U.S. patent application number 09/734459 was filed with the patent office on 2001-08-09 for laser gas replenishment method.
Invention is credited to Albrecht, Hans-Stephan, Schroeder, Thomas, Vogler, Klaus Wolfgang.
Application Number | 20010012309 09/734459 |
Document ID | / |
Family ID | 27494560 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012309 |
Kind Code |
A1 |
Albrecht, Hans-Stephan ; et
al. |
August 9, 2001 |
Laser gas replenishment method
Abstract
A method and apparatus is provided for stabilizing output beam
parameters of a gas discharge laser by maintaining a constituent
gas of the laser gas mixture at a predetermined partial pressure
using a gas supply unit and a processor. The constituent gas of the
laser gas mixture is provided at an initial partial pressure and
the constituent gas is subject to depletion within the laser
discharge chamber. Injections of the constituent gas are performed
each to increase the partial pressure by a selected amount in the
discharge chamber preferably less than 0.2 mbar per injection. A
number of successive injections is performed at selected intervals
to maintain the constituent gas substantially at the initial
partial pressure for maintaining stable output beam parameters. The
amount per injection and/or the interval between injections may be
varied based on the measured value of the driving voltage and/or a
calculated amount of the constituent gas in the discharge chamber.
The driving voltage is determined to be in one of multiple driving
voltage ranges that are adjusted based on the aging of the system.
Within each range, gas injections and gas replacements are
preferably performed based on total applied electrical energy to
the discharge and/or alternatively, on time and/or pulse count.
Inventors: |
Albrecht, Hans-Stephan;
(Goettingen, DE) ; Vogler, Klaus Wolfgang;
(Goettingen, DE) ; Schroeder, Thomas; (Goettingen,
DE) |
Correspondence
Address: |
Andrew V. Smith
Limbach & Limbach L.L.P.
2001 Ferry Building
San Francisco
CA
94111
US
|
Family ID: |
27494560 |
Appl. No.: |
09/734459 |
Filed: |
December 11, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09734459 |
Dec 11, 2000 |
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09447882 |
Nov 23, 1999 |
|
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|
60171717 |
Dec 22, 1999 |
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60124785 |
Mar 17, 1999 |
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Current U.S.
Class: |
372/55 ;
372/59 |
Current CPC
Class: |
H01S 3/036 20130101;
G03F 7/70025 20130101; G03F 7/70575 20130101; H01S 3/225 20130101;
H01S 3/134 20130101 |
Class at
Publication: |
372/55 ;
372/59 |
International
Class: |
H01S 003/22; H01S
003/223 |
Claims
What is claimed is:
1. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including a constituent gas which is
subject to depletion; a plurality of electrodes connected to a
power supply circuit for providing a driving voltage to said
electrodes as a pulsed discharge to energize said laser gas
mixture; a resonator surrounding said discharge chamber for
generating a pulsed laser beam; a gas supply unit connected to said
discharge chamber; and a processor for controlling gaseous flow
between said gas supply unit and said discharge chamber, wherein
said gas supply unit and said processor are configured to permit
between 0.0001 mbar and 0.2 mbar of said constituent gas to inject
into said discharge chamber at selected intervals, and wherein an
amount of constituent gas injected is varied based on a value of
the driving voltage applied to achieve a predetermined output pulse
energy.
2. The laser system of claim 1, wherein the processor determines
within which of a plurality of ranges the driving voltage currently
lies within and determines said amount of constituent gas to be
injected based on this driving voltage range determination.
3. The laser system of claim 2, wherein said ranges are adjusted
based on aging of at least one component of the laser system.
4. The laser system of claim 3, wherein said at least one component
is one of an optical component and a component of the discharge
chamber.
5. The laser system of claim 2, wherein said amount of constituent
gas to be injected differs between at least two voltage ranges.
6. The laser system of claim 1, wherein said amount of constituent
gas injected is varied based on a calculated amount of the
constituent gas in the gas mixture after a previous injection.
7. The laser system of claim 6, wherein said calculated amount of
said constituent gas in the gas mixture is based on a measured
pressure in an accumulator from which constituent gas was
previously injected.
8. The laser system of claim 7, wherein said calculated amount is
also based on measured temperatures in said accumulator and said
discharge chamber.
9. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including a constituent gas which is
subject to depletion; a plurality of electrodes connected to a
power supply circuit for providing a driving voltage to said
electrodes to generate a pulsed discharge to energize said laser
gas mixture; a resonator surrounding said discharge chamber for
generating a pulsed laser beam; a gas supply unit connected to said
discharge chamber; and a processor for controlling gaseous flow
between said gas supply unit and said discharge chamber, wherein
said gas supply unit and said processor are configured to permit
between 0.0001 mbar and 0.2 mbar of said constituent gas to inject
into said discharge chamber at selected intervals, and wherein said
intervals are varied based on a value of the driving voltage
applied to achieve a predetermined output pulse energy.
10. The laser system of claim 9, wherein the processor determines
within which of a plurality of ranges the driving voltage currently
lies within and determines said interval between injections based
on this driving voltage range determination.
11. The laser system of claim 10, wherein said ranges are adjusted
based on aging of at least one component of the laser system.
12. The laser system of claim 11, wherein said at least one
component is one of an optical component and a component of the
discharge chamber.
13. The laser system of claim 10, wherein said interval between
injections differs between at least two voltage ranges.
14. The laser system of claim 9, wherein the interval between
injections is varied based on a calculated amount of the
constituent gas in the gas mixture after a previous injection.
15. The laser system of claim 14, wherein said calculated amount of
said constituent gas in the gas mixture is based on a measured
pressure in an accumulator from which constituent gas was
previously injected.
16. The laser system of claim 15, wherein said calculated amount is
also based on measured temperatures in said accumulator and said
discharge chamber.
17. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including a constituent gas which is
subject to depletion; a plurality of electrodes connected to a
power supply circuit for providing a driving voltage as a pulsed
discharge to energize said laser gas mixture; a resonator
surrounding said discharge chamber for generating a pulsed laser
beam; a gas supply unit connected to said discharge chamber; and a
processor for controlling gaseous flow between said gas supply unit
and said discharge chamber, wherein said gas supply unit and said
processor are configured to permit between 0.0001 mbar and 0.2 mbar
of said constituent gas to inject into said discharge chamber at
selected intervals, and wherein an amount of constituent gas
injected is varied based on a calculated amount of constituent gas
injected in a previous injection.
18. The laser system of claim 17, wherein said gas supply unit of
said laser system includes an accumulator, and said calculated
amount of said constituent gas in the gas mixture is based on a
measured pressure in said accumulator from which constituent gas
was previously injected.
19. The laser system of claim 18, wherein said calculated amount is
also based on measured temperatures in said accumulator and said
discharge chamber.
20. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including a constituent gas which is
subject to depletion; a plurality of electrodes connected to a
power supply circuit for providing a driving voltage as a pulsed
discharge to energize said laser gas mixture; a resonator
surrounding said discharge chamber for generating a pulsed laser
beam; a gas supply unit connected to said discharge chamber; and a
processor for controlling gaseous flow between said gas supply unit
and said discharge chamber, wherein said gas supply unit and said
processor are configured to permit between 0.0001 mbar and 0.2 mbar
of said constituent gas to inject into said discharge chamber at
selected intervals, and wherein said intervals are varied based on
a calculated amount of constituent gas injected in a previous
injection.
21. The laser system of claim 20, wherein said gas supply unit of
said laser system includes an accumulator, and said calculated
amount of said constituent gas in the gas mixture is based on a
measured pressure in said accumulator from which constituent gas
was previously injected.
22. The laser system of claim 21, wherein said calculated amount is
also based on measured temperatures in said accumulator and said
discharge chamber.
23. A method for controlling a composition of a gas mixture within
a discharge chamber of a gas discharge laser system, comprising the
steps of: monitoring an input driving voltage of a pulse power
circuit of the laser; monitoring a second parameter indicative of
the concentration of a constituent gas of the gas mixture;
determining an amount of constituent gas between 0.0001 mbar and
0.2 mbar to be injected into said discharge chamber based on a
value of the driving voltage applied to achieve a predetermined
output pulse energy; and injecting said amount of said constituent
gas into said discharge chamber at selected intervals when a
predetermined value of said second parameter is reached.
24. The method of claim 23, further comprising the step of
determining within which of a plurality of ranges the driving
voltage currently lies within, and wherein said amount of
constituent gas determining step determines said amount based on
this driving voltage range determination.
25. The method of claim 24, further comprising the step of
adjusting said ranges based on aging of at least one component of
the laser system.
26. The method of claim 25, wherein said at least one component is
one of an optical component and a component of the discharge
chamber.
27. The method of claim 24, wherein said amount of constituent gas
to be injected differs between at least two voltage ranges.
28. The method of claim 23, further comprising the step of varying
said amount of constituent gas injected based on a calculated
amount of the constituent gas in the gas mixture after a previous
injection.
29. The method of claim 28, wherein said calculated amount of said
constituent gas in the gas mixture is based on a measured pressure
in an accumulator from which constituent gas was previously
injected.
30. The method of claim 29, wherein said calculated amount is also
based on measured temperatures in said accumulator and said
discharge chamber.
31. A method for controlling a composition of a gas mixture within
a discharge chamber of a gas discharge laser system, comprising the
steps of: monitoring an input driving voltage of a pulse power
circuit of the laser; monitoring a second parameter indicative of
the concentration of a constituent gas of the gas mixture;
determining an interval value of said second parameter between
which injections into the discharge chamber of said constituent gas
between 0.0001 mbar and 0.2 mbar are performed based on a value of
the driving voltage applied to achieve a predetermined output pulse
energy; and injecting said constituent gas into said discharge
chamber at said interval when a predetermined value of said second
parameter based on said predetermined interval is reached.
32. The method of claim 31, further comprising the step of
determining within which of a plurality of ranges the driving
voltage currently lies within, and wherein said interval
determining step includes determining said interval between
injections based on this driving voltage range determination.
33. The method of claim 32, further comprising the step of
adjusting said ranges based on aging of at least one component of
the laser system.
34. The method of claim 33, wherein said at least one component is
one of an optical component and a component of the discharge
chamber.
35. The method of claim 32, wherein said interval between
injections differs between at least two voltage ranges.
36. The method of claim 31, further comprising the step of varying
said interval between injections based on a calculated amount of
the constituent gas in the gas mixture after a previous
injection.
37. The method of claim 36, wherein said calculated amount of said
constituent gas in the gas mixture is based on a measured pressure
in an accumulator from which constituent gas was previously
injected.
38. The method of claim 37, wherein said calculated amount is also
based on measured temperatures in said accumulator and said
discharge chamber.
39. A method for controlling a composition of a gas mixture within
a discharge chamber of a gas discharge laser system, comprising the
steps of: monitoring a parameter indicative of the concentration of
a constituent gas of the gas mixture; determining a next amount of
constituent gas between 0.0001 mbar and 0.2 mbar to be injected
into said discharge chamber based on a calculated amount of said
constituent gas injected in a previous injection; and injecting
said next amount of said constituent gas into said discharge
chamber at selected interval amounts of said second parameter.
40. The method of claim 39, wherein said next amount of said
constituent gas is between 0.001 mbar and 0.02 mbar.
41. The method of claim 40, further comprising the step of
monitoring an input driving voltage of a pulse power circuit of the
laser, and determining said amount of constituent gas based further
on a value of said input driving voltage.
42. The method of claim 40, further comprising the step of
measuring a pressure within an accumulator from which constituent
gas was previously injection, and wherein said calculated amount of
said constituent gas in the gas mixture is based on the measured
pressure in the accumulator from which constituent gas was
previously injected.
