JPH0643978B2 - Gas sensor and manufacturing method thereof - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a gas sensor provided with a metal oxide thin film having semiconductor characteristics and a method for manufacturing the same.

2. Description of the Related Art Semiconductor Gas Sensors currently used are mainly manufactured by sintering. However, since the manufacturing method by sintering has complicated processes and includes various fluctuation factors that affect the performance of the product, it is difficult to say that it is satisfactory in terms of product reliability, stability, durability, etc. . Moreover, the product by sintering is
Since the size cannot be kept below a certain level, there is also a drawback that the sensitivity is low. Development of thin-film type semiconductor sensors to replace sintered products is also in progress, but no practical alternatives to sintered products have been obtained.

One of the reasons why thin film semiconductor sensors are difficult to put into practical use is
Generally, the thin film type semiconductor sensor is extremely excellent in hydrogen detection ability, but it can be mentioned that methane can hardly be detected. Therefore, for example, by oxidizing a substrate made of silicon is formed an insulating film made of SiO _2, a method of forming a SnO ₂ film doped with P _t thereon (Japanese Patent Application No. Sho 54-240
94)), and a method of doping P or B into a silicon substrate on which an SiO ₂ insulating film is formed (JP-A-57-1784).
However, since it is difficult to uniformly dope the dopant atoms, it is difficult to obtain a desired effect.

Means for Solving the Problems The present inventor has conducted various experiments and studies in view of the current state of the art as a result, and as a result, it is possible to use at least one of a metal and a metal oxide as a vapor deposition material source. Evaporation method (PV
When a vapor deposition layer is formed on a substrate under specific conditions according to D), a thin film having a specific crystal structure is formed at the interface to be in contact with the gas phase. It has been found that not only does it exhibit excellent properties as a sensor for hydrocarbons such as methane, ethane, propane and butane, and gas phase components such as oxygen. That is, the present invention provides the following gas sensor element and a method for manufacturing the same.

I. In a metal oxide thin film gas sensor that performs gas detection by using impedance change of a metal oxide thin film formed on the surface of an insulating substrate, (i) the metal oxide thin film is perpendicular to or almost perpendicular to the surface of the insulating substrate. Is formed as columnar crystals having a diameter of 0.001 to 10 μm in the direction, and (ii) the average length of non-fused portions between adjacent columnar crystals is 30% or more of the columnar crystal length. Gas sensor; [however, the crystal orientation and crystallinity of the gas detection surface of the oxide thin film gas sensor in which the metal is tin is determined by irradiating CuK with X
The strongest diffraction line intensity in the case of line diffraction is I ₁ , and the second,
When the third and fourth strongest diffraction line intensities are I ₂ , I ₃ and I ₄ , respectively, the line intensity of (a) (211) plane or (110) plane is the strongest, and I ₂ / I ₁ ≦ 0.6 and the full width at half maximum of I ₁ is 0.58 or more, or (b) I ₁ is the line intensity of the (110) plane or the (101) plane and I ₂ is
It is the line intensity of the (101) plane or the (110) plane, and I ₂ / I ₁ ≧ 0.
5, I ₃ / I ₂ <0.6 and the half width of I ₁ is 0.54
Or more, (c) I ₁ is the line intensity of the (110) plane or the (211) plane, and I ₂ is
It is the line intensity of the (101) plane or the (110) plane, and I ₂ / I ₁ ≧ 0.
5, I ₃ / I ₂ <0.6 and the full width at half maximum of I ₁ is 0.58
Or more, (d) I ₁ , I ₂ and I ₃ are line intensities of any of (110) plane, (101) plane and (211) plane, and I ₃ / I ₁ ≧
0.5, I ₄ / I ₃ <0.6, and the half-width of I ₁ is 0.
61 or more, or (e) I ₁ , I ₂ , I ₃ and I ₄ are (110) plane and (1
The linear intensity of any one of the (01) plane, the (211) plane, and the (301) plane, and I ₄ / I ₁ ≧ 0.5, I ₅ / I ₄ <0.6, and I
₁ is 0.73 or more, or (f) I ₁ is the line intensity of the (301) plane, and I ₂ / I ₁ ≦ 0.
6 and the half width of I ₁ is 0.60 or more].

