ND Filter and Method for Manufacturing ND Filter

Abstract

To provide an ND filter, etc., with which the flatness of transmittance distribution and antireflection performance in a specific wavelength region such as visible light are both achieved at a higher level.SOLUTION: An ND filter 1 according to the present invention comprises a substrate 2, and an optical film 4 arranged on one or more film-forming surfaces of the substrate 2. The optical film 4 includes a light absorption layer 10 composed of an admixture of Ti and Nb (Ti+Nb). The optical film 4 may be arranged directly, or indirectly via an intermediate film, to the film-forming surfaces of the substrate 2.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a neutral density (ND) filter and a method for manufacturing the ND filter.

Background Art

[0002] As an ND filter having a flat transmittance distribution of light within the visible range, the one described in Japanese Patent No. 4623349 (Patent Document 1) is known. This ND filter (the first embodiment, hereinafter, FIG. 1) has an optical multilayer on a substrate with a hard coat layer. In the optical multilayer, the 1st and 3rd layers from the substrate side are Nb layers, and the 2nd and 4th layers are SiO2 layers. The Nb layer is formed by the DC sputtering method as described in

[0023] . In Example 1 in which a Si layer is arranged as an adhesion layer instead of the hard coat layer with respect to this first embodiment (hereinafter, FIG. 6), in the wavelength range of 400 nm (nanometer) or more and 700 nm or less, the transmittance distribution is flat and the reflectance is about 1.9%.

[0003] Also, as a similar ND filter, the one described in Japanese Patent Application Laid-Open No. 2004-295015 (Patent Document 2) is known. This ND filter (embodiment, FIG. 1(B)) includes a first optical film on a transparent substrate. In the first optical film, the 1st and 5th layers from the substrate side are SiO2 layers, the 3rd layer is an Al2O3 layer, and the 2nd and 4th layers are Ti+TiO x layers. Each layer of this first optical film is formed by vacuum evaporation as described in

[0016] or less. In particular, for the Ti+TiO x layer, control related to the degree of reaction of oxygen with respect to Ti, that is, the value of x, is relatively easy by vacuum evaporation. On the other hand, even if it is considered to form the first optical film by sputtering, since the reactivity of oxygen with respect to Ti is high in sputtering, it is difficult to control the value of x. Therefore, Ti+TiO xSputtering cannot be used to form the first optical film including the layer. Also, in

[0015] , as a modification example of the metal material of the light absorption film of this ND filter, metals selected from Cr, Nb, Ni, NiCr, NiFe, and mixtures thereof are listed instead of Ti. However, as a modification example, a mixture of Ti and Nb is not listed, and no specific examples such as the layer configuration and formation of the mixture are shown at all.

[0004] Furthermore, as a similar ND filter, the one described in Japanese Patent No. 4981456 (Patent Document 3) is known. This ND filter (NbO x Example,

[0045] , FIG. 13) has a total of 11 layers. The odd-numbered layers counted from the substrate side are SiO2 layers, and the even-numbered layers are NbO x layers. The NbO x layer is formed by vapor deposition instead of sputtering, similar to the case of the above-mentioned TiO x . The transmittance of this ND filter in the wavelength range of 400 nm or more and 700 nm or less is flat, and the reflectance distribution is up to 4 - 5%.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] In each of the above-mentioned ND filters, there is room for reducing the reflectance of light in the visible region, and in particular, it is required to ensure a reflectance lower than about 1.9% according to Patent Document 1, that is, to ensure better antireflection performance. In addition, in each of the above ND filters, it is difficult to achieve both flatness of the transmittance distribution in the visible region and reduction of reflectance within a realistic cost range while maintaining the material of the layer.

[0007] Therefore, the main object of the present invention is to provide an ND filter and a method for manufacturing an ND filter in which flatness of the transmittance distribution and antireflection performance in a specific wavelength range such as the visible region are achieved at a higher level. Another main object of the present invention is to provide an ND filter and a method for manufacturing an ND filter with a low manufacturing cost.

Means for Solving the Problems

[0008] The invention according to claim 1 is an ND filter comprising a substrate and an optical film disposed on the side of a film-forming surface which is one or more surfaces of the substrate, wherein the optical film includes a light absorption layer made of a mixture of Ti and Nb, Three layers each physical film thickness of the light absorption layers is 3.2 nm or more, 7.6 nm or less and is characterized by this. Claim 2 The invention according to is an ND filter comprising a substrate and an optical film disposed on the side of a film-forming surface which is one or more surfaces of the substrate, wherein the optical film includes a light absorption layer made of a mixture of Ti and Nb, Four layers each physical film thickness of the light absorption layers is 3.9 nm or more and 39.5 nm or less and is characterized by this. Claim 3 The invention according to is characterized in that, in the above invention, the optical film includes a dielectric layer made of a dielectric. Claim 4 The invention according to is characterized in that, in the above invention, the element number ratio of Ti and Nb in the light absorption layer is in the range of Ti:Nb = 15:85 to 75:25.

