Method for manufacturing gas barrier films
The alternating layers of amorphous and polycrystalline films in the gas barrier film trap protons and prevent continuous grain boundary extension, addressing the low barrier performance issue in existing laminated structures, resulting in enhanced hydrogen gas barrier efficiency.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- DENSO CORP
- Filing Date
- 2022-05-19
- Publication Date
- 2026-06-02
AI Technical Summary
Existing laminated polycrystalline films for hydrogen gas barriers suffer from low performance due to grain boundaries and crystal defects that create penetration paths for protons, leading to inadequate barrier properties.
A gas barrier film comprising alternating layers of amorphous films, primarily composed of AlN or Al2O3, and polycrystalline films, such as TiO2 or TiN, formed using atomic layer deposition to trap protons and prevent continuous grain boundary extension, thereby enhancing barrier performance.
The film effectively suppresses proton diffusion and penetration, achieving high hydrogen gas barrier performance by trapping protons in grain boundaries and eliminating continuous grain boundary paths, thus improving overall barrier efficiency.
Smart Images
Figure 0007868399000002 
Figure 0007868399000003 
Figure 0007868399000004
Abstract
Description
Technical Field
[0001] The present invention relates to a gas barrier manufacturing method with high barrier performance against hydrogen gas. membrane
Background Art
[0002] Patent Document 1 discloses a gas barrier film having barrier performance against hydrogen gas. This gas barrier film has a structure in which each of a plurality of first polycrystalline films and each of a plurality of second polycrystalline films are alternately laminated. The first polycrystalline film and the second polycrystalline film are composed of different alloy nitride compounds.
Prior Art Documents
Patent Documents
[0003]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0004] In the laminated structure of polycrystalline films as described above, grain boundaries present in each polycrystalline film have a function of proton traps that capture protons (i.e., hydrogen ions). However, as described below, there is a possibility that a penetration path is formed in which protons penetrate through the gas barrier film in the film thickness direction of the gas barrier film. That is, when the upper polycrystalline film is formed in contact with the surface of the lower polycrystalline film, there are grain boundaries extending in the thickness direction of the lower polycrystalline film, and the upper polycrystalline film is formed so as to inherit the grain boundaries. As a result, the grain boundaries extending in the thickness direction present in each polycrystalline film may be in a continuous state throughout the thickness direction of the gas barrier film. The grain boundaries in this state become the above penetration path. Therefore, in the laminated structure of polycrystalline films, in the above case, the barrier performance against hydrogen gas is low.
[0005] Furthermore, this problem is not limited to grain boundaries; it also occurs when large crystal defects capable of proton movement exist continuously along the thickness direction of the gas barrier film, extending throughout the entire thickness direction of the gas barrier film.
[0006] In view of the above, the present invention provides a gas barrier with high hydrogen gas barrier performance. membrane The purpose is to provide a manufacturing method. [Means for solving the problem]
[0007] To achieve the above objective, the invention described in claim 1 teeth, A gas barrier covering the surface (11) of a metal substrate (10) used in an environment where high-pressure hydrogen gas is present, as a component for a hydrogen station that supplies high-pressure hydrogen gas. Method for manufacturing membranes and 、 The aforementioned gas barrier film is It comprises one or more amorphous films (21) and one or more polycrystalline films (22), Each of the one or more amorphous films is a film mainly composed of AlN or a film mainly composed of Al2O3. the law of nature, The manufacturing method includes forming one amorphous film (21a) from the one or more amorphous films so as to be in contact with the metal substrate, and alternately stacking each of the one or more amorphous films and each of the one or more polycrystalline films by atomic layer deposition.
[0008] The gas barrier film of the present invention comprises one or more polycrystalline films. Therefore, protons can be trapped by the grain boundaries present in the polycrystalline films, thereby suppressing the diffusion of protons within the polycrystalline films.
[0009] Furthermore, the gas barrier film of the present invention comprises one or more amorphous films, each of which is either a film mainly composed of AlN or a film mainly composed of Al2O3. Amorphous films mainly composed of AlN or Al2O3 have high electrical resistance. Therefore, the proton conductivity of this amorphous film is lower than that of an amorphous film with lower electrical resistance. Thus, the amorphous film used in the gas barrier film of the present invention has high gas barrier performance against hydrogen gas.
