Cross-Reference to Related Applications

fool] This application claims priority to United. States Provisional Patent Application Serial Number 61/653,978 filed on May 31, 2012, entitled "'SYSTEM AND METHOD FOR PROTECTING A SEED LASER IN AN EUV LIGHT SOURCE WITH A B AGG AOM", Attorney Docket No, PA1161PRV, and also claims priorit to United States Utility Patent Application Serial Number 13/737,858 filed on January 9, 2013, entitled "SYSTEM AND METHOD FOR PROTECTING A SEED LASER IN AN EUV LIGHT SOURCE WITH. A. BRAGG AOM", Attorney Docket No. PA1161US, the entire contents of which are hereby incorporated herein by reference,

Field of the Invention

[002] The present invention relates generally to laser produced plasma extreme ultraviolet light sources. More specifically, the invention relates to a method .and apparatus for the use of seed lasers as such light sources.

Background of the Invention

[003] The semiconductor industry continues to develop lithographic

technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet ("EUV") light (also sometimes referred, to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 1.0 and 120 nanometers (ran). EUV lithography is currently generally considered to include EUV light at wavelengths in the range of 10 - 14 nm, and is used to produce extremely small features, for example, sub-32 nm features,, in substrates such as silicori. wafers, To be commerciall useful, it is desirable that these systems be highly reliable and -provide cost effective throughput and reasonable process latitude,

[O ] Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements, e.g,, xenon, lithium, tin, indium, antimony, tellurium, akiminum, etc., with one or more emission Ime(s) in the EUV range. In one such method, often termed: lase 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 desired line-emitting element, with a laser beam at an irradiation site. The line-emitting element may be in pure form or alloy form, for example, an allo that is a liquid at desired temperatures, or may be mixed or dispersed with another material such as a li uid.

[005] In some prior art LPP systems, dro lets in a droplet stream are irradiated by a separate laser pulse to form a plasma from each droplet, Alternatively, some prior art systems have been disclosed in which each droplet is sequentially illuminated by more than one light pulse. In some cases, each droplet may be. exposed to a so-called "pre-pu!se" to heat, expand, gasify, vaporize, and/or ionize the target material and/or generate a weak plasma, followed, by a so-called "main pulse" to generate a stron plasma and convert most or all of the pre-pulse affected material into plasma and thereby produce an EUV light emission. It will be appreciated that more than one pre-pulse may be used and more than one main pulse may be used, and that the functions of the pre-pulse and main pulse may overlap to some extent.

[006] Since EUV output power i an LPP system generally scales with the drive laser power that irradiates the target material, in some cases it may also be considered desirable to employ an arrangement including a relatively low-power oscillator, or "seed, laser/" and one or more amplifiers to amplify the pulses from the seed laser. The use of a large amplifier allows for the use of die seed laser while still providing the relatively high power pulses used in the LPP process.

[0071 However, the irradiation of the droplets by the laser pulses .may result in refiecfcions and thus light propagating back toward the seed laser. Further, the seed laser may include sensitive- optics, and, since the pulses from the seed. laser have been amplified, this back-propagating light may be of a large enough intensity to damage the relatively fragile seed laser.

[OOSj For example, in some cases the. amplifiers) may have a signal gain, on the order of 100,000 (i.e., 10 ^s ). In such a case, a typical protection device of the prior art, such ^' as a polarization discriminating optical isolator, which may for example stop approximately 93 to 99 percent of the back-propagating light, may be insufficient to protect die seed laser from damage.

[009] Accordingly, it is desirable to have an improved system and method for protecting the seed laser in such an EUV light source.

Summary of the Invention

{0010] Disclosed herein are a method and apparatus fo protecting the seed laser(s) in a laser produced plasma (LPP) extreme ultraviolet (EUV) light system using a Bragg acousto-opiic modulator (AOM).

[0011] One embodiment describes a system comprising: a laser source producing laser light on a first, path; a Bragg acous o-o tic modulation device switehable between a first state in which the laser light is deflected from the first path onto a second path toward an irradiation site at which a target material may be irradiated by the laser light and a second state in which the laser light is transmitted a direction, other than onto the second path; a delay device on the second path between the Bragg device and the irradiation site for delaying the laser light; and means for switching the Bragg device from the first state to the second state af er the laser light has been deflected onto the second path such that any reflections of the laser light from the irradiation site are transmitted in a direction other than along the first path, and thus ^' prevented from reaching the laser source,

[0012] J another embodiment, a system is described comprising: a pre-pulse seed laser that produces laser light; a first Bragg acousto-optic modulation device swltc able between a first state in which the laser light from the pre-pulse seed laser Is deflected onto a first beam path and a second state in which the laser light is transmitted in a direction other Ulan onto the first beam path; a mai pulse seed laser that produces laser light; a second Bragg acousto-optic modulation device sw ^' itchable between a first state lit which the laser light from the main pulse seed laser is deflected onto a second beam path and a second state in which the laser light Is. transmitted in a direction other than onto the second beam path; a combiner for directing the laser light from the pre-pulse seed laser on the first beam path and the laser light from the main pulse laser on the second beam path onto a common beam path toward an irradiation site at which a target material may be irradiated by the laser light; means for switching the first Bragg device from the first state to the -second state after the laser light from the pre- pulse seed laser has been deflected onto the first beam path such that arty reflections of the laser Sight from the irradiation site are prevented f ont reaching the pre-pulse seed laser; and means for switching the second Bragg device from the first state to the second state after the laser light from the main pulse seed laser has been deflected onto the second beam path such tha any reflections of the laser light from the irradiation site are prevented from reaching, the mai pulse seed laser.

