Droplet metrology using tunable-wavelength laser

Abstract

Disclosed is an apparatus for and method of adjusting spectral properties of a laser curtain including a focal position of a laser curtain created by a tunable-wavelength laser with respect to a position of a mass of target material in which the wavelength of the laser radiation output by the tunable-wavelength laser is changed to cause a chromatic shift in the spectral property.

Description

DROPLET METROLOGY USING TUNABLE- WAVELENGTH LASERCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 608,380 which was filed on 11 December 2023 and US application 63 / 684,612 which was filed on 19 August 2024 which are incorporated herein in their entirety by reference.FIELD

[0002] The present disclosure relates to light sources which produce extreme ultraviolet light by excitation of a target material, in particular to measuring characteristics of droplets of target material in such sources.BACKGROUND

[0003] Extreme ultraviolet (“EUV”) light, 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 an amplified light beam that can be referred to as a drive laser. 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 EUV light source, EUV light may be produced in a two-step process in which a droplet of target material travelling to an irradiation site is first struck by a pre-pulse that conditions the droplet for subsequent phase conversion at the irradiation site. Conditioning in this context may include altering the shape of the droplet, e.g., flattening the droplet, or the distribution of the droplet, e.g., at least partially dispersing some of the droplet as a mist. For example, a conditioning pulse hits the droplet to modify the distribution of the target material and a main pulse hits the target to transform it to an EUV light-emitting plasma.

[0006] When the target material is in the form of a droplet in a stream of droplets it is important to detect the position of the droplets very precisely for efficient and debris-minimized operation of the light source. In some systems the droplet scatters the light from the conditioning pulse or the main pulse and the scattered light is used to detect the droplet. For example, U.S. Patent No. 7,372,056, issued May 13, 2008, and titled “LPP EUV Plasma Source Material Target Delivery System,” discloses the use ofa droplet detection radiation source and a droplet radiation detector that detects radiation scattered by a droplet of target material.

[0007] All patent applications, patents, and printed publications cited herein are incorporated herein by reference in their entireties, except for any definitions, subject matter disclaimers or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.

[0008] U.S. Patent No. 8,158,960, issued April 17, 2012, and titled “Laser Produced Plasma EUV Light Source,” discloses the use of a droplet position detection system which may include one or more droplet imagers that provide an output indicative of the position of one or more droplets, e.g., relative to the irradiation region. The imager(s) may provide this output to a droplet position detection feedback system, which can compute a droplet position and trajectory, from which a droplet position error can be computed. The droplet position error may then be provided as an input to a controller, which can, for example, provide a position, direction and / or timing correction signal to the system to control a source timing circuit and / or to control a beam position and shaping system, e.g., to change the location and / or focal power of the laser pulses being delivered to the irradiation region.

[0009] U.S. Patent No. 9,241,395, issued January 19, 2016, and titled “System and Method for Controlling Droplet Timing in an LPP EUV Light Source,” discloses a droplet illumination module that generates one or two laser curtains for detecting the droplets. A droplet detection module detects each droplet as it passes through either a first curtain or a second curtain, determines when the source laser should generate a pulse so that the pulse arrives at the irradiation site at the same time as the droplet, and sends a signal to the source laser to fire at the correct time.

[0010] U.S. Patent No. 9,497,840, issued November 15, 2016, and titled “System and Method for Creating and Utilizing Dual Laser Curtains from a Single Laser in an LPP EUV Light Source,” discloses the use of two laser curtains and sensors that detect the position of the droplets of target material as they pass through the curtains.

[0011] International Publication No. W02022243006A1, published November 24, 2022, and titled “Metrology System for Extreme Ultraviolet Light Source,” discloses a metrology system that includes a light apparatus configured to generate an optical probe propagating along a probe optical axis that intersects a target axial path at a probe region, the target axial path extending primarily along an X axis of an X, Y, Z coordinate system. The detection apparatus is configured to detect produced light at a plurality of distinct wavelengths, each wavelength associated with a distinct location along an X- transverse axis of the X, Y, Z coordinate system,

[0012] In general in such systems the position of the focus of the laser beam that generates the light curtain is determined during an initial set up of the system to be coincident with the point that droplets traverse the curtain. There is typically no provision of any mechanism or means for adjusting the position of the focus after the system has been deployed in the field. This opens up the possibility that the position of the droplets the system is supposed to detect will shift with respect to the position of thefocus. This means that the droplets will not pass through the portion of the laser curtain having the narrowest cross section and the greatest irradiance, thus degrading the overall performance of the system. It is in this context that the need for the presently disclosed subject matter arises.SUMMARY

[0013] 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.

[0014] According to an aspect of an embodiment there may be disclosed an apparatus for detecting a characteristic of at least one droplet in a stream of droplets of target material, the apparatus comprising an illumination system arranged to illuminate with a beam of radiation at a position in a trajectory of the at least one droplet after exiting a droplet generator nozzle, the illumination system including a tunable-wavelength laser configured to generate the beam of radiation, a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection, and a controller arranged to receive the signal and adapted to selectably cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation.

[0015] The characteristic of the at least one droplet may be a presence of the droplet. The detection system may be arranged and adapted to perform a detection of radiation that has been scattered by the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection. The detection system may be arranged and adapted to perform a detection of radiation that has been reflected by the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection.

[0016] The stream may be one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and the position may be at a distance along the stream less than the coalescence length away from the droplet generator nozzle. The stream may be one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and the position may be at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.

[0017] The signal may be characterized by a signal width indicative of a resolution of the detection and the controller may be adapted to cause the tunable -wavelength laser to alter its wavelength to move a focal position of the beam of radiation to decrease the signal width. The controller may be adapted to cause the tunable-wavelength laser to alter its wavelength to improve contrast of the detection. The contrast may be given by the relationwherein SI is the peak amplitude of the signal from a smaller of two droplets that are closely spaced in a direction along the trajectory of the at least one droplet and S 12 is an amplitude of a dip between the peak amplitude of the smaller of the two droplets and a the peak amplitude of a the signal from the larger of the two droplets.

[0018] The controller may be adapted to cause the tunable-wavelength laser to alter its wavelength to increase an amplitude of the signal.

[0019] The detection system may generate a signal indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.

[0020] According to another aspect of an embodiment there is disclosed an apparatus for detecting multiple characteristics of a target material, the apparatus comprising a first illumination system arranged to illuminate a first position in a trajectory of the target material after exiting a nozzle with a first beam of first radiation, the first illumination system including a tunable-wavelength laser configured to generate the first beam of first radiation, a first detection system arranged to receive the first radiation that has interacted with the target material when the target material traverses the first position and to generate a first signal indicative of a first detection of the target material, a first controller system arranged to receive the first signal and adapted to cause the tunable-wavelength laser to selectably alter its wavelength to move a focal position of the first beam of radiation, a second illumination system arranged to illuminate a second position in a trajectory of the target material after exiting the nozzle with a second beam of second radiation, the second position being further from the nozzle than the first position, and a second detection system arranged to receive the second radiation that has interacted with the target material when the target material traverses the second position and to generate a second signal indicative of a second detection of the target material.

[0021] The first radiation and the second radiation may interact with the target material by being scattered by the target material. The target material may be formed as droplets which undergo coalescence as the droplets travel in a stream away from the nozzle up to a coalescence length from the nozzle and the first position may be at a distance along the stream less than the coalescence length away from the nozzle. The target material may be formed as droplets which undergo coalescence as the droplets travel in a stream away from the nozzle up to a coalescence length from the nozzle and the second position may be at a distance along the stream greater than the coalescence length away from the nozzle.

[0022] The first signal may be characterized by a signal width indicative of a resolution of the first detection and the first controller may be adapted to cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation first to decrease the signal width. The firstcontroller may be adapted to cause the tunable-wavelength laser to alter its wavelength to improve contrast of the detection. The target material may be formed as droplets which travel in a stream away from the nozzle and wherein the contrast may be given by the relationS12C = 1 -SI wherein SI is the peak amplitude of the signal from the smaller of two droplets that are closely spaced in a direction along the trajectory of the droplet and S 12 is an amplitude of a dip between the peak amplitude of the smaller of the two droplets and the peak amplitude of the signal from the larger of the two droplets.

