JP2007154274A - Fluoride film depositing method using cluster beam and optical element using fluoride film obtained by the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for depositing a fluoride film by a cluster beam which is low in optical absorption not only in a visible but in UV region as well even without execution of high-temperature heating of a substrate, is dense and is high in environmental durability. <P>SOLUTION: The fluoride film is formed as follows by using the film deposition method by a vacuum vapor deposition method and the cluster beam. When depositing the fluoride film, a fluoride film material is vacuum evaporated from a resistance heating board 3 while a synthetic quartz substrate 7 is irradiated with the cluster ion beam from a cluster ion beam source 5 to thereby form the fluoride film on the synthetic quartz substrate 7. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an element manufactured by a thin film forming method using clusters. Specifically, the present invention relates to the manufacture of optical elements using fluoride films.

Conventionally, various thin film forming methods have been used in the manufacture of devices using fluoride films.
A fluoride film has a larger band gap energy than an oxide film and has less optical absorption even for light having a short wavelength in the ultraviolet region. Therefore, it can be used for an optical element such as an antireflection film or a mirror for ultraviolet light. .
MgF _{2 and the} like are very useful in optical design because they have a low refractive index with respect to visible light.
Currently, vacuum deposition methods, ion-assisted deposition methods, ion plating methods, sputtering methods, and the like have been proposed as these forming methods.
However, for example, in vacuum deposition, substrate heating is necessary to obtain a film excellent in denseness, mechanical strength, environmental durability, and the like.
However, this substrate heating often causes problems such as deformation of the substrate, film peeling or cracking due to an increase in film stress, and a reduction in production throughput.

Further, for example, in the ion assist deposition method, ion plating method, sputtering method, etc., a dense film having excellent mechanical strength can be obtained without heating the substrate.
However, in these methods, high energy ions and electrons cause fluorine dissociation and color center damage, and it is difficult to obtain a film with low optical absorption. As other thin film forming methods, there are methods using a cluster beam as disclosed in Patent Document 1, Patent Document 2, and the like.
When the cluster beam collides with the substrate and the film, the cluster is decomposed. Therefore, by increasing the cluster size (the number of constituent particles), each constituent particle can have low energy.
Therefore, there is a high possibility that dissociation or the like occurs in the substrate or film.
For oxide films, for example, Patent Document 3 proposes a thin film forming method using a cluster beam.
Japanese Patent No. 3352842 Japanese Patent Publication No. 07-065166 JP 2004-43874 A

However, the method for forming a thin film using the cluster beam in the above-described conventional example is a method for forming an oxide film.
That is, in the conventional method for forming a thin film using a cluster beam, an excellent method for forming a fluoride film such as low optical absorption and high environmental durability has not yet been achieved. For example, in the 65th JSAP Autumn Meeting Preliminary Proceedings P543, LaF ₃ and MgF ₂ are vaporized by an electron gun and formed while irradiating a cluster beam of SF _{6 on} a substrate or film.
However, this method is a film having extremely high optical absorption such as an extinction coefficient of 10 ⁻³ , and is difficult to use as an optical element.
Thus, a satisfactory method for forming a fluoride film such as low optical absorption and high environmental durability has not yet been achieved, and its realization is desired.
In view of the above problems, the present invention provides a method for forming a fluoride film by a cluster beam that has low optical absorption, is dense, and has high environmental durability, not only in the visible but also in the ultraviolet region, without performing high-temperature heating of the substrate. It is intended to provide.
Another object of the present invention is to provide an optical element composed of a fluoride film obtained by the above-described method for forming a fluoride film.

