JP2025103782A - Multilayer film, optical member, and method for manufacturing multilayer film - Google Patents

Abstract

To provide a multilayer film that fully exhibits a self-purification function by a photocatalytic reaction on a surface, even in a case where a film thickness of a low refractive index layer of a surface is made equal to or more than a specific thickness for reducing a reflection rate of light, and can maintain hydrophilicity over a long period in a dark place.SOLUTION: In a multilayer film, a layer containing cerium oxide contains cerium oxide that includes a cubic system polycrystalline structure and a columnar structure, and a film thickness of the layer containing the cerium oxide is equal to or more than 85 nm and less than 800 nm, let the layer as a whole containing the cerium oxide be an area (A), and let an oxygen vacancy rate of the cerium oxide in the area (A) be an oxygen vacancy rate (VA), the oxygen vacancy rate (VA) is equal to or more than 0.05% and less than 10%, a film thickness of a layer containing silicon oxide or a layer containing magnesium fluoride is equal to or more than 50 nm and less than 240 nm, and a refractive index of a layer containing a low refractive index layer with respect to light of a wavelength 500 nm is equal to or less than 1.65.SELECTED DRAWING: Figure 1A

Description

This disclosure relates to a multilayer film with excellent self-cleaning properties and hydrophilicity, an optical component having the multilayer film, and a method for manufacturing the multilayer film.

Optical components such as optical lenses, mirrors, and optical filters have a film formed from an inorganic material to increase or decrease the light transmittance or reflectance. Films formed from inorganic materials generally have high surface free energy immediately after film formation, and therefore are highly hydrophilic. However, the surface free energy decreases in a relatively short period of time due to self-reactions or the adhesion of dirt from humans or the environment, and the hydrophilicity decreases.

For example, if the hydrophilicity of the surface of optical products used in automobiles, security cameras, and optical products used outdoors, such as eyeglass lenses, or the protective covers for these products, is reduced, visibility may be impaired if water droplets adhere to them, and the optical products and protective covers mentioned above may not be able to fully function.

As a means of solving the above-mentioned problems, a hydrophilic film in which a thin film of silicon dioxide is formed on a thin film of crystalline titanium dioxide is used (Patent Document 1, Patent Document 2, Non-Patent Document 1). When the surface of crystalline titanium dioxide is irradiated with near-ultraviolet light, active oxygen is generated by the photocatalytic function, and the generated active oxygen decomposes the organic matter on the surface of the hydrophilic film. As a result, the hydrophilicity of the hydrophilic film is restored, and the dirt is washed away by rainfall, and the film is self-purified. In addition, by placing silicon dioxide on top of titanium dioxide, even if the light irradiation is stopped, it does not become hydrophobic in a short time like a thin film of titanium dioxide alone, and it is known that the hydrophilicity continues for about 1 to 2 weeks even in a dark place.

However, as shown in Table 2 of Patent Document 1, when the silicon dioxide thin film formed on the crystalline titanium dioxide thin film is less than 50 nm, there is a problem that the reflectance increases and the transmittance decreases because the refractive index of titanium dioxide is large. Therefore, although it can be applied to vehicle door mirrors and the like where a high reflectance does not cause any problems, there is a problem that it is not suitable for optical components such as lenses equipped with an anti-reflection film.
Furthermore, when the silicon dioxide thin film formed on the surface of the crystalline titanium dioxide thin film is 50 nm or more, there is a problem that the self-cleaning function by the photocatalytic reaction is not fully exhibited. In addition, although the hydrophilicity maintaining performance in a dark place is improved compared to the case where only crystalline titanium dioxide is used, there is still a problem that it is not sufficient.

In addition, as described in the examples of Patent Document 2 and Non-Patent Document 1, when a crystalline titanium dioxide layer is used as a photocatalyst, in order to crystallize the thin film made of titanium dioxide, the thin film or the substrate on which the thin film is provided may have to be heated to a high temperature, which poses the problem that resin substrates with low heat resistance cannot be used. Some wet processes for forming titanium dioxide films do not require high-temperature heating. However, when using wet processes, it is difficult to precisely control the film thickness in 1 nm increments, form a thin film with a uniform film thickness, and form a film on a substrate other than a substrate with a simple shape such as a flat plate, and there are inherent problems such as a short shelf life of the coating liquid.

JP 2000-053449 A Japanese Patent Application Publication No. 09-057912

K. Miyashita, et al, Journal of the Ceramic Society of Japan 110 [5] 450-454 (2002)

Considering application to optical components, there is a demand for a multilayer film that fully exhibits the self-cleaning function through photocatalytic reaction even when the low refractive index layer is made thick so that the low refractive index layer on the surface of the optical component can be designed as an anti-reflection film. There is also a demand for a multilayer film that maintains its hydrophilicity for a longer period in a dark place.

The object of the present disclosure has been made in consideration of the above problems, and is to provide a multilayer film that can fully exhibit a self-cleaning function by photocatalytic reaction on the surface and can maintain hydrophilicity for a long period of time in a dark place, even when the film thickness of the low refractive index layer on the surface is set to a specific thickness or more in order to reduce the reflectance of light. The object of the present disclosure is to provide an optical member having the multilayer film. In addition, the object of the present disclosure is to provide a method for manufacturing the multilayer film.

In order to solve the above problems, the present disclosure provides:
A layer containing cerium oxide and a low refractive index layer provided directly or via another layer on the layer containing cerium oxide,
the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride,
the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure and a columnar structure,
The thickness of the layer containing cerium oxide is 85 nm or more and 800 nm or less,
When the entire layer containing the cerium oxide is defined as a region (A) and the oxygen deficiency rate of the cerium oxide in the region (A) is defined as an oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
The low refractive index layer has a thickness of 50 nm or more and 240 nm or less,
The multilayer film is characterized in that the refractive index of the low refractive index layer for light with a wavelength of 500 nm is 1.65 or less.

The present disclosure also relates to an optical member having the above multilayer film.

The present disclosure also provides
A step (A) of forming a layer containing cerium oxide on a substrate directly or via another layer by a vacuum deposition method;
and (B) forming a low refractive index layer by a vacuum deposition method on the layer containing cerium oxide directly or via another layer,
the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride,
the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure and a columnar structure,
The thickness of the layer containing cerium oxide is 85 nm or more and 800 nm or less,
When the entire layer containing the cerium oxide is designated as region (A) and the oxygen deficiency rate of the cerium oxide in the region (A) is designated as oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.5% or more and 10% or less,
The method for producing a multilayer film is characterized in that the low refractive index layer has a thickness of 50 nm or more and 240 nm or less.

According to one aspect of the present disclosure, even when the film thickness of the low refractive index layer on the surface is set to a specific thickness or more in order to reduce the reflectance of light, it is possible to obtain a multilayer film that exhibits sufficient self-cleaning function due to a photocatalytic reaction on the surface and can maintain hydrophilicity for a long period of time even in a dark place, an optical member having the multilayer film, and a method for manufacturing the multilayer film.

1 is a schematic cross-sectional view showing a configuration according to a first embodiment of the present disclosure. FIG. 1 is a schematic cross-sectional view for explaining a first embodiment according to the present disclosure. FIG. 11 is a schematic cross-sectional view showing a configuration according to a second embodiment of the present disclosure. FIG. 1 is a schematic diagram illustrating an example of an application example according to the present disclosure. FIG. 13 is a schematic diagram showing another example of an application according to the present disclosure.

Hereinafter, preferred embodiments of the multilayer film, the optical member having the multilayer film, and the method for forming the multilayer film according to the present disclosure will be described.
Furthermore, the present disclosure is not limited to the following embodiments.
In the present disclosure, the description of a numerical range such as [XX to YY] or [XX to YY] means a numerical range including the lower and upper limits, which are the endpoints, unless otherwise specified. Furthermore, when a numerical range is described in stages, the upper and lower limits of each numerical range can be arbitrarily combined.

In this disclosure, the term "multilayer film" refers to a structure including two or more layers formed on the surface of a substrate. The multilayer film according to this disclosure can be provided on the substrate directly or via another layer. Hereinafter, the layers constituting the multilayer film may also be referred to as a film or thin film.

In this disclosure, the "substrate" is a solid article.

In this disclosure, the term "optical component" refers to an optical component that includes the above multilayer film. Examples of such optical components include optical filters, optical lenses, light-collecting lenses, optical films, optical prisms, eyeglass lenses, photographic lenses, surveillance camera covers, vehicle-mounted camera covers, vehicle-mounted sensor covers, vehicle door mirrors, platform door sensors, light-collecting lens covers, sheet glass, light-collecting lenses, display cover glass, touch panels, and various films.

Before specifically explaining the multilayer film according to the present disclosure, in order to facilitate understanding of the present disclosure, we will first describe below the presumed mechanism by which the effect is achieved. However, the following explanation is merely a hypothesis, and the present disclosure is in no way limited by the following hypothesis.

The inventors have found that when a layer containing cerium oxide, which has a cubic polycrystalline structure with specific oxygen vacancies, is placed on a substrate directly or through another layer, and a layer containing magnesium fluoride or silicon oxide of a specific thickness is placed on the layer containing cerium oxide directly or through another layer, a hydrophilicity recovery function due to the self-purifying properties of the photocatalyst is expressed and the ability to maintain hydrophilicity in a dark place is dramatically improved. The following is thought to be the mechanism behind the above.

When a near-ultraviolet light-responsive photocatalyst film is irradiated with near-ultraviolet light, electrons and holes are generated by photoexcitation. If the generated electrons and holes reach the surface of the film, a chemical reaction occurs, and the hydrophilic recovery function due to self-purification is expressed. The cerium oxide in the cerium oxide-containing layer of the present disclosure has an appropriate amount of oxygen vacancies. The absence of an excess of oxygen vacancies suppresses the recombination of holes and electrons generated by photoexcitation. As a result, the photocatalytic activity is enhanced. In addition, the decrease in the mobility of electrons and holes generated by photoexcitation due to a shortage of oxygen vacancies is suppressed. As a result, the decrease in the self-purification function of the photocatalyst is suppressed.

Furthermore, due to the presence of oxygen vacancies, water molecules (H ₂ O) and oxygen molecules (O ₂ ) derived from the thin film or the environment are captured by the oxygen vacancies, and active species are generated even in an environment without ultraviolet light, contributing to maintaining the hydrophilicity of the surface in a dark place. In addition, in the layer containing cerium oxide of the present disclosure, oxygen vacancies are present, particularly at the polycrystalline interface. This improves electrical conductivity, promoting the separation of electrons and holes generated by photoexcitation, making it easier for the electrons and holes to move smoothly along the interface and reach the surface. As a result, it is believed that the hydrophilicity recovery function due to the self-purifying properties of the photocatalyst is expressed, and the hydrophilicity maintenance ability in a dark place is dramatically improved.

