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

Abstract

Provided is a gas sensor FET having improved gas detection sensitivity. The gas sensor FET comprises: 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, exposed to the atmosphere, of the catalyst metal layer film has a structure in which a plurality of dendritic structures in which a plurality of fine structure parts smaller than a columnar part are formed on the surface of the columnar part are two-dimensionally arranged. The plurality of dendritic structures are electrically connected to each other.

Description

Gas sensor FET, gas sensor, and method for manufacturing gas sensor FET

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

[0002] Gas sensors for detecting specific gases or impurity gases have been known. One known gas sensor is a field effect transistor (FET) gas sensor. Patent Document 1 describes an example of a FET gas sensor.

[0003] JP 2018-115888 A

[0004] The FET gas sensor uses a semiconductor element called a gas sensor FET, which is relatively small in size and 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.

[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 microstructure portions 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, which includes a semiconductor substrate and a gas detection section formed on the semiconductor substrate and constituting a gate electrode layer, the gas detection section having a catalyst metal layer film including a side exposed to an atmosphere, the method including a catalyst metal layer deposition step of depositing the catalyst metal layer film by sputtering at a sputtering pressure of 5 Pa or more.

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

[0011] 1 is a diagram showing an example of the structure of the gas sensor FET according to Example 1. FIG. 2 is a diagram showing an example of a gate electrode layer of the gas sensor FET according to Example 1. FIG. 3 is a flow chart showing an example of a manufacturing method of the gas sensor FET according to Example 1. FIG. 4 is a flow chart showing an example of a method of forming the gate electrode layer according to Example 1. FIG. 5 is a diagram for explaining that the formation of the gate electrode layer is performed after the formation of a protective film. FIG. 6 is a diagram showing the relationship between the sputtering pressure during the formation of a catalyst metal layer and the non-surface area of ​​the catalyst metal layer film. FIG. 7 is a diagram showing enlarged structural images of a plurality of Pt catalyst metal layer films formed under different sputtering pressures during film formation. FIG. 8 is a diagram showing an example of measurement of the relationship between the non-surface area of ​​the catalyst metal layer film and the amount of variation in gate threshold voltage with respect to the concentration of a detected gas in the gas sensor FET according to Example 1. FIG. 9 is a diagram showing an example of measurement of the amount of variation in gate threshold voltage with respect to the film thickness of the catalyst metal layer film in the gas sensor FET according to Example 1. FIG. 10 is a diagram showing the structure of the gate electrode layer of the gas sensor FET according to Example 2 and the variation in gate threshold voltage with respect to the change in the concentration of a detected gas in the atmosphere. FIG. 11 is a diagram showing the structure of the gate electrode layer of the gas sensor FET according to Example 3 and the variation in gate threshold voltage with respect to the change in the concentration of a detected gas in the atmosphere. 1 is a diagram showing the structure of a gate electrode layer of a gas sensor FET according to Example 4 and the fluctuation of a gate threshold voltage due to a change in the concentration of a detected gas in the atmosphere. 2 is a diagram showing the structure of a gate electrode layer of a gas sensor FET according to Example 5 and the fluctuation of a gate threshold voltage due to a change in the concentration of a detected gas in the atmosphere. 3 is a diagram for explaining the amount of threshold voltage fluctuation. 4 is a diagram showing the relationship between the structure of a gate electrode layer and the amount of gate threshold voltage fluctuation.

[0012] (Background of the study by the inventors) Technologies for generating electricity using hydrogen, such as fuel cells, are attracting attention in an effort to realize a carbon-free society. With growing environmental awareness among people around the world, there are high expectations for improvements in technology for these fuels that do not emit carbon dioxide. Meanwhile, in recent years, it has become known that PEFCs (Polymer Electrolyte Fuel Cells), which are used in home fuel cell systems and automotive fuel cells, experience cell degradation due to impurity gases such as carbon monoxide, hydrogen sulfide, and hydrocarbon compounds in high-concentration hydrogen gas, 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, 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 the formation of a corridor-structured Pt film on the gate electrode increases the surface area of ​​the Pt film, inducing the adsorption of hydrogen gas.

