Abstract:
An EUV light source is disclosed which may comprise at least one optical element having a surface, such as a multi-layer collector mirror; a laser source generating a laser beam; and a source material irradiated by the laser beam to form a plasma and emit EUV light. In one aspect, the source material may consist essentially of a tin compound and may generate tin debris by plasma formation which deposits on the optical element and, in addition, the tin compound may include an element that is effective in etching deposited tin from the optical element surface. Tin compounds may include SnBr 4 , SnBr 2  and SnH 4 . In another aspect, an EUV light source may comprise a molten source material irradiated by a laser beam to form a plasma and emit EUV light, the source material comprising tin and at least one other metal, for example tin with Gallium and/or Indium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation-in-part application of U.S. patent application Ser. No. 11/174,442 entitled SYSTEMS AND METHODS FOR REDUCING THE INFLUENCE OF PLASMA-GENERATED DEBRIS ON THE INTERNAL COMPONENTS OF AN EUV LIGHT SOURCE which is a continuation-in-part of application of U.S. patent application Ser. No. 10/900,839, entitled EUV LIGHT SOURCE, filed on Jul. 27, 2004, and is a continuation-in-part of application of U.S. patent application Ser. No. 10/803,526, entitled HIGH REPETITION RATE LPP EUV LIGHT SOURCE, filed on Mar. 17, 2004, and is a continuation-in-part application of U.S. patent application Ser. No. 10/798,740, entitled COLLECTOR FOR EUV LIGHT, filed on Mar. 10, 2004, the disclosures of each of which are hereby incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates to extreme ultraviolet (“EUV”) light generators providing EUV light from a plasma created from a source material and collected and directed to a focus for utilization outside of the EUV light source generation chamber, e.g., for semiconductor integrated circuit manufacturing photolithography e.g., at wavelengths of around 50 nm and below. 
   BACKGROUND OF THE INVENTION 
   Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers. 
   Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, indium, antimony, tellurium, aluminum, etc., with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. 
   Heretofore, various systems in which the line-emitting element is presented for irradiation/electric discharge have been disclosed. Many diverse forms and states have been attempted, to include, presenting the element in pure form, e.g. pure metal, presenting the element as a compound, e.g. a salt, or in a solution, e.g. dissolved in a solvent such as water. Moreover, systems have been disclosed in which the line-emitting substance is presented as a liquid, gas, vapor and/or solid and can be in the form of a droplet, stream, moving tape, aerosol, particles in a liquid stream, gas jet, etc. 
   In addition to the large variation in source materials/material forms described above, a plethora of techniques have been developed to create a plasma of the source material. For example, a number of discharge plasma production (DPP) techniques have been developed such as capillary discharge, z-pinch, dense plasma focus, electrodeless z-pinch, star-pinch, etc. In a similar manner, for laser produced plasma systems, there are a number of choices available such as laser type, wavelength, pulse energy, etc. 
   From the above discussion, it is apparent that there are a relatively large number of possible EUV light source configurations. With this in mind, a number of factors warrant consideration when designing an EUV light source, especially a light source intended for high volume, production photolithography. One such factor is conversion efficiency, C, which is typically defined as the ratio of output EUV power, B, to input power, A, (i.e. the power required to operate a drive laser or create a gas discharge), C=B/A. For at least some LPP setups, conversion efficiency is a function of the type of laser used, the nature of the line emitting element and the coupling between laser beam and target. This coupling, in turn, may depend on the composition of the target, with some target materials allowing deep penetration of the laser beam into the target resulting in relatively high conversion efficiency. 
   Another factor that is often considered when designing a high volume EUV light source is the generation and mitigation of debris which may damage EUV light source optics such as a laser input window, collector mirror and/or metrology equipment. Thus, for at least some source materials, the production of a plasma may also generate undesirable by-products in the plasma chamber (e.g. debris) which can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements. This debris can include out-of-band photons, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of source material. This debris may also include chamber material from secondary sputtering and for the case of electric discharge type systems, electrode material. For this reason, it is often desirable to employ one or more techniques to minimize the types, relative amounts and total amount of debris formed for a given EUV output power. When the target size, e.g. droplet diameter, and/or target makeup, e.g. chemistry, are chosen to minimize debris, the targets are sometimes referred to as so-called “mass limited” targets. 
   The high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them. Thus, debris reduction and/or suitable techniques to reduce the impact of debris may need to be considered in the design of a high volume EUV light source. 
