Radiated fields of rectangular air-coupled micromachined transducers

The radiated fields of surface micromachined capacitive transducers, with single element sizes in the 2-5 mm range, have been measured experimentally. The transducers have been fabricated using a low temperature CMOS compatible process, and consist of a single metallised PECVD silicon nitride membrane (of typically 1 μm thickness), separated by a 1-2 μm air gap from a lower electrode on the silicon substrate. The radiated fields were plotted using a miniature micromachined detector, which was scanned through the radiated fields for both wide bandwidth excitation and tone burst operation. The fields of single elements were seen to have similarities to those expected from the theory for rectangular plane pistons in air over the 200 kHz - 2 MHz frequency range. Larger transducers were also fabricated from arrays of single elements, and the fields again scanned experimentally in air. The results illustrate the usefulness of this approach to the creation of highly directional ultrasonic beams in air. Comparison to theoretical fields in each case demonstrates that the devices are well characterized in terms of their emitted beam patterns