U.S. patent application number 10/321557 was filed with the patent office on 2003-06-26 for pulse-width extending optical systems, projection-exposure apparatus comprising same, and manufacturing methods using same.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Kudo, Yuji.
Application Number | 20030117601 10/321557 |
Document ID | / |
Family ID | 26401031 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030117601 |
Kind Code |
A1 |
Kudo, Yuji |
June 26, 2003 |
Pulse-width extending optical systems, projection-exposure
apparatus comprising same, and manufacturing methods using same
Abstract
This invention pertains to systems for extending the pulse
length of pulsed sources of optical radiation. These systems reduce
peak optical pulse power without reducing average optical power.
The pulse-width extending systems split optical pulses into pulse
portions, introduce relative delays among the pulse portions, and
then redirect the pulse portions (or portions thereof) along a
common axis. Such pulse-width extending systems are especially
useful in projection-exposure apparatus for the manufacture of
semiconductor devices where short wavelength, high power optical
sources tend to damage optical components.
Inventors: |
Kudo, Yuji; (Kawasaki-shi,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. Box 19928
Alexandria
VA
22320
US
|
Assignee: |
Nikon Corporation
|
Family ID: |
26401031 |
Appl. No.: |
10/321557 |
Filed: |
December 18, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10321557 |
Dec 18, 2002 |
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09455271 |
Dec 6, 1999 |
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6549267 |
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09455271 |
Dec 6, 1999 |
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08802951 |
Feb 21, 1997 |
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Current U.S.
Class: |
355/53 ; 355/67;
355/69; 355/71 |
Current CPC
Class: |
G03B 27/42 20130101;
H01S 3/0057 20130101; G03F 7/7055 20130101; Y10S 372/70
20130101 |
Class at
Publication: |
355/53 ; 355/67;
355/71; 355/69 |
International
Class: |
G03B 027/42 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 1996 |
JP |
8-059970 |
Claims
What is claimed is:
1. A light source apparatus comprising: a light source unit
supplying a light pulse having a wavelength not more than 193 nm;
and a pulse-width extending optical system arranged in an optical
path of the light pulse supplied from the light source unit, which
extends a duration of the light pulse supplied from the light
source unit while decreasing a peak power of the light pulse, the
pulse-width extending optical system including an optical surface
having a multi-layer coating to divide the light pulse into at
least two light pulses.
2. A light source apparatus according to claim 1, wherein the
pulse-width extending optical system comprises a beam dividing
member to divide the light pulse into multiple pulse portions, and
a beam deflecting system disposed to direct at least one of the
pulse portions to the beam dividing member, wherein the beam
dividing member includes the optical surface having the multi-layer
coating.
3. A light source apparatus according to claim 2, wherein the beam
deflecting system comprises a relay optical system to form an image
of the optical surface on the optical surface.
4. A light source apparatus according to claim 2, wherein the beam
deflecting system comprises a relay optical system disposed to
produce a magnification of about +1 or -1.
5. A light source apparatus according to claim 1, wherein the
pulse-width extending optical system comprises a relay optical
system to form an image of the optical surface on the optical
surface.
6. A light source apparatus according to claim 1, wherein the
pulse-width extending optical system comprises a relay optical
system disposed to produce a magnification of about +1 or -1.
7. A light source apparatus according to claim 1, wherein the light
source unit comprises an excimer laser source.
8. A light source apparatus according to claim 1, wherein the
pulse-width extending optical system provides a pulse length of at
least 3 m with respect to the divided pulse portions.
9. A light source apparatus comprising: a light source unit
supplying a light pulse having a wavelength not more than 193 nm;
and a pulse-width extending optical system arranged in an optical
path of the light pulse supplied from the light source unit, which
extends a duration of the light pulse supplied from the light
source unit while decreasing a peak power of the light pulse, the
pulse-width extending optical system including a polarizing beam
splitter to divide the light pulse into at least two light
pulses.
10. A light source apparatus according to claim 9, wherein: the
pulse-width extending optical system further comprises a beam
converting member changing a polarized beam condition and at least
one reflecting member.
11. A light source apparatus according to claim 9, wherein the
pulse-width extending optical system further comprises a beam
deflecting system disposed to direct at least one of the pulse
portions to the polarizing beam splitter, wherein the polarizing
beam splitter includes an optical dividing surface having a
multi-layer coating.
12. A light source apparatus according to claim 11, wherein the
beam deflecting system comprises a relay optical system to form an
image of the optical dividing surface on the optical dividing
surface.
13. A light source apparatus according to claim 11, wherein the
beam deflecting system comprises a relay optical system disposed to
produce a magnification of about +1 or -1.
14. A light source apparatus according to claim 9, wherein the
pulse-width extending optical system comprises a relay optical
system to form an image of an optical surface on the optical
surface.
15. A light source apparatus according to claim 9, wherein the
pulse-width extending optical system comprises a relay optical
system disposed to produce a magnification of about +1 or -1.
16. A light source apparatus according to claim 9, wherein the
light source unit comprises an excimer laser source.
17. A light source apparatus according to claim 9, wherein the
pulse-width extending optical system provides a pulse length of at
least 3 m with respect to the divided pulse portions.
18. A light source apparatus comprising: a light source unit
supplying a light pulse having a wavelength not more than 193 nm;
and a pulse-width extending optical system arranged in an optical
path of the light pulse supplied from the light source unit, which
extends a duration of the light pulse supplied from the light
source unit while decreasing a peak power of the light pulse, the
pulse-width extending optical system including a beam converting
member changing a polarized beam condition.
19. A light source apparatus according to claim 18, wherein the
beam converting member includes a half wavelength plate.
20. A light source apparatus according to claim 18, wherein the
light source unit comprises an excimer laser source.
21. A light source apparatus according to claim 18, wherein the
pulse-width extending optical system provides a pulse length of at
least 3 m with respect to divided pulse portions.
22. A light source apparatus comprising: a light source unit
supplying a light pulse having a wavelength not more than 193 nm;
and a pulse-width extending optical system arranged in an optical
path of the light pulse supplied from the light source unit, which
extends a duration of the light pulse supplied from the light
source unit while decreasing a peak power of the light pulse, the
pulse-width extending optical system including a light split member
having a light split surface with a reflectance R such that
29.3%<R<50% or a transmittance T such that
29.3%<T<50%.
23. A light source apparatus according to claim 22, wherein the
pulse-width extending optical system comprises a beam dividing
member to divide the light pulse into multiple pulse portions, and
a beam deflecting system disposed to direct at least one of the
pulse portions to the beam dividing member, wherein the beam
dividing member includes the light split surface having a
multi-layer coating.
