Fabrication of silicon microring resonator with smooth sidewalls

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Chen et al.: Fabrication of silicon microring resonator with smooth sidewalls Fig. 6 Cross-section view of the fabricated silicon waveguide. Fig. 7 Silicon microring resonator with smooth sidewalls. than 1:1 under chlorine plasma, and it needs around 400-nm-thick resist for etching 300-nm-thick silicon. Moreover, we observed that in some runs, the e-beam resist became delaminated by the Cl 2 plasma, as shown in Fig. 4. Due to the higher etch selectivity of Si to HSQ than that of Si to maN-2403, thinner e-beam resist can be applied when using HSQ as mask material to improve EBL resolution and reduce mask-edge roughness. The etch selectivity of silicon to HSQ is higher than 5:1, so in our experiment, the thickness of HSQ is 90 nm, and that of maN-2403 is 300 nm for 320-nm-thick silicon etching by optimized ICP processes. Using our Cr and maN-2403 mask etch process, the etched surface roughness of the silicon microring resonators is relatively low, normally less than 10 nm. This is due in part to the formation of the CF x polymer on the silicon sidewall, which tends to reduce striation roughness associated with ion bombardment. The process with the HSQ masks permits considerable process latitude due to the high selectivity of silicon to HSQ. However, it is worthy of notice that only Cl 2 was used as the etching gas since C 4 F 8 corrupts HSQ under plasma, and the striation roughness induced by ion bombardment on the silicon sidewalls should be taken into consideration. Using the normal process, the etched surface roughness was 20 to 30 nm. In our process, we optimized the etch recipe based on HSQ mask etching and achieved etched surface roughness of less than 10 nm, as shown in Fig. 5, where Fig. 5b is a top view of the edge of the silicon waveguide, indicating roughness less than 10 nm. The roughness was demonstrated by inspecting the ripple edge fluctuation on top of the waveguide structure in a vertical etched profile with high-resolution SEM. Using this method, it is essential to make sure that the Table 1 Comparison of various fabrication processes. Process complexity Process latitude Sidewall roughness Cr mask Complicated Excellent Rough maN-2403 mask Simple Poor Smooth HSQ mask Simple Good Smooth sidewall angle of the waveguide is close to 90 deg, and that there is no etch undercut below the etch mask, as shown in Fig. 6, so that the sidewall roughness can be demonstrated by the edge fluctuation on top of the waveguide. Since the edge fluctuation in Fig. 5b is around 5 nm, sidewall roughness less than 10 nm was achieved. The advantage of the SEM inspection is that it can provide an approximate value of the roughness for the whole waveguide structure, which is essential for evaluating the optical scattering loss, but the limitation is that the definite value of the roughness cannot be detected by this method. A comparison was carried out for the preceding three processes considering process complexity, process latitude, and sidewall roughness of the resonator. The results are shown in Table 1. Last, by controlling the mask-edge roughness and the etched surface roughness, silicon microring resonators were fabricated with an average sidewall roughness of less than 10 nm, as shown in Fig. 7, which was demonstrated by inspecting the edge fluctuation of the waveguide structure in a vertical etched profile with high-resolution SEM. These high-quality results could be achieved with both maN-2403- and HSQ-based processes, where the process with maN-2403 features the advantage of CF x sidewall protection etching, while that with HSQ takes advantage of the higher EBL resolution with thinner e-beam resist. 4 Conclusion The process details of the fabrication of silicon microring resonators based on EBL and ICP etching were presented. Silicon sidewall roughness of less than 10 nm was achieved by accurate control of the mask-edge roughness and the etched surface roughness. Processes employing three different mask materials were discussed, analyzed, and compared to achieve reduced process complexity, enhanced process latitude, and low sidewall roughness of the fabricated resonators. Acknowledgments This work is partially supported by the National Basic Research Program of China 2006CB708310 and the Natural Science Foundation of China 60578048. The work of DSC was supported in part by the CNRS. J. Micro/Nanolith. MEMS MOEMS 043060-4 Oct–Dec 2009/Vol. 84 Downloaded from SPIE Digital Library on 28 Dec 2009 to 162.105.81.60. Terms of Use: http://spiedl.org/terms
