Enhancing quality factor by etch holes in piezoelectric-on-silicon lateral mode resonators

We report a unique method of using etch-holes to greatly improve the unloaded quality factor (Q_u) of VHF-band low impedance laterally vibrating AlN Thin-film Piezoelectric-on-silicon (TPoS) MEMS resonators. We have validated the proposed method experimentally by applying it to fabricated devices operating at their 5th order modes with resonant frequencies of 105 MHz. The largest improvement in Q_u resulting from adding holes was found to be 5.7 times on average using the proposed method. The experimental results are corroborated by finite-element (FE) simulations which show that the holes re-distribute the strain energy in the resonator body. The re-distribution of strain energy consequently greatly suppresses the axial-direction deformation in the supporting beam tethers and undercut anchoring regions. Having less energy in the supporting tethers and undercut anchoring regions leads to a reduction of anchor loss and thus enhances Q_u particularly when Q_u is limited by anchor loss. It was also found that this effect of enhancing Q_u through the proposed approach remains significant for different supporting tether lengths. Experimental results show that the increase in Q_u going from plain resonators to resonators with holes is consistent regardless of tether lengths variation (i.e. the measured variation in Q_u for different tether lengths is insignificant compared to the increase from adding etch holes). In comparison, no increase in Q_u is observed when driving the same set of devices in their fundamental modes (22 MHz) as the added holes do not suppress axial deformation in the supporting beam tethers and undercut anchoring regions as indicated by FE simulations. As such, the enhancement in Q_u using the proposed approach is specific to the mode, thus offering the benefit of selectivity.