24. A light source apparatus according to claim 23, wherein the
beam deflecting system comprises a relay optical system to form an
image of the light split surface on the light split surface.
25. A light source apparatus according to claim 23, wherein the
beam deflecting system comprises a relay optical system disposed to
produce a magnification of about +1 or -1.
26. A light source apparatus according to claim 22, wherein the
pulse-width extending optical system comprises a relay optical
system to form an image of the light split surface on the light
split surface.
27. A light source apparatus according to claim 22, wherein the
pulse-width extending optical system comprises a relay optical
system disposed to produce a magnification of about +1 or -1.
28. A light source apparatus according to claim 22, wherein the
light source unit comprises an excimer laser source.
29. A light source apparatus according to claim 22, wherein the
pulse-width extending optical system provides a pulse length of at
least 3 m with respect to divided pulse portions.
Description
[0001] This is a continuation of application Ser. No. 08/802,951,
filed Feb. 21, 1997, which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention pertains to pulse-width extending optical
systems and a projection-exposure apparatus using such optical
systems. More specifically, the invention pertains to optical
systems that extend the duration of light pulses emitted by a
pulsed laser, and to projection-exposure apparatus using such
optical systems. The invention also pertains to methods for
manufacturing semiconductor devices using such projection-exposure
apparatus.
BACKGROUND OF THE INVENTION
[0003] FIG. 16 is a block diagram of a conventional
microlithography projection-exposure apparatus comprising a pulsed
laser. This projection-exposure apparatus comprises a laser 1 that
emits light pulses. A beam-shaping optical system 2 shapes the
cross section of the beam. The beam then enters a fly-eye lens 3.
The fly-eye lens 3 divides the incident laser beam into multiple
secondary light sources, one such secondary light source being
formed at the rear focal point of the fly-eye lens 3. An aperture 4
limits the beam, and a condenser lens 5 uniformly illuminates a
mask 6 with the beam. Typically the mask 6 contains a
high-resolution pattern with extremely small features, e.g.,
patterns for semiconductor integrated circuits. A projection
optical system 7 projects the pattern of the mask 6 on a wafer 8.
The projected pattern may be either demagnified (reduced) or
magnified (enlarged).
[0004] The resolution of the pattern of the mask 6 as projected on
the wafer 8 depends on the wavelength of the light from the laser
1. The laser 1 emits light having as short a wavelength as possible
in order to form high-resolution patterns on the wafer 8.
[0005] Many lasers emitting short wavelengths of light emit pulses
of light. The peak optical powers and intensities of pulsed lasers
are very much larger than their average optical powers. (Optical
power is defined as optical energy per unit time; optical intensity
is optical power per unit area.) For example, for an ArF excimer
laser which emits light at a wavelength of 193 nm and which has a
beam cross section of 20 mm by 5 mm, typical peak pulse intensities
during pulses are on the order of 10 MW/cm.sup.2.
[0006] Short-wavelength radiation tends to cause changes in optical
materials. These changes include increased absorption by the
materials and radiation-induced changes in refractive index. These
changes are frequently irreversible. In addition, such changes are
more readily produced by high power and high-intensity radiation in
comparison with radiation of similar average power but lesser peak
values. For this reason, systems using short-wavelength lasers
often suffer from radiation-induced changes to their optical
elements.
[0007] Conventional projection-exposure apparatus using
short-wavelength lasers also exhibit astigmatism caused by
variations in the refractive indices of the lens material of the
projection optical system. Such astigmatism significantly degrades
the resolution of the projection optical systems.
[0008] For example, the ArF excimer laser (emission wavelength of
193 nm) is a suitable short-wavelength laser. Only a few refractive
optical materials are appropriate for use in optical systems with
this short wavelength. The most commonly used materials are
synthetic fused quartz and fluorite. Both of these materials show a
gradual decline in transmissivity when irradiated by light of
intensities greater than certain threshold intensities. In order to
prevent a decline in transmissivity, the optical systems of
projection-exposure apparatus frequently enlarge the diameter of
the light beam so as to reduce the intensity of optical pulses on
the lenses.
SUMMARY OF THE INVENTION
[0009] This invention provides pulse-width extending optical
systems, projection-exposure apparatus comprising such systems, and
manufacturing methods using such projection-exposure apparatus. The
pulse-width extending systems lower laser peak powers in the
optical system without reducing average laser power. For
convenience, optical pulse width is defined as the time during
which an optical pulse has an intensity greater than one-half of
the maximum value of the intensity. It is also convenient to define
an optical pulse length as the distance traveled by an optical
pulse in a time equal to its pulse width.
[0010] In a preferred embodiment of a pulse-width extending optical
system according to the invention, a beamsplitter splits an
incident laser pulse into two or more pulses. The pulses propagate
along optical paths such that they are delayed with respect to each
other. A beamsplitter then receives the delayed pulses and directs
them along a common output optical path. In the example
embodiments, one or more beamsplitters split and combine the
pulses.
[0011] Because the split pulses are delayed, the peak power at the
output is reduced because optical pulse energy from the original
pulse arrives over a time period that is longer than the original
pulse duration. The delay among the pulses is set by causing the
split pulses to travel different optical paths. To effectively
reduce the laser power, the optical path differences are preferably
greater than the pulse length.
[0012] Pulse-width extending optical systems according to the
invention also preferably comprise relay systems operable to ensure
that the delayed pulses maintain appropriate beam cross-sections
and do not become large because of the natural divergence of light
beams. The relay systems provide an additional benefit. If a relay
system inverts a beam image, then beam uniformity is improved
because the relay system overlaps a beam and an inverted image.
[0013] According to another aspect of the present invention,
projection-exposure apparatus are provided comprising pulse-width
extending optical systems as summarized above. In such a
projection-exposure apparatus, the pulse-width extended radiation
is directed to a multi-source image mechanism to form multiple
images of the pulse-width extended radiation. A condenser then
illuminates the mask substantially uniformly using the multi-source
images. This projection-exposure apparatus can be advantageously
used for manufacturing semiconductor devices.
[0014] The foregoing and other objects, features, and advantages of
the invention will become more apparent from the following detailed
description of the example embodiments which proceeds with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DIAGRAMS
[0015] FIG. 1 is an optical block diagram of a pulse-width
extending optical system according to a first example embodiment of
the invention.
[0016] FIG. 2 is a plot of laser power as a function of time for
the laser of the pulse-width extending optical system of FIG.