[0009] Claim 5In the method for manufacturing an ND filter, by simultaneously sputtering Ti and Nb in a film formation chamber in which a substrate is placed, an optical absorption layer made of a mixture of Ti and Nb is formed on the substrate, Three layers wherein each physical film thickness is 3.2 nm or more 7.6 nm or less and is formed in such a state. Claim 6 In the method for manufacturing an ND filter, by simultaneously sputtering Ti and Nb in a film formation chamber in which a substrate is placed, an optical absorption layer made of a mixture of Ti and Nb is formed on the substrate, Four layers wherein each physical film thickness is 3.9 nm or more and 39.5 nm or less and is formed in such a state. Claim 7 In the invention described above, in the optical absorption layer, the element number ratio of Ti and Nb is controlled by the ratio of the area of the exposed surface in the film formation chamber of the Ti target and the area of the exposed surface in the film formation chamber of the Nb target. Claim 8 In the invention described above, a Ti + Nb target in which a Ti target and an Nb target are combined is used as a target, and the optical absorption layer is formed.

Advantages of the Invention

[0010] The main advantage of the present invention is to provide an ND filter and a method for manufacturing an ND filter in which the flatness of the transmittance distribution in a specific wavelength range such as the visible range and the antireflection performance are compatible at a higher level. Another main advantage of the present invention is to provide an ND filter and a method for manufacturing an ND filter with low manufacturing costs.

Brief Description of the Drawings

[0011]

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Mode for Carrying Out the Invention

[0012] Hereinafter, examples of embodiments according to the present invention will be described with reference to the drawings as appropriate. Note that the present invention is not limited to the following examples.

[0013] ≪Configuration of ND Filter, etc.≫ The ND filter according to the present invention is a filter in which the distribution of the transmittance of light in a specific wavelength range is flat, and the light is transmitted almost uniformly. For example, the difference between the maximum value and the minimum value of the transmittance [%] in the wavelength range is preferably within 15 points, more preferably within 10 points, and still more preferably within 5 points. Examples of the specific wavelength range include the visible range or a wavelength range within the visible range. The visible range or the wavelength range within the visible range is, for example, both 400 nm or more and 700 nm or less, and the lower limit thereof may be 380 nm, 390 nm, 410 nm, 420 nm, etc., and the upper limit thereof may be 760 nm, 780 nm, 800 nm, etc. The visible range and the wavelength range within the visible range do not have to match. Alternatively, the specific wavelength range may belong to the ultraviolet range or the infrared range instead of or together with the visible range, or may belong to a combination thereof. Hereinafter, an example will be described in which the specific wavelength range as the object to be uniformly transmitted is 400 nm or more and 700 nm or less. Note that, according to this description, the specific wavelength range is not limited to 400 nm or more and 700 nm or less.

[0014] As illustrated in FIG. 1, the 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 film-forming surface of the substrate 2. The film-forming surface is one side of the substrate 2. Note that the film-forming surface may be both sides of the substrate 2. When the optical film 4 is formed on both sides of the substrate 2, the configurations of the optical films 4 on each side may be the same as or different from each other. Preferably, the configurations are the same as each other. Alternatively, the optical film 4 according to the present invention may be formed on one side, and an optical multilayer film or an optical single-layer film not belonging to the present invention may be formed on the other side.

[0015] The substrate 2 may be made of any material such as a resin such as polycarbonate or glass as long as it is transparent (appropriately including translucent). Preferably, it is glass containing an alkali element, and more preferably tempered glass, for example, chemically strengthened glass. Since the substrate 2 made of tempered glass has a compressive stress layer formed on its surface, even if a crack occurs on the surface, the growth of the crack is suppressed by the compressive stress, and it is more resistant to impact than a normal (untempered) glass substrate 2. Furthermore, the substrate 2 is preferably non-windable and non-rollable. As long as the substrate is not flexible enough to be wound up without undergoing plastic deformation, the optical film 4 and the like can be formed more stably, and the ND filter 1 can be made more robust.

[0016] The optical film 4 is a film including one or more layers, and includes at least one light absorption layer 10 that absorbs light in a specific wavelength range (specific light), and has a 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, on at least one of the substrate side and the air side from the optical film 4, one or more other films such as at least any one of a hard coat film, an antifouling film, an antireflection film, and a conductive film may be provided. Also, these hard coat films, conductive films, etc. may be treated as being included in the optical film 4.

[0017] The light absorption layer 10 is made of a mixture of Ti (titanium) and Nb (niobium), both of which are metals. That is, the optical film 4 includes the light absorption layer 10 which is a layer made of a mixture of Ti and Nb (Ti + Nb). The ratio of Ti to Nb in the light absorption layer 10 may be any, but is preferably from 15:85 (Ti elemental ratio of 15% or more) to 75:15 (75% or less) in terms of elemental number ratio. The light absorption layer 10 is formed by evaporation, sputtering, or the like, and is preferably formed by sputtering. In addition, in the optical film 4, one or more light absorption layers other than Ti + Nb may be included.

[0018] Regarding absorption for uniform transmission of visible light, since the absorption [%] can be simply expressed as "100 - (transmittance [%] + reflectance [%])", it can be grasped by the fact that at least one of the spectral transmittance distribution and the spectral reflectance distribution in the visible region is flat. When the reflectance is small, it can be grasped by the fact that the spectral transmittance distribution is flat. The flatness of absorption is evaluated by the difference between the maximum value and the minimum value of absorption, and the flatness of the spectral transmittance distribution is evaluated by the difference between the maximum value and the minimum value of transmittance. In both cases, the smaller the difference, the higher the flatness. There is a need for high flatness as it provides uniform light attenuation. Also, for example, the light used in the imaging element of a camera with an ND filter 1 is the transmitted light of the ND filter 1. However, since the reflected light in the ND filter 1 causes noise in the imaging element and the optical system, there is a demand to reduce the reflectance of the ND filter 1 as much as possible. In addition, in order to achieve a desired transmittance for the entire ND filter 1, the absorption of visible light by the light absorption layer 10 may have a distribution corresponding to the distribution of absorption, transmittance, and reflectance in other layers of the optical film 4 or other films or the substrate 2.

