Synthetic fused silica member

Details Synthetic fused silica member Synthetic fused silica member

2000-03-07

2001-12-25

Sample, David R. (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C430S005000

Reexamination Certificate

active

06333284

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a synthetic fused silica member used, for example, in an exposure apparatus (optical aligner) for producing IC in which a light source providing light having a wavelength of 200 nm or less such as fluorine excimer laser or the like is used, especially a synthetic fused silica substrate for photomask.
2. Description of the Related Art
Along with a recent increase in the degree of integration of semiconductor device, there has been required for improving accuracy of micromachining in so-called photolithography technique. In order to make fine patterns, it is necessary to use a light source providing a light having shorter wavelength, and higher energy. Therefore, high transmittance and durability to light having short wavelength have been required in optical member to be used.
Conventionally, there have been used synthetic fused silica or fluorite (CaF
2
) as a photomask, lens or the like in an exposure apparatus for producing semiconductor IC or the like in which a light source providing light having short wavelength, as in ultraviolet region or vacuum ultraviolet region, such as excimer laser or the like is used. Because, only synthetic fused silica and fluoride crystal as represented by fluorite can be practically used as the material having high transmittance to light having wavelength such as KrF (248 nm), ArF (193 nm) excimer laser.
The characteristics of the synthetic fused silica are different depending on methods for producing it. For example, there are the synthetic fused silica having high hydroxyl group-content which is produced from oxyhydrogen flame by “a direct method” and those having low hydroxyl group-content produced by “a soot method” wherein silica sintered body is molten.
As described above, along with increase in a degree of integration of semiconductor devices, a light source providing a light having a shorter wavelength, represented by KrF (248 nm) or ArF (193 nm) excimer laser has been used, and as a result, a photon energy of the light has been increased. When a synthetic fused silica is irradiated with a light having a short wavelength and high photon energy, structural defects which can be substantial damages may be caused in the glass, and the transmittance may be lowered. There is proposed the material in which significant lowering of transmittance is not caused, even when it is irradiated with a light having short wavelength and high photon energy (see, for example, in Japanese Patent KOKAI Hei 7-291635).
However, recently, optical member for light in the vacuum ultraviolet region which has shorter wavelength as 200 nm or less, especially for fluorine excimer laser (157 nm), have been developed. If the optical member used for such a light source having short wavelength can be made of a synthetic fused silica, it would be quite advantageous, since the technology is an extension of the conventional optical exposure technology, and only slight change of processes, for example, change of light source is necessary. Namely, the conventional photolithography technology can be prolonged.
However, the above-mentioned range of the wavelength is close to essential absorption edge of transmittance of the synthetic fused silica. Accordingly, if it is pure SiO
2
, it is in principle transparent in the range of the wavelength up to 125 nm (see Material Technology-Highly Functional Glass, published by Tokyo University publishing department). However, as described above, an actual synthetic fused silica contains hydroxyl groups, chlorine, metal impurity, or the like depending on the method for production, and absorption due to them has a subtle affect on the transmittance.
On the other hand, fluorite may be used as other material than synthetic fused silica that is excellent in transmittance in the vacuum ultraviolet region. However, it has a coefficient of thermal expansion that is double-digits higher than that of synthetic fused silica. For example, when it is used as a substrate for a photomask, accuracy of dimension gets worse due to affect of thermal expansion, which may lead to blur of patterns. Furthermore, it is disadvantageous that water cannot be used in the steps for polishing a member made of fluorite to be transparent, since fluorite is deliquescent. Accordingly, it is difficult to be machined.
SUMMARY OF THE INVENTION
The present invention has been accomplished to solve the above-mentioned problems. An object of the present invention is to provide a synthetic fused silica member which can efficiently transmit a light having a wavelength of 200 nm or less, especially light from fluorine excimer laser (157 nm), and is not liable to sustain damage causing lowering of transmittance.
To achieve the above mentioned object, the present invention provides a synthetic fused silica member used at a wavelength of 200 nm or less which has a hydroxyl group-content of 1 to 50 ppm and a fluorine-content of 100 to 1000 ppm, and contains no chlorine.
When the synthetic fused silica which has a hydroxyl group-content and a fluorine-content in the above range, and contains no chlorine is used as an optical member for light having a wavelength of 200 nm or less, especially for fluorine excimer laser (wavelength of 157 nm), it can transmit the light well, and it is durable for long hours of use.
In that case, the synthetic fused silica member can be produced so that birefringence may be 2 nm/cm or less.
Thereby, light can be uniformly transmitted, and therefore, blur of patterns is not caused on exposure, for example, in photolithography.
The synthetic fused silica of the present invention is very useful when it is used as a synthetic fused silica substrate for photomask used at a wavelength of 200 nm or less. It can be used in an optical system wherein a fluorine excimer laser is used as a light source.
Accordingly, the present invention is practically very useful from a practical standpoint, since it can prolong the conventional photolithography technique.
According to the present invention, the synthetic fused silica member contains a desired amount of hydroxyl groups and a desired amount of fluorine, and no chlorine, and therefore it can efficiently transmit light having a wavelength of 200 nm or less, especially the fluorine excimer laser (157 nm), and is durable for long hours of use. Furthermore, since it may have birefringence of 2 nm/cm or less, it can transmit the light uniformly, and does not cause blur of pattern, for example, in photolithography.
Accordingly, a technology such as photo lithography or the like using a fluorine excimer laser as a light source can be conducted without changing design used in conventional methods, namely prolongation of the technology can be achieved, and therefore, the present invention is significantly excellent from the practical standpoint.
DESCRIPTION OF THE INVENTION AND EMBODIMENT
The present invention will now be described more in detail. However, the invention is not limited thereto.
The inventors of the present invention have made a lot of experiments, and found that if a synthetic fused silica member has a hydroxyl group-content of 1 to 50 ppm and a fluorine-content of 100 to 1000 ppm, and contains no chlorine, and preferably has birefringence of 2 nm/cm or less, there can be improved a transmittance and resistance to the light having a wavelength of 200 nm or less, especially a fluorine excimer laser, and therefore the member can be used as an optical member for the light, and the exposure technology can be prolonged. Then, the inventors have further studied the conditions to complete the present invention.
The synthetic fused silica member of the present invention used for the light having a wavelength of 200 nm or less has a hydroxyl group-content of 1 to 50 ppm, preferably 1 to 30 ppm. If the hydroxyl group-content is more than 50 ppm, transmittance at 157 nm is 10% or less, and is not practically useful. It is preferable that the hydroxyl group-content is as low as possible, since transmittance in the wavelength region of 200

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