[00133 In still another embodiment, a method of protecting a laser pulse source from pulse refle ti ns is described, comprising generating a laser pulse on a first path from a laser source; passing the laser pulse through a Bragg acousto- optk modulation device, the Bragg device switchabie between a first state in which the pulse is deflected onto a second path toward an irradiation site at which a target material, may be irradiated by the laser pulse and a second state in which the laser pulse is transmitted i a direction other than onto the second path, while the Bragg device is in the first state; and switching the Bragg device from the first state- to the second state after the laser pulse has been deflected onto the second path such that any reflections of the lase pulse from the irradiation site are transmitted in a direction other than along the first path, and thus prevented from reaching the laser source.

Brief Description of the Drawings

{0014] Figure 1 is an illustration of some of the components of an embodimen of an LPP BUY system.

[0015] Figure 2 is an illustration of some of the components of a seed laser module that may b used in an LPP EUV system.

[0016] Figure 3 is another illustration of some of the components of an embodiment of an LPP EUV system,

[0017] Figure 4 is another illustration of some of the components of an embodiment of an LPP EUV system. [0018] Figure 5 is another illustration of some of the components of an

embodiment of an LPF EUV system.

[0019] Figure 6 is another illustration of some of the components of an

embodiment of an LPP EUV system.

[0020] Figure 7 is another illustration of some of the components of an

embodiment of a.n LPP EUV system.

[0021] Figure 8 is a flowchart of one embodiment of a method of protecting a seed laser as described herein.

Detailed Description of the Invention

[0022] Th present application describes a method and apparatus for protecting the seed laser source(s) in a laser produced plasma (LPP) extreme ultraviolet (EUV) ligh system.

[002.3] In one embodiment, a method of protecting a seed laser source involves using a Brag aeousto-optic modulator (AO ) as a switch on the beam path from tire seed laser to other optical components and ultimately to the irradiaticm site. When pulses from the seed laser are generated, power is applied to the Bragg AOM and the pulses are thus deflected onto the desired beam path rather than passing straight through the Bragg AOM. Once the pulses have passed through the Bragg AOM, power to the Bragg AOM ceases, so that any reflections from the Irradiation site will pass straight through the Bragg AOM and will not be deflected back to the seed laser, it desired, such reflections may pass through the Bragg AOM into a "beam dump" or cooling component. [0024] Figure 1 is a simplified schematic view of some of the components of one embodiment of an LPP EUV light source 20. As shown in Figure 1, the EUV light source 10 includes a laser source 12 for generating a beam of laser pulses and delivering the beam along one or more beam paths from the laser source 12 and into a chamber 14 to illuminate a respective target,, such as a droplet, at an irradiation region 16. Examples of laser arrangements that may be suitable for use in the system 12 shown in Figure 1 are described in more detail below.

[0025] As also shown m Figure 1, the EUV light source 10 may also include a target material delivery system 26 that, for example, delivers droplets of a target materia! into the interior of chamber 14 to the irradiation region 16, where the droplets will interact with one or more laser pulses to ultimately produce plasma and generate an EUV emission, Various target material delivery systems have been presented in die prior art, and their relative advantages will be apparent to those of skill in the art,

[0026] As above, the target material is an EUV emitting element thai may include, but is not necessarily limited to, a materia! that includes tin, lithium., xenon or combinations thereof. The target material may be in the form of liquid droplets, or alternatively may be solid particles contained within liquid droplets. For example,, the element tin may be presented as a target material as pure tin, as a tin compound, such as SnBn, SnBn , Snhh , as a tin alloy, e.g., tin-gailium alloys, tin-indium alloys, or tin-indiimvgai!ium. alloys, or a combination thereof. Depending on the material used, the target material ma be presented to. the irradiation region 16 at various temperatures including room temperature or near room temperature (e.g., tin alloys or SnBri)., at a temperature above room temperature (e.g., pure tin), or at temperatures belo room temperature (e.g., Sni'h). In some cases, these compounds may be relatively volatile, suc as SnBr*. Similar alloys and .compounds of EUV emitting elements other than tin., and the relative advantages of such materials and those described above will be apparent to those of skill in the art

IQ0273 Returning to Figure 1, the EUV light source 10 may also include an optical element 18 such as a near-normal incidence collector mirror havin a reflective surface in the form of a. prolate spheroid (i.e., an ellipse rotated about its major axis), such that the optical element 18 has a first focus within or near the

irradiation region 16 and a second focus at a so-called intermediate region 20, where the EUV light may be output from the. EUV light source 10 and input to a device utilizing EUV light such as an integrated circuit lithography tool (not shown).. As shown in Figure % the optical element 18 is formed with an aperture to allow the laser light pulses generated by the laser source 12 to pass through and reach the irradiation region 16.

