JPH05322828A - Humidity sensor and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a thin humidity sensor of good performance, in which thin- film element section has good adhesion to the substrate. CONSTITUTION:The title humidity sensor 1 is formed by successively forming a layer 18 having fine alumina projections and second thin layer 11 constituted by successively forming a first thin-film electrode 14a, first thin-film layer 12 composed mainly of at least one kind selected from the oxides of alkaline earth metals, and second thin film layer 11 composed mainly of the oxide of a metal other than the alkaline earth metal elements, alkali metal elements, and precious metal elements and a second thin film electrode 14b on one surface of a compact substrate 15 having a good electrical insulating property and forming thin film heaters 16 on the other surface of the substrate 15. The sensor 1 is always maintained in a high-temperature state of >=200 deg.C and detects humidity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a humidity sensor and a method of manufacturing the same, and more particularly, to a thin (small) humidity sensor that can handle a wide temperature range and a method of manufacturing such a humidity sensor.

[0002]

2. Description of the Related Art Conventionally, various methods have been used to detect humidity in the atmosphere and moisture in various gases, but recently, in various home electric appliances, etc. Humidity detecting means has been added to control devices, and individual element type humidity sensors have been widely used. Due to its detection principle, this solid element type humidity sensor uses resistance change (impedance change), capacitance change, frequency change, heat conduction change, and ultrasonic wave. And those using infrared rays, etc., among which resistance change type and capacitance change type are mainly used.

These resistance change type and capacitance change type humidity sensors are roughly classified into polymer type sensors (sensors using a polymer film) and metal oxide type sensors. The polymer type sensor detects the relative humidity in the atmosphere in both the resistance change type and the capacitance change type, and its material characteristics (property of the polymer film used)
It is not suitable for the measurement of low temperature atmosphere below 0 ° C and high temperature atmosphere above 50 ° C. This is because freezing may occur in a low temperature atmosphere of 0 ° C. or lower, and the polymer film may deteriorate in a high temperature atmosphere of approximately 50 ° C. or higher. Normal,
This type of sensor is a non-heating type, but in the case of polymer type sensors, the output is likely to fluctuate due to dust on the element surface (polymer film surface) due to dust, oil smoke, etc. in the use atmosphere, and long-term stability Is bad. Therefore, the environmental conditions of use of the polymer sensor are naturally limited.

On the other hand, as the metal oxide type sensor, both the resistance change type and the capacitance change type which are operated at room temperature are widely used, but this type of sensor is also used in the same environment as the polymer type sensor described above. There are drawbacks that the temperature is limited and that the output is easily changed due to dirt on the element surface due to dust, oil smoke, etc. in the use atmosphere. Therefore, by utilizing the heat resistance of the constituent material (metal oxide), a so-called refresh mechanism is provided to temporarily heat the element to 400 to 500 ° C to burn off the dirt on the surface of the element (metal oxide). Sensors have also been developed.

However, when the refresh mechanism is operated to temporarily heat the element (refreshing), the humidity sensor is used for a while after the refreshing (until the initial stable level is restored again). It cannot function and cannot detect humidity during that time. In addition, in the metal oxide sensor, a sensor configured to detect the humidity by utilizing the large hygroscopic ability of the metal oxide at a low temperature and measuring the resistance change associated with the adsorption and desorption of water is widely used. There is.

As an attempt to solve the above-mentioned drawbacks (problems of narrow environmental temperature and long-term stability) of the room temperature operation type sensor (including the sensor having a refresh mechanism), a high temperature operation type metal oxide is used. Physical sensors have been developed. In general, the amount of change in impedance of a metal oxide with respect to humidity in the high temperature range is smaller than that at room temperature, but it shows a change enough to detect a change in humidity, and is sufficient even in a high temperature atmosphere of 100 ° C or higher. It is possible to detect humidity. In such a high temperature operation type metal oxide sensor, the element is always several hundred degrees Celsius.
Since it is heated to 0 ° C., no special refreshing is necessary and the element surface is always kept clean.
Therefore, the detection level is stable for a long period of time.