43. The method of claim 42, further comprising the step of
measuring a temperature in said accumulator and a temperature in
said discharge chamber, and wherein said calculated amount is also
based on the measured temperatures in said accumulator and said
discharge chamber.
44. A method for controlling a composition of a gas mixture within
a discharge chamber of a gas discharge laser system, comprising the
steps of: monitoring a parameter indicative of the concentration of
a constituent gas of the gas mixture; determining an interval value
of said parameter between which injections of the constituent gas
between 0.0001 mbar and 0.2 mbar into said discharge chamber are
performed, said interval value being based on a calculated amount
of said constituent gas injected in a previous injection; and
injecting said constituent gas into said discharge chamber at said
interval value of said parameter.
45. The method of claim 44, wherein said injection of said
constituent gas is between 0.001 mbar and 0.02 mbar.
46. The method of claim 45, further comprising the step of
monitoring an input driving voltage of a pulse power circuit of the
laser, and determining said interval value based further on a value
of said driving voltage.
47. The method of claim 46, further comprising the step of
measuring a pressure in an accumulator from which constituent gas
was previously injected, and wherein said calculated amount of said
constituent gas in the gas mixture is based on said measured
pressure in said accumulator from which constituent gas was
previously injected.
48. The method of claim 47, further comprising the step of
measuring a temperature in said accumulator and a temperature
within said discharge chamber, and wherein said calculated amount
is also based on the measured temperatures in said accumulator and
said discharge chamber.
49. A gas discharge laser system, comprising: a discharge chamber
containing a laser gas mixture including a constituent gas which is
subject to depletion; a plurality of electrodes connected to a
power supply circuit for providing a driving voltage to said
electrodes as a pulsed discharge to energize said laser gas
mixture; a resonator surrounding said discharge chamber for
generating a pulsed laser beam; a gas supply unit connected to said
discharge chamber; and a processor for controlling gaseous flow
between said gas supply unit and said discharge chamber, wherein
said gas supply unit and said processor are configured to permit an
amount of the gas mixture between 5% and 70% of the total gas
mixture to be exchanged during a partial new fill procedure at
selected intervals, and wherein the partial new fill procedure is
initiated based on a value of the driving voltage applied to
achieve a predetermined output pulse energy being above a threshold
voltage.
50. The laser system of claim 49, wherein the amount of the gas
mixture exchanged in the partial new fill procedure is between 5%
and 50%.
51. The laser system of claim 49, wherein the amount of the gas
mixture exchanged in the partial new fill procedure is between 20%
and 50%.
52. The laser system of claim 49, wherein the amount of the gas
mixture exchanged in the partial new fill procedure is
substantially 1 bar.
53. A method for controlling a composition of a laser gas mixture
in a discharge chamber of a gas discharge laser system, comprising
the steps of: monitoring an input driving voltage of a pulse power
circuit of the laser; injecting a selected amount of a constituent
gas into the discharge chamber at selected intervals when the
driving voltage applied to achieve a predetermined output pulse
energy is at or below a first threshold value; and exchanging an
amount of the gas mixture between 5% and 70% of the total gas
mixture during a partial new fill procedure when the driving
voltage is above a second threshold value higher than said first
threshold value.
54. The method of claim 53, wherein the selected amount of the
constituent gas injected in the injecting step is between 0.001
mbar and 0.02 mbar.
55. The method of claim 54, further comprising a second injecting
step, which is performed when the driving voltage is between the
first and second threshold values, wherein an amount of constituent
gas injected is between 0.02 mbar and 0.2 mbar.
56. The method of claim 53, further comprising a second injecting
step, which is performed when the driving voltage is between the
first and second threshold values, wherein an amount of constituent
gas injected is between 0.001 and 0.02 mbar at selected intervals
that are smaller than said selected intervals of injections in the
first injecting step.
57. The method of claim 53, wherein the amount of the gas mixture
exchanged in the partial new fill procedure is between 5% and
50%.
58. The method of claim 53, wherein the amount of the gas mixture
exchanged in the partial new fill procedure is between 20% and
50%.
59. The method of claim 53, wherein the amount of the gas mixture
exchanged in the partial new fill procedure is substantially 1
bar.
60. A method for controlling a composition of a laser gas mixture
in a discharge chamber of a gas discharge laser system, comprising
the steps of: monitoring an input driving voltage of a pulse power
circuit of the laser; injecting a selected amount of a constituent
gas between 0.0001 mbar and 0.2 mbar into the discharge chamber at
first intervals, when the driving voltage applied to achieve a
predetermined output pulse energy is at or below a first threshold
value; and injecting a same selected amount of the constituent gas
into the discharge chamber at second intervals smaller than said
first intervals to increase a concentration of said constituent gas
in the discharge chamber at said smaller intervals compared with
said first intervals, when the driving voltage applied to achieve a
predetermined output pulse energy is above the first threshold
value.
61. The method of claim 60, wherein the amount injected is between
0.001 mbar and 0.02 mbar.
62. A method for controlling a composition of a laser gas mixture
in a discharge chamber of a gas discharge laser system, comprising
the steps of: monitoring an input driving voltage of a pulse power
circuit of the laser; injecting a first amount of a constituent gas
between 0.0001 mbar and 0.2 mbar into the discharge chamber at
selected intervals, when the driving voltage applied to achieve a
predetermined output pulse energy is at or below a first threshold
value; and injecting a second amount larger than the first amount
of the constituent gas into the discharge chamber at said selected
intervals to increase a concentration of said constituent gas in
the discharge chamber at smaller intervals compared with said
injecting said first amount at said selected intervals, when the
driving voltage applied to achieve a predetermined output pulse
energy is above the first threshold value.
63. The method of claim 62, wherein the amount injected is between
0.001 mbar and 0.02 mbar.
64. A method for controlling a composition of a laser gas mixture
in a discharge chamber of a gas discharge laser system, comprising
the steps of: monitoring an input driving voltage of a pulse power
circuit of the laser; injecting a first amount of a constituent gas
between 0.0001 mbar and 0.2 mbar into the discharge chamber at
selected intervals, when the driving voltage applied to achieve a
predetermined output pulse energy is at or below a first threshold
value; injecting a second amount of gas into the discharge chamber
together with releasing a similar amount of gas to reduce a
contaminant concentration in the gas mixture, when the driving
voltage applied to achieve a predetermined output pulse energy is
above the first threshold value and below a second threshold value;
and injecting a third amount of gas larger than the second amount
into the discharge chamber together with releasing a similar amount
of gas to reduce a contaminant concentration in the gas mixture
more so than in said second amount injection and release step, when
the driving voltage applied to achieve a predetermined output pulse
energy is above the second threshold value.
65. The method of claim 64, wherein the amount injected is between
0.001 mbar and 0.02 mbar.
66. A method for controlling a composition of a laser gas mixture
in a discharge chamber of a gas discharge laser system, comprising
the steps of: monitoring an input driving voltage of a pulse power
circuit of the laser; injecting a first amount of a constituent gas
between 0.0001 mbar and 0.2 mbar into the discharge chamber at
first intervals, when the driving voltage applied to achieve a
predetermined output pulse energy is at or below a first threshold
value; and injecting a second amount of gas into the discharge
chamber together with releasing a similar amount of gas at second
intervals to reduce a contaminant concentration in the gas mixture,
when the driving voltage applied to achieve a predetermined output
pulse energy is below a second threshold value; and injecting the
second amount of gas into the discharge chamber together with
releasing a similar amount of gas at third intervals smaller than
said second intervals to reduce a contaminant concentration in the
gas mixture at said smaller intervals than in said second amount
injection and release step at said second intervals, when the
driving voltage applied to achieve a predetermined output pulse
energy is above the second threshold value.
67. The method of claim 66, wherein the amount injected is between
0.001 mbar and 0.02 mbar.
Description
PRIORITY
[0001] This application claims the benefit of priority to U.S.
provisional patent application No. 60/171,717, filed Dec. 22, 1999,
and this application is a Continuation-in-Part of U.S. patent
application Ser. No. 09/447,882, filed Nov. 23, 1999, which claims
the benefit of U.S. provisional patent application No. 60/124,785,
filed Mar. 17, 1999, wherein the above application are assigned to
the same assignee as the present application and are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method and apparatus for
stabilizing output beam parameters of a gas discharge laser. More
particularly, the present invention relates to maintaining an
optimal gas mixture composition over long, continuous operating or
static periods using very small gas injections.
[0004] 2. Discussion of the Related Art
[0005] Pulsed gas discharge lasers such as excimer and molecular
lasers emitting in the deep ultraviolet (DUV) or vacuum ultraviolet
(VUV) have become very important for industrial applications such
as photolithography. Such lasers generally include a discharge
chamber containing two or more gases such as a halogen and one or
two rare gases. KrF (248 nm), ArF (193 nm), XeF (350 nm), KrCl (222
nm), XeCl (308 nm), and F.sub.2 (157 nm) lasers are examples.
[0006] The efficiencies of excitation of the gas mixtures and
various parameters of the output beams of these lasers vary
sensitively with the compositions of their gas mixtures. An optimal
gas mixture composition for a KrF laser has preferred gas mixture
component ratios around 0.1% F.sub.2/1% Kr/98.9% Ne (see U.S. Pat.
No. 4,393,505, which is assigned to the same assignee and is hereby
incorporated by reference). A F.sub.2 laser may have a gas
component ratio around 0.1% F.sub.2/99.9% Ne or He or a combination
thereof (see U.S. patent application Ser. No. 09/317,526, which is
assigned to the same assignee and is hereby incorporated by
reference). Small amounts of Xe may be added to rare gas halide gas
mixtures, as well (see U.S. patent application Ser. No. 60/160,126,
which is assigned to the same assignee and is hereby incorporated
by reference; see also R. S. Taylor and K. E. Leopold, Transmission
Properties of Spark Preionization Radiation in Rare-Gas Halide
Laser Gas Mixes, IEEE Journal of Quantum Electronics, pp.
2195-2207, vol. 31, no. 12 (December 1995). Any deviation from the
optimum gas compositions of these or other excimer or molecular
lasers would typically result in instabilities or reductions from
optimal of one or more output beam parameters such as beam energy,
energy stability, temporal pulse width, temporal coherence, spatial
coherence, discharge width, bandwidth, and long and short axial
beam profiles and divergences.
[0007] Especially important in this regard is the concentration (or
partial pressure) of the halogen, e.g., F.sub.2, in the gas
mixture. The depletion of the rare gases, e.g., Kr and Ne for a KrF
laser, is low in comparison to that for the F.sub.2. FIG. 1 shows
laser output efficiency versus fluorine concentration for a KrF
laser, showing a decreasing output efficiency away from a central
maximum. FIG. 2 shows how the temporal pulse width (pulse length or
duration) of KrF laser pulses decrease with increasing F.sub.2
concentration. FIGS. 3-4 show the dependence of output energy on
driving voltage (i.e., of the discharge circuit) for various
F.sub.2 concentrations of a F.sub.2 laser. It is observed from
FIGS. 3-4 that for any given driving voltage, the pulse energy
decreases with decreasing F.sub.2 concentration. In FIG. 3, for
example, at 1.9 kV, the pulse energies are around 13 mJ, 11 mJ and
10 mJ for F.sub.2 partial pressures of 3.46 mbar, 3.16 mbar and
2.86 mbar, respectively. The legend in FIG. 3 indicates the partial
pressures of two premixes, i.e., premix A and premix B, that are
filled into the discharge chamber of a KrF laser. Premix A
comprised substantially 1% F.sub.2 and 99% Ne, and premix B
comprised substantially 1% Kr and 99% Ne. Therefore, for the graph
indicated by triangular data points, a partial pressure of 346 mbar
for premix A indicates that the gas mixture had substantially 3.46
mbar of F.sub.2 and a partial pressure of 3200 mbar for premix B
indicates that the gas mixture had substantially 32 mbar of Kr, the
remainder of the gas mixture being the buffer gas Ne. FIG. 5 shows
a steadily increasing bandwidth of a KrF laser with increasing
F.sub.2 concentration.
