Narrowband ladder-type MEMS filter based on high-Q thin-film piezoelectric-on-silicon MEMS resonators

Abstract

This article reports a narrowband ladder-type microelectromechanical system (MEMS) filter based on thin-film piezoelectric-on-silicon (TPoS) MEMS resonators with high quality factors ($Qs$). The reported second-order ladder-type MEMS filter consists of three single-port TPoS MEMS resonators. Dependencies of the resonator’s $Q$ and effective electromechanical coupling factor ($k_{eff}^2$), as well as the filter’s bandwidth on the device thickness of the TPoS MEMS resonators are investigated using finite element method (FEM) analysis. The mechanical $Q$ of the resonator increases while its $k_{eff}^2$ decreases as the thickness of the silicon device layer become thicker from $20\mu m$ to $60\mu m$, resulting in a lower insertion loss and narrower bandwidth for the designed filter. Measurement results illustrate that the fabricated TPoS resonator with silicon device layer of $60\mu m$ achieves a high $Q$ of 50258 and $k_{eff}^2$ of 0.094% at its resonant frequency of 25.9 MHz, which agree well with the FEM simulated values. Thanks to the high $Q$ and low $k_{eff}^2$ of the TPoS MEMS resonator, the reported ladder-type MEMS filter achieves a narrow percent bandwidth of 0.059%, a low insertion loss of 0.71 dB, and a 20-dB shape factor of 1.33 under proper termination impedance. It provides a promising way for achieving narrowband MEMS filters for channel selection at radio frequency in wireless communications.