The high temperature operation resistance change type humidity sensor includes
Moisture-sensitive material obtained by cutting ceramic powder such as ZrO ₂ , TiO ₂ and CaZrO ₃ into a paste with a binder, molding it into a sheet and firing it into chips The body was being used. As an example of such a high temperature operation type humidity sensor, Japanese Patent Laid-Open No.
No. 138001 discloses a sensor having the structure shown in FIG. The humidity sensor 8 shown in FIG. 6 is formed on both surfaces of a metal oxide chip substrate 81 (which is referred to as a humidity sensitive element, which changes an electric resistance value with a change in humidity). A coil heater 83 is arranged around an element having a pair of porous electrodes 82, 82 and lead wire portions 84, 84 made of platinum or the like connected to these electrodes. In the humidity sensor 8 having this structure, the coil heater 83 needs to surround the element so as not to come into contact with the humidity sensitive body, or an insulating layer needs to be formed on the surface of the coil heater 83.

Since the humidity sensor 8 shown in FIG. 6 has a structure for indirectly heating the element (humidity sensor), the heater thermal efficiency is poor. Further, since the coil heater is used, it is difficult to increase the resistance value of the heater. Therefore, a relatively large current is required to heat the element (humidity sensor) to a desired operating temperature, resulting in high power consumption. Become. Therefore, when incorporating this type of element into a device, an expensive circuit component having a large capacity is required.

Further, Japanese Patent Laid-Open No. 1-158340 discloses a humidity sensor as shown in FIG. FIG. 7 shows the humidity sensor 9 in a partially cutaway manner.
Has a structure in which the heater 92, the electrode 94, and the temperature measuring element 93 are embedded in one humidity sensing element 91. In the humidity sensor 9, the heater 92 also serves as the other electrode facing the electrode 94. In the sensor having such a structure, the heater 92 directly heats the moisture sensitive body 91, so that the thermal efficiency is increased. The humidity sensor having such a structure can be manufactured by stacking a plurality of ceramic green sheets each having a heater 92, a temperature measuring element 93, and an electrode 94 formed on the surface thereof, and firing the stacked sheets. Connection to a heater or the like embedded in the body 91 can be performed using the through hole 95.

In the conventional high temperature operation type humidity sensor as shown in FIGS. 6 and 7, both the molded body made of the metal oxide powder is fired to obtain the moisture sensitive body. The metal oxide is
Generally, it exhibits a high resistance value, although it depends on the composition and the impurity concentration.
Therefore, in this type of humidity sensor, the resistance value increases as the thickness of the humidity sensing element increases, which makes circuit design difficult. Since the humidity sensing of the high temperature type humidity sensor is basically based on the change of the resistance value of the humidity sensitive body, it is necessary to set the resistance value (impedance) of the humidity sensitive body to 1 MΩ or less in practice. It is desirable to use a thin moisture sensitive material.

Further, when the thickness of the moisture sensitive body is large, the following problems are likely to occur. That is, the surface layer portion of the moisture sensitive body easily absorbs moisture in the atmosphere to cause a resistance change, but on the other hand, moisture does not easily reach the inside (deep part) of the moisture sensitive body, so that almost no resistance change occurs. Absent. When such a state occurs, the element resistance (the resistance of the moisture sensitive body) gradually changes, and the output is not stable. In order to avoid such a state, the moisture sensitive body is made porous so that the atmospheric gas containing moisture can easily reach the inside of the moisture sensitive body.

From the above viewpoints, it is desirable to make the moisture sensitive body itself thin, but in recent years, there has been an aim to reduce the size of electronic circuits and electronic devices, and even in high temperature operating humidity sensors. Miniaturization and low power consumption type (low current type) have been demanded. From this point of view, it is desired to make the moisture sensitive film thinner.

By the way, generally, when a sheet-shaped sintered body of metal oxide is produced, a doctor blade method is used. According to this method, however, the thickness of the sheet is limited to several tens of μm, no matter how thin. Generally, a sintered body of about 100 to 200 μm is used. With such a thickness, miniaturization (thinning) of the element is not sufficient. Even if a doctor blade method is used to obtain a sintered body of about several tens of μm, such a thin sintered body may cause problems in strength, dimensions, handling, etc. Is difficult

Further, the following problems will occur.
That is, it is not possible to achieve the downsizing (thinning) of the element simply by thinning the moisture sensitive body, and the electrode portions formed on both surfaces of the moisture sensitive body must also be formed in a thin film shape. However, when a thin-film electrode is formed of a Pt-based metal (alloy) by a sputtering method, etc., the porosity of the electrode is higher than that of a thick electrode formed by sintering the same kind of material. It gets quite small. Therefore, in the element formed by forming thin film electrodes on both sides of a thin film moisture sensitive material made of a conventional moisture sensitive material, the amount of moisture reaching the moisture sensitive material through the thin film electrode is much smaller, and the sensitivity of the element is reduced. Is considerably smaller.

