WO2020179189A1

WO2020179189A1 - Antireflection film, optical element, and method for forming antireflection film

Info

Publication number: WO2020179189A1
Application number: PCT/JP2019/049550
Authority: WO
Inventors: 穣澁谷; 和生川俣; 涼橋本
Original assignee: 株式会社タムロン
Priority date: 2019-03-06
Filing date: 2019-12-18
Publication date: 2020-09-10
Also published as: JP7349798B2; JP2020144208A

Abstract

The antireflection film according to the present invention is provided in an optical element having a convex optical surface. The maximum inclination angle of an inclination angle θ of the convex optical surface, which is shown by an angle formed by a normal line and an optical axis at a measurement location, is 25° or more. The antireflection film is characterized by having a multilayer structure composed of a plurality of optical thin films provided on the convex optical surface side, wherein, in each of the optical thin films, when a point at which the convex optical surface and the optical axis intersect at right angles is defined as a center, at least part of the film provided in a region where the inclination angle θ of the convex optical surface is 25° or more is thicker than the physical film thickness of the film provided in the center of the convex optical surface.

Description

Antireflection film, optical element, and method for forming antireflection film

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

Conventionally, an optical element having a multi-layered antireflection film on a convex optical surface is known. Each optical thin film constituting the multilayer structure is usually formed by a vacuum deposition method or a sputtering method. In the optical thin film formed by such a method, the physical film thickness dx at an arbitrary measurement location generally follows the equation of dx=d0cosθ, and the physical film thickness at the peripheral portion is thinner than at the center. Here, d0 is the physical film thickness of the film provided at the center of the convex optical surface, and θ is the inclination angle of the convex optical surface indicated by the angle formed by the normal line and the optical axis at the measurement location. ..

Since the physical film thickness of the peripheral part is thinner than that of the center, the reflection characteristics of the peripheral part shift to the short wavelength side. Further, since the incident angle of light with respect to the optical surface becomes large in the peripheral portion, the reflection characteristic of the peripheral portion shifts to the short wavelength side. Therefore, when an antireflection film in which an optical thin film whose physical film thickness in the peripheral portion is thinner than the center is laminated is provided on an optical element whose convex optical surface has a maximum inclination angle θ of 25° or more, Due to the influence of both the physical film thickness and the incident angle, there is a problem that the reflection characteristic of the peripheral portion is largely shifted to the short wavelength side, and a ghost is generated.

In order to improve such optical characteristics, in recent years, an optical thin film having a uniform physical film thickness from the center to the periphery has been proposed. For example, in Patent Document 1, a film is formed while irradiating ions or plasma onto a convex surface of an optical element that rotates about the optical axis in a posture in which the lens optical axis is inclined by 70° with respect to the vapor deposition source. Membrane methods are disclosed. In Patent Document 2, when vapor deposition particles from a single evaporation source are attached to the convex surface of an optical element that revolves around other axes while rotating about the optical axis, the vapor deposition particles with a predetermined shape are shielded by a shielding plate. A film forming method for shielding the adhesion of the particles is disclosed. In Patent Document 3, a sputtering process step of forming sputtered particles emitted from a metal target on a concave surface of an optical element to form a metal film, and irradiating the metal film with an ion beam to emit the metal film A film forming method is disclosed in which a resputtering treatment step of depositing sputtered particles on the concave surface of the optical element again and an oxidation treatment step of irradiating the metal film with an oxygen radical beam to perform an oxidation treatment are performed.

However, when an antireflection film in which an optical thin film having a uniform physical film thickness is laminated is provided on the optical element, the reflectance of light having a small incident angle φ can be suppressed to be low in a region having a small inclination angle θ. In the region where the inclination angle θ is large, there is a problem that the reflectance of light having a large incident angle φ cannot be suppressed low. Here, the incident angle φ is an angle formed by the normal line at the measurement point on the optical surface of the optical element and the incident direction of light.

Further, in the film forming method disclosed in Patent Document 1, since the physical film thickness is made uniform by rotating the optical element around the optical axis, it is possible to form a film on two or more optical elements at the same time. There is a problem that productivity is low. In the film forming method disclosed in Patent Document 2, since it is necessary to change the shape of the shielding plate depending on the size and shape of the optical element and the degree of contamination of the chamber affects the film formation, a desired film thickness can be obtained for a long time. There is a problem that it is difficult to obtain an optical thin film having The film forming method disclosed in Patent Document 3 has a problem that it cannot be applied to a convex optical surface of an optical element. This is because, when the metal film formed on the convex optical surface by the sputtering process is irradiated with an ion beam, the sputtered particles emitted from the convex optical surface move in a direction away from the convex optical surface, so that the sputtered particles are This is because the film cannot be formed again on the convex optical surface.

On the other hand, an optical thin film having a thicker physical film thickness in the peripheral portion than in the central portion has also been proposed. For example, in Patent Document 4, it is composed of three layers of optical thin films, the first and third layers of optical thin films have uniform physical film thickness in the central portion and the peripheral portion, and the second layer optical thin film has a central portion. There is disclosed an antireflection film having a larger physical film thickness in the peripheral portion than that. Each optical thin film that constitutes this antireflection film is formed by a diaphragm plate when vapor deposition particles from a plurality of evaporation sources adhere to a convex optical element that revolves around the other axis while rotating about the optical axis. The film thickness distribution is controlled by controlling the amount of evaporation from the evaporation source to limit the amount of deposition particles. The antireflection film disclosed in Patent Document 4 has a reflectance of 0.3% or less when light (ultraviolet rays) having a main wavelength of 193.4 nm is incident on the center of the lens at an incident angle of 0 to 35°. The reflectance when the ultraviolet rays are incident on the peripheral portion of the lens at an incident angle of 10 to 40° can be set to 0.3% or less, and the optical characteristics at all positions are determined by considering the incident angle of light. It is said to have such optical characteristics as an antireflection effect.

Japanese Unexamined Patent Publication No. 2006-91600 JP, 2010-138477, A Japanese Unexamined Patent Publication No. 2012-128321 Japanese Unexamined Patent Publication No. 2003-7585

However, although Patent Document 4 discloses antireflection performance when light having only a specific wavelength is incident, it does not disclose antireflection performance when light having a wide wavelength band is incident. .. Further, in the film forming method disclosed in Patent Document 4, a first evaporation source arranged directly below the revolution axis of the lens and a second evaporation source arranged directly below the revolution trajectory of the lens are used for a lens that revolves around its axis. A film is formed by the source and the third evaporation source. Therefore, the positional relationship between the lens and each evaporation source is important, and it is essential to arrange the lenses in a row on the orbit of the lens, that is, on the concentric circles of the revolution axes of the lens, which is inconvenient in terms of low productivity. There is.

Therefore, the present invention provides a wide wavelength band when light with a wide wavelength band is incident at a large incident angle in a region where the maximum tilt angle is 25° or more and the convex optical surface of the optical element has a large tilt angle. It is an object of the present invention to provide an antireflection film capable of suppressing light reflection low and an optical element provided with the antireflection film. A further object of the present invention is to provide a film forming method having excellent productivity of such an antireflection film.

Therefore, the inventors of the present invention have made the following inventions as a result of extensive studies to solve the above-mentioned problems.

That is, the antireflection film according to the present invention has a convex optical surface in which the maximum inclination angle θ of the convex optical surface indicated by the angle between the normal line and the optical axis at the measurement location is 25° or more. An antireflection film provided on an optical element, comprising a multilayer structure composed of a plurality of optical thin films provided on the convex optical surface side, each optical thin film has a point where the convex optical surface and the optical axis are orthogonal to each other. When centered, at least a part of the film provided in the region where the inclination angle θ of the convex optical surface is 25 ° or more is thicker than the physical film thickness of the film provided at the center of the convex optical surface. And

The optical element according to the present invention is an optical element having a convex optical surface having a maximum tilt angle of 25° or more, and is characterized in that the above-mentioned antireflection film is provided on the convex optical surface.

