JPH07198648A - Gas detecting film and manufacture thereof - Google Patents

Abstract

PURPOSE:To enhance the stability of the electric characteristic of a gas detecting film against aging-effect by using SnO2 mainly having a tetragonal crystal structure and having another crystal structure the content rate of which is less than a specific rate, so as to give a property with which thermal strain can hardly be effected by a heat, to an SnO2 film having a multilayer structure. CONSTITUTION:SnO2 mainly has a tetragonal crystal structure and also has another crystal structure (cubic crystal or orthorhombic crystal structure) having a content rate less than 1/10 (the rate between a first peak measured by an X-ray diffraction process and belonging to the tetragonal crystal structure and a first peak belonging to other crystal structures such as cubic crystal structure or orthorhombic crystal structure is less than 1/10). The first peaks give a highest diffraction intensity. Further, it is preferable that SnO2 in the upper and lower layers of a multilayer structure has not an orientation. However, SnO2 in the lower layer may have a preferential orientation more or less. Having no orientation means such that the intensity ratio of the diffraction line in view of the plane indices of SnO2 is around the strengthening ratio when it is measured in an X-ray diffraction process.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a gas detection film for detecting the presence of a specific gas in an atmosphere and a method for producing the same.

[0002]

2. Description of the Related Art A metal oxide is used as a gas detection substance,
(1) A heater film is provided on the back surface of the metal oxide semiconductor via an electrode and an insulating substrate, or (2) a heater coil also serving as an electrode and an electrode is provided inside the metal oxide semiconductor, and the heater film and / or Alternatively, a gas sensor is known in which the resistance value of a metal oxide semiconductor heated by a heater coil changes due to gas adsorption on the surface.

A typical one of the gas sensors is a thin film gas sensor. As shown in FIGS. 1 (a) and 1 (b), the gas sensor film 51 and its resistance value change on one side of the heat resistant substrate 1 are schematically shown. The electrodes 41 and 42 for detecting the are formed, and the heater film 20 is formed on the opposite side. Note that (a) is a cross-sectional view and (b) is a perspective view. Also, 61 and 62 are power supply lines to the heaters, 71 and 7.
Reference numeral 2 represents a signal extraction line of the gas detection film 51.

On the other hand, FIG. 2 shows an outline of a typical gas sensor of another type. Here, a pair of coil electrodes 4 are shown.
A metal oxide sintered body (gas detection substance 52) having a 2 to 3 (mm) angle is held between 3 and 44, and a change in resistance value during this period is detected. In addition, this type of gas sensor
One of the pair of electrodes (for example, the electrode 43) also serves as a heater coil, which heats the gas detection substance 52. In the figure, 12 is a base and 8 is an electrode pin.

The type shown in FIG. 2 has a problem in responsiveness because it consumes a large amount of power and has a large heat capacity. On the other hand, the gas sensor of the type as shown in FIG. 1 has good power consumption and responsiveness because the gas detection substance is a thin film, but has a problem of large change over time. Causes of this include changes in crystal structure, increase in particle size, and surface contamination (surface coating). Driving (heating) of the gas sensor causes a decrease in adsorption area or a decrease in chemical activity due to the above causes. It has the drawback of causing changes.

By the way, since the gas detecting element detects gas by utilizing the adsorption / desorption reaction on the surface of the metal oxide,
The surface physical properties [particle size (specific surface area)] play a major role, but it is important to prepare a film having a stable crystal structure in order to determine the surface physical properties. Therefore, one of the inventors of the present invention previously applied for a “gas sensor” having a multi-layer structure in which the metal oxide semiconductor thin film has a different form, and is disclosed in JP-A-4-212048
Published as an issue.

[0007]

[Purpose] An object of the present invention is to provide SnO ₂ thin films having a multi-layered structure different in the above-described form with a property of being less susceptible to thermal strain, and to improve electrical characteristics and stability with time. .

[0008]

According to a first aspect of the present invention, gas detection is performed by utilizing a resistance change of a tin oxide semiconductor thin film formed on an insulating substrate and having a multi-layer structure having different forms. The gas detection film is characterized in that the crystal structure of SnO ₂ is mainly tetragonal and the content of other crystal structures (cubic and orthorhombic) is 1/10 or less.

