Micro-optoelectromechanical systems for space-based instruments

NASA's Origin Program and the European Cosmic Vision program bring into focus one of the ultimate goals of astronomy: helping us to find our place in the universe by explaining how our world and others came into being. The study of how galaxies, stars, and planetary systems form and evolve calls for a new generation of orbiting space telescopes and other astronomical instruments. Micro-optoelectromechanical systems (MOEMS) controlled remotely, and that work at cryogenic temperatures, will be crucial in these instruments. MOEMS devices allow incoming light intensity to be filtered and objects to be selected with programmable slit masks. They also allow phase and wavefront control adjustment via microdeformable mirrors, and spectral tailoring via programmable diffraction gratings. Major applications for these devices are multi-object spectroscopy (MOS), wavefront correction, and programmable spectrographs. For several years our group has designed, fabricated, and characterized MOEMS devices for astronomical instruments. This technology will provide the key to small, light, low-cost, and scientifically efficient instruments, paving the way for tomorrow's observational astronomy.

Infrared astronomical MOS in ground-based and orbiting telescopes is used extensively to investigate the formation and evolution of galaxies. One application for our MOEMS is to provide programmable slit masks at the entrance of the spectrograph. For the James Webb Space Telescope near-IR spectrograph, a microelectromechanical systems-based programmable microshutter array has been developed by NASA. We are studying the ability of micromirror arrays (MMAs) to fulfill the performance requested for future MOS instruments in space. To this end, we have designed a silicon-based MMA to operate at cryogenic temperatures, and a commercial array for a space evaluation program.

The Astronomical Observatory of Marseille-Provence, France, and the Swiss Federal Institute of Technology in Lausanne are collaborating on MIRA—a next-generation MMA—by developing a process to generate reflective slit masks. MIRA comprises an array of mirrors made by coating single-crystal 100×200μm silicon surfaces with gold. Our first MIRA prototype comprised 5×5 micromirrors. The micromirrors can be tilted by electrostatic actuation, yielding a 20^° mechanical tilt angle. Mirror tilting worked before, during, and after cryogenic cooling at 92K. We were able to determine that the surface quality of the micromirrors was not compromised at cryogenic temperatures. Our next-generation MMA comprises 2048 (32×64) micromirrors and has been designed to allow individual mirror addressing. We fabricated it using fusion and eutectic wafer-level bonding (see Figure 1). Without coatings, these mirrors exhibit a peak-to-valley deformation less than 5nm and provide a tilt angle of 20^° for an actuation voltage of 120V. Individual addressing is achieved using a line-column algorithm based on optimized voltage-tilt hysteresis (i.e., the pull-in, pull-out behavior) of the electrostatic actuator. A first experiment of the line-column algorithm was demonstrated by individually tilting several micromirrors.¹ Devices are currently packaged, wire-bonded, and integrated to a dedicated electronics to demonstrate the individual actuation of all micromirrors on an array. An operational test of this large array at cryogenic temperatures (<100K) is scheduled in early 2012.

Figure 1. Array of 2048 (32×64) micromirrors, each with dimensions 100×200μm. Mirrors are individually addressable via a line-column algorithm based on optimized voltage-tilt hysteresis of the electrostatic actuator.

In parallel with this development, the European Space Agency (ESA) is working with the Astrophysics Laboratory of Marseille to assess a commercial digital micromirror device (DMD) made by Texas Instruments for space applications, such as the ESA EUCLID mission. The DMD features 2048×1080 mirrors on a 13.68μm pitch. For MOS applications in space, the device should work in vacuum, at low temperature, and each MOS exposure would last typically for 1500s with micromirrors held in a static state, either on or off. A specific thermal/vacuum test chamber has been developed for test conditions down to 233K at 10⁻⁵mbar vacuum. Imaging capability for resolving each micromirror has also been developed for determining degradation in any single mirror (see Figure 2). Our tests reveal that the DMD remains fully operational.² A 1038-hour-life test in space survey conditions, total ionizing dose radiation, thermal cycling, and vibrations/shocks has also been successfully completed. We encountered no problems that would affect the ability of the DMD to meet operational requirements for space.

We are also developing a DMD-based spectrograph demonstrator. We wish to access the largest field of view with the highest contrast, using our 2048×1080 mirror DMD. Figure 3 shows a typical mask pattern. Our solution is an all-reflective spectrograph design with F/4 on the DMD component, with two arms in parallel for spectroscopy and imaging.

Figure 2. Schematic diagram and photograph of the digital micromirror device (DMD) space evaluation setup.

Figure 3. A typical mask pattern for multi-object spectroscopy, using the 2048×1080mirror DMD.

In summary, we have successfully fabricated micromirror arrays and used them in tests to apply mask patterns in a multi-object spectrograph. These developments show the suitability of these components for instruments in space. The next steps will be to test our large arrays and the MOS demonstrator. Testing is crucial in assessing the performance of this new family of instruments, and is necessary to develop operational procedures for investigating astronomical objects. Our demonstrator will be used in the Italian National Galileo Telescope, located in the Canary Islands.