JP4862829B2 - Absorption-type multilayer ND filter - Google Patents

Description

  The present invention relates to an absorptive multilayer ND filter that attenuates transmitted light in the visible wavelength range, and more particularly to an improvement in an absorptive multilayer ND filter having a resin film as a substrate and excellent in environmental resistance.

  As an ND filter (Neutral Density Filter), a reflection type ND filter that reflects and attenuates incident light and an absorption type ND filter that absorbs and attenuates incident light are known. When an ND filter is incorporated in a lens optical system in which reflected light is a problem, an absorption ND filter is generally used. In this absorption ND filter, an absorbing substance is mixed in the substrate itself (color glass ND filter). ) Type, and there is a type in which the substrate itself does not absorb and the thin film formed on its surface has absorption. In the latter case, the thin film is composed of a multilayer film (absorption type multilayer film) to prevent reflection on the surface of the thin film, and has an antireflection effect as well as a function of attenuating transmitted light. In addition, an absorption multilayer ND filter used in a small and thin digital camera has a small installation space, so that the substrate itself needs to be thin, and a resin film is an optimal substrate.

As an absorption-type multilayer ND filter in which the thin film is composed of a multilayer film, Patent Document 1 discloses an ND filter in which the multilayer film is composed of an oxide dielectric film and an absorption film layer. A metal film such as Ti is employed as the layer. As the absorption film layer, an ND filter using a metal oxide film such as TiOx or Ta ₂ Ox that intentionally introduces oxygen during film formation and absorbs oxygen deficiency is also known.

Here, when the absorption film layer is formed of a metal film that does not intentionally introduce oxygen at the time of film formation, the metal film has a higher extinction coefficient than the metal oxide film such as TiOx or Ta ₂ Ox. Therefore, in order to obtain the same extinction coefficient, the thickness of the absorption film layer can be reduced by using a metal film.

  And when forming an absorption-type multilayer film on a flexible resin film substrate, considering the warp of the resin film substrate, cracking of the film, film formation time, etc., the film compared with the above metal oxide film such as TiOx It is advantageous to employ a metal film that can be set to a thin thickness for the absorption film layer.

  However, since the metal film or the metal oxide film such as TiOx that is not completely oxidized easily oxidizes and its extinction coefficient decreases, the metal film or the metal that is not completely oxidized as the absorption film layer. It is known that the transmittance of the ND filter employing an oxide film increases with time, particularly in the case of a metal film.

  In such an ND filter employing a metal film or a metal oxide film that is not completely oxidized, the absorption film layer may oxidize and increase its transmittance in a high temperature and high humidity environment. It was a problem.

  By the way, it is considered that oxygen that oxidizes the metal film and the metal oxide film that is not completely oxidized is supplied in the air, or from a resin film substrate or an oxide dielectric film. In particular, when the metal film has a thickness of 10 nm or less, it is easily affected by oxidation.

In order to prevent oxidation of the metal film or the like, a method has been proposed in which heat treatment is performed in the air or in an oxygen atmosphere so that the vicinity of the interface of the metal film or the like is oxidized in advance and the oxidation does not proceed to the inside of the metal film or the like. For example, Patent Document 2 proposes a thin-film ND filter in which an absorption film layer and a dielectric film are laminated on a transparent substrate. The absorption film layer is formed by vapor deposition using a metal material Ti as a raw material, and oxygen is added. An absorbing film layer containing an oxide TiOx of a metal material introduced in a state in which a mixed gas containing the gas is introduced and the degree of vacuum is maintained constant between 1 × 10 ⁻³ Pa and 1 × 10 ⁻² Pa. It is used. In addition, a method has been proposed in which an absorption film layer and a dielectric film are laminated on a transparent substrate, and then heated in an oxygen atmosphere containing 10% or more of oxygen to saturate changes in optical characteristics.

  Patent Document 3 relates to a thin-film ND filter in which one or more transparent dielectric films and an absorption film layer are laminated on a light-transmitting substrate, and the transmittance of the absorption film layer is increased by oxidation. A method of adopting a difficult lower metal nitride film has been proposed.

