Gas sensor FET, gas sensor, and method of manufacturing gas sensor fet

Abstract

To provide a gas sensor FET with improved gas detection sensitivity.SOLUTION: A gas sensor FET is provided, comprising a semiconductor substrate, and a gate electrode layer formed on the semiconductor substrate and configured to function a gas detection unit. The gate electrode layer includes a catalytic metal layer film having a side exposed to the atmosphere. The side of the catalytic metal layer film exposed to the atmosphere has a structure consisting of multiple two-dimensionally arranged dendritic structures, each having formed thereon a columnar portion having a plurality of fine structures that are smaller and finer than the columnar portion. The multiple dendritic structures are electrically connected to each other.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a gas sensor FET, a gas sensor, and a method for manufacturing a gas sensor FET. [Background technology]

[0002] Conventionally, gas sensors for detecting specific gases or impurity gases have been known. One type of gas sensor is a FET gas sensor. Patent Document 1 describes an example of a FET gas sensor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-115888 A Summary of the Invention [Problem to be solved by the invention]

[0004] FET type gas sensors use a semiconductor element called a gas sensor FET. Gas sensor FETs are relatively small in size and are suitable for miniaturizing gas sensors.

[0005] However, the gas sensor FET tends to have low gas detection sensitivity, and there is still room for improvement in this gas detection sensitivity.

[0006] An object of the present invention is to further increase the gas detection sensitivity of a gas sensor FET. [Means for solving the problem]

[0007] One representative embodiment of the present invention is a gas sensor FET comprising a semiconductor substrate, and a gate electrode layer formed on the semiconductor substrate and functioning as a gas detection unit, the gate electrode layer having a catalyst metal layer film including a side exposed to an atmosphere, the side of the catalyst metal layer film exposed to the atmosphere having a structure in which a plurality of dendritic structures are two-dimensionally arranged, each of which has a columnar portion on its surface and a plurality of microstructure portions that are smaller and finer than the columnar portion, and the plurality of dendritic structures are electrically connected to each other.

[0008] Moreover, one representative embodiment of the present invention is a gas sensor comprising: the gas sensor FET arranged on a substrate; a reference FET arranged on the substrate for referencing a gate threshold voltage; and a heater arranged on the substrate for bringing the temperature of the substrate close to a predetermined temperature range.

[0009] Moreover, one representative embodiment of the present invention is a method for manufacturing a gas sensor FET, comprising a semiconductor substrate, and a gas detection material portion formed on the semiconductor substrate and constituting a gate electrode layer, the gas detection material portion having a catalytic metal layer film including a side exposed to an atmosphere, the method including a catalytic metal layer deposition step of depositing the catalytic metal layer film by sputtering at a sputtering pressure of 5 Pa or more. Effect of the Invention

[0010] According to the embodiment of the present invention, the gas detection sensitivity of the gas sensor FET can be further improved. [Brief description of the drawings]

[0011] [Figure 1] 1 is a diagram showing an example of the structure of a gas sensor FET according to a first embodiment; [Diagram 2] FIG. 2 is a diagram showing an example of a gate electrode layer of the gas sensor FET according to the first embodiment. [Diagram 3] FIG. 4 is a flowchart showing an example of a method for manufacturing the gas sensor FET according to the first embodiment. [Figure 4]4 is a flow chart showing an example of a method for forming a gate electrode layer according to the first embodiment. [Diagram 5] 13 is a diagram for explaining that a gate electrode layer is formed after a protective film is formed. FIG. [Figure 6] FIG. 4 is a diagram showing the relationship between the sputtering pressure during formation of a catalytic metal layer and the non-surface area of ​​the catalytic metal layer. [Figure 7] FIG. 13 is a diagram showing enlarged structural images of a plurality of Pt catalytic metal layer films formed under different sputtering pressures. [Figure 8] 5 is a graph showing a measurement example of the relationship between the non-uniform area of ​​the catalytic metal layer film in the gas sensor FET according to Example 1 and the amount of change in gate threshold voltage with respect to the concentration of the detected gas. FIG. [Figure 9] 5 is a graph showing a measurement example of the amount of change in gate threshold voltage with respect to the film thickness of a catalyst metal layer in the gas sensor FET according to Example 1. FIG. [Figure 10] 11 is a diagram showing the structure of a gate electrode layer of the gas sensor FET according to Example 2, and a graph showing the fluctuation of the gate threshold voltage due to a change in the concentration of the detected gas in the atmosphere. FIG. [Figure 11] 13 is a diagram showing the structure of a gate electrode layer of the gas sensor FET according to Example 3 and the fluctuation of the gate threshold voltage due to the change in the concentration of the detected gas in the atmosphere. FIG. [Figure 12] 13 is a diagram showing the structure of a gate electrode layer of the gas sensor FET according to Example 4 and fluctuations in gate threshold voltage due to changes in the concentration of a detected gas in the atmosphere. FIG. [Figure 13] FIG. 13 is a diagram showing the structure of a gate electrode layer of the gas sensor FET according to Example 5 and fluctuations in gate threshold voltage due to changes in the concentration of a detected gas in the atmosphere. [Figure 14] FIG. 13 is a diagram for explaining a threshold voltage fluctuation amount. [Figure 15] 11 is a diagram showing the relationship between the configuration of a gate electrode layer and the amount of gate threshold voltage fluctuation. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] (Details of the study by the present inventors) Toward the realization of a carbon-free society, technologies that use hydrogen to generate electricity, such as fuel cells, are attracting attention. With growing environmental awareness among people around the world, there are hopes for improvements in technology for these fuels that do not emit carbon dioxide. Meanwhile, in the case of PEFCs (Polymer Electrolyte Fuel Cells), which have been used in recent years as home fuel cell systems and automotive fuel cells, it is known that impurity gases such as carbon monoxide, hydrogen sulfide, and hydrocarbon compounds in high-concentration hydrogen gas cause cell deterioration, and it is said that the lifespan of the cells will be shortened unless refresh operations are performed.

