A Compact Hybrid Silicon/Electro-Optic Polymer Resonant Cavity Modulator Design

Transcription

1 A Compact ybid Silicon/lecto-Optic Polyme Resonant Cavity Modulato Design K. K. McLauchlan and S. T. Dunham lectical ngineeing Depatment, Univesity of Washington, Seattle, WA, USA ; ABSTRACT The design and simulation of a novel esonant cavity optical modulato incopoating a hybid silicon/electo-optic polyme slot waveguide stuctue is pesented in this wok. The device utilizes the electo-optic polyme in the cavity egion to povide an active mateial fo modulation and includes distibuted Bagg eflectos in single mode silicon waveguide egions at each end of the cavity to ceate a naow esponse peak at the esonant wavelength. Simulation esults show that this electo-optic modulato design can simultaneously attain a lage modulation depth, shot device length and a low dive voltage, all of which ae expected to be necessay fo futue high speed integated optics devices. The high opeating fequency and complex natue of the stuctue lead to a need fo full 3D simulations in ode to obtain accuate popagation chaacteistics, paticulaly concening scatteing losses. oweve, 3D simulations ae vey computationally expensive, especially duing design optimization. Theefoe, the peiodicity of the device has been exploited to allow a cascade matix appoach to be employed to educe the necessay computational esouces equied fo accuate simulation of the popagation chaacteistics. The design and fabication pocess have been chosen to allow fo the majoity of the fabication to be completed befoe the electo-optic polyme is intoduced into the pocess, which enables the use of well-established CMOS pocessing techniques, and should acceleate the tansition to hybid silicon/electo-optic polyme devices in futue integated optics applications. Keywods: Optical modulation, lecto-optic polyme, Integated optics, Silicon photonics 1. INTRODUCTION Thee has been an inceasing inteest in high-density integated optics in ecent yeas. One of the moe pomising ways of achieving this is though the use of silicon-on-insulato (SOI) wafes, which ae also CMOS compatible, theeby allowing fo the use of the well-established pocessing methods fo silicon. In ode to ealize fully integated optics, all optical components must be not only CMOS compatible, but also have small dimensions and excellent opeational chaacteistics. Optical modulatos ae one component in which it is challenging to simultaneously achieve small size along with a stong, high-speed esponse in silicon devices. The V π -L poduct, which combines the length of the active egion with the voltage necessay to achieve a phase shift of π adians, is a common figue of meit fo Mach-Zehnde (MZ) modulato stuctues. A silicon MZ modulato has been demonstated that exhibits a 1Gz modulation bandwidth, but the V π -L poduct is still 8V-cm [1]. A Faby-Peot (FP) esonant cavity device in SOI has been demonstated that is less than 2µm in length, but an applied voltage of 3.7V only esulted in a modulation depth of the tansmitted intensity of 53% [2], which may not be sufficient fo accuate detection of modulation. A MZ modulato demonstated with an electo-optic (O) polyme exhibited a V π -L poduct of 2.2V-cm, which is even bette than lithium niobate MZ modulatos, which ae the cuent industy standad [3]. An O polyme/silicon micoing esonato has been demonstated that combines the benefits of both silicon and the fast, stong esponse of O polymes. This stuctue has a ing adius of 1µm and equied an applied voltage of 2V in ode to change the output intensity by 5dB [4]. Recent advances in the design of O polymes have esulted in polymes with both a fast esponse and vey lage O coefficients, which allow fo the fast switching with low applied voltages that will be necessay fo integated optics applications. These O polymes should allow fo excellent opeational chaacteistics in an optical modulato, but due to the elatively low index of efaction of the polymes, device footpints ae geneally quite lage and O polymes ae not CMOS compatible fo integated applications. The stuctue poposed hee combines the benefits of both silicon and O polymes to ceate a high-speed device with a lage modulation depth, low dive voltage, and vey compact size that allows all of the silicon pocessing to be completed using standad CMOS pocessing befoe the O polyme is intoduced into the stuctue. Tuning the Optic Response of Photonic Bandgap Stuctues III, edited by Paul V. Baun, Shaon M. Weiss, Poc. of SPI Vol. 6322, 63223, (26) X/6/$15 doi: / Poc. of SPI Vol

