EUV Light Source Target Metrology

Abstract

A system and method for performing target metrology in an extreme ultraviolet light source is disclosed in which illumination for a target imaging / detection system is selected to have a central wavelength less than 400 nm. In some embodiments, the radiation is further selected to have a linewidth of at least 4 nm. In some embodiments, the radiation is obtained as the second harmonic of a Ti:sapphire laser or as a harmonic of a Nd:YAG laser.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63 / 322,754, "EUV Light Source Target Metrology," filed on March 23, 2022, and is incorporated by reference in its entirety into this specification.

[0002] This disclosure relates to light sources that produce extreme ultraviolet radiation by excitation of a target material, and in particular to measurement, eg, detection, of the target material within such light sources. [Background technology]

[0003]

[0003] Extreme ultraviolet ("EUV") light, for example, electromagnetic radiation having wavelengths of approximately 50 nm or less (sometimes referred to as soft x-rays), including light with wavelengths of about 13 nm, is used in photolithography processes to create extremely small features in and on substrates, for example silicon wafers. The term "light" is used herein to refer to electromagnetic radiation, whether or not this electromagnetic radiation is typically in the visible portion of the spectrum.

[0004]

[0004] Methods for generating EUV light include, but are not limited to, converting the physical state of a target material from an initial state to a plasma state. The target material includes elements that have emission lines in the EUV range, such as xenon, lithium, or tin. In one such method, often referred to as laser-produced plasma ("LPP"), the required plasma is generated by irradiating the target material, for example in the form of droplets, streams, or clusters, with an amplified light beam, which may be referred to as a drive laser. For this process, the plasma is typically generated in a sealed vessel, such as a vacuum chamber, and monitored using various types of metrology instruments.

[0005]

[0005] One of the goals in efficiently producing EUV light is to achieve a proper relative position of the conditioning beam and the target, also referred to as aligning the conditioning beam and the target. Generally, to operate the light source efficiently and with minimal debris, it is important to align the target and conditioning beam to within a few micrometers. Thus, a great deal of effort has been expended to determine the position of the target relative to the conditioning beam or reference frame. See, for example, U.S. Patent No. 7,372,056, issued on May 13, 2008, entitled "LPPE UV Plasma Source Material Target Delivery System," U.S. Patent No. 8,158,960, issued on April 17, 2012, entitled "Laser Produced Plasma EUV Light Source," U.S. Patent No. 9,241,395, issued on January 19, 2016, entitled "System and Method for Controlling Droplet Timing in an LPPE UV Light Source," and U.S. Patent No. 9,497,840, issued on November 15, 2016, entitled "System and Method for Creating and Utilizing Dual Laser Curtains from a Single Laser in an LPPE UV Light Source."

[0006]

[0006] All patent applications, patents, and publications cited herein are incorporated herein by reference in their entirety, except for any definitions, disclaimers or disclaimers of subject matter, and except to the extent that the incorporated material conflicts with an express disclosure herein. In the event of a conflict, the language disclosed herein will control.

[0007]

[0007] In some EUV sources, the target is conditioned with a radiation pulse that includes a first portion (the "pedestal") and a second portion (the main pulse or heating pulse). The first and second portions may be close to each other in time, i.e., the first and second portions may be part of a single radiation pulse, with no gap or region of no radiation between the first and second portions.

[0008]

[0008] A first portion of the radiation pulse interacts with the target to modify the absorption properties of the target. For example, this interaction can modify the absorption properties by reducing the density gradient of the target and by increasing the volume of the target that interacts with the radiation pulse at the surface that receives the radiation pulse, thereby increasing the amount of radiation that the target can absorb. This process is called rarefaction, and the pulse used to cause rarefaction is called a rarefaction pulse.

[0009]

[0009] In this manner, the interaction between the target and the first portion of the radiation pulse conditions the target. The second portion of the radiation pulse has sufficient energy to convert target material within the target into a plasma that emits EUV light. Because the conditioning by the first portion increases the amount of radiation the target can absorb, this conditioning can result in a larger portion of the target being converted into a plasma that emits EUV light. Additionally, this conditioning can reduce the reflectivity of the target, thus reducing the amount of back reflection back to the light source generating the radiation pulse.

[0010]

[0010] Furthermore, the spatial distribution of the target material can be modified to increase the size of the target material in a direction transverse to the amplified light beam before the first portion of the pulse interacts with the target material. For example, the target can be expanded from a droplet to a flat disk using a separate pulse of radiation (a "pre-pulse") that interacts with the target material. Increasing the size of the target material prior to interaction with the amplified light beam can increase the portion of the target material exposed to the amplified light beam, thereby increasing the amount of EUV light generated for a given amount of target material. That is, the increased target material volume can absorb the radiation pulse more efficiently, and the larger EUV emission volume can generate an increased amount of EUV light.

[0011]

[0011] Although tailored pulses help improve conversion efficiency, changing the density of the target material poses challenges for imaging the target material for metrology purposes. It is in these circumstances that the need for the presently disclosed subject matter arises. Summary of the Invention

[0012]

[0012] The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments or to limit the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

[0013]

[0013] According to one aspect of an embodiment, a target metrology system for an extreme ultraviolet light source is disclosed, the target metrology system including a light source emitting illumination laser radiation having a central wavelength of less than 400 nm and positioned to illuminate a portion of a target material supply path with the illumination laser radiation, and a radiation receiver positioned to receive the illumination laser radiation after the illumination laser radiation illuminates the portion of the target material supply path.