[0008] In industrial applications, it is advantageous to have an
excimer or molecular fluorine laser capable of operating
continuously for long periods of time, i.e., having minimal
downtime. It is desired to have an excimer or molecular laser
capable of running non-stop year round, or at least having a
minimal number and duration of down time periods for scheduled
maintenance, while maintaining constant output beam parameters.
Uptimes of, e.g., greater than 98% require precise control and
stabilization of output beam parameters, which in turn require
precise control of the composition of the gas mixture.
[0009] Unfortunately, gas contamination occurs during operation of
excimer and molecular fluorine lasers due to the aggressive nature
of the fluorine or chlorine in the gas mixture. The halogen gas is
highly reactive and its concentration in the gas mixture decreases
as it reacts, leaving traces of contaminants. The halogen gas
reacts with materials of the discharge chamber or tube as well as
with other gases in the mixture. Moreover, the reactions take place
and the gas mixture degrades whether the laser is operating
(discharging) or not. The passive gas lifetime is about one week
for a typical KrF-laser.
[0010] During operation of a KrF-excimer laser, such contaminants
as HF, CF.sub.4, COF.sub.2, SiF.sub.4 have been observed to
increase in concentration rapidly (see G. M. Jurisch et al., Gas
Contaminant Effects in Discharge-Excited KrF Lasers, Applied
Optics, Vol. 31, No. 12, pp. 1975-1981 (Apr. 20, 1992)). For a
static KrF laser gas mixture, i.e., with no discharge running,
increases in the concentrations of HF, O.sub.2, CO.sub.2 and
SiF.sub.4 have been observed (see Jurisch et al., above).
[0011] One way to effectively reduce this gas degradation is by
reducing or eliminating contamination sources within the laser
discharge chamber. With this in mind, an all metal, ceramic laser
tube has been disclosed (see D. Basting et al., Laserrohr fur
halogenhaltige Gasentladungslaser" G 295 20 280.1, Jan. 25,
1995/Apr. 18, 1996 (disclosing the Lambda Physik Novatube, and
hereby incorporated by reference into the present application)).
FIG. 6 qualitatively illustrates how using a tube comprising
materials that are more resistant to halogen erosion (plot B) can
slow the reduction of F.sub.2 concentration in the gas mixture
compared to using a tube which is not resistant to halogen erosion
(plot A). The F.sub.2 concentration is shown in plot A to decrease
to about 60% of its initial value after about 70 million pulses,
whereas the F.sub.2 concentration is shown in plot B to decrease
only to about 80% of its initial value after the same number of
pulses. Gas purification systems, such as cryogenic gas filters
(see U.S. Pat. Nos. 4,534,034, 5,136,605, 5,430,752, 5,111,473 and
5,001,721 assigned to the same assignee, and hereby incorporated by
reference) or electrostatic particle filters (see U.S. Pat. No.
4,534,034, assigned to the same assignee and U.S. Pat. No.
5,586,134, each of which is incorporated by reference) are also
being used to extend KrF laser gas lifetimes to 100 million shots
before a new fill is advisable.
[0012] It is not easy to directly measure the halogen concentration
within the laser tube for making rapid online adjustments (see U.S.
Pat. No. 5,149,659 (disclosing monitoring chemical reactions in the
gas mixture)). Therefore, it is recognized in the present invention
that an advantageous method applicable to industrial laser systems
includes using a known relationship between F.sub.2 concentration
and a laser parameter, such as one of the F.sub.2 concentration
dependent output beam parameters mentioned above. In such a method,
precise values of the parameter would be directly measured, and the
F.sub.2 concentration would be calculated from those values. In
this way, the F.sub.2 concentration may be indirectly
monitored.
[0013] Methods have been disclosed for indirectly monitoring
halogen depletion in a narrow band excimer laser by monitoring beam
profile (see U.S. Pat. No. 5,642,374, hereby incorporated by
reference) and spectral (band) width (see U.S. Pat. No. 5,450,436,
hereby incorporated by reference). Neither of these methods is
particularly reliable, however, since beam profile and bandwidth
are each influenced by various other operation conditions such as
repetition rate, tuning accuracy, thermal conditions and aging of
the laser tube. That is, the same bandwidth can be generated by
different gas compositions depending on these other operating
conditions.
[0014] An advantageous technique monitors amplified spontaneous
emission (ASE), and is described in U.S. patent application Ser.
No. 09/418,052 (assigned to the same assignee and hereby
incorporated by reference). The ASE is very sensitive to changes in
fluorine concentration, and thus the fluorine concentration may be
monitored indirectly by monitoring the ASE, notwithstanding whether
other parameters are changing and effecting each other as the
fluorine concentration in the gas mixture changes.
[0015] It is known to compensate the degradation in laser
efficiency due to halogen depletion by steadily increasing the
driving voltage of the discharge circuit to maintain the output
beam at constant energy. To illustrate this, FIG. 7 shows how at
constant driving voltage, the energy of output laser pulses
decreases with pulse count. FIG. 8 then shows how the driving
voltage may be steadily increased to compensate the halogen
depletion and thereby produce output pulses of constant energy.
[0016] One drawback of this approach is that output beam parameters
other than energy such as those discussed above with respect to
FIGS. 1-5 affected by the gas mixture degradation will not be
correspondingly corrected by steadily increasing the driving
voltage. FIGS. 9-11 illustrate this point showing the driving
voltage dependencies, respectively, of the long and short axis beam
profiles, short axis beam divergence and energy stability sigma.
Moreover, at some point the halogen becomes so depleted that the
driving voltage reaches its maximum value and the pulse energy
cannot be maintained without refreshing the gas mixture.
[0017] It is desired to have a method of stabilizing all of the
output parameters affected by halogen depletion and not just the
energy of output pulses. It is recognized in the present invention
that this is most advantageously achieved by adjusting the halogen
and rare gas concentrations themselves.
[0018] There are techniques available for replenishing a gas
mixture by injecting additional rare and halogen gases into the
discharge chamber between new gas fills and to methods including
readjusting the gas pressure, e.g., by releasing gases from the
laser tube (see especially U.S. patent application Ser. Nos.
60/124,785, and 09/379,034, and also U.S. patent application Ser.
No. 09/418,052; and U.S. Pat. Nos. 5,396,514 and 4,977,573, each of
which is assigned to the same assignee and hereby incorporated by
reference). A more complex system monitors gas mixture degradation
and readjusts the gas mixture using selective replenishment
algorithms for each gas of the gas mixture (see U.S. Pat. No.
5,440,578, hereby incorporated by reference). One technique uses an
expert system including a database of information and graphs
corresponding to different gas mixtures and laser operating
conditions (see the '034 application, mentioned just above). A data
set of driving voltage versus output pulse energy, e.g., is
measured and compared to a stored "master" data set corresponding
to an optimal gas composition such as may be present in the
discharge chamber after a new fill. From a comparison of values of
the data sets and/or the slopes of graphs generated from the data
sets, a present gas mixture status and appropriate gas
replenishment procedures, if any, may be determined and undertaken
to reoptimize the gas mixture. Early gas replenishment procedures
are described in the '573 application (mentioned above).
[0019] Most conventional techniques generally produce some
disturbances in laser operation conditions when the gas is
replenished. For example, strong pronounced jumps of the driving
voltage are produced as a result of macro-halogen injections
(macro-HI) as illustrated in FIG. 12 (macro-HI are distinguished
from micro-halogen injections, or .mu.HI, as described in the '785
application). The result of a macro-HI is a strong distortion of
meaningful output beam parameters such as the pulse-to-pulse
stability. For this reason, in some techniques, the laser is
typically shut down and restarted for gas replenishment, remarkably
reducing laser uptime (see U.S. Pat. No. 5,450,436).
[0020] The '785 application referred to above provides a technique
wherein gas replenishment is performed for maintaining constant gas
mixture conditions without disturbing significant output beam
parameters. The '785 application describes a gas discharge laser
system which has a discharge chamber containing a gas mixture
including a constituent halogen-containing species, a pair of
electrodes connected to a power supply circuit including a driving
voltage for energizing the first gas mixture, and a resonator
surrounding the discharge chamber for generating a laser beam.
[0021] A gas supply unit is connected to the discharge chamber for
replenishing the gas mixture including the constituent
halogen-containing species. The gas supply unit includes a gas
inlet port having a valve for permitting a small amount of gas to
inject into the discharge chamber to mix with the gas mixture
therein. A processor monitors a parameter indicative of the partial
pressure of the first constituent gas and controls the valve at
successive predetermined intervals to compensate a degradation of
the constituent halogen-containing species in the gas mixture.
[0022] The partial pressure of the halogen containing-species in
the gas mixture is increased by an amount preferably less than 0.2
mbar, as a result of each successive injection. The gaseous
composition of the injected gas is preferably 1%-5% of the
halogen-containing gas and 95%-99% buffer gas, so that the overall
pressure in the discharge chamber increases by less than 20 mbar,
and preferably less than 10 mbar per gas injection.
[0023] The processor monitors the parameter indicative of the
partial pressure of the halogen-containing gas and the parameter
varies with a known correspondence to the partial pressure of the
halogen gas. The small gas injections each produce only small
variations in partial pressure of the halogen gas in the gas
mixture of the laser tube, and thus discontinuities in laser output
beam parameters are reduced or altogether avoided.
[0024] The constituent gas is typically a halogen containing
molecular species such as molecular fluorine or hydrogen chloride.
The constituent gas to be replenished using the method of the '785
application may alternatively be an active rare gas or gas
additive. The monitored parameter may be any of time, shot count,
driving voltage for maintaining a constant laser beam output
energy, pulse shape, pulse duration, pulse stability, beam profile,
bandwidth of the laser beam, energy stability, temporal pulse
width, temporal coherence, spatial coherence, amplified spontaneous
emission (ASE), discharge width, and long and short axial beam
profiles and divergences, or a combination thereof. Each of these
parameters varies with a known correspondence to the partial
pressure of the halogen, and then halogen partial pressure is then
precisely controlled using the small gas injections to provide
stable output beam parameters.
[0025] The gas supply unit of the '785 application preferably
includes a small gas reservoir for storing the constituent gas or
second gas mixture prior to being injected into the discharge
chamber (see U.S. Pat. No. 5,396,514, which is assigned to the same
assignee and is hereby incorporated by reference, for a general
description of how such a gas reservoir may be used). The reservoir
may be the volume of the valve assembly or an additional
accumulator. The accumulator is advantageous for controlling the
amount of the gas to be injected. The pressure and volume of the
gases to be injected are selected so that the overall pressure in
the discharge chamber will increase by a predetermined amount
preferably less than 10 mbar, and preferably between 0.1 and 2
mbar, with each injection. As above, the halogen partial pressure
preferably increases by less than 0.2 mbar and preferably far less
such as around 0.02 mbar per injection. These preferred partial
pressures may be varied depending on the percentage concentration
of the halogen containing species in the gas premixture to be
injected.
[0026] Injections may be continuously performed during operation of
the laser in selected amounts and at selected small intervals.
Alternatively, a series of injections may be performed at small
intervals followed by periods wherein no injections are performed.
The series of injections followed by the latent period would then
be repeated at predetermined larger intervals. A comprehensive
algorithm is desired for performing gas actions in order to better
stabilize the gas composition in the laser tube, and
correspondingly better stabilize significant parameters of the
output beam of the excimer or molecular fluorine laser system.
