Gas sensor

Abstract

To provide a gas sensor with high sensitivity.SOLUTION: A gas sensor 100 according to the present disclosure comprises a first semiconductor thin film having gas permeability, and a pair of metallic thin films formed on both upper and lower sides of the first semiconductor thin film. The first semiconductor thin film has a semiconductor upper surface and a semiconductor lower surface extending in the horizontal direction, and a semiconductor end surface connecting the semiconductor upper surface and the semiconductor lower surface, the metallic thin films have a metal upper surface and a metal lower surface extending horizontally, and a metal end face connecting the metal upper surface and the metal lower surface, the semiconductor upper surface is in contact with the metal lower surface of one of the metallic thin films, and the semiconductor lower surface is in contact with the metal upper surface of the other metallic thin film, thereby laminating the first semiconductor thin film and the pair of metallic thin films on top of each other, and the metal end face of one metallic thin film and the semiconductor end face are arranged so that they are on the same surface as the metal end face of the other metallic thin film, thereby forming a detection surface.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present disclosure relates to gas sensors. [Background technology]

[0002] As a method for forming a nanogap for a molecular device or a biosensor, for example, the method described in JP 2006-234799 A (Patent Document 1 below) is known. A biosensor is a detector that detects specific molecules that constitute a living organism, such as an enzyme or an antibody. There are chemical, optical, and electrical methods for detecting specific molecules, and among these, electrical detection methods are the most accurate because they allow rapid detection with a small amount of specific molecules. After injecting a solution containing a biological substance into the nanogap, the specific substance can be detected through changes in the electrical properties at both ends of the nanogap. The smaller the gap size of the nanogap, the higher the sensitivity, allowing for more effective detection.

[0003] For example, a planar nanogap for a biosensor is formed as follows. First, a doped backgate thin film, an insulating film, a first Au layer, and a hard mask are sequentially formed on a silicon substrate. Next, a pattern is formed using the hard mask, and the first Au layer is anisotropically etched to form a pattern to be used as one electrode of the planar nanogap. Next, a SAM is formed on the side of the first Au layer to form a gap between the first and second Au layers. Next, a second Au layer is formed on the insulating film exposed by etching to form the other electrode of the planar nanogap. Next, the hard mask is removed to obtain a pattern in which a SAM is formed between two electrodes (the first and second Au layers). Next, the SAM formed between the first and second Au layers is removed to form a planar nanogap corresponding to the length of the SAM. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2006-234799 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, since the above biosensor does not have a sensitive film, it has a problem of low sensitivity as a gas sensor. [Means for solving the problem]

[0006] The gas sensor of the present disclosure is a gas sensor including a first semiconductor thin film having gas permeability, and a pair of metal thin films formed on both the top and bottom sides of the first semiconductor thin film, wherein the first semiconductor thin film has a semiconductor upper surface and a semiconductor lower surface extending in the horizontal direction, and a semiconductor end surface connecting the semiconductor upper surface and the semiconductor lower surface, the metal thin film has a metal upper surface and a metal lower surface extending in the horizontal direction, and a metal end surface connecting the metal upper surface and the metal lower surface, the first semiconductor thin film and the pair of metal thin films are stacked on each other by the semiconductor upper surface being in contact with the metal lower surface of one of the metal thin films and the semiconductor lower surface being in contact with the metal upper surface of the other of the metal thin films, and the metal end surface of one of the metal thin films, the semiconductor end surface, and the metal end surface of the other of the metal thin films are arranged side by side so as to be flush with each other, thereby forming a detection surface. Effect of the Invention

