Abstract:
An DPP EUV source is disclosed which may comprise a debris mitigation apparatus employing a metal halogen gas producing a metal halide from debris exiting the plasma. The EUV source may have a debris shield that may comprise a plurality of curvilinear shield members having inner and outer surfaces connected by light passages aligned to a focal point, which shield members may be alternated with open spaces between them and may have surfaces that form a circle in one axis or rotation and an ellipse in another. The source may have a temperature control mechanism operatively connected to the collector and operative to regulate the temperature of the respective shell members to maintain a temperature related geometry optimizing the glancing angle of incidence reflections from the respective shell members, or a mechanical positioner to position the shell members.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. Ser. No. 10/742,233 filed Dec. 18, 2003, which is a continuation-in-part of U.S. Ser. No. 10/409,254 filed Apr. 8, 2003, now U.S. Pat. No. 6,972,421, which is a continuation-in-part of U.S. Ser. No. 10/384,967 filed Mar. 8, 2003, now U.S. Pat. No. 6,904,073, U.S. Ser. No. 10/189,824 filed Jul. 3, 2002, now U.S. Pat. No. 6,815,700, U.S. Ser. No. 10/120,655 filed Apr. 10, 2002, now U.S. Pat. No. 6,744,060, U.S. Ser. No. 09/875,719 filed Jun. 6, 2001 now U.S. Pat. No. 6,586,757 and U.S. Ser. No. 09/875,721 filed Jun. 6, 2001, now U.S. Pat. No. 6,566,668, U.S. Ser. No. 09/690,084 filed Oct. 16, 2000, now U.S. Pat. No. 6,566,667; and claims the benefit of patent application Ser. Nos. 60/422,808 filed Oct. 31, 2002 and 60/419,805 filed Oct. 18, 2002; all of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to EUV and soft-x-ray light sources utilizing a discharge between electrodes to form the light emitting plasma. 
     BACKGROUND OF THE INVENTION 
     It is well known to produce extreme ultraviolet (“EUV”) light from plasmas created, e.g., by applying a high voltage across electrodes to produce a discharge, e.g., in a gas medium, e.g., containing an active material, e.g., Xenon, to produce light at EUV wavelengths, e.g., for xenon at 13.5 nm (also referred to as soft-x-ray). Such EUV light sources are commonly referred to as discharge produced plasma (“DPP”) EUV (soft-x-ray) light sources. 
     U.S. Pat. No. 5,763,930, issued to Partlo on Jun. 9, 1998, U.S. Pat. No. 6,051,841, issued to Partlo on Apr. 18, 2000, U.S. Pat. No. 6,064,072, issued to Partlo et al. on May 16, 2000, U.S. Pat. No. 6,452,199, issued to Partlo et al. on Sep. 17, 2002, U.S. Pat. No. 6,541,786, issued to Partlo on Apr. 1, 2003, and U.S. Pat. No. 6,586,757, issued to Melnychuck et al. on Jul. 1, 2003, along with pending U.S. applications Ser. Nos. 09/752,818, 10/120,655, entitled PULSE POWER SYSTEM FOR EXTREME ULTRAVIOLET AND X-RAY LIGHT, filed on Apr. 10, 2002, with inventors Ness et al., Published on Nov. 7, 2002, Pub. No. US/2002-0163313-A1, Ser. No. 10/189,824, filed on Jul. 3, 2002, entitled PLASMA FOCUS LIGHT SOURCE WITH IMPROVED PULSE POWER SYSTEM, with inventors Melnychuk et al., published on Jan. 9, 2003, Publication No. US/2003-0006383-A1, Ser. No. 10/384,967, filed on Mar. 8, 2003, entitled HIGH POWER DEEP ULTRAVIOLET LASER WITH LONG LIFE OPTICS, with inventors Yager, et al., Ser. No. 10/409,254, filed on Apr. 8, 2003, entitled EXTREME ULTRAVIOLET LIGHT SOURCE, with inventors Melnychuk et al. all discuss aspects of EUV light sources particularly utilizing DPP to create the plasma producing the light, and the disclosures of each of these are hereby incorporated by reference. 
     Current EUV collection optics consist, e.g., of several nested shells with common focal points, e.g., at some common ambient temperature. Typically these shells are formed, e.g., from nickel, and feature relatively thin walls, e.g., approximately 1 mm thick. A consequence of EUV light generation is high thermal loads on components close to the EUV source point. In the case of optical components, these thermal loads can, e.g., distort critical surfaces shifting focal points. 
     A very efficient manner for transmitting EUV light is, e.g., via “glancing angle of incidence” reflectors. Typically the nested collector shells will feature, e.g., at least two distinct reflecting surfaces, e.g., flat or curved surfaces, enabling light emitted at large angles from a discharge produced plasma to be collected and delivered to an intermediate focal point or plane at a relatively small angle, i.e., numerical aperture. 
     Avoiding distortions and maintaining focus plane or point is an aspect of EUV light source design that can use some improvement. 
     Electrode lifetime is another EUV light source issue that needs attention. Electrode lifetimes of 100M shots at a 10% output degradation are believed to be minimum requirements for a DPP EUV system. Current technology allows for more on the order of less than 30M shots with around the noted degradation. A byproduct of EUV light emission by means of a DPP produced pinched plasma is high thermal loads on the structures and elements in close proximity to the pinch formation. This can lead to several detrimental effects on performance and on component life, e.g., in the case of the central electrode, thermal loads may be so severe that the outer surface of the electrode could excessively erode, e.g., through material vaporization. Erosion eventually forces replacement of the electrode for a number of reasons, including effects on the plasma formation and inability to withstand the pressure of cooling water circulating in the interior of the electrode structure. 
     At this time, EUV electrode lifetimes are an order of magnitude away from lifetime figures quoted by the lithography industry. As such, replacement costs and machine downtime during electrode replacement constitute a large portion of “cost of ownership” for DPP EUV sources. 
     SiC—BN is known to be used in the defense industry as armor plating. SiC doping with BN is common for SiC-graphite systems, e.g., coated fibers with BN. TiW is known to be used for contacts in the semiconductor industry and is a common machined material, e.g., for PVD targets. 
     Another important consideration for DPP EUV light sources is the need to substantially decrease deleterious effects of electrode debris, arising from a discharge produced plasma EUV light source, impinging upon system optics, e.g., the collector optical elements. 
     Another important aspect of DPP EUV light sources is the need to make the most efficient use possible of the energy injected into the DPP apparatus, in order to maximize the light output for a given energy input. Very high energy light output is required and there are limits, e.g., on the ability to deliver very high energy pulses to the discharge electrodes at the required repetition rates, e.g., due to timing and heat dissipation requirements. 