SUMMARY OF THE INVENTION
[0027] It is an therefore an object of the invention to provide an
improved excimer or molecular laser system, wherein the gas mixture
status may be precisely and periodically determined and smoothly
adjusted.
[0028] It is a further object of the invention to provide a
technique which automatically compensates gas mixture degradation
without disturbing laser operation conditions when the gas is
replenished.
[0029] It is another object of the invention to provide an improved
excimer or molecular laser system capable of running continuously
while maintaining stable output beam parameters.
[0030] In accord with the above objects, a gas replenishment
technique is provided for an excimer or molecular fluorine laser
system. The technique encompasses several aspects of the present
invention, each contributing to achieving the above objects. In a
first aspect, it is recognized that the fluorine concentration in
the laser gas mixture has a known correspondence to the value of
the driving voltage, when the driving voltage is being adjusted to
maintain a constant pulse-to-pulse output beam energy, constant
energy dose or moving average energy dose, optimum energy
stability, etc. Thus, a particular gas replenishment action is
performed first based on the value of the driving voltage for each
gas action, and then based on a counter that counts total
accumulated electrical input to the discharge, time and/or pulse
count.
[0031] For example, the amount of gas including a
halogen-containing species and/or the total amount of gas injected
may be based on the driving voltage. Whether the gas action is a
partial or mini gas replacement or only a gas injection is also
determined based on the driving voltage. It may be determined that
no gas action will be presently performed. Also, the interval
between the previous gas action and the next gas action may be
adjusted.
[0032] Another factor that is preferably taken in account in
determining the above particulars of the next gas action is the
specific amount of halogen that was injected during the previous
gas action. That amount may be determined based on measurements of
the gas pressure in an accumulator (see the '785 application) from
which the gas was injected during the previous gas action (and
optionally also based on the pressure in the laser tube). The
temperatures of the gas mixtures in the laser tube and the
accumulator may also be taken into account.
[0033] On a larger overall scale, or macro scale, the determination
of which gas actions are to be performed, if any, may be based on
which of several ranges of driving voltages that the driving
voltage is presently at. For example, if the driving voltage is
presently in a first range, then partial gas replacement (PGR) will
be performed for cleaning the gas mixture, and causing the driving
voltage to vary out of the first range.
[0034] If the driving voltage is presently in a second range below
the first range, then enhanced .mu.HIs together with periodic
mini-gas replacements (MGR) are performed, preferably subject to
adjustments as described above from injection to injection and/or
from MGR to MGR, until the driving voltage varies out of the second
range. Enhanced .mu.HIs may include injections of larger amounts of
halogen than ordinary .mu.HIs, or the injections may be performed
more often or at reduced intervals than ordinary .mu.HIs would be
performed.
[0035] If the driving voltage is presently in a third range below
the second range, then ordinary .mu.HIs together with periodic
mini-gas replacements (MGR) are performed, preferably subject to
adjustments as described above from injection to injection and/or
from MGR to MGR, until and unless the driving voltage varies out of
the third range.
[0036] If the driving voltage is presently in a fourth range below
the third range, then no gas actions are performed. Alternatively,
a gas replacement action may be performed, e.g., to reduce the
fluorine concentration in the gas mixture. More than MGR may be
performed, or more than one amount of gas may be injected (and
correspondingly released) during MGRs, as well, or the interval
between MGRs may be adjusted.
[0037] In addition, after a new fill of the laser tube, the system
of the present invention is adjusted depending on the age of the
tube and/or the optics of the laser resonator. The driving voltage
ranges may adjusted within which the particular types of gas
actions are performed as described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a graph of the output efficiency of an excimer or
molecular laser versus F.sub.2-concentration.
[0039] FIG. 2 is a graph of integrated pulse width of an excimer or
molecular laser versus F.sub.2-concentration
[0040] FIG. 3 shows several graphs of output beam energy of a KrF
excimer laser versus driving voltage for various gas mixture
component partial pressures.
[0041] FIG. 4 shows several graphs of output beam energy of an
excimer or molecular fluorine laser versus driving voltage for
various F.sub.2-concentrations.
[0042] FIG. 5 is a graph of the bandwidth of an excimer laser
versus F.sub.2-concentration.
[0043] FIG. 6 illustrates how F.sub.2 depletion rates vary for
excimer or molecular fluorine lasers depending on discharge chamber
composition.
[0044] FIG. 7 is a graph of pulse energy versus pulse count for an
excimer or molecular laser operating at constant driving
voltage.
[0045] FIG. 8 is a graph of driving voltage versus pulse count for
an excimer or molecular laser operating at constant output pulse
energy.
[0046] FIG. 9 shows a first graph of the long axis beam profile
versus driving voltage and a second graph of the short axis beam
profile versus driving voltage for an excimer or molecular laser
operating at constant output pulse energy.
[0047] FIG. 10 is a graph of the divergence of the short axis of an
output beam versus driving voltage of an excimer or molecular laser
operating at constant output pulse energy.
[0048] FIG. 11 is a graph of output pulse energy stability versus
driving voltage of an excimer or molecular laser operating at
constant output pulse energy.
[0049] FIG. 12 illustrates the strong pronounced discontinuities in
the driving voltage when large halogen partial pressures increases
are rapidly effected in the discharge chamber due to halogen
injections.
[0050] FIG. 13a shows a schematic block diagram of an excimer or
molecular laser in accord with a preferred embodiment.
[0051] FIG. 13b shows a schematic diagram of the gas control unit
of the excimer or molecular laser of FIG. 13a.
[0052] FIG. 14a schematically shows gas lines for halogen
injections into the discharge chamber of the laser of FIG. 13 using
an accumulator.
[0053] FIG. 14b shows a computer display connected to the processor
of FIG. 13a indicating that the processor is controlling the gas
replenishment process.
[0054] FIG. 15 is a graph of driving voltage versus time also
showing periodic halogen injections for a system in accord with a
preferred embodiment.
[0055] FIG. 16 is a graph of driving voltage versus time also
showing periodic halogen injections and mini gas replacements for a
system in accord with a preferred embodiment.
[0056] FIG. 17 is a graph of pulse energy stability (sigma, upper
graph) versus time and moving averages (over 40 pulse intervals,
maximum and minimum) for a laser system operating at 2 kHz in
accord with a preferred embodiment.
[0057] FIG. 18 is qualitative graph of driving voltage versus time
also showing periodic micro-halogen injections (:HI) for a system
in accord with a preferred embodiment.
[0058] FIG. 19 is a graph of energy stability variation versus
pulse count for a system in accord with a preferred embodiment.
[0059] FIG. 20 is a graph of beam divergence versus pulse count for
a system in accord with a preferred embodiment.
[0060] FIG. 21 is a qualitative graph of driving voltage versus
pulse count also showing periodic halogen injections, mini gas
replacements and partial gas replacements for a system in accord
with a preferred embodiment.
[0061] FIG. 22 is a flow diagram for performing halogen injections,
mini gas replacements and partial gas replacements in accord with a
preferred embodiment.
[0062] FIG. 23 is a further qualitative graph of driving voltage
versus pulse count also showing periodic halogen injections, mini
gas replacements and partial gas replacements for a system in
accord with a preferred embodiment.
[0063] FIG. 24 is a further flow diagram for performing halogen
injections, mini gas replacements and partial gas replacements in
accord with a preferred embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0064] FIG. 13a shows a schematic block diagram of a preferred
embodiment of an excimer or molecular fluorine laser. The laser
system of FIG. 13a includes a laser tube 1 including an electrode
or discharge chamber and a gas flow vessel, wherein the gas flow
vessel typically includes a blower and heat exchanger or cooling
unit. The laser tube 1 contains a laser gas mixture, and a pressure
gauge P is preferably provided for monitoring the pressure in the
laser tube 1. A resonator surrounds the tube 1 and includes a rear
optics module 2 and a front optics module 3.
[0065] The rear optics module 2 includes a resonator reflector
which may be a highly reflective mirror, a grating or a highly
reflecting surface of another optical component such as an etalon
or a prism. A wavelength calibration module is preferably included
with the rear optics module. Preferred wavelength calibration units
or devices and techniques are disclosed in U.S. Pat. No. 4,905,243
and U.S. patent application Ser. Nos. 09/136,275, 09/167,657 and
09/179,262, each of which is assigned to the same assignee as the
present application and is hereby incorporated by reference.
[0066] The front optics module 3 preferably includes a resonator
reflector which is preferably an output coupler. The resonator
reflector of the front optics module may alternatively be a highly
reflecting mirror and other means for output coupling the beam 13
may be used, such as a beam splitter or other angled partially
reflecting surface within the resonator. The front optics module 3
also may include a line narrowing and/or selection unit and/or a
wavelength tuning unit.
[0067] Alternatively, the line narrowing and/or selection unit
and/or wavelength tuning unit may be included with the rear optics
module. Such optical elements as one or more beam expanding
elements such as beam expanding prism(s) and/or lens arrangements,
one or more dispersive elements such as dispersive prism(s) and/or
a grating, one or more etalons, birefringent plate(s), or grism(s)
may be included for line narrowing, selection and/or tuning. 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 application
Ser. Nos. 09/317,695, 09/130,277, 09/244,554, 09/317,527,
09/073,070, 60/124,241, 60/140,532, and 60/140,531, 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, and
4,829,536, are each hereby incorporated by reference into the
present application, as describing line narrowing, selection and/or
tuning elements, devices and/or techniques, among others known to
those skilled in the art, which may be used in a laser system
according to the preferred embodiment.
[0068] Wavelength, pulse energy, and gas control information, as
well as other information about the laser system is received by a
processor 11. The processor 11 controls the wavelength of the
output beam 13 by controlling the line tuning module based on the
wavelength information the processor 11 receives, the electrical
pulse power and discharge module ("pulse power module") 5 based on
pulse energy information it receives, and gas control elements 6-10
and 12 based on information it receives relating to the gas mixture
status, and on data saved in its database(s) (see the '653
application, above).
[0069] A beam portion is preferably received by an energy monitor 4
which measures the energy and/or angular distribution and/or other
beam parameters of the received beam portion of the output beam 13.
Data corresponding to the energy of the beam portion is then sent
to the processor 11 which is connected to the energy monitor 4. The
processor 11 then uses this information to perform processing
relating to the energy of the output beam 13.
[0070] The pulse power module 5 provides energy to the gas mixture
via a pair of electrodes 14 within the discharge chamber 1.
Preferably, a preionization unit (not shown) is also energized by
the pulse power module for preionizing the gas mixture just prior
to the main discharge. The energy of the output beam 13 of the
laser system has a known dependence on the "driving voltage" of the
pulse power module. The driving voltage is adjusted during laser
operation to control and stabilize the energy of the output beam
13. The processor 11 controls the driving voltage based on the
energy information received from the energy monitor 4. In accord
with the present invention, the processor 11 also controls and
stabilizes the status of the gas mixture and thus indirectly
controls and stabilizes other laser output beam parameters such as
energy stability, temporal pulse width, spatial and temporal
coherences, bandwidth, and long and short axial beam profiles and
divergences by controlling the status of the gas mixture within the
laser tube 1.
[0071] FIG. 13b shows a detailed schematic of the gas control box
10 of FIG. 13a. The gas control box 10 is connected to the laser
tube 1 for supplying gas based on control signals received from the
processor 11. The processor 11 regulates the delivery of gases or
mixtures of gases to the laser tube 1 via a valve assembly 6 or
system of valves. The valve assembly preferably has a reservoir or
compartment 7 having a known volume and having a pressure gauge P
attached for measuring the pressure in the compartment 7. The
compartment as well as the laser tube preferably also each have
means, such as a thermocouple arrangement, for measuring the
temperature of the gases within the compartment and tube. The
compartment 7 may be 20 cm.sup.3 or so in volumetric size (by
contrast, the laser tube 1 may be 42,000 cm.sup.3 volumetrically).
