U.S. patent application number 10/112493 was filed with the patent office on 2002-09-19 for laser resonator for improving narrow band emission of an excimer laser.
This patent application is currently assigned to Lambda Physik AG.. Invention is credited to Albrecht, Hans-Stephan, Heist, Peter, Volger, Klaus Wolfgang.
Application Number | 20020131468 10/112493 |
Document ID | / |
Family ID | 22443925 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020131468 |
Kind Code |
A1 |
Albrecht, Hans-Stephan ; et
al. |
September 19, 2002 |
Laser resonator for improving narrow band emission of an excimer
laser
Abstract
An apparatus and method are provided for bandwidth narrowing of
an excimer laser to .DELTA..lambda..apprxeq.6 pm or less with high
spectral purity and minimized output power loss. Output stability
with respect to pulse energy, beam pointing, beam size and beam
output location is also provided. The excimer laser includes an
active laser medium for generating a spectral beam at an original
wavelength, means for selecting and narrowing the broadband output
spectrum of the excimer laser, a resonator having at least one
highly reflecting surface, and an output coupler. Means for
adapting a divergence of the resonating band within the resonator
is further included in the apparatus of the invention. The
divergence adapting causes the spectral purity to improve by
between 20% and 50% and the output power to reduce by less than
10%. A method according to the invention includes selecting and
aligning the divergence adapting means.
Inventors: |
Albrecht, Hans-Stephan;
(Gottingen, DE) ; Heist, Peter; (Jena, DE)
; Volger, Klaus Wolfgang; (Gottingen, DE) |
Correspondence
Address: |
Andrew V. Smith
Sierra Patent Group, Ltd.
P.O. Box 6149
Stateline
NV
89449
US
|
Assignee: |
Lambda Physik AG.
|
Family ID: |
22443925 |
Appl. No.: |
10/112493 |
Filed: |
March 28, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10112493 |
Mar 28, 2002 |
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09923632 |
Aug 6, 2001 |
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6404796 |
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09923632 |
Aug 6, 2001 |
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09130277 |
Aug 6, 1998 |
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6285701 |
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Current U.S.
Class: |
372/57 |
Current CPC
Class: |
H01S 3/08081 20130101;
H01S 3/08 20130101; H01S 3/225 20130101; H01S 3/139 20130101; H01S
3/1305 20130101; H01S 3/08036 20130101 |
Class at
Publication: |
372/57 |
International
Class: |
H01S 003/22; H01S
003/223 |
Claims
What is claimed is:
1. A method of controlling a spectral parameter of an output beam
from an excimer or molecular fluorine laser including a laser
active medium, a resonator, an output coupler, a processor, an
energy detector, a spectrometer and a wavelength selection unit,
comprising the steps of: operating the excimer or molecular
fluorine laser including an optical component having a curved
surface within the resonator for providing the output beam with an
improved spectral purity; measuring a spectral parameter of the
beam with a spectrometer, the spectral parameter being selected
from the group consisting of spectral purity and bandwidth; sending
signals to the processor based on the measuring of the spectral
parameter; and adjusting the optical component within the resonator
for controlling the spectral parameter of the beam based on the
spectral parameter signals sent to the processor.
2. A method of controlling a spectral parameter of an output beam
from an excimer or molecular fluorine laser including a laser
active medium, a resonator, an output coupler, one or more
processors, an energy detector, a spectrometer and a wavelength
selection unit, comprising the steps of: operating the excimer or
molecular fluorine laser including an optical component having a
curved surface within the resonator for providing the output beam
with improved spectral purity; measuring beam energy and a spectral
parameter with the energy detector and spectrometer, respectively,
the spectral parameter being selected from the group consisting of
spectral purity and bandwidth; sending signals to the one or more
processors indicative of the beam energy and spectral parameter;
and adjusting the optical component within the resonator for
controlling the spectral parameter of the beam based on the
spectral parameter signals sent to at least one of the one or more
processors.
3. The method of any of claims 1 or 2, wherein said adjusting of
said optical component further for adapting a divergence of the
output beam.
4. The method of any of claims 1 or 2, wherein the optical
component includes a refractive portion which refracts the
beam.
5. The method of any of claim 1 or 2, further comprising the step
of adjusting a geometry of an aperture for further improving the
spectral purity of the beam.
6. The method of any of claims 1 or 2, wherein the inclusion of the
optical component having the curved surface within the resonator
and the performance of the aligning step causes the spectral purity
of the output beam to improve by between 20% and 50% and the output
power to reduce by less than 10%.
7. The method of any of claims 1 or 2, wherein the adjusting step
includes adjusting a curvature of said curved surface of the
optical component.
8. The method of any of claims 1 or 2, wherein the optical
component is a resonator reflector of said resonator.