First Embodiment
The first embodiment is directed to a multilayer film.
The multilayer film of the present disclosure is
A layer containing cerium oxide and a low refractive index layer provided directly or via another layer on the layer containing cerium oxide,
the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride,
the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure and a columnar structure,
The thickness of the layer containing cerium oxide is 85 nm or more and 800 nm or less,
When the entire layer containing the cerium oxide is defined as a region (A) and the oxygen deficiency rate of the cerium oxide in the region (A) is defined as an oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
The low refractive index layer has a thickness of 50 nm or more and 240 nm or less,
The low refractive index layer has a refractive index of 1.65 or less for light having a wavelength of 500 nm.

The multilayer film of the present disclosure has a layer containing cerium oxide and a low refractive index layer directly or via another layer on the layer containing cerium oxide, and the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride. FIG. 1A is a schematic cross-sectional view showing a first embodiment of the multilayer film of the present disclosure provided on a substrate. FIG. 1A shows a configuration example in which a layer containing cerium oxide 13 of the present disclosure is formed as a low refractive index layer on a substrate 11, and a layer containing magnesium fluoride 14 is further formed on the layer containing cerium oxide 13. In FIG. 1A (and FIG. 1B described later), a layer containing silicon oxide 15 may be formed on the layer containing cerium oxide 13 instead of the layer containing magnesium fluoride 14. Note that FIGS. 1A, 1B, and 2 are schematic representations of the configuration of the multilayer film of the present disclosure. Therefore, the areas and film thicknesses of each layer are not shown in exact proportions.

The substrate 11 will now be described.
The substrate 11 may be any material capable of laminating other layers 12 and the layer 13 containing cerium oxide of the present disclosure, and may be made of glass, ceramics, resin, metal, or the like. The shape of the substrate is not limited, and may be, for example, flat, curved, concave, convex, or film-like. The substrate 11 may also have a hard coat layer or a barrier layer. In addition, the size and thickness of the substrate 11 are not particularly limited, and may be appropriately set depending on the application.

The cerium oxide-containing layer 13 according to the present disclosure will be described. In the multilayer film according to the present disclosure, the entire layer containing cerium oxide is designated as region (A), and the oxygen deficiency rate of cerium oxide in region (A) is designated as oxygen deficiency rate (V _A ), where V _A is 0.05% or more and 10% or less. If the oxygen deficiency rate is less than 0.05%, the mobility of electrons and holes excited by light decreases, and the self-purification function by the photocatalyst decreases. If it exceeds 10%, the electrons and holes excited by light are more likely to recombine, and the self-purification function by the photocatalyst decreases.

In the present disclosure, the oxygen deficiency rate can be defined as (B-A)/B x 100 (%), where A is the oxygen content of cerium oxide and B is the theoretical oxygen content of _CeO2 . For example, the oxygen deficiency rate of _CeO2 according to the stoichiometric composition is 0%, and the oxygen deficiency rate when it is assumed that all oxygen atoms in _CeO2 are absent (when it is Ce) is 100%.

In the multilayer film of the present disclosure, the oxygen deficiency rate (V _A ) of the cerium oxide in the region (A) of the cerium oxide-containing layer 13 is preferably 0.05% or more and 0.6% or less. When the oxygen deficiency rate is 0.05% or more, the self-purifying function and the ability to maintain hydrophilicity are further improved.

In the multilayer film of the present disclosure, the oxygen deficiency rate (V _A ) of the cerium oxide in the region (A) of the cerium oxide-containing layer 13 is preferably 0.1% or more and 0.3% or less. When the oxygen deficiency rate is in this range, the self-purifying function and the ability to maintain hydrophilicity are further improved.

In the multilayer film of the present disclosure, the cerium oxide-containing layer 13 contains cerium oxide (CeO _x ) having a cubic polycrystalline structure. A layer made of cerium oxide with a single crystal structure is prone to cracking. A layer made of cerium oxide with an amorphous structure is significantly reduced in the self-purifying function by photocatalysis and in the ability to maintain hydrophilicity in a dark place.

In addition, the definition of "polycrystalline structure" in this disclosure means that a peak specific to cerium oxide appears in an X-ray diffraction (XRD) measurement of the film after deposition.

In the multilayer film of the present disclosure, the thickness of the cerium oxide-containing layer 13 is 85 nm or more and 800 nm or less. If the thickness of the cerium oxide-containing layer 13 is less than 85 nm, the photocatalytic self-purification function and the ability to maintain hydrophilicity in a dark place are significantly reduced. If the thickness of the cerium oxide-containing layer 13 exceeds 800 nm, cracks are likely to occur, and the light scattering due to the heterogeneity, surface roughness, and crystal interfaces of polycrystals may become too large, which may adversely affect the optical properties.

In the multilayer film of the present disclosure, the cerium oxide-containing layer 13 contains cerium oxide having a columnar structure. The columnar structure promotes the separation of electrons and holes excited by light, and also makes it easier for the electrons and holes to reach the surface. This enhances the self-purification function of the photocatalyst.

Here, we define the "columnar structure" in this disclosure. The columnar structure refers to the inclusion of polycrystals such as cylindrical structures, prismatic structures, frustum-shaped structures, rod-shaped structures, and fibrous columnar structures in the film. They may be solid or hollow. The longitudinal direction of the columns generally grows from the substrate side of the film toward the outside air side of the film, and includes those that grow straight vertically, those that grow at an angle, those that grow while curving, those that grow branched out like branches, and those in which multiple columnar crystals fuse together during growth.

The layer 13 containing cerium oxide of the present disclosure may be disposed directly on the substrate 11, or may be disposed via another layer 12 described below.

In the multilayer film of the present disclosure, the film thickness of the low refractive index layer is preferably 50 nm or more and 240 nm or less. In the multilayer film of the present disclosure, the refractive index of the low refractive index layer for light with a wavelength of 500 nm is preferably 1.65 or less.

The layer 14 containing magnesium fluoride according to the present disclosure will be described. In the multilayer film of the present disclosure, the layer 14 containing magnesium fluoride has a thickness of 50 nm or more and 240 nm or less. If the layer 14 containing magnesium fluoride has a thickness of less than 50 nm, the reflectance of the multilayer film may be too high. On the other hand, if the layer 14 containing magnesium fluoride has a thickness of more than 240 nm, the self-purifying function by photocatalysis may not be exhibited on the surface of the multilayer film. In addition, in the multilayer film of the present disclosure, the layer 14 containing magnesium fluoride has a refractive index of 1.65 or less for light with a wavelength of 500 nm. If the refractive index of the layer 14 containing magnesium fluoride exceeds 1.65, the reflectance of the multilayer film may be too high.

In the multilayer film of the present disclosure, as shown in Fig. 1B, when a region of the cerium oxide-containing layer 13 located 8 nm or less from the interface between the cerium oxide-containing layer 13 and the magnesium fluoride-containing layer 14 is defined as region (B) and the oxygen deficiency rate of the cerium oxide in region (B) is defined as oxygen deficiency rate ( _VB ), the oxygen deficiency rate ( _VB ) is preferably 0.5% or more and 30% or less. In the multilayer film of the present disclosure, when a region of region (A) excluding region (B) is defined as region (C) and the oxygen deficiency rate of the cerium oxide in region (C) is defined as oxygen deficiency rate ( _VC ), the oxygen deficiency rate ( _VC ) is preferably 0% or more and 10% or less. In the multilayer film of the present disclosure, the oxygen deficiency rate ( _VB ) is preferably larger than the oxygen deficiency rate ( _VC ).

It is preferable that the content ratio of magnesium fluoride in the total amount of materials constituting the layer 14 containing magnesium fluoride is 65 mass% or more. If it is within the above range, the ability to maintain hydrophilicity in a dark place is further improved.

As described above, in FIG. 1A and FIG. 1B, a layer 15 containing silicon oxide may be disposed in place of the layer 14 containing magnesium fluoride. The layer 15 containing silicon oxide according to the present disclosure will be described below.

In the multilayer film of the present disclosure, the silicon oxide-containing layer 15 has a refractive index of 1.65 or less for light with a wavelength of 500 nm. If the refractive index of the silicon oxide-containing layer 15 exceeds 1.65, the reflectance of the multilayer film may become too high. In the multilayer film of the present disclosure, the silicon oxide-containing layer 15 is 50 nm or more and 240 nm or less. If the thickness of the silicon oxide-containing layer 15 is less than 50 nm, the reflectance of the multilayer film may become too high. On the other hand, if the thickness of the silicon oxide-containing layer 15 exceeds 240 nm, the self-purifying function by the photocatalyst may not be exhibited on the surface of the multilayer film.

In the multilayer film of the present disclosure, as shown in Fig. 1B, a region of the cerium oxide-containing layer 13 located 8 nm or less from the interface between the cerium oxide-containing layer 13 and the silicon oxide-containing layer 15 is defined as region (B), and the oxygen deficiency rate of the cerium oxide in region (B) is defined as oxygen deficiency rate ( _VB ), and the oxygen deficiency rate ( _VB ) is preferably 0.5% or more and 30% or less. In the multilayer film of the present disclosure, a region of region (A) excluding region (B) is defined as region (C), and the oxygen deficiency rate of the cerium oxide in region (C) is defined as oxygen deficiency rate ( _VC ), and the oxygen deficiency rate ( _VC ) is preferably 0% or more and 10% or less. In the multilayer film of the present disclosure, the oxygen deficiency rate ( _VB ) is preferably greater than the oxygen deficiency rate ( _VC ).

The silicon oxide-containing layer 15 is a layer containing silicon oxide (SiO _x ). The content of silicon oxide in the silicon oxide-containing layer 15 is preferably 65 mass % or more with respect to the entire silicon oxide-containing layer 15. If the content of silicon oxide in the silicon oxide-containing layer 15 is within the above range, the ability to maintain hydrophilicity in a dark place is further improved.

The silicon oxide-containing layer 15 preferably has a composition of silicon oxide of SiO _x , where x is 1.5 or more and 2.0 or less. If the value of x in the composition of silicon oxide SiO _x is within the above range, the refractive index of the silicon oxide-containing layer 15 can be made 1.65 or less. In addition, a film that is more transparent in the wavelength range from visible light to near infrared light can be obtained.

The silicon oxide-containing layer 15 may contain aluminum oxide in addition to silicon oxide (SiO _x ). In this case, the content of aluminum oxide in the silicon oxide-containing layer 15 is preferably 0.1 mass % or more and 10 mass % or less with respect to the entire silicon oxide-containing layer 15. When the silicon oxide-containing layer 15 contains 0.1 to 10 mass % aluminum oxide, the durability of the multilayer film, such as scratch resistance and moisture resistance, can be improved while maintaining the photocatalytic self-purification function of the multilayer film and its ability to maintain hydrophilicity in a dark place.

