1.  Field of the Invention
The present invention relates to a moisture sensor and, more particularly, to a moisture permeable electrode in a moisture sensor having a moisture sensitive polymer film. 
2.  Description of the Prior Art
A thin-film moisture sensor has been developed, which includes a moisture sensitive polymer film made of acetate, cellulose, polystyrenesulfonate, polyacrylate, etc.  The thin-film moisture sensor detects, for example, the humidity and the dew condensation, and has wide application in, for example, air conditioning apparatus, automobiles, medical care, agriculture, forestry and stockbreeding.  The amount of water absorbed in the polymer film increases/decreases in reversible manner depending on the water contents in the atmosphere.  This results in a variation of the ionic conductivity or the dielectric constant of the moisture sensitive polymer film.  The thus created variation of the dielectric constant or the ionic conductivity is utilized to detect the humidity. 
The thin-film moisture sensor generally includes a comb-shaped electrode formed on a substrate, and a moisture sensitive polymer film formed on the comb-shaped electrode.  A moisture permeable coating film is disposed on the moisture sensitive polymer film, when required.  Instead of the coating film, a moisture permeable conductive film can be formed on the moisture sensitive polymer film, which functions as a counter electrode. 
The above-mentioned moisture permeable conductive film must satisfy the following requirements. 
(1) The conductive film must show strong adhesion to the moisture sensitive polymer film.  PA1 (2) Cracks will not be formed in the conductive film, or the conductive film will not be peeled from the moisture sensitive polymer film even when the polymer film swells or shrinks due to the moisture absorption and the moisture desorption.  PA1 (3) The conductive film must show a sufficient moisture permeability.  PA1 (4) The conductive film must have a sufficiently low resistance value as compared with the impedance of the moisture sensitive polymer film.  PA1 (5) The conductive film should be formed on the moisture sensitive polymer film at a temperature which does not damage the polymer film. 
A thin-film moisture sensor has been proposed, which includes a moisture permeable conductive film made of a sintered body such as ruthenium oxide (RuO.sub.2), or a metal thin-film such as a gold (Au) film.  The sintered body electrode is not suited for the thin-film moisture sensor because the sintered body is formed on the polymer film at a temperature exceeding the thermal resistivity of the moisture sensitive polymer film.  The Au thin-film is generally formed on the moisture sensitive polymer film through the use of a vacuum evaporation technique or a sputtering method.  However, the substrate temperature can not be set at a considerably high level due to the thermal characteristics of the polymer film.  Therefore, the Au thin-film does not show a desirable adhesion to the moisture sensitive polymer film.  Furthermore, there is a considerably high possibility that the Au thin-film forms cracks therein or peels from the polymer film when the thin-film moisture sensor is disposed in a high humidity atmosphere and the moisture sensitive polymer film swells.