U.S. patent application number 09/971796 was filed with the patent office on 2002-02-21 for device for on-line control of output power of vacuum-uv laser.
Invention is credited to Govorkov, Sergei V., Hua, Gongxue.
Application Number | 20020021735 09/971796 |
Document ID | / |
Family ID | 27385512 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020021735 |
Kind Code |
A1 |
Govorkov, Sergei V. ; et
al. |
February 21, 2002 |
Device for on-line control of output power of vacuum-uv laser
Abstract
A beam delivery system for a laser emitting at a relevant
wavelength of less than 200 nm is provided. The system includes a
sealed enclosure connected to the laser and surrounding the path of
the beam as it exits the laser resonator. The enclosure extends
between the laser output coupler and a photodetector sensitive at
the wavelength of the relevant laser emission. The interior of the
enclosure, and thus the beam path between the output coupler and
the detector, is substantially free of species that strongly
photoabsorb radiation at the relevant laser emission wavelength. A
beam splitting element diverts at least a portion of the beam for
measurement by the detector. The beam splitting element preferably
includes a beam splitting mirror, holographic beam sampler or
diffraction grating. In addition, optics are preferably provided
for filtering a visible portion of the diverted beam, so that
substantially only a VUV portion of the diverted beam is received
at the detector. The filtering optics preferably include a
diffraction grating, holographic beam sampler or one or more
dichroic mirrors.
Inventors: |
Govorkov, Sergei V.; (Boca
Raton, FL) ; Hua, Gongxue; (Fort Lauderdale,
FL) |
Correspondence
Address: |
Andrew V. Smith
Sierra Patent Group, Ltd.
P.O. Box 6149
Stateline
NV
89449
US
|
Family ID: |
27385512 |
Appl. No.: |
09/971796 |
Filed: |
October 3, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09971796 |
Oct 3, 2001 |
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09598522 |
Jun 21, 2000 |
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09971796 |
Oct 3, 2001 |
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09343333 |
Jun 30, 1999 |
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6219368 |
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60140530 |
Jun 23, 1999 |
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60119973 |
Feb 12, 1999 |
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Current U.S.
Class: |
372/105 |
Current CPC
Class: |
G03F 7/70558 20130101;
G03F 7/70808 20130101; B23K 26/128 20130101; H01S 3/225 20130101;
G01J 1/4257 20130101; H01S 3/0014 20130101; H01S 3/134 20130101;
B23K 26/12 20130101; G03F 7/70025 20130101; B23K 26/705 20151001;
B23K 2103/04 20180801 |
Class at
Publication: |
372/105 |
International
Class: |
H01S 003/08 |
Claims
What is claimed is:
1. A method of delivering VUV laser light portion from a main beam
which is generated by a VUV laser for use at an application process
to a detector for monitoring a parameter of the beam, comprising
the steps of: sealing off a beam path of the VUV laser light
portion within an enclosure optically coupled with the detector;
preparing the interior of the enclosure for transmitting the main
beam and VUV light portion for delivery to the detector such that
said interior is substantially free of VUV photoabsorbing species,
and wherein said VUV light portion that is delivered to the
detector is directed along a beam path within said enclosure and is
thereby protected from being substantially attenuated by said VUV
photoabsorbing species; separating said VUV light portion for
delivery to the detector from the main beam; and detecting the VUV
light portion separated from said main beam at said separating step
and delivered to the detector along said beam path and not
substantially attenuated by said VUV photoabsorbing species.
2. The method of claim 1, wherein said VUV laser is a molecular
fluorine laser, said method further comprising the step of
filtering a red beam portion from said VUV light portion.
3. The method of claim 2, wherein said filtering step is performed
after said separating step.
4. The method of claim 2, wherein said filtering step includes
dispersing the red beam portion from the VUV light portion.
5. The method of claim 2, wherein said filtering step includes
reflecting the VUV light portion while transmitting the red beam
portion using a dichroic mirror.
6. The method of claim 2, wherein said filtering step and said
separating step are performed simultaneously.
7. The method of claim 6, wherein said filtering and separating
steps include dispersing the beam.
8. The method of claim 6, wherein said filtering and separating
steps include dispersing the beam using a holographic beam
sampler.
9. The method of claim 6, further comprising the step of
redirecting the VUV light portion to the detector after said
filtering step.
10. The method of claim 2, wherein said preparing step includes
flowing an inert gas through said enclosure.
11. The method of claim 10, wherein said preparing step further
includes evacuating said enclosure prior to said inert gas flowing
step.
12. The method of claim 11, wherein said evacuating and flowing
steps are performed a plurality of times, with a final flowing step
being performed and maintained during operation of the VUV
laser.
13. The method of claim 12, further comprising the step of
redirecting the VUV light portion to the detector after the
separating step.
14. The method of claim 2, wherein detecting step is performed
after said separating and filtering steps.
15. A method of delivering a laser beam which is generated by an
excimer or molecular fluorine laser for use at an application
process, comprising the steps of: sealing off a beam path of the
laser beam within an enclosure; disposing at least one optical
component within said enclosure; preparing an interior of the
enclosure for transmitting the laser beam such that said interior
is substantially free of contaminant species, and wherein said beam
is directed along a beam path within said enclosure and is thereby
protected from being substantially disturbed by said contaminant
species; and interacting said beam with said at least one optical
component within said enclosure, wherein said beam is thereby
directed along said beam path within said enclosure and not
substantially disturbed by said contaminant species.
16. The method of claim 15, wherein said at least one optical
component includes a diffraction grating.
