JP2013015827A - Nd filter and optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ND filter capable of decreasing aging in optical characteristics caused by transfer of oxygen in a thin film to improve environmental stability, and to reduce impact of aging to an imaging result in an optical device using the ND filter.SOLUTION: An ND filter includes: a substrate 13 having light permeability; and a refraction factor gradient thin film 12 including a metallic compound for absorbing light in at least one portion of visible wavelength region. The refraction factor gradient thin film 12 has a composition containing at least the metallic compound and a chemical compound different from the metallic compound, consecutively changing in a film thickness direction. Use of an optical system employing the ND filter 14 of this invention helps reduce aging of the image obtained at the imaging device.

Description

  The present invention relates to an ND filter. The present invention also relates to an optical device used in an optical system that captures an image with an image sensor using an ND filter.

  The aperture stop is provided to control the amount of incident light on a solid-state image sensor such as a silver salt film or a CCD or CMOS sensor, and the aperture is further reduced as the field becomes brighter. It has become.

  Therefore, when shooting a clear or high-brightness scene, the aperture is in a so-called small aperture state, and is susceptible to the effects of aperture hunting and light diffraction, resulting in degradation of image performance. There is.

  As countermeasures against this, the amount of light is controlled by arranging an ND filter in the vicinity of the diaphragm, or by directly attaching the ND filter to the diaphragm blades. We are trying to make it larger.

  In recent years, as the sensitivity of the image sensor increases, the density of the ND filter is increased to further reduce the light transmittance, and even when a highly sensitive image sensor is used, the aperture of the diaphragm becomes too small. Improvements have been made to prevent it.

  A transparent substrate such as glass may be used as the substrate for forming the ND filter. However, when there is a high demand for processability to an arbitrary shape, miniaturization and weight reduction, for example, PET (polyethylene terephthalate) Various plastic materials such as PEN (polyethylene naphthalate) and PC (polycarbonate) have been used. Among these, arton (made by JSR: product name) and Zeonex (made by Nippon Zeon: product name) are representative from the comprehensive viewpoints including heat resistance, flexibility, and cost factors. Norbornene-based resins, polyimide-based resins, and the like are suitable substrates.

  Absorption type ND filters include those that mix organic dyes or pigments that absorb light into the substrate and knead them, and those that apply organic dyes or pigments that absorb light. The type has a fatal defect that the wavelength dependency of the spectral transmittance is too large. Therefore, at present, the most common production method is to produce an ND filter by forming a multilayer film on a transparent substrate such as plastic or glass by a vacuum film formation method such as an evaporation method or a sputtering method. .

  In the absorption type ND filter constituted by the multilayer film as described above, a metal film or a metal oxide film having a relatively large extinction coefficient is used for an absorption layer that exhibits light absorption. However, it is known that the metal film and the metal oxide film are easily oxidized, and as a result, the extinction coefficient decreases, so that the absorption decreases and the transmission increases. Such oxygen is supplied from the surrounding environment such as the substrate and the atmosphere, and from the dielectric layer sandwiching the absorption layer.

  Among these, particularly in a thin film, oxygen easily moves near the interface where the composition changes greatly. As such an environmental improvement measure, Patent Document 1 proposes a method of manufacturing an absorption film using a metal nitride film that is less affected by oxidation. Patent Document 2 discloses a method of improving environmental performance by heating in an oxygen atmosphere to saturate oxidation from a so-called aging effect.

JP 2003-322709 A JP 2003-043211 A

  However, in the method of using a lower metal nitride film in the absorption layer and suppressing the variation in transmittance as disclosed in Patent Document 1, the lower metal nitride film is less consumed than the metal film or the metal oxide film. Since the extinction coefficient is small, the film thickness is increased, which causes a problem that the film stress is increased and the productivity is deteriorated.

  In addition, in the case of the method of suppressing the transmission change by the aging effect shown in Patent Document 2, when it is necessary to make the absorption layer thin due to a stress problem or a thermal problem in the process, etc. In many cases, it is difficult to oxidize and stabilize only the vicinity of the interface of the absorption layer, and the oxidation proceeds to the inside, making it difficult to achieve a high concentration.

  As described above, an object of the present invention is to provide an ND filter that suppresses a change in optical characteristics over time caused by movement of oxygen in a thin film and improves environmental stability. Another object of the present invention is to provide an optical device that improves the image quality stability of a captured image by using such an ND filter.

  In order to solve the above-described problems, an ND filter according to the present invention includes a light-transmitting substrate and a refractive compound that includes a metal compound that absorbs light in at least a part of a visible wavelength region on the substrate. A refractive index gradient thin film, wherein the gradient refractive index thin film has a composition comprising at least the metal compound and a compound different from the metal compound continuously changing in the film thickness direction. And

  In order to solve the above problems, an optical device of the present invention is characterized by using the above-described ND filter in an optical system.

  According to the present invention, there is no interface in the thin film, and a structure having a very close composition at the front and rear positions in the thin film can maintain a stable state in which oxygen migration hardly occurs. Thus, an ND filter with improved environmental stability can be obtained. In addition, by using such an ND filter in an optical device, an optical device with improved stability of a captured image can be provided.

It is an example of the refractive index distribution of the gradient refractive index thin film which concerns on this invention. It is an example of composition of an optical filter concerning the present invention. It is a modification of the structure of the optical filter which concerns on this invention. It is an example of a refractive index profile of the gradient refractive index thin film according to the present invention. It is an electron microscope figure of this multilayer film and a refractive index gradient thin film. It is an example of the spectral transmittance characteristics of TiO and _Ti 2 _{O 3} according to the present invention. It is a schematic plan view of the sputtering apparatus used for implementation of this invention. 2 is a spectral reflectance characteristic of an optical filter manufactured according to Example 1. FIG. 6 is a configuration diagram of an optical filter as Comparative Example 1. FIG. It is a spectral reflectance characteristic of the ND filter produced by the comparative example 1. It is a comparison figure of the spectral transmission characteristic of Example 1 and Example 2. FIG. It is the schematic of a microarray structure of a pillar array shape. It is an example of arrangement | sequence of a fine periodic structure. 6 is a configuration diagram of an optical filter manufactured according to Example 3. FIG. 6 is a configuration example of an optical filter described in Example 3. It is a refractive index profile of the refractive index gradient thin film in Example 3. The substrate is arranged on the left side, and the antireflection structure is arranged on the right side. 6 is a spectral reflectance characteristic of an optical filter manufactured according to Example 3. It is explanatory drawing of the light quantity aperture stop apparatus using the ND filter of Example 4. FIG. FIG. 6 is an explanatory diagram of an optical system of an optical photographing apparatus using an ND filter of Example 4. It is explanatory drawing of the optical measuring apparatus using the ND filter of Example 5. FIG.

  The ND filter according to the present invention has a refractive index gradient thin film including a light-transmitting substrate and a light-absorbing metal oxide film. An antireflection structure may be provided on the gradient refractive index thin film.

  As the substrate, those having strength and optical characteristics as the substrate of the optical filter and capable of functioning as a base for forming the gradient refractive index thin film and the antireflection structure are used. As a substrate, a substrate made of a glass material, PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PES (polyethersulfone), PC (polycarbonate), PO (polyolefin), PI (polyimide) and PMMA (PMMA) A substrate made of a resin material selected from polymethyl methacrylate) or the like can be used.

  The gradient refractive index thin film is a thin film having light absorption, and the light absorption of the gradient refractive index thin film in the thickness direction is set according to the function and characteristics of the target optical filter. When at least about 1% is absorbed with respect to a predetermined wavelength of incident light, it can be said that it has light absorptivity for the wavelength.

  The gradient refractive index thin film is composed of a metal compound that absorbs light in at least a part of the visible wavelength region. By continuously changing the composition of the gradient refractive index thin film, the adjacent composition becomes closer and oxygen moves. Is reduced.

The gradient refractive index thin film has a refractive index change that changes continuously and periodically in its thickness direction. This refractive index change is
(1) a portion where the continuous refractive index change becomes the maximum rate of change per film thickness with respect to the film thickness direction in the gradient refractive index thin film;
(2) A portion where the refractive index change is gentler than the refractive index change in the portion of (1) in the vicinity of the substrate of the gradient refractive index thin film and in the vicinity of the medium of the gradient refractive index thin film that is adjacent to the substrate. When,
(3) On the substrate side, a portion where the refractive index changes so as to approach the refractive index of the substrate, until the end of the refractive index change on the substrate side;
(4) It is preferable that the antireflection structure side includes a portion where the refractive index changes so as to approach the refractive index of the antireflection structure to the end point on the antireflection structure side of the refractive index change.

