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] BACKGROUND 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, causing the oxygen adsorbed on the surface of the gas sensing layer to take 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, returning the electrons captured by the adsorbed oxygen to the gas sensing layer, making it easier for electricity to flow through the gas sensing layer. Semiconductor gas sensors can detect gas by measuring changes 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 electrodes are formed. However, because the heater and the electrodes are arranged apart, heat transfer from the heater to the gas sensing layer between the electrodes is inefficient, and there is a problem in that the entire substrate needs to be heated 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 current situation, and has as its object to provide a semiconductor gas sensor capable of efficiently heating the gas sensing 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 over 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 using 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, the end of the electrode wiring layer opposite the electrode layer and the end of the heater wiring layer opposite the heater layer may be arranged near one side of the substrate, and the wiring layer and the measuring device may be electrically connected by inserting or removing one side of the substrate into or from a female connector having a plurality of terminals corresponding to the end.

[0010] In addition, a concave-convex structure layer may be provided below the gas detection layer, formed on the substrate, on 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 this 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 concave-convex 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] Furthermore, 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 has been added and which is biased toward a negative charge. More specifically, the gas sensing layer may be formed of tungsten trioxide to which nickel has been added. The impurity added to tungsten trioxide (hereinafter also referred to simply as tungsten oxide) may be any element having a valence of five or less, and may be, in addition to nickel (Ni), for example, silicon (Si), indium (In), aluminum (Al), or two or more elements. 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 has been 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 addition, in the semiconductor gas sensor of the present invention, a plurality of the gas detection 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 detection layers, sandwiching the heater layer therebetween. [Effects of the Invention]

[0017] In the semiconductor gas sensor of the present invention, the heater layer is disposed between the pair of electrode layers, so that the gas sensing layer between the electrode layers can be efficiently heated by the heater layer. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic plan view showing an embodiment of a semiconductor gas sensor; [Figure 2] FIG. 2 is a schematic exploded perspective view showing the embodiment. [Figure 3] FIG. 2 is a schematic cross-sectional view corresponding to the position AA in FIG. 1. [Figure 4] 10A and 10B are schematic diagrams illustrating a connection state between the gas sensor and the female connector. [Figure 5] FIG. 10 is a schematic plan view showing another embodiment of the gas sensor. [Figure 6] FIG. 10 is a schematic plan view showing still another embodiment of the gas sensor. [Figure 7] FIG. 10 is a schematic cross-sectional view 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 concave-convex structure layer. [Figure 10] 1 is a scanning electron microscope image of the surface of a concave-convex 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] 10 is a graph showing the results of using a test gas sensor and a conventional gas sensor to detect acetone. [Figure 13] 10 is a graph showing the response characteristics of a test gas sensor to acetone. [Figure 14] 10 is a graph showing the response characteristics of a test gas sensor to ethanol. [Figure 15] 10 is a graph showing the response characteristics of a gas sensor of a comparative example to acetone. [Figure 16] 10 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] 1 is a graph showing the response characteristics of the gas sensor of Reference Example 1 to ethanol. [Figure 19] 10 is a graph showing the response characteristics of the gas sensor of Reference Example 2 to acetone. [Figure 20] 10 is a graph showing the response characteristics of the gas sensor of Reference Example 2 to ethanol. [Figure 21] 10 is a graph showing the response characteristics of the gas sensor of Reference Example 3 to acetone. [Figure 22] 10 is a graph showing the response characteristics of the gas sensor of Reference Example 3 to ethanol. [Figure 23] FIG. 10 is a schematic plan view showing still another embodiment of the gas sensor. [Figure 24] FIG. 10 is a schematic plan view showing still another embodiment of the gas sensor. [Figure 25] FIG. 10 is a schematic plan view showing still another embodiment of the gas sensor. DETAILED DESCRIPTION OF THE INVENTION

[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 arranged spaced apart from each other. The heater layer 7 is disposed between the pair of electrode layers 3, 3. The heater layer 7 is provided spaced apart from the electrode layer 3.

[0021] The gas sensor 1 includes, on a substrate 2, 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. 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 that are simultaneously formed 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. 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 substrate 2 facing the side 2a 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 ceramic, 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, for example, 10 mm in length, 10 mm in width, and 1 mm in thickness or more. In this embodiment, the substrate 2 is a rectangular ceramic substrate measuring 30 mm in length, 12 mm in width, and 1.5 mm in thickness.

