JPH11304755A - Limiting current-type oxygen sensor and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a limiting current-type oxygen sensor in which an irregularity in the characteristic of a sensor element is reduced and which obtains a stable characteristic. SOLUTION: In a limiting current-type oxygen sensor, the oxygen ion conduction of a solid electrolyte is used, an electrode 23 which is used to ionize oxygen is covered with a coating film 25 so as to limit the passage of oxygen gas, and a limiting current characteristic is obtained. In the limiting current-type oxygen sensor, the coating film 25 is manufactured by using a vapor deposition method in such a way that the angle in the longitudinal direction of particles in its film texture is set within ±5 deg. with reference to a preset angle. Thereby, it is possible to manufacture the limiting current-type oxygen sensor whose air permeability is stable and in which an irregularity in a characteristic is small.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor used for controlling the combustion of an environmental monitor, an oxygen deficiency alarm, a boiler, an engine, and the like, and more particularly to a limiter for controlling the diffusion of oxygen gas to obtain a limiting current characteristic. The present invention relates to a current-type oxygen sensor and a method for manufacturing the same.

[0002]

2. Description of the Related Art A limiting current type oxygen sensor comprises a solid electrolyte for transporting oxygen ions and a gas diffusion rate limiting section for limiting the supply of oxygen to limit the transport of ions. For example, as shown in FIG. 9, a cathode 2 and an anode 3 are provided on both surfaces of a solid electrolyte plate 1 as a limiting current type oxygen sensor of this type.
And a structure in which the surface of the cathode 2 is covered with a ceramic cap 5 having a fine hole 4 or a porous material having air permeability. When a voltage is applied between the cathode 2 and the anode 3 formed on both surfaces of the solid electrolyte plate 1, oxygen gas existing in the atmosphere at the interface between the cathode 2 and the solid electrolyte plate 1 is ionized to generate oxygen ions. The generated oxygen ions are transported through the solid electrolyte toward the anode 3 according to the voltage gradient, and are gasified again at the anode 3 and released into the atmosphere. At this time, a current flows between the electrodes according to the amount of oxygen transported. The current increases as the applied voltage increases and the oxygen concentration in the atmosphere increases. However, if the cathode 2 is covered with the cap 5 or a porous body as described above and the supply amount of the atmosphere gas is limited, the flowing current is reduced to a voltage. Instead, it depends only on the oxygen concentration of the atmosphere. That is, when the solid electrolyte plate 1 has a sufficiently high oxygen ion transport capacity compared to the amount of oxygen flowing into the vicinity of the cathode 2, the flowing current is limited by the oxygen gas passage speed of the material covering the cathode 2. .

The limiting current type oxygen sensor having such a structure is stable and highly reliable over a long period of time, and is therefore used for a precision oxygen concentration analyzer and the like.
Further, by changing the size of the oxygen gas inflow hole, it is possible to provide a high-precision oxygen sensor corresponding to a wide oxygen concentration region. However, since this sensor requires processing and assembly for each chip, mass production is difficult, and the manufacturing cost is high, so its use has been limited.

In view of the above, in recent years, several limiting current type oxygen sensors having a structure excellent in mass productivity have been proposed by applying thin film and thick film forming techniques used in semiconductor manufacturing.
These sensor elements are roughly classified into a sandwich type or a planar type, but all obtain a limiting current characteristic by limiting the inflow of oxygen gas that reaches the interface between the cathode that ionizes oxygen and the solid electrolyte. The mechanism is the same as that of the device using the solid electrolyte plate.

As a typical sandwich type thin film oxygen sensor, for example, as shown in FIG. 10, a sensor in which a cathode 2, a solid electrolyte layer 7 and an anode 3 are sequentially laminated on a substrate 6 having air permeability has been proposed. (For example, Japanese Patent Publication No. 5-22177). In this method, since the substrate 6 serving as the diffusion-controlling layer needs to have appropriate air permeability, the material and the type of the substrate 6 are limited, and it is difficult to form a composite with other sensors and circuit elements on the same substrate. . Further, since the pore size of the substrate 6 is large, there is also a problem that good characteristics cannot be obtained.

As a structure which is not restricted by the substrate, in a planar type, for example, as shown in FIG. 11, a cathode 2 and an anode 3 are formed in a comb shape on an upper surface of a solid electrolyte layer 8 formed on a dense substrate 10. A structure that restricts oxygen gas reaching the cathode 2 by forming and covering the cathode 2 (or the anode 3) with the air-permeable coating film 9, or a structure in which the cathode film itself is a diffusion-controlling layer (For example, Japanese Patent Application Laid-Open No. Sho 62-198748, Shinshu University, Nakao et al .; Transactions of the Institute of Electrical Engineers of Japan, Vol. 115, No. 9).

