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 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.

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 lowtemperature silicon nitride membrane technology, postprocessed 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. optimum method of integrating transducers and electronics on the same substrate. This paper presents the fabrication of cMUTs using a low-temperature silicon nitride membrane technology, thus enabling transducers to be fabricated directly onto a silicon substrate containing prefabricated electronics. This results in an inherent lowparasitic connectivity to individual transducer elements within an array, whilst retaining full flexibility in controlling the fundamental device properties which determine the transducers' subsequent acoustical performance.

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h devices have been fabricated using a CMOS-Ahsrrucr -Silicon micromachined ultrasonic compatihle process. Devices of up to Imm in size have been tested, with silicon nitride membranes of Ipm and 2pm thickness. The work has ultrasonic receivers, as a function of membrane investigated the response of these devices as thickness and lateral dimensions. I t is shown that the resultant surface micromachined transducers can operate over a wide bandwidth in air, without the resonant behaviour associated with previous devices.