Fabrication of long-range surface plasmon-polariton waveguides in lithium niobate onsiliconGreg Mattiussi, Nancy Lahoud, Robert Charbonneau, and Pierre Berini Citation: Journal of Vacuum Science & Technology A 25, 692 (2007); doi: 10.1116/1.2740294 View online: http://dx.doi.org/10.1116/1.2740294 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/25/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Characterization of long-range surface plasmon-polariton in stripe waveguides using scanning near-field opticalmicroscopy J. Appl. Phys. 102, 123110 (2007); 10.1063/1.2826910 Long-range surface plasmon-polariton waveguides and devices in lithium niobate J. Appl. Phys. 101, 113114 (2007); 10.1063/1.2739300 Long-range surface plasmon-polariton mode cutoff and radiation in embedded strip waveguides J. Appl. 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Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47Fabrication of long-range surface plasmon-polariton waveguidesin lithium niobate on siliconGreg MattiussiaEpocal Inc., Ottawa, Ontario K1G 6C6, CanadaNancy LahoudSpectalis Corporation, Ottawa, Ontario K1S 4E6, CanadaRobert CharbonneauaDefence Research and Development Canada, Ottawa, Ontario K1A 0Z4, CanadaPierre BerinibSchool of Information Technology and Engineering, University of Ottawa, Ottawa, Ontario K1N 6N5,Canada and Spectalis Corporation, Ottawa, Ontario K1S 4E6, CanadaReceived 14 March 2007; accepted 23 April 2007; published 17 May 2007This article describes the fabrication of long-range surface plasmon-polariton waveguidescomprisedofathinnarrowgoldstripeburiedinlithiumniobatecladdings. Thewaveguidesarefabricatedviadirect bondingandthinningof lithiumniobatewafers, resultinginbulkqualitycladdings. The assembly is direct bonded onto a silicon support wafer. Processing details are givenalong with results of physical characterization conducted on structures that underwent intermediateprocess steps and on completed structures. 2007 American Vacuum Society. DOI: 10.1116/1.2740294I. INTRODUCTIONSurface plasmon-polaritons SPPs are transverse mag-neticpolarizedsurfacewavesthat propagatealongthesur-faceofamediumhavingahighfreechargecarrierdensityboundedbyadielectric.1,2Adielectric-metal interface, forexample, supportsSPPsat optical wavelengths. Plasmonicstructuresinvolvingmetal planes, lms, andparticleshavebeen studied over time,3,4due to their fascinating and poten-tially useful properties.The surface plasmon waveguide sketched in cross-sectional viewinFig. 1a, consistingofathinmetal lmsurroundedbydielectric, isofparticularinterest inthisar-ticle. This metal stripe waveguide was studiedtheoretically5,6and experimentally.79The symmetric 1=3 waveguidesupports as afundamental mode, along-rangelow-loss surfaceplasmon-polaritonLRSPP wave,identied as thessb0mode.5The existence of this mode wasveriedexperimentally,7andits attenuationwas measuredand compared with theory.8,9The metal stripe waveguide op-eratinginthismodewasproposedasthefoundationofanintegrated optics technology,5and since then, numerous ele-mentsanddevicesbasedonitsusehavebeenreported, in-cludingpassives,1013Bragggratings,14,15andthermo-opticdevices.1619Metal stripesonsubstratesexposedtoair20,21also support SPPs but they are typically not long range as theLRSPP is in general supported by a highly symmetric struc-ture 13.5,6The need for the structure to be symmetric adds complex-ityfromthefabricationstandpoint sinceanuppercladdingmust be deposited such that it is index matched to the lowercladding. This can be achieved by using the same depositiontechniquetoformbothcladdings, ashasbeendoneinthecase of spin-coated benzocyclobutene11and sputteredsilica.12Opticalcrystalsandferroelectrics,suchasLiNbO3,possess interesting nonlinear, electro-optic, and