[0010] Furthermore, in the gas barrier film of the present invention, each of one or more amorphous films and each of one or more polycrystalline films are laminated. There are no grain boundaries and crystal defects in the amorphous film. Therefore, even when grain boundaries or crystal defects exist in the polycrystalline film in a state of extending along the thickness direction of the polycrystalline film, it is possible to avoid the state of continuously extending in the thickness direction of the gas barrier film over the entire thickness direction of the gas barrier film. That is, it is possible to avoid the formation of a penetration path through which protons penetrate the gas barrier film in the thickness direction of the gas barrier film.
[0011] From these, it is possible to provide a gas barrier film having high gas barrier performance against hydrogen gas.
[0012] The reference numerals with parentheses attached to each component etc. show an example of the correspondence relationship between the component etc. and the specific components etc. described in the embodiments described later.
Brief Description of Drawings
[0013] [Figure 1] It is a cross-sectional schematic view of the gas barrier film and the metal substrate of the first embodiment. [Figure 2] It is a diagram showing each manufacturing process of the manufacturing method of the gas barrier film of FIG. 1. [Figure 3] It is a diagram showing the relationship between the AlN single film thickness / TiO2 single film thickness and the hydrogen permeation coefficient in each of Examples 1-3, 5 and Comparative Example 1.
Modes for Carrying Out the Invention
[0014] Hereinafter, embodiments of the present invention will be described based on the drawings. In each of the following embodiments, parts that are the same or equivalent to each other will be described with the same reference numerals.
[0015] (First Embodiment) The gas barrier film 20 of the present embodiment shown in FIG. 1 is used for a member used in an environment where hydrogen gas is present. Examples of such a member include a member for a hydrogen station that supplies high-pressure hydrogen gas.
[0016] The gas barrier film 20 is a film that covers the surface 11 of the metal substrate 10. The gas barrier film 20 has gas barrier performance against H2 (that is, hydrogen) gas. The gas barrier film 20 is formed in contact with the surface 11 of the metal substrate 10 in order to suppress hydrogen embrittlement of the metal substrate 10. The gas barrier film 20 has gas barrier performance not only against hydrogen gas but also against other gases such as H2O.
[0017] The metal substrate 10 is made of an iron-based metal material mainly composed of iron such as stainless steel. The metal substrate 10 has a shape such as a pipe or a heat exchanger used in an environment where high-pressure hydrogen is present.
[0018] The gas barrier film 20 includes one or more amorphous films 21 and one or more polycrystalline films 22. Each of the one or more amorphous films 21 and each of the one or more polycrystalline films 22 are alternately laminated. In the gas barrier film 20 shown in FIG. 1, the one or more amorphous films 21 are a plurality of amorphous films 21, and the one or more polycrystalline films 22 are a plurality of polycrystalline films 22. One of the plurality of amorphous films 21, an amorphous film 21a, is in contact with the surface of the metal substrate 10. Another one of the plurality of amorphous films 21, an amorphous film 21b, constitutes the surface of the gas barrier film 20.
[0019] Although not shown, the one or more amorphous films 21 may be only one amorphous film 21, and the one or more polycrystalline films 22 may be only one polycrystalline film 22. In this case, "each of the one or more amorphous films 21" refers to one amorphous film 21, and "each of the one or more polycrystalline films 22" refers to one polycrystalline film 22. Also, in this case, one amorphous film 21 is either in contact with the surface of the metal substrate 10 or constitutes the surface of the gas barrier film 20.
[0020] The amorphous film 21 is a film composed of amorphous material. Amorphous refers to a state of matter that does not have a crystalline structure, and is also called non-crystalline. The amorphous nature of the material constituting the film can be confirmed, for example, by performing electron diffraction measurements on the film. If the measurement result is a halo pattern, the material constituting the film is amorphous.
[0021] Each of the one or more amorphous films 21 is either an AlN film or an Al2O3 film. An AlN film is a film that contains AlN as its main component. Containing AlN as its main component means that the AlN content of the film is 90 atm% or more of the total film. An Al2O3 film is a film that contains Al2O3 as its main component. Containing Al2O3 as its main component means that the Al2O3 content of the film is 90 atm% or more of the total film.
[0022] In the case of multiple amorphous films 21, it is preferable that all of the multiple amorphous films 21 have the same main component. Some of the multiple amorphous films 21 may be AlN films, and other parts of the multiple amorphous films 21 may be Al2O3 films.
