Plasma Etching in VLSI Fabrication: Principles, Processes, and Applications

This lecture by Dr. G. S. Virdi, Ex-Chief Scientist at CSIR–Central Electronics Engineering Research Institute, Pilani, presents a comprehensive overview of plasma etching, one of the most critical steps in VLSI fabrication. The lecture explains the fundamental concepts of dry etching, distinguishing between isotropic and anisotropic processes, and discusses various plasma etching systems such as RIE, MRIE, ECR, and ICP. It also covers plasma parameters, temperature control, process mechanisms, and the advantages and limitations of plasma etching. This presentation is particularly beneficial for engineering and science students, M.Tech and M.Sc scholars, and researchers working in the field of microelectronics and semiconductor device fabrication.

1. PlasmaEtching
Dr.G.S.Virdi
Ex.Chief Scientist
CSIR -Central Electronics Engineering Research Institute
Pilani-333031,India
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2. Outline
•WhatisplasmaEtching
•Parametersinplasma
•Isotropicand Anisotropic
•Typesofplasmaetchingsystem
•Reactiveplasmaetchingandequipment
•Processesofplasmaetching
•Advantagesanddisadvantagesofplasmaetching
•Howplasmaetchingworks
•Whyuseplasmaetching
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3. WhenPlasmaEtchingappliesin
Microelectronic
Bytheearly1970s,plasmaetchingwasfirst widely
adaptedtodevicemanufacturing,which wasmainly
oxygenplasma.Atthistime,itwas primarilyusedfor
photoresistashing.
Bythelate1970s,itwaswidelyrecognizedthat plasma
etchingcanofferthepossibilityof anisotropyetching,
thenrapidlyincreasing microelectronicindustrydrives
thetransitionfrom wetetchingtoplasmaetchingand
continuously promotesitsdevelopment.
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4. WhatisPlasmaEtching
•Alsoknownasdryetching
•OneofthemostimportantprocessesinIC
manufacturing
•Akeyprocessforremovingmaterialfromsurface
•hasanumberofadvantageoverchemicaletching
•usesagasthatissubjectedtoanintenseelectric
field
•theonlycommerciallyusabletechnologyfor
anisotropyremovalofmaterialfromsurface
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5. Typesofplasma etching system
1.Noneplasmabased
usesspontaneousreactiontoappropriate
reactivegasmixture
2.Plasmabased
usesradiofrequency(RF)powertodrive
chemicalreaction
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6. Noneplasmabased
•Isotropicetchingofsilicon
•Typicallyfluorine-containinggases
(interhalogens)thatis readilyto etchsi
•Highly selectivitymakinglayers
•Noneedprocessingplasmaequipment
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7. Plasmabased
RFpower isusedto drivechemical
reactive
Plasmatakesplaceofelevated
temperature orveryreactivechemicals
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8. Isotropicetching
•Aprocessthatetchesatthesamerateinall
direction
•Verygoodpatterntransfer
•Poorselectivity
•Canetchanything
•Highetchrate
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9. Anisotropicetching
•Aprocessthatetches onlyinonedirection.
•Preferableforsmallerfeaturedimension
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10. IsotropicVs.Anisotropic
•Isotropic:verticalandhorizontal
•Anisotropy:muchhigherverticalratethan
horizontal
Isotropic Anisotropic
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11. Isotropicvs.Anisotropic
•hf=thicknessofthethin
film
l=lateraldistanceetched
underneath Af =degree of
anisotropic
Af=1-l/hf
Anisotropic
Af =degree of
isotropic hf=l
•Af=0 isotropic
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12. Plasmaetching
Therearefiveclassesofplasmaetching
mechanisms
–Sputteretching
–Chemicaletching
–Acceleratedion-assistedetching
–Sidewall-protectedion-assistedetching
–Reactiveionetching(RIE)
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13. Sputteretching
•Highlyanisotropic
•Apurelyphysicalprocess
•Verysimilartoionimplantationbutlowenergy ions
areusedto avoidimplantation damage
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14. Chemicaletching
•isotropic
•Plasmaisusedtoproducechemicallyreactive
species(atomsradicalandions)frominert
moleculargas.
•Productionofgasbyproductsisextremely
important
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15. Acceleratedion–assistedetching
•Likethesputteretchingions are accelerated by
thesheath potential
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16. Sidewall-protectedion-assisted
etching
•Canbeanisotropic
•Involvesadditionalspeciestocreatea
protectivesidewallbarrier.