acousto-opticproperties,whichwouldbeinterestingtoexploitwithinthecontextofLRSPPs. However, suchmaterialsarenoteasilyor readily deposited and are available essentially only in bulkor wafer form.A promising approach for fabricating matched crystallinecladdings is direct bonding22,23and thinning. Direct bondingcanbe carriedout at lowtemperatures, does not involveadhesives, and the materials to be bonded do not need to belattice matched. Direct bonding requires i at surfaces hav-ingdeviationslessthanabout5 nmand iisurfaceactiva-tionthroughappropriatecleaningandtreatment. A gooddi-rectbondresultsinaseamlessmaterialinterfacefreefrommacroscaledefects. Variousmaterialscanbedirectbonded,includingLiNbO3toitself24andtoother materials.23Fur-thermore, LiNbO3can be polished to obtain a thinnerlayer,24,25or crystal ion slicing can be used to exfoliate a thinlayer from the host wafer.26,27Crystal ion slicing can also becombinedwithdirect bondingtoproduce a thinlayer ofLiNbO3 on another substrate.28Thin LiNbO3 layers obtainedin such manner2428retain their bulk properties.Inthis article, we describe indetail the fabricationoflong-range surface plasmon waveguides consisting of a thinAustripeburiedinLiNbO3. Thefabricationapproachin-volves direct bonding and thinning of LiNbO3wafers toform the claddings. Results of physical characterization con-ductedonstructureshavingundergoneintermediateprocesssteps are given throughout the article. A brief account of theaFormerly with Spectalis Corporation, Ottawa, Ontario K1S 4E6, Canada.bAuthor towhomcorrespondenceshouldbeaddressed; electronicmails:berini@site.uottawa.ca and pierreberini@spectalis.com692 692 J. Vac. Sci. Technol. A 254, Jul/Aug 2007 0734-2101/2007/254/692/9/$23.00 2007 American Vacuum Society Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47fabricationapproachhasalreadybeenreported,29andpre-liminary optical measurements on fabricated waveguideswererecentlypublished.30,31Thearticleisorganizedasfol-lows:Sec. IIgivesthefullprocessow, Sec. IIIgivesanddiscussesresultsofintermediateprocesssteps,Sec.IV pre-sents nished devices, and Sec. V gives concluding remarks.II. PROCESS FLOWA. Device structureThe device of interest is sketched in Fig. 1b. The wave-guideconsists of theAustripeboundedbyz-cut LiNbO3claddings. The top Au electrode and the Al ground plane areused to apply a voltage to the stack and thus a modulation ofthessb0modepropagatingalongthestripeviatheelectro-optic effect present in LiNbO3. The main transverse electriceld component of the ssb0mode propagating along the wave-guideis orientedperpendicular totheAustripe5andthusalongthezaxesofthecladdings, sothestrongest electro-opticcoefcientofLiNbO3canbeexploitedbyapplyingavertical electric eld as shown. The mode interacts primarilywith ne, the extraordinary index of LiNbO3, which is modu-lated by the applied electric eld E according to ne,E=nen, where n=ne3r33E/ 2. For LiNbO3at 0=1550 nm, ne=2.1377 and r33=29.9 pm/ V.Thetopandbottomz-cut LiNbO3claddingscanbeori-entedsuchthattheircrystalorientationsareeitheroppositetoeachotherorinthesamedirection, assuggestedinFig.1b. Withthetopandbottomcladdingsalignedoppositely,theappliedvoltageincreasestheindexofoneoftheclad-dingsbynwhiledecreasingtheindexoftheotherbythesameamount,thusinducinganindexasymmetryof2ninthe waveguide structure. This asymmetrycuts off the ssb0mode6and forces it to radiate into the high-index cladding31as in thermo-optic structures,18,19thus resulting in broadbandintensity modulation. With the top and bottomcladdingsalignedinthesamedirection, theappliedvoltageincreasesordecreasestheindexofbothcladdingsidenticallybyn,thus imparting a phase modulation on the ssb0mode.Basedonoptical modeling,30it wasdeterminedthat theburied Au stripe should have a thickness of t =2022 nm anda width of w=0.952m in order for the waveguide to havea reasonably low attenuation few dB/mm and a manageablemode size. The thickness required for the LiNbO3 claddingsis then 1015m.It is desirabletofabricatethewholestructureonaSiwafer anticipating the possible integration of the electro-optic device with underlying electronics.B. Flow of the main process stepsFigure 2 summarizes the main process steps developed tofabricatedevicesassketchedinFig. 