Semiconductor gas sensor

Abstract

To efficiently heat a gas detection layer between electrode layers by a heater layer.SOLUTION: A gas sensor 1 comprises: a pair of electrode layers 3, 3 formed on an insulative substrate 2; a heater layer 7 formed on the substrate 2; and a gas detection layer 5 formed so as to stride over the substrate 2, the electrode layers 3, 3 and the heater layer 7. The heater layer 7 is disposed between the pair of electrode layers 3, 3.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a semiconductor gas sensor. [Background technology]

[0002] 2. Description of the Related Art Conventionally, semiconductor gas sensors having a gas detection layer made of an oxide semiconductor are known (see, for example, Patent Document 1).

[0003] This type of semiconductor gas sensor is used by heating the gas sensing layer to a temperature of several hundred degrees, so that the oxygen adsorbed on the surface of the gas sensing layer takes away electrons in the gas sensing layer, making it difficult for electricity to flow through the gas sensing layer. When the gas sensing layer is exposed to a reducing gas such as acetone or carbon monoxide in this state, the reducing gas reacts with the oxygen on the surface of the gas sensing layer, and the electrons captured by the adsorbed oxygen are returned to the gas sensing layer, making it easier for electricity to flow through the gas sensing layer. The semiconductor gas sensor can detect gas by measuring the change in the electrical resistance of the gas sensing layer.

[0004] In the gas sensor of Patent Document 1, a heater and a pair of electrodes spaced apart from the heater are formed on one surface of an insulating substrate, and a gas sensing layer is formed in the region where the heater and the electrodes are formed. However, because the heater and the electrodes are disposed apart, the transfer of heat from the heater to the gas sensing layer between the electrodes is inefficient, and there is a problem in that it is necessary to heat the entire substrate in order to heat the gas sensing layer between the electrodes to a desired temperature. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 4-109157 [Patent Document 2] Patent No. 5240767 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to improve the above-mentioned current situation, and has an object to provide a semiconductor gas sensor capable of efficiently heating a gas detection layer between a pair of electrodes. [Means for solving the problem]

[0007] The semiconductor gas sensor of the present invention comprises a pair of electrode layers formed on an insulating substrate, a heater layer formed on the substrate, and a gas detection layer formed across the substrate, the electrode layers and the heater layer, and the heater layer is disposed between the pair of electrode layers.

[0008] In the semiconductor gas sensor of the present invention, for example, a pair of electrode wiring layers connected to the pair of electrode layers and a pair of heater wiring layers connected to both ends of the heater layer are provided on the substrate, and the electrode layers, the heater layer, the electrode wiring layer and the heater wiring layer are formed of conductive layer patterns formed simultaneously on the substrate from the same material, and the wiring width of the heater wiring layer may be formed larger than the wiring width of the heater layer.

[0009] Furthermore, an end portion of the electrode wiring layer opposite the electrode layer and an end portion of the heater wiring layer opposite the heater layer may be arranged in the vicinity of one side of the substrate, and the wiring layer and a measuring device may be electrically connected by inserting and removing the one side of the substrate into a female connector having a plurality of terminals corresponding to the ends.

[0010] In addition, a concave-convex structure layer may be provided below the gas detection layer, the concave-convex structure layer being formed on the substrate, the electrode layer and across the heater layer, the concave-convex structure layer having a porous structure, and the gas detection layer may have a porous structure resulting from the porous structure of the concave-convex structure layer.

[0011] In such an embodiment, the concave-convex structure layer may be made of, for example, an insulating material.

[0012] The concave-convex structure layer may be made of, for example, an insulating metal oxide.

[0013] The uneven structure layer may be formed of an insulating aluminum compound layer obtained by making an alumina layer porous by hot water treatment, for example.

[0014] In the semiconductor gas sensor of the present invention, the gas sensing layer may be formed of, for example, tungsten trioxide to which an impurity is added and which is biased to a negative charge. More specifically, the gas sensing layer may be formed of tungsten trioxide to which nickel is added. The impurity added to tungsten trioxide (hereinafter also simply referred to as tungsten oxide) may be an element having a valence of five or less, and may be, for example, silicon (Si), indium (In), aluminum (Al), or two or more elements other than nickel (Ni). The oxide semiconductor forming the gas sensing layer may be a metal oxide other than tungsten trioxide, such as tin dioxide or titanium dioxide, or may be an oxide semiconductor to which an impurity is added.

[0015] In the semiconductor gas sensor of the present invention, a plurality of sets of the pair of electrode layers, the heater layer and the gas sensing layer may be provided on the substrate.

[0016] In the semiconductor gas sensor of the present invention, a plurality of the gas sensing layers covering one of the heater layers may be formed in different regions on the substrate, and a pair of electrode layers may be formed on each of the gas sensing layers, sandwiching the heater layer therebetween. Effect of the Invention

