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Piezoelectric power conversion has emerged as a promising technology for power-dense, efficient dc–dc conversion absent of magnetic components for energy storage. However, two practical challenges of piezoelectric devices have limited prototype demonstrations to relatively low power. First, spurious resonant modes plague low impedance resonator design, restricting operating load range, and lowering efficiency. Second, the mechanical nature of piezoelectric energy storage prevents traditional methods of removing heat. This work presents a spurious-free piezoelectric resonator design and a FOM for measuring a power resonator's spurious modes. Additionally, this work develops thermal enhancements at both the piezoelectric device and converter level to improve heat extraction. These advances in acoustic and thermal performance overcome the challenges for moving piezoelectric power conversion to high power, demonstrated by a paralleled piezoelectric dc–dc converter capable of outputting 3.2 kW at 97.7% efficiency, and a piezoelectric resonator achieving a component power density of 5.7 kW/cm$^{3}$.

This work demonstrates the spectrum-clean shear-horizontal surface acoustic wave (SH-SAW) resonators with large electromechanical coupling coefficient ( ${k}_{\text {eff}}^{{2}}{)}$ and substantial admittance ratio ( ${A}_{R}{)}$ in a lithium niobate on insulator (LNOI) platform in the gigahertz range. The solidly mounted LN waveguide is acoustically engineered through optimal selection of wavelength ( $\boldsymbol {\lambda }$ ) and electrode thickness for eliminating the undesired Rayleigh SAW mode while enhancing the ${k}_{\text {eff}}^{{2}}$ of the SH-SAW. Consequently, the transverse spurious modes are suppressed using repetitive cross-interdigital transducers (X-IDT), with minimal tradeoffs to other key performance parameters. As a proof-of-concept, the SH-SAW resonators are fabricated on a 1.2-/0.5-/500- $\boldsymbol {\mu }\text{m}$ YX-LN/SiO2/Si functional substrate with gold electrodes, exhibiting a resonant frequency of 1.17 GHz, a ${k}_{\text {eff}}^{{2}}$ of 43.3%, an ${A}_{R}$ of 74.3 dB, and a spectrum-clean response at $\boldsymbol {\lambda }$ of $2.6~ \boldsymbol {\mu }\text{m}$ with only a few minor spurious modes away from the main mode, offering significant potential for applications in wideband RF circuits.

This work reports a millimeter wave (mmWave) thin-film bulk acoustic resonator (FBAR) in sputtered scandium aluminum nitride (ScAlN). This paper identifies challenges of frequency scaling sputtered ScAlN into mmWave and proposes a stack and new fabrication procedure with a sputtered Sc0.3 Al0.7 N on Al on Si carrier wafer. The resonator achieves electromechanical coupling ( ${k} ^{2}$ ) of 7.0% and quality factor ( ${Q}$ ) of 62 for the first-order symmetric (S1) mode at 21.4 GHz, along with ${k} ^{2}$ of 4.0% and ${Q}$ of 19 for the third-order symmetric (S3) mode at 55.4 GHz, showing higher figures of merit (FoM, ${k} ^{2} \cdot {Q}$ ) than reported AlN/ScAlN-based mmWave acoustic resonators. The ScAlN quality is identified by transmission electron microscopy (TEM) and X-ray diffraction (XRD), identifying the bottlenecks in the existing piezoelectric-metal stack. Further improvement of ScAlN/AlN-based mmWave acoustic resonators calls for better crystalline quality from improved thin-film deposition methods. [2023-0151]

This work reports an acoustic filter at 23.5 GHz with a low insertion loss (IL) of 2.38 dB and a 3-dB fractional bandwidth (FBW) of 18.2%, significantly surpassing the state-of-the-art. The device leverages electrically coupled acoustic resonators in 100 nm 128° Y-cut lithium niobate (LiNbO3) piezoelectric thin film, operating in the first-order antisymmetric (A1) mode. A new film stack, namely transferred thin-film LiNbO3 on silicon (Si) substrate with an intermediate amorphous silicon (a-Si) layer, facilitates the record-breaking performance at millimeter-wave (mmWave). The filter features a compact footprint of 0.56 mm2. In this letter, acoustic and EM consideration, along with material characterization with X-ray diffraction and verified with cross-sectional electron microscopy are reported. Upon further development, the reported filter platform can enable various front-end signal-processing functions at mmWave. [2023-0129]

Piezoelectric power converters, where acoustic resonators replace the inductors as energy storage elements, promise much higher power density and higher efficiency compared to conventional circuits. Recently, lithium niobate (LiNbO3) piezoelectric resonators have been integrated within power converter circuits, showing good conversion efficiency, thanks to their high quality factor ( ${Q}$ ) and electromechanical coupling ( ${k}_{t}^{\,{2}}$ ). However, the converter output power range is limited by large spurious modes near resonance. This work reports a near-spurious-free LiNbO3 thickness shear (TS) resonator, demonstrating a high ${Q}$ of 3500 and ${k}_{t}^{\,{2}}$ of 45% at 5.94 MHz, with a fractional suppressed region of 35%. First, we identify the best LiNbO3 crystal orientation for efficient TS resonators. Then, we propose a novel acoustic design without busbars for spurious suppression, which is extensively simulated, fabricated, and characterized. Further analysis is done to identify existing spurious modes in our proposed design, specifically the effect of dicing on our TS resonator design. Upon optimization, LiNbO3 TS resonators could potentially empower a new design space for low-loss and compact power converters.

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