Synthetic quartz glass optical material and optical member

Details Synthetic quartz glass optical material and optical member Synthetic quartz glass optical material and optical member

2002-01-29

2004-02-10

Sample, David R (Department: 1731)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

Reexamination Certificate

active

06689705

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a synthetic quartz glass optical member that transmits a F
2
excimer laser radiation such as a lens, a prism, a filter, a photomask, etc., and to an optical material for the same.
PRIOR ART
The technique of photolithography, which transfers a circuit pattern on a mask to a wafer by using light, is economically superior to other techniques using electron beam or an X-ray radiation, and is thereby widely used as an exposure technique for producing integrated circuits.
With the recent demand for further miniaturized and highly integrated LSIs, radiations with shorter wavelengths are being used as the light source, and aligners using i-lines 365 nm in wavelength capable of forming patterns with line widths in a range of from 0.5 to 0.4 &mgr;m or those using KrF excimer laser radiations capable of forming patterns with line widths in a range of from 0.18 to 0.35 &mgr;m have been put to practical use. Moreover, an aligner using an ArF excimer laser emitting radiations 193 nm in wavelength and capable of forming patterns with line widths in a range of from 0.10 to 0.20 &mgr;m is now placed under development.
As an apparatus for the next age lithography, there are studied an electron beam direct patterning technology, a X-ray proximity ion lithography, and a F
2
excimer laser exposure technology. Among them, electron beam direct writing technology has a problem concerning its throughput, and the X-ray proximity ion lithography suffers a problem in forming the mask. Thus, the F
2
excimer laser exposure technology is attracting most attention as the next age exposure technology.
From the view point of transmittance, resistance against laser radiations, uniformity, etc., a quartz glass, and particularly, a high purity synthetic quartz glass has been used as the optical material for use in the conventional excimer lasers using KrF, ArF, etc. A quartz glass exhibits high optical transmittance in the wavelength region for the KrF and ArF laser radiations, and the resistance against laser radiation can be increased by optimizing the conditions of production. Hence, an optical material, and particularly such usable as a projection lens, is already available.
However, since a F
2
excimer laser generates a radiation at a wavelength of 157 nm, i.e., a wavelength further shorter than that of an ArF excimer laser, it was found impossible to obtain a sufficiently high transmittance with the synthetic quartz glass used conventionally for a KrF or an ArF excimer laser. Hence, the optical material usable for the F
2
excimer laser was limited only to fluorite. This was a great limitation in designing the apparatus.
On the other hand, in JP-A-Hei4-195101 (the term “JP-A-” as referred herein signifies “an unexamined published Japanese patent application”) is disclosed a technology for reducing or extinguishing the defects that absorb the radiation in the wavelength region of from 155 to 400 nm, and for preventing the generation of defects even in case a high energy ultraviolet radiation is irradiated for a long duration of time. However, the effect of the technology concerning the resistance against a laser radiation is confirmed only for an ArF excimer laser radiation, and there is no description concerning the resistance against a radiation of a F
2
excimer laser.
Furthermore, JP-A-Hei8-67530 discloses a technology for increasing the stability against an ArF excimer laser radiation by doping a quartz glass with 1% by weight or more of fluorine and 10 ppm or more of OH groups. This publication also shows a considerable improvement in the transmittance for ultraviolet radiation in the vicinity of 157 nm, i.e., in the wavelength region of a F
2
excimer laser radiation.
Problems the Invention is to Solve
Surely, the optical stability of a quartz glass against an ArF excimer radiation can be improved by incorporating fluorine into the quartz glass. However, in case of a F
2
excimer laser radiation with a further shorter wavelength as compared with the ArF excimer laser radiation, it has been found that the technology above is not capable of sufficiently suppressing the generation of defects attributed to the irradiation of an excimer laser radiation.
The present inventors studied the physical properties and the damage behavior of a quartz glass under the irradiation of a F
2
excimer laser radiation in case the ultraviolet radiation is changed to that of a F
2
excimer laser. As a result, the glass characteristics suitable for a quartz glass material for optical use together with a F
2
excimer laser has been discovered. The present invention has been accomplished based on these findings.
More specifically, an object of the present invention is to provide a synthetic quartz glass optical material having a high optical transmittance for a radiation 157 nm in wavelength emitted from F
2
excimer laser and a high resistance against irradiation of a F
2
excimer laser radiation, yet having a uniformity suitable for such a fine patterning using a F
2
excimer laser, and to provide an optical member using the same.
Means for Solving the Problems
The problems above are solved by any of the constitutions (1) to (8) described below.
(1) A synthetic quartz glass optical material for F
2
excimer lasers having an OH group concentration of 0.5 ppm or lower, a fluorine concentration of 0.1 to 2 mol %, a hydrogen molecule density of 5×10
16
molecules/cm
3
or lower, a difference between the maximum and minimum fluorine concentrations within 20 mol ppm, and a difference between the maximum and minimum refraction indices of 2×10
−5
or lower.
(2) A synthetic quartz glass optical material for F
2
excimer lasers having an OH group concentration of 0.5 ppm or lower, a fluorine concentration of 0.1 to 2 mol %, a hydrogen molecule density of 5×10
16
molecules/cm
3
or lower, a difference between the maximum and minimum fluorine concentrations within 100 mol ppm, a difference between the maximum and minimum fictive temperatures within 50° C., and a difference between the maximum and minimum refraction indices being set to 2×10
−5
or lower by relatively forming a fluctuation in refractive indices in accordance with the fictive temperature, in such a manner that the fluctuation in refractive indices attributed to the distribution in the concentration of fluorine be cancelled.
(3) A synthetic quartz glass optical material for F
2
excimer lasers as described in constitution (2) above, wherein the maximum value in the distribution of fictive temperature is 920° C. or lower.
(4) A synthetic quartz glass optical material for F
2
excimer lasers as described in one of constitutions (1) to (3) above, wherein the internal transmittance for a radiation 157 nm in wavelength emitted from F
2
excimer lasers is 70% or higher.
(5) A synthetic quartz glass optical material for F
2
excimer lasers as described in one of constitutions (1) to (3) above, wherein the internal transmittance for a radiation 163 nm in wavelength is 90% or higher.
(6) A synthetic quartz glass optical material for F
2
excimer lasers as described in one of constitutions (1) to (3) above, wherein the drop in transmittance for a radiation 157 nm in wavelength after irradiating thereto 3×10
6
pulses of F
2
excimer laser radiation at an energy density per pulse of 10 mJ/cm
2
is 5% per 10 mm or less.
(7) A synthetic quartz glass optical material for F
2
excimer lasers as described in one of constitutions (1) to (3) above, wherein the birefringence measured at a wavelength of 633 nm is 0.5 nm/cm or lower.
(8) A synthetic quartz glass optical member for F
2
excimer lasers formed by using a synthetic quartz glass optical material as described in one of constitutions (1) to (7) above.
Embodiments for Practicing the Invention
It is required that the synthetic quartz glass according to the present invention is a synthetic quartz glass obtained by a so-called soot process. A synthetic quartz glass obtained by the soot process can achieve a combination of req

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