JPH0650923A - Humidity sensor element - Google Patents

Abstract

PURPOSE:To facilitate reaching of moisture to a moisture sensitive body and to make it possible to perform excellent humidity detection under the high temperature state wherein moisture adsorbing amount is less by forming a mesh-shaped electrode on a thin-film moisture sensitive body. CONSTITUTION:A first thin-film electrode 2 comprising Pt or the like, a thin-film moisture sensitive body 3 comprising ZeO2 or the like and Mg0 or the like, whose electric resistance is changed with humidity change, and a second thin- film electrode 4 are sequentially laminated on one surface of a substrate 1 having the thickness of about 100-800mum comprising electric insulating ceramics. A thin-film heater 5 comprising Pt and Pt group metal is formed on the other surface of the substrate 1. At this time, the electrode 4 is formed in the mesh state by a photoresist method after the vapor deposition by a sputtering method and the like. The diameter of the hole in the mesh is set at about 1-10mum, and the numerical aperture is set at 5-50%. As a result, the reaching of moisture to the moisture sensitive body 3 is made easy, and the humidity can be detected sufficiently accurately even at the high temperature of 200 deg.C or more, wherein moisture adsorption amount is less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a humidity sensor element,
More specifically, the present invention relates to a thin (small) humidity sensor element that exhibits good sensitivity even in a high temperature range.

[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 and the like. Humidity detection means have been provided to control equipment. Therefore, the solid-state type humidity sensor element has been widely used. Due to its detection principle, this solid-state humidity sensor element uses a resistance change (impedance change), a capacitance change, a frequency change, a heat conduction change, and an ultrasonic wave. And those using infrared rays and the like, but among them, 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 sensors (sensors using a polymer film) and metal oxide sensors. The polymer type sensor detects the relative humidity in the atmosphere in both of the resistance change type and the capacitance change type. Also, it is not suitable for measurement in high temperature atmosphere above about 50 ℃. 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. Normally, 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 dirt on the element surface (polymer film surface) due to dust, oil smoke, etc. in the use atmosphere, and long-term Poor stability. Therefore,
The environmental conditions of use of polymer sensors are naturally limited.

On the other hand, in the metal oxide type sensor, the resistance change type and the capacitance change type are mainly operated at room temperature. In particular, a metal oxide is widely used, which utilizes a large hygroscopic ability of a metal oxide at a low temperature and measures a resistance change associated with adsorption / desorption of water to detect humidity. This type of sensor also has a limitation in use environment temperature like the above-mentioned polymer type sensor, and has a drawback 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 taking advantage of the heat resistance of the constituent material (metal oxide), the element is temporarily heated to 400 to 500 ° C.
A sensor having a so-called refreshing mechanism that heats up the element (metal oxide) to burn off dirt on the surface of the element (metal oxide) has also been developed.

However, when the refresh mechanism is activated to temporarily heat the element (refreshing), the humidity sensor element is refreshed and for a while after refreshing (until the initial stable level is restored again). It cannot function as, and cannot detect humidity during that time.

As an attempt to solve the above-mentioned drawbacks (problems of narrow operating environment temperature and long-term stability) in the room temperature operation type sensor (including the sensor having the refresh mechanism), the high temperature operation type metal oxide is used. Physical sensors have been developed. In general, the amount of change in impedance of the metal oxide with respect to humidity in a high temperature region is smaller than the amount of change in normal temperature. The reason for this is as follows. That is, in the low temperature range (including normal temperature), the resistance changes mainly due to physical adsorption by Van der Waals force, whereas in the high temperature range, B is mainly produced from A and H ₂ O (however, A Is a metal oxide having a high hygroscopic property, and B is a hydrate of A). The main reason for this is that the resistance change mechanism differs between the low temperature region and the high temperature region.

As described above, the high temperature operation type humidity sensor element has a smaller resistance change amount than the room temperature operation type humidity sensor element. Therefore, if sufficient moisture does not reach the moisture sensitive body, the amount of change in resistance becomes even smaller and accurate humidity detection becomes difficult.

As the electrode structure of the sensor element in the high temperature operation type humidity sensor, a counter electrode type is widely used. In this case, the moisture sensitive body is directly exposed to the atmosphere and is therefore easily detected. However, in the case of the counter electrode type, a thin film moisture sensitive body has a large resistance, and it is difficult to downsize the humidity sensor element.

