U.S. patent application number 09/826296 was filed with the patent office on 2001-10-18 for discharge unit for a high repetition rate excimer or molecular fluorine laser.
This patent application is currently assigned to Lambda Physik AG. Invention is credited to Berger, Vadim, Bragin, Igor, Rebhan, Ulrich, Stamm, Uwe.
Application Number | 20010030986 09/826296 |
Document ID | / |
Family ID | 26826406 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010030986 |
Kind Code |
A1 |
Bragin, Igor ; et
al. |
October 18, 2001 |
Discharge unit for a high repetition rate excimer or molecular
fluorine laser
Abstract
A laser for an excimer or molecular fluorine laser includes an
electrode chamber connected with a gas flow vessel and having a
pair of main electrodes and a preionization unit each connected to
a discharge circuit. A spoiler is provided within the electrode
chamber and is shaped to provide a more uniform gas flow through
the discharge area between the main electrodes, to shield one of
the preionization units from one of the main electrodes, and to
reflect acoustic waves generated in the discharge area into the gas
flow vessel for absorption therein. A spoiler unit may include a
pair of opposed spoiler elements on either side of the discharge
area. One or both main electrodes includes a base portion and a
center portion which may be a nipple protruding from the base
portion. The center portion substantially carries the periodic
discharge current such that the discharge width is and may be
significantly less than the width of the base portion. The
curvatures of both main electrodes may conform to the curvature of
the gas flow through the discharge chamber to further improve
aerodynamic performance. A plurality of low inductive conducting
ribs are connected to the grounded main electrode and shaped to
provide a more uniform flow of gases through openings defined
between adjacent ribs.
Inventors: |
Bragin, Igor; (Gottingen,
DE) ; Berger, Vadim; (Gottingen, DE) ; Stamm,
Uwe; (Gottingen, DE) ; Rebhan, Ulrich;
(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: |
26826406 |
Appl. No.: |
09/826296 |
Filed: |
April 3, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09826296 |
Apr 3, 2001 |
|
|
|
09453670 |
Dec 3, 1999 |
|
|
|
60128227 |
Apr 7, 1999 |
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Current U.S.
Class: |
372/57 ;
372/55 |
Current CPC
Class: |
H01S 3/225 20130101;
H01S 3/036 20130101; H01S 3/0971 20130101; H01S 3/038 20130101 |
Class at
Publication: |
372/57 ;
372/55 |
International
Class: |
H01S 003/22; H01S
003/223 |
Claims
What is claimed is:
1. An excimer or molecular fluorine laser, comprising: an electrode
chamber connected with a gas flow vessel defining a laser tube
having a laser gas mixture therein; a pair of elongated main
electrodes in the electrode chamber separated by a discharge area;
a preionization unit in the electrode chamber; a discharge circuit
for energizing the gas mixture; a resonator for generating a laser
beam; a spoiler integrated with the chamber, said spoiler being
spaced from each main electrode and shaped to provide an uniform
gas flow through the discharge area; and a dielectric insulator for
isolating one of the main electrodes, wherein the spoiler is
integrated with the dielectric insulator.
2. An excimer or molecular fluorine laser, comprising: an electrode
chamber connected with a gas flow vessel defining a laser tube
having a gas mixture therein; a pair of elongated main electrodes
in the electrode chamber separated by a discharge area; an
insulating frame for isolating one of the main electrodes, said
frame being aerodynamically shaped to provide a more uniform gas
flow through the discharge area; a preionization unit in the
electrode chamber; a discharge circuit for energizing the gas
mixture; and a resonator for generating a laser beam.
3. The laser of claim 2, wherein said aerodynamic shape is such
that opposing walls of said frame are inclined toward each
other.
4. The laser of claim 3, wherein the ends of said opposed walls
furthest from said gas flow vessel are inclined toward each
other.
5. The laser of claim 2, wherein said frame is shaped such that
said electrode chamber is trapezoidally shaped.
6. The laser of any of claims 2, 3 or 5, wherein the frame is
configured to dampen acoustic waves generated in the discharge
area.
7. The laser of any of claims 2, 3 or 5, wherein the frame is
configured to reflect acoustic waves generated in the discharge
area into the gas flow vessel to be absorbed by gas flow components
therein.
Description
PRIORITY
[0001] This patent application is a divisional application which
claims the benefit or priority to parent U.S. patent application
Ser. No. 09/453,670, filed Dec. 3, 1999, which claims the benefit
of priority to U.S. provisional patent application No. 60/128,227,
filed Apr. 7, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a discharge unit for an
excimer or molecular gas laser, particularly having a narrow
discharge width and aerodynamic gas flow.
[0004] 2. Discussion of the Related Art
[0005] Pulsed gas discharge lasers, emitting in the deep
ultraviolet region (DUV) and/or vacuum ultraviolet region (VUV),
are important tools for a wide range of industrial applications.
For example, microlithography applications currently use a line
narrowed excimer laser (e.g., ArF, KrF, XeCl, KrCl or XeF) or a
molecular fluorine (F.sub.2) laser having high efficiency and
stability at high repetition rates (e.g., 1000 Hz or more).
