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Seminar Report
On
Biogas as an alternate fuel for IC Engines
Submitted By: Guided By :Arun Kumar Dr. G.A. Harmain
EN. No. 197/06
Roll-25, 7th Sem. Professor
Mechanical Engg. Deptt. Mechanical Engg. Dept
NIT Srinagar. NIT Srinagar.
NATIONAL INSTITUTE OF TECHNOLOGY
HAZRATBAL, SRINAGAR- 190006 (J&K)
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CERTIFICATE
It is certified that the seminar report entitled Biogas as an alternate
fuel for IC Engines is the work carried by Arun Kumar under my
guidance and supervision. He has fulfilled all the requirements as per
status of NIT for the submission of this report.
Dr. G.A. Harmain
Professor
Mechanical Engg. Deptt.
NIT Srinagar.
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CONTENTS
SERIAL NO. TOPICS PAGE NO.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
ABSTRACT
INTRODUCTION
HISTORY
MANUFACTURING PROCESSES OF
BIOGAS
PRESENT THEORIES AND PRACTICES
NEEDS AND NECESSITIES
APPLICATIONS
COMPARISON WITH OTHER FUELS
ADVANTAGES
COMPATIBILITY WITH ENGINES
REPORTS
PERFORMANCE
CONCLUSIONS
BIBLIOGRAPHY
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1. ABSTRACT
Biogasasthenameitselfindicatesthatthisgasismadeby usingbiogenic resources. Earlier
it wasnot knownto usthathow muchitcanbebeneficial in future. Astimehascome
showingapathtowards renewablesource requirementbeingtheshortage of fossil fuels.
Economy of Indiais very poorif we focus ontheexistence of fossil fuel in India. Inthe
coming years Indiaisgoingto establishbiogasplants onagreatscale. Asbiogascanbe
manufacturedin Indiaeconomically as raw material to it, isinabundance. In IC enginesit
canbeused very comfortably. Thisgasis free fromharmful emissionsaftercombustion. We
in Indiaareusing CNG and LPG gases forthe IC enginestoday also so thereis
No problem ofdealing withthegaseous fuel. Thisgasmainly has Methaneand Carbon
dioxideasitsmainconstituents. Inthe 16thcentury thisgas wasused forheating ofbathe
waterin Persia. Engine working onbiogas will emitalmostno harmful gas whichcauses
greenhouseeffectintheenvironment. Asbiogenic wastein Indiaare occupyinga lot of
spaceandcausing foul smell, so by using waste forbiogasa lot of landissavedalso
environmentbecomescleannearthosesites. Biogasisproducedextractingchemical energy
fromthe organicmaterial. Also humanexcretacanbeusedas raw material.
Properties ofthisgas for IC enginearemoresuitablethangasolineanddiesel. Some
modificationintheenginehasto bedoneif wantto operate onSI engine or CI engine. There
isaneed ofadvancedsparktimingtechnique. Thereisalso requirement of Carbondioxide
elimination. So somemoreprocessesareinvolved forthepurification ofbiogas forachieving
better fuel qualities. Thisgasis resistantto knockingintheengine. Andhighercompression
ratioscanbeachieved which leadsto greaterengineefficiency. Thisgas wasearlierused for
cookingpurposesandstreet-lighting. Dungusedinitsplantaccounts for 21% oftotal rural
energy in India. Indian Governmentintroduced largescalebiogasproductionin 1981 through
National project. 2 millionbiogasplants werein operationin 1995. Thisgasisequivalent
to CNG buteconomical than CNG. In India CNG isinabundancethan LPG so more CNG is
beingused rightnow.
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2. INTRODUCTION
Thegasisdrawninto thecylinderstogether withthecombustionair. Theconnectionto the
intakemanifoldmay takedifferent forms. Owingto thehighignitiontemperature ofbiogas, a
diesel enginemustalwaysbe operated withamixture ofbiogasanddiesel oil. A spark-
ignitionengine will also operate on 100% biogas. Biogasburns less rapidly thandiesel fuel.
Consequently, enginesdesigned for lessthan 2000 rpmarethebetterchoice. Spark-ignition
engines runabouttwiceas fastasdiesel engines, thus leadingto lowerefficiency when
operating onbiogas.
Any internal combustionengine, exceptatwo-stroke, canbeadaptedto run onbiogas. On
spark-ignitiongasolineengines (hereafter referredto asgasolineengines), abiogasandair
mixerisneededinadvance ofthecarburetornearthechoke. Thebiogasisintroduced viaa
fivemmdiametertubeconnectedto thebiogassupply throughacontrol valve. Theengineis
started ongasolineandthenswitched overto biogasaftertheengineis running. Theengine
canbeswitchedbackto gasolineifthereisashortage ofbiogas. Forsmooth running ofthe
engine, thebiogas flow shouldbesteady; thiscanbedone onstationary enginesbycounterbalancingthegascap. Sheafferand Roland, acompany thathasdevelopedbiogas
systems foruseinthe UnitedStates recommendsusinggasolineengines. They only use
biogas for fuel, butthey keeppropanebottledgasasabackupincasethereisashortage of
biogas. Thecompany also recommendsthatenginesthatare runcontinuously have oncein
week oil and filterchanges.
Becausetheuse ofbiogasto runenginesandtheuse oftheexcessengineheatto heat
digestersare oftenthemostimportant factorsinmakingbiogassystemsprofitable, what
followsarethreedifferent reports onusingbiogasasanengine fuel.
L. John Fry'saccount ofhisuse ofbiogasto runengines ona farmduringasix yearperiodis
one ofthemoreimpressive onesto be foundinbiogas literature. The followingsectionis
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adapted fromhisbook, Practical Building of Methane Power Plants for Rural Energy
Independence.
