PZT-on-silicon RF-MEMS Lamb wave resonators and filters

Lamb-wave piezoelectric RF-MEMS resonators have demonstrated promising performance, such as low motional impedance and high Q-factor. Lamb-wave resonators are still in the perfectioning state and therefore there is a great demand for further understanding of various issues such as reducing the anchor-loss, spurious mode suppression using various designs and simulation techniques such as finite-element methods. AlN, ZnO and recently PZT thin-films are the prevalent piezoelectric materials utilized in the resonators. Of these, PZT has the highest electromechanical coupling-factor which is a big opportunity to reduce the size of the filters and keep the motional impedance low. PZT suffers from a high feed-through due to its high dielectric permittivity which, as a consequence, drastically reduces the stopband rejection. In this thesis, to overcome this problem, we propose a feed-through cancellation method in the presence of specific grounding resistances (non-zero grounding) at the input- and output-sides, which always exist and prevent perfect grounding. Particularly, these grounding resistances have been considered in the designs. Using the proposed technique, the stopband rejection of the resonator is improved by more than 20 dB. Two different bandpass filter configurations (i.e. mechanically coupled and differentially readout) are presented in this thesis. The mechanically-coupled filter consists of two mechanically-coupled resonators with identical designed wave-length. One resonator is used to actuate the whole coupled structure and the other one is used to pick up the signal. The mechanically-coupled filter is actuated using two differential input signals and on the other side, the output signal is also picked up using a differential readout. As both actuation and readout are done using a differential technique, the feed-through signals trough air and substrate are not eliminated. Therefore, the filter shows a low stopband rejection. To overcome this problem, a new concept of bandpass filter is proposed in this thesis called, differential readout filter. Two different filters at two different resonance frequencies around 380MHz and 700MHz are presented. This technique showed a considerable improvement in the performance of RF-MEMS filters compared to the existing Lamb wave filters. Conventional electromechanical filters are using mechanical and/or electrical couplings, however, this new technique is based on the phase change of two un-coupled resonators below and above their resonances. This technique resolves the design issue associated with high feed-through at high frequencies as well as exploiting piezoelectric materials with high-dielectric constant like PZT.