[0006] An electrode chamber design and electrode configuration of a
conventional discharge unit are illustrated in FIG. 1. The
electrode chamber of FIG. 1 houses a pair of elongated main
electrodes 2, 4. The main electrodes 2, 4 are separated by a gap or
discharge area 6 through which a gas mixture is flowed. A set of
high voltage capacitors or "peaking" capacitors Cp is preferably
positioned as close as possible to the main discharge electrodes 2,
4, and as uniformly as possible over the length of the electrodes
2, 4. One or two or more preionization units are used to preionize
the gas mixture in the discharge area 6 prior to the main
discharge.
[0007] One of the main electrodes, in this case electrode 2, is
connected to a pulsed high voltage generator. The high voltage
generator typically includes a thyratron or a solid state switch
for providing a fast and powerful charge to the peaking capacitors
8 up to the electrical breakdown voltage of the gas discharge gap
6. The other main electrode 4 is usually connected to ground
potential. Fast and powerful discharge of the peaking capacitors
Cp, followed by electrical breakdown of the active laser gases in
the gas mixture provides the necessary pumping of the gas
mixture.
[0008] The peaking capacitors in both cases are disposed outside of
the electrode chamber (that is not necessary, but common, because
it easily avoids exposure of the peaking capacitors to the
aggressive halogen gas). One of the main discharge electrodes, the
ground electrode, is connected directly to the metal body of the
electrode chamber. The other or high voltage electrode is connected
to the peaking capacitors and is separated from the grounded metal
body of the electrode chamber by means of a dielectric (e.g.,
ceramic) insulator.
[0009] The gas mixture is characterized as being strongly
electronegative and maintained at an elevated pressure (e.g., a few
bars). The gas mixture for an excimer laser includes an active rare
gas such as krypton, argon or xenon, a halogen containing species
such as fluorine or HCl, and a buffer gas such as neon or helium. A
molecular fluorine laser includes molecular fluorine and a buffer
gas such as neon and/or helium.
[0010] A typical preionization arrangement includes two
preionization units 10 each including a conducting electrode inside
a dielectric tube. The preionization units 10 are connected to a
pulsed high voltage source and preionize the gas mixture by forming
a uniform surface glow discharge. The preionization units 10 are
typically positioned in the vicinity of the discharge area 6
between the main electrodes 2, 4 and provide an initial ionization
of the laser gas during the charging of the peaking capacitors Cp
by the high voltage pulsed generator. UV-preionizers typically
include arrays of electrical sparks, sometimes stabilized by
dielectric surfaces, or other configurations of barrier or corona
discharge sources. Soft x-ray radiation sources are also sometimes
used.
[0011] Examples of preionization arrangements which could be used
for UV-preionization are shown in FIGS. 2a and 2b. FIG. 2a shows a
corona preionization arrangement including two corona units 10a.
Each corona unit 10a shown includes an cylindrical electrode 16
surrounded by a dielectric tube 18. An external electrode 20a
provides a potential difference for each preionization unit 10a.
The UV radiation emitted by the preionization units 10a preionizes
gaseous components within the discharge area 6.
[0012] FIG. 2b shows a cross section of a UV-spark preionization
arrangement wherein the preionization units 10b include separate
pins 22 surrounded by dielectrics 24. These pins 22 are fed-through
the chamber and connected to a pulsed power source outside the
chamber. A plurality of spark gaps 26 are formed due to a potential
difference between an electrode 20b in proximity to the pins 22 and
produces preionization of the gas in the discharge area 6.
[0013] Besides the discharge unit having a pulser circuit and a
laser tube including an electrode chamber such as that illustrated
in FIG. 1, the laser tube of the discharge unit further includes a
gas vessel 11 having a gas flow system or blower 12 and a heat
exchanger 14 as illustrated in FIG. 3. A vane 15 is also shown
extending from the blower 12 generally to the electrode 4 of the
discharge chamber. The blower 12 forces the gas to flow generally
as indicated by the arrows in FIG. 3. The gas mixture is naturally
heated as it is excited by the electrical discharge in the
discharge area 6. The heat exchanger 14 cools the heated gas after
it exits the electrode chamber. The portion of the gas mixture
which participates in a laser pulse is replaced by fresh gas before
the next laser pulse occurs. Although not shown, a gas supply unit
also typically supplies fresh gas to the system from outside gas
containers to replenish each of the components of the gas mixture.
In particular, halogen containing gas is typically supplied because
the halogen concentration in the gas mixture tends to deplete
rapidly during operation, while it is desired to maintain a
constant or near constant halogen concentration in the gas mixture.
Means for releasing some of the gas mixture is also typically
provided so that the gas pressure can be controlled and to expel
contaminated gases.
[0014] Above, various components of a pulsed gas discharge laser
such as an excimer or molecular laser have been discussed with
respect to their design and arrangement within the electrode
chamber. The design and placement of the electrode chamber itself
relative to the gas vessel 11, the placement of the peaking
capacitors Cp, and the insulation of the high voltage electrode 2
are further considerations in effective discharge unit design.
