U.S. patent application number 13/107247 was filed with the patent office on 2011-09-08 for two-stage laser system for aligners.
This patent application is currently assigned to KOMATSU, LTD.. Invention is credited to Tatsuya Ariga, Takahito Kumazaki, Kotaro Sasano, Osamu WAKABAYASHI.
Application Number | 20110216800 13/107247 |
Document ID | / |
Family ID | 33312622 |
Filed Date | 2011-09-08 |
United States Patent
Application |
20110216800 |
Kind Code |
A1 |
WAKABAYASHI; Osamu ; et
al. |
September 8, 2011 |
TWO-STAGE LASER SYSTEM FOR ALIGNERS
Abstract
The invention relates to a two-stage laser system well fit for
semiconductor aligners, which is reduced in terms of spatial
coherence while taking advantage of the high stability, high output
efficiency and fine line width of the MOPO mode. The two-stage
laser system for aligners comprises an oscillation-stage laser (50)
and an amplification-stage laser (60). Oscillation laser light
having divergence is used as the oscillation-stage laser (50), and
the amplification-stage laser (60) comprises a Fabry-Perot etalon
resonator made up of an input side mirror (1) and an output side
mirror (2). The resonator is configured as a stable resonator.
Inventors: |
WAKABAYASHI; Osamu;
(Hiratsuka-Shi, JP) ; Ariga; Tatsuya;
(Hiratsuka-Shi, JP) ; Kumazaki; Takahito;
(Hiratsuka-Shi, JP) ; Sasano; Kotaro;
(Hiratsuka-Shi, JP) |
Assignee: |
KOMATSU, LTD.
Tokyo
JP
|
Family ID: |
33312622 |
Appl. No.: |
13/107247 |
Filed: |
May 13, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11566235 |
Dec 3, 2006 |
7957449 |
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13107247 |
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10554537 |
Oct 24, 2005 |
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PCT/JP2004/005490 |
Apr 16, 2004 |
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11566235 |
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Current U.S.
Class: |
372/50.22 |
Current CPC
Class: |
G03F 7/70025 20130101;
H01S 3/083 20130101; H01S 3/2308 20130101 |
Class at
Publication: |
372/50.22 |
International
Class: |
H01S 5/50 20060101
H01S005/50 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 22, 2003 |
JP |
2003-116924 |
Aug 22, 2003 |
JP |
2003-298286 |
Claims
1. A two-stage laser system for aligners, comprising an
oscillation-stage laser and an amplification-stage laser, wherein:
laser light output from said oscillation-stage laser is injected
into said amplification-stage laser and is amplified therein, said
amplified laser light is output from said amplification
stage-laser, said oscillation-stage laser and said amplification
stage laser each comprises a chamber filled with a laser gas, said
oscillation-stage laser oscillates laser light having divergence,
and said amplification-stage laser comprises a Fabry-Perot etalon
type resonator, wherein said resonator is configured as a stable
resonator.
2-45. (canceled)
Description
ART FIELD
[0001] The present invention relates generally to a two-stage laser
system for aligners, and more particularly to a two-stage laser
system well fit for semiconductor aligners, which is reduced in
terms of spatial coherence.
BACKGROUND ART
[0002] So far, two-stage laser systems comprising an
oscillation-stage laser and an amplification-stage laser adapted to
amplify laser light emitted out of the oscillation-stage laser have
been known so far in the art for the purpose of obtaining high
outputs. Two modes, MOPA (Master Oscillator Power Amplifier) and
MOPO (Master Oscillator Power Oscillator) are known for
double-chamber laser systems. The former is a mode having no
resonator in the amplification stage, and the latter is a mode
having an unstable resonator in the amplification stage. The MOPA
mode and the MOPO mode have merits and demerits over each
other.
MOPA
[0003] (a) Low spatial coherence (merit). That is, given the same
share quantity (pinhole-to-pinhole spacing) in the beam transverse
direction, the visibility of interference fringes is low. Notice
that the share quantity and visibility will be explained later.
[0004] (b) Low energy stability (demerit). This is because output
fluctuations are sensitive to fluctuations of synchronous
excitation timing between the chambers. [0005] (c) Output
efficiency is lower than that of the MOPO mode; laser (seed) energy
from the oscillation-stage laser must be more than that of the MOPO
mode (demerit). [0006] (d) Thick spectral line width (demerit).
This is because the latter half of a laser pulse from the
oscillation-stage laser contains a lot more roundtrips, and so the
spectral line width is too narrow to amplify the tail of that
latter half.
MOPO
[0006] [0007] (a) High spatial coherence (demerit). That is, given
the same share quantity (pinhole-to-pinhole spacing) in the beam
transverse direction, the visibility of interference fringes is
high. [0008] (b) High energy stability (merit). This is because
output fluctuations are insensitive to fluctuations of synchronous
excitation timing between the chambers. [0009] (c) Output
efficiency is higher than that of the MOPA mode; laser (seed)
energy from the oscillation-stage laser can be less than that of
the MOPA mode (merit). [0010] (d) Fine spectral line width (merit).
This is because the latter half of a laser pulse from the
oscillation-stage laser contains a lot more roundtrips, and so the
spectral line width is narrow enough to amplify the tail of that
latter half.
[0011] As described above, the MOPO mode is more favorable than the
MOPA mode saving (a) spatial coherence; in other words, it will be
more suitable as a light source for semiconductor aligners such as
excimer laser or F.sub.2 laser, if proper action is taken to reduce
the spatial coherence.
[0012] However, the MOPO mode has now been found to have problems
in conjunction with the use of an unstable resonator as mentioned
above. The problems will now be discussed at great length.
[0013] In what follows, the "oscillation-stage laser" will be
tantamount to the "line narrowing oscillation-stage laser". A MOPA
system, and a MOPO system is basically made up of at least one
oscillation-stage laser and one amplification stage or
amplification-stage laser. When there is no resonator in the
amplification-stage laser, that amplification-stage laser is herein
called the amplification stage with no resonance of light. A system
having a resonator in the amplification stage is called a MOPO
system. When there is a resonator in the amplification stage, the
amplification stage functions as an amplification-stage laser with
resonance of light. Accordingly, when the amplification stage is
compared with the amplification-stage laser, higher efficiency
amplification is achievable with the amplification-stage laser than
with the amplification stage, given equal excitation energy.
[0014] So far, the amplification-stage laser of an excimer laser
MOPO system has incorporated an unstable resonator using a concave
mirror having a seed light-introduction hole in its center as an
input side mirror and a convex mirror as an output side mirror.
Such a concave mirror/convex mirror combination of the unstable
resonator constitutes a telephoto optical system having a
geometrical magnification factor. Having an optical magnification
of about 20, the unstable resonator is used for the purpose of
efficiently obtaining high-output, high-coherence laser light in
the MOPO system. Notice that the unstable resonator has so far been
used primarily as a light source for physicochemical
researches.
[0015] A system having an unstable resonator in an
amplification-stage laser has been proposed as a light source for
semiconductor aligners, as set forth in patent publication 1.
Although this unstable resonator has an optical magnification
reduced down to about 10, the inventors' experimentation has
suggested that the spatial coherence is not reduced down to any
sufficient level.
[0016] That is, the object of using the unstable resonator in a
conventional MOPO system is to provide efficient amplification of
seed light. A concave mirror that forms a part of the unstable
resonator is located in the amplification-stage laser to inject the
seed light all over the amplification-stage laser gain area,
thereby providing efficient amplification of the seed light.
[0017] Patent Publication 1 [0018] Patent No. 2820103
[0019] Non-Patent Publication 1 [0020] "Basics and Applications of
Lasers", translated by Hitoshi Mochizuki and two others, pp. 30-33
(published from Maruzen Co., Ltd. on Jan. 20, 1986)
[0021] Non-Patent Publication 2 [0022] Sov. J. Quantum Electron.
16(5), May 1986, pp. 707-709
[0023] One of the specifications of much importance in a laser
system for aligners is in-plane low coherence (spatial coherence)
in a laser light profile section. This spatial coherence capability
(coherence) is evaluated by comparison of the coherence of a
partial beam profile at a given constant distance (share quantity)
A in the beam profile. That distance indicated by A is a value
determined by element-to-element spacing, etc. in a fly-eye lens
used to eliminate brightness variations in an illumination system
in a semiconductor aligner such as a stepper. Then, the spatial
coherence at two points in the share quantity A is evaluated by
visibility defined by the following formula:
Visibility=(maximum fringe intensity I.sub.max-minimum fringe
intensity I.sub.min)/(maximum fringe intensity I.sub.max+minimum
fringe intensity I.sub.min) (1)
[0024] Notice here that the "fringe intensity" means the intensity
of interference fringes upon interference of light from two
pointes. FIG. 71 is indicative in schematic of interference fringes
of light from two points at a given share quantity A and their
maximum and minimum fringe intensities I.sub.max and I.sub.min, and
FIG. 72 is indicative in schematic of interference fringes of light
from two points at a given share quantity and their maximum and
minimum fringe intensities I.sub.max and I.sub.min, with an added
laser portion. More specifically, FIG. 72 is a schematic
representation of an optical arrangement for the evaluation of
spatial coherence of a laser light source by a Young's
interferometer as well as interference fringes of light from two
points at a given share quantity (=pinhole-to-pinhole distance) and
their maximum and minimum fringe intensities I.sub.max and
I.sub.min. Generally, the spatial coherence is determined depending
on the size and intensity distribution of a light source, as viewed
from the position of a pinhole that is a point of measurement.
[0025] FIG. 73 is indicative of the results of measurement of the
visibility of a line narrowing laser and the results of measurement
in the case of using the line narrowing laser as an
oscillation-stage laser to provide amplification in an unstable
resonator amplification-stage laser, as obtained in the inventors'
experimentation. These results teach that it is required to satisfy
the condition that the share quantity from a semiconductor aligner
be equal to or greater than A and the visibility be equal to or
less than Vt. Usually, the visibility of a single line narrowing
laser satisfies this condition. However, when an unstable resonator
having a magnification factor of 5 was used in the
amplification-stage laser in this experimentation, the share
quantity providing a visibility equal to or less than Vt increased
up to B. B.apprxeq.5.times.A; the share quantity providing the
desired visibility equal to or less than Vt increases by the
magnification factor of the unstable resonator. In other words, in
the arrangement of FIG. 72 wherein the laser portion is added to
FIG. 71, the spatial coherence is evaluated while a beam-expanding
optical system comprising a combined concave mirror and convex
mirror is located between the laser light source and the pinhole.
In this case, the size of the light source, as viewed from the
pinhole, decreases by the beam magnification factor. Thus, the
share quantity providing the same visibility equal to or less than
Vt increases by the magnification factor.
[0026] In view of the fact that when the unstable resonator is used
in the amplification-stage laser, the share quantity increases by a
quantity corresponding to the magnification of that unstable
resonator, the inventors have made further experiments, using a
MOPO system with a stable resonator the optical magnification of
which is set at 1 using plane mirrors as both input- and
output-side mirrors. As a result, it has been found that the share
quantity A equivalent to that obtained with a single
oscillation-stage laser, i.e., that of seed light can be achieved
with a MOPO system using that resonator (FIG. 73). That is, the
inventors have now discovered that as the unstable resonator is
used in the amplification-stage laser of the MOPO system, it causes
the share quantity to increase by the optical magnification factor
of the unstable resonator, and that if a stable resonator is used,
this can then be averted. As described later, this finding is one
of the rudiments of the invention.
[0027] From another angle of view, why the spatial coherence and
the share quantity increase with the use of the unstable resonator
is now explained.
[0028] FIG. 74 is illustrative in (beam profile) section of laser
light emitted out of an oscillation-stage laser. Consider now the
coherence of laser light P1 and P2' spaced away by a distance A1
and laser light P1 and P2 spaced away by a distance A2 in the beam
profile. As shown in FIG. 75, the laser light P1 and P2' at a short
distance are put in order or substantially equal in terms of wave
phase. With increasing distance, however, there is a little wave
phase shift even at the same wavelength; laser light P1 and P2 at a
relatively long distance are less likely to interfere spatially. In
other words, a long pinhole-to-pinhole distance allows for a
decrease in the visibility of interference fringes.
[0029] In the prior art, the amplification-stage laser resonator
was an unstable resonator. As shown in FIG. 78(a), the unstable
resonator comprises an input side concave mirror and an output side
convex mirror, and is of the type that is capable of geometrically
expanding the section of seed light. Accordingly, when both the
amplification-stage laser and the oscillation-stage laser are of
much the same size in excitation section (discharge section), seed
light from the oscillation-stage laser is such that a partial beam
portion having a radius 3A is cut out of the general beam section,
as shown in FIG. 76. In the section cut out in this way, the closer
the laser light P2 is to the laser light P3, the higher the
visibility of interference fringe becomes. Although there is a low
visibility at a distance A3, the visibility of interference fringe
at distance A4 becomes higher with increasing coherence.
[0030] As described above, the prior art amplification-stage laser
resonator is an expander system; laser light is expanded while high
coherence is maintained. As a consequence, the post-amplification
laser light P3 diverges to the position of P3' as shown in FIG. 77,
while high coherence is maintained intact. Thus, even when the
specifications for coherence are met at a distance A5 in the
oscillation-stage laser, high coherence is still maintained even at
a distance A6 beyond the distance A5 by the expansion of the beam
of seed light after amplification, offering a problem that the
specifications for low coherence are not met.
[0031] FIG. 78 is illustrative of how seed light (explained with
reference to FIGS. 76 and 77) diverges in the amplification-stage
laser using an unstable resonator. A laser light section at a
position Z1 of the input side concave mirror (FIG. 78(b))
corresponds to FIG. 76, and a laser light section at a position Z2
of the output side convex mirror (FIG. 78(c)) corresponds to FIG.
77. In the prior art, the amplification-stage laser resonator was
an unstable resonator. As shown in FIG. 81(a), the unstable
resonator comprises an input side cylindrical concave mirror and an
output side cylindrical convex mirror. Reference is now made to a
resonator of the type that is capable of geometrically expanding
the section of seed light in a longitudinal direction. Seed light
oscillated out of the oscillation-stage laser is such that, as
shown in FIG. 79, the visibility at a distance A3 (share quantity)
between laser light P1 and P3 is higher in the section of injection
of seed light than in the section of a beam in the
amplification-stage laser amplified by the unstable resonator shown
in FIG. 80. Given visibility equivalent to that in the
oscillation-stage laser, the distance A4 between laser light P1 and
P3 becomes long by the magnification factor of the unstable
resonator, meaning that the spatial coherence becomes high.
[0032] With the prior art two-stage laser system for aligners that
relies upon the MOPO mode, the spatial coherence distance becomes
long in proportion to the magnification factor at which the beam of
seed light is expanded by the unstable resonator, because the
unstable resonator is used in the amplification-stage laser. Thus,
the prior art two-stage laser is less than satisfactory for light
sources for semiconductor aligners.
DISCLOSURE OF THE INVENTION
[0033] In view of such problems with the prior art as described
above, the primary object of the invention is to provide a
two-stage laser system well fit for semiconductor aligners, which
is reduced in terms of spatial coherence while taking advantage of
the high stability, high output efficiency and fine line width of
the MOPO mode.
[0034] According to the invention, this object is accomplished by
the provision of a two-stage laser system for aligners, comprising
an oscillation-stage laser and an amplification-stage laser,
wherein:
[0035] laser light output from said oscillation-stage laser is
injected into said amplification-stage laser and is amplified
therein,
[0036] said amplified laser light is output from said
amplification-stage laser,
[0037] said oscillation-stage laser and said amplification-stage
laser each comprises a chamber filled with a laser gas,
[0038] said oscillation-stage laser oscillates laser light having
divergence, and
[0039] said amplification-stage laser comprises a Fabry-Perot
etalon type resonator, wherein said resonator is configured as a
stable resonator.
[0040] Preferably in this case, the resonator comprises an input
side mirror in which laser light oscillated out of said
oscillation-stage laser is entered and an output side mirror
through which amplified laser light outputs, wherein the input side
mirror comprises a total-reflection mirror having a
total-reflection mirror coating externally of an area through which
laser light oscillated out of the oscillation-stage laser is
introduced in the resonator, and the output side mirror comprises a
planar, partial reflecting mirror.
[0041] The substrate of the input side mirror could be provided in
its substantially central portion with a hole or slit shaped in
such a way as to introduce laser light oscillated out of the
oscillation-stage laser in the resonator.
[0042] The substrate of the input side mirror is formed of a
transparent substrate, and a total-reflection mirror coating is
applied to a peripheral area of the surface of the transparent
substrate other than an area at a substantially central portion of
the surface of the transparent substrate, wherein said area is
shaped in such a way as to introduce laser light oscillated out of
the oscillation-stage laser in the resonator, or a slit area
including said shape.
[0043] The laser light oscillated out of the oscillation-stage
laser could be introduced in the resonator from a periphery of the
input side mirror or a peripheral portion thereof that is not
applied with a total-reflection mirror coating.
[0044] Alternatively, the resonator could comprise an input side
mirror in which the laser light oscillated out of the
oscillation-stage laser is entered and an output side mirror
through which the amplified laser light outputs, wherein the input
side mirror comprises a partial reflecting mirror and the output
side mirror comprises a planar, partial reflecting mirror.
[0045] The laser light oscillated out of the oscillation-stage
laser could be introduced in the resonator from a periphery of the
input side mirror.
[0046] The output side mirror in the resonator in the
oscillation-stage laser and the input side mirror in the
amplification-stage laser could be formed on each side surface of
the same substrate.
[0047] The input side mirror comprises a plane mirror, a concave
mirror or a cylindrical concave mirror.
[0048] Alternatively, the resonator could comprise an output side
mirror in which the laser light oscillated out of the
oscillation-stage laser is entered and through which the amplified
laser light outputs, and a rear side mirror, wherein the substrate
of the output side mirror is formed of a transparent substrate, an
area of the output side mirror, through which the amplified laser
light outputs, has a partial reflection capability, and the rear
side mirror comprises a planar total-reflection mirror.
[0049] The resonator could comprise an output side mirror in which
the laser light oscillated out of the oscillation-stage laser is
entered and through which the amplified laser light outputs, and a
right-angle prism, wherein the substrate of the output side mirror
is formed of a transparent substrate, an area of the output side
mirror, through which the amplified laser light outputs, has a
partial reflection capability, and the right-angle prism comprises
a total-reflection right-angle prism capable of reflecting all
incident light.
[0050] The laser light oscillated out of the oscillation-stage
laser is introduced in the resonator from a periphery of the output
side mirror or a peripheral portion of the output side mirror
having no partial reflection capability.
[0051] The output side mirror could be a partial reflecting
mirror.
[0052] In this case, the laser light oscillated out of the
oscillation-stage laser could be introduced in the resonator from a
periphery of the output side mirror.
[0053] The output side mirror could comprise a plane mirror, a
concave mirror or a cylindrical concave mirror.
[0054] In the two-stage laser system for aligners according to the
invention, the resonator could comprise an input side mirror in
which the laser light oscillated out of the oscillation-stage laser
is entered, wherein the input side mirror comprises a partial
reflecting mirror, and an output side mirror, wherein the output
light of the oscillation-stage laser is entered in the resonator
through the partial reflecting mirror, and the optical axis of the
resonator is in substantial alignment with the optical axis of the
oscillation-stage laser.
[0055] The resonator could comprise a total-reflection rear side
mirror and an output side mirror, wherein a beam splitter is
located between the rear side mirror and a rear side laser window
and on the optical axis of said resonator, the laser light
oscillated out of the oscillation-stage laser is incident on the
beam splitter, and the optical axis of laser light reflected from
the beam splitter is in substantial alignment with the optical axis
of the resonator.
[0056] The resonator could comprise a total-reflection rear side
mirror and an output side mirror, wherein a beam splitter is
located between the rear side mirror and a front side laser window
and on the optical axis of the resonator, the laser light
oscillated out of the oscillation-stage laser is incident on the
beam splitter, and the optical axis of laser light reflected from
the beam splitter is in substantial alignment with the optical axis
of the resonator.
[0057] The resonator could comprise a total-reflection rear side
mirror and an output side mirror, wherein a beam splitter, on which
the laser light oscillated out of the oscillation-stage laser is
incident, is located externally of the resonator and on the optical
axis of the resonator, the laser light oscillated out of the
oscillation-stage laser is incident on the beam splitter, the
optical axis of laser light reflected from the beam splitter is in
substantial alignment with the optical axis of said resonator, and
the laser light is entered in the resonator through the output side
mirror.
[0058] A front mirror in the oscillation-stage laser could comprise
a partial reflecting mirror, and be shared by an input side mirror
in which the laser light oscillated out of the oscillation-stage
laser is entered.
[0059] In the two-stage laser system for aligners according to the
invention, the optical axis of laser light oscillated out of the
oscillation-stage laser and entered in the amplification-stage
laser could be set at an angle with respect to the optical axis of
the resonator in the amplification-stage laser.
[0060] A length about twice as long as the length of the resonator
in the amplification-stage laser could be set longer than a
time-based coherent length corresponding to the spectral line width
of the oscillation-stage laser.
[0061] The two-stage laser system for aligners according could
further comprise between the oscillation-stage laser and the
amplification-stage laser a conversion optical system having at
least one of a function of compressing the beam shape of laser
light oscillated out of the oscillation-stage laser and a function
of magnifying the divergence of laser light oscillated out of the
oscillation-stage laser.
[0062] Preferably, the divergence of laser light entered in the
amplification-stage laser should satisfy the following
conditions:
.theta. h .gtoreq. Tan - 1 [ { ( Ha - Hs ) / 2 } ( 1 / L ) / ( P c
/ L ] = Tan - 1 { ( Ha - Hs ) / ( 2 P c ) } ( 2 ) .theta. v
.gtoreq. Tan - 1 [ { ( Va - Vs ) / 2 } ( 1 / L ) / ( P c / L ] =
Tan - 1 { ( Va - Vs ) / ( 2 P c ) } ( 3 ) ##EQU00001##
Here .theta.v and .theta.h are the angles of divergence of laser
light entered in the amplification-stage laser in the vertical and
horizontal directions, respectively, P is an effective pulse width,
c is the velocity of light, L is a resonator length, Vs and Hs are
the beam diameters of laser light entered in the
amplification-stage laser in the vertical and horizontal
directions, respectively, and Va and Ha are the beam diameters of
output light in the vertical and horizontal directions,
respectively.
[0063] The invention also provides a two-stage laser system for
aligners, comprising an oscillation-stage laser and an
amplification-stage laser, wherein:
[0064] laser light output from said oscillation-stage laser is
injected into said amplification-stage laser and is amplified
therein,
[0065] said amplified laser light is output from said
amplification-stage laser,
[0066] the oscillation-stage laser and the amplification-stage
laser each comprises a chamber filled with a laser gas,
[0067] the oscillation-stage laser oscillates laser light having
divergence,
[0068] the amplification-stage laser comprises a ring resonator
comprising an input/output partial reflecting mirror and a
plurality of total-reflection mirrors for reflecting laser light
entered via the partial reflecting mirror back to a position of the
partial reflecting mirror, and
[0069] the partial reflecting mirror and the plurality of
total-reflection mirrors are each formed of a plane.
