Abstract:
A plasma generating system is disclosed having a source of target material droplets, e.g. tin droplets, and a laser, e.g. a pulsed CO 2  laser, producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation. For the device, the droplet source may comprise a fluid exiting an orifice and a sub-system producing a disturbance in the fluid which generates droplets having differing initial velocities causing at least some adjacent droplet pairs to coalesce together prior to reaching the irradiation region. In one implementation, the disturbance may comprise a frequency modulated disturbance waveform and in another implementation, the disturbance may comprise an amplitude modulated disturbance waveform.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 11/827,803, filed on Jul. 13, 2007, and published on Jan. 15, 2009, as U.S. 2009/0014668A1, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE HAVING A DROPLET STREAM PRODUCED USING A MODULATED DISTURBANCE WAVE, Attorney Docket Number 2007-0030-01; the contents of which are incorporated herein by reference. 
         [0002]    The present application is related to U.S. patent application Ser. No. 11/358,988 filed on Feb. 21, 2006, published on Nov. 16, 2006, as U.S. 2006/0255298A1, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, Attorney Docket Number 2005-0085-01; U.S. patent application Ser. No. 11/067,124 filed on Feb. 25, 2005, now U.S. Pat. No. 7,405,416, issued on Jul. 29, 2008, entitled METHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, Attorney Docket Number 2004-0008-01; U.S. patent application Ser. No. 11/174,443 filed on Jun. 29, 2005, now U.S. Pat. No. 7,372,056, issued on May 13, 2008, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, Attorney Docket Number 2005-0003-01; U.S. patent application Ser. No. 11/358,983 filed on Feb. 21, 2006, now U.S. Pat. No. 7,378,673, issued on May 27, 2008, entitled SOURCE MATERIAL DISPENSER FOR EUV LIGHT SOURCE, Attorney Docket Number 2005-0102-01; U.S. patent application Ser. No. 11/358,992 filed on Feb. 21, 2006, now U.S. Pat. No. 7,598,509, issued on Oct. 6, 2009, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, Attorney Docket Number 2005-0081-01; U.S. patent application Ser. No. 11/174,299 filed on Jun. 29, 2005, now U.S. Pat. No. 7,439,530, issued on Oct. 21, 2008, entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM, Attorney Docket Number 2005-0044-01; U.S. patent application Ser. No. 11/406,216 filed on Apr. 17, 2006, now U.S. Pat. No. 7,465,946, issued on Dec. 16, 2008, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0003-01; U.S. patent application Ser. No. 11/580,414 filed on Oct. 13, 2006, now U.S. Pat. No. 7,491,954, issued on Feb. 17, 2009, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0025-01, and U.S. patent application Ser. No. 11/644,153 filed on Dec. 22, 2006, published on Jun. 26, 2008, as U.S. 2008/0149862A1, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, Attorney Docket Number 2006-0006-01; U.S. patent application Ser. No. 11/505,177, filed on Aug. 16, 2006, published on Feb. 21, 2008, as U.S. 2008/0043321A1, entitled EUV OPTICS, Attorney Docket Number 2006-0027-01; U.S. patent application Ser. No. 11/452,558 filed on Jun. 14, 2006, now U.S. Pat. No. 7,518,787, issued on Apr. 14, 2009, entitled DRIVE LASER FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0001-01; U.S. Pat. No. 6,928,093, issued to Webb, et al., on Aug. 9, 2005, entitled LONG DELAY AND HIGH TIS PULSE STRETCHER; U.S. application Ser. No. 11/394,512, Attorney Docket Number 2004-0144-01 filed on Mar. 31, 2006, now U.S. Pat. No. 7,415,056, issued on Aug. 19, 2008, entitled CONFOCAL PULSE STRETCHER; U.S. application Ser. No. 11/138,001 (Attorney Docket Number 2004-0128-01) filed on May 26, 2005, published on Nov. 24, 2005, as U.S. 2005/0259709A1, entitled SYSTEMS AND METHODS FOR IMPLEMENTING AN INTERACTION BETWEEN A LASER SHAPED AS A LINE BEAM AND A FILM DEPOSITED ON A SUBSTRATE; and U.S. application Ser. No. 10/141,216, filed on May 7, 2002, now U.S. Pat. No. 6,693,939, issued on Feb. 17, 2004, entitled, LASER LITHOGRAPHY LIGHT SOURCE WITH BEAM DELIVERY Attorney Docket Number 2002-0039-01; U.S. Pat. No. 6,625,191 issued to Knowles, et al., on Sep. 23, 2003, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, U.S. application Ser. No. 10/012,002, Attorney Docket Number 2001-0090-01; U.S. Pat. No. 6,549,551 issued to Ness, et al., on Apr. 15, 2003, entitled INJECTION SEEDED LASER WITH PRECISE TIMING CONTROL, U.S. application Ser. No. 09/848,043, Attorney Docket Number 2001-0020-01; and U.S. Pat. No. 6,567,450 issued to Myers, et al., on May 20, 2003, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, U.S. application Ser. No. 09/943,343, Attorney Docket Number 2001-0084-01; U.S. patent application Ser. No. 11/509,925 filed on Aug. 25, 2006, now U.S. Pat. No. 7,476,886, issued on Jan. 13, 2009, entitled SOURCE MATERIAL COLLECTION UNIT FOR A LASER PRODUCED PLASMA EUV LIGHT SOURCE, Attorney Docket Number 2005-0086-01, the entire contents of each of which are hereby incorporated by reference herein. 
     
    
