S-Band Micromechanical Resonant Impedance Transformers Based on Aluminum Nitride FBARs

This work presents an intensive study of passive voltage amplification offered by one-port microelectromechanical systems (MEMS) piezoelectric resonators used as matching network (MN) in $S$ -band RF wake-up receiver (WuRX) front-ends. Through theoretical modeling and numerical analysis, a large electromechanical coupling coefficient ( ${k}_{t}^{2}$ ), high quality factor ( $Q$ ), low transducer capacitance ( $C_{o}$ ), and light capacitive loading ( $C_{L}$ ) are the keys to obtain high passive voltage gain at gigahertz (GHz) frequencies. To evaluate the performance of GHz MEMS-based MNs, several thin-film bulk acoustic wave resonators (FBARs) with different transducer capacitances at 2.3 GHz were designed and fabricated using a 200-mm commercial aluminum nitride (AlN) piezoelectric MEMS-CMOS process. The fabricated resonators achieve a high figure-of-merit (FoM = ${k}_{t}^{2} \boldsymbol {\cdot }{Q}$ ) from 90 to 175, resulting in a high passive gain of around 21 dB at a reasonable reactive load of 100 fF. Based on a small FBAR device with a $C_{o}$ of only 67 fF, the highest gain achieved in this study is 23.4 dB based on a complex load ( $C_{L} =50$ fF and $R_{L} =50\,\,\text{k}\Omega$ ) while attaining good input matching ( $S_{11} < -10$ dB). In addition, the effect of spurious modes on the passive voltage gain is numerically investigated by the 2-D finite element method (FEM), which is in good agreement with the experimental results.