17. The method of claim 16, wherein said interacting step includes
the step of dispersing said beam such that only a selected portion
of a spectral distribution of said beam continues to propagate
along said beam path and other portions of said spectral
distribution of said beam are dispersed away from said beam
path.
18. A method of delivering a sub-200 nm lithographic exposure
radiation portion from a main beam which is generated by a
lithographic exposure radiation source for use at an application
process to a detector for monitoring a parameter of the beam,
comprising the steps of: sealing off a beam path within an
enclosure optically coupled with the detector; preparing the
interior of the enclosure for transmitting the main beam and the
sub-200 nm lithographic exposure radiation portion for delivery to
the detector such that said interior is substantially free of
sub-200 nm photoabsorbing species, and wherein said exposure
radiation portion that is delivered to the detector is directed
along a beam path within said enclosure and is thereby protected
from being substantially attenuated by said sub-200 nm
photoabsorbing species; separating said exposure radiation portion
for delivery to the detector from the main beam; and detecting the
exposure radiation portion separated from said main beam at said
separating step and delivered to the detector along said beam path
and not substantially attenuated by said sub-200 nm photoabsorbing
species.
19. The method of claim 18, wherein said exposure radiation source
is a molecular fluorine laser, said method further comprising the
step of filtering a red beam portion from said exposure radiation
portion.
20. The method of claim 18, wherein said preparing step includes
evacuating said enclosure.
21. The method of claim 18, wherein said preparing step includes
flowing an inert gas through said enclosure.
22. The method of claim 21, wherein said preparing step further
includes evacuating said enclosure prior to said inert gas flowing
step.
23. The method of claim 22, wherein said evacuating and flowing
steps are performed a plurality of times, with a final flowing step
being performed and maintained during operation of the exposure
radiation source.
24. The method of claim 18, further comprising the step of
redirecting the exposure radiation portion to the detector after
the separating step.
25. A method of delivering sub-200 nm lithographic exposure
radiation which is generated by a lithographic exposure radiation
source for use at an application process, comprising the steps of:
sealing off a beam path of the sub-200 nm lithographic exposure
radiation within an enclosure; preparing an interior of the
enclosure for transmitting the exposure radiation such that said
interior is substantially free of sub-200 nm photoabsorbing
species, and wherein said exposure radiation is directed along a
beam path within said enclosure and is thereby protected from being
substantially attenuated due to the presence of said sub-200 nm
photoabsorbing species, and wherein said exposure radiation is
thereby directed along said beam path within said enclosure and not
substantially attenuated by said sub-200 nm photoabsorbing
species.
26. A method of delivering sub-200 nm lithographic exposure
radiation which is generated by a lithographic exposure radiation
source for use at an application process, comprising the steps of:
sealing off a beam path of the sub-200 nm lithographic exposure
radiation within an enclosure; disposing at least one optical
element within said enclosure; preparing an interior of the
enclosure for transmitting the exposure radiation such that said
interior is substantially free of contaminant species, and wherein
said exposure radiation is directed along a beam path within said
enclosure and is thereby protected from being substantially
disturbed by said contaminant species; and interacting said
exposure radiation with said at least one optical component within
said enclosure, wherein said exposure radiation is thereby directed
along said beam path within said enclosure and not substantially
disturbed by said contaminant species.
27. The method of claim 26, wherein said at least one optical
component includes a diffraction grating.
28. The method of claim 27, wherein said interacting step includes
the step of dispersing said exposure radiation such that only a
selected portion of a spectral distribution of said exposure
radiation continues to propagate along said beam path and other
portions of said spectral distribution of said exposure radiation
are dispersed away from said beam path.
Description
PRIORITY
[0001] This application is a divisional application filed under 37
C.F.R. 1.53(b) which claims the benefit of priority to U.S. patent
application Ser. No. 09/598,522, filed Jun. 21, 2000, which claims
the benefit of priority to U.S. provisional patent application No.
60/140,530, filed Jun. 23, 1999, which is hereby incorporated by
reference, and which is also a Continuation-in-Part of U.S. patent
application Ser. No. 09/343,333, filed Jun. 30, 1999, now U.S. Pat.
No. 6,219,368, which claims the benefit of priority to U.S.
provisional patent application No. 60/119,973, filed Feb. 12,
1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to on-line control of the output power
of a molecular fluorine laser beam, and particularly to a technique
for redirecting VUV light of the beam to a VUV detector, while
filtering visible light from the redirected beam.
[0004] 2. Discussion of the Related Art
[0005] The molecular fluorine laser emitting at 157 nm has an
advantageously short wavelength, or high photon energy. Because of
this, very small structures, such as sub-0.18 micron structures and
even sub-0.10 micron structures, may be formed by photolithographic
exposure on semiconductor substrates. TFT annealing and
micro-machining applications may also be performed advantageously
at this wavelength.
[0006] For the applications mentioned above, on-line monitoring and
control of the output power of the laser may be advantageously
performed such that the energy stability of the output beam and
overall performance of the laser may be enhanced. For this purpose,
an energy or power detector may be configured to receive a split
off portion of the output beam. The input voltage and other
conditions such as the gas mixture composition may be actively
adjusted depending on the measured pulse energy, energy dose or
moving average energy in order to provide high stability.
[0007] There are several factors inhibiting use of conventional
light detectors for on-line monitoring of VUV laser output. First,
laser radiation below 200 nm is strongly absorbed in the
atmosphere, e.g., by water vapor, oxygen, hydrocarbons, and
fluorocarbons. Specifically, at 157 nm, the extinction length of a
molecular fluorine laser beam is around 1 mm or less in ambient air
due mostly to the presence of oxygen and water vapor in the air.
