JP2015200805A - Optical filter, light quantity diaphragm device, and imaging equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical filter having an increased accuracy of preventing reflection at an interface between a transparent substrate and a film layer or between a transparent substrate and an air layer in order to solve problems of an existing fine concavo-convex periodic structure that an ideal arrangement is difficult to achieve and low reflection characteristics are not utilized enough in a recent situation where imaging equipment such as digital cameras and video cameras is becoming smaller and smaller and reflecting light on the surface of an ND filter used as a light quantity adjusting member can cause a ghost image.SOLUTION: A fine concavo-convex periodic structure is formed on a surface of a transparent substrate to have a pitch and a height not larger than the wavelengths of visible light, and the structure is characterized in having an index of refraction which continuously varies from the base material side to the surface layer side.

Description

  The present invention relates to an optical functional film having an excellent antireflection effect, and particularly relates to an optical filter, a light amount diaphragm device, and an imaging device using an antireflection film whose refractive index is continuously changed.

  2. Description of the Related Art Conventionally, an aperture device is incorporated in an optical apparatus such as a digital camera or a video camera in order to adjust the amount of light. This diaphragm device adjusts the amount of light incident on a solid-state imaging device such as a CCD by opening and closing diaphragm blades, and is narrowed down more when the object field is bright. Therefore, when shooting a clear or high-luminance subject, the aperture becomes a small aperture to attenuate the amount of light, but if it is left as it is, the light may be diffracted by the aperture and cause deterioration of the captured image. . In addition, as the sensitivity of a solid-state imaging device such as a CCD becomes higher, it is necessary to further attenuate the amount of light, and this tendency of image degradation is remarkable.

  Therefore, as a countermeasure for this problem, a film-like ND (Neutral Density) filter is attached to the diaphragm blade as a light quantity adjusting member to attenuate the light quantity in a state where the diaphragm opening is large. Specifically, an ND filter made of another member different from the diaphragm blade is attached to a part of the diaphragm blade with an adhesive, and when the subject has high brightness, the ND filter is positioned on the optical axis, Limit. For this reason, it is possible to avoid narrowing down until the aperture opening becomes too small, and to maintain the aperture opening at a constant size. Further, an ND filter having a density gradient is used as a light amount adjustment function, and the light amount may be further adjusted by moving the ND filter on the optical axis. In addition, various diaphragm devices have been proposed in which the ND filter is independently provided with an optical action without bonding the ND filter to the diaphragm blades.

As the ND filter in the diaphragm device as described above, a filter in which a multilayer film is formed on a transparent substrate by vacuum deposition or the like is generally used. For example, as shown in Patent Document 1, an antireflection layer made of a gradient refractive index film whose refractive index continuously changes from the base material side to the surface layer side is provided on the base material. A film in which the refractive index of the portion in contact with the substrate surface is higher than the refractive index of the substrate is used. Further, in Patent Document 2, on the fine uneven periodic structure on the transparent substrate, and the Al ₂ O ₃ film is an antireflection film for reducing reflection is the light-absorbing layer for reducing the permeability In order to increase the antireflection effect by laminating TixOy films alternately, a SiO ₂ film, which is a low refractive material, is used as the outermost layer in order to increase the antireflection effect. λ = 500-600 nm) deposited ND filters are disclosed.

JP 2004-012657 A JP 2008-077684 A

In recent years, downsizing of imaging devices such as digital cameras and video cameras has progressed, and reflected light on the surface of an ND filter used as a light amount adjusting member may cause ghost. Usually, an ND film is formed on one side of a transparent substrate made of PET resin or the like, and an AR film is formed on the other side as an antireflection film, or an ND film is formed on both sides of the transparent substrate. FIG. 5 shows a film configuration diagram of the ND film. Al ₂ O ₃ films and TixOy films are alternately stacked on a transparent substrate, an MgF ₂ film is deposited on the outermost layer, and an ND film is formed. Yes. FIG. 7 is an explanatory diagram of reflection at the interface of the ND filter. As an example, the ND film 2 is formed on one side of the transparent substrate, and the AR film 3 is formed on the other side. Since reflection occurs at the interface between substances having different refractive indexes, the interface a between the air and the AR film 3, the interface b between the AR film 3 and the transparent substrate 1, the interface c between the transparent substrate 1 and the ND film 2, and the ND film 2 and the air. Light reflection occurs on the four surfaces of the interface d. In addition, if the ND film 2 and the AR film 3 are multilayer films, there is reflection also at the interface of each layer therein. Among them, it is most effective to reduce the reflectance of the interface b between the AR film 3 and the transparent substrate 1 and the interface c between the transparent substrate 1 and the ND film 2. However, in practice, it is difficult to arrange the fine irregular periodic structures in an ideal equal pitch configuration, and there is a problem that the reflection cannot be reduced to the theoretical reflectivity as an actual product.

