A Miniature Capacitive Ultrasonic Imager Array

This paper describes the development of a miniature capacitive micromachined ultrasonic transducer (CMUT) array suitable for minimally invasive medical imaging and diagnosis. In contrast to conventional laboratory-scale CMUT platforms, which are generally integrated on a silicon substrate thicker than 550 mum, this imager array is integrated on a probe shaped silicon substrate with a typical shank dimension of 60 mum(width) times 40 mum(thickness) times 4-10 mm(length) for 1-D arrays, and 0.4-2.3 mm(width) times 100 mum(thickness) times 6-12 mm(length) for 2-D arrays. Such miniature CMUT arrays are suitable for implantation into tissue through a fine incision or by being placed inside an organ for close-range imaging. In a close-range diagnosis made possible by using such miniature CMUT arrays, ultrasound of a higher frequency can be used and the conflict associated with the penetration depth and image resolution can be resolved. This imager array was fabricated using a two-layer polysilicon surface micromachining process followed by a double-sided deep silicon etching for substrate shaping. The total mask count was eight. The central frequency of ultrasound transmitted by a circular 46 mum-diameter transducer was 3.8 MHz, while its fractional bandwidth was 116% in water. A simple transducer-fluid model was used to predict the acoustic characteristics of this device in water. Preliminary B-mode imaging using a 21-element 1-D array was demonstrated.