[0019] Figure 2 is a graph showing the distribution of the refractive index, which is one of the optical constants, in a Ti layer composed of a single Ti and an Nb layer composed of a single Nb (within a wavelength range of about 300 nm or more and 900 nm or less). Figure 3 is a graph showing the distribution of the extinction coefficient, which is one of the optical constants, in the Ti layer and the Nb layer (within the same wavelength range). In a Ti+Nb layer composed of Ti+Nb, when the element number ratio is Ti:Nb = 50:50, the distribution of the refractive index becomes an average of the distribution of the Ti layer and the distribution of the Nb layer, and the same applies to the distribution of the extinction coefficient. And for the amount by which Ti wins over Nb in the element number ratio, the distributions of the refractive index and the extinction coefficient shift towards the distribution of the Ti layer from the case of Ti:Nb = 50:50, and for the amount by which Ti is inferior to Nb in the element number ratio, the distributions of the refractive index and the extinction coefficient shift towards the distribution of the Nb layer from the case of Ti:Nb = 50:50. In the Ti layer, due to the tendency of the optical constants in a specific wavelength range, the transmittance increases on the short-wavelength side rather than the long-wavelength side in the specific wavelength range. In the Nb layer, due to the tendency of the optical constants in a specific wavelength range, the transmittance decreases on the short-wavelength side rather than the long-wavelength side in the specific wavelength range. In the Ti+Nb layer, the increasing and decreasing tendencies of the transmittances of the Ti layer and the Nb layer are relaxed by the mixing of Ti and Nb with opposite tendencies to each other, and the transmittance distribution approaches flatness. The light absorption layer 10 of the present invention utilizes such properties of Ti+Nb.

[0020] Furthermore, the optical film 4 may have one or more low refractive index layers 12 made of a low refractive index material. Examples of such a low refractive index material include silicon oxide (especially SiO2). Also, as the low refractive index material, other dielectric materials, or metal materials or metal oxide materials may be used. Preferably, the low refractive index layer 12 is a dielectric layer made of a dielectric. The low refractive index layer 12 and the light absorption 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. Since the light absorption layer 10 has a higher refractive index than most dielectric materials, it can be mainly treated as a high refractive index layer. When the layer closest to the substrate side (the layer closest to the substrate) in the optical film 4 is regarded as the first layer, the first layer may be the low refractive index layer 12 or the light absorption layer 10. Preferably, the odd-numbered layers are the low refractive index layer 12 and the even-numbered layers are the light absorption layer 10. If the first layer is the low refractive index layer 12, the second layer of the light absorption layer 10 is not directly formed on the surface on the substrate 2 side, and the surface on the substrate 2 side comes into contact with the low refractive index layer 12, resulting in better adhesion of the optical film 4.

[0021] Also, the optical film 4 may have one or more high refractive index layers other than the light absorption layer 10. The high refractive index layer is formed from a high refractive index material. Examples of such a high refractive index material include, for example, dielectric materials, or metal materials or metal oxide materials. More specific examples include zirconium oxide (especially ZrO2), titanium oxide (especially TiO2), tantalum oxide (especially Ta2O5), niobium oxide (especially Nb2O5), silicon nitride (especially SiN x, at least one of Si3N4) and hafnium oxide (especially HfO2) can be mentioned. From the viewpoint of the ease of manufacturing the optical film 4 (simplification of the manufacturing apparatus), it is preferable that there is a common part between the high refractive index material and the material of the light absorption layer 10 (for example, Nb2O5 and Nb). Similarly, it is preferable that there is a common part between the low refractive index material and the high refractive index material (for example, SiO2 and SiN x ) is preferable. In addition, in the optical film 4, a plurality of types of high refractive index layers may be included, and the same applies to the low refractive index layer.

[0022] 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 attached to the front of the camera lens or the like, or may be incorporated in the optical system of the camera (built into the camera). In addition, the ND filter for a camera may be for an in-vehicle camera, for a security camera, or for a camera attached to a medical device.

[0023] As described above, the ND filter 1 of the present invention includes the substrate 2 and the optical film 4 disposed on the side of the film formation surface which is one or more surfaces of the substrate 2 (it may be directly disposed with respect to the film formation surface of the substrate 2 or indirectly disposed via an intermediate film). The optical film 4 includes a light absorption layer 10 made of a mixture of Ti and Nb (Ti + Nb). Therefore, the light absorption layer 10 is formed by Ti + Nb in which the optical characteristics of Ti and Nb are averaged, and the ND filter 1 in which the flatness of the transmittance distribution and the antireflection performance are compatible at a high level is provided. In addition, the optical film 4 includes a low refractive index layer 12 as a dielectric layer made of a dielectric. Therefore, the ND filter 1 becomes an alternating film of the light absorption layer 10 and the low refractive index layer 12, and the flatness of the transmittance distribution and the antireflection performance are compatible at an even higher level.

[0024] ≪Manufacturing apparatus of ND filter etc.≫ Next, an embodiment of an apparatus for manufacturing the above-described ND filter 1 will be described. Furthermore, the manufacturing apparatus for the ND filter 1 according to the present invention is not limited to the following form.

