Laser chamber with discharge system having acoustic scattering surfaces

Abstract

An apparatus for generating laser radiation from discharges in a discharge chamber in which the discharges produce acoustic waves which would disrupt operation of the apparatus at certain repetition rates if reflected back to their origin in which a surface of an electrode adjacent the preionizer tube or a surface of the preionizer tube or both are provided with acoustic scattering features which scatter acoustic waves impinging on the electrode and preionizer tube surfaces.

Description

LASER CHAMBER WITH DISCHARGE SYSTEMHAVING ACOUSTIC SCATTERING SURFACESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 607,175, filed December 7,2023, titled LASER CHAMBER WITH DISCHARGE SYSTEM HAVING ACOUSTIC SCATTERING SURFACES, which is incorporated herein by reference in its entirety.FIELD

[0002] The disclosed subject matter relates to laser discharge systems in which discharges in a discharge region used to produce laser radiation at the same time produce acoustic disturbances which may be undesirably reflected back to the discharge region or affect the refractive index inside the discharge chamber.BACKGROUND

[0003] Photolithography is a process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. A photolithography radiation source provides the deep ultraviolet (DUV) radiation (wavelengths in a range of about 100 nanometers (nm) to about 400 nm) used to expose a photoresist on the wafer. Often, the radiation source is a laser source and the radiation is a pulsed laser beam. The radiation beam is passed through a beam delivery unit, then a reticle or a mask, and then projected onto a silicon wafer coated with photoresist. In this way, a chip design is patterned onto a photoresist that is then etched and cleaned.

[0004] In some photolithography radiation sources a laser beam is produced by causing discharges in a discharge region between electrodes (anode and cathode) of one or more laser discharge chambers. One challenge in the design and use of these systems is that the discharge which produces the laser radiation also generates strong acoustic waves inside the laser discharge chamber. These acoustic waves create gas density modulations that propagate within the laser discharge chamber. Surfaces in the laser discharge chamber may reflect these acoustic waves back into the discharge region and adversely affect the laser’s performance. In particular these reflected waves may result in a round trip time-of-flight resonance depending on the inter-pulse delay or discharge repetition rate at which the laser system is operated. They can also affect propagation of the ensuing light pulses via uneven modulation of the index of refraction within the discharge region.

[0005] In other words, acoustic disturbances in the discharge chambers are defined by pressure fronts originating from the plasma discharge in the discharge region between the electrodes, propagating away from the discharge region, and bouncing off all solid surfaces within the chamber. Reflected acoustic fronts can re-enter discharge region and affect propagation of the following light pulses due to uneven modulation of the index of refraction within the discharge region.

[0006] It would be advantageous to mitigate the adverse effects of these acoustic disturbances and improve the uniformity of the pressure distribution in the discharge region. It is in this context that the need for the subject matter of the present disclosure arises.SUMMARY

[0007] The following presents a succinct summary of one or more embodiments in order to promote a basic understanding of the presently disclosed subject matter. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify as key or critical any elements of any embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts relating to one or more embodiments in a streamlined form as a prelude to the more detailed description that is presented later.

[0008] According to one aspect of an embodiment there is disclosed a discharge system for a discharge chamber of a laser, the discharge system comprising an electrode having surface, the surface being elongated in a first direction and configured to extend parallel and adjacent to a discharge region of the discharge chamber, and a plurality of electrode acoustic scattering features disposed on the surface.

[0009] The plurality of electrode acoustic scattering surfaces may comprise a plurality of rounded sawtooth ridges each extending from the surface and arranged in a row extending in the first direction. Respective ones of the plurality of rounded sawtooth ridges may extend from the surface to a height with a magnitude varying in dependence on a distance of the rounded sawtooth ridge from an end of the row in the first direction. The height may vary sinusoidally in dependence on the distance of the rounded sawtooth ndge from the end of the row in the first direction. Respective ones of the plurality of rounded sawtooth ridges may have a width in the first direction with a magnitude varying depending on a distance of the rounded sawtooth ridge from an end of the row in the first direction. The width may vary sinusoidally in dependence on the distance of the rounded sawtooth ridge from the end of the row in the first direction.

[0010] A rounded portion of respective ones of the plurality of rounded sawtooth ridges may have a radius of curvature with a magnitude depending on a distance of the rounded sawtooth ridge from an end of the row in the first direction. The radius of curvature may vary sinusoidally in dependence on the distance of the rounded sawtooth ridge from the end of the row in the first direction.

[0011] The discharge system may further comprise a preionizer configured as a generally cylindrical hollow tube extending axially in the first direction and having a tube wall with an outer surface, at least a portion of the outer surface being provided with a plurality of preionizer acoustic scattering features. The tube may comprise a dielectric material The laser may be configured to generate a discharge in a discharge region and wherein the preionizer acoustic scattering features may then be provided on at least a portion of the outer surface arranged to face the discharge area. The preionizer acoustic scattering features may be provided on substantially all of the outer surface.

[0012] According to another aspect of an embodiment there is disclosed a discharge chamber for a laser system, the discharge chamber comprising a first electrode having a first electrode discharge surface, a second electrode having a second electrode discharge surface, the first electrode discharge surface being arranged facing and spaced away from the second electrode discharge surface to define a discharge gap and a preionizer including a preionizer tube extending laterally adjacent the first electrode, a surface of the preionizer tube facing the discharge gap being provided with a plurality of preionizer acoustic scattering features dimensioned and arranged to scatter acoustic waves from the discharge gap. The first electrode may have a shoulder region adjacent to the preionizer and provided with first electrode acoustic scattering features dimensioned and arranged to scatter acoustic waves from the discharge gap.

[0013] According to another aspect of an embodiment there is disclosed a preionizer for a discharge chamber of a laser system, the preionizer comprising a tube configured as a generally cylindrical hollow tube extending axially in a first direction and having a tube wall with an outer surface, at least a portion of the outer surface being provided with a plurality of preionizer acoustic scattering features, the acoustic scattering features comprising a first plurality of first axial segments each being coaxial with the tube and having a circular cross section with a first radius interspersed with a second plurality of second axial segments each being coaxial with the tube and having a circular cross section with a second radius different from the first radius, and an electrode positioned at least partially within the tube. The tube may comprise a dielectric material. The laser system may be configured to generate a discharge in a discharge region and wherein the acoustic scattering features are provided on at least a portion of the outer surface arranged to face the discharge area. The acoustic scattering features may be provided on substantially all of the outer surface.