[0016] The specific surface area of ​​these Pt gallery structures is 0.1 to 10 m depending on the degree of oxidation of the base film. ２ / g. The detection mechanism of this sensor utilizes the fact that hydrogen gas adsorbed on a Pt catalyst film is protonated, and the protons accumulate near the gate insulating film, generating an electric field that 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 as the detection film. This sensor also describes a crystal grain size of about 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 the purpose of detecting hydrogen and the like include the following. Patent Document 3 (JP 2012-013579 A) describes a chemical sensor technology using a sensitive film. Not limited to FET-type sensors, sensors that utilize properties such as changes in resistance or mass depending on the substance to which the sensitive film reacts can be produced by changing the type 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 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. Among these, FET-type, capacitor-type, and diode-type sensors are classified as work function sensors. Work function sensors can be manufactured using a process that uses a semiconductor substrate, and are therefore expected to achieve lower costs, smaller sizes, and lower power consumption compared to sensors classified as other types.

[0019] As described in Table 2 of Non-Patent Document 3, FET gas sensors are further classified into several types. Among them, the catalytic metal gate FET (referred to as 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 (Japanese Patent Laid-Open Publication No. 2005-283578) describes an 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 of Patent Document 3 uses the fact that when the base metal is oxidized, the Pt layer, which is a catalytic metal, undergoes a structural change, increasing the number of grain boundaries, 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. 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 effects of the embodiments of the present invention is the enhancement of gas detection sensitivity in the gas sensor FET, but other effects include a wider range of options for the sensing metal layer, a wider dynamic range, stable manufacturing, and lower costs. Hereinafter, embodiments 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 FET1 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 a part of each of the two n-type semiconductors 12. When gas is adsorbed to the gate electrode layer 13, the threshold voltage of the FET changes. Gas can be detected by measuring this change in threshold voltage. In other words, the gate electrode layer 13 functions as a gas detection unit.

[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 the semiconductor substrate 11, and an oxide layer film (for example, TiO ２ A catalyst metal layer film (e.g., Pt film, specific surface area: about 40 m) 133 having a structure characteristic of this embodiment is formed on the catalyst metal layer film. ２ 1. The oxide layer film 133 corresponds to the adhesive layer film in a gate electrode layer having a conventional corridor structure. That is, the gate electrode layer 13 has the oxide layer film 133 disposed on the side opposite to the side exposed to the atmosphere in contact with the catalyst metal layer film 134.

[0026] The gate electrode layer 13 according to Example 1 has a characteristic structure, as shown in the upper and middle sections of FIG. 2 , in which a plurality of tree-like structures 1341 are arranged two-dimensionally. Each tree-like structure 1341 is formed by forming a plurality of fine structures 1341b, each of which is smaller and finer than the columnar portion 1341a, on the surface of the columnar portion 1341a. The tree-like structures 1341 are electrically connected to each other. 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 FIG. 2 shows an x-z ​​cross section of the tree-like structure 1341 (a half cross section of the tree-like structure) and an x-y cross section of the tree-like structure 1341 (a quarter cross section of the tree-like structure). As can be seen from these figures, in the tree-like structure 1341, a large number of thin plate-shaped, leaf-shaped, or granular-shaped 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 corresponding to the gas sensor in the gate electrode layer 13 compared to 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 for 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 in terms of parts other than the gate electrode. Therefore, here, we will first explain an outline of the manufacturing process for the gas sensor FET of 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. This flowchart includes only the main steps, and omits detailed steps.

[0030] 3, first, in step S1, a semiconductor substrate is prepared, that is, a semiconductor substrate (also called a wafer or a silicon wafer) is cut out from a silicon ingot and polished.

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

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

[0033] Next, in step S4, an exposure process is carried out, that is, a process is carried out 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 step of removing the etching photoresist is carried out, 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.

[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 is carried out.

[0037] Next, in step S8, a step 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. This is because the gate electrode layer needs to be exposed to the atmosphere so that it can function as a gas sensor. As will be described later, after the protective film is formed, part of the protective film needs to be removed, and the gate electrode layer needs to be formed in the area where the protective film is not present as a final step.

[0038] 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 process proceeds to step S9. If it is determined that all the necessary patterns have not been formed (S8: No), the process returns to step S2, and the steps from forming the oxide film to the doping are carried out again.

