WAVEGUIDES fabricated by electron beam irradiation

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1 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 16, NO. 5, MAY Thermooptic Interferometric Switches Fabricated by Electron Beam Irradiation of Silica-on-Silicon Ary Syahriar, Richard R. A. Syms, and Thomas J. Tate Abstract The thermal stability of channel optical waveguide devices fabricated by electron beam irradiation of plasmaenhanced chemical vapor deposition (PECVD) silica-on-silicon is investigated. The degree of stability is dependent on the starting material and on the use of thermal annealing prior to irradiation. High-temperature postprocessing is shown to reduce modal confinement, increasing losses in waveguide bends and the coupling coefficient in directional couplers. A low-temperature cladding process based on a thick MgF 2 layer is described, and low-loss thermooptic Mach Zehnder interferometric switches are demonstrated. Index Terms Integrated optics, optical waveguides, silica-onsilicon, optical switching. I. INTRODUCTION WAVEGUIDES fabricated by electron beam irradiation [1] [3] offer an alternative to topographic guides [4] in silica-on-silicon planar lightwave circuits (PLC s). Comparable structures can be fabricated by optical irradiation, either at IR wavelengths (using densification by a CO 2 laser [5]) or at UV wavelengths (by color-center formation [6] or photopolymerization [7]). In each case, the potential advantage over the topographic method is that a buried channel guide may be formed directly, avoiding the difficulties associated with etching and cladding of rib structures. Electron beam irradiation is now relatively advanced. Substrate absorption and OH contamination have been minimized, and low (0.1 db/cm) and spectrally flat propagation loss has been obtained in materials deposited by plasmaenhanced chemical vapor deposition (PECVD) [8]. A range of channel guide power-splitters has been demonstrated, using a simple process based on irradiation through an electroplated Au surface mask [9]. The most significant difficulties remain the small index change (, depending on the material [8]), and the low stability of the induced changes. While thermal instability should not compromise lifetimes at working temperatures [3], it restricts the thermal load allowed in subsequent processing, especially deposition of the cladding used to isolate the guided mode from the heater electrode in a thermooptic switch [4]. For low guides, a thick layer of silicate glass is used. Sputtering is one method of deposition, but is normally too slow. PECVD is faster, Manuscript received August 5, 1997; revised January 26, The authors are with the Optical and Semiconductor Devices Section, Department of Electrical and Electronic Engineering, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2BT U.K. Publisher Item Identifier S (98) but would require several hours at 350 C, followed by annealing [10]. An alternative is a spin-coated, ultraviolet (UV)-cured polymer. A few polymers with suitable refractive indices exist [11]; however, there can be difficulties with adhesion of these materials. Consequently, most measurements of irradiated guides have been obtained with the device clad by index-matching oil. In this paper, we first consider the effect of thermal processing on irradiated components. We then show that good performance can be obtained in thermooptic switches using a relatively thin cladding of MgF 2. This material can be deposited very simply by evaporation on the flat surface obtained after guide formation, and has allowed the first demonstration of viable switching components made by the irradiation process. II. STABILITY OF IRRADIATION-INDUCED CHANGES The macroscopic effects of irradiation in silica glasses (namely, volumetric and refractive index changes [12]) are caused by formation of defects, and their subsequent relaxation is due to reaction with diffusing molecular species [13]. A variety of defects may be created; those studied most often (namely, center, nonbridging oxygen-hole center, and peroxy radical defects) are paramagnetic, and their densities may be determined by electron spin resonance [14]. Their relative concentrations vary from material to material, depending on the availability of their particular precursors. The size and stability of the induced changes is also strongly material-dependent. In past works, we have investigated waveguide formation in PECVD silica-on-silicon layers supplied by LETI (France), BT Labs (U.K.) and BNR Europe Ltd., now known as Nortel (England). Each material was deposited by a