Four valves 8a-8d are shown as controlling the flow of gases
contained in external gas containers into the compartment 7. Of
course, more or less than four such valves may be provided. Another
valve 32 is shown controlling the access of a vacuum pump vp to the
compartment 7 which is shown connected through a halogen filter hf.
Another valve 34 is shown controlling the flow of gases between the
compartment 7 and the laser tube 1. A further valve or valves (not
shown) may be provided along the line 35 from valve 34 to the tube
1 for controlling the atmosphere in the line 35, e.g., using a pump
for evacuating the line 35.
[0072] Small amounts of a gas or gas mixture are preferably
injected from the compartment 7 into the discharge chamber 1 as
.mu.HIs or enhanced .mu.HIs, or during a PGR or MGR action. As an
example, the gas supply connected to the valve assembly 6 through
gas line 36a may be a premix A including 1% F.sub.2:99% Ne, and
that through gas line 36b may be a premix B including 1% Kr:99% Ne,
for a KrF laser. For an ArF laser, premix B would have Ar instead
of Kr, and for a F.sub.2 laser premix B is not used. Thus, by
injecting premix A and premix B into the tube 1 via the valve
assembly, the fluorine and krypton concentrations in the laser tube
1, respectively, may be replenished. Gas lines 36c and 36d may be
used for different additional gas mixtures. Although not shown, the
tube 1 preferably has additional means for releasing gas, or
alternatively, the gas is released through the valve assembly, such
as via valves 34 and 32.
[0073] New fills, partial and mini gas replacements and gas
injection procedures, e.g., enhanced and ordinary micro-halogen
injections, and any and all other gas replenishment actions are
initiated and controlled by the processor 11 which controls the
valve assembly 6 and the pump vp based on various input information
in a feedback loop.
[0074] An exemplary method according to the present invention is
next described for accurately and precisely replenishing the
fluorine concentration in the laser tube 1 in small amounts such
that significant output beam parameters are not significantly
disturbed, if at all, with each gas injection. The processor 11,
which is monitoring a parameter indicative of the fluorine
concentration in the laser tube 11, determines that it is time for
a micro-halogen injection (.mu.HI).
[0075] The processor 11 then sends a signal that causes valve 8a to
open and allow premix A to fill the compartment 7 to a
predetermined pressure, e.g., 5 bar. Then, valve 8a is closed and
valve 34 is opened allowing at least some of the premix A that was
filled into the compartment 7 to release into the laser tube 1.
[0076] If the pressure in the tube was 3 bar prior to the injection
and the tube has 42,000 cm.sup.3, and the injection is such that
the pressure in the accumulator was reduced to 3 bar after the
injection, then 2.times.20/40,000 bar would be the pressure
increase in the tube 1 as a result of the injection, or 1 mbar. If
the premix A contains 1% F.sub.2:99% Ne, then the increase in
partial pressure of the F.sub.2 in the laser tube as a result of
the injection would be approximately 0.01 mbar.
[0077] The above calculation may be performed by the processor 11
to determine more precisely how much F.sub.2 was injected, or prior
to injection, the pressure in the compartment 7 may be set
according to a calculation by the processor 11 concerning how much
F.sub.2 should be injected based on the status information of the
monitored parameter received by the processor 11, or based on
pre-programmed criteria. A correction for difference in temperature
between the gas in the compartment 7 and that in the tube 1 may
also be performed by the processor 11 for more accuracy, or the
temperature of the gas in the compartment 7 may be preset, e.g., to
the temperature within the laser tube 1.
[0078] Preferably, an amount of gas premix corresponding to smaller
than 10 mbar total gas pressure, or 0.1 mbar F.sub.2 partial
pressure, increase in the tube 1 is injected from the compartment
7. Even more preferably, less than 5 mbar or even 2 mbar total gas
pressure (0.05 or 0.02 mbar F.sub.2 partial pressure) increase in
the laser tube 1 results from the gas injection.
[0079] The compartment 7 may simply be the valve assembly 6 itself,
or may be an additional accumulator (described in detail below).
The compartment 7 is also configured so that the small amounts of
gas may be injected at successive very short intervals, to
compensate a degradation of a halogen gas and/or another gas or
gases within the discharge chamber 1 of an excimer or molecular
laser such as a KrF, ArF or F.sub.2 laser.
[0080] There may be more than one compartment like compartment 7,
as described above, each having different properties such as
volumetric space. For example, there may be two compartments, one
for .mu.HIs and the other for enhanced .mu.HIs. There may be more
than two, for still further versatility in the amounts of halogen
to be injected in a gas action, and for adjusting the driving
voltage ranges corresponding to different gas action algorithms.
Different premixes may be injected from the different compartments.
Also, the exemplary method described using premixes of particular
gas compositions, but many different gas compositions could be used
in accord with the present invention. For example, gas compositions
having higher fluorine (or hydrogen chloride) percentage
concentrations could be used such as 5% or 2% instead of 1%. There
also may be an additional valve connected to a 100% buffer gas
container.
[0081] Advantageously, the processor 11 and gas supply unit are
configured to permit the delivery or injection of very small
amounts of one or more gases or gas mixtures to the discharge
chamber 1. The injection of the small amounts of the gas or gas
mixture result in gas pressure increases in the discharge chamber 1
below 10 mbar, and preferably between 0.1 and 2 mbar. Each gas in
the gas mixture within the discharge chamber 1 may be separately
regulated so the gas composition within the discharge chamber may
be precisely controlled. For example, similar injections of Kr, Ar
or Xe may be performed for replenishing those gases in the laser
tube 1.
[0082] Because the amount of gas injected during a gas injection or
replacement procedure is small, laser output beam parameters do not
vary greatly with each injection. The injections are preferably
carried out periodically at predetermined intervals corresponding
to known depletion amounts of the gases. For example, if the
halogen partial pressure in the gas mixture of an F.sub.2 laser is
known, under current operating conditions, to be around 3 bar after
a new fill and to deplete by 0.1 mbar per X minutes or Y shots,
then halogen injections including, e.g., 1 mbar (pressure increase
in tube 1) of a premix including 1% F.sub.2 could be performed
every X/10 minutes or Y/10 shots, in accord with the present
invention, to maintain the concentration of the halogen, or halogen
injections of 2 mbar of the premix may be performed every X/5
minutes, and so on. Also, micro-halogen injections (.mu.HI) of 1
mbar of premix A including 1% F.sub.2 and 99% Ne buffer may be
injected every X/5 minutes for 100 minutes followed by a period of
100 minutes when no injections are performed. Many variations are
possible within the spirit of the present invention including
irregular gas actions as determined by the processor.
[0083] In contrast with the present invention, if, e.g., a 50 mbar
(pressure increase in tube 1) premix A injection (again having 1%
F.sub.2 such that the F.sub.2 partial pressure increase in the tube
1 is 0.5 mbar and corresponds to around a 17% increase in the
F.sub.2 concentration in the tube 1) is performed every 5X minutes
or 5Y shots, or at any time, the large injection amount will cause
output beam parameters of the laser beam to noticeably and
undesirably fluctuate in response. For example, the pulse energy or
driving voltage can fluctuate by 10% or more when the large
injection is performed. If the laser is not shut down, or
industrial processing interrupted, when the large injection is
performed, then imprecise industrial processing will occur due to
disturbances in meaningful output beam parameters.
[0084] The halogen injection algorithm of the present invention may
be considered to extend a total halogen injection over a longer
period of time or number of pulse counts. Over the period of the
several halogen injections, the high voltage and the F.sub.2
concentration do not change significantly so that significant
changes in pulse energy and pulse energy stability, among other
meaningful output beam parameters, are eliminated. Again, some of
these other output beam parameters are listed above and each will
be extremely stable using the method of the present invention.
[0085] FIG. 14a schematically shows another configuration of gas
lines for halogen injections into the discharge chamber 1 of the
laser of FIG. 13a using an accumulator 6a. The accumulator 6a is
connected to the laser tube 1 via laser head valve LH. The
accumulator 6a is also connected to a gas line 12a via halogen
valve H connected to a gas bottle 13 including the halogen or
halogen premix. For example, the gas bottle 13 may be filled with a
gas mixture including an F.sub.2 mixture (e.g., 5% F.sub.2/95% Ne
or a 5% HCl/1% H.sub.2 in neon mixture or a 1% F.sub.2:99% Ne
premix, among other possibilities). A pump is shown connected to
each of the accumulator 6a and the laser tube 1 via a vacuum valve
V. The tube 1 is shown valve-connected to additional gas lines and
valves including a buffer gas via valve B, a rare gas or rare gas
premix via valve R (used with KrF, ArF, XeCl and XeF excimer
lasers, e.g.) and an inert gas via valve I. The inert gas valve I
or another valve not depicted may be used for valve connecting to a
source of Xe to be used as an additive in the gas mixture within
the tube. Again, one or more additional accumulators may be added
to the system.
[0086] The accumulator 6a has the particular advantage that the
small amounts of gas including the F.sub.2 within the F.sub.2
premix to be injected with each halogen injection in accord with
the present invention may be precisely controlled. The accumulator
is easily pumped to low pressure. A precise amount of F.sub.2 gas
or F.sub.2 gas premix is released into the accumulator 6a and the
amount of F.sub.2 is determined according to the total gas pressure
within the accumulator, the known volumes of the accumulator 6a and
the laser tube 1 and the known concentration of the F.sub.2 or the
F.sub.2 percentage concentration in the premix gas. A F.sub.2
partial pressure increase in the laser tube 1 after the injection
is determined based on the amount of F.sub.2 known to be in the
accumulator 6a prior to (and possible after) the injection.
[0087] Based on this determination and/or other factors such as the
interval between the previous and current gas actions (measured in
time or pulse count, e.g.) and/or the value of the driving voltage
at the time of the previous, present and/or next gas action, the
interval between the current and next gas action and/or the amount
of halogen containing gas or total gas to be injected in the next
gas action may be determined so that a precise amount of each gas,
particularly the halogen-containing gas, may be injected in the
next gas action. Also, the type of gas action to be performed may
be determined based on these or other factors.
[0088] FIG. 14b shows how a display monitor attached to the
processor 11 might look as the laser system is operating. The laser
tube is shown to have an internal pressure of 2064 mbar, while the
pressure within the gas manifold (corresponding to the compartment
7 of FIG. 13a or the accumulator 6a of FIG. 14a) shows an internal
pressure of 4706 mbar. As discussed, the precise amount of gas
injected into the laser tube can be calculated based in part on
these pressure readings. Again, the temperature may be taken into
account for making an even more precise determination.
[0089] Various gas actions and procedures will now be described.
The procedures are potentially applicable to all gas discharge
lasers, although excimer lasers (e.g., KrF, ArF, XeCl and XeF) and
the F.sub.2 laser would benefit greatly by the present invention.
The KrF-laser is used as a particular example below.
[0090] The process begins with a new fill which is performed prior
to operating the laser system. For a new fill, the laser tube 1 is
evacuated and a fresh gas mixture is then filled in. A new fill of
a KrF-laser would typically result in a gas mixture having
approximately the following partition of gases:
F.sub.2:Kr:Ne=0.1%:1.0%:98.9%. If the gas mixture within the KrF
laser discharge chamber has a typical total pressure of around
p=3000 mbar, then the partial pressures of F.sub.2 and Kr would
typically be around 3 mbar and 30 mbar, respectively. A new fill
for a F.sub.2 laser would produce the following typical partition
of gases: F.sub.2:Ne=0.1%:99.9%. For the F.sub.2 laser, He or a
mixture of He and Ne may be used as the buffer instead of only Ne
(see the '526 application, above).