     FIELD 
       [0003]    The present disclosure relates to extreme ultraviolet (“EUV”) light sources that provide EUV light from a plasma that is created from a target material and collected and directed to an intermediate region for utilization outside of the EUV light source chamber, e.g. by a lithography scanner/stepper. 
       BACKGROUND 
       [0004]    Extreme ultraviolet 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. 
         [0005]    Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more 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 having the required line-emitting element, with a laser beam. 
         [0006]    One particular LPP technique involves irradiating a target material droplet with one or more pre-pulse(s) followed by a main pulse. In this regard, CO 2  lasers may present certain advantages as a drive laser producing “main” pulses in an LPP process. This may be especially true for certain target materials such as molten tin droplets. For example, one advantage may include the ability to produce a relatively high conversion efficiency e.g., the ratio of output EUV in-band power to drive laser input power. 
         [0007]    In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source element, such as 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. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a distance from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. In more quantitative terms, one arrangement that is currently being developed with the goal of producing about 100W at the intermediate location contemplates the use of a pulsed, focused 10-12 kW CO 2  drive laser which is synchronized with a droplet generator to sequentially irradiate about 40,000-100,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of droplets at a relatively high repetition rate (e.g. 40-100 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position (i.e. with very small “jitter”) over relatively long periods of time. 
         [0008]    For a typical LPP setup, target material droplets are generated and then travel within a vacuum chamber to an irradiation site where they are irradiated, e.g. by a focused laser beam. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber (e.g. debris) that can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements. These debris can include high-energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of is source material. For this reason, it is often desirable to use so-called “mass limited” droplets of source material to reduce or eliminate the formation of debris. The use of “mass limited” droplets also may result in a reduction in source material consumption. Techniques to achieve a mass-limited droplet may involve diluting the source material and/or using relatively small droplets. For example, the use of droplets as small as 10-50 μm is currently contemplated. 
         [0009]    In addition to their effect on optical elements in the vacuum chamber, the plasma by-products may also adversely affect the droplet(s) approaching the irradiation site (i.e. subsequent droplets in the droplet stream). In some cases, interactions between droplets and the plasma by-products may result in a lower EUV output for these droplets. In this regard, U.S. Pat. No. 6,855,943 (hereinafter the &#39;943 patent) which issued to Shields on Feb. 15, 2005 and is entitled “DROPLET TARGET DELIVERY METHOD FOR HIGH PULSE-RATE LASER-PLASMA EXTREME ULTRAVIOLET LIGHT SOURCE” discloses a technique in which only some of the droplets in a droplet stream, e.g., every third droplet, is irradiated to produce a pulsed EUV light output. As disclosed in the &#39;943 patent, the nonparticipating droplets (so-called buffer droplets) advantageously shield the next participating droplet from the effects of the plasma generated at the irradiation site. However, the use of buffer droplets may increase source material consumption and/or vacuum chamber contamination and/or may require droplet generation at a frequency much higher (e.g. by a factor of two or more) than required without the use of buffer droplets. On the other hand, if the spacing between droplets can be increased, the use of buffer droplets may be reduced or eliminated. Thus, droplet size, spacing and timing consistency (i.e. jitter) tend to be on the top of the list of factors to be considered when designing a droplet generator for an LPP EUV light source. 
