Capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (CMUT) is a relatively new concept in the field of ultrasonic transducers. Most of the commercial ultrasonic transducers today are based on piezoelectricity. CMUTs are the transducers where the energy transduction is due to change in capacitance. CMUTs are constructed on silicon using micromachining techniques. A cavity is formed in a silicon substrate, and a thin layer suspended on the top of the cavity serves as a membrane on which a metallized layer acts an electrode, together with the silicon substrate which serves as a bottom electrode.

If an AC signal is applied across the biased electrodes, the vibrating membrane will produce ultrasonic waves in the medium of interest. In this way it works as a transmitter. On the other hand, if ultrasonic waves are applied on the membrane of a biased CMUT, it will generate alternating signal as the capacitance of the CMUT is varied. In this way, it works as a receiver of ultrasonic waves.

As CMUTs are micromachined devices, it is easier to construct 2D and 3D arrays of transducers using this technology. This means large numbers of CMUTs could be included in a transducer array providing larger bandwidth compared to other transducer technologies. To achieve a high frequency operation using CMUTs is easier due to its smaller dimensions. The frequency of operation depends on the cell size (cavity of membrane), and on the stiffness of the material used as a membrane. As it is built on silicon, the integration of electronics would be easier for the CMUTs compared to other transducer technologies. The properties to use in high frequency with large bandwidth makes it a good choice to use as a transducer in medical imaging, especially in an intravascular ultrasound (IVUS). Because of its broader bandwidth, it could be used in second-harmonic imaging. Also some experiments have been performed to use CMUTs as hydrophones.

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