Methane (biogas withoutcarbondioxide)makesanexcellent fuel forinternal combustion
enginesbecauseit:
1)hasa very high octane rating,
2) leaves little orno carbondepositsincylinders or onpistons,
3)greatly reducestheamount ofsludgebuild-upinthe oil, whichmeans longer operating
timesbetween oil changes,
4)doesnotdilutethe oil onthecylinder wallsduringenginestart-upas liquid fuelsdo, and
thuspromotes longerengine life
5)hasno tetra-ethyl leadinitto foul sparkplugsandpollutetheair,
6)mixesbetter withairthangasoline, resultinginabetterexplosioninthecylinder,
7) resultsin less valveburning,
8)burnsclean, with fewerpollutantsthanmany other fuels.
Thereisadirect relationshipbetweenpressureandtemperature. Whenpressuregoesup, so
doestemperature; whenpressuregoesdown, so doestemperature. Thisisexactly what
happensinsidethecylinders ofgasolineanddiesel engines.
Ingasolineenginesa fuel-airmixtureis letinto thecylinder, thepistonpushesupand
compressesthemixture, thesparkplug fires, thereisanexplosion, andthehotgases formed
by theburning fuel expandandpushthepistondown. Atthe very bottom ofthepiston's
travel, thecylinderspacehasitsgreatest volume. Atthe very top ofthepiston'stravel, the
cylinderspaceisassmall asitcanbe. The ratio ofthe largest volumeto thesmallest volume
iscalledthecompression ratio. Ifthecompression ratio is fourto one, the fuel-airmixture
will becompressedby a factor of four. Or, to lookatitanother way, theexplodinggases will
expand fourtimestheir original volume.
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Now, astheprocess ofcompressionand firing repeatsandcontinues, thecylinder wallsheat
up, andthisincreasesthetemperature oftheincoming fuel-gasmixture. Asthismixtureis
compressedby thepiston, itbecomeshotterthanit wouldinacoldengineandmay reachits
ignitiontemperaturebeforethepistonhas finishedcompressingit. Boom, the fuel-air
mixturesexplodestoo soon (predetonates). Thisiscommonly called knock. Itstealspower
fromtheenginebecausethepistonmustcontinueupwardagainstthe force oftheexplosion
pushingitdown. Obviously, themorethe fuel-airmixtureiscompressed, thegreater will be
itstendency to predetonate, sincegreatercompression will meanhigherpressuresand
temperatures.
It wouldseemthat whatis wantedinanengineisa low compression ratio, right? Wrong. As
waspointed outabove, thecompression ratio isalso theexpansion ratio, andthemorethe
explodinghotgasesareallowedto expand, themorethey will fall intemperature. Inessence,
thismeansthatthegreatertheexpansion ofthesehotgasesinthecylinderspace, themore
efficienttheengine will becauseit will convertmore ofthatheatinto themotion ofthe
piston. Thetrade-offisbetweenthe knocking ofpredetonationandthermal (heat)efficiency.
Sparkengine fuelssuchasgasolineare ratedby their octanenumber. The octane rating ofa
fuel isameasure ofhow well itavoidspredetonation. Methanehasan octanenumber of 120
ormore. Thismeansthatitcaneasilybeusedinhighcompressionengines, becauseit rarely
predetonates.
Biogas, whichismethanemixed withcarbondioxide, hasa lower octane ratingthanmethane
(butstill over 100). Carbondioxidealso actsto decreasemethane'sability to detonate whenit
isignited, so notasmuchpowerisavailable fromthemethaneinun-scrubbedbiogasasis
frompuremethane, givenequal volumes ofmethane. The factisthatanythingexcept oxygen
mixed withmethane will diluteit, becausenotasmuchmethanecangetinto thecylinder,
andclearly this will further reducethepoweravailable fromeachpowerstrokeinthe
cylinder. Removingthecarbondioxide will increasethepoweravailable.
Thetrace ofhydrogensulfidethatisinbiogasshould onlybe removedifitispresentin
amounts (by volume)greaterthan 0.1 percent. Butthenthere wouldbeno way to smell gas
leaks--because ofall thegasesinbiogas, only hydrogensulfidehasany smell (rotteneggs).
Hydrogensulfidetroublescanbepartly overcomeby replacingthestandardengine valves
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withheat resistant valvesandchangingthethermostatinthecoolingsystemso thatthe water
circulatesat 65 degreescentigrade (150 F) ratherthan 50 degreescentigrade (120 F).
Sometimeseventheseprecautionsarenottakenandtheengine runs just fine.
Testsshow thatby usingthebest fuel-air ratiosandaveraging outputs, 100 percentmethane
outperformsa 50 percentmethane/50 percentcarbondioxidemixtureby approximately 86percentinthesameengine, all otherconditionsbeingequal. Lookedatanother way, diluted
methane (biogas)hasto provide 1.86 timestheenergy inputto providethesameenergy
outputthatpuremethanecan. Usingagasolineenginedesigned for research whichhada
variablecompression ratio (4:1 to 16:1), it was foundthat outputpeakedatacompression
ration 15:1, a fuel-air ratio of 1:10 (10 percentmethaneto 90 percentairby volume), and
withthetimingsetso thattheengine fired30 percentbeforetopdeadcenter.
Ordinary four-cycle, spark-ignitiongasolineenginescanbeeasily convertedto run on
biogas, butthey tendnotto havethehighcompression ratios whichcanbeused withbiogas.
Very small enginessuchasmotorcycleengines often requirea fuel mix ofgasolineand oil.
Thesetwo-cyclespark-ignitionenginesarenot very suitable forbiogas, butthey canbeused.
Lubricationmaybeaproblem, becausetheseenginesgetsome oftheirpiston lubrication
fromthe oil inthe fuel mixture, of whichbiogashasnone. Operatingatwo-cycleengineasa
dual fuel (biogasand oil)enginemightbeanexperiment worthtrying, especially ifthe
capacity ofthebiogasdigesteristoo small to provideenoughgas fora largerengine.
Anothercommonenginetypeisthediesel. Diesel enginesdo nothavesparkplugs. What
happensinadiesel engineisthatairiscompressedand whenthepiston reachesthe right
placeinthecylinderspace, thediesel fuel issquirted (injected)into thecylinderandtheheat
whichhasbeendevelopedby compressingtheairignitesthe fuel-airmixture, causingan
explosion withoutneed ofaspark.