[0028] The optical element 18 should have an appropriate surface for collecting the EUV light and directing it to the intermediate region 20 for subsequent delivery to the device utilizing the EU V light. For example, optical element 18 ^• might have a graded multi-layer coating with 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.

[0029] it will be appreciated by those of skill in the art that optical elements other U an a prolate spheroid mirror may be used as optical element 18, For example, optical element 18 may alternatively be a parabola rotated about its major axis or ^• may be configured to deliver a beam having a ring-shaped cross section to an intermediate location. In other embodiments, optical element IS may utilize coatings and layers other than or in addition to those described herein. Those of skill In the art will be able to select an appropriate shape, and composition for optical element 18 in a particular situation.

[0030] As shown in Figure 1, the EUV ligh source 10 may include a focusing unit 22 which includes one or more optical elements for focusing the laser beam to a focal spot at the irradiation site. EUV light source 10 may also include a beam conditioning unit 24, having one or more-optical elements, between the laser source 12 and the focusing unit 22, for expanding, steering -and/or shaping the. laser beam, and/or shaping the laser pulses. Various focusing units and. beam conditioning units are know in the art, and may be appropriately selected by those of skill in the art,

[0031] As noted above. In some cases an LPP EUV system uses one or more seed lasers to generate laser pulses, which may then be amplified to become the laser beam that irradiates the target material at irradiation site 16 to form a plasma that produces the EUV emission. Figure 2 is a simplified schematic view of one embodiment of a seed laser module 3.0 that may be used, as part of the laser light source in an LPP EUV system.

[0032] ^' As illustrated in Figure 2, seed laser module 30 includes two seed lasers, a pre-pulse seed laser 32 and a main pulse seed laser 34, One of- skill in the art will appreciate that where such an embodiment containing two seed lasers is used., the target material may ^' be irradiated first by one or more pulses from the pre- pulse seed laser 32 and then by one or more pulses from the main pulse seed laser 34,

[0033] Seed laser module 30 is shown as having a "folded" arrangement rather than arranging the components in a straight line. In practice, such an

arrangement is typical in order to limit the size of the module, To achieve this, the beams- produced by the laser pulses of pre-pulse seed laser 32 and mam pulse seed laser 34 are directed onto desired beam paths by a plurality of optical components 36. Depending upon the particular configuration desired, optical components 36 may be such elements as lenses, filters, prisms, mirrors or any other element which may be used to direct the beam in a desired direction. I some cases., optical components 36 may perform other functions as _: well, such as altering the polarization of the passing beam,

[0034] As is known to those of skill in the art, the seed lasers contain relatively fragile optical components, such as the output coupler, polarizer, rear mirror, grating, aeoiisio-optieal modulation (AOM) switches, etc, Thus, it is desirable to prevent any light that may be reflected from the target material at the irradiation site from reaching and damaging these components,

[0035] In the embodiment of Figure 2, the beams from each seed laser are firs passed through art electro-optic modulator 38 (EOM), The EOMs 38 are used with the seed lasers as pulse shaping units to trim the pulses generated by the seed lasers to pulses having shorter duration and faster fail-time, A. shorter pulse duration and relatively fast fall-time ma increase EUV output and light source efficiency because of a short interaction time between the pulse- and a target, and because tmneeded portions of the pulse do not deplete amplifier gain. While two separate pulse shaping units (EOMs 38) are shown, alternatively a common pulse shaping unit may be used to trim both pre-pulse and main pulse seeds-.

[0036] The beams from the seed lasers are then passed through acousto-optic modulators (AOMs) 40 and 42, As will, be explained below, the AOMs- 40 and 42 act as "switches" or "shutters/' which opera e to divert any reflections of the laser pulses from the target material from reaching the seed lasers; as above, seed lasers typically contain sensitive optics, and the AOMs 40 and 42 thus prevent any reflections from causing damage to the seed laser elements. In the embodiment shown here, the beams from each seed laser pass through two AOMs; however,, in some embodiments, the beams from each seed laser may be passed through only a single AOM on each path.

[0:037] After passing through the AOMs 40 and 42, the two beams are

"combined" by beam combiner 4-4. Since the pulses from each seed laser are generated at different times, this really means that the two temporally separated beams are placed on a common beam path 46 for further processing and use.

[0038] After being placed on the common beam path, the beam from one of the seed lasers (again, there will only foe one at a lime) passes through a beam delay unit 48 such as is known in the art and as will, be explained further below, Next, the beam, is directed through a pre-amp!ifier 50 and then through a beam expander 52. Following this, the beam passes through a thin film polarizer 54, and is then directed onward by optical component 56, whic again is an element which directs the beam to the next stage in the LPP EUV system and may perform other functions as well. From optical component 56, the beam typically passes to one or more optical amplifiers and other components, as will he illust ted be low .