Second, contaminants such as oil vapors and other organic
substances generated, for instance, by vacuum pumps and plastic
enclosures tend to form films on optical surfaces causing strong
absorption. Third, the molecular fluorine laser generates, in
addition to 157 nm light, radiation in the red part of the visible
spectrum, between 600 and 800 nm, due to emission by excited atomic
fluorine species in the laser gas mixture. This red emission is
sensed by most optical detectors whose sensitivity tends to be
higher in the visible part of the spectrum, as compared to that in
the VUV range, i.e., at 157 nm.
SUMMARY OF THE INVENTION
[0008] It is therefore an object of the invention to provide a
method and apparatus for detecting output power of a molecular
fluorine laser beam without the beam being substantially absorbed
as it propagates to the detector.
[0009] It is a further object of the invention to provide a method
and apparatus for detecting the VUV output of a molecular fluorine
laser while any accompanying visible output of the laser is
substantially suppressed before reaching the detector.
[0010] In accord with the above objects, a beam delivery system for
a laser emitting at a relevant wavelength of less than 200 nm is
provided. The system includes a sealed enclosure surrounding the
path of the beam as it exits the laser resonator. The enclosure
extends between the laser output coupler and a photodetector
sensitive at the wavelength of the relevant laser emission. The
interior of the enclosure, and thus the beam path between the
output coupler and the detector, is substantially free of species
that strongly photoabsorb radiation at the relevant laser emission
wavelength. A beam splitting element diverts at least a portion of
the beam for measurement by the detector.
[0011] The beam splitting element preferably includes a beam
splitting mirror, holographic beam sampler or diffraction grating.
In addition, optics are preferably provided for filtering a visible
portion of the diverted beam, so that substantially only a VUV
portion of the diverted beam is received at the detector. The
filtering optics preferably include a diffraction grating,
holographic beam sampler or dichroic mirrors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 schematically illustrates a molecular fluorine laser
system in accord with a preferred embodiment.
[0013] FIG. 2 schematically illustrates a beam path enclosure in
accord with a first preferred embodiment.
[0014] FIG. 3 shows plots of measured laser output power versus
time for a molecular fluorine laser system including a beam path
enclosure having an evacuated interior and an enclosure purged with
a steady flow of inert gas in accord with a preferred
embodiment.
[0015] FIG. 4 schematically illustrates a beam path enclosure in
accord with a second preferred embodiment.
[0016] FIG. 5 schematically illustrates a beam path enclosure in
accord with a third preferred embodiment.
[0017] FIG. 6 schematically illustrates a beam path enclosure in
accord with a fourth preferred embodiment.
[0018] FIGS. 7a and 7b schematically illustrate alternative beam
splitter configurations to the first and third preferred
embodiments of FIGS. 2 and 5, respectively.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] The preferred embodiments described below provide means of
on-line monitoring of the output power of a vacuum UV laser,
specifically a molecular fluorine laser, operating in a wavelength
range below 200 nm. Preferred and alternative embodiments described
below further provide means of minimizing variations of sensitivity
of VUV laser energy monitor due to absorption, as well as
suppressing a visible red portion of the output. The former is
generally achieved by providing a hermetic enclosure which is
preferably purged with an inert gas. The latter is preferably
provided by one of three techniques including the use of a
diffraction grating, a dichroic thin-film dielectric mirror
arrangement, or a holographic beam sampler.
[0020] Referring to FIG. 1, a VUV laser system, preferably a
molecular fluorine laser for deep ultraviolet (DUV) or vacuum
ultraviolet (VUV) lithography, is schematically shown. Alternative
configurations for laser systems for use in such other industrial
applications as TFT annealing and/or micromachining, e.g., are
understood by one skilled in the art as being modified from the
system shown in FIG. 1 to meet the requirements of that
application. For this purpose, alternative VUV laser system and
component configurations are described at U.S. patent applications
Ser. Nos. 09/317,695, 09/317,526, 09/317,527, 09/343,333,
60/122,145, 60/140,531, 60/162,735, 60/166,952, 60/171,172,
60/141,678, 60/173,993, 60/166,967, 60/172,674, and 60/181,156, and
U.S. patent application of Kleinschmidt, Ser. No. not yet assigned,
filed May 18, 2000, for "Reduction of Laser Speckle in
Photolithography by Controlled Disruption of Spatial Coherence of
Laser Beam," and U.S. Pat. No. 6,005,880, each of which is assigned
to the same assignee as the present application and is hereby
incorporated by reference.
[0021] The system shown in FIG. 1 generally includes a laser
chamber 2 having a pair of main discharge electrodes 3 connected
with a solid-state pulser module 4, and a gas handling module 6.
The solid-state pulser module 4 is powered by a high voltage power
supply 8. The laser chamber 2 is surrounded by optics module 10 and
optics module 12, forming a resonator. The optics modules 10 and 12
are controlled by an optics control module 14.
[0022] A computer 16 for laser control receives various inputs and
controls various operating parameters of the system. A diagnostic
module 18 receives and measures various parameters of a split off
portion of the main beam 20 via optics for deflecting a small
portion of the beam toward the module 18, such as preferably a beam
splitter module 21, as shown. The beam 20 is preferably the laser
output to an imaging system (not shown) and ultimately to a
workpiece (also not shown). The laser control computer 16
communicates through an interface 24 with a stepper/scanner
computer 26 and other control units 28.
[0023] The laser chamber 2 contains a laser gas mixture and
includes a pair of main discharge electrodes and one or more
preionization electrodes (not shown). Preferred main electrodes 3
are described at U.S. patent applications Ser. Nos. 09/453,670,
60/184,705 and 60/128,227, each of which is assigned to the same
assignee as the present application and is hereby incorporated by
reference. Other electrode configurations are set forth at U.S.