[0023] If all of the multiple amorphous films 21 are AlN films, the thickness of each of the multiple amorphous films 21 is 15 nm or less. This is because amorphous films 21 can be obtained by forming them at a thickness of 15 nm or less.
[0024] Each of the one or more polycrystalline films 22 has multiple crystal grains and crystal grain boundaries, which are the boundaries between the crystal grains, and has gas barrier properties against hydrogen gas. Each of the one or more polycrystalline films 22 is a metal oxide film. A metal oxide film is a film that mainly contains metal oxides. "Mainly contains metal oxides" means that the metal oxide content of the film as a whole is 90 atm% or more. Examples of metal oxides include TiO2 and TiO x(However, 0 < x < 2), examples include HfO2. A film containing TiO2 as the main component is a TiO2 film.
[0025] In addition, each of one or more polycrystalline films 22 may be a metal nitride film containing a metal nitride such as TiN as the main component, or a metal oxynitride film containing a metal oxynitride such as TiON as the main component. "Containing as the main component" has the same meaning as in the case of a metal oxide film.
[0026] In the case of a plurality of polycrystalline films 22, it is preferable that all of the plurality of polycrystalline films 22 have the same main component. A part of the plurality of polycrystalline films 22 and another part may have different main components.
[0027] Next, a method for manufacturing the gas barrier film 20 of the present embodiment will be described. The method for manufacturing the gas barrier film 20 includes a step of preparing the metal substrate 10 and a step of forming the gas barrier film 20.
[0028] In the step of preparing the metal substrate 10, as shown in Fig. 2(a), a metal substrate 10 having a surface 11 is prepared. In order to improve the adhesion of the gas barrier film 20 to the metal substrate 10, it is preferable to perform a pretreatment on the surface 11 of the metal substrate 10.
[0029] In the step of forming the gas barrier film 20, each of one or more amorphous films 21 and each of one or more polycrystalline films 22 are formed by ALD as follows. ALD is an abbreviation for Atomic layer deposition, and is also called atomic layer deposition method or atomic vapor deposition method.
[0030] First, the metal substrate 10 is placed inside a reaction vessel not shown in the ALD, specifically, inside a vacuum chamber. Then, heating is performed so that the temperature inside the reaction vessel and the temperature of the metal substrate 10 reach the film formation temperature, which is 500°C here.
[0031] Next, as shown in Figure 2(b), a single amorphous film 21 is formed on the surface 11 of the metal substrate 10 by ALD. When forming an AlN film as the amorphous film 21, a single AlN film of a predetermined thickness is formed by repeatedly introducing TMA (i.e., trimethylaluminum) and NH3 alternately into the reaction vessel.
[0032] Next, as shown in Figure 2(c), a single layer of polycrystalline film 22 is deposited on the surface of the amorphous film 21 by ALD. When depositing a TiO2 film as the polycrystalline film 22, a single layer of TiO2 film of a predetermined thickness is deposited by repeatedly introducing TiCl4 and H2O alternately into the reaction vessel.
[0033] When manufacturing a gas barrier film 20 having a structure in which only one layer each of an amorphous film 21 and a polycrystalline film 22 are stacked, the metal substrate 10 is then removed from the reaction vessel. This completes the production of the gas barrier film 20. In this case, the polycrystalline film 22 may be deposited before the amorphous film 21.
[0034] Furthermore, when manufacturing a gas barrier film 20 having a structure in which multiple amorphous films 21 and multiple polycrystalline films 22 are alternately stacked, the deposition of the amorphous films 21 and the deposition of the polycrystalline films 22 are repeatedly performed alternately.
[0035] In this way, a gas barrier film 20 is manufactured in which one or more amorphous films 21 and one or more polycrystalline films 22 are alternately stacked. Note that each of the one or more amorphous films 21 and each of the one or more polycrystalline films 22 may be formed by a CVD method other than ALD or by a sputtering method.
[0036] As described above, the gas barrier film 20 of this embodiment comprises one or more polycrystalline films 22. Therefore, protons can be trapped by the grain boundaries present in the polycrystalline films 22, thereby suppressing the diffusion of protons within the polycrystalline films.
