Advances in ultrasonic capacitive transducer technology
This thesis describes the development and characterisation of a variety of novel capacitive ultrasonic transducers for use in air-coupled and immersion applications. The first sections of this thesis examine the fabrication and initial characterisation of novel fully micromachined transducers. These devices, produced using a silicon surface micromachining process, consisted of a large laterally dimensioned (up to 5mm square), silicon nitride membrane, typically O.S-2I1m in thickness, above a nominal 211m air gap. Typical operation in air was observed in the MHz region. The effects of membrane thickness and size were studied, as well as the effect of applied bias voltage. Differences between batches of devices were investigated. Their dynamic membrane displacements were measured using interferometric techniques. When driven with transient voltages typical displacements of a few nm were observed. Their radiated fields were also investigated, and compared to plane piston approximations, calculated with a Matlab™ program. Good agreement was found. This thesis also describes the development and characterisation of three novel aircoupled focusing transducers. Two of these devices utilised shaped backplates, one being cylindrically concave, and the other conically concave, to produce lines of focus. The third device utilised an off-axis parabolic mirror to produce a point of focus. The development and characterisation of each is described. Their frequency responses were measured, and found to be comparable with similar planar transducers. Experiments were devised to measure their vertical and lateral resolutions, the latter typically being found to be approximately a wavelength of their centre operating frequency (-500kHz). Their radiated peak sound pressure fields were measured, and for the cylindrical and conical device compared to theoretical plane piston approximations calculated with a Matlab™ program, with good agreement of form being found. Finally, all three transducers were used to create 3-D images of a two pence coin, using amplitude and time of arrival data. Reasonable images resulted. The final section. examines the development of an immersion transducer. This was characterised as both source and receiver, showing increases in both bandwidth and frequency response than when used in air. The effects of backplate surface properties on the characteristics of the device were studied. using a range of roughened, polished, and photolithographic metallic backplates. In addition, the effect of applied bias was also investigated. Sound pressure fields were measured, using both broadband and narrowband tone burst excitations. and compared to theoretical predictions from plane piston theory. These were calculated using a Matlab™ program, with good agreement found. The transducer was then used to produce pulse-echo C-scan images of artificially induced machined defects in Perspex and aluminium plates. Excellent images resulted.