  By the way, in the method described in Patent Document 2 in which the vicinity of the interface of the metal film or the like is previously oxidized by heat treatment in the air or in an oxygen atmosphere, particularly in a thin metal film having a thickness of 10 nm or less, oxidation proceeds instantaneously to the inside. Therefore, it has been difficult to form an oxide film only near the interface. In addition, when heat treatment is performed in the air or in an oxygen atmosphere, warpage and cracks may occur due to differences in the thermal expansion coefficient between the formed absorption multilayer film and the resin film substrate and relaxation of residual stress. Furthermore, since a resin film as a substrate is usually a long one when an ND filter is manufactured, a resin film having an absorption multilayer film formed by, for example, a sputtering roll coater is wound around a take-up roller. In addition, there is a problem that an extremely large apparatus is required to perform a uniform heat treatment while being attached.

On the other hand, in the method described in Patent Document 3, which employs a lower metal nitride film in which the transmittance is not easily increased by oxidation, the drawback of increasing the transmittance is solved, but the lower metal nitride film is more effective than the metal film. Since the attenuation coefficient is small, there is a problem that when the film thickness is increased, film cracking occurs when the ND filter is pressed.
Japanese Patent Application Laid-Open No. 5-93811 JP 2003-43211 A JP 2003-322709 A

  The present invention has been made paying attention to such problems, and the problem is that the absorption type multilayer ND filter using a metal film as the absorption film layer is caused by oxidation of the metal film by an external atmosphere. Accordingly, an object of the present invention is to provide an absorptive multilayer ND filter excellent in environmental resistance, in which the transmittance does not increase with time.

  As a result of intensive studies conducted by the present inventors in order to solve the above problems, the following solution techniques have been found. That is, Ni, Cr, Ti, Nb, Ta, etc. have been conventionally used as the metal material constituting the absorption film layer. However, all of these metal materials tend to oxidize and become transparent, that is, the extinction coefficient becomes small. Therefore, the transmittance of the absorption multilayer ND filter increases in a high-temperature and high-humidity environment. It was extremely difficult to maintain the spectroscopic characteristics immediately after the film for a long period of time.

  Therefore, the present inventors tried to solve the above problems by applying chemically stable Pt as the material of the absorption film layer. That is, Pt is an extremely stable metal whose extinction coefficient hardly changes due to oxidation. Pt is an extremely expensive metal, but has a high extinction coefficient. Therefore, in order to obtain the same absorption, for example, the film thickness may be about half or less than that of Ni. This will be specifically described below.

First, Table 1 shows the film configuration of an absorption multilayer ND filter having an average transmittance of 6.3% according to a conventional example composed of a Ni film and a SiO ₂ film, and FIG. When this absorptive multilayer ND filter was allowed to stand for 24 hours in a high-temperature and high-humidity environment at a temperature of 80 ° C. and a humidity of 90%, the transmittance might increase as shown in FIG. This is because the extinction coefficient is reduced by oxidation of the Ni film.

他方、Ｐｔ膜とＳｉＯ_２膜で構成される平均透過率６．３％の吸収型多層膜ＮＤフィルターの膜構成を表２に、その分光透過特性を図３に示す。

On the other hand, the film configuration of an absorption multilayer ND filter composed of a Pt film and a SiO ₂ film and having an average transmittance of 6.3% is shown in Table 2, and its spectral transmission characteristics are shown in FIG.

この吸収型多層膜ＮＤフィルターにおいては、極めて安定なＰｔを金属膜の材料に適用しているため、高温高湿環境下に長時間放置した場合でも酸化による消衰係数の変化はほとんどない。しかし、酸化物誘電体膜に透明なＳｉＯ_２膜を用いた場合、図３に示すように短波長になるほど透過率が高くなる傾向があり、可視波長域（波長４００〜７００ｎｍ）における分光透過特性のフラット性［＝（最大透過率−最小透過率）／平均透過率］が約６５％も低下し好ましくなかった。そこで、物理的気相成長法によるＳｉＯ_２の成膜時に酸素の導入量を減少させ、完全に酸化させない若干着色したＳｉＯｘ（１．５＜ｘ＜２）膜を酸化物誘電体膜に用いたところ、図４に示すように上記分光透過特性のフラット性を改善することが可能となり、１０％以下の非常に良好なフラット性が得られることを発見した。更に、この吸収型多層膜ＮＤフィルターを、温度８０℃、湿度９０％の高温高湿環境下に２４時間放置した場合でも、図５に示すように透過率は変化しなかった。本発明はこのような技術的検討を経て完成されている。

