Gas sensor and method for manufacturing a gas sensor

Abstract

To provide a gas sensor which may offer improved response.SOLUTION: A gas sensor 10 is provided, comprising an electrode unit 50 with one or more electrode pairs 70, each consisting of a first electrode 71 and a second electrode 72 facing each other in a first direction via a gap, a sensitive film 60 provided between the first electrode 71 and the second electrode 72 of each electrode pair 70, and a voltage application unit 80 configured to apply voltage to the electrode unit 50 across the first electrode 71 side and the second electrode 72 side. The sensitive film 60 is a dense body.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a gas sensor and a method for manufacturing a gas sensor. [Background technology]

[0002] Conventional chemical sensors using MEMS have an electrode pair, and the shortest distance between these electrodes is several micrometers. While such sensors exhibit sensitivity, their responsiveness is poor, requiring a long time for concentration detection, making real-time measurements over periods of several seconds to tens of seconds difficult. On the other hand, as described in Patent Document 1, there are reports of gas sensors using nanogap electrodes with improved responsiveness by setting the distance between electrodes to approximately several nanometers to 100 nanometers. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-32746 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, while the sensor described in Patent Document 1 showed a fast response when exposed to the detection gas, it took a long time for the sensor characteristics to recover in actual use. Furthermore, the mechanism for improving this response was not clear, resulting in insufficient knowledge about what kind of sensor configuration would achieve a fast response.

[0005] This disclosure provides a gas sensor that can improve responsiveness. [Means for solving the problem]

[0006] One of the gas sensors disclosed herein is One or more electrode pairs having a first electrode and a second electrode, an electrode portion in which the first electrode and the second electrode are spaced apart from each other while facing in a first direction in the electrode pair, A sensitive film disposed between the first electrode and the second electrode in the electrode pair, A voltage application unit that applies a voltage between the first electrode side and the second electrode side with respect to the electrode portion, Comprising The sensitive film is characterized by being a dense body.

[0007] A method for manufacturing a gas sensor according to one aspect of the present disclosure is The method for manufacturing the above gas sensor, The sensitive film is formed by physical vapor deposition or chemical vapor deposition.

[0008] A method for manufacturing a gas sensor according to one aspect of the present disclosure is The sensitive film is subjected to a heat treatment after film formation.

Advantages of the Invention

[0009] The technology according to the present disclosure can improve responsiveness.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a side sectional view schematically showing a gas sensor of the first embodiment. [Figure 2] FIG. 2 is a partially enlarged view of the gas sensor of FIG. 1. [Figure 3] FIG. 3 is a plane SEM image of an electrode pair of a gas sensor. [Figure 4] FIG. 4 is a plane SEM image of a sensitive film of a gas sensor. [Figure 5] FIG. 5 is a schematic diagram for explaining an electrode pair and an interelectrode volume. [Figure 6] FIG. 6 is an explanatory diagram for explaining a method of measuring width and height at a cross section of a first electrode. [Figure 7] FIG. 7 is a process diagram for explaining a manufacturing process of a gas sensor. [Figure 8] Figure 8 is a process diagram illustrating the manufacturing process of the gas sensor, following Figure 7. [Figure 9] Figure 9 is an explanatory diagram illustrating the sensor evaluation device in general terms. [Figure 10] Figure 10(A) is an explanatory diagram showing the time change of the output in the gas sensor of Comparative Example 2, and Figure 10(B) is an explanatory diagram showing the time change of the output in the gas sensor of Experimental Example 4. [Figure 11] Figure 11(A) is an explanatory diagram illustrating the tin oxide depletion layer before the gas sensor is exposed to the test gas, and Figure 11(B) is an explanatory diagram illustrating the tin oxide depletion layer after the gas sensor is exposed to the test gas. [Modes for carrying out the invention]

[0011] Embodiments of the present disclosure are listed and illustrated below. The features [1] to [4] illustrated below may be combined in any way that is not contradictory.

[0012] [1] The gas sensor of the present invention is An electrode portion comprising one or more electrode pairs having a first electrode and a second electrode, wherein the first electrode and the second electrode in the electrode pair are facing each other in a first direction and spaced apart from each other, A sensitive membrane is disposed between the first electrode and the second electrode in the electrode pair, A voltage application unit that applies a voltage between the first electrode side and the second electrode side to the electrode portion, Equipped with, The aforementioned sensitive membrane is a dense body.

[0013] In the gas sensor described in [1] above, densifying the sensitive film facilitates electron movement in the depletion layer, thereby improving the response speed (rise rate and return rate) of the output electrical signal.

[0014] [2] In the gas recovery described above, At least some of the particles constituting the sensitive membrane are electrically coupled to each other.

[0015] In the gas sensor described in [2] above, the contact resistance between particles is reduced by electrically conductive coupling between the particles constituting the sensitive film, which facilitates the formation of electron conduction paths and facilitates electron movement in the depletion layer.

[0016] [3] A method for manufacturing the gas sensor described above, The sensitive film is formed by physical vapor deposition or chemical vapor deposition.

[0017] In the gas sensor described in [3] above, a homogeneous sensitive film can be formed, enabling a stable output of electrical signals.

[0018] [4] A method for manufacturing the gas sensor described above, The aforementioned sensitive film is subjected to a heat treatment after film formation.

[0019] In the gas sensor described in [4] above, bonding becomes easier between the particles constituting the sensitive film, reducing the contact resistance between particles and facilitating electron movement in the depletion layer.