1.
[0017] FIG. 3 is a plot of laser power as a function of time for
multiple split laser pulses without temporal overlap.
[0018] FIG. 4 is a plot of power as a function of time for multiple
split laser pulses in which the optical path delay is one-half the
laser pulse-width.
[0019] FIG. 5 is an optical block diagram of a pulse-width
extending optical system according to a second example embodiment
of the invention.
[0020] FIG. 6 is an optical block diagram of a pulse-width
extending optical system according to a third example embodiment of
the invention.
[0021] FIG. 7 is an optical block diagram of a pulse-width
extending optical system according to a fourth example embodiment
of the invention.
[0022] FIG. 8 is an optical block diagram of a pulse-width
extending optical system according to a fifth example embodiment of
the invention.
[0023] FIG. 9 is an optical block diagram of a pulse-width
extending optical system according to a sixth example embodiment of
the invention.
[0024] FIG. 10 is a plot of the optical beam cross-section for the
delayed optical pulses of the optical system of the second example
embodiment.
[0025] FIG. 11 is a plot of the optical beam cross-section for an
asymmetric optical pulse from an excimer laser according to the
sixth example embodiment.
[0026] FIG. 12 is a plot of the beam cross-section of alternating
delayed optical pulses showing that even-numbered pulses are
inverted with respect to odd-number pulses in a delaying optical
system with a relay optical system such as that of the sixth
example embodiment.
[0027] FIG. 13 is an optical block diagram of a pulse-width
extending optical system according to a seventh example embodiment
of the invention.
[0028] FIG. 14 is an optical block diagram of a pulse-width
extending optical system according to an eighth example embodiment
of the invention.
[0029] FIG. 15 is an optical block diagram of a projection-exposure
apparatus comprising a pulse-width extending optical system
according to a ninth example embodiment of the invention.
[0030] FIG. 16 is an optical block diagram of a prior-art
projection-exposure apparatus employing a pulsed laser.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0031] The duration of a light pulse can be increased by splitting
it into multiple sub-pulses, causing the sub-pulses to propagate
differing distances thereby acquiring differing delays, and then
directing the sub-pulses back along a common axis. By increasing
the duration of the pulse, the peak pulse power decreases. If pulse
splitting and recombining introduces insignificant optical loss,
then the recombined pulses have approximately the same total energy
as the initial light pulse. Thus, the average optical power is
unchanged, but the peak power decreases. By decreasing the peak
optical power, optical-material degradation caused by high powers
is reduced. This is especially important for projection-exposure
apparatus utilizing pulsed lasers where optical-material
degradation degrades the performance of the projection optical
system in such an apparatus.
[0032] FIG. 1 is an optical block diagram of a pulse-width
extending optical system according to a first example embodiment.
FIG. 2 shows a representative plot of the intensity, as a function
of time, of the laser pulses as emitted by the laser 1 of the
FIG.-1 embodiment.
[0033] The pulse-width extending optical system of FIG. 1 comprises
a laser 1 that emits pulses of nearly linearly polarized light. The
polarization orientation of the pulses emitted by the laser 1 is
either perpendicular or parallel to the plane of the page of FIG.
1.
[0034] A light pulse emitted by the laser 1 strikes a dielectric
beamsplitter 10, which has a partially transmitting, dielectric
multi-layer coating. The dielectric beamsplitter 10 divides the
laser pulse into a first output pulse that is reflected by the
dielectric beamsplitter 10, and a first circulating pulse that is
transmitted through the dielectric beamsplitter 10. The first
output pulse from the dielectric beamsplitter 10 then strikes a
mirror 14 that directs the pulse to an output. The first
circulating pulse is transmitted by the dielectric beamsplitter 10
and is then reflected sequentially by mirrors 11, 12, 13 before
returning to the dielectric beamsplitter 10. The mirrors 11, 12, 13
and the dielectric beamsplitter 10 form a circulating optical path
("circulator") for the circulating pulse.
[0035] The dielectric beamsplitter 10 then divides the returning
pulse into a second output pulse and a second circulating pulse.
The second output pulse is the portion of the first circulating
pulse that is transmitted through the dielectric beamsplitter 10.
The mirror 14 reflects the second output pulse to an output. The
first circulating pulse is partially reflected by the dielectric
beamsplitter 10, forming the second circulating pulse. The second
circulating pulse follows the same optical path as the first
circulating pulse. The mirrors 11, 12, 13 sequentially reflect the
second circulating pulse and return it to the dielectric
beamsplitter 10.
[0036] The dielectric beamsplitter 10 once again divides the
returning pulse into a third output pulse and a third circulating
pulse. The third output pulse follows the optical path of the
second output pulse and the first output pulse; the mirror 14
reflects this pulse to an output. The third circulating pulse
similarly follows the optical path of the first and second
circulating pulses. The mirrors 11, 12, 13 reflect the third
circulating pulse and redirect it to the dielectric beamsplitter
10. The dielectric beamsplitter 10 once again divides the returning
pulse in the same way as the prior circulating pulses were
divided.
[0037] In this way, pulses circulate along the circulating optical
path, from mirror 11 to mirror 12 to mirror 13 and back to the
dielectric beamsplitter 10. The dielectric beamsplitter 10
transmits a portion of the circulating pulse so the successive
circulating pulses decrease in power. The output pulses are all
directed along substantially the same axis as the first output
pulse. Therefore, downstream optical systems at the output will
redirect all output pulses in same way. If the mirrors 11, 12, 13
and the dielectric beamsplitter 10 are perfect (i.e., exhibit no
loss of light energy reflecting therefrom), then all of the light
emitted by the laser 1 is eventually reflected to the output by the
mirror 14.
[0038] The dielectric beamsplitter 10 thus splits a single incident
laser pulse into multiple pulses that travel differing distances.
The dielectric beamsplitter 10 also recombines the multiple pulses
and directs them in a single direction. The circulating optical
path of the circulator is formed by the disposition of the three
mirrors 11, 12, 13 so as to provide a predetermined
optical-path-length difference or form a pulse-delay optical path.
The mirrors 11, 12, 13 have the function of an optical system that
provides an optical-path-length difference or forms a pulse-delay
optical system.
[0039] If the pulse width of the pulses from the laser 1 is
.delta.(sec), then the pulse travels a distance ("pulse length")
during a time equal to the pulse width of
3.times.10.sup.8.times..delta.[m], wherein it is assumed that the
pulse travels in a medium having a refractive index n=1. If the
optical path length of the circulator (i.e., the dielectric
beamsplitter 10 and the mirrors 11, 12, 13) is longer than the
pulse length, then the pulses delivered to the mirror 14 through
the dielectric beamsplitter 10 will have no temporal overlap. FIG.
