Control of conversion efficiency in an EUV source

Abstract

Disclosed is an apparatus for and method of controlling the conversion efficiency of an extreme ultraviolet (EUV) radiation source by timing irradiation laser pulses such that they strike droplets of the target material while the droplet of target material has a specified shape and / or orientation with respect to the pulses.

Description

CONTROL OF CONVERSION EFFICIENCY IN AN EUV SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 739,227, filed December 27, 2024, titled CONTROL OF CONVERSION EFFICIENCY IN AN EUV SOURCE, which is incorporated herein by reference in its entirety.FIELD

[0002] The present disclosure relates to light sources which produce extreme ultraviolet (EUV) radiation by converting the state of a target material to a plasma state, in particular to controlling the amount of energy produced in the conversion process.BACKGROUND

[0003] EUV radiation, for example, 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 nm, is used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers.

[0004] Methods for generating EUV light include, but are not limited to, altering the physical state of the target material into a plasma state. The target material includes an element, for example, xenon, lithium, or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) EUV production, the required plasma is produced by irradiating a target material, for example, in the form of a droplet, stream, or cluster of target material, with one or more pulses of amplified light beams. The plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

[0005] In the LPP EUV light source, EUV light may be produced in a multiple step process in which a droplet of target material en route to an irradiation site is struck by one or more laser pulses that condition and transform the droplet into a target. The target material is suitable for subsequent heating and phase conversion at the irradiation site by a high-energy pulse, called a main pulse, that is suitable for vaporizing and ionizing the target. In this context, the conditioning performed by the initial laser pulses may include altering the shape of the droplet, e.g., flattening the droplet, or altering the distribution of the droplet, e.g., rarefaction by at least partially dispersing some of the droplet with a diminution in density. For example, in a two-step process a conditioning pulse, called a pre-pulse, hits the droplet to modify the distribution of the target material and the main pulse then hits the resulting target at an irradiation site to convert the target to an EUV radiation-emitting plasma. In a three -step process a first conditioning pulse (pre-pulse) hits the droplet to modify the shape of the target material to create a target, a second conditioning pulse, called a rarefaction pulse (or rarefication pulse), thenhits the target to change its density, and the main pulse then hits the rarified target at an irradiation site to convert the rarified target to an EUV radiation-emitting plasma

[0006] These conditioning pulses, including the pre-pulse and the rarefaction pulse, may come from respectively dedicated laser radiation sources or can be produced by a common laser source that also produces the main pulse or some combination of these. Other pulses such as pedestal pulses may be used for further conditioning. This multiple irradiation pulse process is repeated many times over with each iteration producing a pulse of EUV radiation at a rate referred to as the repetition rate. Future lithography needs will drive a demand for EUV sources having an increased EUV pulse-to-pulse brightness energy and / or EUV sources capable of operating at faster repetition rates.

[0007] One factor to be considered in studying the target material conversion process is the effect that the plasma generated at the irradiation site has on droplets as they approach the irradiation site. The exposure of the incoming next droplet to the plasma pressure distribution deforms the droplet and causes the droplet shape to evolve in an oscillatory pattern. For example, in some systems, after a plasma event, the next incoming droplet is first deformed into an oblate spheroid. The droplet then contracts back to a round spheroid before assuming the shape of a prolate spheroid. It then reverts back to a round spheroid again, restarting the cycle. This effect is an example of a next-droplet interaction (“NDI”). These descriptions of the droplet shape are approximations because the generated targets no longer have a truly round shape due to the loss of radial -axial symmetry in the target expansion.

[0008] When the pre-pulse hits the droplet when the droplet is in a deformed state, the target material distribution is perturbed in a manner different from the unperturbed droplet case. This difference in the way the pre-pulse interacts with the droplet impacts EUV power output. The result is a change in capacity for EUV power generation in the form of a reduced output power and / or an increased variability of output power and / or a reduced controllability of output power.

[0009] It is in this context that the need for the presently disclosed subject matter arises.SUMMARY

[0010] The following presents a summary of one or more embodiments in order to facilitate a basic understanding of the disclosed subject matter. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify any elements as being key or critical nor set limits on the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments as a prelude to the more detailed description that is presented later.