[0014] According to another aspect of an embodiment there is disclosed a discharge chamber comprising a cathode having a cathode shoulder with a modulated shoulder shape and a preionization tube proximate to the cathode shoulder, the preionization tube having a modulated tube shape having a first section with first diameter extending a first longitudinal distance along the preionization tube and a second section with a second diameter extending a second longitudinal distance along the preionization tube.

[0015] The discharge chamber may further comprise a plurality of beads disposed along the preionization tube, the beads and the preionization tube forming the modulated tube shape. The modulated shoulder shape may comprise protrusions of varying height and disposed with varying periodicity along the cathode shoulder.

[0016] Further embodiments, features, and advantages of the subject matter of the present disclosure, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the presently disclosed subject matter and, together with the description, further serve to explain the principles of the presently disclosed subject matter and to enable a person skilled in the relevant art(s) to make and use the presently disclosed subject matter. The drawings are not to scale unless otherwise indicated.

[0018] FIG. 1 is a schematic diagram of an overall broad conception of a photolithography system.

[0019] FIG 2 is a schematic diagram of an overall broad conception of an illumination system such as might be used in the photolithography system of FIG. 1.

[0020] FIG. 3 is a cross section of a discharge chamber such as might be used in the illumination system of FIG. 2.

[0021] FIG. 4 is a cross section of a portion of the discharge chamber of FIG. 3.

[0022] FIG. 5 is a perspective view of an arrangement of components in the discharge chamber of FIG. 3.

[0023] FIG. 6A is a cross section of part of the length of a preionizer system and a discharge in a discharge region.

[0024] FIG. 6B is an end-on cross section of the preionizer system and discharge of FIG. 6A.

[0025] FIG 7 is a diagram of a portion of a wall for a preionizer tube of a preionizer system according to an aspect of an embodiment.

[0026] FIG 8 is a projection of a portion of an exterior surface of a preionizer tube wall provided with an arrangement of acoustic scattering features to create an acoustic scattering surface according to an aspect of an embodiment.

[0027] FIG 9 is a projection of a portion of an exterior surface of a preionizer tube wall provided with another arrangement of acoustic scattering features to create an acoustic scattering surface according to an aspect of an embodiment.

[0028] FIG 10 is a side view of a preionizer tube with acoustic scattering features according to an aspect of an embodiment.

[0029] FIG. 11 is a perspective view of the preionizer tube of FIG. 10.

[0030] FIGS. 12A - 12D are side views of examples of embodiments of acoustic scattering features according to aspects of embodiments.

[0031] FIG. 13A is a side view of a preionizer tube having a surface modulation according to an aspect of an embodiment.

[0032] FIG 13B is a side view of a preionizer tube having a surface modulation according to an aspect of an embodiment.

[0033] FIG 14A is a diagram of a surface modulation according to an aspect of an embodiment.

[0034] FIG. 14B is a diagram of a surface modulation according to an aspect of an embodiment.

[0035] FIG. 15 is a diagram showing a possible relative placement of components in a discharge system according to an aspect of an embodiment.

[0036] FIG 16A is a perspective view of a cathode structure according to an aspect of an embodiment.

[0037] FIG. 16B is a perspective view of a cathode structure and preionizer tube according to an aspect of an embodiment

[0038] FIG. 17 is a perspective view of a portion of the surface of the cathode structure and preionizer tube according to an aspect of an embodiment.

[0039] Further features and advantages of the presently disclosed subject matter, as well as the structure and operation of various embodiments of the presently disclosed subject matter, are described in detail below with reference to the accompanying drawings. It is noted that the scope of the presently disclosed subject matter is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings presented herein.DETAILED DESCRIPTION

[0040] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details associated with it below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments. This summary is not an extensive overview of all contemplated embodiments and is not intended to single out as key or critical any elements of any embodiments nor delineate the scope of any or all embodiments.

[0041] Systems such as those described herein may render benefits in a wide range of applications and implementations. For the sake of having a specific nonlimiting example to facilitate description, one such application is in semiconductor photolithography. FIG. 1 shows a photolithography system 100 that includes an illumination system 105. As described more fully below, the illumination system 105 includes a radiation source that produces a pulsed radiation beam 110 and directs it to a photolithography exposure apparatus 115 such as a scanner that patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125 constructed to hold wafer 120 and connected to a positioner 130 configured to accurately position the wafer 120 in accordance with certain parameters.

[0042] The pulsed radiation beam 110 may have a wavelength in the DUV range, for example, with a wavelength of 248 nm or 193 nm. The scanner 115 includes an optical arrangement 135 having, for example, one or more condenser lenses, a mask, and an objective arrangement. The objective arrangement includes a projection lens and enables an image transfer to occur from the mask to photoresist on the wafer 120. The illumination system 105 adjusts the range of angles for the pulsed radiation beam 110 impinging on the mask.

[0043] The scanner 115 can include, among other features, a lithography controller 140 that controls how layers are printed on the wafer 120. The lithography controller 140 may include a memory that stores information such as process recipes that determine the parameters of the beam including a length of the exposure on the wafer 120 based on, for example, the mask used, as well as other factors that affect exposure. During lithography, a burst of pulses of the pulsed radiation beam 110 illuminates the same area of the wafer 120 to constitute an illumination dose.

[0044] The photolithography system 100 also preferably includes a control system 145. In general, the control system 145 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 145 can be centralized or be partially or wholly distributed throughout the photolithography system 100.