[0020] <First Embodiment> 1-1. Gas Sensor Configuration Hereinafter, a first embodiment embodying the present invention will be described with reference to Figures 1 to 11. The gas sensor 10 of the first embodiment shown in Figure 1 is an example of the gas sensor of this disclosure. In the following description, for the sake of explanation, the vertical direction shown in Figures 1 and 2 will be defined as the vertical direction, but it does not have to coincide with the vertical direction in the actual arrangement of the gas sensor 10.

[0021] The gas sensor 10 comprises a substrate 20, insulating layers 31-35, a heating element 40, an electrode section 50, a sensitive film 60, a voltage application section 80 (see Figure 9), and an output section 90 (see Figure 9). The electrode section 50 comprises one or more electrode pairs 70 having a first electrode 71 and a second electrode 72. Preferably, the electrode section 50 comprises four or more electrode pairs 70, and more preferably, 100 or more electrode pairs 70.

[0022] Figure 3 illustrates an electrode section 50 provided with multiple electrode pairs 70. As shown in Figure 3, in the electrode pair 70, the first electrode 71 and the second electrode 72 are separated from each other in a first direction with a gap in between. The first direction is parallel to the upper surface of the substrate 20 and is the direction in which the first electrode 71 and the second electrode 72 extend. The first direction is also the direction in which the first electrode 71 and the second electrode 72 face each other. The first direction is also perpendicular to the direction in which the multiple first electrodes 71 are arranged and perpendicular to the direction in which the multiple second electrodes 72 are arranged.

[0023] The substrate 20 is made of, for example, a silicon wafer. The material of the substrate 20 is not particularly limited, but is a semiconductor such as silicon. When a ceramic substrate such as sapphire, zirconia, or alumina is used for the substrate 20, the formation of the insulating layers 31 to 34 described later can be omitted. The planar shape of the substrate 20 is not particularly limited, and can be, for example, rectangular or circular. The size of the substrate 20 is not particularly limited, and can be a rectangular substrate with sides of 0.1 mm to 10 mm, or a circular substrate with a similar area. The thickness of the substrate 20 is not particularly limited, and can be, for example, 400 μm to 500 μm.

[0024] The substrate 20 may have a space 21 formed by cutting out a part of itself. The space 21 is, for example, a cavity that opens and penetrates both the front and back surfaces of the substrate 20, or a recess that opens on only one of the front or back surfaces of the substrate 20. The shape of the opening 22 of the space 21 and the cross-sectional shape of the inside of the space 21 are not particularly limited, but are usually simple shapes such as rectangles and circles. The size of the space 21 is not particularly limited, but if the space 21 is a cavity, it is preferable that the opening area of ​​one of the openings on the front or back of the substrate 20 is larger. This opening area is not particularly limited, but the larger opening area is 0.01 mm². 2 ~4mm 2 Preferably, 0.25 mm 2 ~2mm 2 It is more preferable to do so. If the space 21 is a recess, the opening area can be the same range as that of a cavity.

[0025] The electrode portion 50, described later, is insulated from the substrate 20 by insulating layers 31-35 provided on the substrate 20. The insulating layers 31-35 may be formed over the entire surface of the substrate 20, or they may be formed over only a portion of the substrate 20. The insulating layers 31, 33, and 35 are laminated on one side surface (top surface) of the substrate 20. The substrate 20 is provided with a space portion 21. When the substrate 20 has a space portion 21, the insulating layers 31, 33, and 35 cover the opening 22 of the space portion 21 and are supported by the substrate 20. The insulating layers 31, 33, and 35 may be formed to cover the entire surface of the opening 22, or they may be formed to cover a portion of the opening 22 as long as they can be supported by the substrate 20. The insulating layers 32 and 34 are laminated on the other side surface (bottom surface) of the substrate 20. The insulating layers 31-35 only need to have sufficient insulating properties, and their material is not particularly limited. For example, examples of materials for the insulating layers 31-35 include SiO2, Si3N4, and SiO2. x N y Examples of silicon compounds include (where x and y are arbitrary values). The shape and thickness of the insulating layers 31 to 35 are not particularly limited. A single insulating layer may be provided instead of insulating layers 31, 33, and 35. By forming a silicon oxide film on the substrate 20, leakage current between the electrodes of the electrode pair 70 can be suppressed.

[0026] The gas sensor 10 is provided with multiple heating elements 40. The heating elements 40 are provided on an insulating layer 33 and covered by an insulating layer 35. Leads (not shown) for supplying power from an external circuit are connected to the heating elements 40 via contact parts (not shown). Contact pads (not shown) are provided on the surface of the contact parts for the heating elements. The heating elements 40 generate heat when a voltage is applied. This causes the sensitive film 60 to heat up and become activated, enabling the measurement of gas concentration. The position of the heating elements 40 is not particularly limited as long as it is inside the insulating layers 31, 33, and 35, but if a space 21 is formed in the substrate 20, it is preferable to place the heating elements 40 in a position corresponding to the space 21. By placing the heating elements 40 in a position corresponding to the space 21, it is possible to suppress the heat from the heating elements 40 from dissipating into the substrate 20. In addition, heat can be efficiently transferred to the sensitive film 60, and the temperature of the sensitive film 60 can be controlled with greater precision. The position corresponding to the space 21 means a positional relationship in which at least a part of the heating element 40 overlaps with the space 21 in the thickness direction of the substrate 20, and it is more preferable that the entire heating element 40 overlaps with the space 21. The heating element 40 is conductive, and its material is not particularly limited. For example, platinum, platinum alloy, nickel alloy, chromium alloy, and nickel-chromium alloy can be used for the heating element 40. Among these, it is preferable to use platinum or nickel-chromium alloy for the heating element 40, as these have a large temperature coefficient of resistance and their resistance value and temperature coefficient of resistance do not change easily even with long-term repeated use.

