Radius of Curvature Measurement Using Piezoelectric Micromachined Ultrasonic Transducers

This work presents the concept, basic theory, fabrication and testing results of the non-contact radius of curvature (RoC) measurement of a surface using piezoelectric micromachined ultrasonic transducers (pMUT). The system is composed of a minimum of three side-by-side transducer arrays composed of $4\times 20$ pMUTs with $208\mu \mathrm{m}$ in diaphragm radius made of $2\mu \mathrm{m}$ -thick AlN and a natural frequency of 462.83 kHz are utilized for the measurement scheme, and the extracted electromechanical coupling factor for AlN pMUT is 1.58%. Dual-electrode bimorph (DEB) pMUTs were fabricated using a CMOS compatible fabrication process with two $1\ \mu \mathrm{m}$ -thick layers of active piezoelectric AlN and 130nm Mo electrode layers. Experiments show good measurement accuracy for 3D-printed curved surfaces up to RoC of 200mm at a distance of about 5cm away. These results indicate that the proposed method have the potential to measure surface curvature inside a media that light cannot penetrate such as beams and columns inside the buildings, and implants inside the human bodies.