Therefore, the present inventor previously proposed a humidity sensor element having the structure shown in FIG. 8 (Japanese Patent Application No. 4-76136).
This humidity sensor element 10 is a thin humidity sensor formed by stacking a layer 11 made of a metal oxide whose electric resistance changes with a change in humidity and a layer 12 made of an oxide of an alkaline earth element. A thin film electrode (14) is formed on both surfaces of a film (moisture-sensitive thin film) 13, and the entire element is thin.
When the moisture-sensitive thin film 13 having such a layered structure is used, a relatively dense thin film electrode 14, 14 is formed on both surfaces thereof by a method such as sputtering.
Even if the above is provided, since the layer 12 made of an oxide of an alkaline earth element arranged in the moisture-sensitive thin film 13 has a large hygroscopic ability, it is possible to perform highly sensitive humidity detection.

However, if the thin humidity sensor element described above is simply formed on the substrate, the adhesion between this element (the thin film electrode on one surface of the element) and the substrate will not be good. The adhesiveness between the substrate and the thin film electrode is very important in terms of sensor characteristics and manufacturing. Also, the heating means of the humidity sensor (element) must be devised so as to have good thermal efficiency.

Therefore, an object of the present invention is that it is thin and can perform excellent humidity detection in a wide temperature range,
It is an object of the present invention to provide a humidity sensor having low power consumption and good adhesion between a substrate and a thin film element formed on the substrate, and a method for manufacturing such a humidity sensor.

[0018]

As a result of earnest research in view of the above object, the present inventors have found that after forming fine alumina projections on one surface of a dense insulating substrate, (a) the first A thin moisture-sensitive thin film in which a thin-film electrode, (b) a layer made of a metal oxide whose electric resistance changes with a change in humidity, and a layer made of an oxide of an alkaline earth element are laminated. And (c)
By forming the second thin-film electrode in that order and forming a thin-film heater on the other surface of this substrate, it is possible to make a thin humidity sensor as a whole. It was discovered that the adhesion with the electrode is good. Further, it has been discovered that the layer made of an oxide of an alkaline earth element in the moisture-sensitive thin film has a large hygroscopic ability (moisture adsorbing ability), and therefore humidity can be detected with good sensitivity. Furthermore, since the heater is formed on the substrate, the thermal efficiency is improved and the power consumption can be reduced, and the present invention has been completed.

That is, in the humidity sensor of the present invention, fine alumina protrusions are formed on one surface of a dense and highly electrically insulating substrate, and (a) the first thin film electrode is formed thereon. And (b) a first thin-film layer containing at least one kind of oxide of an alkaline earth metal element as a main component, and an oxide of a metal other than an alkaline earth metal element, an alkali metal element system, and a noble metal element. A moisture sensitive thin film formed by laminating a second thin film layer as a component, and (c) a second thin film electrode is laminated in that order, and on the other surface of the substrate, (d) A thin film heater is formed, and by the operation of the heater,
It is characterized in that it is constantly kept at a high temperature of 200 ° C. or higher, and the resistance value of the moisture-sensitive thin film changes according to the change of humidity in the atmosphere.

Further, in the method of the present invention for manufacturing the humidity sensor having the above-mentioned structure, the aluminum thin film is formed on one surface of the dense insulating substrate, and the aluminum thin film is treated with hot water or high temperature steam. To form a boehmite thin film having fine protrusions, which is baked in an oxidizing atmosphere to form alumina fine protrusions on the surface of the substrate, and then on the substrate surface on which the alumina fine protrusions are formed. , (A) a first thin film electrode, (b) a first thin film layer containing at least one kind of oxide of an alkaline earth metal element as a main component, an alkaline earth metal element, an alkali metal element system, and A moisture-sensitive thin film formed by laminating a second thin film layer containing an oxide of a metal other than a noble metal element as a main component, and (c) a second thin film electrode are laminated in that order.

[0021]

The present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 is a schematic sectional view showing a humidity sensor according to an embodiment of the present invention. In this humidity sensor 1, a fine alumina projection layer 18 is formed on one surface of a dense and electrically insulating substrate 15, and (a) the first thin film electrode 14a is formed thereon. And (b) a layer made of a metal oxide (hereinafter referred to as a humidity sensitive resistor layer) 11 that changes an electric resistance value with a change in humidity and a layer 12 made of an oxide of an alkaline earth element. Moisture sensitive thin film 13 formed by stacking,
(c) The second thin film electrode 14 is laminated in that order.
A thin film heater 16 is formed on the other surface of the substrate 15. A heater formed on the surface of the substrate 15
Since 16 is meandering, the heater 16 is shown in the sectional view of FIG.
The cross-sections of are shown spaced apart.

