Acoustic Bragg reflectors for Q-enhancement of unreleased MEMS resonators

Abstract

In this thesis, the author introduces the first fully unreleased Micro-Electro-Mechanical (MEM) resonator, and the design of acoustic Bragg reflectors (ABRs) for energy localization and quality factor (Q)- enhancement for unreleased resonators. Two of the greatest challenges in MEMS are those of packaging and integration with CMOS technology. Development of unreleased MEMS resonators at the transistor level of the CMOS stack will enable direct integration into front-end-of-line (FEOL) processing, making these devices an attractive choice for onchip signal generation and signal processing. The demonstrated first fully unreleased resonator exhibits a resonance at 39 GHz with a Q of 129, corresponding to the 1st harmonic longitudinal resonance of the unreleased resonator, a silicon Resonant Body Transistor (RBT) fully clad in Si0 2. A spurious mode occurs at 41 GHz, which is in good correspondence with simulation results. The Q of 129 at 39 GHz is about 4 times lower than that of its released counterpart. Enhanced with the ABRs, the unreleased resonator is able to maintain high Q, and suppress spurious modes. Analysis on the ABR design for unreleased resonators covers design principles, fabrication variations, and comparison to released devices. In the end, it is demonstrated that the ABR is more favorable than the phononic crystal for acoustic energy localization for unreleased resonators, providing a 9 times higher Q.