AN INTEGRATED PVDF ULTRASONIC SENSOR WITH IMPROVED SENSITIVITY USING POLYIMIDE

This paper presents an integrated ultrasonic transducer structure with improved sensitivity which contains a MOSFET-BJT (bipolar junction tr ansistor) sense amplifier. The extended-gate electrode of the MOSFET i s placed over an epitaxial layer isolated from a silicon substrate and is padded up with a polyimide (PI) dielectric layer, which significan tly reduces the extended-gate capacitance and therefore increases the sensor sensitivity. The effects of PI preparation conditions on the re sulting film are examined. The frequency response of the sense circuit and the impulse response of the proposed structure used as a receiver are tested. With a 5.2 mu m thick PI layer, a sensitivity improvement of over 13 dB is achieved when compared with the normal transducer st ructure. These results match the predictions of voltage-transfer analy sis and circuit simulation. (C) 1997 Elsevier Science S.A.