Abstract:
A system and method for an extreme ultraviolet light chamber comprising a collector mirror, a cooling system coupled to a backside of the collector mirror operative to cool a reflective surface of the collector mirror and a buffer gas source coupled to the extreme ultraviolet light chamber.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 61/168,033 filed on Apr. 9, 2009 and entitled “Extreme Ultraviolet Light Output,” which is incorporated herein by reference in its entirety for all purposes. This application also claims priority from U.S. Provisional Patent Application No. 61/168,012 filed on Apr. 9, 2009 and entitled “System, Method and Apparatus for Laser Produced Plasma Extreme Ultraviolet Chamber with Hot Walls and Cold Collector Mirror,” which is incorporated herein by reference in its entirety for all purposes. This application also claims priority from U.S. Provisional Patent Application No. 61/168,000 filed on Apr. 9, 2009 and entitled “System, Method and Apparatus for Droplet Catcher for Prevention of Backsplash in a EUV Generation Chamber,” which is incorporated herein by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     The present invention relates generally to laser produced plasma extreme ultraviolet systems, methods and apparatus, and more particularly, to systems, methods and apparatus for droplet management in a laser produced plasma extreme ultraviolet system. 
     Laser produced plasma (LPP) extreme ultraviolet (EUV) systems produce a plasma by irradiating a droplet of a plasma target material with a source laser. The resulting plasma emits light at a desired wavelength, in this instance, EUV (e.g., less than about 50 nm wavelength and including light at a wavelength of about 13.5 nm or less). 
     Unfortunately irradiating the droplet of the plasma target material can result in debris from the droplet. The debris can be deposited on the collector mirror and other inner surfaces of the EUV chamber. The debris deposited on the collector mirror can reduce the amount of EUV light output. 
     Further, some of the droplets of the target material are not irradiated by the source laser and as a result may produce splashes and other micro-particles and debris that can become deposited on the inner surfaces of the LPP chamber. 
     In view of the foregoing, there is a need for providing better control of the micro-particles and debris generated during the process of operating in a LPP EUV chamber. 
     SUMMARY 
     Broadly speaking, the present invention fills these needs by providing an improved EUV light chamber in an LPP EUV system. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, computer readable media, or a device. Several inventive embodiments of the present invention are described below. 
     One embodiment provides an extreme ultraviolet light chamber comprising a collector, a cooling system coupled to a backside of the collector operative to cool a reflective surface of the collector and a buffer gas source coupled to the extreme ultraviolet light chamber. 
     The chamber can also include a target material condenser system coupled to the extreme ultraviolet light chamber. The chamber can also include multiple baffles located between the collector and an output of the extreme ultraviolet light chamber. The chamber can also include a heat source coupled to at least a portion of the baffles. The heat source can be capable of heating at least a portion of the baffles to a temperature greater than the reflective surface of the collector. The heat source can be capable of heating at least a portion of the baffles to a melting temperature of a target material. 
     At least a first portion of each one of the baffles can be substantially aligned to an irradiation region. At least a second portion of at least one of the baffles is substantially not aligned to the irradiation region. The baffles can begin at an edge of a transmissive region and the baffles extend to an inner surface of the extreme ultraviolet light chamber. Each one of the baffles is separated from an adjacent baffle by a corresponding space. Each one of the corresponding spaces between the adjacent baffles can have an equal width or a different width. 
     The chamber can also include a target material condenser system coupled to the extreme ultraviolet light chamber. The target material condenser system can include a vacuum source coupled to the extreme ultraviolet light chamber. 
     Another embodiment provides an extreme ultraviolet light chamber including a collector and multiple baffles located between the collector and an output of the extreme ultraviolet light chamber. At least a first portion of each one of the baffles is substantially aligned to an irradiation region and at least a second portion of at least one of the plurality of baffles is substantially not aligned to the irradiation region. 
     Another embodiment provides a method of generating an extreme ultraviolet light including outputting droplets of target material from a droplet generator in an extreme ultraviolet laser chamber, focusing a source laser on a selected one of the droplets in an irradiation region, irradiating the selected one of the droplets, collecting an extreme ultraviolet light emitted from the irradiated droplet in a collector, a reflective surface of the collector is cooled, a target material residue is deposited on the reflective surface of the collector, the target material residue being emitted from the irradiated droplet, a hydrogen containing gas is injected into the extreme ultraviolet laser chamber and a first quantity of target material residue on the reflective surface of the collector is converted to a hydride and the hydride of the first quantity of target material residue is evaporated from the reflective surface of the collector, the evaporated hydride of the first quantity of target material residue is removed from the extreme ultraviolet laser chamber. 
     The method can also include collecting a second quantity of target material residue on a set of baffles located between the collector and an output of the extreme ultraviolet laser chamber. The method can also include heating at least a portion of the baffles to a melting temperature of the target material residue. The liquefied target material residue can be captured in a target material condenser system. 
     The method can also include heating non-critical inner surfaces of the extreme ultraviolet laser chamber to a temperature greater than the temperature of the reflective surface of the collector. The non-critical inner surfaces of the extreme ultraviolet laser chamber include surfaces other than the collector. Removing the evaporated hydride of the first quantity of target material residue from the extreme ultraviolet laser chamber can include decomposing the evaporated hydride on the heated non-critical inner surfaces of the extreme ultraviolet laser chamber. 
     The method can also include heating non-critical inner surfaces of the extreme ultraviolet laser chamber to a temperature equal to or greater than a melting temperature of the target material residue. The liquefied target material residue can be captured in a target material condenser system. 
