JP2020030237A - Antireflection film, optical element and method for depositing antireflection film - Google Patents

Abstract

To provide: an antireflection film having low reflectance, small unevenness of reflected color and excellent durability; an optical element including the same; and a film deposition method excellent in the productivity of such an antireflection film.SOLUTION: An antireflection film 1 includes a multilayer structure provided on the side of the convex optical surface 11a of an optical element 11 having a convex optical surface 11a having the maximum inclination angle of 25° or more. In each optical thin film 2 constituting the multilayer structure, a filling ratio at any point is 90% or more; a L value in a CIE1976 Labcolor system satisfies the following conditional expressions (1); a difference Δa between a values in any two points and a difference Δb between b values in the two points satisfy the following conditional expressions (2). L<5....(1), (Δa+Δb)<5....(2)SELECTED DRAWING: Figure 1

Description

  The present invention relates to an antireflection film, an optical element including the same, and a method for forming the antireflection film.

  2. Description of the Related Art An optical element having a multilayered antireflection film on a convex optical surface is known. It is desired that the antireflection film has low reflectance in a desired wavelength band and small unevenness of the reflected color. However, the anti-reflection film provided on the optical element having the convex R-shaped optical surface may cause unevenness of the reflection color, impair the aesthetic appearance, and degrade the product quality. For example, in the peripheral portion of the convex optical surface, the film thickness becomes thinner than in the central portion, and the reflection characteristic shifts to the shorter wavelength side. Therefore, even if the reflection color is green at the central portion of the convex optical surface, the reflection color becomes red at the peripheral portion, which may cause unevenness of the reflection color. The occurrence of unevenness in the reflection color in the anti-reflection film is caused by a convex optical surface having a portion where the inclination angle, which is the angle between the optical axis and the normal to the position, is 25 ° or more, ie, the maximum inclination angle is 25 ° or more This is particularly remarkable in the case of an optical element having a convex optical surface.

  Each optical thin film constituting the multilayer structure is usually formed by a vacuum evaporation method or a sputtering method. In the case of an optical thin film formed by such a method, generally, the film thickness d at an arbitrary measurement point generally follows the equation of d = d0 cos θ. Here, d0 is the film thickness at the center of the convex optical surface, and θ is the inclination angle of the optical surface indicated by the angle between the normal and the optical axis at the measurement point. For example, the thickness of the optical thin film formed at the position where the inclination angle θ of the convex optical surface is 25 ° is 0.91d0, the position at 60 ° is 0.5d0, and the position at 80 ° Is 0.17d0. That is, the thickness of the optical thin film becomes thinner as the inclination angle θ of the convex optical surface becomes larger. For example, in the case of a convex optical surface in which the inclination angle gradually increases from the central portion toward the peripheral portion, the thickness of the optical thin film becomes thinner at a portion closer to the peripheral portion than at the central portion. When the film thickness is small, the reflection characteristic shifts to the short wavelength side. Therefore, when the thickness of the optical thin film is not uniform over the entire convex optical surface, the anti-reflection film has uneven reflection color. Even in practical use, the reflectance at the peripheral portion of the antireflection film increases at a specific wavelength, which leads to the occurrence of ghost.

  Conventionally, a technique for making the film thickness uniform has been proposed in order to prevent uneven reflection color of the antireflection film. For example, Patent Literature 1 discloses a technique of forming a film while irradiating ions or plasma to a convex lens that rotates about the lens optical axis in a posture in which the lens optical axis is inclined at 70 ° with respect to the evaporation source. Have been. According to Patent Document 1, it is described that a film having excellent uniformity in film thickness and excellent in denseness can be formed by the effect of ion assist or plasma assist.

  Further, Patent Document 2 discloses a sputtering process in which sputter particles emitted from a metal target are deposited on a concave surface of an optical element to form a metal film, and the metal film is irradiated with an ion beam to form a metal film. A film forming method is disclosed in which a re-sputtering process of emitting the released sputter particles again on the concave surface of the optical element and an oxidizing process of irradiating the metal film with an oxygen radical beam to perform an oxidizing process are disclosed. I have. According to Patent Literature 2, sputter particles released from the metal film at the center of the concave surface are deposited on the peripheral portion of the concave surface, so that a metal film having a uniform thickness can be formed on the concave surface.

JP-A-2006-91600 JP 2012-128321 A

  However, when the antireflection film having a multilayer structure in which a plurality of optical thin films are stacked on the convex optical surface side of the optical element by the film forming method disclosed in Patent Documents 1 and 2, the following disadvantages are caused. is there.

That is, the optical thin film formed by the film forming method disclosed in Patent Literature 1 has an inconvenience that it is not suitable for an antireflection film due to the presence of voids. More specifically, Example 2 of Patent Document 1 shows an optical thin film formed by an ion-assisted film forming method using LaTiO ₃ as a vapor deposition source and using oxygen ions. The optical thin film of Example 2 has a refractive index for light having a wavelength of 550 nm of 1.91 at the top of the lens and 1.93 at the periphery of the lens. It is known that LaTiO ₃ is a material that can achieve a refractive index of 2.10 for light having a wavelength of 550 nm. Based on these numerical values, the porosity and the filling rate of the optical thin film of Example 2 are calculated by the Lorentz-Lorentz relational expression, and the porosity is 15% by volume and the filling rate is 85% by volume. Such an optical thin film having many voids is sparse and therefore has low physical strength and low physical durability. In addition, when moisture enters the voids, a change in spectral characteristics and a further decrease in durability occur. When LaTiO ₃ is used as a high refractive index material, it is optically disadvantageous if the refractive index of the high refractive index material is low. Further, the film forming method disclosed in Patent Document 1 forms a uniform film thickness by rotating a convex lens about an optical axis. Therefore, film formation cannot be performed simultaneously on two or more lenses, and there is also a disadvantage that productivity is low.

  Further, the film forming method disclosed in Patent Document 2 has a disadvantage that it cannot be applied to a convex optical surface of an optical element. This is because, when an ion beam is applied to the metal film formed on the convex optical surface in the sputtering process, the sputtered particles emitted from the convex optical surface travel in a direction away from the convex optical surface. Therefore, the sputtered particles cannot be formed again on the convex optical surface.

  Therefore, an object of the present invention is to provide an antireflection film having a low reflectance, a small unevenness of a reflection color, and excellent durability, and an optical element including the same. Further, an object of the present invention is to provide a film forming method excellent in productivity of such an antireflection film.

In order to solve the above problem, an antireflection film according to the present invention is an antireflection film having a multilayer structure provided on the convex optical surface side of an optical element having a convex optical surface having a maximum inclination angle of 25 ° or more. Each of the optical thin films constituting the multilayer structure has a filling ratio of 90% or more at an arbitrary position, and the L value in the L ^* a ^* b ^* color system of CIE1976 is represented by the following conditional expression (1). ) Is satisfied, and Δb satisfies the following conditional expression (2) as the difference a between the a values at any two places and the difference between the b values at the two places.
L <5 ... (1)
(Δa ² + Δb ² ) ^1/2 <5 (2)

  In order to solve the above problems, an optical element according to the present invention is characterized in that the above-described antireflection film is provided on a convex optical surface having a maximum inclination angle of 25 ° or more.

  Furthermore, in order to solve the above problem, the method for forming an antireflection film according to the present invention includes forming an antireflection film having a multilayer structure on the convex optical surface side where the maximum tilt angle of the optical element is 25 ° or more. A film forming method for forming a film by depositing a film forming material from a film forming source on the convex optical surface side of the optical element while rotating the optical element, The convex optical surface of the rotating optical element is irradiated with ions from an ion source or plasma from a plasma source from a direction inclined with respect to an optical axis to the convex optical surface, thereby depositing the convex optical surface. While removing the film material, the film is densified, and by the region of the convex optical surface closer to the ion source or the plasma source, the convex optical surface farther from the ion source or the plasma source The ions to the region or the An irradiation step of blocking the incidence of plasma, wherein the film forming step and the irradiation step are performed to form each optical thin film constituting the multilayer structure on the convex optical surface side of the optical element. It is characterized by.

  According to the present invention, it is possible to provide an antireflection film having a low reflectance, a small unevenness of a reflection color, and excellent durability, and an optical element having the antireflection film. Further, according to the present invention, it is possible to provide a film forming method excellent in productivity of such an antireflection film.

It is a typical sectional view showing the antireflection film concerning the present invention. FIG. 1 is a schematic diagram of a film forming apparatus that performs a method of forming an antireflection film according to the present invention. FIG. 3 is an enlarged view of a main part of the film forming apparatus shown in FIG. 2. It is a figure which shows the optical element provided with the antireflection film of Examples 1-5 and Comparative Examples 1-4. FIG. 1 is a schematic diagram of a film forming apparatus that performs a method of forming an antireflection film according to the present invention. (A) is a front view of the film forming apparatus, and (b) is a bottom view of the optical element supporting device as viewed from below. 4 is a graph showing a film thickness distribution of optical thin films of Examples 1 to 5 and Comparative Examples 1 to 4. (A) shows the film thickness distribution of the optical thin films of Examples 1 to 5, and (b) shows the film thickness distribution of the optical thin films of Comparative Examples 1 to 4. 4 is a graph showing the filling rates of optical thin films of Examples 1 to 5 and Comparative Examples 1 to 4. (A) shows the filling rate of the optical thin films of Examples 1 to 5, and (b) shows the filling rate of the optical thin films of Comparative Examples 1 to 4. 4 is a graph showing the reflection characteristics of the antireflection film of Example 1. 9 is a graph showing the reflection characteristics of the antireflection film of Example 2. 9 is a graph showing the reflection characteristics of the antireflection film of Comparative Example 1. 5 is a graph showing a change in reflection characteristics of the antireflection film of Example 1. 7 is a graph showing a change in reflection characteristics of the antireflection film of Example 2. 9 is a graph showing a change in reflection characteristics of the antireflection film of Comparative Example 2.