   Another factor that may be considered when selecting a source material is the temperature at which the source material may need to be processed. For example, pure lithium and tin both have relatively high melting points, which in some cases may forbid their practical use in applications in which piezo-electric materials are employed to produce a uniform stream of source material droplets. Other factors that may influence the choice of a target material include the materials toxicity and the materials compatibility (e.g. corrosiveness, etc.) with the source material dispenser. 
   Specific examples of EUV light source configurations include U.S. Pat. No. 6,831,963 which discloses the use of tin bromides in solution and at room temperature to produce a debris-free EUV emission, and U.S. Patent application No. 2005/0167617 which discloses the use of tin bromides due to their higher vapor pressure as compared with pure tin, and primarily describes the use of tin halogenide vapor as a source material for a gas discharge EUV light source. Another example of EUV light source configuration is disclosed in an article by Guenther Derra et al., entitled, Tin Deliver Systems for Gas Discharge Sources, that was presented at an SPIE EUV source workshop on Feb. 27, 2005. In the Derra et al. article, the use of stannane gas (SnH 4 ) in a gas discharge EUV light source is disclosed. Also, PCT application WO2004/104707 to Zink et al. entitled, Method and Device for Cleaning at least One Optical Component, discloses an EUV irradiation device in which an optical component becomes contaminated due to an inorganic substance introduced by a radiation source and the device includes a supply device to introduce a reactant for the purpose of removing the deposits. WO2004/104707 discloses that the radiation source may include tin and the reactant may include a halogen or halogen compound. Still, despite these disclosures, a suitable light source configuration for high volume EUV lithography has yet to be developed. 
   With the above in mind, Applicants disclose a laser produced plasma EUV light source, and corresponding methods of use. 
   SUMMARY OF THE INVENTION 
   In a first aspect, an EUV light source is disclosed which may comprise at least one optical element having a surface, such as a multi-layer collector mirror; a laser source generating a laser beam; and a source material irradiated by the laser beam to form a plasma and emit EUV light. For this aspect, the source may consist essentially of a tin compound and may generate tin debris by plasma formation which deposits on the optical element and the tin compound may include an element that is effective in etching deposited tin from the optical element surface. Tin compounds may include SnBr 4 , SnBr 2  and SnH 4 . 
   In one embodiment, the EUV light source may comprise a heater to heat Tin that has deposited on the element surface to a temperature greater than 150° C. to increase a rate of chemical reaction between the deposited Tin and the etchant. For example, the heater may be an ohmic heater, a radiative heater, a radio-frequency heater and/or a microwave heater. 
   In another aspect, the EUV light source may comprise a droplet generator system creating droplets of the source material. In one particular embodiment, the droplet generator system may heat the source material to a temperature above room temperature, for example, the droplet generator system may heat SnBr 4  to a temperature above 35 degrees C. to generate droplets and/or may heat SnBr 2  to a temperature above 216 degrees C. to generate droplets. In another embodiment, the droplet generator system may cool the source material to a temperature below room temperature, for example, the droplet generator system may cool SnH 4  to a temperature below minus 52 degrees C. to generate droplets. 
   In one or more of the above described embodiments, the laser source may comprise a CO 2  laser source producing radiation having a wavelength of about 10.6 μm. 
   In one aspect of an embodiment, an EUV light source may comprise a source material comprising at least one tin bromide, a heater for heating the source material above the melting point of the tin bromide; and a CO 2  laser source generating a laser beam for irradiating the liquid source material to produce a plasma therefrom and emit EUV radiation. For example, the source material may comprise SnBr 2  or SnBr 4 . 
   In another aspect, an EUV light source may comprise a source material comprising SnH 4 , a cooling system for cooling the source material below the boiling point of SnH 4 ; and a CO 2  laser source generating a laser beam for irradiating the liquid source material to produce a plasma therefrom and emit EUV radiation. 
   In yet another aspect, an EUV light source may comprise a laser source generating a laser beam; and a molten source material irradiated by said laser beam to form a plasma and emit EUV light, said source material comprising tin and at least one other metal, the source material having a melting point lower than pure tin. For example, the molten source material may comprise tin with Gallium, Indium or Gallium and Indium. One such alloy has tin in the range of 13 to 19 percent by weight, Indium in the range of 19 to 25 percent by weight and Gallium in the range of 59 to 65 percent by weight. In one implementation, the source material melting point is lower than 150° C. and may be at or about room temperature. 
