We fabricated grating couplers in silicon-on-insulator waveguides with focused-ion-beam. First devices were very lossy, but by using selective etchant and a hard mask we obtained efficiencies comparable to traditional fabrication techniques

Baets, Roel

Schrauwen, Jonathan

Van Thourhout, Dries

English

Ghent University Academic Bibliography

Focused-ion-beam fabricated vertical ﬁber couplerson silicon-on-insulator waveguidesJonathan Schrauwen, Dries Van Thourhout and Roel BaetsGhent University - IMEC, Department of Information Technology,Photonics Research Group, Gent, BelgiumAbstract— We fabricated grating couplers in silicon-on-insulator waveguides with focused-ion-beam. First devices werevery lossy, but by using selective etchant and a hard maskwe obtained efﬁciencies comparable to traditional fabricationtechniques.I. INTRODUCTIONFocused-ion-beam (FIB) is known as a very ﬂexible micro-fabrication technique. It has since long been used in micro-electronics for device modiﬁcation and mask repair. Recently,commercial systems with beam diameters under 10nm havebecome available, and the ability of FIB to do nanofabricationis being considered [1]. In photonics, FIB would be the idealtool to do fast prototyping, high resolution device modiﬁcation,resonant cavity tuning, slanted facet etching, and much more.One of the inherent problems however, is the high accelerationvoltage (30kV) needed to focalize the beam. It causes ionimplantation, crystal damage and amorphization deep intothe processed material. All these effects lead to attenuationof light, which makes the fabrication of low-loss photoniccomponents difﬁcult. FIB has been used to study variousphenomena involving light in waveguides [2], [3] and allowsthe direct fabrication of structures such as photonic crystals[4], [5]. But to our knowledge there are few or no reports inliterature about low-loss photonic devices fabricated in silicon-on-insulator by means of FIB. In this work we report on apossible way to circumvent the problem of optical attenuationin devices fabricated with FIB in silicon-on-insulator (SOI).By using I2 as selective etch gas and Al2O3 as protectivehard mask we obtained a device that features the same lossesas one fabricated with traditional techniques. This proves thatFIB can be used to fabricate low-loss photonic devices.The silicon-on-insulator platform is a good candidate forfuture ultra-compact photonic integrated circuits due to itscompatibility with CMOS technology and the possibility ofmaking low-loss waveguides with compact bends. One in-convenience however, is the large mode-size mismatch withsingle-mode optical ﬁber, causing very inefﬁcient ﬁbre-to-chip coupling for butt-coupled devices. This problem canbe solved by using tapers or grating couplers. The latterare shallow gratings that diffract the light into a verticallypositioned ﬁber. In previous work we have fabricated theseshallow grating couplers with a CMOS compatible process(248nm deep UV lithography and ICP etching) and optimizedthe parameters for optimal coupling efﬁciency and 1550nmoperation [6]. The optimal design is a grating with about 25Fig. 1. Two identical ﬁber couplers fabricated with focused-ion-beam (FIB)on a 10μm wide and 220nm thick Si waveguide.periods, lattice constant of 630nm, slit width of 315nm andetch depth of 70nm. It couples about 25% [7] of the lightfrom the waveguide into a single-mode ﬁber that is inclined10 degrees to avoid back diffraction.II. DIRECT FABRICATIONA ﬁrst approach is the direct fabrication of gratings into10μm wide waveguides. For this process no mask is needed:blanking of the ion beam and correct scanning copy the wantedstructure into the substrate. For submicrometer structures how-ever, the gaussian proﬁle of the beam [8] and redepositioncause edges to be rounded and sidewalls to be nonvertical.Due to this effect the gratings we fabricated with FIB havemore rounded slits than the ones fabricated with traditionalICP etching. This is shown in the insets of Figure 2. We haveperformed FDTD calculations to verify whether this has anegative effect on the performance. The result is that if slitwidth and etch depth are optimized the theoretical efﬁciencyand bandwidth are close to the traditional ones.To determine the coupling efﬁciency we use a ﬁbre-to-ﬁbretransmission measurement for TE-polarization. The structureconsists of an input coupler, a 10μm wide 220nm thickwaveguide and an output coupler. This is shown in Figure1. We assume that both couplers are identical. Figure 2 showsthe measured ﬁbre-to-chip coupling efﬁciencies for directlyfabricated grating couplers. We scanned the ion beam on 25slits of 200nm wide, with a grating period of 