JPH10197476A - Limiting current-type oxygen sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a limiting current-type oxygen sensor which can be mass- produced through a simple manufacturing process and can be easily compounded with a peripheral circuit, etc. SOLUTION: A limiting-current type oxygen sensor is composed of a cathode 31 formed on a substrate 10, a solid-state electrolyte layer 20 having a columnar crystal structure, and an anode 32 formed on the layer 20. Since columnar crystals are formed in the columnar crystal structure of the electrolyte layer 20, gaps elongated in the thickness direction of the layer 20 are formed between the crystal grains and oxygen on the surface side is introduced to the cathode 31 through the gaps. Therefore, the electrolyte layer 20 functions as an ionic conductor and a diffusion controlled layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a limiting current type oxygen sensor used for an environmental monitor, an oxygen deficiency alarm, a combustion control of a boiler, an engine or the like.

[0002]

2. Description of the Related Art A limiting current type oxygen sensor comprises a solid electrolyte made of, for example, stabilized zirconia for transporting oxygen ions, and a gas diffusion rate limiting portion for limiting oxygen supply and controlling ion transport. As shown in FIG. 4, the limiting current type oxygen sensor that is put into practical use has electrode layers (cathode, anode) 31 and 32 formed on both surfaces of a solid electrolyte plate 22,
The cathode 31 is provided with thin diffusion holes 4 for diffusing oxygen.
1 is covered with a ceramic cap 40 provided. When a voltage is applied between the anode 32 and the cathode 31, oxygen gas in the atmosphere is ionized at the interface between the cathode 31 and the solid electrolyte plate 22 to generate oxygen ions. Oxygen ions are supplied to the solid electrolyte plate 2 according to the voltage gradient.
2 is transported toward the anode 32 and the anode 32
Gasifies again. At this time, a current flows between the anode 32 and the cathode 31 according to the transport amount of oxygen ions.

The current increases as the applied voltage increases and the oxygen concentration in the atmosphere increases. However, when the cathode 31 is covered with the cap 40 as described above and the supply amount of the atmosphere gas is restricted, the current increases. The current depends only on the oxygen concentration of the atmosphere without depending on the voltage. That is,
When the amount of oxygen flowing into the cathode 31 is smaller than the oxygen ion transport capacity of the solid electrolyte plate 22, the flowing current is limited by the oxygen gas passing speed through the diffusion hole 41 of the cap 40 covering the cathode 31. The limiting current type oxygen sensor having such a structure has a feature of being stable and highly reliable for a long period of time, and is therefore used for a precision oxygen concentration analyzer and the like.

In recent years, several limiting current type oxygen sensors having a structure excellent in mass productivity have been proposed by applying thin film or thick film manufacturing techniques used in semiconductor manufacturing. As a typical example, as shown in FIG. 5, a cathode 31, a solid electrolyte layer 23, and an anode 32 are sequentially laminated on a porous substrate 11 having air permeability, and the surroundings are covered with a gas shielding film 12. (Japanese Patent Laid-Open No. 5-221)
No. 77). According to this, the amount of gas passing through the porous substrate 11 becomes rate-determining and indicates a limiting current.

Further, as shown in FIG. 6, first, a porous layer 14 is provided on a dense substrate 13 such as silicon, and a cathode 31, a solid electrolyte layer 23 and an anode 32 are sequentially laminated thereon, and the surroundings are gaseous. After covering with the shielding film 12, a part of the back surface side of the substrate 13 is removed by etching to remove the porous layer 1.
There is also proposed a structure in which a through-hole 15 reaching the nozzle 4 is provided, or a gas is introduced from a part of the porous layer 14 (not shown).

[0006]

However, the limiting current type oxygen sensor (FIG. 4) which has been put into practical use conventionally is as follows.
Since each chip is processed and assembled, mass production is difficult, the production cost is high, and its use is limited.

A limiting current type oxygen sensor as shown in FIG. 5 requires a porous substrate having air permeability, but such a porous substrate is expensive and its type is limited. In particular, it is difficult to form peripheral circuits for amplification, judgment, compensation, etc., which are found in recent semiconductor sensors, on the same substrate. In addition, when a cleaning step or a photoresist step is used, it is difficult to remove impurities adhered to the pores of the porous substrate in a developing step, and peeling of the film and deterioration of ion conduction characteristics occur.

Further, the limiting current type oxygen sensor shown in FIG. 6 is for avoiding such a problem of the porous substrate, but requires sophisticated and complicated steps such as pattern formation and anisotropic etching. However, there is a disadvantage that the number of layers increases and the entire process becomes complicated.