         [0010]    One technique for generating droplets involves melting a target material, e.g. tin, and then forcing it under high pressure through a relative small diameter orifice, e.g. 5-30 μm. Under most conditions, naturally occurring instabilities, e.g. noise, in the stream exiting the orifice may cause the stream to break up into droplets. In order to synchronize the droplets with optical pulses of the LPP drive laser, a repetitive disturbance with an amplitude exceeding that of the random noise may be applied to the continuous stream. By applying a disturbance at the same frequency (or its higher harmonics) as the repetition rate of the pulsed laser, the droplets can be synchronized with the laser pulses. In the past, the disturbance has typically been applied to the stream by driving an electro-actuatable element (such as a piezoelectric material) with a waveform of a single frequency such as a sinusoidal waveform, triangular waveform, square waveform or their equivalent. 
         [0011]    As used herein, the term “electro-actuatable element” and its derivatives, means a material or structure which undergoes a dimensional change when subjected to a voltage, electric field, magnetic field, or combinations thereof and includes but is not limited to piezoelectric materials, electrostrictive materials and magnetostrictive materials. 
         [0012]    In general, for the application of single frequency, non-modulated waveform disturbances, the spacing between droplets increases as the disturbance frequency decreases (i.e. holding other factors such as pressure and orifice diameter constant). However, as disclosed in “Drop Formation From A Vibrating Orifice Generator Driven By Modulated Electrical Signals” (G. Brenn and U. Lackermeier, Phys. Fluids 9, 3658 (1997) the contents of which are incorporated by reference herein), for disturbance frequencies below about 0.3ν/(πd), where ν is the stream velocity and d is the diameter of the continuous liquid stream, more than one droplet may be generated for each disturbance period. Thus, for 10 μm liquid jet at a stream velocity of about 50 m/s, the calculated frequency minimum below which more than one drop per period may be produced is about 480 kHz (note: it/is currently envisioned that a droplet repetition rate of 40-100 kHz and velocities of about 30-50 m/s may be desirable for LPP EUV processes). The net result is that is for the application of single frequency, non-modulated waveform disturbances, the spacing between droplets is fundamentally limited and cannot exceed approximately 3.33 πd. As indicated above, it may be desirable to supply a sufficient distance between adjacent droplets in the droplet stream to reduce/eliminate the effect of the debris from the plasma on approaching droplet(s). Moreover, because the limitation on spacing is proportional to stream diameter, and as a consequence droplet size, this limitation can be particularly severe in applications such as LPP EUV light sources where relatively small, mass-limited, droplets are desirable (see discussion above). 
         [0013]    With the above in mind, Applicants disclose a laser produced plasma, EUV light source having a droplet stream produced using a modulated disturbance waveform, and corresponding methods of use. 
       SUMMARY 
       [0014]    In one aspect, a device is disclosed which may comprise a plasma generating system having a source of target material droplets, e.g. tin, and a laser, e.g. pulsed CO 2  laser, producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation. For the device, the droplet source may comprise a fluid exiting an orifice and a sub-system producing a disturbance in the fluid which generates droplets having differing initial velocities causing at least some adjacent droplet pairs to coalesce together prior to reaching the irradiation region. 
