T.J. Robertson

Cost-effective and manufacturable route to the fabrication of high-density 2D micromachined ultrasonic transducer arrays and (CMOS) signal conditioning electronics on the same silicon substrate

The ability to fabricate Capacitive Micromachined Ultrasonic Transducers (cMUTs) and signal conditioning electronics together on the same silicon substrate offers significant benefits to overall transducer performance, and simplifies connectivity within a dense 2D transducer array. This integration of cMUTs and electronics has been demonstrated utilising a low-temperature silicon nitride membrane technology, post-processed onto CMOS-ASIC substrates. Successful results from fabricated arrays of cMUTs and front-end analogue amplifiers confirm the integration to be relatively simple and highly manufacturable.

Novel, wide bandwidth, micromachined ultrasonic transducers

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. This process allows inherent control of parameters such as membrane size and thickness, cavity size and the intrinsic stress in the membrane to be achieved. Devices fabricated using this process exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz when driven from a standard ultrasonic voltage source. Experiments have been performed with devices containing silicon nitride membranes of variable thicknesses over a 2 /spl mu/m thick air cavity and with device dimensions of up to 5 mm square. This is much larger than has been reported for a device with a single membrane. Calibration measurements using 1/8 inch microphones in air, and miniature PVDF hyrdophones in water, have been performed. The dependence on d.c. bias voltage is examined, involving static membrane deflection measurements and received peak voltages. Pulse-echo and pitch-catch mode operation have been achieved. Interferometric measurements of membrane displacement have been performed in air to illustrate the membrane deflection characteristics. Operation in liquids is also discussed.

Surface metrology using reflected ultrasonic signals in air

Abstract An ultrasonic signal has been generated in air, using a capacitive transducer to produce a broad bandwidth transient signal. This was then focused using an off-axis parabolic mirror onto the surface of solid materials. The reflected signal was detected in terms of received amplitude and time of arrival, and images formed of the surface profile by scanning the focal region over metallic surfaces. Measurements of small steps formed from slip gauges have shown that 5 μm vertical features can be detected. The field of the transducer assembly has been measured with a miniature detector, and the width of the focal region studied. Scans of the surfaces of coins are used to illustrate the technique for non-contact surface profiling.

The characterization of capacitive micromachined ultrasonic transducers in air

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.

Capacitive air-coupled cylindrical transducers for ultrasonic imaging applications

This paper examines the development and characterization of an air-coupled ultrasonic capacitance transducer with a cylindrical geometry. Focusing is achieved using a shaped backplate and an attached thin polymer membrane. The design and construction of these transducers is described, together with the results of measurements to examine the frequency characteristics in pulse-echo mode. The sound pressure field is then examined, by scanning with a miniature detector, and compared to theoretical models. Further experiments are described to determine the lateral resolution and depth of field. Results from imaging experiments of surface features are presented, using two orthogonal devices in a pitch-catch mode.

A conical air-coupled capacitance transducer for surface imaging

This paper describes the construction and operation of an air-coupled capacitance transducer with a conical backplate. This was designed to produce a focal region over an extended distance along the transducer axis. Measurements were performed to examine both the frequency response of the transducer in pulse-echo mode, and the lateral resolution for imaging purposes. The radiated field was measured and compared to theory, and the extent of the focal region determined. Images of surface topography are presented, to illustrate the range of application of the transducer.

An air-coupled line-focused capacitive ultrasonic transducer

A new type of air-coupled, line-focusing transducer is described. This capacitive broadband device, with an active area measuring 19.5 mm/spl times/45 mm, consists of a metallised 3.5 mm thick polymer film mounted over a brass backplate cylindrically concave along its length, with a radius of 28 mm. Device bandwidth (-6 dB) was found to be typically 200 kHz to 900 kHz, with a focal line width of /spl sim/0.68 mm. A miniature air-coupled receiver was used to scan the radiated field of these devices, excited in both tone-burst and broadband mode, and the results compared to theoretical sound pressure plots using an impulse response method. A pair of devices has been used in a novel crossed arrangement to image the surface profile of solids.

A model for the radiated field of a plane piston after reflection from a curved surface

A theoretical model is described for the field resulting from reflection of a plane piston source from a curved surface. The approach is based on an impulse response model, which treats the surface of the mirror as a grid of point reflectors to give an overall reflective response. The model was verified by comparison to experimental measurements in air, using a polymer-filmed capacitive transducer source fitted with an optical-grade mirror, at frequencies up to 1 MHz. The design parameters of such a device were investigated in order to determine the effect on the focal area, and to indicate how this model could be used to optimize the focusing effect of such a system.

The propagation of ultrasound within a gas jet

A study is presented in which the directional characteristics of an ultrasonic signal have been modified due to propagation within an axial jet. The radiated ultrasonic field from a transducer positioned within the air jet has been studied at frequencies above 100 kHz for the first time. The effects of nozzle shape, nozzle diameter, and variations in air jet velocity and temperature have been investigated. At high air flow velocities, divergence of the ultrasonic beam was observed. This was attributed to the increased acoustic velocities in the direction of the flow. An effective waveguide was also demonstrated by cooling the air jet to below-ambient temperatures, so that the acoustic velocity in the air jet was lower than that in the surrounding ambient atmosphere. The result is likely to be of use in air-coupled ultrasonic materials inspection.

Radiated fields of rectangular air-coupled micromachined transducers

The radiated fields of surface micromachined capacitive transducers, with single element sizes in the 2-5 mm range, have been measured experimentally. The transducers have been fabricated using a low temperature CMOS compatible process, and consist of a single metallised PECVD silicon nitride membrane (of typically 1 /spl mu/m thickness), separated by a 1-2 /spl mu/m air gap from a lower electrode on the silicon substrate. The radiated fields were plotted using a miniature micromachined detector, which was scanned through the radiated fields for both wide bandwidth excitation and tone burst operation. The fields of single elements were seen to have similarities to those expected from the theory for rectangular plane pistons in air over the 200 kHz - 2 MHz frequency range. Larger transducers were also fabricated from arrays of single elements, and the fields again scanned experimentally in air. The results illustrate the usefulness of this approach to the creation of highly directional ultrasonic beams in air. Comparison to theoretical fields in each case demonstrates that the devices are well characterized in terms of their emitted beam patterns.