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] Japanese Patent Application Publication No. 2018-115888 Summary of the Invention [Problem to be solved by the invention]

[0004] FET 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 has a catalyst metal layer film including a side exposed to the atmosphere; the side of the catalyst metal layer film exposed to the atmosphere has 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 microstructures that are smaller and finer than the columnar portion; and the plurality of dendritic structures are electrically connected to each other.

[0008] Furthermore, 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] Furthermore, one representative embodiment of the present invention is a method for manufacturing a gas sensor FET, comprising: a semiconductor substrate; and a gas sensing material portion formed on the semiconductor substrate and constituting a gate electrode layer, the gas sensing material portion having a catalyst metal layer film including a side exposed to an atmosphere, the method comprising a catalyst metal layer deposition step of depositing the catalyst metal layer film by sputtering at a sputtering pressure of 5 Pa or more. [Effects 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 explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating an example of the structure of a gas sensor FET according to Example 1. FIG. [Figure 2] FIG. 2 is a diagram showing an example of a gate electrode layer of the gas sensor FET according to the first embodiment. [Figure 3] FIG. 3 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. FIG. [Figure 5] 10A and 10B are diagrams for explaining that a gate electrode layer is formed after a protective film is formed. [Figure 6] FIG. 10 is a diagram showing the relationship between the sputtering pressure during formation of a catalytic metal layer and the specific surface area of ​​the catalytic metal layer. [Figure 7] FIG. 10 is a diagram showing enlarged structural images of a plurality of Pt catalytic metal layer films formed under different sputtering pressures. [Figure 8] 10 is a graph showing an example of measurement of the relationship between the specific surface area of ​​the catalyst metal layer film and the amount of change in gate threshold voltage with respect to the concentration of detected gas in the gas sensor FET according to Example 1. FIG. [Figure 9] 4 is a graph showing an example of measurement 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. FIG. [Figure 10] 10 is a diagram showing the structure of a gate electrode layer of the gas sensor FET according to Example 2 and a graph showing fluctuations in gate threshold voltage due to changes in the concentration of a detected gas in the atmosphere. FIG. [Figure 11] 10 is a diagram showing the structure of the 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] 10 is a diagram showing the structure of the gate electrode layer of the gas sensor FET according to Example 4 and the fluctuation of the gate threshold voltage due to changes in the concentration of the detected gas in the atmosphere. FIG. [Figure 13] 10 is a diagram showing the structure of the gate electrode layer of the gas sensor FET according to Example 5 and the fluctuation of the gate threshold voltage due to changes in the concentration of the detected gas in the atmosphere. FIG. [Figure 14] FIG. 10 is a diagram for explaining the amount of gate threshold voltage fluctuation. [Figure 15] FIG. 10 is a diagram showing the relationship between the configuration of a gate electrode layer and the amount of gate threshold voltage fluctuation. DETAILED DESCRIPTION OF THE INVENTION

[0012] (Background of the study by the 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 high 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 for 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 degradation, and it is said that the cell's lifespan will be shortened unless it is refreshed.

[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, semiconductor FET sensors formed from a laminated film of platinum (Pt) and titanium (Ti) (for example, as described in Patent Document 2: JP 2009-300267 A and Non-Patent Document 1: Applied Physics Express 3 (2010) 047201) have been shown to be capable of detecting impurities in high-concentration hydrogen to a certain extent. The sensing film of this sensor undergoes a structural change in the upper Pt film due to oxidation of the Ti layer introduced as an adhesion layer, resulting in the formation of numerous grain boundaries. This structure, with numerous grain boundaries, is called a corridor structure. It is believed that when a corridor-structured Pt film 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 varies from 0.1 to 10 m depending on the degree of oxidation of the base film. 2The detection mechanism of this sensor is that hydrogen gas adsorbed on the Pt catalyst film is protonated, and the protons accumulate near the gate insulating film, generating an electric field, which gate This utilizes a change in threshold voltage. Non-Patent Document 2 also describes a similar sensor that uses island-shaped porous Pt formed from a solution as the detection membrane. The crystal grain size is also described as being around 0.5 μm, which is thought to have a specific surface area roughly equivalent to that of the above-mentioned gallery structure sensor.

