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Proceedings Paper

High-efficiency silicon immersion grating by electron-beam lithography
Author(s): Yuji Ikeda; Naoto Kobayashi; Hiroshi Terada; Akinori Shibayama; Akira Ozawa; Chikako Yasui; Sohei Kondo; Tae-Soo Pyo; Hideyo Kawakita
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Paper Abstract

Silicon immersion grating have opened the new possibilities of building compact high-resolution cryogenic spectrometers for the near-infrared (NIR) region from 1.4 to 5.5μm. We are developing a silicon immersion grating for a next-generation NIR high-resolution spectrometer attached to the Subaru 8.2 m telescope. Since a long time, the anisotropic wet etching technique using photolithography has used for the fabrication of silicon immersion gratings (e.g., Wiedemann & Jennings1 and Keller et al.2). Here, we present an alternative technique using electron-beam (EB) lithography, which does not employ either any photolithography masks or UV light source. This technique uses "direct" EBs to expose the resist. The EBs are precisely controlled by using a closed-looped system comprising a laser interferometer. As compared to photolithographic technique, this technique results in more accurate groove pitches and lower surface roughness near the edge of the mask. We fabricate a sample grating with a groove pitch of 30 µm and a blaze angle of 69° on a 10mm × 20mm × 2mm flat substrate by EB lithography. Our detailed optical testing of the grating using visible laser shows good optical performances: extremely low scattered light (< 0.9%), less production of ghost light (< 0.01%), and high relative diffraction efficiency (~ 88%). We plan to fabricate the final immersion grating by fixing the etched grating substrate to a silicon prism using the optical-contact method. We are in the middle of R&D of this process and found that much tighter optical-contact than usual seems to be required because of the high incidence angle to the contact surface. Our first sample immersion grating shows a significant reflection loss at the contact surface when it is used in an immersion mode with a NIR laser beam. We are trying to improve the polishing process of the thin silicon substrate for better contact.

Paper Details

Date Published: 10 July 2008
PDF: 12 pages
Proc. SPIE 7014, Ground-based and Airborne Instrumentation for Astronomy II, 701469 (10 July 2008); doi: 10.1117/12.788126
Show Author Affiliations
Yuji Ikeda, Photocoding (Japan)
Naoto Kobayashi, Institute of Astronomy, Univ. of Tokyo (Japan)
Hiroshi Terada, Subaru Telescope, National Astronomical Observatory of Japan (United States)
Akinori Shibayama, NTT-AT Nanofabrication Corp. (Japan)
Akira Ozawa, NTT-AT Nanofabrication Corp. (Japan)
Chikako Yasui, Institute of Astronomy, Univ. of Tokyo (Japan)
Sohei Kondo, Institute of Astronomy, Univ. of Tokyo (Japan)
Tae-Soo Pyo, Institute of Astronomy, Univ. of Tokyo (Japan)
Hideyo Kawakita, Kyoto Sangyo Univ. (Japan)

Published in SPIE Proceedings Vol. 7014:
Ground-based and Airborne Instrumentation for Astronomy II
Ian S. McLean; Mark M. Casali, Editor(s)

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