Abstract:
A method and apparatus if disclosed which may comprise a high power high repetition rate gas discharge laser UV light source which may comprise: a gas discharge chamber comprising an interior wall comprising a vertical wall and an adjacent bottom wall; a gas circulation fan creating a gas flow path adjacent the interior vertical wall and the adjacent bottom wall; an in-chamber dust trap positioned a region of low gas flow, which may be along an interior wall and may comprise at least one meshed screen, e.g., a plurality of meshed screens, which may comprise at least two different gauge meshed screens. The dust trap may extend along the bottom interior wall of the chamber and/or a vertical portion of the interior wall. The dust trap may comprise a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; and the second meshed screen intermediate the first meshed screen and the interior wall. The chamber may comprise a plurality of dust collecting recesses in at least one of the vertical interior wall and the bottom wall of the chamber which may be selected from a group comprising a one-part recess and a multi-part recess, which may comprise two sections angled with respect to each other. The dust trap may comprise a pressure trap positioned between a portion of a main insulator and an interior wall of the chamber. The chamber may comprise a gas circulating fan comprising a cross-flow fan with a fan cutoff that may comprise a vortex control pocket. The chamber may comprise a preionization mechanism comprising a preionization tub containing a ground rod within an elongated opening in the preionization tube that may comprise a compliant member, an automatic preionization shut-off mechanism, a preionization onset control mechanism and/or a focusing element. The chamber may comprise an elongated baffle plate that may comprise a plurality of pyramidal structures including varying numbers of generally pyramidal elements and oriented in groups of varying numbers of generally pyramidal elements and oriented along and transverse to the longitudinal axis. Acoustic resonances within the chamber may also be reduced by introducing an artificial jitter into the timing of the laser discharges varying the inter-pulse period randomly or in a repeating pattern from pulse to pulse within a burst.

Description:
FIELD OF THE INVENTION 
   The present invention relates to gas discharge lasers, e.g., those used in the production of high power high output pulse repetition rate high stability UV light sources, e.g., DUV light, e.g., for the exposure of integrated circuit photoresists in integrated circuit lithography manufacturing processes. 
   BACKGROUND OF THE INVENTION 
   It is known that in gas discharge lasers, e.g., utilizing fluorine in the laser gas, e.g., KrF, ArF and F 2  gas discharge lasers, there is a great propensity for the production of debris, e.g., in the form of metal fluorides, e.g., due to the interaction of fluorine with metallic components within the laser gas discharge chamber. This can occur particularly during gas discharge, and e.g., with metal materials in the electrodes between which the electric discharge occurs to cause the gas discharge, a chemical and electrical phenomenon that generates radiation. Such gas discharge lasers may be used particularly at or about a selected desired center-wavelength, e.g., for KrF gas discharge lasers at about 248 nm and for ArF gas discharge lasers at about 193 nm. This debris can, over time, plate out on such things a optical components of the laser chamber, e.g., chamber windows, which can cause reduced output power for a number of reasons, e.g., undesired reflection of laser light off of the optic and/or blockage of transmission of laser light through the optic. This can cause, e.g., the need to operate the laser at, e.g., an undesired elevated discharge voltage, e.g., resulting in reduction in laser chamber lifetime. In addition, under some conditions depending on fluence levels and wavelength, among other things, the plated debris can cause, e.g., localized high absorption on an optical element, resulting in earlier than normal failure of the optical element under, e.g., DUV light at high fluence. More importantly, however, dust entrained in the flowing gas can cause, e.g., scatter loss. This phenomena akin to “white-out” on a weather context, can cause the photons generated in a gas discharge between the electrodes of the gas discharge laser to so scatter that they do not reach the mirrors in the laser resonance cavity in sufficient quantities to cause adequate lasing in the excited gas medium during the discharge. This can be significant enough when the dust content is high enough, that no lasing occurs at all in a given pulse or pulses. This phenomenon increases in frequency and likelihood as the dust accumulates in the chamber over the live of the chamber, e.g., measured in billions of shots, and eventually can lead to, or at least be a significant contributor to, what is referred to in the industry as old age syndrome (“OAS”), the onset of which generally requires chamber replacement to maintain, e.g., the required output laser pulse energy (dose), and may also be impacted by such other requirements as pulse to pulse parameter stability requirements being engendered by increasingly demanding requirements, e.g., from lithography tool makers. 
   It is known in the art of gas discharge laser systems to provide for a debris/dust trap external to the laser gas discharge chamber, with input and output ports from the chamber and returning to the chamber for chamber gas to flow out of the chamber, through the debris trap, and back into the chamber. For example, applicants&#39; assignee has sold gas discharge laser systems with a so-called metal fluoride trap (“MFT”) having a trap inlet and a trap outlet, e.g., near an output window for generated laser light, to flush the area of that window with cleaned gas, as shown, e.g., in U.S. Pat. No. 5,018,161, entitled COMPACT EXCIMER LASER, issued to Akins et al. on May 21, 1991, e.g., as also shown in e.g., the 7000 series and XLA series lasers. Such an external trap may be electrostatic, requiring extra cost and power consumption added to the economics of utilizing such laser systems. Also U.S. Pat. No. 5,373,523, entitled EXCIMER LASER APARATUS, issued to Fujimoto on Dec. 13, 1994 shows an external dust trap on the side of a laser gas discharge chamber. U.S. Pat. No. 6,570,899, entitled GAS LASER DEVICE, issued to Yabu et al. on May 27, 2003, based upon an application Ser. No. 09/648,630, filed on Aug. 28, 2000, illustrates another form of external debris trap. 