[0039] Various wavelength tunable seed lasers that are suitable for use as both pre-puise and main pulse seed lasers are known in the. art ^' For example, in one embodiment a seed laser may be a CC¼s laser having a sealed filling gas including COi at sub-atmospheric pressure, for example, 0,05 to 0.2 atmospheres, and pumped by a radio-frequency discharge. In some embodiments, a grating may be used to help define the optical cavity of the seed laser,, and the grating may be rotated to tune the seed laser to a selected rotational line.

[0040] Figures 3 through 7 are simplified, schematics of various embodiments of a portion of a laser source 100 for use in the light source 12 shown i Figure 1, each of a different configuration. Some of the elements shown in these figures correspond to elements that appear in Figures 1 and 2 above,

[00411 Figure 3 shows a first example of a laser source 100 having, components that provide protection for a seed laser from reflected light. As shown, the device 100 includes a single seed laser 104 which produces a light output on beam path 106 which passes through a switch 108, a beam delay unit 110., an amplifier 112, and a beam conditioning unit 114, and subsequentl interacts with a target material at an irradiation site 116.

[0042] The amplifier 112 may have one or more amplifying units, each having a sealed gas and excitation source, and the beam conditioning unit 114 may have one or more optical components for expanding, steering, pulse shaping, focusing and/or shaping the beam,

[0043] Switch 108 is reconfigurable between a first, open state which allows light to flow substantially unimpeded through the switch along beam, path 106 and a second, dosed state which deflects and/or diffuses a substantial, portion of light from the beam path 106. In some cases, two such switches may be serially positioned adjacent to one another along the beam path to increase the amount of the light which is deflected from the beam path, when the switches are closed. Thus, if the first switch, deflects 90% of the incident light and passes the remaining 10% to the second switch, and the second switch similarly deflects 90% of he light incident upon it. the combined effect of the two switches will be to deflect 99% of the light incident upon the .first switch,

[0044] Beam delay unit 110 such as is known in the art is located on beam path 106 between the switch 108 and amplifier 112, Beam delay unit 110 has a beam folding optical arrangement including optical components such as mirrors, prisms, etc., such that light passing through the unit travels an optical. delay distance, d«f«by. Using an estimated light speed of about 3 x 10 ^s meters per second, each meter of beam delay adds an additional approximately 3.33 ns of travel, time for the light on the beam path. 106,

[0045] in one embodiment, the beam delay unit 110 is. sized with, a length such that the round trip time from the switch 108 to the irradiation site 116 and back, including a suitable margin of safety, exceeds- the close time of the switch 108, so that switch 108 has time to close to- prevent light reflected from the target material at the irradiation site from reaching seed laser 104. Such a round trip time would thus include twice the travel time from the switch 108 to the beam delay unit 110, twice the one-way travel time in the beam delay unit 110, twice the travel time from the beam delay unit 110 to the amplifier .1.12, twice the one way travel time in the amplifier 112, twice the tr vel time from the amplifier 112 to the beam conditioning unit 114, twice the one way travel time i the beam conditioning unit 114 and twice the travel time from the beam conditioning tinit 114 to the irradiation site 116,

[004.6] There are various types of switches that may switch between a first open state which allows light to flow through the switch along beam path 106 and a second,, closed state which deflects and/or diffuses most of the light. One such typ of switch is an acousto-optie modulator (AOM), An AOM uses the acousto- optic effect, in which an acoustic (sound) wave, within a material causes a change in the optical characfceristtcs of the material, to diffract and shift the frequency of light passing through the AOM.

[0047] An AOM is typically activated by a piezoelectric transducer (PZT) attached to the AOM, Power (typically KJF power) is applied to the PZT as an oscillating electric signal, which causes the PZT to vibrate and. creates the acoustic wave in the AOM, When no power is applied/ there is thus no acoustic wave, and light is transmitted directly through the AOM; when power is applied, the acoustic wave is present and the incident light beam is deflected in "deflection mode" and shifted in frequency, Due to the desired switchin speeds, power is typically applied to the PZT at the direction of a processor or controller.

[0048] It is because of Hie frequency shift that AOMs. are often used in pairs, as shown in Figure 2. This allows the shift in frequency by a first AOM to be couriered by the use of a second AOM in which the acoustic wave is applied from the opposite side of the AOM so that it produces a similar shift in frequency but in the opposite direction, thus resulting in a signal of the original frequency. This can be important in maintaining the appropriate center frequency of a signal for proper amplification by an amplifier. However, in some circumstances only a single AOM may be used. One of skill in the art will appreciate that the frequency shift is a function of the diffraction order; a +1 diffracted order adds to the frequency, while a -1 diffracted order subtracts from the frequency. The sign of the order is determined, by the direction of the acoustic wave in the crystal and the angle of incidence of the beam in the crystal. One of skill in die art will also apprecia e when a pair of AOMs should be used and when a single AOM may be appropriate. [0049] There are different types of AOMs; two such types are a Raman-Nath AOM and a Bragg AOM. A. Raman-Math AO -was initially selected for use as switch 108 i some prior art embodiments (as AOMs 40 in Figure 2 might e as well) due to expected advantages over the Bragg AOM _. , which was believed to be unsuitable for this purpose,

