Phase shift micro-beamforming of CMUT arrays using the spring-softening effect

In phased array systems, a partial beamforming can be carried out inside the probe to reduce the number of signals to be processed by the main system. Such technique, known as micro-beamforming, involves the application of a delay-and-sum beamforming to small groups of adjacent array elements using dedicated electronics. In this paper we show that, due to the electrostatic spring-softening effect, the phase response of Capacitive Micromachined Ultrasonic Transducers (CMUTs) can be controlled by varying the bias voltage. Based on this phenomenon, we propose a phase shift micro-beamforming method of CMUT arrays. We analyze the performance obtainable by applying this method to a typical phased array configuration for medical imaging. We show that, with relatively high quality factor CMUTs that still allow broadband immersion operation, it is possible to obtain up to 90° phase shifts by varying the bias voltage. Further, by inverting the sign of the bias voltage, we show that it is possible to obtain an additional 180° phase shift. Simulation results show that, with a 40% -3 dB one-way fractional bandwidth CMUT, the grating lobe level of the transmission radiation patterns, obtained with the proposed micro-beamforming technique, may be kept below -23 dB using broadband excitation signals for maximum steering angles of ±45°.