激光直写技术

Laserwritingsystemsandtechnologiesforfabricationofbinaryandcontinuous-reliefdiffractiveopticalelementsA.G.Poleshchuk*,V.P.KorolkovInstituteofAutomationandElectrometrySBRAS,Novosibirsk,630090,RussiaABSTRACTPrecisionlaser-writingsystemsoperatedinpolarcoordinatesanddirectwritingtechnologiesforfabricationofdiffractiveopticalelementsandcomputergeneratedhologramshavebeendescribed.Thesesystemscanmanufacturecontinuous-reliefandbinarymicrostructureswithminimumfeaturesizesoflessthan0.6µmandlaserbeampositioningaccuracyof0.05µmover300-mmsubstrates.Hardwareandsoftwareofthesystempermittowriteondifferenttypesofphotosensitiveandthermalrecordingmaterials.Severalexamplesoffabricateddiffractiveelementshavebeenpresented.Keywords:diffractiveopticalelements,directlaserwriting,microlensarrays,aspherictesting,computer-generatedholograms.1.INTRODUCTIONAtpresent,oneofthemostpromisingtrendsindevelopmentofopticsisawideapplicationofcomputer-generatedholograms(CGH)ordiffractiveopticalelements(DOE).Theseelementsarethinreliefphasestructuresandhavebeenalreadywidelyusedinopticalcomputingdevices,opticalmemory,displays,sensors,etc.Theirpotentialitiesdependinmanyrespectsonthefabricationtechnologywhichshouldensuredesiredtechnicalparameters.Untilrecently,DOEswerefabricatedbymeansofimagepatterngeneratorsspecializedfortasksofsemiconductorindustry.Theyexposeresistfilmcoveringamaskinglayerbygeneratingofimagesintheshapeofrectangles,trapezes,andstraightlinesorientedalongtwoorthogonalaxesx-yofthescanningsystem.ApplicationofthesesystemsforfabricationofmosttypesofDOEsleadstolightscatteringatcornersofmicroelementsand,consequently,additionalnoisesanddistortionsintheimagearegenerated.TherearesometypesofDOEsforwhosedescriptionthepolarcoordinatesystemismorepreferredthantheCartesianone.Forexample,CGHsdesignedtotestandcertifyasphericwavefrontsoftheprimarymirrorsinmoderntelescopesmusthavethecirculardiffractivestructurefabricatedwithanaccuracyofnoworsethan0.1µmatminimalperiodoflessthan1µm,andoverallsizesofseveralhundredmillimeters1.TheflatnessoftheopticalsubstratefortheCGHsmustbenoworsethanλ/20.Suchelementscanbecurrentlyfabricatedwiththedesiredprecisionandqualityonlybymeansofwriterswithcircularscanningoffocusedbeam(eitherlaserorelectronbeam)orbydiamondturning2.Inthepresentpaperweconsideronlycircularlaserwritingsystems(CLWS)andsomeoftheproposedmethodsforfabricationofthediffractiveelements.FirstCLWSwascreatedinthemiddle1970s3inordertofabricateaxiallysymmetriccomputer-generatedhologramstotestasphericoptics.ThisworkstimulateddevelopmentofvarioustypesofCLWSsandfabricationtechnologies4,5,6,7specializedforDOEs.DevelopmentofthemethodsforDOEfabricationinthepolarcoordinatesystembeganattheInstituteofAutomationandElectrometryoftheSiberianBranchoftheRussianAcademyofSciences(IA&ESBRAS)insecondhalfoftheseventies8.Themaineffortwasfocusedoncreationofhigh-precisionCLWSsanddevelopmentofthethermochemicaltechnologyforresistlessmanufacturingmasksinthinchromiumfilms9.Atpresent,CLWSsdevelopedbytheIA&ESBRAStogetherwiththeTechnologicalDesignInstituteSBRASareusedforDOEfabricationinRussiaandinsomeothercountries(Italy,Germany,andChina)10,11,12.Ourdiffractiveelementswereappliedforcertificationof6.5-mand8.4-mmainmirrorsoftelescopesmadeintheStewardObservatoryMirrorLabofArizonaUniversity(USA)1,13.Oureffortswerealsodirectedonthedevelopmentofnewmethodsformanufacturingofcontinuous-reliefDOEs14.*[e-mail];tel:[phone],Fax:[phone];:Laser-assistedMicro-andNanotechnologies,editedbyVladislavPanchenko,OlegLouchev,SergeiMalyshev,Proc.ofSPIEVol.6732,67320X,(2007)·0277-786X/07/$18·doi:10.1117/12.751930Proc.ofSPIEVol.673267320X-1DownloadedfromSPIEDigitalLibraryon30Oct2010to61.175.193.50.TermsofUse:(asshowninFig.2).Thefirstwayisamaskfabricationandthenphotolithographicprocessing.Themaskscanbeamplitudeorgrayscale.ThiswayhasagoodpreferenceforthemassproductionofDOEswithaveragequalityandspecifications.Thesecondwayisadirectlaserwritingofbinaryorcontinuous-relief(blazed)profiles.ThiswayismorepreferableforfabricationofuniqueDOEswithexcellentopticalspecifications,e.g.forwavefrontshaping.Bellowwewillmoreconcentrateonthesecondway–applicationoflaserwritingsystemsfordirectfabricationofhigh-precisionDOEs.Fig.1.MethodsforfabricationofbinaryandblazedDOEs.3.SPECIALIZEDLASERWRITINGSYSTEMFig.2showsasimplifiedfunctionaldiagramofthedevelopedCLWS.Wewillconsiderbrieflyitsprincipalunitsforbettercomprehensionofthewritingmethods.Rotationunit.Thesubstratewiththedepositedrecordinglayerisfixedonthefaceplateoftheairbearingspindlebyavacuumchuck.Theangularencoderandthesystemoffrequencymultiplicationandphase-lockcontrolformcloc