First, the substrate 15 of the humidity sensor is made of a material that is dense and has a good electric insulation. Specifically, it is preferable to use a substrate made of Al ₂ O ₃ , SiO ₂ or the like.

In particular, when alumina or an alumina-based composite material is used as the substrate 15, the coefficient of thermal expansion of the fine protrusion layer 18 of alumina and the substrate 15 can be made substantially equal.
The bondability between alumina and the fine protrusion layer 18 of alumina is further improved.

Alumina fine protrusion layer on one surface of the substrate 15
Although 18 can be formed by the method described later, according to this method, the projection size (average diameter) in the alumina fine projection layer 18 can be set to several tens of angstroms to about 500 angstroms. Further, the alumina fine protrusions can be formed densely on the substrate 15.
If the thin film electrode 14a is formed on the alumina fine protrusion layer 18 having such a protrusion size, the substrate 15 and the thin film electrode are formed.
The adhesiveness with 14a can be remarkably improved. Further, since the projection size in the alumina fine projection layer 18 can be kept within a narrow distribution, it is possible to set the porosity of the thin film electrode 14a formed thereon to a desired degree. If the porosity of the thin film electrode 14a can be adjusted, the porosity of the moisture-sensitive resistor layer 11 formed thereon can also be easily adjusted.

By the way, the adhesion between the substrate 15 and the thin film electrode 14a becomes very important in terms of manufacturing and sensor characteristics. When forming a thin film electrode on a substrate such as alumina by sputtering, generally, the larger the surface roughness of the substrate, the better the adhesion between the substrate and the thin film electrode. However, if the surface roughness of the substrate is simply increased by mechanical treatment and the thin film electrode 14a is formed on this, it becomes difficult to control the porosity of the thin film electrode 14a. That is,
The distribution of hole diameters in the thin film electrode 14a becomes wider. Also,
It is easy to form a porous thin film electrode 14a having pores with a relatively large diameter. On the thin film electrode 14a in such a state,
After all, the moisture-sensitive thin film 13 having pores of relatively large diameter is easily formed. If a large-diameter hole is formed in the moisture-sensitive thin film 13 in this way, a short circuit occurs between the electrodes, resulting in a defective product. On the other hand, if the surface roughness of the substrate 15 is reduced and the thin film electrode is to be provided on the surface roughness, the adhesion of the thin film electrode to the substrate is reduced.

However, the alumina fine protrusion layer 18 as described above is provided on the substrate 15, and the thin film electrode 14a is formed thereon.
By forming the above, it becomes possible to improve the adhesion between the thin film electrodes and the substrate while controlling the porosity of the electrodes 14a and 14b (and the moisture sensitive thin film 13 formed therebetween).

The first thin film electrode 14a provided on the alumina fine protrusion layer 18 on the substrate 15 and the second thin film electrode 14b formed on the upper surface of the moisture sensitive thin film 13 described later are both Pt and Pt type. It can be formed from metals or their alloys.
The thickness of each of the two electrodes is preferably about 500 to 2000 angstroms. The thin film electrodes 14a and 14b can be formed by a sputtering method or the like.

The sputtering conditions for forming the thin film electrode are preferably such that the substrate temperature is 100 to 400 ° C. and the sputtering output is 1.2 to 2.0 W / cm ² .

On the first thin film electrode 14a, a moisture sensitive thin film 13 is formed by laminating a moisture sensitive resistor layer 11 and a layer 12 made of an oxide of an alkaline earth element. In this embodiment, each of the moisture-sensitive resistor layer 11 and the layer 12 made of an oxide of an alkaline earth element in the moisture-sensitive thin film 13 is one layer.
As described below, the present invention is not limited to this.

The moisture sensitive resistor layer 11 in the moisture sensitive thin film 13 is formed of the same material as the moisture sensitive substance (moisture sensitive resistor) used in the conventional high temperature operation type metal oxide sensor. In the present invention, an oxide of a metal other than an alkaline earth metal element, an alkali metal element system, and a noble metal element is used. In particular,
ZrO ₂ , TiO ₂ , CeO ₂ , La ₂ O ₃ , Nb ₂ O ₅ , Y ₂ O
₃ , Ta ₂ O ₅ , Cr ₂ O ₃ , Ni ₂ O ₃ , Fe ₂ O ₃ , ZnO,
V ₂ O ₅ or the like, or a mixture of these oxides can be used. However, in terms of sensitivity, ZrO ₂ , Ti
O ₂ , Ta ₂ O ₅ , Nb ₂ O ₅ and ZnO are preferred. With the humidity-sensitive resistor layer made of the above-mentioned materials, it becomes possible to detect humidity well even at an ambient temperature exceeding 200 ° C.