In this absorptive multilayer ND filter, extremely stable Pt is applied to the material of the metal film, so that even when left in a high temperature and high humidity environment for a long time, there is almost no change in the extinction coefficient due to oxidation. However, when a transparent SiO ₂ film is used as the oxide dielectric film, the transmittance tends to increase as the wavelength becomes shorter as shown in FIG. 3, and the spectral transmission characteristics in the visible wavelength region (wavelength 400 to 700 nm). The flatness [= (maximum transmittance−minimum transmittance) / average transmittance] was lowered by about 65%, which was not preferable. Therefore, reducing the amount of oxygen introduced during the film formation of the SiO ₂ by physical vapor deposition, completely some not oxidized colored SiOx (1.5 <x <2) film was used as an oxide dielectric film However, as shown in FIG. 4, it was possible to improve the flatness of the spectral transmission characteristics, and it was found that a very good flatness of 10% or less can be obtained. Further, even when this absorption multilayer ND filter was left in a high-temperature and high-humidity environment at a temperature of 80 ° C. and a humidity of 90% for 24 hours, the transmittance did not change as shown in FIG. The present invention has been completed through such technical studies.

That is, the invention according to claim 1
In an absorption multilayer ND filter comprising an absorption multilayer film in which an oxide dielectric film and a metal film are alternately laminated on at least one surface of a substrate made of a resin film,
The metal film is composed of Pt alone or a Pt alloy, and the oxide is formed by SiOx (1.5 <x <2) formed by controlling the amount of oxygen introduced during physical vapor deposition. A flat spectral transmission characteristic defined by a value obtained by dividing the difference between the maximum transmittance and the minimum transmittance in the visible wavelength range (wavelength 400 to 700 nm) of the absorption multilayer film by the average transmittance while the dielectric film is formed. The reflectivity is 10% or less, and the reflectance of the absorption multilayer film in the visible wavelength region is 5% or less.

The invention according to claim 2
In the absorption multilayer ND filter according to the invention of claim 1,
The outermost layer of the absorption multilayer film and the innermost layer in contact with the substrate are each composed of an oxide dielectric film,
The invention according to claim 3
In the absorptive multilayer ND filter according to claim 1 or 2,
The target used when forming the oxide dielectric film by physical vapor deposition is at least one target selected from Si single crystal, Si polycrystal or SiC ceramics,
The invention according to claim 4
In the absorptive multilayer ND filter according to claim 1, 2, or 3,
Each film thickness of the metal film composed of Pt alone or Pt alloy is 1 nm to 10 nm, and each film thickness of the oxide dielectric film composed of SiOx (1.5 <x <2) is 10 nm to 100 nm. Each of the ranges is set.

The absorption multilayer ND filter according to the present invention is
The metal film is composed of Pt alone or a Pt alloy, and the oxide is formed by SiOx (1.5 <x <2) formed by controlling the amount of oxygen introduced during physical vapor deposition. A flat spectral transmission characteristic defined by a value obtained by dividing the difference between the maximum transmittance and the minimum transmittance in the visible wavelength range (wavelength 400 to 700 nm) of the absorption multilayer film by the average transmittance while the dielectric film is formed. The reflectivity is 10% or less, and the reflectance of the absorption multilayer film in the visible wavelength region is 5% or less.

  And, in the absorption type multilayer ND filter according to the present invention, extremely stable Pt is applied to the metal film material. Therefore, even if it is exposed to a high-temperature and high-humidity environment, the spectral optical characteristics immediately after the film formation are obtained. Can be maintained over a long period of time, and the flatness of the spectral transmission characteristic in the visible wavelength region (wavelength 400 to 700 nm) is also as good as 10% or less.

  Hereinafter, embodiments of the present invention will be described in detail.

  First, an absorptive multilayer film ND filter according to the present invention comprises an absorptive multilayer film comprising an absorptive multilayer film in which an oxide dielectric film and a metal film are alternately laminated on at least one surface of a substrate made of a resin film. In the film ND filter, the metal film is composed of Pt alone or a Pt alloy, and SiOx (1.5 <x <2) is formed by controlling the amount of oxygen introduced during film formation by physical vapor deposition. ), And the oxide dielectric film is defined by the difference between the maximum transmittance and the minimum transmittance in the visible wavelength region (wavelength 400 to 700 nm) of the absorption multilayer film divided by the average transmittance. The flatness of the spectral transmission characteristics is 10% or less, and the reflectance of the absorption multilayer film in the visible wavelength region is 5% or less.

