Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1999-04-27
2002-04-23
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000
Reexamination Certificate
active
06377337
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus for exposing photosensitive substrates, such as silicon plates and glass, to light through patterns designed for devices, such as semiconductors including an IC, an LSI, etc., a liquid crystal panel, a magnetic head, a CCD (image sensor), and so on.
2. Description of the Related Art
In manufacturing an IC, an LSI, a liquid crystal element, etc., by photolithography, a projection exposure apparatus (projection aligner) is employed. The projection exposure apparatus is arranged to perform an exposure by projecting through a projection optical system a pattern of a photomask or a reticle (hereinafter referred to as a “mask”) onto a substrate, such as a silicon plate or a glass plate, which is coated with a photoresist or the like (hereinafter referred to as a “wafer” in general).
FIG. 1
schematically illustrates the arrangement of a conventional projection exposure apparatus. In
FIG. 1
, there are illustrated a KrF excimer laser
191
used as a light source, an illumination optical system
192
, illumination light
193
, a mask
194
, exposure light
195
on the object side, a projection optical system
196
, exposure light
197
on the image side, a photosensitive substrate (wafer)
198
, and a substrate stage
199
which holds the photosensitive substrate
198
.
In the conventional projection exposure apparatus, a laser beam emitted from the excimer laser
191
is led to the illumination optical system
192
. At the illumination optical system
192
, the laser beam is converted into the illumination light
193
having a light intensity distribution, a luminous distribution, etc., which are predetermined. The illumination light
193
falls on the mask
194
. A circuit pattern which is to be eventually formed on the photosensitive substrate
198
is beforehand formed on the mask
194
with chromium or the like. The incident illumination light
193
passes through the mask
194
and is diffracted by the circuit pattern to become the object-side exposure light
195
. The projection optical system
196
converts the exposure light
195
into the image-side exposure light
197
to image the circuit pattern on the photosensitive substrate
198
at a predetermined magnification with sufficiently small aberrations. As shown in an enlarged view at the lower part of
FIG. 1
, the image-side exposure light
197
converges on the photosensitive substrate
198
at a predetermined NA (numerical aperture=sin &thgr;) to form the image there. To have the circuit pattern formed in a plurality of shot areas on the photosensitive substrate
198
, the substrate stage
199
is arranged to be movable stepwise to vary the relative positions of the photosensitive substrate
198
and the projection optical system
196
.
However, with the conventional projection exposure apparatus using the KrF excimer laser arranged as described above, it is difficult to form a pattern image of a line width not greater than 0.15 &mgr;m.
The reason for this difficulty is as follows. The resolution of the projection optical system is limited by a trade-off between an optical resolution and the depth of focus due to the wavelength of the exposure light. The resolution R of the resolving pattern of the projection exposure apparatus and the depth of focus DOF can be expressed by the following Rayleigh's formulas (1) and (2):
R
=
k1
λ
NA
(
1
)
DOF
=
k2
λ
NA
2
(
2
)
In the above formulas, &lgr; represents the wavelength of the exposure light, NA represents a numerical aperture indicative of the brightness of the optical system on the light exit side, and k
1
and k
2
represent constants which are normally between 0.5 and 0.7.
According to the formulas (1) and (2), in order to make the resolution R smaller for a higher degree of resolution, it is necessary either to make the wavelength &lgr; smaller for a shorter wavelength or to make the value NA larger for a higher degree of brightness. At the same time, however, the depth of focus DOF required for a necessary performance of the projection optical system must be kept at least at a certain value. This requirement imposes some limitation on the increase of the brightness value NA.
There is another known exposure method which does not depend on the formulas (1) and (2).
FIG. 2
is a schematic diagram for explaining such an exposure method. Referring to
FIG. 2
, a coherent light beam emitted from a laser beam source
151
is divided by a half-mirror
152
into two light fluxes. Mirrors
153
a
and
153
b
are arranged to deflect the two light fluxes respectively at some angles to cause the two light fluxes to join together on a photosensitive substrate
154
in such a way as to form interference fringes there. The photosensitive part of the photosensitive substrate
154
is allowed to sense light according to a distribution of light intensity made by the interference fringes. Then, a periodic protrusion-and-recess pattern is formed according to the distribution of the light intensity by a developing process.
The resolution R obtained by the above exposure method is expressed by the following formula (3), wherein the resolution R is assumed to be the width of each of lines and spaces, i.e., the width of each of the bright and dark parts of the interference fringes, &thgr; represents the angle of incidence on the substrate
154
of the two light fluxes
151
a
and
151
b,
and NA=sin &thgr;.
R
=
λ
4
sin
θ
=
λ
4
NA
=
0.25
λ
NA
(
3
)
As is understandable by comparing the formulas (3) and (1) with each other, the constant k
1
becomes 0.25 (k
1
=0.25) according to the exposure method shown in FIG.
2
. Considering that the value of the constant k
1
of the conventional projection exposure method is between 0.5 and 0.7, the resolution obtainable by the exposure method shown in
FIG. 2
is more than two times as high as the resolution obtainable by the conventional exposure method. According to the exposure method shown in
FIG. 2
, assuming that &lgr; is 0.248 &mgr;m and NA is 0.6, for example, the resolution R becomes 0.10 &mgr;m.
However, the exposure method shown in
FIG. 2
presents a serious problem in that a circuit pattern composed of diverse shapes like semiconductor element patterns hardly can be obtained by carrying out an exposure according to that method, because only such a line-and-space pattern that has a uniform pitch over its whole area is obtainable according to the method of making an exposure through the interference of light fluxes as shown in FIG.
2
.
This problem can be solved, for example, by a known multiple exposure method whereby the projection exposure by the method of FIG.
1
and the two-light-flux interference exposure by the method of
FIG. 2
are alternately made in combination for one and the same area of a photosensitive substrate one after another without carrying out any developing process on the substrate at intervals between these exposures.
However, the two-light-flux interference exposure in the conventional multiple exposure method necessitates either the use of the apparatus shown in
FIG. 2
, in addition to the projection exposure apparatus, or the use of a special mask such as a phase-shifting mask in using the projection exposure apparatus.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a projection exposure apparatus arranged to permit a multiple exposure to be simply carried out.
To attain the above object, in accordance with an aspect of the invention, there is provided a projection exposure apparatus having a multiple exposure mode, which comprises an illumination system and a projection system, the projection system including means for automatically or manually supplying into an optical path a filter which blocks a zero-order light beam among a plurality of light beams coming from a mask illuminated by the illumination system, wherein an exposure step in the multi
Sugita Mitsuro
Suzuki Akiyoshi
Adams Russell
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Hung Henry
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