Film-coated lens, lens unit and camera module

Abstract

To provide: a lens with an anti-reflection film showing excellent durability under a high-temperature environment and a high-temperature and high-humidity environment while satisfying a desired low-reflectance condition in a wide wavelength band area; a lens unit; and a camera module.SOLUTION: A lens with a film has an anti-reflection film on its front surface. The anti-reflection film is formed by alternately laminating: a first film that is formed of a material having a first refractive index; and a second film that is formed of a material having a second refractive index higher than the first refractive index. Both an internal stress of the first film and an internal stress of the second film indicate compression stresses of 0-50 MPa.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention particularly relates to a film-equipped lens, a lens unit, and a camera module that are provided in an on-board camera mounted on a vehicle such as an automobile. [Background technology]

[0002] In recent years, automobiles have been equipped with on-board cameras to assist with parking and prevent collisions through image recognition, and attempts have also been made to apply these to autonomous driving. Camera modules such as these on-board cameras generally include a lens unit having a lens group consisting of multiple lenses arranged along an optical axis, a lens barrel that houses and holds this lens group, and an aperture member arranged between at least one of the lenses in the lens group (see, for example, Patent Document 1).

[0003] The surfaces of the lenses that make up such lens units are generally provided with an anti-reflection coating (AR coating) by, for example, vapor deposition to increase their transmittance. Furthermore, when the anti-reflection coating has a multi-layer structure, it is generally possible to satisfy the desired low reflectance condition over a wide wavelength range by alternately laminating films made of low-refractive-index materials and films made of high-refractive-index materials. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-231993 Summary of the Invention [Problem to be solved by the invention]

[0005] In the case of multilayer anti-reflection coatings, the internal stress of each layer must also be considered. This is because, when films of low-refractive index materials and high-refractive index materials are alternately stacked, the internal stress of each layer can reduce the film's durability in high-temperature and high-humidity environments, potentially preventing the desired optical characteristics from being achieved. For example, in an anti-reflection coating composed of alternating layers of SiO2, a low-refractive index material, and ZrO2, a high-refractive index material, the internal stress of the SiO2 film is compressive, while the internal stress of the ZrO2 film is tensile, resulting in an excessively high tensile stress throughout the film. In other words, the internal stress of the entire film (total stress) is biased in one direction. Therefore, when such a film is exposed to a high-temperature environment, such as during high-temperature testing, and the lens expands, it may not be able to withstand the deformation and cracks may form on the surface (the surface of the anti-reflection coating). These cracks can cause ghosting and adversely affect the optical characteristics.

[0006] Furthermore, although the problem of high-temperature durability (heat resistance) of anti-reflection coatings can be resolved to some extent by improving the film-forming conditions, such improved film-forming conditions may in turn result in a trade-off in that the durability at high temperatures and high humidity may be deteriorated. For example, anti-reflection coatings formed under such film-forming conditions may swell and protrude due to high humidity, resulting in deterioration of optical properties and poor appearance.

[0007] The present invention has been made in consideration of the above circumstances, and aims to provide a film-coated lens, a lens unit, and a camera module that include an anti-reflection film that exhibits excellent durability in high temperature and high temperature / high humidity environments while satisfying the desired low reflectance conditions in a wide wavelength band. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present invention provides a film-coated lens that is provided in a lens barrel and has an anti-reflection film formed on its surface, The anti-reflection film is formed by alternately laminating a first film made of a material having a first refractive index and a second film made of a material having a second refractive index higher than the first refractive index, and is characterized in that the internal stress of the first film and the second film both exhibit a compressive stress of 0 to 50 MPa.

[0009] The present inventors have investigated how the internal stress of each layer (each film) constituting a multilayer antireflection coating affects the durability of the film under high-temperature, high-humidity environments, and have found that in a laminate structure formed by alternately stacking films of low-refractive-index material and high-refractive-index material, if the internal stress of both the low-refractive-index material film and the high-refractive-index material film is maintained to be a compressive stress of 0 to 50 MPa, in other words, if the melting points of both the low-refractive-index material film and the high-refractive-index material film are both 2100°C or less and the Young's modulus of both the low-refractive-index material film and the high-refractive-index material film are both 110 MPa or less, the film will exhibit excellent durability under high-temperature and high-humidity environments. This will be explained in detail below.