[0014]

[0014] The radiation receiver may generate an output signal indicative of a position of the target in the portion of the target material supply path. The target metrology system may further include a position detection feedback system arranged to receive the output signal of the radiation receiver and adapted to calculate a target position or a target trajectory, or both, based at least in part on the output signal. The radiation receiver may include a target imager. The target imager may include a camera. The target imager may include a CCD array. The target imager may generate a shadow graph.

[0015]

[0015] The illumination laser radiation may have a central wavelength of about 266 nm. The light source may include a frequency-doubled Ti:sapphire laser arranged to receive the pump beam and emit illumination laser radiation at a central wavelength in the range of about 250 nm to about 450 nm. The light source may include a frequency-doubled Ti:sapphire laser arranged to receive the pump beam and emit illumination laser radiation at a central wavelength of about 380 nm.

[0016]

[0016] The light source may further include a diode-pumped solid-state (DPSS) laser arranged to generate a pump beam, the pump beam having a central wavelength of about 532 nm. The DPSS laser may include a Nd:YAG laser and a frequency multiplier arranged to receive the beam and generate the pump beam.

[0017]

[0017] The Ti:sapphire laser may include a dichroic optical element arranged to receive the pump beam, a Ti:sapphire crystal, and an output coupler. The Ti:sapphire laser may include a frequency doubling element arranged to receive from the output coupler an output coupler radiation having a center wavelength of about 760 nm, and to frequency doubling the output coupler radiation to generate an illumination laser radiation with a center wavelength of about 380 nm and a wavelength tuning range of 250 nm to 450 nm. The frequency doubling element may include a nonlinear optical element. The nonlinear optical element may include a lithium borate crystal.

[0018]

[0018] The target metrology system may further include a fiber beam delivery system positioned and adapted to carry the illumination laser radiation from the light source to a portion of the target material feed path. The fiber beam delivery system may include a multi-order fiber.

[0019]

[0019] The illumination laser radiation may be pulsed to have a pulse length in the range of 1 to 10 nanoseconds. The illumination laser radiation may have a linewidth of 4 nm or greater.

[0020]

[0020] The light source may include a Nd:YAG laser and the illumination laser radiation may be obtained as a harmonic of the Nd:YAG laser.

[0021]

[0021] According to another aspect of an embodiment, a target metrology method for an extreme ultraviolet light source is disclosed that includes illuminating a portion of a target material supply path with illumination laser radiation having a central wavelength of less than 400 nm, receiving the illumination laser radiation as received illumination after the illumination laser radiation illuminates the portion of the target material supply path, and imaging target material present in the portion of the target material supply path based on the received illumination.

[0022]

[0022] Imaging the target material may include generating an output signal indicative of a position of the target material present in the portion of the target material supply path. The method may further include calculating a target position or a target trajectory, or both, based at least in part on the output signal. Imaging the target material may include using a target imager. The target imager may include a camera. The target imager may include a CCD array. Imaging the target material may include generating a shadowgraph.

[0023]

[0023] The illumination laser radiation may have a central wavelength of about 266 nm. Illuminating the portion of the target material supply path with illumination laser radiation having a central wavelength of less than 400 nm may be performed at least in part by using a frequency-doubled Ti:sapphire laser arranged to receive the pump beam and emit illumination laser radiation at a central wavelength of about 380 nm or less.

[0024] The target metrology method may further include pumping the Ti:sapphire laser with a diode-pumped solid-state (DPSS) laser arranged to generate a pump beam, the pump beam having a center wavelength of about 532 nm. The DPSS laser may include a Nd:YAG laser and a frequency multiplier arranged to receive the beam and generate the pump beam.

[0025]

[0025] The Ti:sapphire laser may include a dichroic optical element arranged to receive the pump beam, a Ti:sapphire crystal, and an output coupler. The Ti:sapphire laser may include a frequency doubling element arranged to receive the output coupler radiation from the output coupler, the output coupler radiation having a central wavelength of about 760 nm and a wavelength tuning range of up to 960 nm, and to frequency-double (by doubling or tripling) the output coupler radiation to generate an illumination laser radiation at a predetermined central wavelength within a tuning range of about 250 nm to 450 nm (e.g., about 380 nm). The frequency doubling element may include a nonlinear optical element that may include a lithium borate crystal.

[0026]

[0026] Illuminating the portion of the target material feed path with illumination laser radiation having a central wavelength of less than 400 nm may be performed at least in part using a fiber beam delivery system for conveying the illumination laser radiation from a light source to the portion of the target material feed path. The fiber beam delivery system may include a multi-order fiber.

[0027]

[0027] The illumination laser radiation may be pulsed to have a pulse length in the range of 1 to 10 nanoseconds. The illumination laser radiation may have a linewidth of 4 nm or greater.

[0028]

[0028] Illuminating a portion of the target material supply path with illumination laser radiation having a central wavelength of less than 400 nm may include using an Nd:YAG laser, and the illumination laser radiation may be obtained as a harmonic (e.g., the fourth harmonic) of the Nd:YAG laser.

