第四章_光刻

null第四章 光 刻第四章 光 刻理学院 刘德雄 2011年3月6本章主要内容本章主要内容Photolithography(光刻)掌握掌握光刻胶的组成 +PR 和 –PR的区别 描述光刻的步骤 四种对准和曝光系统 Explain relationships of resolution and depth of focus to wavelength and numerical aperture. 光刻概述 Photolithography光刻概述 Photolithography临时性地涂覆光刻胶到硅片上 转移图形到光刻胶上 IC制造中最重要的工艺 占用40 to 50% 芯片制造时间 决定着芯片的最小特征尺寸 IC FabricationIC FabricationEDA: Electronic Design Automation PR: Photoresist 光致抗蚀剂，光刻胶，光阻材料光刻需要光刻需要高分辨率 High Resolution 光刻胶高光敏性 High PR Sensitivity 精确对准 Precision Alignment 精确的工艺参数控制 Precise Process Parameters Control 低缺陷密度 Low Defect DensityPhotoresist－ PR－光刻胶Photoresist－ 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PhotoresistPhotoresistNegative and Positive PhotoresistsNegative and Positive PhotoresistsPhotoresist Composition光刻胶基本组成Photoresist Composition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Negative ResistNegative Resist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 Negative PhotoresistNegative Photoresist负胶的缺点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 Comparison of PhotoresistsComparison of Photoresists正胶 Positive Photoresist正胶 Positive Photoresist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 Positive Photoresist正胶 Positive Photoresist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。对光刻胶的对光刻胶的要求高分辨率 – Thinner PR film has higher the resolution – Thinner PR film, the lower the etching and ion implantation resistance 高抗蚀性 好黏附性 更宽的工艺容差 – Higher tolerance to process condition change 光刻胶的物理特性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. －Etch resistance －Ion implantation blocking 光刻工艺 Photolithography Process光刻工艺 Photolithography Process光刻基本步骤 • 涂胶 Photoresist coating • 对准和曝光 Alignment and exposure • 显影 Development过去工艺：光刻步骤过去工艺：光刻步骤现代工艺：光刻步骤现代工艺：光刻步骤光刻工序光刻工序1、清洗硅片 Wafer Clean1、清洗硅片 Wafer Clean2、预烘和底胶涂覆 Pre-bake and Primer Vapor2、预烘和底胶涂覆 Pre-bake and Primer Vapor3、光刻胶涂覆 Photoresist Coating3、光刻胶涂覆 Photoresist Coating4、前烘 Soft Bake4、前烘 Soft Bake5、对准 Alignment5、对准 Alignment6、曝光Exposure6、曝光Exposure7、后烘 Post Exposure Bake7、后烘 Post Exposure Bake8、显影 Development8、显影 Development9、坚膜 Hard Bake9、坚膜 Hard Bake10、图形检测 Pattern Inspection10、图形检测 Pattern InspectionWafer CleanWafer Clean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 Wafer Clean Process（新方法）Wafer Clean Process（新方法）光刻2－预烘光刻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 Usually performed in-situ with pre-bake PR涂覆前用冷却板冷却圆片预烘和底胶蒸气涂覆预烘和底胶蒸气涂覆Wafer Cooling硅片冷却Wafer Cooling硅片冷却Wafer need to cool down 水冷冷却板 温度影响光刻胶粘性 – Affect PR spin coating thickness 影响光刻胶涂覆厚度光刻3－涂胶 Spin Coating光刻3－涂胶 Spin Coating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粘性 ViscosityFluids stick on the solid surface Affect PR thickness in spin coating Related to PR type and temperature 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光刻胶厚度与旋转速率和粘性的关系动态旋转速率动态旋转速率时间转速光刻胶旋涂机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 ~ 500 rpm 缓变上升至 ~ 3000 - 7000 rpm实验室匀胶机实验室匀胶机Photoresist Spin CoaterPhotoresist Spin CoaterEBR: Edge bead removal边缘修复滴胶滴胶光刻胶吸回光刻胶吸回Photoresist Spin CoatingPhotoresist Spin CoatingPhotoresist Spin CoatingPhotoresist Spin CoatingPhotoresist Spin CoatingPhotoresist Spin CoatingEdge Bead Removal (EBR)Edge Bead Removal (EBR)PR spread to the edges and backside PR could flakes off during mechanical handling and causes particles Front and back chemical EBR Front optical EBREdge Bead RemovalEdge Bead RemovalEdge Bead RemovalEdge Bead RemovalReady For Soft BakeReady For Soft BakeOptical Edge Bead RemovalOptical Edge Bead Removal• After