[0007] Also, in a sensor having a sandwich structure, as shown in FIG. 12, for example, as shown in FIG. 12, there is a report that a solid electrolyte layer 11 is used as a diffusion-controlling layer to obtain a limiting current characteristic (Sato et al .; Divisional Research Group CS-97-41, November 1997).

[0008]

The air-permeable coating film serving as the diffusion-controlling layer is usually produced by a thin film forming means such as printing, thermal spraying, or vapor deposition. It is desirable that the air permeability can be controlled over a wide range with good reproducibility. This is because a desired limit current characteristic cannot be obtained if the air permeability varies. Sputtering and vacuum deposition are methods that can form an ideal diffusion-controlled layer because the thickness of a thin film can be accurately controlled and the film thickness is excellent in in-plane uniformity. However, the air permeability of the film changes depending on the type of the film forming apparatus and the mounting position of the substrate with respect to the target, and the air permeability significantly changes depending on the position in the substrate. There was a problem of being big. ± 2 for the measurement system
Calibration in the range of about 5% is possible, but beyond this range calibration becomes impossible and the sensor becomes unsuitable for use.

The present invention has been made in view of the above points, and an object of the present invention is to provide a limiting current type oxygen sensor capable of reducing a variation in characteristics of a sensor element and obtaining stable characteristics, and a method of manufacturing the same. Aim.

[0010]

A limiting current type oxygen sensor according to the present invention utilizes oxygen ion conduction of a solid electrolyte formed on a substrate and covers an electrode for ionizing oxygen with a coating film. In the limiting current type oxygen sensor in which the passage of oxygen gas is restricted to obtain a limiting current characteristic, the coating film is formed by a vapor deposition method, and a growth direction of particles of the film structure and a normal line of the substrate. It is characterized in that it is manufactured so that the angle with the direction is within ± 5 ° with respect to a preset angle.

Further, a method of manufacturing a limiting current type oxygen sensor according to the present invention utilizes oxygen ion conduction of a solid electrolyte formed on a substrate and covers an electrode for ionizing oxygen with a coating film. In a method of manufacturing a limiting current type oxygen sensor in which the passage of oxygen gas is limited to obtain a limiting current characteristic, a plurality of oxygen sensors manufactured in the same process, the growth direction of the particles of the coating film and the A step of forming the coating film using an evaporation method so that a variation in an angle between the substrate and a normal direction of the substrate falls within a range of ± 5 °.

The present invention is based on the fact that the present inventors have found that the air permeability of a coating film produced by a vapor deposition method differs depending on the direction in which particles fly from a vapor deposition source. The difference in the film structure depending on the flying direction, that is, the direction in which the vapor deposition material is incident on the substrate affects the air permeability, and causes a characteristic difference depending on the film forming apparatus and the mounting position. Therefore, in any of the film forming apparatuses, the incident angle of the flying particles from the evaporation source to the substrate is limited to be within a certain range, and the variation in the growth direction of the crystal grains of the formed film is within a range of ± 5 degrees. When the film formation was performed so as to be within the range, it was possible to obtain an almost constant limit current value not exceeding ± 25%, which is the calibratable range, in all the sensor elements. In addition, it was confirmed that when the variation in the crystal grain growth direction exceeded ± 5 degrees, the variation in the limit current value exceeded ± 25% that could be calibrated.

Generally, as shown in FIG. 1, the structure of a vapor-deposited film is composed of columnar, needle-like, or plate-like crystal particles extending in the incident direction. Between the angle α formed by the normal and the angle β formed by the growth direction of the crystal grains and the normal,

[0014]

Tanβ = (1/2) tanα

It is empirically known that the relationship holds (AGDirks and HJ Leamy, Thin Solid Films, V
ol. 47 (1997) 219). Therefore, in order for the angle between the growth direction of the crystal grains and the normal to fall within a range of β ± 5 degrees with respect to a desired design value β, the angle of the incidence direction of the structure of the deposited film on the substrate with respect to the substrate normal is required. It is sufficient to determine the positional relationship between the substrate and the target material such that is within the range of the following Expression 3.

[0016]

[Equation 3] tan ^-1 {2 * tan (β-5 °)} ≦ α ≦ tan ^-1 ｛2
* tan (β + 5 °)｝

Thus, the coating layer for oxygen diffusion can be formed while suppressing the variation in the growth direction of the crystal grains with respect to the substrate to within ± 5 °, and the air permeability is stabilized, and as a result, the variation in characteristics is small. An oxygen sensor can be manufactured.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 FIG. 2 is a perspective view, partially omitted, showing a schematic configuration of a limiting current type oxygen sensor according to Embodiment 1 of the present invention. In this oxygen sensor, a solid electrolyte layer 22 such as zirconia is formed on a dense insulator substrate 21, and an anode 23 and a cathode 24 are formed in a comb shape on the solid electrolyte layer 22. Further, on the cathode 23 and the anode 24, This is a planar type oxygen sensor covered with a coating film 25 for restricting passage of oxygen gas. The angle of the coating film 25 in the longitudinal direction (growth direction) of the columnar, needle-like, or plate-like crystal grains 26 with respect to the normal direction of the substrate 21 is β ± 5 when the design value is β. It is formed to be within the range of °.

