Narrow band gas discharge laser with gas additive

The present invention provides a very narrow band pulse excimer laser capable of producing pulses at a rate in the range of about 500 to 2000 Hz with enhanced energy dose control and reproducibility. Very small quantities of a stablizing additive consisting of oxygen or a heavy noble gas (xenon or radon for KrF lasers, or krypton, xenon or radon for ArF lasers), are added to the gas mixture. Tests performed show substantial improvements in energy stability with the addition of about 30 ppm of xenon to a KrF laser. Tests show improved performance for the ArF lasers with the addition of about 6-10 ppm of Xe or 40 ppm of Kr. In a preferred embodiment very narrow bandwidth is achieved on a KrF laser by reducing fluorine partial pressure to less than 0.10 percent and by increasing the reflectance of the output coupler to greater than 25 percent. In a preferred embodiment, prior art fused silica beam expansion prisms used in the prior art line-narrowing module were replaced with calcium fluoride prisms.

BACKGROUND OF THE INVENTION
 Excimer lasers are currently becoming the workhorse light source for the
 integrated circuit lithography industry. A typical prior art KrF excimer
 laser is depicted in FIG. 1 and FIG. 9. A pulse power module 2 provides
 electrical pulses lasting about 100 ns to electrodes 6 located in a
 discharge chamber 8. The electrodes are about 28 inches long and are
 spaced apart about 3/5 inch. Typical lithography lasers operated at a high
 pulse rate of about 1,000 Hz to 4,000 Hz. For this reason it is necessary
 to circulate a laser gas (about 0.1 percent fluorine, 1.3 percent krypton
 and the rest neon which functions as a buffer gas) through the space
 between the electrodes. This is done with tangential blower 10 located in
 the laser discharge chamber. The laser gasses are cooled with a heat
 exchanger also located in the chamber. Commercial excimer laser systems
 are typically comprised of several modules that may be replaced quickly
 without disturbing the rest of the system. Principal modules are shown in
 FIG. 1 and include:
 Laser Chamber 8,
 Pulse Power Module 2, consisting of three submodules
 Output coupler 16,
 Line Narrowing Module 18
 Wavemeter 20
 Computer Control Unit 22
 Peripheral Support Sub systems
 Blower 10
 The discharge chamber is operated at a pressure of about three atmospheres.
 These lasers operate in a pulse mode at about 600 Hz to about 1,000 Hz,
 the energy per pulse being about 10 mJ and the duration of the laser
 pulses is about 15 ns. Thus, the average power of the laser beam is about
 6 to 10 Watts and the average power of the pulses is in the range of about
 700 KW. A typical mode of operation is referred to as the "burst mode" of
 operation. In this mode, the laser produces "bursts" of about 50 to 150
 pulses at the rate of 1,000 pulses per second. Thus, the duration of the
 burst is about 50 to 150 milliseconds. Prior art lithograph, excimer
 lasers are equipped with a feedback voltage control circuit which measures
 output pulse energy and automatically adjusts the discharge voltage to
 maintain a desired (usually constant) output pulse energy. It is very
 important that the output pulse energy be accurately controlled to the
 desired level.
 It is well known that at wavelengths below 300 nm there is only one
 suitable optical material generally available for building the stepper
 lens used for chip lithography. This material is fused silica. An all
 fused silica stepper lens will have no chromatic correction capability.
 The KrF excimer laser has a natural bandwidth of approximately 300 pm
 (full width half maximum). For a refractive system (with NA&gt;0.5)--either a
 stepper or a scanner--this bandwidth has to be reduced to below 1 pm.
 Current prior art commercially available laser systems can provide KrF
 laser beams at a nominal wavelength of about 248 nm with a bandwidth of
 about 0.8 pm (0.0008 nm). Wavelength stability on the best commercial
 lasers is about 0.25 pm. With these parameters stepper makers can provide
 stepper equipment to provide integrated circuit resolutions of about 0.3
 microns. To improve resolution a narrower bandwidth is required. For
 example, a reduction of a bandwidth to below 0.6 pm would permit
 improvement of the resolution to below 0.25 microns.
 Argon fluoride, ArF excimer lasers which operate at a wavelength of about
 193 nm using a gas mixture of about 0.08 to 0.12% fluorine, 3.5% argon and
 the rest neon, are beginning to be used for integrated circuit
 lithography. F.sub.2 lasers produce laser radiation at wavelengths of
 about 159 nm. The gas mixture typically is 0.1% percent fluorine and the
 rest helium or neon.
