Optical interferometric measurements of the static/dynamic response characteristics of MEMS ultrasonic transducers

One of the most critical aspects of developing and optimizing capacitive micro-machined transducer systems involves the introduction of appropriate stress levels in the membrane structures during the manufacturing process. Subtle variations in the elastic modulus levels and mechanical coupling can dramatically alter the dynamic vibratory response of the MEMS for ultrasonic applications. In this effort, two different optical interferometric NDE approaches were used to evaluate the static and dynamic characteristics of individual MEMS elements in an ultrasonic transducer array system for variations of applied stress. The interferometric techniques provided a detailed microscopic characterization of the physical motions and local microscopic positions of the MEMS transducer membranes. It was found that the flexural response levels of individual MEMS membrane structures due to increased electrostatic forces was directly coupled to the dynamic response of the micro-transducer, and could potentially be used for optimizing the efficiency and dynamic motion extent of the MEMS transducer array. The optical interferometric techniques both proved to be valuable micro-NDE characterization tools, and were perfectly suited for characterizing the dynamic and static responses of the MEMS ultrasonic transducer systems.