WO2001029942A1 - Energy stabilized gas discharge laser - Google Patents

Abstract

An excimer or molecular fluorine laser, such as a KrF- or ArF-laser, or a molecular fluorine (F2) laser, particularly for photolithography applications, has a gas mixture including a trace amount of a gas additive. The concentration of the gas additive in the gas mixture is optimized for improving energy stability and/or the overshoot control of the laser output beam. The concentration is further determined and adjusted at new fills and/or during laser operation based on its effect on the output pulse energy in view of constraints and/or aging on the discharge circuit and/or other components of the laser system. Attenuation control is also provided for increasing the lifetimes of components of the laser system by controlling the concentration of the gas additive over time. A specific preferred concentration of xenon is more than 100 ppm for improving the energy stability and/or overshoot control. The laser system may be equipped with an internal gas supply unit including an internal xenon gas supply, or a xenon generator for supplying xenon gas from condensed matter xenon.

Description

TITLE: Energy Stabilized Gas Discharge Laser

Priority

The present application is a Continuation-in-Part application that claims

the benefit of priority to U.S. patent applications no. 09/498, 1 21 , filed

February 4, 2000, and 09/484,81 8, filed January 1 8, 2000, and also claims

the benefit of priority to provisional patent applications no. 60/1 60, 1 26, filed October 18, 1 999, and 60/1 27,062, filed March 31 , 1 999, and

60/1 78,620, filed January 27, 2000, each application being hereby

incorporated by reference into the present application.

Background of the Invention

1 . Field of the Invention

The invention relates to gas discharge lasers, particularly to excimer and

molecular fluorine lasers having gas mixtures with optimal concentrations of

specific component gases, such as halogen containing species, active rare gases, buffer gases, and a xenon additive for improving pulse-to-pulse and

peak-to-peak energy stabilities, energy dose stability and burst energy

overshoot control, and increasing the lifetimes of laser system components.

2. Discussion of the Related Art

The term "excimer laser" describes gas lasers in which the lasing

medium contains excimers (e.g. Ar₂*), exciplexes (e.g. ArF*) or trimers (e.g.

Kr₂F*). The feature common to all is a gas discharge in which highly excited

molecules that have no stable ground state are created. The following

invention primarily concerns excimer lasers in which the lasing medium is

composed of halogen-containing, particularly fluorine-containing exciplexes

(e.g. ArF* and KrF*). In addition, the present invention relates to molecular

fluorine (F₂) lasers.

In a number of scientific, medical and industrial applications for

excimer and molecular gas lasers, it is important that the radiation pulses

emitted have a stable (constant) energy. In gas lasers, the fact that gas

discharge conditions and characteristics can change has an impact on the

achievement of a constant energy from pulse to pulse of the emitted

radiation. Characteristics and conditions of the gas discharge are dependent

upon a number of parameters that with adequate control can allow

significant improvements toward exact reproducibility. The result is that the

energy of the emitted laser radiation pulses is not maintained exactly constant from pulse to pulse. It is desired to have an excimer or molecular

fluorine laser that demonstrates greater pulse-to-pulse stability.

Energy stability is described by various characteristics of the laser

beam depending on the application. One of these characteristics is the

standard deviation sigma of a distribution of energies of a large number of

laser pulses. As many applications use laser output not continuously but in

bursts of light pulses, other parameters are also used for stability {see U.S.

patent no. 5,463,650, which is hereby incorporated by reference into the

present application, and particularly the background discussion therein).

Specific application of the excimer or molecular fluorine laser beam in optical

lithography as an illumination source for wafer scanners, the energy dose

stability is significant (see U.S. patent no. 5, 140,600, which is assigned to

the same assignee as the present application, and The Source™ (Cymer,

Inc.), Vol. 1 , Issue 2 (Summer 1 999), each of which is hereby incorporated

by reference into the present application).

Another significant characteristic is peak-to-peak stability. For

measuring the peak-to-peak energy stability values, laser pulse energies are

accumulated over some interval. The absolute difference between the

maximum and minimum energies related to the average laser pulse energy is

defined as the peak-to-peak stability.

Of particular interest in burst mode applications, the energy overshoot,

as illustrated in Fig. 1 , is a significant characteristic. Energy overshoot, or spiking, is observed when the laser is operated with constant high voltage at

the discharge chamber in burst mode and the first few pulses have higher

energies than pulses later in the burst (see U.S. patents no. 5,710,787 and

5,463,650, hereby incorporated by reference). The energy overshoot

(designated "ovs" in Fig. 1 ) is defined as the difference between the energy

of the first pulse in a burst and the steady state energy in the entire burst.

The quality of the gas discharge and also the pulse energy of the

emitted laser radiation pulses are dependent upon and are sensitive to

variations in gas discharge conditions such as characteristics of the external

electrical circuit, the composition and shape of the gas discharge electrodes,

the type and quality of pre-ionization, etc. The purity of the gas mixture in

the laser gas discharge chamber and the composition of the gas are also very

important. Even tiny impurities of certain kinds are known to be very

detrimental to the energy of the emitted radiation pulses, the stability of their

energy (the consistency of energy per laser pulse from one firing to the next),

the intensity distribution in the laser beam profile, the life of the laser gas and

the life of individual optical and other laser components. Such impurities in

the gas can be present in the gas mixture from the very beginning or they

may form during operation of the laser, e.g. through interactions between

reactive components of the laser gas mixture (e.g. of the halogen) and the

laser chamber material or through diffusion from the materials or chemical

reactions in the gas mixture. For example, during operation of a KrF-excimer laser, such contaminants as HF, CF₄, COF₂, SiF₄ 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. 1 2, pp.

1 975-1 981 (April 20, 1 992)). For a static KrF laser gas mixture, i.e., with no

discharge running, increases in the concentrations of HF, O₂, CO₂ and SiF₄

have been observed (see Jurisch et al., above).

It is known that the addition of certain substances to the gas mixture

can improve particular characteristics of the emitted radiation. For example,

U.S. Patents no. 5,307,364 and 5,982,800 (hereby incorporated by

reference) suggest that small amounts of oxygen be added to the gas

mixture to achieve greater reproducibility of emitted radiation during laser

operation. Oxygen, however, is not an inert gas, and its effects on other

parameters of the excimer laser, such as the uniformity of the emission

intensity curve and the life of the gas mixture are not yet fully understood

and may be in fact detrimental. Oxygen, especially atomic oxygen and ozone

which can form in the gas discharge, are extremely chemically reactive, and

their effects on the laser gas mixture can be quite detrimental, especially

during long periods of operation. Due to the presence of oxygen, other

stable impurities such as OF₂ and FONO form in the excimer laser gas

mixture. These can have a considerable absorption effect on the laser

irradiation or the pre-ionization radiation. Tests recommended by the current

state of technological developments in which the energy of excimer laser radiation impulses is stabilized through the addition of gases to the gas

mixture have shown disadvantageous effects on other characteristics of the

laser and the emitted radiation.

Filling an excimer or molecular fluorine laser with a gas mixture of

precise composition and maintaining that composition is known to be

advantageous for determining significant output beam parameters. For

example, KrF-excimer laser gas mixtures typically comprise around 1 % Kr,

0.1 % F₂ and a 98.9% Ne buffer. For the ArF-excimer laser, the composition

is around 1 % Ar, 0.1 % F₂ and 98.9% buffer. The molecular fluorine laser

typically has around 0.1 % F₂ and 99.9% buffer gas.

The introduction of very small quantities (#0.1 Torr) of xenon in

excimer and molecular fluorine laser gas mixtures has been proposed as

increasing the photopreionization yield. See 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. 21 95-2207, Vol.

31 , No. 1 2 (December 1 995). Taylor et al. demonstrate an enhancement of

spark pre-ionization intensity by the action of a Xenon additive to the gas

mixture. An advantageous result of this enhancement of the preionization

density is an improvement of the homogeneity of the excimer laser

discharge. Taylor et al. describe qualitatively, however, that if the xenon

concentration is too high, then absorption of laser radiation would occur and

degrade the output laser beam. The conclusion of Taylor et al. then is that only a small amount of xenon added to an excimer laser gas mixture would

enhance the preionization intensity and improve the discharge.

More recently, the use of xenon in ArF excimer lasers has been

reported by Wakabayashi et al. See Wakabayashi et al., Billion Level Durable

ArF Excimer Laser with Highly Stable Energy, SPIE's 24^th Annual International

Symposium on Microlithography, Santa Clara, May 1 4-1 9, 1 999.

Wakabayashi et al. describe similar results as Taylor et. al (see above),

namely, an improvement of the preionization density resulting in an increased

output energy at the same input discharge voltage of the ArF-excimer laser.

The optimal concentration of xenon in the ArF-excimer laser gas mixture is

described as 10 ppm, or the peak of the output energy versus xenon

concentration curve shown at Fig. 6 of Wakabayashi et al.

Summary of the Invention

It is recognized in the present invention that an advantageous value of

the concentration of an additive, such as a noble gas, e.g., preferably xenon

and alternatively krypton to an ArF-excimer laser gas mixture, as well

preferably xenon or argon to a KrF-laser, argon or krypton to a XeCI- or XeF-

laser, and xenon, argon or krypton to a F₂-laser gas mixture, wherein the

concentration selected depends not only on its effect on the photo-

preionization yield and the output energy, but also on the energy stability and

overshoot control of the laser. It is therefore an object of the invention to provide an excimer or

molecular fluorine laser having a gas mixture including an appropriate

concentration of the gas additive based at least in part on the effect of the

concentration of the additive on improving the energy stability of the output

laser beam. The energy stability is determined based on both the stability of

the first pulse or first few pulses after a pause for a laser operating in burst

mode, and also on the overall stability of the output energy of the laser.

It is a further object of the invention to provide the appropriate

concentration of the additive gas based on the effect of the concentration of

the additive on improving the overshoot control of the laser.

It is a further object of the invention to provide an excimer or

molecular fluorine laser with energy attenuation control to increase the

lifetimes of optical and laser tube components.