[0029] 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 the accompanying drawings. [Brief description of the drawings]

[0030] [Figure 1] FIG. 1 is a schematic, not to scale, illustration of the overall broad concept of a laser-produced plasma EUV radiation source. [Diagram 2]

[0031] 1 is a schematic, not to scale, diagram of a target material metrology system used in a laser produced plasma EUV radiation source. [Figure 3A]

[0032] FIG. 2 is a diagram, not to scale, showing certain conditions of a target material for a laser-produced plasma EUV radiation source. [Figure 3B]

[0032] FIG. 1 is a diagram, not to scale, showing certain conditions of a target material for a laser-produced plasma EUV radiation source. [Figure 3C]

[0032] FIG. 1 is a diagram, not to scale, showing certain conditions of a target material for a laser-produced plasma EUV radiation source. [Figure 4]

[0033] 1 is a not-to-scale diagram of an illumination source for a target material metrology system used in a laser produced plasma EUV radiation source in accordance with an aspect of an embodiment; FIG. [Diagram 5]

[0034] 1 is a not-to-scale diagram of an illumination source for a target material metrology system used in a laser produced plasma EUV radiation source in accordance with an aspect of an embodiment; FIG.

[0031]

[0035] Further features and advantages of the disclosed subject matter, as well as the structure and operation of various embodiments of the disclosed subject matter, are described in detail below with reference to the accompanying drawings. It should be noted that the applicability of the disclosed subject matter is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032]

[0036] Various embodiments will now be described with reference to the drawings, in which like reference numerals are used to refer to like components throughout the specification. In the following description, for purposes of explanation, numerous specific details are set forth in order to facilitate a thorough understanding of the embodiments. However, it will be apparent that in some or all cases, any of the embodiments described below can be practiced without employing the specific design details described below.

[0033]

[0037] Referring first to FIG. 1, a schematic diagram of an exemplary EUV radiation source, e.g., a laser-produced plasma EUV radiation source 10, according to one aspect of an embodiment of the subject matter disclosed herein is shown. As shown, the EUV radiation source 10 can include a pulsed or continuous laser source 22, e.g., a pulsed gas discharge CO2 laser source that generates a beam 12 of radiation, typically at a wavelength less than 20 μm, e.g., in the range of about 10.6 μm to about 0.5 μm. The pulsed gas discharge CO2 laser source can have DC or RF excitation operating at high power and high pulse repetition rate. The EUV radiation source 10 can also include one or more modules, such as a tuned laser 23 that emits a beam 25 of tuned radiation, as described above.

[0034]

[0038] The EUV radiation source 10 also includes a target delivery system 24 for delivering the target material in the form of droplets or a continuous liquid stream. In this example, the target material is liquid, but could also be solid, for example. The target material could be comprised of tin or a tin compound, but other materials could also be used. In the illustrated system, the target material delivery system 24 introduces the target material target 14 inside the vacuum chamber 26 to an irradiation region 28 where the target material can be irradiated to generate a plasma. In some cases, an electric charge can be imparted to the target material to allow the target material to be guided toward or away from the irradiation region 28. It is noted that, as used herein, the irradiation region is the region where irradiation of the target material takes place, even if no irradiation is actually taking place. The EUV light source can also include a beam focusing and steering system 32.

[0035]

[0039] In the illustrated system, the components are arranged such that the target 14 moves substantially horizontally along the target material feed path. The direction from the laser source 22 towards the illumination area 28, i.e., the nominal direction of propagation of the beam 12, may be considered as the Z-axis. The path that the target 14 follows from the target material delivery system 24 to the illumination area 28 may be considered as the X-axis, with the target moving in the -X direction. Thus, the perspective of FIG. 1 is perpendicular to the XZ plane. Also, while a system is depicted in which the target 14 moves substantially horizontally, one skilled in the art will appreciate that other arrangements in which the target moves vertically, or at an angle between 90 degrees (horizontal) and 0 degrees (vertical) relative to gravity, may also be used.

[0036]

[0040] EUV radiation source 10 may also include an EUV source controller system 60, which may also include a laser firing control system 65, along with laser beam steering system 32. EUV radiation source 10 may also include a detector, such as a target position detection system, which may include one or more target imagers 70 that generate an output indicative of an absolute or relative position of the target droplet with respect to irradiation area 28, and provide this output to target position detection feedback system 62.

[0037]

[0041] The target position detection feedback system 62 may use the output of the target imager 70 to calculate the target position and target trajectory, from which a target error may be calculated. The target error may be calculated for each target, or as an average, or on some other basis. The target error may then be provided as an input to the light source controller 60. In response, the light source controller 60 may generate a control signal, such as a laser position, direction, or timing correction signal, and provide the control signal to the laser beam steering system 32. The laser beam steering system 32 may use the control signal to change the position and / or focus power of the laser beam focus within the chamber 26. The laser beam steering system 32 may also use the control signal to change the geometry of the interaction of the beam 12 with the target 14. For example, the beam 12 may strike the target 14 off-center or at an angle of incidence other than head-on.

[0038]

[0042] As shown in FIG. 1, the target material delivery system 24 may include a target delivery control system 90. The target delivery control system 90 operates in response to a signal, such as the target error described above or some amount derived from the target error provided by the light source controller 60, to adjust the path of the target 14 through the illumination region 28. This may be accomplished, for example, by repositioning the point at which the target delivery mechanism 92 releases the target 14. The target release point may be repositioned, for example, by tilting the target delivery mechanism 92 or by shifting (e.g., moving laterally) the target delivery mechanism 92. The target delivery mechanism 92 preferably extends into the chamber 26, and the target material and gas from a gas source (not shown) are supplied externally to place the target material in the target delivery mechanism 92 under pressure.

