JP4689306B2 - Reflective optical element - Google Patents

Description

  The present invention relates to a technique for manufacturing a reflective optical element by press-molding a metal plate.

Conventionally, flight simulators, head-mounted displays, projectors, and the like are known as image systems using reflective optical elements. Among them, the surface of an Al (aluminum) rolled material as disclosed in Japanese Patent Application Laid-Open No. 7-243027 (Patent Document 1) as a proposal for a reflective optical element material or a reflective optical element mainly composed of a metal material. A reflective material obtained by continuously vapor-depositing an Al-based metal on the surface, and a stainless steel plate that has been polished to a mirror surface as disclosed in JP-A-8-36222 (Patent Document 2) is processed into a curved surface by press molding or bulge molding. A method of manufacturing a mirror, Al or an Al alloy or stainless steel as disclosed in Japanese Patent Application Laid-Open No. 9-120705 (Patent Document 3) is formed into a curved shape by spatula drawing or hydraulic forming, and then by various polishing methods A method for obtaining a mirror for illumination is known.
Japanese Patent Laid-Open No. 7-243027 JP-A-8-36222 Japanese Patent Laid-Open No. 9-120705 JP 2002-316226 A

  However, the conventional metal optical element material has crystal anisotropy parallel to the rolling direction of the material, and the mechanical characteristics differ between the rolling direction of the material and the direction orthogonal to the rolling direction. Moreover, since it is compressed in the plate thickness direction, the hardness of the material surface is very high. Therefore, when manufacturing a molded product such as a concave mirror by pressing, it is difficult to obtain high shape accuracy because the transferability of the press mold differs in the rolling direction and the direction orthogonal to the rolling direction. Because of the high hardness, it was difficult to sufficiently transfer the smoothness of the surface of the press die to the surface of the molded product.

  Therefore, as can be seen in Patent Documents 1 to 3, most of the metal reflective optical elements are finally subjected to polishing and mirror finishing processing by taking a long process to ensure shape accuracy and surface roughness.

  A material that focuses only on surface roughness has been developed. In the reflective material in which an Al-based metal is continuously deposited on the surface of an Al rolled material disclosed in Patent Document 1, the crystal grain size is reduced and the surface roughness is improved. ing. Certainly, by reducing the crystal grain size, the surface roughness of the material itself is lowered and the reflectance is improved. However, when vapor deposition is performed so that the crystal grain size is reduced in the vapor deposition process, the vapor deposition layer becomes hard due to the effect of the Hall Petch, and the smoothness of the surface of the press die is sufficiently transferred to the surface of the molded product in the press process. It is difficult to form a free curved surface having a small curvature radius and a surface crack that is liable to occur.

  As another method, there is a method using an expensive commercially available optical aluminum material. JP 2002-316226 A (Patent Document 4) glosses high-purity aluminum and low-purity aluminum plywood with roller burnishing. And embossing with ultra-high stress to obtain a metal optical element having a diameter of about 1 to 2 mm is disclosed. In this method, since the material is originally smooth and a high load can be applied to a very small area, it can provide yield strength that is higher than the work hardening of the material. It was not possible to transfer sufficiently, and the surface roughness was 10 nm or more. Moreover, since this aluminum optical element has a strong anisotropy with respect to the original material, in order to satisfy the required accuracy of the optical element, it was only possible to obtain a small molded product of φ2 mm or less.

  Therefore, the present invention has been made in view of the above-described problems, and the object thereof is to easily obtain high shape accuracy and high surface smoothness when an optical element is formed by pressing a metal plate. Is to do.

In order to solve the above-described problems and achieve the object, a reflective optical element according to the present invention includes a base material made of a metal plate material, and a metal film made of metal having a crystal structure formed on the base material. a metal plate provided with bets, a reflective optical element manufactured by press molding, among the crystal structures, the normal of the crystal plane appears on the surface of the metal film, the target of the reflective optical element If the angle of the normal of the surface shape was theta, average .theta.a the optical effective surface within the reflective optical element of the angle theta is characterized by satisfying 0 ° ≦ θa ≦ 40 °.

  In the reflective optical element according to the present invention, the metal film has an average crystal grain size of 0.5 μm or less.

Further, in the reflective optical element according to the present invention, the surface of the metal film is formed is an oxide film, and the thickness of the oxide film is 10nm or less.

