A review on the state of the art of silicon oxynitride deposition at the MESA Research Institute will be given. The recent progress in the application of silicon oxynitride in communication devices will be discusse

Driessen, Alfred

Lambeck, Paul V.

Ridder, René M. de

Wörhoff, Kerstin

English

Worhoff, Kerstin

de Ridder, R.M.

Lambeck, Paul

Driessen, A.

NARCIS 

Silicon oxynitride in integrated optics

University of Twente Research Information

1l:lSam - 11:30am FK3 Silicon oxynitride in integrated optics Kerstin Worhoff, RenC M. de Ridder, Paul V. Lambeck, and Alfred Driessen MESA Research Institute, University of Twente, P.O. Box 2 17, 7500 AE Enschede, The Netherlands, e-mail: K. Worhoff@el.utwente.nl n N-Ha, dep [at%] Si-Hasd,, [at%] N-Ha,,, [at%] Si-Ha,,,, [at%] ~ 'i.4a8P 3.4 < meas. limit 0.4 < meas limit < meas limit 1.527p 6.1 < meas. limit 1.3 Abstract - A review on the state of the art of silicon oxyni- tride deposition at the MESA Research Institute will be given. The recent progress in the application of silicon oxynitride in communication devices will be discussed. I .  INTRODUCTION Integrated optics (IO) devices for application in telecom- munication are highly demanding on low insertion loss, effi- cient fiber-to-chip coupling, high compactness (small bending radii) and polarization independent operation. In most operating communication devices developed within the last decade, low-index contrast waveguiding structures have been applied El]. The large channel cross-section of these waveguides matches well with the standard optical fiber, but due to the low index contrast large bending radii are re- quired for low bend losses leading to a low integration den- sity. The compactness of 10  devices is significantly increased when applying high-index contrast waveguiding structures. Although for these structures the fiber-to-chip coupling effi- ciency is generally low, this parameter can be significantly improved by a proper design of the fiber-to-chip interface. Silicon oxynitride (SiON) grown by chemical vapor deposi- tion (CVD) is a material which is well-suited for the realiza- tion of high contrast waveguides, since the range over which the refractive index of this material can be tuned (1.46-2) is large. In this contribution we will shortly review on the state of the art of our SiON deposition process and the important properties of this material. Then, the development and reali- zation of a SON-based high contrast channel structure having the potential of filfilling all the above given demands arising from the telecommunication application will be discussed. Finally, results of the application of this structure in a WDM add-drop device will be shown. 11. SILICON OXYNITRIDE DEPOSITION High-quality silicon oxynitride waveguides are deposited either by Plasma Enhanced Chemical Vapor Deposition (PECVD) [2] and Low Pressure Chemical Vapor Deposition (LPCVD) [3].  For the growth of the PECVD layers a parallel-plate Elec- trotech 210 machine with a low frequency (187.5 kHz) RF- generator is applied. The layers are deposited at 60 W plasma power, 650 mTorr pressure, 300°C temperature and N,O, NH, and 2%SiH4 i N, gas sources of which the gas flows can be varied up to a maximum flow of 1400 sccm, 36 sccm and 3000 sccm, respectively. The refractive indices of the layers are tuned by changing the flow ratio between the gases. In principle, the tuning range of the PECVD process compre- 0-7807-4947-4/98/$10.00 01998 IEEE 1.696pl 17.8 hends the SiON index range from 1.46 (SiO,) to 2.0 (Si3N4). However, for applications highly-demanding on the accurate adjusting of a specified refractive index, the range from 1.46 to 1.7 is most suited, since in that range the sensitivity of the refractive index to the flow ratio change is low. The deposi- tion rates of the PECVD process are high, 30-50 nm/min. The LPCVD waveguides are deposited in a Tempress hot- wall quarts-tube reactor from SiH,Cl,, 0, and NH, with maximum flows of 100 sccm, 5 sccm and 120 sccm, respec- tively, at temperatures between 800-900°C and pressures be- tween 50-200 mTorr. The LPCVD process is suited for the deposition of waveguides with refractive indices between 1.7- 2. The deposition rates of this process are adjustable between 2-10 nm/min, enabling controlled growth of even very thin layers of several nanometers. The uniformity over the wafer and the run-to-run repro- ducibility of the layer thickness (6d) and the refractive index (An) are shown in Table 1. The material birefringence, AnTM- TE = nTM -nrE, of the SiON layers grown by both CVD proc- esses, shown in the right column of Table 1, is stress-induced. Table I :  Uniformity and reproducibility of thickness (a) and refactive index (An) and material birefiingence for SON layers with dicerent refractive indices; P-PECVD, L-LPCVD 1.4 4.5 1 < meas limit 3 I L.J 2.008LI 4 1 2 1  4 I 0.7 I -8.5 For layers grown by both CVD processes, optical losses of slab-type waveguides of below 0.2 dB/cm have been obtained within the visible range of light. At wavelengths around 1550 nm, the optical losses are 1.8 dB/cm and 10 dB/cm for PECVD layer with n=l.488 and n=l.696, respectively. This strong increase is due to absorption caused by vibrational overtones of the N-H and Si-H bonds. The N-H and Si-H content of different SiON layers, shown in Table 2, has been measured by €R-spectroscopy. After annealing at 1150°C the hydrogen content is strongly decreased and the optical losses 370 of the layer with n=1.488 is reduced to below 0.2 dB/cm at 1550 nm wavelength. 