MEMS-based programmable reflective slit mask for multi-object spectroscopy

Multi-object spectroscopy is a powerful tool for space and ground-based telescopes for the study of the formation of galaxies. This technique requires a programmable slit mask for astronomical object selection. We are developing MEMS-based programmable reflective slit masks for multi-object spectroscopy that consist of micromirror arrays on which each micromirror of size 100 x 200 μm2 is electrostatically tilted providing a precise angle. The main requirements for these arrays are cryogenic environment capabilities, precise and uniform tilt angle over the whole device, uniformity of the mirror voltage-tilt hysteresis and a low mirror deformation. A first generation of MEMS-based programmable reflective slit masks composed of 5 x 5 micromirrors was tested in cryogenic conditions at 92 K. Then, first prototypes of large arrays were microfabricated and characterized, but the reliability of these arrays had to be improved. To increase the reliability of these devices, a third generation of micromirror arrays composed of 64 x 32 micromirrors is under development. This generation was especially designed for individual actuation of each mirror, applying a line-column algorithm based on the voltage-tilt hysteresis of the actuator. The fabrication process was optimized and is now based on multiple wafer level bonding steps. Microfabricated devices have micromirror with a peak-to-valley deformation less than 3 nm. The mirrors can be tilted at 20° by an actuation voltage lower than 100 V. First experiments showed that our micromirrors are well suited for the line-column addressing of each micromirror.