[0017] Since the semiconductor gas sensor of the present invention has the heater layer disposed between the pair of electrode layers, the gas sensing layer between the electrode layers can be efficiently heated by the heater layer. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic plan view showing an embodiment of a semiconductor gas sensor. [Diagram 2] FIG. 2 is a schematic exploded perspective view showing the embodiment. [Diagram 3] 2 is a schematic cross-sectional view corresponding to the position AA in FIG. 1. [Figure 4] 5 is a schematic diagram for explaining a connection state between the gas sensor and a female connector. FIG. [Diagram 5] FIG. 11 is a schematic plan view showing another embodiment of the gas sensor. [Figure 6] FIG. 11 is a schematic plan view showing still another embodiment of the gas sensor. [Figure 7] FIG. 11 is a schematic cross-sectional view for illustrating still another embodiment of the gas sensor. [Figure 8] 5A to 5C are schematic cross-sectional views for explaining an example of a manufacturing process of the embodiment. [Figure 9] 1 is a scanning electron microscope image showing a fracture surface of a relief structure layer. [Figure 10] 1 is a scanning electron microscope image of the surface of a relief structure layer. [Figure 11] 1 is a scanning electron microscope image of the surface of a gas sensing layer formed on a concave-convex structure layer. [Figure 12] 1 is a graph showing the results of using a test gas sensor and a conventional gas sensor to detect acetone. [Figure 13] 13 is a graph showing the response characteristics of a test gas sensor to acetone. [Figure 14] 1 is a graph showing the response characteristics of a verification gas sensor to ethanol. [Figure 15] 13 is a graph showing the response characteristics of a gas sensor of a comparative example to acetone. [Figure 16] 13 is a graph showing the response characteristics of a gas sensor of a comparative example to ethanol. [Figure 17] 1 is a graph showing the response characteristics of the gas sensor of Reference Example 1 to acetone. [Figure 18] 4 is a graph showing the response characteristics of the gas sensor of Reference Example 1 to ethanol. [Figure 19] 13 is a graph showing the response characteristics of the gas sensor of Reference Example 2 to acetone. [Figure 20] 13 is a graph showing the response characteristics of the gas sensor of Reference Example 2 to ethanol. [Figure 21] 13 is a graph showing the response characteristics of the gas sensor of Reference Example 3 to acetone. [Figure 22] 13 is a graph showing the response characteristics of the gas sensor of Reference Example 3 to ethanol. [Figure 23] FIG. 11 is a schematic plan view showing still another embodiment of the gas sensor. [Figure 24] FIG. 11 is a schematic plan view showing still another embodiment of the gas sensor. [Diagram 25] FIG. 11 is a schematic plan view showing still another embodiment of the gas sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] An embodiment of a semiconductor gas sensor of the present invention will be described with reference to the drawings. Fig. 1 is a schematic plan view showing one embodiment. Fig. 2 is a schematic exploded perspective view showing the same embodiment. Fig. 3 is a schematic cross-sectional view corresponding to the position AA in Fig. 1.

[0020] As shown in Figs. 1 and 2, the semiconductor gas sensor 1 of this embodiment includes an insulating substrate 2, a pair of electrode layers 3, 3 and a heater layer 7 formed on the substrate 2, and a gas detection layer 5 made of an oxide semiconductor formed across the substrate 2, the electrode layer 3 and the heater layer 7. The gas sensor 1 is a so-called substrate-type semiconductor gas sensor. The pair of electrode layers 3, 3 are disposed at a distance from each other. The heater layer 7 is disposed between the pair of electrode layers 3, 3. The heater layer 7 is disposed at a distance from the electrode layer 3.

[0021] The gas sensor 1 includes a pair of electrode wiring layers 8, 8 provided for each electrode layer 3, 3, and a pair of heater wiring layers 9, 9 connected to both ends of a heater layer 7, on a substrate 2. The electrode layer 3, the heater layer 7, the electrode wiring layer 8, and the heater wiring layer 9 are formed as conductive layer patterns formed simultaneously on the substrate 2 using the same material.

[0022] An end 8a of the electrode wiring layer 8 opposite to the electrode layer 3, and an end 9a of the heater wiring layer 9 opposite to the heater layer 7 are arranged side by side along one side 2a of the substrate 2 in the vicinity of the one side 2a. As shown in Fig. 4, when the end 8a of the electrode wiring layer 8 and the end 9a of the heater wiring layer 9 on the side of the one side 2a of the substrate 2 are inserted into a female connector 11, terminals 11a provided on the female connector 11 for the ends 8a, 9a of the wiring layers 8, 9 come into contact with the ends 8a, 9a, and the gas sensor 1 is electrically connected to a measuring device (not shown).

[0023] The insulating substrate 2 is formed of an insulating material such as ceramics, glass, or quartz. The size and shape of the substrate 2 are not particularly limited. The size of the substrate 2 can be large enough for a user to handle by hand, for example, 10 mm in length × 10 mm in width × 1 mm in thickness or more. In this embodiment, the substrate 2 is a rectangular ceramic substrate with dimensions of 30 mm in length × 12 mm in width × 1.5 mm in thickness.

[0024] The electrode layer 3 and the electrode wiring layer 8 are intended to detect a change in the electrical resistance value of the gas detection layer 5. The pair of electrode layers 3, 3 are disposed at a position close to the other side 2b opposite to the side 2a of the substrate 2 and spaced apart from each other along the width direction of the substrate 2. The electrode wiring layer 8 extends from the electrode layer 3 toward the side 2a of the substrate 2 along the length direction of the substrate 2.

[0025] The heater layer 7 and the heater wiring layer 9 are intended to heat the gas sensing layer 5 between the pair of electrode layers 3, 3. The heater layer 7 is disposed along the longitudinal direction of the substrate 2 between the pair of electrode layers 3, 3. One of the heater wiring layers 9 is disposed between the pair of electrode wiring layers 8, and extends along the longitudinal direction of the substrate 2 from the end of the heater layer 7 on the side 2a toward the side 2a.

[0026] The other heater wiring layer 9 has a portion extending in the width direction of the substrate 2 in a region closer to the other side 2b than the electrode layer 3, and a portion extending along the length direction of the substrate 2 in a region outside the electrode layer 3 and the electrode wiring layer 8 in the width direction of the substrate 2, and is provided in a generally L-shape in plan view. The portion of the other heater wiring layer 9 extending in the width direction of the substrate 2 on the other side 2b side is connected to the end of the heater layer 7 on the other side 2b side.