[0013] In addition, hydrogen engines used in distributed power sources use hydrogen gas containing impurities such as carburizing gas as fuel, which also requires precise management of the combustion state after measuring the impurity concentration of the gas.

[0014] Therefore, a technology to detect impurities in high-concentration hydrogen is needed. Conventional gas sensor technology was able to measure high-concentration hydrogen, but it was difficult to detect impurities contained in high-concentration hydrogen.

[0015] In contrast, it has been found that a semiconductor FET sensor formed from a laminated film of Pt (platinum) and Ti (titanium) (for example, as described in Patent Document 2: JP 2009-300267 A and Non-Patent Document 1: Applied Physics Express 3 (2010) 047201) is capable of detecting impurities in high-concentration hydrogen to a certain extent. In the detection film of this sensor, the Ti layer introduced as an adhesion layer is oxidized, causing a structural change in the upper Pt film, forming a large number of grain boundaries. A structure in which a large number of grain boundaries are formed in this way is called a corridor structure. It is believed that when a Pt film with a corridor structure is formed on the gate electrode, the surface area of ​​the Pt film increases, inducing the adsorption of hydrogen gas.

[0016] The specific surface area of ​​these Pt gallery structures ranges from 0.1 to 10 m depending on the degree of oxidation of the undercoat. 2 / g. The detection mechanism of this sensor utilizes the fact that hydrogen gas adsorbed on the Pt catalyst film is protonated, and the protons are accumulated near the gate insulating film, generating an electric field, which changes the threshold voltage of the FET. Non-Patent Document 2 also describes a similar sensor in which island-shaped porous Pt formed from a solution is used for the detection film. The crystal grain size of this sensor is also described as about 0.5 μm, which is considered to be approximately the same specific surface area as the above-mentioned corridor structure sensor.

[0017] Other gas sensors for the purpose of hydrogen detection and the like include the following. Patent Document 3 (JP Patent Publication 2012-013579 A) describes a technology for a chemical sensor using a sensitive film. Not limited to FET type sensors, sensors that utilize properties such as a change in resistance or mass depending on the substance the sensitive film reacts to can be produced with different types of sensitive film, making it possible to detect various chemical substances. The chemical sensor in Figure 1 of Patent Document 3 is an example of a FET type sensor, and Figures 7 and 8 of Patent Document 3 show examples of FET type sensors that use a reference electrode.

[0018] As described in Non-Patent Document 3 (Sensors and Actuators B 330 (2021) 129240), there are several detection principles for hydrogen sensors, which are a type of gas sensor. Among these, FET type, capacitor type, and diode type sensors are classified as work function type sensors. Work function type sensors can be manufactured by a process using a semiconductor substrate, so they are expected to achieve lower cost, smaller size, and lower power consumption than sensors classified into other types.

[0019] As described in Table 2 of Non-Patent Document 3, FET-type gas sensors are further classified into several types. Among them, the catalytic metal gate FET (called the Catalytic Metal Gate FET in Non-Patent Document 2: Japanese Journal Of Applied Physics 37 (1998) 1100) has a simple structure similar to that of a normal MOSFET, and is therefore particularly suitable for miniaturization and cost reduction.

[0020] Patent Document 4 (JP Patent Publication 2005-283578 A) describes a FET gas sensor as one type of gas sensor. The FET gas sensor has a simple structure and is therefore particularly suitable for miniaturization and cost reduction.

[0021] The Pt / Ti corridor structure FET sensor described in Patent Document 3 works by utilizing the fact that when the base metal is oxidized, the Pt layer, which is a catalytic metal, undergoes a structural change and the number of grain boundaries increases, thereby increasing the number of gas adsorption sites to be sensed and enabling efficient detection. However, the combinations of catalytic metals and adhesive metal layers that similarly undergo structural changes are limited, and in reality there are very few options for effective catalytic metal layers.

[0022] Even if grain boundaries are effectively formed, the specific surface area is less than 10 m 2 / g is the limit. Furthermore, the specific surface area varies depending on the degree of oxidation of the adhesive metal layer during manufacturing, so there are concerns about stable manufacturing. For this reason, it has been impossible to select a catalyst metal layer that matches the detection gas or a highly sensitive catalyst metal layer.

[0023] Under such a background, the present inventors have invented a gas sensor FET with higher gas detection sensitivity as a result of intensive research. One of the effects of the embodiment of the present invention is high sensitivity of gas detection in the gas sensor FET, but other effects include a wider range of options for the detection metal layer, a wider dynamic range, and stable and low cost manufacturing. Hereinafter, the embodiment of the present invention will be described.

[0024] (Embodiment 1) <Example 1> Fig. 1 is a diagram showing an example of the structure of a gas sensor FET according to Example 1. As shown in Fig. 1, the gas sensor FET 1 has two n-type semiconductors 12 constituting a drain and a source formed at a distance from each other on a p-type semiconductor substrate 11 made of silicon or the like, and a gate electrode layer 13 formed so as to cover each part of the two n-type semiconductors 12. When gas is adsorbed to the gate electrode layer 13, the threshold voltage of the FET changes. The gas can be detected by measuring the change in this threshold voltage.

[0025] 2 is a diagram showing an example of a gate electrode layer of the gas sensor FET according to the first embodiment. In the gate electrode layer 13 shown in FIG. 2, a gate insulating film 132 is formed on a semiconductor substrate 11, an oxide layer film (e.g., a TiO2 film, 10 nm thick) 133 is formed thereon, and a catalyst metal layer film (e.g., a Pt film, specific surface area of ​​about 40 m2) having a structure characteristic of this embodiment is formed thereon. 2 1, an oxide layer 133 (20 nm thick) is formed on the gate electrode layer 132. The oxide layer 133 corresponds to an adhesive layer in a gate electrode layer having a conventional corridor structure.