[0027] The electrode portion 50 (one or more electrode pairs 70) is provided on the surface of the insulating layer 35. One end of the first electrode 71 and one end of the second electrode 72 are arranged opposite each other to form a nanogap. In this first embodiment, a nanogap refers to a distance of 100 nm or less between one end of the first electrode 71 and one end of the second electrode 72.

[0028] The first electrode 71 and the second electrode 72 are formed from one or more types of metals. Examples of metals that make up the first electrode 71 and the second electrode 72 include gold (Au) and platinum (Pt). The first electrode 71 and the second electrode 72 have a two-layer structure, for example, a lower electrode and an upper electrode. For example, the lower electrode is made of titanium (Ti) and the upper electrode is made of platinum (Pt).

[0029] The first electrode 71 and the second electrode 72 are connected to electrode contacts (not shown) via lead portions (not shown) for supplying power from an external circuit. Contact pads (not shown) are provided on the surface of the electrode contacts.

[0030] The sensitive film 60 is provided on the upper surface of the first electrode 71, the upper surface of the second electrode 72, and the upper surface of the insulating layer 35. The sensitive film 60 is positioned between the first electrode 71 and the second electrode 72 in the electrode pair 70. The sensitive film 60 is preferably made of a metal oxide. The sensitive film 60 is preferably made of an oxide semiconductor. The oxide semiconductor is preferably an n-type semiconductor. The n-type semiconductor is preferably SnO2, ZnO, In2O3, Fe2O3, ITO (tin-doped indium), or WO3, which have easily changeable resistance values.

[0031] The sensitive film 60 is preferably a dense material. A dense material is an object with a porosity of less than 10%. The surface of the sensitive film 60 is, for example, the surface of a dense material as shown in Figure 4. By densifying the sensitive film 60, electron movement in the depletion layer becomes easier, and the response speed (rise rate and return rate) of the electrical signal output from the output unit 90 can be improved.

[0032] At least some of the particles constituting the sensitive film 60 are electrically coupled (bonded) to each other. Electrical coupling between particles is defined as a state in which at least half of the surface area of ​​a particle, excluding its edges, is in contact with the surface of other particles, and a path for electrons that have conducted within the particle to conduct is formed, covering at least half of the particle's surface area. Electrical coupling between particles allows electricity to flow between them (a state in which current can flow). Electrical coupling between the particles constituting the sensitive film 60 reduces the contact resistance between particles, facilitating electron movement in the depletion layer.

[0033] The voltage application unit 80 applies a voltage to the electrode unit 50 between the first electrode 71 side and the second electrode 72 side. The voltage application unit 80 includes, for example, a voltage control circuit (such as a constant voltage circuit) that performs the operation of applying a voltage to the electrode unit 50 based on power supplied from a power source, and a control unit (such as a CPU) that controls the voltage control circuit.

[0034] The output unit 90 outputs the gas concentration detected using the electrode unit 50 as an electrical signal. The output unit 90 may also have an amplifier or the like to amplify the electrical signal.

[0035] The gas detected by the gas sensor 10 is preferably flammable. The resistance of the gas sensor 10 can be changed by the flammable gas. Preferred flammable gases include ketones (such as acetone), alcohols (such as ethanol), carbon monoxide, nitrogen oxides (NO, N2O), aldehydes (such as acetaldehyde), sulfide gases (such as hydrogen sulfide), and VOC gases (such as toluene).

[0036] The gas sensor 10 is preferably of the resistance measurement type or impedance measurement type.

[0037] 1-2. Conditions for electrical indicators in the electrode section In the gas sensor 10, the electrical index at the electrode section 50 will be explained. The value obtained by dividing the voltage V applied to the electrode section 50 by the inter-electrode volume is defined as the electrical index (hereinafter also simply referred to as the index) X at the electrode section 50. The inter-electrode volume B is the product of the inter-electrode distance D and the electrode area S. That is, the index X is expressed by the following equation (1). X=V / B=V / (D×S)…Formula (1) For example, in the case where there is one electrode pair 70 as shown in Figure 5, the volume of the space SP between the first electrode 71 and the second electrode 72 corresponds to the inter-electrode volume B.

[0038] The electrode-to-electrode distance D is the distance (shortest distance) in the first direction between the first electrode 71 and the second electrode 72 constituting the electrode pair 70 when the electrode section 50 is composed of one electrode pair 70. On the other hand, when the electrode section 50 is composed of one electrode pair 70, the electrode-to-electrode distance D is the minimum of the distances (shortest distances) in the first direction between the first electrode 71 and the second electrode 72 in all electrode pairs 70.

[0039] The electrode area S is the electrode area of ​​one electrode (for example, the first electrode 71) of the electrode pair 70 when the electrode section 50 is composed of one electrode pair 70. On the other hand, if the electrode section 50 is composed of multiple electrode pairs 70, the electrode area S is the sum of the electrode areas of one electrode (for example, the first electrode 71) of all the electrode pairs 70.