Examples of laser designs are illustrated in cross-sectional views
at the FIGS. 4a and 4b.
[0015] The discharge unit illustrated at FIG. 4a includes a
dielectric frame or one or two or more dielectric insulators 28
(see Industrial Excimer Lasers: Fundamentals, Technolgy and
Maintenance, Dirk Basting, Ed., 2.sup.nd edition (1991); Litho
laser tube of Lambda Physik, GmbH). Each dielectric insulator 28 is
mechanically connected to the gas vessel 11 that is connected to
the grounded discharge electrode 4. The dielectric frame or
insulator(s) 28 electrically isolate the high voltage electrode 2.
That is, the roof 31 connected to the high voltage electrode 2 is
insulated from the grounded main electrode 4 by the dielectric
insulator(s) 28.
[0016] Where the electrode chamber, e.g., as shown in FIG. 4a,
meets the gas vessel 11, an arrangement 30 of conducting ribs are
connected electrically to the grounded electrode 4. The rib
arrangement 30 of the discharge unit includes several rectangular
ribs 32 separated by openings to permit gas flow from the gas
vessel 11 into the electrode chamber and into the discharge area 6.
The relationship between the rectangular ribs 32 and the opening
separating them are illustrated at Fig. The ribs 32 serve as low
inductive current conductors in the discharge circuitry. A lower
inductivity of the discharge electrical current loop is
advantageous as better matching may be provided between the wave
impedance of the electrical discharge loop and the gas discharge
impedance.
[0017] The discharge unit of FIG. 4a advantageously allows the
discharge loop to exhibit a characteristically low inductivity.
However, the gas flow through the discharge area 6, and especially
near the grounded electrode 4, has a high curvature producing
turbulences that complicate the gas exchange in the discharge area
6.
[0018] Another consideration arises with respect to the nearly
rectangular interior shape of the electrode chamber. Powerful and
symmetric energy dissipation in the gas discharge area 6,
particularly when the system is operating at a high repetition
rate, can lead to acoustical resonances and amplification of the
level of standing acoustical waves. Modulation of the gas density
by the acoustical disturbances can have an adverse influence on the
uniformity of the gas discharge and ultimately on significant laser
output parameters.
[0019] One way to reduce the level of these acoustical disturbances
is to introduce acoustical dampers into the field of the acoustical
waves. These dampers may be used as obstacles for the acoustical
waves. However, the dampers can also have an adverse influence on
the uniformity of the gas flow. In addition, the dampers would have
large surface areas which are subject to attack by aggressive
halogens in the gas mixture.
[0020] FIG. 4b shows an alternative discharge unit design to that
illustrated at FIG. 4a (see U.S. Pat. Nos. 4,891,818 to Levatter
and 5,771,258 to Morton et al.). A dielectric insulator plate 33
separates the high voltage electrode from the metal walls of the
electrode chamber. The main electrodes 2, 4 are immersed in the gas
flow vessel 11. Electrical current return bars similar to the
rectangular ribs 32 of the arrangement of FIG. 4a may once again
cross the gas flow and shorten the discharge loop from the grounded
discharge electrode 4 to the walls of the laser tube. The gas
exchange conditions are improved over those discussed above with
respect to the arrangement of FIG. 4a.
[0021] The improved gas exchange conditions provided by the
arrangement of FIG. 4b are advantageous because satisfactory laser
operation may be achieved at lower gas flow rates, and strong and
uniform gas flow permits satisfactory operation at higher
repetition rates (see U.S. Pat. No. 5,247,534 to Muller-Horsche,
assigned to the same assignee as the present invention, and hereby
incorporated by reference). However, the connection of the high
voltage electrode 2 via the dielectric plate 33 implies the use of
a plurality of concentrated feedthroughs 34. This gives rise to an
undesirably higher inductivity of the electrical discharge current
loop.
[0022] FIG. 4f shows an alternative design. The insulators 128
shown in FIG. 4f conform with the gas flow.
[0023] Another consideration of discharge unit design is the main
electrodes 2, 4 themselves. Features of the main electrodes 2, 4
including their size, shape and proximity to each other and to
other elements within the electrode chamber such as the
preionization units determine important discharge conditions such
as the shape and uniformity of the static electrical field in the
discharge area 6 and the width of the discharge area 6.
[0024] In line narrowed lasers, used as illuminating sources for
microlithography, some additional considerations amplify the
desirability of minimizing the discharge width. One of these is the
design of the resonator assembly. The discharge width should be
reduced to a value commensurate with the effective aperture size of
the line narrowing resonator. For example, an effective aperture of
a linewidth narrowing resonator might be on the order of 3 to 4 mm
or less, and is typically around 2 mm. Thus, the discharge width
should be comparable to or less than this 3 to 4 mm
specification.
[0025] A narrower discharge width is also more suitable for laser
operation at higher repetition rates (e.g., 1 kHz or more). Yet
another advantage to having a narrow discharge width is that the
exchange of gases in the discharge area is simplified.
