Optical: systems and elements – Polarization without modulation – Depolarization
Reexamination Certificate
2000-09-01
2002-04-02
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Polarization without modulation
Depolarization
C359S350000, C359S355000, C359S490020, C355S067000, C501S054000
Reexamination Certificate
active
06366404
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection optical system, a production method thereof, and a projection exposure apparatus using it. More particularly, the invention concerns a projection optical system used for transferring a predetermined mask pattern onto a substrate by use of a light source of an ultraviolet region suitable for semiconductor fabrication, a production method thereof, and a projection exposure apparatus using it.
2. Related Background Art
An example of the conventional projection exposure apparatus for semiconductor fabrication is one having the structure as illustrated in FIG.
16
A and FIG.
16
B.
Specifically, the projection exposure apparatus
800
illustrated in
FIG. 16A
is constructed in such structure that rays from a light source
501
such as a mercury-arc lamp or the like are collected by an ellipsoidal mirror
502
and that thereafter they are converted into a bundle of parallel rays by a collimator lens
503
. Then this parallel beam travels through a fly's eye lens
504
, which is an aggregate of optical elements
504
a
of a square section as illustrated in
FIG. 16B
, to form a plurality of light source images on the exit side thereof. An aperture stop
505
having a circular aperture is disposed at the position of the light source images. Beams from the plurality of light source images are condensed by a condenser lens
506
to uniformly illuminate a reticle R as an object to be illuminated, in superimposed fashion.
A pattern on the reticle R kept under uniform illumination by the illumination optical system in this way is projected onto a wafer W coated with a resist, by a projection optical system
507
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
800
of
FIG. 16A
is designed to perform exposure by the so-called step-and-repeat method in which the wafer stage is successively moved two-dimensionally in order to implement exposure in a next shot area after completion of exposure in one shot area on the wafer.
In recent years proposals have been made about such scanning exposure methods that a rectangular or arcuate beam was radiated onto the reticle R and that the reticle R and wafer W located in conjugate with each other with respect to the projection optical system
507
were moved in a fixed direction whereby the pattern of the reticle R could be transferred in high throughput onto the wafer W.
In the projection exposure apparatus in either of the above methods, it is desirable that optical members used in their optical systems have high transmittance at the wavelength of the light source used. The reason is as follows: the optical systems of the projection exposure apparatus are constructed of a combination of many optical members; even if optical loss per lens is little the total transmittance will be decreased greatly when the optical loss is added up by the number of optical members used. If an optical member inferior in transmittance were used, it would absorb the exposure light to increase the temperature of the optical member itself and thus cause heterogeneity of refractive index and, in turn, local thermal expansion of the optical member would deform polished surfaces. This would degrade the optical performance.
On the other hand, the projection optical systems are required to have high homogeneity of refractive index of the optical members in order to achieve a finer and sharper projection exposure pattern. The reason is that variations in refractive index will cause a lead and a lag of light and this will greatly affect the imaging performance of the projection optical system.
Thus silica glass or calcium fluoride crystals high in transmittance in the ultraviolet region and excellent in homogeneity are generally used as materials for the optical members used in the optical systems of the projection exposure apparatus in the ultraviolet region (not more than the wavelength of 400 nm).
Proposals of decreasing the wavelength of the light source have been made recently in order to transfer a finer mask pattern image onto the wafer surface, that is, in order to enhance the resolution. For example, decrease of wavelength into a shorter range is under way from the g-line (436 nm) and the i-line (365 nm), which have been used heretofore, to KrF (248 nm) and ArF (193 nm) excimer lasers.
In the projection exposure using such shorter-wavelength excimer lasers, since the purpose is to obtain the finer mask pattern, the materials used are those with higher performance as to the homogeneity of transmittance and refractive index.
SUMMARY OF THE INVENTION
With use of such materials having the high homogeneity of transmittance and refractive index, however, there were cases wherein desired resolution was not obtained after the optical system was assembled in combination of plural materials.
An object of the present invention is, therefore, to provide a projection optical system having high imaging performance, a production method thereof, and a projection exposure apparatus capable of achieving high resolution.
The inventors have conducted intensive and extensive research in order to accomplish the above object and first found out that birefringence of the materials of the optical members greatly affected the imaging performance of the projection optical system and the resolution of the projection exposure apparatus. Then the inventors discovered that the imaging performance close to designed performance of the projection optical system and the resolution close to designed performance of the projection exposure apparatus were attained if the magnitude of birefringence, i.e., birefringence values (absolute values) of the materials of the optical members were not more than 2 nm/cm and if distribution of birefringence was symmetric with respect to the center in the optical members, and disclosed it in Japanese Patent Application Laid-Open No. 8-107060.
With increase in demands for much higher resolution of the projection exposure apparatus, however, there were cases wherein satisfactory imaging performance of the projection optical system and satisfactory resolution of the projection exposure apparatus were not attained even with employment of the above conventional design concept if light of the shorter wavelength was used as a light source or if an optical member having a large diameter and a large thickness was used.
Thus the inventors have conducted further research and, as a result, discovered that the cause of failing to obtain the projection optical system and projection exposure apparatus of desired optical performance even with use of the optical members having good homogeneity of transmittance and refractive index was that distribution states of birefringence of the optical members differed among the optical members, the different birefringence distributions were added up in the overall optical system where the projection optical system was constructed in combination of a plurality of optical members, and this resulted in disturbing the wavefront of the light in the overall optical system, thereby greatly affecting the imaging performance of the projection optical system and the resolution of the projection exposure apparatus.
Describing the above in more detail, the conventional ways of evaluating the birefringence of optical members were nothing but arguments about whether the magnitude (absolute values) was high or low, and there was no concept of the above distribution of birefringence in the optical members at all, either. For example, for measuring the birefringence of a silica glass member, it was common recognition to those skilled in the art to measure the birefringence at several points near 95% of the diameter of the member and employ a maximum as a birefringence value in the member. However, the inventors precisely measured the distribution of birefringence of silica glass members and found that the actual distribution of birefringence was n
Hiraiwa Hiroyuki
Tanaka Issey
Nikon Corporation
Oliff & Berridg,e PLC
Schuberg Darren
LandOfFree
Projection optical system, production method thereof, and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Projection optical system, production method thereof, and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Projection optical system, production method thereof, and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2835512