[0040] A specific method for deriving the electrode area in each electrode section 50 will now be described. First, one first electrode 71 is selected (if there is only one electrode pair 70, the first electrode 71 of that electrode pair 70 is selected), and a cutting plane is set. For example, the first electrode 71 of an electrode pair 70 for which the electrode distance D is specified is selected. For example, the cutting plane of the electrode section 50 is defined as a plane that is perpendicular to the surface of the substrate 20 and passes through a position at a distance (preferably 25 nm) between one end (the end on the second electrode 72 side) of the selected first electrode 71 and the other end (the end opposite to the second electrode 72) that is between 10 nm and 100 nm. In the example in Figure 3, the AA cross section is the cutting plane. The area of ​​each first electrode 71 that appears on the cutting plane is defined as the electrode area. The electrode area S is the total area of ​​the cross-sections of all first electrodes 71 at the cutting plane obtained by cutting all first electrodes 71 at any position between 10 nm and 100 nm in the first direction.

[0041] An example of a method for measuring the electrode area appearing on a cross-section is described below. The width W of the first electrode 71 is defined as the shortest length at which the cross-section of the first electrode 71 overlaps with a straight line that intersects with the side surface of the first electrode 71 and is parallel to the surface of the substrate 20. Here, the side surface is the portion on both the left and right sides of the cross-section that is inclined at 45° or less with respect to a straight line perpendicular to the surface of the substrate 20 (also called a perpendicular line), and the portion below that. If both the left and right sides of the cross-section are curved, the side surface is the portion where the tangent to the curved portion is inclined at 45° or less with respect to the perpendicular line. The height H of the first electrode 71 is defined as the shortest length at which the cross-section of the first electrode 71 overlaps with a straight line that intersects with the top surface of the first electrode 71 and is parallel to the surface of the substrate 20. The product of the width W and height H of the first electrode 71 is defined as the electrode area appearing on the cross-section of the first electrode 71. Here, the top surface is the portion on the left-right center side of the cross-section, excluding the side surface.

[0042] For example, in the cross-sectional view of the first electrode 71 as shown in FIG. 6(A), a straight line L1 parallel to the plate surface of the substrate 20 has the shortest overlap with the cross-sectional view, and the overlap length W1 is the width. Also, a straight line L2 perpendicular to the plate surface of the substrate 20 has the shortest overlap with the cross-sectional view, and the overlap length H1 is the height. Similarly, in the cross-sectional view of the first electrode 71 as shown in FIG. 6(B), a straight line L3 parallel to the plate surface of the substrate 20 has the shortest overlap with the cross-sectional view, and the overlap length W2 is the width. Also, a straight line L4 perpendicular to the plate surface of the substrate 20 has the shortest overlap with the cross-sectional view, and the overlap length H2 is the height. Similarly, in the cross-sectional view of the first electrode 71 as shown in FIG. 6(C), a straight line L5 parallel to the plate surface of the substrate 20 has the shortest overlap with the cross-sectional view, and the overlap length W3 is the width. Also, a straight line L6 perpendicular to the plate surface of the substrate 20 has the shortest overlap with the cross-sectional view, and the overlap length H3 is the height.

[0043] Note that the electrode area S may be obtained by selecting a predetermined number (for example, 100) of the first electrodes 71 among the plurality of first electrodes 71, taking the average value of the cross-sectional areas of the selected first electrodes 71, and multiplying the obtained average value by the number of the first electrodes 71 included in the electrode portion 50.

[0044] The index (index X represented by formula (1)) in the electrode portion 50 is greater than 4.2×10 16 V / m 3 and less than 2.0×10 25 V / m 3 It is preferably less than 6.0×10 19 V / m 3 and greater than 2.0×10 25 V / m 3 More preferably, it is less than 6.0×10 20 V / m 3 and greater than 2.0×10 25 V / m 3 With such a configuration, the response speed (rise speed and return speed) of the electrical signal output by the output unit 90 can be improved.

[0045] 1-3. Conditions of the electrode area The electrode area S is 2.5×10 -15 m2 The above 2.1 × 10 -11 m 2 The following is true: 2.1 × 10 -14 m 2 The above 2.1 × 10 -12 m 2 The following is preferable. Note that the electrode area S is the first electrode 71 of the electrode pair 70 when the electrode section 50 is composed of one electrode pair 70, and the sum of the electrode areas of the first electrode 71 of all electrode pairs 70 when the electrode section 50 is composed of multiple electrode pairs 70. With this configuration, the response speed (rise rate and return rate) of the electrical signal output by the output section 90 can be improved.

[0046] 1-4. Conditions for the distance between electrodes The minimum inter-electrode distance among all electrode pairs 70 in the electrode section 50 is preferably 5 nm to 100 nm. With this configuration, a tunnel current effect can be obtained between the electrodes, further improving the response speed (rise rate and return rate) of the electrical signal output from the output section 90.

[0047] 1-5. Conditions for the total width of the first electrode The sum of the widths W of the first electrodes 71 in all electrode pairs 70 in the electrode section 50 (or the width W of a single first electrode 71 if there is only one electrode pair 70) is preferably 20 nm or more. This configuration makes it possible to reduce the resistance of electron transfer and improve the response speed (rise rate and return rate) of the output electrical signal.