The moisture-sensitive resistor layer 11 preferably has a thickness of about 0.2 to 5 μm, more preferably about 1 to 2 μm. If the moisture-sensitive resistor layer 11 has a thickness of less than 0.2 μm, short-circuiting of electrodes is likely to occur. On the other hand, when the thickness exceeds 5 μm, the object of thinning the element to reduce the size cannot be achieved, and the impedance becomes large.

Since the moisture-sensitive resistor layer 11 is formed thin as described above, it is preferable to use the vapor phase growth method for forming it. However, the present invention is not limited to this, and other methods may be used as long as the humidity-sensitive resistor layer 11 can be formed to the above-described thickness. Specifically, it can be formed by a sputtering method, a vapor deposition method such as a CVD method, or a sol-gel solution method. Particularly, the sputtering method (high-frequency sputtering method) is preferable. According to this method, the moisture sensitive resistor layer having a thickness of about 1 μm can be easily formed. When forming a metal oxide film (forming a metal oxide layer) by a high frequency sputtering method, the film formation rate is usually about 40 to 100 angstrom / min.
If the sputtering is performed for 4 hours, a moisture sensitive resistor layer having a thickness of about 1 μm can be obtained. The moisture-sensitive resistor layer formed by the high frequency sputtering method is preferably heat-treated in the atmosphere for the purpose of supplementing oxygen vacancies in the metal oxide in the layer and removing strain generated in the layer. preferable.

When the moisture-sensitive resistor layer 11 is formed by sputtering, the substrate temperature is preferably 200 to 400 ° C. and the sputter output is preferably 1.5 to 2.5 W / cm ² . The gas pressure at that time is preferably 5 to 20 mmTorr.

The layer 12 made of an alkaline earth oxide is
It is preferably formed to a thickness of about 50 to 5000 angstroms. When the thickness of the alkaline earth oxide layer 12 is less than 50 angstroms, the effect of providing the alkaline earth oxide layer 12 in the humidity sensing film 13 does not sufficiently appear, and the sensitivity of the element is good. do not become. Further, according to the research conducted by the present inventors, even if the layer 12 has a thickness exceeding 5000 angstroms, good sensitivity cannot be obtained. If the alkaline earth element oxide layer 12 is too thick, the water adsorbed to this layer must travel a long distance to reach the moisture-sensitive resistor layer 11, and as a result, the moisture-sensitive resistor layer 11 This is probably because a sufficient amount of water cannot be reached. In the case of forming a humidity sensing film by laminating a plurality of alkaline earth element oxide layers 12 and a plurality of humidity sensitive resistor layers 11, the total of the plurality of alkaline earth element oxide layers 12 is The thickness is preferably about 50 to 5000 angstroms.

As the oxide of the alkaline earth element, Mg
It is preferable to use O, BaO, CaO, and SrO, and these may be mixed and used. The formation of the layer 12 made of the oxide of the alkaline earth element also applies to the moisture-sensitive resistor layer 11 described above.
It can be formed by a method similar to.

On the other hand, the thin film heater 16 can be formed on the other surface of the substrate 15 by a method such as screen printing using a platinum paste or photolithography. In particular, according to the photolithography method, the heater can be patterned easily and accurately, and the production efficiency becomes very high. Further, since the patterning accuracy is high, the variation in the resistance value of the heater is very small.

Each thin film electrode and thin film heater
As the lead wire 17 connected to the wire 16, a Pt wire, a wire made of a Pt-based alloy, or the like can be used.

With the structure shown in FIG. 1, even if the thin film electrode 14b formed by sputtering or the like has a small porosity (in the case of an electrode thin film such as Pt formed by sputtering, the pore diameter is generally several tens of angstroms). The moisture is a moisture-sensitive thin film due to the large water adsorption capacity of the alkaline earth element oxide layer 12 provided in the moisture-sensitive thin film 13.
Captured within 13. As a result, the moisture reaches the moisture-sensitive resistor layer 11 in the moisture-sensitive thin film 13, and the resistance change corresponding to the amount of moisture occurs in the moisture-sensitive resistor layer 11.