In accordance with the above objects, an excimer laser, such as a KrF-

or ArF-laser, or a molecular fluorine (F₂) laser, preferably for high repetition

rate operation such as above 1 kHz, is provided with a gas mixture including

a small amount of a gas additive. The gas additive is preferably xenon. For

the ArF-excimer laser, for stability reasons, the initial concentration of the

gas additive is selected and may be adjusted in accordance with selected

values of one or more of energy stability, overshoot control, and pulse

energy. The xenon concentration selection may be further based on the

additional criteria of output pulse energy control. For example, the pulse

energy may be attenuated, e.g., to advantageously lengthen the laser pulses,

by decreasing the fluorine concentration in the gas mixture, and then the loss

of energy may be compensated by adding an appropriate amount of xenon to

the gas mixture. The pulse energy or energy dose may be regulated by

controlling the amount of xenon in the gas mixture.

A gas discharge laser such as an excimer or molecular fluorine laser in

accord with the present invention includes a laser tube including an electrode

chamber containing a pair of elongated main electrodes and one or more

preionization electrodes, and a gas flow vessel. The laser tube is filled with a

gas mixture including a laser active gas or gases, a buffer and a trace amount

of an additive gas for improvement of burst energy overshoot control and/or

a characteristic energy stability such as standard deviation sigma, and/or

peak-to-peak, pulse-to-pulse and/or dose stability, and/or adjustment of the

output energy level of the laser, such as for energy attenuation control or for

balancing the energy stability and/or overshoot control.

The preferred laser system is equipped with an internal gas supply unit

including a supply of the additive gas, preferably a xenon supply. An output

beam parameter stabilization algorithm is provided for the laser system which

maintains optimal concentration of all of the gas mixture constituents

including the halogen containing species, F₂ or HCI, and the gas additive, preferably xenon, as well as for the active rare gases Ar and Kr for the ArF-

laser and the KrF-laser, respectively. Preferred gas control, composition and

stabilization algorithms are described at U.S. patent applications no.

09/379,034, 60/1 24,785, 60/1 59,525, 09/41 8,052, 09/31 7,526 and

60/1 27,062 and U.S. patents no. 4,393,505 and 4,977,573, each of which

is assigned to the same assignee and is hereby incorporated by reference into

the present application, wherein the algorithms disclosed in the above

patents and patent applications are modified in accord with the present

invention to include the injection and control of the gas additive into the gas

supply in the discharge chamber. Such parameters as energy stability,

energy dose stability, output pulse energy and driving voltage (and/or

amplified spontaneous emission (ASE) and/or features of the temporal or

spatial pulse shape and/or one or more other parameters such as total

accumulated energy input to the discharge, bandwidth, moving average

energy dose, temporal or spatial coherence, discharge width, and long and

short axial beam profiles and divergences, time, pulse count or a combination

thereof) may be monitored and parameters of the output beam mentioned

above and/or others are stabilized in accord with the present invention.

The control of the amount of the gas additive in the gas mixture is also

preferably used to increase the lifetimes of laser components. The

characteristic output power range is initially set to be higher than the desired

output power of the laser system, within the range of operating driving voltages. Then, the power is attenuated by adding more of the gas additive,

preferably xenon, into the gas mixture until the output power is reduced to

the desired level. As the laser components age, the amount of

additive/xenon is reduced to achieve the desired output power with each

new fill.

The gas additive may be added to the gas mixture from a gas

container including a premix including the preferred xenon gas additive.

Alternatively, xenon gas can be obtained from xenon containing crystals that

are heated to dissociate the xenon containing crystals. In this embodiment, a

xenon generator is filled with xenon-containing crystals and a heating

element and temperature controller are used to control the xenon gas pressure.

Although xenon is the preferred gas additive, other gas additives may

be used in accord with the present invention. Argon may be used as the gas

additive for a KrF laser. Krypton may be used as the gas additive for an ArF

laser. Argon and/or krypton may be used as the gas additive for a XeCI or

XeF laser. Argon, Krypton and/or Xenon may be used for a F₂ laser. NO

may be used for a XeCI laser (e.g., 0.1 % NO in Ne). NO₂, N₂O₄, FONO or

FNO may be used for a XeCI or F₂ laser.

Another element or molecule, such as a metal, e.g., W or Pt, may be

added that would react to form one or more metal fluoride or metal chloride

species, i.e., preferably WF, WF₂, PtF, PtF₂ or alternatively WF_X or PtF_x, wherein x is preferably between three and sixteen, within the gas mixture.

The metals may be added to one or more electrodes preferably of the

preionization unit or another metal component of the laser tube, if any.

Other candidate metals include chromium, and aluminum. Silicon, carbon,

hydrogen fluoride, ozone, mercury, hafnium, metals and alloys having high

vapor pressure similar to mercury and hafnium, such as are typically liquids

at standard temperature and pressure (STP) may be used. Some metal

oxides such as molecular combinations of oxygen and one or more of

chromium, fluorine or aluminum, are other preferred candidate elements or

molecular species that may be used and/or that are or will form halides (i.e.,

fluorides or chlorides), may be used as the gas additive, wherein xenon is

herein described as being preferred.

Some particular preferred molecular combinations, either neutral or

ionized or combinations of neutral and ionized species, that may be added or

that may be formed by an additive reacting with the fluorine or chlorine

already in the gas mixture include HF, HF, CF_X (particularly CF₄), CrOF₂,

CrOF, CrO₂F, CrO₂F₂, CrO₂, CrO, Cr, CrF₂, CrF, SiF₄, SiF, OF, O₂F, OF₂, Al,

AIO, AI₂O, AI₂O₂, AIF, and AIF₂. Other possibilities include N, N₂, N_x, C, C₂,

C_x, H, H₂, H_x, O, O_x where x is a small integer above 3, such as 3-1 6, and

combinations of any of these elements and/or molecules, as well as air itself.

Any of the above mentioned elements or molecules or combinations thereof

may be added to the gas mixture, preferably in trace amounts, such as less than 500-1000 ppm, or less than 0.1 %, in accord with the present

invention.

In addition, more than one gas additive may be added to the gas

mixture. For example, two or more of the additive mentioned above may be

added to the gas mixture for controlling the pulse energy, energy dose,

energy stability and/or overshoot control, either separately or in combination.

One gas additive, or combination of gas additives, may be used to control

one of these parameters or others, and another gas additive, or combination

of gas additives, may be used to control another of the above parameters.

Brief Description of the Drawings

Fig. 1 illustrates energy overshoot, or spiking, for a laser operating in

burst mode.

Fig. 2 shows a xenon gas generator in accord with the present

invention.

Fig. 3a shows a pulse-to-pulse energy stability over a large number of

bursts of 240 pulses for a conventional KrF laser system.

Fig. 3b shows the energy overshoot of a conventional burst mode

operation KrF laser as a percentage over the steady state output energy over

entire bursts including around 240 pulses. Fig. 4a shows a pulse-to-pulse energy stability over the same number

of bursts of 240 pulses as Fig. 2a for a KrF laser system in accord with the

present invention.

Fig. 4b shows the energy overshoot of a burst mode operation KrF

laser in accord with the present invention as a percentage over the steady

state output energy over entire bursts including around 240 pulses.

Fig. 5 shows the dependence on xenon concentration of the energy

overshoot of a KrF laser operating in burst mode as a percentage over the

steady state output energy of an entire burst.

Fig. 6a shows a measured dependence on xenon concentration from

30 to 520 ppm of the energy stability of a KrF laser operating in burst mode

as the inverse of the percentage deviation from the steady state output

energy of the laser.

Fig. 6b shows a measured dependence on xenon concentration from 0

to 30 ppm of the energy stability of a KrF laser operating in burst mode as

the percentage deviation from the steady state output energy of the laser.

Fig. 6c shows a measured dependence on xenon concentration from

30 to 520 ppm of the output pulse energy of a KrF laser at constant

discharge voltage.

Fig. 6d shows a measured dependence on xenon concentration from 0

to 30 ppm of the output pulse energy of a KrF laser at constant discharge

voltage. Fig. 6e shows measured dependences of the output energy and energy

stability sigma on the xenon concentration in an ArF laser gas mixture, where

the output energy dependence is shown by depicting the discharge voltage

needed to maintain 5 mJ output energy.

Fig. 7 shows a preferred embodiment of a KrF, ArF or F₂ laser system

in accord with the present invention.

Detailed Description of the Preferred Embodiments

Preferred embodiments of the invention including procedures and laser

systems having improved discharge homogeneity and energy stability,

particularly in ArF and KrF excimer lasers and F₂ lasers are described below.

The preferred embodiments implement the spirit of the present invention into

working laser systems and are generally related to providing a gas additive,

which is preferably xenon and/or may be one of the other gas additives

mentioned above, to excimer and/or molecular fluorine laser gas mixtures

particularly for controlling and/or stabilizing the pulse energy, energy stability,

energy dose control and/or energy overshoot of these laser systems.

The invention and description below are particularly drawn to

controlling and/or stabilizing these parameters of these laser systems when

the laser operates in burst pattern operation, although the present invention

may be applied to continuous output laser systems, as well. The invention

may be applied to other excimer lasers such as XeCI, XeF and KrCI lasers. and other additives such as Ar, Kr, and others enumerated above, may be

advantageous gas additives in some embodiments with some of these laser

systems. The present invention is particularly drawn to lasers operating at

high repetition rates such as 1 or 2 kHz pulse repetition frequency or higher.

Below are particularly described aspects of the invention including the

use of a certain amount of xenon as an additive to the conventional gas

mixture of an excimer or molecular fluorine laser, to design an apparatus

which enables the accurate injection of the xenon to the laser gas mixture, to

use gas injection and replenishment algorithms which allow the maintenance

of the optimum gas mixture and xenon partial pressure in the gas mixture by

computer controlled gas actions, and to use a fast energy detector to

determine and control the optimum xenon partial pressure in the gas mixture,

along with other gaseous constituents such as the halogen in the gas mixture

(see the '034 and '785 applications incorporated by reference above).

In a preferred embodiment of the invention, both according to a

preferred method and laser system, a particular amount of xenon is initially

filled along with the usual constituents of the gas mixture (see the '505 and

'573 patents, and the '526 and '785 applications, mentioned above) into the

laser tube during a new fill. It is recognized in the present invention that

adding xenon to the gas mixture effects more than one aspect of the laser

system. Thus, the particular "optimal" amount of xenon initially filled into

the laser tube depends on the type of laser being used and the result of adding the xenon that is desired. For example, the output energy of the laser

at a particular operating discharge voltage may be advantageously enhanced

or attenuated depending on the amount of xenon that is added to the gas

mixture. Additionally, energy stability and overshoot control may be

advantageously improved to a degree that depends on the amount of xenon

that is added. Also, a particular amount of xenon may be added according to

a balance of these effected aspects of the laser system.

