Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2002-12-11
2004-07-13
Thomas, Alexander S. (Department: 1772)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S364000
Reexamination Certificate
active
06761951
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to synthetic quartz glass blanks having a good transmittance and low deterioration during laser irradiation from which can be obtained optical elements such as lenses, prisms, mirrors and windows for use with excimer lasers, and particularly ArF excimer lasers.
2. Prior Art
Higher levels of integration in VLSI circuits have led to exposure patterns of increasingly small linewidth. This has created a need for exposure light sources of shorter wavelength in the lithography systems used to form circuit patterns on semiconductor wafers. The i-line (wavelength, 365 nm), once the light source of choice in lithography steppers, has been largely supplanted by the KrF excimer laser (248 nm), and today ArF excimer lasers (193 nm) are starting to see industrial use.
This trend toward shorter wavelength light sources has also created a need for higher precision in the optical components (e.g., lenses, windows, prisms) used in exposure tools. Some of the many important concerns that exist regarding such components, particularly when used with ArF excimer lasers, include refractive index homogeneity, improving the transmittance and reducing the scattering of laser light, and stability to excimer laser irradiation.
Of these concerns, the refractive index homogeneity &Dgr;n is the most critical and most difficult to achieve. The hydroxyl group concentration and its distribution have a large influence on the refractive index distribution in quartz glass. That is, a hydroxyl group concentration of 10 ppm reportedly narrows the refractive index distribution in quartz glass by 1×10
−6
. It can readily be seen from this that a very high-homogeneity synthetic quartz glass body having a hydroxyl group concentration distribution of only 10 ppm would be needed to obtain a synthetic quartz glass blank in which &Dgr;n=1×10
−6
.
Two methods are commonly used for making synthetic quartz glass: a direct method in which a silica-forming starting material is flame hydrolyzed, forming fine particles of silica which are then melted and deposited to effect growth; and a soot method in which a silica-forming starting material is flame hydrolyzed, forming fine particles of silica which are deposited to effect growth, then later vitrified to form a clear glass. However, obtaining a synthetic quartz glass body of such high homogeneity directly by either of these methods is technically very difficult. To obtain a synthetic quartz glass body of higher homogeneity, it is thus necessary to subject the synthetic glass ingot obtained by either method to homogenizing treatment.
The most efficient and effective way to homogenize quartz glass is to carry out the zone melting process disclosed in JP-A 7-267662 in the ingot growth direction and in a direction perpendicular thereto. This approach has a number of advantages. For example, the molten portion of the ingot is mechanically agitated, enabling efficient homogenization to be carried out and thus making it possible to narrow, for example, the distribution in the hydroxyl group concentration. In addition, during homogenization, the quartz glass ingot is treated without being brought into contact with anything other than the burner flame, minimizing the diffusion of external impurities to the ingot and thus holding down the decline in UV light transmittance.
Generally, when homogenizing treatment by a zone melting process is used to improve the uniformity of the hydroxyl group concentration, a wider variation in hydroxyl group concentration prior to such treatment results in less efficient homogenization. The efficiency of homogenization declines also with increasing hydroxyl group concentration. At higher hydroxyl group concentrations in particular, the variation in concentration is generally wider, detracting even further from the efficiency of homogenization. For this reason, a hydroxyl group concentration of 1,000 ppm or less is preferred in synthetic quartz glass ingots subjected to homogenization.
Other properties which, like the refractive index homogeneity &Dgr;n, are of critical importance in synthetic quartz glass blanks for optical elements used in ArF excimer laser exposure systems, are the transmittance of the glass to UV light and its stability to laser irradiation.
The most important transmittance to UV light is the transmittance to the 193 nm wavelength light used in an ArF excimer laser. The transmittance of quartz glass to light at this wavelength decreases as the content of impurities rises. Typical impurities include alkali metals such as sodium, and other metallic elements such as copper and iron. By using a silane or silicone starting material of very high purity to produce the synthetic quart glass, the concentration of such metallic impurities present within the quartz glass can be brought down to below the level of detection by a highly sensitive detector (<1 ppb). However, because sodium and copper have relatively large coefficients of diffusion to quartz glass, the diffusion and admixture of such external impurities often occurs during homogenization and heat treatment. Special care must be taken to avoid such contamination during these treatment operations.
Stability of the quartz glass to excimer laser irradiation is a very important factor, particularly as an ArF excimer laser reportedly causes five times more damage than a KrF excimer laser.
When quartz glass is irradiated with ArF excimer laser light, one effect that arises is the cleavage of Si—O—Si bonds by the very intense energy of the light, forming the paramagnetic defects commonly known as E′ centers which absorb 215 nm light. Another effect, commonly referred to as “laser compaction,” is a rearrangement of the network structure of quartz glass that increases the density of the glass.
The former effect lowers the transmittance of the quartz glass to 193 nm light, and the latter effect raises the refractive index and increases the birefringence. All of these changes in optical characteristics are undesirable for an exposure system.
It is known that reducing the number of intrinsic defects in quartz glass and setting the hydrogen concentration in the glass to at least a certain level are both highly effective for improving the stability of the quartz glass to laser irradiation.
Intrinsic defects present in quartz glass include defects characterized by too much or too little oxygen for the Si—O—Si structure making up the quartz glass. Well-known examples include oxygen deficient defects (Si—Si, which absorbs at 245 nm) and oxygen surplus defects (Si—O—O—Si, which absorbs at 177 nm). However, such defects, or at least those which are measurable by spectrophotometric means, are excluded from optical-grade synthetic quartz glass to begin with. Of greater concern are more subtle defects, such as those in which the Si—O—Si bond angle falls outside the range of stability, as in the case of excessively stretched or compressed Si—O—Si bonds.
To remove such unstable structures, JP-A 7-61823 discloses a process in which the growth rate of quartz glass produced by the direct method is held to a level of not more than 2 mm per hour.
Although this process does appear to work, because the growth rate is very slow, it has a poor productivity and is not very cost-effective. Moreover, with regard to the general production conditions, it is empirically known that a slow growth rate tends to increase the hydroxyl group concentration in the resulting quartz glass. Two examples are cited in JP-A 7-61823, but the synthetic quartz glass obtained in both had hydroxyl group concentrations of 1,200 ppm, which is considerably higher than 1,000 ppm.
Because, as noted above, hydroxyl groups have a large impact on the refractive index of quartz glass, a lower hydroxyl group concentration is preferred for obtaining a more uniform refractive index distribution. Homogenization of the resulting quartz glass body is not called for in the art disclosed in JP-A 7-61823. However, in cases where homoge
Fujinoki Akira
Nishimura Hiroyuki
Otsuka Hisatoshi
Shimakawa Takayuki
Shirota Kazuo
Shin-Etsu Chemical Co. , Ltd.
Thomas Alexander S.
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