U.S. patent application number 12/639339 was filed with the patent office on 2012-04-12 for laser system.
This patent application is currently assigned to CYMER, INC.. Invention is credited to Daniel J.W. Brown, William N. Partlo, Richard L. Sandstrom.
Application Number | 20120087386 12/639339 |
Document ID | / |
Family ID | 46329659 |
Filed Date | 2012-04-12 |
United States Patent
Application |
20120087386 |
Kind Code |
A1 |
Brown; Daniel J.W. ; et
al. |
April 12, 2012 |
LASER SYSTEM
Abstract
An apparatus/method may comprise a line narrowed pulsed
lithography laser light source which may comprise: a seed pulse
providing laser system which may comprise: a first pulsed seed
laser producing seed pulses at a rate of X kHz; a second pulsed
seed laser producing seed pulses at a rate of X kHz; an
amplification system which may comprise: a first amplifier gain
system which may comprise a first and a second pulsed gas discharge
amplifier gain medium, each with a nominal center wavelength in the
UV range, and each operating at 1/2X kHz on output pulses from the
first seed laser; a second amplifier gain system which may comprise
a first and a second pulsed amplifier gain medium, each with a
nominal center wavelength in the UV range, and each operating at
1/2X kHz on output pulses from the second seed laser.
Inventors: |
Brown; Daniel J.W.; (San
Diego, CA) ; Partlo; William N.; (Poway, CA) ;
Sandstrom; Richard L.; (Encinitas, CA) |
Assignee: |
CYMER, INC.
San Diego
CA
|
Family ID: |
46329659 |
Appl. No.: |
12/639339 |
Filed: |
December 16, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11982103 |
Oct 31, 2007 |
7643529 |
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12639339 |
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11787180 |
Apr 13, 2007 |
7885309 |
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11982103 |
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11584792 |
Oct 20, 2006 |
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11787180 |
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60814293 |
Jun 16, 2006 |
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60814424 |
Jun 16, 2006 |
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60732688 |
Nov 1, 2005 |
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Current U.S.
Class: |
372/55 ;
372/69 |
Current CPC
Class: |
G03F 7/70025 20130101;
H01S 3/0057 20130101; H01S 3/08009 20130101; H01S 2301/02 20130101;
H01S 3/2383 20130101; H01S 3/2251 20130101; G03F 7/7055 20130101;
G03F 7/70341 20130101; H01S 3/10092 20130101; H01S 3/005 20130101;
H01S 3/225 20130101; H01S 3/08004 20130101; G03F 7/70041 20130101;
H01S 3/083 20130101; H01S 3/2258 20130101; H01S 3/2333
20130101 |
Class at
Publication: |
372/55 ;
372/69 |
International
Class: |
H01S 3/22 20060101
H01S003/22; H01S 3/09 20060101 H01S003/09 |
Claims
1-8. (canceled)
9. An apparatus comprising: a plurality of pulsed laser light
sources, each comprising: a seed pulse providing laser system
comprising a pulsed seed laser system producing a beam of seed
pulses, and a beam splitter receiving the output of the seed laser
system and providing a split output of every pulse from the beam
splitter; and an amplifier gain system comprising a first gas
discharge amplifier gain medium operating at Y kHz on a respective
output from the beam splitter and a second gas discharge amplifier
gain medium operating at Z kHz on a respective output from the beam
splitter, where Y+Z=X; and at least one pulse combiner providing an
output of laser pulses from the pulsed laser light sources at a
pulse repetition rate of NX, where N is the total number of pulsed
laser light sources in the apparatus.
10. The apparatus of claim 12 wherein the seed pulse providing
system comprises an excimer gas discharge laser and a line
narrowing module.
11. The apparatus of claim 12 wherein the beam splitter comprises a
geometric beam splitter transmitting at least one spatial portion
of the pulse beam on one path and at least a second portion of the
pulse beam on a second path.
12. An apparatus comprising: a pulsed laser light source
comprising: a seed pulse providing laser system comprising a pulsed
seed laser system producing a beam of seed pulses; and a beam
splitter receiving the output of the seed laser system and
providing a split output of every pulse from the beam splitter; and
an amplifier gain system comprising a first gas discharge amplifier
gain medium operating at Y kHz on a respective output from the beam
splitter and a second gas discharge amplifier gain medium operating
at Z kHz on a respective output from the beam splitter, where
Y+Z=X; and a pulse combiner combining the output pulses from the
first and second amplifier gain systems to produce an output laser
light pulse beam at a pulse repetition rate of X kHz; wherein the
beam splitter comprises a partially reflective optic reflecting a
portion of each pulse in the pulse beam on one path and
transmitting a portion of each pulse in the pulse beam on a second
path.
13. An apparatus comprising: a pulsed laser light source
comprising: a seed pulse providing laser system comprising a pulsed
seed laser system producing a beam of seed pulses; and a beam
splitter receiving the output of the seed laser system and
providing a split output of every pulse from the beam splitter; and
an amplifier gain system comprising a first gas discharge amplifier
gain medium operating at Y kHz on a respective output from the beam
splitter and a second gas discharge amplifier gain medium operating
at Z kHz on a respective output from the beam splitter, where
Y+Z=X; and a pulse combiner combining the output pulses from the
first and second amplifier gain systems to produce an output laser
light pulse beam at a pulse repetition rate of X kHz; wherein the
beam splitter comprises a stimulated optical path modulator that
directs a first portion of a pulse in the pulse beam in a first
path during a first time period and a second portion of a pulse on
a second path during a second time period, or that directs
alternating pulses on alternating ones of the first path and the
second path in alternating time periods.
14. The apparatus of claim 12 wherein each gas discharge amplifier
gain medium comprises a power amplifier.
15. The apparatus of claim 12 wherein each gas discharge
amplification gain medium comprises a power oscillator.
16. The apparatus of claim 12 wherein each gas discharge
amplification gain medium comprises a ring power amplification
stage.
17. The apparatus of claim 12 wherein the pulse combiner comprises
a beam combining optical element forming a laser light source
output beam comprising a first spatially defined component from the
first amplifier gain medium and a second spatially defined
component from the second amplifier gain medium.
18. The apparatus of claim 12 wherein the pulse combiner comprises
a beam combining optical element forming a laser light source
output beam comprising a first component from the first amplifier
gain medium and a second component from the second amplifier gain
medium each exiting the pulse combiner along a path comprising a
single optical axis.
19-25. (canceled)
26. The apparatus of claim 12 wherein each amplification stage is
alternatively seeded by the pulsed seed laser system.
27. The apparatus of claim 13 wherein each gas discharge amplifier
gain medium comprises a power amplifier.
28. The apparatus of claim 13 wherein each gas discharge
amplification gain medium comprises a power oscillator.
29. The apparatus of claim 13 wherein each gas discharge
amplification gain medium comprises a ring power amplification
stage.
30. The apparatus of claim 13 wherein the pulse combiner comprises
a beam combining optical element forming a laser light source
output beam comprising a first spatially defined component from the
first amplifier gain medium and a second spatially defined
component from the second amplifier gain medium.
31. The apparatus of claim 13 wherein the pulse combiner comprises
a beam combining optical element forming a laser light source
output beam comprising a first component from the first amplifier
gain medium and a second component from the second amplifier gain
medium each exiting the pulse combiner along a path comprising a
single optical axis.
Description
FIELD OF THE DISCLOSED SUBJECT MATTER
[0001] The subject matter disclosed is related to high power gas
discharge laser systems for DUV light sources, e.g., used in
integrated circuit photolithography, e.g., in highly line narrowed
versions, e.g., for immersion lithography and other lithography
uses requiring high power and/or requiring longer component life in
lower power applications, or broad band versions used for treatment
of material on a workpiece substrate, e.g., laser annealing for low
temperature poly-silicon processing ("LTPS"), such as thin beam
sequential lateral solidification ("tbSLS"), and more particularly
to a seed laser and amplification gain medium system with an
improved power amplification stage providing higher gain and
reduced ASE and coherency busting, e.g., for reduction in
speckle.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] The present application is a Divisional of U.S. application
Ser. No. 11/982,103, filed on Oct. 31, 2007, which is a
Continuation-in-Part of U.S. application Ser. No. 11/787,180, filed
on Apr. 13, 3007, entitled LASER SYSTEM, Attorney Docket No.
2006-0083-04, which was a Continuation-in-Part of U.S. application
Ser. No. 11/584,792, filed on Oct. 20, 2006, entitled LASER SYSTEM,
Attorney Docket No. 2006-0083-01, which claimed priority to U.S.
Application No. 60/732,688, filed on Nov. 1, 2005, entitled 200 W
GAS DISCHARGE EXCIMER OR MOLECULAR FLUORINE MULTICHAMBER LASER,
Attorney Docket No. 2005-0094-01, and to U.S. Application No.
60/814,293 filed on Jun. 16, 2006, entitled 200 WATT DUV GAS
DISCHARGE LASER SYSTEM Attorney Docket No. 2005-0103-01, and to
U.S. Application No. 60/814,424, filed on Jun. 16, 2006, entitled
LONG LIVED MO IN MOPO CONFIGURED LASER SYSTEM, Attorney Docket No.
2006-0012-01, and was a Continuation-in-Part of U.S. patent
application Ser. Nos. 11/521,904, filed on the Sep. 14, 2006,
entitled LASER SYSTEM, Attorney Docket No. 2005-0103-02; and
11/522,052, filed on Sep. 14, 2006, entitled LASER SYSTEM, Attorney
Docket No. 2005-0104-01; and 11/521,833, filed on Sep. 14, 2006,
entitled LASER SYSTEM, Attorney Docket No. 2005-0105-01; and
11/521,860, filed on Sep. 14, 2006, entitled LASER SYSTEM, Attorney
Docket No. 2006-0007-01; and 11/521,834, filed on Sep. 14, 2006,
entitled LASER SYSTEM, Attorney Docket No. 2006-0012-02; and
11/521,906, filed on Sep. 14, 2006, entitled LASER SYSTEM, Attorney
Docket No. 2006-0013-01; and 11/521,858, filed on Sep. 14, 2006,
entitled LASER SYSTEM, Attorney Docket No. 2006-0018-01; and
11/521,835, filed on Sep. 14, 2006, entitled LASER SYSTEM, Attorney
Docket No. 2006-0020-01; and 11/521,905, entitled LASER SYSTEM,
filed Sep. 14, 2006, Attorney Docket No. 2006-0071-01, the
disclosures of each of which are hereby incorporated by
reference.
[0003] The present application is related to U.S. application Ser.
No. 11/447,380, entitled DEVICE AND METHOD TO STABILIZE BEAM SHAPE
AND SYMMETRY FOR HIGH ENERGY PULSED LASER APPLICATIONS, filed on
Jun. 5, 2006, Attorney Docket No. 2006-0039-01, and is related to
U.S. application Ser. No. 10/881,533, entitled METHOD AND APPARATUS
FOR GAS DISCHARGE LASER OUTPUT LIGHT COHERENCY REDUCTION, filed on
Jun. 29, 2004, and published on Dec. 29, 2005, Pub. No.
20050286599, the disclosures of which are hereby incorporated by
reference. The present application is also related to U.S. Pat.
Nos. 6,549,551, issued on Apr. 15, 2003, to Ness et al, entitled
INJECTION SEEDED LASER WITH PRECISE TIMING CONTROL; and 6,567,450,
issued on May 20, 2003, to Myers et al, entitled VERY NARROW BAND
TWO CHAMBER HIGH REP RATE GAS DISCHARGE LASER SYSTEM; and
6,625,191, entitled VERY NARROW BAND TWO CHAMBER HIGH REP RATE GAS
DISCHARGE LASER SYSTEM, issued on Sep. 23, 2003 to Knowles et al;
and 6,865,210, issued on Mar. 8, 2005, to Ershov et al, entitled
TIMING CONTROL FOR TWO CHAMBERED GAS DISCHARGE LASER SYSTEM; and
6,690,704, entitled CONTROL SYSTEM FOR TWO CHAMBER GAS DISCHARGE
LASER SYSTEM, issued on Feb. 10, 2004 to Fallon et al; and 6561263,
issued on May 6, 2003, to Morton et al., entitled DISCHARGE LASER
HAVING ELECTRODES WITH SPUTTER CAVITIES AND DISCHARGE PEAKS; and
U.S. Pat. No. 6,928,093, entitled LONG DELAY AND HIGH TIS PULSE
STRETCHER, issued to Webb et al. on Aug. 9, 2005; the present
application is also related to co-pending U.S. application Ser.
Nos. 10/781,251, filed on Feb. 18, 2004, entitled VERY HIGH ENERGY,
HIGH STABILITY GAS DISCHARGE LASER SURFACE TREATMENT SYSTEM,
Attorney Docket No. 2003-0105-02; and 10/884,547, filed on Jul. 1,
2004, entitled LASER THIN FILM POLY-SILICON ANNEALING SYSTEM,
Attorney Docket No. 2004-0062-01, published on Jun. 30, 2005, Pub.
No. US-2005-0141580; and 11/173,988, entitled ACTIVE BANDWIDTH
CONTROL FOR A LASER, filed on Jun. 30, 2005, Attorney Docket No.
2005-0001-01; and 11/169,203, entitled HIGH PULSE REPETITION RATE
GAS DISCHARGE LASER, filed on Jun. 27, 2005, Attorney Docket No.
2004-0094-01; and 11/095,293, entitled GAS DISCHARGE LASER OUTPUT
LIGHT BEAM PARAMETER CONTROL, filed on Mar. 31, 2005, Attorney
Docket No. 2004-0119-01; and 11/095,976, entitled 6 KHZ AND ABOVE
GAS DISCHARGE LASER SYSTEM, filed on Mar. 31, 2005; and 11/201,877,
filed on Aug. 11, 2005, Attorney Docket No. 2004-0063-01, entitled
LASER THIN FILM POLY-SILICON ANNEALING OPTICAL SYSTEM, Published on
Dec. 8, 2005, Pub. No. US-2005-0269300; and 11/254,282, entitled
METHOD AND APPARATUS FOR GAS DISCHARGE LASER BANDWIDTH AND CENTER
WAVELENGTH CONTROL, Attorney Docket No. 2005-0076-01; and
11/346,519, filed on Feb. 1, 2006, entitled, VERY NARROW BAND, TWO
CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, Attorney Docket
No. 2001-0090-16; and 11/323,604, filed on Dec. 29, 2005, entitled
MULTI-CHAMBER GAS DISCHARGE LASER BANDWIDTH CONTROL THROUGH
DISCHARGE TIMING; and 11/363,116, entitled VERY HIGH REPETITION
RATE NARROW BAND GAS DISCHARGE LASER SYSTEM, filed on Feb. 27,
2006, Attorney Docket No. 2003-0129-04; and 10/881,533, entitled
METHOD AND APPARATUS FOR GAS DISCHARGE LASER OUTPUT LIGHT COHERENCY
REDUCTION, filed on Jun. 30, 2004, Attorney Docket No.
2003-0120-01; and 10/847,799, entitled LASER OUTPUT LIGHT PULSE
STRETCHER, filed on May 18, 2004, Attorney Docket No. 2003-0121;
and U.S. patent application Ser. No. 11/394,512, entitled CONFOCAL
PULSE STRETCHER, filed on Mar. 31, 2006, Attorney Docket No.
2004-0144-01; the disclosures of each of which are incorporated
herein by reference.
BACKGROUND
[0004] Laser systems such as applicants' assignee's laser systems,
e.g., 7XXX and XLA-1XX, XLA-2XX and XLA-3XX model laser systems,
with the 7XXX, being single chamber laser systems, e.g., ArF or KrF
excimer laser systems, and the XLA models being multi-chamber laser
systems, e.g., master oscillator-power amplifier ("MOPA")
configured laser systems, e.g., excimer MOPAs, may be used for such
applications as above noted, e.g., laser annealing of materials on
a workpiece substrate and as integrated circuit photolithography
DUV light sources. The latter of which systems currently operating
at 6 kHz and can produce about 60-90 watts delivered to, e.g., a
photolithography tool in a burst of hundreds of pulses with about
10-15 mJ per pulse. This operation is relatively close to maximum
due, e.g., to high pulse energy and concomitant optical damage
problems. Also limitations exist such as in the form of fan size
and speed needed to circulate the gas in the lasing chamber(s).
[0005] In order to preserve the endless march of Moore's law and
with practical and economical EUV light source production perhaps
not arriving quite on time, the photolithography tool makers have
turned to a technology known as immersion lithography, whereby a
fluid is interposed between the wafer and the mask/retical and
projection lens assembly.
[0006] While much of the photolithography tool may still be
utilized for immersion techniques, the DUV 193 nm light source will
need to be significantly changed, in order to meet higher
throughput requirements, and also for beam stability reasons
(bandwidth, dose, center wavelength, etc.).
[0007] A further motivation for implementing a very high average
power, e.g., 100 W, laser system, e.g., as an XLA laser system or
other power amplification stage, is that further advances in
lithographic resolution can be achieved through a "double exposure"
technique. Scanner makers will no doubt want to produce a stepper
that can provide double exposure with no loss in wafer throughput.
Thus, wafer stage speed (scan speed) would also have to increase by
approximately 2.times.. Thus, the laser average power must also
increase by around 2.times. or perhaps even more to maintain
throughput.
[0008] It is also possible that improved performance in the optics,
e.g., with the application of optical coatings and/or angle of
incidence changes could be used to increase the overall laser
system output, although studies by applicants' employer have
indicated that there is not enough margin there to get to the
required overall output energy levels and/or certain optics could
not safely be modified in the effort. Thus this is not currently an
available option because as currently perceived a relatively small
percentage change in overall output energy is obtainable by this
route alone, i.e., somewhere between ten to twenty percent.
[0009] Various assumptions and constraints may be applicable
regarding illuminator component lifetimes and capabilities and the
like which result in the conclusion that for a 6 kHz.times.33 mJ
solution, pulse duration must increase by a factor of 4, and for a
12 kHz.times.17 mJ solution, pulse duration must increase by a
factor of 2. Also, since the same degree of high polarization will
be required from the laser light source, one can not use
polarization coupling to combine separate laser beams to form a
laser system output light pulse beam input to the scanner (though
polarization coupling may be used elsewhere) and a 2.times.
increase in power density through various components will cause
increased depolarization. Whether or not the scanner (e.g., in the
illuminator) can accommodate a change in laser beam size is also an
issue. Some scanners may also not be able to accommodate a laser
light source in the form of two input beams, e.g., as side-by-side
laser beams with, e.g., interleaved pulsing. Applicants assume that
laser spectral requirements will remain the same as for the
equivalent lens used in single exposure systems.
[0010] One possible solution to the requirements noted above, a
single XLA running at 6 kHz and with a 20-30 mJ pulse energy output
from the power amplifier, has a number of problems in the effective
implementation, not the least of which is the difficulties in
getting to 6 kHz operation in an excimer seed to excimer amplifier
gain medium multi-chamber laser system arrangement, for which
applicants' assignee has proposed certain design approaches
discussed in one or more co-pending applications noted above. In
addition, the most likely deterrent to such an approach is
unacceptably high energy density on certain critical optical
elements in the XLA system at 20-30 mJ output pulse energy.
Alternatively one could try to implement a single MOPA XLA
operating at 12 kHz with a 17 mJ output pulse energy from the power
amplifier, however, getting to 12 kHz poses a number of problems,
e.g., an approximately 8.times. increase in chamber blower power,
significantly accentuated chamber acoustic impacts on output pulse
parameters, and difficulties in maintaining chamber robustness at
high voltage with such a high repetition rate. Similarly, a single
MOPO XLA (with a power oscillator in place of the power amplifier)
operating at 12 kHz with a 17 mJ output pulse energy from the
amplifier would face much the same detrimental impacts to effective
operation. A Single MOPA XLA tic-toc (e.g., a master
oscillator--single aperture--seeding a plurality of
amplifiers--multiple apertures--and recombined back to a single
output--single aperture) with excimer seed operating the MO at 12
kHz and each amplifier gain medium operating at 6 kHz with each
having 17 mJ output pulse energy would suffer from the same
problems, however, only in the MO. A single MOPA XLA tic-toc with
solid state seed operating at 12 kHz (tic-toc to 2 multi-pass PA's
at 6 kHz each, 17 mJ output pulse energy from each) is a
possibility, however, this would require a high average power solid
state seed laser, e.g., with about a 12 W average power output,
which is not currently available. Two "standard" six kHz XLAs could
be used side by side to tic-toc a total of a 12 kHz of 17 mJ output
pulse energy laser pulses, if acceptable from a cost standpoint for
very high power (around 200 W) lithography laser light sources,
e.g., for immersion lithography. Cost of consumables may be
acceptable, e.g., for each individual laser system, but the overall
cost of operation of the entire system essentially doubles. Other
problems need also be addressed, however the above noted are what
applicants currently believe to be the "show stoppers" to the
various noted configurations meeting the requirements for
performance and cost of operation for very high power laser
operations, e.g., for immersion lithography laser light
sources.
[0011] Applicants' employer's competitor GigaPhoton has utilized
multi-chamber seed laser/amplifier laser systems in a master
oscillator power oscillator configuration, as shown, e.g., in U.S.
Pat. Nos. 6,721,344, entitled INJECTION LOCKING TYPE OR MOPA TYPE
OF LASER DEVICE, issued on Apr. 13, 2004 to Nakao et al; 6,741,627,
entitled PHOTOLITHOGRAPHIC MOLECULAR FLUORINE LASER SYSTEM, issued
on May 25, 2004 to Kitatochi et al, and 6,839,373, entitled
ULTRA-NARROWBAND FLUORINE LASER APPARATUS, issued on Jan. 4, 2005
to Takehisha et al. However, not without certain problems not faced
by a power amplifier (i.e., a fixed amplification path one or more
passes through the amplification medium as opposed to laser
oscillation). These may include, e.g., two critical challenges in
the application of the injection locking method, e.g., to
lithography. They are related to ASE and coherence.
[0012] Fork, et al. Amplification of femtosecond optical pulses
using a double confocal resonator, Optical Letters, Vol. 14, No. 19
(October 1989) refers to a multipass power amplifier. U.S. Pat. No.
6,816,520, entitled SOLID STATE SYSTEM AND METHOD FOR GENERATING
ULTRAVIOLET LIGHT, issued on Nov. 9, 2004 to Tolloch et al.,
relates to mixing schemes for 193 nm light generation with a solid
state seed to an excimer laser; U.S. Pat. No. 6,373,869, entitled
SYSTEM AND METHOD FOR GENERATING COHERENT RADIATION AT VACUUM
ULTRAVIOLET WAVELENGTHS USING EFFICIENT FOUR WAVE MIXING, issued to
Jacob on Apr. 16, 2002, relates to mixing schemes for 193 nm light
generation. U.S. Publication No. 20050185683A1 relates to frequency
shifting to get 193 nm light. U.S. Pat. No. 5,233,460, entitled
METHOD AND MEANS FOR REDUCING SPECKLE IN COHERENT LASER PULSES,
issued to Partlo et al. on Aug. 3, 1993 discusses misaligned
optical delay paths for coherence busting on the output of gas
discharge laser systems such as excimer laser systems. U.S. Pat.
No. 6,191,887, entitled LASER ILLUMINATION WITH SPECKLE REDUCTION,
issued to Michaloski et al. on Feb. 20, 2001, relates to coherence
busting for speckle reduction in a multiple delay path pulse
stretcher. U.S. Pat. No. 5,940,418, entitled SOLID-STATE LASER
SYSTEM FOR ULTRA-VIOLET MICRO-LITHOGRAPHY, issued to Shields on
Aug. 17, 1999 relates to MOPO/PA configurations where a solid state
laser is the MO for a solid state laser PO or PA but refers to an
article as describing the production of 193 nm light using an
excimer laser, a dye laser and a birefringent BBO crystal for
frequency multiplication harmonic generation, Muckenheim et al.,
"Attaining the wavelength Range 189-197 by frequency mixing in
B-BaB.sub.2O.sub.4," Appl. Phys. B 45 (1988), pp. 259-261. U.S.
Pat. No. 6,031,854, entitled DIODE PUMPED CASCADE LASER FOR DEEP UV
GENERATION, issued to Ming on Feb. 29, 2000 relates to a solid
state cascade laser in which the output of a diode pumped solid
state laser is used to pump another solid state laser to produce
DUV light; U.S. Pat. No. 6,320,886, entitled LASER DEVICE, issued
to Dawber on Nov. 20, 2001 relates to a solid state optical
parametric generator ("OPG") that is pumped by light produced by a
pump source 4 that is disclosed also to be a solid state laser, and
where the OPG is in a resonance cavity. U.S. Pat. No. 6,477,188,
entitled LIGHT SOURCE, issued to Takaoka on Nov. 5, 2002, relates
to solid state lasers seeding and/or pumping other solid state
lasers or OPGs or OPOs. U.S. Pat. No. 6,590,698, entitled
ULTRAVIOLET LASER APPARATUS AND EXPOSURE APPARATUS USING SAME,
issued to Ohtsuki on Jul. 8, 2003, relates to a solid state feed of
a seed into distributed fiber-optic amplifiers. U.S. Pat. No.
6,654,163, entitled OPTICAL AMPLIFIER ARRANGEMENT FOR SOLID STATE
LASER, issued to Du on Nov. 25, 2003, relates to an amplifier gain
medium that can be a gas discharge or solid state laser seeded from
an undisclosed type of laser. U.S. Pat. No. 6,721,344, entitled
INJECTION LOCKING TYPE OR MOPA TYPE OF LASER DEVICE, issued to
Nakao et al. on Apr. 13, 2004 discloses an F.sub.2 gas discharge
laser in a MOPA or MOPO configuration with a gas discharge master
oscillator seeding a gas discharge amplifier. U.S. Pat. No.
