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第六讲湿法腐蚀/刻蚀outline•Si--Anisotropic----KOH,TMAH,EPW--Isotropic----HNA•SiO2-Glass,PSG--Isotropic----HF,BHF•Si3N4--Isotropic----BoiledH3PO4•ExampleSiliconEtching(Anisotropic)•KOH•EPW•TMAHKOHetching•Relationshipbetweenetchingrateandtemperature?•Relationshipbetweenetchingrateandconcentration?•SelectivityofKOHetching?•What’scornercompensation?•Etchingstopmethod?KOHetching•Relationshipbetweenetchingrateandtemperature?•Relationshipbetweenetchingrateandconcentration?•SelectivityofKOHetching?•What’scornercompensation?•Etchingstopmethod?KOHetching•Relationshipbetweenetchingrateandtemperature?•Relationshipbetweenetchingrateandconcentration?•SelectivityofKOHetching?•What’scornercompensation?•Etchingstopmethod?通常{111}面腐蚀速率最慢,与{100}比可达400:1KOHetching•Relationshipbetweenetchingrateandtemperature?•Relationshipbetweenetchingrateandconcentration?•SelectivityofKOHetching?•What’scornercompensation?•Etchingstopmethod?KOHetching•Relationshipbetweenetchingrateandtemperature?•Relationshipbetweenetchingrateandconcentration?•SelectivityofKOHetching?•What’scornercompensation?•Etchingstopmethod?EtchStops•Often,itisrequiredthatoneetcharegionofsiliconandstoponawelldefined“etch-stop”thatthenstopstheetchabruptly.•Thereareseveraletchstoptechniques,includingconcentration-dependent,electrochemical,anddielectric.•Theseetchstopsallowonetocontrolthethicknessofamicrostructureaccurately(1µm),andhaveveryuniformandreproduciblecharacteristicsHeavilydopedHeavilydopedLightlydopedEtchStops控制时间控制掺杂浓度浓硼掺杂自停止腐蚀技术:KOH、EPW腐蚀,在掺杂浓度小于阈值时,腐蚀速率为常数,大于阈值时,腐蚀速率急剧降低—重掺杂导致腐蚀停止。湿法腐蚀工艺理论模型:三种应变模型—重掺杂应力使表面SiO2生长速率超过腐蚀速率复合模型—硅表面高浓度空穴存在使氧化反应中产生的电子复合,还原反应因为缺少电子减慢,腐蚀速率降低电化学模型—利用能带理论解释。缺点:应力大,压阻系数突变,IC不兼容解决办法:PN结自停止电钝化装置:工作电极(WE):接硅辅助电极(CE):腐蚀液中(Pt)参考电极(RE):测硅的电势,(SCE饱和甘汞电极)I—V曲线反应了不同材料、导电类型的普遍特征(Vocp:开路电势;Vpp:钝化势)电钝化电钝化腐蚀机制:腐蚀反应分三个区A区:Vocp附近把Si开始转化为硅复合物Si(OH)22+和H2,无电荷转移,电流为零B区:VPP附近,开始钝化,电流减小C区:VOFP附近,氧化物形成,电流迅速接近于零电钝化相关参数的影响:晶向、掺杂浓度、温度、腐蚀液配比光照的影响:产生电子空穴对N型:Vpp降低,Vocp不变;P型:Vpp降低,Vocp向正方向飘移尽量避免光照利用电钝化可以进行停止控制,问题是?PN结自停止腐蚀PN结自停止腐蚀:P衬底生长N外延层(结构层)SiO2,SiN掩膜,恒压源加在N区和CE之间,电压略大于VppN区接正极,PN结反偏,电压落在结上,P衬底被正常腐蚀;P衬底被腐蚀掉后,PN结被破坏,外加电压加在N与腐蚀液间,且大于N型硅的钝化势,N型硅被钝化,腐蚀停止三极系统N型外延层对腐蚀液电位难于精确控制,影响N层厚度均匀性。需增加参考电极(RE)—三极系统但P型区电位由于缺陷等原因导致短路,引起边界电流,钝化P区。即使理想的PN结也会因双极效应使腐蚀停在离PN结界面几微米处四电极系统—精度可到0.2微米•适当恒压源加在衬底,使腐蚀电势处于P区的Vocp附近•Ve使PN结反偏,外延层电位略大于N区的钝化势Vpp四极系统DielectricEtchStop•Silicon-On-Insulator(SOI)wafersarebecomingandabundantbecauseoftheiruseinICmanufacturing.1•TheSiontheinsulator(whichisusuallyasilicondioxide)canbeeithersingle-crystal,orpolycrystalline•ThesiliconoxideactsasanexcellentetchstopbecauseitdoesnotetchappreciablyinetchantlikeKOHorEDP.