Etching, Diffusion, and Ion Implantation in Semiconductor Fabrication

This lecture by Dr. G. S. Virdi, Ex-Chief Scientist, CSIR-Central Electronics Engineering Research Institute, Pilani, provides a comprehensive overview of three key processes in semiconductor device fabrication—etching, diffusion, and ion implantation.

The lecture begins with an in-depth discussion of etching techniques, comparing wet and dry (plasma) methods, their mechanisms, selectivity, anisotropy, and industrial applications. It further elaborates on reactive ion etching (RIE) as a critical step in achieving precise pattern transfer in micro- and nano-fabrication.

Subsequently, the lecture explores impurity doping techniques, explaining how diffusion and ion implantation alter the electrical properties of semiconductors to form p–n junctions, MOSFET structures, and other device regions. The fundamental principles, process steps, dopant sources, and advantages of each technique are clearly presented.

This lecture serves as a valuable resource for engineering and science students, as well as researchers working in microelectronics, semiconductor processing, and IC fabrication technologies.

1. Etching-Diffusion & Ion Implantation
Dr.G.S.Virdi
Ex.Chief Scientist
CSIR-Central Electronics Engineering Research Institute
Pilani—33303 1,India

2. Etchingistheprocesswhereunwantedareasof
filmsareremovedbyeitherdissolvingthemina
wetchemicalsolution(WetEtching)orbyreacting
themwithgasesinaplasmatoformvolatile
products(DryEtching).
Resistprotectsareaswhicharetoremain.In
somecasesahardmask,usuallypatternedlayers
ofSiO
2orSi
3N
4,areusedwhentheetchselectivity
tophoto-resistislowortheetchingenvironment
causesresisttodelaminate.
Thisispartoflithography-patterntransfer.
Etching
Dr.G.S.VIRDI

3. Toremovematerialfromareasidentifiedbythelithographyprocess
Tocreatestructuresforfunctionaluse
Toremoveoxidelayersbelowfeaturestoallowformotion
NecessityofEtching
Dr.G.S.VIRDI

4. Etchingisdoneeitherin“dry”or“wet”methods:
•Wetetchingusesliquidetchantswithwafersimmersedinetchantsolution
•Wetetchischeapandsimple,buthardtocontrol(notreproducible),notpopularfor
Nanoforpatterntransferpurpose
•Dryetchingusesgasphaseetchantsinaplasma,bothchemicalandphysical
(sputteringprocess)
•Dryplasmaetchcanbeusedformanydielectricmaterialsandsomemetals(Al,Ti,
Cr,Ta,W…).
•Forothermetals,ionmilling(Ar
+
)canbeused,butwithlowetchingselectivity.(asa
result,formetalsthatcannotbedry-etched,itisbettertopatternthemusingliftoff)
Generally,chemicaletchinghashighselectivity,physicaletching
(sputtering,milling)haslowselectivity.
EtchingMethods
Dr.G.S.VIRDI

5. Etchingisconsistedof3processes:
•Masstransportofreactants(throughaboundarylayer)tothesurfacetobeetched
•Reactionbetweenreactantsandthefilmtobeetchedatthesurface
•Masstransportofreactionproductsfromthesurfacethroughthesurfaceboundary
layer
EtchingBasic
Dr.G.S.VIRDI

6. FiguresofMerit
Selectivity
Blue:layertoremain
1. Apoorlyselectiveetchremovesthetoplayer,
butalsoattackstheunderlyingmaterial.
2. Ahighlyselectiveetchleavestheunderlying
materialunharmed
Isotropy
Red:maskinglayer;
yellow:layertoberemoved
1.Aperfectlyisotropicetchproducesround
sidewalls.
2.Aperfectlyanisotropicetchproducesvertical
sidewalls