[0050] It has previously been thought tha in using an AOM as switch 108, the AOM should fee " ^' off/' or "open/' to allow the seed pulse to pass,- and then turned "on" or "closed" to divert any reflected light This avoids any deflection of the seed pulse which may cause distortion. With respect to the reflected light., in general a Raman-Math AOM operates more efficiently than a Bragg AOM in deflecting light when the AOM is "On" i.e., in '" ^' deflection mode." Raman-Nath AOMs were also .thought to better maintain the integrity of the incident light beam. Finally, a Raman-Math AOM switches from ' ^' 'open" to "closed" position faster than a Bragg AOM. For this reason, it has previously been thought best to use a Raman-Nath AOM as switch 108, by letting the seed pulse pass while the AOM is '''off" and applyin power to the AOM once the seed pul.se has passed so that any reflected light returning is deflected.

[00513 A description of a device using a Raman-Nath AOM as switch 108 may be found in U.S. patent application 13/077 757, publication number 2012/0092746, filed on March 13, 2011 and commonly owned by the assignee of the present application. As described therein, in a typical Raman-Nath AOM, a fully-open to fully-closed switch time for a beam 3 millimeters (mm) in diameter might be in the range of about 300-500 ns. Thus, i some cases a close time of about 400 ns might be assumed for design purposes, so that a round trip time from, the switch 108 to the irradiation site 116 and back of about 800-1000 ns may be considered sufficient to allow the switch 108 to close. [005.2] Suppose that in the operation of the device 100 shown in Figure 3, a pulse of light is first emitted from seed laser 104 a time t = 0, having, for example, a pulse duration of about 100 ns. The trailing edge of the pulse exits switch 108 at about t = 100 ns at which time switch 108 is activated to close. Assuming a 100 m beam delay path in delay unit 110, this results in a pulse delay of approximately 333 ns. Adding a light travel time of about 150. na from the beam delay unit 110 to the irradiation site 116, the. leading edge of the p.ulse will reach the irradiation site 116 at about t *= 483 ns, and. the trailing edge of the pulse will reach the irradiation site 6 at about 583 ns.

[0053] If switch 108 has: a close time of about 400 ns or so, it should be fully closed by t - 500 ns. Any reflections from the droplet will reach the closed switch 108 at a time 483 ns after d e time of the reflection, i.e., 150 ns to travel from the irradiation site back to the pulse dela unit 110 and 333 ns of delay by the pulse delay unit 110. Thus, any reflection from the leading edge of the pulse will reach switch 108 at approximately t = 966 ns, while a reflection from the trailing edge, of the pulse should reach switch 108 at approximately t - 1066 ns. Assuming thai- switch 108 is closed by t - 500 ns, this provides a factor of safety of at least approximately 466 ns.

[0054] However, since the selection of a ^' Raman-Nath AO.M as switch 108, it has been found in practice that the Raman-Nath AOM suffers from a number of problems in actual use. As above, the Raman-Nath AOM is "closed/' and diverts the incident light beam when it is "on," i.e., receiving power. Since switch 108 is intended to be closed most of the time, this means, thai the Raman-Nath AOM must receive power most of the time; in operation, the Saman- th AOM is typically receiving power about 90% of the time or more. At such a 90% or higher duty cycle, a Raman-Math AOM will generally not have a long life, and thus must be replaced relatively often.

[0055] Such a high duty cycle also results in about 100 watts of average powe being delivered into the Raman-Nat AOM material (a crystal) by the acoustic wave, heating it and producing severe cylindrical iensing of the beam which effectively drops transmission through the switch and introduces aberrations in tile beam (for example, astigmatism and/or unwanted focus), I is also know that the increase in temperature of the Raman-Nath AOM increases the optical absorption of the switch, and this may potentially cause a state of optical thermal runaway at a lower optical power than expected,.

[0056] Another issue is the frequency with whic the switch 108 must he turned on and off. The seed lasers may operate a frequencies ranging from

approximately 50 KHz to 100 KHz or more., which means switch 108 must also cycle at that rate. At a pulse rate of SO KHz a Raman-Nath AOM works, but at higher frequencies such as 100 KHz It becomes less reliable. Also, at such a high frequency there is beam distortion,

[0057] in addition,, the diversion efficienc of a Raman-Nath AOM degrades easily with the duty cycle and hard use. In some cases, after substantial use Raman-Nath AOMs have been observed to pass 10% of the incident beam even when in the -"closed" state, rather than the 3% passed when they were new, Wi h two such AOMs in series, this would result in the contrast ratio, as above the ratio of the incident light to the transmitted light, dropping from 1000 to 1 down to 100 to 1, i.e., instead of allowing only 0.1% of the incident light to pass, ten times more, or 1%, of the light would pass even the two "closed" switches. [0058] Finally, because a Raman-Nath operates as a "closed" switch and diverts the reflected light when the acoustic wave is applied, if power to the Raman- Nath switches is lost,, the. switches revert to the "open" function and allow the reflected light to pass through. Thus, in such a situation any reflections will pass through the switch and may damage the seed laser(s).