In view of the above, the present invention avoids the problem of using a porous substrate having air permeability, and does not add a layer for controlling the diffusion other than the electrodes and the solid electrolyte layer. It is an object of the present invention to provide a limiting current type oxygen sensor which is small and simple, is suitable for mass production, and can be easily integrated with peripheral circuits and other sensors.

[0010]

In order to achieve the above object, in a limiting current type oxygen sensor according to the present invention, a dense substrate having no air permeability and a lower electrode formed on the substrate are provided. Layer, a gas-permeable oxygen ion conductive solid electrolyte layer formed to cover the lower electrode layer,
And an upper electrode layer provided on the solid electrolyte layer.

The oxygen ion conductive solid electrolyte layer having air permeability may be formed of a columnar or plate-like solid electrolyte layer as in the second aspect of the present invention.

An oxygen ion conductive solid electrolyte layer is formed so as to cover the lower electrode layer, and this solid electrolyte layer has air permeability. That is, the oxygen gas on the surface side reaches the lower electrode layer through the permeable solid electrolyte layer and is ionized. The oxygen ions are transported to the surface side in the solid electrolyte layer, and are gasified by the upper electrode layer on the surface side. As described above, the solid electrolyte layer functions not only as an ion conductor layer but also as a gas diffusion rate-controlling layer. Therefore, it is not necessary to use a porous substrate having air permeability, and the problem can be avoided. In addition, the number of manufacturing steps is small and simple, which is suitable for mass production.
Furthermore, it is easy to combine with peripheral circuits and other sensors.

When the oxygen-conductive solid electrolyte layer having gas permeability is formed of a solid electrolyte layer having a columnar or plate-like crystal structure, the solid electrolyte layer can be easily and easily manufactured by, for example, an RF sputtering method. In such a solid electrolyte layer having a columnar or plate-like crystal structure, a gap between the columnar or plate-like crystal particles to grow at right angles to a film surface is formed in a vertical direction (film thickness direction). It is stretched and has air permeability in the film thickness direction. Although the terms “film” and “layer” are used in this specification,
These mean thin films or thin layers and are used interchangeably. Through this gap in the film thickness direction, the oxygen gas on the surface side is guided to the lower electrode layer covered with the solid electrolyte layer and ionized. The oxygen ions are transported to the upper electrode layer at right angles to the film surface through the inside of the crystal grains, where they are gasified and discharged to the surface side. Since the gaps between the crystal grains serve as diffusion controlling holes, the solid electrolyte layer functions not only as an ionic conductor layer but also as a gas diffusion controlling layer.

[0014]

Next, embodiments of the present invention will be described in detail with reference to the drawings. In FIG. 1, a limiting current type oxygen sensor according to the present invention includes a cathode (lower electrode layer) 3 on the surface of a dense substrate 10 having no air permeability.
1 is formed, an oxygen ion conductive solid electrolyte layer 20 having air permeability is formed so as to cover the solid electrolyte layer 1, and then an anode (upper electrode layer) 32 is provided on the solid electrolyte layer 20. Although not shown, a heater pattern is formed on the back surface of the substrate 10 so that the substrate 10 is uniformly heated from the back surface.

As the substrate 10, a dense alumina substrate having excellent heat resistance and mechanical strength is used. First
In this step, Pt was sputtered on the substrate 10, and then a pattern of a cathode 31 and a lead portion (not shown) connected to the cathode 31 was formed by using a photolithography technique.

The substrate 10 is made of silicon carbide, silicon nitride, aluminum nitride, titanium nitride, boron carbide, titanium oxide, stabilized zirconia, magnesia, beryllia, titanium, which is also excellent in heat resistance and mechanical strength. Barium acid,
Heat-resistant ceramics such as strontium titanate, calcium titanate and potassium titanate, and enamel materials coated with these, diamond, sapphire, quartz, spinel, rutile, lithium niobate, lithium tantalate, silicon, gallium arsenide, sulfide A single crystal of zinc, cadmium tellurium, or the like, various kinds of glass substrates, or the like can be used. The electrode layer 31 (and 32) is also made of Pt-P in addition to Pt and Pt-Rh which are heat-resistant materials.
Several materials such as d and Pt-Ag can be used. The method of forming the electrode layers 31 (32) is not limited to the sputtering method as described above, but also includes a physical vapor deposition method such as vacuum deposition and ion plating, a chemical thin film forming method such as a CVD method, or plasma spraying. A thick film manufacturing process such as printing, spin coating, or the like can be employed.

