Thin-Piezo on Single-Crystal Silicon Reactive Etched RF MEMS Resonators

Abstract

This paper demonstrates how a single crystal silicon wafer can be used to fabricate thin-film piezoelectric-on-silicon (TPoS) resonators by utilizing a modified version of Single Crystal Silicon Reactive Etch and Metallization (SCREAM) process. The developed process enables the fabrication of MEMS resonators with varied device layer thicknesses ranging from sub-micrometer to tens of micrometers (one thickness per die) from a single bulk silicon wafer, while avoiding the need of costly silicon-on-insulator (SOI) substrates. The thin-film piezoelectric on single-crystal silicon reactive etched technique allows batch fabrication of TPoS resonators, while also retaining the same number of photolithography steps. To maintain a good resonator body sidewall roughness, a conformal Al2O3 thin film was deposited by atomic layer deposition to act as the sidewall protection layer. Through the developed process, resonators with varied silicon layer ranging from $0.1~\mu \text{m}$ to $47~\mu \text{m}$ have been successfully implemented. The measured results under different ZnO-to-Si thickness ratios have been studied, in terms of motional impedance ( $R_{\mathrm {m}}$ ), quality factor ( $Q$ ), and resonance frequency. It is noted that TPoS MEMS resonators operating in fundamental and higher lateral extensional modes exhibit their best performance under an optimal ZnO-to-Si thickness ratio. Resonators fabricated by the modified TPoS process with a Si device layer thickness of 4- $20~\mu \text{m}$ exhibits optimal performance. The highest $Q$ of 1,567 for a disk resonator and the lowest motional impedance of 791 $\Omega $ for a square plate resonator were achieved with Si layer thicknesses of $20~\mu \text{m}$ and $4~\mu \text{m}$ , respectively.