[0059] For these reasons, Bragg AOMs have now been considered as alternatives to the Raman-Nath AOMs in spite of the advantages of Raman-Nath AOMs discussed above. In particular, by using Bragg AOMs to pass the seed laser pulses while "an " i.e., in "deflection mode/' a number of advantages ^' are obtained ove the prior use of Raman-Nath AOMs. While there is some distortion of the seed laser pulse b the Bragg AOM when "on," it has been found that the loss in beam energy in this case is- acceptable, as the seed laser pulses may be sufficiently amplified even after passing through an operating Bragg AOM,

[00.60] By using the Bragg AOMs to pass the seed laser pulses in the "on" position -or "deflection mode," rather tha in the "off" mode as with the Raman- Nath AOMs, the Bragg AOMs are on only about! to 3% of the time, rather than the 90%+ of the time that the Raman-Nath AOMs must be powered in operation. Because much less power is applied to the Bragg AOMs, there is less thermal tensing and distortion of the beam, and a longer life cycle of the Bragg AOMs, as well as a greater number of available commercial products due to the lower operating parameters,

[0061] Finally, if power to the Bragg AOMs is lost the Bragg AOMs return to their "off position, transmitting the reflected beam directly rather than deflecting it back along the beam path,, and thus preventing the reflections from reaching the seed laser. The fact that the Bragg AOMs are not as fast as the .Raman-Nath AOMs may be compensated for by increasing the optical delay in a beam delay unit (such as beam delay unit 48 in Figure 1 or beam delay unit 110 in Figure 3) to a longer period if necessary.

(Q062J When the Bragg AOMs are operated in di version mode,, the beam(s) passing through them are diverted. This is illustrated in Figure 2, where the first Bragg AOM 40■tJ rough which a beam passes alters the directio of ^' the beam,, and the second Bragg AOM 42 returns, the beam to a path that is parallel to, but offset front, the path of the beam before the first Bragg AOM. It may thus be seen that if a reflectio reaches a Bragg AOM 42, and die Bragg AOM 42 has been switched off and is no longer in diversion mode,, the beam will not proceed along the beam path back to the respective seed laser, but will exit Bragg AOM 42 in the direction in which it enters. For additional safety, a "beam, dump ^* " element (not shown), which may include a cooling element, may be added to contain such a reflected beam.

[0063] When a Bragg AOM is excited by a 40 MHz acoustic wave, the diversion of the. beam is in a direction 77 rnii!iradians (mrad) away from the direction of the incident beam. Where two Bragg AOMs such as Bragg AOMs.40 and 42 are located 525 millimeters (mm) a art his results in the path of the beam out of the second Bragg AOM.42 being laterally offset 30 mm from the path of the beam into the first Bragg AOM 40.

|GQ64] Other embodiments of a device having components, that provide protection of the seed laser(s) from reflected light are shown in Figures 4 through 7, In these embodiments, Bragg AOMs may again be used as swi ches that may be dosed to protect the seed lasers as described above. [0065] In Figure A, a. laser source 200 again has switch 108 and beam delay unit .110 to provide protection for the seed laser 10 from reflections; laser source 200 as shown also has an optional optical isolator 202 that may he included for additional protection against reflections if desired. As in laser source 100 in Figure 3, device 200 again includes a seed laser 104 producing a light output on beam path 106 which passes through switch 108, beam delay unit 110, amplifier 112, and beam conditioning unit 11.4, and subsequently interacts with a target material at an irradiation site 116,

[0066] In the embodiment of Figure 4, seed laser 104 may include one or more polanzer(s) and/or Brewster's windows (not shown) such that light exiting seed laser 104 has a primary polarization direction. As -shown, an optical isolator 202 includes a phase retarding optical component 204, such as a quarter wave assembly, and a polarizer 206 that is aligned parallel to the primary polarization direction of the seed laser. In this configuration, light exiting the oscillator on beam path 106 with the primary polarization direction will pass through switch 108 and beam delay unit 110, and then through the polarizer 206 and be altered by the phase retarding optical component 204 so that it exits optical isolator 202 with circular polarization.

[Θ067] This light on beam path 06 will continue through the amplifier 112 and beam conditioner-unit 114, and then reflect from the target material where an additional phase retardation due to plasma reflection will occur, resulting in an elliptic-ally polarized state. The reflected light will pass back through the beam conditioning unit 114 and amplifier 112, and then be incident upon the phase retarding optical component 204. Upon passing through the phase retarding optical component 204, the reflected light will be altered- again, exiting, the phase- retarding optical component 204 in a polarization state such that when it reaches polarizer 206, most of the light perhaps 93% or so, will be absorbed or reflected.., and the remaining approximately 6 to 7% of the light can leak through polarizer

206.