     SUMMARY OF THE INVENTION 
     A DPP EUV source is disclosed which may comprise a debris mitigation apparatus employing a metal halogen gas producing a metal halide from debris exiting the plasma. The EUV source may have a debris shield that may comprise a plurality of curvilinear shield members having inner and outer surfaces connected by light passages aligned to a focal point, which shield members may be alternated with open spaces between them and may have surfaces that form a circle in one axis of rotation and an ellipse in another. The electrodes may be supplied with a discharge pulse shaped to produce a modest current during the axial run out phase of the discharge and a peak occurring during the radial compression phase of the discharge. The light source may comprise a turbomolecular pump having an inlet connected to the generation chamber and operable to preferentially pump more of the source gas than the buffer gas from the chamber. The source may comprise a tuned electrically conductive electrode comprising: a differentially doped ceramic material doped in a first region to at least select electrical conductivity and in a second region at least to select thermal conductivity. The first region may be at or near the outer surface of the electrode structure and the ceramic material may be SiC or alumina and the dopant is BN or a metal oxide, including SiO or TiO 2 . The source may comprise a moveable electrode assembly mount operative to move the electrode assembly mount from a replacement position to an operating position, with the moveable mount on a bellows. The source may have a temperature control mechanism operatively connected to the collector and operative to regulate the temperature of the respective shell members to maintain a temperature related geometry optimizing the glancing angle of incidence reflections from the respective shell members, or a mechanical positioner to position the shell members. The shells may be biased with a voltage. The debris shield may be fabricated using off focus laser radiation. The anode may be cooled with a hollow interior defining two coolant passages or porous metal defining the passages. The debris shield may be formed of pluralities of large, intermediate and small fins attached either to a mounting ring or hub or to each other with interlocking tabs that provide uniform separation and strengthening and do not block any significant amount of light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FlGS.  1  and  1 A show a schematic view of a discharge-produced plasma EUV (soft-x-ray) light source and the major components of an embodiment of such a system; 
         FIG. 2  shows a schematic view of an embodiment of an electrode for DPP EUV light production; 
         FIG. 3  shows an embodiment of a collector system for an EUV light source, adapted to, e.g., collect the light in a cone of emission from a light producing plasma; 
         FIG. 4  shows a schematic cross-sectional view of the grazing angle of incidence operation of the embodiment of a collector shown in  FIG. 3 ; 
         FIG. 5  shows an embodiment of the present invention including an electrode replacement system according to an embodiment of the present invention; 
         FIG. 6  shows a closer view of the embodiment of  FIG. 4 ; 
         FIG. 7  shows the embodiments of  FIGS. 5 and 6  with a gate valve sealing mechanism in place for electrode replacement; 
         FIG. 8  shows a schematic view of a process for fabricating materials useful in electrodes for DPP according to an embodiment of the present invention; 
         FIG. 9  shows a cross-sectional view of a center electrode (anode) according to an embodiment of the present invention; 
         FIG. 10  shows a perspective cut-away view of an electrode assembly according to an embodiment of the present invention; 
         FIG. 11  shows a closer perspective cut-away view of a portion of the electrode assembly shown in  FIG. 10  and the center electrode (anode) shown in  FIG. 9 ; 
         FIG. 12  shows a top view of the electrode assembly shown in  FIGS. 10 and 11 ; 
         FIGS. 12   a - c  show cross-sectional views of the electrode assembly of  FIGS. 10-12  with the cross-sections taken along lines A-A, B-B and C-C in  FIG. 12 , respectively; 
         FIG. 13  shows a cross-sectional view of the electrode assembly of  FIGS. 10-12   c  with a center electrode (anode) assembly included; 
         FIG. 14  shows a cold plate portion of the assembly of  FIGS. 10-13  showing cooling channels according to an embodiment of the present invention; 
         FIG. 15  shows a perspective view of a debris shield according to an embodiment of the present invention; 
         FIG. 16  shows a schematic view of a process for making a debris shield according to an embodiment of the present invention; 
         FIGS. 17A-H  sow another debris shield according to an embodiment of the present invention; and, 
         FIGS. 18A and 18B  show simulations of the generation of a plasma pinch according to aspects of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning now to  FIGS. 1 and 1A  there is shown a discharge produced plasma (“DPP”) EUV and soft-x-ray light source  20  according to an embodiment of the present invention. The EUV light source may include, e.g., a housing  22 , defining a discharge chamber  24 . Attached, e.g., through a sealed opening in one wall of the chamber  22  may be, e.g., a pair of electrodes  26 , which may include, e.g., generally cylindrical electrodes including, e.g., an outer electrode  28 , which may be, e.g., the cathode, and an inner electrode  30 , which may be, e.g., an anode or vice-versa, but for purposes of this disclosure the former designations wilt be used. The inner electrode  30  may be insulated from the outer electrode  28 , e.g., by an insulator  70  as shown in  FIG. 2 , and together, when supplied by a very high voltage and with a very fast rise time pulse of electrical energy, e.g., from a solid stair pulse power module  139 , shown in  FIG. 7 , produce a discharge between the electrodes  28 ,  30 , e.g., through an ionized gas, e.g., comprising helium. The discharge may be facilitated in its initiation by a preionizer  206 , e.g., as shown in  FIGS. 10-12   c . The discharge may, e.g., initially form a magnetic field that extends generally radially from the inner electrode near the insulator  70  and preionizer  206  as shown at  82  in  FIG. 2  and then extend more axially as it transmits along the outer surface  208  of the inner electrode (anode)  30 , as shown schematically at  84  in  FIG. 2 . The axially extending magnetic field  84 , forms a high density plasma pinch  32  confined briefly by the magnetic field  84  comprising a source material, e.g., xenon, e.g., delivered to the pinch site through a source delivery tube  60  and, e.g., delivered into a center electrode (anode) tip depression  34  in the tip of the center electrode  30 . 
     Light emitted from the plasma pinch may, e.g., be collected by, e.g., a grazing angle of incidence collector  40  after passing through, e.g., a debris shield  36  that can, e.g., trap debris, e.g., ionized xenon particles emitted from the plasma during the light generation process or electrode material, e.g., tungsten debris from the electrodes, and potentially damaging, e.g., to the reflecting surface(s) in the collector  40 . The light focused by the collector  40 , which may be, e.g., a single curved surface reflecting rays of the EUV light by grazing angle of incidence reflection to a focal point or plane called the intermediate focus  42 , may also pass through, e.g., a spectral purity filter operative to filter out substantially all of the light except at, e.g., 13.5 nm and a relatively narrow bandwidth around 13.5 nm. 
     An aspect of an embodiment of the present invention contemplates compensating for thermal loads on the collector  40  in order to produce more consistent high EUV energy delivered to the intermediate focus  42 . 
     Turning now to  FIGS. 3 and 4  there is shown a perspective cut-away view of a collector  40  according to an embodiment of the present invention, and also a schematic view of an example of the operation of the collector  40  according to an embodiment of the present invention. As can be seen in  FIG. 4 , a ray tracing of some exemplary limiting rays, each of the shells  102  of the collector  40 , having a first shell portion  102   a  and a second shell portion  102   b , has a limiting ray  104  and a limiting ray  104 ′ and is arranged to reflect the limiting rays  104 ,  104 ′ at a grazing angle of incidence from portion  102   a  to portion  102   b , each of which portions  102   a ,  102   b  could be flat or curved. At portion  102   b  the grazing angle of incidence reflection will focus the light in rays  104 ,  104 ′ toward the intermediate focus  42 . For purposes of this application, the light may be broadband and need to pass through a filter of some sort, e.g., the spectral purity filter  50  shown in  FIG. 1 . As can be seen from  FIG. 4 , there is very little real estate, i.e., physical space, available to bolster the thickness of individual shells  102 , including their component parts  102   a ,  102   b . To do so, would impede the transmission of the adjacent, e.g., next outer shell. Modifying shell geometry to allow thicker walls increases the respective glancing angles, thereby reducing transmission efficiency of the design. Some rays of light  104 ″ and  104 ″′ for example, do not enter the cone of entrance of the collector  40 , or do not do so at an appropriate grazing angle of incidence, usually less that about 2°, depending on the wavelength λ of the emitted light and the material of the reflecting surface, and are, therefore, not collected by the collector. 
     The collector  40 , as shown in  FIG. 3  can be composed of a plurality of nested shells  102 , each smaller in diameter than the other from outside in. The shells may be made up of a plurality of portions, e.g., two portions  102   a  and  102   b , with portion  102   a  closest to the pinch site  32 . Each shell  102  portion  102   a  may, e.g., be angled to reflect the light rays in a portion of the cone of incidence of the plasma generated light that is incident on the collector  40  shells  102  and to reflect that light to the portion  102   b . At the portion  102   b  a further grazing angle of incidence reflection can occur, which can, e.g., reflect the incident EUV light at an angle focused at the intermediate focus  42 . 
     The shells  102  can, e.g., be mounted to a collector hub  90  which may have, e.g., collector hub extensions  92  extending from the hub  90  along the axial length of the collector  40 . Also attached to the hub  90  may be a plurality, e.g. four radial struts  94 . Each of the shells  102  may be connected to the struts  94 , e.g., by welding or brazing. The structure of the collector  40  and the mountings of the shells  102  to the struts  94  may be reinforced by a radial collector fairing  100 . 
     According to an aspect of an embodiment of the present invention a maximum heat load that the collector  40  is likely to be expected to see can be derived. The geometry of the collector  40  and its constituent shells  102  and their portions  1 O 2   a ,  102   b  can be created so that, an aspect of desired performance, e.g., focus is, e.g., achieved solely at this temperature. That is, at some known preselected temperature there will be a known geometry of the parts of the collector, resulting in a desired operating parameter, e.g., focus at the intermediate focus  42 , selection of a particular λ, etc. Heating elements (e.g., temperature control mechanism  14  of  FIG. 1A ) can be attached to individual shells  102  of the collector  40 , or, e.g., to the hub  90  and/or its extensions  92 , and can, e.g., be utilized to maintain this ideal geometry regardless of duty cycle or repetition rate, which otherwise could cause the temperature of the collector  40  to vary over time, such varying temperature can, e.g. warp the shell portions  102   a ,  102   b  and/or modify their positional relationship to each other. According to another aspect of an embodiment of the present invention, cooling (e.g., temperature control mechanism  16  of  FIG. 1A ) could be used to maintain the desired fixed temperature, e.g., with the inclusion of Peltier coolers (not shown), rather than heater element, e.g., a model Drift 0.8 (40 mm sq.) 172 watt made by Kryotherm. Shell geometry can thereby be certified at a non-elevated temperature. 
     In either event, the collector shells  102  may be equipped, e.g., with biomorph piezoelectric actuators such as a model PL122-140 series made by Physik Instrumente, which may be, e.g., bonded, e.g., by brazing, to the exterior surfaces of each shell portion  102   a ,  102   b . Applying a voltage to the piezoelectric actuators can, e.g., distort the shell portion  102   a ,  102   b , effectively altering the focal point of the shell  102 , e.g., to the intermediate focus  42 . 