7. FiguresofMerit
Dr.G.S.VIRDI

8. Isotropicvs.Anisotropic
Generallyspeaking,chemicalprocess(wetetch,plasmaetch)leadstoisotropicetch;whereas
physicalprocess(directionalenergeticbombardment)leadstoanisotropicetch.
Isotropic:
•Besttousewithlargefeatureswhensidewallslopedoesnotmatter,andtoundercutthe
mask(foreasyliftoff).
•Largecriticaldimension(CD,i.e.featuresize)loss,generallynotfornano-fabrication.
•Quick,easy,andcheap.
Anisotropic:
•Bestformakingsmallfeatureswithverticalsidewalls,preferredpatterntransfermethod
fornano-fabricationandsomemicro-fabrication.
•Typicallymorecostly.
Dr.G.S.VIRDI

9. WetChemicalEtching:
Regionsinthewaferare“dissolved”awaybychemicalreactions.
Techniquecannotproducesharp“sidewalls,”sinceetchingisisotropic.
Wetchemicaletchingisusedforproductswithfeaturesizesgreaterthan2µm
WetChemicalEtching/WetEtching
Etchingrate:
•Theetchratecanbecontrolledbyanyofthethreeserialprocesses(reactantstransporttothesurface,
reaction,reactionproductstransportfromthesurface).
•Preferenceistohavereactionratecontrolledprocessbecause
oEtchratecanbeincreasedbytemperature
oGoodcontroloverreactionrate–temperatureofaliquidiseasytocontrol
•Masstransportcontrolwillresultinnon-uniformetchrate:edgeetchesfaster.
Dr.G.S.VIRDI

10. WetChemicalEtching/WetEtching
Advantages:
Damage-freefinishtowafersurfacewheresurfacemorphologyistypicallysmooth
andshiny
Fastetchrateespeciallyforblanketetch(μm/min)
Etchingisonlychemical:greatselectivity
Simpleanddirectetchingprocesssincesimpleresistcanbeusedasetchmask
processoccuratatmosphericenvironment
Cheapercost
Highetchselectivityeasilyavailableforetchants,resistandetchedmaterials
goodetchuniformityacrosswafer
Disadvantages:
Isotropicetching
Nocontrolforprecisionetching
Notwellsuitedfornanostructures.
Poorprocesscontrol,
Notwellreproducible.
Dr.G.S.VIRDI

11. ApplicationofWetProcess
Siliconetching:
Forsemiconductormaterials,wetchemicaletchingusuallyproceedbyoxidation
followedbythedissolutionoftheoxidebyachemicalreaction.
Forsilicon,themostcommonlyusedetchantsaremixtureofnitricacid(HNO
3)and
hydrofluoricacid(HF)inwateroraceticacid(CH
3COOH)
Si+4HNO
3→SiO
2+2H
2O+4NO
2
HydrofluoricacidisusedtodissolvetheSiO
2layer
SiO
2+6HF→H
2SiF
6+2H
2O
Silicondioxideetching:
DilutesolutionofHFwithorwithouttheadditionofammoniumfluoride(NH
4F)isused
forwetetching.
Dr.G.S.VIRDI

12. SiliconnitrideandPoly-siliconEtching:
SiliconnitridefilmsareetchableatroomtemperatureinconcentratedHForbuffered
HFandinaboilingH
3PO
4solution.
Selectiveetchingofnitridetooxideisdonewith85%H
3PO
4at180
o
Cbecausethis
solutionattackssilicondioxideveryslowly.Siliconrateforsiliconnitrideis10nm/min
butlessthan1nm/minforsilicondioxide
GalliumArsenideEtching:
ThemostcommonlyusedetchantsaretheH
2SO
4-H
2O
2-H
2OandH
3PO
4-H
2O
2-H
2O.
Foranetchantwithan8:1:1volumerationofH
2SO
4:H
2O
2-H
2O,theetchrateis0.8
µm/minfor<111>Gafaceand1.5µm/minforallotherfaces.
ApplicationofWetProcess
Dr.G.S.VIRDI