In the embodiment shown in FIG. 1, the moisture-sensitive thin film 13 is 1
One moisture-sensitive resistor layer 11 and one layer 12 made of an oxide of an alkaline earth element are laminated, and the moisture-sensitive resistor layer 11 is formed on the outer side (the side far from the substrate 15). However, the present invention is not limited to this. For example, a moisture sensitive thin film
The moisture sensitive resistor layer 11 in 13 and the layer 12 made of one oxide of an alkaline earth element may be laminated in the opposite manner.
Further, in order to improve the adhesion between the moisture-sensitive thin film 13 and the thin-film electrode 14b, in the embodiment shown in FIG. 1, a moisture-sensitive resistor layer is further formed on the alkaline earth element oxide layer 12. A laminated structure in which 11 is provided may be used, or the moisture-sensitive resistor layer 11 and the alkaline earth element oxide layer 12 may be alternately laminated a plurality of times.

However, considering the role of the alkaline earth element oxide layer 12, it is preferable to dispose the alkaline earth element oxide layer 12 on the atmosphere side (surface layer portion of the humidity sensing film) as much as possible. That is, it is preferable to dispose an alkaline earth element oxide layer on at least one surface layer of the humidity sensing film.

Next, a method of forming the alumina fine protrusion layer 18 provided on the substrate 15 will be described. First, a pure aluminum thin film is formed on the substrate 15. The thickness of the aluminum thin film has a direct influence on the boehmite protrusion size by the subsequent hot water treatment (or high temperature steam treatment), and therefore must be precisely controlled. The thickness of the aluminum thin film is preferably in the range of 100 to 3000 angstroms, but it is actually determined in consideration of the thickness of the thin film electrode or the like.

Next, if the aluminum thin film is quickly subjected to hot water treatment or exposed to high-temperature steam before being oxidized, aluminum hydroxide (boehmite) is formed with fine projections. Although it is unclear why the protrusions are formed, the tendency is that the protrusion size becomes smaller when the treatment is performed slowly on the low temperature side for a long time. Further, the smaller the aluminum film thickness, the smaller the projection size. In case of hot water treatment, the temperature is 80 ~ 100 ℃
Therefore, 5 to 30 minutes is suitable as the processing time. Also,
In the treatment with steam, steam at 100 to 300 ° C can be used for 1 to 10
It is appropriate to allocate.

After forming the boehmite protrusions by the above method, the boehmite protrusions are fired in an oxidizing atmosphere (for example, in the air) to change into stable alumina (fine alumina protrusions). Usually, 1000 to 1500 ° C (for example, about 1300
If it is baked at (° C.) for 1 to 5 hours (for example, about 1 hour), the boehmite protrusions change into alumina protrusions. At this time, the protrusion size hardly changes (from the size of the boehmite protrusion).

The alumina fine projections obtained by the above method are extremely fine and uniform, and when a thin film electrode is formed on this, the substrate 15 (substrate) of the thin film electrode 14a is formed.
Adhesion to the alumina fine projection layer 18) provided on 15 is sufficiently increased.

Hereinafter, the present invention will be described in more detail with reference to specific examples.

Example 1 A humidity sensor having the structure shown in FIG. 1 was manufactured by the following procedure. On a 2 mm x 2 mm x 0.2 mm alumina substrate (purity 97%), a thin film of pure aluminum was formed to a thickness of about 500 angstrom by RF sputtering, and the pure aluminum boiled before the surface of the aluminum thin film was oxidized. It was immersed in water to form a boehmite (aluminum hydroxide) film. The surface of the formed boehmite film had an average surface roughness of about 100 Å and very fine protrusions. Bake this at 1300 ° C for 1 hour in the air,
An alumina film having fine protrusions was obtained.

A Pt thin film electrode having a thickness of about 2000 is formed as a first electrode on the fine projection film by the RF sputtering method.
The film was formed in Angstrom. The sputtering conditions at that time were as follows: sputter output 1.7 W / cm ² , substrate temperature 200
The gas pressure was 10 mmTorr.

Next, a ZrO ₂ layer (moisture-sensitive resistor layer) was formed (layered) on the Pt thin film electrode by the high frequency magnetron sputtering method to a thickness of about 2 μm. Further, an MgO layer having a thickness of 500 angstrom was formed on the ZrO ₂ layer by a high frequency magnetron sputtering method.

Here, for the purpose of complementing oxygen vacancies in the ZrO ₂ layer and MgO layer and removing film strain, the obtained laminate was heated in the atmosphere at 600 ° C. for 2 hours.

Further, a Pt thin film electrode was formed on the MgO layer of the above laminated body by sputtering, and a lead wire was connected to each of the Pt thin film electrodes.

