Mgf2 Optical Thin Film Including Amorphous Silicon Oxide Binder, Optical Element Provided With the Same, and Method for Producing Mgf2 Optical Thin Film

Abstract

An MgF2 optical thin film is formed on an optical surface of a base material. The MgF2 optical thin film includes MgF2 particles and an amorphous silicon oxide-based binder which exists on the surfaces of the MgF2 particles and between the MgF2 particles. Owing to this amorphous silicon oxide-based binder, the optical thin film can have high mechanical strength and high adhesion to the base material, while having excellent environment resistance and a lower refractive index.

Description

TECHNICAL FIELD [0001] The present invention relates to a singlelayered or multilayered optical thin film formed of MgF2, a method for producing the same, an optical element having the MgF2 optical thin film, and an optical system for optical instruments such as cameras, microscopes, binoculars, and exposure apparatuses which is provided with the optical element. BACKGROUND ART [0002] Surfaces of individual lenses, which construct an optical system such as a camera lens and an objective lens of a microscope, are coated with an antireflection film in order to reduce the reflection. In general, the optical thin film such as the antireflection film is produced by the dry method (dry process), in which the vacuum vapor deposition method, the sputtering method, the CVD method (Chemical Vapor Deposition) or the like is used. [0003] In order to obtain a high performance optical thin film having a low reflectance in a wide wavelength band or a wide angle band, it is known that a multilayere...

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Benefits of technology

[0028] According to a first aspect of the present invention, there is provided an MgF2 optical thin film, comprising MgF2 minute particles; and an amorphous silicon oxide-based binder which exists between the MgF2 minute particles.

[0029] According to the present invention, the MgF2 minute particles, which are excellent in the environment resistance (durability), are used as the main constitutive substance of the film, and the MgF2 minute particles are bonded or connected to one another by the amorphous silicon oxide-based binder. Therefore, the strong bonding or connection can be provided between the MgF2 minute particles and between the MgF2 minute particles and the base material. The mechanical strength of the film and the adhesive force between the film and the base material are improved. That is, it is considered that the MgF2 optical thin film has such a structure that the voids (gaps) are present between the MgF2 minute particles, and the voids are filled with the amorphous silicon oxide-based binder. SiO2, which is somewhat inferior in the environment resistance, can be used as the amorphous silicon oxide-based binder. However, it is enough to use a small amount of the binder as compared with the MgF2 minute particles. Therefore, it is possible to sufficiently secure the environment resistance as the entire thin film.

[0030] In the MgF2 optical thin film of the present invention, the MgF2 minute particles may be connected by the amorphous silicon oxide-based binder, and the amorphous silicon oxide-based binder, disposed on surfaces of MgF2 minute particles, among the MgF2 minute particles, which exist at an outermost portion of the MgF2 optical thin film, may have a thickness which is not more than 5% of a wavelength of light to be radiated. Further, in the MgF2 optical thin film of the present invention, the amorphous silicon oxide-based binder, which exists between adjacent MgF2 minute particles, among the MgF2 minute particles, may have a thickness which is smaller than a particle diameter of the MgF2 minute particles. When the thickness of the binder is adjusted as described above, it is possible to lower the refractive index. In particular, when the refractive index of the outermost layer of the optical thin film is sufficiently lowered, the antireflection film is obtained, which has the low reflectance in a wide wavelength band or zone and in a wide angle band or zone.

[0031] In the present invention, it is unnecessary that the amorphous silicon oxide-based binder exists in all of the spaces between the MgF2 minute particles. It is enough that the amorphous silicon oxide-based binder exists in a part of the spaces between the MgF2 minute particles to maintain the thin film of the MgF2 minute particles. It is unnecessary that the amorphous silicon oxide-based binder completely exists in the spaces between the MgF2 minute particles and the base material. It is enough that the amorphous silicon oxide-based binder exists in a part of the spaces between the MgF2 minute particles and the base material, and thus the amorphous silicon oxide-based binder reinforces the bonding between the MgF2 minute particles and the base material.

[0032] In the MgF2 optical thin film of the present invention, the MgF2 minute particles may have an average particle diameter of 1 nm to 100 nm (not less than 1 nm and not more than 100 nm). In this case, owing to the high crystallization property of the MgF2 minute particles, the adhesion or agglutination between the MgF2 minute particles can be suppressed to secure the voids between the MgF2 minute particles so that the MgF2 optical thin film having a porous structure may be successfully obtained. When the porous structure is provided, it is possible to obtain the MgF2 optical thin film having the lower refractive index. The porous structure may have a percentage of voids or porosity of not more than 50%, in view of the retention of the film strength.

[0033] In the MgF2 optical thin film of the present invention, the amorphous silicon oxide-based binder may be formed of amorphous silica. In this case, the refractive index of amorphous silica is low, i.e., 1.42. Therefore, it is possible to secure the low refractive index of the entire thin film even when the composite is formed together with the MgF2 minute particles.

Problems solved by technology

However, in the case of the film using MgF2 as described in International Publication No. 02 / 18982A1, the film strength and the adhesive force with respect to the base material are insufficient, although the refractive index of the optical thin film can be lowered to be not more than 1.30.

In the case of the films in which the film strength and the adhesive force with respect to the base material can be secured as described in Japanese Patent No. 3272111, Japanese Patent Application Laid-open No. 11-6902, Japanese Patent Application Laid-open No. 7-48527, Japanese Patent Application Laid-open No. 8-122501 and Japanese Patent Application Laid-open No. 2000-169133, the problem arises such that the refractive index of the film cannot be lowered sufficiently, and the environment resistance is insufficient.

There has been the problem such that it is not possible to obtain any optical thin film which is capable of sufficiently decreasing the refractive index and of securing the mechanical strength of the film, the adhesive force with respect to the base material, and the environment resistance.

However, in the case of the wet process, it is difficult to perform the multilayered coating.

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