Optical filter and method for manufacturing optical filter

Abstract

An optical filter according to the present disclosure comprises a substrate having crystallinity and a multilayer film formed on a crystal orientation surface of the substrate, the multilayer film transmitting light in a specific wavelength region and being two or more layers of film including a crystalline first film and a crystalline second film that transmits light in a specific wavelength region and has a refractive index different from that of the first film, the first film positioned closest the substrate side being formed directly on the crystal orientation surface, the ratio of the refractive index of the first film and the refractive index of the second film with respect to light in a region that is half to two times the center wavelength of the specific wavelength region being 1.01-2.50, each film of the multilayer film having crystallinity, the crystal structure of the first film and the crystal structure of the second film being the same, and the lattice mismatch between the first film and the second film being 0.1-5.0%.

Description

Optical filter and method for manufacturing optical filter

[0001] This application claims priority to Japanese Patent Application No. 2024-121330, filed on July 26, 2024, the contents of which are incorporated herein by reference.

[0002] Optical filters that transmit light in a specific wavelength range are used in a variety of applications. Optical filters are divided into two types: absorption optical filters, such as those disclosed in Patent Document 1, and reflective optical filters. Absorption optical filters transmit light in a specific wavelength range by absorbing some wavelengths. Reflection optical filters are formed by stacking multiple thin films. In reflection optical filters, reflection and transmission are repeated at each interface between the thin films, causing optical interference within the optical filter. As a result, only light in the specific wavelength range passes through the optical filter, and the remaining light is reflected without passing through the optical filter.

[0003] Patent No. 4981456

[0004] However, conventional optical filters have focused on the selectivity of light in a specific wavelength range that passes through the optical filter, and have not controlled the absorption or reflection characteristics of the remaining light. As a result, it has not been possible to utilize the energy of the light absorbed or reflected by the optical filter, and there remains a problem in terms of improving the energy efficiency of incident light.

[0005] The present disclosure has been made to solve the above-mentioned problems, and aims to provide an optical filter that can improve the utilization efficiency of the energy of incident light, and a method for manufacturing an optical filter.

[0006] In order to solve the above-mentioned problems, the optical filter according to the present disclosure comprises a crystalline substrate and a multilayer film formed on a crystal orientation plane of the substrate, the multilayer film being two or more layers including a crystalline first film that transmits light in a specific wavelength region and is laminated on the first film, the crystalline second film that transmits light in the specific wavelength region and has a refractive index different from that of the first film, the first film being located closest to the substrate being formed directly on the crystal orientation plane, the ratio of the refractive index of the first film to the refractive index of the second film for light in a region from half to twice the center wavelength of the specific wavelength region being 1.01 or more and 2.50 or less, each film of the multilayer film being crystalline, the crystalline structure of the first film being identical to the crystalline structure of the second film, and the lattice mismatch between the first film and the second film being 0.1% or more and 5.0% or less.

[0007] The method for manufacturing an optical filter according to the present disclosure is a method for manufacturing an optical filter for manufacturing the above-mentioned optical filter, and includes a first film formation process for forming the first film on the substrate by a mist CVD method, and a second film formation process for forming the second film on the substrate by a mist CVD method.

[0008] According to the optical filter and the method for manufacturing the optical filter of the present disclosure, it is possible to improve the efficiency of use of the energy of incident light.

[0009] FIG. 1 is a schematic configuration diagram showing an application example of an optical filter according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram showing an optical filter according to an embodiment of the present disclosure. FIG. 3 is a schematic diagram showing an example of a manufacturing apparatus for an optical filter according to an embodiment of the present disclosure. FIG. 4 is a flowchart showing the steps of a manufacturing method for an optical filter according to an embodiment of the present disclosure. FIG. 5 is a diagram showing film formation conditions for a first film according to Example 1 of the present disclosure. FIG. 6 is a diagram showing film formation conditions for a second film according to Example 1 of the present disclosure. FIG. 7 is a diagram showing a multilayer film formed on a substrate according to Example 1 of the present disclosure. FIG. 8 is a diagram showing design values ​​for thicknesses of each layer of the multilayer film according to Example 1 of the present disclosure. FIG. 9 is a diagram showing optical characteristics of an optical filter according to Example 1 of the present disclosure. FIG. 10 is a diagram showing film formation conditions for a first film according to Example 2 of the present disclosure. FIG. 11 is a diagram showing film formation conditions for a second film according to Example 2 of the present disclosure. FIG. 12 is a diagram showing elements that can form an oxide film by the mist CVD method of the present disclosure.

[0010] An optical filter 10 according to an embodiment of the present disclosure and a method for manufacturing the optical filter 10 will be described below with reference to FIGS.

[0011] As shown in FIG. 1 , the optical filter 10 is applied to, for example, a drying oven 1. The drying oven 1 includes a heater 2 and an optical filter 10. The optical filter 10 is disposed between the heater 2 and an object to be dried 3. The heater 2 irradiates light toward the object to be dried 3. The wavelength of the light emitted from the heater 2 has a distribution similar to a blackbody radiation distribution in which the power of light at the center wavelength of a specific wavelength range is greatest, with the distribution narrow on the shorter wavelength side and wide on the longer wavelength side of the center wavelength. The optical filter 10 transmits light in the specific wavelength range from the light (incident light 4 a) irradiated from the heater 2. The light in the specific wavelength range can pass through the optical filter 10 without being absorbed or reflected for the most part. The target wavelength range is an asymmetric region from the center wavelength, from a region close to the center wavelength to a region far from the center wavelength.

[0012] When the optical filter 10 is applied to the drying furnace 1, the specific wavelength range is a wavelength range of light that is easily absorbed by the liquid (solvent, water, etc.) contained in the object to be dried 3. The light that has passed through the optical filter 10 (transmitted light 4b) is irradiated onto the object to be dried 3. The liquid contained in the object to be dried 3 is evaporated by the energy of the transmitted light 4b. For example, when the object to be dried 3 is a coating film applied to a film or a substrate, the drying furnace 1 selectively irradiates infrared light of a wavelength that is easily absorbed by the solvent in the coating film using the optical filter 10. This causes the solvent in the coating film to evaporate without heating the film or substrate.

