Glass manufacturing – Control responsive to condition sensing means
Reexamination Certificate
2001-07-03
2003-07-22
Derrington, James (Department: 1731)
Glass manufacturing
Control responsive to condition sensing means
C269S271000, C269S268000, C269S268000
Reexamination Certificate
active
06595030
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a device for generating an optically homogeneous streak-free quartz glass body having a large diameter. In accordance with the invention, bodies made of synthetic quartz glass and having a high homogeneity of their optical properties such as refractive index distribution, transmission, and fluorescence can be manufactured, in particular for use with wavelength under 250 nm.
The transmission and radiation stability required for an application in the UV-range, for example, in excimer lasers or in the microlithography at wavelengths below 250 nm, demands an extremely high purity of the manufactured quartz glass bodies which generally is ensured by a procedure via the gas-phase. In a flame hydrolysis method, a gaseous Si-starting-compound together with oxygen and a fuel gas is converted in a flame to yield SiO
2
-particles which are vitreously molten to the surface of a quartz glass body heated by a burner.
The increasing demands in the field of microlithography to increase the numerical aperture at a simultaneously high homogeneity of the optical components resulted in the manufacture of larger and larger quartz glass blanks. The quartz glass cylinders conventionally produced by non-displaceably fixing a single burner relative to a surface on which the quartz glass cylinders are molten are, however, limited with respect to the maximum diameters that can be attained. A transformation of the glass cylinders obtainable by this method into disks of expensive and involving the possibility of a negative variation of material, which unfavorably affects transmission and laser stability. Thus, a widening of the deposition range is required for a direct deposition on the desired final format as it is obtained by the conventional use of a plurality of burners or by moving a burner relative to the glass body in a plane normal to the axis of the quartz glass body or by a combination of both methods.
A particularly demanding technological request is to satisfy the requirements for a considerably larger area, which requirements the quartz glass body has to satisfy as they relate to the homogeneity of the optical properties even with larger diameters. This particularly is applicable to the homogeneity of the refractive index distribution with the currently used wavelengths of the mercury vapor lamp (365 nm-i-line and 436 nm-G-line) in microlithography.
Below 250 nm, however, particular consideration has to be given to a constant high transmission to be maintained over the entire optically usable area as well as an excellent damage behavior to be ensured over this area. When these short wavelengths are used, then the irradiation takes place already in the vicinity of the band edge of the glass. Hence, the smallest variations of the structure and the concentration in the glass arising from the manufacture take effect on the glass. Such variations lead to a displacement of the band edge and hence to the transmission below 200 nm with transmission variations in an order of size of some 0.001 cm
−1
and more (given as decimal absorption coefficient) which is as it relates to microlithography of relevance. As a result of the light scattering, increasing with &lgr;
4
, the diffusion rate for quartz glass, depending on the specific glass structure, lies also in an order of size of some 0.001 cm
−1
.
Additionally, there are numerous intrinsic defects in synthetic glass known, such as Cl-inclusions, OH-groups, oxygen defects etc., having absorption bands in a range between 150 nm and 250 nm. In view these strong dependencies of the transmission below 250 nm from the material properties, significant local transmission variations within a quartz glass body will arise with the conventional manufacturing methods.
Japanese Patent No. 01-024032 describes the deposition of quartz glass in a rotating cylindrical vessel wherein a burner performs a rotating or linear movement to feed a gaseous Si compound as well as the fuel gases. In a further embodiment, a second burner, which exclusively is gas-fueled, is used as an auxiliary heater. The deposition of SiO
2
particles in a vessel unavoidably leads to strong variations of the flow conditions which, in turn, lead to variations in the deposition conditions during the process. As a result, there are intense streak formations substantially at right angles to the deposition direction.
In Japanese Patent No. 06-234531, a burner providing O
2
-compounds, H
2
-compounds, and an Si-compound is moved in a plane relative to a quartz glass ingot being constructed in accordance with the temperature distribution measured at the ingot head. The temperature distribution detected by an IR- camera is fed into a computer adapted to control an x,y-table, upon which the ingot is mounted.
EP 0 735 006 describes the movement of a quartz glass ingot in the x-direction and y-direction at right angles to the deposition direction of the synthetic quartz glass. The resulting quartz glass contains streaks which substantially lie at right angles to the deposition direction.
U.S. Pat. No. 5,696,038 describes a relative and oscillating motion between a source of SiO
2
-particles and the quartz glass body being constructed, the motion taking place at right angles to the width of the body at a definite periodicity. There are described in detail cycle periods, which allow a definite minimum layer thickness deposition per cycle, which have a definite minimum duration, and show helically shaped paths or which limit the formation of streaks. The deposition of SiO
2
is performed in a flat vessel which is heated from top by a plurality of burners fixed in the upper part of a furnace. The manufactured material contains streaks at right angles to the deposition direction.
European Patent Publication No. EP 0 850 199 relates to those problems caused by the air streams which result from the motion of quartz glass body in a configuration according to U.S. Pat. No. 5,696,038. A method and device is claimed for maintaining a constant air stream about the growing quartz body while the latter performs oscillating movements. The deposition of SiO
2
is performed in a flat vessel in analogy to U.S. Pat. No. 5,696,038, which is heated from top by a plurality of non-displaceable burners. The manufactured material also contains streaks at right angles to the deposition direction.
Moreover, with all described prior art solutions, there is, due to the unfavorable flow conditions, the danger of introducing microstructural defects or glass bubbles in the material, apart from streaks.
European Patent Publication No. EP 0 720 970 discloses a quartz glass having a structure temperature of less than or equal to 1200 K, an OH-content of at least 1000 ppm, and an internal transmission of at least 99.6% at 193 nm. However, it does not discuss the steadiness of the transmission over the face of a quartz glass body.
U.S. Pat. No. 5,696,624 describes protection for a quartz glass of an internal transmission of greater than 99.9% per 1 cm sample thickness across a diameter of at least 150 mm before and after bombardment with 106 pulses of an ArF-laser. It is well-known that the internal transmission in the UV range can only be exactly measured by large expenditures. Any kind of surface contamination and surface defects (for example, by polishing) results in a severe reduction of the UV transmission that with highly transparent material makes up a multifold of the measuring effect of the internal transmission. For this reason, the purification of the samples gets a predominant significance. There is no reference to that problem in the above-cited publication. Generally, surface effects can be eliminated by carrying out thickness-dependent series of measurements.
In the measuring method described in U.S. Pat. No. 5,696,624 two samples having lengths of 2 mm and 12 mm were used. There is used a dual-beam spectrophotometer, the publication has, however, no reference to a purging of the spectrometer with a purging gas being non-absorbing in the UV-range, for exa
Bauch Hartmut
Hack Hraban
Martin Rolf
Menzel Andreas
Rudiger Frank
Derrington James
Glas Schot
Jordan and Hamburg LLP
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