  An object of the present invention is to solve the above-mentioned problems and improve the accuracy of the antireflection effect at the interface between the transparent substrate and the film layer or the transparent substrate and the air layer, which is difficult to reduce the reflection by the conventional method. To provide a filter.

  In order to solve the above-described problems, the technical feature of the optical filter according to the present invention is that a fine uneven periodic structure having a pitch and height below the visible light wavelength is formed on the surface of the transparent substrate, and the fine uneven periodic structure is formed. A characteristic is that the refractive index continuously changes from the base material side to the surface layer side.

  According to the optical filter of the present invention, a fine uneven periodic structure composed of uneven portions having a pitch and height below the visible light wavelength is formed on the substrate of the optical filter, and the fine uneven cycle period from the substrate to the film or the air layer. By setting the refractive index in the structure to be small, reflected light generated at the interface between the transparent substrate and the film or between the transparent substrate and the air layer can be efficiently reduced.

Examples of photographic optics used in video cameras Configuration diagram of fine irregular periodic structure in this example Schematic diagram of fine periodic structure Fine periodic structure formation process ND filter membrane configuration diagram Vacuum chamber configuration diagram Where the reflectance occurs on the film Example of ND filter configuration

(Example 1)
The present invention will be described in detail based on examples. FIG. 1 is a configuration diagram of a photographing optical system, in which a solid-state image pickup device 27 including a lens 21, a light amount diaphragm device 22, lenses 23 to 25, a low-pass filter 26, a CCD, and the like are sequentially arranged. In the light quantity diaphragm device 22, a pair of diaphragm blades 29 a and 29 b are movably attached to the diaphragm blade support plate 28. An ND filter 30 for dimming the amount of light passing through the substantially rhombic opening formed by the diaphragm blades 29a and 29b is bonded to the diaphragm blade 29a.

  FIG. 3 is a schematic diagram of a fine uneven periodic structure having a Moth eye structure used in this embodiment. On the surface of the ND filter 30 in FIG. 1, as shown in FIG. 3, an infinite number of conical projections 32 are arranged at equal intervals on a transparent substrate 31 made of PET (polyethylene terephthalate) having a thickness of about 100 μm. An uneven periodic structure 33 is provided.

  A PET (polyethylene terephthalate) resin film is used as the transparent substrate 31 in the present embodiment. However, in addition to PET, it is also possible to use a film-like synthetic resin substrate having transparency and mechanical strength. Examples include PEN (polyethylene naphthalate), acrylic resin, polycarbonate, polyimide resin, norbornene resin, polystyrene, polyvinyl chloride, polyarylate, polysulfone, polyethersulfone, polyetherimide, and the like. The thickness of the transparent substrate 31 is preferably as thin as possible while maintaining the rigidity of the ND filter 30. Specifically, the thickness is preferably 300 μm or less, more preferably 50 to 100 μm.

  With respect to the height direction of each projection 32, the volume of the projection 32 gradually increases from the top to the bottom of the projection 32, and the corresponding effective refractive index also increases from the top to the bottom of the projection 32. It is distributed continuously toward. Therefore, it has a smooth effective refractive index distribution from the air layer toward the transparent substrate 31, and when light is incident on the fine concavo-convex periodic structure 33, there is no abrupt refractive index difference, so that the light is almost reflected. Without reaching the transparent substrate 31.

  In order to obtain a good antireflection effect, it is preferable that the protrusion 32 has a conical shape as shown in FIG. 3, but instead of the conical shape, a quadrangular pyramidal protrusion or other polygonal pyramid shape. Further, a fine uneven periodic structure 33 having an inverted conical recess may be used.

  The pitch P of the concave / convex pattern of the fine concave / convex periodic structure 33 is preferably smaller than the wavelength (λ = 400 to 700 nm) in the visible light region, and is set to 300 nm in this embodiment. When the pitch P is increased and becomes a wavelength in the visible light region, for example, 550 nm, a light diffraction phenomenon occurs and causes image degradation. As the pitch P is decreased, the wavelength at which the antireflection effect is obtained is also expanded to the lower wavelength side. However, as the pitch P is decreased, it becomes difficult to form the fine uneven periodic structure 33. Therefore, the pitch P is preferably 100 to 300 nm. .

