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2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaSemiconductor Semiconductor Manufacturing TechnologyManufacturing TechnologyMichael Quirk&Julian Serda Michael Quirk&Julian Serda October 2001 by Prentice HallOctober 2001 by Prentice HallChapter 13Chapter 13 Photolithography:Surface Photolithography:Surface Preparation to Soft BakePreparation to Soft Bake 2000 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian Serda 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaObjectivesAfter studying the material in this chapter,you will be able to:1.Explain the basic concepts for photolithography,including process overview,critical dimension generations,light spectrum,resolution and process latitude.2.Discuss the difference between negative and positive lithography.3.State and describe the eight basic steps to photolithography.4.Explain how the wafer surface is prepared for photolithography.5.Describe photoresist and discuss photoresist physical properties.6.Discuss the chemistry and applications of conventional i-line photoresist.7.Describe the chemistry and benefits of deep UV(DUV)resists,including chemically amplified resists.8.Explain how photoresist is applied in wafer manufacturing.9.Discuss the purpose of soft bake and how it is accomplished in production.2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaWafer Fabrication Process FlowImplantDiffusionTest/SortEtchPolishPhotoCompleted waferUnpatterned waferWafer startThin FilmsWafer fabrication(front-end)Used with permission from Advanced Micro DevicesFigure 13.1 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPatterning ProcessPhotomaskReticleCritical Dimension GenerationsLight SpectrumResolutionOverlay AccuracyProcess LatitudePhotolithography Concepts 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPhotomask and Reticle for MicrolithographyPhotograph provided courtesy of Advanced Micro Devices4:1 Reticle1:1 MaskPhoto 13.1 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaThree Dimensional Pattern in PhotoresistLinewidthSpaceThicknessSubstratePhotoresistFigure 13.2 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaSection of the Electromagnetic SpectrumVisibleRadio wavesMicro-wavesInfraredGamma raysUVX-raysf(Hz)1010101010101010101046810121416221820(m)420-2-4-6-8-14-10-1210101010101010101010365436405248193157ghiDUVDUVVUV(nm)Common UV wavelengths used in optical lithography.Figure 13.3 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaImportant Wavelengths for Photolithography ExposureTable 13.1 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaImportance of Mask Overlay AccuracyPMOSFETNMOSFETCross section of CMOS inverterTop view of CMOS inverterThe masking layers determine the accuracy by which subsequent processes can be performed.The photoresist mask pattern prepares individual layers for proper placement,orientation,and size of structures to be etched or implanted.Small sizes and low tolerances do not provide much room for error.Figure 13.4 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPhotolithography ProcessesNegative ResistWafer image is opposite of mask imageExposed resist hardens and is insolubleDeveloper removes unexposed resistPositive ResistMask image is same as wafer imageExposed resist softens and is solubleDeveloper removes exposed resist 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaNegative LithographyUltraviolet lightIslandAreas exposed to light become crosslinked and resist the developer chemical.Resulting pattern after the resist is developed.WindowExposed area of photoresistShadow on photoresistChrome island on glass maskSilicon substrateSilicon substratePhotoresistPhotoresistOxideOxidePhotoresistPhotoresistOxideOxideSilicon substrateSilicon substrateFigure 13.5 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPositive LithographyFigure 13.6 photoresistsilicon substrateoxideoxidesilicon substratephotoresistUltraviolet lightIslandAreas exposed to light are dissolved.Resulting pattern after the resist is developed.Shadow on photoresistExposed area of photoresistChrome island on glass maskWindowSilicon substrateSilicon substratePhotoresistPhotoresistOxideOxidePhotoresistPhotoresistOxideOxideSilicon substrateSilicon substrate 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaRelationship Between Mask and ResistDesired photoresist structure to be printed on wafer WindowSubstrateIsland