4,982,406, entitled SELF INJECTION-LOCKING LASER TECHNIQUE, issued
to Facklam on Jan. 1, 1999, relates to a laser system that has
so-called "self-injection locking" and appears to disclose a number
of prior art systems, that inject a seed beam into an amplifier
laser. U.S. Pat. No. 4,019,157, entitled METHOD AND APPARATUS FOR
TUNING HIGH POWER LASERS, issued to Hutchinson on Apr. 19, 1977,
relates to a pulsed gas laser (CO.sub.2) seeded with a CW laser
beam from a seed laser disclosed to be a CW CO.sub.2 laser. U.S.
Pat. No. 4,227,159, entitled COMMON-RESONATOR PRE-LOCKED LASER,
issued to Barrett on Oct. 10, 1980 relates to a dye laser
simultaneously pumped in a resonator cavity by an argon ion laser
and a solid state Nd:YAG frequency doubled laser. U.S. Pat. No.
4,019,157, entitled METHOD AND APPARATUS FOR TUNING HIGH POWER
LASERS, issued to Hutchinson on Apr. 19, 1977, relates to high
power gas lasers which are seeded by a beam from a low power laser.
U.S. Pat. No. 4,264,870, entitled AUTOMATIC LOCKING SYSTEM FOR AN
INJECTION LOCKED LASER, issued to Avicola on Apr. 28, 1981, relates
to an injection locked oscillator which is an optically pumped dye
laser that is provided with a seed laser pulse from a "master
oscillator" but this MO actually acts to create a population
inversion in the ILO cavity at a wavelength selected by the
wavelength of the master oscillator pulse prior to stimulated
emission lasing in the ILO resulting from the pumping of the ILO
flash lamp. U.S. Pat. No. 4,490,823, entitled INJECTION-LOCKED
UNSTABLE LASER, issued to Komine on Dec. 25, 1984, relates to a
laser system that has an optical switch to form the cavity first to
include line narrowing in a stable resonator and thereafter to
switch to an unstable resonator with the line narrowing package not
in the cavity any longer. U.S. Pat. No. 4,606,034, entitled
ENHANCED LASER POWER OUTPUT, issued to Eden et al. on Aug. 12,
1986, relates to population inversion created by a "seed" pulse
before stimulated emission is caused in the amplifier by the
amplifier being pumped. U.S. Pat. No. 4,689,794, entitled
INJECTION-LOCKING A XENON CHLORIDE LASER AT 308.4 NM, issued to
Brosman on Aug. 25, 1987, relates to an injection locked excimer
gas discharge laser system, e.g., a XeCl laser which either uses
line narrowing or an injection of a low level amount of radiation
into the cavity to essentially do preionization so the gain
achieved by the main pumping need not be so high.
[0013] Partlo et al, Diffuser speckle model: application to
multiple moving diffusers, Appl. Opt. 32, 3009-3014 (1993),
discusses speckle reduction techniques.
[0014] Ti:sapphire (Titanium-sapphire) lasers emit near-infrared
light, tunable in the range from 650 to 1100 nanometers. These
lasers are tunable and can generate ultrashort pulses.
Titanium-sapphire refers to the lasing medium, a crystal of
sapphire (Al.sub.2O.sub.3) that is doped with titanium ions. A
Ti:sapphire laser is usually pumped with another laser with a
wavelength of 514 to 532 nm, for which argon lasers (514.5 nm) and
frequency-doubled Nd:YAG, Nd:YLF, and Nd:YVO lasers (527-532 nm)
may be used as discussed at
http://en.wikipedia.org/wiki/Ti-sapphire_laser.
[0015] Second harmonic generation (SHG, also called frequency
doubling) is a nonlinear optical process, in which photons, e.g.,
at a given wavelength, interacting with a nonlinear material are
effectively "combined" to form new photons with twice the energy,
and therefore twice the frequency and half the wavelength of the
initial photons. Only under special circumstances, the rate of
conversion of photons to the higher-energy photons is significant.
The two fundamental requirements for efficient nonlinear power
conversion are that the pump intensity is high over a certain
propagation length, and that the involved beams preserve a certain
phase relationship over that length. Under properly optimized
conditions, it is possible to obtain more than 50% conversion
efficiency (sometimes even more than 80%) by focussing an intense
laser beam into a suitable nonlinear crystal. This is widely used,
for example to generate green light at 532 nm from the near
infrared output of a Nd:YAG laser at 1064 nm. Some common materials
used for second harmonic generation are potassium titanyl phosphate
(KTP), lithium triborate (LBO), cesium lithium borate (CLBO),
lithium tantalate, and lithium niobate.
[0016] As mentioned above, a high conversion efficiency requires
that the input light and the second harmonic light are kept in
phase. This is not the case without special measures, because the
speed of light in a material generally varies with wavelength due
to dispersion of the index of refraction. In some nonlinear
crystals, a particular combination of crystal orientation and
crystal temperature can be found where, due to birefringence, the
fundamental and second harmonic light both see the same index of
refraction, and so remain in phase as they propagate. In other
nonlinear materials, where this is not possible, periodic poling is
used to keep the waves approximately in phase. This technique,
called quasi-phase matching, is commonly used for lithium niobate
and lithium tantalate, and greatly expands the options for
efficient frequency doubling at various wavelengths and
temperatures.
http://en.wikipedia.org/wiki/Second_harmonic_generation.
[0017] Acousto-optic ("AO") crystals are often used in optical
systems to modulate, frequency shift, or diffract a laser beam. In
the case of frequency shifting, the beam interacts with an acoustic
wave that moves inside the crystal, Bragg-reflecting from the wave.
The frequency of the reflected beam is the sum of the frequency of
the original beam and the frequency of the acoustic wave. Depending
on its direction of motion, the acoustic wave can contribute either
a positive or negative frequency component. In an AO phase shifter,
the beam to be phase shifted reflects first from one AO crystal
that adds a radio-frequency (RF) component, then from a second AO
crystal that subtracts an RF component of the same magnitude,
restoring the beam's original frequency. The phase delays between
the two RF signals can be varied, adding a controllable phase shift
to the beam, as is discussed at
http://lfw.pennnet.com/Articles/Article_Display.cfm?Section=ARTCL&ARTICLE-
_ID=221417&VERSION_NUM=3&p=12
[0018] Published International Application WO 97/08792, published
on Mar. 6, 1997 discloses an amplifier with an intracavity optical
system that has an optical path that passes each pass of a sixteen
pass path through the same intersection point at which is directed
a pumping source to amplify the light passing through the
intersection point.
[0019] R. Paschotta, Regenerative Amplifiers, found at
http://www.rp-photonics.com/regenerativeamplifiers.html (2006)
discusses the fact that a regenerative amplifier, may be considered
to be an optical amplifier with a laser cavity in which pulses do a
certain number of round trips, e.g., in order to achieve strong
amplification of short optical pulses. Multiple passes through the
gain medium, e.g., a solid state or gaseous lasing medium may be
achieved, e.g., by placing the gain medium in an optical cavity,
together with an optical switch, e.g., an electro-optic modulator
and/or a polarizer. The gain medium may be pumped for some time, so
that it accumulates some energy, after which an initial pulse may
be injected into the cavity through a port which is opened for a
short time (shorter than the round-trip time), e.g., with the
electro-optic (or sometimes acousto-optic) switch. Thereafter the
pulse can undergo many (possibly hundreds) of cavity round trips,
being amplified to a high energy level, often referred to as
oscillation. The electro-optic switch can then be used again to
release the pulse from the cavity. Alternatively, the number of
oscillations may be determined by using a partially reflective
output coupler that reflects some portion, e.g., around 10%-20% of
the light generated in the cavity back into the cavity until the
amount of light generated by stimulated emission in the lasing
medium is such that a useful pulse of energy passes through the
output coupler during each respective initiation and maintenance of
an excited medium, e.g., in a pulsed laser system.
[0020] Uppal et al, Performance of a general asymmetric Nd: glass
ring laser, Applied Optics, Vol. 25, No. 1 (January 1986) discusses
an Nd:glass ring laser. Fork, et al. Amplification of femtosecond
optical pulses using a double confocal resonator, Optical Letters,
Vol. 14, No. 19 (October 1989) discloses a seed laser/power
amplifier system with multiple passes through a gain medium in a
ring configuration, which Fork et al. indicates can be "converted
into a closed regenerative multi pass amplifier by small
reorientations of two of the four mirrors that compose the
resonator [and providing] additional means . . . for introducing
and extracting the pulse from the closed regenerator". This
reference refers to an open-ended amplifier portion with fixed
number of passes through the amplifier portion (fixed by the optics
and, e.g., how long it takes for the beam to walk off of the lens
and exit the amplifier portion) as a "resonator".
[0021] Mitsubishi published Japanese Patent Application Ser. No.
JP11-025890, filed on Feb. 3, 1999, published on Aug. 11, 2000,
Publication No. 2000223408, entitled SEMICONDUCTOR MANUFACTURING
DEVICE, AND MANUFACTURING OF SEMICONDUCTOR DEVICE, disclosed a
solid state seed laser and an injection locked power amplifier with
a phase delay homogenizer, e.g., a grism or grism-like optic,
between the master oscillator and amplifier. United States
Published application 20060171439, published on Aug. 3, 2006,
entitled MASTER OSCILLATOR--POWER AMPLIFIER EXCIMER LASER SYSTEM, a
divisional of an earlier published application 20040202220,
discloses as master oscillator/power amplifier laser system with an
optical delay path intermediate the master oscillator and power
amplifier which creates extended pulses from the input pulses with
overlapping daughter pulses.
[0022] As used herein the term resonator and other related terms,
e.g., cavity, oscillation, output coupler are used to refer,
specifically to either a master oscillator or amplifier portion, a
power oscillator, as lasing that occurs by oscillation within the
cavity until sufficient pulse intensity exists for a useful pulse
to emerge from the partially reflective output coupler as a laser
output pulse. This depends on the optical properties of the laser
cavity, e.g., the size of the cavity and the reflectivity of the
output coupler and not simply on the number of reflections that
direct the seed laser input through the gain medium a fixed number
of times, e.g., a one pass, two pass, etc. power amplifier, or six
or so times in the embodiment disclosed in Fork, et al.
Amplification of femtosecond optical pulses using a double confocal
resonator, Optical Letters, Vol. 14, No. 19 (October 1989) and not
on the operation of some optical switch in the cavity. In some of
the literature an oscillator in which the round trip through the
amplification gain medium, e.g., around a loop in a bow-tie or
racetrack loop, is not an integer number of wavelengths, may be
referred to as an amplifier, e.g., a power amplifier, while also
constituting an oscillator laser. The term power amplification
stage and more specifically ring power amplification stage is
intended herein to cover both of these versions of a power
oscillator, i.e., whether the path through the gain medium is an
integer multiple of the laser system nominal center wavelength or
not and whether the literature, or some of it, would refer to such
an "oscillator" as a power amplifier or not. The closed loop path
or oscillation loop as used herein refers to the path through the
amplification gain medium, e.g., an excimer or similar gas
discharge laser amplification stage, around which the seed laser
pulse light oscillates in the amplification stage.
[0023] Yb3+ fiber lasers are inherently tunable, as discussed in J
Nilsson et al "High-power wavelength-tunable cladding-pumped
rare-earth-doped silica fiber lasers," Opt. Fiber Technol. 10, pp
5-30 (2004).
SUMMARY
[0024] An apparatus and method are disclosed which may comprise a
line narrowed pulsed lithography laser light source which may
comprise: a seed pulse providing laser system which may comprise: a
first pulsed seed laser producing seed pulses at a rate of X kHz; a
second pulsed seed laser producing seed pulses at a rate of X kHz;
an amplification system which may comprise: a first amplifier gain
system which may comprise a first and a second pulsed gas discharge
amplifier gain medium, each with a nominal center wavelength in the
UV range, and each operating at 1/2X kHz on output pulses from the
first seed laser; a second amplifier gain system which may comprise
a first and a second pulsed amplifier gain medium, each with a
nominal center wavelength in the UV range, and each operating at
1/2X kHz on output pulses from the second seed laser; a first and a
second line narrowing module each selecting a desired bandwidth
for, respectively, the first and second seed laser; a pulse
combiner combining the output pulses from the first and second
amplifier gain systems to produce an output laser light pulse beam
at a pulse repetition rate of 2X kHz. The first and second pulsed
seed laser systems each may comprise a gas discharge laser, e.g., a
single seed with a single aperture or a plurality of seeds with a
plurality of apertures may seed a plurality of amplifiers or a
plurality of groups of amplifiers constituting many apertures and
with the output all combined through a single aperture output. The
system and method may comprise a seed laser output beam divider
providing output pulses of the first pulsed seed laser to the
respective first amplifier gain medium and second amplifier gain
medium of the first amplifier gain system and a second seed laser
output beam divider providing output pulses of the second pulsed
seed laser respectively to the first amplifier gain medium and the
second amplifier gain medium of the second amplifier gain system.
Each pulsed amplifier gain medium may comprise a gas discharge
laser system. X kHz may be at least 4 kHz. Each respective
amplifier gain medium may comprise a ring power amplification
stage. The apparatus and method may comprise a narrow band pulsed
lithography laser light source which may comprise: a seed pulse
providing laser system which may comprise a single pulsed solid
state seed laser with a nominal center wavelength outside of the UV
range and operating at 2X kHz; an amplification system which may
comprise: a first amplifier gain system which may comprise a first
and a second pulsed gas discharge amplifier gain medium, each with
a nominal center wavelength in the UV range, and each operating at
1/2X kHz on output pulses from the first seed laser system; a
second amplifier gain system which may comprise a first and a
second pulsed amplifier gain medium, each with a nominal center
wavelength in the UV range, and each operating at 1/2X kHz on
output pulses from the second seed laser system; a seed pulse
divider providing pulses from the solid state seed laser to a first
amplifier gain system pulse divider and to a second amplifier gain
system pulse divider, the first amplifier gain system pulse divider
providing pulses to the first and second pulsed amplifier gain
medium of the first amplifier gain system and the second amplifier
gain system pulse divider providing pulses to the first and second
amplifier gain medium of the second amplifier gain system; a pulse
combiner combining the output pulses from the first and second
amplifier gain systems to produce a light source output laser light
pulse beam at a pulse repetition rate of 2X kHz. The apparatus and
method may comprise a frequency converter converting the wavelength
of the output of the seed laser to a UV wavelength suitable for
amplification in the amplifier gain medium. Each pulsed amplifier
gain medium may comprise a gas discharge laser system. X kHz may be
at least 6 kHz. Each respective amplifier gain medium may comprise
a ring power amplification stage. The apparatus and method may
comprise a line narrowed pulsed lithography laser light source
which may comprise: a first pulsed seed laser system producing seed
pulses at a rate of X kHz; a second pulsed seed laser system
producing seed pulses at a rate of X kHz, an nth pulsed seed laser
system producing seed pulses at a rate of X kHz; a first amplifier
gain system which may comprise A amplifier gain mediums each with a
nominal center wavelength in the UV range, and each operating at
(1/A)*X kHz on respective Ath output pulses from the first seed
laser system; a second amplifier gain system which may comprise A
pulsed amplifier gain mediums, each with a nominal center
wavelength in the UV range, and each operating at (1/A)*X kHz on
respective Ath output pulses from the second seed laser system; an
nth amplifier gain system comprising A pulsed amplifier gain
mediums, each with a nominal center wavelength in the UV range, and
each operating at (1/A)*X kHz on respective Ath output pulses from
the nth seed laser system; n line narrowing modules each selecting
a desired bandwidth for, respectively, the first through nth seed
laser system; a pulse combiner combining the output pulses from the
n amplifier gain systems to produce an output laser light pulse
beam at a pulse repetition rate of nX kHz. Each pulsed seed laser
system may comprise a gas discharge laser with a nominal center
wavelength essentially the same as the nominal center wavelength of
the respective amplifier gain medium. The apparatus and method may
comprise a seed laser beam divider providing output pulses of a
respective pulsed seed laser to one of n amplifier gain system beam
dividers; the respective n amplifier gain system beam divider
providing each Ath pulse to a respective one of the A amplifier
gain mediums in the respective amplifier gain system. Each pulsed
amplifier gain medium may comprise a gas discharge laser system. X
kHz is at least 4 kHz. Each respective amplifier gain medium may
comprise a ring power amplification stage. The apparatus and method
may comprise a line narrowed pulsed lithography laser light source
which may comprise: a first pulsed solid state seed laser system
producing seed pulses at a rate of X kHz; a second pulsed solid
state seed laser system producing seed pulses at a rate of X kHz,
an nth pulsed solid state seed laser system producing seed pulses
at a rate of X kHz; a first amplifier gain system comprising A
pulsed gas discharge amplifier gain mediums, each with a nominal
center wavelength in the UV range, and each operating at (1/A)*X
kHz on respective Ath output pulses from the first seed laser
system; a second amplifier gain system comprising A pulsed
amplifier gain mediums, each with a nominal center wavelength in
the UV range, and each operating at (1/A)*X kHz on respective Ath
output pulses from the second seed laser system; an nth amplifier
gain system comprising A pulsed amplifier gain mediums, each with a
nominal center wavelength in the UV range, and each operating at
(1/A)*X kHz on respective Ath output pulses from the nth seed laser
system; a pulse combiner combining the output pulses from the
amplifier gain systems to produce an output laser light source
pulse beam at a pulse repetition rate of nX kHz. The apparatus and
method may comprise a seed laser beam divider providing output
pulses of the respective pulsed seed laser to each of the A
amplifier gain mediums in the respective nth amplifier gain system.
Each pulsed amplifier gain medium may comprise a gas discharge
laser system. X kHz may be at least 6 kHz. Each respective
amplifier gain medium may comprise a ring power amplification
stage. The apparatus and method may comprise a broad band pulsed
laser light source which may comprise a first pulsed seed laser
system producing seed pulses at a rate of X kHz; a second pulsed
seed laser system producing seed pulses at a rate of X kHz; an nth
pulsed seed laser system producing seed pulses at a rate of X kHz;
a first amplifier gain system comprising A pulsed gas discharge
amplifier gain mediums, each with a nominal center wavelength in
the UV range, and each operating at (1/A)*X kHz on respective Ath
output pulses from the first seed laser system; a second amplifier
gain system comprising A pulsed amplifier gain mediums, each with a
nominal center wavelength in the UV range, and each operating at
(1/A)*X kHz on respective Ath output pulses from the second seed
laser system; an nth amplifier gain system comprising A pulsed
amplifier gain mediums, each with a nominal center wavelength in
the UV range, and each operating at (1/A)*X kHz on respective Ath
output pulses from the nth seed laser system; a pulse combiner
combining the output pulses from the amplifier gain systems to
produce an output laser light pulse beam at a pulse repetition rate
of nX kHz. The pulsed seed laser systems may comprise a gas
discharge laser with a nominal center wavelength essentially the
same as the nominal center wavelength of the amplifier gain medium.
The apparatus and method may comprise a seed laser beam divider
providing output pulses of the pulsed seed laser to a respective
one of the n amplifier gain system beam dividers; the respective
nth amplifier gain system beam divider providing each Ath pulse to
a respective one of the A amplifier gain mediums in the respective
amplifier gain system. Each pulsed amplifier gain medium may
comprise a gas discharge laser system. X kHz may be at least 6 kHz.
Each respective amplifier gain medium may comprise a ring power
amplification stage. The apparatus and method may comprise a line
narrowed pulsed lithography laser light source which may comprise:
a narrow band seed pulse providing laser system which may comprise
a pulsed seed laser system producing a beam of seed pulses with a
bandwidth of .ltoreq.0.2 pm FWHM or .ltoreq.0.5 pm E95%, at a rate
of X kHz; a beam splitter receiving the output of the seed laser
system and providing a split output of every pulse from the pulse
splitter; an amplifier gain system comprising a first gas discharge
amplifier gain medium and operating at Y kHz on respective outputs
from the pulse splitter and a second gas discharge amplifier gain
medium operating at Z kHz on respective outputs from the pulse
splitter, where Y+Z=X; a pulse combiner combining the output pulses
from the first and second amplifier gain systems to produce an
output laser light pulse beam at a pulse repetition rate of X kHz.
The apparatus and method may comprise a plurality of N light
sources; a pulse combiner providing an output of line narrowed
laser pulses at a pulse repetition rate of NX. The seed pulse
providing system may comprise a solid state laser, e.g., a fiber
laser, e.g., selected from a group comprising Nd-based, Er-based
and Yb-based solid state lasers, or a gas discharge laser and a
line narrowing module. The beam splitter may comprise a geometric
beam splitter transmitting at least one spatial portion of the
pulse beam on one path and at least a second portion of the pulse
beam on a second path. The beam splitter may comprise a partially
reflective optic reflecting a portion of each pulse in the pulse
beam on one path and transmitting a portion of each pulse in the
pulse beam on a second path. The beam splitter may comprise a
stimulated optical path modulator directing a first portion of a
pulse in the pulse beam in a first path during a first time period
and a second portion of a pulse on a second path during a second
time period, or directing separate pulses on respective ones of a
plurality of separate path in respective time periods. The gas
discharge amplifier gain medium may comprise a power amplifier.
Each gas discharge amplification gain medium may comprise a power
oscillator. The apparatus and method may comprise: the pulse
combiner comprising a beam combining optical element forming a
laser light source output beam comprising a first spatially defined
component from the first amplifier gain medium and a second
spatially defined component from the second amplifier gain medium.
The pulse combiner may comprise a beam combining optical element
forming a laser light source output beam comprising a first
component from the first amplifier gain medium and a second
component from the second amplifier gain medium exiting the pulse
combiner along a path comprising a single optical axis. The pulse
combiner may comprise comprising an OPuS pulse stretcher.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 shows schematically and in block diagram form a very
high average power laser light source according to aspects of an
embodiment of the disclosed subject matter;
[0026] FIG. 2 illustrates schematically and in block diagram form a
very high average power laser light source according to aspects of
an embodiment of the disclosed subject matter;
[0027] FIG. 3 shows schematically in block diagram form an example
of a very high average power laser light source according to
aspects of an embodiment of the disclosed subject matter;
[0028] FIG. 4 shows partly schematically and partly in block
diagram form, by way of example an immersion laser lithography
system according to aspects of an embodiment of the disclosed
subject matter;
[0029] FIG. 5 shows schematically and in block diagram form a solid
state seed laser to gas discharge amplifier laser system according
to aspects of an embodiment of the disclosed subject matter;
[0030] FIG. 6 shows in block diagram form a solid state seed
laser/amplifier laser system according to aspects of an embodiment
of the disclosed subject matter;
[0031] FIG. 7 shows schematically and in block diagram form
conversion of the output of a seed laser, e.g., with a frequency
converter along with a beam divider, followed by coherency busting
according to aspects of an embodiment of the disclosed subject
matter;
[0032] FIG. 8 shows schematically and in block diagram form a
version of the embodiment of FIG. 7 according to aspects of an
embodiment of the disclosed subject matter;
[0033] FIG. 9 shows schematically a coherency buster according to
aspects of an embodiment of the disclosed subject matter;
[0034] FIG. 10 shows schematically and partly in block diagram form
an injection seeded DUV gas discharge master oscillator/amplifier
gain medium laser system solid state master oscillator according to
aspects of an embodiment of the disclosed subject matter;
[0035] FIG. 11 shows schematically and partly in block diagram form
an injection seeded DUV gas discharge master oscillator/amplifier
gain medium laser system solid state master oscillator according to
aspects of an embodiment of the disclosed subject matter;
[0036] FIG. 12 shows schematically and partly in block diagram form
an injection seeded DUV gas discharge master oscillator/amplifier
gain medium laser system solid state master oscillator according to
aspects of an embodiment of the disclosed subject matter;
[0037] FIG. 13 shows schematically and partly in block diagram form
an injection seeded DUV gas discharge master oscillator/amplifier
gain medium laser system solid state master oscillator according to
aspects of an embodiment of the disclosed subject matter;
[0038] FIG. 14 illustrates in schematic and partly block diagram
form an example of very high average output power laser system
power amplification stage according to aspects of an embodiment of
the disclosed subject matter;
[0039] FIG. 15 shows partly schematically and partly in block
diagram for an example of elements of a coherence busting scheme
and the results of aspects of the scheme according to aspects of an
embodiment of the disclosed subject matter;
[0040] FIG. 16 illustrates in schematic and partly block diagram
form an example of very high average output power laser system
according to aspects of an embodiment of the disclosed subject
matter;
[0041] FIG. 17 illustrates relative speckle intensity for a various
E-O deflector voltages related to relative timing between the EO
and the pulse generation in the seed laser according to aspects of
an embodiment of the disclosed subject matter;
[0042] FIG. 18 illustrates pointing shift relative to E-O voltage
according to aspects of an embodiment of the disclosed subject
matter;
[0043] FIG. 19 illustrates an example of the timing of an E-O
deflection voltage and a seed laser pulse spectrum according to
aspects of an embodiment of the disclosed subject matter;
[0044] FIGS. 20A and B illustrate the effect of beam combining
according to aspects of an embodiment of the disclosed subject
matter;
[0045] FIG. 21 illustrates schematically and in partly block
diagram form a beam combiner with divergence control according to
aspects of an embodiment of the disclosed subject matter;
[0046] FIG. 22 illustrates the effect of beam sweeping/painting on
coherency according to aspects of an embodiment of the disclosed
subject matter;
[0047] FIG. 23 illustrates schematically and in partly block
diagram form a very high power solid state seed laser and gain
amplifier laser system according to aspects of an embodiment of the
disclosed subject matter;
[0048] FIG. 24 shows schematically and in cartoon fashion the
effects of multiple coherence busting schemes;
[0049] FIG. 25 illustrates schematically and partly in block
diagram format a regenerative/recirculating power gain oscillator
power amplification stage according to aspects of an embodiment of
the disclosed subject matter;
[0050] FIG. 26 illustrates schematically a coherency reduction
scheme according to aspects of an embodiment of the disclosed
subject matter;
[0051] FIG. 27 illustrates schematically and partly in block
diagram form a solid state seed laser/gain amplifier laser system
according to aspects of an embodiment of the disclosed subject
matter;
[0052] FIG. 28 illustrates schematically and partly in block
diagram form a solid state seed laser/gain amplifier laser system
according to aspects of an embodiment of the disclosed subject
matter;
[0053] FIG. 29 illustrates normalized output pulse shapes from
laser systems according to aspects of an embodiment of the
disclosed subject matter;
[0054] FIG. 30 represents schematically E-O cell laser steering
input voltages according to aspects of an embodiment of the
disclosed subject matter;
[0055] FIG. 31 represents schematically in block diagram form a
laser steering system according to aspects of an embodiment of the
disclosed subject matter;
[0056] FIG. 32 represents schematically E-O cell laser steering
voltages input signals according to aspects of an embodiment of the
disclosed subject matter;
[0057] FIG. 33 illustrates results of simulated beam pulse
recombination results;
[0058] FIG. 34 illustrates exemplary coherency busting test results
according to aspects of an embodiment of the disclosed subject
matter;
[0059] FIG. 35 illustrates schematically a coherency buster
according to aspects of an embodiment of the disclosed subject
matter;
[0060] FIG. 36 illustrates exemplary coherency busting test results
according to aspects of an embodiment of the disclosed subject
matter;
[0061] FIG. 37 illustrates exemplary coherency busting test results
according to aspects of an embodiment of the disclosed subject
matter;
[0062] FIG. 38 illustrates a simulated E-O supply voltage with
respect to a seed pulse intensity over time, according to aspects
of an embodiment of the disclosed subject matter;
[0063] FIG. 39 illustrates a test E-O supply voltage with respect
to a seed pulse intensity over time, according to aspects of an
embodiment of the disclosed subject matter;
[0064] FIG. 40 illustrates a E-O cell drive circuit according to
aspects of an embodiment of the disclosed subject matter;
[0065] FIG. 41 illustrates schematically and partly in block
diagram form a solid state seed laser with about 193 nm output
light according to aspects of an embodiment of the disclosed
subject matter;
[0066] FIG. 42 illustrates schematically and partly in block
diagram form a sold state seed laser with about 193 nm output light
according to aspects of an embodiment of the disclosed subject
matter;
[0067] FIG. 43 illustrates various frequency up-conversion
schemes;
[0068] FIG. 44 illustrates schematically and in block diagram form
a broad band light source and laser surface treatment system using
the DUV laser light according to aspects of an embodiment of the
disclosed subject matter;
[0069] FIG. 45 illustrates plots of ASE and output energy vs. seed
pulse to amplifier delay time according to aspects of an embodiment
of the disclosed subject matter;
[0070] FIG. 46 shows schematically a seed injection mechanism and
beam expander according to aspects of an embodiment of the subject
matter disclosed;
[0071] FIG. 47 shows schematically a coherence buster optical delay
path according to aspects of an embodiment of the disclosed subject
matter;
[0072] FIG. 48 shows schematically a coherence buster optical delay
path according to aspects of an embodiment of the disclosed subject
matter.