  The metal film of the absorption multilayer film is composed of Pt alone or a Pt alloy as described above, and examples of the Pt alloy include a Pt—Pd alloy, a Pt—Pd—Ru alloy, and a Pt—Pd—Cu alloy. . The oxide dielectric film of the absorption multilayer film is composed of a SiOx (1.5 <x <2) film, and the target used when depositing the SiOx by physical vapor deposition is a Si single crystal, One or more kinds of targets selected from Si polycrystal or SiC ceramics can be used. When SiC ceramics containing C is used as the target, C is hardly exhausted because it is almost exhausted.

  Here, the oxide dielectric film is formed by physical vapor deposition using one or more targets selected from Si single crystal, Si polycrystal, or SiC ceramics and controlling the amount of oxygen gas introduced. To do. The amount of oxygen gas introduced during film formation is controlled by dividing the difference between the maximum transmittance and the minimum transmittance in the visible wavelength region (wavelength 400 to 700 nm) of the absorption multilayer film by the average transmittance [i.e. The flatness of the spectral transmission characteristic defined by (maximum transmittance−minimum transmittance) / average transmittance] is 10% or less, and the reflectance of the absorption multilayer film in the visible wavelength region is 5% or less. It is necessary to be done. Examples of the physical vapor deposition method include vacuum deposition, ion beam sputtering, magnetron sputtering, and ion plating.

  Next, as an example of the absorption multilayer ND filter according to the present invention, the film configuration of the absorption multilayer ND filter having an average transmittance of 6.3% is shown in Table 2, and the spectral transmission characteristics thereof are shown in FIG. Show. In this absorption-type multilayer ND filter, a SiOx film formed by a magnetron sputtering method using a film-forming material (target) containing SiC as a main component and controlling the amount of oxygen gas introduced for the oxide dielectric film. 1.5 <x <2) is applied, and a Pt target is used as the metal film, and a Pt film formed by DC sputtering without introducing oxygen is applied.

The reason why the oxygen-deficient oxide dielectric film composed of the SiOx (1.5 <x <2) film is used is the absorption type composed of a transparent SiO ₂ film and a Pt film. In the multilayer ND filter, as described above, the transmittance on the short wavelength side in the visible wavelength region (wavelength 400 to 700 nm) increases, and the spectrum defined by [(maximum transmittance−minimum transmittance) / average transmittance]. Since the flatness of the transmission characteristics has a detrimental effect, the object is to eliminate the above-described detrimental effect by using a SiOx film having a lower transmittance (higher absorptance) on the short wavelength side than on the long wavelength side in the visible wavelength region.

  Then, the film is formed by adjusting the film forming conditions so that the flatness [= (maximum transmittance−minimum transmittance) / average transmittance] of the spectral transmission characteristics is 10% or less. Specifically, the amount of oxygen introduced during the formation of the SiOx film is controlled by an oxygen introduction valve, and is adjusted by impedance monitoring so that the impedance between the cathodes becomes a set value. When the set value of impedance is high, the amount of oxygen introduced is reduced and a colored SiOx film having a large extinction coefficient is obtained. Further, when the impedance setting value is lowered, the amount of oxygen introduced is increased, and the transparent SiOx film having a small extinction coefficient is obtained.

The impedance monitor applies a phenomenon in which the impedance between the target electrodes changes depending on the amount of oxygen introduced. This impedance monitor is formed with a desired film in a transition region between the metal mode and the oxide mode. Thus, it is used to control and monitor the amount of oxygen introduced to form an oxide dielectric film at high speed. For example, in a film obtained using a target containing SiC as a main component, the X value of SiOx changes close to 2 as the oxygen partial pressure during film formation increases (the amount of oxygen introduced during film formation increases). Then, the colored film changes to a transparent film (SiO ₂ ). As described above, with respect to the oxide dielectric film, the above-described target is used as a film forming material, and the film is formed by physical vapor deposition such as vacuum deposition, ion beam sputtering, magnetron sputtering, ion plating, In addition, the extinction coefficient can be adjusted by controlling the amount of oxygen introduced during film formation using an impedance monitor and reducing the oxygen gas introduced during film formation.

  The Pt film combined with the SiOx film is formed by physical vapor deposition such as DC sputtering using Pt alone or a Pt alloy as a target.

