Uniform tilt-angle micromirror array for multi-object spectroscopy

We report on micromirror arrays being developed for the use as reflective slit mask in Multi Object Spectrographs for astronomical applications. The micromirrors are etched in bulk single crystal silicon whereas the cantilever type suspension is realized by surface micromachining. One micromirror element is 100μm x 200μm in size. The micromirrors are actuated electrostatically by electrodes located on a second chip. The use of silicon on insulator (SOI) wafers for both mirror and electrode chip ensures thermal compatibility for cryogenic operation. A system of multiple landing beams has been developed, which passively locks the mirror at a well defined tilt angle when actuated. The mechanical tilt angle obtained is 20o at a pull-in voltage of 90V. Measurements with an optical profiler showed that the tilt angle of the actuated and locked mirror is stable with a precision of one arc minute over a range of 15V. This locking system makes the tilt angle merely independent from process variations across the wafer and thus provides uniform tilt angle over the whole array. The precision on tilt angle from mirror to mirror measured is one arc minute. The surface quality of the mirrors in actuated state is better than 10nm peak-to-valley and the local roughness is around 1nm RMS.