[0027] The wiring width of heater wiring layer 9 is formed to be larger than that of heater layer 7. In this embodiment, the wiring width of heater layer 7 is about 0.5 mm, and the wiring width of heater wiring layer 9 is about 3 to 8 mm. The wiring width of electrode layer 3 is about 2 to 3 mm, and the wiring width of electrode wiring layer 8 is about 1 mm. The distance between electrode layer 3 and heater layer 7 in the width direction of substrate 2 is about 0.5 mm.

[0028] The arrangement and shape of the layers 3, 7, 8, and 9 formed on the substrate 2 are not limited to this, and any arrangement and shape can be adopted as long as the heater layer 7 is disposed between a pair of electrode layers 3, 3.

[0029] For example, as shown in FIG. 5, the gas sensor 1 may be configured such that the heater layer 7 is disposed extending in the width direction of the substrate 2, and a pair of electrode layers 3, 3 are disposed to sandwich the heater layer 7 in the length direction of the substrate 2.

[0030] The layers 3, 7, 8, and 9 can be formed of, for example, a nichrome layer formed on the substrate 2 by a sputtering method using a metal mask having openings corresponding to the regions where the electrode layer 3 and the heater layer 7 are to be formed. The layers 3, 7, 8, and 9 have a thickness of, for example, 1 μm. The material of the electrode layer 3 is not particularly limited, and can be, for example, nichrome, platinum, gold, silver, titanium, nickel, aluminum, ruthenium, tantalum, titanium, copper, platinum, niobium, zirconium, or alloys of these elements, or alloys of these elements and carbon, or carbon alone. The electrode layer 3 can be a single layer film or a multilayer film in which multiple films are stacked.

[0031] 1 to 3, the gas detection layer 5 is formed straddling the substrate 2, the pair of electrode layers 3, 3, and the heater layer 7. In this embodiment, the gas detection layer 5 covers, on the upper surface of the substrate 2, a portion of the electrode layer 3 that is closer to the heater layer 7, and a portion of the heater layer 7 that is sandwiched between the electrode layers 3, 3. The gas detection layer 5 may cover the entire electrode layer 3, or may cover the entire heater layer 7. The gas detection layer 5 may also be formed straddling the electrode wiring layer 8 and the heater wiring layer 9.

[0032] The gas sensing layer 5 is formed of an oxide semiconductor film, for example, titanium dioxide having a thickness of about 150 nm. Note that the material of the gas sensing layer 5 is not limited to titanium dioxide, and may be other oxide semiconductors such as tungsten trioxide, tin dioxide, zinc oxide, indium oxide, or the like, or may be doped (added) with an impurity element.

[0033] The gas sensing layer 5 is preferably manufactured by a vapor deposition method, since the formation area and film thickness can be controlled with high precision. In this embodiment, the formation area of ​​the gas sensing layer 5 is the same as the formation area of ​​the relief structure layer 4. Here, the vapor deposition method may be a so-called physical vapor deposition method (PVD method) such as a sputtering method, a vacuum deposition method, a reactive plasma deposition method, or an ion plating method, or a so-called chemical vapor deposition method (CVD method). However, the manufacturing method of each layer is not limited to the vapor deposition method, and may be a printing method such as a screen printing method or an inkjet printing method.

[0034] An adhesive layer may be formed on the surface of the substrate 2 to prevent peeling between the substrate 2 and the layers 3, 7, 8, and 9. The material for such an adhesive layer may be any material that has good adhesion to the substrate 2 and the layers 3, 7, 8, and 9, and may be, for example, chromium, titanium, or tungsten.

[0035] Since the gas sensor 1 has the heater layer 7 disposed between the pair of electrode layers 3, 3, the heater layer 7 can efficiently heat the gas sensing layer 5 between the electrode layers 3, 3. This allows the gas sensor 1 to reliably heat the gas sensing layer 5 to a desired operating temperature, for example, an operating temperature of 200 degrees or higher.

[0036] Incidentally, the conventional gas sensor disclosed in Patent Document 1 has a substrate size of 1 mm square, and the wiring patterns of the heater and electrodes are formed by photolithography, which causes a problem of high manufacturing costs. In addition, when making the gas sensor smaller, as the wiring width and pitch of the wiring pattern formed by the conductive layer become narrower, defects such as disconnections and short circuits become more likely to occur, resulting in a problem of low yield. Such problems can be solved by scaling up the substrate size to a size that the user can handle by hand, for example, to a size of about several centimeters, and increasing the wiring width.

[0037] Scaling up a gas sensor increases its heat dissipation ability. However, in conventional gas sensors, the heater is placed around a pair of electrodes and is separated from the gas sensing layer between the electrodes, so that the heater must be operated at a high temperature of 500 degrees or more to heat the gas sensing layer to an operating temperature of 200 degrees or more. Such operating conditions increase power consumption, raise safety concerns due to exposure to high temperatures, and shorten the life of the gas sensor.

[0038] In the gas sensor 1 of this embodiment, the heater layer 7 is disposed between the electrode layers 3, 3, so that the gas sensing layer 5 between the electrode layers 3, 3 can be efficiently and reliably heated to a desired operating temperature. Therefore, even if the gas sensor 1 has a configuration that is inexpensive and easy to mass-produce, for example, in which the electrode layer 3 and the heater layer 7 are formed by pattern deposition on a substrate 2 that is large enough for a user to handle by hand, the gas sensing layer 5 between the electrode layers 3, 3 can be efficiently and reliably heated by the heater layer 7.