     According to an aspect of an embodiment of the present invention each shell  102  may, e.g., have two discrete parts  102   a ,  102   b , each having its own curvature and/or angular relationship to the other and to the hub  90 . Focus can, e.g., be maintained by altering the relationship between the two halves  102   a ,  102   b  along the optical axis. This may be done, e.g., using positioning motors (not shown) or piezoelectric elements (not shown), dependent, e.g., upon degree of motion requirements. The motors or piezoelectric elements may be mounted, e.g., external to the vacuum environment with manipulators (not shown) linked, e.g., via a bellows (not shown) to the shells  102 . The shells  102 , may in turn, be interconnected, e.g., at the joints  106 , e.g., by a thin connecting member, which does not block a significant amount of the light transmitting the collector, such that, e.g., manipulation of, e.g., the joint  106  on the outermost shell  102 , as with an actuator as noted above, can serve to manipulate all of the shells  102  at the same time. 
     According to another aspect of an embodiment of the present invention, enabling rapid electrode replacement can leverage electrode lifetime as a “cost of ownership” issue. This may be accomplished, e.g., utilizing a quick electrode replacement assembly as shown in  FIGS. 5-7 . 
     According to another embodiment of the present invention, the shells  102  may be connected to a bias voltage (not shown) to deflect charged ions of the same polarity as the bias voltage from, e.g., the reflecting surface(s) of the shell  102  and, e.g., toward the roughened surfaces, for debris collection. 
     At this time, EUV electrode lifetimes are an order of magnitude away from lifetime figures quoted by the lithography industry. As such, replacement costs and machine downtime during electrode replacement constitute a large portion of “cost of ownership” for DPP EUV sources. The electrode  26  is located within a large vacuum chamber  24  that also may house, e.g., collection optics  40 , spectral purity filters  50 , debris traps  32 , etc. Breaking the seal on the vacuum chamber  24 , e.g., to access the electrode  26 , can, e.g., expose the interior environment of the vacuum chamber  24  to ambient room conditions, e.g., humidity, lack of cleanliness, etc. Upon resealing the vacuum chamber  24 , the time to pump down to operating vacuum conditions can adversely impact overall performance (it contributes to the cost of ownership) and may also be difficult due to debris and water vapor attached to the interior walls of chamber due to the exposure to the external environment. 
     Even in a perfect environment, pump down time is on the order of 5-10 minutes given a chamber  24  of volume currently considered necessary to house the required optical components. Quicker pump down times could be achieved, e.g., by adding additional high vacuum pumps, but a significant cost, approximately $20-$30K each dependent upon model chosen. The critical down time element, however, will likely be removing through the pump down the water vapor trapped within the chamber  24 . 
     According to an aspect of an embodiment of the present invention, e.g., the addition of a sealed flange adjacent to the electrode  26  could eliminate the need for venting the vessel and subsequent pump down following resealing. This location is not favorable for location of such a sealed flange however. By necessity, the collection optics  40  (and therefore the debris trap  36 ) must be located in close proximity to the point of the pinch  32 . Additionally, the area in close proximity to the pinch  32 , e.g., can be subjected to temperatures in excess of 2000 degrees C. and can, e.g., also be prone to “plating up” with the metal vaporized from the surface of the electrode  26  during light emission. According to an aspect of an embodiment of the present invention, therefore, the employment of a bellows  122  can be utilized to increase, e.g., the distance between the tip of the electrode  26  and the first optical component, e.g., the debris trap  36 , e.g., in order to facilitate replacement of the electrode  26 . Such a bellows  122  may also have some utility in regard to the optical alignment of the pinch  32  location, which has been observed by applicants to vary, e.g., with rep rate and gas mixture, and thermal effects on the collection optics, e.g., warping of the collector  40 , affecting collector  40  focal length. 
     By collapsing the bellows  122 , a gap between the electrode  26  and first optical component, e.g., debris shield  32 , large enough to accommodate a sealing mechanism, e.g., a “gate valve”  130  can be established. This gate valve  130  would perform the function of sealing the vessel  22  during swapping of the electrode  26 . 
     Due to the large thermal loads, e.g., originating from the pinch  32  location, the bellows  122 , e.g., must be of a sufficiently large in diameter to survive. However, the diameter of the bellows  122  also dictates, e.g., the size of the gate valve  130  required to seal the chamber  24 , since, e.g., it must withdraw from the aperture of the gate valve  130  during normal operation. Locating the bellows  122  in a “shadow” from the electrode  26 , e.g., as shown in  FIG. 6  can, e.g., considerably lessen the thermal load to which the bellows  122  will be exposed. The “shielded” bellows  122 , e.g., positioned between the chamber  24  wall  132  and a flange  134  on the electrode assembly  160 , between the bellows  122  and the location of the pinch  32  can facilitate limiting the size of the bellows  122 , e.g., a smaller diameter, which can also be true for the gate valve  130 . 
     The gate valve  130  must be free of elastomers as is common to environments in which the EUV optical components are located as any out gassing from any such elastomers would profoundly reduce optical component lifetimes. A drawback of non-elastomeric seals, however, is the stringent requirement as regards surface finish and flatness of the sealing surface. Within the EUV DPP environment these surfaces must be located to, e.g., minimize plating up by the vaporized metal discharged from the surface of the electrode. A further embodiment of the invention could include a replaceable seal surface  136  that could be replaced should the presently installed one become compromised. A further embodiment of this invention would be the inclusion of a dry nitrogen purge point in the vicinity of the seal flange  126 . If the seal surface becomes contaminated to a degree that vacuum integrity cannot be maintained within the vessel (during electrode servicing), the leak can be detected and the chamber filled with dry nitrogen to prevent the formation of water vapor and intrusion of debris contained in the ambient environment. 
     Elements of the EUV light source  20  can be quite large and as a consequence, quite heavy. Individual sections of the vacuum vessel  22  could weigh in excess of 400 lbs. According to an aspect of an embodiment of the present invention independently mounting each module, e.g., vacuum vessel  22  section(s)/DPP electrodes  32 /DPP commutator  140 , on a common set of linear rails (not shown) enables these sections can be unbolted and slid apart for servicing as shown in  FIG. 7 . The linear rails can, e.g., serve the dual purpose of easing module handling and alignment during reassembly in the course of electrode exchanges. 
     According to an aspect of an embodiment of the present invention, materials for the electrode  26  must be carefully considered, along with techniques for their fabrication and their specific structural aspects, taking into consideration the harsh environment in which the electrodes must function, and particularly structural and thermal loads that must be accommodated. Silicon carbide, SiC, is an example of a material with beneficial properties according to an embodiment of the present invention, in that SiC can, e.g., be tuned for high thermal and electrical conductivity. The electric conductivity of this material, and like materials generically referred to as refractory metal carbide ceramics, can also be changed by adding certain refractory impurities, as explained in more detail below. 
     In addition to tuning SiC, and like materials, the electrical conductivity of Alumina, aluminum oxide (AlO 2 ) can also be tuned, e.g., with the addition of, e.g., Titania. The resulting conductive doped ceramic could, e.g., be able to withstand sputter damage and thermal damage better than any metal. In addition, Titanium Tungsten (TiW) ceramic metal combinations (“ceremets”) may also work like SiC and related materials. TiW is conductive and does not need metal doping for electrical conductivity, however, it possesses a more limited thermal conductivity. TiW machines well, and is best suited for use according to aspects of an embodiment of the present invention if produced by vacuum hot pressing. Aluminum oxide-Titanium oxide, alumina-titania (AlN—TiO 2 ), systems could also work for lower temperature systems. 
     Applicants have discovered that metallic electrodes  26 , particularly the inner electrode (anode)  30  have a great tendency to melt and/or ablate at the surfaces of the electrodes  26 , particularly in the vicinity of the pinch  32 , i.e., on the anode  30 . Observed damage to the surface of used electrode  26  suggests to applicants that the plasma formed in the pinch  32  can impart significant thermal and ionic energy onto the surfaces of the electrode  26 , and particularly the anode  30 . Even Tungsten thorium (W—Th) alloys appear to melt, e.g., at about 3500° K, and can be sputtered easily. 
     Covalent materials tend to be electrically insulating and to better resist ionic damage. A doped ceramic, such as SiC or Alumina could be tuned for both electrical and thermal conductivity. SiC doped, e.g., with BN decomposes at 2700° K, and can be modified to have a thermal conductivity near that of pure aluminum. BN doping levels in SiC can be as high as 30% by weight. Since thermal shock resistance trends proportionally with material thermal conductivity, strength and fracture toughness and is inversely proportional to the expansion coefficient, SiC—BN composites could exhibit very high thermal shock resistance. The thermal shock resistance of Alumina is 200° C. (ΔT° C.) and BN—SiC composites exhibits a 630-1200° C. (ΔT° C.) with BN doping at, e.g., 30%. 
