Morphology and Microstructure Evolution of Multilayer Au/Cr/Si Thin Films Subject to Annealing
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Morphology and Microstructure Evolution of Multilayer Au/Cr/Si Thin Films Subject to Annealing David Miller1, Cari Herrmann1,2, Hans Maier3, Steve George2, Conrad Stoldt1, Ken Gall1 1 Department of Mechanical Engineering2 Department of Chemistry and BiochemistryUniversity of Colorado, Boulder, CO 80309 3 Lehrstuhl für Werkstoffkunde (Materials Science), University of Paderborn, 33095 Paderborn, Germany ABSTRACT Au/Cr/Si microcantilevers were studied in their as-deposited condition and annealed state, with emphasis on a thermal treatment of 225 °C for 24 hours. Change in beam curvature was monitored during isothermal hold as a function of time. Secondary grain growth was observed in the gold, which contained non-uniformly distributed twins and dislocation defects. Diffusional transport of the chromium layer was observed during annealing. Nodules arranged in the “rolling hill” topography were observed at the free surface, both before and after annealing. Nanometer thick coatings of alumina grown by atomic layer deposition improved the uniformity of both microstructure evolution and curvature evolution during high-temperature annealing. INTRODUCTION Au/Cr/Si devices were originally researched for use in hybrid (high frequency) electronics [1], and later for scanning tunneling microscope mounts, as well as X-ray lithography masks. More recent microsystems application areas include optics, electronics, chemical & biological sensing, micro-actuation, as well as fabrication process monitoring. Behavior in these micromechanical multilayer devices differs from that in traditional thin films studies (“thick substrate”) owing to the comparable thickness of the various layers and the resulting difference in stress distributions. The relatively large displacement, large curvature, and lesser internal stress seen in multilayer micro-devices makes them different from conventional thin film specimens and renders them extremely sensitive to change in microstructure. Also, the metallic layers often exist having a limited thermal history, i.e. the as-deposited structure, as necessitated in microsystem applications. Multimorph structures are typically exposed to thermo-mechanical loading during their fabrication, subsequent post-processing, and application environment; therefore significant change in microstructure and mechanical curvature will come to exist. Such changes will influence design performance as well as device reliability. Arrays of Au/Cr/Si microcantilevers were used to better understand the long-term performance and reliability of thin film Au/Cr/Si composite structures. Structural curvature (mechanical shape), chemical composition, and film morphology were studied as a function of time and temperature. The goal of the research was to identify and correlate changes in morphology and microstructure to the observed changes in curvature.
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EXPERIMENTAL Multimorph cantilever beams were fabricated using the Multi-User MEMS Process (MUMPS), provided by the MEMSCAP Corporation [2]. Interposed arrays of fixed-fre
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