Electrostatically operated micromirrors for Hadamard transform spectrometer

The paper presents the development of a linear micromirror array which can be used as a switchable entrance mask for a double-array Hadamard transform spectrometer. In addition to the detector array, the double-array spectrometer has a linear multislit array realized by independently switchable micromirrors at the entrance side. Two different switch positions of the electrostatically operated mirrors allow the reflection of light into or away from the spectrometer. With this arrangement (mirror array, concave grating, and array detector) and the use of the Hadamard transform principle it is possible to increase the signal-to-noise ratio and the resolution of the system compared to conventional spectrometers of the same size. In order to use a micromirror array as an entrance mask the dimensions of the switching elements must be similar to those of entrance slits and the mirror surface must have a high reflectivity in the desired spectral range. To obtain a high resolution a given pitch between the mirrors must be realized. Using thin-film technology we developed a linear array with 18 tiltable mirrors. The mirrors consist of a multilayer system of aluminum and silicon nitride deposited on a structured sacrificial layer of polyimide. The aluminum has a comparatively high reflectivity in the UV/VIS region and the silicon nitride yields good mechanical properties for realizing stress reduced mirrors and bending structures. After etching of the sacrificial layer, the mirrors can be switched electrostatically. The angle between on and off position is about 5/spl deg/. Low switching voltages in the range of 15-45 V can be realized.