[0017] Other gas sensors for purposes such as hydrogen detection include the following: Patent Document 3 (JP 2012-013579 A) describes a chemical sensor technology using a sensitive membrane. Not limited to FET-type sensors, sensors that utilize properties such as changes in resistance or mass depending on the substance the sensitive membrane reacts to can be produced with different types of sensitive membranes, making it possible to detect a variety of chemical substances. The chemical sensor in Figure 1 of Patent Document 3 is an example of an 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), hydrogen sensors, which are a type of gas sensor, have several detection principles. Of these, FET-type, capacitor-type, and diode-type sensors are classified as work function sensors. Because work function sensors can be manufactured using a process that uses a semiconductor substrate, they are expected to be cheaper, more compact, and consume less power than sensors classified as other types.

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

[0020] Patent Document 4 (Japanese Patent Laid-Open Publication No. 2005-283578) 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 gallery structure FET sensor of Patent Document 3 generates a catalytic metal when the base metal is oxidized. layer The Pt layer undergoes a structural change, increasing the grain boundaries, which increases the number of gas adsorption sites to be sensed, enabling efficient detection. layer Therefore, the number of effective options for the catalytic metal layer is very limited.

[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. As a result, it has been impossible to select a catalyst metal layer that matches the detection gas or a highly sensitive catalyst metal layer.

[0023] Under these circumstances, the present inventors have conducted extensive research and have devised a gas sensor FET with higher gas detection sensitivity. One of the advantages of the embodiments of the present invention is the enhancement of gas detection sensitivity in the gas sensor FET. Other advantages include a wider range of options for the sensing metal layer, a wider dynamic range, stable manufacturing, and lower costs. The following describes the embodiments of the present invention.

[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, which form a drain and a source, formed at an interval 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 FET gate The threshold voltage changes. gate By measuring the change in the threshold voltage, the gas can be detected.

[0025] 2 is a diagram showing an example of a gate electrode layer of the gas sensor FET according to Example 1. 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: approximately 40 m) having a structure characteristic of this example is further formed thereon. 2 The oxide layer film 133 corresponds to an adhesive layer film in a gate electrode layer having a conventional corridor structure.

[0026] The gate electrode layer 13 according to Example 1 has a characteristic structure, as shown in the upper and middle sections of Figure 2, in which a plurality of tree-like structures 1341 are two-dimensionally arranged, each of which is formed by forming a plurality of fine structures 1341b smaller and finer than the columnar portion 1341a on the surface of the columnar portion 1341a. Here, the column axis direction of the columnar portion 1341a is defined as the z-axis direction, one direction in a plane perpendicular to the z-axis direction is defined as the x-axis direction, and the direction perpendicular to the x-axis direction in the same perpendicular plane is defined as the y-axis direction. The lower section of Figure 2 shows an x-z ​​cross section (half cross section of the tree-like structure) of the tree-like structure 1341 and an x-y cross section (one-quarter cross section of the tree-like structure) of the tree-like structure 1341. As can be seen from these figures, in the tree-like structure 1341, a large number of thin plate-shaped or granular microstructure portions 1341b, which resemble the thin leaves or petals of a tree and extend in the z-axis direction so as 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 the dendritic structure 1341 increases the specific surface area of ​​the catalytic metal in the gate electrode layer 13, which corresponds to the gas sensor, compared to a conventional gallery structure, thereby improving 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 now be described.

[0028] <Manufacturing method of 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 in terms of the parts other than the gate electrode. Therefore, here, we will first provide an overview of the manufacturing process for the gas sensor FET according to this embodiment, and then we will mainly explain the method for forming the gate electrode layer of the gas sensor FET according to this embodiment.

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

[0030] 3, first, in step S1, a step of preparing a semiconductor substrate is carried out, 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 carried out.

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

[0032] Next, in step S3, a photoresist application step is performed, that is, a step of applying 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, an etching photoresist removal step is performed, that is, a step 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 carried out, that is, a step of diffusing impurities by supplying an impurity gas to the surface of the semiconductor substrate.