[0013] Furthermore, most of the remaining light that does not pass through the optical filter 10 is reflected by the optical filter 10. The light reflected by the optical filter 10 (reflected light 4c) is returned to the heater 2. As a result, the energy of the reflected light 4c is reused for operating the heater 2, etc.

[0014] Although the optical filter 10 is described here as being used in the drying oven 1, the present invention is not limited to this. The optical filter 10 can also be used in applications other than the drying oven 1.

[0015] (Optical Filter) The following describes the configuration of the optical filter 10. The optical filter 10 selectively transmits light in a specific wavelength range. The optical filter 10 may also be referred to as, for example, a "laminated structure" or an "optical multilayer film."

[0016] For example, when the optical filter 10 is applied to the drying oven 1 described above, the specific wavelength range is 0.1 μm or more and 12.0 μm or less, preferably 1.0 μm or more and 8.0 μm or less, and more preferably 2.5 μm or more and 3.5 μm or less. Note that, although the embodiment is described here using as an example a case where the specific wavelength range is in the infrared range and the optical filter 10 selectively transmits light in the infrared range, this is not limiting. The specific wavelength range can be changed as appropriate and is set as appropriate depending on the application to which the filter is applied.

[0017] As shown in FIG. 2 , the optical filter 10 includes a substrate 5 and a multilayer film 13. The substrate 5 is crystalline. The substrate 5 transmits light in a specific wavelength region. The multilayer film 13 is formed on a crystal orientation plane 5a of the substrate 5. The "crystal orientation plane" here refers to a plane that forms the surface of a crystal, and is also referred to as a "crystal plane." The multilayer film 13 is a film consisting of two or more layers, including a first film 11 and a second film 12 stacked on the first film 11. Hereinafter, the multilayer film 13 formed by stacking the first film 11 and the second film 12 may be simply referred to as the multilayer film 13. The first film 11 and the second film 12 have different refractive indices for light in at least a specific wavelength region. The first film 11 and the second film 12 are alternately stacked. In the following, the stacking direction of the first film 11 and the second film 12 will be simply referred to as the "stacking direction." In this manner, the optical filter 10 is composed of a multilayer film 13 having a two-layer or more structure. The number of layers in the multilayer film 13 that constitutes the optical filter 10 can be changed as appropriate, and the specific wavelength range is set as appropriate depending on the application to which it is applied.

[0018] Both the first film 11 and the second film 12 are crystalline films that transmit light in a specific wavelength range. "Crystalline" here refers to the ability to confirm crystalline peaks, for example, by X-ray diffraction measurement. For example, both the first film 11 and the second film 12 are crystalline oxide films. In this case, the first film 11 may be referred to as a first crystalline oxide film, and the second film 12 may be referred to as a second crystalline oxide film. The closer each film is to a single crystal, the more uniform the refractive index distribution and the higher the optical properties. The greater the lattice matching, the larger the single crystals will be in the thickness direction and in the in-plane direction of the film. Conversely, if lattice mismatching continues in the film thickness direction, a crystal interface will result, preventing the formation of large single crystals and resulting in unwanted reflections at the crystal interface. Since the thickness of each single crystal is not uniform, the surface roughness will increase, affecting the crystallinity of the upper layer. The mismatched region has a different refractive index from the lattice-matched single crystal portion, and the refractive index gradually changes from the single crystal region toward the interface, preventing the desired interface reflection characteristics from being achieved. Therefore, each film is required to have high crystallinity, meaning that the thickness direction where there is lattice mismatch from the interface is short, and each crystal is wide so that boundaries between crystals do not appear. In particular, to minimize the lattice mismatch with the substrate 5, it is preferable to adjust the crystal orientation plane 5a of the first film 11, which is in contact with the substrate 5, so that the lattice constant of the crystalline substrate 5 is close to that of the first film 11. Furthermore, it is preferable to select a material for the first film 11 whose lattice constant is closer to that of the substrate 5 than that of the second film 12. Furthermore, the first film 11, which is located closest to the substrate 5, is formed directly on the crystal orientation plane 5a.

[0019] The thickness of the optical filter 10 in the stacking direction (hereinafter simply referred to as "thickness") is, for example, 1.0 μm or more and 100.0 μm or less. The thickness of the optical filter 10 is preferably 1.0 μm or more and 50.0 μm or less, and more preferably 1.0 μm or more and 25.0 μm or less. The thickness of one layer of the first film 11 is, for example, 0.01 μm or more and 2.00 μm or less. The thickness of one layer of the first film 11 is preferably 0.01 μm or more and 6.00 μm or less, and more preferably 0.01 μm or more and 10.00 μm or less. The thickness of one layer of the second film 12 is, for example, 0.01 μm or more and 2.00 μm or less. The thickness of one layer of the first film 11 is preferably 0.01 μm or more and 6.00 μm or less, and more preferably 0.01 μm or more and 10.00 μm or less. The first film 11 on the substrate 5 not only functions as the optical filter 10, but may also function to control the crystallinity of the film. The thickness of the optical filter 10, the thickness of one layer of the first film 11, and the thickness of one layer of the second film 12 can be changed as appropriate. The thickness of the optical filter 10, the thickness of one layer of the first film 11, and the thickness of one layer of the second film 12 are set as appropriate depending on the application to which the optical filter 10 is applied. The thickness of the optical filter 10 and the thickness of each film in the stacking direction are measured, for example, by obtaining a cross-sectional SEM (Scanning Electron Microscope) image.

[0020] In this embodiment, for example, two crystalline oxide films that satisfy the following condition (1) are selected as the first film 11 and the second film 12. (1) The ratio of the refractive index of the first film 11 to the refractive index of the second film 12 for light in a region from half to twice the central wavelength of the specific wavelength region is 1.01 or more and 2.50 or less. Here, if a wavelength 0.5 times the central wavelength of the specific wavelength region is defined as the "first wavelength" and a wavelength 2.0 times the central wavelength of the specific wavelength region is defined as the "second wavelength," the "region from half to twice the central wavelength of the specific wavelength region" refers to the "region from the first wavelength to the second wavelength." Furthermore, if the lower of the refractive indices of the first film 11 and the second film 12 is defined as the "first refractive index" and the higher of the refractive indices is defined as the "second refractive index," the "ratio of the refractive index of the first film 11 to the refractive index of the second film 12" refers to the "second refractive index relative to the first refractive index," i.e., "(second refractive index) / (first refractive index)."