  In the reflective optical element according to the present invention, the oxide film formed on the surface of the metal film is a mixed crystal of amorphous and fine crystals.

  In the reflective optical element according to the present invention, the metal film is made of a single metal having a purity of 98 mol% or more.

  In the reflective optical element according to the present invention, the surface roughness Ra of the surface of the metal film is 10 nm or less.

In the reflective optical element according to the present invention, when the shape error from the target surface shape of the reflective optical element is expressed by a PV value, PV ≦ 5 μm.

  According to the present invention, when an optical element is formed by pressing a metal plate, high shape accuracy and high surface smoothness can be easily obtained.

  Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view showing a plate 10 that is a material for manufacturing a curved mirror. The plate 10 is formed by forming a high-purity metal film 3 on one side of a metal base 1 by vapor deposition. . An undercoat 2 is applied to the surface of the base material 1 in order to improve the adhesion between the base material 1 and the high-purity metal film 3. On the surface of the high-purity metal film 3, an oxide film 5 oxidized by oxygen in the surrounding atmosphere in the vapor deposition process is formed. The ambient atmosphere during the deposition of the high-purity metal film 3 may be in air, in an inert gas, in a vacuum, or the like, but is also included in the air, in an inert gas, in a vacuum, or the like. Since the surface of the high-purity metal film 3 is oxidized by a small amount of oxygen, the oxide film 5 is always formed although its thickness varies. The oxide film 5 is in an amorphous state and is not crystallized.

  FIG. 2 is a cross-sectional view of a final molded product (reflection optical element) manufactured by press-molding the plate material 10 shown in FIG. 1 with a molding die. FIG. 2 shows a state in which a concave mirror as a reflective optical element is molded.

  FIG. 3 is a diagram showing a crystal structure inside the high-purity metal film 3 in the plate 10 shown in FIG. In the present embodiment, for example, Al (aluminum) having a face-centered cubic lattice crystal structure is used as the metal material for forming the high-purity metal film 3. Such a material is deposited to form a columnar crystal structure as shown in FIG.

  FIG. 4 is a view showing a crystal structure inside the high-purity metal film 3 after the plate material shown in FIG. 1 is press-molded by a molding die. In this embodiment, since the high-purity metal film 3 is deposited at room temperature, the deposited high-purity metal film 3 is relatively soft before being formed. It is in a state that is not hardened. Therefore, the crystal structure inside the high-purity metal film 3 is likely to slip, rotate, etc., can be easily deformed following the smooth surface of the mold, and the surface of the mold is smooth. Very well transcribed.

  Hereinafter, a more detailed description will be given with reference to FIGS.

  Conventionally, when a large reflective optical element is to be manufactured by press molding using a rolled plate material, it is necessary to give the optical material a sufficient yield load that is higher than work hardening at the time of rolling. The influence of the texture cannot be ignored, and it is difficult to mold a large reflective optical element having a diameter of 3 mm or more because the shape accuracy cannot be obtained. In addition, since a molding process stress that exceeds work hardening is required, a large processing machine capable of obtaining a large load is required as the optical element is increased in size, leading to an increase in waste of cost and resources.

  Therefore, in the present embodiment, a high-purity metal film 3 having a structure different from that of the base material 1 is formed on the surface of the base material 1. As a result, even if the surface roughness of the base material 1 is somewhat rough, as described above, the high-purity metal film 3 in a state where the snow has piled up and has not been compacted has a smooth surface of the mold. The surface of the high-purity metal film 3 after forming has an ultra-smooth surface roughness that is not affected by the base material because it is easily deformed by copying. Moreover, in this embodiment, the board | plate material which annealed the board | plate material which is not rolled or the rolled board | plate material as the base material 1, and removed the mechanical anisotropy of the board | plate material produced by rolling is used. Thereby, it is prevented that the degree of deformation differs depending on the direction during press molding, and the shape accuracy of the molded product can be increased.

