Glass manufacturing – Processes – Forming product or preform from molten glass
Reexamination Certificate
2000-09-27
2003-01-14
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes
Forming product or preform from molten glass
C065S024000, C065S374130, C065S374140, C065S374150
Reexamination Certificate
active
06505484
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of forming silica glass into a desired shape by heating and pressing a synthetic silica bulk, and a forming apparatus thereof.
More particularly, the invention concerns a method of producing silica glass formed products with excellent optical characteristics, for example, suitable for optical members such as reticle (photomask) substrates, imaging optics, etc. in a high yield, by pressing synthetic silica bulks made of raw materials of silicon compounds such as silicon tetrachloride, silanes, organic silicons, and the like or synthetic silica bulks doped with a component to vary the index of refraction, such as Ge, Ti, B, F, Al, or the like, and a forming apparatus for carrying out the method.
2. Related Background Art
Reduction projection exposure systems (or photolithography systems) are mainly used for transfer of integrated circuit patterns such as ICs, LSIs, and so on. Requirements for projection optical systems used in the systems of this type are a wider exposure area with increase in integration of integrated circuits and higher resolving power throughout the entire exposure area. For enhancing the resolving power of the projection optical systems, it is conceivable to decrease the exposure wavelength to shorter wavelengths or to increase the numerical aperture (NA) of the projection optical systems.
The exposure wavelengths have been decreasing toward shorter wavelengths, e.g., from the g-line (436 nm) to the i-line (365 nm) and further to the KrF (248 nm) excimer laser and the ArF (193 nm) excimer laser. For further increase in integration of integrated circuits, research is now under way on methods for using a light source of the F
2
(157 nm) excimer laser, X-rays, or an electron beam. Among these, the reduction projection exposure apparatus using the F
2
excimer laser, which can be fabricated by taking advantage of the design concepts heretofore, is hastily moving into the limelight.
In general, in the case of optical glasses used as optical members of illumination optical systems or projection optical systems in the reduction projection exposure apparatus using the light sources of longer wavelengths than the i-line, the light transmittance decreases quickly in the wavelength region under the i-line and most optical glasses become untransmissive, particularly, to radiation in the wavelength region of not more than 250 nm. For that reason, silica glass, or single crystals of fluorides such as calcium fluoride, barium fluoride, and so on are usable as materials for lenses constituting the optical systems in the reduction projection exposure apparatus using the light source of excimer laser. These materials are essential materials to correction for chromatic aberration in the imaging optical system of excimer laser.
A reticle can be named as another significant element for printing circuits onto a wafer in the reduction projection exposure apparatus. The materials used for the reticle need to have transmission characteristics including endurance to the excimer lasers, of course, and another significant issue is thermal expansion due to heating of the substrate; therefore, the silica glass with good transmission characteristics and with a small coefficient of thermal expansion is used. Further important properties of the materials used for the reticle are chemical resistance and anti-etching characteristics thereof in view of the production processes.
A silica glass production method called a direct process is a method of mixing and burning a combustion gas (usually, oxygen gas) and a combustible gas (hydrogen gas or natural gas) in a silica glass burner, ejecting a silicon tetrachloride gas of high purity as a feed gas diluted with a carrier gas (oxygen gas, hydrogen gas, an inert gas, or the like), from the burner, making the feed gas react with water made by combustion of oxygen gas and hydrogen gas in the surroundings (to effect hydrolysis) to create silica glass particles, depositing the silica glass particles onto a target of a silica glass plate or the like undergoing rotation, swinging, and pulling-down motion, which is located below the burner, simultaneously melting the particles by heat of combustion of oxygen gas and hydrogen gas, and further vitrifying the deposits to obtain a silica glass ingot. This method permits the silica glass ingot to be obtained in relatively large diameter.
Another production method of silica glass ingot except for the direct process is a VAD (vapor-phase axial deposition) process making use of the technology used for production of optical fibers. This is a method of consolidating a porous silica glass preform by an atmospheric heat treatment.
The silica glass ingot fabricated by the production methods as described above is further cut into a glass block (synthetic silica bulk) having predetermined size and shape. Then this synthetic silica bulk is formed into a silica glass formed product of desired shape and size at high temperatures in a forming vessel of graphite and the formed product is used as an optical member such as the reticle or the like.
As one of such silica glass forming methods, Japanese Patent Application Laid-Open No. S56-129621 discloses the forming method of forming the bulk at the temperature of not less than 1700° C. and in a helium gas atmosphere under the absolute pressure of 0.1 to 760 Torr in the graphite forming vessel and thereafter quenching the formed product down to 1100 to 1300° C. Japanese Patent Application Laid-Open No. S57-67031 discloses the forming method in which the graphite forming vessel is of two or more-part split vertical structure. Further, Japanese Patent Publication of examined Application (KoKoku Publication) No. H04-54626 discloses the method of forming at 1600 to 1700° C. by use of the graphite forming vessel having the structure for relaxing stress caused by the difference between thermal expansion coefficients of the silica glass and the forming vessel.
SUMMARY OF THE INVENTION
The conventional forming methods described above, however, had the problem that bubbles were created in the synthetic silica bulk (silica glass) in the middle of pressing at high temperatures and a lot of bubbles remained in the silica glass formed product after the forming. This silica glass formed product with many bubbles remaining inside cannot be used as an optical member. Particularly, in the case of the synthetic silica bulks doped with the component to vary the refractive index, such as Ge, Ti, B, F, Al, or the like, because the viscosity thereof is high, a lot of bubbles tend to remain inside the silica glass formed product after the forming.
In general, in cases wherein the synthetic silica bulk (silica glass) is formed by pressing in the forming vessel, there occurs great difference in shrinkage between the synthetic silica bulk and the forming vessel during cooling down to the room temperature after the forming at high temperatures, because the constitutive materials of the silica glass bulk and the forming vessel have their respective coefficients of thermal expansion largely different from each other. For that reason, in the conventional methods, unwanted stress was exerted on the synthetic silica bulk and the forming vessel, so as to result in cracking of the silica glass formed product formed by pressing, or even breakage of the forming vessel in certain cases.
There also arises the problem that the constitutive materials of the synthetic silica bulk and the forming vessel react with each other at high temperatures. For example, where the graphite forming vessel is used as a forming vessel, the synthetic silica bulk reacts with graphite at high temperatures to create silicon carbide. Therefore, the surface of the silica glass formed product after the forming became rough, depending upon the forming temperatures, and cracking occurred from the rough surface in some cases.
An object of the present invention is, therefore, to provide a method of producing silica glass formed products with excel
Fujiwara Seishi
Jinbo Hiroki
Komine Norio
Colaianni Michael
Nikon Corporation
Oliff & Berridg,e PLC
LandOfFree
Forming method of silica glass and forming apparatus thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Forming method of silica glass and forming apparatus thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Forming method of silica glass and forming apparatus thereof will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3068576