The refractive index gradient thin film changes its refractive index continuously or stepwise in the film thickness direction so as to approach the refractive index of the substrate, and the substrate and the structure adjacent to the atmosphere or the gradient refractive index thin film (for example, an antireflection structure). The difference in refractive index between and may be reduced.
When this refractive index change in the film thickness direction is divided into a plurality of regions in the film thickness direction, the gradient refractive index thin film has a region where the spectral transmission characteristic in the visible wavelength region becomes higher as the wavelength becomes longer, and the visible wavelength region The spectral transmission characteristic of the light source has a region that becomes lower as the wavelength becomes longer. Preferably, the composition of the gradient refractive index thin film changes continuously in the film thickness direction. By continuously changing the composition, an effect of improving the antireflection effect and environmental stability can be obtained. In addition, when the structure adjacent to the gradient refractive index thin film is an antireflection structure, the gradient refractive index thin film reduces the refractive index difference between the substrate and the antireflection structure. The refractive index difference between the refractive index gradient thin film on the substrate side end and the refractive index difference between the refractive index gradient thin film on the antireflection structure side end is set to be smaller than the refractive index difference between the substrate and the antireflection structure. Particularly preferably, the refractive index of the substrate is changed so as to continuously connect the refractive index of the antireflection structure. Further, the reflection on both sides of the substrate may be reduced by providing at least one of the antireflection structure and the gradient refractive index thin film on the opposite surface of the gradient refractive index thin film provided on one side of the substrate.

Note that the substrate side end point of the above refractive index change is, for example, a point indicated by A in FIG. 1, and the end point on the antireflection structure side is a point indicated by B. In the example shown in FIG. 1, at the terminal portion including the substrate side end point (or starting point) A of the change in the refractive index distribution, the refractive index of the gradient refractive index film approaches the refractive index of the substrate at the terminal portion including this point A. Has changed. Similarly, in the terminal portion including the antireflection structure end point (or starting point) B of the change in the refractive index distribution, the refractive index of the refractive index gradient film approaches the refractive index of the antireflection structure at the terminal portion including the point B. Has changed. The point A may be located at the substrate side interface. Point B may also be located at the interface on the antireflection structure side. Further, since the vicinity of A and B in FIG. 1 is the vicinity of an adjacent medium, the portion C in which the refractive index change is the maximum refractive index change per film thickness that continuously changes the refractive index between A and B. The refractive index change is more gradual than the refractive index change. Since the refractive index change is gentle in the vicinity of the adjacent medium, the composition can be further stabilized and the environmental stability can be improved at a portion where oxidation is more likely to occur than in the structure of the gradient index thin film.
In addition, when an antireflection structure is provided, reflection due to a difference in refractive index from the antireflection structure can be reduced by bringing the refractive index close to the refractive index of the antireflection structure.

Note that the substrate-side end point of the above refractive index change is, for example, the point indicated by A in FIG. 1, and the end point on the anti-reflective fine periodic structure side is the point indicated by B. In the example shown in FIG. 1, the refractive index of the gradient refractive index film changes so as to approach the refractive index of the substrate at the terminal portion including the substrate side end point (or starting point) A of the change in the refractive index distribution. Similarly, in the terminal portion including the antireflection structure end point (or starting point) B of the change in the refractive index distribution, the refractive index of the refractive index gradient film changes so as to approach the refractive index of the antireflection structure. The point A may be located at the substrate side interface. Point B may also be located at the interface on the antireflection structure side. In addition, reflection can be greatly reduced if there is a continuous change or minute refractive index difference, and the refractive index changes smoothly from the larger refractive index to the adjacent structure, that is, the refractive index of the substrate or fine periodic structure. Even if it approaches, you may approach from the one where a refractive index is small. The difference a between the refractive index of the substrate side end in the film thickness direction of the gradient refractive index thin film and the refractive index of the substrate, and the refractive index of the end on the fine periodic structure side in the film thickness direction of the gradient refractive index thin film and the fine periodic structure It is only necessary that the sum (a + b) of the refractive index difference b of the body is smaller than the refractive index difference between two structures adjacent to both surfaces of the gradient refractive index thin film.
That is, the refractive index changes in the film thickness direction so that the refractive index of the gradient refractive index thin film reduces the refractive index difference between the refractive index of the substrate and the refractive index of the material constituting the microstructure. The relationship between the refractive index difference | A−B | and (a + b) between A and the refractive index B of the fine periodic structure is that | A−B |> a + b. This relationship is the same in the case of the substrate, the other gradient refractive index thin film, and the antireflection structure in FIGS.

  Depending on the film formation method, there may be a portion where the refractive index in the thickness direction is constant at the first portion of the thin film formed on the substrate. For example, as described later, when forming a gradient refractive index thin film on a substrate, the blending ratio of a plurality of thin film forming materials is changed to form a continuous change in refractive index in the film thickness direction. At that time, after the film formation is started at a constant film formation material concentration, the blending ratio of the plurality of thin film formation materials can be changed after a certain time has elapsed. In that case, a portion where the refractive index does not change in the thickness direction as described above may occur.

  The refractive index at the end point of the refractive index change on the substrate side is the same as the refractive index of the substrate, or the refractive index within the range of the refractive index difference allowed in the characteristics of the target ND filter with respect to the refractive index of the substrate. Any rate is acceptable. Similarly, the refractive index at the end point of the refractive index change on the antireflection structure side is the same as the refractive index of the antireflection structure, or the objective in the wavelength or wavelength region of transmitted light with respect to the refractive index of the antireflection structure. It is sufficient that the refractive index is within the range of the refractive index difference allowed in the characteristics of the ND filter. These refractive index differences are preferably 0.05 or less. Therefore, even when the portion where the refractive index does not change in the thickness direction described above is in contact with the interface on the substrate side, the refractive index of the portion where the refractive index does not change is 0. 0 relative to the refractive index of the substrate. The refractive index difference is preferably within 05. This also applies to the case where there is a portion where the refractive index does not change in the thickness direction in contact with the interface on the antireflection structure side.

The width of the change in the refractive index in the thickness direction of the gradient refractive index thin film can be set in various ways depending on the characteristics of the target optical filter, the type of material for forming the gradient refractive index thin film, or a combination thereof. For example, when changing from a region made of SiO ₂ to a region made of TiO ₂ using three kinds of elements in the thickness direction of the gradient refractive index thin film, the thickness is changed within a range of about 1.47 to 2.70. be able to.
The thickness of the gradient refractive index film can be appropriately selected according to the intended function. The thickness of the gradient refractive index film can be 10 to 4000 nm, more preferably 100 to 1000 nm.

  Any antireflection structure may be used as long as it has an antireflection function required to obtain the desired optical characteristics of the ND filter. The antireflection structure includes a fine structure having a surface on which a large number of fine protrusions are arranged at a pitch shorter than the wavelength of visible light, or a surface provided with repeated irregularities at a pitch shorter than the wavelength of visible light. It is possible to use a microstructure having This fine structure reduces the difference in refractive index from the atmosphere and adjacent media by randomly forming protrusions such as needles and pillars, and protrusions or recesses of a concavo-convex structure finely formed in a staircase shape. Also included. As this fine structure, one selected from known fine structures according to the purpose can be used. For example, it has a periodic structure consisting of a large number of protrusions arranged with a repetition period shorter than the wavelength of visible light transmitting through the substrate, or a periodic structure consisting of an uneven structure with a repetition period shorter than the wavelength of visible light transmitting through the substrate. If it is a fine periodic structure, it can be produced with good reproducibility using a method such as optical nanoimprint. As another antireflection structure, an antireflection film formed of a single layer or a plurality of thin films can be used. In a single-layer or multiple-layer antireflection film, the gradient refractive index thin film has a refractive index in the film thickness direction so as to reduce the refractive index difference between the refractive index of the layer adjacent to the gradient refractive index thin film and the refractive index of the substrate. Change.

  The substrate, the gradient refractive index thin film whose refractive index continuously changes in the film thickness direction, and the antireflection structure that exhibits the antireflection effect in the desired wavelength region are arranged adjacent to each other in this order. By doing so, the reflectance of light in the ND filter can be remarkably reduced. Therefore, in the present invention, a thin film whose refractive index changes continuously in the film thickness direction, preferably continuously and periodically, and the relationship between the refractive indexes of the substrate, the gradient refractive index thin film, and the antireflection structure is described above. (3) and (4) are set.

