Fabrication of CMUTs with a low temperature wafer bonding technology

Abstract

Low temperature process is important for capacitive micromachined ultrasonic transducers (CMUTs) to integrate with integrated circuits (ICs) in order to reduce parasitic capacitance and improve its device performance. This paper presents a novel low temperature fabrication technology for CMUTs. Transition metal layers (the Ti/Au layer and Ti/Sn/Au layer) were used for bonding between the silicon substrate and silicon membrane. The bonding process can be done under a temperature less than 350°C. Additionally, the use of the device layer of a silicon-on-insulator SOI wafer as the membrane can improve the membrane uniformity and performance consistence of CMUT cells in comparison with other low temperature processes using sacrificial release processes. Therefore, this fabrication technology can benefit from both integration with ICs and device performance. Using this approach, we have fabricated CMUTs with a membrane diameter of 100 μm and membrane thickness of 2 μm. The fabrication process and some measured results obtained from the fabricated CMUTs are presented in detail.