Fabrication of 32×32 2 D CMUT Arrays on a Borosilicate Glass Substrate With Silicon- Through-Wafer Interconnects Using Non- Aligned and Aligned Anodic Bonding

Abstract

2D arrays are crucial for developing compact and efficient 3D ultrasound systems. Capacitive micromachined ultrasonic transducer (CMUT) arrays, providing convenient integration with supporting electronics, are advantageous for implementing such systems. Fabricating 2D CMUT arrays and integrated circuits (ICs) separately and then combining them in the packaging stage provides flexibility in design and integration. The integrated system can be used for beam-steering and electronic focusing in 3D space. Previously, fabrication processes were reported for implementing 2D CMUT arrays on glass substrates with copper through-glass-via (Cu-TGV) interconnects using anodic bonding and silicon through-glass-via (Si-TGV) interconnects using a sacrificial-release process. Both approaches had challenges, such as voids in Cu-vias, microcracks in laser-drilled glass, mechanical stress in CVD nitride layers, and low fill factor due to fabrication limitations. These challenges can be overcome by combining Si-TGV interconnects with an anodic bonding process. We developed a Si-TGV wafer with a backside glass layer to make it compatible with anodic bonding. We designed and fabricated $32\times 32~2$ D CMUT arrays with a single cell per element to increase the fill factor and to produce high pressure. We measured an output pressure as high as 4.75 MPa $_{\textbf {pp}}$ at 1.8 MHz by focusing the array at 8 mm (F $\#1$ ). Four arrays, tiled next to each other in a $2\times 2$ grid, focusing at 15 mm produced up to 8.65 MPa $_{\textbf {pp}}$ pressure at 1.8 MHz. We achieved 99.9% element yield measured in a single array.[2024-0078]