In order to solve the above-described problems, the present invention provides a method for forming a fluoride film using a cluster beam configured as follows, and an optical element configured by the fluoride film obtained by the method. is there.
The method for forming a fluoride film using a cluster beam of the present invention is characterized by having the following steps.
That is, it is characterized by having a step of forming a fluoride film on the substrate by evaporating the fluoride film material by vacuum evaporation by resistance heating while irradiating the substrate with the cluster beam.
In the present invention, any one of GdF ₃ , LaF ₃ , NdF ₃ , DyF ₃ , PdF ₃ , MgF ₂ , AlF ₃ , NaF ₃ , LiF, CaF ₂ , and BaF ₂ is used as the fluoride film material. It is characterized by that.
Alternatively, any one of SrF ₂ , ErF ₃ , SmF ₃ , CeF ₃ , Na ₃ AlF ₆ , Na ₅ Al ₃ F ₁₄ , YbF ₃ , ErF ₃ , and TbF ₃ is used.
In that case, you may make it use the mixture with either of them.
Further, in the present invention, when forming a fluoride film on the substrate, LaF ₃ is used as the fluoride film material,
When the number of the fluoride molecules deposited on the substrate is x, and the number of atoms constituting the cluster beam irradiated to the substrate and the fluoride film is a, respectively,
The following relational expression (1) is satisfied, and a fluoride film made of the film material is formed on the substrate.

Further, according to the present invention, when a fluoride film is formed on the substrate, the fluoride film material is MgF ₂ , LiF, BaF ₂ , GdF ₃ , NdF ₃ , DyF ₃ , PdF ₃ , CaF ₂ , SrF ₂ , ErF ₃ , SmF ₃ , CeF ₃ , YbF ₃ , TbF ₃ , or ErF ₃ is used,
When the number of the fluoride molecules deposited on the substrate is x and the number of atoms constituting the cluster beam irradiated on the substrate and the fluoride film is a, respectively,
The following relational expression (2) is satisfied, and a fluoride film made of the film material is formed on the substrate.

Further, the present invention uses MgF ₂ as the fluoride film material when forming a fluoride film on the substrate,
When the number of the fluoride molecules deposited on the substrate is x and the number of atoms constituting the cluster beam irradiated on the substrate and the fluoride film is a, respectively,
The following relational expression (3) is satisfied, and a fluoride film made of the film material having an average surface roughness of 1.0 nm or less is formed on the substrate.

Further, the present invention uses any one of AlF ₃ , NaF ₃ , Na ₃ AlF ₆ , and Na ₅ Al ₃ F ₁₄ as the fluoride film material when forming a fluoride film on the substrate,
When the number of the fluoride molecules deposited on the substrate is x and the number of atoms constituting the cluster beam irradiated on the substrate and the fluoride film is a, respectively,
The following relational expression (4) is satisfied, and a fluoride film made of the film material is formed on the substrate.

Further, the present invention is characterized in that the energy of each atom constituting the cluster beam irradiated to the substrate and the fluoride film is 3 to 50 eV / atm.
In addition, the present invention is characterized in that the cluster beam is composed of rare gas atoms.
In addition, the present invention is characterized in that after the film is formed, the fluoride film is irradiated with UV light to reduce optical absorption of the fluoride film by photo-modification.
The optical element of the present invention is characterized in that it is composed of a fluoride film obtained by any of the above-described methods for forming a fluoride film.

According to the present invention, it is possible to realize a method for forming a fluoride film by a cluster beam that has low optical absorption, is dense, and has high environmental durability, not only in the visible but also in the ultraviolet region, without heating the substrate at a high temperature. Can do.
In addition, an optical element composed of a fluoride film obtained by the above-described method for forming a fluoride film can be realized.

With the above configuration, it is possible to realize a method for forming a fluoride film by a cluster beam that has low optical absorption, is dense, and has high environmental durability, not only in the visible but also in the ultraviolet region, without heating the substrate at a high temperature. .
This is based on the following knowledge of the present inventors.
As a result of intensive studies, the present inventors have found a film forming method using a vacuum deposition method and the following method using a cluster beam.
Specifically, by using a vacuum evaporation method using resistance heating, the fluoride material is evaporated to such an extent that dissociation of fluorine defects and the like is negligible with respect to the optical characteristics, and the film is deposited on the substrate. . At that time, a cluster beam is irradiated.
For example, if the fluoride film material is LaF ₃ , the number of LaF ₃ film molecules deposited on the substrate is x, and the number of atoms constituting the cluster beam irradiated to the substrate and the film at that time is a, respectively.
Between these,