Diesel fuelsdo nothave octane ratings; they havecetane ratings. The kind ofmeasurementis
different fordiesel becausethequalitiesneeded fordiesel fuel are very differentthanthe
qualitiesneeded forgasoline fuel. Ingasolineenginesthe fuel shouldnotburnuntil itis lit
withaspark. Indiesel enginestheinjected fuel shouldburnassoonasitentersthecylinder.
Thatis why cetanenumbersareall abouthow easily the fuel ignites onits owninthe
cylinder.
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3. HISTORY
Hearingabouthow biogascontrolspollutionandimprovessanitation, Mr. Parayno visited
thebiogassystemat Maya Farmsanddecidedthenandthereto have onebuilt forhis
piggery. Mr. Parayno enjoys recounting whathappenedduringthe longdry summer of 1977
whenthehydroelectricplantin Central Luzoncouldnotgenerateenoughpower. He
extendedthebiogaspipeto thestoreandtransferredsome ofthemantle lamps fromthe
piggery. Whentheelectricpower wasshut off, as frequently happened, hehadthe only
brightly litstoreinthearea. Thisbroughtinmany customers. Mr. Parayno isnow thinking
aboutusingthegasto runanengineanda 2.5 KVA electricgenerator (Maramba, 1978).
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.
y The Adams Golf DiXX Digital Instruction Putteruses MEMS, specifically a MicroInertial NavigationSystem to analyze factors of the swingmotion, includingpath,
tempo, speedandhand vibration levels.
Companies withstrong MEMSprogramscome inmany sizes. The larger firmsspecialize in
manufacturinghigh volume inexpensivecomponents orpackagedsolutions forendmarkets
suchasautomobiles, biomedical, andelectronics. Thesuccessful small firmsprovide valuein
innovativesolutionsandabsorb theexpense ofcustom fabrication withhighsalesmargins.on
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Siliconisthematerial usedto createmostintegratedcircuitsusedinconsumerelectronicsin
themodern world. Theeconomies ofscale, ready availability ofhigh-quality materialsand
ability to incorporate electronic functionality make silicon attractive for a wide variety of
MEMSapplications. Siliconalso hassignificantadvantagesengenderedthroughitsmaterial
properties. Insinglecrystal form, siliconisanalmostperfect Hookeanmaterial, meaningthat
it has linear relationshipbetween applied stress and strain. As well as making for highly
repeatablemotion, thisalso makessilicon very reliableasitsuffers very little fatigueandcan
have service lifetimes in the range ofbillions to trillions of cycles withoutbreaking. The
basic techniques forproducing all siliconbased MEMS devices aredeposition ofmaterial
layers, patterning of these layers by photolithography and then etching to produce the
requiredshapes.
Polymers
Even though theelectronics industry providesaneconomy ofscale forthesilicon industry,crystallinesilicon isstill acomplexand relatively expensivematerial to produce. Polymers
on the other hand can be produced in huge volumes, with a great variety of material
characteristics. MEMSdevicescanbemade frompolymersby processes suchas injection
molding, embossing or stereolithography and are especially well suited to microfluidic
applicationssuchasdisposablebloodtestingcartridges.
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Metals
Metalscanalso beused to create MEMSelements. Whilemetalsdo nothavesome of the
advantagesdisplayedby silicon in terms ofmechanical properties, whenused within their
limitations, metalscanexhibit very highdegrees of reliability. Metalscanbedepositedby
electroplating, evaporation, and sputteringprocesses.Commonly used metals include gold,
nickel, aluminium, chromium, titanium, tungsten, platinum, andsilver.
4 . General Design Methodology
MEMSdesignprocessbegins with the identification ofthegeneral operatingprinciplesand
overall structural elements, thenproceeds onto analysis and simulation, and finally ontooutlining ofthe individual steps in the fabricationprocess. This is oftenan iterativeprocess
involvingcontinuousadjustments to the shape, structure, and fabrication steps. Thedesign
processisnotanexactanalytical sciencebut ratherinvolvesdevelopingengineeringmodels,
many for thepurpose of obtainingbasicphysical insights. Computer-basedsimulation tools
using finite-elementmodelingareconvenient foranalyzingcomplexsystems. A number of
availableprograms, suchas ANSYS
(ANSYS, Inc., of Canonsburg, Pennsylvania) and CoventorWare (Coventor, Inc., of
Cary, North Carolina), can simulatemechanical, thermal, and electrostatic structures. Any
MEMSsimulationsoftwareuseseither oftwo approaches:
4.1 System level (or behavioral or reduced order or lumped parameter) modeling:
Thisapproachcapturesthemaincharacteristics ofa MEMSdevice. Itprovidesaquickand
easy method to predict themainbehavior ofa MEMSdevice. The requirement is that the
devicecanbedescribedby sets of ordinary differential equationsandnonlinear functionsata
blockdiagram level. This approach originated fromcontrol systemengineering. Themulti
domainproblemisavoidedsince, typically, thesimulationtoolsarephysically dimensionless
only theuserinterpretstheinputand output ofthe variousblocksinaphysically meaningful
way.
4.2 Finite element modeling (FEM):
This approach originated from mechanical engineering where it was used to predict
mechanical responsesto a load, suchas forcesandmoments, appliedto apart. Thepartto be
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simulated is broken down into small, discrete elements a process called meshing. Each
element has a number of nodes and its corners at which it interacts with neighboring
elements. Theanalysiscanbeextended to nonmechanical loads, forexample, temperature.
Additionally, finiteelementsimulationtechniqueshavebeensuccessfully appliedto simulate
electromagnetic fields, thermodynamicproblemssuchassqueeze filmdamping, and fluidics.
FEM results inmore realistic simulation results than behavioral modeling, but it ismuch
morecomputationally demandingandhenceitisdifficultto simulateentiresystems.
