JP4988282B2 - Optical filter - Google Patents

Description

  The present invention relates to an optical filter used in a photographing apparatus such as a camera or an optical apparatus.

  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 against this problem, a film-like ND (Neutral Density) filter is attached to the diaphragm blade as a light quantity adjusting member, and the quantity of light is attenuated while the diaphragm aperture 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, a multilayer film made of a metal film, a metal oxide, and a dielectric film is formed on a transparent resin film such as PET by a vacuum deposition method. Further, in Patent Document 2, a flat transmittance characteristic and a surface antireflection characteristic are obtained by alternately providing a light absorbing film made of one kind of metal oxide having a light absorbing property and a transparent dielectric film. An ND filter that satisfies the back-surface antireflection characteristic is disclosed.

JP-A-10-133253 JP 2003-344612 A

  However, 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. Normally, as shown in FIG. 15A, an ND film 2 is formed on one side of a transparent substrate 1 made of PET resin or the like, and an AR coating 3 is formed on the other side as an antireflection film, or FIG. As shown in FIG. 2B, ND films 2 are formed on both surfaces of the transparent substrate 1.

FIG. 16 shows a film configuration diagram of the ND film 2. Al ₂ O ₃ films 11 and TiOx films 12 are alternately stacked on the transparent substrate 1, and an MgF ₂ film 13 is deposited on the seventh outermost layer. The ND film 2 having a seven-layer structure is formed.

  FIG. 17 is an explanatory diagram of reflection at the interface of the ND filter. As shown in FIG. 15A, the ND film 2 is formed on one surface and the AR coating 3 is formed on the other surface. Since reflection occurs at the interface between substances having different refractive indexes, the interface a between the air and the AR coat 3, the interface b between the AR coat 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. If the ND film 2 or the AR coat 3 is a multilayer film, there is reflection at the interface of each layer in the film.

  An object of the present invention is to provide an optical filter that solves the above-mentioned problems and improves the antireflection effect at the interface between the transparent substrate and the vapor deposition layer, which is difficult to reduce reflection by the conventional method.

Technical characteristics of the optical filter according to the present invention for achieving the above object is to have the following pitch and height visible light wavelength to form a fine uneven periodic structure having a reflection preventing function on a surface of the transparent substrate, An ND film in which a light absorption layer and a dielectric layer made of metal or metal oxide are alternately laminated is formed on the fine uneven periodic structure , and an antireflection film is formed on the outermost layer of the ND film. It is in.

Also, the technical characteristics of the optical filter according to the present invention, have a duplex in the visible light wavelength or less of the pitch and height of the transparent substrate to form a fine uneven periodic structure having a reflection preventing function, at least one surface of said transparent substrate An ND film in which a light absorption layer made of metal or metal oxide and a dielectric layer are alternately laminated is formed on the fine concavo-convex periodic structure , and an antireflection film is formed on the outermost layer of the ND film . There is.

  According to the optical filter of the present invention, the substrate and the inorganic formed on the substrate by forming a fine concavo-convex periodic structure composed of concavo-convex portions having a pitch and height below the visible light wavelength on the substrate of the optical filter. Reflected light generated at the interface with the hard film can be reduced.

  The present invention will be described in detail based on the embodiment shown in FIGS.

  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. 2 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. 2, an infinite number of conical protrusions 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. 2, but instead of the conical shape, the quadrangular pyramid-shaped protrusion 34 shown in FIG. The fine uneven periodic structure 33 may be used as another polygonal pyramid shape, or the inverted conical recess portion 35 shown in FIG.

  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 becomes smaller, the wavelength at which the antireflection effect can be obtained is also expanded to the lower wavelength side. However, as the pitch P is made smaller, the formation of the fine uneven periodic structure 33 becomes more difficult, so the pitch P is preferably 100 to 300 nm. .

  Further, the larger 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, the greater the antireflection effect becomes. 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 33 to be formed, heat and pressure are applied to the surface of the substrate 31 to form fine unevenness. The periodic structure 33 is transferred. Alternatively, the fine uneven periodic structure 33 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 of a solidifiable material on the surface of the substrate 31 and a mold having a shape opposite to that of the fine uneven periodic structure 33 to be formed is adhered to solidify the solidifiable material.

