Gas sensor

Abstract

To provide a resistance change type gas sensor excellent in responsiveness.SOLUTION: There is provided a resistance change type gas sensor 1 including an insulating layer 9, a pair of counter electrodes 11 and 12 provided on the surface of the insulating layer with a gap G therebetween, and a gas sensitive film 14 provided on the gap and surfaces of the pair of counter electrodes. An insulating film 16 is interposed in at least part of a region excluding a portion adjacent to the gap in an interface between the pair of counter electrodes and the gas sensitive film.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a resistance change type gas sensor for detecting the concentration of a gas present in a detection atmosphere.

Background Art

[0002] As a gas alarm, an alcohol checker, a humidity sensor, and an odor sensor, a resistance change type gas sensor is known in which a pair of opposing electrodes are provided with a gap on the surface of an insulating layer, and a gas sensitive film is provided on the surface of the opposing electrodes. In particular, it has been reported that when the gap between each pair of opposing electrodes is narrowed to the nanoscale (nm level), high responsiveness and recovery characteristics can be obtained (Patent Document 1).

[0003] The gas sensor of Patent Document 1 arranges a relatively large first electrode and a second electrode at an interval, applies a solution containing metal oxide nanoparticles to this interval, dries it, and then performs heat treatment to form a nanoscale gap between the nanoparticles.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in the case of the gas sensor of Patent Document 1, a sensitive film is formed in a narrow region including a nanoscale gap. However, forming the sensitive film in a narrow region is difficult in practice and inferior in productivity due to misalignment of the photomask or printing misalignment during film formation of the sensitive film material. On one hand, when the sensing film is formed up to the surfaces of the first electrode and the second electrode other than the nanoscale gap, a conduction path is formed also between the first electrode and the second electrode far from these gaps and the film reacts with the gas, making it difficult to exhibit the originally high responsiveness of the nanoscale gap. That is, an object of the present invention is to provide a resistance change type gas sensor excellent in responsiveness.

Means for Solving the Problems

[0006] In order to solve the above problems, the gas sensor of the present invention is a resistance change type gas sensor including an insulating layer, a pair of opposing electrodes provided with a gap opened on the surface of the insulating layer, and a gas sensing film provided on the gap and the surfaces of the pair of opposing electrodes, wherein an insulating film is interposed in at least a part of a region excluding a part adjacent to the gap at an interface between the pair of opposing electrodes and the gas sensing film.

[0007] According to this gas sensor, by interposing an insulating film in at least a part of a region excluding a part adjacent to the gap at an interface between a pair of opposing electrodes and a gas sensing film, the gas sensing film itself can be provided in a wide range of regions including the gap and the surfaces of the pair of opposing electrodes. Thereby, even if there is a deviation of a photomask or a printing deviation during film formation of the material of the gas sensing film, the gas sensing film can be surely provided near the gap contributing to gas detection. As a result, a conduction path is generated between the opposing electrodes near the gap, and the responsiveness can be improved. Further, even if a gas sensing film is formed on the surfaces of the opposing electrodes other than near the gap, an insulating film is interposed in this part. For this reason, it is possible to suppress the opposing electrodes far from the gap from becoming a conduction path and reacting with the gas, and to suppress a decrease in responsiveness.

[0008] In the gas sensor of the present invention, when viewed in a direction along a line segment connecting the tips of the opposing electrodes where the gap is minimized, the length G1 of the gap and the distance G2 between the portions of the opposing electrodes where the insulating film is interposed may satisfy the relationship G1 < G2. According to this gas sensor, since the area of the portion of the counter electrode where the insulating film is interposed becomes larger, it is possible to further suppress the counter electrodes far from the gap from becoming a conduction path and reacting to the gas, and it is possible to further suppress a decrease in responsiveness.

[0009] In the gas sensor of the present invention, the insulating film may be made of an oxide of the metal or alloy constituting the counter electrode, or a nitride of the metal or alloy. According to this gas sensor, there is no need to form the insulating film from another material, and the production efficiency is improved.

[0010] In the gas sensor of the present invention, the counter electrode has one or more thin wire portions adjacent to the gap and a base portion connected to the rear end side of the thin wire portion, and the insulating film may be formed only on the surface of the base portion or only on the surface of the base portion and the surface of the lead portion connected to the base portion. According to this gas sensor, if the thin wire portion is made fine and the gap is narrowed to the nanoscale (nm level), high responsiveness and recovery characteristics can be obtained.