A fluoride film made of a fluoride film material is formed on the substrate so as to satisfy the following relational expression.
Further, the energy of atoms constituting the cluster beam is 3 to 20 eV / atm.
Further, by using a chemically stable rare gas for the constituent atoms of the cluster beam, even if these are taken into the film, the optical absorption of the film is not increased. By these, first, dissociation such as fluorine deficiency in the material during evaporation can be prevented.
Further, damage caused by ions and electrons occurring during film formation can be suppressed. Furthermore, despite the fact that the substrate is not heated at high temperature, it is possible to obtain a fluoride film that has low optical absorption not only in the visible but also in the ultraviolet region, is dense, smooth, and has environmental durability.
The fluoride film thus formed can be used effectively for optical elements, semiconductor elements, and the like.

Next, a film forming apparatus used in the above-described method for forming a fluoride film using a cluster beam according to the present invention will be described.
FIG. 1 shows an example of a thin film forming apparatus in this embodiment.
In FIG. 1, 1 is a vacuum chamber, 2 is a vacuum valve, 3 is a resistance heating board, 4 is a resistance heating mechanism, 5 is a class ion beam source, 6 is a substrate dome, 7 is a synthetic quartz substrate, and 8 is a film thickness monitor. , 9 are shutters.
In the film forming apparatus according to the embodiment of the present invention, the vacuum chamber 1 is provided with a resistance heating mechanism 4 for evaporating the fluoride film material and a resistance heating board 3 into which the material is put. The vacuum chamber 1 is evacuated to 2.0 × 10 ⁻⁵ Pa through a vacuum valve 2.
Further, in the vacuum chamber 1, a rotatable substrate dome 6 for placing the substrate 7 is provided, and a film thickness monitor 8 and a shutter 9 are provided for controlling the film thickness.
Then, when the film material evaporated from the resistance heating board 3 of the resistance heating mechanism 4 is formed on the synthetic quartz substrate 7, the cluster ion beam source 5 irradiates the cluster beam toward the substrate 7 while applying the thin film. It is configured to form.

FIG. 2 shows a detailed view of the cluster ion beam source 5.
In FIG. 2, 10 is a nozzle, 11 is a cluster generation chamber, 12 is a skimmer, 13 is a valve, 14 is an ionization chamber, 15 is a valve, 16 is a filament, 17 is an acceleration electrode, 18 is an acceleration chamber, and 19 is a size separation chamber. It is.
The cluster ion beam source 5 in the embodiment of the present invention includes a cluster generation chamber 11, a cluster ionization chamber 14 and an acceleration chamber 18, and a cluster size separation chamber 19. The generation chamber 11 has a nozzle 10 for clustering the introduced gas by adiabatic expansion, and the cluster is taken out to the ionization chamber 14 and the acceleration chamber 18 via the skimmer 12.
In the ionization chamber 14, the clusters are ionized by electron collision using the filament 16 and accelerated by the acceleration electrode 17 in the acceleration chamber 18.
The accelerated cluster ion beam is applied to the substrate 7 after eliminating clusters of unnecessary size by a magnetic field deflection by a magnet in the size separation chamber 19.
Here, the cluster generation chamber 11 and the ionization chamber 14 are operated and exhausted through valves 13 and 15 so that the degree of vacuum of the vacuum chamber 1 does not increase excessively.

The thin film is formed as follows by the thin film forming apparatus having the above configuration.
Using such a cluster ion beam source 5, Ar gas is ejected from the nozzle 10, and A
r clusters are generated by adiabatic expansion. This Ar cluster is ionized by electron impact.
Then, this cluster ion is extracted as a beam and accelerated by the acceleration electrode 17.
In the vacuum chamber 1, the fluoride film material is vacuum evaporated from the resistance heating board 3 while irradiating the above-described cluster ion beam onto the synthetic quartz substrate 7, thereby forming a fluoride thin film on the synthetic quartz substrate 7.