[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 a gate electrode layer. This is because the gate electrode layer needs to be exposed to the atmosphere so that it can function as a gas sensor. As described below, after forming the protective film, it is necessary to remove part of the protective film and form the gate electrode layer in the area where the protective film is not present as a final process.

[0038] If it is determined that all 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 necessary patterns have not been formed (S8: No), the processing steps return to step S2, and the steps from forming the oxide film to the doping are performed again.

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

[0040] Next, in step S10, a wiring process is performed. That is, a process of wiring between contacts is performed by connecting wiring material or the like. The wiring material used here is an oxidation-resistant material. As will be described later, after wiring is performed, a protective film must be formed, a portion of the protective film must be removed, and a gate insulating film must be formed there. When forming this gate insulating film, the semiconductor substrate is subjected to a strong oxidation process. This is to prevent the previously placed wiring material from being altered by oxidation. Examples of oxidation-resistant materials include tungsten, which is resistant to oxidation, and high-melting-point metal materials such as LaB6 and CuMn, or oxidation-resistant metal materials.

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

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

[0043] Next, in step S13, a gate electrode layer is formed. For example, a gate insulating film (having a thickness of, for example, 15 nm) is formed on the 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 for 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 chart showing an example of a method for forming a gate electrode layer according to Example 1. 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, which functions as the detection film of the sensor, particularly the catalyst metal layer film, must be exposed to the outside. Therefore, when 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 then 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 is carried out in which 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 removed.

[0047] Next, in step S131, a gate insulating film is formed on the semiconductor substrate 11. Specifically, a gate insulating film 132 is formed by using a known method such as sputtering or plasma CVD in the area where the protective film 14 has been removed on the semiconductor substrate 11. 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 plasma CVD or the like using tetraethoxysilane (TEOS) or monosilane as raw materials at a temperature of about 200°C to 500°C. Since the gate insulating film 132 is exposed to strong oxidizing conditions, there is a risk that the pre-formed wiring material, 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. Examples of oxidation-resistant metal materials include high-melting-point metals such as tungsten, lanthanum hexaboride (LaB6), and copper manganese (CuMn), as well as oxidation-resistant alloys.

[0049] Next, in step S132, an oxide layer film is formed. This oxide layer film 133 corresponds to the adhesive layer film in a conventional Pt / Ti corridor gate electrode. In this embodiment, this oxide layer film 133 is not necessarily required, but it can be used in combination with the catalyst metal layer film 134 to adjust the detectable gas and detection sensitivity. Examples of the oxide layer film 133 include a TiO2 film, a YSZ film, and a GDC layer film. This film formation is performed by, for example, a sputtering method. For example, a magnetron sputtering device is used to perform sputtering using a 2-inch diameter target such as TiO2, YSZ (8% Y2O3, 92% ZrO2), or GDC layer (10% GD2O3, 90% CeO2) 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 catalytic metal layer film is formed. A catalytic metal layer film 134 made 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 sputtering film formation using a 2-inch diameter Pt target under conditions of an Ar gas pressure of 7.0 Pa, a DC discharge power of 50 W, or a high-frequency discharge power of 50 W. By performing sputtering under high-pressure conditions, a specific surface area of ​​40 m 2 / g or more, a catalytic metal layer film 134 having a dendritic structure is formed. As this catalytic metal layer film 134, for example, a Pt film having a thickness of 25 nm is formed.

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

[0052] Furthermore, when the underlying oxide layer was replaced with a solid electrolyte film, yttria-stabilized zirconia (YSZ) (thickness: 10 nm, for example), the gate threshold voltage fluctuation settled quickly, demonstrating good detection characteristics. The YSZ film was deposited using a 4-inch diameter Y2O3 (8 mol%)-ZrO2 (92 mol%) target with a magnetron sputtering system at a high-frequency discharge power of 200 W. This is thought to be because hydrogen gas adsorbs onto the Pt catalytic metal layer, converts to protons, and is quickly transported to the gate insulating film.

[0053] Next, in step S134, a wiring step is performed, that is, a step of connecting the gate electrode layer 13 and the 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 performing an oxidation or reduction surface treatment on the formed catalyst metal layer film after the step of forming the catalyst metal layer film.

