This paper describes the development of a capacitive micromachined ultrasonic transducer (CMUT) array for minimally invasive photoacoustic imaging (PAI). Integrated on a miniature silicon bar that is approximately 100 mu m thick and 2.8-5 mm x 8-18 mm in area, this CMUT array can be implanted into a tissue or placed inside an organ without causing major tissue disruption. Close proximity to the target tissue allows this CMUT array to pick up a relatively weak ultrasound signal generated in a photoacoustic process and to provide diagnostic information inaccessible from a noninvasive transducer. For invasive PAI, silicon-based CMUTs offer an additional significant advantage: The silicon and the dielectric membrane of CMUTs are relatively transparent to near infrared, and the shadowing problem associated with the piezoelectric ultrasonic transducers can be minimized or avoided. A two-layer polysilicon surface micromachining process was used to fabricate this device, followed by a double-sided deep-silicon-etching process for shaping the silicon substrate into a thin probe. Experimental characterization found that the center frequency of the CMUT devices with a 46-mu m-diameter 1.0-mu m-thick polysilicon membrane was 5.0 MHz, with a fractional bandwidth of 116% in reception mode. The PAI of the nerve cord of a lobster using this miniature CMUT array was demonstrated.
