Micromachined sapphire GHz lateral overtone bulk acoustic resonators transduced by aluminum nitride

This paper introduces a new class of piezoelectric-transduced bulk acoustic wave resonators formed by a micro-machined c-plane sapphire (Al₂O₃) membrane (~750 nm). The thin film sapphire (TFS) is fully suspended in air and made to vibrate in the GHz range by a sputtered aluminum nitride (AlN) film. For the first time, the realization of the TFS is achieved via a layer transfer process from a single crystal c-plane sapphire wafer. In order to demonstrate the superior intrinsic material quality of the sapphire membrane, a lateral overtone bulk acoustic resonator (LOBAR) configuration, recently introduced by our group, was employed. The LOBAR is engineered to minimize the effects of mechanical energy dissipation and extract the ultimate limit set by phonon-phonon dissipation in the TFS. In addition to the conventional rectangular design, an annular LOBAR geometry is introduced in this paper. This design permits to lower the device impedance (~3k Ω), while attaining a high quality factor (Q). The measured responses exhibit f·Q of 4.1·10¹² Hz and 4.6·10¹² Hz at 1 and 2 GHz, respectively in the annular configuration with a 9% of transducer to sapphire coverage ratio. The conventional rectangular LOBAR with coverage ratio of 0.57% exhibits a Q of 5,440 at 2.8 GHz, which translates to the highest f·Q (1.53·10¹³ Hz) ever reported for AlN-based suspended resonators.