SOI—Based 3dB MMI Splitter

SOI—Based 3dB MMI Splitter SOI—Based 3dB MMI Splitter 第21卷第11期 2000年11月 半导体学报 CHINESEJOURNAIOFSEMICONDUCTORS Vo1.21,No.11 Nov.,2000 c WEIH0hg 卜胁衄MMISplitte zhen(~),YUJinzh0ng(~中),IIUThongli( ZHANGXia0一feng(张晓蝗),SHIWei(~伟).andFANGChangshui(N吕水). (1StateKeyLaboratoryintegratedt~le,’lroniesinstituteSem&onductors, 1hChineseAcademy.}Seience~,Be}i}gl00083-China, (8AlAroelectroniCenter,InstituteofSemtconductor.~,TheC8i~eseAcademyofSciences,Bingl00083-CMna) (3StateKeylaboratoryofCrystaljldatertals?ShandongUni~wrsity,dinah830lO0China) AbstractAtypeofSOl—basedMMI3dBsplitterhasbeendemonstrated.Thegeometrywas analyzedanddesignedbyeffectiveindexmethodandguidemodemethodThefabricationtoler— ancewasanalyzedtoo.Thedevicewasfabricatedandnear—fieldoutputwasobtained.Thede— viceshowslargewidthtolerance,lowlossandlowpoweruniformity. Keywords:SOl PACC:4282 CLCnumberTN25 lIntr0ducti0n integratedOptlCS MM工 AArttde1D.02534177(2000)l卜l055—05 j. 3dBsplitterisoneofthekeydevicesintheintegratedoptics.YbranchandXcross branchareboththeconventionalstructurestorealize3dBsplitter.however,whichare difficulttofabricatebecauseofthesinai1branchangle.Inrecentyears,MuhiModeInter ference(MMI)couplers.basedontheselfimagingeffect,areacceptedpopularlydueto theiradvantages,suchaslow—loss,compactsizeandlargefabricationtolerance..Cou— piersbasedonMMIstructurecanbeobtainedinvariousmaterials.suchasInP.SiO2.and GaAs.Theirexcellentpertormancehasbeenreported{as].SiliconOnInsulator【SOI) technologyisconsideredbeingapromisingtechnologytoguidewavephotonicdevices operatingintheinfrared(>1.2#m).TheSO1一CMOSintegratedcircuit,beingthefuture low—power,high—speedelectronictechnology,ensurestheavailabilityofhigh—qualitylow— COStsubstrates.BecauseofthelargeindexstepbetweenSiandSiO2(An,2.0),thin claddinglayer(<1.0#m)isusedinSO1waveguidestructures,makingthemcompatible *ProjectSupportedbyNationalNaturalScienceFoundationofChiaaUnderGrantNo89896260and89990540 WEIHong—zhen()mbornin1969HeiscurrentlyaPhD.candidateHisresearchinterestsRreinintegra edoptoelectronlcdevices.Email:weihz@redsemiaccn Received15March2000.revisednlR~uscriptreceived24April2000 ? ??f l056半导体学报21卷 withVISItechnology.IncontrasttOthesilicawaveguidetechnology,5—10?nthick claddingcoverlayerisneededduetothelowindexstep.Therefore,developmentof6pti— calandoptoeleetroniedevicesinSOItechnologymakesitpossibletOfabricatelow—COSt, monolithicoptoekctr0nicscircuits.Inthisletter,wepresentthedesign,performance analysisandfabricationofintegrated1×23dBMMIsplitterwithSOItechnology.The fabricationtoleranceisalsoanalyzed.Theprocesstechnologyisfullycompatiblewiththe siliconCMOSintegratedcircuitprocess. 2Design AnMMIcouplerisbasedontheself—imagingpropertyofamultimodewaveguide. Schematicdiagramofthe3dBMMIsplitterisshowninFig.1.TheMMIsplitterconsists ofasinglemodeinputwaveguide,aplanarmultimodewaveguideandtwosingle—modeout— putwaveguides.BecauseofthelargeindexstepofSiandSiOz,theribstructureShouldbe appliedtOtheSO1waveguidestOloadthefundamentalmodeunderacertaincondition[. SchematicoftheribS01waveguideisshowninFig.2. 