JP2014016568A - Absorption type multilayer film nd filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an absorption type multilayer film ND filter having a low reflection property and small wavelength dependence of transmittance.SOLUTION: An absorption type multilayer film ND filter comprises an absorption type multilayer film, which has a five layer structure comprising three oxide dielectric film layers D, Dand Dand two absorption film layers Aand Aalternately laminated on top of each other, provided on each surface of a resin substrate S. A ratio T2/T4 of thickness T2 of the second layer counting from the resin substrate S, or the absorption film layer A, to thickness T4 of the fourth layer, or the absorption film layer A, on each side shall be in a range between 0.60 and 0.95. A thickness ratio T3/T5 of the third layer, or the oxide dielectric film layer D, to the fifth layer, or the oxide dielectric film layer D, on each side shall be in a range between 0.5 and 0.9. The fifth layer, or the oxide dielectric film layer D, is made of SiOcontaining a nanoporous material.

Description

  The present invention relates to an absorptive multilayer ND filter that attenuates transmitted light in the visible light region, and is particularly excellent in low reflectivity, in which oxide dielectric film layers and metal layers are alternately laminated on both surfaces of a resin film substrate. The present invention relates to an absorption type multilayer ND filter.

  In a camera system such as a digital still camera or a video camera, an ND (Neutral Density) filter that attenuates light is used to prevent saturation of a CCD or CMOS element, which is an imaging element, due to excessive light incidence. . In particular, in the camera system, since the reflected light inside the camera is regarded as a problem, an absorption ND filter is generally used as the ND filter incorporated in the lens optical system.

  This absorption type ND filter has a thin film formed on the surface, although there is no function to absorb the substrate itself other than the type in which the substrate itself is mixed with an absorbing material (colored glass ND filter) and the type in which the absorbing material is applied to the substrate. There is a type that absorbs. In the case of the type that absorbs with the latter thin film, in order to prevent reflection on the surface of the thin film, the thin film is composed of a multilayer film (absorptive multilayer film) and has a function of attenuating transmitted light and an antireflection function. There is.

As an absorptive multilayer ND filter composed of such multilayers (absorptive multilayers), physical properties such as vacuum deposition and sputtering are applied on transparent resin film substrates such as PET (polyethylene terephthalate) and PC (polycarbonate). There is known an ND filter in which an absorption film layer and an oxide dielectric film layer such as SiO ₂ are alternately stacked by film formation by a chemical vapor deposition method.

As the absorption film layer, an absorption film layer made of a metal oxide such as TiO _x or Ta ₂ O _x having absorption due to oxygen deficiency is known. This metal oxide can be formed by intentionally introducing oxygen, but its extinction coefficient is lower than that of an absorption film layer made of metal formed without intentionally introducing oxygen, Quality problems such as substrate warpage, which will be described later, may occur.

  Therefore, in Patent Document 1, a metal such as Ti or Cr is used as a material for the absorption film layer instead of the metal oxide, and the absorption film layer made of the metal film and the oxide dielectric film layer are alternately formed. Laminated ND filters have been proposed. Patent Documents 2 and 3 describe an absorption multilayer ND filter using a film forming material in which Ti or W is added to Ni as a metal film. These ND filters shown in Patent Documents 1 to 3 can be made thinner to obtain the same extinction coefficient than the absorption film layer made of the metal oxide described above. Therefore, it is possible to efficiently produce an absorption multilayer ND filter having desired transmission characteristics and reflection characteristics.

  That is, the absorption-type multilayer ND filter is cut into a predetermined shape and size to form an ND filter chip, and then incorporated into the above-described digital camera or the like. However, the digital camera or the like is small and thin and has a small installation space. Since the substrate is narrow, a resin film that is thin and flexible per se is the optimum substrate for the substrate of the ND filter. When forming an absorption multilayer film on such a resin film substrate, the metal film can reduce the film thickness of the absorption film layer compared to the metal oxide film, so that the film formation time can be shortened, and thus the film formation time. It is possible to suppress problems such as warpage of the resin film substrate and cracking of the film that can be caused by lengthening the thickness.