[0021] Regarding (1), the ratio of the refractive index of the first film 11 to the refractive index of the second film 12 for light in a specific wavelength region is preferably 1.20 or more and 2.50 or less, and more preferably 1.50 or more and 2.50 or less.

[0022] Furthermore, it is preferable that the first film 11 and the second film 12 satisfy the following conditions (2) to (4): (2) The absorptance of the first film 11 for light in a specific wavelength range is 0.01% or more and 20.00% or less in the range from half to twice the center wavelength of the specific wavelength range, and the absorptance of the second film 12 for light in the specific wavelength range is 0.01% or more and 20.00% or less in the range from half to twice the center wavelength of the specific wavelength range. (3) Each film of the multilayer film 13 has crystallinity. (4) The crystal structure of the first film 11 and the crystal structure of the second film 12 are identical, and the lattice mismatch between the first film 11 and the second film 12 is 0.1% or more and 5.0% or less.

[0023] Regarding (2), the absorptance of the first film 11 for light in the specific wavelength range is preferably 0.01% (measurement limit) to 50.00% inclusive, more preferably 0.01% (measurement limit) to 20.00% inclusive, in the range from one-third to three times the central wavelength of the specific wavelength range. The absorptance of the second film 12 for light in the specific wavelength range is preferably 0.01% (measurement limit) to 50.00% inclusive, more preferably 0.01% (measurement limit) to 20.00% inclusive, in the range from one-third to three times the central wavelength of the specific wavelength range.

[0024] Regarding (3), it is desirable that the multilayer film 13 made up of the first film 11 and the second film 12 has high crystallinity as a whole.

[0025] Regarding (4), lattice mismatch is also called lattice mismatch. Lattice mismatch is the degree of difference in lattice constants. The lattice mismatch here means, for example, the degree of difference between the lattice constant (a1) of the first film 11 and the lattice constant (a2) of the second film 12. The lattice mismatch can be expressed, for example, by the following formula:

[0026]

[0027] The lattice mismatch between the first film 11 and the second film 12 is preferably 1.0% or more and 5.0% or less, and more preferably 1.0% or more and 3.0% or less.

[0028] Furthermore, it is more preferable that the first film 11 and the second film 12 satisfy the following conditions (5) to (8): (5) The first film 11 and the second film 12 can each be formed as a single film with high crystallinity. (6) The first film 11 and the second film 12 have a structure that is stable against temperature changes. (7) The surfaces of the first film 11 and the second film 12 are flat. (8) Neither the first film 11 nor the second film 12 are made of materials that have a negative impact on the environment.

[0029] Regarding (5), it is preferable that at least one of the first film 11 and the second film 12 is in the most thermally stable phase, and it is more preferable that both the first film 11 and the second film 12 are in the most thermally stable phase.

[0030] Regarding (6), the term "stable" here means that the first film 11 and the second film 12 do not change into another phase that does not retain the refractive index. It is considered preferable that the structure of both the first film 11 and the second film 12 does not change when heated or cooled.

[0031] Regarding (7), for example, the surface roughness of the first film 11 and the surface roughness of the second film 12 are preferably on the order of several nanometers. Specifically, they are set to be smaller than the wavelengths in the near-infrared to visible light range (wavelengths of approximately 1 μm), which are shorter than the specific wavelength range. The surface roughness of the first film 11 is 1 nm to 30 nm, preferably 1 nm to 20 nm, and more preferably 1 nm to 10 nm. The surface roughness of the second film 12 is 1 nm to 30 nm, preferably 1 nm to 20 nm, and more preferably 1 nm to 10 nm. The surface roughness of the first film 11 and the second film 12 is measured, for example, using a scanning probe microscope.

[0032] Regarding (8), it is preferable that the first film 11 and the second film 12 are made of a material that has a low environmental impact. For example, it is considered preferable that the first film 11 and the second film 12 are made of a material that suppresses the generation of pollutants and greenhouse gases during production and disposal.

[0033] (Method of Manufacturing Optical Filter) Next, a method of manufacturing the optical filter 10 will be described. The optical filter 10 is manufactured by respectively forming a first film 11 and a second film 12. Examples of film formation methods include mist CVD (Chemical Vapor Deposition), electron beam evaporation, ion beam evaporation, sputtering, HVPE (Hydride Vapor Phase Epitaxy), and plasma CVD. Of these, the mist CVD method is the most preferable. The mist CVD method is a technique in which a raw material solution 32a is atomized or converted into droplets to generate mist 32b, which is then heated in a film formation chamber 43 to thermally react with the mist 32b on the substrate 5 to form a film. Below, a case in which the first film 11 and the second film 12 are formed using the mist CVD method will be described.

[0034] (Mist CVD Apparatus) An example of the configuration of a mist CVD apparatus will be described with reference to Fig. 3. The mist CVD apparatus includes a mist generation source 31, a container 33, an ultrasonic vibrator 34, an inlet pipe 35, a first switching valve 36a, a second switching valve 36b, a carrier gas source 37, a carrier gas pipe 38, a first flow rate control valve 39, a dilution gas source 40, a dilution gas pipe 41, a second flow rate control valve 42, a film formation chamber 43, a holding table 44, and a heater 45.

[0035] The mist source 31 contains a raw solution 32a. The bottom of the mist source 31 is disposed within a container 33. Water is stored within the container 33. An ultrasonic vibrator 34 is also provided within the container 33. The ultrasonic vibrator 34 vibrates the raw solution 32a, generating mist 32b. A preferred example of a method for atomizing the raw solution 32a is to vibrate the raw solution 32a using the ultrasonic vibrator 34, but the raw solution 32a may be atomized by other methods. In this embodiment, the mist source 31 includes a first mist source 31a containing the raw solution 32a for the first film 11 and a second mist source 31b containing the raw solution 32a for the second film 12. The mist source 31 is connected to a film formation chamber 43 via an inlet pipe 35. In this embodiment, a first switching valve 36a is provided at the connection between the first mist generation source 31a and the inlet pipe 35, and a second switching valve 36b is provided at the connection between the second mist generation source 31b and the inlet pipe 35. By opening either the first switching valve 36a or the second switching valve 36b and closing the other, it is possible to connect only one of the first mist generation source 31a and the second mist generation source 31b to the film formation chamber 43.

