Porous film, optical element, optical system, interchangeable lens, optical device, and method for manufacturing porous film

Abstract

Provision of porous membrane. The porous film contains silica particles, has a refractive index of 1.1 to 1.25, and a contact angle with water of 40° or more.

Description

Technical Field

[0001] The present invention relates to a porous film, an optical element, an optical system, an interchangeable lens, an optical device, and a method for producing a porous film.

Background Art

[0002] For example, Patent Document 1 discloses an antireflection low-refractive-index film having a refractive index of 1.28 to 1.38. Such an antireflection film is required to have not only a low refractive index but also excellent environmental resistance.

Prior Art Literature

Patent Literature

[0003]

Patent Document 1

Summary of the Invention

[0004] According to a first aspect, the porous film is a porous film containing silica particles, has a refractive index of 1.1 to 1.25, and has a contact angle to water of 40° or more. According to a second aspect, the porous film is a porous film containing silica particles, has a refractive index of 1.1 to 1.25, and has trimethylsilyl groups on a surface thereof. According to a third aspect, the porous film is a porous film containing silica particles, has a refractive index of 1.1 to 1.25, and has a surface treated with a silane coupling agent. According to a fourth aspect, a method for producing a porous film comprises the steps of: preparing a mixed solution by mixing a silicon compound with a solvent containing a tertiary amine, water, and methoxypropanol (PGME); stirring the mixed solution; applying the stirred mixed solution onto a substrate to form a coating film; and heating the coating film to form the porous film.

Brief Description of the Drawings

[0005] [Figure 1]This is a flowchart illustrating a method for manufacturing a porous membrane in one embodiment. [Figure 2] This is a perspective view of an imaging device in one embodiment. [Figure 3] This is a front view of another example of an imaging device in one embodiment. [Figure 4] This is a rear view of another example of the imaging device in one embodiment. [Figure 5] This figure shows the refractive index of porous films in comparative examples and examples. [Figure 6] This figure shows the scattering of porous films in comparative examples and examples for light at wavelengths of 350 nm and 544 nm. [Figure 7] This figure shows the contact angles with water of the porous membranes in the comparative example and the example. [Figure 8] This figure shows the results of IR measurements performed on the porous membranes of the comparative example and the example, confirming the presence or absence of an absorption band around 1259 cm⁻¹. [Modes for carrying out the invention]

[0006] -Embodiment- A porous membrane according to one embodiment will be described with reference to the drawings. The porous membrane of this embodiment is composed of silica particles (SiO2 particles), has a low refractive index, and is a porous membrane with excellent environmental resistance.

[0007] The porous membrane of this embodiment is composed of a gel network of silica particles and has a structure with numerous pores of several nanometer size within the membrane. The refractive index of the porous membrane of this embodiment is in the range of 1.1 to 1.25, more preferably in the range of 1.17 to 1.23. In this specification, refractive index refers to the refractive index for light with a wavelength of 550 nm. The porous membrane of this embodiment has a contact angle with water of 40° or more, more preferably 45° or more. To achieve this contact angle, the porous membrane has trimethylsilyl groups on its surface. Furthermore, the porous membrane of this embodiment has a scattering of 1000 ppm or less, more preferably 900 ppm or less, at a wavelength of 350 nm.

[0008] The following describes a manufacturing method for producing the porous membrane mentioned above. The porous particles of this embodiment are formed by hydrolyzing and dehydrating a silicon compound under a base catalyst. Tetramethyl orthosilicate (TMOS) is used as the silicon compound. This tetramethyl orthosilicate is added to a solvent containing a tertiary amine, water, and methoxypropanol (PGME) and stirred. For example, triethylamine can be used as the tertiary amine. A predetermined amount of nitric acid may be added to the solvent in the container to extend the liquid life. Stirring is carried out at room temperature. If the temperature is too high, the reaction rate is too fast, making it difficult to control the refractive index of the final porous film. Conversely, if the temperature is too low, the reaction rate is too slow, making the final porous film brittle. Therefore, the temperature during stirring is preferably 15 to 30°C, and more preferably 20 to 25°C. The stirring time is also a condition that affects the refractive index of the formed porous film. The stirring time is set appropriately according to the desired refractive index, but can be in the range of 12 to 100 hours, for example. The longer the stirring time, the lower the refractive index of the porous membrane tends to be. Due to stirring, tetramethyl orthosilicate undergoes hydrolysis as follows, forming silica particles in the solution. Si(OMe)4 + 2H2O → SiO2 + 4MeOH

