Limiting current gas sensor

Abstract

To provide a limiting current gas sensor that can control the degree of diffusion rate limitation and therefore the measurement range, has a low resistance value, and can be operated at low temperatures.SOLUTION: A limiting current gas sensor (1) includes an insulating layer (9), a cathode electrode (11) and an anode electrode (12) provided on the surface of the insulating layer with a gap (G) therebetween, and a solid electrolyte layer (14) provided on the gap, and on the surfaces of the cathode electrode, and the anode electrode. The solid electrolyte layer has a thickness of 1 μm or less, and is provided with at least one first opening (14h1) communicating with the surface of the cathode electrode and at least one second opening (14h2) communicating with the surface of the anode electrode. The total length of the interface between the solid electrolyte layer and the cathode electrode at all of the first openings is taken as L1 and the total length of the interface between the solid electrolyte layer and the anode electrode at all of the second openings is taken as L2, then L1<L2 is satisfied.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a limiting current gas sensor for detecting the concentration of a gas present in a target atmosphere. [Background technology]

[0002] A limiting current oxygen sensor is known as a sensor for detecting the concentration of oxygen gas for purposes such as preventing oxygen deficiency and controlling combustion. It has electrodes on the top and bottom surfaces of an ion-conductive solid electrolyte such as zirconium oxide, and measures the limiting current value by diffusing oxygen gas that reaches one electrode (cathode).

[0003] In order to miniaturize the element, a thin-film limiting current oxygen sensor has been developed in which electrodes and a solid electrolyte layer are sequentially stacked on an insulating substrate (Patent Document 1). The gas sensor in Patent Document 1 has a cathode electrode layer and an anode electrode layer formed on a substrate, and these electrode layers are covered with a solid electrolyte layer. The solid electrolyte layer has a columnar or plate-like crystal structure, which allows gas to pass through the gaps between the columnar crystal grains extending in the vertical direction (film thickness direction) by diffusion-controlling the rate of gas flow. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-197477 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the case of the gas sensor of Patent Document 1, it is difficult to form columnar crystals, and it is also difficult to control the size of the gaps between the crystals, which in turn makes it difficult to control the diffusion rate. In addition, because there are gaps between the columnar crystals in the horizontal direction, there is little contact between the crystals, and oxygen is ionized even in areas far from the closest part between the pair of electrodes, which increases the resistance value and the operating temperature. That is, an object of the present invention is to provide a limiting current type gas sensor capable of controlling the degree of diffusion rate limitation and, consequently, the measurement range, and having a low resistance value and being drivable at low temperatures. **Means for Solving the Problems**

[0006] In order to solve the above problems, the limiting current type gas sensor of the present invention includes an insulating layer, a cathode electrode and an anode electrode provided with a gap on the surface of the insulating layer, and a solid electrolyte layer provided on the surfaces of the gap, the cathode electrode and the anode electrode. In the limiting current type gas sensor, the thickness of the solid electrolyte layer is 1 μm or less, and the solid electrolyte layer is provided with at least one first opening communicating with the surface of the cathode electrode and at least one second opening communicating with the surface of the anode electrode. When the total length of the interfaces between the solid electrolyte layer and the cathode electrode in all the first openings is L1, and the total length of the interfaces between the solid electrolyte layer and the anode electrode in all the second openings is L2, it is characterized by satisfying L1 < L2.

[0007] According to this limiting current type gas sensor, by providing the first opening and the second opening that communicate with the surfaces of the cathode electrode and the anode electrode respectively from the solid electrolyte layer, and controlling the sizes and numbers of the first opening and the second opening, the degree of diffusion rate limitation in the limiting current type gas sensor, and consequently, the measurement range can be easily controlled. Furthermore, by appropriately designing the first opening and the second opening, a limiting current type gas sensor with a low resistance value and drivable at low temperatures can be obtained. Also, by setting L1 < L2, since L2 becomes larger, the oxygen discharge due to the oxygen pumping action becomes larger than the oxygen introduction on the cathode side, and it becomes easier to obtain a limiting current at which the current value saturates.

