JPH01196558A - Humidity sensor - Google Patents

Abstract

PURPOSE:To obtain a highly sensitive humidity sensor with excellent environmental resistance, by a method wherein a moisture-sensitive thin film is formed by sputtering on the top of a lower electrode and a water permeable electrode thereon. CONSTITUTION:A lower electrode 11 is made of conducting ceramics (e.g. 10mum thick) on an electrically insulating substrate 10 by a thick film printing or sputtering. A moisture-sensitive thin film 12 with the thickness of several mum comprising an insulating material of SiO2, Si3N4, AlO3, ZrO2, MgO or TiO2 or a mixture thereof is formed on the top of the electrode by a sputtering method. A steam permeating upper electrode 13 with the thickness of several 1,000Angstrom is provided on the top of the moisture-sensitive thin film 12 by a vacuum evaporation or the like and the top and bottom of the moisture-sensitive thin film 12 are sandwiched by the electrodes 11 and 13. This achieves higher environmental resistance and responsiveness with a thinner film of the moisture-sensitive material.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a humidity sensor that detects humidity by utilizing the fact that the impedance of an insulating material changes due to adsorption of water molecules.

[Conventional technology] Humidity sensors currently in practical use mainly use hygroscopic polymers or porous ceramics made by firing metal oxides as moisture-sensitive materials, and they absorb static electricity due to adsorption of water molecules. Humidity is detected using changes in capacitance and impedance. First, humidity sensors that use hygroscopic polymers to detect changes in capacitance are highly accurate because the hygroscopic polymers have extremely small temperature coefficients, are fast in response, and have a small hysteresis of less than 3% RH. Humidity measurement is possible. However, hygroscopic polymers deteriorate in atmospheres containing organic solvents or in high-temperature atmospheres, so they have the disadvantage of being subject to restrictions on the atmosphere in which they are used.

On the other hand, a humidity sensor based on a method of detecting changes in impedance using porous ceramics has excellent environmental resistance and is not subject to any particular restrictions, so it can be used over a wide range of applications. However, humidity sensors using porous ceramics have the disadvantage that they are inferior to humidity sensors using hygroscopic polymers in terms of measurement accuracy such as response, reproducibility, and hysteresis.

Therefore, in order to compensate for the above-mentioned drawbacks, it has been considered to use a thermally and chemically stable highly insulating material as a moisture-sensitive material, but normally the resistance value is so high that it cannot be measured without using an insulation resistance meter. It has high resistance. Therefore, Figures 8 and 9
With the structure of a conventional humidity sensor as shown in the figure, it is impossible to actually detect humidity.

For example, FIG. 8 shows a bulk type material, in which the moisture sensitive material 1 is mainly a porous sintered body of metal oxide, and is sandwiched between two electrode plates 2. The thickness d of the moisture sensitive material 1 is m
The area S of the electrode plate 2 is in units of m m2 to cm2. When the above-mentioned highly insulating material is used as the moisture-sensitive material 1, the area S of the electrode plate 2 may be increased by reducing the thickness d of the moisture-sensitive material 1 to the limit as long as mechanical strength is obtained. , the resistivity is so large that the impedance cannot be measured.

On the other hand, FIG. 9 shows a method using thick film printing, in which electrodes 4a and 4b are provided on an insulating substrate 3, and the upper portions of these are covered with a moisture-sensitive material 5. In this case, electrode 4a
If the distance d between and 4b is made smaller, the resistance value will decrease, but
Since the practical limit is about 0.2 mm, the moisture-sensitive material 5
It is impossible to measure changes in impedance when using highly insulating materials.

[Object of the Invention] The object of the present invention is to eliminate the problems related to the above-mentioned conventional example,
The purpose of the present invention is to form a moisture-sensitive thin film using a sputtering method to provide a highly accurate humidity sensor that has excellent resistance to environments such as temperature and atmosphere.

[Summary of the Invention] The gist of the present invention for achieving the above-mentioned object is to provide a lower electrode formed on an insulating substrate, a moisture-sensitive thin film formed by sputtering an insulating material on the upper part of the lower electrode, and a moisture-sensitive thin film formed on the lower electrode by sputtering. A humidity sensor comprising a moisture-permeable upper electrode formed on top of a wet thin film, the lower electrode comprising a metal film formed by thick film printing or a conductive ceramic film formed by sputtering or thick film printing. This is a humidity sensor characterized by the following.