  Since the optical constants (refractive index, extinction system number) of the Pt film differ depending on the Pt film deposition conditions (sputtering power, Ar introduction amount, deposition rate, residual gas, film thickness, etc.), the spectral transmission characteristics are flat. The impedance setting value in the SiOx film when the property [= (maximum transmittance−minimum transmittance) / average transmittance] is 10% or less cannot be specifically expressed. In the examples shown below, the extinction coefficient in the SiOx film when the flatness of spectral transmission characteristics [= (maximum transmittance−minimum transmittance) / average transmittance] is 10% or less is “0.1 ± 0. However, since the optical constant (refractive index, extinction system number) of the Pt film varies depending on the film forming conditions as described above, the optimum extinction coefficient of the SiOx film to be combined also depends on the optical constant of the Pt film. Resulting in.

  Therefore, an absorption multilayer film is formed using the impedance setting value as a parameter, and the process of measuring the spectral transmission characteristics of the obtained absorption multilayer film to obtain flatness is repeated, and the flatness becomes 10% or less. Obtain the impedance setting value.

  For example, an absorption multilayer film composed of a SiOx film and a Pt film shown in Table 2 is formed.

The first time, the SiO ₂ film and the Pt film are configured such that the impedance setting value during SiOx film formation is reduced (introduction of oxygen is increased) and the extinction coefficient becomes substantially zero.

The extinction coefficient of a SiOx single layer film (in this case, a SiO ₂ single layer film) formed under the same conditions as this film formation condition was measured by a spectroscopic ellipsometry method. The coefficient was “k <0.001”.

  Next, the spectral optical characteristics of the obtained absorption multilayer ND filter are measured with a self-recording spectrophotometer, and the spectral transmission characteristics (see FIG. 6) and spectral reflection characteristics (see FIG. 7) are confirmed. The flatness [= (maximum transmittance−minimum transmittance) / average transmittance] is confirmed from the transmission characteristics. With the extinction coefficient “k <0.001”, the flatness of the spectral transmission characteristic is about 65% as shown in FIG. 6, and when such an ND filter is inserted, the color tone of the image is greatly increased. It is not preferable to change.

  Therefore, based on the result of the first time, in the second film formation of the absorption-type multilayer film, the impedance setting value at the time of SiOx film formation is slightly increased (oxygen introduction is slightly decreased) and the slightly colored SiOx film (1. 5 <x <2) and the Pt film constitute an absorption multilayer film.

  When the extinction coefficient of the SiOx single layer film formed under the same conditions as this film forming condition was measured by a spectroscopic ellipsometry method, the extinction coefficient at the measurement limit was “k = 0.05”.

  Next, the spectral optical characteristics of the obtained absorption multilayer ND filter are measured with a self-recording spectrophotometer, and the spectral transmission characteristics (see FIG. 6) and spectral reflection characteristics (see FIG. 7) are confirmed. The flatness [= (maximum transmittance−minimum transmittance) / average transmittance] is confirmed from the transmission characteristics. In the extinction coefficient “k = 0.05”, the flatness of the spectral transmission characteristic is about 32% as shown in FIG. 6, and the color tone of the image changes by the insertion of the ND filter. This is not preferable.

  Therefore, based on the result of the second time, in the third film formation of the absorption-type multilayer film, the impedance setting value at the time of SiOx film formation is further increased (oxygen introduction is extremely reduced), and the slightly colored SiOx film (1 .5 <x <2) and the Pt film constitute an absorption multilayer film.

  When the extinction coefficient of the SiOx single layer film formed under the same conditions as this film forming condition was measured by a spectroscopic ellipsometry method, the extinction coefficient at the measurement limit was “k = 0.1”.

  Next, the spectral optical characteristics of the obtained absorption multilayer ND filter are measured with a self-recording spectrophotometer, and the spectral transmission characteristics (see FIG. 6) and spectral reflection characteristics (see FIG. 7) are confirmed. The flatness [= (maximum transmittance−minimum transmittance) / average transmittance] is confirmed from the transmission characteristics. With the extinction coefficient “k = 0.1”, the flatness of the spectral transmission characteristic was about 6% as shown in FIG. If the flatness is 10% or less, it cannot be determined that the color tone has changed even if the image is viewed.

  In this way, a plurality of absorption-type multilayer ND filters having different optical characteristics are prepared by repeatedly changing the impedance setting value during SiOx film formation, and the flatness of the spectral transmission characteristics [= (maximum transmittance−minimum transmittance) / The film forming conditions for which the [average transmittance] is 10% or less may be adjusted.

