Experimental Characterization and Optimization of Palladium-Coated Laterally Vibrating Resonators for High-Sensitivity Hydrogen Sensing

Abstract

This paper presents the experimental characterization and optimization of palladium (Pd)-coated aluminum nitride (AlN) laterally vibrating resonators (LVRs) for high-sensitivity hydrogen (H₂) detection. The sensing mechanism relies on hydrogen-induced biaxial compressive stress in the Pd film, which shifts the resonant frequency through stress-frequency transduction, enabling compact, selective, and high-resolution H₂ monitoring for leak-detection applications. Pd-coated LVRs operating from 123 to 396 MHz were fabricated on a single 4-inch wafer, enabling a controlled study of the combined effects of operating frequency, Pd thickness, and residual stress on electromechanical performance and sensing response. Quality factors reaching $Q = 1625$ and electromechanical coupling coefficients reaching $k_{t}^{2} = 1.94\%$ were achieved, representing a $2.7\times $ improvement in Q over prior ScAlN-based implementations. Sensitivity increases monotonically with frequency (15 to 54 Hz/ppm at 123–396 MHz for 100 nm Pd) and with Pd thickness ( $8.5\times $ from 100 to 200 nm). Residual stress is identified as a previously underreported optimization lever, producing a $2.6\times $ sensitivity enhancement for the same Pd thickness. The 317 MHz device with 150 nm Pd is identified as the optimal design point ( $Q = 1411$ , $k_{t}^{2} = 0.92\%$ ), achieving a peak sensitivity of 772 Hz/ppm in the low-concentration regime with $t_{90} \approx 10$ min, establishing Pd-coated AlN LVRs among the most sensitive resonant H₂ sensors reported to date.[ 2026-0081]