Capacitive-based relative humidity (RH) sensor ICs utilize open-cavity packages to allow transmission of environmental moisture into the package. To capture the water vapor in the air, a polymer layer is placed over the sensing electrodes within the cavity to act as a dielectric to the capacitive sensor. Water vapor interacts with the polymer, and alters the dielectric constant. This change in dielectric constant creates a change in capacitance for the sensing electrode beneath the polymer which is then converted internally into a relative humidity output.

Due to the presence of this cavity, RH sensors are not only exposed to moisture but also multiple other undesired chemicals. When additional chemicals are introduced into the sensing polymer, they change the dielectric properties of the polymer and can alter the RH performance of the RH sensor. These unwanted chemicals are referred to as contaminants. The impact on sensor accuracy is variable and is dependent on the type, concentration and exposure period of contamination.

Figure 1-1 Capacitive Humidity Sensor Structure