  In this embodiment, as shown in FIG. 5, hot pressing is performed using an upper mold 42 in which a fine uneven periodic groove 41 having a shape opposite to the fine uneven periodic structure 33 is formed, and a lower mold 43 having a flat surface. Thus, the fine irregular periodic groove 41 is transferred onto the transparent substrate 31. The fine uneven periodic groove 41 of the upper mold 42 can be formed by forming a resist pattern on a mold base material by electron beam drawing and etching the base material by reactive ion etching, for example.

  In the hot press in the present embodiment, the temperature is maintained at about 110 ° C., which is often higher than the glass transition point of PET by a heater (not shown). Then, the transparent substrate 31 is sandwiched between the upper die 42 and the lower die 43, and pressure is applied in the direction of the arrow in the figure, so that the transparent substrate 31 softened by heat is formed on the inner surface of the upper die 42 by pressure. The uneven periodic groove 41 is transferred.

  When the surface shape of the fine concavo-convex periodic structure 33 thus formed was measured by AFM, a concavo-convex pattern having a height of 350 nm and a pitch of 300 nm was formed.

  Moreover, the fine uneven | corrugated periodic groove | channel 41 of the upper mold | type 42 can use what was produced by the other method. For example, using electron beam drawing or laser interferometry used in semiconductor manufacturing, or forming a predetermined fine concavo-convex periodic structure 33 on a glass material, Ni plating is performed, and this plating layer is peeled off. An electroforming method for forming a mold can also be used.

  FIG. 6 shows a cross-sectional view of the vapor deposition jig 51, and the transparent substrate 31 and the vapor deposition pattern forming mask 52 are fixed to the vapor deposition jig 51 through pins or the like.

  FIG. 7 shows the configuration of a vacuum deposition chamber 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 fine uneven periodic structure 33 formed in FIG. The figure is shown. In the chamber 61, a vapor deposition source 62 and a rotatable vapor deposition umbrella 63 are provided. In this vapor deposition umbrella 63, a vapor deposition pattern forming mask 52 having an opening at a film formation site and a transparent substrate 31 are set in FIG. A vapor deposition jig 51 shown is arranged. The transparent substrate 31 fixed to the vapor deposition jig 51 rotates around the Z axis together with the vapor deposition umbrella 63 to form a film.

FIG. 8 shows a film configuration diagram of an ND film 71 which is an inorganic hard film having a concentration of 1.0 (transmittance of 10%) formed on the fine uneven periodic structure 33 by the above-described method. On the fine uneven periodic structure 33 on the transparent substrate 31, an Al ₂ O ₃ film 72, which is an antireflection film for reducing the reflectivity, is formed on the _{first, third} , and fifth layers, and the second, fourth, and sixth layers are formed. TiOx films 73, which are light absorption layers for reducing the transmittance, are alternately stacked. Further, in order to enhance the antireflection effect, a SiO ₂ film 74, which is a low refractive material, is applied to the seventh outermost layer as an optical film thickness n × d (n: refractive index, d: physical film thickness) at λ / 4 (λ = 500 to 600 nm) is vapor-deposited to form an ND film 71 having a seven-layer structure. Instead of the SiO ₂ film 74, an MgF ₂ film may be used.

As the antireflection film, a transparent dielectric can be used, and 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, the reflectance can be reduced by controlling the film thickness of the Al ₂ O ₃ film 72. The light transmittance varies depending on the total film thickness of the TiOx film 73, and the light transmittance decreases as the total film thickness of the TiOx film 73 increases. In addition, the flatness of the light transmittance within the wavelength range of λ = 400 to 700 nm varies depending on x of the above-described TiOx 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 71 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 ND film 71 is formed by a vacuum evaporation method as described above. Then, after the outermost SiO ₂ film 74 is formed, the transparent substrate 31 is taken out from the vacuum vapor deposition machine, and if necessary, the transparent substrate 31 on which the ND film 71 is formed is used as the back surface and is again placed on the vapor deposition jig 51. Set and deposit in a vacuum deposition machine.