 Gas discharge laser typically use a preionizer technique for preionizing
 the gas between the electrodes prior to the main electrical discharge.
 Examples of these preionizers are spark gap preionizers and corono
 discharge preionizers. Spark gaps produce ions with a discharge between
 two electrodes like an automatic spark plug. A corono discharge preionizer
 produce ions by creating a corono of ions adjacent to a conductor at high
 voltage. A typical corono discharge preionizer is described in U.S. Pat.
 No. 5,337,330 which is incorporated herein by reference. The ionization
 produced by these preionizers produces ultraviolet radiation which in turn
 reacts with the laser gas to generate a substantial ion population in the
 laser gas between the electrodes. Typically spark gap preionizers produces
 higher energy ultraviolet radiation than corono discharge preionizers, but
 the radiation from the corono discharge preionizers tends to be much more
 uniform.
 It is known that the addition of about 10 to 50 ppm of oxygen to an excimer
 laser gas mixture can be used to stabilize the efficiency and performance
 of the laser. These additives improve the preionization efficiency of the
 laser. See, for example, U.S. Pat. No. 5,307,364. Small quantities of
 xenon have been proposed as a gas additive for CO.sub.2 lasers. See Japan
 Patent Number JP 60180185 issued in 1984 based on a patent application
 filed on Feb. 27, 1984. In a 1995 article entitled, Tranmission Properties
 of Spark Preionization Radiation in Rare-Gas Halide Laser Gas Mixes, (IEEE
 Journal of Quantum Electronics, Vol. 31, No. 12, December 1995), the
 author discusses gas additives to enhance preionization of rare-gas halide
 lasers. This paper deals with lasers utilizing spark gap preionization.
 Spark gap preionization is known to produce high energy photons which in
 turn preionizes laser gas between the laser electrodes. The author points
 out that the ionization potential of xenon is too high (i.e., greater than
 the preferred ionization potential of &lt;10 ev); however, the author
 suggests that it might be possible to use small quantities of xenon in
 lasers which have transmission windows at vacuum ultraviolet wavelengths
 &lt;103 nm or photon energies in excess the 12.1 ionization potential of
 xenon. The article infers that spark gap photons have energies less than
 10 ev and suggests that higher energy photons such as x-rays could be used
 to excite xenon if used as an additive.
 The actual performance of integrated circuit lithography equipment then
 depends critically on maintaining minimum bandwidth of the laser
 throughout its operational lifetime, and also minimizing the laser's
 energy variation from pulse-to-pulse.
 Therefore, a need exists for a reliable, production quality excimer laser
 system, capable of long-term factory operation and having accurately
 controlled pulse energy stability, wavelength, and a bandwidth.
 SUMMARY OF THE INVENTION
 The present invention provides a very narrow band pulse gas discharge laser
 capable of producing pulses at a rate in the range of about 500 to 4000 Hz
 with enhanced energy dose control and reproducibility. Very small
 quantities of a stablizing additive consisting of oxygen or a heavy noble
 gas (xenon or radon for KrF lasers, or krypton, xenon or radon for ArF
 lasers or F.sub.2 lasers), are added to the gas mixture. Tests performed
 show substantial improvements in energy stability with the addition of
 about 30 ppm of xenon to a KrF laser. Tests show improved performance for
 the ArF lasers with the addition of about 6-10 ppm of Xe, 40 ppm of Kr or
 about 3 to 10 ppm oxygen. In a preferred embodiment very narrow bandwidth
 is achieved on a KrF laser by reducing fluorine partial pressure to less
 than 0.10 percent and by increasing the reflectance of the output coupler
 to greater than 25 percent. When operating in a burst mode performance of
 the laser is improved by pre-pulsing the laser; i.e., providing a few
 pulses prior to the start of the burst. In a preferred embodiment, prior
 art fused silica beam expansion prisms used in the prior art
 line-narrowing module were replaced with calcium fluoride prisms.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Preferred embodiments of the present invention are described below.
 DESCRIPTION OF LASER
 FIG. 1 shows the principal elements of a commercial excimer laser system of
 the type used today in integrated circuit lithography.
 The Chamber
 The discharge chamber 8 is a vessel, designed to hold several atmospheres
 of corrosive gases. These vessel., are designed to known safety standards,
 such as those specified by ASME. The two electrodes separated by a gap of
 1.2 to 2.5 cm define the discharge region. The cathode is supported by an
 insulating structure since it is connected to the high voltage, while the
 anode is attached to the metal chamber as it is at ground potential.