[0007] According to the present disclosure, a highly sensitive gas sensor can be provided. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram of a gas sensor. [Diagram 2] FIG. 2 is a cross-sectional view showing a state in which electrode layers and first gas-sensitive films are alternately formed on an insulating substrate. [Diagram 3] FIG. 3 is a cross-sectional view showing a state in which a cross section is formed in which the end face of the electrode layer and the end face of the first gas-sensitive film are exposed. [Figure 4]FIG. 4 is a cross-sectional view showing a state where a second gas-sensitive film is formed on the cross section. [Diagram 5] FIG. 5 is an enlarged cross-sectional view showing a part of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Outline of the embodiment] First, embodiments of the present disclosure will be listed and described. (1) A gas sensor according to the present disclosure includes a first semiconductor thin film having gas permeability, and a pair of metal thin films formed on both the top and bottom sides of the first semiconductor thin film, wherein the first semiconductor thin film has a semiconductor upper surface and a semiconductor lower surface extending in a horizontal direction, and a semiconductor end surface connecting the semiconductor upper surface and the semiconductor lower surface, the metal thin film has a metal upper surface and a metal lower surface extending in a horizontal direction, and a metal end surface connecting the metal upper surface and the metal lower surface, the first semiconductor thin film and the pair of metal thin films are stacked on top of each other by the semiconductor upper surface being in contact with the metal lower surface of one of the metal thin films and the semiconductor lower surface being in contact with the metal upper surface of the other of the metal thin films, and the metal end surface of one of the metal thin films, the semiconductor end surface, and the metal end surface of the other of the metal thin films are arranged side by side so as to be flush with each other, thereby forming a detection surface.

[0010] When forming a sensing surface by exposing a cross section after alternately depositing a metal thin film and an insulating thin film, the reaction points are only in the vicinity of the sensing surface, resulting in low sensitivity as a gas sensor. In this respect, according to the gas sensor of the present disclosure, the first semiconductor thin film has gas permeability, so that the first semiconductor thin film that serves as the reaction points is not limited to the sensing surface but extends horizontally from the sensing surface, thereby increasing the sensitivity as a gas sensor. In addition, the first semiconductor thin film can improve the selectivity of the gas that can be detected, so that both sensitivity and selectivity can be achieved.

[0011] (2) The gas sensor described in (1) above preferably further comprises a second semiconductor thin film formed on at least the sensing surface. Since a second semiconductor thin film, which acts as a reaction site, is formed on the detection surface, the sensitivity of the gas sensor can be further increased.

[0012] (3) In the gas sensor described in (2), it is preferable that the second semiconductor thin film comprises a sensing thin film formed on the sensing surface, and a protective thin film formed on the metal upper surface. The sensing thin film can increase the sensitivity of the gas sensor, and the protective thin film can protect the upper metal surface.

[0013] [Details of the embodiment] Specific examples of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0014] (Overall configuration of the gas sensor 100) 1, the gas sensor 100 of this embodiment includes an insulating substrate 10, a laminate 11 laminated on the surface of the insulating substrate 10, and a first wiring 15 and a second wiring 16 disposed on the surface of the insulating substrate 10 and connected to the laminate 11. The first wiring 15 is connected to a positive terminal of an ammeter 13 via a power source 12. The negative terminal of the ammeter 13 is connected to the second wiring 16.

[0015] The insulating substrate 10 is made of, for example, silicon (Si). As shown in Fig. 2, the insulating substrate 10 includes a base 10B having a hollow portion 10A in the center, an insulating layer 10C made of a plurality of insulating films formed on the base 10B, and a heater 10D embedded inside the insulating layer 10C. A laminate 11 is laminated on the upper surface of the insulating layer 10C. The heater 10D is capable of heating the laminate 11.

[0016] (Configuration of laminate 11) The laminate 11 includes, from the bottom up, a first electrode layer 31, a first gas-sensitive film 21, a second electrode layer 32, a second gas-sensitive film 22, and a third electrode layer 33. The first electrode layer 31, the second electrode layer 32, and the third electrode layer 33 correspond to the metal thin films of the present disclosure, and the first gas-sensitive film 21 and the second gas-sensitive film 22 correspond to the first semiconductor thin films of the present disclosure.