[0024] The electrode layer 3 and the electrode wiring layer 8 are intended to detect changes in the electrical resistance value of the gas detection layer 5. The pair of electrode layers 3, 3 are disposed at positions closer to the side 2b opposite to the side 2a of the substrate 2 and spaced apart from each other in 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 in the longitudinal 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 between the pair of electrode layers 3, 3 along the longitudinal direction of the substrate 2. One of the heater wiring layers 9 is disposed between the pair of electrode wiring layers 8, and extends from the end of the heater layer 7 on the side 2a toward the side 2a along the longitudinal direction of the substrate 2.

[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 the wiring width 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 extends in the width direction of the substrate 2, and a pair of electrode layers 3, 3 are arranged 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, for example, by a nichrome layer deposited on the substrate 2 by sputtering 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 thickness of the layers 3, 7, 8, and 9 is, for example, 1 μm. The material of the electrode layer 3 is not particularly limited, and examples thereof include nichrome, platinum, gold, silver, titanium, nickel, aluminum, ruthenium, tantalum, titanium, copper, platinum, niobium, zirconium, alloys of these elements, alloys of these elements with carbon, and elemental carbon. 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 on the substrate 2, the pair of electrode layers 3, 3, and the heater layer 7, straddling the same. 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 the entire heater layer 7. The gas detection layer 5 may also be formed on the electrode wiring layer 8 or the heater wiring layer 9, straddling the same.

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

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

[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 examples of such materials include chromium, titanium, and tungsten.

[0035] The gas sensor 1 has the heater layer 7 disposed between the pair of electrode layers 3, 3, and therefore 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°C or higher.

[0036] The conventional gas sensor disclosed in Patent Document 1 has a substrate size of 1 mm square, and the wiring patterns for the heater and electrodes are formed by photolithography, resulting in high manufacturing costs. Furthermore, as the width and pitch of the wiring pattern formed by the conductive layer become narrower when making the gas sensor smaller, defects such as breaks and short circuits become more likely to occur, resulting in low yields. These problems can be solved by scaling up the substrate size to a size that can be handled by the user, for example, to a few centimeters, and increasing the wiring width.

[0037] Scaling up a gas sensor improves its heat dissipation capacity. 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. This means that the heater must be operated at a high temperature of 500°C or higher to heat the gas sensing layer to an operating temperature of 200°C or higher. Such operating conditions increase power consumption, raise safety concerns due to exposure to high temperatures, and shorten the lifespan 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, such as forming the electrode layer 3 and the heater layer 7 by pattern deposition on a substrate 2 that is small enough for a user to handle, 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, on a substrate 2, 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. These layers 3, 7, 8, 9 are formed of conductive layer patterns formed simultaneously using the same material. Therefore, the gas sensor 1 can reduce the number of manufacturing steps and manufacturing costs.

[0040] The wiring width of the heater wiring layer 9 is larger than that of the heater layer 7. Therefore, the gas sensor 1 can locally heat the gas sensing layer 5 between the electrode layers 3, 3 while reducing heat generation from the heater wiring layer 9 by making the electrical resistance of the heater wiring layer 9 smaller than that of the heater layer 7, 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 near one side 2a of the substrate 2. This allows for 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 these ends 8a, 9a. Furthermore, since the gas sensor 1 does not require wiring processing such as soldering, an easily replaceable gas sensor 1 can be manufactured inexpensively, and the gas sensor 1 can be used, for example, as a disposable chip.

[0042] Here, the gas sensor 1 can locally heat the gas detection layer 5 between the electrode layers 3, 3, thereby suppressing a temperature rise at the ends 8a, 9a of the wiring layers 8, 9, particularly at 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 at the portion 1a of the gas sensor 1 that contacts the female connector 11 (see FIG. 1 ), without providing a heat dissipation opening in the substrate 2 or connecting the substrate 2 to a pin member by wire soldering or the like.

[0043] The female connector 11 may be made of a resin material with a heat resistance temperature of about 100°C, but in the gas sensor 1 of this embodiment, the temperature of the portion 1a that contacts the female connector 11 can be kept below 100°C when the heater layer 7 is heated to about 320°C. In this way, the gas sensor 1 can be mass-produced at low cost, and can be easily replaced by inserting it into or removing it from the female connector 11 without damaging the female connector 11.

[0044] 6 is a schematic plan view showing yet 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 a substrate 2. A pair of electrode layers 3, 3 is formed on each gas sensing layer 5, sandwiching the heater layer 7 therebetween. An electrode wiring layer 8 is connected to each electrode layer 3. An end 8a of each electrode wiring layer 8, together with an end 9a of a heater wiring layer 9, is located near one side 2a of the substrate 2. In other words, the gas sensor 1 of this embodiment includes multiple sets of pairs of electrode layers 3, 3, heater layers 7, and gas sensing layers 5 on a single substrate 2.

