Fabrication of Capacitive Micromachined Ultrasonic Transducers With High-k Insulation Layer Using Silicon Fusion Bonding

With its excellent yield and potential for mass production, a capacitive micromachined ultrasonic transducer (CMUT) is a promising alternative solution to conventional piezoelectric ultrasound transducers. However, as CMUTs require high bias voltage for operation, reducing the voltage is a critical issue in the industry to overcome the problems of reliability and the need for high-voltage driving circuitry. One of the promising methods to reduce the high bias voltage is to increase the dielectric constant by replacing the insulation layer with a high-k material. Here, we present a new fabrication method for the high-k insulation layer CMUT that maintains the reliability and advantages of silicon wafer-bonded CMUT. Notably, our proposed process eliminates the need for additional photolithography steps to replace the insulation layer with high-k material compared to the conventional CMUT fabrication. In contrast to the conventional CMUT, which employs silicon dioxide film for insulation, our high-k CMUT exhibits a reduction in pull-in voltage of 11.3%. These results suggest the potential for enhanced sensitivity in ultrasonic imaging applications. [2024-0153]