     Still another embodiment provides a method of generating an extreme ultraviolet light including outputting droplets of target material from a droplet generator in an extreme ultraviolet laser chamber, focusing a source laser on a selected one of the plurality of droplets in an irradiation region, irradiating the selected one of the plurality of droplets, collecting an extreme ultraviolet light emitted from the irradiated droplet in a collector and collecting a quantity of target material residue on a set of baffles located between the collector and an output of the extreme ultraviolet laser chamber. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic view of a laser-produced-plasma EUV light source, in accordance with embodiments of the disclosed subject matter. 
         FIG. 2A  is a schematic of the components of a simplified target material dispenser that may be used in some or all of the embodiments described herein in accordance with embodiments of the disclosed subject matter. 
         FIGS. 2B and 2C  are more detailed schematics of some of the components in a EUV chamber in accordance with embodiments of the disclosed subject matter. 
         FIG. 3  is a flowchart diagram that illustrates the method operations performed in generating EUV, in accordance with embodiments of the disclosed subject matter. 
         FIG. 4  is a flowchart diagram that illustrates the method operations performed in removing microparticles on the collector mirror, in accordance with embodiments of the disclosed subject matter. 
         FIGS. 5A-5F  illustrate a mid vessel baffle assembly in an EUV chamber, in accordance with an embodiment of the disclosed subject mater. 
         FIG. 6  illustrates a mid vessel baffle assembly in an EUV chamber, in accordance with an embodiment of the disclosed subject mater. 
         FIG. 7  is a flowchart diagram that illustrates the method operations performed in capturing and removing the third portion of the microparticles in the baffle assembly, in accordance with embodiments of the disclosed subject matter. 
         FIG. 8  is a block diagram of an integrated system including the EUV chamber, in accordance with embodiments of the disclosed subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     Several exemplary embodiments for an improved catch system and method for capturing the unused droplets in an LPP EUV system will now be described. It will be apparent to those skilled in the art that the present invention may be practiced without some or all of the specific details set forth herein. 
     One LPP technique involves generating a stream of target material droplets and irradiating some or all of the droplets with light pulses, e.g. zero, one or more pre-pulse(s) followed by a main pulse. In more theoretical terms, LPP light sources generate EUV radiation by depositing light or laser energy into a target material having at least one EUV emitting element (e.g., xenon (Xe), tin (Sn) or lithium (Li)), creating a highly ionized plasma with electron temperatures of several 10&#39;s of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. 
     A near-normal-incidence mirror (a “collector mirror”) is positioned at a relatively short distance (e.g., 10-50 cm) from the plasma to collect, direct and focus the EUV light to an intermediate location or focal point. The collected EUV light can then be relayed from the intermediate location to a set of scanner optics and ultimately to a target, such as a semiconductor wafer, in a photolithography process. 
     The collector mirror includes a delicate and relatively expensive multi-layer coating to efficiently reflect EUV light. Keeping the surface of the collector mirror relatively clean and protecting the surface from unwanted plasma-generated debris is a challenge facing the EUV light source developers. 
     In an exemplary arrangement that is currently being developed with the goal of producing about 100 W at the intermediate location. A pulsed, focused 10-12 kW CO 2  drive laser (or suitable other laser such as an excimer laser) is synchronized with a droplet generator to sequentially irradiate about 10,000-200,000 tin droplets per second. This arrangement needs to produce a stable stream of droplets at a relatively high repetition rate (e.g., 10-200 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position over relatively long periods of time. 
       FIG. 1  is a schematic view of a laser-produced-plasma EUV light source  20 , in accordance with embodiments of the disclosed subject matter. The LPP light source  20  includes a light pulse generation system  22  for generating a train of light pulses and delivering the light pulses into an EUV chamber  26 . Each light pulse  23  travels along a beam path from the light pulse generation system  22  and into the EUV chamber  26  to illuminate a respective target droplet at an irradiation region  28 . 
     Suitable lasers for use in the light pulse generation system  22  shown in  FIG. 1 , may include a pulsed laser device, e.g., a pulsed gas discharge CO 2  laser device producing radiation at about 9.3 μm or about 10.6 μm, e.g., with DC or RF excitation, operating at relatively high power, e.g., about 10 kW or higher and high pulse repetition rate, e.g., about 50 kHz or more. In one particular implementation, the laser in the light pulse generation system  22  may be an axial-flow RF-pumped CO 2  laser having a MOPA configuration with multiple stages of amplification and having a seed pulse that is initiated by a Q-switched Master Oscillator (MO) with low energy and high repetition rate, e.g., capable of 100 kHz operation. From the MO, the laser pulse may then be amplified, shaped, and focused before reaching the irradiation region  28 . 
     Continuously pumped CO 2  amplifiers may be used for the light pulse generation system  22 . For example, a suitable CO 2  laser device having an oscillator and three amplifiers (O-PA1-PA2-PA3 configuration) is disclosed in co-owned U.S. Pat. No. 7,439,530, issued on Oct. 21, 2008, entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM, the entire contents of which are hereby incorporated by reference herein. 
     Alternatively, the laser in the light pulse generation system  22  may be configured as a so-called “self-targeting” laser system in which the droplet serves as one mirror of the optical cavity. In some “self-targeting” arrangements, a master oscillator may not be required. Self targeting laser systems are disclosed and claimed in co-owned U.S. Pat. No. 7,491,954, issued on Feb. 17, 2009, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, the entire contents of which are hereby incorporated by reference herein. 