alignment and exposure • Wafer edge expose (WEE) • Exposed photoresist at edge dissolves during development Optical Edge Bead RemovalOptical Edge Bead RemovalDeveloper Spin OffDeveloper Spin Off光刻4－前烘光刻4－前烘蒸发光刻胶中的溶剂 Evaporating most of solvents in PR 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Solvents help to make a thin PR but absorb radiation and affect adhesion Soft baking time and temperature are determined by the matrix evaluations前烘要适度前烘要适度过多的烘烤使光刻胶聚合，感光灵敏度变差Over bake: polymerized, less photo-sensitivity 烘烤不够影响黏附性和曝光 Under bake: affect adhesion and exposure Soft BakeSoft BakeHot plates Convection oven Infrared oven Microwave ovenBaking SystemsBaking SystemsHot PlatesHot PlatesWidely used in the industry Back side heating, no surface “crust” In-line track systemWafer CoolingWafer CoolingNeed to cool down to ambient temperature Water-cooled chill plate Silicon thermal expansion rate: 2.5x10-6/°C For 8 inch (200 mm) wafer, 1 °C change causes 0.5 um difference in diameterAlignment and ExposureAlignment and ExposureIC制造的最关键过程Most critical process for IC fabrication Most expensive tool (stepper) in an IC fab. Most challenging technology Determines the minimum feature size Currently 0.09 um and pushing to 0.045 umAlignment and Exposure ToolsAlignment and Exposure Tools接触式曝光机 Contact printer 接近式曝光机 Proximity printer 投影式曝光机 Projection printer 步进式曝光机 StepperContact PrinterContact PrinterSimple equipment Use before mid-70s Resolution: capable for sub-micron Direct mask-wafer contact, limited mask lifetime ParticlesContact PrinterContact PrinterContact PrintingContact PrintingProximity PrinterProximity Printer~ 10 um from wafer surface No direct contact Longer mask lifetime Resolution: > 3 um Proximity PrinterProximity PrinterProximity PrintingProximity PrintingProjection PrinterProjection PrinterWorks like an overhead projector Mask to wafer, 1:1 Resolution to about 1 umProjection SystemProjection SystemStepperStepperMost popular used photolithography tool in the advanced IC fabs Reduction of image gives high resolution 0.25 um and beyond Very expensiveStep-&-Repeat Alignment/ExposureStep-&-Repeat Alignment/ExposureExposure Light SourceExposure Light Source短波长Short wavelength 高亮度High intensity 稳定Stable 高压汞灯High-pressure mercury lamp 激光源Excimer laser汞灯光谱分析 Spectrum of the Mercury Lamp汞灯光谱分析 Spectrum of the Mercury LampPhotolithography Light SourcesPhotolithography Light SourcesStanding Wave EffectStanding Wave Effect光线发生干涉、反射 Interference of the incident and reflection lights 周期性的出现过曝光或者曝光不够Periodically overexposure and underexposure 影响曝光效果Affects photolithography resolution.Standing Wave IntensityStanding Wave IntensityStanding Wave Effect on PhotoresistStanding Wave Effect on Photoresist光刻5－后烘PEB光刻5－后烘PEB光刻胶成玻璃状温度Photoresist glass transition temperature Tg Baking temperature higher than Tg 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 平衡驻波效应 平滑光刻胶侧墙，提高分辨率PEB Minimizes Standing Wave EffectPEB Minimizes Standing Wave Effect光刻6－显影Development光刻6－显影Development显影液溶剂溶解掉光刻胶中软化部分 从掩膜版转移图形到光刻胶上 三个基本步骤: – 显影 – 漂洗 – 干燥Development: ImmersionDevelopment: Immersion显影漂洗旋转干燥DevelopmentDevelopmentDevelopment ProfilesDevelopment Profiles光刻7－坚膜Hard Bake光刻7－坚膜Hard Bake蒸发光刻胶中所有溶剂 Evaporating all solvents in PR Improving etch and implantation resistance Improve PR adhesion with surface Polymerize聚合 and stabilize photoresist 光刻胶流动填充针孔PR flow to fill pinhole光刻胶热流动填充针孔光刻胶热流动填充针孔坚膜Hard Bake坚膜Hard Bake热板最为常用 检测后可在烘箱中坚膜 坚膜温度: 100 to 130 °C 坚膜时间：1 to 2 minutes 坚膜温度通常高于前烘温度，对于相同光刻胶坚膜的控制坚膜的控制坚膜不足 －光刻胶不能充分聚合 －造成较高的光刻胶刻蚀速率 －黏附性变差 过坚膜 －光刻胶流动造成分辨率变差光刻胶流动Photoresist Flow光刻胶流动Photoresist Flow过坚膜会引起太多的光刻胶流动，影响光刻的分辨率正常坚膜过坚膜问题问题如果涂胶时用错光刻胶，会发生什么问题？ 