Japanese Patent Laid-Open No. 05-093811 JP 2006-058854 A JP 2006-009694 A

Incidentally, when an optical element having a flat surface such as an ND filter is incorporated in a camera or the like, lower reflection characteristics are required. This is because the flat ND filter causes multiple reflection inside the camera, which causes ghost in the video information. In order to reduce the reflection, it is generally effective to lower the refractive index of the outermost surface layer at the boundary with air. Therefore, MgF ₂ which is a material having a lower refractive index is usually used for the outermost surface layer. Often done.

Although the fluorine compound such as MgF ₂ can be formed by a vapor deposition method, fluorine disappeared by a film formation method using plasma such as a sputtering method, and the film could not be formed satisfactorily. Therefore, in the case of stacking the absorption multilayer film with a highly productive film forming apparatus combining the sputtering method and the roll-to-roll method, conventionally, SiO ₂ is used as the material of the outermost surface layer. However, regarding the refractive index, MgF ₂ is 1.38, whereas SiO ₂ is as high as 1.45, and sufficient low reflection characteristics could not be obtained.

  Further, it is desirable that the ND filter attenuates light at a substantially constant rate over all wavelengths of visible light so that the hue of the image does not change when incorporated in the lens system. As described above, in the ND filter, it is desired to reduce the wavelength dependency of the transmittance of visible light as much as possible. Therefore, conventionally, various types of materials are combined and the number of layers has been increased. However, since the number of targets is limited in the sputtering method, the characteristics of the obtained ND filter are limited, and a sufficiently satisfactory characteristic cannot be obtained.

Thus, it can be efficiently produced by a film forming method using plasma such as sputtering, has a low reflection characteristic comparable to that of an ND filter using MgF ₂ , and has a wavelength dependency of transmittance. A low absorption multilayer ND filter has been strongly desired.

In order to solve the above-mentioned problems, the present inventor has intensively studied to obtain an ND filter exhibiting low reflection characteristics by a simple method while using SiO ₂ having a higher refractive index than that of MgF ₂ as a dielectric material. . As a result, in the absorption multilayer ND filter composed of a plurality of absorption film layers and a plurality of oxide dielectric film layers, the ratio of the film thicknesses of the absorption film layers and the film thickness of the oxide dielectric film layers In addition, the outermost surface layer is made of a low-refractive index SiO _x film containing nanoporous material, and the wavelength dependence of the transmittance is reduced while ensuring the same reflection characteristics as when MgF ₂ is used. The present inventors have found that this can be done and have completed the present invention.

That is, the absorption multilayer ND filter according to the present invention is a resin having a five-layer absorption multilayer film in which three oxide dielectric film layers and two absorption film layers are alternately laminated one by one. Absorption-type multilayer ND filters provided on both sides of the substrate, each having a film thickness T2 of the second absorption film layer and a film thickness T4 of the fourth absorption film layer counted from the resin substrate side. The ratio T2 / T4 is in the range of 0.60 to 0.95, and the ratio T3 / T5 of the film thickness of the third oxide dielectric film layer and the fifth oxide dielectric film layer is The fifth oxide dielectric film layer is in the range of 0.5 to 0.9, and is characterized in that it is made of SiO _x containing nanoporous material.

  According to the present invention, a high-quality ND filter having low reflection characteristics in the visible light wavelength range of about 400 nm to about 700 nm and having little transmittance wavelength dependency can be obtained at a low cost by a relatively simple manufacturing process. It becomes possible to provide.

It is typical sectional drawing of one specific example of the absorption type multilayer ND filter which concerns on this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic front view of a sputtering roll coater device that can suitably produce an absorption multilayer ND filter according to the present invention. It is a graph which shows the spectral transmittance and spectral reflectance in wavelength 400nm -700nm of the absorption type multilayer ND filter of an Example. It is a graph which shows the spectral transmittance and spectral reflectance in wavelength 400nm-700nm of the absorption type multilayer film ND filter of a comparative example.

