Abstract
This work reports on the design and characterization of a mechanically-coupled CMOS-MEMS filter centered at several MHz with a narrow bandwidth and reasonable insertion loss after filter termination performed in a 4-port network analyzer. To implement a bandpass filter, the proposed filter structure produces two physical resonance modes, therefore forming a filter passband with a desired bandwidth by the use of the mechanical coupler at proper coupling locations. By the use of a conventional filter design, the metal stacking layers in a foundry-oriented CMOS platform were used to fabricate the device structures, making great progress aligned with existing fabrication lines. To further reduce the motional impedance, the high-velocity coupled array and gap-reduction mechanism were adopted to create larger transduction areas and tiny gap spacing for capacitive transducers, respectively. However, due to the existence of undesired parasitics from a typical two-port configuration, the resonance response would be dwarfed and masked by the background feedthrough floor. Furthermore, the matching condition is also limited by shunt capacitance at the I/O ports for filter transmission, that significantly barricades the proper filter termination. To solve this issue, a pure motional response of the proposed filter can be extracted once the de-embedding scheme is carried out. To attain the lower material loss, the silicon dioxide structure was developed in this work as well. Ideally, the SiO2 can provide better electromechanical coupling and higher quality factor as compared with the metal stacking counterparts. Based on the electrical isolation of the oxide structure, the filter devices can be operated in differential configuration via the balun function on the network analyzer. With that, the feedthrough parasitic can be evidently alleviated without any post-data processing, thus creating 30dB noise-floor improvement in filter performance.