JP7701708B2 - ND filter and manufacturing method thereof - Google Patents

Description

The present invention relates to an ND (Neutral Density) filter and a method for manufacturing an ND filter.

As an ND filter having a flat distribution of light transmittance within the visible range, the one described in Japanese Patent No. 4623349 (Patent Document 1) is known.
This ND filter (first embodiment, [0015] below, FIG. 1) has an optical multilayer body on a substrate with a hard coat layer. In the optical multilayer body, the first and third layers from the substrate side are Nb layers, and the second and fourth layers are SiO2 _layers . The Nb layer is formed by a DC sputtering method as described in [0023].
In contrast to the first embodiment, in Example 1 in which a Si layer was arranged as an adhesion layer instead of a hard coat layer (see [0048] below, FIG. 6), the transmittance distribution was flat and the reflectance was approximately 1.9% in the wavelength range of 400 nm (nanometers) or more and 700 nm or less.

A similar ND filter is known, for example, as disclosed in Japanese Patent Application Laid-Open No. 2004-295015 (Patent Document 2).
This ND filter (embodiment, FIG. 1(B)) has a first optical film on a transparent substrate. In the first optical film, the first and fifth layers from the substrate side are _SiO2 layers, the third layer is an _Al2O3 layer, and the _second and fourth layers are Ti+ _TiOx layers. Each layer of this first optical film is formed by vacuum deposition as described below in [0016]. In particular, the Ti+ _TiOx layer is relatively easy to control the reactivity of oxygen with Ti, that is, the value of x, by vacuum deposition. On the other hand, even if it is considered to form the first optical film by sputtering, it is difficult to control the value of x because the reactivity of oxygen with Ti is high in sputtering. Therefore, sputtering cannot be used to form the first optical film including the Ti+ _TiOx layer.
Also, in [0015], as an example of a modification of the metal material of the light absorbing film of this ND filter, metals selected from Cr, Nb, Ni, NiCr, NiFe and mixtures thereof are given instead of Ti. However, a mixture of Ti and Nb is not given as an example of the modification, and no specific examples of the structure and formation of the layer of the mixture are given.

Furthermore, a similar ND filter is known, as described in Japanese Patent No. 4981456 (Patent Document 3).
The film of this ND filter (Example of NbO _x , [0045], FIG. 13) has a total of 11 layers, with the odd-numbered layers counting from the substrate side being SiO ₂ layers, and the even-numbered layers being NbO _x layers. The NbO _x layers are formed by deposition, not sputtering, as in the case of TiO _x described above. The transmittance of this ND filter in the wavelength range of 400 nm to 700 nm is flat, and the reflectance distribution is a maximum of 4 to 5%.

Patent No. 4623349 JP 2004-295015 A Patent No. 4981456

In each of the above-mentioned ND filters, there is room for reducing the reflectance for light in the visible range, and there is a particular demand for ensuring a reflectance below the approximately 1.9% disclosed in Patent Document 1, that is, for ensuring superior anti-reflection performance.
Furthermore, in each of the above-mentioned ND filters, it is difficult to achieve both a flat transmittance distribution in the visible range and a reduced reflectance while maintaining the layer materials and staying within a realistic cost range.

Therefore, a main object of the present invention is to provide an ND filter and a manufacturing method for an ND filter which achieves both a higher level of flatness of transmittance distribution in a specific wavelength range such as the visible range and anti-reflection performance.
Another main object of the present invention is to provide an ND filter and a method for manufacturing an ND filter which can be manufactured at low cost.

The invention described in claim 1 is an ND filter comprising a substrate and an optical film arranged on the side of a deposition surface, which is one or more surfaces of the substrate, the optical film including three light absorbing layers made of a mixture of Ti and Nb, and each of the light absorbing layers has a physical film thickness of 3.2 nm or more and 7.6 nm or less .
The invention described in claim 2 is an ND filter comprising a substrate and an optical film arranged on the side of a deposition surface, which is one or more surfaces of the substrate, the optical film including four light absorption layers made of a mixture of Ti and Nb, and the physical film thickness of each of the light absorption layers being 3.9 nm or more and 39.5 nm or less .
The invention described in claim 3 is characterized in that in the above invention, the optical film includes a dielectric layer made of a dielectric material.
The invention described in claim 4 is characterized in that, in the above invention, the element ratio of Ti and Nb in the light absorbing layer is within the range of Ti:Nb=15:85 to 75:25.

The invention described in claim 5 is a method for manufacturing an ND filter, characterized in that Ti and Nb are simultaneously sputtered in a film formation chamber in which a substrate is placed, thereby forming a light absorption layer made of a mixture of Ti and Nb on the substrate in three layers , each with a physical film thickness of 3.2 nm or more and 7.6 nm or less .
The invention described in claim 6 is a method for manufacturing an ND filter, characterized in that Ti and Nb are simultaneously sputtered in a film formation chamber in which a substrate is placed, thereby forming a light absorption layer made of a mixture of Ti and Nb on the substrate in four layers , each with a physical film thickness of 3.9 nm or more and 39.5 nm or less .
The invention described in claim 7 is characterized in that in the above invention, the elemental ratio of Ti and Nb in the light absorbing layer is controlled by a ratio between an area of an exposed surface of a Ti target in a film formation chamber and an area of an exposed surface of a Nb target in a film formation chamber.
The invention described in claim 8 is characterized in that, in the above invention, the light absorbing layer is formed using a Ti+Nb target in which a Ti target and a Nb target are combined.

The main effect of the present invention is to provide an ND filter and a manufacturing method for an ND filter that achieves both a higher level of flatness of transmittance distribution in a specific wavelength range such as the visible range and anti-reflection performance.
Another major effect of the present invention is to provide an ND filter and a method for manufacturing an ND filter that can be manufactured at low cost.

