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High-efficiency silicon immersion grating by electron-beam lithography

Yuji Ikeda (Photocoding), Naoto Kobayashi(Univ. Tokyo), Hiroshi Terada(Subaru Telescope), Akinori Shibayama, Akira Ozawa(NTT-AT N), Chikako Yasui, Sohei Kondo(Univ. Tokyo), Tae-Soo Pyo(Subaru Telescope), and Hideyo Kawakita(Kyoto-Sangyo Univ.)
2008年6月27日 SPIE conference
"Astronomical Instrumentation" @Marseille
■ アブストラクト
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 5.5um. 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 & Jennings and Keller et al.). 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-loopedsystem 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 30um and a blaze angle of 69 deg. on a 10mm x 20mm x 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.