A method for forming an antireflection film according to the present invention is a method for forming an antireflection film for forming the above-described antireflection film, wherein the convex optical of the optical element is rotated while rotating the optical element. A film forming step of forming a film by depositing a film forming material from a film forming source on the surface side, and an ion from an ion source or a plasma from a plasma source on the convex optical surface side of the rotating optical element. By irradiating from a direction inclined with respect to the optical axis, the film-forming material deposited at the center of the convex optical surface side is densified while being removed more than the peripheral portion of the convex optical surface side, An irradiation step of shielding the ion or the plasma from being incident on the ion source or the region of the convex optical surface far from the plasma source by the region of the convex optical surface near the source or the plasma source. Each of the optical thin films constituting the multilayer structure is formed by performing the film forming step and the irradiation step.

The antireflection film of the present invention is provided with a multilayer structure composed of a plurality of optical thin films provided in an optical element having a convex optical surface having a maximum tilt angle of 25° or more, and each optical thin film is the convex optical surface. At least a part of the film provided in the region where the inclination angle θ of the surface is 25 ° or more is thicker than the physical film thickness of the film provided at the center of the convex optical surface. According to the present invention, since each optical thin film has the above-mentioned film thickness distribution, light having a wide wavelength band is large in a region where the inclination angle θ of the convex optical surface of the optical element having a maximum inclination angle of 25 ° or more is large. It is possible to provide an antireflection film capable of suppressing reflection of light in a wide wavelength band to be low when entering at an incident angle and an optical element including the antireflection film.

Then, in the method for forming an antireflection film of the present invention, the above-mentioned antireflection film can be formed by forming the optical thin film of each layer by performing the above-described film forming step and irradiation step. Therefore, according to the present invention, it is possible to provide a method for forming an antireflection film having excellent productivity.

It is a typical sectional view showing the antireflection film concerning the present invention. It is a schematic diagram of the film forming apparatus which carries out the film forming method of the antireflection film which concerns on this invention. It is a principal part enlarged view of the film-forming apparatus shown in FIG. It is the schematic of the film forming apparatus which carries out the film forming method of the antireflection film which concerns on this invention. (A) is a front view of the film-forming apparatus, (b) is a bottom view of the optical element supporting device as viewed from below. It is a figure which shows the optical element which provides the antireflection film of Examples 1, 3 and Comparative Example 1. It is a figure which shows the optical element which provides the antireflection film of Examples 2, 4 and Comparative Example 2. It is a graph which shows the film thickness distribution of each optical thin film which constitutes the antireflection film of Examples 1 and 2. (A) shows Example 1 and (b) shows Example 2. 6 is a graph showing the film thickness distribution of each optical thin film constituting the antireflection film of Examples 3 to 4. (A) shows Example 3 and (b) shows Example 4. It is a graph which shows the film thickness distribution of each optical thin film which constitutes the antireflection film of Comparative Examples 1 and 2. (A) shows Comparative Example 1 and (b) shows Comparative Example 2. 3 is a graph showing the spectral reflectance of the antireflection film of Example 1. (A) is the spectral reflectance of incident light having an incident angle φ of 0°, 10°, 20° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 0°. (B) shows the spectral reflection of incident light having incident angles φ of 20°, 30°, and 40° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 35°. Shows the ratio, (c) of the incident light having an incident angle φ of 40°, 50°, 60° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 60°. Shows spectral reflectance. 5 is a graph showing the spectral reflectance of the antireflection film of Example 2. (A) is the spectral reflectance of incident light having an incident angle φ of 0°, 10°, 20° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 0°. (B) shows the spectral reflection of incident light having incident angles φ of 20°, 30°, and 40° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 35°. Shows the ratio, (c) of the incident light having an incident angle φ of 50°, 60°, 70° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 70°. Shows spectral reflectance. 7 is a graph showing the spectral reflectance of the antireflection film of Example 3. (A) shows the spectral reflectance of incident light having incident angles φ of 0 °, 10 °, and 20 °, measured at a location corresponding to a location where the inclination angle θ of the optical surface on the antireflection film is 0 °. (B) shows the spectral reflection of incident light having incident angles φ of 20°, 30°, and 40° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 35°. Shows the ratio, (c) of the incident light having an incident angle φ of 50°, 60°, 70° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 70°. Shows spectral reflectance. 9 is a graph showing the spectral reflectance of the antireflection film of Example 4. (A) shows the spectral reflectance of incident light having incident angles φ of 0 °, 10 °, and 20 °, measured at a location corresponding to a location where the inclination angle θ of the optical surface of the antireflection film is 0 °. , (B) are the spectral reflectances of incident light having incident angles φ of 20°, 30°, and 40° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 35°. (C) is a spectrum of incident light having incident angles φ of 50°, 60°, and 70° measured at a position corresponding to a position where the tilt angle θ of the optical surface on the antireflection film is 70°. The reflectance is shown. It is a graph which shows the spectral reflectance of the antireflection film of Comparative Example 1. (A) is the spectral reflectance of incident light having an incident angle φ of 0°, 10°, 20° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 0°. (B) shows the spectral reflection of incident light having incident angles φ of 20°, 30°, and 40° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 35°. Indicates the rate. It is a graph which shows the spectral reflectance of the antireflection film of Comparative Example 1. (A) is the spectral reflectance of incident light having incident angles φ of 40°, 50°, and 60° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 60°. (B) shows the spectral reflection of incident light having incident angles φ of 50°, 60°, and 70° measured at a position corresponding to a position where the inclination angle θ of the optical surface on the antireflection film is 70°. Indicates the rate.

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

1. Anti-reflection film The anti-reflection film according to the present invention is a convex optical surface in which the maximum inclination angle θ of the inclination angle θ of the convex optical surface indicated by the angle between the normal line and the optical axis at the measurement point is 25 ° or more. An antireflection film provided on an optical element having a multi-layer structure composed of a plurality of optical thin films provided on the convex optical surface side, and each optical thin film is a point at which the convex optical surface and the optical axis are orthogonal to each other. When at least a part of the film provided in the region where the inclination angle θ of the convex optical surface is 25° or more is thicker than the physical film thickness of the film provided in the center of the convex optical surface. Characterize.

The antireflection film is provided on a convex optical surface where the maximum tilt angle of the optical element is 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 the maximum inclination angle of 25° or more means an optical surface in which there is a portion where the inclination angle θ is 25° or more when the inclination angle θ is measured in the convex optical surface. For example, when the convex optical surface having a maximum inclination angle of 25° or more is an optical surface whose inclination angle gradually increases from the center toward the peripheral portion, the inclination angle θ of the peripheral portion of the convex optical surface is θ. Is 25 ° or more. The convex optical surface may be a surface having a curvature or a free-form surface. With a convex optical surface having a maximum inclination angle of 25° or more and being a free-form surface, the angle γ/2 is 25° when the angle formed by the normals of any two points on the convex optical surface is γ. The above 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 antireflection film 1 shown in FIG. 1 is provided on the optical element 11 having a convex optical surface 11a having a maximum tilt angle of 90°, that is, a peripheral tilt angle θ of 90°. However, the maximum tilt angle of the antireflection film is not limited to 90°, and can be any angle as long as it is 25° or more. Hereinafter, the convex optical surface 11a may be abbreviated as "optical surface 11a". The antireflection film 1 shown in FIG. 1 has a multilayer structure in which seven layers of optical thin films 2 are laminated, and the number of layers can be any number as long as it is two or more layers. In each of the seven layers, the optical thin film 2 of the first layer, the optical thin film 2b of the second layer, the optical thin film 2c of the third layer, the optical thin film 2d of the fourth layer, in order from the side closer to the optical surface 11a, Also referred to as the fifth layer optical thin film 2e, the sixth layer optical thin film 2f, and the seventh layer optical thin film g. The hatching of the optical thin film 2 in FIG. 1 is omitted.

Each optical thin film forming the multilayer structure is a film provided in a region in which an inclination angle θ of the convex optical surface is 25° or more when centered on a point where the convex optical surface of the optical element and the optical axis are orthogonal to each other. At least a part of the film is thicker than the physical film thickness of the film provided at the center of the convex optical surface.