[0039] The gas sensor 1 includes a pair of electrode layers 3, 3, a heater layer 7, a pair of electrode wiring layers 8, 8 provided for each of the electrode layers 3, 3, and a pair of heater wiring layers 9, 9 connected to both ends of the heater layer 7, all on a substrate 2. These layers 3, 7, 8, 9 are formed of conductive layer patterns formed simultaneously from the same material. Therefore, the gas sensor 1 can reduce the number of manufacturing steps and the manufacturing cost.

[0040] In addition, the wiring width of the heater wiring layer 9 is formed to be larger than the wiring width of the heater layer 7. Therefore, in the gas sensor 1, the electric resistance of the heater wiring layer 9 is made smaller than that of the heater layer 7, so that the gas sensing layer 5 between the electrode layers 3, 3 can be locally heated while reducing heat generation of the heater wiring layer 9, thereby reducing power consumption.

[0041] In the gas sensor 1, an end 8a of the electrode wiring layer 8 opposite the electrode layer 3 and an end 9a of the heater wiring layer 9 opposite the heater layer 7 are arranged in the vicinity of one side 2a of the substrate 2. This allows easy electrical connection between the gas sensor 1 and a measuring device by inserting and removing the one side 2a of the substrate 2 into and from a female connector 11 having terminals 11a corresponding to the ends 8a, 9a. Furthermore, since the gas sensor 1 does not require wiring processing such as soldering, the gas sensor 1 can be manufactured inexpensively and is easily replaceable, and the gas sensor 1 can be used as a disposable chip, for example.

[0042] Here, since the gas sensor 1 can locally heat the gas detection layer 5 between the electrode layers 3, 3, it is possible to suppress a temperature rise in the ends 8a, 9a of the wiring layers 8, 9, particularly a temperature rise in the end 9a of the heater wiring layer 9 connected to the heater layer 7. This makes it possible to suppress a temperature rise during measurement in the portion 1a (see FIG. 1) of the gas sensor 1 that contacts the female connector 11, without providing a heat dissipation opening in the substrate 2 or connecting the substrate 2 to a pin member by wires such as soldering.

[0043] While a resin member having a heat resistance of about 100° C. is sometimes used for the female connector 11, in the gas sensor 1 of this embodiment, the temperature of the portion 1a in contact with the female connector 11 can be kept below 100° C. when the heater layer 7 is heated to about 320° C. In this manner, the gas sensor 1 can be mass-produced at low cost, and can be easily replaced by being inserted into and removed from the female connector 11 without damaging the female connector 11.

[0044] 6 is a schematic plan view showing still another embodiment of the gas sensor. In this embodiment, two gas sensing layers 5 covering one heater layer 7 are formed in different regions on the substrate 2. A pair of electrode layers 3, 3 are formed on each gas sensing layer 5 with the heater layer 7 sandwiched therebetween. An electrode wiring layer 8 is connected to each electrode layer 3. An end 8a of each electrode wiring layer 8 is disposed near one side 2a of the substrate 2 together with an end 9a of a heater wiring layer 9. In other words, the gas sensor 1 of this embodiment includes a plurality of sets of a pair of electrode layers 3, 3, a heater layer 7, and a gas sensing layer 5 on one substrate 2.

[0045] When the two gas sensing layers 5 are made of different semiconductor materials, each gas sensing layer 5 can be configured to detect a different gas. When the two gas sensing layers 5 are made of the same semiconductor material, the surface area of ​​the gas sensing layer 5 can be increased, thereby improving the sensor sensitivity (detection sensitivity).

[0046] In addition, three or more gas detection layers 5 covering one heater layer 7 may be formed in different regions on the substrate 2, and a pair of electrode layers 3, 3 may be formed for each gas detection layer 5, sandwiching the heater layer 7 therebetween.

[0047] Incidentally, for semiconductor gas sensors, a technique is known in which the surface area of ​​the gas sensing layer is increased by forming the gas sensing layer from an aggregate of many columnar crystals, thereby improving the sensor sensitivity and gas selectivity (see, for example, Patent Document 2). However, in conventional gas sensors, in order to increase the surface area of ​​the gas sensing layer, it is necessary to form the oxide semiconductor constituting the gas sensing layer into columnar crystals, which places limitations on the types of oxide semiconductor that can be formed.

[0048] Next, an embodiment will be described in which the surface area of ​​the gas sensing layer can be increased regardless of the type of oxide semiconductor in the gas sensing layer, thereby improving the sensor sensitivity and gas selectivity.

[0049] Fig. 7 is a schematic cross-sectional view for explaining still another embodiment of the gas sensor. This embodiment differs from the embodiment shown in Fig. 1 etc. in the configuration of the gas detection layer 5, but the other configurations are the same as those of the embodiment shown in Fig. 1 etc. The cross-sectional position in Fig. 7 corresponds to the position AA in Fig. 1.

[0050] The gas sensor 1 of this embodiment includes a concave-convex structure layer 4 formed under a gas detection layer 5, straddling the substrate 2, the electrode layer 3, and the heater layer 7. The concave-convex structure layer 4 has a porous structure. The gas detection layer 5 has a porous structure resulting from the porous structure of the concave-convex structure layer 4.

[0051] The uneven structure layer 4 is formed of an insulating metal oxide having a porous structure with a large number of base pores 4a. In this embodiment, the uneven structure layer 4 is formed of an aluminum compound layer obtained by making an alumina layer porous by hot water treatment. In this embodiment, the uneven structure layer 4 covers, on the upper surface of the substrate 2, a portion of the electrode layer 3 that is closer to the heater layer 7 and a portion of the heater layer 7 that is sandwiched between the electrode layers 3, 3. The uneven structure layer 4 may cover the entire electrode layer 3 or the entire heater layer 7. The uneven structure layer 4 may be formed over the electrode wiring layer 8 or the heater wiring layer.