[0011] According to an aspect of an embodiment there is disclosed an apparatus for generating extreme ultraviolet radiation by converting a target material to a plasma state, the apparatus comprising a droplet generator for generating a droplet of target material, a shape of the droplet undergoing a cyclic evolution in a repeatable manner as the droplet travels a path to an irradiation site and a source of laser radiation for generating a beam of laser radiation, the source being configured such that the beam irradiates the droplet when the droplet reaches a position on the path where a target material distributionof the droplet corresponds to a predetermined target material distribution.

[0012] The beam may be a pulsed beam and the source of laser radiation may generate a pulse of laser radiation so that the pulse reaches the position when the droplet reaches the position. The source of laser radiation may be a pre-pulse laser configured to generate the pulse as a pre-pulse which conditions the droplet.

[0013] The cyclic evolution may be caused by a next droplet interaction.

[0014] The apparatus may further comprise a deformation laser arranged to generate a deformation beam that strikes the droplet before the droplet reaches the position and wherein the cyclic evolution is caused by the deformation beam.

[0015] The predetermined target material distribution may be a prolate spheroid. The predetermined target material distribution may be an oblate spheroid.

[0016] According to another aspect of an embodiment there is disclosed an apparatus for generating extreme ultraviolet radiation by converting a target material from a liquid state to a plasma state, the apparatus comprising a droplet generator for generating a plurality of droplets of target material and a source of laser radiation for generating a plurality of pulses of laser radiation, each droplet of the plurality of droplets being illuminated by a respective one of the pulses of laser radiation, wherein for each droplet the source of laser radiation times generation of the respective pulse so that the pulse strikes the target at a time when the droplet when a target material distribution of the droplet corresponds to a predetermined target material distribution.

[0017] The predetermined target material distribution may be an oblate spheroid. The predetermined target material distribution may be a prolate spheroid. The target material may comprise tin.

[0018] The source of laser radiation may be a pre-pulse laser configured to generate the pulse as a pre-pulse which conditions the droplet.

[0019] According to another aspect of an embodiment there is disclosed a method of generating extreme ultraviolet radiation by converting a target material to a plasma state, the apparatus comprising generating a droplet of target material, a shape of the droplet undergoing a cyclic evolution in a repeatable manner as the droplet travels a path to an irradiation site and generating a beam of laser radiation such that the beam irradiates the droplet when the droplet reaches a position on the path where a target material distribution of the droplet corresponds to a predetermined target material distribution.

[0020] The beam may be a pulsed beam and generating a beam of laser radiation may comprise generating a pulse of laser radiation so that the pulse reaches the position when the droplet reaches the position.

[0021] The pulse may be a pre-pulse which conditions the droplet.

[0022] The target material distribution of the droplet may evolve to the predetermined target material distribution due to a next droplet interaction.

[0023] The may further comprise generating a beam of deformation laser radiation and thetarget material distribution of the droplet evolves to the predetermined target material distribution due to interaction with the beam of deformation radiation.

[0024] The target material may comprise tin.

[0025] Further embodiments, features, and advantages of the subject matter of the present disclosure, as well as the structure and operation of the various embodiments are described in detail below with reference to accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the presently disclosed subject matter and, together with the description, further serve to explain the principles of the presently disclosed subject matter and to enable a person skilled in the relevant art(s) to make and use the presently disclosed subject matter. The drawings are not to scale unless otherwise indicated.

[0027] FIG. 1 is a schematic diagram of an overall broad conception for a laser-produced plasma EUV radiation source system.

[0028] FIG. 2 is a schematic diagram of a target material being delivered to an irradiation site.

[0029] FIG. 3 is a graphical diagram illustrating certain principles of target shape evolution.

[0030] FIG. 4 is a graphical diagram illustrating certain principles of target shape oscillation.

[0031] FIG. 5 is a flow chart of a method of controlling conversion efficiency according to an aspect of an embodiment.

[0032] FIG. 6 is a schematic diagram of a target material being delivered to an irradiation site according to an aspect of an embodiment.

[0033] Further features and advantages of various embodiments, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It is noted that teachings contained herein are not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art provided with the teachings provided herein.DETAILED DESCRIPTION

[0034] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details of nonlimiting examples are set forth in order to promote a thorough understanding of one or more embodiments.

