High-Temperature Acoustic and Electric Characterization of Ferroelectric Al.Sc.N Films

This work presents the experimental measurements and analysis of high-temperature electric and acoustic characterization of ferroelectric film bulk acoustic resonators (FBARs) based on sputtered aluminum scandium nitride (Al0.7 Sc0.3N) films. We recently reported a decreasing trend of the coercive field versus temperature and observed a three-fold reduction of the coercive field from 3MV/cm at room temperature to 1MV/cm at 600K. This work further studies the detailed electro-acoustic properties of Al0.7 Sc0.3 N thin films and FBARs at elevated temperatures. Such studies are critical given the high-power operation and self-heating issues related to 5G acoustic filters. Here, the polarization-dependent capacitance behavior of the metal-ferroelectric-metal (MFM) capacitor is studied in detail at various temperatures up to 600K. At 600K, we measured the DC I-V curves and showed clear resistance switching at a reduced voltage of 100 V compared to room temperature. Furthermore, the resonance frequency of FBARs is tested at varying temperatures up to 600K. We applied +/-100V DC bias and concluded that under the same DC bias conditions, a frequency tuning of 3% is measured at 600K, which is about 3 times larger than at room temperature. The FBARs demonstrate two operating states: metal-polar and N-polar and the electromechanical coupling coefficient ( $k_{\mathbf {t}}^{\mathbf {2}}$ ) can be tuned with DC bias. This unique behavior paves a path forward for $k_{\mathbf {t}}^{\mathbf {2}}$ and frequency modulation in ferroelectric resonator elements.