different PECVD process. SiH 4, NO, and N 2 precursors were used at 400 C in a Coyote type tube reactor at BT [15]; SiH 4 and N 2 O were used at 350 C in a low-frequency parallel plate reactor at BNR [16]. No details are available for the LETI process, but the material was essentially as described in [10]. The properties of these materials are summarized in Table I. PECVD silica may contain peaks in its absorption spectrum due to contamination by impurities such as hydrogen and nitrogen introduced by particular precursors. For example, increased loss at 1.39 m wavelength is associated with absorption by the first harmonic of the O H bond vibration, while absorption at 1.5 m corresponds to the third harmonic of the Si H bond or the second harmonic of the N H bond [10]. Typical TE fiber-device-fiber insertion loss spectra measured in channel guides fabricated by electron beam irradiation of LETI and BNR materials are shown in Fig. 1. In each case, /98$ IEEE

2 842 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 16, NO. 5, MAY 1998 TABLE I PROPERTIES OF SILICA-ON-SILICON MATERIALS INVESTIGATED FOR WAVEGUIDE FABRICATION Fig. 1. Spectral variation of channel guide insertion loss, for PECVD SiO 2 supplied by LETI and BNR Europe Ltd., and annealed under different conditions prior to irradiation. Fig. 2. Variation of normalized compaction with anneal time at 800 C, for PECVD SiO 2 supplied by LETI and BNR Europe Ltd., and annealed under different conditions prior to irradiation. irradiation was performed at 25 kev through a 7 m opening in a 1- m thick Au surface mask, and the overall device length was 3.4 cm. However, the data shown are from samples that were subjected to different thermal treatments prior to irradiation, in an effort to reduce propagation loss. For the as-deposited LETI silica, high loss may be seen at both 1.39 and 1.5 m wavelength. Annealing for 20 min at 1150 C removes much of the O H absorption, but the N H peak is still strongly evident. No spectrum is shown for as-deposited BNR silica; this material has an unusual response, and requires a preirradiation anneal step to obtain a positive index change suitable for guide fabrication [9]. Instead, baseline data are shown for material annealed for 20 min at 800 C. In this case, strong O H absorption is present, but the N H peak is absent. The hydroxyl contamination is largely removed by annealing at temperatures above 1050 C [8], and the material then appears to allow the lowest loss of any we have investigated; however, it also offers the weakest

3 SYAHRIAR et al.: THERMOOPTIC INTERFEROMETRIC SWITCHES 843 Fig. 3. Variation of insertion loss with transition length for back-to-back S-bends formed by irradiation of BNR material, before and after annealing at 800 C. Fig. 4. Variation of cross-coupled power with interaction length for directional couplers formed by irradiation of BNR material, before and after annealing at 800 C. and least stable index changes, while the largest and most stable changes are obtained in the lossier films. Whether the presence of hydrogen alone is responsible for these effects, or whether nitrogen is also required, is unclear. However, intermediate stability was obtained in waveguides formed by electron irradiation of bulk silica formed by flame hydrolysis of SiCl 4, which might be expected to be nitrogenfree [17]. Similarly, hydrogen loading is well known to allow large index changes in fiber gratings recorded holographically in germanosilicate glass by UV lasers [18]. There are also differences from material to material in the decay of the irradiation-induced effects caused by subsequent annealing. The changes can easily be observed by measuring the surface compaction (which is strongly correlated with the index change [19]) near a channel guide in accelerated lifetime tests. Fig. 2 shows the variation of compaction with anneal time, obtained when the materials from Fig. 1 were heated in O 2 at 800 C. In each case, compaction reduces with time. The as-deposited LETI sample is highly stable, with an annealout characteristic that can be modeled by a single exponential function [3]. In contrast, the annealed LETI material shows an initial rapid drop followed by a slower decay. Both BNR samples show this behavior, and the effect is exaggerated as more of the hydroxyl contamination is removed. A simple hypothesis is that one defect is predominant in the as-deposited LETI sample, and two (in different proportions) in the others. Furthermore, manipulation of the hydroxyl content appears to alter their relative populations, with the result that attempting to reduce propagation