[0035] FIG. 1 is a schematic diagram depicting an example of an EUV radiation source, e.g., an LPP EUV radiation source 10 according to one aspect of an embodiment. As shown, the LPP EUV radiation source 10 may include a pulsed or continuous laser source 22, which may for example be apulsed gas discharge CO2 laser source producing a beam 12 of main pulses of radiation at a wavelength generally below 20 pm, for example, in the range of about 10.6 pm to about 0.5 pm or less. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and at a high pulse repetition rate. The LPP EUV radiation source 10 may also include one or more modules such as a pre-pulse laser 23 emitting a beam 25 of conditioning radiation as explained above.

[0036] The LPP EUV radiation source 10 also includes a target delivery system 24 for delivering target material in the form of liquid droplets or a continuous liquid stream. In this example, the target material is a liquid. The target material may be made up of tin or a tin compound, although other materials could be used. In the system depicted the target material delivery system 24 introduces droplets 14 of the target material into the interior of a vacuum chamber 26 to an irradiation site 28 where the target material may be irradiated to produce plasma. As explained in more detail below, the irradiation site 28 in general coincides with the primary focus of a collector mirror 30. In some cases, an electrical charge is placed on the target material to permit the target material to be steered toward or away from the irradiation site 28. It should be noted that as used herein an irradiation site is a site where target material irradiation is to occur and is an irradiation region even at times when no irradiation is actually occurring. The LPP EUV light source 10 may also include a laser beam steering system 32.

[0037] The LPP EUV radiation source 10 may also include an EUV light source controller system 60, which may also incorporate a laser firing control system 65. The EUV LPP radiation source 10 may also include a detector such as a target position detection system which may include one or more droplet imagers 70 that generate an output indicative of the absolute or relative position of a target droplet, e.g., relative to the irradiation region 28, and provide this output to a target position detection feedback system 62.

[0038] The target position detection feedback system 62 may use the output of the droplet imager 70 to compute a target position and trajectory, from which a target error can be computed. The target error can be computed on a droplet-by-droplet basis, or on average, or on some other basis. The target error may then be provided as an input to the EUV light source controller 60. In response, the EUV light source controller 60 can generate a control signal such as a laser position, direction, or timing correction signal and provide this control signal to the laser beam steering system 32. The laser beam steering system 32 can use the control signal to change the location and / or focal power of the laser beam focal spot within the chamber 26. The laser beam steering system 32 can also use the control signal to change the geometry of the interaction of the beam 12 and the droplet 14. For example, the beam 12 can be made to strike the droplet 14 off-center or at an angle of incidence other than directly head-on.

[0039] As shown in FIG. 1, the target material delivery system 24 may incorporate a target delivery control system 90. The target delivery control system 90 is operable in response to a signal, for example, the target error described above, or some quantity derived from the target error provided by the EUV light source controller 60, to adjust the paths of the target droplets 14. This may be accomplished, for example, by repositioning the point at which a target delivery mechanism 92 releasesthe droplets 14. The droplet release point may be repositioned, for example, by tilting the target delivery mechanism 92 or by shifting the target delivery mechanism 92. The target delivery mechanism 92 extends into the chamber 26 and is preferably externally supplied with target material and with gas from a gas source to place the target material in the target delivery mechanism 92 under pressure.

[0040] Continuing with FIG. 1, the radiation source 10 may also include one or more optical elements. In the following discussion, a collector 30 is used as an example of such an optical element, but the discussion applies to systems incorporating other optical elements as well. The collector 30 may be a normal incidence reflector, for example, implemented as a multilayer mirror (MLM) fabricated by depositing many pairs of Mo and Si layers on a substrate with additional thin barrier layers, for example B4C, ZrC, SisN4 or C, deposited at each interface between layer pairs to effectively block thermally induced interlayer diffusion, but the collector 30 may be formed of other layers of material in other embodiments. The collector 30 may be in the form of a prolate ellipsoid, with a central aperture to allow the laser beam 12 to pass through and reach the irradiation region 28. The collector 30 may be, e.g., in the shape of an ellipsoid that has a first focus at the irradiation region 28 and a second focus at a so-called intermediate point 40 (also called the intermediate focus) where the EUV radiation may be output from the LPP EUV radiation source 10 and input to, e.g., an integrated circuit lithography scanner or stepper 50. The scanner or stepper 50 uses the radiation, for example, to process a silicon wafer workpiece 52 in a known manner using a reticle or mask 54. The silicon wafer workpiece 52 is then additionally processed in a known manner to obtain integrated circuit devices.