[0039]

[0043] Continuing with reference to FIG. 1, the EUV radiation source 10 may also include one or more optical elements. The following description uses the collector 30 as an example of such an optical element, but the description also applies to other optical elements. The collector 30 may be a normal incidence reflector implemented as a multi-layer mirror (MLM) fabricated, for example, by depositing multiple pairs of Mo and Si layers on a substrate with additional thin barrier layers, such as B4C, ZrC, Si3N4, or C, at each interface to effectively block thermally induced interlayer diffusion. The collector 30 may take the form of a prolate ellipsoid with a central opening to allow the beam 12 to pass through and reach the irradiation area 28. Collector 30 may, for example, have the shape of an ellipsoid having a first focus at illumination region 28 and a second focus at a so-called midpoint 40 (also referred to as intermediate focus 40), where EUV radiation may be output from EUV radiation source 10 and may, for example, be input to an integrated circuit lithography scanner or stepper 50 which uses this radiation to process a silicon wafer workpiece 52 in a known manner, for example using a reticle or mask 54. Silicon wafer workpiece 52 may then be further processed in a known manner to obtain an integrated circuit device.

[0040]

[0044] FIG. 2 is a not-to-scale view of a portion of EUV radiation source 10 showing laser source 22, target 14, irradiation region 28, chamber 26, target delivery control system 90, and target delivery mechanism 92. Elements such as collector 30 shown in FIG. 1 are not shown in FIG. 2 for clarity. The arrangement of FIG. 2 also includes a controller 100 that combines the functions of, for example, source controller 60, target position detection feedback system 62, and laser firing control system 65 of FIG. 1. The arrangement shown in FIG. 2 also includes a target material metrology system 105 that includes an illumination source 110 and an illumination receiver 120. The illustrated arrangement is for backlight or brightfield imaging, where the target is illuminated from behind against radiation receiver 120. For this reason, illumination source 110 may also be referred to as a backlight module (BLM). Those skilled in the art will appreciate that target material metrology system 105 may be configured as a frontlight or darkfield system, where the target is illuminated from the front and the illumination receiver receives radiation reflected by the target.

[0041]

[0045] As used herein, the term "imaging" and its synonyms refer to acquiring a camera image, acquiring a shadowgram, acquiring a shadowgraph, and simply detecting the presence and contours of objects within the field of view, i.e., detection. Thus, depending on the application, the illumination receiver 110 may be embodied as a camera, a CCD array, etc., and corresponds to the target imager 70 of FIG.

[0042]

[0046] As an example, a conventional illumination source may generate radiation having a wavelength of 905 nm for target metrology. Such an illumination module is configured for target metrology where the target is in the form of a droplet. For this reason, the receiving module is also called a target forming camera (DFC). This system has a wavelength of about 6.9 g / cm 3 It is designed to image a target made of liquid tin, which has a density of about 3.5 × 10 22 Sn atoms / cm 3The resolution limit of such a system, when implemented with a practical optical geometry at an EUV source, is on the order of a few microns.

[0043]

[0047] One of the challenges posed by the use of such an arrangement arises from the presence of so-called "satellite" droplets. The target material leaving the droplet generator is initially in the form of a continuous stream. This continuous stream splits into a stream of very small microdroplets immediately after leaving the nozzle of the droplet generator. The microdroplets in the droplet / target stream then coalesce as they travel towards the irradiation area and fully coalesce after traveling a distance called the coalescence length. This is the situation of target 14 in FIG. 3A.

[0044]

[0048] Ideally, all microdroplets will coalesce to become a target for full coalescence while moving along the desired trajectory at the desired interval, but some microdroplets may not coalesce. These microdroplets that remain in the stream beyond the coalescence distance are called satellite droplets. These are shown as microdroplets 15 in FIG. 3B. Characteristics such as the number and frequency of microdroplets can indicate system performance indicators such as droplet generator health and are also useful in the process of optimizing droplet generator performance (droplet generator tuning). For this reason, it can be beneficial to detect the presence of such droplets. Thus, while a resolution limit of a few microns is usually sufficient to enable detection of fully coalesced targets, a resolution limit of 1 μm or less is required for detection of satellite droplets resulting from incomplete coalescence.

[0045]

[0049] The rarefaction pulse may also result in smaller pellets 16 of target material being ejected from the target 17, resulting in smaller masses of target material as shown in FIG. 3C, requiring higher resolution to be detected.

[0046]

[0050] Additionally, the disk-shaped distribution 17 as shown in FIG. 3C may include areas (e.g., a thinned center portion) that are too sparse (not dense enough) to be recognized by conventional target material metrology systems. The density of the target material is primarily selected to optimize conversion of the target material to generate the plasma. A CO2 laser with a wavelength of 10.59 μm produces a Sn density of approximately 1×10 18 Sn atoms / cm 2 (1 / 10000th the density of liquid Sn), which results in a plasma density close to the critical density. As mentioned above, the optimal target material density can be achieved by conditioning the target with a pedestal pulse, which among other things redistributes the mass of the target into a disk-like shape with a middle portion of the desired density. However, it is difficult, if not impossible, to detect and / or image this density of tin or the like using conventional target material metrology sources. Therefore, it would be further beneficial if the illumination source could also generate radiation at wavelengths that allow it to "see" the low density target material.