[0045] FIG. 2 shows a pulsed laser source that produces a pulsed laser beam as the radiation beam 110 as an example of an illumination system 105. FIG. 2 shows a two-chamber laser system as a nonlimiting example but it will be understood that the principles explained herein are equally applicable to a single chamber laser system or a laser system having more than two chambers. The gas discharge laser system may include, e.g., a solid state or gas discharge master oscillator (“MO”) seed laser system 200, an amplification stage, e.g., a power ring amplifier (“PRA”) stage 205, relay optics 210, and laser system output subsystem 215. The seed system 200 may include, e.g., an MO chamber 220 which includes a pair of electrodes 222 and 224.

[0046] The MO seed laser system 200 may also include a master oscillator output coupler(“MO OC”) 230, which may comprise a partially reflective mirror, forming an MO discharge chamber 220 with an oscillator cavity, defined in part by a reflective grating (not shown) in a line narrowing module (“LNM”) 235, that oscillates to form the seed laser output pulse. The MO seed laser system 200 may also include a line-center analysis module (“LAM”) 240. A MO wavefront engineering box (“WEB”) 245 may serve to redirect the output of the MO seed laser system 200 toward the amplification stage 205, and may include, e g., a multi prism beam expander (not shown) and an optical delay path (not shown). The beam path through the LNM 235, the MO discharge chamber 220, the MO OC 230, and the LAM 240 defines an optical axis 237 for each of these components.

[0047] The amplification stage 205 may include, e g., a PRA discharge chamber 250, which also may be an oscillator, e.g., formed by seed beam injection and output coupling optics (not shown) that may be incorporated into a PRA WEB 255. The beam may be redirected back through the gain medium in the PRA discharge chamber 250 by a beam reverser (“BR”) 260. The PRA WEB 255 mayincorporate a partially reflective input / output coupler (not shown) and a maximally reflective mirror for the nominal operating wavelength (e.g., at around 193 nm for an ArF system) and one or more prisms. The PRA discharge chamber 250 may also include a pair of electrodes 252 and 254.

[0048] A bandwidth analysis module (“BAM”) 265 may receive the output laser radiation beam of pulses from PRA discharge chamber 250 and pick off a portion of the radiation beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser output radiation beam of pulses then passes through the PRA WEB 255 to an optical pulse stretcher (“OPuS”) 270 and an autoshutter, in this case a combined autoshutter metrology module (“CASMM”) 275, which may also be the location of a pulse energy meter. One purpose of the OPuS 270 may be, e.g., to convert a single output laser pulse into a pulse train. Secondary pulses created from the original single output pulse may be delayed with respect to each other. By distributing the original laser pulse energy into a train of secondary pulses, the effective pulse length of the laser can be expanded and at the same time the peak pulse intensity reduced. The OPuS 270 may accordingly be arranged to receive the laser beam from the PRA WEB 255 and direct its output to the CASMM 275.

[0049] The beam path through the BR 260, the PA discharge chamber 250, and the BAM 265 defines an optical axis 267 for each of these components.

[0050] The PRA discharge chamber 250 and the MO discharge chamber 220 are configured as chambers in which electrical discharges between the electrodes cause lasing gas discharges in a lasing gas to create an inverted population of high energy molecules or excimers, including, e.g., ArF, KrF, F2, XeF, and / or XeCl, to produce relatively broad band radiation that may be line narrowed to a relatively very narrow bandwidth and center wavelength selected in the LNM 235.

[0051] Turning now to FIG. 3 there is shown a laser discharge chamber 300 which may serve, for example, as the PRA discharge chamber 250 or the MO discharge chamber 220. The chamber 300 may be composed, e.g., of an upper chamber body 305 and a lower chamber body 310, which may, when connected to each other by suitable means, e.g., by bolting, serve to define a chamber interior 315. The upper chamber body 305 and lower chamber body 310 also define a chamber interior vertical wall 320 with the lower chamber body 310 defining a chamber interior horizontal bottom wall 325.

[0052] In this specification including the claims the terms “up,” “down,” “upper,” “lower,”“top,” “bottom,” “vertical,” “horizontal,” and like terms are intended to connote relative orientation only and not any absolute orientation such as orientation with respect to gravity unless otherwise indicated or clear from context.

[0053] Contained within the chamber interior 315 is, e.g., a gas discharge system comprising two elongated (along the X axis in and out of the plane of the figure) opposing electrodes, a cathode 330 and an anode 335, defining between them an elongated gas discharge gap or region 340, wherein, in response to a sufficient voltage being imposed across the cathode 330 and anode 335, a discharge occurs in the gas discharge region 340 resulting in the production of radiation at or near a characteristic center wavelength, that is optically directed along the optical axis of the output laser radiation pulsegenerally aligned to the longitudinal optical axis of the laser discharge chamber 300 along the X axis (in and out of the plane of the figure) as indicated by the inset.

[0054] Also within the chamber interior 315 maybe, e.g., an anode support bar 345. The anode335 may be electrically connected to the upper chamber body 305 through a plurality of current returns 306, with the upper chamber body 305, along with lower chamber body 310, being kept at a common voltage, e.g., at ground voltage.

[0055] The cathode 330 may, e.g., be connected to an electrical discharge high voltage feed through an assembly 350, e.g., by a high voltage feed through 355, which passes through a main insulator 360. The main insulator 360 may keep the cathode 330 electrically isolated from the upper chamber body 305. Also within the chamber interior 315 may be, e.g., a preionizer 365, which may include, for example, a preionizer tube, adjacent to the cathode 330. The preionizer 365 may be configured as an elongated hollow tube made of dielectric material that is aligned parallel to the discharge electrodes and positioned near the discharge region 340. A conductive preionization electrode (typically made of copper or brass), is positioned in the bore of the tube and used to create a potential difference between the preionization electrode and one of the main discharge electrodes. This potential difference extends across the dielectric tube radially and results in a substantially uniform emission of photons that are emitted from the outer surface of the tube.

[0056] More information about preionizers may be found in U.S. Patent No. 7,542,502, titled “Thermal -expansion Tolerant, Preionizer Electrode for a Gas Discharge Laser,”, issued June 2, 2009.

[0057] All patent applications, patents, and printed publications cited herein are incorporated herein by reference in their entireties, except for any definitions, subject matter disclaimers, or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.