[0041] For a right-handed reference coordinate system, Z is the direction along which the beam 12 propagates and is also the direction from the collector 30 to the irradiation region 28 and the intermediate point 40. X is along the droplet propagation direction. Y is orthogonal to the XZ plane. To make this a right-handed coordinate system, the trajectory of the droplets 14 is taken to be in the -X direction. The view of FIG. 1 is thus normal to the XZ plane. It will be understood by one having ordinary skill in the art that the droplets 14 may travel at any angle with respect to gravity between and including 90° (horizontal) and 0° (vertical either up or down) and any angle in between.

[0042] In the example shown, the target material is in the form of a stream of droplets released by the target delivery mechanism 92, also referred to as a droplet generator. The droplet 14 can be converted by a main pulse in this form. Alternatively, the droplet 14 can be preconditioned for ionization with a pre-pulse that can, for example, change the geometric distribution of the droplet 14 to produce a target. Note that herein, the quantity of target material as dispensed by the target generator is referred to as a droplet before it is struck by a pre-pulse and referred to as a target after it has been struck by a pre -pulse.

[0043] FIG. 2 is a schematic diagram showing the system for irradiating droplets in more detail. As shown, a target delivery control system 90 causes a target delivery mechanism 92 to release a stream of target material droplets 14. Although the droplets 14 are shown as being of uniform size and spacing one for ordinary skill in the art will appreciate that in fact the target material upon leaving thetarget delivery mechanism 92 is in the form of a stream which breaks up into micro droplets which then ultimately coalesce into the droplets of the size at which they hit by a pre-pulse. FIG. 2 shows a three-pulse system in which the droplets 14 are first hit by a conditioning pulse from a pre-pulse laser 23 which conditions the droplet, for example, by altering the shape of the droplet to create a target. The target is then struck by a rarefaction pulse from a rarefaction laser 21 to alter the distribution of the target material in the target, which is then hit by a main pulse from a main pulse laser 22 at an irradiation site 28.

[0044] As mentioned, the creation of the plasma generated by conversion of a target at the radiation site 29 generates a pressure which affects at least the immediately trailing droplet 14, this phenomenon being referred to as next droplet interaction or NDI. The plasma pressure causes the shape of the close-by incoming droplet 14 to evolve in an oscillatory motion. Specifically, as shown in FIG.3, after a next droplet interaction from a previous target at time To, the droplet 14 starts to evolve. This is depicted in FIG. 3 as first into an oblate spheroid, that is, a shape resembling an ellipse that is rotated about its minor axis, at time Ti. The droplet 14 then rebounds back to a round spheroid at time T2 continuing to a prolate spheroid, that is, a shape resembling an ellipse that is rotated about its major axis, at time T3. The shape of the droplet then reverts back to a round spheroid again at time T4. This cycle repeats approximately sinusoidally in time as the droplet 14 travels to the position on the droplet path at which it will be struck by a pre-pulse at time T5. The shape of the droplet evolves cyclically.

[0045] This process is also shown in FIG. 4. In FIG. 4, the horizontal axis is time in arbitrary units and the vertical axis is position in arbitrary units. A main pulse strikes a leading target 50 at a time TMP. This preceding target 50 will then be converted into a plasma 60. A trailing droplet / target 70 is exposed to a next droplet interaction in the form of plasma pressure from the plasma event caused by the target conversion. As above, next droplet interaction perturbs the droplet 70 which perturbs the shape of the droplet 70 so that the shape of the droplet evolves cyclically as depicted in FIG. 4 first into an oblate spheroid, then a round spheroid, then a prolate spheroid, then a round spheroid, with this cycle repeating until the droplet is struck by a pre-pulse at time TPP. The target 70 is then struck by a prepulse at time TPP and then by a rarefaction pulse as described above at time TRP and then by a main pulse at time TMP2. Again, the terms oblate spheroid, round spheroid, and prolate spheroid are used as approximations to convey the evolution of the shape of the droplet 70.

[0046] Thus, in this circumstance, the pre-pulse hits the droplet 70 while the droplet 70 is in a perturbed state. As a result, the generated target shape is also perturbed in the sense that the target shape deviates from a more symmetric shape that would be obtained by striking an unperturbed droplet. This affects the distribution of target material in the target that is presented to the main pulse for conversion.