9. The method of claim 8, wherein the optical component is the
output coupler.
10. The method of any of claims 1 or 2, wherein the adjusting step
is automatically initiated by the processor when a spectral
parameter signal is sent to the processor.
11. The method of any of claims 1 or 2, wherein the adjusting step
is manually performed.
12. A method of controlling a spectral parameter of an output beam
from an excimer or molecular fluorine laser including a laser
active medium, a resonator, an output coupler, a processor, an
energy detector, a spectrometer and a wavelength selection unit,
comprising the steps of: operating the excimer or molecular
fluorine laser including an optical component within the resonator
for providing the output beam with an improved spectral purity;
measuring a spectral parameter of the beam with a spectrometer, the
spectral parameter being selected from the group consisting of
spectral purity, wavelength and bandwidth; sending signals to the
processor based on the measuring of the spectral parameter; and
adjusting the optical component within the resonator for
controlling the spectral parameter of the beam based on the
spectral parameter signals sent to the processor.
13. A method of controlling a spectral parameter of an output beam
from an excimer or molecular fluorine laser including a laser
active medium, a resonator, an output coupler, one or more
processors, an energy detector, a spectrometer and a wavelength
selection unit, comprising the steps of: operating the excimer or
molecular fluorine laser including a first optical component within
the resonator for providing the output beam with improved spectral
purity; measuring beam energy and a spectral parameter with the
energy detector and spectrometer, respectively, the spectral
parameter being selected from the group consisting of spectral
purity, wavelength and bandwidth; sending signals to the one or
more processors indicative of the beam energy and spectral
parameter; and adjusting the optical component within the resonator
for controlling the spectral parameter of the beam based on the
spectral parameter signals sent to at least one of the one or more
processors.
14. A method of controlling a spectral parameter of an output beam
from a molecular fluorine laser having a wavelength around 157 nm
for use as source radiation for producing structures on IC chips,
the molecular fluorine laser including a molecular fluorine laser
active medium, a resonator, an output coupler, a processor, an
energy detector, a spectrometer and a wavelength selection unit,
comprising the steps of: operating the molecular fluorine laser
including at least one wavelength selection optical component of
said wavelength selection unit within the resonator for controlling
a spectral parameter of the output beam having said wavelength
around 157 nm, the spectral parameter being selected from the group
consisting of spectral purity and bandwidth; measuring the spectral
parameter of the beam with the spectrometer; sending signals to the
processor based on the measuring of the spectral parameter; and
adjusting the at least one wavelength selection optical component
of said wavelength selection unit within said resonator for
controlling the spectral parameter of the output beam based on said
signals sent to the processor.
15. The method of claim 14, wherein said at least one wavelength
selection optical component includes a resonator reflector.
16. The method of claim 14, further comprising the step of
adjusting the beam energy of the output beam based on the beam
energy signals sent to the processor.
17. An excimer or molecular fluorine laser, comprising an active
laser medium for emitting an output beam; a resonator defining an
optical path intersecting said active medium; at least one
line-narrowing optical component for narrowing the bandwidth of the
output beam; an adjustable aperture within the resonator for
controlling spectral purity of the output beam; an energy detector
and a spectrometer each for receiving a portion of the spectral
beam; and one or more processors for receiving signals from each of
the energy detector and the spectrometer, and wherein the
adjustable aperture is configured to be adjustable based on an
adjustment signal received from at least one of the one or more
processors, the adjustment signal being determined based at least
on spectral information received from the spectrometer.
18. An excimer or molecular fluorine laser, comprising: an active
laser medium for generating a spectral beam at an original central
wavelength; a resonator including a first reflecting surface and a
second reflecting surface, an optical path intersecting said active
medium being defined for said resonator for generating a laser
beam, at least one of said first and second reflecting surfaces
being a curved surface including an adjustable curvature; a
wavelength selector for selecting a wavelength band from the
spectral beam including a beam expander and a grating, the grating
also serving as said first reflecting surface; an energy detector
and a spectrometer each for receiving a portion of the spectral
beam; one or more processors for receiving signals from each of the
energy detector and the spectrometer, and wherein the curvature of
the curved surface is automatically adjusted when a signal is
received from at least one of the one or more processors based on
information received from the spectrometer.
19. The laser of claim 18, further comprising an aperture for
adapting a divergence of the resonating beam to improve spectral
purity of the laser beam.
20. An excimer or molecular fluorine laser, comprising an active
laser medium for emitting an output beam; a resonator defining an
optical path intersecting said active medium; at least one
line-narrowing optical component for narrowing the bandwidth of the
output beam, including at least one adjustable optic for
controlling the wavelength of the output beam; an energy detector
and a spectrometer each for receiving a portion of the beam; and
one or more processors for receiving signals from each of the
energy detector and the spectrometer, and wherein the adjustable
optic is configured to be adjustable based on an adjustment signal
received from at least one of the one or more processors, the
adjustment signal being determined based at least on spectral
information received from the spectrometer.
