Arun Report for Biogas

8/8/2019 Arun Report for Biogas

1/37

1

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)


2/37


3/37

3

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.


4/37

4

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

5

6


5/37

5

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.


6/37

6

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


7/37

7

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.


8/37

8

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


9/37

9

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.


10/37

10

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).


11/37

11

.

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


12/37

12

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.


13/37

13

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


14/37

14

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)


15/37

15

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.


16/37

16

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.


17/37

17

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.


18/37

18

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.).


19/37

19

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


20/37

20

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.


21/37

21

. 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


22/37

22

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.).


23/37

23

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.).


24/37

24

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


25/37

25

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


26/37

26

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.


27/37

27

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).


28/37


29/37

29

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


30/37

30

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


31/37

31

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


32/37

32

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-


33/37


34/37

34

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


35/37

35

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


36/37

36

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.


37/37

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).

Documents

Arun Report for Biogas