[0023] The controller may be adapted to cause the tunable-wavelength laser to alter its wavelength to increase the amplitude of the signal. The target material may be formed as droplets and wherein the detection system generates a signal indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.

[0024] According to another aspect of an embodiment there is disclosed a method of detecting a characteristic of a droplet of target material in a stream of droplets of target material for generating extreme ultraviolet radiation, the method comprising using a tunable-wavelength laser to generate a beam of metrology radiation used to illuminate a position in the stream between a droplet generator nozzle and an irradiation region, detecting metrology radiation that has interacted with the droplet when the droplet traverses the position, generating a signal indicative of the detection of the droplet, and causing the tunable-wavelength laser to selectably alter its wavelength to move a focal position of the beam of radiation based on the signal.

[0025] The stream may be one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and the position may be at a distance along the stream less than the coalescence length away from the droplet generator nozzle. The stream may be one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and the position may be at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.

[0026] The signal may be characterized by a signal width indicative of a resolution of the detection and causing the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation may be performed to decrease the signal width. The wavelength of the tunable-wavelength laser may be altered to improve contrast of the detection. The contrast may be given by the relationS12C = 1 -SIwherein S 1 may be the peak amplitude of the signal from the smaller of two droplets that are closely spaced and S 12 may be an amplitude of a dip between the peak amplitude of the signal from the smaller of two droplets and the peak amplitude of the signal from the larger of the two droplets.

[0027] The wavelength of the tunable-wavelength laser may be altered to increase an amplitude of the signal. The signal may be indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.

[0028] According to another aspect of an embodiment there is disclosed a method of detecting a characteristic of a droplet of target material in a stream of droplets for generating extreme ultraviolet radiation, the method comprising generating a laser curtain at a position downstream of a droplet generator nozzle producing the stream using a beam of radiation from a tunable-wavelength laser, detecting a presence of a droplet in the laser curtain by detecting radiation from the beam scattered by the droplet when the droplet traverses the laser curtain, generating a signal indicative of the presence of the droplet, and causing the tunable-wavelength laser to change the wavelength of the beam of radiation based on the signal.

[0029] The stream may be one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and the position may be at a distance along the stream less than the coalescence length away from the droplet generator nozzle. The stream may be one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and the position may be at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.

[0030] The signal may be characterized by a signal width indicative of a resolution of the detection and causing the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation may be performed to alter the signal width. The wavelength of the tunable-wavelength laser may be altered to improve contrast of the detection. The contrast may be given by the relationwherein S 1 may be the peak amplitude of the signal from the smaller of two droplets that are closely spaced in a longitudinal direction of the two droplets and S12 may be an amplitude of a dip between the peak amplitude of the signal from the smaller of the wo droplets and the peak amplitude of the signal from the larger of the two droplets.

[0031] The wavelength of the tunable-wavelength laser may be altered to increase an amplitude of the signal. The signal may be indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.

[0032] According to another aspect of an embodiment there is disclosed a system comprising a tunable- wavelength laser that produces laser radiation used to generate a laser curtain, a scattered light detector, a droplet generator having a droplet generator nozzle adapted to direct droplets in a stream through the laser curtain, and one or more processors configured to perform operations of obtaining a signal based on laser radiation that has interacted with a droplet in the laser curtain, the signal having a width representing a measurement resolution, and tuning the tunable-wavelength laser to adjust a wavelength of the laser radiation to move a focal location of a focus of the laser curtain thereof to increase the measurement resolution.

[0033] The operations further may further comprise moving the focal location to an optimal location by adjusting the wavelength to minimize the width. At least some components of an optical system through which laser radiation from the tunable wavelength laser passes to produce the laser may have a larger dispersion than fused silica to increase the movement of the focus for a given wavelength adjustment.

[0034] The droplets in the stream may undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and the position may be at a distance along the stream less than the coalescence length away from the droplet generator nozzle.

[0035] The droplets in the stream may undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and the position may be at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.

[0036] According to another aspect of an embodiment there is disclosed an apparatus for determining a position of a droplet in a stream of droplets of target material, the apparatus comprising an illumination system arranged to illuminate with radiation a position in a trajectory of the droplet after exiting a droplet generator nozzle, the illumination system including a tunable- wavelength laser configured to generate the radiation, a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection, and a controller arranged to receive the signal and adapted to cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation and to determine the position of the droplet based on which wavelength of the tunable- wavelength laser optimizes an amplitude of the signal.

[0037] According to another aspect of an embodiment there is disclosed an apparatus for detecting a characteristic of at least one droplet in a stream of droplets of target material, the apparatus comprising an illumination system arranged to generate a beam of radiation to create a laser curtain at a position in a trajectory of the at least one droplet after exiting a droplet generator nozzle, the illumination system including a tunable-wavelength laser configured to generate the beam of radiation, and a relay optical system in a beam path of the beam having a first portion forming an intermediate image and a second portion adapted to magnify and relay the intermediate image to a position for the laser curtain, adetection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection, and a controller arranged to receive the signal and adapted to selectably cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation.

[0038] The first portion may include a first lens arranged to collimate an incoming beam and a second lens arranged to form an intermediate waist and the second portion may include a third lens arranged to relay the waist to the position for the laser curtain. The first lens, the second lens, and third lens may comprise an optical material having high dispersion. The optical material may be fused silica. The optical material may be Ohara Optical Glass Type S-NPH3.

[0039] According to another aspect of an embodiment there is disclosed an apparatus for detecting a characteristic of at least one droplet in a stream of droplets of target material, the apparatus comprising an illumination system arranged to generate a beam of radiation to create a laser curtain at a position in a trajectory of the at least one droplet after exiting a droplet generator nozzle, the illumination system including a tunable-wavelength laser configured to generate the beam of radiation, and an optical system arranged to divide the beam into a first beam portion having a first wavelength component and a second beam portion having a second wavelength component and to cause the first beam portion to propagate along a first path to a position for the laser curtain and to cause the second beam portion to propagate along a second path to the position for the laser curtain, a portion of the second path being noncoincident with the first path, a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection, and a controller arranged to receive the signal and adapted to selectably cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation.

[0040] The optical system may include a pair of dispersing prisms. The dispersing prisms may comprise fused silica. The dispersing prisms may comprise Ohara Optical Glass Type S-NPH3.

[0041] The first path may be through a first plurality of glass blocks and the second path may be through a second plurality of glass blocks with the second plurality of glass blocks includes glass blocks not included in the first plurality of glass blocks. The first plurality of glass blocks and the second plurality of glass blocks may comprise fused silica. The first plurality of glass blocks and the second plurality of glass blocks may comprise Ohara Optical Glass Type S-NPH3. The first path may be through a first portion of a tilted glass plate and the second path may be through a second portion of the tilted glass plate. The tilted glass plate may comprise fused silica. The tilted glass plate may comprise Ohara Optical Glass Type S-NPH3.

[0042] 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 DRAWING

[0043] FIG. 1 is a schematic, not-to-scale view of an overall broad conception for a laser-produced plasma EUV radiation source system.

[0044] FIG. 2 is a schematic, not-to-scale view of a target material metrology system.

[0045] FIG. 3 is a schematic, not-to-scale view of a target material delivery system.

[0046] FIG. 4 is a diagram illustrating certain principles of target material stream break up and droplet coalescence.

[0047] FIG. 5A is a diagram of an arrangement for detecting droplets in a droplet stream.

[0048] FIG. 5B is a diagram of a laser curtain for detecting droplets in a droplet stream.

[0049] FIG. 6 is a graphical representation of examples of signals from a detector in a target material metrology system.

[0050] FIG. 7 A is a diagram of droplets of target material traversing a laser curtain in a target material metrology system.

[0051] FIG. 7B is a graphical representation of the signal from a detector detecting the droplets of target material traversing a laser curtain as in FIG. 7 A in a target material metrology system.

[0052] FIG. 8 is a partially schematic, not-to-scale block diagram of a system for detecting a target material droplet according to an aspect of an embodiment.

[0053] FIG. 9 is a flow chart of a method of controlling the coincidence of a droplet of target material and a focal position in a laser curtain from a tunable-wavelength laser in a target metrology system according to an aspect of an embodiment.

[0054] FIG. 10A is a diagram of a side view of an optical relay system according to an aspect of an embodiment.

[0055] FIG. 10B is a diagram of a top view of an optical relay system according to an aspect of an embodiment.

