Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-09-30
2004-08-24
Kim, Peter B. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C053S077000, C359S857000
Reexamination Certificate
active
06781671
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF FOREIGN PRIORITY DATE
This application claims right of benefit of prior filing date of Japanese Patent Application No. H13-305521 (2001), filed Oct. 1, 2001, entitled “Imaging Optical System and Exposure Apparatus,” the content of which is incorporated herein by reference in its entirety.
DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention pertains to an imaging optical system and exposure apparatus which may be favorably used during manufacture for example of semiconductor elements, liquid crystal display elements, image pickup elements, CCD elements, thin-film magnetic heads, and/or other such microdevices through use of photolithographic techniques.
2. Background of the Invention
Minimum linewidths in circuit patterns grow smaller with each successive generation of technology in the semiconductor and like fields of art. With each such generation of technology, greater resolving power has therefore been demanded of the exposure apparatuses used in such fields of art, and the light used for exposure (hereinafter “actinic light”) has moved to shorter and shorter wavelengths. Note that except where otherwise specified, “light” and “radiation” are used interchangeably herein and without intention to limit either to wavelengths which are visible or invisible or the like, both terms being used to refer to that portion of the electromagnetic spectrum where wavelength is less than about 1 mm and in particular including a range from roughly the infrared to the x-ray region, inclusive. “Actinic” light or radiation as used herein refers to light or radiation used for exposure without regard to whether such exposure occurs by a chemical, physical, or other process. “Exposure” as used herein refers to any change due to receipt of such actinic light or radiation at the wafer or other such substrate or workpiece.
One candidate under consideration as a next-generation exposure apparatus is an EUVL (Extreme Ultra Violet Lithography) apparatus making use of EUVL technology and employing light of wavelengths on the order of 5 nm to 20 nm. Because there are no materials having transmittances that would allow adequate formation of an optical system from refractive elements at this range of wavelengths, such optical systems must be constructed exclusively from reflective surfaces. Many types of optical systems have been proposed as EUVL projection optical systems.
From the laws of physics, the limit of resolution of an exposure apparatus is given by the following formula:
(Linewidth at limit of resolution)=
k×&lgr;/NA
. . . where &lgr; is actinic light wavelength, NA is the numerical aperture of the projection optical system, and k is a constant which depends on the characteristics of the apparatus and photosensitive resin, as well as on various other conditions. The laws of physics establish a minimum value of 0.25 for k, but practical considerations associated with actual apparatuses make 0.4 a more reasonable value for k. At present, the wavelength of light employed by most EUVL apparatuses is 13.4 nm. At the present time, EUVL apparatuses are typically contemplated for use in applications demanding minimum linewidths on the order of 50 nm. Using these values and the above formula, it can be determined that the projection optical system of an EUVL apparatus capable of achieving minimum linewidths of 50 nm should have an NA of
(Required NA)>
k
×&lgr;/(minimum linewidth)=0.4×13.4/50=0.11
Moreover, to accommodate minimum linewidths of 30 nm, which represents the generation after the 50 nm minimum linewidth generation, an NA of 0.18 or higher would be required.
And yet, of the many EUVL optical systems proposed to date, there is virtually none which achieves an NA on the order of 0.1 to 0.18 over an exposure area of practical size while at the same time permitting adequate correction of aberration. This is because until recently, EUVL was only contemplated for use in applications demanding minimum linewidths of 100 nm. The recent thinking is that the 100 nm minimum linewidth generation and the 70 nm minimum linewidth generation can be accommodated by exposure technologies other than EUVL. This has consequently ratcheted forward the generation contemplated for handling using EUVL technology, but the truth of the matter is that as of the present time there are but few optical systems which have the resolving power capable of accommodating minimum linewidths of 50 nm and smaller. Of the several types of optical system proposed to date, while there are extremely few cases where aberration is adequately corrected in the context of an NA on the order of 0.1 to 0.18 in a system whose design parameters have been disclosed, some examples are disclosed at Japanese Patent Application Publication Kokai No. H10-90602 (1998) and Japanese Patent Application Publication Kokai No. H9-211332 (1997).
Now, with EUVL, because the pattern to be transferred is formed on a mask which is used in reflection, illuminating light must be incident on the mask at an oblique angle with respect thereto. This is because if illuminating light were incident perpendicularly on a reflection mask, the optical path of an illumination system which illuminates the mask and the optical path of an imaging system which is arrived at after reflection from the mask would overlap, and optical elements of the illumination system would occlude the optical path of the imaging system, and/or optical elements of the imaging system would occlude the optical path of the illumination system.
In causing a light beam to be obliquely incident thereon, both of the foregoing two patent publications disclose that the light beam be inclined in such a direction so as to cause it to be constricted as it goes from the mask surface to the projection optical system. Or stating this another way, the entrance pupil of the projection optical system is to the projection optical system side of the mask.
The basis for being able to state this in either of the foregoing two ways will be explained through use of a drawing. In a projection optical system or other such optical system without vignetting, the entrance pupil is defined as the image of the applicable aperture stop as produced by that portion of the optical system between the object surface and that aperture stop. Now, a ray passing through the center of the aperture stop is called a principal ray, and from the foregoing definition it is clear that the entrance pupil will be located at the intersection of the optical axis and an extension of a principal ray passing through the image plane. This is shown diagrammatically in FIG.
6
.
FIG.
6
(
a
) shows an example of a design solution in which a light beam is inclined in such direction as to cause it to be constricted with respect to the optical axis as it goes from the mask surface (object surface) to a projection optical system. In such a case, the intersection of the principal ray and the optical axis will be to the right, or to the projection lens side, of the object plane, and the entrance pupil will be to the projection optical system side of the mask plane (object plane). Conversely, in FIG.
6
(
b
), a light beam is inclined in such direction as to cause it to diverge with respect to the optical axis as it goes from the mask surface (object surface) to the projection optical system, in which case the intersection of the principal ray and the optical axis will be to the left, or on the side opposite the projection lens side, of the object plane, and the entrance pupil will be on the side opposite the projection optical system side of the mask plane (object plane).
The former situation (i.e., a situation such as that shown in FIG.
6
(
a
)) creates the following difficulties from the standpoint of illumination optical system design. For efficient, distortion-free transfer of the mask pattern to the surface being exposed it is, in general, required that the exit pupil of the illumination optical system be conjugate to the entrance pupil o
Finnegan Henderson Farabow Garrett & Dunner LLP
Kim Peter B.
Nikon Corporation
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