Argon Fluoride Laser

In a first embodiment for the ArF-excimer laser, for improving the

energy stability, the concentration of xenon is greater than 10 ppm, and is as

high as substantially 300-500 ppm or more. It will be shown below that the

energy stability and overshoot control each improve with xenon

concentration for concentrations of more than 500 ppm. It will be further

shown below that the output energy at a particular discharge voltage has a

maximum around 10 ppm, or that the required discharge voltage for

producing output pulses at a particular energy (e.g., 5 mJ) has a minimum

around 1 0 ppm. However, at higher concentrations of xenon, such as

greater than 1 00 ppm, e.g., the energy stability and overshoot control are

advantageously improved in accord with the present invention.

The preferred xenon concentration in this first embodiment for the ArF

laser is balanced by the attenuating influence of the xenon additive on the pulse energy at these concentrations above 10 ppm. The upper limit for a

particular laser system depends on limitations of the discharge circuit

including the power supply, components of the pulser circuit and especially

the discharge electrodes. That is, a particular pulse energy somewhere in a

range from a few mJ to over 1 0 mJ is specified for a particular industrial

application of the laser, and xenon cannot be added in amounts too high that

the laser system is unable to generate pulses at that specified energy.

Preferably then, in this embodiment where the energy stability and/or

overshoot control is sought to be maximized, the xenon concentration in the

gas mixture is adjusted in accordance with the specified output power level

and constraints of laser system components such as the power supply,

pulser module and electrodes. Those system components are preferably

configured to produce a higher output energy than would be desired when no

xenon is added to the gas mixture, and then xenon is added to the gas

mixture to attenuate the pulse energy to the desired value. Advantageously,

the pulse energy is at the desired value, and the energy stability and/or

overshoot is also at an improved, preferably selected, value. The system

components may also be conventionally configured, and the xenon, e.g.,

more than 100 ppm, is added and the driving voltage increased to adjust the

output energy to the selected value while again having advantageously

improved energy stability and/or overshoot control in accord with the present

invention. Krypton Fluoride Laser

In a third embodiment, this time for the KrF-excimer laser, where it is

desired to improve the energy stability and/or overshoot control of the output

beam, the concentration of the preferred gas additive, i.e., xenon, is more

than substantially 1 2 ppm, and preferably more than substantially 20 ppm,

but less than substantially 2000 ppm, and preferably less than substantially

600 ppm. As with the first embodiment for the ArF laser, the upper limit on

the xenon concentration is imposed by limitations on the power supply,

pulser circuit and discharge electrodes. As improvements of these

components are achieved, the xenon concentration upper limit can be raised.

In a fourth embodiment, for the KrF laser, for balancing output pulse energy

and the improvement to energy stability and/or overshoot control due to the

xenon additive, the preferred xenon concentration range from which the

particular xenon concentration is selected is a range between 1 00 and 500

ppm.

As discussed above, absorption and energy attenuation as a result

thereof can serve to put an upper limit on the concentration of xenon in the

gas mixture because it can significantly reduce the output energy of laser

pulses at a particular driving discharge voltage. When the system can no

longer compensate the attenuation due to additional xenon by increasing the

driving voltage to maintain the specified output pulse energy, then the upper limit xenon concentration is reached. It is preferred to have as much xenon

as possible in the gas mixture, within the constraints on the system

components for delivering the desired output energy, for improving the

energy stability and overshoot control.

Lifetime Extension of Laser Components

This attenuating effect of the gas additive in the gas mixtures of

excimer and molecular fluorine lasers can be used advantageously in accord

with the following embodiment of present invention to increase the lifetimes

of laser components, including resonator optics components. Variations in

the quality of the various laser components (e.g., optical components in the

resonator such as prisms, gratings, etalons and windows, as well as the laser

chamber) can lead to variations in the output power of the laser system of up

to 20-40%. In addition, aging of components over their lifetimes leads to a

reduction of the maximum available output power over time. This leads to

operations at higher driving input voltages to achieve the same output power.

The dynamic range of the operating voltage is however limited putting an

upper limit on the lifetimes of the laser components.

The dependence of output power on xenon partial pressure may be

advantageously used in accord with the present invention to extend these

component lifetimes. The system is initially configured to have an excess of

laser power when the components are new. That is, the operating range of voltages is above that typically required for generating output laser pulses at

specified energies (e.g., between a few mJ to over 10 mJ). At this time, a

certain amount of xenon is added to the mixture so that the output power is

at the desired value within the operating voltage range.

For example, a nominal 1 0W ArF laser having a < 0.6 pm FWHM

bandwidth at 2 kHz repetition rate may be designed to deliver a maximum

power of 30 Watts. The typical dynamic operating range of the driving

discharge voltage would then allow the conventional laser to operate at a

minimum of 1 5 W, which is 5W above the desired 10 W power for a laser

with new components. In accord with the present invention, however, a gas

additive such as xenon may be added to the gas mixture in selected amounts

to attenuate the laser power and bring the output power into the desired

range for the operating range of the driving voltage of the laser system.

As the optical and laser tube components age, the xenon partial

pressure in the gas mixture is adjusted with each new fill to a different value

to achieve the same desired output power within the operating voltage

range. The xenon concentration can also be adjusted between new fills

according to gas control procedures described below.

An exemplary procedure in accord with this embodiment of the

present invention for increasing component lifetimes is as follows. After a

new gas fill of an excimer or molecular fluorine laser (without xenon), the

laser is started with a nominal high voltage at the operating point of the laser and the output power or energy is measured by an energy monitor, which is

typically internally configured with the laser system (see discussion regarding

Fig. 7). The power for the new laser will be measured to be higher than

desired, in accord with this embodiment, and so a certain amount (e.g., 1 0

ppm) of xenon is added to the gas mixture, and the power is measured

again. The addition of xenon may be repeated and the output power

measured a number of times until the output power is reduced to within the

desired value within the operating range of the driving voltage.

Alternatively and advantageously, the expert system including a

computer database and processor (see the '034 application, mentioned

above) can store values of xenon amounts added after previous new fills

and/or from previous experience with other lasers, and an estimated initial

amount of xenon to be added with a present new fill can be estimated.

Then, an initial amount of xenon can be added which is closer to the actual

desired amount than described above, after which the repeated steps of

adding small amounts (e.g., 10 ppm) of xenon and measuring the power can

be performed. In this way, the overall procedure will consume less time.

In accord with this embodiment of the present invention, the amount

of xenon added to the gas fill will generally decrease as the components age

and the maximum output power of the laser system decreases. Since, as

discussed above, the lifetimes of the laser components ends when the

system can no longer achieve the desired output power even when operating at the maximum driving voltage, the advantage of adjusting the xenon

concentration to control the output power is clearly set forth in this

procedure of the present invention. The result is that the lifetimes of the

components is advantageously increased (e.g., more than 1 00%).

Gas Replenishment

The gas additive concentration not only can be adjusted at a new fill,

but can also be adjusted between new fills using gas replenishment

procedures in accord with the present invention. For this purpose, a source

of xenon is preferably integrated with the excimer or molecular fluorine laser

system. That is, an internal xenon supply is provided with the laser system.

Alternatively, a certain amount of xenon is mixed in a premix with an inert

gas of a conventional supply of gases in gas supply bottles or containers that

are external to the laser. After the initial predetermined amount of xenon is

first filled into the discharge chamber at a new fill, gas replenishment

techniques are preferably used in accord with the present invention to

maintain the optimal xenon concentration in the gas mixture and/or to adjust

the predetermined amount. An outline of preferred techniques is set forth in

the '785 and '034 applications, referred to above, which are drawn

particularly to halogen (and rare gas) replenishment, but may be modified to

include xenon concentration control and/or replenishment in accord with the

present invention. Excimer lasers of the usual type contain a gas mixture with a total

pressure that is usually less than 5 bar. The bulk of the mixture, typically 90

to 99%, consists of a so-called buffer gas. Helium and neon are typical

buffer gases. The buffer gas serves to transfer energy. The atoms of the

buffer gas do not become part of the emitting, highly excited molecules in

the gas discharge. The rare gas, which forms highly excited excimers,

exciplexes or trimers in rare gas-halogen lasers, is found in much lower

concentrations, typically in the range of 1 to 9%. The concentration of the

halogen donor is typically 0.1 to 0.2%; particularly diatomic halogen

molecules such as F₂ or HCI or other halogen-containing molecules can be

used as halogen-donors. The molecular fluorine laser does not include an

active rare gas in its gas mixture.

The present invention is an excimer or molecular fluorine laser system

wherein the laser tube is configured to receive injections with high accuracy

of predetermined small amounts of xenon as an additive to the gas mixture.

Means for stabilizing the optimum xenon partial pressure are also provided.

The particular techniques including micro-injections and gas replacements

and pressure adjustments are disclosed in the '034 and '785 applications

discussed above.

The xenon may be injected in pure form or as a constituent gas in a

premix including an amount of an inert gas such as Ar, Ne, He, or Kr. In the

case of the ArF and KrF lasers, a premix of 0.05% Xe in Ar and Kr, respectively, is preferred. In another preferred configuration, 1 .4% Xe in Ne

is used as a premix. The present invention is not however limited to the

particular premix concentrations of xenon and buffer and/or other gases. As

discussed below, it is preferred that the xenon gas supply be internal to the

laser system, although the xenon may alternatively be supplied from external

gas sources.

The xenon is injected in intervals and amounts determined based on an

expert system including a processor which receives monitored values of

output beam parameters and values such as energy and energy stability and

on values of the high voltage. Very small amounts and short intervals are

possible because the gas supply system is so configured (see the '785

application and the '514 patent, mentioned above).

Other parameters such as beam profile, temporal and spatial

coherences, discharge width, time, shot or pulse count, pulse shape, pulse

duration, pulse stability, bandwidth of the laser beam, or a combination of

two or more of these parameters may be used. The expert system generally

compares monitored values with stored values to determine whether, which

type and to what extent that gas replenishment procedures are to be

performed, including whether and to what extent xenon injection or

replenishment is to be performed based on the monitored parameters.