  Further, when the ratio (aspect ratio) D / P between the height D of the convex portion or the concave portion of the fine uneven periodic structure 33 and the pitch P of the fine uneven periodic structure 33 is large, the antireflection effect becomes large. Is preferably 0.2 or more, more preferably 1 or more. However, this height D is also preferably smaller than the wavelength in the visible light region.

  In order to form the protrusions 32 constituting the fine uneven periodic structure 33, for example, using a mold having a shape opposite to the fine uneven periodic structure to be formed, heat and pressure are applied to the surface of the substrate to form the fine uneven period. Transcribe the structure. Alternatively, the fine uneven periodic structure can be formed directly on the surface of the substrate 31 using a semiconductor manufacturing technique. Further, there is a method in which a film is formed with a photo-curing resin on the surface of the substrate, and a mold having a shape opposite to that of the fine concavo-convex periodic structure to be formed is adhered to solidify the photo-curing resin.

  In this embodiment, as shown in FIG. 4, a method of filling an aluminum mold 4 with a photocurable resin 5 that is a monomer such as acrylic, placing a transparent substrate 1 thereon, and then releasing the mold by light irradiation. Now, a method for transferring the fine uneven periodic grooves on the transparent substrate 1 will be described.

  A method of manufacturing a structure in which the refractive index in the fine uneven periodic structure is continuously changed by this method will be described.

  As an example of manufacturing a structure in which the refractive index is continuously changed, a method of adding metal oxide particles to a photocurable resin is used. The particle size of the metal oxide particles is preferably less than that because the fine uneven periodic structure is several hundred nm, more preferably several tens nm, and most preferably several nm. The case where the metal oxide particles are selected as zirconium oxide (particle size: 10 to 20 nm) is taken as an example.

  The principle of changing the refractive index is that it is possible to control the characteristic n0 having a refractive index higher than the refractive index n1 of the photocurable resin by adjusting the blending ratio wt% of the metal oxide particles added to the photocurable resin. Become.

  By the method described above, the refractive index changing region can be freely set by selecting a photo-curing resin.

  For example, when zirconium oxide is added to urethane acrylate in an amount of 0 wt% to 70 wt%, the refractive index can be changed in the range of 1.52 (n1) to 1.66 (n0).

  Utilizing the properties described above, the refractive index is continuously varied by changing the compounding ratio of the metal oxide particles in the photocurable resin. As the means, a method of dispersing metal oxide particles by vibration or centrifugation can be mentioned. In addition, it is better to select a photocurable resin so that the refractive index on the transparent substrate side matches the refractive index of the transparent substrate as much as possible.

  When the surface shape of the fine concavo-convex periodic structure with the refractive index continuously changing as shown in FIG. 2 was measured by AFM, a concavo-convex pattern having a height of 350 nm and a pitch of 300 nm was formed.

  FIG. 6 shows a vacuum deposition apparatus for forming an ND film, which is an inorganic film made of a laminated film such as a metal film, a metal oxide, or a dielectric film, on the surface of the fine irregular periodic structure formed in FIG. The block diagram of the chamber is shown. A vapor deposition source 62 and a rotatable vapor deposition umbrella 63 are provided in the chamber 61, and a vapor deposition pattern forming mask 52 having an opening at a film formation site and a vapor deposition jig 51 are disposed in the vapor deposition umbrella 63. . The vapor deposition jig 51 rotates with the vapor deposition umbrella 63 about the Z axis to form a film.

FIG. 5 shows an ND film configuration diagram which is an inorganic hard film formed on a transparent substrate or a fine uneven periodic structure by the above-described method. On the transparent substrate or the fine uneven periodic structure, the first, third, and fifth layers have an Al ₂ O ₃ film 72 that is an antireflection film for reducing the reflectance, and the second, fourth, and sixth layers have transmittance. A TixOy film 73 that is a light absorption layer for reduction is alternately laminated. Further, in order to enhance the antireflection effect, an MgF ₂ film, which is a low refractive material, is applied to the outermost layer as the eleventh layer at an optical film thickness n × d (n: refractive index, d: physical film thickness) of λ / 4 (λ = 500 to 600 nm) is deposited to form an ND film having an 11 structure.

