Uniform tilt-angle micromirror array for multi-object spectroscopy

We report on micromirror arrays being developed for the use as re”ective slit mask in Multi Object Spectrographsfor astronomical applications. The micromirrors are etched in bulk single crystal silicon whereas the cantilevertype suspension is realized by surface micromachining. One micromirror element is 100 µ m x 200 µ m in size. Themicromirrors 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 systemof multiple landing beams has been developed, which passively locks the mirror at a well de“ned tilt angle whenactuated. The mechanical tilt angle obtained is 20 at a pull-in voltage of 90V. Measurements with an opticalpro“ler showed that the tilt angle of the actuated and locked mirror is stable with a precision of one arc minuteover a range of 15V. This locking system makes the tilt angle merely independent from process variations acrossthe wafer and thus provides uniform tilt angle over the whole array. The precision on tilt angle from mirror tomirror measured is one arc minute. The surface quality of the mirrors in actuated state is better than 10nmpeak-to-valley and the local roughness is around 1nm RMS.Keywords: micromirror, multi object spectroscopy, MOEMS, mirror array, DRIE