3 shows the output-pulse power as a function of time for a
circulator in which the output pulses have no temporal overlap.
[0040] For example, if the pulse width .delta. is 10 nsec, the
pulse length is 3 m. If the optical path length of the circulator
is greater than 3 m, then the recombined pulses have no temporal
overlap. FIG. 3 shows that the circulator extends the optical pulse
width and the optical power (energy per unit of time) is
significantly reduced.
[0041] Even if the optical path length of the circulator is less
than or equal to the pulse length, if each pulse is sufficiently
delayed or advanced with respect to other pulses, then the peak
power of the recombined beam is still smaller than the peak power
of the original laser pulse entering the circulator. The peak
optical power can be effectively reduced by setting the optical
path difference to half the pulse length. As shown in FIG. 2, the
pulse width is defined as the time during which the pulse has a
power greater than one-half of its peak power. The half-pulse
length is the distance light travels during a time equal to
one-half pulse width.
[0042] FIG. 4 shows optical power as a function of time for a
series of pulses exiting the circulator when the optical path
length is one-half of the pulse length. FIG. 4 shows that the peak
power of each pulse occurs at times for which the peak powers of
adjacent pulses have fallen to less than one-half of their peak
values. This demonstrates that, even when the optical path is equal
to one-half the pulse length, the optical power is significantly
reduced.
[0043] The power in each pulse formed by the dielectric
beamsplitter 10 depends on the reflectivity of the dielectric
beamsplitter 10. The beamsplitter 10 has a dielectric multi-layer
film that is partially transmitting; absorption and other losses in
the beamsplitter are negligible. If R is the reflectivity of the
dielectric beamsplitter 10, then transmissivity T is T=1-R. The
optical power of each pulse is found using the following
equations:
E.sub.1=E.multidot.R (1)
E.sub.2=E.multidot.(1-R).multidot.(1-R) (2)
E.sub.3=E.multidot.(1-R).multidot.R.multidot.(1-R) (3)
E.sub.n=E.multidot.(1-R).multidot.R.sup.n-2.multidot.(1-R) (4)
[0044] wherein E is the power of the pulse emitted by the laser;
E.sub.1 is the power of the first output pulse; E.sub.2 is the
power of the second output pulse formed by transmission of the
first circulating pulse through the beamsplitter; E.sub.3 is the
power of the third output pulse formed by transmission of the
second circulating pulse through the beamsplitter; E.sub.n is the
power of the nth output pulse formed by transmission of the (n-1)th
circulating pulse.
[0045] As shown in FIGS. 3 and 4, the powers of the first and
second output pulses are the largest. As is apparent from the
foregoing equations (1)-(4), the relative magnitudes of the pulses
depend on the reflectivity of the beamsplitter 10. In order to
minimize the maximum optical power of the combined pulses, the
reflectivity R of the dielectric beamsplitter 10 should be chosen
so that E.sub.1 (the power of the first output pulse) and E.sub.2
(the power of the second output pulse) are nearly equal. Using the
foregoing equations (1)-(4), the reflectivity R is determined by
the following equation:
E.multidot.R=E.multidot.(1-R).multidot.(1-R)
[0046] Therefore, the reflectivity R of the dielectric beamsplitter
10 should be about 38.2 percent. With this value of reflectivity R,
the ratio of E (the power of the pulse emitted by the laser) to
E.sub.n (the power in the nth output pulse) is given by the
following equations:
first output pulse E.sub.1/E=38.2% (5)
second output pulse E.sub.2/E=38.2% (6)
third output pulse E.sub.3/E=14.6% (7)
fourth output pulse E.sub.4/E=05.6% (8)
fifth output pulse E.sub.5/E=02.1% (9)
[0047] It is difficult to keep the powers of the first and second
pulses exactly equal because the reflectivity R will generally vary
slightly from the ideal value due to manufacturing errors. If the
pulse powers are chosen so that the first and second output pulses
have powers less than 50 percent of the power of the original laser
pulse, then the reflectivity R can have a broad range of values.
Using the foregoing equations (1)-(4) and setting both E.sub.1 and
E.sub.2 to be less than 50 percent of E, the reflectivity R must
satisfy the following conditions:
E.multidot.R<0.5E (10)
E.multidot.(1-R).multidot.(1-R)<0.5E (11)
[0048] These inequalities are readily solved to find the
appropriate range of values for the reflectivity R of the
dielectric beamsplitter 10:
29.3% <R<50% (12)
[0049] Furthermore, in the first example embodiment, the laser
pulses were assumed to be linearly polarized light, but they could
be unpolarized, circularly polarized, or randomly polarized light.
However, in these cases, the reflectivity of the dielectric
beamsplitter 10 should be the same for all polarizations. For
example, the reflectivity for both s-polarization and
p-polarization should be equal. However, even if the s-polarization
and p-polarization reflectivities are different, a similar
reduction in power is obtained if the average of these
reflectivities satisfies the inequality (12). The relative
percentage of s-polarization and p-polarization in the first pulse
of light will differ from the percentage of s-polarization and
p-polarization in the second pulse of light, but the optical power
will be reduced.
[0050] FIG. 5 shows a block diagram of a pulse-width extending
optical system according to a second example embodiment of the
invention. This embodiment is similar to the first example
embodiment, but the action of the transmissivity and the
reflectivity of the half mirror comprising the means for splitting
light and the means for synthesizing light are reversed. The second
example embodiment is explained below pointing out the differences
with respect to the first example embodiment.
[0051] FIG. 5 shows certain features of the second example
embodiment. A laser 1 emits a light pulse that strikes a dielectric
beamsplitter 20 comprising a partially reflecting dielectric
multi-layer film. The beamsplitter 20 splits the laser pulse into a
first transmitted pulse and a first circulating pulse. The first
transmitted pulse is output. The first circulating pulse is then
sequentially reflected by mirrors 21, 22, 23, 24. The mirror 24
reflects the first circulating pulse to the dielectric beamsplitter
20.
[0052] The dielectric beamsplitter 20 then splits the first
circulating pulse. The transmitted portion of the first circulating
pulse becomes a second circulating pulse; the reflected portion is
a second output pulse that is directed to the output. The second
circulating pulse follows the same optical path as the first
circulating pulse and is sequentially reflected by the mirrors 21,
22, 23, 24. The mirror 24 reflects the second circulating pulse to
the dielectric beamsplitter 20.
