Patent Publication Number: US-2006017387-A1

Title: Inductively-driven plasma light source

Description:
RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. Ser. Nos. 10/888,434, 10/888,795 and 10/888,955, all filed on Jul. 9, 2004. This application claims priority to and incorporates by reference in their entirety U.S. Ser. Nos. 10/888,434, 10/888,795 and 10/888,955. 
    
    
     FIELD OF THE INVENTION  
      The invention relates to methods and apparatus for generating a plasma, and more particularly, to methods and apparatus for providing an inductively-driven plasma light source.  
     BACKGROUND OF THE INVENTION  
      Plasma discharges can be used in a variety of applications. For example, a plasma discharge can be used to excite gases to produce activated gases containing ions, free radicals, atoms and molecules. Plasma discharges also can be used to produce electromagnetic radiation (e.g., light). The electromagnetic radiation produced as a result of a plasma discharge can itself be used in a variety of applications. For example, electromagnetic radiation produced by a plasma discharge can be a source of illumination in a lithography system used in the fabrication of semiconductor wafers. Electromagnetic radiation produced by a plasma discharge can alternatively be used as the source of illumination in microscopy systems, for example, a soft X-ray microscopy system. The parameters (e.g., wavelength and power level) of the light vary widely depending upon the application.  
      The present state of the art in (e.g., extreme ultraviolet and x-ray) plasma light sources consists of or features plasmas generated by bombarding target materials with high energy laser beams, electrons or other particles or by electrical discharge between electrodes. A large amount of energy is used to generate and project the laser beams, electrons or other particles toward the target materials. Power sources must generate voltages large enough to create electrical discharges between conductive electrodes to produce very high temperature, high density plasmas in a working gas. As a result, however, the plasma light sources generate undesirable particle emissions from the electrodes.  
      It is therefore a principal object of this invention to provide a plasma source. Another object of the invention is to provide a plasma source that produces minimal undesirable emissions (e.g., particles, infrared light, and visible light). Another object of the invention is to provide a high energy light source.  
      Another object of the invention is to provide an improved lithography system for semiconductor fabrication. Yet another object of the invention is to provide an improved microscopy system.  
     SUMMARY OF THE INVENTION  
      The present invention features a plasma source for generating electromagnetic radiation.  
      The invention, in one aspect, features a light source. The light source includes a chamber having a plasma discharge region and containing an ionizable medium. The light source also includes a magnetic core that surrounds a portion of the plasma discharge region. The light source also includes a pulse power system for providing at least one pulse of energy to the magnetic core for delivering power to a plasma formed in the plasma discharge region. The plasma has a localized high intensity zone.  
      The plasma can substantially vary in current density along a path of current flow in the plasma. The zone can be a point source of high intensity light. The zone can be a region where the plasma is pinched to form a neck. The plasma can be a non-uniform plasma. The zone can be created by, for example, gas pressure, an output of the power system, or current flow in the plasma.  
      The light source can include a feature in the chamber for producing a non-uniformity in the plasma. The feature can be configured to substantially localize an emission of light by the plasma. The feature can be removable or, alternatively, be permanent. The feature can be located remotely relative to the magnetic core. In one embodiment the feature can be a gas inlet for producing a region of higher pressure for producing the zone. In another embodiment the feature can be an insert located in the plasma discharge region. The feature can include a gas inlet. In some embodiments of the invention the feature or insert can include cooling capability for cooling the insert or other portions of the light source. In certain embodiments the cooling capability involves pressurized subcooled flow boiling. The light source also can include a rotating disk that is capable of alternately uncovering the plasma discharge region during operation of the light source. At least one aperture in the disk can be the feature that creates the localized high intensity zone. The rotating disk can include a hollow region for carrying coolant. A thin gas layer can conduct heat from the disk to a cooled surface.  
      In some embodiments the pulse of energy provided to the magnetic core can form the plasma. Each pulse of energy can possess different characteristics. Each pulse of energy can be provided at a frequency of between about 100 pulses per second and about 15,000 pulses per second. Each pulse of energy can be provided for a duration of time between about 10 ns and about 10 μs. The at least one pulse of energy can be a plurality of pulses.  
      In yet another embodiment of the invention the pulse power system can include an energy storage device, for example, at least one capacitor and/or a second magnetic core. A second magnetic core can discharge each pulse of energy to the first magnetic core to deliver power to the plasma. The pulse power system can include a magnetic pulse-compression generator, a magnetic switch for selectively delivering each pulse of energy to the magnetic core, and/or a saturable inductor. The magnetic core of the light source can be configured to produce at least essentially a Z-pinch in a channel region located in the chamber or, alternatively, at least a capillary discharge in a channel region in the chamber. The plasma (e.g., plasma loops) can form the secondary of a transformer.  
      The light source of the present invention also can include at least one port for introducing the ionizable medium into the chamber. The ionizable medium can be an ionizable fluid (i.e., a gas or liquid). The ionizable medium can include one or more gases, for example, one or more of the following gases: Xenon, Lithium, Nitrogen, Argon, Helium, Fluorine, Tin, Ammonia, Stannane, Krypton or Neon. The ionizable medium can be a solid (e.g., Tin or Lithium) that can be vaporized by a thermal process or sputtering process within the chamber or vaporized externally and then introduced into the chamber. The light source also can include an ionization source (e.g., an ultraviolet lamp, an RF source, a spark plug or a DC discharge source) for pre-ionizing the ionizable medium. The ionization source can also be inductive leakage current that flows from a second magnetic core to the magnetic core surrounding the portion of the plasma discharge region.  
      The light source can include an enclosure that at least partially encloses the magnetic core. The enclosure can define a plurality of holes in the enclosure. A plurality of plasma loops can pass through the plurality of holes when the magnetic core delivers power to the plasma. The enclosure can include two parallel (e.g., disk-shaped) plates. The parallel plates can be conductive and form a primary winding around the magnetic core. The enclosure can, for example, include or be formed from a metal material such as copper, tungsten, aluminum or one of a variety of copper-tungsten alloys. Coolant can flow through the enclosure for cooling a location adjacent the localized high intensity zone.  
      In some embodiments of the invention the light source can be configured to produce light for different uses. In other embodiments of the invention a light source can be configured to produce light at wavelengths shorter than about 100 nm when the light source generates a plasma discharge. In another embodiment of the invention a light source can be configured to produce light at wavelengths shorter than about 15 nm when the light source generates a plasma discharge. The light source can be configured to generate a plasma discharge suitable for semiconductor fabrication lithographic systems. The light source can be configured to generate a plasma discharge suitable for microscopy systems.  
      The invention, in another aspect, features an inductively-driven light source.  
      In another aspect of the invention, a light source features a chamber having a plasma discharge region and containing an ionizable material. The light source also includes a transformer having a first magnetic core that surrounds a portion of the plasma discharge region. The light source also includes a second magnetic core linked with the first magnetic core by a current. The light source also includes a power supply for providing a first signal (e.g., a voltage signal) to the second magnetic core, wherein the second magnetic core provides a second signal (e.g., a pulse of energy) to the first magnetic core when the second magnetic core saturates, and wherein the first magnetic core delivers power to a plasma formed in the plasma discharge region from the ionizable medium in response to the second signal. The light source can include a metallic material for conducting the current.  
      In another aspect of the invention, a light source includes a chamber having a channel region and containing an ionizable medium. The light source includes a magnetic core that surrounds a portion of the channel region and a pulse power system for providing at least one pulse of energy to the magnetic core for exciting the ionizable medium to form at least essentially a Z-pinch in the channel region. The current density of the plasma can be greater than about 1 KA/cm 2 . The pressure in the channel region can be less than about 100 mTorr. In other embodiments, the pressure is less than about 1 Torr. In some embodiments, the pressure is about 200 mTorr.  
      In yet another aspect of the invention, a light source includes a chamber containing a light emitting plasma with a localized high-intensity zone that emits a substantial portion of the emitted light. The light source also includes a magnetic core that surrounds a portion of the non-uniform light emitting plasma. The light source also includes a pulse power system for providing at least one pulse of energy to the magnetic core for delivering power to the plasma.  
