A three-mask process for fabricating vacuum-sealed capacitive micromachined ultrasonic transducers using anodic bonding


This paper introduces a simplified fabrication method for vacuum-sealed capacitive micromachined ultrasonic transducer (CMUT) arrays using anodic bonding. Anodic bonding provides the established blessings of wafer-bondingbased CMUT fabrication processes, together with method simplicity, control over plate thickness and properties, high fill factor, and ability to implement giant vibrating cells. Likewise to these, compared with fusion bonding, anodic bonding can be performed at lower processing temperatures, i.e., 350°C versus 110zero°C; surface roughness requirement for anodic bonding is more than ten times more relaxed, i.e., five-nm rootmean- square (RMS) roughness as opposed to zero.5 nm for fusion bonding; anodic bonding will be performed on smaller contact area and hence improves the fill factor for CMUTs. Although anodic bonding has been previously used for CMUT fabrication, a CMUT with a vacuum cavity might not are achieved, mainly as a result of gas is trapped inside the cavities throughout anodic bonding. In the approach we have a tendency to gift during this paper, the vacuum cavity is achieved by gap a channel in the plate structure to evacuate the trapped gas and subsequently sealing this channel by conformal silicon nitride deposition in the vacuum environment. The plate structure of the fabricated CMUT consists of the only-crystal silicon device layer of a silicon-on-insulator wafer and a thin silicon nitride insulation layer. The presented fabrication approach employs only three photolithographic steps and combines the advantages of anodic bonding with the benefits of a patterned metal bottom electrode on an insulating substrate, specifically low parasitic series resistance and low parasitic shunt capacitance. In this paper, the developed fabrication scheme is described in detail, as well as process recipes. The fabricated transducers are characterized using electrical input impedance measurements in air and hydrophone measurements in immersion. A repr- sentative design is employed to demonstrate immersion operation in conventional, collapse-snapback, and collapse modes. In collapsemode operation, an output pressure of one.67 MPa pp is shown at 7 MHz on the surface of the transducer for sixty-Vpp, three-cycle sinusoidal excitation at 30-V dc bias.

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