         [0015]    For this aspect, the ratio of initial droplets to coalesced droplets may be two, three, four or more and in some cases ten or more. In one embodiment, the subsystem may comprise a signal generator and an electro-actuatable element, e.g. at least one piezoelectric crystal, and in a particular embodiment, the sub-system may comprise a capillary tube and is the disturbance may be created in the fluid by vibrating, e.g. squeezing, the capillary tube. In one implementation, the disturbance may comprise a frequency modulated disturbance waveform and in another implementation, the disturbance may comprise an amplitude modulated disturbance waveform. 
         [0016]    In an implementation of this aspect, the disturbance may comprise a carrier wave having a carrier wave frequency and a modulation wave having a frequency comprising a carrier wave frequency subharmonic. In a particular implementation of this aspect, the laser may be a pulsed laser having a pulse repetition rate and the disturbance may comprise a modulated disturbance waveform having a modulation frequency equal to the pulse repetition rate. 
         [0017]    In another aspect, a device is disclosed which may include a plasma generating system comprising a source of target material droplets and a laser producing a beam irradiating the droplets at an irradiation region, the plasma producing EUV radiation. For this aspect, the droplet source may comprise a fluid exiting an orifice and a sub-system producing a disturbance in the fluid, the disturbance comprising at least two characteristic frequencies. 
         [0018]    In a further aspect, a device is disclosed which may include a means for forcing a fluid through an orifice, a means operable on the fluid to generate a first droplet and a second droplet, the first droplet having a different initial velocity than the second droplet causing the first and second droplet to coalesce together prior to reaching an irradiation region, and a means for irradiating the droplets at the irradiation region to form a plasma. In one implementation, the means operable on the fluid may generate a third droplet having an initial velocity to cause the first, second and third droplets to coalesce together prior to reaching the irradiation region. In one embodiment, the means operable on the fluid may comprise one electro-actuable element and in another embodiment, the means operable on the fluid may comprise a plurality of electro-actuable elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a simplified, schematic view of a laser produced plasma EUV light source; 
           [0020]      FIG. 2  shows a schematic a simplified droplet source; 
           [0021]      FIGS. 2A-2D  illustrate several different techniques for coupling an electro-actuable element with a fluid to create a disturbance in a stream exiting an orifice; 
           [0022]      FIG. 3  illustrates the pattern of droplets resulting from a single frequency, non-modulated disturbance waveform; 
           [0023]      FIG. 4  illustrates the pattern of droplets resulting from an amplitude modulated disturbance waveform; 
           [0024]      FIG. 5  illustrates the pattern of droplets resulting from a frequency modulated disturbance waveform; 
           [0025]      FIG. 6  shows photographs of tin droplets obtained for a single frequency, non-modulated waveform disturbance and several frequency modulated waveform disturbances; 
           [0026]      FIG. 7  illustrates a droplet pattern achievable using a modulated waveform disturbance in which droplet pairs reach the irradiation region allowing one droplet to shield subsequent droplet pairs from plasma debris; and 
           [0027]      FIG. 8  illustrates a droplet pattern achievable using a modulated waveform disturbance in which droplet pairs reach the irradiation region with a first droplet reflecting light into a self-directing laser system to initiate a discharge which irradiates the second droplet to produce an EUV emitting plasma. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    With initial reference to  FIG. 1  there is shown a schematic view of an EUV light source, e.g., a laser-produced-plasma, EUV light source  20  according to one aspect of an embodiment. As shown in  FIG. 1 , and described in further details below, the LPP light source  20  may include a system  22  for generating a train of light pulses and delivering the light pulses into a chamber  26 . As detailed below, each light pulse may travel along a beam path from the system  22  and into the chamber  26  to illuminate a respective target droplet at an irradiation region  28 . 