[0056] Furthermore, 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 catalyst metal layer film 134, and then depositing an insulating layer such as SiN / SiO or AlN / AlO by sputtering or the like, each with a thickness of, for example, 100 nm or more.

[0057] <Sputtering pressure during catalytic metal layer deposition and catalytic metal layer deposition ratio Relationship with surface area> Here, the sputtering pressure during the formation of the catalytic metal layer and the thickness of the catalytic metal layer are ratio The relationship with surface area will now be explained.

[0058] Figure 6 shows the relationship between the sputtering pressure during catalytic metal layer formation and the catalytic metal layer thickness. ratio 7 is a diagram showing the relationship between the surface area and the Pt catalytic metal layer film, and Fig. 7 is a diagram showing enlarged images of the structures 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 catalytic metal layer formation is less than 2 Pa, the catalytic metal layer film 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 catalytic metal layer film has a columnar structure in which multiple columnar portions are arranged two-dimensionally, as shown in image P2. However, the multiple columnar portions have smooth surfaces and are arranged with almost no gaps between them, so the specific surface area remains relatively low. On the other hand, when the sputtering pressure is 5 Pa or higher, the catalytic metal layer film has a columnar structure, but the surfaces of the columnar portions have a dendritic structure, as shown in image P3. Therefore, the specific surface area becomes relatively large. It is thought that the dendritic structure grows and the specific surface area also increases as the sputtering pressure increases.

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

[0061] FIG. 8 is a graph showing the catalytic metal layer film of the gas sensor FET according to Example 1. ratio table 8 is a graph showing an example of measurement of the relationship between the area and the amount of change in gate threshold voltage with respect to the concentration of 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 gate threshold voltage V th DV of th and (hereinafter, the gate threshold voltage fluctuation amount DV th This shows an example of measurement of the relationship between the electrode thickness and the gate electrode thickness (also called the electrode thickness). In this example, a detection film structure obtained by forming a Pt catalyst metal layer film with a columnar dendritic structure on a gate insulating film was 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 Figure 8, the specific surface area of ​​the catalytic metal layer membrane 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 corridor structure, the specific surface area of ​​the catalytic 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 voltage Vcc is 0.4 to 0.9 V. Compared with the case where the gate electrode layer has a conventional Pt / Ti corridor structure (D1), in the embodiment of this example where the gate electrode layer has a Pt catalyst metal layer film with a columnar dendritic structure, the specific surface area of ​​the catalyst metal layer film is 20 m 2 / g or more, 150m 2 / g or less, the gate threshold voltage fluctuation DV th It can be seen 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 catalyst metal layer> Next, an example of measurement of the variation in gate threshold voltage with respect to the thickness of the catalyst metal layer film will be described.

[0064] 9 is a graph 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 used as the gate electrode layer of the gas sensor FET. The atmosphere is nitrogen gas with a 3% hydrogen gas concentration (3% H2 / N2). The catalyst metal layer film is made of Pt and is formed by sputtering at 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 th In 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 in adhesion and film peeling due to the increased aspect ratio of the columnar dendritic structure. Therefore, based on the experience of the present inventors, it is believed that the thickness of a catalyst metal layer film having a columnar dendritic structure is preferably up to about 500 nm.

[0066] Furthermore, in the manufacture of gas sensor FETs, while the conventional Pt / Ti corridor structure results in structural variations depending on the degree of oxidation of the Ti layer, the columnar dendritic structure of this embodiment results in structural variations that depend solely on the film formation conditions. Therefore, this embodiment enables stable and uniform manufacture of gas sensor FETs. Furthermore, this embodiment also enables the catalyst metal layer having a columnar dendritic structure to be formed directly on the gate insulating film without the need for an adhesive layer below the catalyst metal layer, and the manufactured FET can be used as a gas sensor.