5 .Fabrication Issues in MEMS
Silicon micromachining has been a key factor for the vast progress of MEMS. Silicon
micromachiningcomprises oftwo technologies: bulkmicromachining, in whichstructuresare
etchedinto siliconsubstrate, andsurfacemicromachining, in whichthemicromechanical layers
are formed from layersand filmsdeposited onthesurface. Bulkmicromachiningandsurface
micromachiningarethetwo majormicromachiningprocesses ofsilicon; silicon waferbondingis usually necessary for silicon microfabrication. LIGA and three-dimensional (3D)
microfabricationshavebeenused forhigh-aspect ratio and3D microstructures fabrication for
MEMS
Siliconmicromachiningcombinesadding layers ofmaterial overasilicon wafer withetching
(selectively removingmaterial)precisepatternsinthese layers orintheunderlyingsubstrate.
Theimplementationisbased onabroadportfolio of fabricationprocesses, includingmaterial
deposition, patterning, and etching techniques. Lithography plays a significant role in the
delineation ofaccurateandprecisepatterns. Thesearethetools of MEMS (see Figure 2)
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Figure 2 Illustration ofthebasicprocess flow inmicromachining: Layersaredeposited;
photoresistis lithographicallypatternedandthenusedasamaskto etchtheunderlying
materials. Theprocessis repeateduntil completion ofthemicrostructure.
5.1 Bulk micromachining of silicon: -Thebulk micromachining technique canbe divided into wet etching and dry etching of
silicon according to thephase of etchants. Liquid etchants, almost exclusively relying onaqueous chemicals are referred to as wet etching, while vapor and plasma etchants are
referredto asdry etching.
Bulk micromachining is a fabrication technique which builds mechanical elements by
starting with a silicon wafer, and then etching away unwantedparts, andbeing left with
useful mechanical devices. Typically, the waferis
photo patterned, leavingaprotective layer on theparts ofthe wafer that you want to keep.
The waferisthensubmersedinto a liquidetchant, likepotassiumhydroxide, whicheatsaway
any exposedsilicon. Thisisa relatively simpleandinexpensive fabricationtechnology, and
is well suited forapplications whichdo not requiremuchcomplexity, and whichareprice
sensitive.
Today, almost all pressure sensors are built with Bulk Micromachining. Bulk
Micromachinedpressure sensors offer several advantages over traditional pressure sensors.
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They cost less, are highly reliable, manufacturable, and there is very good repeatability
between devices. All new cars on the market today have several micromachinedpressure
sensors, typically used to measuremanifoldpressure in theengine. Thesmall sizeandhigh
reliability of micromachined pressure sensors make them ideal for a variety of medical
applicationsas well.
Bulkmicromachiningisthe oldestparadigm ofsiliconbased MEMS. The wholethickness of
a silicon wafer is used forbuilding the micro-mechanical structures.Silicon is machined
using variousetchingprocesses. Anodicbonding ofglassplates oradditional silicon wafers
is used for adding features in the third dimension and for hermetic encapsulation. Bulk
micromachining has been essential in enabling high performance pressure sensors and
accelerometersthathavechangedtheshape ofthesensorindustry inthe 80'sand 90's.
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Figure 3 Bulksiliconmicromachining:
(a) Isotropicetching; (b) Anisotropicetching; (c) Anisotropicetching withburiedetch-stoplayer; (d) Dielectricmembrane releasedby back-sidebulketching; (e) Dopantdependent
wetetching. (f) Anisotropicdry etching.
Withbulk-micromachinedsiliconmicrostructures, the wafer-bondingtechnique isnecessaryfor theassembled MEMSdevices. Surfacemicromachining, however, canbeused to build
themonolithic MEMSdevices.
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5.2 Surface Micromachining :-
Surfacemicromachiningdoesnotshape thebulksiliconbut insteadbuildsstructures onthesurface ofthesiliconby depositingthin films of sacrificial layersand structural layersand
by removingeventually thesacrificial layersto releasethemechanical structures (Figure 4).Theprimeadvantage ofsurface-micro-machinedstructures is theireasy integration with IC
components, since the wafer isalso the working for IC elements. Surfacemicromachiningrequiresacompatibleset ofstructural materials, sacrificial materialsandchemical etchants.
Siliconmicrostructures fabricatedby surfacemicromachining areusually planar structures(oraretwo dimensional). Othertechniquesinvolvingtheuse ofthin-filmstructural materials
releasedby the removal ofanunderlyingsacrificial layerhavehelpedto extendconventionalsurface micromachining into the third dimension. By connectingpolysiliconplates to the
substrate and to each other with hinges, 3D micromechanical structures canbe assembledafter release
Figure 4.
Processingsteps of
typical surface micromachining
Figure no 5 (Basic MEMS Processes.).
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6.1 MEMS Thin Film Deposition Processes
One ofthebasicbuildingblocks in MEMSprocessing is theability to depositthin films of
material. In this text we assume a thin film to have a thickness anywherebetween a few
nanometers to about 100 micrometer. The film can subsequently be locally etched using
processesthe Lithography and Etching.
MEMSdepositiontechnology canbeclassifiedintwo groups:
1. Depositionsthathappenbecause ofachemical reaction:I. Chemical Vapor Deposition (CVD)
II. ElectrodepositionIII. Thermal oxidation
Theseprocessesexploitthecreation ofsolidmaterialsdirectly fromchemical reactionsingasand/or liquidcompositions or withthesubstratematerial. Thesolidmaterial isusually notthe
only product formedby the reaction. Byproductscan includegases, liquidsandeven other
solids.
2. Depositionsthathappenbecause ofaphysical reaction:I. Physical Vapor Deposition (PVD)
II. CastingCommon forall theseprocessesarethatthematerial depositedisphysically moved onto the
substrate. In other words, there is no chemical reaction which forms the material on the
substrate. This isnotcompletely correct forcastingprocesses, though it ismoreconvenient
to think ofthemthat way.
Thisisby no meansanexhaustive listsincetechnologiesevolvecontinuously.
I. Chemical Vapor Deposition (CVD)In thisprocess, the substrate isplaced inside a reactor to which a number of gases are
supplied.