630nm. To varythe etch depth we etched several gratings with different doses,ranging from 4 1017 to 40 1017 Ga+/cm2. By making FIBcross-sections we veriﬁed the etch depths, as shown in theinsets of Figure 2. The ion beam at 10keV cannot be focalizedas well as the one at 30keV and produces broader and roundergrating slits.1011-4244-0096-1/06/$20.00 ©2006 IEEEP20 18:30 – 20:30Fig. 2. Very inefﬁcient ﬁber couplers fabricated directly into Si with 10keV(black) and 30keV (grey) Ga ions. The insets show the cross-sections throughtwo grating periods: the 10keV beam is broader and makes a rounder proﬁle,but still these gratings are the most efﬁcient.The efﬁciency of the directly etched gratings are all lowerthan 1%, which is one order of magnitude lower than thevalues calculated with FDTD. This is most probably due tothe damage of the Ga ions in the Si crystal, and was alsonoticed for gratings in other materials [9]. Figure 3 showsthe trajectories of 250 Ga ions accelerated to 10keV and30keV, and projected perpendicularly on a Si target. Theywere calculated with SRIM [10], a freely available MonteCarlo simulation tool. The Si atoms that reach the surface aftersubsequent binary collisions and have enough energy te escapethe surface potential are sputtered; the others are displaced andgenerate vacancies and substitutional Si atoms in addition tothe implanted Ga ions. Figure 3 shows that the thickness ofthis damaged layer is more than 50nm for 30keV Ga ions,whereas for 10keV ions it is about half as thick. This explainsthe somewhat higher coupling efﬁciency of gratings fabricatedwith 10keV. But due to the bad focalization in this case, thedimensions and the sidewall angle of the slits are difﬁcult tocontrol. Even though we did not further optimize the width ofthe etched slits, we believe that Figure 2 is sufﬁcient proof tosay that directly etched gratings in thin Si waveguides are tooinefﬁcient for practical use.III. ETCH ENHANCEMENT AND HARD MASKIt was formerly shown that optically active material is lessdamaged when chemical etch enhancement gasses are usedduring FIB etching [11]. I2 is known as etch enhancement gasfor Si: the molecules attach to the Si surface and facilitate theextraction of Si atoms from the collision cascade, resultingin chemical etch enhancement up to a factor of 10. However,the dimensions of an etched hole become difﬁcult to controlwhen chemical enhancement is used, so we need a hardFig. 3. Trajectories of 250 Ga ions accelerated to 10 and 30keV, perpendic-ularly projected on a Si target. Etching with 10keV ions produces a shallowerdamaged layer and less optical losses.Fig. 4. Proﬁle of a grating etched in Si with I2 as selective etch gas andAl2O3 as hard mask. The proﬁle is asymmetric due to the gas injectionsystem.etch mask to controllably make grating couplers. As hardmask one could choose a metal, but thin metal ﬁlms areoften grainy, which makes homogeneous etching with FIBimpossible due to the crystal orientation dependence of theetch yield. Therefore we chose Al2O3: it is an amorphousmaterial that can homogeneously be etched. In addition to thisI2 reduces the etch yield of Al2O3, so the etch selectivity ofSi can be enhanced with this etch mask.A layer of 50nm Al2O3 was evaporated on the prede-ﬁned Si waveguides using electron beam evaporation in aOxygen rich atmosphere. First the grating was etched intothis layer with FIB, using triﬂuoroacedtamide (TFA) as etchenhancer. The parameters for this etch process were: dose12.3 1017 Ga+/cm2, beam energy 30keV, beam current 50pA.The process was optimized for complete mask breakthrough.In principle the second etch step, copying of the gratinginto the silicon, can be performed with conventional etchingtechniques, but for process simplicity and speed we choseto perform it also with FIB. The ion beam was scanned ona rectangle covering the entire grating, with I2 as selectiveetchant, 30keV acceleration voltage and 50pA beam current.We optimized the etch dose and measured a maximum cou-102Fig. 5. The coupling efﬁciency of three gratings fabricated with I2 andAl2O3. The width of the scanned rectangles in the ﬁrst etch step was variedfrom 280nm to 350nm to vary the central frequency.pling efﬁciency of 24% (by ﬁtting a gaussian to the curve)with an etch dose of 0.4 1017 Ga+/cm2 and a slit widthof 280nm for the ﬁrst step. Cross-sections of the fabricatedstructured showed that the Si is slightly etched in the ﬁrstetch step. Figure 4 shows that the second etch step produces asomewhat asymmetric slit proﬁle, which could not be changedby changing the way of scanning the ion beam during etching.It is probably caused by the asymmetry in the gas injectionsystem. The original