     Electrical conductivity could be tuned close to the surface of the electrode  26 , since the bulk conductivity of the ceramic there may be inadequate. As for alumina materials, surface conductivity could be enhanced by metal oxide doping (SnO, TiO2) without significant adverse changes to the other beneficial properties of the material. 
     SiC—BN or Alumina-Titania systems can be synthesized in many ways. 
     Plasma spray or liquid phase sintering can be used with mixed source powders. For optimum material density, which can be critical for an electrode that will not crack or explode, one could modify a particulate enhanced CVD growth process such as the one used by Trex in Kauai. Based on the available literature and information, e.g., as found in the web page of Trex Enterprises, www.trexenterprises.com referring to a “CVC technology for the production of economical ultra high purity silicon carbide parts, it is applicants&#39; understanding that Trex adds SiC powders into a SiC CVD process, using, e.g., Methyl-Chloro-Silane (“MCS”), to assist with keeping high bulk density at high growth rates. According to an aspect of an embodiment of the present invention, part of this SiC powder could be substituted with BN powder, thereby allowing the nano-particulate BN to incorporate uniformly into the ceramic. Since the Trex&#39;s process is done at low pressures (100 Torr), the electrode will have relatively low dissolved gasses and have a high density. The Trex SiC material is close to 100% dense, which is the ideal. Such a process is illustrated schematically in  FIG. 8 , e.g., showing schematically the synthesis of SiC—BN composite via particulate enhanced CVD, i.e., the thermal decomposition of Methly-Chloro Silane  140  onto BN particles  142  in the presence of a SiC surface  144 , e.g., in a reducing environment, e.g., 100 mTorr, 1400° C., with an MCS in the presence of H 2 . 
     Other synthesis methods can be, e.g., for Alumina—TiW: A) sintering in a reducing environment to create non-stoichiometric oxides on ceramic surfaces, i.e., which are oxygen deficient and conductive; B) placing alumina in titania and sinter/diffusing the two together; C) depositing alternating layers of the 2 materials and then firing the system at &gt;1900° C. or D) vacuum hot pressing, which, e.g., could be used for pure W, since hot presses of refractory metals can contain high dissolved gas levels. High dissolved gas levels promote electrode pitting and volcanic type eruptions in metal electrodes. 
     Turning now to  FIG. 9  three is shown an electrode  26 , e.g., an anode  30 , which may, e.g., have an outer surface  150  which may, e.g., be formed of doped alumina or SiC—BN, which could also be, e.g., undoped TiW. 
     A discharge produced plasma focus light source  20  for EUV, e.g., for use in microlithography also brings about other requirements regarding the electrodes  26 , particularly in regard to cooling and fabrication requirements. The coaxial electrode set of a cathode  28  and an anode  30 , shown schematically in  FIG. 2  can be, e.g., exposed to high average heat flux (&gt;1 kW/cm 2 ) and extremely high transient heat flux (&gt;1 MW/cm 2 ) during pulsed operation. This can require, e.g., the use of refractory metals and specialized alloys, e.g., as discussed above, in conjunction with the best available cooling techniques. High vacuum and structural integrity joints can also be required between dissimilar metals with varying thermal expansion coefficients. 
     Referring to  FIGS. 9-16  there is shown an embodiment of the present invention including an electrode assembly  160 . The electrode assembly  160  can, e.g., include a cathode (outer electrode) assembly  162  and an anode assembly  220 . The cylindrical anode  30  (inner electrode) has been tested by applicants in a number of geometric embodiments with various outer diameters. The smallest cooled device was tested with an OD of 0.625 inches and another was tested with an OD of 0.725 inches. Larger electrodes up to 1″ OD or even greater are contemplated to be needed in the future. The cooling of larger electrodes, however, can be less challenging due to the larger area over which the heat flux is distributed. The joints between dissimilar metals, however, become more difficult in larger diameter electrodes, due to the larger relative change in dimensions with temperature, e.g., during fabrication and operation. Conversely the smaller diameter electrodes may be easier to fabricate but more difficult to cool during operation. The design can also be complicated in general by the need to deliver a plasma source, e.g., a gas, currently contemplated to be, e.g., xenon, through the center of the electrode. The delivery may also be of a metal in a solid or liquid state. The electrode  26  along with the plasma source being delivered can be considered to be a consumable and is therefore also cost sensitive. 
     Typically a mixture of braze and fusion welding techniques can be useful in assembling electrodes of the type contemplated in an embodiment of the present invention. The type of joint and order of fabrication may be determined, e.g., by the specific design. Where possible easily weldable stainless steels, for example 304L or 316L, can be used to fabricate the electrode assembly  160 . This can, e.g., keep the cost of materials down, simplify the machining and assembly and improve the yield of finished parts. Due to the high surface temperature transients the electrode  26  proper can, e.g., be fabricated from a refractory metal such as tungsten or its pseudo alloys including W—Cu, W—La, W—Th and W—Re. This can, however, present the problem of joining a brittle, refractory metal with a low coefficient of thermal expansion (CTE) of ˜4.5 ppm/° C. to, e.g., steel, which has a relatively high CTE of ˜16.6 ppm/° C. Such a joint may be required to be, e.g., high vacuum compatible and able to simultaneously withstand internal coolant pressures, e.g., in excess of 1000 psig. The design may further be complicated by the need to machine, e.g., deep annular cooling channels in the tungsten, which ordinarily cannot be turned or milled, even with so called ‘machinable’ types of pseudo alloys, and must be created, e.g., by electrical discharge machining (“EDM”) and grinding processes. High precision machining of most parts is required to ensure adequate and uniform cooling of the electrode assembly  160  with the added requirements, e.g., of a tightly constrained cooling volume. 
     According to an aspect of an embodiment of the present invention applicants currently contemplate that the tungsten to steel joint be brazed, e.g., using gold and nickel alloys such as NIORO® (82% Au-18% Ni) at temperatures of ˜1000° C., e.g., in a vacuum furnace at pressures of, e.g., in the range of 10-6 Torr. Gold has been selected by applicants because of its ability to wet tungsten well and its high ductility, which can, e.g., result in lower residual stresses in the joint. The specific design of the joint according to an aspect of an embodiment of the present invention can be such that the steel provides an annular mounting slot for the tungsten. During heating in the furnace the more rapidly expanding steel can strain the tungsten elastically from its inner diameter. This can have, e.g., the dual benefit of reducing the residual stresses in the tungsten upon cooling and also centering the tungsten accurately in the steel base. Lower residual stress is of critical importance to avoid cracking of the tungsten. 
     Another possible technique according to an aspect of the present invention is to utilize copper backcasting. Applicants contemplate a process in which includes pouring molten oxygen free copper around, e.g., a refractory metal electrode blank. The finished part can then be machined from the resulting assembly. Although oxygen free copper has a CTE of 17 ppm/° C., it is soft and ductile with a yield stress of only 10 ksi (˜25% of austenitic stainless steels such as 304L) and, therefore, it can yield locally at the joint and greatly reduce the compressive stress on the tungsten. It can be subsequently annealed if desired to further lower the residual stresses. One particular advantage of such a process is the excellent vacuum and structural properties of the joint. Such a joint produced by such a process can be, e.g., generally less prone to leakage, which can otherwise be a problem with brazed assemblies. 
     A principal drawback to this technique is, e.g., the lack of strength in the copper. Copper, e.g., does not cope well with threaded details or high local bearing forces exerted by metal seals, which according to aspects of an embodiment of the present invention could be considered essential for the present application. However, such problems can be avoided, e.g., by careful design and are not considered by applicants to be limiting in the use of the technique to fabricate DPP EUV electrodes according to aspects of an embodiment of the present invention. Plansee, including its American subsidiary, Schwartzkopf, among others, is a source for joints commercially fabricated according to the just referenced process(es). 
     Tungsten electrodes larger than ˜0.75″ OD brazed to steel can have, e.g., a high risk of cracking due to residual stresses at the joint interface. A technique to avoid this can be, e.g., to use a transition insert in the joint. Material selection for the transition insert can require, e.g., a material with a CTE close to that of tungsten but also with good ductility to better deal with the higher stresses, e.g., which can eventually be realized at the boundary with the steel. Good machinability is also a helpful property. According to an aspect of an embodiment of the present invention applicant contemplate using molybdenum, with a CTE of 5.35 ppm/° C., due to its ability to meet required criteria and the fact that it lends itself well to brazing with similar techniques. This can be especially useful for contemplated larger diameter tungsten electrodes, which, e.g., suggests utilizing this concept in the engineering design. 
     According to aspects of an embodiment of the present invention, the electrode assembly  160  may comprise a outer electrode assembly  162 , which may have an electrode assembly side wall  164  connected to an electrode assembly mounting flange  166  for mounting the electrode assembly  160  to the SSPPM  139  with mounting screws  168 . The generally cylindrical side wall  164  may be connected to or integral with a circular cold plate  170  which may have machined in it a central opening for insertion of a cathode base  210  and a plurality of cooling channels  172 ,  174  and  214 ,  216 . 
