JPS6328481B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a humidity sensor element using an Al oxide thin film. Humidity sensors have traditionally been used in some areas for humidity observation, but recently their use has rapidly expanded to include home appliances and the like. Some sensors using Al films are already known. The representative ones are as follows. (a) Patent Application No. 1983-83507 (moisture content detection element using anodized aluminum film) (b) Prototype of moisture sensing element (Tokyo Metropolitan Industrial Technology Center Research Report No. 6; 1976, p. 51) (~56 pages) (c) Sealing effect of anodized Al
Effects near the Barrier−Porous Layer
Interface of Anodic Aluminas；Solid Sate
Science, Sep., 1970) The manufacturing method will be explained very simply with reference to FIG. The figure is a plan view of this type of sensor element. In the figure, 1 is a ceramic substrate, and 2 is a comb-shaped thin film electrode formed by vacuum deposition or sputtering. That is, after forming a thin film using a valve metal such as Ti by vapor deposition or sputtering over the front surface of the substrate 1, the desired comb-shaped electrodes are formed by photo-etching. Next, an aluminum thin film is formed on the entire surface of the comb-shaped electrode portion by mask vapor deposition, and then an aluminum oxide thin film 3 (indicated by loosely hatched lines) is formed by anodic oxidation using an acid such as sulfuric acid or oxalic acid. Next, after the sensor element is completed, an Au terminal portion 4 is provided by a mask vapor deposition method in order to solder the external lead wire 5. Thereafter, the element is immersed in boiling pure water for about 30 minutes to perform a so-called sealing process, and finally the lead wire 5 is soldered to the Au terminal part 4. Although the sensor made as described above has many features, it also has the following drawbacks. (1) Hysteresis with respect to humidity is large. In other words, the conductance between the lead wires 5 and 5 in Fig. 1 increases as the humidity rises, but conversely, when the humidity decreases, the conductance value passes through a different path than when it rose, and the difference increases. is large. A so-called hysteresis phenomenon occurs. (2) In low humidity, sensitivity gradually decreases over time. That is, the conductance value and its slope with respect to humidity between the lead wires 5 decrease with time. (3) Sensitivity increases and becomes unstable in high humidity. In other words, in high humidity where the relative humidity is 80%RH or higher,
Sensitivity increases gradually and becomes unstable. An object of the present invention is to provide a method for manufacturing an Al oxide thin film sensor element with excellent characteristics, which can be put to practical use by eliminating or significantly improving the above-mentioned drawbacks. In order to achieve the above object, the method for manufacturing a humidity sensor element using an anodic oxide thin film of Al according to the present invention provides a method for manufacturing a humidity sensor element using an anodic oxide thin film of Al, which is provided over the entire surface of a thin valve metal electrode formed on an insulating substrate. A humidity sensor element in which the surface of Al oxide produced by anodic oxidation of an Al thin film is coated with a surfactant, which is characterized by subjecting it to high-temperature heat treatment in a vacuum immediately before coating with the active agent. . Next, the configuration of the present invention will be explained. The present invention is filed by the same applicant,
That is, this patent application No. 56-008601 (hereinafter referred to as the previous application) relates to an improvement in the manufacturing method of a humidity sensor element disclosed as an embodiment of the manufacturing method. Therefore, we will first discuss the previous application very briefly. The humidity sensor element of the previous application uses a surfactant to coat the surface of Al oxide, which is produced by anodizing a thin Al film provided over the entire surface of a thin valve metal electrode formed on an insulating substrate. It is characterized by being coated. In your invention, you have investigated in detail the reasons for the above-mentioned drawbacks (1) to (3), and therefore clarified a method for improving them, and as a result, provided the sensor element according to the previously filed invention, and An example of the manufacturing method was described, and the effects of the sensor element were also described. Since it would be complicated to repeat the above explanation over and over again, it will be omitted, and the manufacturing method according to the present invention will be described in detail immediately. As described above, this manufacturing method belongs to an improvement of the manufacturing method detailed as an example of the manufacturing method of the temperature sensor according to the previous application, so there are many essentially similar points. That is, there are many points that overlap with the manufacturing method of the previous application, and it also indirectly shows the structure of the sensor of the previous application. Next, the present invention will be explained using the following examples. Referring to Figure 1, high-purity alumina porcelain is used as the ceramic substrate 1, and the dimensions are 10 mm x 15 mm x
It is 0.6mm. Wash and dry this thoroughly. Next, the substrate is placed in a high frequency sputtering tank, and the surface
Sputter the Ta film. The film thickness is 2000 Å. Next, a comb-shaped electrode 2 is formed by chemical etching.
The width of the comb teeth was 50 μm, and the interval was 50 μm. Next, thin diagonal lines 3 are formed using mask evaporation method in a high vacuum.
Al thin film (thickness 5000 Å to 7000 Å) is applied to the entire surface of the area indicated by
Å) and Au electrode terminals shown as 4 are formed. Next, an insulating band several mm wide is made using synthetic resin between the Al film 3 and the gold electrode terminal 4. Next, the electrode terminal 4 of the substrate is made common, and the part 3 is suspended in an electrolytic solution of dilute sulfuric acid or oxalic acid to serve as an anode. In this case, the resin layer is immersed up to the vicinity of the center line. This is done in order to prevent current concentration near the surface during anodic oxidation and to ensure that subsequent oxidation is performed uniformly over the entire surface. Next, a piece of highly pure Ta is suspended in the electrolyte to serve as a cathode. First, a constant current is passed between the positive and negative electrodes to sufficiently form the anodizing material, and then a constant voltage forming process is performed using a slightly higher voltage to complete the anodic forming process. In this case, Ta was used as the valve metal. Although the definition of valve metal is not clear today,
It is said to be a metal that produces an insulator through anodization. For example, Se, Ti, Ta, Nb, Hf, etc. The necessity of using a valve metal in the anodization process will be briefly explained with reference to FIG. The figure is an enlarged sectional view of the comb-shaped electrode portion of FIG. 1. In the figure, the first
Components that are the same as those in the drawings are given the same reference numerals and descriptions thereof will be omitted. Anodic oxidation starts from the surface of the Al layer, then reaches the comb-like teeth of Ta, and the surface becomes an insulating layer of oxidized Ta, and oxidation of this part ends, and thus the reaction gradually spreads to the deep part of the Al layer. 3 is oxidized Al
The layer 2 ₁ is metal Ta and the layer 2 ₂ is oxidized Ta. Therefore, the comb-shaped electrode 2 is not made of valve metal, but for example
When Au is used, since no oxide film 2 ₂ is formed, the anodizing current flows into 2, and therefore the deep part of the Al layer is not easily oxidized. Therefore, the circuit between 2 and 2 in FIG. 2 remains short-circuited. Next, Ta was used as the valve metal because the oxide film 2 ₂ is extremely thin and dense compared to other valve metals, and oxidation does not progress further after it is made into a finished product. This is because it has the advantage of Therefore, when used later as a sensor element, it exhibits excellent characteristics in terms of stability, sensitivity, etc. After completion of chemical formation, the product is thoroughly washed and subjected to stabilization heat treatment at 300°C to 350°C for about 2 hours. Next, the pores are thoroughly sealed in boiling pure water. Finally, the nonionic surfactant polyoxyethylene alkyl phenyl ether (Polyoxyethylen)
Alkyl Phenyl Ether;