[0073] FIG. 49 shows schematically and in block diagram form a
laser system according to aspects of an embodiment of the disclosed
subject matter;
[0074] FIG. 50 shows schematically and in block diagram form a
laser system according to aspects of an embodiment of the disclosed
subject matter.
DETAILED DESCRIPTION
[0075] According to aspects of an embodiment of the disclosed
subject matter there are certain performance requirements necessary
of a very high power amplification stage cavity for, e.g., a 200 w
laser system, e.g., with two amplifier gain medium chambers in
parallel. They should produce linear polarization (>98%). Each
amplification stage should produce, and survive, .gtoreq.100 W
average output energy, e.g., at 193 nm wavelength of ArF, or less
stringently at longer wavelengths, e.g., 248 for KrF and 351 for
XeF or 318 for XeCl, though even more stringent for F.sub.2 at 157
nm. Each amplification stage in one embodiment may operate at about
6 kHz or above. The amplification stage optics must survive energy
densities associated with 17 mJ/(losses).apprxeq.29 mJ per pulse
near the amplification stage output. According to aspects of an
embodiment of the disclosed subject matter, the amplification
stage(s) can exhibit full seeding (at or near saturation) with
relatively small seed laser energy. According to aspects of an
embodiment of the disclosed subject matter seed laser energy may be
no more than around 1 .mu.J, though the system overall output power
in such cases may be less than 200 W. Applicants believe that the
amplification stage should also support a moderately large angular
distribution, e.g., to maintain the same angular spread of the seed
laser, in order to avoid inadvertently improving coherence by,
e.g., removing coherence cells, e.g., with a range of angles of
within a few m Rad. Protection of the seed laser from reverse
traveling radiation is also an important operational requirement.
When properly seeded, ASE levels produced by the amplification
stage, according to aspects of an embodiment of the disclosed
subject matter, should be less than 3.times.e-4 of total
output.
[0076] According to aspects of an embodiment of the disclosed
subject matter applicants expect that (1) the gain cross-section
will be similar to existing ArF chambers, e.g., applicants'
assignee's XLA ArF laser system power amplifier ("PA") chambers,
(2) the gain length will also be similar to existing ArF chambers,
(3) the gain duration will also be similar to existing ArF
chambers.
[0077] According to aspects of an embodiment of the disclosed
subject matter, applicants propose, e.g., a single MO/gain
amplification medium XLA tic-toc with a solid state seed laser
operating at 12 kHz with about a 1 mJ seed laser output light pulse
energy and the two amplification stages each operating at around a
17 mJ output pulse energy. In addition, according to aspects of an
embodiment of the disclosed subject matter, applicants propose the
utilization of a regenerative gain media in which the oscillating
laser generated light photons pass through the gain media at least
twice per oscillation, e.g., a ring power amplification stage,
which can enable the generation of up to ten times the output pulse
energy in the ring power amplification stage compared, e.g., to a
power amplifier ("PA") in a MOPA configuration. With such a ring
power amplification stage, in the tic-toc mode, the MO energy
required may be significantly lower, e.g., in the .mu.J to tens of
.mu.Js range. For testing purposes applicants have simulated the
input from a solid state 193 nm seed laser using a line-narrowed
ArF laser.
[0078] Applicants have studied ASE vs. MO-PO timing difference for
the different values of the above noted parameters with results as
indicated in FIG. 45. Similarly a study of MOPO energy vs. MO-PO
timing as a function of these same parameters also illustrated in
FIG. 45.
[0079] In order to meet the requirements noted above, including,
e.g., the constraints of known lithography laser light source
technology, applicants propose, according to aspects of an
embodiment of the disclosed subject matter, a number of overall
architectures that are believed to provide workable ways to address
the requirements and constraints noted above. The first may be to
provide two multi-chamber laser systems along the lines of
applicants' assignee's XLA XXX laser system series, e.g., with two
dual chamber laser oscillator/amplifier arrangements whereby each
is configured to run at around 6 kHz producing output pulses at
about 17 mJ with interleaved firing times to produce a single
approximately 12 kHz system producing about 17 m per pulse.
[0080] Thus, e.g., according to aspects of an embodiment of the
disclosed subject matter, illustrated schematically and in block
diagram form in FIG. 1, a very high average power laser system,
e.g., a 200 W immersion lithography laser light source 20 may
comprise a plurality of oscillator/amplifier laser system output
light pulse beam sources, e.g., 22, 24, each of which comprising,
e.g., a master oscillator portion comprising master oscillator
chambers 30, such as those being sold by applicants' assignee Cymer
Inc. as part of an existing XLA XXX model multi-chamber laser
system. Also included in each oscillator/amplifier laser system 22,
24 may be a power amplifier portion 32, e.g., comprising an
amplifier gain medium. Each of the two oscillator/amplifier laser
systems 22, 24 provide an output light pulse beam to a beam
combiner 40, e.g., in an overleaving fashion.
[0081] Thus, e.g., with each laser system 22, 24 operating at 6 kHz
and 17 mJ output laser light pulse beam pulse energy the combined
output from the beam combiner 40 could be a 12 kHz 17 mJ output
resulting in about a 200 W average power laser system. It will also
be understood that the embodiment of FIG. 1 may also be implemented
with, e.g., a further plurality of identical oscillator amplifier
laser systems 26, 28 to produce a 400 W average power laser system.
Alternatively, each of the oscillator/amplifier systems 22, 24, 26,
28 could, e.g., operate at less than 6 kHz, e.g., each at 4 kHz
and/or with a higher overall oscillator/amplifier system 22, 24,
26, 28 output laser light pulse beam pulse energy, e.g., up to
around 33 mJ, to the extent that optical damage limits and cost of
operation and other factors will allow, for various combinations of
ultimate output 100 pulse repetition rate and pulse energy for a
similar variety of average output power values from the system
20.
[0082] Referring now to FIG. 2 there is illustrated schematically
and in block diagram form a very high average power tic-toc seed
laser/amplifier system 50 according to aspects of an embodiment of
the disclosed subject matter. The seed laser amplifier system 50
may include, e.g., a seed laser portion 30, e.g., a solid state
seed laser such as a Nd:YAG or a Nd:YLF or a Ti:Sapphire or a fiber
laser or other solid state laser, e.g., operating at around 12 kHz
with a 1-2 mJ output energy pulse and a pair of amplifier portions
32, each being supplied with, e.g., the alternating output pulses
from the seed laser portion 30, e.g., through a beam splitter 52,
discussed in more detail elsewhere in the present application. The
pulse could be supplied in other than an alternating fashion,
depending on the repetition rates of the amplification stages. Each
of the amplifier portions 32 can then be run at, e.g., around 6 kHz
for a 200 W output with only a 17 mJ output from each of the
amplifier portions 32.
[0083] Referring to FIG. 3 there is shown schematically in block
diagram form an example of a very high average power multiple
tic-toc seed laser/amplifier system 70 according to aspects of an
embodiment of the disclosed subject matter. The system 70 may
include, e.g., a first and a second seed laser 72 each supplying
seed laser pulses to a pair of amplifier portions, e.g., amplifier
gain media 74, through a beam splitter 52 and with the output of
each combined in a beam combiner 78 to provide a laser light source
system output laser light pulse beam 100 with an average output
power of at or above 200 W. The seed lasers could be, e.g., solid
state lasers operating at, e.g., around 12 kHz and the amplifier
portions could be, e.g., gas discharge lasers, e.g., excimer or
molecular fluorine lasers operating at around 6 kHz. Alternatively,
e.g., the seed lasers 72 could be excimer lasers, e.g, KrF, ArF,
XeCl, XeF or molecular fluorine lasers operating at about 6 kHz
with the respective pairs of tic-toc amplifier portions each
operating at 3 kHz for a total of 12 kHz and 17 mJ per lithography
or LTPS laser light source system output laser light pulses and a
resultant average power of around 200 W. Frequency conversion, as
discussed in more detail elsewhere in the present application may
be needed to shift the wavelength of the seed laser(s) 72, e.g.,
solid state lasers, up to the wavelength of the gas discharge laser
amplifier portions 74. The beam combiner 78 may be a single beam
combiner as shown or cascading combiners as shown in the combiners
40, 42 in FIG. 1.
[0084] It will also be understood by those skilled in the art that
various combinations and permutations of the arrangement
illustrated in FIG. 3 may be utilized. For example there may be a
plurality of A seed lasers 72 operating at X kHz with each seeding
a plurality of B amplifier portions 74, each operating at XB kHz
and the combination providing AX system output laser light source
output pulses in the output beam 100 of FIG. 3. Then, depending on
the necessary average system output power, the pulse energy for the
output of each of the plurality of amplifier portions 74 may be
determined, e.g., with A=2 and B=2, as illustrated in FIG. 3 and
X=6 kHz the overall output beam 100 can have a 12 kHz output and
with 17 mJ pulses out of the amplifier portions one gets around 200
w of average output power. The same may be said for the possible
arrangements of FIG. 1.
[0085] It will be noted that a tic-toc amplifier LTPS or immersion
lithography light source, e.g., seeded by a master oscillator
running at, e.g., twice the repetition frequency of the, e.g., two
amplifier chambers, could be two excimer laser chambers in a
MO/amplification gain medium configuration. For example, each
amplification medium could have a recirculating/regenerative ring
power amplification stage, each of which is alternatively seeded by
a master oscillator running at twice the repetition rate of either
amplification stage excimer laser chamber. Such systems can be run
at any of the desired wavelengths, e.g., DUV wavelengths, e.g.,
with the MO and PA/PO operating at 157 nm (F.sub.2), 193 nm (ArF),
248 nm (KrF), 308 nm (XeCl) or 351 nm (XeF). Further, such systems
could include solid state or excimer seed lasers operating at a
higher pulse repetition rate seeding a plurality of power
amplification stages, e.g., two, in tic-toc configuration, such as
ring power amplification stages.
[0086] In FIG. 4 there is shown partly schematically and partly in
block diagram form, by way of example an immersion laser
lithography system 80 according to aspects of an embodiment of the
disclosed subject matter. The system 80 may include, e.g., a very
high average power output laser light pulse beam source 20 such as
shown in FIG. 1 or 50 such as shown in FIG. 2 or 70 such as shown
in FIG. 3, supplying line narrowed pulses at 200 W or above average
power to a scanner 90, such as those made by ASML, Canon, or Nikon.
The scanner 90 may incorporate an illuminator 92, a reticle 94 and
a wafer stage 96 carrying a wafer 98 for exposure by the radiation
from the light source 20. On the wafer stage 96 may be a liquid
source 102, e.g., with the liquid being water having a different
index of refraction than the ambient around the reticle 94 and
stage 96, and a liquid drain 104, supplying the liquid 106 to cover
the wafer 98 for immersion lithography.
[0087] It will also be understood that for purposes of coherence
busting, either for excimer or other gas discharge seed lasers
supplying excimer or other gas discharge laser amplifier portions
or for solid state seed lasers, use of multiple amplifier portions
with the beams combined as noted elsewhere in the present
application may have beneficial effects in busting up the coherency
and therefore, assisting in reducing the effects of the speckle,
e.g., in integrated circuit photolithography or LTPS or tbSLS
processing. It will also be understood that one or more of the
various coherence busting techniques and/or combinations thereof
disclosed herein may be utilized inside of the scanner 90, whether
that scanner 90 is an immersion scanner or not.
[0088] Turning now to FIG. 5 there is shown schematically and in
block diagram form a solid state seed laser to gas discharge
amplifier laser system 120 according to aspects of an embodiment of
the disclosed subject matter. The system 120 may include, e.g., a
solid state pulsed seed laser 122, e.g., an Nd:YAG or an ND:YLF
tunable solid state laser 122. The output of the laser 122 may pass
through a coherence buster/frequency multiplier 126, which may,
e.g., be a single optical element, e.g., capable of both frequency
shifting the output of the seed laser 122 and beam steering, as is
explained in more detail elsewhere in this application with respect
to coherency busting, or could be a frequency shifter along with a
coherency buster in series, e.g., as shown in FIG. 6. The system
may also have, e.g., an amplifier gain medium such as a PA or PO
124, or, e.g., a ring power amplification stage 124, e.g., with the
output 100 supplied to a scanner 90 (shown in FIG. 5).
[0089] It will be understood that with various tuning mechanisms
may be used, e.g., operating temperature, as is know in the art,
the solid state laser, e.g., a 1064 ns wavelength Nd:YAG
(neodymium-doped yttrium aluminum garnet
(Nd:Y.sub.3Al.sub.5O.sub.12)), or 1053 ns Nd:YLF (neodymium doped
yttrium lithium fluoride) or a 1054 Ti:Sapphire laser (tunable from
about 650 to 1100 nm, which can also produce ultra-short pulses of
about _ nm in length), and/or by line selection. This can take
advantage of different transition spectra for the solid state laser
122 harmonic frequency generation and/or frequency
addition/subtraction or other frequency shifting techniques, e.g.,
Raman shifting, which may be utilized to change the output
frequency of the seed laser 122. The desired frequency/wavelength
for amplification in the amplifier portion 124 may be attained,
e.g., within an acceptable .DELTA..lamda. from the nominal center
wavelengths of around 351 for XeF, 248 for KrF, 193 for ArF and 157
for molecular fluorine to have acceptable amplifying lasing occur
in the amplifier portion 124, e.g., by amplified stimulated
emission lasing, as is well understood in the art. As noted above,
coherency busting of the type discussed elsewhere herein may be
used inside the scanner 90 or other application tool, e.g., another
micro-lithography tool or a thin beam laser annealing tool.
[0090] Turning to FIG. 6 there is shown in block diagram form a
solid state seed laser/amplifier laser system 120 according to
aspects of an embodiment of the disclosed subject matter similar to
that of FIG. 6 wherein, e.g., a frequency multiplier 130 and a
coherence buster 132 may be utilized to provide appropriate seed
pulses to the amplifier laser portion 124 to accommodate, e.g., the
high coherency of the seed laser output laser light pulse beam
pulses and also their frequency shift to the desired
frequency/wavelength for amplification, e.g., in the gas discharge
amplification gain medium of the amplification stage 124.
[0091] Turning to FIG. 7 there is shown schematically and in block
diagram form conversion of the output of a seed laser, e.g., with a
frequency converter 130 along with a beam divider 140, followed by
coherency busting in one axis, e.g., the long axis of the laser
beam or a first axis if the beam is not an elongated rectangle and
the short axis or a second orthogonal axis if the beam is not an
elongated rectangle, with a respective vertical axis coherency
buster 142 and horizontal axis coherency buster 144, as explained
in more detail herein. The outputs of the coherency busters 142,
144 may be combined in a beam combiner 146, which, as noted
elsewhere, may also serve a coherency busting role, e.g., as shown
in connection with FIG. 35, and/or FIGS. 20 A and B, and provided
as seed laser pulses to the amplifier gain medium portion 148.
[0092] Turning to FIG. 8 there is shown schematically and in block
diagram form a version of the embodiment of FIG. 7 in which, e.g.,
the frequency conversion in a frequency converter 130 occurs after
the coherency busting, i.e., intermediate the beam combiner 146 and
the amplifier portion 148.
[0093] Turning now to FIG. 9 there is shown in schematic form a
pulse stretcher 160, which can be, e.g., a version of the optical
pulse stretcher ("OPuS") sold with applicants' assignee's laser
systems however with, e.g., much shortened delay paths, as
explained elsewhere (a so called mini-OPuS) not designed for pulse
stretching per se, i.e., enough stretching for significant pulse
elongation in the spatial and temporal domains, e.g., increasing
the T.sub.is by 4.times. or more and creating overlapping pulses,
as in applicants' assignee's currently sold OPuS pulse stretchers,
as are more fully described in, e.g., U.S. Pat. No. 6,928,093,
entitled LONG DELAY AND HIGH TIS PULSE STRETCHER, issued to Webb et
al. on Aug. 9, 2005 referenced above. However, the same
folding/inverse imaging effects on the beam for coherency busting
purposes, or also as explained in regard to the beam mixer of FIG.
35, can be achieved.
[0094] The coherency buster 160 may have an input beam 162 incident
on a beam splitter 164, e.g., a partially reflective mirror 164 for
the pertinent wavelength. Part of the beam 162 that is reflected
into the delay path comprised of a plurality of mirrors, e.g.,
mirrors 166a, is negatively imaged, e.g., twice, and on the final
leg of the delay path passes through a pulse trimmer 170. It will
be understood that such optical coherence busters may have more or
less than four mirrors, e.g., six mirrors, but are illustrated
schematically with four for convenience and clarity. A portion of
the light exiting the pulse trimmer 170 is reflected into the
output beam 172 and a portion reenters the delay path. The delay
path may be much shorter than the seven to ten meters or so of,
e.g., a 4.times. OPuS, such that the second and third passes
through the delay path do not overlap the pulses entering and
leaving the coherency buster 160, but rather do not even
substantially stretch the pulses. The pulse trimmer 160 may be
used, e.g., to shorten the ultimate output pulse 172, e.g., by
cutting off a portion of the pulse circulating in the coherency
buster delay path using the pulse trimmer 170, or much or all or
substantially all of the second and subsequent passes through the
delay path. The pulse trimmer 170 may be, e.g., a Pockels cell or
other suitable fast acting light transmission switch, e.g., a light
beam modulator/deflector, e.g., an electro-optic or acousto-optic
device, e.g., a crystal that changes refractive index when excited
by a field, e.g., an electric field, an acoustic field or a
magnetic field.
[0095] In addition to traditionally used integrated circuit
photolithography laser light sources, e.g., ArF and KrF, a
fiber-laser-based solid-state 351 nm MO can also be realized
according to aspects of an embodiment of the disclosed subject
matter. Such a master oscillator architecture may be a simpler more
robust solution than a bulk-solid-state laser. Such a laser may be
utilized in the field of IC lithography as a very high power (200w
or greater) laser surface processing light source architecture that
may, e.g., incorporate the fiber-laser-based master oscillator and
also in a very low power MO output pulse laser system (e.g., with
the MO operating at the .mu.J output pulse energy level for greater
efficiency and lower cost of use and other advantages noted in the
above referenced co-pending application filed on the same day as
the present application), or in a very high power broad band
application such as for LTPS or tbSLS use, e.g., as a 351 nm laser
system. The use of pulse trimming with stimulated optical
switches/modulators may be particularly useful between the master
oscillator and amplification stage since the pulse energy may be
lower there according to aspects of an embodiment of the disclosed
subject matter.
[0096] According to aspects of an embodiment of the disclosed
subject matter the generation of 351 nm radiation, e.g., coherent
351 nm radiation, can be done with a solid-state configuration
having, e.g., a solid-state drive laser (or lasers) that drive
linear or nonlinear frequency conversion stages. Generation of 351
nm laser radiation can be, as illustrated, attained by third
harmonic conversion of the output of a Nd:YLF laser operating at
1053 nm. In order to use this approach as a seed laser for an XeF
excimer amplifier/oscillator, however, one must ensure that the
nominal center wavelength of the, e.g., Nd:YLF seed laser master
oscillator matches the gain spectrum of XeF (two lines at 351.12
and 351.26 nm). An alternative approach could be to use an Yb-doped
fiber laser as the fundamental drive laser seed pulse source. Yb3+
fiber lasers are inherently tunable, as discussed in J Nilsson et
al "High-power wavelength-tunable cladding-pumped rare-earth-doped
silica fiber lasers," Opt. Fiber Technol. 10, pp 5-30 (2004), to
allow operation between 1050 and 1065 nm. Fiber lasers offer some
simplifications in design that may be of particular benefit in
applications requiring ultra-reliability, such as LTPS and
microlithography. Applicants propose using a pulsed fiber laser
system as the source of moderate peak power (5-50 kW)
high-repetition-rate (multi-kHz, e.g., up to about 12-15 kHz) 1054
nm narrowband pulsed radiation. Such a laser could be constructed
using standard Yb.sup.3+ pulsed fiber laser technology--either a
q-switched fiber oscillator, a pulsed diode source that is fiber
amplified, or a CW source (fiber oscillator or diode) that is
modulated (internally or externally) and is fiber amplified. For
example a CW solid state laser, e.g., a diode laser, with a very
narrow bandwidth (very high spectral purity), e.g., a broadband
laser, e.g., matched to the fiber laser, to provide a very narrow
band seed to the pulsed solid state fiber laser for amplification
and the production of a very narrow band pulsed solid state seed to
the power amplification stage(s). Appropriate LMA (large-mode area)
fiber technology may be used to minimize spectral degradation due
to nonlinear effects in the fiber comprising the fiber laser
amplification oscillator or any subsequent amplification stages.
Using such approaches allows spatial beam quality to be maintained
(there are techniques for ensuring single-mode operation in large
mode area fibers) while reducing the peak power in the core of the
fiber. After the 1054 nm radiation is generated, it can, e.g., be
frequency upconverted directly to, e.g., about 351.2 nm, using two
stages of nonlinear frequency conversion (second harmonic
generation ("SHG") of 1054 to 527 nm then sum frequency generation
("SFG") with the residual fundamental to 351.2 nm (with
.about.+/-0.1 nm bandwidth).
[0097] Turning now to FIGS. 10-13 there are shown schematically and
partly in block diagram form a plurality of injection seeded DUV
gas discharge master oscillator/amplifier gain medium laser system
solid state master oscillators 200 according to aspects of an
embodiment of the disclosed subject matter. The master oscillator
200 may include, e.g., a Yb.sup.3+ doped fiber oscillator amplifier
210, e.g., with a diode pump 212 and a seed laser, e.g., a 1054 nm
CW seed diode laser 214, which may have a spectral purity of 100
MHz FWHM in the fundamental.
[0098] Referring to FIG. 10 the master oscillator oscillation
cavity may be formed by a rear cavity fully reflective mirror 220
and a partially reflective output coupler 222, which may be 90%
reflective at the nominal 1054 nm center wavelength of the fiber
oscillator 210. The master oscillator 200 may employ a Q switch 224
to allow for the output pulse energy of the master oscillator 210
to accumulate in the oscillation cavity until sufficiently high in
energy before the Q-switch 224 is opened, as is well known in the
art. The output of the master oscillator 200 may thus be pulsed by
the frequency of operation of the Q-switch, e.g., at a rate of
about 12 kHz. The output of the fiber oscillator laser 210 may be
passed through a second harmonic generator 230, followed by a
frequency adder 232, to add the original frequency to the second
harmonic to generate a third harmonic, i.e., a wavelength of about
352 nm suitable for amplification, perhaps with some slight
shifting to closer to 351, in, e.g., a XeF gas discharge laser
power amplifier or power oscillator or ring power amplification
stage amplifying gain medium (not shown in FIGS. 10-13).
[0099] Turning to FIG. 11 there is shown schematically and partly
in block diagram form a solid state master oscillator 200 according
to aspects of an embodiment of the disclosed subject matter. In
this embodiment an external amplitude modulator 240, e.g., an
acousto-optic or electro-optic switch or other suitable mechanism,
may be used to pulse the CW seed 214 into the fiber amplifier 210
to produce a pulsed output of the master oscillator 200.
[0100] In the embodiment of FIG. 12 the 1054 seed may utilize,
e.g., a pulsed seed diode 250 to produce a pulsed output out of the
master oscillator 210, e.g., at around 12 kHz. In the embodiment of
FIG. 13 a tunable CW Yb.sup.3+ master oscillator 260 may be
switched into the fiber amplifier 210 with an external amplitude
modulator, such as is discussed above, to get a pulsed seed laser
output from the master oscillator 200. The fiber amplifier 210 may
utilize pump diodes 212 to pump the fiber amplifier 210.