      In another aspect of the invention, a light source includes a chamber having a plasma discharge region and containing an ionizable medium. The light source also includes a magnetic core that surrounds a portion of the plasma discharge region. The light source also includes a means for providing at least one pulse of energy to the magnetic core for delivering power to a plasma formed in the plasma discharge region. The plasma has a localized high intensity zone.  
      In another aspect of the invention, a plasma source includes a chamber having a plasma discharge region and containing an ionizable medium. The plasma source also includes a magnetic core that surrounds a portion of the plasma discharge region and induces an electric current in the plasma sufficient to form a Z-pinch.  
      In general, in another aspect the invention relates to a method for generating a light signal. The method involves introducing an ionizable medium capable of generating a plasma into a chamber. The method also involves applying at least one pulse of energy to a magnetic core that surrounds a portion of a plasma discharge region within the chamber such that the magnetic core delivers power to the plasma. The plasma has a localized high intensity zone.  
      The method for generating the light signal can involve producing a non-uniformity in the plasma. The method also can involve localizing an emission of light by the plasma. The method also can involve producing a region of higher pressure to produce the non-uniformity.  
      The plasma can be a non-uniform plasma. The plasma can substantially vary in current density along a path of current flow in the plasma. The zone can be a point source of high intensity light. The zone can be a region where the plasma is pinched to form a neck. The zone can be created with a feature in the chamber. The zone can be created with gas pressure. The zone can be created with an output of the power system. Current flow in the plasma can create the zone.  
      The method also can involve locating an insert in the plasma discharge region. The insert can define a necked region for localizing an emission of light by the plasma. The insert can include a gas inlet and/or cooling capability. A non-uniformity can be produced in the plasma by a feature located in the chamber. The feature can be configured to substantially localize an emission of light by the plasma. The feature can be located remotely relative to the magnetic core.  
      The at least one pulse of energy provided to the magnetic core can form the plasma. Each pulse of energy can be pulsed at a frequency of between about 100 pulses per second and about 15,000 pulses per second. Each pulse of energy can be provided for a duration of time between about 10 ns and about 10 μs. The pulse power system can an energy storage device, for example, at least one capacitor and/or a second magnetic core.  
      In some embodiments, the method of the invention can involve discharging the at least one pulse of energy from the second magnetic core to the first magnetic core to deliver power to the plasma. The pulse power system can include, for example, a magnetic pulse-compression generator and/or a saturable inductor. The method can involve delivering each pulse of energy to the magnetic core by operation of a magnetic switch.  
      In some embodiments, the method of the invention can involve producing at least essentially a Z-pinch or essentially a capillary discharge in a channel region located in the chamber. In some embodiments the method can involve introducing the ionizable medium into the chamber via at least one port. The ionizable medium can include one or more gases, for example, one or more of the following gases: Xenon, Lithium, Nitrogen, Argon, Helium, Fluorine, Tin, Ammonia, Stannane, Krypton or Neon. The method also can involve pre-ionizing the ionizable medium with an ionization source (e.g., an ultraviolet lamp, an RF source, a spark plug or a DC discharge source). Alternatively or additionally, inductive leakage current flowing from a second magnetic core to the magnetic core surrounding the portion of the plasma discharge region can be used to pre-ionize the ionizable medium. In another embodiment, the ionizable medium can be a solid (e.g., Tin or Lithium) that can be vaporized by a thermal process or sputtering process within the chamber or vaporized externally and then introduced into the chamber.  
      In another embodiment of the invention the method can involve at least partially enclosing the magnetic core within an enclosure. The enclosure can include a plurality of holes. A plurality of plasma loops can pass through the plurality of holes when the magnetic core delivers power to the plasma. The enclosure can include two parallel plates. The two parallel plates can be used to form a primary winding around the magnetic core. The enclosure can include or be formed from a metal material, for example, copper, tungsten, aluminum or copper-tungsten alloys. Coolant can be provided to the enclosure to cool a location adjacent the localized high intensity location.  
      The method can involve alternately uncovering the plasma discharge region. A rotating disk can be used to alternately uncover the plasma discharge region and alternately define a feature that creates the localized high intensity zone. A coolant can be provided to a hollow region in the rotating disk.  
      In another embodiment the method can involve producing light at wavelengths shorter than about 100 nm. In another embodiments the method can involve producing light at wavelengths shorter than about 15 nm. The method also can involve generating a plasma discharge suitable for semiconductor fabrication lithographic systems. The method also can involve generating a plasma discharge suitable for microscopy systems.  
      The invention, in another aspect, features a lithography system. The lithography system includes at least one light collection optic and at least one light condenser optic in optical communication with the at least one collection optic. The lithography system also includes a light source capable of generating light for collection by the at least one collection optic. The light source includes a chamber having a plasma discharge region and containing an ionizable medium. The light source also includes a magnetic core that surrounds a portion of the plasma discharge region and a pulse power system for providing at least one pulse of energy to the magnetic core for delivering power to a plasma formed in the plasma discharge region. The plasma has a localized high intensity zone.  
      In some embodiments of the invention, light emitted by the plasma is collected by the at least one collection optic, condensed by the at least one condenser optic and at least partially directed through a lithographic mask.  
      The invention, in another aspect, features an inductively-driven light source for illuminating a semiconductor wafer in a lithography system.  
      In general, in another aspect the invention relates to a method for illuminating a semiconductor wafer in a lithography system. The method involves introducing an ionizable medium capable of generating a plasma into a chamber. The method also involves applying at least one pulse of energy to a magnetic core that surrounds a portion of a plasma discharge region within the chamber such that the magnetic core delivers power to the plasma. The plasma has a localized high intensity zone. The method also involves collecting light emitted by the plasma, condensing the collected light; and directing at least part of the condensed light through a mask onto a surface of a semiconductor wafer.  
      The invention, in another aspect, features a microscopy system. The microscopy system includes a first optical element for collecting light and a second optical element for projecting an image of a sample onto a detector. The detector is in optical communication with the first and second optical elements. The microscopy system also includes a light source in optical communication with the first optical element. The light source includes a chamber having a plasma discharge region and containing an ionizable medium. The light source also includes a magnetic core that surrounds a portion of the plasma discharge region and a pulse power system for providing at least one pulse of energy to the magnetic core for delivering power to a plasma formed in the plasma discharge region. The plasma has a localized high intensity zone.  
      In some embodiments of the invention, light emitted by the plasma is collected by the first optical element to illuminate the sample and the second optical element projects an image of the sample onto the detector.  
      In general, in another aspect the invention relates to a microscopy method. The method involves introducing an ionizable medium capable of generating a plasma into a chamber. The method also involves applying at least one pulse of energy to a magnetic core that surrounds a portion of a plasma discharge region within the chamber such that the magnetic core delivers power to the plasma. The plasma has a localized high intensity zone. The method also involves collecting a light emitted by the plasma with a first optical element and projecting it through a sample. The method also involves projecting the light emitted through the sample to a detector.  
      Another aspect of the invention features an insert for an inductively-driven plasma light source. The insert has a body that defines at least one interior passage and has a first open end and a second open end. The insert has an outer surface adapted to couple or connect with an inductively-driven plasma light source in a plasma discharge region. In other embodiments, the outer surface of the insert is directly connected to the plasma light source. In other embodiments, the outer surface of the insert is indirectly connected to the plasma light source. In other embodiments, the outer surface of the insert is in physical contact with the plasma light source.  
      The at least one interior passage can define a region to create a localized high intensity zone in the plasma. The insert can be a consumable. The insert can be in thermal communication with a cooling structure.  
      In one embodiment, the outer surface of the insert couples or connects to the plasma light source by threads in a receptacle inside a chamber of the plasma light source. In another embodiment, the insert can slip fit into a receptacle inside a chamber of the plasma light source and tighten in place due to heating by the plasma (e.g., in the plasma discharge region).  
