Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-06-06
2003-10-28
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000
Reexamination Certificate
active
06639652
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an illumination system with a light source such as a high pressure Hg lamp, for example, wherein light is emitted in a range having a certain size or area. More particularly, the invention concerns an illumination system suitably usable in a projection exposure apparatus for the manufacture of semiconductor devices or liquid crystal panels, for example, in which illumination of a rectangular region is required for a reticle to be used in an exposure sequence.
Projection exposure apparatuses use an illumination system for uniformly illuminating a surface to be illuminated, by use of a light source such as a high pressure Hg lamp wherein light is emitted from a predetermined range (a range having a certain size or area) with a predetermined distribution, as disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 333795/1994. Referring to
FIG. 29
, an example using such an illumination system will be described. Denoted in the drawing at
1
is a light source, and denoted at
2
is light collecting means for collecting light emitted from the light source
1
with a predetermined angular distribution. In this example, the collecting mirror uses an elliptical mirror. The light source
1
is disposed adjacent to a first focal point of the elliptical mirror, and the light emitted from the light source
1
is collected adjacent to a second focal point
00
.
In the example of
FIG. 29
, an optical integrator is used to provide Koehler illumination, by which uniform illumination is accomplished. Namely, the light collected in the vicinity of the second focal point
00
of the elliptical mirror (collecting means
2
) is transformed by a collimator lens
3
into parallel light which then enters an optical integrator
4
in a parallel beam. The optical integrator
4
serves to produce a number of light convergence points about a light exit end thereof. In the example of
FIG. 29
, these light convergence points function as secondary light sources which with and, through an optical system
5
, an illumination region
6
is uniformly illuminated.
In order that the illumination is made in accordance with the shape of the illumination region, because of Koehler illumination, a fly's eye lens (as the optical integrator
4
) may be designed so that the emission angle from the integrator corresponds to the illumination position. For example, if the illumination region has a square shape, a fly's eye lens comprising an array of element lenses having a square sectional shape, as shown in
FIG. 30
, may be used. If the illumination region has a rectangular shape, a rectangular fly's eye lens such as shown in
FIG. 31A
, comprising element lenses of rectangular shape having the same sectional aspect ratio as that of the illumination region, may be used.
This is because, when parallel light is projected on a rectangular fly's eye lens, as schematically shown in
FIGS. 31B and 31C
, the incidence height of light rays upon the light entrance face is higher in the lengthwise section in
FIG. 31B
than in the widthwise section in
FIG. 31C
, such that the largest value of the angle defined between the light ray emitted from the exit face and the optical axis becomes larger in the lengthwise direction. Namely, øb>øc. By performing Koehler illumination while using this emitted light, a rectangular region can be illuminated efficiently.
As regards constituent elements of optical systems where the system is used in a projection exposure apparatus, denoted in
FIG. 29
at
7
is a projection lens system. Denoted at
8
is a stop of the projection lens system
7
, and denoted at
9
is a wafer substrate.
However, when a fly's eye lens is used as an optical integrator, as shown in
FIG. 32
, there is a certain limit to the incidence angle of light that can be emitted from each element lens of the fly's eye lens. More specifically, while a light ray being incident with a certain incidence angle “a” can be emitted from the light exit face of the fly's eye lens, another light ray being incident with an incidence angle “b” larger than a certain angle cannot be emitted from the fly's eye lens because it is eclipsed by the side face of the element lens.
If light rays collected by the light collecting means
2
are completely converged at a convergence point, as described above, a completely parallel light flux can be projected on the fly's eye lens by using a collimator lens and, therefore, all the light rays emitted from the light source can be projected out of the fly's eye lens. Thus, the secondary light sources can be produced very efficiently. However, in a light source such as a high pressure Hg lamp, light rays are emitted from a predetermined range having a certain size or area, and all the light rays emitted from the light source cannot be completely collected at the second focal point only by use of a single elliptical mirror. In other words, the light rays reflected by the elliptical mirror necessarily have a particular angular distribution and a particular positional distribution, on a plane orthogonal to the optical axis and adjacent to the second focal point of the elliptical mirror.
As described, when a light source such as a high pressure Hg lamp is used, the light rays emitted from the light source cannot be collected at a single point and, therefore, all the light rays cannot be transformed into a parallel light flux even by use of a collimator lens. As a result of it, upon the light entrance surface of a fly's eye lens, the light rays necessarily have a particular angular distribution. Therefore, the relationship between the incidence angle of light upon the fly's eye lens and the capability of emission thereof from the lens must be considered. If there exist some rays that cannot be emitted from the fly's eye lens, the illuminance may be lowered by the lens.
Next, an illumination system is discussed, which is arranged so that light rays (light source image) distributed at the light convergence point are imaged upon a fly's eye lens to assure uniform illumination on a surface to be illuminated. This, however, does not narrow the scope of the present invention. The way of making light rays, being distributed about the second focal point, to enter a fly's eye lens can be chosen freely. It can be done within the scope of the present invention to make light enter a fly's eye lens while using a collimator lens such as shown in FIG.
29
.
As described above, when a fly's eye lens comprising plural element lenses is used in relation to an illumination region of rectangular shape, the largest value for the incidence angle of light that can be emitted from the lens differs between that in the lengthwise direction and that in the widthwise direction of the rectangular shape. More specifically, the largest incidence angle &thgr;
b
of light that can be emitted, with respect to the lengthwise direction of
FIG. 31B
, is larger than the largest incidence angle &thgr;
c
of the light that can be emitted, with respect to the widthwise direction of FIG.
31
C. Nevertheless, as regards the light fluxes being collected by conventional light collecting means toward the convergence point, because the elliptical mirror which is used conventionally as the light collecting means has a shape being revolutionally symmetrical with respect to the optical axis, as shown in
FIG. 4
, the light rays emitted from the first focal point of the elliptical mirror define a cone about the optical axis, with its apex placed at the convergence point
00
. Although those to be taken into account are light rays which are not collected at the second focal point, since those light rays emitted from positions away from the first focal point go beside the cone and pass near the second focal point, only the cone defined by the light rays emitted from the first focal point is discussed here as a representative example. Also, in the following description, while only the light rays emit
Mori Ken-ichiro
Takahashi Kazuhiro
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