[0026] The characteristic structure of the gate electrode layer 13 according to the first embodiment is a columnar tree structure in which a plurality of tree structures 1341, each of which is formed by forming a plurality of fine structures 1341b smaller and finer than the column structure 1341a on the surface of the column structure 1341a, are two-dimensionally arranged, as shown in the upper and middle parts of Fig. 2. Here, the column axis direction of the column structure 1341a is the z-axis direction, one direction in a plane perpendicular to the z-axis direction is the x-axis direction, and the direction perpendicular to the x-axis direction in the same perpendicular plane is the y-axis direction. The lower part of Fig. 2 shows the xz cross section of the tree structure 1341 (half cross section of the tree structure) and the xy cross section of the tree structure 1341 (1 / 4 cross section of the tree structure). As can be seen from these figures, in the tree-like structure 1341, a large number of thin plate-shaped or granular microstructures 1341b, which resemble the thin leaves or petals of a tree and extend in the z-axis direction to include a component in the direction in which the film grows during film formation, are formed so as to almost completely fill the surface of the columnar portion 1341a.

[0027] The shape of such dendritic structures 1341 increases the specific surface area of ​​the catalytic metal in the gate electrode layer 13 that corresponds to the gas sensor, as compared with the conventional gallery structure, thereby improving the gas detection sensitivity. A method for manufacturing a gas sensor FET having such a dendritic catalytic metal film in the gate electrode layer 13 will be described below.

[0028] <Method of manufacturing gas sensor FET> The method for manufacturing the gas sensor FET according to this embodiment is characterized by the method for forming the gate electrode layer, and is not particularly different from the method for manufacturing a general MOSFET with respect to the parts other than the gate electrode. Therefore, here, an outline of the manufacturing process for the gas sensor FET according to this embodiment will be first described, and then the method for forming the gate electrode layer of the gas sensor FET according to this embodiment will be mainly described.

[0029] 3 is a flow chart showing an example of a method for manufacturing the gas sensor FET according to Example 1. In this flow chart, only main processes are included, and detailed processes are omitted.

[0030] 3, first, in step S1, a step of preparing a semiconductor substrate is performed. That is, a step of cutting a semiconductor substrate (also called a wafer or a silicon wafer) from a silicon ingot and polishing it is performed.

[0031] Next, in step S2, a process of forming an oxide film is performed, that is, a process of forming an insulating oxide film necessary for forming an insulating film between wiring layers of FETs on the surface of the semiconductor substrate is performed.

[0032] Next, in step S3, a photoresist application process is performed, that is, a process of applying a photoresist onto the oxide film formed on the surface of the semiconductor substrate is performed.

[0033] Next, in step S4, an exposure process is performed, that is, a process is performed in which the semiconductor substrate is covered with a mask, and light such as ultraviolet light is irradiated onto the semiconductor substrate to transfer a required pattern onto the oxide film.

[0034] Next, in step S5, a development step is performed, that is, a step of developing the photoresist, leaving the photoresist on the oxide film to which the pattern has been transferred, and removing the other photoresist is performed.

[0035] Next, in step S6, a process of removing the etching photoresist is performed, that is, a process of removing the photoresist and removing unnecessary portions of the oxide film from which the photoresist was removed in step S5 is performed.

[0036] Next, in step S7, a step of diffusing impurities (doping) is performed, that is, a step of diffusing impurities by supplying an impurity gas to the surface of the semiconductor substrate is performed.

[0037] Next, in step S8, a process of determining whether all necessary patterns have been formed is performed. Note that the formation of the necessary patterns does not include the formation of the gate electrode layer here. This is because the gate electrode layer needs to be exposed to the atmosphere so as to function as a gas sensor, and therefore, as described below, after forming the protective film, a part of the protective film needs to be removed, and the gate electrode layer needs to be formed in the part without the protective film as a final process.

[0038] In this determination, if it is determined that all the necessary patterns have been formed and the chip is complete except for the gate electrode layer portion (S8: Yes), the processing steps proceed to step S9. If it is determined that all the necessary patterns have not been formed (S8: No), the processing steps return to step S2, and the process from forming the oxide film to the doping process is performed again.

[0039] In step S9, a contact forming step is performed, that is, a step of performing a predetermined process on necessary portions so that the chip surface is ready for wiring, and forming contacts is performed.

[0040] Next, in step S10, a wiring process is performed. That is, a process of wiring between contacts is performed by connecting wiring materials or the like. The wiring materials used here are materials having anti-oxidation properties. As described later, after wiring is performed, a protective film is formed, a part of the protective film is removed, and a gate insulating film is formed thereon. When forming this gate insulating film, a strong oxidation process is performed on the semiconductor substrate. At this time, this is to prevent the wiring material that has been placed earlier from being altered by oxidation. Examples of materials having anti-oxidation properties include tungsten, which is difficult to oxidize, high-melting point metal materials such as LaB6 and CuMn, and anti-oxidation metal materials.

[0041] Next, in step S11, a step of forming a protective film is performed, that is, a step of covering the entire chip with a protective film in order to prevent the chip from being contaminated or deteriorated by external gas, atmosphere, etc. is performed.

[0042] Next, in step S12, a process of removing a portion of the protective film where the gate electrode layer is to be formed is performed. For example, the protective film covering the region where the gate electrode layer is to be formed is removed by dry etching or wet etching.

[0043] Next, in step S13, a process of forming a gate electrode layer is performed. For example, a gate insulating film (having a thickness of, for example, 15 nm) is formed on a semiconductor substrate, an oxide film is formed thereon, and a catalyst metal layer is further formed thereon to form the gate electrode layer.

[0044] <Method of forming gate electrode layer> Here, an example of a method for forming the gate electrode layer according to this embodiment will be described.