1b. InorderfortheLiNbO3claddingstobesuccessfullywaferbonded, the Austripe must be embedded into the surface of a LiNbO3 waferandplanarizedsuchthattheresultingsurfaceisadequatelysmooth deviations less than about 5 nm. Hence, the processFIG. 1. a Metal stripewaveguidesketchedincross-sectional view. Themetal stripe has thickness t, width w, and permittivity 2, and is bounded bydielectric claddings of permittivities 1 and 3. b Cross-sectional sketch ofthe device based on direct-bonded z-cut LiNbO3 claddings.FIG. 2. Flow of main process steps to embed an Au stripe in thin claddingsof LiNbO3 CMP: chemical-mechanical polish.693 Mattiussi et al.: Fabrication of long-range surface plasmon-polariton waveguides 693JVST A - Vacuum, Surfaces, and Films Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47ow begins with a series of steps aimed at obtaining an em-beddedplanarAustripeonarst LiNbO3wafer that willeventuallybecometheupper cladding. OncetheAustripehas been embedded, the wafer is ipped and direct bonded toasecondLiNbO3waferwhichwillbecomethelowerclad-ding. This second wafer then undergoes grinding and polish-ing to the thickness desired for the lower cladding15m. Alisthendepositedontothebottomsurfaceofthis wafer followedbyathinlayer of SiO2. TheLiNbO3stack is then direct bonded to a SiO2 layer on a Si wafer. Therst wafer then undergoes grinding and polishing to thethickness desired for the upper cladding 15m. Finally,the top Au electrode is dened.LiNbO3andSihavedifferentcoefcientsofthermalex-pansion, but all process steps occur at low temperature. TheLiNbO3 to LiNbO3 direct bonding step occurs at room tem-perature andis followedbyanannealingstepat 300 Cearlyintheprocess ow. All subsequent processingstepsoccurattemperaturesbelow150 C. ThedirectbondingoftheLiNbO3/ Al / SiO2stacktoSiO2/ Si alsooccursat roomtemperature and is followed by a low temperature anneal.C. Flow for embedding the Au stripeFigure 3 shows two approaches to embedding theAustripe. The approach sketched in Fig. 3a is based on usingthesameresistpatterntoetchthetrenchinLiNbO3andtoliftofftheexcess Au, whiletheapproachdescribedinFig.3b is based on chemical-mechanical polishing CMP. TheapproachofFig. 3awaschosensinceall oftherequiredsteps were readily available. Chemical etching of LiNbO3 isdifcult so ion beam milling was used to etch the trench intowhich the Au was subsequently deposited.D. Process detailsDetails pertaining to the process ow described above aresummarized in Table I.III. RESULTSA. Embedded Au stripesFigure4shows AFMscansoftrenchprolesinLiNbO3etchedunderthesameionbeammillingconditions 3 min,25 incident angle, 500 V, 400 mA. Figure 4a showstrenches on either side of a large tens of micrometers mesaand Fig. 4b shows a narrow nominally 1mwide trench.There is clearly a difference in etch depth between these twostructures.Figure4b shows byextrapolatingthetrenchsidewallprole that the milling rate reaches its peak open area valueapproximately 1.75maway from the edge of the photore-sist. Evidently, theproximityofthephotoresistreducestheeffective mill rate. This could be due to a number of factors:i sputtered resist fromthe sidewalls deposited into thetrench, iishadowingeffectsduetothehighangleofinci-dence of the incoming ions cause the center of the trench linetobe exposedduringthe entire millingtime but not theedges, and iii the milling process may be removing exces-sive amounts of photoresist. According to factor iii, as theresist is