[0009] The stirred solution is applied to a substrate, and a coating film is formed by a film deposition process. The film deposition process is performed, for example, using a spin coater. By appropriately setting the conditions when using the spin coater, the thickness of the coating film can be set to any desired thickness. In the deposited coating film, silica particles are linked together to form a gel network. This coating film is heated to cure. The heating conditions at this time can be set so that the heating temperature is in the range of 140 to 180°C and the heating time is in the range of 1 to 5 hours. Specifically, the heating temperature can be set to, for example, 160°C and the heating time to, for example, 3 hours. If the heating time is too long, the porous film that is ultimately formed will become brittle, so temperature control is important. Through the heat treatment, the gel network undergoes dehydration condensation, forming a porous film with many pores of several nanometers in size. After heating, the coating film is cooled by standing at room temperature for a predetermined time, and the formation of the porous film is completed.

[0010] The surface of the porous film formed as described above has a large amount of OH groups. In high temperature and high humidity environments, the OH groups on the surface of the porous film condense with each other, causing changes in the refractive index and film thickness of the porous film. Therefore, when a large amount of OH groups are present, the porous film has poor environmental resistance. In this embodiment, the surface of the porous film is treated with a silane coupling agent to reduce the amount of OH groups. The silane coupling agent treatment is performed using hexamethyldisilazane (HMDS). This silane coupling agent treatment may be performed using gas phase treatment, liquid phase treatment, or mist treatment. When performing gas phase treatment, the substrate on which the porous film is formed is left at room temperature for a predetermined time in an environment (sealed container) where hexamethyldisilazane has been vaporized. After that, it is heated at a predetermined temperature for a predetermined time. When performing liquid phase treatment, the substrate on which the porous film is formed is immersed in a solution of hexamethyldisilazane, left for a predetermined time under ultrasonic conditions, and then heated at a predetermined temperature for a predetermined time. When performing mist treatment, the substrate with the porous film formed on it is placed in a container, and the container is filled with hexamethyldisilazane in mist form. After a predetermined time has elapsed, the substrate is removed from the container, washed, and then heated at a predetermined temperature for a predetermined time.

[0011] The silane coupling agent treatment described above causes the OH groups on the surface of the porous membrane to bond (couple) with the trimethylsilyl groups of the silane coupling agent. In other words, trimethylsilyl groups are formed on the surface of the porous membrane. As a result, the contact angle with water of the porous membrane becomes relatively larger compared to before the silane coupling agent treatment, resulting in the value described above. That is, the amount of OH groups on the surface of the porous membrane is reduced by the silane coupling agent treatment, and the changes in refractive index and film thickness of the porous membrane caused by OH groups in high temperature and high humidity environments are suppressed, so the porous membrane has high environmental resistance.

[0012] The method for manufacturing the porous membrane described above will be explained using the flowchart shown in Figure 1. In step S1, tetramethyl orthosilicate (TMOS) is added to a solvent containing a tertiary amine, water, and methoxypropanol (PGME), and the mixture is stirred at room temperature (stirring treatment), before proceeding to step S2. The stirring time (reaction time) is determined based on the refractive index required for the porous membrane to be produced.