In the antireflection film according to the present invention, since each optical thin film forming the multilayer structure has the above-mentioned film thickness distribution, a region where the inclination angle θ of the convex optical surface of the optical element having the maximum inclination angle of 25° or more is large. When light with a wide wavelength band enters at a large incident angle, the reflection of light can be suppressed to a low level over a wide wavelength band.

Here, the antireflection performance will be described in detail. Normally, when light is incident on the convex optical surface of an optical element, light having a small incident angle φ is incident on a region where the tilt angle θ of the convex optical surface is small, while light is incident on a region where the tilt angle θ is large. Light with a large incident angle φ is incident. The larger the incident angle φ of light, the more the spectral characteristics shift to the shorter wavelength side. On the other hand, in the antireflection film according to the present invention, at least a part of the film provided in the region where the inclination angle θ of the convex optical surface is 25 ° or more is the physical film thickness of the film provided at the center of the convex optical surface. By being thicker than, the shift of the spectral characteristics to the short wavelength side can be canceled out. As a result, according to the antireflection film according to the present invention, when light having a wide wavelength band is incident at a large incident angle in a region where the inclination angle θ of the convex optical surface is large, the light is reflected over a wide wavelength band. Can be kept low.

Specifically showing the antireflection performance, the antireflection film according to the present invention has, for example, a wavelength in the region where the inclination angle θ of the convex optical surface of the optical element having the maximum inclination angle of 25° or more is 35°. When the light of 420 nm or more and 680 nm is incident at the incident angle φ of 20° or more and 40° or less, the reflectance of the light of the above wavelength can be suppressed to 1% or less. When light of the above wavelength is incident on the region where the inclination angle θ of the convex optical surface is 60° at an incident angle φ of 40° or more and 60° or less, the reflectance of light can be suppressed to 10% or less. .. When light of the above wavelength is incident on the region where the inclination angle θ of the convex optical surface is 70° at an incident angle φ of 70°, the reflectance of light can be suppressed to 20% or less.

Considering the effective region of the optical element, at least a part of the film provided in the region where the inclination angle θ of the convex optical surface is 25 ° or more and less than 80 ° is located at the center of the convex optical surface. It is preferably thicker than the physical film thickness of the film to be provided.

Then, in each optical thin film, the entire film provided in the region where the inclination angle θ of the convex optical surface is 25 ° or more and less than 75 ° is thicker than the physical film thickness of the film provided at the center of the convex optical surface. It is more preferable that it is a thing. The antireflection film formed by laminating such an optical thin film can more reliably obtain the effect of suppressing the reflection of light to a low level over the wide wavelength band described above.

Further, for each optical thin film, when the physical film thickness of the film provided at the center of the convex optical surface is d0 and the physical film thickness at an arbitrary measurement point of the optical thin film is dx, the inclination of the convex optical surface is set. Among the films provided in the region where the angle θ is 25° or more, the physical film thickness of a portion thicker than the physical film thickness of the film provided in the center of the convex optical surface satisfies the following conditional expression. preferable.
1<dx/d0≦1.3

In this conditional expression, in the film provided in the region where the inclination angle θ of the convex optical surface is 25° or more, the physical film thickness is thicker than the physical film thickness d0 of the film provided in the center of the convex optical surface. Then, the physical film thickness dx at an arbitrary measurement point is within the range of 1 to 1.3 times the physical film thickness d0 of the film provided at the center, and there is an excessively thick physical film portion at that part. It means that it does not exist. In that case, it is possible to more reliably suppress the reflection of light incident on a region where the inclination angle θ of the convex optical surface of the optical element is large at a large incident angle, and it is also possible to suppress the occurrence of ghosts. .. On the other hand, when the above conditional expression is not satisfied, that is, when there is a portion of dx/d0≦1 in the film provided in the region where the inclination angle θ of the convex optical surface of the optical element is 25° or more, It may not be possible to suppress the reflection of the light incident on the region, which is not preferable. In addition, when the film provided in the region where the inclination angle θ of the convex optical surface of the optical element is 25° or more exists in a portion where 1.3<dx/d0 exists, the spectral characteristics described above are shifted to the short wavelength side. Is excessively canceled to shift to the longer wavelength side, and a blue ghost may occur, which is not preferable.

The film thickness of the optical thin film can be measured with a cross-section SEM or a contact-type film thickness meter. 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.

By the way, in order to further reduce the reflectance, the antireflection film according to the present invention preferably includes an optical thin film having a high refractive index layer and an optical thin film having a low refractive index layer. Further, the antireflection film may include an intermediate refractive index layer having a refractive index intermediate between those of the high refractive index layer and the low refractive index layer. The high-refractive index layer, the low-refractive index layer, and the intermediate-refractive index layer can be appropriately combined to form an antireflection film.

The high refractive index layer preferably contains 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 . Such a high refractive index layer can realize a high refractive index of 2.0 or more. Further, the low refractive index layer preferably contains SiO ₂ alone or a mixture of SiO ₂ and Al ₂ O ₃ . Such a low refractive index layer can reduce the refractive index to 1.50 or less. The low refractive index layer as the final layer preferably contains SiO ₂ . Further, the intermediate refractive index layer preferably contains a metal oxide such as Al ₂ O ₃ , Y ₂ O ₃ , YbF ₂ , or a mixture such as Al ₂ O ₃ + La ₂ O ₃ . Such an intermediate refractive index layer can realize a refractive index of 1.50 or more and 2.0 or less. For example, in the antireflection film 1 shown in FIG. 1, the optical thin film 2a of the first layer, the optical thin film 2c of the third layer, the optical thin film 2e of the fifth layer, and the optical thin film 2f of the seventh layer are composed of SiO ₂ and have a low refractive index. The optical thin film 2b as the second layer, the optical thin film 2d as the fourth layer, and the optical thin film 2f as the sixth layer may be layers, and may be high refractive index layers made of TiO ₂ .

Each optical thin film constituting the antireflection film described above can be formed by a film forming method described later. Since ions or plasma is used in the film forming method, the optical thin film contains an element that constitutes ions or plasma. For example, when Ar is plasmatized to form 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, when the optical thin film contains 1×10 22 atomic%/cm ³ or more of Ar, the optical thin film may not be densified, which is not preferable.

2. Method of Forming Antireflection Film Next, an embodiment of the method of forming the antireflection film described above will be described. A method for forming an antireflection film according to the present invention is a method for forming an antireflection film for forming the above-described antireflection film, wherein the convex optical of the optical element is rotated while rotating the optical element. A film forming step of forming a film by depositing a film forming material from a film forming source on the surface side, and an ion from an ion source or a plasma from a plasma source on the convex optical surface side of the rotating optical element. By irradiating from a direction inclined with respect to the optical axis, the film forming material deposited at the center of the convex optical surface side is densified while being removed more than the peripheral portion of the convex optical surface, and the ion source Alternatively, an irradiation step of shielding the ion or the plasma from being incident on the ion source or the region of the convex optical surface far from the plasma source by the region of the convex optical surface near the plasma source. The optical thin film forming the multilayer structure is formed by performing the film forming step and the irradiation step.

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

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

The optical element support device 41 is hung from the ceiling wall of the film forming chamber 31, is a rotatable disk-shaped support base 42, and is hung on the peripheral portion of the support base 42, and is a rotatable disk-shaped optical element holder. And 43. The support base 42 rotates by the drive of a first motor (not shown). The axis of rotation 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. Further, the optical element holder 43 revolves around the rotation axis L1 of the support base 42 as a rotation axis when driven by the first motor. Six optical element holders 43 are arranged at equal intervals on the support base 42. However, in FIG. 2, only two optical element holders 43 are shown and the other optical element holders 43 are omitted.

