System and method for optimizing target rarefaction for enhancing EUV radiation from a laser-produced plasma

Abstract

A method of operating an EUV radiation source includes generating a stream of droplets of target material. The method includes illuminating a droplet using a first laser pulse to adjust a shape of the droplet into a target. The method includes illuminating the target using a second laser pulse to rarefy the target. The method includes illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation. The method includes measuring an EUV performance metric as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse. The method includes determining a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measuring.

Description

SYSTEM AND METHOD FOR OPTIMIZING TARGET RAREFACTION FOR ENHANCING EUV RADIATION FROM A LASER-PRODUCED PLASMACROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 730,444, filed December 10, 2024, titled SYSTEM AND METHOD FOR OPTIMIZING TARGET RAREFACTION FOR ENHANCING EUV RADIATION FROM A LASER-PRODUCED PLASMA, which is incorporated herein by reference in its entirety.FIELD

[0002] The present disclosure relates to radiation sources for lithographic apparatuses.BACKGROUND

[0003] Illumination generated by a radiation source can be used by tools used for semiconductor manufacturing processes. Examples of such exposure apparatuses include a lithographic apparatus, a metrology or inspection apparatus (e.g., a mask inspection apparatus, an actinic mask inspection apparatus, or the like), among others.

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of radiation can determine the minimum size of features that are to be formed on the substrate, with smaller wavelengths allowing for smaller features. For example, a lithographic apparatus that uses extreme ultraviolet (EUV) radiation (having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm) can print smaller features on a substrate as compared to a lithographic apparatus that uses radiation with a wavelength of 193 nm. A drawback of operating at EUV wavelengths is that many optical materials can absorb EUV illumination, thereby attenuating EUV illumination intensity. Reflective optics can help mitigate attenuation. EUV lithography can implement one or more reflective surfaces to guide EUV radiation along an optical path.

[0005] A mask inspection apparatus (e.g., actinic mask inspection apparatus) is an apparatus that can be used to measure dimensions or detecting defects in masks or mask blanks. Mask blanks used in EUV lithography generally have a multilayer structure that functions as a Bragg reflector. Layers of the multilayer structure can be altematingly molybdenum and silicon. The projected pattern in a lithographic process can become deformed if a defect is present in the multilayer structure. Therefore, a mask inspection is an important, and often necessary, step of mass-production lithographicprocesses to check whether a defect is present in a mask. EUV mask inspection can be used for several purposes and in several different stages of lithographic fabrication.

[0006] Firstly, mask inspection can be used for the detection of phase defects that occur in mask blanks. Such phase defects can occur during the manufacturing of the multilayer stack of the mask blank. If undetected, phase defects are reproduced on all chips that correspond to the part of the mask containing the phase defects. Such phase defects can be correctly detected by using the same or similar actinic EUV wavelength as the lithography tool (e.g., 13.5 nm). Secondly, at a stage associated with quality control of EUV patterned masks, patterned mask inspection masks can be inspected. Mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Fourthly, the mask inspection can be used for optical proximity correction (OPC) evaluation or during a mask repair process to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0007] Hence, systems and methods to generate EUV illumination are an important aspect to lithographic tools used for semiconductor manufacturing processes. State of the art illumination systems have limitations that impede advancements in lithographic fabrication and next generation chip production. Improved illumination sources are desired, capable of generating stable EUV illumination at high brightness.SUMMARY

[0008] Embodiments of the present disclosure provides a system and method for a high power and stable plasma-produced EUV radiation source.

[0009] In some embodiments, a method of operating an EUV radiation source comprises generating a stream of droplets of target material. The method can also comprise illuminating a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target. The method can also comprise illuminating the target using a second laser pulse to rarefy the target. The method can also comprise illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation. The method can also comprise measuring an EUV performance metric as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse. The method can also comprise determining a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measuring.

[0010] In some embodiments, a non-transitory computer-readable medium stores a set of instructions that is executable by at least one processor of an EUV radiation source to cause the EUV radiation source to perform operations. The EUV source is configured to use stream of droplets of targetmaterial. The operations can comprise illuminating a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target. The operations can also comprise illuminating the target using a second laser pulse to rarefy the target. The operations can also comprise illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation. The operations can also comprise measuring an EUV performance metric as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse. The operations can also comprise determining a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measuring.

