JP2001305089A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To heighten the moisture resistance of a gas sensor with a metallic oxide semiconductor as a gas detector. SOLUTION: The metallic oxide semiconductor serving as the gas detector is composed of tin oxide, and both pentavalent transition metal and trivalent transition metal are added to the semiconductor to allow the stable adsorption of oxygen in the atmosphere to the surface of the semiconductor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor using a thin-film or thick-film metal oxide semiconductor for detecting flammable gas including methane gas for city gas, propane gas and CO gas. More particularly, the present invention relates to a material composition of a metal oxide semiconductor serving as a gas detector aimed at improving the moisture resistance.

[0002]

2. Description of the Related Art Generally, a semiconductor gas sensor detects the presence or absence of a gas by utilizing a change in the resistance value of a metal oxide semiconductor. When the resistance value of a gas sensor using such a metal oxide semiconductor as a gas detector when there is no detection gas is Ra and the resistance value when there is a detection gas is Rg, Ra / Rg
Is used as the sensor sensitivity, and the sensor sensitivity of a gas sensor using a metal oxide semiconductor as a gas detector is affected by moisture in the atmosphere. Oxygen in the atmosphere is negatively adsorbed as O ^2-on the surface of the metal oxide semiconductor as a gas detector at a high temperature, thereby reducing the number of conduction electrons in the metal oxide semiconductor. Has a high resistance Ra,
When the humidity of the atmosphere increases, the moisture in the air is a hydroxyl group OH ^-
As the amount of oxygen adsorbed on the oxygen adsorption site increases and the number of oxygen that can be adsorbed by the metal oxide semiconductor decreases, the resistance Ra of the metal oxide semiconductor in the air decreases. On the other hand, when the detection gas is present, the oxygen adsorbed on the metal oxide semiconductor by the gas is to be desorbed, and the humidity is high hydroxyl OH ^- in many adsorption amount, the amount of that amount adsorbed oxygen In addition to the decrease, the hydroxyl group adsorbed on the metal oxide semiconductor is not desorbed by the detection gas, so that the decrease in the resistance Rg of the metal oxide semiconductor in the detection gas is also small and a relatively large resistance is obtained. As shown. Therefore, the value of the sensor sensitivity Ra / Rg decreases, and the sensitivity of the gas sensor deteriorates.

In order to solve such a problem of moisture resistance as a gas sensor of a metal oxide semiconductor, hitherto,
Means have been taken to form various moisture-resistant coatings on the sensor, but a sufficient effect has not always been obtained.

[0004]

SUMMARY OF THE INVENTION According to the present invention, in order to enhance the moisture resistance of a gas sensor using a metal oxide semiconductor as a gas detector as described above, oxygen in the atmosphere is stably applied to the surface of the metal oxide semiconductor. It is an object to provide a gas sensor using a metal oxide semiconductor that can be adsorbed as a gas detector.

[0005]

In order to solve the above-mentioned problems, the present invention provides a gas sensor using a metal oxide semiconductor as a gas detector, wherein the metal oxide semiconductor serving as the gas detector is a pentavalent transition metal. characterized in that it comprises a M ₁ a trivalent transition metal M ₂ (the invention of claim 1).

In the above-mentioned gas sensor, the metal oxide semiconductor is a pentavalent transition metal M ₁ and at least one of vanadium, niobium and tantalum and a trivalent transition metal M 1.
It is characterized in that chromium is included as ₂ (invention of claim 2).

Further, in the gas sensor according to claim 1 or 2, the metal oxide semiconductor is a pentavalent transition metal M
₁ in a total amount of 0.5 to 5.0 atomic%, and chromium in an atomic ratio to a pentavalent transition metal (Cr / M ₁ ) of 0.5 to 0.5 atomic%.
2.0 is included (the invention of claim 3).

Furthermore, in the gas sensor according to any one of the first to third aspects of the present invention, the metal oxide semiconductor is formed of a thin or thick metal oxide semiconductor. 4 invention).

As described above, by including a pentavalent and trivalent transition metal in the metal oxide semiconductor serving as a gas detector, the surface of the metal oxide semiconductor can be maintained without significantly changing the carrier concentration of the metal oxide semiconductor. The bonding force between the surface of the metal oxide semiconductor and oxygen can be increased by the bonding force between the oxygen and the hydroxyl group, thereby improving the moisture resistance of the gas sensor.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Tin oxide (SnO ₂ ) is used as a metal oxide semiconductor, and Sn on the surface of the tin oxide is changed to Sc.
Bonding force between surface atoms and oxygen and between surface atoms and hydroxyl groups when substituted with a transition metal from (scandium) to Cu (copper) (evaluation based on "effective covalent charge" indicating the degree of covalent bonding) Was calculated by the DV-Xα simulation method. The result is shown in FIG. In addition,
The DV-Xα simulation method is described in Hirohiko Adachi, "First Calculation of Electronic Structure" (published by Kyoritsu Shuppan, 1998) and Hirohiko Adachi, "Introduction to Quantum Materials Chemistry" (1991)
(Published in the year Kyoritsu Shuppan).