Pat. Nos. 5,729,565 and 4,860,300, each of which is assigned to the
same assignee, and alternative embodiments are set forth at U.S.
Pat. Nos. 4,691,322, 5,535,233 and 5,557,629, all of which are
hereby incorporated by reference. The laser chamber 2 also includes
a preionization arrangement (not shown). Preferred preionization
units are set forth at U.S. patent applications Ser. Nos.
60,162,845, 60/160,182, 60/127,237, 09/535,276 and 09/247,887, each
of which is assigned to the same assignee as the present
application, and alternative embodiments are set forth at U.S. Pat.
Nos. 5,337,330, 5,818,865 and 5,991,324, all of the above
preionization units being hereby incorporated by reference.
[0024] The solid-state pulser module 14 and high voltage power
supply 8 supply electrical energy in compressed electrical pulses
to the preionization and main electrodes within the laser chamber 2
to energize the gas mixture. The preferred pulser module and high
voltage power supply are described at U.S. patent applications Ser.
Nos. 60/149,392, 60/198,058, and 09/390,146, and U.S. patent
application of Osmanow, et al., serial number not yet assigned,
filed May 15, 2000, for "Electrical Excitation Circuit for Pulsed
Laser", and U.S. Pat. Nos. 6,005,880 and 6,020,723, each of which
is assigned to the same assignee as the present application and
which is hereby incorporated by reference into the present
application. Other alternative pulser modules are described at U.S.
Pat. Nos. 5,982,800, 5,982,795, 5,940,421, 5,914,974, 5,949,806,
5,936,988, 6,028,872 and 5,729,562, each of which is hereby
incorporated by reference. A conventional pulser module may
generate electrical pulses in excess of 3 Joules of electrical
power (see the '988 patent, mentioned above).
[0025] The laser resonator which surrounds the laser chamber 2
containing the laser gas mixture includes optics module 10
including line-narrowing optics for a line narrowed excimer or
molecular fluorine laser, which may be replaced by a high
reflectivity mirror or the like if line-narrowing is not desired.
Exemplary line-narrowing optics of the optics module 10 include a
beam expander, an optional etalon and a diffraction grating, which
produces a relatively high degree of dispersion, for a narrow band
laser such as is used with a refractive or catadioptric optical
lithography imaging system. For a semi-narrow band laser such as is
used with an all-reflective imaging system, the grating is replaced
with a highly reflective mirror, and a lower degree of dispersion
may be produced by a dispersive prism.
[0026] The beam expander of the line-narrowing optics of the optics
module 10 typically includes one or more prisms. The beam expander
may include other beam expanding optics such as a lens assembly or
a converging/diverging lens pair. The grating or highly reflective
mirror is preferably rotatable so that the wavelengths reflected
into the acceptance angle of the resonator can be selected or
tuned. The grating is typically used, particularly in KrF and ArF
lasers, both for achieving narrow bandwidths and also often for
retroreflecting the beam back toward the laser tube. One or more
dispersive prisms may also be used, and more than one etalon may be
used.
[0027] Depending on the type and extent of line-narrowing and/or
selection and tuning that is desired, and the particular laser that
the line-narrowing optics of the optics module 10 is to be
installed into, there are many alternative optical configurations
that may be used. For this purpose, those shown in U.S. Pat. Nos.
4,399,540, 4,905,243, 5,226,050, 5,559,816, 5,659,419, 5,663,973,
5,761,236, and 5,946,337, and U.S. patent applications Ser. Nos.
09/317,695, 09/130,277, 09/244,554, 09/317,527, 09/073,070,
60/124,241, 60/140,532, 60/147,219 and 60/140,531, 60/147,219,
60/170,342, 60/172,749, 60/178,620, 60/173,993, 60/166,277,
60/166,967, 60/167,835, 60/170,919, 60/186,096, each of which is
assigned to the same assignee as the present application, and U.S.
Pat. Nos. 5,095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991,
5,898,725, 5,901,163, 5,917,849, 5,970,082, 5,404,366, 4,975,919,
5,142,543, 5,596,596, 5,802,094, 4,856,018, 5,970,082, 5,978,409,
5,999,318, 5,150,370 and 4,829,536, are each hereby incorporated by
reference into the present application.
[0028] Optics module 12 preferably includes means for outcoupling
the beam 20, such as a partially reflective resonator reflector.
The beam 20 may be otherwise outcoupled such as by an
intraresonator beam splitter or partially reflecting surface of
another optical element, and the optics module 12 would in this
case include a highly reflective mirror. The optics control module
14 controls the optics modules 10 and 12 such as by receiving and
interpretting signals from the processor 16, and initiating
realignment or reconfiguration procedures (see the '241, '695, 277,
554, and 527 applications mentioned above).
[0029] The laser chamber 2 is sealed by windows transparent to the
wavelengths of the emitted laser radiation 14. The windows may be
Brewster windows or may be aligned at another angle to the optical
path of the resonating beam. The beam path between the laser
chamber and each of the optics modules 10 and 1 2 is sealed by
enclosures 17 and 19, and the interiors of the enclosures is
substantially free of water vapor, oxygen, hydrocarbons,
fluorocarbons and the like which otherwise strongly absorb VUV
laser radiation.
