MEMS原理-05刻蚀工艺

05EtchingProcess刻蚀工艺OutlinesEtchingProcessEtchingIntroductionDryEtchingProcessWetEtchingProcessCleaningProcessIntroductionSummaryEtchingIntroductionDryEtchingvs.WetEtchingComparisonofProcessEtchingIntroductionDryEtchingvs.WetEtchingComparisonofProcessEtchingIntroductionIsotropicvs.AnisotropicEtchingEtchingIntroductionAnisotropicEtchingofSilicon•Differinghybridized(sp3)orbitalorientationondifferentcrystalplanescausesdrasticdifferencesinetchrate.•Typically,etchratesare:(100)(110)(111).•The(111)familyofcrystallographicplanesarenormallythe“stop”planesforanisotropicetching.•Thereare8(111)planesalongthe±x±y±zunitvectors.•Intersectionsoftheseplaneswithplanarbottomsproducethestandardanisotropicetchingstructuresfor(100)Siwafers:–V-grooves–pyramidalpits–pyramidalcavitiesEtchingIntroductionAnisotropicEtchingofSiliconDryEtchingWhatisaplasma?Dryetchingcanbeassociatedwiththeuseofaplasma.Plasmaisconsideredasafourthstateofmatter.Astheionizationisallbutnaturalforthemoleculesformingthegas,itischemicallyhighlyreactive.Thismakesplasmaveryinterestinginmicrotechnologyprocessing.Gasesaremadeofmoleculeswithnolinkingforcebetweenthem(byoppositiontoliquids,andevenmoretosolids!).Agasnaturallyhasasmallionizationrate,thismeansasmallquantityofionizedmoleculescomparedtothetotalquantityofmoleculesinagivenvolume.Theseionsarecreatedandionsrecombinesveryfastinsidethegas.Aplasmacanbeseenasahighlyionizedgas,sothattheglobalcloudhasaneutralelectriccharge,butatalocalscale,youcanseeelectricchargesforanotsoshorttime.SchematicsofagasanditscomparisonwithplasmaDryEtchingEtchingMechanismDryEtchingGasPhaseEtchingDryEtchingRF-Plasma-BasedDryEtching•Aplasmaisfullyorpartiallyionizedgascomposedofequalnumbersofpositiveandnegativechargesandadifferentnumberofunionizedmolecules.•Aplasmaisproducedwhenanelectricfieldofsufficientmagnitudeisappliedtoagas,causingthegastobreakdownandbecomeionized.DryEtchingDryEtchingChemistriesDryEtchingMethodsofDryEtchingDryEtchingTypesofDryEtchingProcessesDryEtchingTypesofDryEtchingProcessesDryEtchingDryetchingSi/SiO2inF-basedGasesandPlasma•ProminentetchchemistryinICs&MEMS•CF4doesnotetchSi(doesnotchemisorb)butF2gaswilletchSiwithetchproductsSiF2andSiF4•PlasmaisneededtogenerateFthatmustpenetrateSiF2-likesurfaceDryEtchingDependenceofEtchRateandSi/SiO2SelectivityonO2/CF4RatioEvenwithplasmatheetchrateisslow-insufficientFconcentrationAddingO2totheplasmacanincreaseFconcentrationO+CF3COF2+FthenO+COF2􀃆CO2+2F•Etchratemaximizesaround12%O2•EtchratedecreasesathigherO2concentrations-DilutionofFconcentrationwithoverlyabundantO2-TrendissimilarforSiO2•EtchrateishigherforSi•Si/SiO2selectivityisgood•IsotropicetchingDryEtchingDependenceofEtchRateandSiO2/SiSelectivityonH2/CF4Ratio•AddingH2drasticallylowersSietchrate-lowersFconcentration(H++F+e-􀃆HF)-Nearly0at40%H2•However,etchrateofSiO2remainsnearlyconstant•Allowsetchselectivitytobeincreasedtremendously•Mechanismforincreasedselectivityhastwocomponents-depositionofanon-volatileresidue-roleofO2inetchingofSiO2DryEtchingIncreaseofDegreeofAnisotropyFormationofSidewallPassivatingFilms•Formationofnonvalatilefluorocarbonsthatdepositonthesurfaces(Polymerization)•Thedepositcanonlyberemovedbyphysicalcollisionswithincidentions•Fluorocarbonfilmsdepositsonallsurfaces,buttheionvelocityisnearlyvertical.Asaresult,astheetchingproceedsthereislittleionbombardmentofthesidewallsandthefluorocarbonfilmaccumulates•Addinghydrogenencouragestheformationofthefluorocarbonfilmsbecausehydrogenscavengefluorine,creatingacarbon-richplasma(samethinghappenedwhenC2F6isusedinsteadofCF4)•LessaccumulationisobservedonSiO2thanSisurfaces•TradeoffbetweenSi/SiO2selectivityandAnisotropyDryEtchingControllingPolymerizationthroughF/C-Ratio•HigherF/C-ratioleadstomoreetching•LowerF/C-ratioleadstomorepolymerization•Canbedeterminedbythegasused•AddingH2consumesF–leadstopolymerization•AddingO2consumesC–leadstoetchingDryEtchingDryEthcingOrganicFilms•O2plasmascanremoveorganicfilmswithhighselectivity•AddingCF4canincreaseetchrateandlowersvariation-butselectivitycanbereducediftoomuchisaddedDryEtchingDamageinReactiveIonEtching(RIE)SF6/O2/He，SF6/CHF3/HeDryEtchingProcessFluorin-basedDryEtchingProcessDryEtchingProcessWetEtching•Mixturesofacids,bases,andwater-HF,H3PO4,H2SO4,KOH,H2O2,HCl,..•Canbeusedtoetchmanymaterials-Si,SiO2,Si3N4,PR,Al,Au,Cu,…•EtchRate:-widerange•EtchSelectivity-typicallyquitehigh-sensitivetocontamination•EtchGeometry:-typicallyisotropic,somespecialcasesareanisotropicWetEtchingHydrofluoricAcidforSiO2•Selective(roomtemperature)-etchesSiO2andnotSi-willalsoattackAl,Si3N4,..•Ratedependsstronglyonconcentration-maximum:49%HF(“concentrated)~2μm/min-controlled:5to50:1(“timed”)~0.1μm/min•Dangerous!-notastrongacid-deceptive(looksjustlikewater)-penetrateskin(adsorption)andattacksslowly-willtargetbones•EtchGeometry-completelyisotropic(usedtoundercut/release)•Reactions:SiO2+6HF􀃆H2SiF6(aq)+2H2OWetEtchingBufferedHFforSiO2BufferedHF(BHF),alsocalledBufferedoxideetch(BOE)additionofNH4FtoHFsolution-controlthepHvalue-replenishthedepletionofthefluorideionstomainta