Optical: systems and elements – Having significant infrared or ultraviolet property – Having ultraviolet absorbing or shielding property
Reexamination Certificate
2000-11-24
2002-10-29
Sikder, Mohammad (Department: 2872)
Optical: systems and elements
Having significant infrared or ultraviolet property
Having ultraviolet absorbing or shielding property
C359S350000, C359S355000
Reexamination Certificate
active
06473227
ABSTRACT:
INDUSTRIAL FIELD OF APPLICATION
The present invention relates to a silica glass optical material, a method for producing the same, and to a projection lens; in further detail, it relates to a silica glass optical material for a projection lens to be used in vacuum ultraviolet radiation lithography using radiation from 155 to 195 nm in wavelength (mainly, in an aligner for producing ultra high density integrated circuits using an excimer laser or an excimer lamp as the light source), to a method for producing the same, and to the projection lens.
Conventionally, ultraviolet radiation using a mercury vapor lamp, such as g-line and i-line, had been used as the light source for producing patterns of integrated circuits on a silicon wafer. However, as the semiconductor devices become finer, the aforementioned g-line and i-line found limits in resolution. Accordingly, excimer lasers which emit radiation with shorter wavelength attracted attention, and a photolithographic device using KrF excimer laser (248 nm) has been developed and put into practice. However, a higher degree of integration of the semiconductor devices is expected to be achieved in the near future, and this requires a light source capable of producing fine patterns with line widths of 0.1 &mgr;m or still finer. As the light sources capable of satisfying the requirements above, there can be mentioned high power output vacuum ultraviolet radiation from 155 to 195 nm in wavelength. Thus, efforts are devoted mainly to the development of an ArF excimer laser (193 nm), as well as an ArCl excimer laser (175 nm), a F
2
excimer laser (157 nm), etc. However, because the high power output vacuum ultraviolet radiation are far higher in power as compared with those used conventionally in photolithographic devices, the optical materials subjected to the irradiation may suffer abrupt damage such as a drop in transmittance, an increase in refractive index, generation of strain, generation of fluorescence, occasional generation of micro-cracks, etc., and this may make the material practically unfeasible concerning their function as a projection lens.
In the light of such circumstances, the development of an optical material that suffers less damage by the irradiation of the aforementioned high power output ultraviolet radiation emitted by an excimer laser or an excimer lamp has been keenly demanded.
As a material that satisfies the aforementioned requirements, there is known a material disclosed in JP-A-Hei6-227827. More specifically, the optical material disclosed in the publication above is a transparent quartz glass produced by heating a porous quartz glass body formed by depositing fine quartz glass particles obtained by flame hydrolysis and growing it, characterized in that the transparent quartz glass contains 10 ppm or less of OH, 400 ppm or more of a halogen, and that it contains hydrogen.
As an optical material that meets to the demands above, the present inventors have proposed, in JP-B-Hei6-48734 (the term “JP-B-” as referred herein signifies “an examined published Japanese patent application”), an optical material for laser radiation having a gaseous hydrogen concentration of at least 5×10
16
(molecules/cm
3
) or higher and an OH group concentration of 100 wtppm or higher. Furthermore, in JP-B-Hei6-27013, the present inventors proposed a synthetic silica glass optical body having a hydrogen gas concentration of at least 5×10
16
(molecules/cm
3
) or higher, an OH group concentration of 100 wtppm or higher, and substantially free from distribution in fluctuation of refractive index by canceling out the distribution in fluctuation of refractive index based on the concentration distribution of OH groups by the distribution in fluctuation of refractive index based on the fictive temperature.
The conventional silica glass optical materials above were satisfactory when used with an excimer laser or an excimer lamp emitting radiation from 195 to 250 nm in wavelength, or with an excimer radiation 195 nm or shorter in wavelength applied to thin members such as photomasks.
However, in drawing circuit patterns using the excimer radiation above, a projection lens made of silica glass is used, and such a projection lens becomes a large optical element exceeding a size 200 mm in diameter and 30 mm in thickness. Thus, if the silica glass optical material as above is used in a projection lens, non-uniform distribution likely occurs in the concentration of hydrogen molecules and OH groups, and this leads to inferior optical characteristics ascribed to the change in transmittance and refractive index. If OH groups should be contained in the silica glass optical material in such a high concentration as to exceed 100 wtppm, the durability becomes inferior due to a drop in the initial transmittance in the vacuum ultraviolet region. That is, the optical material proposed in the published patent application suffered problems of low initial transmittance in the wavelength region of from 155 to 195 nm and of insufficient durability.
The optical material disclosed in JP-A-Hei6-227827 utilizes halogen, however, among the halogens, Cl and the like are apt to generate defects upon irradiation by ultraviolet radiation, and it suffers a serious problem of deteriorating the performance of the optical material such as transmittance in the targeted spectral region.
SUMMARY OF THE INVENTION
It is an object of the present invention is to provide a silica glass optical material having a high initial transmittance with respect to vacuum ultraviolet radiation in a wavelength region of from 155 to 195 nm, a high precision, high durability, and an excellent homogeneity.
It is a further object of the invention to provide a method for producing the silica glass optical material.
It is another object of the invention to provide a projection lens made from the silica glass optical material.
The present inventors have intensively conducted studies, and it has been found that there can be obtained a synthetic silica glass optical material having high transmittance, high homogeneity, and an excellent durability, by increasing the purity of the optical material than that disclosed in the published patent application above while controlling the concentration of the OH groups and the hydrogen molecules in a certain range, and by making the concentration distribution thereof uniform while particularly selecting fluorine from the halogens and controlling the concentration of fluorine to a specific range with the distribution thereof being axially symmetrical. The present invention has been accomplished based on these findings.
The problems above can be solved by one of the compositions described in (1) to (14) below.
(1) A silica glass optical material for a projection lens to be used in vacuum ultraviolet radiation lithography using radiation from 155 to 195 nm in wavelength, wherein, said silica glass optical material is of ultrahigh purity, contains from 1 to 10 wtppm of OH groups, from 100 to 10,000 wtppm of F, and from 1×10
17
to 1×10
19
molecules/cm
3
of H
2
, and has a distribution in concentration of F that is axially symmetrical to the central axis.
(2) A silica glass optical material of (1) above, wherein the value of F/OH is in a range of from 50 to 1000.
(3) A silica glass optical material of (1) or (2) above, wherein the material is of ultrahigh purity, i.e., containing a maximum of 1 wtppb each of Li, Na, and K, a maximum of 0.5 wtppb each of Ca and Mg, and a maximum of 0.1 wtppb each of Cr, Fe, Ni, Mo, and W.
(4) A silica glass optical material described in one of (1) to (3), wherein the material contains a maximum of 1×10
17
molecules/cm
3
of H
2
O.
(5) A silica glass optical material described in one of (1) to (4), the distribution in concentration of F that is axially symmetrical to the central axis is such that the concentration gradually increases or decreases from the central portion towards the outer peripheral portion of the silica glass optical material.
(6) A silica glass optical material described in (5), wher
Heraeus Quarzglas GmbH & Co. KG
Milde & Hoffberg LLP
Sikder Mohammad
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