[0025] FIG. 4 is a schematic top view of the manufacturing apparatus 101 according to this form. The manufacturing apparatus 101 is a drum-type sputtering film-forming apparatus (carousel-type sputtering apparatus) that forms an optical film 4 on one side of one or more substrates 2. The manufacturing apparatus 101 includes a vacuum chamber 102 as a film-forming chamber and a cylindrical drum 104 rotatably arranged around its own axis at the central part thereof. On the outer peripheral cylindrical surface of the drum 104, the substrate 2 to be film-formed is held with the film-forming surface facing outward.

[0026] On one surface of the vacuum chamber 102, a first sputtering source 110 is arranged. The first sputtering source 110 includes a sputtering cathode 112 for setting a first target T1, a pair of anti-deposition plates 114, and a sputtering gas inlet 116 into which sputtering gas is introduced with an appropriate flow rate adjustment. The sputtering cathode 112 is connected to an external DC power source (not shown). The anti-deposition plate 114 is arranged so as to separate the space between the first target T1 and the corresponding part of the drum 104 from the internal part of the other vacuum chamber 102. The sputtering gas inlet 116 flows sputtering gas toward the space separated by the anti-deposition plate 114.

[0027] On another surface of the vacuum chamber 102, a second sputtering source 120 is arranged. The second sputtering source 120 includes, similarly to the first sputtering source 110, a sputtering cathode 122 for setting a second target T2, a pair of anti-deposition plates 124, and a sputtering gas inlet 126.

[0028] Furthermore, on another surface of the vacuum chamber 102, a radical source 130 is arranged. The radical source 130 includes a radical gas inlet 134 through which a gas can be introduced after adjusting the flow rate by a valve 132, and a gun 136 capable of generating plasma when a voltage is applied by a power source for acceleration voltage (not shown). The gas introduced into the interior of the vacuum chamber 102 from the radical gas inlet 134 is radicalized by the plasma generated by the gun 136 and irradiated in a beam shape toward the substrate 2.

[0029] In addition, exhaust portions 140 are provided on both sides of the radical source 130. In each exhaust portion 140, the interior of the vacuum chamber 102 is evacuated. Note that the arrangement and the number of installations of at least any one of the first sputtering source 110, the second sputtering source 120, the radical source 130, and each exhaust portion 140 are not limited to those described above.

[0030] An operation example of the manufacturing apparatus 101 (an example of a method for manufacturing the ND filter 1) will be mainly described with reference to FIG. 5 in the case where the light absorption layer 10 is a Ti+Nb layer and the low refractive index layer 12 of the SiO2 layer is provided on the odd-numbered layers counted from the substrate 2. First, the substrate 2 is set on the drum 104, and a Ti+Nb target TTa or a Ti+Nb target TTb or the like is set as the first target T1, and Si is set as the second target T2 (step S1). FIG. 6(A) is a schematic diagram of the surface of the drum 104 facing the Ti+Nb target TTa. The Ti+Nb target TTa has a Ti part 150a as a target of Ti made of Ti and an Nb part 152a as a target of Nb made of Nb. Both the Ti part 150a and the Nb part 152a are in a block shape and are combined in a state of being arranged in the direction of a virtual tangent (tangent direction) to the cylindrical surface of the drum 104. The vertical sizes (heights) of the Ti part 150a and the Nb part 152a are the same. In the Ti+Nb target TTa, the size (width) of the Ti part 150a in the tangent direction is the same as the width of the Nb part 152a, and the width of the Ti part 150a: the width of the Nb part 152a = 50:50. Considering that the heights of the Ti part 150a and the Nb part 152a are equal, the area of the surface of the Ti part 150a facing the drum 104: the area of the surface of the Nb part 152a facing the drum 104 = 50:50. The surface of the Ti part 150a facing the drum 104 (the surface on the drum 104 side) is the exposed surface in the vacuum chamber 102 in the Ti target, and the surface of the Nb part 152a facing the drum 104 (the surface on the drum 104 side) is the exposed surface in the vacuum chamber 102 in the Nb target. FIG. 6(B) is a schematic diagram of the surface of the drum 104 facing the Ti+Nb target TTb. The Ti+Nb target TTb has a Ti part 150b made of Ti and an Nb part 152b made of Nb. The Ti part 150b is formed in the same manner as the Ti part 150a except for the width, and the Nb part 152b is formed in the same manner as the Nb part 152a except for the width. In the Ti+Nb target TTb, the width of the Ti part 150b: the width of the Nb part 152b = 70:30, and the area of the surface of the Ti part 150b facing the drum 104: the area of the surface of the Nb part 152b facing the drum 104 = 70:30. The surface of the Ti part 150b facing the drum 104 (the surface on the drum 104 side) is the exposed surface in the vacuum chamber 102 in the Ti target, and the surface of the Nb part 152b facing the drum 104 (the surface on the drum 104 side) is the exposed surface in the vacuum chamber 102 in the Nb target. Furthermore, at least one of the shapes and heights of the targets TTa and TTb for Ti + Nb may be different from each other. Targets for Ti + Nb with area ratios other than (50:50, 70:30) may be used.