A specific example of the absorption multilayer ND filter according to the present invention will be described below with reference to FIG. This absorption multilayer ND filter has five layers in which three oxide dielectric film layers D ₁ , D ₂ and D ₃ and two absorption film layers A ₁ and A ₂ are alternately stacked. The absorption multilayer film having the structure is provided on both surfaces of the resin substrate S. In each of the first surface and the second surface of the resin substrate S, and the thickness T2 of the absorbent layer A ₁ of the second layer counted from the resin substrate S side of the fourth layer of the absorber layer A ₂ having a thickness T4 the ratio T2 / T4 is in the range of 0.60 to 0.95, the oxide of the third layer thickness T3 of the oxide dielectric film layer _{D 2} of the fifth layer dielectric layer _{D 3} The ratio T3 / T5 of the film thickness T5 is in the range of 0.5 to 0.9. Further, the fifth oxide dielectric film layer D ₃ is made of SiO _x containing nanoporous material.

  The absorption multilayer ND filter having such a configuration has a spectral transmission characteristic defined by a value obtained by dividing the difference between the maximum transmittance and the minimum transmittance in the visible wavelength range (wavelength of about 400 nm to about 700 nm) by the average transmittance. Has excellent optical properties of 5% or less. Furthermore, the maximum reflectance in the visible wavelength region (wavelength of about 400 nm to about 700 nm) is 2.0% or less. Hereafter, each component which comprises this absorption type multilayer ND filter of this invention is demonstrated in detail.

(1) Oxide dielectric film layer (SiO _x layer including SiO _x layer and outermost layer nanoporous layer)
Oxide dielectric film layers D ₁ , D ₂ , D ₃ (SiO _x layer and outermost layer) located in the first layer, the third layer, and the fifth layer (outermost surface layer) counting from the resin substrate S side The SiO _x layer including nanoporous surface layer) is formed by using an ion beam sputtering method or a magnetron sputtering method using a target mainly composed of SiC or Si and controlling the amount of oxygen gas introduced. Can do. As a result, the extinction coefficient at 400 nm of each oxide dielectric film layer can be set to 0 to 0.07.

In order to reduce the surface reflection of the absorption type multi-layer film ND filter, the refractive index of the oxide dielectric film layer D ₃ of the fifth layer located on the outermost surface layer as described above is preferably as low as possible. Therefore, the oxide dielectric film layer D ₃ of the fifth layer is formed by SiO _x layer comprising nanoporous. Thus, it extinction coefficient of the fifth layer of the oxide dielectric film layer D ₃ is reduced as compared with the oxide dielectric film layer D _1, D ₂ the other two, to realize a low refractive index It becomes possible. In particular, oxides of the fifth layer dielectric layer D ₃ is preferably extinction coefficient is 0 to 0.01.

Note that the SiO _x layer is a layer in which various silicon oxide such as SiO or SiO ₂ is mixed in the amorphous, as the refractive index, a higher value than the SiO ₂ in accordance with oxygen deficiency of SiO ₂ It has the feature of showing. This SiO _x layer is formed by performing sputtering in an atmosphere having a small amount of oxygen. Moreover, the layer containing nanoporous means the layer of the porous body containing innumerable micropores of nanometer order.

The SiO _x layer containing nanoporous material can be formed by sputtering at a higher Ar gas pressure than when forming the first and third layers. This is because sputtering is performed in an atmosphere containing excessive gas by increasing the Ar gas pressure during sputtering, so that more Ar and oxygen gas are taken into the SiO _x film. As a result, it is considered that many fine bubbles having a refractive index of 1 are formed, the extinction coefficient approaches 0, and the refractive index is lowered.

  That is, if there is a void in the dielectric layer, the refractive index of the void is 1, so the refractive index of the entire layer is smaller than its bulk value. Furthermore, by setting the size of the voids to nanoporous having a size of several nanometers that does not cause visible light scattering, it is possible to achieve a desired low reflectance while preventing haze reduction due to visible light scattering.

Among these first, third and fifth oxide dielectric film layers D ₁ , D ₂ and D ₃ , the film thickness T ₃ of the third oxide dielectric film layer D ₂ and the fifth layer the ratio T3 / T5 in thickness T5 of the oxide dielectric film layer _{D 3} eyes, must be within the range of 0.5 to 0.9. Thus, while limiting the film thickness ratio of the oxide dielectric film layers D ₂ and D ₃ and limiting the film thickness ratio of the absorption film layers A ₁ and A ₂ as described later, the wavelength of the transmittance The dependency can be reduced. The inventor speculates that the reason is due to the light interference effect at the interface of each layer. That is, when the ratio T3 / T5 is lower than 0.5, the transmittance in the wavelength region of 550 nm to 700 nm increases. On the other hand, when the ratio T3 / T5 exceeds 0.9, the transmittance in the wavelength region of 400 nm to 550 nm is increased, and the high-quality ND filter having a low wavelength dependency of the transmittance, which is a feature of the present invention. Cannot be obtained.