   In one aspect of an embodiment, the molten source material consists essentially of metals. Also, the molten source material may comprise a eutectic tin alloy. In one embodiment, the laser source may comprise a CO 2  laser source producing radiation having a wavelength of about 10.6 μm. In a particular embodiment, the EUV light source may further comprise at least one optical element having a surface, the light source may generate tin debris by plasma formation, and the light source may comprise an reactant supply source for providing a reactant effective in etching deposited tin from the surface. A heater may be provided to heat tin deposits on the element surface to a temperature greater than 150° C. to increase a rate of chemical reaction between the deposited tin and the reactant. The reactant may be selected from the group of reactants consisting of HBr, HI, I 2 , Br 2 , Cl 2 , HCl, H 2  and combinations thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic, not to scale, view of an overall broad conception for a laser-produced plasma EUV light source according to an aspect of the present invention; 
       FIG. 2  shows a schematic, not to scale, view of a droplet generator system for creating droplets of a source material such as SnBr 4  and or SnBr 2 ; 
       FIG. 3  shows a schematic, not to scale, view of a droplet generator system for creating droplets of a source material such as SnH 4 ; 
       FIG. 4  shows a prior art, binary constitution diagram for Gallium Tin alloys; 
       FIG. 5  shows a prior art, binary constitution diagram for Indium Tin alloys; and 
       FIG. 6  shows a prior art, ternary constitution diagram for Gallium Tin Indium alloys. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   With initial reference to  FIG. 1  there is shown a schematic view of an exemplary EUV light source, e.g., a laser produced plasma EUV light source  20  according to one aspect of an embodiment of the present invention. As shown, the LPP light source  20  may include a pulsed or continuous laser source  22 , e.g., a pulsed gas discharge CO 2  laser source producing radiation at 10.6 μm, e.g. with DC or RF excitation operating at high power and high pulse repetition rate. For example, a suitable CO 2  laser source having a MO-PA1-PA2-PA3 configuration is disclosed in co-pending U.S. patent application Ser. No. 11/174,299 filed on Jun. 29, 2005, and entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM, the entire contents of which are hereby incorporated by reference herein. 
   Depending on the application, other types of lasers may also be suitable. For example, a solid state laser, an excimer, a molecular fluorine laser, 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, an excimer laser having a single chamber, an excimer laser having more than two chambers, e.g., an oscillator chamber and two amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator/power oscillator (MOPO) arrangement, a power oscillator/power amplifier (POPA) arrangement, or a solid state laser that seeds one or more CO 2 , excimer or molecular fluorine amplifier or oscillator chambers, may be suitable. Other designs are possible. 
   The light source  20  may also include a target delivery system  24 , e.g., delivering target(s), e.g. target(s) of a source material, e.g. SnH 4 , SnBr 2 , SnBr 4  or combinations thereof, in the form of liquid droplets or a continuous liquid stream. The target(s) may be delivered by the target delivery system  24 , e.g., into the interior of a chamber  26  to an irradiation site  28  where the target will be irradiated and produce a plasma. In some cases, the targets may include an electrical charge allowing the targets to be selectively steered toward or away from the irradiation site  28 . 
   Continuing with  FIG. 1 , the light source  20  may also include one or more optical elements such as a collector  30 , e.g., a normal incidence reflector, e.g. a SiC substrate coated with a Mo/Si multilayer with additional thin barrier layers deposited at each interface to effectively block thermally-induced interlayer diffusion, in the form of a prolate ellipsoid, with an aperture to allow the laser light to pass through and reach the irradiation site  28 . The collector  30  may be, e.g., in the shape of a ellipsoid that has a first focus at the irradiation site  28  and a second focus at a so-called intermediate point  40  (also called the intermediate focus  40 ) where the EUV light may be output from the light source  20  and input to, e.g., an integrated circuit lithography tool (not shown). 
   The light source  20  may also include an EUV light source controller system  60 , which may also include a laser firing control system  65 , along with, e.g., a laser beam positioning system (not shown). The light source  20  may also include a target position detection system which may include one or more droplet imagers  70  that provide an output indicative of the position of a target droplet, e.g., relative to the irradiation site  28  and provide this output to a target position detection feedback system  62 , which can, e.g., compute a target position and trajectory, from which a target error can be computed, e.g. on a droplet by droplet basis or on average. The target error may then be provided as an input to the light source controller  60 , which can, e.g., provide a laser position, direction and timing correction signal, e.g., to a laser beam positioning controller (not shown) that the laser beam positioning system can use, e.g., to control the laser timing circuit and/or to control a laser beam position and shaping system (not shown), e.g., to change the location and/or focal power of the laser beam focal spot within the chamber  26 . 