  However, when an ND filter having a light-absorbing property in a gradient refractive index thin film is used, the structure in which the gradient refractive index thin film is simply disposed between the substrate and the fine periodic structure has high image quality such as color balance. It is very difficult to adjust and improve the spectral transmission characteristics required for the conversion. Therefore, in a preferred configuration of the present invention, the region where the spectral transmission characteristic becomes higher as it goes to the longer wavelength side and the region where the spectral transmission characteristic becomes lower as it goes toward the longer wavelength side with respect to the incident light on the substrate By disposing in the film thickness direction of the gradient refractive index thin film, it is possible to obtain an ND filter in which the flatness of the spectral transmission characteristic and the spectral transmission characteristic of the gradient refractive index thin film is improved.

  In addition, by providing the refractive index gradient thin film on each of both surfaces of the substrate, the respective film stresses can be offset and warpage of the substrate can be prevented.

  The configuration of the ND filter according to the present invention can be used for an ND filter used in an optical device in which deterioration with time is a problem, thereby improving environmental stability and preventing image deterioration.

  The substrate, the gradient refractive index thin film whose refractive index continuously changes in the film thickness direction, and the antireflection structure that exhibits the antireflection effect in the desired wavelength region are arranged adjacent to each other in this order. By doing so, the reflectance of light in the optical filter can be significantly reduced. In order to adjust and improve the spectral transmission characteristics required for high image quality, such as color balance in optical filters with a light-absorbing refractive index gradient thin film, stepwise or continuously in the film thickness direction Preferably, a thin film whose refractive index changes continuously and periodically is used, and the relationship between the refractive indices of the substrate, the gradient refractive index thin film, and the antireflection structure is set as described in (1) to (4) above. Is preferred. With this preferable configuration, it is possible to obtain an absorption type optical filter that significantly reduces reflection and improves the flatness of spectral transmission characteristics.

Example 1
The absorption type ND filter configured as shown in FIG. 2 will be described in detail below.

  In addition, the refractive index in the embodiment and each example described below can be specified as the refractive index of light having a wavelength of 540 nm from the constituent materials of the substrate, the gradient refractive index thin film, and the antireflection structure.

  As shown in FIG. 2, the gradient refractive index thin film 12 is disposed on one side (upper surface) side of the substrate 13. Further, when the antireflection structure 111 is disposed on the gradient refractive index thin film 12 and the antireflection structure 112 is also disposed on the back surface of the substrate 13, an antireflection effect can be further obtained. The gradient refractive index thin film 12 has absorption in at least a part of the film.

  In the case of the configuration shown in FIG. 2, since reflection on the back surface of the substrate becomes large, some antireflection structure 112 is often required on this surface as well. As such antireflection structures 111 and 112, as shown in FIGS. 3A to 3B, the fine periodic structures 151 and 152 having an antireflection effect, a single layer, or a plurality of layers Examples thereof include antireflection films 161 and 162 formed of a thin film. Furthermore, as shown in FIGS. 3C to 3D, a configuration in which the fine periodic structure 15 and the antireflection film 16 are used in combination. An optimal configuration may be selected as appropriate. With such a configuration, for example, regardless of which direction of the filter is directed to the image sensor side, generation of ghost light due to reflection of the filter can be suppressed. It can also be placed in the system.

  3A to 3D, the configuration as shown in FIG. 3A is more desirable from the viewpoint of reducing reflection. Therefore, in Example 3 of the present invention described later, a fine periodic structure was formed on both sides of the substrate as the antireflection structure.

  Here, for example, a function having the same effect as that of the antireflection films 161 and 162 having a multilayer structure as shown in FIG. 3B can be incorporated into the gradient refractive index thin film 12. In that case, a refractive index profile for preventing the interface reflection with the outside air is required by increasing and decreasing the refractive index periodically and continuously a plurality of times within a predetermined region near the interface of the surface layer. Therefore, it can be considered that the antireflection structure is separately provided on the gradient refractive index thin film. In addition, when creating an antireflection film, an antireflection film whose refractive index changes periodically and continuously is used on the gradient refractive index thin film, using a material different from the material used to create the gradient refractive index thin film. May be.

<About gradient refractive index thin film>
The gradient refractive index thin film 12 is prepared by mixing these two types and adjusting the refractive index continuously in the film thickness direction while adjusting the deposition rate of the SiO ₂ and TiO x films by the meta-mode sputtering method. In order to obtain the desired absorption characteristics, it was prepared.

  Examples of the gradient refractive index thin film having such a continuous refractive index profile are shown in FIGS. In FIG. 1, a refractive index gradient thin film and a fine periodic structure are laminated in this order from a substrate having a relatively high refractive index. And it has a change that the refractive index continuously increases or decreases from the substrate side with respect to the film thickness direction, and approaches the refractive index of each adjacent structure as it goes to the interface at both ends of the gradient refractive index thin film. Taking changes.

  Various methods have been studied for designing such a gradient refractive index thin film, and unlike a continuous change, it is a step-type refractive index distribution in which the refractive index gradually changes stepwise. However, it has been found that by adjusting this refractive index distribution, it is possible to obtain substantially the same optical characteristics as a film having a continuous index change. However, for reflection reduction, etc., it is possible to obtain more ideal characteristics by having a continuous refractive index change, and since the interface in the thin film disappears and the film composition before and after becomes very close, Effects such as improved adhesion strength of the film and improved environmental stability appear. From such a viewpoint, it is better to select a refractive index distribution in which the refractive index continuously changes.

  An example of the refractive index profile of such a gradient refractive index thin film is shown in FIG. FIG. 1 shows a case where a substrate having a relatively high refractive index is laminated in the order of a refractive index gradient thin film and an antireflection structure such as a fine periodic structure. And it has a change that the refractive index continuously increases or decreases from the substrate side with respect to the film thickness direction, and approaches the refractive index of each adjacent structure as it goes to the interface of both ends of the gradient refractive index thin film. Taking changes. In the configuration shown in FIG. 1, a substrate is arranged on the left side of FIG. 1 and an antireflection structure is arranged on the right side as a structure. When the antireflection structure is not formed, the refractive index is changed so as to approach an adjacent medium such as air, water, or an organic medium. At this time, it is preferable that the refractive index change is moderate in the vicinity of adjacent media. Since the refractive index change is gradual, the composition before and after the film thickness direction approaches and the environmental stability is further improved. Further, in the case where the oxygen supply is easily performed in the vicinity of the adjacent medium, the influence of the change with time can be further reduced. Therefore, in the vicinity of A and B in FIG. 1, the film thickness is larger than the refractive index change of the portion C in which the refractive index change is the maximum refractive index change per film thickness that continuously changes the refractive index between A and B. The change in the refractive index is moderate, that is, a small change. The portion C where the maximum refractive index change per film thickness can be appropriately designed between A and B in consideration of absorption characteristics and the like.

  With the configuration of the optical filter of the present invention, it is possible to reduce internal reflection in the gradient refractive index thin film by improving the environmental stability and reducing the difference in refractive index per film thickness.

The gradient refractive index thin film is a thin film whose refractive index changes continuously in the direction perpendicular to the film surface, that is, in the film thickness direction, preferably continuously and periodically. A film whose refractive index continuously and periodically changes in the film thickness direction can also be called a rugate film, a rugate filter, or the like. FIG. 5 shows a schematic diagram of electron micrographs of the multilayer film and the gradient refractive index thin film. FIG. 5A is a schematic diagram of a cross section in the film thickness direction of the multilayer film, and FIG. 5B is a schematic diagram of a cross section of the gradient refractive index thin film. For example, when the dark portion is SiO ₂ and the light (white) portion is TiO ₂ , the multilayer film is clearly separated at the interface of the film, whereas the gradient refractive index thin film is a multilayer film. Unlike the film, the film interface is not clearly separated. Further, the contrast becomes strong in the portion where the refractive index change of the refractive index gradient thin film is large.

  A plot obtained by analyzing the depth direction analysis with the concentration (intensity) on the vertical axis and the depth (corresponding parameters such as film thickness) on the horizontal axis is referred to as a depth profile.

  In the depth direction analysis for examining the composition distribution from the surface to the inside of the measurement sample, a method of analyzing the surface while scraping the surface using accelerated ions is often used for analysis of micron order or less. This method is called ion sputtering, and is known as X-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES or ESCA), and is an optical component having a structure in which a layer is formed on the substrate surface. It is often used for evaluation of electronic parts and functional materials.

  In these X-ray photoelectron spectroscopy, a sample is irradiated with X-rays in an ultra-high vacuum to detect emitted electrons (photoelectrons). The emitted photoelectrons are caused by the inner-shell electrons of the target atoms, and the energy is determined for each element. Therefore, it is possible to perform qualitative analysis by knowing the energy value. Thus, by evaluating the change in the composition in the film thickness direction in the gradient refractive index thin film and obtaining the depth profile, it is possible to confirm whether a desired refractive index distribution can be obtained.