     Depending on the application, other types of lasers may also be suitable for use in the light pulse generation system  22 , e.g., an excimer or molecular fluorine laser operating at high power and high pulse repetition rate. Other examples include, a solid state laser, e.g., having a fiber, rod or disk shaped active media, a MOPA configured excimer laser system, e.g., as shown in U.S. Pat. Nos. 6,625,191, 6,549,551, and 6,567,450, the entire contents of which are hereby incorporated by reference herein, an excimer laser having one or more chambers, e.g., an oscillator chamber and one or more amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator/power oscillator (MOPO) arrangement, a master oscillator/power ring amplifier (MOPRA) arrangement, a power oscillator/power amplifier (POPA) arrangement, or a solid state laser that seeds one or more excimer or molecular fluorine amplifier or oscillator chambers, may be suitable. Other designs are possible. 
     Referring again to  FIG. 1 , the EUV light source  20  may also include a target material delivery system  24 , e.g., delivering droplets of a target material into the interior of a chamber  26  to the irradiation region  28 , where the droplets  102 A,  102 B will interact with one or more light pulses  23 , e.g., one or more pre-pulses and thereafter one or more main pulses, to ultimately produce a plasma and generate an EUV emission  34 . The EUV chamber  26  is maintained at a near vacuum (e.g., between about 50 mT and 1500 mT) for the plasma formation. The target material may include, but is not necessarily limited to, a material that includes tin, lithium, xenon, etc., or combinations thereof. The EUV emitting element, e.g., tin, lithium, xenon, etc., may be in the form of liquid droplets and/or solid particles contained within liquid droplets  102 A,  102 B. 
     By way of example, the element tin may be used as pure tin, as a tin compound, e.g., SnBr 4 , SnBr 2 , SnH 4 , as a tin alloy, e.g., tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or a combination thereof. Depending on the material used, the target material may be presented to the irradiation region  28  at various temperatures including room temperature or near room temperature (e.g., tin alloys, SnBr 4 ), at an elevated temperature, (e.g., pure tin) or at temperatures below room temperature, (e.g., SnH 4 ), and in some cases, can be relatively volatile, e.g., SnBr 4 . More details concerning the use of these materials in an LPP EUV light source is provided in co-owned U.S. Pat. No. 7,465,946, issued Dec. 18, 2008, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, the contents of which are hereby incorporated by reference herein. 
     Referring further to  FIG. 1 , the EUV light source  20  includes a collector mirror  30 . The collector mirror  30  is a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid (i.e., an ellipse rotated about its major axis). The actual shape and geometry can of course change depending on the size of the chamber and the location of focus. The collector mirror  30  can include a graded multi-layer coating in one or more embodiments. The graded multi-layer coating can include alternating layers of Molybdenum and Silicon, and in some cases one or more high temperature diffusion barrier layers, smoothing layers, capping layers and/or etch stop layers. 
     The collector mirror  30  also includes an aperture  32 . The aperture  32  allows the light pulses  23  generated by the light pulse generation system  22  to pass through to the irradiation region  28 . The collector mirror  30  can be a prolate spheroid mirror that has a first focus within or near the irradiation region  28  and a second focus at an intermediate region  40 . The EUV light  34  is output at or near the intermediate region  40  from the EUV light source  20  and input to a device  42  utilizing EUV light  34 . By way of example, the device  42  that receives the EUV light  34  can be an integrated circuit lithography tool. 
     It is to be appreciated that other optics may be used in place of the prolate spheroid mirror  30  for collecting and directing EUV light  34  to an intermediate location for subsequent delivery to a device utilizing the EUV light. By way of example the collector mirror  30  can be a parabola rotated about its major axis. Alternatively, the collector mirror  30  can be configured to deliver a beam having a ring-shaped cross-section to the intermediate location  40  (e.g., co-pending U.S. patent application Ser. No. 11/505,177, filed on Aug. 16, 2006, entitled EUV OPTICS, the contents of which are hereby incorporated by reference). 
     The EUV light source  20  may also include an EUV controller  60 . The EUV controller  60  can include a firing control system  65  for triggering one or more lamps and/or laser devices in the light pulse generation system  22  to thereby generate light pulses  23  for delivery into the chamber  26 . 
     The EUV light source  20  may also include a droplet position detection system including one or more droplet imagers  70 . The droplet imagers  70  can capture images using CCD&#39;s or other imaging technologies and/or backlight stroboscopic illumination and/or light curtains that provide an output indicative of the position and/or timing of one or more droplets  102 A,  102 B relative to the irradiation region  28 . The imagers  70  are coupled to and output the droplet location and timing data to a droplet position detection feedback system  62 . The droplet position detection feedback system  62  can compute a droplet position and trajectory, from which a droplet error can be computed. The droplet error can be calculated on a droplet by droplet basis or on average droplet data. The droplet position error may then be provided as an input to the EUV controller  60 . The EUV controller  60  can provide a position, direction and/or timing correction signal to the light pulse generation system  22  to control a source timing circuit and/or to control a beam position and shaping system to change the trajectory and/or focal power or focal point of the light pulses being delivered to the irradiation region  28  in the chamber  26 . 
     The EUV light source  20  can also include one or more EUV metrology instruments for measuring various properties of the EUV light generated by the source  20 . These properties may include, for example, intensity (e.g., total intensity or intensity within a particular spectral band), spectral bandwidth, polarization, beam position, pointing, etc. For the EUV light source  20 , the instrument(s) may be configured to operate while the downstream tool, e.g., photolithography scanner, is on-line, e.g., by sampling a portion of the EUV output, e.g., using a pickoff mirror or sampling “uncollected” EUV light, and/or may operate while the downstream tool, e.g., photolithography scanner, is off-line, for example, by measuring the entire EUV output of the EUV light source  20 . 