[0030] After a portion of the output beam 20 passes the outcoupler
of the optics module 12, that output portion impinges upon beam
splitter module 21 which includes optics for deflecting a portion
of the beam to the diagnostic module 18, or otherwise allowing a
small portion of the outcoupled beam to reach the diagnostic module
18, while a main beam portion 20 is allowed to continue as the
output beam 20 of the laser system. Preferred optics include a
beamsplitter or otherwise partially reflecting surface optic. The
optics may also include a mirror or beam splitter as a second
reflecting optic. More than one beam splitter and/or HR mirror(s),
and/or dichroic mirror(s) may be used to direct portions of the
beam to components of the diagnostic module 18. A holographic beam
sampler, transmission grating, partially transmissive reflection
diffraction grating, grism, prism or other refractive, dispersive
and/or transmissive optic or optics may also be used to separate a
small beam portion 22 from the main beam 20 for detection at the
diagnostic module 18, while allowing most of the main beam 20 to
reach an application process directly or via an imaging system or
otherwise. The output beam 20 may be transmitted at the beam
splitter module while a reflected beam portion 22 is directed at
the diagnostic module 18, or the main beam 20 may be reflected,
while a small portion 22 is transmitted to the diagnostic module
18. The portion of the outcoupled beam which continues past the
beam splitter module 21 is the output beam 20 of the laser, which
propagates toward an industrial or experimental application such as
an imaging system and workpiece for photolithographic
applications.
[0031] An enclosure 23 seals the beam path of the beams 22 and 20
such as to keep the beam paths free of photoabsorbing species. The
enclosure 23 and beam splitting module 21 will be described in more
detail below with respect to FIGS. 2-7. For example, the beam
splitting module 21 preferably also includes optics for filtering
visible red light from the beam 22 so that substantially only VUV
light is received at a detector of the diagnostic module 18. Also,
an inert gas purge is preferably flowing through the enclosure
23.
[0032] The diagnostic module 18 preferably includes at least one
energy detector. This detector measures the total energy of the
beam portion that corresponds directly to the energy of the output
beam 20. An optical configuration such as an optical attenuator,
e.g., a plate or a coating, or other optics may be formed on or
near the detector or beam splitter module 21 to control the
intensity, spectral distribution and/or other parameters of the
radiation impinging upon the detector (see U.S. patent applications
Ser. Nos. 09/172,805, 60/172,749, 60/166,952 and 60/178,620, each
of which is assigned to the same assignee as the present
application and is hereby incorporated by reference).
[0033] One other component of the diagnostic module 18 is
preferably a wavelength and/or bandwidth detection component such
as a monitor etalon or grating spectrometer (see U.S. patent
applications Ser. Nos. 09/416,344, 60/186,003, 60/158,808, and
60/186,096, and Lokai, et al., serial number not yet assigned,
"Absolute Wavelength Calibration of Lithography Laser Using
Multiple Element or Tandem See Through Hollow Cathode Lamp", filed
May 10, 2000, each of which is assigned to the same assignee as the
present application, and U.S. Pat. Nos. 4,905,243, 5,978,391,
5,450,207, 4,926,428, 5,748,346, 5,025,445, and 5,978,394, all of
the above wavelength and/or bandwidth detection and monitoring
components being hereby incorporated by reference.
[0034] Other components of the diagnostic module may include a
pulse shape detector or ASE detector, such as are described at U.S.
patent applications Ser. Nos. 09/484,818 and 09/418,052,
respectively, each of which is assigned to the same assignee as the
present application and is hereby incorporated by reference, such
as for gas control and/or output beam energy stabilization. There
may be a beam alignment monitor, e.g., such as is described at U.S.
Pat. No. 6,014,206 which is hereby incorporated by reference.
[0035] The processor or control computer 16 receives and processes
values of some of the pulse shape, energy, amplified spontaneous
emission (ASE), energy stability, energy overshoot for burst mode
operation, wavelength, spectral purity and/or bandwidth, among
other input or output parameters of the laser system and output
beam. The processor 16 also controls the line narrowing module to
tune the wavelength and/or bandwidth or spectral purity, and
controls the power supply and pulser module 4 and 8 to control
preferably the moving average pulse power or energy, such that the
energy dose at points on the workpiece is stabilized around a
desired value. In addition, the computer 16 controls the gas
handling module 6 which includes gas supply valves connected to
various gas sources.
[0036] The laser gas mixture is initially filled into the laser
chamber 2 during new fills. The gas composition for a very stable
excimer laser in accord with the preferred embodiment uses helium
or neon or a mixture of helium and neon as buffer gas, depending on
the laser. Preferred gas composition are described at U.S. Pat.
Nos. 4,393,405 and 4,977,573 and U.S. patent applications Ser. Nos.
09/317,526, 09/513,025, 60/124,785, 09/418,052, 60/159,525 and
60/160,126, each of which is assigned to the same assignee and is
hereby incorporated by reference into the present application. The
concentration of the fluorine in the gas mixture may range from
0.003% to 1.00%, and is preferably around 0.1%. An additional gas
additive, such as a rare gas, may be added for increased energy
stability and/or as an attenuator as described in the '025
application, mentioned above. Specifically, for the F2-laser, an
addition of Xenon and/or Argon may be used. The concentration of
xenon or argon in the mixture may range from 0.0001% to 0.1%. For
an ArF-laser, an addition of xenon or krypton may be used also
having a concentration between 0.0001% to 0.1%.