II. A metal or metal oxide is used as a target, and the insulating substrate temperature is set to 5 in an inert sputter gas containing oxygen.
0 to 700 ° C., sputter gas pressure 1 × 10 ⁻¹ to 1 × 10
^A method for producing a metal oxide thin film gas sensor, which comprises forming a metal oxide thin film on an insulating substrate at a rate of 10 Å / min or more under the condition of ^-4 Torr.

As the substrate of the gas sensor of the present invention, a silicon substrate, a ceramics substrate, a glass substrate or the like is used. When a silicon substrate is used, the surface of the silicon substrate is S
forming an insulating layer of iO _2. Examples of the metal oxide provided as a thin film semiconductor layer on the substrate include tin oxide, zinc oxide, tungsten oxide, titanium oxide, iron oxide, magnesium oxide, molybdenum oxide, niobium oxide, tantalum oxide, vanadium oxide, zirconium oxide. , Chromium oxide and the like. In order for these oxides to exhibit the characteristics as a semiconductor, it is necessary that some of the oxygen atoms are lost from the complete oxide, that is, a form having a lattice defect.
The existence of such lattice defects can be confirmed by measuring the conductivity.

The most important thing in the gas sensor of the present invention is that the metal oxide thin film has an average representative diameter of 0.001 μm in a direction perpendicular to or almost perpendicular to the insulating substrate surface (about 0 to 30 degrees with respect to the normal to the substrate surface).
It is formed as an aggregate structure of columnar crystals of m to 10 μm, and adjacent columnar crystals must not be completely fused or adhered to each other, and observed with an electron microscope having a resolution of 10Å. In addition, the average length of the non-fused portion or non-adhered portion between the columnar crystals is at least 3 of the columnar crystal length.
It is 0%. Even if apparently columnar crystals are formed, if the separation between the crystals does not satisfy the above requirements, the sensitivity to gas is extremely low,
It cannot be a good gas sensor. Further, it goes without saying that when the metal oxide does not exhibit any columnar crystal structure at all, it becomes less sensitive to gas and cannot be used as a gas sensor. The average value of the ratio of the height of columnar crystals / average representative diameter in the metal oxide in the present invention is preferably 5 or more, and particularly preferably 10 or more. When this ratio is less than 5, the separation state between crystals is often insufficient, and the sensitivity of the gas sensor tends to decrease. In the gas sensor of the present invention, depending on the manufacturing conditions, a dendrite structure (that is, dendrite microcrystals) may be formed in the columnar crystals of the metal oxide thin film, and in this case, the sensitivity as a gas sensor is high. Was further improved.

As is clear from the examples described below, the columnar crystals in the metal oxide thin film in the gas sensor of the present invention have extremely uniform diameters and lengths. Therefore, for example, the average diameter is 0.1 μm
In this case, 70% or more of the crystals are within the range of 0.1 ± 0.02 μm, and there are almost no crystals having a diameter of 1/2 or less or 2 times or more of the average diameter.