[0017] In the stacking direction, the distance between the first electrode layer 31 and the second electrode layer 32 (the thickness of the first gas-sensitive film 21) is on the order of nanometers, and may be, for example, 1 nm or more and less than 1000 nm, less than 100 nm, or several tens of nm. The first wiring 15 shown in FIG. 1 is connected to the first electrode layer 31 or the second electrode layer 32. The second wiring 16 is connected to the second electrode layer 32 or the third electrode layer 33. The distance between the second electrode layer 32 and the third electrode layer 33 is also on the order of nanometers.

[0018] The material of each of the electrode layers 31, 32, 33 and each of the wirings 15, 16 is not particularly limited, but may be any material having electrical conductivity, and may be one or more metals selected from metals such as aluminum (Al), copper (Cu), gold (Au), platinum (Pt), etc. The material of the insulating layer 10C is not particularly limited, but may be any material having no electrical conductivity, and may be silicon oxide (SiO 2 ) and other oxides can be used.

[0019] Each of the gas-sensitive films 21 and 22 is configured to change its own electrical characteristics (e.g., resistance value) in response to a specific gas. The material of the gas-sensitive films 21 and 22 may be appropriately selected depending on the gas to be detected, and is not particularly limited. For example, an oxide semiconductor may be used. As the oxide semiconductor, tin oxide (SnO 2 ), zinc oxide (ZnO), tungsten oxide (WO 3 ), indium oxide (In 2 O 3 ), titanium dioxide (TiO 2 ), and vanadium pentoxide (V 2 O 5) or the like can be employed. In the gas sensor 100, when a specific gas reacts with the gas-sensitive films 21, 22 and the resistance values ​​of the gas-sensitive films 21, 22 change, the current value of the ammeter 13 connected to the power source 12 changes in the gas sensor 100. This enables the gas sensor 100 to detect the specific gas.

[0020] 5, each electrode layer 31, 32, 33 has an upper surface 31A, 32A, 33A and a lower surface 31B, 32B, 33C extending in the horizontal direction (direction perpendicular to the stacking direction), and an end surface 31C, 32C, 33C connecting the upper surface 31A, 32A, 33A to the lower surface 31B, 32B, 33B. The upper surfaces 31A, 32A, 33A correspond to the metal upper surface of the present disclosure, the lower surfaces 31B, 32B, 33B correspond to the metal lower surface of the present disclosure, and the end surface 31C, 32C, 33C correspond to the metal end surface of the present disclosure.

[0021] Each of the gas-sensitive films 21, 22 has an upper surface 21A, 22A and a lower surface 21B, 22B extending in the horizontal direction, and an end surface 21C, 22C connecting the upper surface 21A, 22A and the lower surface 21B, 22B. The upper surface 21A, 22A corresponds to a semiconductor upper surface in the present disclosure, the lower surface 21B, 22B corresponds to a semiconductor lower surface in the present disclosure, and the end surface 21C, 22C corresponds to a semiconductor end surface in the present disclosure.

[0022] In addition, the gas-sensitive films 21 and 22 are gas-permeable, and gas that reaches the end faces 21C and 22C can enter the inside of the gas-sensitive films 21 and 22. This allows the gas sensor 100 to detect gas not only at the end faces 21C and 22C, but also at the inside of the gas-sensitive films 21 and 22, which act as reaction points. "Gas-permeable" means that the gas-sensitive film has multiple pores with a pore size of 5 nm or more. There is no upper limit to the pore size, but it is preferable that the pore size is 100 nm or less.

[0023] As shown in FIG. 3, the laminate 11 includes a first horizontal surface 11A extending horizontally in the first electrode layer 31, a second horizontal surface 11C extending horizontally in the third electrode layer 33, and a cross-section 11B connecting the first horizontal surface 11A and the second horizontal surface 11C and inclined with respect to the stacking direction.

