Wafer Cleaning Techniques in Semiconductor Fabrication

This lecture, authored by Dr. G. S. Virdi, Ex-Chief Scientist at CSIR–Central Electronics Engineering Research Institute, Pilani, focuses on the essential process of wafer cleaning used in semiconductor and microelectronic device fabrication.
The lecture explains the need for cleaning silicon wafers after every key step such as diffusion, oxidation, etching, and photolithography. It covers in detail the RCA (SC1/SC2) methods, sulphuric peroxide cleaning (SPM), megasonic and dry cleaning techniques, and advanced methods involving ozone and ultrapure water.
The importance of removing organic, metallic, and particle contamination to achieve high-yield, high-performance devices is highlighted.
This lecture is highly beneficial for engineering and science students, M.Sc./M.Tech researchers, and professionals involved in semiconductor fabrication and materials processing.

1. WaferCleaning
CSIR-Central Electronics Engineering Research Institute
Pilani—33303 1,India
Dr.G.S.Virdi
Ex.Chief Scientist

2. Device/WaferFabrication
•FabricationProcesses
–Step1:WaferSurfacePreparationandCleaning
–Step2:PhotoresistDeposition
–Step3:PhotoresistSoftBaking
–Step4:Lithography:AlignmentandPhotoresistExposure
(More)
–Step5:PhotoresistDevelopment
recap
–Step6:PhotoresistHardbaking
–Step7:DevelopmentInspection
–Step8:Etching/Deposition(More)
–Step9:PhotoresistRemoval/Stripping
–Step10:FinalInspection

3. WaferCleaning
•WaferCleaningsimplymeans
–“Getridofparticlesand
contamination”
Thisisthemost
importantstep

4. How?
Since1960s…….
SC1(RCA):NH
4OH-H
2O
2-H
2O(1:1:5to1:2:7)@
70-80ºC
29%30%
AtahighpH,SC1attacksorganicandparticles
contaminationbyoxidation
SC2:HC1-H
2O
2-H
2O(1:1:6to1:2:8)@70-80ºC
37%30%
AtalowpH,SC2canremovemetalcontaminationby
formingasolublecomplex
SPM:H
2SO
4(98%)-H
2O
2(30%)(4:1)
*SPM:Sulphuricperoxidemixture

5. Particles
Themosteffectivecommercialmethodisthe
Megasoniccleaningprocess+SC1--->remove
organicandinorganicparticlesattemperature
of40ºC

6. ParticleCleaning
•UseofhighpressureNitrogen gas
fromahandheldgun (Blow-off)
•Mechanicalwafersurface
scrubbers
–Expensive
•High-pressurewatercleaning

7. MetalContamination
•Sources:RIEetching,chemicalsolution,
ionimplantation
•Problems:
–Induceleakagecurrentofp-njunctions
–Reduceminoritycarrierlifetime
–faultbuiltupduringregrow
•WetCleaning:diluteHF(0.5%)-
H
2O
2(10%)andSC1SC2

8. OrganicContamination
•Sources:Vapour,photoresist,containers,
chemical,fingerprints(oil),
carbon(compound)
•Problems:
–Incompletecleaningofsurface,leaving
contaminationssuchasnativeoxideormetal
impurities
–micromasking-->RIE process
–photoresististhemaincontaminationsourcein
ICprocesses
•Cleaningprocess:
–Ozone-injectedultrapurewater(strongoxidising
agentO
3)
–Acetoneandalcohol(simpleway)
NativeOxide:ownoxideofthesolid;e.g.SiO2inthecaseofsiliconand
Al2O3inthecaseofaluminum.

9. SurfaceMicroroughness
•Surfacemicroroughnessisanimportant
factorinthemanufactureofhigh
performanceandqualitydevices
•Forgrowing100Åthinfilm,thesurface
requirementshouldbeatomicallyflat
•Sources:SC1cleaning,NH
4OH(etchant),
H
2O
2(oxidant)
•Problems:lowperformance,lowyield
•Problemsolving:
–reducetheproportionofNH
4OH,temperature,
time
–useSPMandSC2

10. NativeOxide
•Sources:Oxidation,exposuretime,
organiccontamination,metallicimpurities
•7daysofexposuretocleanroomairfor
silicon---->6.7Å
•Problems:
–Uncontrollableultrathinoxidegrowth,high
contactresistance,hardforepitaxialgrowths
(MBEandMOCVD)
–Aproblemforhighperformancedevices.
•Problemsolving:
–shortenprocessingtime
–HF(0.5%)-H
2O
2(10%)
–Etching
ownoxideofthesolid;e.g.SiO2inthecaseofsiliconandAl2O3inthecaseofaluminum.
End
of
Secti
on

