Fabrication of a micro humidity sensor with polypyrrole using the CMOS process

This study investigates the fabrication of a micro humidity sensor integrated with circuitry using the complimentary metal oxide semiconductor (CMOS) process. The area of the humidity sensor chip is about 1 mm². The advantages of the humidity sensor include small volume, high sensitivity, and integrated with circuitry on a chip. The sensor, which is a capacitive type, comprises the improved interdigital electrodes that can enhance its sensitivity. The sensitive film of the humidity sensor is polypyrrole, which is prepared by the chemical polymerization method. The humidity sensor needs a post-CMOS process to coat the sensitive film. The post-CMOS process uses a wet etching to etch the sacrificial layers, and then the polypyrrole is coated on the humidity sensor. The sensor produces a change in capacitance when the sensitive film absorbs or desorbs vapor. The humidity sensor consists of a ring oscillator circuit that is utilized to convert the capacitance variation into the oscillation frequency output.