[0047] One parameter of interest in the design and implementation of LPP EUV sources is the conversion efficiency (“CE”). Conversion efficiency may be defined as the ratio of EUV power output per main pulse power input. It has been determined that CE exhibits a dependence on the distribution of target material in the target that is presented to the main pulse for conversion. Thus, CE depends inpart on the shape of the target that is presented to the main pulse, that is, the shape the main pulse “sees.” In particular, for a droplet whose shape is evolving as it travels, it has been determined that the CE for that droplet that is in one particular stage or phase of its evolution when it was struck by a pre-pulse can be higher than for a droplet that was in a different phase of its evolution when struck by the pre-pulse. The CE for a droplet that was in a particular stage of its evolution when it was struck by a pre-pulse is also observed to be higher than for a droplet not subject to an immediately preceding plasma event, i.e., an unperturbed “NDI-free” droplet having a stable shape.

[0048] Thus, according to an aspect of an embodiment a system for irradiating droplets having an evolving perturbation with a pre-pulse and a main pulse is configured so that the pre-pulse is positioned and / or timed so that it strikes a droplet while the droplet is in a particular phase of its evolution to yield a higher conversion efficiency when that droplet is then later subjected to a main pulse.

[0049] For a particular implementation, the desired stage in the evolution of the droplet shape as presented to the pre-pulse may be an oblate shape as described above. In other implementations the desired stage might be a different shape, such as a prolate shape or a shape having a particular degree of oblateness or prolateness. The underlying principle is to leverage droplet distortion to obtain an advantage. In other words, the optimal shape may be determined empirically for a given set of conditions and the position when that optimal shape emerges along its path the irradiation site can also be determined, Then the pre-pulse laser is arranged so it delivers the pre-pulse to the droplet when the droplet is at that position.

[0050] In general, CE depends on several factors including: (1) the deformation of the droplet (NDI) (2) target expansion recipe, including factors such as target expansion time and target expansion speed and (3) target rarefaction recipe. All of the three will impact the final target material density distribution prior to the main pulse. More specifically, as an example, an oblate target can yield a higher CE given a fixed target expansion recipe and target rarefaction recipe. A different target shape may yield a higher CE given a different fixed target expansion recipe and different target rarefaction recipe. Other factors may also affect the identification and selection of the desired shape such as main pulse laser wavelength or other laser-related parameters. The pre-pulse is thus positioned and timed so that it strikes a droplet when the droplet is in its desired deformation phase.

[0051] FIG. 5 is a flow chart describing a method of controlling conversion efficiency according to an aspect of an embodiment. A droplet is created in a step S10. This step can be implemented, for example, with a droplet generator as described above. Then, as the droplet is traversing its trajectory toward the irradiation site, the droplet will be exposed to plasma pressure in a step S20. As noted above, this will cause the shape of the droplet to oscillate. Then, in a step S30, the droplet is struck with a pre-pulse when the target is most likely to be in its optimal shape. As noted above one shape which may be an optimal shape is an oblate spheroid. It can be determined when thedroplet is in an optimal shape, for example, by determining empirically for a given set of conditions the position in its trajectory when the droplet is likely to be in this shape.

[0052] In the example above the droplet shape distortion is caused intrinsically by NDI. Droplet shape distortion can also be induced extrinsically, for example, by using an additional laser pulse that initiates the shape evolution of the droplets prior to when they are struck by a pre-pulse. This is shown in FIG. 6, which depicts an arrangement similar to that depicted in FIG. 2 except the arrangement depicted in FIG. 6 additionally includes a deformation laser 80 having a power output calibrated to cause the shape of the droplet 14 to start evolve before being struck by the pre-pulse laser.

[0053] One way in which a system can be configured to ensure that a droplet has a desired shaped when it is subjected to the pre-pulse is by determining experimentally when in its trajectory the droplet is likely to be in that shape. Then, striking the droplet while it is in this shape is accomplished by supplying the pre-pulse to that position in the trajectory. Another way in which the system can be configured to ensure that the droplet is in its desired shape when subjected to the pre-pulse is by making provision for capturing an image of the droplet and striking the droplet when it is in the desired shape.

[0054] Some of the above description is in terms of functional block diagrams with some functions allocated to some blocks and other functions allocated to other blocks. It will be understood that the allocations are arbitrary and that different divisions and allocations of functionality are possible so long as the overall functions are carried out as described above.

[0055] The above description includes examples of multiple embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for each of these embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of elements of the various embodiments are possible based on the disclosure. Accordingly, the described embodiments are intended to be representative of and encompass all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

[0056] Furthermore, to the extent that the terms “includes” or “incorporates” are used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. Also, although elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated.

