Glass manufacturing – Processes – Reshaping or surface deformation of glass preform
Reexamination Certificate
1999-07-26
2001-04-03
Silverman, Stanley S. (Department: 1731)
Glass manufacturing
Processes
Reshaping or surface deformation of glass preform
C065S064000, C065S292000
Reexamination Certificate
active
06209354
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a unidirectionally highly homogeneous massive formed article of synthetic silica glass as a raw material for making a highly homogeneous formed article of synthetic silica glass for optical use and, in particular, to a unidirectionally highly homogeneous massive formed article of synthetic silica glass useful as a raw material for preparing highly homogeneous synthetic silica glass formed article for optical use, which is free of any cord and which can easily be homogenized in the whole directions and from which cords can easily be removed.
The present invention also relates to a highly homogeneous synthetic silica glass formed article for optical use, which is free of any cord, in particular, a method for preparing a synthetic silica glass formed article for optical parts suitably used in optical systems which are used in exposure devices for photolithography such as deep UV lithography wherein ultraviolet rays emitted from, for instance, an excimer laser, which is quite highly homogeneous and free of any cord in the three directions perpendicular to one another and which has high transparency to these ultraviolet rays and is stable to irradiated ultraviolet light rays.
The present invention likewise relates to a method for preparing a synthetic silica glass formed article which is suitable for use in making homogeneous optical parts of synthetic silica glass used as optical systems for ultraviolet rays such as lenses and prisms constituting optical systems for ultraviolet rays, which is free of any cord and highly homogeneous and which has optically high transmittance to ultraviolet rays.
BACKGROUND OF THE PRIOR ART
In the production of semiconductor devices, the lithography technique for forming integrated circuit patterns on semiconductor wafers has strikingly been advanced. In particular, the wavelength of the light rays used in the lithography has increasingly been reduced and changed, for instance, from x-rays emitted from mercury lamps (365 nm) to light rays emitted from excimer lasers such as KrF excimer laser (248 nm) as the degree of integration of LSI's increases. Such reduction in the wavelength of light rays used in the lithography requires the use of silica glass optical parts having high transmittance to ultraviolet rays as optical parts incorporated into the optical system used in the lithography instead of the conventional multicomponent glass optical parts, in particular, the use of chemically purified synthetic silica glass optical parts having highly improved transmittance to ultraviolet rays. Moreover, such optical parts incorporated into these optical systems of high precision should satisfy more strict requirements such as quite excellent transmittance to ultraviolet rays and very high optical homogeneity.
In general, the glass sometimes shows abrupt change in its refractive index appearing in the streak-like or lamellar configuration. Such a heterogeneous structure is generally called “cord” and is distinguished from the homogeneity in the refractive index. In addition, the glass has heterogeneous structures which are accompanied by a change in the refractive index of the glass and such heterogeneous structures are caused due to the heterogeneity of compositions and the heterogeneity in thermal and other conditions during the production thereof. In the optical glass, such a heterogeneous structure including such a cord is a defect which should be removed since the structure would distort the optical path. Furthermore, the optical glass must also satisfy the strict requirements for the homogeneity in the refractive index to an extent depending on the precision required for specific optical systems in order to obtain a highly precise transmitting wave surface.
In the production of optical glass for general use, the homogenization of the refractive index is carried out by compulsorily stirring a melt of molten glass and then gradually cooling the melt under quite strict temperature control in order to satisfy these requirements.
However, it is difficult to heat the silica glass in a crucible up to a high temperature at which the silica glass is in a molten state having a low viscosity because of the nature peculiar thereto. On the contrary, the silica glass has a sufficiently high viscosity even at a high temperature and is insufficient in the flyability. For this reason, it is in fact difficult to obtain silica glass having satisfactory optical homogeneity through a mixing operation such as stirring.
As means for homogenizing such silica glass, for instance, U.S. Pat. No. 2,904,713 discloses a method for homogenizing the whole composition of silica glass which comprises locally heating the silica glass while supporting both ends thereof by a lathe for processing silica glass to partially form a molten zone within the silica glass, twisting, expanding and contracting the molten zone through application of an external force thereto so that the ends approach or are separated from one another while rotating the ends in different manners to move the molten zone over the entire length of the silica glass.
U.S. Pat. No. 3,485,613 discloses a method for performing homogenization of silica glass, called lateral zone melting method, which comprises setting the viscosity of the molten portion of the silica glass, i.e., the molten zone at a level of not more than 1013 P and moving the molten zone over the entire length of the silica glass while twisting the molten zone of the silica glass by rotating both ends of the silica glass in the same direction and at different numbers of revolutions or in different directions.
These techniques for homogenizing silica glass do not intend to eliminate the cord present in silica glass, but U.S. Pat. No. 5,086,352 and Japanese Un-examined Patent Publication (hereunder referred to as “J.P. KOKAI”) No. Hei 3-88742 disclose that the cord of silica glass can be removed through these homogenization techniques for silica glass.
Moreover, DEOS No. 42 04 406 A1 discloses a method for eliminating cords of natural and synthetic silica glass in the three directions perpendicular to one another, in which the homogenization of the silica glass while establishing a second rotation axis perpendicular to a first rotation axis for homogenization is carried out by molding a silica glass formed article subjected to a first homogenization in, for instance, an inverted T-shaped mold of graphite having a square cross section to form a rod-like synthetic silica glass having a square cross section and a longitudinal axis in the direction perpendicular to the rotation axis, for the first homogenization treatment, of the rod-like silica glass formed article subjected to the first homogenization, carrying out a second homogenization while using the longitudinal axis as a second rotation axis to completely eliminate cords in the three directions perpendicular to one another in quite high efficiency.
In addition, the method for homogenizing a silica glass material along the two axes perpendicular to one another and for removing cords as disclosed in the foregoing DEOS No. 42 04 406 A1 permits, for the first time, the economical production of a highly homogeneous silica glass formed article free of cord, which may be used in, for instance, the photolithography.
Incidentally, the synthetic silica glass has been used as a silica glass raw material for optical use because of high purity, in particular, a low content of metallic impurities and excellent light transmittance to light rays falling within the ultraviolet region and is prepared according to the direct method or the soot method. For instance, the vapor phase axial deposition method (VAD method) basically comprises the steps of subjecting a volatile silicon compound to the flame hydrolysis, depositing, as a layer, silica fine particles formed during the flame hydrolysis on a substrate which is rotated and then transparentizing the deposit of silica particles. A lamellar heterogeneous structure is thus formed during growing a synthetic silica
Englisch Wolfgang
Fujinoki Akira
Kataoka Masaatsu
Sugama Akihiko
Ruller Jacqueline A.
Scully Scott Murphy & Presser
Shin-Etsu Quartz Products Co. Ltd.
Silverman Stanley S.
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