[0010] First, as shown in the table in Figure 4, a 26.33 nm thick SiO layer with a refractive index of 1.54 was placed on a resin lens with a refractive index of 1.54. On top of that, ZrO2 (melting point 2700°C; Young's modulus 210 MPa) with a refractive index of 2.00 and SiO2 (melting point 1700°C) with a refractive index of 1.42, which is lower than ZrO2, were alternately layered to create a laminated structure (thicknesses from bottom to top were 6.75 nm (ZrO2), 14.28 nm (SiO2)). When optimization was performed by a specified simulation at a specified light incident angle for an anti-reflection coating (a seven-layer laminate structure) having thicknesses of 33.76 nm (SiO2), 33.76 nm (ZrO2), 9.52 nm (SiO2), 76.30 nm (ZrO2), and 85.69 nm (SiO2)), the spectral characteristic curve L1 shown in the spectral characteristic diagram of Figure 6 (a spectral characteristic diagram showing the relationship between the reflectance (%) of the anti-reflection coating and the wavelength (nm) of incident light) was obtained.

[0011] As can be seen from the spectral characteristic curve L1, this anti-reflection coating, which is made by alternating layers of low-refractive-index material (SiO2) and high-refractive-index material (ZrO2), can nearly meet the desired low reflectance requirement (reflectance of 1.5 or less) over the specified wide wavelength range (400 nm to 700 nm). However, as mentioned above, the internal stress of the SiO2 film is compressive, while the internal stress of the ZrO2 film is highly tensile. This results in excessive tensile stress throughout the film (internal stress of the entire film is biased in one direction). Therefore, when the lens expands due to exposure to a high-temperature environment, such as during high-temperature testing, the lens may be unable to withstand the deformation and cracks may form on the surface (surface of the anti-reflection coating). These cracks can cause ghosting and adversely affect the optical characteristics.

[0012] The occurrence of cracks due to such internal stress imbalance can be prevented to some extent by reducing the number of film layers and reducing the overall stress of the film. For example, as shown in the table in Figure 5, an antireflection coating with a layered structure consisting of a resin lens with a refractive index of 1.54, a 25.97 nm thick SiO layer with a refractive index of 1.56, a 11.86 nm thick ZrO2 layer with a refractive index of 2.04, and a 123.89 nm thick SiO2 layer with a refractive index of 1.43, which is lower than the ZrO2 layer, was layered on top of that (a total of three layers). However, as can be seen from the spectral characteristic curve L2 in the spectral characteristic diagram in Figure 6, this three-layer antireflection coating did not completely achieve a reflectance of 1.5 or less within the specified incident wavelength range (400 nm to 700 nm). In other words, this three-layer structure results in an overall high reflectance, resulting in poor spectral characteristics.

[0013] Therefore, the inventors focused on the internal stress of each layer (each film) constituting the laminated structure of the anti-reflection film, which is formed by alternately laminating films of low refractive index material and films of high refractive index material, and as a result of trial and error involving various tests, they identified a stress form that satisfies the desired low reflectance condition in a wide wavelength band (specifically, even when the number of layers is ensured to almost satisfy the desired low reflectance condition (reflectance of 1.5 or less) in a specified wide wavelength band (400 nm to 700 nm)), while preventing cracks or film swelling / bulging (enabling the film to follow stress changes in high temperature environments and high temperature / high humidity environments). That is, the inventors have confirmed that, at room temperature, if the stress configuration is maintained such that the internal stress of both the low-refractive index material film and the high-refractive index material film exhibits a compressive stress of 0 to 50 MPa, specifically, for example, by using materials capable of realizing such a stress configuration as the low-refractive index material and the high-refractive index material, and by setting film-forming conditions such that the internal stress (compressive stress) of each layer (each film) is reduced to 0 to 50 MPa as necessary, excellent film durability can be obtained in a high-temperature, high-humidity environment. In this case, it is desirable to keep the difference in internal stress (compressive stress) between each layer constituting the laminate structure as low as possible and to smooth the compressive stress distribution throughout the entire laminate structure as much as possible (or to reduce the stress of the entire anti-reflection film).

