Characterization of improved Capacitive Micromachined Ultrasonic Transducers (CMUTS) using ALD high-Κ dielectric isolation

Use of high-κ dielectric, atomic layer deposition (ALD) hafnium oxide (HfO₂) as an isolation layer material is demonstrated as an improvement over traditional plasma enhanced chemical vapor deposition (PECVD) silicon nitride (Si₃N₄) for Capacitive Micromachined Ultrasonic Transducers (CMUTs) fabricated by a low temperature, CMOS compatible, sacrificial release method. ALD HfO₂ dielectric properties are characterized to optimize CMUT design. Performance improvements are evaluated through parallel plate modeling which showed high gains especially for vacuum gaps of 50 nm and below. Experiments are performed on parallel fabricated test CMUTs with 50 nm gap and 16.5 MHz center frequency to measure and compare pressure output and receive sensitivity for both materials. Results show 6 dB improvement in receive sensitivity (Pa/V) with the collapse voltage reduced by one half, while in transmit mode, half the input voltage is needed to achieve the same maximum output pressure, both as predicted by the models.