[0070] In this case, between the oscillation-stage laser and the
amplification-stage laser there could be located a conversion
optical system having a function of compressing the beam shape of
laser light oscillated out of the oscillation-stage laser.
[0071] The optical path length in the ring resonator could be set
longer than a time-based coherent length corresponding to the
spectral line width of the oscillation-stage laser.
[0072] Further, the invention provides a two-stage laser system for
aligners, comprising an oscillation-stage laser and an
amplification-stage laser, wherein:
[0073] laser light output from said oscillation-stage laser is
injected into said amplification-stage laser and is amplified
therein,
[0074] said amplified laser light is output from said
amplification-stage laser,
[0075] the oscillation-stage laser and the amplification-stage
laser each comprises a chamber filled with a laser gas,
[0076] said amplification-stage laser comprises a Fabry-Perot
etalon resonator, wherein the resonator is configured as a stable
resonator, and
[0077] the optical axis of laser light oscillated out of the
oscillation-stage laser and entered in the amplification-stage
laser is set at an angle with respect to the optical axis of the
resonator in the amplification-stage laser.
[0078] Further, the invention provides a two-stage laser system for
aligners, comprising an oscillation-stage laser and an
amplification-stage laser, wherein:
[0079] laser light output from said oscillation-stage laser is
injected into said amplification-stage laser and is amplified
therein,
[0080] said amplified laser light is output from said
amplification-stage laser,
[0081] the amplified laser light leaves as output, wherein the
oscillation-stage laser and the amplification-stage laser each
comprises a chamber filled with a laser gas,
[0082] the amplification-stage laser comprises a Fabry-Perot etalon
resonator, wherein the resonator is configured as a stable
resonator, and
[0083] a length about twice as long as the length of the resonator
in the amplification-stage laser is set longer than a time-based
coherent length corresponding to the spectral line width of the
oscillation-stage laser.
[0084] Further, the invention provides a two-stage laser system for
aligners, comprising an oscillation-stage laser and an
amplification-stage laser, wherein:
[0085] laser light output from said oscillation-stage laser is
injected into said amplification-stage laser and is amplified
therein,
[0086] said amplified laser light is output from said
amplification-stage laser,
[0087] the oscillation-stage laser and the amplification-stage
laser each comprises a chamber filled with a laser gas,
[0088] the amplification-stage laser comprises a Fabry-Perot etalon
resonator, wherein the resonator is configured as a stable
resonator,
[0089] the optical axis of laser light oscillated out of the
oscillation-stage laser and entered in the amplification-stage
laser is set at an angle with respect to the optical axis of the
resonator in the amplification-stage laser, and
[0090] a length about twice as long as the length of the resonator
in the amplification-stage laser is set longer than a time-based
coherent length corresponding to the spectral line width of the
oscillation-stage laser.
[0091] Further, the invention provides a two-stage laser system for
aligners, comprising an oscillation-stage laser and an
amplification-stage laser, wherein:
[0092] laser light output from said oscillaton-stage laser is
injected into said amplification-stage laser and is amplified
therein,
[0093] said amplified laser light is output from said
amplification-stage laser,
[0094] the oscillation-stage laser and the amplification-stage
laser each comprises a chamber filled with a laser gas,
[0095] the amplification-stage laser comprises a resonator
comprising a rear side mirror and an output side mirror,
[0096] the reflecting surfaces of the rear side mirror and the
output side mirror are each formed of a plane,
[0097] the normal lines to the rear side mirror and the output side
mirror are set at an angle with respect to the optical axis of
laser light oscillated out of the oscillation-stage laser and
entered in the amplification-stage laser and at an angle with one
another, and
[0098] laser light oscillated out of the oscillation-stage laser is
entered in the resonator from a side on which a distance between
both mirrors is longer.
[0099] Preferably in this case, the resonator should be positioned
such that laser light reflected at the rear side mirror or the
output side mirror on which laser light oscillated out of the
oscillation-stage laser is first incident is reflected toward a
side on which the distance between both mirrors is shorter.
[0100] Preferably, the rear side mirror and the output side mirror
should be mutually set in such a way as to make an angle of 0.01
mrad to 0.2 mrad inclusive.
[0101] In this case, too, a length about twice as long as the
length of the resonator in the amplification-stage laser should be
set longer than a time-based coherent length corresponding to the
spectral line width of the oscillation-stage laser.
[0102] Throughout the above two-stage laser systems for aligners,
the laser light oscillated out of the oscillation-stage laser could
be introduced in the resonator from any side position of the
resonator.
[0103] Throughout the above two-stage laser systems for aligners,
each of the mirrors that form the resonator could be held by a
mirror holder capable of moving each mirror in a substantially
vertical direction to the optical axis direction of the
resonator.
[0104] Throughout the above two-stage laser systems for aligners,
the oscillation-stage laser could further comprise line narrowing
means for line narrowing the oscillated laser light so as to be
configured as a KrF excimer laser, an ArF excimer laser, and a
molecule fluorine (F.sub.2) laser.
[0105] Alternatively, the laser system could be configured as a
molecule fluorine (F.sub.2) laser system comprising wavelength
select means for selecting one oscillation line from laser light
oscillated in the oscillation-stage laser.
[0106] Still alternatively, the laser system could be configured as
a molecule fluorine (F.sub.2) laser system comprising wavelength
select means for selecting one oscillation line from laser light
produced on the output side of the amplification-stage laser.
[0107] In the two-stage laser system for aligners according to the
invention, oscillation laser light having divergence is used as the
oscillation-stage laser and the amplification-stage laser comprises
a Fabry-Perot etalon resonator where the resonator is configured as
a stable resonator or, alternatively, oscillation laser light
having divergence is used as the oscillation-stage laser and the
amplification-stage laser comprises a ring resonator comprising an
input/output partial reflecting mirror and a plurality of
total-reflection mirrors for reflecting laser light entered via the
partial reflecting mirror back to the position of the partial
reflecting mirror wherein the partial reflecting mirror and the
plurality of total-reflection mirrors are each formed of a plane.
Thus, the two-stage laser system for aligners according to the
invention has the features of the MOPO mode that output
fluctuations are insensitive to fluctuations of synchronous
excitation timing between the chambers, high energy stability and
high output efficiency are achievable, laser (seed) energy from the
oscillation stage can be kept lower, the spectral line width is
narrow because of the latter half of a laser pulse from the
oscillation-stage laser makes a lot more roundtrips, and the line
width is narrow because the tail of the latter half can be
amplified, and has the features of the MOPA mode as well that the
spatial coherence is low; that is, given the same share quantity
(pinhole-to-pinhole space) in the beam transverse direction, the
visibility of interference fringes and the spatial coherence are
low.
[0108] If the optical axis of laser light oscillated out of the
oscillation-stage laser and entered in the amplification-stage
laser is set in such a way as to make an angle with respect to the
optical axis of the resonator in the amplification-stage laser,
then the spatial coherence is much more reduced.
[0109] If the length about twice as long as the length of the
resonator in the amplification-stage laser is set longer than the
time-based coherent length corresponding to the spectral line width
of the oscillation-stage laser or the length of the optical path
through the ring resonator is set longer than the time-based
coherent length corresponding to the spectral line width of the
oscillation-stage laser, it is then possible to prevent any
interference fringe pattern from occurring on the beam profile of
laser light produced out of the amplification-stage laser. It is
thus possible to maintain the symmetry of the beam profile and hold
back its fluctuations and, hence, provide uniform illumination of
masks in an aligner. Thus, the invention provides a two-stage laser
system well fit especially for semiconductor aligners.
[0110] The invention is in no sense limited to the use of the
oscillation laser light having divergence as the oscillation-stage
laser. For instance, if the optical axis of laser light oscillated
out of the oscillation-stage laser and entered in the
amplification-stage laser is set in such a way as to make an angle
with respect to the optical axis of the resonator in the
amplification-stage laser, it is then possible to obtain a
two-stage laser system that does not only have the above features
of the MOPO mode but also is reduced in terms of spatial coherence
so that it lends itself well to semiconductor aligners.
[0111] Further, if the reflecting surfaces of the rear side mirror
and the output side mirror are each formed of a plane, the normal
lines to the rear side mirror and the output side mirror are set in
such a way as to make an angle with respect to the optical axis of
laser light oscillated out of the oscillation-stage laser and
entered in the amplification-stage laser and with each other as
well, and the laser light oscillated out of the oscillation-stage
laser is entered in the resonator from the side on which the
distance between both mirrors is longer, it is then possible to
obtain a two-stage laser system that does not only have the above
features obtained by setting the optical axis of laser light
entered in the amplification-stage laser in such a way as to make
an angle with respect to the optical axis of the resonator in the
amplification-stage laser but also has an increased laser output
and an extended pulse width and ensures the degree of flexibility
in the injection of laser light entered in the amplification-stage
laser with a decrease in the peak intensity of the
oscillation-stage laser, and so is best suited for use with
semiconductor aligners.
[0112] Still other objects and advantages of the invention will in
part be obvious and will in part be apparent from the
specification.
[0113] The invention accordingly comprises the features of
construction, combinations of elements, and arrangement of parts
which will be exemplified in the construction hereinafter set
forth, and the scope of the invention will be indicated in the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0114] FIG. 1 is illustrative in schematic of the fundamental
construction of the two-stage laser system for aligners according
to the invention.
[0115] FIG. 2 is illustrative of how the divergence needed for seed
light in the invention is defined.
[0116] FIG. 3 is illustrative of the construction of one embodiment
of the resonator mirrors used in the amplification-stage laser.
[0117] FIG. 4 is illustrative of the construction of another
embodiment of the resonator mirrors used in the amplification-stage
laser.
[0118] FIG. 5 is illustrative of the construction of yet another
embodiment of the resonator mirrors used in the amplification-stage
laser.
[0119] FIG. 6 is illustrative of the construction of a further
embodiment of the resonator mirrors used in the amplification-stage
laser.
[0120] FIG. 7 is illustrative of the construction of a further
embodiment of the resonator mirrors used in the amplification-stage
laser.
[0121] FIG. 8 is illustrative of the construction of a further
embodiment of the resonator mirrors used in the amplification-stage
laser.
[0122] FIG. 9 is illustrative of the conversion optical system
interposed between the oscillation-stage laser and the
amplification-stage laser.
[0123] FIG. 10 is illustrative of one exemplary conversion optical
system.
[0124] FIG. 11 is illustrative of another exemplary conversion
optical system.
[0125] FIG. 12 is illustrative of the two-stage laser system for
aligners according to the invention using another conversion
optical system.
[0126] FIG. 13 is a general representation of one embodiment of the
two-stage laser system for aligners comprising the basic features
of the invention.
[0127] FIG. 14 is illustrative of part of one specific embodiment
of the two-stage laser system for aligners of FIG. 13.
[0128] FIG. 15 is illustrative of share quantity of low coherence
versus visibility relations in the arrangement of FIG. 14.
[0129] FIG. 16 is illustrative of the two-stage laser system for
aligners, which comprises one modification to the input side mirror
in the amplification-stage laser.
[0130] FIG. 17 is illustrative of another modification to the input
side mirror in the resonator used in the amplification-stage
laser.
[0131] FIG. 18 is illustrative of yet another modification to the
input side mirror in the resonator used in the amplification-stage
laser.
[0132] FIG. 19 is illustrative of the two-stage laser system for
aligners, which comprises another modification to the input side
mirror in the amplification-stage laser.
[0133] FIG. 20 is illustrative of the principles of operation when
the optical axis of the resonator in the amplification-stage laser
and the optical axis of seed light make an angle.
[0134] FIG. 21 is illustrative of the two-stage laser system for
aligners, which comprises yet another modification to the input
side mirror in the amplification-stage laser.
[0135] FIG. 22 is illustrative of a modification to the input side
mirror usable in the embodiment of FIG. 19. FIG. 23 is illustrative
of a modification to the input side mirror usable in the embodiment
of FIG. 20.
[0136] FIG. 24 is illustrative of a modification to the input side
mirror usable in the embodiment of FIG. 21.
[0137] FIG. 25 is illustrative of part of another embodiment of the
two-stage laser system for aligners according to the invention.
[0138] FIG. 26 is illustrative of the amplification-stage laser
that forms a part of the two-stage laser system for aligners shown
in FIG. 25.
[0139] FIG. 27 is illustrative of one exemplary mirror holder.
[0140] FIG. 28 is illustrative of part of yet another embodiment of
the two-stage laser system for aligners according to the
invention.
[0141] FIG. 29 is illustrative of another embodiment of the
amplification-stage laser in the two-stage laser system for
aligners according to the invention.
[0142] FIG. 30 is illustrative of one embodiment of the resonator
mirrors used in the amplification-stage laser of FIG. 29.
[0143] FIG. 31 is illustrative of a two-stage laser system for
aligners as in FIG. 16, wherein the input side mirror and the
output side mirror in the resonator in the amplification-stage
laser are arranged in nonparallel relations.
[0144] FIG. 32 is illustrative of under what conditions laser
output can be effectively taken while laser light makes a given
frequency of roundtrips in the resonator in the amplification-stage
laser.
[0145] FIG. 33 is illustrative, as in FIG. 32, of under what
conditions laser output can be effectively taken while laser light
makes a given frequency of roundtrips in the resonator in the
amplification-stage laser.
[0146] FIG. 34 is illustrative, as in FIG. 32, of under what
conditions laser output can be effectively taken while laser light
makes a given frequency of roundtrips in the resonator in the
amplification-stage laser.
[0147] FIG. 35 is illustrative of how to calculate the relations
between the position and the angle of injection of seed light at
the position of the input side mirror (rear side mirror) necessary
to effectively take laser output out of the output side mirror
without deviation from the discharge area.
[0148] FIG. 36 is illustrative of the embodiment of FIG. 31 as
viewed from the side opposite to the chamber, wherein the edge of
the input side mirror is in alignment with the end of the discharge
area.
[0149] FIG. 37 is illustrative of one area out of which laser
output can be effectively taken in a parallel arrangement of the
input side mirror and the output side mirror, wherein the edge of
the input side mirror is in alignment with the end of the discharge
area.
[0150] FIG. 38 is illustrative of an area out of which laser output
can be effectively taken with the input side mirror inclined.
[0151] FIG. 39 is illustrative of why higher laser output and more
extended pulse width are achievable in a nonparallel arrangement
than in a parallel arrangement of the input side mirror and the
output side mirror.
[0152] FIG. 40 is a view for studying the angle relation of
inclination in a nonparallel arrangement of two plane mirrors that
form together the resonator in the amplification-stage laser.
[0153] FIG. 41 is illustrative in schematic of the range for
obtaining a proper inclination.
[0154] FIG. 42 is illustrative of another embodiment wherein seed
light is entered as is the case with the two-stage laser system for
aligners of FIG. 25 according to the invention.
[0155] FIG. 43 is illustrative of a modification to the input side
mirror usable in the embodiment of FIG. 31.
[0156] FIG. 44 is illustrative of the arrangement corresponding to
FIG. 31 but with the use of the input side mirror of FIG. 43.
[0157] FIG. 45 is illustrative of a modification to the output side
mirror usable in the embodiment of FIG. 42.
[0158] FIG. 46 is illustrative of the arrangement corresponding to
FIG. 42 but with the use of the output side mirror of FIG. 46.
[0159] FIG. 47 is illustrative of another arrangement wherein seed
light is entered as in FIG. 21 of the two-stage laser system for
aligners according to the invention.
[0160] FIG. 48 is a top view illustrative of one embodiment wherein
seed light is injected in the resonator in the amplification-stage
laser from its side opposite to the laser exit side.
[0161] FIG. 49 is a top view illustrative of another embodiment
wherein seed light is injected in the resonator in the
amplification-stage laser from its side opposite to the laser exit
side.
[0162] FIG. 50 is indicative of the reflection characteristics of
CaF.sub.2 to P-polarized light.
[0163] FIG. 51 is a top view illustrative of yet another embodiment
wherein seed light is injected in the resonator in the
amplification-stage laser from its side opposite to the laser exit
side, and shows the construction of the window member in that
case.
[0164] FIG. 52 is a top view illustrative of a further embodiment
wherein seed light is injected in the resonator in the
amplification-stage laser from its side opposite to the laser exit
side, and shows the construction of the prism in that case.
[0165] FIG. 53 is a top view illustrative of an embodiment
corresponding to FIG. 48 but with the injection of seed light from
between the output side mirror and the chamber in the
amplification-stage laser.
[0166] FIG. 54 is a top view illustrative of an embodiment
corresponding to FIG. 49 but with the injection of seed light from
between the output side mirror and the chamber in the
amplification-stage laser.
[0167] FIG. 55 is a top view illustrative of an embodiment
corresponding to FIG. 51 but with the injection of seed light from
between the output side mirror and the chamber in the
amplification-stage laser, and shows the construction of the window
member in that case.
[0168] FIG. 56 is a top view illustrative of an embodiment
corresponding to FIG. 52 but with the injection of seed light from
between the output side mirror and the chamber in the
amplification-stage laser.
[0169] FIG. 57 is a top view illustrative of one embodiment wherein
seed light is guided directly in the laser chamber in the
amplification-stage laser.
[0170] FIG. 58 is a top view illustrative of another embodiment
wherein seed light is guided directly in the laser chamber in the
amplification-stage laser.
[0171] FIG. 59 is a top view illustrative of one embodiment wherein
seed light is guided to the amplification-stage laser in the back
surface injection mode.
[0172] FIG. 60 is illustrative of relations between the input side
mirror (the reflectivity of the rear mirror) and laser output after
injection and synchronization.
[0173] FIG. 61 is illustrative of one example indicative of an
effective enabling area with respect to the angle and position,
.theta.in and Xin, of injection of seed light in the case where the
seed light is injected from the back surface of the input mirror in
the input side mirror.
[0174] FIG. 62 is a top view illustrative of one embodiment of the
mode of introducing seed light in the amplification-stage laser via
a beam splitter in the resonator in the amplification-stage
laser.
[0175] FIG. 63 is a top view illustrative of another embodiment of
the mode of introducing seed light in the amplification-stage laser
via a beam splitter in the resonator in the amplification-stage
laser.
[0176] FIG. 64 is a top view illustrative of one embodiment wherein
seed light is injected in the amplification-stage laser through the
output side mirror by way of a beam splitter.
[0177] FIG. 65 is illustrative of part of another embodiment of the
two-stage laser system for aligners according to the invention.
[0178] FIG. 66 is illustrative of part of yet another embodiment of
the two-stage laser system for aligners according to the
invention.
[0179] FIG. 67 is illustrative in schematic of the construction of
one embodiment of the two-stage laser system for aligners according
to the invention, wherein a ring resonator is used in the
amplification-stage laser.
[0180] FIG. 68 is illustrative in schematic of the construction of
another embodiment of the two-stage laser system for aligners
according to the invention, wherein a ring resonator is used in the
amplification-stage laser.
[0181] FIG. 69 is illustrative of the principles of why an
interference fringe pattern occurs in the two-stage laser system
for aligners.
[0182] FIG. 70 is indicative of relations between the length twice
as long as the resonator length L of the amplification-stage laser
and the visibility of an interference fringe pattern.
[0183] FIG. 71 is illustrative in schematic of interference fringes
of light from two points in a given share quantity as well as the
maximum and minimum fringe intensities.
[0184] FIG. 72 is a view, as in FIG. 71, of a laser portion added
to it.
[0185] FIG. 73 is illustrative of the results of measurement of the
visibility of a line narrowing laser, the results of measurement of
visibility in the case where laser light is amplified in an
unstable resonator amplification-stage laser, and the results of
measurement of visibility in the case where laser light is
amplified in a stable resonator amplification-stage laser.
[0186] FIG. 74 is indicative of the section (beam profile) of laser
light emitted out of the oscillation-stage laser.
[0187] FIG. 75 is a view for studying coherence between laser light
and laser light spaced away by a different distance in the beam
profile.
[0188] FIG. 76 is illustrative of how to cut a beam portion out of
seed light from the oscillation-stage laser.
[0189] FIG. 77 is illustrative of how the post-amplification laser
light diverges while high coherence is kept intact in the case
where the laser is expanded in a prior art amplification-stage
laser resonator.
[0190] FIG. 78 is illustrative in further detail of how the seed
light explained with reference to FIGS. 76 and 77 diverges in the
amplification-stage laser using an unstable resonator.
[0191] FIG. 79 is a view similar to FIG. 76.
[0192] FIG. 80 is a view similar to FIG. 77.
[0193] FIG. 81 is illustrative in further detail of how the seed
light explained with reference to FIGS. 79 and 80 diverges in the
amplification-stage laser using an unstable resonator.
BEST MODE FOR CARRYING OUT THE INVENTION
[0194] First of all, the principles of the two-stage laser system
for aligners according to the invention are now explained.
[0195] As described with reference to FIG. 73, it has been found
that in the MOPO system comprising an oscillation-stage laser and
an amplification-stage laser in which laser light (seed light)
oscillated out of the oscillation-stage laser is entered for
amplification, and which comprises a resonator comprising an input
side mirror and an output side mirror, if the resonator in the
amplification-stage laser is configured as a stable resonator, it
is then possible to achieve a low spatial coherence equivalent to
that of the oscillation-stage laser.
[0196] FIG. 1 is illustrative in schematic of the basic arrangement
of the two-stage laser system for aligners according to the
invention. The two-stage laser system for aligners according to the
invention is a MOPO system as described above, which comprises an
oscillation-stage laser (MO: Master Oscillator) 50, and an
amplification-stage laser (PO: Power Oscillator) 60 in which seed
light oscillated out of the oscillation-stage laser 50 is entered
and amplified to produce laser light. The amplification-stage laser
60 is equipped with a Fabry-Perot etalon resonator comprising an
input side mirror (rear side mirror) 1 and an output side mirror
(front side mirror).sub.2, between which a chamber 3 filled with a
laser gas is located. The amplification-stage laser 60 further
comprises discharge electrodes, etc. for exciting the laser gas in
the chamber 3 to form a gain area.
[0197] In the oscillation-stage laser 50, typically, a chamber 53
filled with a laser gas is provided in a laser resonator comprising
a rear side mirror that also serves as an optical element in a line
narrowing module 51 constructed from, for instance, an expanding
prism and a grating (diffraction grating), and a front mirror 52.
The oscillation-stage laser 50 further comprises discharge
electrodes, etc. for exciting the laser gas in the chamber 53 to
form a gain area.
[0198] Although not essential, between the oscillation-stage laser
50 and the amplification-stage laser 60, there is located a
conversion optical system 70 for reducing the sectional area of a
seed light beam entered from the oscillation-stage laser 50 in the
amplification-stage laser 60 or converting the angle of divergence
of seed light from the oscillation-stage laser 50.
[0199] In the laser system of the invention, if the resonator
comprising the input side mirror 1 and the output side mirror 2 in
the amplification-stage laser 60 is constructed of a stable
resonator as described above, it is then possible to achieve a low
spatial coherence equivalent to that of the oscillation-stage
laser.
[0200] The stable resonator in the laser system must satisfy the
following condition (a):
0.ltoreq.(1-L/R1)(1-L/R2).ltoreq.1 (a)
Here R1 is the radius of curvature of the input side mirror (rear
side mirror) 1, R2 is the radius of curvature of the output side
mirror 2, and L is a mirror-to-mirror spacing, provided that the
radius of curvature of a concave mirror is defined as positive and
the radius of curvature of a convex mirror as negative.