     The EUV light source  20  can also include a droplet control system  90 , operable in response to a signal (which in some implementations may include the droplet error described above, or some quantity derived therefrom) from the EUV controller  60 , to e.g., modify the release point of the target material from a target material dispenser  92  and/or modify droplet formation timing, to correct for errors in the droplets  102 A,  102 B arriving at the desired irradiation region  28  and/or synchronize the generation of droplets  102 A,  102 B with the light pulse generation system  22 . 
       FIG. 2A  is a schematic of the components of a simplified target material dispenser  92  that may be used in some or all of the embodiments described herein in accordance with embodiments of the disclosed subject matter. The target material dispenser  92  includes a conduit or reservoir  94  holding a fluid form of the target material  96 . The fluid target material  96  can be a liquid such as a molten metal (e.g., molten tin), under a pressure, P. The reservoir  94  includes an orifice  98  allowing the pressurized fluid target material  96  to flow through the orifice  98  establishing a continuous stream  100 . The continuous stream  100  subsequently breaks into a stream of droplets  102 A,  102 B. The target material dispenser  92  further includes a sub-system producing a disturbance in the fluid having an electro-actuatable element  104  that is operable, coupled with the fluid target material  96  and/or the orifice  98  and a signal generator  106  driving the electro-actuatable element  104 . 
     More details regarding various droplet dispenser configurations and their relative advantages may be found in co-pending U.S. patent application Ser. No. 12/214,736, filed on Jun. 19, 2008, entitled SYSTEMS AND METHODS FOR TARGET MATERIAL DELIVERY IN A LASER PRODUCED PLASMA EUV LIGHT SOURCE, U.S. patent application Ser. No. 11/827,803, filed on Jul. 13, 2007, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE HAVING A DROPLET STREAM PRODUCED USING A MODULATED DISTURBANCE WAVE, co-pending U.S. patent application Ser. No. 11/358,988, filed on Feb. 21, 2006, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, co-owned U.S. Pat. No. 7,405,416, issued Jul. 29, 2008, entitled METHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY; and co-owned U.S. Pat. No. 7,372,056, issued on May 13, 2008, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM; the contents of each of which are hereby incorporated by reference. 
     The droplets  102 A,  102 B are between about 20 and about 100 μm in diameter. The droplets  102 A,  102 B are produced by pressurizing target material  96  through the orifice  98 . By way of example, the orifice  98  can have a diameter of less than about 50 μm in one embodiment. The droplets  102 A,  102 B are launched at a velocity of about 30 to 70 m/s. Due to the high velocity of the droplets  102 A,  102 B, the droplet stay on the nearly straight droplet path  209  and do not impinge on the collector mirror  30 , whether the droplets stream is produced in horizontal, vertical, or some other orientation. In one embodiment, not all the droplets  102 A,  102 B produced by the target material dispenser  92  in continuous mode are used for plasma generation. If the EUV source works with a duty cycle of less than 100% a portion of the droplets  102   c  will pass the irradiation region  28  and can be collected thereafter. If the unused droplets  102   c  are allowed to impact the opposite wall of the EUV source chamber they will produce a large amount of fast moving fragments with broad spatial distribution. Significant portions of these fragments  231  will be deposited on the EUV collector mirror  30  and diagnostic ports and devices  70 , thus affecting their performance. 
     Another source of the debris is the irradiation region  28 . When irradiated with intense light pulses the droplets  102 A,  102 B are heated on one side that results in rapid asymmetric material expansion and EUV light emissions  230 . As described above the EUV light emissions  230  are collected in the collector mirror  30 . As a result of the expansion a significant amount of droplet material is accelerated in the direction away from the light pulse  23  with velocities comparable to the velocity of the droplets  102 A,  102 B as they are output from the target material dispenser  92 . This material is traveling away from the irradiation region  28  until it strikes some surface, at which point it can be reflected or backsplashed in various directions. The backsplashed target material  231  may be deposited on the collector mirror  30 . 
       FIGS. 2B and 2C  are more detailed schematics of some of the components in a EUV chamber in accordance with embodiments of the disclosed subject matter. As described above, the target material dispenser  92  outputs a stream of droplets  102 A,  102 B, however, not all of the droplets are irradiated (i.e., used) to generate the EUV  34 . By way of example unused droplets  102 C are not irradiated by the incoming light pulses  23 . 
     The unused droplets  102 C are captured in a first catch  210  so as to minimize any backsplash of the unused droplets within the EUV chamber  26 . The backsplash  236  can be in the form of microparticles or liquid droplets. The unused droplets  102 C strike the bottom  211  of the first catch  210 . Microparticles  236  can reflect multiple times from the bottom and off the walls of first catch  210  and a portion of the micro particles  222 , as shown in  FIG. 2C , can escape back into the EUV chamber  26  and a portion of the microparticles  231  can deposit on various surface such as on the collector mirror  30 . Microparticles  220  are shown in phantom to illustrate some of the backsplash of microparticles that are captured or prevented by the catch  210 . 
     The first catch  210  can be an elongated tube having a cross section that can be circular, oblong, oval, rectangular, square, or any other suitable shape. As shown in  FIG. 2C , the first catch  210  includes an open end  224  oriented toward the target material dispenser  92 . The open end  224  can be substantially centered on the droplet path  209 . The first catch  210  also includes a centerline  223  that may or may not be aligned to the droplet path  209  as will be described in more detail below. 