devices were designed to operate at acentral wavelength of 1550nm, with the ﬁbers positioned atan angle of 10 degrees. The ﬁrst devices made with FIB hadcentral wavelengths near 1600nm however. The two step etchprocess allows for a good control of slit width and depth. Thewidth of the scanned rectangles in the ﬁrst step determines theﬁnal slit width, whereas the dose of the second step determinesthe etch depth. Figure 5 shows the optimization of the slitwidth in the ﬁrst etch step. As expected from simulations thecentral wavelength shifts to smaller values as the slits becomebroader. By changing the width of the scanned rectangles from280nm to 350nm, without changing the areal dose, the centralwavelength shifts from 1611nm to 1576nm and the efﬁciencydrops only 1%. The complete etch process for one gratingcoupler with 25 slits takes about 5 minutes.IV. CONCLUSIONSWe have fabricated a grating coupler in a 10μm wide Siwaveguide using focused-ion-beam. The ion induced crystaldamage and the resulting optical losses can be minimizedby using I2 as selective etch gas and Al2O3 as hard mask.This resulted in a ﬁbre-to-chip coupling efﬁciency of 24%,which is comparable to devices fabricated with conventionaltechniques. This proves that the ion induced optical losses canbe circumvented, and opens the way to fast FIB prototyping,high resolution device modiﬁcation, resonant cavity tuning,slanted facet etching, and much more.V. AKNOWLEDGEMENTSThis work was partly supported by the European unionthrough the Network of Excellence ePIXnet, by the BelgianIAP-PHOTON network and the Fund for Scientiﬁc Research(FWO). The authors would also like to thank F. Van Laereand D. Taillaert for useful discussions and help with themeasurement setup.REFERENCES[1] J. Gierak, E. Bourhis, M. N. M. Combes, Y. Chriqui, I. Sagnes,D. Mailly, P. Hawkes, R. Jede, L. Bruchhaus, L. Bardotti, B. Prevel,A. Hannour, P. Melinon, A. Perez, J. Ferre, J. P. Jamet, A. Mougin,C. Chappert, and V. Mathet, “Exploration of the ultimate patterningpotential achievable with focused ion beams,” Microelectr. Eng., vol.78-79, pp. 266–278, 2005.[2] C. Peeters, E. Fluck, A. M. Otter, M. L. M. Balistreri, J. P. Korterik,L. Kuipers, and N. F. van Hulst, “Photon scanning tunneling microscopyof tailor-made photonic structures,” Appl. Phys. Lett., vol. 77, no. 1, pp.142–144, 2000.[3] D. J. Moss, V. G. Ta’eed, B. J. Eggleton, D. Freeman, S. Madden,M. Samoc, B. Luther-Davies, S. Janz, and D. X. Xu, “Bragg gratingsin silicon-on-insulator waveguides by focused ion beam milling,” Appl.Phys. Lett., vol. 85, no. 21, pp. 4860–4862, 2004.[4] S. Cabrini, A. Carpentiero, R. Kumar, L. Businaro, P. Candeloro,M. Prasciolu, A. Gosparini, C. Andreani, M. De Vittorio, T. Stomeo,and E. Di Fabrizio, “Focused ion beam lithography for two dimensionalarray structures for photonic applications,” Microelectr. Eng., vol. 78-79,pp. 11–15, 2005.[5] L. Vogelaar, W. Nijdam, H. A. G. M. van Wolferen, R. M. de Ridder,F. B. Segerink, E. Fluck, L. Kuipers, and N. F. van Hulst, “Largearea photonic crystal slabs for visible light with waveguiding defectstructures: Fabrication with focused ion beam assisted laser interferencelithography,” Adv. Materials, vol. 13, no. 20, pp. 1551–+, 2001.[6] D. Taillaert, P. Bienstman, and R. Baets, “Compact efﬁcient broadbandgrating coupler for silicon-on-insulator waveguides,” Opt. Lett., vol. 29,no. 23, pp. 2749–2751, 2004.[7] P. Dumon, W. Bogaerts, D. Van Thourhout, D. Taillaert, R. Baets,J. Wouters, S. Beckx, and P. Jaenen, “Compact wavelength router basedon a Silicon-on-insulator arrayed waveguide grating pigtailed to a ﬁberarray,” Opt. Express, vol. 14, no. 2, pp. 664–669, 2006.[8] A. Lugstein, B. Basnar, G. Hobler, and E. Bertagnolli, “Current den-sity proﬁle extraction of focused ion beams based on atomic forcemicroscopy contour proﬁling of nanodots,” J. Appl. Phys., vol. 92, no. 7,pp. 4037–4042, 2002.[9] K. E. Zinoviev and C. Dominguez, “Diffraction grating couplers milledin Si3N4 rib waveguides with a focused ion beam,” Opt. Express, vol. 13,no. 21, pp. 8618–8624, 2005.[10] www.srim.org, “Stopping and range of ions in matter.”[11] Y. Sugimoto, M. Taneya, H. Hidaka, and K. Akita, “Reduction ofInduced Damage in Gaas Processed by Ga+ Focused-Ion-Beam-AssistedCl-2 Etching,” J. Appl. Phys., vol. 68, no. 5, pp. 2392–2399, 1990.1032006 3rd IEEEInternationalConference on13 - 15 September 2006Crowne Plaza Ottawa Hotel,Ottawa, ON, CanadaIEEE Catalog Number: 06EX1276CISBN Number: 1-4244-0096-1Library of Congress: 2005938119SPONSORED BY IEEE LASERS AND ELECTRO-OPTICS SOCIETYGroupIVPhotonicsForewordLeos WebsiteAcknowledgementAuthor IndexTable of ContentsCommittee ListSearch Help© Copyright