[0031] Next, the inside of the vacuum chamber 102 is evacuated (step S2). Subsequently, the drum 104 is rotated so that the substrate 2 held by the drum 104 sequentially and repeatedly passes at high speed inside each of the first sputtering source 110, the second sputtering source 120, and the radical source 130 (step S3). Next, the substrate 2 is cleaned (step S4). That is, with oxygen (O2) gas 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 irradiated onto the moving substrate 2 for a predetermined time. Even if organic substances or the like are attached to the surface of the substrate 2, the organic substances or the like are decomposed and peeled off by the radical oxygen and the ultraviolet rays generated by the plasma, and the surface of the substrate 2 is cleaned. Such cleaning improves the adhesion of the film to be formed later.

[0032] Subsequently, the optical film 4 is formed (step S5). In the optical film 4 of the operation example, since the first layer in contact with the substrate 2 is the SiO2 layer, first, the SiO2 layer is formed mainly by repeating the deposition and oxidation of Si by the second sputtering source 120 and the radical source 130. That is, with the rotation of the drum 104 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, so that 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 introduced from the radical gas inlet 134 of the radical source 130, a high-frequency voltage is applied to the gun 136 to generate radical oxygen, which is irradiated onto the moving substrate 2 on which Si is deposited, and the Si is oxidized. The film thickness of the SiO2 layer is controlled by the duration of sputtering when the input power to the sputter cathode 112 is constant and the film formation rate, which is the physical film thickness formed per unit time, is constant. Therefore, when the time corresponding to the desired film thickness has elapsed, the voltage application to the sputter cathode 112 is stopped and the formation of the first SiO2 layer is completed.

[0033] Next, the Ti+Nb layer, which is the light absorption layer 10, is formed mainly by the deposition of Ti and Nb by the first sputter source 110 (light absorption layer formation step, formation of the Ti+Nb layer by sputtering). That is, with the rotation of the drum 104 maintained, a rare gas (here, Ar gas) is introduced from the sputter gas inlet 126 of the first sputter source 110, and a DC voltage is applied to the sputter cathode 122, so that 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 deactivated. Note that the rotation of the drum 104 may be temporarily decelerated or once stopped after the formation of the first layer and before the formation of the second layer or the like. Sputtering with Ar acts evenly on the drum 104 facing surface of the first target T1 in a non-negligible time. Therefore, when the Ti+Nb target TTa is used as the first target T1, Ti and Nb come out according to the area ratio of the drum 104 facing surfaces of the Ti part 150a and the Nb part 152a and are deposited on the film formation surface of the substrate 2, that is, a Ti+Nb layer with an elemental ratio of Ti:Nb = 50:50 is formed as the light absorption layer 10. Also, when the Ti+Nb target TTb is used as the first target T1, a Ti+Nb layer with an elemental ratio of Ti:Nb = 70:30 is formed as the light absorption layer 10 in the same manner. The film thickness of the Ti+Nb layer can be controlled by time in the same manner as the SiO2 layer. When the time corresponding to the desired film thickness has elapsed, the voltage application to the sputter cathode 112 is stopped and the formation of the second Ti+Nb layer is completed.

[0034] Then, by appropriately repeating the formation of the SiO2 layer and the Ti+Nb layer, the optical film 4 is formed. The SiO2 layer and the Ti+Nb layer cover the entire film-forming surface of the substrate 2. When the formation of the optical film 4 is completed, the drum 104 is stopped, and after being appropriately cooled, the substrate 2 with the optical film 4, that is, the ND filter 1, is taken out (step S6). In addition, an antifouling film or the like may be further applied to the upper (outer) side of the optical film 4 by the manufacturing apparatus 101 or another apparatus.

[0035] Thus, in the method for manufacturing the ND filter 1 of the present invention, by simultaneously sputtering Ti and Nb in the vacuum chamber 102 in which the substrate 2 is placed, the light absorption layer 10 made of a mixture of Ti and Nb (Ti+Nb) is formed on the substrate 2. Therefore, Ti+Nb, which is difficult to deposit, can be easily and stably manufactured, and an ND filter in which the flatness of the transmittance distribution and the antireflection performance in a specific wavelength range are compatible at a higher level is manufactured at a low manufacturing cost. Further, in the method for manufacturing the ND filter 1 of the present invention, the element number 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 in the Ti portions 150a, 150b, etc. and the area of the exposed surface in the vacuum chamber 102 in the Nb portions 152a, 152b, etc. Therefore, by adjusting the area ratio, Ti+Nb having a desired element ratio can be easily obtained, and the optical characteristics of the ND filter 1 can be easily finely adjusted. Furthermore, in the method for manufacturing the ND filter 1 of the present invention, using the targets TTa, TTb, etc. for Ti+Nb in which the Ti portions 150a, 150b, etc. and the Nb portions 152a, 152b, etc. are combined, the light absorption layer 10 is formed. Therefore, by simply setting the targets TTa, TTb, etc. for Ti+Nb in a normal sputtering apparatus, the Ti+Nb layer is formed, and an ND filter in which the flatness of the transmittance distribution and the antireflection performance in a specific wavelength range are compatible at a higher level is manufactured at an even lower manufacturing cost.

Example

[0036] Next, preferred embodiments of the present invention and comparative examples not belonging to the present invention will be described separately in the first stage (Examples 1 to 4, Comparative Examples 1 to 2) and the second stage (Examples 5 to 8, Comparative Examples 3 to 4). Note that the present invention is not limited to the following examples. Also, depending on the understanding of the present invention, the following examples may substantially become comparative examples, or the following comparative examples may substantially become examples.

