Abstract
This study presents a CMOS micromachined capacitive ultrasonic sensor that can be used in water. By using CMOS MEMS technology, the sensor can be integrated with the amplifier. By placing sensing circuit beneath the sensor, we can effectively reduce the parasitic capacitance of the circuit and save the sensor area. Purpose of this study is to increase the ultrasonic frequency of the sensor. We will increase the structural resonance frequency and bandwidth of the amplifier to 15 MHz. Increasing the frequency will enhance the resolution of photoacoustic imaging. This study shows the measured ultrasonic signals by the capacitive sensors fabricated by post CMOS fabrication. The sensor size is 42 μm. By using post-CMOS process, we etch the metal layer (Metal 3) through the 2 μm × 2 μm etching holes, and then refill the etching holes by silicon dioxide. Four membranes constitute a single sensor. The total capacitance value is 70 fF. The resonance frequency of the ultrasonic signals in water is 15 MHz. For the 3D photoacoustic imaging of a carbon fiber, the lateral resolution is 362.5 μm, and the axial resolution is 300.7 μm. For the 3D photoacoustic imaging of an hair, the lateral resolution is 507 μm, and the axial resolution is 305 μm.