  Further, in the present embodiment, on the surface of the high-purity metal film 3 after forming, as shown in FIG. 5, the surface of the ideal mirror surface, which is the ideal shape of the target mirror, is the crystal plane D forming the surface. In order to obtain a desired smooth surface, it is desirable to match as much as possible. When the high-purity metal film 3 has a face-centered cubic lattice crystal structure like aluminum as in the present embodiment, it is ultimately the case where the high-purity metal film 3 is represented by the Miller index of the face-centered cubic lattice crystal. It has been found by experiments of the present inventor that the (1,1,2) plane coincides with an ideal mirror surface. Therefore, it is ideal that the normal line of the crystal plane D constituting the surface of the mirror which is an actual molded product shown in FIG. 5 is coincident with the normal line of the (1, 1, 2) plane (ideal mirror plane). is there.

  Therefore, when the inventors of the present application confirmed by experiment, the normal line of the crystal plane (its mirror index is (hkl)) D constituting the surface of the actually formed mirror is (1,1,2) plane. It has been found that a mirror surface having desired smoothness required for a high-precision reflective optical element can be obtained if the average value θa in the optical effective plane of the angle θ formed with the normal satisfies the following expression.

0.7 ≦ cos θa = | (h + k + 2l) / √ {6 × (h ² + k ² + l ² )} | ≦ 1 (1)
This equation can be rewritten as the following equation.

0 ° ≦ θa ≦ 40 ° (2)
This represents an important structural feature of the optical element which is the molded product obtained in the present embodiment. In other words, if the angle formed by the normal of the crystal plane that forms the surface of the final molded product and the normal of the ideal mirror surface satisfies the above equation, the required surface roughness of the high-precision reflective optical element can be satisfied. As already described, since the amorphous oxide film 5 is formed on the surface of the high-purity metal film 3, the crystal plane as shown in FIG. 5 appears as it is on the surface of the mirror that is a molded product. However, since the amorphous oxide film 5 is a very thin layer, the shape of the crystal plane appears almost as it is on the surface of the oxide film 5.

  In the present embodiment, the average crystal grain size of the surface layer (high-purity metal film 3) of the formed reflective optical element is set to 0.5 μm or less.

  As described above, since the average crystal grain size of the high-purity metal film 3 is small, the columnar crystals constituting the high-purity metal film 3 are very finely sheared, slid, and rotated, and formed into a fine structure. The surface is created with an ultra-smooth surface roughness. When a conventional material (rolled material) is processed, subgrain boundaries exist inside the crystal, the crystal grain size itself is as large as several tens to several hundreds μm, and has a shape extending in the rolling direction. Such crystal grains have a strong anisotropy in the rolling direction and a high dislocation density inside the crystal. For this reason, the molding surface cannot obtain an ultra-smooth surface roughness, and the shape accuracy of the molded product also deteriorates.

  In the present embodiment, the thickness of the oxide film on the surface of the molded reflective optical element (the surface of the high-purity metal film 3) is set to 10 nm or less.

  The oxide film 5 present on the outermost surface is a thin film having a wide band gap. As the thickness increases, the reflected light on the surface and the reflected light on the metal surface on the lower surface affect the optical aberration. In addition, the light absorbed in the oxide film increases, which affects the reflectance. Therefore, it is necessary for the reflective optical element that the oxide film thickness is 10 nm or less.

  In the present embodiment, the oxide film on the surface of the formed reflective optical element (the surface of the high-purity metal film 3) is a mixed crystal of amorphous and fine crystals.

  By being a mixed crystal of amorphous and fine crystals, the starting point of corrosion is dispersed finely and uniformly, thereby making it possible to prevent local corrosion.

  In this embodiment, as a material for creating the reflective optical element having the above-described features, first, a plate material 10 composed of a high-purity metal film 3 having a different structure from that of the base material 1, which is a high-purity metal film 3, the average value θa in the optically effective plane of the angle θ formed by the normal of D (the mirror index is (hkl)) and the normal of the (1, 1, 2) plane, A plate material satisfying the following formula is used.

0.55 ≦ cos θa = | (h + k + 2l) / √ {6 × (h ² + k ² + l ² )} | ≦ 1 (3)
This equation can be rewritten as the following equation.

0 ° ≦ θa ≦ 57 ° (4)
This means that the surface of the columnar crystal of the high purity metal film 3 is already slightly oriented to the ideal mirror surface. By performing press molding using such a plate material 10 as a raw material, a molded product that finally satisfies the expressions (1) and (2) can be obtained.

  In the present embodiment, the high-purity metal film 3 has a columnar crystal metal structure of 50 nm or more perpendicular to the surface thereof.