[0101] According to aspects of an embodiment of the disclosed
subject matter applicants have determined certain characteristics
desirably evidenced by a seed laser, e.g., a solid state seed
laser, for a very high average power laser system, e.g., for
photolithography or LTPS applications, including, e.g., pulse
energy, pulse duration and timing jitter, which can drive the
selection of a seed laser, e.g., a solid state seed laser to the
choice(s) of Nd:YAG, Nd:YLF, Ti:Sapphire, and fiber lasers, as
discussed elsewhere.
[0102] According to aspects of an embodiment of the disclosed
subject matter applicants have also studied certain amplification
stage resonator cavity properties. On the one hand may be a
flat-flat cavity with simple beam splitter input/output coupling,
which is simple of construction, though perhaps more wasteful of
seed laser energy than is practical in a production system. On the
other hand may be a recirculating or regenerative power oscillator,
e.g., a ring power amplification stage, e.g., with a beam
splitter/mirror input/output coupler and multiple passes through
the gain medium per each oscillation within the oscillator cavity
of the power amplification stage. It will be understood by those
skilled in the art, as noted above, terms like oscillator, cavity
and the like used in reference to, e.g., a MOPO configured laser
system mean that the amplification portions of the laser system,
seeded by a seed laser portion, lases due to stimulated emission
from the seed beam pulse oscillating in the cavity. This is
distinguished from what may be referred to as a power amplifier,
such as the PA portions of applicants' assignee's MOPA configured
XLA XXX series laser systems. By contrast the amplification occurs
in a power amplifier by stimulated emission during a gas discharge
in the amplification gain medium of the amplifier portion of the
laser system as the seed laser pulse is directed through the
amplification gain medium in an excited state a fixed number of
times by an optical arrangement, e.g., a two pass optical system as
used in applicants' assignee's current XLA XXX series laser
systems. In some of the literature, however, an oscillating
amplifier wherein the number of passes through the gain medium in
the oscillation path, e.g., a bow-tie or racetrack loop path which
is not an integer multiple of the nominal center wavelength of the
laser output may be considered to be a "power amplifier" rather
than a "power oscillator." Therefore for purposes of this
application and the appended claims the use of the term "ring power
amplification stage" is intended to cover either type of power
oscillator, regardless of the relation of the cavity size to the
wavelength.
[0103] The flat-flat configuration may use a traditional
polarization input/output coupling e.g., with a polarizing beam
splitter and a quarter wave plate and partially reflective output
coupler, e.g., as described in more detail below with respect to
FIGS. 23 and 28. This may make more efficient use of the seed laser
energy but could also be more susceptible to, e.g., thermal effects
at high pulse energy and/or high average output power. Other
input/output coupling could also be employed as explained in more
detail in the above referenced co-pending provisional application
filed concurrently with the present application, Attorney Docket
No. 2006-0012-01.
[0104] Turning now to FIGS. 14 and 16 there are illustrated in
schematic and partly block diagram form examples of very high
power, e.g., around 200 W or better average output power, laser
systems, 280, and 450, respectively, according to aspects of an
embodiment of the disclosed subject matter. These laser systems
280, 450 may be used, e.g., for immersion lithography use or for
LTPS use, or the like, which may include, e.g., in the case of FIG.
14 a ring power oscillator amplification stage configured laser
system 280. The system 280 may include a seed laser 286, which may
provide seed laser pulses at, e.g., around 0.1 mJ or less and a
pulse repetition rate of, e.g., around 6 kHz, in a seed laser
output light pulse beam 288 of laser output light pulses. The beam
288 from the seed laser 286 may pass through a seed injection
coupling mechanism 300 into an amplifier gain medium portion 290 of
the laser system 280.
[0105] The amplifier gain portion 290 may comprise a ring power
amplification stage chamber 292 containing a pair of gas discharge
electrodes 294 one of which is seen in the view of FIG. 14. The
chamber 292 may also comprise an input chamber section 296 and a
beam reverser chamber section 298, each of which may be formed with
or attached to, e.g., by suitable leak proof means, the chamber
292, such that, e.g., the optics in the input section 296 and in
the beam reverser section 298 can be beneficially exposed to
fluorine in the lasing gas mixture enclosed in the chamber sections
292, 296, 298.
[0106] The seed injection mechanism may include, e.g., a beam
splitter/input-output coupler 302 which may be coated with a
coating or otherwise selected or made to be partially reflective to
the seed laser light, e.g., at a nominal center wavelength of
around 193 nm for ArF, 248 nm for KrF, 318 for XeCl or 351 for XeF
laser systems, and a maximally reflective mirror 304 that is
maximally reflective at the selected nominal center wavelength for
the respective ArF, Krf, XeCl or XeF or the like gas discharge
laser systems.
[0107] The beam reverser 310 may be similar to the power amplifier
beam reversers, e.g., sold in applicants' assignee's XLA MOPA
configured laser systems, XLA XXX systems, or as discussed in more
detail in the above referenced co-pending patent application filed
on the same date as the present application Attorney docket No.
2006-0012-02. In the input section 296 optically accessible through
an input window 312 may be placed a beam expander 320, which may be
comprised of a prism 322 and a prism 324, which together may narrow
the beam 288 on its way into the chamber 292 and conversely expand
it on its way out of the chamber 292, the expansion on the way out
serving to, e.g., protect the optical elements, e.g., the
input/output coupler 300 and the narrowing of the beam 288 on the
way into the chamber 292 serving to, e.g., narrow the beam 340
entering the amplification gain medium to approximately the width
of the discharge between the electrodes 294 in a direction
generally perpendicular to the separation of the electrodes
294.
[0108] Baffles 330 may serve to, e.g., protect the optics in the
input section 296 and the beam reverser section 298 of the chamber
292 from damage resulting from, e.g., debris circulating with the
lasing gas mixture in the chamber 292.
[0109] Inside the cavity of the ring power amplification stage 290
the beam 288 may take a first direction recirculating oscillation
path 340 and return on a second direction recirculating oscillation
path 342 to the seed injection mechanism 300 where the partially
reflective input/output coupler acts as a traditional output
coupler for an oscillator laser cavity and reflects part of the
oscillating laser light photons to the Rmax mirror 304 and back
along the path 340. Thus the oscillation in the cavity formed by
the seed injection mechanism 300 and the beam reverser 310 is a
multi-pass oscillation path such that the oscillating photons pass
through the gain medium between the electrodes 294 more than once,
in the illustrated case twice, in different directions and on
different paths for each oscillation loop. Such multi-pass
oscillation, as noted, is distinct from the photons in a power
amplifier making a fixed number of passes through the gain medium,
e.g., two in applicants' assignee's XLA XXX laser systems, without
oscillating along such power amplifier light path. When the
oscillation in the recirculating/regenerative path 340, 342 builds
up enough pulse energy a laser system output laser light pulse beam
100 is produced from the seeded power oscillator laser system 280.
The seed laser 286 could be either a gas discharge, e.g., excimer
or fluorine laser or a solid state laser.
[0110] FIG. 16 illustrates schematically and partly in block
diagram form a ring power amplification stage laser system 490
configured similarly to applicants' assignee's XLA XXX
multi-chambered MOPA laser systems with the PA replaced by a ring
power amplification stage 490 according to aspects of an embodiment
of the disclosed subject matter. The laser system 450 may be
comprised of an excimer gas discharge laser seed laser 452 which
may comprise a master oscillator laser chamber 454, with a line
narrowing module 456 having a reflective element, e.g., a
wavelength and bandwidth selective grating, forming a rear cavity
mirror and a partially reflective output coupler 458 forming the
other end of the master oscillator 452 oscillation cavity. The
master oscillator 452 seed laser output laser light pulse beam of
pulses leaving the output coupler 458 may pass through a metrology
module (line center analysis module "LAM") 470, which may sample a
portion of the output of the MO chamber 454, using a beam splitter
472, and also, in addition to a wavemeter (not shown) for measuring
nominal center wavelength of the master oscillator seed laser
output laser light pulse beam pulses may comprise an MO laser
output light pulse beam pulse energy monitor 474 and an ASE monitor
476, such as a fluorescence detector. The ASE detector, e.g., a
broad band photodetector, may serve to detect the presence of a
high enough intensity of broadband light to indicate the timing of
the discharge in the amplification gain medium is off such that
significant lasing in band is not occurring (the seed pulse is not
timed to be in the cavity of the amplification stage during the
discharge) and essentially only broad band lasing is occurring
during the discharge in the amplification stage.
[0111] The master oscillator seed laser 452 output laser light
pulse beam may then pass to a turning mirror 480 and from there to
a seed injection mechanism 300 input to an amplifier gain medium
portion 490, which may comprise a ring power amplification stage
chamber 492, having a chamber input section 494 and a chamber beam
reverser section 496. It will be understood by those skilled in the
art that this schematic view of the laser system 450 does not
reflect various aspects of the optical path of the beam from the MO
452 to the PO chamber 442, which are drawn schematically to conform
to the plane of the paper and not the optical realities of the
optical path between the two and into the amplification stage
chamber 492.
[0112] The seed injection mechanism 300 may include, e.g., a
partially reflective input/output coupler 302, e.g., a beam
splitter similar to those sold with applicants' assignee's laser
systems, e.g., as part of an optical pulse stretcher ("OPuS"), and
a maximally reflective mirror Rmax 304 for the given nominal center
wavelength, with the partially reflective output coupler 302
serving as an input/output coupler as noted above and specifically
as the output coupler for the ring power amplification stage 490
oscillation cavity (defined also by the beam reverser 310). The
seed laser output laser light pulse beam from the MO 452 may pass
into the ring power amplification stage chamber 492 through an
input window 500 and also pass through a beam expander 510 as noted
above with respect to FIG. 14. The input section 494 of the ring
power amplification stage chamber 492 may also house the beam
expander 510, consisting of, e.g., a prism 512 and a prism 514.
Other forms of seed injection mechanisms may include those
discussed in the above referenced co-pending provisional
application filed on the same as the provisional application from
which this application claims priority and the other co-pending
applications claiming priority to that provisional application or
the provisional application from which this present application
claims priority.
[0113] The output of the ring power amplification stage oscillator
490 may be the overall system output laser light pulse beam of
laser pulses, however, as illustrated in FIG. 16, this beam
(eventually output beam 100 to the utilization tool, e.g., the
scanner) passes also through a metrology unit (bandwidth analysis
module "BAM") 340, where output laser light pulse beam bandwidth
may be measured, e.g., for each pulse in the beam, and through a
pulse stretcher, e.g., a 4.times. OPuS 520 which may include, e.g.,
a first delay path 522, which the laser system output beam enters
through beam splitter 526 and a second delay path 524 entered
through beam splitter 528 (the delay paths formed by mirrors 530).
Leaving the OPuS 520 the output beam 100 passes through a shutter
540 which may also have a beam splitter 542, e.g., to take off a
portion of the laser system output laser light pulse beam 100 to
measure, e.g., pulse energy.
[0114] With the beam expander 320 in FIGS. 14 and 510 in FIG. 16
placed inside the ring power amplification stage oscillation cavity
there is, e.g., a reduction of the energy density on the maximum
reflector 304 and partial reflector 302 that make up the
input/output coupler 300 of the ring cavity of the amplification
stage 290, 490 is achieved. With the beam reverser 310 moved to
inside the cavity, the space vacated can house the BAM (or SAM).
The use of protective optical coatings, e.g., with protectively
coated calcium fluoride ("PCCF") optics can be eliminated, e.g., on
the beam reverser 310 and beam expander 320, 510. There could also
be no need for PCCF on the amplification stage chamber window 500
and the output window 500 could be at a 47 degree orientation.
[0115] A ring power amplification stage in certain applications can
actually allow the use of much less energy from the MO,
approximately 1-100 uJ instead of the present value of about 1-3
mJ. For example in regular photolithography where about one third
to one half of the 200 W is actually required from, e.g., a
non-immersion very high average power laser lithography light
source system, a reduction in MO energy requirement of about 3 to
10 times could lead to a corresponding increase in LNM lifetime,
based on current LNM lifetime models. In addition, such small MO
energy could likely allow use of a low MO chamber pressure and
partial pressure of fluorine, necessary for operation at greater
than 1 mJ, e.g., at around 1.5-3 mJ, with beneficial results from a
chamber lifetime perspective. And also, since the power
amplification stage reaches strong saturation with 100 .mu.J of MO
energy and below, e.g., down to about 5 .mu.J or so, output energy
stability will be dominated by the good ring power amplification
stage characteristics and not the less than ideal MO energy
stability characteristics. The present Cymer XLA XXX MOPA systems
are dominated by the MO energy instabilities. Other output laser
beam parameters, e.g., pointing stability, profile stability, and
ASE stability may also be beneficially impacted by a configuration
according to aspects of an embodiment of the disclosed subject
matter utilizing reduced MO energy output.
[0116] According to aspects of an embodiment of the disclosed
subject matter applicants propose to use a 6 mirror coherency
busting mechanism (for convenience herein optical pulse delay paths
are indicated schematically as having four mirrors per delay path)
which has been developed by applicants' assignee for additional
path delay inside either or both of the 1.sup.st or 2.sup.nd pulse
stretchers in the OPuS used with applicants' assignee's XLA model
multi-chamber laser systems. Such a delay path can, e.g., produce
-1 imaging for each sub-pulse. This is illustrated schematically
and in cartoon fashion, e.g., in FIG. 20A wherein is illustrated
the summation of these "flipped" sub-pulses. The flipped sub-pulses
shown, e.g., in FIG. 20B can be used, e.g., for improved profile
uniformity and symmetry. A 6 mirror design can convert pointing
shifts into a divergence increase which may, e.g., be beneficial in
a ring arrangement for ASE reduction. The standard 4 mirror design
does not. It will be understood that the delay path for this
coherency busting purpose need not be as long as the actual OPuS
used for pulse stretching to get a much increased pulse T.sub.is,
and overlapping pulses. Rather the coherency busting mechanism, a
so-called "mini-OPuS", among other characteristics can fold the
pulses a certain number of times. This is illustrated by the pulse
580, with the corner (pre-flip) designated 582 and the pulses 584,
586, 588. In addition, due to misalignment of mirrors in the delay
path, a "hall of mirrors" effect due to subportions of the beam
being misaligned, may also reduce the coherency in the seed laser
pulse, and, e.g., so long as the delay path exceeds the spatial
coherency length of the beam. In this regard, a four mirror
mini-OPuS, e.g., with confocal spherical mirrors for ease of
alignment, may serve as a satisfactory coherency buster, even
without beam flipping in both axes as explained elsewhere in this
application.
[0117] According to aspects of an embodiment of the disclosed
subject matter it may be necessary to combine two separate laser
beams at various points within a system according to aspects of an
embodiment of the disclosed subject matter. If only half of the
entrance to a 6 mirror pulse stretcher is illuminated, the
sub-pulses flip between top and bottom as shown, e.g., in FIG. 20B.
The summation of these "flipped" sub-pulses can lead to a filled
in, full size profile, e.g., as illustrated in the pulse stretching
simulation shown in FIG. 33, with the curve 562 showing the pulse
before entering the delay path and curve 564 (black) after one
delay path and 566 (red) after a second delay path. Laser
divergence may then be used to fill in the center portion 568,
e.g., after some propagation, e.g., over about 1 m or so.
[0118] Turning now to FIG. 26 there is shown a schematic
representation of the effects of coherence busting according to
aspects of an embodiment of the disclosed subject matter. Utilizing
an imaging delay path, e.g., a pulse stretcher, e.g., a so-called
optical pulse stretcher ("OPuS"), e.g., a 4.times. T.sub.is six
mirror OPuS sold with the above noted applicants' assignee's laser
systems, and illustrated in United States patents and co-pending
applications noted above, or a modified version thereof with a
shorter delay path used, e.g., for folding the beam on itself
and/or for delay exceeding the coherence length as discussed
herein, the so-called mini-OPuS, one can achieve a degree of
coherence busting, e.g., between the MO and amplifier gain medium,
e.g., a PA or a PO or a ring power amplification stage. Other forms
of coherence busting e.g., as illustrated in FIG. 35 could be used
alone or in combination with such a "mini-OPuS," e.g., as
illustrated in FIG. 33 and elsewhere herein or as the mini-OPuS
itself.
[0119] According to aspects of an embodiment of the disclosed
subject matter, the pointing/divergence sensitivity of a pulse
stretcher, e.g., a 4 mirror 6 mirror pulse stretcher, e.g., a
regular OPuS such as a 4.times. T.sub.is OPuS, or a so-called
mini-OPuS, or the delay path discussed in more detail in regard to
FIG. 35, can be put to advantage, e.g., by adding active mirror
control with feedback from, e.g., a pointing/divergence sensor,
illustrated, e.g., in FIGS. 21 and 23. Such advantages include
creating or sustaining, e.g., a hall of mirrors effect whereby,
e.g., the laser output light pulse beam being smoothed in the delay
path actually becomes something like a plurality of beams of very
slightly different pointing and thus angle of incidence on the
various mirrors of the pulse stretcher and/or down stream of the
delay path(s). Applicants assignee has observed this in pulse
stretchers where it is very difficult to perfectly align the
mirrors, e.g., of the currently used 4.times. T.sub.is OPuS pulse
stretcher, thus creating the hall of mirrors effect that reduces
the coherence of the laser output light pulse beam exiting the
pulse stretcher. Thus the beam 860 forms a plurality of separate
beams 82. In FIG. 26 this is also illustrated schematically and as
a result of a flat-flat cavity 850 with slightly misaligned mirrors
forming the rear of the cavity 852 and an output coupler 854, but
the same effect has been observed in an OPuS by applicants employer
with the coherence busting effect noted above. The cavity
illustrated in FIG. 26 may also have a polarizing input coupler 858
and a quarter wave plate 856.
[0120] FIG. 26 illustrates a reduction in coherency, e.g., when
using both the reflectivity of an OC and an Rmax, e.g., in a
flat-flat cavity with, e.g., a polarizing input coupling from a
seed laser source of seed laser pulses. The angles have been
exaggerated for clarity of illustration. There are, e.g., multiple
rays produced by a static fan out, i.e., "hall of mirrors" effect,
e.g., created between the OC and the Rmax. The theoretical energy
weighting of these rays, assuming no transmission losses through
the cavity and perfect reflectivity is shown below.
TABLE-US-00001 Ray Number Fractional Energy Normalized Energy 1 0.2
= 0.200 0.3125 2 0.8*0.8 = 0.640 1.000 3 0.8*0.2*0.8 = 0.128 0.2000
4 0.8*0.2*0.2*0.8 = 0.0256 0.0400 5 0.8*0.2*0.2*0.2*0.8 = 0.00512
0.0080 6 0.8*0.2*0.2*0.2*0.2*0.8 = 0.00102 0.0016
One may assume that each ray is incoherent from all others, e.g.,
where the path length between the OC and the Rmax is maintained to
be longer than the temporal coherence length and, e.g., with
non-overlapping stretching, i.e., of much less than the pulse
length. Each ray may also be assumed, e.g., to be angled slightly
different from all others since, e.g., perfect alignment is
believed to be extremely difficult, especially in the vertical
direction. Applicants believe that about 37 .mu.rad of angle
difference in the vertical direction is needed to create
uncorrelated speckle. Summing the normalized energy weighting to
give the equivalent number of independent pulses and taking the
square root to give the reduction in standard deviation, the sum
from the above is 1.56. The square root is 1.25 and thus the
standard deviation when using both OC and Rmax reflections is
predicted to be 0.551/1.25=0.440, which comports well with a value
that applicants have measured, i.e., 0.427.
[0121] Static fan out, otherwise referred to herein as a hall of
mirrors effect, believed to be essentially unavoidable with manual
alignment, produces a single pulse speckle contrast with
amplification in an amplification gain medium that is 2.50.times.
smaller than the seed laser alone. This reduction is the equivalent
of 6.3 uncorrelated sub-pulses. Some of this contrast reduction is
due to the weak line content from the XeF power oscillator used for
testing the effects of the oscillation amplification stage, but
most is believed to be due to the static fan out effect. Likely,
many of the sub-pulses created by the OPuS-like static fan out
characteristics of the OC-Rmax (OC-rear cavity mirror) reflections
are all amplified to nearly equal intensities and thus create more
equivalent independent pulses than shown in the above table.
[0122] Tilt angle required to produce uncorrelated speckle patterns
may be significant. The first big jump in equivalent pulses, from
1.0 to 1.55, is believed by applicants to be mostly due to the poor
pulse-to-pulse repeatability of the speckle patterns when running
as a MOPO. Even without changing the mirror tilt at all, two pulses
are correlated no better than 30-35%. With seed only, this
pulse-to-pulse correlation has been found to be about 85-90%. The
long slow rise in equivalent pulse number does not even reach a
value of 2.0 until about 400 .mu.rad of mirror tilt as illustrated,
e.g., in FIG. 37. This result could mean, e.g., there may be a need
for a large angular sweep, of about .+-.500-1000 .mu.rad, e.g., to
create several uncorrelated speckle patterns in a single pulse.
[0123] Through experimentation relating to coherence applicants'
employer has learned that, e.g., sub-pulses produced by a pulse
stretcher are incoherent and lead to a different fringe pattern if
their angles are slightly shifted. The pin hole fringe pattern
shifts maximum to minimum when input angle is .lamda./2d.
[0124] A plot of pointing shift (inferred by applicants from
speckle shift measurements) v. E-O cell applied voltage is shown in
FIG. 18. According to aspects of an embodiment of the disclosed
subject matter applicants propose to sweep the pointing of the seed
laser within a single pulse in order to reduce the speckle contrast
within. This may be done, e.g., with electro optical elements,
e.g., elements 712 and 714 shown illustratively in the schematic
and partly block diagram illustration of aspects of an embodiment
of the disclosed subject matter found in FIG. 23. Using vertical
expansion prior to input of a seed laser pulse into an excimer
power oscillator, e.g., a XeF chamber, placed as close to an input
coupler, e.g., a beam splitter, and with a clear aperture of the
E-O deflector at around 3.2 mm in diameter, the deflector may have
to be upstream of the vertical expansion (not shown in FIG. 23). To
minimize any translation in the oscillator cavity, e.g., the XeF
cavity 730, e.g., associated with the angular tilt from the E-O
deflector, it may be desirable to place the E-O deflector as close
to the amplifier cavity as possible.
[0125] Use of a solid state laser source for lithography has been
proposed in the past and not pursued for two reasons. Solid state
lasers are not considered capable of the high average power
required for lithography and a solid state laser produces single
mode output which is highly (perfectly) coherent. According to
aspects of an embodiment of the disclosed subject matter applicants
propose to address the low average power problem with, e.g., a
hybrid solid state seed/excimer amplifier combination. The high
coherence properties of the solid state seed can be addressed in a
number of ways according to aspects of embodiments of the disclosed
subject matter, e.g., by creating sub-pulses, e.g., that are
separated in time longer than the coherence length along with
changing the seed laser pointing, e.g., over very short time
scales, e.g., within a single laser pulse, or a combination of
both. Coherency busting has been found by applicants to be of
benefit in dual chamber gas discharge (e.g. excimer) seed/gas
discharge (e.g., excimer) amplifier portion lasers as well.
[0126] De-phasing of a speckle pattern can be seen from a diffuser
670 to occur with a .lamda./2d where d is the illumination length
of a slit aperture or diameter of a circular aperture, e.g., as
illustrated schematically and in cartoon fashion in FIG. 22.
Incoherence of a speckle pattern can also be seen to occur from
each sub-pulse produced by a pulse stretcher, which can, e.g., be
further exploited by, e.g., intentionally misaligning each pulse
stretcher, e.g., a mirror(s) in the pulse stretcher, by a very
slight amount. It can be very difficult to precisely align the
mirrors in, e.g., a 4.times. T.sub.is OPuS type of pulse stretcher,
and they are slightly out of alignment almost all the time, without
having to intentionally misalign them. This amount of "ordinary"
misalignment is an amount sufficient to achieve a desired level of
speckle reduction and is illustrated schematically in FIG. 26, as
discussed above.
[0127] The effective number of equivalent independent laser pulses
can be seen to be equal to the T.sub.is magnification of the each
pulse stretcher. Each OPuS pulse stretcher of the kind noted above
may have a multiplication of around .about.2.4.times.. With, e.g.,
three stages of pulse stretching, the number of independent
sub-pulses will be (2.4).sup.3=13.8. Since speckle contrast scales
with the number of independent sub-pulses, N, as 1/ N, pulse
stretchers can provide an output speckle contrast of 1413.8=26.9%
with an input speckle contrast of 100%. Since this may still be too
high a speckle contrast, according to aspects of an embodiment of
the disclosed subject matter a mechanism(s) may be provided to
reduce the speckle contrast into or out of the pulse stretcher(s).
The same can be said for the so-called mini-OPuSs discussed
elsewhere.
[0128] Pulse trimming has been demonstrated, e.g., with the
utilization of electro-optics, e.g., at 193 nm. Rather than
polarization rotation, used in some other forms of pulse trimming,
electro-optics can be used for beam steering, e.g., steering a seed
laser light pulse beam within a single pulse in the beam.
Utilization of such, e.g., at the output of the seed laser, can
result in, e.g., according to aspects of an embodiment of the
disclosed subject matter, the electro-optic material(s) only
needing to be subject to a low average power seed laser beam. By,
e.g., randomly and/or continuously changing the beam steering,
e.g., within a single laser pulse, the angular acceptance of the
power amplification stage can be "painted" or filled in for each
laser pulse. As a result, a main pulse can have a divergence set,
e.g., by the PO/power amplification stage optical configuration and
not, e.g., by the seed laser characteristics. A greatly reduced
coherence for the laser system output laser light pulse can be the
result.
