Ring PMUT array based miniaturized photoacoustic endoscopy device

The first successful validation of a forward-looking Piezoelectric Micromachined Ultrasound Transducer (PMUT) ring array designed for photoacoustic endoscopic imaging applications is presented. PMUT ring arrays were fabricated with a 0.5 mm inner diameter, to allow insertion of an optical fiber for light delivery, and ~2.5 mm overall outer diameter. Each ring array consisted of 6 elements, with a total of 102 PMUT cells, or 17 cells per element. Each PMUT cell has a 100 μm diameter multi-layered diaphragm having a ~700 nm thick c-axis oriented aluminium nitride (AlN) thin film as the piezoelectric layer over a Si (handle layer) / SiO₂ (1 μm) / Si (10 μm) / SiO₂ (100 nm) / TiO₂ (40 nm) / Pt (150 nm) substrate to act as an ultrasound receive element. The resonant frequency was ~ 6 MHz in water. The output end of an optical fiber, coupled to a pulsed laser diode (PLD), was fitted with a 2.5 mm ferrule. The PMUT ring array was concentrically mounted on the ferrule to obtain a miniaturized endoscopic PAI device. The observed photoacoustic bandwidth was ~75%, and a strong photoacoustic signal of ~13 mV peak-to-peak output was observed from a light absorbing target kept 5 mm away from the PMUT array. 2D and 3D photoacoustic images of the targets were obtained via raster scanning of the phantom sample. In the future, the functionality of the PMUT ring array will be enhanced through multichannel acquisition to obtain variable acoustic focus and limited view photoacoustic images in real time