  Hereinafter, embodiments of an antireflection film, a method of forming the same, and an optical element including the antireflection film according to the present invention will be described.

1. Anti-reflection film The anti-reflection film according to the present invention is an anti-reflection film having a multilayer structure provided on the convex optical surface side of an optical element having a convex optical surface having a maximum tilt angle of 25 ° or more, Each of the optical thin films constituting the multilayer structure has a filling rate of 90% or more at an arbitrary position, and the L value in the L ^* a ^* b ^* color system of CIE1976 satisfies the following conditional expression (1). Δb satisfies the following conditional expression (2): the difference Δa between the a values at the two places and the difference between the b values at the two places.
L <5 ... (1)
(Δa ² + Δb ² ) ^1/2 <5 (2)

  The antireflection film is provided on a convex optical surface having a maximum inclination angle of the optical element of 25 ° or more, and has a multilayer structure in which n layers (n is an integer of 2 or more) of optical thin films are laminated. The convex optical surface having a maximum inclination angle of 25 ° or more refers to an optical surface in which there is a portion having a maximum inclination angle of 25 ° or more when the inclination angle is measured in the convex optical surface. For example, when the convex optical surface having the maximum tilt angle of 25 ° or more is an optical surface whose tilt angle gradually increases from the center to the peripheral portion, the tilt angle of the peripheral portion of the convex optical surface is 25 ° or more. The convex optical surface may be a surface having a curvature or a free-form surface. A convex optical surface having a maximum inclination angle of 25 ° or more and being a free-form surface is defined as an angle γ / 2 ≧ 25 °, where γ is an angle between normals of two arbitrary points on the convex optical surface. Optical surface. At this time, the line segment forming the angle γ / 2 can be regarded as the optical axis of the optical element having the convex optical surface.

  FIG. 1 shows an antireflection film according to an embodiment of the present invention. The anti-reflection film 1 shown in FIG. 1 is provided on an optical element 11 having a convex optical surface 11a having a maximum inclination angle of 90 °, that is, a peripheral part having an inclination angle of 90 °. Hereinafter, the convex optical surface 11a may be referred to as “optical surface 11a”. The antireflection film 1 has a multilayer structure in which seven optical thin films 2 are stacked. The seven-layer optical thin film 2 includes, in order from the side closer to the optical surface 11a, a first optical thin film 2a, a second optical thin film 2b, a third optical thin film 2c, a fourth optical thin film 2d, Also referred to as a five-layer optical thin film 2e, a sixth-layer optical thin film 2f, and a seventh-layer optical thin film g. The hatching of the optical thin film 2 in FIG. 1 is omitted.

  Each of the optical thin films constituting the multilayer structure has a filling rate of 90% or more at an arbitrary position. A filling rate of 90% or more means that the porosity is less than 10% and the number of voids is small. An optical thin film having a filling factor of 90% or more can obtain high physical strength. Further, since the porosity is less than 10%, it is possible to suppress the intrusion of moisture through the voids and the deterioration. Therefore, the antireflection film in which the optical thin films are laminated in n layers can obtain excellent durability. Further, since this optical thin film has a filling rate of 90% or more, it is possible to realize a refractive index close to the original refractive index of the material. On the other hand, when the filling rate of the optical thin film is less than 90%, the physical strength is low, and the optical thin film may be deteriorated by infiltration of moisture. An antireflection film having such an optical thin film laminated thereon is not preferable because excellent durability cannot be obtained. Further, when the filling rate of the optical thin film is less than 90%, a refractive index close to the original refractive index of the material cannot be realized. When an antireflection film is formed by stacking such optical thin films, it is necessary to increase the number of stacked optical thin films, which is inefficient and is not preferable.

Here, the filling factor of the optical thin film can be measured by, for example, X-ray diffraction method (XRD), and a formula (H. Angus Macleod (2010), ") quoted from a thin-films optical filters shown in the following formula: Thin-Films Optical Filters ", USA: CRC Press, pp. 570-572.) Or Lorentz-Lorentz relational expression.
_{p = (n eff -n air)} / (n s -n air)
However, in the above equation,
p: Filling rate of optical thin film n _eff : Effective refractive index of optical thin film n _air : Refractive index of _air n _s : Original refractive index of film when there is no void in film or recommended value of manufacturer

Further, in the antireflection film, the L value in the L ^* a ^* b ^* color system of CIE1976 satisfies the following conditional expression (1), the difference Δa between the a values at any two places, and the b value at the two places. Δb satisfies the following conditional expression (2). Thereby, it is possible to realize an antireflection film having a low reflectance and a small uneven reflection color. Here, the conditional expression (1) means that the L value is less than 5 at any part of the anti-reflection film, and that the anti-reflection film is an anti-reflection film whose reflection is suppressed over the whole. When the conditional expression (1) is not satisfied, it is not preferable because there are portions where the L value is 5 or more in the antireflection film, and the antireflection film has high reflectance or uneven reflection color. Then, in any condition of the antireflection film, the conditional expression (2) is calculated from the difference a of the a value Δa and the difference b of the b value at any two places (Δa ² + Δb ² ) ^1/2 is 5 Less than that, which means that the antireflection film has a small uneven reflection color. When the conditional expression (2) is not satisfied, there is a portion where (Δa ² + Δb ² ) ^1/2 is 5 or more in the antireflection film, which is not preferable because the reflection color is uneven.

In each optical thin film, it is preferable that the minimum value d (min) and the maximum value d (max) of the film thickness satisfy the following conditional expression (3).
cos (5θ / 6) ≦ d (min) / d (max) ≦ 1.0 (3)
Here, θ is the inclination angle of the convex optical surface at the measurement location.

  Conditional expression (3) indicates that each optical thin film constituting the multilayer structure of the anti-reflection film has a ratio of a position where the film thickness is minimum to a position where the film thickness is minimum is cos (5θ / 6) or more. It means that thickness uniformity is excellent. Specifically, it is preferable that the thickness of the optical thin film satisfies 0.935 ≦ d (min) / d (max) ≦ 1 at a position where the inclination angle of the convex optical surface is 25 °. It is preferable that the thickness of the optical thin film satisfies 0.643 ≦ d (min) / d (max) ≦ 1 at a position where the inclination angle of the convex optical surface is 60 °. At the point where the inclination angle of the convex optical surface is 90 °, the thickness of the optical thin film preferably satisfies 0.259 ≦ d (min) / d (max) ≦ 1.

  An optical thin film satisfying conditional expression (3) has excellent film thickness uniformity. Therefore, it is possible to further reduce the unevenness of the reflection color and to realize an antireflection film in which ghost is suppressed. Further, it is possible to realize an antireflection film having a low reflectance not only at the center but also at the periphery of the convex optical surface. When the optical thin film does not satisfy the conditional expression (3), the unevenness of the reflection color of the antireflection film may not be reduced or a ghost may occur because the film thickness uniformity is insufficient. Absent.

In each optical thin film, it is more preferable that the minimum value d (min) and the maximum value d (max) of the film thickness satisfy the following conditional expression (4).
cos (θ / 2) ≦ d (min) / d (max) ≦ 1.0 (4)

  Specifically, it is preferable that the thickness of the optical thin film satisfies 0.976 ≦ d (min) / d (max) ≦ 1 at the position where the inclination angle of the convex optical surface is 25 °. It is preferable that the thickness of the optical thin film satisfies 0.866 ≦ d (min) / d (max) ≦ 1 at the position where the inclination angle of the convex optical surface is 60 °. At the point where the inclination angle of the convex optical surface is 90 °, the thickness of the optical thin film preferably satisfies 0.707 ≦ d (min) / d (max) ≦ 1.

  The optical thin film that satisfies the conditional expression (4) has more excellent film thickness uniformity than the optical thin film that satisfies the conditional expression (3) but does not satisfy the conditional expression (4). Therefore, the antireflection film in which the optical thin films satisfying the conditional expression (4) are stacked can surely suppress the occurrence of unevenness of the reflection color and can further reduce the reflectance.

  The thickness of each optical thin film can be measured by a cross-sectional SEM or a contact-type thickness gauge. Alternatively, the reflectance of the optical thin film can be measured by an ellipsometer or the like, and the film thickness or the refractive index can be calculated from the reflectance by simulation.

  Incidentally, the antireflection film preferably includes an optical thin film that is a high refractive index layer and an optical thin film that is a low refractive index layer in order to further reduce the reflectance. Further, the antireflection film may include an intermediate refractive index layer having a refractive index between the high refractive index layer and the low refractive index layer. The antireflection film can be formed by appropriately combining the high refractive index layer, the low refractive index layer, and the intermediate refractive index layer.