The fundamental principle oftheprocessisthatachemical reactiontakesplacebetweenthe
sourcegases. Theproduct ofthat reaction isasolidmaterial withcondenses onall surfaces
insidethe reactor.
The two most important CVD technologies in
MEMSarethe Low Pressure CVD (LPCVD)and
Plasma Enhanced CVD (PECVD). The LPCVD
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processproduces layers withexcellentuniformity of thicknessandmaterial characteristics.
Themainproblems withtheprocessarethehighdepositiontemperature (higherthan 600C)
andthe relatively slow deposition rate.
The PECVD processcan operateat lowertemperatures (downto 300 C)thanksto theextra
energy suppliedto thegasmoleculesby theplasmainthe reactor.
Figure no 6 (Diagram showing a LPCVD Reactor.).
However, the quality of the films tends to be inferior to processes running at higher
temperatures.
Secondly, most PECVD depositionsystemscan only depositthematerial on oneside ofthe
wafers on 1 to 4 wafersatatime. LPCVD systemsdeposit films onbothsides ofat least 25
wafersatatime. A schematicdiagram ofatypical LPCVD reactorisshownin figure 6.
When do I want to use CVD?
CVDprocessesareideal to use when you wantathin film withgoodstepcoverage. A variety
ofmaterialscanbedeposited withthis technology, however, some ofthemare lesspopular
with fabsbecause of hazardousbyproducts formed duringprocessing. The quality of the
material varies fromprocessto process, howeveragood rule ofthumbisthathigherprocess
temperature yieldsamaterial withhigherquality and lessdefects.
II. Electrodeposition: -This process is also known as "electroplating" and is typically restricted to electrically
conductive materials. There arebasically two technologies forplating: Electroplating and
Electrolessplating. In theelectroplatingprocess the substrate isplaced ina liquid solution
(electrolyte). When an electrical potential is applied between a conducting area on the
substrateandacounterelectrode (usually platinum) in the liquid, achemical redoxprocess
takesplace resultinginthe formation ofa layer ofmaterial onthesubstrateandusually some
gasgenerationatthecounterelectrode.
In the electroless plating process a more
complex chemical solution is used, in
whichdepositionhappensspontaneously on
any surface which formsasufficiently high
electrochemicalpotential withthesolution.
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. Figure no 7(Schematic Diagram of a typical Setup for Electroplating).
Thisprocessisdesirablesinceitdoesnot requireany external electrical potential andcontact
to the substrate duringprocessing. Unfortunately, it is also more difficult to control with
regards to film thickness and uniformity. A schematic diagram of a typical setup for
electroplating is shown in the figure 7. Figure no 7(Schematic Diagram of a typical
Setup for Electroplating).
When do I want to use Electrodeposition ?
Theelectrodepositionprocessis well suitedto make films ofmetalssuchascopper, goldand
nickel? The filmscanbemade inany thickness from ~1 m to >100 m. Thedeposition is
bestcontrolled whenused withanexternal electrical potential, however, it requireselectrical
contactto thesubstrate when immersed inthe liquidbath. Inany process, thesurface ofthesubstratemusthaveanelectrically conductingcoatingbeforethedepositioncanbedone.
III. Thermal oxidationThis is one ofthemostbasicdepositiontechnologies. Itissimply oxidation ofthesubstrate
surfaceinan oxygen richatmosphere. Thetemperatureis raisedto 800 C-1100 C to speed
up the process. This is also the only deposition
technology which actually consumes some of thesubstrate as it proceeds. The growth of the film is
spurned by diffusion of oxygen into the substrate,
which means the film growth is actually downwards
into the substrate. Figure no 8 (
Schematic Diagram of a typical Wafer Oxidation
Furnace).
As the thickness of the oxidized layer increases, the diffusion of oxygen to the substrate
becomes more difficult leading to a parabolic relationship between film thickness and
oxidationtime for filmsthickerthan ~100nm.
Thisprocessisnaturally limitedto materialsthatcanbe oxidized, anditcan only form films
thatare oxides ofthatmaterial. Thisistheclassical processusedto formsilicondioxide ona
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siliconsubstrate. A schematicdiagram ofa typical wafer oxidation furnace isshown in the
figure 8.
When do I want to use thermal oxidation?
Whenever you can! This is a simple process, which unfortunately produces films with
somewhat limitedusein MEMScomponents.
It is typically used to form films thatareused forelectrical insulation or thatareused for
otherprocesspurposes laterinaprocesssequence.
I. Physical Vapor Deposition (PVD):PVD coversanumber ofdepositiontechnologiesin whichmaterial is released fromasource
and transferred to the substrate. The two most important technologies are evaporation and
sputtering.
When do I want to use PVD?
PVD comprises the standard technologies fordeposition ofmetals. It is farmorecommon
than CVD formetalssince itcanbeperformedat lowerprocess riskandcheaper in regards
to materials cost. The qualities of the films are inferior to CVD, which formetals means
higher resistivity and forinsulatorsmoredefectsand traps. Thestepcoverage isalso notas
goodas CVD.
The choice of deposition method (i.e. evaporation vs. sputtering) may in many casesbe
arbitrary, andmay dependmore on whattechnology isavailable forthespecificmaterial at
thetime.
A. EvaporationInevaporation thesubstrate isplaced insidea vacuumchamber,
in whichablock (source) of thematerial to bedeposited isalso
located. Thesourcematerial is thenheated to thepoint where it
startsto boil andevaporate.
Figure no 9 (Schematic Diagram for e-beam evaporation.).
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The vacuum is requiredto allow themoleculesto evaporate freely inthechamber, andthey
subsequently condense on all surfaces. This principle is the same for all evaporation
technologies, only themethodusedto theheat (evaporate)thesourcematerial differs.
Therearetwo popularevaporationtechnologies, whicharee-beamevaporationand resistive
evaporationeach referringto theheatingmethod. Ine-beamevaporation, anelectronbeamis
aimedatthesourcematerial causing local heatingandevaporation. In resistiveevaporation, a
tungstenboat, containing the sourcematerial, is heated electrically with a high current to
make the material evaporate. Many materials are restrictive in terms of what evaporation
method canbe used (i.e. aluminum isquite difficult to evaporateusing resistive heating),
which typically relates to the phase transition properties of that material. A schematicdiagram ofatypical system fore-beamevaporationisshowninthe figure9 .