[0053] The dielectric beamsplitter 20 then splits the second
circulating pulse into a third output pulse and a third circulating
pulse. The third output pulse is the portion of the second
circulating pulse reflected by the dielectric beamsplitter 20; the
third output pulse is directed to the output. The portion of the
second circulating pulse transmitted by the beamsplitter 20 becomes
the third circulating pulse. The third circulating pulse follows
the same optical path as the first and second circulating pulses
and returns to the dielectric beamsplitter 20.
[0054] The dielectric beamsplitter 20 again divides the circulating
pulse into a fourth output pulse and a fourth circulating pulse.
The fourth output pulse is reflected to the output; the fourth
circulating pulse follow the same optical path as the other
circulating pulses before returning to the beamsplitter 20.
[0055] As will be readily apparent, there are still more
circulating pulses and output pulses than the four discussed above.
The circulating optical path of the circulator is formed by the
disposition of the four mirrors 21, 22, 23, 24 so as to provide a
predetermined optical-path-length difference or form a pulse-delay
optical path. The mirrors 21, 22, 23, 24 have the function of an
optical system that provides an optical-path-length difference or
forms a pulse-delay optical system. The magnitude of the output
pulses decreases with every reflection by the dielectric
beamsplitter 20 because reflection by the dielectric beamsplitter
20 directs a portion of each circulating pulse to the output. If
the dielectric beamsplitter 20 exhibits no loss and the mirrors 21,
22, 23, 24 are perfect (i.e., 100-percent reflective), then the
energy of the original laser pulse is delivered to the output
without any loss.
[0056] In the second example embodiment, the action of the
transmission and reflection of the dielectric beamsplitter 20 is
opposite that of the first example embodiment. In the second
example embodiment, the transmissivity T of the dielectric
beamsplitter 20 needed to make the power of the first output pulse
and the power of the second output pulse equal is equal to the
reflectivity R of the dielectric beamsplitter 10 needed to make the
powers of the first and second output pulses equal. Therefore, the
transmissivity T of dielectric beamsplitter 20 is 38.2 percent.
[0057] Similarly, to make the power E.sub.1 of the first output
pulse and the power E.sub.2 of the second pulse of light less than
or equal to 50 percent of the power E of the original laser pulse,
the transmissivity T of the dielectric beamsplitter 20 should
satisfy the following condition:
29.3%<T<50% (13)
[0058] As in the first example embodiment, if the pulses of light
from the laser 1 of the second example embodiment are unpolarized,
circularly polarized, or randomly polarized, then approximately the
same effect is achieved if the transmissivity averaged over the s-
and p-polarizations satisfies condition (13), above.
[0059] FIG. 6 shows a block diagram of a pulse-width extending
optical system corresponding to a third example embodiment of the
invention. In contrast to the first and second example embodiments
in which a single beamsplitter both divides light pulses and
directs multiple pulses to the output, the third example embodiment
utilizes a first polarizing beamsplitter 30 for splitting light and
a second polarizing beamsplitter 33 for directing light to an
output. The third example embodiment is explained below while
pointing out differences relative to the first and second example
embodiments.
[0060] In the pulse-width extending optical system in FIG. 6, a
light pulse emitted by the laser 1 is split into p-polarized and
s-polarized components by the first polarizing beamsplitter 30. The
first polarizing beamsplitter 30 completely transmits p-polarized
light while completely reflecting s-polarized light. The
p-polarized component of the pulse transmitted by the first
polarizing beamsplitter 30 is also transmitted by the second
polarizing beamsplitter 33 and is directed to the output. In
contrast, the s-polarized component of the pulse is reflected by
the first polarizing beamsplitter 30 and is then reflected
sequentially by mirrors 31, 32 before reaching the second
polarizing beamsplitter 33. The second polarizing beamsplitter 33
then reflects the s-polarized pulse portion to the output.
[0061] In this way, the first polarizing beamsplitter 30 serves to
divide the input light pulse along two optical paths. The second
polarizing beamsplitter 33 serves to recombine the beams and direct
the recombined beam to the output.
[0062] The p-polarized and the s-polarized portions of the original
laser pulse travel different optical paths. As stated above, if
this optical path difference between the two portions is longer
than the laser pulse length, then the two split pulses will have no
temporal overlap at the output. Also, as noted above, even if the
optical path difference is set to one-half the pulse length, the
power at the output is significantly reduced.
[0063] Unlike the first and second example embodiments, the
original laser pulse in the third example embodiment is split into
two pulses and each of these pulses becomes one of only two output
pulses. There is no series of output pulses obtained from a series
of circulating pulses.
[0064] The third example embodiment splits input pulses that are
either unpolarized, circularly polarized, or randomly polarized
into two pulses having pulse powers that are approximately equal,
each having 50 percent of the power of the input laser pulse.
[0065] If the original laser pulse is linearly or elliptically
polarized, then the power in the p-polarized and s-polarized pulses
will not generally be equal. It is desirable to arrange the
polarization axis of the first polarizing beamsplitter 30 so that
the powers of the pulses are nearly equal. If the original laser
pulse is linearly polarized and its polarization orientation is
either parallel or perpendicular to the plane of the page of FIG.
6, then it is desirable to orient the polarizing axis of the first
polarizing beamsplitter 30 at an angle of 45 from the plane of the
page and perpendicular to the emission direction of the laser
1.
[0066] In the third example embodiment, two non-polarizing
beamsplitters can be used instead of the polarizing beamsplitters
30, 33, respectively. However, in such an alternative case, some
optical power would likely be lost. Referring to FIG. 6, the pulse
portion that would be transmitted by a non-polarizing beamsplitter
at 30 generally would not be fully transmitted by a non-polarizing
beamsplitter at 33; a portion of the pulse would be reflected
downward (i.e., toward the bottom of the drawing) away from the
output and thus would be lost. Similarly, the pulse portion
reflected by a non-polarizing beamsplitter at 30 would not be fully
reflected by the non-polarizing beamsplitter at 33; the transmitted
portion would continue toward the bottom of the drawing and thus
would be lost.
[0067] FIG. 7 is an optical block diagram of the pulse-width
extending optical system according to the fourth example embodiment
of this invention. The fourth example embodiment is similar to the
first example embodiment, except that the fourth example embodiment
has a halfwave retarder 41 in the optical path of the circulating
optical system. The fourth example embodiment is explained below
pointing out the differences relative to the first example
embodiment.