Next, as a second step, a yttria partially stabilized zirconia solid electrolyte layer 20 having oxygen ion conductivity is formed so as to cover the cathode 31. For this, an RF gas sputtering method is used, and an argon gas containing 10% oxygen is used as a sputtering gas. In order to allow the solid electrolyte layer 20 to have air permeability in the vertical direction (in the direction of the film thickness, that is, in the direction from the surface to the cathode 31), the temperature of the substrate 10 is kept low and the columnar or plate-like crystal is formed by sputtering at a low pressure. The film grows in a structure. For example, the substrate 10 is water-cooled via a substrate holder to suppress a temperature rise during film formation.
The pressure of the sputtering gas was 10 mTorr. The sputter gas pressure is changed in the range of 1 mTorr to 20 mTorr so as to conform to the specification of the measured oxygen concentration required for the sensor. The film thickness is also related to the amount of passed oxygen. Under these film growth conditions, the optimum film thickness is around 10 μm to 20 μm when the measured oxygen concentration is around 20%, and 5 μm around 5%.
〜１０10 μm.

In the third step, the substrate 10 and the zirconia solid electrolyte layer 2 are formed in the same manner as the cathode 31.
An anode 32 was formed by forming a Pt film on the anode 0. By setting the film thickness between 0.1 μm and 1 μm, the electrode layer 32 having appropriate air permeability and conductivity can be obtained.

Here, a lift-off method, which is one of the photolithographic methods, is applied as a method for forming the patterns of the electrode layers 31 and 32. In the lift-off method, masking is performed using a resist on a substrate or the like before film formation, and after forming an electrode material thereon, the masking portion together with the resist is removed. In this case, when a porous substrate is used as in the conventional case (FIG. 5), the resist penetrates into the pores of the substrate, and there is a disadvantage that the resist residue is difficult to be removed even when the development is performed. When the dense substrate 10 was used, no fluorescence due to the residual resist was observed under a fluorescence microscope (fluorescence was observed in a part of the porous substrate, and when there was such a residual resist, a film was laminated thereon). Then, the film may be peeled off in the subsequent steps, and this may cause a temporal change in sensor characteristics.)

The limiting current type oxygen sensor thus prepared is heated by a heater pattern on the back surface of the substrate 10 to set the anode 32 to an operating temperature of 450 ° to 600 °.
-A flat limit current region was shown when the applied voltage between the cathodes 31 was 0.5 V to 1.5 V. When the applied voltage was 1 V, the output current increased depending on the oxygen concentration, showing good oxygen sensor characteristics.

This indicates that the solid electrolyte layer 20 functioned as a diffusion-controlling layer. That is, when the yttria partially stabilized zirconia solid electrolyte layer 20 having a columnar or columnar crystal structure is enlarged, it has a structure as shown in FIGS. 2 and 3 (FIG. 2 is a cross-sectional view as viewed from the side. And FIG. 3 is viewed from the front side). Since the crystal grains grow in a columnar or plate-like shape in the film thickness direction, adjacent particles are in close contact with each other, but a gap 21 is generated in a part thereof. It extends and provides a vent between the surface and the internal cathode 31. Then, oxygen on the front side is guided to the cathode 31 through the gap 21, ionized by the cathode 31, transported to the front side through the columnar or plate-like crystal grains, and oxygenated by the anode 32 on the front side. I do. This gap 21 functions as a hole for diffusion control.

[0022]

As described above, in the limiting current type oxygen sensor of the present invention, two electrode layers and a permeable solid electrolyte layer are sequentially formed on a non-permeable dense substrate. Therefore, the manufacturing cost can be reduced, the manufacturing yield can be improved, and mass production is possible. Further, it is possible to avoid problems when a porous substrate is used, maintain performance as an oxygen sensor, and improve reliability. Since the substrate to be used can be selected from more types, it can be used in a wide range of applications and can be easily combined with peripheral circuits and other sensors.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view schematically showing a crystal structure.

FIG. 3 is an enlarged plan view schematically showing a crystal structure.

FIG. 4 is a cross-sectional view schematically showing a conventional example.

FIG. 5 is a cross-sectional view schematically showing another conventional example.

FIG. 6 is a sectional view schematically showing still another conventional example.

[Explanation of symbols]

10, 13 Dense substrate 11 Porous substrate 12 Gas barrier film 14 Porous layer 15 Through hole 20 Columnar crystal structure solid electrolyte layer 21 Gap 22 Solid electrolyte plate 23 Solid electrolyte layer 31 Cathode 32 Anode 40 Cap 41 Diffusion hole

Claims (2)

[Claims] A dense substrate having no air permeability; a lower electrode layer formed on the substrate; a gas-permeable oxygen ion conductive solid electrolyte layer formed to cover the lower electrode layer; A limiting current type oxygen sensor, comprising: an upper electrode layer provided on the solid electrolyte layer. 2. The limiting current type oxygen sensor according to claim 1, wherein the oxygen ion conductive solid electrolyte layer having air permeability is a solid electrolyte layer having a columnar or plate-like crystal structure.