Epitaxial growth

B.Sc., electronics

1. IC’S AND INSTRUMENTATION
Prof K.Manikantan
Government Arts College
Udhagamandalam

2. Epitaxial Growth
•ThewordepitaxyisderivedfromGreekwordepimeaning'upon'andthepasttenseofthe
wordteinonmeaning'arranged'.So,onecoulddescribeepitaxyas,arrangingatomsinsingle
crystalfashionuponasinglecrystalsubstrate,sothattheresultinglayerisanextensionofthe
substratecrystalstructure.Thebasicchemicalreactionusedfortheepitaxialgrowthofpure
siliconisthehydrogenreductionofsilicontetrachloride.

3. Epitaxialfilmswithspecificimpurityconcentrationarerequired.Thisis
accomplishedbyintroducingphosphine<PH3>forthen-typeandbi.borane
B2H6forp.typedopingintothesilicon.
Tetrachloridehydrogengasstream.Theprocessiscarriedoutinareaction
chamberconsistingofalongcylindricalquartotubeencircledbyanRF
inductioncoil.Figureshowsthediagrammaticrepresentationofthesystem
used.Thesiliconwafersareplacedonarectangulargraphiterodcalledaboat.
Thisboatisthenplacedinthereactionchamberwherethegraphiteisheated
inductivelytoatemperature1200'C.Thevariousgasesrequiredforthegrowth
ofdesiredepitaxiallayersareintroducedintothesystemthroughacontrol
console.

5. Oxidation
SiO2hasthepropertyofpreventingthediffusionofalmostallimpuritiesthroughit.It
servestwoveryimportantpurposes.
1.SiO2isanextremelyhardprotectivecoatingandisunaffectedbyalmostallreagents
excepthydrofluoricacid.Thusitstandsagainstanycontamination.
2.ByselectiveetchingofSiO2,diffusionofimpuritiesthroughcarefullydefinedwindows
intheSiO2canbeaccomplishedtofabricatevariouscomponents.
Thesiliconwafersarestackedupinaquartzboatandtheninsertedintoquartzfurnace
tube.TheSi-wafersareraisedtoahightemperatureintherangeof950to1150Candatthe
sametime,exposedtoagascontainingO.orH.Oorboth.
ThechemicalreactionisSi+2H20SiO2+2H2Thisoxidationprocessiscalledthermal
oxidationbecausehightemperatureisusedtogrowtheoxidelayer.
Thethicknessofthefilmisgovernedbytime,temperatureandthemoisturecontent.The
thicknessofoxidelayerisusuallyintheorderof0.02to2μm.

6. Photolithography
Withthehelpofphotolithography,ithasbecomepossibletoproducemicroscopicallysmall
circuitanddevicepatternsonSi-wafers.
Asmanyas10,000transistorscanbefabricatedona1cmx1cmchip.Theconventional
photolithographicprocessusesultravioletlightexposureanddevicedimensionorlinewidth
assmallas2I1IIlcanbeobtained.
However,withtheadventoflatesttechnologyusingX.rayorelectronbeamlithographic
techniques,ithasbecomepossibletoproducede.vicedimension7
downtosubmicronrange(<1μm).Photolithographyinvolvestwoprocesses,namely:Making
ofaphotographicmask.
Photoetching
Themakingofaphotographicmaskinvolvesthefollowingsequenceofoperations-firstthe
preparationofinitialartworkandsecondly,itsreduction.
TheinitiallayoutorartworkofanICisnormallydoneatascaleseveralhundredtimeslarger
thanthefinaldimensionsofthefinishedmonolithiccircuit.Thisisbecause,foratinychip,
largertheartwork,moreaccurateisthefinalmask.Forexample,itisoftenrequiredtomake
anopeningofwidthabout1mil(2511m).

8. Thisinitiallayoutisthendecomposedintoseveralmasklayers,eachcorrespondingtoa
processstepinthefabricationschedule,e.g.,amaskforbasediffusion,anotherforcollector
diffusion,anotherformetallizationandsoon.
Forphotographicpurpose,artworkshouldnotcontainanylinedrawingsbutmustbeof
alternateclearandopaqueregions.ThisisaccomplishedbytheuseofclearMylarcoatedwith
asheetofredphotographicallyopaquemylar(tradename-Rubylith).
PhotoetchingisusedfortheremovalofSi02fromdesiredregionssothatthedesired
impuritiescanbediffused.Thewaferiscoatedwith1\filmofphotosensitiveemulsion
(KodakPhotoresistKPR).
Thethicknessofthefilmisintherangeof5000-10000AasshowninFig(a).Themask
negativeofthedesiredpattern)aspreparedbystepsdescribedearlierisplacedoverthe
photoresistcoatedwaferasshowninFig(b).Thisisnowexposedtoultravioletlight.sothat
KPRbecomespolymerizedbeneaththetransparentregionsofthemask.

9. Themaskisthenremovedandthewaferisdevelopedusing1chemical
(trichloroethylene)whichdissolvestheunexposedunpolymerizedregionsonthe
photoresistandleavesthepatternasshowninFig(c).
Thepolymerizedphotoresistisnextfixedor,,"red.sothatitbecomesimmuneto
certainchemicalscalledetchantsusedinsubsequentprocessingsteps.Thechipis
immersedintheetchingsolutionofhydrofluoricacid,whichremovestheSi02from
theareaswhicharenotprotectedbyKPRasshowninFig(d)afterdiffusionof
impuritiesthephotoresistisremovedwithachemicalsolvent(hotH2SO4)and
mechanicalabrasion.