      In some embodiments, at least a surface of the at least one interior passage of the insert includes a material with a low plasma sputter rate (e.g., carbon, titanium, tungsten, diamond, graphite, silicon carbide, silicon, ruthenium, or a refractory material). In other embodiments, a surface of at least one interior passage of the insert includes a material with both a low plasma sputter rate and a high thermal conductivity (e.g., highly oriented pyrolytic graphite (HOPG) or thermal pyrolytic graphite (TPG)). In another embodiment, a surface of at least one interior passage of the insert can be made of a material having a low absorption of EUV radiation (e.g., ruthenium or silicon).  
      The interior passage geometry of the insert can be used to control the size and shape of the plasma high intensity zone. The inner surface of the passage can define a reduced dimension of the passage. The geometry of the inner surface of the passage can be asymmetric about a midline between the two open ends. In another embodiment, the geometry of the inner surface can be defined by a radius of curvature which is substantially less than the minimum dimension across the passage. In another embodiment, the geometry of the inner surface can be defined by a radius of curvature between about 25% to about 100% of the minimum dimension across the passage.  
      The invention, in another aspect, features an insert for an inductively-driven plasma light source. The insert has a body defining at least one interior passage and has a first open end and a second open end. The insert also has a means for coupling or connecting with an inductively-driven light source in a plasma discharge region.  
      The insert can be defined by two or more bodies. The insert can have at least one gas inlet hole in the body. In another embodiment, the insert can have at least one cooling channel passing through the body. In one embodiment, the insert is replaced using a robotic arm.  
      The invention, in another aspect, features a light source. The light source includes a chamber having a plasma discharge region and containing an ionizable medium. The light source also includes a magnetic core that surrounds a portion of the plasma discharge region. The light source also includes a power system for providing energy to the magnetic core for delivering power to a plasma formed in the plasma discharge region, wherein the plasma has a localized high intensity zone. The light source also includes a filter disposed relative to the light source to reduce indirect or direct plasma emissions.  
      The filter can be configured to maximize collisions with emissions which are not traveling parallel to radiation emanating from the light source (e.g., from the high intensity zone). The filter can be configured to minimize reduction of emissions traveling parallel to radiation emanating from the light source (e.g., from the high intensity zone). In one embodiment, the filter is made up of walls which are substantially parallel to the direction of radiation emanating from the high intensity zone, and has channels between the walls. A curtain of gas can be maintained in the vicinity of the filter to increase collisions between the filter and emissions other than radiation.  
      In another embodiment, the filter can have cooling channels. The surfaces of the filter which are exposed to the emissions can comprise a material with a low plasma sputter rate (e.g., carbon, titanium, tungsten, diamond, graphite, silicon carbide, silicon, ruthenium, or a refractory material). In another embodiment, the surfaces of the filter which are exposed to the emissions can comprise a material with both a low plasma sputter rate and a high thermal conductivity (e.g., highly oriented pyrolytic graphite or thermal pyrolytic graphite).  
      In another aspect, the invention relates to a method for generating a light signal. The method includes introducing an ionizable medium capable of generating a plasma into a chamber. The method also includes applying energy to a magnetic core that surrounds a portion of a plasma discharge region within the chamber such that the magnetic core delivers power to the plasma. The plasma has a localized high intensity zone. The inventive method also includes filtering emissions emanating from the localized high intensity zone of the plasma.  
      In one embodiment, the method includes positioning the filter relative to the high intensity zone (e.g., a source of light) to reduce direct or indirect emissions. The method can include maximizing collisions with emissions which are not traveling parallel to radiation emanating from the high intensity zone. The method can include minimizing reduction of emissions traveling parallel to the radiation emanating from the high intensity zone.  
      In one embodiment, this method can include locating walls which are substantially parallel to the direction of radiation emanating from the high intensity zone and positioning channels between the walls. The surfaces of the filter which are exposed to the emissions can comprise a material with a low plasma sputter rate (e.g., carbon, titanium, tungsten, diamond, graphite, silicon carbide, silicon, ruthenium, or a refractory material). In another embodiment, the surfaces of the filter which are exposed to the emissions can comprise a material with both a low plasma sputter rate and a high thermal conductivity (e.g., highly oriented pyrolytic graphite or thermal pyrolytic graphite).  
      The invention, in another aspect, features a light source. The light source includes a chamber having a plasma discharge region and containing an ionizable material. The light source also includes a magnetic core that surrounds a portion of the plasma discharge region. The light source also includes a power system for providing energy to the magnetic core for delivering power to a plasma formed in the plasma discharge region and having a localized high intensity zone. The light source also includes means for minimal reduction of emissions traveling substantially parallel to the direction of radiation emitted from the high intensity zone. The light source also includes means for maximal reduction of emissions traveling other than substantially parallel to the direction of the radiation emitted from the high intensity zone.  
      The invention, in another aspect, features an inductively-driven plasma source. The plasma source includes a chamber having a plasma discharge region and containing an ionizable medium. The plasma source also includes a system for spreading heat flux and ion flux over a large surface area. This system uses at least one object, located within the plasma chamber, where at least the outer surface of the object moves with respect to the plasma. At least one of the objects is in thermal communication with a cooling channel.  
      In another embodiment, the outer surface of at least one of the objects can include a sacrificial layer. The sacrificial layer can be continuously coated on the outer surface. The sacrificial layer can be made from a material which emits EUV radiation (e.g., lithium or tin).  
      In another embodiment, the objects can be two or more closely spaced rods. The space between the rods can define a region to create a localized high intensity zone in the plasma. In another embodiment, a local geometry of the at least one object can define a region to create a localized high intensity zone in the plasma.  
      In general, in another aspect, the invention relates to a method for generating an inductively-driven plasma. The method includes introducing an ionizable medium capable of generating a plasma in a chamber and applying energy to a magnetic core surrounding a plasma discharge region in the chamber. The method also includes spreading the heat flux and ion flux from the inductively-driven plasma over a large surface area. The method includes locating at least one object within a region of the plasma and moving at least an outer surface of the at least one object with respect to the plasma. The method also includes providing the at least one object with a cooling channel in thermal communication with the at least one object. In this method, the plasma can erode a sacrificial layer from the outer surface of the object. In another embodiment, the method can include continuously coating the outer surface of the at least one object with the sacrificial layer. The sacrificial layer can be formed of a material which emits EUV radiation (e.g., lithium or tin).  
      The method can further include placing the at least one object in such a way as to create a localized high intensity zone in the plasma. The method can also involve locating a second object relative to the first object in order to define a region to create a localized high intensity zone in the plasma.  
      The invention, in one aspect, features a light source. The light source includes a chamber having a plasma discharge region and containing an ionizable medium. The light source also includes a magnetic core that surrounds a portion of the plasma discharge region. The light source also includes a pulse power system for providing at least one pulse of energy to the magnetic core for delivering power to a plasma formed in the plasma discharge region. The plasma has a localized high intensity zone. The light source includes a magnet located in the chamber to modify a shape of the plasma. In one embodiment, the magnet is inside the plasma discharge region and can create the localized high intensity zone. The magnet can be a permanent magnet or an electromagnet. In another embodiment, the magnet can be located adjacent the high intensity zone.  
      The invention, in another aspect, relates to a method for operating an EUV light source. EUV light is generated in a chamber using a plasma. A consumable is provided which defines a localized region of high intensity in the plasma. The method also includes replacing (e.g., with a robotic arm) the consumable based on a selected criterion without exposing the chamber to atmospheric conditions. In some embodiments, the selected criterion is one or more of a predetermined time, a measured degradation of the consumable, or a measured degradation of a process control variable associated with operation of the light source. In some embodiments, the selected criterion is a measured degradation of a process control variable associated with operation of a system (e.g., lithography system, microscopy system, or other semiconductor processing system).  
      The method can also include maintaining a vacuum in the chamber during replacement of the consumable. The plasma light source can be an inductively-driven plasma light source. The consumable can be an insert.  
      The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other objects, feature and advantages of the invention, as well as the invention itself, will be more fully understood from the following illustrative description, when read together with the accompanying drawings which are not necessarily to scale.  
       FIG. 1  is a cross-sectional view of a magnetic core surrounding a portion of a plasma discharge region, according to an illustrative embodiment of the invention.  
