Effect of Process Parameters on TED-Based Q-Factor of MEMS

Abstract

Continued advances in microelectromechanical systems (MEMS) technology have led to development of numerous applications including, but not limited to: automotive, communication, information technology, deep-space, medical, safety, national security, etc. These developments are being made possible because of creative designs and novel packaging based on use of some of the most sophisticated analytical and experimental tools available today. These tools are also employed to overcome limitations due to inherent behavior of materials fabricated into miniature shapes subjected to extremely harsh operating conditions while satisfying very challenging specifications/requirements of their applications. Thermoelastic internal friction is present in all structural materials and has been found experimentally in miniature silicon resonators (e.g., microgyroscopes, accelerometers, as well as biological, chemical, and other sensors/actuators) that rely on vibrations of either sensing elements or application-specific elastic suspensions that resonate. Regardless of their applications, sensors are always designed to provide the most sensitive responses to the signals they are developed to detect and/or monitor. One way to describe this sensitivity is to use the Quality (Q) factor. Most recent experimental evidence indicates that as the physical sizes of sensors decrease (especially because of continued advances in fabrication, e.g., by surface micromachining) the corresponding Q-factors become more and more dependent on thermoelastic damping (TED). This form of damping depends on material properties such as coefficient of thermal expansion, thermal conductivity, specific heat, density, and modulus of elasticity. It is also related to such design/operating parameters as resonator dimensions and temperature. This paper reviews a theoretical analysis of the effects that thermoelastic internal friction has on the Q-factor of microscale resonators and shows that the internal friction relating to TED is a fundamental damping mechanism in determination of quality of high-Q resonators over a range of operating conditions. Furthermore, the analysis also shows that the Q of resonators can be critical to the development of modern sensors. Microscale resonators are often used as basic sensing elements in the modern micromachined sensors. These sensors are frequency-modulated devices and exhibit a change in output frequency that is related to measurements and/or control of a physical variable. Accuracy and precision of these measurements/controls are inherently dependent on the frequency stability of the sensor/device output. This, in turn, greatly depends on damping in the resonating element itself.

Advances in optoelectronic methodology for micro- and nano-scale measurements

“…Consequently, material properties have a direct influence on thermal management characteristics of a microswitch and its behavior under actual operating conditions 17 . In fact, fabrication tolerances and accuracy of material properties have profound influence on dynamics as well as the thermomechanical performance of microswitches 17,20 . Selected analytical, computational, and experimental solutions used to determine micromechanical characteristics of the representative MEMS, described in this Section, are reviewed in Section 3.…”

Dynamic behavior of microstructures

“…The switch fabrication methods are particularly important because they influence the material type, surface finish, texture, and overall size of the microswitch components (especially electrical interfaces of a microswitch). Consequently, material properties have a direct influence on thermal management characteristics of a microswitch and its behaviour under actual operating conditions [23]. In fact, fabrication tolerances and accuracy of material properties have profound influence on dynamics as well as the thermomechanical performance of microswitches [9,24].…”