The film thicknesses of the first, third, and fifth oxide dielectric film layers D ₁ , D ₂ , and D ₃ are each preferably ₃ to 150 nm. If each film thickness is less than 3 nm, the contribution as an optical thin film is small, and it becomes difficult to control the film thickness. Furthermore, it cannot be expected to suppress a phenomenon in which the transmittance increases due to oxidation of the absorption film layer in a high temperature and high humidity environment. On the other hand, if each thickness of the oxide dielectric film layer is thicker than 150 nm, the desired performance as an optical thin film in the visible light region may not be exhibited effectively, and it is only preferable to reduce the production efficiency. Absent.

(2) Absorption film layer (metal layer)
The absorption film layers A ₁ and A ₂ located in the second layer and the fourth layer as counted from the resin substrate S side are composed of metal films. This metal film can be formed by physical vapor deposition using metal as a raw material and stopping the introduction of oxygen gas. The physical vapor deposition method can be selected from a vacuum deposition method, an ion beam sputtering method, a magnetron sputtering method, or an ion plating method. The absorption film obtained by the physical vapor deposition method has a refractive index at a wavelength of 400 nm of 1.5 to 3.0 and an extinction coefficient of 1.5 to 6.0.

The metal film formed without intentionally introducing oxygen at the time of film formation is TiO _x or Ta ₂ O _x having absorption due to oxygen deficiency formed by intentionally introducing oxygen at the time of film formation as described above. It is known that it has a high extinction coefficient as compared with metal oxide films such as. And when forming an absorption type multilayer film by alternately laminating an oxide dielectric film layer and an absorption film layer on a flexible resin film substrate, the resin film substrate that can be caused by a long film formation time Taking into account the warpage and cracking of the film, it is desirable to employ a metal film having a high extinction coefficient because the film thickness can be made thinner than that of the metal oxide film.

However, the metal film easily oxidizes and the extinction coefficient decreases, and the ND filter using the metal absorption film has a problem that the transmittance increases with time. Therefore, in the absorption multilayer ND filter according to the present invention, the oxide dielectric film layer adjacent to the metal absorption film is composed of the SiO _x film or the SiO _x film containing nanoporous, thereby oxidizing the metal absorber film layer. Is suppressed.

When the film thicknesses of the second and fourth absorption film layers A ₁ and A ₂ are T2 and T4, respectively, the ratio T2 / T4 needs to be in the range of 0.60 to 0.95. is there. Thus, by limiting the film thickness ratio together with the film thickness ratio of the oxide dielectric film layers D ₂ and D ₃ , the wavelength dependency of the transmittance is lowered as described above. That is, when the ratio T2 / T4 is lower than 0.60, the reflectance near the wavelength of 400 nm and the reflectance near 700 nm corresponding to both ends of the visible light region increase. On the other hand, when the ratio T2 / T4 exceeds 0.95, the reflectance near 550 nm, which is the center of the visible light region, increases, and it becomes difficult to ensure the low reflection characteristic that is a feature of the present invention.

The thicknesses of the second and fourth absorption film layers A ₁ and A ₂ are preferably 3 nm to 20 nm. If each film thickness is less than 3 nm, the metal film may be completely oxidized to the inside, which is not preferable. On the other hand, when each film thickness exceeds 20 nm, the spectral transmittance may be remarkably lowered.

The material of the metal film constituting the absorption film layers A ₁ and A ₂ is preferably Ni simple substance or Ni-based alloy which is a metal that is relatively difficult to oxidize. In the case of a Ni-based alloy, in particular, a Ni-based alloy containing one or more elements selected from the group consisting of Ti, Al, V, W, Ta, and Si is more preferable. Furthermore, when Ti is contained, the content is within a range of 5 to 15% by mass, when Al is contained, the content is within a range of 3 to 8% by mass, and when V is contained, the content is 3 to 9%. In the range of mass%, when W is included, the content is within the range of 18 to 32 mass%, when Ta is included, the content is within the range of 5 to 12 mass%, and when Si is included, the content is Is preferably in the range of 2 to 6 mass%.