11. Wet-chemicalcleaning
Techniques
•Sulfuricacid:
–Themostcommonchemicalcleaningsolutionis
hot(90-125ºC)sulfuricacid,removingmost
inorganicresiduesandparticles.
•Sulfuricacid+H
2O
2:(Veryeffective)
–Oxidantsareaddedtoremoveorganicresidues.
C+O
2--->CO
2(gas)
–Photoresiststripper
•Sulfuricacid+(NH
4)
2SO
4:
–AdrawbackofaddingH
2O
2(strongoxidiser).
Precautionneeded.
–H
2O
2decaysrapidly
•Standardprocedures:
–Chemicalcleaning-->DIwaterrinsing--
>Drying(Nitrogenblow-off+baking)

12. CleaningTechniques
•ImmersionCleaning:common,easy
–Expensive(lotofchemicalsolutions)
–Presentpotentialrecontamination
–Cannotreachessmalleranddeeperpattern/structure
•SprayCleaning:
–Chemicalcostsaredecreased(Spray,lesschemical)
–Freefromrecontamination
–Cleaningefficiencyimprovedduetohighpressureof
thesprayassistsincleaningsmallpatternsandholes
–Immediatesprayrinsinginonestation(savetime)
•DryCleaning:(torectifysome problems inwettech.I.e.
Particlegeneration/drying)
–Ultraviolet-ozoneclean,vaporclean,plasma,
thermal

13. WaterRinsing
•Wetcleaningchemicalscanalsobecontaminantsif
leftonthesurface.
•Therefore,chemicalcleaningsfollowwithDIwater
rinsing
•DIwaterrinsingalsoservesafteretching
•Overflowrinsers
•ContinuoussupplyofDIwater
•Enhancedbyastreamofnitrogenbubbles
•Minimum5min
•flowrate=5timesthevolumeoftherinser/min
•DIwater18ohm
•15-18ohmonexit side(Cleaned)
•Rinsetimeisdeterminedbymeasuringther
D
es
I
istivity
N
2

14. WaterRinsing
•Cascadedrinser
–Twoorthreeoverflowrinsersconnectedtoeach
other
–Waterenteronlytheendrinserandcascadesthrough
thedownstreamrinsers
–Veryefficientwhenseveralboatsofwafersarebeing
rinsedsimultaneously
•Sonicassistedcleaning/rinsing
–Addingultrasonic/megasonic
–Cavitationeffect
–Speedupwettingprocess
N
2
DI
water

15. Drying
•Nitrogenblowoff:Removeresiduewaterdroplets
–Remainingwatermayinterferewithanysubsequent
operation
•Spin-rinsedryers(SRDs)
–Completedryingisaccomplishedinacentrifugelike
equipment.
–StartwithrinsingofDIwater(slowrpm),then
HeatedN
2injection(highrpm)
•VacuumDehydrationbaking
–Beforeapplyingphotoresist(PR)
–Hydrophilic:Whenexposedtomoistures
–PRadhereswellonahydrophobicsurface
–PRcannotadhereonahydrophilicsurface
–Temperature~200ºCfor30min
–Canbeusedifdelayinsubstratepreparation
(growing,sputtering)

16. WaferPriming
•Aprocesswherewafersareexposedtoavapourof
HMDStoprimethewafersurfacepriorto
photoresistcoating.(Silicon)
•Primersformbondswithsurfaceandproducea
polar(electrostatic)surface
•Resistadhesionfactors
•Moisturecontentonsurface
•wettingcharacteristicsofre
•typeofprimer
•delayinexposureandsoftb
s
a
u
ke
rface
surface
smoothness
•stressfromcoatingprocess
•surfacecontamination

17. WaferPriming
•Typesofpriming
–Immersionpriming(simplestway)
•lackofcontrolandnotfreefromcontamination
•Expensive
–Spinpriming
–vapourpriming
•Freefromcontamination
•Cheap
•ForGaAswafer:
–Nonecessarily,becauseGaAsalreadyha
surface
PR
Primer
wafer

18. StandardCleaning Procedures
1.AhotH
2SO
4:H
2O
2(2:1to3:1)at
120ºCmixtureisusedtoremovethe
greasycontamination,whichmaybe
fromthecassetteorresiduesfromthe
photoresistlayers.
2.SC1
3.AftertheSC1process,a15s
immersionin1%HF-H
2Osolution
maybebeneficialforremovingany
traceimpurity.

19. AdvancedCleaning Procedures
1.H
2O+O
3:organiccontamination
2.NH
4OH+H
2O
2(0.05:1:5):Particle,organic
andmetallicimpurities
3.HF+H
2O
2(0.5%:10%):Nativeoxide,
metallicimpurities
4.Ultrapurewater:Rinsing
IMEC
1.H
2SO
4+H
2O
2(4:1)@90ºCfor10min
2.HF(0.5%)/IPA(0.1%)@roomtemperature
for2min
4.Ultrapurewater:Rinsing
IPA:isopropylalcohol

20. CleaningComparison
Cleaningmethods
thegateoxide
Yieldsof
IMEC+RCA(spray)+HF(bath)80%
IMEC+RCA(spray) 65%
RCA(bath) 60%
IMEC(bath) 83%

21. Thank You…