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Focused-ion-beam fabricated vertical fiber couplers on silicon-on insulator waveguides

Jonathan Schrauwen

Dries Van Thourhout

Roel Baets

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Focused-ion-beam fabricated vertical fiber couplers on silicon-on-insulator waveguides

APPLIED PHYSICS LETTERS 89, 141102 2006Focused-ion-beam fabricated vertical fiber couplers on silicon-on-insulatorwaveguidesJonathan Schrauwen,a Dries Van Thourhout, and Roel BaetsPhotonics Research Group, Department of Information Technology, Ghent University-IMEC, 9000 Gent,BelgiumReceived 26 April 2006; accepted 8 August 2006; published online 2 October 2006Focused-ion-beam technology is a versatile micro- and nanostructuring tool, in which material issputtered locally by a finely focused ion beam. But for photonic applications this process has theinherent inconvenience of generating optical losses due to ion induced damage. Processoptimization is needed to reduce these losses. In this letter the authors present the focused-ion-beamfabrication of shallow gratings in silicon-on-insulator waveguides for the coupling of light to avertically positioned fiber. Using Al2O3 as mask and I2 as selective etchant this resulted in a 24%fiber-to-chip coupling efficiency, which is comparable to similar devices fabricated withconventional etching techniques. © 2006 American Institute of Physics. DOI: 10.1063/1.2356311Microfabrication with focused ion beam FIB consistsof hitting a substrate locally with high energy ions; in mostcommercial systems these are gallium ions with energiesaround 30 keV. If the substrate is crystalline this processinduces lattice damage, makes the top layer amorphous, andimplants ions deeper into the substrate.1,2,15 These effectscause optical losses and make the direct fabrication of low-loss photonic devices nontrivial. FIB has been used to studyvarious phenomena involving light in waveguides3,4 and al-lows the direct fabrication of structures such as photoniccrystals.5,6 But there are few or no reports in literature aboutlow-loss photonic devices fabricated in silicon-on-insulatorby means of FIB.The silicon-on-insulator SOI platform is a promisingcandidate for future ultra-compact photonic integrated cir-cuits because of its compatibility with complementary metal-oxide semiconductor CMOS technology.7 The high indexcontrast in this material system allows for the fabrication ofshort waveguide bends and thus circuits with a high degreeof integration. The SOI wafers we use have a thin top siliconlayer of 220 nm and a bottom oxide thickness of 1 m. Oneof the difficulties of a high contrast platform is the couplingof light from optical fibers due to the large mode-size mis-match. This problem can be solved by using tapers or gratingcouplers. The latter are shallow gratings in broad waveguidesthat diffract light out of the waveguides into vertically posi-tioned single-mode fibers. In previous work we have fabri-cated these shallow grating couplers with a CMOS compat-ible process 248 nm deep UV lithography and inductivelycoupled plasma etching and optimized the parameters foroptimal coupling efficiency 25% and more8 and 1550 nmoperation.9 However, it would be interesting to be able tomake structures in situ, anywhere on a wafer. This wouldallow fast prototyping, trimming, and fault testing. FIB is theideal tool for this. In this letter we present a method to makegrating couplers with FIB, featuring a comparable efficiencybut much larger flexibility than those of traditional fabrica-tion methods. We compare different FIB etching schemesand optimize for lowest ion damage and best coupling effi-ciency. First grating couplers were etched directly into sili-aElectronic mail: jonathan.schrauwen@intec.ugent.be0003-6951/2006/8914/141102/3/$23.00 89, 14110Downloaded 20 Oct 2006 to 157.193.172.109. Redistribution subject tocon waveguides using different acceleration voltages, withand without added gas. The efficiencies of these couplers arerather low, however. Therefore