[0008] In the limiting current type gas sensor of the present invention, the first opening and the second opening may be arranged adjacent to the gap. External oxygen gas becomes oxygen ions on the cathode electrode and migrates through the solid electrolyte layer on the gap surface to reach the anode electrode. Therefore, in this limiting current gas sensor, by arranging the first opening and the second opening adjacent to the gap, oxygen ions generated on the cathode electrode migrate a short distance through the gap to the anode electrode, thereby achieving a limiting current gas sensor with even lower resistance and operable at lower temperatures.

[0009] In the limiting current gas sensor of the present invention, the gap may have a width of less than 1000 nm. According to this limiting current gas sensor, oxygen ions generated on the cathode electrode travel a shorter distance through the gap to the anode electrode, resulting in a limiting current gas sensor with even lower resistance and operable at even lower temperatures.

[0010] In the limiting current gas sensor of the present invention, the cathode electrode and the anode electrode may each be comb-shaped, and the comb teeth of the cathode electrode and the anode electrode may be arranged to face each other. In this limiting current gas sensor, the comb teeth of the cathode electrode and the anode electrode are close to each other, and the gap is narrowed accordingly, so that the resistance is further reduced and a limiting current gas sensor that can be operated at a lower temperature is obtained. [Effects of the Invention]

[0011] According to the present invention, it is possible to obtain a limiting current gas sensor that can control the degree of diffusion rate limitation and therefore the measurement range, has a low resistance value, and can be operated at low temperatures. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a plan view of a gas sensor according to an embodiment of the present invention; [Figure 2] FIG. 2 is a cross-sectional view taken along line AA in FIG. [Figure 3] FIG. 2 is a plan view showing a heater inside the gas sensor. [Figure 4]FIG. 2 is a partially enlarged view showing the detailed configuration of a cathode electrode and an anode electrode. [Figure 5] FIG. 4 is a perspective view showing a second opening. [Figure 6] 3A to 3C are diagrams illustrating a manufacturing process of the gas sensor. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a plan view of a gas sensor 1 according to an embodiment of the present invention, FIG. 2 is a cross-sectional view taken along line AA in FIG. 1, FIG. 3 is a plan view showing a heater 20 inside the gas sensor 1, and FIG. 4 is a partially enlarged view showing the detailed configuration of a cathode electrode 11 and an anode electrode 12.

[0014] As shown in FIG. 1, the limiting current gas sensor (gas sensor) 1 is a limiting current gas sensor including an insulating layer 9, a cathode electrode 11 (11b, 11t) and an anode electrode 12 (12b, 12t) provided on the surface of the insulating layer 9 with a gap G therebetween, and a solid electrolyte layer 14. As shown in FIG. 2, the solid electrolyte layer 14 is provided in the gap G and on the surfaces of the cathode electrodes 11 (11b, 11t) and the anode electrodes 12 (12b, 12t).

[0015] As shown in FIG. 3, a heater 20 is embedded inside the insulating layer 9, and the heater 20 heats the solid electrolyte layer 14 to a temperature equal to or higher than the activation temperature. The heating portion 21, which is the main body of the heater 20, is formed in a spiral pattern, and a pair of lead portions extending from both ends of the heating portion 21 are connected to heater electrodes 51 and 52, respectively. The heater electrodes 51 and 52 are exposed on the upper surface of the gas sensor 1 through predetermined contact holes (not shown), and are heated by passing electricity between the heater electrodes 51 and 52 from an external power source. The heater electrodes 51 and 52 are made of, for example, gold (Au), and one of the heater electrodes 51 and 52 is a ground electrode.

[0016] The heat generating portion 21 is disposed inside a region (thin film portion) corresponding to a recess 31 constituting a diaphragm of the substrate 30, which will be described later. This allows the heat generating portion 21 to be insulated from the surroundings, so that the temperature can be increased or decreased in a short time, thereby reducing the heat capacity of the gas sensor 1 and, in turn, the power consumption.