The process is completed by treating with 0.1 to 1% by volume solution of [Formula]) and drying to form an active layer with a thickness of about 0.5 μm (previous application).
In the present invention, recent research by the present inventors has revealed that the sensor characteristics are significantly improved if the following step is added before treatment with the activator. Next, this point will be explained in detail. Conventionally, it has been said that the best heat treatment conditions for Al oxide films are in the air at a temperature of 100°C to 200°C for several hours to about 20 hours. (See Document 1 below) Γ Document 1 Magazine name: Humidity and Moisture magazine
1965.1.p.372 Author: ACJason Publisher: Reinhold Publishing Co.,
USA However, in our experiments, almost no stabilizing effect was observed in the sensor element according to the previously filed invention (its manufacturing method is as described above). Next, when the processing conditions are set to 250° C. to 350° C. for 1 to 2 hours, the sensitivity increases somewhat, but returns to the original value with the passage of time. If the temperature is further increased to 400°C or higher, the Ta electrode etc. will oxidize and the characteristics will deteriorate significantly. However, the processing conditions were as follows: degree of vacuum (10 ^-2 to 10 ^-4 ) Torr, processing temperature (100 to 300) °C, and processing time (1-
2) It was confirmed that the sensor characteristics were significantly improved by immersing the sensor in the surfactant immediately after the treatment, as described below. In particular, excellent characteristics can be obtained when a low frequency of (5 to 10) Hertz is used as the voltage applied to the sensor element. Next, I will briefly explain the reason. Al films obtained by vacuum evaporation are generally a collection of microcrystals, and some of them react with water to form oxides during the anodic oxidation process, so they contain impurities. Therefore, even if the sealing treatment as detailed in the previous application is performed, it cannot be said that a dense alumite layer is necessarily formed on the surface. _The film thus formed on the _surface has a so _- called It is pseudo-boehmite. Furthermore, during anodic oxidation, a large amount of SO ₄ ^--ions or
C ₂ O ₄ ^-- ions remain. These contribute to adsorption with water and ion conductivity, and become a factor that makes sensor characteristics unstable. Therefore, if we perform vacuum treatment at high temperature,
After the dehydration reaction and most of the aforementioned ions and gasified impurities are removed, the surface is activated and can easily adsorb moisture, gas, etc. in the air, so immediately immerse it in a surfactant. When processed,
The activator penetrates evenly into the oxide film surface, pores, etc. It is thought that the characteristics of the surface of the oxide film were significantly improved in this way, resulting in a dramatic improvement in the characteristics. The processing conditions are a temperature of 400℃ or higher and a degree of vacuum.
When the temperature exceeds 10 ^-5 Torr, evaporation and agglomeration of thin films such as Al and Ta (agglomeration,
agglomeration), resulting in severe deterioration.
Further, the processing time may be 3 hours or more, but if it is too long, the characteristics will deteriorate. In practice, the shorter the treatment time, the better; one to two hours is sufficient in most cases. Furthermore, as it was explained in the previous application that a nonionic surfactant is desirable, the description will be omitted for the sake of brevity. Next, the effect will be briefly described. (1) Sensor characteristics, that is, temperature characteristics, are greatly improved. In particular, the improvement is remarkable when excited at an extremely low frequency of 10 Hz or less. The straight line in FIG. 3 is the characteristic of the sensor element of the present invention, and the curved line is the characteristic of the sensor element according to the previous application and the conventional sensor element. The measurement circuit is as shown in FIG. In the figure, the resistor Rs indicated by 10 is a standard resistance, the Rx resistor indicated by 11 is a sensor resistor, 12 is an amplifier, and 13 and 14 are negative feedback resistors. Now, when an AC voltage of input voltage Vi is applied between input AB, and if the amplification degree of amplifier 12 is A, then
The output voltage V ₀ across the output terminals CD is clearly: V ₀ =(Rx/Rx+Rs)Vi・A (1) In the above measurement, Rs=1000MΩ, A=100 (resistors 13 and 14 were appropriately selected) Vi=0.8V (frequency 6Hz) were selected. In other words, it is clear that the sensor produced by the manufacturing method of the present application has excellent characteristics. Next, since the present application relates to an improvement on the invention of the previous application, the sensor element according to the method of the present application has the following effects as compared to conventional elements. That is, (2) The increase in conductance with humidity is large, especially when the relative humidity is 60% or more. (3) The hysteresis phenomenon in the characteristic curve showing the relationship between humidity and conductance is small. (4) If the thickness of the layer coated with surfactant is 1 μm or less, the response speed is fast. (5) If the thickness of the activator layer is 1 μm or less,
It does not easily dissolve even at high temperatures.