[0039] In step S9, a contact forming step is carried out, that is, a predetermined process is carried out at necessary locations 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 carried out. That is, a process of wiring between contacts is carried out by connecting wiring materials or the like. The wiring material used here is made of a material having anti-oxidation properties. As will be described later, after wiring is carried out, a protective film is formed, a portion of the protective film is removed, and a gate insulating film is formed thereon. When this gate insulating film is formed, a strong oxidation treatment is carried out on the semiconductor substrate. At this time, this is to prevent the previously placed wiring material from being altered by oxidation. Examples of materials having anti-oxidation properties include tungsten and LaB, which are resistant to oxidation. ６ , CuMn, or other high melting point metal materials 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, the protective film covering the region where the gate electrode layer will be formed is removed by dry etching or wet etching.

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

[0044] <Method of Forming Gate Electrode Layer> Here, an example of a method of forming a 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 Example 1. Fig. 5 is a diagram for explaining that the formation of the gate electrode layer is performed after the formation of a protective film. 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 form an opening in the protective film, and then form the gate electrode layer in the opening (see Fig. 5).

[0046] First, as a premise, it is assumed that a protective film 14 is provided on a semiconductor substrate 11 as shown in Fig. 5. In step S130, as shown in Fig. 4, 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 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 plasma CVD or the like using tetraethoxysilane (TEOS) or monosilane as raw materials under conditions of about 200°C to 500°C. In this case, since the gate insulating film 132 is exposed to strong oxidizing conditions, there is a risk that the wiring material, such as Al, formed beforehand may be oxidized. Therefore, in the gas sensor FET according to this embodiment, it is preferable that the wiring material is made of an oxidation-resistant metal material. Examples of oxidation-resistant metal materials include tungsten and lanthanum hexaboride (LaB ６ ), high melting point metals such as copper manganese (CuMn) and oxidation resistant alloys are effective.

[0049] Next, in step S132, a process of forming an oxide layer film is carried out. This oxide layer film 133 is an oxide layer film that corresponds to an adhesive layer film in a conventional gate electrode with a 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 in combination with the catalyst metal layer film 134. As the oxide layer film 133, for example, TiO ２ A film, a YSZ film, a GDC layer film, or the like is formed. For example, a sputtering method is used for this film formation. For example, a 2-inch diameter TiO ２ , YSZ (8% Y ２ O ３ , 92% ZrO2), GDC layer (10% Gd ２ O ３ , 90% CeO ２ ) or the like, sputtering is performed under the 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 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 and a DC 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 catalyst layer 134 having a specific surface area of ​​40 m. ２ The catalytic metal layer 134 is formed to have a dendritic structure with a conductivity of 0.1 μm / g or more. 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 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, favorable fluctuations in gate threshold voltage were confirmed when detecting hydrogen in a 3% hydrogen atmosphere.

[0052] Furthermore, when the lower oxide layer film is made of a solid electrolyte film, yttria-stabilized zirconia (YSZ) film (thickness: for example, 10 nm), the gate threshold voltage fluctuation settles quickly, and good detection characteristics are shown. ２ O ３ (8mol%)-ZrO ２ Using a (92 mol%) target, film formation is performed using a magnetron sputtering device with a high-frequency discharge power of 200 W. This is thought to be because hydrogen gas is adsorbed onto the Pt catalytic metal layer film, 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 the gate electrode layer 13 and the contact with a wiring material is performed.

[0054] By carrying out the above steps, the gas sensor FET1 according to this embodiment is completed. The gate electrode layer 13, which functions as a 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 after the step of forming the catalytic metal layer film, in which the formed catalytic metal layer film is subjected to an oxidation or reduction surface treatment to activate the catalytic effect.

[0056] In addition, for a reference FET in which the gate electrode layer 13 does not have an externally exposed portion, after the catalyst metal layer film 134 is formed, a Si ２ N ３ / SiO ２ and AlN / Al ２ O ３ The insulating layers can be manufactured by depositing the above insulating layers, each having a thickness of 100 nm or more, by sputtering or the like.

[0057] <Relationship Between Sputtering Pressure During Deposition of Catalyst Metal Layer and Non-Surface Area of ​​Catalyst Metal Layer> Here, the relationship between the sputtering pressure during deposition of the catalyst metal layer and the non-surface area of ​​the catalyst metal layer will be described.

