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 the surfaces of the pair of counter electrodes. Each of the counter electrodes includes a tip portion 11t, 12t adjacent to the gap, and a rear end portion 11b, 12b arranged at positions farther from the gap than the tip portions and electrically connected to the tip portion and lead portion 11L, 12L. The tip portions are made of metal or alloy, different from that forming the rear end portions. The rear end portions have lower responsiveness to the gas to be measured than the tip portions.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 a smell sensor, a resistance change type gas sensor is known in which a pair of opposed 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 opposed electrodes. In particular, it has been reported that when the gap between the opposed 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 heat-treats it 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 due to misalignment of the photomask or printing misalignment during film formation of the sensitive film material, and the productivity is also poor. 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 conductive path is formed between the first electrode and the second electrode far from these gaps and 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 having excellent 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. Each of the opposing electrodes includes a tip portion adjacent to the gap, and a rear end portion disposed at a position farther from the gap than the tip portion and electrically connected to the tip portion and the lead portion. The tip portion and the rear end portion are made of different metals or alloys, and the rear end portion is characterized in that it has lower reactivity to the gas to be measured than the tip portion.

[0007] According to this gas sensor, by making the rear end portion a metal or alloy having lower reactivity to the gas to be measured than the tip portion, 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 the photomask or printing deviation during the film formation of the material of the gas sensing film, the gas sensing film can be surely provided near the gap that contributes to gas detection. As a result, a conductive path is generated between the tip portions near the gap, and the responsiveness can be improved. Moreover, even if the gas sensing film is formed on the surface of the rear end portion other than near the gap, the chemical reaction of the gas to be measured at this site is suppressed. For this reason, it is possible to suppress the reaction of the gas with the conductive path formed between the rear end portions far from the gap, and to suppress the decrease in responsiveness.

[0008] In the gas sensor of the present invention, the tip portion may be made of a single metal selected from Au, Ag, Pt, Pd, Rh, Ru, Os, Ir, or an alloy containing the metal. According to this gas sensor, the reactivity of the tip portion with respect to the gas to be measured is further increased.

[0009] In the gas sensor of the present invention, when viewed in the direction along the 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 rear end portions sandwiching the gap may satisfy the relationship G1 < G2. According to this gas sensor, since the region of the rear end portion of the opposing electrodes having low reactivity with respect to the gas to be measured becomes wider, it is further suppressed that the region between the rear end portions far from the gap becomes a conduction path and reacts with the gas, and a decrease in responsiveness can be further suppressed.

Advantages of the Invention

[0010] According to this invention, a resistance change type gas sensor excellent in responsiveness can be obtained.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a plan view of the 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 the heater 20 inside the gas sensor 1, and FIG. 4 is a partial enlarged view showing the detailed configuration of a pair of opposing electrodes 11 and 12.

[0013] 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) and 12 (12b, 12t) provided with a gap G opened on the surface of the insulating layer 9, and a gas sensitive film 14.

[0014] Here, a metal oxide such as tin oxide constituting the gas sensitive film 14 has a property that its resistance value changes when a gas to be measured contacts its surface, and a resistance change type gas sensor (semiconductor type gas sensor) utilizes this property.

[0015] As shown in FIG. 2, the gas sensitive film 14 is provided on the gap G and the surfaces of a pair of opposing electrodes 11 (11b, 11t) and 12 (12b, 12t).

[0016] 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 certain 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.

[0017] The heating part 21 which is the main body part of the heater 20 is formed in a spiral pattern, and a pair of lead parts extending from both ends of the heating part 21 are connected to the 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 energizing 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.

[0018] Note that a heating part 21 is disposed inside a region (thin film part) corresponding to a recess 31 that constitutes a diaphragm of a substrate 30 described later. As a result, since the heating 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.

[0019] 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.

[0020] 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.

[0021] Also, 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, for example, by microfabrication technology (micromachining process: MEMS) using a silicon semiconductor substrate. By providing the 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.

[0022] Next, the configuration of the pair of opposing electrodes 11 and 12 will be described. As shown in FIG. 1, the counter 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. The thin wire portions 11t and 12t correspond to the "tip portions" in the claims, and the base portions 11b and 12b correspond to the "rear end portions" in the claims. In addition, 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 are configured to output the resistance change between the detection electrodes 41 and 42 to the outside. The detection electrodes 41 and 42 are formed of, for example, aluminum (Al) or gold (Au).

[0023] 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. In addition, 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.

[0024] 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. In addition, if the thin wire portions 11t and 12t are made finer and the gap G is narrowed to the nanoscale (nm level), high responsiveness and recovery characteristics can be obtained.

