A MEMS Ultrasonic Transducer for Monitoring of Steel Structures

Ultrasonic methods can be used to monitor crack propagation, weld failure, or section loss at critical locations in steel structures. However, ultrasonic inspection requires a skilled technician, and most commonly the signal obtained at the time of any inspection is not preserved for later use. A preferred technology would use a MEMS device permanently installed at a critical location, polled remotely, capable of on-chip signal processing that could exploit a signal history. We first review questions related to wave geometry, signal levels, flaw localization, and electromechanical design issues for microscale transducers. We then describe the design, characterization, and initial testing of a MEMS transducer to function as a detector array. The device is approximately 1-cm square and was fabricated by the MUMPS process. The chip has 23 sensor elements to function in a phased array geometry, each element containing 180 polysilicon diaphragms with a diameter of 85 microns and an unloaded natural frequency near 3.5 MHz. We report on characterization studies including capacitance-voltage measurements and admittance measurements. We then report on initial experiments, using a conventional piezoelectric transducer for excitation, with successful detection of signals in an on-axis transmission experiment, or the equivalent of a thickness-gauging application.