 Preionization is done by corona discharge preionizers located on either
 side of the discharge region. Due to the corrosive nature of the gas the
 chambers use particular metals chosen to resist fluorine attack. The
 fluorine gas however, still reacts with the chamber internal parts such as
 chamber walls and electrodes; thus consuming fluorine and generating metal
 fluoride contaminants.
 Since the laser is pulsed (500 to 4000Hz), it is essential to clear the
 discharge region between pulses--a task preferably performed by a
 tangential blower, which is magnetically coupled to an external drive
 source. Heat is extracted from the laser gas by means of a water-cooled
 finned heat exchanger inside the chamber. Metal fluoride dust is trapped
 by means of an electrostatic precipitator not shown. A small amount of
 laser gas is extracted from the chamber and is passed over negatively
 charged high field wires to trap the dust. The dust-free gas is then
 released over the windows to keep them clean. The gas is driven through
 the precipitator by the differential pressure built up inside the laser
 chamber due to the high velocity flow.
 Pulse Power Module
 This preferred embodiment utilizes a solid state pulsed power module
 (SSPPM) circuit shown in FIG. 2. The 20 KV power supply of prior art
 thyratron systems is replaced by a 1 kV supply. The thyratron switch is
 replaced by an SCR switch that does not feed C.sub.p directly, but instead
 switches the energy of C.sub.0 into a pulse compression circuit formed by
 C.sub.1, C.sub.2, C.sub.3, a step-up transformer, and three saturable
 inductors. The operation of this circuit is as follows. The DC charge
 stored on C.sub.0 is switched through the SCR and the inductor L.sub.0
 into C.sub.1. The saturable inductor, L.sub.1, holds off the voltage on
 C.sub.1 for approximately 2.5 ns and then becomes conducting, allowing the
 transfer of charge from C.sub.1 to C.sub.2. The second saturable inductor,
 L.sub.2, holds off the voltage on C.sub.2 for approximately 500 ns and
 then allows the charge on C.sub.2 to flow through the primary of 1:20
 step-up transformer. The output from the step-up transformer is stored on
 C.sub.3 until the saturable inductor L.sub.3 becomes conducting in
 approximately 100-150 ns. The charge is then finally transferred through
 L.sub.3 into C.sub.p and laser discharge occurs.
 Spectral Narrowing
 As stated earlier, bandwidth (FWHM) of a free running KrF excimer laser is
 approximately 300 pm. Currently, excimer steppers utilize lasers
 spectrally narrowed to between 0.8 and 3 pm, FWHM, depending on the NA of
 the lens. It should be noted that the integrated energy spectrum and the
 spectral width at 95% energy are more critical to stepper performance than
 the FWHM value. However, most users find it convenient to talk about FWHM
 instead of spectral width at 95% energy.
 Spectral narrowing of a KrF laser is complicated by its short pulse
 duration (10 to 15 ns, FWHM) and UV wavelength. The short pulse results in
 very high intra-cavity power (.about.1 MW/cm.sup.2), and the short
 wavelength can thermally distort optical materials due to their high
 absorption co-efficient at 248 nm. Also, the total number of round trips
 through the resonator (which includes the line narrowing optical elements)
 for a typical laser is small, about 3 to 4. If the single pass linewidth
 through the resonator is denoted by ).DELTA..lambda..sub.1, then the final
 linewidth ).DELTA..lambda..sub.f after n passes is given by:
 ##EQU1##
 Therefore, the single pass linewidth of the optical system should be, at
 most, a factor of two higher than the final linewidth. In fact, time
 resolved spectral measurements by Applicants fellow workers indicate that
 the spectral linewidth could decrease by a factor of two from the start of
 the pulse to the tail of the pulse. Therefore, the efficiency of
 converting the broadband spectrum to line narrowed spectrum (i.e. from 300
 pm to &lt;1 pm) of the optical system must be very high.
 The common technique of line-narrowing the KrF laser is by introducing
 wavelength dispersive optical elements in the resonator. Three types of
 dispersive elements can be used: prisms, etalons and gratings. The use of
 a high dispersive grating in a Littrow configuration is the simplest and
 most effective spectral line narrowing technique. Because the grating is a
 dispersive element, the: line-width is proportional to the beam
 divergence. To get narrow line-width, a small beam divergence is required.
