Gas replenishment method and apparatus for excimer lasers

An excimer gas laser using a fluorine/krypton/neon gas mixture is provided with separate fluorine/krypton/neon and krypton/neon gas sources for use in replenishing the gas mixture. A bleed-down mechanism is also provided for draining a portion of the gas mixture from the excimer laser. A control mechanism controls operation of the separate fluorine/krypton/neon and krypton/neon sources and the bleed-down mechanism to selectively vary the gas mixture within the excimer laser to maintain an overall optimal laser efficiency. Preferably, the control system monitors operational parameters of the excimer laser including gain, wavelength, bandwidth and pulse rate, to determine whether the gas mixture within the excimer laser may have changed from an optimal mixture. The control system controls operation of the separate fluorine/krypton/neon and krypton/neon sources to compensate for changes in the operation parameters of the laser to thereby maintain high overall laser efficiency. Alternatively, gas replenishment is controlled subject to pre-determined empirically-based gas replenishment strategies.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention generally relates to gas lasers and, in particular, relates 
to methods and apparatus for replenishing gas mixtures within an excimer 
gas laser. 
2. Description of Related Art 
Many gas lasers, particularly excimer lasers, maintain a mixture of two or 
more gasses in a discharge chamber for use in generating a laser beam. A 
typical excimer laser for example may include a gas mixture composed of 
fluorine, krypton, and neon. The efficiency of the laser depends, in part, 
on the particular composition of the gas mixture. A deviation from an 
optimal composition may decrease the efficiency of the laser, thereby 
resulting in an output beam of less power. Moreover, a non-optimal gas 
composition may affect the ability of the laser to maintain a desired 
output frequency or to maintain a desired pulse rate. A substantial 
deviation in the gas mixture from an optimal mixture may also affect the 
durability and reliability of the laser, including causing an increase in 
corrosion or wear within the laser itself. 
The composition of the gas mixture may change as a function of time 
depending upon several factors. In particular, for fluorine/krypton 
excimer lasers, the amount of fluorine tends to be depleted while the 
excimer laser is operated. Fluorine, being a highly reactive halogen, 
tends to react with materials within the excimer laser by an amount 
sufficient to lower the amount of fluorine relative to krypton. 
For example, a typical fluorine/krypton excimer laser may include a gas 
mixture composed of 0.1 percent fluorine, 1.0 percent krypton, and 98.9 
percent neon. During operation of the excimer laser, fluorine becomes 
depleted thereby changing the relative compositions described above. The 
krypton and neon components, being substantially non-reactive noble 
gasses, are not as significantly depleted as the fluorine. 
On possible way to correct for depletion of the fluorine in the laser mix 
is to completely replace the laser mix with a fresh fill containing the 
correct relative concentrations of gasses. Although a complete replacement 
of the gas mixture is an effective way to compensate for a deviation in 
the relative composition of the gasses, such is not a particularly cost 
effective or efficient method for compensating for gas depletion. Indeed, 
excimer laser pre-mixes have become quite expensive and, particularly for 
large scale excimer lasers having considerable gas chamber volume, the 
cost of completely flushing the gas chamber and replacing it with new gas 
can be substantial. 
Accordingly, methods have been proposed for compensating for gas component 
depletion without requiring a complete replacement of the gas mixture. To 
this end, some fluorine/krypton excimer lasers are provided with a means 
for adding fluorine to an existing gas mixture, without requiring complete 
replacement of the gas mixture. During normal operation only fluorine is 
depleted. Hence, it would be simplest to add only fluorine. However, 
fluorine is extremely reactive and dangerous. Accordingly, the fluorine 
must be diluted, typically by neon. Hence, a fluorine source having 
fluorine diluted by neon (typically 1.0% fluorine) is provided for 
replenishing depleted fluorine. The fluorine/neon mixture is pumped into 
the gas chamber in an attempt to compensate for depleted fluorine. With 
such a technique, the period between complete replacement of the gas 
mixture can be extended substantially and the overall cost effectiveness 
of the excimer laser system is improved. However, by providing a 
replenishment source which includes a mixture of fluorine and neon, the 
pressure of the laser gas mix will steadily rise due to the addition of 
neon. Bleed-down is required when the addition of the fluorine/neon gas 
mixture increases the overall pressure within the chamber above a desired 
amount, and a portion of the gas mixture must be released to lower the 
pressure. When the gas pressure is lowered during bleed-down, some amount 
of krypton is lost. As the cycles of gas injection and bleed-down proceed, 
the concentration of krypton drops steadily, impairing the efficiency of 
the laser. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a method and apparatus for 
avoiding the depletion of krypton which occurs during injection/bleed 
cycles used to replenish fluorine consumed during operation of a 
fluorine/krypton excimer laser. In accordance with this object a method is 
provided for adding krypton to a fluorine/neon injection mixture whereby 
an amount of krypton lost during a bleed down operation is substantially 
replenished during a gas injection operation. 
