Fabricating capacitive micromachined ultrasonic transducers with wafer-bonding technology

TLDR

The study introduces a wafer‑bonding method for fabricating capacitive micromachined ultrasonic transducers (CMUTs). The method defines the transducer membrane on an SOI wafer and the cavity on a prime wafer, then bonds them in a vacuum via silicon direct bonding to form the CMUT. This process produces large, repeatable CMUTs with vacuum‑sealed cavities, sub‑0.1 μm gap control, 5 % predictable membrane mechanics, and a reduced 15‑step fabrication, yielding predictable center frequency, bandwidth, and collapse voltage.

Abstract

Introduces a new method for fabricating capacitive micromachined ultrasonic transducers (CMUTs) that uses a wafer bonding technique. The transducer membrane and cavity are defined on an SOI (silicon-on-insulator) wafer and on a prime wafer, respectively. Then, using silicon direct bonding in a vacuum environment, the two wafers are bonded together to form a transducer. This new technique, capable of fabricating large CMUTs, offers advantages over the traditionally micromachined CMUTs. First, forming a vacuum-sealed cavity is relatively easy since the wafer bonding is performed in a vacuum chamber. Second, this process enables better control over the gap height, making it possible to fabricate very small gaps (less than 0.1 μm). Third, since the membrane is made of single crystal silicon, it is possible to predict and control the mechanical properties of the membrane to within 5%. Finally, the number of process steps involved in making a CMUT has been reduced from 22 to 15, shortening the device turn-around time. All of these advantages provide repeatable fabrication of CMUTs featuring predictable center frequency, bandwidth, and collapse voltage.