Fabrication of High-Efficiency CMUTs With Reduced Parasitics Using Embedded Metallic Layers

The transmit and receive sensitivity of the capacitive micromachined ultrasonic transducer (CMUT) is proportional to the active device capacitance formed by the vacuum gap of the device, and an insulation layer between the gap and the device electrode. In the sacrificial release process of CMUT fabrication, this insulation layer cannot be made arbitrarily thin due to conformality issues. In this paper, we propose and prove the applicability of a micromachining technique by which metallic sacrificial islands are embedded inside grooves etched on the substrate, yielding topology free surfaces. This obviates the conformality requirement, and enables the growth of a thinner insulation layer which reduces the effective gap height, and, hence, improves sensitivity. Embedded metalic layers, which provide a flat surface for subsequent process steps, have also been used as the back electrode of the CMUT, which facilitated the manufacturing of devices with reduced stray capacitance on thermally oxidized wafers. CMUT devices were fabricated using the proposed technique, and their parameters were measured to justify the performance improvement. While the dc bias requirement is reduced by 19%, the output sensitivity of the device is 10% higher than that of the conventional CMUT, and spurious capacitance is decreased by 70%.