A high sensitivity capacitive micromachined ultrasonic transducer (CMUT) with stepped cavity structure

Compared to traditional piezoelectric transducers, capacitive micromachined ultrasonic transducers (CMUTs) offer advantages such as wide bandwidth, low power consumption, small size, and better impedance matching with fluids, enabling more applications in a wide range of temperature and humidity. This paper investigates the design, simulation, fabrication, and experimental validation of a CMUT with a stepped cavity structure. Finite element analysis (FEA) was used to optimize electrode size, cavity height, and cavity radius, establishing an optimal structure 2-stage-Optimal, whose electromechanical coupling coefficients are superior to conventional structures. A 16 × 16 element CMUTs array with 2-stage-Optimal structure was fabricated, and the electrical impedance characteristics and resonance frequency of the fabricated CMUTs chip were measured. Simple impedance matching was performed to optimize its transmission efficiency. Under a DC bias voltage of 80 % collapse voltage, the CMUTs chip's center frequency is approximately 0.9 MHz, with a −6 dB bandwidth of about 82.6 %, a maximum transmission sensitivity of 4.3 kPa/V, and a maximum reception sensitivity of 101.5 mV/kPa, or −199.8 dB (ref. 1 V/μPa). The study shows that the 2-stage-Optimal stepped cavity structure CMUT exhibit high sensitivity characteristics.