[0017] 2, the gas sensor 1 includes a substrate 30 made of single crystal silicon. The substrate 30 has insulating layers (an upper insulating layer 9 and a lower insulating layer 39) on its upper and lower surfaces. The upper insulating layer 9 is formed on the front surface of the substrate 30, while the lower insulating layer 39 is formed on the back surface of the substrate 30. The upper insulating layer 9 corresponds to the "insulating layer" in the claims. Then, by removing a portion of the substrate 30 so that the upper insulating layer 9 is partially exposed (approximately square when viewed from above), a diaphragm structure is formed with a recess (opening in the substrate 30) 31 as shown in Figure 2.

[0018] The upper insulating layer 9 and the lower insulating layer 39 may be formed of a single material, or may be formed to form multiple layers using different materials. In this embodiment, the upper insulating layer 9 and the lower insulating layer 39 are made of multiple layers in which silicon oxide (SiO2) and silicon nitride (Si3N4) are stacked.

[0019] The gas sensor 1 is approximately several mm in length and width (for example, 3 mm×3 mm), and can be manufactured by micromachining technology (micromachining: MEMS) using a silicon semiconductor substrate, for example. By providing an insulating layer 9 that forms a diaphragm on the substrate 30 and arranging the cathode electrode 11 and the anode electrode 12 on the upper surface of this insulating layer 9, the gas sensor 1 can be made smaller and consume less power.

[0020] Next, the configurations of the cathode electrode 11 and the anode electrode 12 will be described. As shown in Figures 1 and 4, the cathode electrode 11 and the anode electrode 12 each have one or more thin wire portions 11t, 12t adjacent to the gap G, and base portions 11b, 12b connected to the rear end sides of the thin wire portions 11t, 12t. In addition, lead portions 11L and 12L are connected to the base portions 11b and 12b, respectively. The lead portions 11L and 12L are connected to detection electrodes 41 and 42, respectively. The detection electrodes 41 and 42 are exposed on the upper surface of the gas sensor 1 through predetermined contact holes (not shown), and a change in resistance between the detection electrodes 41 and 42 is output to the outside. The detection electrodes 41 and 42 are made of, for example, gold (Au).

[0021] More specifically, as shown in FIG. 4, the base portions 11b and 12b are formed in the shape of rectangular plates and face each other. A plurality of thin wire portions 11t, 12t extend along the longitudinal direction from the opposing sides of base portions 11b, 12b, respectively. A gap G is formed between the thin wire portions 11t, 12t at their closest positions. The thin wire portions 11t are spaced apart from one another in a direction perpendicular to the longitudinal direction. Similarly, the thin wire portions 12t are spaced apart from one another in a direction perpendicular to the longitudinal direction. Therefore, the plurality of thin wire portions 11t, 12t are arranged in a comb shape, with one end of each being connected to the opposing sides of the base portions 11b, 12b.

[0022] The thin wire portions 11t and 12t extend from the upper surfaces of the base portions 11b and 12b to the side walls that form the opposing sides of the base portions 11b and 12b, respectively, and further extend on the upper surface of the insulating layer 9 so as to be close to each other. The thin wire portions 11t and 12t can be formed by depositing a Pt film using, for example, a lift-off process. Furthermore, by making the fine wire portions 11t and 12t fine and narrowing the gap G to the nanoscale (nm level), high response and recovery characteristics can be obtained.

[0023] Among the cathode electrode 11 and the anode electrode 12, the thin wire portions 11t and 12t are adjacent to the gap G, and the base portions 11b and 12b are arranged at positions farther from the gap G than the thin wire portions 11t and 12t and are electrically connected to the thin wire portions 11t, 12t and the lead portions 11L, 12L. The thin wire portions 11t and 12t can be made of a noble metal such as Pt.