loss by annealing prior to irradiation seems to be directly at odds with maintaining device stability. Since there is evidence that electron irradiation itself reduces both O H and N H absorption [3], one explanation is that these groups provide a source of atomic hydrogen which can diffuse and react with other defects to stabilize them against alternative annealing reactions (with, for example, oxygen). As the defects are annealed, the index changes are reduced. There are two serious consequences for waveguide components. Reduced confinement results in increased loss, especially in bends. Fig. 3 shows the variation of fiber-devicefiber insertion loss with transition length, for a set of backto-back sinusoidal S-bends formed by irradiation of low-loss Fig. 5. Variation of cross-coupled power with interaction length for directional couplers formed by irradiation of BNR material, using different index matching oil claddings. BNR material [9]. Irradiation was performed as described above to give a weak index change of to a depth of 5 m. In each case, the overall device length was 3.4 cm, the bends introduced a fixed offset of 150 m, and the test wavelength was m. Before annealing, the insertion loss tends to that of a straight guide at long transition lengths, and rises rapidly at short lengths (or small radii). After annealing at 800 C for 30 s, the minimum tolerable transition length is much larger. The TM mode loss is also higher than the TE loss, suggesting increased substrate absorption. Reduced confinement also alters the coupling coefficient in directional couplers. For example, Fig. 4 shows the variation of the normalized cross-coupled power with interaction length for a set of couplers fabricated and tested as above, with a fixed waveguide gap of 6 m. Before annealing, the coupling characteristic follows the conventional sinusoidal variation (allowing for residual coupling in the input and output regions). After annealing at 800 C, there is a significant decrease in the period of the power transfer characteristic, indicating an increase in the coupling coefficient. It should be emphasized that these relatively dramatic results were obtained in the least stable material identified

4 844 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 16, NO. 5, MAY 1998 (c) Fig. 6. Mode profile of planar waveguide formed by electron beam irradiation, assuming an MgF 2 cladding, schematic of channel guide, and (c) schematic of thermooptic switch. to date. Furthermore, irradiated components are certainly not ephemeral at room temperature; all results in this paper were obtained from devices at least three years old. However, performance variations of this type suggest that thermal loading due to subsequent processing must be minimized if lowloss components with predictable characteristics are to be obtained. III. CLADDING OF IRRADIATED WAVEGUIDES A reduction of the cladding layer index reduces the cladding thickness required and increases the range of possible materials and deposition processes. However, it increases the asymmetry of the guide, and any polarization-dependence of modal properties. The most pronounced effects are observed in directional couplers. Fig. 5 shows the variation of the crosscoupled power with interaction length for the coupler of Fig. 4, using different overlay oils. For a cladding index of 1.45, the power transfer characteristics of the TE and TM modes are virtually indistinguishable. As the cladding index is reduced, some polarization splitting occurs. However, the effect is relatively small even for an index as low as 1.40, because the guides are almost completely buried in silica. It was therefore decided to investigate the use of MgF 2 ( ) as a cladding for irradiated guides. The required thickness was determined using an elementary model. Fig. 6 shows the field profile at m for an asymmetric slab guide with buffer, core, and cladding indices of 1.458, 1.464, and 1.39, respectively, and a core thickness of 5 m. The mode profile is highly asymmetric, and the rapid decay of the evanescent field in the cladding suggests that losses should reduce for MgF 2 thicknesses above 1 m. This thickness can be deposited relatively rapidly (30 mins) by thermal evaporation, although the sample must be heated to eliminate water of crystallization and obtain a consolidated film. It was found that a sample temperature of 145 C (achieved by radiant heating) allowed MgF 2 thicknesses of 1 m, although severe cracking occurred at 1.4 m. MgF 2 cladding layers were used to construct -junctionbased thermooptic Mach Zehnder interferometric modulators with the transverse geometry of Fig. 6 and the layout of Fig. 6(c). The 10-mm long heater electrodes were constructed above one arm of each interferometer by patterning a Å thick layer of sputtered Ti metal into 50 m wide strips fed by 4 mm wide bus bars, and a dummy electrode was