Therefore, a laminated type electrode is considered, which sandwiches the humidity sensitive element from above and below. In this case, as compared with the counter electrode, the electrode area can be increased without increasing the size of the humidity sensor element, and thus the resistance can be reduced. In the laminated type electrode, the humidity sensitive material adsorbs the moisture that has entered from the upper electrode and the resistance changes, but when the electrode is formed using a sputtering method or the like, the porosity of the electrode is small and Since the moisture in the atmosphere does not reach easily, there is little change in resistance. Therefore, it is difficult to obtain good humidity detection sensitivity and the response is slow.

Therefore, an object of the present invention is to provide a humidity sensor element using a laminated electrode, which has a mesh-like thin film electrode formed by a vapor phase growth method such as a sputtering method, so that a good humidity can be detected. It is to provide a humidity sensor element exhibiting sensitivity.

[0011]

As a result of earnest research in view of the above problems, the present inventor has found that the upper electrode portion of the humidity sensor element is formed in a mesh shape using a photoresist, so that the humidity sensor can be formed. It has been discovered that moisture can be easily reached and that humidity can be detected with sufficient accuracy even at a high temperature of 200 ° C. or higher where the amount of adsorbed moisture is small. The present invention has been completed based on the above findings.

That is, the humidity sensor element of the present invention has two elements.
A humidity sensor that is always operated at a high temperature of 00 ° C. or higher, and has a structure in which a first thin film electrode, a thin film moisture sensitive material, and a second thin film electrode are sequentially laminated on a ceramic substrate. In the element, the second thin film electrode is mesh-shaped.

[0013]

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 element according to an embodiment of the present invention. This humidity sensor element 10
Is a first thin film electrode 2 on one surface of a substrate 1 made of an electrically insulating ceramic, a thin film moisture sensitive element 3 whose electric resistance changes with humidity, and a second thin film Thin film electrodes 4 are sequentially laminated. A thin film heater 5 is formed on the other surface of the substrate 1.

In a preferred embodiment, the thin film moisture sensitive body 3 comprises a thin film layer 3a made of a metal oxide (an oxide of a metal other than an alkaline earth metal) which changes its electric resistance value with a change in humidity. , A thin film layer 3b made of an oxide of an alkaline earth metal.

The layer structure of the humidity sensor element of the present invention will be described in detail below. [1] Substrate The substrate 1 is formed of ceramic having good electric insulation. Further, since it is heated to 400 to 500 ° C. by the thin film heater 4, it is necessary to have a large thermal shock resistance,
Further, it is necessary to have sufficient strength in consideration of handling and the like. Alumina (Al ₂ O ₃ ), alumina-silica (3
Al ₂ O ₃ .2SiO ₂ ) and the like are preferable.

The substrate 1 preferably has a thickness of 100 to 800 μm. If the substrate 1 is less than 100 μm,
Due to insufficient strength, handling during the manufacturing process is difficult. On the other hand, if the thickness exceeds 800 μm, the heating efficiency of the thin film heater 4 becomes too low and the power consumption increases. A more preferable thickness is 200 to 500 μm.

The substrate 1 can be formed by a known sintering method after being formed by a doctor blade method or the like.
For example, the above ceramic material such as alumina has a thickness of 30 to 7
A slurry is generated by adding 0% molding binder, and this is molded into a predetermined thickness by a doctor blade method or the like, dried and then sintered.

The ceramic forming the substrate 1 may be dense or porous. However, in order to increase the amount of moisture adsorbed on the moisture sensitive body, it is preferable to form the ceramic porous body having good electric insulation.

When the substrate 1 is formed of a ceramic porous body, its pore size is preferably 0.1 to 0.8 μm. When the pore diameter is less than 0.1 μm, the pore diameter of the laminated moisture sensitive body is further reduced, the amount of adsorbed water is reduced, and it is difficult to obtain good humidity detection sensitivity. On the other hand, when the pore size exceeds 0.8 μm, the pore size of the moisture sensitive body becomes too large, which may cause a short circuit between both thin film electrodes due to the structure of the sensor element. A more preferable pore size is 0.
It is 2 to 0.5 μm.

The porosity of the substrate 1 is preferably 10 to 30%. If the porosity is less than 10%, the amount of moisture adsorbed on the thin film moisture-sensitive material formed thereon is small, and good humidity detection sensitivity cannot be obtained. On the other hand, when the porosity exceeds 30%, the strength of the substrate 1 becomes insufficient, and
A thickness of mm or more is required. In addition, since the porosity of the moisture sensitive body also becomes large, a short circuit easily occurs between the thin film electrodes on both sides of the moisture sensitive body.