       FIG. 2  is a schematic electrical circuit model of a plasma source, according to an illustrative embodiment of the invention.  
       FIG. 3A  is a cross-sectional view of two magnetic cores and a feature for producing a non-uniformity in a plasma, according to another illustrative embodiment of the invention.  
       FIG. 3B  is a blow-up view of a region of  FIG. 3A .  
       FIG. 4  is a schematic electrical circuit model of a plasma source, according to an illustrative embodiment of the invention.  
       FIG. 5A  is an isometric view of a plasma source, according to an illustrative embodiment of the invention.  
       FIG. 5B  is a cutaway view of the plasma source of  FIG. 5A .  
       FIG. 6  is a schematic block diagram of a lithography system, according to an illustrative embodiment of the invention.  
       FIG. 7  is a schematic block diagram of a microscopy system, according to an illustrative embodiment of the invention.  
       FIG. 8A  is a cutaway view of an isometric view of a plasma source illustrating the placement of an insert, according to an illustrative embodiment of the invention.  
       FIG. 8B  is a blow-up of a region of  FIG. 8A .  
       FIG. 9A  is a cross-sectional view of an insert having an asymmetric inner geometry, according to an illustrative embodiment of the invention.  
       FIG. 9B  is a cross-sectional view of an insert, according to an illustrative embodiment of the invention.  
       FIG. 9C  is a cross-sectional view of an insert, according to an illustrative embodiment of the invention.  
       FIG. 10  is a schematic diagram of the placement of a filter, according to an illustrative embodiment of the invention.  
       FIG. 11A  is a schematic view of a filter, according to an illustrative embodiment of the invention.  
       FIG. 11B  is a cross-sectional view of the filter of  FIG. 11A .  
       FIG. 12A  is a schematic side view of a system for spreading heat and ion flux from a plasma over a large surface area, according to an illustrative embodiment of the invention.  
       FIG. 12B  is a schematic end-view of the system of  FIG. 12A .  
       FIG. 13  is a cross-sectional diagram of a plasma chamber, showing placement of magnets to create a high intensity zone, according to an illustrative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
       FIG. 1  is a cross-sectional view of a plasma source  100  for generating a plasma that embodies the invention. The plasma source  100  includes a chamber  104  that defines a plasma discharge region  112 . The chamber  104  contains an ionizable medium that is used to generate a plasma (shown as two plasma loops  116   a  and  116   b ) in the plasma discharge region  112 . The plasma source  100  includes a transformer  124  that induces an electric current into the two plasma loops  116   a  and  116   b  (generally  116 ) formed in the plasma discharge region  112 . The transformer  124  includes a magnetic core  108  and a primary winding  140 . A gap  158  is located between the winding  140  and the magnetic core  108 .  
      In this embodiment, the winding  140  is a copper enclosure that at least partially encloses the magnetic core  108  and that provides a conductive path that at least partially encircles the magnetic core  108 . The copper enclosure is electrically equivalent to a single turn winding that encircles the magnetic core  108 . In another embodiment, the plasma source  100  instead includes an enclosure that at least partially encloses the magnetic core  108  in the chamber  104  and a separate metal (e.g., copper or aluminum) strip that at least partially encircles the magnetic core  108 . In this embodiment, the metal strip is located in the gap  158  between the enclosure and the magnetic core  108  and is the primary winding of the magnetic core  108  of the transformer  124 .  
      The plasma source  100  also includes a power system  136  for delivering energy to the magnetic core  108 . In this embodiment, the power system  136  is a pulse power system that delivers at least one pulse of energy to the magnetic core  108 . In operation, the power system  136  typically delivers a series of pulses of energy to the magnetic core  108  for delivering power to the plasma. The power system  136  delivers pulses of energy to the transformer  124  via electrical connections  120   a  and  120   b  (generally  120 ). The pulses of energy induce a flow of electric current in the magnetic core  108  that delivers power to the plasma loops  116   a  and  116   b  in the plasma discharge region  112 . The magnitude of the power delivered to the plasma loops  116   a  and  116   b  depends on the magnetic field produced by the magnetic core  108  and the frequency and duration of the pulses of energy delivered to the transformer  124  according to Faraday&#39;s law of induction.  
      In some embodiments, the power system  136  provides pulses of energy to the magnetic core  108  at a frequency of between about 1 pulse and about 50,000 pulses per second. In certain embodiments, the power system  136  provides pulses of energy to the magnetic core  108  at a frequency of between about 100 pulses and 15,000 pulses per second. In certain embodiments, the pulses of energy are provide to the magnetic core  108  for a duration of time between about 10 ns and about 10 μs. The power system  136  may include an energy storage device (e.g., a capacitor) that stores energy prior to delivering a pulse of energy to the magnetic core  108 . In some embodiments, the power system  136  includes a second magnetic core. In certain embodiments, the second magnetic core discharges pulses of energy to the first magnetic core  108  to deliver power to the plasma. In some embodiments, the power system  136  includes a magnetic pulse-compression generator and/or a saturable inductor. In other embodiments, the power system  136  includes a magnetic switch for selectively delivering the pulse of energy to the magnetic core  108 . In certain embodiments, the pulse of energy can be selectively delivered to coincide with a predefined or operator-defined duty cycle of the plasma source  100 . In other embodiments, the pulse of energy can be delivered to the magnetic core when, for example, a saturable inductor becomes saturated.  
      The plasma source  100  also may include a means for generating free charges in the chamber  104  that provides an initial ionization event that pre-ionizes the ionizable medium to ignite the plasma loops  116   a  and  116   b  in the chamber  104 . Free charges can be generated in the chamber by an ionization source, such as, an ultraviolet light, an RF source, a spark plug or a DC discharge source. Alternatively or additionally, inductive leakage current flowing from a second magnetic core in the power system  136  to the magnetic core  108  can pre-ionize the ionizable medium. In certain embodiments, the ionizable medium is pre-ionized by one or more ionization sources.  
      The ionizable medium can be an ionizable fluid (i.e., a gas or liquid). By way of example, the ionizable medium can be a gas, such as Xenon, Lithium, Tin, Nitrogen, Argon, Helium, Fluorine, Ammonia, Stannane, Krypton or Neon. Alternatively, the ionizable medium can be finely divided particle (e.g., Tin) introduced through at least one gas port into the chamber  104  with a carrier gas, such as helium. In another embodiment, the ionizable medium can be a solid (e.g., Tin or Lithium) that can be vaporized by a thermal process or sputtering process within the chamber or vaporized externally and then introduced into the chamber  104 . In certain embodiments, the plasma source  100  includes a vapor generator (not shown) that vaporizes the metal and introduces the vaporized metal into the chamber  104 . In certain embodiments, the plasma source  100  also includes a heating module for heating the vaporized metal in the chamber  104 . The chamber  104  may be formed, at least in part, from a metallic material such as copper, tungsten, a copper-tungsten alloy or any material suitable for containing the ionizable medium and the plasma and for otherwise supporting the operation of the plasma source  100 .  
      Referring to  FIG. 1 , the plasma loops  116   a  and  116   b  converge in a channel region  132  defined by the magnetic core  108  and the winding  140 . In one exemplary embodiment, pressure in the channel region is less than about 100 mTorr. In other embodiments, the pressure is less than about 1 Torr. In some embodiments, the pressure is about 200 mTorr. Energy intensity varies along the path of a plasma loop if the cross-sectional area of the plasma loop varies along the length of the plasma loop. Energy intensity may therefore be altered along the path of a plasma loop by use of features or forces that alter cross-sectional area of the plasma loop. Altering the cross-sectional area of a plasma loop is also referred to herein as constricting the flow of current in the plasma or pinching the plasma loop. Accordingly, the energy intensity is greater at a location along the path of the plasma loop where the cross-sectional area is decreased. Similarly, the energy intensity is lower at a given point along the path of the plasma loop where the cross-sectional area is increased. It is therefore possible to create locations with higher or lower energy intensity.  