[0056] FIG. 11 is a diagram of a side view of an optical relay system according to another aspect of an embodiment.

[0057] FIG. 12 is a perspective view of an optical relay system according to another aspect of an embodiment.

[0058] FIG. 13 is a diagram of a side view of an optical relay system according to another aspect of an embodiment.

[0059] FIG. 14 is a diagram of a side view of an optical relay system according to another aspect of an embodiment.

[0060] FIG. 15 is a diagram of a side view of an optical relay system according to another aspect of an embodiment.

[0061] 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. Itis 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

[0062] 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.

[0063] With initial reference to FIG. 1, there is shown a schematic view of 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 a pulsed gas discharge CO2 laser source producing a beam 12 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 conditioning laser 23 emitting a beam 25 of conditioning radiation as explained above.

[0064] 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 region 28 where the target material may be irradiated to produce plasma. As explained in more detail below, the irradiation region 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 region 28. It should be noted that as used herein an irradiation region is a region 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.

[0065] 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.

[0066] 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 errormay 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.

[0067] 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 releases the 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.

[0068] 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, Si iNr 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.

[0069] 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 andincluding 90° (horizontal) and 0° (vertical either up or down) and any angle in between.

[0070] 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 ionized by a main pulse in this form. Alternatively, the droplet 14 can be preconditioned for ionization with a conditioning pulse that can, for example, change the geometric distribution of the droplet 14. Note that herein, the form of the target material is referred to as a droplet even if one or more conditioning pulses have altered the target material from a true droplet form.

[0071] Thus, it may be necessary both to hit the droplet 14 accurately with the conditioning pulse to ensure the droplet 14 is in the desired form (disk, cloud, etc.), and to hit it again accurately with the main pulse to promote efficient production of EUV radiation. The ionization pulses may also be directed so that they deliberately miss some droplets. These unused droplets travel to a target material catch 80.

[0072] One droplet detection metrology utilizes darkfield illumination, where the scatter from a droplet passing through a laser curtain is collected. The metrology device detects the droplet crossing at a specific location in space to provide a trigger to the system controls to enable efficient production of EUV light. An example of such a system is shown schematically in FIG. 2, in which a droplet detection controller 122 causes a droplet illumination module (DIM) 124 to illuminate a droplet 14. More particularly, the DIM 124 establishes a laser curtain 126 across the trajectory of the droplet 14. As used herein, the term “laser curtain” is used to refer to a volume of space illuminated by laser radiation. A droplet detection module (DDM) 128 detects the radiation backscattered by the droplet 14 to permit the droplet detection controller 122 to derive information such as the position of the droplet 14. A beam dump 130 is arranged to receive radiation that has not been scattered by the droplet 14. As explained more fully below, the detection process described above in connection with FIG. 2 may be used to detect the droplets and tune the operation of the droplet generator.

[0073] In some implementations it is advantageous to provide multiple laser curtains. Such an implementation is shown in FIG. 3. Referring now to FIG. 3, there is shown a capillary 210 terminating in a nozzle 220 and protruding from a nozzle body 270. An electro-actuatable element 200 is positioned around a lengthwise portion of the capillary 210. A shroud 215 is arranged to protect the droplet stream from being exposed to gas flows in the chamber 26. In some embodiments, the shroud 215 may be part of a cold flow ring. In some embodiments, the shroud 215 is attached to the cold flow ring.

[0074] The electro-actuatable element 200 transduces electrical energy from a waveform generator 230 to apply varying pressure to the capillary 210. This introduces a velocity perturbation in the stream 240 of molten target material 240 exiting the nozzle 220. The target material ultimately coalesces into droplets which are illuminated by the DIM 124. The DDM 128 detects radiation that interacts with, i.e., is scattered by the target material. The detection may be used to determine certain characteristics of the droplet stream in a first detection region DR1. A control unit 260 may use the detection data from the DDM 128 to generate a feedback signal to control operation of the wave generator 230 and tune operation of the target delivery mechanism (e.g., droplet generator) 92.

[0075] As used herein, references to the radiation interacting with a droplet encompass circumstances in which the droplet scatters the radiation as well as circumstances in which the droplet at least partially blocks or obstructs the radiation, with reflection being an example of scattering.

[0076] The arrangement of FIG. 3 includes a second laser curtain arrangement made up of a DIM 150 creating a laser curtain 152. The second laser curtain arrangement is positioned to create a second laser curtain to detect droplets in a different portion of the droplet stream. In particular, as explained in more detail below, droplets in the stream merge and coalesce as they travel away from the droplet generator nozzle until, at some distance away from the droplet generator nozzle known as the coalescence length, they are essentially fully coalesced. The second laser curtain is arranged to detect droplets in a portion of the stream that is within the coalescence length of the droplet generator nozzle.

[0077] Thus, in the arrangement shown in FIG. 3, a droplet coalescence metrology (DCM) module 154 detects radiation scattered by target material droplets in a second detection region DR2. A beam dump 156 is arranged to collect radiation that has not been scattered by the droplet. In the arrangement shown, DR1 is beyond the coalescence length L of the droplet stream and so may be expected to detect primarily fully coalesced droplets and satellites as explained below. DR2 is within the coalescence length of the droplet stream and so may be expected to detect primarily droplets that have not yet fully coalesced. In general the shroud 215 is arranged to protect the not yet fully coalesced droplets. In some embodiments, at least one of the DIM 150, the beam dump 156, and the DCM module 154 is mounted on the shroud 215. The DIM 150 and DCM module 154 may be positioned adjacent apertures in the shroud 215 as shown. The control unit 260 may also use the detection data from the DCM module 154 to generate a feedback signal to control operation of the wave generator 230 and tune operation of the target delivery mechanism 92.

[0078] Droplet generator tuning as mentioned above refers to the process of adjusting certain operational parameters of the droplet generator to control its performance. The design of the droplet generator makes available certain “levers” that can be manipulated to control its operation. For example, control of the drive waveform applied to the electro-actuatable element 200 can be used to control aspects of the droplet coalescence process. These aspects can be observed, such as the coalescence length L in FIG. 3, the number of stray droplets (“satellites”), and the velocity profile of the droplet stream to determine whether operation of the droplet generator is satisfactory or whether it needs to be tuned to improve its performance by adjusting these operational parameters. The tuning is in-line when it can be performed without taking the droplet generator offline.

[0079] For example, the control unit 260 may control the relative phase of the components of a hybrid driving signal applied to the electro-actuatable element 200, that is, a driving signal that is a combination of a two or more components having different frequencies, different waveforms, or both. The control unit 260 may also control the respective amplitudes of the lower frequency periodic component and the amplitude of the higher frequency periodic component. This control may be based on a control input 265 (see FIG. 3) which may originate from another controller or be based on a user input. The relativephase of the lower frequency periodic component and the higher frequency periodic component may be adjusted to affect the coalescence length L. The amplitude of the lower frequency periodic component may be adjusted to control droplet coalescence. The amplitude of the higher frequency arbitrary periodic component may be adjusted to control droplet velocity jitter, i.e., stability.

[0080] The imposition of the hybrid waveform described above decomposes the overall droplet coalescence process into a succession of subcoalescence steps or regimes evolving as a function of distance from the droplet generator nozzle 220. FIG. 4 illustrates these principles. It should be noted that the difference in the relative sizes of the capillary 210 and the droplets 14 has been greatly reduced in FIG. 4 for simplification. For example, in a first regime, that is, when the target material first exits the nozzle 220, the target material is in the form of a velocity-perturbed steady stream 14a. In a second regime, the stream breaks up into a series of microdroplets 14b having varying velocities. In the third regime, measured either in time of flight or by distance from the nozzle 220, the microdroplets 14b coalesce into droplets of intermediate sizes, referred to as subcoalesced droplets 14c, having varying velocities with respect to one another. In the fourth regime the subcoalesced droplets 14c coalesce into droplets 14 having the desired final size. The number of subcoalescence steps can vary. The distance from the droplet generator nozzle 220 to the point at which the droplets reach their final coalesced state is the coalescence length L. In the coalescence process, there may be droplets that do not coalesce with others. These are the satellite droplets.

[0081] The droplets in a given regime may not all be the same size. Some of the droplets may be generated primarily in response to the lower frequency periodic component of the drive signal and others of the droplets may be generated primarily in response to the higher order periodic component. Droplets generated primarily by the lower frequency periodic component will have a different size than droplets generated by the higher order periodic component.

