Fabrication of Capacitive Micromachined Ultrasonic Transducers via Local Oxidation and Direct Wafer Bonding

We present the successful fabrication of capacitive micromachined ultrasonic transducers (CMUTs) with an improved insulation layer structure. The goal is to improve device reliability (electrical breakdown) and device performance (reduced parasitic capacitance). The fabrication is based on consecutive thermal oxidation steps, on local oxidation of silicon (LOCOS), and on direct wafer bonding. No chemical-mechanical polishing step is required during the device fabrication. Aside from the advantages associated with direct wafer bonding for CMUT fabrication (simple fabrication, cell shape flexibility, wide gap height range, good uniformity, well-known material properties of single-crystal materials, and low intrinsic stress), the main vertical dimension (electrode separation) is determined by thermal oxidation only, which provides excellent vertical tolerance control ( <;10 nm) and unprecedented uniformity across the wafer. Thus, we successfully fabricated CMUTs with gap heights as small as 40 nm with a uniformity of ±2 nm over the entire wafer. This paper demonstrates that reliable parallel-plate electrostatic actuators and sensors with gap heights in the tens of nanometer range can be realized via consecutive thermal oxidation steps, LOCOS, and direct wafer bonding without chemical-mechanical polishing steps.