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.)
"Astronomical Instrumentation" @Marseille
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.