  However, even if it is based on the first to third test results, the optimum extinction coefficient of the SiOx film is not necessarily “about 0.1 ± 0.01” for the third time. This is also combined with the Pt film used for the metal film because the optical constants (refractive index, extinction coefficient) differ depending on the film forming conditions (sputtering power, film forming speed, Ar gas partial pressure, residual gas, etc.). This is because the optimum extinction coefficient of the SiOx film depends on the optical constant of Pt.

  Here, from the viewpoint of suppressing oxidation of the metal film, the outermost layer of the absorption multilayer film composed of the oxide dielectric film and the metal film is preferably composed of an oxide dielectric film, and resin In consideration of adhesion to the film substrate (PET), the innermost layer of the absorption multilayer film composed of the oxide dielectric film and the metal film is also preferably composed of the oxide dielectric film.

  Further, it is preferable that each thickness of the oxide dielectric film constituting the absorption multilayer film is set in a range of 10 nm to 100 nm, and each thickness of the metal film is set in a range of 1 nm to 10 nm. . This is because if the thickness of each oxide dielectric film is less than 10 nm, the interference effect as a multilayer film may be reduced, and if it exceeds 100 nm, film cracking may occur. On the other hand, if the thickness of each metal film is less than 1 nm, the desired film formation may be difficult because the film formation time is short. If the film thickness exceeds 10 nm, the transmittance is less than 1% and the desired transmittance is obtained. This is because it may not be possible to obtain. In order to obtain a thin metal film, the in-line sputtering method or roll coating method has an inversely proportional relationship between the film thickness and the substrate transport speed, and therefore, for example, it can be obtained under the condition of a substrate transport speed of 1 m / min. If the film thickness of the Pt film is 4 nm, a Pt film having a film thickness of 2 nm can be obtained by setting the substrate transport speed to 2 m / min.

  Next, although the material of the resin film which comprises a board | substrate is not specifically limited, What is transparent is preferable, When considering mass productivity, it is preferable that it is a flexible board | substrate which can perform the dry type sputtering roll coating mentioned later. The flexible substrate is excellent in that it is cheaper, lighter and more deformable than a conventional glass substrate. As specific examples of the resin film constituting the substrate, the resin film selected from polyethylene terephthalate (PET), polyethersulfone (PES), polyarylate (PAR), polycarbonate (PC), polyolefin (PO) and norbornene. Or a composite of a single resin film selected from the above resin materials and an acrylic organic film covering one or both sides of the single resin film. In particular, as for norbornene resin materials, representative examples include ZEONOR (registered trademark) of Nippon Zeon Co., Ltd. and Arton (registered trademark) of JSR Corporation.

  Examples of the present invention will be specifically described below.

  An absorption multilayer ND filter according to Example 1 having an average transmittance of 6.3% was manufactured.

  A film-forming material (target) containing SiC as a main component was used for the oxide dielectric film of the absorption multilayer ND filter, and a Pt film-forming material (target) was used for the metal film.

  A sputtering roll coater [manufactured by Sumitomo Metal Mining Co., Ltd.] was used as the film forming means, and a turbo molecular pump was used as the exhaust pump.

  The SiOx film was formed by introducing Ar gas at 150 sccm, sputtering by a dual magnetron sputtering method at a sputtering power of 10 kW, and controlling oxygen introduction by an impedance monitor. The smaller the impedance control set value is, the more oxygen is introduced.

  Here, in the dual magnetron sputtering method, in order to form an insulating film at high speed, a medium frequency (40 kHz) pulse is alternately applied to two targets to suppress the occurrence of arcing, thereby forming an insulating layer on the target surface. It is the sputtering method which prevents.

  In addition, the impedance monitor controls and monitors the amount of oxygen introduced so that the film formed as described above becomes a desired film in the transition region between the metal mode and the oxide mode. Used for high speed film formation.

  A film obtained using a target composed mainly of SiC has an X value of SiOx that is close to 2 as the oxygen partial pressure during film formation increases (the amount of oxygen introduced during film formation increases). It changes from a colored film to a transparent film.

  On the other hand, the metal film is formed by DC sputtering using a Pt target, and oxygen is not introduced. Further, Ar gas was introduced at 150 sccm, and film formation was performed at a sputtering power of 500 W.

  The film forming conditions of the absorption multilayer film were adjusted (searched) by the repeated test described below.

  First, an absorption multilayer film composed of a SiOx film and a Pt film shown in Table 2 is formed.

The first time, the SiO ₂ film and the Pt film are configured such that the impedance setting value during SiOx film formation is reduced (introduction of oxygen is increased) and the extinction coefficient becomes substantially zero. The optical constants (refractive index and extinction coefficient) of the Pt film are shown in Table 3 below.