   These types of external traps are also bulky, and tend to fill and become clogged and require replacement, or potentially allow undetected operation over, e.g., several billion laser output pulses (“shots”) of operation with “dirty” gas wherein unwanted OAS events, e.g., zero or low pulse energy lasing are occurring. In addition, they may not be capable of removing debris from the gas circulating within the laser gas discharge chamber fast enough, e.g., at elevated repetition rates of 4K and above, and especially at, e.g., the 6K and 8K and above levels, to prevent detrimental effects on the discharge due to debris presence in the gas between the electrodes at the time of discharge, which, e.g., is variable from discharge to discharge with resultant detrimental effects on such things as bandwidth and wavelength stability, beam shape and spatial coherence stability, etc. 
   As laser light pulse output repetition rates have increased, along with tighter and tighter controls required on such things as center wavelength, bandwidth and dose and the stability of such characteristics of the laser output light pulses have become necessary for keeping up with the demands of, e.g., integrated circuit lithography light sources, it has become even more important to effectively, efficiently and quickly remove debris, e.g., metal fluoride dust and the like from the circulating gas. 
   Debris removal from the gas, e.g., between discharges in the gas discharge chamber between the electrodes can, e.g., require very high fan motor speeds that both add temperature to the chamber gas and vibrations that can interfere with meeting laser output light parameter requirements and/or interfere with maintaining stability over time and over different duty cycles and over different output light pulse repetition rates. More debris in the gas can increase rates of deposition of the debris on optical elements, e.g., chamber windows contributing to reductions in performance and/or failures of the optical elements requiring more frequent replacements that are desirable. For these reasons there is a need for an improved debris removal system and method for very high repetition rate narrow band gas discharge lasers. According to aspects of an embodiment of the present invention applicants have proposed additional, low cost, easily implemented and very reliable means for debris removal from the gas circulating within the gas circulation flow path in the laser gas discharge chamber. This also, e.g., extends the life of the MFT, whose function is mostly to maintain a supply of cleaned gas to the chamber window regions. 
   It is also known in the art of gas discharge laser light sources to utilize preionization of the gas discharge region between gas discharge electrodes that produce the chemical and electrical changes in the gas between the electrodes. Each discharge of electrical energy between the electrodes causes laser light emission and/or amplification, e.g., in an oscillator resonance cavity or an amplification chamber, e.g., amplifying a narrow banded beam output from an oscillator chamber, e.g., in a master oscillator, power amplifier (“MOPA”) configuration. Preionization may be done, e.g., in lasers sold by applicants&#39; assignee with one or more preionization tubes positioned near the gas discharge region. The preionization tubes emit, via, e.g., a corona discharge UV and X-ray radiation which creates electrons via photoionization in the gas between the electrodes assisting the onset of the electric discharge in the gas between the electrodes. Applicants have determined that photoionization in the gas discharge region is less than ideal because most of the electrons are formed in the region of the preionization tube(s) and not in the gas discharge region. Such spatial nonuniformity of the electron distribution is believed by applicants to contribute to adverse effects on energy stability, especially early in a burst of laser output pulses. Applicants, therefore, according to aspects of an embodiment of the present invention propose certain methods and apparatus for improvement of preionization. Applicants propose a number of other improvements for the preionization. 
   In addition, it is known that acoustic effects in the laser can interfere with proper formation of the discharge in the discharge region, e.g., uniformity in the horizontal or vertical axes of the discharge, which can be caused by a variety of sources of acoustic wavefronts produced in and transmitted through the gas discharge chamber, including, e.g., from the gas circulation fan. Applicants herein propose ways to mitigate or eliminate most of such harmful effects, e.g., on the shape of the discharge. 
   Another problem facing the operation of gas discharge lasers, particularly as the requirements, e.g., for lithography light sources call for ever narrower bandwidths, bandwidth stability and center wavelength stability shot to shot, power (dose) stability shot to shot or at least over a plurality of shots on average, e.g., within a burst of shots, is in chamber acoustic effects on the repeatability shot to shot. Such requirements for repeatability may, e.g., require essentially exactly the same gas discharge conditions. In addition to the variability of the gas debris content mentioned above there are other possible sources of variability, e.g. two principal sources of these acoustic variations in the gas discharge chamber, shock waves generated from the spinning of and, to a degree, vibrations within the gas circulating fan and acoustic waves created by a prior discharge reflecting back to the discharge in time with a subsequent discharge, and, usually also aligned to the longitudinal axis of the discharge so as to substantially effect the entire length of the subsequent discharge. 
   According to aspects of an embodiment of the present invention applicants have proposed certain methods and apparatus for the mitigation of these detrimental effects on high repetition rate (e.g., 4 KHz+), high power (e.g., 40 W+), narrow banded, e.g., &lt;about 0.25 pm bandwidth at full width half max (“FWHM”) for ArF and &lt;1.2 pm E95% for ArF and less than about 0.35 pm FWHM and 1.5 pm E95% for KrF, laser light sources. Along with the above are requirements, e.g., for tighter dose stability requirements, e.g., ±about 0.3% for dense lines and less than that for isolated lines, wavelength stability, e.g., ±0.1 pm 3σ, and bandwidth stability, e.g., about ±0.05 pm FWHM 3σ, all of which will become even more stringent requirements as feature sizes (“critical dimensions” “CDs”) continue to decrease with resulting decreases in k 1 , along with increasing throughput and therefore dose requirements. 