we have investigated differ-ent protective hard masks and chose Al2O3 as the best can-didate. Waveguide loss measurements were performed toprove the protective ability of Al2O3. Finally we combinedthis hard mask with I2 as selective etchant and obtained afiber-to-chip coupling efficiency of 24%.We use an FEI Nova 600 dualbeam that generates a10 nm wide beam of 30 keV Ga+ ions. This beam is focusedonto 10 m wide and 220 nm thick Si waveguides on SOI,with the purpose of etching a shallow grating pattern. Themachine is equipped with a commercial gas injection systemthat can deliver I2, H2O, and trifluoroacetamide TFA closeto the beam spot through a fine needle. To determine thecoupling efficiency we use a fiber-to-fiber transmission mea-surement for TE polarization. The structure consists of aninput coupler, a 10 m wide waveguide, and an output cou-pler. We assume that both couplers are identical.A first approach is to use FIB to directly etch the gratingin Si. We have tried this with 10 and 30 keV Ga+ ions, withand without I2 as selective etchant. The results are presentedin Fig. 1. Each curve in the graph is the most efficient froma set with different etch doses. The dose variation rangedfrom 41017 to 401017 Ga+/cm2 in the case of directetching and from 0.61017 to 3.61017 Ga+/cm2 for the I2etching process. The 30 keV etches were performed with abeam current of 50 pA, the 10 keV etches with a current of120 pA for the etching without gas, and 50 pA for I2 etching.By making cross sections we verified that the expected opti-mal grating profile was covered in this dose range. Westrived for the optimized gratings of previous work: 25 lineswith a width of 315 nm, a spacing of 315 nm, and an etchdepth of 70 nm. But investigation of the cross sectionsshowed that the structures are more “rounded” than the grat-ings fabricated with conventional techniques. This is causedby the Gaussian10 profile of the beam and redeposition. Weperformed finite-difference time-domain simulations with el-lipselike grating slits rather than square ones and showed thatthere is little effect on the maximum obtainable efficiency ifetch depth and filling factor are optimized. Figure 1 showsthat the gratings fabricated without I2 are very inefficient,© 2006 American Institute of Physics2-1 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp141102-2 Schrauwen, Van Thourhout, and Baets Appl. Phys. Lett. 89, 141102 2006although they have cross sections that look more or less likethe optimal ones. This very low efficiency of directly etchedgratings was already noticed11 and is caused by the ion in-duced damage in the material. Figure 1 also shows that thegratings etched with I2 are much more efficient about a fac-tor of 20, although the cross section does not look lessrounded see inset of Fig. 1. This is explained as follows: I2molecules other halogens can also be used are adsorbed onthe Si surface and facilitate the extraction of Si atoms fromthe collision cascade by the formation of volatile species.12So smaller doses are needed when I2 gas is used, and less iondamage is generated. Similar effects were seen on opticallyactive material.13 Figure 1 shows a maximum fiber-to-chipcoupling efficiency of about 11%. This is still far below theefficiency of gratings made with classical etching techniquesabout 25% Ref. 8. The reason for this are the nonoptimaldimensions of the gratings. The inconvenience of the en-hanced etching process is the loss of dimensional control:one cannot etch 90° sidewalls nor easily control the width ofa hole. This is why we chose to work with a hard etch maskrather than to further optimize the direct etching with I2.To choose the right material as etch mask we have cal-culated the sputter yields for several materials using the pro-gram SRIM stopping and range of ions in matter14. Severalmetals have a lower sputter yield than Si Cu, Ni, but evapo-rated films of these materials have grains. Since the etchyield depends on crystal orientation, the etching of thesegrainy films is inhomogeneous. Therefore