The silicon oxide-containing layer 15 may contain cerium oxide in addition to silicon oxide (SiO _x ). In this case, the content of cerium oxide in the silicon oxide-containing layer 15 is preferably 0.1% by mass to 10% by mass with respect to the entire silicon oxide-containing layer 15. By having the silicon oxide-containing layer 15 contain 0.1% by mass to 10% by mass of cerium oxide, the self-purification function by photocatalysis can be enhanced while maintaining the hydrophilicity maintaining ability of the multilayer film.

Second Embodiment
The second embodiment is directed to a multilayer film. The multilayer film of the second embodiment differs from the multilayer film of the first embodiment in that the multilayer film is provided on another layer 12 provided on a substrate 11. The components other than the other layer 12 are as described above, and therefore will not be described.
Fig. 2 is a schematic cross-sectional view showing a second embodiment of the multilayer film of the present disclosure. In Fig. 2, the multilayer film according to the present disclosure is provided on another layer 12 provided on a substrate 11. That is, the second embodiment shows a configuration example in which another layer 12 is formed on the substrate 11, a layer 13 containing cerium oxide of the present disclosure is further formed on the other layer 12, and a layer 15 containing silicon oxide is further formed as a low refractive index layer on the layer 13 containing cerium oxide.

The substrate 11 and the layer 13 containing cerium oxide of the present disclosure are as described in the first embodiment above.

In FIG. 2, the layer 14 containing magnesium fluoride described in the first embodiment may be disposed instead of all or part of the layer 15 containing silicon oxide. Also, the layer 13 containing cerium oxide of the present disclosure may be disposed directly on the substrate 11 without another layer 12 therebetween.

As the other layer 12, a layer containing a metal, a fluoride, an oxide, a carbide, a sulfide, a halide, a nitride, or a complex anion compound (oxynitride, oxysulfide, oxyhalide, oxyfluoride, oxynitride, etc.) can be disposed. Specific examples of the other layer 12 include metal layers containing elements such as aluminum (Al), chromium (Cr), gold (Au), silver (Ag), copper (Cu), silicon (Si), germanium (Ge), titanium (Ti), and nickel (Ni), layers containing fluorides such as magnesium fluoride (MgF ₂ ) and calcium fluoride (CaF ₂ ), and layers containing silicon oxide (SiO _x ), aluminum oxide (Al ₂ O _x ), yttrium oxide (Y ₂ O _x ), zirconium oxide (ZrO _x ), hafnium oxide (HfO _x ), zinc oxide (ZnO _x ), tantalum oxide (Ta ₂ O _x ), niobium oxide (Nb ₂ O _x ), indium oxide (In ₂ O _x ), tin oxide (SnO _x ), tungsten oxide (WO _x ), cerium oxide (CeO _x ), and titanium oxide (TiO _{x )} . ), lanthanum titanate (La _x Ti _y O _z ), aluminum titanate (La _x Al _y O _z ), a layer containing an oxide such as alumina-added silicon dioxide (SiO ₂ +Al ₂ O ₃ ), a layer containing a sulfide such as ZnS, a layer containing a nitride such as silicon nitride (Si _x N _y ), gallium nitride (GaN), a layer containing a carbide such as tungsten carbide (WC), a composite anion compound such as silicon oxynitride (SiO _x N _y ), lead titanium oxyfluoride (Pb _w Ti _x O _y F _z ), or the like can be used. The other layer 12 may be a single layer or a multilayer of two or more layers. When the other layer 12 is a multilayer of two or more layers, the other layer 12 may be formed by combining a plurality of types of layers among the layers exemplified above. The other layer 12 may also be a layer containing a mixture of two or more types of compounds contained in the layers exemplified above.

The method for forming the other layer 12 is not particularly limited. For example, the method for forming the other layer 12 may be a dry film formation method such as a sputtering method, a vacuum deposition method, or an ion plating method, or a wet film formation method such as a dipping method, a coating method, a spray method, a spin coating method, a bar coating method, a printing method, or a flow coating method.

By adjusting the composition, refractive index, film thickness, number of layers, etc. of the other layers 12 according to the purpose and function, it is possible to create a multilayer film with specific functions added, such as an anti-reflection layer, a half-mirror layer, a light absorption layer, an alkali diffusion prevention layer, an adhesion layer, an antistatic layer, a heater layer, etc.

<Application Examples>
An application example of the present disclosure is an optical member.
The optical member of the present disclosure is characterized by having the multilayer film described above.
Fig. 3 and Fig. 4 are schematic diagrams each showing a configuration of an embodiment of the optical member of the present disclosure. Fig. 3 shows a lens cover for a surveillance camera, in which a multilayer film of the present disclosure is formed on the surface of a dome-shaped resin substrate 21. Fig. 4 shows glasses, which are composed of a glasses lens 31, which is an embodiment of the optical member of the present disclosure, and a glasses frame 32. The multilayer film of the present disclosure is formed on both surfaces of the glasses lens 31.

The multilayer film of the present disclosure can be used as an optical thin film such as an anti-reflection film, various optical filter multilayer films, and optical mirror multilayer films. It can also be used for optical components such as optical filters, optical lenses, light collecting lenses, optical films, optical prisms, eyeglass lenses, photographic lenses, surveillance camera covers, vehicle-mounted camera covers, vehicle-mounted sensor covers, vehicle door mirrors, sheet glass, light collecting lenses, display cover glass, touch panels, and various films, as well as covers to protect the optical components. In addition, by coating layers having compositions, refractive indexes, film thicknesses, and number of layers according to the purpose and function of the substrate 11 on surfaces other than the surfaces on which the above-mentioned layers are provided, it is possible to make optical components with specific functions added thereto, such as a mirror layer, a half mirror layer, a light absorbing layer, a transparent heater layer, and an anti-reflection layer.

Third embodiment
The third embodiment relates to a method for manufacturing a multilayer film.
The method for producing a multilayer film according to the present disclosure includes the steps of:
A step (A) of forming a layer containing cerium oxide on a substrate directly or via another layer by a vacuum deposition method;
and (B) forming a low refractive index layer by a vacuum deposition method on the layer containing cerium oxide directly or via another layer,
the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride,
the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure and a columnar structure,
The thickness of the layer containing cerium oxide is 85 nm or more and 800 nm or less,
When the entire layer containing the cerium oxide is defined as a region (A) and the oxygen deficiency rate of the cerium oxide in the region (A) is defined as an oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
The low refractive index layer has a thickness of 50 nm or more and 240 nm or less.
The multilayer film has been described above, so a part of the description will be omitted.

The cerium oxide-containing layer 13 formed in the above step (A) contains cerium oxide having a cubic polycrystalline structure with a specific oxygen deficiency rate, and the thickness of the cerium oxide-containing layer 13 formed in the above step (A) is 85 nm to 800 nm. The thickness of the silicon oxide-containing layer 14 or the magnesium fluoride-containing layer 15 formed in the above step (B) is 50 nm to 240 nm. The refractive index of the silicon oxide-containing layer 14 and the magnesium fluoride-containing layer 15 formed in the above step (B) for light with a wavelength of 500 nm is 1.65 or less.

The method for producing a multilayer film disclosed herein includes a step (A) of forming a layer containing cerium oxide on a substrate directly or via another layer by vacuum deposition, and a step (B) of forming a layer containing silicon oxide or a layer containing magnesium fluoride on the layer containing cerium oxide directly or via another layer by vacuum deposition. The temperature of the substrate during vacuum deposition is preferably a temperature at which cerium oxide crystallizes. Although it depends on the heat resistance temperature of the substrate used and other film formation conditions, the temperature can usually be selected in the range of 0°C or higher and 500°C or lower.

The evaporation method in vacuum deposition is not limited as long as it is a method that evaporates the film-forming material. For example, evaporation means such as an electron gun, resistance heating, or laser can be used. In addition, the above evaporation means can be used in combination with ion assist, plasma assist, etc., as necessary.

In the method for producing a multilayer film of the present disclosure, prior to step (A), the total value of the partial pressure of water molecules (H ₂ O) and oxygen molecules (O ₂ ) in the atmosphere is preferably 2×10 ^-2 Pa or less, more preferably 5.9×10 ^-4 Pa or less. Moreover, during film formation in region (B), the total value of the partial pressure of water molecules (H ₂ O) and oxygen species (in the present invention, oxygen species refers to oxygen atoms, oxygen molecules, and oxygen ions) is preferably 2×10 ^-2 Pa or less on average, more preferably 7.8×10 ^-3 Pa or less on average.

Even if the degree of vacuum in the deposition apparatus is the same (e.g., 7×10 ⁻⁴ Pa), the partial pressure ratio of gas remaining in the vacuum atmosphere varies greatly depending on the state of the deposition apparatus, for example, the degree of dirt, whether there is a small vacuum leak or not, whether prior baking is performed, etc. As a result, the partial pressure of gas before and during the deposition of the cerium oxide-containing layer changes, which may affect the characteristics of the obtained cerium oxide-containing layer 13. For example, the partial pressure of water molecules can be reduced by cleaning the area around the evaporation source and the wall surfaces in the apparatus before deposition so as to reduce the amount of adsorbed substances (e.g., water molecules).

In addition, when using hearth liners made of pure copper, which is the most commonly used material for forming films using electron guns, the high thermal and electrical conductivity of the material results in high energy loss, and the electron gun output must be increased for evaporation to occur, resulting in a higher temperature around the evaporation source. This can result in a larger amount of adsorbed gas (such as water molecules) being desorbed. By using a hearth liner made of molybdenum or tantalum instead of pure copper, the electron gun output during film formation can be reduced. Molybdenum and other materials also have the advantage of containing less gas than regular oxygen-free copper.

If a cryochiller is installed in addition to a rotary pump and a diffusion pump in the vacuum exhaust system, the partial pressure of water molecules in the atmosphere is likely to decrease. In addition, if the area near the wall of the vacuum deposition system is heated using a heater (wall heater) that heats the wall of the vacuum deposition system, or if baking is performed in advance, the partial pressure of water molecules in the atmosphere is likely to decrease.

Before starting the deposition process, it is important to check that there are almost no leaks in the deposition equipment in order to stabilize the oxygen partial pressure inside the equipment. This is because even a small leak can cause the air around the equipment to flow into the equipment, increasing the oxygen partial pressure in the atmosphere and potentially affecting the properties of the resulting cerium oxide-containing layer.

When heating the substrate, there is a time lag until the actual temperature of the substrate becomes the same as the temperature measured by the thermocouple. Although it depends on the substrate heat setting temperature, the material of the substrate holder, the thickness of the substrate, etc., it is necessary to wait about 10 to 20 minutes after the thermocouple temperature reaches the set temperature. In addition, since a certain amount of moisture is adsorbed on the substrate, by heating the substrate sufficiently, more moisture adsorbed on the substrate before film formation can be desorbed. During film formation, energy from radiant heat from the evaporation source is also added, making it easier for moisture to desorb from the substrate. Therefore, when heating the substrate, the film formation process was started after 15 minutes or more had passed since the thermocouple used to measure the substrate temperature indicated the set temperature ±2°C.