         [0029]    Suitable lasers for use as the device  22 ′ shown in  FIG. 1  may include a pulsed laser device, e.g., a pulsed gas discharge CO 2  laser device producing radiation at 9.3 μm or 10.6 μm, e.g., with DC or RF excitation, operating at relatively high power, e.g., 10 kW or higher and high pulse repetition rate, e.g.; 50 kHz or more. In one particular implementation, the laser may be an axial-flow RF-pumped CO 2  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 entering the LPP chamber. Continuously pumped CO 2  amplifiers may be used for the system  22 ′. For example, a suitable CO 2  laser device having an oscillator and three amplifiers (O-PA1-PA2-PA3 configuration) is disclosed in U.S. patent application Ser. No. 11/174,299 filed on Jun. 29, 2005, now U.S. Pat. No. 7,439,530, issued on Oct. 21, 2008, and entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM, Attorney Docket Number 2005-0044-01, the entire contents of which have been previously incorporated by reference herein. Alternatively, the laser 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 U.S. patent application Ser. No. 11/580,414 filed on Oct. 13, 2006, now U.S. Pat. No. 7,491,954, issued on Feb. 17, 2009, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0025-01, the entire contents of which have been previously incorporated by reference herein. 
         [0030]    Depending on the application, other types of lasers may also be suitable, e.g., an excimer or molecular fluorine laser operating at high power and high pulse repetition rate. Examples include, a solid state laser, e.g., having a fiber 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, 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 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. 
         [0031]    As further shown in  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 will interact with one or more light pulses, e.g., one or more pre-pulses and thereafter one or more main pulses, to ultimately produce a plasma and generate an EUV emission. The target material may include, but is not necessarily limited to, a material that includes tin, lithium, xenon 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. For 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 source is provided in U.S. patent application Ser. No. 11/406,216 filed on Apr. 17, 2006, now U.S. Pat. No. 7,465,946, issued on Dec. 16, 2008, entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0003-01, the contents of which have been previously incorporated by reference herein. 
         [0032]    Continuing with  FIG. 1 , the EUV light source  20  may also include an optic  30 , e.g., a collector mirror in the form of a truncated ellipsoid having, e.g., a graded multi-layer coating with alternating layers of Molybdenum and Silicon.  FIG. 1  shows that the optic  30  may be formed with an aperture to allow the light pulses generated by the system  22  to pass through and reach the irradiation region  28 . As shown, the optic  30  may be, e.g., an ellipsoidal mirror that has a first focus within or near the irradiation region  28  and a second focus at a so-called intermediate region  40  where the EUV light may be output from the EUV light source  20  and input to a device utilizing EUV light, e.g., an integrated circuit lithography tool (not shown). It is to be appreciated that other optics may be used in place of the ellipsoidal mirror for collecting and directing light to an intermediate location for subsequent delivery to a device utilizing EUV light, for example the optic may be parabolic or may be configured to deliver a beam having a ring-shaped cross-section to an intermediate location, see e.g. U.S. patent application Ser. No. 11/505,177 filed on Aug. 16, 2006, published on Feb. 21, 2008 as U.S. 2008/0043321A1, entitled EUV OPTICS, Attorney Docket Number 2006-0027-01, the contents of which are hereby incorporated by reference. 
         [0033]    Continuing with reference to  FIG. 1 , the EUV light source  20  may also include an EUV controller  60 , which may also include a firing control system  65  for triggering one or more lamps and/or laser devices in the system  22  to thereby generate light pulses for delivery into the chamber  26 . The EUV light source  20  may also include a droplet position detection system which may include one or more droplet imagers  70  that provide an output indicative of the position of one or more droplets, e.g., relative to the irradiation region  28 . The imager(s)  70  may provide this output to a droplet position detection feedback system  62 , which can, e.g., compute a droplet position and trajectory, from which a droplet error can be computed, e.g., on a droplet by droplet basis or on average. The droplet error may then be provided as an input to the controller  60 , which can, for example, provide a position, direction and/or timing correction signal to the system  22  to control a source timing circuit and/or to control a beam position and shaping system, e.g., to change the location and/or focal power of the light pulses being delivered to the irradiation region  28  in the chamber  26 . 
         [0034]    The EUV light source  20  may 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 . 
         [0035]    As further shown in  FIG. 1 , the EUV light source  20  may 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 controller  60 , to e.g., modify the release point of the target material from a droplet source  92  and/or modify droplet formation timing, to correct for errors in the droplets arriving at the desired irradiation region  28  and/or synchronize the generation of droplets with the pulsed laser system  22 . 