Using an energy detector, the output energy and energy stability of the

laser emission may measured, and in burst operation the energy overshoot may be particularly measured as the first or first few pulses of bursts of

pulses. If the measured values differ from preset reference values or desired

values, the amount of xenon in the laser gas mixture may be increased by

xenon gas injection or reduced by gas release preferably in combination with

gas injections or by mini or partial gas replacement (see the '785

application). By monitoring and controlling such parameters as laser pulse

energy, energy stability and/or burst overshoot after the gas actions are

performed, it is possible to determine whether the optimum concentration of

xenon is in the gas mixture. By monitoring these and/or other parameters,

such as ASE or temporal pulse shape (see the '052 and '062 applications

mentioned above) in combination, it is possible to know both the halogen

concentration and the xenon concentration in the gas mixture at any time,

even though the concentrations of the xenon and the halogen may both

effect some parameters such as pulse energy.

If after the gas actions are performed, it is determined that the

optimum concentration of Xenon is not in the gas mixture after the laser

parameters are measured, then corresponding gas actions are carried out and

the control measurements of the laser parameters are repeated until the

optimum xenon concentration is reached.

Condensed Matter Xenon Supply ln another embodiment of the present invention, the objects of the

invention are met wherein xenon-containing condensed matter is added to

the gas discharge chamber of the laser, or is in physical relation to this

chamber in a manner that makes gas transport or gas diffusion possible.

Such solids supply the necessary traces of xenon or xenon-containing

compound to achieve the energy-stabilizing effect on the emitted laser

irradiation impulses that is the goal of this invention.

In this embodiment of the present invention, xenon is preferably

supplied using a solid xenon containing species such as XeF₂, rather than

directly using a gaseous supply of xenon premix as described above. Referring to Fig. 2, a xenon gas generator 20 comprises a small container 22

which can be filled with xenon containing crystals (such as XeF₂). The

container 22 can be connected to the laser tubel by at least one gas line 23.

A valve or valves V1 , V3 can be used to separate the container 22 from the

laser tube 1 . A separate receptacle 26 maybe used wherein the dissociated

xenon and fluorine gases may be mixed prior to injection into the laser tube

1 . Buffer gas can be used to flush the xenon fluorine mix into the laser tube

1 via valve V3. For this purpose a buffer filling line is connected through

valve V2 to the receptacle 26. The receptacle 26 may be used for accurate

control of the amount of xenon being injected. For this purpose, the

pressures of each of the receptacle 26 and laser tube 1 are monitored prior

to injection. The receptacle 26 and use thereof may be similar to or the same one as that described for gas replenishment of the halogen and active

rare gases in the '514 patent and/or the '785 application, incorporated by

reference above.

The container 22 is preferably equipped with a heating element 24 and

a temperature control device such as a conventional temperature controller

(not shown). The container 22 is preferably heated to a preset temperature

that will result in dissociation of the xenon-containing molecules of the

crystals. For example, XeF₂ would dissociate into xenon gas and F₂ gas.

The generated gas is then filled into the laser tube 1 , either directly or

through the receptacle 26, as described above. The amount of released

xenon depends on the temperature applied to the solid xenon compound.

That is, the xenon pressure or partial pressure can be adjusted by controlling

the temperature within the container 22. Any losses of xenon due to partial

gas replacement can be automatically compensated by xenon release from

the heated solid compound. The released amount of fluorine would not be

sufficient for the laser. Thus, fluorine and other rare gases would be filled

into the laser tube in the usual way from gas tanks and/or premix bottles as

describe above.

An exemplary procedure for partial gas replacement in accord with the

present invention is as follows. First, valve V1 is closed. A portion of the

laser gas is released from the laser tube 1 in the usual way (e.g., see U.S.

patent no. 4,977,573, which is assigned to the same assignee and is hereby incorporated by reference into the present application). Next, the halogen

and active rare gases and buffer gas are filled into the laser tube 1 from gas

tanks. Then, valve V1 is opened and the reduced xenon pressure is

compensated, again either directly or through the receptacle 26 as described

above. It may be of advantage to also connect the gas generator 20 with

two gas lines to the laser tube and cycle some of the laser gas through the

generator 20. In this way, stabilization of the xenon pressure can be more

quickly achieved and not so limited by the diffusion rate of xenon from the

generator 20 to the laser tube 1

The xenon or the xenon-containing substance can be injected directly

into the gas mixture or added to one of the gas components before filling, for

instance to Ne, Kr, Ar, He or F₂. If xenon or a xenon-containing substance is

added to the gas discharge chamber in the form of a solid in accord with this

embodiment in order to create the aforementioned low xenon concentration

in the gas discharge, condensed xenon fluorides (for instance XeF₂, XeF₄,

XeF₆) are particularly envisaged for this purpose and are introduced into the

laser chamber beforehand or form during operation of the laser. It has been

shown that measurable amounts of such substances (xenon fluorides) can

accumulate inside the laser chamber during operation (condensation), if the

laser is operated with a fluorine-containing gas mixture in which xenon or

xenon-containing compounds are present (e.g. XeF*). In this case, the

aforementioned condensable xenon fluoride is formed during operation of the laser and remains in the laser chamber. It later provides the aforementioned

traces of xenon in the gas mixture, even when the laser is no longer supplied

with xenon from external sources. The remains of the xenon-containing solid

in the gas discharge chamber supply the necessary concentrations in the ppm

range for stabilization of the impulse energy during further laser operation

(without further addition of xenon for several subsequent gas fills).

The invention is therefore also implemented when an excimer or

molecular fluorine laser is prepared and operated in such way that it is

operated with a gas mixture containing fluorine in the presence of xenon and

is thereafter operated without the addition of further xenon (in ppm range), but because of the previous operation, there is still a sufficient trace of

xenon in the gas mixture.

Experimental Results

Fig. 3a shows a pulse-to-pulse energy stability over a large number of

bursts each including about 240 pulses for a KrF laser system without any

xenon additive in its gas mixture. The KrF laser was operated at 2KHz and

the bursts followed a 0.8 second pause. The pulse energy stability is

depicted as a percentage deviation from the steady-state average. The

pulse-to-pulse energy stability of the KrF laser having a conventional gas mix

without a xenon additive is shown in Fig. 3a to vary from a minimum around

5% to over 1 5%. The stability is particularly poor over the first 70 pulses or so, where it fluctuates between 1 0% and 1 5%. After the first 70 pulses,

the stability settles into a range between about 7% and 1 2%.

Fig. 3b shows the energy overshoot of the laser of Fig. 3a as a

percentage over the steady state average output energy over entire bursts

each including around 240 pulses. The overshoot of the KrF laser having a

conventional gas mix without a xenon additive is shown in Fig. 3b to be

around 30% for the first pulse or pulses and rapidly decreases to around

1 0% after 5-10 pulses, and to around 5% after around 25 pulses. The

overshoot is then shown to decrease somewhat more smoothly over the

remainder of the burst. At the last 50-100 pulses, the overshoot effect on

the pulse energies is finally reduced substantially to zero, i.e., the steady-

state value is reached.

Fig. 4a shows a pulse-to-pulse energy stability as in Fig. 3a over a large number of bursts each including about 240 pulses for a laser system in

accord with the present invention as a percentage over the steady-state

average. The laser system of the present invention whose output pulse

energies were measured and plotted in Fig. 4a was the same used for Fig. 3a

and had a gas mixture including about 35 ppm of a xenon additive to an

otherwise typical KrF laser gas mixture. The KrF laser was again operated at

2KHz and measured bursts followed an 0.8 second pause. The pulse-to-pulse

energy stability for the KrF laser having 35 ppm of a xenon additive in its gas

mix is shown in Fig. 4a to vary from a minimum just below 4% to no more than 1 2%. Except for a few peaks in the first 60-70 pulses, the stability is

shown to be below 10% over these first pulses where overshoot is typically

most pronounced. After the first 60-70 pulses, the stability settles into a

range between about 3% and 8%.

Fig. 4b shows the energy overshoot of a burst mode operation KrF

laser in accord with the present invention as a percentage over the steady

state output energy over entire bursts each including around 240 pulses. As

with Fig. 4a, the laser system of the present invention whose output pulse

energies were measured and plotted in Fig. 4b had a gas mixture including

about 35 ppm of a xenon additive to an otherwise typical KrF laser gas

mixture. Again, the KrF laser was operated at 2KHz and the measured burst

followed a 0.8 second pause. The overshoot is shown in Fig. 4b to be

around 9-1 0% for the first pulse or pulses and rapidly decreases to around

3% after 5-1 0 pulses, and to around 2% after around 20 pulses. The

overshoot is then shown to decease over the remainder of the burst, and at

the last 50-1 00 pulses, the overshoot effect on the pulse energies is

decreased substantially to zero.

At least two major improvements in the output energy stability are

observed for the laser having the Xe-additive in its gas mixture used for

measuring the data of Figs. 4a-4b over the laser not having the xenon

additive used for measuring the data of Figs. 3a-3b. First, the pulse-to-pulse

energy stability shown in Fig. 4a for the laser using the gas mixture with the 35 ppm xenon additive is at all points less than 1 2% deviation, and less than

1 0% deviation for most of the laser pulses at the beginning of the burst, and

less than 8% after 100 pulses. The stability demonstrated by the laser in

Fig. 4a is a significant improvement compared to the laser of Fig. 3a

operating without xenon, wherein the energy stability is observed to be as

high as 1 8% for some pulses, is at around 1 5% for pulses at the beginning

of the burst and remains around 10% after the first 100 pulses. Second, the

burst overshoot defined as the average deviation of the first pulse in the

burst from the steady-state energy value is reduced from 30% for the laser

of Fig. 3b operating without xenon to less than 1 0% for the laser of Fig. 4b

operating with xenon.

Fig. 5 shows the dependence of the energy overshoot on the xenon

partial pressure in the gas mixture of a KrF laser. Fig. 5 indicates a strong

improvement of the overshoot already at very small xenon concentrations.

That is, the overshoot decreases from 32% for no xenon additive, as already

indicated at Fig. 3b, to 1 2-1 3% at around 1 7 ppm xenon. A reduction of the

overshoot to 8% is observed at around 37 ppm xenon, and a further

reduction to 2-3% is shown for a 67 ppm xenon concentration.

In obtaining the experimental results that follow in Table 1 , a KrF

excimer laser was operated at a repetition rate of 1 kHz. A laser of the type

Lambda Physik Litho/P was being used. The total gas pressure was 3 bar

absolute. The individual components in the gas mixture were present in approximately the following concentrations: 0. 1 % F₂, 1 % Kr, 98.9% neon

and a trace of xenon in the range of 1 0 - 500 ppm. Pre-ionization was

carried out with UV sparks although corona preionization is also typically

used in KrF, as well as ArF and F₂ laser systems. The applied high voltage

used during the testing was of the order of 1 5 kV.

