Epitaxially-grown thick polysilicon for baw disk resonator gyroscopes with very low dissipation

This paper reports, for the first time, on characterization of gyroscopic modes in solid epi-poly disk resonators, showing Qs exceeding 1M at ~3MHz with a maximum f.Q of 4.2e×10¹², and on fabrication challenges associated with implementation of solid disk BAW gyroscopes on thick (45μm) epitaxially-grown polysilicon. Our work reveals that the isotropic nature of epi-poly not only enables degenerate n=2 wine-glass modes, but more importantly produces balanced n=3 modes with extremely low anchor loss, both of which are not seen in <;100> single crystalline silicon (SCS). Unlike <;111> SCS, degenerate modes in epi-poly substrates are insensitive to crystallographic plane misalignments, offering a greater potential for mass production. The n=3 modes of epi-poly disks provide better substrate decoupling compared to their n=2 counterparts, leading to higher robustness against vibration and stress variation for gyro applications. A reduced-temperature HARPSS process is developed to fabricate an epi-poly n=3 solid-disk BAW gyro having nano-gap electrodes with Q of 350k at 3MHz and linear scale factor in mode-matched operation.