Phononic Crystals for Acoustic Confinement in CMOS-MEMS Resonators

This work presents the first implementation of phononic crystals (PnCs) in a standard CMOS process to realize high-Q RF MEMS resonators at GHz frequencies without the need for any post-processing or packaging. An unreleased acoustic resonant cavity is defined using a PnC comprising back-end of line (BEOL) materials such as routing metals and low-k intermetal dielectric. A CMOS-MEMS resonant body transistor (RBT) with electrostatic driving and piezoresistive sensing is implemented within this cavity. This results in a 10x enhancement in Q at resonance and improved suppression of spurious modes off-resonance as compared to first-generation CMOS-integrated devices. The first PnC-confined RBT is demonstrated in IBM's 32nm SOI process at 2.8 GHz with Q of 252, spanning a footprint of 5 mu m x 7 mu m.