[0052] Such a concave-convex structure layer 4 can be formed, for example, by forming an amorphous alumina film with a thickness of about 30 nm on the substrate 2 after the electrode layer 3 is formed by atomic layer deposition (ALD) or physical vapor deposition (PVD), and then immersing the substrate 2 in hot water at about 75 to 85°C for several minutes to several tens of minutes. The porous aluminum compound layer formed by this hot water treatment has a thickness (height) of, for example, about 140 to 150 nm. In this way, the concave-convex structure layer 4 having a porous structure can be formed by simply carrying out a very simple process such as hot water treatment, and the manufacturing cost can be reduced.

[0053] Some of the base pores 4a of the relief-structure layer 4 open to the surface of the relief-structure layer 4 and reach the electrode layer 3 and the heater layer 7. The gas sensing layer 5, which will be described later, is electrically connected to the electrode layer 3 through the base pores 4a.

[0054] The concave-convex-structure layer 4 having a porous structure is not limited to an aluminum compound layer made porous by hot water treatment, and may be formed of other insulating metal oxides. The concave-convex-structure layer 4 may also be formed of an insulating material other than a metal oxide, such as mesoporous silica.

[0055] The gas sensing layer 5 is formed on the concave-convex structure layer 4, and has a porous structure (numerous sensing layer pores 5a) resulting from the porous structure (numerous base pores 4a) of the concave-convex structure layer 4. The gas sensing layer 5 having such a porous structure can be formed by stacking an oxide semiconductor on the concave-convex structure layer 4. In this embodiment, tungsten trioxide (WO3, hereinafter also simply referred to as tungsten oxide) is used as the material for the gas sensing layer 5.

[0056] The material of the gas detection layer 5 is not limited to tungsten oxide, but may be other oxide semiconductors such as tin dioxide, titanium dioxide, zinc oxide, indium oxide, etc., or may be doped (added) with an impurity element.

[0057] The porous structure of the gas sensing layer 5 depends on the porous structure of the concave-convex structure layer 4, and has a thickness (film thickness) of, for example, about 20 to 1000 nm. In addition, by changing the film thickness of the gas sensing layer 5, it is possible to control the porous structure of the gas sensing layer 5 (the opening shape, opening width, pore depth, etc. of the sensing layer pores 5a). In this way, it is possible to form a gas sensor 1 including a gas sensing layer 5 having a porous structure suited to the application.

[0058] An adhesive layer for preventing peeling between the substrate 2 and the concave-convex structure layer 4 may be formed on the surface of the substrate 2. Such an adhesive layer may be a surface treatment layer formed by performing a surface treatment on the surface of the substrate 2 that can improve adhesion to the concave-convex structure layer 4. Examples of such surface treatments include plasma treatment, corona treatment, flame treatment, etching treatment, steam treatment, and ion beam treatment.

[0059] The gas sensor 1 of this embodiment has the gas sensing layer 5 with a porous structure, and therefore the surface area of ​​the gas sensing layer 5 can be significantly increased compared to a flat surface, thereby improving the sensor sensitivity (detection sensitivity).

[0060] Furthermore, since the porous structure (detection layer pores 5a) of the gas detection layer 5 originates from the porous structure (base pores 4a) of the underlying concave-convex structure layer 4, the gas detection layer 5 having a porous structure can be easily and reliably formed by stacking an oxide semiconductor material on the concave-convex structure layer 4. In other words, the gas sensor 1 forms the gas detection layer 5 having a porous structure even if the oxide semiconductor layer forming the gas detection layer 5 does not have a specific crystal structure that increases the surface area, such as a columnar crystal. That is, the gas sensor 1 can increase the surface area of ​​the gas detection layer 5 regardless of the type and crystal structure of the oxide semiconductor of the gas detection layer 5, and can thereby improve the sensor sensitivity and gas selectivity.

[0061] In addition, it is possible to control the porous structure (the opening shape, opening width, pore depth, etc. of the sensing layer pores 5a) of the gas sensing layer 5 by changing the film thickness of the gas sensing layer 5. In this way, it is possible to form a gas sensor 1 including a gas sensing layer 5 having a porous structure suited to the application.

[0062] Next, a manufacturing example of a gas sensor 1 having a concave-convex structure layer 4 will be described with reference to Fig. 8. A pair of electrode layers 3, 3, a heater layer 7, a pair of electrode wiring layers 8, 8, and a pair of heater wiring layers 9, 9 were formed on an insulating substrate 2, a ceramic substrate having a thickness of about 1.5 mm, by a sputtering method using a metal mask (see Fig. 8(1) and Fig. 1). For example, the layers 3, 7, 8, 9 are formed of nichrome having a thickness of about 1 µm.

[0063] Next, using a metal mask having an opening pattern corresponding to the formation regions of the concave-convex structure layer 4 and the gas sensing layer 5, an alumina layer 6 having a thickness of about 30 nm was formed as a metal oxide film for forming the concave-convex structure layer 4 (see FIG. 8(2)). The thickness of the alumina layer 6 may be larger or smaller than 30 nm.

[0064] The substrate 2 on which the layers 3, 7, 8, and 9 and the alumina layer 6 were formed was immersed in hot water at about 75 to 85°C for about 7 minutes to make the alumina layer 6 porous, thereby forming a concave-convex structure layer 4 having a large number of base pores 4a (porous structure). The substrate 2 was then dried. The porous concave-convex structure layer 4 had a thickness of about 200 to 300 nm (see FIG. 8(3)).