[0013] In step S2, the stirred solution is applied to a substrate fixed on the rotating table of the spin coater, and then the rotating table is rotated to form a coating film, after which the process proceeds to step S3. In step S3, the formed coating film is cured by heating, for example, at a heating temperature of 160°C for 3 hours (heat curing treatment) to form a porous film, and the process proceeds to step S4. In step S4, the surface of the porous film is treated with a silane coupling agent to reduce the amount of OH groups on the surface of the porous film. The silane coupling agent treatment is performed using hexamethyldisilazane (HMDS) by one of the following methods: gas phase treatment, liquid phase treatment, or mist treatment. As a result, the porous film of this embodiment is obtained.

[0014] The porous membrane obtained in this manner can be suitably used as an antireflective film. The antireflective film may be a single-layer film or a multi-layer film. When the antireflective film is a multi-layer film, it is known that the higher the refractive index of the membrane material used, or the use of a low refractive index membrane as the outermost layer, can improve optical performance or reduce the number of layers in the multi-layer film even with the same optical performance. In particular, simulation has revealed that by forming only the outermost layer as a low refractive index membrane having a refractive index of 1.30 or less, optical performance can be extremely improved. Since the porous membrane of the present embodiment has a low refractive index of 1.1 to 1.25, it can be suitably used as a component of an antireflective film, and particularly can be suitably used as the outermost layer of a multi-layer film constituting an antireflective film. Note that the outermost layer means the layer farthest from the substrate among the multi-layer film.

[0015] The optical element provided with the above-described antireflective film can be suitably used, for example, as a lens or the like. Examples of optical systems including such lenses include objective lenses, condenser lenses, imaging lenses, interchangeable lenses for cameras, and the like. These can be used in imaging devices such as interchangeable-lens cameras and non-interchangeable-lens cameras, and optical devices such as microscopes. Note that optical devices are not limited to the above-described imaging devices and microscopes, but also include video cameras, teleconverters, telescopes, binoculars, monoculars, laser rangefinders, projectors, and the like. An example of an imaging device is described below.

[0016] Figure 2 is a perspective view of an imaging device having an optical system provided with an antireflective film including the porous membrane of the present embodiment. The image pickup apparatus 1 is a so-called digital single-lens reflex camera (interchangeable-lens camera), and the photographing lens 103 (optical system) includes a lens provided with an antireflection film containing the porous membrane of the present embodiment. A lens barrel 102 is detachably attached to a lens mount (not shown) of a camera body 101. Then, light that has passed through the photographing lens 103 of the lens barrel 102 forms an image on a sensor chip (solid-state image pickup element) 104 of a multichip module 106 disposed on the back side of the camera body 101. The sensor chip 104 is a bare chip such as a so-called CMOS image sensor, and the multichip module 106 is, for example, a COG (Chip On Glass) type module in which the sensor chip 104 is bare-chip mounted on a glass substrate 105.

[0017] FIG. 3 is a front view of another example of an image pickup apparatus having an optical element provided with an antireflection film containing the porous membrane of the present embodiment, and FIG. 4 is a rear view of the image pickup apparatus shown in FIG. 3. This image pickup apparatus CAM is a so-called digital still camera (non-interchangeable-lens camera), and the photographing lens WL (optical system) includes a lens provided with an antireflection film containing the porous membrane of the present embodiment.

[0018] In the image pickup apparatus CAM, when a power button (not shown) is pressed, a shutter (not shown) of the photographing lens WL is opened, light from a subject (object) is condensed by the photographing lens WL, and forms an image on an image pickup element disposed on an image plane. A subject image formed on the image pickup element is displayed on a liquid crystal monitor LM disposed behind the image pickup apparatus CAM. After a photographer determines the composition of the subject image while viewing the liquid crystal monitor LM, the photographer presses down a release button B1 to capture the subject image with the image pickup element, and records and stores the captured image in a memory (not shown). The image pickup apparatus CAM is provided with an auxiliary light emitting unit EF that emits auxiliary light when the subject image is dark, a function button B2 used for setting various conditions of the image pickup apparatus CAM, and the like.