[0045] When the two gas sensing layers 5 are made of different semiconductor materials, they can be configured to detect different gases. When the two gas sensing layers 5 are made of the same semiconductor material, the surface area of ​​the gas sensing layers 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 on 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, increasing the surface area of ​​the gas sensing layer requires forming 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 illustrating yet another embodiment of the gas sensor. This embodiment differs from the embodiment shown in Fig. 1 etc. in the configuration of the gas sensing 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 below the gas sensing 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 sensing layer 5 also has a porous structure resulting from the porous structure of the concave-convex structure layer 4.

[0051] The concave-convex structure layer 4 is made of an insulating metal oxide having a porous structure with numerous base pores 4a. In this embodiment, the concave-convex structure layer 4 is made of an aluminum compound layer obtained by making an alumina layer porous by hot water treatment. In this embodiment, the concave-convex 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 concave-convex structure layer 4 may cover the entire electrode layer 3 or the entire heater layer 7. The concave-convex structure layer 4 may also 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 depositing a film of amorphous alumina to a thickness of about 30 nm on the substrate 2 after the electrode layer 3 has been formed thereon 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 simply by performing the extremely simple process of hot water treatment, thereby reducing manufacturing costs.

[0053] 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 electrode layer 3 and the heater layer 7. The gas detection layer 5, which will be described later, is electrically connected to the electrode layer 3 via the base pores 4a.

[0054] The porous concave-convex-structure layer 4 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, or the like, 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 (such as the opening shape, opening width, and pore depth of the sensing layer pores 5a). This makes it possible to form a gas sensor 1 equipped with a gas sensing layer 5 having a porous structure suited to the application.

[0058] An adhesive layer may be formed on the surface of the substrate 2 to prevent peeling between the substrate 2 and the concave-convex structure layer 4. Such an adhesive layer may be formed as 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 a 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 of the gas sensing layer 5 (sensing layer pores 5a) is derived from the porous structure (base pores 4a) of the underlying concave-convex-structure layer 4, the gas sensing 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 sensing layer 5 having a porous structure even if the oxide semiconductor layer forming the gas sensing 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 sensing layer 5 regardless of the type or crystal structure of the oxide semiconductor of the gas sensing layer 5, thereby improving the sensor sensitivity and gas selectivity.

[0061] Furthermore, it is possible to control the porous structure (such as the opening shape, opening width, and pore depth of the sensing layer pores 5a) of the gas sensing layer 5 by changing the film thickness of the gas sensing layer 5. This makes it possible to form a gas sensor 1 equipped with a gas sensing layer 5 having a porous structure suited to the application.

[0062] Next, with reference to Fig. 8, a manufacturing example of a gas sensor 1 having a concave-convex structure layer 4 will be described. 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 sputtering 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 greater or less 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 warm 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 Figure 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. The concave-convex structure layer 4 shown in Figs. 9 and 10 was formed by depositing an alumina layer 6 on a 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, which is made of an insulating aluminum compound layer obtained by making the alumina layer 6 porous through 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 deposited on the concave-convex structure layer 4 by reactive plasma deposition using a metal mask with an opening pattern corresponding to the formation region of the concave-convex structure layer 4 (the formation region of the alumina layer 6), thereby forming a gas sensing layer 5 with numerous sensing layer pores 5a (porous structure). The gas sensing layer 5 is electrically connected to the electrode layer 3 through the base pores 4a. Here, a 100-nm-thick film of tungsten oxide (WO3) doped with 0.5 at% (atomic percent) indium (In) was used as the oxide semiconductor material. The gas sensing layer 5 was then sintered.

[0068] In this way, 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, reactive plasma vapor deposition method) using a metal mask with an opening pattern, patterning by etching or lift-off after deposition of each layer is not required, thereby reducing manufacturing costs.

[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 will have high resistance and the measurement sensitivity will decrease. 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), it will take a long time to form the gas sensing layer 5, resulting in a decrease in production efficiency.

[0071] It should be noted that the manufacturing cost can be reduced by providing regions for a plurality of gas sensors 1 on one substrate 2, forming a plurality of gas sensors 1 at the same time, and then separating each gas sensor 1 into individual pieces.