The gas sensor of the present invention is manufactured as follows. When a ceramic plate and a glass plate are used as the substrate, an insulating layer made of SiO ₂ is formed directly on the surface of the substrate when a silicon plate is used, and then a metal oxide is formed by the PVD method. A thin film semiconductor layer is formed. The conditions during the vapor deposition operation may vary depending on the vapor deposition method, the type of substrate, the type of metal and metal oxide as the source of the vapor deposition material, and the like.
In the case of the sputtering method belonging to the D method, the substrate temperature is 5
The atmospheric pressure of the protective gas such as Ar, He, and N ₂ containing oxygen is from 1 × 10 ⁻¹ to 1 × 10 ⁻⁴ Torr, and the film forming rate is 10 Å / min or more. When the substrate temperature is lower than 50 ° C., the metal oxide composed of lumpy fine particles that do not show a clear crystal structure is formed on the substrate in close contact with each other, so that a gas sensor cannot be obtained. When the substrate temperature is higher than 700 ° C., solid diffusion between crystals and recrystallization progress during the formation of a thin film of metal oxide, and a granular structure similar to a lump or granular solid produced by melting of a metal. Are formed, and the columnar structure is not formed. Inert gas pressure is 1 × 1
When it is ^0-4 Torr or less, fine fibrous crystals having a high density and a fibrous shape are obtained, and the sensitivity as a gas sensor becomes insufficient. Even within the atmospheric gas pressure range adopted in the present invention, in the case of 1 × 10 ^{−1 to} 9 × 10 ⁻³ Torr, the columnar structure of the metal oxide crystals formed on the substrate surface is the most prominent. It has high sensitivity to both hydrocarbons such as methane and hydrogen. On the other hand, the atmospheric gas pressure is 9 × 10 ^-3 to 1
In the case of ^{x10 -4} Torr, the crystal diameter decreases, the sensitivity to hydrocarbons such as methane decreases significantly, and the sensitivity to hydrogen tends to be maximized. When the film formation rate of the metal oxide is less than 10 Å / min, a thin film composed of lumpy ultrafine particles adhered to each other can be obtained, which has poor adhesion to the substrate and is high as a gas sensor. It has no sensitivity. When the film formation rate is 10 to 400 Å / min, the above-mentioned dendrite structure is easily formed in the clear columnar structure, and good sensitivity to both hydrocarbons and hydrogen is exhibited. When the film formation rate exceeds 400 Å / min, the dendrite structure is hardly formed, and the columnar structure becomes slightly close to the lump structure, and the sensitivity to hydrogen is good, but the sensitivity to hydrocarbons is wide. descend. The high frequency output and the distance between the target and the substrate mainly contribute to the film formation rate. Therefore, in the present invention, it is preferable that the high frequency output is about 10 W to 10 KW and the target-substrate distance is about 10 mm to 500 mm.

As the vapor deposition material source, tin, zinc, tungsten, titanium, iron, magnesium, molybdenum, niobium, tantalum, zirconium, titanium oxide, iron oxide, tin oxide, zinc oxide, tungsten oxide, magnesium oxide, molybdenum oxide, niobium oxide, Metals and metal oxides such as tantalum oxide, zirconium oxide and chromium oxide are used.

When using a metal such as tin as a vapor deposition material source, it is preferable to set the oxygen / inert gas ratio to about 1/5 to 5/1 (volume ratio), while using an oxide such as tin oxide. In this case, the oxygen / inert gas ratio is preferably about 1/50 to 1/2 (volume ratio).

If necessary, the gas sensor element of the present invention formed by vapor deposition can be further annealed to improve its stability and durability. The annealing treatment is carried out, for example, by holding in a dry air atmosphere at 500 ° C. for about 4 hours.

When the above element is used as a gas sensor, for example, a platinum electrode may be formed on the thin film semiconductor layer and a predetermined lead wire may be connected thereto according to a conventional method.

Effects of the Invention According to the present invention, the following effects are achieved.

(1) A thin-film semiconductor gas sensor can be obtained without requiring a doping step and using a catalyst.
The obtained gas sensor has the ability to detect not only hydrogen but also hydrocarbons such as methane and oxygen. Further, its gas sensitivity is extremely high, and even a trace amount of gas can be detected.

(2) Since the doping is not performed and the catalyst is not used, there is no deterioration of the sensor due to re-diffusion of the dopant, poisoning of the catalyst, sintering, etc., so that the durability of the sensor is improved.

(3) Since each polycrystal has an independent and thermally stable structure, it is possible to prevent sintering of the metal oxide, which is the main cause of deterioration over time of conventional sensors. Also improves the durability of the sensor.

Examples Hereinafter, the features of the present invention will be further clarified by examples.

Example 1 A silicon wafer (2 mm × 3 mm) as a substrate was heated at 1000 ° C. for 2 hours in an atmosphere containing oxygen and water vapor to form an SiO ₂ insulating layer on the surface, and then parallel plate type high frequency magnetron sputtering was performed. Using the apparatus, a vapor deposition operation was performed using a SnO ₂ sintered body as a target material. The conditions during vapor deposition are as shown in Table 1 below.

