Effects of heterostructure stacking on acoustic dissipation in coupled-ring resonators

We describe a systematic study on quantifying the effects of heterostructure layer stacking upon measured dissipation and quality factors (Qs) of AlN-on-Si coupled-ring breathing mode micromechanical resonators. For the first time, we design and fabricate resonators of the same lateral dimensions but with different stacking layers, namely, Si, AlN-on-Si, and Al-on-AlN-on-Si. With both optical and electrical readout techniques, we measure the fundamental radial-extensional mode and its Q values and compare the observed and simulated energy dissipation. For the same device geometry, the results show that the bare Si device has the highest Q (11,304). Adding an AlN layer leads to a reduction in Q for the AlN-on-Si device by a factor of ~1.46 compared to bare Si device. Top layer metallization further compromises the Q by a factor of ~1.85. Modeling of the dissipation processes suggests that neither the thermoelastic damping (TED) nor the anchor loss is the limiting loss factor. A piezoelectric-specific loss mechanism known as charge redistribution loss and interface loss are both possible limiting mechanisms.