JP2000085025A - Production of optical thin film, and optical thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To widen the selectivity of a substrate shape and to obtain an optical thin film of uniform quality and thickness easily by a method in which optical thin film material sol is applied on a substrate, after a gel film being deposited, the temperature and pressure of liquid for immersion are brought into a critical state or above to vaporize the liquid, and unnecessary components in the gel film are removed. SOLUTION: A double-convex fluorite lens is set in a holder, immersed in a SiO2 sol solution, pulled up at a constant speed, and a SiO2 gel film is formed. The lens with the SiO2 gel film formed is transferred quickly together with the holder into the chamber 7 of a critical drier, the chamber 7 is sealed, a valve 13b is opened, liquid carbon dioxide which was heated in advance is introduced from a liquid tank 5 into the chamber 7. Next, the temperature and pressure of the chamber are brought into the critical temperature and critical pressure of carbon dioxide or above, the liquid carbon dioxide from which unnecessary components are eluted is released into a separator 9 held at a critical point or below, the sequence of operations is repeated five times, and the SiO2 gel film is dried completely.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical component, particularly an optical thin film such as an antireflection film or a reflection film used for an optical component for a laser or the like having a high irradiation energy density, and a method for producing the optical thin film.

[0002]

2. Description of the Related Art At present, almost all optical elements are formed with optical thin films such as an antireflection film and a reflection enhancement film.
As a method for producing an optical thin film, a vacuum evaporation method, a sputtering method, an ion plating method, a CVD method, and the like are widely used according to the characteristics of the optical thin film intended.
Since these methods generally form a film under reduced pressure or in a closed system, there is a problem that the apparatus becomes large-scale. In contrast to these methods, a sol-gel method has recently been used as a unique method of forming a thin film by a wet method. The characteristics of the sol-gel method include the following. 1) A porous film can be obtained because of the gel state. Further, densification can be performed by firing. 2) The firing temperature is several hundred to 1,000 ° C. lower than that of ordinary ceramics. 3) Since the raw material can be easily purified, a high-purity film can be formed. 4) Since the raw materials are mixed in a low-viscosity liquid state, they are excellent in homogeneity, and are advantageous for preparing composite oxides and the like. Further, a compound film having high stoichiometry can be obtained. 5) The apparatus is simple and suitable for forming a film on a large area substrate or a substrate having a complicated shape. Further, simultaneous film formation on both sides or partial film formation is possible.

As starting materials for film formation by the sol-gel method, metal halides, metal oxyhalides, and metal organic compounds are used. Among them, metal alkoxides are most often used. The reasons are that there is no need to remove nonvolatile components such as halides, and that the hydrolysis / condensation polymerization process can be easily controlled. In metal alkoxides, the rate of hydrolysis reaction can be controlled by changing the type of alkoxyl group. For example, the longer the alkyl group and the more branches,
Steric hindrance increases stability against hydrolysis. By utilizing this property, it is possible to eliminate the influence of the difference in the hydrolysis rate of different metal alkoxides, which is a problem when preparing a composite oxide. Generally, metal alkoxides lack an affinity for water, so that a uniform solvent requires the use of a common solvent. In addition, when water is directly added to a highly reactive metal alkoxide, local hydrolysis occurs and precipitation occurs.For this reason, it is necessary to dilute the metal alkoxide and water to prevent this. Each of them is often diluted with a common solvent such as alcohol.

The sol is deposited on the substrate surface by (1) a spray method of spraying the sol, (2) a spin coating method of dropping the sol onto a rotating substrate and spreading the sol by centrifugal force.
(3) A dip coating method in which a substrate is immersed in a sol and pulled up is used. The sol-gel film formed by such a characteristic process has recently attracted attention as a method for forming an optical thin film having high laser durability. This is mainly because the sol-gel film is porous. It is said that the damage of the optical thin film by light is caused by the absorption of light energy by the film material and the absorbed energy is converted into heat energy. It hardly generates stress against impact such as generation of laser, and shows higher laser durability than a dense film.