[0068] The pulse-width extending optical system used in the FIG.-7
embodiment comprises a laser 1 that emits a nearly linearly
polarized light pulse. For purposes of describing this embodiment,
the direction of polarization of the laser pulses is parallel to
the plane of the page of FIG. 7. Light polarized in this direction
will be referred to as "p-polarized." It will be readily apparent
to those skilled in the art that other polarization directions can
be accommodated.
[0069] A p-polarized pulse from the laser 1 enters a dielectric
beamsplitter 40 that splits the pulse into a reflected pulse and a
transmitted pulse. The reflected pulse is reflected by a mirror 14
to an output. The transmitted pulse is reflected sequentially by
mirrors 11, 12 before entering the halfwave retarder 41.
[0070] The halfwave retarder 41 is made of an optically anisotropic
material and can rotate the orientation of linearly polarized
light. Linearly polarized light, polarized at an angle with respect
to the axis of a halfwave retarder, exits the halfwave retarder
linearly polarized at an angle--with respect to the axis of the
halfwave retarder. This is equivalent to a rotation of the
direction of polarization by 2. For example, if light enters a
halfwave retarder polarized at +45 degrees with respect to the axis
of the retarder, then the light exits polarized at -45 degrees, or
equivalently, with its polarization rotated by 90 degrees. This and
other properties of halfwave retarders are well-known.
[0071] The halfwave retarder 41 is oriented so that its axis is at
an angle of 45 degrees with respect to the plane of the drawing and
to the path of the pulse between mirror 40 and the mirror 13. The
halfwave retarder 41 therefore rotates the direction of
polarization of light passing through it by 90 degrees. Thus, every
pulse entering the halfwave retarder 41 as p-polarized exits as
s-polarized; s-polarized pulses entering the halfwave retarder 41
similarly exit as p-polarized. Therefore, the first circulating
pulse is rotated from p-polarization to s-polarization by halfwave
retarder 41 before the first circulating pulse returns to the
dielectric beamsplitter 40.
[0072] The dielectric beamsplitter 40 then transmits a portion of
the first circulating pulse. This portion is the second output
pulse. The mirror 14 reflects the second output pulse toward the
output. The dielectric beamsplitter 40 reflects a portion of the
first circulating pulse, forming a second circulating pulse. The
mirrors 11, 12, 13 reflect the second circulating pulse, returning
it to the beamsplitter 40 after the second circulating pulse passes
through the halfwave retarder 41 (which changes the polarization of
the pulse from s-polarization back to p-polarization).
[0073] The dielectric beamsplitter 40 transmits a portion of the
second circulating pulse, forming a third output pulse. The mirror
14 reflects the third output pulse to the output. The portion of
the second circulating pulse reflected by the dielectric
beamsplitter 40 becomes a third circulating pulse. The third
circulating pulse is reflected by the mirrors 11, 12, 13 and
transmitted by the halfwave retarder 41 which changes the third
circulating pulse from p-polarization to s-polarization and then
returns to the dielectric beamsplitter 40.
[0074] In this way the pulses that are reflected by the dielectric
beamsplitter 40 travel the same optical path via the mirrors 11,
12, 13, and the halfwave retarder 41. The light pulses reflected by
the dielectric beamsplitter 40 alternate polarization between
p-polarization and s-polarization. After a pulse has traveled the
optical path, a portion of the pulse is transmitted to the output
by the dielectric beamsplitter 40. The magnitude of a circulating
pulse declines with each reflection by the dielectric beamsplitter
40 because a portion of the pulse is transmitted to the output.
Eventually, the energy in the original laser pulse is delivered to
the output.
[0075] The power of the circulating and output pulses depends upon
the reflectivity of the dielectric beamsplitter 40. The dielectric
beamsplitter 40 generally has different reflectivities for
s-polarized and p-polarized pulses. For purposes of explanation,
the reflectivity of the dielectric beamsplitter 40 to p-polarized
pulses is R.sub.p and the reflectivity to s-polarized pulses is
R.sub.s. The powers E.sub.1, E.sub.2, and E.sub.3 of the first
three output pulses is given by equations (14)-(16):
E.sub.1=E.multidot.R.sub.p (14)
E.sub.2=E.multidot.(1-R.sub.p).multidot.(1-R.sub.p) (15)
E.sub.3=E.multidot.(1-R.sub.p).multidot.R.sub.s.multidot.(1-R.sub.p)
(16)
[0076] In the fourth example embodiment, the halfwave retarder 41
is placed in the optical path of the circulating optical system.
The halfwave retarder 41 alternately changes the polarization of
the pulses so that the power E.sub.1 of the first output pulse, the
power E.sub.2 of the second output pulse, and the power E.sub.3 of
the third output pulse can be made early equal. If equations
(14)-(16) are solved such that E.sub.1=E.sub.2=E.sub.3, then the
reflectivities R.sub.s and R.sub.p required may be determined. The
solution is R.sub.p=29.3% and R.sub.s=58.6%.
[0077] With these reflectivities, the powers E.sub.1, E.sub.2, and
E.sub.3 of the first three output pulses are approximately 29.3
percent of the power E of the original laser pulse. In this way,
the fourth example embodiment achieves lower peak power in the
output pulses than the first example embodiment in which the powers
E.sub.1 and E.sub.2 of the first two output pulses are
approximately 38.2 percent of the power E of the original laser
pulse.
[0078] It is also possible to provide for errors in the
reflectivities of the dielectric beamsplitter 40. For example, the
reflectivities can be selected such that the powers E.sub.1,
E.sub.2, and E.sub.3 of the first three output pulses are 40
percent or less than the power E of the original laser pulse. Using
the foregoing equations (14)-(16) to determine the magnitudes of
E.sub.1, E.sub.2, and E.sub.3, the reflectivities R.sub.s and
R.sub.p must satisfy the following inequalities:
E.multidot.R.sub.p<0.4E (17)
E.multidot.(1-R.sub.p).multidot.(1-R.sub.s)<0.4E (18)
E.multidot.(1-R.sub.p).multidot.R.sub.s.multidot.(1-R.sub.p)<0.4E
(19)
[0079] It will be readily apparent that, if the original laser
pulse is s-polarized instead of p-polarized, interchanging R.sub.s
and R.sub.p in expressions (14)-(19) gives the correct
expression.
[0080] FIG. 8 is an optical block diagram of a pulse-width
extending optical system according to a fifth example embodiment of
the invention.