When placing the substrate in the deposition device, a jig, usually called a holder or holder, is used to set the substrate in the device. If a clean, washed jig is used and made of a material with low gas adsorption and desorption, such as SUS316LN, the amount of gas released, such as moisture, during film formation can be reduced. On the other hand, dirty jigs, porous materials, jigs made of brass or synthetic resin, or galvanized jigs, etc., release a lot of gas, making it easier for gas components such as moisture and oxygen to be released from the jig during film formation.

The reason for cleaning the inner wall surface, installing a cryochiller, checking for leaks, controlling the substrate heating time, selecting the jig material, etc. is that when forming the cerium oxide-containing layer 13, even under the same gas introduction conditions (for example, when no gas is introduced or when the amount of oxygen introduced is 1×10 ⁻² Pa), the partial pressure of oxygen molecules and water molecules in the atmosphere will be different. This may affect the oxygen deficiency rate of the obtained cerium oxide-containing layer. Therefore, if these are not performed, even if other film formation conditions, etc. are the same as those in the examples of the present application, a multilayer film including the cerium oxide-containing layer 13 of the present disclosure may not be obtained.

In the method for producing a multilayer film according to the present disclosure, when the entire layer containing cerium oxide is defined as region (A) and the oxygen deficiency rate of cerium oxide in region (A) is defined as oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less. In addition, in the method for producing a multilayer film according to the present disclosure, as shown in FIG. 1B , when a region of the cerium oxide-containing layer located 8 nm or less from the interface between the cerium oxide-containing layer and the low refractive index layer (the layer containing magnesium fluoride or the layer containing silicon oxide) is defined as region (B) and the oxygen deficiency rate of cerium oxide in region (B) is defined as oxygen deficiency rate (V _B ), the oxygen deficiency rate (V _B ) is 0.5% or more and 30% or less; and when a region of region (A) excluding region (B) is defined as region (C) and the oxygen deficiency rate of cerium oxide in region (C) is defined as oxygen deficiency rate (V _C ), the oxygen deficiency rate (V _C ) is 0% or more and 10% or less, and the oxygen deficiency rate (V _B ) is preferably greater than the oxygen deficiency rate (V _C ).

The above method can be used to effectively manufacture the multilayer film disclosed herein.

The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited in any way to the following examples.

The materials used to fabricate and evaluate the multilayer films in the examples are listed below.

(Base material)
Flat plates of the materials listed below were used as substrates. When the substrate temperature during deposition was 100°C or higher, substrates other than resin were used. When the substrate temperature was other than this, all of the substrates listed below were used. The silicon and sodium chloride substrates were used for analyzing the oxygen deficiency rate and measuring the film thickness using an electron microscope.
・Borosilicate glass: 3mm thick
・Synthetic quartz: thickness 3 mm
・Polycarbonate resin: thickness 2mm
・Polymethylmethacrylate resin: thickness 2 mm
・Silicon: Thickness 1mm
・Sodium chloride: 2 mm thick
(film forming material)
The following materials were used:
・Ce: Granular, purity 99.9%
_CeO2 : cylindrical, purity 99.9%
_La2O3 : _cylindrical , purity 99.9%
_Sm2O3 : _cylindrical , purity 99.9%
・SiO: Granular, purity 99.9%
・SiO ₂ : Granular, purity 99.9%
・_Al2O3 : _Granular , purity 99.95%
・MgF ₂ : Granular, purity 99.9%
・_Ti3O5 : _Granular , purity 99.9%
・TaO: granular, purity 99.9%
・Cr: Granular, purity 99.9%
(Gas for ion-assisted deposition)
_O2 : gas, purity 99.999%
Ar: gas, purity 99.999%
(Gas for checking vacuum leaks)
He: gas, purity 99.9999%
(Etching gas used during measurement)
Ar: gas, purity 99.9999%
(Reagents, etc.)
・Pure water: JIS K0557 A4
Stearic acid: JIS K8585 special grade, purity 99.9%
Heptane: JIS K9701 special grade, purity 99.9%

(Fabrication of multilayer film)
The film formation method and film formation conditions common to the examples and comparative examples in the production of the multilayer film will be described below. A vacuum deposition apparatus (dome diameter Φ1300 mm, deposition distance 1100 mm) was used as the film formation apparatus.
In order to obtain the same multilayer film on all substrates placed in the deposition device, the temperature distribution, deposition rate of the film-forming material, and ion-assisted incident ion distribution were adjusted so that they were the same for all substrates placed on the dome and the monitor substrate. To achieve this adjustment, measurements were made using Faraday cups, thermocouples, and film thickness measuring devices, and the positioning of compensation plates and sheathed heaters and heater output were repeatedly optimized.
Before deposition, the area around the evaporation source, the dome, and the walls inside the device were polished and wiped clean with an organic solvent (isohexane).

Next, the film-forming material and various clean substrates were placed in a vacuum deposition apparatus.
The deposition material for film formation by the electron gun was placed in a cleaned and degassed molybdenum hearth liner and evaporated. A cleaned and degassed SUS316LN substrate holder was used.
If the only test piece obtained was a multilayer film, analysis and evaluation would be difficult. Therefore, various substrates were set up in a mechanism that allowed the substrate to be replaced for each layer, enabling not only multilayer films but also single-layer films for analysis and evaluation to be obtained at the same time.

Thereafter, the chamber was evacuated until the partial pressure of water molecules and oxygen molecules reached the vacuum level required to start film formation. A cryochiller (PFC-1102HC, manufactured by Polycold) was used in addition to a rotary pump and a diffusion pump as a vacuum exhaust device.
Before starting the film formation process, it was confirmed that there was almost no leakage in the vacuum deposition apparatus using He gas for leakage detection and a quadrupole mass spectrometer (Qulee with YTP-H, manufactured by ULVAC, Inc.) as a leakage detector. It was confirmed that the mass spectrometer did not detect He gas for leakage detection and that the ratio of nitrogen molecules (mass number 28) to oxygen molecules (mass number 32) was clearly different from that in the atmosphere (N ₂ :O ₂ = 79%:21%).

Before starting the deposition of the cerium oxide-containing layer 13, the partial pressure of water molecules and the partial pressure of oxygen molecules in the atmosphere were measured and calculated using an ionization vacuum gauge and a quadrupole mass spectrometer.
In addition, during the formation of region (B) of layer 13 containing cerium oxide, the partial pressure of water molecules and the partial pressure of oxygen species (oxygen atoms, oxygen molecules, oxygen ions) were measured and calculated every second using an ionization vacuum gauge and a quadrupole mass spectrometer, and these partial pressures were summed and averaged to calculate the average partial pressure during the formation of region (B).
The substrate temperature during the film formation was −15° C. to 600° C. The film formation process was started after 15 minutes or more had elapsed since the substrate temperature had reached the set temperature ±2° C.

Thereafter, a multilayer film was formed on the set substrate by vacuum deposition of the film-forming material as shown in Tables 1-1 to 1-4 to obtain a test piece.
Unless otherwise specified, layers containing cerium oxide were deposited at a deposition rate of 0.5 nm/s. The dome rotation speed during deposition of each _layer was 20 rpm. Films consisting of two components, such as _{CeO2 + Al2O3} or _SiO2 + _Al2O3 , were deposited by a binary deposition method in which two types of film-forming materials are placed on two heating sources and evaporated simultaneously. When _MgF2 and SiO were used as film-forming materials, a tantalum boat and resistance heating were used as the heating source.

In each of Examples 1 to 66 and Comparative Examples 1 to 22, there was no substantial difference between the multilayer films obtained by changing the type of substrate, so only one example is shown in Tables 1-1 to 1-4. In addition, "crystalline" in Tables 1-1 to 1-4 means that the layer containing cerium oxide contains cerium oxide having a cubic polycrystalline structure, and "amorphous" means that the layer containing cerium oxide does not contain cerium oxide having a cubic polycrystalline structure. In Tables 1-1 to 1-4, "columnar" means that the layer containing cerium oxide contains cerium oxide having a columnar structure, and "non-columnar" means that the layer containing cerium oxide does not contain cerium oxide having a columnar structure.

The individual conditions for fabricating the multilayer film in each of the examples and comparative examples will be described below.
[Example 1]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.53×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Then, a MgF ₂ film (low refractive index layer) was formed on the layer using MgF ₂ as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the surface layer 8 nm side of the layer containing cerium oxide (region (B): the region of the layer containing cerium oxide located 8 nm or less from the interface between the layer containing cerium oxide and the layer containing silicon oxide or the layer containing magnesium fluoride) was 3.11×10 ⁻³ Pa on average. Other film formation conditions were as described in (Fabrication of multilayer film).

[Example 2]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 105°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.50×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.06×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 3]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 110°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.50×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed on the layer using SiO and Al ₂ O ₃ as film forming materials, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.05×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 4]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 115°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.49×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Next, a MgF ₂ film (low refractive index layer) was formed on the layer using MgF ₂ as a film forming material, and a multilayer film was produced. Note that the total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.03×10 ⁻³ Pa on average. Other film formation conditions were as described in (Production of multilayer film).

[Example 5]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 140°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.48×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.00×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 6]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 120°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.47×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed on the layer using SiO and CeO ₂ as film forming materials, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.98×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 7]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 125°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.45×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Next, a MgF ₂ film (low refractive index layer) was formed on the layer using MgF ₂ as a film forming material, and a multilayer film was produced. Note that the total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.96×10 ⁻³ Pa on average. Other film formation conditions were as described in (Production of multilayer film).

[Example 8]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 130°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.45×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.94×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 9]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 135°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.44×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed on the layer using SiO and Al ₂ O ₃ as film forming materials, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.92×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 10]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 140°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.39×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Next, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.84×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 11]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 145°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.38×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.80×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 12]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 150°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.37×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed on the layer using SiO and CeO ₂ as film forming materials, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.78×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 13]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 155°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.34×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Next, a MgF ₂ film (low refractive index layer) was formed on the layer using MgF ₂ as a film forming material, and a multilayer film was produced. Note that the total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.73×10 ⁻³ Pa on average. Other film formation conditions were as described in (Production of multilayer film).

[Example 14]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.33×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.71×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 15]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 165°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.26×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed on the layer using SiO and Al ₂ O ₃ as film forming materials, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 2.57×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 16]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 150°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.71×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 0.7 nm/s using CeO ₂ as a film formation material. Next, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film formation material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 4.89×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 17]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 175°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.78×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 1.0 nm/s using CeO ₂ as a film formation material. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film formation material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 7.23×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 18]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 180°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.96×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 1.5 nm/s using CeO ₂ as a film formation material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed thereon using SiO and CeO ₂ as film formation materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.20×10 ⁻² Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 19]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using Ce and CeO ₂ as film-forming materials. Then, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 5.84×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 20]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.83×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using Ce and CeO ₂ as film-forming materials. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 5.76×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 21]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 200°C and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.03×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 0.65 nm/s using Sm ₂ O ₃ and CeO ₂ as film formation materials. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed thereon using SiO and Al ₂ O ₃ as film formation materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.71×10 ⁻³ Pa on average. Other film formation conditions were as described in (Fabrication of multilayer film).

