Resistivity humidity sensors based on hydrogenated amorphous carbon films

We have studied the humidity dependence of the electrical properties in hydrogenated amorphous carbon (a-C:H) films. The films were prepared in two stages combining the techniques of physical deposition in vapor phase evaporation (PAVD) and plasma pulsed nitriding. They were deposited over printed circuit boards made of synthetic resin FR2, predesigned to measure DC and AC transport. The Raman spectrum showed a broad peak whose two components are characteristics of a-C:H. By treating the sample in N2-H2 plasma, the impedance became intrinsically sensitive to the relative humidity (RH) of the surrounding media. We observed that when RH increased, the electrical impedance (Z) of the films diminished. The complex impedance spectroscopy method was used to analyze the interaction between the water molecules and the a-C:H film. The main contribution to the humidity dependency of Z came from the resistivity component. The present work provides a fundamental support to develop humidity sensors based on the variation of the impedance of a-C:H films.