In a preferred embodiment, the invention provides a method for varying a 
gas mixture within a gas chamber of a gas laser, with said gas mixture 
including a halogen, a first noble gas and a second noble gas and with 
said gas mixture having a desired composition. The method comprises the 
steps of: 
selectively pumping a halogen/first noble gas/second noble gags mixture 
into said chamber; 
selectively pumping a first noble gas/second noble gas mixture into said 
chamber; and 
selectively releasing a portion of said gas mixture within said chamber; 
with each of the foregoing steps performed by amounts sufficient to adjust 
an actual composition of said gas mixture within said chamber toward the 
desired composition. 
In the preferred embodiment, the desired laser gas mixture is about 0.1 
percent fluorine, 1.0 percent krypton, and 98.9 percent neon. Two source 
gas mixtures are used to achieve this mixture. The first source gas 
mixture contains 1 percent fluorine, 1 percent krypton, and 98 percent 
neon. The second source mixture contains only 1 percent krypton and 99 
percent neon. During an initial fill of the laser, approximately 10 
percent of the gas is provided by the first source mixture, and 90 percent 
by the second source mixture to achieve the desired laser gas mixture of 
about 0.1 percent Fluorine 1.0 percent krypton, and 98.9 percent neon. 
When it is determined by suitable means that more fluorine must be added to 
replenish fluorine lost during operation of the laser, a quantity of the 
second mixture is pumped into the chamber. Periodically, although not 
necessarily after each injection, some gas is bled out of the chamber to 
reduce the overall pressure to the desired amount. By adding krypton as 
part of the injection mixture with a suitable fluorine/krypton ratio, the 
bleed-down process removes only as much krypton as was added during the 
injections. Hence, the concentration of krypton remains substantially 
constant. 
In accordance with another embodiment of the invention an apparatus is 
provided for varying a gas mixture within a gas chamber of a gas laser, 
wherein the gas chamber contains a gas mixture including a halogen, a 
first noble gas and a second noble gas and wherein the gas mixture has a 
desired composition. The apparatus comprises: 
first pump means for selectively pumping a halogen/first noble gas/second 
noble gas mixture into said chamber; 
second pump means for selectively pumping a first noble gas/second noble 
gas mixture into said chamber; 
bleed-down means for selectively releasing a portion of said gas mixture 
within said chamber; and 
control means for controlling operation of said first pump means, said 
second pump means and said bleed-down means to vary an actual gas 
composition within said gas chamber toward the desired composition. 
In the preferred embodiment of the apparatus, means are provided for 
determining a difference between a desired composition and the actual 
composition of the gas mixture within the chamber. The determination means 
detects a change in operating efficiency of the excimer laser from a 
initial efficiency level. The control means may include an expert system 
which monitors changes in operational parameters of the laser to determine 
optimal gas mixture replenishment parameters, including when, and by how 
much, the gas mixtures are to be added to the chamber. 
In both the method and apparatus of the invention, the provision of 
separate sources for fluorine/krypton/neon and krypton/neon allows 
depletion of krypton to be compensated for without requiring a complete 
replacement of the gas within the excimer laser. Furthermore, by providing 
a pair of gas sources in combination with a bleed-down capability, the 
actual composition of the gas mixture within the excimer laser can be 
varied in a refined and precise manner to maintain an optimal gas mixture 
composition for extended periods of time. Thus, the time period between 
complete flushing and replacement of the gas mixture is extended and the 
overall cost of gasses required for use in the chamber is minimized. 
Thus, the general objects of the invention set forth above are achieved. 
Other objects and advantages of the invention will be apparent from the 
detailed description of the invention set forth below.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 2, preferred embodiments of the invention will now 
be described. 
