Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
2003-06-19
2004-10-26
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With field curvature shaping
C359S796000
Reexamination Certificate
active
06809876
ABSTRACT:
BACKGROUND OF THE INVENTION
CROSS-REFERENCE
This application is a Continuation Application of International Application No. PCT/JP02/07266 which was filed on Jul. 17, 2002 claiming the conventional priority of Japanese patent Application No. 2001-218045 filed on Jul. 18, 2001.
1. Field of the Invention
The present invention relates to an optical element having a multilayer film which minimally absorbs the light in the vacuum ultraviolet region and to an exposure apparatus provided with the optical element.
2. Description of the Related Art
The fluoride material has such an excellent optical characteristic that the material is transparent over a wide light wavelength range from the infrared to the vacuum ultraviolet region. In particular, almost all of oxide materials are opaque at wavelengths of not more than 180 nm, while a large number of fluoride materials are transparent. Therefore, the fluoride is necessary and indispensable especially for optical thin films and optical element materials to be used for the vacuum ultraviolet region.
In recent years, the high integration and the high densification are progressively advanced for the semiconductor integrated circuit. In order that the line width of the semiconductor integrated circuit is thinned and the pattern is made to be further minute, it is demanded to further improve the photolithography resolution of the reduction projection exposure apparatus for producing the semiconductor circuit. In order to improve the photolithography resolution of the reduction projection exposure apparatus, the wavelength of the exposure light source has been hitherto shortened to the g-ray, the i-ray (wavelength: 365 nm), and the KrF excimer laser (wavelength: 248 nm). It is inevitable that the wavelength will be progressively shortened to the ArF laser (wavelength: 193 nm) and the F
2
laser (wavelength: 157 nm) in future. The fluoride, which is also transparent with respect to the exposure light beam having the shortened wavelength, is used for the optical element such as lenses and prisms and the optical thin film such as antireflection films and polarization films with which the surface of the optical element is coated.
As much as several tens of optical elements, which are directed to a variety of ways of use, are arranged between the laser light source and the wafer on which the semiconductor circuit is exposed in the reduction projection exposure apparatus as described above. The surfaces of the optical elements are coated with fluoride thin films depending on their purposes respectively. It is a matter of course that the materials for the optical elements themselves and the fluoride thin films absorb the light. Therefore, the amount of light, which finally arrives at the wafer surface, is considerably decreased. In order to improve the exposure performance and the productivity, it is necessary that the decrease in light amount is minimized as far as possible.
As a result of diligent researches and developments for many years, the defect and the content of impurity, which cause the light absorption, are suppressed as far as possible in relation to the optical element materials themselves. Owing to the advancement of the polishing technique, the scattering on the element surface is lowered as well. On the other hand, the optical thin film has been hitherto formed by means of various PVD methods including, for example, the vacuum deposition based on the resistance heating and the electron beam dissolution, the vacuum deposition combined with the ion assist, the ion plating, the sputtering, and the ion beam sputtering.
Fluorine is deficient as compared with the stoichiometric composition in the fluoride thin film produced by any film formation technique other than the resistance heating type vacuum deposition method. As a result, the absorption edge wavelength is shifted toward the long wavelength side as compared with the ideal crystal, and any absorption band also appears due to the defect and the impurity. Therefore, in such a fluoride thin film, the light absorption is increased in the vacuum ultraviolet region.
That is, in the case of the electron beam dissolution type vacuum deposition, the deposition material of fluoride is evaporated by using the electron radiation energy. However, the separation is caused by the energy between the fluorine atom and the metal atom in a certain proportion. Therefore, the fluorine deficiency occurs in the vapor deposition film. In the case of the ion assist vacuum deposition and the ion plating, the thin film surface, at which the growth is continued on the substrate, undergoes the ion beam radiation and the plasma radiation. As a result, the fluorine divergence, which is caused by the collision of charged particles on the growth film surface, also occurs in addition to the fluorine divergence caused on the side of the vapor deposition material. Further, in the case of the sputtering and the ion beam sputtering, when the fluoride target is subjected to the sputtering by means of the ion impact, only the light fluorine atoms are selectively subjected to the sputtering due to the selective sputtering phenomenon. As a result, the fluorine deficiency occurs on the target surface.
Therefore, in the case of the respective film formation processes other than the resistance heating type vacuum deposition method, any fluorine-based gas is introduced into the film formation vessel during the film formation in order to supplement the fluorine deficiency of the deposited film. However, it is complicated to control the reaction, and a film, which is based on the stoichiometric composition, is not necessarily obtained with ease.
On the contrary, in the case of the resistance heating type vacuum deposition method, fluoride crystal grains, which serve as the raw material, are placed on a vapor deposition boat made of a high melting point metal such as molybdenum, tungsten, and tantalum, and the vapor deposition boat is heated by applying the electric power to evaporate the raw material fluoride crystals. The fluoride, which is evaporated in accordance with the reversible physical change of the evaporation caused by the heating, does not cause the fluorine deficiency. The evaporated fluoride is allowed to fly onto a substrate having a relatively low temperature installed at a position opposed to the vapor deposition boat. The fluoride is deposited and solidified on the substrate while effecting the adsorption and the desorption, and thus the fluoride is grown as a thin film. The fluoride thin film, which is formed on the substrate as described above, hardly causes the fluorine deficiency.
As described above, the fluoride thin film, which is produced by the resistance heating type vacuum deposition method, is more excellent than the fluoride thin films produced by the other film formation techniques in that the fluorine deficiency is not caused and the fluoride thin film has the stoichiometric composition. On the other hand, the fluoride thin film has the columnar or prism-shaped structure. Therefore, the fluoride thin film has such a drawback that the film is porous and it has a large surface area, as compared with the single crystals and the bulk polycrystals as well as the films produced by the other film formation methods.
This drawback results from the fact that the substrate (fluoride optical element) has the low temperature. However, if the substrate is heated to a high temperature of several hundred degrees centigrade or more, the optical constant of the optical element itself is changed. Further, the shape of the lens surface, which has been strictly finished, is also changed, and the intended function as the optical element is consequently lost. Therefore, the substrate temperature is suppressed to be low.
If the substrate temperature is relatively high after the particles evaporated from the evaporation boat are allowed to fly onto the substrate surface to effect the adsorption, then the structure of the formed film is consequently dense, and the structural irregularity is scarcely caused as well, becaus
Nikon Corporation
Oliff & Berridg,e PLC
Schwartz Jordan M.
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