[0037] Halogen and rare gas injections, total pressure adjustments
and gas replacement procedures are performed using the gas handling
module 6 preferably including a vacuum pump, a valve network and
one or more gas compartments. The gas handling module 6 receives
gas via gas lines connected to gas containers, tanks, canisters
and/or bottles. Preferred gas handling and/or replenishment
procedures of the preferred embodiment, other than as specifically
described herein, are described at U.S. Pat. Nos. 4,977,573 and
5,396,514 and U.S. patent applications Ser. Nos. 60/124,785,
09/418,052, 09/379,034, 60/171,717, and 60/159,525, each of which
is assigned to the same assignee as the present application, and
U.S. Pat. Nos. 5,978,406, 6,014,398 and 6,028,880, all of which are
hereby incorporated by reference. A Xe gas supply may be included
either internal or external to the laser system according to the
'025 application, mentioned above.
[0038] Referring now to FIG. 2, a first preferred embodiment of a
beam delivery system includes the enclosure 23, mentioned briefly
above, which seals the beam paths of the beams 20 and 22 everywhere
after the beam is outcoupled from the laser system until the beam
20 reaches the application process 30. The enclosure 23 is
maintained substantially free of VUV photoabsorbing species such as
water vapor, oxygen, hydrocarbons and fluorocarbons preferably by a
method as set forth at U.S. patent application No. 09/343,333,
incorporated by reference above.
[0039] Briefly, the preferred method, as described in more detail
in the '333 application, is a method wherein the enclosure 23 is
first pumped down to a rough vacuum, e.g., using a mechanical
roughing pump, such as a rotary vane pump. Next, an inert gas is
purged into the enclosure 23. The vacuum/purging steps are
preferably performed some optimal number of times, such as from one
to ten times, balancing the removal of photoabsorbing impurities in
the enclosure 23 with the time it takes to perform those steps.
Then, an inert gas is flowed at a slight overpressure (e.g.,<50
mbar) using gas inlets 32a and 32b and gas outlet 34. The VUV laser
is operated with the inert gas flowing at the slight overpressure.
The above method is preferred as being time and cost efficient. Two
alternative methods which may, however, be used for keeping the
beam path substantially free of photoabsorbing species are pumping
the enclosure 23 to high vacuum, and flowing an inert gas at high
flow rate through the enclosure 23.
[0040] The application process 30 may include a separate housing
for the workpiece and/or additional optical equipment such as an
imaging system, or may be the workpiece itself. Two reflectors 36a
and 36b, preferably both being beam splitters or one reflector 36a
being a beam splitter and the other reflector 36b being a mirror,
are shown for splitting off beam 22 and allowing the substantial
portion of the beam 20 to pass through unhindered towards the
application process 30. The beam 22 is directed ultimately to the
detector 38, preferably via a collecting lens, grids and a diffuser
(collectively 40), and a signal 42 corresponding to the energy
measured is sent to a processor (not shown) or other data
acquisition equipment using a vacuum feedthrough 44. A visible red
light portion of the beam 22 is preferably first filtered such that
substantially only the VUV portion of the beam 22 reaches the
detector 38, as described in more detail below.
[0041] The reflectors 36a and 36b preferably each comprise uncoated
plates made of excimer grade CaF.sub.2, MgF.sub.2 quartz, fused
silica, doped fused silica, LiF, BaF.sub.2, or other material that
is mostly transparent to VUV radiation. In this case, the
reflectivity of each reflector 36a and 36b is preferably
approximately 3-15%, e.g., 8%. Additional dielectric coatings can
be deposited onto preferred reflectors 36a and 36b in order to
reduce or increase reflectivity. However, uncoated surfaces allow
the preferred reflectors 36a and 36b to have longer lifetimes than
those with coated surfaces.
[0042] The incidence angles of the beam onto the preferred
reflectors 36a and 36b are preferably relatively small, in order to
reduce the dependence of the reflectivity on the polarization of
the incident laser beam, as explained below. The reflectivity of
the uncoated surface for p- and s-polarized beams is described by
Fresnel's formulas:
R.sub.s=sin.sup.2(.phi.-.phi.')/sin.sup.2( +.phi.'),
R.sub.p=tan.sup.2(.phi.-.phi.')/tan.sup.2(.phi.+.phi.'),
[0043] where incident and refracted angles .phi. and .phi.' are
approximately related through the formula:
sin(.phi.)=n.multidot.sin(.phi.'),
[0044] where n is the refractive index of the material.
[0045] Thus, for the angles that approach Brewster's angle
.phi..sub.B=arctan(n), the reflectivity of the p-component
decreases to zero, while s-components experience an increase in
reflectivity. For example, for materials such as CaF.sub.2 or
MgF.sub.2 with refractive indices of approximately 1.5, Brewster's
angle .phi..sub.B is approximately 56.degree. . At 45.degree.
incidence, the ratio of reflectivities for s- and p-polarized beams
is still as high as 10.5. One should preferably avoid such contrast
since in the case of p-polarized laser output, small changes of
polarization state can cause large errors in energy readings.
Therefore, the incidence angles are preferably limited to less than
22.5.degree..
[0046] The reflectors 36a and 36b direct the beam 22 at an
appropriate angle to the diffraction grating 46. The grating 46
shown is a reflection grating 46. An alternative configuration may
include a transmission grating. A grism may also alternatively be
used preferably made of CaF.sub.2 or another of the VUV transparent
materials set forth above.
[0047] The grating 46 provides separation of the VUV beam from the
red portion of the beam 22. The incidence and reflection angles
.theta..sub.i and .theta..sub.r into/from the diffraction grating
46 are related through the formula:
sin(.theta..sub.i)-sin(.theta..sub.r)=m.lambda./d
[0048] where .lambda. is the wavelength, m is diffraction order
(m=0, +/-1, +/-2 . . . ) and d is the periodic spacing of the
grooves of the grating. For example, a typical grating with the
groove density of 1200 grooves/mm and an incidence angle of
11.degree., zeroth- and first-order reflected beams at 157 nm will
be at -11.degree. and zero.degree., respectively. At the same time,
the nearest angles of reflection for the red light of the
wavelength of approximately 700 nm will be around -11.degree. and
40.5.degree. for zeroth-and-first orders, respectively. Thereafter,
one can separate the VUV and red portions of the beam 22 by using
an aperture in front of the detector as shown in FIGS. 2, 4, 5 or
6.