•Thethicknessofthesiliconontopoftheinsulatorcanbecontrolledveryaccuratelywithbetterthan5%uniformity.•SOIwafersaretypicallymuchmoreexpensivethanstandardSiwafers.DielectricSingle-CrystalSi(SOI)DielectricPolycrystallineSiSiliconEtching(Anisotropic)•KOH•EPW•TMAHEDP/EPW湿法腐蚀系统EPW腐蚀系统:乙二胺(NH2(CH2)2NH2)E邻苯二酚(C6H2(OH)2)P水W电离反应:NH2(CH2)2NH2+H2O—NH2(CH2)2NH3++(OH)-氧化还原:Si+2(OH)-+4H2O—Si(OH)62-+2H2络合过程:Si(OH)62-+C6H2(OH)2—Si(C6H4O2)32-+6H2OEthyleneDiamine,Pyrochatechol,WaterEPW反应装置•反应温度:115度,低温下将在硅表面产生不可溶解的残留物,使表面粗糙•带回流:防蒸发•带搅拌EDP(EPW)(115°C)Etchrate(100)=0.75µm/minAlmaybeetchedR(100)R(110)R(111)Etchratio(100)/(111)=35Etchmasks:SiO2~0.2nm/min,Si3N4~0.1nm/minBorondopedetchstop,50×slowerCarcinogenic,corrosiveIstheresafealternative?SiliconEtching(Anisotropic)•KOH•EPW•TMAHTMAH•TMAH,Tetramethylammoniumhydroxide,10-40wt.%(90°C)–Etchrate(100)=0.5-1.5µm/min–Alsafe,ICcompatible(PHvaluerelated,13to12,1um/minto1nm/min)–Etchratio(100)/(111)=10-35–Etchmasks:SiO2,Si3N4~0.05-0.25nm/min–Borondopedetchstop,upto40×slowerASEIssues•Choosingamethod–Desiredshapes–Etchdepthanduniformity1–Surfaceroughness–Processcompatibility–Safety,cost,availability•Etchratevariationduetowetetchset-up–Lossofreactivespeciesthroughconsumption–Evaporationofliquids(concentrationchange)–Poormixing(etchproductblocksdiffusionofreactants)–Contamination–Appliedpotentialoutline•Si--Anisotropic----KOH,TMAH,EPW--Isotropic----HNA•SiO2-Glass,PSG--Isotropic----HF,BHF•Si3N4--Isotropic----BoiledH3PO4•ExampleISEApplications•Creatingstructureswithroundsurfaces•Thinning/RemovingsiliconwafersSiIC’sIsotropically-EtchedDiaphragmReleasedShellsBoronDopedSisubstrateEtchSiusingHNAandanitridemaskBorondopeshellstodesireddepthReleaseshellsinEDP/KOHEtchhastobetimed,noselfstopHardtocontrol,non-uniformNotreproducibleHNA(HF+HNO3+AceticAcid(HAC))•Creatingstructureswithroundsurfaces•Thinning/Removingsiliconwafers•去除表面损伤•清洗炉管湿法腐蚀工艺反应过程:Si+2h+——Si2+Si2++2OH-——Si(OH)2氢被释放形成SiO2HF溶解SiO2形成水溶液H2SiF6Si+HNO3+6HF=H2SiF6+H2NO2+H2O+H2HNO3+H2O=2H2NO2+2OH-+2h+湿法腐蚀工艺H2O、HAC的作用:稀释剂或缓冲剂HAC的作用是控制HNO3的溶解度,在使用时间内使氧化速率保持常数(h+的固容度一定)湿法腐蚀工艺不同组分对腐蚀速率的影响:•HF高、HNO3低腐蚀速率由HNO3浓度控制开始阶段困难,易变,在一定周期内硅表面缓慢生长氧化层,腐蚀受氧化-还原反应控制,趋于依赖晶向•HF低、HNO3高:腐蚀速率受HF溶解形成的SiO2的速率控制反应有自钝化特点,表面覆盖SiO2(30~50A)基本限制来自去除硅的复合物腐蚀各向同性、抛光作用湿法腐蚀工艺•HF:HNO3=1:1与稀释剂有关稀释剂低于10%,对其不敏感稀释剂10%~30%,随稀释剂速率增加降低稀释剂高于30%,微小的浓度变化导致腐蚀速率很大变化•多种比例可供选择,用于腐蚀硅•HF溶液对硅也有溶解作用,但速率很慢,小于1A/Min湿法腐蚀工艺腐蚀剂HFHNO3HAC备注掩膜速率(um/min)10
本文标题:第六讲 湿法腐蚀
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