21. A molecular fluorine laser for generating a 157 nm laser beam
for providing improved resolvability of structures on IC chips as a
lithographic processing tool, comprising: an molecular fluorine
laser medium for emitting radiation at an original central
wavelength around 157 nm; a resonator including a first reflecting
surface and a second reflecting surface, an optical path
intersecting said active medium being defined for said resonator
for generating the laser beam; a wavelength selector including at
least one wavelength selection optical element for selecting a
wavelength band from the spectral distribution of the emitted
radiation; an energy detector and a spectrometer each for receiving
a portion of the spectral beam; and one or more processors for
receiving signals from each of the energy detector and the
spectrometer, and wherein the wavelength selection optical
component is automatically adjusted for adjusting a spectral
parameter of the selected wavelength band when a signal is received
from at least one of the one or more processors based on
information received from the spectrometer.
22. The laser of claim 21, wherein an adjustment of the beam energy
of the output beam is automatically initiated by the processor
based on information received from the energy detector.
Description
PRIORITY
[0001] This application is a 37 C.F.R. 1.53(b) continuation
application of U.S. patent application Ser. No. 09/923,632, filed
Aug. 6, 2001, which is a continuation of U.S. patent application
Se. No. 09/130,277, filed Aug. 6, 1998.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a resonator designed for
narrow-linewidth emission, and particularly to a resonator for an
excimer laser having optical components for improving spectral
purity, reducing spectral bandwidth and optimizing output power for
emitting a high resolution photolithographic beam.
[0004] 2. Discussion of the Related Art
[0005] To increase the capacities and operation speeds of
integrated circuits, manufacturers are inclined to design smaller
internal structures for devices and other components of these
chips. The reduction in size of a structure produced on a silicon
wafer is limited by the ability to optically resolve the structure.
This resolution ability depends directly upon the
photolithographical source radiation and optics used.
[0006] Excimer lasers emitting pulsed UV-radiation are becoming
increasingly important instruments in specialized material
processing. The term "excimer" was first utilized as an
abbreviation for "excited dimer", meaning two or more identical
atoms comprising a molecule which only exists in an excited state,
such as He.sub.2 and Xe.sub.2. Today, the term "excimer" has a
broader meaning in the laser world and encompasses such rare gas
halides as XeCl (308 nm), KrF (248 nm), ArF (193 nm), KrCl (222
nm), and XeF (351 nm). Several mercury-halides are also used as
active gases in excimer lasers, such as HgBr. Even N.sub.2,
N.sub.2.sup.+, CO.sub.2 and F.sub.2 (157 nm) may be used as active
media within excimer laser discharge chambers. As is apparent, many
excimer lasers radiate at ultraviolet wavelengths making them
desirable for use as lithography tools. The KrF-excimer laser
emitting around 248 nm and the ArF-excimer laser emitting around
193 nm are rapidly becoming the light sources of choice for
photolithographic processing of integrated circuit devices (IC's).
The F.sub.2-laser is also being developed for such usage and emits
light around 157 nm.
[0007] To produce smaller feature sizes on IC chips, stepper and
scanner machines are using expensive large scale submicron
projection objectives for imaging a reticle onto a wafer surface
with high diffracting-limited precision. The objectives operate at
deep ultraviolet (DUV) wavelengths, such as the emission
wavelengths of excimer lasers. For example, the KrF-excimer laser
emitting around 248 nm is currently being used as a DUV radiation
source. To reach greater resolution limits, the large field
objective lenses are designed and optimized in view of various
possible and discovered imaging errors. The design optimization of
the objectives is, however, inadequate to meet the precision
demands of sub-quarter micron lithographic technology.
[0008] One way to improve the resolvability of structures on IC
chips is to use more nearly monochromatic source radiation, i.e.,
radiation having a reduced bandwidth, .DELTA..lambda.. Other
strategies include using shorter absolute wavelength, .lambda.,
radiation such as that emitted around 193 nm and 157 nm by ArF- and
F.sub.2-lasers, respectively, and increasing the numerical aperture
(NA) of the projection lens.
