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We are developing micromirror arrays (MMA) for future generation infrared multiobject spectroscopy (MOS)
requiring cryogenic environment. So far we successfully realized small arrays of 5×5 single-crystalline silicon
micromirrors. The 100μm ×200μm micromirrors show excellent surface quality and can be tilted by electrostatic
actuation yielding 20° mechanical tilt-angle. An electromechanical locking mechanism has been demonstrated
that provides uniform tilt-angle within one arc minute precision over the whole array. Infrared MOS requires
cryogenic environment and coated mirrors, silicon being transparent in the infrared. We report on the influence
of the reflective coating on the mirror quality and on the characterization of the MMA in cryogenic environment.
A Veeco/Wyko optical profiler was used to measure the flatness of uncoated and coated mirrors. The uncoated
and unactuated micromirrors showed a peak-to-valley deformation (PTV) of below 10nm. An evaporated 10nm
chrome/50nm gold coating on the mirror increased the PTV to 35nm; by depositing the same layers on both
sides of the mirrors the PTV was reduced down to 17nm. Cryogenic characterization was carried out on a
custom built interferometric characterization bench onto which a cryogenic chamber was mounted. The chamber
pressure was at 10e-6 mbar and the temperature measured right next to the micromirror device was 86K. The
micromirrors could be actuated before, during and after cryogenic testing. The PTV of the chrome/gold coated
mirrors increased from 35nm to 50nm, still remaining in the requirements of < lambda/20 for lambda=1μm.
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Severin Waldis, Frederic Zamkotsian, Patrick Lanzoni, Wilfried Noell, Nico de Rooij, "Micromirrors for multiobject spectroscopy: optical and cryogenic characterization," Proc. SPIE 6887, MOEMS and Miniaturized Systems VII, 68870B (13 February 2008); https://doi.org/10.1117/12.764381