     The outer electrode (cathode) base  210  may have machined into it a plurality of cooling channels  184  and inlet tube  182  openings and outlet tube  180  openings, forming, e.g., four channels  184 , each with an inlet tube  182  and an outlet tube  180 , for cooling the cathode  28 . The coolant may enter from a coolant inlet  173  to an inlet plenum  172 , which is connected to a pair of opposing inlet plenums  176  and  178  (shown in  FIG. 12   c  and  FIG. 14 ). Each of two of the four long tubes  180  is connected to the inlet plenum  176  or  178 . Each of tow of the four short outlet tubes  182  is connected to a respective channel  184  and to an outlet plenum  214  or  216 , each of which are connected to a coolant outlet  175 . 
     The cathode base  210  may also be machined to contain a central opening  218  that forms the cathode inner wall  163 . 
     The materials for the electrode assembly  160 , including the outer electrode (cathode) assembly  162  and the inner electrode (anode) assembly  220 , can be, e.g., stainless steel type 304L, except for the anode  30 , which may be made of sintered tungsten, or materials discussed above. A critical dimension may be the separation between the partition  256  and the point where the inner walls  250  and  254  meet at the top of the electrode  30  and must be selected based upon the desired amount of coolant flow that needs to pass this point between the partition  256  and the electrode walls  250 ,  254  for adequate cooling. 
     According to an aspect of an embodiment of the present invention a simple open channel cooling arrangement may be utilized to cool the anode  30 , e.g., wherein the coolant flows up one inner wall  250  of the inner electrode (anode)  30 , formed by the anode  30  having a hollow interior  252  of the electrode  30 , and then flow down the other inner wall  254  of the inner electrode (anode)  30 , which may be facilitated by the imposition of a heat pipe partition  256  between the inner walls  250 ,  254  within the hollow interior  252 . Heat transfer can be achieved by convection at the boundary between electrode inner walls  250 ,  254  and coolant passing between the inner walls  250 ,  254  and the partition  256 . Applicants have determined that best thermal results can be achieved in this application, e.g., with the coolant flowing up the interior inner wall  254  and down the outer inner wall  250 . 
     Another consideration is, e.g., the thin walled (0.010″) partition  256 , which according to an aspect of an embodiment of the present invention can separate the inlet  260  of the inner electrode (anode)  30  cooling system, leading to the passage between the partition  256  and inside inner wall  254  from the exhaust channel  270  of the heat exchanger for the cooling of the inner electrode  30  exhausting the passage between the partition  256  and the outside inner wall  250 . This partition  256 , according to an aspect of an embodiment of the present invention may be, e.g., better loaded in tension by the coolant pressure rather than compression, e.g., to avoid buckling, which can be the consequence of the just described flow path. Such a scheme can, e.g., also enable the design to utilize the full yield strength of the material of the partition  256 , e.g., 304L. Applicants have tested a prototype electrode  30  cooled this way with flow rates up to 37 lpm and with entry pressures of &gt;800 psig. Applicants believe that such a design may be capable, e.g., of withstanding inlet water pressures well in excess of 1000 psig and above and thermal loads corresponding to source plasma discharge repetition rates above 3 kHz, and above. 
     Annular channels, however, e.g., may need a high heat transfer coefficient. according to an aspect of an embodiment of the present invention, e.g., the limited area exposed to the coolant, e.g., can require very efficient heat transfer and thus a high heat transfer coefficient. Also, according to an aspect of the present invention, higher temperatures, e.g., at the inner walls  250 ,  254 , can, e.g., require the need to deliver high flow rates of coolant at high pressure, e.g., to suppress coolant boiling, particularly sheet or bulk boiling as opposed to nucleate boiling, which may actually improve heat transfer from the inner walls  250 ,  254  to the coolant. 
     According to another aspect of an embodiment of the present invention, applicants contemplate the use of a porous metal heat exchanger within the hollow interior of the electrode  30 . In such an embodiment (not shown), e.g., a porous metal media may, e.g., be bonded to the inside walls  250 ,  254  of the electrode  30 , e.g., particularly in the region of the tip  34  containing the pinch opening of the electrode  30 , e.g., by brazing. This can result, e.g., in what amounts to a large extended fin on the anode  30 , for cooling purposes. The conductive heat transfer from the inner walls  250 ,  254  into this extended porous surface area, e.g., can be more efficient than the convective heat transfer across the simple wall of the annular channels into the coolant. The extended porous surface area, e.g., then can have a much greater area from which to exhaust its heat into the coolant. The result can be, e.g., better heat transfer using less coolant. Such a structure could also replace the partition  256  and inner walls  250 , and  254  throughout the entire hollow portion  252  of the electrode  30 . A possible drawback with porous metal heat exchanger, e.g., can be a high inherent pressure drop across the porous medium. At high source repetition rates this could, e.g., require high inlet pressures and result in large mechanical stresses in the braze joints and flow partition associated only with the pumping of coolant, which will need to be addressed. Another potential drawback can be, e.g., the temperature drop across the walls of the electrode  30 , due, e.g., to the more effective heat transfer into the coolant, which magnifies the temperature drop across the walls of the electrode  30 , which can produce high stress loads across the shell walls of the electrode  30 . 
     Higher stress levels may lead to structural failure in the tungsten shell walls of the electrode. Another design criteria may be, e.g., alternating stresses in the electrode  30  shell walls, e.g., due to operation of the electrode  30  in a burst mode, which can, e.g., have different consequences than static stress loads. This results in, e.g., the requirements that the material of the electrode  30  be both tensily strong and also tough. The distribution of heat flux incident on the electrode  30  may also be of consideration in determining, e.g., electrode lifetimes, e.g., in different modes of operation, including repetition rate, pinch temperature, duty cycle, etc. Applicants have, however, tested, e.g., porous tungsten electrodes, e.g., obtained from Thermacore up to repetition rates of 2 kHz in a negative polarity configuration without failure. Another possibility according to an aspect of an embodiment of the present invention could be, e.g., the use of porous copper cooled electrodes  30 , e.g., made using a porous copper foam, e.g., available commercially from Porvair, which can be, e.g., machined by electrical discharge machining (“EDM”) into useful geometries for application according to embodiments of the present invention. Another possibility according to aspects of an embodiment of the present invention is to employ uniformly deposited silver to selected optimized brazing thicknesses for brazing using, e.g., electroplating or ionic fusion techniques. Such an approach may, e.g., realize the full potential of porous metal cooling as discussed above for the inner electrode (anode)  30 . 
     According to an aspect of an embodiment of the present invention, high heat flux cooling may be realized by using, e.g., micro channels. According to this embodiment, e.g., coolant may be pumped at high inlet pressure through a series of small passages, micro-channels. Typically these passages may be, e.g., tubular or rectangular and have overall dimensions of 0.020 inches or less. The ratio of channel surface area to coolant volume in such an arrangement may be favorable and this technique, similar to that used currently to cool laser diodes and other high heat flux electronic devices and power semiconductors, may be utilized in cooling the electrodes  30  of the present invention. Applicants have tested prototype micro-channel-cooled electrodes  30  up to repetition rates of 2 kHz and believe that much higher repetition rates are achievable with this cooling technique. This technology does present relatively high pressure drops from inlet to outlet, however, and may result also in stress in the assembly during source operation, e.g., brazing a relatively stiff micro channel insert inside the hollow portion  252  of a tungsten shell anode  30 , as discussed above, may, e.g., result in additional constraints and stresses in the anode assembly  220 , which will have to be accounted for in the overall design. 
     The outer electrode (cathode)  28 , e.g., as shown in  FIGS. 10-15 , can include a cathode assembly  162  that can, e.g., be generally annular in shape. The cathode  28  itself, within the cathode assembly  162 , may have the form of a 15° conical inner surface  163 , e.g., that faces the inner electrode  30 , e.g., with a clearance varying from, e.g., 0.19 inches at the base to 0.46 inches at the upper edge. The upper edge may be covered by a cathode lid  212 . The outer electrode (cathode)  28  may be, e.g., much larger than the inner electrode (anode)  30 , and is, therefore, less challenging to cool. According to an embodiment of the present invention, also erosion of the outer electrode  28  can be less of a problem than with the inner electrode  30  and therefore material selection and thus fabrication can also be somewhat more simple. This outer electrode (cathode)  28  may, therefore, not be considered to be a consumable. 
     According to an aspect of an embodiment of the present invention, the outer electrode (cathode)  28  may, e.g., be fabricated, e.g., from Glidcop® AL-15, a proprietary oxide dispersion strengthened copper available from OMG Metals Inc. according to an aspect of the present invention, this material was selected, e.g., for its high thermal and electrical conductivity, and also, e.g., combined with good mechanical strength and reasonable machinability. Such a Glidcop® outer electrode  28  can, e.g., be brazed into, e.g., a 304L stainless steel base  210 . 