[0009] The smallest structure resolvable on an IC chip depends on
the "critical dimension" (CD) of the photolithography equipment
being used: 1 CD = K 1 NA ; where
[0010] NA is a measure of the acceptance angle of the projection
lens, .lambda. is the wavelength of the source radiation, and
K.sub.1 is a constant around approximately 0.6-0.8. Simply
increasing the numerical aperture NA to reduce the critical
dimension CD simultaneously reduces the depth of focus DOF of the
projection lens by the second power of NA: 2 DOF = K 2 ( NA ) 2 ;
where
[0011] K.sub.2 is a constant around approximately 0.8-1.0. This
complicates wafer adjustment and adds further strain on the demand
for improved chromatic correction of the projection lenses.
Additionally, increasing the numerical aperture NA to reduce the
critical dimension CD for achieving smaller structures requires a
decrease in the bandwidth .DELTA..lambda. of laser emission
according to: 3 = K 3 ( NA ) 2 ; where
[0012] K.sub.3 is a constant dependent on parameters associated
with the projection lens(es). Each of the above assumes that such
other laser parameters as repetition rate, stability, and output
power remain constant.
[0013] Some techniques are known for selecting and for narrowing
laser emission bandwidths including using optically dispersive
elements such as etalons, gratings and prisms, as well as modified
resonator arrangements. See U.S. Pat. No. 5,095,492 to Sandstrom
(disclosing a dispersive grating having a concave radius of
curvature); U.S. Pat. No. 5,559,816 to Basting et al. (disclosing a
technique using the polarization properties of light); U.S. Pat.
No. 5,150,370 to Furuya et al. (disclosing a fabry-perot etalon
within the laser resonator); U.S. Pat. Nos. 5,404,366, 5,596,596
and E.U. Patent Pub. No. 0 472 727, each to Wakabayashi et al.
(disclosing a concave outcoupler and a fixed aperture within the
laser resonator); U.S. Pat. No. 4,829,536 to Kajiyama et al.
(disclosing angularly offset etalons).
[0014] Using this available knowledge, the bandwidth of laser
emission, e.g., which is naturally around 500 pm for a KrF-excimer
laser, can be reduced to .DELTA..lambda..apprxeq.0.8 pm, sufficient
to meet the demands of current projection lenses (NA.apprxeq.0.53)
for producing quarter micron ship structures. Further improvements
in projection objectives (NA .apprxeq.0.8) combined with a further
reduction in laser emission bandwidths
(.DELTA..lambda..apprxeq.0.4-0.6 pm) are expected to reduce the
critical dimension CD using KrF-excimer laser sources down to
CD.apprxeq.0.18 microns. See J. Mulkens et al., Step and Scan
Technology for the 193 nm Era, Third International Symposium on 193
nm Lithography, Onuma, Japan (Jun. 29-Jul. 2, 1997).
[0015] The drawback to this significant bandwidth and CD reduction
is a correspondingly significant reduction in available laser
output power. Narrow band efficiencies of twenty to forty percent
of broadband output power are typical. There is thus a need for
efficient spectral narrowing methods which minimize power loss.
[0016] FIG. 1 shows a conventional excimer laser arrangement. A
laser tube 1 contains a laser active medium (not shown) for
emitting a characteristic wavelength upon excitation pumping of the
laser active medium. A wavelength selection and narrowing assembly
2 includes a dispersive grating 3 and at least one expanding and/or
dispersive prism 4. The grating 3 also serves to reflect
substantially all of the laser light incident upon it at a
wavelength dependent angle. A narrow band of the light dispersed
once through the prism 4 and incident upon the grating 3 is
reflected off of the grating 3 and back along the optical path of
the arrangement, while all other wavelengths are reflected away
from the optical path. The arrangement is completed with an output
coupling mirror 5 which reflects a portion of the resonating band
and allows the rest to continue unreflected ultimately defining the
output beam of the system.
[0017] The excimer laser arrangement of FIG. 2 includes all of the
elements of FIG. 1 except the output coupling mirror 5, and further
includes a beam splitter 6 and a highly reflective mirror 8. The
beam splitter 6 serves as an output coupler reflecting the narrow
band laser emission 9 from the optical path of the resonating beam.
A highly reflective mirror 8 is used instead of the partially
reflecting output coupling mirror 5 of the arrangement of FIG.
1.
SUMMARY OF THE INVENTION
[0018] The present invention sets forth an apparatus and method for
bandwidth narrowing of an excimer laser to
.DELTA..lambda..apprxeq.0.6 pm or less with high spectral purity
and minimized output power loss. Additional and/or modified optical
elements within the laser resonator are used. Output stability with
respect to pulse energy, beam pointing, beam size and beam output
location are also improvements of the present invention.
