Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
2000-10-06
2004-03-16
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With field curvature shaping
C359S726000
Reexamination Certificate
active
06707616
ABSTRACT:
1. Field of the Invention
The present invention relates to a projection exposure apparatus used in a lithography process in a line of manufacturing a semiconductor device, an imaging device such as a CCD etc, a liquid crystal display device, or a device such as a thin-film magnetic head etc. The present invention relates also to a catadioptric optical system used as a projection optical system of the this type of projection exposure apparatus. The present invention relates further to a projection exposure method using this type of projection exposure apparatus in the lithography process.
2. Related Background Art
A reduction type projection exposure apparatus is widely used for manufacturing an integrated circuit such as an LSI. In the reduction type projection exposure apparatus, a reduced image of a mask pattern is formed on a wafer classified as a photosensitive substrate through the projection optical system. Over the recent years, a pattern of the integrated circuit, which is projection-exposed on a semiconductor substrate, has become increasingly hyperfine, and it has been required that a resolution of the projection exposure apparatus be further enhanced.
The further enhancement of the resolution of the projection exposure apparatus entails increasing a numerical aperture (NA) of the projection optical system and decreasing a wavelength of exposure radiation. It is, however, difficult in terms of a geometry of the optical system that the numerical aperture NA of the projection optical system is set to a predetermined value or greater. Further, if NA of the projection optical system is increased, a depth of focus usable decreases. As a result, it is difficult to actualize a theoretically attainable resolution. Therefore, in fact, what is strongly demanded is a reduction in the wavelength of exposure radiation in order to enhance the resolution of the projection exposure apparatus.
A KrF excimer laser of which a wavelength is 248 nm and an ARF excimer laser having a wavelength of 193 nm, have hitherto been proposed as radiation (light) sources for exposure. Further, utilization of a radiation (light) source having a wavelength as short as 180 nm or less is on the examination.
For example, an F
2
laser having an oscillation wavelength of 157 nm exists as a radiation source for emitting an exposure radiation of which a wavelength is 180 nm or smaller. In the case of using this type of radiation source, however, the following problems arise. First, there are an extremely limited kind of glasses that transmit beams having the wavelength of 180 nm or under. Second, there are very few substances appropriate to be taken for an anti-reflection film, and hence it is difficult to form a preferable anti-reflection film. Consequently, a radiation reflectance on each lens surface increases, with the result that a sharp drop in radiation quantity occurs especially in a high-resolution projection optical system requiring a larger number of lenses.
Further, there are limited types of refracting optical members usable for short wavelength beams having a wavelength as short as 180 nm. Such being the case, a chromatic aberration occurred depending on a wavelength width of the laser beams must be eliminated (corrected) under such a condition that only the limited types of glass materials (optical materials) can be used for the refracting optical member.
By the way, in the case of the F
2
laser, there exist neither a refracting optical member nor a reflecting optical member with a small loss of radiation quantity in its oscillation wavelength band. Consequently, it seems more difficult to narrow the band on a large scale with respect to the F
2
laser than in the case of a KrF excimer laser and an ArF excimer laser. An intensity of the F
2
laser that has already been utilized is small, and consequently an intensity of the exposure radiation, and more essentially, an exposure speed tend to further decrease by narrowing the band. In other words, the F
2
laser has a larger influence caused by the decrease in the exposure speed due to the narrowed band than other types of lasers.
A natural wavelength width of the F
2
laser having a wavelength of 157.6 nm is, however, by far smaller than a natural wavelength width of the ARF excimer laser etc, and hence a full width half maximum of the F
2
laser can be reduced down to 20 pm or under by slightly narrowing the band. For instance, Lambda Physik, Inc. (Germany) utilized an F
2
laser of which a full width half maximum is 10 pm and an output is 10 W as NovaLine TMF500 by comparatively simply narrowing the band. Then, in the projection exposure apparatus using the F
2
laser as a radiation source for exposure, it is desired that laser beams having a full width half maximum of this degree in order to increase an exposure efficiency and avoid an intricacy of the laser device due to the narrowed band. In this case, in the projection optical system, the chromatic aberration must be eliminated in a broader band over a wavelength width on the order of 10 pm-20 pm than in the prior art. Note that the band can be narrowed to such a degree that the full width half maximum is approximately 2 pm by adding a band narrowing element with respect to the F
2
laser.
As explained above, however, the types of the glass materials utilizable are strictly limited in the wavelength region of 180 nm or under, so that it is difficult to sufficiently eliminate the chromatic aberration in the dioptric optical system. Further, an internal absorption and a surface reflection might occur in the lens elements in this wavelength region, and therefore, if the number of the lens elements increases, a radiation transmittance in the optical system conspicuously declines.
For example, the short wavelength beams of 180 nm or smaller such as the F
2
laser beams exhibit a high absorption rate by air (oxygen). Hence, in the projection exposure apparatus using, e.g., the F
2
laser as an exposure radiation source, it is required that the air in the optical system be replaced with a gas hard to absorb an exposure radiation (illumination radiation), i.e., an inert gas such as helium etc in order to prevent the decline of the transmittance of the optical system by avoiding the radiation absorption by the air.
SUMMARY OF THE INVENTION
It is a first object of the present invention to restrain a decrease in loss of radiation quantity down to its minimum while attaining a high resolution in the case of using an exposure radiation having a wavelength of 180 nm or smaller.
It is a second object of the present invention to attain a high resolution while restraining a decrease in loss of radiation quantity by correcting well a chromatic aberration with limited kinds of optical materials in the case of using an exposure radiation having a wavelength of 180 nm or smaller.
It is a third object of the present invention to ensure a high transmittance of a projection optical system by avoiding well a radiation absorption with a gas exhibiting a low degree of cleanness in the case of using an exposure radiation having a wavelength of 180 nm or smaller.
It is a fourth object of the present invention to preferably form a hyperfine pattern by restraining a decline of an image forming performance even when trying to increase a numerical aperture and enlarge an exposure area.
To accomplish the first object, according to a first aspect of the present invention, a projection exposure apparatus comprises an illumination optical system for illuminating a mask formed with a pattern with beams of radiation, and a projection optical system for forming an image of the pattern on a workpiece on the basis of beams from the mask. Then, the illumination optical system supplies an illumination radiation having a center wavelength of 180 nm or smaller, and the projection optical system includes at least one concave mirror, fifteen or less pieces of refracting lenses, and four or more aspherical surfaces.
In the projection exposure apparatus according to the first aspect, the refracting lenses may all be form from the same
Takahashi Tomowaki
Tsukamoto Hiroyuki
Miles & Stockbridge P.C.
Nikon Corporation
Sugarman Scott J.
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