The second aspect of the present invention is: (a) the upper layer has a fine particle structure, the lower layer has a columnar structure having a two-layer structure, and (b) the upper layer has a continuous film structure, and the lower layer has a fine particle structure. It is a continuous film structure and the lower layer is a two-layer structure having a columnar structure. (D) A multi-layer structure selected from the group consisting of a three-layer structure consisting of an upper layer having a continuous film structure, an intermediate layer having a fine particle structure, and a lower layer having a columnar structure. In the gas detecting film made of the tin oxide semiconductor thin film, the crystal structure of SnO ₂ is mainly tetragonal, and other crystal structures (cubic,
The content of orthorhombic crystals is 1/10 or less (the ratio of the first peak attributed to the tetragonal crystal to the cubic and the first peak attributed to the orthorhombic crystal by the X-ray diffraction method is 1/10 or less. ) It is related with the said gas detection film characterized by the above. In addition, the said 1st peak has shown the thing with the largest diffraction line intensity in JCPDS.

SnO ₂ in the upper and lower layers in the multilayer structure
Preferably has no orientation, but the lower layer may have some preferred SnO ₂ (101) orientation. Having no orientation means that the diffraction line intensity ratio from each plane index of SnO ₂ is JCP by the X-ray diffraction method.
This means that the value is close to the DS intensity ratio (bulk value).

A third aspect of the present invention relates to a method for producing a gas detecting film, characterized in that an amorphous layer containing SnO ₂ and / or SnO is produced and annealed at 400 ° C. or higher in an oxygen atmosphere for crystallization. . The content of β-Sn microcrystals in the amorphous layer is preferably 1% or less. The method for forming the amorphous layer is
Vapor deposition, sputtering, ion plating, CVD
Although a known thin film forming method utilizing a physical method such as a gas phase chemical reaction method or the like can be used, a “thin film vapor deposition apparatus” invented by Ota, one of the present inventors (Japanese Patent Publication No. 1-533).
51). Examples of the substrate include silicon, glass, quartz, ceramics such as alumina, Ni, Cu, Al, Cr and the like. In the case of conductivity, it is necessary to interpose an insulating film with the metal oxide semiconductor thin film. . Materials for electrodes in the gas sensor include Pt, Au, Pd, Rh, Ir, Ni, Cr, Mo,
Examples of the material of the heater film in the gas sensor include Pt, SiC, TaN ₂ , and NiC.
r, PtIr, PtRh, and the like.

The idea of the present invention is not limited to the type shown in FIG. 1, but the gas sensor having a micro heater structure proposed by the present inventors (Japanese Patent Laid-Open No. 167645/1989) has been proposed. It can also be applied to a gas detection film.

[0013]

EXAMPLES Next, the present invention will be described in more detail with reference to examples. As the thin film forming apparatus in this embodiment, the one disclosed in Japanese Patent Publication No. 1-53351 is used.

Example 1 Metallic tin (4N, manufactured by Kojundo Chemical Co., Ltd.) was used as a vapor deposition material, and oxygen gas was introduced into a vacuum chamber which had been evacuated to a level of 10 ⁻⁴ (Pa) in advance and 10 ⁻¹ ( In the state of (Pa) level, a current of about 70 (A) is applied to the filament to generate thermoelectrons, and a voltage of about 110 (V) is applied to the grid to generate plasma. A thermal oxide film [SiO ₂ (1μ
m)] with high resistance Si wafer [thickness t = 0.5 (m
m)] was used. The vapor deposition material was placed on a W (tungsten) boat for evaporation and evaporated by a resistance heating method, and the film formation rate was monitored by a crystal oscillator and controlled by an applied voltage in the resistance heating. The film thickness is 3000
Å Table 1 shows the results of analysis of the composition of the as-deposited SnO ₂ thin film produced by this film-forming method by the Mossbauer effect.

[0015]

[Table 1]

The result of the X-ray diffraction analysis at that time is shown in FIG. Composition in the as-deposited, when it is SnO ₂ thin film formed at a relatively slow deposition rate 5 Å / s depending on the deposition rate is approximately SnO ₂ 100% amorphous to increase the deposition rate SnO _2, SnO two- When the film formation rate is further increased by containing a kind of amorphous, β-S
n crystallites are also contained. When these samples are annealed in oxygen at 700 ° C. and analyzed by the X-ray diffraction method, the results are shown in FIGS. In addition, SnO ₂ (11
Table 2 shows the intensity ratio of the diffraction line from each surface index when the diffraction line intensity of 0) is set to 100.