[0011] In some embodiments, a radiation source comprises a droplet generator, a first laser device, a second laser device, and a control system. The droplet generator can generate a stream of droplets of target material. The first laser device can illuminate a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target. The first laser device can also illuminate the target using a second laser pulse to rarefy the target. The second laser device can illuminate the rarefied target using a third laser pulse generate EUV radiation based on ionization of the rarefied target using the third laser pulse. The control system can include one or more processors to analyze measurements of an EUV performance metric of the radiation source as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse. The one or more processors can also determine a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measurements.

[0012] In some embodiments, a lithographic apparatus comprises a radiation source and a projection system. The radiation source can comprise a droplet generator, a first laser device, a second laser device, and a control system. The radiation source can generate a beam of EUV radiation to illuminate a pattern of a patterning device. The droplet generator can generate a stream of droplets of target material. The first laser device can illuminate a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target. The first laser device can also illuminate the target using a second laser pulse to rarefy the target. The second laser device can illuminate the rarefied target using a third laser pulse generate EUV radiation based on ionization of the rarefied target using the third laser pulse. The control system can include one or more processors to analyze measurements of an EUV performance metric of the radiation source as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse. The one or more processors can also determine a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measurements. The projection system can direct the beam to project an image of the pattern onto a substrate.BRIEF DESCRIPTION OF FIGURES

[0013] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0014] FIG. 1 shows an example lithographic system comprising a lithographic apparatus and a radiation source, consistent with embodiments of the present disclosure.

[0015] FIG. 2 shows an example timeline for generating EUV radiation in a laser-produced plasma source, consistent with embodiments of the present disclosure.

[0016] FIG. 3 shows a graph of to illustrate an example of variability of EUV performance with respect to recipe setpoint(s), consistent with embodiments of the present disclosure.

[0017] FIG. 4 shows a graph to illustrate an example of variability of EUV performance with respect to incident position of a laser on a target, consistent with embodiments of the present disclosure.

[0018] FIG. 5 shows a graph to illustrate an example of variability of EUV performance with respect to laser pulse duration, consistent with embodiments of the present disclosure.

[0019] FIG. 6 shows a flowchart of an example method for initializing and operating an EUV source, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0020] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent an exhaustive set of implementations. Instead, they are merely examples of apparatuses, systems, and methods consistent with aspects related to subject matter that may be recited in the appended claims. For example, although some embodiments are described in the context of EUV- based lithographic apparatuses, the present disclosure is not so limited. Unless infeasible, embodiments described herein can be implemented in any type of lithographic apparatus.

[0021] Electronic devices are constructed of circuits formed on a substrate. The substrate is typically of a semiconductor material (e.g., silicon) and is often referred to as a wafer by persons of skill in the art. Many circuits may be formed together on the same piece of silicon and are called integrated circuits or ICs. With advancements in technology, the size of these circuits has decreased dramatically so that many more of them can fit on the substrate. For example, an IC chip in a smart phone can be as small as a fingernail and yet may include over 2 billion transistors, the size of each transistor being less than 1 / 1, 000th the width of a human hair.

[0022] Yield is a metric that characterizes failure rate in device fabrication, which relates to cost and efficiency. Yield can be defined as a ratio of all the wafers that are produced by a fab to the number of wafers that were introduced to the fab. Or yield can be the number of working chips that successfully result from the device fabrication process performed on a wafer to the number of potential chips thatcan be fabricated from that wafer in the ideal case of zero failure. As some wafers or chips fail during fabrication, the overall yield is less than 100%. For example, to obtain a 75% yield for a 50-step process (where a step can be indicative of the number of layers formed on a wafer), each individual step should typically have a yield greater than 99.4%. In contrast, if each of the individual steps has a yield of 95%, the compounding errors at each step result in an overall process yield as low as 7-8%. Every wafer or chip lost during fabrication is a sunk cost and lost time for the fab.