[0024] Next, the solid electrolyte layer 14 will be described. The thickness of the solid electrolyte layer 14 is 1 μm or less. This is because if the thickness of the solid electrolyte layer 14 becomes too thick exceeding 1 μm, the solid electrolyte layer 14 may crack or peel due to the internal stress of the solid electrolyte layer 14.

[0025] Also, as shown in FIG. 2, the solid electrolyte layer 14 is provided with at least one first opening 14h1 communicating with the surface of the cathode electrode 11 and at least one second opening 14h2 communicating with the surface of the anode electrode 12. Furthermore, when the total length of the interfaces between the solid electrolyte layer 14 and the cathode electrode 11 in all the first openings 14h1 is L1, and the total length of the interfaces between the solid electrolyte layer 14 and the anode electrode in all the second openings 14h2 is L2, L1 < L2 is satisfied.

[0026] Here, as shown in FIG. 5, taking one second opening 14h2 as an example, the length of the interface between the solid electrolyte layer 14 and the anode electrode 12 (12B) in the second opening 14h2 is defined as L21. The reason for defining the length (circumferential length) L21 of the interface of the second opening 14h2 is that the region where the three phases of the solid electrolyte layer 14, the anode electrode 12 (12B), and the gas phase contact in the second opening 14h2 becomes a triple-phase interface, and it is the site where the reaction occurs in which oxygen in the gas diffused from the second opening 14h2 to the anode electrode 12 (12B) is ionized. And the total length of the lengths (circumferential lengths) L21 of the interfaces of all the second openings 14h2 is L2.

[0027] Here, the L21 of each of the second openings 14h2 can be obtained by measuring, with a microscope or the like, the line on the circumference where the anode electrode 12 (12B) is in contact with the solid electrolyte layer 14 when viewing the second opening 14h2 from above (the surface of the solid electrolyte layer 14). The same applies to the first opening 14h1 as to the second opening 14h2. Incidentally, as will be described later, when the first opening 14h1 and the second opening 14h2 are formed by photolithography and etching, the etching rate that spreads in the lateral direction decreases as the etching becomes deeper. Therefore, as shown in FIG. 5, the openings on the surface side of the first opening 14h1 and the second opening 14h2 are wide, and the openings on the side of the cathode electrode 11 and the anode electrode 12 tend to be narrow.

[0028] Also, the external oxygen gas diffused into the cathode electrode 11 becomes oxygen ions on the cathode electrode 11, moves through the solid electrolyte layer 14 on the surface of the gap G, reaches the anode electrode 12, receives electrons, and diffuses to the outside as oxygen gas. Therefore, the reason for making L1 < L2 is that since L2 becomes large, the oxygen discharge due to the oxygen pumping action becomes larger than the oxygen introduction on the cathode side, and it becomes easier to obtain the limiting current at which the current value saturates. Incidentally, in the example of FIG. 2, five second openings 14h2 are provided and one first opening 14h1 is provided so that L1 < L2.

[0029] In this way, by providing the first opening 14h1 and the second opening 14h2 that communicate with the surfaces of the cathode electrode 11 and the anode electrode 12 respectively from the solid electrolyte layer 14 and controlling the sizes and numbers of the first opening 14h1 and the second opening 14h2, the degree of diffusion control in the limiting current type gas sensor, and thus the measurement range, can be easily controlled. Furthermore, by appropriately designing the first opening 14h1 and the second opening 14h2, a limiting current type gas sensor that can be driven at low temperature with a low resistance value can be obtained. Incidentally, the first opening 14h1 and the second opening 14h2 can be accurately fabricated by etching a part of the solid electrolyte layer 14 using, for example, photolithography in a semiconductor process.