     The base  210  can, e.g., interface the outer electrode  28  with portions of a DPP pulsed power unit  139  shown in  FIG. 7 . Applicants have brazed the outer electrode  28  to the base  210 , e.g., using either a nickel based alloy, e.g., Nibsi® (Nickel/boron/silicon), available from Morgan Crucible Company plc, and, more recently, NIORO® braze material, available from Morgan Crucible Company plc, which was also used for brazing in the inner electrode  30 , as discussed above, also using similar braze preparation and furnace processes as for the inner electrode also as described above. 
     As mentioned above, the task of cooling the outer electrode  28  is simplified over the inner electrode  30  due to its larger size and use of a high thermal conductivity material. Relatively large open channel water galleries  184  may, e.g., be machined into the Glidcop® outer electrode  28  body  210 , which may be supplied and exhausted, e.g., through 316L tubing formed, e.g., of short tubes  182 , e.g., brazed into openings in the body  210 , and also connected to an inlet manifold  214  and exhaust manifold  216 . According to an embodiment of the present invention, e.g., four or more of such galleries  184  may be provided to ensure uniform coolant flow, and thus more uniform cooling at all locations. Inlet and exhaust plumbing, e.g., inlet plenum  214  and outlet plenum  216  (shown in more detail in  FIG. 14 ) may be arranged, e.g., to have similar flow resistance for each gallery  184 , thus a similar amount of coolant flows through each one. According to an aspect of the present invention, applicants anticipate that open channels  184  with a high flow rate of coolant and sufficient backpressure to prevent boiling, as discussed above, can, e.g., suffice for cooling the outer electrode  28 . However, if needed, e.g., the porous media or micro-channels cooling discussed above may be employed. 
     According to an aspect of an embodiment of the present invention there may be included, e.g., an integral cold plate  170  machined into the top surface of the 304L cathode assembly  162 . The flow channels of this cold plate  170  (shown in more detail in  FIG. 16 ) may include, e.g., the actual inlet manifold  214  and exhaust manifold  216  for the electrode  28 . This may be done, e.g., to cool a pulsed power output switch, LS3 (not shown), which may be located, e.g., below the electrode assembly  160 , as well as to, e.g., equalize the flow to each cooling gallery  182 . The channels, e.g.,  214 ,  216 , may be, e.g., milled into the top of the cathode assembly  162  and may also be, e.g., sealed, e.g., with fusion welded plates. 
     The outer electrode  28  may be formed of the walls of the cooling channels forming the cooling galleries  184  and may have braised to its top end and to the cathode base  210  a cathode lid  212  that serves to seal the coolant galleries  184 . 
     The cathode assembly  163  may be joined to the anode assembly  220  by screws  231  and may be insulated from one another by, e.g., an overlap center insulator  222 , which can, e.g., be made of, e.g., pyrolitic boron nitride or alumina and can, e.g., extend axially along the outer walls of the inner electrode  30  and by an elastomer free electrode insulator  224 , which may be made from, e.g., pyrolitic boron nitride or alumina, and may, e.g., be held in place by an insulator retainer clip  242  and its set screw  244 . Sealing may be provided between the insulator  224  and the cathode base  210  and anode assembly  220  by a pair of elastomer free metal C sealing rings  230 , one between the insulator  224  and the cathode base  210  and one between the insulator and the anode assembly  220 , inserted into respective opposing grooves. 
     According to another aspect of an embodiment of the present invention, debris mitigation is considered an import consideration for an operative long-lived DPP EUV light source. According to one aspect of an embodiment of the present invention, the center electrode  28  of a Discharge produced plasma EUV light source may be, e.g., fabricated from a high temperature, possibly refractory, material, as noted above, and also may be, e.g., possessive of a strong magnetic permeability. according to this aspect of an embodiment of the present invention, debris eroded from the electrode  30 , as a result of, e.g., plasma bombardment, surface melting or ablation, surface boiling, etc., may, e.g., be substantially magnetic as well. according to an aspect of the present invention applicants contemplate, e.g., the generation of a suitably large magnetic field at least about 50 mTesla (“mT”) and e.g., within a range of 50 mT to 1 T, in the optical path between the location, e.g., of the plasma and, e.g., the collector optical element. In this way, e.g., debris may be deflected and then collected, e.g., quasi-permanently by, e.g., a suitably placed stationary magnetic (not shown), e.g., arranged around the perimeter of the optical path. According to an aspect of this embodiment of the present invention, this collector magnetic field may be, e.g., generated by electromagnets, in which event, debris can be, e.g., swept out during a regeneration cycle, e.g., during which the electromagnets may be de-energized. A suitable cooled high temperature refractory magnetic metal may be, e.g., cobalt. 
     According to another aspect of an embodiment of the present invention, applicants contemplate the use of shaping of the current pulses to the electrodes  30 ,  28 , e.g., to make optimum use of the current, e.g., in compressing the active element (plasma source), typically by, e.g., peaking the current during the magnetic compression phase of the discharge. According to an aspect of this embodiment, the SSPPM may include, e.g., an additional saturable inductor in the last stage of the DPP SSPPM. A possible optimum waveform can, e.g., start out with a modest current during the axial rundown phase and then peaks during the radial compression phase. 
     Applicants have simulated this discharge using an electrode geometry somewhat closer to the schematic diagram of  FIG. 2 , and in addition the simulation was done with He rather than, e.g., Xe. However, the simulation provides enough detail in the simulation to understand the dynamics of the operation of aspects of an embodiment of the invention. In the simulation, e.g., a simulation of the discharge was conducted using the SSPPM currently available in applicants&#39; employers gas discharge laser products, having an inductance for the last stage compression head satuable inductor of 6 nH, which is about as low as can currently be achieved with available materials and geometries, and will give the fastest discharge across the electrode as is possible, i.e., the fastest rise time of the discharge pulse. This simulation is shown in  FIG. 18A , with the time scale elongated for illustrative purposes. As can be seen in  FIG. 1 , in a similar simulation of the use of first a 12 nH inductance and, with the switching on of an additional saturable inductor going relatively rapidly to a 6 nH inductance total, the discharge initially goes to about 20 kAmps and gently rises to about 40 KA, before settling back a few Kamps until the point when the inductance goes to, e.g., 6 nH at which time a rapid spike to about 85 Kamps occurs followed by a drop to 0 in about 20 ns according to the simulation. During the axial rundown phase of the discharge by the peaking capacitor across the electrode, i.e., from about 50 ns to about 240 ns in the simulation of  FIG. 18   a , the discharge is generally horizontally disposed between the inner surface of the outer electrode  28  and the outer surface of the inner electrode  30 . Increasingly angling slightly upwardly along the inner electrode  30  until reaching a region of the inner electrode  30  generally adjacent the lower most extension of the depression at the tip  34  of the inner electrode  30 . Therefore acting, e.g., to initiate the discharge, e.g., at  82  and move the discharge, e.g., at  80  towards the electrode  26  tip, and requiring less current to sustain the traveling discharge. During the radial compression phase, e.g., between 240 ns and 260 ns in the simulation of  FIG. 18   a , when the plasma forms at the tip  34  of the electrode  26 , discharge is rapidly increased in current flow to the electrode  26 , which, e.g., rapidly transfers significantly increased amounts of kinetic energy to the plasma, e.g., through the rapidly increasing magnetic field confining the plasma, e.g., resulting in a better pinch  32 . This is illustrated in a further simulation shown in  FIG. 18B . The better pinch has a number of beneficial properties, e.g., keeping the active source gas ions within the pinch longer to, e.g., induce more energy transfer to the ions, e.g., resulting in more x-ray generation from the pinch  32 . 
     This shape of the delivered current can allow, e.g., as much as 3-5 times greater current during the pinch formation, e.g., in the radial rundown, with a concomitant increase in compression, while overall, the electrode dissipates the same amount of energy from the peaking capacitor in the SSPPM, thus also maintaining a thermal energy budget in the electrodes  30 ,  28  during the entire pulse that is no different that the conventional discharge shown in the simulation of  FIG. 18   a . The conventional saturable inductor can, e.g., be included in the SSPPM  139  compression head circuit, e.g., having twice the currently conventional saturable inductance, e.g., 12 nH as shown in the simulation of  FIG. 18A , and be saturated as usual. An additional saturable inductor, e.g., in parallel with the conventional saturable inductor, to make the parallel inductance smaller, can then be biased to saturate, e.g., as shown in the simulation of  FIG. 18A , e.g., rapidly increasing the discharge current at the very end of the discharge, e.g., as shown in the simulation of  FIG. 18   a . Applicants&#39; simulations of a plasma fluid utilizing simulation software have confirmed the advantages of the proposed driver configuration. 