   As shown in  FIG. 1 , the light source  20  may include a target delivery control system  90 , operable in response to a signal (which in some implementations may include the target error described above, or some quantity derived therefrom) from the system controller  60 , to e.g., modify the release point of the target droplets as released by the target delivery mechanism  92  to correct for errors in the target droplets arriving at the desired irradiation site  28 . 
     FIG. 2  shows in greater detail a target delivery mechanism  92 ′ for delivering target(s) of source material having, for example, liquid SnBr 2 , SnBr 4  or combinations thereof, into a chamber  26 ′ for irradiation therein to produce a plasma. For example, for liquid SnBr 4  which melts at about 31 degrees Celsius, the mechanism  92 ′ may function to heat SnBr 4  to a range of about 35-45 degrees Celsius, and generate a stream of SnBr 4  droplets. When liquid SnBr 2  which melts at about 216 degrees Celsius is used, the mechanism  92 ′ may function to heat SnBr 2  to a range of about 220-230 degrees Celsius, and generate a stream of SnBr 2  droplets 
   For the specific embodiment shown in  FIG. 2 , the target delivery mechanism  92 ′ may include a cartridge  143  holding a molten source material, e.g. tin bromide (SnBr 2  or SnBr 4 ), under pressure, e.g. using inert gas, e.g. Argon gas, to pass the source material through a set of filters  145  which may be for example, fifteen and seven microns, respectively, which trap solid inclusions, e.g. solid compounds like oxides, nitrides; metal impurities and so on, of seven microns and larger. From the filters  145 , the source material may pass through an open/close thermal valve  147  to a dispenser  148 . For example a Peltier device may be employed to establish the valve  147 , freezing source material between the filters  145  and dispenser  148  to close the valve  147  and heating the frozen source material to open the valve  147 . 
   For the mechanism  92 ′, one or more modulating or non-modulating source material dispenser(s)  148  may be used. For example, a modulating dispenser may be used having a capillary tube formed with an orifice. The dispenser  148  may include one or more electro-actuatable elements, e.g. actuators made of a piezoelectric material, which can be selectively expanded or contracted to deform the capillary tube and modulate a release of source material from the dispenser  148 . A heater  150  may be used to maintain the source material in a molten state while passing through the dispenser  148 . Examples of modulating droplet dispensers can be found in co-pending U.S. patent application Ser. No. 11/067,124 filed on Feb. 25, 2005, entitled METHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, co-pending U.S. patent application Ser. No. 11/174,443 filed on Jun. 29, 2005, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, co-pending U.S. patent application Ser. No. 11/174,443 filed on Feb. 21, 2006, and entitled, SOURCE MATERIAL DISPENSER FOR EUV LIGHT SOURCE, the entire contents of each of which are hereby incorporated by reference herein. An example of non-modulating droplet dispenser can be found in co-pending U.S. patent application Ser. No. 11/358,983 filed on Feb. 21, 2006, and entitled, LASER PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, the entire contents of each of which are hereby incorporated by reference herein. 
   For the arrangement shown in  FIG. 2 , the chamber  26 ′ may be heated to a temperature of about 100 degrees Celsius, including all inside components such as pumps, pump lines up to a scrubber (not shown), which can be used to scrub bromide gases. As shown, a differentially pumped and gate valve isolated Residual Gas Analyzer (RGA)  152  may be used to ensure the absence of bromide gases prior to opening the chamber  26 ′.  FIG. 2  also shows that a tin bromide recovery system  154  may be used to recover the relatively volatile tin bromide and maintain a suitable gas pressure in the chamber  26 ′. As shown, the recovery system  154  may include a pair of gate valves  156   a,b  spaced apart to establish an isolation chamber  158 , a tin bromide reservoir  160  and a cooling device  162 , e.g. Peltier device to solidify the tin bromide. The two-valve system  154  may be differentially pumped and include scrubbers (not shown) to allow the reservoir  160  to be emptied without breaking chamber  26 ′ pressure or releasing hazardous gases, e.g. bromides. 