[0037] [Examples 1 to 4 and Comparative Examples 1 to 2] ≪Configurations, etc. 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 a flat and transparent substrate 2 made of whiteboard glass (B270 manufactured by Corning) using the above-described manufacturing apparatus 101. The optical constants (refractive index and extinction coefficient) of this substrate 2 are shown in FIG. 7. Further, as shown in the following [Table 1], the optical film 4 has a configuration of a total of seven layers in which the first layer closest to the substrate 2 is an SiO2 layer (low refractive index layer 12) and the SiO2 layer and the Ti+Nb layer (light absorption layer 10) are alternately laminated. The film thickness of each layer in the optical film 4 of Example 1 was designed such that the transmittance was around 25% in a specific wavelength range (400 nm or more and 700 nm or less), 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 perpendicularly incident (incident angle 0°) from the air side (outer side) with respect 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 perpendicularly incident from the substrate 2 side with respect to the interface between the substrate 2 and the optical film 4 (the first layer). In these design elements, the elements of the maximum surface reflectance of 1% or less and the maximum interface reflectance of 1% or less were given priority, and ensuring the flatness at a transmittance of 25% was made secondary to these elements.

[0038] More specifically, in the film formation of the optical film 4 in Example 1, the inside of the vacuum chamber 102 was -4 2×10 Pa (Pascal) at the start of film formation. Also, the rotation speed of the drum 104 was 100 rpm (revolutions per minute). Furthermore, in the cleaning of the substrate 2, with oxygen (O2) gas introduced from the radical gas inlet 134 of the radical source 130 at a flow rate of 500 sccm (Standard Cubic Centimeter per Minute), a high-frequency voltage was applied to the gun 136 at an input power of 3 kW (kilowatt) to generate radical oxygen, which was 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 performed on the vacuum chamber 102, the drum 104, or the substrate 2. Even considering the heat generated by the operation of the radical source 130 and the like, the maximum temperature of the substrate 2 during the entire process was 150°C. When forming the SiO2 layer, Ar gas was introduced from the sputter gas inlet 126 of the second sputter source 120 at 300 sccm, and a DC voltage with an input power of 8 kW was applied to the sputter cathode 122. As a result, Si on the surface of the second target T2 (purity 99.9% or higher) was deposited on the surface of the substrate 2 by sputtering with Ar. At the same time, with O2 gas introduced from the radical gas inlet 134 of the radical source 130 at 150 sccm, a high-frequency voltage was applied to the gun 136 at an input power of 3 kW to generate radical oxygen, which was then irradiated onto the moving substrate 2 on which Si had been deposited, thereby oxidizing the Si. Then, when forming the Ti+Nb layer, Ar gas was introduced from the sputter gas inlet 116 of the first sputter source 110 at 300 ccm, and a DC voltage with an input power of 6 kW was applied to the sputter cathode 112. As a result, Ti and Nb on the surface of the first target T1 (each purity of Ti and Nb 99.9% or higher) were deposited on the surface of the substrate 2 in a mixed state by sputtering with Ar. Here, the radical source 130 was turned off. In Example 1, the above-mentioned Ti+Nb target TTa was used as the first target T1, and in each Ti+Nb layer, Ti and Nb were made to have an elemental ratio of Ti:Nb = 50:50 (the same applies hereinafter). Incidentally, the rotation of the drum 104 may be temporarily speeded up or stopped once.

[0039]

Table 1

[0040] The ND filter 1 of Example 2 was formed in the same manner as in Example 1, except for the area ratio of the Ti part and the Nb part 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 Ti:Nb = 80:20. The ND filter 1 of Example 3 was formed in the same manner as in Example 1, except for the area ratio of the Ti part and the Nb part in the first target T1 and the element number ratio of the Ti+Nb layer, as shown in the following [Table 3]. The Ti+Nb layer of Example 3 has Ti:Nb = 20:80. The ND filter 1 of Example 4 was formed in the same manner as in Example 1, except for the area ratio of the Ti part and the Nb part in the first target T1 and the element number ratio of the Ti+Nb layer, as shown in the following [Table 4]. The Ti+Nb layer of Example 4 has Ti:Nb = 70:30.

[0041]

Table 2

Table 3

Table 4

[0042] ≪Configuration, etc. of Comparative Examples 1 to 2≫ The ND filter of Comparative Example 1 was formed in the same manner as in Example 1, except that each light absorption 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 absorption layer was a Nb layer. The configuration of the optical film of Comparative Example 2 is shown in the following [Table 6].

[0043]

Table 5

Table 6

[0044] ≪Transmittance, reflectance, etc.≫ Figures 8 to 13 are graphs showing the transmittance, surface reflectance, and interface reflectance in the 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 to 2, respectively. According to these figures, in Examples 1 to 4 and Comparative Examples 1 to 2, as a result of prioritizing the reduction of reflectance, the surface reflectance and the interface reflectance are 1% or less over the entire specific wavelength range.

[0045] Furthermore, in Comparative Example 1 (Ti:Nb = 100:0), in the specific wavelength range (400 nm or more and 700 nm or less), the flatness of the transmittance is relatively poor. The maximum value of the transmittance in the specific wavelength range related to the flatness of the transmittance in Comparative Example 1 is 27.082%, the minimum value is 24.040%, and the difference between the maximum value and the minimum value (the width of the transmittance ΔT) is 3.042. The smaller ΔT is, the stronger (better) the flatness of the transmittance distribution in the specific wavelength range becomes. Also, in Comparative Example 2 (Ti:Nb = 0:100), the maximum value of the transmittance in the specific wavelength range is 26.171%, the minimum value is 24.137%, and ΔT = 2.035. Similar to Comparative Example 1, the flatness of the transmittance is relatively poor.