[0036] The carrier gas source 37 is connected to the mist generation source 31 via a carrier gas pipe 38. The carrier gas source 37 supplies a carrier gas 37a to the mist generation source 31. The carrier gas 37a transports the resulting mist 32b to the film formation chamber 43. In this embodiment, a carrier gas source 37 is provided for each mist generation source 31. The carrier gas 37a is not particularly limited, and may be, for example, an inert gas such as oxygen, air, ozone, or nitrogen. A first flow rate control valve 39 is provided in the carrier gas pipe 38. The first flow rate control valve 39 controls the flow rate of the carrier gas 37a delivered from the carrier gas source 37.

[0037] The dilution gas source 40 is connected to the introduction pipe 35 by a dilution gas pipe 41. The dilution gas pipe 41 is provided upstream of the first selector valve 36a or the second selector valve 36b. The dilution gas source 40 supplies a dilution gas 40a to the introduction pipe 35. In this embodiment, a dilution gas source 40 is provided for each mist generating source 31. The dilution gas 40a is not particularly limited, but may be, for example, an inert gas such as oxygen, air, ozone, or nitrogen. The dilution gas pipe 41 is provided with a second flow rate control valve 42. The second flow rate control valve 42 controls the flow rate of the dilution gas 40a delivered from the dilution gas source 40.

[0038] A substrate 5 used as a base is placed in the film formation chamber 43. A multilayer film 13 consisting of a first film 11 and a second film 12 is laminated on the substrate 5. The film formation chamber 43 is, for example, a quartz tube. A holding table 44 is installed in the film formation chamber 43. The holding table 44 holds the substrate 5 in a fixed position. The holding table 44 is made of, for example, quartz. The surface of the holding table 44 on which the substrate 5 is placed is inclined with respect to the horizontal plane and faces the inlet for the mist 32b. A heater 45 is also attached to the periphery of the film formation chamber 43. The heater 45 heats the film formation chamber 43 to a temperature suitable for film formation depending on the type of raw material solution 32a. The pressure within the film formation chamber 43 is not particularly limited, but atmospheric pressure is preferable. The substrate 5 used as a base is appropriately selected depending on the type of film to be formed on the substrate 5. For example, a substrate 5 having a crystalline structure similar to that of the film to be formed on the substrate 5 is considered preferable.

[0039] The configuration of the mist CVD apparatus is not limited to the above. For example, in the present embodiment, the first mist generating source 31a and the second mist generating source 31b are provided. However, only one mist generating source 31 may be provided. In this case, when forming the first film 11, the raw material solution 32a for the first film 11 is contained in the mist generating source 31. When forming the second film 12, the raw material solution 32a in the mist generating source 31 is replaced, and the raw material solution 32a for the second film 12 is contained in the mist generating source 31. In addition, the carrier gas 37a and the dilution gas 40a are appropriately selected depending on the type of raw material solution 32a in the mist generating source 31.

[0040] (Method of Manufacturing Optical Filter) Next, the steps of a method of manufacturing the optical filter 10 using the mist CVD method will be described with reference to the flow shown in Fig. 4. The method of manufacturing the optical filter 10 includes a first film formation step (step S1), a second film formation step (step S2), and a determination step (step S3).

[0041] (First Film Formation Process) In the first film formation process, a first film 11 is formed on the substrate 5 by mist CVD. Note that "film formation on the substrate 5" refers not only to direct film formation on the substrate 5 but also to film formation on a film already formed on the substrate 5 (such as the first film 11 or second film 12). In the first film formation process, an operator opens the first switching valve 36a and closes the second switching valve 36b to generate mist 32b from the first mist generating source 31a. This mist 32b is sent into the film formation chamber 43 by the carrier gas 37a. At this time, a dilution gas 40a is sent to the carrier gas 37a to adjust the concentration of the carrier gas 37a. The operator then heats the film formation chamber 43 to a set temperature using the heater 45, causing the mist 32b to react on the substrate 5 to form the first film 11. After the first film formation process, a second film formation process is performed.

[0042] (Second Film Formation Process) In the second film formation process, a second film 12 is formed on the substrate 5 by mist CVD. In the second film formation process, an operator opens the second switching valve 36b, closes the first switching valve 36a, and generates mist 32b from the second mist generation source 31b. This mist 32b is sent into the film formation chamber 43 by the carrier gas 37a. At this time, a dilution gas 40a is sent to the carrier gas 37a to adjust the concentration of the carrier gas 37a. Then, an operator heats the inside of the film formation chamber 43 to a set temperature using the heater 45, causing the mist 32b to react on the substrate 5 and forming the second film 12. After the second film formation process, a determination process is performed.

[0043] (Determination Step) In the determination step, it is determined whether the multilayer film 13 consisting of the first film 11 and the second film 12 has been formed to the target thickness. If the multilayer film 13 has not been formed to the target thickness (step S3; NO), the process returns to the first film formation step. As a result, the first film 11 and the second film 12 are alternately stacked, and the multilayer film 13 is formed to the target thickness. If the multilayer film 13 has been formed to the target thickness (step S3; YES), the formation of the multilayer film 13 is completed. Through the above steps, the manufacture of the optical filter 10 is completed.

[0044] Although the first and second film formation processes have been described in terms of the case where the first film formation process is performed first and then the second film formation process, this is not limiting. The second film formation process may be performed first and then the first film formation process. Furthermore, if the multilayer film 13 has been stacked to a desired thickness, the formation of the multilayer film 13 may be completed after the first film formation process without performing the second film formation process. Furthermore, after the optical filter 10 is completed, the multilayer film 13 may be peeled off from the substrate 5, if necessary.

[0045] (Operational Effects) The optical filter 10 configured as described above and the method for manufacturing the optical filter 10 provide the following operational effects.

