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CHAPTER1INTRODUCTION1.1MATERIALSPROCESSINGChemicallyreactiveplasmadischargesarewidelyusedtomodifythesurfaceprop-ertiesofmaterials.Plasmaprocessingtechnologyisvitallyimportanttoseveralofthelargestmanufacturingindustriesintheworld.Plasma-basedsurfaceprocessesareindispensableformanufacturingtheverylargescaleintegratedcircuits(ICs)usedbytheelectronicsindustry.Suchprocessesarealsocriticalfortheaerospace,automotive,steel,biomedical,andtoxicwastemanagementindustries.Materialsandsurfacestructurescanbefabricatedthatarenotattainablebyanyothercommer-cialmethod,andthesurfacepropertiesofmaterialscanbemodifiedinuniqueways.Forexample,0.2-mm-wide,4-mm-deeptrenchescanbeetchedintosiliconfilmsorsubstrates(Fig.1.1).Ahumanhairis50–100mmindiameter,sohundredsofthesetrencheswouldfitendwisewithinahumanhair.Uniquematerialssuchasdiamondfilmsandamorphoussiliconforsolarcellshavealsobeenproduced,andplasma-basedhardeningofsurgicallyimplantedhipjointsandmachinetoolshaveextendedtheirworkinglifetimesmanyfold.Itisinstructivetolookcloseratintegratedcircuitfabrication,whichisthekeyapplicationthatwedescribeinthisbook.Asaveryincompletelistofplasmapro-cesses,argonoroxygendischargesareusedtosputter-depositaluminum,tungsten,orhigh-temperaturesuperconductingfilms;oxygendischargescanbeusedtogrowSiO2filmsonsilicon;SiH2Cl2=NH3andSi(OC2H5)4=O2dischargesareusedfortheplasma-enhancedchemicalvapordeposition(PECVD)ofSi3N4andSiO2films,1PrinciplesofPlasmaDischargesandMaterialsProcessing,byM.A.LiebermanandA.J.Lichtenberg.ISBN0-471-72001-1Copyright#2005JohnWiley&Sons,Inc.respectively;BF3dischargescanbeusedtoimplantdopant(B)atomsintosilicon;CF4=Cl2=O2dischargesareusedtoselectivelyremovesiliconfilms;andoxygendis-chargesareusedtoremovephotoresistorpolymerfilms.Thesetypesofsteps(depositorgrow,dopeormodify,etchorremove)arerepeatedagainandagaininthemanufactureofamodernIC.Theyaretheequivalent,onamicrometer-sizescale,ofcentimeter-sizemanufactureusingmetalandcomponents,boltsandsolder,anddrillpressandlathe.FormicrofabricationofanIC,one-thirdofthetenstohundredsoffabricationstepsaretypicallyplasmabased.Figure1.2showsatypicalsetofstepstocreateametalfilmpatternedwithsub-micrometerfeaturesonalargearea(300mmdiameter)wafersubstrate.In(a),thefilmisdeposited;in(b),aphotoresistlayerisdepositedoverthefilm;in(c),theresistisselectivelyexposedtolightthroughapattern;andin(d),theresistisdeveloped,removingtheexposedresistregionsandleavingbehindapatternedresistmask.In(e),thispatternistransferredintothefilmbyanetchprocess;themaskprotectstheunderlyingfilmfrombeingetched.In(f),theremainingresistmaskisremoved.Ofthesesixsteps,plasmaprocessingisgenerallyusedforfilmdeposition(a)andetch(e),andmayalsobeusedforresistdevelopment(d)andremoval(f).Theetchprocessin(e)isillustratedasleadingtoverticalsidewallsalignedwiththeresistmask;thatis,themaskpatternhasbeenfaithfullytransferredintothemetalfilm.Thiscanbeaccomplishedbyanetchprocessthatremovesmaterialintheverticaldirectiononly.Thehorizontaletchrateiszero.Suchanisotropicetchesareeasilyproducedbyplasmaprocessing.Ontheotherhand,onemightimagineFIGURE1.1.Trenchetch(0.2mmwideby4mmdeep)insingle-crystalsilicon,showingtheextraordinarycapabilitiesofplasmaprocessing;suchtrenchesareusedfordeviceisolationandchargestoragecapacitorsinintegratedcircuits.2INTRODUCTIONthatexposingthemaskedfilm(d)toaliquid(orvaporphase)etchantwillleadtotheundercutisotropicprofileshowninFigure1.3a(comparetoFig.1.2e),whichisproducedbyequalverticalandhorizontaletchrates.Manyyearsago,featurespa-cings(e.g.,betweentrenches)weretensofmicrometers,muchexceedingrequiredfilmthicknesses.Undercuttingwasthenacceptable.Thisisnolongertruewithsubmicrometerfeaturespacings.Thereductioninfeaturesizesandspacingsmakesanisotropicetchprocessesessential.Infact,strictlyverticaletchesaresome-timesnotdesired;onewantscontrolledsidewallangles.Plasmaprocessingistheonlycommercialtechnologycapableofsuchcontrol.AnisotropyisacriticalprocessparameterinICmanufactureandhasbeenamajorforceindrivingthedevelopmentofplasmaprocessingtechnology.TheetchprocessappliedtoremovethefilminFigure1.2disshowninFigure1.2easnotremoving,eitherthephotoresistortheunderlyingsubstrate.ThisselectivityisanothercriticalprocessparameterforICmanufacture.WhereasFIGURE1.2.Depositionandpatterntransferinmanufacturinganintegratedcircuit:(a)metaldeposition;(b)photoresistdeposition;(c)opticalexposurethroughapattern;(d)photoresistdevelopment;(e)anisotropicplasmaetch;(f)remainingphotoresistremoval.1.1MATERIALSPROCESSING3wetetcheshavebeendevelopedhavingessentiallyinfiniteselectivity,highlyselec-tiveplasmaetchprocessesarenoteasilydesigned.Selectivityandanisotropyoftencompeteinthedesignofaplasmaetchprocess,withresultsasshowninFigure1.3b.ComparethistotheidealizedresultshowninFigure1.2e.Assumingthatfilm-to-substrateselectivityisacriticalissue,onemightimaginesimplyturningofftheplasmaafterthefilmhasbeenetchedthrough.Thisrequiresagoodendpointdetectionsystem.Eventhen,variationsinfilmthicknessandetchrateacrosstheareaofthewaferimplythattheetchcannotbestoppedattherightmomentevery-where.Hence,dependi