Chen et al.: Fabrication of silicon microring resonator with smooth sidewalls References 1. P. P. Absil, J. V. Hryniewicz, B. E. Little, R. A. Wilson, L. G. Joneckis, and P.-T. Ho, “Compact microring notch filters,” IEEE Photon. Technol. Lett. 124, 398–400 2000. 2. B. E. Little, S. T. Chu, H. A. Haus, J. Foresi, and J.-P. Laine, “Microring resonator channel dropping filters,” J. Lightwave Technol. 15, 998–1005 1997. 3. V. R. Almeida, C. A. Barrios, R. R. Panepucci, and M. Lipson, “Alloptical control of light on a silicon chip,” Nature (London) 431, 1081–1083 2004. 4. Q. F. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature (London) 435, 325–327 2005. 5. P. P. Absil, J. V. Hryniewicz, B. E. Little, P. S. Cho, R. A. Wilson, L. G. Joneckis, and P.-T. Ho, “Wavelength conversion in GaAs microring resonators,” Opt. Lett. 25, 554–556 2000. 6. Z. Xia, Y. Chen, and Z. Zhou, “Dual waveguide coupled microring resonator sensor based on intensity detection,” IEEE J. Quantum Electron. 441, 100–107 2008. 7. J. Niehusmann, A. Vorkel, P. H. Bolivar, T. Wahlbrink, W. Henschel, and H. Kurz, “Ultrahigh-quality-factor silicon-on-insulator microring resonator,” Opt. Lett. 29, 2861–2863 2004. 8. T. B. Jones, M. Hochberg, C. Walker, and A. Scherer, “High-Q ring resonators in thin silicon-on-insulator,” Appl. Phys. Lett. 85, 3346– 3347 2004. Yao Chen received his BS degree from Huzhong University of Science and Technology, China, in 2004, and he is now a PhD candidate in the Silicon Photonics and Microsystems Group at Wuhan National Laboratory for Optoelectronics. He has been working with Prof. Zhou, Prof. Summers, and Prof. Yu on a joint project and focusing on fabrication of silicon microring resonator and silicon-based photonic crystals. His research interests include fabrication for optoelectronics, semiconductor manufacturing technology, and MEMS technology. Junbo Feng received his BS degree from Huzhong University of Science and Technology, China, in 2004, and he is now a PhD candidate in the Silicon Photonics and Microsystems Group at Wuhan National Laboratory for Optoelectronics. His research interests include design and fabrication of photonic devices, semiconductor manufacturing technology, and MEMS technology. Zhiping Zhou received his PhD EE degree from Georgia Institute of Technology GT in Atlanta in 1993. He was a founder and the vice president of production at Hengnan Transistor Factory in China 1971 to 1978 and a guest scientist of NIST 1987 to 1989. From 1993 to 2005, he was with the Microelectronics Research Center at GT, where he engaged in research and development in the areas of semiconductor photonic devices. He is now a Changjiang professor at Peking University, China, and also an adjunct professor at the School of Electrical and Computer Engineering of GT. He is a senior member of IEEE, a member of SPIE, a member of OSA, and a life member of PSC. Jun Yu received his BS degree from Huazhong University of Science and Technology HUST in 1970, after which he worked as an assistant professor in HUST. Subsequently, he was a visiting research scholar at Moscow University 1989 to 1990 and at Northwestern University 1998 to 1999. He became a professor at HUST in 1996 and is currently the director of the Microelectronics Research Institute and vice director of the Beijing National Biochip Engineering Center. His multidisciplinary interests overlap the areas of novel ferroelectric materials and devices, MEMS, biochips and silicon thin-film solar cells. He is a senior member of the Chinese Institute of Electronics, a member of AAAS, and a member of MRS. He has earned numerous awards, holds 5 patents, and has over 130 publications. Christopher J. Summers received his BS and PhD in physics from Reading University in the UK. He has held positions at Bell Labs, GTE Labs, and McDonnell Douglas Research Labs. In 1981, he joined the Georgia Tech Research Institute and in 1993 became director of the Phosphor Technology Center of Excellence. He is currently a professor at the Georgia Tech School of Materials Science and Engineering and an adjunct faculty member in the Schools of Physics and Electrical and Computer Engineering. He has a research history in electro-optical materials characterization, molecular beam epitaxy of infrared and electronic materials, and the synthesis of phosphor materials for displays and solid-state lighting. His current research interests are in propagation through 2-D photonic crystals, such as self-collimation phenomena, giant refraction and superprism effects, and the use of 3-D photonic crystals infiltrated by atomic layer deposition for studying luminescence modification and tunability. He has received numerous awards and honors, holds 10 patents, and has over 300 publications. David S. Citrin was awarded a BA degree in physics, from Williams College, Massachusetts 1985, after which he earned MS 1987 and PhD 1991 degrees in physics from the University of Illinois at Urbana- Champaign, where he conducted theoretical research on the optical properties of quantum wires. Subsequently, Citrin was a postdoctoral research fellow at the Max Planck Institute for Solid State Research, Stuttgart, Germany 1992 to 1993, where he worked on exciton radiative decay in low-dimensional semiconductor structures. From 1993 to 1995, he was Center fellow at the Center for Ultrafast Optical Science at the University of Michigan, where his work addressed ultrafast phenomena in quantum wells and microcavities. He was an assistant professor of physics at Washington State University from 1995 to 2001, and received several professional awards. Citrin is presently a professor in the School of Electrical and Computer Engineering at the Georgia Institute of Technology. Areas of special focus include terahertz science and technology, semiconductor devices, nanoscale science and engineering, plasmonics, nonlinear optics, and ultrafast phenomena. J. Micro/Nanolith. MEMS MOEMS 043060-5 Oct–Dec 2009/Vol. 84 Downloaded from SPIE Digital Library on 28 Dec 2009 to 162.105.81.60. Terms of Use: http://spiedl.org/terms
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Fabrication of silicon microring resonator with smooth sidewalls

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