JP4556763B2 - Method for producing fluorine-based optical polymer film and reflection mirror for ultraviolet region - Google Patents

Description

  The present invention relates to an antireflection film or a reflection film applied to the surface of a substrate of an optical element such as a mirror, a lens, a prism, and a filter that is mainly suitable for use in the ultraviolet region.

In image forming apparatuses and projection exposure apparatuses, it is desired to use ultraviolet light having a shorter wavelength in order to reduce the line width and increase the degree of integration. Conventionally, an optical system of such an apparatus is coated with a metal film or a dielectric film in order to increase the reflectance and transmittance with respect to ultraviolet light. These coatings are performed after the film thickness and refractive index are appropriately designed according to the wavelength and application of the light used, but the material is magnesium fluoride (MgF ₂ ) because of its low ultraviolet light absorption. And metal fluorides such as calcium fluoride (CaF ₂ ) are often used.

Conventionally, a vacuum deposition method, a sputtering method, an ion plating method, or the like is used for coating these metal fluoride films (Non-Patent Document 1).
"Far ultraviolet optical properties of MgF2 films deposited by ion-beam sputtering and their application as protective coatings for Al", Juan I. Larruquert, Ritva AM Keski-Kuha, Optics Communications, 215 (2003) 93-99

  In any of the above methods, the metal fluoride film produced has a deficiency of fluorine atoms or ions (fluorine deficiency), thereby causing light absorption and scattering that cannot occur from the original material properties. Such absorption and scattering become a big problem in the ultraviolet region rather than the visible region, and may not be used as an optical component.

  In addition, the film formation of the fluoride film by the above method is performed in a state where the base material is heated. However, when the thermal expansion coefficients of the fluoride film and the base material are greatly different, the respective shrinkage amounts are reduced when cooling after the film formation. A difference occurs and a large stress is generated in the film. As a result, there are problems such as poor adhesion between the manufactured fluoride film and the substrate, and the film is likely to crack.

  The present invention provides a method for producing a fluorine-based optical polymer film that functions as an antireflection film or a reflection film that suppresses fluorine deficiency as much as possible and hardly causes peeling.

A method for producing a fluorine-based optical film according to the present invention made to solve the above-mentioned problems is a step of applying an RF voltage in a state where a fluorine-based gas is introduced into a vacuum vessel to convert the gas into plasma, A step of applying a direct-current voltage between a heating evaporation source disposed in the vacuum vessel and a processing stage holding a substrate, and a step of vaporizing and ionizing metal fluoride by the heating evaporation source, Thereby , a fluoropolymer film is formed on the surface of the substrate.

  In addition to the fluorine-based gas, an organic gas may be introduced.

  The optical film according to the present invention can be used as an antireflection film for coating on the surface of lenses, prisms, and filters by appropriately setting the thickness thereof, and can also be used as a reflection film for coating on the surface of mirrors. You can also

Therefore, as a substrate for coating the fluorine-based optical film according to the present invention, in the case of lenses, prisms, and filters, conventionally used synthetic quartz glass, calcium fluoride (CaF ₂ ), lithium fluoride (LiF) In the case of mirrors, conventionally used metals such as aluminum can be used as well.

The metal fluoride that is the basic material of the optical film preferably has a refractive index as low as possible. Magnesium fluoride (MgF ₂ ), calcium fluoride (CaF ₂ ), aluminum fluoride (AlF ₃ ), lithium fluoride (LiF) or the like is preferable. Among these, MgF ₂ is preferable because it has the lowest refractive index.

As the fluorine-based gas, an organic substance such as carbon tetrafluoride (CF ₄ ) or fluorocarbon, or an inorganic substance such as fluorine gas (F ₂ ) or nitrogen trifluoride (NF ₃ ) can be used.

As the organic gas used additionally, an unsaturated organic substance that can be a polymer precursor, for example, an alkene such as ethylene (CH ₂ ═CH ₂ ), propylene (CH ₃ CH═CH ₂ ), or the like can be used.

By using the method of the present invention, it is possible to produce a fluorine-based optical film with very little fluorine deficiency, while reducing the space filling density of the film. Therefore, the stress generated in the film at the time of cooling after film formation can be reduced, and peeling of the optical film from the substrate can be effectively prevented. In addition, since the refractive index can be made smaller than that of a conventional metal fluoride film, an optical film with less loss can be obtained in either case of a reflection film or an antireflection film.

  Further, when an organic gas is additionally used, the space filling density of the film can be further reduced and the refractive index can be further reduced. Thereby, the loss at the time of permeation | transmission or reflection can be suppressed rather than the conventional metal fluoride film | membrane, and it can utilize more suitably as an antireflection film or a reflecting film.

  One embodiment of an apparatus for carrying out the method for producing a fluorine-based optical film of the present invention will be described with reference to FIG. FIG. 1 shows a composite ion plating apparatus. A processing table 12 for fixing an object to be coated is provided in the upper part of the processing chamber 11 which is a vacuum vessel, and a heating boat 13 for heating the coating film main agent is provided immediately below the processing table 12. It is desirable to employ a resistance heating method using a high resistance material for the heating boat 13. A DC voltage source DC for applying a DC voltage is provided between the heating boat 13 and the processing table 12, and a plasma generating RF electrode 14 is disposed between the processing table 12 and the heating boat 13. .