[0023] Speed, or throughput, has been a traditionally important metric alongside yield. Throughput is a measurable quantity that characterizes the manufacture speed of a fab (e.g., number of IC units produced per unit time). Throughput has become even more important in view of recent global chip shortages. As there are multiple steps in the fabrication of a chip device (e.g., multiple steps for multiple layers), each step can have a characteristic throughput. For example, a throughput value can be assigned to how quickly a lithographic system can conduct an illumination optimization process. Innovations in the design or functions of source optimizers can increase throughput or resolve problems in another aspect while mitigating adverse impact to throughput.

[0024] To achieve high-yielding lithographic prints of device structures, small wavelength illumination (e.g., EUV wavelength) can be used to print structures that are smaller compared to lithography systems that use larger wavelengths (the smaller the wavelength, the smaller the features that can be fabricated). In a plasma-based EUV source, a plasma can be created by delivering a high power laser to a target material (e.g., a droplet of tin). The droplet is precisely manipulated in shape and density via a series of laser pulses. At the final stage of the droplet manipulation, a main laser pulse ionizes the target material, the volume of which emits radiation with a wavelength(s) in the EUV range. There is room for improvement regarding the droplet manipulation to maximize the brightness of the illumination generated. Reproducibility of the ionization process is also important, as it provides for a stable brightness. Instabilities can cause errors in lithographic projection, thereby reducing yield. Various embodiments described herein provide devices and methods for stably increasing the brightness of laser-produced plasma illumination sources material beyond the limits of current the state of the art.

[0025] Objects and advantages of the disclosure can be realized by the elements and combinations as set forth in embodiments described herein. However, embodiments of the present disclosure are not necessarily required to achieve such exemplary objects or advantages. Some embodiments can achieve a different feature or enhancement without necessarily achieving any expressly stated object or advantage.

[0026] As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component can comprise A or B, then, unless specifically stated otherwise or infeasible, the component can comprise A, or B, or A and B. As a second example, if it is stated that a component can comprise A, B, or C, then, unlessspecifically stated otherwise or infeasible, the component can comprise A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0027] Relative dimensions of components in drawings may be exaggerated for clarity. Within the following description of drawings, the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described.

[0028] The term “patterning device” can be considered synonymous with similar terms of art, such as “reticle” or “mask.” The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a pattern on a cross section of a radiation beam. The radiation beam then can recreate the pattern in a target portion of a substrate.

[0029] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic, or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein can be considered as synonymous with the more general term “projection system.”

[0030] The terms “rarefaction,” “rarefication,” “rarefied,” “disperse,” or the like, can be used interchangeably in the present disclosure when referring to effects where material is dispersed, made thin, or made less dense. For example, the term “rarefaction pulse” can alternatively be referred to as “rarefication pulse,” “dispersal pulse,” “density-reducing pulse,” or the like.

[0031] Illumination can be understood to be a form of radiation. Hence, the terms “radiation” and “illumination” can be used herein interchangeably.

[0032] FIG. 1 shows an example lithographic system comprising a radiation source SO and a lithographic apparatus LA, consistent with embodiments of the present disclosure. The radiation source SO can generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA can comprise an illumination system IL, a support structure MT, a projection system PS, and a substrate table WT. Support structure MT can support a patterning device MA (e.g., a mask). Substrate table WT can support a substrate W.

[0033] Illumination system IL can condition EUV radiation beam B before EUV radiation beam B is incident upon the patterning device MA. Illumination system IL can comprise a facetted field mirror device 10 and a facetted pupil mirror device 11. Faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. Illumination system IL can comprise other mirrors or devices in addition to, or instead of, faceted field mirror device 10 and faceted pupil mirror device 11.

[0034] After being conditioned, EUV radiation beam B can interact with patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project patterned EUV radiation beam B’ onto substrate W. Projection system PS cancomprise a plurality of mirrors (e.g., mirrors 13 and 14) that project patterned EUV radiation beam B’ onto substrate W held by substrate table WT. Projection system PS can apply a reduction factor to patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on patterning device MA. For example, a reduction factor of 4 or 8 can be applied. Although projection system PS is illustrated as having two mirrors 13 and 14 in FIG. 1, projection system PS can include a different number of mirrors (e.g., six or eight mirrors).