  Various methods have been studied for designing such a gradient refractive index thin film, and unlike a continuous change, it is a step-type refractive index distribution in which the refractive index gradually changes stepwise. However, it has been found that by adjusting this refractive index distribution, it is possible to obtain substantially the same optical characteristics as a film having a continuous index change. However, for reflection reduction and the like, it is possible to obtain more ideal characteristics by having a continuous refractive index change. Furthermore, since the interface disappears in the thin film and the composition of the film before and after becomes very close, effects such as improved adhesion strength of the film and improved environmental stability appear. From such a viewpoint, it is better to select a refractive index distribution in which the refractive index continuously changes.

Although the range of the refractive index may be limited due to the recent development of film forming techniques such as sputtering and vapor deposition, an arbitrary refractive index can be obtained at least within the range.
For example, in the sputtering method, two types of materials are discharged simultaneously, the discharge power of each material, that is, the input power to the target is changed, and the mixing ratio is changed to change the refractive index between the two substances. It is possible to produce an intermediate refractive index material. Two or more kinds may be mixed.

  In the case of such a sputtering method, if one material is made to have low power, the discharge may become unstable. In the case of meta mode sputtering, problems such as being in reaction mode occur. Therefore, in order to realize all the refractive indexes between two substances, it is necessary to control the film thickness by adjusting factors other than the input power in parallel, for example, by controlling the film formation amount by the mask method. In this case, however, the mechanism and control of the apparatus are complicated.

  As described above, in the meta mode sputtering method, the refractive index was changed within a range in which discharge can be stably maintained and controlled. In addition to continuously changing the refractive index in the film thickness direction, it is possible to change the extinction coefficient by changing x of TiOx in the film thickness direction. As described above, in the configuration of this embodiment, the refractive index and the extinction coefficient are changed by continuously changing the composition ratio of the three elements of Ti, Si, and O in the film thickness direction of the gradient refractive index thin film. Can be changed continuously. Even when other materials are used or when the types of materials constituting the gradient refractive index thin film are increased, the same adjustment is possible. In addition, the composition can be continuously changed by continuously changing the density of the thin film.

  In addition to continuously changing the refractive index in the film thickness direction, the absorption characteristic in the gradient refractive index thin film 12 was adjusted by changing the x of TiOx in the film thickness direction and changing the extinction coefficient. . In Examples 2 and 3, which will be described later, the spectral transmission characteristics in the visible wavelength region of 400 nm to 700 nm are flat characteristics with small dispersion as the total film.

As an example, in TiO in which x is equal to 1, the spectral transmission characteristic in the visible wavelength region tends to become a characteristic that gradually increases as it goes to the long wavelength side as shown in FIG. In Ti ₂ O ₃ where x is equivalent to 1.5, the spectral transmission characteristic in the visible wavelength region tends to become gradually lower as the wavelength becomes longer, as shown in FIG. 6B. is there. Therefore, as described above, the spectral transmission characteristics are adjusted to be flat as a whole by providing one or more combinations in which regions having opposite dispersion shapes are arranged in the film thickness direction of the gradient refractive index thin film 12. A similar tendency is exhibited when the ratio of metal and oxygen in a metal oxide used in a general optical thin film changes. Since the wavelength dependence of the transmittance of two or more types of metal oxides having different extinction coefficients, which is a coefficient related to the transmittance, cancels out the change with each other, a flat transmission can be obtained in a multilayer film configuration. The idea of obtaining a rate characteristic is disclosed in Japanese Patent No. 3359114. A film can be designed to improve the flatness by utilizing this characteristic of the metal oxide. Here, since the refractive index also changes by changing the value of x, it is necessary to determine and control the film formation ratio with SiO ₂ based on the basic data obtained in advance. Specific means for changing the value of x in the film thickness direction can be controlled by adjusting the power of the oxidation source or adjusting the amount of gas introduced depending on the film forming method.

  Further, in the case where reflection on the film surface layer farthest from the substrate or reflection on the back surface of the substrate becomes a problem, an antireflection structure is formed on the surface layer side of the refractive gradient thin film 12 as in the ND filter 14 shown in FIG. The bodies 111 and 112 may be formed. As the antireflection structures 111 and 112, for example, a moth-eye structure which is a fine periodic structure having an antireflection effect, an antireflection film composed of a single layer or multiple layers, and the like can be considered. Furthermore, it is possible to obtain substantially the same effect by continuously forming a refractive index distribution that is increased or decreased several times like the refractive index distribution in an antireflection film having a multilayer structure, unlike the multilayer film structure. It is. Here, for example, a function having the same effect as that of the antireflection structure 111 or 112 as shown in FIG. 4 can be incorporated into the gradient refractive index thin film. In that case, the refractive index is continuously increased or decreased several times within a predetermined region near the interface of the surface layer, and a refractive index profile for preventing interface reflection with the outside air is required. Therefore, it can be considered that the antireflection structure is separately provided on the gradient refractive index thin film. However, in the present Example 1, since it was an object to reduce the change in the optical characteristics accompanying the movement of oxygen or the like in the gradient refractive index thin film, this is because the intention to remove other characteristic change factors as much as possible. No simple antireflection structure is formed, the antireflection structures 111 and 112 shown in FIG. 1 are not provided, and the substrate 11 and the refractive index gradient thin film 12 alone are used.

<Sputtering equipment configuration>
FIG. 7 is a cross-sectional plan view taken along a plane orthogonal to the rotation axis of the substrate transfer device of the sputter deposition apparatus used for the production of the gradient refractive index thin film.

  As a sputter deposition apparatus, a rotatable cylindrical substrate transfer device 52 that holds a substrate 51 on which a thin film is formed is provided in a vacuum chamber 53, and an outer peripheral portion of the substrate transfer device 52 and a vacuum chamber 53 outside thereof are provided. An apparatus in which two sputter regions 54 and 55 and a reaction region 57 are provided in an annular space between the two is used. A substrate is carried in from the region 59.

  The substrate 51 was mounted on the substrate transfer device 52 so that the surface on which the film is formed faces outward. The sputter regions 54 and 55 are equipped with AC double (dual) cathode type targets 54a and 55a. A high frequency power source 56 is disposed outside the vacuum chamber 53. The shape of the target material is not limited to a flat plate type, and may be a cylindrical cylindrical type. In addition to these, in the separate region 58, for example, an ion gun grid having a grid electrode by high-frequency excitation, or a neutralizer that emits low-energy electrons that neutralize positive ions in order to prevent charge accumulation of positive ions on the substrate. Etc. can also be provided. The sputtering apparatus used in the present invention may be provided with, for example, three or more sputtering areas, and can be implemented with a configuration other than the above apparatus.

Using the sputtering apparatus shown in FIG. 7, a refractive index gradient thin film was formed with a structure in which a Si target was disposed in the sputtering region 54, a Ti target was disposed in the sputtering region 55, and oxygen was introduced into the reaction region 57. The substrate 51 fixed to the substrate transfer device 52 is rotated at high speed to form an ultrathin film of Si and Ti on the substrate 51 in the sputtering regions 54 and 55, and then the ultrathin film of Si and Ti is oxidized in the reaction region 57. . As a result, an oxide film of Si and Ti was formed, and by repeating this operation, a mixed film of the Si oxide film and the Ti oxide film was produced. Further, the refractive index gradient thin film whose refractive index continuously changes in the film thickness direction was formed by continuously changing the sputtering rate and oxidation rate in each sputtering region during film formation. It is also possible to produce a mixed film equivalent to SiO ₂ and TiOx by controlling the sputtering rate and oxidation rate of Si and Ti based on the deposition conditions of SiO ₂ and TiOx alone. is there. When the refractive index is continuously changed from the refractive index of the SiO ₂ film alone to the refractive index of the TiOx film alone, the discharge may become unstable if the input power is lowered. At the time of control, not only control of input power but also a mask mechanism provided on the cathode was used.

Example 1
The absorption type ND filter configured as shown in FIG. 2 will be described in detail below. In this embodiment, the gradient index thin film 12 having absorption at least in part of the thin film is disposed on one side of the substrate 13. The antireflection structure is not provided.

  For the substrate 13 on which such an ND filter 14 is formed, SFL-6 glass having a thickness of 1.0 mm and a refractive index of 1.81 was used. In particular, because of the single-sided film structure as in this example, a glass material was used for the purpose of reducing the influence of water absorption on the substrate.

The gradient refractive index thin film 12 was prepared by mixing these two types by adjusting the deposition rate of the SiO ₂ and TiO x films by the meta mode sputtering method. By continuously changing the composition in the film thickness direction, the refractive index was continuously changed in the film thickness direction so as to obtain desired absorption characteristics. The thickness of the gradient refractive index thin film 12 was adjusted to be 200 nm.