13. IndryEtching,materialremovalreactionsoccurinthegasphase.
Itcanbeplasmaornon-plasmabased.
Advantages
Eliminateshandlingofdangerousacidsandsolvents
Usessmallamountsofchemicals
Isotropicoranisotropicetchprofiles
Faithfulpatterntransferintounderlyinglayers(littlefeaturesizeloss)
DirectionaletchingwithoutusingthecrystalorientationofSi
Highresolutionandcleanliness
Lessundercutting
Nounintentionalprolongationofetching
Betterprocesscontrol
Easeofautomation
Disadvantages:
Somegasesarequitetoxicandcorrosive
Re-depositionofnovolatilecompounds
Needforspecialisedexpensiveequipment
DryEtching
Dr.G.S.VIRDI

14. TypesofDryEtching
•Non-plasmabased-usesspontaneousreactionofappropriatereactive
gasmixture.
•Plasmabased-usesradiofrequency(RF)powertodrivechemical
reaction.
Dr.G.S.VIRDI

15. 15
15
Non-Plasma based Dry Etching
•XeF
2isawhitepowder,withvaporpressure
3.8Torrat25
o
C.
•Typicaletchrate1μm/min
•Heatisgeneratedduringexothermicreaction
2XeF
2+Si2Xe(g)+SiF
4(g)
PopularforMEMSapplication.
IsotropicetchingofSi
TypicallyF-containinggases(fluoridesorinterhalogens)thatreadilyetchSi
Highselectivitytomaskinglayers
Noneedforplasmaprocessingequipment
Highlycontrollableviatemperatureandpartialpressureofreactants
Xenondi-fluoride(XeF
2)etchingofSi:
MEMS:microelectromechanicalsystems
Dr.G.S.VIRDI

16. WhatisaPlasma?
Aplasmaisapartiallyionizedgasmadeupofequalpartspositivelyandnegatively
chargedparticles.
Plasmasaregeneratedbyflowinggasesthroughanelectricormagneticfield.
Plasmaconsistsof:ionized
atoms/molecules+freeelectrons,
freeradicals(neutral).
Variousreactionsandspecies
presentinaplasma
Dr.G.S.VIRDI

17. PlasmaEtching
•Twocomponentsexistedinplasma
oIonicspeciesresultsindirectionaletching.
oChemicalreactivespeciesresultsinhighetchselectivity.
•Controloftheratioofionic/reactivecomponentsinplasmacanmodulatethedry
etchingrateandetchingprofile.
Dr.G.S.VIRDI

18. Chemical
Process Physica l Process
Wet
etching
Plasma
etching
Reactive
Ion
etching
High
density
plasma
etching
Ion
milling
&
Sputter
etching
Selectivity
Pressure
Energy(power)
Anisotropicity
PlasmaEtchingTypes
•Chemicaletching:freeradicalsreactwithmaterialtoberemoved.E.g.plasmaetchingathigh
pressurecloseto1Torr.
•Physicaletchingorsputtering:ionicspecies,acceleratedbythebuilt-inelectricfield(self-bias),
bombardthematerialstoberemoved.E.g.sputtercleaningusingArgasinsputterdeposition
system.
•Ionenhancedetching:combinedchemicalandphysicalprocess,highermaterialremovalrate
thaneachprocessalone.E.g.reactiveionetching(RIE),whichisthemostwidelyuseddry
etchingtechnique.

19. PlasmaEtching(Chemical)
•Inaplasma,reactiveneutralchemical
species(freeradicals,e.g.Fatomsor
molecularspeciesCF
3)aremainly
responsibleforthechemicalreactiondue
totheirmuchgreaternumberscompared
toions.
•Thosefreeradicalsaremoreabundant
thanionsbecause:
1)theyaregeneratedatlowerthreshold
energy(e.g.<8eV;incomparison,Aris
ionizedat15.7eV);and
2)they(unchargedradicals)havelonger
lifetimeintheplasma.
•Theneutralradicalsarriveatcathode
surfacebydiffusion(thusnon-
directional).
Duetotheirincompletebonding,
freeradicalsarehighlyreactive
chemicalspecies.
Chemicaletching
Dr.G.S.VIRDI