[0068] The light which leaks through the polarizer 206, which maybe substantial due to the large gain of the .amplifier 112, e.g., 300 to 350 watts or more, will pass through the beam delay unit 110. and reach the closed switch 108. However, due to the reduction in reflected light by optical isolator 202 of over 90%, switch 108 need not block as much light to protect oscillator 104 as in the embodime of

Figure 3.

[0069] Figure 5 shows another embodiment of a laser source 300 having: seed lasers and components that provide protection from reflected light. As shown in Figure 5, device 300 includes a pre-pulse seed laser 302 which produces a light output on beam path 304 that is reflected by beam combiner 306 onto common beam path 106, and a main pulse seed laser 308 which produces a light output which passes through beam combiner 306 onto common beam path 106,

[0070] For example, beam combiner 306 may be a diffraction grating, dichroic beam combiner, prism, volume Bragg grating, polarization discriminating beam combiner or partially reflecting beam combiner or Other suitable optical component for placing the beams on common beam path 106. Also a above, although, the beam combiner 306 is shown reflecting the pre-pulse seed and transmitting the main pulse seed, it is to be appreciated that the beam combiner 306 could he arranged, to reflect the output of the main pulse seed laser and transmit the output of the pre-pulse seed laser.

[0071] Once on common bea path ^" 106, the seed outputs pass through switch 108, beam delay unit 1 0, optical isolator 202, amplifier 1.12, beam conditioning unit 114, and subsequently interact with a. target material at an. irradiation site 116, all as previously described above with reference to Figure 4,

[0072] In one application of device 300, switch 108; is initially opened allowing a laser pulse from. p.re-pulse seed laser 302 to pass through switch 108 and is thereafter closed to block "pie-pulse" ^' -reflections- from the droplet. After a predetermined period that is related to the pre-pwlse duration and the length of the path from switch 108 to the droplet, switch 108 can be opened to allow a laser pulse from the main pulse seed laser 308 to pass through and switch 108 is thereafter closed to block "main pulse" reflections from the droplet. The process can then be repeated to irradiate another target material droplet,

[0073] Figure 6 shows anothe embodiment of a laser source 400 having seed lasers and component providing protection for the seed lasers from reflections, in this configu ation, as in device 300 in- Figure 5, the device 400 includes a pre- pnise seed laser 302 which produces- a light output on. beam path 304 thai is reflected by beam combiner 306 -onto common beam path 106, and a main pulse seed laser 308 which produces a light output which passes through beam, combiner 306 onto common beam path 106, Similarly, once on common beam path 106, the mai pulse seed output passes through beam dela unit 110, optical isolator 202, amplifier 112, beam conditioning unit 114 and subsequently interacts with a target material at an irradiation site 116.

[0074] In Figure 6, however, switch 108 is located between main pulse seed laser 308 and beam combiner 306, rather than between beam combiner 308 and. beam delay unit 110 as in device 300 in Figure 5, Thus, the operation of device 400 in Figure 6 is slightl different than that of device 300 in Figure 5. In one method of operation of device 400, a laser pulse from the pre~pulse seed laser is generated. and directed to the droplet Switch 108 is initially closed to protect the main- pulse seed laser from "pre-pulse" reflections, i.e., reflections of the pre-puise from the droplet.

[0075] After a predetermined period that is related to the pre-pulse duration and the length of the path from the switch 108 to the droplet as described above, the switch 108 can be opened to allow a laser pulse from the main pulse seed laser to pass through the switch. Switch 108 is thereafte again closed to block "main pulse" reflections from the droplet The process can then be repeated to irradiate another target material droplet. By coordin ing the effects of beam combiner 306 and optical isol or 202. reflections from the droplets are prevented from bein further reflected onto beam path 304 and back to pre-pulse seed laser 302.

[0076] Figure 7 shows another embodiment of a laser source 500 having a pre- pulse seed laser, main pulse seed laser and a seed protection unit. In this embodiment,- the device 500 again includes a pre-pulse seed laser 302 which produces a light output on beam path 304 that is reflected by beam combiner 306 onto common beam path 106., and a. main pulse seed laser 308 which produces a light output which passes through beam combiner 306 onto common beam path 106. Similarly, once on ^' common beam, path 106, the mai pulse seed outpu passes through beam delay unit 110 _. , optical isolator 202, amplifier 132, beam conditioning unit 114 and subsequently interacts with a target material at an irradiation site 116.

[0077] in this embodiment, there are two switches 108a and 108b located between, the respective seed lasers and beam combiner 306. The ou tput of the pre-pulse seed laser 302 passes through switch 108b before being directed onto common beam, path 106 b beam combiner 306, while the output of mai pulse seed laser 308 passes through switch 108a before passing through beam combiner 306 onto common beam path 106.