[0019] An apparatus according to the present invention is an
excimer laser including an active laser medium for generating a
broadband spectral beam at an original wavelength, means for
narrowing the wavelength and/or selecting a spectral line of the
generated broadband spectral beam, a resonator and a means for
outcoupling the resonating band. Means for adapting or matching the
divergence of the intracavity rays is further included in the
apparatus according to the present invention for optimizing the
combination of output power, spectral purity and bandwidth of the
output beam of the excimer laser. A method according to the present
invention includes selecting and aligning the divergence adapting
or matching means such that the combination of output power,
spectral purity and bandwidth of the output beam of the excimer
laser is optimized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 shows a conventional excimer laser arrangement
including output coupling via a partially reflective resonator
mirror.
[0021] FIG. 2 shows a conventional excimer laser arrangement
including a phase retardation prism and output coupling via a beam
splitter.
[0022] FIG. 3 shows an excimer laser arrangement according to a
first embodiment of the present invention.
[0023] FIG. 4 shows an excimer laser arrangement according to a
second embodiment of the present invention.
[0024] FIG. 5A shows an excimer laser arrangement according to a
third embodiment of the present invention.
[0025] FIG. 5B shows an excimer laser arrangement according to a
fourth embodiment of the present invention.
[0026] FIG. 6 shows an excimer laser arrangement according to a
fifth embodiment of the present invention.
[0027] FIG. 7 shows a calculated output spectrum for the excimer
laser arrangement of FIG. 1.
[0028] FIG. 8 shows a calculated output spectrum for the excimer
laser arrangement of FIGS. 5A and 5B.
[0029] FIG. 9 shows a calculated output spectrum for an excimer
laser arrangement wherein a cylindrical lens is placed between the
laser active medium and a beam expander.
[0030] FIG. 10 shows a calculated output spectrum for the excimer
laser arrangement of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0031] Spectral laser emissions propagate with a full-angle of beam
divergence after n round trips within the resonator of an excimer
laser approximately according to: 4 0 4 a L ( n ) 1 / 2 ; where
[0032] a is the aperture radius (or its geometrical equivalent
corresponding to the geometry of the aperture), L is the length of
the resonator, and n is the number of round trips an average photon
emitted from the laser active medium traverses within the resonator
before outcoupling through, e.g., the outcoupling mirror 5 of the
arrangement of FIG. 1, or the beam splitter 6A of the arrangement
of FIG. 2. See S. Kawata, I. Hikima, Y. Ichihara, and S. Watanabe,
Spatial Coherence of KrF Excimer Lasers, Appl. Opt., vol. 31, page
387 (1992). When dispersive elements are used for wavelength
narrowing, the angular divergence corresponds to a finite
bandwidth:
.theta..sub.0.apprxeq..DELTA..lambda..sub.0.
[0033] Different parts of the lateral laser beam comprise different
spectral portions of the spectrally narrowed beam. See R.
Sandstrom, Measurements of Beam Characteristics Relevant to DUV
Microlithography on a KrF Excimer Laser, SPIE: Microlithography
III, vol. 1264, 505, 511 (1990) (showing in FIG. 8 the variation in
spectral content of a previously horizontally dispersed beam as a
vertical slit mask selects out portions of the beam, scanning
horizontally from beam center to the right). The side wings, or
outer wavelengths, of the spectral band of the excimer laser
contain a significant amount of energy which cannot be ignored due
to its effect of diminishing the spectral purity and output power
of the output beam. See Sandstrom (above), at 507-11. These side
wings also contribute to the width of the band.
[0034] Furthermore, spectral narrowing generally occurs in any
laser resonator according to the spectral narrowing effect: 5 L 0 (
n ) 1 / 2 ; where
[0035] n is the number of round trips, .DELTA..lambda..sub.0 is the
bandwidth narrowed by optical components within the resonator, and
.DELTA..lambda..sub.L is the laser emission bandwidth. Effective
spectral narrowing of a laser beam typically requires a large
number n of round trips. Since an excimer laser usually has a few
round trips, the natural spectral narrowing associated with other
types of lasers is not achieved. Further, in an excimer laser, the
side wings are amplified at the expense of the center of the band,
since they are within the divergence/acceptance angle of the beam
during the few round trips traversed by the beam within the
resonator.
[0036] At a given dispersive power, the degree of divergence
compensation has to be carefully adapted to the number of round
trips, since they, together with the resulting bandwidth, spectral
purity and output power of the emitted beam, are interdependent.
Spectral purity is a measure of the spectral energy distribution
within a narrow central region around the line center, e.g., within
a 2 pm limit. The spectral purity may also be defined as the energy
within a specified wavelength interval divided by the total
energy.