[0049] Collecting lens and diffuser, of the assembly 40 which also
includes grids, described below, should be preferably made of one
of the materials mentioned above as a choice for preferred
beam-splitters 36a and 36b. The diffuser serves to attenuate the
beam and also to decrease dependence of the overall sensitivity on
the beam alignment. The attenuator grids are preferably fine-pitch
stainless steel meshes. These serve as additional diffusers and
attenuators, and additionally provide shielding of the detector
against electromagnetic interference. Additional beam shaping
optics, such as an aperture 47, may be includes, e.g., as set forth
at U.S. patent application Ser. No. 60/172,749, which is assigned
to the same assignee and is hereby incorporated by reference.
[0050] Preferably, optical components 40 and detector 38 are
encased into the enclosure 23, as shown, or in a separately
hermetically sealed housing with inert gas purging, having an
entrance window for the beam 22. It has been observed that when
such enclosure is evacuated, there tends to occur a build-up of
hydrocarbon film on the optical elements exposed to the UV beam.
This is likely caused by polymerization of organic molecules
present in low-grade vacuum. Instead of providing high-vacuum
enclosure, it is preferred to arrange purging, as described above
(see the '333 application) with clean inert gas (such as nitrogen,
helium, argon, neon and others) at a flow rate preferably around 5
liters/min or less.
[0051] Experimentally, it has been observed that purging improves
stability of the laser output by at least an order of magnitude, as
shown in FIG. 3. FIG. 3 shows the output power of a laser in accord
with the preferred embodiment of FIGS. 1 and 2. Plot 1 shows the
output power when inert gas purging is used, and plot 2 shows the
output power when an evacuated housing is used. Plot 1 shows the
output power stabilized around 2.2 W over about 2.5 hours, while
plot 2 shows the output power decreasing from around 2.8 W to
around 2.5 W over the same period. Thus the energy stability
observed with inert gas purging is far better than with an
evacuated housing.
[0052] The gas flow path is also preferably arranged in such a way
as to minimize or avoid any "dead", un-purged spaces in the
enclosure 23 of FIG. 2. For example, an additional gas inlet 32b is
preferably provided as shown in FIG. 2 to the chamber encasing the
detector and separated from the main beam path by grid attenuators.
The collecting lens and diffuser are preferably mounted so that
there are vent holes around them. Among mentioned above inert
gases, it is preferred to use ultra-high purity argon, for the
reason of its relatively low cost, as compared to helium and neon.
Nitrogen of ultra-high purity grade typically contains higher
levels of impurities as compared to UHP-helium and neon and,
therefore, is less suitable for purging.
[0053] The detector 38 may be one of, but is not limited to, a
silicon photodiode, pyroelectric, thermopile, electron phototube,
photomultiplier, CCD-detector, or diamond detector as set forth in
U.S. patent application Ser. No. 60/122,145, which is assigned to
the same assignee as is hereby incorporated by reference.
Preference is based on the lifetime, sensitivity, time resolution
and cost.
[0054] The diffraction grating 46 is preferably aluminum-coated and
protected with the thin layer of MgF.sub.2, and may be otherwise as
may be known to one skilled in the art of UV diffraction gratings.
The grating may be one of those described at U.S. patent
application Ser. No. 60/167,835, which is assigned to the same
assignee, and U.S. Pat. No. 5,999,318, each of which is hereby
incorporated by reference. Sides of the grating should be carefully
protected from stray UV light by appropriate shields, for example
made of aluminum foil. The purpose of the shields is to prevent
degradation and outgassing of organic materials beneath the
aluminum layer which are commonly used in the process of
replication of gratings.
[0055] Referring to FIG. 4, the second embodiment is preferably the
same or similar to the first embodiment shown and described with
respect to FIG. 2, except that the second embodiment shown at FIG.
4 utilizes a holographic beam sampler 48 (for example: HBS-series
from Gentec Electro-optics, Sainte-Foy, Quebec, Canada).
Holographic beam sampler 48 is preferably a transmissive
diffraction grating formed on a transparent substrate (see above
for the choice of materials transparent in the VUV range).
Advantages of the holographic beam sampler 48 include: (1) only
very small portion of the beam energy is split off
(typically.about.0.1%), therefore, insertion losses are very low,
e.g., as compared to.about.8% for conventional beam-splitter, and
(2) wavelength separation is achieved at the same time, since the
diffraction angle for the red portion of the beam is different from
that of the VUV component, thus making the design simple and
robust. A disadvantage of the holographic sampler 48, however, is
its higher cost. The choice of preferred materials for the
diffractive beam sampler is dictated by its transparency in VUV
range and radiation hardness. Examples of such materials are
CaF.sub.2, MgF.sub.2, quartz, fused silica, doped fused silica,
LiF, BaF.sub.2.
[0056] The VUV portion of the beam 20 that is diffracted at the
holographic beam sampler 48 is directed to a reflector 50 such as a
VUV mirror or beamsplitter. The reflector directs the VUV light
toward the assembly 40 and detector 38 The reflector 50 is designed
for maximum reflectance at VUV wavelengths. The reflector 50 may be
at least partly transmissive at visible wavelengths to prevent or
minimize red light reflection towards the detector. A copper shield
may be provided around the reflector 50 to absorb this red light,
e.g., so that the red light is not otherwise reflected within the
enclosure towards the detector 38. An example of such an
arrangement of the reflector 50 is described at U.S. patent
application Ser. No. 60/166,952, which is assigned to the same
assignee and is hereby incorporated by reference.