   SUMMARY OF THE INVENTION 
   A method and apparatus if disclosed which may comprise a high power high repetition rate gas discharge laser UV light source which may comprise: a gas discharge chamber comprising an interior wall comprising a vertical wall and an adjacent bottom wall; a gas circulation fan creating a gas flow path adjacent the interior vertical wall and the adjacent bottom wall; an in-chamber dust trap positioned a region of low gas flow, which may be along an interior wall and may comprise at least one meshed screen, e.g., a plurality of meshed screens, which may comprise at least two different gauge meshed screens. The dust trap may extend along the bottom interior wall of the chamber and/or a vertical portion of the interior wall. The dust trap may comprise a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; and the second meshed screen intermediate the first meshed screen and the interior wall. The chamber may comprise a plurality of dust collecting recesses in at least one of the vertical interior wall and the bottom wall of the chamber which may be selected from a group comprising a one-part recess and a multi-part recess, which may comprise two sections angled with respect to each other. The dust trap may comprise a pressure trap positioned between a portion of a main insulator and an interior wall of the chamber. The chamber may comprise a gas circulating fan comprising a cross-flow fan with a fan cutoff that may comprise a vortex control pocket. The chamber may comprise a preionization mechanism comprising a preionization tub containing a ground rod within an elongated opening in the preionization tube that may comprise a compliant member, an automatic preionization shut-off mechanism, a preionization onset control mechanism and/or a focusing element. The chamber may comprise an elongated baffle plate that may comprise a plurality of pyramidal structures including varying numbers of generally pyramidal elements and oriented in groups of varying numbers of generally pyramidal elements and oriented along and transverse to the longitudinal axis. Acoustic resonances within the chamber may also be reduced by introducing an artificial jitter into the timing of the laser discharges varying the inter-pulse period randomly or in a repeating pattern from pulse to pulse within a burst. 

   
     BRIEF DESCRIPTION OF TIE DRAWINGS 
       FIG. 1  shows a cross-sectional partly schematic view of a gas discharge laser chamber according to aspects of an embodiment of the present invention, with the cross-section taken transverse to an optical axis of laser output light beams produced in the gas discharge laser chamber; 
       FIGS. 2A-D  show aspects of an embodiment of the present invention employing in-chamber debris catching and holding; 
       FIGS. 3A-C  show aspects of another embodiment of the present invention employing in chamber debris catching and holding; 
       FIG. 4  shows schematically an aspect of an embodiment of the present invention regarding in-chamber debris catching and holding; 
       FIG. 5  shows schematically another aspect of an embodiment of the present inventions regarding in-chamber debris catching and holding; 
       FIGS. 6A-C  show in greater detail aspects of an embodiment of the present invention shown, e.g., in  FIGS. 3A-B ; 
       FIG. 7  shows aspects of an embodiment of the present invention for acoustic resonance mitigation; 
       FIGS. 8-8B  and  FIGS. 9 and 10  show aspects of an embodiment of the present invention relating to an improved preionization ground rod construction; 
       FIGS. 11-13  show aspects of embodiments of the present invention relating to improved preionization and in-chamber debris catching and holding; 
       FIG. 14  shows a baffle plate according to aspects of an embodiment of the present invention; and, 
       FIG. 15  shows aspects of an embodiment of the present invention relating to improved preionization. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Turning now to  FIG. 1  there is shown a gas discharge laser system gas discharge chamber  20  according to an aspect of an embodiment of the present invention. The chamber  20  may be composed, e.g., of a chamber upper half  22  and a chamber lower half  24 , which may, when connected to each other by suitable means, e.g., by bolting, serve to define a chamber interior  26 . The chamber upper half  22  and chamber lower half also define, e.g., a chamber interior vertical wall  28  and the chamber lower half  24  defines a chamber interior horizontal bottom wall  29 . 
   Also contained within the chamber  26  is, e.g., a gas discharge system comprising two elongated opposing electrodes, cathode  30  and anode  32 , defining between them a gas discharge region  34 , wherein in response to sufficient voltage being present across the cathode  34  and anode  32  the gas between the electrodes in the also elongated discharge region  34  conducts and certain chemical and electrical reactions take place in the ionized plasma of the discharge that result in the production of radiation, e.g., at or near a characteristic center wavelength, that is optically directed along the optical axis of an output laser light pulse generally aligned to the longitudinal axis of the electrodes  30 ,  32 . 
   Also within the chamber  26  may be, e.g., an anode support  36 , which may be made, e.g., of a suitable dielectric material, e.g., a ceramic material, and an anode support bar  38 , which may be made of a suitable conductive material, e.g., brass. the anode may be connected to the chamber upper half  22  through a plurality of current returns  46 , with the chamber top  22 , along with the chamber bottom  24 , e.g. kept as a common voltage, e.g., at ground voltage. 
   The cathode  32  may, e.g., be connected to an electrical discharge high voltage feed through assembly  40 , e.g., by a high voltage feed through  42 , which passes through a main insulator  44 . The main insulator  44  may keep the cathode electrically isolated from the chamber upper half  22 . 
