Novel, wide bandwidth, micromachined ultrasonic transducers

Abstract

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.

Capacitive Micromachined Ultrasonic Transducers: Technology and Application

“…In the proposed low-temperature processes for cMUTs [20,28,29], the structural material for the membrane's fabrication is silicon nitride, deposited by plasma enhanced chemical vapor deposition (PECVD).…”

“…Most of these techniques are based on surface micromachining techniques [2,20], others are modified BiCMOS processes [21], and others combine bulk and surface micromachining for the realization of the membranes and of the back plate [22].…”

Visualizing acoustic displacements of capacitive micromachined transducers using an interferometric microscope

“…In addition, there may be variability in the fabrication processes. In order to fully understand the behavior of many micromachined mechanical devices, such as those used to generate and detect ultrasound, 1,2 it is thus necessary to be able to measure their dynamic displacement behavior. Metrology techniques such as scanning probe and optical microscopy are well established for measuring static displacements of such devices, for instance when a dc bias is applied to a membrane or cantilever.…”

All-optically integrated photoacoustic and optical coherence tomography: A review

“…In addition, the axial resolution of the PAI system is inversely proportional to the bandwidth of ultrasonic transducer. 36 Therefore, improving the bandwidth of ultrasonic transducer is a key factor for improving the quality of PAI. However, owing to the limitations of piezoelectric materials and fabrication techniques, ultrasonic transducers usually have limited bandwidth.…”