we chose Al2O3,an amorphous material, as etch mask. The difference in sput-ter yields of Si and Al2O3 can be enhanced by using I2 gas. Athin film of Al2O3 was evaporated on Si with an electronbeam evaporation system. The purpose of this hard mask istwofold: it has to enhance dimensional control and it has toprotect Si from ion damage in the regions where it does notneed to be etched, e.g., during focalization of the beam. Ascan be calculated with the SRIM software the penetrationdepth of 30 keV Ga ions in Al2O3 is smaller than 30 nm, sowe chose a thickness of 50 nm for optimal protection. Figure2 shows some experiments demonstrating the protective ca-pacity of Al2O3. A section of 20 m on a multimode 10 mwide Si waveguide was etched with a perpendicularly inci-dent focused ion beam. The doses were kept low enough tomake the etched depth very small 10 nm, so the trans-mission through these implanted waveguides is a measurefor the optical losses generated by FIB etching. Figure 2Downloaded 20 Oct 2006 to 157.193.172.109. Redistribution subject toshows the losses for 10 and 30 keV implanted waveguides,where the losses are presented in units of dB/m. As ex-pected the 30 keV implantation produces more lossywaveguides, because the implantation depth is greater. Theoptical loss is of the order of 1 dB/m, about four orders ofmagnitude bigger than that for nonimplanted waveguides.This explains why the directly etched gratings from Fig. 1are so inefficient. For doses above 31017 Ga+/cm2 notransmission could be measured in our setup. We did thesame experiment with waveguides that were covered with50 nm of Al2O3. The inset in Fig. 2 shows the trajectory of500 Ga+ ions that hit a 50 nm layer of Al2O3, after beingaccelerated to 30 keV. Not one of them can penetrate to theSi, so as expected no significant dependence of the losses onthe implantation dose was measured. This proves the protec-tive capacity of the Al2O3 mask.Finally we want to use this protective mask as a hardetch mask for making grating couplers in Si. FIB is used forthe structuring of the hard mask in a first step. Afterwardsone could use classic etching techniques, such as reactive ionetching, to etch the Si, but for process simplicity and speedFIG. 1. Shallow grating couplers etched with FIB inSOI waveguides. By using I2 as etch enhancement gasthe efficiency is greatly improved. The top left insetshows a FIB cross section through one grating slitetched with I2.FIG. 2. Losses measured in waveguides that were etched with relativelysmall doses of Ga+ ions. Higher energy implantation generates damagedeeper into the layer and thus more loss. By adding Al2O3 as protectivelayer no significant losses were measured. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp141102-3 Schrauwen, Van Thourhout, and Baets Appl. Phys. Lett. 89, 141102 2006we chose to do the second step also with FIB. Figure 3presents the measured coupling efficiencies for differentetching schemes. The gray curve is a structure where onlythe first etch step is performed: the hard mask etch. We var-ied the dose and etch gas to optimize breakthrough. The bestresults were obtained by etching with TFA gas, dose of12.31017 Ga+/cm2, beam energy of 30 keV, and beam cur-rent of 50 pA. A cross section after this etch step Fig. 4ashows that also the Si is etched, which explains the reason-able coupling efficiency of this grating. Figure 3 shows fourdifferent processes for the second etch step: 30 and 5 keV,with and without I2. This second etch step was performed byscanning a rectangle covering the entire grating. The 30 keVetch without gas 4.11017 Ga+/cm2, 300 pA produced avery lossy coupler as expected, but by adding I2 to the pro-cess and doing some dose optimization we reached a cou-pling efficiency of 24% 0.41017 Ga+/cm2, 50 pA. Thisefficiency is comparable to those of couplers etched withtraditional methods. The total etch time for one grating cou-FIG. 3. Gratings fabricated with Al2O3 as mask using a two step etch pro-cess. The gray curve shows the coupling efficiency of a grating