7-电子束光刻Electron-beam-lithography-1

Electronbeamlithography(EBL)1.Overviewandresolutionlimit.2.Electronsource(thermionicandfieldemission).3.Electronoptics(electrostaticandmagneticlens).4.Aberrations(spherical,chromatic,diffraction,astigmation).5.EBLsystems(raster/vectorscan,round/shapedbeam)Note:inthetextbook,e-beamlithographyandfocusedionbeamareputwithinonechapter,becausetheyareboth“chargedbeam”.HereIwillintroducethemseparately,inordertogiveyouaclearerconceptofeach.ECE730:Fabricationinthenanoscale:principles,technologyandapplicationsInstructor:BoCui,ECE,UniversityofWaterloo;~bcui/Textbook:Nanofabrication:principles,capabilitiesandlimits,byZhengCuiE-beamlithography(EBL)overview(directwritingwithafocusede-beam)•Electronbeamisfocusedtospotsize5nmusingelectronoptics.•Verysmallwavelength:resolutionlesslimitedbydiffraction.•Generatepatternbydirectwriting:neneedofmaskormold.•Sequentialpixel-by-pixelwriting:lowthroughput,unsuitableformassproduction.)(226.1nmVForEBLat30kVaccelerationvoltage=0.007nmForelectron:(ViselectronkineticenergyineV)Forlight:)(24.1mVeVhcForanelectronwithkineticenergyof1eV,theassociatedDeBrogliewavelengthis1.23nm,aboutathousandtimessmallerthana1eVphoton.(Note:electronrestmassenergyismc2=511keV,sorelativityisunimportantfor50keV)2•Typicalenergyforbreakingabond:10eV•Buttypicalenergyofthee-beam:10-100kV(problemsofaberrationatlowenergythatleadstolargebeamspotsizeandlowresolution,sousehighenergyforEBL)•Bondisbrokenbysecondary(includingAuger)electronswithlowenergy.Exposureofresist3E-beamlithographyfacts•Developedin1960salongwithscanningelectronmicroscope(SEM).•Breakthroughmadein1968whenapolymercalledPMMA(polymethylmethacrylate)wasdiscoveredtohavehighresolution.•Fastgrowthin1990swhen“nano”begantobecome“hot”andcomputerbecamemoreavailableforautomaticlithographycontrol.•Sincearound2000,focusedionbeam(FIB)patterningbegantocompetewithEBLinsomeapplications.•TodayEBLisstillthemostpopularnano-patterningtechniquesforacademicresearchandprototyping.4SEM/EBLsystemcomponents•Anelectrongunorelectronsourcethatsuppliestheelectrons.•Anelectroncolumnthat'shapes'andfocusestheelectronbeam.•Amechanicalstagethatpositionsthewaferundertheelectronbeam.•(optional)Awaferhandlingsystemthatautomaticallyfeedswaferstothesystemandunloadsthemafterprocessing.•Acomputersystemthatcontrolstheequipment.5EBLsystems:mostresearchtoolsarebasedonSEMSEMconversion•ConventionalSEM(30kV)•AlmostnoSEMmodification•Addbeamblanker•Addhardwarecontroller•Lowcost:$100KDedicatedEBLsystem•BasedonSEMsystem•Withperfectintegration•Interferometerstage•Focuscorrection(lasersampleheightcontrol)•Cost$1-2ME-beamwriter•Highenergycolumn(100kV)•Dedicatedelectronoptics•Highreproducibility•Automaticandcontinuous(overfewdays)writing•Highcost($5M)NPGSsystemRaithsystemVistecsystemBeamblanker:isaDCbias(42Vbetweentwoparallelplateelectrodes)perpendiculartoelectronpath,sothatelectronsaredeflectedawayfromtheaxisandthus“turnedoff”/blocked/blankedbytheaperturebelow.Thebeamneedstobeblankedsothatitwon’texposetheresistduringitsmovingtonextpatternlocation.Electronbeamlithography(EBL)1.Overviewandresolutionlimit.2.Electronsource(thermionicandfieldemission).3.Electronoptics(electrostaticandmagneticlens).4.Aberrations(spherical,chromatic,diffraction,astigmation).5.EBLsystems(raster/vectorscan,round/shapedbeam)Electronguns/sourceC:cathodeforemittingelectronsE:extractionelectrodeA1,A2:cathodelenselectrodetofocustheemittedelectronsSchematicstructureofelectrongunThreetypesofelectronguns:•Thermionicemissiongun(W,LaB6,not-sharptip).•Fieldemissiongun(cold,verysharpWtip,tunnelingcurrent).•Schottkygun(fieldassistedthermionicemission,sharptip).8Electronscanbeemittedfromafilament(emitterorcathode)bygainingadditionalenergyfromheatorelectricfield.•Whetheritisfieldemissionornotdependsontheelectricfieldnearthetipapex,whichdetermineswhethertunnelingisimportantornot.•Sharpertipleadstohigherelectricfieldneartipapex,sofieldemission(bytunneling)playsamajorrole,itisthuscalledfieldemissiongun(FEG).•Eventhermionicemissionreliesontheelectricfieldfromtheextractionelectrode,butherethermionicemissionplaysamajorrole.9Electrongun:thermionicemission(tungstenhairpinfilaments)•ThelongtimesourceofchoicehasbeentheWhairpinsource•Workingathightemperature,someelectronshavethermalkineticenergyhighenoughtoovercometheenergybarrier(workfunction)ButkTstillworkfunction4eV).At2000oC,kT=1.3810-232273/1.610-19=0.20eV.•Escapedelectronisthenextractedbytheelectricfieldgeneratedbythenearbyelectrode.•CurrentdensityJcdependsonthetemperatureandcathodeworkfunction.•Cheaptomakeanduse($12.58ea)andonlyamodestvacuumisrequired.Lasttensofhours.workfunctioneVconductionbandvacuumlevelthermionicelectronicThermionicelectronsSchematicmodelofthermionicemissionWorkfunc-tion(eV)VacuumlevelWfilamentForagoodintroduction,goto::thermionicemission(LaB6tip)(HereworkfunctionisnotedasEA,insteadof)Lowworkfunction,highmeltingpoint/Tisgood.BesidesW,singlecrystalLaB6isanotherpopulartipmaterialf