  In such a structure, compared to conventional skin-passed materials, it can move sufficiently until the transition piles up, and as described above, intragranular slip occurs in a suitable shear direction, and free to slide and rotate. Therefore, the spreadability is good. Furthermore, it has an orientation perpendicular to the surface of the high purity metal film 3, and the structure parallel to the surface of the high purity metal film 3 is isotropic. For this reason, the shape accuracy is stable, and a molded product as intended can be obtained.

  In the present embodiment, the high-purity metal film 3 is a high-purity metal thin film made of a single metal having a purity of 98 mol% or more.

  From the general formula: τ = Gb / L (τ: shear force, G: shear elastic modulus, b: shear displacement, L: distance between impurities), it can be seen that the molding pressure decreases and the processing becomes easier as the purity increases. .

  In the present embodiment, a protective film, an enhanced reflection film, or a film in which these are laminated in a composite manner is formed on the surface of the metal reflective optical element obtained by molding.

  By this film formation, the durability against humidity is remarkably improved, and the effect of the increased reflection film on the ultra-smoothed surface is remarkable.

  Further, in the present embodiment, the plate material 10 as a molding material satisfying the formula (3) or the formula (4) is molded only by press molding without using any lubricant, and the formula (1) or (2) By molding a molded product that satisfies the above conditions, it is possible to obtain a surface layer that is ultra-flattened to Ra: 10 nm or less and a reflective optical element that also has a surface accuracy (strain: PV) of 5 μm or less.

  In particular, the press mold is made of cemented carbide, tool steel, ceramics, etc., and the surface thereof is a metal oxide, metal carbide, metal nitride, high-density carbon, noble metal base film, or The film is covered with a laminated film in which these are combined. These combined effects can contribute to improved shape accuracy, reduced friction, and improved mold release.

  From such a viewpoint, the optimum conditions under which a high-quality reflective optical element can be obtained at a low price are intensively studied, and examples and comparative examples are described below. Incidentally, the degree of orientation of the crystal surface on the surface of the reflective optical element of this embodiment is investigated for each crystal grain unit on the outermost surface of the deposited aluminum by a backscattered electron diffraction image (EBSP) in an arbitrary field of view of 30000 times. .

  Further, the crystal grain size of the high-purity metal film 3 of the reflective optical element is observed with a scanning electron microscope (SEM) in a cross section, and an arbitrary portion of the observation surface is orthogonally crossed vertically and horizontally, and d length (vertical average crystal Particle size) = L (actual size of vertical line) / n (number of crystal grains on vertical line) / a (magnification) is calculated in the same way, and d average is calculated, d average = (d vertical + d horizontal) / 2 It calculated in. The thickness of the oxide film 5 was measured by observing the cross section of the reflective optical element with a transmission electron microscope (TEM). Similarly, approximate values can be obtained in units of several nm by Auger electron spectroscopy (AES). The structure of the oxide film 5 was checked for the presence of reciprocal lattice points by electron diffraction. When it is difficult to visually determine, there is a method of obtaining a radial distribution function from an electron beam diffraction chart and comparing the existence probability between two atoms. Alternatively, it can be confirmed by determining whether a halo or a slight diffraction peak is recognized by thin film X-ray diffraction. Furthermore, the notation of the surface roughness of the macroscopic surface indicates roughness data (Ra) when measured in an arbitrary approximately 300 μm square using New View 100 which is a non-contact optical interference measuring device. Moreover, the shape data used Zygo Mark4 which is a non-contact optical interference measuring apparatus, and showed the shape accuracy (PV) of the whole optical effective surface.

  Next, the columnar crystal structure on the surface of the material for the reflective optical element can be produced at low cost by using a high-purity metal material on a commercially available rolled plate at a temperature of 80 ° C. or less (preferably room temperature) for PVD or CVD. In this way, a large area material can be obtained at a time. In this case, annealing is performed to eliminate the anisotropy of the rolled sheet.

  In addition, cutting out the material from the ingot obtained by the unidirectional solidification method is one means, and pouring the molten metal into a plate-shaped mold, cooling it slowly, developing columnar crystals on the surface, Getting is one way.