[0017]

[Table 2]

From these results, the crystal growth by annealing depends on the composition at the as-depot, and only the peaks attributed to SnO ₂ tetragonal crystals are observed at the film forming rate of 22 Å / s or less,
In addition, the diffraction line intensity ratio from each surface index is almost the same as the intensity ratio (bulk value) of JCPDS, but some SnO ₂ (101) in the sample containing β-Sn such as 40Å / s.
Is observed, and another crystal structure (orthorhombic) is generated. The results of SEM observation of these films are shown in FIGS. The composition of Azdepo is SnO ₂ : Sn
When O = 32: 68, a two-layer structure is formed by annealing in oxygen at 700 ° C. A heat cycle test (room temperature ⇔ 450 ° C., 2 minute cycle) was performed on these samples, and the change over time is shown in FIG. Gas sensitivity S in the figure is Ra / Rg
(Ra: resistance value in air, Rg: resistance value in gas). It can be seen that the samples other than the two-layer structure are deteriorated by the heat cycle test for about 150 hours.

From these results, the composition control of the amorphous layer (SnO ₂ :
SnO = 32: 68) makes it possible to fabricate a two-layer structure with good reproducibility. In the case where β-Sn is contained in the as-deposited film, tetragonal crystals and orthorhombic crystals are mixed after annealing, Further, it can be seen that a structure in which the preferential orientation of SnO ₂ (101) is slight is likely to be subjected to thermal strain.

Example 2 In the film forming method described in Example 1, the film was formed at a film forming rate of 22 Å / s and annealed (700 ° C. in oxygen) to form a SnO ₂ thin film having a two-layer structure. At this time, differences in the time-dependent characteristics and crystal structures of the films on the respective substrates in the substrate processing methods A and B were examined. The treatment method A is a treatment method without washing (sonication of trichlene, acetone, and ethanol for 5 minutes each), and the treatment method B is a treatment method of cleaning (sonication with trichlene, acetone, and ethanol for 5 minutes each). Is. FIG. 11 shows the aging characteristics of these films. The substrate that has been cleaned, that is, the film that has been processed by the processing method B is deteriorated by the heat cycle test for about 275 hours, but the one that has not been cleaned, that is, the film of the substrate that has been processed by the processing method A is
Has good aging characteristics. When each sample is analyzed by the X-ray diffraction method, it becomes as shown in FIG. In the case where the substrate is cleaned, that is, the sample of the processing method B, 2θ = 2
Orthorhombic SnO ₂ (111) and 2 at 9.9 (deg.)
Orthorhombic SnO ₂ (021) at θ = 35.8 (deg.)
It was observed. One of the causes of deterioration over time is that the sample that has undergone substrate cleaning has a mixture of tetragonal and orthorhombic crystals, that is, a structure that is susceptible to strain due to heat due to the presence of layers with different internal stresses. It is considered to be a factor.

Example 3 The SnO ₂ thin film (two-layer structure) formed and annealed without cleaning the substrate prepared in Example 2 was analyzed by an X-ray diffraction method with the upper and lower layers completely separated. When it went, it became like FIG. It was found that the diffraction line intensity ratio from each plane index in the upper layer was almost equal to the bulk value, but in the lower layer, it was found that there was some preferential orientation of SnO ₂ (101). Such SnO ₂ (10
1) By providing a non-oriented upper layer on a lower layer having a preferential orientation, a gas detection film having good aging characteristics can be obtained.

[0022]

[Effect] According to the present invention, the SnO ₂ thin film having a multi-layered structure is provided with the property of being less susceptible to strain due to heat, thereby stabilizing the electric characteristics and improving the stability over time, and at the same time, the SnO ₂ having the multi-layered structure. It was possible to provide a reproducible and stable manufacturing method of a thin film.

[Brief description of drawings]

FIG. 1 shows a gas sensor having a thin gas-sensitive film,
(A) is the sectional view, (b) is the perspective view.

