Hermetically Encapsulated Fully Differential Breathe Mode Ring Resonators For Timing Applications PDF Download

As modern electronic devices continue to miniaturize and integrate more functionality, silicon-based MEMS timing references are gaining more and more attention and are replacing quartz crystals in the $5 billion market. They offer a lot of advantages such as a small foot-print, low cost, low power consumption, etc. However, the performance (phase noise in particular) of MEMS-based timing references still needs improvement to outperform the well-established quartz crystals. This work presents a fully-differential breathe-mode ring resonators with a quality factor as high as 473,000 at 10MHz by minimizing air damping, anchor loss and thermo-elastic dissipation. This quality factor approaches the theoretical maximum quality factor set by the phonon-to-phonon scattering in the material. It is among the highest reported in literature. MEMS oscillators based on this breathe-mode ring resonator produce a -120dBc/Hz close-to-carrier (@1kHz offset) phase noise performance. In addition, ultra-narrow bandwidth breathe-mode ring filters are designed and fabricated based on this high Q breathe-mode ring resonator using mechanical coupling, achieving less than 0.05% bandwidth. However, due to the large transduction gap size (1.5μm) dictated by the "epi-seal" fabrication process, the motional impedance of the breathe-mode ring resonator is relatively high, causing poor oscillator phase noise performance. In this work, a modified "epi-seal" fabrication process employing surface micromachining combined with bulk micro-machining is performed, achieving a transduction gap size as small as 260nm, thereby reducing the motional impedance by six times and improving the phase noise performance by 15dB theoretically. In the last section, integrated CMOS-MEMS structures, including 10 and 25 MHz breathe-mode ring resonators and 850 kHz double ended tuning fork resonators integrated with CMOS trans-impedance amplifiers (TIAs), are designed and fabricated in the InvenSense Nasiri fabrication platform. This platform offers a great way to help bring good concepts to quick realization by opening the commercially proven MEMS fabrication platform to the public.