[0019] Higher antireflection performance is required for optical systems used in these cameras and the like. In order to achieve this, it is effective to use the porous membrane of the present embodiment for an antireflection film.

[0020] An example of the porous membrane according to the above-described embodiment will now be explained. [Examples] In this embodiment, the porous membrane is formed by the following procedure. 54.43 grams of 1-methoxy-2-propanol (PGME) (Fujifilm Wako Pure Chemical Industries) are placed in a resin bottle. Next, 36.1 μL of triethylamine (Tokyo Chemical Industries) and 1.731 mL of pure water are measured out using a micropipette and added to the resin bottle. The mixture is then stirred using a magnetic stirrer at a rotation speed of 600 rpm for 5 minutes to form a base solvent. In the following examples, Examples 1-4, 7-10, and 13-16 are for reference only.

[0021] To the above base solvent, 7.31 grams of tetramethyl orthosilicate (TMOS) (Tokyo Chemical Industries) are added and the mixture is stirred at room temperature for a specified time. Further, 27.2 grams of 1-methoxy-2-propanol (PGME) are added to dilute the mixture to a PGME content of 70 wt% to obtain the coating solution. If nitric acid is added to extend the life of the coating solution, 11 μL of nitric acid (1.42) (Fujifilm Wako Pure Chemical Industries) may be added dropwise. The coating solution is placed in a syringe and dropped onto the substrate through a 5.0 μm mesh syringe filter. The substrate with the coating solution is fixed to the spin coater's rotary table, and the rotary table is accelerated to 3000 rpm in 5 seconds, maintained at that speed for 30 seconds, and then decelerated and stopped in 5 seconds. The rotation of the rotary table is controlled according to a preset program. The coating film formed on the substrate by the spin coater is heated in an oven at a heating temperature of 160°C for 3 hours. After heating, the material is left to stand at room temperature for 24 hours. Following these steps, a porous film is formed on the substrate. In the following explanation, this state will be referred to as a test piece.

[0022] The porous membrane of the test piece is treated with a silane coupling agent. As mentioned above, there are three methods for silane coupling agent treatment: gas phase treatment, liquid phase treatment, and mist treatment. The conditions for each treatment are described below. <Vapor phase treatment> Place the test piece and 0.614 μL of hexamethyldisilazane (HMDS) (Tokyo Chemical Industries) in a sealed container with a capacity of approximately 1 L, and leave it to stand at room temperature for 24 hours. After that, remove the test piece from the sealed container and heat treat it at a temperature of 60°C for 30 minutes.

[0023] <Liquid phase treatment> A diluted HMDS solution is prepared by diluting hexamethyldisilazane (HMDS) with methanol to 30 wt%. The test piece is immersed in this diluted HMDS solution and treated for 20 minutes while applying ultrasound. After treatment, the test piece is ultrasonically washed in methanol for 1 minute, and then heat-treated at a heating temperature of 60°C for 30 minutes.

[0024] <Mist treatment> The test piece is heat-treated at a temperature of 70°C for 30 minutes. After heat treatment, the test piece is placed in a container, and the container is filled with hexamethyldisilazane (HMDS) mist using a nebulizer. After generating mist with the nebulizer for 5 minutes, the mist generation is stopped and the container is left to stand for 5 minutes, after which the test piece is removed from the container. The removed test piece is heat-treated at a temperature of 70°C for 30 minutes. The heat-treated test piece is immersed in methanol and ultrasonically cleaned for 2 minutes. After that, the test piece is washed with pure water and then heat-treated at a temperature of 70°C for 30 minutes.

