Design and Experimental Characterization of CMUTs Based on Eutectic Bonding Technology

Capacitive micromachined ultrasonic transducers (CMUTs) have widespread application in fields of ultrasonic imaging, ultrasonic therapy and 3D ultrasonic gesture recognition. Integration of CMUTs with integrated circuits (ICs) is a key approach to reduce its parasitic capacitance and improve the signal to noise ratio. However, current fusion bonding-based fabrication technologies need high temperature (>1 000 ℃), which impedes the integration of CMUTs with ICs. Developing eutectic bonding-based fabrication technology is an effective approach to solve this problem. Based on this low-temperature fabrication process, this paper designs CMUTs with circular and hexagonal cavities and their corresponding array structures. Finite element method (FEM) and theoretical formulas are used for the analyses of their collapse voltages, resonant frequencies and deflection of the membrane under thermal stress and atmospheric pressure. The analytical results indicate that the collapse voltages and resonant frequency of CMUTs are within the expected range, and the membranes will not collapse under the co-action of thermal stress and atmospheric pressure. Experimental testing on morphology, structure parameters, capacitances and impedance-frequency characteristics of the fabricated CMUTs chips are carried out. The results indicate that the chip morphology, structure parameters, capacitance and impedance- frequency characteristics are consistent with those expected, and the CMUTs chips can work normally. These results demonstrate the feasibility of the structure design and fabrication process of the CMUTs. These researches provide a foundation for further integrated design and fabrication of CMUTs with ICs.