  This is because if the Ti content is 5% by mass or more, the ferromagnetic characteristics can be remarkably weakened, and DC magnetron sputtering film formation can be performed even with a cathode on which a normal magnet having a low magnetic force is disposed. On the other hand, if the Ti content exceeds 15% by mass, a large amount of intermetallic compounds may be formed, which is not preferable. The reason why the contents of Al, V, W, Ta, and Si, which are elements other than Ti, are within the above ranges is the same as in the case of Ti.

In the ND filter of one specific example of the present invention, the fifth layer is the outermost surface layer, but may be further laminated thereon. For example, a fluororesin film having a thickness of about 2 nm to 10 nm may be formed on the SiO _x layer that is the oxide dielectric layer D ₃ located in the fifth layer. Thereby, a lower refractive index can be realized. This fluororesin film can be formed, for example, by a coating method such as spray coating, dip coating, or spin coating. However, since the film thickness is 10 nm or less, dip coating can be preferably used.

Specifically, a fluororesin layer can be formed by dipping a commercially available fluororesin solution (DS-5110Z130 manufactured by Harves Co., Ltd.) as a fluororesin solution for 10 seconds and then air drying for 24 hours. This fluororesin coating can also be expected to have the effect of preventing deterioration of the refractive index of the SiO _x film layer containing nanoporous material over time.

(3) Resin Substrate The resin substrate used in the present invention is preferably transparent, and is a long resin film substrate capable of dry roll coating by a roll-to-roll method described later in consideration of mass productivity. It is preferable that This is because the resin film substrate has excellent characteristics such as being cheaper, lighter and more flexible than a conventional glass substrate.

  Specific examples of such a resin film substrate were selected from the group consisting of polyethylene terephthalate (PET), polyethersulfone (PES), polyarylate (PAR), polycarbonate (PC), polyolefin (PO), and norbornene. Examples thereof include a single resin film made of a resin, or a composite in which an acrylic organic film is coated on one or both sides of a single resin film made of the above resin. Typical examples of resin films using norbornene as a material include ZEONOR (trade name) of Nippon Zeon Co., Ltd. and Arton (trade name) of JSR Corporation.

(4) Method for Producing Absorption-type Multilayer Film ND Filter Next, a method for producing the absorption-type multilayer film ND filter of the present invention by a roll-to-roll method will be described. FIG. 2 is a front view showing a schematic configuration of a roll-to-roll physical vapor deposition apparatus (sputtering roll coater apparatus). The physical vapor deposition apparatus shown in FIG. 2 is provided in a sputtering chamber 10 equipped with a vacuum pump (not shown), and includes a first roll 11 for winding and unwinding a long resin film F, and The second roll 12, the water-cooled can roll 13 around which the resin film F conveyed from one of the first roll 11 and the second roll 12 to the other is wound, and the first through the water-cooled can roll 13. Four guide rolls 14, 15, 16, and 17 that define a transport path of the resin film F transported between the roll 11 and the second roll 12 are provided.

  The 1st roll 11 and the 2nd roll 12 are provided with the powder clutch, and, thereby, the tension balance of the resin film F is maintained. Further, the transport direction of the resin film F is determined by the rotation direction of the water-cooled can roll 13, and in this specification, the water-cooled can roll 13 rotates counterclockwise on the paper surface of FIG. The transport direction in which the scale-shaped resin film F is drawn out from the first roll 11 and wound up on the second roll 12 side is defined as the forward rotation direction, and conversely, the water-cooled can roll 13 rotates clockwise on the paper surface of FIG. A reverse direction is defined as a conveyance direction in which the long resin film F rotates and is drawn out from the second roll 12 and wound up on the first roll 11 side.

In the sputtering chamber 10, a sputtering cathode (dual magnetron sputtering cathode) 18 and a sputtering cathode 19 are disposed so as to face the outer peripheral surface of the water-cooled can roll 13. A target for forming an oxide dielectric film layer (for example, a SiO _x film layer) is attached to the sputtering cathode 18. On the other hand, a target for forming a metal absorption film layer (for example, Ni alloy layer) is attached to the sputtering cathode 19.

