Fabrication of MEMS ZnO dome-shaped-diaphragm transducers for high frequency ultrasonic imaging [biomedical imaging]

This paper describes the microfabrication technique for a dome-shaped-diaphragm transducer (DSDT) for 200 MHz cellular microstructure imaging. The DSDT uses a piezoelectric ZnO film to generate acoustic waves and is fabricated on a Si substrate. The DSDTs have been tested with a pulse echo method using a quartz target, and shown to produce an echo signal at 210 MHz with 20% bandwidth. The DSDT fabrication uses spherical balls to precisely shape wax molds, onto which parylene is deposited as a support layer for the DSDT. The ZnO film is sputter-deposited on the parylene dome diaphragm. E-solder silver epoxy is placed and cured on the back surface to function both as an acoustic backing and as a structural support. Quarter wavelength thick parylene is deposited on the front side of the wafer for acoustic matching. The fabrication technique for the DSDTs is meant for low-cost mass production of the devices for HF biomedical imaging. Moreover, the technique allows for a precise control of the radius and curvature of the DSD by adjusting the size of the front-to-backside thru holes using different radii of the spherical balls.