1 is a schematic cross-sectional view of an ND filter according to the present invention. 1 is a graph showing the distribution of refractive index (within a wavelength range of approximately 300 nm to 900 nm) among optical constants of a Ti layer, which is a layer made of Ti alone, and an Nb layer, which is a layer made of Nb alone. 1 is a graph showing the distribution of extinction coefficients (within the same wavelength range) among the optical constants of a Ti layer and an Nb layer, respectively. FIG. 2 is a schematic top view of an ND filter manufacturing apparatus. 3 is a flowchart of the operation example of FIG. 2. 1A is a schematic diagram of the drum-side surface of a Ti+Nb target, and FIG. 1B is a schematic diagram of the drum-side surface of another Ti+Nb target. 1 is a graph showing the optical constants (refractive index and extinction coefficient) of the substrates according to each of the examples and comparative examples. 1 is a graph showing the transmittance, surface reflectance, and interface reflectance in a specific wavelength range and an adjacent range (380 nm or more and 750 nm or less) in Example 1. 11 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Example 2. 11 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Example 3. 11 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Example 4. 11 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Comparative Example 1. 13 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Comparative Example 2. This is a graph in which the element ratio of Ti (Ti ratio, %) in the Ti+Nb layer is plotted on a plane with the horizontal axis representing the transmittance width ΔT in a specific wavelength range (400 nm or more and 700 nm or less) on the vertical axis, and Examples 1 to 4 and Comparative Examples 1 and 2 are plotted. 13 is a graph showing the transmittance, surface reflectance, and interface reflectance in a specific wavelength range and an adjacent range (380 nm or more and 750 nm or less) in Example 5. 13 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Example 6. 13 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Example 7. 13 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Example 8. 13 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Comparative Example 3. 13 is a graph showing the transmittance, surface reflectance, and interface reflectance in the same wavelength range in Comparative Example 4. This is a graph in which the element ratio of Ti (Ti ratio, %) in the Ti+Nb layer is plotted on a plane with the horizontal axis representing the transmittance width ΔT in a specific wavelength range (400 nm or more and 700 nm or less) on the vertical axis, and Examples 5 to 8 and Comparative Examples 3 to 4 are plotted.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.
However, the present invention is not limited to the following examples.

<ND filter configuration, etc.>
The ND filter according to the present invention is a filter in which the distribution of transmittance for light within a specific wavelength range is flat, and transmits the light almost uniformly. For example, the difference between the maximum and minimum values of the transmittance [%] within the wavelength range is preferably within 15 points, more preferably within 10 points, and even more preferably within 5 points.
The specific wavelength range may be, for example, the visible range or a wavelength range within the visible range. The visible range or the wavelength range within the visible range may be, for example, 400 nm or more and 700 nm or less, and the lower limit may be 380 nm, 390 nm, 410 nm, 420 nm, etc., and the upper limit may be 760 nm, 780 nm, 800 nm, etc. The visible range and the wavelength range within the visible range may not be the same.
Furthermore, the specific wavelength range may be, instead of or in addition to the visible range, an ultraviolet range or an infrared range, or a combination of these.
In the following description, the specific wavelength range to be uniformly transmitted is 400 nm to 700 nm inclusive. However, this description does not limit the specific wavelength range to 400 nm to 700 nm inclusive.

As illustrated in FIG. 1, an ND filter 1 according to the present invention includes a substrate 2 and an optical film 4 .
The optical film 4 is formed on the deposition surface of the substrate 2. The deposition surface is one side of the substrate 2. The deposition surface may be both sides of the substrate 2. When the optical film 4 is formed on both sides of the substrate 2, the configuration of the optical film 4 on each side may be the same or different, but preferably the configurations are the same. Alternatively, the optical film 4 according to the present invention may be formed on one side, and an optical multilayer film or optical single layer film not belonging to the present invention may be formed on the other side.

The substrate 2 may be made of any material, including resin such as polycarbonate or glass, as long as it is transparent (including translucent as appropriate), but is preferably glass containing an alkali element, and more preferably reinforced glass, for example chemically reinforced glass.
The reinforced glass substrate 2 has a compressive stress layer formed on its surface, so that even if a crack occurs on the surface, the growth of the crack is suppressed by the compressive stress, and the substrate 2 is more resistant to impact than a normal (non-reinforced) glass substrate 2.
Moreover, it is preferable that the substrate 2 cannot be wound up or rolled. If the substrate is not flexible enough to be wound up without causing plastic deformation, the optical film 4 and the like can be formed more stably, and the ND filter 1 can be made more robust.

The optical film 4 is a film including one or more layers, and has one or more light absorbing layers 10 that absorb light in a specific wavelength range (specific light), and has the function of realizing uniform transmission (ND) of the specific light.
When the optical film 4 includes a plurality of layers, it becomes an optical multilayer film.
In addition, at least one other film such as a hard coat film, an antifouling film, an antireflection film, and a conductive film may be provided on at least one of the substrate side and the air side of the optical film 4. Furthermore, these hard coat films, conductive films, etc. may be treated as being included in the optical film 4.

The light absorbing layer 10 is made of a mixture of metals Ti (titanium) and Nb (niobium). That is, the optical film 4 includes the light absorbing layer 10, which is a layer made of a mixture of Ti and Nb (Ti+Nb).
The ratio of Ti and Nb in the light absorbing layer 10 may be any ratio, but is preferably Ti:Nb=15:85 (Ti element ratio of 15% or more) to 75:15 (Ti element ratio of 75% or less) in terms of the number of elements.
The light absorbing layer 10 is formed by deposition or sputtering, and is preferably formed by sputtering.
The optical film 4 may include one or more light absorbing layers other than Ti+Nb.