Results of the experiment were as follows:

Table 1

Concentration Pulse Energy S Sttaannidard Deviation

(in ppm) (relative units) (%)

0 1 1 .32

49 0.94 0.91

82 0.88 0.81

1 22 0.84 0.81

1 63 0.80 0.86

204 0.75 0.83

334 0.66 0.88

484 0.58 0.94

The above experimental results show that for the excimer laser used

and under the given operating conditions, there is an optimum standard

deviation of 0.81 % (as is usual, the standard deviation is calculated as the square root of the mean of the squared deviation of the variables from their

mean). At the optimal stability, the pulse energy fell slightly, but in a large

number of applications, the advantage of stabilization of the energy from the

emitted laser beam impulses outweighs this slight decrease in output energy,

which is compensated by increasing the high voltage in those applications.

Fig. 6a shows a measured dependence on xenon concentration of the

energy stability sigma of an ArF laser, operating in burst mode and having a

bandwidth less than 0.6 pm, as an inverse of the percentage deviation from

the steady state output energy of the laser for xenon concentrations above

around 30 ppm. Fig. 6b shows a measured dependence on xenon

concentration of the energy stability sigma of an ArF laser as the percentage

deviation from the steady state output energy for concentrations below 30

ppm. The energy stability is shown in Fig. 6b to improve drastically when

only a few ppm of xenon are added to the gas mixture, and is shown in Fig.

6a to improve steadily with increasing xenon concentration thereafter.

Fig. 6c shows a measured dependence on xenon concentration from

30 to 520 ppm of the output pulse energy of the ArF laser of Figs. 6a-6b at

constant discharge voltage. Fig. 6d shows a measured dependence on xenon

concentration from 0 to 30 ppm of the output pulse energy of the ArF laser

of Figs. 6a-6c. The pulse energy is shown at Fig. 6d to improve drastically

when a few ppm of xenon are added to the gas mixture. The pulse energy is then shown to attentuate substantially linearly from about 5.7 mJ at 30 ppm

xenon to around 1 mJ at just over 500 ppm xenon.

Adding traces of the rare gas xenon has no observed negative effects

on the quality of the gas discharge. Only unstable XeF* or stable XeF₂, XeF₄

or XeF₆ is formed in the gas discharge.

Fig. 6e illustrates the influence of xenon on the output energy and the

energy stability of an ArF excimer laser used for 1 93 nm lithography at

xenon concentrations below 40 ppm. The dependence of the output energy

is shown by depicting the high voltage needed to maintain 5 mJ output

energy. The dependence of the output energy on the Xenon concentration is

qualitatively similar to the results obtained by Wakabayashi et. al. (see

above). Fig. 6e shows that the xenon concentration which produces

maximum output energy, or which requires the lowest high voltage to

maintain the 5 mJ output energy, is at or just slightly below around 1 0 ppm.

At 10 ppm, the required high voltage is around 1 8.9 kV. At xenon

concentrations above and below 10 ppm, the high voltage required to

maintain the 5 mJ output energy increases. For example, at 0 ppm and

again at around 28 ppm, the required high voltage is around 1 9.6 kV. At

xenon concentrations above 28 ppm, the high voltage continues to increase.

The output energy stability is, however, improved at xenon

concentrations above 1 0 ppm, and continues to improve at xenon

concentrations as high as 35 ppm. As shown in Fig. 6e, the energy stability sigma is around 3.3% at 0 ppm xenon Concentration. The energy sigma

improves to around 2.4% between around 1 7 ppm to 21 ppm. The energy

sigma improves further to 2.1 % around 28 ppm. Based on these data, it is

recognized in the present invention that the optimal xenon concentration is

above that which produces the lowest required high voltage to maintain the

5 mJ output energy, but below that which produces a required high voltage

that is significantly increased from that minimum. The optimal xenon

concentration, for an embodiment of the invention which seeks to improve

the combination of the energy stabilty sigma and the output pulse energy, is

thus based on both of the plots shown in Fig. 6e, i.e., the high voltage and

the energy sigma versus xenon concentration graphs. The optimal xenon

concentration for the laser system of this preferred embodiment is thus

between around 10 ppm and 30 ppm for the ArF laser.

As Fig. 6e shows, the energy stability is significantly improved to

below 2.8% and is at less than 2.7% for a xenon concentration of 1 2 ppm.

The energy stability improves at still higher xenon concentrations. Thus, in

accord with the present invention, an ArF laser is provided having a xenon

concentration of 1 2 ppm or higher. This improved energy stability is

particularly advantageous for an excimer laser for use in combination with an

imaging system for photolithographic applications. A similar improvement of

the energy stability is expected for the 1 57 nm molecular fluorine (F₂) laser

when small amounts of a gas additive, such as xenon, are added to the gas mixture. The specific optimal concentration of xenon is based on a similar

study of sigma and high voltage versus xenon concentration graphs

measured using a molecular fluorine laser.

Preferred Laser System

A preferred embodiment of a KrF, ArF or F₂ laser system in accord

with the present invention is shown in Fig. 7. Fig. 7 shows various modules

of an excimer or molecular fluorine laser for deep ultraviolet (DUV) or vacuum

ultraviolet (VUV) lithography using radiation around 248 nm, 1 93 nm or 1 57

nm, respectively. The discharge chamber 1 contains a laser gas mixture and

includes a pair of main discharge electrodes 1 a, 1 b and one or more

preionization electrodes (not shown). Exemplary electrode configurations are

described at U.S. provisional patent application no. 60/1 28,227, which is

assigned to the same assignee as the present application and which is hereby

incorporated by reference into the present application. Exemplary

preionization assemblies are described in U.S. patent applications no.

09/247,887, 60/1 60, 1 82 and 60/1 62,645, each of which is assigned to the

same assignee as the present application and which is hereby incorporated

by reference into the present application.

The laser resonator which surrounds the discharge chamber 1

containing the laser gas mixture includes a line narrowing module 2 for a line

narrowed excimer or molecular fluorine laser, which may be replaced by a high reflectivity mirror or the like if line-narrowing is not desired, and an

outcoupling module 3. Depending on the type and extent of line-narrowing

and/or selection and tuning that is desired, and the particular laser that the

line-narrowing module is to be installed into, there are many alternative line-

narrowing configurations that may be used. For this purpose, those shown

in U.S. patents no. 4,399,540, 4,905,243, 5,226,050, 5,559,81 6,

5,659,41 9, 5,663,973, 5,761 ,236, and 5,946,337, and U.S. patent

applications no. 09/31 7,695, 09/1 30,277, 09/244,554, 09/31 7,527,

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. patents

no. 5,095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991 , 5,898,725,

5,901 , 1 63, 5,91 7,849, 5,970,082, 5,404,366, 4,975,91 9, 5, 142,543,

5,596,596, 5,802,094, 4,856,01 8, and 4,829,536, all of which are hereby

incorporated by reference into the present application.

The discharge chamber is sealed by windows 8 transparent to the

wavelengths of the emitted laser radiation 1 4. After a portion of the output

beam passes the outcoupler 3, that output portion impinges upon a beam

splitter 6 which reflects a portion of the beam to a second beam splitter 7.

A portion of the beam impinging the second beam splitter then reflects to a

fast energy detector 5 and the remainder traverses the beam splitter and is

received by a bandwidth and wavelength meter 4. The portion of the

outcoupled beam which traverses the beam splitter 6 is the output emission of the laser, which propagates toward an industrial or experimental

application such as a light source for photolithographic applications.

A pulse power module 9 and high voltage power supply 10 supply

electrical energy to the main electrodes 1 a, 1 b to energize the gas mixture.

The preferred pulse power module and high voltage power supply are

described at U.S. patent applications 08/842,578, 08/822,451 , and

09/390, 1 46, each of which is assigned to the same assignee as the present

application and which is hereby incorporated by reference into the present

application.

A processor or control computer 1 1 receives and/or processes values

of the energy, energy stability, wavelength, and bandwidth of the output

beam and controls the line narrowing module to tune the wavelength, and

controls the power supply components 9 and 1 0 to control the energy. In

addition, the processor 1 1 controls the gas supply unit which includes gas

supply valves 1 2 and a gas additive supply 1 3, which may be internal or

external to the laser system. For the KrF laser, a gas additive supply of

preferably xenon is internal to the laser system. For the ArF laser, a gas

additive supply of preferably xenon is maintained external to the laser

system, such as along with the external gas supply (not shown) of the other

gases of the system such as the halogen containing gas, the active rare gas

and the buffer gas via gas tubing 1 7. Alternatively, the ArF laser may have

an internal supply of xenon or another gas additive, or an external supply of a gas additive other than xenon. The KrF laser may have an external supply of

xenon or another gas additive, or an internal supply of xenon. The xenon

and/or other gas additive is connected to the gas supply valves via

appropriate gas tubing 1 5. The gas supply valves are connected to the laser

tube via other gas tubing 1 6 which is/are preferably connected to a vacuum

pump 1 8 or other low pressure source.

As described in the '875 application with respect to the halogen

containing and other gases of the system, when the processor determines

that a xenon injection is to be performed, a compartment is first filled with

the xenon to a prescribed pressure (for this purpose, U.S. Patent no.

5,396,514, which is assigned to the same assignee as the present

application is hereby incorporated into the present application by reference;

see also the '785 application referred to above). Then the xenon is injected

into the tube 1 . By way of this method, considering the pressure in the tube

1 and that in the compartment filled with xenon, as well as the volume of

the compartment, it is possible to determine more precisely how much xenon

has been injected into the tube. The system also includes means for

releasing gas including xenon from the tube 1 should it be desired to reduced

the pressure in the tube 1 , or should a partial pressure of one of the gases

such as the xenon be determined to be too high, or if a gas replenishment

action such as partial gas replacement or mini gas replacement such as are described in the '785 application is to be performed, or if a new fill is to be

performed.

As mentioned, the gas compartment of the laser preferably contains a

source or supply of xenon 1 3. The xenon source 1 3 is connected with gas

tubings 1 5 and if necessary additional valves to the gas supply valves 1 2.

The standard gas mixture is supplied to the laser by external gas supply via

the gas supply tubings 1 7.