[0058] Fig. 6 is a diagram showing the relationship between the sputtering pressure during catalyst metal layer deposition and the non-surface area of ​​the catalyst metal layer film, and Fig. 7 is a diagram showing enlarged structural images of a plurality of Pt catalyst metal layer films deposited 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 two-dimensionally arranged, 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 Fluctuation in Gate Threshold Voltage with Respect to Specific Surface Area of ​​Catalyst Metal Layer Film> Here, a measurement example of fluctuation 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 an example of measurement of the relationship between the specific surface area of ​​the catalytic 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. 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 gate threshold voltage V ｔｈ The amount of change DV ｔｈ and (hereinafter, the gate threshold voltage fluctuation amount DV ｔｈ This figure shows an example of measurement of the relationship between the electrode thickness and the electrode capacitance (also called the electrode capacitance). 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 is used as the gate electrode layer of a gas sensor FET. The thickness of the catalyst metal layer film is 25 nm.

[0062] As shown in FIG. 8, when the specific surface area of ​​the catalytic metal layer film is 20 m ２ / g or more, the gate threshold voltage fluctuation amount DV ｔｈ becomes 1.0 V or more. When the specific surface area of ​​the catalyst metal layer film is 120 m ２ / g, the gate threshold voltage fluctuation amount DV ｔｈ In the case of the conventional Pt / Ti gallery structure, the specific surface area of ​​the catalytic metal layer is 0.5 to 10 m, as shown by D1 in FIG. ２ / g, and the gate threshold voltage fluctuation amount DV ｔｈ In comparison 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 amount 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 Fluctuation in Gate Threshold Voltage with Respect to Film Thickness of Catalyst Metal Layer Film> Next, a measurement example of fluctuation in gate threshold voltage with respect to film 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 the columnar tree structure according to this Example on the 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%H ２ / N ２ The catalytic metal layer film was made of Pt and was 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 tree 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 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] Second Embodiment 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 , the gas sensor FET according to 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 (SiO ２ However, in the structure K1, the oxide layer corresponding to the adhesive layer film is made of titanium oxide (TiO ２ ), and in structure K2, the oxide layer is yttria-stabilized zirconia (YSZ). The sensing gas is nitrogen (N ２ ) in hydrogen (H ２ )

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

[0070] Graphs G0 to G2 in FIG. 10 show the atmosphere as ２ 100%, and then ２ Gate threshold voltage fluctuation DV when the concentration is changed to 25% 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 can be confirmed.

[0071] Third Embodiment 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 ｔｈ 11 , the gas sensor FET according to Example 3 uses Ni with a columnar dendritic structure as the catalytic metal layer of the gate electrode, and silicon oxide (SiO ２ ) is used. No oxide layer film corresponding to the adhesive layer film is provided. The detection gas is nitrogen (N ２ ) in hydrogen (H ２ Ni is oxidized in the atmosphere, making it difficult for it 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, similar to Example 1, 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. 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, a Ti layer or an adhesive layer equivalent to Ti oxide is not required, and the Ni columnar dendritic structure can be directly formed and used as the gate electrode. Note that, because the Ni columnar dendritic structure 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 atmosphere as ２ 100%, and then ２ 3% concentration, then again N ２ Gate threshold voltage V when changed to 100% ｔｈ As can be seen from graph G3, a threshold voltage fluctuation similar to that of a Pt-based catalyst metal layer is confirmed. It is difficult to form a catalyst metal layer such as Ni into a columnar dendritic structure using conventional manufacturing methods, and the fact that this is possible is largely due to the method of forming the gate electrode layer by sputtering according to this embodiment.

[0076] Example 4 Example 4 will 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 The gas sensor FET according to Example 4 uses a gate electrode in which a Pt catalyst metal layer film having a columnar dendritic structure is laminated on a titanium oxide film, and the detected gas is hydrogen sulfide (H ２ The thickness of the Pt catalyst metal layer film having a columnar dendritic structure is 25 nm.

[0078] Graph G4 in FIG. 12 shows the gate threshold voltage V ｔｈGraph G4 shows the fluctuation of the gate threshold voltage. It can be seen from this graph G4 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 a conventional gas sensor FET with a catalytic metal / oxide gallery structure, and almost the same gate threshold voltage fluctuation is obtained.