[0065] Fig. 9 is a scanning electron microscope image showing a fracture surface of the concave-convex-structure layer 4. Fig. 10 is a scanning electron microscope image of the surface of the concave-convex-structure layer 4. Note that the concave-convex-structure layer 4 shown in Figs. 9 and 10 was formed by forming the alumina layer 6 on the glass substrate 2 under the same conditions as above, and then performing a hot water treatment under the same conditions as above.

[0066] 9 and 10, a large number of base pores 4a are formed in the concave-convex-structure layer 4 made of an insulating aluminum compound layer obtained by making the alumina layer 6 porous by hot water treatment. The surface of the concave-convex-structure layer 4 has an uneven shape. Some of the base pores 4a of the concave-convex-structure layer 4 open to the surface of the concave-convex-structure layer 4 and reach the substrate 2.

[0067] Next, an oxide semiconductor material was laminated on the concave-convex structure layer 4 by reactive plasma deposition using a metal mask having an opening pattern corresponding to the formation region of the concave-convex structure layer 4 (the formation region of the alumina layer 6), to form a gas sensing layer 5 having a large number of sensing layer pores 5a (porous structure). The gas sensing layer 5 is electrically connected to the electrode layer 3 via the base pores 4a. Here, as the oxide semiconductor material, tungsten oxide (WO3) doped with indium (In) at 0.5 at% (atomic percent) was formed to a film thickness of 100 nm. Then, the gas sensing layer 5 was sintered.

[0068] In this manner, by forming the metal oxide film for forming the uneven structure layer 4 and the gas sensing layer 5 by a vapor deposition method (here, a reactive plasma vapor deposition method) using a metal mask having an opening pattern, patterning by an etching method or a lift-off method is not required after the deposition of each layer, and the manufacturing cost can be reduced.

[0069] Fig. 11 is a scanning electron microscope image of the surface of the porous gas sensing layer 5. As can be seen from Fig. 7, a porous structure (sensing layer pores 5a) is formed on the surface of the gas sensing layer 5 due to the porous structure of the relief structure layer 4. This increases the surface area of ​​the gas sensing layer 5 and improves the sensor sensitivity.

[0070] The thickness of the gas sensing layer 5 is not particularly limited, but is preferably 50 nm or more and 1000 nm or less. If the thickness of the gas sensing layer 5 is thinner than 50 nm, the gas sensing layer 5 has high resistance and the measurement sensitivity is reduced. If the thickness of the gas sensing layer 5 is thicker than 1000 nm, when the gas sensing layer 5 is formed by a vapor deposition method (e.g., reactive plasma vapor deposition method), it takes a long time to form the gas sensing layer 5, and production efficiency is reduced.

[0071] In addition, by providing a plurality of regions for the gas sensors 1 on one substrate 2 and simultaneously forming the plurality of gas sensors 1, the individual gas sensors 1 can be separated, thereby reducing the manufacturing cost.

[0072] Next, an example of a measurement result using a gas sensor including a gas sensing layer 5 having sensing layer pores 5a resulting from base pores 4a of the concave-convex structure layer 4 will be described. In this example, in order to verify the performance of the concave-convex structure layer 4 and the gas sensing layer 5, a verification gas sensor was used in which a pair of comb-shaped electrode layers were formed facing each other on a glass substrate, and the concave-convex structure layer 4 and the gas sensing layer 5 were formed so as to cover the areas where the comb-shaped portions were formed. The verification gas sensor used had a gas sensing layer 5 made of tungsten oxide containing 0.5 at % indium, and the concave-convex structure layer 4 and the gas sensing layer 5 formed under the same conditions as the manufacturing process described with reference to FIG. 8.

[0073] The measurement gas used had an acetone concentration of 25 to 1000 ppb (parts per billion). As a comparative example, a conventional gas sensor was used. The gas sensor was heated to, for example, 300° C., and the sensor resistance value (R air ) and the sensor resistance value (R gas) and the sensitivity (=R air / R gas ) was sought.

[0074] Figure 12 is a graph showing the results of using the verification gas sensor and a conventional gas sensor to detect acetone. In Figure 12, the vertical axis indicates sensitivity (arbitrary units) and the horizontal axis indicates the acetone concentration of the measurement gas. As can be seen from Figure 8, the verification gas sensor (see "Verification Example") achieved a measurement sensitivity that was about 10 times higher than that of the conventional gas sensor (see "Conventional Sensor") (see the measurement result for an acetone concentration of 1000 ppb). Furthermore, the verification gas sensor had a sensitivity of about 8 for an acetone concentration of 500 ppb, demonstrating high detection sensitivity for acetone. Furthermore, the verification gas sensor was also able to detect acetone at a concentration of 25 ppb, demonstrating its high detection sensitivity for acetone.

[0075] It is known that the acetone concentration in the breath of diabetic patients is elevated, and attempts have been made to detect this elevated acetone concentration using a gas sensor to enable the diagnosis of diabetes. In this diabetes diagnosis, the gas sensor is required to have the ability to detect acetone at low concentrations of about 500 to 1000 ppb, and to be selective for components other than acetone (especially ethanol) contained in the breath.

[0076] The detection sensitivity of the above-mentioned gas sensor for verification was verified for acetone and ethanol. Figures 13 and 14 are graphs showing the response characteristics of the gas sensor for verification for acetone and ethanol. Measurement gases used were one with an acetone concentration of 1000 ppb (1 ppm) and one with an ethanol concentration of 1000 ppb. In each of the graphs in Figures 13 and 14, the horizontal axis indicates relative time (seconds) and the vertical axis indicates sensitivity (arbitrary units).

