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 MgF 2 optical thin film is formed on an optical surface of a base material. The MgF 2 optical thin film includes MgF 2 particles and an amorphous silicon oxide-based binder which exists on the surfaces of the MgF 2 particles and between the MgF 2 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.

1 . An MgF 2  optical thin film comprising: 
 MgF 2  minute particles; and    an amorphous silicon oxide-based binder which exists between the MgF 2  minute particles.    
     
     
         2 . The MgF 2  optical thin film according to  claim 1 , wherein the MgF 2  minute particles are connected by the amorphous silicon oxide-based binder, and the amorphous silicon oxide-based binder, disposed on surfaces of MgF 2  minute particles, among the MgF 2  minute particles, which exist at an outermost portion of the MgF 2  optical thin film, has a thickness which is not more than 5% of a wavelength of light to be radiated.  
     
     
         3 . The MgF 2  optical thin film according to  claim 1 , wherein the amorphous silicon oxide-based binder, which exists between adjacent MgF 2  minute particles, among the MgF 2  minute particles, has a thickness which is smaller than a particle diameter of the MgF 2  minute particles.  
     
     
         4 . The MgF 2  optical thin film according to  claim 1 , wherein the MgF 2  minute particles have an average particle diameter of 1 nm to 100 nm.  
     
     
         5 . The MgF 2  optical thin film according to  claim 1 , wherein the MgF 2  optical thin film has a porous structure.  
     
     
         6 . The MgF 2  optical thin film according to  claim 5 , wherein the porous structure has a percentage of voids of not more than 50%.  
     
     
         7 . The MgF 2  optical thin film according to  claim 1 , wherein voids are present between the MgF 2  minute particles, and the voids are filled with the amorphous silicon oxide-based binder.  
     
     
         8 . The MgF 2  optical thin film according to  claim 1 , wherein the amorphous silicon oxide-based binder is formed of amorphous silica.  
     
     
         9 . The MgF 2  optical thin film according to  claim 1 , wherein a strength of the optical thin film, which is measured by a microindentation test method, is not less than 30 MPa.  
     
     
         10 . The MgF 2  optical thin film according to  claim 1 , wherein a refractive index of the MgF 2  optical thin film is 1.20 to 1.41.  
     
     
         11 . A multilayered optical thin film comprising a plurality of stacked optical thin films, wherein the MgF 2  optical thin film as defined in  claim 1  is stacked as an outermost layer of the stacked optical thin films.  
     
     
         12 . A multilayered optical thin film comprising a plurality of stacked optical thin films, wherein a plurality of MgF 2  optical thin films, each of the films being as defined in  claim 1 , are included in the multilayered optical thin film.  
     
     
         13 . The multilayered optical thin film according to  claim 11 , wherein a plurality of MgF 2  optical thin films are disposed adjacently to each other; and a difference in refractive index between the adjacent MgF 2  optical thin films is 0.02 to 0.23.  
     
     
         14 . A multilayered optical thin film comprising a plurality of stacked optical thin films which includes the MgF 2  optical thin film as defined in  claim 1  and an optical thin film formed by a dry process.  
     
     
         15 . An optical element comprising a base material which has a refractive index of 1.4 to 2.1; and the MgF 2  optical thin film as defined in  claim 1  which is stacked on at least one of optical surfaces of the base material; wherein at least one of the optical surfaces is formed to have one of a flat surface and a curved surface.  
     
     
         16 . The optical element according to  claim 15 , wherein at least one of the optical surfaces of the base material is formed to have the curved surface form having such a shape that (effective lens diameter D)/(lens radius R) is 0.5 to 2.  
     
     
         17 . The optical element according to  claim 15 , wherein the optical element is used for a light beam having a wavelength region of 400 nm to 800 nm.  
     
     
         18 . An optical element comprising a substrate, and a multilayered antireflection film which is formed on the substrate and which is constructed of a stack of at least three types of layers having different refractive indexes respectively; 
 wherein an uppermost layer, of the multilayered antireflection film, which makes contact with a medium, is the MgF 2  optical thin film as defined in  claim 1 , the MgF 2  optical thin film having a refractive index of not more than 1.30 at a design center wavelength λ 0 ; and    remaining layers, of the multilayered antireflection film, other than the uppermost layer are constructed by stacking a layer having a refractive index of not less than 2 at the design center wavelength λ 0  and a layer having a refractive index of 1.38 to 1.7 at the design center wavelength λ 0 .    
     
     
         19 . The optical element according to  claim 18 , wherein a layer, among the layers, which makes contact with the substrate, has a refractive index of 1.38 to 1.7 at the design center wavelength λ 0 , and a second layer, which is counted from the medium, has a refractive index of not less than 2 at the design center wavelength λ 0 .  
     