[0057] Additionally, all or a portion of any aspect and / or embodiment may be utilized with all or a portion of any other aspect and / or embodiment, unless stated otherwise. Features, materials, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example are to be understood to be applicable to any other aspect, embodiment or example described in this section or elsewhere in this specification unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[0058] Furthermore, certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations, one or more features from a claimed combination can, in some cases, be excised from the combination, and the combination may be claimed as a subcombination or variation of a subcombination.

[0059] Moreover, while operations may be depicted in the drawings or described in the specification in a particular order, such operations need not be performed in the particular order shown or in sequential order, or that all operations be performed, to achieve desirable results. Other operations that are not depicted or described can be incorporated in the example methods and processes. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the described operations. Further, the operations may be rearranged or reordered in other implementations. Those skilled in the art will appreciate that in some embodiments, the actual steps taken in the processes illustrated and / or disclosed may differ from those shown in the figures. Depending on the embodiment, certain of the steps described above may be removed, others may be added.

[0060] Furthermore, the features and attributes of the specific embodiments disclosed above may be combined in different ways to form additional embodiments, all of which fall within the scope of the present disclosure. Also, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products.

[0061] For purposes of this disclosure, certain aspects, advantages, and novel features are described herein. Not necessarily all such advantages may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the disclosure may be embodied or carried out in a manner that achieves one advantage or a group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.

[0062] Conditional language, such as “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and / or steps. Thus, such conditional language is not generally intended to imply that features, elements, and / or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, and / or steps are included or are to be performed in any particular embodiment.

[0063] Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey thatan item, term, etc. may be either X, Y, or Z. Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z.

[0064] Language of degree used herein, such as the terms “approximately,” “about,” “generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately”, “about”, “generally,” and “substantially” may refer to an amount that is within less than 10% of, within less than 5 % of, within less than 1% of, within less than 0.1 % of, and within less than 0.01 % of the stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by less than or equal to 15 degrees, 10 degrees, 5 degrees, 3 degrees, 1 degree, or 0.1 degree.

[0065] The scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments in this section or elsewhere in this specification, and may be defined by claims as presented in this section or elsewhere in this specification or as presented in the future. The language of the claims is to be interpreted broadly based on the language employed in the claims and not limited to the examples described in the present specification or during the prosecution of the application, which examples are to be construed as nonexclusive.

[0066] Aspects and implementations of the present disclosure can be further described using the following numbered clauses:1. An apparatus for generating extreme ultraviolet radiation by converting a target material to a plasma state, the apparatus comprising:a droplet generator for generating a droplet of target material, a shape of the droplet undergoing a cyclic evolution in a repeatable manner as the droplet travels a path to an irradiation site; anda source of laser radiation for generating a beam of laser radiation, the source being configured such that the beam irradiates the droplet when the droplet reaches a position on the path where a target material distribution of the droplet corresponds to a predetermined target material distribution.2. The apparatus of clause 1 wherein the beam is a pulsed beam and wherein the source of laser radiation generates a pulse of laser radiation so that the pulse reaches the position when the droplet reaches the position.3. The apparatus of clause 2 wherein the source of laser radiation is a pre -pulse laser configured to generate the pulse as a pre-pulse which conditions the droplet.4. The apparatus of clause 1 wherein the cyclic evolution is caused by a next droplet interaction. 5. The apparatus of clause 1 further comprising a deformation laser arranged to generate a deformation beam that strikes the droplet before the droplet reaches the position and wherein the cyclic evolution is caused by the deformation beam.6. The apparatus of clause 1 wherein the predetermined target material distribution is a prolate spheroid.7. The apparatus of clause 1 wherein the predetermined target material distribution is an oblate spheroid.8. Apparatus for generating extreme ultraviolet radiation by converting a target material from a liquid state to a plasma state, the apparatus comprising:a droplet generator for generating a plurality of droplets of target material; anda source of laser radiation for generating a plurality of pulses of laser radiation, each droplet of the plurality of droplets being illuminated by a respective one of the pulses of laser radiation, wherein for each droplet the source of laser radiation times generation of the respective pulse so that the pulse strikes the target at a time when the droplet when a target material distribution of the droplet corresponds to a predetermined target material distribution.9. The apparatus of clause 8 wherein the predetermined target material distribution is an oblate spheroid.10. The apparatus of clause 8 wherein the target material comprises tin.11. The apparatus of clause 8 wherein the source of laser radiation is a pre -pulse laser configured to generate the pulse as a pre-pulse which conditions the droplet.12. A method of generating extreme ultraviolet radiation by converting a target material to a plasma state, the apparatus comprising:generating a droplet of target material, a shape of the droplet undergoing a cyclic evolution in a repeatable manner as the droplet travels a path to an irradiation site; andgenerating a beam of laser radiation such that the beam irradiates the droplet when the droplet reaches a position on the path where a target material distribution of the droplet corresponds to a predetermined target material distribution.13. The method of clause 12 wherein the beam is a pulsed beam and generating a beam of laser radiation comprises generating a pulse of laser radiation so that the pulse reaches the position when the droplet reaches the position.14. The method of clause 13 wherein the pulse is a pre-pulse which conditions the droplet.15. The method of clause 12 wherein the target material distribution of the droplet evolves to the predetermined target material distribution due to a next droplet interaction.16. The method of clause 12 further comprising generating a beam of deformation laser radiation and the target material distribution of the droplet evolves to the predetermined target material distribution due to interaction with the beam of deformation radiation.17. The method of clause 12, wherein the target material comprises tin.

