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

This paper demonstrates how a single crystal silicon wafer can be used to fabricate thinfilm 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 0.1μm to 47μm have been successfully implemented. The measured results under different ZnO-to-Si thickness ratios have been studied, in terms of motional impedance (Rm), 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 μm exhibits optimal performance. The highest Q of 1,567 for a disk resonator and the lowest motional impedance of 791 Ω for a square plate resonator were achieved with Si layer thicknesses of 20 μm and 4 μm, respectively.