 Hence in a preferred design, 2 slits and a 3 prism-beam expander is
 inserted in the laser resonator. The principal elements of a preferred
 line-narrowing module are shown in FIG. 7. These include 3 prisms; 30, 32
 and 34, a tuning mirror, 3C and an eschelle grating, 38. The mirror is
 pivoted to change the wavelength of the laser.
 Improved Spectral Performance
 Applicants and their fellow workers have designed, built and tested laser
 KrF excimer laser equipment capable of meeting linewidth specifications of
 0.50 pm at FWHM with 95% of the energy of the laser beam within 2 pm.
 These results have been demonstrated on new, mid-age and old discharge
 chambers for 80 million pulses proving that the system is capable of
 continuous performance within these specifications over the normal life of
 the equipment with usual maintenance. These results represent an
 approximately 50% improvement over the prior art narrow band excimer laser
 technology.
 In order to achieve this improved performance Applicants have improved both
 the laser equipment and the operating parameters of the laser.
 Reduction of Fluorine Consumption
 In preferred embodiments of the present invention which have been built and
 tested by Applicants, great care was taken to eliminate materials from the
 discharge chamber that consume fluorine. Fluorine consumption in a
 discharge chamber is due to fluorine reaction with materials in the
 chamber. These reactions typically produce contaminants, which result in
 deterioration of laser performance. In order to minimize fluorine
 consumption, this preferred embodiment include the following specific
 features:
 The chamber walls are aluminum coated with nickel.
 The electrodes are brass.
 All metal O-rings are used as seals.
 Insulators are all ceramic and fluorine compatible.
 Alumina is applicants preferred insulator material.
 An electrostatic filter is provided as in prior art designs to filter
 contaminants produced during operation.
 The fan unit is driven using a magnetically coupled motor located outside
 the sealed discharge chamber using a prior art technique.
 During manufacture, parts are precision cleaned to remove potential
 contaminants.
 After assembly, the chamber is passivated with fluorine.
 Reduction of Nominal Fluorine Concentration
 This preferred embodiment requires substantial changes in operating
 procedures and parameters of the laser system in order to achieve the
 desired very narrowband output. The fluorine concentration in reduced from
 0.1% (30 kPa) to about 0.06% (18 kPa). The total gas pressure is about 300
 kPa. (The Kr concentration is maintained at the prior art level of about
 1.3% and the remainder of the laser gas is neon.) During operation,
 fluorine will be gradually depleted. Constant pulse energy is obtained by
 gradually increasing the laser operating voltage in accordance with prior
 art techniques. Injections of a mixture of fluorine and neon are made
 periodically (typically at intervals of about 1 to 4 hours) to make up for
 the depletion of fluorine in accordance with techniques well known in the
 excimer laser prior art. During this procedure, the fluorine concentration
 is preferably maintained within the range of between about 0.055% and
 0.065% and the operating voltage is maintained within a corresponding
 range appropriate to maintain constant pulse energy. For example, in a
 preferred embodiment this range was 670 Volts to 790 Volts.
 Increase in Reflectivity of Output Coupler
 In this preferred embodiment of the present invention the reflectivity of
 the output coupler has been increased from about 10% which was typical of
 prior art narrow band excimer lasers to about 30%. This was done to help
 make up for the loss of laser efficiency resulting from the reduced
 fluorine concentration.
 Switch to Calcium Fluoride Prisms
 The change in the reflectivity of the output coupler from 10% to 30% had
 the effect of approximately doubling the light passing through the
 line-narrowing module. The additional heat generated by this additional
 illumination in the prior art fused silica prisms caused thermal
 distortion in the prisms. To solve this problem the fused silica prisms
 were replaced with calcium fluoride prisms. Calcium fluoride has higher
 thermal conductivity and could handle the additional energy without
 unacceptable distortion.
 Fluorine Reduction
 FIG. 10 describes the relationship between operating voltage, fluorine
 concentration and pulse energy. This graph shows that as the fluorine
 concentration decreases the voltage must be increased to maintain the
 desired output of 10 mJ per pulse. However, in this particular embodiment
 the upper limit on the operating voltage is 800 Volts. Note that with a
 10% R output coupler the lowest fluorine concentration corresponding to an
 output of 10 mJ would be 25 kPa at which point the operating voltage would
 have risen to 800 Volts. However with a 30% R output coupler, the fluorine
 concentration could be reduced to as low as about 20 kPa while still
 maintaining a 10 mJ pulse energy with the operating voltage at slightly
 under 800 Volts. FIG. 11 shows actual test results of reducing the
 fluorine concentration on line width (measured at FWHM and at 95% pulse
 energy) for both continuous pulses at 1000 Hz and for 500 pulse bursts and
 1000 Hz. For this particular test the output coupler had a 25%
 reflectivity. Typical laser pulse shapes for prior art KrF systems and
 these very narrowband KrF lasers are compared in FIGS. 12A and 12B. Note
 that with the very narrowband lasers, energy is shifted to the latter part
 of the pulse, which represents photons, which have had the benefit of more
 trips through the line-narrowing module. As a result, the integrated pulse
 spectral linewidth of the laser is reduced.