[0048] 1-6. Method for Manufacturing Gas Sensors A method for manufacturing the gas sensor 10 embodying the present invention will be described with reference to Figures 7 and 8. The method for manufacturing the gas sensor 10 includes the steps of forming an electrode portion 50 on a substrate 20 by a lift-off method, and forming a sensitive film 60 in the gap portion of the electrode portion 50.

[0049] First, insulating layers 31, 33, 35 and a heating element 40 are formed on the cleaned substrate 20 (silicon wafer). The cleaned substrate 20 is placed in a heat treatment furnace and subjected to thermal oxidation treatment to form a silicon oxide film (see Figure 7(A)) which will become the insulating layer 31 (first insulating layer) over the entire surface of the substrate 20. Note that in Figures 7(A) to (C) and 8(A) to (D), only the configuration of the upper surface of the substrate 20 is shown, and the explanation of the formation of insulating layers 32 and 34 is omitted. Next, a silicon nitride film is formed on the insulating layer 31 by plasma CVD using, for example, SiH4 and NH3 as source gases, as shown in Figure 7(A), to form the insulating layer 33 (lower insulating layer). After that, as shown in Figure 7(B), a heating element 40 is formed on the surface of the insulating layer 33 by, for example, a sputtering method. For example, the heating element 40 is composed of a Ti layer and a Pt layer on top of it. Next, the resist is patterned using photolithography, and the pattern of the heating element 40 is formed by etching.

[0050] The method for forming the heating element 40 is not particularly limited. For example, the components that will become the heating element 40 are deposited on the surface of the insulating layer 33, and then unnecessary parts are removed by various etching methods similar to those exemplified in the method for forming the space 21. Next, as shown in Figure 7(C), an insulating layer 35, for example a silicon nitride film, is formed on the surface to form the insulating layer 35 (upper insulating layer). In this way, the insulating layers 33, 35 (second insulating layer) and the heating element 40 disposed inside the insulating layers 33, 35 (second insulating layer) are formed. It should be noted that the components that will become the insulating layers 31 to 35 can also be deposited on the surface of the substrate 20, or the insulating layers that have been formed in advance can be bonded to the surface of the substrate.

[0051] Next, as shown in Figure 8(A), a photoresist composition is applied to the insulating layer 35 by spin coating and dried to form a resist film 37. Note that in Figures 8(A) to (D), the components below the insulating layer 35 and the heating element 40 are not shown. The resist film 37 is formed to a thickness of, for example, 20 nm to 40 nm. The resist film 37 is formed using a photoresist composition for electron beam lithography. The mask pattern is fabricated by exposing the resist film 37 by electron beam lithography and developing it. The mask pattern is formed such that the gap length of the electrode pair 70 is, for example, 20 nm.

[0052] Next, as shown in Figure 8(B), a metal film 70A is formed to cover substantially the entire surface of the insulating layer 35 and the resist film 37. The metal film 70A is made of gold (Au) or platinum (Pt). The thickness of the metal film 70A is preferably 5 nm to 20 nm, for example, it is formed to 15 nm. Such a metal film 70A is formed, for example, by electron beam deposition. The mask pattern is peeled off, and at the same time, the metal film 70A overlapping that portion is removed. As a result, as shown in Figure 8(C), an electrode portion 50 (multiple electrode pairs 70) with the metal film 70A stacked on top of it is formed. For example, the distance between electrodes (nanogap length) in the electrode pair 70 is, for example, 20 nm. The width of the first electrode 71 is formed to be, for example, 15 nm. In addition to the electron beam lithography method described above, the electrode pairs 70 may also be formed using nanoimprint lithography, a method in which a pattern is transferred to the resist by pressing a mold (die) that serves as the master plate. Furthermore, after the electrodes are formed, annealing (for example, heat treatment at 360°C) may be performed in a vacuum.

[0053] As shown in Figure 1, when a space 21 is to be provided in the substrate 20, it can be formed, for example, by removing a part of the substrate 20 by etching. In this case, the etching method is not particularly limited, and wet etching or dry etching may be used. Furthermore, the etching method may be anisotropic etching or isotropic etching. When forming a cavity in the substrate 20, wet etching using an anisotropic etching solution is preferred.

[0054] Next, as shown in Figure 8(D), a sensitive film 60 is formed on the upper surface of the first electrode 71, the upper surface of the second electrode 72, and the upper surface of the insulating layer 35. The sensitive film 60 is preferably formed by physical vapor deposition (PVD) or chemical vapor deposition (CVD). As an example of physical vapor deposition (PVD), sputtering is used. The sensitive film 60 is formed, for example, by an RF sputtering apparatus using a tin oxide (SnO2) target for sputtering.

[0055] The sensitive film 60 may be subjected to heat treatment after film formation. This causes the particles contained in the sensitive film 60 to bond together, forming a dense sensitive film 60. In this heat treatment, a heater (heating element 40) installed in the gas sensor is used to change the temperature by applying an electric current. This makes it easier for the particles constituting the sensitive film 60 to bond together, reduces the contact resistance between particles, and facilitates the movement of electrons in the depletion layer.

[0056] 1-7. Effects of the First Embodiment The following explanation concerns an example of the effects of this configuration. In the first embodiment, the gas sensor 10 has a dense sensing film 60. This facilitates the movement of electrons in the depletion layer, improving the response speed (rise rate and return rate) of the output electrical signal.