Next, a description will be given of a specific method for manufacturing an oxygen sensor in which β is set to 0 °, that is, the crystal grains 26 are oriented in the normal direction. Dense high-purity alumina (purity 99
% Or more) zirconia (8% mol Y
_After a solid electrolyte layer 22 made of a ₂ O ₃ —ZrO ₂ ) film was formed to a thickness of 10 μm by sputtering, a pair of comb-shaped electrodes (a cathode 23 and an anode 24) were formed on the surface of the zirconia film. Pt was used as the electrode material, and film formation by sputtering and pattern formation by a lift-off method were applied. Comb electrode 2
3, 24 have a thickness of 0.5 μm, a width of 50 μm, and an electrode spacing of 2
0 μm, and the sensing range was 2 mm × 2 mm.

A coating film 25 was formed on the comb electrodes 23 and 24. In forming the coating film 25, as shown in FIG.
Using a planar magnetron type sputtering apparatus equipped with a 6-inch target 31 and a dome type substrate holder 32 arranged so that the surface of the substrate 21 faces the center of the target,
The target material was an alumina sintered body. The distance between the center of the target and the substrate was 200 mm, and the substrate holder 32 had a spherical surface with the center of the target 31 as the center of the sphere. Accordingly, the center of the substrate 21 faces the center of the target 31, and the incident angle of the flying particles at that point is vertical. When the substrate 21 is a 3-inch disk, the substrate 2
The incident angle at the outer periphery of 1 is about 10 ° as shown in FIG.

Seven substrates 21 are attached to the apparatus,
A sensor element coated with an alumina film of 2 μm as the coating film 25 was prepared and evaluated under the following conditions. The gold furnace was heated to 500 ° C. in a nitrogen atmosphere containing 21% oxygen, and a voltage of 0.1 to 2 V was applied to measure the output current. 20 elements extracted from 7 sheets are 30-40μ
The limiting current value of A is shown. The measurement system equipped with the present oxygen sensor element has a calibration range of ± 25%, and all the sensors were non-defective. When the cross section of the element on the outermost periphery of the substrate was observed using a scanning electron microscope (SEM),
It was inclined about 5 degrees from the grain growth direction.

Embodiment 2 FIG. 4 is a partially omitted perspective view of an oxygen sensor according to Embodiment 2 of the present invention. In this oxygen sensor, a cathode 42 is formed on a dense insulator substrate 41, a solid electrolyte layer 43 also serving as a coating film for restricting passage of oxygen gas is formed thereon, and an anode 44 is further formed thereon. It is a sandwich type oxygen sensor formed. The solid electrolyte layer 43 also serving as a coating film is formed by forming the columnar, acicular, or plate-like crystal particles 45 of the coating film with respect to the substrate 41 in the longitudinal direction (growth direction).
When the design value is β, the angle between the normal and the normal direction is β
It is formed so as to fall within a range of ± 5 °.

Next, the detailed manufacturing process of the oxygen sensor of this embodiment, in which β = 0 ° as in the previous embodiment, will be described. In this example, a sandwich-type sensor element using the solid electrolyte layer 43 as a diffusion-controlling layer (coating film) was manufactured using an ECR ion beam sputtering apparatus. First, P is lifted off on a dense high-purity alumina substrate 41.
The cathode 42 of t was formed. Next, a 10 μm zirconia film was deposited as the solid electrolyte layer 43 using an ion beam sputtering apparatus. In this film forming step, as shown in FIG. 5, the substrate 41 was arranged by the ECR ion gun 51 so as to be perpendicular to the incident direction of the zirconia particles flying from the target 52. In the ion beam sputtering apparatus, since the pressure of the atmosphere is low and the mean free path of gas molecules is long, the distance between the substrate 41 and the ion source can be made sufficiently longer than the distance between the target substrate and the planar type sputtering apparatus. Therefore, the incident angle of the flying particles can be made almost perpendicular to the substrate surface at all positions on the substrate. Such an effect can also be expected in an ion plating or laser ablation film forming apparatus.

Next, a Pt anode 44 was formed on the zirconia film in the same manner as the cathode 42. Cathode 42,
The Pt films for both the anodes 44 were formed by a planar magnetron type sputtering device, and each had a thickness of 0.5 μm.
０．７0.7 μm. 2 extracted from the fabricated device
The limiting current value for 0 pieces was 0.4 to 0.5 mA at an oxygen concentration of 21%, and it was found that the variation of the limiting current value can be suppressed to a narrow range also in the present method.