The high refractive index layer preferably contains at least one metal oxide selected from the group consisting of TiO ₂ , Nb ₂ O ₅ , ZrO ₂ , La ₂ O ₃ , Ta ₂ O ₅ , and HfO ₂ . The high refractive index layer containing these metal oxides can realize a high refractive index of 2.0 or more. It is preferable that the low refractive index layer as the final layer contains SiO ₂ . When the low refractive index layer include both or SiO ₂ and _Al 2 _{O 3} containing SiO ₂ alone can reduce the refractive index to 1.50 or less. The intermediate refractive index layer preferably contains a metal oxide such as Al ₂ O ₃ , Y ₂ O ₃ and YbF ₂ or a mixture such as Al ₂ O ₃ + L ₂ O ₃ . The refractive index of the intermediate refractive index layer is 1.50 or more and 2.0 or less. For example, in the antireflection film 1 shown in FIG. 1, the first optical thin film 2a, the third optical thin film 2c, and the fifth optical thin film 2e are formed as intermediate refractive index layers made of Al ₂ O ₃ . The two-layer optical thin film 2b, the fourth-layer optical thin film 2d, and the sixth-layer optical thin film 2f are high-refractive-index layers made of LaTiO _3, and the seventh-layer optical thin film 2f, which is the final layer, is made of SiO _2. It may be a refractive index layer.

Each optical thin film constituting the above-described antireflection film can be formed by a film forming method described later. Since ions or plasma are used in the film forming method, the optical thin film contains an element constituting ions or plasma. For example, in the case where Ar is formed into a plasma and formed into a film, it can be confirmed by secondary ion mass spectrometry (SIMS) that the optical thin film contains 1 × 10 ¹⁹ atomic% / cm ³ or more of Ar. However, it is not preferable that the optical thin film contains Ar of 1 × 10 ^{22 at} % / cm ³ or more because the optical thin film may not be dense.

  The antireflection film having the above configuration can have excellent antireflection characteristics. For example, in the case of an antireflection film for the visible region, a wavelength of 420 nm or more and 680 nm or less at an incident angle of 0 ° at an arbitrary position. And an average reflectance of 1% or less can be achieved. The antireflection film has an average reflectance of 1% or less with respect to the light at any location regardless of the central portion and the peripheral portion, and has excellent antireflection characteristics. If the reflectance exceeds 1%, the antireflection film may have insufficient antireflection characteristics. Furthermore, by performing optimal design for light in the ultraviolet or near-infrared range, an antireflection film with an average reflectance of 1% or less for light in the ultraviolet or near-infrared range can be realized. Can be.

2. Next, an embodiment of a method for forming an anti-reflection film will be described. The method for forming an antireflection film according to the present invention is a method for forming an antireflection film having a multilayer structure on a convex optical surface side where the maximum tilt angle of the optical element is 25 ° or more, A film forming step of forming a film by depositing a film forming material from a film forming source on the convex optical surface side of the optical element while rotating the optical element; By irradiating the surface side with ions from an ion source or plasma from a plasma source from a direction inclined with respect to the optical axis, while removing the film-forming material deposited on the convex optical surface side, the film is removed. Densification and incidence of the ions or plasma to the region of the convex optical surface farther from the ion source or the plasma source by the region of the convex optical surface closer to the ion source or the plasma source Irradiation process to shield Comprising, by performing the said film forming step said irradiation step, forming each optical thin film constituting the multilayer structure on the optical surface of the convex of the optical element.

  The method for forming an antireflection film according to the present invention can be performed by, for example, a film forming apparatus shown in FIGS. The film forming apparatus shown in FIGS. 2 and 3 is one of the embodiments. In particular, for the film formation process, various forms, for example, sputtering, CVD, and the like can be applied as a film formation source, and the invention is not limited to the film formation process of this embodiment. First, an embodiment of the film forming apparatus will be described.

  The film forming apparatus 21 shown in FIG. 2 includes an optical element support device 41 having a planetary rotation mechanism, a vapor deposition source 51 as a film forming source, and an ion gun 61 in a film forming chamber 31 capable of holding the inside thereof in a vacuum. Is provided. In this embodiment, the ion gun 61 for irradiating ions is used, but a plasma gun for irradiating plasma may be used instead of the ion gun 61.

  The optical element supporting device 41 is hung from the ceiling wall of the film forming chamber 31 and is rotatable, and has a disk-shaped support base 42. The optical element holder is hung around the periphery of the support base 42, and is rotatable. 43. The support base 42 rotates by driving a first motor (not shown). The rotation axis of the support base 42 is L1. The optical element holder 43 rotates by driving a second motor (not shown). The rotation axis of the optical element holder 43 is L2. The optical element holder 43 revolves around the rotation axis L1 of the support base 42 as a rotation axis by driving the first motor. Six optical element holders 43 are arranged on the support base 42 at equal intervals. However, in FIG. 2, only two optical element holders 43 are described, and the description of the other optical element holders 43 is omitted.

  In the present embodiment, the optical element holder 43 is suspended from the support base 42 such that the film forming surface 43a on which the optical element 11 is mounted and on which the film is formed faces obliquely downward. The direction of the film forming surface 43 a is adjusted by the angle adjusting mechanism 44. FIG. 2 shows a state where the film forming surface 43a is inclined by 20 ° with respect to the vertical direction. The plurality of optical elements 11 are arranged concentrically around the rotation axis L2 of the optical element holder 43 with the optical surface 11a on which the antireflection film is formed facing outward on the film forming surface 43a. For example, seven optical elements 11 can be arranged at equal intervals around the rotation axis L2 of the optical element holder 43, and 14 optical elements 11 can be arranged at equal intervals around the periphery. However, in FIG. 2, only two optical elements 11 are described, and the description of the other optical elements 11 is omitted. The optical element 11 attached to the film formation surface 43a takes a posture in which the optical axis OA is inclined with respect to the vertical direction. In this embodiment, since the film formation surface 43a is flat, the optical axis OA of each optical element 11 and the rotation axis L2 of the optical element holder 43 are parallel, but need not be.

In the present embodiment, as shown in FIG. 2, the evaporation source 51 is provided at the bottom of the film forming chamber 21 and inside the orbit of the optical element holder 43. However, the position of the evaporation source 51 is not limited to this position. For example, when performing side sputtering, the evaporation source 51 can be provided at a position in the horizontal direction with respect to the optical element holder 43. The vapor deposition source 51 forms a vapor deposition material which is a film forming material by using an electron gun, resistance heating, a sputtering source, sputtering by an ion gun or a plasma gun, heat vapor deposition by a plasma gun, a chemical vapor deposition method, ion plating, or the like. Can be membrane. As the evaporation source 51, for example, various optical materials such as TiO ₂ , Nb ₂ O ₅ , ZrO ₂ , La ₂ O ₃ , Ta ₂ O ₅ , HfO ₂ , SiO ₂ , and Al ₂ O ₃ can be used. . The deposition material from the deposition source 51 rises while spreading, and deposits on the optical surface 11a of the optical element 11, the optical element holder 43, and the like. The deposition material is incident on the optical surface 11a side of the optical element 11 from various directions, but mainly from a vertically downward direction. That is, the deposition material mainly enters the optical surface 11a of the optical element 11 from a direction inclined with respect to the optical axis OA. For example, at a certain moment, the deposition material enters the optical surface 11a side of the optical element 11 from a direction D1 inclined at an angle α with respect to the optical axis OA. The angle α can be adjusted by changing the position of the evaporation source 51, the direction of the film forming surface 43a, and the like. FIG. 2 shows a state where the angle α is 70 °. The deposition rate of the vapor deposition material on the optical surface 11a side is determined by the pressure (degree of vacuum) in the film forming chamber 31, the position of the vapor deposition source 51, and the film deposition conditions of the vapor deposition source 51 (resistance heating temperature, evaporation area, electron It can be controlled by the electron beam size of the gun, emission current, acceleration voltage, etc.

In the present embodiment, the ion gun 61 is also located at the bottom of the film forming chamber 21, inside the orbit of the optical element holder 43, and with respect to the rotation axis L 1 of the support base 42. It is provided at the opposite position. However, the position of the ion gun 61 is not limited to this position as long as self-shielding can be performed as described later. The ion gun 61 irradiates ions at high speed. In this embodiment, one or more rare gases selected from the group consisting of He, Ne, Ar, Xe, and Xr and O ₂ are introduced into the ion gun 61 as appropriate, and the rare gas and oxygen gas are ionized by the ion gun 61. And irradiate. The ions irradiated by the ion gun 61 have high straightness because they are accelerated. The ion gun 61 irradiates ions in a predetermined direction. For example, at a certain moment, the ion gun 61 irradiates the ions such that the ions are incident on the optical surface 11a side of the optical element 11 from a direction D2 inclined at an angle β with respect to the optical axis OA. The angle β can be adjusted by changing the position of the ion gun 61 and the irradiation angle. FIG. 2 shows a state in which the angle β is 70 °. The irradiation energy of the ion gun 61 can be controlled by accelerating voltage, beam current, beam voltage, film forming pressure, gas introduction type, gas introduction amount, and the like.

  Next, an embodiment of a method for forming an antireflection film will be described. Here, a method of forming the antireflection film 1 shown in FIG. 1 will be described. By repeating the following film forming step and irradiation step simultaneously or alternately, first, the first-layer optical thin film 2a is formed, and subsequently, the second-layer optical thin film 2b to the seventh-layer optical thin film 2g are formed. Are formed in order. Hereinafter, formation of the first-layer optical thin film 2a will be described in detail.