20. •Duetotheirincompletebonding(incompleteoutershells),freeradicals(neutral,
e.g.CF
3andFfromCF
4plasma)arehighlyreactivechemicalspecies.
•Freeradicalsreactwithfilmtobeetchedandformvolatileby-products.
Purechemicaletchisisotropicornearly
isotropic,andtheetchingprofiledepends
onarrivalangleandstickingcoefficients
offreeradicals.
PlasmaEtching(Chemical)
Dr.G.S.VIRDI

21. Advantages:
Lowerchemicalcosts
Reducedenvironmentalimpact
Greatercleanliness
Greaterpotentialforproduction-lineautomation.
Disadvantages:
Plasmaetchhaslowerselectivitythanwetetching
Purechemicaletchisisotropicornearlyisotropic
HighRFlevelscancausedamagetothewafer
PlasmaEtching(Chemical)
Dr.G.S.VIRDI

22. PlasmaEtching(Physical)
Physicallybombardthefilmstobeetchedwithenergizedchemically
inertionsoratoms
Materialisremovedbyionbombardmentofthesubstrate.This
processismostoftenusedtopre-cleansubstratespriortodeposition.
Gasdischargeisusedtoenergizechemicallyinertionsoratoms(e.g.
Ar)
Highlyanisotropicetching
Damagetounderlyingmaterial—maychangedeviceproperties
RarelyusedinVLSI
Dr.G.S.VIRDI

23. SputteringEtching/Ion Milling
•Physicalmillingwhenusingheavyinert
gases(Ar).
•Plasmaisusedtogenerateionbeam(Ar
+
),
whichisextractedandacceleratedtoetch
thesample.(i.e.sampleoutsideofplasma)
•Thustheiondensity(determinedbyplasma
source)andionenergy(determinedbyDC
accelerationvoltage–biasbyappliedDC
voltage,notbyRFbiasasinhighdensity
plasmaetchingsystem),canbecontrolled
independently.
•Highaccelerationvoltage(>1kV),leadingto
millrate10-30nm/min.
•UsedwheneverRIEisnotpossible(dueto
thelackofvolatilespeciesformation).
UsuallyemployedtoetchCu,Ni,Au,
superconductingmaterialscontaining
metals…
•Lowpressure10
-4
Torr(>1orderlowerthan
RIE),solargemeanfreepathandlessenergy
lossduetocollision.(suchlowpressure
cannotsustainaplasma,soionmillingisnot
plasmaetching)
Dr.G.S.VIRDI

24. Figure10-8Problemsassociatedwithsputteretching(oranyetchingthathasahigh
degreeofphysical/ionicetching):a)trenchingatbottomofsidewalls;b)redepositionof
photoresistandothermaterials;c)chargingandionpathdistortion.
24
•Poorselectivity(2:1or1:1),veryanisotropic.
•Sputteringratedependsonsputteryieldswhichcanbeafunctionofincidentangle.
•Problemsincludefaceting(sputteryieldisafunctionofincidentangle),trenching,re-
deposition,chargingandionpathdistortion,radiationdamage.
•Notpopular,etchestooslow,thoughreactivegas(CF
4,CCl
4,O
2)canbeaddedtoslightly
improveselectivityandetchingrate.
SputteringEtching/Ion Milling
Dr.G.S.VIRDI

25. ReactiveIonEtching(RIE)------Combinationofchemicalandphysicaletching
Directionaletchingduetoionassistance.
PlasmaEtching(Chemical+Physical)

26. InRIEprocesses,thewaferssitonthe
poweredelectrodesubstratesinalow
pressurehalogen-richenvironment.
Thisplacementsetsupanegative
biasonthewaferwhichaccelerates
positivelychargeions(chemicallyinert
ions)towardthesurface.
Moreover,glowdischargeisusedto
producechemicallyreactivespecies
(atoms,radicals,orions)
Therefore,thematerialcanbe
removedbybothchemicalmeansand
ionbombardmentofthesubstrate
surface.
ReactiveIonEtcing(RIE)