[0129] According to aspects of an embodiment of the disclosed
subject matter an injection controlled amplifier laser system,
e.g., with a plane cavity and flat rear mirror, may have suitable
energy stability, e.g., for seed pulse inject energies in the range
of 0.0085 to 0.99 mJ. This energy of the beam may be, e.g.,
incident on the rear mirror of, e.g., a power amplification stage,
which may form the input coupler from the seed laser. This
reflector may have, e.g., about a 90% reflection and about 10%
transmission. Therefore, the seeding energy entering the
amplification stage cavity itself may be, e.g., about an order of
magnitude smaller than what is incident onto the back reflector.
With a ring cavity, especially with a partially reflecting seed
injection mechanism according to aspects of an embodiment of the
disclosed subject matter, discussed elsewhere herein, e.g., the
input seed energy may be much less wasted, e.g., about 80% is
injected to the amplification stage. An Rmax and OC can be in an
F.sub.2 containing environment, and thus more robust, though, e.g.,
if polarization coupling is used, coupling efficiency may still be
less than optimum for certain applications. A suitable
architecture, e.g., in a MOPA configuration may be a 2-channel
("tic-toc") solid state seed laser, e.g., a 3.sup.rd harmonic
Nd:YLF MO or Nd:YAG system (tuned, e.g., to 351 nm) along with a
pair of two 3-pass XeF PA modules. Such a system in a MOPO, e.g., a
master oscillator/power amplification stage (such as a ring power
oscillator amplification stage) configuration is also considered as
an effective alternative. Such a two channel MOPO approach may be
similar to the MOPA configuration, i.e., with two seeded power
oscillators. Various coupling techniques could be used, e.g., MO
coupling using a polarization technique or a seed inject mechanism.
Efficiency v. E.sub.mo for differing PO/PA configurations has been
found to be better for a MOPO or a three pass MOPA, though four
pass MOPAs were not tested. Exemplary pulse width (FWHM) has been
found to be for an MOPO about 17.3 ns, for a MOPA, single pass,
about 13.9 ns and for a MOPA triple pass about 12.7 ns.
[0130] Applicants have examined speckle patterns for decorrelation
with angular shift, e.g., in a MOPO output beam, e.g., with a
Nd-YLF seed laser and a XeF power oscillator (e.g., a flat-flat
polarization coupled arrangement). With the relative timing between
the XeF discharge and the seed laser pulse adjusted and angular and
spatial adjustment also made for maximum suppression of the weak
line (353) produced by the XeF gain.
[0131] The maximum intensity of the seed pulse has been observed to
occur during the initial, very low level, fluorescence of the
amplification stage. This very low level fluorescence (and thus
gain) is believed to be enhanced by this seed light, as observed in
MOPO output. Adjustment of the timing of the seed earlier than or
later than, e.g., about 20 or so ns before the amplification stage
firing can, e.g., lead to an increase in weak line output.
[0132] According to aspects of an embodiment of the disclosed
subject matter coherence busting may be accomplished by beam
steering, e.g., with electro-optical elements, e.g., pointing of
the seed beam during a single pulse using, e.g., a ConOptics E-O
deflector assembly matched for the desired nominal center
wavelength. Such E-O devices may be like those used in CD and DVD
writers that use a doubled Ar-ion line near 351 nm having E-O
deflectors used to modulate the beam. With a pointing coefficient
of, e.g., about 0.6 .mu.rad/volt and with a capacitance of 50 pF,
even a full mrad of deflection requires only 1,700V. A drive
circuit useful for pulse trimming, e.g., as illustrated
schematically in FIG. 40 (discussed in more detail elsewhere in the
present application) can be used, e.g., with a resister in series
to produce a controlled sweep rate, e.g., during a single pulse.
The seed pulse duration can be around 15 ns, so the rate of rise is
well within the capabilities of such a driver, for reasonable
pointing changes, such as up to a m Rad. With a pumping diode
current of around 30 A and 4 A to the oscillator pump diode, the
seed laser output laser light pulse beam pulse energy was
determined to be 1.2 mJ, sufficient for seeding a gas discharge
laser, e.g., a XeF gas discharge laser.
[0133] A plot of speckle contrast (average speckle
cross-correlation versus mirror tilt--input angle change) for a
MOPO configuration is shown by way of example in FIG. 36. A similar
plot for only a seed laser pulse passing through an amplifier gain
medium in an oscillator configuration, but without excitation of
the amplifier gain medium is illustrated by way of example as plot
590 in FIG. 37, which also shows by way of example a plot 592 of
equivalent independent pulses. A similar plot is shown in FIG. 34,
for the seed laser pulse only in the PO, with curve 596 being the
equivalent independent pulses, curve 594 being the normalized
standard deviation and curve 598 being the cross correlation.
Similar to the MOPO case, it takes about 150-250 .mu.rad of tilt to
produce completely or essentially completely uncorrelated speckle
patterns and about two equivalent independent pulses. But, as
described above, the starting speckle contrast for no shift may be
smaller than with the OC reflection only by a factor of about 1.25.
Thus according to aspects of an embodiment of the disclosed subject
matter applicants have discovered that, e.g., a MOPO single pulse
speckle contrast may be significantly lower than a seed-only case,
because, e.g., static fan out of the rays produced by the multiple
OC-Rmax-OC-Rmax reflections, e.g., because each of these
reflections exit at the illustrated separate slightly different
angle, producing uncorrelated speckle patterns as shown by way of
example in FIG. 26.
[0134] According to aspects of an embodiment of the present
application applicants believe that this discovery may be utilized
to greatly simplify the necessary coherence busting scheme. Instead
of creating the electro-optic capability of, e.g., steering and/or
more rapidly modulating ("hybrid painting," in the case of using
both), e.g., the entire divergence space, e.g., in one or both axes
(e.g., requiring high frequency devices), one can slightly misalign
the seed to the PO, e.g., in one axis or the other or both, to
exploit this spreading static ray out effect, the so-called hall of
mirrors effect. It may then also be possible to use, e.g., only a
linear sweep of pointing along one axis or the other or both, e.g.,
where the one axis is the other axis in the case of spreading only
in one axis, with, e.g., a greatly reduced requirements on the E-O
drive electronics. In the simplest case, misalignment spreading
(beam fan-out so-called hall of mirrors effect, may be employed in
only one axis and "singly painting" in the other, e.g., with a saw
tooth signed to a tilt mirror and without AC creating hybrid
painting. More complex permutations and combinations of these
coherency busting techniques may also be applicable.
[0135] FIG. 38 gives an example of an idealized high frequency
painting E-O voltage signal superimposed on a ramped (time varying)
E-O DC voltage signal in relation to the intensity of the seed
pulse being "painted", e.g., into a delay path or into the
amplifying gain medium, e.g., a PA or PO or other power
amplification stage. The ramp voltage may be created, e.g., by a
fast R-C decay of an E-O cell capacitance as illustrated
schematically in the circuit of FIG. 40. Due to certain constraints
on a test circuit that applicants have so far built and tested,
e.g., limited RF frequency, impedance mismatch, E-O load cell
capacitance mismatch and the like, the actual voltages delivered by
the "painting" circuit are shown in FIG. 39, as best as could be
measured considering difficulties with probe loading, etc. These
are approximately 25% of the needed RF frequency (e.g., about 100
MHz as opposed to 400 MHz) and 10% of the needed peak to peak
voltage (e.g., around .+-.200 kV as opposed to .+-.2000 kV). The
painting voltages could, of course, be better optimized, however,
the test circuit was used to demonstrate the effectiveness of
"painting" the seed beam into the amplifier gain medium for
coherency/speckle reduction, e.g., with hybrid painting using both
time varying DC steering and AC modulation, e.g., one in one axis
and the other in a second axis, e.g., orthogonally related to each
other.
[0136] Applicants' experimental measurements have determined that
with no ramp and no AC voltage the 2D speckle contrast overall is
76.8% and varies from the horizontal to the vertical axis. With
painting using the ramp alone the speckle contrast overall was
29.4%, again varying in the two axes. Painting with the AC alone
gave a speckle contrast overall of 59.9%, again varying in the two
axes. With the ramp and AC voltages applied the spectral contrast
was 28.1% overall and varying in both axes. This was using a less
optimized circuit than the one of FIG. 40, which was not available
for the testing and the actual tested circuit test results are
shown in FIG. 39.
[0137] Applicants believe that a more optimized circuit, shown by
way of example in FIG. 40, will even improve further the reduction
in speckle contrast. The circuit 1100 of FIG. 40 may include, e.g.,
an E-O cell, such as noted above, with an E-O cell capacitance 1104
and an impedance matching inductor 1110, and an N:1 step-up
transformer 1120. Also included as illustrated may be, e.g., a DC
power supply 1122 charging a capacitor 1126 through a large
resistor 1130 and an RF frequency generator connected to a fast
acting switch, e.g., a transistor 1140 (in reality a bank of
transistors in parallel), through a 50.OMEGA. resistor. Also the
capacitor 1126 discharges through a small resistor 1142 when the
switch 1140 is closed.
[0138] According to aspects of an embodiment of the disclosed
subject matter "painting" may also be done upstream of the
amplifier gain medium, e.g., by tilting a mirror upstream of the
amplification, e.g. a piezo-electrically adjustable mirror. The
results with the seed only, both with OC only and with OC plus Rmax
reflections, look very similar to those measured by applicants such
as with tilting a mirror, e.g., through a diffuser as illustrated
in FIG. 22. As with the previous measurements, the OPuS-like
characteristics of the OC-Rmax reflections can be seen to lead to
single-pulse speckle contrast values reduced by the equivalent
number of sub-pulses produced. The angular tilt required to produce
uncorrelated speckle patterns was determined to be about 200-250
.mu.rad, again similar to the results with tilting the mirror,
e.g., downstream from the power amplification stage.
[0139] Applicants have performed characterizations of a solid state
MO./power amplification stage using an excimer seed laser, e.g.,
greatly attenuated to simulate the expected pulse energy of, e.g.,
a 193 nm solid state laser. The pulse duration produced, however,
did not match that expected from a 193 nm solid state laser.
Applicants believe that proper simulation of the seed pulse
duration should further reduce the total seed laser energy required
for MO/power amplification stage operation. Using a pulse trimmer,
e.g., a Pockels cell to which was applied a step voltage, e.g.,
timed to trim the later portion of the excimer seed pulse shape
(1/4.lamda. voltage=2.5 kV), and due to the rise time of the
excimer seed laser pulse and the fall time of the Pockels cell, the
shortest practical pulse shape attained was about 9 ns FWHM and
.about.15 ns foot-to-foot. Trimming the later portion of the seed
pulse was determined to have virtually no impact on the MO/power
amplification stage output pulse characteristics, e.g., intensity,
even with approximately 25% of the seed pulse energy eliminated.
However, as noted elsewhere in the present application pulse
trimming may further reduce speckle by eliminating a portion(s) of
the output pulse with the greatest coherency (least speckle
contrast).
[0140] Required limits on ASE as currently understood are believed
to be attainable with around 5 uJ of seed laser energy and below,
e.g., with a long seed pulse shape. Saturation test results have
shown applicants that output energy can be attained and the same
ASE upper limit levels can be achieved with only 3.75 uJ of seed
laser energy when using a short duration seed pulse. Further
reductions in seed pulse duration might be possible, resulting in
even smaller seed energy requirements. However such further
reductions in seed energy may be unnecessary since applicants
envision using .about.10 uJ of solid state 193 nm seed energy.
Shorter pulse durations may prove difficult since, e.g., two stages
of mini-OPuS may be used, e.g., between seed laser and the power
amplification stage, with a requirement that the delay length of
each mini-OPuS be greater than the seed laser pulse duration, the
resulting stretched pulse then being approximately 10 ns FWHM.
[0141] According to aspects of an embodiment of the disclosed
subject matter it is contemplated to apply a time changing voltage
on a timescale similar to the seed pulse duration, e.g., by
applying a DC voltage level until triggered, at which point the
high voltage may be shorted to ground, e.g., via a stack of fast
MOSFETS, e.g., illustrated schematically in FIG. 40 as a single
transistor 1130. A plot of the applied voltage and the seed laser
pulse shape are shown in FIG. 19. Placing a series resister between
the E-O cell terminal and voltage supply can be used to control,
e.g., the voltage slope applied to the E-O cell. The 50 pF
capacitance of the E-O cell in series with, e.g., a 200.OMEGA.
resister gives an initial slope of about 10.sup.11 .mu.rad/s. The
voltage across the E-O cell drops, e.g., as seen in FIG. 19 from
the DC level to nearly zero in a time similar to the seed pulse
duration. By changing the relative timing between the E-O cell
pulser and the seed laser one can, e.g., change the amount of
pointing sweep that occurs during the seed pulse. In addition, one
can change the value of the initial DC voltage to affect a greater
or lesser pointing sweep during the seed pulse. Applicants have
tested this fast pointing capability, e.g., with the seed laser
only and reflecting from an OC only, therefore, with no OPuS effect
from the multiple reflections from the OC and Rmax and no effects
due to MOPO operation. Without optimizing for relative timing
between the E-O cell and the seed pulse, applicants captured
speckle patterns for a range of timing between the two. Applicants
applied three difference levels of DC voltage to the E-O cell in
order to change the maximum available pointing slope. The results
showed a minimum speckle intensity normalized standard deviation at
about 57 ns relative timing. Without any angular shift during the
seed pulse, at both small and large relative timing values, below
and above 57 ns the speckle contrast is high. This correlates with
values found by applicants during static testing. When, e.g., the
relative timing places the E-O Cell voltage slope coincident with
the seed pulse, the speckle pattern of a single pulse is smeared in
the vertical direction, in a dramatic and satisfactory way.
[0142] One can normalize these contrast values to the maximum value
in order to evaluate the percentage reduction in contrast, e.g.,
brought about by the dynamic pointing shift. At the optimum
relative timing point the speckle contrast was found to be reduced
to about 40% of its peak. Using the 1/ {square root over (N)}
assumption for equivalent number of independent pulses the data can
be used to derive the number of pulses required to achieve this
level of speckle contrast reduction. At the optimum relative
timing, and with 3 kV applied to the E-O cell, the contrast
reduction was found to be equivalent to 6 pulses. Even higher
voltage levels (and thus even larger pointing shift during a single
pulse) could improve this result. Applicants performed similar
measurements with the seed laser pulse entering the power
amplification stage cavity, but no discharges between the
amplification stage electrodes and noted that reflections from the
OC and the Rmax in the XeF cavity, from the OPuS effect, beam
spreading alone, indicated that the maximum speckle contrast was
reduced by the amount predicted by the OPuS effect (N=1.56 with a
20% OC, giving 1/J=0.80. Thus 70% contrast becomes 56%). The effect
of smearing, even though the initial speckle contrast is lower,
appears not to change when adding the secondary reflections from
the full XeF cavity. The equivalent pulse for speckle reduction is
still about 6.
[0143] Applicants performed similar measurements with amplification
stage cavity electrodes discharging and thus implicating the
effects of the amplification within the amplification stage cavity,
which indicated as shown in FIG. 17 the decrease in the impact on
speckle reduction through seed beam sweeping. With such a
configuration, the effect was found to be just over half of the
equivalent number of pulses produced, i.e., about 3, when operating
as a MO/amplification stage, also found was a rather large
reduction in peak speckle contrast, with no smearing. Previous
measurements of MO/amplification stage operation showed a reduction
equivalent to about 6 pulses. These results show a reduction
equivalent to about 8 pulses. Applicants suspect that the
amplification stage cavity may discriminate against off-axis ray
angles, e.g., in a flat-flat cavity, and thus the spray of angles
sent into the cavity may not all be equally amplified (this could
be corrected, e.g., with a true stable cavity, e.g., employing a
curved OC and a curved Rmax). Another explanation may be that not
all of the seed pulse takes part in controlling the amplification
stage characteristics. Maybe only, e.g., the first 5 ns of the seed
pulse's 10-15 ns pulse duration controls the amplification stage
and thus the E-O sweep is not fast enough to occur within that
smaller window. This may also be corrected, e.g., by using a
smaller resister and a shorter sweep.
[0144] Referring to FIG. 35, a beam mixer 1050 is shown for
operation on a beam 1052 (which for illustrative purposes has been
shown as having an upper white half and a lower black half). As
explained in greater detail below, the beam mixer 1050 can be used
to alter the intensity profile of a beam, e.g. improving intensity
symmetry along a selected axis of a beam, and can be used to reduce
beam coherency, or both. For the embodiment shown, the beam mixer
1050 includes a beam splitter 1054 and mirrors 1056a-c.
[0145] For the arrangement shown in FIG. 35, the beam can be
initially incident upon the beam splitter 1054 whereupon a portion
of the beam may be directed, via reflection, toward mirror 1056a
and the remainder 1066 is transmitted (, e.g., with substantially
no change in direction) through the beam splitter 1054 and exits
the beam mixer 1050 on an output beam path. In one setup, a beam
splitter 1054 reflecting about forty to sixty percent of the
incident light, e.g. fifty percent, may be used. For this setup,
about fifty percent of the initial beam incident upon the beam
splitter 1054 is directed toward the mirror 1056a. For the beam
mixer 1050, mirrors 1056a-c may typically be flat, maximum
reflectivity mirrors. As shown in FIG. 35, mirror 1056a may be
positioned and oriented to receive light from the beam splitter
1054 at an angle of incidence of approximately thirty degrees. As
further shown, mirror 1056b may be positioned and oriented to
receive light reflected from mirror 1056a at an angle of incidence
of approximately thirty degrees, and mirror 1056c may be positioned
and oriented to receive light reflected from mirror 1056b at an
angle of incidence of approximately thirty degrees.
[0146] Continuing with FIG. 35, light reflected from mirror 1056c
can be made to be incident upon the beam splitter 1054 at an angle
of incidence of about forty-five degrees. For a fifty percent
reflectivity beam splitter, about half of the light from mirror
1056c is reflected onto the output beam path 1066 and about half of
the light from mirror 1056c passes through the beam splitter 1054
on a beam path toward mirror 1056a, as shown. Thus, the output beam
path includes a combined beam containing the portion of the initial
beam 1052 that passed through the beam splitter 1054 and the
portion of light from mirror 1056c that is reflected from the beam
splitter 1054. Similarly, the light on the path from the beam
splitter 1054 to mirror 1056a includes a combined beam containing
the portion of the initial beam 1052 that is reflected by the beam
splitter 1054 and the portion of light from mirror 1056c that is
transmitted through the beam splitter 1054.
[0147] The beam entering the beam mixer 1050 in FIG. 35 is shown
illustratively as having a rectangular cross-section that defines a
long axis 1058. This type of beam is typical of a laser beam
produced by an excimer laser with the long axis corresponding to
the direction from one discharge electrode to the other. A typical
beam may have dimension of about 3 mm by 12 mm. Moreover, for the
output of an excimer laser, the intensity profile in one axis,
e.g., the long axis 1058 is typically unsymmetrical, whereas the
intensity profile in the other axis, e.g., the short axis (i.e. the
axis normal to the long axis 1058) is approximately Gaussian.
Although the beam mixer 1050 shown is particularly suitable for
improving symmetry of a high power excimer discharge laser, it is
to be appreciated that it can be used in conjunction with other
types of laser systems and for other applications, for example, the
beam mixer may be used to reduce coherency in a beam generated by a
solid state laser.
[0148] FIG. 35 shows that the beam extends along the axis 1058 from
a first edge 1060 to a second edge 1062. FIG. 35 also shows that
the mirrors 1056a-c establishing a spatially inverting path which
has a beginning 1064 and an end 1066. As FIG. 35 illustrates, the
inverting path may be characterized in that a part of the beam near
the first beam edge 1060 at the beginning 1064 of the inverting
path translates to the second beam edge at the end 1066 of the
inverting path. More specifically, for the mixer 1050 shown, a
photon at the `top` of the beam which strikes mirror 1056a
translates and leaves mirror 1056c at the `bottom` of the beam.
Since the inverting path constitutes a delay path, there will be
some temporal stretching of the pulse, however, this can be
minimized by minimizing the delay path, e.g., to a length of about
a ns or so with suitable delay path time, etc., as noted elsewhere,
the beam mixer 1050 could form a coherence buster mini-OPuS, e.g.,
as discussed in regard to FIG. 47.
[0149] The beam mixer 1050 may be placed in between the seed beam
laser portion and the amplifier laser portion of a MOPA or MOPO
configured multi-chambered laser system, or other master oscillator
amplification gain medium arrangements, e.g., with a power
amplification stage, such as a ring power amplification stage, such
as that shown in FIGS. 15, 16, 23, 25 27 and 28. Specifically it
may be substituted for the mini-OPuS (376 and/or 380) of FIG. 15.
As an example, either or both of the short delay path pulse
stretchers 376, 378 inserted between the MO 372 as shown in FIG. 15
and the PO amplifier portion 394 of the multi-chamber laser system
of FIG. 15 may have substituted for it the beam mixer 1050 of FIG.
35.
[0150] FIG. 15 shows partly schematically and partly in block
diagram form an example of a coherence busting scheme 360 and the
results of aspects of the scheme according to aspects of an
embodiment of the disclosed subject matter, e.g., in terms of beam
divergence and thus coherence busting. The illustrated system may
incorporate, e.g., an oscillator/amplifier laser 370, e.g.,
including a solid state or excimer seed laser 372, and an
oscillator amplifier laser 394, or other power amplification stage,
e.g., a ring power amplification stage. The amplifier gain medium
394 may be, e.g., an excimer laser arranged in a power oscillator
configuration, e.g., with a fully reflective rear cavity mirror 396
and an input/output coupler, e.g., a partially reflective mirror
398. It will be understood that other seed laser/amplification
stage arrangements, some of which are discussed herein, may also be
used with the schematically illustrated coherence busting scheme
shown by way of example in FIG. 15.
[0151] At the output of the seed laser 372 is illustrated a
representation of the seed laser output laser light pulse beam
pulse coherency 374 containing a single dot indicative of
relatively high coherency. The output of the seed laser 372 may be
passed through one or more coherency busters, e.g., 376, 378, e.g.,
as shown by example in FIG. 9, or 1050 illustrated in FIG. 35
(discussed in more detail in the co-pending application noted
above, Attorney Docket No. 2005-0039) or other optical arrangements
such as disclosed in US20050286599, referenced above, or one or
more mini-OPuS coherence busting mechanisms discussed above, or
combinations thereof. A possible embodiment according to aspects of
an embodiment of the disclosed subject matter may be the use of a
confocal OPuS, e.g., one like that disclosed in the co-pending U.S.
patent application Ser. No. 10/847,799, entitled LASER OUTPUT LIGHT
PULSE STRETCHER, filed on May 18, 2004, Attorney Docket No.
2003-0121, referenced above, with, e.g., two confocal spherical
mirrors and four passes of delay path, i.e., from the beam splitter
to mirror No. 1 to mirror No. 2 back to mirror No. 1 and back to
mirror No. 2 and then returned to the beam splitter, passing
through, e.g., an offset correction optic, e.g., as discussed in
the co-pending U.S. patent application Ser. No. 11/394,512,
entitled CONFOCAL PULSE STRETCHER, filed on Mar. 31, 2006, Attorney
Docket No. 2004-0144-01, referenced above. This version of a
so-called "mini-OpuS" may comprise two pulse stretchers in series,
e.g., with a delay path offset selected to interleave the high
frequency peaks in the temporal pulse intensity curve of the output
of the master oscillator, such that individual mini-peaks
superimposed on the general humped or multi-humped shape of the
output pulse from the MO become interleaved in the treated pulse,
with advantages in reducing speckle. This may be achieved by, e.g.,
a delay offset of about 2 ns for a first one ns and then three ns
delay line mini-OPuS pair or about a 1 ns delay between a 3 ns and
4 ns delay line mini-OPuS pair in series or for a 4 ns and 5 ns
delay line mini-OPus pair in series. It will be understood that the
pulse itself will not be stretched sufficiently to overlap other
pulses, but rather sill essentially not be stretched at all, since
the delay path is so much shorter than the ten or so meters of
delay path in the normal pulse stretching OPuSs currently sold by
applicants' assignee.
[0152] The preferred embodiment uses a first delay something more
than lns due to increased alignment problems with the shorter delay
and increased aberrations in the pulse as stretched in a shorter
delay path. Each of the delay paths is, however longer than the
coherence length of the pulse and the second delay path is longer
than the first, to achieve coherence busting effects discussed
herein.
[0153] The mini-OPuS pulse stretchers may be selected and arranged
to, e.g., fold the beam on itself or fan it out in first one axis,
e.g., in a first mini-OPuS 376, resulting in the coherency
representation 378 and then in another orthogonally related axis,
e.g., in a second mini-OPuS 380, resulting, e.g., in the coherency
representation 390. A pulse trimmer/pulse steerer 392, e.g., and
electro-optical ("E-O") element 392 may sweep (paint) the seed beam
into the input/output coupler 400 of the amplifier portion 394
resulting in the blurring in one axis as shown in the pulse
coherency representation out of the power oscillator 410 (and also
the coherence representation 410 into the amplification gain stage
394). The "regular" or "standard" OPuS, e.g., a 4.times. T.sub.is
OPuS (roughly tem meters of delay path), which may contain, e.g., 2
delay paths 412, 420 initiated by a first beam splitter 414 and a
second beam splitter 422, similarly may be arranged to fold the
beam on itself in first one axis and then a second resulting, e.g.,
in the pulse coherency representations of, respectively, 414 and
424. The final coherency representation 424 shows schematically
that the coherency of the seed beam has been greatly reduced, i.e.,
the beam has been smeared in its passage from the seed laser 372 to
the amplifier gain medium 394a and as amplified in the amplifier
gain medium 394 and subsequently further having its coherency
busted in the 4.times. regular OPuS 412, 420.