     FIG. 2  illustrates the generation of tin bromide, e.g. SnBr 2  or SnBr 4 , droplets for irradiation by a laser beam  164 , which as described above may be a CO 2  laser source producing radiation at 10.6 μm. Irradiation of the tin bromide creates a plasma which may include tin ions and bromide ions, and in some cases a portion of the droplet will be evaporated (but not ionized). This unused gaseous portion may be evacuated from the chamber via pumping and thus the unused portion of the droplet may not contribute to optical surface contamination. Tin ions formed in the plasma may reach the surface of an optical element, e.g. the surface of a collector mirror and deposit on the surface as metallic tin. Bromide ions, on the other hand, will be available and may etch tin, e.g. react with and remove tin deposits from the surface. The EUV light source may also include one or more heaters (not shown) to heat Tin that has deposited on the surface of an optical element, e.g. collector, laser input window, metrology window, to a temperature greater than 150° C. to increase a rate of chemical reaction between the deposited Tin and the etchant, e.g. bromide. For example, the heater may be an ohmic heater, a radiative heater, a radio-frequency heater and/or a microwave heater. 
   Thus, the use of tin bromide, may provide; a good coupling between tin and the laser, e.g. CO 2  laser source at 10.6 μm, resulting in a good conversion efficiency, reduced operating temperature for the droplet generator, e.g. SnBr 4  as compared to tin, an inherent debris mitigation mechanism (bromide etching), and may in some cases be used as a mass limited source material to decrease debris and material consumption. 
     FIG. 3  shows in greater detail a target delivery mechanism  92 ″ for delivering target(s) of source material having, for example, liquid SnH 4  into a chamber  26 ″ for irradiation therein to produce a plasma. For example, for liquid SnH 4  which boils at about minus 52 degrees Celsius, the mechanism  92 ″ may function to cool SnH 4  to a range of about minus 55 to minus 70 degrees Celsius, and generate a stream of SnBr 4  droplets. 
   For the specific embodiment shown in  FIG. 3 , the target delivery mechanism  92 ″ may include a cryogenic reservoir  300  holding a source material, e.g. tin hydride (SnH 4 ), in a liquid state and under pressure, e.g. using pressurized gas, e.g. inert gas, e.g. Argon gas. For example, nitrogen gas from a liquid nitrogen dewar may be circulated around the reservoir  300  to maintain the SnH 4  at a temperature of about minus 55 to minus 70 degrees Celsius. Other gases may be used to cool the SnH 4  such as Helium. Alternatively, or in addition thereto, a Peltier cooling system may be used to cool the SnH 4 . The argon gas pressure may be used to pass the source material through a set of filters  145 ′ which may be for example, fifteen and seven microns, respectively, which trap solid inclusions, e.g. solid compounds like oxides, nitrides; metal impurities and so on, of seven microns and larger. From the filters  145 , the source material may pass through an open/close valve  147 ′ to a dispenser  148 ′. For the mechanism  92 ″, one or more modulating or non-modulating source material dispenser(s) as described above may be used. For the arrangement shown in  FIG. 3 , the chamber  26 ″ may include a scrubber (not shown), a may be differentially pumped. As shown, a differentially pumped and gate valve isolated Residual Gas Analyzer (RGA)  152 ′ may be used to ensure the absence of gases prior to opening the chamber  26 ′. 
     FIG. 3  illustrates the generation of SnH 4 , droplets for irradiation by a laser beam  164 ′, which as described above may be a CO 2  laser source producing radiation at 10.6 μm. Irradiation of the SnH 4  creates a plasma which may include tin ions and hydrogen ions, and in some cases a portion of the droplet will be evaporated (but not ionized). This unused gaseous portion may be evacuated from the chamber via pumping and thus the unused portion of the droplet may not contribute to optical surface contamination. Tin ions formed in the plasma may reach the surface of an optical element, e.g. the surface of a collector mirror and deposit on the surface as metallic tin. Hydrogen ions, on the other hand, will be available and may etch tin, e.g. react with and remove tin deposits from the surface. The EUV light source may also include one or more heaters (not shown) to heat Tin that has deposited on the surface of an optical element, e.g. collector, laser input window, metrology window, to a temperature greater than 150° C. to increase a rate of chemical reaction between the deposited Tin and the etchant, e.g. hydrogen. For example, the heater may be an ohmic heater, a radiative heater, a radio-frequency heater and/or a microwave heater. 
   Thus, the use of SnH 4 , may provide a good coupling between tin and the laser, e.g. CO 2  laser source at 10.6 μm, resulting in a good conversion efficiency, reduced operating temperature for the droplet generator, e.g. SnH 4  as compared to tin, an inherent debris mitigation mechanism (hydrogen etching), and may in some cases be used as a mass limited source material to decrease debris and material consumption. 