In the present embodiment, the optical element holder 43 is suspended from the support base 42 so that the film forming surface 43a on which the optical element 11 is attached and the film is formed faces diagonally downward. The orientation of the film forming surface 43a is adjusted by the angle adjusting mechanism 44. FIG. 2 shows a state in which the film-forming surface 43a is tilted by 20 ° with respect to the vertical direction. On the film-forming surface 43a, a plurality of optical elements 11 are concentrically arranged 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. 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 on the outer circumference thereof. However, in FIG. 2, only two optical elements 11 are shown and the other optical elements 11 are omitted. As described above, in the present embodiment, the optical element support device 41 is provided with six optical element holders 43, and 14 optical elements 11 can be arranged in each optical element holder 43, so that a maximum of 84 optics can be arranged. It is possible to form a film on the element 11 at the same time. The optical element 11 attached to the film formation surface 43a has a posture in which the optical axis OA is inclined with respect to the vertical direction. In the present 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 they do not necessarily have to be parallel.

In the present embodiment, as shown in FIG. 2, the vapor deposition 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 vapor deposition source 51 is not limited to this position. For example, when performing side sputtering, the vapor deposition source 51 can be provided at a horizontal position with respect to the optical element holder 43. The thin-film deposition source 51 forms a thin-film deposition material that is a film-forming material by using an electron gun, resistance heating, a sputtering source, sputtering with an ion gun or a plasma gun, heating vapor deposition with a plasma gun, a chemical vapor deposition method, ion plating, or the like. Can be membrane. As the vapor deposition 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 vapor deposition material from the vapor deposition source 51 rises while spreading and is deposited on the optical surface 11 a of the optical element 11, the optical element holder 43, and the like. The vapor deposition substance enters the optical surface 11a side of the optical element 11 from various directions, but mainly enters from the vertically downward direction. That is, the vapor 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 vapor deposition material is incident on the optical surface 11a side of the optical element 11 from the direction D1 inclined at the angle α with respect to the optical axis OA. The angle α can be adjusted by changing the position of the vapor deposition source 51, the direction of the film formation surface 43a, and the like. FIG. 2 shows a state in which the angle α is 70°. The deposition rate of the vapor deposition material on the optical surface 11a side is determined by the pressure (vacuum degree) in the film deposition chamber 31, the position of the vapor deposition source 51, the film deposition conditions of the vapor deposition source 51 (temperature of resistance heating, 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 the bottom of the film forming chamber 21, is inside the revolution trajectory of the optical element holder 43, and is the vapor deposition source 51 with respect to the rotation axis L1 of the support substrate 42. It is provided on the opposite side. However, the position of the ion gun 61 is not limited to this position as long as self-shielding can be performed, as will be described later. The ion gun 61 irradiates ions at high speed. In the present embodiment, one or more rare gases or nitrogen selected from the group of He, Ne, Ar, Xe, and Xr and O ₂ are appropriately introduced into the ion gun 61, and these gases 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 so that the ions are incident on the optical surface 11a side of the optical element 11 from the 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 the acceleration voltage, the beam current, the beam voltage, the film forming pressure, the gas introduction species, the 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. First and second optical thin films 2a are formed on the optical surface 11a of the optical element 11 by repeating the following film forming step and irradiation step simultaneously or alternately, and subsequently, the second layer optical thin film is formed. The optical thin film 2g of the seventh layer is sequentially formed from 2b. Hereinafter, formation of the optical thin film 2a as the first layer will be described in detail.

(Film forming process)
The film forming process is performed as follows. In this embodiment, the optical thin film 2a of the first layer is formed by using Al ₂ O ₃ as the vapor deposition source 51. The vapor deposition source 51 is heated to evaporate the vapor-deposited substance (Al ₂ O ₃ ) in a state where the optical element 11 is rotated around the rotation axis L1 of the support substrate 42 and the rotation axis L2 of the optical element holder 43 as rotation axes. A film is formed by depositing the vapor-deposited substance on the optical surface 11a of the optical element 11. The vapor deposition substance is incident on the optical surface 11a from various directions, but is mainly incident on the optical surface 11a from a direction D1 inclined with respect to the optical axis OA. Since the optical element 11 is rotating as described above, the film thickness distribution of the film formed on the optical surface 11a of the optical element 11 substantially conforms to the above equation of dx=d0 cos θ. That is, the film formed on the optical surface 11a by the film forming process has the thickest physical film thickness at the center of the optical surface 11a, and the physical film thickness increases as the inclination angle θ increases and approaches the peripheral portion of the optical surface 11a. It is thin.

(Irradiation process)
The irradiation step is performed as follows. Ion irradiation is performed by the ion gun 61 in a state where the optical element 11 is 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 vapor deposition material deposited on the optical surface 11 a of the optical element 11, energy is imparted to the deposited vapor deposition material. As a result, at the position where the ions collide, the ions act as assisting the film formation, and the film formed on the optical surface 11a is densified. Further, the deposited material deposited on the optical surface 11a is removed, and the film is thinned. Some of the ions that have collided with the vapor 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 straightness is higher than that of the vapor-deposited substance from the thin-film deposition source 51. Therefore, the ions enter the optical surface 11a from the direction D2 that is 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 there is a portion having an inclination angle of 25° or more. Therefore, the region on the optical surface 11a near the ion gun 61 shields the ion from entering the region on the optical surface 11a far from the ion gun 61 (the region R surrounded by the double chain line in FIG. 3). It 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. Here, the central portion of the optical surface 11a means a circular area within a predetermined distance from the center of the optical surface 11a of the optical element 11. On the other hand, in the peripheral portion of the optical surface 11a, the incident region and the shield region where the incident of ions is shielded are switched with the rotation of the optical element 11. As a result, a large amount of ions are incident on the central portion of the optical surface 11a as compared with the peripheral portion, and the amount of the vapor-deposited substance removed from the optical surface 11a is large. For example, 20% or more of the vapor deposition material deposited on the optical surface 11a is removed at the central portion of the optical surface 11a, whereas the removal amount at the peripheral portion of the optical surface 11a is only about several percent. In this 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 is moved up, down, left and right as the optical element holder 43 rotates. Also changes. However, since the ions irradiated by the ion gun 61 have high straightness, even if the ion irradiation position on the optical surface 11a changes with the rotation of the optical element holder 43, self-shielding is sufficiently generated. , It is possible to scrape a large amount of the vapor-deposited substance deposited on the central portion of the optical surface 11a.

As described above, the ion irradiation in the irradiation step reduces the thickness and densifies the film formed on the optical surface 11a in the film forming step. Then, by repeating the above-mentioned film forming step and irradiation step simultaneously or alternately, the film formed on the optical surface 11a gradually becomes thicker, and the film formed at the center of the optical surface 11a becomes thicker. Also, the physical film thickness of the film provided in the region where the inclination angle θ of the optical surface 11a is 25° or more becomes thicker. At this time, the conditions of the film forming process and the irradiation process are appropriately changed, and which of the film forming process and the irradiation process is given priority, and the process is repeated while keeping a balance. 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. Regarding the film thickness distribution of the optical thin film 2a of the first layer, the physical film thickness of the film provided in the region where the inclination angle θ of the optical surface 11a is 25 ° or more is larger than that of the film provided at the center of the optical surface 11a. It's getting thicker.

After that, the film forming step and the irradiation step are repeated while appropriately changing the material of the vapor deposition source 51, and the remaining optical thin films 2b to 2g are formed on the optical thin film 2a of the first layer. To do. The optical thin films 2b to 2g of the second to seventh layers have the same film thickness distribution as the optical thin film 2a of the first layer. As described above, the antireflection film 1 having a multilayer structure composed of optical thin films 2a to 2g can be formed on the optical surface 11a of the optical element 11 shown in FIG.

When the film forming process and the irradiation process are alternately performed, it is preferable to perform as follows. First, a film is formed by a film forming process. An irradiation step is performed before the film thickness reaches 10 nm, and the surface layer of the film is scraped off to reduce the thickness of the film to form a sublayer. By repeating the film forming process and the irradiation process, the sub-layers are laminated to form the first-layer 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 layer portion of the sub layer, while the effect of densification does not occur in the deep layer 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 forming step and the irradiation step at the same time rather than alternately. When simultaneously performed, only the dense layers are laminated, and the optical thin film 2a of the first layer that is uniform in the depth direction can be obtained.

