Fabrication and Sub-Assembly of Electrostatically Actuated Silicon Nitride Microshutter Arrays

We have developed a new microshutter array (MSA) subassembly. The MSA and a silicon substrate are flip-bonded together. The MSA has a new back side fabrication process to actuate the microshutters electrostatically, and the new silicon substrate has light shields. The microshutters with a pixel size of 100 x 200 μm2 are fabricated on silicon with thin silicon nitride membranes. The microshutters rotate 90o on torsion bars. The selected microshutters are actuated, held, and addressed electrostatically by applying voltages on the electrodes the front and back sides of the microshutters. The substrate has the light shield to block lights around the microshutters. Also, electrical connections are made from the MSA to a controller board via the substrate. INTRODUCTION In the Goddard Space Flight Center, we have developed a new MSA sub-assembly. The sub-assembly consists of the MSA and a silicon substrate. The MSA has a new back side fabrication process to actuate the microshutters electrostatically, and the new silicon substrate has light shields. In the past, a large MSA array size of 175x384 had been developed, fabricated, and installed in the James Webb Space Telescope (JWST) as multi-object aperture selectors [1, 2]. The JWST MSAs were magnetically opened and electrostatically held open. The magnetic operation, however, involved moving magnets thus complicated actuation schemes and increased the payload. Instead, the electrostatic actuation is simple. Thus, the weight of the MSA assembly is far reduced. The microshutters were made with silicon nitride membranes on silicon with a pixel size of 100 x 200 μm2 and rotate 90o on torsion bars. Figure 1 shows a scanning electron microscope (SEM) image of the MSA; on the front side, microshutter blades are suspended on the 2 μm wide torsion bars, the gap around the microshutters is 2 μm wide, aluminum is patterned to create electrodes, and molybdenum nitride is patterned on top of the microshutter blade. Also, the JWST microshutters had the light shields patterned with aluminum overhang around the openings as shown in Figure 2. The purpose of the light shield is to block light passing through the perimeters around the microshutters. The light shield step required 3 lithography steps: a sacrificial layer, partially exposed bumps pattern on the sacrificial layer, and an aluminum pattern over the sacrificial layer. In order to avoid these three lithography steps plus an aluminum deposition, we decided to develop a new silicon substrate with the light shields. The silicon substrate was first developed and used as a mechanical support for the MSA and an electrical interface between the MSA and the external control electronics on a printed circuit board as shown in Figure 3. For the first time, we present the new MSA indium-flip-chip bonded to the silicon substrate with light shields that block light leak around the openings around the microshutters. The front side of the MSA is flip-chip bonded to the aluminum electrodes on the silicon oxide surface of the silicon substrate. The front electrodes are connected to the substrate electrodes via indium bumps while the back side electrodes are wire bonded to the substrate electrodes. Figure 1: A SEM image of the front side image of the MSA. On the microshutters, the patterned microshutter blade, torsion bar and strips of molybdenum nitride are shown. Figure 2: A SEM image of the MSA with aluminum light shields that prevent light passing through the perimeters around the microshutters. There are 1 μm wide and 0.25 μm deep dimples of the light shield bumps to prevent the stiction of the microshutters to the light shields. A close-up photo of the bumps are shown. FABRICATION OF ELECTROSTATICALLY ACTUATED SILICON NITRIDE MICROSHUTTER ARRAYS L. Oh , M. Li ,K. Kim, D. Kelly, A. Kutyrev, S. Moseley SGT Inc. NASA/Goddard Space Flight Center ASRC Federal Corp. University of Maryland