[0014] As an example, a material with lower internal stress than ZrO2 (softer and with a lower melting point than ZrO2), such as Nb2TiO7, is used as the high-refractive index material (or its main component). Nb2TiO7 has a low melting point (approximately 1490°C) and a low Young's modulus (approximately 100 MPa), allowing it to conform well to substrates such as resin lenses even at high temperatures. In addition, a mixture of SiO2 and Al2O3 (melting point approximately 2000°C), which is denser and more heat-resistant than SiO2, is used as the low-refractive index material (or its main component). Adding Al2O3 to SiO2 not only creates a hard but also dense crystalline structure, improving adhesion to adjacent high-refractive index layers and substrates such as resin lenses, and improving conformity to adjacent high-refractive index layers and lens substrates at high temperatures. By combining these high-refractive-index materials with low-refractive-index materials, it is possible to realize a stress pattern in which the internal stress of both the low-refractive-index material film and the high-refractive-index material film exhibits a compressive stress of 0 to 50 MPa. (By setting the melting points of both the low-refractive-index material film and the high-refractive-index material film to 2100°C or less and the Young's moduli of both the low-refractive-index material film and the high-refractive-index material film to 110 MPa or less, it is possible to realize a stress pattern in which the internal stress of both the low-refractive-index material film and the high-refractive-index material film exhibits a compressive stress of 0 to 50 MPa.) Furthermore, the film-forming conditions for reducing the internal stress (compressive stress) of each layer (each film) to 0 to 50 MPa may be set, for example, by controlling the amount of oxygen introduced. Alternatively, when each layer constituting the laminate structure is formed using an ion-assisted or plasma-assisted method, this may be achieved by controlling the parameters of the assisted process (e.g., gas flow rate, irradiation time, applied power, etc.).

[0015] In the above configuration, the anti-reflection coating is provided, for example, by vapor deposition, within at least the optically effective range (effective diameter) of the lens. In the above configuration, the anti-reflection coating may have another film interposed between the alternatingly stacked first and second films. In the above configuration, the coated lens with the anti-reflection coating may be made of glass or resin. However, as mentioned above, this anti-reflection coating has excellent heat resistance that can accommodate thermal deformation of the lens, making it particularly suitable for resin lenses that tend to expand and contract due to temperature changes. In the above configuration, the anti-reflection coating may be provided not only on the surface of the lens facing the object side, but also on the back surface of the lens facing the image side.

[0016] The present invention also provides a lens unit having the above-mentioned film-coated lens, and a camera module having this lens unit. Such a lens unit and camera module can also provide the same effects as the above-mentioned film-coated lens. [Effects of the Invention]

[0017] According to the present invention, in a laminated structure formed by alternately laminating films of low refractive index material and films of high refractive index material, the internal stress of both the low refractive index material film and the high refractive index material film is a compressive stress of 0 to 50 MPa, and therefore it is possible to provide a coated lens, lens unit, and camera module that includes an anti-reflection film that satisfies the desired low reflectance condition over a wide wavelength range and exhibits excellent durability in high temperature and high temperature / high humidity environments. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic cross-sectional view of a lens unit with a film-coated lens according to an embodiment of the present invention. [Figure 2] 2 is a schematic cross-sectional view of a camera module including the lens unit of FIG. 1. [Figure 3] FIG. 2 is a table showing detailed data on the laminate structure of the seven-layer heat-resistant anti-reflection coating formed on the coated lens of the lens unit of FIG. 1. [Figure 4] FIG. 1 is a table showing detailed data on the stack structure of a conventional seven-layer antireflection film. [Figure 5] FIG. 1 is a table showing detailed data on the stack structure of a conventional three-layer antireflection film. [Figure 6] 6 is a diagram showing spectral characteristics of the respective layered structures of FIGS. 3, 4 and 5. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The lens unit of the present embodiment described below is particularly intended for use as a camera module for an in-vehicle camera or the like, and is fixedly installed on the exterior surface of the vehicle, with wiring drawn into the vehicle and connected to a display or other device. Hatching of multiple lenses is omitted in Figures 1 and 2 described below.