Advantages of the Invention

[0011] According to this invention, a resistance change type gas sensor excellent in responsiveness can be obtained.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments 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 A-A of 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 a detailed configuration of a pair of opposing electrodes 11 and 12.

[0014] As shown in FIG. 1, the gas sensor 1 is a resistance change type gas sensor including an insulating layer 9, a pair of opposing electrodes 11 (11b, 11t), 12 (12b, 12t) provided with a gap G opened on the surface of the insulating layer 9, and a gas sensitive film 14.

[0015] Here, metal oxides such as tin oxide constituting the gas sensitive film 14 have the property that their resistance values change when the gas to be measured contacts their surfaces, and resistance change type gas sensors (semiconductor type gas sensors) utilize this property.

[0016] As shown in FIG. 2, the gas sensitive film 14 is provided on the gap G and the surfaces of the pair of opposing electrodes 11 (11b, 11t), 12 (12b, 12t). Furthermore, an insulating film 16 is interposed in at least a part of the region excluding the portion adjacent to the gap G among the interfaces between the pair of opposing electrodes 11 and 12 and the gas sensitive film 14.

[0017] In this example, as shown in FIG. 3, a heater 20 is embedded inside the insulating layer 9. When the gas sensitive film 14 is heated to a predetermined temperature by the heater 20, oxygen in the atmosphere is adsorbed on the surface of the gas sensitive film 14, and the gas sensitive film 14 maintains a constant resistance value. However, depending on the type of the gas sensitive film 14, the heater 30 may be omitted, so the heater 30 is not an essential configuration of the present invention.

[0018] The heat generating part 21, which is the main body of the heater 20, is formed in a spiral pattern, and a pair of lead parts extending from both ends of the heat generating part 21 are respectively connected to the heater electrodes 51 and 52. 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 applying current between the heater electrodes 51 and 52 from an external power source. The heater electrodes 51 and 52 are formed of, for example, aluminum (Al) or gold (Au), and one of the heater electrodes 51 and 52 is a ground electrode.

[0019] Note that the heat generating part 21 is disposed inside a region (thin film part) corresponding to a recess 31 that constitutes a diaphragm of a substrate 30 to be described later. Thereby, since the heat generating part 21 is thermally insulated from the surroundings, the temperature can be raised or lowered in a short time, and the heat capacity of the gas sensor 1, and thus the power consumption, can be reduced.

[0020] As shown in FIG. 2, the gas sensor 1 includes a substrate 30 made of a silicon semiconductor. Insulating layers (upper insulating layer 9 and lower insulating layer 39) are provided on the upper and lower surfaces of the substrate 30. The upper insulating layer 9 is formed on the 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 part of the substrate 30 so that the upper insulating layer 9 is partially exposed (substantially square when viewed from above), a diaphragm structure with a recess (opening of the substrate 30) 31 as shown in FIG. 2 is formed.

[0021] Note that the upper insulating layer 9 and the lower insulating layer 39 may be formed of a single material, or may be formed of a plurality of layers using different materials. In the present embodiment, the upper insulating layer 9 and the lower insulating layer 39 are composed of a plurality of layers formed by laminating silicon oxide (SiO2), silicon nitride (Si3N4), and silicon oxide (SiO2) in this order.

[0022] Further, the gas sensor 1 has a size of about several millimeters (for example, 3 mm × 3 mm) in both length and width, and can be manufactured by, for example, micromachining technology (micromachining process: MEMS) using a silicon semiconductor substrate. By providing an insulating layer 9 for forming a diaphragm on the substrate 30 and disposing a pair of opposing electrodes 11 and 12 on the insulating layer 9, miniaturization and power saving of the gas sensor 1 can be achieved.

[0023] Next, the configuration of the pair of opposing electrodes 11 and 12 will be described. As shown in FIG. 1, the opposing electrodes 11 and 12 each have one or more thin wire portions 11t and 12t adjacent to the gap G, and base portions 11b and 12b connected to the rear end sides of the thin wire portions 11t and 12t. Further, lead portions 11L and 12L are connected to the base portions 11b and 12b. The lead portions 11L and 12L are connected to the 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 the resistance change between the detection electrodes 41 and 42 is output to the outside.