  The unevenness of the fine uneven periodic structure 33 is transferred to the deposited film in the ND film 71 up to about the second to third layers, but after that, the unevenness is gradually smoothed. Therefore, several layers close to the fine uneven periodic structure 33 are at the boundary surface, and the same antireflection effect as that of the fine uneven periodic structure 33 can be expected. Further, this does not have a special influence on the characteristics as the ND film 71.

Further, as shown in FIG. 9A, a single layer film of SiO ₂ is formed on the back surface as an antireflection film 75 by λ / 4 (λ = 540 nm), or as shown in FIG. Also, the ND film 71 is deposited. Then, after the film formation on the back surface is completed, the transparent substrate 31 is taken out from the vacuum evaporation machine, and the plurality of ND filters 30 formed on the transparent substrate 31 are punched into individual shapes.

  The manufactured ND filter 30 was attached to the diaphragm blades of the light quantity diaphragm device, and the captured image was evaluated. As shown in Table 1, the ND filter 30 in which the fine uneven periodic structure 33 is formed on the transparent substrate 31 has a low reflectance and no ghost is seen, but the fine uneven periodic structure 33 is not formed. A ghost was observed for the ND filter.

Table 1
Fine irregular periodic structure Ghost Maximum reflectance (λ = 400-700nm)
Yes No 2%
No Yes 6%

  In Example 2, the ND filter 30 in which the ND film 71 is formed on the fine uneven periodic structure 33 provided on both surfaces of the transparent substrate 31 shown in FIG. Similar to the first embodiment, the transparent substrate 31 is made of a PET resin film having a thickness of 100 μm, and an upper mold 42 and a lower mold 43 ′ in which fine uneven periodic grooves 41 as shown in FIG. 11 are formed. The fine uneven periodic grooves 41 of the upper mold 42 and the lower mold 43 ′ are transferred to both surfaces of the transparent substrate 31 by hot pressing under the same temperature and pressure conditions as in the first embodiment. Thus, when the surface shape of the transparent substrate 31 to which the fine uneven periodic structure 33 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 31 having the fine uneven periodic structure 33 formed on both sides by the above-described method, the ND film 71 having a concentration of 0.3 is formed on both surfaces of the fine uneven periodic structure 33, respectively. 6 (transmittance 25%) of the ND filter 30 was formed. The method for forming the ND film 71 in the second embodiment is the same as that in the first embodiment.

  The ND filter 30 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 30 in which the fine uneven periodic structure 33 is formed on the transparent substrate 31, the reflectance is small and no ghost is seen, but the ND filter 30 in which the fine uneven periodic structure 33 is not formed. A slight ghost was seen for the filter.

Table 2
Fine irregular periodic structure Ghost Maximum reflectance (λ = 400-700nm)
Yes No 1%
No Yes (minor) 4%

  In Example 3, after forming the fine uneven periodic structure 33 on the transparent substrate 31 in the same procedure as in Example 1, the density 1.0 (transmittance) is formed on the fine uneven periodic structure 33 as shown in FIG. An ND film 83 which is an inorganic hard film having a uniform density portion 81 and a gradation density portion 82 of 10%) is formed.

As shown in FIG. 13, when a vapor deposition pattern forming mask 52 is set on the vapor deposition jig 51 at a predetermined interval from the transparent substrate 31, and vapor deposition is performed in a vacuum vapor deposition machine, the vapor deposition jig 51 is placed in the chamber 61 shown in FIG. The ND film 83 having a gradation density distribution as shown in FIG. 14 can be formed while rotating around the Z axis as indicated by the arrow.

  The ND film 83 has a gradation density portion 82 in which the film thickness gradually decreases. In the gradation density portion 82, the film thickness of each layer differs depending on the position. Normally, even if the ND filter 30 having such a gradation density part 82 has a film configuration in which reflection is suppressed by the uniform density part 81, the film thickness is changed in the gradation density part 82, so that reflection is large. The position which becomes will occur. This is because the reflection is totally suppressed by utilizing the interference of the reflected light of each layer. In the third embodiment, the fine uneven periodic structure 33 formed on the surface of the transparent substrate 31 causes the interface of each layer. Since reflection is suppressed by giving a gradient to the refractive index, an increase in reflectance due to position can be suppressed.

