X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
2002-11-27
2004-08-24
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
C378S070000
Reexamination Certificate
active
06782074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical member composed of a fluoride crystal as a raw material for producing an optical element for constructing an optical system of an optical instrument such as a camera, a microscope, a telescope as well as of a projection exposure apparatus for photolithography such as a stepper, and a method for evaluating the same. The present invention also relates to an optical system and a projection exposure apparatus incorporated with an optical element produced of the optical member.
2. Description of the Related Art
In recent years, the lithography technique for drawing an integrated circuit pattern on a wafer is rapidly advanced. The demand to increase the degree of integration of the integrated circuit is growing year after year. In order to realize the high degree of integration, it is necessary to enhance the resolving power of the projection optical system of the projection exposure apparatus. The resolving power of the projection optical system is determined by the wavelength of a light beam to be used and NA (numerical aperture) of the projection optical system. That is, the resolving power can be increased by further shortening the wavelength of the light beam to be used (realization of short wavelength) and/or further increasing NA of the projection optical system (realization of large diameter).
At first, a description will be made about the realization of the short wavelength of the light beam. The wavelength of the light source to be used for the projection exposure apparatus has been already changed to the g-ray (wavelength: 436 nm) and the i-ray (wavelength: 365 nm). It is investigated to use light beams having shorter wavelengths in the future, including, for example, the KrF excimer laser light beam (wavelength: 248 nm) and the ArF excimer laser light beam (wavelength: 193 nm). However, if commonly used multicomponent optical glass is used as a lens material for an image-forming optical system such as a projection optical system to be used for the light beam as described above, the transmittance is considerably lowered.
Therefore, silica glass or fluoride crystal, for example calcium fluoride crystal is generally used as an optical member for the optical system of the projection exposure apparatus which uses the excimer laser as the light source. In order to satisfy the image formation performance required for the optical member to be used for the optical system of the excimer laser projection exposure apparatus, it is desirable to use a single crystal in the case of a crystalline material.
As the performance of the projection exposure apparatus is highly enhanced, a calcium fluoride single crystal having a large diameter, i.e., a diameter of about &phgr;120 mm to &phgr;350 mm is recently required in order to increase NA. Such a calcium fluoride single crystal has a small refractive index and a small dispersion (dependency of the refractive index on the wavelength) as compared with the commonly used optical glass and the silica glass. Therefore, a merit is also obtained such that the chromatic aberration can be corrected by using the calcium fluoride single crystal together with the optical member composed of a material such as silica glass. It is also possible to obtain a single crystal having a large diameter exceeding &phgr;120 mm.
The calcium fluoride single crystal, which has the advantages as described above, has been hitherto used as lens materials for cameras, microscopes, and telescopes other than as the optical material for the projection exposure apparatus. Recently, single crystals of barium fluoride and strontium fluoride, which are fluoride single crystals other than the calcium fluoride single crystal, attract the attention as optical materials for the next generation, because they belong to the same cubic system and they have similar properties.
A variety of single crystal growth methods are known as the method for producing the fluoride single crystal, including, for example, the melt method such as the Tammann method and the Bridgman method (also referred to as the Stockbarger method or the pull-down method). A method for producing the calcium fluoride single crystal based on the Bridgman method will be described below by way of example.
FIG. 2
conceptually shows a growth apparatus for the calcium fluoride single crystal based on the Bridgman method.
In order to produce the calcium fluoride single crystal for the purpose of the use in the ultraviolet or vacuum ultraviolet region, a calcium fluoride raw material having high purity, which is produced by means of chemical synthesis, is generally used as the raw material. If any powder is used as the raw material to grow the crystal, the volume is greatly decreased when the raw material is melted. In order to avoid such an inconvenience, the crystal is generally grown by using a raw material obtained by semi-melting the powder once or obtained by crushing the product obtained by semi-melting the powder once.
At first, a crucible, which is filled with a semi-molten material or a crushed material thereof, is set in the growth apparatus. The interior of the growth apparatus is maintained in a vacuum atmosphere of 10
−3
to 10
−4
Pa. Subsequently, the temperature in the growth apparatus is raised to a temperature which is not less than the melting point of calcium fluoride (1370° C. to 1450° C.) to melt the raw material.
At the crystal growth (grain growth) stage, the crucible is moved downwardly at a speed of about 0.1 to 5 mm/h, and thus the crystal growth is gradually advanced from the lower portion of the crucible. The crystal growth comes to an end when the uppermost portion of the melt is crystallized. The grown crystal (ingot) is gradually cooled to a temperature in the vicinity of the room temperature so that the crystal (ingot) is not broken. After that, the interior of the growth apparatus is open to the atmospheric air, and the ingot is taken out.
A crucible made of graphite is generally used for the crystal growth. The crucible is pencil-shaped with its tip having a conical configuration. Therefore, the crystal growth is started from the tip having the conical configuration disposed at the bottom of the crucible. The crystallization is gradually advanced, and the ingot is finally obtained.
A seed crystal is sometimes introduced into the tip portion in order to control the crystal plane orientation of the ingot. However, in general, when a large fluoride crystal is produced by means of the Bridgman method, it is considered that the crystal growth orientation does not obey any law, and the crystal direction of the ingot is randomly determined every time when the crystal growth is performed. Especially, in the case of a large ingot having a diameter exceeding &phgr;120 mm, it is extremely difficult to control the crystal plane orientation.
A large residual stress exists in the ingot taken out from the crucible after the crystal growth. Therefore, a simple heat treatment is performed while retaining the ingot shape as it is. The ingot of the calcium fluoride single crystal obtained as described above is cut and processed into an appropriate size depending on an objective product.
When an optical element, in which the crystal plane orientation causes no problem, is produced, the ingot is cut horizontally to have a parallel plate-shaped configuration (cut into round slices) in order to cut out raw materials from the ingot more efficiently. A heat treatment is applied to the cut raw materials in order to obtain desired image formation performance (uniformity of refractive index and reduction of stress induced birefringence).
When an optical element, in which the crystal plane orientation should be considered, is produced, for example, when the optical axis is made perpendicular to the {111} crystal plane, then the {111} crystal plane of the fluoride single crystal ingot is measured. The raw material is cut out so that the {111} plane resides in the two parallel pla
Bruce David V.
Nikon Corporation
Oliff & Berridg,e PLC
Song Hoon
LandOfFree
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