   Also within the chamber interior  26  may be, e.g., a preionizer  50 , e.g., in the vicinity of the cathode  30 . The preionizer  50  may include, e.g., a ground rod  52  that may be made of a suitable conductive material, e.g., brass (shown in more detail in  FIGS. 7 ,  11  and  12 ), and may, e.g., be substantially enclosed within the interior of a, e.g., hollow cylindrical preionizer tube  54 , which may be made of a suitable dielectric material, e.g., a ceramic. The preionizer  50  may have, e.g., a cylindrical centerline axis that is generally parallel to the longitudinal centerline axis of the cathode  30  and anode  32  and, therefore, also the discharge region  34 . The preionizer  50  may, in operation, e.g., function based upon the difference in voltage between the cathode  34  and the ground rod  52 . In addition the preionizer  50  may operate due to the difference in electrical potential between a conducting shim  56 , shown in more detail in  FIGS. 11 and 12 , which occupies the space between the preionizer  50  and the main insulator  44  along substantially the length of the preionizer  50 , or at least along substantially the entire length of the cathode  30 , and which is in electrical contact with the cathode  30 . 
   Also within the gas discharge chamber  26  may be, e.g., a gas circulation system comprising, e.g., a gas circulation fan  60 , which may be, e.g., a generally cylindrical crossflow fan  60 . The fan  60  serves to move gas within the chamber interior  26 , generally in a circular fashion as seen, e.g., in the cross-sectional view of, e.g.,  FIG. 1 , in order to, e.g., remove from the discharge region  34  between successive gas discharges the gas that contains, e.g., ionized particles and debris and depleted F 2 , to replenish the discharge region, e.g., with fresh gas, e.g., containing F 2  before the next successive gas discharge. The gas circulation system may also include a plurality of heat exchangers  70  in the generally circular gas flow path to remove heat added to the gas, e.g., by the discharges and the operation of the fan  60 . 
   The gas circulation system may also have, e.g., a plurality of curved baffles  80  and a flow directing vane  62 , which may serve, e.g., to shape the generally circular gas flow path out of the discharge region  34  toward the heat exchangers  70  and ultimately the intake of the fan  60  and from the output of the fan  60  to the discharge region  34 , respectively. 
   The chamber  26  may also have a metal fluoride trap  90  as is known in the art, although an aspect of an embodiment of the present invention is to replace the MFT where possible. 
   Also contained in the chamber interior  26 , e.g., along the horizontal bottom interior wall  29  of the bottom  24  of the chamber  20  may be, e.g., a dust trap  100 . It will be understood that by dust is meant the various forms of debris, e.g., mostly metal fluoride material, that circulates with the gas circulation and appears to the naked eye to be dust-like or lint-like. 
   The dust trap  100  according to an aspect of an embodiment of the present invention may be illustrated by reference to  FIGS. 2A-D  as an example. As shown in  FIGS. 2A-D , the dust trap  100  may be comprised of at least one meshed element, e.g., a wire mesh  102  having a first gauge, e.g., [ —— ] and a first pitch, e.g., [ —— ] and may be comprised of vertical mesh members  102   a  and horizontal mesh members  102   b , e.g., made of wire of, e.g., a suitable metallic material, e.g., brass. Each wire may have, e.g. a wire gauge, i.e., diameter, e.g., [ —— ]. The meshed horizontal mesh members  102   b  and vertical mesh members  102   a  may form mesh openings, illustrated according to an aspect of an embodiment of the present invention to be rectilinear, but to be understood to be of any suitable meshed shape, e.g., triangular and/or of square or rectangular shape may be employed. 
   The dust trap  100  may also comprise, e.g., a second meshed element  104 , which may be similar to the first mesh element  102 , but may comprise a second gauge and pitch smaller than the first, e.g., [ —— ] and [ —— ]. It will also be understood that the second meshed element  104  may also have horizontal mesh wires  104   b  and vertical mesh wires  104   a  having a respective wire gauge or gauges, and forming mesh openings  108  of any possible suitable shape. The first and second meshed elements  102 ,  104  may have differing shaped mesh openings. It is also possible according to an aspect of an embodiment of the present invention to have more than two different meshed elements varying, e.g., in gauge, pitch and opening shape and wire gauge size. 
   Turning now to  FIGS. 2C  and D there is illustrated, by way of example, two of the many possible arrangements of meshed elements, e.g.,  102 , 104  according to aspects of an embodiment of the present invention. In  FIG. 2C  there is shown an embodiment in which meshed elements  102 ,  104 , e.g., of generally the same thickness, but with differing, e.g., gauge, wire gauge size, opening shape or the like, are layered one on top of the other, e.g., in a four layer embodiment. It will also be understood that each of the meshed elements  102 ,  104  as illustrated in  FIG. 2C  may be different in one or more of the respects noted above, or that a pattern may be repeated, e.g., every third layer, e.g., in a six layer embodiment. That is to say that the dust trap  100  may comprise, e.g., six layers, with the first three differing from each other e.g., narrowing in opening size and the next three being a repeat of the first three. It will also be under stood that the dust trap  100  could comprise, e.g., only the first three such layers just described. 
   Turning to  FIG. 2D  there is shown aspects of an alternative embodiment according to the present invention. In  FIG. 2D  there is illustrated, by way of example, an embodiment in which meshed elements, e.g.,  102 ,  104  of differing thicknesses in the vertical direction (as that direction is shown for illustrative purposes in  FIG. 2D ) are layered to form the dust trap  100 ′. It will be understood, that, as with  FIG. 2C , the various mesh elements  102 , 104 , may be the same or differ from each other in aspects noted above apart from the varying thickness. As illustrated in  FIG. 2D , the dust trap  100 ′ comprises two alternating thin ( 104 ) and thick ( 102 ) mesh elements placed on top of an additional thick meshed element  102 . It will also be understood that, for convenience, the thick meshed elements shown in  FIG. 2D  are illustrated to be the larger meshed elements  102  illustrated in  FIG. 2A , but this need not be the case, and one or more of the thicker meshed elements illustrated in  FIG. 2D  may be of smaller gauge and/or pitch and/or wire gauge size. 