[0201] Such a stable resonator has a multimode as in the resonator
used in the oscillation-stage laser 50, and by using such a stable
resonator at the amplification-stage laser 60, it would be possible
to achieve a low spatial coherence equivalent to that of the
oscillation-stage laser 50.
[0202] However, only with the use of such a stable resonator at the
amplification-stage laser 60, seed light is not geometrically
magnified in the stable resonator, when the amplified laser light
leaves the stable resonator, failing to bury the gain area of the
laser gas in the amplification-stage laser 60 with seed light for
efficient amplification.
[0203] Therefore, light having divergence is used as the seed light
oscillated out of the oscillation-stage laser 50 in the invention,
and using the divergence of that seed light, the gain area of the
laser gas is buried with the seed light that diverges while a
plurality of reflections occur in the stable resonator, so that
high-efficient amplification can take place.
[0204] Gas lasers used as light sources in semiconductor aligners,
for instance, fluorine molecule (F.sub.2) laser, KrF excimer laser
and ArF excimer laser make multimode oscillations. In general, the
oscillated laser light diverges to some degrees, and the use of
such a gas laser for the oscillation-stage laser 50 will allow the
seed light entering the amplification-stage laser 60 to have
divergence. It is noted, however, that the angle of divergence of
that seed light could be controlled through an optical system as
described later; the angle of divergence of the seed light entering
the amplification-stage laser 60 could be controlled within the
range as desired.
[0205] Therefore, the minimum divergence required for the seed
light is defined depending on the pulse width in the
amplification-stage laser 60, as follows.
[0206] FIGS. 2(a) and 2(b) are illustrative in vertical and
horizontal sections, respectively, of the definition of divergence
required for the seed light. As shown, the resonator in an
amplification-stage laser 60 comprises an input side mirror 1 and
an output side mirror 2, between which there is positioned a
chamber 3 filled with a laser gas. In this embodiment, the laser
gas within the chamber 3 is excited by discharge between an upper
electrode 4 and a lower electrode 5 to form a gain area. Further
between the input side mirror 1 and the output side mirror 2, there
is located an aperture (opening) 6 that will determine the beam
size of output laser light.
[0207] Upon entrance from the input side mirror 1, seed light
reflects plural times in the resonator in the amplification-stage
laser 60 within the duration of pulse width, so that the gain area
(discharge area) can be buried with the seed light having
divergence. Therefore, the seed light of an effective pulse width P
in the amplification-stage laser 60 must have the angles of
divergence, .theta.v and .theta.h, in the vertical and horizontal
directions, as defined below. Here, P is the effective pulse width,
c is the velocity of light, L is a resonator length, Vs and Hs are
beam diameters in the vertical and horizontal directions of the
seed light, and Va and Ha are beam diameters in the vertical and
horizontal direction of amplified light, provided that each beam
diameter is measured at a position of a peak strength of
1/e.sup.2.
.theta. h .gtoreq. Tan - 1 [ { ( Ha - Hs ) / 2 } ( 1 / L ) / ( P c
/ L ] = Tan - 1 { ( Ha - Hs ) / ( 2 P c ) } ( 2 ) .theta. v
.gtoreq. Tan - 1 [ { ( Va - Vs ) / 2 } ( 1 / L ) / ( P c / L ] =
Tan - 1 { ( Va - Vs ) / ( 2 P c ) } ( 3 ) ##EQU00002##
Here .theta.v and .theta.h are the angles of divergence of laser
light entered in the amplification-stage laser in the vertical and
horizontal directions, respectively, P is the effective pulse
width, c is the velocity of light, L is the resonator length, Vs
and Hs are the beam diameters of laser light entered in the
amplification-stage laser in the vertical and horizontal
directions, respectively, and Va and Ha are the beam light of
output light in the vertical and horizontal directions,
respectively.
[0208] For instance, when Vs=0 mm, Va=16 mm, L=1,000 mm and the
effective pulse width (the width of laser pulses actually injected
in the amplification-stage laser 60) P=10 ns, seed light having a
divergence of 1 mrad.ltoreq..theta.v will be needed, and when Hs=1
mm, Ha=3 mm, L=1,000 mm and the effective pulse width P=10 ns, seed
light having a divergence of 0.3 mrad.ltoreq..theta.h will be
required.
[0209] While, in the above discussions, both the input and output
side mirrors 1 and 2 of the resonator in the amplification-stage
laser 60 are assumed to comprise plane mirrors, yet it is required
that the output side mirror 2 that is the reflecting mirror for
reflecting the seed light first be a plane mirror (R2=.infin.) to
bury the gain area using the divergence of the seed light. On the
contrary, the input side mirror 1 could be either a plane mirror or
a concave mirror included in the range that satisfies the above
formula (a), that is, the range capable of meeting L.ltoreq.R1.
[0210] When the above formulae (2) and (3) are satisfied,
narrow-banded laser light from the oscillation-stage laser 50 fills
the gain area (discharge area) by the divergence effect in the
amplification-stage laser 60, as shown in FIGS. 2(a) and 2(b). In
the area filled with the laser light from the oscillation-stage
laser 50, induction emission takes place while keeping the spectral
characteristics of the oscillation-stage laser 50 intact, so that
the amplification-stage laser 60 oscillates with narrow-band laser
spectra having similar characteristics to those of the
oscillation-stage laser 50.
[0211] Some embodiments of the construction of the resonator used
with the amplification-stage laser 60 are now explained with
reference to FIGS. 3-8, throughout which (a1) is a front view, (a2)
is a longitudinal section, (a2') is a cross section and (a3) a rear
view as viewed from the input side of the input side mirror 1, and
(b1) is a front view and (b2) is a sectional view as viewed from
the input side of the output side mirror 2. Notice here that an
arrow added to (a2) is indicative of an input direction of seed
light and an arrow added to (b2) is indicative of an output
direction of output laser light.
[0212] In the embodiment of FIG. 3, the input side mirror 1 is
formed of a plane substrate having a seed light-introduction
circular hole 7 in its center portion. The plane substrate is
applied on its (output side) back surface with a high-reflectivity
(total-reflection) mirror coating 8. The (input side) front surface
of the plane substrate may or may not have an antireflection
coating 9. The output side mirror 2 is formed of a plane substrate
that is applied on its (input side) front surface with a partial
reflecting mirror coating 10 and on its (output side) back surface
with an antireflection coating 11.
[0213] Throughout the invention, it is noted that a wedge could be
provided on the seed light input side surface of the input side
mirror 1 substrate (to position a wedge surface obliquely, not
vertically, with respect to incident light), so that the seed light
is reflected at that surface in such a way as not to go back to the
oscillation-stage mirror 50. Likewise throughout the invention, a
wedge could be provided on the plane substrate of the output side
mirror 2 to keep feeble back-surface reflection against going back
into the resonator.
[0214] In the embodiment of FIG. 4, the input side mirror 1 is
formed of a plane substrate that has a seed light-introduction
longitudinal slit 7' in its center portion. The plane substrate is
applied on its (output side) back surface with a high-reflectivity
mirror coating 8. The (input side) front surface of the plane
substrate may or may not have an antireflection coating 9. The
output side mirror 2 is formed of a plane substrate that is applied
on its (input side) front surface with a partial reflecting mirror
coating 10 and on its (output side) back surface with an
antireflection coating 9. In this embodiment, the longitudinal slit
7' is of shape and size substantially equal to the sectional shape
of the input side mirror 1 for the seed light from the
oscillation-stage laser 50 or seed light deformed by a conversion
optical system 70 interposed between the oscillation-stage laser 50
and the amplification-stage laser 60.
[0215] In the embodiment of FIG. 5, the input side mirror 1 is
formed of a transparent plane substrate that is applied on its
(output side) back surface with a high-reflectivity mirror coating
8 except a central, longitudinal area 7''. The (input side) front
surface of the plane substrate may or may not have an
antireflection coating 9. The output side mirror 2 is formed of a
plane substrate that is applied on its (input side) front surface
with a partial reflecting mirror coating 10 and on its (output
side) back surface with an antireflection coating 11. In this
embodiment, a slit 7'' is provided in the high-reflectivity mirror
coating 8 instead of providing the seed light-introduction slit 7'
in the substrate (FIG. 4).
[0216] In the embodiment of FIG. 6, the input side mirror 1 is
constructed as in FIG. 3, and a modification is made to the partial
reflecting mirror coating 10 of the output side mirror 2. The
output side mirror 2 is formed of a plane substrate that has a
mirror coating 10' of relatively low reflectivity at its (input
side) center surface portion, with a mirror coating 10 of
relatively high reflectivity applied around the coating 10'. On the
portion having the mirror coating 10' of relatively low
reflectivity in this embodiment, the seed light introduced from a
circular hole 7 in the input side mirror 1 is directly incident
without undergoing repeated reflections between the input side
mirror 1 and the output side mirror 2; the reflectivity of that
portion remains relatively low because the amplified laser light is
weakened by a reduction in the length of the seed light passing
through a gain area.
[0217] Generally, the partial reflecting mirror coating applied on
the output side mirror 2 has the optimum reflectivity at which
laser outputs reach a maximum. As the partial reflecting mirror
coating 10 on the output side mirror 2 is allowed to have the
optimum reflectivity, satisfactory laser output efficiency might be
obtained. As discussed above, however, there is a decrease in
energy at the center of output beam shape depending on the distance
where the seed light obtains gains, which will otherwise cause the
section of laser light (discharge direction) to have an uneven
output profile.
[0218] In the coating on the output side mirror 2 according to this
embodiment, the mirror coating 10' at the center of the output side
mirror 2 and the mirror coating 10 applied around it are varied in
reflectivity in such a way as to obtain as uniform an output
profile as possible. This works somewhat against laser output
efficiency, because both the coatings have often difficulty in
having the optimum reflectivity at which laser outputs reach a
maximum. However, the output profile across the section of an
output laser beam becomes satisfactory.
[0219] In the above embodiment, by way of illustration but not by
way of limitation, the reflectivity of the mirror coating 10' at
the center of the output side mirror 2 is set lower than that of
the mirror coating 10' around it.
[0220] As a matter of course, the reflectivity at which the above
maximum laser output is not obtained is higher or lower than the
above optimum reflectivity. In other words, the mirror coating has
a plurality of reflectivities at which there are obtained given
laser outputs lower than the above maximum laser outputs. In the
above embodiment, therefore, even when the reflectivity of the
mirror coating 10' at the center of the output side mirror 12 is
set higher than that of the mirror coating 10 applied around it,
effects equivalent to those of the above embodiments will often be
obtained.
[0221] In the embodiments of FIGS. 7 and 8, the input and output
side mirrors 1 and 2 are constructed as in FIG. 4. However, the
(output side) back surface of the substrate of the input side
mirror 1 has a cylindrical concave shape rather than a planar
shape. In FIG. 7, the generating line of that cylindrical concave
surface directs vertically (in the longitudinal direction of the
longitudinal slit 7'), and in FIG. 8, the generating line directs
horizontally (in the direction vertical to the longitudinal
direction of the longitudinal slit 7'). In either case, the radius
of curvature of that cylindrical concave surface is set in such a
way as to satisfy the above condition L.ltoreq.R1.
[0222] Specifically but not exclusively, FIGS. 3-8 are illustrative
of mirror arrangements for the resonator used with the
amplification-stage laser 60. Combinations of them or modifications
to them could be used. For instance, the surface of the input side
mirror 1 to be provided with the high-reflectivity mirror coating 8
could be configured in a spherical concave form.
[0223] A partial reflecting mirror coating could be provided all
over the output side surface of the input side mirror 1. In this
case, fabrication could be facilitated because of no need of
providing such a circular hole 7 as depicted in FIG. 3 or such a
slit 7'' provided with the antireflection coating 9 as depicted in
FIG. 5. It is noted, however, that there is a lowering in the
efficiency of utilization of the seed light, because a part of the
seed light is reflected upon incidence and is not injected in the
amplification-stage laser 60.
[0224] The conversion optical system 70 interposed between the
oscillation-stage laser 50 and the amplification-stage laser 60 is
now explained. As already described, this conversion optical system
70 is provided if required, and has primarily both or either one of
two functions.
[0225] Generally, as the energy density of the seed light entering
the amplification-stage laser 60 becomes too low, it is difficult
to obtain any sufficient amplification factor at the
amplification-stage laser 60. In that case, it is desired that the
conversion optical system 70 be provided such that the beam
diameter of the seed light is reduced to increase the energy
density before the seed light enters the amplification-stage laser
60, as depicted in FIG. 9.
[0226] Such a seed light beam reduction optical system uses such
beam diameter reduction prisms 71 and 72 as depicted in FIG. 10. In
this case, each prism 71, 72 comprises a rectangular refracting
prism wherein as input light enters vertically an entrance side
plane, it passes almost vertically through the entrance side plane
and is refracted at an exit side oblique surface, so that the
section of the beam in the paper is reduced. With a plurality of,
preferably, an even number of, such prisms 71 and 72, therefore,
the beam diameter of the seed light in a one-dimensional direction
(longitudinally or transversely) or a two-dimensional direction
(longitudinally or transversely) can be reduced to increase the
energy density.
[0227] Alternatively, the seed light beam reduction optical system
could use such a telephoto optical system as depicted in FIGS.
11(a) and 11(b). In the optical system of FIG. 11(a), a positive
lens 73 having a longer focal length and a positive lens 74 having
a shorter focal length are located at a co-focal point, enabling
the beam diameter to be reduced by the focal length ratio. In the
optical system of FIG. 11(b), a positive lens 75 having a longer
focal length and a negative lens 76 having a shorter focal length
are again located at a co-focal point. In this case, too, the beam
diameter is reduced by the focal length ratio (absolute value).
[0228] One function of the conversion optical system 70 is to
reduce the beam diameter of such seed light as mentioned above,
thereby increasing the energy density, and another function is to
enlarge the angles of divergence, .theta.v and .theta.h, of the
seed light entering the amplification-stage laser 60 in such a way
as to satisfy formulae (2) and (3) in the case where the divergence
of the seed light oscillated out of the oscillation-stage laser 50
does not satisfy formulae (2) and (3). To perform the second
function, i.e., to tweak the angles of divergence, .theta.v and
.theta.h, of the seed light, such a telephoto optical system as
depicted in FIGS. 11(a) and 11(b) is used to adjust the distance
between the positive lenses 73 and 74, and the distance between the
positive lens 75 and the negative lens 76.
[0229] By the way, gas lasers used as light sources for
semiconductor aligners, for instance, fluorine molecule (F.sub.2)
laser, KrF excimer laser and ArF excimer laser, are excited by
discharge between discharge electrodes 54 and 55 to form a gain
area; the section of the seed light from the oscillation-stage
laser 50 has a longitudinally slender shape (because the discharge
electrodes 54 and 55 are provided to sandwich it from above and
below). When the seed light has a longitudinally slender shape in
section, its horizontal direction divergence is likely to satisfy
the relation of formula (2); however, its vertical direction
divergence becomes small, often failing to satisfy the relation of
formula (3). In that case, a negative cylindrical lens 77 having a
cylindrical concave surface 78 with the generating line in the
horizontal direction, negative refracting power in the vertical
direction alone, divergence in the vertical direction and no
refracting power in the horizontal direction, as shown in the
three-view drawing of FIG. 12(b), is provided as the conversion
optical system 70 to be interposed between the oscillation-stage
laser 50 and the amplification-stage laser 60, as depicted in FIG.
12(a), thereby enabling both the angles of divergence, .theta.v and
.theta.h, of the seed light entering the amplification-stage laser
60 in the vertical and horizontal directions to satisfy the
relations of the above formulae (2) and (3).
[0230] One exemplary construction of the two-stage laser system for
aligners comprising the essential features of the invention is now
explained with reference to FIG. 13 showing the generation
construction thereof.
[0231] When the MOPO system according to the invention is a
fluorine molecule (F.sub.2) laser system, a chamber 53 in the
oscillation-stage laser 50 and a chamber 3 in the
amplification-stage laser 60 are each filled with a laser gas
comprising fluorine (F.sub.2) gas and buffer gas comprising helium
(He), neon (Ne) or the like. When the MOPO system is a KrF excimer
laser system, the chamber 53 in the oscillation-stage laser 50 and
the chamber 3 in the amplification-stage laser. 60 are each filled
with a laser gas comprising krypton (Kr) and fluorine (F.sub.2)
gases and buffer gas comprising helium (He), neon (Ne) or the like.
When the MOPO system is an ArF excimer laser system, the chamber 53
in the oscillation-stage laser 50 and the chamber 3 in the
amplification-stage laser 60 are each filled with a laser gas
comprising argon (Ar) and fluorine (F.sub.2) gases and buffer gas
comprising helium (He), neon (Ne) or the like. The laser chamber 53
in the oscillation-stage laser 50 has a discharge portion
comprising a pair of discharge electrodes 54 and 55, and the laser
chamber 3 in the amplification-stage laser 60 has a discharge
portion comprising a pair of discharge electrodes 4 and 5. These
discharge portions have a pair of cathodes 55, 5 and a pair of
anodes 54, 4 located vertically in a parallel direction to the
paper. High voltage pulses are applied to these pairs of electrodes
54 and 55, and 4 and 5 from the associated power sources 56 and 16,
thereby producing discharges between these electrodes.
[0232] At both ends of axial extension from the pairs of electrodes
54 and 55, and 4 and 5 in the chambers 53 and 3 in the
oscillation-stage laser 50 and amplification-stage laser 60, there
are located window members 57 and 17, each formed of a material
transparent to laser oscillated light such as CaF.sub.2. Exposed
surfaces of the window members 57 and 17 opposite to the interiors
of the chambers 53 and 3 are located parallel with each other and
at such a Brewster angle with respect to laser light as to reduce
reflection losses. The window members 57 and 17 are also positioned
in such a way as to place the P-polarized component of laser light
in the horizontal direction.
[0233] A cross-flow fan, not shown in FIG. 13, is housed in each
chamber 53, 3 to circulate the laser gas therein and send the laser
gas into the discharge portion. Each of the oscillation-stage laser
50 and the amplification-stage laser 60 comprises a F.sub.2 gas
supply system and a buffer gas supply system for supplying F.sub.2
gas and buffer gas to the chamber 53, 3, and a gas evacuation
system for evacuating the laser gas in the chamber 53, 3. In FIG.
13, these are collectively designated as a gas supply/evacuation
control valve 58 and a gas supply/evacuation control valve 18.
Notice that the KrF laser system and the ArF laser system comprise
a Kr gas supply system and an Ar gas supply system, respectively.
The gas pressures in the chambers 53 and 3 are monitored by
pressure sensors P1 and P2, respectively, and gas pressure
information is sent to a utility controller 81 that controls the
gas supply/evacuation valves 58 and 18, thereby controlling the gas
composition and pressure in the oscillation-stage chamber 53 and
the amplification-stage chamber 3, respectively.
[0234] Laser outputs change with gas temperatures. To this end, gas
temperature control is carried out. The gas temperature is
monitored by temperature sensors T1 and T2 added to the respective
chambers 53 and 3, and temperature signals are sent to the utility
controller 81 that controls the flow of coolant water by coolant
water flow control valves 59 and 19, respectively. As a result, the
amounts of exhaust heat in the respective heat exchangers 34 and 44
in the chambers 53 and 3 are controlled for temperature
control.
[0235] The oscillation-stage laser 50 comprises a line narrowing
module (LNM) 51 constructed from an expanding prism and a grating
(diffraction grating), and a laser resonator is constructed of an
optical element in the line narrowing module 51 and a front mirror
52. Although not shown, a line narrowing module using etalon and a
total-reflection mirror instead of the expanding prism and grating
could be used.
[0236] A part of laser light emitted out of the oscillation-stage
laser 50 and the amplification-stage laser 60 is split by means of
a laser splitter not shown and guided to monitor modules 34 and 45,
respectively, which monitor the laser light characteristics of the
oscillation-stage laser 50 and the amplification-stage laser 60,
respectively, such as outputs, line widths and center wavelengths.
In FIG. 13, the monitor modules 35 and 45 are installed in both the
oscillation-stage laser 50 and the amplification-stage laser 60,
although it is acceptable to use either one of them.
[0237] Center wavelength signals from the monitor modules 35 and 45
are sent to a wavelength controller 82 that drives the optical
element in the line narrowing module 51 through a driver 83 to make
a selection from wavelengths and performs wavelength control in
such a way that the center wavelength of the oscillation-stage
laser 50 becomes the desired wavelength. Notice that the above
wavelength control could be carried out by issuing commands from
the wavelength controller 82 to the driver 83 on the basis of
wavelength information from the monitor module 45 to which a part
of laser light emitted out of the amplification-stage laser 60 is
guided in such a way that the wavelength of laser light emitted out
of the oscillation-stage laser 50 becomes the given wavelength.
[0238] Laser output signals from the monitor modules 35 and 45 are
sent to an energy controller 84. Then, applied voltage is
controlled by way of a synchronous controller 85, and control is
done in such a way that the energy of the oscillation-stage laser
50 and the amplification-stage laser 60 has the desired value. The
output signals of the monitor module 45 could be sent to the energy
controller 84 as shown at (1) in FIG. 13, or alternatively outputs
at an output monitor provided on the side of a semiconductor
aligner 100, although not shown, could be supplied to the energy
controller 84 as shown at (2).
[0239] After passing through the monitor module 35, the laser light
(seed laser light) from the oscillation-stage laser 50 passes
through a beam steering unit 86 comprising a reflecting mirror,
etc. and then through the conversion optical system 70. Then, the
laser light is guided to the amplification-stage laser 60 for
injection. The conversion optical system 70 comprises a mechanism
wherein, as previously described, the angle of divergence of the
laser light from the oscillation-stage laser 50 is controlled to
such a value as to allow the oscillation-stage laser light to be
injected in the amplification-stage laser 60 at a given angle of
divergence. With the MOPO system of the invention, the stable
resonator made up of an input side mirror (rear side mirror) 1 and
an output side mirror (front side mirror) 2 is used on the
amplification-stage laser 60 in such a way that amplification could
take place even with a limited input. The input side mirror 1 is
holed at 7 (FIG. 3). Passing through the hole 7, the laser is
reflected as shown by an arrow in FIG. 13, and the injected seed
laser light is expanded by the hole, passing effectively through
the discharge portion to increase the power of the laser light.
Finally, the laser leaves the output side mirror 2.
[0240] Instead of providing a spatial opening for the hole 7 in the
input side mirror 1, it is acceptable to use a mirror substrate
with only a hole portion applied with an antireflection coating
(see FIG. 5).
[0241] By way of a power source 56 built up of a charger 31/switch
32/MPC (magnetic pulse compression circuit) 33 and a power source
16 built up of a charger 41/switch 42/MPC (magnetic pulse
compression circuit) 43, high voltage pulses are applied to a pair
of discharge electrodes 54 and 55 in the oscillation-stage laser 50
and a pair of discharge electrodes 4 and 5 in the
amplification-stage laser 60, respectively, to give rise to
discharge between the electrodes 54 and 55 and between the
electrodes 4 and 5. This discharge in turn causes excitation of the
laser gases filled in the laser chambers 53 and 3,
respectively.