As the antireflection film, a transparent dielectric can be used. In addition to the Al ₂ O ₃ film 72, SiO ₂ , SiO, MgF ₂ , ZrO ₂ , TiO ₂ or the like can be used. The light absorbing layer can be made of a material having a characteristic of absorbing a wavelength in the visible region. In addition to the TiOx film 73, metals such as Ti, Ni, Cr, NiCr, NiFe, and Nb, alloys, and oxides can be used. Can be used.

Then, by monitoring the reflectance during vapor deposition, it is possible to reduce the reflectance by controlling the film thickness of the Al ₂ O ₃ film. The light transmittance varies depending on the total thickness of the TixOy film, and the light transmittance decreases as the total thickness of the TixOy film increases. Further, the flatness of the light transmittance within the wavelength range of λ = 400 to 700 nm varies depending on y of the above TixOy film composition, and the light transmittance distribution becomes flat when appropriately selected. Note that the number of layers to be stacked and the materials used vary depending on the target characteristics.

Such an ND film can be formed by a film forming method such as a vacuum evaporation method, a sputtering method, or an ion plating method. In this embodiment, the film was formed by a vacuum evaporation method as described above. Then, after forming the outermost MgF ₂ film, the transparent substrate is taken out from the vacuum deposition machine, and if necessary, the transparent substrate on which the ND film is formed is set as the back surface on the deposition jig 51 and vacuumed. Vapor deposition is performed in a vapor deposition machine.

  After the film formation is completed, the transparent substrate is taken out from the vacuum vapor deposition machine, and a plurality of ND filters formed on the transparent substrate are pressed into individual shapes.

  For the ND filter manufactured in this way, a plurality of constituent samples as shown in FIG. First, as shown in Table 1, as a per experiment, a sample in which a fine uneven periodic structure was formed on one surface on a transparent substrate was manufactured, and the reflectance was confirmed. While the maximum reflectance of the transparent substrate is 9%, the maximum reflectance is reduced to 5% when the fine uneven periodic structure is formed on one side. As an improvement measure, when a fine concavo-convex periodic structure in which the refractive index was continuously changed as in this example was formed on one side, the maximum reflectance was reduced to 4% or less, and an effect was observed.

Table 1 shows a comparison result between the maximum reflectance (λ = 400 to 700 nm) and the conventional example in the example of the present invention when there is a fine uneven periodic structure.

(Example 2)
In Example 2, an ND filter in which a fine uneven periodic structure is provided on one side of a transparent substrate shown in FIG. 8A and ND films are formed on both sides of the transparent substrate is manufactured. As in Example 1, the transparent substrate 1 is made of a PET resin film having a thickness of 100 μm and a fine uneven periodic structure as shown in FIG. In the same manner as in Example 1, an aluminum mold 4 is filled with a photo-curing resin 5 that is a monomer such as acrylic, and a transparent substrate 1 is placed on the aluminum mold 4 and irradiated with light to release the mold. The groove is transferred to one side of the transparent substrate 1 by a transfer method. Thus, when the surface shape of the transparent substrate 1 to which the fine uneven periodic structure was transferred was measured by AFM, it was confirmed that unevenness having a height of 350 nm and a pitch of 300 nm was formed.

  By using the transparent substrate 1 having a fine uneven periodic structure formed on one side by the above-described method, an ND film 2 having a concentration of 0.5 is formed on both surfaces of the fine uneven periodic structure, thereby providing a concentration of 1.0 ( An ND filter having a transmittance of 10% was formed. The method for forming the ND film 2 in the second embodiment is the same as that in the first embodiment.

  The ND filter produced in this way was attached to the diaphragm blade of the light quantity diaphragm device, and the captured image was evaluated. As shown in Table 2, with respect to the ND filter in which the fine uneven periodic structure was formed on the transparent substrate 1, the reflectance on the fine uneven periodic structure surface was reduced, and no ghost was seen. Furthermore, when the fine uneven periodic structure in which the refractive index of this example was continuously changed was adopted, the reflectance was 0.50% or less, and a significant improvement in reflectance was observed.

  Therefore, further improvement of characteristics can be expected by forming the fine uneven periodic structure having the configuration of the present embodiment on both surfaces of the transparent substrate. Further, the ND concentration is effective not only for a single concentration but also for a configuration in which the concentration changes continuously.

Table 2 shows a comparison result between the ghost, the maximum reflectance (λ = 400 to 700 nm) and the conventional example in the example of the present invention when there is a fine uneven periodic structure. The two values of the maximum reflectance are the maximum reflectance of light traveling in the direction shown in FIG.