A new fill of the laser is controlled automatically by the control

computer 1 1 . In the present invention xenon gas from the xenon source 1 3

is injected into the discharge chamber 1 with high accuracy during the new

fill. The injection may be carried out in a preferred version of the invention

just after having reduced the pressure in the discharge chamber 1 to a preset

low value pressure, e.g., around 20 - 30 mbar, before the new gas fill is

started. In another preferred example of the invention the xenon injection is

carried out at the end of the new fill when the standard gas mixture has

already been filled.

The present invention including the addition of xenon to the gas

mixture at predetermined concentrations is particularly advantageous when

operating at high repetition rates. That is, performance of the laser at

moderate repetition rates (e.g., well below 1 kHz such as from 1 to 300 or

500 Hz) is not observed to change as advantageously with the addition of Xe

in the mixture as when operating at high repetition rates such as 1 kHz and above. The behavior of the laser at high repetition rates (about I kHz and

higher) with the addition of the xenon is significantly improved and the

power of the laser at the higher repetition rates was nearly linear and the

pulse to pulse energy stability (standard deviation) was better.

Proper operation of the laser at the high repetition rates depends on

the various factors. Repetitive and very intense periodic gas discharges in the

discharge chamber 1 is improved by continuous refreshing of the gas in the

area between the electrodes. Intense gas flow between the electrodes 1 a,

1 b is not the only important condition, though, and the present invention

demonstrates that the gas mixture composition including maintaining precise

constituent gas concentrations is important.

The pulse to pulse energy stability of the laser output radiation also

strongly depends on the kinetics of the gas discharge processes, of the laser

excitation, on the specific features of the building up processes of the laser

pulse as well as on the ordinary stability of the electrical pulse generator,

used for the pumping of the electrical discharge. The increase of the

intensity of the preionization of the gas, which is advantageously achieved in

the present invention by adding trace amounts of xenon to the gas mixture

according to prescribed concentrations, provides significant improvement to

the pulse to pulse stability.

It has been shown in our experiments that the objects of the invention

have been met. Additions of small amounts of xenon improves the laser operation, particularly when maintained at precise concentrations, with

particular advantage at the high repetition rates. That is, the improvement of

laser performance at high repetition rates, with particular reference to the

pulse to pulse energy stability (standard deviation), is advantageously

achieved.

Objects of the Invention Met

The several embodiments of the present invention set forth above

meet the objects of the invention by controlling the concentration of a xenon

additive to the gas mixture of an excimer or molecular fluorine laser to

control the pulse energy, energy stability and overshoot control of the laser.

The laser comprises an apparatus for supplying xenon to the laser gas

mixture and procedures to inject and control the appropriate xenon amount in

the gas mixture of the gas discharge vessel of the laser. The present

invention achieves an optimal balance between the highest energy stability

and overshoot control and the energy output of the laser depending on

constraints imposed by other components of the laser system and desired

beam parameter specifications. The present invention may also be used to

increase the lifetimes of its components as set forth above.

These features of the present invention are achieved in the present

invention based on an investigation of the energy stability, overshoot control

and output power dependencies of the Xenon concentration in the laser chamber. Experimental data are now detailed to illustrate the advantages

explained above with regard to the particular embodiments of the present

invention.

The amount of traces of xenon or xenon-containing compound in the

gas mixture of the excimer laser in this invention only refers to such fluorine-

containing excimer laser gas mixtures that do not contain larger amounts of

xenon for other reasons, for instance because the exciplexes contain xenon

(e.g. XeF or XeCI). The concentrations of xenon added to the gas mixture in

accord with the present invention (less than 2000 ppm for KrF and

significantly less for ArF lasers) do not make such xenon-containing gas

mixtures in excimer lasers the subject this invention.

The optimal concentration of traces of xenon in the gas mixture

referred to in this invention depends on the characteristics and conditions of

the excimer laser in individual cases and cannot be prescribed for every type

of excimer laser in terms of the optimal values: optimal xenon concentration

for each type of laser must be determined experimentally. For example, the

invention delivers particularly good results when the excimer laser is operated

at a relatively high repetition rate, particularly at a repetition rate greater than

100 Hz, and especially when greater than 500 Hz.

As is described above, the concentration of the xenon or the

substance that supplies xenon is not advantageously increased indefinitely

into the gas mixture, but reaches an optimal value that is dependent on various laser parameters and also varies among types of lasers with respect

to the gas mixture used, type of preionization, configuration of the electrical

gas discharge (in particular the electrode geometry and the condition of the

electrodes), and the external electrical circuit. The concentration may simply

be optimized empirically for each laser type.

Those skilled in the art will appreciate that the just-disclosed preferred

embodiments are subject to numerous adaptations and modifications without

departing from the scope and spirit of the invention. Therefore, it is to be

understood that, within the scope and spirit of the invention, the invention

may be practiced other than as specifically described above. The scope of

the invention is thus not limited by the particular embodiments described

above. Instead, the scope of the present invention is understood to be

encompassed by the language of the claims that follow, and structural and

functional equivalents thereof.

Claims

What is claimed is:

1 . An ArF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and more than 1 00 ppm of xenon;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam.

2. The ArF laser of Claim 1 , wherein the gas mixture includes less than

2000 ppm of xenon.

3. An ArF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and an energy attenuating gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam,

wherein the concentration of the gas additive in the gas mixture is

adjusted to control the energy stability of the laser beam.

4. The laser of Claim 3, wherein the concentration of the gas additive in the

gas mixture is also adjusted to control the energy overshoot of the laser

beam when the laser is operating in burst mode.

5. An ArF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and an energy attenuating gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture; a resonator surrounding the discharge chamber for generating an

output laser beam,

wherein the concentration of xenon in the gas mixture is adjusted to

control the energy overshoot of the laser beam when the laser is operating in

burst mode.

6. The laser of any of Claims 4-5, wherein the energy overshoot is controlled

to be less than 20% by adjusting the concentration of the gas additive.

7. The laser of any of Claims 4-5, wherein the energy overshoot is controlled

to be less than 1 0% by adjusting the concentration of the gas additive.

8. The laser of any of Claims 4-5, wherein the energy overshoot is controlled

to be less than 5% by adjusting the concentration of the gas additive.

9. The laser of any of Claims 3-5, wherein the concentration of the gas

additive in the gas mixture is also adjusted to control one of the pulse

energy and the energy dose of the laser beam.

10. An ArF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and a gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam,

wherein the concentration of the gas additive in the gas mixture is

actively adjusted during laser operation to control the pulse energy.

1 1 . The ArF laser of Claim 10, wherein the concentration of the gas

additive in the gas mixture is adjusted to control the pulse energy in a

range between substantially 3.5 mJ and 1 5 mJ.

1 2. The ArF laser of Claim 1 0, wherein the concentration of the gas

additive in the gas mixture is adjusted to control the pulse energy in a

range between substantially 4.0 mJ and 5.5 mJ.

1 3. The laser of Claim 1 2, wherein the energy overshoot is controlled to

be less than 20% by adjusting the concentration of the gas additive.

1 4. The laser of Claim 1 2, wherein the energy overshoot is controlled to

be less than 10% by adjusting the concentration of the gas additive.

1 5. The laser of Claim 1 2, wherein the energy overshoot is controlled to

be less than 5% by adjusting the concentration of the gas additive.

1 6. A system for photolithographically forming a structure on a silicon

wafer, comprising:

an ArF laser system including:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and 1 2 ppm or more of xenon;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture; and

a resonator surrounding the discharge chamber for generating an

output laser beam; and an imaging system including optics for imaging said laser beam onto

said silicon wafer.

1 7. The photolithographic etching system of Claim 1 6, wherein said ArF

laser further includes a gas handling unit for periodically replenishing the

gas mixture during operation of the laser.

1 8. The photolithographic etching system of Claim 1 7, wherein said ArF

laser further includes a processor for monitoring one or more parameters

of the laser beam and replenishing said gas mixture based on detected

values of the one or more parameters.

1 9. The photolithographic etching system of Claim 1 6, wherein said

imaging system comprises all-reflective optics and said ArF laser further

includes a line-narrowing unit for reducing the bandwidth of the ArF laser

to less than 100 pm.

20. The photolithographic etching system of Claim 1 6, wherein said

imaging system comprises a refractive optic and said ArF laser further

includes a line-narrowing unit for reducing the bandwidth of the ArF laser

to less than 0.8 pm.

21 . The photolithographic etching system of Claim 1 6, further comprising

one of a mask and a reticle for forming said structure on said wafer.

22. The photolithographic etching system of Claim 1 6, further comprising

a monitor etalon for wavelength calibration.

23. An ArF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and an attenuating gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture; and a resonator surrounding the discharge chamber for generating an

output laser beam, wherein said output laser beam has a specified energy,

and wherein the discharge circuit is configured to apply an operating driving

voltage to the electrodes only in a range at or above a minimum driving

voltage which when applied to the electrodes would produce a laser beam

above said specified energy without the attenuating gas additive in the gas

mixture.

24. An ArF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and 1 7 ppm or more of a xenon attenuator; a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture; and

a resonator surrounding the discharge chamber for generating an

output laser beam, wherein the energy of output pulses of the laser is less

than the energy would be if the xenon were not included in the gas mixture.

25. The laser of Claim 24, wherein the xenon concentration is 30 ppm or

more.

26. An ArF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and xenon;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam; and

a gas control unit including a supply of xenon gas internal to the laser

housing.

27. The ArF laser of Claim 26, wherein said gas control unit includes a

xenon generator including a supply of condensed matter xenon in a

controlled environment for supplying the xenon gas.

28. A KrF laser, comprising:

a discharge chamber initially filled with a gas mixture including

krypton, fluorine, a buffer gas and more than 100 ppm of xenon;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam.

29. The KrF laser of Claim 28, wherein the gas mixture includes less than

2000 ppm of xenon.

30. A KrF laser, comprising:

a discharge chamber initially filled with a gas mixture including

krypton, fluorine, a buffer gas and an attenuating gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam,

wherein the concentration of the attenuating gas additive in the gas

mixture is adjusted to control the energy stability of the laser beam.

31 . The laser of Claim 30, wherein the concentration of gas additive in the

gas mixture is also adjusted to control the energy overshoot of the laser

beam when the laser is operating in burst mode.