     FIG. 4  shows a binary constitution diagram for Gallium Tin alloys. As shown, alloys of Gallium Tin have a melting point lower than pure tin and alloys are available which can be processed below 150 degrees C. allowing the use of piezoelectric actuators, and alloys are available that are liquid at room temperature allowing these alloys to be processed without heating the source material. Gallium debris, like tin debris, which deposits on optical surfaces such as the collector material may be, for example, using one or more of the following etchants; HBr, HI, Br 2 , Cl 2 , HCl, H 2 . Heat may be used to accelerate etching of both gallium and tin. For example, an HBr etchant may be used with heating of the optical component surface to a temperature in the range of 300-400 degrees Celsius. Use of the Eutectic alloy having a composition of about 12.5 weight percent Sn, balance Ga may be preferable in some applications due to its low melting point and compatibility (e.g. corrosiveness) with source material dispenser materials. 
   Thus, the use of Gallium-Tin alloys may provide a good coupling between tin and the laser, e.g. CO 2  laser source at 10.6 μm, resulting in a good conversion efficiency, reduced operating temperature for the droplet generator, a debris mitigation mechanism, e.g. etching of both tin and Gallium, and in some cases be used as a mass limited source material to decrease debris and material consumption. 
     FIG. 5  shows a binary constitution diagram for Indium Tin alloys. As shown, alloys of Indium Tin have a melting point lower than pure tin and alloys are available which can be processed below 150 degrees C. allowing the use of piezoelectric actuators. Indium debris, like tin debris, which deposits on optical surfaces such as the collector material may be, for example, using one or more of the following etchants; HBr, HI, Br 2 , Cl 2 , HCl, H 2 . Heat may be used to accelerate etching of both indium and tin. For example, an HBr etchant may be used with heating of the optical component surface to a temperature in the range of 300-400 degrees Celsius. Use of the Eutectic alloy having a composition of about 49.1 weight percent Sn, balance In may be preferable in some applications due to its low melting point and compatibility (e.g. corrosiveness) with source material dispenser materials. 
   Thus, the use of Indium-Tin alloys may provide; a good coupling between tin and the laser, e.g. CO 2  laser source at 10.6 μm, resulting in a good conversion efficiency, reduced operating temperature for the droplet generator, a debris mitigation mechanism, e.g. etching of both tin and Indium, and in some cases be used as a mass limited source material to decrease debris and material consumption. 
     FIG. 6  shows a prior art, ternary, liquidus projection, constitution diagram of for Gallium Tin Indium alloys. As shown, alloys of Indium Gallium Tin are available having a melting point lower than pure tin, alloys are available which can be processed below 150 degrees C. allowing the use of piezoelectric actuators, and alloys are available that are liquid at room temperature allowing these alloys to be processed without heating the source material. 
   As indicated above, both Gallium and Indium debris, like tin debris, which deposits on optical surfaces such as the collector material may be, for example, using one or more of the following etchants; HBr, HI, I 2 , Br 2 , Cl 2 , HCl, H 2 . Heat may be used to accelerate etching of gallium, indium and tin. For example, an HBr etchant may be used with heating of the optical component surface to a temperature in the range of 300-400 degrees Celsius. Use of the an alloy having a composition of about 16+/−3 weight percent Sn, 22+/−3 weight percent In, balance Ga may be preferable in some applications due to its low melting point and compatibility (e.g. corrosiveness) with source material dispenser materials. 
   Thus, the use of Gallium-Tin-Indium alloys may provide a good coupling between tin and the laser, e.g. CO 2  laser source at 10.6 μm, resulting in a good conversion efficiency, reduced operating temperature for the droplet generator, a debris mitigation mechanism, e.g. etching of both tin and Gallium, and in some cases be used as a mass limited source material to decrease debris and material consumption. 
   As used herein, the term “metal” and its derivatives includes all elements of the periodic table of elements except: Hydrogen, Helium, Carbon, Nitrogen, Oxygen, Fluorine, Neon, Phosphorus, Sulfur, Chlorine, Argon, Selenium, Bromine, Krypton, Iodine, Xenon and Radon. 
   It will be understood by those skilled in the art that the aspects of embodiments of the present invention disclosed above are intended to be preferred embodiments only and not to limit the disclosure of the present invention(s) in any way and particularly not to a specific preferred embodiment alone. Many changes and modification can be made to the disclosed aspects of embodiments of the disclosed invention(s) that will be understood and appreciated by those skilled in the art. The appended claims are intended in scope and meaning to cover not only the disclosed aspects of embodiments of the present invention(s) but also such equivalents and other modifications and changes that would be apparent to those skilled in the art.