[0037] Furthermore, the gas barrier film 20 of this embodiment comprises one or more amorphous films 21, each of which is either an AlN film or an Al2O3 film. AlN films and Al2O3 films have high electrical resistance. Therefore, the proton conductivity of this amorphous film 21 is lower than that of an amorphous film with lower electrical resistance. Thus, the amorphous film 21 used in the gas barrier film 20 of this embodiment has high gas barrier performance against hydrogen gas.
[0038] Furthermore, in the gas barrier film 20 of this embodiment, one or more amorphous films 21 and one or more polycrystalline films 22 are stacked. The amorphous films do not contain grain boundaries or crystal defects. For example, when forming an amorphous film 21 in contact with a polycrystalline film 22, even if there are grain boundaries and crystal defects in the polycrystalline film that extend along the thickness direction of the polycrystalline film, such grain boundaries and crystal defects are not carried over and formed in the amorphous film 21. Therefore, even if grain boundaries or crystal defects exist in the polycrystalline film 22 in a state that extends along the thickness direction of the polycrystalline film 22, it is possible to avoid them extending continuously in the thickness direction of the gas barrier film 20 throughout the entire thickness direction of the gas barrier film 20. In other words, it is possible to avoid the formation of penetrating paths through which protons move through the gas barrier film 20 in the thickness direction of the gas barrier film 20.
[0039] Based on these findings, it is possible to provide a gas barrier film with high gas barrier performance against hydrogen gas.
[0040] Furthermore, one method for manufacturing the gas barrier film 20 in this embodiment includes forming each of the one or more amorphous films 21 and each of the one or more polycrystalline films 22 using ALD. This allows the gas barrier film 20 to be formed on the surface 11 of the metal substrate 10 even if the metal substrate 10 has a complex shape. Moreover, this allows the gas barrier film 20 to be manufactured by switching the type of gas used for film formation using the same film deposition apparatus.
[0041] The gas barrier film 20 of this embodiment provides the following further effects.
[0042] (1) One of the amorphous films 21a is in contact with the metal substrate 10.
[0043] Unlike this case, when the polycrystalline film 22 is formed in contact with the metal substrate 10, the polycrystalline film 22 is formed in such a way that it inherits the crystal structure of the metal substrate 10. As a result, the crystal grains contained in this polycrystalline film become larger, and the grain boundaries extend in the direction of the film thickness so as to penetrate the polycrystalline film. Protons move through the polycrystalline film via these grain boundaries.
[0044] In contrast, by forming an amorphous film 21 in contact with the metal substrate 10, it is possible to avoid a state where the grain boundaries extend in the film thickness direction so as to penetrate the film. Therefore, compared to the case where a polycrystalline film 22 is formed in contact with the metal substrate 10, the gas barrier performance against hydrogen gas can be improved.
[0045] (2) One of the amorphous films 21b constitutes the surface of the gas barrier film 20.
[0046] Unlike this case, if the polycrystalline film 22 constitutes the surface of the gas barrier film 20, protons can penetrate through the grain boundaries of the polycrystalline film 22. In contrast, if the amorphous film 21 constitutes the surface of the gas barrier film 20, the amorphous film 21 does not have grain boundaries, so the penetration of protons from the grain boundaries can be avoided.
[0047] Furthermore, this allows for a reduction in the area on which hydrogen molecules adhere to the gas barrier film 20 compared to the case where the polycrystalline film 22 constitutes the surface of the gas barrier film 20. Therefore, the probability of hydrogen molecules becoming protons and penetrating the gas barrier film can be reduced. Thus, the gas barrier performance against hydrogen gas can be improved.
[0048] (3) Regarding the combination of the amorphous film 21 and the polycrystalline film 22, each of the one or more amorphous films 21 is preferably an AlN film, and each of the one or more polycrystalline films 22 is preferably a metal oxide film. According to this, the material constituting the amorphous film 21 and the material constituting the polycrystalline film 22 are in a relationship where the types of materials are different, namely nitride and oxide. Therefore, when the amorphous film 21 and the polycrystalline film 22 are formed alternately, it is possible to suppress the mixing of the material constituting the amorphous film 21 and the material constituting the polycrystalline film 22.
[0049] (4) Regarding the combination of the amorphous film 21 and the polycrystalline film 22, each of the one or more amorphous films 21 is preferably an AlN film, and each of the one or more polycrystalline films 22 is preferably a TiO2 film. This is because the AlN film, which is an amorphous film, has high gas barrier performance against hydrogen gas as a single film, and the TiO2 film, which is a polycrystalline film, can achieve the proton capture effect due to grain boundaries.