[0035] Substrate W can include previously formed patterns. Where this is the case, lithographic apparatus LA can align the image formed by patterned EUV radiation beam B’ with the pattern that was previously formed on substrate W.

[0036] In some embodiments, a relative vacuum at a pressure below atmospheric pressure is provided in radiation source SO, in illumination system IL, or in projection system PS. The rarified gas can allow one or more features. For example, a small amount of gas (e.g. hydrogen) allows for better management of tin -based contamination in tin-based EUV sources.

[0037] Lithographic apparatus LA and radiation source SO described herein can be used for performing patterning process for a circuit layout. A circuit layout patterning method can comprise receiving a substrate with a photoresist layer. The method can also comprise directing EUV radiation from a radiation source to the photoresist layer to form a patterned photoresist layer. The method can also comprise developing and etching the patterned photoresist layer to form the circuit layout.

[0038] Radiation source SO can be a laser produced plasma (LPP) source. A laser system 1 (e.g., a CO2 laser) can be arranged to deposit energy via a laser beam 2 into a target material (sometimes referred to as a fuel). Droplet generator 3 can provide the target material in the form of liquid droplets. The target material can be tin (Sn). Although tin is referred to in the following description, any material having an emission spectrum suitable for forming radiation beam B can be used. The target material may, for example, be in liquid form, and may, for example, be a metal or alloy. Droplet generator 3 can comprise a nozzle configured to direct droplets of the target material along a trajectory towards a plasma formation region 4 (laser is focused here, hence it is sometimes referred to as primary focus (PF)). Laser beam 2 can be incident upon the droplets at plasma formation region 4. The deposition of laser energy into the target material can create a plasma 7 at plasma formation region 4. Radiation (e.g., EUV radiation) can be emitted from plasma 7 during de-excitation and recombination of electrons with ions of plasma 7.

[0039] The EUV radiation from plasma 7 can be collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector (sometimes referred to more generally as a normal -incidence radiation collector). Collector 5 can comprise a multilayer mirror structure arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). Collector 5 can have an ellipsoidal configuration having two focal points. A first one of the focal points can be at the plasma formation region 4. A second one of the focal points can be at an intermediate focus 6.

[0040] Laser system 1 can be spatially separated from radiation source SO. Where this is the case, laser beam 2 can be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors or a beam expander, or other optics. Laser system 1, radiation source SO, and the beam delivery system can together be considered to be a radiation system.

[0041] Radiation that is reflected by collector 5 can form EUV radiation beam B. EUV radiation beam B can be focused at intermediate focus 6 to form an image at intermediate focus 6 of plasma 7 at plasma formation region 4. The image at intermediate focus 6 can act as a virtual radiation source for illumination system IL. The radiation source SO can be arranged such that intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of radiation source SO.

[0042] Lithographic apparatus LA can also comprise a control system 15, detector 16, and detector 17. Control system 15 can comprise one or more processors and circuitry to control various functions of lithographic apparatus LA (e.g., feedback signals, droplet generation of droplet generator 3, laser pulse generation of laser system 1, or the like). A processor can comprise a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), an optical processor, programmable logic controllers, a microcontroller, a microprocessor, a digital signal processor, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic (GAL), a Complex Programmable Logic Device (CPLD), a Field-Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), any type circuitry capable of data processing, or any combination of any number thereof. The processor can be a virtual processor. The virtual processor can include one or more processors distributed across multiple machines or devices coupled via a network.

[0043] Detector 16 can be disposed past intermediate focus 6 (downstream) for monitoring one or more properties of radiation beam B (e.g., monitor brightness, wavelength, pulse characteristics, or the like). Detector 17 can be disposed to have line of sight to plasma region 4 (e.g., proximal to a lip of collector 5). Detector 17 can be used for monitoring one or more properties of the plasma generated at plasma region 4 (e.g., monitor plasma volume, plasma shape, radiation brightness, radiation wavelength, pulse characteristics, or the like). Detectors 16 or 17 can comprise one or more a suitable photon-sensitive devices according to the properties being monitored (e.g., an image capture device such as a camera, a single-pixel sensor such as a photodiode, or the like).