     FIGS. 3A-C  illustrate an aspect of an embodiment of the present invention wherein dust catcher/collector elements, e.g.,  130 ,  132 ,  134  may, e.g., be formed in the interior wall portions, e.g.,  28 ,  29  of the chamber  20 . According to an aspect of an embodiment of the present invention, the elements  130 ,  134  may comprise a first portion  130   a ,  134   a  and a second portion  130   b ,  134   b  extending in a direction angled from the respective first portion  130   a ,  134   b  and the elements  132  may have only a single portion formed, e.g., in the respective portion, e.g., walls  28 ,  29  of the chamber  20 . 
     FIG. 3C  illustrates schematically the fact that the dust catchers/collectors, e.g.,  130 ,  132  may be randomly sized, positioned and oriented along, e.g., a wall of the chamber  20 , e.g., wall  28 , e.g., with the flow from top to bottom of  FIG. 3C  as illustrated. For example, two piece catcher/collectors  130  may be spaced at differing heights along, e.g., wall  28  (which it will be understood could just as well be, e.g., bottom  29 ) and having differing widths horizontally (as that direction is illustrated in  FIG. 3C  to be horizontal on the page) and can even be oriented, e.g., diagonally. It will also be understood that it is possible according to aspects of an embodiment of the present invention for the illustrated wall, e.g., wall  28  to be rotated along with the illustrated catcher/collectors  130 ,  132  so that flow is still from top to bottom of the illustration, as so rotated, but is incident on the openings  130 ,  132  at a ninety degree (or perhaps, e.g., even a forty-five degree) angle to that illustrated in  FIG. 3C . Also illustrated is the possibility of intermingling the two piece catcher/collectors, e.g.,  130  with the single piece ones  132 . 
     FIG. 3B  shows an aspect of an embodiment of the present invention in which, e.g., a dust trap  100  may be combined with dust catcher and collector elements, e.g.,  132 ,  134 .  FIG. 1  illustrates an aspect of an embodiment of the present invention in which a dust trap  100  rests on (or is suitably attached to), e.g., the horizontally extending (as that direction is shown in  FIG. 1  for illustrative purposes) floor  29  of the chamber and  FIG. 5  illustrates an aspect of an embodiment of the present invention where a dust trap  100 ″ is positioned along both the bottom  28  and vertical side wall  28  of the chamber  20 . 
   In operation the above-described dust collectors/traps have the effect of minimizing the equilibrium particle density of the debris in the gas flow without significantly increasing the impedance within the chamber interior  26  to gas circulation. This is important because at higher gas discharge repetition rates, e.g., beyond 4 KHz the blower power and heat budget becomes a critical performance issue as well as blower wear, e.g., bearing wear at higher speeds and eventual requirements for blower replacement due to excessive vibration. The collectors/traps may also be assisted in their operation by the changing gas flow patterns as the fan is cycled through on and off periods and as flow recommences in the on cycle. In operation with the collectors, e.g.,  130 ,  132 ,  134  and or traps, e.g.,  100 ,  100 ′,  100 ″ in and/or along the walls of the chamber, essentially in the boundary flow region, the boundary layer flow, along what would be the otherwise smooth-wall, may, e.g., be disrupted in such a way that particles of debris (so-called dust) are accelerated into the openings, e.g.,  130 ,  132 ,  143  in the wall and/or meshed elements in the dust traps, e.g.,  100 ,  100 ′ and  100 ″, and once inside essentially cannot return to the gas flow inside the chamber interior  26 . For example once inside the meshed elements of the traps  100 ,  100 ′,  100 ″, e.g., having entered a mesh opening in the outermost layer, the particle may, e.g., be slowed by eddy currents, or otherwise, and then is further slowed in each succeeding meshed layer such that the particles then remain within the meshed structure. Applicants also suspect that flow can be created, e.g., around the wire in the mesh, e.g., orthogonal to the flow direction, which also accelerates particles in the direction of the next succeeding internal mesh layer. 
   This so-called boundary layer effect is taken advantage of by applicants according to aspects of an embodiment of the present invention. With regard to the dust trap  100 , 100 ′,  100 ″, as illustrated very schematically and not to proportion in  FIG. 4 , the meshed element, e.g.,  102  openings, e.g.,  106  presented to the flow  120 , e.g., in the boundary layer present an opportunity for debris to precipitate out of the flow as illustrated at  124 , pass through, if appropriate, openings, e.g.,  108  in an underlying meshed element layer, and then perhaps be collected in a pocked formed, e.g., by openings, e.g.,  106  in a still further underlying meshed element layer, e.g.,  102 . Applicants believe that the flow in the boundary layer near the boundaries of the gas circulation path is slow enough to encourage debris to enter the exposed openings, e.g.,  106  as illustrated in  FIG. 4  and fast enough to also encourage, e.g., eddy currents  122  to form, which further encourages the precipitation  124 . 
     FIGS. 6A-C  illustrate the operation dust traps  100 - 100 ″ according to aspects of an embodiment of the present invention. It will be understood by those skilled in the art of fluid dynamics that the gas circulation in the generally circular circulation path referenced above is of relatively high velocities, e.g., measured at points in the cross-section of the circulation path on the order of tens of meters per second, determined, e.g., by the amount of gas flow necessary through the discharge region to adequately and properly clear the discharge region between shots. More toward the boundaries of the container in which the gas is circulating, e.g., the wall sections  28 ,  29 , due, e.g., to friction of the moving gas along these boundary portions, the gas flow slows down until its profile at the surface of the boundary element, e.g., interior wall  28  or  29  to zero or almost zero. 
