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We have developed a single wafer vacuum encapsulation for MEMS resonators, using a thick (20 µm) polysilicon encapsulation to package micromechanical resonators in a pressure < 1 Pa. The encapsulation is robust enough to withstand standard back-end processing steps, such as wafer dicing, die handling, and injection molding of plastic. We have continuously monitored the pressure of encapsulated resonators for more than 10,000 hours and have seen no measurable change of pressure inside the encapsulation at ambient temperature. We have subjected packaged resonators to > 600 cycles of-50-80°C and no measurable change in cavity pressure was seen. We have also performed accelerated leakage tests by driving hydrogen gas in and out of the encapsulation at elevated pressure. Two results have come from these hydrogen diffusion tests. First, hydrogen diffusion rates through the encapsulation at temperatures 300-400°C have been determined. Second, the package was shown to withstand multiple temperature cycles between room and 300-400°C without showing any adverse affects. The high robustness and stability of the encapsulation can be attributed to the clean, high temperature environment during the sealing process.

Thermoelastic dissipation (TED) is analyzed for complex geometries of micromechanical resonators, demonstrating the impact of resonator design (i.e. slots machined into flexural beams) on TED-limited quality factor. Clarence Zener first described TED for simple beams in 1937. This work extends beyond simple beams into arbitrary geometries, verifying simulations that completely capture the coupled physics that occur. Novel geometries of slots engineered at specific locations within the flexural resonator beams are utilized. These slots drastically affect the thermal-mechanical coupling and have an impact on the quality factor, providing resonators with quality factors higher than those predicted by simple Zener theory. The ideal location for maximum impact of slots is determined to be in regions of high strain. We have demonstrated the ability to predict and control the quality factor of micromechanical resonators limited by thermoelastic dissipation. This enables tuning of the quality factor by structure design without the need to scale its size, thus allowing for enhanced design optimization.

Abstract:The use of advanced embedded system technologies such as microelectromechanical system (MEMS) sensors and wireless communications hold great promise for measuring the response of civil structures to ambient and external disturbances. In this paper, the design of a high-performance planar piezoresistive MEMS accelerometer is discussed in detail. The piezoresistive accelerometer possesses superior performance characteristics including low noise densities when measuring local structural responses characterized by high-frequency content. A low-cost wireless sensing unit, designed for automated structural monitoring, is utilized to record and wirelessly transmit measurements obtained by the piezoresistive accelerometer. To validate the performance of the wireless monitoring system including the interfaced piezoresistive accelerometer, a five degree-offreedom laboratory test structure is utilized. Introduction:The field of structural engineering can gain great benefit by embracing new technologies being developed in related fields such as microelectromechanical system (MEMS) sensors and wireless communications. One area where advanced technology can have an immediate impact is in improving the current state-of-practice of structural monitoring systems. The importance of structural monitoring is growing due to the recognition that monitoring is an effective vehicle for advancing our understanding of civil structures and their response to external loads. For example, research directed towards developing performance-based earthquake engineering design criteria can benefit from wide-spread use of monitoring systems because they provide empirical data for validation of structural and statistical models. Structural monitoring systems are also required for the acquisition of real-time response time-histories that are used by structural health monitoring algorithms to rapidly identify potential damages in a structural system.