Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-06-28
2004-12-14
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S066000, C355S071000
Reexamination Certificate
active
06831731
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a projection optical system and an exposure apparatus provided with a projection optical system. In particular, the present invention relates to a suitable catadioptric type projection optical system in an exposure apparatus used when fabricating microdevices such as semiconductors or the like in a photolithographic process.
2. Description of Related Art
In recent years, the miniaturization in semiconductor device fabrication and. semiconductor chip packaging fabrication is increasing, and a projection optical system with a higher resolution is required for a photolithographic exposure device. To satisfy this resolution requirement, the wavelength of the exposure light is shortened, and the NA (the numerical aperture of a projection optical system) is increased. However, when the wavelength of the exposure light is shortened, the types of optical glass that are able to be used are limited due to light absorption.
For example, when using light in a vacuum ultraviolet region with a wavelength of 200 nm or less, an F
2
laser (wavelength 157 nm) in particular, as the exposure light, a fluoride crystal such as calcium fluoride (fluorite: CaF
2
) and barium fluoride (BaF
2
) must be used as a radiation transmissive optical material in the projection optical system. In reality, a design that forms a projection optical system with only fluorite is assumed in an exposure apparatus using an F
2
laser. Fluorite is a cubic system that was thought to be optically isotropic and to have substantially no birefringence. Further, in prior visible light experiments, only low birefringence (random occurrences caused by internal stress) has been observed in fluorite.
However, at a symposium (2nd International Symposium on 157 nm Lithography) concerning lithography held on May 15, 2001, John H. Burnett et al. of the U.S. NIST announced that he confirmed both in theory and by experiment that fluorite has an intrinsic birefringence.
According to this presentation, fluorite birefringence is nearly zero in the crystal axis direction [111] and the equivalent axes [−111], [1−11] and [11−1], and in the crystal axis [100] and equivalent axes [010] and [001], but other directions have a value which is not substantially zero. In particular, the six crystal axis directions [110], [−110], [101], [−101], [001] and [01−1] have a maximum birefringence of 6.5 nm/cm for a wavelength of 157 nm and 3.6 nm/cm for a wavelength of 193 nm. These values of birefringence are substantially greater than 1 nm/cm, the permissible value of random birefringence. However, for the portion that is not random the effect of birefringence may accumulate through multiple lenses.
Previously, the birefringence of fluorite was not considered in designing a projection optical system, so from the perspective of working ease, the crystal axis [111] and the optical axis are generally aligned. In such a case, in a projection optical system, the NA (numerical aperture) is comparatively large, so the crystal performance may deteriorate due to the effect of birefringence because the light ray that is somewhat tilted from the crystal axis [111] also passes through the lens.
However, Burnett et al. revealed in the presentation mentioned above, a method of compensating for the effect of birefringence by aligning the optical axis of a pair of fluorite lenses with the crystal axis [111] and rotating a pair of fluorite crystals 60° relatively with the optical axis at the center. It is possible to alleviate the effect of birefringence with this method, but the effect of the compensation is not sufficient because the effect of birefringence is not actively compensated for due to the effect of birefringence in the opposite direction.
Also, when using F
2
laser light (157 nm wavelength) as an exposure light, the outgas from the photoresist caused by exposure is unavoidable. Therefore, unless extraordinary steps are taken, it is impossible to avoid a contamination of the lenses caused by outgas in a conventional projection optical system having a large numerical aperture.
SUMMARY OF THE INVENTION
The present invention addresses the above-described problems. One object of the present invention is to provide a projection optical system having excellent optical performance that is substantially not affected by birefringence even when using optical materials with intrinsic birefringence such as fluorite, for example, and to provide an exposure apparatus having the projection optical system. A further object of the present invention is to provide a projection optical system capable of effectively avoiding contamination of the lenses caused by outgas from the photoresist, and to provide an exposure apparatus as part of the projection optical system.
In order to address the above-described problems, a first aspect of the present invention provides a projection optical system capable of forming a reduced image of a first surface onto a second surface, and includes a plurality of lenses and at least one concave reflective mirror, wherein the projection optical system, when used in an exposure apparatus to scan expose the first surface onto the second surface while moving the first surface and the second surface along a scanning direction, forms a slit-shaped or arc-shaped exposure area at the second surface when not scanning; and satisfies the conditional expression
0.5<(
Dw·Nw
)/
Ew
<1.4 (1)
where Dn is a working distance of the second surface side, Nw is a numerical aperture of the second surface side, and Ew is a length in the direction orthogonal to the scanning direction in the slit-shaped or arc-shaped exposure area. It should be noted that a slit shape in the present invention refers to a shape extending in a direction across a scanning direction, for example, a rectangular, trapezoidal or hexagonal shape extending in a direction across a scanning direction.
According to a preferred embodiment of the first aspect of the invention, a projection optical system has a slit-shaped or arc-shaped exposure area that does not intersect the optical axis of the projection optical system. The projection optical system is provided with a refractive type first optical imaging system to form a first intermediate image of a first surface; a second optical imaging system, having at least one negative lens and a concave reflective mirror, to form the first intermediate image into a second intermediate image of nearly the same magnification near the first intermediate image forming position based on the light beam from the first intermediate image; a refractive type third optical imaging system to form a reduced image of the second intermediate image onto a second surface based on the light beam from the second intermediate image; a first optical path folding mirror arranged in the optical path between the first optical imaging system and the second optical imaging system; and a second optical path folding mirror arranged in the optical path between the second optical imaging system and the third optical imaging system. In this case, the effective area of the first optical path folding mirror and the effective area of the second optical path folding mirror preferably have a reflective surface formed across the whole of the planar surface. It is preferable that the effective area of the first optical path folding mirror and the effective area of the second optical path folding mirror not have a spatial overlap, and be arranged such that the whole light beam from the first surface is guided to the second surface.
Further, according to a preferred embodiment of the first aspect of the invention, all lenses comprising the first optical imaging system and the third optical imaging system are arranged along a single straight line along the optical axis. Furthermore, in the first aspect of
Niisaka Shunsuke
Omura Yasuhiro
Owa Soichi
Ozawa Toshihiko
Shiraishi Naomasa
Fuller Rodney
Nikon Corporation
Oliff & Berridg,e PLC
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