Effect of Thin Aluminum Coatings on Structural Damping of Silicon Microresonators
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1222-DD01-06
Effect of Thin Aluminum Coatings on Structural Damping of Silicon Microresonators Guruprasad Sosale1, Sairam Prabhakar1, Luc Frechette2, and Srikar Vengallatore1 1 2
Department of Mechanical Engineering, McGill University, Montreal, H3A 2K6, Canada Département de Génie Mécanique, Université de Sherbrooke, Sherbrooke, Canada
ABSTRACT Quantifying the effects of thin metallic coatings on the damping factors of micro- and nanomechanical resonators is important for the design of high-performance devices for sensing and communications. This study presents experimental results for the increase in damping caused by aluminum films coated on cantilevered single-crystal silicon beams. The monolithic silicon beams (100 to 125 µm thick) can operate at the ultimate limits of dissipation established by thermoelastic damping with quality factors ranging from 104 to 105. However, coating these beams with 60 to 100 nm of aluminum can increase the damping by factors of three to five. These results provide guidelines for designing composite micromechanical resonators, and establish the foundation of a new approach for accurate measurement of internal friction in substrate-bonded thin films.
INTRODUCTION Micromachined resonators are widely used as the components of microelectromechanical systems (MEMS) used for sensing, communications, and energy harvesting. Designing microresonators with high quality factors (or, equivalently, low structural damping) is essential for many applications. In general, energy dissipation in microresonators is due to a combination of several mechanisms including viscous damping due to the surrounding fluid (air), support losses, thermoelastic damping, and internal friction. Some of these losses can be minimized, or eliminated, by selecting appropriate operating conditions and structural designs, engineering the microstructure, and reducing the density of crystallographic defects. If successfully employed, these strategies can ensure that the damping in the resonator is reduced to the ultimate limits mandated by the second law of thermodynamics. Several groups have demonstrated that vacuum-operated single-crystal silicon resonators, with carefully designed support architectures, can exhibit quality factors limited only by thermoelastic damping (see, for example, Ref. [1]). For many applications, however, monolithic single-crystal silicon resonators cannot satisfy the demands for performance and multiple functionalities. Instead, composite structures consisting of layered thin films deposited on ceramic substrates are preferred. Common examples include coating polycrystalline metallic films (such as aluminum, copper, gold, or silver films with thickness ranging from 50 nm to 500 nm) on silicon and silicon carbide beams to improve electrical conductivity, enhance optical reflectivity, and alter surface chemistry. Such beams are now commercially available for several applications including atomic force microscopy. In principle, the ultimate limits on damping in such layered structures are establ
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