      Constricting the flow of current in a plasma is also sometimes referred to as producing a Z-pinch or a capillary discharge. A Z-pinch in a plasma is characterized by the plasma decreasing in cross-sectional area at a specific location along the path of the plasma. The plasma decreases in cross-sectional area as a result of the current that is flowing through the cross-sectional area of the plasma at the specific location. Generally, a magnetic field is generated due to the current in the plasma and, the magnetic field confines and compresses the plasma. In this case, the plasma carries an induced current along the plasma path and a resulting magnetic field surrounds and compresses the plasma. This effect is strongest where the cross-sectional area of the plasma is minimum and works to further compress the cross-sectional area, hence further increasing the current density in the plasma.  
      In one embodiment, the channel  132  is a region of decreased cross-sectional area relative to other locations along the path of the plasma loops  116   a  and  116   b . As such, the energy intensity is increased in the plasma loops  116   a  and  116   b  within the channel  132  relative to the energy intensity in other locations of the plasma loops  116   a  and  116   b.  The increased energy intensity increases the emitted electromagnetic energy (e.g., emitted light) in the channel  132 .  
      The plasma loops  116   a  and  116   b  also have a localized high intensity zone  144  as a result of the increased energy intensity. In certain embodiments, a high intensity light  154  is produced in and emitted from the zone  144  due to the increased energy intensity. Current density substantially varies along the path of the current flow in the plasma loops  116   a  and  116   b . In one exemplary embodiment, the current density of the plasma is in the localized high intensity zone is greater than about 1 KA/cm 2 . In some embodiments, the zone  144  is a point source of high intensity light and is a region where the plasma loops  116   a  and  116   b  are pinched to form a neck.  
      In some embodiments, a feature is located in the chamber  104  that creates the zone  144 . In certain embodiments, the feature produces a non-uniformity in the plasma loops  116   a  and  116   b . The feature is permanent in some embodiments and removable in other embodiments. In some embodiments, the feature is configured to substantially localize an emission of light by the plasma loops  116   a  and  116   b  to, for example, create a point source of high intensity electromagnetic radiation. In other embodiments, the feature is located remotely relative to the magnetic core  108 . In certain embodiments, the remotely located feature creates the localized high intensity zone in the plasma in a location remote to the magnetic core  108  in the chamber  104 . For example, the disk  308  of  FIGS. 3A and 3B  discussed later herein is located remotely relative to the magnetic core  108 . In certain embodiment, a gas inlet is located remotely from the magnetic core to create a region of higher pressure to create a localized high intensity zone.  
      In some embodiments, the feature is an insert that defines a necked region. In certain embodiments, the insert localizes an emission of light by the plasma in the necked region. In certain other embodiments, the insert includes a gas inlet for, for example, introducing the ionizable medium into the chamber  104 . In other embodiments, the feature includes cooling capability for cooling a region of the feature. In certain embodiments, the cooling capability involves subcooled flow boiling as described by, for example, S. G. Kandlikar “ Heat Transfer Characteristics in Partial Boiling, Fully Developed Boling, and Significant Void Flow Regions of Subcooled Flow Boiling”Journal of Heat Transfer Feb.  2, 1998. In certain embodiments, the cooling capability involves pressurized subcooled flow boiling. In other embodiments, the insert includes cooling capability for cooling a region of the insert adjacent to, for example, the zone  144 .  
      In some embodiments, gas pressure creates the localized high intensity zone  144  by, for example, producing a region of higher pressure at least partially around a portion of the plasma loops  116   a  and  116   b . The plasma loops  116   a  and  116   b  are pinched in the region of high pressure due to the increased gas pressure. In certain embodiments, a gas inlet is the feature that introduces a gas into the chamber  104  to increase gas pressure. In yet another embodiment, an output of the power system  136  can create the localized high intensity zone  144  in the plasma loops  116   a  and  116   b.    
       FIG. 2  is a schematic electrical circuit model  200  of a plasma source, for example the plasma source  100  of  FIG. 1 . The model  200  includes a power system  136 , according to one embodiment of the invention. The power system  136  is electrically connected to a transformer, such as the transformer  124  of  FIG. 1 . The model  200  also includes an inductive element  212  that is a portion of the electrical inductance of the plasma, such as the plasma loops  116   a  and  116   b  of  FIG. 1 . The model  200  also includes a resistive element  216  that is a portion of the electrical resistance of the plasma, such as the plasma loops  116   a  and  116   b  of  FIG. 1 . In this embodiment, the power system is a pulse power system that delivers via electrical connections  120   a  and  120   b  a pulse of energy to the transformer  124 . The pulse of energy is then delivered to the plasma by, for example, a magnetic core which is a component of the transformer, such as the magnetic core  108  of the transformer  124  of  FIG. 1 .  
      In another embodiment, illustrated in  FIGS. 3A and 3B , the plasma source  100  includes a chamber  104  that defines a plasma discharge region  112 . The chamber  104  contains an ionizable medium that is used to generate a plasma in the plasma discharge region  112 . The plasma source  100  includes a transformer  124  that couples electromagnetic energy into two plasma loops  116   a  and  116   b  (generally  116 ) formed in the plasma discharge region  112 . The transformer  124  includes a first magnetic core  108 . The plasma source  100  also includes a winding  140 . In this embodiment, the winding  140  is an enclosure for locating the magnetic cores  108  and  304  in the chamber  104 . The winding  104  is also a primary winding of magnetic core  108  and a winding for magnetic core  304 .  
      The winding  140  around the first magnetic core  108  forms the primary winding of the transformer  124 . In this embodiment, the second magnetic core and the winding  140  are part of the power system  136  and form a saturable inductor that delivers a pulse of energy to the first magnetic core  108 . The power system  136  includes a capacitor  320  that is electrically connected via connections  380   a  and  380   b  to the winding  140 . In certain embodiments, the capacitor  320  stores energy that is selectively delivered to the first magnetic core  108 . A voltage supply  324 , which may be a line voltage supply or a bus voltage supply, is coupled to the capacitor  320 .  
      The plasma source  100  also includes a disk  308  that creates a localized high intensity zone  144  in the plasma loops  116   a  and  116   b . In this embodiment, the disk  308  is located remotely relative to the first magnetic core  108 . The disk  308  rotates around the Z-axis of the disk  308  (referring to  FIG. 3B ) at a point of rotation  316  of the disk  308 . The disk  308  has three apertures  312   a ,  312   b  and  312   c  (generally  312 ) that are located equally angularly spaced around the disk  308 . The apertures  312  are located in the disk  308  such that at any angular orientation of the disk  308  rotated around the Z-Axis only one (e.g., aperture  312   a  in  FIGS. 3A and 3B ) of the three apertures  312   a ,  312   b  and  312   c  is aligned with the channel  132  located within the core  108 . In this manner, the disk  308  can be rotated around the Z-axis such that the channel  132  may be alternately uncovered (e.g., when aligned with an aperture  312 ) and covered (e.g., when not aligned with an aperture  312 ). The disk  308  is configured to pinch (i.e., decrease the cross-sectional area of) the two plasma loops  116   a  and  116   b  in the aperture  312   a . In this manner, the apertures  312  are features in the disk of the plasma source  100  that create the localized high intensity zone  144  in the plasma loops  316   a  and  316   b.  By pinching the two plasma loops  116   a  and  116   b  in the location of the aperture  312   a  the energy intensity of the two plasma loops  116   a  and  116   b  in the location of the aperture  312   a  is greater than the energy intensity in a cross-section of the plasma loops  116   a  and  116   b  in other locations along the current paths of the plasma loops  116   a  and  116   b.    
      It is understood that variations on, for example, the geometry of the disk  308  and the number and or shape of the apertures  312  is contemplated by the description herein. In one embodiment, the disk  308  is a stationary disk having at least one aperture  312 . In some embodiments, the disk  308  has a hollow region (not shown) for carrying coolant to cool a region of the disk  308  adjacent the localized high intensity zone  144 . In some embodiments, the plasma source  100  includes a thin gas layer that conducts heat from the disk  308  to a cooled surface in the chamber  104 .  