[0072] Next, an example of measurement results using a gas sensor including a gas sensing layer 5 having sensing layer pores 5a resulting from the base pores 4a of the concave-convex-structure layer 4 will be described. Here, 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 doped with 0.5 at % indium, and the concave-convex-structure layer 4 and the gas sensing layer 5 were 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). A conventional gas sensor was used as a comparative example. The gas sensor was heated to, for example, 300°C, and the sensor resistance (R air ) and the sensor resistance value (R gas) and measure the sensitivity (=R air / R gas ) was sought.

[0074] Figure 12 is a graph showing the results of using the test gas sensor and a conventional gas sensor to detect acetone. In Figure 12, the vertical axis represents sensitivity (arbitrary units), and the horizontal axis represents the acetone concentration of the measured gas. As can be seen from Figure 8, the test gas sensor (see "Test Example") achieved a measurement sensitivity that was approximately 10 times higher than that of the conventional gas sensor (see "Conventional Sensor") (see the measurement result for an acetone concentration of 1,000 ppb). Furthermore, the test gas sensor achieved a sensitivity of approximately 8 for an acetone concentration of 500 ppb, demonstrating high detection sensitivity for acetone. Furthermore, the test 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 be able 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 test gas sensor to acetone and ethanol was verified. Figures 13 and 14 are graphs showing the response characteristics of the test gas sensor to acetone and ethanol. The measurement gases used were an acetone concentration of 1000 ppb (1 ppm) and an ethanol concentration of 1000 ppb. In each of the graphs in Figures 13 and 14, the horizontal axis represents relative time (seconds) and the vertical axis represents sensitivity (arbitrary units).

[0077] 13, the test gas sensor had a detection peak of approximately 24.0 for acetone at a concentration of 1000 ppb, and a detection peak of approximately 25.0 for ethanol at a concentration of 1000 ppb. These results demonstrate that the test gas sensor having a gas sensing layer 5 made of indium-doped tungsten oxide exhibits high detection sensitivity for 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 the manufacturing process described with reference to FIG. 8, in which (1) a pair of interdigital electrode layers was formed facing each other on the substrate 2, and then (2) the step of forming the alumina layer 6 and (3) the hot water treatment step were omitted, and (4) the gas sensing layer 5 was formed on the substrate 2 and the pair of interdigital 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 comparative gas sensor to acetone and ethanol. The measurement conditions were 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 Figure 15, the gas sensor of the comparative example had a detection peak of approximately 12.5 for acetone at a concentration of 1000 ppb. Also, as shown in Figure 16, the gas sensor of the comparative example had a detection peak of approximately 18.7 for ethanol at a concentration of 1000 ppb. Comparing the response characteristics of the test gas sensor shown in Figures 9 and 10 revealed that the test 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 robustness than a gas sensing layer without 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 sensing layer (tungsten oxide). The impurities added to the 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, which has a gas sensing layer made of tungsten oxide with nickel added. FIGS. 19 and 20 are graphs showing the response characteristics to acetone and ethanol of the gas sensor of Reference Example 2, which has a gas sensing layer made of tungsten oxide with silicon added. FIGS. 21 and 22 are graphs showing the response characteristics to acetone and ethanol of the gas sensor of Reference Example 3, which has a gas sensing layer made of tungsten oxide with aluminum added. The measurement conditions were the same as those used to measure the response characteristics of the verification gas sensor shown in FIGS. 13 and 14. In each of the graphs in FIGS. 17 to 22, the horizontal axis represents relative time (seconds), and the vertical axis represents sensitivity (arbitrary units).

[0084] As shown in Figure 17, the gas sensor of Reference Example 1, which has a gas sensing layer made of tungsten oxide and nickel added, has a detection peak of about 3.9 for acetone at a concentration of 1000 ppb, indicating very high detection sensitivity for acetone. On the other hand, as shown in Figure 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] This result indicates that the gas sensor 1 having nickel-added tungsten oxide as the gas sensing 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, which has a gas sensing layer 5 made of tungsten oxide and silicon added, 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, which has a gas sensing layer made of tungsten oxide with aluminum added, had detection peaks of about 3.0 for both 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 gas sensors having a gas sensing layer of tungsten oxide (WO3) doped with nickel or silicon exhibit better detection sensitivity to acetone than gas sensors having tungsten oxide doped with aluminum. It was also found that gas sensors having a gas sensing layer made of tungsten oxide doped with nickel exhibit 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 within the scope of 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 additions, omissions, substitutions, and other modifications of the configurations 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 also have a plurality of conductive layer patterns, and may be formed of, for example, 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 a single 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 a single 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. Ends 8a of each electrode wiring layer 8 and ends 9a of each heater wiring layer 9 are located near one side 2a of the substrate 2.