Next, a Pt thin film was formed on the opposite surface of the substrate by sputtering to a thickness of about 3000 angstroms, and then a heater having a meandering pattern was formed by photolithography, a lead wire was connected, and a humidity sensor was connected. did.

A circuit shown in FIG. 2 was formed using this humidity sensor. Here, the DC voltage power source E was used as the power source of the heater 16 in the sensor 1. The signal voltage power supply 31 is AC 70Hz, 7V _0-P , and the signal detection load resistance R _L is 500 k.
Ω. An AC power supply is used as the signal voltage power supply 31 in order to prevent polarization of adsorbed water.

By changing the humidity in the atmosphere around the sensor using the circuit of FIG. 2, the sensor (operating) temperature is set to 400 to 50.
The relationship between the absolute humidity in the atmosphere and the impedance at 0 ° C was investigated. Results are shown in FIG.

Further, 40% relative humidity at 10 ° C. (hereinafter,
It is described as 10 ° C / 40% RH. Here, RH indicates relative humidity. The amount of water in the air at 10 ° C / 40% RH is 2.
It is 92 g / kg. ) Sensor impedance R at
_The humidity detection sensitivity S was calculated from 1 and the sensor impedance R _{2 at} 35 ° C./80% RH (24.64 g / kg) according to the following equation (1). S = log (R ₁ / R ₂ ) ... (1) The humidity detection sensitivity S at each temperature is shown in FIG.

As can be seen from FIG. 3, in the humidity sensor of this embodiment, the sensor element impedance fluctuates sufficiently in accordance with the humidity even in the high temperature range of 400 to 500 ° C., and has a good sensitivity. I understand. In particular, it can be seen that the sensitivity is best when the operating temperature is 450 ° C.

Using the circuit shown in FIG. 2, the operating temperature is
The response time of the sensor was measured at 400-500 ° C. In this response time measurement, when the absolute humidity of the atmosphere was rapidly (stepwise) changed from 2.92 g / kg to 6.69 g / kg,
Rapidly (stepwise) from 7.16 g / kg to 14.31 g / kg
The 90% response time (T ₉₀ ) for each of the changes and the rapid (stepwise) change from 14.31 g / kg to 24.64 g / kg was determined. The results are shown in Fig. 4.

As can be seen from FIG. 4, the operating temperature is 450 ° C.
Alternatively, good response characteristics are obtained at 500 ° C. As described above, from the viewpoint of sensitivity and response characteristics, it seems that the operating temperature of the sensor of this example is about 450 ° C. optimal.

Examples 2 to 6 and Comparative Example 1 A fine alumina projection layer was formed on an alumina substrate in the same manner as in Example 1 except that the thickness of the aluminum thin film was changed. Here, the size of the boehmite protrusions formed (the size of the finally obtained alumina fine protrusions) greatly changes depending on the thickness of the aluminum thin film. Therefore, the thickness of the aluminum thin film was selected to be a desired thickness, and the alumina microprojection layer having the (average) size shown below was formed on the substrate. Example 2: 75 angstroms Example 3: 200 angstroms Example 4: 330 angstroms Example 5: 405 angstroms Example 6: 500 angstroms

A humidity sensor having the structure shown in FIG. 2 was manufactured in the same manner as in Example 1 using each of the above substrates.

The operating temperature of each humidity sensor is 450 ° C.
The humidity was detected in an atmosphere with an absolute humidity of 2.92 g / kg to 24.64 g / kg. And 10 ℃ / 40% RH
Based on (2.92 g / kg), the sensitivity at each measurement point was calculated using the above-mentioned formula (1). Results are shown in FIG.

A humidity sensor was manufactured in the same manner as in Example 1 except that the alumina fine protrusion layer was not provided on the alumina substrate (Comparative Example 1). With respect to this humidity sensor, the humidity was detected in the same manner as in Example 2, and the sensitivity was calculated. The results are also shown in FIG.

As can be seen from FIG. 5, the sensitivity increases as the protrusion size increases. Further, the humidity sensor of each example has better sensitivity than the humidity sensor of Comparative Example 1.

Example 7 In order to see the effect of improving the film adhesion according to the present invention, a substrate having fine alumina projections was prepared in the same manner as in Example 1 and a Pt thin film electrode was formed. Moisture sensitive resistor layer 11 on this
In the step of forming the (moisture-sensitive thin film) by the sputtering method, the influence of the substrate temperature and the sputtering power on the peeling of the humidity-sensitive resistor layer 11 (moisture-sensitive thin film) was examined.