尚、この成膜条件と同一条件で成膜したＳｉＯｘ膜（ＳｉＯ_２単層膜）の消衰係数を分光エリプソメトリー法により測定したところ、測定限界の消衰係数「ｋ＜０．００１」であった。

When the extinction coefficient of the SiOx film (SiO ₂ single layer film) formed under the same conditions as this film forming condition was measured by the spectroscopic ellipsometry method, the extinction coefficient “k <0.001” at the measurement limit was obtained. there were.

  Next, the spectral optical characteristics of the obtained absorption multilayer ND filter were measured with a self-recording spectrophotometer. The spectral transmission characteristics are shown in FIG. 6, and the spectral reflection characteristics are shown in FIG. Further, when the flatness [= (maximum transmittance−minimum transmittance) / average transmittance] was determined from the spectral transmittance characteristics of FIG. 6, it was about 65%. When such an ND filter is inserted, the color tone of the image changes significantly, which is not preferable.

  Therefore, based on the first test result, in the second absorption multilayer film formation, the impedance setting value at the time of SiOx film formation is slightly increased (oxygen introduction is slightly decreased) to slightly color the SiOx film (1. 5 <x <2) and the Pt film constitute an absorption multilayer film.

  When the extinction coefficient of the SiOx single layer film formed under the same conditions as this film forming condition was measured by a spectroscopic ellipsometry method, the extinction coefficient at the measurement limit was “k = 0.05”.

  Next, the spectral optical characteristics of the obtained absorption multilayer ND filter were measured with a self-recording spectrophotometer. The spectral transmission characteristics are shown in FIG. 6, and the spectral reflection characteristics are shown in FIG. Further, when the flatness [= (maximum transmittance−minimum transmittance) / average transmittance] was determined from the spectral transmittance characteristics of FIG. 6, it was about 32%. Even in this case, the color tone of the image changes due to the insertion of the ND filter.

  In the third film formation of the absorption multilayer film, the impedance setting value at the time of SiOx film formation is further increased (oxygen introduction is extremely reduced), and a slightly colored SiOx film (1.5 <x <2) and Pt An absorptive multilayer film was formed with the film.

  When the extinction coefficient of the SiOx single layer film formed under the same conditions as this film forming condition was measured by a spectroscopic ellipsometry method, the extinction coefficient at the measurement limit was “k = 0.1”.

  Next, the spectral optical characteristics of the obtained absorption multilayer ND filter were measured with a self-recording spectrophotometer. The spectral transmission characteristics are shown in FIG. 6, and the spectral reflection characteristics are shown in FIG. Further, when the flatness [= (maximum transmittance−minimum transmittance) / average transmittance] was determined from the spectral transmittance characteristics of FIG. 6, it was about 6%. If the flatness is 10% or less, it cannot be determined that the color tone has changed even if the image is viewed.

  Therefore, when a theoretical simulation of the optical thin film is performed based on the third data, the range of the extinction coefficient of the SiOx film (1.5 <x <2) in which the flatness of the spectral transmittance is 10% or less is “ About 0.1 ± 0.01 ”.

  Although it is in a very narrow extinction coefficient range, it has been confirmed that the reproducibility of the extinction coefficient by impedance control during the formation of the SiOx film is high and does not cause any particular problem.

[Comparative example]
An absorption multilayer ND filter according to a comparative example having an average transmittance of 6.3% composed of the Ni film and the SiO ₂ film shown in Table 1 was manufactured. The film forming conditions of the absorption multilayer film are the same as those in the first embodiment. However, even in an absorption type multilayer ND filter composed of a SiO ₂ film and a Ni film in which the impedance setting value during SiOx film formation is reduced (oxygen introduction is increased) and the extinction coefficient becomes almost zero. Similarly to the absorption multilayer ND filter according to Example 1, the flatness of the spectral transmittance can be reduced to 10% or less.

  The spectral optical characteristics of the absorption multilayer ND filter according to the obtained comparative example were measured with a self-recording spectrophotometer. The spectral transmission characteristics are shown in FIG. 1, and the spectral reflection characteristics are shown in FIG.

"Evaluation"
The absorptive multilayer ND filters according to Example 1 and the comparative example were left in an environmental test machine set at 80 ° C. and 90%, respectively, and their environmental resistance was investigated.