[0047]

[0051] For strong emission lines, the absorption by the target material is proportional to the emission line intensity; that is, the higher the emission line intensity, the higher the absorption. This trend means that the shorter the wavelength, the higher the absorption can be expected. For a conventional illumination source that produces a 905 nm backlight, the relative intensity is 30, but light with a wavelength of less than 400 nm produces an absorption two orders of magnitude higher than the absorption for 905 nm.

[0048]

[0052] Essentially, the diluted tin target material is more opaque to radiation having a wavelength less than about 400 nm, and therefore the diluted tin target can be detected and / or imaged using radiation having a wavelength less than about 400 nm.

[0049]

[0053] For ease of explanation, the present specification describes specific wavelengths, although one of ordinary skill in the art will appreciate that other wavelengths may be used that meet the criteria of being sufficiently absorbed by the diluted target material such that the diluted target material can be imaged, e.g., detected.

[0050]

[0054] Furthermore, the use of radiation with a narrow linewidth, typically the full width at half maximum (FWHM) of the optical spectrum, leads to a noisy background due to spikes and rings that arise from the use of coherent radiation, which also limits the resolution.

[0051]

[0055] Therefore, it is generally desirable to use illumination sources that produce radiation with shorter wavelengths and broader linewidths, while at the same time, there is a need to use illumination sources that can produce very short pulses of radiation that can "freeze" the motion of a target moving at 100 m / s.

[0052]

[0056] Thus, there is a need for a light source capable of detecting and / or imaging a target material such as tin at a density optimal for conversion. According to one aspect of the embodiment, this need is addressed by using an illumination source having a wavelength less than 400 nm, i.e., at the top of the UV wavelength range of 100-400 nm. As an example, the illumination source may include a Ti:sapphire laser. More specifically, as an example, a rarefied low density Sn target may be imaged using the 380 nm second harmonic of a Ti:sapphire laser. Such a light source may be configured to have a broad linewidth of about 4 nm. Here and elsewhere, the term "about" applied to a value is intended to refer to a range that includes the value and other values ​​that fall within the customary tolerance of that value.

[0053]

[0057] Figure 4 illustrates one embodiment of an illumination source 500 for generating radiation with desired characteristics. In the arrangement of Figure 4, a Ti:sapphire laser 510 (also called a Ti:Al2O3 laser, titanium sapphire laser, or Ti:sapph) is pumped at 532 nm by a (DPSS) diode-pumped solid-state laser 585. In this example, this solid-state laser 585 may be a frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, where 532 nm represents the second harmonic of the fundamental 1064 nm radiation emitted by the neodymium-doped laser material.

[0054]

[0058] The Ti:sapphire laser 510 includes a Ti:sapphire crystal 520 in which each of the ends in the direction of the beam (i.e., the end where the beam enters the crystal and the end where the beam exits the crystal) is cut at a Brewster angle to minimize reflection losses. For the propagation of a laser beam from air to a laser crystal (having a refractive index n), the Brewster angle θ B is θ B = arctan(n). The refractive index, n, of Ti:sapphire is approximately 1.76, and the Brewster angle is approximately 60.4°.

[0055]

[0059] The Ti:sapphire laser 510 also includes an output coupler 530 with a reflectivity of 60%-80% at 760 nm, and a dichroic optic 540 with a high reflectivity at 760 nm and an anti-reflection coating for 532 nm. The Ti:sapphire laser 510 also includes a focusing lens 550. The Ti:sapphire laser 510 can be operated in a gain-switching manner, i.e., emitting laser radiation with a wavelength tuning range up to 960 nm (e.g., about 760 nm) when the gain is above the oscillation threshold, and no output when the gain is below the oscillation threshold.

[0056]

[0060] The frequency of the 760 nm radiation, tunable from about 720 nm to about 960 nm, generated by the Ti:sapphire crystal 520 is multiplied, e.g., doubled, to generate 380 nm radiation by the frequency multiplier 560. The frequency multiplier 560 may be part of the Ti:sapphire laser 510 as shown, or may be positioned outside the Ti:sapphire laser 510. The frequency multiplier 560 may be implemented as a nonlinear optical crystal, such as a lithium triborate (LiB3O5 or LBO) crystal used for second harmonic generation (SHG). SHG, also called frequency doubling, is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material to "combine" and generate a new photon with twice the energy of the original photon (in other words, twice the frequency and half the wavelength). It has a wavelength tuning range of about 720 nm to 960 nm and, by SHG and third harmonic generation (THG), can produce UV radiation having a wavelength in the range of about 250 nm to 450 nm.

[0057]

[0061] As shown, the output of the frequency multiplier 560 is coupled to a fiber beam delivery system 570. Beam homogenization can be achieved by using multi-order fiber in the fiber beam delivery system 570, resulting in overlapping of multiple beam modes with broad linewidths and therefore reduced coherence.

[0058]

[0062] 4, the Ti:sapphire laser 510 is pumped by 532 nm pulses generated by a DPSS laser 580, which may be implemented as a frequency doubled Nd:YAG laser 585. The 532 nm wavelength of these pulses is a SHG of the 1064 nm fundamental of the Nd:YAG laser material, and the frequency multiplier 590 may be implemented as a nonlinear optical crystal such as beta barium borate (β-BaB2O4 or BBO) for SHG.