       FIG. 4  illustrates an electrical circuit model  400  of a plasma source, such as the plasma source  100  of  FIGS. 3A and 3B . The model  400  includes a power system  136  that is electrically connected to a transformer, such as the transformer  124  of  FIG. 3A . The model  400  also includes an inductive element  212  that is a portion of the electrical inductance of the plasma. The model  400  also includes a resistive element  216  that is a portion of the resistance of the plasma. A pulse power system  136  delivers via electrical connections  380   a  and  380   b  pulses of energy to the transformer  124 . The power system  136  includes a voltage supply  324  that charges the capacitor  320 . The power system  136  also includes a saturable inductor  328  which is a magnetic switch that delivers energy stored in the capacitor  320  to the first magnetic core  108  when the inductor  328  becomes saturated.  
      In some embodiments, the capacitor  320  is a plurality of capacitors that are connected in parallel. In certain embodiments, the saturable inductor  328  is a plurality of saturable inductors that form, in part, a magnetic pulse-compression generator. The magnetic pulse-compression generator compresses the pulse duration of the pulse of energy that is delivered to the first magnetic core  108 .  
      In another embodiment, illustrated in  FIGS. 5A and 5B , a portion of a plasma source  500  includes an enclosure  512  that, at least, partially encloses a first magnetic core  524  and a second magnetic core  528 . In this embodiment, the enclosure  512  has two conductive parallel plates  540   a  and  540   b  that form a conductive path at least partially around the first magnetic core  524  and form a primary winding around the first magnetic core  524  of a transformer, such as the transformer  124  of  FIG. 4 . The parallel plates  540   a  and  540   b  also form a conductive path at least partially around the second magnetic core  528  forming an inductor, such as the inductor  328  of  FIG. 4 . The plasma source  500  also includes a plurality of capacitors  520  located around the outer circumference of the enclosure  512 . By way of example, the capacitors  520  can be the capacitor  320  of  FIG. 4 .  
      The enclosure  512  defines at least two holes  516  and  532  that pass through the enclosure  512 . In this embodiment, there are six holes  532  that are located equally angularly spaced around a diameter of the plasma source  500 . Hole  516  is a single hole through the enclosure  512 . In one embodiment, the six plasma loops  508  each converge and pass through the hole  516  as a single current carrying plasma path. The six plasma loops also each pass through one of the six holes  532 . The parallel plates  540   a  and  540   b  have a groove  504  and  506 , respectively. The grooves  504  and  506  each locate an annular element (not shown) for creating a pressurized seal and for defining a chamber, such as the chamber  104  of  FIG. 3A , which encloses the plasma loops  508  during operation of the plasma source  500 .  
      The hole  516  in the enclosure defines a necked region  536 . The necked region  536  is a region of decreased cross-section area relative to other locations along the length of the hole  516 . As such, the energy intensity is increased in the plasma loops  508 , at least, in the necked region  536  forming a localized high intensity zone in the plasma loops  508  in the necked region  536 . In this embodiment, there also are a series of holes  540  located in the necked region  536 . The holes  540  may be, for example, gas inlets for introducing the ionizable medium into the chamber of the plasma source  500 . In other embodiments, the enclosure  512  includes a coolant passage (not shown) for flowing coolant through the enclosure for cooling a location of the enclosure  512  adjacent the localized high intensity zone.  
       FIG. 6  is a schematic block diagram of a lithography system  600  that embodies the invention. The lithography system  600  includes a plasma source, such as the plasma source  500  of  FIGS. 5A and 5B . The lithography system  600  also includes at least one light collection optic  608  that collects light  604  emitted by the plasma source  500 . By way of example, the light  604  is emitted by a localized high intensity zone in the plasma of the plasma source  500 . In one embodiment, the light  604  produced by the plasma source  500  is light having a wavelength shorter than about 15 nm for processing a semiconductor wafer  636 . The light collection optic  608  collects the light  604  and directs collected light  624  to at least one light condenser optic  612 . In this embodiment, the light condenser optic  624  condenses (i.e., focuses) the light  624  and directs condensed light  628  towards mirror  616   a  (generally  616 ) which directs reflected light  632   a  towards mirror  616   b  which, in turn, directs reflected light  632   b  towards a reflective lithographic mask  620 . Light reflecting off the lithographic mask  620  (illustrated as the light  640 ) is directed to the semiconductor wafer  636  to, for example, produce at least a portion of a circuit image on the wafer  636 . Alternatively, the lithographic mask  620  can be a transmissive lithographic mask in which the light  632   b , instead, passes through the lithographic mask  620  and produces a circuit image on the wafer  636 .  
      In an exemplary embodiment, a lithography system, such as the lithography system  600  of  FIG. 6  produces a circuit image on the surface of the semiconductor wafer  636 . The plasma source  500  produces plasma at a pulse rate of about 10,000 pulses per second. The plasma has a localized high intensity zone that is a point source of pulses of high intensity light  604  having a wavelength shorter than about 15 nm. Collection optic  608  collects the light  604  emitted by the plasma source  500 . The collection optic  608  directs the collected light  624  to light condenser optic  612 . The light condenser optic  624  condenses (i.e., focuses) the light  624  and directs condensed light  628  towards mirror  616   a  (generally  616 ) which directs reflected light  632   a  towards mirror  616   b  which, in turn, directs reflected light  632   b  towards a reflective lithographic mask  620 . The mirrors  616   a  and  616   b  are multilayer optical elements that reflect wavelengths of light in a narrow wavelength band (e.g., between about 5 nm and about 20 nm). The mirrors  616   a  and  616   b , therefore, transmit light in that narrow band (e.g., light having a low infrared light content).  
       FIG. 7  is a schematic block diagram of a microscopy system  700  (e.g., a soft X-ray microscopy system) that embodies the invention. The microscopy system  700  includes a plasma source, such as the plasma source  500  of  FIGS. 5A and 5B . The microscopy system  700  also includes a first optical element  728  for collecting light  706  emitted from a localized high intensity zone of a plasma, such as the plasma  508  of the plasma source of  FIG. 5 . In one embodiment, the light  706  emitted by the plasma source  500  is light having a wavelength shorter than about 5 nm for conducting X-ray microscopy. The light  706  collected by the first optical element  728  is then directed as light signal  732  towards a sample  708  (e.g., a biological sample) located on a substrate  704 . Light  712  which passes through the sample  708  and the substrate  704  then passes through a second optical element  716 . Light  720  passing through the second optical element (e.g., an image of the sample  728 ) is then directed onto an electromagnetic signal detector  724  imaging the sample  728 .  
       FIGS. 8A and 8B  are cutaway views of another embodiment of an enclosure  512  of a plasma source  500 . In this embodiment, the hole  516  is defined by a receptacle  801  and an insert  802 . The receptacle  801  can be an integral part of the enclosure  512  or a separate part of the enclosure  512 . In another embodiment, the receptacle  801  can be a region of the enclosure  512  that couples to the insert  802  (e.g., by a slip fit, threads, friction fit, or interference fit). In any of these embodiments, thermal expansion of the insert results in a good thermal and electrical contact between the insert and the receptacle.  
      In other embodiments, an outer surface of the insert  802  is directly connected to the plasma source  500 . In other embodiments, the outer surface of the insert  802  is indirectly connected to the plasma source  500 . In other embodiments, the outer surface of the insert  802  is in physical contact with the plasma source  500 .  
       FIG. 9A  is a cross section view of one embodiment of an insert  802  and the receptacle  801  in an enclosure (e.g., the enclosure  512  of  FIG. 8A ). The insert  802  has a body  840  that has a first open end  811  and a second open end  812 . The plasma loops  508  enter the first open end  811 , pass through an interior passage  820  of the insert  802 , and exit the second open end  812 . The interior passage  820  of the body  840  of the insert  802  defines a necked region  805 . The necked region  805  is the region that defines a reduced dimension of the interior passage  820  along the length of the passage  820  between the first open end  811  and second open end  812  of the insert  802 . The energy intensity is increased in the plasma loops  508  in the necked region  805  forming a localized high intensity zone.  
