Micromachined Capacitive Transducer Arrays for Medical Ultrasound Imaging

The fabrication and characterization of microand theoretical results on spurious mode excitation in the sented. f f e p o r d s Ultrasonic transducer, array imaging, microNitrids W highly doped silicon machining, CIDSS talk I. INTRODUCTIOX Capacitive micromachined ultrasonic transducers (cMUT) have been recently emerging as an attractive alternative to conventional piezoelectric transducers [l]. They offer a larger set of parameters for optimization of transducer ics integration [2]. The fabrication process and theoretical performance as well as ease of fabrication and electronmodeling of single cMUT devices were reported earlier [3], 141, [2]. Many applications, especially immersion imaging applications, demand better understanding and improvement of cMUTs both in terms of individual device performance and array behavior. Cross coupling through the array structure and the fluid medium is one of the most important factors affecting the array operation 151. The level of these disturbances can be quantified by radiation pattern measurements, electrical measurements on the aray elements, and by detection of structural displacements using optical probes 151. Several l-D and 2-D cMUT arrays as well as test structures consisting of single isolated elements have been fabricated for experimental charact,erization. Radiation pattern measurements and optical probe measurements x e performed using these structures. The results are related to Lamb waves propagating in the silicon wafer and to the Stoneley-type waves propagating a t the fluid-silicon wafer interface. Images of radiating transducers and aluminum targets are also obtained using a 64 element l-D array around 4 MHz to show the feasibility of array imaging by cMUTs. 111 this paper, the fabrication process of l-D and 2-D cMUT arrays is briefly explained. Then, the experimental Nitride an pauemed amorphous silicon Nitride deposmon and pvitcming Mctalicauon md patterning 0 Silicon substrate Amorphous sillcon Silicon nirnde Aiummurn Fig. 1 . Fabrication process of cMUTs array. The process flow for a single capacitor device is shown in Fig. 1. The fabrication process starts with 4 inch silicon wafers heavily doped with 4 hour phosphorus gas phase drive-in a t 1000°C to achieve good conductivit,y at the at 800°C as an insulator and an etc.h stop in the sacriwafer surface. A thin layer of LPCVD nitride is deposited ficial etch to be performed later. .4 sacrificial polysilicon layer is subsequently deposited and dry-etch patterned into 0-7803-4095-7/98/$10.00