[0058] Also within the chamber interior 315 may be a gas circulation system comprising a gas circulation fan 370, which may be, e.g., a generally cylindrical crossflow fan 370. The fan 370 serves to move gas within the chamber interior 315, generally in a circular fashion as seen in the cross-sectional view of FIG. 3, in order to remove gas, that contains ionized particles and debris and is depleted of F2, from the discharge region 340 between successive discharges, and thus to replenish the discharge region 340 with fresh gas before the next gas discharge. The gas circulation system may also include a plurality of heat exchangers 375 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 370.

[0059] The gas circulation system may also have a plurality of curved baffles 380 and a flow directing vane 385, which may serve to shape the generally circular gas flow path out of the discharge region 340 toward the heat exchangers 375 and ultimately the intake of the fan 370, and from the output of the fan 370 to the discharge region 340, respectively. The upper chamber body 305 may also have an attached metal fluoride trap 390 in fluid communication with the chamber interior 315.

[0060] FIG. 4 is a schematic view that further illustrates how the electrodes and adjacent components are situated in the gas discharge laser chamber. As shown in FIG. 4, anode 335 and cathode 330 are disposed in an opposing relationship so that elongated cathode discharge surface 332 faces elongated discharge anode surface 337. Anode 335 is mounted on an anode support bar 345. In one example, the length L (see FIG. 5) of each of the anode 335 and the cathode may for example lie in a range of about 20 inches to 30 inches. As noted above, the width of the discharge area and the length of the electrodes may be varied to suit the needs of particular applications. Also visible in FIG. 4 is preionizer 365.

[0061] FIG. 5 is a perspective view of an example of the arrangement of components in the laser discharge chamber 300. As can be seen in FIG. 5, the elongate cathode discharge surface 332 of the cathode 330 faces an elongate anode discharge surface 337 of the anode 335 across a gap defining the discharge region 340. When a discharge occurs in the discharge region 340, radiation generated by the discharge propagates in the direction indicated by arrow 400. Also shown is an optical axis 410 for the laser discharge chamber 300. As depicted, the optical axis 410 maybe regarded as extending parallel to an X axis of a right-handed cartesian coordinate system. The elongate cathode 330, the elongate anode 335, and the gap defining the discharge region 340 all extend parallel to the optical axis 410. Also visible in FIG. 5 is preionizer 365 which also extends parallel to the optical axis 410. FIG. 5 also shows in phantom the main insulator 360 which also extends parallel to the optical axis 410. Also visible in FIG. 5 is the anode support bar 345 which also extends parallel to the optical axis 410.

[0062] The discharge occurring in the discharge region 340 generates acoustic waves inside the discharge chamber 300. The acoustic waves generated by the discharge propagate outward from the discharge region 340, reflect off of internal surfaces of the laser discharge chamber 300, and then return to the discharge region 340 where they distort the laser beam produced by subsequent pulses.

[0063] One measure that may be employed to mitigate the adverse effects of these acoustic disturbances involves arranging acoustic baffles on the internal surfaces of the walls of the discharge chamber. As a result acoustic waves that are reflected by the walls do not return to the discharge region simultaneously in a coherent fashion. Rather the baffles randomize, i.e., scatter, the acoustic reflections. This mitigates resonance reflections because the return waves arrive at different parts of the discharge region at different times, reducing their overall adverse effect on laser performance. This measure is in general effective to suppress resonances at 4 kHz and below, but at higher repetition rates the reflecting surfaces that affect performance most significantly are located closer to, and above and below, the discharge region, rather than located at a distance laterally displaced at a greater distance from the discharge region.

[0064] The potential for distortion is thus the most significant when the acoustic wave returns at a time coinciding with the start of a new discharge. Thus the amount of distortion in general depends on the relationship between the repetition rate of the pulses and the round trip time-of-flight distance of acoustic waves travelling at the speed of sound from the discharge area to reflecting surfaces within thelaser discharge chamber and back again. At higher repetition rates, for example, in the range of 5.8 kHz to 6 kHz, the reflecting surfaces more proximate to the discharge region dominate the resonant acoustic distortion effect. These include surfaces above and below the discharge region such as surfaces of the preionizer, the main insulator, and the anode support bar facing the discharge region.

[0065] As mentioned, the preionizer is characteristically configured as an electrode arranged in a cylindrical tube made of a dielectric material such as a ceramic material. The preionizer tube typically has a smooth external surface that gives rise to the type of coherent acoustic reflections that can lead to resonances in laser performance characteristics (e g. bandwidth) at higher repetition rates, e.g., repetition rates near or in excess of 6kHz.

[0066] In accordance with an aspect of an embodiment, at least part of the surface of the preionizer tube is engineered or modulated by providing it with acoustic scattering features that function as acoustic baffles that interact with acoustic waves impinging on the surface by scattering the acoustic waves, essentially randomizing the reflected waves so they do not return to the discharge region in a coherent fashion. The acoustic scattering features are provided on at least the part of the exterior surface of the preionizer tube configured to be arranged to face the discharge region although it will be understood that the acoustic scattering features may also be provided on other parts, including the entirety, of the exterior surface of the tube. In the specification, including the claims, the surface on which the acoustic scattering features are provided is referred to as the acoustic scattering surface. Herein, “provided with” is intended to refer to processes such as molding that create the acoustic scattering features during manufacture of the preionizer tube as well as processes that add the acoustic scattering features to the wall / exterior surface of the preionizer tube after manufacture of the preionizer tube.

[0067] The acoustic scattering features can be created by any one of a number of possible methods. For example, the acoustic scattering features may be implemented by molding the material making up the preionizer tube so that its surface includes the acoustic scattering features. As another example, the acoustic scattering features may be machined out of the exterior of the wall of the preionizer tube using a cutting tool . Other methods including creating a layer of material on the exterior wall of the preionizer tube where the layer is provided with the acoustic scattering features may be used.

[0068] It will be appreciated by one of ordinary skill in the art that various parameters relating to the acoustic scattering features such as their shape, depth, area, distribution (including spacing), percent surface coverage, and density (number of features per unit area) may be varied according to the considerations of a particular application. For example, regarding depth, it may be assumed that the range of acoustic wavelengths of interest is defined by the physical size (width and height) of the cross section of the laser discharge. Effects from acoustic waves having wavelengths much smaller than the discharge cross section can be assumed to average to zero. In addition, acoustic waves having wavelengths much larger than the discharge cross section can be assumed to create a pressure gradient that is insufficient to have a significant adverse effect on laser performance.

