A SOLID-STATE RELATIVE-HUMIDITY MEASUREMENT SYSTEM

There is a great demand for a compact, reliable, accurate, and inexpensive relative humidity sensor. We report the implementation of a solid-state capacitive relative humidity (RH) sensing system which employs a polymer as the moisture sensitive material. The sensing mechanism is based on the fact that the dielectric permittivity of the insulating polymer, polyimide (PI), is linearly related to the ambient relative humidity. Therefore, the capacitance of a parallel plate device using PI as the dielectric is a linear function of ambient RH (because PI does not swell during moisture absorption). PI is used extensively in integrated circuit (IC) fabrication as an interlevel dielectric, passivation layer, and planarizer. We use standard IC processing techniques to fabricate the integrated sensor capacitor. In order for such a sensor to be useful in the field, a voltage, current, or frequency output is desirable. We have designed and built (in discrete form) a capacitance-to-voltage converter circuit for this application. The performance of this system is reported. This RH sensor system can be built by any research group with access to a simple thin film facility. © 1990 IEEE