[Example 22]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.84×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using Ce and CeO ₂ as film-forming materials. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 5.76×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 23]
The vacuum deposition apparatus was set to a substrate temperature of 250°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 250±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. When the layer containing cerium oxide was formed, ion assist was performed using an RF ion source. The ion assist conditions were an acceleration voltage value of 700V, an acceleration current value of 700mA, and an O ₂ gas flow rate of 60sscm. In addition, when the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was formed, the ion gun and gas introduction were stopped. When the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was formed, the ion gun and gas introduction were stopped. Next, a _MgF2 film (low refractive index layer) was formed thereon using _MgF2 as a film forming material to produce a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the surface 8 nm side (region (B)) of the layer containing cerium oxide was 3.08× ^10-3 Pa on average. The other film formation conditions were as described in (Production of multilayer film).

[Example 24]
The vacuum deposition apparatus was set to a substrate temperature of 350°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 350±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.49×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. When the layer containing cerium oxide was formed, oxygen was introduced by Auto Pressure Control (APC). The amount of oxygen gas introduced was 1.9×10 ⁻² Pa. In addition, when the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was formed, the film was formed 1 minute after the introduction of oxygen gas was stopped. Next, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material, and a multilayer film was produced. During deposition of the layer containing cerium oxide in the top 8 nm region (region (B)), the total value of the partial pressure of water molecules and the partial pressure of oxygen species was 3.03×10 ⁻³ Pa on average. The other deposition conditions were as described in (Fabrication of multilayer film).

[Example 25]
The vacuum deposition apparatus was set to a substrate temperature of 300°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 300±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.45×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. When the layer containing cerium oxide was formed, ion assist was performed using an RF ion source. The ion assist conditions were an acceleration voltage value of 700V, an acceleration current value of 700mA, an O ₂ gas flow rate of 35sscm, and an Ar gas flow rate of 7sccm. In addition, when forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the ion gun and gas introduction were stopped. Next, a _SiO2 + _Al2O3 film (low refractive index layer) was formed thereon using _SiO2 and _Al2O3 as film _- forming materials to produce a multilayer film. The total value _of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the surface 8 nm (region (B)) of the layer containing cerium oxide was 2.95× ^10-3 Pa on average. The other film formation conditions were as described in (Production of multilayer film).

[Example 26]
The vacuum deposition apparatus was set to a substrate temperature of 300°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 300±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.39×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. When the layer containing cerium oxide was formed, ion assist was performed using an RF ion source. The ion assist conditions were an acceleration voltage value of 700V, an acceleration current value of 700mA, an O ₂ gas flow rate of 30sscm, and an Ar gas flow rate of 10sccm. In addition, when the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was formed, the ion gun and gas introduction were stopped to form the film. Subsequently, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material, and a multilayer film was produced. During deposition of the layer containing cerium oxide, 8 nm from the surface (region (B)), the total value of the partial pressure of water molecules and the partial pressure of oxygen species was 2.83×10 ⁻³ Pa on average. The other deposition conditions were as described in (Fabrication of multilayer film).

[Example 27]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.45×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 2.0 nm/s using CeO ₂ as a film formation material. Next, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film formation material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.99×10 ⁻² Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 28]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using Ce and CeO ₂ as film-forming materials. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 5.84×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 29]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Next, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.08×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 30]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.08×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 31]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.50×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed on the layer using SiO and Al ₂ O ₃ as film forming materials, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.05×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 32]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.47×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Next, a MgF ₂ film (low refractive index layer) was formed on the layer using MgF ₂ as a film forming material, and a multilayer film was produced. Note that the total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.98×10 ⁻³ Pa on average. Other film formation conditions were as described in (Production of multilayer film).

[Example 33]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.45×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film forming material to prepare a multilayer film. Note that the total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.95×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 34]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.42×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed on the layer using SiO and CeO ₂ as film forming materials, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.89×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 35]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.38×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Next, a MgF ₂ film (low refractive index layer) was formed on the layer using MgF ₂ as a film forming material, and a multilayer film was produced. Note that the total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.80×10 ⁻³ Pa on average. Other film formation conditions were as described in (Production of multilayer film).

[Example 36]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.37×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.80×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 37]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.31×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed on the layer using SiO and Al ₂ O ₃ as film forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.66×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 38]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 200°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 9.84×10 ⁻⁵ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 0.7 nm/s using La ₂ O ₃ and CeO ₂ as film formation materials. Then, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film formation material, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.80×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 39]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using Ce and CeO ₂ as film-forming materials. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface 8 nm (region (B)) of the layer containing cerium oxide was 5.84×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 40]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.27×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film-forming material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed thereon using SiO and CeO ₂ as film-forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface 8 nm (region (B)) of the layer containing cerium oxide was 2.59×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 41]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface 8 nm (region (B)) of the layer containing cerium oxide was 3.08×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 42]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.37×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed thereon using SiO and CeO ₂ as film forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.79×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 43]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.08×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 44]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.45×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.94×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 45]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed on the layer using SiO and Al ₂ O ₃ as film forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 3.08×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 46]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.45×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.95×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 47]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 3.07×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 48]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.41×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed thereon using SiO and CeO ₂ as film forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.87×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 49]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.07×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 50]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.37×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface 8 nm (region (B)) of the layer containing cerium oxide was 2.78×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 51]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed thereon using SiO ₂ and Al ₂ O ₃ as film forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 3.08×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 52]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.45×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.95×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 53]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.50×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO ₂ as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface 8 nm (region (B)) of the layer containing cerium oxide was 3.05×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 54]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.44×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed thereon using SiO ₂ and CeO ₂ as film forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.94×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 55]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.37×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film forming material. Then, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.79×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 56]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.22×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 2.49×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 57]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.08×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on each substrate at a film formation rate of 3.0 nm/s using CeO ₂ as a film formation material. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed thereon using SiO and Al ₂ O ₃ as film formation materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.54×10 ⁻² Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 58]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 160°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using Ce and CeO ₂ as film-forming materials. Then, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.51×10 ⁻² Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 59]
The vacuum deposition apparatus was turned off to heat the substrate, and the wall heater was set to 50°C to evacuate. The substrate temperature before film formation was 28°C. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.22 x ^10-4 Pa. A layer containing cerium oxide was formed on various substrates using _CeO2 as a film-forming material. Then, a _SiO2 film (low refractive index layer) was formed on the layer using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface 8 nm (region (B)) of the layer containing cerium oxide was 2.48 x ^10-3 Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 60]
The vacuum deposition apparatus was turned off to heat the substrate, and the wall heater was set to 50° C. to evacuate the vacuum. The substrate temperature before film formation was 31° C. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.45×10 ⁻⁴ Pa. A layer containing cerium oxide was formed on various substrates using CeO ₂ as a film-forming material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed on the layer using SiO and CeO ₂ as film-forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.95×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 61]
The vacuum deposition apparatus was set to a substrate temperature of 100°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 100±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 9.54×10 ⁻⁵ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Next, a MgF ₂ film (low refractive index layer) was formed on the layer using MgF ₂ as a film forming material, and a multilayer film was produced. Note that the total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.94×10 ⁻³ Pa on average. Other film formation conditions were as described in (Production of multilayer film).

[Example 62]
The vacuum deposition apparatus was set to a substrate temperature of 100°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 100±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.36×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 2.78×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 63]
The vacuum deposition apparatus was set to a substrate temperature of 270°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 270±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using Ce and CeO ₂ as film-forming materials. Then, a SiO ₂ +Al ₂ O ₃ film (low refractive index layer) was formed on the layer using SiO and Al ₂ O ₃ as film-forming materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 5.84×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 64]
The vacuum deposition apparatus was set to a substrate temperature of 270°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 270±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.84×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 1.0 nm/s using CeO ₂ as a film formation material. Next, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film formation material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 7.50×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 65]
The vacuum deposition apparatus was set to a substrate temperature of 500°C and a wall heater of 200°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 500±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.37×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. Next, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO as a film-forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 2.79×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Example 66]
The vacuum deposition apparatus was set to a substrate temperature of 500°C and a wall heater of 200°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 500±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 0.7 nm/s using _{Y 2} O ₃ and CeO ₂ as film formation materials. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed thereon using SiO and CeO 2 as film formation materials, to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 8.18×10 ⁻³ Pa on average. Other film formation conditions were as described in (Fabrication of multilayer film).

[Comparative Example 1]
The vacuum deposition apparatus was set to a substrate temperature of 300°C and a wall heater of 50°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 8.20×10 ⁻³ Pa. Then, a layer containing cerium oxide was formed on each substrate at _a film formation rate of 0.2 nm/s using CeO ₂ as a film formation material. Then, a SiO ₂ film (low refractive index layer) was formed on the layer using SiO 2 as a film formation material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 6.68×10 ⁻² Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Comparative Example 2]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 6.15×10 ⁻³ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 0.2 nm/s using CeO ₂ as a film formation material. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed on the layer using SiO ₂ and CeO ₂ as film formation materials to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 5.01×10 ⁻² Pa on average. Other film formation conditions were as described in (Preparation of multilayer film).

[Comparative Example 3]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.53×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation speed of 0.2 nm/s using CeO ₂ as a film formation material. When forming the layer containing cerium oxide, O ₂ gas was introduced using an Auto Pressure Control (APC) device so that the degree of vacuum was 1.8×10 ⁻² Pa. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.80×10 ⁻² Pa on average. Subsequently, a _MgF2 film (low refractive index layer) was formed thereon using _MgF2 as a film forming material to prepare a multilayer film. The other film forming conditions were as described in (Preparation of multilayer film).

[Comparative Example 4]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.53×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation rate of 0.2 nm/s using CeO ₂ as a film formation material. When forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, ion assist was performed using an RF ion source. The ion assist conditions were an acceleration voltage value of 700 V, an acceleration current value of 700 mA, and an O ₂ gas flow rate of 50 sscm. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 3.11×10 ⁻³ Pa on average. Subsequently, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film forming material to prepare a multilayer film. The other film forming conditions were as described in (Preparation of multilayer film).

[Comparative Example 5]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 150°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.91×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation speed of 0.9 nm/s using Ce and CeO ₂ as film formation materials. When forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the film formation speed was changed to 0.1 nm/s. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.07×10 ⁻² Pa on average. Subsequently, a _SiO2 + _Al2O3 film (low refractive index layer ₎ was formed thereon using SiO and _Al2O3 as _film forming materials to prepare a multilayer film. Other film forming conditions were as described in (Preparation of multilayer film).