[0082] FIG. 5A is a diagram of the droplet metrology system for detecting droplets in the coalescence region, that is, the portion of the droplet stream that is within the coalescence length L of the droplet generator nozzle 220 (FIG. 4). The following description refers to this coalescence droplet metrology system as an example but it will be appreciated that the description may be applied to metrology systems arranged to detect droplets at other positions. As shown in FIGS. 5A and 5B, the DIM 150 illuminates a YZ plane at a predetermined distance from the droplet generator nozzle 220 with a laser curtain 152. The predetermined distance is selected to be between pre-sub-coalescence completion and main coalescence completion. For example, in an implementation main coalescence may complete at about 300 mm. To leave a margin the predetermined distance could be set to be no further than about 250 mm from the droplet generator nozzle. As for a lower end value for the predetermined distance, the subcoalescence length may be on the order of about 30 mm. To leave a margin the predetermined distance could be set to be no closer than about 50 mm. Otherwise, there would be a risk of failing to detect main droplets at the DCM. Detection of droplets at the DCM, however, may be necessary formain coalescence optimization. Thus, the predetermined distance between the droplet generator nozzle 220 and the laser curtain 152 would be from about 50 mm to about 250 mm, for example, 70 mm.

[0083] As shown in FIG. 5 A the planar projection of the laser curtain 152 is narrowest in the X direction near its focus which coincides with the droplet 14 in an ideal case and diverges with distance away from the focus in the Y direction.

[0084] Laser radiation scattered by the droplets 14 is collected by the DCM module 154 which includes a radiation sensing element, e.g., a photodiode. The radiation sensing element converts the optical signal to an electrical signal. When there is only one droplet within the laser curtain 152, then the electrical signal will have a single peak as shown in FIG. 6, which is a graph of metrology signal amplitude versus, time with a superposed representation of the droplet within the laser curtain accountable for generating each signal.

[0085] The signal provided by the DCM module 154 is processed, e.g., digitized and filtered, and then analyzed to perform peak detection and pattern recognition to enable detection and / or measurement of some or all of various parameters such as droplet presence, droplet size, droplet spacing, and droplet stability as shown in FIG. 6. These parameters are then used to tune the droplet generator to obtain stable and optimized coalescence at the target coalescence length / position.

[0086] In general, and as shown in FIG. 6, there will be droplets of various sizes that pass through a laser curtain placed closer to the droplet generator than the coalescence length. For example, there may be subcoalesced droplets such as droplets 250 produced by a higher frequency component of the drive signal and larger diameter droplets 252 produced by a lower frequency component of the drive signal that have not yet merged with other droplets. The droplets may include even larger diameter droplets 254. The droplets may also include smaller diameter droplets 256 that have not merged and are either satellites or in the process of becoming satellites. Each of these droplets results in a corresponding metrology signal shown on the bottom axis of FIG. 6. In general, the amplitude of the metrology signal will be proportional to the size of the droplet crossing the laser curtain with larger droplets producing higher amplitude peaks and smaller droplets producing lower amplitude peaks as shown.

[0087] Another parameter of interest as shown is droplet spacing which is the time between successive peaks. This influences coalescence behavior. Droplet stability, that is, the jitter in the amplitude of the signal peak, can also be measured and used for inline tuning of the droplet generator. Here, “inline” means performed while the droplet generator is operating and is not offline. The signal from the DCM module 154 (FIG. 3) can also be used to detect satellites.

[0088] Design of the DCM module involves meeting both field of view and resolution criteria. As will be explained in more detail below it is desirable to have a larger field of view to avoid missing the detection of droplets. It is also, however, desirable to have a fine resolution so that the presence of two droplets in the field of view at the same time is not misinterpreted as being the presence of only a single droplet.

[0089] Field of view, which encompasses the detection region referred to above, is determined by the waist size of the laser beam used to establish the laser curtain. The beam waist, or beam focus, of a laser beam is the location where the beam radius is at a minimum. The waist radius is the beam radius at that location. The beam waist is where the beam intensity on-axis is the largest. The Rayleigh Length ZR, or Rayleigh range, of the laser beam is a measure of how the beam expands and intensity drops off with displacement from the beam focus. It is the distance from the beam waist where the beam radius is increased by a factor of the square root of 2 and the beam cross-sectional area is doubled. The Rayleigh Length is given by the equationwhere coo is the beam waist size and X is the beam wavelength. The beam waist thus determines the Rayleigh Length for a given wavelength and so the lateral extent of the field of view of the DCM module.

[0090] In determining the size of the field of view to implement it is also advantageous to take into account that accessibility and space constraints may make it difficult or impossible to adjust or align a DCM module that has been deployed to a customer site. It is thus desirable to design the DCM module so that it has a sufficient field of view to enable measurements in view of anticipated variations in system tolerances. As an example, in an implementation a total net tolerance might be expressed as a given value in the Y and Z directions, that is, radially. At a high-level this is based on the sum of the droplet generator nozzle position tolerance and the metrology alignment tolerance. In these circumstances it might be advantageous to design the field of view to be in a range of a scaling factor of 1 to 1.5 times this tolerance, for example 1.3 times, to provide a margin for unknown tolerances that might be encountered, for example, for components which later are determined to be incapable of meeting the desired tolerances, or for outliers. Using a scaling factor of 1.3 the scaled tolerance could be, for example, about + 1.2mm. Thus, while typically droplets will be within a certain tolerance of the center of the field of view, the system will still be able to provide good measurements in extreme cases in which the droplets are closer to the edge. Thus, in some embodiments the DCM module is designed to have a field of view of approximately ± 1.2 mm (2.4 mm diameter) in the YZ plane.

[0091] In other words, in some embodiments the laser curtain waist coo is set such that the Rayleigh Length ZR is equal to half of the field of view, e.g., about 1.2 mm and coo is set to be about 24.3 pm. These conditions result in minimum laser divergence over the field of view and gives the best resolution at the edge of the field of view with a concession of decreased resolution at the center of the field of view. Resolution at the center of the field of view can be improved with a narrower laser curtain, but the increased divergence of the laser curtain results in decreased resolution at the edge of the field of view.

[0092] Further as regards resolution, one design goal is to resolve (separately detect) closely spaced droplets. If two droplets are too close together in the direction of travel then it is possible that they will both be in the laser curtain at the same time. This is shown in FIG. 7A in which droplet 14 and droplet 14’ are separated only by a distance Ax which is less than the waist size of laser beam creating the laser curtain 152. This results in an overlapping signal as shown in FIG. 7B in which the lower peak is from detection of the droplet 14’ and the higher peak is from detection of the droplet 14. The peaks are separated in time by an interval At which is equal to the amount of time it takes the droplet 14 to travel the distance Ax. The “dip” between droplet signals can be quantified as the contrast C which may be expressed in terms of a percentage. In other words, the contrast may be expressed as a percentage of the dip in amplitude of the respective signals from one of the two droplets. Thus if the peak amplitude of the signal from the smaller of two droplets that are closely spaced in the X direction is SI and S12 is the amplitude of the dip between that signal and peak of the signal from the larger of the two droplets then the contrast is given by the equation

[0093] Another factor to be taken into account in considering resolution is the impact on the field of view in the Y direction, where, due to laser curtain divergence, the minimum observable droplet spacing is increased. Thus the resolution is limited by the laser curtain width at the position the droplets pass through it. To account for this the resolution performance is assumed to be the worst-case tolerance, that is, the resolution tolerance at the edge of the field of view in the Y direction (along laser curtain divergence).

[0094] The need to design the droplet metrology system for worst-case tolerances arises in part from a lack of flexibility to adjust the focus of the laser curtain in the field. More design flexibility could be obtained if it were possible to adjust the position of the laser focus in a deployed system despite the limited ability to implement mechanical methods for doing so.

[0095] According to an aspect of an embodiment adjusting the position of the laser curtain focus is enabled in units deployed in the field without the use of any mechanical positional adjustment means by using a tunable-wavelength laser and moving the focus using a chromatic focus shift. It is thus possible to adjust the laser curtain focus position to the actual position of the droplets thereby reducing signal width and improving resolution. In an embodiment the signal width is measured from the droplets and a control loop is established to reduce the signal width by adjusting the wavelength of the radiation emitted by the tunable-wavelength laser to adjust the laser curtain focus position, i.e., the position of the beam waist. This reduces signal width and improves resolution.