[0091] The new fill procedure can be performed using separate gas
lines delivering pure or premixed gases. Typical gas premixes used
regularly in semiconductor industry fabs are premixes A and B,
where: premix A has 1% F.sub.2/1% Kr/Ne and premix B has 1%
Kr/Ne.
[0092] After the new fill is performed, the halogen gas begins to
react with components of the laser tube 1 that it comes into
contact with, whether the laser is operating or not. "Gas
replenishment" is a general term which includes gas replacement
(PGRs and MGRs each subject to varying amounts and compositions of
injected and released gases) and gas injections (.mu.HIs and
enhanced .mu.HIs again each subject to varying amounts and
compositions of injected gases), performed to bring the gas mixture
status back closer to new fill status.
[0093] Any gas replenishment procedures are performed taking into
account that each gas in the gas mixture depletes at a different
depletion rate due to the halogen depletion just described and the
gas replenishment procedures performed in response. For the narrow
band KrF-laser, e.g., F.sub.2-depletion occurs at a rate of between
about 0.1% to 0.3% (and sometimes up to nearly 1%) per million
shots, whereas Kr depletion occurs about 10 to 50 times more
slowly. The Ne buffer is less important, but may also be considered
as part of an overall gas replenishment operation, e.g., to
maintain a desired pressure in the tube 1.
[0094] Separate gas actions are preferably performed to replenish
each constituent gas of the gas mixture. For the KrF-laser, for
example, the F.sub.2 may be replenished by halogen or halogen/rare
gas or premix A injections and the Kr replenished by rare gas or
premix B injections. Other gas additives such as Xe may be
replenished by Xe gas or still further premixes C, D, etc. The
individual depletion rates also depend on operating conditions of
the laser such as whether the laser is in broadband or narrow band
mode, the operating energy level, whether the laser is turned off
or is in continuous, standby or other burst pattern operation, and
the operating repetition rate. The processor 11 is programmed to
consider all of these variations in laser operation.
[0095] The gas mixture status is considered sufficiently stable in
the present invention when deviations in fluorine and krypton
content are below 5%, and preferably below 3%. Without any gas
replenishment actions, after 100 million shots the partial
pressures of F.sub.2 and Kr might degrade by between 30% and 100%
and between 0.5% and 5%, respectively.
[0096] To compensate for the various depletion rates of the gases
in the discharge chamber, the present invention performs a variety
of separate and cross-linked gas replenishment procedures, which
take into account the variety of individual degradation rates by
referring to a comprehensive database of different laser operating
conditions. A preferred technique is disclosed in the '653
application already mentioned above. The behavior of the particular
laser in operation and related experiences with gas degradation
under different operating conditions are stored in that database
and are used by a processor-controlled "expert system" to determine
the current conditions in the laser and manage the gas
replenishment or refurbishment operations. A history of gas actions
performed during the current operation of the laser may also be
used in accord with the present invention.
[0097] As mentioned above, series of small gas injections (referred
to as enhanced and ordinary micro gas or halogen injections, or
.mu.HI) can be used to return any constituent gas of an excimer or
molecular laser, particularly the very active halogen, to its
optimal concentration in the discharge chamber without disturbing
significant output beam parameters. However, the gas mixture also
degrades over time as contaminants build up in the discharge
chamber. Therefore, mini gas replacements (MGR) and partial gas
replacements (PGR) are also performed in the preferred methods. Gas
replacement generally involves releasing some gas from the
discharge chamber, including expelling some of the contaminants.
MGR involves replacement of a small amount of gas periodically at
longer intervals than the small .mu.HIs are performed. PGR involves
still larger gas replacement and is performed at still longer
periodic intervals generally for "cleaning" the gas mixture. The
precise intervals in each case depend on consulting current laser
operating conditions and the expert system and comprehensive
database. The intervals are changes of parameters which vary with a
known relationship to the degradation of the gas mixture. As such,
the intervals may be one or a combination of time, pulse count or
variations in driving voltage, pulse shape, pulse duration, pulse
stability, beam profile, coherence, discharge width or bandwidth.
In addition, the accumulated pulse energy dose may used as such an
interval. Each of .mu.HI, MGR and PGR may be performed while the
laser system is up and running, thus not compromising laser
uptime.
[0098] Three exemplary gas replenishment methods for stabilizing an
optimum gas mixture are described below. Many other methods are
possible including combinations of the ones described below. The
methods and parameters used may also be varied during the laser
operation depending on the laser operating conditions and based on
the data base and the expert system. The processor and gas supply
unit are configured to perform many methods based on a
comprehensive database of laser operating conditions and gas
mixtures statuses.
[0099] Each method involves well-defined very small gas actions
with small, successive gas injections preferably by injecting a
premix of less than 10 mbar and more preferably between 0.1 and 2
mbar including a concentration including preferably 5% or less of
the halogen containing species in order not to disturb the laser
operation and output beam parameters. Whatever the composition of
the premix, it is the amount of the halogen in the premix that is
most significant. That is, the preferred amount of the halogen
containing species that is injected in the small gas actions
preferably corresponds to less than 0.1 or 0.2 mbar and more
preferably between 0.001 and 0.02 mbar partial pressure increase in
the laser tube 1.
[0100] The first exemplary gas stabilization method involves
performing gas injections based on operation time. The method takes
into account whether or not the laser is operating, i.e., whether
the laser system is up and performing industrial processing, in
standby mode, or simply shut off. The first method is thus useful
for maintaining either an active or a passive gas composition
status. Time-correlated .mu.HI, MGR and PGR are performed according
to a selectable time interval based on operating conditions. For
example, .mu.HIs may be performed after time intervals t.sub.1,
MGRs after time intervals t.sub.2, and PGRs after time intervals
t.sub.3.
[0101] In accord with the present invention, the time intervals
t.sub.1, t.sub.2 and t.sub.3 are adjusted in real time as are the
amounts and/or compositions of gases injected during the gas
actions. Preferably, the time intervals and gas amounts and
compositions are adjusted from gas action to gas action. In
addition, the driving voltage ranges within which particular gas
actions are performed are preferably also adjusted, at least at
each new fill based on the aging of the tube and optical components
of the laser resonator. Such ranges may be adjusted during
operation, even between new fills, e.g., based on beam-induced
effects on the optical components of the line-narrowing module (see
for a general explanation of such effects U.S. patent application
Ser. No. 60/124,804, assigned to the same assignee and hereby
incorporated by reference).
[0102] Below, detailed graphs are described for an operating laser
system in accord with the present invention. Typically, gas actions
occur after several hours if the laser is in the standby-mode
without pulsing or pulsing with low repetition rate (<100 Hz).
If the laser is completely switched off (power-off-mode), a battery
driven internal clock is still running and the expert system can
release an adequate, time controlled number of injections during
the warm-up phase after re-starting the laser. The number and
amount of the injections can be also related to certain driving
voltage start conditions which initiate a preferred sequence of gas
actions to reestablish optimum gas quality.
[0103] FIGS. 15 and 16 are graphs of driving voltage versus time
also illustrating the intervals of periodic .mu.HI and periodic
.mu.HI and MGR, respectively, for a fully operating system in
accord with the present invention. FIG. 15 includes a plot of
driving voltage versus time (A) wherein .mu.HIs are performed about
every 12 minutes, as indicated by the vertical lines (some of which
are designated for reference with a "B") on the graph, for a
narrowband laser running in 2000 Hz burst mode at 10 mJ output beam
energy. The vertical axis only corresponds to graph A. As is shown
by graph A, the small .mu.HIs produce no noticeable discontinuities
in the driving voltage.
[0104] FIG. 16 is a plot (labelled "A") of driving voltage versus
time wherein .mu.HIs are performed about every 12 minutes, as
indicated by the short vertical lines on the graph (again, some of
which are designated for reference with a "B" and the vertical axis
doesn't describe the halogen injections in any way), and MGR is
performed about every 90 minutes, as indicated by the taller
vertical lines on the graph (some of which are designated with a
"C" for reference and again the vertical axis is insignificant in
regard to the MGRs shown), for a narrowband laser running in 2000
Hz burst mode at 10 mJ output beam energy. Again, the driving
voltage is substantially constant around 1.8 KV and no major
changes, e.g., more than %5, are observed.
[0105] A comparison of FIGS. 15 and 16 with FIG. 8 reveals that the
present invention advantageously avoids the conventional approach
which drastically increases the driving voltage as the gas mixture
degrades. By avoiding discontinuities, fluctuations or changes in
the driving voltage in this way, disturbances of meaningful output
beam parameters are also avoided.
[0106] FIG. 17 includes a graph (labelled "A") of pulse energy
stability versus time of the laser pulses by values of standard
deviation (SDEV) and moving average stabilities (.+-.MAV) as
percentages of the absolute pulse energy for a system in accord
with the present invention. The graphs labelled "B" and "C" show
the moving average for groups of 40 pulses each. During this run,
micro-halogen injections were performed resulting in very stable
continuous laser operation without any detectable deviations caused
by the gas replenishment actions.
[0107] The second exemplary gas stabilization method involves
performing gas injections based on shot or pulse count using a shot
or pulse counter. After certain numbers of laser pulses, e.g.,
N(.mu.HI), N(MGR), and N(PGR), depending again on the mode of
operation of the laser, .mu.HI, MGR and PGR can be respectively
performed. Typically, the .mu.HIs amount to about 0.5 . . . 2.0
mbar of fluorine premix (e.g., 1-5% F.sub.2:95-99% Ne) for the KrF,
ArF, XeF or F.sub.2 lasers (Ne being replaceable with He or a mix
of He and Ne) or HCl premix (e.g., 1-5% HCl:1% in Ne or He) for
XeCl or KrCl laser and are released after several hundred thousand
or even after millions of laser shots. Each .mu.HI just compensates
the halogen depletion since the last gas action and typically
corresponds to less than 0.1 mbar of the halogen containing species
and more preferably between 0.001 and 0.02 mbar partial pressure
increase in the laser tube 1 per, e.g., 1 million shots. The actual
amounts and shot intervals vary depending on the type of laser, the
composition of the discharge chamber, the original gas mixture
composition and operating mode, e.g., energy, or repetition rate,
being used.
[0108] A third exemplary method is similar to those described above
using time or pulse count, and this method instead uses accumulated
energy applied to the discharge. Use of this parameter, and
advantages thereof, are set forth in the '525 application. The
total input electrical energy to the discharge is maintained in a
counter for that purpose, and gas actions are performed after
certain intervals or amounts of this input electrical energy are
applied.
[0109] Also, in accord with a preferred embodiment, the intervals
of any of the exemplary methods are dynamically adjusted from
injection to injection, as are the amounts of halogen injected with
each gas action. The interval between the current and next
injection is set based on any one or a combination of parameters
such as the driving voltage or any of the output beam parameters
described above. In addition, the amount of halogen injected in the
current injection and/or the interval between the previous and
current injection may be taken into account.
[0110] The amount of halogen injected in any .mu.HI or enhanced
.mu.HI may be determined in accord with the present invention by
measuring the pressure in the accumulator (see FIGS. 13b and 14a)
and the laser tube at the time of the injection, and/or just
before, and/or just after the injection. The temperatures of the
gases in the accumulator and tube may be measured as well. The
interior volumes of the tube and accumulator are known in advance.
The well-known formula PV=Nk.sub.BT is used to calculate the amount
of halogen injected into the tube during any injection.
