Low Stress, High Reflectivity Thin Films for MEMS Mirrors
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Low Stress, High Reflectivity Thin Films for MEMS Mirrors Yoosuf N. Picard*†, David P. Adams*, Olga B. Spahn*, Steven M. Yalisove†, Daryl J. Dagel*, Joseph Sobczak* * Sandia National Laboratories Albuquerque, NM 87105 † Materials Science and Engineering Dept., University of Michigan Ann Arbor, MI 48109 ABSTRACT Au/Cr and Au/Ti films were deposited on Si (100) substrates using DC planar magnetron sputtering to assess residual stress in high reflectivity coatings. The dependence of stress on argon sputter pressure, component materials, and film thickness is discussed. Stress evolution as a function of thickness for individual Cr, Ti and Au films is also investigated to identify highstress components of these two-layer coatings. Near-zero stress Au/Ti films were achieved with a particular set of sputtering parameters. Using the same process conditions, films were deposited onto pre-released MEMS mirrors having a number of different shapes and sizes. Optical interferometry demonstrates minimal change in the bow of 500, 250, and 125 µm diameter mirrors, consistent with a λ/40 flatness (λ = 1319 nm). INTRODUCTION The need for micromirrors that handle power levels approaching 1 Watt has motivated this research of MEMS mirror materials. In particular, minimizing thermal absorption by maximizing film reflectivity has led to research of high-bandwidth, reflective thin films for IR wavelength radiation. However, coatings having a moderate compressive or tensile stress can lead to significant micromirror curvature [1]. Maintaining flatness of a ~2 micron-thick MEMS mirror to within λ/40 requires near-zero average film stress and minimal through-thickness film stress. Coated MEMS mirrors must also remain flat over the lifetime of the device. In the present study, metal films were sputtered on Si substrates to assess the influence of growth conditions (sputter gas pressure and deposition rate) on the residual stress of the film. Once these growth conditions were optimized to achieve near-zero stress, thin film coatings were deposited on a micromirror test pattern. Optical interferometry was used to assess mirror curvature changes induced by the coating. EXPERIMENTAL All thin film mirror coatings were deposited using a Unifilm PVD-300 DC planar magnetron sputtering system with 75-mm diameter targets. To assess residual stress, films were deposited on the native oxide of 50-mm diameter, 75-micron thick Si (100) substrates (Virginia Semiconductor). Before deposition, hydrocarbon contaminants were cleaned off the substrate surface by baking in an ultraviolet oxygen cleaning system for 30 minutes, heating within the sputter chamber at 50°C for 10 minutes and 150°C for 15 minutes. Samples were allowed to cool to ambient temperature prior to deposition. System base pressures prior to deposition were 1.0E-07 Torr or better, and the Ar sputter gas was controlled to a particular pressure between 2U3.11.1
40 mTorr during deposition. The Unifilm sputter system utilizes a programmed substrate rastering scheme to obtain >98% film thickness u
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