     
         20 . An optical system comprising a plurality of optical elements which are arranged between an object and an image plane, wherein at least one of the plurality of optical elements is the optical element as defined in  claim 15 .  
     
     
         21 . The optical system according to  claim 20 , wherein Rn×Rm≦0.002° holds in an entire visible region provided that Rn represents a reflectance of normal incidence on an n-th ghost-generating surface in the optical system, and Rm represents a reflectance of normal incidence on an m-th ghost-generating surface.  
     
     
         22 . The optical system according to  claim 21 , wherein the multilayered antireflection film is applied to at least one of the n-th and m-th ghost-generating surfaces.  
     
     
         23 . The optical system according to  claim 21 , wherein the multilayered antireflection film is applied to a surface to which a flat surface or a concave surface is opposite as viewed from a diaphragm of the optical system.  
     
     
         24 . The optical system according to  claim 20 , which is an imaging optical system or an observation optical system.  
     
     
         25 . A method for producing an MgF 2  optical thin film, comprising: 
 a step of preparing a sol solution in which MgF 2  minute particles are dispersed;    a step of preparing a binder solution which contains a component capable of forming an amorphous silicon oxide-based binder by a reaction;    a step of preparing a coating liquid by mixing the sol solution and the binder solution;    a step of forming a film by coating the coating liquid on a base material and by performing drying; and    a step of performing a heat treatment after forming the film.    
     
     
         26 . A method for producing an MgF 2  optical thin film, the method comprising: 
 a step of preparing a sol solution in which MgF 2  minute particles are dispersed;    a step of preparing a binder solution which contains a component capable of forming an amorphous silicon oxide-based binder by a reaction;    a step of forming a porous film by coating the sol solution on a base material and by performing drying;    a step of coating the binder solution on the porous film and impregnating the binder solution into the porous film; and    a step of performing a heat treatment after the impregnation.    
     
     
         27 . The method for producing the MgF 2  optical thin film according to  claim 25 , wherein the sol solution is prepared by synthesizing the MgF 2  minute particles by reacting a magnesium compound and a fluorine compound in a solvent.  
     
     
         28 . The method for producing the MgF 2  optical thin film according to  claim 27 , wherein the sol solution is prepared by mixing the magnesium compound and the fluorine compound in the solvent and performing at least one of a pressurizing treatment and a heat treatment.  
     
     
         29 . The method for producing the MgF 2  optical thin film according to  claim 27 , wherein the magnesium compound is magnesium acetate, the fluorine compound is hydrofluoric acid, and the solvent is methanol.  
     
     
         30 . The method for producing the MgF 2  optical thin film according to  claim 28 , wherein a molar ratio of fluorine contained in the fluorine compound existing in the solvent to magnesium contained in the magnesium compound existing in the solvent is 1.9 to 2.0.  
     
     
         31 . The method for producing the MgF 2  optical thin film according to  claim 25 , wherein the component, which is capable of forming the amorphous silicon oxide-based binder, is an organic silicon compound.  
     
     
         32 . The method for producing the MgF 2  optical thin film according to  claim 31 , wherein the organic silicon compound is silicon alkoxide, a polymer thereof, or polysilazane.  
     
     
         33 . The method for producing the MgF 2  optical thin film according to  claim 25 , wherein an SiO 2 -converted concentration of silicon in the coating liquid or the binder solution to be coated on the porous film is not more than 5% by weight.  
     
     
         34 . The method for producing the MgF 2  optical thin film according to  claim 25 , wherein the coating liquid or the sol solution is coated on the base material by a spin coat method or a dip coat method.  
     
     
         35 . The method for producing the MgF 2  optical thin film according to  claim 25 , wherein the coating liquid or the sol solution is coated on the base material in an atmosphere of relative humidity of 5% to 40% by a spin coat method.  
     
     
         36 . The method for producing the MgF 2  optical thin film according to  claim 25 , wherein the coating liquid or the sol solution is coated on the base material by a spin coat method by rotating the base material at a maximum number of revolutions of 500 rpm to 9,000 rpm within 3 seconds after supplying the coating liquid or the sol solution to the base material.  
     
     
         37 . The method for producing the MgF 2  optical thin film according to  claim 25 , wherein the MgF 2  optical thin film having a desired refractive index is produced by adjusting an SiO 2 -converted concentration of silicon in the binder solution or the coating liquid which is to be coated on the porous film and with which the porous film is to be impregnated.  
     
     
         38 . The method for producing the MgF 2  optical thin film according to  claim 27 , wherein a plurality of pieces of the MgF 2  optical thin film having desired refractive index is produced by adjusting a molar ratio of fluorine contained in the fluorine compound to magnesium contained in the magnesium compound of the sol solution.  
     
     
         39 . The method for producing the MgF 2  optical thin film according to  claim 25 , wherein the MgF 2  minute particles have an average particle diameter of 1 nm to 100 nm.  
     