     FIG. 5  illustrates an aspect of an embodiment of the present invention wherein a dust trap  100 ″ lies along a wall, e.g., wall  28  and the bottom  29  of the chamber interior  26 . 
   Applicants have found that after relatively short operating times the floating debris being circulated with the laser gas in the gas circulation path essentially all collects in the dust collector, e.g., a dust collector  100  on the floor of the chamber  20  and does not thereafter significantly migrate or get shocked back into the gas circulation flow path. In the past, applicants believe that dust tended to precipitate to the bottom of the chamber, but that various acoustic and other shock wave and vibrational disturbances then periodically entrained the dust, or significant quantities of it, back into the gas circulation path. A dust trap, e.g.,  100 ,  100 ′,  100 ″ according to aspects of an embodiment of the present invention have been found by applicants to virtually eliminate the reintrainment of dust collected in the dust traps, e.g.,  100 ,  100 ′ and  100 ″ according to aspects of embodiments of the present invention. 
   Similarly, the catcher and collector elements,  130 ,  132  and  134  are believed to work alone or in conjunction with a dust trap  100 ,  100 ′,  100 ″, e.g., as shown in  FIG. 3B  in a similar fashion, e.g., as illustrated in  FIGS. 6A-C , wherein flow and currents  150 , e.g., eddy currents from the boundary layer flow  154  and into and within the openings  130 ,  132  and  134  in the walls, e.g.,  28 ,  29  of the chamber  20 , cause the deposition of debris  156  and, especially in the shapes such as  130  and  134 , discourage or prevent its subsequent removal. It will also be understood that the openings  132  (basically grooves in the chamber interior walls, e.g.,  28  and  19 ) may be of other shapes in cross section and the openings  130 ,  132  and  134  may be of irregular sized and spacing horizontally and/or vertically. It will also be understood that the openings, e.g.,  130 ,  132 ,  134  may be formed in a plate separate from the wall(s) e.g.,  28 ,  29  and attached, e.g., by bolting, to the respective wall(s), e.g.,  28 ,  29 . The dust collectors, e.g.,  100 ,  100 ′,  100 ″ may also be bolted to the respective wall(s), either at the periphery of the meshed elements, e.g., through a frame (not shown) holding the respective meshed element(s) or through the mesh itself, or both. 
   Turning now to  FIG. 7 , there is shown aspects of an embodiment of the present invention wherein an element acting as a cutoff for the fan, e.g., an anode support  36 , may have formed therein a pocket  200 . The pocket may serve, e.g., to improve flow efficiency in the chamber  26  by, e.g., improving the control of the fan vortex location allowing a greater pressure drop across the fan for a given volume of flow. This can improve the effectiveness of the replenishment of fresh gas between the electrodes between shots while at the same time not significantly raising acoustic effects from the fan operation, e.g., generated in the vortex and/or from increased fan vibrations (from, e.g., higher speed) to raise the fan pressure drop without the just described vortex control. 
   Also shown in  FIG. 7  are additional embodiments of debris traps placed in low gas flow regions, e.g., traps  280 , forming trap pockets  282 , e.g., in pockets formed between, e.g., the main insulator  42  and the top half  22  of the chamber (also shown in  FIGS. 11 and 12 ). In this embodiment the dust traps  280  could be, e.g., generally u-shaped honeycombed structures that can fit, e.g., between the main  42  insulator wings as shown in the Figures general along most or all of the longitudinal extension of the main insulator  42  along the longitudinal axis of the discharge region  34  within the chamber interior  26 . Gas pressure and circulation will tend to move debris toward the structures  280  and once the debris passes through the honeycombed openings, e.g., into the pockets  282  it will be difficult for the debris to be caused to return back through the honeycombed structures  280  to return into the gas flow. 
   Turning now to  FIGS. 8-8B  there is shown, partially cut-away, a preionizer ground rod  54  according to aspects of an embodiment of the present invention, which may have, e.g., a ground rod elongated thick section  180  and a ground rod thin section  182  on either end of the thick section  180 , which is, e.g., for arc prevention reasons as is known in the art. The ground rod may have, e.g., a terminus section  210 , which may serve, e.g., as a connecting pin, which may be, e.g., inserted into a ground rod mounting fitting, e.g., in the respective chamber wall and held in place, e.g., by a set screw (not shown). One end of the ground rod  54  may have a mounting flange  214 . At least one longitudinal flexure member  216  (two are illustrated in  FIG. 8 ) may be placed at a terminus of the ground rod  54 , shaped and sized to fit within the interior of the preionizer ceramic tube  52  (shown, e.g., in  FIGS. 7A  and B. The flexure member  216  may comprise, e.g., a plurality of slots  218 , also illustrated in  FIG. 8A  and the cross-section of  FIG. 8B  along the cross-section lines  8 B- 8 B in  FIG. 8A . The slots  218  serve to form an accordion-like structure that will compress under compressive stress to the ground rod  54 , e.g., due to thermal stressing of the ground rod  54  or other causes, without also inducing, e.g., bending torque in the ground rod  54 , which applicants have found to cause catastrophic preionizer tube failures. 