(Deposition process)
The film forming process is performed as follows. In the present embodiment, the first optical thin film 2a is formed using Al ₂ O ₃ as the evaporation source 51. With the optical element 11 being rotated about the rotation axis L1 of the support base 42 and the rotation axis L2 of the optical element holder 43, the evaporation source 51 is heated to evaporate the evaporation material (Al ₂ O ₃ ). The deposition material is deposited on the optical surface 11a of the optical element 11 to form a film. The vapor deposition material enters the optical surface 11a from various directions, but mainly enters the optical surface 11a from a direction D1 inclined with respect to the optical axis OA. Since the optical element 11 rotates as described above, the film thickness distribution of the film formed on the optical surface 11a of the optical element 11 substantially corresponds to the above-described expression of d = d0 cos θ. That is, the film formed on the optical surface 11a has the largest thickness at the central portion, and becomes thinner from the central portion to the peripheral portion.

(Irradiation process)
The irradiation step is performed as follows. The ion gun 61 irradiates ions with the optical element 11 rotated with the rotation axis L1 of the support base 42 and the rotation axis L2 of the optical element holder 43 as rotation axes. When the ions collide with the deposited material deposited on the optical surface 11a of the optical element 11, energy is imparted to the deposited material. As a result, at the place where the ions collide, the ions act as assisting film formation, and the film formed on the optical surface 11a is densified. Further, the deposition material deposited on the optical surface 11a is removed, and the thickness of the film is reduced. Some of the ions that have collided with the deposition material remain in the film in some form.

  Since the ions irradiated by the ion gun 61 are intentionally accelerated by the accelerating voltage, the ions have higher rectilinearity than the deposition material from the deposition source 51. Therefore, the ions enter the optical surface 11a from the direction D2 inclined with respect to the optical axis OA of the optical element 11. The optical surface 11a of the optical element 11 is an optical surface having a deep R such that a portion having an inclination angle of 25 ° or more exists. Therefore, the region of the optical surface 11a on the side closer to the ion gun 61 blocks incidence of ions on the region of the optical surface 11a farther from the ion gun 61 (region R surrounded by a double-dashed line in FIG. 3). You. This is called “self-shielding”. Self-shielding does not occur at the center of the optical surface 11a, but occurs at the periphery. Specifically, the central portion of the optical surface 11a is an incident area where ions are incident regardless of the rotation of the optical element 11. On the other hand, in the peripheral portion of the optical surface 11a, the incident area and the shielding area where the incidence of ions is shielded are switched with the rotation of the optical element 11. As a result, more ions are incident on the central portion of the optical surface 11a than on the peripheral portion, and the removal amount of the deposition material from the optical surface 11a is increased. For example, at the center of the optical surface 11a, 20% or more of the deposition material deposited on the optical surface 11a is removed, while the removal amount at the peripheral portion of the optical surface 11a is only about several percent. In the present embodiment, since the optical element 11 is arranged around the rotation axis L2 of the optical element holder 43, the ion irradiation position on the optical surface 11a moves up, down, left and right with the rotation of the optical element holder 43. Also change. However, since the ions irradiated by the ion gun 61 have high rectilinearity, even if the position of ion irradiation on the optical surface 11a changes up, down, left, and right with the rotation of the optical element holder 43, sufficient self-shielding occurs. In addition, a large amount of deposition material deposited on the central portion of the optical surface 11a can be removed.

As described above, by the ion irradiation in the irradiation step, the film formed on the optical surface 11a in the film forming step is densified and reduced in thickness. By repeating the film forming step and the irradiation step simultaneously or alternately, the film formed on the optical surface 11a gradually increases in thickness, and the film thickness increases from the central portion on the optical surface 11a side. It is uniformed over the periphery. At this time, the conditions of the film forming step and the irradiation step are appropriately changed, and the film forming step and the irradiation step are repeatedly performed while keeping a balance as to which one is preferentially performed. As described above, the first-layer optical thin film 2a made of Al ₂ O ₃ and having a desired film thickness can be formed on the convex optical surface 11a of the optical element 11. The optical thin film 2a of the first layer has a uniform film thickness from the central portion to the peripheral portion, and is densified to have a filling rate of 90% or more at an arbitrary portion.

  Thereafter, while appropriately changing the material of the evaporation source 51, the film forming step and the irradiation step are repeatedly performed while appropriately changing the conditions, and the remaining optical thin films 2b to 2g are formed on the first optical thin film 2a. I do. Like the optical thin film 2a of the first layer, the optical thin films 2b to 2g of the second to seventh layers have a uniform thickness from the central portion to the peripheral portion, and the filling rate of an arbitrary portion is reduced. 90% or more. As described above, the antireflection film 1 having the multilayer structure can be formed on the optical surface 11a of the optical element 11 shown in FIG.

  When the film forming step and the irradiation step are performed alternately, it is preferable to perform the following steps. First, a film is formed by a film forming process. An irradiation step is performed before the film thickness reaches 10 nm, and the sublayer is formed by shaving off the surface layer of the film and reducing the film thickness. By repeating the film forming step and the irradiation step, the sub-layers are stacked to form the first optical thin film 2a having a desired film thickness. On the other hand, if the irradiation step is performed after the film thickness exceeds 10 nm, the effect of densification occurs in the surface portion of the sub-layer, but the effect of densification does not occur in the deep portion of the sub-layer. Therefore, the obtained optical thin film 2a of the first layer is not preferable because a dense layer and a non-dense layer are laminated and become inhomogeneous in the depth direction. From the above, it is more preferable to perform the film formation step and the irradiation step at the same time rather than alternately. When they are performed at the same time, only a dense layer is laminated, and a first-layer optical thin film 2a uniform in the depth direction can be obtained.

  In the apparatus configuration of the present embodiment, when the optical element 11 approaches the evaporation source 51, a film forming process is mainly performed, and the film on the optical surface 11a becomes thick. On the other hand, when the optical element 11 approaches the ion gun 61, an irradiation step is mainly performed, and the film on the optical surface 11a becomes thin. Then, the rate at which the film becomes thicker in the film formation step, that is, the deposition rate of the deposition substance can be controlled by the above-described film formation conditions. Further, the rate at which the film becomes thinner in the irradiation step, that is, the rate at which the deposition material is removed by ions can be controlled by the above-described ion irradiation energy.

  Here, referring to FIG. 2, the angle α between the incident direction D1 of the deposition material on the optical surface 11a and the optical axis OA of the optical element 11, and the incident direction D2 of ions on the optical surface 11a and the optical axis The angle β formed with the OA will be described. First, the angle α will be described. The deposition material from the deposition source 51 rises with spreading. When the optical element 1 is located directly above the vapor deposition source 51, the amount of vapor deposition material deposited on the optical surface 11a is the largest. For example, the angle between the optical axis OA of the optical element 11 at that position and the vertical direction is defined as an angle α. In the present embodiment, the angle α is set to 70 °, but the angle α is preferably from 0 ° to 90 °, more preferably from 45 ° to 90 °. When the angle α is 0 °, the optical axis OA of the optical element 11 coincides with the vertical direction. When the angle α exceeds 90 °, the optical axis OA of the optical element 11 faces upward from the horizontal direction. The deposition amount of the deposition material on the central portion of the optical surface 11a is reduced, and the film forming speed is undesirably reduced. On the other hand, in order to uniformly form a film on the plurality of optical elements 11 irrespective of the mounting position in the optical element holder 43, the angle α is preferably 0 ° or more and as small as possible. An example of a small angle α is a film forming apparatus having a plane planetary rotation mechanism for forming an optical multilayer filter on a flat glass. However, if the angle α is less than 45 °, the deposition amount of the deposition material at the central portion of the optical surface 11a increases in the film forming process, while the deposition amount at the peripheral portion decreases excessively. In that case, in order to make the film thickness uniform from the central portion on the optical surface 11a side to the peripheral portion, it is necessary to cut more the film at the central portion of the optical surface 11a in the irradiation process, May be required. From the above, in order to further reduce the time required for the irradiation step, the angle α is more preferably 45 ° or more and 90 ° or less.

  Next, the angle β will be described. Since the ions emitted from the ion gun 61 have straightness, when the optical element 11 is located at a position facing the irradiation port of the ion gun 61, the amount of ions incident on the optical surface 11a is the largest. For example, an angle between the optical axis OA of the optical element 11 at that position and a line segment connecting the center of the optical surface 11a and the ion gun 61 is defined as an angle β. In the present embodiment, the angle β is set to 70 °, but the angle β is preferably 45 ° or more and 90 ° or less. When the angle β is 45 ° or more, uniformity of the film can be secured. For example, in the case of a convex optical surface 11 having a particularly large maximum inclination angle such as a hemispherical shape (the maximum inclination angle is 90 °), the angle β is preferably set to 60 ° or more. If the angle β is less than 45 °, self-shielding during ion irradiation becomes insufficient, and it is difficult to secure uniformity of the film, which is not preferable. On the other hand, if the angle exceeds 90 °, the amount of ion irradiation to the central portion of the optical surface 11a decreases, and it becomes difficult to cut the film at the central portion, which is not preferable.