     The backsplash can be reduced or minimized by using a first catch  210  having a relatively large aspect ratio L/W, e.g. greater than about 3 and preferably greater than about 8, where L is the first catch length and W is the largest inside dimension normal to L, of the first catch. Upon striking the inner wall of the first catch  210 , the unused droplets  102   c  reduce their velocity and the unused droplets can be captured in the first catch, as shown. 
     As shown in  FIG. 2B , the irradiated droplets can also produce microparticles  232  after being irradiated. The microparticles  232  can be distributed around the EUV chamber  26 . A portion of the microparticles  231  can be deposited on the collector mirror  30 . A portion of the microparticles  232  can be captured in an optional second catch  240 . The first catch  210  and second catch  240  can also be heated or cooled. 
     Parts, or all of the first and second catches  210 ,  240 , may have double walls. The space between the double walls can be filled with, or designed to pass one or more heat exchange fluids, such as water, tin, gallium, tin-gallium alloy, etc., for the efficient thermal management (i.e., heating or cooling) of the catch  210 ,  240 . 
       FIG. 3  is a flowchart diagram that illustrates the method operations  300  performed in generating EUV  34 , in accordance with embodiments of the disclosed subject matter. The operations illustrated herein are by way of example, as it should be understood that some operations may have sub-operations and in other instances, certain operations described herein may not be included in the illustrated operations. With this in mind, the method and operations  300  will now be described. 
     In an operation  305 , a light pulse  23  is directed to the irradiation region  28  in the EUV chamber  26 . In an operation  310 , a selected one of a stream of droplets  102 A,  102 B is delivered to the irradiation region  28  at substantially the same time the light pulse  23  arrives at the irradiation region and EUV light  34  is generated from the irradiated droplet in an operation  315 . 
     In an operation  320 , a first portion of microparticles  231 , a second portion of microparticles  232  and a third portion of microparticles  233  are expelled from the irradiated droplet. The first portion of the microparticles  231  are expelled out of the irradiation region  28  and toward the collector mirror  30 . The second portion of the microparticles  232  are expelled out of the irradiation region  28  and toward the catches  210 ,  240 . The third portion of the microparticles  233  are expelled out of the irradiation region  28  and toward a secondary region  235 B of the EUV chamber  26 . The EUV chamber  26  is divided into a primary region  235 A and a secondary region  235 B. The primary region  235 A includes the collector mirror  30  and the irradiation region  28 . The secondary region  235 A includes that portion of the EUV chamber  26  between the outlet  40 A and the irradiation region  28 . 
     In an operation  325 , the second portion of the microparticles  232  and the unused droplets  102 C of the stream of droplets  102 A,  102 B are captured in the first and/or second catches  210 ,  240  as described above. Capturing the second portion of the microparticles  232  and the unused droplets  102 C substantially limits any backsplash of microparticles and droplets  236 . 
     In an operation  330 , the first portion of the microparticles  231  collect on the collector mirror  30 . In an operation  335 , the third portion of the microparticles  233  impact on any surfaces in the secondary region  235 B of the EUV chamber  26 . 
     The third portion of the microparticles  233  is divided into a fourth portion  233 A and a fifth portion  233 B. In an operation  340 , the fourth portion of the microparticles  233 A impinge on and collect on the surfaces  236 A in the secondary region  235 B of the EUV chamber  26 . The fifth portion of the microparticles  233 B impinge on and reflect off the surfaces  235 A in the secondary region  235 B of the EUV chamber  26  in an operation  345 . In an operation  350 , the fifth portion of the microparticles  233 B eventually escape from the EUV chamber  26  though the outlet  40 A. 
     In an operation  355 , the EUV from the irradiation region  28  is collected by the mirror collector  30 . The mirror collector  30  focuses the EUV  34  to an intermediate location  40  in an operation  360  and in an operation  365 , the EUV  34  is output from the EUV chamber though the outlet  40 A and the method operations can end. 
       FIG. 4  is a flowchart diagram that illustrates the method operations  400  performed in removing microparticles  231  on the collector mirror  30 , in accordance with embodiments of the disclosed subject matter. The operations illustrated herein are by way of example, as it should be understood that some operations may have sub-operations and in other instances, certain operations described herein may not be included in the illustrated operations. With this in mind, the method and operations  400  will now be described. 
     In an operation  405 , an etchant and/or buffer gas is introduced into the EUV chamber  26 . By way of example, Argon (Ar), Helium (He) and/or Hydrogen (H) buffer gas can be included in the EUV chamber  26  to slow down or stop the fast ions and microparticles  231  emitted by the plasma so that the fast ions and microparticles  231  do not damage the collector mirror  30 . Hydrogen and more particularly H radicals can be used for cleaning/etching of optical surfaces in the EUV chamber  26 . The hydrogen gas pressure inside the EUV chamber can be between about 300 to about 800 mT and the gas flow into and out of the EUV chamber is between about 50 and about 100 standard liters per minute (SLM). 
     In an operation  410 , a light pulse  23  is directed to the irradiation region  28  in the EUV chamber  26 . In an operation  415 , a selected one of a stream of droplets  102 A,  102 B is delivered to the irradiation region  28  at substantially the same time the light pulse  23  arrives at the irradiation region and a plasma is generated from the irradiated droplet in an operation  420 . Irradiating the droplet generates target material residue including droplet fragments and microparticles  231 ,  232 ,  233  and fast moving ions emitted outward from the irradiation region  28 . 