[0020] FIG. 1 shows a lens unit 11 according to one embodiment of the present invention. As shown, the lens unit 11 of this embodiment includes a cylindrical lens barrel 12, e.g., made of resin or metal; a plurality of glass or resin lenses, e.g., five lenses consisting of a first lens 13, a second lens 14, a third lens 15, a fourth lens 16, and a fifth lens 17, arranged within a stepped inner storage space S of the lens barrel 12; and an aperture member (not shown). The aperture member is either an "aperture diaphragm" that limits the amount of transmitted light and determines the F-number, which is an index of brightness, or an "optical diaphragm" that blocks light rays that cause ghosting or aberrations. An in-vehicle camera equipped with such lens unit 11 includes the lens unit 11, a circuit board (not shown) having an image sensor, and an installation member (not shown) for installing the circuit board in a vehicle such as an automobile.

[0021] The multiple lenses 13, 14, 15, 16, and 17 fixed to and supported by the lens barrel 12 are arranged with their optical axes aligned, and the lenses 13, 14, 15, 16, and 17 are arranged along a single optical axis O to form a group of lenses L used for imaging. Of these, the two lenses, the fourth and fifth lenses 16 and 17, which are positioned closest to the image side (the innermost side of the inner storage space S), are, for example, cemented lenses.

[0022] At the object side end (the upper end in Figure 1) of the lens barrel 12, a crimped portion 23 is provided by crimping the end radially inward, and this crimped portion 23 fixes the first lens 13, which is positioned closest to the object side of the lens group L, to the object side end of the lens barrel 12.

[0023] Furthermore, an inner flange portion 24 having an opening with a diameter smaller than that of fifth lens 17 is provided at the image side end (the lower end in FIG. 1) of lens barrel 12. This inner flange portion 24 and crimped portion 23 hold the multiple lenses 13, 14, 15, 16, and 17 that make up lens group L within lens barrel 12, as well as the diaphragm member.

[0024] The outer peripheral surface of first lens 13, which is positioned closest to the object, has a tapered portion with a reduced diameter on the image side of lens 13, and an O-ring 26 serving as a seal is provided at this tapered portion, sealing the gap between the outer peripheral surface of lens 13 and the inner peripheral surface of lens barrel 12 at the object side end of lens barrel 12. This prevents water, dust, and other fine particles from entering lens barrel 12 from the object side end of lens unit 11.

[0025] The inner and outer diameters of the lens barrel 12 decrease in stages from the object side to the image plane side. Specifically, the lens barrel 12 has a large-diameter section 12A that houses and holds the first and second lenses 13 and 14, and a small-diameter section 12B that houses and holds the third, second, and fifth lenses 15, 16, and 17. Corresponding to the stepped shape of the lens barrel 12, the outer diameters of the lenses 13, 14, 15, 16, and 17 decrease from the object side to the image plane side. Basically, the outer diameters of the lenses 13, 14, 15, 16, and 17 are approximately equal to the inner diameters of the portions of the lens barrel 12 that support (hold) the lenses 13, 14, 15, 16, and 17. The outer flange 25, which is used when installing the lens barrel 12 in an in-vehicle camera, is provided on the outer peripheral surface of the lens barrel 12 in a brim-like shape.

[0026] In the present embodiment, the first lens 13, which is positioned closest to the object in the lens group L, has a lens front surface 13a facing the object side and a lens back surface 13b facing the image side, and is a coated lens having an anti-reflection coating 30 formed at least on the lens front surface 13a. Note that, although the coating form of the first lens 13 made of resin as a coated lens will be described below, the first lens 13 may also be a glass lens, and the other lenses 14, 15, 16, and 17 constituting the lens group L may also be formed as coated lenses having the same or similar coating form.