[0037] FIG. 3 shows an arrangement of a resonator of an excimer
laser in accordance with a first embodiment of the present
invention. The arrangement includes an active laser medium 1 for
emitting light having a characteristic wavelength. The arrangement
further includes wavelength selecting and narrowing optics 2
comprising a grating 3 and at least one prism 4. The grating 3
serves as one of the two reflecting resonator surfaces of the first
embodiment. The grating 3 reflects substantially all light incident
upon it, each wavelength at a different angle. The other reflecting
surface is a convex-curved, preferably cylindrical, surface of an
output coupling mirror 15. The output coupler 15 preferably
transmits a portion of the light incident upon it and reflects the
rest. Alternatively, the outermost radial portion of the resonating
band simply misses the output coupling mirror 15, which has a
smaller radius than the resonating beam at that point. In either
event or in another conventional way, the portion of the resonating
band that exits the resonator along the predetermined optical path
after encountering the output coupling mirror 15 defines the
emitted output beam 16 of the laser. The resonator may thus be
operated as an unstable resonator, or alternatively, additional
optics may be used to stabilize the resonator.
[0038] As mentioned above, the lateral laser beam comprises a
spectrum of wavelengths, ordered according to wavelength, as an
effect of traversing the wavelength selection and narrowing optics
2. The optics of the arrangement of the first embodiment are
aligned such that the center of the resonating band strikes
approximately at the center of the mirror 15. That center portion
is reflected back along the optical path of the resonator.
[0039] Other wavelengths above and below the center of the band,
are reflected off the mirror 15 at angles away from the optical
path wherein these angles are enhanced due to the convex nature of
the mirror 15. Wavelengths that are sufficiently removed from the
center of the band are reflected at such a high angle that they no
longer are accepted by apertures within the resonator, such as
those surrounding the laser active medium 1. These wavelengths will
not be part of any subsequently emitted output beam 16. Since a
smaller geometrical region of the resonating beam will be accepted
by natural apertures of the resonator, the resonating band of the
first embodiment comprises a narrower range of wavelengths than a
resonating band of a conventional resonator.
[0040] The resonator of the first embodiment is an unstable
resonator if a light ray initially propagating parallel to the
optical axis of the laser cavity could not be reflected back
between the two mirrored surfaces 3 and 15 indefinitely without
escaping from between the mirrors 3 and 15, other than by
outcoupling. That is, the angle a ray of the resonating beam makes
with the optical axis will increase with the number of round trips
the ray makes within the resonator. If the grating 3 is flat and no
additional focusing optics are provided in the arrangement of the
first embodiment, then the first embodiment will include an
unstable resonator by virtue of the convex output coupler 15.
[0041] The angle of a light ray incident upon the output coupler 15
over which this light reflected from the outcoupling mirror 15 is
dispersed out of the resonating beam is the acceptance angle of the
beam. The smaller the radius of curvature of the convex outcoupling
mirror 15, the smaller the acceptance angle of the beam.
Consequently, the smaller the radius of curvature of the
outcoupling mirror 15, the narrower the band of wavelengths that
the ultimate emitted beam 16 will comprise. The radius of curvature
is preferably constant over the surface of the mirror 15, but may
change with distance from the center, or along a diameter. The
focal length of the mirror 15 is preferably in the range of several
meters, as are most curved components of the present invention.
[0042] The narrowing of the beam 16 is not achieved without a
price. Generally, with all else being the same, the narrower the
bandwidth of the beam, the weaker the output power of that beam 16.
At some point, the beam 16 can be so narrowed that its output power
is not sufficient to perform adequate lithography. However, the
radius of curvature of the convex mirror 15 can be selected
appropriately, such that the output power of the beam 16 is
sufficient, while the desired bandwidth narrowing is achieved.
[0043] Thus, a first advantage of the first embodiment over a
conventional arrangement having a, e.g., flat outcoupling mirror 5,
such as that of FIG. 1, is that the spectral purity of the beam 16
of the first embodiment is enhanced over the prior art. A second
advantage is that the bandwidth of the beam may be more greatly
narrowed than a prior art beam while maintaining adequate output
power, since the side wings of the resonating beam are dispersed
out by the outcoupling mirror 16 and do not absorb power being
amplified. Available power is then focusable on amplifying a more
useful narrow central portion of the emission band 16. In a
preferred embodiment, the radius of curvature of the outcoupling
mirror 15 is adjustable to optimize the combination of output
power, bandwidth and spectral purity of the emitted beam 16.
[0044] The present invention is capable of improving spectral
purity by between 20% and 50%, or more while power loss is kept to
less than 10%. Power loss may be defined as the difference between
the power of a state of the art laser and the power of a laser
according to the present invention, divided by the power of a state
of the art laser. Typical line narrowing efficiency is between 0.2
and 0.4, and particularly is around 0.3 for lasers used in
lithography.
[0045] FIG. 4 shows an arrangement of a resonator of an excimer
laser in accordance with a second embodiment of the present
invention. The second embodiment has an active laser medium 11,
wavelength selection and narrowing optics 12 including a grating 3,
which preferably serves also as one of two reflecting surfaces of
the resonator, and at least one prism 14, and an output coupler 25.
