Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1999-01-27
2003-12-16
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S067000, C250S492100
Reexamination Certificate
active
06665052
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an illumination optical system and, more particularly, to an illumination optical system suitably usable in an illumination optical device of a projection exposure apparatus for the manufacture of semiconductor devices, for example.
In projection exposure apparatuses for the manufacture of semiconductor devices, a circuit pattern formed on a reticle (mask) is projected onto a semiconductor substrate such as a wafer through a projection optical system, by which projection exposure is performed.
In conventional projection exposure apparatuses, light from a light source having a predetermined luminance distribution or from an image of such a light source is projected on a fly's eye lens. With secondary light sources as defined in the neighborhood of the light exit surface of the fly's eye lens, a condenser optical system illuminates a reticle having a circuit pattern formed thereon. In this manner, the surface to be illuminated (such as a reticle or a wafer) is illuminated uniformly.
It is known that, in the projection through a condenser optical system, as a condition for providing a uniform numerical aperture of an illumination optical system upon the surface to be illuminated, the fly's eye lens should satisfy the sine condition. If the focal length of small lens elements constituting the fly's eye lens is F, the largest incidence height of the illumination optical system parallel to the optical axis of the small lens elements is h, and the angle defined, with respect to the optical axis, by an emitted light ray having been incident at the largest incidence height is &thgr;, then the sine condition can be expressed by:
h=F·
sin &thgr;.
Generally, a fly's eye lens comprises small lens elements of a rod-like shape having convex spherical surfaces at its opposite ends, being disposed two-dimensionally. With such a fly's eye lens, the sine condition as above cannot be satisfied sufficiently and, therefore, there is a tendency that due to insufficient satisfaction of the sine condition the illuminance decreases from the center of the surface being illuminated toward its periphery.
Further, an anti-reflection film used in a lens has a characteristic that it reflects lights more with a larger incidence angle of light. Since generally light rays passing through the periphery of a lens have a larger incidence angle, also there is a tendency that the illuminance upon the surface being illuminated decreases from the center toward the periphery.
In consideration of this, in conventional projection exposure apparatuses, in order to prevent non-uniformness of illuminance wherein the illuminance at the periphery of the surface being illuminated is lower than that at the center thereof and thereby to attain uniform illuminance upon the surface being illuminated, a condenser optical system is used to positively produce distortion aberration such as shown in
FIG. 15A
, to thereby prevent a decrease in quantity of light impinging on the peripheral portion.
When a condenser optical system effective to produce distortion aberration of
FIG. 15A
is used, due to an unchangeable quantity of Helmholz-Lagrange, the secondary light sources as can be observed from various points on the surface being illuminated differ in size, between points on the axis and off the axis, such as shown in FIG.
15
B. For example, in
FIG. 15B
, the secondary light sources as can be observed at different points have the same shape, at any point within the picture field. However, if the largest light intensity is 100 and comparing diameters of contour lines of light intensity
20
, it is seen that the diameter is larger at points off the axis, as compared with the diameter at a point on the axis. Namely, in conventional projection exposure apparatuses, a condenser lens is used to positively produce distortion aberration and, as a result of this, secondary light sources observed on the axis and off the axis, upon the surface being illuminated, differ in size (&sgr; value).
In recent projection exposure apparatuses for the manufacture of semiconductor devices, enlargement of integration of VLSI has required a higher image uniformness over the whole surface being illuminated, than required before. In order to meet this, the illumination optical system should provide uniformness of illuminance upon the surface to be illuminated as well as uniformness, over the whole surface to be illuminated, in size of secondary light sources as can be observed from various points on the surface being illuminated.
Here, if the numerical aperture of an illumination optical system is NA
il
and the numerical aperture of a projection optical system is NA
po
, the secondary light sources (effective light sources) can be expressed by a value:
&sgr;=NA
il
/NA
po
For example, in accordance with results of experiments made to a projection exposure apparatus under the condition that the wavelength is 248 nm and the image side numerical aperture NA is 0.6, it has been found that, when &sgr; changes by 0.1, the linewidth of an isolated line of 0.3 micron linewidth changes by about 20 nm. From the uniformness of linewidth within the picture field as currently being required, the effect of a secondary light source (&sgr;) difference in the surface to be illuminated, upon the linewidth, should be kept low of about 5 nm or less. In order to attain this, if the largest diameter of those of the secondary light sources observed at various points on the surface being illuminated, which can be regarded as having the same intensity ratio, is denoted by &sgr;
max
while the smallest diameter thereof is denoted by &sgr;
min
, preferably the balance difference of &sgr;
max
and &sgr;
min
should be not greater than 3%, at the maximum.
However, with illumination optical systems conventionally used in projection exposure apparatuses, it is difficult to satisfy both the uniformness of illuminance and the uniformness of illumination numerical aperture, upon the surface being illuminated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an illumination optical system and/or a projection exposure apparatus by which both the uniformness of illuminance and the uniformness of illumination numerical aperture, upon the surface being illuminated, can be improved.
In accordance with a first aspect of the present invention, there is provided an illumination optical system, comprising: secondary light source forming means for forming secondary light sources by use of light from a light source; and optical means for illuminating a surface to be illuminated, with light from the secondary light sources; wherein the secondary light sources as seen from points on the surface illuminated have substantially the same numerical aperture and wherein illuminance on the surface illuminated is substantially uniform.
In accordance with a second aspect of the present invention, there is provided an illumination optical system, comprising: secondary light source forming means for forming secondary light sources by use of light from a light source; optical means for illuminating a surface to be illuminated, with light from the secondary light sources, wherein said optical means is arranged to assure that the secondary light sources as seen from points on the surface illuminated have substantially the same numerical aperture; and illuminance uniforming means effective to make the illuminance on the surface illuminated uniform.
In accordance with a third aspect of the present invention, there is provided an illumination optical system, comprising: a fly's eye lens for forming secondary light sources by use of light from a light source, and optical means for illuminating a surface to be illuminated, with light from the secondary light sources, wherein said fly's eye lens has a plurality of small lens elements each comprising an aspherical surface lens.
In accordance with a fourth aspect of the present invention, there is provided an illumination optical
Adams Russell
Fitzpatrick ,Cella, Harper & Scinto
Kim Peter B.
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