Abstract
This work presents the CMOS micromachined capacitive sensors for ultrasound detection in water. By using the CMOSMEMS technology, the sensor combined integrated circuit is possible. The CMOS fabrication can effectively reduce the parasitic capacitance to enhance the signal to noise ratio. Convenient routing, which is desired for making large two-dimensional arrays, is achieved by placing the detection circuits beneath sensing membranes. The sensing membranes are released by a post-CMOS metal etch and sealed by using either 2 um silicon dioxide or parylene-D . The microstructure has a suspended plate of 60 um in diameter that produces a sensing capacitance of 34.8 fF. The single detection element is formed by nine membranes with a total capacitance value of 312.2 fF. An alternating voltage bias is applied to the membranes for stabilizing the sensed signals which would otherwise attenuate over time due to trapped charges between electrodes. By this method, measured signal is 456.1 and 683.5 mVpp with 14 mm between the sensor and the ultrasonic sound emitter. Resonant frequencies of the sensors in water are 8.8 and 6 MHz, respectively.