On the back surface of the transparent substrate 31 on which the ND film 83 is laminated, a single layer film of SiO ₂ is deposited as an antireflection film 75 by depositing λ / 4 (λ = 540 nm). After the antireflection film 75 on the back surface is formed, the transparent substrate 31 is taken out from the vacuum deposition machine, and the plurality of ND filters 30 formed on the transparent substrate 31 are punched out into individual shapes.

  The ND filter 30 produced in this way was attached to the diaphragm blades of the light quantity diaphragm device, and the captured image was evaluated. Table 3 shows the evaluation. For the ND filter 30 in which the fine uneven periodic structure 33 is formed on the transparent substrate 31, the reflectance is reduced and no ghost is seen, but the fine uneven periodic structure 33 is not formed. A ghost was seen for the ND filter.

Table 3
Fine irregular periodic structure Ghost Maximum reflectance (λ = 400-700nm)
Yes No 4%
No Yes 15%

1 is a configuration diagram of a photographic optical system of Example 1. FIG. It is a schematic diagram of a fine uneven | corrugated periodic structure. It is a schematic diagram of the fine uneven | corrugated periodic structure of a modification. It is a schematic diagram of the fine uneven | corrugated periodic structure of another modification. It is explanatory drawing which forms a fine uneven | corrugated periodic structure by hot press. It is sectional drawing of a vapor deposition jig. It is a block diagram of a chamber. It is a film | membrane structure figure of ND film | membrane. It is a cross-sectional schematic diagram of an ND filter. 6 is a schematic cross-sectional view of an ND filter of Example 2. FIG. It is explanatory drawing which forms a fine uneven | corrugated periodic structure by hot press. 6 is a schematic cross-sectional view of an ND filter of Example 3. FIG. It is sectional drawing of a vapor deposition jig. It is a top view of the transparent substrate which formed the ND film | membrane. It is a cross-sectional schematic diagram of the ND filter of a prior art example. It is a film | membrane structure figure of ND film | membrane. It is explanatory drawing of the light reflection in the interface of ND filter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 21 Lens 22 Light quantity stop apparatus 27 Solid-state image sensor 28 Aperture blade support plate 29a, 29b Aperture blade 30 ND filter 31 Transparent substrate 32, 34 Protrusion part 33 Fine uneven | corrugated periodic structure 35 Concave part 41 Fine uneven | corrugated periodic groove 42 Upper mold | type 43, 43 ' Lower mold 51 Deposition jig 52 Deposition pattern forming mask 61 Chamber 62 Deposition source 63 Deposition umbrella 71, 83 ND film 72 Al ₂ O ₃ film 73 TiOx film 74 SiO ₂ film 75 Antireflection film 81 Uniform density part 82 Gradation density part

Claims (6)

It has a visible light wavelength or less of the pitch and height to form a fine uneven periodic structure having a reflection preventing function on a surface of the transparent substrate, the light absorbing layer and a dielectric made of a metal or metal oxide onto the fine irregularities periodic structure An optical filter comprising: an ND film in which body layers are alternately stacked; and an antireflection film is formed on the outermost layer of the ND film . Have a visible light wavelength or less of the pitch and height to form a fine uneven periodic structure having a reflection preventing function on both surfaces of the transparent substrate, a metal or metal oxide on at least the transparent substrate of one surface of the fine concavo-convex periodic structure on An optical filter comprising: an ND film in which a light absorption layer and a dielectric layer are alternately laminated; and an antireflection film is formed on the outermost layer of the ND film .   The optical filter according to claim 1, wherein the transparent substrate is made of a synthetic resin. The optical filter according to claim 1 , wherein the visible light transmission density of the ND film has a region where the density continuously changes. A light amount diaphragm comprising: aperture blades for forming an opening; and the optical filter according to any one of claims 1 to 4 for adjusting a light amount of light passing through the opening. apparatus. 6. An imaging apparatus comprising: an optical system; a light quantity stop device according to claim 5 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.