Examples of the present invention will be described below. In each of the following examples, a fluoride film was formed as follows using the thin film forming apparatus shown in FIGS.
[Example 1]
In Example 1, a fluoride film was formed as follows.
First, in the cluster ion beam source 5, Ar gas is ejected from the nozzle 10 to generate Ar clusters by adiabatic expansion. This Ar cluster is ionized by electron impact. Then, this cluster ion is extracted as a beam and accelerated by the acceleration electrode 17.
In the vacuum chamber 1, LaF ₃ is vacuum evaporated from the resistance heating board 3 while irradiating the above-described cluster ion beam onto the synthetic quartz substrate 7 to form a film thickness of 260 nm on the synthetic quartz substrate 7.
A LaF ₃ film was formed.
At that time, when the number of LaF ₃ film molecules is x and the number of Ar atoms constituting the cluster beam irradiated to the substrate and the film is a,
a / x = 3.4
It is.

FIG. 3 shows the spectral characteristics of the synthetic quartz substrate on which the LaF ₃ film is formed when the energy of atoms constituting the cluster beam in this embodiment is 3 to 20 eV / atm.
It can be seen from FIG. 3 that the optical loss is homogeneous and hardly visible.
The extinction coefficient of this LaF ₃ film was as small as 0.0001 for 193 nm light.
FIG. 4 shows the results of conducting a high temperature and high humidity environment durability test of this LaF ₃ film at 60 ° C. and 90% for 1000 hours.
The wavelength shift was less than 1 nm.
Moreover, as a result of measuring the surface roughness of the LaF ₃ film by AFM, the average surface roughness was 0.7 n.
m.
Thus, without heating the substrate, it was possible to obtain a dense LaF ₃ film having low optical absorption not only in the visible but also in the ultraviolet region, and having high environmental durability.
Also,
a / x = 5.0
In the same manner, a LaF ₃ film having a uniform extinction coefficient with respect to 193 nm light as small as 0.0001 and an average surface roughness of about 0.7 nm could be obtained.

Moreover, in FIG. 5, as a comparative example with Example 1,
a / x = 2.0
The spectral characteristics of the synthetic quartz substrate on which the LaF ₃ film is formed are shown.
From FIG. 5, it can be seen from the reflection characteristics that the film is inhomogeneous and porous.

[Example 2]
In Example 2, a fluoride film was formed as follows.
First, in the cluster ion beam source 5, Ar gas is mixed and ejected from the nozzle 10, and an Ar cluster is generated by adiabatic expansion.
This Ar cluster is ionized by electron impact. Then, this cluster ion is extracted as a beam and accelerated by the acceleration electrode 17.
In the vacuum chamber 1, while the synthetic quartz substrate 7 is irradiated with the cluster ion beam, MgF ₂ is vacuum evaporated from the resistance heating board 3 to form a film thickness of 430 n on the synthetic quartz substrate 7.
m MgF ₂ film was formed.
At that time, when the number of MgF ₂ film molecules is x and the number of Ar atoms constituting the cluster beam irradiated to the substrate and the film is a, respectively,
a / x = 2.1
It is.

FIG. 6 shows the spectral characteristics of the synthetic quartz substrate on which the MgF ₂ film is formed when the energy of atoms constituting the cluster beam in this embodiment is 3 to 20 eV / atm.
It can be seen from the reflection characteristics shown in FIG.
The extinction coefficient of this Mg ₂ film was as small as 0.0001 with respect to 193 nm light.
The average refractive index of this MgF ₂ film at a wavelength of 193 nm was 1.40.

Moreover, in FIG. 7, as a comparative example with Example 2,
a / x = 1.5
The spectral characteristic of the synthetic quartz substrate in which the MgF2 film | membrane in the case of ₁ is formed is shown.
It can be seen from the reflection characteristics shown in FIG. 7 that the film is inhomogeneous and porous.
Further, the average refractive index of the MgF ₂ film at a wavelength of 193 nm was as low as 1.36 and was not dense.

