| This research effort developed electret-biased MEMS-scale ultrasonic sensor arrays meant to discriminate between elastic waves generated by an impact with a hard material and elastic waves generated by impact with a soft material. The effort investigated two main technical areas; (1) How to integrate electrets into MEMS fabrication processes so that sensor arrays could be easily realized, and (2) How to use these sensor arrays to discriminate between elastic wave sources with different frequency content. The effort explored multiple electret materials, fabrication processes, and polarization techniques, leading to the development of an in situ electret charging process. It also explored multiple MEMS transducer designs, leading to placing actual sensor elements onto structures and capturing impact events.;The electret integration portion of the effort demonstrated an in situ charging process that allowed the entire MEMS fabrication to be completed prior to polarizing the electret film. The process was based on the fabrication of a micro-scale "charging grid" suspended a short distance above the electret material and separated by a charging gas. Energizing the grid with a high voltage led to ionization of the gas in the gap, and the migration of charge to the surface of the electret polymer. This process and the resulting performance of charged films were characterized. The process could be easily performed after entire device fabrication.;The MEMS sensor array portion of the effort built suspended metal resonant structures separated by an air gap above the electret film and charging grids. These structures were designed to respond at particular frequencies of interest in an ultrasonic system. After fabrication, devices were polarized and attached to test articles for impact tests. Waveforms captured from the impacts demonstrated how the resonant structures can be employed to discriminate between the different impacts, and identified the challenges that exist in developing sensor arrays for impact scenario.;In summary, the effort resulted in a new technique for electret integration into MEMS devices, the demonstration of array processing of ultrasonic signals, and the micromechanical analysis of impact waveforms. The resulting device was able to discriminate between impacts with materials of differing acoustic impedance. |
