Glass manufacturing – Processes – With measuring – sensing – inspecting – indicating – or testing
Reexamination Certificate
2002-02-13
2004-11-02
Chin, Peter (Department: 1731)
Glass manufacturing
Processes
With measuring, sensing, inspecting, indicating, or testing
C065S032100
Reexamination Certificate
active
06810687
ABSTRACT:
INDUSTRIAL FIELD OF APPLICATION
The present invention relates to a synthetic quartz glass member for optics having excellent optical transmittance, homogeneity, and stability with respect to the irradiation of a high intensity ultraviolet radiation, in particular, to those of KrF and ArF excimer lasers. More specifically, the synthetic quartz glass members according to the present invention relate to those favorably applicable to lenses of aligners and other optical components for use in lithography using excimer lasers for producing semiconductor chips, as well as for prisms used in narrowing bands of excimer lasers, other general use optical members utilized in KrF and ArF excimer laser radiations, lenses, beam splitters, prisms, etc.
PRIOR ART
Recently, practical LSIs are being produced by KrF excimer laser lithography, and developments targeted to the realization of ArF excimer laser lithography for the next generation exposure technology are in progress. Furthermore, improvements are also made on KrF lithography, such that the performance of patterning finer circuit patterns or the processing capability (throughput) per hour can be improved. More specifically, fine circuit patterns are now produced by employing improved exposure methods such as modified illumination method and phase shift masks, and this fundamentally requires an aligner equipped with a lens made from a material of higher performance. That is, a lens material having higher homogeneity and low birefringence is strongly demanded for accurate focusing. Furthermore, in order to improve the throughput, the quantity of exposure light for use in patterning is being increased by elevating the repetition frequency or the energy of the excimer laser. In such a case, the lens material tends to be optically damaged due to the increased quantity of light transmitted through the lens material of the aligner. At any rate, a quartz glass for use as the lens material with higher quality is required every year with the increase in performance of the aligner. The means now being taken based on prior art for improving homogeneity or for increasing laser resistance is found insufficient.
The technique for obtaining a quartz glass of high homogeneity has been studied in various fields, and many patents applications have been made thereon. In general, the homogeneity of an optical quartz glass signifies the uniformity in refractive index. Generally, a retardation in phase is measured by an interferometer using a He—Ne laser emitting a radiation 632.8 nm in wavelength to calculate the distribution in refractive indices. A highly uniform quartz glass lens material is necessary because an extremely high imaging performance is required on a lens for use in lithography. The refractive index changes depending on the impurities incorporated in the quartz glass, e.g., chlorine or hydroxyl groups, etc., or on the fictive temperature. Thus, if the quartz glass contains impurities distributed therein, the distribution of refractive indices changes depending thereon. For instance, a portion containing SiOH groups in a larger quantity yields a relatively low refractive index. The fictive temperature changes depending on the cooling rate in thermally treating quartz glass, and a distribution in refractive index is also formed due to the generation of a distribution in fictive temperature within the quartz glass. In JP-A-Hei3-88743 (the term “JP-A” as referred herein signifies “an unexamined published Japanese patent application”) is disclosed a method for producing a glass of high homogeneity by positively combining the factors above inducing fluctuation in refractive index. Furthermore, the same published patent application describes a method of imparting high laser resistance to the quartz glass by thermally treating the glass in a hydrogen-containing atmosphere to thereby enable the doping of hydrogen molecules. That is, by doping glass with hydrogen, the technique enables obtaining a glass of high homogeneity while maintaining high resistance against an excimer laser radiation. Accordingly, since a high resistance against laser radiation can be imparted by applying hydrogen doping to a highly homogeneous quartz glass obtained by controlling the distribution of impurities such as SiOH and chlorine, it has been believed that such a material could be favorably used as a lens material for use in excimer laser lithography. In fact, however, there has been found practical problems such that (1) an even higher resistance against laser radiation is required recently in view of increased repetition times and higher energy used in laser, and (2) homogeneous doping of hydrogen molecules is difficult in view of the recent demand on larger materials having even higher NA. Thus, a material sufficiently satisfying the requirements cannot be obtained by a method described in the published patent application above. In particular, taking the diffusion coefficient of hydrogen molecules into consideration, it is extremely difficult to realize a large material uniformly doped with hydrogen molecules by performing heat treatment in an atmosphere containing hydrogen. In practice, there are problems as such that, by calculation, such a process requires an extremely long time of treatment or a hydrogen addition treatment at a considerably high temperature. Furthermore, from the studies to present, it has been found that hydrogen molecules also influence the distribution in refractive indices, and that uniformly doped hydrogen molecules make an important factor.
Homogenous hydrogen doping is important in obtaining a highly homogeneous quartz glass yet having high resistance against laser radiation, however, particularly in case of a large material, a long time duration is necessary to sufficiently diffuse hydrogen deep into the inside of the material. This results in a serious problem of impairing the productivity. Applying hydrogen doping at higher temperatures at which the diffusion coefficient of hydrogen becomes higher can shorten the time necessary for diffusing hydrogen. However, a quartz glass exposed to hydrogen at high temperatures tend to newly generate defects, and this is known to impair the resistance against laser radiation of the quartz glass. JP-A-Hei6-166522 describes the resistance against the irradiation of an ArF excimer laser of a hydrogen-doped material obtained by thermally treating synthetic quartz glass member trial produced materials at various temperatures under a gaseous hydrogen atmosphere. It shows that the material subjected to a heat treatment at high temperatures exceeding 600° C. yields an extremely impaired resistance against laser radiation. This is attributed to the fact that reductive defects generate inside the quartz glass due to the hydrogen molecules introduced at higher temperatures, which induce absorption in the ultraviolet region in case an excimer laser is irradiated thereto. The absorption thus induced is ascribed to paramagnetic defects generally denoted as E′ centers, and yields an absorption band having its center wavelength in the range of from 210 to 215 nm. Thus, in order to suppress the generation of E′ centers, i.e., to suppress the generation of induced absorption, the published patent application above describes the application of a heat treatment at a temperature not higher than 600° C. As described above, however, the diffusion coefficient for hydrogen decreases at lower temperatures, and this increases the process time in case of processing a large material as to impair the productivity. Accordingly, in order to introduce more hydrogen molecules into the quartz glass, the published patent above describes a treatment carried out under a high pressure of 1 atm or higher, preferably, 50 atm or higher. However, in general, the distribution of hydrogen molecule concentration for a quartz glass thus obtained yields, in accordance with the principle of a diffusion phenomenon, higher concentration values at the peripheral portions as compared with those at the central portion. In case of a treatmen
Fujinoki Akira
Nishimura Hiroyuki
Yokota Toru
Chin Peter
Heraeus Quarzglas GmbH & Co. KG.
Lopez Carlos
Tiajoloff & Kelly
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