FIG. 1 shows the type of transition metal (surface impurity) substituted for Sn of SnO _{2 on} the horizontal axis, and the value of effective covalent charge on the vertical axis. The broken line O indicates the bonding force between the surface atoms of SnO ₂ and oxygen, and the broken line OH indicates the bonding force between the surface atoms of SnO ₂ and the hydroxyl groups.

As is apparent from FIG. 1, vanadium (V) and chromium (Cr) among the substituted transition metals have a higher binding force between the surface atoms and oxygen, and a lower bond between the surface atoms and the hydroxyl group than the other substituted transition metals. It can be seen that the difference from the bonding force is significant. This is because, when the substituted transition metals are V and Cr, when the effective binding charge is divided into a binding component and an anti-binding component, the binding component is particularly greatly increased when the surface atom is bonded to oxygen.

[0013] The tin oxide, which is the parent material, is Sn
In contrast to (4+), vanadium is a donor type with V (5+), and chromium is an acceptor type with Cr (3+). Therefore, by simultaneously containing these with almost equivalent atomic weights, the carrier concentration of the parent is increased. Without changing, it is possible to increase the binding force of the adsorbed oxygen.

Embodiment 1 Next, an embodiment in which the present invention is applied to a gas sensor having a thin film structure based on such a principle will be described. FIGS. 2 and 3 show the configuration of a gas sensor according to Embodiment 1 of the present invention.

In these figures, reference numeral 1 denotes a substrate made of Si (silicon), on which a thermally oxidized insulating film 2 is formed, and a pair of Pts at a predetermined interval on the insulating film 2.
A thin-film electrode layer 3 made of (platinum) is provided, and at least one of pentavalent transition metals nadium (V), niobium (Nb), and tantalum (Ta) is provided across the electrode layers 3 and 3. Gas detector layers 5 made of a tin oxide (SnO ₂ ) film doped with a seed and chromium (Cr), a trivalent transition metal, are joined via contact layers 4, 4. Further, a selective combustion layer 6 is formed on the gas detector layer 5 for improving the sensitivity to the detected gas and for improving the gas selectivity.

Such a gas sensor is manufactured as follows. The substrate 1 is made of Si having a thickness of 400 μm on which a thermal oxide insulating film 2 is formed to a thickness of 0.5 μm. After the substrate 1 is degreased with an organic solvent or the like, it is washed with a rinser dryer. After that, Pt was applied by RF magnetron sputtering equipment.
The electrode layer 3 is formed. At this time, the film forming conditions are argon (Ar) gas 1 Pa, substrate temperature 300 ° C., and RF power 2 W / cm ² , and the film thickness is 200
Finish to nm.

After forming the Pt electrode layer, an ohmic contact layer 4 made of SnO ₂ doped with antimony (Sb) is formed thereon. This film formation is performed by reactive sputtering using an RF magnetron sputtering device. Using SnO ₂ having 1% by mass of Sb as a target, a substrate temperature of 30 in Ar + O ₂ gas at a pressure of 2 Pa.
The film is formed to a thickness of 100 nm under the conditions of 0 ° C. and RF power of 2 W / cm ² . Thereby, the contact layer 4 becomes Sb
The doped SnO ₂ film.

Next, an SnO ₂ film doped with V of the pentavalent transition metal M ₁ and Cr of the trivalent transition metal M ₂ is applied to the gas detector layer 5.
Formed by a sputtering apparatus. At this time, the target was V of the pentavalent transition metal M ₁ at 2 atomic% and Cr of the trivalent transition metal at the ratio of the number of atoms to V (Cr /
V) is SnO ₂ containing 1.0. Using this target, the pressure of the A + O ₂ gas is 2 Pa and the substrate temperature is 15 Pa.
The film was formed at 0 to 300 ° C. and RF power of 2 W / cm ² until the film thickness became 500 nm. As a result, the gas detector layer 5 composed of a SnO ₂ thin film in which V of the pentavalent transition metal M ₁ and Cr of the trivalent transition metal M ₂ are doped at substantially the same atomic weight is formed.

Further, on the gas detector layer 5, an alumina thin film containing palladium (Pd) to be a selective combustion layer 6 is formed. The formation of the alumina thin film containing Pd is as follows.
Vacuum evaporation is performed using Pd-added alumina powder as an evaporation source. The film thickness was 1000-5000 °.

