1. Field of the Invention
The present invention relates to a humidity sensor for use in an air conditioning system or the like, and a method for producing the humidity sensor.
2. Description of the Background Art
A variety of humidity sensors have been developed. The usual sensing principles of humidity sensing in which they operate are:
(1) Conductivity variation due to adsorption of water to porous ceramics is utilized.
(2) Dielectric constant variation or conductivity variation due by adsorption of water to high polymers;
(3) Ion conductivity variation of an electrolyte; and
(4) Thermal conductivity of a sensing material.
In such humidity sensors, an electric property change with respect to humidity variation in the atmosphere is utilized, that is, the humidity variation is detected in terms of an electric signal having a different value.
In FIG. 1, there is shown a conventional humidity sensor comprising a substrate 1, a lower electrode 2 formed on the substrate 1, a sensing film 3 formed on the lower electrode 2 and an upper electrode 4 formed on the sensing film 3, which are successively laminated one on another. One end of the upper electrode 4 is extended down onto the substrate 1, and a bonding pad 5 is connected to the extended upper electrode 4 on the substrate 1 for coupling to an external circuit (not shown).
In this case, the sensing film 3 usually detects the humidity in the atmosphere through the upper electrode 4, and the upper electrode 4 is made thin in so as to promote the permeation of the humidity. The sensing film 3 is approximately several thousands .ANG. to several .mu.m in thickness in order to have a humidity sensing ability with a sufficient capacity. Hence, the upper electrode 4 is formed in a stair shape to reach the bonding pad 5. In the thin upper electrode 4, a crack is apt to take place in its side wall portion 4a, and thus disconnection may be caused by the crack.
In FIG. 2, there is shown another conventional humidity sensor having basically the same structure as that of the aforementioned conventional humidity sensor shown in FIG. 1. In this case, a thin upper electrode 4 is provided with a vertical electrode 6 for reinforcing the step portion thereof, i.e., the side wall portion and the adjacent end portion of the upper main flat portion. The vertical electrode 6 is formed sufficiently thick by the vapor deposition method or the like, and the crack in the side wall portion can be effectively prevented.
However, in this sensor, the humidity sensing property of the sensing film 3 is liable to be deteriorated by heat. The vertical electrode 6 is formed so as to have the same or greater thickness as or than that of the sensing film 3. This is carried out for a long period of time in a high temperature step, which can readily cause the deterioration of the humidity sensing property of the sensing film 3.
In FIG. 3, there is shown still another conventional humidity sensor, in which a pair of lower electrodes 2a and 2b, separated from each other, are provided on a substrate 1. Then, a sensing film 3 and an upper electrode 4 are consecutively formed on the lower electrodes 2a and 2b formed on the substrate 1. A pair of bonding pads 5a and 5b for coupling with an external circuit are attached to the opposite end portions of the lower electrodes 2a and 2b.
In this case, no side wall or no vertical electrode of the above described conventional humidity sensors is required, but there is a series of two capacitors formed between the upper electrode 4 and the two lower electrodes 2a and 2b, the sensing film 3 being interposed therebetween. A capacity variation of the sensing film 3 is picked up. The detecting efficiency of the sensing film 3 having the same area as the above described conventional humidity sensors is fairly reduced. (For instance, the capacity is reduced to one fourth, because of a series of two capacitors).
In conventional humidity sensors, as described above, high detecting efficiency, prevention of a crack in the step portion of the upper electrode and the high reliability of the detection, cannot be attained at the same time.