With this configuration, when the oxide dielectric film layer is formed, the water-cooled can roll 13 is rotated counterclockwise to convey the long resin film F in the forward rotation direction. As a result, the resin film F is drawn out from the first roll 11, guided to the water-cooled can roll 13 through the guide rolls 14 and 15, and is adhered to the outer peripheral surface of the water-cooled can roll 13 and cooled by the sputtering cathode 18. An oxide dielectric film layer (eg, a SiO _x film layer) is formed. The oil film F on which the oxide dielectric film layer is formed is wound around the second roll 12 through the guide rolls 16 and 17.

  When the formation of the oxide dielectric film layer is completed and almost all of the resin film F is wound on the second roll 12, the rotation direction of the water-cooled can roll 13 is reversed and the resin film F is conveyed in the reverse direction. To do. As a result, the resin film F is drawn out from the second roll 12 and guided to the water-cooled can roll 13 through the guide rolls 17 and 16, where it is adhered to the outer peripheral surface of the water-cooled can roll 13 and cooled by the sputtering cathode 19. An absorption film layer (for example, a Ni alloy layer) is formed on the oxide dielectric film layer. The resin film F on which the absorption film layer is formed is wound around the first roll 11 through the guide rolls 15 and 14. Hereinafter, the oxide dielectric film layer and the absorption film layer are formed on one surface of the long resin film F by repeating the oxide dielectric film layer formation process and the absorption film layer formation process described above. An absorption multilayer film laminated alternately is obtained.

  When laminating an absorption multilayer film similar to the above on the other side of the long resin film F, the roll of the resin film F having the absorption multilayer film laminated on one side is once removed from the roll coater device. Then, after rewinding so that the front and back are reversed, the apparatus is again applied to the first roll 11 and laminated in the same manner as described above. The film thickness of the absorption multilayer film can be controlled by controlling the conveyance speed of the long resin film F.

[Example]
Using the roll-to-roll physical vapor deposition apparatus (sputtering roll coater apparatus) shown in FIG. 2, three oxide dielectric film layers and two absorption film layers are alternately formed one by one. An absorptive multilayer ND filter having the laminated absorptive multilayer film on both sides of the film was produced. Ni-7.5 mass% Ti alloy target [Ti-Ni alloy target with a Ti addition ratio of 7.5 mass% manufactured by Sumitomo Metal Mining Co., Ltd.] as a target for forming the absorption film layer The film was formed by DC magnetron sputtering using Ar gas.

On the other hand, as a target for depositing SiO _x that is an oxide dielectric film or SiO _x containing nanoporous material, a dual magnetron sputtering in which Ar gas is introduced using a Si target (manufactured by Sumitomo Metal Mining Co., Ltd.). Was formed. In order to form SiO _x containing Si _x or nanoporous film from Si, the oxygen introduction amount was controlled by an impedance monitor, and the extinction coefficient was adjusted by the oxygen introduction amount.

  Here, in dual magnetron sputtering, in order to form an insulating film at high speed, medium frequency (40 kHz) pulses are alternately applied to two targets to suppress arcing and prevent formation of an insulating layer on the target surface. It is a sputtering method. The impedance monitor applies a phenomenon in which the impedance between the target electrodes changes depending on the amount of oxygen introduced, so that the film to be formed becomes a film of a desired mode in a transition region between the metal mode and the oxide mode. The amount introduced is controlled and monitored, and is used to form an oxide dielectric film layer at high speed.

  Since the thickness of the film obtained by the film formation method is determined by the transport speed of the resin film substrate and the sputtering power, the sputtering power is set to 7.5 kW when forming the oxide dielectric film layer, and the absorption film Sputtering power was set to 1 kW when forming the layer. And the conveyance speed of the resin film board | substrate was adjusted so that it might become a desired physical film thickness. A PET film was used as the resin film substrate, and five layers on one side of the film structure shown in Table 1 below were first formed on one side. Thereafter, the roll of the film is removed and rewound so that the front and back are reversed. The film is again mounted on the roll coater and formed on the other surface in the same manner as described above. Five layers were deposited.

