Abstract
This work presents a capacitive ultrasonic sensor chip fabricated by the 0.18-m CMOS-MEMS technology. The unique feature of this work is integrating the circuit and the sensor on the same chip, which can efficiently reduce the parasitic capacitance. The study combines capacitive ultrasonic sensors with peak-detection circuits. Ultrasonic waves produced by the photoacoustic effect are received by the sensing pixels, followed by signal amplification, peak detection, and collection of all the detected values. These values correspond to the brightness of the image. The time for image production is significantly reduced. The CMOS MEMS technology allows convenient signal processing to enhance scalability of the array and sensor miniaturization to increase image resolution. The structure is successfully released by wet etching and sealed by 2.4-μm Parylene C. The fabricated capacitive ultrasonic sensor is tested in water. The measured resonant frequency, band width and sensitivity are 3.8 MHz, 2.8 MHz, and 494.505 mV??⁄MPa/V. In the experiment we successfully detect and hold the peak values by using input signals with frequencies from 1 MHz to 10 MHz and amplitudes from 0.1V to 1V. The errors are within ±20% for inputs from 0.1V to 0.5V. The errors reduce to less than ±12% when input voltage is from 0.5V to 1 V. The peak detection circuit design can be improved by the calibration. Therefore, a better image resolution can achieved.