   Turning to  FIGS. 9 and 10  there is shown an alternative embodiment for a ground rod  54  similar to that shown in  FIGS. 8-8B , according to aspects of an embodiment of the present invention in which at least one of the thin sections  260  of the ground rod  54  may, e.g., be formed into a relatively flexible and elastic member  250  that is sized to also fit within the preionizer tube  52 . The flexible member  250  may include, e.g., a plurality of longitudinally extending portions  252 , laterally extending portions  256  and elbow joints  254 . It will be understood that this member  250  may be formed by machining the thinned section  260  of the ground rod  54  to a still smaller thickness diameter and bending the still narrowed portion of thinned section  250  to form the s-shaped bends of the elastic member  250 . This flexible section will also allow for compression in the longitudinal direction without inducing bending moment into the ground rod  52 . 
   Turning now to  FIG. 11  there is shown a preionization control system  290  according to aspects of an embodiment of the present invention, which may comprise, e.g., a preionization cut-off circuit comprising, e.g., an RC network of resistor  292 , which may be, e.g., a [______] Ω resistor and a capacitor  294 , which may be, e.g., a [______] F capacitor. Applicants have determined that a certain level of preionization in the operation of gas discharge lasers of the type described above in a burst mode is necessary during the first few pulses in the burst, but that, if maintained at that level during the rest of the burst, actually may be detrimental to proper laser light source operation, e.g., detrimental to producing DUV light as desired, after a first number of pulses in the burst, e.g., [______] pulses. Therefore, the RC network of the preionizer controller  290  may be timed to charge capacitor  294 , which may be connected in parallel with resistor  292  between the preionizer ground rod  54  at  296  and ground, i.e., the potential of the anode  32 . The connection of the ground rod  54  to the potential of the anode may be done, e.g., by insulating the ground rod  54  from connection to the chamber body, e.g., chamber top half  22 , with which it is normally in electrical contact in current lasers sold by applicants&#39; assignee and inserting the RC network  290  between the ground rod  54  and the anode  32 . 
   It will be understood that in operation the ground rod  54  at first will be connected to ground through the resistor  292 . However, over time as the cathode  30  is successively brought to high voltage for successive pulses in the burst, the capacitor  294  will charge according to the selected time constant of the RC network  290 , until at some point in the burst the potential difference between the capacitor  294  and the cathode  30  will be such that the potential difference between the cathode  30  and the ground rod  54  of the preionizer  50  cannot generate a corona discharge any longer during that burst. Between bursts the resistor  292  can be selected to be of such a value that all or substantially al of the charge on the capacitor  294  bleeds to ground and the ground rod  54  is effectively connected to ground awaiting the next burst. 
   It will also be understood that, while only one preionization tube  50  is illustrated in  FIG. 11 , for simplicity, two such preionization tubes  50  may be used, with, e.g., only one having the preionization control circuit  290  just described and the other being connected, e.g., directly to ground. Therefore, in operation, e.g., this preionizer will continue to act as a preionizer all through the burst. In this fashion, e.g., preionization can be made, e.g., to be at a high electron density, and e.g., on both sides of the discharge, early in a burst, e.g., for the first [______] number of pulses and at a lower electron density during the remainder of the pulses in the burst. 
   Turning now to  FIG. 12  there is shown an improved preionizer  50  according to an aspect of an embodiment of the present invention. The improved preionizer  50  may incorporate, e.g., a mirror  300  which may be, e.g., a section of a cylindrical ellipse  302 , having, e.g., a first focus at the centerline longitudinal axis of the preionizer  50 , e.g., the centerline longitudinal axis of the ground rod  54  of the preionizer  50  and a second focus in a desired portion of the gas discharge region  34  between the electrodes  30 ,  32 . this selected portion may be, e.g., along the longitudinal centerline axis of the gas discharge, and, e.g., at a selected position between the electrode  30 ,  32 , i.e., vertically displaced (as that direction is illustrated in  FIG. 12 ) within the discharge region  34 , e.g., mid-way between cathode  30  and anode  32 . In this manner, as illustrated by example in  FIG. 13 , the density distribution of electrons from the preionization can be shifted more into the center of the gas discharge region, e.g., enhancing the effectiveness of the preionization. The mirror  300  may be formed as a part of, e.g., the main insulator  44 , e.g., by grinding and polishing out the elliptical surface and may be reflective due to a highly polished surface or by the addition of a suitable coating, e.g., [______]. 
   Currently a manner of effecting preionization in gas discharge lasers is to utilize the preionizer as essentially a capacitive element, e.g., with the dielectric of the preionization tube  52  acting as the dielectric in a capacitor formed, e.g., between the cathode  30  and the ground rod  54 . As a high voltage, e.g., a high negative voltage, is applied to the cathode  30  at some point the differential in voltage between the cathode  30  and anode  32  causes an electrical discharge between the two as the gas medium becomes conductive. The preionizer tube is selected in radial dimension and shape to initiate a corona discharge on the surface of the preionization tube  52  at a time just shortly before the peak anode to cathode voltage is reached to seed the gas medium with electrons to help initiate and speed up the formation of the electrical discharge between the electrodes  30 ,  32 . 
   In some circumstances of operation, however, e.g., in a molecular fluorine laser and/or one operating a reduced high voltage, the initiation and/or duration of the electrical discharge may tend to hang up more than normal before the actual discharge occurs and the high voltage begins ringing between the electrodes  30 , 32 . Thus, e.g., the preionization effect may be mis-timed enough to interfere with the timing and adequacy of preionization. 
