Design and fabrication of ultrathin silicon-nitride membranes for use in UV-visible airgap-based MEMS optical filters

摘要

MEMS-basedairgapopticalfiltersarecomposedofquarter-wavethickhigh-indexdielectricmembranesthatareseparatedbyairgaps.Themainchallengeinthefabricationofthesefiltersistheintertwinedopticalandmechanicalrequirements.Thethicknessofthelayersdecreaseswithdesignwavelength,whichmakestheopticalperformanceintheUVmoresusceptibletofabricationtolerances,suchasthicknessandcompositionofthedepositedlayers,whiletheabilitytosustainacertainlevelofresidualstressbythestructuralstrengthbecomesmorecritical.Silicon-nitridehasacomparativelyhighYoung'smodulusandgoodopticalproperties,whichmakesitasuitablecandidateasthemembranematerial.However,boththemechanicalandopticalpropertiesinasilicon-nitridefilmstronglydependonthespecificsofthedepositionprocess.Adesigntrade-offisrequiredbetweenthemechanicalstrengthandtheindexofrefraction,bytuningthesiliconcontentinthesilicon-nitridefilm.However,alsothebenefitofahighindexofrefractioninasilicon-richfilmshouldbeweighedagainsttheincreasedUVopticalabsorption.Thisworkpresentsthedesign,fabrication,andpreliminarycharacterizationofoneandthreequarter-wavethicksilicon-nitridemembraneswithaone-quarterairgapanddesignedtogiveaspectralreflectanceat400nm.ThePECVDsilicon-nitridelayerswereinitiallycharacterized,andthedatawasusedfortheopticalandmechanicaldesignoftheairgapfilters.ACMOScompatibleprocessbasedonpolysiliconsacrificiallayerswasusedforthefabricationofthemembranes.Opticalcharacterizationresultsarepresented...