[2] First Thin Film Electrode The first thin film electrode 2 is formed of Pt, a Pt group metal, or an alloy thereof. The thickness of each electrode 2 is 500 ~
It is preferably about 2000 Å. The thin film electrode 2 can be formed by a sputtering method or the like.

[3] Thin-Film Moisture Sensitive Body The thin-film moisture sensitive material 3 formed on the first thin-film electrode 2 is
Thin film layer 3a whose electric resistance value changes with changes in humidity
(It can also be called a moisture sensitive resistor layer) and a thin film layer 3b formed thereon of an oxide of an alkaline earth metal. Although one thin film layer 3a and one thin film layer 3b in the moisture sensitive thin film 3 are shown in FIG. 1, the present invention is not limited to this, and each may have a plurality of layers.

(A) Thin film layer (first thin film layer) 3a The thin film layer 3a is formed of an oxide of a metal other than an alkaline earth metal. Specifically, 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 and the like, and mixtures thereof can be used. However, from the viewpoint of sensitivity, ZrO ₂ , TiO ₂ , Ta ₂ O ₅ , Nb ₂ O ₅ , and ZnO are preferable. With a thin film layer made of such a metal oxide,
Humidity can be satisfactorily detected even at an ambient temperature exceeding 200 ° C.

The thickness of the thin film layer 3a is preferably about 0.2 to 5 μm, more preferably about 1 to 2 μm. If the thickness of the thin film layer 3a is less than 0.2 μm, a short circuit may occur. On the other hand, if the thickness exceeds 5 μm, the purpose of thinning the humidity sensor element cannot be achieved.

Since the thin film layer 3a is thin as described above, it is preferably formed by the vapor phase growth method. Specifically, it is formed by a vapor deposition method such as a sputtering method or a CVD method. Further, as a forming method other than the vapor phase growth method, a sol-gel method, a solution method or the like can be used.

As the vapor phase growth method, a sputtering method such as a high frequency sputtering method is particularly preferable. According to this method, the thin film layer 3a having a thickness of about 1 μm can be easily formed. When a metal oxide film is formed by a high frequency sputtering method, the film formation rate is usually about 40 to 100 angstrom / min. Therefore, if the sputtering is performed for 2 to 4 hours, the thickness can be about 1 μm. In addition,
When the thin film layer 3a is formed by the high frequency sputtering method, heat treatment in the atmosphere is performed for the purpose of complementing the oxygen deficiency of the metal oxide generated in the layer and removing the strain generated in the layer. It is preferable to carry out.

In the above description, the case where the thin film layer 3a is formed on the substrate made of dense ceramic has been described. However, in order to increase the amount of moisture adsorbed on the thin film layer 3a, good electrical insulation is obtained. It is preferable to form it on a substrate composed of a porous ceramic body. At that time, the porosity of the thin film layer 3a is determined by the fine structure of the substrate 1 (pore diameter, porosity).
Can be controlled by. Specifically, the thin film layer 3a is 0.05
It is preferable to have a pore size of ˜0.7 μm and a porosity of 5 to 25%. As a result, the thin film layer 3a exhibits a good water adsorption property, so that the humidity sensitivity of the humidity sensor element becomes very good.

(B) Thin film layer (second thin film layer) 3b Examples of alkaline earth metal oxides include MgO, BaO, and Ca.
O, SrO or mixtures thereof are preferred.

The thin film layer 3b made of an oxide of an alkaline earth metal is preferably formed to a thickness of about 50 to 5000 angstroms. If the thickness of the thin film layer 3b is less than 50 Å, the effect of providing the thin film layer 3b cannot be sufficiently obtained, and the sensitivity of the humidity sensor element will not be good. On the other hand, if the thin film layer 3b has a thickness exceeding 5000 angstroms,
The water adsorbed to this layer has to travel a long distance to reach the thin film layer 3a, and as a result, a sufficient amount of water cannot reach the thin film layer 3a, so that the sensitivity of the humidity sensor element is rather lowered. .

The plurality of thin film layers 3b and the plurality of thin film layers 3
When a and a are laminated to form the thin film moisture sensitive body 3, the upper limit of the total thickness of the plurality of thin film layers 3b is preferably set to about 5000 angstroms.

The thin film layer 3b can also be formed by a vapor phase growth method such as a sputtering method as in the case of the thin film layer 3a.

[4] Second thin film electrode The second thin film electrode 4 is formed on the thin film moisture sensitive body 3 in a mesh shape. The material is Pt, Au, or RuO ₂
A thin film or the like is preferable.

