The development of a silicon-only capacitive dew point sensor is described in this paper. The principle of operation is based on the change in capacitance of an interdigitated parallel plate capacitor due to the condensation of water below dew point temperature. The proposed sensor element is simple in fabrication, with only one lithography step and no metal or polymer usage. The experiments show good repeatability and baseline stability, whereas detection accuracy needs to be further improved. The influence of condensed water on the capacitance is investigated at different frequencies and cooling rates. It is expected that replacing air with water increases the capacitance by a factor of 80, due to the ratio of relative permittivity of water to ambient air. However, at low frequencies, the conducting properties of water in combination with the ultra-thin oxide coating on the electrodes result in a more complex capacitance behavior and an increase of up to four orders of magnitude has been measured. This additional and more rapid increase in capacitance may be utilized for faster dew point detection. In the current stage of development, the accuracy of the dew point temperature reading is equal to or better than the reference sensor (+/- 2 degrees C), with a response time of < 40 s, and is expected to be further improved with a more accurate test setup.

In this work we study a silicon parallel plate capacitor as a dew point sensor. Accumulation of water between the parallel plates below dew point temperature leads to an increase in capacitance. The expected 80-fold increase in capacitance (ratio of relative permittivity of water to air) holds true at high frequencies while at low frequencies an increase up to four orders of magnitude has been measured. We report that the condensed water models more as a conductor than as a dielectric at low frequencies, which, along with the native oxide of silicon, explains the unexpected increase in capacitance.