Studies of air-coupled capacitive ultrasonic transducers with TiO ₂ high-k dielectric backplate electrode coatings

Capacitive ultrasonic transducers for operation in air typically consist of a rigid backplate electrode and a movable flexible membrane electrode. The backplate is often contoured or textured to modify the frequency response and sensitivity of the device, and the membrane electrode is typically a metallized dielectric polymer film. The compliance of the air in the gap between the two electrodes provides the restoring force for the membrane. The sensitivity of these devices would be increased if the dielectric constant k of the membrane electrode could be increased, but suitable high-k materials are often not found in membrane form. Also, most polymer membrane materials are relatively fragile; metal foils may be used as membrane electrodes in harsh environments, but lack the additional dielectric effect of the polymer membranes. Hence, the inclusion of a high-k dielectric layer on the backplate electrode should increase the device capacitance and hence its sensitivity. Earlier work investigated the use of HfO₂ high-k layers in this regard. This latest work has investigated the effect of using TiO₂ high-k layers on the backplate electrode of an air-coupled capacitive ultrasonic transducer for enhanced operation in air.