FIG. 1 illustrates a portion of an excimer laser 10 having a gas lasing 
chamber 12 filled with a mixture of fluorine, krypton and neon. A pair of 
windows 14 and 16 are provided on opposing ends of chamber 12 for emitting 
a coherent laser beam generated within chamber 12. A wave meter 18 is 
positioned adjacent to window 14 for receiving the laser beam transmitted 
through window 14. Wave meter 18 includes mechanisms for determining the 
precise wavelength and band-width of the laser beam emitted from chamber 
12. Wave meter 18 also includes mechanisms for determining the power or 
energy of the laser beam. Wave meter 18 outputs signals representative of 
the energy and the wavelength along output lines 20 and 22 respectively. 
The energy signal, output along line 20, is received by a laser control 
mechanism 23. Laser control mechanism 22 controls operation of excimer 
laser 10 to adjust the energy of the laser beam generated within laser 
chamber 12. Laser control mechanism 23 also preferably operates to pulse 
the laser beam at a desired pulse rate. 
The wavelength signal output from wave meter 18 along line 22 is received 
by a spectral-narrowing mechanism 24, which adjusts the wavelength of the 
laser beam generated within chamber 12. Thus, with the provision of 
wavemeter 18 and spectral-narrowing mechanism 24, the wavelength of the 
laser beam generated by excimer laser 10 may be controlled. Preferably, 
control mechanism 23 operates to pulse the laser beam generated within 
chamber 12 to achieve a pulsed, rather than a continuous, laser beam. The 
components of excimer laser 10, thus far described, may be of conventional 
design and fabrication and will not be described in further detail herein. 
However, a preferred excimer laser assembly is set forth in U.S. Pat. No. 
4,959,840, entitled "Compact Excimer Laser Including an Electrode Mounted 
in Insulating Relationship to Wall of the Laser", assigned to the assignee 
of the present application. A preferred wave meter for use with the 
excimer laser is set forth in U.S. Pat. No. 5,025,445, entitled "System 
for, and Method of, Regulating the Wavelength of a Light Beam", also 
assigned to the assignee of the present application. U.S. No. 4,959,840 
and U.S. Pat. No. 5,025,445 are both incorporated herein by reference. 
In addition to the components thus far described, excimer laser 10 includes 
a gas replenishment apparatus, generally denoted 30. Gas replenishment 
apparatus 30 includes a fluorine/krypton/neon gas pump 32 a krypton/neon 
gas pump 34, and a bleed-down mechanism 36. Fluorine/krypton/neon gas 
source 32 is connected to chamber 12 via an input port 38. 
Fluorine/krypton/neon pump 32 includes appropriate mechanisms for pumping 
selected amounts of a fluorine/krypton/neon gas mixture into chamber 12. 
Krypton/neon gas pump 34 is connected to chamber 12 via an inlet port 40 
and includes appropriate mechanisms for pumping a desired amount of 
krypton/neon gas mixture into chamber 12. Bleed-down mechanism 36 is 
connected to an output port 42 of chamber 12 and includes mechanisms for 
selectively releasing a desired amount of gas mixture from within chamber 
12. A control mechanism 44 is connected to each of pumps 32 and 34 and 
bleed-down mechanism 36 for controlling operation thereof. Control 
mechanism 44 may include, as a component, an expert system 46 described 
below. 
Preferably, chamber 12 is initially filled with a pre-mix of fluorine, 
krypton and neon, with the pre-mix having the composition of 0.1 percent 
fluorine, 1.0 percent krypton, and 98.9 percent neon, with the percentages 
based on partial pressure. A pre-mix source 48 is connected to chamber 12 
through an inlet port 50 for initially filling chamber 12 with the 
pre-mix. Alternatively, a premix source need not be provided. Rather, 
chamber 12 can be initially filled using sources 32 and 34. 
With the relative gas chamber mixture described above, excimer laser 10 
preferably generates a laser beam having wavelength of 248 nano-meters. 
Wave meter 18 and line narrowing mechanism 24 operate in combination to 
maintain the wavelength of the laser beam at the desired wavelength of 248 
nano-meters with a bandwidth of no more than 1 pico-meter (full-width 
half-maximum). 
Although initially provided with a pre-mix having the precise composition 
noted above, the mixture within chamber 12 varies as a function of time 
while excimer laser 10 is operated. In particular, a portion of the 
fluorine is depleted during operation, possibly by reaction with other 
materials or compositions within laser chamber 12. The krypton and neon 
are also depleted, though to a lesser extent, during operation of the 
laser, perhaps by leakage from chamber 12. As a result, the relative 
composition of the mixture within chamber 12 varies as a function of time 
and can deviate, somewhat substantially, from the original pre-mix 
composition. A substantial variation from the composition of the gas 
mixture can affect the efficiency of the laser, including causing a 
general lowering in the laser gain and a deviation in the precise 
wavelength and bandwidth achievable. A variation in the composition may 
also affect the ability to pulse the laser beams at a desired rate. 