[0024] The first electrode layer 31 includes a thin layer 36 overlapping the first horizontal surface 11A in a plan view seen from the stacking direction, a thick layer 38 overlapping the second horizontal surface 11C, and a gradually changing layer 37 connecting the thin layer 36 and the thick layer 38. The thick layer 38 is thicker than the thin layer 36 and is thicker than the second electrode layer 32 and the third electrode layer 33. The gradually changing layer 37 gradually changes in thickness as it is displaced in the horizontal direction.

[0025] 5, the upper surface 31A of the first electrode layer 31 is in contact with the lower surface 21B of the first gas-sensitive film 21. The upper surface 21A of the first gas-sensitive film 21 is in contact with the lower surface 32B of the second electrode layer 32. The upper surface 32A of the second electrode layer 32 is in contact with the lower surface 22B of the second gas-sensitive film 22. The upper surface 22A of the second gas-sensitive film 22 is in contact with the lower surface 33B of the third electrode layer 33. In this way, the first electrode layer 31, the first gas-sensitive film 21, the second electrode layer 32, the second gas-sensitive film 22, and the third electrode layer 33 are stacked one on top of the other.

[0026] The first horizontal surface 11A is the upper surface 36A of the thin layer 36 in the first electrode layer 31. The cross section 11B is an inclined surface in which an end surface 37A of the gradually changing layer 37 in the first electrode layer 31, an end surface 21C of the first gas-sensitive film 21, an end surface 32C of the second electrode layer 32, an end surface 22C of the second gas-sensitive film 22, and an end surface 33C of the third electrode layer 33 are arranged side by side so as to be flush with each other. The second horizontal surface 11C is the upper surface 33A of the third electrode layer 33. The cross section 11B corresponds to the detection surface of the present disclosure.

[0027] The first electrode layer 31 is the bottom layer in the laminate 11, and is a specific electrode layer having a first horizontal surface 11A continuing to the cross section 11B. The angle of a corner 39 formed by the first horizontal surface 11A and the cross section 11B (or an end surface 37A) is an obtuse angle greater than 90 degrees.

[0028] The gas sensor 100 includes a third gas-sensitive film 23 formed over the first horizontal surface 11A, the cross section 11B, and the second horizontal surface 11C of the laminate 11. The third gas-sensitive film 23 includes a first protective thin film 23A formed on the first horizontal surface 11A, a detection thin film 23B formed on the cross section 11B, and a second protective thin film 23C formed on the second horizontal surface 11C. The detection thin film 23B can increase the sensitivity of the gas sensor 100, and the protective thin films 23A and 23C can protect the upper surfaces 36A and 33A. The third gas-sensitive film 23 is made of the same material as the first gas-sensitive film 21 and the second gas-sensitive film 22.

[0029] (Method of manufacturing the gas sensor 100) The method for manufacturing the gas sensor 100 is not particularly limited, but only a general semiconductor process can be used. As shown in FIG. 2, a metal thin film (Pt) and a semiconductor thin film (SnO 2 Specifically, a first electrode layer 31, a first gas-sensitive film 21, a second electrode layer 32, a second gas-sensitive film 22, and a third electrode layer 33 are formed on the insulating layer 10C of the insulating substrate 10 in this order from the bottom.

[0030] 3, ion etching is performed from the upper side of the third electrode layer 33 to form the cross section 11B and the first horizontal surface 11A, thereby flatly exposing the end faces 31C, 32C, and 33C of the electrode layers 31, 32, and 33 and the end faces 21C and 22C of the gas-sensitive films 21 and 22. Next, as shown in FIG. 4, the third gas-sensitive film 23 is formed in the region extending from the first horizontal surface 11A through the cross section 11B to the second horizontal surface 11C.

[0031] (Functions and Effects of Gas Sensor 100) Since the cross section 11B is formed by ion etching to form a nano-gap electrode, a gas sensor 100 with high response can be realized. Since the gas-permeable gas-sensitive films 21 and 22 are arranged between the electrode layers 31, 32, and 33, a gas sensor 100 with high sensitivity and high selectivity can be realized. That is, the gas to be detected is taken into the gas-sensitive films 21 and 22 from the cross section 11B, so that the reaction points spread horizontally, and the sensitivity of the gas sensor 100 is increased. In addition, when the gas-sensitive film is a porous body, the porosity, hole shape, hole size, etc. can be appropriately adjusted to select the gas to be detected. Furthermore, since the third gas-sensitive film 23 is formed on the cross section 11B, the detection surface can be expanded, and a gas sensor 100 with even higher sensitivity can be realized.