27. •RIEisananisotropic(duetodirectionalionbombardment)andhighlyselective(dueto
chemicalreaction)etchingprocess.
AnisotropicProfile
HigherEtchRatethaneither
process
Higherselectivityratiothan
physicaletch
Smallerfeaturesizespossible
ToGreatercontrol
widthsandedge
overline
profilesis
possiblewithoxides,nitrides,
poly-siliconandaluminum.
WidelyusedinVLSIfabrication
ReactiveIonEtcing(RIE)

28. IonEnergyvs.PressureforaPlasma

29. Chemical—PhysicChem.+Phys.
Purelychemicaletching
(usingonlyreactive
neutralspecies)
Isotropicetching
Chemical+physicaletching
(usingreactiveneutralspecies
andionicspecies)
Anisotropicetching
Physicaletching
(usingionicspecies)

30. Impuritydopingistheintroductionofcontrolledamountofimpurity
dopantintosemiconductors.
Themaingoalofdopingischangingtheelectricalpropertiesof
semiconductor.
ImportanceofDoping
Formationofp-njunctionandfabricationofdevicesduringwafer
fabrication.
Alterthetypeandlevelofconductivityofsemiconductormaterials.
Formbases,emitters,andresistorsinbipolardevices,aswellas
drainsandsourcesinMOSdevices.
Dopepoly-siliconlayers.
ImpurityDoping

31. Diffusionandionimplantationarethetwokeymethodsofimpurity
doping
Diffusion:DopantatomsmovefromthesurfaceintoSibythermal
meansviasubstitutionalorinterstitialdiffusionmechanisms.
Ionimplantation:DopantatomsareforcefullyaddedintoSiintheform
ofenergeticionbeaminjection.
Comparisonof(a)diffusionand(b)ionimplantationtechniquesfortheselective
introductionofdopantsintothesemiconductorsubstrate.
DopingTechniques

32. Figure1:Comparisonofthermaldiffusionandionimplantationforselectivelyintroducing
impuritiesintothesurfaceregionofasemiconductorwafer.ImpurityconcentrationCvaries
withdepthx
DiffusionandIonimplantation

33. DopantSources

34. WhatisDiffusion?
Simplediffusionofasubstance(blue)duetoaconcentrationgradientacrossasemi-
permeablemembrane(pink).
Basically,theprocesshappensasaresultoftheconcentration
gradient.
Diffusionprocessiscarriedoutinsystemscalled“diffusionfurnaces”.
Itisfairlyexpensiveandveryaccurate.
Therearethreemainsourcesofdopants:gaseous,liquid,andsolids
thegaseoussourcesaretheonemostwidelyusedinthistechnique
(Reliableandconvenientsources:BF
3,PH
3,AsH
3).
Inthisprocess,thesourcegasreactswithoxygenonthewafersurface
resultinginadopantoxide.Next,itdiffusesintoSilicon,formingan
uniformdopantconcentrationacrossthesurface.

35. DiffusionSteps
Therearetwomainstepsofdiffusionasfollows.Thesestepsareusedtocreatedoped
regions.
Pre-deposition(fordosecontrol)
Inthisstep,desireddopantatomsare
controllablyintroducedontothetargetfrom
methodssuchasgasphasediffusions,and
solidphasediffusions.
Drive-in(forprofilecontrol)
Oncethedopantatomshavearrivedonthewafer
surface,theyneedtoberedistributedintothebulk.
Thisprocessiscalleddrive-in.Inthisstep,the
introduceddopantsaredrivendeeperintothe
substancewithoutintroducingfurtherdopant
atoms.