     According to another aspect of an embodiment of the present invention, a source gas may be desirable for use, e.g., xenon, e.g., for the generation of EUV light at a particular λ, e.g., 13.5 nm, but also absorb the same light to a high enough degree to interfere with overall light production output. Therefore, applicants contemplate the use of a buffer gas, e.g., argon and helium, that is less absorbent of the produced light at the desired λ, and to, e.g., differentially remove the source gas and the buffer gas from the EUV light production vessel. According to an embodiment of the present invention, a turbopump (not shown) can be configured specifically for the higher molecular weight of a source gas, e.g., xenon, while reducing the pumping capacity with respect to, e.g., argon and helium. This can be accomplished, e.g., by altering the pump operating characteristics, e.g., internal clearances, blade angle and speed, and also, e.g., eliminating the Holweck (molecular drag) stage of the pump. The turbomolecular pump design, therefore, can be set up to exhibit preferential pumping of a higher atomic weight gas (or molecular weight if appropriate), e.g., xenon, over a much lower molecular weight gas, e.g., helium, based, e.g., on molecular velocities. 
     Turning now to  FIG. 15 , there is shown a debris shield  300  according to an embodiment of the present invention. Such a debris shield  300  enables simplified fabrication of the debris shield  300  while still achieving the functional solution of preventing debris from the light source reaching the collector mirrors. Simplified fabrication techniques may be employed, according to this embodiment of the present invention, e.g., to make the debris shield  300 , and at the same time be more cost effective than some other proposed fabrication techniques, e.g., fabricating columnar structures. The fabrication structure and technique also broadens the possible array of materials from which such a simplified fabrication debris shield  300  may be made. 
     The design of the debris shield  300  according to this embodiment of the present invention can, e.g., be made up of coplanar layers  302  that can, e.g., be arranged to allow the photons to be emitted from the plasma source  32  and pass to the collector  40  as in a columnar structure. The debris would need to navigate through these layers  302  in order to reach the mirror(s) of the collector  40 . The number of layers  302  required would be determined by monitoring how much debris is actually able to exit the outer layer of the debris shield  300 , formed by an outer surface  306  of the outer-most layer  302 , i.e., outer-most from the plasma pinch  32 . 
     As can be seen in  FIG. 15 , each layer  302  is made up of a plurality of light passages  304  extending between an outer surface  306  of the respective layer  302  and an inner surface  308  of the respective layer  302 . The respective curvilinear outer surfaces  306  of each layer  302  may have, e.g., an arc  316 , e.g., having a first radius of curvature about a focal point, e.g., at the center of the plasma, which may, e.g., be a fixed point with respect to the electrode  30 , where the plasma is controlled to essentially be located for each discharge, of a point that it dynamically determined, e.g., pulse to pulse from the positioning of the actual plasma, e.g., its center of gravity. This arc  316  may be an arc of a circle centered at the focus point. Each respective inner surface  308  may have the same or a similar concentric arc centered at the same focus, except with a smaller radius of curvature, depending upon the thickness of the layer  302 . The surfaces are co-planar in the sense that the curvatures about the two axes of rotation can, e.g., remain the same throughout the structure, i.e., from outer surface of a layer to inner surface of a layer to outer surface of the next layer disposed toward the pinch  32 . 
     The outer surface  306  of each respective layer  302  may have, e.g., an arc  318 , e.g., forming an ellipse centered on a first focal point, which may coincide the center of the circle formed by the arc  316 , or of a concentric circle with the center of the circle forming the arc  316 . This may be determined, e.g., by the shape of the collector mirror(s) being utilized for the collector  40 . 
     Each of the light passages  304  may be uniform in shape between the outer surface  306  and the inner surface  308  in each respective layer  302 , or tapered toward one or both of the centers of the shapes comprising arcs  316  and  318 . 
     The layers may be formed of, e.g. a metal, e.g., titanium or tungsten, a ceramic or refractory metal, e.g. SiO 2 , Alumina AlO 2  or Titania, TiO 2 , or other ceramic metal combinations. 
     According to another aspect of this embodiment of the present invention, there may be gaps between each layer  302 , as shown in  FIG. 15 . The respective layers  302  may, e.g., be attached to each other by, e.g., connector posts  320 , which may correspond to each of the four corners of the light passages  304  throughout the interface space between the respective layers, or periodically spaced connector posts  320 , as shown in  FIG. 15 . Also as shown in  FIG. 15 , the layers  302  may be divided into sections, e.g., either of the entire size shown in  FIG. 15  or sub-sections such as section  330  shown in  FIG. 15 . In this fashion, e.g., an entire solid of rotation can be fabricated to fit all around or substantially all around the plasma pinch focus  32 . 
     Potentially the debris could fall to the bottom of the shield  300  rather than build up in the holes of the proposed design. Debris removal could, e.g., be an added feature, e.g., allowing longer intervals between replacements. 
     It will also be understood that the debris shield  300  may be formed, e.g., with the openings  304  formed only of opposing side walls along one of the arcs  316  or  318 , e.g., either in each layer  302  or in the openings between adjacent layers  302 , if there are such openings. That is, the passages  304  need not have four walls  312 , which may still provide enough debris trapping and be structurally sound, but, e.g., facilitate fabrication and/or facilitate the debris shield  300  having one arc 2= 316 ,  318  that is a part of a circle and the other that is, e.g., part of an ellipse. 
     Turning now to  FIG. 16 , there is shown another embodiment of the present invention regarding the fabrication and structure of a debris shield  400 .  FIG. 16  shows an example of an “Out-of-focus laser machining” technique useful, e.g., for producing DPP or other EUV debris shields, e.g., with light transmission passages focused to a focal point, or other applications for tapered array structures with a common focus. 
     Such a debris shield  400  may, e.g., require tapered channels pointing to a common focus  402 . Laser machining can, e.g., be carried out with sufficiently high laser intensity utilizing an unfocused laser beam. Accordingly, applicants have discovered that the correct shape for a debris shield and its light passages may, e.g., be made by using a focusing lens  404  behind a grid-like mask  406 . In the arrangement of  FIG. 18 , e.g., high enough laser power and suitably short laser wavelength to do the laser machining out-of-focus may be provided, e.g. utilizing applicants&#39; assignee&#39;s XLA dual chambered MOPA configured lasers. The general set-up is shown in  FIG. 16  can include, e.g., a parallel laser beam  410  (which need not necessarily be wholly parallel), which can be, e.g., incident from the right as shown in  FIG. 18 . The laser beam  410  can, e.g., first be incident on the mask  406 , which can be a grid or mesh, e.g., in order to make square or circular channels  412 . The mask, which may be made of, e.g., W or Mo, can, e.g., be coated on the side facing the laser beam  410  with a reflective coating, e.g., a thin film of, e.g., aluminum to enhance the reflectivity and avoid degradation of the mask  406  by the laser beam  410 . The mask  406  can also, e.g., be tilted very slightly to avoid back-reflection into the laser amplifier/oscillator. Also, if the mesh (not shown) is made from wires with round cross section, the back-reflection problem can be reduced. The lens  404 , or, more generally, the focusing optic, can, e.g., generate an array of convergent beamlets  414  that already have the required taper. 
     A work-piece  420 , which may be, e.g., a section of a spherical solid, constructed and positioned to have a center at the focus  402 , can be placed at the correct distance between the lens  404  and the laser focus  402 . Even if the laser beam  410 , e.g., is not be intense enough to machine the entire debris shield  400  at once, e.g., a focused laser beam, e.g., scanned, e.g., across the mask  406  can have the desired effect. The entire set-up, i.e., mask  406 , lens  404  and work-piece  420 , may, e.g., be moved laterally, perpendicularly as shown in  FIG. 16  in front of the laser beam  410 , and the channels, e.g., can then be machined consecutively. Scanning over the entire surface of the workpiece may be, e.g., controlled to be faster than the channel  412  drilling, such that, e.g., no additional stresses are induced in parts of the workpiece by partially finished drilling. To make the scanning reproducible, it could, e.g., be motorized and/or controlled by piezoelectric actuators that can, e.g., drive the lateral motion of the entire setup  404 ,  406  and  420  with respect to the laser beam. Alternatively, the laser beam may be scanned across the lens  404  by means of actuator-controlled deflection optics (not shown) that can, e.g., conserves the direction of incidence of the laser beam and the laser focus position. The laser beam  410  intensity can be controlled to be high enough so that that it will ablate the workpiece, even out-of-focus but at the same time not damage the lens  404  and reflecting mask  406 . Therefore, in most cases, e.g., a short (ultraviolet) wavelength of the laser beam  410  would be best suited. Applicants believe that it is better to put the mask  406  in front of the lens  404 , e.g., as shown in  FIG. 16 , e.g., in order to avoid laser-sputtered material from the mask  406  being incident on the lens and damaging the lens. The sputtered material most often is emitted toward the direction from which the laser light is incident. Another option, e.g., to increase the laser intensity at the workpiece  420  may be, e.g., to go to femtosecond laser machining, e.g., using a Ti:sapphire laser at 745 nm or 772 nm, e.g., with subsequent frequency tripling or quadrupling, respectively, and then amplifying this pulse, e.g., using a KrF or ArF excimer gas discharge laser amplifier. 
