A CMOS Compatible Ultrasonic Transducer Fabricated With Deep Reactive Ion Etching

This paper describes design, fabrication, and test of an integrated micromachined ultrasound transducer (MUT). This MUT can work as an emitter and a receiver of ultrasonic signals in air and is intended to be used in applications requiring both acoustic signal generation and sensing. Among these applications there can be detection or distance measurements of different objects where the nature either of these objects or of their surroundings does not allow the use of light-based methods. The device has been fabricated in a 0.8 mum CMOS process combined with deep reactive ion etching, integrating in the same chip a suspended membrane (plate) and the associated interface electronics. The plate is thermally actuated at its resonance frequency (40 kHz) during emission. During reception, a piezoresistive bridge placed on the membrane is used for monitoring its deflections. The main advantage of the design using an actuator and a sensor integrated in one multilayer structure is its simplicity. Comparing with capacitive transducers where two-electrode structure is used, considerations in terms of mechanical stability and membrane damping as well as the fabrication process are not critical. The integrated solution allows the use of an amplifier at the output of the piezoresistive Wheatstone bridge which results in the sensitivity of 35 mV/Pa; the maximum acoustic pressure generated by the transducer at 10 mm is 5 mPa