Glass manufacturing – Processes – With shaping of particulate material and subsequent fusing...
Reexamination Certificate
2000-11-10
2003-04-22
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes
With shaping of particulate material and subsequent fusing...
C065S017600, C065S033200, C065S104000, C065S117000, C065S378000, C065S397000, C065S426000, C065S427000, C065SDIG008, C065SDIG001, C501S905000
Reexamination Certificate
active
06550277
ABSTRACT:
The invention concerns a quartz glass body for an optical component for transmitting UV radiation with a wavelength of 250 nm and less, in particular a wavelength of 157 nm, as well as a process for the manufacture of the quartz glass body whereby fine quartz glass particles are formed by means of flame hydrolysis of a silicon compound, deposited and vitrified.
Optical components of synthetic quartz glass are used in particular for the transmission of high energy UV laser radiation, for example in exposure optics of microlithographic apparatus for the manufacture of highly integrated circuits in semiconductor chips. Modern microlithographic devices use excimer lasers which generate high energy pulsed UV radiation with a wavelength of 248 nm (KrF lasers), 193 nm (ArF lasers), or 157 nm (F
2
lasers). However, with such short wave radiation, structural defects and corresponding absorptions come into play, which are characteristic of the type and quality of the respective quartz glass bodies. Suitability of a quartz glass with regard to high base transmission and radiation resistance depends on the structural properties of the glass resulting from local stochiometric deviations, and on the chemical composition of the glass. For example, high hydrogen content may contribute to a correction of defects and therefore to a slower increase of radiation induced absorption.
It has been observed that despite similar chemical or structural properties of a quartz glass the suitability as an optical component can vary if the quartz glass has been produced by different manufacturing methods. On the other hand, chemical or structural differences may be present but cannot be linked unequivocally with the observable transmission or damage characteristics in applications. Therefore, a quartz glass body for an optical component according to the invention is best characterized by its method of manufacture.
An optical component of this kind for the transmission of UV radiation with a wavelength of less than 250 nm and a method for its manufacture is known for example from EP 691 312 A1. The component described therein is obtained by means of a four stage process in which first a porous soot body is doped or dehydrated by means of a halogen compound (fluoride or chlorine compounds), resulting in the desired low OH content of the glass. This soot body is then vitrified and treated (in the vitrified state) with hydrogen or oxygen. The hydrogen or oxygen treatment takes place at relatively low temperatures (max. 500° C.) and can be therefore effective only at a relatively shallow surface depth of the quartz glass body since the diffusion velocity of hydrogen or oxygen in vitrified glass is very low. As a result, only the outer portions of a glass body produced in this manner have the required quality for applications involving very short wave UV radiation. In addition, the penetration depth of hydrogen or oxygen doping can only be determined by means of expensive analytical methods. The process according to EP 691 312 A1 is therefore by no means economical.
Quartz glass absorption in the 140-200 nm wavelength range is substantially determined by the so-called Urbach edge (drastically increasing absorption below about 155 nm. C.f. I. T. Godmanis, A. N. Trukhin, K. Huibner, “Exciton-Phonon Interaction in Crystalline and Vitreous SiO
2
,” Phys. Stat. Sol. (b), 116 (1983), 279-287) and the absorption bands of the oxygen deficiency centers (ODC I bands at 164 nm, see: L. Skulja “Optically Active Oxygen Deficiency Related Centers in Amorphous Silicon Dioxide,” Journal of Non-Crystalline Solids, 239 (1998) 16-48). It is necessary for this reason to push the absorption edge to as short a wavelength as possible and to keep the concentration of oxygen deficiency centers as low as possible.
An additional absorption band (at about 160 nm) in quartz glass is caused by the OH content. Increase of the OH content shifts the band toward the longer wavelengths (cf. H. Imai, K. Arai, H. Honoso, Y. Abe, T. Arai, H. Imagawa, “Dependence of defects induced by excimer laser on intrinsic structural defects in synthetic glasses,” Phys. review, B, 44 (1991) 4812-4817). Therefore, a more desirable low OH content of a porous soot body of synthetic quartz glass can be achieved by for example a heat treatment with a halogen compound. However, in quartz glass of low OH content another absorption band appears, based on the ODC of the ≡Si—Si≡type. This compound, and therefore the ODC I absorption band at 164 nm, can be removed by a hydrogen or oxygen treatment according to the following reactions:
≡Si—Si≡+H
2
→≡Si—H+H—Si≡ (1)
or
≡Si—Si≡+½O
2
→≡Si—O—Si≡ (2)
The object of the present invention is therefore to provide a quartz glass body for an optical component suitable for the transmission of UV radiation of a wavelength of 250 nm or shorter, especially of a wavelength of 157 nm, in that it has maximum homogeneity and in that absorptions at the abovementioned wavelengths are avoided. Furthermore, it is the object of the invention to provide a rational, economically effective, process for the manufacture of such a quartz glass body by overcoming the disadvantages of the prior art and by allowing bulk integration of oxygen or hydrogen into the quartz glass.
With respect to quartz glass body for an optical component, the object is achieved according to the invention as concerns the in that the quartz glass body has an OH content of not more than 10 ppm by weight, a glass structure substantially free of oxygen defect centers and that its base transmission in the 155-250 nm range is at least 80% at a radiation penetration depth of 10 mm with and the relative change of the base transmission over the usable area of the quartz glass body is a maximum 1%.
The quartz glass body according to the present invention is distinguished by a combination of characteristics that together make the quartz glass suitable in the long term as an optical component for the transmission of UV radiation of a wavelength of 250 nm and shorter, especially of a wavelength of 157 nm. Suitability as an optical component is given by the base transmission of at least 80% in the wavelength range in question. The transmission is achieved in a glass structure substantially free of oxygen defect centers and a low OH content. Of course, the quartz glass body according to the invention is distinguished by the absence of bubbles and streaks. A substantial advantage of the quartz glass body according to the invention is that the high base transmission is assured over the entire volume of the quartz glass body because during the manufacturing process this parameter is already being addressed by means of a heat treatment step before the vitrification in that defect centers are avoided or balanced out by the addition of a dopant. The high transmission uniformity of the quartz glass body according to the invention is characterized by a relative transmission change of a maximum of one percent (≦1%) established by measurements of the transmission over the usable area of the quartz glass body.
What is meant by base transmission is the internal transmission T after deduction of the Fresnel reflection loss according to the formula T=10
−kd
where k is the decadic extinction coefficient and d is the depth of radiation penetration in the test sample.
It is furthermore advantageous for the quartz glass body as an optical component for the transmission of UV radiation of 250 nm or shorter if the strain double refraction is less than 10 nm/cm and the inhomogeneity in the refraction index &Dgr;n is less than 20×10
−6
. Such a quartz glass body has very high homogeneity. What is meant by inhomogeneity of the refraction index is the difference between the highest and the lowest refraction values within a sample. The refractive index differentials are measured interferometrically at a wavelength of 633 nm. The strain double refraction as a further indicator
Uebbing Bruno
Vydra Jan
Colaianni Michael
Heraeus Quarzglas GmbH & Co. KG
Tiajoloff Andrew L.
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