15 Multilevel silicon diffractive optics for terahertz waves

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MultilevelsilicondiffractiveopticsforterahertzwavesE.D.Walsbya)DepartmentofElectrical&ComputerEngineering,UniversityofCanterbury,Christchurch,NewZealand

S.Wang,J.Xu,andT.YuanDepartmentsofPhysics,AppliedPhysicsandAstronomy,RensselaerPolytechnicInstitute,Troy,NewYork

R.BlaikieandS.M.DurbinDepartmentofElectrical&ComputerEngineering,UniversityofCanterbury,Christchurch,NewZealand

X.-C.ZhangDepartmentsofPhysics,AppliedPhysicsandAstronomy,RensselaerPolytechnicInstitute,Troy,NewYork

D.R.S.CummingDepartmentofElectronicsandElectricalEngineering,UniversityofGlasgow,Glasgow,UnitedKingdom

͑Received28May2002;acceptedfor9September2002͒

AmultilevelmicrofabricationprocesshasbeendevelopedtoproducesiliconFresnellensesforterahertzwaves.Arepeatedbinaryfabricationprocesswasusedtocreatelenseswithuptoeightlevelsincomplexityandtheselenseshavebeencomparedtobothlesscomplexstructuresandrefractiveopticlenses.ThemicrofabricationrequireddeepreactiveionetchingandmultilevelresistprocessingusingSU8photoresist.Atthedesignfrequencyof1THzaneight-levellensshowedsignificantimprovementinintensityatthefocusandhadmuchreducedenergylostintosidelobescomparedwithrefractivelenses.©2002AmericanVacuumSociety.͓DOI:10.1116/1.1518021͔

I.INTRODUCTIONDiffractiveopticsaremostbeneficialinmanyopticalsys-temswheretheycanperformtasksotherwisenotpossiblewithconventionalrefractiveoptics.1Asofyettheuseof

diffractiveopticshasnotbeenwidelyexploredinthetera-hertz͑THz͒frequencyrange.Thisexcitingfieldhasmanyapplicationsinwhichthediversitysuppliedbyadiffractiveopticstructureismuchdesired.2Currentlybeammanipula-

tionatTHzfrequenciesisrestrictedtoparabolicmirrorsandrefractiveopticswhichusuallyaremadefromhighdensitypolyethyleneorsilicon.Theformerisoftenusedasitisaninexpensivematerialandcanbemachinedeasilyusingacomputerizedlathe.Howeveratanyfrequencythereisal-waysadesiretousediffractiveFresnellensesastheyoffermanyadvantagesincludinglowfnumber,reducedformfac-torandtheabilitytoeasilycreatelensarraysorotherarbi-trarybeamformation.Becausesiliconmicroelectricalmechanicalstructure͑MEMS͒technologyisrelativelymature,itisanobviouscandidateformakingdiffractiveTHzopticalelements.Usingsiliconasamaterialhasotherbenefits:itshighrefractiveindexmeansthedepthofthestructuringisminimized;also,thefactthatthelensescanbecreatedonconventionalwafersmeanshighqualitymaterialinwhichtofabricatethemal-readyexists.Siliconallowstheformationofalargediameteroptic–orlargearraysofsmalleroptics–withextremelyhighsurfacequalityandaverysmallthickness.Binaryopticfabricationisawell-knowntechniqueforfabricatingmultipleleveldiffractivestructures.3Thereisa

desiretocreatestructuresofgreatercomplexity,astheper-formancegainoversimpletwolevelstructuresorzoneplatesissignificant.Thedimensionsanddepthofthesestructuresis

dependentonthefrequencyusedandthefocallengthde-sired.Varyingcombinationsofthiswillcreatedifferentfab-ricationchallengesasitwillrequiretheuseofadifferentrangeofmicrofabricationprocesses.4Thisworkdetailsthe

productionofsiliconFresnellensesdesignedtooptimallyperformat1THz.Atthisfrequencythewavelengthis300␮mandthesiliconetchdepthrequiredtoproducea␲phase

shiftis124␮m.Constructingamultiplelevelstructureof

thistotaldepthintroducessomeuniquechallengesinetch-ing,planarizationandlithography.Fresnellensesoftwo,four,andeightlevelshavebeenfabricatedwithadiameterof30mmandafocallengthof26mm.Thesewerecomparedtoasimplezone-plateofthesamedimensionsandrefractivelensesmadefromsiliconandpolyethyleneinapulsedTHzimagingsystem.Analyzingthesedifferentstructuresontheirfocalplanegivesanexcel-lentcomparisonofthetemporalandspatialresponse.Theremainderofthisarticleisstructuredasfollows.Thenextsectiondetailsthefabricationtechniquedesignedtofab-ricatemultiple-levelTHzFresnellenses.Followingthistest-ingresultsarepresentedforvariousdiffractivelensesshow-ingtheperformanceenhancementsmultiple-levellensesgiveoversimpletwo-levelopticsandzoneplates.Atestcompari-sonwithconventionalrefractiveopticsindicatessimilarin-tensityandanimprovementinaberrationatthefocus.

II.FABRICATIONTECHNIQUEA.LithographyInbinaryopticfabricationeachlithographyandetchingstageincreasesthestructurecomplexitybyafactorof2.Hencetheeight-levelstructuresthatweremadetookatotalofthreelithographyandetchingstagestocomplete.Eachstageinvolvesusingamaskwithfeaturesofhalfthedimen-sionandhalftheetchdepthofthepreviousstage.Henceasa͒Authortowhomcorrespondenceshouldbeaddressed;electronicmail:

e.walsby@elec.canterbury.ac.nz

27802780J.Vac.Sci.Technol.B20„6…,NovÕDec20021071-1023Õ2002Õ20„6…Õ2780Õ4Õ$19.00©2002AmericanVacuumSociety