[Embodiments of the invention] The present invention will be explained in detail based on illustrated embodiments.

FIG. 1 shows a first embodiment of a humidity sensor according to the present invention, in which (a) is a sectional view and (b) is an electrical equivalent circuit diagram thereof. A lower electrode 11 with a thickness of 10 pm, for example, made of a metal film created by thick film printing or a conductive ceramic created by sputtering or thick film printing is provided on the electrically insulating substrate 10, and on top of the lower electrode 11 is provided. 5i
02, S i3 N4 , AlO2, Z r02 , M
gO1Ti02. Alternatively, a moisture-sensitive thin film 12 made of an insulating material such as a mixed substance containing these materials and having a thickness of several grams is prepared by a sparging method. On the top of the moisture-sensitive thin film 12, a water vapor permeable upper electrode 13 with a thickness of several thousand layers is provided by vacuum deposition or the like, and the moisture-sensitive thin film 12 has a structure in which the upper and lower parts are sandwiched between the two electrodes 11 and 13. It has become.

The sputtering method basically involves performing a discharge in a low-vacuum argon atmosphere, and atoms or atomic groups of a substance (target) placed on the cathode side fly out to the outside due to collisions with argon ions. ) side is attached to the substrate 10. Therefore, if the film forming substance is placed on the cathode side and the film forming substrate 10 is placed on the anode side, and discharge is performed in an argon atmosphere of 10 (~l O'Tart), a thin film can be formed on the film forming substrate 10. The film thickness can be controlled only by physical parameters such as the degree of vacuum, vapor pressure, and power output, and the adhesion between the thin film and the lower electrode 11 on the substrate 10 is also superior to that of normal vapor deposition. The composition of the membrane can also be the same as that of the membrane-forming material.

There are many variations of sputtering methods, but
5i02 or Si3N as in this example.

When creating insulating films such as
Sputtering and reactive sputtering, in which a thin film is created by a type of chemical reaction by injecting oxygen or nitrogen in addition to argon as an atmosphere and using silicon, metal materials, etc. as targets, are mainly performed.

With the above configuration, the thickness of the moisture-sensitive thin film 12 is 0.1
Since it is in the range of 1-1O1L, the impedance value is sufficiently measurable even under normal conditions. This is the resistance value R=p-d/
S (where ρ is specific resistance, d is electrode spacing, and S is cross-sectional area)
Therefore, even if the material has a large specific resistance ρ, it will have a measurable resistance value if the cross-sectional area S is sufficiently larger than the electrode spacing d.

Since the moisture-sensitive thin film 16 is formed by sputtering, the film thickness is highly stable, and even when mass-produced, the film can be manufactured with high precision and stable specifications. In addition, since 11 is created by thick film printing or conductive ceramics with a small expansion coefficient are used, the humidity-sensitive thin film 12 and the lower electrode 11 can be matched in expansion coefficient, so that the moisture-sensitive thin film 12 can be peeled off. This reduces the occurrence of cracks and increases durability.

In addition, by reducing the thickness of the moisture-sensitive thin film 12,
Affected by the capacitance component due to the dielectric constant of the adsorbed water molecules,
Its impedance value decreases. Therefore, conceptually, it is an equivalent circuit in which a resistor R and a capacitor C are connected in parallel as shown in FIG. 1(b), and its impedance value Z is expressed by the following equation.

I Z l = 1/((1/R) 2 +
(ωC) 2) 4 Figure 2 is a graph showing the humidity sensitivity characteristics and hysteresis characteristics of this humidity sensor. is obtained, and by taking the logarithm of the impedance, it is possible to obtain approximately linear characteristics. On the other hand, the hysteresis was approximately 2% RH, indicating good reproducibility.

Figure 3 shows the response, and it takes about 8 to 90% response.
It takes 9 seconds to reach this point and has improved responsiveness compared to conventional humidity sensors using porous ceramics.

Note that these data use 5i02 as the insulating moisture-sensitive thin film 12, but Si3N4. AN□0
3. Similar characteristics can be obtained with MgO1Zr02, TiO2, or a mixture containing these. In addition, the lower electrode 11 is made of Au made by thick film printing.
-Pt, Pt, Au, Au-Pd, Ag-Pd, Ru
It is appropriate to use conductive ceramics such as O2.