In the apparatus configuration of the present embodiment, when the optical element 11 approaches the vapor deposition source 51, the film forming step 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, the irradiation process is mainly performed, and the film on the optical surface 11a becomes thin. The rate at which the film becomes thicker in the film forming step, that is, the deposition rate of the vapor deposition material can be controlled by the above-described film forming conditions. In addition, the rate at which the film becomes thin in the irradiation step, that is, the rate at which the vapor deposition material is removed by ions can be controlled by the ion irradiation energy described above.

Here, referring to FIG. 2, the angle α formed by the incident direction D1 of the vapor-deposited substance on the optical surface 11a and the optical axis OA of the optical element 11, and the incident direction D2 and the optical axis of the ions on the optical surface 11a. The angle β formed with the OA will be described. First, the angle α will be described. The vapor deposition material from the vapor deposition source 51 rises in a spread manner. When the optical element 1 is located directly above the vapor deposition source 51, the deposition amount of the vapor deposition substance on the optical surface 11a is the largest. For example, the angle between the optical axis OA of the optical element 11 and the vertical direction at that position is defined as the angle α. In the present embodiment, the angle α is 70 °, but the angle α is preferably 0 ° or more and 90 ° or less, and more preferably 45 ° or more and 90 ° or less. 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. It is not preferable because the deposition amount of the vapor deposition material on the center of the optical surface 11a is reduced and the film formation rate is slowed down. On the other hand, in order to form a film evenly on a plurality of optical elements 11 regardless of the mounting position on the optical element holder 43, it is preferable that the angle α is 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 plate glass. However, if the angle α is less than 45°, the deposition amount of the vapor deposition material in the central portion of the optical surface 11a increases while the deposition amount in the peripheral portion becomes excessively small in the film forming process. In that case, in order to make the physical film thickness provided in the region where the inclination angle θ of the optical surface 11a is 25° or more larger than that of the film provided in the center of the optical surface 11a, the optical surface 11a should be formed in the irradiation step. It is necessary to remove more of the film provided in the central part of the, and the irradiation process may take a long time. From the above, in order to shorten 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 explained. Since the ions emitted from the ion gun 61 have straightness, the amount of the ions incident on the optical surface 11a is the largest when the optical element 11 is located at a position facing the irradiation port of the ion gun 61. For example, the angle formed by the optical axis OA of the optical element 11 at that position and the line segment connecting the center of the optical surface 11a and the ion gun 61 is defined as the angle β. In the present embodiment, the angle β is 70 °, but the angle β is preferably 45 ° or more and 90 ° or less. Since the angle β is 45° or more, at least a part of the film provided in the region where the inclination angle θ of the optical surface 11a is 25° or more is more than the physical film thickness of the film provided in the center of the optical surface 11a. It is possible to realize a film thickness distribution that is thick. 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°, the self-shielding at the time of ion irradiation becomes insufficient and it is difficult to realize the above film thickness distribution, which is not preferable. On the other hand, when the angle exceeds 90°, the ion irradiation amount to the central portion of the optical surface 11a decreases, and it becomes difficult to scrape 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 about an axis different from the optical axis OA as a rotation axis. Specifically, the optical element 11 is rotated around the rotation axis L1 of the support base 42 and around the rotation axis L2 of the optical element holder 43 by the optical element support device 41 provided with the 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 in the vertical direction with the rotation of the optical element holder 43 with the rotation axis L2 as the axis. , Move left and right. At this time, the film thickness formed by the film forming step differs depending on the mounting position of the optical element 11 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 ion incident region and the shielding region on the optical surface 11a differs depending on the mounting position of the optical element 1. However, since the film forming step and the irradiation step 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 subjected to regardless of the mounting position of the optical element 11. The antireflection film 1 can be formed evenly. Therefore, the film forming method of the present embodiment is suitable for mass production of the antireflection film 1. The optical element 11 may be rotated so that self-shielding occurs during the irradiation step, and the rotating method is not limited to this.

Then, according to the film forming method of the present embodiment, the antireflection film 1 having the same film thickness distribution as the optical thin film 2 of each layer can be simultaneously formed on a maximum of 84 optical elements 11. , Excellent productivity can be obtained.

Furthermore, 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. For example, 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. As shown in FIG. 4A, the general-purpose sputtering apparatus includes an optical element supporting device 101, a vapor deposition source 111, and an ion gun 121 in a film forming chamber whose inside can be kept in vacuum. As the vapor deposition source 111 and the ion gun 121, the same ones as the vapor deposition source 51 and the ion gun 61 shown in FIG. 4 can be used.

The optical element support device 101 includes a disk-shaped support base 102 and an optical element holder 103 arranged on the support base 102 and having a diameter smaller than that of 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. 4B, the support base 102 rotates about the center of the rotation axis L1, 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 formation 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. 4A, the vapor deposition source 111 is provided, for example, at the bottom of the film forming chamber and directly below the rotation trajectory of the optical element holder 103. The ion source 121 is provided, for example, at the bottom of the film forming chamber and directly below the rotation trajectory of the optical element holder 103, at a position different from that of the vapor deposition source 111. The ion gun 121 emits ions obliquely upward. The ion gun 121 is capable of irradiating ions toward the optical surface 11a side of the optical element 11 from a direction inclined at an angle β of 45 ° or more and 90 ° or less with respect to the optical axis OA. The position of is adjusted. The film forming method of the present embodiment described above can also be performed by the apparatus configuration shown in FIG. 4 as described above. Then, it is possible to cause self-shielding when performing the irradiation process described above.

3. Optical Element The optical element according to the present invention is characterized by including the above-described antireflection film on the convex optical surface side having a maximum inclination angle of 25° or more. According to the present invention, by providing the above-mentioned antireflection film, when light having a wide wavelength band is incident on a region having a large inclination angle θ at a large incident angle φ, the reflection of light is lowered over a wide wavelength band. An optical element that can be suppressed can be provided. Examples of the optical element include a photographing optical element and a projection optical element. Specifically, the lens includes, for example, an interchangeable lens of a single-lens reflex camera, a lens mounted on a digital camera (DSC), and a mobile phone. Lenses for digital cameras, lenses for irradiation system projectors, free-curved lenses for headlights of cars, laser processing lenses and axicon lenses, pickup lenses for DVDs, CDs, Blu-rays, mobile phones and smartphones Various lenses such as lenses used in cameras can be mentioned.

The optical thin film of each layer constituting the antireflection film provided in the optical element can be formed by the film forming step of depositing the vapor deposition substance and the irradiation step of irradiating with ions or plasma as described above. Therefore, when the optical thin film of the first layer is formed on the convex optical surface described above, ions, plasma, electrons and the like may collide with the convex optical surface in the irradiation step. When the optical element is made of a specific glass material, for example, when it is made of a glass material such as FCD1 containing fluorine, when accelerated electrons or the like collide with a convex optical surface, light absorption occurs in the optical element. Is not preferable.

Therefore, it is preferable that the optical element according to the present invention is provided with a protective layer between the convex optical surface and the antireflection film to prevent ions, plasma, or electrons from being incident on the convex optical surface. .. By forming the optical thin film of the first layer on the protective layer, it is possible to prevent electrons and the like from directly colliding with the optical element and prevent absorption of light. 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 having FCD1 (refractive index 1.497), it is preferable that the protective layer is made of SiO ₂ having almost the same refractive index. Since the refractive indexes 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 it is possible to obtain the same optical characteristics as when the protective layer does not exist despite the presence of the protective layer. At this time, even if the film thickness of the protective layer is non-uniform, the influence on the optical characteristics can be prevented. Therefore, the protective layer can be formed by a normal vacuum deposition method that does not use ions or plasma. For example, the protective layer can be formed by performing the above-described film forming process using the vapor deposition source 51. The thickness of the protective layer is preferably 0.5 nm or more, more preferably 5 nm or more. If the thickness of the protective layer is less than 0.5 nm, the coating on the convex optical surface becomes insufficient, and the rare gas or nitrogen adheres to the convex optical surface when forming the optical thin film of the first layer. May not be prevented, which is not preferable.