[0046] In this embodiment, the optical filter 10 includes a crystalline substrate 5 and a multilayer film 13 formed on a crystal orientation plane 5a of the substrate 5. The multilayer film 13 is a two-layer film including a crystalline first film 11 that transmits light in a specific wavelength range and a crystalline second film 12 that is stacked on the first film 11 and transmits light in the specific wavelength range and has a refractive index different from that of the first film 11. The first film 11, which is located closest to the substrate 5, is formed directly on the crystal orientation plane 5a. The ratio of the refractive index of the first film 11 to the refractive index of the second film 12 for light in a range from half to twice the center wavelength of the specific wavelength range is 1.01 or more and 2.50 or less. Each film in the multilayer film 13 is crystalline. The crystal structure of the first film 11 and the crystal structure of the second film 12 are identical, and the lattice mismatch between the first film 11 and the second film 12 is 0.1% or more and 5.0% or less.

[0047] This allows the optical filter 10 to transmit or reflect most of the light in the specific wavelength region without absorbing it. As a result, the energy of light that passes through the optical filter 10 is utilized at the light's destination, and the light reflected by the optical filter 10 is utilized at the light's reflection destination. This allows most of the incident energy to be utilized effectively, improving the utilization efficiency of the incident energy. Furthermore, the greater the separation of transmittance and reflectance between the specific wavelength region and other wavelength regions, the better the energy utilization performance of the drying oven 1. Furthermore, the greater the refractive index ratio of the film and the greater the total number of films, the clearer the separation of transmittance and reflectance.

[0048] In this embodiment, the absorptance of the first film 11 for light in the specific wavelength region is 0.01% or more and 20.00% or less in the region from half to twice the central wavelength of the specific wavelength region, and the absorptance of the second film 12 for light in the specific wavelength region is 0.01% or more and 20.00% or less in the region from half to twice the central wavelength of the specific wavelength region.

[0049] As a result, the optical filter 10 can transmit most of the light in the specific wavelength range without absorbing it. Also, the optical filter 10 can reflect light on the shorter and longer wavelength sides than the specific wavelength range without absorbing it. Therefore, most of the incident energy can be utilized at the destination of the transmitted light.

[0050] In this embodiment, at least one of the first film 11 and the second film 12 is in the most thermally stable phase.

[0051] This suppresses changes in the crystal structure of the first film 11 and the second film 12, thereby maintaining good transmittance, reflectance, and absorptance for light in a specific wavelength range. Furthermore, the occurrence of defects in the optical filter 10 is suppressed, thereby improving the quality of the optical filter 10.

[0052] In this embodiment, the specific wavelength region of the optical filter 10 is the infrared region.

[0053] In this case, the optical filter 10 can transmit most of the light in the infrared region without absorbing it. Also, the optical filter 10 can reflect light with wavelengths shorter and longer than the infrared region without absorbing it. Therefore, in the infrared region and the region near the infrared region, most of the incident energy can be utilized at the light transmission destination.

[0054] The manufacturing method of the optical filter 10 of this embodiment includes a first film formation process (step S1) of forming a first film 11 on a substrate 5 by a mist CVD method, and a second film formation process (step S2) of forming a second film 12 on the substrate 5 by a mist CVD method.

[0055] This allows the film formation speed to be faster than when the first film 11 and the second film 12 are formed by electron beam evaporation, sputtering, or the like, while maintaining high crystallinity of the multilayer film 13 made up of the first film 11 and the second film 12. Furthermore, since film formation can be performed under atmospheric pressure, there is no need to form the first film 11 and the second film 12 in a vacuum space in the film formation chamber 43. This allows the lead time and manufacturing time to be shortened, while also reducing manufacturing costs.

[0056] Example 1 Next, Example 1 of the optical filter 10 described in the above embodiment and the manufacturing method of the optical filter 10 will be described with reference to FIGS. 5 to 9. The inventors formed both the first film 11 and the second film 12 by mist CVD. The first film 11 was made of α-Fe ２ O ３The second film 12 is a single crystal film of α-Ga (α-phase iron oxide). ２ O ３ (α-phase gallium oxide) single crystal film. ２ O ３ (α-phase iron oxide) single crystal film and α-Ga ２ O ３ The optical filter 10 having the multilayer film 13 formed by laminating a single crystal film (of α-phase gallium oxide) satisfies the conditions (1) to (8) of the above-described embodiment.

[0057] For example, α-Fe ２ O ３ The refractive index of α-Ga is 3.00. ２ O ３ The refractive index of α-Fe is known to be 2.00. ２ O ３ and α-Ga ２ O ３ Both have a corundum structure. ２ O ３ is the most thermally stable phase, and α-Ga ２ O ３ is a thermally metastable phase. ２ O ３ and α-Ga ２ O ３ The lattice mismatch with the SiO 2 film is about 1.0%.

[0058] The inventors used a c-plane sapphire substrate as the base substrate 5 on which the multilayer film 13 is formed, and placed it in the film formation chamber 43. The inventors formed a first film 11 and a second film 12 on the c-plane sapphire substrate. The film formation conditions for the first film 11 and the second film 12 will be described below.

[0059] First, the film formation conditions for the first film 11 shown in Fig. 5 will be described. The inventors used a raw material solution 32a (precursor) containing Fe(acac) ３ The inventors used a solution with a concentration of 0.050 mol / L for the raw material solution 32a. The inventors also used HCl as the solvent with a concentration of 3.0 vol%. The inventors also used O ２The flow rate of the carrier gas 37a was set to 5.0 L / min. ２ The flow rate of the dilution gas 40a was set to 0.5 L / min. As described above, the inventors used a c-plane sapphire substrate as the base substrate 5. The inventors also set the film formation temperature to 500°C.

[0060] Next, the film formation conditions for the second film 12 shown in Fig. 6 will be described. The inventors used a raw material solution 32a (precursor) containing Ga(acac) ３ The inventors used a solution with a concentration of 0.050 mol / L for the raw material solution 32a. The inventors also used HCl as the solvent with a concentration of 3.0 vol%. The inventors also used O ２ The flow rate of the carrier gas 37a was set to 3.0 L / min. ２ The flow rate of the dilution gas 40a was set to 0.5 L / min. As described above, the inventors used a c-plane sapphire substrate as the base substrate 5. The inventors also set the film formation temperature to 500°C.

