Abstract
This papers investigates device approaches towards the confinement of acoustic modes in unreleased UHF MEMS resonators. Acoustic mode confinement is achieved using specially designed mechanically coupled acoustic cavities known as acoustic Bragg Grating Coupler structures to spatially localize the vibration energy within the resonators and thereby improve the motional impedance (R_x) and mechanical quality factor (Q). This enhancement in the mechanical response is demonstrated with numerical simulations using distinct unreleased resonator technologies involving dielectric transduction mechanisms. These initial investigations show improvements in the Q as well as enhanced vibrational amplitudes within the resonator domains (i.e. translating to improved R_x values) in the case of coupled cavities as opposed to single cavity designs. An initial approach to fabricate the devices in a CMOS compatible dual-trench technology are presented.
Highlights
Modern RF and future mmWave communication devices rely on low phase noise oscillators and high performance band-pass transmission filters
The stringent requirements on the frequency selectivity for some of RF transceiver applications are traditionally achieved by Surface Acoustic Wave (SAW) resonators, bulk acoustic wave resonators (BAW) and/or partially released thin-film bulk acoustic wave resonators (FBAR)
For BAW and FBAR technology, the film thickness dictates the resonant frequency which restricts devices operating at different frequencies being manufactured on the same die/wafer and monolithic integration
Summary
Modern RF and future mmWave communication devices rely on low phase noise oscillators and high performance band-pass transmission filters. For BAW and FBAR technology, the film thickness dictates the resonant frequency which restricts devices operating at different frequencies being manufactured on the same die/wafer and monolithic integration. The majority of MEMS resonators still require a final release-step to create freely suspended vibrating structures (Wang and Weinstein 2011). This release step adds processing complexity to the manufacturing of monolithically integrated MEMS and CMOS. To overcome the need of post-processing release steps, the technology described in Wang and Weinstein (2012a) using deep-trench capacitors (DT) has enabled a new route towards the integration of MEMS resonators and the CMOS. This paper builds upon this technology as well as replicated phonic-crystal (PnC) structures to propose a device methodology comprising multiple coupled acoustic cavities for acoustic mode confinement in order to achieve high Q factors
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