5 SYAHRIAR et al.: THERMOOPTIC INTERFEROMETRIC SWITCHES 845 TABLE II LOSS PARAMETERS OF WAVEGUIDES AND DEVICES WITH DIFFERENT MgF 2 BUFFER THICKNESSES Fig. 8. Interferometer switching characteristics obtained using a square wave heater drive at 500 Hz and 1 khz. Fig. 7. Variation of normalized transmission with heater power for a thermooptic Mach Zehnder interferometric modulator formed by irradiation of BT material. placed above the unheated arm to avoid any phase or amplitude imbalance. Metal strips were also placed over straight sections of waveguide for comparison purposes. The waveguides were formed in BT silica, which has characteristics intermediate between those of LETI and BNR material. Irradiation parameters chosen to obtain essentially polarization independent insertion losses of 1dBat m for 3.4 cm lengths of straight guide with an oil cladding. Insertion losses for interferometers measured under similar conditions were 2.0 db (TE) and 2.5 db (TM), with the difference being ascribed to slight birefringence. Any additional TE/TM differential in straight guide propagation loss or in interferometer insertion loss was then assumed to be due to polarization-dependent absorption by the metal overlay. With no cladding, very large ( 25 db) TM interferometer insertion losses were obtained. Losses decreased dramatically as the cladding thickness increased, and the majority of the TE/TM loss differential was eliminated with 1 m of MgF 2 (Table II). Switching performance was essentially similar to devices demonstrated by other technologies. Fig. 7 shows the variation of transmission with heater power, which follows the conventional sinusoidal form. The lack of a phase bias in the curve suggests that there is no phase shift between the two interferometer arms, although the relatively poor extinction ratio (10 db) suggests unequal splitting in the -junctions. Full switching was obtained at a power of 0.5W. Fig. 8 and shows switch characteristics obtained using a square wave heater drive at 500 Hz and 1 khz, respectively. Minimum switching times of 0.5 ms are slightly shorter than results obtained with topographic guides with a much thicker silica cladding [4]. With an MgF 2 thickness of 1.4 m, the cladding layer cracked catastrophically. The TE/TM loss differential then approached the value obtained with a bare device, but absolute losses were much higher (due to scattering). IV. CONCLUSIONS In conclusion, we have demonstrated that the thermal stability of waveguides formed by electron beam irradiation of PECVD silica-on-silicon varies from material to material, and is affected by thermal preprocessing designed to reduce propagation loss. Increased stability does appear to be correlated with hydrogen contamination. We have also shown that thin layers of evaporated MgF 2 can be used as a cladding for such waveguides. Because process times and temperatures are low, this coating method avoids degradation of device performance by annealing. Polarization effects that might be expected from the low refractive index of MgF 2 appear limited, because the guides are almost entirely buried in silica. This process has allowed irradiated waveguides to be used as the basis of lowloss thermooptic switching components for the first time, and should greatly improve the future prospects for electron beam irradiation as a simple PLC fabrication technology. ACKNOWLEDGMENT The authors gratefully acknowledge the supply of PECVD silica-on-silicon material by BNR Europe Ltd. (now Nortel) and BT labs, and deposition of the MgF 2 cladding layers by Dr. P. Judge. REFERENCES [1] A. J. Houghton and P. D. Townsend, Optical waveguide formed by low energy electron irradiation of silica, Appl. Phys. Lett., vol. 29, pp , 1976.