In this embodiment, the refractive index change in the film thickness direction of the gradient refractive index thin film 12 is designed to be the minimum increase or decrease so as not to be complicated. In order to have absorption as shown in FIG. 8, a refractive index profile is formed in which there is one peak that raises and lowers the refractive index from the refractive index of the substrate 13. When the maximum value of the refractive index change per film thickness is set, the one with an inflection point on the substrate 13 (high refractive index) side is refracted from a complicated profile having multiple peaks and valleys. It is easy to change the refractive index gradually with respect to the medium (including the substrate 13) adjacent to the gradient index thin film 12. The substrate 13 is made of glass and has a refractive index higher than that of air or general plastic. Therefore, the inflection point is refracted closer to the substrate 13 within the gradient refractive index thin film 12 in order to give a predetermined absorption, that is, closer to the substrate 13 than the center position in the film thickness direction of the gradient refractive index thin film. There is a mountain that increases and decreases the rate. From the mountain through the portion where the maximum change rate per film thickness with respect to the film thickness direction in the gradient refractive index thin film as shown by point C in FIG. 1, the refractive index changes gradually toward the atmosphere. Configured. The end point of the gradient refractive index thin film 12 was configured to be a SiO ₂ thin film from the viewpoint of antireflection and environmental stability. Therefore, the refractive index at the end point of the gradient refractive index thin film 12 is approximately 1.47.

  The spectral transmittance characteristics of the ND filter produced in Example 1 are shown in FIG. The ND filter of Example 1 was put into a high temperature test under a dry nitrogen atmosphere, and the transmittance at a wavelength of 540 nm after 1000 hours was compared before and after the test. The transmittance was changed from 17.2% to 17.4%, and the rate of increase was about 0.1%. Here, the change in transmittance is evaluated as an index of environmental stability of the ND filter, and the smaller the change in transmittance, the better the environmental stability.

  In this example, in order to clarify the effect of this configuration, Ti that is considered to be relatively easily oxidized is used. However, by using a material that is more difficult to oxidize, such as Nb or Ni, It is possible to further reduce the change in transmittance.

  In this embodiment, a single-concentration ND filter was created. When creating a gradation ND filter, a mask having a shielding plate capable of adjusting an angle formed with the mask surface is used. And it can form by using the method of forming a gradation density distribution on a board | substrate by shielding a part of film | membrane material with respect to the surface of a target with a mask.

(Comparative Example 1)
In order to consider the effect of environmental stability in Example 1, a spectral transmittance characteristic substantially the same as that in FIG. 8 is formed with a film thickness of 200 nm using a multilayer structure, and other materials and processes are as much as possible. The ND filter 4 manufactured so as to be the same as in Example 1 is described below.

As in Example 1, as shown in FIG. 9, an optical multilayer film 2 in which seven layers of SiO ₂ and TiOx were alternately laminated was formed on one side of the substrate 3. As shown in FIG. 10, the ND filter 4 having a spectral transmittance characteristic substantially the same as that of the ND filter 14 manufactured in Example 1 was manufactured. For the substrate 3 on which such an ND filter 4 is formed, SFL-6 glass having a refractive index of 1.81 and a thickness of 1.0 mm was used. SiO ₂ and TiOx were formed by a meta mode sputtering method.

  The manufactured ND filter was put into a high temperature test under a dry nitrogen atmosphere, and when the increase rate of the transmittance at a wavelength of 540 nm after 1000 hours was confirmed, the transmittance was 16.9 before and after the test. From 1% to 17.6%, a change of about 0.4%. In particular, when compared with the ND filter produced in Example 1, the result showed a very large change.

(Example 2)
In the same manner as in Example 1, SFL-6 glass having a thickness of 1.0 mm was used for the substrate 13, and an ND filter composed of the substrate 13 and the gradient refractive index thin film 12 was produced. The graded refractive index thin film 12 is mixed with the two types while adjusting the film formation rate of the SiO ₂ and TiO x films by the metamode sputtering method so that the film thickness becomes 200 nm, and the composition can be continuously changed. Thus, the refractive index was continuously changed in the film thickness direction and adjusted to obtain a desired absorption characteristic. In addition to this, by changing the x of TiOx, the refractive index gradient thin film 12 is divided into a region where the spectral transmittance in the visible wavelength region becomes higher as it goes to the longer wavelength side and a region that becomes lower as it goes to the longer wavelength side. It was set as the structure provided in. In this way, in the spectral transmittance in a specific wavelength region, by providing regions having opposite dispersion shapes in the same film, for example, in the case of an ND filter, a flatter spectral transmission characteristic with smaller wavelength dispersion is obtained. It becomes possible to obtain.

Further, in the present embodiment, like the first embodiment, the minimum refractive index increase / decrease is designed so that the refractive index profile of the gradient refractive index thin film 12 is not complicated. The refractive index profile is such that there is one peak that raises and lowers the refractive index from the refractive index of the substrate 11. The composition ratio of the mixed film equivalent to SiO ₂ and TiO was adjusted so that the refractive index gradient thin film 12 had a refractive index of 1.81 close to the substrate 13 at the interface with the substrate 11. As the distance from the substrate 11 increases in the film thickness direction, the composition ratio corresponding to TiO with respect to SiO ₂ is gradually increased, and when the refractive index becomes 2.1, the composition ratio of these two kinds of materials is continuously increased. The change was changed to a change in the acid value of TiO, and the change was continuously made to be equivalent to Ti ₂ O ₃ . After that, when the composition corresponding to the mixed film of SiO ₂ and Ti ₂ O ₃ was obtained, the composition ratio corresponding to SiO _{2 was} gradually increased with respect to Ti ₂ O ₃ , so that these two kinds were changed from the change in the acid value. Instead of a continuous change in the composition ratio of the materials, the end point of the gradient refractive index thin film 12 was configured to be SiO ₂ from the viewpoint of antireflection and environmental stability. Therefore, the refractive index at the end point of the gradient refractive index thin film 12 is approximately 1.47.

Thus, the region showing spectral transmission strongly influenced by TiO and the region showing spectral transmission strongly influenced by Ti ₂ O ₃ were formed in the gradient refractive index thin film. As a result, it is possible to obtain desired transmission characteristics by mixing regions having different dispersion characteristics as illustrated in FIG. 6 in the visible wavelength region. In this example, these were adjusted so that the spectral transmission characteristics were flatter than those in Example 1 in the visible wavelength region.

  FIG. 11 shows the spectral transmittance characteristics of the ND filter manufactured as described above. From FIG. 11, it can be confirmed that the flatness of the spectral transmittance in the visible wavelength region is improved as compared with the ND filter manufactured in Example 1.

This was put into a high temperature test under a dry nitrogen atmosphere, and the transmittance at a wavelength of 540 nm after 1000 hours was compared before and after the test. The transmittance increased from 15.7% to 15.9%, and the increase rate was about 0.1%.
(Example 3)

<About antireflection structure>
After formation of the gradient refractive index thin film 12 on the substrate 13, a fine period as a submicron pitch antireflection structure having an antireflection effect is formed on the gradient refractive index thin film 12 by an optical nanoimprint method using a UV curable resin. Structures 151 and 152 were formed.

The fine periodic structure has been produced with the recent improvement of fine processing technology.
A fine periodic structure having an antireflection effect, which is one of such structures, can be called a moth-eye structure or the like. By making the shape of the structure into a shape in which the refractive index change becomes continuous in a pseudo manner, the reflection due to the difference in refractive index between substances is reduced.

  FIG. 12 is a perspective view from above showing a schematic example of a fine periodic structure having an antireflection effect in which cones are arranged in a pillar array on a substrate. Similarly, it is possible to form a fine periodic structure arranged in a hole array. Such a structure is often generated on the surface of a substance, for example, as a means different from an antireflection film formed by laminating a single thin film or a plurality of thin films by a vacuum film formation method or the like.

  Various methods have been proposed for producing such a fine periodic structure. In this example, an optical nanoimprint method using a UV curable resin was used.

  As shown in FIG. 12, the fine periodic structure has a pillar array shape in which cones are periodically arranged, and considering the application of the ND filter, the reflectance in the visible wavelength region can be reduced at least. It was designed to have a height of 350 nm and a period of 250 nm. Further, regarding the matrix arrangement of the protrusion structures, a square arrangement as shown in the plan view of FIG. 13A and a three-way (hexagonal) arrangement as shown in the plan view of FIG. 13B can be considered. The three-way arrangement is said to have a higher antireflection effect because the exposed surface of the substrate material is less. Therefore, in this example, a three-way array of pyramids was used.

  The production of a filter having a refractive index gradient film formed on both sides of the substrate as shown in FIG. 14 will be described below.

