An Air-Coupled Capacitive Micromachined Ultrasound Transducer for Noncontact Nondestructive Evaluation

We report a high-efficiency air-coupled capacitive micromachined ultrasound transducer (CMUT) for nondestructive evaluation applications. Air-coupled ultrasound inspection is attractive for materials unsuitable for contact or immersion inspections. However, this method is not widely adopted due to strong interface scattering and air attenuation of acoustic energy. Air-coupled CMUTs' efficiency must be improved for commercial inspection applications. We have developed a large-gap CMUT transmitter/sensor device that has high transduction efficiency in air. The CMUT has a specially patterned insulation layer that enables large acoustic output at high driving voltage without dielectric breakdown or charging failures typical for similar devices. Paired CMUT transmitter/sensor tests showed significantly higher transduction efficiency than state-of-the-art commercial transducers. This indicates improvement of air-coupled CMUTs is achievable and may lead to widespread use of CMUTs for air inspections.