      In this embodiment, the insert  802  has threads  810  on an outer surface  824  of the insert  802 . The receptacle  801  has a corresponding set of threads  810  to mate with the threads  810  of the insert  802 . The insert  802  is inserted into the receptacle  801  by rotating the the insert  802  relative to the receptacle  801 , thereby mating the threads  810  of the insert  802  and the receptacle  801 . In other embodiments, neither the insert  802  nor the receptacle  801  have threads  810  and the insert  802  can be slip fit into the receptacle  801  using a groove and key mechanism (not shown). The heat from the plasma causes the insert  802  to expand and hold it firmly in place within the receptacle  801 . In this embodiment, the insert  802  is a unitary structure. In another embodiment, insert  802  can be defined by two or more bodies.  
      In this embodiment, the insert  802  defines a region that creates a high intensity zone in the plasma. The size of the high intensity zone, in part, determines the intensity of the plasma and the brightness of radiation emitted by the zone. The brightness of the high intensity zone can be increased by reducing its size (e.g. diameter or length). Generally, the minimum dimension of the necked region  805  along the passage  820  of the insert  802  determines the size of the high intensity zone. The local geometry of an inner surface  803  of the passage  820  in the insert  802  also determines the size of the high intensity zone. In some embodiments, the geometry of the inner surface  803  is asymmetric about a center line  804  of the insert  802 , as shown in  FIG. 9A .  
      The inner surface  803  of the insert  802  is exposed to the high intensity zone of the plasma. In some embodiments, the insert  802  is formed such that at least the inner surface  803  is made of a material with a low plasma sputter rate, allowing it to resist erosion by the plasma. For example, this can include materials like carbon, titanium, tungsten, diamond, graphite, silicon carbide, silicon, ruthenium, or a refractory material. It is also understood that alloys or compounds including one or more of those materials can be used to form the insert  802  or coat the inner surface  803  of the insert  802 .  
      In another embodiment, it is recognized that material from the inner surface  803  of the insert  802  interacts with the plasma (e.g., sputtered by the plasma) and is deposited on, for example, optical elements of a light source. In this case, it is desirable to form the insert such that at least the inner surface  803  comprises or is coated with a material which does not absorb the EUV light being emitted by the light source. For example, materials that do not absorb or absorb a minimal amount of the EUV radiation include ruthenium or silicon, or alloys or compounds of ruthenium or silicon. This way, material sputtered from the inner surface  803  of the insert  802  and deposited on, for example, the optical elements, does not substantially interfere with the functioning (e.g., transmission of EUV radiation) of the optical elements.  
      In this embodiment, the insert  802  is in thermal communication with the receptacle  801  in order to dissipate the heat from the plasma high intensity zone. In some embodiments, one or more cooling channels (not shown) can pass through the body  840  of the insert  802  to cool the insert  802 . In some embodiments it is desirable to form the insert  802  such that at least the inner surface  803  is made of a material with a low plasma sputter rate and a high thermal conductivity. For example, this can include highly oriented pyrolytic graphite (HOPG) or thermal pyrolytic graphite (TPG). It is also understood that alloys or compounds with those materials can be used.  
      In this embodiment, the insert  802  includes a gas inlet  806  for, for example, introducing the ionizable medium into the chamber, as described previously herein.  
       FIG. 9B  illustrates another embodiment of an insert  802 . In this embodiment, the geometry of the inner surface  803  is symmetric about a center line  804  of the insert  802 . As stated earlier, the local geometry of the inner surface  803  of the interior passage  820  of the insert  802  determines the size of the high intensity zone. The size of the high intensity zone determines, in part, the brightness of the radiation emanating from the high intensity zone. Characteristics of the geometry of inner surface  803  factor into this determination. Characteristics include, but are not limited to, the following. The minimum dimension of the necked region  805  constrains the high intensity zone along the y-axis. The necked region  805  can be, but does not need to be, radially symmetric around the axis  813  of the insert  802 . A length  809  of the necked region  805  also serves to constrain the high intensity zone. A slope of the sidewall  808  of the necked region  805  also determines the size of the high intensity zone. In addition, varying the radius of curvature  807  of the inner surface  803  changes the size of the high intensity zone. For example, as the radius of curvature  807  is decreased, the high intensity zone also decreases in size.  
       FIG. 9C  illustrates another embodiment of the insert  802 . In this embodiment, the slope of the sidewall  808  is vertical (perpendicular to the z-axis), making the length  809  of the necked region  805  uniform in the radial direction. Again, it is understood that the local geometry of the inner surface  803  of the insert  802  need not be radially symmetric around the axis  813  of the insert  802 . In some embodiments, the local geometry shown in  FIG. 9C  that defines the inner surface  803  is a plurality of discrete posts positioned within the insert  802  along the inner surface  803  of the insert  802 .  
      Other shapes, sizes and features are contemplated for the local geometry of the inner surface  803  of the insert  802 . Portions of the inner surface  803  can be concave or convex, while still having a radius  807  that defines the high intensity zone. The slope of the sidewall  808  of the necked region  805  can be positive, negative, or zero. The local geometry of the inner surface  803  can be radially symmetric about the axis  813  of the insert  802  or not. The local geometry of the inner surface  803  of the insert  802  can be symmetric about the center line  804  or not.  
      In some embodiments, applications using a plasma source (e.g., the plasma source  100  of  FIG. 1  include an enclosure (e.g., the enclosure  512  of  FIG. 8A ) that includes an insert (e.g., the insert  802  of  FIG. 9A ). In these applications, the insert  802  is a consumable component of the plasma source  100  that can be removed or replaced by an operator. In some embodiments, the insert  802  can be replaced using a robotic arm (not shown) that engages or interfaces with the insert  802 . In this manner, the robotic arm can remove an insert  802  and replace it with a new insert  802 . It may be desirable to replace inserts  802  that have become worn or damaged during operation of the plasma source.  
      By way of example, a coating of material (e.g. ruthenium) on the inner surface  803  of the insert  802  may erode or be sputtered as plasma loops  508  pass through the interior passage  820  of the insert  802 . In some embodiments, as the inner surface  803  of the insert  802  is eroded or sputtered by the plasma loops  508 , its ability to define the localized high intensity zone can be compromised. A new insert  802  can be placed into a chamber  104  of the plasma source  100  through a vacuum load lock (not shown) installed in the chamber  104 . After the new insert  802  is placed in the chamber  104 , the robotic arm can be used to install the new insert  802  into the receptacle  801  of the enclosure  512 . For example, if the receptacle  801  and the insert  802  have mating threads  810 , the robotic arm can rotate the insert  802  relative to the receptacle  801  to install the insert  802  by mating the matching threads  810 . In this manner, by robotically replacing the insert  802 , uptime of the plasma source is improved. Robotically replacing the insert  802  while maintaining a vacuum in the chamber  104 , further improves uptime of the plasma source.  
       FIG. 10  is a schematic diagram of a filter  902  used in conjunction with a plasma source (not shown). The plasma source has a light emitting region  901  (e.g., the localized high intensity zone of the plasma source  500  of  FIGS. 5A and 5B ). The filter  902  is disposed relative to the light emitting region  901  to reduce emissions from the light emitting region  901  and from other locations in the plasma source. Emissions include, but are not limited to, particles sputtered from surfaces within the plasma source, ions, atoms, molecules, charged particles, and radiation. In this embodiment, the filter  902  is positioned between the light emitting region  901  and, for example, collection optics  903  of a lithography system (e.g., the lithography system  600  of  FIG. 6 ). The role of the filter  902  is to allow radiation from the light emitting region  901  to reach the collection optics  903 , but not allow (or reduce), for example, particles, charged particles, ions, molecules or atoms to reach the collection optics  903 .  
      The filter  902  is configured to minimize the reduction of emissions traveling substantially parallel to the direction of radiation  904  emanating from the light emitting region  901 . The filter  902  is also configured to trap emissions which are traveling in directions substantially not parallel  905  (e.g., in some cases orthogonal) to the direction of radiation  904  emanating from the light emitting region  901 . The particles, charged particles, ions, molecules and atoms which are not traveling substantially parallel to the direction of radiation  904  emanating from the light emitting region  901  collide with the filter  902  and cannot reach, for example, the collection optics  903 . The particles, charged particles, ions, molecules and atoms which are initially traveling substantially parallel to the direction of radiation  904  emanating from the light emitting region  901  undergo collisions with gas atoms, ions or molecules and be deflected so that they begin to travel in a non-parallel direction thereby becoming trapped at the filter. In some embodiments, the filter  902  is capable of substantially reducing the number of particles, charged particles, ions, molecules and atoms which reach, for example, collection optics  903 , while not substantially reducing the amount of radiation which reaches, for example, the collection optics  903 .  
