| A directional source of audio sound created using a parametric array, sometimes called an audio spotlight, generates a sound beam that is much narrower than the sound beam generated by a conventional source. These directional sources require the transmission of a modulated high-intensity ultrasonic carrier wave. Capacitive transducers are well-suited for parametric array audio applications because they can efficiently generate high-intensity ultrasound with a relatively wide bandwidth. CMUTs with vacuum-sealed cavities are particularly advantageous because they lack squeeze-film damping, which increases bandwidth but reduces displacement, and because their sealed cavities and permanently attached membranes make them relatively robust. In this paper, we present the basic design constraints of CMUTs intended to generate low-frequency high-intensity airborne ultrasound. In addition, we describe a new method for fabricating these CMUTs that results in uniform cavity depths and a thick insulating oxide layer. Measurement of a fabricated device's input impedance and small-signal displacement demonstrates the success of the new fabrication method and shows good agreement with theory. |