2 2. RSONANT CAVITY MODULATOR STRUCTUR This esonant cavity modulato design incopoates a set of distibuted Bagg eflectos on each side of a hybid slot waveguide cavity. The seies of Bagg eflectos ae ceated with cicula holes appopiately spaced in a single-mode silicon waveguide to incease the eflectivity on each side of the esonant cavity, as illustated in Fig. 1. The esonant cavity beaks the peiodicity of the Bagg eflecto and esults in a shap tansmission peak at the esonant wavelength, detemined by the length of the cavity egion. The slot waveguide in the cavity egion guides light in the naow lowe index of efaction egion between two highe index egions, in this case silicon idges [5]. The slot waveguide is consideed hybid as it incopoates an O polyme as the low index mateial in which the light will be guided. Because light is confined pimaily to the O polyme, as shown in Fig. 2, applying an electic field acoss the polyme will change its index of efaction. The stength of this esponse will depend upon the mateial chaacteistics of the O polyme, the applied field duing device opeation, as well as the success of the poling pocess duing fabication, which aligns the chomophoes in the polyme and allows fo the index change. Since the silicon idges can also seve as electodes, the hybid slot waveguide stuctue should allow fo high-speed modulation will small applied voltages. P Polyme / P Fig. 1. Top view of the hybid silicon/electo-optic polyme esonant cavity modulato. The total length of the device is less than 3µm, and the bulk of the pocessing can be completed using standad CMOS fabication techniques. Polyme Poled Polyme Si2 Fig. 2. Coss-section of a finite element simulation of the hybid slot waveguide stuctue showing the field confinement in the low index O polyme egion athe than the highe index silicon idges. Fo the design of this device, the dimensions of the hybid waveguide wee optimized to achieve the lagest change in the popagation constant, as shown in Fig. 3. This change in popagation constant, β, is detemined fo a given index of efaction of the O polyme (n 1.6 at λ 1.55µm) and an assumed O coefficient fo the poled polyme ( 33 1pm/V) with q. 1 shown below: Poc. of SPI Vol

3 2π β β1 β 2 ( n1 n2 ) (1) λ whee λ is the wavelength in fee space, n 1 is the index of efaction with no applied voltage, and n 2 is the index of efaction with an applied voltage, V a, and electode spacing, w eo, as given by n n V a 2 n1 (2) 2weo 7 s- w.1nm 6 - w.15nm w.2nm w.25nm $ Width of the Polyme Region w, (nm) Fig. 3. The change in popagation constant calculated fom finite element simulations showing that thee is a lage change in popagation constant fo a given applied voltage as the width of the polyme egion is educed and that the width of the silicon idges should be wide enough to pomote confinement of the field in the polyme, but not lage enough to stongly confine a guided mode individually. The hybid slot waveguide stuctue educes the necessay applied voltage fo a given change in popagation constant since the silicon idges, which ae necessay to facilitate guiding of light in the low index polyme, also act as electodes. The esulting electode spacing is significantly educed fom that equied fo an all-polyme waveguide, whee the electodes must be placed sufficiently fa away fom the waveguide to pevent excessive popagation loss. Also illustated in Fig. 3, as the minimum featue size in moden CMOS fabication facilities continues to decease, the dimensions of the O polyme slot in the hybid waveguide can also be deceased, which will allow fo a lowe opeating voltage to achieve a simila change in popagation constant and theefoe the esonant wavelength. 