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.