Glass manufacturing – Processes – With shaping of particulate material and subsequent fusing...
Reexamination Certificate
2000-09-28
2002-12-31
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes
With shaping of particulate material and subsequent fusing...
C065S021100, C065S413000, C501S054000
Reexamination Certificate
active
06499315
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical synthetic quartz glass having excellent light transmissivity, optical homogeneity, and optical stability with respect to the ultraviolet ray irradiation, particularly to the excimer laser irradiation, which is an ultraviolet ray laser, a production method thereof, and an optical member for an excimer laser using the synthetic optical glass.
2. Description of the Related Art
With a higher integration degree of the LSI, processing accuracy of the sub-micron order is required in the photolithography for drawing an integrated circuit pattern on a silicon wafer. Therefore, an exposing device for drawing a circuit pattern is also improved. For example, a finer line width in drawing is achieved by a shorter wavelength of the power source of the exposing device or adopting the resolution enhancement techniques using interference of light. That is, the light source has a short wavelength between the g ray (wavelength 436 nm) to the i ray (wavelength 365 nm) of a mercury lamp, and the exposing device of the diffraction system adopts resolution enhancement techniquies utilizing the interference of light, such as a off-axis illumination method or a phase shift mask method. The demand for finer drawing is accelerated these days so that an excimer laser having a shorter wavelength is used in place of the mercury lamp. Examples of the excimer laser include a KrF laser (wavelength 248 nm) and an ArF laser (wavelength 193 nm). Owing to the use of the excimer laser as the light source, a higher quality is required for optical members including a lens used in the exposing device. For example, if the light transmissivity is poor, the focal length of the lens or the other characteristics suffer adverse effects by the heat generation of the lens due to the light absorption, or if the optical homogeneity is poor, problems such as deterioration of the image formation characteristics are involved.
Conventionally, an optical synthetic quartz glass is used as the material of the optical members for an exposing device for drawing a circuit pattern by a photolithography. The synthetic quartz glass can be produced in the direct method where a vapor of a highly pure silicon compound such as silicon tetrachloride (SiCl
4
) is introduced directly to the oxygen-hydrogen flame and glass fine particles obtained by the flame hydrolysis are deposited on a heat resistant substrate to be a molten glass for obtaining a transparent glass or in the soot method where the glass fine particles are deposited on a heat resistant substrate as a porous material and heated in an electric furnace to be a molten glass. In either case, by the use of a highly pure material, the transmissivity in the ultraviolet ray region is improved as well as the optical homogeneity can be maintained. In the case of an optical material made from the above-mentioned synthetic quartz glass, damage caused by the light did not have to be considered if the i ray of the mercury lamp is used as the light source of the exposing device, however, it should be dealt with if the excimer laser is used instead. The problem is derived from the pulse energy per one shot of the excimer laser, which is a pulse laser, is extremely large compared with a continuous light source such as the i ray (mercury lamp, CW laser, etc.). Damage caused by the excimer laser on a synthetic quartz glass may differ remarkably depending on the production method or the production conditions thereof. The damage refers to the deterioration of the laser transmissivity caused by the absorption in the ultraviolet region by the ultraviolet ray laser irradiation and the rise of the refractive index caused by the permanent compaction (contraction of the glass). The insusceptibility to the damage is called the laser resistance. As mentioned above, the absorption in the ultraviolet region occurs subject to the damage. This is considered to be because of the paramagnetic defect derived from the intrinsic defect in the quartz glass by the light reaction. The existence of the paramagnetic defect derived from the laser has been observed and identified by the ESR spectrum. As examples thereof, the structures such as El center (Si.) and NBOHC (Si—O.) are known. Such a paramagnetic defect, in general, has an; absorption band. For example, E′ center has it at 215 nm. Furthermore, although the species of the defect has not been identified yet, absorption is observed also at 260 nm subject to the excimer laser irradiation. These absorption bands can be comparatively broad and strong. For example, in the case of the use as the light transmissive material for a KrF laser (wavelength 248 nm) or an ArF laser (wavelength 193 nm), deterioration in the laser transmissivity caused thereby poses a serious problem. In addition to the absorption in the ultraviolet region, a permanent compaction occurs. The compaction is derived from the transition to a more stable structure in a part of the quartz glass due to the atom recombination subject to a strong laser energy irradiation. Accordingly, the density in the irradiated portion heightens to raise the refractive index of the quartz glass material, resulting in a major influence on the image formation characteristics. Furthermore, due to the local density rise in the laser-irradiated portion, the stress is generated at the interface between the non-irradiated portion and the irradiated portion with the distortion so as to raise the birefringence index and affect the optical characteristics.
Various methods have been proposed in order to solve the above-mentioned problems. Example thereof include a method of having particular production conditions of a quartz glass, and a method of applying a heat treatment to a produced synthetic quartz glass in a particular atmosphere. As an example of the former method, Japanese Unexamined Patent Publication Nos. 6-199531 and 6-287022 disclose a production method with a hydrogen-excessive condition, paying attention to the gas balance in the synthetic quartz glass production. By having the hydrogen molecules dissolved as mentioned above, the paramagnetic defect caused by the laser irradiation can be compensated by the hydrogen molecules so that the generation of the paramagnetic defect can be restrained and thus the laser transmissivity can be ensured without generating an absorption band in the ultraviolet ray region. In the method of dissolving hydrogen molecules, the laser resistance improves with a larger amount of dissolved hydrogen molecules in producing a synthetic quartz glass. However, since the amount of the hydrogen molecules to be dissolved remarkably varies according to not only the gas amount of a combustion gas and a combustion-supporting gas but the surface temperature or the surface area of the substrate to be deposited during the growth in the direct method, it is difficult to control the factor. Therefore, hydrogen molecules dissolved more than intended may result in a strongly reducing property to generate an oxygen lacking defect or a reduced species of silicon atoms (═Si:), which provides a precursor of the paramagnetic defect and deteriorates the laser resistance. Although the above-mentioned method of dissolving hydrogen molecules is advantageous in terms of the improvement of the laser resistance, with the laser irradiation for a very long time to the synthetic quartz glass having hydrogen molecules dissolved, the ultraviolet ray absorption occurs due to the dissolved hydrogen molecule consumption. Since the absorption is derived from the paramagnetic defect, a production method of a quartz glass for minimizing the paramagnetic defect structure is discussed. Examples of structures to cause the paramagnetic defect include (i) one derived from a glass structure, such as an unstable SiO
2
network portion, (ii) an unordered structure generated from the deviation in the stoichiometric ratio, such as Si—Si and Si—O—O—Si, (iii) a structure excluding silica, such as SiCl and SiOH, and (iv) an unordered st
Fujinoki Akira
Nishimura Hiroyuki
Otsuka Hisatoshi
Finnegan Henderson Farabow Garrett & Dunner LLP
Hoffmann John
Shin-Etsu Quartz Products Co., Ltd
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