3. SIMULATION 3.1 Simulation Method A cascade matix method has been employed to allow fo finite element modeling of lage 3D stuctues, which is geneally vey difficult due to the significant computational esouces/time equied to pefom the full 3D analysis. With the cascade matix method, individual sections of the device ae each simulated sepaately, and the popagation chaacteistics, T, of each section ae then used to build up the entie device, as shown in q. 3 and Fig. 4 [6]. With this method, a fast 3D analysis of a 5µm stuctue, which would be infeasible fo a full 3D finite element simulation, can be completed in less than two hous. The esonant cavity modulato stuctue lends itself well to the cascade method due to the peiodicity of the device. As shown below, thee ae only fou unique sections that need to be analyzed to detemine the tansmission chaacteistics of the device. This also allows fo vey fast design vaiation and optimization, as a change in the stuctue equies only a single section be simulated again, athe than the entie device. T tot TgTdTdTd TdTtTcT ctctctt TdTdTdTd Tg (3) Poc. of SPI Vol

4 Fig. 4. Individual sections used fo finite element simulations of the stuctue. Due to the peiodicity of the device, only fou unique simulations wee equied. A compaison of the simulation esults of a manageable section of the Bagg eflecto is shown below. As the figue illustates, thee is excellent ageement between the cascade matix method and the full finite element simulation, which equied moe than 7 hous to complete, while the individual sections equied appoximately 15 minutes each. m -D -3 Wavelen9th ct1? Fig. 5. A compaison of the simulations fo a 7-peiod Bagg eflecto illustate excellent ageement between the full simulation and cascade stuctue. 3.2 Analysis of the simulation esults of an ideal stuctue The figues of meit fo this device include the achievable modulation depth, which elates the diffeence between the outputs at the detection wavelength with and without an applied field, the full-width at half maximum (FWM), the maximum tansmission (the thoughput at the esonant wavelength), and the oveall length of the device. Thee is a geneal tade-off between the cavity length and the modulation depth, which limits the minimum size of the device fo a given applied voltage. This elationship is illustated in Fig. 6a, whee the modulation depth inceases with both inceasing cavity length and numbe of peiods in the Bagg eflecto. Inceasing the numbe of peiods in the Bagg eflecto inceases the modulation depth and deceases the FWM up to a satuation point because each additional peiod not only inceases the eflectivity, but it also adds loss to the system, which significantly educes the tansmission at the esonant wavelength. This tansmission limit will be detemined by the detection capabilities of the system fo a given application, which will limit the numbe of eflecto peiods that can be included in the system, as shown in Fig 6b. Reducing the size of the holes in the Bagg eflecto potentially inceases the cost of fabication, but also impoves though-put up to the point whee the diffeence in scatteing loss between the two diametes becomes negligible fo a given waveguide stuctue, as is also illustated in Fig. 6b fo hole diametes of 1nm and 75nm in a 6nm silicon waveguide. Poc. of SPI Vol

5 1 qn 8 7 m -D so C 4 D o 2 1. Lc5,_ - 1:1) i2o L25z. L5O; >< d2nm B d175nm 4 dl5onm e d125nm dloonm * d75nm Mumbo of Poioth in bq fleflocw Modulation Depth fo lnceasin9 Numbe of Peiods Fig. 6. Figue 6a demonstates the incease in achievable modulation depth fo a given hole diamete (1nm) as the cavity length and numbe of peiods ae inceased. Figue 6b illustates the limits to impoved pefomance by compaing the tade-offs between thoughput and modulation depth fo vaious hole diametes. Inceasing the applied voltage also inceases the change in popagation constant in the hybid slot waveguide, and is theefoe an altenative method of inceasing the modulation depth. Analysis of the poduct of the voltage necessay to achieve a given modulation depth compaed to the length of the cavity, the V MD -L poduct, shows that the stength of each appoach is simila, which leaves flexibility in design choices depending upon the desied application. 