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17. Reactiveionetching
Process inwhichchemicaletchingis accompaniedbyno
undercutting sinceside-wallarenotexposed(ie.Ionassisted
etching)
Bombardment opensareasfor
reactions Lowselectivity
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18. Parametersinplasma
•Temperatures:etchingrate anddirectivity
•Pressure:iondensityandiondirectivity
•Densities:chargedandneutralparticles
•Power:iondensityandionkineticenergy
•Othervariable:gasflowrate,loading,
reactormaterialandmaskingmaterial
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19. Plasmaetchprocesstemperature
control
•Processtemperatureisthesinglemost important
parameterin theplasmaprocess.
•Processtemperaturehasprimarycontrol overetch
rateandhasa secondaryeffecton etchuniformity
•thetemperatureatwhichtheprocess
operateshasamajorinfluenceon
processingrates.
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20. Plasmatemperaturecontrol
(cont.)
•Thehighertheprocesstemperature,the fasterthe
processingrate.
•Processtemperaturecontrolis mandatory when
processingtemperaturesensitive devices.
•Uncontrolledprocesstemperaturecancause distortion.
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21. Lowpressure
-Tohelp etching by productdiffuseoutthe via
-The meanfreepathlengthsofgasmolecules and
ion arelongerand thisreduces scatteringcollision
thatcancausealossin profile control
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22. HowPlasmaetchingworks
•Chamberisevacuated
•Chamberisfilled withgases
•RFenergyis appliedtopair ofelectrodes
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23. HowPlasmaetchingworks
•Applied energy accelerateselectron increasing kinetic
energy
Electroncollide withneutralgasmoleculesformingions
andmoreelectrons
Plasmadischargeisbulkregionanddarkorsheathregion
nearelectrodes
Bulkregion is semi-neutralornearlyequal tonumber of
electronsandions
Sheathregionisnearlyallofthepotentialdrop;
accelerates“+”ionsfrombulkregionwhichbombardthe
substrate
Maintainedat1Pa(75mtorr)to750Pa(56torr)withgas
densityof 27x10ˆ14to2x10^17molecules/cm^3
•
•
•
•
•
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24. Reactiveplasmaetchingand
equipment
•Barrelreactor
•ReactiveIonEtcher(RIE)
•Magnetic-Enhanced RIE(MRIE)
•electroncyclotronresonanceplasmaetcher
(ECR)
•inductivelycoupledplasma (ICP)
•Clusteredplasmaprocessing
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25. BarrelReactor
•The maincomponentsofthe barrelreactor is the
cylindricalvacuum chamberwitha pairofRF
electrodesconcentricallyinside
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26. RIEsystem
•Asymmetricalparallelplatesystem
•Plasma,sheathandboundarylayer
•Combinationphysicalandchemicaletching
•Advantage:Anisotropyetching
•Disadvantage: low
etchrate low
selectivity Surface
damage
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27. MRIEsystem
•Advantage: largeanisotropic
high etch ratereducedsurface
damageetchingrelative
independentofloading
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28. ERCsystem
•
•
Higherplasmadensityatlowpressure
Controlthedensityof
reactiveionandtheir
kineticenergyseparated
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29. ICPsystem
•Simplesystem
•Almostsameprocess
resultasthatfrom ERC
system
•TwoRFpower
generatorstocontrol
ionenergyand ion
densityseparately
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30. Clusteredplasmasystem
Useawaferhandletopasswafersfromone processchamber
toanotherinavacuum environment
canalsoincreasethroughput,andprovidean advantageof
highchipyield,sincewaferis exposedtolesscontamination.
minimizeparticulatecontamination
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31. Processesofplasmaetching
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32. AdvantagesofPlasmaetching
•Easyto controland reproduce
•Operatedinlowpressurechamber
•Noliquidwaste
•Usessmallamountsofchemicals
•Eliminates handlingofdangerousacids
•canbe highlyselective tounderlyinglayers
•causeslittle damagetothephotoresist
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33. Disadvantagesofplasmaetching
•thewafercanbedamagedfromtheRF
radiation.
•Tendencyforisotropicetching
•poorpatterntransfer
•Somegasesarequitetoxic
•Expensiveequipment
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34. When do we want touse plasma
etching
•Thefirstthing you shouldnoteaboutthis
technologyisthatitisexpensive torun
comparedtowetetching.
•Needverticalsidewallsfordeepetchingin the
substrate
•Adoptedplasmaetchingtoachievesmall
features
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35. Why useplasma etching?
•Fast
•Selective
•Lowdamage
•Anisotropic
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36. References
•GlowDischargeProcesses
•http://personal.cityu.edu.hk/~appkchu/AP41
•An introduction to plasmaetching forVLSI
circuittechnology
•PlasmaetchingprocessesforULSI
semiconductorcircuits
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37. Thank You…
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