       FIGS. 11A and 11B  illustrate one embodiment of a filter  902 . The filter  902  comprises a plurality of thin walls  910  with narrow channels  911  between the walls  910 . In this embodiment, the walls  910  are arranged radially around the center  912  of the filter  902 . In some embodiments, the walls  910  are formed such that at least the surfaces of the walls exposed to the emissions (surfaces within the channels  911 ) comprise or are coated with a material which has a low plasma sputter rate. For example, this can include materials like carbon, titanium, tungsten, diamond, graphite, silicon carbide, silicon, ruthenium, or a refractory material. In this embodiment, radiation from a light emitting region (e.g., the light emitting region  901  of  FIG. 10 ) is directed toward an inside region  930  of the filter  902  along the positive direction of the y-axis.  
      In this embodiment, the filter  902  includes at least one cooling channel  920 . The walls  910  are in thermal communication with the at least one cooling channel  920 . The filter  902  includes an inlet  924   a  and an outlet  924   b  for flowing coolant through the channel  920 . The cooling channel  920  dissipates heat associated with, for example, particles, charged particles, ions, molecules or atoms impacting the walls  910 . In some embodiments, the walls  910  are formed such that at least the surfaces of the walls exposed to the emissions are made from a material which has a low plasma sputter rate and a high thermal conductivity. For example, this can include materials like highly oriented pyrolytic graphite or thermal pyrolytic graphite. In some embodiments, multiple cooling channels  920  are provided to cool the filter  902  due to exposure of the filter  902  to particles, charged particles, ions, molecules and atoms. Cooling the filter  902  keeps it at a temperature which will not compromise the structural integrity of the filter  902  and also prevent excessive thermal radiation from the filter  902 .  
      In another embodiment, a curtain of buffer gas is maintained in the vicinity of the filter  902 . This buffer gas can be inert and have a low absorption of EUV radiation (e.g., helium or argon). Emissions such as particles, charged particles, ions, molecules and atoms which are initially traveling in a direction substantially parallel to the direction of radiation (e.g., the direction of radiation  904  of  FIG. 10 ) emanating from the light emitting region  901  collide with gas molecules. After colliding with the gas molecules, the particles, charged particles, ions, molecules and atoms travel in directions substantially not parallel  905  to the direction of radiation  904  emanating from the light emitting region  901 . The particles, charged particles, ions, molecules and atoms then collide with the walls  910  of the filter  902  and are trapped by the surfaces of the walls  910 . The radiation emanating from the light emitting region  901  is not affected by the gas molecules and passes through the channels  911  between the walls  910 .  
      In other embodiments (not shown) the walls  910  are configured to be substantially parallel to each other to form a Venetian blind-like structure (as presented to the light emitting region  901 ). In other embodiments (not shown), the walls  910  can be curved to form concentric cylinders (with an open end of the cylinders facing the light emitting region  901 ). In other embodiments, the walls can be curved into individual cylinders and placed in a honeycomb pattern (as presented to the light emitting region  901 ).  
      Another embodiment of a plasma source chamber  104  is shown in  FIGS. 12A and 12B . In this embodiment, objects  1001   a  and  1001   b  (generally  1001 ) are disposed near a high intensity zone  144  of a plasma. Surfaces  1002   a  and  1002   b  (generally  1002 ) of the objects  1001   a  and  1001   b , respectively, are moving with respect to the plasma. The moving surfaces  1002  act to spread the heat flux and ion flux associated with the plasma over a large surface area of the surfaces  1002  of the objects  1001 . In this embodiment, the objects  1001  are two rods. The rods  1001  are spaced closely together along the y-axis near the plasma discharge region and have a local geometry  1010  that defines the localized high intensity zone  144 . By using multiple objects  1001  spaced closely together along with a local geometry  1010  in at least one object  1001 , the high intensity zone is constrained in two dimensions.  
      In some embodiments, however, a single object  1001  is used to spread the heat flux and ion flux associated with the plasma and to define the localized high intensity zone relative to another structure. It is understood that various alternate sizes, shapes and quantities of objects  1001  can be used.  
      In this embodiment, at least one object  1001  is in thermal communication with cooling channels  1020 . Coolant flows through the channels  1020  to enable the surfaces  1002  of the objects  1001  to dissipate the heat from the plasma. By moving the surface  1002  of the objects  1001  with respect to the plasma (e.g., rotating the rods  1001  around the z-axis), the plasma is constantly presented with a newly cooled portion of the surface  1002  for dissipating heat. In another embodiment, the surface  1002  of the at least one object  1001  is covered with a sacrificial layer. This allows ion flux and heat flux from the plasma to erode the sacrificial layer of the surface  1002  of the at least one object  1001  without damaging the underlying object  1001 . By moving the surface  1002  with respect to the plasma, the plasma is presented with a fresh surface to dissipate the ion flux and heat flux. Plasma ions collide with the surface  1002  of the at least one object  1001 . These collisions result in, for example, the scattering of particles, charged particles, ions, molecules and atoms from the surface  1002  of the at least one object  1001 . In this manner, the resulting particles, charged particles, ions, molecules and atoms are most likely not traveling towards, for example, the collection optics (not shown). In this way, the at least one object  1001  has prevented the ion flux from the plasma from interacting with, for example, collection optics (not shown).  
      In one embodiment, the surface  1002  of the at least one object  1001  is continuously coated with the sacrificial layer. This can be accomplished by providing solid material (not shown) to the at least one object  1001  being heated by the plasma. Heat from the plasma melts the solid material melts allowing it to coat the surface  1002  of the at least one object  1001 . In another embodiment, molten material can be supplied to the surface  1002  of the at least one object  1001  using a wick. In another embodiment, part of the surface  1002  of the at least one object  1001  can rest in a bath of molten material, which adheres to the surface  1002  as it moves (e.g., rotates). In another embodiment, the material can be deposited on the surface  1002  of the at least one object  1001  from the gas phase, using any of a number of well known gas phase deposition techniques. By continuously coating the surface  1002  of the at least one object  1001 , the sacrificial layer is constantly replenished and the plasma is continuously presented with a fresh surface  1002  to dissipate the ion flux and heat flux, without harming the underlying at least one object  1001 .  
      In another embodiment, at least the surface  1002  of the at least one object  1001  can be made from a material which is capable of emitting EUV radiation (e.g., lithium or tin). Plasma ions colliding with the surface  1002  cause atoms and ions of that material to be emitted from the surface  1002  into the plasma, where the atoms and ions can emit EUV radiation, increasing the radiation produced by the plasma.  
       FIG. 13  is a cross-sectional view of another embodiment of the plasma source chamber  104 . In this embodiment, one or more magnets (generally  1101 ) are disposed near the high intensity zone  144  of the plasma. The at least one magnet  1101  can be either a permanent magnet or an electromagnet. By placing at least one magnet  1101  in the plasma chamber  104 , the magnetic field generated by the at least one magnet  1101  defines a region to create a localized high intensity zone  144 . It is understood that a variety of configurations and placements of magnets  1101  are possible. In this embodiment, the magnets  1101  are located within the channel  132  in the plasma discharge region  112 . In another embodiment, one or more magnets  1101  can be located adjacent to, but outside of the channel  132 . In this manner, using a magnetic field, rather than a physical object (e.g., the objects  1001  of  FIGS. 12A and 12B  and the disk  308  of  FIGS. 3A and 3B ) to define a region to create a localized high intensity zone  144  in the plasma reduces the flux of particles, charged particles, ions, molecules and atoms that result from collisions between the plasma ion flux and the physical object.  
      Variations, modifications, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the invention is to be defined not by the preceding illustrative description but instead by the spirit and scope of the following claims.