Air-coupled CMUTs beyond 2 MHz

The present work sets out the design, fabrication, and characterization of air-coupled capacitive micromachined ultrasonic transducers (CMUTs) operating beyond 2 MHz for high-resolution sensing. The development of these devices is founded upon a robust CMUT process at Fraunhofer IPMS that utilizes a sacrificial-cavity approach. The plates are arranged in a hexagonal configuration and are connected in parallel, with the objective of enhancing transmit power and receive sensitivity. Laser-Doppler vibrometry (LDV) was employed to confirm uniform plate motion with phase variations at the array edges and a central-plate peak deflection of 475 nm at 2.26 MHz. Frequency response analysis demonstrates a small deviation between transmit and receive optimal frequencies, thereby supporting pulse-echo operation with the same device. Furthermore, the analysis reveals the presence of a higher, air-damped mode near 5.6 MHz. Utilizing a compact control system for sensor evaluation up to ±200 V and pulse drive from 0.01 MHz to 25 MHz, we demonstrate the efficacy of reliable pulse-echo measurements at a distance up to 40 mm with the implementation of averaging. Our results establish practical megahertz-range air-coupled CMUT sensing and provide an accessible instrumentation path for laboratory and feasibility studies, enabling applications in small-object detection, robotic gripping, non-destructive evaluation, and surface characterization.