A highly reflective mirror may alternatively perform the reflective
function of the grating 3. The output coupler 25 may be similar to
the conventional output coupler 5 of FIG. 1, or it may be similar
to the output coupler 15 having a reflecting surface with a convex
radius of curvature of FIG. 3, or may be another operable output
coupler 25.
[0046] The laser active medium 11 is contained within a housing
having a first optical window 17A and a second optical window 17B
to facilitate entrance and exit of the resonating beam. The windows
17A and 17B each comprise one or more conventionally UV transparent
materials such as crystalline quartz, CaF.sub.2 and/or MgF.sub.2,
for example.
[0047] At least one surface of at least one, and preferably both,
windows 17a and 17B is curved. Both surfaces of one or both windows
17A and 17B may be curved, but preferably only the outer surfaces
are curved as shown in FIG. 4. The effective total radius of
curvature of all curved surfaces of the windows 17A and 17B is
selected to match or adapt the divergence of the resonating beam
and optimize the combination of laser output power, bandwidth and
spectral purity. By matching or adapting the divergence of the
resonating beam, the angles of light rays of the resonating beam
relative to the optical axis are changed to minimize the angle of
the light rays relative to the optical axis. The divergence is
matched or adapted in the present invention to optimize spectral
purity and bandwidth. Divergence adapting is provided in the
present invention by focusing elements and/or by the cutting of
rays with an angle relative to the optical axis greater than a
specified angle by one or more apertures.
[0048] The prism 14 of the second embodiment has a first curved
surface 18A and a second curved surface 18B through which the
resonating beam enters and exits the prism 14. Alternatively, only
one surface 18A or 18B may be curved. Another arrangement of the
wavelength narrowing and selection optics 12 is possible wherein
the resonating beam enters and exits the prism through the same
curved surface. The curvature of each surface 18A and 18B is
preferably convex. Alternatively, one may be concave or the radius
of curvature may change with position on one or both surfaces 18A
and 18B.
[0049] A first advantage of the second embodiment over a
conventional arrangement such as that shown in FIG. 1 is that the
radius of curvature of each of the surfaces 17A, 17B, 18A and 18B
may be selected to match the divergence of the laser beam, and
optimize output power, bandwidth and spectral purity. A second
advantage is that the surfaces 18A and 18B of the prism 14 and/or
the surfaces 17A and 17B of the housing containing the active laser
medium 11 may be used to expand or narrow the resonating beam,
depending on what is needed in the arrangement considering the
properties and alignment of the other optics in the
arrangement.
[0050] FIG. 5A shows an arrangement of a resonator of an excimer
laser in accordance with a third embodiment of the present
invention. The third embodiment includes an active laser medium 21,
wavelength selection and narrowing optics 22 including a grating 3,
which preferably serves also as one of two reflecting surfaces of
the resonator, and at least one prism 24, and an output coupler 35.
A highly reflective mirror may alternatively perform the reflective
function of the grating 3. The output coupler 35 may be similar to
the conventional output coupler 5 of FIG. 1, or it may be similar
to the output coupler 15 having a reflecting surface with a convex
radius of curvature of FIG. 3, or may be another operable output
coupler 35. An output beam 16 is transmitted past the output
coupler 35. The prism 24 and housing of the active laser medium 21
may be configured as in the either of the first or the second
embodiments of FIGS. 3 and 4, respectively, or otherwise
conventionally.
[0051] The third embodiment also includes an aperture 19 located
within the resonator arrangement of FIG. 5A. The aperture 19 is
preferably located near the grating 3 as shown in FIG. 5A, but may
be located at various locations along the optical path of the
resonating beam. More than one aperture may be placed along the
optical path of the resonating band. The aperture 19 is preferably
adjustable to optimize the combination of the output power, the
bandwidth and the spectral purity of the output beam 16. The
aperture 19 is blocking highly divergent beams, i.e., beams having
a large angle relative to the optical axis of the resonator,
primarily to improve spectral purity. When the aperture 19 is
located close to the grating 3, the output power is not
significantly affected by the presence of the aperture 19. An
advantage of the third embodiment is that the spectral purity,
bandwidth and output power of the output beam 16 are optimized over
those of a conventional arrangement such as that described in FIG.
1.