[Comparative Example 6]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 150°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation speed of 1.0 nm/s using Ce and CeO ₂ as film formation materials. When forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the film formation speed was changed to 0.1 nm/s. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.17×10 ⁻² Pa on average. Next, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film formation material, to prepare a multilayer film. Other film formation conditions were as described in (Fabrication of multilayer film).

[Comparative Example 7]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 150°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.83×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation speed of 1.1 nm/s using Ce and CeO ₂ as film formation materials. When forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the film formation speed was changed to 0.1 nm/s. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.27×10 ⁻² Pa on average. Then, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film formation material, to prepare a multilayer film. Other film formation conditions were as described in (Fabrication of multilayer film).

[Comparative Example 8]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 150°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.79×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation speed of 1.2 nm/s using Ce and CeO ₂ as film formation materials. When forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the film formation speed was changed to 0.1 nm/s. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.36×10 ⁻² Pa on average. Then, a SiO ₂ +CeO ₂ film (low refractive index layer) was formed on it using SiO and CeO ₂ as film formation materials, to produce a multilayer film. Other film formation conditions were as described in (Fabrication of multilayer film).

[Comparative Example 9]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 150°C, and evacuated. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.75×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate at a film formation speed of 1.4 nm/s using Ce and CeO ₂ as film formation materials. When forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the film formation speed was changed to 0.1 nm/s. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide was 1.57×10 ⁻² Pa on average. Next, a MgF ₂ film (low refractive index layer) was formed thereon using MgF ₂ as a film formation material, to prepare a multilayer film. Other film formation conditions were as described in (Fabrication of multilayer film).

[Comparative Example 10]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.53×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film-forming material. When the layer containing cerium oxide was formed, oxygen was introduced by APC. The amount of oxygen gas introduced was 5.0×10 ⁻² Pa. During the film formation of the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen species was 5.00×10 ⁻² Pa on average. Next, a SiO ₂ film (low refractive index layer) was formed thereon using SiO as a film-forming material, and a multilayer film was produced. Other film formation conditions were as described in (Fabrication of multilayer film).

[Comparative Example 11]
The vacuum deposition apparatus was set to a substrate temperature of 250°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 250±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.52×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. When the layer containing cerium oxide was formed, ion assist was performed using an RF ion source. The ion assist conditions were an acceleration voltage value of 700V, an acceleration current value of 700mA, and an O ₂ gas flow rate of 80sscm. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface 8 nm (region (B)) of the layer containing cerium oxide was 3.09×10 ⁻² Pa on average. Subsequently, a _SiO2 + _Al2O3 film (low refractive index layer ₎ was formed thereon using SiO and _Al2O3 as _film forming materials to prepare a multilayer film. Other film forming conditions were as described in (Preparation of multilayer film).

[Comparative Example 12]
The vacuum deposition apparatus was set to a substrate temperature of 250°C and a wall heater of 100°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 250±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.52×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. When the layer containing cerium oxide was formed, ion assist was performed using an RF ion source. The ion assist conditions were an acceleration voltage value of 700V, an acceleration current value of 700mA, and an O ₂ gas flow rate of 70sscm. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface 8 nm (region (B)) of the layer containing cerium oxide was 3.02×10 ⁻² Pa on average. Also. Subsequently, a _MgF2 film (low refractive index layer) was formed thereon using _MgF2 as a film forming material to prepare a multilayer film. The other film forming conditions were as described in (Preparation of multilayer film).

[Comparative Example 13]
Instead of the layer containing cerium oxide having a columnar structure in Example 34, the substrate temperature was changed to 600° C. so that a layer containing cerium oxide having a non-columnar structure was obtained. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. The film formation of the layer containing cerium oxide was performed with ion assistance using an RF ion source. The ion assistance conditions were an acceleration voltage value of 900 V, an acceleration current value of 1000 mA, an O ₂ gas flow rate of 6 sscm, and an Ar gas flow rate of 34 sccm. In addition, when forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the O ₂ gas flow rate of the ion source was changed to 2 sscm, and the Ar gas flow rate was changed to 38 sccm to form the film. During deposition of the layer containing cerium oxide in the top 8 nm region (region (B)), the total value of the partial pressure of water molecules and the partial pressure of oxygen species was 3.08×10 ⁻³ Pa on average. Except for this, a multilayer film was prepared in the same manner as in Example 34.

[Comparative Example 14]
Instead of the layer containing cerium oxide having a columnar structure in Example 36, the substrate temperature was changed to 600° C. so that a layer containing cerium oxide having a non-columnar structure was obtained. Before starting the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.51×10 ⁻⁴ Pa. The film formation of the layer containing cerium oxide was performed with ion assistance using an RF ion source. The ion assistance conditions were an acceleration voltage value of 1000 V, an acceleration current value of 1000 mA, an O ₂ gas flow rate of 3 sscm, and an Ar gas flow rate of 37 sccm. In addition, when forming the surface layer side 8 nm (region (B)) of the layer containing cerium oxide, the O ₂ gas flow rate of the ion source was changed to 0 sscm, and the Ar gas flow rate was changed to 40 sccm to form the film. During deposition of the layer containing cerium oxide in the top 8 nm region (region (B)), the total value of the partial pressure of water molecules and the partial pressure of oxygen species was 3.07×10 ⁻³ Pa on average. Except for this, a multilayer film was prepared in the same manner as in Example 36.

[Comparative Example 15]
Instead of the cubic polycrystalline CeO2 _layer of Example 19, an amorphous _CeO2 layer was obtained by changing the substrate temperature to -18°C, and in addition, the wall heater was turned off before evacuation and film formation. Before starting the film formation process of the cerium oxide-containing layer, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.38 x ^10-5 Pa. In addition, during film formation of the surface 8 nm (region (B)) of the cerium oxide-containing layer, the total value of the partial pressure of water molecules and the partial pressure of oxygen species was 4.84 x ^10-4 Pa on average. A multilayer film was produced in the same manner as in Example 19 except for the above.

[Comparative Example 16]
Instead of the cubic polycrystalline CeO2 _layer of Example 20, an amorphous _CeO2 layer was obtained by changing the substrate temperature to -18°C, and in addition, the wall heater was turned off before evacuation and film formation. Before starting the film formation process of the cerium oxide-containing layer, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87 x ^10-4 Pa. In addition, during film formation of the surface 8 nm (region (B)) of the cerium oxide-containing layer, the total value of the partial pressure of water molecules and the partial pressure of oxygen species was 5.84 x ^10-3 Pa on average. A multilayer film was produced in the same manner as in Example 20 except for the above.

[Comparative Example 17]
Instead of forming the SiO ₂ film in Example 24, a SiO film was formed at a film formation rate of 1.3 nm/s using SiO. The SiO film had a large refractive index and a large amount of light loss due to light absorption, so it was not suitable as an optical thin film.

[Comparative Example 18]
The vacuum deposition apparatus was set to a substrate temperature of 400°C and a wall heater of 180°C, and evacuated. When 15 minutes or more had elapsed since the substrate temperature reached 400±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 2.87×10 ⁻⁴ Pa. Then, a layer containing cerium oxide was formed on each substrate using CeO ₂ as a film forming material. Then, a Y ₂ O ₃ film was formed on the layer using Y ₂ O ₃ as a film forming material to prepare a multilayer film. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation of the surface layer 8 nm (region (B)) of the layer containing cerium oxide was 5.84×10 ⁻³ Pa on average. Other film formation conditions were as described in (Preparation of multilayer film). Since the refractive index was larger than that of the _{Y 2} O ₃ film, it was not suitable as an optical member.

(Evaluation of oxygen deficiency rate)
The obtained multilayer film test piece was introduced into an X-ray photoelectron spectroscopy apparatus (ESCA-300 manufactured by Scienta) and evacuated to a high vacuum. The thin film above the cerium oxide-containing layer 13 of the multilayer film was removed by etching with argon ions. Then, XPS spectrum measurement of the Ce 3d orbital and etching with argon ions were repeated. The analysis area of the multilayer film was changed and a total of three locations were analyzed. From the obtained results, the oxygen deficiency rate (V _A ), oxygen deficiency rate (V _B ), and oxygen deficiency rate (V _C ) of the cerium oxide-containing layer (13) were calculated.

For oxygen vacancy rates below the lower limit of quantification by X-ray photoelectron spectroscopy, observation, projection, and measurement of oxygen vacancies were performed using a scanning transmission electron microscope (HF5000, manufactured by Hitachi High-Technologies). The multilayer film to be measured was pretreated by (1) splitting the obtained multilayer film together with the substrate to expose the cross section, (2) processing with a focused ion beam (FIB), and (3) etching in the vertical direction with an ion beam, using either or a combination of (1) to (3) above to collect and finely process the measurement area. In addition, to minimize changes in oxygen vacancies, processing was performed immediately after film formation in an atmosphere that combined low temperature, argon gas, and a vacuum pump.

Then, under the condition of an acceleration voltage of 200 kV, HAADF-STEM images and ABF-STEM images showing the presence or absence of oxygen atoms in the layer containing cerium oxide were obtained stepwise in the depth direction of the film thickness. To assist in the observation of oxygen and oxygen vacancies, which are light elements, LAADF-STEM images of the same area were also obtained. A total of five images were obtained by changing the analysis area of the layer film. Each of the obtained images was converted into a mapping image by software processing, and the number of oxygen sites and the number of oxygen vacancies were counted to determine the oxygen deficiency rate (V _A ), (V _B ), and (V _C ) of the layer containing cerium oxide.

For the multilayer films obtained in Examples 19 to 22, 28, 38, and 39 and Comparative Examples 5 to 9, the oxygen deficiency rate (V _A ) was also measured by X-ray absorption fine structure (XAFS). CeO ₂ and Ce(NO ₃ ) ₃ -6H ₂ O were used as standard samples, the measured absorption edge was the Ce-L ₃ absorption edge (5723.0 eV), and the measurement was performed by a fluorescence yield method using a multi-element silicon drift detector. The oxygen deficiency rate was calculated from the fitting results of the obtained XANES spectrum. There was almost no difference from the oxygen deficiency rate value measured by X-ray photoelectron spectroscopy or electron microscope, and it was confirmed that the measurement of the oxygen deficiency rate was valid.

(Thickness (film thickness) measurement and refractive index measurement)
The thickness and refractive index of each layer of the multilayer film in the examples and comparative examples were determined using a spectroscopic ellipsometry (JA WOOLLAM ESM300). The wavelength of the light source was set to 192 nm to 1000 nm, and the incidence angle was measured in 5° increments in the range of 45° to 65°. In addition, in order to improve the accuracy of the analysis, the transmittance was also measured. The obtained reflection and transmittance data were analyzed by combining models such as Cauchy, Gaussian, Tauc-Lorentz, and effective medium approximation (surface roughness), and the film thickness and refractive index were obtained. Since it is a multilayer film, the number of layers and film thickness are complicated, and analysis may be difficult. In such a case, a single layer film obtained separately by film formation of the same batch was measured and analyzed, and the analysis was performed using the obtained data such as the refractive index.