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

[0068] FIG. 10 shows the structure of the gate electrode layer of the gas sensor FET according to Example 2 and the gate threshold voltage V th 10 shows the fluctuation of the FET gas sensor of Example 2. As shown in structures K1 and K2 of FIG. 10, the gas sensor FET of Example 2 uses a Pt (25 nm thick) film with a columnar dendritic structure as the catalyst metal layer film in the gate electrode layer, and silicon oxide (SiO2) as the gate insulating film. However, in structure K1, the oxide layer corresponding to the adhesion layer film is titanium oxide (TiO2), and in 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 th The structure K0 is a Pt / Ti corridor structure, with an adhesion layer made of titanium oxide and a gate insulating film made of 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, the amount of Pt used is larger than that of the conventional gate Threshold voltage fluctuations have been confirmed.

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

[0072] FIG. 11 shows the structure of the gate electrode layer of the gas sensor FET according to Example 3 and the gate threshold voltage V th11 shows the fluctuation of the . The gas sensor FET of Example 3 uses Ni with a columnar dendritic structure as the catalytic metal layer of the gate electrode, as shown in structure K3 of FIG. 11, 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 function as a catalyst, but it can be used for detection in a reducing atmosphere such as hydrogen.

[0073] For the Ni with a columnar dendritic structure, sputtering was performed using a magnetron sputtering device with a Ni target of 2 inches in diameter under conditions of an Ar gas pressure of 7.0 Pa and a high frequency power of 50 W, as in Example 1. By performing sputtering under high pressure conditions, a 40 m 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, there is no need for a Ti layer or an adhesive layer equivalent to Ti oxide; instead, a Ni columnar dendritic structure layer can be directly formed and used as the gate electrode. Note that because the Ni columnar dendritic structure layer is prone to natural oxidation, it is subjected to a reduction treatment at 200°C in a hydrogen atmosphere immediately before measurement.

[0075] Graph G3 in FIG. 11 shows the gate threshold voltage V when the atmosphere is initially N2 100%, then changed to H2 3% concentration, and then changed back to N2 100%. th As can be seen from graph G3, P Touch Similar to the metal catalyst layer gate It is difficult to form a catalytic metal layer such as Ni into a columnar dendritic structure using conventional manufacturing methods, and this is made possible largely by the method of forming the gate electrode layer by sputtering according to this 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 Example 4 and the gate threshold voltage V th 10 shows the fluctuations due to changes in the concentration of the detected gas in the atmosphere. The gas sensor FET of Example 4 uses a gate electrode in which a Pt catalyst metal layer film with a columnar dendritic structure is laminated on a titanium oxide film, and the detected gas is hydrogen sulfide (HS). The thickness of the Pt catalyst metal layer film with a columnar dendritic structure is 25 nm.

[0078] Graph G4 in FIG. 12 shows the gate threshold voltage V th This graph G4 shows that the gate threshold voltage fluctuates due to hydrogen sulfide, an impurity gas, even in high-concentration hydrogen. Regarding the amount of catalytic metal used for gas detection, the amount of Pt used is 50% or less compared to conventional gas sensor FETs with catalytic metal / oxide gallery structures, and roughly the same gate threshold voltage fluctuation is obtained.

[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 Example 5 and the gate threshold voltage V th The gate electrode structure of the gas sensor FET according to Example 5 is the same as that of Example 4, but the gate threshold voltage V th Specifically, a gas sensor FET similar to that in Example 4 was placed in a 100% hydrogen atmosphere, 100 ppm of carbon monoxide was introduced into the atmosphere, and then the atmosphere was returned to 100% hydrogen, 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 porous structure catalyst metal layer.

[0082] By utilizing the above-described effects of the gas sensor FET according to Example 5, it becomes possible to detect impurity gases in high-concentration hydrogen gas, which are the cause of deterioration of fuel cell catalysts. 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 extending the life of the fuel cell. Furthermore, for hydrogen engines used in automobiles and distributed power sources, optimization of combustion efficiency and stable operation can be achieved by utilizing the detection data of impurity gases in high-concentration hydrogen for engine control in real time.

[0083] <Features of the manufacturing method of the gas sensor FET according to this embodiment> Next, the features of the manufacturing method of the gas sensor FET according to this embodiment will be described. The manufacturing method of the gas sensor FET according to this embodiment is based on the manufacturing method of 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 must 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 the gate electrode layer portion that functions as the gas detection film, particularly the catalytic metal layer film, needs to be exposed to the outside, so 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] Figure 14 shows gate Threshold voltage fluctuation DV th 14 is a graph showing the relationship between the gate threshold voltage and the channel current of an FET. In an 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 and the state changes to 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. The gate threshold voltage fluctuation amount in the relationship shown in FIG.