[0045] Fig. 4 is a flow diagram showing an example of a method for forming a gate electrode layer according to the first embodiment. Fig. 5 is a diagram for explaining that the gate electrode layer is formed after the protective film is formed. As described above, the gate electrode layer portion that operates as the detection film of the sensor, particularly the catalyst metal layer film, needs to be exposed to the outside. Therefore, in forming the gate electrode layer including the gate insulating film, it is necessary to first form a protective film (passivation film), then provide an opening in the protective film, and form the gate electrode layer in the opening (see Fig. 5).

[0046] First, it is assumed that a protective film 14 is provided on a semiconductor substrate 11 as shown in Fig. 5. As shown in Fig. 4, in step S130, a process of removing a part of the protective film 14 provided on the semiconductor substrate 11, which corresponds to a region where a gate electrode layer is to be formed, is carried out.

[0047] Next, in step S131, a process of forming a gate insulating film on the semiconductor substrate 11 is performed. Specifically, a gate insulating film 132 is formed by using a known method such as a sputtering method or a plasma CVD method in a portion on the semiconductor substrate 11 from which the protective film 14 has been removed. The thickness of the gate insulating film 132 is, for example, about 50 to 100 nm.

[0048] For example, the gate insulating film 132 is formed by using a plasma CVD method or the like under conditions of about 200° C. to 500° C. using tetraethoxysilane (TEOS) or monosilane as raw materials. In this case, since it is exposed to strong oxidizing conditions, there is a risk that the wiring material formed beforehand, such as Al, may be oxidized. Therefore, in the gas sensor FET according to this embodiment, the wiring material is preferably made of an oxidation-resistant metal material. As the oxidation-resistant metal material, for example, high-melting point metals such as tungsten, lanthanum hexaboride (LaB6), copper manganese (CuMn), and oxidation-resistant alloys are effective.

[0049] Next, in step S132, a process of forming an oxide layer film is performed. This oxide layer film 133 is an oxide layer film corresponding to an adhesive layer film in a gate electrode with a conventional Pt / Ti corridor structure. In this embodiment, this oxide layer film 133 is not necessarily required, but can be used to adjust the detection gas and detection sensitivity by combining it with the catalyst metal layer film 134. As this oxide layer film 133, for example, a TiO2 film, a YSZ film, a GDC layer film, etc. are formed. For example, a sputtering method is used for this film formation. For example, a magnetron sputtering device is used to perform sputtering formation using targets such as TiO2, YSZ (8% Y2O3, 92% ZrO2), and GDC layer (10% GD2O3, 90% CeO2) with a diameter of 2 inches under conditions of an Ar gas pressure of 0.5 Pa and a high-frequency discharge power of 50 W.

[0050] Next, in step S133, a process of forming a catalytic metal layer film is performed. A catalytic metal layer film 134 of Pt, Ni, or the like having a columnar dendritic structure according to this embodiment is formed. A high-pressure sputtering method is used for this film formation. For example, a magnetron sputtering device is used to perform sputter formation using a Pt target of 2 inches φ under conditions of an Ar gas pressure of 7.0 Pa, a direct current discharge power of 50 W or a high-frequency discharge power of 50 W. By performing sputtering under high pressure conditions, it is possible to form a film with a specific surface area of ​​40 m 2 / g or more, a catalytic metal layer 134 having a dendritic structure is formed. As the catalytic metal layer 134, for example, a Pt film having a thickness of 25 nm is formed.

[0051] Here, an example of a 25 nm thick Pt catalytic metal layer film has been described. However, compared to a conventional Pt / Ti corridor structure Pt catalytic metal layer film (film thickness: e.g., 15 nm), even with approximately half the amount of Pt used, favorable gate threshold voltage fluctuation has been confirmed in hydrogen detection in a 3% hydrogen atmosphere.

[0052] Furthermore, when the lower oxide layer was made into a solid electrolyte film, yttria-stabilized zirconia (YSZ) film (film thickness: 10 nm, for example), the gate threshold voltage fluctuation settled quickly, and good detection characteristics were shown. The YSZ film was formed using a 4-inch φ Y2O3 (8 mol%)-ZrO2 (92 mol%) target with a high-frequency discharge power of 200 W using a magnetron sputtering device. This is thought to be because hydrogen gas is adsorbed onto the Pt catalytic metal layer, becomes protons, and is quickly transported to the gate insulating film side.

[0053] Next, in step S134, a wiring step is performed, that is, a step of connecting gate electrode layer 13 and a contact with a wiring material is performed.

[0054] By carrying out the above steps, the gas sensor FET 1 according to this embodiment is completed. The gate electrode layer 13, which is the gas detection portion, can be configured so as to be exposed to the atmosphere without being blocked by the protective film 14.

[0055] In addition, the manufacturing method of the gas sensor FET may include a surface treatment step of activating the catalytic effect by subjecting the formed catalytic metal layer film to an oxidation or reduction surface treatment after the step of forming the catalytic metal layer film.

[0056] In addition, a reference FET in which the gate electrode layer 13 does not have an externally exposed portion can be manufactured by depositing the above-mentioned catalytic metal layer film 134, and then depositing an insulating layer such as Si2N3 / SiO2 or AlN / Al2O3, each having a thickness of, for example, 100 nm or more, by sputtering or the like.

[0057] <Relationship between sputtering pressure during catalytic metal layer formation and non-surface area of ​​catalytic metal layer> Here, the relationship between the sputtering pressure during formation of the catalytic metal layer and the non-surface area of ​​the catalytic metal layer will be described.

[0058] Fig. 6 is a diagram showing the relationship between the sputtering pressure during deposition of the catalytic metal layer and the non-surface area of ​​the catalytic metal layer, and Fig. 7 is a diagram showing enlarged structural images of a plurality of Pt catalytic metal layer films formed under different sputtering pressures.