of photoresistQuartzChromeIslandMask pattern required when using negative photoresist(opposite of intended structure)Mask pattern required when using positive photoresist(same as intended structure)Figure 13.7 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaClear Field and Dark Field MasksSimulation of contact holes(positive resist lithography)Simulation of metal interconnect lines(positive resist lithography)Clear Field MaskDark Field MaskFigure 13.8 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaEight Steps of PhotolithographyTable 13.2 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaEight Steps of Photolithography8)Develop inspect5)Post-exposure bake6)Develop7)Hard bakeUV LightMask4)Alignment and ExposureResist2)Spin coat3)Soft bake1)Vapor primeHMDSFigure 13.9 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPhotolithography Track SystemPhoto courtesy of Advanced Micro Devices,TEL Track Mark VIIIPhoto 13.2 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaVapor PrimeThe First Step of Photolithography:Promotes Good Photoresist-to-Wafer AdhesionPrimes Wafer with Hexamethyldisilazane,HMDSFollowed by Dehydration BakeEnsures Wafer Surface is Clean and Dry 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaSpin CoatProcess Summary:Wafer is held onto vacuum chuckDispense 5ml of photoresistSlow spin 500 rpmRamp up to 3000 to 5000 rpmQuality measures:timespeedthicknessuniformityparticles and defectsVacuum chuckSpindle connected to spin motorTo vacuum pumpPhotoresist dispenserFigure 13.10 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaSoft bakeCharacteristics of Soft Bake:Improves Photoresist-to-Wafer AdhesionPromotes Resist Uniformity on WaferImproves Linewidth Control During EtchDrives Off Most of Solvent in PhotoresistTypical Bake Temperatures are 90 to 100C For About 30 SecondsOn a Hot PlateFollowed by Cooling Step on Cold Plate 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaAlignment and ExposureProcess Summary:Transfers the mask image to the resist-coated waferActivates photo-sensitive components of photoresistQuality measures:linewidth resolutionoverlay accuracyparticles and defectsUV light sourceMaskResistFigure 13.11 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPost-Exposure BakeRequired for Deep UV ResistsTypical Temperatures 100 to 110C on a hot plateImmediately after ExposureHas Become a Virtual Standard for DUV and Standard Resists 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPhotoresist DevelopmentProcess Summary:Soluble areas of photoresist are dissolved by developer chemicalVisible patterns appear on wafer-windows-islandsQuality measures:-line resolution-uniformity-particles and defectsVacuum chuckSpindle connected to spin motorTo vacuum pumpDevelop dispenserFigure 13.12 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaHard BakeA Post-Development Thermal BakeEvaporate Remaining SolventImprove Resist-to-Wafer AdhesionHigher Temperature(120 to 140C)than Soft Bake 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaDevelop InspectInspect to Verify a Quality PatternIdentify Quality Problems(Defects)Characterize the Performance of the Photolithography ProcessPrevents Passing Defects to Other AreasEtchImplantRework Misprocessed or Defective Resist-coated WafersTypically an Automated Operation 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaVapor PrimeWafer CleaningDehydration BakeWafer PrimingPriming TechniquesPuddle Dispense and SpinSpray Dispense and SpinVapor Prime and Dehydration Bake 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaEffect of Poor Resist Adhesion Due to Surface ContaminationFigure 13.13 Resist liftoff 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaHMDS Puddle Dispense and SpinPuddle formationSpin wafer to remove excess liquidFigure 13.14 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaHMDS Hot Plate Dehydration Bake and Vapor PrimeWaferExhaustHot plateChamber coverProcess Summary:Dehydration bake in enclosed chamber with exhaustHexamethyldisilazane(HMDS)Clean and dry wafer surface(hydrophobic)Temp 200 to 250CTime 60 sec.Figure 13.15 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaThe Purpose of Photoresist in Wafer FabTo transfer the mask pattern to the photoresist on the top layer of the wafer surfaceTo protect the underlying material during subsequent processing e.g.etch or ion implantation.2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaSuccessive