A Dual-Electrode Piezoelectric Micromachined Ultrasonic Transducer for Ring-Down-Based Helium Concentration Measurement

Compact sensing systems can support the preparation and dilution of helium–air mixtures. This study presents a dual-electrode piezoelectric micromachined ultrasonic transducer (PMUT) for helium concentration calibration using its transient response. The inner electrode excites the membrane, and the outer ring electrode detects the response, enabling the amplitude-envelope area, fitted decay rate, and band-summed spectral index to be extracted from the same waveform. The PMUT comprises an aluminum nitride (AlN) piezoelectric layer and molybdenum electrodes on a silicon-on-insulator (SOI) substrate and was fabricated using a complementary metal–oxide–semiconductor (CMOS)-compatible process. Over 60–80 vol% helium, the fitted decay rate increased with an endpoint-based sensitivity of approximately 5.8 × 102 s−1 per vol%. For the measured concentrations within the 30–90 vol% interval, the spectral index exhibited a linear relationship with R2 = 0.9888 and an endpoint-based sensitivity of −0.21 V2 per vol%; the actual calibration points spanned 33.3–81.8 vol%. The envelope area decreased at higher helium fractions. These results establish concentration calibration in helium–air mixtures using a single PMUT and fixed-frequency excitation without frequency sweeps during measurement.