[0242] At the respective power sources 56 and 16, capacitors are
charged by the chargers 31 and 41. As the switches 32 and 42 are
held on, energy charged in the capacitors is transferred as voltage
pulses to the magnetic pulse compression circuits 33 and 43 where
they are compressed for application to the pair of electrodes 54
and 55 and the pair of electrodes 4 and 5. Although not shown, the
power sources 56 and 16 are each provided with a step-up
transformer that could be used to boost up voltage pulses.
[0243] The switches 32 and 42 are put on or off in response to
operating commands (trigger signals) from the synchronous
controller 85.
[0244] The synchronous controller 85 sends trigger signals to the
power source 56 built up of a charger 31/switch 32/MPC (magnetic
pulse compression circuit) 33 and the power source 16 built up of a
charger 41/switch 42/MPC (magnetic pulse compression circuit) 43
such that discharge is generated at the amplification-stage laser
60 at a timing of injecting the laser emitted out of the
oscillation-stage laser 50 in the amplification-stage laser 60. As
there is a discharge timing lag between the oscillation-stage laser
50 and the amplification-stage laser 60, the laser light emitted
out of the oscillation-stage laser 50 will be not efficiently
amplified. The synchronous controller 85 gleans information about
when discharge starts to occur at the oscillation-stage laser 50
and the amplification-stage laser 60 from discharge sensors 36 and
46, respectively, and laser output information from the energy
controller 85 to set a delay time between the trigger signals sent
to the power source 56 for the oscillation-stage laser 50 and the
trigger signals sent to the power source 16 for the
amplification-stage laser 60.
[0245] The utility controller 81, energy controller 84 and
wavelength controller 82 are connected to the main controller 80.
The main controller 80 is connected to the aligner 100. In response
to commands from the aligner 100, the main controller 80 is
operable to allocate the respective controls to the respective
controllers 81, 84 and 82, allowing the respective controllers 81,
84 and 82 to execute the respective controls.
[0246] The laser light emitted out of the oscillation-stage laser
50 is aligned by the beam steering unit 86 made up of two mirrors
such that it passes through a discharge area in the
amplification-stage laser 60. The two mirrors that form the beam
steering unit 86 are driven by a driver 87 for angle control, so
that the direction of travel of the laser light issuing from the
oscillation-stage laser 50 is controlled.
[0247] How to control the beam steering unit 86 is now specifically
explained. For instance, suppose here that the direction of travel
of laser light given out of the oscillation-stage laser 50 was not
aligned such that it passed through the discharge area in the
amplification-stage laser 60. A part or the whole of the laser
light emitted out of the oscillation-stage laser 50 will then be
cut off or reflected in an undesired direction, for instance, by
the discharge electrodes 4 and 5 in the amplification-stage laser
60, failing to leave the amplification-stage laser 60 or causing
laser power to become smaller than the desired value. To avert
this, the beam steering unit 86 is controlled in such a way as to
have a maximum laser light output while that output is monitored on
the monitor module 45. This is explained with reference to FIG. 13.
The results of monitoring by the monitor module 45 are sent to the
wavelength controller 82 that, on the basis of output results
received from the monitor module 45, gives a command to the driver
87 to drive and control the beam steering unit 86 such that the
output reaches a maximum, thereby controlling the direction of
travel of the laser light issuing from the oscillation-stage laser
50.
[0248] Part of one specific embodiment of the two-stage laser
system for aligners of such construction is shown in FIG. 14. FIGS.
14(a) and 14(b) are an top view and a side view of that part. An
ArF excimer laser of 193 nm wavelength is used for each of the
oscillation- and amplification-stage lasers 50 and 60, and a
resonator built up of a planar input side mirror 1 and a planar
output side mirror 2 as shown in FIG. 3 is used in the
amplification-stage laser 60. By way of the beam steering unit 86
made up of two mirrors, the seed light from the oscillation-stage
laser 50 is injected in the input side mirror 1 with a hole 7 in
the resonator in the amplification-stage laser 60. The input side
mirror 1 is applied on its reflecting surface or its chamber 3 side
surface with a total-reflection coating, and the output side mirror
2 is a partial reflecting mirror. The hole 7 has a diameter of
about 2 mm (=Vs=Hs), the amplification-stage laser 60 has a
discharge size of Ha=16 mm and Va=3 mm, and the resonator length L
is about 1 m. The effective injection pulse width of the
oscillation-stage laser 50 is P=20 ns.
[0249] With such an arrangement, there were obtained data of such
low coherence (share quantity versus visibility relations) as shown
in FIG. 15. FIG. 15 shows not only the share quantity versus
visibility relations obtained from the oscillation-stage laser 50
(oscillator) in this embodiment (using the resonator of the
invention; similar to FIG. 73) but also the share quantity versus
visibility relations in the case of using a prior art unstable
resonator.
[0250] Under the above conditions, the angle of divergence in the
horizontal direction, .theta.h, must satisfy the requirement of
0.05 mrad<.theta.h, and the angle of divergence in the vertical
direction, .theta.v, must satisfy the requirement of 1.2
mrad<.theta.v. In the oscillation-stage laser 50 according to
the above embodiment, the angle of divergence in the horizontal
direction, .theta.h, is 1 mrad and the angle of divergence in the
vertical direction, .theta.v, is 3 mrad; they satisfy the above
conditions (2) and (3). For this reason, any conversion optical
system 70 is not used in this embodiment.
[0251] From these results, it has been found that low coherence
equivalent to that in a prior art MORA laser system is achievable
while maintaining line widths and energy stability comparable to
those in the prior art MOPA laser system using an unstable
resonator.
[0252] By the way, the introduction of the seed light from the
oscillation-stage laser 50 in the resonator in the
amplification-stage laser 60 are achievable by the hole 7, 7'
provided in the center portion of the input side mirror 1, the slit
7'' formed in the center portion of the high-reflectivity mirror
coating 8 applied on the input side mirror 1, and a partial
reflecting mirror coating applied all over the input side surface
of the input side mirror 1, as described with reference to FIGS.
3-8. However, there are available some other modifications, as
described below.
[0253] FIG. 16 is illustrative of part of one exemplary
modification. FIG. 16(a) is a top view, FIG. 16(b) is a side view,
and FIG. 16(c) is a view of the input side mirror 1 in the
amplification-stage laser 60, as viewed from its chamber 3 side. In
the input side mirror 1 in the amplification-stage laser 60
according to this modification, two high-reflectivity, rectangular
plane mirrors 1.sub.1 and 1.sub.2 are arranged side by side on the
same plane with a gap 21 between their edges. The two plane mirrors
1.sub.1 and 1.sub.2 are located such that the gap 21 is narrower
than a discharge area 22 formed by discharge electrodes 4 and 5 in
the amplification-stage laser 60. In other words, the seed light 23
is introduced through the slit 21 formed between the two plane
mirrors 1.sub.1 and 1.sub.2. The high-reflectivity mirror planes of
the two plane mirrors 1.sub.1 and 1.sub.2 lie within the same
plane, and so the two plane mirrors 1.sub.1 and 1.sub.2 have the
same function as one mirror having a slit. In this embodiment, the
same function that the input side mirror 1 of FIG. 4 has is
achieved by use of two high-reflectivity mirrors 1.sub.1 and
1.sub.2. Even with such an arrangement, it is possible to obtain
line widths and energy stability equivalent to those of a prior art
MOPO system using an unstable resonator and low coherence
comparable to that in a prior art MORA laser system.
[0254] In the input side mirror 1 of FIG. 16, the two
high-reflectivity (total-reflection), rectangular plane mirrors
1.sub.1 and 1.sub.2 are arranged side by side on the same plane
with the gap 21 formed between their edges. However, the
longitudinal slit 7' formed in the center portion of the input side
mirror 1, as shown in FIG. 5, could be configured as the same slit
that forms the gap 21 in the embodiment of FIG. 17, as shown in
FIGS. 17 and 18.
[0255] In FIGS. 17 and 18, (a) is a front view of the input side
mirror 1 as viewed from its output side (the chamber 3 side), (b)
is a longitudinal section, and (c) and (d) are illustrative of in
what position relation the input side mirror 1 is located with
respect to a discharge area 22, as viewed from the output side of
the input side mirror 1.
[0256] In the embodiment of FIGS. 17 and 18, the input side mirror
1 is formed of a CaF.sub.2 or other plane substrate transparent to
ultraviolet light. The output side surface (FIG. 17(a)) of the
plane substrate is applied with a high-reflectivity
(total-reflection) mirror coating 8 except a central slit-form area
and a peripheral edge, and an antireflection coating 9 is applied
on a slit-form area 21 at the center portions of the input and
output side surfaces and on the peripheral edges thereof. FIG. 17
shows an example of the plane substrate having a rectangular shape,
and FIG. 18 shows an example of the plane substrate having a
circular shape.
[0257] With the input side mirror 1 of FIG. 16 (FIG. 16(c)), there
is difficulty in the application of coating as far as the mirror
end faces (peripheral portions) for the purpose of holding the
mirror during vapor deposition. In addition, it is not easy to
process the ends of a CaF.sub.2 or other substrate into right-angle
faces with high precision; usually, there is a chip off the ends
during fabrication. Without chip-free application of a
high-reflectivity (total-reflection) coating 8 as far as the ends,
the substrate ends having a decreased reflectivity will cause
losses leading to an oscillation efficiency drop.
[0258] If such an input side mirror 1 as shown in FIGS. 17 and 18
is used with the laser system shown in FIGS. 16(a) and 16(b), on
the other hand, it will be easy to process the ends of the
high-reflectivity (total-reflection) coating 8; it will be possible
to apply the high-reflectivity coating 8 as far as the boundary
between the seed light 23 and the amplified laser light in the
amplification-stage laser 60.
[0259] The size of the input side mirror 1 shown in FIGS. 17 and 18
should desirously be such that, as shown in FIGS. 17(c) and 17(d)
and FIGS. 18(c) and 18(d), the longitudinal length of the center
slit-form area 21 applied with the antireflection coating 9 is
longer than the discharge area 22 defined by the discharge
electrodes 4 and 5 in the amplification-stage laser 60.
[0260] As shown, the input side mirror 1 is located externally of
the laser chamber 3. Thus, even when the distance between the
discharge electrodes 4 and 5 is designed to become longer, the
laser light from the laser chamber 3 is unlikely to lie off the
input side mirror 1 as long as that distance is within the range of
the longitudinal size of the center slit-form area 21 applied with
the antireflection coating 9.
[0261] FIGS. 17(c) and 18(c) show that the longitudinal direction
of the center slit-form area 21 applied with the antireflection
coating 9 substantially matches that of the discharge area 22
defined by the discharge electrodes 4 and 5 in the
amplification-stage laser 60, and FIGS. 17(d) and 18(d) illustrate
that the longitudinal direction of the center slit-form area 21
applied with the anti-reflection coating 9 is substantially
orthogonal to that of the discharge area 22 defined by the
discharge electrodes 4 and 5 in the amplification-stage laser
60.
[0262] In the input side mirror 1 shown in FIGS. 17 and 18, it is
noted that the slit-form area 21 at the end faces of the input- and
output-side surfaces as well as the peripheral portion of the plane
substrate may not have the antireflection coating 9. With such an
arrangement wherein only two sites are provided with the
high-reflectivity (total-reflection) coating 8 and there is no
antireflection coating 9, the robustness of the input side mirror 1
to laser light can be improved due to no possibility of any coating
deterioration.
[0263] In the exemplary laser system of FIG. 16, the
total-reflection mirror 1 (FIG. 16(c), FIG. 17 and FIG. 18) and the
partial reflecting mirror 2 are each formed of a plane mirror;
however, the invention is not always limited to them as long as the
stable resonator is set up by both mirrors 1 and 2.
[0264] FIG. 19 is illustrative of part of another embodiment as in
FIG. 16. In this embodiment, the input side mirror 1 in the
amplification-stage laser 60 is built up of one high-reflectivity
(total-reflection), hole-free plane mirror. One such input side
mirror 1 is decentered in the horizontal direction with respect to
the seed light from the oscillation-stage laser 50, and located
such that its edge is positioned within or near the discharge area
22 defined by the discharge electrodes 4 and 5 in the
amplification-stage laser 60. The seed light 23 is introduced in
the amplification-stage laser 60 from outside along that edge. With
this arrangement, it is possible to prevent pits from occurring in
the profile of laser light produced out of the amplification-stage
laser 60 (there are spots of weak intensity in a center beam
portion). Notice that the optical axis of the seed light 23 could
be slightly inclined with respect to the optical axis of the input-
and output-side mirrors 1 and 2 in such a way as to fill the
discharge area with the seed light.
[0265] As a result of experimentation, the inventors have now found
that if seed light is injected in the amplification-stage laser 60
in such a way as to fill the discharge area therewith while the
optical axis C of the seed light 23 is slightly inclined with
respect to the optical axis D of the input- and output-side mirrors
1 and 2, it is then possible to obtain much lower coherence so that
the efficiency of amplification and oscillation at the
amplification-stage laser 60 can be much more enhanced.
[0266] A possible reason for this is now explained with reference
to FIG. 20. FIG. 20 is indicative of the principles of how the
amplification-stage laser 60 operates upon entrance of the seed
light 23 from the end of the discharge area 22 while the optical
axis C of the seed light 23 is slightly inclined with respect to
the optical axis D of the input- and output-side mirrors 1 and 2.
FIGS. 20(a) and 20(b) are a top view and a side view of the
resonator in the amplification-stage laser 60, respectively.
[0267] As shown in the top view of FIG. 20(a), the narrow-banded
seed light 23 leaving the oscillation-stage laser 50 (see FIG. 19)
passes through the end of the input side mirror (total-reflection
mirror) 1, and is injected in the amplification-stage laser 60 from
the side of the discharge area 22. This seed light 23 enters the
discharge area 22 while its optical axis C is at a slight angle
.alpha. (of e.g., about 0.5 mrad) with respect to the optical axis
of the resonator in the amplification-stage laser 60, and is
amplified through the discharge area 22, entering the output side
mirror (partial reflecting mirror) 2. A part of laser light
amplified upon entrance in the output side mirror 2 is produced as
laser light K1 after passing through the output side mirror 2.
Another part of laser light amplified upon entrance in the output
side mirror 2 is reflected by the output side mirror 2.
[0268] This reflected light again passes through the discharge area
22 for amplification, and then goes back to the discharge area for
amplification after entering the input side mirror 1 and reflection
thereat. The amplified laser light enters the output side mirror 2,
and a part of it is produced as laser light K2 after passing
through it while another is reflected back to the discharge area.
After such resonation is repeated, laser light K3 is produced as
the output of the amplification-stage laser 60. Here the angle of
incidence of the seed light 23 on the output side mirror 2 and the
angles of incidence and reflection of the amplified light on and at
the input- and output-side mirrors 1 and 2 are indicated by a with
respect to the optical axis D of the resonator in the
amplification-stage laser 60. In this connection, FIG. 20(a) is
also illustrative in schematic of the intensity distributions of
output laser light K1, K2 and K3.
[0269] In this way, the seed light 23 is subjected to multiple
reflections between the output side mirror (partial reflecting
mirror) 2 and the input side mirror (total-reflection mirror) 1 in
a zigzag fashion, as shown in the top view of FIG. 20(a). This
state will give birth to effects equivalent to the case where a
plurality of point light sources (S1, S2 and S3) are provided to
the output side mirror 2. Spatial coherence becomes low with
increasing light source size. Consequently, with the optical axis C
of the seed light 23 at a slight angle with respect to the optical
axis D of the input- and output-side mirrors 1 and 2, the
amplification and oscillation of laser light having low spatial
coherence could be possible in the amplification-stage laser
60.
[0270] With the resonator in the amplification-stage laser 60 shown
in FIG. 20, there are mutual misalignments in the exit positions of
output laser light K1, K2 and K3 (in the embodiment of FIG. 20,
such misalignments occur at a given spacing of, e.g., about 1 mm,
in the horizontal direction), and so the profile (energy
distribution) of the laser light leaving the output side mirror 2
comes close to a top hat form (a rectangular wave-form
distribution), allowing the energy density within a laser light
plane to become lower than that of a Gaussian beam. As a result, it
is possible to reduce damages to the optical elements in the
amplification-stage laser 60 (such as window member 17, input side
mirror 1 and output side mirror 2) as well as to optical elements
for shaping the laser beam leaving the amplification-stage laser 60
(such as a total-reflection mirror, a beam expander or the like
located in a beam delivery unit for connecting together the
two-stage laser system for aligners and an aligner).
[0271] Throughout the invention, the angle of inclination, .alpha.
(in rad), of the optical axis D of the resonator in the
amplification laser 60 with respect to the optical axis C of the
seed light 23 should desirously satisfy the relation of the
following condition:
0.0005.ltoreq.2.alpha.L.ltoreq.0.0015 (4)
Here L is the length of the resonator in the amplification-stage
laser 60.
[0272] Although described in detail later, it is preferable that
the optical path difference due to the resonator in the
amplification-stage laser 60 (an optical path difference between
laser light K1 and K2 or between K2 and K3) is set longer than the
time-based coherent length corresponding to the spectral line width
of the narrow-banded seed light 23 produced out of the
oscillation-stage laser 50, because laser light K1, K2 and K3 do
not interfere one another with the result that there are no
interference fringes on the beam profile of the laser light
produced out of the amplification-stage laser 60. This in turn
leads to not only improvements in the symmetry of the beam profile
of the output laser beam but also reductions in its fluctuations.
Thus, it is possible to provide uniform illumination to masks in
the aligner and the subjects to be exposed to light (e.g.
wafers).
[0273] Further, if the seed light 23 is injected in the discharge
area 22 while its optical axis C is slightly inclined with respect
to the optical axis D of the resonator in the amplification-stage
laser 60 as described above, then the discharge area 22 in the
amplification-stage laser 60 can then be filled in it with the seed
light 23 or its amplified light even at a small angle of divergence
of the seed light 23. This in turn allows for the oscillation of
the amplification-stage laser 60 by amplified resonation.
[0274] In this embodiment, the input- and output-side mirrors 1 and
2 are each formed of a plane mirror; however, the invention is not
necessarily limited to them as long as the stable resonator is made
up of both mirrors. For instance, if the input side mirror 1 or the
output side mirror 2 is formed of a cylindrical concave mirror,
further reductions of spatial coherence are then possible. That is,
as the cylindrical concave mirror is located such that the
generating line direction substantially matches the center axis of
the discharge direction, it gives rise to a lot more resonance
modes, resulting in further reductions of spatial coherence in the
vertical direction to the discharge direction.
[0275] FIG. 21 is illustrative of part of yet another embodiment,
as in FIG. 16. In this embodiment, the input side mirror 1 in the
amplification-stage laser 60 is made up of one high-reflectivity
(total-reflection), hole-free plane mirror. One such input side
mirror 1 is decentered in the vertical direction to the seed light
from the oscillation-stage laser 50 or upward in FIG. 21. The seed
light 23 is then introduced in the discharge area from outside
along an edge of the decentered input side mirror 1 that lies on
the opposite side with respect to the direction of decentration.
With this arrangement, it is possible to prevent pits from
occurring on the profile of laser light produced out of the
amplification-stage laser 60 (there are spots of weak intensity in
a center beam portion). It is acceptable to fill the discharge area
with the seed light 23 while the optical axis of the seed light 23
is slightly inclined with respect to the optical axis of the input-
and output-side mirrors 1 and 2. As previously stated, this
arrangement allows for a lot lower coherence, so that the
amplification and oscillation of the amplification-stage laser 60
can occur with efficiency.
[0276] In the embodiments of FIG. 19 and FIG. 21, the input side
mirror 1 is made up of one high-reflectivity (total-reflection),
hole-free plane mirror 1. However, it is noted that another input
side mirror 1 could be achieved by applying an antireflection
coating 9 to a seed light-incident area of the output side surface
of a CaF.sub.2 or other plane substrate transparent to ultraviolet
light and a high-reflectivity (total-reflection) mirror coating 8
to the remaining area, as shown in FIG. 22, FIG. 23 and FIG. 24,
respectively, with (a) being a view as viewed from the chamber 3
side and (b) being a sectional view. With each of the input side
mirrors 1 shown in FIGS. 16-21, there is difficulty in applying
coating as far as the ends of the mirror for the purpose of keeping
the mirror during vapor deposition. It is also not easy to process
the ends of the CaF.sub.2 or other substrate into right-angle faces
with high accuracy. Usually, there is a chip off the ends during
fabrication. Without chip-free application of a high-reflectivity
(total-reflection) coating 8 as far as the ends, the substrate ends
having a decreased reflectivity will cause losses leading to an
oscillation efficiency drop. If each of such input side mirrors 1
as shown in FIG. 22, FIG. 23 and FIG. 24 is used, processing of the
ends of the high-reflectivity (total-reflection) coating 8 will
then be facilitated, so that the high-reflectivity coating 8 will
be applied as far as the boundary between the seed light 23 and the
amplified laser light in the amplification-stage laser 60.
[0277] Referring here to FIGS. 23(c) and 23(d), there is shown in
what relation the input side mirror 1 is positioned with respect to
the discharge area 22 in the two-stage laser system shown in FIG.
19 or FIG. 21. Specifically, FIGS. 23(c) and 23(d) are views as
viewed from the output side of the input side mirror 1 (the side on
which the seed light 23 is incident). FIG. 23(c) shows that the
direction of the end of an area applied with a high-reflectivity
(total-reflection) coating 8 on the side of the mirror, which is
not its peripheral edge side, substantially matches the
longitudinal direction of the discharge area 22 defined by the
discharge electrodes 4 and 5, and FIG. 23(d) shows that the
direction of the end of the area applied with the high-reflectivity
(total-reflection) coating 8 on the side of the mirror, which is
not its peripheral edge side, is substantially orthogonal to the
longitudinal direction of the discharge area 22 defined by the
discharge electrodes 4 and 5. In FIGS. 23(c) and 23(d), it is
preferable that the area ratio of an area X where the area of the
input side mirror 1 applied with the high-reflectivity
(total-reflection) coating 8 makes an intersection with the
discharge area 22 and an area Y where the area of the input side
mirror 1 applied with the antireflection coating 9 makes an
intersection with the discharge area 22 is at least X<Y. The
reason is that when X>Y, the light oscillated from the
amplification-stage laser 60 goes back to the oscillation-stage
laser 50, doing damage to the optical elements in the
oscillation-stage laser 50 (especially the front mirror 52), and
bring about a drop of the laser output produced out of the front
mirror 52 in the oscillation-stage laser 50 (the output of the seed
light 23).
[0278] The size of the input side mirror 1 shown in FIG. 22, FIG.
23 and FIG. 24 should desirously be such that the length of the end
of the area applied with the high-reflectivity (total-reflection)
coating 8 on the side of the mirror, which is not its peripheral
edge side, is longer than the length of the discharge area 22
defined by the discharge electrodes 4 and 5 in the
amplification-stage laser 60 in its longitudinal direction.
