Determination of mechanical properties of a MEMS directional sound sensor using a nanoindenter

We use a nanoindenter to measure the stiffness of mechanical components of a microelectromechanical directional sound sensor. The results validate analytical and numerical linear elastic models, identify the physical structures associated with each resonant frequency, and provide an estimate of the maximum sound pressure the sensor can tolerate. Because the sensor has bending and twisting components that act as springs in series, the overall compliance is the sum of several terms, each of which varies with the location of the loading force along the sensor's surface. By fitting a curve to a plot of the measured overall stiffness vs. location of the loading force, we quantify the separate compliance terms and thereby estimate the resonant frequencies of the corresponding vibrational modes. The frequencies estimated by this method for the two modes are in reasonably good agreement with the measured resonant frequencies. Finally, we establish a minimum failure strength of the sensor, from which we estimate that it can tolerate a sound pressure level greater than about 162 dB without damage.