FIG. 4 shows the second embodiment, (a) is a sectional view, (b)
is a plan view. Basically the same as FIG. 1(a),
A lower electrode 15 made of a metal film or conductive ceramic, a moisture-sensitive thin film 16 made of an insulating material made by sputtering, and an upper electrode 17 permeable to water vapor are sequentially laminated on an electrically insulating substrate 14. However, in practical use, the upper and lower electrodes 15 and 17 are used for signal transmission and reception with the outside.
are provided with connection ends 15s and 17s, respectively, to which conducting wires (not shown) are connected, respectively.

Note that in FIG. 4(b), the upper and lower electrodes 15.17 are shown offset for ease of viewing, but the upper and lower electrodes 15.17 are shown offset.
17 has the same shape except for the connecting ends 15s and 17s,
Of course, it is preferable to arrange them so that they overlap at the same position.

By the way, in FIGS. 1 and 4, the moisture-sensitive thin film 12.1
Since 6 is a porous membrane so that adsorption of water molecules is sufficiently small, the upper electrode 1 formed on the moisture-sensitive thin film 12.16
3.17 needs to be made appropriately thick, but water vapor permeability is reduced and responsiveness deteriorates. Therefore, the upper electrode 13
．． It is necessary to minimize the number of electrodes to be routed for extracting signals from 17. FIG. 5 shows a third embodiment that takes this into consideration, where (a) is a schematic structural diagram and (b) is its equivalent circuit diagram. This embodiment basically has the same structure as that in FIG. 1(a), but the lower electrode 11 is divided into two parts, lla and llb, and all signals are transmitted and received from the outside by the lower electrode 11. Theoretically, two parallel circuits of resistors Ra, Rb and capacitors Ca, Cb are connected in series, as shown in FIG. 5(b).

FIG. 6 is a plan view of the fourth embodiment. In the embodiment of FIG. 4, the lower electrode 15 is divided into two parts 15a and 15b, and the connection ends 15s and 15S' each have a signal input terminal. It has a structure with conductive wires attached for transmission and reception. Therefore, conceptually, as shown in FIG. 7, signals are transmitted and received from the outside via the two lower electrodes 15a and 15b.

Furthermore, by making this humidity sensor thinner, it is possible to create it on the same substrate as sensors for temperature, pressure, light, etc., and it can be applied to multifunctional sensors.

[Effects of the Invention] As explained above, the humidity sensor according to the present invention uses an insulating material as the moisture-sensitive material and thins the moisture-sensitive material by sputtering, so it has excellent resistance to environments such as humidity and atmosphere. In addition to improving responsiveness, it also makes it easier to control the film thickness, ensuring uniform standards and high reliability even in mass production.Furthermore, the lower electrode can be made of a metal film created by thick film printing or a conductive ceramic. Since the coefficient of expansion matches that of the substrate, peeling of the moisture sensitive film and occurrence of cracks can be suppressed and durability can be improved.

[Brief explanation of the drawing]

The drawings show an embodiment of the humidity sensor according to the present invention, FIG. 1(a) is a sectional view of the first embodiment, FIG. 1(b) is an equivalent circuit diagram,
FIG. 2 is a moisture sensitivity characteristic and hysteresis characteristic diagram, FIG. 3 is a response characteristic diagram, and FIG. 4(a) is a sectional view of the second embodiment.
(1)) is a plan view, FIG. 5(a) is a sectional view of the third embodiment, (b) is an isolateral plan view, FIG. 6 is a plan view of the fourth embodiment, and FIG. 7 is a plan view of the fourth embodiment. It is a side view, and FIG. 8 is a configuration diagram of a conventional example.
FIG. 9 is a sectional view of a conventional example. Reference numeral 10.14 is a substrate, 11.15.15a and 15b are lower electrodes, 12.16 is a moisture-sensitive thin film, and 13.17 is an upper electrode.

Claims (1)

[Claims] 1. A humidity sensor consisting of a lower electrode formed on an insulating substrate, a moisture-sensitive thin film formed by sputtering an insulating material on the upper part of the lower electrode, and a moisture-permeable upper electrode formed on the upper part of the humidity-sensitive thin film. The humidity sensor is characterized in that the lower electrode is made of a metal film formed by thick film printing or a conductive ceramic film formed by sputtering or thick film printing.