COMPARATIVE EXAMPLE In Example 1, as shown in FIG. 6, when the substrate holder 33 of the sputtering apparatus for forming a coating film is changed from a spherical one to a flat one, the maximum incident angle on the outer periphery of the substrate becomes It is about 25 degrees. Seven substrates were attached in the same manner as in Example 1, the sensor element was coated, and the limiting current value was measured. The extracted 20 limit current values vary widely from 30 to 90 μA, and are within the range of 30 to 60 μA that can be calibrated.
There were two elements that deviated from A. The crystal growth direction (longitudinal direction of the grains) of these two devices was inclined by about 10 degrees.

The above is the normal direction of the substrate (β = 0 °)
FIG. 7 shows an example in which the crystal particles of the coating film are grown, but when the design value β is set to an arbitrary angle, for example, as shown in FIG. 7 (ion beam sputtering) and FIG. 8 (planar magnetron type sputtering) As described above, if the direction of the coating material particles coming from the targets 52 and 31 is set to be inclined by α with respect to the normal direction of the substrates 41 and 21, the coating material particles are covered within a range of ± 5 degrees with respect to an arbitrary angle β. An overcoat can be created.

[0027]

As described above, according to the present invention, the variation in the growth direction of the crystal grains with respect to the substrate is suppressed to within ± 5 ° to form a coating layer for oxygen diffusion, whereby the air permeability is stabilized. However, there is an effect that a limiting current type oxygen sensor with little variation in characteristics can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the direction of vapor deposition particles of a coating film of a limiting current type oxygen sensor according to the present invention and its flying direction.

FIG. 2 is a partially omitted perspective view of the limiting current type oxygen sensor according to the first embodiment of the present invention.

FIG. 3 is a view for explaining a sputtering apparatus used for manufacturing the sensor.

FIG. 4 is a partially omitted perspective view of a limiting current type oxygen sensor according to a second embodiment of the present invention.

FIG. 5 is a view for explaining an ion beam sputtering apparatus used for manufacturing the sensor.

FIG. 6 is a view for explaining a sputtering apparatus used for manufacturing a limiting current type oxygen sensor of a comparative example.

FIG. 7 is a view for explaining a sputtering apparatus used for manufacturing a limiting current type oxygen sensor according to another embodiment of the present invention.

FIG. 8 is a view for explaining a sputtering apparatus used for manufacturing a limiting current type oxygen sensor according to still another embodiment of the present invention.

FIG. 9 is a diagram showing a conventional limiting current type oxygen sensor.

FIG. 10 is a diagram showing a conventional sandwich type limiting current type oxygen sensor.

FIG. 11 is a view showing a conventional planar type limiting current type oxygen sensor.

FIG. 12 is a diagram showing a conventional sandwich type limiting current type oxygen sensor.

[Explanation of symbols]

1: solid electrolyte plate, 2, 23, 42: cathode, 3, 2
4,44 ... anode, 6,10,21,41 ... substrate,
7, 8, 11, 22, 43 ... solid electrolyte layer, 25 ... coating film.

Claims (3)

[Claims] 1. An oxygen ion conduction of a solid electrolyte formed on a substrate is used, and an electrode for ionizing oxygen is covered with a coating film to restrict the passage of oxygen gas to obtain a limiting current characteristic. In the limiting current type oxygen sensor as described above, the coating film is formed by an evaporation method, and an angle between a growth direction of particles of the film structure and a normal direction of the substrate is ± a predetermined angle. A limiting current type oxygen sensor which is manufactured so as to be within 5 °. 2. A limiting current characteristic is obtained by utilizing oxygen ion conduction of a solid electrolyte formed on a substrate and limiting the passage of oxygen gas by covering an electrode for ionizing oxygen with a coating film. In the method for manufacturing a limiting current type oxygen sensor, the variation in the angle between the growth direction of the particles of the coating film and the normal direction of the substrate is ± 5 for a plurality of oxygen sensors manufactured in the same process. A method for manufacturing a limiting current type oxygen sensor, comprising a step of forming the coating film using an evaporation method so as to fall within the range of °. 3. A design value of an angle between a growth direction of particles of the coating film and a normal direction of the substrate is β, and α is a flying direction of the particles of the coating film during vapor deposition. Tan ^-1 {2 * tan (β-5 °)} ≦ α ≦ tan ^-1 ｛2
3. The method for manufacturing a limiting current type oxygen sensor according to claim 2, wherein the positional relationship between the substrate on which the coating film is formed and the target material is determined so that * tan (β + 5 °)｝.