[0059] As shown in FIG. 6, when the sputtering pressure during the formation of the catalyst metal layer is less than 2 Pa, the catalyst metal layer has a nearly flat surface as shown in image P1. In this case, the specific surface area is relatively low. When the sputtering pressure is 3 Pa, the catalyst metal layer has a columnar structure in which a plurality of columnar parts are arranged two-dimensionally as shown in image P2, but the plurality of columnar parts with smooth surfaces are arranged almost without gaps, and the specific surface area remains relatively low. On the other hand, when the sputtering pressure is 5 Pa or more, the catalyst metal layer has a columnar structure, but the surface of the columnar parts has a dendritic structure as shown in image P3, and thus has a columnar dendritic structure. Therefore, the specific surface area becomes relatively large, and it is considered that the higher the sputtering pressure, the more the dendritic structure grows and the larger the specific surface area becomes.

[0060] <Measurement example of gate threshold voltage variation with specific surface area of ​​catalytic metal layer> Here, a measurement example of the variation in gate threshold voltage with respect to the specific surface area of ​​the catalyst metal layer film will be described.

[0061] 8 is a diagram showing a measurement example of the relationship between the specific surface area of ​​the catalytic metal layer film in the gas sensor FET according to Example 1 and the amount of change in the gate threshold voltage with respect to the concentration of the detected gas. In the example of FIG. 8, the specific surface area of ​​the catalytic metal layer film in the gas sensor FET according to Example 1 and the amount of change in the gate threshold voltage V th The change in DV th (hereinafter, the gate threshold voltage fluctuation amount DV th This shows a measurement example of the relationship between the capacitance and the gate electrode of the gas sensor FET. In this measurement example, a detection film structure obtained by forming a Pt catalyst metal layer film with a columnar dendritic structure on a gate insulating film is adopted as the gate electrode layer of the gas sensor FET. The thickness of the catalyst metal layer film is 25 nm.

[0062] As shown in FIG. 8, the specific surface area of ​​the catalytic metal layer is 20 m 2 / g or more, the gate threshold voltage fluctuation amount DV th becomes 1.0 V or more. And, when the specific surface area of ​​the catalyst metal layer film is 120 m 2 / g, the gate threshold voltage fluctuation DV th In the case of the conventional Pt / Ti gallery structure, the specific surface area of ​​the catalyst metal layer is 0.5 to 10 m2, as shown by D1 in Figure 8. 2 / g, and the gate threshold voltage fluctuation DV th The specific surface area of ​​the catalyst metal layer is 20 m2 or less in the embodiment of the present invention, in which the gate electrode layer has a Pt catalyst metal layer film with a columnar dendritic structure (D1), compared with the case where the gate electrode layer has a conventional Pt / Ti corridor structure (D2), 2 / g or more, 150m 2 / g or less, the gate threshold voltage fluctuation DV th It is clear that a voltage of 1.0 V or more and 3.0 V or less can be expected, and that a sufficiently practical gas detection sensitivity is obtained.

[0063] <Measurement example of the change in gate threshold voltage with respect to the thickness of the catalytic metal layer> Next, a measurement example of the variation in gate threshold voltage with respect to the thickness of the catalyst metal layer will be described.

[0064] 9 is a diagram showing a measurement example of the amount of change in gate threshold voltage with respect to the thickness of the catalyst metal layer film in the gas sensor FET according to Example 1. In this measurement example, a detection film structure obtained by forming the catalyst metal layer film having a columnar dendritic structure according to this Example on a gate insulating film is adopted as the gate electrode layer of the gas sensor FET. The atmosphere is nitrogen gas with a hydrogen gas concentration of 3% (3%H2 / N2). The catalyst metal layer film is Pt, and is formed by sputtering with a sputtering pressure of 7 Pa.

[0065] As can be seen from FIG. 9, as the thickness of the catalyst metal layer film having the columnar dendritic structure increases, the area of ​​the adsorption site in the gate electrode layer of the gas sensor FET increases, and the gate threshold voltage fluctuation amount DV thIn principle, the larger the thickness of the catalyst metal layer film, the larger the area of ​​gas adsorption sites, and the higher the sensitivity can be expected. However, catalyst metal layer films having a columnar dendritic structure are prone to deterioration of adhesion and film peeling due to the increase in the aspect ratio of the columnar dendritic structure. Therefore, according to the experience of the present inventors, it is considered that the thickness of a catalyst metal layer film having a columnar dendritic structure is preferably up to about 500 nm.

[0066] In addition, in the manufacture of gas sensor FETs, in the case of a conventional Pt / Ti corridor structure, there is structural variation due to the degree of oxidation of the Ti layer, whereas in the case of the columnar dendritic structure according to the present embodiment, the structural variation depends simply on the film formation conditions. Therefore, according to this embodiment, gas sensor FETs can be manufactured stably and uniformly. Furthermore, according to this embodiment, it is possible to form a catalyst metal layer having a columnar dendritic structure directly on a gate insulating film without introducing an adhesive layer under the catalyst metal layer, and to use the manufactured FET as a gas sensor.

[0067] <Example 2> Next, a second embodiment will be described.

[0068] FIG. 10 shows the gate threshold voltage V th 10 shows the variation of the capacitance. As shown in the structures K1 and K2 of FIG. 10, the gas sensor FET according to the second embodiment uses a Pt (25 nm thick) film with a columnar dendritic structure as the catalyst metal layer film in the gate electrode layer, and uses silicon oxide (SiO2) as the gate insulating film. However, in the structure K1, the oxide layer corresponding to the adhesive layer film is titanium oxide (TiO2), and in the structure K2, the oxide layer is yttria stabilized zirconia (YSZ). The detection gas is hydrogen (H2) in nitrogen (N2).

