Glass-SOI-Based Hybrid-Bonded Capacitive Micromachined Ultrasonic Transducer With Hermetic Cavities for Immersion Applications

Capacitive micromachined ultrasonic transducer (CMUT) can be widely used in ultrasonic devices. After a discussion of the advantages and challenges for the existing CMUT technology especially for immersion applications, this paper presents a glass-silicon-on-insulator (SOI)-based and hybrid-bonded CMUT with hermetic cavities for the first time. It is understood that while anodic bonding can provide good mechanical strength, it is poor in providing sealing. Hence, Au-Au thermal compression bonding is needed. With the hybrid-bonded vacuum-sealing-line, the bond pad can be opened without damaging the vacuum level of the hermetic cavities, which effectively addresses the difficulty in cavity vacuum sealing. Due to structure design and the insulation characteristic of the glass wafer, the CMUT also has highlight in the aspect of no additional stray parasitic capacitances. Furthermore, the low-temperature compatibility of this hybrid-bonded CMUT improves the performance uniformity for both cells and dies. Capacitance-voltage test of device certificates the feasibility of electrical connections and confirms the low parasitic capacitance of device. Topography of the prototype is measured by using white light interferometer to verify the hermetic performance. Frequency response characterization, including dynamic vibration mode, resonance frequency, vibrating deflection and velocity, are carried out with a laser Doppler vibrometer. The immersion test is carried out and demonstrates the transmitting and receiving of ultrasound from fabricated devices. This glass-SOI-based and hybrid-bonded hermetic CMUT shows a great potential for immersion applications.