Synthetic quartz glass and method for preparing the same

Details Synthetic quartz glass and method for preparing the same Synthetic quartz glass and method for preparing the same

2001-03-01

2003-06-10

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C065S017400, C065S017600, C065S111000

Reexamination Certificate

active

06576578

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a synthetic quartz glass for optical components to be used for an apparatus employing ultraviolet lights having wavelengths of from 150 to 200 nm as a light source, and a process for producing it, particularly to a synthetic quartz glass to be used as optical components such as a lens, a prism, a photomask, a pellicle and a material for windows, to be used for light within a range of from the vacuum ultraviolet region and the ultraviolet region, such as an ArF excimer laser (wavelength: 193 nm), a F
2
laser (wavelength: 157 nm), a low pressure mercury lamp (wavelength: 185 nm) or a Xe
2
★ excimer lamp (wavelength: 172 nm).
BACKGROUND ART
A synthetic quartz glass has such characteristics that it is a transparent material within a wavelength range of as wide as from the near infrared region to the ultraviolet region, it has an extremely small thermal expansion coefficient and is excellent in dimensional stability, and it contains substantially no metal impurity and has a high purity. Accordingly, a synthetic quartz glass has been mainly used for optical components of a conventional optical apparatus employing g-line or i-line as a light source.
Along with high-integration of LSI in recent years, techniques to draw finer and thinner lines have been required in an optical lithography technology to draw an integration circuit pattern on a wafer, and accordingly use of light having a shorter wavelength as an exposure light source has been promoted. For example, for a light source of a stepper for optical lithography, an ArF excimer laser (hereinafter referred to simply as an ArF laser) or a F
2
laser is now to be used, as advanced from conventional g-line (wavelength: 436 nm) and i-line (wavelength: 365 nm).
Further, a low pressure mercury lamp or a Xe
2
★ excimer lamp is used for an apparatus such as optical CVD, an apparatus for cleaning silicon wafers or an ozone-generation apparatus, and it is being developed to apply it to the optical lithography technology in future.
It is necessary to use a synthetic quartz glass for a gas filled tube of a lamp to be used for a low pressure mercury lamp or an excimer lamp, or an optical element to be used for an optical apparatus employing the above-mentioned short wavelength light source.
A synthetic quartz glass to be used for such optical systems, is required to have light transmittance within a wavelength range of from the ultraviolet region to the vacuum ultraviolet region, and it is required that the light transmittance at the service wavelength will not decrease after irradiation of light.
With a conventional synthetic quartz glass, if it is irradiated with light from a high energy light source such as an ArF laser or a F
2
laser, a new absorption band will be formed in a ultraviolet region, and such has been problematic for an optical component to be used for constituting an optical system employing an ArF laser or a F
2
laser as a light source.
If an ArF laser or a F
2
laser is applied for a long time, an absorption band (hereinafter referred to as a 214 nm absorption band) having a wavelength of 214 nm at the center, so-called an E′ center (≡Si.), and an absorption band (hereinafter referred to as a 260 nm absorption band) having a wavelength of 260 nm at the center, so-called NBOHC (non-crosslinked oxygen radical: ≡Si—O.), will be formed.
As a technique to suppress formation of such absorption bands, a method of incorporating at least 100 ppm of OH groups and at least 5×10
16
molecules/cm
3
of hydrogen molecules into a synthetic quartz glass which contains substantially no reduction type defects or oxidation type defects, has been proposed (JP-A-3-101282). It is disclosed that hydrogen molecules in the synthetic quartz glass have a function to mend defects such as E′ centers or NBOHC formed by the ultraviolet ray irradiation, and the OH groups have a function to reduce the concentration of defect precursors which become E′ centers or NBOHC when irradiated with ultraviolet rays.
However, as a result of a detailed research on the change in light transmittance of a synthetic quartz glass by ultraviolet ray irradiation, the present inventors have found that in the synthetic quartz glass, an absorption band (hereinafter referred to as a 163 nm absorption band) having a wavelength of 163 nm at the center, will be formed in addition to the 214 nm absorption band and the 260 nm absorption band. When it is used as an optical component for an apparatus employing light with a wavelength of at least 200 nm as a light source, the service wavelength and the 163 nm absorption band are apart, whereby there will be no substantial influence of the decrease in light transmittance due to formation of the 163 nm absorption band. However, in a case where it is used as an optical component for an apparatus employing light with a wavelength of from 150 to 200 nm as a light source, the light transmittance in the vicinity of the service wavelength will decrease due to formation of the 163 nm absorption band.
The present invention has an object to provide a synthetic quartz glass which is to be used for an apparatus employing ultraviolet rays having wavelengths of from 150 to 200 nm as a light source and which has high light transmittance at a wavelength of from 150 to 200 nm and is excellent in ultraviolet ray resistance (the light transmittance in the vicinity of the service wavelength will not decrease even when irradiated with light employing ultraviolet rays with wavelengths of from 150 to 200 nm as a light source), and a process for its production.
DISCLOSURE OF THE INVENTION
The present invention provides a synthetic quartz glass to be used for light with a wavelength of from 150 to 200 nm, wherein the OH group concentration in the synthetic quartz glass is at most 100 ppm, the hydrogen molecule concentration is at most 1×10
17
molecules/cm
3
, reduction type defects are at most 1×10
15
defects/cm
3
, and the relation between the change &Dgr;k
163
in the absorption coefficient at a wavelength of 163 nm and the change &Dgr;k
190
in the absorption coefficient at a wavelength of 190 nm, as between before and after irradiation of ultraviolet rays with a wavelength of at most 250 nm, satisfies 0<&Dgr;k
163
<&Dgr;k
190
.
BEST MODE FOR CARRYING OUT THE INVENTION
It is important that the OH group concentration is at most 100 ppm (meant for weight ppm), and in a case where it is used as an optical component for an apparatus employing light in a vacuum ultraviolet region having a wavelength of at most 180 nm as a light source, the OH group concentration is preferably at most 50 ppm, particularly preferably at most 10 ppm. The lower the OH group concentration, the higher the light transmittance.
When a synthetic quartz glass containing hydrogen molecules is irradiated with ultraviolet rays, the 163 nm absorption band will be formed. The 163 nm absorption band is attributable to reduction type defects (≡Si—Si≡ bonds) and will substantially lower the light transmittance of ultraviolet rays with wavelengths of at most 200 nm. With a view to suppressing formation of the 163 nm absorption band, it is important that the hydrogen molecule concentration in the synthetic quartz glass is at most 1×10
17
molecules/cm
3
, particularly preferably at most 3×10
16
molecules/cm
3
.
To accomplish the object of the present invention, it is necessary to suppress formation of the 163 nm absorption band. The degree for suppression of formation of the 163 nm absorption band can be evaluated from the relation between the change &Dgr;k
163
in the absorption coefficient at a wavelength of 163 nm and the change &Dgr;k
190
in the absorption coefficient at a wavelength of 190 nm, as between before and after irradiation of ultraviolet rays with a wavelength of at most 250 nm. Namely, in the present invention, it is important that the relation of 0<&Dgr;k
163
<&Dgr;k
190
is satisfied.
Further, when irradiated with ultraviolet rays,

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