  Then, at 24 hours, each absorption multilayer ND filter was taken out from the environmental testing machine, and the spectral transmission characteristics after 24 hours were measured with a self-recording spectrophotometer, and the environmental resistance was examined from the change over time. . In addition, the change of the spectral transmittance | permeability by the environmental resistance test of the absorption type multilayer ND filter which concerns on Example 1 and a comparative example is shown in FIG. 5 and FIG. 2, respectively.

  From the graph of FIG. 2, it is confirmed that the absorption multilayer ND filter according to the comparative example tends to increase the transmittance of the spectral transmission characteristic, particularly the short wavelength side in the visible wavelength range, by the environmental resistance test. It was done. This is because the Ni film as the absorption film layer is oxidized and the extinction coefficient is lowered.

  On the other hand, it was confirmed from the graph of FIG. 5 that the absorption-type multilayer ND filter according to Example 1 had no change in spectral transmission characteristics due to the environmental resistance test. This is presumably because the Pt film as the absorption film layer is extremely stable.

  According to the absorptive multilayer ND filter of the present invention, the spectral optical characteristics immediately after film formation can be maintained for a long period of time even when exposed to a high-temperature and high-humidity environment, and the visible wavelength range (wavelength 400) The flatness of spectral transmission characteristics at ~ 700nm) is also good at 10% or less, so it has industrial applicability for small and thin digital cameras that require long-term reliability in harsh environments. is doing.

Graph showing the spectral transmittance characteristics of the Ni film and the average transmittance 6.3% of the absorption type multi-layer film ND filter according to a conventional example (Comparative Example) constituted by SiO ₂ film. Ni film and graphs showing a conventional example (Comparative Example) spectral transmission characteristics before and after high-temperature and high-humidity environmental test average transmittance 6.3% of the absorption type multi-layer film ND filter according to composed of SiO ₂ film. The graph which shows the spectral transmission characteristic of the absorption-type multilayer ND filter with an average transmittance of 6.3% composed of a Pt film and a SiO ₂ film. The graph which shows the spectral transmission characteristic of the absorption type multilayer ND filter which concerns on this invention of the average transmittance | permeability of 6.3% comprised with a Pt film | membrane and a SiOx (1.5 <x <2) film | membrane. The graph which shows the spectral transmission characteristic before and behind the high temperature, high humidity environment test in the absorption type multilayer film ND filter which concerns on this invention of the average transmittance | permeability 6.3% comprised with a Pt film | membrane and a SiOx (1.5 <x <2) film | membrane. Figure. The graph which shows the spectral transmission characteristic of the absorption type multilayer ND filter which concerns on this invention (Example 1) with an average transmittance of 6.3%. The graph which shows the spectral reflection characteristic of the absorption type multilayer ND filter which concerns on this invention (Example 1) with an average transmittance of 6.3%. Graph showing the spectral reflection characteristics of the Ni film and the average transmittance 6.3% of the absorption type multi-layer film ND filter according to a conventional example (Comparative Example) constituted by SiO ₂ film.

Claims (4)

In an absorption multilayer ND filter comprising an absorption multilayer film in which an oxide dielectric film and a metal film are alternately laminated on at least one surface of a substrate made of a resin film,
The metal film is composed of Pt alone or a Pt alloy, and the oxide is formed by SiOx (1.5 <x <2) formed by controlling the amount of oxygen introduced during physical vapor deposition. A flat spectral transmission characteristic defined by a value obtained by dividing the difference between the maximum transmittance and the minimum transmittance in the visible wavelength range (wavelength 400 to 700 nm) of the absorption multilayer film by the average transmittance while the dielectric film is formed. An absorptive multilayer film ND filter, wherein the reflectivity is 10% or less, and the reflectance of the absorptive multilayer film in the visible wavelength region is 5% or less.   2. The absorptive multilayer ND filter according to claim 1, wherein an outermost layer of the absorptive multilayer film and an innermost layer in contact with the substrate are each composed of an oxide dielectric film.   The target used when forming the oxide dielectric film by physical vapor deposition is at least one target selected from Si single crystal, Si polycrystal, or SiC ceramics. 3. The absorption multilayer ND filter according to 1 or 2.   Each film thickness of the metal film composed of Pt alone or Pt alloy is 1 nm to 10 nm, and each film thickness of the oxide dielectric film composed of SiOx (1.5 <x <2) is 10 nm to 100 nm. The absorptive multilayer ND filter according to claim 1, wherein each of the ranges is set to a range.

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