每种光刻胶都有不同的敏感性和粘性，都需要不同的旋转速率、斜坡速率、旋转时间、烘干时间和温度、曝光强度和时间、显影液和显影条件，因此图形转移将失败。光刻－图形检测Pattern Inspection光刻－图形检测Pattern Inspection• 检查发现问题，剥去光刻胶，重新开始 – 光刻胶图形是暂时的 – 刻蚀和离子注入图形是永久的 • 光刻工艺是可以返工的 • 刻蚀和注入以后就不能再返工 • 检测手段：SEM、光学显微镜问题问题为什么不能用光学显微镜检查0.25um尺寸的图形? 因为特征尺寸 (0.25 mm = 2500Å) 小于可见光的波长，可见光波长为3900 Å (紫光) to 7500 Å (红光) 图形检测图形检测未对准问题：重叠和错位 - Run-out, Run-in, 掩膜旋转，圆片旋转，X方向错位，Y方向错位 临界尺寸Critical dimension (CD) 表面不规则：划痕、针孔、瑕疵和污染物未对准问题未对准问题临界尺寸Critical Dimension临界尺寸Critical Dimension图形检测图形检测通过图形检测，即可进入下一步工艺 刻蚀或离子注入光刻间全部流程光刻间全部流程未来趋势 Future Trends未来趋势 Future Trends更小特征尺寸 Smaller feature size 更高分辨率 Higher resolution 减小波长 Reducing wavelength 采用相移掩膜 Phase-shift mask光衍射光衍射光衍射影响分辨率光衍射的减小光衍射的减小波长越短，衍射越弱 光学凸镜能够收集衍射光并且增强图象数值孔径 Numerical Aperture NA数值孔径 Numerical Aperture NANA：表示凸镜收集衍射光的能力 NA = 2 r0 / D r0 ： 凸镜的半径 D ：目标与凸镜的距离 NA越大，凸镜收集更多的衍射光，产生更尖锐的图形分辨率 Resolution分辨率 Resolution能重复得到最小特征尺寸 The achievable, repeatable minimum feature size 被光源的波长和系统的孔径确定 Determined by the wavelength of the light and the numerical aperture of the system. The resolution can be expressed as：R=K1λ/NA K1 is the system constant, λis the wavelength of the light, NA = 2 r0 /D, is the numerical aperture NA: capability of lens to collect diffraction light 举例1举例1K1 = 0.6， R=K1λ/NA 提高分辨率提高分辨率Increase NA – Larger lens, could be too expensive and unpractical – Reduce DOF（焦深） and cause fabrication difficulties Reduce wavelength – Need develop light source, PR and equipment – Limitation for reducing wavelength – UV to DUV, to EUV, and to X-Ray Reduce K1 – Phase shift maskWavelength and Frequency of Electromagnetic WaveWavelength and Frequency of Electromagnetic WaveRF: Radio frequency; MW: Microwave; IR: infrared; and UV: ultraviolet焦深 Depth of Focus焦深 Depth of Focus焦深 Depth of focus焦深 Depth of focusThe range that light is in focus and can achieve good resolution of projected image Depth of focus can be expressed as: DOF=K2λ/[2(NA)2] 举例2举例2K2 = 0.6， DOF=K2λ/[2(NA)2] 0.3630.161焦深 Depth of Focus焦深 Depth of FocusSmaller numerical aperture, larger DOF – Disposable cameras with very small lenses – Almost everything is in focus – Bad resolution Prefer reduce wavelength than increase NA to improve resolution High resolution, small DOF Focus at the middle of PR layer Focus on the Mid-Plain to Optimize the ResolutionFocus on the Mid-Plain to Optimize the Resolution表面平坦化 Surface Planarization Requirement表面平坦化 Surface Planarization RequirementHigher resolution requires – Shorter l – Larger NA Both reduces DOF Wafer surface must be highly planarized. CMP is required for 0.25 mm feature patterning.I-line and DUVI-line and DUVMercury i-line, 365 nm – Commonly used in 0.35 mm lithography DUV KrF excimer laser, 248 nm – 0.25 mm, 0.18 mm and 0.13 mm lithography ArF excimer laser,193 nm – Application: < 0.13 mm F2 excimer laser 157 nm – Still in R&D, < 0.10 mm applicationI-line and DUVI-line and DUVSiO2 strongly absorbs UV when l < 180 nm Silica lenses and masks can’t be used 157 nm F2 laser photolithography – Fused silica with low OH concentration, fluorine doped silica, and calcium fluoride (CaF2), – With phase-shift mask, even 0.035 mm is possible Further delay next generation lithographyNext Generation Lithography (NGL)Next Generation Lithography (NGL)极度紫外光刻 Extreme UV (EUV) lithography X射线光刻 X-Ray lithography 电子束光刻 Electron beam (E-beam) lithographyFuture