  As shown in FIG. 14, in this embodiment, the refractive index gradient thin film 221 is disposed on one side (upper surface) side of the substrate 23, and the antireflection structure 211 is disposed on the refractive index gradient thin film 221. Thereafter, the gradient refractive index thin film 222 (another gradient refractive index thin film) and the antireflection structure 212 (another antireflection structure) were also arranged on the back side of the substrate 23 in the same manner. The function of having the desired absorption in the desired wavelength region in the ND filter 24 is provided to both the refractive index gradient thin films 221 and 222. In some cases, it is possible to obtain similar characteristics even with only one of the gradient refractive index thin films 221 and 222. As such antireflection structures 211 and 212, as shown in FIGS. 15 (a) and 15 (b), the fine periodic structures 251 and 252 having an antireflection effect, a single layer, or a plurality of layers Examples thereof include antireflection films 261 and 262 formed as thin films. Furthermore, as shown in FIG. 15C, a configuration in which the fine periodic structure 25 and the antireflection film 26 are used in combination. An optimal configuration may be selected as appropriate.

Among FIGS. 15A to 15C, it is more preferable to adopt the configuration as shown in FIG. 15A from the viewpoint of reducing reflection. Therefore, in this embodiment, as shown in FIG. 15A, the fine periodic structures 251 and 252 are formed on both surfaces of the substrate 23 as the antireflection structure.
An appropriate amount of UV curable resin was dropped onto a quartz substrate as a mold having a hole array shape obtained by reversing the previously designed shape. Thereafter, the resin is cured by irradiating UV light with the quartz mold pressed against the substrate to be imprinted, and as shown in FIG. Structures 251 and 252 were manufactured. Various UV curable resins can be used, but here, Toyo Gosei PAK-01 (trade name) was used to adjust the refractive index to 1.50 after polymerization and curing.

  Here, in order to improve the adhesion between the gradient refractive index thin film and the fine periodic structure, primer treatment was performed, and an adhesion layer was provided between the refractive index gradient thin film and the fine periodic structure. The primer solution is based on KBM-503 (trade name) manufactured by Shin-Etsu Chemical Co., Ltd., which is a surfactant, and an appropriate amount of IPA (isopropyl alcohol) or nitric acid is added. The refractive index of the cured adhesion layer after coating is 1 What was adjusted to be .45 was used. This was dropped onto a gradient refractive index thin film through a 0.2 μm PTFE (polytetrafluoroethylene) filter, and applied to form an ultrathin film by spin coating. If it is necessary to further strengthen the adhesion, TEOS (tetraethyl orthosilicate) or the like may be further added to the above-described primer solution components. In order to more uniformly apply the primer solution, it is more preferable to subject the substrate to hydrophilic treatment with UV ozone before applying the primer solution. Furthermore, when it is provided on both sides of the substrate, the concentration may be adjusted appropriately, and coating may be performed by dip coating, or after coating one side by spin coating, the front and back sides of the substrate are changed, and the other side is coated again by spin coating. However, the latter was selected in this embodiment. The difference in refractive index between the adhesion layer and the adjacent structure is preferably 0.05 or less.

  In addition, in the case of a filter having absorption in the entire visible wavelength region such as an ND filter, it often has absorption in the ultraviolet region. Therefore, depending on the wavelength of the UV light to be used, when light is irradiated from the substrate side of the filter, the ND filter absorbs at least part of the light, and sufficient light may not reach the resin. Therefore, in such a case, it is necessary to irradiate UV light from the mold side, and it is necessary to select a mold of a material that sufficiently transmits the wavelength of the necessary UV light.

  Further, in consideration of the process of optical nanoimprinting, if imprinting is performed on one side of the substrate 23 and then imprinting on the other side, damage such as chipping or cracking is imparted to the initially formed fine periodic structure. It is assumed that. Therefore, a method for arranging imprinting molds on both sides of the substrate and performing optical nanoimprinting on both sides simultaneously was selected. In this case, productivity can be improved by arranging two UV light sources on both sides of the substrate.

  For the substrate 23 on which the ND filter 24 of Example 3 is formed, a PET film having a thickness of 0.1 mm was used so that the refractive index was about 1.60. In this embodiment, a PET film is used. However, the present invention is not limited to this, and a glass-based material or a resin material such as PO, PI-based, PEN, PES, PC, or PMMA-based material may be used.

In the refractive index profile of FIG. 14A, x of TiOx is fixed at about 1.5 from the interface point P0 to the point P1 on the substrate side, and it is continuously changed by changing the composition ratio with SiO _2. Refractive index change was formed.
Next, as the point P1 passes through the point P2 and approaches the point P3, x of TiOx is continuously changed from 1.5 to 1.0. At the same time, the composition ratio with SiO ₂ is changed, the composition ratio of SiO ₂ is increased with respect to TiOx as it approaches from point P1 to point P2, and further the composition ratio of SiO ₂ with respect to TiOx as it approaches point P3 from point P2. A continuous refractive index change was formed by reducing the.
Furthermore, from the point P3 to the interface point P4 on the antireflection structure side, x of TiOx is fixed at about 1.0, and a continuous refractive index change is formed by changing the composition ratio with SiO ₂ . .
In the vicinity of the point P1, spectral transmission greatly influenced by Ti ₂ O ₃ is shown, and in the vicinity of the point P3, spectral transmission greatly influenced by TiO is shown. Therefore, by configuring in this way, regions having different dispersion characteristics as exemplified in FIG. 6 in the visible wavelength region are mixed in the gradient refractive index thin film, and the influence degree is adjusted by the film thickness and the composition ratio. Thus, desired transmission characteristics can be obtained. In this example, these were adjusted so that the spectral transmission characteristics were flat in the visible wavelength region.

  In this embodiment, as described above, the gradient refractive index thin film 22 has a refractive index profile as shown in FIG. It is possible to form the profile shown in FIG. 16 (b) in which a plurality of peaks and valleys in FIG. 16 (a) are formed. Designed to increase or decrease. Further, a difference in refractive index tends to occur at the interface between the substrate and the antireflection structure. In the area close to the substrate and the antireflection structure from the viewpoint of antireflection, a film design in which the refractive index change is gentle was performed. From the viewpoint of preventing reflection, it is preferable to design so as not to cause a refractive index difference as much as possible as in the conceptual diagram shown in FIG. However, in order to obtain desired absorption, a region having a high refractive index is required. Therefore, it is preferable that the refractive index gradient thin film gradually increases in refractive index from the side closer to the substrate and gradually approaches the refractive index of the antireflection structure through the inflection point toward the antireflection structure.

On the other hand, if the refractive index is different at the interface between the substrate and the gradient refractive index thin film and the interface between the gradient refractive index thin film and the fine periodic structure, reflection occurs according to the difference in refractive index. Therefore, when reflection at these interfaces becomes a problem, it is desirable to make the difference in refractive index as small as possible. In this embodiment, the refractive index difference between the two interfaces is adjusted to 0. 0 by adjusting the rate ratio of SiO ₂ and TiO x immediately after the start of film formation of the gradient refractive index thin film and just before the film formation. It adjusted so that it might become 05 or less. The thickness of the gradient refractive index thin film 221 was adjusted to 200 nm. As the thickness of the refractive index gradient thin film becomes thinner, the rate of change of the refractive index from the substrate to the antireflection structure becomes steep. Therefore, a thicker film is preferable from the viewpoint of preventing reflection. When it is necessary to further reduce the reflection, it can be dealt with by increasing the thickness to about 400 nm.

As in the first and second embodiments, first, the refractive index gradient thin film 221 is mixed on one side of the substrate 23, and the two types are mixed while adjusting the deposition rate of the SiO ₂ and TiO x films by the meta mode sputtering method. Then, the refractive index was continuously changed in the film thickness direction to adjust and produce a desired absorption characteristic. Thereafter, the front and back sides of the substrate were changed, and a mixed film of SiO ₂ and TiOx was similarly produced again. Film formation was performed such that the spectral transmission characteristics were flat as a whole film.
Here, the gradient refractive index thin film 221 is formed so as to contain a large amount of Ti ₂ O ₃ so that the spectral transmission characteristic in the visible wavelength region becomes higher as it becomes longer, and the gradient refractive index thin film 222 is formed in the visible wavelength region. Alternatively, it may be formed so as to contain a large amount of TiO so that the spectral transmission characteristic of the light becomes lower as it goes to the longer wavelength side. That is, the film design may be performed so that the spectral transmission characteristics in the visible wavelength region of 400 nm to 700 nm are flat characteristics with small dispersion as the total refractive index gradient thin film on both sides. When a point where the spectral transmission characteristics change in the film thickness direction is provided so that x of TiOx is continuously changed from 1.5 to 1.0, the degree of freedom in design with respect to flatness can be improved. . Further, the change of the spectral transmission characteristic may be reversed in the film thickness direction so that x is continuously changed from 1.0 to 1.5.