Absorption for uniform transmission of visible light can be understood by the flatness of at least one of the spectral transmittance distribution and the spectral reflectance distribution in the visible range, since absorption [%] is simply expressed as "100 - (transmittance [%] + reflectance [%])", and when the reflectance is small, it can be understood by the flatness of the spectral transmittance distribution. The flatness of absorption is evaluated by the difference between the maximum and minimum absorption values, and the flatness of the spectral transmittance distribution is evaluated by the difference between the maximum and minimum transmittance values, and the smaller the difference in either case, the higher the flatness. There is a need for high flatness as it brings about uniform light reduction.
Furthermore, for example, the light used in an imaging element of a camera equipped with an ND filter 1 is light that passes through the ND filter 1, and since reflected light from the ND filter 1 causes noise in the imaging element and optical system, there is a demand to reduce the reflectance of the ND filter 1 as much as possible.
Furthermore, in order to achieve the desired transmittance for the ND filter 1 as a whole, the absorption of visible light by the light absorbing layer 10 may be distributed in accordance with the distribution of absorption, transmittance, and reflectance in other layers or other films of the optical film 4, or in the substrate 2.

FIG. 2 is a graph showing the distribution of refractive index (within a wavelength range of approximately 300 nm to 900 nm) among the optical constants of a Ti layer, which is a layer made of Ti alone, and an Nb layer, which is a layer made of Nb alone.
FIG. 3 is a graph showing the distribution of extinction coefficients (in the same wavelength range) among the optical constants of the Ti layer and the Nb layer.
In the Ti+Nb layer made of Ti+Nb, when the element ratio is Ti:Nb=50:50, the refractive index distribution is the average of the distribution of the Ti layer and the distribution of the Nb layer, and the same is true for the distribution of the extinction coefficient. Since Ti is superior to Nb in the element ratio, the distributions of the refractive index and the extinction coefficient are closer to the distribution of the Ti layer than in the case of Ti:Nb=50:50, and since Ti is inferior to Nb in the element ratio, the distributions of the refractive index and the extinction coefficient are closer to the distribution of the Nb layer than in the case of Ti:Nb=50:50.
In the Ti layer, due to the tendency of the optical constants in the specific wavelength range, the transmittance is higher on the short wavelength side of the specific wavelength range than on the long wavelength side.
In the Nb layer, due to the tendency of the optical constants in the specific wavelength range, the transmittance is lower on the short wavelength side of the specific wavelength range than on the long wavelength side.
In the Ti+Nb layer, the tendency of the transmittance of the Ti layer and the Nb layer to increase or decrease is alleviated by the mixture of Ti and Nb, which have opposite tendencies, and the transmittance distribution approaches a flatter value. The light absorbing layer 10 of the present invention utilizes such a property of Ti+Nb.

Furthermore, the optical film 4 may have one or more low-refractive index layers 12 made of a low-refractive index material. An example of such a low-refractive index material is silicon oxide (particularly SiO ₂ ). Other dielectric materials, metal materials, or metal oxide materials may also be used as the low-refractive index material. Preferably, the low-refractive index layer 12 is a dielectric layer made of a dielectric material.
The low refractive index layer 12 and the light absorbing layer 10 in the optical film 4 (optical multilayer film) are preferably arranged alternately. The number of layers in such an optical film 4 is not particularly limited. The light absorbing layer 10 has a higher refractive index than most dielectric materials, so it can be mainly treated as a high refractive index layer. When the layer closest to the substrate in the optical film 4 (the layer closest to the substrate) is the first layer, the first layer may be the low refractive index layer 12 or the light absorbing layer 10, but preferably the odd-numbered layer is the low refractive index layer 12 and the even-numbered layer is the light absorbing layer 10. If the first layer is the low refractive index layer 12, the second light absorbing layer 10 is not directly formed on the surface on the substrate 2 side, and the surface on the substrate 2 side is in contact with the low refractive index layer 12, so that the adhesion of the optical film 4 is better.

The optical film 4 may also have one or more high refractive index layers other than the light absorbing layer 10. The high refractive index layer is formed from a high refractive index material. Examples of such high refractive index materials include dielectric materials, metal materials, or metal oxide materials, and more specific examples include at least one of zirconium oxide (particularly ZrO ₂ ), titanium oxide (particularly TiO ₂ ), tantalum oxide (particularly Ta ₂ O ₅ ), niobium oxide (particularly Nb ₂ O ₅ ), silicon nitride (particularly SiN _x , Si ₃ N ₄ ), and hafnium oxide (particularly HfO ₂ ). From the viewpoint of ease of fabrication of the optical film 4 (simplification of the manufacturing device), it is preferable that the high refractive index material and the material of the light absorbing layer 10 have a common portion (for example, Nb ₂ O ₅ and Nb). Similarly, it is preferable that the low refractive index material and the high refractive index material have a common portion (for example, SiO ₂ and SiN _x ).
Incidentally, the optical film 4 may include a plurality of types of high refractive index layers, and the same applies to the low refractive index layers.

The ND filter 1 including the substrate 2 on which such an optical film 4 is disposed is preferably used for a camera.
The ND filter for a camera may be one that is attached later, such as in front of the lens of the camera, or may be one that is incorporated into the optical system of the camera (built into the camera).
Moreover, the ND filter for a camera may be for a vehicle-mounted camera, a security camera, or a camera attached to a medical device.

In this way, the ND filter 1 of the present invention includes the substrate 2 and the optical film 4 arranged on the side of the deposition surface, which is one or more surfaces of the substrate 2 (it may be arranged directly on the deposition surface of the substrate 2 or indirectly via an intermediate film), and the optical film 4 includes a light absorbing layer 10 made of a mixture of Ti and Nb (Ti+Nb). Therefore, the light absorbing layer 10 is formed of Ti+Nb in which the optical properties of Ti and Nb are averaged, and an ND filter 1 is provided in which both the flatness of the transmittance distribution and anti-reflection performance are achieved at a high level.
The optical film 4 also includes a low refractive index layer 12 as a dielectric layer made of a dielectric material. Therefore, the ND filter 1 is an alternating film of the light absorbing layer 10 and the low refractive index layer 12, and achieves both flatness of the transmittance distribution and anti-reflection performance at a higher level.