[0057] In the gas sensor 10 of the first embodiment, at least some of the particles constituting the sensitive film 60 are electrically coupled. This allows for electrical coupling between the particles constituting the sensitive film 60, thereby reducing the contact resistance between the particles and facilitating electron movement in the depletion layer.

[0058] In the first embodiment, the gas sensor 10 is manufactured by forming the sensitive film 60 using either physical vapor deposition or chemical vapor deposition. This allows for the formation of a homogeneous sensitive film 60, enabling stable output of electrical signals.

[0059] The manufacturing method of the gas sensor 10 in the first embodiment involves applying a heat treatment to the sensitive film 60 after film formation. This facilitates bonding between the particles constituting the sensitive film 60, reduces contact resistance between particles, facilitates the formation of electron conduction paths, and facilitates electron movement in the depletion layer. [Examples]

[0060] The present invention will be described in more detail below with reference to examples. 1. Manufacturing of gas sensors The gas sensors in Experimental Examples 1-6 and Comparative Examples 1 and 2 shown in Table 1 were manufactured by the process described below. Experimental Example 1-6 corresponds to the example. The gas sensors in Experimental Examples 1-6 and Comparative Examples 1 and 2 used a 3mm x 5mm silicon substrate (hereinafter simply referred to as the substrate). Insulating layers were formed on both the front and back surfaces of the substrate. The insulating layer consisted of a first insulating layer made of silicon oxide (SiO2) and a second insulating layer made of silicon nitride (Si3N4) laminated on the surface of the first insulating layer.

[0061] [Table 1]

[0062] The substrate has a space formed on the side where the insulating layer is formed, and the area of ​​the opening of the space is 1 mm². 2 The insulation layer has a heating element formed at a position corresponding to the empty space. A lead portion for the heating element (not shown) for power supply is connected to this heating element. The lead portion for the heating element has a contact portion for connecting to an external circuit. The heating element and the lead portion for the heating element have a two-layer structure consisting of a Pt layer and a Ti layer.

[0063] One or more pairs of electrodes are formed on the surface of the insulating layer at positions corresponding to the heating element. A sensitive film is formed on the surface of the insulating layer at the positions corresponding to the heating element. The sensitive film is formed on the surface of the insulating layer so as to be in contact with the upper surface of the electrodes. Electrode leads are connected to the electrodes. The electrode leads have electrode contacts for connecting to an external circuit.

[0064] The electrodes in Experimental Example 1-6 have a lower electrode formed on the surface of an insulating layer and an upper electrode formed on the upper surface of this lower electrode. The lower electrode is made of Ti. The upper electrode is made of Pt. The thickness of the lower electrode is 3 nm. The thickness of the upper electrode is 10 nm. The electrodes of Comparative Examples 1 and 2 have a lower electrode formed on the surface of an insulating layer and an upper electrode formed on the upper surface of the lower electrode. The lower electrode is made of Ti. The upper electrode is made of Pt. The thickness of the lower electrode is 20 nm. The thickness of the upper electrode is 40 nm.

[0065] (1) Cleaning of the circuit board A 400 μm thick silicon substrate was immersed in a cleaning solution and cleaned.

[0066] (2) Formation of an insulating layer The cleaned silicon substrate was placed in a heat treatment furnace, and a 100 nm thick silicon oxide film, which would serve as the insulating layer (first insulating layer), was formed over the entire surface of the substrate by thermal oxidation treatment.

[0067] (3) Formation of insulating layer and heating element (including heating element lead portion) A silicon nitride film was formed on the first insulating layer of the substrate by plasma CVD using SiH4 and NH3 as source gases. A silicon nitride film with a thickness of 400 nm was formed on one side (top surface) of the substrate to form an insulating layer (lower insulating layer). A silicon nitride film with a thickness of 200 nm was formed on the other side (bottom surface) of the substrate to form an insulating layer (lower insulating layer). Subsequently, a heating element was formed on the surface of the second insulating layer using a DC sputtering apparatus. Specifically, a Ti layer with a thickness of 25 nm was formed on the surface of the second insulating layer, and then a Pt layer with a thickness of 250 nm was formed on the surface of this Ti layer to form the heating element. Next, the resist patterning was performed by photolithography and the pattern of the heating element was formed by etching. After that, a silicon nitride film with a thickness of 400 nm, which would become the upper insulating layer of the second insulating layer, was formed in the same manner as for the lower insulating layer. In this way, the second insulating layer and the heating element disposed inside the second insulating layer were formed.

[0068] (4) Formation of contact portion for heating element Using a dry etching method, the insulating layer was etched to create holes in the area corresponding to the heat-generating element contact, exposing the area that would become the heat-generating element contact (part of the heat-generating element). Subsequently, a 20 nm thick Ti layer was formed using a DC sputtering apparatus, followed by a 40 nm thick Pt layer. After sputtering, the resist patterning was performed using photolithography to form the heat-generating element contact area.

[0069] (5) Formation of contact pads A 50 nm thick Cr layer was formed on the substrate using a DC sputtering apparatus, and a 1 μm thick Au layer was formed on its surface. Subsequently, the resist patterning was performed by photolithography, and contact pads were formed on the surfaces of the electrode contact area and the heating element contact area by etching.