  As a pretreatment for forming the reflective optical element material (plate material 10) prepared by the above method, no various types of polishing were performed. Molding is performed using a curved surface mold with a surface roughness Ra ≦ 10 nm under a molding temperature lower than the recrystallization temperature and in any of air, vacuum, or inert atmosphere, without using any lubricant. The forming load and the forming speed were controlled only by press forming the material for the reflecting optical element so that the surface roughness of the reflecting element was Ra ≦ 10 nm and the shape accuracy of the optical effective surface was PV ≦ 5 μm with respect to the shape of the mold. .

  It should be noted that the following examples do not limit the present invention, and any change in the plate material for an optical element, its constituent materials, means, manufacturing method, and molded product in consideration of the above-described purpose. It is included within the scope of the invention.

<Example 1>
A columnar crystal structure with a film thickness of 2 μm was formed at room temperature on a commercially available JIS standard pure aluminum material surface not subjected to skin pass or the like with aluminum having an atmosphere of 2 × 10 ⁻⁵ torr and a purity of 99.99 mol%. As a pre-molding treatment, this material was not subjected to any polishing. Molding is performed using a curved surface mold with a surface roughness Ra ≦ 10 nm under a molding temperature lower than the recrystallization temperature and in any of air, vacuum, or inert atmosphere, without using any lubricant. An optical reflection element was obtained from this material only by press molding. The surface roughness of this reflective element was Ra: 6 nm, and the shape accuracy of the optically effective surface was PV: 1.1 μm with respect to the mold shape. From the EBSP result of this reflective optical element, since the (113) plane was oriented on the surface of the reflective optical element, the expression (1) or (2) was satisfied. The crystal grain size in the molded cross section was 0.30 μm from SEM observation. The oxide film thickness was 5 nm from TEM observation, and the structure of this oxide film was confirmed to be a mixed crystal of amorphous and crystal by electron diffraction. A SiO ₂ protective film was formed on the obtained molded product to improve durability.

<Example 2>
Molding was performed in the same manner as in Example 1 except that a commercially available rolled aluminum plate was annealed and used for the plate material as the molding material. As a result, a molded product similar to that in Example 1 could be obtained.

<Example 3>
A columnar crystal structure with a film thickness of 1.5 μm is formed at room temperature on the surface of a commercially available superplastic aluminum alloy material not subjected to skin pass or the like, with an atmosphere of 2 × 10 ⁻⁵ torr and a purity of 99.9 mol% of material. did. As a pre-molding treatment, this material was not subjected to any polishing. Molding is performed using a curved surface mold with a surface roughness Ra ≦ 10 nm under a molding temperature lower than the recrystallization temperature and in any of air, vacuum, or inert atmosphere, without using any lubricant. An optical reflection element was obtained from this material only by press molding. The surface roughness of the reflecting element was Ra: 8 nm, and the shape accuracy of the optically effective surface was PV: 1.6 μm with respect to the mold shape. From the EBSP result of this reflective optical element, since the (422) plane was oriented on the surface of the reflective optical element, the formula (1) or (2) was satisfied. The crystal grain size in the molded cross section was 0.17 μm from SEM observation. The oxide film thickness was 4 nm from TEM observation, and the structure of this oxide film was confirmed to be a mixed crystal of amorphous and crystal by electron beam diffraction. A SiO ₂ protective film was formed on the obtained molded product to improve durability.

<Example 4>
A columnar crystal structure having a film thickness of 2 μm was formed at room temperature on a commercially available JIS standard oxygen-free copper material surface that was not subjected to skin pass or the like with copper having an atmosphere of 2 × 10 ⁻⁵ torr and a purity of 99.99 mol%. . As a pre-molding treatment, this material was not subjected to any polishing. Molding is performed using a curved surface mold with a surface roughness Ra ≦ 10 nm under a molding temperature below the recrystallization temperature and any conditions in the air, vacuum or inert atmosphere, without using any lubricant, An optical reflection element was obtained from this material only by press molding. The surface roughness of this reflective element was Ra: 5 nm, and the shape accuracy of the optically effective surface was PV: 0.7 μm with respect to the shape of the mold. From the EBSP result of this reflective optical element, since the (331) plane was oriented on the surface of the reflective optical element, the expression (1) or (2) was satisfied. The crystal grain size in the molded cross section was 0.27 μm from SEM observation. The oxide film thickness was 7 nm from TEM observation, and the structure of this oxide film was confirmed to be a mixed crystal of amorphous and crystal by electron diffraction. An Al-enhanced reflection film and a SiO ₂ protective film were formed on the obtained molded product to improve infrared reflectivity and durability.