[0059]

[0063] The pulse duration of the Ti:sapphire laser 510 depends on the pulse duration of the applied pump laser pulse. Since the pump laser has a duration of a few nanoseconds, the pulse duration of the Ti:sapphire laser is also a few nanoseconds. Its central wavelength and linewidth depend strongly on the 760 nm membrane of the output coupler 530. The linewidth at 380 nm can be more than 4 nm, which is 100 times wider than the linewidth of the fourth harmonic at 266 nm. This wide linewidth allows the visualization of satellite droplets wherever they appear within the field of view.

[0060]

[0064] The sub-micron resolution enabled by this arrangement is useful for measuring targets in the 20-27 μm diameter range as well as sub-micron satellite droplets. Information obtained from these measurements can be used to calculate Sn volume balances, i.e. the amount of Sn contained in each target, which is useful for on-line adjustment of the droplet generator and for droplet generator qualification, both of which can extend the lifetime of the droplet generator.

[0061]

[0065] FIG. 5 is a diagram of another arrangement for generating target material metrology radiation having a wavelength of less than 400 nm, according to another aspect of the embodiment. As shown in FIG. 5, the output of a 532 nm Q-switched Nd:YAG laser source 620 (SHG of the fundamental wavelength 1064 nm of the Nd:YAG laser) is directed to a frequency multiplier 630, which may be implemented as a BBO nonlinear crystal, to generate a fourth harmonic laser radiation at 266 nm. A mirror 640 separates the 266 nm laser radiation from the emission radiation at other wavelengths. For example, the mirror 640 may have high reflection efficiency at 266 nm and high transmission efficiency at 532 nm and 1064 nm. As shown, the 266 nm output of the frequency multiplier 630 is coupled to a fiber beam delivery system 650. Beam homogenization can be achieved by using a multi-order fiber, resulting in overlapping of multiple beam modes with wide linewidths, thus reducing coherence.

[0062]

[0066] The present disclosure is made with functional building blocks illustrating the implementation of certain functions and relationships between these functional building blocks. The boundaries of these functional building blocks are arbitrarily defined herein for convenience of description. Alternative boundaries can be defined so long as the certain functions are appropriately performed. For example, the functions of a control module can be divided among multiple systems or can be performed at least in part by an overall control system.

[0063]

[0067] The above description includes examples of one or more embodiments. Of course, for purposes of describing the above-described embodiments, it is not possible to describe every conceivable combination of components or methods, but one of ordinary skill in the art will recognize that many further combinations and permutations of the various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

[0064]

[0068] Furthermore, wherever used in the detailed description or the claims, the term "including" is intended to be inclusive, similar to the interpretation of the term "comprising" when employed as a transitional term in the claims. Furthermore, although elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural is also intended unless the singular is clearly stated. Also, all or a portion of any aspect and / or embodiment can be utilized with all or a portion of any other aspect and / or embodiment, unless stated otherwise.

[0065]