[0046] In contrast, in Example 1 (Ti:Nb = 50:50), the maximum value of the transmittance in the specific wavelength range is 25.395%, the minimum value is 24.830%, and ΔT = 0.564, and the flatness of the transmittance in the specific wavelength range is excellent. Also, in Example 2 (Ti:Nb = 80:20), the maximum value of the transmittance in the specific wavelength range is 26.159%, the minimum value is 24.410%, and ΔT = 1.749, and the flatness of the transmittance in the specific wavelength range is excellent. Furthermore, in Example 3 (Ti:Nb = 20:80), the maximum value of the transmittance in the specific wavelength range is 25.677%, the minimum value is 24.669%, and ΔT = 0.992, and the flatness of the transmittance in the specific wavelength range is excellent. Furthermore, in Example 4 (Ti:Nb = 70:30), the maximum value of the transmittance in the specific wavelength range is 25.488%, the minimum value is 24.834%, ΔT = 0.654, and it is excellent in the flatness of the transmittance in the specific wavelength range.

[0047] FIG. 14 is a graph plotting Examples 1 to 4 and Comparative Examples 1 to 2 on a plane with the elemental ratio of Ti (Ti ratio, %) in the Ti + Nb layer on the horizontal axis and ΔT on the vertical axis. In FIG. 14, the curve connecting each plot, that is, the function ΔT with the Ti ratio obtained from each plot as the independent variable, is also shown. In FIG. 14 related to Examples 1 to 4 and Comparative Examples 1 to 2 where light in the specific wavelength range is transmitted with a transmittance of 25% as a reference (ND4), for the following reasons, 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, where a transmittance of 25%, that is, an optical density OD≒0.602 is used as a reference, the range of the Ti ratio in which the width ΔT of the transmittance falls within ±1.8% of the reference in terms of OD and sufficient flatness is obtained is generally 15% or more and 75% or less. To describe this point in more detail, the OD below 1.8% of the reference OD is 0.602×(1 - 0.018)=0.5912, and when converted to transmittance, it is 25.633%. On the other hand, the OD above 1.8% of the reference OD is 0.602×(1 + 0.018)=0.6128, and when converted to transmittance, it is 24.389%. Therefore, the width ΔT of the transmittance when the OD falls within ±1.8% of the reference is 25.633 - 24.389 = 1.244. In FIG. 14, horizontal double arrows are shown at the part where ΔT = 1.244. And in FIG. 14, within the range of the Ti ratio indicated by the double arrows, ΔT becomes 1.244 or less, the minimum value of that range is approximately 15% (the left dotted arrow), and the maximum value of that range is approximately 75% (the right dotted arrow). Therefore, in the ND filters 1 of Examples 1 to 4 and the like, when the element number ratio of Ti and Nb in the light absorption layer 10 is in the range of Ti:Nb = 15:85 to 75:25, while the reflectance in a specific wavelength range is at a high level of 1% or less, 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.

[0048] [Examples 5 to 8 and Comparative Examples 3 to 4] ≪Configuration etc. of Examples 5 to 8≫ Examples 5 to 8 were formed in the same manner as Examples 1 to 4 in order, except that the optical film 4 had a total of 9 layers and the transmittance in a specific wavelength range was around 1%. However, the Ti ratios of the light absorption layer 10 in Examples 5 to 8 are 80%, 40%, 20%, and 70% in order. The configurations of Examples 5 to 8 are shown in the following [Table 7] to [Table 10] in order.

[0049]

Table 7

Table 8

Table 9

Table 10

[0050] ≪Configuration etc. of Comparative Examples 3 to 4≫ Comparative Examples 3 to 4 were formed in the same manner as Comparative Examples 1 to 2 in order, except that the optical film had a total of 9 layers and the transmittance in a specific wavelength range was around 1%. The configurations of Comparative Examples 3 to 4 are shown in the following [Table 11] to [Table 12] in order.

[0051]

Table 11

Table 12

[0052] ≪Transmittance, Reflectance, etc.≫ Figs. 15 to 20 are graphs showing the transmittance, surface reflectance, and interface reflectance in the 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 to 4, respectively. According to these figures, in Examples 5 to 8 and Comparative Examples 3 to 4, as a result of prioritizing the reduction of reflectance, the surface reflectance and the interface reflectance are 1% or less throughout the specific wavelength range.

[0053] Furthermore, in Comparative Example 3 (Ti:Nb = 100:0), the maximum value of the transmittance in the specific wavelength range is 1.147%, the minimum value is 0.946%, and ΔT = 0.201, and the flatness of the transmittance is relatively poor. Also, in Comparative Example 4 (Ti:Nb = 0:100), the maximum value of the transmittance in the specific wavelength range is 1.131%, the minimum value is 0.947%, and ΔT = 0.185, and like Comparative Example 1, the flatness of the transmittance is relatively poor.