 Burst Mode Operation
 As indicated in the Background section of this specification, a typical
 mode of operation of a KrF laser is a "burst mode" in which bursts of
 about 125 pulses are produced at the rate of 1000 pulses per second. The
 burst lasts for about 125 milliseconds and there typically is a "dead
 time" of a fraction of a second between bursts. Applicants' KrF laser
 contains about 0.017 cubic meters of laser gas and the flow rate of gas
 between the electrodes produced by blower 10 is about 0.228 cubic meters
 per second. This would imply a total gas circulation time of about 75
 milliseconds; however, flow in the chamber is far from uniform and
 portions of the gas circulates much quicker. The speed of the gas between
 the electrodes is about 20 meters per second and the Applicants estimate
 that the fastest gas makes the trip in about 20 milliseconds. Applicants
 have discovered a "slug effect" generated by the first or the first few
 pulses in a burst. This slug effect is shown in FIG. 13 that is a plot of
 pulse energy for each of the 123 pulses of a typical burst of 123 pulses
 averaged over 50 bursts. There is a large drop-off after the first pulse
 and another large dip after the 21st pulse, i.e. about 21 milliseconds
 following the first pulse. This dip is extremely reproducible and the
 timing of the dip is in proportion to the fan speed. Applicants do not
 know the exact cause of this first 40 milliseconds of very reproducible
 erratic performance but have identified it as the "slug effect" and
 believe it is attributable to chemical effects generated when "clean"
 laser gas passing between the electrodes is blasted with 16,000 to 20,000
 volts during the first pulse or the first few pulses. The gas passing
 between the electrodes during the first 20 milliseconds is substantially
 all "clean" laser gas but after about 20 milliseconds, gas electrocuted
 during the first pulse begins to pass back between the electrodes. After
 about 39 milliseconds into the burst, the gas in the laser is thoroughly
 mixed and the slug effect disappears.
 Gas Additives
 Applicants, through their experimentation have discovered that substantial
 improvements in laser performance can be realized by the addition of very
 small quantities of selected gases. Prior art teaches that about 10 to 50
 ppm oxygen improves energy stability. However, these quantities of oxygen
 produce a decrease in the power output which tends to outweigh the
 improvements in stability. Applicants have discovered that quantities of
 oxygen smaller than 10 ppm provide significantly improved stability
 without significant detrimental effects. Applicants have also discovered
 that the addition of very small quantities of heavy noble gases provides
 substantial improvements without significant detrimental effects.
 Xenon Additive
 The effect of xenon additives on operating voltage and efficiency are given
 in FIG. 8A. The rate of laser efficiency decrease is about 0.15% per 1 ppm
 of xenon which is not good; but the energy stability was noticeably
 improved for all xenon concentrations and exhibited a slight maximum
 around 30 ppm. This maximum is not apparent from the drawing. All
 subsequent tests were performed with a xenon concentration of 30 ppm.
 The energy versus voltage characteristic is given in FIG. 8B. The energy is
 lower with xenon over the entire range.
 The burst transients are compared in FIGS. 8C and 8D. With xenon the energy
 transient is reduced, especially for the first ten pulses, which makes it
 easier on the energy algorithm. A major improvement with xenon is found in
 the energy stability, which is reduced, for all pulse numbers. This is in
 contrast to the effect of oxygen which only works on the reentrant. In
 fact, this chamber does not exhibit any reentrant, so the reentrant effect
 of xenon could not be confirmed with this chamber. For a blower speed of
 4200 rpm the reentrant should occur at about 20 ms. (Note, subsequent
 tests with chambers that do exhibit a reentrant effect confirm that 30 ppm
 xenon does produce at least a small reduction in the reentrant effect.
