Thermoelastically actuated MEMS ultrasonic transducer

Ultrasound based techniques provide a method for noncontact distance measurement utilizing a thin vibrating diaphragm as the acoustic source. Devices based on different operating principles have been reported including electrostatic and piezoelectric. The conventional pulse-echo technique is limited by a finite pulse decay time that restricts the measurement range to greater than a few centimeters. Proximity sensing on the order of millimeters can be achieved using continuous wave form techniques such as the acoustic impedance method or simultaneous excitation at multiple frequencies. Using silicon microfabrication technology, a MEMS ultrasonic transducer has been fabricated for millimeter range proximity sensing. The salient features of the sensor/actuator include a 0.01 mm thick, 1 mm diameter composite diaphragm that is thermoelastically driven at ultrasonic frequencies using a circular heater at its center. Dynamic Joulean heating of the diffused surface heater induces bending moments in the diaphragm due to the temperature gradient across the membrane cross section. Electromechanical transduction of the membrane vibration is achieved via four diffused silicon piezoresistive strain gauges located at the edge of the diaphragm. The design, fabrication and preliminary characterization of this sensor/actuator are presented.