  Further, in the film forming method of the present embodiment, the optical element 11 rotates around an axis different from the optical axis OA as a rotation axis. Specifically, the optical element 11 is rotated about the rotation axis L1 of the support base 42 and the rotation axis L2 of the optical element holder 43 by the optical element support device 41 having a planetary rotation mechanism. Therefore, the optical element 11 moves (rotates) in the horizontal direction with the rotation of the support base 42 about the rotation axis L1 and moves in the vertical direction with the rotation of the optical element holder 43 about the rotation axis L2. , Move left and right. At this time, the film thickness formed in the film forming process differs depending on the position where the optical element 11 is mounted on the optical element holder 43, that is, the distance from the rotation axis L2 to the optical element 11. Further, the range of the incident area and the shield area of the ions on the optical surface 11a in the irradiation step differs depending on the mounting position of the optical element 1. However, since the film formation process and the irradiation process are performed in a state where the optical element 11 is moved in the horizontal direction, the vertical direction, and the horizontal direction, the plurality of optical elements 11 are The antireflection film 1 can be uniformly formed. Therefore, the film forming method of the present embodiment is suitable for mass production of the antireflection film 1. Note that the optical element 11 only needs to be rotated so that self-shielding occurs during the irradiation step, and the method of rotation is not limited to this.

  Further, in the present embodiment, the film forming method using the film forming apparatus 21 shown in FIG. 2 has been described, but the present invention is not limited to this. Conventionally, a general-purpose sputtering apparatus generally used for forming a thin film may be used. A film forming method using a general-purpose sputtering apparatus will be briefly described with reference to FIG. The general-purpose sputtering apparatus includes an optical element support device 101, a vapor deposition source 111, and an ion gun 121 in a film forming chamber whose inside can be maintained in a vacuum, as schematically shown in FIG. The same vapor deposition source 111 and ion gun 61 as those shown in FIG. 2 can be used for the vapor deposition source 111 and the ion gun 121.

  The optical element support device 101 includes a disk-shaped support base 102 and an optical element holder 103 disposed on the support base 102 and having a smaller diameter than the support base 102. The optical element support device 101 is suspended from the ceiling wall of the film forming chamber so that the film forming surface 103a of the optical element holder 103 faces the bottom of the film forming chamber. As shown in FIG. 5B, the support base 102 rotates around the rotation axis L1 around its center, and two or more optical element holders 103 are arranged around the rotation axis L1. The optical element holder 103 rotates with its center as the rotation axis L2. On the film forming surface 103a of the optical element holder 103, two or more optical elements 11 can be arranged around the rotation axis L2. At this time, the optical axis OA of the optical element 11 coincides with the vertical direction. Further, as shown in FIG. 5A, the evaporation source 111 is provided, for example, at the bottom of the film forming chamber and immediately below the rotation trajectory of the optical element holder 103. The ion source 121 is provided, for example, at the bottom of the film formation chamber and immediately below the rotation trajectory of the optical element holder 103 and at a position different from the vapor deposition source 111. The ion gun 121 irradiates ions obliquely upward. The ion gun 121 is directed toward the optical surface 11a of the optical element 11 so that ions can be irradiated from a direction inclined at an angle β of 45 ° to 90 ° with respect to the optical axis OA. Position has been adjusted. The above-described apparatus configuration shown in FIG. 5 can also perform the film forming method of the present embodiment described above. Then, self-shielding can be caused when performing the above-described irradiation step.

3. Optical Element An optical element according to the present invention is provided with the above-described antireflection film on a convex optical surface having a maximum inclination angle of 25 ° or more. According to the present invention, by providing the above-described antireflection film, it is possible to provide an optical element having high appearance productivity and less ghost. Examples of the optical element include a photographing optical element and a projection optical element. Specifically, as the lens, for example, an interchangeable lens of a single-lens reflex camera, a lens mounted on a digital camera (DSC), or mounted on a mobile phone Lenses for digital cameras, projector lenses for illumination systems, free-form lenses for car headlights, laser processing lenses and axicon lenses, DVD, CD, Blu-ray pickup lenses, mobile phones and smartphones Various lenses such as a lens used for a camera can be given.

By the way, as described above, each optical thin film constituting the antireflection film is formed by a film forming step of depositing a deposition material and an irradiation step of irradiating ions or plasma. Therefore, when depositing the first-layer optical thin film on the above-mentioned convex optical surface, ions, plasma, electrons, etc. may collide with the convex optical surface. If the optical element is made of a specific glass material, for example, a glass material such as FCD1 containing fluorine, light absorption occurs in the optical element when accelerated electrons or the like collide with the convex optical surface. Is not preferred. Therefore, the optical element according to the present invention preferably includes a protective layer between the convex optical surface and the antireflection film for preventing the incidence of ions, plasma, or electrons on the convex optical surface. . By forming the first layer of the optical thin film on the protective layer, it is possible to prevent electrons and the like from directly colliding with the optical element and to prevent light absorption. The protective layer is preferably made of a material having the same refractive index as the optical element. For example, when the optical element is made of a glass material of FCD1 (refractive index: 1.497), it is preferable that the protective layer is made of SiO ₂ having substantially the same refractive index. Since the refractive indices are almost the same, it can be considered that there is almost no interface between the glass material and the protective layer. Therefore, there is no optical interference effect, and the same optical characteristics as when no protective layer is present can be obtained despite the presence of the protective layer. At this time, even if the thickness of the protective layer is not uniform, it is possible to prevent the influence on the optical characteristics. Therefore, the protective layer can be formed by a normal vacuum deposition method without using ions or plasma. For example, the protective layer can be formed by performing the above-described film forming process using the evaporation source 51. The thickness of the protective layer is preferably at least 0.5 nm, more preferably at least 5 nm. When the thickness of the protective layer is less than 0.5 nm, the coating on the convex optical surface becomes insufficient, and the deposition of the rare gas element on the convex optical surface when forming the first optical thin film is performed. It is not preferable because it cannot be prevented.

Furthermore, an antifouling film or a hard film can be formed as a functional film on the surface of the antireflection film. For example, an antifouling film with a fluorine coating or a hard film made of diamond-like carbon (DLC) or SiO _x N _y can be provided. The thickness of the functional film is preferably 10 nm or less in order to prevent the influence on the optical characteristics.

  Next, the present invention will be specifically described with reference to Examples and Comparative Examples. However, the present invention is not limited to the following embodiments.

  In the present embodiment, the anti-reflection film composed of seven layers of optical thin films 2a to 2g is formed on the convex optical surface 11a of the optical element 11 by performing the above-described film forming method using the film forming apparatus 21 shown in FIG. 1 was formed.

(Optical element)
The optical element 11 having the shape shown in FIG. 4 was used. In the optical element 11 shown in FIG. 4, when the inclination angle was measured at a position where the distance D from the optical axis OA was 15 mm on the convex optical surface 11a, the inclination angle was 60 °. From this, the maximum inclination angle of the convex optical surface 11a in the optical element 11 used in the present embodiment is 60 ° or more. In this embodiment, the optical element 11 made of TAF1 is used.

(Deposition conditions)
First, two or more optical elements 11 were arranged around the rotation axis L2 of the optical element holder 43, and the optical element holder 43 was inclined to maintain a posture in which the film forming surface 43a was inclined by 20 ° with respect to the vertical direction. The optical element 11 was rotated by rotating the optical element holder 43 while rotating the support base 42. Oxygen gas at a flow rate of 20 sccm was introduced into the film forming chamber 31 to adjust the degree of vacuum to 1.5 × 10 ⁻² Pa. Then, while the optical element 11 was heated to a temperature of 250 ° C., the above-described film forming step and irradiation step were repeated to form the respective optical thin films 2a to 2g.

In the film forming process, the following was used as the evaporation source 51. The first, third, and fifth optical thin films 2a, 2c, and 2e were formed using Al ₂ O ₃ . TiO ₂ and La ₂ O ₃ were used for forming the second, fourth and sixth optical thin films 2b, 2d and 2f. SiO ₂ was used for forming the optical thin film 2 g of the seventh layer. Since the film formation surface 43a of the optical element holder 43 is inclined by 20 ° with respect to the vertical direction, the evaporation material evaporated from the evaporation source 51 is mainly inclined at an angle α = 70 ° with respect to the optical axis OA. The light was incident on the optical surface 11a of the optical element 11 from the direction D1.

  In the irradiation step, Ar gas at a flow rate of 40 sccm was introduced at the time of ion irradiation by the ion gun 61, and the Ar gas and oxygen gas were ionized and irradiated by the ion gun 61. At this time, the acceleration voltage of the ion gun 61 was set to 1.5 kV. The irradiated ions enter the optical surface 11a of the optical element 11 from a direction D2 inclined at an angle β = 70 ° with respect to the optical axis OA.

In this embodiment, the antireflection film 1 composed of seven optical thin films 2a to 2g is formed on the convex optical surface 11a of the optical element 11 in exactly the same manner as in the first embodiment except that the evaporation source 51 is changed. Was formed. In the present embodiment, the following was used as the evaporation source 51. The first, third, fifth and seventh optical thin films 2a, 2c, 2e and 2g were formed using SiO ₂ . TiO ₂ was used for forming the second, fourth and sixth optical thin films 2b, 2d and 2f.

  In the present embodiment, first, the antireflection film 1 composed of seven layers of optical thin films 2a to 2g was formed on the convex optical surface 11a of the optical element 11 exactly in the same manner as in the second embodiment. Thereafter, a functional film (antifouling film) made of perfluorocarbon was formed to a thickness of 5 nm on the optical thin film 2g of the seventh layer which is the final layer of the antireflection film 1. Resistance heating was used to form the functional film. In forming the functional film, the ion source 61 was not used.