[0090] As shown in FIG. 15, when the gate electrode layer has a Pt—Ti—O corridor structure, the gate threshold voltage fluctuation amount DV th 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 catalyst metal layer with a Pt columnar dendritic structure having a thickness of 15 nm is laminated on a YSZ film, the gate threshold voltage fluctuation amount DV th The voltage is about 1.05 V. In addition, the gate electrode layer has a structure of TiO x When a catalyst metal layer with a Pt columnar dendritic structure with a thickness of 15 nm is laminated on the film, the gate threshold voltage fluctuation DV thFurthermore, when the gate electrode layer has a structure consisting of only a catalytic metal layer with a columnar dendritic structure without an oxide layer, the gate threshold voltage fluctuation amount DV th is about 0.8V.

[0092] From these results, it can be seen that when the gate electrode layer has the structure according to this embodiment, the amount of Pt used is about half that of the Pt—Ti—O corridor structure, but the gate electrode layer has a conductivity equal to or greater than that of the Pt—Ti—O corridor structure. gate Threshold voltage fluctuation DV 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 this embodiment, a catalyst metal layer film having a columnar dendritic structure is used for the gate electrode layer of the gas sensor FET, which increases the specific surface area of ​​the catalyst metal layer film and thereby realizes a gas sensor FET with improved gas detection sensitivity. In particular, a highly sensitive gas sensor FET suitable for detecting hydrogen gas and 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 only 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 10m 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 present invention, the catalyst metal layer film is made of a porous catalyst metal, which can be realized by film formation alone, regardless of the surface condition. When film formation methods such as sputtering are used, film formation under appropriate high-pressure conditions results in the formation of numerous fine structures in the horizontal and diagonal directions in addition to the usual columnar structure in the z-axis direction. Using this catalyst metal layer film as the sensing film enables efficient gas detection. Furthermore, the underlying oxide layer and the resulting changes in film quality, which were necessary in conventional gallery-structure sensors made of catalyst metal / oxide, 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 multiple dendritic structures are arranged two-dimensionally is formed by high-pressure sputtering, which improves the specific surface area relative to the amount of catalytic metal used, thereby reducing material costs and improving gas detection sensitivity.

[0097] Furthermore, according to this embodiment, an adhesive layer film to be inserted between the gate insulating film and the catalyst metal layer film on the semiconductor substrate is not necessarily required, which makes it 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 gate threshold voltage, this corresponds to a range in which the amount of change in gate threshold voltage, which is directly linked to the gas detection sensitivity, is considered to be sufficiently higher than when the gate electrode has a conventional gallery structure.

[0099] The sputtering pressure when forming the catalytic metal layer is preferably 5 Pa or more. This is because, from 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 found to be the optimum specific surface area of ​​the catalytic metal layer, 20 m 2Although it is not possible to uniquely determine a suitable upper limit for the sputtering pressure, a value of 50 Pa or less would be a realistic guideline based on the specifications of the sputtering apparatus, the knowledge and experience of the present inventors, and the like.

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

[0101] Furthermore, based on the inventors' previous research results, it is also envisioned that the catalytic metal layer film may be composed 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 realistic examples.

[0102] Furthermore, based on the research results of the present inventors 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 examples.

[0103] Furthermore, based on the results of the inventors' research to date, it is assumed that the thickness of the metal oxide film will 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 a gas sensor FET according to an embodiment of the present invention, a reference FET for referencing the gate threshold voltage, and a heater, all arranged on the same substrate, is also an embodiment of the present invention. For example, the gas sensor FET, heater, and 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. The gate threshold voltages of the reference FET and the gas sensor FET are then monitored, and gas detection is performed based on the comparison results. This gas sensor can perform highly reliable gas detection that is less susceptible to changes in environmental temperature.

[0106] The above describes 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...Catalyst metal layer film, 1341...Dendrite 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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