etched byonly the first step. The second step was varied, and the most efficient grat-ings were produced with 30 keV ions and I2 etch gas. The inset shows across section of a grating etched with mask and I2.FIG. 4. Cross section through two slits of a grating etched a with TFA formask break through and b followed by a second etch step with I2 gas and30 keV ions 0.91017 Ga+/cm2. The white dotted lines were added toincrease the contrast.Downloaded 20 Oct 2006 to 157.193.172.109. Redistribution subject topler is about 5 min. Because of the lower damage depth ofthe 5 keV etch without gas 4.11017 Ga+/cm2, 70 pA, theproduced gratings were more efficient than the 30 keV etchwithout gas, but we could not reach very high efficiencies byadding I2 to this process 0.31017 Ga+/cm2, 70 pA. Thismight be caused by the lower chemical enhancement atlower acceleration voltages. Figure 4b shows a cross sec-tion of a grating that was etched with 30 keV ions and I2,with a dose of 0.91017 Ga+/cm2. The asymmetry could notbe removed by changing the scanning of the ion beam, so wethink that it is caused by the directional gas injection of oursystem.To conclude we have optimized the FIB etching of shal-low grating couplers in silicon-on-insulator waveguides. Di-rectly etched gratings are very lossy due to ion induced dam-age in the Si, but by adding I2 as chemical enhancement andAl2O3 as mask we reached a coupling efficiency of 24%,which is comparable to those of similar devices fabricatedwith classical etching techniques. This proves that FIB canbe used for the fabrication of low-loss photonic devices.This work was partly supported by the European Unionthrough the Network of Excellence ePIXnet, by the BelgianIAP-PHOTON Network, and the Fund for Scientific Re-search. The authors would also like to thank F. Van Laereand D. Taillaert for useful discussions and help with the mea-surement setup.1S. Rubanov and P. R. Munroe, J. Microsc. 214, 213 2004.2A. A. Tseng, J. Micromech. Microeng. 14, R15 2004.3C. Peeters, E. Fluck, A. M. Otter, M. L. M. Balistreri, J. P. Korterik, L.Kuipers, and N. F. Van Hulst, Appl. Phys. Lett. 77, 142 2000.4D. J. Moss, V. G. Ta’eed, B. J. Eggleton, D. Freeman, S. Madden, M.Samoc, B. Luther-Davies, S. Janz, and D. X. Xu, Appl. Phys. Lett. 85,4860 2004.5S. Cabrini, A. Carpentiero, R. Kumar, L. Businaro, P. Candeloro, M. Pras-ciolu, A. Gosparini, C. Andreani, M. De Vittorio, T. Stomeo, and E. DiFabrizio, Microelectron. Eng. 78-79, 11 2005.6L. Vogelaar, W. Nijdam, H. A. G. M. Van Wolferen, R. M. De Ridder, F.B. Segerink, E. Fluck, L. Kuipers, and N. F. Van Hulst, Adv. Mater. Wein-heim, Ger. 13, 1551 2001.7W. Bogaerts, R. Baets, P. Dumon, V. Wiaux, S. Beckx, D. Taillaert, B.Luyssaert, J. Van Campenhout, P. Bienstman, and D. Van Thourhout, J.Lightwave Technol. 23, 401 2005.8P. Dumon, W. Bogaerts, D. Van Thourhout, D. Taillaert, R. Baets, J.Wouters, S. Beckx, and P. Jaenen, Opt. Express 14, 664 2006.9D. Taillaert, P. Bienstman, and R. Baets, Opt. Lett. 29, 2749 2004.10A. Lugstein, B. Basnar, G. Hobler, and E. Bertagnolli, J. Appl. Phys. 92,4037 2002.11K. E. Zinoviev and C. Dominguez, Opt. Express 13, 8618 2005.12K. Edinger and T. Kraus, Microelectron. Eng. 57-58, 263 2001.13Y. Sugimoto, M. Taneya, H. Hidaka, and K. Akita, J. Appl. Phys. 68, 23921990.14J. F. Ziegler, Nucl. Instrum. Methods Phys. Res. B 219-220, 1027 2004.15J. Gierak, E. Bourhis, M. N. M. Combes, Y. Chriqui, I. Sagnes, D. Mailly,P. Hawkes, R. Jede, L. Bruchhaus, L. Bardotti, B. Prevel, A. Hannour, P.Melinon, A. Perez, J. Ferre, J. P. Jamet, A. Mougin, C. Chappert, and V.Mathet, Microelectron. Eng. 78-79, 266 2005. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Focused-Ion-Beam Fabricated Vertical Fiber Couplers on Silicon-on-Insulator Waveguides

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Focused-ion-beam fabricated vertical fiber couplers on silicon-on-insulator waveguides

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Ghent University Academic Bibliography

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