[0069] For comparison, the upper part of Fig. 10 shows the gate electrode structure K0 of a conventional gas sensor FET and the gate threshold voltage V thThe structure K0 is a Pt / Ti corridor structure, the adhesion layer is titanium oxide, and the gate insulating film is silicon oxide.

[0070] Graphs G0 to G2 in FIG. 10 show the gate threshold voltage fluctuation DV when the atmosphere is initially set to N2 100% and then changed to H2 25% concentration. th As can be seen from the graphs G1 and G2, even though the amount of Pt used in the Pt catalyst metal layer film is smaller than that of the conventional gallery structure, a larger threshold voltage fluctuation than the conventional one is confirmed.

[0071] <Example 3> Next, a third embodiment will be described.

[0072] FIG. 11 shows the gate threshold voltage V th 11 shows the variation of Ni. As shown in structure K3 of FIG. 11, the gas sensor FET of Example 3 uses Ni with a columnar dendritic structure as the catalytic metal layer of the gate electrode, and silicon oxide (SiO2) as the gate insulating film. No oxide layer film corresponding to the adhesive layer film is provided. The detection gas is hydrogen (H2) in nitrogen (N2). Ni is oxidized in the air, making it difficult to perform catalytic action, but it can be used for detection in a reducing atmosphere such as hydrogen.

[0073] For the columnar dendritic structure of Ni, similarly to Example 1, sputtering film formation was performed using a magnetron sputtering device with a Ni target of 2 inches in diameter under conditions of Ar gas pressure of 7.0 Pa and high frequency power of 50 W. By performing sputtering under high pressure conditions, 2 A Ni catalyst metal layer film having a columnar dendritic structure and a specific surface area of ​​1 / g or more is formed.

[0074] Here, a 7-nm-thick Ni columnar dendritic structure is used as the catalytic metal layer film for the gate electrode of the gas sensor FET. Unlike the conventional Pt / Ti corridor structure, a Ti layer or an adhesive layer equivalent to Ti oxide is not required, and the Ni layer with the columnar dendritic structure can be directly formed and used as the gate electrode. Note that since the Ni layer with the columnar dendritic structure is prone to natural oxidation, it is subjected to reduction treatment at 200°C in a hydrogen atmosphere immediately before the measurement.

[0075] Graph G3 in FIG. 11 shows the gate threshold voltage V th As can be seen from graph G3, the threshold voltage fluctuation is confirmed to be similar to that of the Pt-based catalyst metal layer. It is difficult to form a catalyst metal layer such as Ni into a columnar dendritic structure by the conventional manufacturing method, and the fact that this is possible is largely due to the method of forming the gate electrode layer by sputtering according to the present embodiment.

[0076] <Example 4> A fourth embodiment will now be described.

[0077] FIG. 12 shows the structure of the gate electrode layer of the gas sensor FET according to the fourth embodiment and the gate threshold voltage V th 10 is a graph showing the change due to the change in the concentration of the detected gas in the atmosphere. The gas sensor FET according to Example 4 uses a gate electrode in which a Pt catalytic metal layer film having a columnar dendritic structure is laminated on a titanium oxide film, and the detected gas is hydrogen sulfide (H2S). The thickness of the Pt catalytic metal layer film having the columnar dendritic structure is 25 nm.

[0078] Graph G4 in FIG. 12 shows the gate threshold voltage V thThis graph G4 shows that the gate threshold voltage fluctuates due to hydrogen sulfide, an impurity gas, even in high-concentration hydrogen. As for the amount of catalytic metal used for gas detection, it is possible to obtain roughly the same gate threshold voltage fluctuation with less than 50% of the amount of Pt used compared to a conventional gas sensor FET with a catalytic metal / oxide gallery structure.

[0079] <Example 5> A fifth embodiment will now be described.

[0080] FIG. 13 shows the structure of the gate electrode layer of the gas sensor FET according to the fifth embodiment and the gate threshold voltage V th The gate electrode structure of the gas sensor FET according to the fifth embodiment is the same as that of the fourth embodiment, but the gate threshold voltage V th Specifically, a gas sensor FET similar to that in Example 4 was placed in an atmosphere of 100% hydrogen concentration, 100 ppm of carbon monoxide was introduced into the atmosphere, and the atmosphere was returned to 100% hydrogen concentration, and the fluctuation of the gate threshold voltage was measured.

[0081] As can be seen from graph G5 in FIG. 13, the gate threshold voltage V th The gas detection sensitivity can be improved by increasing the thickness (film thickness) of the catalyst metal layer having a porous structure.

[0082] By utilizing the above-mentioned effect of the gas sensor FET according to the fifth embodiment, it becomes possible to detect impurity gases in high-concentration hydrogen gas, which are the cause of deterioration of the catalyst in fuel cells. For example, calculations can be made from the integrated value of the exposed impurity gases and reflected in the refresh operation of the PEFC, thereby contributing to the extension of the life of the fuel cell. Furthermore, for hydrogen engines used in automobiles and distributed power sources, the detection data of impurity gases in high-concentration hydrogen can be utilized in real-time engine control, enabling optimization of combustion efficiency and stable operation.

[0083] <Features of the manufacturing method of the gas sensor FET according to the present embodiment> Next, the features of the method for manufacturing the gas sensor FET according to this embodiment will be described. The method for manufacturing the gas sensor FET according to this embodiment is based on a method for manufacturing a general MOSFET, but differs significantly in the following three points.

[0084] The first difference is that the catalytic metal layer of the gate electrode needs to have a porous structure to increase the specific surface area of ​​the catalytic metal layer, and therefore the catalytic metal layer is formed by sputtering.

[0085] The second difference is that since the portion of the gate electrode layer that functions as the gas detection film, particularly the catalytic metal layer film, needs to be exposed to the outside, the gate electrode layer is formed by removing part of the protective film after the protective film is formed.