         [0036]      FIG. 2  illustrates the components of a simplified droplet source  92  in schematic format. As shown there, the droplet source  92  may include a reservoir  94  holding a fluid, e.g. molten tin, under pressure. Also shown, the reservoir  94  may be formed with an orifice  98  allowing the pressurized fluid  98  to flow through the orifice establishing a continuous stream  100  which subsequently breaks into a plurality of droplets  102   a, b.    
         [0037]    Continuing with  FIG. 2 , the droplet source  92  shown further includes a sub-system producing a disturbance in the fluid having an electro-actuatable element  104  that is operable coupled with the fluid  98  and a signal generator  106  driving the electro-actuatable element  104 .  FIGS. 2A-2D  show various ways in which one or more electro-actuatable elements may be operable coupled with the fluid to create droplets. Beginning with  FIG. 2A , an arrangement is shown in which the fluid is forced to flow from a reservoir  108  under pressure through a tube  110 , e.g. capillary tube, having an inside diameter between about 0.5-0.8 mm, and a length of about 10 to 50 mm, creating a continuous stream  112  exiting an orifice  114  of the tube  110  which subsequently breaks up into droplets  116   a,b . As shown, an electro-actuatable element  118  may be coupled to the tube For example, an electro-actuatable element may be coupled to the tube  110  to deflect the tube  110  and disturb the stream  112 .  FIG. 2B  shows a similar arrangement having a reservoir  120 , tube  122  and a pair of electro-actuatable elements  124 ,  126 , each coupled to the tube  122  to deflect the tube  122  at a respective frequency.  FIG. 2C  shows another variation in which a plate  128  is positioned in a reservoir  130  moveable to force fluid through an orifice  132  to create a stream  134  which breaks into droplets  136   a,b . As shown, a force may be applied to the plate  128  and one or more electro-actuatable elements  138  may be coupled to the plate to disturb the stream  134 . It is to be appreciated that a capillary tube may be used with the embodiment shown in  FIG. 2C .  FIG. 2D  shows another variation in which a fluid is forced to flow from a reservoir  140  under pressure through a tube  142  creating a continuous stream  144  exiting an orifice  146  of the tube  142  which subsequently breaks up into droplets  148   a,b . As shown, an electro-actuatable element  150 , e.g. having a ring-like shape, may be positioned around the tube  142 . When driven, the electro-actuatable element  142  may selectively squeeze the tube  142  to disturb the stream  144 . It is to be appreciated that two or more electro-actuatable elements may be employed to selectively squeeze the tube  142  at respective frequencies. 
         [0038]    More details regarding various droplet dispenser configurations and their relative advantages may be found in U.S. patent application Ser. No. 11/358,988 filed on Feb. 21, 2006, published on Nov. 16, 2006, as U.S. 2006/0255298A1, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, Attorney Docket Number 2005-0085-01; U.S. patent application Ser. No. 11/067,124 filed on Feb. 25, 2005, now U.S. Pat. No. 7,405,416, issued on Jul. 29, 2008, entitled METHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, Attorney Docket Number 2004-0008-01; and U.S. patent application Ser. No. 11/174,443 filed on Jun. 29, 2005, now U.S. Pat. No. 7,372,056, issued on May 13, 2008, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, Attorney Docket Number 2005-0003-01, the contents of each of which are hereby incorporated by reference. 
         [0039]      FIG. 3  illustrates the pattern of droplets  200  resulting from a single frequency, sine wave disturbance waveform  202  (for disturbance frequencies above about 0.3ν/(σd)). It can be seen that each period of the disturbance waveform produces a droplet and the resulting droplets are spaced by one disturbance waveform wavelength.  FIG. 3  also illustrates that the droplets do not coalesce together, but rather, each droplet is established with the same initial velocity. 