[0005]

Since the sol-gel method is a wet method, it must necessarily undergo a step of drying the sol. At this time, a large change in volume of the sol occurs, and the density and the density of the gel after drying are increased. This may cause uneven film thickness or cracks. This is because the solvent contained in the film has a surface tension, and the molecular chain of the substance constituting the film contracts due to the surface tension as the volume decreases due to the evaporation of the solvent. Such a phenomenon is easily affected by the shape of the substrate and the drying environment, and not only limits the degree of freedom in selecting the shape of the substrate, but also requires strict control of the film forming environment.

[0006] Further, since the optical thin film formed by the sol-gel method is porous, if heat treatment (baking) is not performed, many substances such as water, alcohol, alkoxide and the like (hereinafter referred to as unnecessary components) are often present. It is adsorbed, and a film loss due to absorption may occur depending on the wavelength of the light source used. If the film loss is large, the light durability of the film is reduced due to the energy absorption of the incident light, so that baking for removing these adsorbed components is required. However, in order to carry out baking, there is a problem that the film must be dried slowly in advance so as not to cause rapid shrinkage of the film, and a special heating facility is required.

Accordingly, the present invention has been made to solve the conventional problems, and when an optical thin film is formed by using a sol-gel method, the selectivity of the substrate shape is increased and unnecessary components are reduced. It is an object of the present invention to provide a method for easily forming an optical thin film having a uniform film quality and thickness.

[0008]

According to the present invention, there is provided an optical thin film material sol coated on a substrate, a gel film is deposited, and then immersed in a liquid. A method for producing an optical thin film (claim 1), comprising removing unnecessary components in a gel film while evaporating and drying a liquid.

The present invention also provides "an optical thin film characterized by having at least an optical thin film manufactured by the method for manufacturing an optical thin film according to claim 1 (claim 2)". In addition, the present invention provides a method wherein “the optical thin film has a thickness of
An optical thin film according to claim 2 (claim 3) for use in a wavelength range of 0 nm is provided.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION A method for manufacturing an optical thin film according to an embodiment of the present invention comprises applying an optical thin film material sol onto a substrate,
After depositing the gel film, the gel film is immersed in a liquid, the temperature and pressure of the liquid are set to critical levels or more, and the liquid is vaporized and dried. Examples of the optical substrate include optical glass and plastic lenses.

Examples of the optical thin film material sol include sols of metal halides, metal oxyhalides, metal alkoxides, and metal organic compounds described in the prior art. A composite oxide may be added to the sol for adjusting the refractive index.
The critical state means that when a gas is pressurized at a critical temperature, the pressure (p) -volume (v) curve shows an inflection point, and (∂p
/ ∂v) T = 0, ( ∂ 2 p / ∂ 2 v) T = 0 (T is a point at which the temperature), means a state in which the gas is changed continuously liquid, the critical pressure of the pressure at that time, The volume is called the critical volume. Carbon dioxide and Freon gas can relatively easily reach the temperature and pressure conditions (supercritical state) above the critical point. Therefore, when the optical substrate on which the gel film is formed is immersed in liquid carbon dioxide or chlorofluorocarbon (liquid), the water or alcohol in the gel film diffuses into liquid carbon dioxide or chlorofluorocarbon (liquid) and simultaneously carbon dioxide or chlorofluorocarbon (liquid). Penetrates. In this state, the temperature and pressure are raised to the critical point or more, and the mixture is slowly vaporized and dried, whereby the deformation of the gel film can be prevented.