     According to another aspect of an embodiment of the present invention, debris removal may be effected using an electrochemical reaction. Applicants contemplate taking advantage of the fact that tungsten reacts directly with fluorine, F 2 , or a molecule containing fluorine, e.g., NF 3 , at room temperature to form tungsten fluoride, WF 6 . Excess Tungsten atoms, e.g., from a tungsten electrode  30 , according to an embodiment of the present invention can be removed from the source output, e.g., by combining the source output with a halogen gas such as fluorine or chlorine to form, e.g. a metal halide. As an example, a volatile gas may, e.g., be formed by reaction of undesired debris particles, e.g., tungsten atoms, ions and clusters, in the presence of, e.g., reactive halogen gases forming molecules like WF 6  or WCl 6 . In strong contrast to the pure tungsten particles that have a high sticking probability on solid surfaces (like collector optics) these molecular compounds have a very low sticking probability on solid surfaces and are thus preferentially pumped away and removed from the vessel. This volatile gas, e.g., inserted into the output light emitted from the EUV plasma source thus can, e.g., provide an environment where increased atomic collisions may occur between the undesirable Tungsten atoms and the “scrubber” halogen gas. This could then cause, e.g., the Tungsten atoms to combine with the gas(es) to form a compound such as Tungsten Fluoride (WF 6 ) or Tungsten Chloride (WCl 6 ) and to be removed from the vessel. 
     According to another aspect of an embodiment of the present invention, electrode lifetime can be increased and/or replacement costs can be decreased in a variety of different ways. The inner electrode  30  could be, e.g., made to be screwed by including threaded connections on the electrode outer wall and on the anode assembly  220 . The electrode  30  could be made to be continuously fed, e.g., by external means, e.g., a fitting extending through the wall of the vessel  22 , which could be threaded both for movement of the electrode as it wears over time and to provide a tortuous path for pressure sealing of the vessel. The electrode,  30  could be mounted on a sleeve that is so threaded. The electrode  30  could be replaced with a plurality of electrodes, e.g., arranged in an array and either fired to share the discharge pulses or filed, e.g., seriatim, or left unfired for a time and then placed into the discharge circuit. The shape of the electrode  30  could be selected to foster longer life. Thermoelectric cooling could be substituted for water cooling. 
     Turning now to  FIGS. 17A-H  there is shown another debris shield according to an embodiment of the present invention.  FIG. 17A  shows a perspective view of a debris shield  450  according to aspects of an embodiment f the present invention. The debris shield  450  may comprise a mounting ring  452  having an opening defining a collection aperture, e.g., extending over a portion of a spherical surface of light expanding from a plasma source at a focus and covering, e.g., approximately 1-2 steradians. In the center of the opening may be a hub  454 , having side walls containing slots  455  and, e.g., tapering toward the focus. The mounting ring  452  may also have slots  453  (shown in  FIG. 17C ). 
     A plurality of thin, e.g., about 0.25 cm thick long fins  456  may be engagingly mounted to the slots  455  and/or  453 , respectively in the mounting ring  452  or the hub  454 . It will be understood, that slots may only be needed in one or the other of the mounting ring  452  and hub  454  and/or that the slots may be a plurality of short sots, ass opposed to those shown in, e.g.,  FIGS. 17   a  and  17 D extending the length of the hub  454 , and may, e.g. be keyed to certain long fins  456 , e.g., for particular positioning around the hub  454 , i.e., each long fin  456  may have a particular slot or slots vertically displaced along a radius of the outer surface of the hub  454  tapered portion into which a particular long fin  456  and only that long fin  456  can engage. The same may be true for slots  453  on the mounting ring  452 . 
     Intermediate the long fins  456 , forming, according to an aspect of an embodiment of the present invention, e.g., a grouping of, e.g., five fins, comprised of, e.g., two long fins  456 , an intermediate fin  458 , e.g., between the adjacent two long fins  456 , and two short fins  470 , each intermediate the intermediate fin  458  and adjacent long fins  456 . 
     As shown in more detail in  FIG. 17E , the long fins  456  may have an intermediate fin tab receiving slot  457  and a short fin tab receiving slot  459 . In turn, the intermediate fins, as shown in  FIG. 17F  may have a long tab  460 , which can, e.g., engage a respective intermediate tab receiving slot  457  on an adjacent long fin  456 . Also, e.g., there may be mounted, e.g., intermediate the intermediate fin  458  and adjacent long fins  456  a pair of short fins  470 . Each short fin  470  may have, e.g., a short fin tab, which may, e.g., engagingly fit into a respective short fin tab receiving slot  459  in a respective adjacent long fin  456 . Each of the intermediate fins  458  and short fins  470  may also have, e.g., separator/strengthening fins, respectively  460   a ,  472   a  that, may, e.g., rest against adjacent respective intermediate fins  458  or long fns  454 , as the case may be. It will be seen that the tabs  460 ,  460   a ,  472 ,  472   a  can, e.g., extend along the radius to the focus of the debris shield, e.g., at the center of the plasma pinch  32 , so as to not block any significant amount f the light emitting from the pinch  32  and passing through the debris shield  450 . As can be seen in the top view of  FIG. 17  B, the tabs  460 ,  460   a ,  472 ,  472   a  are visible extending along respective radii to the focus. 
     The debris shield  450  may have a mounting ring top locking ring  484  and a mounting ring bottom locking ring  488  respectively held in place on the mounting ring by screws  486  and  490 , e.g., to hold the mounting ting facing sides of the respective fins  454 ,  456  and  470  to the mounting ring  452 , regardless f whether or not slots  453  exist on the mounting ring  452 . Similarly, the hub  454  may have a top locking plate  480 , which may be held in place, e.g., by a locking plate nut  482  and a bottom locking nut  483 . 
     It will be understood that in operation the thin, e.g., 0.25 cm thick fins  456 ,  458 ,  470  can serve to collect debris plating onto the surfaces of the fins  56 ,  458 ,  470 , and the interlocking tabs  460 ,  472  and separator tabs  460   a ,  472   a  can strengthen and uniformly separate the fins  456 ,  458  and  470  in the groups of the structure and prevent warping, e.g., due to thermal exposure of the debris shield  450 . 
     According to another aspect of an embodiment of the present invention there is known that metallic compounds may be used as a source for a discharge produced plasma, and that powdered forms of such a metallic compound, e.g., tin may be a reliable method of delivering the source for formation of the plasma. However, reliable methods of delivering the right quantities of such material. Applicants have discovered such a method. According to an aspect of an embodiment of the present invention applicants propose to provide particles of metal in the form, e.g. of a powder with the particles generally, e.g., as small as possible, e.g., on the order of 1μ in diameter. By puffing the powdered compound, e.g., tin, into a gas feedstock used in a pulse plasma discharge, the powder can be delivered to the plasma formation site. The feedstock, i.e., carrier, gas may be, e.g., a benign gas, e.g., neon, e.g., serving solely as a carrier, or, e.g., may be an active gas, e.g., xenon, which could also assist in the formation of the plasma, and/or in initiation of the breakdown of the plasma discharge. The method of, e.g., atomizing the, e.g., tin into the feedstock may, e.g., include a method in which the feedstock gas passes through or over a quantity of the powdered metal, e.g., tin, which may be, e.g., agitated, e.g., shaken up, e.g., with a piezoelectric actuator, e.g., sufficiently to cause fine particles of the metal o become airborne in the feedstock gas stream, which may then be directed, e.g., through a hollow anode, to the plasma formation site. 
     It will be understood that in this fashion a precision metered amount of the powdered material may be inserted into the feedstock gas flow, e.g., a certain density pre unit time, and this amount may be modulated, as desired, e.g., by modulating the amount of agitation, e.g., by modulating the voltage applied to the piezoelectric actuator. It will also be understood, that control may also be exercised by modifying the feedstock gas flow rate past the agitated powdered material. Modulation may be effected to, e.g, limit debris formation in the plasma. Modulation may also be effected, e.g., by periodically interrupting flow of the feedstock gas, e.g., also by periodically injecting a pure feedstock gas flow, e.g., without any inserted material, e.g., with a cross-flow geometry. Also, e.g., if larger particles are used, debris mitigation may be effected using, e.g., a mesh whose holes prevent passage within the feedstock gas of particles above a certain selected size. 
     The above described embodiments of the present invention are not to be considered the only embodiments of the inventions disclosed in the present application and the embodiments are subject to many changes and modifications that will be understood by those skilled in the art and their equivalents and still remain within the scope of the app-ended claims, which alone should be considered to define the scope of the inventions as claimed.