milled away, more LiNbO3 is exposed and subjectedto the milling process. Because this extra LiNbO3 is exposedgraduallythroughoutthemillingtime, thetrenchdepthde-creases smoothlytowards the edges. This alsoleads toatrenchthatiswiderthanexpected. A modiedresistproleor a hard mask process would correct this problem.A trench prole such as that shown in Fig. 4b is unsat-isfactory, sinceauniformthicknessof Ausubsequentlyde-posited and lifted off yields an embedded stripe having edgesthat protrude above the surface of the LiNbO3, as is apparentfromthescanningelectronmicroscopy SEMmicrographsin top and cross-sectional views given as Figs. 4c and 4d.Theseprotrusionstheninterferewiththesubsequent directbonding process. Indeed, there were issues with bondingLiNbO3 wafers that featured embedded stripes such as these.An Arionsputteretch,subsequentlyappliedtothesurface,appearedtoselectivelyremovetheAuwithrespect totheLiNbO3, reducing the topography sufciently to carry out thedirect bonding step successfully.B. Direct bonding and thinning of LiNbO3Thejoiningof twowafersoccursat roomtemperature,followingtheapplicationofaproprietarysurfacechemistrydesignedtoavoidtheformationofbubblesat theinterfacetypical of direct bonding processes based on the reduction ofhydrogenbondstothoseofanoxideaccompaniedwiththeformationofH2O. Anannealisappliedafterthejoininginorder to increase the bond strength i.e., interface energy toFIG. 3. Process ows for creating a metal-lled trench in LiNbO3. a Self-alignedprocess owmakes useof thesameresist maskfor etchingthetrenchandforperformingthelift-offsubsequentto Audeposition. b Al-ternate process ow makes use of a mask for etching the trench and then ofa CMP step to planarize a blanket deposition of Au.694 Mattiussi et al.: Fabrication of long-range surface plasmon-polariton waveguides 694J. Vac. Sci. Technol. A, Vol. 25, No. 4, Jul/Aug 2007 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47alevel that issufcient fortheassemblytosurvivesubse-quentprocessingsuchaslithography,metaldeposition,wa-fer polishing, dicing, andendfacet polishing. This directbonding process was applied to join LiNbO3 to LiNbO3 andSiO2 to SiO2.Theformationof bubbles was not observedduringthebonding process. However, in some cases of LiNbO3toLiNbO3bonding, someunbondedareaswereobservedasNewtons rings that are thought to be caused by particles ofvarious sizes trappedat thebondinginterface. Theoccur-renceofparticlesshoulddecreaseasthisprocessingstepisimproved.ThedevicestructuredepictedinFig. 1b callsfor twodirect bonds: one to join two LiNbO3 wafers together to formthe claddings and another to join the LiNbO3assembly to aSi wafer viaaSiO2SiO2bondFig. 2, bottomdiagram.Hence, intermediate structures, such as those depicted in Fig.5, were created in order to exercise the bonding step, and thegrinding and polishing of LiNbO3wafers down to thinlayers.Figure 5 shows SEMmicrographs of some successfuldirect-bondedstructures. Figure 5a shows LiNbO3/ SiO2direct bonded to SiO2/ Si, Fig. 5b shows LiNbO3/ Al / SiO2direct-bonded to SiO2/ Si, and Fig. 5c shows a thin16mlayerofLiNbO3obtainedbychemical-mechanicalpolishing of a full thickness LiNbO3 wafer direct bonded to aSi wafer as in Fig. 5b. In the case of Fig. 5a, the thin layerof SiO2 was deposited on the LiNbO3 wafer before bonding,andinthecaseofFigs.5band5c,an AllayerfollowedbyathinSiO2layerweredepositedontheLiNbO3waferbefore bonding.Experiments have demonstrated a basic mechanical resil-iency of LiNbO3 lms on Si fabricated in this manner. Afterformationof 25mthicklms of LiNbO3on0.4mofTABLE I. Details of the process ow adopted to fabricate the devices.Module Name Step Description Comments1 Stripeformation1.1 Clean incomingwafersIncoming wafers: z-cut, optical grade LiNbO3,100 mm diameter, 0.5 mm thick, TTV15m, warp30mClean: SC-11.2 Stripe