[0054] In contrast, in Example 5 (Ti:Nb = 80:20), the maximum value of the transmittance in the specific wavelength range is 1.133%, the minimum value is 0.955%, and ΔT = 0.178, and it is excellent in the flatness of the transmittance in the specific wavelength range. Also, in Example 6 (Ti:Nb = 40:60), the maximum value of the transmittance in the specific wavelength range is 1.110%, the minimum value is 0.968%, and ΔT = 0.142, and it is excellent in the flatness of the transmittance in the specific wavelength range. Furthermore, in Example 7 (Ti:Nb = 20:80), the maximum value of the transmittance in the specific wavelength range is 1.105%, the minimum value is 0.964%, and ΔT = 0.141, and it is excellent in the flatness of the transmittance in the specific wavelength range. Still further, in Example 8 (Ti:Nb = 70:30), the maximum value of the transmittance in the specific wavelength range is 1.107%, the minimum value is 0.967%, and ΔT = 0.140, and it is excellent in the flatness of the transmittance in the specific wavelength range.

[0055] FIG. 21 is a graph plotting Examples 5 to 8 and Comparative Examples 3 to 4 on a plane with the elemental ratio of Ti (Ti ratio, %) in the Ti + Nb layer on the horizontal axis and ΔT on the vertical axis. In FIG. 14, the curve connecting each plot, that is, the function ΔT with the Ti ratio obtained from each plot as the independent variable, is also shown. In FIG. 21 related to Examples 5 to 8 and Comparative Examples 3 to 4 where light in a specific wavelength range is transmitted with a transmittance of 1% as a reference (ND100), for the following reasons, 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, where a transmittance of 1%, that is, an optical density OD = 2, is used as a reference, the range of the Ti ratio in which the width ΔT of the transmittance falls within ±1.8% of the reference in terms of OD and sufficient flatness is obtained is generally 15% or more and 75% or less. To describe this point in more detail, the OD below 1.8% of the reference OD is 2×(1 - 0.018) = 1.964, and when converted to transmittance, it is 1.0864%. On the other hand, the OD above 1.8% of the reference OD is 2×(1 + 0.018) = 2.036, and when converted to transmittance, it is 0.9204%. Therefore, the width ΔT of the transmittance when the OD falls within ±1.8% of the reference is 1.0864 - 0.9204 = 0.166. In FIG. 21, horizontal double arrows are shown at the part where ΔT = 0.166. And in FIG. 21, within the range of the Ti ratio indicated by the double arrows, ΔT becomes 0.166 or less. The minimum value of this range is approximately 15% (the left dotted arrow), and the maximum value of this range is approximately 75% (the right dotted arrow). Incidentally, this range of the Ti ratio is the same as that in the case of Examples 1 to 4 (ND4) as described above. Therefore, in the ND filter 1 such as Examples 5 to 8, when the elemental number ratio of Ti and Nb in the light absorption layer 10 is within the range of Ti:Nb = 15:85 to 75:25, while the reflectance in a specific wavelength range is at a high level of 1% or less, 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. Also, in the ND filter 1 having OD values based on criteria different from those of Examples 1 to 8, it was confirmed by simulation that when the range of the Ti ratio is 15 to 75%, ΔT falls within ±1.8% of the criteria and is preferably excellent in the flatness of the transmittance distribution.

Explanation of symbols

[0056] 1... ND filter, 2... substrate, 4... optical film, 10... light absorption layer, 12... low refractive index layer (dielectric layer), 101... manufacturing apparatus (for ND filter 1), 102... vacuum chamber (film formation chamber), 150a, 150b... Ti part (Ti target), 152a, 152b... Nb part (Nb target), TTa, TTb... target for Ti + Nb.

Claims

1. A substrate, An optical film disposed on the side of the film-forming surface which is one or more surfaces of the substrate, Comprising: The optical film includes three light absorption layers made of a mixture of Ti and Nb, The physical film thickness of each of the light absorption layers is 3.2 nm or more and 7.6 nm or less. An ND filter characterized by this.

2. A substrate, An optical film disposed on the side of the film-forming surface which is one or more surfaces of the substrate, Comprising: The optical film includes four light absorption layers made of a mixture of Ti and Nb, The physical film thickness of each of the light absorption layers is 3.9 nm or more and 39.5 nm or less. An ND filter characterized by this.

3. The optical film includes a dielectric layer made of a dielectric. The ND filter according to claim 1 or claim 2, characterized by this.

4. The elemental number ratio of Ti and Nb in the light absorption layer is in the range of Ti:Nb = 15:85 to 75:

25. The ND filter according to any one of claims 1 to 3, characterized by this.

5. By simultaneously sputtering Ti and Nb in a film-forming chamber where the substrate is placed, a light absorption layer made of a mixture of Ti and Nb is formed on the substrate in a state where each of the three physical film thicknesses is 3.2 nm or more and 7.6 nm or less. A method for manufacturing an ND filter, characterized by this.

6. By simultaneously sputtering Ti and Nb in a film-forming chamber where the substrate is placed, a light absorption layer made of a mixture of Ti and Nb is formed on the substrate in a state where each of the four physical film thicknesses is 3.9 nm or more and 39.5 nm or less. A method for manufacturing an ND filter, characterized by this.

7. The elemental number ratio of Ti and Nb in the light absorption layer is controlled by the ratio of the area of the exposed surface in the film-forming chamber of the Ti target to the area of the exposed surface in the film-forming chamber of the Nb target. The method for manufacturing an ND filter according to claim 5 or claim 6, characterized by this.

8. Using a Ti+Nb target in which a Ti target and an Nb target are combined as the target, the light absorption layer is formed. The method for manufacturing an ND filter according to any one of claims 5 to 7, characterized by this.

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