B Sputtering
Sputteringisatechnology in whichthematerial is released fromthesourceat
much lowertemperaturethanevaporation. Thesubstrateisplacedina vacuumchamber with
the sourcematerial, named a target, and an inertgas (such as argon) is introduced at low
pressure. Gasplasmaisstruckusingan RF powersource, causingthegasto becomeionized.
The ions are accelerated towards the surface of the target, causing atoms of the sourcematerial to break off fromthetargetin vapor formandcondense onall surfacesincludingthe
substrate. As forevaporation, thebasicprinciple ofsputtering is thesame forall sputtering
technologies. Thedifferencestypically relateto themanorin whichtheionbombardment of
thetargetis realized. A schematicdiagram ofatypical RF sputteringsystemisshowninthe
figure10.
Figure no 10 (Schematic Diagram of
Sputtering System.).
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II. CastingIn thisprocess the material to be deposited is dissolved in liquid form in a solvent. The
material can be applied to the substrate by spraying or spinning. Once the solvent is
evaporated, athin film ofthematerial remains onthesubstrate. Thisisparticularly useful for
polymermaterials, whichmay beeasily dissolvedin organicsolvents, and itisthecommon
methodused to apply photoresist to substrates (inphotolithography). The thicknesses that
canbecast
onasubstrate rangeall the way fromasinglemonolayer ofmolecules (adhesionpromotion)
to tens ofmicrometers. In recent years, thecastingtechnology hasalso beenappliedto form
films of glass materials on substrates. The spin castingprocess is illustrated in the figure
below.
When do I want to use casting?
Casting is a simple technology which can be used for a variety of materials (mostlypolymers). Thecontrol on film thicknessdepends onexactconditions, butcanbesustained
within +/-10% ina wide range. If youareplanningto usephotolithography you will beusing
casting, which is an integral part of that
technology. There are also other interesting
materials such aspolyimide and spin-on glass
whichcanbeappliedby casting.
Figure no 11 (Schematic Diagram of Spin
Casting System.).
a) PhotolithographyLithography in MEMS context is typically the transfer of a pattern to a photosensitive
material by selectiveexposureto a radiationsourcesuchas light. A photosensitivematerial is
amaterial thatexperiencesachange in itsphysical properties whenexposed to a radiation
source. Ifaphotosensitivematerial isselectively exposedto radiation (e.g. by maskingsome
of the radiation) thepattern of the radiation on the material is transferred to thematerial
exposed, as
theproperties oftheexposedandunexposed regionsdiffers. Thisexposed regioncanthenbe
removed ortreatedprovidingamask fortheunderlying
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substrate. Photolithography istypically used withmetal or otherthin filmdeposition, wetand
dry etching.
b) Lithography:Pattern Transfer
Lithography in the MEMScontext is typically the transfer ofapattern to aphotosensitive
material by selectiveexposureto a radiationsourcesuchas light. A photosensitivematerial is
amaterial thatexperiencesachange in itsphysical properties whenexposed to a radiation
source. If weselectively exposeaphotosensitivematerial to radiation (e.g. by maskingsome
of the radiation) thepattern of the radiation on the material is transferred to thematerial
exposed, as the
properties of the
exposedandunexposed
regions differs (asshownin figure)
Fig 12:Transfer of a
pattern to a
photosensitive
material
This discussion will focus on optical lithography, which is simply lithography using a
radiation source with wavelength(s) in the visible spectrum. In lithography for
micromachining, thephotosensitivematerial usedistypically aphotoresist (also called resist,
otherphotosensitivepolymersarealso used). When resistisexposedto a radiationsource of
aspecifica wavelength, thechemical resistance ofthe resistto developersolutionchanges. If
the resist isplaced inadevelopersolutionafterselectiveexposure to a lightsource, it will
etchaway one ofthetwo regions (exposed orunexposed). Iftheexposedmaterial isetched
away by thedeveloperandtheunexposed regionis resilient, thematerial isconsideredto be
apositive resist (shown in figure 13a). Iftheexposedmaterial is resilient to thedeveloper
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and theunexposed region isetchedaway, it isconsidered to beanegative resist (shown in
figure 13b).
Lithography is the principal mechanism for pattern definition in micromachining.
Photosensitive compounds areprimarily organic, and do not encompass the spectrum of
materialsproperties ofinterestto micro-machinists. However, asthetechnique iscapable of
producing fine features in an economic fashion, aphotosensitive layer is often used as a
temporary mask whenetchinganunderlying layer, so thatthepatternmay be transferredto
the underlying layer . Photoresist may also be used as a template forpatterning material
deposited after lithography . The resist is subsequently etched away, and the material
deposited on the resist is "lifted off". The deposition template (lift-off) approach for
transferringapattern from resistto another layeris lesscommonthanusingthe resistpattern
as an etch mask. The reason for this is that resist is incompatible with most MEMSdepositionprocesses, usually becauseitcannot withstandhightemperaturesandmay actasa
source ofcontamination
Figure 13: a) Patterndefinitioninpositive resist , b)Patterndefinitioninnegative resist.
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Once thepatternhasbeen transferredto another layer, the resist isusually stripped.
Thisis oftennecessary asthe resistmay beincompatible with furthermicromachiningsteps.
Italso makesthetopography moredramatic, whichmay hamper further lithography steps.
c) Etching ProcessIn orderto forma functional MEMSstructure onasubstrate, itisnecessary to etchthethin
filmspreviously deposited and/or the substrate itself. In general, there are two classes of
etchingprocesses:
1.Wetetching wherethematerial isdissolved whenimmersedinachemical solution
2. Dry etching where thematerial is sputtered ordissolved using reactive ions or a vapor
phaseetchant
In the following, we will briefly discuss the mostpopular technologies for wet and dry
etching.
a)
Wet etchingThis is thesimplestetching technology. All it requires isacontainer witha liquidsolution
that will dissolve the material in question. Unfortunately, there are complications since
usually amaskisdesiredto selectively etchthematerial. Onemust findamaskthat will not
dissolve or at least etchesmuch slower than thematerial to bepatterned. Secondly, some
single crystal materials, such as silicon, exhibit anisotropic etching in certain chemicals.