The present embodiment is similar to the second embodiment except that the material of the optical element 11 is changed and that a seven-layer optical thin film 2a to 2g is formed after forming a protective film on the convex optical surface 11a. The antireflection film 1 was formed in exactly the same manner as in Example 1. In this embodiment, the optical element 11 made of the FCD 1 is used. First, oxygen gas is introduced into the film forming apparatus 21 to adjust the degree of vacuum to 1.5 × 10 ⁻² Pa, and SiO ₂ is used as the evaporation source 51 in exactly the same manner as the above-described film forming step. ₂ was formed on the optical surface 11a. In forming the protective film, the ion source 61 was not used. Thereafter, in exactly the same manner as in Example 2, an antireflection film 1 comprising seven layers of optical thin films 2a to 2g was formed on the protective layer.

  In the present embodiment, the convex optical surface of the optical element 11 is exactly the same as the embodiment 2 except that the inclination of the film forming surface 43a of the optical element holder 43, the output of the ion gun, and the film forming conditions are changed. An antireflection film 1 composed of seven optical thin films 2a to 2g was formed on 11a. The inclination of the film forming surface 43 was changed so as to incline by 45 ° with respect to the vertical direction. Accordingly, the angle α between the incident direction of the deposition material on the optical surface 11a and the optical axis OA is 45 °, and the angle β between the incident direction of ions on the optical surface 11a and the optical axis OA is 45 °. Became. The output of the ion gun was set to an acceleration voltage of 700 V. Then, in order to balance the film forming step and the irradiation step, the film forming conditions in the film forming step were changed, and the deposition rate of the deposition material was adjusted.

Comparative example

[Comparative Example 1]
In this comparative example, an antireflection film 1 composed of seven layers of optical thin films 2a to 2g was formed on a convex optical surface 11a of an optical element 11 using a general film forming apparatus different from that in Example 1.

In this comparative example, BMC1300 manufactured by SYNCHRON Co., Ltd., which is a general-purpose film forming apparatus, was used as the film forming apparatus. This film forming apparatus includes a dome in which the optical element 11 is arranged, and a deposition source 51 and an ion source 61 identical to those used in the first embodiment, in a film forming chamber in which the inside can be kept in a vacuum.
This dome has a dome shape unlike the optical element holder used in the first embodiment. The dome is suspended from the ceiling wall of the film forming chamber in an upwardly convex posture, and rotates around its center as a rotation axis. In the dome, the inner concave surface is a film forming surface, and the optical element 11 is arranged on the film forming surface. In the dome used in this comparative example, 300 or more optical elements 11 can be arranged around the rotation axis of the film formation surface. Since the film forming surface is concave, the optical element 11 attached to the film forming surface has a posture in which its optical axis OA is inclined by 5 to 30 ° with respect to the vertical direction depending on the arrangement.

  Then, the film forming step and the irradiation step were performed in the same manner as in Example 1. The film forming process is performed except that the evaporation material evaporated from the evaporation source 1 mainly enters the optical surface 11a of the optical element 11 from the direction D1 inclined at an angle α = 5 to 30 ° with respect to the optical axis OA. The operation was performed in exactly the same manner as in Example 1. The irradiation step was performed in the same manner as in Example 1 except that the ions irradiated by the ion gun were incident on the optical surface 11a of the optical element 11 from the direction D2 inclined at an angle β = 10 to 40 ° with respect to the optical axis OA. And performed exactly the same.

[Comparative Example 2]
In the present comparative example, the optical element 11 is exactly the same as the first embodiment except that the mounting position of the optical element 11 in the optical element holder 43 is changed and the irradiation condition of the ion gun 61 is changed. An antireflection film 1 composed of seven layers of optical thin films 2a to 2g was formed on the convex optical surface 11a. In this comparative example, first, one optical element 11 was arranged on the rotation axis L2 of the optical element holder 43. Then, the installation position of the ion gun 61 was changed such that the ions irradiated by the ion gun 61 were incident on the optical surface 11a of the optical element 11 from the direction of the angle β = 20 ° with respect to the optical axis OA. Accordingly, the angle β between the direction of incidence of the ions on the optical surface 11a and the optical axis OA has become 20 °. Furthermore, in ion irradiation by the ion gun 61, oxygen gas was ionized and irradiated without introducing Ar gas. At this time, the acceleration voltage of the ion gun 61 was 700 V.

[Comparative Example 3]
In this comparative example, seven layers of optical thin films were formed on the convex optical surface 11a of the optical element 11 in exactly the same manner as in Comparative Example 2, except that the mounting position of the optical element 11 in the optical element holder 43 was changed. An antireflection film 1 made of 2a to 2g was formed. In this comparative example, as in the first embodiment, two or more optical elements 11 are placed around the rotation axis L2 of the optical element holder 43 and at a position where the rotation axis L2 does not coincide with the optical axis OA of the optical element 11. Placed.

[Comparative Example 4]
In this comparative example, the reflections composed of seven layers of optical thin films 2a to 2g were formed on the convex optical surface 11a of the optical element 11 in exactly the same manner as in Example 2 except that the irradiation conditions of the ion gun 61 were changed. The prevention film 1 was formed. In this comparative example, similarly to Comparative Example 2, when performing ion irradiation by the ion gun 61, oxygen gas was ionized and irradiated without introducing Ar gas, and the acceleration voltage was set to 700V.

〔Evaluation item〕
With respect to the obtained antireflection films 1 of Examples 1 to 5 and Comparative Examples 1 to 4, the following evaluation was performed.
(1) Film Thickness Distribution of Optical Thin Film The film thickness of each of the optical thin films 2a to 2g constituting the antireflection film 1 was measured by a cross-sectional SEM to obtain a film thickness distribution. The measurement of the film thickness was performed on the optical thin films 2a to 2g at locations corresponding to the locations where the inclination angles were 0 °, 25 °, 35 °, 45 °, and 60 ° on the optical surface 11a. The thickness distribution of the optical thin films 2a to 2g of each layer was almost the same if the material was the same, regardless of the order of the layer. Then, the average film thickness distribution of the optical thin films 2a to 2g was determined from the film thickness distribution for each material of the optical thin film. FIG. 6 shows the results. The broken line in FIG. 6 indicates d (min) / d (max) = cos (5 / 6θ). θ is the inclination angle of the measurement point.

(2) Filling rate of optical thin film First, the reflectance of each layer of the obtained optical thin films 2a to 2g was measured using a reflection spectral thickness meter FE3000 manufactured by Otsuka Electronics Co., Ltd. Subsequently, the refractive index was calculated from the obtained reflectance data and the physical film thickness obtained in (1). In addition, for each material used for forming the optical thin films 2a to 2g of each layer, a single-layer film was formed on the optical surface 11a in the same manner as the film formation. For a single layer film for each material, the refractive index and the film thickness of each layer were calculated using an eprisometer M-2000 of JA Woollam, and the refractive index and the film thickness were calculated for each optical thin film 2a to FE3000 and SEM. It was confirmed that they matched the refractive index and the film thickness of 2 g. The calculation of the refractive index and the film thickness was performed on portions of the single-layer film corresponding to the portions having the inclination angles of 0 °, 25 °, 35 °, 45 °, and 60 ° on the optical surface 11a. Then, the equation of Thin-films optical filters described above _{(p = (n eff -n air} ) / (n s -n air)), were calculated filling factor of the optical thin film layers. The calculation of the filling rate was performed on the portions of the optical thin films 2a to 2g corresponding to the portions having the inclination angles of 0 °, 25 °, 35 °, 45 °, and 60 ° on the optical surface 11a. Here, in the equation of Thin-films optical filters, n _eff (refractive index of the optical thin film) is a refractive index for each material of the optical thin films 2a to 2g obtained by simulation. n _air (refractive index of air) is 1. As for _ns (the original refractive index of the film when there is no void in the film), Al ₂ O ₃ is 1.64, La ₂ O ₃ + TiO ₂ is 2.10, and SiO ₂ is 1.48. And Nb ₂ O ₅ is 2.30. These can be checked from the manufacturer's catalog values and literature. FIG. 7 shows the result of the optical thin film having the lowest filling rate among the optical thin films of the materials of the optical thin films 2a to 2g. The broken line in FIG. 7 indicates that the filling rate is 90%.

(3) L ^* a ^* b ^* color system The antireflection films 1 of Examples 1 to 5 and Comparative Examples 1 to 4 were measured by a reflection spectral thickness meter FE-3000 manufactured by Otsuka Electronics Co., Ltd. The reflectance was roughly measured, and the L value, a value, and b value in the CIE1976 L ^* a ^* b ^* color system were determined. The measurement of the L value, the a value, and the b value is performed on a portion of the antireflection film 1 corresponding to a portion having an inclination angle of 0 °, 25 °, 35 °, 45 °, 60 ° on the optical surface 11a. went. Then, two of the five measured points are selected, and from the difference a of the a value Δa and the difference b of the b value at the two points, (Δa ² + Δb ² ) ¹ which is the left side of the conditional expression (2) described above. ^{/ 2} was calculated, and the maximum value was calculated. Table 1 shows the results.