(Determination of Composition)
The compositions of layers consisting of two components, such as _SiO2 + _CeO2 or _SiO2 + _{Al2O3 ,} in the multilayer films of the examples and comparative examples were measured using a wavelength dispersive X-ray fluorescence spectrometer (ZSX Primus II, manufactured by Rigaku Corporation). When quantitative analysis of each layer was difficult, measurements were performed on a single layer film obtained separately by film formation in the same batch.

(Confirmation of structure)
The cross section of the multilayer film formed on the Si substrate was observed with a super-high resolution field emission scanning electron microscope (JSM-IT800, manufactured by JEOL Ltd.) to obtain backscattered electron images and secondary electron images. From the obtained images, it was confirmed whether the structure was columnar or other structure (non-columnar). At that time, the film thickness of each layer was also confirmed. There was almost no difference from the film thickness value obtained by the ellipsometer, and it was confirmed that the film thickness measurement by the ellipsometer was appropriate. In addition, the composition of each layer was also measured by energy dispersive X-ray spectroscopy (EDX), and there was almost no difference from the value obtained by the above (composition measurement), and it was confirmed that the above composition measurement was appropriate.

(Measurement of Crystallinity)
The multilayer films of the examples and comparative examples were measured by an XRD diffractometer (Smart Lab, manufactured by Rigaku Corporation) in the range of 2θ = 20° to 100° by the focusing method at a step of 0.01° and a speed of 5°/min, and the layers were identified and their crystallinity was confirmed by the diffraction line intensity. Since it is a multilayer film, the diffraction intensity of MgF ₂ , SiO ₂ + CeO ₂ , etc. may interfere with the analysis. In that case, a single layer film obtained separately by film formation of the same batch was also measured and analyzed. A multilayer film having a diffraction peak due to a layer containing cerium oxide was judged to be crystalline, and a multilayer film having no diffraction peak due to a layer containing cerium oxide was judged to be amorphous.

(Hydrophilicity Maintenance Evaluation)
The multilayer films of the examples and comparative examples were left in a dark place (temperature 23±2°C, humidity 60±15% RH) for 60 days, and then the contact angle with water was measured. After the measurement, the substrate was left in a dark place for an additional 240 days, and after leaving it in a dark place for a total of 300 days, the contact angle with water was measured. The contact angle meter used was a CA-X150 model manufactured by Kyowa Interface Science Co., Ltd., and 2.5 mL of pure water was dropped onto the test piece from a microsyringe, and the contact angle 5 seconds after the drop was obtained by the θ/2 method.
The hydrophilicity of a surface can be quantified by the contact angle with water. Generally, a contact angle of less than 20° is called hydrophilic, and a contact angle of less than 10° is called superhydrophilic. Following this, a contact angle of less than 10° was rated as [A], a contact angle of 10° or more but less than 20° was rated as [B], and a contact angle of 20° or more was rated as [C].

(Self-purification performance evaluation)
Stearic acid was applied to the multilayer films of the examples and comparative examples using a heptane solution (0.3 mass%) of stearic acid in accordance with JIS R1753-1, and dried in a dryer at 70°C for 30 minutes. Thereafter, the contact angle of the test piece coated with stearic acid was measured in the same manner as in the method described in (Evaluation of hydrophilicity maintenance), and it was confirmed that the contact angle was 20° or more. If the contact angle was not 20° or more, the application of stearic acid, drying, and contact angle measurement were repeated until the contact angle became 20° or more. Thereafter, the multilayer film coated with stearic acid was irradiated with ultraviolet light for 2 hours and 30 hours, and then the contact angle was measured again to determine the water contact angle after ultraviolet light irradiation. A black light blue fluorescent lamp (FL20SBL-B, manufactured by Hotalux Co., Ltd.) was used as the ultraviolet light source. The test piece was irradiated with ultraviolet light so that the illuminance was 2.0 mw/ ^cm2 .
As in the evaluation in the above (Evaluation of Hydrophilicity Maintenance), a contact angle of less than 10° was rated as [A], a contact angle of 10° or more and less than 20° was rated as [B], and a contact angle of 20° or more was rated as [C].

The results obtained in (hydrophilicity maintenance evaluation) and (self-cleaning performance evaluation) are shown in Tables 1-1 to 1-4. Note that in each example and comparative example, the evaluation results were the same regardless of the type of substrate.

(Evaluation of Optical Properties)
The transmittance and reflectance of the multilayer films of the examples and comparative examples at an incidence angle of 5° were measured. A UV-Vis-NIR spectrophotometer (UH4150 manufactured by Hitachi High-Technologies) was used as the measuring device, and measurements were made in 1 nm increments over the measurement wavelength range of 450 nm to 1200 nm. The average transmittance (%) and reflectance (%) obtained for each wavelength were calculated to obtain the average transmittance (%) and average reflectance (%). The average light loss was calculated by subtracting the average transmittance (%) and average reflectance (%) from 100%. When the average light loss was less than 1%, it was rated as [A], and when it was 1% or more, it was rated as [C].

[Comparative Example 23]
Instead of forming a layer containing cerium oxide as in Example 23, a _TiO2 film was formed. In this case, _Ti3O5 was used as _the film forming material. The other film forming conditions were the same as those in Example 23 to form a multilayer film. The crystallinity of _{the TiO2} layer was evaluated to be anatase type polycrystal. The structure was evaluated to be a columnar structure. The hydrophilicity retention performance was evaluated to find that the water contact angle after 60 days was 58.7° and after 300 days was 63.4°. The self-purifying performance was evaluated to find that the water contact angle after 2 hours of irradiation was 77.5° and after 30 hours of irradiation was 72.5°.

[Comparative Example 24]
Instead of forming a layer containing cerium oxide as in Example 24, a _TiO2 film was formed. In this case, _Ti3O5 was used as _the film forming material. The other film forming conditions were the same as those in Example 24 to form a multilayer film. The crystallinity of _{the TiO2} layer was evaluated to be polycrystalline with anatase crystals as the main component and trace amounts of rutile crystals present. The structure was evaluated to be columnar. The hydrophilicity retention performance was evaluated to find that the water contact angle after 60 days was 39.4° and after 300 days was 61.6°. The self-purifying performance was evaluated to find that the water contact angle after 2 hours of irradiation was 72.3° and after 30 hours of irradiation was 63.4°.

[Comparative Example 25]
Various substrates were set in holders made of brass and 99.5% aluminum, and the vacuum deposition apparatus was set to OFF with the substrate heating and wall heaters turned OFF, and the vacuum was evacuated to 1.3×10 ⁻³ Pa or less. The substrate temperature before film formation was 24° C. A Cr layer (1 nm) was formed as the first layer on each substrate. A layer containing cerium oxide was formed using an oxygen-free copper hearth liner and CeO ₂ as a film-forming material. Before starting the film-forming process of the layer containing cerium oxide, the total partial pressure of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 8.94×10 ⁻⁴ Pa. Subsequently, a SiO ₂ film (low refractive index layer) was formed thereon using SiO ₂ as a film-forming material, and a multilayer film was produced. The total value of the partial pressure of water molecules and the partial pressure of oxygen species during the film formation in the 8 nm area on the surface side of the layer containing cerium oxide (region (B): the region of the layer containing cerium oxide located 8 nm or less from the interface between the layer containing cerium oxide and the layer containing silicon oxide or the layer containing magnesium fluoride) was 8.03×10 ⁻³ Pa on average. The obtained multilayer film was evaluated for crystallinity, structure, optical properties, and oxygen deficiency rate in the same manner as the multilayer films of Examples 1 to 68. As a result of the evaluation, the layer containing cerium oxide had a crystalline columnar structure, and the oxygen deficiency rates were V _A 0.02%, V _B 0.01%, and V _C 0.02%. The refractive index of the SiO ₂ layer was 1.46. The optical loss of the multilayer film was 1.4%, and the optical properties were rated as [C].

[Example 67]
The flat glass having the multilayer film obtained in Example 3 was processed and attached to the outside of a near-infrared sensor of a commercially available vehicle to form a protective cover for the sensor.

[Example 68]
The substrate used was a dome-shaped transparent substrate made of polymethyl methacrylate resin (Acrylite (registered trademark) manufactured by Mitsubishi Chemical) with an acrylic hard coat, and the substrate temperature of the vacuum deposition apparatus was set to 50°C, and the wall heater was set to 50°C to evacuate. When 15 minutes or more had elapsed since the substrate temperature reached 50±2°C and before the start of the film formation process of the layer containing cerium oxide, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.49×10 ⁻⁴ Pa. Using SiO as the film-forming material, a SiO ₂ film (200 nm) was formed as the first layer at a deposition rate of 0.3 nm/s.

Next, _{a Ta2O5} film (14 nm) was formed on the second layer at a deposition rate of 0.2 nm/s using _TaO as the film-forming material. A _SiO2 film (35 nm) was formed on the first layer at a deposition rate of 0.7 nm/s using SiO2 as the film-forming material for the third layer. A layer (256 nm) containing cerium oxide was formed on the fourth layer at a deposition rate of 0.3 nm/s using Ce and _CeO2 as the film-forming materials. A _SiO2 (95%) + _CeO2 (5% ₎ film (79 nm, refractive index 1.52) was formed on the fifth layer at a deposition rate of 0.5 nm/s using SiO _{and CeO2} as the film-forming materials. A _SiO2 film (10 nm, refractive index 1.46) was formed on the sixth layer, using SiO2 as the film-forming material, to produce a multilayer film. During the film formation, the substrate was rotated in planetary rotation to produce a multilayer film.

In addition, when the first, second, and fourth layers were formed, ion assist was performed using an RF ion source. At that time, for the first layer, ion assist was performed under the conditions of an acceleration voltage value of 280 V, an acceleration current value of 280 mA, and an _O2 gas flow rate of 40 sccm, and for the second layer, ion assist was performed under the conditions of an acceleration voltage value of 500 V, an acceleration current value of 500 mA, and an _O2 gas flow rate of 60 sccm. For the fifth layer, ion assist was performed under the conditions of an acceleration voltage value of 500 V, an acceleration current value of 500 mA, and an _O2 gas flow rate of 40 sccm, and when forming the surface layer side 8 nm of the fifth layer, the film formation speed was changed to 0.5 nm/s, and the _O2 gas flow rate of the ion source was changed to 0 sccm and the Ar gas flow rate to 40 sccm.

The obtained dome-shaped resin substrate with a multilayer film and various substrates formed at the same time were evaluated for the composition, crystallinity, structure, oxygen deficiency rate, and optical properties of the thin film in the same manner as the multilayer films of Examples 1 to 68. As a result of the evaluation, the composition of the SiO ₂ +CeO ₂ layer was SiO ₂ (95%) + CeO ₂ (5%), the film thickness was 79 nm, and the refractive index was 1.52. The refractive index of the SiO ₂ layer was 1.46. In addition, the layer containing cerium oxide had a crystalline columnar structure, and the oxygen deficiency rates were V _A 0.05%, V _B 0.52%, and V _C 0.03%, and the light loss was less than 1.5%. Subsequently, the obtained dome-shaped resin substrate with a multilayer film was attached to a surveillance camera for use as a surveillance camera cover.

