Product method of synthetic silica glass and thermal...

Details Product method of synthetic silica glass and thermal... Product method of synthetic silica glass and thermal...

2001-03-28

2004-05-11

Griffin, Steven P. (Department: 1731)

Glass manufacturing

Processes

With shaping of particulate material and subsequent fusing...

C065S017600, C065S111000, C065S117000

Reexamination Certificate

active

06732546

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a production method of synthetic silica glass and a thermal treatment apparatus and, more particularly, to a method of producing synthetic silica glass useful as a material for optical components and others used together with ultraviolet lasers, and a thermal treatment apparatus used therein.
BACKGROUND ART
Projection exposure systems (steppers) having the structure as illustrated in FIG.
22
A and
FIG. 22B
have been used heretofore in the photolithography technology for printing a microscopic pattern of an integrated circuit on a wafer of silicon or the like.
In the projection exposure apparatus illustrated in
FIG. 22A
, light from a light source
601
such as a mercury arc lamp or the like is collected by an ellipsoidal mirror
602
and thereafter the light is converted into a collimated beam by a collimator lens
603
. Then this collimated beam travels through a fly's eye lens
604
consisting of an assembly of optical elements
604
a
of a rectangular cross section as illustrated in
FIG. 22B
, to form a plurality of light source images on the exit side thereof. An aperture stop
605
having a circular aperture is disposed at this light source image position. Beams from the plurality of light source images are condensed by a condenser lens
606
to uniformly illuminate a reticle R as an object to be illuminated, on a superimposed basis.
A pattern on the reticle R under the uniform illumination by the illumination optical system as described above is projected and printed on a wafer W coated with a resist by a projection optical system
607
consisting of a plurality of lenses. This wafer W is mounted on a wafer stage WS, which is arranged to move two-dimensionally, and the projection exposure apparatus of
FIG. 22A
carries out the exposure in the so-called step-and-repeat method in which after completion of exposure in one shot area on the wafer, the wafer stage is two-dimensionally moved in order for exposure in a next shot area.
Another method proposed in recent years is a scanning exposure method capable of transferring the pattern of the reticle R onto the wafer W at high throughputs by illuminating the reticle R with a beam of rectangular shape or arcuate shape and scanning the reticle R and the wafer W arranged in conjugate relation with respect to the projection optical system
507
, in a certain direction.
In the field of the projection exposure apparatus having such structure, there are desires for higher resolution with recent increase in integration density of LSI (large scale integration). Explaining this with an example of DRAM (dynamic random access memory) out of VLSI (very large scale integration) being a kind of LSI, the capacity thereof increases with development from LSI to VLSI in the following manner; 1K→256K→1M→4M→16M→64M→256M→1G. With this increase in the capacity, the processing line width of patterns required of the projection exposure apparatus decreases as 10 &mgr;m→2 &mgr;m→1&mgr;m→0.8 &mgr;m→0.5 &mgr;m→0.35 &mgr;m→0.25 &mgr;m→0.18 &mgr;m, respectively.
In order to enhance the resolution of the projection exposure apparatus, the optical members used in their optics need to have high transmittances for the exposure light used. This is because the optics of the projection exposure apparatus are composed of combination of many optical members and even if an optical loss per lens is small accumulation of such losses in the number of optical members used will lead to great decrease in the total transmittance. If optical members with low transmittances are employed they will absorb the exposure light to increase temperatures of the optical members and make refractive indexes inhomogeneous and polished surfaces thereof will undergo deformation because of local thermal expansion of the optical members. This will cause degradation of optical performance.
On the other hand, in the case of projection optical systems, there are demands for high homogeneity of refractive indexes of the optical members in order to obtain finer and clearer projection exposure patterns. This is because the dispersion of refractive indexes causes a lead or lag of light and this largely affects the imaging performance of the projection optical system.
Therefore, silica glass or calcium fluoride crystals with high transmittances for the ultraviolet light and with excellent homogeneity are generally used as materials for the optical members used in the optics of the projection exposure apparatus utilizing the ultraviolet light (of the wavelengths not more than 400 nm). Particularly, in the projection exposure apparatus with the excimer laser used in volume production lines of large capacity VRAM of not less than 16M, 0.25 &mgr;m microprocessors, and so on, synthetic silica glass of high purity is used as a material of optical elements for ultraviolet lithography (lens elements used in the illumination optical system or in the projection optical system).
The flame hydrolysis (also called a direct process) is known as a production method of synthetic silica glass. The flame hydrolysis is a method of ejecting a silicon compound as a source material and a combustion gas containing oxygen and hydrogen from a burner to burn the silicon compound in oxyhydrogen flame, thereafter depositing resultant fine particles of silica glass on a target opposed to the burner, and, at the same time as it, vitrifying the silica glass particles to obtain an ingot form of synthetic silica glass.
In general, the flame hydrolysis employs a fabrication system having the structure similar to the so-called Verneuil furnace and the synthesis is carried out with maintaining the in-system temperature at high temperatures of not less than 1000° C. The ingot form of silica glass obtained in this method is quickly cooled from the high temperature region of not less than 1000° C. to ordinary temperatures by natural cooling, is then subjected to cutting and rounding if necessary, and thereafter is subjected to a thermal treatment step of annealing (slow cooling treatment) or the like, thereby yielding a block material. The block material thus obtained is inspected as to radial index homogeneity, thereafter is processed in lens shape, and is further coated with a coating, so as to be able to be used as an optical member for ultraviolet lithography.
Meanwhile, decrease in wavelengths of light sources used for attainment of higher resolution has been proposed in recent years in the projection exposure apparatus, and, for example, the wavelengths have been decreased to the KrF excimer laser (248 nm) and the ArF excimer laser (193 nm), in place of the g-line (436 nm) and the i-line (365 nm) which have been utilized heretofore.
Since the projection exposure systems using such excimer lasers of short wavelengths are intended for attainment of finer mask patterns, their optics are constructed using materials with higher properties as to the homogeneity of transmittance and refractive index.
There were, however, cases wherein a desired resolution was not attained even if the optics were fabricated by assembling a plurality of materials with high and homogeneous transmittances and refractive indexes.
DISCLOSURE OF THE INVENTION
The present invention has been accomplished in view of the problem of the prior art described above and an object of the invention is to provide a production method of synthetic silica glass useful as a material for the optical members (optical elements) and others forming the optics of the projection optical apparatus (steppers) and capable of attaining sufficiently high imaging performance in the optics and sufficiently high resolution in the projection exposure apparatus even in use with a light source of a short wavelength such as the KrF excimer laser or the ArF excimer laser, and to provide a thermal treatment apparatus used therein.
The inventors have conducted intensive and extensive research in order to accomplish the above object and first found that the imaging perfo

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