In the gas sensor having a thin film structure formed by such a method, an appropriate heater is formed on the back surface of the substrate 1 or
Alternatively, it is put on a separate small heater and operated.
In order to measure the resistance of the gas detector layer 5, an appropriate Pt wire 7 is connected to the Pt electrode layer 3, a voltage is applied, and the Pt electrode layer 3 is connected to an ordinary resistance measuring circuit for use.

[0021] In a similar preparation method to Comparative Example 1 Example 1, the gas sensing layer film, S
Samples made of only nO ₂ , doped only with V, and doped only with Cr were produced as a group of Comparative Example 1 with respect to Example 1.

With respect to the gas sensors of Example 1 and Comparative Example 1 thus manufactured, the resistance change of the gas sensor was measured by changing the humidity in an atmosphere containing 0.2% of methane gas, and the humidity dependence of the gas detection sensitivity of the gas sensor was measured. FIG. 4 shows the results of determining the properties. The operating temperature of the gas sensor at this time is 4
The temperature was set to 00 ° C.

Referring to FIG. 4, the gas sensor of the comparative example made of only SnO ₂ shows a large humidity dependency, whereas the gas sensor of the first embodiment (SnO ₂
It is understood that the humidity dependency is greatly suppressed in the case where V and Cr are doped. The gas sensor of the comparative example doped only with V and the gas sensor of the comparative example doped only with Cr follow this.

Next, FIG. 5 shows the results of measuring the sensor sensitivity of the gas sensor of the comparative example using only SnO ₂ and the sensor sensitivity of the gas sensor of Example 1 of the present invention every month. From this result, the sensor sensitivity of the gas sensor of the comparative example is remarkably reduced in a relatively high humidity season (June to September), and shows a large seasonal dependency. It can be seen that the dependence is small. From this, it can be clearly understood that the gas sensor of the present invention suppresses the humidity dependency of the sensor sensitivity.

Embodiment 2 An embodiment in which the present invention is applied to a gas sensor having a thick film structure will be described.

FIGS. 6 and 7 show the configuration of a thick film gas sensor according to an embodiment of the present invention.

In these figures, reference numeral 10 denotes an insulating substrate made of alumina. Two electrodes 20 are provided on one main surface of the substrate 10 at an interval. The gas detector layer 50 is selectively formed on the substrate 10 and the electrode 20.
The oxidized combustion layer 60 is provided so as to completely cover the gas detector layer 50, and the lead wires 7 are respectively drawn out from the electrodes 20. An electrode 20A is formed on the other main surface of the substrate 10,
The heater 8 is selectively formed on the electrode 20A and the substrate 10, and lead lines 90 are respectively drawn from the electrode 20A.

The substrate 10 is made of an alumina substrate having a thickness of 0.5 m.
The heater 80 is formed by polishing to 3 mm in length, 3 mm in length, and 3 mm in width. The heater 80 is formed by a resistor made of ruthenium oxide.

The thick-film gas sensor thus configured is
It is manufactured by the thick film method as follows.

First, a platinum paste is screen-printed on each of two main surfaces of the substrate 10 in a predetermined electrode pattern, dried, and fired at a temperature of 1100 ° C. to form the electrodes 20 and 20A.

Next, the SnO ₂ powder was dried in dry air for 73 hours.
A heat treatment was performed at 0 ° C. for 2 hours, and after this treatment, SnO ₂
Vanadium oxide (V ₂ O ₅ ), niobium oxide (Nb)
₂ O ₅ ), at least one of tantalum oxide (Ta ₂ O ₅ ) powder and chromium oxide (Cr ₂ O ₃ ) powder,
At least one of Ta has a total amount of 0.5 to 5.0 atomic percent and Cr has a ratio of the number of atoms to the pentavalent transition metal M ₁ (Cr / M ₁ ) of 0.5 to 2.0. The mixture is added and mixed so as to be contained in SnO ₂ , and an appropriate amount of ethyl silicate, ethyl cellulose, carbitol or the like is added and kneaded to prepare a paste for a gas detector layer. The paste for the gas detector layer is screen-printed on a predetermined pattern of the gas detector layer 50 across the electrodes 20 on the substrate 10 and dried at 120 ° C. for 2 hours.

Further, an aqueous solution of chloroplatinic acid (H ₂ PtCl ₆ ) was added to the previously heated SnO ₂ powder so that the platinum content was 2% based on SnO ₂ , and the mixture was kneaded and dried. I do. By heating this powder at 600 ° C. for 3 hours, chloroplatinic acid (H ₂ PtCl ₆ ) is decomposed,
Platinum (Pt) is supported on the SnO ₂ powder. After the SnO ₂ powder carrying platinum is pulverized by a ball mill, ethyl silicate, ethyl cellulose, carbitol, etc. are added in an appropriate amount and kneaded to prepare a paste for an oxidized combustion layer. Then, a predetermined pattern of the oxidation combustion layer 60 is screen-printed and dried at 120 ° C. for 2 hours.