[0069] FIG 6A is a not-to-scale cross section of part of the length of a preionizer tube wall368 and electrode 367 with respect to a discharge region 340. FIG. 6B is an end-on view of the preionizer tube wall 368 and electrode 367 of FIG. 6A. A typical discharge in the discharge region 340 in an ArF excimer laser may have a cross section that is approximately 2 mm (0.079”) high (dimension H in FIG. 6B) by 14 mm wide (0.551”) (dimension W in FIG. 6B). Applying the acoustic quarter wavelength rule, the acoustic scattering feature should be at least as large as one-quarter of the acoustic wavelengths in this range and so an exemplary range for a depth of the acoustic scattering features is about 0.02” to about 0.14”.

[0070] FIG. 7 is a cross section of a lengthwise portion of the preionizer tube wall 368 with an acoustic scattering feature 500. As shown in FIG. 7, as well as in FIG. 6B, the preionizer tube wall 368 may have a nominal thickness, for example, in a range of about 0.150 inches to 0.3 inches. Herein, nominal thickness refers to the thickness in areas having no acoustic scattering features. This is shown as dimension T in FIG. 6B and FIG. 7. A given application may require that the preionizer tube wall thickness at all locations must be, for example, at least 0.125 inches. For a preionizer tube wall with a nominal thickness T of 0.190 inches, for example, the maximum depth (extent F beneath the exterior surface 369 of, and into, the preionizer tube wall 368) of the acoustic scattering feature 500 is not greater than 0.065 inches if a minimum thickness M of 0.125 inches is to be preserved. This depth is more than enough to accommodate effective acoustic scattering features based on the design considerations described above. In some embodiments the acoustic scattering features have a depth into the preionizer tube wall 368 greater than one half the nominal thickness of the preionizer tube wall 368.

[0071] It will be noted that the cross section of the example of an acoustic scattering feature500 as depicted in FIG. 7 is semicircular, with the acoustic scattering feature 500 itself being realized as a hemispherical depression or cavity in the surface of, and into the preionizer tube wall 368. It will be appreciated that the acoustic scattering feature of other shapes may be used as described more fully below.

[0072] FIG 8 is a projection of a portion of the exterior surface 369 of the preionizer tube wall368 provided with acoustic scattering features 500 to create an acoustic scattering surface 510. In general the acoustic scattering surface 510 is provided on at least that portion of the exterior surface369 that will be arranged to face the discharge region 340 (FIG. 5) and so is positioned to receive and reflect acoustic waves from discharges in the discharge region 340. It may in some circumstances be desirable based on considerations such as ease of manufacturing, installation, and maintenance to provide the entire exterior surface 369 with acoustic scattering features 500.

[0073] In the embodiment depicted in FIG. 8 the acoustic scattering features 500 are arranged in aperiodic array with an edge-to-edge spacing of the acoustic scattering features 500 with their nearest neighbor being labeled S and a size of each of the acoustic scattering features 500 being labeled D. In general it is desirable to make S small because the greater S is then the greater the amount of flat surface will be available to reflect acoustic waves. In some embodiments S will preferably be in a range of 0.1inches to 0.5 inches. The range for the dimension D will in general coincide with the range defined above as being relevant to acoustic reflections. Thus D in some embodiments will preferably be in a range of 0.1 inches to 0.3 inches. In some embodiments S and D are selected to cover a percentage of the acoustic scattering surface 510 in a range of 10 percent to 100 percent. In some embodiments the acoustic scattering features may have a density in a range of 3 acoustic scattering features per square inch to 100 acoustic scattering features per square inch.

[0074] In the embodiment of FIG. 8 the acoustic scattering features 500 are again shown as hemispherical depressions or cavities. It will again be appreciated that other shapes may be used. Also, in the embodiment of FIG. 8 all of the acoustic scattering features 500 are of the same size and shape. One of ordinary skill will appreciate that the acoustic scattering features 500 may be of different sizes and shapes.

[0075] In the embodiment of FIG. 8 the acoustic scattering features 500 are arranged in a regular array. The acoustic scattering features 500 may alternatively be positioned randomly in an irregular or aperiodic array as shown in FIG. 9. In the embodiment of FIG. 9 the acoustic scattering features 500 are once again shown as hemispherical depressions or cavities. It will again be appreciated that other shapes may be used. Also, in the embodiment of FIG. 9 all of the acoustic scattering features 500 are again all of the same size and shape. One of ordinary skill will appreciate that at least some of the acoustic scattering features 500 may differ from others in size and shape.

[0076] One of ordinary skill in the art will appreciate that the acoustic scattering features may be created or placed on the preionizer tube in arrangements other than those just described as long as the acoustic scattering features serve the purpose of redirecting acoustic waves in directions other than coherently back towards the part of the discharge from which they originated. For example, the acoustic scattering features may be arranged as a linear series of baffles along the longitudinal axis of the preionizer tube. For example, FIG. 10 is a side view ofa preionizer 365 with acoustic scattering features 500 arranged so that a portion of the surface extending in the axial direction has series of triangular features with one side of the triangle extending in a radial direction with respect to the preionizer 365. FIG. 11 is a perspective view of the preionizer tube 365 of FIG. 10.

[0077] The shape of the acoustic scattering features 500 may also vary. FIG. 12A is a magnified view of the portion of the preionizer tube 366 within dotted box A in FIG. 11. As noted above, the acoustic scattering features 500 are implemented as a line of triangular features with one side of the triangle being arranged parallel to a radius of the preionizer tube of preionizer 366. FIG. 12B shows another arrangement of acoustic scattering features 500 in which each of the triangles has an outwardly pointing vertex at an angle less than 45 degrees and both sides of the triangle are oriented at an angle with respect to the radial direction of the preionizer tube. FIG. 12C depicts an embodiment in which the acoustic scattering features 500 have a rounded shape projecting out from the surface of the preionizer tube. FIG. 12D depicts an embodiment in which each of the acoustic scattering features has a substantially semicircular projection. One of ordinary skill in the art will readily appreciate that othershapes may be used. Also, in the embodiments just described, the acoustic scattering features 500 are all of the same general shape and dimensions. One of ordinary skill in the art will readily appreciate that some of the acoustic scattering features 500 may have shapes and dimensions that are different from the shapes and dimensions of others of the acoustic scattering features 500.