[0025] Further, the base portions 11b and 12b can be formed by depositing, for example, Ti. Further, in this example, the lead portions 11L and 12L also have the same composition (Ti) 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.

[0026] Next, the composition of the counter electrodes 11 and 12 will be described. Among the counter electrodes 11 and 12, the fine 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 fine wire portions 11t and 12t and are electrically connected to the fine wire portions 11t and 12t and the lead portions 11L and 12L. The fine wire portions 11t and 12t are made of a metal or alloy different from that of the base portions 11b and 12b, and the base portions 11b and 12b have lower reactivity to the gas to be measured than the fine wire portions 11t and 12t.

[0027] Specifically, in this example, the fine wire portions 11t and 12t are made of Pt, and the base portions 11b and 12b are made of Ti. Noble metals such as Pt have catalytic activity and promote the chemical reaction of the gas to be measured at the interface between the gas sensitive film 14 and the electrodes 11 and 12. On the other hand, Ti has lower reactivity to the gas to be measured than Pt. In this example, as described above, not only the base portions 11b and 12b but also the lead portions 11L and 12L are made of Ti, which has lower reactivity to the gas to be measured than Pt. However, since the lead portions 11L and 12L are farther from the gap G than the counter electrodes 11 and 12 and the influence of the conductive path described later is relatively small, it is not essential to make the lead portions 11L and 12L have the same composition as the base portions 11b and 12b with the insulating film 16 as the base.

[0028] In this way, by making the base portions 11b and 12b of a metal or alloy having lower reactivity to the gas to be measured than the fine wire portions 11t and 12t, the gas sensitive film 14 itself can be provided in a wide area including the gap G and the surfaces of the pair of counter electrodes 11 and 12. As a result, even if there are misalignments of the photomask or printing misalignments 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 thin wire portions 11t and 12t near the gap G, and the responsiveness can be improved.

[0029] Further, even if the gas-sensitive film 14 is formed on the surfaces of the base portions 11b and 12b other than near the gap G, the chemical reaction of the gas to be measured at this portion is suppressed. For this reason, as shown by the path P2 in FIG. 2, it is possible to suppress the reaction to the gas due to the conductive path between the base portions 11b and 12b far from the gap G, and to suppress the decrease in responsiveness. In other words, by making the thin wire portions 11t and 12t near the gap G more catalytically active and limiting the chemical reaction region, the responsiveness is increased.

[0030] Whether the base portions 11b and 12b are less reactive to the gas to be measured than the thin wire portions 11t and 12t is determined as follows. As model gases of the gas to be measured, (i) a mixed gas of CH4 which is a combustible gas and air at 10 ppm, (ii) a mixed gas of H2 which is a reducing gas and air at 10 ppm, (iii) a mixed gas of NO2 which is an oxidizing gas and air at 10 ppm, and (iv) air gas are each prepared. (A) Using the thin wire portions 11t and 12t or the same material as the thin wire portions 11t and 12t, measure the resistance (sensor resistance) between the electrodes in any one of the above gases (i) to (iii) and (iv). (B) Similarly, using the base portions 11b and 12b or the same material as the base portions 11b and 12b, measure the resistance between the electrodes in the gas selected in (A) from the above gases (i) to (iii) and (iv).

[0031] Let the resistance in (iv) be Ra, and the resistance selected in (A) from (i) to (iii) be Rg. The sensitivity of the sensor is represented by |Rg - Ra| / Ra. If the sensitivity of the sensor is greater for (A) than for (B), it can be determined that the reactivity of the base portions 11b and 12b is low.

[0032] When the thin wire portions 11t and 12t are made of a single metal selected from Au, Ag, Pt, Pd, Rh, Ru, Os, Ir or an alloy containing the metal, the reactivity of the thin wire portions 11t and 12t to the gas to be measured becomes even higher. Moreover, examples of the metal used for the base portions 11b and 12b include Ti, Cr, Ta, W, and Mo.

[0033] When the thin wire portions 11t and 12t are made of a single metal selected from Au, Ag, Pt, Pd, Rh, Ru, Os, Ir or an alloy containing the metal, and the base portions 11b and 12b do not contain these metals or alloys, or contain them in a proportion less than that contained in the thin wire portions 11t and 12t, it is possible to determine that the reactivity to the gas to be measured is low without using the above determination method.

