Operating Principles of CMUTs

When a DC voltage is applied between the metallized membrane and the substrate of a CMUT, the membrane is attracted toward the bulk by the electrostatic force, and induced stress within the membrane balances the attraction. Driving the membrane with an AC voltage superposed on the bias generates ultrasound. If the biased membrane is subjected to ultrasound, a current is produced due to the capacitance change under constant bias voltage. The amplitude of the current is a function of frequency, bias voltage, and device capacitance. The efficiency of CMUTs is determined by the electromechanical transformer ratio, which can be expressed as the product of the device capacitance and the electric field strength across the gap. Planar fabrication enables building a thin membrane above a sub-micron sealed cavity, which is crucial to obtain high electric fields for improved transducer performance.

FIGURE 1. Operation of CMUT as a transmitter and a receiver

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