[0030] In an embodiment of the present invention, the first opening 14h1 and the second opening 14h2 may be disposed adjacent to the gap G. As described above, external oxygen gas becomes oxygen ions on the cathode electrode 11 and moves through the solid electrolyte layer 14 on the surface of the gap G to reach the anode electrode 12 . Therefore, by arranging the first opening 14h1 and the second opening 14h2 adjacent to the gap G, oxygen ions generated on the cathode electrode 11 move a short distance to the anode electrode 12 via the gap G, thereby obtaining a limiting current gas sensor with an even lower resistance value and capable of operating at lower temperatures. The term "adjacent" means that the first opening 14h1 and the second opening 14h2 are present within a range of 2 μm on the outer side of the gap G (on the side away from the gap G).

[0031] If the gap G is less than 1000 nm, oxygen ions generated on the cathode electrode 11 will travel a shorter distance through the gap G to the anode electrode 12, resulting in a limiting current gas sensor with a lower resistance and operable at lower temperatures.

[0032] The cathode electrode 11 and the anode electrode 12 may each be comb-shaped and arranged so that the teeth of the cathode electrode 11 and the anode electrode 12 face each other. Here, facing each other also includes the case where the teeth of the cathode electrode 11 and the anode electrode 12 interdigitate with each other. In this way, the comb teeth of the cathode electrode 11 and the anode electrode 12 are closer to each other, and the gap G is narrower, so that a limiting current gas sensor having a lower resistance and operable at lower temperatures is obtained.

[0033] Next, a manufacturing process of the gas sensor 1 will be described with reference to Fig. 6. Note that the compositions and thicknesses of the films described below are merely examples. FIG. 6 is a cross-sectional view corresponding to FIG.

[0034] First, a silicon substrate 30 (e.g., 400 μm thick) is cleaned with a cleaning solution, and a recess 31 that will become a diaphragm is formed as needed. Then, an upper insulating layer 9 and a lower insulating layer 39 are formed on both sides of the substrate 30. The upper insulating layer 9 and the lower insulating layer 39 can be formed, for example, by forming a silicon oxide (SiO2) film on the substrate 30 by thermal oxidation, then forming a silicon nitride (Si3N4) film by low-pressure CVD, and then forming a silicon oxide (SiO2) film by plasma CVD. The thicknesses of each film can be, for example, 100, 200, and 100 nm, respectively.

[0035] Next, an appropriate adhesion layer (tantalum oxide, etc.) is formed on the surface of the upper insulating layer 9, and a layer (platinum) that will become the heater 20 is formed thereon. The film formation can be performed by, for example, RF sputtering, and the thicknesses of the respective films can be, for example, 20 and 110 nm, respectively. Thereafter, the heater 20 (heat generating portion 21) is patterned into a desired shape on the deposition surface of the heater 20 layer by photolithography. Furthermore, the remaining portion of the upper insulating layer 9 is formed on the surface of the patterned heater 20, and the heater 20 is embedded inside the upper insulating layer 9. This film can be formed, for example, by depositing a silicon oxide (SiO2) film by plasma CVD and then depositing a silicon nitride (Si3N4) film by low-pressure CVD. The thicknesses of each film can be, for example, 100 and 200 nm, respectively.

[0036] Next, the pad portions of the heater 20 connected to the heater electrodes 51 and 52 are exposed by, for example, photolithography and reactive ion etching. Then, a gold film (Au film) of 1 μm is formed on the surface including the pad portion by, for example, DC sputtering, and then the portions other than the portions corresponding to the pad portion are removed by photolithography and wet etching. Similarly, the detection pad portion connected to the detection electrodes 41 and 42 and the detection electrodes 41 and 42 are also formed of an Au film.

[0037] Next, a Pt film or the like that will become the base portions 11b, 12b and the lead portions 11L, 12L is formed to a thickness of, for example, 10 nm, and then patterned into a predetermined shape by photolithography and wet etching.