     In an operation  425 , the buffer gas interacts with the plasma and the fast moving ions and the EUV light  34  photons and generates hydrogen radicals. In an operation  430 , a first portion of microparticles  231  including some fast moving ions are expelled from the irradiated droplet in the irradiation region  28  and toward the collector mirror  30 . In an operation  435 , the first portion of microparticles  231  including some fast moving ions collects on the surface of the collector mirror  30  thereby reducing the reflectivity of the collector mirror. 
     In an operation  440 , the collector mirror  30  is cooled to less than the temperature of the other inner surfaces of the EUV chamber. The collector mirror  30  is cooled to less about 50 degrees C. or cooler. The formation and destruction of SnH4 is related to the temperature. For higher temperatures, the formation rate slows and the destruction rate increases. Restated, tin on a hotter surface is less likely to bond with hydrogen to form SnH4 gas and SnH4 near a hot surface is more likely to breakdown (i.e., decompose) into tin and hydrogen gas to deposit tin on the hot surface and release the hydrogen as a hydrogen gas. Thus the tin removal rate decreases as the collector mirror  30  temperature increases. Conversely, as the temperature of the collector mirror  30  is reduced the removal of the deposited tin increases. Further, a higher relative temperature of the remaining inner surfaces (e.g., vessel walls and baffles) reduces tin removal from those surfaces. The differences in temperature between the cooler collector mirror  30  and the relatively hotter vessel walls lead to tin transport from the cooler collector mirror  30  to the hotter vessel walls via SnH4. Possibly more importantly, this temperature differential substantially prevents tin transport from the vessel walls to the collector mirror  30  by SnH4. 
     By way of example, the collector mirror  30  can include a backside cooling mechanism for cooling the backside of the collector mirror  30 . The backside of the collector mirror  30  is opposite the side of the collector mirror used for collecting and reflecting the EUV light  34 . The backside cooling mechanism can include a cooling jacket  30 A coupled to a cooling fluid source  30 B for circulating a cooling fluid through the cooling mirror jacket  30 A. Alternatively or additionally, a cooling gas flow  30 C can be directed from a cooled gas source  30 D toward the backside of the collector mirror  30 . Other suitable cooling mechanisms or combinations thereof can also be used. 
     In an operation  445 , the hydrogen radicals react with target material deposits  231  on the surface of the collector mirror  30  and produce a hydride of the target material. By way of example if the target material is tin, the hydrogen radicals for a tin-tetrahydride (SnH 4 ). The hydride of the target material exits the surface of the collector mirror as a gas in an operation  450 . The hydride reaction proceeds faster at non-elevated temperatures (e.g., less than about 50 degrees C.). The reaction for tin target material is:
 
Tin(solid)+hydrogen radical&gt;&gt;SnH 4 (gas).
 
     Referring again to the example of a tin containing target material in the EUV chamber  26 , the tin-tetrahydride (SnH 4 ) can absorb the EUV light  34  and can decompose on surfaces in the EUV chamber thus redepositing tin. Therefore, reducing the quantity of the hydride of the target material in the EUV chamber  26  will improve the output of the EUV light  34 . In an operation  455 , the non-critical surfaces in the EUV chamber  26  are maintained at a temperature higher than the temperature of the collector mirror  30  so as to promote the decomposition of the hydride of the target material and deposit the target material on the non-critical surfaces in an operation  460 . The non-critical surfaces include the chamber walls  26 A,  235 A and other surfaces not including optical or detectors. 
     By way of example, the non-critical surfaces  26 A,  235 A can be maintained above the melting point of the target material (e.g., above about 232 degrees C. for tin target material). Maintaining the non-critical surfaces  26 A,  235 A above the melting point of the target material allows liquid target material to form in an optional operation  465 . The liquid target material can then be removed from the non-critical surfaces  26 A,  235 A using gravity flow, wetting, etc. and out of the EUV chamber  26  in a target material condenser system  237 , in an operation  470  and the method operations can end. 
     As described above in operations  345  and  350  of  FIG. 3 , the fifth portion of the microparticles  233 B impinge on and reflect off the surfaces  235 A in the secondary region  235 B of the EUV chamber  26  and eventually escape from the EUV chamber  26  though the outlet  40 A. When the fifth portion of the microparticles  233 B escape from the EUV chamber  26  though the outlet  40 A the fifth portion of the microparticles  233 B can contaminate the subsequent process device  42  that uses the EUV light  34 . This contamination can be exacerbated when the EUV light path  246  is in a substantially vertical orientation and especially exacerbated when the EUV light path  246  is in a substantially vertical orientation and the subsequent process device  42  is below the EUV chamber  26  such that gravity assists the travel and escape of the fifth portion of the microparticles  233 B. 
       FIGS. 5A-5F  illustrate a mid vessel baffle assembly  500  in an EUV chamber  26 , in accordance with an embodiment of the disclosed subject mater.  FIG. 5A  is schematic of a side view of the mid vessel baffle assembly  500 .  FIG. 5B  is a more detailed schematic of a side view of the mid vessel baffle assembly  500 .  FIG. 5C  is a perspective view of the mid vessel baffle assembly  500 .  FIG. 5D  is side view of the mid vessel baffle assembly  500 .  FIG. 5E  is a sectional view of the mid vessel baffle assembly  500 .  FIG. 5F  is further detailed a schematic side view of the mid vessel baffle assembly  500 . 
     Referring first to  FIG. 5A , the baffle assembly  500  is located in a mid vessel region  235 ′ of the EUV chamber  26 . The secondary region  235  of the EUV chamber  26  is divided into two portions: the mid vessel region  235 ′ and an aft vessel region  235 ″. The mid vessel region  235 ′ begins at the irradiation region  28  and extends toward the outlet  40 A of the EUV chamber  26 . The aft vessel region  235 ″ extends between the mid vessel region  235 ′ and the outlet  40 A of the EUV chamber  26 . The mid vessel region  235 ′ and the aft vessel region  235 ″ have no specific length and therefore the mid vessel region  235 ′ can include substantially all of the secondary region  235  of the EUV chamber  26 . 