[0279] As shown, the input side mirror 1 is located externally of
the laser chamber 3. Thus, even when the distance between the
discharge electrodes 4 and 5 is designed to become longer, the
laser light from the laser chamber 3 is unlikely to lie off the
input side mirror 1 as long as that distance is within the range of
the length of the end of the area applied with the
high-reflectivity (total-reflection) coating 8 and the
antireflection coating 8 on the side of the mirror, which is not
its peripheral edge side.
[0280] In the input side mirror 1 shown in FIG. 22, FIG. 23 and
FIG. 24, portions of the plane substrate other than its portion
applied with the high-reflectivity (total-reflection) coating 9 may
not be applied with the anti-reflection coating 8. With such an
arrangement wherein only one site is provided with the
high-reflectivity (total-reflection) coating 8 and there is no
antireflection coating 9, the robustness of the input side mirror 1
to laser light can be improved due to no possibility of any coating
deterioration.
[0281] In the exemplary two-stage laser system as described above,
when the seed light 23 from the oscillation-stage laser 50 is
injected in the amplification stage-laser 60, the seed light 23 is
injected from one mirror (the input side mirror 1) of the mirrors
forming the resonator in the amplification stage-laser 60 while the
seed light 23 is produced as amplified laser light out of another
mirror (the output side mirror 2). In what follows, an account will
be given of some embodiments wherein a mirror for the injection of
seed light 23 from the oscillation-stage laser 50 and a mirror
which the amplified seed light 23 leaves have a sharing mirror
function.
[0282] FIG. 25 is illustrative of one exemplary arrangement for
entrance from the output side mirror 2 of the seed light 23 from an
oscillation-stage laser 50. FIG. 25 is illustrative in side
arrangement of the oscillation-stage laser 50 and the
amplification-stage laser 60. The seed light from the line
narrowing oscillation-stage laser 50 comprising a line narrowing
module 51 is reflected by two 45.degree. right-angle prisms 101 and
102 in this order, entering an exit side mirror (partial reflecting
mirror) 2 that is one mirror forming a resonator in the
oscillation-stage laser 60. A substantial part of the seed light 23
passes through the exit side mirror 2 for injection in the
amplification-stage laser 60, although the remaining slight part is
reflected at the entrance surface of the exit side mirror 2. The
injected seed light 23 passes through the discharge area 22 defined
by discharge electrodes 4 and 5 in the amplification-stage laser 60
for reflection by a rear side mirror (total-reflection mirror) 111
that is another mirror forming the resonator in the
amplification-stage laser 60, whereupon the reflected seed light 23
again passes through the discharge area 22, leaving the output side
mirror 2.
[0283] In this embodiment, too, it is acceptable to inject the seed
light 23 having such divergence as to satisfy the above conditions
(2) and (3) in the amplification-stage laser 60. Further, if the
seed light is injected in such a way as to fill the discharge area
while the optical axis C of the seed light 23 is slightly inclined
with respect to the optical axis D of the rear- and output-side
mirrors 111 and 2, much lower coherence is then achievable so that
efficient amplification and oscillation take place at the
amplification-stage laser 60.
[0284] FIG. 26 is illustrative of the amplification-stage laser 60
in the embodiment of FIG. 25, wherein the seed light 23 is entered
in the discharge area 22 in the amplification-stage laser 60 from
its end while the optical axis C of the seed light 23 is slightly
inclined with respect to the optical axis D of the rear side mirror
111 and the output side mirror 2. Specifically, FIGS. 26(a) and
26(b) are a top view and a side view of the resonator in the
amplification-stage laser 60, respectively.
[0285] As shown in the top view of FIG. 26(a), the narrow-banded
seed light 23 produced out of the oscillation-stage laser 50 is
reflected by two 45.degree. right-angle prisms 101 and 102 in this
order (see FIG. 25), entering the exit side mirror 2 that is one
mirror that forms the resonator in the amplification-stage laser
60. A substantial part of the seed light 23 transmits through the
exit side mirror 2, although the remaining slight part is reflected
at the entrance surface of the exit side mirror 2 (as indicated by
a broken line). The transmitted seed light 23 is injected from the
side of a discharge area 22 in the amplification-stage laser
60.
[0286] Entering the discharge area 22 while the optical axis C of
the seed light 23 is set at a slight angle of inclination, .alpha.,
with respect to the optical axis D of the resonator in the
amplification-stage laser 60, the seed light 23 is amplified in the
discharge area 22, entering a rear side mirror 111 where it is
subjected to total reflection. The reflected light again passes
through the discharge area 22 for amplification, and a part of the
amplified laser light transmits through the exit side mirror
(partial reflecting mirror) 2 and is produced as laser light K1.
The remaining part of the amplified laser light is reflected by the
exit side mirror 2, going back to the discharge area 22 for
amplification.
[0287] Then, the amplified laser light is again incident on the
rear side mirror 111 where it is subjected to total reflection. The
reflected light again passes through the discharge area 22 for
amplification, and a part of the amplified laser light transmits
through the exit side mirror (partial reflecting mirror) 2 and is
produced as laser light K2. The remaining part of the amplified
laser light is reflected by the exit side mirror 2, going back to
the discharge area 22. By repetition of such resonation, laser
light K3 is produced as the output of the amplification-stage laser
60.
[0288] Here, the angle of incidence of the seed light 23 on the
output side mirror 2 and the angles of incidence and reflection of
the amplified light on and at the rear side mirror 111 and output
side mirror 2 are each set at an angle, .alpha., with respect to
the optical axis D of the resonator in the amplification-stage
laser 60. In this way, the seed light 23 is subjected to zigzag
multiple reflections between the output side mirror (partial
reflecting mirror) 2 and the rear side mirror (total-reflection
mirror) 111, as shown in the top view of FIG. 26(a). Thus, much
lower spatial coherence is achievable on the same principles as
described with reference to the principles of operation of FIG. 20.
Notice here that the output side mirror (partial reflecting mirror)
2 has a reflectivity of, e.g., 30%. Then, the efficiency of
incidence of the seed light 23 on the amplification-stage laser 60
will become 70%.
[0289] The advantage of this mode is that uniform coatings can be
applied all over the surfaces of the rear side mirror 111 and
output side mirror 2 that form together the resonator in the
amplification-stage laser 60; it is not necessary to apply such
partial coatings as shown in FIG. 17, FIG. 18, FIG. 22, FIG. 23,
and FIG. 24. This leads to another advantage that the mirrors are
easy and less expensive to fabricate, and the quality and
robustness of the coatings are improved as well. Since the output
side mirror (partial reflecting mirror) 2 has a higher
reflectivity, it is acceptable to apply no coating to the injection
site, when the efficiency of injection of the seed light 23 becomes
worse.
[0290] In this embodiment, the rear side mirror 111 and the output
side mirror 2 are each formed of a plane mirror; however, the
invention is by no means limited to it as long as the stable
resonator is set up by both mirrors. For instance, if the rear side
mirror 111 or the output side mirror 2 is configured as a
cylindrical concave mirror, much lower spatial coherence is then
achievable. Specifically, as the cylindrical concave mirror is
located such that its generating line direction substantially
matches the center axis of the discharge direction, it results in a
lot more resonance modes so that much lower coherence is achievable
in the vertical direction to the discharge direction.
[0291] On the other hand, the energy of laser light in the
resonator in the amplification-stage laser 60 will become higher
than that of the laser light produced out of the output side mirror
2 after amplification. This will offer a problem in conjunction
with the robustness of the rear side mirror 111 and output side
mirror 2 to laser light. However, this problem can be solved by
timed movement of the effective portions of these mirrors; that
robustness can be much more improved, as exemplified in FIG.
27.
[0292] FIG. 27 is illustrative of mirror holders for holding the
rear side mirror 111 and the output side mirror 2, respectively, as
viewed from directions indicated by arrows E and F in FIG. 26(a).
Specifically, FIG. 27(a) is illustrative of a mirror holder 210
with a moving stage attached to it, as viewed from the rear side
mirror 111 side (the E side of FIG. 26(a)), and FIG. 27(b) is
illustrative of a mirror holder 211 with a moving stage attached to
it, as viewed from the output side mirror 2 side (the F side of
FIG. 26(a)). These mirror holders 210 and 211, each with the moving
stage attached to it, are fixed to a plate for the fixation of the
resonator in the amplification-stage laser 60, not shown.
[0293] The mirror holder 210 for holding the rear side mirror 111
is now explained. The rear side mirror 111 is fixed to a mirror
holder portion 206, and the mirror holder portion 206 is movably
fixed to a mirror holder stage plate 203 via mirror holder guides
204 and 205. The mirror holder portion 206 is movable by the mirror
holder guides 204 and 205 in the horizontal direction (indicated by
an arrow in FIG. 27(a)) with the optical axis remaining
invariable.
[0294] One end of the mirror holder stage plate 203 on a side at a
right angle with the side provided with the mirror holder guides
204 and 205 is provided with a screw-fixing plate 202 having a
female thread portion. At this female thread portion there is held
a knobbed screw 201. The knobbed screw 201 is fixed at its distal
end with a ball 212. The knobbed screw 201 is threaded in place
such that the ball 212 comes into contact with a side portion of
the mirror holder portion 206.
[0295] On the other hand, the other end portion of the mirror
holder stage plate 203 on the side at a right angle with the side
provided with the mirror holder guides 204 and 205 is provided with
a spring-fixing member 208. One end of a spring 209 is fixed to the
spring-fixing member 208. The other end of the spring 209 is
inserted over a projection 207 attached to the mirror holder
portion 206. The spring 209 is designed and located such that its
resilient force allows the mirror holder portion 206 to be forced
against the ball 212 fixed to the distal end of the knobbed screw
201. Notice here that the projection 207 attached to the mirror
holder 206 is located at a position substantially coaxial with the
knobbed screw 201.
[0296] With such an arrangement, as the knobbed screw 201 is
rotated, it allows the rear side mirror 111 to translate
horizontally with its optical axis remaining invariable. The mirror
holder 211 for holding the exit side mirror 2 is constructed as in
the mirror holder 210.
[0297] Preferably in this embodiment, the mirror holders 210 and
211 should be symmetric with respect to a plane vertical to the
paper sheet of FIG. 27 passing through an XX axis such that the
knobbed screws 201 of the mirror holders 210 and 211 are positioned
on the same side as the amplification-stage laser 60 and their
maintenance sides are positioned in the same direction.
[0298] In the embodiment of FIG. 27, if the rear side mirror 111
and the output side mirror 2 are moved using the mirror holders 210
and 211, then the same mirrors 111 and 2 are each used three times
with the output laser light 213, so that the service life of each
mirror can be extended three times. In this embodiment, the mirror
holder portion 206 is operable to move in one direction alone.
However, the invention is not limited to it; for instance, the
mirror holder portion 206 could be located on a two-axis stage. In
FIG. 27, there is not shown the mirror inclination-adjustment
mechanism necessary for the adjustment of the optical axis of the
rear side mirror 111 and the output side mirror 2; however, that
mechanism could be located on the mirror holder 206.
[0299] Another embodiment of the arrangement wherein the mirror for
the injection of the seed light 23 from the oscillation-stage laser
50 and the mirror for producing the amplified laser light out of
the seed light 23 have a sharing mirror function is now explained
with reference to FIG. 28.
[0300] The amplification-stage laser 60 shown in FIG. 28 is used as
the amplification-stage laser 60 in the two-stage laser system
shown in FIG. 25 in place of the amplification-stage laser 60 shown
in FIG. 26. A structural difference with the amplification-stage
laser 60 shown in FIG. 26 is that an optical element for turning
light back using a total-reflection right-angle prism (roof prism)
103 is used in lieu of the rear side mirror (total-reflection)
mirror 11 in the resonator located in the amplification-stage laser
60 of FIG. 26. Other components are the same as in FIG. 26.
[0301] That is, FIG. 28 shows the amplification-stage laser 60
wherein the optical axis C of the seed light 23 is slightly
inclined with respect to the optical axis D of the total-reflection
right-angle prism 103 and the output side mirror 2 to enter the
seed light 23 in the discharge area 22 in the amplification-stage
laser 60 from its end. Specifically, FIGS. 28(a) and 28(b) are a
top view and a side view of the resonator in the
amplification-stage laser 60, respectively.
[0302] In this embodiment, too, the optical axis C of the seed
light 23 is slightly inclined with respect to the optical axis D of
the total-reflection right-angle prism 103 and the output side
mirror 2 to inject the seed light 23 in such a way as to fill the
discharge area, as previously described. Therefore, much lower
coherence is achievable, and so efficient amplification and
oscillation are achievable at the amplification-stage laser 60.
[0303] As shown in the top view of FIG. 28(a), the narrow-banded
seed light 23 produced out of the oscillation-stage laser 50 is
reflected by two 45.degree. right-angle prisms 101 and 102 in this
order (see FIG. 25), entering the output side mirror (partial
reflecting mirror) 2 that is one optical element forming the
resonator in the amplification-stage laser 60. A substantial part
of the reflected light transmits through the exit side mirror 2,
although the remaining slight part is reflected at the entrance
surface of the exit side mirror 2 (indicated by a broken line in
FIG. 28(a). The transmitted seed light 23 is injected from the side
of the discharge area 22 in the amplification-stage laser 60.
[0304] This seed light 23 enters the amplification-stage laser 60
with its optical axis C set at a slight angle of inclination,
.alpha., with respect to the optical axis D of the resonator in the
amplification-stage laser 60. In the discharge area 22, the seed
light 23 is amplified, and then subjected to Fresnel total
reflection at the surfaces 103.sub.1 and 103.sub.2 of the
total-reflection right-angle prism 103 (reflection at an angle of
incidence larger than the critical angle).
[0305] Notice here that this embodiment works differently than the
embodiment of FIG. 26. More specifically, the incident laser light
is totally reflected twice at the surfaces 103.sub.1 and 103.sub.2
of the total-reflection right-angle prism 103, so that the output
laser light goes back the way that it has come. This turned-back
laser light again passes through the discharge area 22 where it is
amplified. A part of the amplified light transmits through the exit
side mirror (partial reflecting mirror) 2, and is produced as laser
mirror K1. The rest is reflected by the exit side mirror 2, going
back to the discharge area 22 where it is amplified.
[0306] Then, the amplified laser light again enters the
total-reflection right-angle prism 103 where it is totally
reflected. The totally reflected light again goes back the way that
it has come, again passing through the discharge area 22 where it
is amplified. A part of the amplified laser light transmits through
the exit side mirror (partial reflecting mirror) 2, and is produced
as laser light K2. The rest is reflected by the exit side mirror 2,
going back to the discharge area 22 where it is amplified. By
repetition of such resonance, laser light K3 is produced as the
output of the amplification-stage laser 60.
[0307] Here the angle of incidence of the seed light 23 on the
output side mirror 2 and the angles of incidence and reflection of
the amplified light on and at the total-reflection right-angle
prism 103 and the output side mirror 2 becomes .alpha.. In this
way, as shown in the top view of FIG. 28(a), the seed light 23 is
subjected to zigzag multi-reflection between the output side mirror
(partial reflecting mirror) 2 and the total-reflection right-angle
prism 103. Thus, lower spatial coherence is achievable on the same
principles as described with reference to FIG. 23 that is
illustrative of the principles of operation.
[0308] With the embodiment explained with reference to FIG. 28, the
additional following advantages are obtainable in addition to the
advantages equivalent to those previously explained with reference
to FIG. 26. In this embodiment, the laser light is turned back by
the total-reflection right-angle prism 103. For this reason, even
when there is an uneven amplification intensity distribution in the
longitudinal direction (discharge direction) of the amplification
gain that is the discharge area 22, the symmetry and stability of
the output laser light in the longitudinal direction are improved
because the laser light passes through both upper and lower regions
of the discharge area 22.
[0309] More specifically, as the frequency of repetition of laser
oscillation grows (e.g., 3,000 to 4,000 Hz), discharge between the
discharge electrodes 4 and 5 causes standing waves to occur due to
acoustic waves, giving rise to uneven amplification gain
distribution and refractive index in the longitudinal direction
(discharge direction). On the contrary, if the laser light is
turned back by the total-reflection right-angle prism 103 for
re-amplification, then it is possible to maintain the
post-amplification uniformity, symmetry and stability of the laser
light. It is also possible to achieve much lower coherence. Notice
here that to obtain such advantages, the ridgeline of the
reflecting surfaces 103.sub.1 and 103.sub.2 of the total-reflection
right-angle prism 103 (the ridgeline of the roof) must be directed
in a substantially vertical direction to the discharge direction
(see FIG. 28(b)).
[0310] In this embodiment, the entrance surface of the
total-reflection right-angle prism 103 may or may not be applied
with an antireflection coating. However, the entrance surface of
the total-reflection right-angle prism 103 must be inclined with
respect to the reflecting surface of the output side mirror 2 for
the purpose of preventing parasitic oscillation with respect to the
amplification resonator and the optical axis C of the seed
light.
[0311] In the above embodiment of the two-stage laser system, when
the seed light 23 is injected from the oscillation-stage laser 50
in the amplification-stage laser 60, the seed light 23 is injected
therein from one mirror (input side mirror 1) forming the resonator
therein, and the seed light 23 is produced as amplified laser light
out of the other mirror (output side mirror 2). The mirror for the
injection of the seed light 23 from the oscillation-stage laser 50
and the mirror out of which the seed light 23 is produced as
amplified laser light have a sharing function. In any case, the
seed light 23 passes through the discharge area 22 just upon
entrance and transmission of the seed light 23 in and through one
mirror that forms the resonator in the amplification-stage laser
60.
[0312] FIG. 29 and FIG. 30 show a modification to the embodiment of
the invention wherein the seed light 23 is injected in the
amplification-stage laser 60 from one mirror (input side mirror 1)
that forms the resonator therein and produced as amplified laser
light out of the other mirror (output side mirror 2). More
specifically, the seed light 23 enters and transmits through the
input side mirror 1 in the amplification-stage laser 60, and
arrives at the output side mirror 2 through an area other than the
discharge area 22, where it is reflected. Then, the reflected light
passes through the discharge area 22.
[0313] If viewed from the discharge area 22, this embodiment will
be equivalent to the case where the mirror for the injection of the
seed light 23 from the oscillation-stage laser 50 and the mirror
out of which the seed light 23 is produced as amplified laser light
have a sharing function. That is, the seed light 23 enters the
discharge area 22 from its output side, where it is amplified, and
then leaves the discharge area 22.
[0314] FIG. 29 is illustrative of one exemplary embodiment of the
amplification-stage laser 60. Specifically, FIGS. 29(a) and 29(b)
are a top view and a side view of the resonator in the
amplification-stage laser 60.
[0315] As shown in the view of FIG. 30(a) as taken from a direction
indicated by an arrow E in FIG. 29(a), the construction of the
input side mirror 1 in this embodiment is the same as that of the
input side mirror 1 shown in FIG. 22, FIG. 23 and FIG. 24. That is,
an antireflection coating 9 is applied to the area of a CaF.sub.2
or other plane substrate transparent to ultraviolet light, which is
to receive the seed light 23, and a high-reflectivity
(total-reflection) coating 8 is applied to the rest.
[0316] On the other hand, the construction of the output side
mirror 2 is shown in the view of FIG. 30(b) as taken from a
direction indicated by an arrow F in FIG. 29(a). A
high-reflectivity (total-reflection) mirror coating 8 is applied to
the area of a CaF.sub.2 or other plane substrate transparent to
ultraviolet light, which is to receive the seed light 23, and a
partial reflecting mirror coating 10 is applied to the rest.
[0317] Referring to FIG. 29, the seed light 23 produced out of the
oscillation-stage laser 50 (see FIG. 19 or FIG. 21) enters and
transmits through a transmitting portion of the input side mirror 1
(the area provided with the anti-reflection coating 9) and then
through an area of the amplification-stage laser 60 other than the
discharge area 22, and enters a total-reflection portion of the
output side mirror 2 (the area applied with the high-reflectivity
(total-reflection) mirror coating 8), at which it is totally
reflected toward the discharge area 22.
[0318] In the amplification-stage laser 60 shown in FIG. 29, the
optical axis C of the seed light 23 is inclined by a slight angle
.alpha. with respect to the optical axis D of the resonator.
[0319] In this embodiment, too, the optical axis C of the seed
light 23 is slightly inclined with respect to the optical axis D of
the resonator to fill the discharge area with the seed light by
injection. Therefore, much lower coherence is achievable, and
efficient amplification and oscillation take place at the
amplification-stage laser 60 as well, as previously described.
[0320] Upon incidence on the total-reflection portion of the output
side mirror 2 and total reflection toward the discharge area 22,
the seed light 23 passes through the discharge area 22 where it is
amplified. Then, the amplified laser light enters the
total-reflection portion of the input side mirror 1 (the area
applied with the high-reflectivity (total-reflection) mirror
coating 8), where it is totally reflected.
[0321] The reflected light again passes through the discharge area
22 where it is amplified, entering the partial reflecting portion
of the output side mirror 2 (the area applied with the partial
reflecting mirror coating 10), where it is amplified. A part of the
amplified laser light transmits through the output side mirror 2,
leaving it as laser light K1. The rest is reflected there, going
back to the discharge area 22.
[0322] The reflected light that has gone back to the discharge area
22 again passes through the discharge area 22 where it is
amplified. Then, the amplified light enters the total-reflection
portion of the input side mirror 1, where it is totally reflected.
Then, the amplified laser light enters the partial reflecting
portion of the output side mirror 2, where it is amplified. A part
of the amplified laser light transmits through the output side
mirror 2, leaving it as laser light K2. The rest is reflected
there, going back to the discharge area 22. By repetition of such
resonance, T3 is produced as the output of the amplification-stage
laser 60.
[0323] Here the angle of incidence of the seed light 23 on the
output side mirror 2 and the angles of incidence and reflection of
the amplified light on and at the input side mirror 1 and the
output side mirror 2 become a with respect to the optical axis D of
the resonator in the amplification-stage laser 60.
[0324] In this way, the seed light 23 is subjected to zigzag
multiple reflections between the output side mirror 2 and the input
side mirror 1, as shown in the top view of FIG. 29(a).
[0325] The advantage of this embodiment is that the seed light 23
can be injected with efficiency in the amplification-stage laser.
FIG. 30(a) is illustrative of in what relation the input side
mirror 1 is positioned to the discharge area and the seed light 23
as viewed from a direction indicated by an arrow E in FIG. 29(a),
and FIG. 30(b) is illustrative of in what relation the output side
mirror 2 is positioned to the discharge area and the seed light 23
as viewed from a direction indicated by an arrow F in FIG.
29(a).
[0326] In this embodiment, the seed light 23 is entered from a
transmitting portion of the input side mirror 1 in the discharge
area 22 in the amplification-stage laser 60 at a position slightly
spaced away from it. As viewed from a direction E (see FIG. 29(a)),
the discharge area 22 is positioned such that it overlaps the
total-reflection portion of the input side mirror 1 and the end of
the discharge area 22 substantially matches the end of the
total-reflection portion (FIG. 30(a)). As viewed from a direction F
(see FIG. 29(a)), the discharge area 22 is positioned such that the
boundary line between the total-reflection portion and the partial
reflection portion of the output side mirror 1 substantially
matches the end of the discharge area 22 and the discharge area 22
overlaps the partial reflection portion (FIG. 30(b)).