[0061] The inventors performed the first film formation step before the second film formation step, and formed a first film 11 as a first layer to be formed directly on the substrate 5. Thereafter, the inventors formed second films 12 and first films 11 alternately on the first film 11 formed as the first layer. As a result, a multilayer film 13 was formed in which the first films 11 and second films 12 were alternately stacked in a total of six layers, and the optical filter 10 shown in FIG. 7 was obtained.

[0062] Alternatively, an m-plane sapphire substrate may be used as the substrate 5, and other film formation conditions may be the same as those shown in Figures 5 and 6. Measurement results when an m-plane sapphire substrate is used as the substrate 5 will now be described.

[0063] After forming the multilayer film 13, the inventors measured the thickness (film thickness) of each film constituting the multilayer film 13 in the stacking direction, for example, by obtaining a cross-sectional SEM image of the multilayer film 13. This resulted in the results shown in FIG. 8 . In FIG. 8 , the films constituting the multilayer film 13 are labeled as the first layer, the second layer, and the third layer in order from the substrate 5 side, and the thickness of each film in the stacking direction is shown. As shown in FIG. 8 , it was confirmed that the first layer formed on the substrate 5 was the thinnest, with a thickness of several tens of nanometers, while the thicknesses of the other layers were several hundred nanometers.

[0064] The inventors irradiated the obtained optical filter 10 with light in the infrared region and measured the optical characteristics of the optical filter 10. The results shown in FIG. 9 were obtained. The horizontal axis of FIG. 9 indicates the wavelength of the light irradiated onto the optical filter 10, and the vertical axis of FIG. 9 indicates the energy at each wavelength. The energy shown on the vertical axis of FIG. 9 is energy normalized by the blackbody radiation intensity at 693°C. The intensity of light in the specific wavelength region is maximized when the heater temperature is 693°C, and FIG. 9 shows a distribution that decreases in accordance with the blackbody radiation distribution. The thickness (film thickness) of each film constituting the multilayer film 13 in the stacking direction was preset so as to efficiently transmit light in a specific wavelength region of 2.5 μm to 3.5 μm inclusive, and to reflect light particularly efficiently near the outside of that region.

[0065] 9, it was confirmed that in the wavelength range of 2.5 μm or more and 3.5 μm or less, the energy of transmitted light 4b was approximately 0.55 to 0.80, the energy of reflected light 4c was approximately 0.05 to 0.35, and the energy of absorbed light was suppressed to approximately 0.001 to 0.050. These results demonstrate that the optical filter 10 of Example 1 can suppress light absorption and transmit or reflect light in the specific wavelength range of 2.5 μm or more and can effectively utilize the energy of light in the specific wavelength range of 2.5 μm or more and 3.5 μm or less at the transmitted or reflected light destination.

[0066] In this embodiment, the first film 11 is made of α-Fe ２ O ３ The second film 12 is a single crystal film of α-Ga ２ O ３In this case, both the first film 11 and the second film 12 have a corundum structure, and the lattice mismatch can be suppressed to approximately 1.0%. Therefore, the optical filter 10 of this embodiment can suppress the absorption rate of light in a specific wavelength range of 2.5 μm or more and 3.5 μm or less in the infrared region, and transmit or reflect most of the light. Furthermore, defects in the optical filter 10 can be reduced.

[0067] In this example, α-Fe was deposited by mist CVD. ２ O ３ and α-Ga ２ O ３ By the mist CVD method, the multilayer film 13 can be formed at a film formation rate of, for example, 1 μm / hr or more. ２ O ３ and α-Ga ２ O ３ It is possible to form a film of α-Fe, and the optical properties as designed can be obtained. ２ O ３ and α-Ga ２ O ３ Each of the films can be formed with high purity.

[0068] In the above-described embodiments, several film formation methods are presented. ２ O ３ Therefore, the second film 12 can be formed only by the mist CVD method. ２ O ３ The optical filter 10 of this embodiment can be formed as described above.

[0069] In this example, α-Fe ２ O ３ The first film formation step for forming the first film 11, which is a single crystal film of α-Ga ２ O ３ This is carried out before the second film formation step of forming the second film 12, and the first film 11 is formed first as the first layer that is directly formed on the substrate 5. This procedure allows the α-Fe, which is the thermally most stable phase, to be formed. ２ O ３ After forming the first layer on the substrate 5, a thermally metastable phase of α-Ga２ O ３ This allows the formation of a film of α-Fe. ２ O ３ and α-Ga ２ O ３ The multilayer film 13 can be formed in a thermally stable manner.

[0070] Example 2 Next, Example 2 of the optical filter 10 described in the above embodiment and the manufacturing method of the optical filter 10 will be described with reference to FIGS. 10 and 11. The inventors formed both the first film 11 and the second film 12 by mist CVD. The first film 11 was made of α-Fe ２ O ３ The second film 12 is a single crystal film of α-Al (α-phase iron oxide). ２ O ３ It is a single crystal film of α-Fe (α-phase aluminum oxide). ２ O ３ (α-phase iron oxide) single crystal film and α-Al ２ O ３ The optical filter 10 having the multilayer film 13 formed by laminating a single crystal film (of α-phase aluminum oxide) satisfies the conditions (1) to (8) of the above-described embodiment.

[0071] For example, it is known that the refractive index of the first film 11 is 3.00, and the refractive index of the second film 12 is 1.70. ２ O ３ (α-phase iron oxide) and α-Al ２ O ３ (α-phase aluminum oxide) both have a corundum structure. ２ O ３ (α-phase iron oxide) and α-Al ２ O ３ (α-phase aluminum oxide) are the most thermally stable phases. ２ O ３ (α-phase iron oxide) and α-Al ２ O ３ The lattice mismatch with α-phase aluminum oxide is about 5.0%.

[0072] The inventors used a c-plane sapphire substrate as the base substrate 5 on which the multilayer film 13 is formed, and placed it in the film formation chamber 43. The inventors formed a first film 11 and a second film 12 on the c-plane sapphire substrate. The film formation conditions for the first film 11 and the second film 12 will be described below.