Subsequently, the printed gas detector layer 50 and the oxidized combustion layer 60 are heated and fired at 630 ° C. for 3 hours to complete the gas detector layer 50 and the oxidized combustion layer 60.

Finally, the electrode 20 on the other main surface of the substrate 10
A heater 80 made of ruthenium oxide is connected between A and 20A, and platinum electrodes 70 and 9 are connected to the electrodes 20 and 20A, respectively, to obtain a thick film gas sensor.

[0035] By the same creation method as Comparative Example 2 Example 2, which gas sensing layer is made of only SnO _2, V, Nb, those doped only at least one kind of Ta and Cr
A sample doped with only one was prepared as a comparative example 2 group with respect to the example 2.

When an experiment similar to that of Example 1 and Comparative Example 2 was performed on the gas sensors of Example 2 and Comparative Example 2, results similar to those of Example 1 and Comparative Example 1 were obtained. Was completed.

Example 3 In the same manner as in Example 1, the pentavalent transition metal V and 3
Thin-film gas sensors using SnO _{2 in} which the doping amount of the valence transition metal Cr was adjusted variously were prepared. The range of adjustment of the doping amounts of V and Cr is as shown in Table 1. For these gas sensors, the resistance Rg in a gas containing 2.2% of methane gas as a detection gas at 2.2% absolute humidity was used. And the resistance Ra in a gas (atmosphere) containing no methane gas were measured to determine the sensor sensitivity (Ra / Rg). Table 1 shows the results.

As is apparent from Table 1, the content of V is 0.5 to 5.0 atomic%, and the content of Cr is 0.5 to 2 at the ratio of the number of atoms to V (Cr / V). In the range of 0.0, the sensor sensitivity (Ra / Rg) becomes about 20 or more, and high sensor sensitivity can be obtained.

The pentavalent transition metal to be doped into SnO _{2 serving} as the gas detector layer in Examples 1 to 3 may be niobium (Nb) or tantalum (Ta) other than V, and these may be doped. For the gas sensor using SnO ₂ as the gas detector layer, experimental results having the same tendency as in the above example were obtained.

[0040]

[Table 1]

[0041]

According to the present invention, SnO _{2 serving} as a gas detector layer is provided with at least one of V, Nb, and Ta;
By using a metal oxide semiconductor containing Cr, the bonding force of adsorbed oxygen can be increased without greatly changing the carrier concentration of the base of the metal oxide semiconductor. A gas sensor with a small decrease in temperature can be obtained. Especially V, Nb, Ta
At least one of the above is contained in a total amount of 0.5 to 5.0 atomic%, and Cr has a ratio of the number of atoms to the pentavalent transition metal M ₁ (M ₁ / Cr) of 0.5 to 2.0. When the metal oxide semiconductor contained in the range is used for the gas detector, the effect of increasing the sensor sensitivity can be obtained.

[Brief description of the drawings]

FIG. 1 shows the effect of substituted transition metals on the adsorption power of SnO ₂ for oxygen and hydroxyl groups.

FIG. 2 is a plan view showing an embodiment of the thin-film gas sensor according to the present invention.

FIG. 3 is a longitudinal sectional view showing an embodiment of the thin-film gas sensor according to the present invention.

FIG. 4 is a diagram showing humidity dependency of sensor sensitivity of gas sensors according to Example 1 of the present invention and Comparative Example 1.

FIG. 5 is a diagram showing the seasonal dependence of the sensor sensitivity of the gas sensor according to Example 1 of the present invention and Comparative Example 1.

FIG. 6 is a plan view showing an embodiment of a thick film gas sensor according to the present invention.

FIG. 7 is a sectional view taken along line BB of FIG. 6;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Si substrate 3 Electrode layer 4 Contact layer 5 Gas detector layer 6 Selective combustion layer

Claims (4)

[Claims] 1. A gas sensor using a metal oxide semiconductor whose resistance value changes in response to a gas to be detected as a gas detector, wherein the metal oxide semiconductor is made of tin oxide and the pentavalent transition metals M ₁ and M 3 the gas sensor which comprises a valence transition metal M _2. 2. The gas sensor according to claim 1, wherein the metal oxide semiconductor comprises at least one of vanadium, niobium, and tantalum as a pentavalent transition metal M _1.
The gas sensor which comprises chromium as valence transition metal M _2. 3. The gas sensor according to claim 1, wherein the metal oxide semiconductor contains a pentavalent transition metal M ₁ in a total amount of 0.5 to 5.0 atomic percent, and contains chromium in an atom relative to the pentavalent transition metal M ₁ . the gas sensor characterized in that it comprises 0.5 to 2.0 by the ratio of the number (Cr / M _1). 4. The gas sensor according to claim 1, wherein said metal oxide semiconductor is formed of a thin or thick metal oxide semiconductor.