[0327] No application of the coating 10 to the transmitting portion
of the input side mirror 1 and the partial reflecting portion of
the output side mirror 1 provides three advantages as described
just below.
[0328] (1) A common material can be used for both mirrors 1 and
2.
[0329] (2) Both mirrors 1 and 2 are easier to fabricate.
[0330] (3) There is no partial reflecting film (coating 10) at the
partial reflecting portion of the output side mirror 2; robustness
is improved because of no coating deterioration (even when there is
no partial reflecting mirror coating 10, Fresnel reflection allows
the output side mirror to work as a partial reflecting mirror).
[0331] When, as in the embodiment of FIG. 20 or FIG. 29, the seed
light 23 is injected from the input side mirror 1 in the
amplification-stage laser with its optical axis C slightly inclined
with respect to the optical axis D of the resonator, too, it is
acceptable to use the total-reflection right-angle prism (roof
prism) employed in the embodiment of FIG. 28 in place of the input
side mirror 1 applied with the high-reflectivity (total-reflection)
mirror coating 8.
[0332] By the way, the inventors have found that when the seed
light 23 is entered in the discharge area 22 in the
amplification-stage laser 60 from its end while, as in FIG. 20,
FIG. 26 and FIG. 29, the optical axis C of the seed light 23 is
slightly inclined with respect to the optical axis D of the plane
input side mirror 1 or the rear side mirror 111 and the plane
output side mirror that form the resonator in the
amplification-stage laser 60, it is possible to run the laser
system more efficiently while keeping low-coherent characteristics,
for instance, by properly inclining the input side mirror 1 or the
rear side mirror 111 with respect to the output side mirror 2.
[0333] First of all, consider the degree of flexibility in the
condition for the injection of seed light 23. Typically in the
arrangement of FIG. 31 analogous to that of FIG. 19, the length, L,
of the resonator is defined by the distance between the input side
mirror (or the rear side mirror) 1 and the output side mirror 2 in
the amplification-stage laser 60, and the effective width of the
discharge area 22 effective for amplification is defined as Wx (mm)
in the section of FIG. 19(a), and Wy (mm) in the section of FIG.
19(b).
[0334] In view of the position and angle of the seed light 23 at
the position of the input side mirror 1, consider the condition
under which the seed light 23 can make a given frequency of
roundtrips in the resonator (input side mirror 1 and output side
mirror 2) in the amplification-stage laser 60 to effectively obtain
it as laser output. For instance, when the seed light 23 enters a
portion of the input side mirror (rear side mirror) 1 applied with
a high-reflective coating, as shown in the schematic section of
FIG. 32 as taken in the vertical direction (x direction) to the
resonator in the amplification-stage laser 60 and the discharge
direction of the discharge area 22, it will be reflected by the
input side mirror (rear side mirror) 1 at whatever angle.
Therefore, the seed light will be incapable of being effectively
taken as laser output.
[0335] Next, consider the incidence of the seed light 23 from a
position near the edge portion of the input side mirror (rear side
mirror) 1. As the angle of incidence is too shallow (or as the seed
light 23 is incident at an angle almost vertical to the output side
mirror 2), the seed light 23 will be incapable of entering the
discharge area 22, and it will be incapable of entering the
high-reflectivity mirror coating area of the input side mirror
(rear side mirror) 1 upon making roundtrips in the resonator; in
any case, the seed light 23 will escape from the system.
[0336] As the angle of incidence is too tight (or the seed light 23
is obliquely incident on the output side mirror 2), conversely, the
seed light 23 will deviate from the discharge area 22 after
reflection at the output side mirror 2; it will not provide any
effective laser output.
[0337] From such points of view, it is possible to derive the
condition necessary for the position and angle of the seed light 23
at the input side mirror (rear side mirror) 1, under which the seed
light 23 makes a given frequency of roundtrips in the resonator in
the amplification-stage laser 60 to provide effective output laser
light.
[0338] As shown in FIG. 35 as an example, assume that a z-axis is
set in a direction that passes the center of the discharge area 22
and runs along the discharge electrodes; an x-axis is set in a
direction vertical to the discharge direction, the origin is set at
the position of the input side mirror (rear side mirror) 1; with
respect to the x-axis, an upper site of the paper is taken as
positive; and with respect to the angles of the seed light 23 and
the mirror 1, 2, the counterclockwise direction is taken as
positive, and when the normal axis to the mirror lies in a z-axis
direction, those angles are taken as zero. Here let Xin and
.theta.in be the position and angle of injection of the seed light
23 at the position of the input side mirror (rear side mirror) 1, L
be the resonator length, 8' be the angle of inclination of the
input side mirror (rear side mirror) 1, and the angle of
inclination of the output side mirror 2 be zero. Then, the
positions Xn and Xn+0.5 of the seed light 23 at an n-th roundtrip
(the position of the input side mirror (rear side mirror) 1) or a
n+0.5-th roundtrip (the position of the output side mirror 2) are
written as
Xn=Xin+2nL.theta.in+2n(n-1)L.theta.' (5)
Xn+0.5=Xin+(2n+1)L.theta.in+2n.sup.2L.theta.' (6)
[0339] From these equations, the position and angle of injection of
the seed light 23 at the position of the input side mirror (rear
side mirror) 1 needed to take effective laser output out of the
output side mirror 2 with no deviation from the discharge area 22
are calculated depending on the frequency of roundtrips to be taken
into account.
[0340] Typically, consider now the embodiment (FIG. 31) wherein the
input side mirror 1 is decentered in the horizontal direction
(x-axis direction), and assume that the seed light makes six
roundtrips under the conditions that the resonator length is L=1 m,
the discharge width is Wx=2.5 mm, the input side mirror 1 is
arranged parallel with the output side mirror 2, and the edge of
the input side mirror 1 is in alignment with the end of the
discharge area 22 (FIG. 36). As long as the polygonal region
condition shown in FIG. 27 is satisfied, it is possible to obtain
effective laser output. Although depending on the size, beam
divergence angle, etc. of the seed light 23 to be injected, it is
desired that the area of this polygonal region be as large as
possible.
[0341] In FIG. 37, the input side mirror 1 is arranged parallel
with the output side mirror 2. When the input side mirror 1 is
inclined by +0.04 mrad, on the other hand, a region capable of
effectively taking laser light may be found from FIG. 38. As can be
seen from FIG. 38, slight inclination of the input side mirror 1
ensures that there is an increase in the area of the region capable
of effectively taking laser light (which results in an increase in
the degree of flexibility in the injection of the seed light 23),
and low coherence is achievable as in the case of a parallel
arrangement. It is thus possible to achieve a laser system having
improved output.
[0342] In this case, the input side mirror 1 is inclined with
respect to the output side mirror 2 in such a direction that in
view of the distance L between the input side mirror 1 and the
output side mirror 2, the seed light 23 oscillated out of the
oscillation-stage laser 50 is incident from the side where the
mirror-to-mirror spacing becomes wide with the inclination of one
mirror.
[0343] With this arrangement wherein the resonator built up of two
plane mirrors in the amplification-stage laser 60 is set such that
one mirror is slightly inclined, not in parallel, with respect to
the other, the width of spectra occurring through discharge at the
amplification-stage laser 60 decreases in gain relative to broad
natural light emissions, with the result that the broadband ratio
becomes lower than that in the arrangement wherein two mirrors are
arranged in parallel. In other words, it is required for the
oscillation-stage laser 50 to have the desired peak intensity so as
to meet the requirement for the desired broadband ratio or lower,
as set forth in Japanese Patent Application No. 2003-130447. As
described above, however, if the resonator is built up of two
non-parallel mirrors, then the peak level can be much more reduced
down.
[0344] In view of the frequency of roundtrips in the resonator,
there is a large difference between when the resonator mirrors are
parallel and when they have a mutual proper inclination, which
ensures that there is an extension of the pulse width of laser
light. In consideration of the service life of a semiconductor
aligner, it is desired that the laser pulse width be as long as
possible.
[0345] This is now considered in detail. When the resonator mirrors
have a mutual proper inclination as shown in FIG. 39(a), the seed
light 23 is obliquely incident on the resonator due to the relation
of the edge portion of the input side mirror 1 to the discharge
area 22. When one mirror is properly inclined as shown in FIG.
39(a), the seed light can make more roundtrips in the resonator in
the amplification-stage laser as compared to when the resonator
mirrors are parallel as shown in FIG. 39(b), so that far higher
laser output is achievable with a further extension of the pulse
width.
[0346] It is noted that when the inclination of the input side
mirror 1 ranges from 0.0 mrad to 0.16 mrad, the ensuing laser
system output surpasses that of the laser arrangement wherein the
high-reflectivity side plane of the input side mirror 1 is parallel
with the partial-reflectivity side plane of the output side mirror
2. It is to be understood that this range also varies with changes
in the resonator length, discharge width and the frequency of
roundtrips to be taken into account, as can be seen from the
aforesaid equations (5) and (6). For instance, given three
roundtrips, output surpassing that of the parallel resonator mirror
arrangement will be obtained in the range of 0.0 mrad to 0.87
mrad.
[0347] In any case, the above range can relatively easily be
derived on the basis of equations (5) and (6), and if the resonator
is designed while this range is taken into consideration, increased
laser output, extended pulse width, the degree of flexibility in
the injection of the seed light 23 and decreased peak intensity of
the oscillation-stage laser will then be achieved.
[0348] From another point of view, consider here how the input side
mirror (rear side mirror) 1 and the output side mirror 2 are
inclined with respect to the optical axis C of the seed light 23
(see FIG. 20) in the case where two plane mirrors forming the
resonator in the amplification-stage laser 60 are located in a
non-parallel fashion.
[0349] Unlike the case of FIG. 35, set a z-axis in the direction of
travel of the seed light 23 and an x-axis in the discharge
direction or in the vertical direction to the discharge direction
with the origin at a position of the input side mirror (rear side
mirror) 1, on which the seed light 23 is to be incident, as shown
in FIG. 40, and assume that with respect to the x-axis, an upper
site of the paper is taken as positive; with respect to the angles
of the seed light 23 and the mirror 1, 2, the clockwise direction
is taken as positive; and when the normal to the mirror 1, 2 lies
in the z-axis direction, those angles are taken as zero. Here let
.theta..sub.R be the angle of inclination of the input side mirror
(rear side mirror) 1 and .theta..sub.F be the angle of inclination
of the output side mirror 2. The seed light 23 is assumed to be
injected in the origin of the x-axis coordinates at an angle of
inclination of zero.
[0350] The angle at which the light travels after reflection at
each mirror 1, 2 is written as
.theta..sub.NF=2N.theta..sub.F-2(N-1).theta..sub.R
.theta..sub.NR=2N.theta..sub.F-2N.theta..sub.R
Here the suffixes "NF" and "NR" represent light rays after an N-th
reflection at the output side mirror 2 and the input side mirror
(rear side mirror) 1, respectively.
[0351] The coordinates for the point of reflection after N
roundtrips are written as
X.sub.NR=2N.sup.2.theta..sub.FL-2N(N-1).theta..sub.RL
X.sub.NF=2N(N+1).theta..sub.FL-2N.sup.2.theta..sub.RL
Here the suffixes "NF" and "NR" represent the points of N-th
reflection at the output side mirror 2 and the input side mirror
(rear side mirror) 1, respectively, and L stands for the length of
the resonator in the amplification-stage laser.
[0352] Unless X.sub.1R>0, the light will not be reflected at the
input side mirror (rear side mirror) 1 after one roundtrip. It is
therefore required to satisfy
.theta..sub.F>0
[0353] Upon N roundtrips, the condition for reflecting light at the
input side mirror (rear side mirror) 1 becomes
X.sub.NR>0
Therefore,
[0354] .theta..sub.R<N/(N-1).times..theta..sub.F
[0355] From the foregoing, the conditions for reflecting light at
the input side mirror (rear side mirror) 1 upon N roundtrips
become
.theta..sub.F>0, and
.theta..sub.R<N/(N-1).times..theta..sub.F
[0356] Now, to make more resonances in the effective amplification
area as compared with (.theta..sub.F=.theta..sub.R), position
variations in the input side mirror (rear side mirror) 1 or the
output side mirror 2 must be reduced with the frequency of
roundtrips in the resonator. Similar results are obtained with any
mirror; reference is then made to the output side mirror 2.
[0357] The above condition becomes
X.sub.N+2 F-X.sub.N+1 F<X.sub.N+1 F-X.sub.NF
Therefore,
[0358] .theta..sub.F<.theta..sub.R
[0359] From a combination of this with the conditions as provided
above, the condition for reflecting light at the output side mirror
1 up to N roundtrips and reducing the position variations
becomes
0<.theta..sub.F<.theta..sub.R.ltoreq.N/(N-1).times..theta..sub.F
(7)
[0360] That is, it is required in FIG. 40 that both the input side
mirror (rear side mirror) 1 and the output side mirror 2 be
inclined in the clockwise direction, and when the angle of
inclination of the output side mirror 2 is .theta..sub.F, the angle
of inclination .theta..sub.R of the input side mirror (rear side
mirror) 1 be somewhat lager than .theta..sub.F. This is tantamount
to the injection of the seed light 23 from the side where the
distance between the mirrors 1 and 2 is longer and they are
mutually open.
[0361] Typically, given N=5 and .theta..sub.F=0.5 mrad,
0.5 mrad<.theta..sub.R<0.625 mrad
so that the angle of aperture between the input side mirror (rear
side mirror) 1 and the output side mirror 2 lies in the range of 0
to 0.125 mrad.
[0362] In the above discussions, absolute values are not attached
to the inequality regarding position variations.
[0363] However, it is more desirous to attach the absolute value to
the inequality for comparison purposes. The reason could be that
when there is no absolute value sign, any shifts of light in the
negative direction of the x-axis are allowable.
[0364] In the state with the absolute value signs attached, from
|X.sub.N+2 F-X.sub.N+1 F|<|X.sub.N+1 F-X.sub.NF|,
.theta..sub.F<.theta..sub.R<(2N+3)/(2N+2).times..theta..sub.F
is derived. From a combination with the condition of
.theta..sub.F>0, the condition for reflecting light at the
output side mirror 2 up to N roundtrips and reducing the absolute
value of position variations becomes
0<.theta..sub.F<.theta..sub.R<(2N+3)/(2N+2).times..theta..sub.F
(8)
[0365] Typically, given N=5 and .theta..sub.F=0.5 mrad,
0.5 mrad<.theta..sub.R<0.542 mrad
That is, the angle of aperture between the input side mirror (rear
side mirror) 1 and the output side mirror 2 falls in the range of 0
to 0.042 mrad.
[0366] In any event, it is found that both the input side mirror
(rear side mirror) 1 and the output side mirror 2 must be inclined
in the same direction with respect to the optical axis C of the
seed light 23, the angle of inclination .theta..sub.R of the input
side mirror (rear side mirror) 1 must be somewhat larger than the
angle of inclination .theta..sub.F of the output side mirror 2, and
the seed light 23 must be injected from the side on which the
distance between the mirrors 1 and 2 is longer and the angle of
aperture between them is larger. It is then preferable that the
angle of aperture between both mirrors 1 and 2 is in the range of
0.01 mrad to 0.2 mrad. It is here noted that when the seed light 23
is injected from the output side mirror 2 side, .theta..sub.R and
.theta..sub.F are interchangeable.
[0367] FIG. 31 is illustrative of an embodiment of the invention
wherein, as in FIG. 16, both the input side mirror 1 and the output
side mirror 2 are inclined in the same direction with respect to
the optical axis C of the seed light 23, the angle of inclination
of the input side mirror 1 is somewhat larger than the angle of
inclination of the output side mirror 2, and the seed light 23 is
injected from the side on which the distance between both the
mirrors 1 and 2 is longer and the angle of aperture between them is
larger. For the purpose of illustration, the seed light 23 makes a
mere 1.5 roundtrips in the amplification-stage laser 60; in actual
applications, however, there are set a lot more roundtrips. For the
purpose of illustration, light is drawn as one single line; in
actual applications, however, the light is a beam having a finite
width and a finite divergence angle. Throughout the following
drawings regarding the inclinations of the input side mirror (rear
side mirror 111) 1 and the output side mirror 2, mirrors and the
degree of inclination of optical axies are exaggerated for
convenience of illustration.
[0368] As in FIG. 19, this is directed to an embodiment wherein the
input side mirror 1 in the amplification-stage laser 60 is formed
of one high-reflectivity (total-reflection), hole-free plane
mirror. Typically, this input side mirror 1 is applied with a
high-reflectivity coating on the entire surface of its side near to
the chamber 3 in the amplification-stage laser 60. The input side
mirror 1 is also typically provided on its entire back surface with
a reflectivity-free coating (anti-reflection coating) and/or a
suitable wedge angle for the purpose of preventing interferences
between the two surfaces. The output side mirror 2 is typically
provided on the entire surface of its side near to the chamber 3 in
the amplification-stage laser 60 with a partial reflecting mirror
coating (having a reflectivity of typically 10% to 50%) in such a
way as to have an optimum reflectivity in the laser system. The
output side mirror 2 is also typically provided on its entire back
surface with a reflectivity-free coating (antireflection coating)
and/or a suitable wedge angle for the purpose of preventing
interferences between the two surfaces.
[0369] The input side mirror 1 is located such that it is
decentered in the horizontal direction (within the plane of the top
view (a) paper) with respect to the seed light 23 from the
oscillation-stage laser 50, and its high-reflectivity side plane is
not parallel with the partial reflectivity side plane of the output
side mirror 2. More specifically in view of the top view (a), that
plane is located in such a way as to have a proper inclination and
the edge of the input side mirror 1 is positioned within or near
the discharge area 22 defined by the discharge electrodes 4 and 5
in the amplification-stage laser 60. In view of the
high-reflectivity side plane of the input side mirror 1 and the
partial reflectivity side plane of the output side mirror 2, the
direction of that inclination is such that at the edge portion of
the input side mirror 1 in which the seed light 23 is to be
introduced, the distance between the two mirrors is longer than
that between the opposite mirrors. Then, to satisfy inequality (7)
or (8) as described above, the angle of inclination of the input
side mirror 1 with respect to the optical axis C of the seed light
23 is somewhat larger than that of the output side mirror 2 on the
same side.
[0370] With this arrangement, it is possible to prevent pits from
occurring in the profile of the laser light produced out of the
amplification-stage laser 60 (spots of weak light intensity in the
center beam portion).
[0371] The value of the "proper inclination .beta." used herein has
previously been specified. More specifically in a laser system
having fixed other factors such as gas pressure, applied voltage,
and energy of the seed light 23, that value is set such that the
laser system output lies in the range G that does not fall short of
the output S of the laser system wherein the high-reflectivity side
plane of the input side mirror 1 is parallel with the partial
reflectivity side plane of the output side mirror 2 (FIG. 20).
[0372] FIG. 42 is illustrative of one exemplary arrangement wherein
the resonator in the amplification-stage laser 60 is made up of two
non-parallel mirrors, and the seed light 23 from the
oscillation-stage laser 50 is entered from the output side mirror
2, as in FIG. 25. FIGS. 42(a) and 42(b) are a top view and a side
view of that arrangement, respectively, and FIG. 42(c) is
illustrative of the output side mirror 2 in the amplification-stage
laser 60, as viewed from the chamber 3 side. In this arrangement,
the seed light 23 is introduced from outside the edge of the output
side mirror 2 along it. In this case, the seed light 23 is entered
from the output side mirror 2, and so the opposite mirror is called
the rear side mirror 111. Typically, the rear side mirror 111 is
applied with a high-reflectivity coating all over the surface of
the side near the chamber 3 in the amplification-stage laser 60.
The entire back surface of the rear side mirror 111 is typically
applied with a reflectivity-free coating (anti-reflection coating)
and/or a suitable wedge angle for the purpose of preventing
interferences between the two surfaces. The output side mirror 2 is
typically provided on the entire surface of its side near the
chamber 3 in the amplification-stage laser 60 with a partial
reflecting mirror coating (having a reflectivity of typically 10%
to 50%) in such a way as to have an optimum reflectivity in the
laser system. The output side mirror 2 is typically provided on its
entire back surface with a reflectivity-free coating
(antireflection coating) and/or a suitable wedge angle for the
purpose of preventing interferences between the two surfaces.
[0373] In this case, the output side mirror 2 is located such that
it is decentered in the horizontal direction (within the plane of
the top view (a) paper) with respect to the seed light 23 from the
oscillation-stage laser 50, and its partial reflectivity side plane
is not parallel with the high-reflectivity side plane of the rear
side mirror 111. More specifically in view of the top view (a),
that plane is located in such a way as to have a proper inclination
and the edge of the output side mirror 2 is positioned within or
near the discharge area 22 defined by the discharge electrodes 4
and 5 in the amplification-stage laser 60. In view of the partial
reflectivity side plane of the output side mirror 2 and the
high-reflectivity side plane of the rear side mirror 111, the
direction of that inclination is such that at the edge portion of
the output side mirror 2 into which the seed light 23 is to be
introduced, the distance between the two mirrors is longer than
that between the opposite mirrors. In this case, the angle of
inclination of the output side mirror 2 with respect to the optical
axis C of the seed light 23 is somewhat larger than that of the
rear side mirror 111 on the same side (contrary to FIG. 40).
[0374] The value of the "proper inclination .beta." used herein,
too, has previously been specified. More specifically, as shown in
FIG. 41, that value is set such that the laser system output lies
in the range G that does not fall short of the output S of the
laser system wherein the high-reflectivity side plane of the rear
side mirror 111 is parallel with the partial reflectivity side
plane of the output side mirror 2 (FIG. 20). In view of the partial
reflectivity side plane of the output side mirror 2 and the
high-reflectivity side plane of the rear side mirror 111, the
direction of that inclination is such that at the edge portion of
the output side mirror 2 into which the seed light 23 is to be
introduced, the distance between the two mirrors is longer than
that between the opposite mirrors.
[0375] One advantage of this arrangement is that smaller seed light
can be used as the seed light 23, because upon injection in the
amplification-stage laser 60, it is the rear side mirror 111 of
high reflectivity that it strikes at first. As shown in FIG. 42(c),
however, a problem with the arrangement is that the output side
mirror 2 is decentered for the entrance of the seed light 23, and
so the beam is of somewhat limited size. In FIG. 42, the seed light
23 is shown to make only two roundtrips in the amplification-stage
laser 60 for the purpose of illustration; in actual applications,
however, there are set a lot more roundtrips. Likewise for the
purpose of illustration, the light is drawn as one single line;
however, it is a beam having a finite width and a finite divergence
angle.
