Abstract
This study presents the design and fabrication of a capacitive micro humidity sensor integrated with a five-stage ring oscillator circuit on chip using the complimentary metal oxide semiconductor (CMOS) process. The area of the humidity sensor chip is about 1 mm2. The humidity sensor consists of a sensing capacitor and a sensing film. The sensing capacitor is constructed from spiral interdigital electrodes that can enhance the sensitivity of the sensor. The sensing film of the sensor is polypyrrole, which is prepared by the chemical polymerization method, and the film has a porous structure. The sensor needs a post-CMOS process to coat the sensing film. The post-CMOS process uses a wet etching to etch the sacrificial layers, and then the polypyrrole is coated on the sensing capacitor. The sensor generates a change in capacitance when the sensing film absorbs or desorbs vapor. The ring oscillator circuit converts the capacitance variation of the sensor into the oscillation frequency output. Experimental results show that the sensitivity of the humidity sensor is about 99 kHz/%RH at 25 °C.
Highlights
Humidity sensors that are used to measure and monitor environmental humidity are important devices in many industrial and biomedical fields
We investigate the fabrication of a micro humidity sensor integrated with a ring oscillator circuit on chip using the complimentary metal oxide semiconductor (CMOS)-micro electromechanical system (MEMS) technique
The experimental results showed that the ring oscillator circuit was normally operation after the wet etching post-process, indicating that the post-process was compatible with the commercial CMOS process. This humidity sensor integrated with a five-stage ring oscillator circuit has been implemented using the commercial 0.35 μm CMOS process and the post-process
Summary
Humidity sensors that are used to measure and monitor environmental humidity are important devices in many industrial and biomedical fields. Technique has become popular for the miniaturization of sensors. Humidity sensors fabricated by the MEMS technique have the advantages of small size, high performance, easy mass-production and low cost [1]. Several studies have recently used the MEMS technique to manufacture micro humidity sensors. Lee et al [2] presented a polyimide humidity sensor with a micro-bridge structure created using front-side etching with XeF2 gas. The humidity sensor was a capacitive type, and the polyimide was locally cured at a temperature over 350 °C for 1 hour by the MEMS micro hotplate. The humidity sensor exhibited a sensitivity of 0.77 pF/%RH and a hysteresis of 0.61 %RH
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