FIG. 2 is a perspective view showing a conventional gas sensor.

FIG. 3 shows an X-ray diffraction chart of the Azdepo film.

FIG. 4 is an X-ray diffraction chart of the annealed film, where (a) is a film formation rate of 5 Å / s and (b) is 12
3 is an X-ray diffraction chart of SnO ₂ films obtained by forming films at a film forming rate of Å / s and annealing at 700 ° C.

FIG. 5 shows an X-ray diffraction chart of a film after annealing, (c) shows a film forming rate of 22 Å / s, and (d) shows 3
3 is an X-ray diffraction chart of SnO ₂ films obtained by forming films at a film forming rate of 0Å / s and annealing at 700 ° C.

FIG. 6 is an X-ray diffraction chart of the annealed film, (e) shows a film formed at a film forming rate of 40 Å / s,
4 is an X-ray diffraction chart of a SnO ₂ film obtained by annealing at 0 ° C.

FIG. 7 is an SEM image of a SnO ₂ film having a multilayer structure obtained by forming a film at a film forming rate of 5 Å / s and annealing.

FIG. 8 is a SEM image of a SnO ₂ film having a multilayer structure obtained by forming a film at a film forming rate of 22 Å / s and annealing.

FIG. 9 is a SEM image of a SnO ₂ film having a multilayer structure obtained by forming a film at a film forming rate of 40 Å / s and annealing.

FIG. 10 shows a time-dependent characteristic (deposition rate dependency) of a SnO ₂ film.

FIG. 11 shows the characteristics over time of the SnO ₂ film on each substrate by the substrate processing method A and the substrate processing method B.

FIG. 12 SnO ₂ after annealing with and without substrate cleaning
The X-ray diffraction chart of a film | membrane is shown, (a) is the substrate processing method A
In the case of, the case (b) shows the case of the substrate processing method B.

FIG. 13 is an X-ray diffraction chart of upper and lower layers of a SnO ₂ bilayer structure, (a) is an upper layer, and (b) is a lower layer.

[Explanation of symbols]

 1 Heat Resistant Substrate 2 Heater Film 3 Insulating Film 8 Electrode Pin 12 Base 41 Electrode 42 Electrode 43 Electrode / Heater Coil 44 Electrode / Heater Coil 51 Gas Sensitive Film (Metal Oxide Semiconductor Thin Film) 52 Gas Sensitive Material 61 Electric Power to Heater Film Supply line 62 Electric power supply line to heater film 71 Signal extraction line of gas sensitive film 72 Signal extraction line of gas sensitive film t Tetragonal o Orthorhombic

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Etsuko Fujisawa 1-3-3 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd.

Claims (6)

[Claims] 1. A is formed on an insulating substrate, and the gas detection film by utilizing a change in resistance tin oxide semiconductor thin film performs gas detection with a multi-layer structure having different form thereof, SnO ₂ The crystal structure of is mainly tetragonal,
A gas detection film having a content of another crystal structure (cubic crystal, orthorhombic crystal) of 1/10 or less. 2. (a) The upper layer has a fine particle structure, the lower layer has a two-layer structure having a columnar structure, (b) the upper layer has a continuous film structure, and the lower layer has a fine particle structure, and (c) the upper layer has a continuous film structure. The lower layer is a two-layer structure having a columnar structure. (D) A tin oxide semiconductor thin film having a multi-layer structure selected from the group consisting of an upper layer having a continuous film structure, a middle layer having a fine particle structure, and a lower layer having a columnar structure. 2. The gas detection film made of, wherein the crystal structure of SnO ₂ is mainly tetragonal and the content of other crystal structures (cubic or orthorhombic) is 1/10 or less. Gas detection film. 3. SnO in upper and lower layers in the multilayer structure
₂ is having no orientation claim 1 or 2, wherein a gas sensing film. 4. The gas detection film according to claim 1, wherein the lower layer has a slight preferential orientation of SnO ₂ (101). 5. A method for producing a gas detection film, which comprises forming an amorphous layer containing SnO ₂ and / or SnO, and crystallizing it by annealing at 400 ° C. or higher in an oxygen atmosphere. 6. The method for producing a gas detection film according to claim 5, wherein the content of β-Sn microcrystals in the amorphous layer is 1% or less.