Recently, the capacitive micromachined ultrasonic transducer (CMUT) has attracted much attention as a small size microphone and ultrasonic sensor. CMUTs are principally constructed with parallel-plate capacitors with a vibrating membrane and fixed substrate, and for air-use, a thin air-gap exists between the membrane and the substrate. The operation principle of the CMUT device is the mechanical vibration of a thin plate under electrostatic forces. Air damping effect in mechanical motion mainly occurs by air-flow in the narrow air-gap between the parallel-plates. This air damping causes a mechanical resistance to the vibration of the membrane, which could lead to the sensitivity deterioration. In order to reduce this air damping, the thin membrane is usually perforated, and the perforation also improves the etching process of inner sacrificial layer. However, since this perforated membrane structure leads to the area (A) reduction of membrane and electrode, it influences the electrical and mechanical properties in CMUT operation. The external bias condition has an important effect both on the air damping and the performances of CMUT. Especially, the damping effect significantly influenced from the geometries of device structure such as area-ratio (AR) of etching holes to electrode and air-gap height (h). In the present study, we have observed the effect of air damping in characterizations of a proposed CMUT under various electrical bias and structure conditions. Damping ratio and sensitivity of CMUT devices were estimated by measuring the vibration amplitude of the membrane.
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