[0026] In combination with design considerations involving the
static field and discharge width parameters as discussed above, the
electrodes 2, 4 should have a minimized width to provide the most
compact and least inductive design possible of the gas discharge
electrical circuit. Analytical expressions for the shapes of the
electrodes 2, 4 have been proposed including a combination of
implicit hyperbolic functions (see T. Y. Yang, Improved
Uniform-Field Electrode Profiles for TEA Laser and High-Voltage
Applications, The Review of Scientific Instruments, vol. 41, no. 4
(April 1973); G. J. Ernst, Uniform-Field Electrodes with Minimum
Width, Optics Communications, vol. 49, no. 4 (Mar. 15, 1984); G. J.
Ernst, Compact Uniform-Field Electrode Profiles, Optics
Communications, vol. 47, no. 1 (Aug. 1, 1983)), and as a solution
of a system of ordinary differential equations (see E. A.
Stappaerts, A Novel Analytical Design Method for Discharge Laser
Electrode Profiles, Appl. Phys. Lett., 40(12) (Jun. 15, 1982)).
[0027] Typical approaches usually propose the electrodes 2, 4 to be
identical, each having a uniform regular shape with a minimal gap
between the middle portions of the electrodes 2, 4 and a gradually
increasing gap away from the middle portions to the edges. During
laser operation, the discharge will begin in these middle portions.
The real width of the gas discharge is also less than the width of
the electrodes 2, 4. For example, the discharge width might be 11
mm while the width of each electrode 2, 4 is around 30 mm. The
actual discharge width depends on many factors including the gas
mixture, the preionization technique used, the electrical circuitry
and the static electric field distribution.
[0028] The outer portions of the electrodes 2, 4, although carrying
little or no discharge current, contribute significantly to the
electrical field distribution in the vicinity of the discharge area
6. The fact that the outer portions of the electrodes 2, 4 carry
little or no discharge current may be used advantageously for other
considerations in the design of the electrodes 2, 4. For example,
the outer portions of the electrodes 2, 4 may comprise dielectric
materials such as ceramics to thereby prevent parasitic discharge
currents and to further restrict the discharge width (see H. Bucher
and H. Frowein, Elektrode fur einen Gasentladungslaser, Deutsches
Patent DE 4401892 A1 (Jul. 27, 1995)).
[0029] A known design choice (see U.S. Pat. No. 5,557,629 to
Mizoguchi et al. and No. 5,535,233 to Mizoguchi et al.) is to
provide at least one of the electrodes 2, 4 with an elliptical
shape such that the outer surface satisfies the relationship: 1 [ x
/ a ] 2 + [ y / b ] 2 = 1 , where 1 < a b < 4.
[0030] Another technique disclosed in the '629 and '233 patents is
shown in FIG. 5. In the design shown in FIG. 5, additional "easing"
electrodes 36 are positioned on either side of the main discharge
electrodes 2, 4.
SUMMARY OF THE INVENTION
[0031] It is an object of the present invention to provide an
efficient discharge unit for line narrowed excimer or molecular
fluorine lasers, operating at high repetition rates, such as are
used as illumination sources in microlithography applications.
[0032] It is also an object of the invention to provide a discharge
unit wherein the discharge circuit design including the placement
of peaking capacitors Cp exhibits a low inductivity.
[0033] It is a further object of the invention to provide a
discharge unit wherein gas flow conditions are optimized such that
the laser gas may flow rapidly and uniformly through the discharge
area between the main electrodes.
[0034] In accord with the above objects, in a first aspect of the
present invention, an electrode chamber of a laser for an excimer
or molecular fluorine laser is connected with a gas flow vessel,
and includes a pair of elongated main electrodes separated by a
discharge area, and a preionization unit. The electrode chamber
also includes a spoiler integrated with the chamber and spaced from
each of the main electrodes. The spoiler is shaped to provide an
aerodynamic gas flow through the discharge area. A spoiler unit may
include a pair of opposed spoiler elements each integrated with the
chamber on either side of the discharge area, wherein each spoiler
element is spaced from the main discharge electrodes and shaped to
provide an aerodynamic gas flow through the discharge area.
[0035] Also in accord with the objects of the invention, in a
second aspect of the present invention, a laser for an excimer or
molecular fluorine laser is provided including an electrode chamber
having a pair of elongated main electrodes separated by a discharge
area, and a preionization unit. In the electrode chamber, at least
one main electrode includes a base portion and a center portion
which may be a nipple protruding from the base portion. The nipple
substantially carries the periodic discharge current such that the
discharge width is reduced to the width of the nipple which may be
significantly less than the discharge width which would be provided
by an electrode comprising only the base portion. The curvature of
the base portion may be similar to the curvature of gas flow
through the discharge chamber to improve aerodynamic
performance.
