Coherent light generators – Particular active media – Gas
Reexamination Certificate
2000-03-03
2002-11-12
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C359S350000, C359S355000, C065S030100
Reexamination Certificate
active
06480518
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
1. Field of Use
The present invention relates to a synthetic quartz glass member for use in the optical system of a lithographic apparatus employing an excimer laser as a light source and, in further detail, it relates to a synthetic quartz glass member for use in the illumination system or projection system, such as a lens, a prism and a beam splitter of an ArF excimer laser lithographic apparatus.
2. Prior Art
In recent years, along with the increasing degree of integration in LSIs, patterns for integrated circuits drawn on wafers have become finer and mass production has been started for a super LSI in which super fine patterns of quarter &mgr;m or less are drawn. For obtaining such a super fine pattern, the wavelength of optical sources for exposing and thereby drawing them have also to be made shorter, and steppers using an excimer laser as a light source have been developed and steppers using a KrF excimer laser (wavelength: 248 nm) as the light source have been put to practical use. Further, steppers using an ArF excimer laser (wavelength: 193 nm) as a light source have attracted attention as steppers for the next generation of apparatus. Quartz glass, fluorite and the like can be mentioned as glass materials showing sufficient transmittance even in a short wavelength region such as of a KrF excimer laser or the ArF excimer laser. Among them, synthetic quartz glass obtained by flame hydrolysis of silicon compounds and the like at high purity into transparent glass is suitable for optical materials to use in lithography using excimer lasers as the light source. Particularly, in the case of using the synthetic quartz glass as the optical material for the ArF excimer laser lithographic apparatus, it is said that a permissible limit for the internal transmittance at 193.4 nm of wavelength is 99.8% as described in Laid-Open Japanese Patent No. 53432/1998. Therefore, in this publication, the Na concentration is defined as 20 wt.-ppb or less. The synthetic quartz glass at the Na concentration of 20 wt.-ppb or less has been produced by a process under severe control for the synthesis conditions of quartz glass in order to prevent Na contamination by a secondary heat treatment such as a homogenizing treatment. In the ingot obtained under strict setting for the production conditions of the synthetic quartz glass, while the homogeneity in the longitudinal direction (optical axis) can be made higher, it is difficult to make the homogeneity higher in the direction perpendicular thereto (hereinafter referred to as a lateral direction). It is further difficult to remove striae in the lateral direction. They are formed when the conditions change even slightly during growing, so that the synthetic quartz glass ingots have to be produced under extremely strict production conditions. However, fringes in the lateral direction cannot be avoided. This is a phenomenon appearing regularly in the growing direction of the synthetic quartz glass ingots which is usually referred to as a layer or a layered structure and recognized as periodical and fine unevenness of the interference fringe observed upon transmission of a light in a direction perpendicular to the growing direction. The layered structure is a fine periodical structural fluctuation formed upon growing soot or glass by rotating a target for depositing the soot which, although it may differ as to density or thinness, cannot be removed completely by merely setting the production conditions. Then, the layered structure gives significant trouble in the case of using the synthetic quartz glass as a projection system, for example, a beam splitter (as described in “APPLIED OPTICS”, Vol. 31, No. 31, pp. 6658-6661).
As mentioned above, the synthetic quartz glass described in Laid-Open Japanese Patent No. 53432/1998 cannot be used quite satisfactorily as a synthetic quartz glass member for use in ArF excimer laser lithography, particularly, as a synthetic quartz glass member for a projection system required for large size in view of its lack of homogeneity. Accordingly, the secondary heat treatment referred to in this publication is essential also in the synthetic quartz glass described in the publication. However, the homogenizing processing step or molding step described above is conducted at an extremely high processing temperature exceeding 1800° C. and the strain removing step also requires a long time for the processing although the processing temperature is relatively as low as about 1100° C. Therefore, thermal diffusion of Na takes place from furnace materials, crucibles and atmospheres to contaminate members during the secondary heat treatment to lower the transmittance. Due to the thermal diffusion of Na, the quartz glass causes a Na concentration gradient from the outer surface to the inside in which the Na concentration is higher near the outer surface and lower in the inside. This Na concentration gradient results in a transmittance distribution of the quartz glass and, when a lens is prepared from the synthetic quartz glass member described above for instance, the transmittance lowers at the outer circumference relative to the central part. As a result, not only uniform intensity of transmission light cannot be obtained but also distribution of the refractive index is caused by light absorption making it difficult to use it as the optical material of an exposure apparatus for use in ArF excimer laser lithography.
For using the synthetic quartz glass member suitably as a transmission material of an exposure apparatus for use in ArF excimer laser lithography, it is important to maintain the resistance to laser irradiation, namely, high transmittance and high homogeneity stably for a long period of time. Generally, when an excimer laser is irradiated to quartz glass, paramagnetic defects referred to as E′-center or NBOH-center (NBOHC) are formed and transmittance in a ultraviolet region is lowered since the defects have an absorption band in the ultraviolet region. Further, shrinking of quartz glass accompanied by laser irradiation, referred to as laser compaction, is also observed. The refractive index is increased owing to the shrinkage, thereby worsening the image focusing characteristics of the lens member in exposure apparatus.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a quartz glass member for use in an ArF excimer laser having high homogeneity, as well as high transmittance to ArF excimer laser irradiation and also excellent resistance to the laser irradiation.
The invention is based on the finding that a synthetic quartz glass member having high transmittance and high homogeneity, as well as having excellent in the resistance to laser irradiation can be obtained by thermally or mechanically removing layered structures, striae in three directions and internal strains of a quartz glass ingot produced by using a silicon compound of high purity and irradiating with continuous ultraviolet rays at a wavelength of 260 nm or less to a synthetic quartz glass member having an Na concentration of 25 to 60 wt.-ppb.
The synthetic quartz glass member according to the invention not only has excellent homogeneity, but also exhibits high internal transmittance for ArF excimer laser radiation and high resistance against laser radiation and therefore, it is suitable as an optical material for ArF excimer laser steppers. In particular, since even a large member exceeding 200 mm in size has superior homogeneity, as well as exhibits high transmittance for ArF excimer laser radiation, it is useful as a material for lenses, beam splitters and the like for steppers.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention for attaining the foregoing object concerns a synthetic quartz glass member for use in ArF excimer laser lithography comprising a high purity synthetic quartz glass and characterized in that layered structures, striae in three directions and internal strains are thermally and mechanically removed, so that the distribution of refractive index (&Dgr;n) in a plane or
Fujinoki Akira
Nishimura Hiroyuki
Oshima Takayuki
Yaginuma Yasuyuki
Heraeus Quarzglas GmbH & Co. KG
Ip Paul
Milde & Hoffberg LLP
Rodriguez Armando
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