Highly integrated 2-D capacitive micromachined ultrasonic transducers

Two dimensional (2-D) silicon based capacitive micromachined ultrasonic transducer (cMUT) arrays are fabricated efficiently using standard integrated circuit (IC) processing techniques. Furthermore, high density interconnects are implemented using through-chip vias which bring the signal from the front surface of the transducer chip to the back side. The transducer chip then can be flip-chip bonded to a signal processing chip. This results in a very compact two chip hybrid package that can be easily sealed for immersion applications. The electrical interconnects are realized by high aspect ratio vias (50 μm in diameter, 550 μm in depth) that are etched into the silicon substrate by a deep reactive ion etcher (DRIE). The vias are then coated by polysilicon which is subsequently doped to achieve good conductivity. The transducer arrays are then built on the front side using the conventional process developed for cMUTs. Measurements indicate that the through wafer vias have a typical resistance of 7 Ω. The parasitic via capacitances are reduced to 2 pF by reverse biasing the p-n junction formed by the n++ doped polysilicon via coating and the near-intrinsic p-type silicon substrate. Ultrasonic measurements on the 2-D array cMUT elements with these interconnects shows more than 100% fractional bandwidth and high sensitivity