     
         40 . A binder-containing MgF 2  sol solution for producing a MgF 2  optical thin film by the production method as defined in  claim 32 , wherein the sol solution contains MgF 2  minute particles having an average particle diameter of 1 nm to 100 nm and one of silicon alkoxide and a polymer thereof.  
     
     
         41 . The multilayered optical thin film according to  claim 12 , wherein a plurality of MgF 2  optical thin films are disposed adjacently to each other; and a difference in refractive index between the adjacent MgF 2  optical thin films is 0.02 to 0.23.  
     
     
         42 . An optical system comprising a plurality of optical elements which are arranged between an object and an image plane, wherein at least one of the plurality of optical elements is the optical element as defined in  claim 18 .  
     
     
         43 . The optical system according to  claim 42 , wherein Rn×Rm≦0.002% holds in an entire visible region provided that Rn represents a reflectance of normal incidence on an n-th ghost-generating surface in the optical system, and Rm represents a reflectance of normal incidence on an m-th ghost-generating surface.  
     
     
         44 . The optical system according to  claim 43 , wherein the multilayered antireflection film is applied to at least one of the n-th and m-th ghost-generating surfaces.  
     
     
         45 . The optical system according to  claim 43 , wherein the multilayered antireflection film is applied to a surface to which a flat surface or a concave surface is opposite as viewed from a diaphragm of the optical system.  
     
     
         46 . The optical system according to  claim 42 , which is an imaging optical system or an observation optical system.  
     
     
         47 . The method for producing the MgF 2  optical thin film according to  claim 26 , wherein the sol solution is prepared by synthesizing the MgF 2  minute particles by reacting a magnesium compound and a fluorine compound in a solvent.  
     
     
         48 . The method for producing the MgF 2  optical thin film according to  claim 47 , wherein the sol solution is prepared by mixing the magnesium compound and the fluorine compound in the solvent and performing at least one of a pressurizing treatment and a heat treatment.  
     
     
         49 . The method for producing the MgF 2  optical thin film according to  claim 47 , wherein the magnesium compound is magnesium acetate, the fluorine compound is hydrofluoric acid, and the solvent is methanol.  
     
     
         50 . The method for producing the MgF 2  optical thin film according to  claim 48 , wherein a molar ratio of fluorine contained in the fluorine compound existing in the solvent to magnesium contained in the magnesium compound existing in the solvent is 1.9 to 2.0.  
     
     
         51 . The method for producing the MgF 2  optical thin film according to  claim 26 , wherein the component, which is capable of forming the amorphous silicon oxide-based binder, is an organic silicon compound.  
     
     
         52 . The method for producing the MgF 2  optical thin film according to  claim 51 , wherein the organic silicon compound is silicon alkoxide, a polymer thereof, or polysilazane.  
     
     
         53 . The method for producing the MgF 2  optical thin film according to  claim 26 , wherein an SiO 2 -converted concentration of silicon in the coating liquid or the binder solution to be coated on the porous film is not more than 5% by weight.  
     
     
         54 . The method for producing the MgF 2  optical thin film according to  claim 26 , wherein the coating liquid or the sol solution is coated on the base material by a spin coat method or a dip coat method.  
     
     
         55 . The method for producing the MgF 2  optical thin film according to  claim 26 , wherein the coating liquid or the sol solution is coated on the base material in an atmosphere of relative humidity of 5% to 40% by a spin coat method.  
     
     
         56 . The method for producing the MgF 2  optical thin film according to  claim 26 , wherein the coating liquid or the sol solution is coated on the base material by a spin coat method by rotating the base material at a maximum number of revolutions of 500 rpm to 9,000 rpm within 3 seconds after supplying the coating liquid or the sol solution to the base material.  
     
     
         57 . The method for producing the MgF 2  optical thin film according to  claim 26 , wherein the MgF 2  optical thin film having a desired refractive index is produced by adjusting an SiO 2 -converted concentration of silicon in the binder solution or the coating liquid which is to be coated on the porous film and with which the porous film is to be impregnated.  
     
     
         58 . The method for producing the MgF 2  optical thin film according to  claim 47 , wherein a plurality of pieces of the MgF 2  optical thin film having desired refractive index is produced by adjusting a molar ratio of fluorine contained in the fluorine compound to magnesium contained in the magnesium compound of the sol solution.  
     
     
         59 . The method for producing the MgF 2  optical thin film according to  claim 26 , wherein the MgF 2  minute particles have an average particle diameter of 1 nm to 100 nm.  
     
     
         60 . A binder-containing MgF 2  sol solution for producing a MgF 2  optical thin film by the production method as defined in  claim 52 , wherein the sol solution contains MgF 2  minute particles having an average particle diameter of 1 nm to 100 nm and one of silicon alkoxide and a polymer thereof.