[0024] More specifically, as shown in FIG. 4, the base portions 11b and 12b are each formed in a rectangular plate shape and face each other. A plurality of thin wire portions 11t and 12t extend along the longitudinal direction from the opposing sides of the base portions 11b and 12b, respectively. The portions where the respective thin wire portions 11t and 12t are closest to each other form the gap G. Also, the plurality of thin wire portions 11t are spaced apart from each other in a direction perpendicular to the longitudinal direction. Similarly, the plurality of thin wire portions 12t are spaced apart from each other in a direction perpendicular to the longitudinal direction. Therefore, the plurality of thin wire portions 11t and 12t are arranged in a comb shape while connecting one end side of each to the opposing sides of the base portions 11b and 12b.

[0025] Also, as shown in FIG. 2, the thin wire portions 11t and 12t each extend from the upper surface of the base portions 11b and 12b to the side walls forming the opposing sides of the base portions 11b and 12b, and further extend so as to be close to each other on the upper surface of the insulating layer 9. These thin wire portions 11t and 12t can be formed by depositing Pt, for example, by a lift-off process. Moreover, if the thin wire portions 11t and 12t are made finer and the length of the gap G (G1 to be described later) is narrowed down to the nanoscale (nm level), high responsiveness and recovery characteristics can be obtained.

[0026] Also, the base portions 11b and 12b can be formed by depositing Cr, for example. Further, in this example, the lead portions 11L and 12L have the same composition (Cr) as the base portions 11b and 12b, but the lead portions 11L and 12L may have a composition different from that of the base portions 11b and 12b.

[0027] Next, the insulating film 16 will be described. As shown in FIG. 2, the insulating film 16 is interposed in at least a part of the region excluding the portion adjacent to the gap G among the interfaces between the pair of opposing electrodes 11 and 12 and the gas-sensitive film 14. In this example, the insulating film 16 is formed by oxidizing the entire opposing electrodes 11 and 12 after forming the thin wire portions 11t and 12t on the base portions 11b and 12b, thereby converting the surfaces of the base portions 11b and 12b and the lead portions 11L and 12L where the thin wire portions 11t and 12t are not formed into chromium(III) oxide films. Therefore, in this example, the insulating film 16 is formed on the entire surface of the portions of the opposing electrodes 11 and 12 excluding the thin wire portions 11t and 12t.

[0028] Note that in this example, the insulating film 16 is also formed on the surfaces of not only the base portions 11b and 12b but also the lead portions 11L and 12L as described above. However, since the lead portions 11L and 12L are farther from the gap G than the opposing electrodes 11 and 12 and the influence of the conductive path to be described later is relatively small, it is not essential to form the insulating film 16 on the surfaces of the lead portions 11L and 12L.

[0029] Thus, by interposing the insulating film 16 in at least a part of the region excluding the portion adjacent to the gap G among the interfaces between the pair of counter electrodes 11 and 12 and the gas sensitive film 14, the gas sensitive film 14 itself can be provided in a wide range of regions including the gap G and the surfaces of the pair of counter electrodes 11 and 12. As a result, even if there is a deviation of the photomask or a printing deviation during the film formation of the material of the gas sensitive film 14, the gas sensitive film 14 can be surely provided near the gap G that contributes to gas detection. As a result, as shown by the path P1 in FIG. 2, a conductive path is generated between the counter electrodes 11 and 12 near the gap G, and the responsiveness can be improved. Further, even if the gas sensitive film 14 is formed on the surfaces of the counter electrodes 11 and 12 other than the vicinity of the gap G, the insulating film 16 is interposed in this portion. For this reason, as shown by the path P2 in FIG. 2, it is possible to suppress the portion far from the gap G between the counter electrodes 11 and 12 from becoming a conductive path and reacting to the gas, and to suppress a decrease in responsiveness.

[0030] Further, as shown in FIG. 4, the direction along the line segment connecting the tips (thin line portions 11t and 12t) of the counter electrodes 11 and 12 where the gap G is minimized is defined as D. At this time, when viewed in the direction D, the length of the gap G of each counter electrode 11 and 12 (corresponding to the distance between the thin line portions 11t and 12t) is defined as G1, and the distance between the portions where the insulating film 16 is interposed in each counter electrode 11 and 12 (in this example, between the base portions 11b and 12b) is defined as G2. Here, the relationship G1 < G2 is satisfied. By doing so, since the region of the portion where the insulating film 16 is interposed in the counter electrodes 11 and 12 becomes wider, it is possible to further suppress the portion far from the gap G between the counter electrodes 11 and 12 from becoming a conductive path and reacting to the gas, and to more effectively suppress a decrease in responsiveness.

