第四章 光刻

第四章光刻理学院刘德雄2011年3月6本章主要内容Photolithography(光刻)掌握光刻胶的组成+PR和–PR的区别描述光刻工艺的步骤四种对准和曝光系统Explainrelationshipsofresolutionanddepthoffocustowavelengthandnumericalaperture.光刻概述Photolithography临时性地涂覆光刻胶到硅片上转移设计图形到光刻胶上IC制造中最重要的工艺占用40to50%芯片制造时间决定着芯片的最小特征尺寸ICFabricationEDA:ElectronicDesignAutomationPR:Photoresist光致抗蚀剂，光刻胶，光阻材料光刻需要高分辨率HighResolution光刻胶高光敏性HighPRSensitivity精确对准PrecisionAlignment精确的工艺参数控制PreciseProcessParametersControl低缺陷密度LowDefectDensityPhotoresist－PR－光刻胶光敏性材料临时性地涂覆在硅片表面通过曝光转移设计图形到光刻胶上类似于照相机胶片上涂覆的光敏材料PhotoresistNegativePhotoresist负性光刻胶－负胶PositivePhotoresist正性光刻胶－正胶曝光后不可溶解曝光后可溶解显影时未曝光的被溶解显影时曝光的被溶解便宜高分辨率NegativeandPositivePhotoresistsPhotoresistComposition光刻胶基本组成聚合物材料感光材料溶剂添加剂聚合物材料固体有机材料光照下不发生化学反应作用：保证光刻胶薄膜的附着性和抗腐蚀性，同时也决定光刻胶薄膜的其它特性（如光刻胶的厚度、弹性和热稳定性）感光材料当被曝光时发生光化学反应而改变溶解性正性光刻胶：由不溶变为可溶负性光刻胶：由可溶变为不溶溶剂使光刻胶在涂到硅片表面之前保持液态允许采用旋涂的方法获得薄层光刻胶薄膜添加剂不同的添加剂获得不同的工艺结果如：染料，降低反射NegativeResist大多数负胶是聚异戊二烯型曝光后变为交联聚合物交联聚合物有很好的抗化学腐蚀特性未曝光部分将溶解在显影液中NegativePhotoresist负胶的缺点聚合物吸收显影液中的溶剂由于光刻胶膨胀而使分辨率降低其主溶剂二甲苯会引起环境和安全问题ComparisonofPhotoresists正胶PositivePhotoresist曝光部分可以溶解在显影液中正影（光刻胶图形与掩膜图形相同）更高分辨率（无膨胀现象）在IC制造应用更为普遍正胶PositivePhotoresist酚醛清漆树脂聚合物乙酸溶剂含在树脂中的交联感光剂光能使感光剂分解并打断交联化学键光刻胶变为可溶于显影液问题正胶比负胶具有更好的分辨率，为什么十九世纪八十年代以前人们普遍使用负胶？答案因为正胶比负胶贵得多，直到器件特征尺寸减小到3um以下时人们才用正胶代替负胶。对光刻胶的要求高分辨率–ThinnerPRfilmhashighertheresolution–ThinnerPRfilm,thelowertheetchingandionimplantationresistance高抗蚀性好黏附性更宽的工艺容差–Highertolerancetoprocessconditionchange光刻胶的物理特性光刻胶必须能够抵挡一定的工艺条件－Coating,spinning,baking,developing.－Etchresistance－Ionimplantationblocking光刻工艺PhotolithographyProcess光刻基本步骤•涂胶Photoresistcoating•对准和曝光Alignmentandexposure•显影Development过去工艺：光刻步骤现代工艺：光刻步骤光刻工序1、清洗硅片WaferClean2、预烘和底胶涂覆Pre-bakeandPrimerVapor3、光刻胶涂覆PhotoresistCoating4、前烘SoftBake5、对准Alignment6、曝光Exposure7、后烘PostExposureBake8、显影Development9、坚膜HardBake10、图形检测PatternInspectionWaferClean去除污染物去除颗粒减少针孔和其它缺陷提高光刻胶黏附性基本步骤–化学清洗–漂洗–烘干光刻1－清洗过去的方法–高压氮气吹扫–旋转洗刷–高压水柱清洗WaferCleanProcess（新方法）光刻2－预烘脱水烘焙去除圆片表面的潮气增强光刻胶与表面的黏附性通常大约100°C与底胶涂覆合并进行光刻2－底胶涂覆增强光刻胶(PR)和圆片表面的黏附性广泛使用:Hexamethyldisilazane(HMDS)在PR旋转涂覆前HMDS蒸气涂覆Usuallyperformedin-situwithpre-bakePR涂覆前用冷却板冷却圆片预烘和底胶蒸气涂覆WaferCooling硅片冷却Waferneedtocooldown水冷冷却板温度影响光刻胶粘性–AffectPRspincoatingthickness影响光刻胶涂覆厚度光刻3－涂胶SpinCoating圆片放置在真空卡盘上高速旋转液态光刻胶滴在圆片中心光刻胶以离心力向外扩展均匀涂覆在圆片表面粘性ViscosityFluidsstickonthesolidsurfaceAffectPRthicknessinspincoatingRelatedtoPRtypeandtemperature旋转速率越高，涂覆越均匀光刻胶厚度与旋转速率和粘性的关系动态旋转速率时间转速光刻胶旋涂机光刻胶在旋转的圆片表面向外扩展圆片吸在真空卡盘上低速旋转~500rpm缓变上升至