 The laser energy is almost independent of repetition rate (see FIG. 8E),
 with the xenon mixture giving consistently lower values. By contrast, the
 improvement in dose stability with xenon is most noticeable at higher
 pulse rates. At 1 kHz the energy stability is probably dominated by
 effects unrelated to discharge stability such as noise in the data
 acquisition and high voltage power supply regulation. We are using two
 paralleled 5000 power supplies with &gt;3V of dither. The dose stability in a
 2 kHz mode is displayed in FIG. 8F. Addition of 30 pm of xenon reduces the
 dose error by about 0.1%. This is a substantial improvement.
 No effect of xenon on any other beam parameters (spatial profiles and
 divergence, linewidth) was observed. Occasionally, it appeared that xenon
 mixtures produce narrower linewidth. However, this was most likely an
 artifact produced by the fused silica beam expanding prisms. It lakes more
 time to generate a xenon mixture, which allows the prisms to cool down.
 Recorded linewidths were 0.65 pm FWHM and 1.90 pm 95%. The linewidths
 would possibly be narrower with CaF.sub.2 prisms because of better thermal
 properties. A comparison of the temporal profiles at 10 mJ of energy is
 displayed in FIG. 8G. The 30 ppm xenon mixture exhibits a typical waveform
 for higher charging voltages (667V 30 ppm Xe, 651V w/o Xe), namely larger
 initial spike and shorter duration. From this, one would expect a larger
 linewidth with xenon, which was not observed. Nevertheless, the
 differences are very small and only reflect one particular shot.
 Unfortunately, no averaged pulse profiles were recorded.
 Explanation of Results of Xenon Tests
 So why does xenon help at all and why in such small concentration? Some
 insight is gained by observing the peaking capacitor voltage Vcp (FIG.
 8H). For the same charging voltage Vco of 650V, gas breakdown occurs 2 ns
 earlier with a xenon mixture. The obvious explanation is improved
 pre-ionization. Xenon can be ionized by light shorter than 93 nm whereas
 krypton and neon have thresholds of 85 nm and 58 nm, respectively (R. S.
 Taylor, IEEE JQE v. 31, p. 2195, 1995). Therefore, xenon can use a large
 part of the corona light that otherwise would just be transmitted. Even at
 30 ppm the xenon concentration is seven orders of magnitude larger than
 typical pre-ionization electron densities. This means, the amount of xenon
 atoms is not a limiting factor. The absorption cross section of xenon is
 1500 cm.sup.-1 which translates into a 50% transmission after 5 cm for 30
 ppm at 315 kPa. This would explain why higher xenon concentrations are
 less efficient, the 90 nm light is already being filtered out very close
 to the PI tube.
 There are other scenarios possible like faster current avalanching due to
 the lower ionization potential of xenon. This, however, is hindered by the
 low concentration. Another possibility is a change in the spectral content
 of the corona light, which may have beneficial effects. In fact, the
 discharge containing xenon visually appears much brighter, primarily due
 to yellow components.
 Better pre-ionization may also help the minimum clearing ratio (a measure
 of gas flow between electrodes between pulses). There is a very slight
 improvement for lower charging voltages (FIG. 8I). At 650V (10 mJ) 3800
 rpm is barely enough to prevent down-stream-arcing and dose stability
 improves when going to 4200 rpm. At 800V arcing is much more severe,
 although largely aggravated by blips.
 Xenon Effects Survives Refills
 Very early in the experiments a strange phenomenon was encountered: the
 beneficial effect of xenon survives refills. Due to this, detailed studies
 on the influence of the exact xenon concentration became difficult, or at
 least time consuming. What is happening is that after the laser was
 operated with a xenon containing mixture and refilled without xenon, the
 energy stability would stay at a low level. Not as good as with xenon, but
 somewhere in between. A number of experiments were conducted to help
 understand the mechanism of this memory effect. The difference in dose
 stability between a truly xenon-free mixture and a pre-conditioned mixture
 is only 0.05%. This difference is too small to allow any hard conclusions,
 so only some general trends can be outlined.
 There are two possibilities; either xenon physically stays in the chamber
 or it altars the chamber in a long lasting way. Such an alteration could
 be smoothing or cleaning of electrodes or windows. A first refill after a
 xenon fill was operated for four hours and 2 million shots without losing
 good stability. Four to five refills, however, with much fewer shots and
 in a shorter time completely bring the chamber back to normal. This rather
 supports the theory that xenon stays in the chamber. The same conclusion
 is drawn from the fact that simply filling with xenon and never firing the
 laser also helps subsequent fills.