[0073] First, the film formation conditions for the first film 11 shown in Fig. 10 will be described. The inventors used a raw material solution 32a (precursor) containing Fe(acac) ３ The inventors used a solution with a concentration of 0.05 mol / L for the raw material solution 32a. The inventors also used HCl as the solvent with a concentration of 3.0 vol%. The inventors also used O ２ The flow rate of the carrier gas 37a was set to 5.0 L / min. ２ The flow rate of the dilution gas 40a was set to 0.5 L / min. As described above, the inventors used a c-plane sapphire substrate as the base substrate 5. The inventors also set the film formation temperature to 500°C.

[0074] Next, the film formation conditions for the second film 12 shown in Fig. 11 will be described. The inventors used Al(acac) as the raw material solution 32a (precursor). ３ The inventors used a solution with a solution concentration of 0.02 mol / L for the raw material solution 32a. The inventors used a mixture of distilled water and methanol in a ratio of 10:90 as the solvent. The inventors circulated air as the carrier gas 37a and set the flow rate of the carrier gas 37a to 2.5 L / min. The inventors circulated air as the dilution gas 40a and set the flow rate of the dilution gas 40a to 4.5 L / min. As described above, the inventors used a c-plane sapphire substrate as the base substrate 5. The inventors set the film formation temperature to 450°C.

[0075] The inventors performed the first film formation process before the second film formation process, and formed a first film 11 as a first layer to be formed directly on the substrate 5. Thereafter, the inventors alternately formed second films 12 and first films 11 on the first film 11 formed as the first layer. This formed a multilayer film 13 in which the first films 11 and the second films 12 were alternately stacked in multiple layers. Note that, in this embodiment as well, the second film formation process may be performed before the first film formation process, and the second film 12 may be formed as a first layer to be formed directly on the substrate 5.

[0076] In this embodiment, the first film 11 is made of α-Fe ２ O ３ The second film 12 is a single crystal film of α-Al ２ O ３ In this case, both the first film 11 and the second film 12 have a corundum structure, and the lattice mismatch can be suppressed to approximately 5.0%. Therefore, according to the optical filter 10 of this embodiment, similar to the first embodiment, the absorption rate of light in a desired specific wavelength range in the infrared range can be suppressed, and most of the light can be transmitted or reflected. Furthermore, defects in the optical filter 10 can be reduced.

[0077] In this example, α-Fe was deposited by mist CVD. ２ O ３ and α-Al ２ O ３ By the mist CVD method, the multilayer film 13 can be formed at a film formation rate of, for example, 1 μm / hr or more. ２ O ３ and α-Al ２ O ３ This allows the deposition of a film that achieves the designed optical properties.

[0078] The first film 11 and the second film 12 constituting the optical filter 10 of this embodiment can be formed by a film formation method other than the mist CVD method, such as a sputtering method, an HVPE method, or a plasma CVD method.

[0079] (Other Embodiments) Although the embodiments of the present disclosure have been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and design changes and the like within the scope that does not deviate from the gist of the present disclosure are also included.

[0080] In the above embodiment, the optical filter 10 is used in the drying furnace 1, but the present invention is not limited to this. The optical filter 10 can be applied to various devices and facilities that recover energy from unused waste heat, such as industrial waste heat. The optical filter 10 may also be applied to plant facilities that generate a large amount of waste heat, such as power plants, incineration facilities, and chemical plants.

[0081] Furthermore, the number of layers in the multilayer film 13 that constitutes the optical filter 10 is not limited to a specific number such as six layers, but can be changed as appropriate.

[0082] Although the embodiment has been described taking as an example a case where the optical filter 10 is configured only by the multilayer film 13 in which the first film 11 and the second film 12 are alternately stacked, the present invention is not limited to this. The optical filter 10 may have, for example, a film that coats the multilayer film 13 or another film between the first film 11 and the second film 12, as long as the intended optical filter 10 is not impaired.

[0083] Furthermore, the combination of the first film 11 and the second film 12 is not limited to the combination in the above-described embodiment. For example, the first film 11 and the second film 12 may be two oxide films that satisfy at least the above condition (1) from a group of oxides that can be formed by the mist CVD method, as shown in Fig. 12. This makes it possible to manufacture optical filters 10 other than those in the above-described embodiments 1 and 2 (optical filters 10 that are transparent in regions other than the infrared region, or optical filters 10 that do not require transmittance) so as to satisfy the optical characteristics (good transmittance and reflectance) in the desired specific wavelength region.

[0084] <Additional Notes> The optical filter 10 described in each embodiment can be understood, for example, as follows.

[0085] (1) An optical filter 10 according to a first aspect comprises a crystalline substrate 5 and a multilayer film formed on a crystal orientation plane 5 a of the substrate 5, the multilayer film 13 being a two-layer film or more including a crystalline first film 11 that transmits light in a specific wavelength region and is laminated on the first film 11, the crystalline second film 12 that transmits light in the specific wavelength region and has a refractive index different from that of the first film 11, the first film 11 being closest to the substrate 5 being formed directly on the crystal orientation plane 5 a, the ratio of the refractive index of the first film 11 to the refractive index of the second film 12 for light in a region from half to twice the center wavelength of the specific wavelength region being 1.01 or more and 2.50 or less, each film of the multilayer film 13 being crystalline, the crystalline structure of the first film 11 being identical to the crystalline structure of the second film 12, and the lattice mismatch between the first film 11 and the second film 12 being 0.1% or more and 5.0% or less.

[0086] This allows the optical filter 10 to transmit or reflect most of the light in the specific wavelength range without absorbing it, so that the energy of the light that passes through the optical filter 10 is utilized at the destination where the light is transmitted, and the light that is reflected by the optical filter 10 is utilized at the destination where the light is reflected.

[0087] (2) The optical filter 10 of a second aspect is the optical filter 10 of (1), wherein the transmittance or absorptance of the first film 11 for light in the specific wavelength region may be 0.01% or more and 20.00% or less in a region from half to twice the central wavelength of the specific wavelength region, and the absorptance of the second film 12 for light in the specific wavelength region may be 0.01% or more and 20.00% or less in a region from half to twice the central wavelength of the specific wavelength region.

[0088] This allows the optical filter 10 to transmit most of the light in the specific wavelength range without absorbing it, and also allows the optical filter 10 to reflect light on the shorter and longer wavelength sides than the specific wavelength range without absorbing it.

