Micromachined Capacitive Ultrasonic Immersion Transducer for Medical Imaging

Piezoceramics have been the dominant transducer technology in ultrasound medical imaging for several decades. Recent progress in surface micromachined capacitive ultrasonic immersion transducers makes them an alternative transducer technology, especially in highly integrated twodimensional arrays. This paper demonstrates that the surface micromachined capacitive ultrasonic immersion transducer performs at a level competitive enough to challenge the established piezoelectric transducers. Single element transducers and a variety of array transducers are fabricated with CMOS compatible micromachining technology. The transducers are observed to operate from 2MHz to 15MHz in immersion operation. Better than 100dB dynamic range is evident around 4.5MHz for a single device with only 6dB of unknown return loss. Similar performance is observed in a pulse echo experiment. The transducer’s beam pattern indicates that the device behaves as a uniform piston transducer. Theoretical analysis shows that it is feasible to build an ideal immersion transducer with 100% of bandwidth and 3dB insertion loss in wide frequency ranges. The study in this paper concludes that micromachined ultrasonic transducers are an attractive alternative to piezoelectric transducers in ultrasound medical imaging. eration of single element immersion cMUTs [3], [4], derived the electrical equivalent circuit model [5], [ 6 ] , improved the microfabrication [7], [8] and discussed the optimization of fabrication process [9]. In this paper, systematic design, analysis, fabrication, and characterization of immersion cMUTs are discussed, especially targeting array transducers. 11. TRANSDUCER FABRICATION A cMUT consists of metalized silicon nitride membranes suspended above heavily doped silicon bulk. A schematic of one element of the device is shown in Figure 1 where t , is the membrane thickness, t , is the air gap thickness, t b is the insulator layer thickness, T is the cell radius, sp is the cell support dimension, VDC is the DC bias voltage, T is the tension within the membrane, and VAC is the AC signal excited or received. A transducer consists of many such elements. When a voltage is placed between the metalized membrane and the bulk, Coulomb forces attract the membrane toward the bulk and stress within the membrane resists the attraction. If the membrane is driven by an alternating voltage, significant ultrasound generation results. Conversely, if the membrane is biased appropriately and subjected to ultrasonic waves, significant detection currents are generated.