[0376] In the embodiment of FIG. 31, the input side mirror 1 is
made up of one high-reflectivity (total-reflection), hole-free
plane mirror. As shown in FIG. 43(a) as viewed from the chamber 3
side and in the sectional view of FIG. 43(b), however, the input
side mirror 1 could be achieved by applying the antireflection
coating 9 to a seed light-incident area of the output side of a
CaF.sub.2 or other plane substrate transparent to ultraviolet light
and the high-reflectivity (total-reflection) mirror coating 8 to
the rest. That is, with the input side mirror 1 used in FIG. 31,
there is difficulty in the application of coating as far as the
mirror end faces for the purpose of holding the mirror during vapor
deposition. In addition, it is not easy to process the ends of the
CaF.sub.2 or other substrate into right-angle faces with high
precision; usually, there is a chip off the ends during
fabrication. Without chip-free application of the high-reflectivity
(total-reflection) coating 8 as far as the ends, the substrate ends
having a decreased reflectivity will cause losses leading to an
oscillation efficiency drop. If such an input side mirror 1 as
shown in FIG. 43 is used, it will be easy to process the ends of
the high-reflectivity (total-reflection) coating 8; it will be
possible to apply the high-reflectivity coating 8 as far as the
boundary between the seed light 23 and the amplified laser light in
the amplification-stage laser 60. Alternatively, the antireflection
coating 9 could be dispensed with; there could be no coating at
all. FIG. 44 is illustrative of an embodiment corresponding to FIG.
31, wherein this input side mirror 1 is used.
[0377] In the embodiment of FIG. 42, the output side mirror 2 is
made up of one partial reflectivity, hole-free plane mirror. As
shown in FIG. 45(a) as viewed from the chamber 3 side and in the
sectional view of FIG. 44(b), however, the output side mirror 2
could be achieved by applying the antireflection coating 9 to a
seed light-incident area of the chamber 3 side of a CaF.sub.2 or
other plane substrate transparent to ultraviolet light and the
partial reflecting mirror coating 10 to the rest for similar
reasons. FIG. 46 is illustrative of an embodiment corresponding to
FIG. 42, wherein this output mirror 2 is used.
[0378] While, in the above embodiments, the input side mirror 1,
the rear side mirror 111 and the output side mirror 2 are all in
rectangular form, it is to be understood that they could have any
desired shape without deviating from the purport of the
invention.
[0379] Further, when the resonator in the amplification-stage laser
60 is built up of two nonparallel mirrors, the input side mirror 1
could be located while decentered in the vertical direction with
respect to the seed light 23, as in FIG. 21. As long as at the edge
portion of the input side mirror 1 in which the seed light 23 is to
be introduced, the distance between the two mirror portions is
longer than that between the opposite mirror portions, the mirrors
1 and 2 could have any desired inclination sections. FIG. 47 is
illustrative of an embodiment wherein the resonator in the
amplification-stage laser 60 is built up of two nonparallel
mirrors, as in FIG. 21, and FIG. 47(b) is a side view of that
embodiment wherein the input side mirror 1 and the output side
mirror 2 are inclined with respect to the optical axis C of the
seed light 23. In this case, there is shading upon injection of the
seed light 23, and so the laser system efficiency becomes somewhat
lower than could be achieved with the above embodiments described
with reference to the top views.
[0380] In this case, too, it is to be understood that the mirrors 1
and 2 are set at such an angle of inclination that the laser system
output lies in the range G that does not fall short of the output S
of the laser system wherein the high-reflectivity side plane of the
input side mirror 1 is parallel with the partial reflectivity side
plane of the output side mirror 2 (FIG. 20). Alternatively, the
injection area could be ensured by coating rather than at the edges
of the mirrors 1 and 2 (see FIGS. 43-46).
[0381] Throughout all the two-stage laser systems for aligners of
the invention described above, the seed light 23 emitted out of the
oscillation-stage laser 50 is introduced in the resonator in the
amplification-stage laser 60 from the side of the input side mirror
1 or the output side mirror 2 that form that resonator. It is
understood, however, that the seed light 23 could be introduced in
the direction of the laser oscillation optical axis of the
amplification-stage laser 60 from any desired position between the
resonator mirrors 1 and 2. In such a case, the mirror that opposes
the output side mirror 2 will in no sense be any input side mirror.
Therefore, that mirror will hereinafter be called the rear side
mirror 111.
[0382] In what follows, embodiments will be explained under the
three following categories: introduction of the seed light 23 from
between the rear side mirror 111 and the chamber 3 (the rear part
of the resonator), introduction of the seed light 23 from between
the output side mirror 2 and the chamber 3, and direct introduction
of the seed light 23 in the chamber 3. The embodiments will be
explained primarily with reference to the structure of the
amplification-stage laser 60, and with reference to top views
unless otherwise stated. Discharge electrodes 4 and 5 (cathode and
anode), not shown, are located in the vertical direction to the
paper, and laser discharge occurs vertically to the paper. In these
embodiments, there is a higher degree of flexibility in the
introduction of the seed light 23 in the direction vertical to the
(cathode-to-anode) discharge direction than in that discharge
direction, and so the seed light 23 is introduced in the direction
vertical to the discharge direction. Notice here that the direction
of introduction of the seed light 23 is not necessarily limited to
that vertical direction.
[0383] FIG. 48 is a top view illustrative of one embodiment wherein
the seed light 23 is injected from the side of the
amplification-stage laser 60 that is opposite to its laser exit
side. The seed light 23 emitted out of the oscillation-stage laser
50 is injected in the amplification-stage laser 60 via one or more
total-reflection mirrors 121. In FIG. 48, the seed light 23 passes
through the second total-reflection mirror 121 and transmits
through a window member 17 opposite to the laser exit side for
injection in the chamber 3. The injected seed light 23 passes the
side of the discharge area (gain area) 22 between the discharge
electrodes 4 and 5 (the underside of the paper) or through the
discharge area 22 and then through the window member 17 on the
output side mirror 2 side, arriving at the output side mirror 2.
Generally, the output side mirror 2 is applied with a partial
reflecting mirror coating 10 at one side and an antireflection
coating 10 on the other or opposite side. Although whether the
partial reflecting mirror coating 10 of the output side mirror 2
directs toward the chamber 3 side or in the laser output direction
is not any essential requirement, that mirror coating 10 is applied
on the chamber 3 side in FIG. 48. Throughout the following
embodiments, the partial reflecting mirror coating 10 and the
antireflection coating 9 of the output side mirror 2 are shown in
FIG. 48 alone.
[0384] The output side mirror 2 could be formed of an optical
substrate with neither the partial reflecting mirror coating 10 nor
the antireflection coating 9. With laser light of, for instance,
193 nm wavelength, the surface reflection of the optical substrate
is about 4%; if the substrate can make use of front- and
back-surface reflection, it is then possible to achieve a 193 nm
wavelength output mirror having a reflectivity of about 8% without
recourse to any coating.
[0385] The seed light 23 reflected at the partial reflecting mirror
coating 10 of the output side mirror 2 is directed toward the rear
side mirror (total-reflection) mirror 111 positioned in the rear of
the laser resonator. Then, the seed light 23 is subjected to
multiple reflections between the output side mirror 2 and the rear
side mirror 111 that form the resonator, filling the discharge area
22.
[0386] As discharge occurs in the discharge area 22 in the
amplification-stage laser 60 during or after the discharge area 22
is filled with the seed light 23, it allows the amplification-stage
laser 60 to oscillate high-output, narrow-banded laser light having
a line width inherited from the seed light 23 from the
oscillation-stage laser 50.
[0387] FIG. 49 is a top view illustrative of one embodiment wherein
the seed light 23 is injected in the amplification-stage laser 60
using surface reflection at a window member 17 that opposes the
laser exit side not located at the Brewster angle. The seed light
23 from the line narrowing oscillation-stage laser 50 is directed
to the window member 17 on the rear side of the resonator via one
or more total-reflection mirrors 121. The directed seed light 23 is
guided by surface reflection at the window member 17 to the rear
side mirror 111. The seed light 23 is guided to the output side
mirror 2 upon reflection at the rear side mirror 111. Finally, the
light 23 is subjected to multiple reflections between the output
side mirror 2 and the rear side mirror 111 that form together the
resonator.
[0388] Usually, the CaF.sub.2 is used for the window member 17. In
most cases, the seed light 23 is P-polarized light. FIG. 50 shows
the reflection capability of CaF.sub.2 to P-polarized light. Here,
the angle of incidence of the seed light 23 on the window member 17
should preferably be substantially equal to the angle of
inclination with which the window member 17 is located (within
.+-.5.degree.). In other words, this injection mode works for the
window member that is not located with the Brewster angle.
[0389] FIG. 51(a) is a top view of an embodiment wherein the seed
light 23 is injected in the amplification-stage laser 60 while a
high-reflectivity (total-reflection) coating 8 is applied to a part
of the window member 17 that opposes the laser exit side. When the
window member 17 is located in the chamber 3 with the Brewster
angle or so, sufficient reflection of the seed light 23 will not be
expected, as shown in FIG. 50. In this case, a high-reflectivity
(total-reflection) coating 8 is applied to a site--capable of
reflecting the seed light 23--of the part of the window member 17
that opposes the laser exit side, as shown in FIG. 51(b). The rest
area J is or is not be applied with an antireflection coating.
Alternatively, only a site H through which the amplification-stage
laser light passes is or is not applied with an antireflection
coating, and other site is applied with a high-reflectivity
(total-reflection) coating 8 in association with the injection of
the seed light 23.
[0390] In this embodiment, the seed light 23 is reflected at the
portion of the high-reflectivity (total-reflection) coating 8 on
the window member 17, and guided to the rear side mirror 111. Then,
the seed light 23 is reflected at the rear side mirror 111 and
guided to the output side mirror 2. Finally, the light is subjected
to multiple reflections between the output side mirror 2 and the
rear side mirror 111.
[0391] FIG. 52(a) is a top view of an embodiment wherein the seed
light 23 is injected in the amplification-stage laser 60 from the
rear portion of the resonator in the amplification-stage laser 60,
in which resonator there is located a beam expander prism system
(beam expander system) 61. In this embodiment, the beam expander
prism systems 61 and 61 are located between one window member 17
and the rear side mirror 111 and between another window member 17
and the output side mirror 2, respectively, for the purpose of
expanding the laser light incident on the rear side mirror 111 and
the laser light incident on the output side mirror 2 in the
amplification-stage laser 60. Each beam expander prism system 61 is
here composed of two triangular prisms 62 and 63. A beam incident
on one surface of the triangular prism 62 at right angles is
incident from within on another surface at a relatively large angle
of incidence, which it leaves in a one-dimensional direction with
an expanded beam diameter. The beam with an expanded beam diameter
is incident on one surface of another triangular prism 63 at right
angles and then incident from within on another surface with a
relatively large angle of incidence, which it leaves in a
one-dimensional direction with an expanded beam diameter.
[0392] In this embodiment, the seed light 23 is directed by one or
more total-reflection mirrors 121 to the beam expander prism system
61. The prism 61 to which the seed light 23 is to be directed is
applied with or without an antireflection coating on a transmitting
area K of the surface 64 on which laser light resonating in the
resonator is to be incident, as shown in FIG. 52(b). The seed light
23 is incident on that transmitting area K. The rest of the surface
64 of the prism 62 is applied with a high-reflectivity
(total-reflection) coating 8. Specifically but not exclusively in
the embodiment of FIG. 52(b), the high-reflectivity
(total-reflection) coating 8 is applied to the vertex side of the
prism 62. Upon reflection at the high-reflectivity
(total-reflection) coating 8 on the prism 62, the seed light 23
passes through the amplification-stage laser 60 and is guided to
the output side mirror 2. Finally, the light is subjected to
multiple reflections between the output side mirror 2 and the rear
side mirror 111 that form together the resonator.
[0393] Specifically but not exclusively in the embodiment of FIG.
52(a), the seed light 23 is guided from the prism 62 closer to the
chamber 3 into the chamber 3. When the beam expander prism system
61 is composed of two or more prisms, the seed light 23 could be
guided from any surface of any prism into the amplification-stage
laser 60.
[0394] FIG. 53 is a top view of an embodiment corresponding to FIG.
48. In this embodiment, the seed light 23 is injected into the
laser chamber 3 from between the output side mirror 2 and the laser
chamber 3 in the amplification-stage laser 60. The seed light 23 is
injected in the amplification-stage laser 60 via one or more
total-reflection mirrors 121. In FIG. 53, the seed light 23 passes
through the second total-reflection mirror 121 between the output
side mirror 2 and the laser chamber 3 and transmits through a
window member 17 for injection in the chamber 3. The injected seed
light 23 passes the side of the discharge area (gain area) 22 (the
underside of the paper) or through the discharge area 22 and then
through the window member 17 on the rear side mirror 111 side,
arriving at the rear side mirror 111 that is a total-reflection
mirror located on the side opposing the output side mirror with the
resonator chamber 3 in the amplification-stage laser 60 sandwiched
between them. Then, the seed light 23 is reflected toward the
output side mirror 2, and further reflected at a partial reflecting
mirror coating 10 (FIG. 48) on the output side mirror 2. Thus, the
seed light 23 is subjected to multiple reflections between the
output side mirror 2 and the rear side mirror 11. Finally, the
discharge area 22 is filled with the seed light 23. Generally, the
output side mirror 2 is applied with the partial reflecting mirror
coating 10 on one side and an antireflection coating 10 on the
other or opposite side. Although whether the partial reflecting
mirror coating 10 on the output side mirror 2 directs toward the
chamber 3 side or in the laser output direction is not any
essential requirement (See the explanation of FIG. 48).
[0395] As discharge occurs in the discharge area 22 in the
amplification-stage laser 60 during or after the discharge area 22
is filled with the seed light 23, it allows the amplification-stage
laser 60 to oscillate high-output, narrow-banded laser light having
a line width inherited from the seed light 23 from the
oscillation-stage laser 50.
[0396] FIG. 54 is a top view of an embodiment corresponding to FIG.
49. In this embodiment, the seed light 23 from the
oscillation-stage laser 50 is directed to a window member 17 on the
output side mirror 2 side via one or more total-reflection mirrors
121. The directed seed light 23 is guided by surface reflection at
the window member 17 to the output side mirror 2. The seed light 23
is guided to the rear side mirror 111 upon reflection at the
partial reflecting mirror coating 10 (FIG. 48) on the output side
mirror 2. Thus, the seed light 23 is subjected to multiple
reflections between the output side mirror 2 and the rear side
mirror 111 that form together the resonator.
[0397] In this case, too, the CaF.sub.2 is usually used for the
window member 17. In most cases, the seed light 23 is P-polarized
light. FIG. 50 shows the reflection capability of CaF.sub.2 to
P-polarized light. Here, the angle of incidence of the seed light
23 on the window member 17 should preferably be substantially equal
to the angle of inclination with which the window member 17 is
located (within .+-.5.degree.). In other words, this injection mode
works for the window member that is not located with the Brewster
angle.
[0398] FIG. 55 is a top view of an embodiment corresponding to FIG.
51. In this embodiment, the seed light 23 is injected in the
amplification-stage laser 60 while a high-reflectivity
(total-reflection) coating 8 is applied to a part of the window
member 17 on the laser exit side. When the window member 17 is
located in the chamber 3 with the Brewster angle or so, sufficient
reflection of the seed light 23 will not be expected, as shown in
FIG. 50. In this case, the high-reflectivity (total-reflection)
coating 8 is applied to a site--capable of reflecting the seed
light 23--of the part of the window member 17 on the laser exit
side, as shown in FIG. 55(b). The rest area J is or is not be
applied with an antireflection coating. Alternatively, only a site
H through which the amplification-stage laser light passes is or is
not applied with an antireflection coating, and other site is
applied with the high-reflectivity (total-reflection) coating 8 in
association with the injection of the seed light 23.
[0399] In this embodiment, the seed light 23 is reflected at the
portion of the high-reflectivity (total-reflection) coating 8 on
the window member 17, and guided to the output side mirror 2. Then,
the seed light 23 is reflected at the output side mirror 2 and
guided to the rear side mirror 111. Thus, the light is subjected to
multiple reflections between the output side mirror 2 and the rear
side mirror 111.
[0400] FIG. 56 is a top view corresponding to FIG. 52. In this
example, however, a beam expander prism system 61 is located
between the window member 17 and the output side mirror 2 only for
the purpose of expanding the beam of laser light incident on the
output side mirror 2 in the amplification-stage laser 60, and there
is no such a bean expander prism system on the rear side mirror
111. The beam expander prism system 61 is here composed of two
triangular prisms 62 and 63. A beam incident on one surface of the
triangular prism 62 at right angles is incident from within on
another surface at a relatively large angle of incidence, which it
leaves in a one-dimensional direction with an expanded beam
diameter. The beam with an expanded beam diameter is incident on
one surface of another triangular prism 63 at right angles and then
incident from within on another surface with a relatively large
angle of incidence, which it leaves in a one-dimensional direction
with an expanded beam diameter.
[0401] The prism 62 to which the seed light 23 from the
oscillation-stage laser 50 is to be directed has such configure as
shown in FIG. 52(b). The seed light 23 is reflected at the
high-reflectivity (total-reflection) coating 8 on the prism 62 is
guided to the rear side mirror 111 through the amplification-stage
laser 60. Thus, the seed light 23 is subjected to multiple
reflections between the output side mirror 2 and the rear side
mirror 111.
[0402] Specifically but not exclusively in the embodiment of FIG.
56, the seed light 23 is guided from the prism 62 closer to the
chamber 3 into the chamber 3. When the beam expander prism system
61 is composed of two or more prisms, the seed light 23 could be
guided from any surface of any prism into the amplification-stage
laser 60.
[0403] An embodiment of directing the seed light 23 directly in the
chamber 3 in the amplification-stage laser 60 is now explained.
[0404] FIG. 57 is a top view of an embodiment wherein the seed
light 23 is injected in the discharge area 22 through a seed
light-injecting window 65 attached to the side of the chamber 3 in
the amplification-stage laser 60. Anti-reflection coatings could be
applied on both surfaces of the seed light-injecting window 65,
although this is not always necessary. The seed light 23 is
injected in the amplification-stage laser 60 by one or more
total-reflection mirrors 121. In FIG. 57, the seed light 23 is
injected into the discharge area 22 in the chamber 3 in the
amplification-stage laser 60 via the second total-reflection mirror
121 in the chamber 3. The injected seed light 23 passes through the
side of the discharge area 22 (the underside of the paper) or the
discharge area 22 and then transmits through the window member 17
on the rear side mirror 111 side, arriving at the rear side mirror
111. The seed light 23 reflected at the rear side mirror 111 goes
toward the output side mirror 2 located in front of the laser
resonator. Thus, the seed light 23 is subjected to multiple
reflections between the partial reflecting mirror coating 10 (FIG.
48) on the output side mirror 2 and the rear side mirror 111, which
form together resonator. Finally, the discharge area 22 is filled
with the seed light 23.
[0405] As discharge occurs in the discharge area 22 in the
amplification-stage laser 60 during or after the discharge area 22
is filled with the seed light 23, it allows the amplification-stage
laser 60 to oscillate high-output, narrow-banded laser light having
a line width inherited from the seed light 23 from the
oscillation-stage laser 50.
[0406] Specifically but not exclusively in the embodiment of FIG.
57, the seed light 23 is injected toward the rear side mirror 111.
For instance, the seed light 23 could be injected toward the output
side mirror 2.
[0407] FIG. 58 shows an embodiment wherein instead of the
total-reflection mirror 121, a total-reflection prism 122 is used
as the member located in the chamber for total reflection of the
seed light 23, and the rest is the same as in FIG. 57. Therefore,
only the total-reflection prism is now explained. The
total-reflection prism 122 is a CaF.sub.2 prism having no coating
at all. As this total-reflection prism 122 is used as a
total-reflection optical element, it allows the service life of
that optical element to be extended because of no deterioration due
to products resulting from the high-reflectivity (total-reflection)
mirror coating in the laser gas or laser system, which are found
with the total-reflection mirror 121.
[0408] FIG. 59 is a top view of an embodiment wherein a partial
reflecting film 10 is coated to the input side mirror 1, so that
the seed light 23 transmits through the input side mirror 1 from
its back surface for the injection of the seed light 23. This will
hereinafter be called the back surface injection mode. The seed
light 23 from the line narrowing oscillation-stage laser 50 is
introduced and entered in the back surface of the input side mirror
1 that is a rear side mirror in the resonator in the
amplification-stage laser via one or more total-reflection mirrors
121, while its optical axis is in substantial alignment with the
optical axis of the resonator in the amplification-stage laser.
This input side mirror 1 is applied with the partial reflecting
film 10, so that a part of the seed light 23 is injected into the
amplification-stage laser resonator and the rest is reflected by
the partial reflecting film 10. Then, the seed light 23 is filled
in between the output side mirror 2 and the input side mirror 1
that form together the resonator. As high voltage is applied
between the electrodes 4 and 5, it causes discharge, which allows
the seed light 23 to be amplified by induction emission and the
resonator to oscillate the amplification-stage laser 60.
[0409] Because the optical axis of the amplification-stage laser 60
is in alignment with the optical axis of the seed light 23, this
mode provides the following merits: (1) alignment is easily
achievable, (2) the tolerance of misalignment of the optical axis
of the seed light 23 is wide, and (3) there is a possibility of
holding back the occurrence of ASE because 0.5 roundtrip is needed
to fill the amplification-stage laser resonator with the seed light
23. However, a problem with such a back surface injection mode is
how the reflectivity of the input side mirror 1 is optimized.
[0410] FIG. 60 is indicative of relations between the input side
mirror 1 (the reflectivity of the rear mirror) and the
post-synchronization laser output, with relative output normalized
at the respective maximum outputs as ordinate and the reflectivity
of the mirror as abscissa. This is the post-synchronization output
when the reflectivity of the input side mirror 1 varies at an
output side mirror's reflectivity of about 30% while the output of
the oscillation-stage laser is kept constant. From this graph, it
has been found that the reflectivity of the mirror producing a
maximum output is about 90%, and the reflectivity producing 1/2 of
the maximum output ranges from about 36% to about 98%. In other
words; it has been found that the optimum value of the reflectivity
of the input side mirror in the back surface injection mode is
about 90%, and the usable reflectivity range for the input side
mirror 1 is about half the output having the optimum value, i.e.,
ranges from about 36% to about 98%.
[0411] FIG. 61 is indicative of an effective enabling region for
the angle and position of injection of the seed light 23, .theta.in
and Xin, when the seed light 23 is injected in the input side
mirror 1 from its back surface with the same coordinate axes and
under the same amplification-stage laser conditions as in FIG. 35
(resonator length L=1,000 mm, discharge width Wx=2.5 mm, and the
input side mirror 1 and the output side mirror are arranged
parallel with six roundtrips). The polygonal region in FIG. 61 is
larger than those found in FIGS. 37 and 38. This means that the
tolerance of the optical axis of the seed light 23 to variations
becomes wider in the back surface injection mode shown in FIG. 59
than in the oblique injection mode shown in FIG. 35. As a result,
laser performance (such as energy stability and synchronous
tolerance) will become better.