(4) Spectral Reflectance The antireflection films 1 of Examples 1 to 5 and Comparative Examples 1 to 4 were measured for spectral reflectance by a reflection spectral thickness meter FE-3000 manufactured by Otsuka Electronics Co., Ltd. In the measurement of the spectral reflectance, positions corresponding to the positions of the antireflection film 1 having the inclination angles of 0 °, 25 °, 35 °, 45 °, and 60 ° on the optical surface 11a were determined as measurement positions. Then, the angle of incidence of the incident light on the central portion of the antireflection film 1 was set to 0 °, and the wavelength range of the incident light was changed within a range from 350 nm to 850 nm. The results are shown in FIGS.

(5) Reliability Test The antireflection films 1 of Examples 1 to 5 and Comparative Examples 1 to 4 were measured for the above-described spectral reflectance, and then subjected to an environment at a temperature of 60 ° C. and a humidity of 90%. For 240 hours. Thereafter, the spectral reflectance was measured again, and the change was examined. Here, the portion of the antireflection film 1 corresponding to the portion where the inclination angle is 0 ° on the optical surface 11a was set as the measurement portion. The results are shown in Table 1 and FIGS.

  Further, the antireflection film 1 of Examples 1 to 4 and Comparative Examples 1 to 4 which were allowed to stand in the above environment was weighed 500 g by a reciprocating abrasion tester TYPE30 manufactured by Shinto Kagaku Co., Ltd. A durability test was performed by reciprocating 100 times at a moving speed of 1200 mm / min for a moving distance of 10 mm with a load applied. The durability test was performed at positions corresponding to the central portion (position at an inclination angle of 0 °) and the peripheral portion (position at an inclination angle of 60 °) of the optical element 11. Thereafter, the presence or absence of scratches generated on the surface of the antireflection film 1 (the functional layer in Example 3) was visually observed, and the durability was evaluated. Table 1 shows the results. In the column of “Durability” in Table 1, a mark “○” means that no flaw was observed in both the central part and the peripheral part. An “x” mark means that a flaw was observed in at least one of the central part and the peripheral part.

〔Evaluation results〕
Hereinafter, evaluation results of each example and comparative example will be described.
(1) Evaluation Results Regarding Film Thickness Distribution of Optical Thin Film The film thickness distributions of the optical thin films 2a to 2g of Examples 1 to 5 and Comparative Examples 1 to 4 will be described with reference to FIG. As shown in FIG. 6A, in the optical thin films 2a to 2g of Examples 1 to 5, the film thickness ratio d (min) / d (max) gradually increases as the inclination angle θ of the measurement point increases. However, the film thickness ratio is not less than cos (5θ / 6) and not more than 1.0 in the range of the inclination angle θ of 0 ° or more and 60 ° or less, and satisfies the conditional expression (3). . In particular, the optical thin films 2a to 2g of Examples 1 to 5 have a film thickness ratio of 0.91 or more even at a measurement point where the inclination angle θ is 60 °. From the above, the optical thin films 2a to 2g of Examples 1 to 5 have excellent film thickness uniformity from the center to the periphery.

  As shown in FIG. 6B, the optical thin films 2a to 2g of Comparative Example 2 have a film thickness ratio of 95% or more even at a measurement point where the inclination angle is 60 °, and have excellent film thickness uniformity. . However, in the optical thin films 2a to 2g of Comparative Example 1, Comparative Example 3, and Comparative Example 4, the film thickness ratio sharply decreases as the inclination angle θ of the measurement point increases, and the inclination angle θ is 25 ° or more. At the measurement point, the film thickness ratio falls below cos (5θ / 6) and does not satisfy the conditional expression (3). In particular, the film thickness ratio is reduced to about 0.50 at the measurement point where the inclination angle is 60 °. From the above, the optical thin films 2a to 2g of Examples 1 to 5 have poor film thickness uniformity.

(2) Evaluation Results Regarding Filling Ratio of Optical Thin Film The filling ratios of the optical thin films 2a to 2g of Examples 1 to 5 and Comparative Examples 1 to 4 will be described with reference to FIG. As shown in FIG. 7A, the optical thin films 2a to 2g of Examples 1 to 5 achieve a filling rate of 90% or more at measurement points where the inclination angle θ is 0 ° or more and 60 ° or less. I have. A high filling rate in the optical thin films 2a to 2g means that there are few voids.

  Further, as shown in FIG. 7B, the optical thin films 2a to 2g of Comparative Examples 1 and 4 are similar to or slightly inferior to Examples 1 to 5, but have an inclination angle θ. The filling rate has reached 90% at the measurement points from 0 ° to 60 °. However, the optical thin films 2a to 2g of Comparative Examples 2 and 3 have low filling rates and do not reach 90%.

^{(3) L * a * b} * with reference to the evaluation results in Table 1 of the color system, ^L * ^a * ^{b *} Table antireflection film 1 of Example 1 to Example 5 and Comparative Examples 1 to 4 The color system will be described. The L value of the antireflection film 1 of Examples 1 to 5 is always less than 5, regardless of the inclination angle θ. From this, it can be understood that the antireflection films 1 of Examples 1 to 5 suppress reflection at any position where the inclination angle θ is 0 ° or more and 60 ° or less. In the antireflection film 1 of Examples 1 to 5, the value of (Δa ² + Δb ² ) ^{1/2 on} the left side of the conditional expression (2) is less than 5. From this, it can be understood that the antireflection films 1 of Examples 1 to 5 have small unevenness of the reflection color over the entire area from the center to the periphery.

On the other hand, in the antireflection films 1 of Comparative Examples 1 to 4, the L value is less than 5 at the measurement point where the inclination angle θ is 0 ° or more and 25 ° or less, but the inclination angle θ is 35 ° or more. The L value exceeds 5 at the measurement location. From this, in the antireflection film 1 of Comparative Examples 1 to 4, reflection is suppressed at a position where the inclination angle θ is 0 ° or more and 25 ° or less, but at a position where the inclination angle is 35 ° or more. It can be seen that the reflection is not suppressed. In particular, in the antireflection films 1 of Comparative Examples 1, 3 and 4, the L value is about 25 at a position where the inclination angle θ is 35 °, and about 40 at a position of 60 °. From this, it can be understood that the antireflection films 1 of Comparative Example 1, Comparative Example 3, and Comparative Example 4 have extremely large reflected light at a position where the inclination angle is 35 ° or more. Further, in the antireflection films 1 of Comparative Examples 1 to 4, the value of (Δa ² + Δb ² ) ^1/2 exceeds 5. From this, it can be understood that the antireflection films 1 of Comparative Examples 1 to 4 have large unevenness of the reflection color. In particular, in the antireflection films 1 of Comparative Examples 1, 3 and 4, the value of (Δa ² + Δb ² ) ^1/2 is around 40. From this, it can be understood that the antireflection films 1 of Comparative Example 1, Comparative Example 3, and Comparative Example 4 have extremely large unevenness of the reflection color.

(4) Results of Spectral Reflectance The spectral reflectances of the antireflection films 1 of Examples 1 to 2 and Comparative Example 1 will be described with reference to FIGS. As shown in FIG. 8, in the antireflection film 1 of Example 1, the reflectance for light having a wavelength of 420 nm or more and 680 nm or less at an incident angle of 0 ° is 0.6% or less regardless of the inclination angle θ. From this, it can be understood that the antireflection film 1 of Example 1 has extremely excellent antireflection characteristics over the entire region from the center to the periphery. As shown in FIG. 9, the antireflection film 1 of Example 2 has a reflectance of 1% or less for light having a wavelength of 420 nm or more and 680 nm or less at an incident angle of 0 ° regardless of the inclination angle θ. From this, it can be understood that the antireflection film 1 of Example 2 also has excellent antireflection characteristics over the entire region from the center to the periphery.

  On the other hand, as shown in FIG. 10, in the antireflection film of Comparative Example 1, in the measurement at an inclination angle of 0 °, the reflectance for light having an incident angle of 0 ° and a wavelength of 420 nm or more and 680 nm or less was 0.5% or less. It is. However, as the inclination angle θ of the measurement point increases, the reflection characteristic shifts to the shorter wavelength side, and the reflection cannot be suppressed at the longer wavelength side. Even at a measurement point where the inclination angle θ is 25 °, the reflectance exceeds 1% for light having a wavelength longer than 575 nm. From this, it can be understood that the antireflection film 1 of Comparative Example 1 has inferior antireflection characteristics.

(5) Results of Reliability Test The reliability of the antireflection film 1 of Examples 1 to 5 and Comparative Examples 1 to 4 will be described with reference to Table 1 and FIGS. First, as shown in Table 1, the antireflection films 1 of Examples 1 to 5 and Comparative Examples 1 and 4 were subjected to an environment before and after the reliability test, that is, in an environment at a temperature of 60 ° C. and a humidity of 90%. The reflection characteristics did not change before and after standing for 240 hours. FIGS. 11 and 12 also show that the antireflection films 1 of Examples 1 and 2 did not change the reflection characteristics. On the other hand, as shown in Table 1, the antireflection films 1 of Comparative Examples 2 and 3 had deteriorated reflection characteristics. FIG. 13 also shows that the antireflection film 1 of Comparative Example 2 had deteriorated reflection characteristics. Then, as shown in Table 1, the antireflection films 1 of Examples 1 to 5 are excellent in durability without being scratched by a durability test. On the other hand, the antireflection films 1 of Comparative Examples 2 and 3 are flawed and have poor durability. From the above, it can be understood that the antireflection films 1 of Examples 1 to 5 are more reliable than the antireflection films 1 of Comparative Examples 2 and 3.