 Contrary to energy stability, the operating voltage is entirely independent
 of the previous history. This means that not a large percentage of the
 xenon can be carried over to the next fill. There are different ways how
 xenon could remain in the laser. Since xenon is a very heavy gas it may
 collect preferably on the bottom or in the MFT when the blower is not
 running. In that case, it should be removable by pumping the chamber to a
 pressure much lower than what is available with the membrane pump. This
 still did not prevent the memory effect. Which would suggest that xenon
 gets trapped due to its large size in porous materials or virtual leaks in
 the chamber.
 Extended Xenon Test
 FIG. 8I presents an extended run in 2 kHz mode with a 30 ppm xenon mixture.
 No data are available without xenon, so this is merely a statement how
 well the chamber can perform. The total pressure did not increase during
 the test. The linewidth increases for the first 2 hours, typical of
 heating of the fused silica LNP. Thereafter, the normal trend of
 decreasing linewidth for decreasing fluorine concentration is observed.
 However, the linewidth continues to decrease and only stabilizes after 3
 injects.
 The voltage continues to increase and also only stabilizes after 3 injects.
 In conjunction with the linewidth data the voltage increase is most likely
 not due to any impurities but simply because the mixture gets leaned out.
 Once the voltage is increased the injection intervals shorten because the
 discharge is no longer blip-free.
 During the test a fantastically low fluorine consumption rate was observed.
 Immediately after the refill, the laser was running for an incredible 15
 hours and 28 million pulses without injection.
 In summary, this test shows that with an addition of a small amount of
 xenon to the gas mixture a KrF chamber can operate within specifications
 for 95% linewidth and dose stability. Very low fluorine consumption was
 observed.
 Heavy Noble Gas Additives in ArF Laser
 Applicants have conducted experiments with very small quantities of Kr and
 Xe added to a typical ArF gas mixture. (A typical mixture is about 0.08 to
 0.12% fluorine, 3.5% argon and the rest neon.) Both Kr and Xe
 substantially reduced the average 3 sigma of the laser. Without the
 additives the 3 sigma for the laser was about 5%. About 6-10 ppm of Xe
 reduced 3 sigma to about 4% (a 20% improvement). For the same improvement
 with Kr about 40 ppm were required.
 As with the KrF laser the additives reduced the output of the laser. For
 the same discharge voltage purse energy was reduced by about 1% for each
 ppm of Xe and about 0.2% for each ppm of Kr. Thus, an 8 ppm of xenon the
 pulse energy would be reduced by 8% and 40 pm of Kr would reduce the
 output by roughly the same amount about 8%.
 Very Small Quantity Oxygen Addition
 FIGS. 14 and 15 show the effect on the slug effect in a KrF laser of adding
 minute quantities of oxygen to the laser gas. FIG. 14 shows a dramatic
 reduction in the energy decrease occurring at about 22 to 35 milliseconds
 into the burst. FIG. 15 shows that the 3-sigma variation is also
 dramatically reduced with the addition of oxygen in the range of about 25
 to 49 parts per million, but 25 ppm produces a reduction of about 10% in
 the pulse energy and 49 ppm produces a reduction of about 20% Applicants
 have determined about 5 ppm provides significant improvement in stability
 without significant detrimental effects.
 Argon Fluoride Laser--Elimination of Gas Refill Syndrome with Oxygen
 Applicants have discovered that the addition of oxygen also improves
 performance of very narrow band ArF lasers. Applicants have identified
 what they call a gas refill syndrome. They have discovered that
 immediately after replacing the laser gas in an ArF very narrow band
 laser, the laser performs very poorly in that the pulse energy and laser
 energy is substantially reduced. However, after setting overnight, the
 next morning the laser performs within specification.
 This gas refill syndrome was eliminated with the addition of an extremely
 small quantity of oxygen such as about 2 to 3 parts per million. Thus, the
 preferred laser gas mixture for the very narrow band ArF excimer laser is:
 3.5 percent argon
 0.1 percent fluorine
 2-3 parts per million oxygen
 remainder neon to 3 atmospheres.
 Additional quantities of oxygen were added but the oxygen addition beyond 5
 ppm had no significant beneficial effect. Recommended ranges of oxygen in
 both KrF and Arf lasers is between about 2 to about 7 ppm. Recommended
 ranges of Xe for KrF lasers is less than about 30 to 40 ppm. Recommended
 ranges of Kr for ArF lasers is less than about 40 ppm and recommended Xe
 ranges is less than about 10 ppm.