4. FABRICATION The esonant cavity modulato design has been ceated to allow fo the majoity of the pocessing to be completed with standad CMOS fabication techniques in ode to facilitate the tansition to oganic polyme devices. Theefoe the entie silicon potion of the device is fabicated befoe the O polyme is intoduced into the stuctue. Also, due to the small citical dimensions, the silicon potions of the waveguide, Bagg gating, and slot waveguide cavity egion must all be defined in a single lithogaphy and etch step, as alignment at this scale would be extemely difficult. In the expeimental demonstation electon-beam lithogaphy will be used; howeve, the minimum featue size has been kept lage enough to allow fo the entie device to be fabicated using moden CMOS technologies. Vaiations can also be expected to occu duing the fabication pocess, including vaiations of the doping concentations, dimensions of the stuctue, filling of the cavity slot with the O polyme, and etching sidewall oughness. The effects of each of these vaiations on device pefomance ae consideed in the following sections. 4.1 Doping concentation vaiation The vaiation of the concentation of the doped silicon idges is not expected to significantly vay the pefomance of the device. The desied doping concentation of 1 18 cm -3 has been chosen so that thee is not an appeciable optical loss due to popagation in the heavily doped idges as shown in Fig. 7 below, and a change of +/- 5% will not significantly change the popagation of light in the idges. The electical chaacteistics will also vay slightly due to the vaiation in doping concentation, but the 3% vaiation in the esulting time constant due to the doping vaiations will not significantly change the device opeation. Poc. of SPI Vol

6 -4 m -D Wavelen9th cnm? Fig. 7. Tansmission esponse fo vaious doping concentations in the silicon egions of the slot waveguide cavity. Since the vaiation is small fo doping concentations up to 1 18 cm -3, this is expected have little effect on modulato opeation. 4.2 Size vaiation Thee may be global vaiations in the dimensions of the waveguides due to the toleances of the fabication pocess, which is assumed to have a minimum line width of 1nm +/- 1%. These vaiations in dimensions will affect the eflectivity of the mios in the Bagg gating egions, as well as the active section of the hybid slot waveguide. Fabication vaiations that cause the width of the polyme egion to be inceased will esult in a slightly lowe change in popagation constant fo a given applied voltage, and a eduction in the width will actually slightly incease the change in popagation constant and impove the modulation depth. oweve, it is the vaiation in the diamete of the holes in the Bagg gating, which affects the eflectivity of the mios fo a given numbe of peiods, that will have the most significant impact on device opeation. Vaiations of dimension that esult in the full 1% ove-exposue will both incease the size of the holes in the Bagg eflecto and decease the width of the waveguide and theefoe incease the eflectivity fo a given numbe of peiods and shift the esonant wavelength by 3nm. oweve, unde-exposue of the stuctue will educe the eflectivity of the Bagg gating, as each hole will have a smalle effect, as illustated in Fig. 8 fo a hole diamete fo 1nm, although this effect could be educed by inceasing the length of the cavity egion. Theefoe the dimensions of the holes in the Bagg eflecto should be chosen so that vaiations in size esult in a toleable change in the modulation depth and tansmission fo the chosen cavity length a 5 4 -D o Ideal exposue B Unde-exposue of 1 O /o 4 Ove-exposue of 1 O /o 1 15 Numbe of peiods in the Ba99 eflecto Fig. 8. The modulation depth fo a stuctue with a 15µm cavity length and stuctue dimensions vaying up to +1% of the minimum linewidth (1nm). Unde-exposue both deceases the diamete of the holes in the Bagg eflecto and inceases the waveguide width, which significantly educes the achievable modulation depth fo a given applied voltage. Local diffeences due to small exposue and/o etching vaiations may also esult in the waveguide dimensions vaying within a single stuctue. These vaiations ae expected to be less than the global vaiations and will have little effect on Poc. of SPI Vol