[0078] In one method. of operation of device 500, switch 108a is initially closed, A laser pulse from pre-pulse seed laser 302 is generated and passes through open switch 108b and is reflected by beam combiner 306 onto common bea path 106 and directed to the droplet. The. switch.108b is then closed to protect the pre- pulse seed, laser from both "pre-pulse" and "mai pulse" reflections from the droplet. After a predetermined period that is related to the pre-pulse duration and the length of the path from, the switch 108a to the droplet the switch 108a can be opened to -allow a laser pulse from the main pulse seed laser to pass through the switch, and is thereaf ter closed to block ^" " ^' main pulse" reflections from the droplet. The process can then be repeated to irradiate another target material, droplet. In some implementations _. .. the switch 108a may be opened to pass a laser pulse from the main pulse, while "pre-pulse" reflections- are still reaching the beam combiner 306, For example, a desired delay between the pre- pulse and main pulse may be such that the switch 108a is open during pre-pulse reflections,

[0079] In some cases, the beam combiner 306 may be a partial reflector that reflects greater than 50 percent and transmits less that 50 percent of incident- light.. For example, if beam combiner 306 is a 90 percent reflector,- then 90 percent of any light that leaks through, the optical isolator 202 would reach closed switch 108b and only about 10 percent would reach the main pulse seed laser, As above, in one embodiment a delay of about 1000 ns between the pre-pulse and main pulse may be suitable., with a pre-pulse duration of about ^' 100 ns and a main pulse duration of about 100 ns. [0080] Figure 8 is a flowchart of one embodiment of a method of protecting a seed laser as described herein. At step 801, a laser pulse is generated, for example, by a seed laser, (As above, the "beam" is a sequence of pulses, each pulse subjected to the same steps,} At step 802, the pulse is then passed through one or more Bragg AOMs operating in "on" or diversio mode, which deflect the pulse, onto the beam path toward the irradiation site. As above, the desired beam path will take the deflection(s) by the Bragg AOMs into account.

[0081] Once the pulse has been deflected onto the beam path by a Bragg AQM, the Bragg ADM is switched from diversion mode to transmit mode at step 803, i.e., power to the Bragg AQM is removed. At step 804, the pulse may then be passed through any other desired optical components, such as the beam delay unit, optical isolator, amplifier(s), beam conditioning units, etc., as described above. Note that steps 803 and 804 will likely occur simultaneously, at least in part, as the pulse will proceed along the beam path while the Bragg AQ switches modes.

[0082] As above, as long as the time it takes for the pulse to reach the irradiation site and return is long enough, the Bragg ADM is able to switch from diversion mode to transmit mode before any light reflected from the irradiation site returns to the Bragg AQM so that at step 805 the reflected light will be transmitted straight through the Bragg AQM and not deflected, Since the light from the seed laser was previously deflected onto the beam path when the Bragg A.OM was in diversion mode, the reflected light is thus not transmitted back along the same path to the seed laser, so that such reflections are prevented from reaching the seed laser and possibly causing damage thereto. Again, the reflected light, may go into a beam dump if desired. [0083] The disclosed method and apparatus has been explained above with reference to several embodiments. Other embodiments will be apparent to those skilled in the art in light of this disclosure. Certain aspects of the described method and apparatus may readily be implemented using configurations other than those described in the embodiments above,, or in conjunction with elements other than those described above. For example _. , different algorithms and/or logic circuits, perhaps more complex than those described herein, may be used, and possibly different types of drive lasers and/or focus lenses.

[0084] Note that as used herein, the term "optical component" and its derivatives includes, but is not necessarily limited to, one or more components which re lect and/or transmit and/or operate on incident light and includes, but is not limited to, one or mor lenses, windows, filters, wedges, prisms, grisms, gradmgs, transmission fibers, etalons, diffusers, homogenizers, detectors and other instrument components, apertures, axicons and mirrors including multi-layer mirrors, near-normal incidence mirrors, grazing incidence mirrors, specular reflectors, diffuse reflectors and combinations thereof. Moreover, unless otherwise specified, neithe the terms '' ^' optic," "optical component" nor their derivatives^ as used herein, are meant to be limited to components which operate solely or to advantage within one or more specific wavelength range(s) such as at the EUV output light wavelength, the irradiation laser wavelength, a wavelength suitable for metrology or some other wavelength. 0085J As noted herein, various variations are possible, A single, seed laser may ^¬ be used in some cases rather than the two seed lasers illustrated in the Figures. A common switch may protect two seed lasers, or either or both, of the seed lasers may have their own switches for protection, A single Bragg AQJVi may be used in some instances, or more than two Bragg AOMs may be used to protect a single seed laser if desired.

[0086] It should also be appreciated that the described method and apparatus can be _. implemented in numerous ways, including as a process, an apparatus, or a system. The methods described herein may be implemented by program instructions for instructing a processor to perform such methods,, and such instructions recorded on a computer readable storage medium- such as a hard disk drive _. , floppy disk, optical disc such as a compact disc (CD) or digital versatile disc (DVD) _. , flash memory, etc, or a computer network wherein the program instructions are sent over optical or electronic communication links. Such program instructions may be executed by means of a processor or controller, or may be incorporated into fixed logic elements. It should be noted that the order of the steps of the methods described herein may be altered and still be within the scope of the disclosure.

[00S7J These and other variations u on the embodiments are intended to be covered by the present disclosure, which is limited only by the appended claims.