Anisotropic etching in contrast to isotropic etching means different etch rates in different
directions in thematerial. Theclassicexample of this is the crystal plane sidewalls
thatappear whenetchingahole ina silicon wafer inachemical suchaspotassium
hydroxide (KOH). The result is a pyramid shaped hole instead of a hole with rounded
sidewalls witha isotropicetchant. Theprinciple ofanisotropicand isotropic wetetching is
illustratedinthe
figurebelow.
Figure no 14 (Difference between Isotropic and
Anisotropic Etching).
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sidewalls. Theprimary technology isbased ontheso-called "Boschprocess", namedafterthe
Germancompany Robert Bosch which filedthe originalpatent, wheretwo differentgas
compositionsarealternatedinthe reactor. The firstgascompositioncreatesapolymer onthe
surface ofthesubstrate, andthesecondgascompositionetchesthesubstrate. Thepolymeris
immediately sputteredaway by thephysical part of theetching, but only on thehorizontal
surfacesandnotthesidewalls. Sincethepolymer only dissolves very slowly inthechemical
part oftheetching, itbuildsup onthesidewallsandprotectsthem frometching. Asa result,
etchingaspect ratios of 50 to 1 canbeachieved. Theprocesscaneasilybeusedto etch
completely throughasiliconsubstrate, andetch ratesare3-4timeshigherthan wetetching.
Sputteretchingisessentially RIE without reactiveions. Thesystemsusedare very similarin
principle to sputtering deposition systems. The big difference is that substrate is now
subjectedto theionbombardmentinstead ofthematerial targetusedinsputterdeposition.
Vapor phase etching is another dry etching method, which can be done with simplerequipment than what RIE requires. In thisprocess the wafer to beetched isplaced insidea
chamber, in which one ormoregasesareintroduced. Thematerial to beetchedisdissolvedat
the surface in a chemical reaction with the gasmolecules. The two most common vapor
phase etching technologies are silicon dioxide etching using hydrogen fluoride (HF) and
siliconetchingusing
xenondiflouride (XeF2), both of whichareisotropicinnature. Usually, caremustbetakenin
thedesign ofa vaporphaseprocessto nothavebi-products forminthechemical reactionthat
condense onthesurfaceandinterfere withtheetchingprocess.
When do I want to use dry etching?
The firstthing youshouldnoteaboutthistechnology isthatitisexpensiveto runcompared
to wet etching. If you are concerned with feature resolution in thin film structures or you
need vertical sidewalls fordeepetchingsinthesubstrate, youhaveto considerdry etching. If
youareconcernedabouttheprice of yourprocessanddevice, youmay wantto minimizethe
use of dry etching. The IC industry has long since adopted dry etching to achieve small
features, butinmany cases featuresizeisnotascritical in MEMS. Dry etchingisanenabling
technology, whichcomesatasometimeshighcost.
Integrated MEMS Technologies
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Since MEMSdevicesarecreated with the same toolsused to create integratedcircuits, in
some cases it is actually possible to fabricate Micromachines and Microelectronics on the
samepiece ofsilicon. Fabricatingmachinesand transistors sideby sideenablesmachines
thatcanhave intelligence. A number ofexcitingproductsarealready takingadvantage of
thiscapability.
7 . Applications of MEMS
Herearesomeexamples of MEMStechnology:
7.1. Pressure Sensors
MEMS pressure microsensors typically have a flexible diaphragm that deforms in the
presence of a pressure difference. The deformation is converted to an electrical signal
appearing at the sensor output. A pressure sensor canbe used to sense the absolute air
pressure within the intake manifold of an automobile engine, so that the amount of fuel
required for each engine cylinder can be computed. In this example, piezoresistors are
patternedacrosstheedges ofa region whereasilicondiaphragm will bemicromachined. The
substrateisetchedto createthediaphragm. Thesensordieisthenbondedto aglasssubstrate,
creatingasealed
Figure no 15
(Picture
showing a
photo
resistive
Pressure
Sensor .).
vacuum
cavity under
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the diaphragm. The die is mounted on apackage, where the topside of the diaphragm is
exposed to the environment. The change in ambient pressure forces the downward
deformation of thediaphragm, resulting inachange of resistance of thepiezoresistors. On-
chipelectronicsmeasurethe resistancechange, whichcausesacorresponding voltagesignal
to appearatthe outputpin ofthesensorpackage .
7.2. Accelerometers
Accelerometersareaccelerationsensors. Aninertial masssuspendedby springsisactedupon
by acceleration forces that cause the mass to be deflected from its initial position. This
deflection is converted to an electrical signal, which appears at the sensor output. Theapplication of MEMStechnology to accelerometersisa relatively new development.
Figure no 16
(MEMS Application in Automobile, showing various MEMS
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devices.).
