fluids in microchannels.