[0036] In a third aspect of the present invention, an electrode
chamber of a discharge unit for an excimer or molecular fluorine
laser in accord with the above objects is connected with a gas flow
vessel and includes a pair of main electrodes and a preionization
unit. A plurality of ribs connected to one of the main electrodes
cross the gas flow preferably between the electrode chamber and the
gas flow vessel. The ribs are separated by openings to permit gas
flow and shaped to provide an aerodynamic flow of gases through the
openings. The shape of the ribs provides a smooth and uniform gas
flow between the gas flow vessel and the electrode chamber and thus
a reduced aerodynamic resistance for the blower over conventional
conducting ribs. The ribs preferably have widths which smoothly
taper from the end which meets the gas flow to the opposite end.
The ribs may be rounded and each end may have a different radius of
curvature.
[0037] In a fourth aspect of the invention, an electrode chamber of
a laser for an excimer or molecular laser is connected with a gas
flow vessel, and includes a pair of elongated main electrodes
separated by a discharge area, and a preionization unit. The
electrode chamber includes a spoiler spaced from each of the main
electrodes and positioned near a preionization electrode to thereby
shield the preionization electrode from one of the main electrodes.
The spoiler is also shaped to provide an aerodynamic gas flow
through the discharge area. A spoiler unit may include a pair of
opposed spoiler elements each positioned electrode on either side
of the discharge area to shield one of two or more preionization
electrodes from a main electrode, wherein each spoiler element is
spaced from the main discharge electrodes and shaped to provide an
aerodynamic gas flow through the discharge area.
[0038] In a fifth aspect of the invention, an electrode chamber of
a laser for an excimer or molecular laser is connected with a gas
flow vessel, and includes a pair of elongated main electrodes
separated by a discharge area, and a preionization unit. The
electrode chamber includes a spoiler shaped to reflect acoustical
waves emanating from the discharge area into the gas flow. The
spoiler is also shaped to provide an aerodynamic gas flow through
the discharge area. A spoiler unit may include a pair of opposed
spoiler elements positioned on either side of the discharge area
shaped to reflect acoustical waves emanating from the discharge
area into the gas flow vessel, wherein each spoiler element is
shaped to provide an aerodynamic gas flow through the discharge
area.
[0039] Combinations of two or more of the features described above
and below are also anticipated in the present invention. For
example, a discharge chamber in accord with one, more than one or
all three of the above aspects would be in accord with the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 illustrates in a cross sectional view an electrode
chamber of a typical discharge unit design and electrode
configuration.
[0041] FIG. 2a illustrates in a cross section view a laser having
an exemplary UV corona preionization unit design.
[0042] FIG. 2b illustrates in a cross sectional view a laser having
an exemplary UV spark preionization unit design.
[0043] FIG. 3 illustrates in a cross sectional view a laser tube
including an electrode chamber connected with a gas flow
vessel.
[0044] FIG. 4a illustrates in a cross section view a laser tube
including an electrode chamber and a gas flow vessel, wherein the
high voltage electrode is insulated by a dielectric insulator, and
the discharge area is adjacent to the gas flow vessel.
[0045] FIG. 4b illustrates in a cross sectional view a laser tube
including an electrode chamber and a gas flow vessel, wherein the
high voltage electrode is insulated by a dielectric plate, and the
discharge area is immersed in the gas flow vessel.
[0046] FIG. 4c illustrates in a top view low inductivity ribs
crossing the gas flow and separated by openings to permit gas
flow.
[0047] FIG. 4d illustrates a cross sectional side view of the ribs
of FIG. 4c separated by openings through which gas enters the
electrode chamber from the gas flow vessel.
[0048] FIG. 4e illustrates a cross sectional side view of the ribs
of FIG. 4c separated by openings through which gas exits the
discharge chamber and flows back into the gas flow vessel.
[0049] FIG. 5 illustrates a gas discharge electrode arrangement of
the prior art including a pair of easing electrodes around each
main discharge electrode.
[0050] FIG. 6 illustrates a laser having an electrode chamber in
accord with first, fourth and fifth aspects of the present
invention.
[0051] FIG. 7a illustrates a laser having an electrode chamber in
accord with a second aspect of the present invention.
[0052] FIG. 7b illustrates a laser having an alternative electrode
chamber in accord with the second aspect of the present
invention.
[0053] FIG. 8 illustrates a laser having another alternative
electrode chamber in accord with the second aspect of the present
invention.
[0054] FIG. 9a illustrates a laser tube is accord with a third
aspect of the present invention.
[0055] FIG. 9b shows a cross sectional view of the ribs crossing
the gas flow of the laser tube of FIG. 9a where the gas flows into
the electrode chamber from the gas flow vessel, wherein the ribs
are separated by openings to permit the gas flow and shaped to
provide aerodynamic gas flow and the ribs further serve as low
inductivity current return bars.
[0056] FIG. 9c shows a cross sectional view of the ribs crossing
the gas flow of FIG. 9a separated by openings to permit gas flow
from the electrode chamber back into the gas flow vessel, wherein
the ribs are aerodynamically shaped and separated by openings
through which gas exits the electrode chamber and flows back into
the gas flow vessel.