Further, the optical element according to the present invention may be provided with an antifouling film or a hard film as a functional film on the surface of the antireflection film. For example, an antifouling film coated with fluorine or a hard film made of diamond-like carbon (DLC) or SiO _x N _y can be provided. The film 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 examples.

In this embodiment, the antireflection film 1 including the 11 optical thin films 2 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. Formed. The optical thin films 2 of the first layer, the third layer, the fifth layer, the seventh layer, the ninth layer, and the eleventh layer constituting the antireflection film 1 are made of SiO ₂ , and the second layer, the fourth layer, and the sixth layer are composed of SiO ₂ . The optical thin films 2 of the layers, the eighth layer and the tenth layer are made of TiO ₂ .

(Optical element)
As the optical element 11, a material having TAF1 and a shape shown in FIG. 5 was used. In the optical element 11 shown in FIG. 5, when the tilt angle θ was measured at an arbitrary position on the convex optical surface 11a, the tilt angle was 65°. From this, the maximum inclination angle of the convex optical surface 11a in the optical element 11 used in this example is 65° or more. The symbol IL in FIG. 5 means the incident direction of incident light.

(Film forming conditions)
First, two or more optical elements 11 are arranged around the rotation axis L2 of the optical element holder 43, and the optical element holder 43 is tilted to maintain a posture in which the film forming surface 43a is tilted by 10 ° with respect to the vertical direction. The optical element 11 was rotated by rotating the optical element holder 43 while rotating the supporting substrate 42. Oxygen gas with a flow rate of 20 sccm was introduced into the film forming chamber 31 to adjust the vacuum degree 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 the irradiation step were repeated to form each of the 11 optical thin films 2.

In the film forming step, as the vapor deposition source 51, SiO ₂ is used for forming the optical thin film 2 of the first layer, the third layer, the fifth layer, the seventh layer, the ninth layer and the eleventh layer, and the second layer, TiO ₂ was used for forming the optical thin films 2 of the fourth layer, the sixth layer, the eighth layer and the tenth layer. Since the film formation surface 43a of the optical element holder 43 is inclined by 10° with respect to the vertical direction, the vapor deposition material evaporated from the vapor deposition source 51 is mainly inclined at an angle α=80° with respect to the optical axis OA. The light is incident on the optical surface 11a of the optical element 11 from the direction D1.

In the irradiation step, when irradiating ions with the ion gun 61, Ar gas having a flow rate of 40 sccm was introduced, and 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 were incident on the optical surface 11a of the optical element 11 from the direction D2 inclined at an angle β = 75 ° with respect to the optical axis OA.

In this example, the antireflection film 1 including the seven optical thin films 2 was formed on the convex optical surface 11a of the optical element 11 different from that in Example 1 under the following film forming conditions. The optical thin films 2 of the first layer, the third layer, the fifth layer and the seventh layer constituting the antireflection film 1 are made of SiO ₂ , and the optical thin films 2 of the second layer, the fourth layer and the sixth layer are TiO ₂ Consists of.

(Optical element)
As the optical element 11, a material having TAF1 and a shape shown in FIG. 6 was used. In the optical element 11 shown in FIG. 6, when the tilt angle θ was measured at an arbitrary position on the convex optical surface 11a, the tilt angle was 80°. From this, the maximum tilt angle of the convex optical surface 11a in the optical element 11 used in this example is 80° or more. It is also apparent from FIG. 6 that the optical element 11 used in this example has a larger maximum tilt angle than the optical element 11 used in Example 1.

(Film forming conditions)
In this example, the optical element 11 was rotated in the same manner as in Example 1 except that the film formation surface 43a of the optical element holder 43 was held in a posture in which it was inclined by 7° with respect to the vertical direction. The atmosphere in the film forming chamber 31 was the same as in Example 1. Then, in the film forming step, the point where the vapor-deposited substance evaporated from the vapor deposition source 51 is incident on the optical surface 11a of the optical element 11 mainly from the direction D1 inclined at an angle α = 83 ° with respect to the optical axis OA. Except for this, the procedure was the same as in Example 1. The irradiation step is the same as that of the first embodiment except that the ions are irradiated from the direction D2 inclined at an angle β = 78 ° with respect to the optical axis OA so as to be incident on the optical surface 11a of the optical element 11. I went to.

In this example, the antireflection film 1 including the seven optical thin films 2 was formed on the convex optical surface 11a of the same optical element 11 as in Example 1 under the following film forming conditions. The optical thin films 2 of the first layer, the third layer, and the fifth layer constituting the antireflection film 1 are made of Al ₂ O ₃ , and the optical thin films 2 of the second layer, the fourth layer, and the sixth layer are TiO ₂ and La. consist of a mixture of the ₂ O _3, the seventh layer made of SiO _2.

(Film forming conditions)
In this embodiment, the rotation condition of the optical element 11 is the same as that in the second embodiment. The atmosphere in the film forming chamber 31 was the same as in Example 2. Then, in the film forming step, Al ₂ O ₃ is used for forming the optical thin film 2 of the first layer, the third layer, and the fifth layer as the vapor deposition source 51, and the second layer, the fourth layer, and the sixth layer are formed. A mixture of TiO ₂ and La ₂ O ₃ was used for film formation of the optical thin film 2, and SiO ₂ was used for film formation of the optical thin film 2 of the seventh layer, and the same procedure was used for Example 2. The irradiation process was performed in exactly the same manner as in Example 2 except that the acceleration voltage of the ion gun 61 was changed to 1.2 kV.

In this example, the antireflection film 1 including the seven optical thin films 2 was formed on the convex optical surface 11a of the same optical element 11 as in Example 2 under the following film forming conditions. The material constituting each optical thin film 2 of the antireflection film 1 is the same as that of the second embodiment.

(Film forming conditions)
In this example, the optical element 11 was rotated in the same manner as in Example 1 except that the film formation surface 43a of the optical element holder 43 was held in a posture in which it was inclined at 5° with respect to the vertical direction. Then, the film forming process was performed in exactly the same manner as in Example 2. The irradiation step is the same as that of the second embodiment except that the ions are irradiated from the direction D2 inclined at an angle β = 80 ° with respect to the optical axis OA so as to enter the optical surface 11a of the optical element 11. I went to.

Comparative example

[Comparative Example 1]
In this comparative example, an antireflection film 1 composed of 11 layers of optical thin films 2 was formed on the convex optical surface 11a of the same optical element 11 as in Example 1 by a film forming method different from that in Example 1. The material constituting each optical thin film 2 of the antireflection film 1 is the same as that of the first embodiment.

(Film forming method)
In this comparative example, BMC1300 manufactured by Syncron 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 in a film forming chamber capable of holding the inside in a vacuum, and the same vapor deposition source 51 and ion source 61 as those used in Example 1. This dome has a dome shape unlike the optical element holder used in Example 1. The dome is hung on the ceiling wall of the film forming chamber in an upwardly convex posture, and rotates about the center of the dome. The inner concave surface of the dome 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 the optical axis OA of the optical element 11 is inclined 5 to 30° with respect to the vertical direction.

Then, the film forming step and the irradiation step were carried out in the same manner as in Example 1. In the film forming process, except that the vapor deposition material vaporized from the vapor deposition source 1 mainly enters the optical surface 11a of the optical element 11 from the direction D1 inclined at the angle α=5 to 30° with respect to the optical axis OA. , The same as in Example 1. In the irradiation step, the first embodiment is performed except that the ions irradiated by the ion gun are 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. It was exactly the same as.

[Comparative Example 2]
In this comparative example, the antireflection film 1 consisting of 11 layers of the optical thin film 2 was formed on the convex optical surface 11a of the same optical element 11 as in Example 2 by the same film forming method as in Comparative Example 1. The material forming each optical thin film 2 of the antireflection film 1 is the same as that of Comparative Example 1.