Specifically, an absorptive multilayer film was formed along the following film formation steps (1) to (6).
(1) After exhausting the inside of the sputtering chamber 10 of the sputtering roll coater apparatus shown in FIG. 2 to about 1 × 10 ⁻⁴ Pa, the water-cooled can roll 13 is rotated counterclockwise, and the first roll 11 to the second roll. The resin film F was conveyed in the forward rotation direction toward 12. The conveyance speed of the resin film F was adjusted to 1.50 m / min. Further, Ar gas was introduced at 150 sccm to the side of the sputtering cathode 18 to which the Si target was attached, the Ar gas pressure was set to 0.3 Pa, the sputtering output was set to 7.5 kW, and the first oxide dielectric film layer D ₁ was deposited. At the time of film formation, the oxygen introduction amount was adjusted by an impedance monitor so that the extinction coefficient of the SiO _x film became a predetermined value.

(2) When the formation of the first oxide dielectric film layer D ₁ is completed, the water-cooled can roll 13 is reversed, and the resin film F is reversed in the reverse direction from the second roll 12 toward the first roll 11. Conveyed. The conveyance speed of the resin film F was adjusted to 1.49 m / min. Further, Ar gas is introduced at 150 sccm to the side of the sputtering cathode 19 to which the Ni alloy target (Ti—Ni alloy target) is attached, the sputtering output is set to 1 kW, and the second absorption film layer A ₁ made of Ni alloy. Was deposited.

(3) by inverting the water-cooled can roll 13 where the deposition of the absorbing layer A ₁ of the second layer is complete, and conveys the resin film F from the first roll 11 in the forward direction toward the second roll 12 . The conveyance speed of the resin film F was adjusted to 0.53 m / min. Further, Ar gas was introduced at 150 sccm to the side of the sputtering cathode 18 to which the Si target was attached, the Ar gas pressure was set to 0.3 Pa, the sputtering output was set to 7.5 kW, and the third oxide dielectric film layer D ₂ was deposited. At the time of film formation, the oxygen introduction amount was adjusted by an impedance monitor so that the extinction coefficient of the SiO _x film became a predetermined value.

(4) by inverting the water-cooled can roll 13 where the third layer of the oxide dielectric film layer formation of D ₂ is completed, the resin film F from the second roll 12 in the reverse direction towards the first roll 11 Conveyed. The conveyance speed of the resin film F was adjusted to 1.17 m / min. Further, Ar gas is introduced at 150 sccm to the side of the sputtering cathode 19 to which the Ni alloy target (Ti—Ni alloy target) is attached, the sputtering output is set to 1 kW, and the fourth absorption film layer A ₂ made of Ni alloy is formed. Was deposited.

(5) When the film formation of the fourth absorption film layer A ₂ is completed, the water-cooled can roll 13 is reversed, and the resin film F is conveyed in the normal rotation direction from the first roll 11 to the second roll 12. . The conveyance speed of the resin film F was adjusted to 0.41 m / min. Further, Ar gas was introduced at 300 sccm to the side of the sputtering cathode 18 to which the Si target was attached, the Ar gas pressure was set to 0.6 Pa, the sputtering output was set to 7.5 kW, and the fifth oxide dielectric film layer D ₃ was deposited. At the time of film formation, the amount of oxygen introduced was adjusted by an impedance monitor so that the SiO _x film containing nanoporous had a predetermined extinction coefficient.

  (6) Take out the long resin film F on which the film formation on one side (front surface) has been completed, invert the front and back and set it on the first roll 11, and repeat the steps (1) to (5) above. An absorptive multilayer film was also formed on the back surface of the long resin film F. The long resin film F having the absorption multilayer film formed on both the front and back surfaces was taken out of the sputtering chamber.

  The absorption type multilayer ND filter produced in this way (see Table 1 for the film structure of the multilayer film) has an absorption layer thickness ratio T2 / T4 of 0.78 at 6.4 nm / 8.2 nm, The ratio T3 / T5 of the dielectric thickness of the outermost layer of the third-layer dielectric was 0.77 at 59 nm / 77 nm.

  Further, when the spectral transmission characteristics of the absorption multilayer ND filter including the transmission characteristics of the resin film (PET film having a film thickness of 100 μm) were measured using a spectrophotometer (V570 manufactured by JASCO Corporation), visible light was obtained. The average transmittance in the range of 400 to 700 nm was 12.5%. Further, when the spectral reflectance of the absorption multilayer ND filter was measured using a self-recording spectrophotometer (V570, manufactured by JASCO Corporation), the average reflectance in the visible light region of 400 nm to 700 nm was 0.6%.

