Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2000-04-14
2004-12-28
Juba, Jr., John (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S565000, C359S599000, C359S900000, C355S067000
Reexamination Certificate
active
06836365
ABSTRACT:
INCORPORATION BY REFERENCE
The disclosures of the following priority applications are incorporated herein by reference in their entireties: Japanese Patent Application No. 11-107747, filed Apr. 15, 1999; Japanese Patent Application No. 11-284498, filed Oct. 5, 1999; and Japanese Patent Application No. 2000-35910, filed Feb. 14, 2000.
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a diffractive optical element and its fabrication method, an illumination device provided with the diffractive optical element, a projection exposure apparatus, and an exposure method. In particular, the invention relates to a device that illuminates a mask pattern for a semiconductor integrated circuit, a liquid crystal device, or the like, and an exposure method using the illumination device and a projection exposure apparatus that is suitable to the illumination device.
2. Description of Related Art
A process that is generally called photolithography is used for circuit pattern formation on a semiconductor substrate or the like. In this process, a reticle (mask) pattern is transferred onto a substrate such as a semiconductor wafer. First, a photosensitive photoresist is coated on the substrate, and a circuit pattern is transferred to the photoresist by an irradiated optical image, formed, e.g., from a transparent part of a reticle pattern. Furthermore, in a projection exposure apparatus, an image of a circuit pattern to be transferred, which was formed on the reticle, is projected and exposed onto the substrate (wafer) via a projection optical system. An illumination optical system of this projection exposure apparatus includes an optical integrator such as a fly eye lens to make an intensity distribution of the illumination light irradiated onto the reticle homogeneous. The following describes the reason why the intensity distribution of the illumination light irradiated onto the reticle is made homogeneous by using an optical integrator such as a fly eye lens.
FIG. 27A
is a schematic diagram of an optical system of a projection exposure apparatus using a fly eye lens. The light beam generated from a light source (e.g., a KrF excimer laser)
100
is guided to a fly eye lens
103
via a beam expander optical system
101
and an oscillating mirror
102
. Furthermore, the light emitted from the fly eye lens passes through an aperture diaphragm and illuminates a reticle
105
via a condenser lens
104
. The pattern on the reticle
105
is then projected by a projection optical system
106
onto a substrate
107
. The surface of the reticle
105
and the input surfaces of the respective lenses that constitute the fly eye lens
103
are located at conjugate positions relative to the condenser lens
104
. Accordingly, the light beam input to the fly eye lens is divided by element lens units of the fly eye lens, and the divided light beams are then overlapped on the reticle surface. Because of this, even if there is a significant distribution of contrast difference in, for example, a Gaussian distribution of the light beam input to the fly eye lens, this distribution does not become significant at the element lens units of the fly eye lens, and is made to be uniform on the reticle surface because they overlap each other, and illumination distribution with extremely high homogeneousness can be obtained on the reticle surface
105
.
A system is conventionally known in which processing such as beam splitting and overlapping thereof is repeated twice, and this system is hereafter called a double fly eye lens system. One example of an optical system of a conventional projection exposure apparatus using a double fly eye lens is shown in
FIG. 27B. A
shape of a light beam from a light source
201
such as an excimer laser is converted into a light beam with an arbitrary cross-sectional shape via an expander
202
. The light beam then is input to a first fly eye lens (second light source)
205
formed of a plurality of optical elements via a mirror
203
and a quartz prism
204
for alleviating polarization of the light beam, and a plurality of second light source images are formed at the output surface of the first fly eye lens
205
. The light beams output from the plurality of second light sources are condensed by a relay lens
206
and are superimposed on each other so as to homogeneously illuminate the input surface of a second fly eye lens
207
. As a result, a number of third light source images that is equal to the product of the number of lens elements of the first fly eye lens and the number of lens elements of the second fly eye lens can be formed. Furthermore, the diameter of the light beam from the third light source is restricted by a diaphragm
208
, condensed by condenser lens groups
209
and
211
(which includes bending mirror
212
), and are superimposed so as to homogeneously illuminate a pattern on a reticle or mask
213
. Here, a field diaphragm
210
to determine an illumination area is arranged among the condenser lens groups
209
and
211
. Furthermore, based on the illumination light that has been homogeneously illuminated, a pattern formed on the reticle or the mask
213
is projected onto a substrate
215
, which is the object of optical exposure, via projection lenses
214
.
The characteristics of a system called a double fly eye lens as compared to a system using only one fly eye lens are described below. Furthermore, in order to simplify the description, the system using only one fly eye lens is called a single fly eye lens system.
(1) With respect to the effect that makes the illumination light that illuminates a reticle homogeneous, the greater the number of divisions of the fly eye lens (i.e., the more lens units within the fly eye lens), the more significant the effect becomes. However, the fabricating cost of the fly eye lens is substantially proportional to the number of divisions of the fly eye lens. Because of this, if many beam splittings are implemented by a single fly eye lens system, the fabricating cost of the lens becomes unacceptable. In the double fly eye lens system, the number of divisions of the first fly eye lens multiplied by the number of divisions of the second fly eye lens becomes the total number of divisions of the optical system. Accordingly, in a double fly eye lens system, there is an advantage that an illumination system with high performance can be obtained without unacceptably high fabricating cost. For example, if the first fly eye lens has 100 divisions and the second fly eye lens has 100 divisions, an illumination system that is equivalent to 10,000 (=100×100) divisions can be obtained at the fabricating cost of two lenses with 100 divisions.
(2) In a single fly eye lens system, the light distribution of the light source is input to the fly eye lens as-is. Therefore, if the light distribution changes with oscillation of the light source or the like, the spatial coherence of the projection exposure apparatus changes, which is not desirable. However, in the double fly eye lens system, the light distribution input to the second fly eye lens has been made homogeneous by the first fly eye lens. Accordingly, the light distribution hardly changes even if the light source is oscillated or the like. Therefore, there is an advantage such that it is difficult to affect the image performance even if oscillation or the like is generated in the light source.
(3) Another advantage of the double fly eye lens system is that the amount of change of the illumination homogeneousness when the aperture diaphragm is replaced, that is, the amount of change from an ideal Koehler illumination state, is less.
In addition to the above considerations, performance capability such as, e.g., resolution, which is demanded for these exposure apparatus has been approaching the theoretical limit. As is generally well known, a setting value of the optimum constant (e.g., numerical aperture of a projection lens, and numerical aperture of an illumination system, or the like) of the optical system varies depending on the pattern of a reticle
Jr. John Juba
Nikon Corporation
Oliff & Berridg,e PLC
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