32. A KrF laser, comprising:

a discharge chamber initially filled with a gas mixture including

krypton, fluorine, a buffer gas and an attenuating gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam,

wherein the concentration of attenuating gas additive in the gas

mixture is adjusted to control the energy overshoot of the laser beam when

the laser is operating in burst mode.

33. The laser of any of Claims 31 -32, wherein the energy overshoot is

controlled to be less than 20% by adjusting the concentration of the gas

additive.

34. The laser of any of Claims 31 -32, wherein the energy overshoot is

controlled to be less than 1 0% by adjusting the concentration of the gas

additive.

35. The laser of any of Claims 31 -32, wherein the energy overshoot is

controlled to be less than 5% by adjusting the concentration of the gas

additive.

36. The laser of any of Claims 30-32, wherein the concentration of the

gas additive in the gas mixture is also adjusted to control one of the pulse

energy and the energy dose of the laser beam.

37. A KrF laser, comprising: a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and a gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam,

wherein the concentration of the gas additive in the gas mixture is

actively adjusted during laser operation to control the pulse energy.

38. The KrF laser of Claim 37, wherein the concentration of the gas

additive in the gas mixture is adjusted to control the pulse energy in a

range between substantially 3.5 mJ and 1 5 mJ.

39. The KrF laser of Claim 37, wherein the concentration of the gas

additive in the gas mixture is adjusted to control the pulse energy in a

range between substantially 4.0 mJ and 5.5 mJ.

40. The laser of Claim 39, wherein the energy overshoot is controlled to

be less than 20% by adjusting the concentration of the gas additive.

41 . The laser of Claim 39, wherein the energy overshoot is controlled to

be less than 10% by adjusting the concentration of the gas additive.

42. The laser of Claim 39, wherein the energy overshoot is controlled to

be less than 5% by adjusting the concentration of the gas additive.

43. A system for photolithographically forming a structure on a silicon

wafer, comprising:

an KrF laser system including:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and 1 2 ppm or more of xenon;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture; and a resonator surrounding the discharge chamber for generating an

output laser beam; and

an imaging system including optics for imaging said laser beam onto

said silicon wafer.

44. The photolithographic forming system of Claim 43, wherein said KrF

laser further includes a gas handling unit for periodically replenishing the

gas mixture during operation of the laser.

45. The photolithographic forming system of Claim 44, wherein said KrF

laser further includes a processor for monitoring one or more parameters

of the laser beam and replenishing said gas mixture on detected values of

the one or more parameters.

46. The photolithographic forming system of Claim 43, wherein said

imaging system comprises all-reflective optics and said KrF laser further

includes a line-narrowing unit for reducing the bandwidth of the KrF laser

to less than 100 pm.

47. The photolithographic forming system of Claim 43, wherein said

imaging system comprises a refractive optic and said KrF laser further includes a line-narrowing unit for reducing the bandwidth of the KrF laser

to less than 0.8 pm.

48. The photolithographic forming system of Claim 43, further comprising

one of a mask and a reticle for forming said structure on said wafer.

49. The photolithographic forming system of Claim 43, further comprising

a monitor etalon for wavelength calibration.

50. A KrF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and an attenuating gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture; and

a resonator surrounding the discharge chamber for generating an

output laser beam, wherein said output laser beam has a specified energy,

and wherein the discharge circuit is configured to apply an operating driving

voltage to the electrodes only in a range at or above a minimum driving

voltage which when applied to the electrodes would produce a laser beam

above said specified energy without the attenuating gas additive in the gas

mixture.

51 . A KrF laser, comprising:

a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and 1 7 ppm or more of xenon for attenuating the pulse

energy of the laser;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture; and

a resonator surrounding the discharge chamber for generating an

output laser beam.

52. The laser of Claim 51 , wherein the xenon concentration is 30 ppm or

more.

53. A KrF laser, comprising: a discharge chamber initially filled with a gas mixture including argon,

fluorine, a buffer gas and xenon;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam; and

a gas control unit including a supply of xenon gas internal to the laser

housing.

54. The KrF laser of Claim 53, wherein said gas control unit includes a

xenon generator including a supply of condensed matter xenon in a

controlled environment for supplying the xenon gas.

55. An excimer or molecular fluorine laser for generating an output beam

at a specified energy, comprising:

a discharge chamber initially filled with a gas mixture including an

active rare gas, a halogen containing gas, a buffer gas and an attenuating

noble gas additive selected from the group consisting of all noble gases other

than said active rare gas and said buffer gas;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating said

output beam at said specified energy,

wherein said specified energy is less than the laser would output

without the attenuating noble gas additive in the gas mixture.

56. The laser of Claim 55, wherein the discharge circuit is configured to

apply an operating driving voltage to the electrodes only in a range at or

above a minimum driving voltage which when applied to the electrodes

would produce a laser beam above said specified energy without the

xenon in the gas mixture.

57. An excimer or molecular fluorine laser system for generating an output

beam at a specified energy, comprising:

a discharge chamber initially filled with a gas mixture including an

active rare gas, a halogen-containing gas, and a buffer gas, and a trace

amount of an attenuating gas additive for attenuating the output beam to

said specified energy;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating said

output beam at said specified energy.

58. The laser of Claim 57, wherein the discharge circuit is configured to

apply an operating driving voltage to the electrodes only in a range at or

above a minimum driving voltage which when applied to the electrodes

would produce a laser beam above said specified energy without the

attenuating gas additive.

59. A molecular fluorine laser, comprising:

a discharge chamber initially filled with a gas mixture including

fluorine, a buffer gas and a trace amount of a noble gas additive selected

from the group consisting of argon, xenon and krypton; a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam around 1 57 nm.

60. The molecular fluorine laser of Claim 59, wherein the gas additive

attenuates the energy of the laser beam.

61 . The molecular fluorine laser of Claim 59, wherein said output laser

beam has a specified energy, and wherein the discharge circuit is configured to apply an operating driving voltage to the electrodes only in

a range at or above a minimum driving voltage which when applied to the

electrodes would produce a laser beam above said specified energy

without the trace amount of the gas additive in the gas mixture.

62. The molecular fluorine laser of Claim 59, further comprising:

a detector for monitoring a parameter indicative of the concentration

of gas additive in the gas mixture; and

a gas control unit for replenishing the gas additive based on the value

of the monitored parameter.

63. The molecular fluorine laser of Claim 62, wherein said gas control unit

includes an internal supply of the gas additive.

64. The molecular fluorine laser of Claim 62, wherein the gas additive is

xenon and said gas control unit includes a xenon generator including a

supply of condensed matter xenon in a controlled environment for supplying the xenon gas.

65. A method of operating an excimer or molecular fluroine laser to

increase the lifetimes of laser components, comprising the steps of: filling the laser with a gas mixture including a trace amount of an

attenuating gas species;

measuring the output energy of the laser system at a predetermined

discharge voltage; and

adding more of the gas species to the gas mixture to reduce the

output energy of the laser system at said predetermined discharge voltage.

66. The method of Claim 65, wherein the predetermined discharge voltage

is the minimum voltage available for the laser.

67. The method of Claim 65, wherein the adding step reduces the output

energy to a selected energy not available without performing said adding

step.

68. The method of any of Claims 65 or 67, wherein the previous steps are

repeated at a later time and less of the attenuating gas species is added

to reduce the output energy to a same energy as after the original steps.

69. The method of Claim 68, wherein the filling and measuring steps are

repeated at a still later time and no gas additive is added because the

output energy is at the same energy as after the original steps.

70. The method of Claim 68, wherein the attenuating species is xenon.

71 . The method of any of Claims 65-67, wherein the attenuating species

is xenon.

72. A method of initializing an excimer or molecular fluorine laser having a

gas mixture including a trace amount of an energy attenuating gas and

emitting an output laser beam, comprising the steps of:

selecting a value of energy stability; and filling a gas mixture into the laser including a selected amount of the

attenuating gas for controlling the energy stability to the selected value.

73. The method of Claim 72, further comprising the steps of:

measuring the energy stability of the laser beam; and

adjusting the concentration of the attenuating gas to control the value

of the energy stability.

74. A method of initializing an excimer or molecular fluorine laser having a

gas mixture including a trace amount of an energy attenuating gas and

emitting an output laser beam in burst mode, comprising the steps of:

selecting a value of energy overshoot; and

filling a gas mixture into the laser including a selected amount of the

energy attenuating gas for controlling the energy overshoot to the selected

value.

75. The method of Claim 74, further comprising the steps of:

measuring the energy overshoot of the laser beam; and

adjusting the concentration of the energy attenuating gas to control

the value of the energy overshoot.

76. A method of initializing an excimer or molecular fluorine laser having a

gas mixture including a trace amount of an attenuating gas additive and

emitting an output laser beam in burst mode, comprising the steps of:

selecting a value of pulse energy at a certain discharge voltage; and

filling a gas mixture into the laser including a selected amount of the

gas additive for attenuating the pulse energy to the selected value.

77. The method of Claim 76, further comprising the steps of:

measuring the pulse energy of the laser beam; and

adjusting the concentration of the gas additive to control the value of

the pulse energy.

78. A method of operating an excimer or molecular fluorine laser, said

laser including a discharge chamber for holding a laser gas mixture, said laser

including an electrical discharge circuit for generating an excitation voltage in

a range between a minimum and maximum voltage to create output laser

pulses and wherein the energy of each output pulse falls within a

predetermined range defined by a minimum and a maximum level, said

method comprising the steps of:

filling the discharge chamber with a gas mixture including a trace

amount of an attenuating gas, with the proportions of the gases in the

mixture being selected such that if the attenuating gas was not present in the mixture, the energy per pulse would exceed the maximum level even if

the laser was excited with the minimum voltage, said attenuating gas

permitting said laser to generate pulses having an energy within the

predetermined range; and

adjusting the constituents of the gas mixture over time as the laser

gas mixture ages, said adjusting step including lowering the concentration of

the attenuating gas in the gas mixture so that as the gas mixture ages, the

energy per pulse can be maintained within the predetermined range.

79. The method of Claim 78, wherein said attenuating gas is xenon.

80. The method of Claim 78, further comprising the steps of:

monitoring the energy of the laser pulses; and

adjusting the voltage to maintain the energy per pulse within the

predetermined range.

81 . The method of any of Claims 78-80, further comprising the step of

adjusting the proportion of the attenuating gas to maintain the energy per

pulse with the predetermined range.