[0070] (6) Formation of the space The substrate was immersed in a TMAH (tetramethylammonium hydroxide) solution, and anisotropic etching of silicon was performed to form a space corresponding to the heating element. The space was formed so that the surface on which the insulating layer was formed was open.

[0071] (7) Formation of a sensitive membrane Tin oxide (SnO2) was used as the metal oxide nanoparticle to be placed in the nanogap portion of the nanogap electrode. The film was deposited using an RF sputtering apparatus with a tin oxide target for sputtering. The substrate was heated to 280°C, and sputtering of tin oxide was performed while using a mask to prevent sputtering in areas other than the area where the sensitive film was to be formed, thereby forming the sensitive film. At this time, films with a thickness of 20 nm or 200 nm were formed. Figure 4 shows an SEM (scanning electron microscope) image of the formed sensitive film. As can be seen from Figure 4, the sensitive film is a dense film with a porosity of 10% or less, where the particles are bonded together. The sensitive film is electrically bonded (bonded).

[0072] Furthermore, to create an even denser sensitive film, a further bonding treatment (heat treatment) may be performed. This heat treatment was carried out by using a heater (heating element 40) installed on the gas sensor, raising the temperature by applying electricity, and heating in clean air at 300°C to 600°C for 10 minutes to 5 hours. Note that it is sufficient for oxygen to be present so that the tin oxide is not reduced, and the atmosphere can be a simulated gas generated from a cylinder or the like.

[0073] 2. Evaluation method for gas sensors Table 1 shows the results of evaluating the manufactured gas sensors using the gas sensor evaluation device 100 shown in Figure 9. The gas sensor evaluation device 100 comprises a sensor evaluation unit 101 and mass flow controllers 102 and 103. The sensor evaluation unit 101 receives an electrical signal, for example, output from the output unit 90 of the gas sensor 10, and performs the evaluation. In Experimental Example 1-6, a sensor with 580 electrode pairs was used to prevent the electric field strength from becoming too high. The gas sensor in Experimental Example 1-6 had an electrode-to-electrode distance (shortest electrode-to-electrode distance) of 20 nm, a first electrode height of 13 nm, and a first electrode width of 27.5 nm. The height and width of the first electrode here were determined by randomly selecting 100 electrode pairs from the 580 electrode pairs and using their average values, as will be described later. The height of the upper electrode (Pt) was 10 nm, and the height of the lower electrode (Ti) was 3 nm. As described later, the heights of Pt and Ti were determined by randomly selecting 100 electrode pairs from 580 electrode pairs and using their average values. In Comparative Examples 1 and 2, the gas sensors had an inter-electrode distance of 20 μm, a first electrode height of 60 nm, and a first electrode width of 50 μm. The upper electrode (Pt) had a height of 40 nm, and the lower electrode (Ti) had a height of 20 nm.

[0074] The distance between electrodes (the shortest distance between electrodes) was measured by planar observation using a scanning electron microscope (SEM) with the electrode section formed. In Experimental Examples 1-6, the width and height of the cross-section of the first electrode were measured by randomly selecting 100 electrode pairs from 580 electrode pairs and using their average values. The cross-section was mirror-polished and measured by observation using a scanning electron microscope (SEM).

[0075] Acetone was used as the detection gas. Purified air was used as the base gas, and a 2000 ppm / N2 balance cylinder (manufactured by Takachiho Kogyo) was used to mix acetone with purified air using mass flow controllers 102 and 103 to achieve the desired concentrations (4, 10, and 100 ppm in this case) for measurement. The measurement flow rate was 200 sccm. The measurement atmosphere was controlled by power control using a heater (heating element) installed on the gas sensor so that the temperature of the gas sensor reached 250°C. The heater temperature was controlled by supplying power according to the heater resistance value, using a pre-calculated correlation between the heater temperature and heater resistance. Figure 10 shows the results of the response measurement when acetone was introduced. The base resistance differed depending on the element, but the base resistance and sensitivity were normalized so that the base resistance value was 1. Measurements were performed by applying a voltage of 2.5V between the electrodes in Experimental Examples 1 and 4, and Comparative Examples 1 and 2.

[0076] Figure 10(A) is an explanatory diagram showing the time change in the output of the gas sensor in Comparative Example 2, and Figure 10(B) is an explanatory diagram showing the time change in the output of the gas sensor in Experimental Example 4. Compared to the gas sensor in Comparative Example 2, the gas sensor in Experimental Example 4 shows a more responsive decrease in sensor resistance when acetone is added, and also shows a faster response when returned to air. The gases supplied to the gas sensor are in the following order: air, acetone 4 ppm, air, acetone 10 ppm, air, acetone 100 ppm. The output voltage of the gas sensor changes according to the concentration. The change in sensor resistance can be output as a value obtained by converting the change in resistance value into a voltage (a voltage value corresponding to the resistance value) using a measurement circuit. In Figure 10, Va is the normalized base resistance, and Vg is the change in resistance value from the base resistance at the time of sensitivity activation.

[0077] In terms of rise time and recovery time, T90 is the time it takes to reach 90% of the initial value (the value when sensitivity is saturated), with the initial value being 100%. The same applies to T50, which is the time it takes to reach 50% of the initial value (the value when sensitivity is saturated), with the initial value being 100%. In Experimental Example 4, the rise response (T90 response speed) returned to its original value within 5 seconds when sensitivity was first expressed, and within 15 seconds during recovery. This is thought to be due to the strong electric field strength between the electrodes.