[0081] The pulse-width extending optical system of the fifth
example embodiment is similar to that of the second example
embodiment of FIG. 5, differing primarily in the placement of a
halfwave retarder 51 in the optical path of the circulating optical
system. In the fifth example embodiment, as in the fourth example
embodiment, the powers E.sub.1, E.sub.2, and E.sub.3 of the first
three output pulses can be made nearly equal. The fifth example
embodiment is explained below pointing out the differences relative
to the second and fourth example embodiments.
[0082] A laser 1 emits a p-polarized laser pulse. As defined above,
p-polarization is the polarization direction in the plane of the
page of FIG. 8 and perpendicular to the direction of propagation of
the laser pulse. The equations describing the magnitudes of the
output pulses are readily found by substituting T.sub.s and T.sub.p
for R.sub.s and R.sub.p, respectively, in the equations pertaining
to the fourth example embodiment. T.sub.s and T.sub.p are the
transmissivities of a dielectric beamsplitter 50 for the s- and
p-polarized pulses, respectively. This is apparent by noting that
the first output pulse of the fifth example embodiment is
transmitted by the dielectric beamsplitter 50 and subsequent output
pulses are formed by reflection of circulating pulses. In contrast,
in the fourth example embodiment, the first output pulse is
reflected by a beamsplitter and subsequent output pulses are the
transmitted portions of circulating pulses. Therefore, the powers
E.sub.1, E.sub.2, and E.sub.3 of the first three output pulses are
equal if the dielectric beamsplitter 50 has transmissivities
T.sub.p=29.3 percent and T.sub.s=58.6 percent.
[0083] It will be readily apparent that, if the original laser
pulses are s-polarized, the transmissivities T.sub.p and T.sub.s of
the dielectric beamsplitter 50 should be interchanged.
[0084] FIG. 9 is an optical block diagram of a pulse-width
extending optical system according to the sixth example embodiment
of the invention. The pulse-width extending optical system of the
sixth example embodiment is similar to that of the second example
embodiment as shown in FIG. 5, differing primarily from the second
example embodiment in the placement of a relay system 61, 62
(preferably a Keplerian relay system) in the optical path of the
circulating optical system. The sixth example embodiment is
explained below pointing out the differences relative to the second
example embodiment.
[0085] As shown in FIG. 9 the relay system 61, 62 is placed in the
optical path of a circulating optical system formed by mirrors 21,
22, 23, 24. The relay system 61, 62 controls the tendency of the
beam cross-section of the laser pulses to increase. As is
well-known, beams of light naturally diverge. The relay system 61,
62 is arranged so that the beamsplitting surface of a dielectric
beamsplitter 20 and the beam-combining surface of the dielectric
beamsplitter 20 are conjugate, i.e., the beamsplitting surface and
the beam combining surfaces are imaged onto each other. In general,
surfaces imaged onto each other are conjugate surfaces.
[0086] When the laser 1 has a large beam divergence such as is
common with excimer lasers, the circulating pulses tend to diverge
and spread out as they propagate around the circulating optical
system.
[0087] FIG. 10 illustrates the divergence of the circulating
pulses. FIG. 10 shows the intensity of the pulses as function of a
coordinate perpendicular to the propagation direction. As shown in
FIG. 10, the first pulse is narrowest; the second pulse is wider
than the first pulse and narrower than the third pulse. It is
apparent that the cross-section of pulses gradually enlarges as the
pulses propagate.
[0088] The relay system 61, 62 confines the pulses and prevents
them from spreading out, regardless of the number of times they
have propagated through the circulating optical system.
Furthermore, the magnification of the relay system 61, 62 is set so
that the first pulse of light and the second pulse of light are
rotated just 180 degrees relative to each other before the
beamsplitter 20 directs them to the output.
[0089] As shown in FIG. 11, even if an excimer laser emits a pulse
having a non-uniform cross-section, the odd-numbered pulses and the
even-numbered pulses are rotated 180 degrees relative to each other
and directed to the output as shown in FIG. 12. In this way, not
only is beam-spreading controlled but the optical power delivered
to the output is delivered with a more uniform distribution.
[0090] In the sixth example embodiment of FIG. 9, the optical path
joining the laser 1 and the dielectric beamsplitter 20 crosses the
optical path joining the mirrors 22 and 23. If it is undesirable to
have these paths cross, crossing may be avoided by arranging the
optical path formed by the mirrors 21, 22, 23, 24 and the relay
system 61, 62 in a plane other than the plane of the page of FIG.
9. Furthermore, it is generally desirable that the relay system 61,
62 have a magnification of either 1 or -1. The focal point is at
the mid-point of the relay system, so optical elements (such as the
mirrors 21, 22, 23, 24) are preferably sufficiently distant from
the focal point so that they are not damaged by the high light
intensity at the focal point.
[0091] FIG. 13 is an optical block diagram of a pulse-width
extending optical system according to a seventh example embodiment
of the invention.
[0092] The pulse-width extending optical system used in the seventh
example embodiment is similar to that of the first example
embodiment of FIG. 1, differing primarily from the first example
only in the placement of a relay system 71, 72 (preferably
Keplerian) in the optical path of the circulating optical system.
In the seventh example embodiment, as in the sixth example
embodiment, the beamsplitting surface of the beamsplitter 10 and
the beam combining surface are nearly conjugate by means of the
relay system 71, 72 which preferably has a magnification of -1. In
this example embodiment, both beam divergence and beam uniformity
are improved.
[0093] FIG. 14 is an optical block diagram of a pulse-width
extending optical system according to an eighth example embodiment
of the invention.
[0094] The pulse-width extending optical system of the eighth
example embodiment is similar to that of the third example
embodiment, differing from the third example embodiment primarily
in the placement of a relay system 81, 82 (preferably Keplerian) in
the longer of the two optical paths. In the eighth example
embodiment, as in the sixth and seventh example embodiments, the
beamsplitting surface of the first beamsplitter 30 and the
directing (combining) surface of the directing component are nearly
conjugate by means of the relay system 81, 82 which has a
magnification of -1.
[0095] FIG. 15 is an optical block diagram of a projection-exposure
apparatus according to a ninth example embodiment of the invention,
the apparatus comprising a pulse-width extending optical system as
described above.
[0096] The projection-exposure apparatus of the ninth example
embodiment comprises an excimer laser 60 that emits laser pulses.
The laser pulses emitted by the excimer laser 60 enter a dielectric
beamsplitter 20 through a relay system 91, 92 (preferably
Keplerian). The relay system 91, 92 is configured so that the
output aperture of the excimer laser 60 and the beamsplitting
surface of the dielectric beamsplitter 20 are nearly conjugate.
