An innovative piezo-MEMS channel-select filter design based on non-monotonic coupled modes

We propose a novel mechanically-coupled filter design which for the first time combines two distinct physical vibration modes, including length-extensional (LE) and degenerate in-plane shear (IPS) modes excited in a single filter structure, fabricated using InvenSense AlN-on-silicon platform. By taking advantage of typical LE-based (LE-LE) and IPS-based (IPS-IPS) monotonic filter designs but without their inherent constraints, the proposed non-monotonic coupled mode (IPS-LE) filter simultaneously features a nodal-point positioned filter coupler and a differential operation, hence enabling narrow-band filtering and significant feedthrough reduction toward channel-select applications. Notably, the proposed filter topology is also capable of addressing the fly-back issue observed from IPS-based filter. As a result, the designed 34.8-MHz piezo-MEMS filter is successfully demonstrated with a flat passband, 0.18%-bandwidth (63 kHz), and 30-dB stopband rejection under proper termination resistances (RQ).