<ND filter manufacturing equipment, etc.>
Next, an embodiment of an apparatus for manufacturing the above-mentioned ND filter 1 will be described.
The manufacturing apparatus for the ND filter 1 according to the present invention is not limited to the following embodiment.

FIG. 4 is a schematic top view of the manufacturing apparatus 101 according to this embodiment.
The manufacturing apparatus 101 is a drum-type sputtering deposition apparatus (carousel-type sputtering apparatus) that deposits an optical film 4 on one surface of one or more substrates 2 .
The manufacturing apparatus 101 includes a vacuum chamber 102 as a film formation chamber, and a cylindrical drum 104 arranged in the center of the vacuum chamber 102 so as to be rotatable about its own axis. The substrate 2 on which a film is to be formed is held on the outer cylindrical surface of the drum 104 with the film formation surface facing outward.

A first sputtering source 110 is disposed on one side of the vacuum chamber 102 .
The first sputtering source 110 includes a sputtering cathode 112 on which a first target T1 is set, a pair of adhesion prevention plates 114, and a sputtering gas inlet 116 through which a sputtering gas is introduced with its flow rate appropriately adjusted.
The sputtering cathode 112 is connected to an external DC power supply (not shown).
The adhesion prevention plate 114 is disposed so as to separate the first target T 1 and the portion of the drum 104 facing it from the other internal portions of the vacuum chamber 102 .
The sputtering gas inlet 116 flows the sputtering gas toward the space partitioned by the deposition prevention plate 114 .

A second sputtering source 120 is disposed on another side of the vacuum chamber 102 .
Similar to the first sputtering source 110, the second sputtering source 120 includes a sputtering cathode 122 on which a second target T2 is set, a pair of adhesion prevention plates 124, and a sputtering gas inlet 126.

Furthermore, a radical source 130 is disposed on the other side of the vacuum chamber 102 .
The radical source 130 has a radical gas inlet 134 capable of introducing gas with a flow rate adjusted by a valve 132, and a gun 136 capable of generating plasma by applying a voltage from an acceleration voltage power supply (not shown).
The gas introduced into the vacuum chamber 102 from the radical gas inlet 134 is converted into radicals by the plasma generated by the gun 136 , and the radicals are irradiated in the form of a beam toward the substrate 2 .

In addition, exhaust units 140 are provided on both sides of the radical source 130. In each exhaust unit 140, the inside of the vacuum chamber 102 is evacuated.
The arrangement and number of at least one of the first sputtering source 110, the second sputtering source 120, the radical source 130, and the exhaust units 140 are not limited to those described above.

An operation example of the manufacturing apparatus 101 (an example of a manufacturing method of the ND filter 1) in which the light absorption layer 10 is a Ti+Nb layer and a low refractive index layer 12 of a SiO ₂ layer is further provided at an odd-numbered layer counting from the substrate 2 will be described mainly with reference to FIG.
First, the substrate 2 is set on the drum 104, and a target TTa for Ti+Nb or a target TTb for Ti+Nb or the like is set as the first target T1, and Si is set as the second target T2 (step S1).
6A is a schematic diagram of the surface of the Ti+Nb target TTa facing the drum 104. The Ti+Nb target TTa has a Ti portion 150a as a Ti target made of Ti and an Nb portion 152a as an Nb target made of Nb. The Ti portion 150a and the Nb portion 152a are both block-shaped and are joined in a state of being aligned in a virtual tangent direction (tangential direction) to the cylindrical surface of the drum 104. The Ti portion 150a and the Nb portion 152a have the same size (height) in the vertical direction. In the Ti+Nb target TTa, the tangential size (width) of the Ti portion 150a is the same as the width of the Nb portion 152a, and the width of the Ti portion 150a: the width of the Nb portion 152a is 50:50. Taking into consideration that the height of the Ti portion 150a and the height of the Nb portion 152a are equal, the area of the surface of the Ti portion 150a facing the drum 104: the area of the surface of the Nb portion 152a facing the drum 104 = 50: 50. The surface of the Ti portion 150a facing the drum 104 (the surface on the drum 104 side) is the exposed surface of the Ti target in the vacuum chamber 102, and the surface of the Nb portion 152a facing the drum 104 (the surface on the drum 104 side) is the exposed surface of the Nb target in the vacuum chamber 102.
6B is a schematic diagram of the surface of the Ti+Nb target TTb facing the drum 104. The Ti+Nb target TTb has a Ti portion 150b made of Ti and an Nb portion 152b made of Nb. The Ti portion 150b is similar to the Ti portion 150a except for the width, and the Nb portion 152b is similar to the Nb portion 152a except for the width. In the Ti+Nb target TTb, the width of the Ti portion 150b: the width of the Nb portion 152b = 70:30, and the area of the surface of the Ti portion 150b facing the drum 104: the area of the surface of the Nb portion 152b facing the drum 104 = 70:30. The surface of Ti portion 150b facing drum 104 (the surface on the drum 104 side) is the exposed surface of the Ti target in the vacuum chamber 102, and the surface of Nb portion 152b facing drum 104 (the surface on the drum 104 side) is the exposed surface of the Nb target in the vacuum chamber 102.
At least one of the shapes and heights of the Ti+Nb targets TTa and TTb may be different from each other. The Ti+Nb targets TTa and TTb may have an area ratio other than that of (50:50, 70:30).