36. PhosphorusDiffusion
Anexampleofthechemicalreactionforphosphorusdiffusionusingaliquidsourceis
4POCl
3+3O
2→2P
2O
5+6Cl
2
TheP
2O
5formsaglassonsiliconwaferandisthenreducedtophosphorusbysilicon:
2P
2O
5+5Si→4P+5SiO
2
Thephosphorusisreleasedanddiffuseintothesilicon,andCl
2isvented.
Schematicdiagramofatypicalopentube
diffusionsystem

37. IonImplantation
Ionimplantationisalow-temperatureprocessusedtochangethechemicaland
physicalpropertiesofamaterial.
Thisprocessinvolvestheaccelerationofionsofaparticularelementtowardsatarget
toalterthechemicalandphysicalpropertiesofthetarget.
Thistechniqueismainlyusedinsemiconductordevicefabrications.
AdvantagesofIonImplantationTechnique
Theadvantagesofionimplantationincludeprecisecontrolofdoseanddepthofthe
profile/implantation.
Itisalow-temperatureprocessthatoperatesclosetoroomtemperature,sothereisno
needforheat-resistantequipment.
Otheradvantagesincludeawideselectionofmaskingmaterialsandexcellentlateral
doseuniformity.

38. Figure17:Schematicoftheionimplantationprocess.Dopantatomsareionizedbybombarding
withelectrons.Thesearethenisolated,accelerated,andthenimpingedonthewafer. Thereis
alsoascanningsystemthatallowstheionbeamtoscanoverthewafersurface.Adaptedfrom
Fundamentalsofsemiconductormanufacturing andprocesscontrol-MayandSpanos.
IonImplantationProcess

39. Inionimplantation,dopantatomsareionized,isolated,acceleratedand
madetoimpingeonthewafersurface.
Ionimplantationequipmentshouldcontainanionsource.
Thesourcematerialisusuallyintheformofagase.g.AsH3,PH3,and
BF3aresomecommonsources.
Similarly,elementalsourceslikeAsandParealsousedassolid
sources.
Thisionsourceproducesionsofthedesiredelement.
Theionsarethenseparatedusingamassanalyzer,whichisa90◦
magnet,whichbendstheionsdependingonthemass.
Afterselection,thedesiredionsarethenacceleratedandmadeto
impingeonthewafersurface
Beamscanningorrasteringisalsopossibleusingelectricfieldcoilsto
deflecttheionbeams.
Theseionsstrikethetarget,whichisthematerialtobeimplanted.
IonImplantationProcess

40. Eachioniseitheranatomoramolecule.
Thepenetrationdepthoftheionsdependontheirenergy(changedby
theacceleratingfield).
Theamountofionsimplantedonthetargetisknownasthedose.
However,sincethecurrentsuppliedfortheimplantationissmall,the
dosethatcanbeimplantedatagiventimeperiodisalsosmall.
Thereforethistechniqueisusedwheresmallerchemicalchangesare
required.
IonImplantationProcess

41. Onemajorapplicationofionimplantationisthedopingofsemiconductors.
Ionimplantationisespeciallyusefulwithdevicescaling.
Itcanalsobeusedtodopesmallregions.
Itisusuallyusedlaterintheprocessflowwhenthermalbudgetsaretightandthehigh
temperatureofthermaldiffusionisnotallowed.
Theconcentrationprofileforionimplantationisshowninfigure18.Themaximum
concentrationisatacertaindepthbelowthesurface,calledrange.Inthermaldiffusion,
themaximumconcentrationisatthesurfaceandtheconcentrationdecreaseswithdepth.
IonImplantationProcess

42. EffectofIonImplantation
Inionimplantation,sincethewafersurfaceisimpactedbyhighenergy
ions,itcancausedamagebyknockingSiatomsfromtheirposition,
causinglocalstructuraldamage.
Thisneedsapostthermalannealingtreatmenttorepairthedamage.
Therearetwowaysofdoingthis.
1.Tubefurnace-lowtemperatureannealing(600-1000◦C).Tominimize
lateraldiffusion.
2.Rapidthermalannealing-highertemperaturesarepossiblebutfor
shortertimes.

43. DiffusionandIonimplantation

44. Thank You…
Dr.G.S.VIRDI