[0412] FIGS. 62 and 63 are each a top view of an embodiment of the
mode wherein the seed light 23 is introduced in the
amplification-stage laser 60 via a beam splitter 112 in the
resonator in the amplification-stage laser 60. In the embodiment of
FIG. 62, the beam splitter 112 coated with a partial reflecting
film 10 is interposed between the rear side mirror 111 coated with
a total-reflection film 8 and a rear side window 17 to introduce
the seed light 23 in the amplification-stage laser 60. In the
embodiment of FIG. 63, the beam splitter 112 coated with the
partial reflecting film 10 is interposed between a front side
window 17 and the output side mirror 2 to introduce the seed light
23 in the amplification-stage laser 60. The seed light 23 from the
line narrowing oscillation-stage laser 50 is guided and directed to
the beam splitter 112 located in the resonator in the
amplification-stage laser 60 via one or more total-reflection
mirrors 121. The beam splitter 112 is provided with the partial
reflecting film 10 at which a part of the seed light 23 is
reflected, and the reflected light is then injected in the
amplification-stage laser resonator while its optical axis is in
substantial alignment with the optical axis of the resonator. The
remaining transmitted seed light 23 is thrown away. Thus, the seed
light 23 is filled in between the rear side mirror 111 and the
input side mirror 2 that form together the resonator. As high
voltage is applied between the electrodes 4 and 5, it allows
discharge to occur, so that the seed light 23 is amplified by
induction emission and the amplification-stage laser 60 is
oscillated by the resonator. In this embodiment, losses due to the
provision of the beam splitter 112 in the resonator in the
amplification-stage laser 60 grow more than in the embodiment of
FIG. 59, resulting in lower output. However, the aforesaid merits
(1), (2) and (3) are still kept intact.
[0413] In the embodiments of FIGS. 62 and 63, the seed light 23 is
introduced in the resonator via the beam splitter 112. In a
modification to them, a partial reflecting film is coated to the
laser window 17 to allow it to have a similar beam splitter role as
mentioned above. In this case, too, the seed light 23 is injected
in the amplification-stage laser resonator while substantially
coaxial with the resonator. Although the seed light 23 is first
introduced in the discharge area direction by means of the beam
splitter 112, it is acceptable to introduce the seed light 23 in
the direction of the rear side mirror 111 or the output side mirror
2. In any case, the light reflected from the mirror toward the
amplification-stage laser is amplified. It is then required,
however, to make the output of the oscillation-stage laser 50
higher, because losses of the seed light 23 grow more than in the
embodiments of FIGS. 62 and 63.
[0414] FIG. 64 is a top view of an embodiment wherein the seed
light 23 is permitted to transmit through the output side mirror 1
by the beam splitter 112 for injection in the amplification-stage
laser resonator. The seed light 23 from the line narrowing
oscillation-stage laser 50 is guided and directed to the output
side mirror 2 in the resonator in the amplification-stage laser 60
via one or more total-reflection mirrors 121 while its optical axis
is in substantial alignment with the optical axis of the
amplification-stage resonator. The beam splitter 112 is provided
with a partial reflecting film 10, and the portion of the seed
light 23 transmitting through the beam splitter 112 is thrown away
while the reflected light is entered in the output side mirror 2.
The portion of the seed light 23 transmitting through the output
side mirror 1 is injected in the amplification-stage resonator. The
remaining portion of the seed light 23 is reflected by the output
side mirror 1. Thus, the seed light 23 is filled in between the
output side mirror 2 and the rear side mirror 111 that form
together the resonator. As high voltage is applied between the
electrodes 4 and 5, it allows discharge to occur, so that the seed
light 23 is amplified by induction emission and the
amplification-stage laser 60 is oscillated by that resonator. In
this embodiment, losses of the seed light 23 grow more than in the
embodiment of FIG. 59, because of poor efficiency of injection
through the beam splitter 112 and the output side mirror 2. It is
thus required to make the output of the oscillation-stage laser 50
higher; however, the aforesaid merits (1), (2) and (3) are still
kept intact.
[0415] By the way, when the diameter of laser light from the
oscillation-stage laser 50 is equal to the diameter of output laser
light from the amplification-stage laser 60 so that the conversion
optical system 70 could be dispensed with, the front mirror 52 in
the oscillation-stage laser 50 and the input side mirror 1 in the
amplification-stage laser 60 could be provided by a common or
sharing mirror. FIG. 65 is a top view illustrative in schematic of
this embodiment, wherein a cascade connection is made between the
oscillation-stage laser 50 and the amplification-stage laser 60 in
such a way as to share the common mirror 52-1. The front surface of
the transparent substrate of the common mirror 52-1 is applied with
a partial reflecting mirror coating to form a partial reflecting
mirror surface for the front mirror 52 in the oscillation-stage
laser 50, and the back surface of the transparent substrate of the
common mirror 52-1 is provided with a high-reflectivity mirror
coating (except the seed light-introduction hole 7'') as shown
typically in FIG. 5, for application to the input side mirror 1 in
the amplification-stage laser 60. It is here noted that the front
surface of the common mirror 52-1 could have a surface
configuration for the front mirror 52 in the resonator for the
oscillation-stage laser 50, and the back surface could be
configured in a planar or concave shape for the input side mirror 1
in the amplification-stage laser 60.
[0416] FIG. 66 is illustrative of a modification to the back
surface injection mode. When the diameter of laser light from the
oscillation-stage laser 50 is substantially equal to that from the
amplification-stage laser 60 so that the conversion optical system
70 could be dispensed with, the front mirror 52 in the
oscillation-stage laser 50 and the amplification-stage laser 60
could be designed to have a common or sharing input side mirror
52-2 coated with a partial reflecting film 10. It is noted that the
partial reflecting film 10 could be applied to the side of the
input side mirror that opposes the side shown. In this embodiment,
a cascade connection is made between the oscillation-stage laser 50
and the amplification-stage laser 60 in such a way as to share the
common mirror 25-2 coated with the partial reflecting film 10 on
its one surface. The line narrowing module 51 and the partial
reflecting surface of the common mirror 25-2 work as a resonator to
oscillate the oscillation-stage laser 50 to produce the seed light
from the partial reflecting surface of the common mirror 52-2. At
the same time, the seed light is entered directly in the resonator
in the oscillation-stage laser 60, which is made up of the common
mirror 52-2 and the output side mirror 2. As high voltage is
applied between the electrodes 4 and 5, it permits discharge to
occur, so that the seed light is amplified by induction emission
and the amplification-stage laser 60 is oscillated by that
resonator. In this case, the reflectivity of the common mirror
could be effective if it comes in the range of FIG. 60.
[0417] In addition to the merits (1), (2) and (3) of the back
surface injection mode, this mode provides additional merits as set
forth just below. Because the common mirror 52-2 having the partial
reflecting film is shared by the front mirror in the
oscillation-stage laser 50 and the input side mirror in the
amplification-stage laser 60, (1) any means for the introduction of
seed light can be dispensed with, making the system compact and
less costly, (2) the seed light can be injected in the resonator in
the amplification-stage laser without losses, so that the
oscillation-stage laser can be kept low in output and compact in
size, and (3) the optical axes of the oscillation- and
amplification-stage lasers are substantially in alignment, so that
they can be easily adjusted with higher stability.
[0418] A difference in the advantage between the embodiments of
FIGS. 65 and 66 is now explained. The output of the
oscillation-stage laser can be lower in the embodiment of FIG. 66
than in that of FIG. 65. This is because the common mirror 52-2
having the partial reflecting film on its one surface is shared by
the front mirror in the oscillation-stage laser 50 and the input
side mirror in the amplification-stage laser 60, so that all the
seed light 23 can be injected in the resonator in the
amplification-stage laser 60 without causing losses of the output
of the oscillation-stage laser 50. Thus, the oscillation-stage
laser 50 can be made smaller and less costly.
[0419] With the above two-stage laser system for aligners according
to the invention, Fabry-Perot etalon type stable resonator or a
resonator with its two mirrors slightly inclined with each other is
used in the amplification-stage laser so as to achieve a spatial
coherence as low as that of the oscillation-stage laser, and light
having divergence is used as the seed light oscillated from the
oscillation-stage laser so as to fill a laser gas gain area with
the seed light for efficient amplification. Even with a ring
resonator using a plurality of plane mirrors in the
amplification-stage laser, too, the desired low spatial coherence
is achievable.
[0420] FIG. 67 is a side view illustrative in schematic of one
embodiment using such a ring resonator. In this embodiment, the
seed light from the oscillation-stage laser 50 is directed through
a reflecting mirror 99 to the conversion optical system 70 where it
is reduced to the desired beam width, entering the
amplification-stage laser 60. The amplification-stage laser 60
comprises a ring resonator built up of an input/output partial
reflecting mirror 91, a total-reflection mirror 92 for the
reflection of seed light transmitting through the partial
reflecting mirror 91, and a total-reflection right-angle prism
(roof prism) 93 having two total-reflection surfaces and operable
to reflect incident light in a direction substantially parallel
with and opposite to the direction of incidence, wherein all
reflecting surfaces are formed of planes. Thus, the gain area
(discharge area) in the chamber 3 positioned between the partial
reflecting mirror 91/total-reflection mirror 92 and the
total-reflection right-angle prism 93 can be filled with the seed
light having divergence while it makes roundtrips in the ring
resonator, which the amplified laser light leaves as output via the
partial reflecting mirror 91.
[0421] FIG. 68 is a plan view illustrative in schematic of another
embodiment using the ring resonator. The seed light from the
oscillation-stage laser 50 is entered in the amplification-stage
laser 60 via a reflecting mirror 99. The amplification-stage laser
60 comprises a ring resonator built up of an input/output partial
reflecting mirror 91 and three total-reflection mirrors 92, 93 and
94 for sequentially reflecting the seed light transmitting through
the partial reflecting mirror 91 back to the partial reflecting
mirror 91, wherein all reflecting surfaces are formed of planes.
Thus, the gain area (discharge area) in the chamber 3 positioned
between the partial reflecting mirror 91/total-reflection mirror 92
and the total-reflection mirrors 94, 95 can be filled with the seed
light having divergence while it makes roundtrips in the ring
resonator, which the amplified laser light leaves as output via the
partial reflecting mirror 91.
[0422] The inventors have further found that if, in a two-stage
laser system having its spatial coherence decreased while taking
advantage of the high stability, high output efficiency and fine
line width of the MOPO system explained in the preamble of the
disclosure, the length of an optical path in the resonator in the
amplification-stage laser is specified as described below, it is
then possible to provide a two-stage laser system more suitable for
use on semiconductor aligners.
[0423] As a result of experiments after experiments, the inventors
have discovered that there is often an interference fringe pattern
in the beam profile configuration of laser light produced out of
the amplification-stage laser, although depending on the length of
the optical path in the resonator in the amplification-stage
laser.
[0424] This interference fringe pattern, if any, renders the
symmetry of the beam profile configuration worse. Further, the
interference fringe pattern moves with time due to changes in the
center wavelength of the seed light 23 produced out of the
oscillation-stage laser 50, changes in the resonator length of the
amplification-stage laser 60 or the like, rendering the stability
of the beam profile worse too.
[0425] The beam profile configuration of the laser light produced
out of the two-stage laser system that is a light source for the
aligner has some considerable influences on the uniform
illumination of masks on the aligner and, hence, on exposure
capability on what is to be exposed (wafers). Further, fluctuations
of the interference fringe pattern give rise to too large
fluctuations of laser light output to control.
[0426] Why the interference fringe pattern occurs is now explained
with reference to FIGS. 69 and 70. FIG. 69 is illustrative in
schematic of a MOPO type two-stage laser system to which the
invention is applied, and laser light characteristics as well.
Specifically, FIG. 69(a) is a schematic illustration of the MOPO
type two-stage laser system to which the invention is applied; FIG.
69(b) is indicative of a spectral profile of narrow-banded laser
light produced out of the oscillation-stage laser; and FIG. 69(c)
is indicative in section of laser light (beam profile
configuration) produced out of the amplification-stage laser.
[0427] For instance, given the narrow-banded laser light (seed
light 23) produced out of the oscillation-stage laser 50 has the
spectral profile shown in FIG. 69(b). The seed light 23 is injected
in the resonator in the amplification-stage laser 60 (which is
built up of, e.g., the input side mirror 1 and the output side
mirror 2), wherein it is amplified and oscillated.
[0428] Here, when the resonator in the amplification-stage laser 60
is built up of a stable resonator or a resonator with its mirrors
slightly inclined with respect to each other and that resonator is
comprised of an input side (total-reflection) mirror 1 and an
output side (partial reflecting) mirror 2, the seed light 23
transmits through the input side mirror 1 and thereafter passes
through the discharge area 22 in the amplification-stage laser 60,
where it is amplified. The amplified light after passing through
the discharge area 22 is incident on the output side mirror 2 that
is a partial reflecting mirror, and a part of the reflected light
is produced as the first laser light L1 through the output side
mirror 2.
[0429] On the other hand, the amplified light reflected by the
output side mirror 2 passes through the discharge area 22 where it
is amplified, entering the input side mirror 1. The amplified light
subjected to total reflection at the input side mirror 1 passes
through the discharge area 22 wherein it is amplified, entering the
output side mirror 1. A part of that light transmits through the
output side mirror 2, leaving it as the second laser light K2. The
remaining amplified light is reflected by the output side mirror 2
toward the amplification area 22. In the resonator in the
amplification-stage laser 60, such resonance occurs repeatedly.
[0430] The first laser light K1 and the second laser light K2
interfere when the optical path difference between the both laser
light K1 and K2 is shorter than a time-based coherent length Lc
corresponding to the spectral width of the seed light 23 produced
out of the oscillation-stage laser 50.
[0431] Here, let .lamda. be the wavelength of the laser light, and
.DELTA..lamda. be the spectral line width. Then, the time-based
coherent length Lc is defined by equation (9) (see non-patent
publication 1).
Lc=.lamda..sup.2/.DELTA..lamda. (9)
[0432] As in the evaluation of spatial coherence, interference
fringe capability on the B-B section of FIG. 69(a) is evaluated in
terms of visibility and optical path difference. The optical path
difference herein is tantamount to the distance that the laser
light (seed light) travels from entering the resonator to leaving
it; that is, it is substantially twice as long as the resonator
length L of the amplification-stage laser 60. Visibility is also
found from the following formula:
Visibility=(maximum fringe intensity I.sub.max of interference
fringe pattern-minimum fringe intensity I.sub.min of interference
fringe pattern)/(maximum fringe intensity I.sub.max of
inter-ference fringe pattern+minimum fringe intensity of I.sub.min
of interference fringe pattern
[0433] FIG. 70 is indicative of relations between the length twice
as long as the resonator length L of the amplification-stage laser
and the visibility of the interference fringe pattern. FIG. 70
teaches that as the length twice as long as the resonator length L
of the amplification-stage laser 60 (that length substantially
matches the optical path difference between the first laser light
K1 and the second laser light K2) becomes long, the visibility of
the interference fringe pattern occurring on the beam profile of
laser light produced out of the amplification-stage laser 60
becomes small. It also teaches that as the length substantially
twice as long as the resonator length L of the amplification-stage
laser 60 becomes longer than the time-based coherence length Lc of
the seed light 23 produced out of the oscillation-stage laser 50,
the interference fringe pattern virtually disappears.
[0434] Referring typically to an ArF laser MOPO type two-stage
laser system for aligners, when the spectral line width (full width
half maximum) of the seed light 23 produced out of the
oscillation-stage laser 50 is .DELTA..lamda.=0.2 pm and the
wavelength is .lamda.=193.4 nm, the time-based coherence length Lc
is found from equation (9) to be Lc=about 0.186 m. Therefore, to
prevent any interference fringe pattern from occurring in the beam
profile of the laser light output, the resonator length L of the
amplification-stage laser 60 must be 0.186/2=0.093 m or longer.
[0435] When the ring resonator is used as the resonator in such an
amplification-stage laser 60 as shown in FIGS. 67 and 68, such
conditions as mentioned below must be satisfied to prevent any
interference fringe pattern from occurring on the bema profile of
the laser light produced out of the amplification-stage laser
60.
[0436] With the ring resonator, the interference fringe pattern can
be held back by making its optical path length longer than the
time-based coherent length Lc corresponding to the spectral line
width of the narrow-banded seed light 23 produced out of the
oscillation-stage laser 50.
[0437] With the embodiment of FIG. 67, the length of the optical
path taken by the seed light 23 (laser light) from the position
where it leaves the partial reflecting mirror 91 upon entrance and
transmission through it until that laser light again arrives at the
partial reflecting mirror 9 via the total-reflection mirror 92 and
the total-reflection right-angle prism 93 should preferably be
longer than the time-based coherent length Lc.
[0438] With the embodiment of FIG. 68, the length of the optical
path taken by the seed light 23 (laser light) from the position
where it leaves the partial reflecting mirror 91 upon entrance and
transmission through it until that laser light again arrives at the
partial reflecting mirror 9 via the total-reflection mirrors 92, 94
and 95 should preferably be longer than the time-based coherent
length Lc.
[0439] That is, if, in FIGS. 67 and 68, the optical path length
difference upon the laser light split by the partial reflecting
mirror 91 crossing again over it is made longer than the time-based
coherent length Lc corresponding to the spectral line width of the
seed light 23 produced out of the oscillation-stage laser, it is
then possible to prevent any interference fringe pattern from
occurring on the beam profile of the laser light produced out of
the amplification-stage laser 60.
[0440] While the two-stage laser system for aligners according to
the invention has been described with referent to its principles
and embodiments, it is to be understood that the invention is by no
means limited to them and various modifications to them are
possible.
[0441] For instance, when the two-stage laser system for aligners
according to the invention is a fluorine molecule (F.sub.2) laser
system, the oscillation-stage laser 50 could comprise, in place of
the line narrowing module 51, a line select module comprising at
least one angle dispersion element and a total-reflection mirror
located in order from its side on which laser light is
incident.
[0442] Specifically, the laser light produced out of the F.sub.2
laser system has two primary oscillation wavelengths
(.lamda..sub.1=157.6299 nm and .lamda..sub.2=157.5233 nm:
non-patent publication 2). The spectral line width (FWHN) of both
lines is about 1 pm. When an alignment optical system in the
aligner is catadioptric system, chromatic aberrations are prevented
even at such spectral line widths as mentioned above.
[0443] In this case, therefore, the oscillation line of stronger
intensity .lamda..sub.1 (=157.6299 nm) among both lines is usually
selected by the aforesaid line select module upon free-run
oscillation.
[0444] It is noted that such a line select module is not
necessarily located in the oscillation-stage laser 50; it could be
located in the output optical path of the output side mirror 60 in
the amplification-stage laser 60.
[0445] Here, when any interference fringe pattern is prevented from
occurring on the beam profile of the laser light produced out of
the amplification-stage laser 60, the resonator length Lc of the
amplification-stage laser 60 is determined as mentioned above,
while comparing with the time-based coherent length Lc of the
oscillation-stage laser 50. Upon determination of the time-based
coherent length Lc from equation (9), the spectral lie width
.DELTA..lamda., of the oscillation-stage laser 60 is determined as
follows.
[0446] Here consider the case where the two-stage laser system for
aligners is a F.sub.2 laser system comprising a line select module
on the rear side of the oscillation-stage laser 50. From a
comparison of the output of laser light (seed light 23) at a
wavelength .lamda..sub.1 (=157.6299 nm) selected by the line select
module with the output of laser light (seed light 23) at a
wavelength .lamda..sub.2 (=157.5233 nm) selected by the line select
module, it is found that the output of laser light having a
wavelength .lamda..sub.2 is merely about 20% lower than that of
laser light a wavelength .lamda..sub.1. In this case, therefore, it
is possible to select the wavelength .lamda..sub.2 by the line
select module. In other words, the above spectral line width
.DELTA..lamda. is that of the oscillation line at the wavelength
.lamda..sub.1 (=157.6299 nm) or .lamda..sub.2 (=157.5233 nm)
selected by the line select module.
[0447] When the two-stage laser system is a F2 laser system
comprising a line select module externally of the output side
mirror 2 in the amplification-stage laser 60, the spectral line
width .DELTA..lamda. is that of the oscillation line at wavelength
.lamda..sub.1 of stronger intensity among two primary oscillation
lines of wavelength .lamda..sub.i=157.6299 nm and wavelength
.lamda..sub.2=157.5233 nm.
POSSIBLE INDUSTRIAL APPLICATIONS
[0448] In the two-stage laser system for aligners according to the
invention, oscillation laser light having divergence is used as the
oscillation-stage laser and the amplification-stage laser comprises
a Fabry-Perot etalon resonator where the resonator is configured as
a stable resonator or, alternatively, oscillation laser light
having divergence is used as the oscillation-stage laser and the
amplification-stage laser comprises a ring resonator comprising an
input/output partial reflecting mirror and a plurality of
total-reflection mirrors for reflecting laser light entered via the
partial reflecting mirror back to the position of the partial
reflecting mirror wherein the partial reflecting mirror and the
plurality of total-reflection mirrors are each formed of a plane.
Thus, the two-stage laser system for aligners according to the
invention has the features of the MOPO mode that output
fluctuations are insensitive to fluctuations of synchronous
excitation timing between the chambers, high energy stability and
high output efficiency are achievable, laser (seed) energy from the
oscillation stage can be kept lower, the spectral line width is
narrow because the latter half of a laser pulse from the
oscillation-stage laser makes a lot more roundtrips, and the line
width is narrow because the tail of the latter half can be
amplified, and has the features of the MOPA mode as well that the
spatial coherence is low; that is, given the same share quantity
(pinhole-to-pinhole space) in the beam transverse direction, the
visibility of interference fringes and the spatial coherence are
low.
[0449] If the optical axis of laser light oscillated out of the
oscillation-stage laser and entered in the amplification-stage
laser is set in such a way as to make an angle with the optical
axis of the resonator in the amplification-stage laser, then the
spatial coherence is much more reduced.
[0450] If the length about twice as long as the length of the
resonator in the amplification-stage laser is set longer than the
time-based coherent length corresponding to the spectral line width
of the oscillation-stage laser or the length of the optical path
through the ring resonator is set longer than the time-based
coherent length corresponding to the spectral line width of the
oscillation-stage laser, it is then possible to prevent any
interference fringe pattern from occurring on the beam profile of
laser light produced out of the amplification-stage laser. It is
thus possible to maintain the symmetry of the beam profile and hold
back its fluctuations and, hence, provide uniform illumination of
masks in an aligner. Thus, the invention provides a two-stage laser
system well fit especially for semiconductor aligners.
[0451] The invention is in no sense limited to the use of the
oscillation laser light having divergence as the oscillation-stage
laser. For instance, if the optical axis of laser light oscillated
out of the oscillation-stage laser and entered in the
amplification-stage laser is set in such a way as to make an angle
with respect to the optical axis of the resonator in the
amplification-stage laser, it is then possible to obtain a
two-stage laser system that does not only have the above features
of the MOPO mode but also is reduced in terms of spatial coherence
so that it lends itself well to semiconductor aligners.
[0452] Further, if the reflecting surfaces of the rear side mirror
and the output side mirror are each formed of a plane, the normal
lines to the rear side mirror and the output side mirror are set in
such a way as to make an angle with respect to the optical axis of
laser light oscillated out of the oscillation-stage laser and
entered in the amplification-stage laser and with each other as
well, and the laser light oscillated out of the oscillation-stage
laser is entered in the resonator from the side on which the
distance between both mirrors is longer, it is then possible to
obtain a two-stage laser system that does not only have the above
features obtained by setting the optical axis of laser light
entered in the amplification-stage laser in such a way as to make
an angle with respect to the optical axis of the resonator in the
amplification-stage laser but also has an increased laser output
and an extended pulse width and ensures the degree of flexibility
in the injection of laser light entered in the amplification-stage
laser with a decrease in the peak intensity of the
oscillation-stage laser, and so is best suited for use with
semiconductor aligners.
* * * * *