[0077] 13, the test gas sensor had a detection peak of about 24.0 for acetone at a concentration of 1000 ppb, and a detection peak of about 25.0 for ethanol at a concentration of 1000 ppb. These results show that the test gas sensor having a gas sensing layer 5 made of tungsten oxide doped with indium exhibits high detection sensitivity to both acetone and ethanol.

[0078] Next, the results of verifying the effect of forming the gas sensing layer 5 on the concave-convex structure layer 4 will be described. As a comparative gas sensor, a sensor was fabricated in which, in the manufacturing process described with reference to Fig. 8, (1) a pair of comb-shaped electrode layers were formed facing each other on the substrate 2, and then (2) the process of forming the alumina layer 6 and (3) the hot water treatment process were not performed, and (4) the gas sensing layer 5 was formed on the substrate 2 and on the pair of comb-shaped electrode layers. The detection sensitivity of the comparative gas sensor to acetone and ethanol was verified.

[0079] 15 and 16 are graphs showing the response characteristics of the gas sensor of the comparative example to acetone and ethanol. The measurement conditions are the same as those used to measure the response characteristics of the verification gas sensor shown in FIGS. 13 and 14.

[0080] As shown in Fig. 15, the gas sensor of the comparative example had a detection peak of about 12.5 for acetone at a concentration of 1000 ppb. Also, as shown in Fig. 16, the gas sensor of the comparative example had a detection peak of about 18.7 for ethanol at a concentration of 1000 ppb. Comparing the response characteristics of the verification gas sensor shown in Figs. 9 and 10, it was found that the verification gas sensor had higher detection sensitivity for both acetone and ethanol than the gas sensor of the comparative example.

[0081] Thus, it was found that the gas sensing layer 5 having the sensing layer pores 5a resulting from the base pores 4a of the relief-structure layer 4 exhibits higher stability than a gas sensing layer not having the sensing layer pores 5a. By applying the gas sensing layer 5 having the sensing layer pores 5a to the gas sensor 1 of the above embodiment in which the heater layer 7 is disposed between the pair of electrode layers 3, 3, the detection performance of the gas sensor 1 can be improved.

[0082] Next, the results of verifying the types of impurities added to tungsten oxide will be described. As gas sensors for verification, three types of gas sensors, Reference Examples 1 to 3, were fabricated, each having the same structure as the gas sensor of the comparative example, but with different impurities added to the gas detection layer (tungsten oxide). The impurities added to tungsten oxide were 0.5 at% nickel (Reference Example 1), 0.5 at% silicon (Reference Example 2), and 0.5 at% aluminum (Reference Example 3).

[0083] 17 and 18 are graphs showing the response characteristics to acetone and ethanol of the gas sensor of Reference Example 1 having a gas sensing layer in which nickel is added to tungsten oxide. FIGS. 19 and 20 are graphs showing the response characteristics to acetone and ethanol of the gas sensor of Reference Example 2 having a gas sensing layer in which silicon is added to tungsten oxide. FIGS. 21 and 22 are graphs showing the response characteristics to acetone and ethanol of the gas sensor of Reference Example 3 having a gas sensing layer in which aluminum is added to tungsten oxide. The measurement conditions are the same as those when the response characteristics of the verification gas sensor shown in FIGS. 13 and 14 were measured. In each of the graphs in FIGS. 17 to 22, the horizontal axis shows relative time (seconds) and the vertical axis shows sensitivity (arbitrary units).

[0084] As shown in Fig. 17, the gas sensor of Reference Example 1 having a gas sensing layer in which nickel is added to tungsten oxide has a detection peak of about 3.9 for acetone at a concentration of 1000 ppb, indicating that the detection sensitivity for acetone is very high. On the other hand, as shown in Fig. 18, the gas sensor of Reference Example 1 has a detection peak of about 2.3 for ethanol at a concentration of 1000 ppb, which is roughly half the detection sensitivity for acetone, indicating a difference in detection sensitivity between acetone and ethanol.

[0085] From this result, it is understood that the gas sensor 1 having nickel-added tungsten oxide as the gas detection layer 5 formed on the concave-convex structure layer 4 is likely to have a difference in detection sensitivity to acetone and ethanol.

[0086] As shown in Fig. 19, the gas sensor of Reference Example 2 having a gas sensing layer 5 in which silicon was added to tungsten oxide had a detection peak of about 4.7 for acetone at a concentration of 1000 ppb. On the other hand, as shown in Fig. 20, the gas sensor of Reference Example 2 had a detection peak of about 3.7 for ethanol at a concentration of 1000 ppb. The gas sensor of Reference Example 2 showed a difference in detection sensitivity for acetone and ethanol.

[0087] As shown in Figures 21 and 22, the gas sensor of Reference Example 3 having a gas detection layer in which aluminum was added to tungsten oxide had detection peaks of about 3.0 for acetone at a concentration of 1000 ppb and ethanol at a concentration of 1000 ppb, and the detection sensitivity for acetone and ethanol was almost the same.

[0088] As described above, it was found that the gas sensor having a gas sensing layer of tungsten oxide (WO3) doped with nickel or silicon exhibits better detection sensitivity to acetone than the gas sensor having tungsten oxide doped with aluminum. It was also found that the gas sensor having a gas sensing layer made of tungsten oxide doped with nickel exhibits a large difference in detection sensitivity to acetone and ethanol.