[0089] (3) The optical filter 10 of a third aspect is the optical filter 10 of (1) or (2), in which at least one of the first film 11 and the second film 12 may be in the most thermally stable phase.

[0090] This suppresses changes in the crystal structure of the first film 11 and the second film 12, thereby maintaining good transmittance, reflectance, and absorptance for light in a specific wavelength range.

[0091] (4) The optical filter 10 of a fourth aspect is the optical filter 10 of any one of (1) to (3), wherein the specific wavelength range may be an infrared range.

[0092] In this embodiment, the optical filter 10 can transmit most of the light in the infrared region without absorbing it, and can also reflect light on the shorter and longer wavelength sides than the infrared region without absorbing it.

[0093] (5) The optical filter 10 of a fifth aspect is the optical filter 10 of any one of (1) to (4), wherein the first film 11 is made of α-Fe ２ O ３ The second film 12 is a single crystal film of α-Ga ２ O ３ It may also be a single crystal film.

[0094] In this embodiment, the first film 11 and the second film 12 both have a corundum structure, and the lattice mismatch can be suppressed to about 1.0%.

[0095] (6) The optical filter 10 of the sixth aspect is the optical filter 10 of any one of (1) to (4), wherein the first film 11 is made of α-Fe ２ O ３ The second film 12 is a single crystal film of α-Al ２ O ３ It may also be a single crystal film.

[0096] In this embodiment, the first film 11 and the second film 12 both have a corundum structure, and the lattice mismatch can be suppressed to about 5.0%.

[0097] (7) A seventh aspect of the manufacturing method of the optical filter 10 is a manufacturing method of the optical filter 10 for manufacturing the optical filter 10 of (1), and includes a first film formation process of forming the first film 11 on the substrate 5 by a mist CVD method, and a second film formation process of forming the second film 12 on the substrate 5 by a mist CVD method.

[0098] This allows the film formation speed to be faster than when the first film 11 and the second film 12 are formed by electron beam evaporation, sputtering, etc., while maintaining high crystallinity of the multilayer film 13 made up of the first film 11 and the second film 12. Furthermore, since it is not necessary to form the first film 11 and the second film 12 in a vacuum space, it is possible to reduce the lead time and manufacturing time while also reducing manufacturing costs.

[0099] (8) An eighth aspect of the manufacturing method of the optical filter 10 is the manufacturing method of the optical filter 10 of (7), wherein the first film 11 is α-Fe ２ O ３ The second film 12 is a single crystal film of α-Ga ２ O ３ The first film formation step may be performed before the second film formation step, and the first film 11 may be formed first as a first layer that is formed directly on the substrate 5 .

[0100] In this embodiment, the thermally most stable phase, α-Fe ２ O ３ is formed on the substrate 5 as a first layer, and then α-Ga, which is a thermally metastable phase, is formed. ２ O ３ This allows the formation of a film of α-Fe. ２ O ３ and α-Ga ２ O ３ The multilayer film 13 can be formed in a thermally stable manner.

[0101] According to the optical filter and the method for manufacturing the optical filter of the present disclosure, it is possible to improve the efficiency of use of the energy of incident light.

[0102] REFERENCE SIGNS LIST 1 Drying furnace 2 Heater 3 Object to be dried 4a Incident light 4b Transmitted light 4c Reflected light 5 Substrate 5a Crystal orientation plane 10 Optical filter 11 First film 12 Second film 13 Multilayer film 30 Manufacturing apparatus 31 Mist generating source 31a First mist generating source 31b Second mist generating source 32a Raw material solution 32b Mist 33 Container 34 Ultrasonic vibrator 35 Inlet pipe 36a First switching valve 36b Second switching valve 37 Carrier gas source 37a Carrier gas 38 Carrier gas pipe 39 First flow rate control valve 40 Dilution gas source 40a Dilution gas 41 Dilution gas pipe 42 Second flow rate control valve 43 Film formation chamber 44 Holding stand 45 Heater

Claims

1. An optical filter comprising: a crystalline substrate; and a multilayer film formed on a crystal orientation plane of the substrate, wherein the multilayer film is two or more layers including a crystalline first film that transmits light in a specific wavelength region, and a crystalline second film that is laminated on the first film and transmits light in the specific wavelength region and has a refractive index different from that of the first film; the first film that is closest to the substrate is formed directly on the crystal orientation plane; the ratio of the refractive index of the first film to the refractive index of the second film for light in a region from half to twice the center wavelength of the specific wavelength region is 1.01 or more and 2.50 or less; each film of the multilayer film is crystalline; the crystalline structure of the first film is identical to that of the second film; and the lattice mismatch between the first film and the second film is 0.1% or more and 5.0% or less.

2. The optical filter according to claim 1, wherein the absorptance of said first film for light in said specific wavelength region is 0.01% or more and 20.00% or less in the region from half to twice the central wavelength of said specific wavelength region, and the absorptance of said second film for light in said specific wavelength region is 0.01% or more and 20.00% or less in the region from half to twice the central wavelength of said specific wavelength region.

3. The optical filter according to claim 1, wherein at least one of the first film and the second film is in the most thermally stable phase.

4. The optical filter according to any one of claims 1 to 3, wherein the specific wavelength range is an infrared range.

5. The first film is α-Fe ２ O ３ The second film is a single crystal film of α-Ga ２ O ３ The optical filter according to claim 1 , wherein the optical filter is a single crystal film of the formula:

6. The first film is α-Fe ２ O ３ The second film is a single crystal film of α-Al ２ O ３ The optical filter according to claim 1 , wherein the optical filter is a single crystal film of the formula:

7. A method for manufacturing an optical filter according to claim 1, comprising: a first film-forming step of forming the first film on the substrate by a mist CVD method; and a second film-forming step of forming the second film on the substrate by a mist CVD method.

8. The first film is α-Fe ２ O ３ The second film is a single crystal film of α-Ga ２ O ３ 8. The method for manufacturing an optical filter according to claim 7, wherein the first film is a single crystal film of the above formula (I), the first film formation step is performed before the second film formation step, and the first film is formed first as a first layer that is formed directly on the substrate.

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