[0154] It will be understood by those skilled in the art that
depending on the initial coherency of the pulse, e.g., out of the
seed laser, e.g., almost completely coherent in the case of solid
state seed lasers to very little coherency, but still coherency
that is desired to be even further reduced, e.g., with an excimer
seed laser the type, number and arrangement of coherency busting
elements may vary. For example, it may only be necessary to do
active coherency busting, e.g., with one form or another of pulse
steering/painting, for solid state seed lasers, and this may in
some cases for some applications prove to need only a ramp or only
AC pulse deflection, i.e., in one axis or the other, or may prove
to need both DC and AC pulse painting (Hybrid painting) along with
OPuS effect coherency busting both between the MO and amplifier
gain medium, e.g., PO or PA or other amplification gain medium
stage, e.g., a ring power amplification stage, and also may need to
employ the effect of the regular OPuS pulse stretcher(s) on the
output of the amplifier gain medium. With an excimer gas discharge
laser MO, with relatively much lower coherency than from a solid
state seed laser, only passive coherency busting, e.g., between the
MO and gain amplifier medium may be needed, e.g., with one or both
of the mini-OPuSs 376, 380 or other passive optical elements as
noted above between the MO and amplifier gain medium.
[0155] One may still need, however, to do beam steering also, e.g.,
with an active beam steering mechanism for even more smearing of
the pulse (more divergence), that may be less essential and need a
smaller sweeping angle. Such a seed laser mini-OPuS is believed to
need approximately only a 1 foot total path delay each and can also
be conveniently built onto the seed laser optical table as is
currently the practice for relay optics in applicants' assignee's
XLA series laser systems.
[0156] FIG. 17 illustrates an exemplary relative speckle intensity
for a 1 kV E-O deflector voltage v. relative timing. The relative
standard deviation curve 550 is for 1 kV and the equivalent pulse
curve is curve 550'. A 2 kV E-O deflector voltage curve 552 and
equivalent pulse curve 552' are also shown as is a 3 kV E-O
deflector voltage curve 554 and equivalent pulse curve 554'. An
example of a point shift vs. E-O voltage curve 560 is shown by way
of example in FIG. 18.
[0157] It will be understood by those skilled in the art that an
apparatus and method is disclosed for reaching very high average
output power, e.g., around 200 W or more with an excimer or
molecular fluorine gas discharge laser system in the DUV range of
wavelengths, e.g., 351 for XeF, 318 for XeCl, 248 for KrF, 193 for
ArF and 157 for F.sub.2, utilizing, e.g., a power oscillator or
other amplification gain stage, e.g., a ring power amplification
stage, with little or no significant ASE interfering with the
in-band desired radiation output of the laser system, e.g., with a
ratio between the ASE and in-band radiation at or below about
5.times.10.sup.-4, e.g., with, e.g., a 100 uJ pulse energy input
into the power amplification stage cavity per pulse. According to
aspects of an embodiment of the disclosed subject matter unwanted
ring power amplification stage light propagates backwards and can
also be sampled for diagnostics and ASE feedback control. Adding a
small amount of line-narrowing, e.g., with prism tuning, can also
help suppress ASE from the power amplification stage. Also
according to aspects of an embodiment of the disclosed subject
matter a PA may be used, e.g., along with a solid state MO, e.g., a
4 pass amplifier with no oscillation but with acceptable
amplification and perhaps even high enough saturation. With such a
design it may be necessary, e.g., for the 4 passes to each traverse
the entire gain cross-section in each of the 4 passes. The cavity
may have 2 prisms on each side of the cavity, in order to, e.g.,
reduce the energy density on the coated cavity optics and also
provide dispersion for ASE reduction.
[0158] In addition, it may not be that the ultimate ASE levels in a
MOPO, or other master oscillator/power amplification stage
configurations, necessarily increase with decreasing MO energy,
such that according to aspects of an embodiment of the disclosed
subject matter decreasing MO output energy even below 10 .mu.J may
not result in unacceptable ASE, even without, e.g., a partially
reflective off axis seed injection mechanism and/or a regenerative
ring power amplification stage configuration. A cavity with beam
expansion and crossing beams may be constructed that does not
exceed the cavity length of today's XLA, e.g., with the beam
expansion prisms far enough away from the chamber to allow lateral
translation for beam crossing, e.g., at a distance of a few
centimeters of the chamber window, dictated by, e.g., beam width
and crossing angle. A separate vessel for the prisms and/or beam
reverser optics could also allow the use of a direct F.sub.2
supply, e.g., at a different concentration than in the lasing gas
mixture, e.g., at around 1% concentration. This could also, e.g.,
avoid contamination from the optics holders.
[0159] The effect of inverse imaging, e.g., in an optical delay
path, e.g., in a mini-Opus with a delay path of only about one
foot, is illustrated in FIG. 20A, e.g. for an input beam 580, in
which a beam corner 582 is designated by the square initially in
the lower right hand corner of the beam 580. For a first sub-pulse
584, e.g., between an entrance beam splitter and a first mini-OPuS
mirror, the beam corner 582 remains the same. In a second sub-pulse
586, e.g., reflected from the first mirror, the beam has been,
e.g., negatively imaged, e.g., to a second mini-OPuS mirror and the
beam corner has moved to the upper left hand corner and then for a
third sub-pulse 588, reflected to a fourth mini-OPuS mirror, where
the beam corner has been negatively imaged back to the bottom right
hand corner, as illustrated in the figure. Combining all of these
sub-pulses into an output pulse, with a relatively short optical
pulse delay such that the pulse is not very significantly stretched
from a T.sub.is standpoint, can still substantially reduce
coherency by this effect of folding the beam on itself a plurality
of times, depending on the number of mirrors in the delay path.
[0160] FIG. 20B illustrates this same effect, e.g., on half of the
beam, e.g., is the beam had been split into two halves before entry
into the delay path of, e.g., two separate sources, e.g., two solid
state seed lasers operating at X kHz in, e.g., a 2X kHz system. As
can be seen the two halves are similarly negatively imaged in each
sub-pulse resulting in even further reduction in coherency in an
overall output pulse formed, e.g., by the combination of the two
half pulses into a single output pulse, e.g., of the shape shown by
way of example in FIG. 20A.
[0161] Turning now to FIG. 21 there is shown schematically and
partly in block diagram form a beam combiner system 600, according
to aspects of an embodiment of the disclosed subject matter. The
beam combiner system 600 may include, e.g., a first amplifier gain
medium portion 602 and a second amplifier gain medium portion 604,
each of which may be, e.g., a PA or PO of ring power amplification
stage, as described elsewhere in the present application. The
output of each of the amplifier portions 602, 604 may pass through
a beam expander 608, which may include a prism 610 and a prism 612,
e.g., magnifying the beam by, e.g., about 2.times.. A turning
mirror 620 may steer a first laser system output light pulse beam
622 from the amplifier 602 to a second turning mirror 624 which may
steer the pulse beam 622 to form a pulse beam 632 onto a beam
splitter for a first pulse stretcher 640 and thence to a beam
splitter 646 for a second pulse stretcher 644. A turning mirror 630
may steer a second laser system output light pulse beam 632 from
the second amplifier 604 to a second turning mirror 634, which may
steer the beam 632 to form a beam 634 to be incident on the beam
splitter 642 and thence the beam splitter 646. The output of the
first OPuS and second OPuS, which may be "mini-OPuSs" as discussed
elsewhere in the present application, may pass through another beam
splitter 650, where, e.g., a small portion of the laser system
output laser light pulse beam may be diverted, e.g., for metrology
purposes, e.g., focused by a focusing lens 652 into a divergence
detector 654, which may be part of a control system (not shown)
providing feedback control signals 656, e.g., to the beam splitters
642, 646 of the first and/or second OPuSs 640, 644 or the turning
mirrors for each of the beams 632, 634 to, e.g., insure the
pointing from both amplifiers remain overlapped in the far field so
that the beam appears to be as one beam, and also, e.g., so that
the two pulse stretchers maintain the pointing chirp introduces=d,
e.g., due to the confocal nature of the OPuS(s).
[0162] FIG. 22 illustrates schematically the impact of changing the
pointing of the beam (sweeping the beam) in terms of
coherency/speckle reduction. A pulse stretcher 662 may receive a
laser system output laser light pulse beam 100 on a beam splitter
664 and, e.g., through changing the angle of the beam splitter
sweep the pointing of the beam 100 across a slit, e.g., with a slit
diameter of d, onto a diffuser 670. The resultant detected speckle
pattern 680 indicates that the sweeping reduces the coherency
contrast and thus speckle.
[0163] Turning now to FIG. 23 there is illustrated by way of
example in schematic and partly block diagram form a very high
power solid state seeded immersion lithography laser light source
700, which may include, e.g., a high pulse repetition rate, e.g. a
12 kHz, solid state seed laser 702. The output of the seed laser
702 may pass through formatting optics 704, which can include,
e.g., a lens 706 and a lens 708, which may be used to, e.g., to
reformat the beam from a round beam to a shape concomitant with the
shape of the gain medium in the amplifier portion. The output laser
light pulse beam from the seed laser 702 may then be passed through
an x axis electro-optical ("E-O") steering mechanism 712, and/or a
y-axis E-O steering mechanism 714 or both, e.g., an E-O cell model
referenced above, each providing, in a respective axis, e.g.,
orthogonal to each other, a sweep of the beam in order to paint a
reasonable percentage of the utilization tool (e.g., scanner or
annealing tool) aperture, e.g., about 1 mrad, along with a high
frequency AC painting voltage, as explained elsewhere in the
present application. The laser output light pulse beam pulses from
the seed laser 702 may then be split in a beam divider to provide
alternating ("tic-toc") input pulses into a respective one of an
amplifier gain medium, e.g., a first power oscillator 730 and a
second power 730. The power oscillators 730 may comprise a ring
power oscillator.
[0164] The beam divider 720 may comprise, e.g., a beam splitter 722
that selectively transmits, e.g., 50% of the output beam from the
seed laser 702 onto a turning mirror 724 and a turning mirror 726,
leading into the second amplifier gain medium 730 and reflects 50%
to a turning mirror 728 leading to the second gain amplifier medium
730, e.g., on each pulse the beam splitter 720 could also comprise,
e.g., an electro-optical or acousto-optical beam deflector
alternating actuated to rend light to folding mirror 728 or folding
mirror 724 on alternate pulses.
[0165] Each respective gain amplifier medium 730 may include, e.g.,
a power amplification stage chamber 732, an input coupler/rear
cavity mirror 734, e.g., a concave mirror with an aperture on the
axis of revolution of the mirror surface admitting the seed laser
beam into the cavity formed by the rear cavity mirror 734 and a
front cavity mirror 736 as are known in the art of unstable
oscillation cavities. It will be understood that the amplifier gain
medium may be in other configurations mentioned in the present
application, e.g., a stable resonator with, e.g., a seed injection
mechanism, discussed in the co-pending and contemporaneously filed
application referenced above, and e.g., a ring power amplification
stage, or a power amplifier, without an oscillator cavity and with
only a fixed traversal path for amplification while the gain medium
is energized (e.g., a population inversion exists) as is known in
the art, without laser oscillation occurring, i.e., without an
output coupler as is known in the art of laser oscillation
cavities. In oscillation cavity environments, e.g., the convex
mirrors could be replaced, e.g., by an input coupler such as the
seed injection mechanism, discussed in more detail elsewhere in the
present application, and the convex mirror 736 replaced with an
output coupler. Beam expanding, beam combining and coherency
busting and divergence measuring of the respective output beams 766
from the first amplifier gain medium 730 and 764 from the second
amplifier gain medium 730, and feedback control may occur as
discussed in regard to FIG. 21 with respective beam expander 740,
comprising, e.g., prisms 742 and 744, beam combiner comprising
mirrors 750, 752 from the first amplifier gain medium 730 and
mirrors 760, 762 from the second amplifier gain medium 730 and
pulse stretchers 640 and 644 and metrology unit 654.
[0166] FIG. 24 illustrates schematically the results of a coherency
busting scheme on an output laser pulse, e.g., in relation to a
scanner acceptance window, e.g., introducing horizontal and
vertical (as illustrated in the plane of the page drawing of FIG.
24) directions. The dot 780 illustrated schematically and by way of
example an initial seed laser output pulse profile 780. The pattern
of pulses 782 illustrate a pattern of sub-pulse profiles 782 after
beam folding in a perfectly aligned beam delay path, or through a
misaligned pulse stretcher(s) or both, or a combination thereof,
and the circles 784 around each represent the effect on the profile
of electro-optical smearing.
[0167] FIG. 25 illustrates schematically and partly in block
diagram form by way of example a ring power amplification stage
oscillator laser system 800 and a seed injection mechanism 812, as
discussed in more detail in the co-pending and contemporaneously
filed patent application discussed above. The laser system 800 may
comprise, e.g., with bow-tie ring power amplification stage 804 and
a seed laser, e.g., a solid state or gas discharge seed oscillator
802. The seed oscillator 802 may be isolated from the oscillator
cavity of the power amplification stage 804 by an isolator to
prevent unwanted lasing from feedback photons, which may be
unnecessary, e.g., with a proper seed injection mechanism 812. The
power amplification stage section 804 may include, e.g., a power
amplification stage chamber 810, a seed injection mechanism 812,
which may include, e.g., an input/output coupler 814 and a
maximally reflective ("Rmax") mirror 816 beam reverser 820,
reflecting the output beam 806 from the seed oscillator 802 into
the amplifier portion chamber 804, and also include a beam
reverser/returner 820, which may include, e.g., a first maximally
reflective mirror 822 and a second mirror 824, e.g., made of a
material, like the Rmax mirror 816, selected to be maximally
reflective for a suitable band around the nominal center wavelength
of the laser system, e.g., 351 for XeF, 318 for XeCl, 248 for KrF,
193 for ArF and 157 for F.sub.2. The seed injection mechanism and
beam returner, as explained in more detail in the co-pending and
contemporaneously filed patent application referenced above, may be
arranged so as to form the oscillation cavity of the power
amplification stage 840 (whether technically speaking an oscillator
or amplifier oscillator stage, i.e., depending on cavity length),
such that on each oscillation along an oscillation path 826, 828
the output beam 806 from the seed laser 802 passes more than once
through the gain medium formed between a pair of discharge
electrodes (not shown in FIG. 25) per oscillation, i.e., along the
path 826 in a first direction and 828 in a second direction
generally opposite to the first direction but through the gain
medium per every oscillation during the formation of the output
laser light pulse beam 100 which eventually leaves the oscillation
cavity 804, as is well know in the art of laser oscillation
creating an output laser light pulse beam from a laser oscillator.
It will be understood that the angle of offset of the beams 826,828
is greatly exaggerated for illustration purposes and could be
around 1 .mu.rad.
[0168] FIG. 27 illustrates schematically and partly in block
diagram form a solid state seed/power amplifier laser system 880
according to aspects of an embodiment of the disclosed subject
matter. The system may incorporate a solid state 12 kHz seed laser
882 and a pair of amplifier gain media, e.g., a pair of power
amplifier chambers 888. An optical interface module 884 may receive
the output of the seed laser 882 and direct it in tic-toc fashion
into the respective amplifier gain medium 888, e.g., on alternating
pulses. The optical interface module 884 may comprise, e.g., a pair
of cylindrical telescopes 886, which may serve to format the beam,
e.g., because the output may be astigmatic with the telescope
serving to remove the astigmatism, and may also include, e.g., an
input optics module 890, each including, e.g., a mirror 902, a
mirror 908 and a mirror 910, which together with mirrors 904 and
906 may form, e.g., a fixed number of passes, e.g., three passes
through the gain medium between electrodes (not shown in FIG. 27)
in an amplifier gain medium configured, e.g., as a three pass power
amplifier ("PA"), that is, no laser oscillation occurs in the
amplifier gain medium. The respective outputs of the respective
power amplifier 888 may be steered by beam turning mirror 930, 932
on the one hand and 934, 936 on the other through a respective
energy sensor. These output beams from the system 880 may be
combined in a beam combiner as discussed elsewhere in the present
application.
[0169] A coherency buster, e.g., an automated two axis angular
adjustment mechanism 910, e.g., modulating the tilt of the
respective mirror 910 in the input optics module 890 may serve a
similar purpose to that of the X and Y axis beam steering electro
optic elements 712, 714 of the embodiment of FIG. 23, e.g., by
sweeping the beam entering the amplifier gain medium from side to
side and/or up and down for greater divergence and thus coherency
busting as discussed elsewhere herein.
[0170] Turning now to FIG. 28 there is illustrates schematically
and in partly block diagram format a seed laser/amplifier gain
medium laser system such as a solid state seed/power amplification
stage laser system 950 according to aspects of an embodiment of the
disclosed subject matter. The system 950 may include, e.g., a seed
laser, e.g., a solid state 12 kHz seed laser 952 the output of
which may enter into an optical interface modules 884, e.g., into a
respective one of a pair of cylindrical telescopes 886, as in the
embodiment of FIG. 27. Input coupling modules 960 may include,
e.g., a polarizing beam splitter 962, an Rmax 964, a quarter wave
plate 966, and an input coupler Rmax mirror 968, which together
function to couple output of the seed laser 952, respective seed
beam 970, 972, into the respective gain amplifier medium, e.g., a
power amplification stage oscillator having an output coupler 982,
by e.g., using a polarization coupling. Turning mirrors 984, 986,
994, 996 serve the same purpose as the respective turning mirrors
in the embodiment of FIG. 27.
[0171] FIG. 29 represents an illustrative normalized MOPO intensity
1000, a normalized single pass PA intensity 1002 and a normalized
two pass PA intensity 1004.
[0172] FIG. 30 represents an illustrative macroscopic steering
pulse 1010, which may comprise a plurality of alternating high and
low DC voltages 1010, 0102, and 1014, which may repeat in some
pattern, e.g., of three different high voltages, as illustrated and
a superimposed alternating current high frequency steering voltage
1016, which may occur, e.g., both at the higher voltage and at the
low voltage. As illustrated, e.g., the high voltages may have
different pulse durations and different low voltage duration
intervals as well. As shown in FIG. 32, these high voltages 1032
may be of the same value and same low voltage duration interval
1036 with superimposed AC 1034.
[0173] FIG. 31 illustrates schematically and in block diagram form
an optical switching and painting system 1020, according to aspects
of an embodiment of the disclosed subject matter, which may
include, e.g., a solid state seed 1022, a frequency converter 1024,
and an optical switch and painter 1026, which may include an
electro-optical beam director that, e.g., deflects the beam into a
first one of an amplifier gain medium 1030 when the pulse, e.g., as
shown in FIG. 32 is high (1032 in FIG. 32 and into the other
amplifier gain medium 1032, when the pulse is low (1036 in FIG. 32)
and also applies the AC beam steering 1034 into each amplifier
1030, 1032. A second frequency shifter 1028 may be intermediate the
beam splitter/painter 1026 and the respective amplifier gain medium
1032, and may be in addition to the frequency shifting of the
element 1024 or in lieu thereof.
[0174] According to aspects of an embodiment of the disclosed
subject matter applicants propose to generate 193 nm laser light
utilizing a solid-state seed laser, e.g., the generation of
coherent 193 nm radiation in a solid-state configuration with a
solid-state seed drive laser (or lasers) that drive linear or
nonlinear frequency conversion stages. One potential seed laser is
the pulsed Yb fiber laser, lasing at around 1060 nm, tunable in the
1050-1080 nm region. Such lasers constitute a mature and powerful
fiber laser technology, which may, e.g., be configured to produce
short temporal duration pulses (1-5 ns) at multi-kilohertz
repetition frequencies. To generate 193 nm using 1060 nm as the
longest wavelength mixing source, according to aspects of an
embodiment of the disclosed subject matter, applicants propose to
use, e.g., sum frequency generation ("SFG") with a long wavelength
and a moderately short wavelength to generate deep ultraviolet
("DUV"). Second harmonic generation ("SHG") to reach 193 nm is not
possible, due to the present lack of a 236.5 nm source as the other
mixing wavelength. However, such a source could be derived by
fourth harmonic generation, ("FHG") of the 946 nm output of a
q-switched diode-pumped Nd:YAG laser (946 n m being a lower
efficiency transition in Nd:YAG.
[0175] The output of the Nd:YAG is essentially a fixed wavelength,
and overall tunability could be provided by tuning the output
wavelength of the Yb fiber laser, e.g., a Yb.sup.+3 fiber laser.
Tunability of the Yb fiber laser output could be obtained via a CW
diode seed laser, e.g., a New Focus Vortex TLB-6021. Such a diode
laser seeders can provide fast wavelength control over limited
wavelength ranges, e.g., via internal PZT control of reflectors, as
desired for lithography source applications and have a high
spectral purity. Nd:YAG lasers are operable at multi-kilohertz
repetition frequencies, ensuring the overall system repetition rate
can meet the repetition rate requirements for a practical excimer
laser injection seeding source.
[0176] To achieve narrow bandwidth operation, both laser sources
need individually to be narrowband. In Nd:YAG systems, this may be
achieved, e.g., by injection seeding with a CW lower power Nd:YAG
laser, e.g., in a non-planar ring oscillator architecture that is
operating, e.g., with a single longitudinal mode output. In the Yb
fiber laser case, the bandwidth could be assured via the CW diode
laser seeder, which typically operates at very narrow linewidths,
e.g., on the order of 100 MHz FWHM. Further, appropriate large-mode
area ("LMA") fiber technology could be used to minimize spectral
degradation due, e.g., to nonlinear effects in the fiber comprising
the fiber laser oscillator or any subsequent amplification
stages.
[0177] To generate 193.4 nm radiation, e.g., as illustrated
schematically and partly in block diagram form in FIG. 41, a system
1200 including, e.g., a pulsed 946 nm Nd:YAG laser 1204 seeded by a
946 nm seed laser, e.g., a 946 nm CW Nd:YAG seed laser 1202, which
the output of the Nd:YAG laser 1204 frequency doubled, in a
frequency converter 1206, which may include, e.g., a frequency
doubler 1208, e.g., a non-linear material such as an LBO or KTP
crystal, followed by either another frequency doubler (not shown)
or a third harmonic generator 1210 and a fourth harmonic generator
1212 (e.g., each done using sum-frequency generation with residual
pump radiation, e.g., using the above noted crystals), either
approach generating the fourth harmonic at 236.5 nm. The 236.5 nm
radiation can then be mixed, e.g., in a sum frequency generation
with the 1060 nm output of the Yb fiber laser in a final nonlinear
crystal mixing stage, sum frequency generator 1240, e.g., a CLBO or
a BBO. That is, e.g., 1/1040 (0.000943)+1/236.5 (0.00423)=1/193.3
(0.005173). The fiber laser 1222 may have a rear oscillation cavity
mirror 1224 and a front window 1226, with a Q-switch 1228.
[0178] CLBO is cesium lithium borate, which is an effective
4.sup.th or 5.sup.th harmonic generator for Nd:YAG output light,
can be phase matched up for 193 nm operation and has a damage
threshold of >26 GW/cm.sup.2. BBO is beta barium borate
(b-BaB.sub.2O.sub.4), which is one of the most versatile nonlinear
optical crystal materials available and most commonly used for
second-or higher-order harmonic generation of Nd:YAG, Ti: Sapphire,
argon ion and alexandrite lasers. CLBO is preferred because of its
higher transparency and high acceptance angle, which may, however,
require cryogenic cooling for phase matching, also being
problematic because CLBO is a hygroscopic material). The
alternative is BBO, which can be phase matched but is being
operated very close to its absorption band edge at .about.190 nm.
BBO also has much a narrower acceptance angle than CLBO, but this
can be managed through optical design, e.g., with anamorphic
focusing. According to aspects of an embodiment of the disclosed
subject matter both lasers 1024, 1022 can be made relatively
powerful, e.g., with peal output power of greater than about 25 KW,
helping to compensate for any inefficiencies in the nonlinear
frequency conversion stages 1206, 1240.
[0179] According to aspects of the disclosed subject matter, the
generation of 193.3 nm with solid state laser(s) for seeding an
excimer amplifier gain medium may also be done, e.g., by the use of
mature drive laser technologies, which may be wavelength tunable in
a similar fashion to current tuning of excimer lasers. A seed laser
system 1200', illustrated schematically and in partly block diagram
form in FIG. 42, may comprise, e.g., an Er fiber laser 1260, e.g.,
lasing at around 1550 nm but tunable in the 1540-1570 nm range. Er
fiber lasers are available, and use similar generic technologies to
Yb fiber lasers. Such an approach is attractive because of the
maturity of fiber and pump diode laser technology for this
wavelength range, applied, e.g., in fiber-based telecommunications,
e.g., erbium-doped fiber amplifiers or EDFAs used as signal
boosters in optical fiber communication.
[0180] According to aspects of an embodiment of the disclosed
subject matter applicants propose to use a pulsed fiber laser
oscillator 1260 as the source of moderate peak power (e.g., 5-50
kW) high-repetition-rate (multi-kHz, e.g., at least 12) 1546.5 nm
narrowband pulsed radiation. That laser 1260 could be constructed
using standard pulsed fiber laser technology, to use a single-mode
CW tunable narrowband diode laser 1262 as an injection seeder for
the fiber laser oscillator 1260 to ensure narrowband, single
wavelength performance, and also to allow the fast wavelength
tunability required for lithography light source applications. An
example of the type of diode laser seeder 1262 is, e.g., a New
Focus Vortex TLB-1647, which uses an external cavity diode
configuration with PZT wavelength actuation for high-speed
wavelength drive over a limited wavelength range, in parallel with
mechanical drive for extended wavelength range operation. Further,
appropriate large-mode area ("LMA") fiber technology could be used
to minimize spectral degradation, e.g., due to nonlinear effects in
the fiber comprising the fiber laser oscillator or any subsequent
amplification stages. Using such approaches can, e.g., allow
spatial beam quality to be maintained, employing techniques for
ensuring single-mode operation in large mode area fibers, while
reducing the peak power in the core of the fiber. After the 1546.5
nm radiation is generated, it may then be frequency upconverted
directly to 193.3 nm, e.g., using five stages of nonlinear
frequency conversion, either second harmonic generation, or sum
frequency generation. This can be achieved through the steps listed
in FIG. 43, one of which is illustrated by way of example in FIG.
