Optical: systems and elements – Lens
Reexamination Certificate
1999-09-03
2001-05-01
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
C359S722000, C359S800000, C359S350000
Reexamination Certificate
active
06226128
ABSTRACT:
This application claims the benefit of Japanese Application No. 10-252554, filed in Japan on Sep. 7, 1998, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-transmitting optical member, and more particularly, to a light-transmitting optical component (such as a lens, prism, plate, etc.) that is used in an optical system (such as a lithography apparatus, CVD device, laser processing device, etc.) with a light source (such as an ArF excimer laser, F
2
laser, solid state laser, etc.) having a wavelength below 200 nm. The present invention also relates to a manufacturing method and an evaluation method for such a light-transmitting optical component.
2. Discussion of the Related Art
An optical lithography process uses a lens (light-transmitting optical member or component) to direct light from an exposure light source through a mask to a wafer coated with a photo-sensitive material, thereby transferring a pattern on the mask onto the wafer. LSIs are being developed with increasingly high density. Generally, the resolving power of the transferred pattern is inversely proportional to the numerical aperture of the lens and directly proportional to the exposure light wavelength. The numerical aperture of the lens increases as the diameter of the lens increases. However, it is difficult to increase the numerical aperture of the lens past a certain limit, because an impractically large diameter is required. For this reason, a reduction in wavelength of the light source is desired in order to allow a further improvement in resolution in an optical lithography process.
Until now, light sources used in optical lithography have utilized ultraviolet light including the i-line (365 mn) of a high pressure mercury lamp and a shorter wavelength light (248 mn) of a KrF excimer laser. Because the KrF excimer laser is capable of high power output with a high laser oscillation frequency, the KrF excimer laser has been widely used as an efficient light source for optical lithography, and research and development continues to increase its efficiency. As a result of this wide use of ultraviolet light, optical lenses with high ultraviolet transmissivity have been increasingly used in the optical systems of optical lithography apparatus.
Recently, to improve the resolution further, it is expected that light sources producing vacuum ultraviolet light of even shorter wavelengths will be used in optical lithography apparatus.
Although high-light-transmittance optical lenses are used in conventional optical lithography apparatus, these conventional lenses cannot provide the practical degree of transparency needed with vacuum ultraviolet light of a shorter wavelength (about 200 nm or less). This problem has prevented the use of vacuum ultraviolet light sources in optical lithography processes. Vacuum ultraviolet light has a high energy, more than about 6.2 eV. Therefore, if the transmittance of a lens is not sufficiently high, the energy which is not transmitted by the lens is converted to heat, and as a result, the imaging performance deteriorates due to thermal expansion of the optical lens. Also, with such an insufficient transmittance, a photo-resist cannot be properly exposed.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a manufacturing method, evaluation method, and optical lithography apparatus for a light-transmitting optical component that substantially obviates the problems due to limitations and disadvantages of the related art. 5 An object of the present invention is to provide an optical lens having superior optical properties required by an optical lithography apparatus using a vacuum ultraviolet light source with a wavelength below 200 nm.
Another object of the present invention is to provide a method for evaluating of a light-transmitting optical component that transmits light with a wavelength below 200 nm to determine whether the optical component can be used in an optical lithography apparatus using vacuum ultraviolet light source.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a light-transmitting optical member for transmitting light with a wavelength less than about 200 nm, the optical member including crystalline calcium fluoride with a potassium content less than about 0.5 ppm, and the optical member having a degradation in a transmittance of less than 5% after irradiation for a predetermined period of time with light of a wavelength less than about 200 nm. another aspect, the present invention provides a method for evaluating a light-transmitting optical member for use with light of a wavelength less than about 200 nm, the method including the steps of measuring a first transmissivity of the optical component, thereafter irradiating the optical component with light of a wavelength less than about 200 nm for a predetermined time period, measuring a second transmissivity of the optical component that was irradiated in the step of irradiating to determine an amount of a transmissivity decrease after the irradiation, and comparing the amount of transmissivity decrease with a predetermined value to determine whether the light transmitting component is usable in an optical lithography apparatus.
In another aspect, the present invention provides a method for manufacturing a light-transmitting optical member that transmits light of a wavelength less than about 200 nm, the method including the steps of melting a calcium fluoride raw material, and gradually cooling the melted raw material with a temperature gradient at a solid-melt interface to crystallize calcium fluoride, wherein a potassium content in the raw material and the temperature gradient are adapted to provide for the potassium content of the resulting crystal to be less than about 0.5 ppm in a pulling-down scheme.
In another aspect, the present invention provides a method for the manufacture of a light-transmitting optical member that transmits light of a wavelength less than about 200 nm, the method including the steps of melting within a crystal growth crucible of calcium fluoride raw material with a potassium content less than 0.5 ppm, a crystal growth step during which gradual cooling and crystal growth are carried out, and a heat treatment step during which, after maintenance of the obtained grown calcium fluoride crystal at a high temperature, the calcium fluoride crystal is gradually cooled.
In another aspect, the present invention provides an optical lithography apparatus including an illumination optical system that emits exposure light of a wavelength less than about 200 nm towards a mask having a pattern, and a projection optical system that projects an image of the pattern on the mask onto a substrate, wherein at least one of the illumination optical system and the projection optical system includes a light-transmitting optical member formed of crystalline calcium fluoride with a potassium content less than about 0.5 ppm.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 4349249 (1982-09-01), Brisner
patent: 4358543 (1982-11-01), Nozawa
patent: 4757354 (1988-07-01), Sato et al.
patent: 5045507 (1991-09-01), Tran
patent: 5696624 (1997-12-01), Komine et al.
patent: 0908716 A1 (1999-04-01), None
patent: 10001310 (1998-01-01), None
Epps Georgia
Lucas Michael A
Morgan & Lewis & Bockius, LLP
Nikon Corporation
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