7 the hybid slot waveguide cavity egion, but will have a somewhat moe significant impact on the tansmission chaacteistics in the eflecto egions as it will slightly change the hole spacing and the amount each hole will contibute to the eflectivity. Fig. 9 shows the modulation depth fo a Bagg eflecto composed of hole diametes andomly vaied by up to +/-2%, and demonstates that these local andom vaiations will have vey little effect on the tansmission chaacteistics of the device a 5 4 -D o ole Vaiations of +!2 /o 4 Ideal ole Dimensions 1 15 Numbe of peiods in the Ba99 eflecto Fig. 9. The modulation depth fo andom vaiations in hole diamete of +/-2%. These vaiations do not significantly change the tansmission chaacteistics of the device. 4.3 Polyme filling vaiation Filling of the cavity duing polyme pocessing is expected to be a cucial step in the fabication pocess, as the achievable change in popagation constant will depend on the ability of the polyme within the slot to be poled. If thee is poo filling of the slot with the polyme duing fabication, thee will be less active mateial available duing opeation. The eduction of the O polyme in the slot will educe the change in index of efaction with an applied field, and theefoe change the coesponding output by educing the modulation depth o equiing a much lage opeating voltage to achieve a simila esponse. Compounding the eduction in the change in index of efaction of the O polyme is inceased loss due to eflection at the point of coupling into the hybid slot waveguide. Because the index of efaction of the unfilled slot is lowe than the index of efaction of the O polyme, the eflection at the bounday will be geate fo the slot that has less polyme filling. Due to the inceased loss and eduction of the change in popagation constant, thee is a significant degadation in the achievable modulation depth fo a given applied voltage fo the lowe filling of the slot, as illustated in Fig. 1. Theefoe, the popeties of the paticula polyme chosen fo an application will need to be investigated to veify that sufficient slot filling is achievable, as this is expected to have the most significant impact on device opeation. m -D -5 a a a Modulation Depth fo lnceasin9 Numbe of Peiods in DBR Fig. 1. Maximum tansmission vesus the modulation depth fo vaious pecentages of filling of the O polyme in the slot of the hybid waveguide. The polyme filling will be an impotant facto to conside duing the fabication pocess, as poo filling can significantly affect the opeational chaacteistics. Poc. of SPI Vol

8 4.4 Sidewall shape and suface oughness vaiations The angle and oughness of the sidewalls of the silicon waveguide will likely affect the insetion loss and modulation depth of the device and may also facilitate coupling into highe ode modes and/o substate adiation modes. An estimate of acceptable sidewall oughness has been calculated accoding to Payne and Lacey [7]. Using this estimation, assuming a Gaussian autocoelation function and the wost-case coelation length, the sidewall oughness must be limited to 8nm in ode to avoid high popagation losses. Fabication of the silicon waveguide will be done with a dy etch pocess, simila to those commonly found in CMOS fabication facilities, as a design goal fo this device was the ability to manufactue it and incopoate it into futue highly integated optics applications. It is expected that a suface oughness of less than 2nm and a sidewall angle of ~9 should be achievable in these cicumstances, as the IBM photonics eseach goup has peviously demonstated this [8]. oweve, with the fabication facilities available at the Washington Technology Cente (WTC), which is the facility that will be used fo fabication of the expeimental stuctue, both sidewall angle and oughness ae expected to be an issue. The eactive ion etching system available at the WTC is a Tion Phantom Reactive Ion tche (RI) with O 2, SF 6, and CF 3 gases. Legtenbeg, et al. have done an extensive study on the etching of silicon with these gases and consideed the effects of vaious gas mixtues, RF powe and pessue, and these esults wee used as a stating point fo detemining the etch chemisty fo the silicon waveguide of the esonant cavity modulato [9]. Relatively good anisotopy and smooth etch suface wee achieved fo silicon stuctues etched in the Tion Phantom RI system with SF 6 at 3sccm, O 2 at 1sccm, CF 3 at 12sccm, RF powe of 1W and a chambe pessue of 1mba. oweve, the initial sidewall oughness was too lage to pevent consideable