10. Diffusion
AnotherimportantprocessinthefabricationofmonolithicICsisthediffusionofimpuritiesin
theSiliconchip.Thisusesahightemperaturefurnacehavingaflattemperatureprofileovera
usefullength(about20"length).
Aquartzboatcontainingabout20cleanedwafersispushedintothehotzonewithtemperature
maintainedatabouta1000°C.Iimpuritiestobediffusedarerarelyusedintheirelemental
forms.Normally,compoundssuchas~03(Boronoxide),BCl3(Boronchloride)areusedfor
BoronandP20S(Phosphorouspentaoxide)andPOCl3(Phosphorousoxychloride)areusedas
sourcesofPhosphorous.
Acarriergoes,suchasdryoxygenornitrogenisnormallyusedforsweepingtheimpurityto
thehightemperaturezone.Thedepthofdiffusiondependsuponthetimeofdiffusionwhich
normallyextendsto2hours.
Thediffusionofimpuritiesnormallytakesplacebothlaterallyaswellusvertically.Therefore,
theactualjunctionprofileswillbecurved110showninFig.However,forthesakeof
simplicity,lateraldiffusionwillbeomittedinallthedrawings.

12. IonImplantation
lonimplantationistheothertechniqueusedtointroduceimpuritiesinasiliconwafer.Inthis
process,siliconwafersareplacedinavacuumchamberandarescannedbyabeamofhigh-
energydopantin"(boronsforp-typeandphosphorusforn-type)asshowninFig..
Theseionsareacceleratedbyenergiesbetween20kVto250kV.Astheionsstrikethesilicon
wafers,theypenetratesomesmalldistanceintothewafer.
ThedepthofpenetrationofanyparticulartypeofIonincreaseswithincreasingaccelerating
voltage.Ionimplantationtechniquehastwoimportantadvantages.
1.Itisperformedatlowtemperatures.Therefore,previouslydiffusedregionshavealesser
tendencyforlateralspreading.
2.Indiffusionprocess,temperaturehastobecontrolledoveralargeareainsidetheoven,
whereasinionimplantationtechnique,acceleratingpotentialandthebeamcurrentare
electricallycontrolledfromoutside.

14. IsolationTechniques
SinceanumberofcomponentsarefabricatedonthesameICchip,itbecomesnecessaryto
provideelectricalisolationbetweendifferentcomponentsandinterconnections.Varioustypes
ofisolationtechniqueshavebeendeveloped.However,weshaJldiscusshereonlytwo
commonlyusedtechniquesnamely:
PNjunctionisolation.
Dielectricisolation
P-NJunctionIsolation
Inthisisolationtechnique,p+typeimpuritiesareselectivelydiffusedIntothen.typeepitaxial
layersoastoreachp-typesubstrateasShowninFig(a).
Thisproducesislandssurroundedbyp.typemoats.Itcanbeseenthattheseregionsare
separatedbytwobackto.backp.njunctiondiodes.Ifthep-typesubstratematerialisheldat
themostnegativepotentialinthecircuit,thediodeswillbereversebiasedprovidingelectric
isolationbetweentheseislands.
Thedifferentcomponentsarefabricatedintheseisolationislands.Theconcentrationofthe
acceptoratomsintheregionbetweenisolationislandsisusuallykeptmuchhigher(p+)than
thep-typesubstrate.Thispreventsthedepletionregionofthereversebiaseddiodefrom
penetratingmoreintop+regionandpossiblyconnectingtheisolationislands.

16. DielectricIsolation
Herealayerofsoliddielectricsuchassilicondioxideorrubycompletelysurroundseach
component,therebyproducingisolation,bothelectricalandphysical.
Thisisolatingdielectriclayeristhickenoughsothatitsassociatedcapacitanceisnegligible.
Also,itispossibletofabricatebothpnpandnpntransistorwithinthesamesiliconsubstrate.
Sincethismethodrequiresadditionalfabricationsteps,itbecomesmoreexpensive.The
techniqueismostlyusedforfabricatingprofessionalgradeICsrequiredforspecialized
applicationsviz,aerospaceandmilitary,wherehighercostisjustifiedbysuperior
performance.Figure(b)showssuchastructure.

17. Metallization
Thepurposeofthisprocessistoproduceathinmetalfilmlayerthatwillservetomake
interconnectionsofthevariouscomponentsonthechip.Aluminiumisusuallyusedforthe
metallizationofmostICsasitoffersseveraladvantages.
1.Itisrelativelyagoodconductor.
2.Itiseasytodepositaluminiumfilmsusingvacuumdeposition.
3.Aluminiummakesgoodmechanicalbondswithsilicon.
4.Aluminiumformslowresistance,non-rectifying(i.e.ohmic)contactwithp-typesiliconand
theheavilydopedn-typesilicon
Thefilmthicknessofabout1μmandconductionwidthofabout2to25μmiscommonlyused.
TheprocesstakesplaceinavacuumevaporationchamberasshowninFig..
Thepressureinthechamberisreducedtotherangeofabout10-6to10-7torr(l-atmosphere.
760torr=760mmHg).Thematerialtobeevaporatedisplacedinaresistanceheatedtungsten
coilorbasket.Averyhighpowerdensityelectronbeamisfocusedatthesurfaceofthematerial
tobeevaporated.

18. ThisheatsupthematerialtoveryhightemperatureandItstartsvaporizing.Thesevaporstravel
instraightlinepaths.Theevaporatedmoleculeshitthesubstrateandcondensetheretoforma
thinfilmcoating.
Afterthethinfilmmetallizationisdone,thefilmispatternedtoproducetherequired
interconnectionsandbondingpadconfiguration.Thisisdonebyphotolithographicprocessand
aluminiumisetchedawayfromunwantedplacesbyusingetchantslikephosphoricacid
(H3PO4).