Table 1 High-frequency output Approx. 180 W Atmosphere Ar1.8 × 10 ^-2 torr + O ₂ 0.2 × 10 ^-2 torr Substrate temperature 250 ° C Substrate-target distance 50 mm Film formation rate Approx. 300Å / min Tin oxide thin film thus obtained ( The scanning electron microscope photograph (100,000 times) of the cross section of 11,000Å) is shown in Fig. 1, and the transmission electron microscope photograph by the ultrathin section method (100,000 times) is shown in the second.
Shown as a diagram.

A platinum electrode (thickness: about 1 mm) was formed on the vapor-deposited thin film-formed product obtained above by sputtering to obtain the gas sensor element shown in FIG. In FIG. 3, (1) shows a silicon wafer, (3) shows an SiO ₂ insulating layer, (5) shows a tin oxide thin film layer, and (7) shows a platinum electrode.

Then, the above gas sensor element is installed in a cell in an electric furnace, and after being annealed by keeping it at 500 ° C. for 4 hours while flowing dry air, (a) dry air, (ii) methane-containing dry air or (iii) The hydrogen-containing dry air was flowed, and the electrical resistance between the electrodes at each temperature was measured. The result is as shown in FIG. As is clear from FIG. 4, it is apparent that the gas sensor of the present invention has not only hydrogen detection ability but also methane detection ability.

In FIG. 4 and the graphs showing the results of each of the following examples, each curve shows the results for the following gases.

Curve (I) ... Dry air, Curve (II) ... Dry air containing 0.35% methane, Curve (III) ... Dry air containing 0.35% hydrogen.

Example 2 A gas sensor was obtained in the same manner as in Example 1 except that the conditions during vapor deposition were changed as shown in Table 2 below.

Table 2 High-frequency output Approx. 180 W Atmosphere Ar5 × 10 ^-3 torr + O ₂ 1 × 10 ^-3 torr Substrate temperature 250 ° C Substrate-target distance 50 mm Film formation rate approx. 150 Å / min Obtained tin oxide thin film (9000 Å) A scanning electron micrograph (50,000 times) of the cross section is shown in FIG.

The performance as a gas sensor is shown in FIG.

The gas sensor obtained in this example with an atmospheric gas pressure of 6 × 10 ⁻³ Torr has a sensitivity to methane (curve II).
But shows little sensitivity to hydrogen (curve III)
Is clearly very high.

Example 3 A gas sensor was obtained in the same manner as in Example 1 except that the conditions during vapor deposition were changed as shown in Table 3 below.

Table 3 High-frequency output Approx. 300W Atmosphere Ar1.8 × 10 ^-2 torr + O ₂ 0.2 × 10 ^-2 torr Substrate temperature 250 ° C Substrate-target distance 30mm Film formation rate approx. 450Å / min Obtained tin oxide thin film (10000Å Fig. 7 shows a transmission electron micrograph (40,000 times) by the ultrathin section method in Fig. 7).

The performance as a gas sensor is shown in FIG.

The gas sensor of this example formed at a film forming rate of 450 Å / min has good sensitivity to hydrogen (curve II).
I), sensitivity to methane is low (curve II).

Comparative Example 1 A gas sensor was manufactured in the same manner as in Example 1 except that the conditions during vapor deposition were changed as shown in Table 4 below.

Table 4 High frequency output Approx. 50W Atmosphere Ar1.8 × 10 ^-3 torr + O ₂ 0.2 × 10 ^-3 torr Substrate temperature 20 ° C Substrate-target distance 50mm Film formation rate 50Å / min Obtained tin oxide thin film (5000Å) Observation of the ultrathin section of No. 2 by a transmission electron microscope revealed that SnO ₂ did not show a clear crystal structure because the substrate temperature during vapor deposition was low.