[Evaluation item]
The following evaluations were performed on the obtained antireflection films 1 of Examples 1 to 4 and Comparative Examples 1 and 2.
(1) Film Thickness Distribution of Optical Thin Films The film thickness distribution of the optical thin films 2 of each layer constituting the antireflection coatings 1 of Examples 1 to 4 and Comparative Examples 1 and 2 was measured by cross-sectional SEM to obtain the film thickness distribution. .. The film thickness is measured at locations where the inclination angles θ are 0 °, 25 °, 35 °, 45 °, 60 °, 70 °, and 80 ° on the optical surface 11a of the optical element 11 of each optical thin film 2. I went to the place to do. The film thickness distribution of the optical thin film 2 in each layer was the same as long as the material was the same, regardless of the layer number from the optical element 11 side. The film thickness distribution of the optical thin film 2 for each material is shown in FIGS. The angle θ on the horizontal axis in the figure is an angle corresponding to the inclination angle θ of the optical surface 11a of the optical element 11 at the measurement location on the antireflection film 1.

(2) Appearance The appearance of the antireflection film 1 of Examples 1 to 4 and Comparative Examples 1 and 2 was evaluated. The appearance was evaluated by visually observing whether or not the antireflection film 1 had unevenness in reflection color. The results are shown in Table 1. In the “Appearance” column in Table 1, “◯” indicates that no reflection color unevenness was observed, and “x” indicates that reflection color unevenness was observed.

(3) Ghost With respect to the antireflection films 1 of Examples 1 to 4 and Comparative Examples 1 and 2, it was evaluated whether or not a ghost occurs. This evaluation was performed by the stray light test. The results are shown in Table 1. In the "ghost" column in Table 1, "○" indicates that no ghost occurred, "△" indicates that a weak ghost occurred, and "x" indicates that a strong ghost occurred. Indicates that.

(4) Spectral Reflection Characteristics The spectral reflectance of the antireflection films 1 of Examples 1 to 4 and Comparative Examples 1 and 2 was measured by a reflection spectroscopic film thickness meter FE-3000 manufactured by Otsuka Electronics Co., Ltd. At this time, the wavelength of the incident light is set to a position corresponding to a position on the optical surface 11a of the optical element 11 on the antireflection film 1 where the inclination angle θ is 0°, 35°, 60°, 70°. The spectral reflectance was measured when the light was incident at an incident angle of φ of 0 °, 10 °, 20 °, 30 °, 40 °, 50 °, 60 °, and 70 ° while changing in the range of 350 nm or more and 1000 nm or less. .. The results are shown in FIGS. 10 to 15 and Table 1. Table 1 shows, for example, in the first embodiment, a portion corresponding to the center of the antireflection film 1, that is, a portion on the optical surface 11a of the optical element 11 where the inclination angle θ is 0 ° (center of the optical surface 11a). The average reflectance was 0.1% and the maximum reflectance was 0.2% when the reflectance of the light incident at the incident angle φ0° was measured while changing the wavelength of the light within the above range. Is shown.

(5) Abrasion resistance The antireflection film 1 of Examples 1 to 4 and Comparative Examples 1 and 2 was evaluated for abrasion resistance. First, using a reciprocating abrasion tester TYPE30 manufactured by Shinto Kagaku Co., Ltd., a moving distance of 10 mm was reciprocated 100 times at a moving speed of 1200 mm/min while a load of 500 g was applied. Then, regarding the optical thin film 2 on the outermost surface of the antireflection film 1, the center (the position where the inclination angle θ of the optical surface 11a corresponds to 0 °) and the peripheral portion (the position where the inclination angle θ of the optical surface 11 corresponds to 60 °). ) The presence or absence of scratches was visually observed. In the "Abrasion resistance" column in Table 1, "○" indicates that no scratches were observed in both the central and peripheral parts, and "x" marks indicate that scratches were not observed in at least one of the central or peripheral parts. Indicates what was observed.

[Evaluation results]
The evaluation results of each example and comparative example will be described below.
(1) Evaluation Results Regarding Thickness Distribution of Optical Thin Film Regarding the thickness distribution of each optical thin film 2 constituting the antireflection film 1 of Examples 1 to 4 and Comparative Examples 1 to 2 with reference to FIGS. 7 to 9. Describe. As shown in FIGS. 7 and 8, each optical thin film 2 of Examples 1 to 4 was measured at the center, that is, the tilt angle θ was 0°, regardless of the material forming the optical thin film. The physical film thickness at the location is the smallest, and the physical film thickness becomes thicker as the tilt angle θ at the measurement location becomes larger, and the physical film thickness becomes the thickest in the region where the tilt angle θ is 60° to 70°. Then, in each of the optical thin films 2 of Examples 1 to 3, dx / d0, which is the ratio of the physical film thickness dx at an arbitrary measurement point to the physical film thickness d0 at the center, is larger than 1 and falls within the range of less than 1.3. There is. Further, in the optical thin film 2 of Example 4, dx / d0 is larger than 1 and falls within the range of less than 1.3 at the measurement point where the inclination angle θ is 0 ° or more and 60 ° or less and the measurement point where the inclination angle θ is 80 °. However, dx/d0 exceeds 1.3 at the measurement point where the inclination angle θ is 70°.

On the other hand, as shown in FIG. 9, each of the optical thin films 2 of Comparative Examples 1 and 2 has the thickest physical film thickness at the center regardless of the material constituting the optical thin film, and the inclination angle of the measurement point. The physical film thickness decreases as θ increases. Then, each of the optical thin films 2 of Comparative Examples 1 and 2 generally follows the equation that the physical film thickness dx at an arbitrary measurement point is dx = d0cosθ.

(2) Evaluation result regarding appearance The appearance of the antireflection film 1 of Examples 1 to 4 and Comparative Examples 1 and 2 will be described with reference to Table 1. The antireflection film 1 of each of Examples 1 to 4 has an excellent appearance with no unevenness of the reflection color being observed. On the other hand, in the antireflection film 1 of Comparative Examples 1 and 2, unevenness of the reflection color was observed and the appearance was inferior. The unevenness of the reflected color was observed center-symmetrically with respect to the center of the antireflection film 1.

(3) Evaluation result regarding ghost With reference to Table 1, the presence or absence of ghost in the antireflection film 1 of Examples 1 to 4 and Comparative Examples 1 and 2 will be described. No ghost was observed in the antireflection film 1 of Examples 1 to 3. In the antireflection film 1 of Example 4, a weak blue ghost was observed. On the other hand, strong ghosts were observed in the antireflection films 1 of Comparative Examples 1 and 2.

(4) Evaluation Results Regarding Spectral Reflection Characteristics With reference to Table 1 and FIGS. 10 to 15, the spectral reflection characteristics of the antireflection films 1 of Examples 1 to 4 and Comparative Examples 1 and 2 will be described. As shown in FIGS. 9 to 13, the antireflection film 1 of Examples 1 to 4 is the center of the antireflection film 1, that is, a position where the inclination angle θ of the optical surface 11a of the optical element 11 is 0 ° (optical). When measured at a location corresponding to the center of the surface 11a), the reflectance of light having a wavelength of 420 nm or more and 680 nm or less and an incident angle φ of 0 ° or more and 20 ° or less was 0.5% or less. Further, when measured at a location corresponding to a location where the inclination angle θ of the optical surface 11a on the antireflection film 1 is 35 °, light having a wavelength in the above range and an incident angle φ of 20 ° or more and 40 ° or less The reflectance was 1% or less. Then, when measured at a location corresponding to a location where the inclination angle θ of the optical surface 11a on the antireflection film 1 is 70 °, light having a wavelength in the above range and an incident angle φ of 50 ° or more and 60 ° or less. Was less than 7%. Furthermore, when measured at a location corresponding to a location where the tilt angle θ of the optical surface 11a on the antireflection film 1 is 70°, the reflectance of light whose wavelength is in the above range and whose incident angle φ is 70°. Was 15% or less. From these results, the antireflection film 1 of Examples 1 to 4 is light having a wavelength of 420 nm or more and 680 nm or less, which is incident on a region where the inclination angle θ of the convex optical surface 11a of the optical element 11 is small at a small incident angle φ. It is clear that it is possible to suppress the reflection of light having a large incident angle φ in a region where the inclination angle θ of the convex optical surface 11a is large, and it is also possible to suppress the reflection of light having the above-described wavelength to be low. That is, it is apparent that the antireflection films 1 of Examples 1 to 3 have excellent antireflection properties for light in a wide wavelength band over the entire area from the center to the periphery.