[0057] Referring to FIG. 5, the third embodiment is the same or
similar to that shown and described with respect to the first
embodiment of FIG. 2, except that the third embodiment of FIG. 5
utilizes dichroic dielectric mirrors 52 in order to achieve
separation of the VUV beam from the red portion of the laser
output. In the third embodiment shown in FIG. 5, one beam-splitter
36a and two dichroic mirrors 52 are preferably used. The dichroic
mirrors 52 are preferably formed by depositing thin quarter-wave
layers of dielectrics with alternating high and low refractive
index, so that VUV beam is mostly reflected and red light is almost
completely transmitted. Other details of dichroic mirrors 52 are
understood by those skilled in the art. Typically, a contrast ratio
between the reflectance of the VUV light and the red light of
better than 30 can be achieved. The choice of the number of mirrors
is determined by the suppression ratio desired for reducing the
signal caused by the red component, e.g., below 1.0% or less. Two
mirrors will typically provide at least two orders of magnitude
contrast ratio.
[0058] Referring to FIG. 6, the fourth embodiment is an alternative
variation of the first embodiment, and as such, is the same as or
similar to the first embodiment of FIG. 2, except that the fourth
embodiment of FIG. 6 includes only one beam splitter 36a and a
grating 46. In the arrangement of FIG. 6, the intensity of the
optical signal to the detector 38 is increased compared to the
first embodiment of FIG. 2. This may be desired if the sensitivity
of the detector 38 is otherwise insufficient at a given output
power of the laser. Alternatively, three or more beamsplitters 36a,
36b, 36c, etc., can be employed, with the advantage is some
circumstances that a reduction in signal to the detector 38 may be
achieved. In doing so, an advantage of reducing the intensity of
the beam at the diffraction grating 46, and the assembly 40,
particularly including the collecting lens and attenuator grids, is
that the lifetimes of these components may be increased. At the
same time, decreasing in the intensity of the beam sample can lead
to a lower signal-to-noise ratio if the noise is dominated by
scattered light inside the housing of the energy detector 38.
Therefore, depending on the output power of the VUV laser and
sensitivity of the detector 38, there is some optimum number of the
beamsplitters 36a, etc. that may be selected. These considerations
apply to the second and third embodiment as well as to the first
embodiment.
[0059] FIGS. 7a and 7b show alternative arrangements of
beam-splitters that can be utilized in the first and third
embodiments, respectively, preferably when the laser output is
polarized. In both FIG. 7a and FIG. 7b, two reflectors 36a and 36b,
preferably both beamsplitters, are used. The reflectors 36a and 36b
are aligned to eliminate deviations due to polarization
fluctuations and preferences based on differing reflectivities for
the orthogonal polarization components.
[0060] Generally, the first beam splitter 36a and the second
reflector 36b are aligned so that the polarization dependence of
the reflectivity of the first beam splitter 36a is compensated by
the polarization dependence of the reflectivity of the second
reflector 36b. For example, the first beam splitter 36a may be
aligned to reflect the p-polarization component of the incident
beam at 10% of the efficiency of the s-polarization component. The
second reflector 36b is then aligned to reflect the component
corresponding to the s-polarization component incident at the first
beam splitter 36a at 10% of the efficiency of its orthogonal
counterpart corresponding to the p-polarization component incident
at the first beam splitter 36a. Thus, the overall dependence on the
polarization of the output beam of the reflectivity of the first
beam splitter 36a-second reflector 36b combination is reduced or
eliminated.
[0061] Preferably, the first reflector 36a of both FIGS. 7a and 7b
is oriented in such a way that in the case that the incident laser
beam is polarized, the beam is p-polarized with respect to this
first beam-splitter 36a. The second reflector 36b of both FIGS. 7a
and 7b is preferably oriented in a perpendicular plane to the first
reflector 36a, so that the p-component of laser beam reflected from
the first reflector 36a is s-polarized with respect to the second
reflector 36b. Preferably, each reflector 36a and 36b reflects the
beam at an incidence angle of substantially 45 degrees.
[0062] An additional advantage of this configuration of the
reflectors 36a and 36b of both FIGS. 7a and 7b is that the
reflectivity of the first reflector 36a for a polarized laser beam
is significantly reduced, typically from about 4% to 0.1%.
Therefore, more of the beam power is available for the application.
At the same time, the above explained advantage of polarization
selectivity of the first reflector 36a is compensated by the
inverse selectivity of the second reflector 36b, since p- and
s-components of the incident beam become s- and p-components,
respectively, at the second reflector 36b.
[0063] While exemplary drawings and specific embodiments of the
present invention have been described and illustrated, it is to be
understood that the scope of the present invention is not to be
limited to the particular embodiments discussed. Thus, the
embodiments shall be regarded as illustrative rather than
restrictive, and it should be understood that variations may be
made in those embodiments by workers skilled in the arts without
departing from the scope of the present invention as set forth in
the claims that follow, and equivalents thereof.
[0064] In addition, in the method claims that follow, the steps
have been ordered in selected typographical sequences. However, the
sequences have been selected and so ordered for typographical
convenience and are not intended to imply any particular order for
performing the steps, except for those claims wherein a particular
ordering of steps is expressly set forth or understood by one of
ordinary skill in the art as being necessary.
* * * * *