42, wherein .omega. refers to 1546.5 nm and 8.omega. becomes 193.3
nm. In FIG. 42 there is shown the generation of the second harmonic
2.omega. of 1546.5 nm in SHG 1208, and the third harmonic
generation, e.g., by adding the base frequency to the second
harmonic to in SFG 1258 to get 3.omega., and frequency doubling 3 W
to get 6.omega. in frequency double 1258, followed by similar such
sum frequency generations as just noted in SFGs 1252 and 1254 to
get, respectively, 7.omega. and 8.omega.. In addition, according to
aspects of an embodiment of the disclosed subject matter relatively
low-power pulsed fiber laser oscillator outputs, e.g., seeded by a
diode laser for spectrum/wavelength control, could then be boosted
in peak power via, e.g., a subsequent stage(s) of fiber
amplification (not shown). Applicants propose also, the development
of an all-fiber solid state drive laser based on this approach.
[0181] Turning to FIG. 44 there is illustrated schematically and in
block diagram form a laser treatment system, e.g., and LTPS or
tbSLS laser annealing system, e.g., for melting and recrystallizing
amorphous silicon on sheets of glass substrates at low temperature.
The system 1070 may include, e.g., a laser system 20 such as
described herein and a optical system 1272 to transform the laser
20 output light pulse beam from about 5.times.12 mm to 10 or so
microns.times.390 mm or longer thin beams for treating a workpiece,
e.g., held on a work piece handling stage 1274.
[0182] MOPO energy vs. MO-PO timing has been examined at different
values of seed laser energy, ArF chamber gas mixture, percentage
reflectivity of output coupler (cavity Q) and seed laser pulse
duration, with the results as explained in relation to FIG. 45.
[0183] ASE vs. MO-PO timing has been examined for different values
of seed laser energy, ArF chamber gas mixture, percentage
reflectivity of output coupler (cavity Q) and seed laser pulse
duration with the results also explained in relation to FIG.
45.
[0184] Turning to FIG. 45 there is shown a chart illustrating by
way of example a timing and control algorithm according to aspects
of an embodiment of the subject matter disclosed. The chart plots
laser system output energy as a function of the differential timing
of the discharge in the seed laser chamber and the amplification
stage, e.g., the ring power amplification stage as curve 600a,
which is referred to herein as dtMOPO for convenience, recognizing
that the amplification stage in some configurations may not
strictly speaking be a PO but rather a PA though there is
oscillation as opposed to the fixed number of passes through a gain
medium in what applicants' assignee has traditionally referred to
as a power amplifier, i.e., a PA in applicants' assignee's MOPA
XLA-XXX model laser systems, due, e.g., to the ring path length's
relation to the integer multiples of the nominal wavelengths. Also
illustrated is a representative curve of the ASE generated in the
amplification stage of the laser system as a function of dtMOPO, as
curve 602a. In addition there is shown an illustrative curve 604a
representing the change in the bandwidth of the output of the laser
system as a function of dtMOPO. Also illustrated is a selected
limit for ASE shown as curve 606a.
[0185] It will be understood that one can select an operating point
on the ASE curve at or around the minimum extremum and operate
there, e.g., by dithering the control selection of dtMOPA to, e.g.,
determine the point on the operating curve 602a at which the system
is operating. It can be seen that there is quite a bit of leeway to
operate around the minimum extremum of the ASE curve 602a while
maintaining output pulse energy on the relatively flat top portion
of the energy curve to, e.g., maintain laser system output pulse
energy and energy 6, and the related dose and dose .sigma.
constant, within acceptable tolerances. In addition as shown, there
can be a concurrent use of dtMOPO to select bandwidth from a range
of bandwidths while not interfering with the E control just
noted.
[0186] This can be accomplished regardless of the nature of the
seed laser being used, i.e., a solid state seed or a gas discharge
laser seed laser system. Where using a solid state seed laser,
however, one of a variety of techniques may be available to select
(control) the bandwidth of the seed laser, e.g., by controlling,
e.g., the degree of solid state seed laser pumping. Such pump power
control may, e.g., put the pumping power at above the lasing
threshold in order to select a bandwidth. This selection of
bandwidth may shift or change the pertinent values of the curve
604a, but the laser system will still be amenable to the type of E
and BW control noted above using dtMOPO to select both a BW and
concurrently an operating point that maintains the output energy of
the laser system pulses at a stable and more or less constant value
in the flat top region of the illustrated energy curve 600. It is
also possible to use a non-CW solid state seed laser and to adjust
the output bandwidth. For example, selection of the output coupler
reflectivity of the master oscillator cavity (cavity-Q) can adjust
the output bandwidth of the seed laser system. Pulse trimming of
the seed laser pulse may also be utilized to control the overall
output bandwidth of the laser system.
[0187] It can be seen from FIG. 45 that either the selected ASE
upper limit or the extent of the portion of the energy curve that
remains relatively flat with changes in dtMOPO may limit the range
of available bandwidth for selection. The slope and position of the
BW curve also can be seen to influence the available operating
points on the ASE curve to maintain both a constant energy output
and a minimum ASE while also selecting bandwidth from within an
available range of bandwidths by use of the selection of a dtMOPO
operating value.
[0188] It is similarly known that the pulse duration of discharge
pulses in a gas discharge seed laser, among other things, e.g.,
wavefront control may be used to select a nominal bandwidth out of
the seed laser and thus also influence the slope and/or position of
the BW curve 604 as illustrated by way of example in FIG. 45.
[0189] According to aspects of an embodiment of the subject matter
disclosed one may need to select an edge optic, that is, an optic
that may have to be used, and thus perhaps coated, all the way to
its edge, which can be difficult. Such an optic could be required,
e.g., between the output coupler, e.g., 162 shown in FIG. 2 and the
maximum reflector, e.g., 164, shown in FIG. 2, together forming a
version of a seed injection mechanism 160, shown in FIG. 2, e.g.,
depending upon the separation between the two, since there may be
too little room to avoid using an edge optic. If so, then the edge
optic should be selected to be the Rmax, e.g., because of the ray
path of the exiting beam as it passes through the OC portion 162.
From a coatings standpoint it would be preferable to have the OC be
the edge optic because it has fewer layers. However, an alternative
design, according to aspects of an embodiment of the subject matter
disclosed has been chose by applicants and is illustrated
schematically and by way of example in FIG. 30, e.g., wherein the
use of an edge optic can be avoided, e.g., if a large enough
spacing is provided between out-going and in-coming ring power
amplification stage beams, e.g., as created by the beam expander,
142 shown in FIG. 2, e.g., prisms 146, 148. For example, about a 5
mm spacing between the two beams has been determined to be
satisfactory enough to, e.g., to avoid the use of any edge
optics.
[0190] As illustrated by way of example in FIG. 46 the laser
system, e.g., system 110 illustrated by way of example in FIG. 2,
may produce a laser system output pulse beam 100, e.g., using a
ring power amplification stage 144 to amplify the output beam 62 of
a master oscillator 22 in a ring power amplification stage 144. A
beam expander/disperser 142, shown in more detail by way of an
example of aspects of an embodiment of the subject matter disclosed
may be comprised of a first expansion/dispersion prism 146a, and a
second expansion/dispersion prism 146b, and a third prism 148.
[0191] The seed injection mechanism 160 may comprise a partially
reflective input/output coupler 162, and a maximally reflective
(Rmax) mirror 164, illustrated by way of example and partly
schematically in FIG. 30 in a plan view, i.e., looking down on the
seed injection mechanism and m expansion/dispersion 160 and the
ring power amplification stage chamber (not shown) into and out of
which, respectively the beams 74 and 72 traverse, that is from the
perspective of the axis of the output beam 62 traveling from the
master oscillator chamber 22, which in such an embodiment as being
described may be positioned above the chamber 144 (the beam 62
having been folded into the generally horizontal longitudinal axis
as shown (the beam also having been expanded in the MOPuS in its
short axis, as described elsewhere, to make it generally a square
in cross-sectional shape.
[0192] With regard to the configuration of the beam expansion
prisms 146a, 146b and 148 inside the ring power amplification stage
cavity a similar arrangement may be provided to that of the beam
expansion on the output of the power amplifier ("PA") stage in
applicants' assignee's XLA-XXX model laser systems, e.g., with a
4.times. expansion, e.g., provided by a 68.6.degree. incident and
28.1.degree. exit, e.g. on a single prism or on two prisms with the
same incident and exit angles. This can serve to, e.g., balance and
minimize the total Fresnel losses. Reflectivity coatings, e.g.,
anti-reflectivity coatings may be avoided on these surfaces since
they will experience the highest energy densities in the system.
According to aspects of an embodiment of the subject matter
disclosed the beam expander/disperser 160 may be implemented with
the first prism 146 split into to small prisms 146a, and 146b,
which may be, e.g., 33 mm beam expander prisms, e.g., truncated, as
shown by way of example in FIG. 30, to fit in the place where one
similarly angled prism could fit, with the split prism having a
number of advantages, e.g., lower cost and the ability to better
align and/or steer the beams 72, 74 in combination with the beam
reverser (not shown in FIG. 30) and the system output beam 100.
[0193] The master oscillator seed beam 62 may enter the seed
injection mechanism 160 through the beam splitter partially
reflective optical element 162, acting as an input/output coupler,
to the Rmax 164 as beam 62a, from which it is reflected as beam 74a
to the first beam expander prism 146a, which serves to de-magnify
the beam in the horizontal axis by about 1/2.times. (it remains
about 10-11 mm in the vertical axis into the plane of the paper as
shown in FIG. 30). The beam 74b is then directed to the second beam
expansion prism 148, e.g., a 40 mm beam expansion prism, where it
is again de-magnified by about 1/2.times. so the total
de-magnification is about 1/4.times. to form the beam 74 entering
the gain medium of the ring power amplification stage (not shown in
FIG. 30. the beam is reversed by the beam reverser, e.g., a beam
reverser of the type currently used in applicants' assignee's
XLA-XXX model laser system PAs and returns as beam 72 to the prism
148, e.g., having crossed in the gain medium in a bow-tie
arrangement or having traveled roughly parallel, perhaps
overlapping to some degree in a version of a race-track arrangement
from prism 148 where the beam 72 is expanded by roughly 2.times.
the beam 72 b is directed to prism 142b and is expanded a further
approximately 2.times. into beam 72a. Beam 72a is partially
reflected back to the Rmax as part of beam 62a and is partially
transmitted as output beam 100, which gradually increases in energy
until an output beam pulse of sufficient energy is obtained by
lasing oscillation in the ring power amplification stage. The
narrowing of the beam entering the amplification gain medium, e.g.,
the ring power amplification stage has several advantageous
results, e.g., confining the horizontal widths of the beam to about
the width of the electrical gas discharge between the electrodes in
the gain medium (for a bow-tie arrangement the displacement angle
between the two beams is so small that they each essentially stay
within the discharge width of a few mm even thought they are each
about 2-3 mm in horizontal width and for the race track embodiment,
the bean 72 or the bean 72 only passes through the gain medium on
each round trip, or the beams may be further narrowed, or the
discharge widened, so that both beams 72,74 pass through the
discharge gain medium in each round trip of the seed beams 72,
74.
[0194] The positioning and alignment of the prisms 146a, 146b and
148, especially 146a and 146b can be utilized to insure proper
alignment of the output beam 100 from the ring power amplification
stage into the laser output light optical train towards the
shutter. The beam leaving the input/output coupler 162 may be fixed
in size, e.g., in the horizontal direction, e.g., by a horizontal
size selection aperture 130, forming a portion of the system
aperture (in the horizontal axis) to about 10.5 mm. Another
aperture, e.g., in the position roughly of the present PA WEB,
e.g., in applicants' assignee's XLA-XXX laser system products, can
size the beam in the vertical dimension since the beam has about a
1 mRad divergence, the sizing may be slightly smaller in each
dimension than the actual beam dimensions wanted at the shutter,
e.g., by about 1 mm. According to aspects of an embodiment of the
subject matter disclosed applicants propose that a system limiting
aperture be positioned just after the main system output OPuS,
e.g., a 4.times. OPuS. A ring power amplification stage aperture
may be located about 500 mm further inside the laser system. This
distance is too great to avoid pointing changes turning into
position changes at the specified measurement plane (present system
aperture). Instead the limiting system aperture can be located just
after the OPuS, and may have a 193 nm reflecting dielectric coating
instead of a stainless steel plate commonly used. This design can
allow for easier optical alignment, while at the same time reduce
heating of this aperture.
[0195] According to aspects of an embodiment of the subject matter
disclosed, applicants propose to implement a relatively stress-free
chamber window arrangement similar to or the same as that discussed
in an above referenced co-pending U.S. patent application, e.g., at
least on the bean reverser side of the chamber, because of the use
of, e.g., a PCCF coated window a this location.
[0196] According to aspects of an embodiment of the subject matter
disclosed, applicants propose to, e.g., place ASE detection, e.g.,
backward propagation ASE detection, in either the LAM or in an MO
wavefront engineering box ("WEB"), or in a so-called MOPuS, which
can, e.g., include elements of the MOWEB from applicants'
assignee's existing XLA-XXX model laser systems along with the
mini-OPuSs discussed elsewhere in this application and in the
co-pending application Attorney Docket No. 2006-0012-02 referenced
herein, as well as, e.g., beam expansion, e.g., using one or more
beam expansion prisms to expand the output beam of the MO in its
short axis, e.g., to form generally a square cross-sectional beam.
The current MO WEB and its beam turning function is represented
schematically as the turning mirror, e.g., 44 shown in FIG. 2. As a
preference, however, the backward propagation detector may be
placed `in` the MO WEB/MOPuS, that is, e.g., by employing a folding
mirror (fold #2), e.g., 44 in FIG. 2, with, e.g., a reflectivity of
R=95% instead of R=100% and monitoring the leakage through this
mirror 44. Some drift and inaccuracy of this reading may be
tolerated, e.g., since it may be utilized as a trip sensor (i.e.
measurements in the vicinity of 0.001 mJ when conditions are
acceptable--essentially no reverse ASE--as opposed to around 10 mJ
when not acceptable--there is reverse ASE), e.g., when the ring
power amplifier is not timed to amplify the seed pulse, but still
creates broad band laser light. Existing controller, e.g., TEM
controller, cabling and ports and the like for new detectors may be
employed. The detector may, e.g., be the detector currently used by
applicants' assignee on existing XLA-XXX model laser systems to
measure beam intensity, e.g., at the laser system output
shutter.
[0197] According to aspects of an embodiment of the disclosed
subject matter one or more mini-OPuS(s), which may be confocal,
such that they are highly tolerant to misalignment and thus of
potentially low aberration, e.g., for the off-axis rays needed in
the proposed short OPuS(s), the so-called mini-OPuS, can have delay
times of 4 ns and 5 ns respectively, where more than one is
employed. These values were chosen so that both OPuSs exhibit low
wavefront distortion with spherical optics in addition to
appropriate delay paths for coherence busting. The low wavefront
requirement may actually prevent significant speckle reduction from
the mini-OPuS(s) unless an angular fan-out from the output of the
mini-OPuS(s) is generated, e.g., by replacing a flat/flat
compensating plate with a slightly wedged plate, so that the
transmitted beam and the delayed beam in the mini-OPuS are slightly
angularly offset from each other. The laser beam, e.g., from the
master oscillator is partially coherent, which leads to speckle in
the beam. Angularly offsetting the reflected beam(s) reentering the
mini-OPuS output with the transmitted beam, along with the delay
path separation of the main pulse into the main pulse and daughter
pulses, can achieve very significant speckle reduction, e.g., at
the wafer or at the annealing workpiece, arising from the reduction
in the coherence of the laser light source pulse illuminating the
workpiece (wafer or crystallization panel). This can be achieved,
e.g., by intentionally misaligning the delay path mirrors, probably
not possible with a confocal arrangement, but also with the
addition of a slight wedge in the delay path prior to the beam
splitter reflecting part of the delayed beam into the output with
the transmitted beam and its parent pulse and preceding daughter
pulses, if any. For example, a 1 milliradian wedge in the plate
will produce an angular offset in the reflected daughter pulse beam
of 0.86 milliradians.
[0198] The optical delay path(s) of the mini-OPuS(s) may have other
beneficial results in terms of laser performance and efficiency.
According to aspects of an embodiment of the disclosed subject
matter, as illustrated schematically in FIG. 47, the laser beam,
e.g., seed beam 500 from the seed source laser (not shown in FIG.
47), may be split into two beams 502, 504 using a partially
reflective mirror (beam splitter) 510. This mirror 510 transmits a
percentage of the beam into the main beam 502 and reflects the rest
of the beam 500 as beam 504 into an optical delay path 506. The
part 502 that is transmitted continues into the rest of the laser
system (not shown in FIG. 47). The part 504 that is reflected is
directed along a delay path 506 including, e.g., mirrors 512, 514
and 516, with mirror 514 being displaced perpendicularly to the
plane of the paper in the schematic illustration, in order to allow
the main beam 502 to reenter the rest of the laser system, e.g., to
form a laser output beam or for amplification in a subsequent
amplification stage. The beam 504 may then be recombined with the
transmitted portion 502 of the original beam 500. The delayed beam
504 may be passed through a wedge (compensator plate) 520
essentially perpendicularly arranged in the path of beam 504. Thus,
the daughter pulse beam(s) 504 from the delay path 506 are slightly
angularly displaced from the main part of the beam in the
transmitted portion 502 in the far field the displacement may be,
e.g., between about 50 and 500 .mu.Rad.
[0199] The length of the delay path 506 delays the beam pulses so
that there is a slight temporal shift between the part of the beam
that is transmitted and the part that is reflected, e.g., more than
the coherence length, but much less than the pulse length, e.g.,
about 1-5 ns. By selecting the appropriate path length, which
determines the delay time, the addition of the two beams can be
such that the energy in the pulse is spread into a slightly longer
T.sub.is, which in combination with later pulse stretching in the
main OPuS(s) can improve laser performance, as well as providing
other beneficial laser performance benefits.
[0200] Two mini-OPuSs may be needed to achieve the desired effect.
The offset time between the pulses from the two mini-OPuSs may be,
e.g., one nanosecond. Based upon optical and mechanical
considerations, the delays selected for the stretchers may be,
e.g., a 3 ns delay path in the first mini-OPuS and a 4 ns delay
path in the second. If the delay is shorter, the optical system,
e.g., if it uses confocal or spherical mirrors can introduce
unacceptable aberrations. If the delay is longer, it may be
difficult to fit the system into the available space in the laser
cabinet. The distance the beam must travel to achieve the 3 ns
delay is 900 mm and to delay by 4 ns is 1200 mm. A confocal optical
system 500, minimizing the sensitivity to misalignment, illustrated
schematically in FIG. 48 may consist of two mirrors 522, 524, whose
focal points are located at the same position in space and whose
center of curvatures are located at the opposite mirror, along with
a beam splitter 526. A compensator plate 530 (e.g., a wedge) can be
added to insure that the reflected beam and the transmitted beam
are slightly misaligned as noted above with respect to FIG. 48. In
this case, the compensator plate is placed in the path of the
delayed beam at an angle for proper functioning.
[0201] The delay path time(s) in the mini-OPuS(s) for coherence
busting and other purposes may be as short as about the temporal
coherence length and as long as practical due to the noted optical
and space considerations, such as misalignment and aberration
tolerance. If there are two or more mini-OPuSs then the delay path
in each must be different in length, e.g., by more than the
coherence length and selected such that there is no significant
coherence reaction (increase) due to the interaction of daughter
pulses from the separate OPuS(s). For example the delay path times
could be separated by at least a coherence length and by not more
than some amount, e.g., four or five coherence lengths, depending
on the optical arrangement.
[0202] According to aspects of an embodiment of the subject matter
disclosed applicants propose to employ a coherence-busting optical
structure that, e.g., generates multiple sub-pulses delayed
sequentially from a single input pulse, wherein also each sub-pulse
is delayed from the following sub-pulse by more than the coherence
length of the light, and in addition with the pointing of each
sub-pulse intentionally chirped by an amount less than the
divergence of the input pulse. In addition applicants propose to
utilize a pair of coherence-busting optical delay structures, where
the optical delay time difference between the pair of optical delay
structures is more than the coherence length of the input light.
Each of the two optical delay structures may also generate
sub-pulses with controlled chirped pointing as noted in regard to
the aspects of the previously described coherence busting optical
delay structure.
[0203] According to aspects of an embodiment of the disclosed
subject matter two imaging mini-OPuSs, which may be confocal, such
that they are highly tolerant to misalignment and thus of
potentially low aberration, e.g., for the off-axis rays needed in
the proposed short OPuSs, the so-called mini-OPuSs, and can have
delay times of 4 ns and 5 ns respectively. These values were chosen
so that both OPuSs exhibit low wavefront distortion with spherical
optics. The low wavefront requirement may prevent significant
speckle reduction from the mini-OPuSs unless an angular fan-out
from the mini-OPuSs is generated, e.g., by replacing a flat/flat
compensating plate with the slightly wedged plate.
[0204] Turning now to FIG. 49, there is shown schematically and in
block diagram form a laser system 1300 according to aspects of an
embodiment of the disclosed subject matter. The laser system 1300
may include a plurality of seed lasers 1302, 1304 and 1306, with
seed laser 1306 comprising an nth seed laser with the intervening
seed lasers between seed laser 1304 and 1306 not shown in FIG. 49.
Each of the seed lasers may be a solid state seed including any of
the versions as discus.
[0205] It will be understood by those skilled in the art that
according to aspects of an embodiment of the disclosed subject
matter, adequate coherence busting may be achieved sufficiently to
significantly reduce the effects of speckle on the treatment of a
workpiece being exposed to illumination from the laser system, such
as in integrated circuit photolithography photoresist exposure
(including the impact on line edge roughness and line width
roughness) or laser heating, e.g., for laser annealing of amorphous
silicon on a glass substrate for low temperature recrystallization
processes. This may be accomplished by, e.g., passing the laser
beam, either from a single chamber laser system or from the output
of a multi-chamber laser system or from the seed laser in such a
multi-chamber laser system before amplification in another chamber
of the multi-chamber laser system, through an optical arrangement
that splits the output beam into pulses and daughter pulses and
recombines the pulses and daughter pulses into a single beam with
the pulses and daughter pulses angularly displaced from each other
by a slight amount, e.g., between, e.g., about 50 .mu.Rad and 500
.mu.Rad and with each of the daughter pulses having been delayed
from the main pulse(s), e.g., by at least the temporal coherence
length and preferably more than the temporal coherence length.
[0206] This may be done in an optical beam delay path having a beam
splitter to transmit a main beam and inject a portion of the beam
into a delay path and then recombining the main beam with the
delayed beam. In the recombination, the two beams, main and
delayed, may be very slightly angularly offset from each other
(pointed differently) in the far field, referred to herein as
imparting a pointing chirp. The delay path may be selected to be
longer than the temporal coherence length of the pulses.
[0207] The angular displacement may be accomplished using a wedge
in the optical delay path prior to the delayed beam returning to
the beam splitter which wedge imparts a slightly different pointing
to the delayed beam (a pointing chirp). The amount of pointing
chirp, as noted above may be, e.g., between about 50 and 500
.mu.Rad.
[0208] The optical delay paths may comprise two delay paths in
series, each with a respective beam splitter. In such an event each
delay path can be different in length such that there is not
created a coherence effect between the main and daughter pulses
from the respective delay paths For example, if the delay in the
first delay path is 1 ns the delay in the second delay path could
be about 3 ns and if the delay in the first delay path is 3 ns the
delay in the second could be about 4 ns.
[0209] The wedges in the two separate delay paths may be arranged
generally orthogonally to each other with respect to the beam
profile, such that the wedge in the first delay path can serve to
reduce coherence (speckle) in one axis and the wedge in the other
delay path can reduce coherence (speckle) in the other axis,
generally orthogonal to the first thus, the impact on speckle,
e.g., contribution to line edge roughness ("LER") and/or line width
roughness ("LWR"), e.g., at the wafer in exposure of photoresist in
an integrated circuit manufacturing process can be reduced along
feature dimensions in two different axes on the wafer.
[0210] According to aspects of an embodiment of the subject matter
disclosed, with, e.g., a 6 mrad cross of the bowtie in a bowtie
ring power amplification stage, the magnification prisms inside the
ring cavity may be slightly different for the in-going and outgoing
beams, and could be arranged so that the beam grows slightly as it
travels around the ring or shrinks slightly as it travels around
the ring. Alternatively, and preferably according to aspects of an
embodiment of the subject matter disclosed, a result of breaking
the larger beam expansion prism into two separate pieces, e.g.,
enabled by larger spacing between out-going and in-coming beams,
e.g., about 5-6 mm, as illustrated by way of example in FIG. 30,
applicants propose to adjust the angles of the two prisms, e.g.,
146, 148 shown schematically in FIG. 4, such that they result in
the same magnification for both out-going and in-coming beams,
e.g., beams 100 and 62, respectively, shown illustratively and
schematically in FIG. 30.
[0211] According to aspects of an embodiment of the subject matter
disclosed applicants propose to place the Rmax, e.g., 164 and the
OC, e.g., 162 portions of the version of the seed injection
mechanism containing an Rmax 164 and an OC 162, e.g., along with
the positioning of the system horizontal axis beam output aperture
on that same stage. This enables, e.g., prior alignment of each as
an entire unit and removes the need for field alignment of the
individual components. This can allow, e.g., for the position of
the Rmax/OC assembly, e.g., 160, shown in FIG. 2 (a seed injection
mechanism) to be fixed, just like the OC location in a applicants'
assignee's single chamber oscillator systems (e.g., XLS 7000 model
laser systems) is fixed. Similarly, such an arrangement can allow
for the achievement of tolerances such that the Rmax/OC are
positioned relative to the system aperture properly without need
for significant ongoing adjustment. The beam expansion prism may be
moveable for alignment of the injection seed mechanism assembly
with the chamber 144 of the amplification gain medium and the
output beam 100 path with the laser system optical axis.