TrendsFuture Trends相移掩膜 Phase Shift Mask相移掩膜 Phase Shift Maskd(nf - 1) = λ/2 nf : Refractive index of phase shift coatingPhase Shift Mask PatterningPhase Shift Mask PatterningEUV 深紫外EUV 深紫外l = 10 to 14 nm Higher resolution Projected application ~ 2010 0.1 mm and beyondX-ray lithographyX-ray lithographySimilar to proximity printer Difficult to find pure X-ray source Challenge on mask making Unlikely will be used in productionX-ray PrintingX-ray PrintingE-BeamE-BeamUsed for making mask and reticles Smallest geometry achieved: 0.014 mm Direct print possible, no mask is required – Low throughput Scattering exposure system (SCALPEL) looks promising – Tool development – Reticle making – Resist development光刻总结光刻总结光刻：临时的图形转移过程 IC生长中最关键的工艺 需要：高分辨率、低缺陷密度 光刻胶：正和负 工艺过程：预烘、底胶旋涂、PR旋涂、前烘、对准曝光、后烘PEB、显影、坚膜、检测 下一代光刻技术：EUV和电子束光刻Etch 刻蚀Etch 刻蚀熟悉刻蚀术语 比较：干法刻蚀、湿法刻蚀 IC工艺中四种被刻蚀的材料和主要的刻蚀剂 IC工艺的刻蚀过程 注意刻蚀工艺中的危险Definition of EtchDefinition of EtchProcess that removes material from surface Chemical, physical or combination of the two Selective or blanket etch Selective etch transfers IC design image on the photoresist to the surface layer on wafer Other applications: Mask making, Printed electronic board, Artwork, etc.栅掩膜对准 Gate Mask Alignment栅掩膜对准 Gate Mask Alignment栅掩膜曝光 Gate Mask Exposure栅掩膜曝光 Gate Mask ExposureDevelopment/Hard Bake/InspectionDevelopment/Hard Bake/InspectionEtch Polysilicon刻蚀多晶硅Etch Polysilicon刻蚀多晶硅Etch Polysilicon 继续Etch Polysilicon 继续Strip Photoresist 剥去光刻胶Strip Photoresist 剥去光刻胶Ion ImplantationIon ImplantationRapid Thermal AnnealingRapid Thermal Annealing刻蚀术语刻蚀术语刻蚀速率 选择比 刻蚀均匀性 刻蚀剖面 湿法刻蚀 干法刻蚀 RIE：反应离子刻蚀刻蚀速率刻蚀速率Δd = d0 - d1 (Å) is thickness change and t is etch time (min) PE-TEOS PSG film, 1 minute in 6:1 BOE at 22 °C, Before etch, t = 1.7 mm, After wet etch, t = 1.1 mm刻蚀均匀性刻蚀均匀性圆片上和圆片间的重复性 Standard Deviation Non-uniformity偏差不均匀性 N points measurements Max-Min Uniformity 最大最小均匀性 刻蚀选择比刻蚀选择比Selectivity is the ratio of etch rates of different materials. Selectivity to underneath layer and to photoresist 举例举例Etch rate for PE-TEOS PSG film is 6000 Å/min, etch rate for silicon is 30 Å/min, PSG to silicon 刻蚀剖面刻蚀剖面刻蚀剖面刻蚀剖面nullWet Etch 湿法刻蚀Wet EtchWet EtchChemical solution to dissolve the materials on the wafer surface The byproducts are gases, liquids or materials that are soluble in the etchant solution. Three basic steps, etch, rinse and dry Wet EtchWet EtchPure chemical process, isotropic profile Was widely used in IC industry when feature size was larger than 3 micron Still used in advanced IC fabs – Wafer clean – Blanket film strip – Test wafer film strip and clean 湿法刻蚀剖面 Wet Etch Profiles湿法刻蚀剖面 Wet Etch ProfilesCan’t be used for feature size is smaller than 3 mm Replaced by plasma etch for all patterned etchWet Etching Silicon DioxideWet Etching Silicon DioxideHydrofluoric Acid (HF) Solution Normally diluted in buffer solution or DI water to reduce etch rate. SiO2 + 6HF ® H2SiF6 + 2H2O Widely used for CVD film quality control BOE: Buffered oxide etch WERR: wet etch rate ratio Wet Etching Silicon or PolyWet Etching Silicon or PolySilicon etch normally use mixture of nitric acid (HNO3) and hydrofluoric acid (HF) HNO3 oxidizes the silicon and HF removes the oxide at the same time. DI water or acetic acid can be used to dilute the etchant, and reduces the etch rate.Isolation FormationIsolation FormationWet Etching Silicon NitrideWet Etching Silicon NitrideHot (150 to 200 °C) phosphoric acid H3PO4 Solution High selectivity to silicon oxide Used for LOCOS and STI nitride strip