Table 1 shows the results of comparison between the product of the present invention and the comparative product (having the same laminated structure except that the alumina fine protrusion layer 18 is not provided).

Table 1 Detachment substrate temperature output (W) of sputter moisture sensitive thin film Conventional product 50 ° C. (water cooling) 100 × ○ 50 ° C. (water cooling) 300 × ○ 150 ° C. 100 Δ ○ 150 ° C. 300 × ○ 300 ° C. 100 ○ ○ 300 ℃ 300 △ ○ (Note) ○: No peeling △: Slight peeling ×: Peeling marked

As is clear from Table 1, the comparative product can obtain a normal film only under limited conditions.
A film with good adhesion was obtained under all the conditions tested.

[0067]

The humidity sensor of the present invention having the above construction has the following effects.

It has a good sensitivity and can perform a good humidity measurement even at a high temperature of about 400 to 500 ° C. Moreover, its responsiveness is also good.

The heater, the electrodes and the moisture sensitive body are all integrated,
In addition, the thinness of the humidity sensor (element) is achieved.

Since the heating efficiency of the heater is good and the heater is made thin, the heater current value can be reduced.

As described above, since the heater, the electrode, and the moisture sensitive material provided on the substrate are all formed into a thin film, sputtering,
Mass production with good yield can be performed by using a method such as photolithography.

Since the alumina micro-projections are provided on the surface of the substrate, the adhesion between the substrate and the electrode (further, the adhesion between the electrode and the moisture sensitive thin film) is improved, the sensor characteristics are improved, and the manufacturing process is improved. Variations are reduced.

The porosity of the thin-film electrode can be controlled, and thus the porosity of the humidity sensitive resistor layer can be easily adjusted. Therefore, the humidity detection sensitivity can be easily adjusted.

Since the humidity sensor element according to the present invention can be miniaturized and does not require a large amount of electric power, it can be used as various consumer appliances such as an indoor air conditioning sensor and a microwave oven cooking sensor.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing a humidity sensor according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a humidity detection circuit used in an example.

FIG. 3 is a graph showing the relationship between the absolute humidity in the atmosphere and the impedance of the humidity sensor of Example 1 at 400 to 500 ° C.

FIG. 4 is a graph showing the relationship between the absolute humidity in the atmosphere and the response time of the humidity sensor of Example 1 at 400 to 500 ° C.

FIG. 5 is a graph showing the relationship between the size of alumina micro-projections provided on the substrate of the humidity sensor and the sensitivity of the sensor.

FIG. 6 is a perspective view showing an example of a conventional high temperature operation type humidity sensor element.

FIG. 7 is a partially cutaway perspective view showing another example of a conventional high temperature operation type humidity sensor element.

FIG. 8 is a schematic cross-sectional view showing an example of a thin type humidity sensor element.

[Explanation of symbols]

 1 Humidity Sensor 8, 9 Bulk Type Humidity Sensor 11 Moisture Sensitive Resistor Layer 12 Alkaline Earth Element Oxide Layer 13 Moisture Sensitive Thin Films 14a, 14b Thin Film Electrode 16 Thin Film Heater 17 Lead Wire 18 Alumina Microprojection Layer 81 , 91 Moisture Sensitive

Claims (3)

[Claims] 1. A fine alumina projection is formed on one surface of a dense and highly electrically insulating substrate, on which (a) a first thin film electrode and (b) an alkaline earth material are formed. A first thin film layer containing at least one oxide of a group metal element as a main component, and a second thin film containing an oxide of a metal other than an alkaline earth metal element, an alkali metal element system, and a noble metal element as a main component. A moisture-sensitive thin film formed by stacking layers, and (c) a second thin-film electrode are stacked in that order, and (d) a thin-film heater is formed on the other surface of the substrate. The humidity sensor is a
A humidity sensor characterized in that the resistance value of the moisture-sensitive thin film is maintained at a high temperature of ℃ or higher and the resistance value of the moisture-sensitive thin film changes according to the change of humidity in the atmosphere. 2. The humidity sensor according to claim 1, wherein
A humidity sensor, wherein the substrate is made of alumina. 3. The method for manufacturing a humidity sensor according to claim 1, wherein a thin film of aluminum is formed on one surface of a dense insulating substrate, and the thin film is treated with hot water or high temperature steam. A method for manufacturing a humidity sensor, comprising forming a boehmite thin film having fine protrusions and then firing the thin film in an oxidizing atmosphere to form fine alumina protrusions on the surface of the substrate.