 Additional Test Results
 Further testing performed by Applicants have confirmed that small
 quantities of oxygen, preferably less than 10 ppm and small quantities of
 xenon also less than about 10 ppm substantially ArF laser performance.
 Extensive testing, however, has not confirmed which is the best additive.
 At very low pulse repetition rates, the oxygen additive performs much
 better than xenon; however, at repetition rates of in excess of 500 pulses
 per second, performance is about the same. Applicants have determined that
 at high repetition rates of about 1000 pulses per second to about 3000
 pulses per second, laser pulse energy is very slightly greater with the
 xenon than with oxygen of like quantities and with equivalent other
 conditions. Tests have also shown in both cases pulse energy output
 decreases with increasing concentrations of either of the additives. Based
 on the 3000 pulses per second operation, Applicants conclude that the
 effect of Xe at higher repetition rates is more pronounced than with
 O.sub.2.
 Applicants' tests have shown that the poor performance with the xenon
 additive at low pulse rates can be greatly minimized by maintaining
 cooling water flow through the chamber heat exchanger. This result causes
 Applicants to believe that the discharge creates a molecular species which
 is detrimental to laser performancew but that by maintaining a certain
 temperature of the chamber, about 60 to 75.degree. C., this effect can be
 reduced substantially.
 FIGS. 16 through 18 show the results of these additional tests. FIG. 16
 shows the results of an ArF laser operating at 2 and 3 kHz pulse rate with
 70.degree. C. gas temperature. FIGS. 17A and B shows that the xenon
 additive provides slightly more stable operation at high repetition rate
 than the oxygen additive. FIG. 18 demonstrates the low repetition rate
 problem with the xenon additive and the effect of having a cold sink in
 the gas path.
 Pre-Pulsing
 As described above in the section entitled "BURST MODE OPERATION" and shown
 in FIGS. 13, 14 and 15, operation of the laser in a burst mode results of
 the laser in a burst mode results in a slug effect which is related to the
 laser gas circulation time. Applicants have determined that some of the
 adverse effects of the slug effect can be minimized by pre-pulsing the
 laser. This involves providing a few pulses prior to the start of each
 burst. In general only a few pre-pulses will produce even better
 performance.
 FIGS. 19A and B shows the effect on an ArF laser operating at a 2000 pulse
 per second pulse rate pre-pulses 10 ms prior to the start of bursts. The
 chart compares no pre-pulses with 1, 2 and 10 pre-pulses. The gas transmit
 time was about 20 ms. FIG. 20 summarizes the test results of pre-pulses
 shown in FIG. 19. The chart indicates that pre-pulses at 10 ms in advance
 of the pulse cuts the slug effect in half. From this data Applicants
 recommend about 1 to 10 pre-pulses 20 ms prior to the burst and at 10 ms
 prior to the burst as an alternative. These pre-pulses will produce a
 slight improvement in energy stability during burst mode operation.
 Similar beneficial effects on energy stability with pre-pulsing in KrF
 lasers has also been observed by the Applicants.
 Although this very narrow band laser has been described with reference to
 particular embodiments, it is to be appreciated that various adaptations
 and modifications may be made to the invention. Although Applicants did
 not test radon in its lasers, they have concluded that very small
 quantities of radon gas would improve energy stability without substantial
 negative effects. Radon should be easier to ionize than any other noble
 gas and it will not form long-lived compounds with fluorine. Therefore, it
 should like xenon in the KrF laser and krypton in the ArF and KrF lasers
 aid preionization. Applicants expect that the best concentration for radon
 would be similar to those discussed above for Xe and Kr. For example,
 sources of oxygen can be pure oxygen or any of the oxygen referred to in
 U.S. Pat. No. 5,307,364. Also, the source of oxygen could be a solid such
 as aluminum oxide or potassium, which could be contained within the
 chamber environment and the oxygen emission, could be controlled with
 temperature. The performance of F.sub.2 lasers should also be improved by
 additives of the types and quantities described above. The F.sub.2 laser
 is substantially similar to the KrF and ArF lasers described above except
 the preferable laser gas is a mixture of about 0.1% F.sub.2 and the rest
 helium. An F.sub.2 laser without the gas additive is described in Ser. No.
 09/237,446, filed Mar. 19,1999 which is incorporated herein by reference.
 Therefore, the scope of the invention is to be limited only by the
 appended claims and their legal equivalent.