111. APPLICATION IN OPTICAL COMMUNICATION A standardized SON-based waveguiding structure for ap- plication in telecommunication devices operating at 1550 nm wavelength has been developed and realized. This structure, which is schematically shown in Fig. 1, has the potential of fulfilling all the above-mentioned requirements of communi- cation devices and is moreover designed for being insensitive to the technological tolerances of the applied processes. n=’l.4637 ~ Fig. I :  SON-based waveguiding structure with parameters The effective refractive index of this structure is 1.4704 and the polarization dependence of the waveguiding channel is 2 ~ 1 0 - ~ .  This low polarization dependence is attained by com- pensating the material birefringence-introduced polarization dependence of the SiON channel geometrically introducing a thin silicon nitride layer [4].  Based on the effective index contrast of this structure, 1.4x10-*, Whispering Gallery Mode bends [5] with a bending radius of 1.6 mm and an optical loss of 0.1 dB/90” have been calculated. A fiber-to-chip coupling loss, due to the mode mismatch between the modal field in the channel waveguide and a standard telecommunication fiber, of 2.5 dB/facet has been calculated. Nevertheless, this overlap loss can be easily reduced by expanding the modal field in the waveguiding channel, e.g. by adiabatically tapering of the channel width. For a channel width of 1 pm, the overlap loss will be reduced to below 0.3 dB/facet. The SiON waveguiding structure is applied for the realiza- tion of a thermo-optically tunable add-drop multiplexer, which is based on Mach-Zehnder interferometer (MZI) filter elements with non-equal branches, as schematically shown in Fig. 2. The MZI filter element is designed for having a 2 nm free spectral range (FSR) at 1550 nm. The filter element has a path length difference of both branches of 820 pm and the MMI-couplers are 35 pm wide and 920 pm long. The bending radius was conservatively chosen to be 7 mm. The angle a is optimized in a way that at given path length difference the dimensions of the device are minimized. Measurements on the first test device with TE-polarized light showed excellent agreement of the FSR with the de- signed value. A cross-talk of -20 dB for the cross-state (1-2’ and 2-1’) and -10 dB for the bar-state (1-1’ and 2-2’) is meas- ured. This asymmetry can be explained by a non-perfect split- ting ratio of the MMI’s. Fig. 2: Schematic view of a MZI wavelengthjlter with MMI 3-dB-couplers having a path length difference of 2(A-B) with bending radius R and the total subtended angle of arc a; The thenno-optical tuning behavior of the filter has also been measured. Applying gold heaters with not-yet optimized dimensions, a heating power of 0.8 W was required for tuning the device over a full free spectral range, giving a tuning sen- sitivity of 0..4 W/nm. IV. CONCLUSION The state of the art of controlled fabrication of highly uni- form low-loss silicon oxynitride waveguides has been dis- cussed. Based on this material, a channel waveguiding struc- ture has been designed and realized. This channel structure enables the realization of low-loss, compact communication devices with low polarization dependency and high fiber- coupling efficiency. The application of this structure in a thermo-optically tuned MZI filter element has been demon- strated. ACKNOWLEDGMENT The authors would like to thank Lucie Hilderink, Gijs Kri- jnen, Peter Linders and Bertrik Sikken for their contributions to the presented research. REFERENCES [ l ]  M. Kxwachi, “Silica waveguides on silicon and their applica- tion to integrated-optic components”, Opt. Quantum Elec- tron., vol. 22, pp. 39 1-4 16, 1990. K. Worhoff, A. Driessen, P.V. Lambeck, L.T.H. Hilderink, P.W.C. Linders, and Th.J.A. Popma, “PECVD silicon oxyni- tride optimized for application in integrated optics”, accepted for publication E-MRS symposium ‘98, Strasbourg, France, 1998. K. Wbrhoff, P.V. Lambeck, H. Albers, O.F.J. Noordman, N.F. van H[ulst, and Th.J.A. Popma, “Optimization of LPCVD sili- con oxynitride growth to large refractive index homogeneity and layer thickness uniformity”, in Proc. SPIE, vol. 3099, pp. K. Worhoff, B.J. Offrein, A. Driessen, P.V. Lambeck, G.L. Bona:, and Th.J.A. Popma, “Birefringence compensation in double-core optical waveguides”, accepted for publication in proceedings ECOC ’98, Madrid, Spain, 1998. M.K. Smit, E.C.M. Pennings, and H .  Blok, “A normalized approach to the design of low-loss optical waveguide bends”, J. Lightwave Technol., vol. I t ,  pp. 1737-1742, 1993. [2] [3] 257-268, 1997. [4] [ 5 ]  371 

A normalized approach to the design of low-loss optical waveguide bends”,

Birefringence compensation in double-core optical waveguides”, accepted for publication

PECVD silicon oxynitride optimized for application in integrated optics”, accepted for publication E-MRS symposium ‘98,

Silica waveguides on silicon and their application to integrated-optic components”,

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Silicon oxynitride in integrated optics

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