Next, the inside of the vacuum chamber 102 is evacuated (step S2).
Next, the drum 104 is rotated, and the substrate 2 held by the drum 104 is made to pass repeatedly at high speed inside the first sputtering source 110, the second sputtering source 120, and the radical source 130 in sequence (step S3).
Next, the substrate 2 is cleaned (step S4). That is, while oxygen ( _O2 ) gas is being introduced from the radical gas inlet 34 of the radical source 130, a high-frequency voltage is applied to the gun 136 to generate radical oxygen, which is then irradiated onto the moving substrate 2 for a predetermined time. By irradiating the radical oxygen in this way, even if organic matter or the like is attached to the surface of the substrate 2, the organic matter or the like is decomposed and peeled off by the radical oxygen and ultraviolet light generated by the plasma, and the surface of the substrate 2 is cleaned. This cleaning improves the adhesion of a film to be formed later.

Next, the optical film 4 is formed (step S5). In the optical film 4 of the operational example, the first layer in contact with the substrate 2 is a SiO ₂ layer, so the SiO ₂ layer is first formed by repeating deposition and oxidation of Si mainly by the second sputtering source 120 and the radical source 130.
That is, while the rotation of the drum 104 is maintained, a rare gas (here, Ar gas) is introduced from the sputtering gas inlet 116 of the second sputtering source 120, and a DC voltage is applied to the sputtering cathode 112, whereby Si on the surface of the second target T2 is deposited on the surface of the substrate 2 by sputtering with Ar.
At the same time, with _O2 gas being introduced from the radical gas inlet 134 of the radical source 130, a high-frequency voltage is applied to the gun 136 to generate oxygen radicals, which are irradiated onto the moving substrate 2 on which Si has been deposited, thereby oxidizing the Si.
The thickness of the _SiO2 layer is controlled by the length of the sputtering time when the power input to the sputtering cathode 112 is constant and the deposition rate, which is the physical thickness of the film formed per unit time, is constant. Therefore, when the time corresponding to the desired film thickness has elapsed, the voltage application to the sputtering cathode 112 is stopped and the deposition of the first _SiO2 layer is completed.

Next, the Ti+Nb layer which is the light absorbing layer 10 is formed mainly by deposition of Ti and Nb by the first sputtering source 110 (light absorbing layer forming step, forming the Ti+Nb layer by sputtering).
That is, while the rotation of the drum 104 is maintained, a rare gas (here, Ar gas) is introduced from the sputtering gas inlet 126 of the first sputtering source 110, and a DC voltage is applied to the sputtering cathode 122, whereby Ti and Nb on the surface of the first target T1 are deposited on the surface of the substrate 2 by sputtering with Ar. Here, the radical source 130 is inoperative. Note that the rotation of the drum 104 may be temporarily changed in speed or stopped after the formation of the first layer and before the formation of the second layer, for example.
The sputtering by Ar acts uniformly on the surface of the first target T1 facing the drum 104 for a time that is not infinitesimal. Therefore, when the target TTa for Ti+Nb is used as the first target T1, Ti and Nb are produced according to the area ratio of the surfaces of the Ti portion 150a and the Nb portion 152a facing the drum 104 and are deposited on the film formation surface of the substrate 2, that is, a Ti+Nb layer having an element ratio of Ti:Nb=50:50 is formed as the light absorbing layer 10. Also, when the target TTb for Ti+Nb is used as the first target T1, a Ti+Nb layer having an element ratio of Ti:Nb=70:30 is formed as the light absorbing layer 10 in the same manner.
The thickness of the Ti+Nb layer can be controlled by time, similar to the _SiO2 layer, and when the time corresponding to the desired thickness has elapsed, the voltage application to the sputtering cathode 112 is stopped and the deposition of the second Ti+Nb layer is completed.

Then, the deposition of the SiO ₂ layer and the Ti+Nb layer are similarly repeated as appropriate to form the optical film 4. The _{SiO 2} layer and the Ti+Nb layer extend over the entire deposition surface of the substrate 2.
When the formation of the optical film 4 is completed, the drum 104 is stopped and appropriately cooled, and then the substrate 2 with the optical film 4, that is, the ND filter 1, is taken out (step S6).
An antifouling film or the like may be further provided on the optical film 4 (outside) by the manufacturing apparatus 101 or another apparatus.

In this way, the manufacturing method of the ND filter 1 of the present invention involves simultaneously sputtering Ti and Nb in the vacuum chamber 102 in which the substrate 2 is placed, thereby forming the light absorbing layer 10 made of a mixture of Ti and Nb (Ti+Nb) on the substrate 2. Therefore, Ti+Nb, which is difficult to vapor-deposit, can be easily and stably manufactured, and an ND filter that achieves both a higher level of flatness of the transmittance distribution in a specific wavelength range and anti-reflection performance can be manufactured at low manufacturing costs.
In addition, in the manufacturing method of the ND filter 1 of the present invention, the element ratio of Ti and Nb in the light absorption layer 10 is controlled by the ratio of the area of the exposed surface in the vacuum chamber 102 of the Ti parts 150a, 150b, etc. to the area of the exposed surface in the vacuum chamber 102 of the Nb parts 152a, 152b, etc. Therefore, by adjusting the area ratio, Ti+Nb relating to the desired element ratio can be easily obtained, and the optical characteristics of the ND filter 1 can be easily fine-tuned.
Furthermore, in the manufacturing method of the ND filter 1 of the present invention, the light absorbing layer 10 is formed using targets for Ti+Nb TTa, TTb, etc., in which the Ti portions 150a, 150b, etc. and the Nb portions 152a, 152b, etc. are bonded. Therefore, the Ti+Nb layer is formed simply by setting the targets for Ti+Nb TTa, TTb, etc. in a normal sputtering device, and an ND filter that achieves both a higher level of flatness of the transmittance distribution in a specific wavelength range and antireflection performance is manufactured at an even lower manufacturing cost.

Next, preferred examples of the present invention and comparative examples not belonging to the present invention will be explained in a first section (Examples 1 to 4, Comparative Examples 1 and 2) and a second section (Examples 5 to 8, Comparative Examples 3 and 4).
The present invention is not limited to the following examples. Depending on how the present invention is understood, the following examples may essentially be comparative examples, and vice versa.