Reductions in CDs Lead to Progressive Improvements in PhotoresistBetter image definition(resolution).Better adhesion to semiconductor wafer surfaces.Better uniformity characteristics.Increased process latitude(less sensitivity to process variations).2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPhotoresistTypes of PhotoresistNegative Versus Positive PhotoresistsPhotoresist Physical PropertiesConventional I-Line PhotoresistsNegative I-Line PhotoresistsPositive I-Line PhotoresistsDeep UV(DUV)PhotoresistsPhotoresist Dispensing MethodsSpin Coat 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaTypes of PhotoresistsTwo Types of PhotoresistPositive ResistNegative ResistCD CapabilityConventional ResistDeep UV ResistProcess ApplicationsNon-critical LayersCritical Layers 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaNegative Versus Positive ResistsNegative ResistWafer image is opposite of mask imageExposed resist hardens and is insolubleDeveloper removes unexposed resistPositive ResistMask image is same as wafer imageExposed resist softens and is solubleDeveloper removes exposed resistResolution IssuesClear Field Versus Dark Field Masks 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPhotoresist Physical CharacteristicsResolutionContrastSensitivityViscosityAdhesionEtch resistanceSurface tensionStorage and handlingContaminants and particles 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaResist ContrastPoor Resist ContrastSloped wallsSwellingPoor contrastResistFilmGood Resist ContrastSharp wallsNo swellingGood contrastResistFilmFigure 13.16 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaSurface TensionLow surface tension High surface tensionfrom low molecular from high molecular forces forcesFigure 13.17 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaComponents of Conventional PhotoresistAdditives:chemicals that control specific aspects of resist materialSolvent:gives resist its flow characteristicsSensitizers:photosensitive component of the resist materialResin:mix of polymers used as binder;gives resist mechanical and chemical propertiesFigure 13.18 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaNegative Resist Cross-LinkingAreas exposed to light become crosslinked and resist the developer chemical.Unexposed areas remain soluble to developer chemical.Pre-exposure-photoresistPost-exposure-photoresistPost-develop-photoresistUVOxidePhotoresistSubstrateCrosslinksUnexposedExposedSolubleFigure 13.19 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaPAC as Dissolution Inhibitor in Positive I-Line ResistResist exposed to light dissolves in the developer chemical.Unexposed resist,containing PACs,remain crosslinked and insoluble to developer chemical.Pre-exposure+photoresistPost-exposure+photoresistPost-develop+photoresistUVOxidePhotoresistSubstrateSoluble resist ExposedUnexposedPACFigure 13.20 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaGood Contrast Characteristics of Positive I-line PhotoresistPositive Photoresist:Sharp wallsNo swellingGood contrastFilmResistFigure 13.21 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaDUV Emission Spectrum*Intensity of mercury lamp is too low at 248 nm to be usable in DUV photolithography applications.Excimer lasers,such as shown on the left provide more energy for a given DUV wavelength.Mercury lamp spectrum used with permission from USHIO Specialty Lighting ProductsFigure 13.22 100806040200248 nmRelative Intensity(%)KrF laser emission spectrumEmission spectrum of high-intensity mercury lamp120100806040200200300 400 500 600Wavelength(nm)Relative Intensity(%)g-line436 nmi-line365 nmh-line405 nmDUV*248 nm 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaChemically Amplified(CA)DUV ResistResist exposed to light dissolves in the developer chemical.Unexposed resist remains crosslinked and PAGs are inactive.Pre-exposure+CA photoresistPost-exposure+CA photoresistPost-develop+CA photoresistUVOxidePhotoresistSubstrateUnchanged ExposedUnexposedAcid-catalyzed reaction(during PEB)PAGPAGPAGPAGH+PAGPAGPAGH+H+PAGPAGFigure 13.23 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaExposure Steps for Chemically-Amplified DUV ResistTable 13.5 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian SerdaSteps of Photoresist Spin Coating3)Spin-off4)Solvent evaporation1)Resist dispense2)Spin-upFigure 13.24 2001 by Prentice HallSemiconductor Manufacturing