[0027] As shown in the table of FIG. 3, the anti-reflection coating 30 is formed on the first lens 13, which is a resin lens with a refractive index of 1.54, and has a layered structure in which films of a low refractive index material and films of a high refractive index material are alternately stacked. In this case, the anti-reflection coating 30 uses, as its high-refractive index material (or its main component), Nb2TiO7 (melting point: approximately 1490°C; Young's modulus: approximately 100 MPa), a material that has lower internal stress than the previously mentioned conventional ZrO2 (it is softer and has a lower melting point than ZrO2), and uses, as its low-refractive index material (or its main component), a mixed material of SiO2 and Al2O3 (melting point: approximately 2000°C; Young's modulus: 110 MPa or less), which is denser and more heat-resistant than SiO2. This achieves a stress pattern in which the internal stress of both the low-refractive index material film and the high-refractive index material film exhibits a compressive stress of 0 to 50 MPa (by setting the melting points of both the low-refractive index material film and the high-refractive index material film to 2100°C or less and the Young's modulus of both the low-refractive index material film and the high-refractive index material film to 110 MPa or less, thereby achieving a stress pattern in which the internal stress of both the low-refractive index material film and the high-refractive index material film exhibits a compressive stress of 0 to 50 MPa). In this case, the film formation conditions may be set so as to further reduce the internal stress (compressive stress) of each layer (each film) within the range of 0 to 50 MPa. Such film formation conditions may be set, for example, by controlling the amount of oxygen introduced, or, when each layer constituting the laminate structure is formed using an ion-assisted or plasma-assisted method, by controlling the parameters of such an assisted process (e.g., gas flow rate, irradiation time, applied power, etc.).

[0028] More specifically, as shown in FIG. 3, the laminated structure of the anti-reflection coating 30 is formed by vapor-depositing a first film having a first refractive index of 1.45 made of a mixed material of SiO2 and Al2O3 (or containing a mixed material of SiO2 and Al2O3 as a main component) with a thickness of 28.95 nm on the first lens 13 having a refractive index of 1.54 as a first layer, and then vapor-depositing a second film having a second refractive index of 2.04 made of Nb2TiO7 (or containing Nb2TiO7 as a main component) on the first layer. The first film is deposited to a thickness of 11.53 nm, and then the first film is deposited thereon to a thickness of 40.98 nm as a third layer, followed by the second film being deposited thereon to a thickness of 40.17 nm as a fourth layer, and then the first film is deposited thereon to a thickness of 9.07 nm as a fifth layer, and then the second film is deposited thereon to a thickness of 68.85 nm as a sixth layer, and finally the first film is deposited thereon to a thickness of 86.53 nm as a seventh layer.

[0029] When the antireflection coating 30, which has a seven-layer structure, was optimized by a predetermined simulation at a predetermined light incidence angle, the spectral characteristic curve L3 shown in FIG. 6 (a spectral characteristic diagram showing the relationship between the reflectance (%) of the antireflection coating and the wavelength (nm) of incident light) was obtained. As can be seen from the spectral characteristic curve L3, the antireflection coating 30, which is formed by alternately stacking a first layer made of a mixed material of SiO and AlO as a low refractive index material and a second layer made of NbTiO as a high refractive index material, can substantially meet the desired low reflectance condition (reflectance of 1.5 or less) over a specified broad wavelength range (400 nm to 700 nm). Furthermore, even when the antireflection coating 30 satisfies the desired low reflectance condition over a wide wavelength range, i.e., even when the number of layers is sufficient to substantially meet the reflectance of 1.5 or less over the specified broad wavelength range of 400 nm to 700 nm, cracks and film swelling / bulging did not occur in high-temperature and high-humidity environments. That is, the above-described laminated structure realizes a stress configuration that allows the film to flexibly follow stress changes under high temperature and high temperature / high humidity environments.