The hole diameter of the second thin film electrode 4 is 1 to 10 μm.
m. Pore sizes smaller than 1 μm are difficult to make by the method of the present invention. When it exceeds 10 μm, the mesh-shaped electrode has defects and cracks due to thermal strain, so that the function of the electrode cannot be fulfilled. The preferable pore size is 2 to 5 μm.

The porosity is 5 to 50%. When it is lower than 5%, the amount of adsorbed water becomes small, so that the response of the moisture sensitive body is slow and the detection sensitivity of humidity is poor. If it exceeds 50%, the holes are brought into contact with each other, so that the mesh-like electrode is defective and cracked due to thermal strain. Therefore, the second thin film electrode 4 does not function as an electrode. A preferable porosity is 10 to 30%.

The second thin film electrode 4 can be formed into a mesh by vapor deposition by a sputtering method, a CVD method, an ion plating method or the like, and then by a photoresist method. Specifically, a resist material having a desired hole diameter and a desired opening ratio is patterned on the deposited thin film, and plasma etching is performed. After that, the resist material is peeled off.

[5] Thin-Film Heater The thin-film heater 4 formed on the opposite surface of the substrate 1 is P
It is composed of t and Pt group metals. Such a thin film heater 5
Can be formed by a screen printing method using a platinum paste, a photolithography method, or the like. Particularly, according to the photolithography method, the heater 5 can be patterned easily and accurately, and the production efficiency is very high. Further, since the patterning accuracy is high, the variation in the resistance value of the heater 5 is very small.

The heater 5 formed on the surface of the substrate 1
Since it is meandering, the cross-sectional view of FIG. 1 shows the heater 5 as being spaced apart in cross-section.

[6] Lead Wire As the lead wire 6 connected to the thin film electrodes 2 and 4 and the thin film heater 5, a lead wire made of Pt or a Pt alloy can be used.

Considering the role of the oxide of the alkaline earth metal, it is preferable to arrange the thin film layer 3b on the atmosphere side (surface layer of the humidity sensing film) as shown in FIG. The order of stacking 3a and the thin film layer 3b may be reversed.
Further, for the purpose of improving the adhesion between the thin film moisture sensitive body 3 and the thin film electrodes 2a, 2b, a laminated structure in which a thin film layer 3a is further provided on the thin film layer 3b may be adopted. 3a and thin film layer 3b may be alternately laminated a plurality of times. Even in this case, it is preferable that the thin film layer 3b made of an oxide of an alkaline earth metal is located at least on the surface layer portion of the thin film moisture sensitive body 3.

The present invention will be described in more detail with reference to the following specific examples. Example 1 A thin film Pt electrode having a thickness of 2000 angstroms was formed on a 4 mm × 4 mm × 0.2 mm alumina substrate formed by a doctor blade method and sintered at 1500 ° C. by a high frequency magnetron sputtering method, and the other surface Similarly, a thin film Pt heater having a thickness of 4000 angstrom was formed by the high frequency magnetron sputtering method.

Next, a ZrO ₂ layer and an MgO layer were formed on the thin film Pt electrode by a high frequency magnetron sputtering method to a thickness of 2 μm and a thickness of 500 angstrom, respectively.

Subsequently, 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 thin film Pt film was formed on the MgO layer of the above laminated body by a sputtering method, and then a resist material was patterned in a mesh shape and plasma etching was performed. Then, the resist material was peeled off to form a mesh-like thin film Pt electrode (thickness: 3000 angstrom, open area ratio: 20%, pore diameter: 1 to 10 μm). Lead wires of Pt were respectively connected to both Pt electrodes and the thin film heater.

A circuit shown in FIG. 2 was formed using the obtained humidity sensor element. Here, the DC power source E was used as the power source of the heater 5 in the humidity sensor element 10. The signal power supply 11 is AC 70Hz, 7V, and the signal detection load resistance R is
It was set to 500 kΩ. An AC power source is used as the signal power source 11 in order to prevent polarization of adsorbed water.

Using the circuit of FIG. 2, the operating temperature of the humidity sensor element 10 is set to 450 ° C. and the humidity in the atmosphere around the humidity sensor element 10 is changed, and the hole diameter (1 to 10 μm) of the upper electrode and the humidity are changed. The relationship with the detection sensitivity S was investigated.
The results are shown in Table 1.