[0050] (5) When each of the one or more amorphous films 21 is an AlN film and each of the one or more polycrystalline films 22 is a TiO2 film, the film thickness of each of the one or more amorphous films 21 is preferably 15 nm or less, and the ratio of the film thickness of one amorphous film 21 to the film thickness of one polycrystalline film 22 is preferably 3 or less. The film thicknesses of each of the one or more amorphous films 21 are preferably the same, but they may also be different. The film thicknesses of each of the one or more polycrystalline films 22 are preferably the same, but they may also be different.
[0051] According to this, as described in the examples below, it is possible to lower the hydrogen permeation coefficient of the gas barrier film 20 and provide a gas gas barrier film 20 with high gas barrier performance against hydrogen gas.
[0052] (6) Each of the one or more polycrystalline films 22 is preferably a film containing TiO x (where 0 < x < 2) as a main component. TiO x(However, 0 < x < 2) is an N-type in which oxygen atoms are deficient and electrons are excessive with respect to TiO2. As a result, protons conducting through the polycrystalline film 22 can be trapped. Therefore, compared with the case where each of one or more polycrystalline films 22 is a TiO2 film, the gas barrier performance against hydrogen gas is improved.
[0053] (7) Each of the one or more polycrystalline films 22 is preferably a film containing TiON as a main component. TiON, similar to TiO x (However, 0 < x < 2), is N-type and can trap protons conducting through the polycrystalline film 22.
[0054] Also, the crystal structure of TiO x (However, 0 < x < 2) has sites where O elements are deficient with respect to the crystal structure of TiO2. Protons can easily pass through these sites where O elements are deficient. In contrast, the crystal structure of TiON is a state where O elements are substituted by N elements with respect to the crystal structure of TiO2. That is, the sites where the above O elements are deficient are filled with N elements. Therefore, it is possible to avoid protons passing through the sites where O elements are deficient. Thus, compared with the case where each of one or more polycrystalline films 22 is a TiO x (However, 0 < x < 2) film, the gas barrier performance against hydrogen gas is improved.
[0055] (8) The present invention is not limited to the above-described embodiments, and can be appropriately changed within the scope described in the claims, and also includes various modified examples and modifications within the equivalent scope. Also, in each of the above embodiments, it is needless to say that the elements constituting the embodiments are not necessarily essential except in cases where it is explicitly stated that they are essential or cases where they are considered to be clearly essential in principle.
Example
[0056] The inventors prepared a metal substrate 10 and formed a gas barrier film 20 on the prepared metal substrate 10 to create the gas barrier films 20 of Examples 1-5 and Comparative Example 1. Subsequently, the hydrogen permeability coefficients of the gas barrier films 20 of Examples 1-5 and Comparative Example 1 were measured.
[0057] The prepared metal base material 10 is made of SUS316, has a thickness of 0.1 mm, and is disc-shaped with a diameter of 35 mm.
[0058] In forming the gas barrier film 20, an amorphous film 21, which is an AlN film, and a polycrystalline film 22, which is a TiO2 film, were repeatedly formed on the surface 11 of the metal substrate 10 by ALD in the order described, with the AlN film being formed last. At this time, the gas barrier film 20 was formed so that the thickness of each of the multiple AlN films and the thickness of each of the multiple TiO2 films that make up the gas barrier film 20 were uniform.
[0059] Table 1 shows the single film thickness (i.e., the thickness of one AlN film), the number of AlN film layers, the single film thickness (i.e., the thickness of one TiO2 film), and the number of TiO2 film layers in the gas barrier film 20 of Examples 1 to 5 and Comparative Example 1.
[0060] The hydrogen permeability coefficient was measured in accordance with "JIS K-7126-1GC". The measurement temperature was 300°C. The hydrogen permeability coefficients of the gas barrier films 20 for Examples 1 to 5 and Comparative Example 1 are shown in Table 1.
[0061] [Table 1] As shown in Table 1, in the gas barrier films 20 of Examples 1 to 5, the thickness of each of the multiple AlN films is between 2.5 nm and 15 nm. Therefore, each of the multiple AlN films is an amorphous film. On the other hand, in the gas barrier film 20 of Comparative Example 1, the thickness of each of the multiple AlN films is 25 nm, which is greater than 15 nm. Therefore, each of the multiple AlN films is a crystalline film.
