Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-11-13
2004-11-23
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S030000
Reexamination Certificate
active
06822727
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an exposure apparatus and, more particularly, to an exposure apparatus used when a semiconductor device or the like is manufactured in a photolithography process.
BACKGROUND OF THE INVENTION
A demagnifying projection exposure apparatus is used widely in the fabrication of semiconductor circuits such as LSI chips. A demagnifying projection exposure apparatus forms a reduced image of a mask pattern on a wafer, which serves as a photosensitive substrate, via projection optics. The patterns of integrated circuits projected upon a semiconductor substrate to expose the same have become increasingly finer in recent years, and there is growing demand for a projection exposure apparatus having higher resolution and alignment precision.
Improved performance of equipment peripheral to the exposure apparatus is sought in order to achieve a reduction in pattern line width. Since this results in higher equipment cost, an important challenge is to reduce the initial cost and running cost of production facilities.
In order to raise the resolution of a projection exposure apparatus, it is necessary to enlarge the numerical aperture (NA) of the projection optics and to shorten the exposure wavelength thereof. However, making the NA of the projection optics greater than a certain value is difficult in terms of the structure of the optical system. Further, when the NA of the projection optics is enlarged, the utilizable focal depth decreases and, as a result, it is difficult to realize a resolution that is possible in theory. For this reason, it is especially required that the wavelength of the exposing light be reduced in order to raise the resolution of the projection exposure apparatus.
A KrF excimer laser having a wavelength of 248 nm and an ArF excimer laser having a wavelength of 193 nm have been proposed and put into practice as exposure light sources. There is also demand for a light source having a shorter wavelength on the order of 180 nm or less, especially an F
2
laser of wavelength 157.6 nm. When the wavelength of exposing light is reduced, however, absorption of the light by the optical components increases and hence, there is a limitation upon the types of glass materials that can be used as the optical components. For example, CaF
2
crystal (fluorite) alone is known as a practical refraction optical material that can be used for dealing with short-wavelength light. As a consequence, it is difficult to fabricate an optical system in which various types of aberration are limited to desired values through use solely of a refraction optical system employed heretofore in projection optics.
Further, a laser light source of wavelength 200 nm or less has a certain width even if the region of oscillation wavelength is narrowed. In order to maintain good contrast and project a mask pattern, therefore, it is required that chromatic aberration be reduced to the pm (picometer) order.
A reflection optical system utilizing a concave reflecting mirror generally is used as an optical system that reduces chromatic aberration. Further, a reflection and refraction-type optical system comprising a combination of a reflection optical system and a refraction optical system is capable of reducing various aberration inclusive, especially of chromatic aberration, without inviting an increase in the number of lenses. A demagnifying projection optical system of reflection-diffraction type has, therefore, been proposed in order to eliminate chromatic aberration produced by the range of wavelengths possessed by laser light.
[Projection Optics of Reflection-refraction-type Optical System]
A projection optical system of the type disclosed in the specification of Japanese Patent Application Laid-Open No. 8-334695, for example, is a so-called off-axis optical system in which an area offset from the optical axis is used as the optical path, the off-axis arrangement affords better image quality because there is less of a decline in quality of light and no shielding of image-forming light flux. It is also easier to fabricate the various optical members.
A similar example is a projection optical system disclosed in the specification of Japanese Patent Application Laid-Open No. 2000-195772. This optical system position a concave reflecting lens in such a manner that chromatic aberration can be corrected for in excellent fashion.
Furthermore, the specification of Japanese Patent Application Laid-Open No. 2001-27727 describes a reflection-refraction-type optical system as a projection optical system using reflection-refraction-type optics. This proposed optical system has a first image-formation optical system of reflection-refraction-type for forming an intermediate image of a first surface, and a second image-formation optical system of refraction type for forming the final image of the first surface on a second surface, telecentrically based upon light from the intermediate image. This is an optical system referred to generally as a single-barrel system. Since this projection optical system has the construction of a reflection-refraction-type optical system, the projection area of a mask pattern formed on the wafer is formed at a position off-center with respect to the center of the projection optics. Physically speaking, the center of the projection optics is the center of the optical components used in the refraction optics or the center of the optical axis of the optical system. It is the optical axis AX indicated in the specification of Japanese Patent Application Laid-Open No. 2001-27727. Further, since an intermediate image resides in the projection optics and the number of times of the reflection is an even number, the visual-field area on the side of the mask also is formed at a position that is off-center on the same side as the pattern projection area mentioned above.
Furthermore, the specification of Japanese Patent Application Laid-Open No. 2001-15405 describes a reflection-refraction-type optical system, which is referred to as a twin-barrel system, as another example of a projection optical system that relies upon reflection-refraction-type optics. This system has the usually disposed projection-optics lens barrel and an additional lens barrel of the same size placed alongside. Both lens barrels are connected by yet another lens barrel. In comparison with the disclosure of Japanese Patent Application Laid-Open No. 2001-27727, therefore, the apparatus is larger in size.
[Improvement in Alignment Precision]
With regard to achieving an improvement alignment precision, the following schemes are available for dealing with the alignment of the wafer and reticle in an exposure apparatus:
1. a TTL scheme for measuring the position of an alignment mark on the wafer via the projection optics;
2. an off-axis scheme for measuring the position of an alignment mark on the wafer directly without the intervention of the projection optics; and
3. a TTR scheme for observing the wafer and the reticle via the projection optics and detecting the relative positional relationship between the two.
An example of the TTL scheme is a method of detecting an alignment mark on the wafer using light having an alignment wavelength of non-exposing light via a projection optical system referred to as TTL-AA (Though The Lens Auto Alignment). The TTL-AA scheme is advantageous for the following reason: The amount by which the wafer stage is driven at the time of alignment measurement and at the time of exposure is small because the length of line (a so-called baseline) connecting the optical axis of the projection optics and the TTL-AA optical axis is made very short. This makes it possible to suppress measurement error that occurs owing to a change in the distance between the optical axis of the projection optics and the optical axis of the TTL-AA system caused by a change in the environment surrounding the wafer stage. In other words, the advantage is small fluctuation of the baseline.
However, when the exposing light is made short-wavelength light of which the light
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Henry Hung
LandOfFree
Exposure apparatus, method of manufacturing semiconductor... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Exposure apparatus, method of manufacturing semiconductor..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Exposure apparatus, method of manufacturing semiconductor... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3320336