[Examples 1 to 4 and Comparative Examples 1 to 2]
Configurations of Examples 1 to 4
The ND filter 1 of Example 1 was formed by forming only the optical film 4 on one side of the substrate 2 (B270 manufactured by Corning Incorporated) made of flat and transparent white plate glass by the above-mentioned manufacturing apparatus 101. The optical constants (refractive index and extinction coefficient) of this substrate 2 are shown in Fig. 7. Also, as shown in the following [Table 1], the optical film 4 has a total of seven layers in which the first layer closest to the substrate ₂ is a SiO2 layer (low refractive index layer 12), and the _SiO2 layers are alternately stacked with Ti+Nb layers (light absorbing layer 10).
The thickness of each layer in the optical film 4 in Example 1 was designed so that the transmittance was about 25% in a specific wavelength range (400 nm to 700 nm), the maximum value of the surface reflectance was 1% or less in the specific wavelength range, and the maximum value of the interface reflectance was 1% or less in the specific wavelength range. Here, the surface reflectance is the reflectance of light that is perpendicularly incident (incident angle 0°) from the air side (outside) to the air contact surface (outer surface of the outermost layer) of the optical film 4, and the interface reflectance is the reflectance of light that is perpendicularly incident from the substrate 2 side to the interface between the substrate 2 and the optical film 4 (first layer). The priority of these design elements is such that the elements of maximum surface reflectance 1% or less and maximum interface reflectance 1% or less are prioritized, and ensuring flatness at a transmittance of 25% is subordinated to these elements.

More specifically, in forming the optical film 4 in Example 1, the inside of the vacuum chamber 102 was set to 2×10 ⁻⁴ Pa (pascals) at the start of the film formation.
The rotation speed of the drum 104 was set to 100 rpm (revolutions per minute).
Furthermore, in cleaning the substrate 2, oxygen ( _O2 ) gas was introduced from the radical gas inlet 134 of the radical source 130 at a flow rate of 500 sccm (Standard Cubic Centimeter per Minute; 500 milliliters per minute), and a high-frequency voltage was applied to the gun 136 with an input power of 3 kW (kilowatts), generating oxygen radicals which were then irradiated onto the moving substrate 2 for 30 seconds.
The manufacturing apparatus 101 was placed in a room temperature environment, and no heating was applied to the vacuum chamber 102, drum 104, or substrate 2. Even taking into account heat generated by the operation of the radical source 130, etc., the maximum temperature of the substrate 2 during the entire process was 150°C.
During the formation of the _SiO2 layer, Ar gas was introduced from the sputtering gas inlet 126 of the second sputtering source 120 at 300 sccm, and a DC voltage with an input power of 8 kW was applied to the sputtering cathode 122, so that Si on the surface of the second target T2 (purity 99.9% or more) was deposited on the surface of the substrate 2 by sputtering with Ar. At the same time, with _O2 gas being introduced from the radical gas inlet 134 of the radical source 130 at 150 sccm, a high-frequency voltage with an input power of 3 kW was applied to the gun 136, generating oxygen radicals, which were irradiated onto the moving substrate 2 on which the Si had been deposited, thereby oxidizing the Si.
Then, when forming the Ti+Nb layer, Ar gas was introduced at 300 ccm from the sputtering gas inlet 116 of the first sputtering source 110, and a DC voltage was applied to the sputtering cathode 112 with an input power of 6 kW, so that Ti and Nb on the surface of the first target T1 (Ti and Nb each having a purity of 99.9% or more) were deposited in a mixed state on the surface of the substrate 2 by sputtering with Ar. Here, the radical source 130 was not operated. In Example 1, the above-mentioned Ti+Nb target TTa was used as the first target T1, and Ti and Nb were arranged to have an element number ratio (hereinafter the same) of Ti:Nb=50:50 in each Ti+Nb layer.
The rotation of the drum 104 may be temporarily changed in speed or stopped.

The ND filter 1 of Example 2 was formed in the same manner as Example 1, except for the area ratio of the Ti portion and the Nb portion in the first target T1 and the element number ratio of the Ti+Nb layer, as shown in the following Table 2. The Ti+Nb layer of Example 2 has a Ti:Nb ratio of 80:20.
The ND filter 1 of Example 3 was formed in the same manner as Example 1, except for the area ratio of the Ti portion and the Nb portion in the first target T1 and the element ratio of the Ti+Nb layer, as shown in the following Table 3. The Ti+Nb layer of Example 3 has a Ti:Nb ratio of 20:80.
The ND filter 1 of Example 4 was formed in the same manner as Example 1, except for the area ratio of the Ti portion and the Nb portion in the first target T1 and the element ratio of the Ti+Nb layer, as shown in the following Table 4. The Ti+Nb layer of Example 4 has a Ti:Nb ratio of 70:30.

<Configurations of Comparative Examples 1 and 2>
The ND filter of Comparative Example 1 was formed in the same manner as in Example 1, except that each light absorbing layer was a Ti layer. The configuration of the optical film of Comparative Example 1 is shown in the following [Table 5].
The ND filter of Comparative Example 2 was formed in the same manner as in Example 1, except that each light absorbing layer was an Nb layer. The configuration of the optical film of Comparative Example 2 is shown in the following [Table 6].

<Transmittance and reflectance, etc.>
8 to 13 are graphs showing the transmittance, surface reflectance, and interface reflectance in a specific wavelength range and its adjacent range (380 nm or more and 750 nm or less) in Examples 1 to 4 and Comparative Examples 1 and 2, respectively.
According to these figures, in Examples 1 to 4 and Comparative Examples 1 and 2, the reduction of reflectance is prioritized, and as a result, the surface reflectance and interface reflectance are 1% or less over the entire specific wavelength range.

