Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1999-06-01
2002-05-21
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S055000, C355S077000, C250S492200, C250S492220, C430S005000, C430S311000, C430S312000
Reexamination Certificate
active
06392742
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an illumination system and a projection exposure apparatus. The present invention may be suitably used in an apparatus, called a stepper, for the manufacture of various devices such as IC's, LSI's, CCD's, liquid crystal panels or magnetic heads, for example, wherein a circuit pattern formed on an original such as a photomask or reticle (hereinafter, “reticle”) is illuminated uniformly with exposure light of ultraviolet rays or deep ultraviolet rays from an illumination system and the pattern is transferred by projection onto the surface of a wafer, which is coated with a photosensitive material.
In projection exposure apparatuses for the manufacture of semiconductor devices, a reticle having an electronic circuit pattern is illuminated with light from an illumination system, and the circuit pattern is projected and printed by a projection optical system onto the surface of a wafer. In order to attain a high resolving power, the illumination should be made to produce a uniform illuminance distribution within the range of illumination upon the reticle surface or the wafer surface.
For example, projection exposure apparatuses (steppers) may use an illumination system having a combination of a collimator lens and an optical integrator comprising small lenses arrayed at a predetermined pitch, by which exposure light is uniformly projected to the surface that is to be illuminated, such as a reticle surface or a wafer surface.
In an illumination system having such an optical integrator, a plurality of secondary light sources corresponding to the number of the small lenses is produced, and lights from these secondary light sources are superposedly projected, in different directions, to the surface to be illuminated, whereby a uniform illuminance distribution is produced.
Japanese Laid-Open Patent Applications, Laid-Open No. 193/1989, No. 295215/1989, No. 271718/1989, No. 48627/1990 and No. 270312/1998 show an illumination system having an internal reflection type integrator and an amplitude division type integrator (as described above), for enhanced uniformness of the illuminance distribution.
FIG. 9
is a schematic view of a portion of an illumination system having an internal reflection type integrator and an amplitude division type integrator, as disclosed in Japanese Laid-Open Patent Applications, Laid-Open No. 270312/1998.
In the drawing, laser light emitted from a laser light source
101
is once focused just before a light entrance surface of a light pipe
110
(internal reflection type integrator) by means of a lens system
107
. After this, the light is diverged and it enters the light pipe
110
while defining a predetermined divergence angle with respect to the inside reflection surface of the light pipe.
The laser light entering the light pipe
110
is propagated while being reflected by the inside surface of the light pipe
110
. Thus, the light pipe
110
produces a plurality of virtual images, of the laser light source
101
, upon a plane (for example, plane
113
) which is perpendicular to the optical axis.
At a light exit surface
110
′ of the light pipe
110
, plural laser beams coming from the virtual images (that is, laser beams which appear as apparently emitted from plural light sources) are superposed one upon another. As a result, a surface light source having uniform intensity distribution is produced at the light exit surface
110
′ of the light pipe
110
.
With an optical system including 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
114
(amplitude division type integrator) are placed in an optically conjugate relation with each other. Thus, the surface light source of uniform intensity distribution produced at the light exit surface
110
′ is imaged on the light entrance surface
106
of the fly's eye lens
114
, whereby light of uniform sectional intensity distribution is incident on the light entrance surface
106
of the fly's eye lens
114
. The fly's eye lens
114
produces a plurality of light sources (secondary light sources) at its light exit surface. By means of a condenser lens (not shown) the lights from these light sources are superposed one upon another on a reticle (not shown), by which the pattern of the reticle as a whole is illuminated with uniform light intensity. In
FIG. 9
, denoted at LF is the imaging light flux of the optical system (
105
,
111
,
112
) and denoted at NA is the numerical aperture of the optical system at the light exit side.
The shape of the light pipe
110
is determined by taking into account the divergence angle of the laser light from the lens system
107
as well as the length and width of the light pipe
110
, so that the laser beams projected from the light sources to various points on the light entrance surface
106
have optical path differences greater than the coherence length of the laser light. This reduces the coherence with respect to time, thereby suppressing speckle (interference fringe) produced on the light entrance surface
106
.
The manufacture of recent large-integration semiconductor devices such as VLSI requires extraordinarily high uniformness of illuminance distribution for the circuit pattern printing. On the other hand, the whole optical system is required to provide an increased transmission factor in order to reduce a loss in the quantity of exposure light. However, in the illumination system shown in
FIG. 9
, in order to produce a uniform surface light source at the light exit surface of the light pipe, the number of times of inside reflection of the divergent light should be large. To this end, the length of the light pipe should be enlarged while holding the diameter fixed. However, this elongation causes a decrease of the transmission factor due to absorption. Therefore, the length cannot be enlarged, beyond a certain limitation.
This means that improving the uniformness of the illuminance distribution may cause a decrease in the transmission factor, and that retaining a good transmission factor may demand shortening the length of the light pipe, which leads to a failure of the uniform surface light source.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an illumination system by which a light pattern having a uniform sectional light intensity distribution can be produced without considerably decreasing the transmission factor of an optical system.
It is another object of the present invention to provide a projection exposure apparatus having an illumination system such as described above.
In accordance with an aspect of the present invention, there is provided an illumination system, comprising: a light source; an emission angle preserving optical element for emitting light from said light source, at a certain emission angle; a collecting optical system for collecting the light from said emission angle preserving optical element; a pattern forming optical system having at least one of a relay optical system and a diffractive optical element, for producing, on a predetermined plane, a light pattern of desired shape having a uniform light intensity distribution, by use of light from said light collecting optical system; multiple-beam producing means; a zooming optical system for projecting the light intensity distribution on the predetermined plane, onto a light entrance surface of said multiple-beam producing means at a predetermined magnification; and illuminating means for superposedly projecting lights from a light exit surface of said multiple-beam producing means, upon a surface to be illuminated.
In one preferred form of this aspect of the present invention, another emission angle preserving optical element having a different divergence angle is used interchangeably in response to a change in projection magnification when the light from the predetermined plane is projected by said zooming o
Adams Russell
Brown Khaled
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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