[0034] Also, as shown in FIG. 4, the direction along the line segment connecting the tips (thin wire portions 11t and 12t) of the opposing 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 (corresponding to the distance between the thin wire portions 11t and 12t) G1 and the distance G2 between the base portions 11b and 12b sandwiching the gap G satisfy the relationship G1 < G2. By doing so, the region of the base portions 11b and 12b with low reactivity to the gas to be measured among the opposing electrodes 11 and 12 becomes wider, so that the region between the base portions 11b and 12b far from the gap G becomes a conduction path and reacts with the gas is further suppressed, and the decrease in responsiveness can be further suppressed.

[0035] 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 taken across the base portion 12b and the lead portion 12L near the center of the A-A line in FIG. 1, and the right half of the figure is a cross-section taken across the heater electrode 51 of the A-A line in FIG. 1. First, the silicon substrate 30 (for example, with a thickness of 400 μm) is cleaned with a cleaning liquid, and after appropriately forming the recess 31 that will serve as a diaphragm, the upper insulating layer 9 and the 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 through 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 sequence.

[0036] Next, a contact layer (such as tantalum oxide) 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, 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 contact layer and the heat-generating portion 21 is patterned into a desired shape on the film formation surface of the heater 20 layer by photolithography. Specifically, after spin-coating and drying 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 the heater pattern is formed, and the surplus portion can be removed by wet etching. The heater pattern of the heat-generating portion 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, the remaining portion of the upper insulating layer 9 is formed on the surface of the heater 20 after patterning, 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 sequence.

[0037] Next, as shown in FIG. 5(a), expose the pad portion of the heater 20 connected to the heater electrodes 51 and 52, for example, by photolithography and reactive ion etching. Then, after forming a 1-μm-thick gold film (Au film) on the surface including this pad portion by, for example, DC sputtering, remove the portions other than the corresponding portions with respect to the pad portion by photolithography and wet etching. Similarly, form the detection pad portions 41p and 42p (only 41p is shown in FIG. 5) connected to the detection electrodes 41 and 42, and the detection electrodes 41 and 42 also with an Au film.

[0038] Next, as shown in FIG. 5(b), after forming a Ti film, for example, 10 nm thick to be the base portions 11b and 12b and the lead portions 11L and 12L, pattern it into a predetermined shape by photolithography and wet etching.

[0039] Next, as shown in FIG. 5(c), form the fine wire portions 11t and 12t by depositing Pt, for example, by a lift-off process. Specifically, for example, after applying a photoresist composition ink by spin coating on the surface including the base portion precursor 12bx, pattern it with an electron beam and develop it to form a mold having the shape of the fine wire portions 11t and 12t. Then, form a conformal layer (such as 3 nm of titanium) on the entire surface, and deposit a platinum film (for example, 10 nm thick) to be the fine wire portions 11t and 12t thereon by RF sputtering. Thereafter, remove the resist mold by wet etching to obtain the fine wire portions 11t and 12t. The shortest gap G can be, for example, 20 nm.

[0040] The fine wire portions 11t and 12t may be formed between the base portion 12bx and the heater 20. In that case, form the fine wire portions before forming the base portion. By forming the fine wire portions before forming the base portion, it becomes difficult for the fine wire portions to be disconnected due to the step of the base portion.

[0041] Next, as shown in FIG. 5(d), with the entire structure including the substrate 30 heated to, for example, 450° C., a metal mask with a predetermined opening (for example, 1.5 mm square) is used, and tin oxide is formed to a thickness of 200 nm and gold is formed to a thickness of 1 nm by RF sputtering to form the gas-sensitive film 14. Note that gold has the role of a catalyst for improving the reaction of the measurement gas. As described above, the gas sensor 1 can be manufactured.

[0042] 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.

Description of Reference Numerals

[0043] 1 Gas sensor 9 Insulating layer 11, 12 Pair of opposing electrodes 11b, 12b Base portion (rear end portion) 11t, 12t Thin wire portion (tip portion) 11L, 12L Lead portion 14 Gas-sensitive film D Direction along the line segment connecting the tips of the opposing 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: Each of the opposing electrodes includes a tip portion adjacent to the gap, and a rear end portion disposed at a position farther from the gap than the tip portion and electrically connected to the tip portion and the lead portion. The tip portion and the rear end portion are made of different metals or alloys, The gas sensor is characterized in that the rear end portion has lower reactivity to the gas to be measured than the tip portion.

2. The gas sensor according to claim 1, wherein the tip portion is made of a single metal selected from Au, Ag, Pt, Pd, Rh, Ru, Os, Ir or an alloy containing the metal.

3. When viewed in the direction along the line segment connecting the tips of the opposing electrodes where the gap is minimized, The gas sensor according to claim 1 or 2, wherein the length G1 of the gap and the distance G2 between the rear end portions sandwiching the gap satisfy the relationship G1 < G2.

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