10 July 2008 High-efficiency silicon immersion grating by electron-beam lithography
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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.
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Yuji Ikeda, Yuji Ikeda, Naoto Kobayashi, Naoto Kobayashi, Hiroshi Terada, Hiroshi Terada, Akinori Shibayama, Akinori Shibayama, Akira Ozawa, Akira Ozawa, Chikako Yasui, Chikako Yasui, Sohei Kondo, Sohei Kondo, Tae-Soo Pyo, Tae-Soo Pyo, Hideyo Kawakita, Hideyo Kawakita, } "High-efficiency silicon immersion grating by electron-beam lithography", Proc. SPIE 7014, Ground-based and Airborne Instrumentation for Astronomy II, 701469 (10 July 2008); doi: 10.1117/12.788126; https://doi.org/10.1117/12.788126

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