[0079] <Fifth Embodiment> A fifth embodiment will 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 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 of Example 4 was placed in an atmosphere of 100% hydrogen concentration, 100 ppm of carbon monoxide was introduced into the atmosphere, and then 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 with a porous structure.

[0082] By utilizing the above-described effects of the gas sensor FET according to the fifth embodiment, it becomes possible to detect impurity gases in high-concentration hydrogen gas, which are a 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 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 the Present Embodiment> Next, features of the manufacturing method of the gas sensor FET according to the present embodiment will be described. The manufacturing method of the gas sensor FET according to the present 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 since 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, 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, the wiring material is exposed to a strong oxidizing environment when the gate insulating film is formed, so 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 investigating the relationship between the structure of the gate electrode layer and the gate threshold voltage fluctuation will be described.

[0088] FIG. 14 shows the threshold voltage fluctuation amount 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 reach the state of C3, the voltage difference before and after gas detection is C4.

[0089] FIG. 15 shows the relationship between 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 is about 0.8 V. When the gate electrode layer has a structure in which a Pt electrode layer (bulk) having 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 having a Pt columnar 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 Pt catalyst metal layer having a columnar dendritic structure and a thickness of 15 nm 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 catalyst metal layer having a columnar dendritic structure 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 the structure according to this 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, 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 gallery-structured FET sensors made of catalytic metal / oxide, the only combination of materials that can be used to form a gallery structure is Pt-TiO ｘ , Pt-MoO ｘ In addition to being limited to the above, the specific surface area is 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 catalyst metal layer films to detect various gases are limited, making it difficult to expand the material options for catalyst 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 catalyst metal layer film having a columnar dendritic structure in which multiple dendritic structures are arranged two-dimensionally is formed by high-pressure sputtering, thereby improving the specific surface area relative to the amount of catalyst metal used, and making it possible to improve gas detection sensitivity while reducing material costs.

[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 ２ Although it is not possible to uniquely determine a suitable upper limit for the sputtering pressure, a pressure of 50 Pa or less would be a realistic guideline based on the specifications of the sputtering apparatus and the knowledge and experience of the present inventors.

[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 research results of the inventors to date, it is also conceivable that the catalyst 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 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 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.

[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 gas sensor FET comprising: a semiconductor substrate; and a gate electrode layer formed on the semiconductor substrate and functioning as a gas detection section, 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, the plurality of microstructures being smaller and finer than a columnar portion on the surface of the columnar portion, the plurality of dendritic structures being electrically connected to each other.

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

3. In 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.

4. The gas sensor FET according to claim 1, wherein the catalyst metal layer has a thickness of 5 nm or more and 500 nm or less.

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

6. A gas sensor FET according to claim 1, wherein 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.

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

8. The gas sensor FET according to claim 7, wherein the oxide layer is made of yttrium-zirconium oxide, gadolinium-cerium oxide, or titanium oxide.

9. The gas sensor FET according to claim 7, wherein the oxide layer has a thickness of 5 nm or more and 30 nm or less.

10. A gas sensor comprising: a gas sensor FET according to claim 1 arranged on a substrate; a reference FET for referencing a gate threshold voltage arranged on the substrate; and a heater arranged on the substrate for heating the temperature of the substrate to a predetermined temperature range.

11. A method for manufacturing a gas sensor FET, comprising a semiconductor substrate and a gas detection section formed on the semiconductor substrate and constituting a gate electrode layer, the gas detection section having a catalytic metal layer film including a side exposed to an atmosphere, the method comprising a catalytic metal layer deposition step of depositing the catalytic metal layer film by sputtering at a sputtering pressure of 5 Pa or more.

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

13. A method for manufacturing a gas sensor FET as described in claim 12, comprising the steps of: removing a portion of a protective film formed on the semiconductor substrate; forming a gate insulating film on the portion of the semiconductor substrate from which the protective film has been removed; and forming the catalytic metal layer film on the formed gate insulating film.

14. A method for producing a gas sensor FET according to claim 13, further comprising a wiring material connecting step of connecting a wiring material to the catalyst metal layer film, and using a material having oxidation resistance as the wiring material.

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