Infoscience

Thesis

Large Micromirror Array Based on a Scalable Technology for Astronomical Instrumentation

Understanding the formation of galaxies and studying the expansion of the Universe, need the spectral characterization of faint astronomical objects such as stars or galaxies. From these spectra, the distance, the velocity and the chemical composition of the objects are obtained in order to map them in the Universe. In order to improve the scientific return of an observation with a telescope, astronomers developed a technique called Multi-Object Spectroscopy (MOS), allowing the measurement of tens of thousands of objects simultaneously. Such measurements are feasible only if overlapping of spectra is avoided and if the spoiling sources such as bright stars and background emission can be removed during the observation. Therefore, in MOS, a field selector is placed at the focal plane of the telescope allowing the selection of the astronomical objects. This field selector can be a simple slit mask or a complex Micro-Electro-Mechanical Systems (MEMS) device. In this thesis, a micromirror array was developed as a reflective field selector for MOS. Arrays of 2048 micromirrors of 100 × 200 µm2, electrostatically tilted by 24° were modeled, microfabricated and characterized. Electromechanical, optical as well as in a cryogenic environment characterizations were performed. The concept of actuation of the micromirror was based on the electrostatic double plate actuator: when a voltage was applied on the electrode placed underneath the micromirror, the micromirror was attracted towards the electrode and tilted using flexure beams. A precise tilt angle was achieved by stopper beams limiting the motion of the micromirror. The micromirrors were electrically addressed in two ways, line by line or individually. To individually address each micromirror of the array, a line-column concept based on the properties of the tilt angle/voltage hysteresis of the micromirror was used. By finite element modeling, the optimized design for such addressing and a better understanding of the effects of process variations on the behavior of the micromirror were studied. The fabrication process involving two wafer-level bonds (gold-silicon eutectic and fusion bonding) were developed to make micromirror arrays of different sizes (32 × 64, 16 × 32, 8 × 4) but also to be suitable for even larger arrays. To cover high-aspect ratio structures for further processing, a 2 µm-thick silicon dioxide layer was transferred from a wafer on top of these structures by wafer-level fusion bonding and thinning down techniques. Wafer-level eutectic bonding was used to assemble the 2048 micromirrors on their chip electrodes. Characterization was performed on micromirror arrays of 2048 micromirrors driven by a 128 channels high-voltage power supply, developed especially for the actuation of electrostatic MEMS devices. The tilt angle as a function of actuation voltage was measured by white light interferometry. These micromirrors tilted by 24° for an actuation voltage of 124 V. Micromirrors demonstrated an optical fill factor of 82% with respect to their surface and 98% along the micro-mirror in the long slit mode. The peak-to-valley micromirror surface deformation was characterized by phase shift interferometry and silicon micromirrors coated with an adhesion layer of 10 nm of titanium and a layer of 50 nm of gold demonstrated a deformation of 9.8 nm at room temperature. For infrared application, the micromirror array was tested in a cryogenic chamber at a temperature of -111 °C and in vacuum environment. Under these conditions, several lines of 32 micromirrors were successfully tilted by 24° and the micromirrors coated with titanium and gold demonstrated a peak-to-valley surface deformation of 27 nm due to the Coefficient of Thermal Expansion (CTE) mismatch between the gold and the silicon. Line addressing of a micromirror array of 32 × 64 micromirrors was demonstrated. Out of the 17 lines of 32 micromirrors that were wire-bonded 14 were successfully actuated. Individual addressing based on the line-column concept was demonstrated on 2 × 2 micromirrors. This work shows the ability of this new type of programmable slit mask to perform objects selection in future MOS for astronomy.

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