Here, the humidity detection sensitivity S is 10 ° C.
Impedance R ₁ of the humidity sensor element 10 at 40% relative humidity (absolute humidity 2.92 g / kg) and humidity sensor element 10 at 80% relative humidity (absolute humidity 24.64 g / kg) at 35 ° C.
The impedance was calculated from the impedance R _{2 of} S = log (R ₁ / R ₂ )

Similarly, the hole diameter of the upper electrode (1 to 1)
0 μm) and the response time of the humidity sensor element 10, and the hole diameter (1 to 10 μm) of the upper electrode and the humidity sensor element 1
The relationship with the element short circuit rate of 0 was investigated. The results are shown in Table 1.

Here, the element short-circuit rate indicates the rate of occurrence of 100 humidity sensor elements 10 which have cracks caused by defects in the electrodes and which have not fulfilled the function of the electrodes, when 100 holes are measured for each hole diameter.

Table 1 Pore size (μm) Humidity sensitivity S Response time (sec) Element short circuit rate (%) 0.3 0.301 170 0 1 0.325 130 0 2 0.330 110 0 3 0.328 100 0 5 0.332 100 0 10 0.330 100 0 15 0.332 100 0.2 20 0.331 100 0.2

From Table 1, the humidity sensor element of this embodiment is
Good humidity detection sensitivity S was obtained when the hole diameter of the upper electrode was 1 μm or more. Further, when the hole diameter of the upper electrode is 1 μm or more, the response of the humidity sensor element becomes fast, and it is particularly remarkable when it is 2 μm or more. Furthermore, the hole diameter of the upper electrode is 1
It can be seen that a short circuit does not occur up to 0 μm, but a short circuit occurs when it exceeds that.

From the above results, the hole diameter of the upper electrode is 1 to 1
It can be seen that in the range of 0 μm, the humidity sensor element shows good humidity sensitivity without short-circuiting and has good response characteristics. In addition, in the case of the pore diameter in this range, the manufacturing yield was also good.

Example 2 A humidity sensor element having the same structure as that used in Example 1 except that the porosity of the upper electrode was changed to 5 to 50% was produced. The hole diameter of the upper electrode was 5 μm.

Using the obtained humidity sensor element, 450
The relationship between the aperture ratio and the humidity detection sensitivity S, the response time, and the element short-circuit rate at the element temperature of ° C was investigated. The results are shown in Table 2.

Table 2 Porosity (%) Humidity sensitivity S Response time (sec) Element short circuit rate (%) 3 0.271 220 0 5 0.288 160 0 10 0.320 120 0 20 0.332 100 0 30 0.330 100 0 50 0.329 100 0 60 0.332 100 0.15 70 0.326 100 0.6

From Table 2, the humidity sensor element of this embodiment is
When the open area ratio of the upper electrode was 5% or more, good humidity detection sensitivity S was obtained, and particularly when it was 10% or more, it was better. Further, when the open area ratio of the upper electrode was 5% or more, the response of the humidity sensor element became fast, and it was particularly remarkable at 10% or more. Further, it is understood that short circuit does not occur until the open area ratio of the upper electrode is 50%, but short circuit occurs when the open area rate exceeds 50%.

From the above results, the open area ratio of the upper electrode is 5 to 5.
It can be seen that in the range of 50%, the humidity sensor element shows good humidity sensitivity without short-circuiting and has good response characteristics. In addition, in the case of the pore diameter in this range, the manufacturing yield was also good.

[0058]

As described in detail above, the humidity sensor element of the present invention has a structure in which a mesh-shaped electrode is formed on a thin film moisture sensitive body, so that moisture easily reaches the moisture sensitive body,
Good humidity detection sensitivity can be obtained even in a high temperature state of 200 ° C. or higher where the amount of adsorbed water is small.

[Brief description of drawings]

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

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

[Explanation of symbols]

 1 Substrate 2 Thin Film Electrode 3 Thin Film Moisture Sensitive Body 3a First Thin Film Layer 3b Second Thin Film Layer 4 Mesh Thin Film Electrode 5 Thin Film Heater 6 Lead Wire 10 Humidity Sensor Element 11 Signal Power Supply

Claims (2)

[Claims] 1. A structure which is always operated at a high temperature of 200 ° C. or higher and has a structure in which a first thin film electrode, a thin film moisture sensitive material and a second thin film electrode are sequentially laminated on a ceramic substrate. In the humidity sensor element, the second thin film electrode is mesh-shaped. 2. The humidity sensor element according to claim 1, wherein the second thin film electrode has a hole diameter of 1 to 10 μm and a diameter of 5 μm.
A humidity sensor element having a porosity of ˜50%.