[Example 3]
In Example 3, a fluoride film was formed as follows.
First, in the cluster ion beam source 5, Ar gas is mixed and ejected from the nozzle 10, and an Ar cluster is generated by adiabatic expansion.
This Ar cluster is ionized by electron impact. Then, this cluster ion is extracted as a beam and accelerated by the acceleration electrode 17.
In the vacuum chamber 1, while the synthetic quartz substrate 7 is irradiated with the cluster ion beam, MgF ₂ is vacuum evaporated from the resistance heating board 3 to form a film thickness of 430 nm on the synthetic quartz substrate 7.
An MgF ₂ film was formed.
At that time, the ratio when the number of MgF ₂ film molecules is x and the number of Ar atoms constituting the cluster beam irradiated to the substrate and the film is a,
a / x
FIG. 8 shows the average surface roughness of the MgF ₂ film against the above.
When the ratio is 3.1 or more, the surface becomes smooth with an average surface roughness of 0.9 nm or less.

FIG. 9 shows an AFM image of the MgF ₂ film when a / x is 3.1. FIG. 10 shows an AFM image of the MgF ₂ film when a / x is 2.1.
In addition, FIG. 11 shows the results of performing a high-temperature and high-humidity environment durability test at 60 ° C. and 90% for 1000 hours on the MgF ₂ film in the cases where a / x is 3.1 and 2.1.
When a / x is 2.1, the wavelength shift is 3 nm or more, but when 3.1, the wavelength shift is less than 1 nm and the film has excellent environmental durability.
Thus, by setting a / x to 3.1 or more, it is possible to obtain a MgF ₂ film that is smoother and has higher environmental durability in addition to being dense.

[Example 4]
In Example 4, a fluoride film was formed as follows.
First, in the cluster ion beam source 5, Ar gas is ejected from the nozzle 10, and Ar
Clusters are generated by adiabatic expansion.
This Ar cluster is ionized by electron impact. Then, this cluster ion is extracted as a beam and accelerated by the acceleration electrode 17.
In the vacuum chamber 1, LaF ₃ or MgF ₂ is vacuum evaporated from the resistance heating board 3 while irradiating the synthetic quartz substrate 7 with the cluster ion beam.
Then, MgF ₂ film, LaF ₃ film, and MgF ₂ film having optical thicknesses of 0.5λ, 0.25λ, and 0.25λ in order of MgF ₂ , LaF ₃ , and MgF ₂ on the synthetic quartz substrate 7 from the substrate side, respectively. Each film was formed. The target center wavelength λ is 193 nm.
At that time, when the number of LaF ₃ film molecules is x, the number of MgF ₂ film molecules is y, and the number of Ar atoms constituting the cluster beam irradiated to the substrate and the film is a,
a / x = 3.4
a / y = 3.1
It is.
FIG. 12 shows the optical characteristics of the antireflection film when the energy of atoms constituting the cluster beam in this example is 3 to 20 eV / atm.
It is an antireflection film with high transmittance.

[Example 5]
In Example 5, a fluoride film was formed as follows.
First, in the cluster ion beam source 5, Ar gas is mixed and ejected from the nozzle 10, and is generated by Ar cluster adiabatic expansion.
This Ar cluster is ionized by electron impact. Then, this cluster ion is extracted as a beam and accelerated by the acceleration electrode 17.
In the vacuum chamber 1, while the synthetic quartz substrate 7 is irradiated with the cluster ion beam, MgF ₂ is vacuum evaporated from the resistance heating board 3 to form a film thickness of 340 nm on the synthetic quartz substrate 7.
An MgF ₂ film is formed.
At this time, when the number of MgF ₂ film molecules is y and the number of Ar atoms constituting the cluster beam irradiated to the substrate and the film is a, the following is performed.