lithography Re-entrant lift-off resist prole, criticaldimension of 0.35m1.3 Plasma descum O2 plasma etch, maximum 10 nm equivalentresist removal1.4 Trench formation Ion beam milling to a depth of 20 nmIncident angle:251.5 Plasma descum Same as 1.31.6 Stripe metaldepositionLine-of-sight evaporation of 20 nm thick Au; Tiash adhesion layer1.7 Lift-off With care to preserve thin Au stripe1.8 Plasma descum Same as 1.32 Claddingformation2.1 Bond rst wafer tosecond waferSurface preparation: megasonic SC-1 clean,proprietary surface treatment.Bonding at room temperature with no signicantapplied pressure.2.2 Anneal At 300 Cfor about 2 hin furnace at 1 atm;temperature ramps:2 C/ min2.3 Lower cladding Grind and polish blank wafer to desired thickness15mClean: SRD+Megasonic DI after grinding;scrubbing+Megasonic SC-1+SRD after polishing2.4 Ground plane 1.0mthick Al with 15 nm thick Ti adhesionlayers above and below2.5 Deposit SiO2250 CPECVD SiO2 0.4mon Al2.6 Planarization Polish PECVD SiO2 for bonding2.7 Bond SiO2 toSiO2/ SiSimilar to 2.12.8 Upper cladding Similar to 2.33 Topelectrode3.1 Lithography Resist thickness appropriate for 1mlift-off;critical dimension: 8m3.2 Deposition 1mthick Au3.3 Lift-off Solvent-based, then clean4 Singulation 4.1 Dicing andpolishingTwo step dicing: rst cut is a near-polish slowcut 150mdeep using a wide blade; second cutgoes through rest of wafer using a narrower blade.Polish end facets to optical quality.695 Mattiussi et al.: Fabrication of long-range surface plasmon-polariton waveguides 695JVST A - Vacuum, Surfaces, and Films Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47plasma-enhanced chemical-vapor deposition PECVD SiO2on a 100 mm diameter Si wafer, the room temperature waferwasplacedonahot plateat 200 Cfor veminutes. Nodelaminationhadoccurred. The experiment was repeated,with the same positive outcome.C. DefectsAlthoughtheprocessingstepsdescribedintheprevioussubsections werecarriedout successfullyonanumber ofwafers, certain issues were identied regarding the quality oftheLiNbO3layers obtained. Themainproblems observedare summarized in Fig. 6.An issue with the lithography process is the generation ofoptical defects on some LiNbO3 wafers. Figure 6a shows alarge optical defect present in the bulk of the LiNbO3 near analignmentmark,observedafterthelithographyprocesswasapplied to this surface of the wafer z face in this case. It isthought that such defects are the result of an excessivebuildupof surfacechargeduringtemperature-rampedpro-cess steps. Whether they are localized reversals of polariza-tion i.e., microdomains or damage caused by arc dischargesis at present unknown.The image shown as Fig. 6a was captured with the mi-croscope objective focused onto the top surface of theLiNbO3, whiletheimageshownasFig. 6b of thesamedefect wascapturedwiththefocusplaneloweredintotheLiNbO3. It isclear that thesedefectsareembeddedinthebulkof the LiNbO3, or start inthe bulkandendontheopposite face of the wafer +z face in this case, since focus-ing away from the top zsurface and into the wafer leadsto a greater denition of the internal structure of the defect.Furthermore, thelithographydoes not seemtohavebeenaffectedbythe presence of the defect, conrmingthat ifthereisasurfacestructuretothedefect,itisoflimitedex-tent. No obvious correlation was observed between the loca-tion of these defects and the patterns on the mask.There are two problems created by the grinding and pol-ishingof LiNbO3wafersintothinlayers: edgebreakawayandpoint defects. Figure6c shows anexampleof edgeFIG. 