512 + 512 Anodically Bonded Row-Column Addressed CMUT Array Fabrication and Characterization

In the pursuit of 3-D super-resolution ultrasound imaging, transducer arrays with apertures on the size of cm and sub $60-\mu \mathrm{m}$ pitch are required, necessitating many elements and high-frequency operation. This work presents the fabrication process development and characterization of a $\lambda / 2$ pitch 512 + 512 row-column addressed (RCA) capacitive micromachined ultrasonic transducer (CMUT) array. The fabrication process was optimized by addressing key challenges in anodic bonding, specifically vertical short circuits, which severely limit the yield. Process optimization was performed to understand these shorts, and a low-voltage 150-V anodic bonding solution was implemented to mitigate them. The complete process flow is detailed. Electrical characterization confirmed reliable CMUT operation with a 99% yield. To evaluate the acoustic performance, a 128 + 128 RCA CMUT array fabricated using the same process flow was integrated into a modular probe and characterized using a Vantage 256 scanner. Imaging performance of the RCA 128+128 CMUT probe was tested using a 3-D hydrogel scatter phantom with a grid of $205-\times 205-\times 80-\mu \mathrm{m}^{3}$ targets and a wire phantom, demonstrating an axial and lateral resolution at -6 dB of 1.28 $\lambda$ (0.18 mm) and 1.55 $\lambda$ (0.22 mm).