[0086] The third difference is that the gate insulating film, which is part of the gate electrode layer, is formed after the protective film is formed, so the gate insulating film must be formed after the wiring material is connected. Therefore, since the wiring material is exposed to a strong oxidizing environment when forming the gate insulating film, the wiring material is made of an oxidation-resistant metal material.

[0087] <Relationship between gate electrode structure and gate threshold voltage fluctuation> Here, the results of an investigation into the relationship between the structure of the gate electrode layer and the amount of gate threshold voltage fluctuation will be described.

[0088] FIG. 14 shows the threshold voltage fluctuation DV th FIG. 14 is a graph showing the relationship between the gate threshold voltage and the channel current of a FET. In a FET having a current-voltage characteristic C1 as shown in FIG. 14, the gate threshold voltage in a steady state, i.e., before gas detection, is set to C2 (here, 10 -5 When the current-voltage characteristics shift due to gas adsorption to the state of C3, the voltage difference before and after gas detection is C4.

[0089] FIG. 15 shows the gate electrode layer configuration and the gate threshold voltage fluctuation amount DV th 15 is a graph showing the relationship between the gate threshold voltage fluctuation amount and the gate electrode layer temperature.

[0090] As shown in FIG. 15, when the gate electrode layer has a Pt-Ti-O corridor structure, the gate threshold voltage fluctuation DV th is about 0.8 V. In addition, when the gate electrode layer has a structure in which a Pt electrode layer (bulk) with a thickness of 15 nm is laminated on a YSZ film, the gate threshold voltage fluctuation amount DV th is about 0.4V.

[0091] In contrast, when the gate electrode layer has a structure in which a catalytic metal layer having a Pt column dendritic structure with a thickness of 15 nm is laminated on a YSZ film, the gate threshold voltage fluctuation amount DV th is about 1.05 V. When the gate electrode layer has a structure in which a 15 nm-thick Pt columnar dendritic structure catalyst metal layer is laminated on a TiOx film, the gate threshold voltage fluctuation DVth is about 0.95 V. When the gate electrode layer has a structure in which only a columnar dendritic structure catalyst metal layer is laminated without an oxide layer, the gate threshold voltage fluctuation DV th is about 0.8V.

[0092] From these results, when the gate electrode layer has a structure according to the present embodiment, the amount of Pt used is about half that of the Pt-Ti-O corridor structure, but the threshold voltage fluctuation amount DV is equal to or greater than that of the Pt-Ti-O corridor structure. th This shows that the specific surface area increases dramatically with the amount of catalyst metal used because the catalyst metal layer has a columnar dendritic structure, making it possible to perform highly sensitive gas detection with a small amount of catalyst metal used.

[0093] As described above, according to the present embodiment, since the catalyst metal layer film having the columnar dendritic structure is used for the gate electrode layer of the gas sensor FET, the specific surface area of ​​the catalyst metal layer film is increased, and a gas sensor FET having improved gas detection sensitivity can be realized. In particular, a highly sensitive gas sensor FET suitable for detecting hydrogen gas or impurity gases in hydrogen gas or nitrogen gas, and a method for manufacturing the same can be provided.

[0094] In addition, in conventional corridor-structured FET sensors made of catalytic metal / oxide, the combination of materials that can be used to form a corridor structure is Pt-TiO x , Pt-MoO x In addition to being limited to the above, the specific surface area is also limited to a maximum of 10 m 2 / g, making it difficult to achieve more efficient gas detection. In addition, the materials that can be used as catalytic metal layer films to detect various gases are limited, making it difficult to expand the material options for catalytic metal layer films.

[0095] On the other hand, according to the manufacturing method of the gas sensor FET of the embodiment of the present invention, the catalyst metal layer film uses a porous catalyst metal that can be realized only by film formation regardless of the surface state. When a film formation method such as sputtering is used, a film is formed under appropriate conditions under high pressure conditions, and in addition to the usual columnar structure in the z-axis direction, many fine structures are formed in the lateral and oblique directions. By using this catalyst metal layer film as a detection film, efficient gas detection is possible. In addition, the lower oxide layer that was necessary in the conventional gallery structure sensor made of catalyst metal / oxide and the resulting fluctuation in film quality are no longer necessary, and the amount of catalyst metal used can be dramatically reduced.

[0096] Furthermore, according to this embodiment, a catalytic metal layer film having a columnar dendritic structure in which a plurality of dendritic structures are arranged two-dimensionally is formed by high-pressure sputtering, so that the specific surface area relative to the amount of catalytic metal used is improved, and it is also possible to improve gas detection sensitivity while reducing material costs.

[0097] Furthermore, according to this embodiment, since it is not necessarily necessary to insert an adhesive layer film between the gate insulating film and the catalyst metal layer film on the semiconductor substrate, it is possible to simplify the structure and reduce the number of steps during manufacturing.

[0098] In the gas sensor FET according to the present embodiment, the specific surface area of ​​the catalyst metal layer film having a columnar dendritic structure is 20 to 150 m. 2 / g is preferable because, based on the measurement results showing the relationship between the specific surface area of ​​the catalyst metal layer film and the amount of change in the gate threshold voltage, this corresponds to a range in which the amount of change in the gate threshold voltage, which is directly related to the gas detection sensitivity, is considered to be sufficiently higher than when the gate electrode has a conventional gallery structure.

[0099] In addition, the sputtering pressure when forming the catalytic metal layer is preferably 5 Pa or more. This is because, based on the measurement results showing the relationship between the sputtering pressure and the specific surface area of ​​the catalytic metal layer, the condition of a sputtering pressure of 5 Pa or more is the most suitable specific surface area of ​​the catalytic metal layer of 20 m 2 Although it is not possible to uniquely determine a suitable upper limit of the sputtering pressure, a realistic guideline would be 50 Pa or less, based on the specifications of the sputtering apparatus, the knowledge and experience of the present inventors, and the like.