[0078] According to another aspect of an embodiment a portion of the surface of the electrode adjacent the preionizer tube is provided with acoustic scattering features. In some embodiments the acoustic scattering features are configured and arranged to diffuse the acoustic pressure disturbances to make them more uniform. This is achieved by shape modulation (engineering) of the surface of the preionizer tube, shape modulation of the portion of the electrode surfaces adjacent the preionizer tube, or shape modulation of both.

[0079] Thus, instead of an arrangement in which the electrode surfaces are flat or planar, and so in which the acoustic pressure front is permitted to reflect straight back towards the discharge region with the same phase, the electrode surfaces are modulated to disturb the phase of the reflected acoustic wave or redirect it at small angle along the electrode rather than perpendicular to the electrodes. In such an arrangement the multiple pressure front reflections will tend to average themselves out, leading to more uniform distribution of the refractive index in the discharge region.

[0080] Modulation of the shape of the surface of the preionizer tube can take several forms.For example, as described more fully below, all or a portion of the length of the preionizer tube can have a dual diameter cross section. Also, as described above, all or a portion of the length of the preionizer tube can be provided with a sawtooth structure. This sawtooth structure can be placed symmetrically around the circumference of the preionizer tube or placed on only part of the surface of the preionizer tube, e.g., just in the comer of the preionizer tube where it interfaces with the cathode at, for example, the cathode shoulder. Any of the arrangements described above for the acoustic scattering surface of the preionization tube may be used.

[0081] Modulation of the shape of the electrode, e.g., the cathode and, in particular, the cathode shoulder can also take several forms. In the case of the cathode shoulder shape modulation, a rounded sawtooth modulation with variable period and height of the teeth (to improve dephasing of the reflections) can be used. In some implementations the radius of curvature of the sawtooth is selected to prevent electric field localization, leading to parasitic or non-uniform discharge in the discharge region. These structures may be created by machining the cathode shoulder in a straightforward way.

[0082] Turning to specific examples, FIG. 13A is a side view of a preionization tube 367 having a shaped surface including a first group of circular segments 520 having a first diameter interspersed with a second group of circular segments 522 having a second diameter less than the first diameter. In the embodiment shown, the segments are arranged with even spacing but one of ordinary skill in the art will appreciate that variable spacing may be used. One of ordinary skill in the art will also appreciate that other circular segments having other diameters may also be interspersed with the circular segments shown.

[0083] FIG. 13B is a side view of another configuration for a shaped surface of a preionization tube 369. FIG. 13B shows a preionization tube 369 having circular protuberances 530. In the arrangement shown in FIG. 13B the protuberances 530 are arranged in a regular array but one of ordinary skill in the art will appreciate that the protuberances may be arranged in an irregular distribution as well. Protuberances such as protuberance 532 may have a cylindrical cross section as shown. Protuberances may have a circular cross sections such as protuberance 534 as shown. Although the protuberances 530 are shown as being all the same size one of ordinary skill in the art will appreciate that protuberances of varying sizes may be used.

[0084] The shape modulation for the surface of the preionization tube may also be implemented as repeated groups of rounded sawtooth ridges as shown in FIG. 14A. The ridges in FIG. 14A may include a ridge 610 having a height Hi and a width Li, a ridge 620 with a height H2 and a width L2, a ridge 630 having a height Hi and a width Li, and a ridge 640 having a height H4 and a width L4. Ridges 630 and 620 are repeated in the group. The group can be repeated as desired to cover as much of the surface of the preionization tube as desired.

[0085] In the arrangement shown, the heights are caused to vary essentially sinusoidally as a function of the lateral position of the ridge in the group . When these ridges are placed on a preionization tube, their heights and widths may vary sinusoidally as a function of distance from the end of the preionization tube. Here and elsewhere, “placed on” encompasses arrangements in which the surface modulation formed from the surface, for example by machining, in addition to arrangements in which the surface modulation is affixed to the surface. As shown in FIG. 14B, the radii of curvature of the ridges 610, 620, 630, and 640, i.e., Ri, R2, Ri, and R , respectively, may also be varied. In some implementations, however, it will be desirable to ensure that the radius of curvature does not become so small as to promote undesirable arching.

[0086] The arrangements shown in FIGS. 14A and 14B are made up of a group having six ridges but one of ordinary skill in the art will appreciate that groups having fewer or more ridges may be used.

[0087] According to another aspect of an embodiment, the cathode may also be provided with one or more surfaces having a modulated shape to control acoustic reflections. One such arrangement is shown in FIG. 15 which shows a cathode 330 having a discharge surface 332 confronting a discharge region 340. The cathode 330 also has a shoulder 700 between cathode surface 710 and cathode surface 720. Also shown in FIG. 15 are the preionizer 365 and the insulators 360. As can be seen, the surfaces 710 and 720 and the shoulder 700 are provided with surface modulation structures 730.

[0088] FIG 16A is a perspective view of the cathode 330 showing the cathode surfaces 710 and 720 as well as cathode shoulder 700 and the discharge surface 332. As can be seen, the cathode 330 extends in a first direction as indicated by the arrow A pointing in the positive X direction in the orientation shown in the figure. The rounded sawtooth surface modulation of FIGS. 14A and 14B can be provided on one or both of the surfaces 710 and 720 as well as on the cathode shoulder 700. Thus,according to an aspect of an embodiment one or both of cathode surface 710 and cathode surface 720 are at least partially provided with acoustic scattering features. FIG. 16B is a perspective view of the cathode 330 showing the cathode surfaces 710 and 720 as well as cathode shoulder 700 and the discharge surface 332 showing a relative position with respect to preionizer 365.