[0057] FIG. 10 illustrates a laser having an electrode chamber
configuration in accord with an alternative embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0058] FIG. 6 shows an aerodynamic discharge unit in accord with a
first aspect of the present invention. The discharge unit of FIG. 6
includes a pair of main electrodes 2, 4 separated by a discharge
area 6 and connected with a set of peaking capacitors Cp. A pair of
preionization units 10 are also shown and preferred. There may be
only a single preionization unit or more than two. Preferred
preionization units are described at U.S. patent application Ser.
Nos. 09/247,887, 60/160,182 and 60/162,845, each of which is
assigned to the same assignee, and at U.S. Pat. Nos. 5,337,330 and
5,719,896, all of which are hereby incorporated by reference.
[0059] The discharge unit includes one or more dielectric
insulators 38 preferably having a similar design as the dielectric
insulators 28 discussed above with respect to FIG. 4a. The
dielectric insulators 38 of the preferred embodiment may also be
curved, e.g., to provide a more aerodynamic electrode chamber. The
insulators 38 may also be straight, but tilted such as to form a
trapezoidally shaped electrode chamber (see FIG. 10, below).
[0060] In contrast with FIG. 4a, a pair of preferred spoilers 40 in
accord with the present invention are shown in FIG. 6. The spoilers
40 are preferably integrated with the chamber at the dielectric
insulators on either side of the discharge area 6. The spoilers 40
may be integrated parts of a single unit, single material
dielectric assembly with the insulators 38, or they may comprise
different materials suited each to their particular functions. That
is, the spoilers 40 and the dielecric insulators 38 may be formed
together to provide an aerodynamic electrode chamber for improved
gas flow uniformity and in accord with other features of the
spoilers to be described below. Alternatively, the spoilers 40 may
be attached to the insulating members 38.
[0061] The spoilers 40 are shaped and positioned for aerodynamic
and uniform gas flow as the gas flows through the electrode chamber
from the gas flow vessel 11 (partially shown), through the
discharge area 6 and back into the gas flow vessel 11. Preferably,
the spoilers 40 are symmetric in accord with a symmetric discharge
chamber design.
[0062] One end 42 of each of the spoilers 40 is preferably
positioned to shield a preionization unit 10 from the main
electrode 4, and is shown in FIG. 6 extending underneath one of the
pre-ionization units 10 between the preionization unit 10 and the
main electrode 4. These ends 42 of the spoilers 40 are preferably
positioned close to the preionization units 10. For example, the
ends 42 may be just a few millimeters from the preionization units
10. By shielding the preionization units 10 from the main electrode
4, arcing or dielectric breakdown between the preionization units
10 and the main electrode 4 is prevented.
[0063] The spoilers 40 serve to remove gas turbulence zones present
in conventional discharge unit electrode chambers which occur due
to the sharp curvature of the gas flow in the vicinity of the
preionization units 10 and of the grounded discharge electrode
4.
[0064] Another advantageous function of the spoilers 40 according
to the present invention is to reduce the level of acoustical
disturbances within the discharge chamber. The spoilers 40 serve as
"mirrors" to reflect the acoustical disturbances into the gas flow
vessel 11 (partially shown here). Thus, shock waves propagating
outwardly from the discharge area 6 impinge upon the oblique
surfaces 44 of the spoilers 40 and reflect into the gas flow vessel
11. Referring back to FIG. 3, internal components of the gas flow
vessel 11, such as the heat exchanger 14 and the blower 12, then
efficiently damp the acoustical waves.
[0065] This additional function of the spoilers 40 in accord with
the present invention reduces the level of the acoustical
disturbances discussed above with respect to the electrode chamber
of FIG. 4a. Thus, additional acoustical dampers are not used and
the adverse impact on gas flow uniformity of using conventional
dampers is avoided.
[0066] FIG. 7a illustrates a second aspect of the present invention
relating to the shape of the main discharge electrodes 46, 48. As
noted above, the shapes of the discharge electrodes 46, 48
significantly effect characteristics of the discharge area 50. In
accord with the preferred embodiment which incorporates the second
aspect of the present invention, at least one, and preferably both,
of the electrodes 46, 48 includes two regions. One of these
regions, the center portion 52, substantially carries the discharge
current and provides a uniform and narrow gas discharge width. The
other region, or base portion 54, in collaboration with other
conductive and dielectric elements within the discharge chamber, as
discussed above, creates preferred electrical field conditions in
and around the discharge area 50 and also contributes to the
smoothness and uniformity of the gas flow in the vicinity of the
discharge electrodes 46, 48.
[0067] The center portion 52 and base portion 54 preferably form an
electrode 46 having a single unit construction, and composed of a
single material. The center and base portions 52, 54 may also
comprise different materials, but the different materials should
have compatible mechanical and thermal properties such that
mechanical stability and electrical conductivity therebetween is
sufficiently maintained. The center portion 52 and the base portion
54 come together at a discontinuity or irregularity in the shape of
the electrodes 46, 48. A significant deviation of the electrical
field occurs at the location of the irregularity in such a way that
gas discharge occurs substantially from/to the center portions
52.