[0025] Figure 5 shows the refractive indices of each example and each comparative example of porous membranes manufactured by the above manufacturing method. Figure 5(a) shows the refractive indices of Comparative Examples 1 to 6. Comparative Examples 1 to 6 are porous membranes that have not been treated with a silane coupling agent on their surface. In Comparative Example 1, the stirring time of the mixed solution of the base solvent and tetramethyl orthosilicate (TMOS) during the production of the porous membrane was 15 hours. Comparative Examples 2 to 6 are porous membranes with stirring times of 18 hours, 21 hours, 24 hours, 48 ​​hours, and 96 hours, respectively. Figure 5(b) shows the refractive indices of Examples 1 to 6. Examples 1 to 6 are porous membranes formed under the same conditions as Comparative Examples 1 to 6, and then treated with a silane coupling agent by gas phase treatment on their surface. Specifically, Examples 1 to 6 are porous membranes with stirring times of 15 hours, 18 hours, 21 hours, 24 hours, 48 ​​hours, and 96 hours, respectively. Figure 5(c) shows the refractive indices of Examples 7 to 12. Examples 7 to 12 are porous membranes formed under the same conditions as Comparative Examples 1 to 6, and then treated with a silane coupling agent by liquid phase treatment on their surface. Specifically, Examples 7 to 12 are porous membranes with stirring times of 15 hours, 18 hours, 21 hours, 24 hours, 48 ​​hours, and 96 hours, respectively. Figure 5(d) shows the refractive indices of Examples 13 to 18. Examples 13 to 18 are porous membranes formed under the same conditions as Comparative Examples 1 to 6, and then treated with a silane coupling agent by mist treatment on their surface. Specifically, Examples 13 to 18 are porous membranes with stirring times of 15 hours, 18 hours, 21 hours, 24 hours, 48 ​​hours, and 96 hours, respectively.

[0026] As shown in Figure 5, the refractive index of the porous membrane decreases with increasing stirring time between the base solvent and tetramethyl orthosilicate (TMOS) during manufacturing. Furthermore, applying a silane coupling agent to the surface increases the refractive index of the porous membrane compared to before the treatment. As shown in Figure 5, the refractive index is less than 1.25 regardless of whether or not the silane coupling agent treatment is applied. Although not shown in Figure 5, if the reaction time is 96 hours or longer, the refractive index of the formed porous membrane becomes less than 1.1.

[0027] Figure 6 shows the relationship between the porous films of each of the above examples and comparative examples and the scattering of light at wavelengths of 350 nm and 544 nm. Figure 6(a) shows the scattering of Comparative Examples 1-6, Figure 6(b) shows the scattering of Examples 1-6, Figure 6(c) shows the scattering of Examples 7-12, and Figure 6(d) shows the scattering of Examples 13-18. The scattering values ​​shown in Figure 6 represent the ratio of scattered light to incident light on the porous film. Scattered light is the sum of forward and backscatter detected using an integrating sphere. The scattering values ​​of the porous films in the examples are all 1000 ppm or less, indicating that the scattering of porous films produced by the manufacturing method of this example is sufficiently small. In other words, the porous films of this example have the characteristics of a smooth surface and a fine internal structure. As a result, the porous films of this example can be used as thin films for optical components in the visible light region.

[0028] Figure 7 shows the contact angles with water of the porous membranes of each of the above examples and comparative examples. Figure 7(a) shows the contact angles with water of Comparative Examples 1-6, Figure 7(b) shows the contact angles with water of Examples 1-6, Figure 7(c) shows the contact angles with water of Examples 7-12, and Figure 7(d) shows the contact angles with water of the porous membranes of Examples 13-18. As shown in Figure 7(a), the porous membranes of Comparative Examples 1-6 have small contact angles of 7.3° to 14.7° because they have not been treated with a silane coupling agent. In contrast, as shown in Figures 7(b) to (d), the contact angles with water of the porous membranes of Examples 1-18, which have been treated with a silane coupling agent on their surface, all exceed 40°, which is much larger than the contact angles of the porous membranes of Comparative Examples 1-6. This is presumed to be because the silane coupling agent treatment forms trimethylsilyl groups on the surface of the porous membrane, which increases the contact angle with water, i.e., reduces the amount of OH groups on the surface of the porous membrane.