Furthermore, in Comparative Example 1 (Ti:Nb=100:0), the flatness of the transmittance is relatively poor in the specific wavelength range (400 nm or more and 700 nm or less). In Comparative Example 1, the maximum value of the transmittance in the specific wavelength range, which is related to the flatness of the transmittance, is 27.082%, the minimum value is 24.040%, and the difference between the maximum value and the minimum value (transmittance width ΔT) is 3.042. The smaller ΔT is, the stronger (better) the flatness of the transmittance distribution in the specific wavelength range is.
In addition, in Comparative Example 2 (Ti:Nb = 0:100), the maximum transmittance in a specific wavelength range was 26.171%, the minimum was 24.137%, and ΔT was 2.035, and similar to Comparative Example 1, the flatness of the transmittance was relatively poor.

In contrast, in Example 1 (Ti:Nb = 50:50), the maximum transmittance in the specific wavelength range was 25.395%, the minimum was 24.830%, and ΔT was 0.564, indicating excellent flatness of the transmittance in the specific wavelength range.
In addition, in Example 2 (Ti:Nb = 80:20), the maximum transmittance in the specific wavelength range was 26.159%, the minimum transmittance was 24.410%, and ΔT was 1.749, which means that the flatness of the transmittance in the specific wavelength range is excellent.
Furthermore, in Example 3 (Ti:Nb = 20:80), the maximum transmittance in the specific wavelength range is 25.677%, the minimum value is 24.669%, and ΔT = 0.992, indicating excellent flatness of the transmittance in the specific wavelength range.
Furthermore, in Example 4 (Ti:Nb = 70:30), the maximum transmittance in the specific wavelength range was 25.488%, the minimum transmittance was 24.834%, and ΔT was 0.654, which means that the flatness of the transmittance in the specific wavelength range was excellent.

14 is a graph plotting the Ti element ratio (Ti ratio, %) in the Ti+Nb layer on a plane with the horizontal axis being the Ti element ratio in the Ti+Nb layer and the vertical axis being ΔT for Examples 1 to 4 and Comparative Examples 1 and 2. In FIG. 14, a curve connecting each plot, that is, a function ΔT with the Ti ratio obtained from each plot as an independent variable, is also shown.
In Fig. 14 relating to Examples 1 to 4 and Comparative Examples 1 to 2 in which light in a specific wavelength range is transmitted based on a transmittance of 25% (ND4), the Ti ratio is 15% or more and 75% or less, in other words, in the Ti+Nb layer, it is preferable that Ti:Nb = 15:85 to 75:25. That is, in ND4, a transmittance of 25%, that is, an optical density OD ≒ 0.602, is used as the standard, and the range of the Ti ratio in which the transmittance width ΔT falls within ±1.8% of the standard at OD and sufficient flatness is obtained is approximately 15% or more and 75% or less. To elaborate on this point, an OD that is 1.8% lower than the standard OD is 0.602 x (1 - 0.018) = 0.5912, which is converted to a transmittance of 25.633%. On the other hand, an OD that is 1.8% higher than the reference OD is 0.602 x (1 + 0.018) = 0.6128, which is converted to a transmittance of 24.389%. Therefore, the transmittance range ΔT when the OD is within ±1.8% of the reference is 25.633 - 24.389 = 1.244. In FIG. 14, a horizontal double-headed arrow is drawn at the portion where ΔT = 1.244. In FIG. 14, ΔT is 1.244 or less in the range of Ti ratios indicated by the double-headed arrows, with the minimum value of the range being approximately 15% (dotted arrow on the left) and the maximum value of the range being approximately 75% (dotted arrow on the right).
Therefore, in the ND filters 1 of Examples 1 to 4, etc., when the element ratio of Ti and Nb in the light absorption layer 10 is within the range of Ti:Nb=15:85 to 75:25, the reflectance in the specific wavelength range is set to a high level of 1% or less, while ΔT is made sufficiently small in light of the reference optical density OD (about 0.6), and excellent flatness of the transmittance in the specific wavelength range is ensured.

[Examples 5 to 8 and Comparative Examples 3 to 4]
Configurations of Examples 5 to 8
Examples 5 to 8 were formed in the same manner as Examples 1 to 4, except that the optical film 4 had a total of nine layers and the transmittance in a specific wavelength range was about 1%. However, the Ti ratios of the light absorbing layer 10 in Examples 5 to 8 were 80%, 40%, 20%, and 70%, respectively.
The configurations of Examples 5 to 8 are shown in the following Tables 7 to 10, respectively.

<Configurations of Comparative Examples 3 to 4>
Comparative Examples 3 and 4 were formed in the same manner as Comparative Examples 1 and 2, respectively, except that the optical film had a total of nine layers and the transmittance in the specific wavelength range was about 1%.
The configurations of Comparative Examples 3 and 4 are shown in the following Tables 11 and 12, respectively.

<Transmittance and reflectance, etc.>
15 to 20 are graphs showing the transmittance, surface reflectance, and interface reflectance in a specific wavelength range and its adjacent range (380 nm or more and 750 nm or less) in Examples 5 to 8 and Comparative Examples 3 and 4, respectively.
According to these figures, in Examples 5 to 8 and Comparative Examples 3 and 4, the reduction of reflectance was prioritized, and as a result, the surface reflectance and interface reflectance were 1% or less over the entire specific wavelength range.

Furthermore, in Comparative Example 3 (Ti:Nb=100:0), the maximum transmittance in the specific wavelength range was 1.147%, the minimum was 0.946%, and ΔT was 0.201, so the flatness of the transmittance was relatively poor.
In addition, in Comparative Example 4 (Ti:Nb = 0:100), the maximum transmittance in a specific wavelength range was 1.131%, the minimum was 0.947%, and ΔT was 0.185, and similar to Comparative Example 1, the flatness of the transmittance was relatively poor.