[0068] The center portion 52 is shaped to provide a uniform gas
discharge having a narrow width. The shape of a preferred center
portion 52 is described by the formula:
[x/a].sup.m+[y/b].sup.n=1, where m+n.gtoreq.5. (1)
[0069] Experiments performed using electrodes 46 and 48 having
center portions shaped according to formula (1) have shown improved
uniformity and width characteristics over center portions 52 having
m=n=2 (see U.S. Pat. Nos. 5,557,629 and 5,535,233, above).
[0070] More specific details of the preferred electrodes 46, 48 are
provided below. The base portions 54 of the main electrodes 46, 48
have a width around 30 mm. The interelectrode gap is preferably 14
to 16 mm. The middle area has a width around 2 mm. The center
portions 52 have a shape preferably as follows:
[x/1].sup.3+[y/0.85].sup.3=1, high voltage electrode 46, (2)
[x/a].sup.m+[y/b].sup.n=1, ground electrode 48, where (3)
[0071] where x and y are in millimeters, y is in the direction of
the interelectrode gap, x is orthogonal to y and is in the plane of
the cross section of the discharge chamber shown in FIG. 7, m is
preferably between 0.5 and 3, n is preferably between 8 and 13, a
is preferably between 0.5 and 1.5, and b is preferably between 0.2
and 0.8. The parameters of the shape of the center portion 52 of
the high voltage electrode 46 may be in a range around the specific
values given above. Qualitatively speaking, the center portions 52
have a reduced curvature at their tips than those described above
having m=n=2.
[0072] The base portions 54 have smooth, regular shapes. The center
portions 52 are positioned between the base portions 54 and the
discharge area 50. As discussed above, the base portions 54 are
shaped to provide a desired electric field distribution in and
around the discharge area 50. In addition and in combination with
the shape and positioning of the dielectric spoilers 40 and the
preionization units 10, the base portions 54 of the electrodes 46,
48 provide an aerodynamic channel for the flowing laser gas. For
example, the base portions 46, 48 may be shaped according to any of
a variety of smooth curves or a combination of several smooth
curves including those described by circular, elliptical,
parabolic, or hyperbolic functions. The curvatures of the base
portions 54 of the electrodes 46 and 48 may be the same or
different, and have the same direction of curvature with respect to
the discharge area 50, i.e., the base portions 54 each curve away
from the discharge area 50 away from the center portion 52.
[0073] FIG. 7b shows profiles of preferred center portions 52. In
plot 1, m+n=5 and in plot 2, m+n=12. FIG. 7c shows half profiles of
the preferred center portions of FIG. 7b.
[0074] An alternative configuration in accord with the second
aspect of the present invention is shown in FIG. 7d. The discharge
chamber of FIG. 7d is preferably the same as that shown and
described with respect to FIG. 7a, except that the base portion 58
of the high voltage main electrode 56 of FIG. 7d has opposite
curvature to the base portion 54 of the electrode 46 shown in FIG.
7a. That is, the base portion 58 of the electrode 56 curves toward
the discharge area 60 away from its corresponding center portion
52, while the base portion 54 of the electrode 48 curves away from
the discharge area 60 away from its corresponding center portion
52. The alternative configuration shown in FIG. 7d provides an even
more aerodynamic channel for gas flow through the discharge area
60.
[0075] FIG. 8 illustrates another alternative configuration of the
main electrodes in accord with the second aspect of the present
invention. The electrodes 55, 57 have a regular shape and no
discontinuity between base and center portions. The shape of the
center portions of the electrodes 55, 57 is preferably similar to
that described above with respect to FIG. 7a. The base portions
taper to the center portions in a triangular shape where the apexes
of the triangular shaped electrodes are the center portions and are
rounded as described above.
[0076] FIGS. 9a-9c illustrate a third aspect of the present
invention. As discussed above, the dielectric insulators 38 of the
electrode chamber isolate the high voltage main electrode 46. The
gas flow is crossed by a first rib configuration 62a where the gas
flow enters the electrode chamber from the gas flow vessel 11 and
by a second rib configuration 62b where the gas flow exits the
electrode chamber and returns the gas back into the gas flow vessel
11. The ribs 62a, 62b, or current return bars, are separated by
openings for the laser gas to flow into and out of the electrode
chamber from/to the gas flow vessel 11. The ribs are preferably
rigid and conducting, and are connected to the grounded main
discharge electrode 48 to provide a low inductivity current return
path. The conducting ribs 64a of the rib configuration 62a are
preferably substantially shaped as shown in FIG. 9b. The conducting
ribs 64b of the rib configuration 62b are preferably substantially
shaped as shown in FIG. 9c. The ribs 64a and 64b of the rib
configurations 62a and 62b, respectively, are asymmetrically
shaped. In contrast, the ribs 32 shown in cross-section in FIGS. 4d
and 4e are rectangularly shaped.