[0212] According to aspects of an embodiment of the subject matter
disclosed applicants propose to employ a coherence-busting optical
structure that generates multiple sub-pulses delayed sequentially
from a single input pulse, wherein also each sub-pulse is delayed
from the following sub-pulse by more than the coherence length of
the light, and in addition with the pointing of each sub-pulse
intentionally chirped by an amount less than the divergence of the
input pulse. In addition applicants propose to utilize a pair of
coherence-busting optical delay structures, where the optical delay
time difference between the pair of optical delay structures is
more than the coherence length of the input light. Each of the two
optical delay structures may also generate sub-pulses with
controlled chirped pointing as noted in regard to the aspects of
the previously described coherence busting optical delay
structure.
[0213] According to aspects of an embodiment of the subject matter
disclosed applicants propose to position a mechanical shutter to
block the MO output from entering the ring, when appropriate,
similar to such as are utilized on applicants' assignee's OPuSs,
e.g., to block them during alignment and diagnosis. The exact
location could be, e.g., just above the last folding mirror prior
to the ring power amplification stage, where the mini-OPuSs are
protected during unseeded ring power amplification stage alignment
and operation.
[0214] Turning now to FIG. 49 there is shown schematically and in
block diagram a laser DUV light source according to aspects of an
embodiment of the disclosed subject matter. The system 1300 may
include, e.g., a plurality of seed laser systems, which may be
solid state lasers, 1302, 1304, 1306, for example as described
elsewhere in the present application, with the seed laser 1306
being an nth seed laser in the system for each seed laser three may
be a corresponding amplification laser system, e.g., 1310, 1320 and
1330, with the amplification laser system 1306 being an nth
amplification laser system. Each amplification laser system 1310,
1320, 1330 may have a plurality of A, in the illustrative case A=2,
amplification gain mediums 1312, 1314, and 1322, 1324 and 1332,
1334, with the amplification gain mediums 1332, 1334 comprising an
exemplary nth amplification gain medium system 1330. Each gain
medium 1312, 1314, 1322, 1324, 1332, 1334 may comprise a gas
discharge laser, such as an excimer or molecular fluorine laser,
and more specifically may comprise a ring power amplification stage
as described elsewhere in the present application and in above
identified co-pending applications filed on the same day as the
present application. Each of the respective A amplification gain
mediums 1312, 1314 and 1322, 1324 and 1332, 1334 may be supplied
with output pulses from the respective seed laser 1302, 1304 and
1306 by a beam divider 1308. The respective amplifier gain mediums
1312, 1314, 1322, 1324 and 1332, 1334 may operate at a fraction of
the pulse repetition rate X of the respective seed lasers, e.g.,
A/X. A beam combiner 1340 may combine the outputs of the amplifier
gain mediums 1312, 1314, 1322, 1324, 1332, 1334 to form a laser
system 1300 output laser light source beam 100 of pulses at a pulse
repetition rate of nX.
[0215] Turning to FIG. 50 there is illustrated schematically and in
block diagram form a laser system 1350 according to aspects of an
embodiment of the disclosed subject matter. which may comprise a
plurality of seed lasers 1352a, 1352b and 1352c which may be solid
state lasers, 1352a, 1352b, 1352c, for example as described
elsewhere in the present application, with the seed laser 1352c
being an nth seed laser in the system 1450. Each of the seed lasers
may feed a pair of respective amplifier gain mediums 1356, 1358,
1360, 1362 and 1364, 1366, with the amplifier gain mediums 1364,
1366 being the nth pair in the system 1350, corresponding to the
nth seed laser 1352c, with a respective beam divider 1354. Each
amplification gain medium may be a gas discharge laser, such as an
excimer or molecular fluorine laser, and more specifically may
comprise a ring power amplification stage as described elsewhere in
the present application and in above identified co-pending
applications filed on the same day as the present application. Each
of the pairs of amplification gain mediums 1356, 1358, 1360, 1362,
and 1364, 1366 may operate at 1/2 the pulse repetition rate X of
the respective seed laser 1252a, 1352b and 1352c, with the seed
lasers 1352a, 1352b and 1352c all operating at the same pulse
repetition rate X, to produce a laser light source output light
beam of pulses 100 at nX, or each may operate at a respective pulse
repetition rate X, X', X'' . . . X.sup.n' some but not all of which
may be equal to others, such that the output pulse rate in the
output pulse beam 100 is .SIGMA. X'+X'' . . . X.sup.n, through a
beam combiner 1370.
[0216] It will be understood by those skilled in the art that
disclosed in the present application is a method and apparatus
which may comprise a line narrowed pulsed lithography laser light
source which may comprise: a seed pulse providing laser system
which may comprise: a first pulsed seed laser producing seed pulses
at a rate of X kHz; a second pulsed seed laser producing seed
pulses at a rate of X kHz; an amplification system which may
comprise: a first amplifier gain system which may comprise a first
and a second pulsed gas discharge amplifier gain medium, each with
a nominal center wavelength in the UV range, and each operating at
1/2X kHz on output pulses from the first seed laser; a second
amplifier gain system which may comprise a first and a second
pulsed amplifier gain medium, each with a nominal center wavelength
in the UV range, and each operating at 1/2X kHz on output pulses
from the second seed laser; a first and a second line narrowing
module each selecting a desired bandwidth for, respectively, the
first and second seed laser; a pulse combiner combining the output
pulses from the first and second amplifier gain systems to produce
an output laser light pulse beam at a pulse repetition rate of 2X
kHz. The first and second pulsed seed laser systems each may
comprise a gas discharge laser with a nominal center wavelength
essentially the same as the nominal center wavelength of the
amplifier gain medium. The system and method may comprise a seed
laser output beam divider providing output pulses of the first
pulsed seed laser to the respective first amplifier gain medium and
second amplifier gain medium of the first amplifier gain system and
a second seed laser output beam divider providing output pulses of
the second pulsed seed laser respectively to the first amplifier
gain medium and the second amplifier gain medium of the second
amplifier gain system. Each pulsed amplifier gain medium may
comprise a gas discharge laser system. X kHz may be at least 6 kHz.
Each respective amplifier gain medium may comprise a ring power
amplification stage. The apparatus and method may comprise a narrow
band pulsed lithography laser light source which may comprise: a
seed pulse providing laser system which may comprise a single
pulsed solid state seed laser with a nominal center wavelength
outside of the UV range and operating at 2X kHz and having a
bandwidth of .ltoreq.0.2 pm FWHM or .ltoreq.0.5 .mu.m E95%; an
amplification system which may comprise: a first amplifier gain
system which may comprise a first and a second pulsed gas discharge
amplifier gain medium, each with a nominal center wavelength in the
UV range, and each operating at 1/2X kHz on output pulses from the
first seed laser system; a second amplifier gain system which may
comprise a first and a second pulsed amplifier gain medium, each
with a nominal center wavelength in the UV range, and each
operating at 1/2X kHz on output pulses from the second seed laser
system; a seed pulse divider providing pulses from the solid state
seed laser to a first amplifier gain system pulse divider and to a
second amplifier gain system pulse divider, the first amplifier
gain system pulse divider providing pulses to the first and second
pulsed amplifier gain medium of the first amplifier gain system and
the second amplifier gain system pulse divider providing pulses to
the first and second amplifier gain medium of the second amplifier
gain system; a pulse combiner combining the output pulses from the
first and second amplifier gain systems to produce a light source
output laser light pulse beam at a pulse repetition rate of 2X kHz.
The apparatus and method may comprise a frequency converter
converting the wavelength of the output of the seed laser to a UV
wavelength suitable for amplification in the amplifier gain medium.
Each pulsed amplifier gain medium may comprise a gas discharge
laser system. X kHz may be at least 6 kHz. Each respective
amplifier gain medium may comprise a ring power amplification
stage. The apparatus and method may comprise a line narrowed pulsed
lithography laser light source which may comprise: a first pulsed
seed laser system producing seed pulses at a rate of X kHz; a
second pulsed seed laser system producing seed pulses at a rate of
X kHz, an nth pulsed seed laser system producing seed pulses at a
rate of X kHz; a first amplifier gain system which may comprise A
amplifier gain mediums each with a nominal center wavelength in the
UV range, and each operating at (1/A)*X kHz on respective Ath
output pulses from the first seed laser system; a second amplifier
gain system which may comprise A pulsed amplifier gain mediums,
each with a nominal center wavelength in the UV range, and each
operating at (1/A)*X kHz on respective Ath output pulses from the
second seed laser system; an nth amplifier gain system comprising A
pulsed amplifier gain mediums, each with a nominal center
wavelength in the UV range, and each operating at (1/A)*X kHz on
respective Ath output pulses from the nth seed laser system; n line
narrowing modules each selecting a desired bandwidth for,
respectively, the first through nth seed laser system; a pulse
combiner combining the output pulses from the n amplifier gain
systems to produce an output laser light pulse beam at a pulse
repetition rate of nX kHz. Each pulsed seed laser system may
comprise a gas discharge laser with a nominal center wavelength
essentially the same as the nominal center wavelength of the
respective amplifier gain medium. The apparatus and method may
comprise a seed laser beam divider providing output pulses of a
respective pulsed seed laser to one of n amplifier gain system beam
dividers; the respective n amplifier gain system beam divider
providing each Ath pulse to a respective one of the A amplifier
gain mediums in the respective amplifier gain system. Each pulsed
amplifier gain medium may comprise a gas discharge laser system. X
kHz is at least 4 kHz. Each respective amplifier gain medium may
comprise a ring power amplification stage. The apparatus and method
may comprise a line narrowed pulsed lithography laser light source
which may comprise: a first pulsed solid state seed laser system
producing seed pulses at a rate of X kHz; a second pulsed solid
state seed laser system producing seed pulses at a rate of X kHz,
an nth pulsed solid state seed laser system producing seed pulses
at a rate of X kHz; a first amplifier gain system comprising A
pulsed gas discharge amplifier gain mediums, each with a nominal
center wavelength in the UV range, and each operating at (1/A)*X
kHz on respective Ath output pulses from the first seed laser
system; a second amplifier gain system comprising A pulsed
amplifier gain mediums, each with a nominal center wavelength in
the UV range, and each operating at (1/A)*X kHz on respective Ath
output pulses from the second seed laser system; an nth amplifier
gain system comprising A pulsed amplifier gain mediums, each with a
nominal center wavelength in the UV range, and each operating at
(1/A)*X kHz on respective Ath output pulses from the nth seed laser
system; a pulse combiner combining the output pulses from the
amplifier gain systems to produce an output laser light source
pulse beam at a pulse repetition rate of nX kHz. The apparatus and
method may comprise a seed laser beam divider providing output
pulses of the respective pulsed seed laser to each of the A
amplifier gain mediums in the respective nth amplifier gain system.
Each pulsed amplifier gain medium may comprise a gas discharge
laser system. X kHz may be at least 6 kHz. Each respective
amplifier gain medium may comprise a ring power amplification
stage. The apparatus and method may comprise a broad band pulsed
laser light source which may comprise a first pulsed seed laser
system producing seed pulses at a rate of X kHz; a second pulsed
seed laser system producing seed pulses at a rate of X kHz; an nth
pulsed seed laser system producing seed pulses at a rate of X kHz;
a first amplifier gain system comprising A pulsed gas discharge
amplifier gain mediums, each with a nominal center wavelength in
the UV range, and each operating at (1/A)*X kHz on respective Ath
output pulses from the first seed laser system; a second amplifier
gain system comprising A pulsed amplifier gain mediums, each with a
nominal center wavelength in the UV range, and each operating at
(1/A)*X kHz on respective Ath output pulses from the second seed
laser system; an nth amplifier gain system comprising A pulsed
amplifier gain mediums, each with a nominal center wavelength in
the UV range, and each operating at (1/A)*X kHz on respective Ath
output pulses from the nth seed laser system; a pulse combiner
combining the output pulses from the amplifier gain systems to
produce an output laser light pulse beam at a pulse repetition rate
of nX kHz. The pulsed seed laser systems may comprise a gas
discharge laser with a nominal center wavelength essentially the
same as the nominal center wavelength of the amplifier gain medium.
The apparatus and method may comprise a seed laser beam divider
providing output pulses of the pulsed seed laser to a respective
one of the n amplifier gain system beam dividers; the respective
nth amplifier gain system beam divider providing each Ath pulse to
a respective one of the A amplifier gain mediums in the respective
amplifier gain system. Each pulsed amplifier gain medium may
comprise a gas discharge laser system. X kHz may be at least 6 kHz.
Each respective amplifier gain medium may comprise a ring power
amplification stage. The apparatus and method may comprise a line
narrowed pulsed lithography laser light source which may comprise:
a narrow band seed pulse providing laser system which may comprise
a pulsed seed laser system producing a beam of seed pulses with a
bandwidth of .ltoreq.0.2 pm FWHM or .ltoreq.0.5 pm E95%, at a rate
of X kHz; a beam splitter receiving the output of the seed laser
system and providing a split output of every pulse from the pulse
splitter; an amplifier gain system comprising a first gas discharge
amplifier gain medium and operating at Y kHz on respective outputs
from the pulse splitter and a second gas discharge amplifier gain
medium operating at Z kHz on respective outputs from the pulse
splitter, where Y+Z=X; a pulse combiner combining the output pulses
from the first and second amplifier gain systems to produce an
output laser light pulse beam at a pulse repetition rate of X kHz.
The apparatus and method may comprise a plurality of N light
sources; a pulse combiner providing an output of line narrowed
laser pulses at a pulse repetition rate of NX. The seed pulse
providing system may comprise a solid state laser, e.g., a fiber
laser, e.g., selected from a group comprising Nd-based, Er-based
and Yb-based solid state lasers, or a gas discharge laser and a
line narrowing module. The beam splitter may comprise a geometric
beam splitter transmitting at least one spatial portion of the
pulse beam on one path and at least a second portion of the pulse
beam on a second path. These various combinations of seed laser
systems and amplification systems may enable the generation of such
high power average output power with pulse repetition rated of 12
kHz or higher and, for example with 17 mJ or less in each of the
output pulses, i.e., without the seed lasers or amplification gain
medium lasers having to run pulse repetition rates where operation
is difficult for any of the reasons noted above. In addition the
outputs of the seed laser(s) may be kept at levels, e.g., lmJ or
less, especially with ring power amplification stages for
amplification gain mediums, such that operation at, e.g., 90 W or
even higher can be maintained with very much increased MO
lifetimes, as noted above, which may be contributed to further
where the seed lasers are solid state. In the latter cases, the
multiple amplification stages in the amplifications systems may
increase the repetition rates, without the problems noted above,
such that operation at 60 w-90 w is easily obtainable with longer
life for both the MO and amplification stages or operation at 200 w
or even well above can be obtained with, e.g., acceptable
repetition rates in individual components of the systems. Another
benefit may be, e.g., lower energy outputs required from individual
seed lasers may allow for beam splitters/direction modulators to
operate with much lower fluences and with, e.g., concomitant longer
live. The beam splitter may comprise a partially reflective optic,
e.g., a beam splitter, reflecting a portion of each pulse in the
pulse beam on one path and transmitting a portion of each pulse in
the pulse beam on a second path, such that the amplification stages
can run at reduced rep pates, and receive only roughly half of the
output energy of the seed laser, but still produce output pulses at
the pulse repetition rate of the seed laser. The beam splitter may
comprise a stimulated optical path modulator such as, e.g., an
elector-optical or acousto-optical beam modulator, directing a
first portion of a pulse in the pulse beam in a first path during a
first time period and a second portion of a pulse on a second path
during a second time period, or directing separate pulses on
respective ones of a plurality of separate path in respective time
periods, e.g., individual pulse time periods. The gas discharge
amplifier gain medium comprising a power amplifier. Each gas
discharge amplification gain medium may comprise a power
oscillator. The apparatus and method may comprise: the pulse
combiner comprising a beam combining optical element forming a
laser light source output beam comprising a first spatially defined
component from the first amplifier gain medium and a second
spatially defined component from the second amplifier gain medium.
The pulse combiner may comprise a beam combining optical element
forming a laser light source output beam comprising a first
component from the first amplifier gain medium and a second
component from the second amplifier gain medium exiting the pulse
combiner along a path comprising a single optical axis. The pulse
combiner may comprise comprising an OPuS pulse stretcher.
[0217] Applicants have simulated through calculations speckle
reduction as relates to the location of coherence lengths within a
single gas discharge (e.g., ArF or KrF excimer) laser system output
pulse after such a pulse has passed through the two OPuS pulse
stretchers sold on laser systems manufactured by Cymer, Inc., used
for pulse stretching to increase the total integrated spectrum
(T.sub.is) to reduce the impact of peak intensity in the laser
output pulse on the optics in the tool using the output light from
the laser system, e.g., a lithography tool scanner illuminator.
There are two OPuS in series, with the first having a delay path
sufficient to stretch the T.sub.is of the output pulse from about
18.6 ns to about 47.8 ns and the second to stretch the pulse
further to about 83.5 ns, e.g., measured at E955 (the width of the
spectrum within which is contained 95% of the energy of the
pulse.
[0218] Starting with the unstretched pulse, applicants divided the
pulse into portions equal to the approximate coherence length,
assuming a FWHM bandwidth of 0.10 pm and a Gaussian shape for the
coherence length function. The impact of the pulse stretching on
the coherence length portions of the pulse after passing through
the first OPuS was to show that a first intensity hump in the
spectrum of the stretched pulse was made up of the coherence length
portions of the main pulse, a second intensity hump was made up of
coherence length portions of the main pulse overlapped with
coherence length portions of a first daughter pulse. A third hump
in the intensity spectrum is the result of overlapping of the first
and second daughter pulses. Looking at the individual coherence
length portions of the two humps applicants observed that the
multiple versions (including daughters) of the coherence length
portions remained sufficiently separated to not interfere with each
other.
[0219] After passage through the second OPuS the simulated spectra,
again only looking at the content of the first three humps in the
stretched pulse, in the simulation (under the second hump were
contributions from the original undelayed pulse, as before, the
first delayed pulse from the first OPuS, as before and the first
delayed pulse from the second OPuS), applicants observed that in
this second pulse the multiple versions of the coherence length
portions were very close together. This is caused by the fact that
the first OPuS has a delay of .about.18 ns and the second has a
delay of .about.22 ns. Thus only .about.4 ns separates the versions
of the coherence length portions, which is still not close enough
for interference.
[0220] Under the third hump applicants observed contributions from
the first delayed pulse from the first OPuS, the second delayed
pulse from first OPuS, the first delayed pulse from the second
OPuS, and the second delayed pulse from second OPuS. Applicants
observed that the separation between some related coherence
portions is larger than for others in the third hump in the
intensity spectrum of the pulse stretched by two OPuSs. This
increase in separation is due to the fact that two round trips
through each OPuS equal .about.36 ns=18*2 and .about.44 ns=22*2.
Thus the separation between coherence lengths grows with each round
trip.
[0221] Applicants concluded that for a mini-OPuS as described in
this application a single mini-OPuS with delay equal to one
coherence length will create a train of pulses that dies out after
about 4 coherence length values. Thus, applicants determined that
for a single mini-OPuS to be effective, the two main OPuSs should
not bring any daughter coherence lengths to within 4 coherence
lengths of each other. But, applicants have observed in the
simulation that the main OPuSs do just that, though only marginally
so. The separation between coherence lengths for the third and
greater humps is sufficient. Applicants believe that the impact of
a single mini-OPuS between MO and amplification gain medium will be
nearly the full expected coherence busting effect. A second
mini-OPuS between MO and PA may not adequately interact with the
two main OPuSs. The empty spaces, not filled with related coherence
length portions of the spectra pulse humps get scarcer when one
combines a single min-OPuS and two regular OPuSs, and the second
may be too much. According to aspects of an embodiment of the
present invention applicants propose the coordinated change of the
regular OPuS delay lengths when the mini-OPuS(s) are installed,
including whether they are part of the laser system or installed
down stream of the regular main OPuSs, e.g., in the lithography
tool itself. Applicants believe that such mini-OPuS(s) can fill in
the valleys of the pulse duration somewhat, leading to an increase
in T.sub.is, e.g., allowing a reduction in the delay lengths of one
of the two main OPuSs for better overall coherence length
separation.
[0222] It will be understood by those skilled in the art that the
aspects of embodiments of the disclosed subject matter disclosed
above are intended to be preferred embodiments only and not to
limit the disclosure of the disclosed subject matter(s) in any way
and particularly not to a specific preferred embodiment alone. Many
changes and modification can be made to the disclosed aspects of
embodiments of the disclosed invention(s) that will be understood
and appreciated by those skilled in the art. The appended claims
are intended in scope and meaning to cover not only the disclosed
aspects of embodiments of the disclosed subject matter(s) but also
such equivalents and other modifications and changes that would be
apparent to those skilled in the art. In additions to changes and
modifications to the disclosed and claimed aspects of embodiments
of the disclosed subject matter(s) noted above others could be
implemented.
[0223] While the particular aspects of embodiment(s) of the LASER
SYSTEM described and illustrated in this patent application in the
detail required to satisfy 35 U.S.C. .sctn.112 is fully capable of
attaining any above-described purposes for, problems to be solved
by or any other reasons for or objects of the aspects of an
embodiment(s) above described, it is to be understood by those
skilled in the art that it is the presently described aspects of
the described embodiment(s) of the disclosed subject matter are
merely exemplary, illustrative and representative of the subject
matter which is broadly contemplated by the disclosed subject
matter. The scope of the presently described and claimed aspects of
embodiments fully encompasses other embodiments which may now be or
may become obvious to those skilled in the art based on the
teachings of the Specification. The scope of the present LASER
SYSTEM is solely and completely limited by only the appended claims
and nothing beyond the recitations of the appended claims.
Reference to an element in such claims in the singular is not
intended to mean nor shall it mean in interpreting such claim
element "one and only one" unless explicitly so stated, but rather
"one or more". All structural and functional equivalents to any of
the elements of the above-described aspects of an embodiment(s)
that are known or later come to be known to those of ordinary skill
in the art are expressly incorporated herein by reference and are
intended to be encompassed by the present claims. Any term used in
the specification and/or in the claims and expressly given a
meaning in the Specification and/or claims in the present
application shall have that meaning, regardless of any dictionary
or other commonly used meaning for such a term. It is not intended
or necessary for a device or method discussed in the Specification
as any aspect of an embodiment to address each and every problem
sought to be solved by the aspects of embodiments disclosed in this
application, for it to be encompassed by the present claims. No
element, component, or method step in the present disclosure is
intended to be dedicated to the public regardless of whether the
element, component, or method step is explicitly recited in the
claims. No claim element in the appended claims is to be construed
under the provisions of 35 U.S.C. .sctn.112, sixth paragraph,
unless the element is expressly recited using the phrase "means
for" or, in the case of a method claim, the element is recited as a
"step" instead of an "act".
[0224] It will be understood also be those skilled in the art that,
in fulfillment of the patent statutes of the United States,
applicant(s) has disclosed at least one enabling and working
embodiment of each invention recited in any respective claim
appended to the Specification in the present application and
perhaps in some cases only one. For purposes of cutting down on
patent application length and drafting time and making the present
patent application more readable to the inventor(s) and others,
applicant(s) has used from time to time or throughout the present
application definitive verbs (e.g., "is", "are", "does", "has",
"includes" or the like) and/or other definitive verbs (e.g.,
"produces," "causes" "samples," "reads," "signals" or the like)
and/or gerunds (e.g., "producing," "using," "taking," "keeping,"
"making," "determining," "measuring," "calculating" or the like),
in defining an aspect/feature/element of, an action of or
functionality of, and/or describing any other definition of an
aspect/feature/element of an embodiment of the subject matter being
disclosed. Wherever any such definitive word or phrase or the like
is used to describe an aspect/feature/element of any of the one or
more embodiments disclosed herein, i.e., any feature, element,
system, sub-system, component, sub-component, process or algorithm
step, particular material, or the like, it should be read, for
purposes of interpreting the scope of the subject matter of what
applicant(s) has invented, and claimed, to be preceded by one or
more, or all, of the following limiting phrases, "by way of
example," "for example," "as an example," "illustratively only,"
"by way of illustration only," etc., and/or to include any one or
more, or all, of the phrases "may be," "can be", "might be," "could
be" and the like. All such features, elements, steps, materials and
the like should be considered to be described only as a possible
aspect of the one or more disclosed embodiments and not as the sole
possible implementation of any one or more
aspects/features/elements of any embodiments and/or the sole
possible embodiment of the subject matter of what is claimed, even
if, in fulfillment of the requirements of the patent statutes,
applicant(s) has disclosed only a single enabling example of any
such aspect/feature/element of an embodiment or of any embodiment
of the subject matter of what is claimed. Unless expressly and
specifically so stated in the present application or the
prosecution of this application, that applicant(s) believes that a
particular aspect/feature/element of any disclosed embodiment or
any particular disclosed embodiment of the subject matter of what
is claimed, amounts to the one an only way to implement the subject
matter of what is claimed or any aspect/feature/element recited in
any such claim, applicant(s) does not intend that any description
of any disclosed aspect/feature/element of any disclosed embodiment
of the subject matter of what is claimed in the present patent
application or the entire embodiment shall be interpreted to be
such one and only way to implement the subject matter of what is
claimed or any aspect/feature/element thereof, and to thus limit
any claim which is broad enough to cover any such disclosed
implementation along with other possible implementations of the
subject matter of what is claimed, to such disclosed
aspect/feature/element of such disclosed embodiment or such
disclosed embodiment. Applicant(s) specifically, expressly and
unequivocally intends that any claim that has depending from it a
dependent claim with any further detail of any
aspect/feature/element, step, or the like of the subject matter of
what is claimed recited in the parent claim or claims from which it
directly or indirectly depends, shall be interpreted to mean that
the recitation in the parent claim(s) was broad enough to cover the
further detail in the dependent claim along with other
implementations and that the further detail was not the only way to
implement the aspect/feature/element claimed in any such parent
claim(s), and thus be limited to the further detail of any such
aspect/feature/element recited in any such dependent claim to in
any way limit the scope of the broader aspect/feature/element of
any such parent claim, including by incorporating the further
detail of the dependent claim into the parent claim.
* * * * *
References