4. a AFMscan of trenchesetchedintoaLiNbO3waferoneithersideofalargecentralmesa.Thestepheight isabout 22 nm. b AFMscanof a nominally 1m wide trenchetched into a LiNbO3 layer. The trenchwidth is measured as 1.14m. cand d SEM micrographs of Au lledtrenchesinLiNbO3,intopandcross-sectional views, respectively.696 Mattiussi et al.: Fabrication of long-range surface plasmon-polariton waveguides 696J. Vac. Sci. Technol. A, Vol. 25, No. 4, Jul/Aug 2007 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47breakaway observed on a wafer having undergone the grind-ing process to form a 14m thin layer of LiNbO3. The layerstructure on this wafer is similar to that of Fig. 5c, consist-ing of a 14m layer of LiNbO3 on 2m of Al on SiO2 ona Si wafer. Both the SiO2 and Al have been worn away fromtheedgeof thewafer, but inarelativelychaoticmanner,implying that sudden delaminating must have occurred. Thisscenarioisalsosupportedbyngersof Al that wereob-served to be missing in other locations, leaving exposed re-gions of oxide. Figure 6d shows radial strings of point de-fectsthatresultfromgrindingandpolishing.Thesedefectsappear to be similar to those produced during the lithographyprocess. They are radial because of the nature of the grindingprocess: an abrasive-edged wheel is rotated above the waferwhile the wafer itself is rotated to ensure uniformity.D. ViasAlthough the device structure of immediate interest Fig.1b does not call for vias, their fabricationthroughtheLiNbO3claddings was explored, anticipating an eventualneed to contact to the Al ground plane from the top surface.Twoetchingapproaches wereexplored: ionbeammillingandsputteringusingtheprimarybeamof asecondaryionmass spectrometry SIMS machine. The former approach issuitable for wafer scale processing.Figures7aand7bshowSEMmicrographsofalargediameter4.4mdeepholeetchedintoaLiNbO3waferbyion beam milling. The hole was dened lithographically us-ing a photoresist mask.Figure8shows thesecondaryionyieldof thevariouscomponents of a 26m thick layer of LiNbO3 on Al on SiO2on Si as in Fig. 5b, as a function of sputtering depth usingthe primarybeamof a SIMSmachine. The capabilityofdetecting when the underlying lm is exposed end pointingis a very useful feature. Indeed, punch through is very obvi-ous in this case, as indicated by the large rise in Al contain-ing components. Figure 7c shows a microscope image of aviafabricatedwiththisapproach. Someoptimizationisre-quiredsincefabricatingthis largerectangular viarequired18 hours.IV. FINISHED DEVICESFigure9showsvariousimagesoffullynisheddevicesconstructed according to Fig. 1b. Figure 9a shows a SEMcross section of an as-diced and unpolished nished device.The total thickness of the LiNbO3 claddings is about 27min this case and the width of the top electrode is 10m. Thebondinginterfacebetweenthe11mthicktopand16mFIG.5. SEMmicrographsofdirect-bondedlayers: aLiNbO3/ SiO2directbonded to SiO2/ Si, b LiNbO3/ Al / SiO2 direct bonded to SiO2/ Si, and cthin 16mlayerofLiNbO3obtainedbychemical-mechanicalpolishingof a LiNbO3 wafer direct bonded to a Si wafer as in b.FIG. 6. a Microscope image 200 of a defect near an alignment feature.The focal plane is alignedwiththe topsurface. b Microscope image200 of thedefect shownina except that thefocal planeislocatedwithintheLiNbO3wafer. c Microscopeimageof aregionwhereedgebreakawayofthestack LiNbO3/ Al / SiO2isevident. TheedgeoftheSiwaferisjustbeyondthelowerrightcorneroftheimage. dPointdefectsoriginating from the CMP of a LiNbO3wafer. Defects are aligned radially,as emphasized by the superposed white dotted lines.697 Mattiussi et al.: Fabrication of long-range surface plasmon-polariton waveguides 697JVST A - Vacuum, Surfaces, and Films Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47thick bottom LiNbO3 claddings is clearly visible due to dic-ingartifactsterminatingattheinterface.Theembedded Austripe is not visible at this magnication.Figure9b showsaSEMcrosssectionof adicedandpolished for SEM device. The total thickness of theLiNbO3 claddings is about 32m in this case and the widthof thetopelectrodeis 10m. Thebondinginterfacebe-tween the top and bottom LiNbO3 claddings is seamless andfreefrommacroscopicdefects. TheembeddedAustripeisnot visible at this magnication.Figure 9c is a high-magnication SEM cross section ofthebottomportionof thedeviceshowninFig. 