[0029] Whether or not trimethylsilyl groups are present on the surface of a porous membrane can be determined by IR (infrared absorption spectroscopy) measurement. Specifically, in IR measurement, the 1259 cm⁻¹ due to the Si-C bond unique to trimethylsilyl groups can be determined. -1 If absorption is observed in the vicinity, the presence of a trimethylsilyl group can be confirmed.

[0030] Figure 8 shows the results of IR measurements of each of the above examples and comparative examples, at 1259 cm². -1 The results of checking for the presence or absence of absorption bands in the vicinity are shown. Figure 8(a) shows the IR measurement results for Comparative Examples 1-6, Figure 8(b) shows the results for Examples 1-6, Figure 8(c) shows the results for Examples 7-12, and Figure 8(d) shows the results for Examples 13-18. For Comparative Examples 1-6, the measurement was 1259 cm². -1 Although no absorption zones were observed in the vicinity, all of the results for Examples 1-18 were 1259 cm². -1 An absorption band was observed in the vicinity. This indicates that trimethylsilyl groups are present on the surface of the porous membranes in Examples 1-18. It is presumed that this results in a large contact angle with water.

[0031] According to the above-described embodiment, the following effects can be obtained: (1) The porous membrane has silica particles, a refractive index of 1.1 to 1.25, and a contact angle with water of 40° or more. As a result, the porous membrane has a low refractive index and high environmental resistance, so it can be used for applications such as thin films for optical components.

[0032] (2) The porous membrane has trimethylsilyl groups, which are formed when the OH groups on its surface are treated with a silane coupling agent. This increases the contact angle of the porous membrane. In other words, because the amount of OH groups on the surface of the porous membrane is reduced, it is possible to suppress changes in refractive index and film thickness of the porous membrane caused by OH groups in high temperature and high humidity environments.

[0033] (3) The porous film exhibits scattering at a wavelength of 350 nm of less than 1000 ppm. As a result, the porous film is a low-scattering film and can be used for applications such as anti-reflective coatings for optical components.

[0034] (4) A mixed solution is prepared by mixing a tertiary amine, water, a solvent containing methoxypropanol (PGME), and a silicon compound, and the mixture is stirred. The mixed solution is then applied to a substrate to form a coating film, and the coating film is heated to form a porous film. This allows for the safe production of a low refractive index porous film using a simple process without the use of hydrofluoric acid or the like.

[0035] The present invention is not limited to the embodiments described above, as long as the features of the present invention are not impaired, and other forms that can be conceivable within the scope of the technical idea of ​​the present invention are also included within the scope of the present invention.

[0036] The disclosures of the following priority application are incorporated herein by reference. Japanese Patent Application No. 2019-63714 (filed March 28, 2019) [Explanation of Symbols]

[0037] 1. CAM…Imaging device 103. WL…Photography lens

Claims

1. A porous membrane having silica particles, The refractive index for light at 550 nm is 1.1 to 1.

174. Having a trimethylsilyl group on its surface, The scattering at a wavelength of 350 nm is less than 1000 ppm. A porous membrane having a contact angle with water of 40° or more.

2. A porous membrane according to claim 1, A porous film in which scattering at a wavelength of 544 nm is 1000 ppm or less.

3. A porous membrane according to claim 1 or 2, The porous membrane is a porous membrane composed of a gel network of silica particles.

4. An optical element comprising an anti-reflective coating composed of a single layer on a substrate, An optical element wherein the single layer film is a porous film according to any one of claims 1 to 3.

5. An optical element comprising an anti-reflective coating composed of multiple layers on a substrate, An optical element in which at least one layer of the multilayer film is a porous film according to any one of claims 1 to 3.

6. The optical element according to claim 5, An optical element in which the outermost layer of the multilayer film is the porous film.

7. An optical element according to any one of claims 4 to 6, An optical element in which the aforementioned substrate is a lens.

8. An optical system comprising the optical element described in any one of claims 4 to 7.

9. An interchangeable lens having the optical system described in claim 8.

10. An optical device having the optical system described in claim 8.

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