Hereinafter, the process of supercritical cleaning and drying will be described in more detail with reference to FIGS. FIG. 1 is a schematic sectional view of a supercritical cleaning / drying apparatus used in the method for manufacturing an optical thin film according to the embodiment of the present invention. The supercritical cleaning / drying apparatus shown in FIG. 1 includes a liquid tank 5 having a valve 13a.
And a heating mechanism 8 and a chamber 7 for controlling the internal temperature
A chamber 7 having a pressure gauge 6 for measuring the pressure in the chamber,
A liquid tank 5 and a chamber 7
Are connected by an inlet tube 11 provided with a valve 13b and a preheating mechanism 10, and the chamber 7 and the separator 9
Are connected by a discharge pipe 12 provided with a valve 13d.

FIG. 3 is a phase diagram of carbon dioxide and a model diagram of supercritical cleaning and drying using carbon dioxide. Carbon dioxide takes a solid, liquid, or gas state due to the relationship between temperature and pressure, but has a temperature of 31.4 ° C. or more and a pressure of 39.5 kg /.
In a region of cm ² or more, the fluid becomes a supercritical fluid, and there is no distinction between a liquid and a gas. The temperature and pressure at which the transition to the supercritical state is referred to as a critical point (x in the figure). Normal drying is shown in FIG.
Supercritical drying is performed in the process of dotted lines a → b → c, while the process is performed in the process of dotted lines a → c shown therein. That is, when the member is left standing in the chamber of the supercritical cleaning / drying apparatus, the member is heated to 100 ° C. or more at atmospheric pressure and then dried in normal water washing.

On the other hand, in the supercritical cleaning using carbon dioxide, using a supercritical cleaning / drying apparatus shown in FIG. 1, first, high-pressure liquid carbon dioxide 5 is introduced into the chamber 7, It is heated above the critical temperature while maintaining the airtight state inside. Then, in the closed chamber 7, the internal pressure of the chamber also increases with the increase of the vapor pressure of carbon dioxide, and exceeds the critical temperature and critical pressure to be in a supercritical state.

When the carbon dioxide is gradually released from the interior of the chamber 7 after a sufficient extraction time of the unnecessary components, the pressure in the chamber 7 is reduced while the temperature in the chamber 7 remains unchanged. And is vaporized and diffused. In the release process, carbon dioxide is continuously changed from a supercritical fluid to a gas without forming a gas / liquid interface, and thus the object to be dried is dried without being affected by surface tension.

The procedure of the method for manufacturing an optical thin film according to the embodiment will be described below. In the embodiment, carbon dioxide that is inexpensive and easily obtains a supercritical state is used as the supercritical fluid. FIG.
It is a dip coater used for the manufacturing method of the optical thin film of an embodiment. First, a biconvex fluorite lens 1 (refractive index: 1.628) having a radius of curvature R = 100 is prepared and set in a dedicated holder 2 of a dip coater, and Si (OC ₂ H ₅ ) ₄ : H
_It was immersed in a SiO ₂ sol solution 3 mixed at a ratio of ₂ O: C ₂ H ₅ OH: HCl = 1: 6: 10.5: 0.06 and pulled up at a constant speed to form a SiO ₂ gel film.

Next, the lens 1 on which the SiO ₂ gel film was formed was immediately transferred together with the holder 2 into the chamber 7 of the critical drying apparatus shown in FIG. 1, and the inside of the chamber 7 was sealed.
The valve 13 b provided on the introduction pipe 11 was opened, and liquid carbon dioxide was introduced from the liquid tank 5 into the chamber 7. Since the introduction pipe 11 is provided with the preheater 10, the liquid carbon dioxide is heated by the preheater 10 before entering the chamber 7.

Next, the temperature in the chamber is raised to a critical temperature of carbon dioxide of 31.4 ° C. or higher by the heating mechanism 8 and the pressure is raised to a critical pressure of 39.5 kg / cm ² or higher.
Hold for minutes. Thereafter, the liquid carbon dioxide from which the unnecessary components were eluted was discharged into the separator 9 kept below the critical point.
Carbon dioxide in the separator 9 was finally released to the atmosphere, and non-volatile components remained in the separator 9. Carbon dioxide as a supercritical fluid was again injected into the chamber, and after maintaining for a certain period of time, carbon dioxide was released into the separator 9. This series of operations was repeated five times.