In this case, the extinction coefficient of the MgF ₂ film when the energy of atoms constituting the cluster beam is 3 to 20 eV / atm was as small as 0.0002 with respect to 193 nm light.
Further, the extinction coefficient after the UVF was irradiated to the MgF ₂ film and the photo-modification was performed was as small as 0.0001 with respect to the light of 193 nm.
FIG. 13 shows the spectral characteristics of the synthetic quartz substrate on which the MgF ₂ film is formed after photo-modification by irradiating UV light.
The average surface roughness of this MgF ₂ film was 0.9 nm.

FIG. 14 shows a result of measuring the surface roughness of an MgF ₂ film having an energy of atoms of ˜3 eV / atm as a comparative example by AFM as a comparative example.
The average surface roughness was as rough as 2.3 nm.
FIG. 15 shows a case where the energy of atoms constituting the cluster beam is set to 200 eV / atm as a comparative example.
The extinction coefficient of the MgF ₂ film was as large as 0.0018 with respect to 193 nm light.
Further, even when the MgF ₂ film was irradiated with UV light and subjected to photo-modification, it was hardly recovered.

[Example 6]
In Example 6, a fluoride film was formed as follows.
First, in the cluster ion beam source 5, Ar gas is ejected from the nozzle 10 to generate Ar clusters by adiabatic expansion.
This Ar cluster is ionized by electron impact. Then, this cluster ion is extracted as a beam and accelerated by the acceleration electrode 17.
In the vacuum chamber 1, while irradiating the synthetic quartz substrate 7 with the cluster ion beam, AlF ₃ is vacuum evaporated from the resistance heating board 3 to form a film thickness of 430 nm on the synthetic quartz substrate 7.
An AlF ₃ film is formed.
At that time, when the number of AlF ₃ film molecules is x and the number of Ar atoms constituting the cluster beam irradiated to the substrate and the film is a,
a / x = 1.4
It is.
In this case, the energy of atoms constituting the cluster beam is 3 to 20 eV / atm.
In this case, a Scotch tape test was performed on the AlF ₃ film formed on the synthetic quartz. However, there was no problem that the film peeled off from the substrate, and a film having good adhesion was formed.
As a comparative example,
a / x = 1.0
The Scotch tape test was similarly performed on the AlF ₃ film formed on the synthetic quartz.
The film may be peeled off from the substrate and cannot be said to be a film with good adhesion.

The figure which shows an example of the thin film formation apparatus used for the formation method of the fluoride film | membrane by the cluster beam in embodiment of this invention. The figure which shows the detailed structure of the cluster ion beam source of the thin film formation apparatus in embodiment of this invention. It shows the spectral characteristics of the synthetic quartz substrate LaF ₃ film is formed in Embodiment 1 of the present invention. Shows the results of high-temperature and high-humidity environment endurance test of LaF ₃ film in the first embodiment of the present invention. It shows the spectral characteristics of the synthetic quartz substrate LaF ₃ film formed in the comparative example with Example 1 of the present invention. It shows the spectral characteristics of the synthetic quartz substrate that MgF ₂ film is formed in Embodiment 2 of the present invention. It shows the spectral characteristics of the synthetic quartz substrate that MgF ₂ film was formed in the comparative example of Example 2 of the present invention. It shows the average surface roughness of the MgF ₂ film in the third embodiment of the present invention. It shows an AFM image of MgF ₂ film when a / x is 3.1 in Example 3 of the present invention. It shows an AFM image of MgF ₂ film when a / x is 2.1 in Example 3 of the present invention. Shows the results of high-temperature and high-humidity environment endurance test of MgF ₂ film in the third embodiment of the present invention. The optical characteristic of the anti-reflective film in Example 4 of this invention is shown. It shows the spectral characteristics of the synthetic quartz substrate that MgF ₂ film is formed in Embodiment 5 of the present invention. It shows the results of the surface roughness of the MgF ₂ film was measured by AFM in the comparative example of Example 5 of the present invention. It shows the spectral characteristics of the synthetic quartz substrate that MgF ₂ film was formed in the comparative example of Example 5 of the present invention.