[0069] The following clauses are used to further describe the embodiments. 1. A target metrology system for an extreme ultraviolet light source, comprising: a light source emitting illumination laser radiation having a central wavelength of less than 400 nm and positioned to illuminate a portion of the target material supply path with the illumination laser radiation; a radiation receiver positioned to receive the illumination laser radiation after the illumination laser radiation illuminates a portion of the target material supply path; Target metrology systems, including: 2. A target metrology system as described in clause 1, wherein the radiation receiver generates an output signal indicative of a position of the target in a portion of the target material supply path. 3. The target metrology system described in clause 2, further comprising a position detection feedback system arranged to receive an output signal of the radiation receiver and adapted to calculate at least one of a target position and a target trajectory based at least in part on the output signal. 4. The target metrology system of claim 1, wherein the radiation receiver includes a target imager. 5. The target metrology system of claim 4, wherein the target imager includes a camera. 6. The target metrology system of claim 4, wherein the target imager includes a CCD array. 7. The target metrology system of claim 4, wherein the target imager generates a shadow graph. 8. The target metrology system of claim 1, wherein the light source is adapted to generate illumination laser radiation having a central wavelength of about 266 nm. 9. A target metrology system as described in clause 1, wherein the light source includes a frequency-doubled Ti:sapphire laser arranged to receive the pump beam and emit illumination laser radiation at a central wavelength in the range of about 250 nm to about 450 nm. 10. A target metrology system as described in clause 1, wherein the light source includes a frequency-doubled Ti:sapphire laser arranged to receive the pump beam and emit illumination laser radiation at a central wavelength of about 380 nm. 11. The target metrology system of claim 1, wherein the light source further includes a diode-pumped solid-state (DPSS) laser arranged to generate a pump beam, the pump beam having a central wavelength of about 532 nm. 12. The target metrology system of claim 11, wherein the DPSS laser includes a Nd:YAG laser and a frequency multiplier arranged to receive the beam and generate the pump beam. 13. The target metrology system of claim 9, wherein the Ti:sapphire laser includes a dichroic optical element positioned to receive the pump beam, a Ti:sapphire crystal, and an output coupler. 14. The target metrology system of claim 9, wherein the Ti:sapphire laser includes a frequency doubling element arranged to receive from the output coupler output radiation having a central wavelength of about 760 nm and to frequency doubling the output coupler radiation to produce illumination laser radiation at a central wavelength of about 380 nm and with a wavelength tuning range of 250 nm to 450 nm. 15. The target metrology system of claim 14, wherein the frequency multiplication element comprises a nonlinear optical element. 16. The target metrology system of claim 15, wherein the nonlinear optical element comprises a lithium borate crystal. 17. The target metrology system of claim 1, further comprising a fiber beam delivery system positioned and adapted to carry illumination laser radiation from the light source to a portion of the target material supply path. 18. The target metrology system of clause 17, wherein the fiber beam delivery system includes a multi-order fiber. 19. The target metrology system of clause 1, wherein the light source is adapted to generate illumination laser radiation having a pulse length in the range of 1 to 10 nanoseconds. 20. The target metrology system of clause 1, wherein the light source is adapted to generate illumination laser radiation having a linewidth of 4 nm or greater. 21. A target metrology system as described in clause 1, wherein the light source includes a Nd:YAG laser and the illumination laser radiation is obtained as a harmonic of the Nd:YAG laser. 22. A target metrology method for an extreme ultraviolet light source, comprising: Illuminating a portion of the target material supply path with illumination laser radiation having a central wavelength of less than 400 nm; receiving the illumination laser radiation as received radiation after the illumination laser radiation illuminates a portion of the target material supply passage; imaging target material present in a portion of the target material supply path based on the received illumination; A target metrology method comprising: 23. The target metrology method of claim 22, wherein imaging the target material includes generating an output signal indicative of a position of the target material present in the portion of the target material supply path. 24. The target metrology method of claim 23, further comprising calculating at least one of a target position or a target trajectory based at least in part on the output signal. 25. The target metrology method of claim 22, wherein imaging the target material includes using a target imager. 26. The target metrology method of claim 25, wherein the target imager includes a camera. 27. The target metrology method of claim 25, wherein the target imager comprises a CCD array. 28. The target metrology method of claim 22, wherein imaging the target material includes generating a shadowgraph. 29. The target metrology method of claim 22, wherein the illuminating laser radiation has a central wavelength of about 266 nm. 30. A target metrology method as described in clause 22, wherein illuminating a portion of the target material supply path with illumination laser radiation having a central wavelength of less than 400 nm is performed in part using a frequency-doubled Ti:sapphire laser arranged to receive the pump beam and emit illumination laser radiation at a central wavelength of about 380 nm. 31. The target metrology method of claim 30, further comprising pumping the Ti:sapphire laser with a diode-pumped solid-state (DPSS) laser arranged to generate a pump beam, the pump beam having a central wavelength of about 532 nm. 32. The target metrology method of claim 31, wherein the DPSS laser includes a Nd:YAG laser and a frequency multiplier arranged to receive the beam and generate the pump beam. 33. The target metrology method of claim 30, wherein the Ti:sapphire laser includes a dichroic optical element positioned to receive the pump beam, a Ti:sapphire crystal, and an output coupler. 34. The target metrology method of claim 30, wherein the Ti:sapphire laser includes a frequency doubling element arranged to receive from the output coupler output radiation having a central wavelength of about 760 nm and to doubling the frequency of the output coupler radiation to produce illumination laser radiation at a central wavelength of about 380 nm. 35. The target metrology method of claim 34, wherein the frequency multiplication element comprises a nonlinear optical element. 36. The target metrology method of claim 35, wherein the nonlinear optical element comprises a lithium borate crystal. 37. A target metrology method as described in clause 22, wherein illuminating a portion of the target material supply path with illumination laser radiation having a central wavelength of less than 400 nm is performed at least in part using a fiber beam delivery system for transporting the illumination laser radiation from a light source to the portion of the target material supply path. 38. The target metrology method of clause 37, wherein the fiber beam delivery system includes a multi-order fiber. 39. The target metrology method of claim 22, wherein the illuminating laser radiation is pulsed to have a pulse length in the range of 1 to 10 nanoseconds. 40. The target metrology method of claim 22, wherein the illuminating laser radiation has a linewidth of 4 nm or greater. 41. A target metrology method as described in clause 22, wherein illuminating a portion of the target material supply path with illumination laser radiation having a central wavelength of less than 400 nm includes using a Nd:YAG laser, and the illumination laser radiation is obtained as a harmonic of the Nd:YAG laser.

[0066]

[0070] These and other implementations are within the scope of the following claims.

Claims

1. 1. A target metrology system for an extreme ultraviolet light source, comprising: a light source emitting illumination laser radiation having a central wavelength of less than 400 nm and positioned to illuminate a portion of the target material supply path with said illumination laser radiation; a radiation receiver positioned to receive the illumination laser radiation after the illumination laser radiation illuminates the portion of the target material supply path; Target metrology systems, including:

2. The target metrology system of claim 1 , wherein the radiation receiver generates an output signal indicative of a position of a target in the portion of the target material supply path.

3. 3. The target metrology system of claim 2, further comprising a position detection feedback system disposed to receive the output signal of the radiation receiver and adapted to calculate at least one of a target position and a target trajectory based at least in part on the output signal.

4. The target metrology system of claim 1 , wherein the radiation receiver comprises a target imager.

5. The target metrology system of claim 4 , wherein the target imager comprises a camera.

6. The target metrology system of claim 4 , wherein the target imager comprises a CCD array.

7. The target metrology system of claim 4 , wherein the target imager generates a shadowgraph.

8. The target metrology system of claim 1 , wherein the light source is adapted to produce the illumination laser radiation to have a center wavelength of about 266 nm.

9. 10. The target metrology system of claim 1, wherein the light source comprises a frequency-doubled Ti:sapphire laser positioned to receive a pump beam and emit the illumination laser radiation at a center wavelength in the range of about 250 nm to about 450 nm.

