Photocopying – Projection printing and copying cameras – Distortion introducing or rectifying
Reexamination Certificate
2001-06-15
2004-07-13
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Distortion introducing or rectifying
C355S053000, C355S067000
Reexamination Certificate
active
06762823
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an illumination system and a scanning exposure apparatus using the same. The present invention is suitably applicable to a projection exposure apparatus, particularly, a scanning exposure apparatus, for use in a lithographic process among production processes for producing various devices such as ICs, LSIs, CCDs, liquid crystal panels, or magnetic heads, for example, for transferring, by projection exposure, a circuit pattern of an original such as a photomask or a reticle (hereinafter, “reticle”), being uniformly illuminated, onto a wafer while scanning the reticle and the wafer in synchronism with each other.
As a microprocessing technology for semiconductor devices such as ICs or LSIs, Japanese Laid-Open Patent Applications, Laid-Open No. 28313/1995, No. 190966/1997, No. 167735/1997, and No. 172901/1998 show scanning exposure apparatus for forming an image of a circuit pattern, formed on a reticle, upon a wafer (photosensitive substrate) through a projection optical system, while scanning the reticle and the wafer in synchronism with each other.
In this type of exposure apparatus, a reticle and a wafer are scanningly moved relative to a slit-like exposure area, by which one shot area on the wafer (and the whole pattern region defined on the reticle) is exposed. After the scanning exposure of one shot is completed, the wafer is moved stepwise to a next shot exposure position, and the scanning exposure of the next shot is initiated. This operation is repeated until exposures of the whole wafer are completed.
In accordance with recent miniaturization of a semiconductor device, the exposure wavelength is made shorter and shorter. Thus, as regards light sources to be used for the exposure, a KrF excimer laser (emission wavelength 248 nm) and an ArF excimer laser (emission wavelength 193 nm) as well as an F
2
excimer laser (emission wavelength 157 nm) have to be taken into account.
The miniaturization of a semiconductor device is a largest factor for supporting the dynamics of the semiconductor industry. The required linewidth has changed rapidly, from a generation requiring resolution of a linewidth of 250 nm (256 MB DRAM) to generations requiring a linewidth of 180 nm, to a linewidth of 130 nm, and to a linewidth of 100 nm.
In the lithography up to the i-line (wavelength 365 nm), resolution finer than the exposure wavelength has not been carried out. However, in the lithography using a KrF excimer laser, although its wavelength is 248 nm, it is applied to the resolution of a linewidth of 180 nm and to 150 nm. It can be said that the resolution less than the exposure wavelength has to be practiced by all means, including advancements in resist materials and super resolution technologies, for example. When various super resolution technologies are used, a linewidth corresponding to a half wavelength, in terms of lines-and-spaces, will be practicable.
However, the super resolution technologies involve many restrictions in dependence upon a circuit pattern formed on a reticle. The most effective way to improve the resolving power is to shorten the exposure wavelength and to enlarge the numerical aperture (NA) of a projection optical system. This fact generates a large motivation to shortening the wavelength, and it leads to development of lithography using an F
2
excimer laser.
When the exposure wavelength is to be shortened to improve the resolving power, for the exposure wavelength region shorter than 200 nm, there is a large limitation with respect to usable optical materials, and there arises a problem that the efficiency of light utilization becomes extraordinarily poor.
When an ArF excimer laser is used as a light source, optical materials usable in the region of that emission wavelength are only quartz and fluorite. When an F
2
excimer laser is used as a light source, only fluorite is a usable optical material. Further, while these materials are usable, there is another problem. That is, for example, while fluorite has a transmission factor of 99.9%/cm or more with respect to the emission wavelength of an ArF excimer laser, even a best sample may show a value of only 99.5%/cm to 99.6%/cm with respect to the emission wavelength of an F
2
excimer laser.
Situations are similar in regard to films (optical thin films). In the emission range of an F
2
excimer laser, use of an oxide is almost impossible, and usable materials are limited only to fluorine series compounds. As for materials of low refractive index, there are only MgF
2
and AlF
3
. As for those of high refractive index, there are only LaF
3
and GdF
3
, for example. Therefore, with respect to an anti-reflection film, for example, a film having attained a transmission factor of about 99.7% will obtain a transmission factor of 99%, at the best.
The performance of a film is an important factor for determining the overall efficiency of lithography, using an F
2
excimer laser. If it is assumed, for example, that there are ten transmissive or reflective surfaces until the light from a laser impinges on a wafer surface, the efficiency per one surface differs between 99% and 98%, for example, as can be readily understood from the relations 0.99
100
=0.366 and 0.98
100
=0.133, the difference of 1% results in a total difference of 2.5 times more.
However, in the emission range of an F
2
excimer laser, because of the material limitations or a difficulty in surface treatment, basically, the performance of a film is not comparable to that when a KrF excimer laser or an ArF excimer laser is used. If a good film to be used with an F
2
excimer laser is produced, the result can be reflected and a film of better performance can be produced for KrF and ArF excimer lasers.
As regards the film formation for use with an F
2
excimer laser, therefore, it is quite important to produce a film having the same performance as that of currently available films used with conventional excimer lasers. Also, it is very important to produce a film having a durability to F
2
excimer laser light.
Particularly, in an exposure optical system of an exposure apparatus, an illumination optical system includes more optical components than a projection optical system, and thus, the former is a key to the efficiency of light utilizations. While the projection optical system has a single function for printing an image of a reticle on a wafer without distortion, the illumination optical system is a multiple-function system having a shaping function for transforming light from a light source into an appropriate size, an integrating function for providing uniform illumination, an additional function for accomplishing various illumination modes, an imaging function for the masking, a function for controlling the light quantity, for example, and so on.
For a better light utilization efficiency of the illumination optical system, the number of optical components should be reduced as much as possible, and each constituent element should be designed to have multiple functions, or it should be simplified. On the other hand, the illumination optical system must meet requirements from the projection optical system, peculiar to the lithography using an F
2
excimer laser.
As an example of such requirements, when the projection optical system is a catadioptric system (an imaging optical system having a combination of mirrors and lenses), there may be cases in which, depending on the structure of the optical system, it is required to attain illumination corresponding to an imaging region of an arcuate shape. In other words, the illumination optical system is required to produce a slit-like illumination region of an arcuate shape.
As regards the catadioptric system, the optical material for a projection optical system which is usable in the emission wavelength range of an F
2
excimer laser is only one, i.e., fluorite. Therefore, with an ordinary dioptric system, chromatic aberration cannot be corrected. For this reason, the catadioptric system will be a good choice for a projection optica
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Henry Hung
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