9b. Thebonding interface between the two SiO2 layers is clearly dis-cerniblesincethelayersareamorphousandhavedifferentmicrostructures, and the anneal step applied after this directbondwasalowtemperatureanneal, insufcient tosigni-cantly reow the layers.Figure9dgivesanoptical microscopeimageofan-isheddeviceintopview.The Austripeembeddedbetweenthin LiNbO3 claddings is clearly visible.Finished device such as these survive a 40 min ramp fromroom temperature to 105 C and back, as applied during thelithography process for dening the top electrode.Optical measurements were performed on a variety of de-vices bybutt couplingapolarization-alignedpolarization-maintaining ber to the input of the structures and capturingthe output with an infrared camera.12End facet polishing wasnecessaryasthedicingprocessleftthefacetsunacceptablyrough, as can be appreciated from Fig. 9a. Figure 10 showsthe output of a successful construction. The mode is stronglyguidedandweakbackgroundradiationisobserved. The Austripe inthis device is nominally2mwide and20 nmthick. The total thickness of the LiNbO3 stack is 25m andthe length of the structure is 4 mm.The background radiation is likely due to uncoupled inputlight and scattering along the trench and metal stripe.Electro-opticmeasurementswerealsoperformed31withtheresults indicating that the wafer-bonded and thinned LiNbO3claddings retained their bulk electro-optic properties after theprocessing, despite the quality issues observed.V. SUMMARY AND CONCLUDING REMARKSAprocessowfor embeddingathinAustripeinz-cutLiNbO3 claddings was discussed in detail. The main processstepsincludeetchingashallowtrenchonthesurfaceof aLiNbO3wafer,directbondingLiNbO3toLiNbO3,grindingandpolishingthickLiNbO3layersdowntoabout 15m,anddirectbondingaLiNbO3stacktoaSiwaferviainter-mediatelayersofSiO2. AlloftheprocessingoccursatlowFIG. 7. a and b SEM micrographs of a large diameter 4.4m deep holeinLiNbO3etchedbyionbeammilling, intopandperspectiveviews, re-spectively. c Large rectangular via etched through a LiNbO3 layer 17mthick to an underlying layer of Al. Etching was performed using the primarybeam of a SIMS machine.FIG. 8. SIMS response as the primary beam sputters through a 26m thickLiNbO3layer on Al as in Fig. 5b. The end-point signal Al and Al +Ocan be used to detect punch through.698 Mattiussi et al.: Fabrication of long-range surface plasmon-polariton waveguides 698J. Vac. Sci. Technol. A, Vol. 25, No. 4, Jul/Aug 2007 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47temperature300 C inordertopreservetheembeddedthin Au stripe and mitigate problems that could be generateddue to the difference in coefcient of thermal expansion be-tween LiNbO3 and Si.Issues were identied with some of the process steps, themain ones being imperfect trench proles etched intoLiNbO3anddefectsintheLiNbO3claddingssubsequenttothe lithography and grinding steps. Possible causes for theseproblemswerediscussedalongwithpossiblesolutions.De-spite these issues, the full process generated working devicesas conrmedbyoptical andelectro-opticmeasurements.31Indeed, the measurements conrm that the wafer-bonded andthinnedLiNbO3claddings retainedtheir bulkoptical andelectro-optic properties after processing.1H. Raether, SurfacePlasmonsonSmoothandRoughSurfacesandonGratings Springer, Berlin, 1988, Vol. 111.2W. L. Barnes, J. Opt. A, Pure Appl. Opt. 8, S87 2006.3W. L. Barnes, A. Dereux, and T. W. Ebbesen, Nature London 424, 8242003.4S. A. Maier and H. A. Atwater, J. Appl. Phys. 98, 011101 2005.5P. Berini, Phys. Rev. B61, 10484 2000.6P. Berini, Phys. Rev. B63, 125417 2001.7R. Charbonneau, P. Berini, E. Berolo, andE. Lisicka-Shrzek, Opt. Lett.25, 844 2000.8R. Nikolajsen, K. Leosson, I. Salakhutdinov, and S. I. Bozhevolnyi, Appl.Phys. Lett. 82, 668 2003.9P. Berini, R. Charbonneau, N. Lahoud, andG. Mattiussi, J. Appl. Phys.98, 043109 2005.10R. Charbonneau, N. Lahoud, G. Mattiussi, and P. Berini, Opt. Express13,977 2005.11A. Boltasseva, T. Nikolajsen, K. Leosson, K. Kjaer, M. S. Larsen, and S.I. Bozhevolnyi, J. Lightwave Technol. 