6 846 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 16, NO. 5, MAY 1998 [2] S. J. Madden, M. Green, and D. Barbier, Optical channel waveguide fabrication based on electron beam irradiation of silica, Appl. Phys. Lett., vol. 57, pp , [3] R. R. A. Syms, T. J. Tate, and J. J. Lewandowski, Near infrared channel waveguides formed by electron beam irradiation of silica layers on silicon substrates, J. Lightwave Technol., vol. 12, pp , [4] M. Kawachi, Silica waveguides on silicon and their application to integrated-optic components, Opt. Quantum Electron., vol. 22, pp , [5] M. Guglielmi, P. Colombo, L. Mancinelli Degli Espositi, G. C. Righini, and S. Pelli, Planar and strip optical waveguides by sol-gel method and laser densification, Proc. SPIE, vol. 1513, pp , [6] M. Svalgeerd, C. A. Poulsen, A. Bjarklev, and O. Poulsen. Direct UV writing of buried singlemode channel waveguides in Ge doped silica films, Electron. Lett., vol. 30, pp , [7] C.-Y. Li, J. Chisham, M. Andrews, S. I. Najafi, J. D. Mackenzie, and N. Peyghambarian, Sol-gel optical coupler by ultraviolet light imprinting, Electron. Lett., vol. 31, pp , [8] R. R. A. Syms, T. J. Tate, and M. F. Grant, Reduction of propagation loss in silica-on-silicon channel waveguides formed by electron beam irradiation, Electron. Lett., vol. 30, pp , [9] R. R. A. Syms, T. J. Tate, and R. Bellerby, Low-loss near-infrared passive optical waveguide components formed by electron beam irradiation of silica-on-silicon, J. Lightwave Technol., vol. 13, pp , [10] G. Grand, J. P. Jadot, H. Denis, S. Valette, A. Fournier, and A. M. Grouillet, Low-loss PECVD silica channel waveguides for optical communication, Electron. Lett., vol. 26, pp , [11] H. M. Presby and C. A. Edwards, Packaging of glass waveguide silicon devices, Opt. Eng., vol. 31, pp , [12] T. A. Dellin, D. A. Tichenor, and E. H. Barsis, Volume, index of refraction and stress changes in electron irradiated silica, J. Appl. Phys., vol. 48, pp , [13] R. A. B. Devine, On the physical models of annealing of radiation induced defects in amorphous SiO 2, Nucl. Instrum. Meth. Phys. Res., vol. B46, pp , [14] W. L. Warren, E. H. Pondexter, M. Offenberg, and W. Müller-Warmuth, Paramagnetic point defects in amorphous silicon dioxide and amorphous silicon nitride films, J. Electrochem. Soc., vol. 139, pp , [15] R. R. A. Syms, J. Lewandowski, M. Nield, and F. Mackenzie, Tuning of silica-on-silicon directional couplers by electron beam irradiation, IEEE Photon. Technol. Lett., vol. 6, pp , [16] P. R. Wensley, R. Bellerby, S. Day, M. F. Grant, and S. A. Rosser, Improved waveguide technology for silica-on-silicon integrated optics, in Proc. ECIO, Neuchatel, Switzerland, Apr , 1993, pp [17] D. Barbier, M. Green, and S. J. Madden, Waveguide fabrication for integrated optics by electron beam irradiation of silica, J. Lightwave Technol., vol. 9, pp , [18] P. J. Lemaire, R. M. Atkins, V. Mizrahi, and W. A. Reed, High pressure H 2 loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO 2 doped optical fibers, Electron. Lett., vol. 29, pp , [19] J. Lewandowski, R. R. A. Syms, M. F. Grant, and S. Bailey, Controlled formation of buried channel waveguides by electron beam irradiation of glassy layers on silicon, Int. J. Optoelectron., vol. 9, pp , Ary Syahriar was born in Madiun, Indonesia in He received the B.Sc. degree in physics from the University of North Sumatra, Medan, Indonesia, in 1986 and the M.Sc. degree in physics from the University of Waterloo, Waterloo, Ont., Canada, in He is currently pursuing the Ph.D. degree in electrical and electronic engineering at Imperial College London, London, U.K. Since 1987, he has been a Research Scientist for the Agency for the Assessment and Application of Technology (BPP Teknologi) of the Republic of Indonesia. His current research interests include passive and active integrated and fibre optic devices for optical communication applications. Richard R. A. Syms was born in Norfolk, VA, in He received the B.A. degree in engineering science in 1979, and the D.Phil. degree in volume holographic optical elements in 1982, both from Worcester College, Oxford, U.K. He has been Head of the Optical and Semiconductor Devices Section in the Department of Electrical and Electronic Engineering, Imperial College London, U.K., since 1992, and Professor of Microsystems Technology since He has published approximately 70 papers and two books on holography, integrated optics, and microengineering. His main research interests are silica-on-silicon integrated optics and silicon-based MEMS. Thomas J. Tate received the Ph.D. degree in physics in 1984 from the Imperial College London, U.K., in the Department of Electrical and Electronic Engineering. He has since worked at Imperial College London in the Department of Electrical and Electronic Engineering, where he is currently Facilities Manager in the Optical and Semiconductor Devices Section. His research interests include surface modification by ion and electron irradiation (semiconductors, superconductors, mechanical systems, and optics), and vacuum processing of materials. A recent shift of area has been towards microengineering and patterning of materials.

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