[0031] Further, in the present embodiment, the insulating film 16 is made of an oxide of a metal or an alloy (in this example, Cr) constituting the counter electrode 16. By doing so, it is not necessary to form the insulating film 16 from another material, and the production efficiency is improved. However, the insulating film 16 may be separately formed from a predetermined insulating material. The insulating film 16 may be made of a nitride of a metal or an alloy.

[0032] Also, in the present embodiment, the counter electrodes 11 and 12 have one or more thin wire portions 11t and 12t adjacent to the gap G1, and base portions 11b and 12b connected to the rear end sides of the thin wire portions 11t and 12t, and the insulating film 16 is formed only on the surfaces of the base portions 11b and 12b, or only on the surfaces of the base portions 11b and 12b and the surfaces of the lead portions 11L and 12L. By doing so, a conductive path is surely generated in the thin wire portions 11t and 12t adjacent to the gap G1, and the responsiveness can be further improved.

[0033] Next, with reference to FIG. 5, the manufacturing process of the gas sensor 1 will be described. Note that the composition and thickness of each of the following films are merely examples. In FIGS. 5(a) to 5(e), the left half of the figure is a cross section across the base portion 12b and the lead portion 12L near the center of the line A - A in FIG. 1, and the right half of the figure is a cross section across the heater electrode 51 of the line A - A in FIG. 1. First, a silicon substrate 30 (for example, having a thickness of 400 μm) is cleaned with a cleaning liquid, and after appropriately forming a concave portion 31 to be a diaphragm, an upper insulating layer 9 and a lower insulating layer 39 are formed on both surfaces of the substrate 30, respectively. 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 treatment, then forming a silicon nitride (Si3N4) film by low - pressure CVD, and then forming a silicon oxide (SiO2) film by plasma CVD. The thickness of each film can be, for example, 100, 200, and 100 nm in this order.

[0034] Next, a contact layer (such as tantalum oxide) is formed on the surface of the upper insulating layer 9, and a layer (platinum) to be the heater 20 is formed thereon. The film formation can be performed, for example, by RF sputtering, and the thickness of each film can be, for example, 20 and 110 nm, respectively. Thereafter, the heater 20 composed of the adhesion layer and the heating part 21 is patterned into a desired shape on the film formation surface of the 20-layer heater by photolithography. Specifically, after forming and drying a resist by spin-coating an ink of a photoresist composition on the film formation surface to form a resist, exposure and development are performed through a glass mask on which a heater pattern is formed, and the surplus part may be removed by wet etching. The heater pattern of the heating part 21 can be, for example, a square spiral shape, with an outer periphery of 0.5 mm square and an L / S of 20 / 20 μm. Furthermore, on the surface of the heater 20 after patterning, the remaining part of the upper insulating layer 9 is formed into a film, and the heater 20 is embedded inside the upper insulating layer 9. This film can be formed, for example, by forming a silicon oxide (SiO2) film by plasma CVD and then forming a silicon nitride (Si3N4) film by low-pressure CVD. The thickness of each film can be, for example, 100 and 200 nm in order.

[0035] Next, as shown in Fig. 5(a), the pad parts of the heater 20 connected to the heater electrodes 51 and 52 are exposed, for example, by photolithography and reactive ion etching. Then, after forming a gold film (Au film) with a thickness of 1 μm, for example, by DC sputtering on the surface including this pad part, the parts other than the corresponding parts to the pad part are removed by photolithography and wet etching. Similarly, the detection pad parts 41p and 42p (only 41p is shown in Fig. 5) connected to the detection electrodes 41 and 42 are also formed of an Au film.

[0036] Next, as shown in Fig. 5(b), after forming a Cr film with a thickness of, for example, 10 nm for the base parts 11b and 12b, the lead parts 11L and 12L, and the detection electrodes 41 and 42, it is patterned into a predetermined shape by photolithography and wet etching. Thereby, the base part precursors 11bx and 12bx, the lead part precursors 11Lx and 12Lx, and the detection electrode precursors 41x and 42x are obtained (only the base part precursor 12bx, the lead part precursor 12Lx, and the detection electrode precursor 41x are shown in the figure).