     The baffle assembly  500  includes a series of passages and structures (described in more detail below) that receive, slow and capture substantially all of the third portion of the microparticles  233  created when a droplet is irradiated in the irradiation region  28 . The baffle assembly  500  can extend from the irradiation region  28  and the collector mirror  30  to the intermediate location  40  or any portion of the secondary region  235  of the EUV chamber  26 . While the baffle assembly  500  can extend from the irradiation region  28  and the collector mirror  30  to the intermediate location  40 , the baffle assembly does not prevent or otherwise occlude the EUV light  34  from passing from the collector mirror  30  through a three dimensional, cone-shaped transmissive region  502  to the intermediate location  40 . 
     The passages in the baffle assembly  500  begin at the edges  504 A,  504 B of the transmissive region  502  and the passages in the baffle assembly  500  extend to the inner surfaces  235 C of the EUV chamber  26 . Exemplary embodiments of the passages and the structures that form them in the baffle assembly  500  are described in more detail below. Referring to  FIG. 5C , the baffle assembly  500  is shown in a three dimensional pictorial view. The baffle assembly  500  is a series of concentric baffles  500 A- 500 H surrounding but not protruding into the transmissive region  502 . The baffle assembly  500  extends substantially from the edges  504 A,  504 B of the transmissive region  502  to the inner surfaces  235 C of the chamber. 
     Referring to  FIGS. 5B and 5F , the baffle assembly  500  is shown in a side view illustrating the series of concentric baffles  500 A- 500 H. A first portion of the baffles  500 A,  500 H are single step baffles and a second portion of the baffles  500 B- 500 G are multiple step baffles. The single step baffles  500 A,  500 H have an initial corresponding baffle angle αA, αH that is the same between the ends of the baffle assembly  500 , near the edges  504 A,  504 B of the transmissive region  502  and the inner surfaces  235 C of the EUV chamber  26 . The initial corresponding baffle angle αA, αH can substantially align the single step baffles  500 A,  500 H with the irradiation region  28 . 
     The single step baffles  500 A are angled such that their attachment point to the wall  235 C of the EUV chamber is aligned with a shadow formed by a first section of the adjacent baffle. Such an angle substantially prevents any direct line exposure of the wall by droplet fragments coming directly from the irradiation region  28 . The droplet fragments (e.g., microparticles) therefore must first strike the first step in the baffle, thus losing energy, before reaching the wall  235 C. Reducing the energy by deflecting the droplet fragments off at least one surface of a baffle before the droplet fragments impact the EUV chamber wall  235 C reduces the possibility that the droplet fragment will deflect off of the EUV chamber wall and back toward the collector mirror  30 . Further, any back deflection of the droplet fragment must then also bounce at least once off a baffle surface before reaching the collector mirror  30 , thus further reducing the energy and increasing the likelihood that the droplet fragment will stick to the surface of the baffles  500 A- 500 G and not return to the collector mirror  30   
     The multiple step baffles  500 B- 500 G have multiple corresponding baffle angles αB-αG and θB-θG such that the angle of the multiple step baffles changes between the edges  504 A,  504 B of the transmissive region  502  and the inner surfaces  235 C of the EUV chamber  26 . The corresponding initial baffle angle αB-αG can be substantially align a corresponding first portion  500 B′- 500 G′ of the baffles  500 A- 500 H with the irradiation region  28 , as shown by the phantom lines extending from the irradiation region toward each of the baffles  500 A- 500 G. The corresponding second baffle angle θB-θG angles a corresponding second portion  500 B″- 500 G″ of the baffles  500 A- 500 H away from the irradiation region  28  so as to promote reflection and capture of microparticles  233  emitted from the irradiation region. 
     It should be understood that while the baffles  500 A- 500 H are shown as being straight and even multiple straight portions, a curved baffle  501 D- 501 H as shown in  FIG. 5B  could be used. The curvature of the curved baffles  501 D- 501 H can vary between the edges  504 A,  504 B of the transmissive region  502  and the inner surfaces  235 C of the EUV chamber  26 . At least an initial portion of the curved baffles  501 A- 501 H can be substantially aligned with the irradiation region  28 . A combination of curved and straight portions of the baffles could be used or a combination of straight and/or curved and/or multiple step baffles could be included in the baffle assembly  500 . The baffles  500 A- 500 H can be substantially evenly spaced as shown in  FIGS. 5C and 5E  or unevenly spaced as shown in  FIGS. 5B and 5F . 
     One or more portions and/or one or more of the baffles  500 A- 500 H can be heated or cooled as may be desired to improve the capture of the microparticles  233  emitted from the irradiation region  28 . The different portions of the baffles  500 A- 500 H can be manufactured of different materials as may be desired for manufacturability, performance and durability or other reasons. By way of example the baffles  500 A- 500 H can be manufactured from one or more of molybdenum, stainless steel (e.g., SS-304, 316, Titanium, nickel, Copper or Aluminum or similar materials). 
     The baffle assembly  500  can also include holes or spaces or portions of selected baffles removed to provide access for diagnostics detectors (e.g., EUV detectors), one or more pin-hole cameras, vacuum periscope, droplet imagining and detection and steering, target material catch, vacuum ports, windows and any other access needed for the access, design, construction and operation of the EUV chamber  26 . 
