Uniform tilt-angle micromirror array for multiobject spectroscopy

We report on micromirror arrays being developed for use as reflective slit masks in multiobject spectrographs for astronomical applications. The micromirrors are etched in bulk single crystal silicon, whereas cantilever-type suspension is realized by surface micromachining. One micromirror element is 100200 m in size. A system of multiple landing beams is developed, which electrostatically clamps the mirror at a well-defined tilt angle when actuated. The mechanical tilt angle obtained is 20 deg at a pull-in voltage of 90 V. Measurements with an optical profiler show that the tilt angle of the actuated mirror is stable with a precision of one arc minute over a range of 15 V. This electrostatic clamping system provides uniform tilt angle over the whole array: the maximum deviation measured between any two mirrors is as low as one arc minute. The surface quality of the mirrors in the actuated state is better than 10 nm peak-to-valley and the local roughness is around 1-nm rms. Cryogenic testing shows that the micromirror device is functional at temperatures below 100 K.