<Comparative Example 1>
99% high-purity aluminum and 95% or less aluminum or an aluminum alloy were made into plywood by hot rolling and further subjected to a gloss treatment by a cold skin pass. As a pre-molding treatment, this material was not subjected to any polishing. Molding is performed using a curved surface mold with a surface roughness Ra ≦ 10 nm under a molding temperature lower than the recrystallization temperature and in any of air, vacuum, or inert atmosphere, without using any lubricant. This material was molded only by press molding. As a result, the surface roughness of this reflective optical element was Ra: 11 nm, and the shape accuracy of the optically effective surface was greatly distorted with PV: 9 μm relative to the shape of the mold, and a highly accurate reflective optical element could not be obtained. . From the EBSP result of this reflective optical element, since the (220) plane was oriented on the reflective optical element surface, the expressions (1) and (2) were not satisfied. The crystal grain size in the molded cross section was 37 μm from SEM observation. The oxide film thickness was 5 nm from TEM observation, and the structure of this oxide film was a complete crystal by electron diffraction.

<Comparative example 2>
In order to further reduce the surface roughness, 99.8% high-purity copper rolled sheet material was subjected to sufficient cold skin pass. As a pre-molding treatment, this material was not subjected to any polishing. Molding is performed using a curved surface mold with a surface roughness Ra ≦ 10 nm under a molding temperature lower than the recrystallization temperature and in any of air, vacuum, or inert atmosphere, without using any lubricant. This material was molded only by press molding. As a result, the surface roughness of this reflective element was Ra: 16 nm, and the shape accuracy of the optically effective surface was greatly distorted with respect to the mold shape, PV: 10 μm, and a highly accurate reflective optical element could not be obtained. From the EBSP result of this reflective optical element, since the (200) plane was oriented on the reflective optical element surface, the expressions (1) and (2) were not satisfied. The crystal grain size in the molded cross section was 28 μm from SEM observation. The oxide film thickness was 13 nm from TEM observation, and the structure of the oxide film was a complete crystal from electron diffraction.

  As described above, according to the above-described embodiment, the metal optical element obtained by plastic processing the plate material 10 composed of the base material 1 and the high-purity metal thin film 3 has an unprecedented improvement from the metallographic viewpoint. By carrying out the process, it was possible to obtain an excellent surface roughness that was not found in the past at low cost.

It is sectional drawing which shows the board | plate material used as the material for manufacturing a curved mirror. It is sectional drawing of the final molded product manufactured by press-molding the board | plate material shown in FIG. 1 with a shaping | molding die. It is a figure which shows the crystal structure inside the high purity metal film in the board | plate material shown in FIG. It is a figure which shows the crystal structure inside the high purity metal film | membrane 2 after press-molding the board | plate material shown in FIG. 1 with a shaping | molding die. It is a conceptual diagram which shows the inclination with respect to the ideal mirror surface of a crystal surface.

Explanation of symbols

1 Base Material 2 High-Purity Metal Film 3 Oxide Film 10 Plate Material

Claims (7)

A reflective optical element produced by press-molding a metal plate comprising a base material made of a metal plate material, and a metal film formed on the base material and made of a metal having a crystal structure,
Of the crystal structure, when the angle between the normal of the crystal plane appearing on the surface of the metal film and the normal of the target surface shape of the reflective optical element is θ, the reflective optical element at the angle θ The average value θa in the optical effective plane of
0 ° ≦ θa ≦ 40 °
The reflective optical element characterized by satisfy | filling.   The reflective optical element according to claim 1, wherein an average crystal grain size of the metal film is 0.5 μm or less.   The reflective optical element according to claim 1, wherein an oxide film is formed on a surface of the metal film, and the thickness of the oxide film is 10 nm or less.   2. The reflective optical element according to claim 1, wherein the oxide film formed on the surface of the metal film is a mixed crystal of amorphous and fine crystals.   The reflective optical element according to claim 1, wherein the metal film is made of a single metal having a purity of 98 mol% or more.   The reflective optical element according to claim 1, wherein the surface roughness Ra of the surface of the metal film is 10 nm or less.   2. The reflective optical element according to claim 1, wherein when the shape error from the target surface shape of the reflective optical element is expressed by a PV value, PV ≦ 5 μm.

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