[0111] For example, if the accumulator has a measured halogen
partial pressure P.sub.a, and temperature T.sub.a, and a volume
V.sub.a, then the accumulator contains N.sub.a fluorine molecules.
If all of the N.sub.a molecules are injected into the laser tube
during the injection, and the tube has a temperature T.sub.T and
volume V.sub.T, then the change in fluorine partial pressure in the
tube as a result of the injection will be
.DELTA.P(F.sub.2).sub.T=P.sub.aV.sub.aT.sub.T/V.sub.TT.sub.a. Since
it is desired to maintain the total number of fluorine molecules in
the tube, then it may be more appropriate to calculate the change
in the number of fluorine molecules in the tube, i.e.,
.DELTA.N(F.sub.2).sub.T=)- P(F.sub.2).sub.TV.sub.T/k.sub.BT.sub.T,
and keep track of that quantity. Then, the amount of halogen and/or
the interval before the next injection is determined based on the
calculated amount of halogen that was injected in the previous
injection, the partial pressure of the halogen in the tube after
the previous injection and/or the amount of halogen that it is
desired to have in the tube after the next injection.
[0112] The overall calculation depends also on the amount of
depletion that the halogen gas has undergone (or will undergo)
between injections. Such depletion is, in principal, known as a
function of many factors, e.g., including time and pulse count (and
possibly any of the parameters enumerated above or others). For
example, a change in halogen partial pressure (or, alternatively,
the number of halogen molecules) in the laser tube in the interval
between injections can be calculated to depend on k.sub.t.times.
.DELTA.t and on k.sub.p.times..DELTA.p, wherein k.sub.t and k.sub.p
are constants that depend on the rate of halogen depletion with
time and pulse count, respectively, and .DELTA.t and .DELTA.p are
the amount of time and the number of pulses, respectively, in the
interval under consideration. The number of pulses .DELTA.p itself
depends on the repetition rate, taking into account also the number
of pulses in a burst and the pause intervals between bursts for a
laser operating in burst mode. Again, other parameters may have an
effect and may be additive terms included with this
calculation.
[0113] Now, from one interval to the next, a calculation could be
performed as follows. The increase (or decrease reflected as a
negative sum) in fluorine partial pressure in the laser tube over
the interval is calculated to be:
.DELTA.P(F.sub.2).sub.interval.about.P(F.sub.2).sub.T
injection-k.sub.t.times..DELTA.t-k.sub.p.times..DELTA.p. Again,
since it is the total number of fluorine molecules that it is
desired to keep constant, then a calculation of the change in the
number of molecules is calculated as:
.DELTA.N(F.sub.2).sub.interval.about.N(F.sub.2).sub.T
injection-k.sub.t.times..DELTA.t-k.sub.p.times..DELTA.p, where the
constants k.sub.t and k.sub.p would differ from the partial
pressure calculation by a units conversion.
[0114] The overall algorithm would seek to maintain the total
number of halogen molecules (or halogen partial pressure) constant.
Thus, the changes in particle number (or partial pressure) would be
summed continuously over many intervals, or preferably all
intervals since the last new fill. That overall sum would be
maintained as close as possible to zero, in accord with the present
invention.
[0115] As discussed, the shot counter can also be used in
combination with time related gas replenishment, and either of the
shot counter or time related gas replenishment can be used in
combination with the total energy applied to the discharge. The
shot counter or total applied energy can be used for different
laser pulse operation modes, e.g., burst patterns, or continuous
pulsing modes at different pulse repetitions wherein a number of
individual shot or input energy counters N.sub.i(HI) are used. All
of these different counters can be stored in the data base of the
expert system. Which of the different counters N.sub.i(HI) is to be
used at any time is determined by the software of the expert
system.
[0116] FIG. 18 illustrates qualitatively a driving voltage free of
discontinuities when small partial pressure increases are effected
in the laser discharge chamber due to .mu.HIs in accord with the
present invention. The driving voltage is shown as being
substantially constant at around 1.7 KV over 150 million pulses,
while .mu.HIs are performed about once every 12 million pulses. The
pulse energy is also maintained at a constant level.
[0117] A comparison of FIG. 18 with the driving voltage graph of
FIG. 12 shows an advantage of the present invention. In FIG. 12 the
driving voltage is observed to increase steadily until a halogen
injection (HI) is performed, and is then observed to drop
precipitously when the halogen is injected in a large amount in
accord with conventional gas replenishment. These disturbances in
the driving voltage curve of FIG. 12 occur because the intervals
for the HIs are too large and the amounts of halogen injected are
thus too large to prevent the disturbances. As can be deduced from
FIGS. 9-11, these large driving voltage disturbances undesirable
affect meaningful output beam parameters. FIG. 18, on the other
hand, shows no fluctuations in the driving voltage in response to
micro-halogen injections performed in accord with the present
invention.
[0118] FIG. 19 is a graph including two plots. The first plot
following the darkened triangles and labelled "convention HI" is
the energy stability variation versus pulse count for a system
using a conventional HI algorithm and shows sharp discontinuities
in the energy stability. For example, the first HI is shown to
produce a leap from 0.95% to 1.10% almost instantaneously in
response to the HI. The second plot following the darkened circles
and labelled ".mu.HI-present invention" is the energy stability
variation versus pulse count for a system using a .mu.HI algorithm
in accord with the present invention wherein discontinuities are
substantially minimized in the energy stability.
[0119] FIG. 20 is a graph also including two plots. The first plot
following the darkened triangles and labelled "conventional HI" is
the beam divergence versus pulse count for a system using a
conventional HI algorithm and shows sharp discontinuities in the
beam divergence. For example, the first HI is shown to produce a
sharp drop from 1.175 mrad to 1.125 mrad almost instantaneously in
response to the HI. The second plot following the darkened circles
and labelled ".mu.HI-present invention" is the beam divergence
versus pulse count for a system using a .mu.HI algorithm in accord
with the present invention wherein discontinuities are
substantially minimized in the beam divergence.
[0120] The expert system can use a different kind of shot counter,
e.g., N(MGR) and/or N(PGR) for other types of gas actions (i.e.,
different from the N(.mu.HI)). MGR and PGR replace or substitute
different gases of the gas mixture in the laser tube by
predetermined amounts. As mentioned, MGR and PGR include a gas
injection accompanied by a release of gases from the laser tube,
whereas .mu.HI do not involve a release of gases. Gas releases can
be performed simply to reduce the pressure in the laser tube, as
well as for expelling contaminants from the gas mixture. Unequal
degradations of the individual gas components within the gas
mixture are nicely compensated using MGR and PGR, and again,
different numbers N.sub.i(MGR) and N.sub.i(PGR) may be used for
different operating modes and conditions as determined by the
expert system. All of these settings, i.e., N.sub.i(.mu.HI),
N.sub.i(MGR), N.sub.i(PGR) and the separately selectable portions
of injections for each gas can be adapted for the aging of the
laser tube, and/or the aging of the resonator optics, taking into
account changing conditions of gas consumption and replenishments
as the laser system components age. The amount of compensation can
be pre-selected by manual settings or based on settings in the data
base of the computer controlled expert system. For MGR, like
.mu.HI, the portions of injected gases amount to a few mbar total
pressure increase in the laser tube (or percent only). The MGR is
combined with a small pressure release of some few to 10 mbar of
the pressure of the tube, preferably bringing the pressure in the
tube back near to the pressure in the tube just after the last new
fill.
[0121] More than one gas may be injected or replaced in the same
gas action. For example, a certain amount of halogen and a certain
amount of an active rare gas and/or a gas additive for an excimer
laser may be injected together into the laser tube. This injection
may be accompanied by a small pressure release as with MGR.
Alternatively, this mixture of the halogen and rare or additive
gases may simply be injected to increase the partial pressure of
each gas within the discharge chamber without any accompanying
release of gases.
[0122] A further exemplary gas stabilization method involves
performing gas injections based on operating driving voltage values
of the laser. This method can be and preferably is advantageously
combined with any of the first, second and third exemplary methods.
That is, the time related t.sub.1(.mu.HI), t.sub.2(MGR),
t.sub.3(PGR) and the pulse and/or input electrical energy to the
discharge counter-related N.sub.i(.mu.HI), N.sub.i(MGR),
N.sub.i(PGR) gas actions, discussed above, are generally adjusted
during operation depending on the value of the operating driving
voltage, and preferably, on the operation band of the driving
voltage.
[0123] Referring to FIG. 21, several driving voltage levels
(HV.sub.i) can be defined wherein particular gas actions are
predetermined to be performed. The processor monitors the driving
voltage and causes the gas supply unit to perform gas injections of
varying degrees and partial and mini gas replacements of varying
degrees depending on the value of the driving voltage, or which
preset range the current operating driving voltage is in (y-axis of
FIG. 21), based on such parameters as time, pulse count and/or
total input electrical energy to the discharge, etc. (see '525
application mentioned above) (x-axis of FIG. 21).
[0124] An example in accord with the present invention is next
described with reference to FIG. 21. The laser system may operate
at driving voltages between HV.sub.min and HV.sub.max. The actual
operating minimum and maximum driving voltages are set to be in a
much smaller range between HV.sub.1 and HV.sub.6, as illustrated by
the broken ordinate axis. An advantage of the present invention is
that the range HV.sub.1 to HV.sub.6 itself may be reduced to a very
small window such that the operating voltage is never varied
greatly during operation of the laser. Where this operating range
itself lies between HV.sub.min and HV.sub.max, i.e., the actual
voltage range (in Volts) corresponding to the range may be
adjusted, e.g., to increase the lifetimes of the optical components
of the resonator and the laser tube, e.g., such as by adjusting an
output energy attenuating gas additive (see the '126
application).
[0125] The coordinate axis of FIG. 21 denotes the gas actions that
are performed, based on one or more accumulated parameters, when
the driving voltage is in each interval. The general order of
performance of the gas actions is from left to right as the gas
mixture ages. However, when each gas action is performed, the
driving voltage is checked, and the next gas action may that
corresponding to the same driving voltage range, or a different one
denoted to the left or the right of that range. For example, after
a PGR is performed (when it is determined that the driving voltage
is above HV.sub.5), the driving voltage may be reduced to between
HV.sub.2 and HV.sub.3, and so the system would return to ordinary
.mu.HI and MGR.sub.1 gas control operations.
[0126] Within the operating range between HV.sub.1 and HV.sub.6,
several other ranges are defined. For example, when the driving
voltage HV is between HV.sub.1 and HV.sub.2 (i.e.,
HV.sub.1<HV<HV.sub.2), no gas actions are performed as there
is a sufficient amount of halogen in the gas mixture. When the
driving voltage is between HV.sub.2 and HV.sub.3 (i.e.,
HV.sub.2<HV<HV.sub.3), MGR.sub.1 and ordinary .mu.HI are
performed periodically based on the accumulated parameter(s) (i.e.,
input electrical energy to the discharge, time, and/or pulse count,
etc.). This is the ordinary range of operation of the system in
accord with the present invention.
[0127] When the driving voltage is between HV.sub.3 and HV.sub.4
(i.e., HV.sub.3>HV> HV.sub.4), one or both of the injection
amounts of the .mu.HIs and the MGRs with corresponding gas releases
is increased. In this example, only the .mu.HIs are increased.
Thus, the range between HV.sub.3 and HV.sub.4 in FIG. 21 is the
range within which enhanced .mu.HIs are performed, and the same MGR
amounts as in the previous range between HV.sub.2 and HV.sub.3 are
maintained.
[0128] Enhanced .mu.HIs may differ from ordinary .mu.HIs in one or
both of two ways. First, the amount per injection may be increased.
Second, the interval between successive .mu.HIs may be
increased.