Furthermore, the transmittance flatness value (T _max −T _min / T _ave ) representing the wavelength dependency of the transmittance was 4.33%. T _max , T _min , and T _ave are as follows.
T _max : Maximum transmittance at 400 nm to 700 nm T _min : Minimum transmittance at 400 nm to 700 nm T _ave : Average transmittance at 400 nm to 700 nm Spectral transmittance of absorption multilayer ND filter in visible light region 400 nm to 700 nm And the graph of a spectral reflectance is shown to Fig.3 (a), (b).

[Comparative example]
Absorptive multilayer ND filter having an absorptive multilayer film on both sides of a film in which three oxide dielectric film layers and two absorptive film layers are alternately laminated in the same manner as in the above embodiment However, in order to obtain the film configuration shown in Table 2 below, film formation was performed at a gas flow rate of 150 sccm and a gas pressure of 0.3 Pa when forming the outermost surface layer. Further, the conveyance speed of the resin film F is adjusted so that the film thickness ratio T2 / T4 of the absorption film layer is 1.0 and the film thickness ratio T3 / T5 of the oxide dielectric film layer is 1.0. I made it.

When the spectral transmittance and the spectral reflectance were measured for the absorption multilayer ND filter of the comparative example in the same manner as in the example, the average transmittance in the visible light range of 400 to 700 nm was 13.1. %, And the average reflectance was 1.57%. Further, the transmittance flatness value (T _max −T _min / T _ave ) representing the wavelength dependency of the transmittance was 17.01%. FIGS. 4A and 4B are graphs of the spectral transmittance and the spectral reflectance of the absorption multilayer ND filter in the visible light region of 400 nm to 700 nm.

D ₁ , D ₂ , D ₃
Oxide dielectric film layer A _{1, A 2} absorption film layer S resin substrate 10 sputtering chamber 11 first roll 12 and the second roll 13 water cooling can roll 14, 15, 16, 17 guide rolls 18 and 19 the sputtering cathode

Claims (5)

Absorptive multilayer ND filter in which an absorption multilayer film having a five-layer structure in which three oxide dielectric film layers and two absorption film layers are alternately laminated is provided on both sides of a resin substrate. The ratio T2 / T4 of the thickness T2 of the second absorption film layer and the thickness T4 of the fourth absorption film layer counted from the resin substrate side is in the range of 0.60 to 0.95. The thickness ratio T3 / T5 of the third oxide dielectric film layer and the fifth oxide dielectric film layer is in the range of 0.5 to 0.9, An absorption multilayer ND filter, wherein the fifth oxide dielectric film layer is made of SiO _x containing nanoporous material.   The three oxide dielectric film layers were formed by ion beam sputtering or magnetron sputtering while controlling the amount of oxygen gas introduced using a target composed mainly of SiC and Si as a raw material. It is an oxide dielectric film having an attenuation coefficient of 0 to 0.07, and the fifth oxide dielectric film layer has a smaller extinction coefficient than the other two oxide dielectric film layers. The two absorption film layers are formed by using an ion beam sputtering method, a magnetron sputtering method or an ion plating method with a metal as a raw material, and have a refractive index of 1.5 to 3.0 at 400 nm and an extinction coefficient of 1.5. The absorption multilayer ND filter according to claim 1, which is a metal absorption film of ˜6.0.   3. The absorption multilayer ND filter according to claim 1, wherein each of the three oxide dielectric film layers has a thickness of 3 to 150 nm.   The absorption multilayer ND filter according to any one of claims 1 to 3, wherein the two absorption film layers are made of Ni alone or a Ni-based alloy.   The Ni-based alloy includes one or more elements selected from the group consisting of Ti, Al, V, W, Ta, and Si. When Ti is included, the content is 5 to 15% by mass. Within the range, when Al is included, the content is within a range of 3 to 8% by mass, when V is included, the content is within a range of 3 to 9% by mass, and when W is included, the content is 18 to In the range of 32% by mass, when containing Ta, the content is within the range of 5-12% by mass, and when containing Si, the content is within the range of 2-6% by mass, The absorption multilayer ND filter according to claim 4.

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