Replenishment apparatus 30 operates to maintain the composition within 
chamber 12 to that of the original pre-mix composition. In other words, 
replenishment apparatus 30 operates to replenish the fluorine, krypton and 
neon as they are depleted from the chamber. 
The separate fluorine/krypton/neon and krypton/neon gas sources are 
provided to allow both fluorine and krypton to be effectively replenished 
within chamber 12. Fluorine/krypton/neon pump 32 provides a mixture of 1 
percent fluorine, 1 percent krypton and 98 percent neon. Krypton/neon 
source 34 provides a mixture of 1 percent krypton and 99 percent neon. 
Bleed-down mechanism 36 allows a portion of the mixture within chamber 12 
to be removed. In the alternate embodiment described above wherein a 
separate pre-mix source is not provided, chamber 12 can be initially 
filled by providing gas from sources 32 and 34 in the ratio of 1/9. In 
other words, by providing 10 percent of the initial gas from source 32 and 
90 percent from source 34, a resulting mixture of 0.1 percent fluorine, 
1.0 percent krypton and 98.9 percent neon is achieved. 
As an example, should fluorine be depleted, fluorine/krypton/neon gas 
source 32 is controlled to add a desired amount of fluorine/krypton/neon 
to chamber 12 to increase the fluorine percentage therein. If, as a 
result, the overall pressure within chamber 12 exceeds a desired amount, 
excess gas may be removed from chamber 12 using bleed-down mechanism 36. 
Since krypton was present in the injected gas source 32 in substantially 
the desired concentration, any subsequent bleed-down will remove an equal 
quantity of krypton, thereby maintaining the correct krypton concentration 
at all times. In general, the operation of fluorine/krypton/neon source 
32, krypton/neon source 34 and bleed-down mechanism 36 is controlled to 
maintain the mixture as close to the original pre-mix composition as 
possible. Also, in general, the ratios of the gasses within source 32 and 
source 34 can be selected to compensate for different depletion rates of 
the component gasses. In addition to toggling operation of the pump and 
bleed-down mechanisms, control mechanisms 44 may also control the rate by 
which gasses are exchanged with chamber 12. In this regard, gas flow 
regulators, not shown in FIG. 1, may be employed for controlling the flow 
rate of gasses into and out of chamber 12. 
To automatically control operation of pumps 32 and 34 and bleed-down 
mechanism 36, control mechanism 44 receives signals from energy monitor 
and wave meter 18 along signal lines 54. From signals received along this 
line, control mechanism 44 determines the extent to which the various gas 
mixture components have been depleted and controls pumps 32 and 34 and 
bleed-down mechanism 36 accordingly to compensate for depletion. Although 
laser energy, wavelength, and bandwidth are exemplary parameters from 
which control mechanism 44 determines deviations within the gas mixture, 
other parameters may also be employed including the pulse rate of the 
laser, the temperature, the pressure of the laser chamber and other 
general operational parameters as well. The various laser operational 
parameters such as energy, wavelength, band-width and the like together 
define an overall laser "efficiency". The relationship of the gas chamber 
composition to the various operational parameters is determined in advance 
by empirical methods. In other words, the gas mixture is selectively 
varied in accordance with changes in the operational parameters to 
determine effective strategies for compensating for changes in the 
parameters. Strategies which are unsuccessful are discarded. Strategies 
which prove to be successful are incorporated into logic in control unit 
44. For example, if it is determined that a substantial drop in the energy 
of the laser is usually a result of fluorine depletion, then such a 
relationship is programmed within the logic of control unit 44, which 
operates to add fluorine in response to a drop in laser energy. 
Control mechanism 44 is preferably a programmable computer provided with 
software and databases providing logic necessary for controlling operation 
of the pumps and bleed-down mechanisms in response to changes in 
operational parameters. Preferably, the programs and databases include an 
expert system 46, which maintains a history of excimer laser 10 and, based 
on prior experience, controls operation of the pump and bleed-down 
mechanisms in an optimal manner. Expert system 46 preferably maintains a 
database containing a history of the operational parameters of the system 
as a function of time as well as a history of the control of the pump and 
the bleed-down mechanisms. In this manner, and with appropriate 
programming, expert system 46 determines to how best to control operation 
of the pumps and bleed-down mechanisms to compensate for undesirable 
changes in the operation parameters of the laser. 
