Anchor Loss Reduction in Resonant MEMS using MESA Structures

The quality factor of an oscillator is inversely proportional to the damping and is a measure of the width of its amplitude vs. forcing frequency response. For sensing and signal processing applications of microelectromechanical systems (MEMS) oscillators, the quality factor (henceforth called Q) affects the sensitivity and performance of such devices. As MEMS vibrates (resonates) some of its vibrational energy is transmitted to the substrate upon which the MEMS is fabricated. A large component of this energy is carried away as surface acoustic waves (SAW). We demonstrate a design that improves the Q of resonant MEMS oscillators by up to 4times by reflecting surface wave energy back to the MEMS. Wave reflection occurs at trenches fabricated in a circle around the MEMS. The trench creates a "mesa" that provides partial mechanical isolation to the MEMS. The loss of energy due to SAW increases almost exponentially with frequency, with a corresponding decrease in Q. Hence the demonstrated design would become even more useful with the increasing need for higher frequency resonators. The mesa structure presented here is a simple idea which can be easily integrated into existing fabrication procedures and can be used for commercial purposes.