         [0040]      FIG. 4  illustrates the pattern of droplets  300  initially resulting from an amplitude modulated disturbance waveform  302 , which however is unlike the disturbance waveform  202  described above in that it is not limited to disturbance frequencies above about 0.3ν/(πd)). It can be seen that the amplitude modulated waveform disturbance  302  includes two characteristic frequencies, a relatively large frequency, e.g. carrier frequency, corresponding to wavelength λ c , and a smaller frequency, e.g. modulation frequency, corresponding to wavelength, λ m . For the specific disturbance waveform example shown in  FIG. 4 , the modulation frequency is a carrier frequency subharmonic, and in particular, the modulation frequency is a third of the carrier frequency. With this waveform,  FIG. 4  illustrates that each period of the disturbance waveform corresponding to the carrier wavelength, λ c  produces a droplet and the resulting droplets are initially spaced by one carrier wavelength, λ c .  FIG. 4  also illustrates that the droplets coalesce together, resulting in a stream of larger droplets  304 , with one larger droplet for each period of the disturbance waveform corresponding to the modulation wavelength, λ m . It can also be seen that the resulting coalesced droplets are spaced by one modulation wavelength, λ m . Arrows  306   a,b  show the initial relative velocity components that are imparted on the droplets by the modulated waveform disturbance  302  and are responsible for the droplet coalescence. 
         [0041]      FIG. 5  illustrates the pattern of droplets  400  initially resulting from a frequency modulated disturbance waveform  402 , which, like the disturbance waveform  302  described above, is not limited to disturbance frequencies above about 0.3 ν/(πd). It can be seen that the frequency modulated waveform disturbance  402  includes two characteristic frequencies, a relatively large frequency, e.g. carrier frequency, corresponding to wavelength λ c , and a smaller frequency, e.g. modulation frequency, corresponding to wavelength, λ m . For the specific disturbance waveform example shown in  FIG. 5 , the modulation frequency is a carrier frequency subharmonic, and in particular, the modulation frequency is about a third of the carrier frequency. With this waveform,  FIG. 5  illustrates that each period of the disturbance waveform corresponding to the carrier wavelength, λ c  produces a droplet and the resulting droplets are initially spaced by one carrier wavelength, λ m .  FIG. 5  also illustrates that the droplets coalesce together, resulting in a stream of larger droplets  44 , with one larger droplet for each period of the disturbance waveform corresponding to the modulation wavelength, λ m . It can also be seen that the resulting coalesced droplets are spaced by one modulation wavelength, λ m . Like the amplitude modulated disturbance (i.e.  FIG. 4 ), initial relative velocity components are imparted on the droplets by the frequency modulated waveform disturbance  402  and are responsible for the droplet coalescence. 
         [0042]    Although  FIGS. 4 and 5  show and discuss embodiments having two characteristic frequencies, with  FIG. 4  illustrating an amplitude modulated disturbance having two characteristic frequencies and  FIG. 5  illustrating a frequency modulated disturbance having two frequencies, it is to be appreciated that more than two characteristic frequencies may be employed and that the modulation may be either angular modulation (i.e. frequency or phase modulation), amplitude modulation or combinations thereof. 
         [0043]      FIG. 6  shows photographs of tin droplets obtained using an apparatus similar to  FIG. 2D  with an orifice diameter of about 70 μm, stream velocity of ˜30 m/s, for a single frequency, non-modulated waveform disturbance having a frequency of 100 kHz (top photo); a frequency modulated waveform disturbance having a carrier frequency of 100 kHz and a modulating frequency of 10 kHz of a relatively strong modulation depth (second from top photo); a frequency modulated waveform disturbance having a carrier frequency of 100 kHz and a modulating frequency of 10 kHz of a relatively weak modulation depth (third from top photo); a frequency modulated waveform disturbance having a carrier frequency of 100 kHz and a modulating frequency of 15 kHz (fourth from top photo) a frequency modulated waveform disturbance having a carrier frequency of 100 kHz and a modulating frequency of 20 kHz (bottom photo). 
         [0044]    These photographs indicate that tin droplets having a diameter of about 265 μm can be produced that are spaced apart by about 3.14 mm, a spacing which cannot be realized at this droplet size and repetition rate using a single frequency, non-modulated waveform disturbance. 
         [0045]    Measurements conducted using the droplet photos indicated a timing jitter of about 0.14% of a modulation period which is substantially less than the jitter observed under similar conditions using a single frequency, non-modulated waveform disturbance. This effect is achieved by averaging the individual droplets instabilities over a number of coalescing droplets. 
