This work reports on the first demonstration of the frequency tuning and intrinsic polarization switching of film bulk acoustic resonators (FBARs), based on sputtered AlScN piezoelectric thin films with Sc/(Al + Sc) ratio of approx. 30%. A box-like ferroelectric hysteresis behavior of 900 nm-thick Al0.7Sc0.3N sputtered films is obtained, showing a coercive electric field at ~3 MV/cm. The fundamental thickness-mode resonance of the bulk acoustic wave (BAW) resonator is measured at 3.17 GHz frequency with an excellent electromechanical coupling coefficient (kt²) of 18.1 %. The FBAR frequency response is studied, in both (i) the linear tuning regime, upon application of DC electric fields below the coercive field; as well as (ii) the polarization switching regime, upon application of electric fields above the coercive field. A large linear tuning range of 215 ppm x μm/V is obtained in case (i), resulting from the high scandium content. The series resonance frequency of the FBARs is switched ON and OFF in (ii) upon application of 350 V unipolar waveform across the Al0.7Sc0.3N thickness. This is the first demonstration of the intrinsically switchable AlN-based FBARs with a large tuning range; and record high kt² reported for AlN-based FBARs to date. Furthermore, this work paves the way for realization of tunable and switchable wideband acoustic filters operating at super high frequency ranges (SHF). [2020-0203]

This letter presents the first demonstration of thin-film ferroelectric Aluminum Scandium Nitride (AlScN)-on-silicon composite resonators, targeting high-overtone resonance modes in the sub-6GHz band with a high figure of merit (FoM). The resonators are based on sputtered ferroelectric AlScN films with Sc/(Al+Sc) ratio of ~30% and thickness of ~ 1 μm. Two types of AlScN thickness-extensional (TE) resonators are co-fabricated on the same SOI platform; with and without a 3.55 μm-thick Si layer in the resonant stack. We show that although the passive Si device layer underneath the thin piezo-stack results in degradation of the electromechanical coupling coefficient (k _t ² ), it boosts the quality factor (Q), provides structural robustness, and improves the overall Q ×k _t ² FoM. The resonant frequency spectrum of the high-overtone TE modes of AlScN-on-Si composite resonator is analyzed and the dependency of k _t ² on the Si device layer properties is studied. A high k _t ² value of 11.7% at the 3 ^rd -order TE resonant frequency of 2.4 GHz is reported, yielding a high k _t ² ×Q _max FoM of 84. The reported FoM shows 2 × improvement compared to the co-fabricated AlScN-only FBARs.

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 (Al _0.7 Sc _0.3 N) 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 Al _0.7 Sc _0.3 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. [2021-0214]

Next-generation RF front-end architectures require a high Figure of Merit (FoM) of Quality factor ( Q ) x electromechanical coupling factor ( k_t ² ) acoustic resonators for K-band filtering applications. Scaling up the frequency of thin-film bulk acoustic wave resonators (FBARs) involves thickness downscaling in both the piezo films and the metal electrodes, causing mechanical and electrical losses. Furthermore, achieving 50Ω impedance matching for the K-band resonators requires device area reduction compared to conventional FBARs operating below 6GHz, which can lower k_t ² and power handling capability. To tackle these issues, we utilize cascaded FBARs (cas-FBARs) with all-epitaxial metal/Al _0.8 Sc _0.2 N/metal layers on an oxide/Si substrate. The fabricated cas-FBARs achieve a measured Q_max × k_t ² of 14.71 at 19.11GHz, marking the highest k_t ² of 10.14% within the K-band FBAR devices published to date. We investigated k_t ² while considering size scaling, crucial for 50Ω impedance matching for 5G devices. We provide a comprehensive characterization of both single FBARs and cas-FBARs, taking into account the impact of resonator sizes on device performance.