[0093] This configuration allows the heating temperature by the heater layer 7 to be different for each gas sensing layer 5. For example, when two gas sensing layers 5 are formed from different semiconductor materials, each gas sensing layer 5 can be heated to a temperature suitable for detecting the target gas, allowing each gas sensing layer 5 to detect a different gas. The two gas sensing layers 5 may be formed from the same semiconductor material. Alternatively, three or more pairs of electrode layers 3, 3, heater layers 7, and gas sensing layers 5 may be formed on one substrate 2, and the temperatures of the heater layers 7 in each pair may be controlled independently of each other.

[0094] The porous uneven-structure layer 4 formed on the insulating substrate 2 may have any structure as long as the porous structure results in the formation of a porous structure in the gas sensing layer 5. Such an uneven-structure layer may be, for example, a porous film of titanium oxide, tin oxide, magnesium oxide, or the like; a porous metal oxide containing multiple metal elements; or a porous layer formed by bonding a large number of microparticles made of an insulating metal oxide. The porous uneven-structure layer 4 may be any structure as long as the gas sensing layer 5 formed on the uneven-structure layer 4 and the electrode layer 3 below the uneven-structure layer 4 can be electrically connected via pores.

[0095] Such a concave-convex-structure layer 4 may be formed as a porous structure by, for example, performing a process (such as etching) to form a large number of base pores 4a in a film (e.g., a film made of an insulating material) formed across the substrate 2 and the electrode layer 3. The large number of base pores 4a in the concave-convex-structure layer 4 may be formed irregularly or regularly. This also applies to the above-described embodiment described with reference to FIGS. 1 to 3. It is not necessary for all of the base pores 4a opening on the upper surface of the concave-convex-structure layer 4 to reach the lower surface of the concave-convex-structure layer 4. That is, the concave-convex-structure layer 4 may be configured such that at least some of the base pores 4a reach the lower surface of the concave-convex-structure layer 4, allowing electrical connection between the upper gas sensing layer 5 and the lower electrode layer 3. [Explanation of symbols]

[0096] 1 gas sensor, 2 substrate, 3 electrode layer, 4 concave-convex 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 over the substrate, the electrode layer, and the heater layer; the heater layer is disposed between the pair of electrode layers, 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 on the substrate using the same material, The wiring width of the heater wiring layer is formed larger than the wiring width of the heater layer. Semiconductor gas sensor.

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

2. The semiconductor gas sensor according to claim 1.

3. A pair of electrode layers formed on an insulating substrate; a heater layer formed on the substrate; a gas sensing layer formed over the substrate, the electrode layer, and the heater layer; the heater layer is disposed between the pair of electrode layers, a concave-convex structure layer formed under the gas detection layer, on the substrate, on the electrode layer, and across the heater layer; the concave-convex structure layer has a large number of base pores, the gas sensing layer has a large number of sensing layer pores formed due to the large number of base pores, and is electrically connected to the electrode layer via the base pores; Semiconductor gas sensor.

4. 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 on the substrate using the same material, an end of the electrode wiring layer opposite to the electrode layer and an end of the heater wiring layer opposite to the heater layer are arranged near one side of the substrate, The electrode wiring layer and the heater wiring layer can be electrically connected to a measuring device by inserting and removing one side of the substrate into and from a female connector having a plurality of terminals corresponding to the end.

4. The semiconductor gas sensor according to claim 3.

5. A pair of electrode layers formed on an insulating substrate; a heater layer formed on the substrate; a gas sensing layer formed over the substrate, the electrode layer, and the heater layer; the heater layer is disposed between the pair of electrode layers, 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 on the substrate using the same material, an end of the electrode wiring layer opposite to the electrode layer and an end of the heater wiring layer opposite to the heater layer are arranged near one side of the substrate, the electrode wiring layer and the heater wiring layer can be electrically connected to a measuring device 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, a plurality of sets of the pair of electrode layers, the heater layer, and the gas sensing layer are provided on the substrate; Semiconductor gas sensor.

6. A pair of electrode layers formed on an insulating substrate; a heater layer formed on the substrate; a gas sensing layer formed over the substrate, the electrode layer, and the heater layer; the heater layer is disposed between the pair of electrode layers, 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 on the substrate using the same material, an end of the electrode wiring layer opposite to the electrode layer and an end of the heater wiring layer opposite to the heater layer are arranged near one side of the substrate, the electrode wiring layer and the heater wiring layer can be electrically connected to a measuring device 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, a plurality of the gas detection 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 detection layers with the heater layer sandwiched therebetween; Semiconductor gas sensor.

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