23, 413 2005.12R. Charbonneau, C. Scales, I. Breukelaar, S. Fafard, N. Lahoud, G. Mat-tiussi, and P. Berini, J. Lightwave Technol. 24, 477 2006.13A. Degiron and D. R. Smith, Opt. Express 14, 1611 2006.14S.Jett-Charbonneau,R.Charbonneau,N.Lahoud,G.Mattiussi,andP.Berini, Opt. Express 13, 4674 2005.15A.Boltasseva,S.I.Bozhevolnyi,T.Sndergaard,T.Nikolajsen,andK.Leosson, Opt. Express 13, 4237 2005.FIG. 9. a SEM cross section of an as-diced unpolished nished device Fig. 1b. The total thickness of the LiNbO3 claddings is about 27m in this caseand the width of the top electrode is 10m. The bonding interface between the 11m thick top and 16m thick bottom LiNbO3 claddings is clearly visibledue to dicing artifacts terminating at the interface. The embedded Au stripe is not visible at this magnication. b SEM cross section of a diced and polisheddevice. Polishing encapsulation materials are visible near the top electrode. The total thickness of the LiNbO3 claddings is about 32m in this case and thewidth of the top electrode is 10m. The bonding interface between the top and bottom LiNbO3 claddings is seamless. The embedded Au stripe is not visibleat this magnication. c High-magnication SEM cross section of the bottom portion of the device shown in b. d Optical microscope image of a nisheddevice in top view.FIG. 10. Infrared image of the output of a nished 4 mm long device. TheAustripeisnominallyw=2mwideandt =20 nmthick. Thetotalthick-ness of both LiNbO3 claddings is 25m and the stripe is buried at about themidpoint of the stack.699 Mattiussi et al.: Fabrication of long-range surface plasmon-polariton waveguides 699JVST A - Vacuum, Surfaces, and Films Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:4716T.Nikolajsen,K.Leosson,andS.I.Bozhevolnyi,Appl.Phys.Lett. 85,5833 2004.17T. Nikolajsen, K. Leosson, andS. I. Bozhevolnyi, Opt. Commun. 244,455 2005.18G. Gagnon, N. Lahoud, G. Mattiussi, and P. Berini, J. Lightwave Technol.24, 4391 2006.19S. Park and S. H. Song, Electron. Lett. 42, 402 2006.20J.-C. Weeber, J. R. Krenn, A. Dereux, B. Lamprecht, Y. Lacroute, andJ.-P. Goudonnet, Phys. Rev. B64, 045411 2001.21B. Lamprecht, J. R. Krenn, G. Schider, H. Ditlbacher, M. Salerno, N.Felidj, A. Leitner, and F. R. Aussenegg, Appl. Phys. Lett. 79, 51 2001.22Q.-Y. TongandU. Gsele, SemiconductorWaferBonding:ScienceandTechnology Wiley Interscience, Toronto, 1999.23J. Haisma, B. A. C. M. Spierings, U. K. P. Biermann, andA. A. vanGorkum, Appl. Opt. 33, 1154 1994.24Y. Tomita, M. Sugimoto, and K. Eda, Appl. Phys. Lett. 66, 1484 1995.25S. McMeekin, R. M. De La Rue, and W. Johnstone, J. LightwaveTechnol. 10, 163 1992.26M. Levy, R. M. Osgood, Jr., R. Liu, L. E. Cross, G. S. Cargill III, A.Kumar, and H. Bakhru, Appl. Phys. Lett. 73, 2293 1998.27T.A.Ramadan,M.Levy,andR.M.Osgood,Jr.,Appl.Phys.Lett. 76,1407 2000.28P. Rabiei and P. Gunter, Appl. Phys. Lett. 85, 4603 2004.29G. Mattiussi, N. Lahoud, R. Charbonneau, andP. Berini, Proc. SPIE5720, 173 2005.30P. Berini, G. Mattiussi, N. Lahoud, andR. Charbonneau, Proc. SPIE6475, 6475oU 2007.31P. Berini, G. Mattiussi, N. Lahoud, and R. Charbonneau, Appl. Phys. Lett.90, 061108 2007.700 Mattiussi et al.: Fabrication of long-range surface plasmon-polariton waveguides 700J. Vac. Sci. Technol. A, Vol. 25, No. 4, Jul/Aug 2007 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP:103.37.201.70 On: Wed, 12 Aug 2015 18:15:47