[0077] FIG. 9b is a cross sectional view of the rib configuration
62a through which the laser gas enters the electrode chamber from
the gas flow vessel 11. The ribs 64a of the rib configuration 62a
each have a wide end 66a which meets the laser gas as it flows from
the gas flow vessel 11, and a narrow end 68a past which the laser
gas flows as it enters the discharge chamber. Preferably, the ribs
64a are smoothly tapered, e.g., like an airplane wing, from the
wide, upstream end 66a to the narrow, downstream end 68a to improve
gas flow past the rib configuration 62a.
[0078] FIG. 9c is a cross sectional view of the rib configuration
62b through which the laser gas exits the electrode chamber and
flows back into the gas flow vessel 11. The ribs 64b of the rib
configuration 62b each have a wide end 66b which meets the laser
gas as it flows from the electrode chamber, and a narrow end 68b
past which the laser gas flows as it enters the gas flow vessel 11.
. Preferably, the ribs 64b are smoothly tapered, e.g., like an
airplane wing, from the wide, upstream end 66b to the narrow,
downstream end 68b to improve gas flow past the rib configuration
62b.
[0079] The aerodynamic ribs 64a and 64b each provide a reduced
aerodynamic resistance to the flowing gas from that provided by the
conventional rectangular ribs 32 of FIGS. 4d and 4e. The ribs 64a
and 64b are thus shaped to improve the uniformity of the gas flow
in accord with the above objects of the invention. A more
homogeneous gas flow results from modifying the conventional ribs
32 into the ribs 64a, 64b of the present invention. The more
homogeneous gas flow results in a more homogeneous gas density in
the discharge area. The more homogeneous gas density in the
discharge area results in a more homogeneous and stable discharge,
ultimately and advantageously providing more stable output beam
parameters.
[0080] FIG. 10 illustrates an electrode chamber in accord with an
alternative embodiment of the present invention. The laser tube
shown includes an electrode chamber and a gas vessel 11. The
electrode chamber has a pair of main electrodes 2, 4 separated by a
discharge area 6, and one or more (two are shown) preionization
electrodes 10. The electrodes 2, 4 are connected to peaking
capacitors Cp. The current return rib configurations 62a and 62b
are preferably as shown and described above with respect to FIGS.
9a-9c. The gas flow vessel 11 has a blower 12 and heat exchanger 14
also as described above.
[0081] The high voltage main electrode 2 is isolated by a
dielectric frame 128 that differs from that discussed above. The
frame 128 has opposing walls inclined toward each other near the
electrode 2 which is furthest from the gas flow vessel 11. That is,
the frame does not form a rectangular electrode chamber such as
that shown in FIG. 4a. Neither is the electrode chamber sunk into
the gas flow vessel 11 like that shown in FIG. 4b. Instead, the
frame 128 shown is configured such that the electrode chamber forms
a trapezoidal shape.
[0082] The shape of the dielectric frame 128 is advantageous
because in addition to isolating the main electrode 2, the frame
128 provides a more uniform flow of the gas mixture through the
discharge area 6. In addition, acoustic waves generated in the
discharge area are reflected from the frame 128 and into the gas
flow vessel 11, where the acoustic waves are preferably absorbed by
the gas flow components 12 and 14. The frame 128 may be configured
to absorb some of the acoustic waves, as well.
[0083] Although not shown, spoilers may be added similar to those
shown and described at FIG. 6. The spoilers may be integrated with
the frame 128. The spoilers may be shaped to provide a still more
uniform gas flow, and to inhibit dielectric breakdown between the
preionization unit(s) 10 and the main electrode 4. The spoilers may
also serve to further dampen and/or reflect acoustic waves
emanating from the discharge area 6.
[0084] It is anticipated that a discharge chamber in accord with
the preferred or alternative embodiments and any of the aspects
described above in accord with the present invention will be
particularly advantageous for use with an excimer or molecular
fluorine laser. For example, a KrF laser would have a gas mixture
including Kr, F.sub.2 and Ne and optionally a Xe or Ar additive. An
ArF laser would have a gas mixture of Ar, F.sub.2 and Ne and/or He,
and optionally a Xe or Kr additive. A F.sub.2 laser would a gas
mixture of F.sub.2 and Ne and/or He. A XeCl, XeF or KrCl laser
would also benefit with the advantages described above. Preferred
gas mixtures and gas control techniques are described at U.S.
patent application Ser. Nos. 60/124,785, 09/418,052, 60/159,525,
09/379,034, 60/160,126, 09/317,526, and 60/127,062, and U.S. Pat.
Nos. 4,393,505, 5,396,514 and 4,977,563, each of which is a
assigned to the same assignee as the present application, and U.S.
Pat. No. 5,978,406, all of which are hereby incorporated herein by
reference.
[0085] The specific embodiments described in the specification,
drawings, summary of the invention and abstract of the disclosure
are not intended to limit the scope of any of the claims, but are
only meant to provide illustrative examples of the invention to
which the claims are drawn. The scope of the present invention is
understood to be encompassed by the language of the claims, and
structural and functional equivalents thereof.
* * * * *