Photomask

PhotomaskBasicsTABLEOFCONTENTSSection1-Introduction1Section2-HowPhotomaskareMade4Section3-PhotomaskTypes7Section4-PartsofaPhotomask10Section5-QualityInspections15PHOTOMASKBASICSSECTION1•INTRODUCTION•WHATISAPHOTOMASK?•Photomasksarehighprecisionplatescontainingmicroscopicimagesofelectroniccircuits.Photomasksaremadefromveryflatpiecesofquartzorglasswithalayerofchromeononeside.Etchedinthechromeisaportionofanelectroniccircuitdesign.Thiscircuitdesignonthemaskisalsocalledgeometry.•WHATAREPHOTOMASKSUSEDFOR?•Photomasksareusedinwaferfabrication,mostlytomakeICs(integratedcircuits).ICsareusedinmanyproductslikecomputers,calculators,cars,cameras,andstereos.Photomasksarealsousedtomakeflatpaneldisplays,thinfilmheads,PCboards,etc.HOWAREPHOTOMASKSUSED?•Thestepsinmaking(fabricating)deviceslikeICsincludedeposition,photolithography,andetching.Duringdeposition,alayerofeitherelectricallyinsulatingorelectricallyconductivematerial(i.e.metal,polysilicon,oxide)isdepositedonthesurfaceofasiliconwafer.Thismaterialisthencoatedwithaphotosensitiveresist.•Aphotomaskisthenusedmuchthesamewayaphotographicnegativeisusedtomakeaphoto.Photolithographyinvolvesprojectingtheimageonthephotomaskontothewafer.(Iftheimageonthephotomaskisprojectedseveraltimessidebysideontothewafer,thisisknownassteppingandthephotomaskiscalledareticle.)•Thewaferisdevelopedandthenetchedtoremovematerialfromtheareasexposedwiththephotomaskimage.•Anewlayerofmaterialandresististhendepositedonthewaferandtheimageonthenextphotomaskisprojectedontoit.Againthewaferisdevelopedandetched.•Thisprocessisrepeateduntilthecircuitiscomplete.•Thecompletecircuitiscalledadevice.Thedeviceandeachlayerofthecircuitarenamed-thephotomaskusedtomakeaparticularlayerwillhaveboththedevice•nameandthelayername.(Thephotomaskwillsometimesalsobereferredtoaslayer.)Thewaferisnowcutintoindividualdevices,alsocalledcomponentsorchips.SECTION2HOWPHOTOMASKSAREMADE•CIRCUITDESIGN:•Thecustomerdesignsthecircuitanddigitizestheinformation.Thecustomerthensendsusthedigitizeddatacontainingthedesignforeachlayer.Thedatacanbesentonafloppydisk,magnetictape,cassetteorviamodem.•DATAPREPARATION:•Maskmakertakesthecustomer'sdataandformatsitforthewrite(lithography)toolsorsystems.Thisincludesfracturingthedata,sizingthedataifneeded,rotatingthedataifneeded,addingfiducialsandinternalreferencemarks,andmakingajobdeck(instructionsfortheplacementofallthedifferentpatternsonthemask).•Fracturingthedatameanstranslatingthecustomerdataintoalanguagethewritetoolcanunderstand.Thewritesystemusesrectanglesandtrapezoids-sothecustomerdataisdividedup(fractured)intotheseshapes.•Thejobdeckwiththefractureddataisputonamagnetictapeandsenttothewriteareaorpulleddirectlytothemachinesusingnetworksoftware.MATERIALUSEDTOMAKEPHOTOMASKS:•Therearefourtypesofmaterialusedtomakephotomasks;quartz(themostcommonlyusedandmostexpensive),LE,sodalime,andwhitecrown.Themasksizescanrangefrom3inchessquareto7inchessquareand7.25inchesround.•Thethicknessofthemasksrangesfrom60milsto250mils.Currentlythemostcommonsizesofmasksusedare5inchessquare90milsthickand6inchessquare250milsthick.•Thequartzorglass(orsubstrate)hasalayerofchromeononeside.ThechromeiscoveredwithanAR(anti-reflective)coatingandaphotosensitiveresist.•Thesubstratewithchrome,AR,andresistisknownasablank.LITHOGRAPHY:•Lithographyistheprocessofwritingthecircuitdesign(geometry)ontothemask.Thelithographyorwriteequipment(E-beamorCore)writesthegeometryontotheplatebyexposingtheresistwithanelectronbeamorlaser.Thisexposurechangesthemolecularcompositionoftheresist.Duringthedevelopingprocessanyresistthathasbeenexposedwillberemoved.•Themaskisnowetched.EtchingremovesthechromeandARwherevertheresisthasbeenremoved.•Strip,thefinalstepinmakingaphotomask,removesalltheresistfromthemask.Note:Areaswherethechromehasbeenremovedarereferredtoasclearorglass.AreaswherethechromeandARremainarereferredtoaschrome,darkoropaque.SECTION3PHOTOMASKTYPES•1XMASTERS•1XMastersareusedonprojectionalignersmademostlybyPerkinElmer.The1XMastermaskhasmanyrepetitionsoftheprimarypattern(theprinciplecircuitdesign).Eachcompletepatternisalsocalledadie.Unlikereticleswhicharestepped,thepatternsonthe1XMasterphotomaskareprojectedontothewaferonlyonce–alltherepetitionsofthedeviceareonthemaskitself.•Examplesof1XMasters:•Notethelargenumberofdiceonthesemasks.Thefiveuniquediesinboththediagramsarethetestpatternsortestdies.•Arrayreferstothearrangementofthediesinrowsandcolumnsonthemask.•5XRETICLES•Thepatternsona5Xreticlearesteppedontothewafer-projectedmanytimesplacingthepatternsnexttooneanother.Thepatternsonthe5Xreticlearereduced5timeswhenprojectedontothewafer.Thismeansthediesonthephotomaskare5times(5X)largerthantheyareonthefinalproduct.Thereareotherkindsofreductionreticles(2X,4X,and10X),butthe5Xisthemostcommonlyused.Reductionreticlesareusedonavarietyofsteppers,themostcommonbeingASM,Canon,Nikon,andGCA.•Examplesof5XReticles:•Notehowmuchlargerthediesareona5Xcomparedtoa1XMaster.•UT1XRETICLES•AUT1XreticleisdesignedforuseonanUltratechstepper.PatternsontheUT1Xreticlearesteppedontothewafer.Thesepatternsarepr