200 MHz self-focused ZnO MEMS ultrasonic transducers for biomedical imaging

Abstract

A dome-shaped piezoelectric micromachined ultrasonic transducer (pMUT) was fabricated using MEMS silicon bulk micromachining. The IC-compatible fabrication process will allow for the pMUT's controlling electronics and amplifiers to be placed directly adjacent to the transducer, minimizing noise. With MEMS micromachining techniques, the design can be easily modified to create a 200 MHz array. This cost-effective batch process for fabrication makes this device an attractive choice for cellular microstucture imaging, skin cancer detection and pathology, and acoustic property measurement of biomolecules. The active element of the transducer was a self-focused, 14 urn ZnO piezoelectric film with a center resonant frequency of approximately 200 MHz. The element was suspended on a dome-shaped membrane in a 2.5 mm aperture hole etched on a (100) silicon wafer. Spherical self-focusing was achieved using a wax molding of a 5mm sphere to obtain an f-number of 1. The pMUT incorporates a one-quarter wavelength, conformally vapor-deposited Parylene matching layer. The transducer membrane is structurally supported by its backing layer, 1 cm thick lossy conductive epoxy. Impedance characteristics measured showed a phase peak at 215 MHz. The pMUT was tested in a pulse-echo arrangement and an echo signal was obtained with moderate noise at 200 MHz and about 28% bandwidth. Experimental results are in good agreement with those predicted by the KLM model. © 2004 IEEE.