  Thereafter, sub-micron pitch fine periodic structures 251 and 252 having an antireflection effect were formed on the gradient refractive index thin films formed on both surfaces of the substrate by an optical nanoimprint method using a UV curable resin. Imprinting molds were placed on both sides of the substrate on which the ND film was formed, and optical nanoimprinting was performed on both sides simultaneously. Between the gradient refractive index thin film and the fine periodic structure corresponding to the gradient refractive index thin film, primer treatment was performed to provide an adhesion layer.

<Characteristics of optical filter>
FIG. 17 shows spectral reflectance characteristics and spectral transmittance characteristics of the ND filter manufactured in Example 3. The density is about 0.4, and the reflectivity is about 0.5% or less in the visible wavelength region despite the low density type in which reflection tends to increase. With this configuration, a very low reflectance can be realized. A spectrophotometer (U4100, manufactured by Hitachi High-Technologies Corporation) was used for the measurement.

  Further, the spectral transmission characteristic is flat in the entire visible region, and this is an index of flatness: {(maximum value of transmittance at 400 to 700 nm) − (minimum value of transmittance at 400 to 700 nm)} ÷ ( When the flatness is defined as an average value of transmittance at 500 to 600 nm, the flatness of the filter manufactured in this example is as small as about 1.6%, and a filter having excellent flatness is obtained. I was able to.

The ND filter produced in Example 3 was put into a high temperature test under a dry atmospheric environment, and the spectral transmittance characteristics at a wavelength of 550 nm after 1000 hours were compared. The transmittance was changed from 40.9% to 41.3% before and after the test, and the increase rate was about 0.10%. Compared with Example 1 and the comparative example, the concentration may be lighter, but the change is as small as 0.004 even in terms of concentration, and an ND filter having excellent environmental stability could be obtained. (Here, the relationship between transmission (T) and concentration (D) is D = −log ₁₀ T.)

  As shown in Examples 1 to 3, the method of manufacturing an ND filter according to the present invention includes a step of providing a gradient refractive index thin film on a substrate, and the gradient refractive index thin film is made of three or more elements. It is possible to use a manufacturing method characterized in that the material is formed by changing the mixing ratio of these materials by the film forming method used and continuously changing the refractive index in the film thickness direction. In order to improve environmental stability and reduce reflection, it is preferable to perform a step of providing an antireflection structure on the gradient refractive index thin film.

  In Examples 1 to 3, a mixed film of SiO2 and TiOx was produced by the metamode sputtering method, and a gradient thin film having a continuous refractive index was formed by changing the mixing ratio in the film thickness direction. The same effect can be obtained by using oxides of various other metal materials such as Nb, Ta, Zr, Al, Ni, Cr, Cu, Mg, W, and Mo. From the relationship such as the refractive index of the structure that forms the interface with the gradient refractive index thin film as described above, any material that can realize the required refractive index may be used. Select the most suitable material.

  Moreover, you may combine the material containing the element of 3 or more types of metals or metalloids. When three or more kinds of materials are combined, the refractive index can be stably tilted, and the extinction coefficient can be easily adjusted, for example, absorption can be reduced.

  Furthermore, when reactive vapor deposition or the like is used, it is possible to form a gradient thin film by controlling the introduced gas and controlling the refractive index and extinction coefficient. A configuration may be adopted in which absorption in a part of the inclined thin film is given in the film thickness direction, or the refractive index may be continuously changed while having overall absorption. The film forming method is not limited to the meta-mode sputtering method, and other sputtering methods and various vapor deposition methods may be used.

  The gradient refractive index thin film formed as in this embodiment becomes a high-density film, and film stress may be a problem. In this case, as in this embodiment, when a substrate such as glass having high rigidity is used, problems such as warpage due to film stress can be reduced. In addition, by providing the gradient refractive index thin films on both surfaces of the substrate, it is possible to produce a stable ND filter by canceling each film stress.

  In particular, the structure in which the refractive index gradient thin film and the fine periodic structure are provided on both surfaces of the substrate used in this example can provide the stability of the substrate against the film stress. In addition, since the anti-reflective structure can be formed by a series of continuous or simultaneous processes on both sides of the fine periodic structure by optical nanoimprinting, the productivity is excellent.

  Further, by using the sputtering method, it is possible to stably form a thin film having a higher density than the vapor deposition method or the like.

  In this embodiment, an oxide is used to control the refractive index, but nitride may be used, and various compounds can be used as the gradient refractive index thin film as long as the refractive index changes continuously and periodically.

  It is also possible to improve adhesion and durability by providing a buffer layer between the substrate and the gradient refractive index thin film and / or between the gradient refractive index thin film and the antireflection structure. In that case, the design considering the buffer layer may be performed. The refractive index of the buffer layer is the same as that of the substrate or antireflection structure adjacent thereto, or the refractive index difference is made close, preferably the refractive index difference is 0.05 or less. Such a buffer layer can be similarly used in the structure having another gradient refractive index thin film shown in FIG. As a material for forming the adhesion layer when providing the adhesion layer as the buffer layer, in addition to the silane coupling agent, inorganic materials such as Cr, Ti, TiOx, TiNx, SiOx, SiNx, AlOx, and SiOxNy and various organic materials are used. Materials. Depending on the material of the layer that enhances adhesion, a material for forming an adhesion layer can be appropriately selected from known materials and used. What is necessary is just to set the film thickness of a contact | adherence layer so that the optical function and adhesiveness of the target filter may be acquired. The adhesion layer may be formed as a thin film of 10 nm or less, for example.

Example 4
FIG. 18 shows a light quantity stop device. A diaphragm of a light amount diaphragm device suitable for use in a photographing optical system such as a video camera or a digital still camera is provided for controlling the amount of light incident on a solid-state imaging device such as a CCD or CMOS sensor. . As the field of view becomes brighter, the diaphragm blades 31 are controlled so as to be further narrowed down. At this time, as a countermeasure against image performance degradation that occurs in a small aperture state, an ND filter 34 is arranged in the vicinity of the aperture so that the aperture of the aperture can be made larger even if the brightness of the object field is the same. ing. Incident light passes through the light amount restrictor 33 and reaches a solid-state imaging device (not shown) to be converted into an electrical signal to form an image.

For example, the ND filter manufactured in the first to third embodiments is disposed at the position of the ND filter 34 in the diaphragm device 33. However, the arrangement location is not limited to this, and it may be arranged so as to be fixed to the diaphragm blade support plate 32.
FIG. 19 shows the structure of the photographing optical system of the optical photographing apparatus. The photographing optical system 41 includes lens units 41 </ b> A to 41 </ b> D, a solid-state imaging device 42 such as a CCD, and an optical low-pass filter 43. The solid-state image sensor 42 receives the images of the light rays a and b formed by the photographing optical system 41 and converts them into electric signals. The photographing optical system 41 has a light amount diaphragm device including an ND filter 44, diaphragm blades 45 and 46, and a diaphragm blade support plate 47.

  A diaphragm of a light amount diaphragm device suitable for use in a photographing system such as a video camera or a digital still camera is provided for controlling the amount of light incident on a solid-state imaging device such as a CCD or CMOS sensor. As the field of view becomes brighter, the diaphragm blades 45 and 46 are controlled so as to be further narrowed down. At this time, as a countermeasure against image performance degradation that occurs in a small aperture state, an ND filter 44 is arranged in the vicinity of the aperture so that the aperture of the aperture can be made larger even if the brightness of the object field is the same. ing.

  Incident light passes through the light quantity diaphragm and reaches the solid-state imaging device, whereby it is converted into an electrical signal and an image is formed. For example, the ND filter manufactured in the first to fourth embodiments is disposed at the position of the ND filter 44 in the diaphragm device. However, the arrangement location is not limited to this, and the arrangement location may be fixed to the diaphragm blade support plate 47.

  According to the configuration of the above embodiment, it is possible to provide an imaging apparatus equipped with an ND filter that can reduce the possibility of a change in a captured image over time. The ND filter 44 using the ND filter using the gradient refractive index thin film prepared in the first to third embodiments has a difference in color balance even when an image photographed after assembling is compared with an image one month after assembling. Could not be recognized.

  On the other hand, the ND filter 4 having the optical multilayer film structure created in Comparative Example 1 in FIG. 9 is mounted as the ND filter 44, and an image photographed by an optical device using the ND filter 44 was evaluated. As a result, the color balance was different when comparing an image taken immediately after the assembly of the optical device with an image taken of the same object after one month had passed since the assembly.