   Applicants believe that a solution to this problem can be found in putting a switch on the ground rod  54 , e.g., to more actively control preionization by controlling when the ground rod  54  is at common potential and thus can cause the corona discharge due to a difference in potential between the cathode  30  and the ground rod  54 . This switch could be, e.g., in one embodiment, a saturable magnetic switch (not shown), such that variations in the voltage applied to the cathode and to the switch could provide a suitable preionization at, e.g., 20-40 ns before the actual occurrence of the discharge, based on a volt-second product that changes this delay time appropriately according to voltage applied, i.e., for a lower voltage a longer delay in switch closure and vise-versa for higher voltage. Alternatively, a non-active inductive element (not shown), e.g., a [______] could be employed, perhaps at the expense of some amplitude of the displacement current (corona discharge) of the preionizer, or alternatively a surface flashover ceramic connector (not shown) could be utilized, e.g., connecting the ground rod  54  to ground to delay the onset of preionization as appropriate. Although this is not as active a control of preionization onset as the use of, e.g., a switch, it still may be effective. 
   Turning now to  FIG. 14  there is shown a baffle plate  320  according to an aspect of an embodiment of the present invention. The baffle plate may have a base plate  322 , along with, e.g., a plurality of generally pyramidal structures, e.g., pyramidal structure  324 , pyramidal structure  326 , and pyramidal structure  328 , which may differ in the size of the generally pyramid shaped elements within each structure  324 ,  326 ,  328  and the number of pyramidal elements in each structure  324 ,  326 ,  328 . The structures such as  324  and  326  may differ from, e.g., structures  328  by, e.g., the orientation of major ridges formed, i.e., from longitudinally extending to transversely extending. The baffle plate  320  may be attached in the chamber  26 , e.g. to a chamber wall, e.g., wall  29 , e.g., by bolts passing through bolt holes  340 . 
   Acoustic effects within the chamber  26  may also be attenuated or removed by, e.g., introducing artificial jitter into the timing of the laser discharge, so that the inter-pulse period varies, e.g., from one pulse to another within, e.g., a burst of pulses, e.g., randomly or with a repeatable pattern, e.g., within the given burst. Variations in the discharge timing so produced, e.g., on the order of 1/width of a resonance peak can, e.g., break up the periodicity and reduce the growth rate of resonance leading to the formation of resonance peaks, e.g., in the output bandwidth of the laser system. By way of example, for the case of a typical, e.g., 20 Hz wide resonance peak, e.g., at about 3000 Hz laser pulse repetition rate, e.g., a few microsecond artificial jitter can, e.g., correlate acoustic disturbances sufficiently. Instead of a truly random pattern, the pattern of jitter error introduced into the trigger delay can be made to repeat, e.g., every Nslit pulses, where Nslit is the number of pulsed in the exposure slit. This insures, e.g., that the number of pulses seen by the resist at any point in the slit remains the same regardless of jitter magnitude and stage speed. For a MOPA arrangement the jitter would need to be introduced in, e.g., the MOPA firing, with the firing of the PA based on the actual time of the jittered firing of the MO. This may be done, e.g., using existing firing control panels on lasers sold by applicant&#39;s assignee, e.g., 7000 series lasers, to manipulate, e.g., trigger-in to discharge timing. 
   Turning now to  FIG. 15  there is shown aspects of an embodiment of the present invention relating to improvements in preionization. Based on experimental inventigation applicants have developed an improved understanding of factors relating to preionization efficiencies and effectiveness. With regard to, e.g., upstream preionization after an initial preionization, e.g., on the first pulse of a series of pulses, e.g., a burst of pulses applicants have noted that there occurs a period of decreased energy efficiency in the discharge for subsequent pulses, apparently, e.g., until gases that have been ionized in, e.g., the first preionization and first main discharge, have been recirculated back through the chamber, e.g., past the heat exchangers and back through the blower (fan). Applicants believe that this is due to the depletion of ionizable species in and around the discharge region after the first pulse in the burst, and applicants investigated this belief and proposed a solution to the energy stability degradation over the burst time that involves keeping the gas upstream of the discharge region from being depleted during preionization. Applicants, therefore have inserted, e.g., a protective cover  350  over the preionization tube  54 . The protective cover  350 , which may be an extension of the conductive shim  56 , or a metallic coating (not shown) may serve, e.g., to prevent the corona discharge from forming on the preionization tube  54  in the region of the extension  350  of the shim  36 (or the metallic coating) and, therefore, the substantial prevention of the occurrence of the ionization of gas specie upstream of the discharge region prior to gas discharge in the gas discharge region by masking this area from the corona discharge on the preionization tube  54 . As shown in  FIG. 15  the mask formed by the extension  350  of the shim  56  (or the corresponding metallic coating, not shown) may extend substantially all of the way around the preionization tube  54 , except in the region of corona discharge access to the discharge region to, e.g., substantially extinguish the corona discharge in areas of the preionization tube that do not fact the electrodes  30 ,  32 . 
   As can be seen from  FIG. 15 , the mask  390  (or corresponding metallic coating, not shown), together with, e.g., the rest of the shim  56  may serve to focus the production of ions from the corona discharge generated by the unexposed remaining portions of, e.g., the preionization tube  54  toward the discharge region  34 , indicated schematically by the dashed lines extending from the mask  350  to the anode bar  32 . It will be understood that according to aspects of an embodiment of the present invention this mask  390  can be used with other forms of preionization besides simply corona discharge, e.g., with spark gap preioniation and/or preionization through a slotted opening in a preionization tube. 
   It will be understood by those skilled in the art that many modification and changes can be made to the aspects of embodiments of the present invention disclosed in this specification without departing from the scope and spirit of the inventions embodied in the aspects of embodiments of the present invention disclosed in this application. Therefore, the appended claims should not be interpreted to be limited to or narrowed in scope in any way by the specifics of aspects of embodiments described herein.