[0030] In this embodiment, the film stress of the antireflection film 30 forming the laminated structure is measured and calculated as follows. 1. A single layer of any evaporation material is deposited on a flat substrate with little warping, such as a silicon wafer. 2. In order to stabilize the film, leave it at room temperature for two weeks after film formation. After two weeks have passed, measure the change in the radius of curvature of the film surface before and after film formation in a room temperature environment, and calculate the internal stress using the following formula. σ f =[E s t s 2 / (1-V s )6t f ]×[1 / R1-1 / R0]...(formula) where σ f is the membrane stress (internal stress), E s is the Young's modulus of the substrate (lens), t s is the thickness of the substrate, V s is the Poisson's ratio of the substrate, t f is the thickness of the film on the substrate, R0 is the radius of curvature of the substrate before film deposition, and R1 is the radius of curvature of the film and lens after film deposition. In this case, the radius of curvature of the film and lens refers to the radius of curvature of the lens after deposition.

[0031] 2 shows a schematic cross-sectional view of a camera module 300 of this embodiment having a lens unit 11 configured as described above, including a first lens 13 with an anti-reflection coating 30. As shown in the figure, this camera module 300 is configured to include the lens unit 11 of FIG. 1 to which a filter 100 is attached.

[0032] The camera module 300 includes an upper case (camera case) 301, which is an exterior component, and a mount (base) 302 that holds the lens unit 11. The camera module 300 also includes a sealing member 303 and a package sensor (imaging element) 304.

[0033] Upper case 301 is a member that exposes the object-side end of lens unit 11 and covers the other portions. Mount 302 is disposed inside upper case 301, and has female threads 302a that mesh with male threads 11a of lens unit 11. Sealing member 303 is a member that is interposed between the inner surface of upper case 301 and outer peripheral surface 12a of barrel 12 of lens unit 11, and is a member that maintains airtightness inside upper case 301.

[0034] Package sensor 304 is disposed inside mount 302 and is positioned to receive the image of the object formed by lens unit 11. Package sensor 304 also includes a CCD, CMOS, or the like, and converts the light that is collected and reaches it through lens unit 11 into an electrical signal. The converted electrical signal is then converted into analog data or digital data, which are components of the image data captured by the camera.

[0035] While one embodiment of the present invention has been described above, various modifications can be made to the present invention without departing from the spirit and scope of the present invention. For example, the shapes of the lenses, lens barrels, and the like are not limited to those described in the above-described embodiment. Furthermore, the anti-reflection coating on the lens may be formed in any form as long as it has the aforementioned function. In the above-described embodiment, a mixture of SiO2 and Al2O3 and Nb2TiO7 was used as a combination of a low-refractive index material film and a high-refractive index material film that can achieve a compressive internal stress of 0 to 50 MPa. However, other combinations of materials, such as a mixture of SiO2 and Al2O3 and Ta2O5 or LaTiO3, are also possible. Furthermore, some or all of the above-described embodiments may be combined, or portions of one of the above-described embodiments may be omitted, without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0036] 11 Lens unit 12 Telescope tube 13, 14, 15, 16, 17 Lens (membrane lens) 30 Anti-reflection coating 300 Camera Module L lens group

Claims

1. A film-coated lens provided in a lens barrel and having an anti-reflection film formed on its surface, The anti-reflection film is formed by alternately laminating first films formed from a material having a first refractive index and second films formed from a material having a second refractive index higher than the first refractive index, and the internal stress of each of the first films and the internal stress of each of the second films exhibits a compressive stress of 0 to 50 MPa in the same direction, and the internal stress is the film stress when a vapor deposition material forming each of the films is formed into a single layer on a flat substrate, and the film is left at room temperature for two weeks after formation, and after the two weeks have passed, the internal stress is measured in a room temperature environment.

2. 2. The lens with a film according to claim 1, wherein the material forming the first and second films is a vapor deposition material.

3. The material forming the first film is SiO ２ and Al ２ O ３ The material forming the second film is a mixture of Nb ２ TiO ７ 3. The lens with a film according to claim 2, wherein the main component is

4. A lens unit comprising a lens group in which a plurality of lenses are arranged along the optical axis of the lenses, and a lens barrel in which the lens group is housed, A lens unit, wherein at least one of the lenses constituting the lens group is the film-coated lens according to any one of claims 1 to 3.

5. A camera module comprising the lens unit according to claim 4.

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