The performance as a gas sensor is shown in FIG. Not only sensitivity to methane (curve II), but also sensitivity to hydrogen (curve II)
It is clear that III) is also low.

[Brief description of drawings]

1, 2, 5, and 7 are electron micrographs showing the crystal structure of the SnO ₂ thin film formed by the method of the present invention, and FIG. 3 is a schematic view showing an example of the gas sensor of the present invention. Sectional drawings, FIG. 4, FIG. 6 and FIG. 8 are graphs showing the gas detecting ability of the gas sensor according to the embodiment of the present invention, and FIG. 9 is a graph showing the gas detecting ability of the gas sensor according to the comparative example. (1) …… Silicon substrate (3) …… SiO ₂ insulating layer (5) …… Tin oxide thin film layer (7) …… Platinum electrode

Front Page Continuation (72) Inventor Osamu Okada 5-1-1 Hirano-cho, Higashi-ku, Osaka City, Osaka Prefecture (56) References JP-A-55-27925 (JP, A) JP-A-58-83245 (JP, A) JP-A-61-191954 (JP, A)

Claims (4)

[Claims] 1. A metal oxide thin film gas sensor for detecting gas by utilizing impedance change of a metal oxide thin film formed on the surface of an insulating substrate, comprising: (i) the metal oxide thin film on the surface of the insulating substrate. It is formed as a columnar crystal having a diameter of 0.001 to 10 μm in a vertical or almost vertical direction, and (ii) the average length of the non-fused portion between adjacent columnar crystals is 30% or more of the columnar crystal length. A gas sensor characterized by: [However, the crystal orientation and crystallinity of the gas detection surface of the oxide thin film gas sensor in which the metal is tin is determined by irradiating CuK to the X direction.
The strongest diffraction line intensity in the case of line diffraction is I ₁ , and the second,
When the third and fourth strongest diffraction line intensities are I ₂ , I ₃ and I ₄ , respectively, the line intensity of (a) (211) plane or (110) plane is the strongest, and I ₂ / I ₁ ≦ 0.6 and the full width at half maximum of I ₁ is 0.58 or more, or (b) I ₁ is the line intensity of the (110) plane or the (101) plane and I ₂ is
It is the line intensity of the (101) plane or the (110) plane, and I ₂ / I ₁ ≧ 0.
5, I ₃ / I ₂ <0.6 and the half width of I ₁ is 0.54
Or more, (c) I ₁ is the line intensity of the (110) plane or the (211) plane, and I ₂ is
It is the line intensity of the (101) plane or the (110) plane, and I ₂ / I ₁ ≧ 0.
5, I ₃ / I ₂ <0.6 and the full width at half maximum of I ₁ is 0.58
Or more, (d) I ₁ , I ₂ and I ₃ are line intensities of any of (110) plane, (101) plane and (211) plane, and I ₃ / I ₁ ≧
0.5, I ₄ / I ₃ <0.6, and the half-width of I ₁ is 0.
61 or more, or (e) I ₁ , I ₂ , I ₃ and I ₄ are (110) plane and (1
The linear intensity of any one of the (01) plane, the (211) plane, and the (301) plane, and I ₄ / I ₁ ≧ 0.5, I ₅ / I ₄ <0.6, and I
₁ is 0.73 or more, or (f) I ₁ is the line intensity of the (301) plane, and I ₂ / I ₁ ≦ 0.
6 and the half width of I ₁ is 0.60 or more]. 2. The gas sensor according to claim 1, wherein the average value of the ratio of the length of the columnar crystals / the average representative diameter is 5 or more. 3. A metal or metal oxide is used as a target, the insulating substrate temperature is 50 to 700 ° C., and the sputtering gas pressure is 1 × 10 ^-1 in an inert sputtering gas containing oxygen.
10 angstroms / under 1 x 10 ^-4 torr
A method for producing a metal oxide thin film gas sensor, comprising forming a metal oxide thin film on an insulating substrate at a speed of not less than a minute. 4. The method for producing a gas sensor according to claim 3, wherein the target is SnO ₂ and the volume ratio of oxygen / inert gas is 1/2 to 1/50.