[Brief explanation of the drawing]

Fig. 1 is a plan view of a film type (including thin film type) sensor element, Fig. 2 is a partially enlarged cross-sectional view of the comb-shaped electrode in Fig. 1, and Fig. 3 is a sensor element according to the present invention and the previous application. FIG. 4 is a graph showing the humidity characteristics with respect to a conventional sensor element. FIG. 4 is a circuit diagram for measuring the humidity characteristics. In the figure, 1... Ceramic substrate, 2... Comb-shaped thin film electrode, 3... Al oxide thin film, 4... Au electrode terminal, 5... Lead wire.

Claims (1)

[Claims] 1. The surface of Al oxide produced by anodizing a thin Al film provided over the entire surface of a thin valve metal electrode formed on an insulating substrate is coated with a surfactant. 1. A method for manufacturing a humidity sensor element using an anodic oxidation thin film of Al, characterized in that the humidity sensor element is subjected to high-temperature heat treatment in a vacuum immediately before coating with the activator. 2. A method for manufacturing a humidity sensor element using an anodic oxide thin film of Al according to claim 1, characterized in that a nonionic surfactant is used as the surfactant according to claim 1. 3. As the nonionic surfactant described in Section 2,
A method for manufacturing a humidity sensor element using an Al anodized thin film according to claim 2, characterized in that polyoxyethylene alkyl phenyl ether is used. 4. The conditions for the high-temperature heat treatment in vacuum described in paragraph 1 are as follows: degree of vacuum (10 ^-2 to 10 ^-4 ) Torr, temperature (100 to 300) °C, and treatment time (1 to 3) hours. A method for manufacturing a humidity sensor element using an anodic oxidized Al thin film according to any one of claims 1 to 3.