A method for producing a coating film on the reflecting surface of the concave mirror using this apparatus will now be described. The substrate of the glass concave mirror S that is the object to be coated is fixed to the lower surface of the processing table 12 with the processing surface facing down. An aluminum reflecting film is formed in advance on the surface of the concave mirror S. After the processing chamber 11 is evacuated to about 10 ⁻⁴ Pa by the pump P, a fluorine-based gas such as carbon tetrafluoride (CF ₄ ) is introduced into the processing chamber 11 from the gas inlet G1. Further, if necessary, an organic gas such as ethylene (CH ₂ = CH ₂ ), methane (CH ₄ ) or the like is introduced into the processing chamber 11 from the gas inlet G2. When the pressure in the processing chamber 11 reaches about 5 × 10 ⁻² Pa, the introduction of these gases (reactive gases) is stopped.

Next, the heating boat 13 is heated to vaporize magnesium fluoride (MgF ₂ ), which is a main component of the coating film previously placed thereon. When the temperature of the heating boat 13 rises sufficiently, a DC voltage of about 100 V is applied between the heating boat 13 and the processing table 12, and a high frequency of 13.56 MHz and about 50 W is applied from the high frequency power source RF to the plasma generating RF electrode 14. Apply current. By applying this DC voltage, glow discharge is generated between the heating boat 13 and the processing table 12, while the reaction gas such as carbon tetrafluoride is turned into plasma by applying RF power to the plasma generating RF electrode 14. To do. Magnesium fluoride vaporized and ionized by heating is accelerated by a DC voltage to form a film on the reflecting surface of the concave mirror S. At that time, a plasma reaction gas is involved in film formation, and a plasma polymerization reaction occurs. Thereby, fluorine deficiency is compensated to form an atomically dense film, and an effect that the filling rate of the film is reduced at a slightly larger scale can be obtained. One reason for this is thought to be due to the following process. That is, monomer molecules of fluorine-based gas and organic gas are once decomposed in plasma, but this decomposition product undergoes an activation process, and thus atoms constituting the polymer chain of the plasma polymerized film generated in the film. The sequence will be different from the sequence of the monomer molecules.

  By continuing the above process for a predetermined time, a reflective film made of a fluorine-based polymer film having a predetermined thickness can be formed on the reflective surface of the concave mirror S.

  Since the optical film formed by the method according to the present invention has a slightly low filling rate as described above, the internal stress generated during cooling after processing is small, and the occurrence of peeling and cracking is suppressed. Moreover, it has the characteristic that a refractive index is lower than the conventional optical film.

FIG. 2 shows the calculation results of the wavelength reflectance by calculation when the reflective film is formed on the aluminum reflecting surface by the conventional method and when the reflective film is formed by the method according to the present invention. In the case of only the aluminum reflecting surface (Al), the reflectance is low as a whole, and waves are generated on the short wavelength side of 400 nm or less (lip ring). When an MgF ₂ film having a thickness of 150 nm (with a refractive index of 1.38) is formed by a conventional method (MgF ₂ -150 nm), a larger lip ring is generated and cannot be used as a reflector. Similarly, in the case of an MgF ₂ film formed by a conventional method and having a thickness of 50 nm (refractive index is also 1.38) (MgF ₂ -50 nm), the lip ring is fairly small, but still slightly at short wavelengths of 400 nm or less. The waves are coming up. On the other hand, when the refractive index of the MgF ₂ film prepared by the method of the present invention is 1.1 and the thickness is 50 nm, the lip ring is almost eliminated and the reflectance is improved as a whole.

BRIEF DESCRIPTION OF THE DRAWINGS The schematic block diagram of the composite ion plating apparatus which is one embodiment of the manufacturing method of the fluorine-type optical polymer film of this invention. Figure calculation result of the wavelength reflectance when generating the MgF ₂ optical film according to the method of conventional methods and the present invention the aluminum reflecting surface.

Explanation of symbols

11 ... Processing chamber (vacuum container)
12 ... Processing table 13 ... Heating boat 14 ... RF electrode for plasma generation

Claims (4)

A step of applying RF voltage in a state where a fluorine-based gas is introduced into a vacuum vessel to convert the gas into plasma;
Applying a DC voltage between a heating evaporation source disposed in the vacuum vessel and a processing stage holding the substrate;
Vaporizing and ionizing metal fluoride by the heating evaporation source;
Have
Thus, a method for producing a fluorine-based optical film , comprising forming a fluorine-based polymer film on the surface of the substrate.   The method for producing a fluorine-based optical film according to claim 1, wherein an organic gas is also introduced in addition to the fluorine-based gas.   The method for producing a fluorine-based optical film according to claim 1, wherein the metal fluoride is magnesium fluoride.   A reflection mirror for an ultraviolet region, wherein the fluorine-based optical film produced by the method according to claim 1 is coated on a reflection surface.

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