[0067] The above-described implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. An apparatus for generating extreme ultraviolet radiation by converting a target material to a plasma state, the apparatus comprising:a droplet generator for generating a droplet of target material, a shape of the droplet undergoing a cyclic evolution in a repeatable manner as the droplet travels a path to an irradiation site; anda source of laser radiation for generating a beam of laser radiation, the source being configured such that the beam irradiates the droplet when the droplet reaches a position on the path where a target material distribution of the droplet corresponds to a predetermined target material distribution.

2. The apparatus of claim 1 wherein the beam is a pulsed beam and wherein the source of laser radiation generates a pulse of laser radiation so that the pulse reaches the position when the droplet reaches the position.

3. The apparatus of claim 2 wherein the source of laser radiation is a pre -pulse laser configured to generate the pulse as a pre-pulse which conditions the droplet.

4. The apparatus of claim 1 wherein the cyclic evolution is caused by a next droplet interaction.

5. The apparatus of claim 1 further comprising a deformation laser arranged to generate a deformation beam that strikes the droplet before the droplet reaches the position and wherein the cyclic evolution is caused by the deformation beam.

6. The apparatus of claim 1 wherein the predetermined target material distribution is a prolate spheroid.

7. The apparatus of claim 1 wherein the predetermined target material distribution is an oblate spheroid.

8. Apparatus for generating extreme ultraviolet radiation by converting a target material from a liquid state to a plasma state, the apparatus comprising:a droplet generator for generating a plurality of droplets of target material; anda source of laser radiation for generating a plurality of pulses of laser radiation, each droplet of the plurality of droplets being illuminated by a respective one of the pulses of laser radiation, wherein for each droplet the source of laser radiation times generation of the respective pulse so that the pulsestrikes the target at a time when the droplet when a target material distribution of the droplet corresponds to a predetermined target material distribution.

9. The apparatus of claim 8 wherein the predetermined target material distribution is an oblate spheroid.

10. The apparatus of claim 8 wherein the target material comprises tin.

11. The apparatus of claim 8 wherein the source of laser radiation is a pre -pulse laser configured to generate the pulse as a pre-pulse which conditions the droplet.

12. A method of generating extreme ultraviolet radiation by converting a target material to a plasma state, the apparatus comprising:generating a droplet of target material, a shape of the droplet undergoing a cyclic evolution in a repeatable manner as the droplet travels a path to an irradiation site; andgenerating a beam of laser radiation such that the beam irradiates the droplet when the droplet reaches a position on the path where a target material distribution of the droplet corresponds to a predetermined target material distribution.

13. The method of claim 12 wherein the beam is a pulsed beam and generating a beam of laser radiation comprises generating a pulse of laser radiation so that the pulse reaches the position when the droplet reaches the position.

14. The method of claim 13 wherein the pulse is a pre-pulse which conditions the droplet.

15. The method of claim 12 wherein the target material distribution of the droplet evolves to the predetermined target material distribution due to a next droplet interaction.

16. The method of claim 12 further comprising generating a beam of deformation laser radiation and the target material distribution of the droplet evolves to the predetermined target material distribution due to interaction with the beam of deformation radiation.

17. The method of claim 12, wherein the target material comprises tin.

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