[0089] FIG. 17 is a rendering of portion 750 of the surface 710 or 720 (FIG. 16A) or of the preionizer 365 (FIG. 16B) according to an aspect of an embodiment. A possible placement ofthe portion 750 is shown by the dashed boxes in FIGS. 16A and 16B. As can be seen in FIG. 17, the portion 750 in the example depicted in FIG. 17 is made up of a grouping of rounded sawtooth ridges with varying heights and widths. It will be understood that this configuration may be used together with any of the other engineered modulating surface structures. In particular, a cathode having surfaces such as cathode surfaces 710 and 720 provided with the rounded sawtooth surface modulation of FIGS. 14A and 14B can be used in conjunction with the preionizer 367 of FIG. 13A or the preionizer 369 of FIG. 13B.

[0090] It will be appreciated that acoustic scattering features on the surface of the preionizer tube wall and / or the cathode comprise engineered surfaces having local deviations from two dimensional planarity so that the acoustic scattering features have height and / or depth and can equivalently be regarded extending into and out of the preionizer tube wall or into and out of the preionizer tube wall surface. The same is true with respect to the cathode.

[0091] Some of the above description is in terms of functional block diagrams with some functions allocated to some blocks and other functions allocated to other blocks. It will be understood that the division between blocks and the allocations are arbitrary and that different divisions and allocations are possible so long as the overall functions are carried out as described above.

[0092] The above description includes examples of multiple embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for each of these embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of elements ofthe various embodiments are possible based on the disclosure. Accordingly, the described embodiments are intended to be representative of and encompass all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

[0093] Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. Also, although elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and / or embodiment may be utilized with all or a portion of any other aspect and / or embodiment, unless stated otherwise.

[0094] Features, materials, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example are to be understood to be applicable to any other aspect, embodiment or example described in this section or elsewhere in this specification unless incompatible therewith.All of the features disclosed in this specification (including any accompanying claims, abstract, and drawings may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The protection is not restricted to the details of any foregoing embodiments. The protection extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0095] Furthermore, certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations, one or more features from a claimed combination can, in some cases, be excised from the combination, and the combination may be claimed as a subcombination or variation of a subcombination.

[0096] Furthermore, the features and attributes of the specific embodiments disclosed above may be combined in different ways to form additional embodiments, all of which fall within the scope of the present disclosure. Also, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products.

[0097] For purposes of this disclosure, certain aspects, advantages, and novel features are described herein. Not necessarily all such advantages may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the disclosure may be embodied or carried out in a manner that achieves one advantage or a group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.

[0098] Conditional language, such as “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and / or steps. Thus, such conditional language is not generally intended to imply that features, elements, and / or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, and / or steps are included or are to be performed in any particular embodiment.

[0099] Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z. Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one ofZ.

[0100] Language of degree used herein, such as the terms “approximately,” “about,” “generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately”, “about”, “generally,” and “substantially” may refer to an amount that is within less than 10% of, within less than 5 % of, within less than 1% of, within less than 0.1 % of, and within less than 0.01 % ofthe stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by less than or equal to 15 degrees, 10 degrees, 5 degrees, 3 degrees, 1 degree, or 0. 1 degree.

[0101] The scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments in this section or elsewhere in this specification, and may be defined by claims as presented in this section or elsewhere in this specification or as presented in the future. The language of the claims is to be interpreted broadly based on the language employed in the claims and not limited to the examples described in the present specification or during the prosecution of the application, which examples are to be construed as nonexclusive.

[0102] The implementations can be further described using the following clauses.1. A discharge system for a discharge chamber of a laser, the discharge system comprising: an electrode having surface, the surface being elongated in a first direction and configured to extend parallel and adjacent to a discharge region of the discharge chamber; and a plurality of electrode acoustic scattering features disposed on the surface.2. The discharge system as in clause 1 wherein the plurality of electrode acoustic scattering features comprises a plurality of rounded sawtooth ridges each extending from the surface and arranged in a row extending in the first direction.3. The discharge system as in clause 2 wherein respective ones of the plurality of rounded sawtooth ridges extend from the surface to a height with a magnitude varying in dependence on a distance of the rounded sawtooth ridge from an end of the row in the first direction.4. The discharge system of clause 3 wherein the height varies sinusoidally in dependence on the distance of the rounded sawtooth ridge from the end of the row in the first direction.5. The discharge system as in clause 2 wherein respective ones of the plurality of rounded sawtooth ridges have a width in the first direction with a magnitude varying depending on a distance of the rounded sawtooth ridge from an end of the row in the first direction.6. The discharge system of clause 5 wherein the width varies sinusoidally in dependence on the distance of the rounded sawtooth ridge from the end of the row in the first direction.7. The discharge system as in clause 2 wherein a rounded portion of respective ones of the plurality of rounded sawtooth ridges have a radius of curvature with a magnitude varying depending on a distance of the rounded sawtooth ridge from an end of the row in the first direction.8. The discharge system of clause 7 wherein the radius of curvature varies sinusoidally in dependence on the distance of the rounded sawtooth ridge from the end of the row in the first direction.9. The discharge system as in clause 1 further comprising a preionizer configured as a generally cylindrical hollow tube extending axially in the first direction and having a tube wall with an outer surface, at least a portion of the outer surface being provided with a plurality of preionizer acoustic scattering features.10. The discharge system as in clause 9, wherein the tube comprises a dielectric material.11. The discharge system as in clause 9 wherein the laser is configured to generate a discharge in the discharge region and wherein the preionizer acoustic scattering features are provided on at least a portion of the outer surface arranged to face the discharge area.12. The discharge system as in clause 9 wherein the preionizer acoustic scattering features are provided on substantially all of the outer surface.13. A discharge chamber for a laser system, the discharge chamber comprising: a first electrode having a first electrode discharge surface; a second electrode having a second electrode discharge surface, the first electrode discharge surface being arranged facing and spaced away from the second electrode discharge surface to define a discharge gap; and a preionizer including a preionizer tube extending laterally adjacent the first electrode, a surface of the preionizer tube facing the discharge gap being provided with a plurality of preionizer acoustic scattering features dimensioned and arranged to scatter acoustic waves from the discharge gap, and the first electrode having a shoulder region adjacent to the preionizer and provided with first electrode acoustic scattering features dimensioned and arranged to scatter acoustic waves from the discharge gap-14. A preionizer for a discharge chamber of a laser system, the preionizer comprising: a tube configured as a generally cylindrical hollow tube extending axially in a first direction and having a tube wall with an outer surface, at least a portion of the outer surface being provided with a plurality of preionizer acoustic scattering features, the acoustic scattering features comprising a first plurality of first axial segments each being coaxial with the tube and having a circular cross section with a first radius interspersed with a second plurality of second axial segments each being coaxial with the tube and having a circular cross section with a second radius different from the first radius; and an electrode positioned at least partially within the tube.15. The preionizer as in clause 14, wherein the tube comprises a dielectric material.16. The preionizer as in clause 14 wherein the laser system is configured to generate a discharge in a discharge region and wherein the acoustic scattering features are provided on at least a portion of the outer surface arranged to face the discharge region.17. The preionizer as in clause 14 wherein the acoustic scattering features are provided on substantially all of the outer surface.18. A discharge chamber comprising: a cathode having a cathode shoulder with a modulated shoulder shape; and a preionization tube proximate to the cathode shoulder, the preionization tube having a modulated tube shape having a first section with first diameter extending a first longitudinal distance along the preionization tube and a second section with a second diameter extending a second longitudinal distance along the preionization tube19. The discharge chamber as in clause 18, further comprising a plurality of beads disposed along the preionization tube, the beads and the preionization tube forming the modulated tube shape.20. The discharge chamber as in clause 18, wherein the modulated shoulder shape comprises protrusions of varying height and disposed with varying periodicity along the cathode shoulder.