[0062] In the gas barrier films 20 of Examples 1 to 5, the thickness of each of the multiple TiO2 films is between 5 nm and 10 nm. The total number of layers of multiple AlN films and multiple TiO2 films is between 17 and 53. The total thickness of the multiple AlN films and multiple TiO2 films is between 0.17 μm and 0.27 μm. The total thickness of Example 2 is 0.17 μm, and the total thickness of Example 4 is 0.27 μm. In addition, in the gas barrier films 20 of Examples 1 to 5, the thickness ratio, i.e., AlN single film thickness / TiO2 single film thickness, which is the ratio of the thickness of one AlN film to the thickness of one TiO2 film, is between 0.33 and 3.
[0063] The hydrogen permeability coefficients of the gas barrier films 20 in Examples 1 to 5 are 1.5E-15 mol / (msPa). 0.5 The results were as follows, and it was confirmed that each of the gas barrier films 20 in Examples 1 to 5 had high gas barrier performance against hydrogen gas.
[0064] The gas barrier films 20 in Examples 1-3, 5 and Comparative Example 1 have a total film thickness of approximately 175 nm, which is equivalent in thickness. The relationship between the film thickness ratio and the hydrogen permeability coefficient in these gas barrier films 20 is shown in Figure 3. From Figure 3, it can be seen that the hydrogen permeability coefficients of Examples 1-3 and 5, where the film thickness ratio is 3 or less, are lower than those of Comparative Example 1, where the film thickness ratio is 5. Therefore, it is preferable that the film thickness of each of the multiple AlN films is 15 nm or less, and the film thickness ratio is 3 or less.
[0065] Furthermore, as can be seen by comparing Example 3 and Example 4 in Table 1, a larger total number of AlN and TiO2 films is preferable. This is because it results in a lower hydrogen permeability coefficient and higher gas barrier performance against hydrogen gas.
[0066] As described above, the gas barrier films 20 of Examples 1 to 5 have high gas barrier performance against hydrogen gas even when the total thickness of the gas barrier film 20 is within the range of 0.17 μm or more and 0.27 μm or less. Compared to the gas barrier film described in Patent Document 1, which has a total thickness within the range of 0.5 μm or more and 2 μm or less, the total thickness of the gas barrier film is smaller, thus reducing manufacturing costs. [Explanation of symbols]
[0067] 10 Metal substrate 20 Gas barrier film 21 Amorphous membrane 22 Polycrystalline film
Claims
1. A method for manufacturing a gas barrier film that covers the surface (11) of a metal substrate (10) used in an environment where high-pressure hydrogen gas is present, as a component for a hydrogen station that supplies high-pressure hydrogen gas, The gas barrier film comprises one or more amorphous films (21) and one or more polycrystalline films (22). Each of the one or more amorphous films is a film containing AlN as the main component or Al ２ O ３ It is a membrane that contains as its main component, The manufacturing method includes forming one amorphous film (21a) from the one or more amorphous films so as to be in contact with the metal substrate, and alternately stacking each of the one or more amorphous films and each of the one or more polycrystalline films by atomic layer deposition.
2. The manufacturing method according to claim 1, wherein one of the one or more amorphous films (21b) constitutes the surface of the gas barrier film.
3. Each of the one or more amorphous films is a film containing AlN as its main component. The manufacturing method according to claim 1, wherein each of the one or more polycrystalline films is a film mainly composed of a metal oxide.
4. The manufacturing method according to claim 3, wherein the metal oxide is TiO2.
5. The thickness of each of the one or more amorphous films is set to 15 nm or less. The manufacturing method according to claim 4, wherein the ratio of the thickness of one amorphous film to the thickness of one polycrystalline film is 3 or less.
6. The manufacturing method according to claim 5, wherein the total thickness of the one or more amorphous films and the one or more polycrystalline films is 0.17 μm or more and 0.27 μm or less.
7. The manufacturing method according to claim 1, wherein each of the one or more polycrystalline films is a film containing TiOx (where 0 < x < 2) as a main component.
8. The manufacturing method according to claim 1, wherein each of the one or more polycrystalline films is a film containing TiON as a main component.