On the other hand, as shown in FIG. 12, the antireflection film 1 of Comparative Example 1 has a wavelength in the above range and an incident angle when measured at a location corresponding to a location where the inclination angle θ of the optical surface 11a is 0 °. The reflectance of light with φ of 0° or more and 20° or less was 0.5% or less. However, when measured at a location corresponding to the location where the inclination angle θ of the optical surface 11a is 35 °, the reflectance of light having a wavelength of 420 nm or more and 550 nm or less and an incident angle φ of 20 ° or more and 40 ° or less is 1. % Or less, the reflectance increases when the wavelength exceeds 550 nm, and exceeds 10% when the wavelength is 680 nm. Further, when measured at a location corresponding to a location where the inclination angle θ of the optical surface 11a is 60 °, the reflectance of light having a wavelength of 420 nm or more and 680 nm or less and an incident angle φ of 30 ° or more and 50 ° or less is 10. It greatly exceeds %, reaching nearly 30%. From these results, the antireflection film 1 of Comparative Example 1 suppresses the reflection of light having a wavelength of 420 nm or more and 680 nm or less, which is incident on a region where the inclination angle θ of the convex optical surface 11a is large and has a large incident angle φ. It is clear that you can not. The spectral reflection characteristics of the antireflection film 1 of Comparative Example 2 were similar to those of the antireflection film 1 of Comparative Example 1.

(5) Results of Evaluation Regarding Abrasion Resistance With reference to Table 1, the abrasion resistance appearances of the antireflection films 1 of Examples 1 to 4 and Comparative Examples 1 and 2 will be described. The antireflection film 1 of each of Examples 1 to 4 has excellent abrasion resistance, with no flaws observed in both the center and the peripheral portion. On the other hand, the antireflection films 1 of Comparative Examples 1 and 2 are inferior in abrasion resistance because scratches are observed on at least one of the central or peripheral portions. From these facts, the reason why the antireflection film 1 of Examples 1 to 4 is excellent in abrasion resistance is that the optical thin film 2 is densified by ion irradiation with an ion gun 61 when the optical thin film 2 is formed. It is considered that this is because the wear resistance was improved.

(6) Overall Review From the above results, the antireflection film 1 of Examples 1 to 4 is for light in a wide wavelength band incident on a region where the inclination angle θ of the convex optical surface of the optical element is large at a large incident angle φ. It has been clarified that it is possible to suppress the reflection to a low level and also to suppress the reflection of light in a wide wavelength band that is incident at a small incident angle φ in a region where the inclination angle θ of the convex optical surface of the optical element is small. .. Then, it was revealed that the antireflection film 1 of each of Examples 1 to 4 had no unevenness in the reflection color, almost no ghost, and excellent abrasion resistance.

INDUSTRIAL APPLICABILITY The antireflection film and the optical element including the same according to the present invention can be applied to various optical elements such as a photographing optical element and a projection optical element that have a deep convex optical surface of R having a maximum tilt angle of 25° or more. It is suitable. The antireflection film forming method according to the present invention is suitable for forming these antireflection films.

1 Anti-reflection film 2 Optical thin film 2a-2g Optical thin film of each layer 11 Optical element 11a Convex optical surface 51,111 Deposition source 61,121 Ion gun (ion source)
D1 Incident direction when the vapor-deposited material is incident on the convex optical surface side D2 Incident direction when ions or plasma are incident on the convex optical surface side OA Optical axis of the optical element α Vaporized material with respect to the optical axis of the optical element Angle formed by the incident direction of β The angle formed by the incident direction of ions or plasma with respect to the optical axis of the optical element

Claims

An antireflection film provided on an optical element having a convex optical surface having a maximum inclination angle of 25° or more of the inclination angle θ of the convex optical surface indicated by the angle formed by the normal line and the optical axis at the measurement point,
A multilayer structure comprising a plurality of optical thin films provided on the convex optical surface side,
When each optical thin film is centered on a point where the convex optical surface and the optical axis are orthogonal to each other, at least a part of the film provided in a region where the inclination angle θ of the convex optical surface is 25° or more, An antireflection film, which is thicker than a physical film thickness of a film provided at the center of the convex optical surface.
In each of the optical thin films, at least a part of the film provided in the region where the inclination angle θ of the convex optical surface is 25 ° or more and less than 80 ° is larger than the physical film thickness of the film provided at the center of the convex optical surface. The antireflection film according to claim 1, which is also thick.
In each of the optical thin films, the entire film provided in a region where the inclination angle θ of the convex optical surface of the optical element is 25° or more and less than 75° is a physical film of a film provided in the center of the convex optical surface. The antireflection film according to claim 2, which is thicker than the thickness.
When the physical film thickness of the film provided at the center of the convex optical surface of each optical thin film is d0 and the physical film thickness at an arbitrary measurement point of the optical thin film is dx, the inclination angle of the convex optical surface is defined as dx. The physical film thickness of a portion of the film provided in the region where θ is 25° or more, which is thicker than the physical film thickness of the film provided in the center, satisfies the following conditional expression: The antireflection film according to any one of items.
1<dx/d0≦1.3
The antireflection film according to any one of claims 1 to 4, wherein the optical thin film contains one or more rare gas elements or nitrogen selected from the group of He, Ne, Ar, Xe, and Xr. ..
The multilayer structure includes an optical thin film which is a high refractive index layer and an optical thin film which is a low refractive index layer,
The high refractive index layer 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 antireflection film according to claim 5.
The multilayer structure includes an optical thin film which is a high refractive index layer and an optical thin film which is a low refractive index layer,
The low refractive index layer, an antireflection film according to claims 1 mixture is intended to include the one or SiO 2 and Al 2 O 3 containing SiO 2 in any one of claims 6.
An optical element having a convex optical surface in which the maximum tilt angle is 25° or more,
An optical element comprising the antireflection film according to any one of claims 1 to 7 on the convex optical surface side.
The optical element according to claim 7, further comprising a protective layer between the convex optical surface and the antireflection film to prevent ions or plasma from being incident on the convex optical surface.
The optical element according to claim 8 or 9, comprising a functional film provided on the surface of the antireflection film.
A method for forming an antireflection film for forming the antireflection film according to claim 1.
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;
On the convex optical surface side of the rotating optical element, by irradiating ions from an ion source or plasma from a plasma source from a direction inclined with respect to the optical axis, the center of the convex optical surface side. The deposited film-forming material is densified while removing more than the peripheral portion on the optical surface side of the convex, and the ion source or the plasma 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 blocking the incidence of the ions or the plasma to the region of the convex optical surface on the side far from the source,
A method of forming an antireflection film, characterized in that each of the optical thin films constituting the multilayer structure is formed by performing the film forming step and the irradiation step.
The irradiation step is to irradiate the convex optical surface side of the optical element with the ions or the plasma from a direction inclined at an angle of 45 ° or more and 90 ° or less with respect to the optical axis. The method for forming an antireflection film according to [4].
The method for forming an antireflection film according to any one of claims 11 to 12, wherein the optical element is rotated about an axis different from the optical axis as a rotation axis.
The ion or the plasma is any one of claims 11 to 13 which is an ion or plasma formed from one or more rare gases or nitrogen selected from the group of He, Ne, Ar, Xe, and Xr. The method for forming an antireflection film according to.