After the last carbon dioxide injection, carbon dioxide in a supercritical state was gradually vaporized, and the SiO ₂ gel film was completely dried. The dried sample was taken out of the chamber, and the optical characteristics were evaluated. The SiO ₂ gel film thus manufactured has a refractive index of 1.28, which is sufficiently smaller than that of the fluorite substrate (1.628), and is a single-layer film, but has a per-surface in a wavelength range of 185 nm to 200 nm. The residual reflectivity was about 0.2%, which proved to have a sufficient antireflection effect.

The thickness of the film can be controlled by adjusting the viscosity of the sol appropriately to produce an arbitrary film. In addition, since unnecessary organic impurities were sufficiently removed, it was confirmed that the absorption loss of the film was small. Wavelength 193 nm,
In a light durability test using an ArF excimer laser having a pulse width of 50 pm, the laser damage threshold value at the irradiation frequency of 1 × 10 ⁷ was 900 mJ, and the laser durability was excellent.

The optical thin film according to the present invention has a thickness of 193n.
m not only for ArF excimer laser
It can be used as an optical thin film for a laser existing in a wavelength range of 300 nm.

[0022]

As described above, in the method of manufacturing an optical thin film according to the present invention, the two-dimensional optical film manufacturing method according to the conventional sol-gel method has a two-dimensional structure in the plane direction of the substrate, which has occurred in the drying stage. Eliminates non-uniformity in film density and thickness caused by differences in gel drying speed and differences in film depth drying speed, enabling film formation on lenses with large curvatures or substrates with complex shapes In addition to the above, drying in a liquid makes it possible to improve the non-uniformity of the film caused by the movement of the gel or the solvent due to gravity.

Further, it is possible to perform a washing step for removing unnecessary components, which has been performed separately from the conventional drying step, at the same time as the drying step, so that an optical thin film having high light durability can be manufactured with less equipment and time than before. It is possible to manufacture. Furthermore, in the conventional sol-gel method, the film is densified due to the surface tension of the solvent accompanying drying, whereas according to the method for producing an optical thin film of the present invention, the surface tension of the solvent is eliminated, so the drying step Does not progress, and it is possible to form an optical thin film having a low density, a porous structure and a low refractive index.

Further, according to the method for producing an optical thin film of the present invention, unnecessary components can be removed simultaneously with vaporization and drying of the gel film, so that the desired (low absorption, light durability) can be obtained with a small amount of equipment and time. Optical thin films can be manufactured.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a supercritical cleaning / drying apparatus used in a method for producing an optical thin film according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of a dip coating apparatus used in the method for manufacturing an optical thin film according to the embodiment of the present invention.

FIG. 3 is a phase diagram of carbon dioxide and a model diagram of supercritical cleaning and drying using carbon dioxide.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Lens 2 ... Holder 3 ... Gel solution 4 ... Pulling-up machine 5 ... Liquid carbon dioxide 6 ... Pressure gauge 7 ... Chamber 8 ... Heating mechanism 9 ... Separator 10 Preliminary heating mechanism 11 Inlet pipe 12 Outlet pipe 13a, 13b, 13c, 13d Valve 14 Liquid tank

Claims (3)

[Claims] An optical thin film material sol is coated on a substrate, a gel film is deposited, and then immersed in a liquid, and the temperature and pressure of the liquid are raised to critical levels or more, and the liquid is vaporized and dried. A method for producing an optical thin film, comprising removing unnecessary components from a film. 2. An optical thin film having at least an optical thin film manufactured by the method for manufacturing an optical thin film according to claim 1. 3. The optical thin film according to claim 2, wherein said optical thin film is used for a wavelength range of 100 nm to 300 nm.