The surveillance camera fitted with the cover was stored in a product presentation box in a dark place for four months. It was then installed outdoors at night on a rainy day, and even when water got on the camera due to rain, the water droplets spread over the cover, maintaining good visibility. Even after the water dried, no water marks remained, and good visibility was maintained. Furthermore, even when it rained eight months after it was installed outdoors, the water droplets spread over the cover, maintaining good visibility.

[Example 69]
The substrate used was a resin substrate (MR-8 manufactured by Mitsui Chemicals) with a silicon-based hard coat applied, and the substrate temperature of the vacuum deposition apparatus was set to 80°C, and the wall heater was set to 80°C to perform evacuation. When 15 minutes or more had elapsed since the substrate temperature reached 80±2°C, and before the film formation process of the layer containing cerium oxide was started, the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules was confirmed to be 1.52×10 ⁻⁴ Pa. An Al ₂ O ₃ film (82 nm) was formed in the first layer of the multilayer film using Al ₂ O ₃ as the film-forming material. In the second layer, a CeO ₂ film (252 nm) was formed using Ce and CeO ₂ as the film-forming materials. In the third layer, a SiO ₂ (95%) + CeO ₂ (5%) film (75 nm, refractive index 1.52) was formed using SiO ₂ and Al ₂ O ₃ as the film-forming materials. For the last fourth layer, a _SiO2 film (15 nm, refractive index 1.46) was formed using _SiO2 as the film forming material to produce a multilayer film. During film formation, the multilayer film was produced at a deposition rate of 0.5 nm/s. The resin substrate with the obtained multilayer film was processed and attached to a frame for glasses to produce glasses.

The obtained eyeglass lens substrate with the multilayer film and the various substrates formed at the same time were evaluated for crystallinity, structure, oxygen deficiency rate, and optical properties in the same manner as the multilayer films of Examples 1 to 68. As a result of the evaluation, the layer containing cerium oxide had a crystalline columnar structure, the oxygen deficiency rate was _VA 0.05%, _VB 0.53%, and _VC 0.03%, and the light loss was less than 1.5%. The prepared eyeglasses were then stored in an aluminum eyeglass case in a dark place for three months. After that, when water droplets were attached to the lens, the water droplets wet and spread on the lens, maintaining good visibility. The contact angle of water at that time was 4.7°. Furthermore, no water marks remained even after the water dried, and good visibility was maintained.

The multilayer film of the present disclosure can be used for optical components such as optical filters, optical lenses, light collecting lenses, optical films, optical prisms, eyeglass lenses, photographic lenses, vehicle door mirrors, sheet glass, light collecting lenses, display cover glass, touch panels, and various films, as well as covers for protecting optical components such as surveillance camera covers, in-vehicle camera covers, and in-vehicle sensor covers.

The optical members of the present disclosure can also be used as optical devices such as digital cameras, digital video cameras, action cameras, endoscopes, lens barrels, eyeglasses, sensors, binoculars, telescopes, surveillance cameras, vehicle-mounted cameras, smartphones, tablet PCs, weather cameras, live cameras, protective goggles, underwater goggles, head-mounted displays, sunglasses, smart glasses, face shields, helmet shields, vehicle mirrors, and bathroom mirrors, as well as covers for protecting these devices.

The present disclosure includes the following embodiments.
(1)
A layer containing cerium oxide and a low refractive index layer provided directly or via another layer on the layer containing cerium oxide,
the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride,
the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure and a columnar structure,
The thickness of the layer containing cerium oxide is 85 nm or more and 800 nm or less,
When the entire layer containing the cerium oxide is designated as region (A) and the oxygen deficiency rate of the cerium oxide in the region (A) is designated as oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
The low refractive index layer has a thickness of 50 nm or more and 240 nm or less,
A multilayer film, characterized in that the refractive index of the low refractive index layer for light with a wavelength of 500 nm is 1.65 or less.
(2)
The multilayer film according to (1), wherein the oxygen deficiency rate (V _A ) is 0.05% or more and 0.6% or less.
(3)
The multilayer film according to (1), wherein the oxygen deficiency rate (V _A ) is 0.1% or more and 0.3% or less.
(4)
The multilayer film according to any one of (1) to (3), wherein a region of the cerium oxide-containing layer located within a range of 8 nm or less from an interface between the cerium oxide-containing layer and the low refractive index layer is designated as region ( _B ), and an oxygen deficiency rate of cerium oxide in region (B) is designated as oxygen deficiency rate (V _B ), the oxygen deficiency rate (V B ) being 0.5% or more and 30% or less.
(5)
The oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
The oxygen deficiency rate (V _B ) is 0.5% or more and 30% or less,
When the region (A) excluding the region (B) is designated as region (C) and the oxygen deficiency rate of cerium oxide in the region (C) is designated as oxygen deficiency rate (V _C ), the oxygen deficiency rate (V _C ) is 0% or more and 10% or less,
The multilayer film according to any one of (1) to (4), wherein the oxygen deficiency rate (V _B ) is greater than the oxygen deficiency rate (V _C ).
(6)
An optical member having the multilayer film according to any one of (1) to (5).
(7)
A step (A) of forming a layer containing cerium oxide on a substrate directly or via another layer by a vacuum deposition method;
and (B) forming a low refractive index layer by a vacuum deposition method on the layer containing cerium oxide directly or via another layer,
the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride,
the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure and a columnar structure,
The thickness of the layer containing cerium oxide is 85 nm or more and 800 nm or less,
When the entire layer containing the cerium oxide is designated as region (A) and the oxygen deficiency rate of the cerium oxide in the region (A) is designated as oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
2. A method for producing a multilayer film, wherein the low refractive index layer has a thickness of 50 nm or more and 240 nm or less.
(8)
a region of the layer containing cerium oxide within a range of 8 nm or less from an interface between the layer containing cerium oxide and the low refractive index layer is designated as region (B), and an oxygen deficiency rate of cerium oxide in region (B) is designated as oxygen deficiency rate (V _B ), the oxygen deficiency rate (V _B ) is 0.5% or more and 30% or less,
When the region (A) excluding the region (B) is designated as region (C) and the oxygen deficiency rate of cerium oxide in the region (C) is designated as oxygen deficiency rate (V _C ), the oxygen deficiency rate (V _C ) is 0% or more and 10% or less,
The method for producing a multilayer film according to (7), wherein the oxygen deficiency rate (V _B ) is greater than the oxygen deficiency rate (V _C ).
(9)
Prior to the step (A), the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules in the atmosphere is set to 2×10 ⁻² Pa or less;
The method for producing a multilayer film according to (7) or (8), wherein the total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the region (B) is 2×10 ⁻² Pa or less on average.

11 Substrate 12 Other layer 13 Layer containing cerium oxide 14 Layer containing magnesium fluoride 15 Layer containing silicon oxide 16 Layer containing silicon dioxide 21 Dome-shaped resin substrate 31 Spectacle lens 32 Spectacle frame

Claims (9)

A layer containing cerium oxide and a low refractive index layer provided directly or via another layer on the layer containing cerium oxide,
the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride,
the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure and a columnar structure,
The thickness of the layer containing cerium oxide is 85 nm or more and 800 nm or less,
When the entire layer containing the cerium oxide is designated as region (A) and the oxygen deficiency rate of the cerium oxide in the region (A) is designated as oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
The low refractive index layer has a thickness of 50 nm or more and 240 nm or less,
A multilayer film, characterized in that the refractive index of the low refractive index layer for light with a wavelength of 500 nm is 1.65 or less. The multilayer film according to claim 1 , wherein the oxygen deficiency rate (V _A ) is 0.05% or more and 0.6% or less. The multilayer film according to claim 1 , wherein the oxygen deficiency rate (V _A ) is 0.1% or more and 0.3% or less. 2. The multilayer film according to claim 1, wherein a region of the cerium oxide-containing layer located within a range of 8 nm or less from an interface between the cerium oxide-containing layer and the low refractive index layer is defined as region (B), and an oxygen deficiency rate of cerium oxide in region ( _B ) is defined as oxygen deficiency rate (VB), the oxygen deficiency rate ( _VB ) being 0.5% or more and 30% or less. The oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
The oxygen deficiency rate (V _B ) is 0.5% or more and 30% or less,
When the region (A) excluding the region (B) is designated as region (C) and the oxygen deficiency rate of cerium oxide in the region (C) is designated as oxygen deficiency rate (V _C ), the oxygen deficiency rate (V _C ) is 0% or more and 10% or less,
The multilayer film according to claim 4 , wherein the oxygen deficiency rate (V _B ) is greater than the oxygen deficiency rate (V _C ). An optical member having the multilayer film according to any one of claims 1 to 5. A step (A) of forming a layer containing cerium oxide on a substrate directly or via another layer by a vacuum deposition method;
and (B) forming a low refractive index layer by a vacuum deposition method on the layer containing cerium oxide directly or via another layer,
the low refractive index layer has a layer containing silicon oxide or a layer containing magnesium fluoride,
the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure and a columnar structure,
The thickness of the layer containing cerium oxide is 85 nm or more and 800 nm or less,
When the entire layer containing the cerium oxide is designated as region (A) and the oxygen deficiency rate of the cerium oxide in the region (A) is designated as oxygen deficiency rate (V _A ), the oxygen deficiency rate (V _A ) is 0.05% or more and 10% or less,
2. A method for producing a multilayer film, wherein the low refractive index layer has a thickness of 50 nm or more and 240 nm or less. a region of the layer containing cerium oxide within a range of 8 nm or less from an interface between the layer containing cerium oxide and the low refractive index layer is designated as region (B), and an oxygen deficiency rate of cerium oxide in region (B) is designated as oxygen deficiency rate (V _B ), the oxygen deficiency rate (V _B ) is 0.5% or more and 30% or less,
When the region (A) excluding the region (B) is designated as region (C) and the oxygen deficiency rate of cerium oxide in the region (C) is designated as oxygen deficiency rate (V _C ), the oxygen deficiency rate (V _C ) is 0% or more and 10% or less,
The method for producing a multilayer film according to claim 7 , wherein the oxygen deficiency rate (V _B ) is greater than the oxygen deficiency rate (V _C ). Prior to the step (A), the total value of the partial pressure of water molecules and the partial pressure of oxygen molecules in the atmosphere is set to 2×10 ⁻² Pa or less;
9. The method for producing a multilayer film according to claim 7, wherein the total value of the partial pressure of water molecules and the partial pressure of oxygen species during film formation in the region (B) is 2×10 ⁻² Pa or less on average.

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