82. A method of operating an excimer or molecular fluorine laser, said

laser including a discharge chamber for holding a laser gas mixture, said laser including an electrical discharge circuit for generating an excitation

voltage in a range between a minimum and maximum voltage to create

output laser pulses and wherein the energy of each output pulse falls

within a predetermined range defined by a minimum and a maximum

level, said method comprising the steps of:

monitoring the energy of the laser output pulses;

adjusting the voltage to maintain the energy per pulse within the

predetermined range; and

adjusting the proportion of the attenuating gas during laser operation

to maintain the energy per pulse with the predetermined range.

83. A method of operating an excimer or molecular fluorine laser, said laser

including a discharge chamber for holding a laser gas mixture, said laser

including an electrical discharge circuit for generating an excitation voltage in

a range between a minimum and maximum voltage to create output laser

pulses and wherein the energy dose falls within a predetermined range

defined by a minimum and a maximum level, said method comprising the

steps of:

filling the discharge chamber with a gas mixture including a trace

amount of an attenuating gas, with the proportions of the gases in the

mixture being selected such that if the attenuating gas was not present in

the mixture, the energy dose would exceed the maximum level even if the laser was excited with the minimum voltage, said attenuating gas permitting

said laser to generate pulses such that the energy dose is maintained within

the predetermined range; and

adjusting the constituents of the gas mixture over time as the laser

gas mixture ages, said adjusting step including lowering the concentration of

the attenuating gas in the gas mixture so that as the gas mixture ages, the

energy dose can be maintained within the predetermined range.

84. The method of Claim 83, wherein said attenuating gas is xenon.

85. The method of Claim 83, further comprising the steps of: monitoring the energy dose; and

adjusting the voltage to maintain the energy dose within the

predetermined range.

86. The method of any of Claims 83-85, further comprising the step of

adjusting the proportion of the attenuating gas to maintain the energy dose

within the predetermined range.

87. A method of operating an excimer or molecular fluorine laser, said

laser including a discharge chamber for holding a laser gas mixture, said

laser including an electrical discharge circuit for generating an excitation voltage in a range between a minimum and maximum voltage to create

output laser pulses and wherein the energy dose falls within a

predetermined range defined by a minimum and a maximum level, said

method comprising the steps of:

monitoring the energy dose;

adjusting the voltage to maintain the energy dose within the

predetermined range; and adjusting the proportion of the attenuating gas during laser operation

to maintain the energy dose with the predetermined range.

88. A method of operating an excimer or molecular fluorine laser, said laser

including a discharge chamber for holding a laser gas mixture, said laser

including an electrical discharge circuit for generating an excitation voltage in

a range between a minimum and maximum voltage to create output laser

pulses and wherein the energy stability falls within a predetermined range

defined by a minimum and a maximum level, said method comprising the

step of:

filling the discharge chamber with a gas mixture including a trace

amount of an attenuating gas, with the proportions of the gases in the

mixture being selected such that if the attenuating gas was not present in

the mixture, the energy stabilty would be below the maximum level, said attenuating gas permitting said laser to generate pulses such that the energy

stability is maintained within the predetermined range.

89. A method of operating an excimer or molecular fluorine laser, said

laser including a discharge chamber for holding a laser gas mixture, said laser

including an electrical discharge circuit for generating an excitation voltage in

a range between a minimum and maximum voltage to create output laser

pulses and wherein the energy overshoot falls within a predetermined range

defined by a minimum and a maximum level, said method comprising the

step of:

filling the discharge chamber with a gas mixture including a trace

amount of an attenuating gas, with the proportions of the gases in the

mixture being selected such that if the attenuating gas was not present in

the mixture, the energy overshoot would exceed the maximum level, said

attenuating gas permitting said laser to generate pulses such that the energy

overshoot is maintained within the predetermined range.

90. The laser of any of Claims 3, 5 or 23, wherein the attenuating gas is

selected from the group consisting of xenon, krypton, WF_X, PtF_x, a

chromium-containing species, an aluminum-containing species, a silicon-

containing species, HF, HF₂, ozone, mercury, hafnium, CRO_x, FO_x, AIO_x,

HF_X , CF₂, CF₄, CF₆, CF₈, CF₃, CrOF₂, CrOF, CrO₂F, CrO₂F₂, CrO₂, CrO, Cr, CrF₂, CrF, SiF₄, SiF, OF, O₂F, OF₂, Al, AlO, AI₂O, AI₂O₂, AIF, AIF₂, N,

N₂, N_x, C, C₂, C_x, H, H₂, H_x, O, and O_x, where x is an integer from 3-1 6.

91 . The laser of Claim 1 0, wherein the gas additive is selected from the

group consisting of xenon, krypton, WF_X, PtF_x, a chromium-containing

species, an aluminum-containing species, a silicon-containing species, HF,

HF₂, ozone, mercury, hafnium, CRO_x, FO_x, AIO_x, HF_X , CF₂, CF₄, CF₆, CF₈,

CF₃, CrOF₂, CrOF, CrO₂F, CrO₂F₂, CrO₂, CrO, Cr, CrF₂, CrF, SiF₄, SiF, OF,

O₂F, OF₂, Al, AlO, AI₂O, AI₂O₂, AIF, AIF₂, N, N₂, N_x, C, C₂, C_x, H, H₂, H_x, O,

and O_x, where x is an integer from 3-1 6.

92. The laser of any of Claims 30, 32 or 50, wherein the attenuating gas is

selected from the group consisting of xenon, argon, WF_X, PtF_x, a chromium-

containing species, an aluminum-containing species, a silicon-containing

species, HF, HF₂, ozone, mercury, hafnium, CRO_x, FO_x, AIO_x, HF_X , CF₂, CF₄,

CF₆, CF₈, CF₃, CrOF₂, CrOF, CrO₂F, CrO₂F₂, CrO₂, CrO, Cr, CrF₂, CrF, SiF₄,

SiF, OF, O₂F, OF₂, Al, AlO, AI₂O, AI₂O₂, AIF, AIF₂, N, N₂, N_x, C, C₂, C_x, H, H₂,

H_x, O, and O_x, where x is an integer from 3-1 6.

93. The laser of Claim 37, wherein the gas additive is selected from the

group consisting of xenon, argon, WF_X, PtF_x, a chromium-containing

species, an aluminum-containing species, a silicon-containing species, HF, HF₂, ozone, mercury, hafnium, CRO_x, FO_x, AIO_x, HF_X , CF₂, CF₄, CF₆, CF₈,

CF₃, CrOF₂, CrOF, CrO₂F, CrO₂F₂, CrO₂, CrO, Cr, CrF₂, CrF, SiF₄, SiF, OF,

O₂F, OF₂, Al, AlO, AI₂O, AI₂O₂, AIF, AIF₂, N, N₂, N_x, C, C₂, C_x, H, H₂, H_x,

O, and O_x, where x is an integer from 3-1 6.

94. The laser of Claim 56, wherein the attenuating gas is selected from

the group consisting of xenon, argon, krypton, WF_X, PtF_x, a chromium-

containing species, an aluminum-containing species, a silicon-containing

species, HF, HF₂, ozone, mercury, hafnium, CRO_x, FO_x, AIO_x, HF_X , CF₂,

CF₄, CF₆, CF₈, CF₃, CrOF₂, CrOF, CrO₂F, CrO₂F₂, CrO₂, CrO, Cr, CrF₂, CrF,

SiF₄, SiF, OF, O₂F, OF₂, Al, AlO, AI₂O, AI₂O₂, AIF, AIF₂, N, N₂, N_x, C, C₂,

C_x, H, H₂, H_x, O, and O_x, where x is an integer from 3-1 6.

95. The method of any of Claims 64, 71 , 73, 75, 77, 81 , 82, 86, 87 or

88, wherein the attenuating gas is selected from the group consisting of

xenon, argon, krypton, WF_X, PtF_x, a chromium-containing species, an

aluminum-containing species, a silicon-containing species, HF, HF₂, ozone,

mercury, hafnium, CRO_x, FO_x, AIO_x, HF_X , CF₂, CF₄, CF₆, CF₈, CF₃, CrOF₂,

CrOF, CrO₂F, CrO₂F₂, CrO₂, CrO, Cr, CrF₂, CrF, SiF₄, SiF, OF, O₂F, OF₂,

Al, AlO, AI₂O, AI₂O₂, AIF, AIF₂, N, N₂, N_x, C, C₂, C_x, H, H₂, H_x, O, and O_x,

where x is an integer from 3-1 6.

96. An excimer or molecular fluorine laser, comprising:

a discharge chamber initially filled with a gas mixture including an

active rare gas, a halogen-containing species, a buffer gas, a trace amount of

a first gas additive and a trace amount of a second gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture;

a resonator surrounding the discharge chamber for generating an

output laser beam.

97. The laser of Claim 96, wherein said first gas additive is xenon.

98. The laser of Claim 97, wherein said second gas additive is oxygen.

99. The laser of Claim 96, wherein the concentration of said first gas

additive is adjusted to control at least one of energy overshoot and

energy stability, and the concentration of said second gas additive is

adjusted for controlling output energy.

1 00. The laser of Claim 96, wherein the concentrations of said first and

second gas additives are adjusted to control at least one of energy

overshoot, energy stability, and output energy.

101 . An excimer or molecular fluorine laser, comprising:

a discharge chamber initially filled with a gas mixture including an

active rare gas, less than 0.1 % fluorine, a buffer gas, and a trace amount of

a gas additive;

a plurality of electrodes in the discharge chamber connected to a

discharge circuit for energizing the gas mixture; and

a resonator surrounding the discharge chamber for generating an

output laser beam,

wherein the fluorine concentration is less than that which would result

in maximum laser output energy and the concentration of the trace amount

of the gas additive is selected to compensate the reduced output energy.

102. The laser of Claim 1 01 , wherein the fluorine concentration is less than

0.08%.

103. The laser of any of Claims 1 01 or 102, wherein the gas additive is

xenon.

104. The laser of any of Claims 1 , 3, 5, 10, 23, 24, 26, 30, 32, 37, 50,

51 , 53, 57 or 58, wherein the laser is configured for operation at a

repetition rate of 1 kHz or higher.

105. The laser of Claim 104, wherein the laser is configured for operation at a repetition rate of 4 kHz or higher.

106. The system of any of Claims 1 6 or 43, wherein the laser is configured for operation at a repetition rate of 1 kHz or higher.

107. The system of Claim 106, wherein the laser is configured for operation at a repetition rate of 4 kHz or higher.