[0100] Furthermore, based on the results of research conducted by the present inventors to date, it is assumed that the catalyst metal layer film will be made of, for example, platinum, nickel, or a mixture of platinum and nickel, and these would be practical examples.

[0101] Furthermore, based on the results of the inventors' research to date, it is also envisaged that the catalytic metal layer film may be made of, for example, at least one of palladium, rhodium, gold, silver, ruthenium, iridium and copper, or a mixed material in which platinum or nickel is added to at least one of the above metals, and these may also be practical examples.

[0102] Furthermore, based on the results of the inventors' research to date, it is assumed that the metal oxide film may be, for example, yttrium-zirconium oxide, gadolinium-cerium oxide, or titanium oxide, and these may be practical specific examples.

[0103] Furthermore, based on the results of research conducted by the present inventors to date, the thickness of the metal oxide film is expected to be, for example, 5 nm or more and 30 nm or less, and this range can be a practical example.

[0104] The electrode structure having a dendritic structure, which is a feature of the present invention, is not limited to FET-type sensors, but can also be applied to, for example, capacitor-type sensors.

[0105] (Embodiment 2) A gas sensor including the gas sensor FET according to the embodiment of the present invention, a reference FET for referencing the gate threshold voltage, and a heater, and arranged on the same substrate, is also an embodiment of the present invention. For example, the gas sensor FET, the heater, and the reference FET are arranged on the same substrate. The entire substrate is heated to a predetermined temperature range by the heater, so that the fluctuation characteristics of the gate threshold voltages of the gas sensor FET and the reference FET approach the expected characteristics and become stable. Then, the gate threshold voltage of the reference FET and the gate threshold voltage of the gas sensor FET are monitored, and gas detection is performed based on the comparison result. With such a gas sensor, it is possible to perform highly reliable gas detection that is not easily affected by changes in the environmental temperature.

[0106] The above describes the embodiments of the present invention, but these embodiments and examples are merely one form of carrying out the present invention, and the present invention is not limited to the above-described forms and can be modified in various ways without departing from the spirit of the present invention. [Explanation of symbols]

[0107] 1... gas sensor FET, 11... semiconductor substrate, 12... n-type semiconductor, 13... gate electrode layer, 14... protective film, 132... gate insulating film, 133... oxide layer film, 134... catalytic metal layer film, 1341... dendritic structure, 1341a... columnar portion, 1341b... fine structure portion

Claims

1. A semiconductor substrate; a gate electrode layer formed on the semiconductor substrate and functioning as a gas detection unit; the gate electrode layer has a catalytic metal layer film including a side exposed to an atmosphere, the catalytic metal layer film has a surface exposed to the atmosphere, and a plurality of dendritic structures are two-dimensionally arranged, each of the dendritic structures having a surface of a columnar portion and a plurality of fine structures smaller and finer than the columnar portion; The plurality of dendritic structures are electrically connected to each other. Gas sensor FET.

2. 2. The gas sensor FET according to claim 1, The microstructure has a leaf shape, a plate shape, or a grain shape. Gas sensor FET.

3. 2. The gas sensor FET according to claim 1, The specific surface area of ​​the catalytic metal layer is 20 m ２ / g or more, 150m ２ / g or less, Gas sensor FET.

4. 2. The gas sensor FET according to claim 1, The thickness of the catalytic metal layer is 5 nm or more and 500 nm or less. Gas sensor FET.

5. 2. The gas sensor FET according to claim 1, The catalytic metal layer film is made of platinum, nickel, or a mixture of platinum and nickel. Gas sensor FET.

6. 2. The gas sensor FET according to claim 1, The catalytic metal layer film is made of at least one metal selected from the group consisting of palladium, rhodium, gold, silver, ruthenium, iridium, and copper, or a mixed material obtained by adding platinum or nickel to the at least one metal. Gas sensor FET.

7. 2. The gas sensor FET according to claim 1, the gate electrode layer has an oxide layer film that is in contact with the catalytic metal layer film and is disposed on the opposite side to the side exposed to the atmosphere; Gas sensor FET.

8. 8. The gas sensor FET according to claim 7, The oxide layer film is yttrium-zirconium oxide, gadolinium-cerium oxide, or titanium oxide; Gas sensor FET.

9. 8. The gas sensor FET according to claim 7, The thickness of the oxide layer is 5 nm or more and 30 nm or less. Gas sensor FET.

10. A gas sensor FET according to claim 1 disposed on a substrate; a reference FET disposed on the substrate for referencing a gate threshold voltage; a heater disposed on the substrate and configured to heat the substrate to a predetermined temperature range; Equipped with Gas sensor.

11. 1. A method for manufacturing a gas sensor FET, comprising: a semiconductor substrate; and a gas detection material portion formed on the semiconductor substrate and constituting a gate electrode layer, the gas detection material portion having a catalyst metal layer including a side exposed to an atmosphere, the method comprising the steps of: A catalytic metal layer forming step of forming the catalytic metal layer film by sputtering at a sputtering pressure of 5 Pa or more. A method for manufacturing a gas sensor FET.

12. 12. The method for manufacturing the gas sensor FET according to claim 11, After the catalytic metal layer forming step, A surface treatment step of activating the catalytic effect by performing an oxidation or reduction surface treatment on the formed catalytic metal layer film. A method for manufacturing a gas sensor FET.

13. 13. The method for manufacturing the gas sensor FET according to claim 12, a gate insulating film is formed on the semiconductor substrate at a portion of the protective film formed on the semiconductor substrate from which the protective film has been removed, and the catalytic metal layer is formed on the gate insulating film formed on the semiconductor substrate; A method for manufacturing a gas sensor FET.

14. 14. The method for manufacturing the gas sensor FET according to claim 13, a wiring material connecting step of connecting a wiring material to the catalytic metal layer film, A material having an anti-oxidation property is used as the wiring material. A method for manufacturing a gas sensor FET.

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