Explanation of symbols

1: Vacuum chamber 2: Vacuum valve 3: Resistance heating board 4: Resistance heating mechanism 5: Cluster ion beam source 6: Substrate dome 7: Substrate 8: Film thickness monitor 9: Shutter 10: Nozzle 11: Cluster generation chamber 12: Skimmer 13: Valve 14: Ionization chamber 15: Valve 16: Filament 17: Acceleration electrode 18: Acceleration chamber 19: Size separation chamber

Claims (10)

A method of forming a fluoride film using a cluster beam,
A method for forming a fluoride film, comprising the step of: evaporating a fluoride film material by vacuum evaporation using resistance heating while irradiating the substrate with the cluster beam to form a fluoride film on the substrate. . The fluoride film material is GdF ₃ , LaF ₃ , NdF ₃ , DyF ₃ , PdF ₃ , MgF ₂ , AlF ₃ , NaF ₃ , LiF, CaF ₂ , BaF ₂ , SrF ₂ , ErF ₃ , CeF ₃ , CeF ₃ , CeF _3, , One of Na ₃ AlF ₆ , Na ₅ Al ₃ F ₁₄ , YbF ₃ , ErF ₃ , TbF ₃ ,
2. The method for forming a fluoride film according to claim 1, wherein the fluoride film is a mixture thereof. When forming a fluoride film on the substrate, LaF ₃ is used as the fluoride film material,
When the number of the fluoride molecules deposited on the substrate is x, and the number of atoms constituting the cluster beam irradiated to the substrate and the fluoride film is a, respectively,
3. The method of forming a fluoride film according to claim 1, wherein a fluoride film made of the film material is formed on the substrate while satisfying the following relational expression (1).
In forming a fluoride film on the substrate, the fluoride film material may be MgF ₂ , LiF, BaF ₂ , GdF ₃ , NdF ₃ , DyF ₃ , PdF ₃ , CaF ₂ , SrF ₂ , ErF ₃ , SmF ₃ , Any one of CeF ₃ , YbF ₃ , TbF ₃ , ErF ₃ is used,
When the number of the fluoride molecules deposited on the substrate is x and the number of atoms constituting the cluster beam irradiated on the substrate and the fluoride film is a, respectively,
3. The method of forming a fluoride film according to claim 1, wherein a fluoride film made of the film material is formed on the substrate while satisfying the following relational expression (2).
When forming a fluoride film on the substrate, MgF ₂ is used as the fluoride film material,
When the number of the fluoride molecules deposited on the substrate is x and the number of atoms constituting the cluster beam irradiated on the substrate and the fluoride film is a, respectively,
3. The fluoride film according to claim 1, wherein a fluoride film made of the film material satisfying the following relational expression (3) and having an average surface roughness of 1.0 nm or less is formed on the substrate. Method for forming a chemical film.
When forming a fluoride film on the substrate, any one of AlF ₃ , NaF ₃ , Na ₃ AlF ₆ , and Na ₅ Al ₃ F ₁₄ is used as the fluoride film material,
When the number of the fluoride molecules deposited on the substrate is x and the number of atoms constituting the cluster beam irradiated on the substrate and the fluoride film is a, respectively,
3. The method of forming a fluoride film according to claim 1, wherein a fluoride film made of the film material is formed on the substrate while satisfying the following relational expression (4).
The fluoride according to any one of claims 1 to 6, wherein the energy of each atom constituting the cluster beam irradiated to the substrate and the fluoride film is 3 to 50 eV / atm. Method for forming a film.   8. The method for forming a fluoride film according to claim 1, wherein the cluster beam is composed of rare gas atoms.   During or after the formation of the fluoride film in the step of forming the fluoride film on the substrate, these fluoride films are irradiated with UV light and optically modified by optical modification of the fluoride film. The method for forming a fluoride film according to any one of claims 1 to 8, further comprising a step of reducing the temperature.   An optical element comprising a fluoride film obtained by the method for forming a fluoride film according to any one of claims 1 to 9.