[0096] FIG. 8 is a functional block diagram of a DCM system 300 in accordance with an aspect of an embodiment. As in the arrangements described above, the DCM system 300 of FIG. 8 includes a targetmaterial delivery mechanism, e.g. a droplet generator nozzle 220, a beam dump 156, and a DCM module 154. The system shown in FIG. 8 also includes a tunable-wavelength laser radiation source 310 which creates a laser curtain 320. Thus the tunable-wavelength laser radiation source 310 includes both a tunable-wavelength laser and the optics needed to create the laser curtain 320. . During operation, the DCM system 300 operates similarly to the systems described above. However, in the system shown in FIG. 8, a controller 330 is capable of measuring the width of the signal generated by the DCM module 154 in response to detecting one or more droplets in the laser curtain 320 and then sending a signal to the tunable- wavelength laser radiation source 310 to alter the wavelength of the radiation output by the tunable-wavelength laser radiation source 310. This causes the focal point and hence the waist of the laser curtain 320 to shift as indicated by the arrow A. The controller 330 adjusts the wavelength and hence the focus position to minimize the width of the signal from the DCM module 154. Thus, the focal point of the laser radiation output by the tunable- wavelength laser radiation source 310 can be adjusted to coincide better with the actual position of the droplet.

[0097] A wavelength adjustment of about + 40 nm can shift the laser radiation focus by about ± 1.2 mm, in other words, about the same as the range of current tolerances. This is just an example based on an existing design. Using different optics can produce the same or greater focal shift for smaller wavelength adjustments. Also, chromatic focal shifts can be accomplished without significant impact on the waist size of the laser beam. The laser curtain optics design can be modified to produce a larger chromatic focus shift for a given wavelength change, thereby simplifying the wavelength tunable laser design by reducing the required wavelength tuning range to achieve the desired focus adjustment range. Additionally, the laser curtain width can be reduced without having to accept a tradeoff between resolution and size of the field of view because the system can maintain a better focus on the droplets.

[0098] Tunable-wavelength lasers operating in the desired range are available from many sources. Some additional measures may be desirable to ensure an optimal laser power level over the full wavelength adjustment range. For example, the amount of laser power required can be reduced by adjusting the DCM optical design. Also, the wavelength range of interest is within the gain bandwidth of erbium doped fiber amplifiers. It is also expected that by maintaining focus on the droplet, the optical signal level will increase, thereby making it possible to reduce the laser power required by about 10% to about 20%. Given the wavelength range is relatively small, it is expected that there will be at most a minimal impact on the sensor responsivity and Mie scattering efficiency.

[0099] FIG. 9 is a flow chart of a procedure for adjusting the position of a laser beam focus to coincide better, i.e., align, with the position of droplets in a droplets stream in an EUV radiation source. In a step S10 a beam of laser radiation is generated having a wavelength I using a tunable-wavelength laser. In a step S20 the laser beam is directed towards a measurement volume for a droplet stream to create a laser curtain for the droplet stream. In some embodiments, this measurement volume will be within the coalescence length of the droplet generator nozzle. In a step S30 the laser radiation scattered by any droplets that are in the measurement volume generated by the laser beam is detected. In a step S40 thewidth of the signal from the detected scattered laser radiation is determined. In a step S50 a determination is made of whether the signal width is sufficiently narrow, that is, narrow enough to indicate acceptable alignment between the droplets and the laser beam focus. If the signal width is sufficiently narrow, then in a step S60 the output wavelength of the tunable laser is left unchanged. If, however, it is determined in step S50 that the signal width is not narrow enough, then, in a step S70 the wavelength X of the output of the tunable laser is altered. The system continues this process until the signal width is sufficiently narrow indicating, as mentioned, an acceptable degree of alignment of the laser beam focus and the droplets.

[0100] The procedures described above can, as mentioned, be performed during initial setup of the system or in association with a maintenance procedure. In other words, after the laser system is deployed in the field, additional adjustments may be made periodically, in particular at service intervals involving one or more maintenance acts that could potentially have a large effect on droplet position (e.g. a droplet generator swap) or droplet position measurement (e.g., a DCM module swap). It is also possible to use the active control loop during normal operation although the anticipated need for doing so can be expected to be less because droplet position changes can typically be expected to be small with respect to the laser curtain divergence.

[0101] Thus, according to an aspect of an embodiment, use of a tunable-wavelength laser makes it possible to improve resolution by reducing the width of the laser curtain and better maintaining the laser curtain focus on droplets. It also offers the possibility of improved signal budgets by providing a tightened focus on droplets (i.e., heightened irradiance). It offers these benefits without the need for including moving parts or provision for focus adjustment within the DCM module, and so is compatible with the tight volume constraints and high temperatures (about 70 °C) which must be accommodated in some applications. It also avoids implementation of a challenging opto-mechanical design which would otherwise be required to maintain alignment while adjusting focus. Wavelength tuning can be done with high resolution thus providing for innately high focus adjustment resolution. The tunable laser is also compatible with near-infrared radiation sensors such as III-V compound materials or organic photodiodes.

[0102] Thus, the DCM provides a signal that enables detection of the presence, size, spacing, and stability of the droplets. To accomplish this, the DCM illuminates a plane at a given distance from the droplet generator nozzle with a laser curtain, and the scatter from droplets is collected and transmitted to a photodiode. The photodiode converts the optical signal to an electrical signal that is digitized and filtered before further processing carries out peak detection and pattern recognition to determine the metrics of interest. A key detail is the impact of the field of view in Y direction, where, due to laser curtain divergence, the minimum observable droplet spacing is increased.

[0103] The laser curtain optics design can be modified to maximize the chromatic focus sensitivity (CFS; focus shift per unit of wavelength change), thereby reducing the required wavelength tuning range to achieve the desired focus shift and, in doing so, simplifying the tunable laser design. Tomaximize the CFS, the optical system can be designed to (1) maximize the chromatic focal shift of elements that add constructively while minimizing the chromatic focal shift of elements that add destructively (e.g. via low dispersion materials or chromatic correction) and / or (2) optimize the optical design to have maximum sensitivity to chromatic focal shift of the lens elements / groups.

[0104] In some embodiments the optical system CFS is sufficient to cover the desired focus adjustment range with the available wavelength tuning range. As one example, the desired focus adjustment range may be + 1.2 mm and the desired wavelength tuning range may be < + 15 nm. Using these examples of parameters the desired CFS would be > 80 pm / nm.

[0105] A high dispersion optical material may be used to maximize the chromatic focal shift of individual lens elements. In the visible wavelength region, the value usually used to define the chromatic dispersion of glass is the normalized Abbe value vd, defined as:where n is the glass refractive index at wavelengths corresponding Fraunhofer’s C, d, and F spectral lines (656.3 nm, 587.56 nm, and 486.1 nm respectively). Abbe values in the visible spectrum typically range between 70 (low) and 20 (high dispersion). Generally speaking optical materials with an Abbe number below about 50 are considered high dispersion. Materials. For example, optical glass type S- NPH3 has an Abbe number of 17.47. The corresponding chromatic focus shift between the shortest and longest wavelengths is given by:

[0106] From this expression it is clear that the chromatic focal shift is inversely proportional to the dispersion, and as the dispersion increases yddecreases) the focal shift 5f increases.

[0107] Thus, in some embodiments, it will be beneficial to use an optical material having a large chromatic dispersion at the operational wavelength of the laser establishing the laser curtain. For example, the laser curtain radiation may have a wavelength in the near infra-red (NIR) region, e.g., of about 1.55 pm. The wavelength range target for a tunable laser having a center wavelength in this range may be 1.55+0.015 pm, or a range of 30 nm.

[0108] In the evaluation of candidate optical materials it is advantageous to use a modified version of the Abbe equation:VH ~ (n1.55—l) / (n1.535— n1.565)where n1 55is the refractive index of the material at 1.55 pm, n1 535is the refractive index of the material at 1.535 pm, and n1 565is the refractive index of the material at 1.565 pm. As an example, the value of vHfor fused silica is 1238 (unitless). The value of vHfor Ohara Optical Glass Type S-NPH3 (Niobophosphate, high-index) is approximately 1116 (unitless). In addition to fused silica and Ohara Optical Glass Type S-NPH3 other suitable candidate materials include glasses made by Schott AG referred to herein as Schott glasses. One of ordinary skill in the art will readily appreciate that other suitable optical materials are available from other suppliers.