  Furthermore, comparing Table 1 and FIG. 7, in the antireflection films 1 of Examples 1 to 5 and Comparative Examples 1 and 4 which are excellent in durability, the optical thin films 2 a to 2 g constituting the antireflection film 1 The filling rate is 94% or more. On the other hand, in the antireflection films 1 of Comparative Examples 2 and 3 having inferior durability, the filling ratio of the optical thin films 2a to 2g is 90% or less. From this, it can be understood that the durability of the antireflection film 1 is related to the filling rate of the optical thin films 2a to 2g constituting the antireflection film 1.

(6) Overall Evaluation From the above results, the antireflection films 1 of Examples 1 to 5 have a low reflectance over the entire region from the central part to the peripheral part, have small unevenness of the reflected color, and have excellent durability. It became clear. Further, it has been clarified that the thickness distribution of the optical thin films 2a to 2g constituting the antireflection film 1 is related to the suppression of reflection and the suppression of uneven reflection color in the antireflection film 1. It has been found that the durability of the antireflection film 1 is related to the filling rate of the optical thin films 2a to 2g. Furthermore, comparing Example 2 and Example 3, the antireflection film 1 formed by the method of the present example suppresses reflection regardless of whether a functional layer is present thereon, and It can be understood that color unevenness can be reduced. In addition, comparing Example 2 and Example 4, the antireflection film 1 formed by the method of the present example can reflect light regardless of whether or not a protective layer exists between the optical surface 11a. It can be understood that the reflection can be suppressed and the unevenness of the reflection color can be reduced.

  Subsequently, the details of the film forming conditions will be discussed. First, the angles α and β will be considered. In both Example 1 and Comparative Example 1, when performing ion irradiation with the ion gun 61, Ar gas was introduced, Ar gas and oxygen gas were ionized, and the accelerating voltage was 1.5 kV. I have. However, Example 1 is different in that the angle α is 70 ° and the angle β is 70 °, whereas Comparative Example 1 is that the angle α is 30 ° and the angle β is 30 °. The optical thin films 2a to 2g of Example 1 have a high filling rate and excellent film thickness uniformity (see FIG. 6A), whereas the optical thin films 2a to 2g of Comparative Example 1 have a filling rate of Is high, but the film thickness uniformity is poor (see FIG. 6B). From this, it can be understood that in order to obtain the optical thin films 2a to 2g having a high filling factor and excellent film thickness uniformity, it is better to set the angle α to 70 ° and the angle β to 70 °. Further, from the fifth embodiment, it can be understood that the angle α may be set to 45 ° and the angle β may be set to 45 °.

  Next, the irradiation atmosphere of the ion gun 61 will be discussed. Example 5 and Comparative Example 4 are identical in that the acceleration voltage is 700 V when the ion gun 61 is used. However, Example 5 was different in that Ar gas was introduced to ionize Ar gas and oxygen gas during ion irradiation, whereas Comparative Example 4 was ionized with oxygen gas without introducing Ar gas. The optical thin films 2a to 2g of Example 5 have excellent film thickness uniformity (see FIG. 6A), whereas the optical thin films 2a to 2g of Comparative Example 4 have poor film thickness uniformity (see FIG. 6A). 6 (b)). From this, it can be understood that in order to obtain the optical thin films 2a to 2g having excellent film thickness uniformity, it is preferable to ionize Ar gas and oxygen gas to perform ion irradiation.

Next, the arrangement position of the optical element 11 with respect to the rotation axis L2 of the optical element holder 43 will be discussed. A comparative example 2 in which the optical element 11 is arranged on the rotation axis L2 of the optical element holder 43 is compared with a comparative example 3 in which the optical element 11 is arranged around the rotation axis L2. As shown in Table 1, in Comparative Example 2, the (Δa ² + Δb ² ) ^1/2 value of the antireflection film 1 was 6.48 although not less than 5, which is better than 38.80 in Comparative Example 3. . In both Comparative Examples 2 and 3, the durability of the antireflection film 1 is insufficient. Therefore, although the antireflection film 1 of Comparative Example 2 has a problem in durability, it has an advantage that unevenness of the reflection color is small. However, arranging the optical elements 11 on the rotation axis L2 of the optical element holder 43 as in Comparative Example 2 is not preferable in that the number of optical elements 11 that can be arranged is limited and productivity is low. On the other hand, if a plurality of optical elements 11 are arranged around the rotation axis L2 of the optical element holder 43 as in Comparative Example 3 and Examples 1 to 5, the plurality of optical elements 11 This is preferable because a film can be formed and productivity can be improved.

  INDUSTRIAL APPLICABILITY The antireflection film and the method of forming the same according to the present invention are suitable for various optical elements such as a photographing optical element and a projection optical element having a deep R convex optical surface having a maximum inclination angle of 25 ° or more. It is.

REFERENCE SIGNS LIST 1 antireflection film 2 optical thin film 2a first-layer optical thin film 2b second-layer optical thin film 2c third-layer optical thin film 2d fourth-layer optical thin film 2e fifth-layer optical thin film 2f sixth-layer optical thin film 2g Optical thin film of the seventh layer 11 Optical element 11a Convex optical surface 51,111 Evaporation source 61,121 Ion gun (ion source)
D1 Incident direction when vapor deposition material enters the convex optical surface side D2 Incident direction when ion or plasma enters the convex optical surface side OA Optical axis of optical element α Vapor deposition substance with respect to optical axis of optical element The angle formed by the incident direction of ions or plasma with respect to the optical axis of the optical element.

Claims (13)

An optical element having a convex optical surface having a maximum inclination angle of 25 ° or more, an antireflection film having a multilayer structure provided on the convex optical surface side,
Each of the optical thin films constituting the multilayer structure has a filling rate of 90% or more at an arbitrary position,
The L value in the L ^* a ^* b ^* color system of CIE1976 satisfies the following conditional expression (1), and the difference a between the a values at any two places and the difference between the b values at the two places are represented by the following conditions: An antireflection film characterized by satisfying the expression (2).
L <5 ... (1)
(Δa ² + Δb ² ) ^1/2 <5 (2) The anti-reflection film according to claim 1, wherein the optical thin film has a minimum value d (min) and a maximum value d (max) of the film thickness satisfying the following conditional expression (3).
cos (5θ / 6) ≦ d (min) / d (max) ≦ 1.0 (3)
Here, θ is the inclination angle of the convex optical surface at the measurement location.   The anti-reflection film according to claim 1, wherein the optical thin film includes at least one rare gas element selected from the group consisting of He, Ne, Ar, Xe, and Xr. 4. The multilayer structure includes an optical thin film that is a high refractive index layer and an optical thin film that is a low refractive index layer,
2. The high refractive index layer includes one or more metal oxides selected from the group consisting of TiO ₂ , Nb ₂ O ₅ , ZrO ₂ , La ₂ O ₃ , Ta ₂ O ₅ , and HfO _2. The anti-reflection film according to claim 3. The multilayer structure includes an optical thin film that is a high refractive index layer and an optical thin film that is a low refractive index layer,
The low refractive index _layer, SiO 2, _Al 2 O ₃ antireflection film as claimed in any one of claims 4 are those containing at least one metal oxide selected from the group of.   The antireflection film according to any one of claims 1 to 5, wherein the reflectance at an arbitrary point with respect to light having a wavelength of 420 nm or more and 680 nm or less at an incident angle of 0 ° is 1% or less. An optical element having a convex optical surface having a maximum inclination angle of 25 ° or more,
An optical element comprising the anti-reflection film according to any one of claims 1 to 6 provided on the convex optical surface.   The optical element according to claim 7, further comprising: a protective layer between the convex optical surface and the antireflection film, for preventing ions or plasma from entering the convex optical surface.   The optical element according to claim 7, further comprising a functional film provided on the antireflection film. A film forming method for forming an antireflection film having a multilayer structure on a convex optical surface side having a maximum inclination angle of an optical element of 25 ° or more,
A film forming step of forming a film by depositing a film forming material from a film forming source on the convex optical surface side of the optical element while rotating the optical element;
The convex optical surface side of the rotating optical element was deposited on the convex optical surface side by irradiating ions from an ion source or plasma from a plasma source from a direction inclined with respect to the optical axis. The film is densified while removing the film forming material, and the convex optical surface on the side far from the ion source or the plasma source is formed by the region of the convex optical surface on the side close to the ion source or the plasma source. An irradiation step of shielding the ions or the plasma from entering the region,
A film forming method of an anti-reflection film, wherein each of the optical thin films constituting the multilayer structure is formed on the convex optical surface side of the optical element by performing the film forming step and the irradiation step.   The irradiating step irradiates the ions or the plasma to the convex optical surface side of the optical element from a direction inclined at an angle of 45 ° to 90 ° with respect to the optical axis. 3. The method for forming an antireflection film according to item 1.   The method according to claim 10, wherein the optical element rotates about an axis different from the optical axis as a rotation axis.   The ion or plasma according to any one of claims 10 to 12, wherein the ion or plasma is an ion or plasma formed from one or more rare gases selected from the group consisting of He, Ne, Ar, Xe, and Xr. A method for forming an antireflection film.

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