Electrophotography – Diagnostics – Document handling
Reexamination Certificate
1998-03-19
2001-09-04
Epps, Georgia (Department: 2873)
Electrophotography
Diagnostics
Document handling
C359S619000, C359S620000
Reexamination Certificate
active
06285855
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an illumination system, an exposure apparatus and a device manufacturing method. More particularly, the invention is concerned with an illumination system, an exposure apparatus and a device manufacturing method, wherein an excimer laser for emitting light in the ultraviolet region is used as a light source, for illuminating uniformly the surface of a wafer or the surface of a reticle where a fine pattern such as an electronic circuit pattern is formed.
In a semiconductor chip manufacturing process, fine patterns formed on different masks are sequentially transferred to and superposed on the surface of a wafer. To this end, an illumination system of an exposure apparatus illuminates a mask or reticle placed at a position optically conjugate with the surface of a wafer, whereby a pattern of the mask is projected and transferred onto the wafer surface through a projection lens.
The quality of an image transferred to the wafer is largely influenced by the performance of the illumination system, e.g., the uniformness of the illuminance distribution upon the mask surface or wafer surface.
Japanese Laid-Open Patent Application, Laid-Open No. 913/1989, No. 295215/1989, No. 271718/1989, or No. 48627/1990 proposes an illumination system wherein the uniformness of the illuminance distribution is improved by the use of an inside reflection type integrator and a wavefront division type integrator.
FIG. 13
is a schematic view of a portion of an illumination system which uses an inside reflection type integrator and a wavefront division type integrator.
In
FIG. 13
, the laser beam emitted by a laser light source
101
is once converged by a lens system
107
at a position close to the light entrance surface of an optical pipe (inside reflection type integrator)
110
, and then it is diverged such that it enters the optical pipe
110
with a predetermined divergence angle defined with respect to the inside reflection surface of the optical pipe
110
.
The laser beam divergently incident on the optical pipe is propagated within the optical pipe
110
while being reflected by the inside surface thereof. Thus, the optical pipe
110
functions to form a plurality of virtual images, related to the laser light source
101
, on a plane (e.g., plane
113
) which is perpendicular to the optical axis.
On the light exit surface
110
′ of the optical pipe
110
, plural laser light fluxes which appear as coming from the virtual images, that is, as emitted from plural apparent or seeming light sources, are superposed one upon another. As a result of this, a surface light source having a uniform light intensity distribution is defined on the light exit surface
110
′ of the optical pipe
110
.
By means of a condenser lens
105
, an aperture stop
111
and a field lens
112
, the light exit surface
110
′ of the optical pipe
110
and a light entrance surface
106
of a fly's eye lens (wavefront division type integrator) are placed in an optically conjugate relation with each other. Thus, the surface light source with a uniform intensity distribution at the light exit surface
110
′ is imaged on the light entrance surface
106
of the fly's eye lens. As a result, such light as having a uniform sectional intensity distribution enters the fly's eye lens. The fly's eye lens serves to define plural light sources (secondary light sources) at its light exit surface. Light beams from these secondary light sources are superposed by a condenser lens (not shown) one upon another, on the surface of a reticle, not shown. Thus, the pattern of the reticle as a whole is illuminated with a uniform intensity.
The illumination system of
FIG. 13
is provided with an aperture stop, disposed just after the fly's eye lens and having a fixed shape and a fixed diameter. Thus, the numerical aperture of the illumination system (the size of the secondary light source) is fixed and, therefore, the state of illumination is unchangeable with the size of the smallest pattern of the reticle.
Further, in the illumination system of
FIG. 13
, if the laser light source
101
comprises such a light source (as a certain type excimer laser) wherein the path of laser beam LB shifts in a direction perpendicular to the optical axis AX, a minute change of the optical path may cause a change in intensity distribution of light fluxes LF, impinging on respective points
106
on the light entrance surface of the fly's eye lens. This results in a change in the illuminance distribution upon the reticle.
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide an illumination system with an inside reflection type integrator and a wavefront division type integrator, wherein the state of illumination is changeable.
An illumination system according to this aspect may include a variable-magnification imaging optical system disposed just before a wavefront division type integrator. However, if the imaging magnification changes, the open angle NA of the light flux LF changes. Particularly, when the magnification decreases, the open NA of the light flux may become larger, excessively beyond the range allowed by the lenses of the fly's eye lens. On that occasion, a portion of the light entering the lens element is eclipsed within the lens element, such that some light does not emit toward the required range (direction). This causes a reduction of the light quantity for illuminating the reticle.
It is accordingly a second object of the present invention to provide an illumination system with a wavefront division type integrator, wherein even in such a case (regardless of whether the system is equipped with an inside reflection type integrator or not) a decrease in the quantity of light illuminating a mask or reticle is substantially prevented.
It is a third object of the present invention to provide an illumination system with an inside reflection type integrator and a wavefront division type integrator, wherein the illuminance distribution upon the surface being illuminated is unchanged even if the path of light from a light source shifts.
In accordance with an aspect of the present invention, to achieve the first object, there is provided an illumination system, comprising: a first optical integrator of an inside reflection type, for reflecting at least a portion of received light, with its inside surface, and for defining a surface light source at or adjacent to a light exit surface thereof; a second optical integrator of a wavefront division type, for dividing the wavefront of received light and for defining a plurality of light sources at or adjacent to a light exit surface thereof; an imaging optical system for imaging the surface light source at or adjacent to a light entrance surface of said second optical integrator; and a collecting optical system for superposing light rays from said plurality of light sources one upon another, on a surface to be illuminated; wherein said imaging optical system has a variable imaging magnification.
In accordance with another aspect of the present invention, to achieve the second object, there is provided an illumination system, comprising: a wavefront division type optical integrator for dividing the wavefront of received light and for defining a plurality of light sources at or adjacent to a light exit surface thereof; a light projecting optical system for projecting light from a light source to a light entrance surface of said optical integrator, and a collecting optical system for superposing light rays from said plurality of light sources one upon another, on a surface to be illuminated; wherein said light projecting optical system has a focal length which is changeable to cause a change of at least one of the size and the intensity distribution of the light, from the light source, upon the light entrance surface of said optical integrator; and wherein said light projection optical system serves to correct a change in an open angle of light, impinging on said wavefro
Canon Kabushiki Kaisha
Epps Georgia
Fitzpatrick ,Cella, Harper & Scinto
Lucas Michael A
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