Design and Characterisation of High-Q Solidly-Mounted Bulk Acoustic Wave Filters

In this paper bulk-acoustic-wave (BAW) resonators with high Q-factor Q_a at anti-resonance are demonstrated based on a redesigned reflector stack and a lateral design with a guard ring at the border of the resonator. Experimentally a clear maximum of Q_a together with optimal suppression of spurious modes in the inductive region is observed for a guard ring width of about 7 mum. It is demonstrated that by measuring resonators of different size and aspect ratio, bulk and edge loss mechanisms can be differentiated. Furthermore, the most relevant part of the dispersion curve for the main acoustic mode can be extracted. The observed trends are confirmed by a recently developed finite element model.