[0109] As mentioned before, the optical system design to establish the laser curtain can be optimized to increase the CFS. For a finite conjugate optical system (i.e., one that relays an object plane at a finite distance (as opposed to an “infinite” distance) to an image plane again at a finite distance), a defining property is magnification (M, defined as M = image height / object height). For a finite conjugate relay system application, longitudinal (or axial) aberrations are scaled by a factor of M2.

[0110] Thus, by forming an intermediate image with residual axial color aberration, the CFS can be increased by a factor of approximately M2using a relay lens to deliver an overall required system magnification. The combined optical system can be designed to deliver the required final beam waist size at the fixed image plane, while introducing a substantial amount of axial color aberration.

[0111] The overall magnification of a system of chained relay optical systems (with magnifications Mi and M2 is given by:Moverall=MrM2

[0112] For the optical relay system under consideration, the required overall magnification is the ratio of the required final waist size at the droplet plane to the beam waist at the output of the delivery fiber optic, MOveraii = -54 / -10.5 = 5.14x.

[0113] FIGS. 10A and 10B show an example of an optical relay system according to an aspect of an embodiment with the view of FIG. 10 A being orthogonal to the view of FIG. 1 OB . In the example shown the optical material is S-NPH3 glass type but other optical materials could be used for some or all of the optical components. A collimating lens 410 which may be a plano-convex aspheric objective lens collimates the beam exiting the delivery fiber (not shown). An imaging lens 420 which may be a planoconvex acylindrical lens forms an intermediate waist in the narrow axis at intermediate plane O’ at which a stop 430 is positioned. A lens 440 which may be a second plano-convex acylindrical lens relays the waist to the desired location at the droplet plane O’ ’ with the required waist in the narrow direction. Although in this example specific lens shapes such as plano-convex aspheric lenses and plano-convex acylindrical lenses are mentioned, one of ordinary skill in the art will readily appreciate that other lens shapes may be used and that additional optical elements such as additional lenses as well as mirrors may be used.

[0114] For some embodiments it may be beneficial to fold the optical path, for example, to facilitate fitting the optical system into the space available for it. Thus, FIG. 11 shows an optical system 450 having a folded three element design with a path folding mirror 460 positioned in the optical path between the plano-convex aspheric objective lens 410 and the plano-convex acylindrical lens 440. FIG. 12 is a solid model view of a portion of the arrangement shown in FIG. 11.

[0115] Another measure that may be adopted to introduce axial chromatic aberration to the laser curtain optical system to achieve a desired CFS is the use of dispersion prisms. FIG. 13 shows an example of such a system. In optical system 500 as depicted in FIG. 13 a dispersion prism 510 spatially separates the beam exiting the delivery fiber (not shown) into three components with respectively three different wavelengths Xi, X2 and X; with X i>X 2>X 3. Although in the arrangement of FIG. 13 the beam is divided into three components, one of ordinary skill in the art will understand that the beam could be separated into just two components or into more than three components. The divided beam then passes to another dispersion prism 520 which further spaces the components and redirects them to be parallel to an optical axis of a positive lens 530 that is selected to have a strong spherical aberration. As is well known, a positive lens is a lens that is thickest at its center and thinner toward its outer portions thus causing light which passes through it to converge. The light leaving the positive lens 530 then exhibits the axial chromatic aberration useful for shifting the position of the focus of the laser curtain. Again, these optical components may be made of an optical material such as Ohara S-NPH3 with an appropriately high Abbe value.

[0116] It is also possible to introduce axial chromatic aberration using an arrangement in which the light is divided into separate components and the divided components are directed along differing paths. FIG. 14 shows an optical system 700 which uses a wavelength dependent glass path adjustment to increase chromatic aberration. As shown in FIG. 14, a light beam from a delivery fiber (not shown) enters glass block 710 where it encounters a partially reflecting mirror / filter 715. This divides out a beam component having wavelength Xi . The remaining portion of the light beam passes to glass block 720 where it encounters partially reflective mirror filter 725. The partially reflective mirror filter 725 separates out a beam component having wavelength X2 which propagates through glass block 740. The remaining portion of the light beam having wavelength X3 passes into block 730 where it encounters a folding mirror 735 which directs the component of the light beaming having wavelength X3 through a glass block 760 and a glass block 770.

[0117] The component having wavelength X3 propagates to a glass block 780, a glass block 790 and a glass block 800 where it encounters a folding mirror 805 which redirects the path of that component. Meanwhile, the component having wavelength X2 passes through a block 810 to block 820 were encounters a beam combiner 825 which combines the component having wavelength X2 with the component wavelength X3. At the same time, the component having wavelength Xi passes into a block 830 where it encounters a beam combiner 835 which combines the component having wavelength Xi with the combination of components having wavelength X2 and wavelength X3. Again, these opticalcomponents may be made of an optical material such as Ohara S-NPH3 with an appropriately high Abbe value.

[0118] It is also possible to introduce axial chromatic aberration using an arrangement using two tilted glass plates. FIG. 15 shows an example of such a system. In optical system 900 as depicted in FIG. 15 part of a beam exiting a delivery fiber (not shown) enters a first side (left hand side in the orientation shown) tilted glass plate 910. When the beam encounters the second side of the tilted glass plate 910 (right hand side in the orientation shown) a component having wavelength Xi exits the tilted glass plate 910 while the remainder of the beam is reflected back into the bulk of the tilted glass plate 910. When the reflected component of the beam encounters the first side of the tilted glass plate 910 it is reflected back towards the second side where a component having wavelength A exits the tilted glass plate 910 with the remainder of the beam being reflected back into the bulk of the tilted glass plate 910. When the reflected component of the beam again encounters the first side of the tilted glass plate 910 it is reflected back towards the second side where it once again encounters the second side of the tilted glass plate 910 and a component having wavelength Is exits the tilted glass plate 910 while the remainder of the beam is reflected back into the bulk of the tilted glass plate 910.

[0119] In tilted glass plate 920 the three components of the original beam are recombined after having traversed different paths having different path lengths to achieve a beam having the required axial chromatic aberration.

[0120] Although in the arrangement of FIG. 15 the beam is divided into three components, one of ordinary skill in the art will understand that the beam could be separated into more than three components. Again, these optical components may be made of an optical material such as Ohara S- NPH3 with an appropriately high Abbe value.

[0121] Other optical components may be used such as diffraction elements, e.g., gratings, metalenses, and computer generated holograms, which are known to have very high levels of dispersion. It is also possible to use gradient index elements, where if an optical is designed to vary the filled-aperture of a gradient index lens, the effective focal length can be varied and the ability to focus via wavelength can be adjusted.

[0122] In general, the embodiments are directed to altering the properties of radiation used for target material illumination in which the illumination properties such as focusing properties are dependent of the wavelength of the radiation, i.e., chromatic dependent.

[0123] The above description is primarily in terms of darkfield illumination arrangements in which the illuminating radiation interacts with the droplets by being scattered, e.g., reflected off the droplets and detected (e.g. FIG. 2). One of ordinary skill in the art will appreciate, however, that that the principles elucidated herein are also applicable to arrangements using brightfield illumination in which the interaction is obscuration of illuminating radiation which is detected.

[0124] The above description is also primarily in terms of adjusting one type of property of the illuminating radiation, i.e., a focusing property, by adjusting the wavelength of the illuminatingradiation, i.e., wavelength tuning. One of ordinary skill in the art will appreciate that wavelength tuning can alternatively or in addition be used to adjust other properties of the illuminating radiation. For example, a prism or grating may be used in conjunction with wavelength tuning to adjust the position of the focal point in a direction transverse to beam wise. Also, the shape and / or size of the focal point may be adjusted, e.g., magnified. Spherochromatic effects may also be exploited to adjust the properties of the illuminating radiation. For example, the Rayleigh Length may be large along the Y direction, i.e., insensitive to droplet positional variation in the Y direction, but relatively smaller in the Z direction. In such cases the signal amplitude could be optimized by adjusting wavelength to increase the size of the focal region especially if the laser focus is small enough it might miss the droplets altogether.