[0032] <Other embodiments> (1) In the above embodiment, the gas sensor provided with the third gas-sensitive film 23 is illustrated, but the gas sensor may not include the third gas-sensitive film 23.

[0033] (2) In addition to the above-described embodiment, the configuration of the laminate may be modified as appropriate. The laminate may have at least a first electrode layer 31, a first gas-sensitive film 21 formed on the first electrode layer 31, and a second electrode layer 32 formed on the first gas-sensitive film 21.

[0034] (3) In addition to the above embodiment, the configuration of the third gas-sensitive film 23 can be appropriately changed. The third gas-sensitive film 23 may be in contact with the first horizontal surface 11A and the cross section 11B, but may not be in contact with the second horizontal surface 11C. The third gas-sensitive film 23 may be in contact with a part of the first horizontal surface 11A and the cross section 11B, or may be in contact with only the cross section 11B. [Explanation of symbols]

[0035] 10: insulating substrate 10A: hollow portion 10B: base 10C: insulating layer 10D: heater 11: laminate 11A: first horizontal surface 11B: cross section (detection surface) 11C: second horizontal surface 12: power supply 13: ammeter 15: first wiring 16: second wiring 21: First gas sensing film (first semiconductor thin film) 21A: Upper surface (semiconductor upper surface) 21B: Lower surface (semiconductor lower surface) 21C: End face (semiconductor end face) 22: Second gas sensing film (first semiconductor thin film) 22A: Upper surface (semiconductor upper surface) 22B: Lower surface (semiconductor lower surface) 22C: End face (semiconductor end face) 23: Third gas sensing film (second semiconductor thin film) 23A: First protective thin film 23B: Detection thin film 23C: Second protective thin film 31: First electrode layer (metal thin film) 31A: Upper surface (metal upper surface) 31B: Lower surface (metal lower surface) 31C: End face (metal end face) 32: Second electrode layer (metal thin film) 32A: Upper surface (metal upper surface) 32B: Lower surface (metal lower surface) 32C: End face (metal end face) 33: Third electrode layer (metal thin film) 33A: Upper surface (metal upper surface) 33B: Lower surface (metal lower surface) 33C: End face (metal end face) 36: Thin layer 36A: Upper surface 37: Creep layer 37A: End face 38: Thick layer 39: Corner 100: Gas sensor

Claims

1. A gas sensor comprising: a first semiconductor thin film having gas permeability; and a pair of metal thin films formed on both upper and lower sides of the first semiconductor thin film, the first semiconductor thin film has a semiconductor upper surface and a semiconductor lower surface extending in a horizontal direction, and a semiconductor end surface connecting the semiconductor upper surface and the semiconductor lower surface; the thin metal film has an upper metal surface and a lower metal surface extending in a horizontal direction, and a metal end surface connecting the upper metal surface and the lower metal surface, the first semiconductor thin film and the pair of metal thin films are stacked on each other by the semiconductor upper surface being in contact with the metal lower surface of one of the metal thin films and the semiconductor lower surface being in contact with the metal upper surface of the other of the metal thin films, A gas sensor, wherein the metal end face of one of the metal thin films, the semiconductor end face, and the metal end face of the other of the metal thin films are arranged side by side so as to be flush with each other, thereby forming a detection surface.

2. 2. The gas sensor according to claim 1, further comprising: a second semiconductor thin film formed on at least the sensing surface.

3. 3. The gas sensor according to claim 2, wherein the second semiconductor thin film comprises a sensing thin film deposited on the sensing surface and a protective thin film deposited on the upper metal surface.

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