[0097] The magnification of the relay system 91, 92 depends on the
size of the dielectric beamsplitter 20 and the beam cross-section
of the laser emission. Even if the laser 60 emits pulses at an
angle to the optical axis of the relay system 91, 92 and not along
the axis, the relay system 91, 92 still guides the pulses from the
laser 60 to the dielectric beamsplitter 20.
[0098] As in the pulse-width extending optical system in the sixth
example embodiment shown in FIG. 9, the pulses entering the
dielectric beamsplitter 20 shown in FIG. 15 are split into pulses
transmitted by the dielectric beamsplitter 20 (output pulses) and
pulses reflected by the dielectric beamsplitter 20 (circulating
pulses). The mirrors 21, 22, 23, 24 define a circulating optical
path; the circulating optical path comprises a second relay system
93, 94. The circulating pulses return to the dielectric
beamsplitter 20 after propagating around the circulating path. The
circulating pulses return to the dielectric beamsplitter 20, and
portions are reflected by the beamsplitter 20. The output portions
are generally directed along the same optical path. The relay
system 93, 94 preferably has a magnification of -1 and is
configured so that the beamsplitting surface of the dielectric
beamsplitter 20 (which acts as a both a splitting mechanism and an
output mechanism) is conjugate to itself with respect to the
circulating path, i.e., the relay system 93, 94 images the
beamsplitter onto itself with a magnification of -1.
[0099] The relay system 91, 92, the dielectric beamsplitter 20, the
mirrors 21, 22, 23, 24, and the relay system 93, 94 form a
pulse-width extending optical system for the laser 60. The
pulse-width extending optical system of the projection-exposure
apparatus of FIG. 15 also prevents beam spatial divergence with the
relay system 93, 94. As discussed previously, this relay system
improves the spatial uniformity of the beam as well.
[0100] If the optical path length of the circulating optical system
is larger than one-half the pulse length, then the pulse-width is
extended and the peak power is reduced. As a result, the
performance degradation of optical elements and systems following
the pulse-width extending system is reduced and component life is
extended.
[0101] The output pulses of this pulse-width extending system are
directed along a common optical path through the dielectric
beamsplitter 20 and to a third keplerian relay system 95 and 96. A
mirror 97 then reflects the pulses into, for example, a
beam-correcting optical system 2 comprising both cylindrical and
spherical lenses. This beam-correcting optical system 2 changes the
beam cross-section; typically, elliptical cross-sections are
reshaped to be more nearly circular. The corrected beam enters a
fly-eye lens 3. The fly-eye lens 3, as known in the art, has
multiple lens elements arranged in parallel along the optical axis
of the optical system. Pulses incident to the fly-eye lens 3 are
formed into several secondary images (secondary light sources) at
the rear focal point of the fly-eye lens 3. (The secondary images
of the incident radiation received by the fly-eye lens 3 produce
more uniform irradiation.)
[0102] The beam-correcting optical system 2 is arranged so that the
surface of the fly-eye lens 3 into which the radiation enters and
the dielectric beamsplitter 20 are nearly conjugate. Even where
there are slight angular differences between the optical axis of
the illumination optical system and the exit direction of the
output pulses from the dielectric beamsplitter 20, the light pulses
from the beamsplitter 20 are accurately guided to the entrance
surface of the fly-eye lens 3.
[0103] If a cylindrical lens is included in the beam-correcting
optical system 2, as indicated in the detailed summary and drawings
of U.S. patent application Ser. No. 08/603,001 (filed on Feb. 16,
1996; incorporated herein by reference), it is desirable to have
the dielectric beamsplitter 20 and the entrance surface of the
fly-eye lens 3 configured so that they are nearly conjugate in both
horizontal and vertical directions.
[0104] Light from the multiple secondary light sources formed by
the fly-eye lens 3 is limited by aperture 4 and is focused by a
condenser lens 5. The condenser lens 5 directs the light to a
mirror 98 that reflects the beam to a mask 6 that is thereby nearly
uniformly irradiated. The mask 6 contains patterns such as those of
a semiconductor integrated circuit. A projection optical system 7
projects the pattern of the mask 6 (as reduced or enlarged) onto a
wafer 8 that has been coated with a resist sensitive to the
radiation from the laser. Radiation from the laser exposes the
resist coating on the wafer 8. After being exposed in the
projection-exposure apparatus shown in FIG. 15, the wafer 8
undergoes further processing, including development and etching, in
which all but the resist pattern are removed. Following the etching
process, other processes such as a resist-removal process are
performed to conclude the process. After the wafer 8 is fully
processed, it is diced (cut into chips), bonded (wires are
attached), and then packaged. The chip is then ready to use.
[0105] The foregoing example refers to the manufacture of large
scale integration (LSI) semiconductor devices, but
projection-exposure apparatus according to the present invention
can be used in other manufacturing processes. For example, the
projection-exposure apparatus can be used in the manufacture of
liquid-crystal-display elements, thin-film magnetic heads, imaging
elements (such as CCDs), and other semiconductor devices.
[0106] The laser 60, the pulse-width extending optical system, the
relay system 95, 96, the beam-correcting optical system 2, the
fly-eye lens 3, the aperture 4, the condenser lens 5, and the
turning mirror 98 form an illumination system that illuminates the
mask 6 spatially uniformly and with reduced peak optical power. By
splitting the laser pulses into multiple pulses, the total laser
power delivered to the wafer 8 is (neglecting losses) the same as
the total power delivered without splitting the pulses. Splitting
the laser pulses and extending the laser pulse-width reduces the
peak optical power, reducing degradation of optical elements.
[0107] In the ninth example embodiment shown in FIG. 15, the
beam-correcting optical system 2 is situated upstream of the
fly-eye lens 3. However, the beam-correcting optical system 2 may
be in other locations. As stated above, as long as the laser 60,
the dielectric beamsplitter 20, and the fly-eye lens 3 are
conjugate, it is possible, for example, to place the
beam-correcting optical system 2 directly behind the laser 60 or in
the optical path between the dielectric beamsplitter 20 and the
lens 95 of the relay system 95, 96.
[0108] Whereas the invention has been described in connection with
multiple example embodiments, it will be understood that the
invention is not limited to those embodiments. On the contrary, the
invention is intended to encompass all alternatives, modifications,
and equivalents as may be included within the spirit and scope of
the invention as defined by the appended claims.
* * * * *