One such accelerometer design isdiscussedby DeVoe and Pisano (2001) . It is a surface
micromachined piezoelectric accelerometer employing a zinc oxide (ZnO) active
piezoelectric film. Thedesign isasimplecantileverstructure, in which thecantileverbeam
servessimultaneously asproofmassandsensingelement. One ofthe fabricationapproaches
developed isasacrificial oxideprocessbased onpolysiliconsurfacemicromachining, with
theaddition ofapiezoelectric layeratopthepolysilicon film. Inthesacrificial oxideprocess,
apassivation layer of silicon dioxide and low-stress silicon nitride is deposited on abare
silicon wafer, followedby 0.5 micron of liquidphase chemical vapor deposited (LPCVD)
phosphorous-dopedpolysilicon. Then, a 2.0-micron layer ofphosphosilicateglass (PSG) is
depositedby LPCVD andpatternedto define regions wheretheaccelerometerstructure will
beanchoredto thesubstrate. The PSG filmactsasasacrificial layerthatisselectively etchedattheendto freethemechanical structures. A second layer of 2.0-micron-thickphosphorus-
dopedpolysilicon is deposited via LPCVD on top of the PSG, andpatternedby plasma
etching to define themechanical accelerometer structure. This layeralso actsas the lower
electrode forthesensing film. A thin layer ofsiliconnitride isnextdepositedby LPCVD,
andactsasastress-compensation layer forbalancingthehighly compressive residual stresses
in the ZnO film. By varying the thickness of theSi3N4 layer, the accelerometer structure
may betunedto control bendingeffects resulting fromthestressgradientthroughthedevice
thickness. A ZnO layer isthendeposited onthe order of 0.5 micron, followedby sputtering
ofa 0.2-micron layer ofplatinum (Pt)depositedto formtheupperelectrode. A rapidthermal
anneal isperformedto reduce residual stressesinthesensing film. Afterwards, the Pt, Si3N4,
and ZnO layers arepatterned in a single ion milling etch step, and the devices are then
releasedby passivating the ZnO film withphotoresist, and immersing the wafer inbuffered
hydrofluoricacid, which removesthesacrificial PSG layer .
7.3. Inertial Sensors
Inertial sensorsareatype ofaccelerometerandare one oftheprincipal commercial products
that utilize surface micromachining. They are used as airbag-deployment sensors in
automobiles, andastilt orshocksensors. Theapplication oftheseaccelerometersto inertial
measurementunits (IMUs) is limitedby theneed to manually alignandassemblethem into
three-axissystems, andby the resultingalignmenttolerances, their lack ofin-chipanalog-to-
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Figure no 17 (Figure Showing BIO MEMS Devices.).
A recently developed MicroStar cross-connect fabric developedby Bell Labs , a micro-
optoelectromechanical systemdevice, isbased on MEMS technology. Themostpervasive
bottlenecks for communications carriers are the switching and cross-connect fabrics that
switch, route, multiplex, demultiplex, and restore traffic in optical networks. The optical
transmissionsystemsmove informationasphotons, butswitchingandcross-connect fabrics
until now havebeen largely electronic, requiring costly and time-consumingbandwidth-
limiting optical-to-electronic-to-optical conversions at every network connection and cross
point. MicroStar iscomposed of 256 mirrors, each one 0.5 mm indiameter, spaced 1 mm
apart, andcovering lessthan 1 squareinch ofsilicon. Themirrorssit withinthe routerso that
only one wavelength can illuminate any onemirror. Eachmirrorcan tilt independently to
passits wavelengthto any of 256 inputand output fibers. Themirrorarraysaremadeusinga
self-assembly processthatcausesa framearoundeachmirrorto lift fromthesiliconsurface
and lock in place, positioning the mirrors high enough to allow a range of movement.
MicroStar ispart of Lucent Technology's Lambda Router cross-connect system aimed at
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helping carriers deliver vast amounts of data unimpeded by conventional bottlenecks.
Figure no 18 (Figure Showing Pressure Sensor Belt on Jet Planes.).
As a final example, MEMS technology has been used in fabricating vaporization
microchambers for vaporizing liquidmicrothrusters fornanosatellites. Thechamberispart of
amicrochannel withaheight of 2-10 microns, madeusingsiliconandglasssubstrates. The
nozzle is fabricated in the silicon substrate just above a thin-film indium tin oxide heater
deposited onglass.
Amongthepresently availableuses of MEMS orthoseunderstudy are:
Global positionsystemsensorsthatcanbeincluded withcourierparcels forconstanttrackingandthatcanalso senseparcel treatmenten route
Sensorsbuilt into the fabric ofanairplane wingso that itcansenseand react to airflow by changing the wing surface resistance; effectively creatingamyriad of tiny
wing flaps
Optical switching devices that can switch light signals over differentpaths at 20-nanosecondswitchingspeeds
Sensor-drivenheatingandcoolingsystemsthatdramatically improveenergy savings
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Buildingsupports with imbeddedsensorsthatcanalterthe flexibility properties ofamaterial based onatmosphericstresssensing
8. CONCLUSION
Each of the three basic microsystems technology processes we have seen, bulk
micromachining, sacrificial surfacemicromachining, andmicromoldingemploysadifferent
set of capital and intellectual resources. MEMS manufacturing firms must choose which
specificmicrosystemsmanufacturingtechniquesto investin .
MEMStechnology hasthepotential to change ourdaily livesasmuchasthecomputerhas.
However, the material needs of the MEMS field are at apreliminary stage. A thorough
understanding of the properties of existing MEMS materials is just as important as the
development ofnew MEMSmaterials.Future MEMSapplications will bedrivenbyprocessesenablinggreater functionality through
higher levels of electronic-mechanical integration and greater numbers of mechanical
components workingalone ortogether to enableacomplexaction. Future MEMSproducts
will demandhigher levels ofelectrical-mechanical integrationandmore intimateinteraction
with the physical world. The high up-front investment costs for large-volume
commercialization of MEMS will likely limit theinitial involvementto largercompanies in
the IC industry. Advancing fromtheirsuccessassensors, MEMSproducts will beembedded
in larger non-MEMS systems, such as printers, automobiles, and biomedical diagnostic
equipment, and will enablenew andimprovedsystems .
BIOBLIOGRAPHY:
1. COURSE MATERIAL FROM SUMAN MASHRUWALA ADVANCEDNICROENGINEERING LAB. IIT BOMBAY.
2. SEARCH ENGINES www.google.com3. ONLINE ENCLYOPEDIA www.wikipedia.com.4. MICROMACHINE DEVICES.5. M. Mehregany andS. Roy, Introductionto MEMS, 2000, Microengineering
AerospaceSystems, El Segundo, CA, Aerospace Press, AIAA, Inc., 1999.
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6. M. Mehregany, K. J. Gabriel, and W. S. N. Trimmer, Integrated fabrication ofpolysiliconmechanisms, IEEE Transactions on Electron Devices ED-35, 719-723
(June 1988).