  In Example 3, a fine uneven periodic structure is provided on one side of the transparent substrate shown in FIG. 8B to replace the AR film. An ND filter having an ND film formed on the opposite surface of the transparent substrate is manufactured. As in Example 1, the transparent substrate 1 is made of a PET resin film having a thickness of 100 μm and a fine uneven periodic structure as shown in FIG. In the same manner as in Example 1, an aluminum mold 4 is filled with a photo-curing resin 5 that is a monomer such as acrylic, and a transparent substrate 1 is placed on the aluminum mold 4 and irradiated with light to release the mold. The groove is transferred to one side of the transparent substrate 1 by a transfer method. Thus, when the surface shape of the transparent substrate 1 to which the fine uneven periodic structure was transferred was measured by AFM, it was confirmed that unevenness having a height of 350 nm and a pitch of 300 nm was formed.

  By using the transparent substrate 1 having a fine concavo-convex periodic structure formed on one side by the above-described method, an ND film 2 having a concentration of 0.5 is formed on one side on the fine concavo-convex periodic structure, thereby providing a concentration 0.5 (transmission). An ND filter with a rate of 32% was formed. The method for forming the ND film 2 in the second embodiment is the same as that in the first embodiment.

  The ND filter produced in this way was attached to the diaphragm blade of the light quantity diaphragm device, and the captured image was evaluated. As shown in Table 3, with respect to the ND filter having the fine uneven periodic structure formed on the transparent substrate 1, the reflectance on the fine uneven periodic structure surface was reduced, and no ghost was seen. Furthermore, when the fine uneven periodic structure in which the refractive index of this example is continuously changed is adopted, the ND film surface reflectance is 0.30% or less, and the fine uneven periodic structure surface reflectance is 3.00% or less. As a result, the reflectance was greatly improved.

  Therefore, further improvement of characteristics can be expected by forming the fine uneven periodic structure having the configuration of the present embodiment on both surfaces of the transparent substrate. Further, the ND concentration is effective not only for a single concentration but also for a configuration in which the concentration changes continuously.

Table 3 shows a comparison result between the ghost, the maximum reflectance (λ = 400 to 700 nm), and the conventional AR film in the example of the present invention when there is a fine uneven periodic structure. The two values of the maximum reflectance are the maximum reflectance of light traveling in the direction shown in FIG.

  It is possible to provide an optical filter that efficiently reduces reflected light, a light amount diaphragm device that includes the optical filter, and an imaging device equipped with the same.

1: Transparent substrate 2: ND film 3: AR film 4: Aluminum mold 5: Photo-curing resin 21: Lens 22: Light quantity reduction device 23: Lens 24: Lens 25: Lens 26: Low-pass filter 27: Solid-state imaging device 28: Diaphragm blade support plate 29: Diaphragm blade 30: ND filter 31: Transparent substrate 32: Projection 33: Fine uneven periodic structure 51: Deposition jig 61: Chamber 62: Deposition source 63: Deposition umbrella

Claims (7)

  A fine uneven periodic structure having a pitch and height below the visible light wavelength is formed on the surface of the transparent substrate, and the fine uneven periodic structure has a characteristic that the refractive index continuously changes from the transparent substrate side to the surface layer side. An optical filter characterized by that.   A fine uneven periodic structure having a pitch and height below the visible light wavelength is formed on both surfaces of the transparent substrate, and an inorganic hard film is formed on at least the fine uneven periodic structure on one side of the transparent substrate or on the back surface thereof. An optical filter characterized by the above.   The optical filter according to claim 1, wherein the transparent substrate is made of a synthetic resin.   The optical filter according to any one of claims 1 to 3, wherein the inorganic hard film is formed by laminating a light absorption layer made of a metal or a metal oxide and a dielectric layer.   The optical filter according to claim 4, wherein the visible light transmission density of the inorganic hard film has a constant density or a region where the density continuously changes.   A light quantity diaphragm device having diaphragm blades for forming an opening and an optical filter for adjusting the light quantity of light passing through the opening, comprising the optical filter according to any one of claims 1 to 5. A light quantity diaphragm device characterized by that.   An imaging apparatus comprising: an optical system; a light quantity diaphragm device according to claim 6 that limits an amount of light that passes through the optical system; and a solid-state imaging device that receives an image formed by the optical system.

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