一 __JMMI 一 [ 1厂 W?他ide M6Trde Waveg~ide Oust W日 FIG.1SchematicDiagramof1×2MMISplitter s .Inr FIG.2SchematicDiagramofSOIRibWaveguide IntheMMIsplitter,fundamentalmodefieldisfedintothemuhimodewaveguideat thecenter.Inthissituation,onlyevensymmetricmodesareexcited.Theimaginginthe multimodewaveguideisobtainedbylinearcombinationsoftheevensymmetricmodes.To formtwo—foldimages,thelengthofthemultinmdewaveguideshoulebe[: r加f… 一—iLlJ wherePindicatestheimagingperiodicity.Fortheshortestdevice,P=1.Wistheeffee— tirewidthofthemulrimodewaveguide,nistherefractiveindexofthemultimodeseetion. …sthefreespacewavelengthofthelight.Thetwooutputwaveguidesaresymmetrically locatedwithequalspacingwidth. AsimplemethodtOsimulatetheMMIdevicesistheeffectiveindexmethodOrthe guide—modemethod.Thesimulatedimagesinthemultimodewaveguideatthedistancede— terminedbyformula(1)areshowninFig.3.Thewidthofthemultimodewaveguideis 40#m.Heightsoftheinnerandouterribare10mand6川11respectively.Thecorrespond— inglengthofthemultimodewaveguideis2713gin.Inprinciple,threeguidemodescan formtwo—foldimagesinthemultimodewaveguide.However,simulatinresuItshowsthat 11期WEIHong—zhen(魏红振):SOI—Based3dBMMISplitterl057 withtheguidemodesincreasing—theimagesbecome narr.wandthe1.ssandcr.sstalk.fthedevlcereduce.06 Solargeconfinement,whichisthedeepetchingoftheo4 ribguides,isusefultoenhancetheperformanceofthe堇03 device.However,theetchingdepthoftheguidesis0 limitedbythesinglemodec.ndt.n.ftheaccess:: guidesThesimulationresultalsoshowsthatthe widthsoftheoutputwaveguideshaveimportantinflu enceonthecharacteristicsoftheMMIcoupler.WithFIG thewidthsincreasing,thelossandpoweruniformityof SimulatedImagesof1×2MM SplittrMultimode thesplitterreducedTaperedwaveguideswereusedtoWidth:?4.vm,Length:L=2713 connectthemultimodewaveguideandtheaccesswaveguides,asshowninFig.1 OneofthemainadvantagesoftheMMIdeviceisthelargefabricationtolerance.Fab— ricationtolerancesrefertothecontrolofthegeometricaldimensionsduringprocessingand itssubsequentimpactondeviceperformance.Fromformula(1).itisobviousthatthe multimodesectionwidthisbyfarthemostcriticalparametertobecontrolledduringthe fabrication.Tolerancecanbeevaluatedbytheinfluenceofasmallchangeinthedimension onthelossandpoweruniformityoftheMMIdevice.Losscanbeevaluatedbyoverlapping theimagingfieldwiththeoutputwaveguidemodefieldThepoweruniformityisdeter minedby: UF一10log(PL/P2)(2) FIG.4ToleranceofMM1Splitterto WidthofMu】dmodeWaveguide whereP1andP2(PL>P)aretheoutputpow— erinthetwobranchesrespective[),.Thesim ulatedtoleranceofthewidthofthemuhimode sectionisshowninFig.4.Lengthofthemul timodesectionis2713,um.Heightsofthein— netandouterribare10and6?nrespectively. Thesimulatedresultshowsthatthewidth toleranceoftheMMIsplittercanberelaxed greatlybyusingwideroutputwaveguides.In general,foragivenwavelengthandtechnolo gy,a[Itolerancecanberelaxedbyusing wideraccesswaveguides.Taperedwaveguidesareusedtoconnectthemultimode waveguidewiththeaccessguides,asshowninFig1. 