[0037] Next, as shown in FIG. 5(c), for example, by a lift-off process, Pt is deposited to form the thin wire portions 11t and 12t. Specifically, for example, after applying a photoresist composition ink by spin coating on the surface including the base portion precursor 12bx, patterning with an electron beam and developing to form a mold having the shape of the thin wire portions 11t and 12t. Then, a conformal layer (such as 3 nm of titanium) is deposited on the entire surface, and a platinum film (for example, 10 nm thick) to form the thin wire portions 11t and 12t is deposited thereon by RF sputtering. Thereafter, the resist mold is removed by wet etching to obtain the thin wire portions 11t and 12t. The length G1 of the shortest gap G can be, for example, 20 nm.

[0038] The thin wire portions 11t and 12t may be formed between the base portion 12bx and the heater 20. In that case, the thin wire portions are formed before forming the base portion. By forming the thin wire portions before forming the base portion, it becomes difficult for the thin wire portions to be disconnected due to the step of the base portion.

[0039] Next, as shown in FIG. 5(d), after immersing the entire structure including the substrate 30 in concentrated nitric acid for, for example, 1 hour, it is washed with pure water. As a result, an insulating film 16 made of chromium(III) oxide is uniformly deposited on the surfaces of the base portion precursors 11bx, 12bx, the lead portion precursors 11Lx, 12Lx, and the detection electrode precursors 41x, 42x. Also, the lower layers of the insulating film 16 become the base portions 11b, 12b made of Cr, the lead portions 11L, 12L, and the detection electrodes 41, 42 made of Au, respectively.

[0040] Next, as shown in FIG. 5(e), with the entire structure including the substrate 30 heated to, for example, 450° C., using a metal mask with a predetermined opening (for example, 1.5 mm square), tin oxide is deposited to a thickness of 200 nm and gold is deposited to a thickness of 1 nm by RF sputtering to form the gas sensitive film 14. Note that gold has a role of a catalyst for improving the reactivity of the measurement gas. As described above, the gas sensor 1 can be manufactured.

[0041] The present invention is not limited to the above-described embodiments, and it goes without saying that the present invention extends to various modifications and equivalents included in the spirit and scope of the present invention. For example, the gas sensor of the present invention can detect combustible gases, reducing gases, oxidizing gases, and the like. Also, in this example, the insulating film 16 is formed on the entire surface of the portions of the counter electrodes 11 and 12 excluding the thin wire portions 11t and 12t. However, for example, the insulating film 16 may be formed only on a part of the base portions 11b and 12b. The insulating film 16 may be formed by sputtering or the like.

Explanation of Reference Numerals

[0042] 1 Gas sensor 9 Insulating layer 11, 12 A pair of counter electrodes 11b, 12b Base portions 11t, 12t Thin wire portions 11L, 12L Lead portions 14 Gas-sensitive film 16 Insulating film D Direction along the line segment connecting the tips of the counter electrodes G Gap

Claims

1. An insulating layer, A pair of opposing electrodes provided with a gap on the surface of the insulating layer, A gas-sensitive film provided on the gap and the surfaces of the pair of opposing electrodes, In a resistive change type gas sensor comprising: A gas sensor, characterized in that an insulating film is interposed in at least a part of a region excluding a part adjacent to the gap at an interface between the pair of opposing electrodes and the gas-sensitive film.

2. When viewed in a direction along a line segment connecting the tips of the opposing electrodes where the gap is minimized, The length G1 of the gap and the distance G2 between the portions of the opposing electrodes where the insulating film is interposed, The gas sensor according to claim 1, characterized in that the relationship G1 < G2 is satisfied.

3. The gas sensor according to claim 1 or 2, characterized in that the insulating film is made of an oxide of a metal or alloy constituting the opposing electrode, or a nitride of a metal or alloy.

4. The opposing electrode has one or more thin wire portions adjacent to the gap and a base portion connected to the rear end side of the thin wire portion, The gas sensor according to claim 1 or 2, characterized in that the insulating film is formed only on the surface of the base portion, or only on the surface of the base portion and the surface of a lead portion connected to the base portion.

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