       FIG. 6  illustrates a mid vessel baffle assembly  500  in an EUV chamber  26 , in accordance with an embodiment of the disclosed subject mater.  FIG. 7  is a flowchart diagram that illustrates the method operations  700  performed in capturing and removing the third portion of the microparticles  233 C in the baffle assembly  500 , in accordance with embodiments of the disclosed subject matter. The operations illustrated herein are by way of example, as it should be understood that some operations may have sub-operations and in other instances, certain operations described herein may not be included in the illustrated operations. With this in mind, the method and operations  700  will now be described. 
     In an operation  705 , a light pulse  23  is directed to the irradiation region  28  in the EUV chamber  26 . In an operation  710 , a selected one of a stream of droplets  102 A,  102 B is delivered to the irradiation region  28  at substantially the same time the light pulse  23  arrives at the irradiation region and a plasma is generated from the irradiated droplet in an operation  715 . 
     In an operation  720 , the third portion of microparticles  233  (as described in  FIGS. 2B and 5A  above) are emitted from the irradiation region  28  toward the baffle assembly  500 . The third portion of microparticles  233  impinge on the baffle  500 D″ and reflect at point  602  toward the baffle  500 E″, in an operation  725 . In an operation  730 , the third portion of microparticles  233  are captured in the baffle assembly  500  as the microparticles  233  reflect off point  604  and come to rest in the space  605  between the baffle  500 D″ and baffle  500 E″. 
     In an optional operation  735 , a portion of the baffle assembly  500  and/or the EUV chamber can be heated. The portion of the baffle assembly  500  and/or the EUV chamber can be heated such as with a heater  622 . The heater  622  can be any suitable heater by way of example the heater  622  can be a resistive heater or a double wall jacket type heater or combinations thereof. The heater  622  can be coupled to a heater control/source  626  that can provide an electrical current to a resistive heater  622  or a heated medium to circulate through the double jacketed heater  622 . 
     In an operation  740 , the accumulated microparticles  610  can be removed to a target material condenser system  237 . Removing the accumulated microparticles  610  can include heating the accumulated microparticles  610  to a melting temperature and removing the accumulated microparticles  610  in liquid form. The target material condenser system  237  can also include a vacuum source to draw the accumulated microparticles  610  in solid or liquid form into the target material condenser system  237  through a withdrawal port  620 . 
       FIG. 8  is a block diagram of an integrated system  800  including the EUV chamber  26 , in accordance with embodiments of the disclosed subject matter. The integrated system  800  includes the EUV chamber  26 , the light pulse generation system  22 , the device  42  utilizing output EUV light  34 , and an integrated system controller  810  coupled to the EUV chamber, the light pulse generation system and the device utilizing output EUV light. The integrated system controller  810  includes or is coupled to (e.g., via a wired or wireless network  812 ) a user interface  814 . The user interface  814  provides user readable outputs and indications and can receive user inputs and provides user access to the integrated system controller  810 . 
     The integrated system controller  810  can include a special purpose computer or a general purpose computer. The integrated system controller  810  can execute computer programs  816  to monitor, control and collect and store data  818  (e.g., performance history, analysis of performance or defects, operator logs, and history, etc.) for the EUV chamber  26 , the light pulse generation system  22  and the device  42 . By way of example, the integrated system controller  810  can adjust the operations of the EUV chamber  26 , the light pulse generation system  22  and/or the device  42  and/or the components therein (e.g., the first catch  210  and/or second catch  240 , target material dispenser  92 , baffle assembly  500 , etc.) if data collected dictates an adjustment to the operation thereof. 
     With the above embodiments in mind, it should be understood that the invention may employ various computer-implemented operations involving data stored in computer systems. These operations are those requiring physical manipulation of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. Further, the manipulations performed are often referred to in terms, such as producing, identifying, determining, or comparing. 
     Any of the operations described herein that form part of the invention are useful machine operations. The invention also relates to a device or an apparatus for performing these operations. The apparatus may be specially constructed for the required purpose, such as a special purpose computer. When defined as a special purpose computer, the computer can also perform other processing, program execution or routines that are not part of the special purpose, while still being capable of operating for the special purpose. Alternatively, the operations may be processed by a general purpose computer selectively activated or configured by one or more computer programs stored in the computer memory, cache, or obtained over a network. When data is obtained over a network the data maybe processed by other computers on the network, e.g., a cloud of computing resources. 
     The embodiments of the present invention can also be defined as a machine that transforms data from one state to another state. The transformed data can be saved to storage and then manipulated by a processor. The processor thus transforms the data from one thing to another. Still further, the methods can be processed by one or more machines or processors that can be connected over a network. Each machine can transform data from one state or thing to another, and can also process data, save data to storage, transmit data over a network, display the result, or communicate the result to another machine. 
     The invention may be practiced with other computer system configurations including hand-held devices, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers and the like. The invention may also be practiced in distributing computing environments where tasks are performed by remote processing devices that are linked through a network. 
     The invention can also be embodied as computer readable code on a computer readable medium. The computer readable medium is any data storage device that can store data, which can thereafter be read by a computer system. Examples of the computer readable medium include hard drives, network attached storage (NAS), read-only memory, random-access memory, CD-ROMs, CD-Rs, CD-RWs, DVDs, Flash, magnetic tapes, and other optical and non-optical data storage devices. The computer readable medium can also be distributed over a network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     It will be further appreciated that the instructions represented by the operations in the above figures are not required to be performed in the order illustrated, and that all the processing represented by the operations may not be necessary to practice the invention. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.