  Since the ND filter of the present invention improves the environmental stability, the quality of captured images can be maintained by using it in an optical photographing apparatus such as a camera or a video camera that is carried and used and has a large environmental change. it can.

  The light quantity restricting device 33 thus produced can remarkably reduce problems such as ghost caused by the reflection of the filter, and can simultaneously realize, for example, an improvement in color balance due to the transmission characteristics. Is possible.

  However, the present invention is not limited to this, and even with other optical devices, the use of the optical filter with reduced reflectivity as produced in Examples 1 to 3 reduces the problems on the device due to the reflection of the filter. It is possible to reduce problems caused by transmission at the same time.

  In particular, when the photographing optical system is provided with the structure including the fine periodic structure and the gradient refractive index thin film described in Example 3 on both sides, it is possible to obtain a good photographed image while suppressing reflection to the CCD and the like. Assembling is possible without considering the direction of installation.

(Example 5)
FIG. 20 shows the function and configuration of an interference microscope that is an optical measurement apparatus. The light source 910 outputs a predetermined wavelength as a light source. Only a certain wavelength component is extracted by the filter 911 from the observation light output from the light source 910. Thereafter, the amount of the observation light is appropriately adjusted via the ND filter 912 that is selectively disposed on the optical path in accordance with the rotational position of the filter holder 913 holding the ND filter 912 having different transmittances. As the light source, a monochromatic wavelength laser light source or the like can also be used as the light source.

  The filter holder 913 is provided with a plurality of ND filters 912 having different transmittances, and an ND filter having any transmittance by the rotational drive of a rotational drive unit 914 that operates based on control from a CPU (not shown) or the like. 912 is selectively disposed on the optical path. If the spot diameter of the light source corresponds to the gradation range, the transmittance may be changed by positioning the gradation ND filter. In that case, the ND filter can be configured to operate like the diaphragm device shown in the fourth embodiment. The polarization angle of the light that has passed through the ND filter 912 is changed through the polarizing plate 915 that is also disposed on the optical path. The polarizing plate 915 changes the polarization angle of the transmitted light to be a desired angle by being rotationally driven by the polarizing plate rotation driving unit 916. The polarizing plate rotation driving unit 916 is also provided by the CPU or the like. Operates based on the control.

  The light passing through the polarizing plate 915 is reflected in the sample direction by the half mirror 917 and then split by the prism 918 into two parallel optical paths depending on the polarization direction. Both of the lights divided into the two optical paths are irradiated to the observation object 920 placed on the focus observation mechanism 921 for adjusting the focus through the objective lens 919.

  The light reflected from the observation object 920 passes through the half mirror 917 through the objective lens 919 and the prism 918, and forms an image on the imaging element 924 such as a CCD by the imaging lens 922. On the optical path between the imaging lens 922 and the image sensor 924, an analyzer 923 is disposed as a rotatable polarization element.

  The output of the image sensor 924 is converted into a digital signal, and the surface structure, refractive index distribution, and the like can be analyzed by analyzing the interference fringes processed and observed by a CPU or the like. In addition, the optical measurement apparatus is not limited to the present embodiment, and the ND filter of the present invention is used, and the optical measurement apparatus, which is an optical apparatus that requires long-term reliability of measurement accuracy, is ND. It is possible to perform measurement while suppressing adverse effects due to changes in the filter over time. As a result, it is possible to improve the stability of image quality such as interference fringes that are images taken by an optical device such as an optical measurement device that is likely to have an adverse effect on the measurement result over a long period of use.

(Other examples)
Even in the optical filters other than the ND filters described in the first to third embodiments, the same effect can be expected as long as the optical filter has an absorption type and has a problem of flatness of transmitted light. For example, it can be applied to a color filter or the like. It is possible to do. By applying the present invention to these optical filters, it is possible to obtain desired transmission characteristics while reducing reflectivity. In addition, by mounting these optical filters, it is possible to obtain various optical devices in which the above-described problems are improved.

111, 112, 211, 212. Antireflection structure 12, 221, 222. Refractive index gradient thin films 3, 13, 23, 51. Substrate 15, 151, 152, 251, 252. Fine periodic structures 16, 161, 162. Antireflection film 31. Diaphragm blade 32. Diaphragm support plate 33. Light quantity diaphragm device 4, 14, 24, 34.44, 912. ND filter 41. Imaging optical system
41A, 41B, 41C, 41D. Lens unit
42. Solid-state image sensor
43. Optical low-pass filter
45, 46. Aperture blade
52. Substrate transfer device 53. Vacuum chambers 54, 55. Sputter regions 54a, 55a. Target 56. High-frequency power source 57. Reaction area

Claims (19)

A light-transmitting substrate, and a gradient refractive index thin film comprising a metal compound that absorbs light in at least a part of the visible wavelength region on the substrate,
The ND filter, wherein the gradient refractive index thin film has a composition comprising at least the metal compound and a compound different from the metal compound continuously changing in the film thickness direction.
A substrate having optical transparency, and a gradient refractive index thin film comprising a metal compound that absorbs light in at least a part of the visible wavelength region on the substrate,
The ND filter, wherein the refractive index gradient thin film has a refractive index continuously changing in a film thickness direction.
The refractive index gradient thin film is in the film thickness direction,
A region where the spectral transmission characteristic in the visible wavelength region becomes higher as it becomes longer,
The ND filter according to claim 1, wherein the ND filter has a region in which the spectral transmission characteristic in the visible wavelength region becomes lower as the wavelength becomes longer.
The change in the constituent material constituting the composition that continuously changes in the film thickness direction of the gradient refractive index thin film includes at least the composition ratio of a plurality of elements constituting the constituent material, and the type of the constituent material 4. The ND filter according to claim 1, wherein the density of the constituent material is any one of the changes.
The refractive index gradient thin film is in the film thickness direction,
2. A portion that changes from a region in which the spectral transmission characteristic in the visible wavelength region becomes higher as it goes to the longer wavelength side to a region that becomes lower as the spectral transmission property in the visible wavelength region goes to the longer wavelength side. The ND filter according to any one of 4.
The change in refractive index in the film thickness direction of the gradient refractive index thin film is
(1) a portion where the continuous refractive index change becomes the maximum rate of change per film thickness with respect to the film thickness direction in the gradient refractive index thin film;
(2) A portion where the refractive index change is gentler than the refractive index change in the portion of (1) in the vicinity of the substrate of the gradient refractive index thin film and in the vicinity of the medium adjacent to the gradient gradient thin film apart from the substrate. It has ND filter as described in any one of Claims 1-5 characterized by the above-mentioned.
On the surface layer side of the gradient refractive index thin film, it has a light reflection reduction effect in the visible wavelength region,
The ND filter according to claim 1, further comprising an antireflection structure.
The antireflection structure is a fine periodic structure formed with a period equal to or less than the wavelength of visible light,
The ND filter according to claim 7, wherein the ND filter is an antireflection film formed of a single layer or a multilayer thin film.
9. The oxygen migration reducing layer is inserted between the antireflection structure and the gradient refractive index thin film or between the substrate and the gradient refractive index thin film. ND filter.
The ND filter according to claim 9, wherein the oxygen migration reducing layer includes a nitride film.
The change in refractive index in the film thickness direction of the gradient refractive index thin film is
(3) The substrate according to any one of claims 1 to 10, wherein the substrate has a portion where the refractive index changes so as to approach the refractive index of the substrate up to the end of the refractive index change on the substrate side. The ND filter according to claim 1.
The ND filter according to claim 11, wherein a refractive index difference between the end point of the refractive index change on the substrate side of the gradient refractive index thin film and the substrate is smaller than 0.05.
The change in refractive index in the film thickness direction of the gradient refractive index thin film is
(4) The antireflection structure side includes a portion where the refractive index changes so as to approach the refractive index of the antireflection structure until the end point of the refractive index change on the antireflection structure side. The ND filter according to any one of claims 7 to 12.
14. The ND filter according to claim 13, wherein, on the antireflection structure side, a refractive index difference between an end point on the antireflection structure side of the refractive index change and the antireflection structure is smaller than 0.05. .
The antireflection structure is provided on both surfaces of the substrate, and the refractive index gradient thin film is provided between each of the antireflection structures on both surfaces of the substrate. The ND filter according to claim 1.
The substance forming the metal oxide is
Among Ti, Nb, Ta, Zr, Al, Ni, Cr, Cu, Mg, Mo, W,
The ND filter according to claim 1, wherein at least one ND filter is included.
The ND filter according to claim 1, wherein the refractive index change of the refractive index gradient film is periodic.
The ND filter according to claim 1, wherein the gradient refractive index thin film is composed of three or more elements.
An optical device using an ND filter in an optical system,
The ND filter according to any one of claims 1 to 18, wherein the ND filter is an ND filter.