CMUT Working Principle
The capacitive effect applied to ultrasonic transducer, acoustic and ultrasonic sensors for the generation of acoustic waves is known for a long time. The application of a variable voltage on a polarized capacitor (biased) terminals creates a variable field of pressure on the electrodes that generates an acoustic wave in the contiguous medium. However, example of capacitive devices in transducing applications is rare due to the requirement of an electric field strengths as high as 100MV/m to obtain sensitivity comparable with piezoelectric ceramics systems. In the recent years, advances in microelectronics and particularly in microfabrication technology have made possible the achievement of high performance capacitive acoustic devices with very small sealed cavities. Then, capacitive sensors and transducers are competitive to outperform their piezoceramic counterparts which constitute the base of the current ultrasonic and acoustic applications and systems.
Cross-section of a CMUT elementary cell
When applying a DC voltage (VDC) to the parallel plate capacitor, an electrostatic force (F) will cause a deflection of the membrane. When this static attractive force is established between the electrodes, superimposing an AC voltage (VAC) leads to vibration of the membrane and hence to generation of ultrasonic waves. If a DC biased cell is submitted to external forces (ultrasonic waves), an output current will occur due to capacitance variations, the cell is working here as a receptor.