[0089] If the gas sensing layers of Reference Examples 1 to 3 are applied to the gas sensing layer 5 of the gas sensor 1 of the above embodiment in which the heater layer 7 is disposed between the pair of electrode layers 3, 3, the gas selectivity of the gas sensor 1 can be improved.

[0090] The present invention is not limited to the above-described embodiment, but can be embodied in various forms. The configuration of each part is not limited to the illustrated embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, the configurations described in the above-described embodiment and modified examples (notes, etc.) may be combined, and addition, omission, substitution, and other modifications of the configuration are possible.

[0091] For example, the heater layer 7 disposed between the pair of electrode layers 3, 3 is not limited to being linear. The heater layer 7 may be curved or bent, or may be zigzag as shown in Fig. 23. The heater layer 7 may include a plurality of conductive layer patterns, and may be formed of a plurality of conductive layer patterns wired in parallel to each other as shown in Fig. 24.

[0092] Furthermore, a plurality of sets of a pair of electrode layers 3, 3, a heater layer 7, and a gas sensing layer 5 may be provided on one substrate 2. For example, the embodiment shown in FIG. 25 has two sets of a pair of electrode layers 3, 3, a heater layer 7, and a gas sensing layer 5 on one substrate 2. In this embodiment, a pair of heater wiring layers 9 connected to both ends of the heater layer 7 is provided for each set. An end 8a of each electrode wiring layer 8 and an end 9a of each heater wiring layer 9 are disposed in the vicinity of one side 2a of the substrate 2.

[0093] Such a configuration allows the heating temperature by the heater layer 7 to be different for each gas sensing layer 5. For example, when the two gas sensing layers 5 are formed of different semiconductor materials, the gas sensing layers 5 can be configured to detect different gases while being heated to a temperature suitable for detecting the target gas for each gas sensing layer 5. The two gas sensing layers 5 may be formed of the same semiconductor material. Furthermore, three or more sets of a pair of electrode layers 3, 3, a heater layer 7, and a gas sensing layer 5 may be formed on one substrate 2, and the temperature of the heater layer 7 of each set may be controlled independently of each other.

[0094] The concave-convex structure layer 4 having a porous structure formed on the insulating substrate 2 may have a structure that allows a porous structure to be formed in the gas detection layer 5 due to the porous structure. Such a concave-convex structure layer may be, for example, a porous film of titanium oxide, tin oxide, magnesium oxide, or the like, a porous metal oxide containing a plurality of metal elements, or a layer having a porous structure formed by bonding a large number of microparticles made of insulating metal oxide. The concave-convex structure layer 4 having a porous structure may be capable of electrically connecting the gas detection layer 5 formed on the concave-convex structure layer 4 and the electrode layer 3 below the concave-convex structure layer 4 through pores.

[0095] Such a concave-convex structure layer 4 may be, for example, a porous structure obtained by subjecting a film (for example, a film made of an insulating material) formed across the substrate 2 and the electrode layer 3 to a process (etching process, etc.) for forming a large number of base pores 4a. The large number of base pores 4a in the concave-convex structure layer 4 may be formed irregularly or regularly. This is the same as in the above embodiment described with reference to Figs. 1 to 3. Note that not all of the base pores 4a opening on the upper surface of the concave-convex structure layer 4 may reach the lower surface of the concave-convex structure layer 4. That is, the concave-convex structure layer 4 may have a configuration in which at least some of the base pores 4a reach the lower surface of the concave-convex structure layer 4, and the gas detection layer 5 on the upper layer side and the electrode layer 3 on the lower layer side can be electrically connected. [Explanation of symbols]

[0096] REFERENCE SIGNS LIST 1 gas sensor, 2 substrate, 3 electrode layer, 4 uneven structure layer, 4a base pore, 5 gas detection layer, 5a detection layer pore, 6 alumina layer, 7 heater layer, 8 electrode wiring layer, 8a end of electrode wiring layer, 9 heater wiring layer, 9a end of heater wiring layer

Claims

1. A pair of electrode layers formed on an insulating substrate; a heater layer formed on the substrate; a gas sensing layer formed across the substrate, the electrode layer and the heater layer; The heater layer is disposed between the pair of electrode layers. A semiconductor gas sensor.

2. a pair of electrode wiring layers connected to the pair of electrode layers and a pair of heater wiring layers connected to both ends of the heater layer are provided on the substrate; the electrode layer, the heater layer, the electrode wiring layer, and the heater wiring layer are formed of conductive layer patterns that are simultaneously formed of the same material on the substrate, The wiring width of the heater wiring layer is formed larger than the wiring width of the heater layer.

2. The semiconductor gas sensor according to claim 1.

3. an end portion of the electrode wiring layer opposite to the electrode layer and an end portion of the heater wiring layer opposite to the heater layer are arranged in the vicinity of one side of the substrate, The wiring layer and the measuring device can be electrically connected by inserting and removing one side of the substrate into and from a female connector having a plurality of terminals corresponding to the end portion.

3. The semiconductor gas sensor according to claim 2.

4. a concave-convex structure layer formed under the gas detection layer, on the substrate, on the electrode layer and across the heater layer; The uneven structure layer has a porous structure, the gas detection layer has a porous structure resulting from the porous structure of the relief structure layer; 4. A semiconductor gas sensor according to claim 1.

5. a plurality of sets of the pair of electrode layers, the heater layer and the gas sensing layer are provided on the substrate; 4. A semiconductor gas sensor according to claim 1.

6. a plurality of the gas sensing layers covering one of the heater layers are formed in different regions on the substrate, and the pair of electrode layers are formed on each of the gas sensing layers with the heater layer sandwiched therebetween; 4. A semiconductor gas sensor according to claim 1.

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