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

Abstract

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. A series of reproducibly textured backplate electrodes were micromachined, consisting of a regular array of small pits etched into a silicon substrate, and then electroded with platinum. A layer of TiO2 high-k dielectric material was then deposited to different depths on a selection of backplates. A metallized 5 μm PET membrane electrode was then used to produce a range of capacitive ultrasonic transducers for operation in air. These devices were then tested and characterized as transmitters and receivers, and the effects of the TiO2 layers investigated. The capacitance of the devices was modeled to include the capacitances of the active pits and the parasitic capacitance of the intermediate areas, and measurements were in good agreement with the predicted values. The effect of TiO2 high-k layer thickness on device sensitivity and fractional bandwidth was investigated experimentally, and both were seen to increase with the TiO2 layer thickness, as expected. These latest devices with a TiO2 layer were less sensitive than earlier devices with equivalent layers of HfO2 high-k dielectric due to the lower breakdown voltage and dielectric characteristics of TiO2.