         [0046]      FIG. 7  shows a droplet pattern  600  produced using a modulated, e.g. multiple frequency, disturbance waveform (see also  FIG. 6  fourth photo from top). As shown, at a selected distance from orifice  604 . As shown, this droplet pattern in which droplet pairs reach the irradiation region allows droplet  608   a  to establish an EUV emitting plasma upon irradiation by the laser  22 ′ while droplet  608   b  shields subsequent droplet pair  610  from plasma debris. 
         [0047]      FIG. 8  illustrates a droplet pattern  700  achievable using a modulated e.g. multiple frequency, disturbance waveform in which droplet pairs reach the irradiation region with a first droplet  702   a  reflecting light into a self-directing laser system  704  to initiate a laser oscillation output laser beam which irradiates the second droplet  702   b  to produce an EUV emitting plasma. 
         [0048]    Self-directing laser system  704  is more fully described in U.S. patent application Ser. No. 11/580,414 filed on Oct. 13, 2006, now U.S. Pat. No. 7,491,954, issued on Feb. 17, 2009, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0025-01, see in particular,  FIG. 5 , the entire contents of which were previously incorporated by reference. Although the following describes a laser system  704  corresponding to  FIG. 5  of the Ser. No. 11/580,414 patent application, it is to be appreciated that this description is equally applicable to the other self-directed lasers disclosed in the Ser. No. 11/580,414 patent application (i.e.  FIGS. 6-16 .) 
         [0049]    Continuing with  FIG. 8 , it can be seen that the self directing laser system  704  may include an optical amplifier  706   a,b,c . For example, the optical amplifier  706  may be a CW pumped, multiple chamber, CO 2  laser amplifier amplifying light at a wavelength of 10.6 μm and having a relatively high two-pass gain (e.g. a two pass gain of about 1,000,000). As further shown, the amplifier  706  may include a chain of amplifier chambers  706   a - c , arranged in series, each chamber having its own active media and excitation source, e.g. electrodes. 
         [0050]    In use, the first droplet  702   a  of target material is placed on a trajectory passing through or near a beam path  710  extending through the amplifier  706 . Spontaneously emitted photons from the amplifier  706  may be scattered by the droplet and some scattered photons may be placed on path  710  where they travel though the amplifier  706 . As shown, an optic  708  may be positioned to receive the photons on path  710  from the amplifier  706  and direct the beam back through the amplifier  706  for subsequent interaction with the second droplet  702   b  to produce an EUV light emitting plasma. For this arrangement, the optic  708  may be, for example, a flat mirror, curved mirror, phase-conjugate mirror or corner reflector. An optical element  714 , e.g., lens may be positioned to collimate light entering the amplifier  706  from the droplet and focus light traveling from the amplifier  706  to the droplet. An optional optical delay  716  may be provided to establish the required time delay between when the first and second droplets reach the irradiation region. One advantage of using different droplets to 1) establish the optical oscillator and 2) generate an EUV emitting plasma is that the size of the droplets may be independently optimized for their specific function (i.e. reflection versus plasma production). 
         [0051]    While the particular embodiment(s) described and illustrated in this patent application in the detail required to satisfy 35 U.S.C. §112 are fully capable of attaining one or more of the above-described purposes for, problems to be solved by, or any other reasons for or objects of the embodiment(s) above described, it is to be understood by those skilled in the art that the above-described embodiment(s) are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the present application. Reference to an element in the following Claims in the singular is not intended to mean nor shall it mean in interpreting such Claim element “one and only one” unless explicitly so stated, but rather “one or more”. All structural and functional equivalents to is any of the elements of the above-described embodiment(s) that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present Claims. Any term used in the Specification and/or in the Claims and expressly given a meaning in the Specification and/or Claims in the present Application shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term. It is not intended or necessary for a device or method discussed in the Specification as an embodiment to address or solve each and every problem discussed in this application, for it to be encompassed by the present Claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the Claims. No claim element in the appended Claims is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited as a “step” instead of an “act”.