[0038] Next, a Pt film is formed by, for example, a lift-off process to form the thin wire portions 11t and 12t. Specifically, for example, a photoresist composition ink is applied by spin coating to a surface including a film that will become the base portion, and then patterned with an electron beam and developed to form a mold that will have the shape of the thin line portions 11t and 12t. Thereafter, an adhesion layer (titanium 3 nm or the like) is formed on the entire surface, and a platinum film (thickness 10 nm, for example) that will become the thin line portions 11t and 12t is formed on top of that by RF sputtering. Thereafter, the resist pattern is removed by wet etching to obtain the thin line portions 11t and 12t. The shortest gap G can be set to, for example, 20 nm.

[0039] The thin wire portions 11t, 12t may be formed between the base portions 11b, 12b and the heater 20. In this case, the thin wire portions are formed before the base portions are formed. By forming the thin wire portions before the base portions are formed, the thin wire portions are less likely to break due to steps in the base portions.

[0040] Next, as shown in FIG. 6(a), the entire structure including the substrate 30 is heated to, for example, 450°C, and a metal mask with a predetermined opening (for example, 1.5 mm square) is used to form a film of, for example, YSZ (Y2O3=8 mol%) to a thickness of 1 nm by RF sputtering, thereby forming a solid electrolyte layer precursor 14x.

[0041] Next, as shown in FIG. 6(b), a photoresist (for example, a positive resist) layer 70 is applied to the entire surface of the solid electrolyte layer precursor 14x. Then, as shown in FIG. 6(c), the photoresist layer 70 is exposed and developed to form patterns 70h1 and 70h2 that will become the first opening 14h1 and the second opening 14h2. The patterns 70h1 and 70h2 penetrate the photoresist layer 70 and reach the surface of the solid electrolyte layer precursor 14x.

[0042] Next, as shown in FIG. 6(d), the portions of the solid electrolyte layer precursor 14x below the patterns 70h1 and 70h2 are etched by, for example, Ar dry etching using an ion milling device to form a first opening 14h1 and a second opening 14h2. After etching, the photoresist layer 70 that is no longer needed is removed by an ashing device or the like, and the gas sensor 1 can be completed.

[0043] The present invention is not limited to the above-described embodiments, and encompasses various modifications and equivalents within the spirit and scope of the present invention. For example, the gas sensor of the present invention can detect flammable gases, reducing gases, oxidizing gases, etc. The size, number, shape, etc. of the first opening and the second opening are not limited, and examples of the shape as viewed from the top surface of the solid electrolyte layer include a circle, an ellipse, a polygon, etc. In the above embodiment, the thin wire portion of the electrode is configured to have a plurality of electrode pairs (comb teeth), but each may have only one thin wire (one pair). [Explanation of symbols]

[0044] 1. Limiting current gas sensor 9 Insulating layer 11 Cathode electrode 12 Anode electrode 14 Solid electrolyte layer 14h1 1st opening 14h2 2nd opening G Gap

Claims

1. an insulating layer; a cathode electrode and an anode electrode provided on the surface of the insulating layer with a gap therebetween; a solid electrolyte layer provided on the surfaces of the gap, the cathode electrode, and the anode electrode; In a limiting current type gas sensor comprising: The thickness of the solid electrolyte layer is 1 μm or less, the solid electrolyte layer is provided with at least one first opening communicating with a surface of the cathode electrode and at least one second opening communicating with a surface of the anode electrode; a limiting current gas sensor, characterized in that, when a total length of an interface between the solid electrolyte layer and the cathode electrode in all of the first openings is defined as L1, and a total length of an interface between the solid electrolyte layer and the anode electrode in all of the second openings is defined as L2, L1 < L2 is satisfied.

2. 2. The limiting current type gas sensor according to claim 1, wherein the first opening and the second opening are disposed adjacent to the gap.

3. 3. The limiting current type gas sensor according to claim 1, wherein the gap has a width of less than 1000 nm.

4. 3. The limiting current gas sensor according to claim 1, wherein the cathode electrode and the anode electrode are each comb-shaped, and the comb teeth of the cathode electrode and the anode electrode are arranged so as to face each other.

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