10. 10. The target metrology system of claim 1, wherein the light source comprises a frequency-doubled Ti:sapphire laser positioned to receive a pump beam and to emit the illumination laser radiation at a center wavelength of about 380 nm.

11. 10. The target metrology system of claim 1, wherein the light source further comprises a diode-pumped solid-state (DPSS) laser positioned to generate the pump beam, the pump beam having a center wavelength of about 532 nm.

12. 12. The target metrology system of claim 11, wherein the DPSS laser includes a Nd:YAG laser and a frequency multiplier positioned to receive the beam and generate the pump beam.

13. 10. The target metrology system of claim 9, wherein the Ti:sapphire laser includes a dichroic optic positioned to receive the pump beam, a Ti:sapphire crystal, and an output coupler.

14. 10. The target metrology system of claim 9, wherein the Ti:sapphire laser includes a frequency doubling element positioned to receive from the output coupler output radiation having a center wavelength of about 760 nm and to doubling the frequency of the output coupler radiation to produce the illumination laser radiation at a center wavelength of about 380 nm and with a wavelength tuning range of 250 nm to 450 nm.

15. The target metrology system of claim 14 , wherein the frequency multiplication element comprises a nonlinear optical element.

16. 16. The target metrology system of claim 15, wherein the nonlinear optical element comprises a lithium borate crystal.

17. The target metrology system of claim 1 , further comprising a fiber beam delivery system positioned and adapted to convey the illuminating laser radiation from the light source to the portion of the target material supply path.

18. The target metrology system of claim 17 , wherein the fiber beam delivery system includes a multi-order fiber.

19. The target metrology system of claim 1 , wherein the light source is adapted to produce the illumination laser radiation to have a pulse length in the range of 1 to 10 nanoseconds.

20. The target metrology system of claim 1 , wherein the light source is adapted to produce the illumination laser radiation having a linewidth of 4 nm or greater.

21. The target metrology system of claim 1 , wherein the light source includes a Nd:YAG laser and the illumination laser radiation is derived as a harmonic of the Nd:YAG laser.

22. 1. A target metrology method for an extreme ultraviolet light source, comprising: Illuminating a portion of the target material supply path with illumination laser radiation having a central wavelength of less than 400 nm; receiving the illumination laser radiation as received radiation after the illumination laser radiation illuminates the portion of the target material supply passage; imaging target material present in the portion of the target material supply path based on the received illumination; A target metrology method comprising:

23. 23. The target metrology method of claim 22, wherein imaging target material comprises generating an output signal indicative of a position of the target material present in the portion of the target material supply path.

24. 24. The target metrology method of claim 23, further comprising calculating at least one of a target position or a target trajectory based at least in part on the output signal.

25. 23. The target metrology method of claim 22, wherein imaging the target material includes using a target imager.

26. 26. The target metrology method of claim 25, wherein the target imager comprises a camera.

27. 26. The target metrology method of claim 25, wherein the target imager comprises a CCD array.

28. 23. The target metrology method of claim 22, wherein imaging the target material comprises generating a shadowgraph.

29. 23. The target metrology method of claim 22, wherein the illuminating laser radiation has a central wavelength of about 266 nm.

30. 23. The target metrology method of claim 22, wherein illuminating a portion of the target material supply path with the illumination laser radiation having a central wavelength of less than 400 nm is performed in part using a frequency doubled Ti:sapphire laser arranged to receive a pump beam and emit illumination laser radiation at a central wavelength of about 380 nm.

31. 31. The target metrology method of claim 30, further comprising pumping the Ti:sapphire laser with a diode pumped solid state (DPSS) laser arranged to produce the pump beam, the pump beam having a center wavelength of about 532 nm.

32. 32. The target metrology method of claim 31, wherein the DPSS laser includes a Nd:YAG laser and a frequency multiplier positioned to receive the beam and generate the pump beam.

33. 31. The target metrology method of claim 30, wherein the Ti:sapphire laser includes a dichroic optic positioned to receive the pump beam, a Ti:sapphire crystal, and an output coupler.

34. 31. The target metrology method of claim 30, wherein the Ti:sapphire laser includes a frequency doubling element positioned to receive from the output coupler an output coupler radiation having a center wavelength of about 760 nm and to doubling the frequency of the output coupler radiation to produce the illumination laser radiation at a center wavelength of about 380 nm.

35. 35. The target metrology method of claim 34, wherein the frequency multiplication element comprises a nonlinear optical element.

36. 36. The target metrology method of claim 35, wherein the nonlinear optical element comprises a lithium borate crystal.

37. 23. The target metrology method of claim 22, wherein illuminating a portion of a target material supply path with the illumination laser radiation having a central wavelength of less than 400 nm is performed at least in part using a fiber beam delivery system to convey the illumination laser radiation from the light source to the portion of the target material supply path.

38. 38. The target metrology method of claim 37, wherein the fiber beam delivery system includes a multi-order fiber.

39. The target metrology method of claim 22, wherein the illuminating laser radiation is pulsed to have a pulse length in the range of 1 to 10 nanoseconds.

40. 23. The target metrology method of claim 22, wherein the illuminating laser radiation has a linewidth of 4 nm or greater.

41. 23. The target metrology method of claim 22, wherein illuminating a portion of a target material supply path with the illuminating laser radiation having a central wavelength of less than 400 nm includes using a Nd:YAG laser, the illuminating laser radiation being obtained as a harmonic of the Nd:YAG laser.

Images