In contrast, in Example 5 (Ti:Nb = 80:20), the maximum transmittance in the specific wavelength range was 1.133%, the minimum was 0.955%, and ΔT was 0.178, indicating excellent flatness of the transmittance in the specific wavelength range.
In addition, in Example 6 (Ti:Nb = 40:60), the maximum transmittance in the specific wavelength range was 1.110%, the minimum transmittance was 0.968%, and ΔT was 0.142, which means that the flatness of the transmittance in the specific wavelength range is excellent.
Furthermore, in Example 7 (Ti:Nb = 20:80), the maximum transmittance in the specific wavelength range was 1.105%, the minimum transmittance was 0.964%, and ΔT was 0.141, which means that the flatness of the transmittance in the specific wavelength range is excellent.
Furthermore, in Example 8 (Ti:Nb = 70:30), the maximum transmittance in the specific wavelength range was 1.107%, the minimum transmittance was 0.967%, and ΔT was 0.140, indicating excellent flatness of the transmittance in the specific wavelength range.

Fig. 21 is a graph plotting the Ti element ratio (Ti ratio, %) in the Ti+Nb layer on a plane with the horizontal axis being the Ti element ratio in % and the vertical axis being ΔT for Examples 5 to 8 and Comparative Examples 3 and 4. Fig. 14 also shows a curve connecting each plot, i.e., a function ΔT with the Ti ratio obtained from each plot as an independent variable.
In Fig. 21 relating to Examples 5 to 8 and Comparative Examples 3 to 4 in which light in a specific wavelength range is transmitted based on a transmittance of 1% (ND100), the Ti ratio is 15% or more and 75% or less, in other words, in the Ti+Nb layer, it is preferable that Ti:Nb = 15:85 to 75:25. That is, in ND100, a transmittance of 1%, that is, an optical density OD = 2, is used as the standard, and the range of the Ti ratio in which the transmittance width ΔT falls within ±1.8% of the standard at OD and sufficient flatness is obtained is approximately 15% or more and 75% or less. To elaborate on this point, an OD that is 1.8% lower than the standard OD is 2 x (1 - 0.018) = 1.964, which is converted to a transmittance of 1.0864%. On the other hand, an OD that is 1.8% higher than the reference OD is 2×(1+0.018)=2.036, which is converted to a transmittance of 0.9204%. Therefore, the transmittance range ΔT when the OD is within ±1.8% of the reference is 1.0864-0.9204=0.166. In FIG. 21, a horizontal double-headed arrow is drawn at the portion where ΔT=0.166. In FIG. 21, in the range of Ti ratio indicated by the double-headed arrow, ΔT is 0.166 or less, the minimum value of the range is about 15% (dotted arrow on the left), and the maximum value of the range is about 75% (dotted arrow on the right). Note that this range of Ti ratio is the same as that of Examples 1 to 4 (ND4) as described above.
Therefore, in the ND filters 1 of Examples 5 to 8, etc., when the element ratio of Ti and Nb in the light absorption layer 10 is within the range of Ti:Nb=15:85 to 75:25, the reflectance in the specific wavelength range is set to a high level of 1% or less, while ΔT is made sufficiently small in light of the reference optical density OD(2), and excellent flatness of the transmittance in the specific wavelength range is ensured.
Furthermore, even in the case of ND filter 1 having a standard OD value different from those of Examples 1 to 8, it was confirmed by simulation that when the Ti ratio range was 15 to 75%, ΔT fell within ±1.8% of the standard, and the transmittance distribution had excellent flatness, which was preferable.

1: ND filter, 2: substrate, 4: optical film, 10: light absorption layer, 12: low refractive index layer (dielectric layer), 101: manufacturing equipment (for ND filter 1), 102: vacuum chamber (film formation chamber), 150a, 150b: Ti section (Ti target), 152a, 152b: Nb section (Nb target), TTa, TTb: targets for Ti+Nb.

Claims (8)

A substrate;
An optical film disposed on a film formation surface side, which is one or more surfaces of the substrate;
It is equipped with
The optical film includes three light absorbing layers each made of a mixture of Ti and Nb;
The ND filter, wherein the physical film thickness of each of the light absorbing layers is 3.2 nm or more and 7.6 nm or less . A substrate;
An optical film disposed on a film formation surface side, which is one or more surfaces of the substrate;
It is equipped with
The optical film includes four light absorbing layers each made of a mixture of Ti and Nb;
The ND filter is characterized in that the physical film thickness of each of the light absorbing layers is 3.9 nm or more and 39.5 nm or less . 3. The ND filter according to claim 1 , wherein the optical film includes a dielectric layer made of a dielectric material. 4. The ND filter according to claim 1, wherein the element ratio of Ti to Nb in the light absorption layer is within a range of Ti:Nb=15:85 to 75:25. A method for manufacturing an ND filter, comprising the steps of: simultaneously sputtering Ti and Nb in a film formation chamber in which a substrate is placed, thereby forming a light absorption layer made of a mixture of Ti and Nb on the substrate in three layers , each having a physical film thickness of 3.2 nm or more and 7.6 nm or less . A method for manufacturing an ND filter, comprising the steps of: simultaneously sputtering Ti and Nb in a film formation chamber in which a substrate is placed, thereby forming a light absorption layer made of a mixture of Ti and Nb on the substrate in four layers , each layer having a physical film thickness of 3.9 nm or more and 39.5 nm or less . 7. The method for manufacturing an ND filter according to claim 5, wherein the element number ratio of Ti and Nb in the light absorption layer is controlled by a ratio of an area of an exposed surface of a Ti target in a film formation chamber to an area of an exposed surface of a Nb target in a film formation chamber . 8. The method for manufacturing an ND filter according to claim 5, wherein the light absorbing layer is formed using a Ti+Nb target in which a Ti target and a Nb target are combined.

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