[0078] In Experimental Example 4, the applied voltage to the electrode was 2.5V, and the electric field strength was 6.0 × 10⁻⁶. 20 V / m 3 This is the result. On the other hand, in Comparative Examples 1 and 2, the distance between electrodes is 20 μm, the rise time response (T90 response speed) is 30 seconds, which is relatively fast, the output does not return to the base resistance (Va) within the measurement time, T50 is 300 seconds or more, and it can be seen that the recovery characteristics are very poor. This is thought to be due to the low electric field strength in Comparative Examples 1 and 2, unlike Experimental Examples 1-6. The electric field strength in Comparative Examples 1 and 2 is 4.2 × 10⁻¹⁰, given that the height of the first electrode is 60 nm and the width of the first electrode is 50 μm. 16 V / m 3 This is the result. Here, the thickness of the SnO2 film was 200 nm.

[0079] In Experimental Examples 2 and 5, the applied voltage to the electrode was 0.25V, and in Experimental Examples 3 and 6, the applied voltage to the electrode was 0.025V. As shown in Table 1, it can be seen that when the applied voltage is reduced to 0.025V, the output return is slightly worse. In other words, this test shows that the electric field strength is 6.0 × 10⁻⁶. 18 V / m 3 It can be seen that the above is preferable.

[0080] The gas concentration dependence of the current measured in Experimental Examples 1-6 indicates that the concentration of oxygen vacancies in tin oxide changes with the gas. That is, as shown in the change from Figure 11(A) to Figure 11(B), under low oxygen partial pressure conditions when the gas sensor is exposed to the test gas, the width of the tin oxide depletion layer decreases from L1 to L2, making electron transfer easier, and thus the resistance decreases.

[0081] <Other Embodiments> The present invention is not limited to the embodiments described above and in the drawings, and the following embodiments, for example, are also included in the technical scope of the present invention. Furthermore, the various features of the embodiments described above and the embodiments described later may be combined in any way as long as they are not contradictory.

[0082] In the first embodiment described above, the method of measuring the area of ​​the cross-section of the first electrode 71 was illustrated by measuring the width W and height H of the cross-section, but other methods may also be used. For example, the area of ​​the cross-section may be measured using analysis software or the like.

[0083] In the first embodiment described above, the electrode distance D was set to the shortest distance between electrodes among multiple electrode pairs. However, the average of the shortest distances between all electrodes in multiple electrode pairs, or the average of the shortest distances between a predetermined number of electrodes in multiple electrode pairs, may also be used.

[0084] In the first embodiment described above, the cross-section was defined as a plane passing through any position within a distance of 10 nm to 100 nm from one end of the first electrode 71 (the end on the second electrode 72 side), and the conditions for index X and electrode area were defined for that cross-section. However, the configuration may satisfy the conditions for index X and electrode area at any cross-section within a distance of 10 nm to 100 nm.

[0085] In the first embodiment described above, the electrode area S was calculated by averaging the cross-sectional areas of a predetermined number (e.g., 100) of the multiple first electrodes 71 and multiplying the results by the number of first electrodes 71. However, the electrode area S may also be obtained by simply summing the cross-sectional areas of all the first electrodes 71 without taking an average.

[0086] In the first embodiment described above, gold (Au), platinum (Pt), and titanium (Ti) were given as examples of materials for the first electrode 71 and the second electrode 72, but materials with high catalytic activity such as palladium (Pd), ruthenium (Ru), rhodium (Rh), and iridium (Ir) may also be used.

[0087] In the first embodiment described above, the heating element 40 and the electrode portion 50 were formed on the insulating layer 33 and the insulating layer 35, respectively, but they may also be formed on an insulating film such as a silicon oxide (SiO2) film or an alumina (Al2O3) film.

[0088] It should be noted that the embodiments disclosed herein are illustrative and not restrictive in all respects. The scope of the present invention is not limited to the embodiments disclosed herein, and is intended to include all modifications within the scope set forth in the claims or equivalents thereof. [Explanation of Symbols]

[0089] 10...Gas sensor 20... Circuit board 21…Space part 22…Opening 31-35...Insulating layer 37…Resist film 40… Heating element 50...Electrode part 60... Sensitive membrane 70… Electrode pair 70A…Metal film 71...1st electrode 72…Second electrode 80...Voltage application section 90...Output section 100...Gas sensor evaluation device 101...Sensor Evaluation Department 102, 103… Mass flow controllers

Claims

1. An electrode portion comprising one or more electrode pairs having a first electrode and a second electrode, wherein the first electrode and the second electrode in the electrode pair face each other in a first direction with a gap between them, A sensitive membrane is disposed between the first electrode and the second electrode in the electrode pair, A voltage application unit that applies a voltage between the first electrode side and the second electrode side to the electrode portion, Equipped with, The distance between the first electrode and the second electrode in the first direction is 5 nm or more and 100 nm or less. The aforementioned sensitive membrane is a dense material with a porosity of less than 10%. A gas sensor characterized in that at least some of the particles constituting the sensitive membrane are electrically coupled.

2. A method for manufacturing a gas sensor according to Claim 1, A method for manufacturing a gas sensor, characterized by forming the sensitive film by physical vapor deposition or chemical vapor deposition.

3. A method for manufacturing a gas sensor according to Claim 2, A method for manufacturing a gas sensor, characterized by subjecting the formed sensitive film to a heat treatment.

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