[0052] FIG. 5B shows an arrangement of a resonator of an excimer
laser in accordance with a fourth embodiment of the present
invention. The fourth embodiment of FIG. 5B includes all of the
elements of the third embodiment of FIG. 5A. Additionally, the
fourth embodiment includes a beam splitter 6C after the output
coupler 35 which transmits an output beam 26 and reflects a portion
of the output of the output coupler 35. The reflected portion is
received by a high resolution spectrometer 29 for determining the
wavelength and waveform characteristics of the output beam 26. A
second beam splitter 6D is inserted to direct a portion of the
output beam 26 toward an energy detector 28. The outputs of each of
the detector 28 and the spectrometer 29 are received by a computer
30 and processed. The computer then determines how the optics of
the arrangement should be modified to optimize the laser output 26
with regard to the combination of output power, bandwidth and
spectral purity. The optics may then be manually or automatically
adjusted in accordance with the computer's
instructions/suggestions. Particularly with respect to the fourth
embodiment, the aperture size may be modified. Generally, the
detector 28, the high resolution spectrometer 29 and the computer
30 may be used with any of the embodiments of the present invention
to help achieve the task of optimizing the combination of the
output power, the bandwidth and the spectral purity of the output
beam, e.g., 26. Alternatively with respect to the fourth
embodiment, a feed back circuit may be used for real time
monitoring of the bandwidth, output power and spectral purity of
the output beam 26 and adjustment of the aperture 19.
[0053] FIG. 6 shows an arrangement of a resonator of an excimer
laser in accordance with a fifth embodiment of the present
invention. The fifth embodiment preferably includes wavelength
selection and narrowing optics 32 including the prism 4 of the
first embodiment of FIG. 3, and the housing for the laser active
material 1 of the first embodiment of FIG. 3. Alternatively, one or
both of these elements 1, 4 may be substituted by another element
disclosed in one or more other embodiments of the present
invention, e.g., the second embodiment. Additionally, the fifth
embodiment includes a curved grating 13 and a curved output
coupling mirror 45. The two curved optical surfaces together form
an unstable resonator configuration. The curvature of each element
13, 45 may be convex or concave, but preferably the output coupling
mirror 45 is convex like the output coupler 15 of the first
embodiment and the grating is concave, like the grating disclosed
as element 40 in U.S. Pat. No. 5,095,492 to Sandstrom. Preferably,
the combination of the curvatures of the output coupler 45 and the
grating 13 cause the resonator of the fifth embodiment to be
unstable to match the divergence for optimizing the combination of
the output power, the bandwidth and the spectral purity of the
output beam 36. In addition, the radius of curvature of either the
grating 13 or the output coupler 45, or both, may be adjustable.
The resonator of the fifth embodiment may be, and preferably is, an
unstable resonator, such as that described with respect to the
first embodiment.
[0054] FIG. 7 shows a peak embodying the spectral distribution of
the output beam 10 of FIG. 1. The output beam 10 is determined to
have a bandwidth of 1.1 pm, calculated as the full-width at
half-maximum (FWHM) of the peak of FIG. 8 embodying the spectral
distribution of the output beam 10.
[0055] FIG. 8 shows a peak embodying the spectral distribution of
the output beam 16 of FIG. 5A, wherein the optical elements of the
arrangement of the third embodiment included those included in the
arrangement of FIG. 1 and an aperture 19 in front of the grating 3.
The aperture 19 used in obtaining the spectrum of FIG. 8 reduced
the bandwidth from 1.1 to 0.5 pm by geometrically halving the
divergent output beam in front of the grating 3.
[0056] FIG. 9 shows a peak embodying the spectral distribution of
the output beam 46 of another arrangement. An additional optical
element 27 used to obtain the spectrum of FIG. 9 was a cylindrical
lens, having a focal length of preferably several meters, placed
between the housing for the laser active material 21 and the prism
24. The prism 24 used was a prism expander such as that described
with respect to the second embodiment of FIG. 4.
[0057] FIG. 10 shows a peak embodying the spectral distribution of
the output beam 16 of the first embodiment of FIG. 3. The bandwidth
was reduced from 1.1 to 0.5 pm by using the convex-curved output
coupler 15 instead of the conventional output coupler 5 of FIG.
1.
[0058] An advantage of all of the above embodiments and
improvements is that the bandwidth of the output beam of the
excimer laser system to be used in microlithographic applications
is reduced, while the overall laser efficiency is influenced only
slightly. The reason is that only the central or principal part of
the resonating beam traverses the main amplification region of the
laser active medium after it has encountered one of the improved or
additional optical elements of the present invention. Moreover, an
improvement in spectral purity and stabilization of the beam
location, pointing and exit positions is observed when one of the
embodiments or improvements of the present invention is used over
that of a conventional arrangement such as that shown in FIG. 1.
The combination of output power, bandwidth and spectral purity is
optimized by using or combining one or more embodiments of the
present invention by decreasing an acceptance angle of the
resonating beam and/or matching or adapting the divergence of the
resonating beam.
* * * * *