A flexible capacitive micromachined ultrasonic transducer (CMUT) array with increased effective capacitance from concave bottom electrodes for ultrasonic imaging applications

A flexible capacitive micromachined ultrasonic transducer (CMUT) array with increased effective capacitance from concave bottom electrodes is proposed for ultrasonic imaging. A CMUT can transmit and receive ultrasound by vibrating its membrane like a drum. DC bias is applied to bring the membrane closer to the bottom electrode for increasing its sensitivity. However, most of the developed CMUTs have flat bottom electrode, which can not comply with the deflected membrane in a concave surface. Since the capacitance is inverse-proportional to the gap distance between the electrodes, this makes only the 25% central area more sensitive to the capacitance change and the other 75% of the area is considered as parasitic capacitance without coverage of the top electrode. Based on the theoretical analysis, when using concave bottom electrode to reduce the gap distance around the membrane edge, the effective capacitance can increase 10 times comparing with using the flat bottom electrode. The concave bottom electrode is formed on top of the reflowed photoresist in convex spherical shape using over-plating technique. By using the concave bottom electrode to increase the effective area of the membrane, it can increase the effective capacitance to improve the fill factor, output pressure, bandwidth, and sensitivity of the transducer.