popagation losses in the waveguide. Theefoe, a sidewall smoothing pocedue will need to be employed in the expeimental fabication pocess to educe the oughness of the eactive ion etch. Two methods have been epoted that have significantly educed the sidewall oughness of silicon waveguides: wet oxidation at 1 C [1] and wet chemical smoothing [11]. Wet oxidation esulted in a lowe popagation loss, but the pocess does not allow fo the pecise contol of waveguide dimensions that is necessay fo this esonant cavity modulato. Theefoe the wet chemical RCA smoothing pocess poposed by Spaacin et al. will be used [11]. The CMOS-compatible RCA pocess involves a seies of oxidation and oxide stipping using shot dips in SC1, a dilute F solution, and SC2. The SC1 dip acts to emove oganics fom the suface by tapping them in an oxide. The F dip then emoves this oxide and leaves an -teminated silicon suface. Finally the SC2 emoves any metals fom the suface and ceates a potective oxide. Fig. 11 shows the esults of the pocess, which illustates smoothing of the sidewall suface, and this should be futhe enhanced with the impoved stuctue definition using electon beam lithogaphy. a V S Mg OS WO I kv 3 SS87l S 7 GIG VLTIF I SOS I I Fig. 11. Initial sidewall oughness fom RI etch pocess is shown in Fig. 11a (left). Fig 11b (ight) shows the sidewalls afte the RCA smoothing pocess and illustates the impovement in the sidewall oughness. Poc. of SPI Vol

9 5. CONCLUSION The design pesented in this wok simultaneously povides a shot device length, small applied voltages, and a fast esponse time using a standad CMOS pocessing technique fo the majoity of the fabication pocess and finally incopoating the O polyme in ode to take advantage of its fast, stong esponse. The design has been analyzed using a cascade matix method of finite element simulations, which easily incopoate design changes and allows fo fast, accuate design optimization. Analysis has also shown that this stuctue can be designed to be obust enough to ovecome the fabication toleances that can be expected in a CMOS pocess. The use of the SOI wafe and standad pocessing should povide a useful optical modulato fo use in futue integated optics applications. ACKNOWLDGMNTS Reseach suppot is gatefully acknowledged fom a National Science Foundation Gaduate Reseach Fellowship and the National Science Foundation Cente on Mateials and Devices fo Infomation Technology Reseach (CMDITR), DMR RFRNCS 1. Liu, R. Jones, L. Liao, D. Samaa-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccla, A high-speed silicon optical modulato based on a metal-oxide-semiconducto capacito, Natue, vol. 427, pp , Feb C. A. Baios, V. R. Almeida, R. R. Panepucci, B. S. Schmidt, and M. Lipson, Compact silicon tunable Faby-Peot esonato with low powe consumption, I Photon. Technol. Lett., vol. 16, pp , Feb Y. Shi, C. Zhang,. Zhang, J.. Bechtel, L. R. Dalton, B.. Robinson, and W.. Steie, Low (sub 1-volt) halfwave voltage polymeic electo-optic modulatos achieved by contolling chomophoe shape, Science, vol. 288, pp , Apil T. Baeh-Jones, M. ochbeg, G. Wang, R. Lawson, Y. Liao, P. Sullivan, L. Dalton, A. Jen, and A. Schee, Optical modulation and detection in slotted silicon waveguides, Optics xpess, vol. 13, pp , July V. R. Almeida, Q. Xu, C. A. Baios, and M. Lipson, Guiding and confining light in void nanostuctues, Optics Lett. vol. 29, pp , June D. M. Keans and R. W. Beatty, Basic Theoy of Waveguide Junctions and Intoductoy Micowave Netwok Analysis, Pegamon Pess, Oxfod, London, F. Payne and J. Lacey, A theoetical analysis of scatteing loss fom plana optical waveguides, Optical and Quantum lecton., vol. 26, pp , Nov IBM Photonics Reseach, domino.eseach.ibm.com/comm/eseach_pojects.nsf/pages/photonics.fabication.html 9. R. Legtenbeg,. Jansen, M. deboe, and M. lwenspoek, Anisotopic eactive ion etching of silicon using SF 6 /O 2 /CF 3 gas mixtues, J. lectochem. Soc., vol. 142, pp , June K. Lee, D. Lim, L. Kimeling, J. Shin, and F. Ceina, Fabication of ultalow-loss Si/SiO 2 waveguides by oughness eduction, Optics Lett., vol. 26, pp , Dec D. Spaacin, S. Specto, and L. Kimeling, Silicon waveguide sidewall smoothing by wet chemical oxidation, J. Lightwave Technol., vol. 23, pp , August 25. Poc. of SPI Vol