[0129] The range between HV.sub.4 and HV.sub.5 (i.e.,
HV.sub.4<HV<HV.sub.5) represents a new range within which one
or both of the injection amounts of the .mu.HIs and the MGRs with
corresponding gas releases is increased. In this example, only the
MGRs are increased as compared with the range HV.sub.3 to HV.sub.4.
Thus, an enhanced amount of halogen gas is injected (with
corresponding release of gases) during each MGR.sub.2 than the
ordinary amount MGR.sub.1 when the driving voltage is in the range
between HV.sub.4 and HV.sub.5. Alternatively or in combination with
replacing the gas in larger amounts, the mini gas replacements
MGR.sub.2 are performed at smaller intervals than the MGR.sub.1 are
performed. In each of the preferred and alternative MGR.sub.2
procedures, the contaminants in the discharge chamber are reduced
at smaller intervals (e.g., of accumulated input energy to the
discharge, pulse count and/or time, among others) compared with the
MGR.sub.1 procedures that are performed at the lower driving
voltage range between HV.sub.3 and HV.sub.4. The .mu.HIs are also
preferably performed periodically in this range to recondition the
gas mixture. It is noted here that several ranges wherein either or
both of the amounts injected during the .mu.HIs and MGRs is
adjusted may be defined each corresponding to a defined driving
voltage range. Also, as mentioned above with respect to monitoring
the pressure (and optionally the temperature) in the accumulator
(and optionally the laser tube), the amount injected may be
adjusted for each injection.
[0130] When the driving voltage is above HV.sub.5 (i.e.,
HV.sub.5<HV<HV.sub.6), a still greater gas replacement PGR is
implemented. PGR may be used to replace up to ten percent or more
of the gas mixture. Certain safeguards may be used here to prevent
unwanted gas actions from occurring when, for example, the laser is
being tuned. One is to allow a certain time to pass (such as
several minutes) after the HV.sub.5 level is crossed before the gas
action is allowed to be performed, thus ensuring that the driving
voltage actually increased due to gas mixture degradation. When the
driving voltage goes above HV.sub.6, then it is time for a new fill
of the laser tube. It is noted here that the magnitudes of the
driving voltages ranges shown in FIG. 21 are not necessarily drawn
to scale.
[0131] FIG. 22 is a flow diagram for performing ordinary and
enhanced .mu.HIs, MGRs and PGRs in accord with the present
invention and the example set forth as FIG. 21. The procedure
starts with a new fill, wherein the discharge chamber is filled
with an optimal gas mixture. The laser can thereafter be in
operation for industrial applications, in stand-by mode or shut off
completely. A driving voltage check (HV-check) is performed after
the current driving voltage (HV) is measured.
[0132] The measured driving voltage (HV) is compared with
predetermined values for HV.sub.1 through HV.sub.6. The processor
determines whether HV lies between HV.sub.1 and HV.sub.2 (i.e.,
HV.sub.1<HV<HV.sub.2) and thus path (1) is followed and no
gas actions are to be performed and the procedure returns to the
previous step. Although not shown, if the HV lies below HV.sub.1,
then a procedure may be followed to decrease the halogen
concentration in the laser tube, such as by releasing some laser
gas and/or injecting some buffer gas from/into the laser tube.
[0133] If the processor determines that the HV lies between
HV.sub.2 and HV.sub.3, then the system is within the ordinary
operating driving voltage band. If it is within the ordinary
operating band, then path (2) is followed whereby ordinary .mu.HIs
and MGR.sub.1 may be performed based preferably on time, input
electrical energy to the discharge and/or pulse count intervals as
predetermined by the expert system based on operating conditions.
Again each gas action may be adjusted depending on the calculated
partial pressure or number of halogen molecules in the laser tube,
as described above.
[0134] The .mu.HIs and MGR.sub.1 performed when path (2) is
followed may be determined in accordance with any method set forth
in U.S. patent application Ser. No. 09/167,653, already
incorporated by reference. If HV is not within the ordinary
operating band, then it is determined whether HV lies below
HV.sub.2 (i.e., HV<HV.sub.2). If HV is below HV.sub.2, then path
(2) is followed and no gas actions are performed.
[0135] If HV lies between HV.sub.3 and HV.sub.4 (i.e.,
HV.sub.3<HV<HV.sub.4), then path (3) is followed and enhanced
.mu.HI and MGR.sub.1 may be performed again based on the value or
values of the time, pulse count and/or applied electrical energy to
the discharge counter(s) being used. The precise amounts and
compositions of gases that are injected and those that are released
are preferably determined by the expert system and will depend on
operating conditions.
[0136] If HV lies between HV.sub.4 and HV.sub.5 (i.e.,
HV.sub.4<HV<HV.sub.5), then path (4) is followed and enhanced
.mu.HI and MGR.sub.2 may be performed depending on checking the
values of the counters. Again, the precise amounts and compositions
of gases that are injected and those that are released are
preferably determined by the expert system and will depend on
operating conditions.
[0137] If HV lies between HV.sub.5 and HV.sub.6 (i.e.,
HV.sub.5<HV<HV.sub.6), then PGR is performed. If HV lies
above HV.sub.6 (i.e., HV.sub.6<HV), then a new fill is
performed.
[0138] After any of paths (2)-(5) is followed and the corresponding
gas actions are performed, and preferably after a specific settling
time, the method returns to the step of determining the operating
mode of the laser and measuring and comparing HV again with the
predetermined HV levels HV.sub.1 through HV.sub.6.
[0139] The setting of all of these different driving voltage levels
and time, applied electrical energy to the discharge and/or pulse
count schedules can be done individually or can refer to the
computer controlled data base where they are stored for different
operation conditions. The operation of the laser at different
HV-levels under different operation conditions such as continuous
pulsing or burst mode may be taken into consideration.
[0140] In another preferred embodiment, a partial new fill
procedure may be performed according to FIGS. 23 and 24. As shown
in FIG. 23, an additional HV range is established which lies above
the PGR range 5 and yet below an additional threshold value
HV.sub.7. The remainder of the graph shown in FIG. 23 is preferably
the same as that shown in FIG. 21, and the discussion of the other
ranges 1-5 will not be repeated here.
[0141] When the processor determines that the high voltage is above
HV.sub.6, then either a new fill or a partial new fill will be
performed depending on whether the high voltage is at or below
HV.sub.7 wherein a partial new fill is to be performed, or is above
HV.sub.7, wherein a total new fill is performed. When a total new
fill is performed, substantially 100% of the gas mixture is emptied
from the discharge chamber and a totally fresh gas mixture is
introduced into the laser chamber. However, when a partial new fill
is performed, only a fraction (5% to 70% or around 0.15 bar to 2
bar, as examples) of the total gas mixture is released. More
particularly preferred amounts would be between 20% and 50% or 0.6
bar to 1.5 bar. A specifically preferred amount would around 1 bar
or around 30% of the gas mixture. Experiments have shown that
implementing a partial new fill procedure wherein 1 bar is
exchanged increases the gas lifetime by as much as five times over
now having the procedure.
[0142] The amount that may be released is an amount up to which a
substantial duration of time is used to get the gas out with a
pump, and so the amount may be more than 50%, and yet may take
substantially less time than a total new fill. Thus, a partial new
fill procedure has the advantage that a large amount of aged gas is
exchanged with fresh gas in a short amount of time, thus increasing
wafer throughput when the laser is being used in lithographic
applications, for example.
[0143] Referring now to FIG. 24, the flow chart is the same as that
shown in FIG. 22, except that when the processor determines that
the high voltage is above HV.sub.6, an additional determination is
made whether the high voltage is at or below HV.sub.7. If the
answer is yes, i.e., that the high voltage is at below HV.sub.7,
then a partial new fill is initiated, whereby less than
substantially 100% of the gas mixture is taken out of the discharge
chamber and replaced with fresh gas. Advantageously, the system is
only taken offline for a short time compared with performing a
total new fill. If the answer is no, i.e., that the high voltage is
above HV.sub.7, then a new fill is performed just as described
above with respect to FIG. 22. As mentioned, experiments have shown
that the gas lifetime can be improved by as much as five times
before the new fill range would be reached when the partial new
fill procedure is implemented.
[0144] It is to be understood that a system not using all of the
ranges 1-6 and the new fill/partial new fill procedures of range 6
may be advantageously implemented. For example, in FIG. 23, a
system that uses only a single one of the ranges with the partial
new fill and new fill may be used, and the gas lifetime improved.
With some ranges removed, the partial new fill range may be moved
to a lower threshold high voltage. Even according to FIG. 21, one
or more of the ranges may not be implemented, and the system may
still improve the performance of the laser system. It is preferred
that all of the ranges and corresponding gas actions be used for
optimum laser system performance.
[0145] The combination of all of these different kinds of gas
control and replenishment mechanisms involves harmonizing many
factors and variables. Combined with the expert system and
database, the processor controlled laser system of the present
invention offers an extended gas lifetime before a new fill is
necessary. In principle, bringing down the laser system for new
fill might be totally prevented. The lifetime of the laser system
would then depend on scheduled maintenance intervals determined by
other laser components such as those for laser tube window or other
optical components exchange. Again, as mentioned above with
reference to the '126 application, even the lifetimes of the laser
tube and resonator components may be increased to increase the
intervals between downtime periods.
[0146] The above described gas replenishment procedures may be
combined with cryogenic or electrostatic gas purification
techniques, whereby contaminants such as rare gas fluorides, i.e.,
AF.sub.n molecules, where A=Kr, Ar or Xe and n=2, 4 or 6) or other
contaminants as mentioned above are removed from the gas mixture.
For this purpose, U.S. Pat. Nos. 4,534,034, 5,001,721, 5,111,473,
5,136,605 and 5,430,752 are hereby incorporated by reference into
the present application. Standard methods typically include using a
cold trap to freeze out contaminants before recycling the gas back
into the discharge chamber. Some of the contaminants being frozen
out are molecular combinations of active gases such as the active
rare and halogen gases of excimer lasers. Thus, a significant
amount of these important laser gases is removed from gas mixture
in the discharge chamber. The result is a rapid decrease in rare
and halogen gas concentrations undesirably affecting output beam
parameters.
[0147] In summary, the present invention provides a method and
procedure for stabilizing an original or optimal gas composition of
a gas discharge laser, and particularly an excimer or molecular
fluorine (F.sub.2) laser. During a long period of operation of the
laser in a running or stand-by mode, the depletion of the laser gas
is continuously monitored by monitoring and controlling the high
voltage, laser pulse shape, ASE, elapsed time after new fill or
other additional laser parameters some of which have been set forth
above, in addition to accumulated electrical energy applied to the
discharge, time and/or pulse count. According to a database of
known histories and trends of key operating parameters for various
lasers operating under various conditions, a processor controlled
procedure is applied to replenish the gas degradation. The
stabilization process involves using a number of tiny gas actions
(micro injections) performed preferably based on specified time,
driving voltage change, input electrical energy to the discharge
and/or shot count intervals, a combination thereof or some other
interval relating to a parameter which changes with a known
relationship to the gas mixture degradation. A careful combination
of .mu.HIs and MGRs of various amounts, and PGRs, are used to
effect very nearly complete stabilization of the laser gas mixture
over a potentially unlimited duration. Most importantly, the gas
actions described herein do not disturb meaningful output beam
parameters or operation of the laser, because they are smooth and
controlled based on an expert system comprising myriad operating
conditions of the laser system. Thus, the laser can operate without
interruption during the gas replenishment actions and industrial
processing can be performed with high efficiency.
[0148] While exemplary drawings and specific embodiments of the
present invention have been described and illustrated, it is to be
understood that 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.
[0149] In addition, in the method claims that follow, the
operations 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 operations, 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.
* * * * *