[0125] According to another aspect, if the dependence of focus position on wavelength is known for a given arrangement , then the droplet position can be inferred based on the wavelength that returns the optimal signal. In other words, the correlation of wavelength vs. focal position provides a basis for determining the droplet position from the wavelength that results in the optimal, e.g. highest amplitude, signal.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] 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 that an 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.

[0136] 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.

[0137] 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.

[0138] The implementations can be further described using the following clauses.1. Apparatus for detecting a characteristic of at least one droplet in a stream of droplets of target material, the apparatus comprising: an illumination system arranged to illuminate with a beam of radiation at a position in a trajectory of the at least one droplet after exiting a droplet generator nozzle, the illumination system including a tunable-wavelength laser configured to generate the beam of radiation; a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection; and a controller arranged to receive the signal and adapted to selectably cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation.2. The apparatus of clause 1 , wherein the characteristic of the at least one droplet is a presence of the droplet.3. The apparatus of clause 1, wherein the detection system is arranged and adapted to perform a detection of radiation that has been scattered by the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection.4. The apparatus of clause 1 , wherein the detection system is arranged and adapted to perform a detection of radiation that has been reflected by the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection.5. The apparatus of clause 1, wherein the stream is one in which the droplets undergo coalescence asthe droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream less than the coalescence length away from the droplet generator nozzle.6. The apparatus of clause 1 , wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.7. The apparatus of clause 1, wherein the signal is characterized by a signal width indicative of a resolution of the detection and the controller is adapted to cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation to decrease the signal width.8. The apparatus of clause 1, wherein the controller is adapted to cause the tunable- wavelength laser to alter its wavelength to improve contrast of the detection.9. The apparatus of clause 8, wherein the contrast is given by the relationwherein S 1 is the peak amplitude of the signal from the smaller of two droplets that are closely spaced in a direction along the trajectory of the at least one droplet and S12 is an amplitude of a dip between the peak amplitude of the signal from the smaller of two droplets and the peak amplitude of the signal from the larger of the two droplets.10. The apparatus of clause 1, wherein the controller is adapted to cause the tunable-wavelength laser to alter its wavelength to increase an amplitude of the signal.11. The apparatus of clause 1 , wherein the detection system generates a signal indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.12. Apparatus for detecting multiple characteristics of a target material, the apparatus comprising: a first illumination system arranged to illuminate a first position in a trajectory of the target material after exiting a nozzle with a first beam of first radiation, the first illumination system including a tunable-wavelength laser configured to generate the first beam of first radiation, a first detection system arranged to receive the first radiation that has interacted with the target material when the target material traverses the first position and to generate a first signal indicative of a first detection of the target material; a first controller system arranged to receive the first signal and adapted to cause the tunable- wavelength laser to selectably alter its wavelength to move a focal position of the first beam of radiation;a second illumination system arranged to illuminate a second position in a trajectory of the target material after exiting the nozzle with a second beam of second radiation, the second position being further from the nozzle than the first position; and a second detection system arranged to receive the second radiation that has interacted with the target material when the target material traverses the second position and to generate a second signal indicative of a second detection of the target material.13. The apparatus of clause 12, wherein the first radiation and the second radiation interact with the target material by being scattered by the target material.14. The apparatus of clause 12, wherein the target material is formed as droplets which undergo coalescence as the droplets travel in a stream away from the nozzle up to a coalescence length from the nozzle and wherein the first position is at a distance along the stream less than the coalescence length away from the nozzle.15. The apparatus of clause 12, wherein the target material is formed as droplets which undergo coalescence as the droplets travel in a stream away from the nozzle up to a coalescence length from the nozzle and wherein the second position is at a distance along the stream greater than the coalescence length away from the nozzle.16. The apparatus of clause 12, wherein the first signal is characterized by a signal width indicative of a resolution of the first detection and the first controller is adapted to cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation first to decrease the signal width.17. The apparatus of clause 12, wherein the first controller is adapted to cause the tunable-wavelength laser to alter its wavelength to improve contrast of the detection.18. The apparatus of clause 17, wherein the target material is formed as droplets which travel in a stream away from the nozzle and wherein the contrast is given by the relationwherein S 1 is the peak amplitude of the signal from the smaller of two droplets that are closely spaced in a direction along the trajectory of the droplet and S 12 is an amplitude of a dip between the peak amplitude of the signal from the smaller of two droplets and the peak amplitude of the signal from the larger of the two droplets.19. The apparatus of clause 12, wherein the controller is adapted to cause the tunable-wavelength laser to alter its wavelength to increase an amplitude of the signal.20. The apparatus of clause 12, wherein the target material is formed as droplets and wherein the detection system generates a signal indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.21. A method of detecting a characteristic of a droplet of target material in a stream of droplets of target material for generating extreme ultraviolet radiation, the method comprising: using a tunable-wavelength laser to generate a beam of metrology radiation used to illuminate a position in the stream between a droplet generator nozzle and an irradiation region; detecting metrology radiation that has interacted with the droplet when the droplet traverses the position; generating a signal indicative of the detection of the droplet; and causing the tunable-wavelength laser to selectably alter its wavelength to move a focal position of the beam of radiation based on the signal.22. The method of clause 21, wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream less than the coalescence length away from the droplet generator nozzle.23. The method of clause 21, wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.24. The method of clause 21, wherein the signal is characterized by a signal width indicative of a resolution of the detection and causing the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation is performed to decrease the signal width.25. The method of clause 21, wherein the wavelength of the tunable-wavelength laser is altered to improve contrast of the detection.26. The method of clause 25, wherein the contrast is given by the relationwherein S 1 is a peak amplitude of a signal from a smaller of two droplets that are closely spaced and S 12 is an amplitude of a dip between the peak amplitude of the signal from the smaller of the two droplets and a peak amplitude of a signal from a larger of the two droplets.27. The method of clause 21, wherein the wavelength of the tunable-wavelength laser is altered to increase an amplitude of the signal.28. The method of clause 21, wherein the signal is indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.29. A method of detecting a characteristic of a droplet of target material in a stream of droplets for generating extreme ultraviolet radiation, the method comprising:generating a laser curtain at a position downstream of a droplet generator nozzle producing the stream using a beam of radiation from a tunable-wavelength laser; detecting a presence of a droplet in the laser curtain by detecting radiation from the beam scattered by the droplet when the droplet traverses the laser curtain; generating a signal indicative of the presence of the droplet; and causing the tunable-wavelength laser to change the wavelength of the beam of radiation based on the signal.30. The method of clause 29, wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream less than the coalescence length away from the droplet generator nozzle.31. The method of clause 29, wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.32. The method of clause 29, wherein the signal is characterized by a signal width indicative of a resolution of the detection and causing the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation is performed to alter the signal width.33. The method of clause 29, wherein the wavelength of the tunable-wavelength laser is altered to improve contrast of the detection.34. The method of clause 33, wherein the contrast is given by the relationwherein S 1 is a peak amplitude of a signal from a smaller of two droplets that are closely spaced in a longitudinal direction of the two droplets and S 12 is an amplitude of a dip between the peak amplitude of the signal from the smaller of the two droplets and a peak amplitude of a signal from a larger of the two droplets.35. The method of clause 33, wherein the wavelength of the tunable-wavelength laser is altered to increase an amplitude of the signal.36. The method of clause 33, wherein the signal is indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.37. A system comprising: a tunable-wavelength laser that produces laser radiation used to generate a laser curtain; a scattered light detector;a droplet generator having a droplet generator nozzle adapted to direct droplets in a stream through the laser curtain; and one or more processors configured to perform operations of obtaining a signal based on laser radiation that has interacted with a droplet in the laser curtain, the signal having a width representing a measurement resolution, and tuning the tunable-wavelength laser to adjust a wavelength of the laser radiation to move a focal location of a focus of the laser curtain thereof to increase the measurement resolution.38. The system of clause 37, the operations further comprising moving the focal location to an optimal location by adjusting the wavelength to minimize the width.39. The system of clause 37, wherein at least some components of an optical system through which laser radiation from the tunable wavelength laser passes to produce the laser have a larger dispersion than fused silica to increase the movement of the focus for a given wavelength adjustment.40. The system of clause 37, wherein the droplets in the stream undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream less than the coalescence length away from the droplet generator nozzle.41. The system of clause 37, wherein the droplets in the stream undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.42. Apparatus for determining a position of a droplet in a stream of droplets of target material, the apparatus comprising: an illumination system arranged to illuminate with radiation a position in a trajectory of the droplet after exiting a droplet generator nozzle, the illumination system including a tunable- wavelength laser configured to generate the radiation; a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection; and a controller arranged to receive the signal and adapted to cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation and to determine the position of the droplet based on which wavelength of the tunable-wavelength laser optimizes an amplitude of the signal.43. Apparatus for detecting a characteristic of at least one droplet in a stream of droplets of target material, the apparatus comprising: an illumination system arranged to generate a beam of radiation to create a laser curtain at a position in a trajectory of the at least one droplet after exiting a droplet generator nozzle, the illumination system includinga tunable-wavelength laser configured to generate the beam of radiation, and a relay optical system in a beam path of the beam having a first portion forming an intermediate image and a second portion adapted to magnify and relay the intermediate image to a position for the laser curtain; a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection; and a controller arranged to receive the signal and adapted to selectably cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation.44. The apparatus of clause 43, wherein the first portion includes a first lens arranged to collimate an incoming beam and a second lens arranged to form an intermediate waist and wherein the second portion includes a third lens arranged to relay the waist to the position for the laser curtain.45. The apparatus of clause 44, wherein the first lens, the second lens, and third lens comprise an optical material having high dispersion.46. The apparatus of clause 45, wherein the optical material is fused silica.47. The apparatus of clause 45, wherein the optical material is Ohara Optical Glass Type S-NPH3.48. Apparatus for detecting a characteristic of at least one droplet in a stream of droplets of target material, the apparatus comprising: an illumination system arranged to generate a beam of radiation to create a laser curtain at a position in a trajectory of the at least one droplet after exiting a droplet generator nozzle, the illumination system including a tunable-wavelength laser configured to generate the beam of radiation, and an optical system arranged to divide the beam into a first beam portion having a first wavelength component and a second beam portion having a second wavelength component and to cause the first beam portion to propagate along a first path to a position for the laser curtain and to cause the second beam portion to propagate along a second path to the position for the laser curtain, a portion of the second path being noncoincident with the first path; a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection; and a controller arranged to receive the signal and adapted to selectably cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation.49. The apparatus of clause 48, wherein the optical system includes a pair of dispersing prisms.50. The apparatus of clause 49, wherein the dispersing prisms comprise fused silica.51. The apparatus of clause 49, wherein the dispersing prisms comprise Ohara Optical Glass Type S- NPH3.52. The apparatus of clause 48, wherein the first path is through a first plurality of glass blocks andwherein the second path is through a second plurality of glass blocks with the second plurality of glass blocks includes glass blocks not included in the first plurality of glass blocks.53. The apparatus of clause 52, wherein the first plurality of glass blocks and the second plurality of glass blocks comprise fused silica. 54. The apparatus of clause 52 wherein the first plurality of glass blocks and the second plurality of glass blocks comprise Ohara Optical Glass Type S-NPH3.55. The apparatus of clause 48, wherein the first path is through a first portion of a tilted glass plate and wherein the second path is through a second portion of the tilted glass plate.56. The apparatus of clause 55, wherein the tilted glass plate comprises fused silica. 57. The apparatus of clause 55, wherein the tilted glass plate comprises Ohara Optical Glass Type S-NPH3.