~3000-7000rpm实验室匀胶机PhotoresistSpinCoaterEBR:Edgebeadremoval边缘修复滴胶光刻胶吸回PhotoresistSpinCoatingPhotoresistSpinCoatingPhotoresistSpinCoatingEdgeBeadRemoval(EBR)PRspreadtotheedgesandbacksidePRcouldflakesoffduringmechanicalhandlingandcausesparticlesFrontandbackchemicalEBRFrontopticalEBREdgeBeadRemovalEdgeBeadRemovalReadyForSoftBakeOpticalEdgeBeadRemoval•Afteralignmentandexposure•Waferedgeexpose(WEE)•ExposedphotoresistatedgedissolvesduringdevelopmentOpticalEdgeBeadRemovalDeveloperSpinOff光刻4－前烘蒸发光刻胶中的溶剂EvaporatingmostofsolventsinPR溶剂能使涂覆的光刻胶更薄，但吸收热量且影响光刻胶的黏附性SolventshelptomakeathinPRbutabsorbradiationandaffectadhesionSoftbakingtimeandtemperaturearedeterminedbythematrixevaluations前烘要适度过多的烘烤使光刻胶聚合，感光灵敏度变差Overbake:polymerized,lessphoto-sensitivity烘烤不够影响黏附性和曝光Underbake:affectadhesionandexposureSoftBakeHotplatesConvectionovenInfraredovenMicrowaveovenBakingSystemsHotPlatesWidelyusedintheindustryBacksideheating,nosurface“crust”In-linetracksystemWaferCoolingNeedtocooldowntoambienttemperatureWater-cooledchillplateSiliconthermalexpansionrate:2.5x10-6/°CFor8inch(200mm)wafer,1°Cchangecauses0.5umdifferenceindiameterAlignmentandExposureIC制造的最关键过程MostcriticalprocessforICfabricationMostexpensivetool(stepper)inanICfab.MostchallengingtechnologyDeterminestheminimumfeaturesizeCurrently0.09umandpushingto0.045umAlignmentandExposureTools接触式曝光机Contactprinter接近式曝光机Proximityprinter投影式曝光机Projectionprinter步进式曝光机StepperContactPrinterSimpleequipmentUsebeforemid-70sResolution:capableforsub-micronDirectmask-wafercontact,limitedmasklifetimeParticlesContactPrinterContactPrintingProximityPrinter~10umfromwafersurfaceNodirectcontactLongermasklifetimeResolution:3umProximityPrinterProximityPrintingProjectionPrinterWorkslikeanoverheadprojectorMasktowafer,1:1Resolutiontoabout1umProjectionSystemStepperMostpopularusedphotolithographytoolintheadvancedICfabsReductionofimagegiveshighresolution0.25umandbeyondVeryexpensiveStep-&-RepeatAlignment/ExposureExposureLightSource短波长Shortwavelength高亮度Highintensity稳定Stable高压汞灯High-pressuremercurylamp激光源Excimerlaser汞灯光谱分析SpectrumoftheMercuryLampPhotolithographyLightSourcesStandingWaveEffect光线发生干涉、反射Interferenceoftheincidentandreflectionlights周期性的出现过曝光或者曝光不够Periodicallyoverexposureandunderexposure影响曝光效果Affectsphotolithographyresolution.StandingWaveIntensityStandingWaveEffectonPhotoresist光刻5－后烘PEB光刻胶成玻璃状温度PhotoresistglasstransitiontemperatureTgBakingtemperaturehigherthanTg光刻胶分子发生热运动过曝光和欠曝光的光刻胶分子发生重分布平衡驻波