[0103] The above-described implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A discharge system for a discharge chamber of a laser, the discharge system comprising: an electrode having surface, the surface being elongated in a first direction and configured to extend parallel and adjacent to a discharge region of the discharge chamber; and a plurality of electrode acoustic scattering features disposed on the surface.

2. The discharge system as in claim 1 wherein the plurality of electrode acoustic scattering features comprises a plurality of rounded sawtooth ridges each extending from the surface and arranged in a row extending in the first direction.

3. The discharge system as in claim 2 wherein respective ones of the plurality of rounded sawtooth ridges extend from the surface to a height with a magnitude varying in dependence on a distance of the rounded sawtooth ridge from an end of the row in the first direction.

4. The discharge system of claim 3 wherein the height varies sinusoidally in dependence on the distance of the rounded sawtooth ridge from the end of the row in the first direction.

5. The discharge system as in claim 2 wherein respective ones of the plurality of rounded sawtooth ridges have a width in the first direction with a magnitude varying in dependence on a distance of the rounded sawtooth ridge from an end of the row in the first direction.

6. The discharge system of claim 5 wherein the width varies sinusoidally in dependence on the distance of the rounded sawtooth ridge from the end of the row in the first direction.7 The discharge system as in claim 2 wherein a rounded portion of respective ones of the plurality of rounded sawtooth ridges have a radius of curvature with a magnitude varying in dependence on a distance of the rounded sawtooth ridge from an end of the row in the first direction.

8. The discharge system of claim 7 wherein the radius of curvature varies sinusoidally in dependence on the distance of the rounded sawtooth ridge from the end of the row in the first direction.

9. The discharge system as in claim 1 further comprising a preionizer configured as a generally cylindrical hollow tube extending axially in the first direction and having a tube wall with an outer surface, at least a portion of the outer surface being provided with a plurality of preionizer acoustic scattering features.

10. The discharge system as in claim 9, wherein the tube comprises a dielectric material.

11. The discharge system as in claim 9 wherein the laser is configured to generate a discharge in the discharge region and wherein the preionizer acoustic scattering features are provided on at least a portion of the outer surface arranged to face the discharge area.

12. The discharge system as in claim 9 wherein the preionizer acoustic scattering features are provided on substantially all of the outer surface.

13. A discharge chamber for a laser system, the discharge chamber comprising: a first electrode having a first electrode discharge surface; a second electrode having a second electrode discharge surface, the first electrode discharge surface being arranged facing and spaced away from the second electrode discharge surface to define a discharge gap; and a preionizer including a preionizer tube extending laterally adjacent the first electrode, a surface of the preionizer tube facing the discharge gap being provided with a plurality of preionizer acoustic scattering features, and the first electrode having a shoulder region adjacent to the preionizer and provided with first electrode acoustic scattering features.

14. A preionizer for a discharge chamber of a laser system, the preionizer comprising: a tube configured as a generally cylindrical hollow tube extending axially in a first direction and having a tube wall with an outer surface, at least a portion of the outer surface being provided with a plurality of preionizer acoustic scattering features, the acoustic scattering features comprising a first plurality of first axial segments each being coaxial with the tube and having a circular cross section with a first radius interspersed with a second plurality of second axial segments each being coaxial with the tube and having a circular cross section with a second radius different from the first radius; and an electrode positioned at least partially within the tube.

15. The preionizer as in claim 14, wherein the tube comprises a dielectric material.

16. The preionizer as in claim 14 wherein the laser system is configured to generate a discharge in a discharge region and wherein the acoustic scattering features are provided on at least a portion of the outer surface arranged to face the discharge region.

17. The preionizer as in claim 14 wherein the acoustic scattering features are provided on substantially all of the outer surface.

18. A discharge chamber comprising :a cathode having a cathode shoulder with a modulated shoulder shape; and a preionization tube proximate to the cathode shoulder, the preionization tube having a modulated tube shape having a first section with first diameter extending a first longitudinal distance along the preionization tube and a second section with a second diameter extending a second longitudinal distance along the preionization tube.

19. The discharge chamber as in claim 18, further comprising a plurality of beads disposed along the preionization tube, the beads and the preionization tube forming the modulated tube shape.

20. The discharge chamber as in claim 18, wherein the modulated shoulder shape comprises protrusions of varying height and disposed with varying periodicity along the cathode shoulder.

Images