3Fabrication TheconventionalSiprocesstechnologyisusedtofabricatetheMMIsplitter.The SIMOXSO1waferwithburiedSiO2andtopsiliconbeingboth0.25btmwasfollowedby 11?noftheepitaxialmonocrysta[silicon.TheribwaveguidesandMMIsplitterswere ?p/j{ 1058半导体学报21卷 LFIG5SEMGraphofOutpu FacetofMMISplkter formedbyone—stepRIE(ReactiveIonEtching)processing usingaCrmask.Theetchingwasperformedat3sccnaSF6 and10sccnlNwith10OWRFpowerof30rain.Inourde— sign,thewidthsoftheaccesswaveguideswere10mn,and severalsplitterswithdifferentdimensionsizeweredesigned toeva1uatethetolerance.Thelengthsofthemuhimode sectioninthesesplittersare2713”m.Widthsofthenluhi— modesectionare39,40,41and42fm1.respectively.Corre spondingly.thesplittersamplerswerenumbered1,2. 3and4.Theribetchingdepthis5tmathroughoutthe chip.TheSEMgraphoftheoutputwaveguidefacetisshowninFig.5.Thetopcladding layerwasformedbyadrywet—drythermaloxidationonthepatternedSO1wafer.Thick hessofthecoverSiO}isabout0.7tma.So,thethicknessofthecore—layeroftheinnerrib sectionisabout10.5”m.Taperedwaveguideswereusedtoconnectthenmhimodesection withtheoutputwaveguide.Thewaferwasthinnedandcleaved. 4ExperimentalResult Thelightfromthefiberatwavelength–1.3”mwascoupledintotheribwaveguides throughthecleavedfacetofaninputguide.ThelightemergingfromtheOUtpUtwave guideswasprojectedontothelightsensitiveareaofaninfraredCCDusingax20micro scopeobjective.ThentheimagewasdisplayedontheTVmonitorwithatotal200magni— ficationofthatofthecleavedontputfacet.TheCCDhada linescanfacility,whichwasusedtomeasuretheintensitv profilesthroughthecenteroftheoutputspots,bywhich wecanestimatethepoweruniformityofthecoupler.The imagesonTVweretakenbyadigitalcamera.Figure6 showsthemeasurednear—fieldimagesofeachoutput waveguidesofsample4.Themeasuredresultofsample 1,2and3arethesameasthatofthesanaple4.There isnoobviousdifferentresultsfordifferentsample.There- suitshowsthatthewidthtoleranceoftheMMIdeviceis largeenoughfortheconventionalSiprocesstechnology. ?FIG6MeasuredNearFieldImage ofTwoOutputWaveguidesin MMISptitter(Sample4) Notincludingthereflectivelossofthetwofacets.theexcesslossofsample4isabout 1?8dB.ThepoweruniformityofthetWOoutputbranchesislessthan0.5dB. 5Conelusion Inconclusion,wehavedemonstratedthe3dBMMIsplitterbasedonSOIinaform suitablefortheOEICapplication.Thesplitterwithdifferentwidthofnluhimodesecti0n weredesignedandfabricated.Theoutputnear—fieldimageswereobtained.Theexcessl0ss 11期WEIHong—zhen(魏红振)et.:SO1Based3dBMMISplitter1059 ofthesplitterisabout1.8dB.Thepoweruniformityislessthan0.5dB.Thewidthtoler anceofdeviceisenoughlargeforconventionalSiprocesstechnology. [1] —’ 2— :3] [4] [5] [6] [7] References M.A.F…ddandMFa[[ahi.IEEEPhotoniesTeehno1.Lett.,1999r11(6):697—6g9. Moh?adR.P&iannandRobertIMacDonald.ApplOpt.,1997,36(21):50975108. PrTeA.B雌se…M…Bachmann.HMelchior.【JB.SoldanoandMKSmit.J.[.~htwaveTeehno[.-1994 12(6):1oo41009 R.MJerkins.J.M.Heaton,D.RWightrJ.TParker,J.C.H.BirbeekandG.W.Smith,App[-PhysLett??199 4 64(6)684686. Q.Lai,MBachmann.W.Huaziker,P.A.BesseandH.Melchior.Electron.Lett.1996,32(17):1576—157 7. RASorer.JSchmidtchenandKPeterrruann,1gEEJQ咖tumE[ecIron?1997?27:19711卯3. LucasBSoldanoandgrikCM.Pennings,J.【JightwaveTechno1.,19g5.13(4):615627.