Study of Energy Loss Mechanisms in AlN-Based Piezoelectric Length Extensional-Mode Resonators

This paper reports on the investigation of anomalous low quality factors (Qs) in AlN thin film-based length extensional (LE)-mode resonators using finite element method (FEM), analytical modeling, and experimental techniques. Different heterostructures of LE resonators having Al/AlN/Si, Al/AlN, AlN/Si, and Si are designed and fabricated using standard MEMS processes. Experimental Qs along with resonances are obtained using electrical and optical readout and are compared with the modeled and analytically obtained Qs. The results show that the thermoelastic damping (TED) in metal electrode, anchor loss, and charge redistribution loss are not the dominant loss mechanisms. With the help of Mason's network modeling, we investigated the dielectric loss, which is also observed to be negligible for these resonators. An internal mechanical loss, originating from the columnar growth of AlN and observed using Mason's model, is proved to be the dominant loss mechanism. This observation is confirmed and reinforced by the COMOSL's FEM analysis of TED in AlN, having columnar growth effect and by experimental results.