MEMS Capacitive Relative Humidity Sensor Design, Fabrication, and Characterization

The motivating principle behind this research is the development of a small, wearable sensor that would use humidity and temperature measurements as metrics for health monitoring. We have successfully fabricated macro-scale devices via pressing of a polymer between two aluminum plates, and the responses of these capacitors were tested in various humidity conditions. The transition from macro to micro scale devices brings up issues with dielectric material choice, characterization methods, and fabrication processes. Dielectric materials investigated for the micro level include various polymers, cellulose/paper, and even just air. We propose to use copy paper as the dielectric as it has shown the most promising results. We have found that the 6 mm x 6 mm device very closely matches the output of the commercial sensor used for reference, changing on average 0.1 pF for every 1% change in relative humidity. Our preliminary results show a need for a rigorous characterization scheme for evaluating figures of merit such as response time, accuracy, and consistency. The ultimate goal is to develop a humidity sensor that can be combined with interface circuits to prototype a universal micro-scale health monitor.