[0139] The above-described implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. Apparatus for detecting a characteristic of at least one droplet in a stream of droplets of target material, the apparatus comprising: an illumination system arranged to illuminate with a beam of radiation at a position in a trajectory of the at least one droplet after exiting a droplet generator nozzle, the illumination system including a tunable-wavelength laser configured to generate the beam of radiation; a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection; and a controller arranged to receive the signal and adapted to selectably cause the tunable- wavelength laser to alter its wavelength to move a focal position of the beam of radiation.

2. The apparatus of claim 1, wherein the characteristic of the at least one droplet is a presence of the droplet.

3. The apparatus of claim 1, wherein the detection system is arranged and adapted to perform a detection of radiation that has been scattered by the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection.

4. The apparatus of claim 1, wherein the detection system is arranged and adapted to perform a detection of radiation that has been reflected by the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection.

5. The apparatus of claim 1, wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream less than the coalescence length away from the droplet generator nozzle.

6. The apparatus of claim 1, wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream greater than the coalescence length away from the droplet generator nozzle.

7. The apparatus of claim 1, wherein the signal is characterized by a signal width indicative of a resolution of the detection and the controller is adapted to cause the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation to decrease the signalwidth.

8. The apparatus of claim 1, wherein the controller is adapted to cause the tunable- wavelength laser to alter its wavelength to increase a contrast of the detection.

9. The apparatus of claim 8, wherein the contrast is given by the relationS12C = 1 -SI wherein SI is a peak amplitude of a signal from a smaller of two droplets that are closely spaced in a direction along the trajectory of the at least one droplet and S12 is an amplitude of a dip between the peak amplitude of the signal from the smaller of the two droplets and a peak amplitude of a signal from a larger of the two droplets.

10. The apparatus of claim 1, wherein the controller is adapted to cause the tunable- wavelength laser to alter its wavelength to increase an amplitude of the signal.

11. The apparatus of claim 1 , wherein the detection system generates a signal indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, and satellite detection.

12. A method of detecting a characteristic of a droplet of target material in a stream of droplets of target material for generating extreme ultraviolet radiation, the method comprising: using a tunable-wavelength laser to generate a beam of metrology radiation used to illuminate a position in the stream between a droplet generator nozzle and an irradiation region; detecting metrology radiation that has interacted with the droplet when the droplet traverses the position; generating a signal indicative of the detection of the droplet; and causing the tunable-wavelength laser to selectably alter its wavelength to move a focal position of the beam of radiation based on the signal.

13. The method of claim 12, wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream less than the coalescence length away from the droplet generator nozzle.

14. The method of claim 12, wherein the stream is one in which the droplets undergo coalescence as the droplets travel away from the droplet generator nozzle up to a coalescence length from the droplet generator nozzle and wherein the position is at a distance along the stream greater thanthe coalescence length away from the droplet generator nozzle.

15. The method of claim 12, wherein the signal is characterized by a signal width indicative of a resolution of the detection and causing the tunable-wavelength laser to alter its wavelength to move a focal position of the beam of radiation is performed to decrease the signal width.

16. The method of claim 12, wherein the wavelength of the tunable-wavelength laser is altered to increase a contrast of the detection.

17. The method of claim 16, wherein the contrast is given by the relationS12C = 1 -SI wherein SI is a peak amplitude of a first signal from a smaller of two droplets that are closely spaced and S 12 is an amplitude of a dip between the peak amplitude of the signal from the smaller of the two droplets and a peak amplitude of a second signal from a larger of the two droplets.

18. The method of claim 12, wherein the wavelength of the tunable-wavelength laser is altered to increase an amplitude of the signal.

19. The method of claim 12, wherein the signal is indicative of at least one of droplet presence, droplet size, droplet spacing, droplet stability, timing of when the droplet traverses the position, and satellite detection.

20. Apparatus for detecting a characteristic of at least one droplet in a stream of droplets of target material, the apparatus comprising: an illumination system arranged to generate a beam of radiation to create a laser curtain at a position in a trajectory of the at least one droplet after exiting a droplet generator nozzle, the illumination system including a tunable-wavelength laser configured to generate the beam of radiation, and a relay optical system in a beam path of the beam having a first portion forming an intermediate image and a second portion adapted to magnify and relay the intermediate image to a position for the laser curtain; a detection system arranged and adapted to perform a detection of radiation that has interacted with the at least one droplet when the at least one droplet traverses the beam position and to generate a signal indicative of the detection; and a controller arranged to receive the signal and adapted to selectably cause the tunable- wavelength laser to alter its wavelength to move a focal position of the beam of radiation.

21. The apparatus of claim 20, wherein the first portion includes a first lens arranged to collimate an incoming beam and a second lens arranged to form an intermediate waist and wherein the second portion includes a third lens arranged to relay the waist to the position for the laser curtain.

22. The apparatus of claim 21, wherein the first lens, the second lens, and third lens comprise an optical material having high dispersion.

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