Although a programmed control mechanism having an expert system is a 
preferred mechanism for controlling the operation of the pumps and the 
bleed-down mechanism, such is not required. As an alternative, no 
automatic control mechanism is provided. Rather, the pumps and bleed-down 
mechanisms are manually controlled by an operator in response to changes 
in operational parameters detected by the operator using the appropriate 
sensors, not shown. Furthermore, although the control mechanisms are 
preferably operated in response to changes in laser parameters detected by 
various sensors, such is not required. Rather, the pumps and depletion 
mechanism may be controlled simply based on previously defined control 
strategies. For example, if it is determined that fluorine and krypton are 
always depleted by certain amounts during operation of the laser, then 
pumps 32 and 34 and depletion mechanism 36 may be controlled in accordance 
with a predetermined strategy for maintaining the fluorine and krypton 
levels without actually monitoring the operation of laser 10. 
Furthermore, although it is anticipated that no mechanism for directly 
measuring the relative compositions within chamber 12 is employed, such a 
mechanism could well be provided, eliminating the need to determine 
changes in the gas mixture from changes in operational parameters of the 
laser. In such a circumstance, control system 44 merely monitors the 
actual measured composition of gasses within chamber 12 and controls pumps 
32 and 34 and bleed-down mechanism 36 appropriately to compensate for any 
variation in the mixture. 
Furthermore, although it is anticipated that pre-mix source 48 provides an 
optimal composition of gasses, such need not be the case. If it is 
determined that the pre-mix composition is non-optimal, control mechanism 
44 may control the pumps and bleed-down mechanism in a manner to achieve 
an optimal mixture which differs from the pre-mix. For example, if it is 
determined that the pre-mix is provided with too low of a percentage of 
fluorine, gas replenishment mechanism 30 may be operated to immediately 
increase the amount of fluorine after chamber 12 is filled with the 
pre-mix. 
As can be appreciated, a wide number of specific embodiments can be 
employed and various strategies and techniques used, all in accordance 
with the general principles of the invention. 
Despite operation of replenishment 30, it is anticipated that the mixture 
within chamber 12 will need to be flushed and replaced periodically. Thus, 
the operation of the replenishment apparatus is not necessarily intended 
to completely prevent the need to flush the chamber and replace the 
gasses. Rather, operation of the replenishment mechanism is primarily 
provided for extending the period between necessary system flushes and for 
maintaining an optimal mixture during that period. 
A method by which the gas mixture within an excimer laser is replenished 
and maintained is generally illustrated in FIG. 2. Initially, at step 200, 
a gas chamber of the excimer laser is filled with a pre-mix having a 
preferably optimal composition of fluorine, krypton and neon. At 102, the 
excimer laser is activated. Operational parameters of the laser are 
detected at step 104. The operational parameters may include the energy, 
frequency and bandwidth of the laser as well as the pulse repetition rate 
of the laser. 
At 106, the system determines a change in the gas mixture causing the 
change in the operational parameters. Next, at 108, the separate 
fluorine/krypton/neon and krypton/neon sources and the bleed-down 
mechanism are controlled to compensate for changes in the gas mixture in 
an attempt to maintain an optimal mixture. Control of the excimer laser 
proceeds in a feedback loop denoted by line 110 in FIG. 2. As such, the 
system continually monitors operational parameters of the laser and 
controls the separate gas sources and depletion mechanism accordingly to 
maintain an optimal overall laser efficiency. 
FIG. 2, thus, represents a general overview of method steps of the 
invention. Actual operation of the system may depend on numerous factors, 
described above, and various gas replenishment strategies may be employed. 
In particular, the system need not detect actual changes in operational 
parameters of the system and instead may control gas replenishment using 
previously determined, empirically-based, replenishment strategies. 
What has been described is a method and apparatus for replenishing a gas 
mixture within an excimer gas laser. The general principles of the 
invention, although described above with reference to a 
fluorine/krypton/neon excimer laser, may be applied to other excimer 
lasers and to other gas lasers as well. In general, principles of the 
invention may be advantageously exploited in any gas laser system 
employing two or more separate gas components which are subject to 
independent depletion. Thus, the exemplary embodiments described herein 
are merely illustrative of the invention and are not intended to limit the 
scope of the invention.