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Microcharacterization of MEMS ultrasonic transducers using laser interferometry

A detailed micro-characterization of a MEMS ultrasonic transducer was done using a scanning heterodyne interferometry technique. Both temporal and spatial measurements were made of the out-of-plane displacement levels of the transducer under normal operating conditions. Spatial resolution levels approaching the optical diffraction limit of 1 mm were achieved, which allowed characterizations of individual micro-transducer elements to be made. The resonance characteristics of individual transducer membranes were evaluated for drive frequencies between 1 MHz and 7 MHz. Although the majority of transducer elements showed nearly identical frequency response characteristics, several of the MEMS elements showed evidence of shifted resonance response features, which dramatically altered their performance level. Displacement levels in excess of 100 nm were observed for peak DC and AC drive voltage input levels. Time-sequenced measurements of the oscillating MEMS structures were also studied, and showed phase-reversal effects near the edges of transducer membranes. The scanning interferometry technique proved to be a very useful NDE tool for micro-characterization, and provided a wealth of information regarding the micro-features of the MEMS ultrasonic transducer which are currently not available with any other advanced NDE.