[0044] FIG. 2 shows an example timeline 200 for generating EUV radiation in a LPP source, consistent with embodiments of the present disclosure. In some embodiments, a graph 202 represents a plot of laser pulse power as a function of time. Arbitrary units are used.

[0045] Droplet generator 3 (FIG. 1) can generate a stream of droplets target material (e.g., liquid tin). With precision timing, each droplet can be ionized as they arrive at plasma formation region 4 (FIG. 1). Consequently, EUV radiation can be generated in pulses (e.g., tens of thousands of pulses per second; embodiment are also applicable to faster or slower repetition rates). It follows that the pulserepetition rate of source SO (FIG. 1) is based on the repetition rate or frequency of droplets in the droplet stream. To generate efficient EUV -emitting plasma from the droplets, it is desirable to create ideal droplet density and shape (or as close to ideal as possible). Just as important, it is desirable to generate a stable EUV-emitting plasma by ensuring that the ideal droplet density and shape is repeatable (e.g., repeatable for each pulse at tens of thousands of pulses per second). Instabilities in EUV-emission can cause irregular dose to downstream lithographic processes, thereby creating errors in pattern transfers, reducing yield. To achieve consistent high-brightness EUV emission from each droplet, a three-laser-pulse method can be used, as illustrated by timeline 200. Timeline 200 shows an example using three laser pulses. Additional pulses can also be used.

[0046] Laser pulses can be supplied by laser system 1 (FIG. 1). A droplet 204 of the droplet stream generated by droplet generator 3 (FIG. 1) can be illuminated by a series of pulses. The series can start with a relatively low energy preliminary pulse of light, called a pre-pulse (PP), from laser system 1 (FIG. 1) as droplet 204 arrives at plasma formation region 4 (FIG. 1). The laser pre-pulse (e.g., first laser pulse) is represented as pre-pulse 212 in FIG. 2. Droplet 204 can absorb the energy of pre-pulse 212, thereby imparting kinetic energy (e.g., heat) on the atoms in droplet 204. Droplet 204, being in a liquid state, can deform based on the energy received from pre-pulse 212. Generally, the speed and shape of target material deformation can depend on properties of the incident laser pulse, such as wavelength (affecting light-matter coupling), laser energy or intensity, pulse duration, or the like. These factors can be relevant not only to pre-pulse 212 but also to one or more of the subsequent laser pulses.

[0047] Pre-pulse 212 can define an initial step in the process of droplet manipulation. At this step, droplet 204 can be flattened and expanded into a thin disc or pancake shape, herein referred to as target 208. The time period between pre-pulse 212 to a next pulse can be referred to as a period of target expansion (or simply “expansion”). The expansion time is denoted as TE. Partially expanded target 206 represents a conceptual snapshot of an intermediate stage as droplet 204 is being transformed into a disc shape.

[0048] The process of selecting the parameter values for pre-pulse 212 (or any of the other laser pulses) can be complex, as fluid dynamics and molecular forces (e.g., van der Waals forces) can govern droplet deformation to first order. If the energy of pre-pulse 212 is high, an undesirable amount of material is ablated from droplet 204, resulting in reduced target material for EUV production and adverse impact to EUV power and stability. On the other hand, if the energy is too low, it takes a longer time to complete target expansion and the unstable fluid dynamics of the expanding target can result in uneven mass distribution. Uneven target material distribution, in particular stray “satellite” micro-droplets, can adversely impact the absorption of the next pulse, thereby adversely impacting the repeatability and efficiency of EUV generated from the droplet (e.g., introducing instability of illumination dosage during lithographic processes). In another example, selecting a different wavelength can change the light-matter interaction, resulting in a differentabsorption regime. In FIG. 2, pre-pulse 212 is illustrated as having lower pulse energy compared to other pulses, indicating selection of a wavelength for pre-pulse pulse 212 (e.g., about 1 pm) that is more readily absorbed, thereby needing fewer photons to achieve a desired deformation speed of droplet 204.

[0049] In some embodiments, arriving at the disc shape of target 208 marks the end of expansion time TE. At this instant, target 208 can be illuminated using a rarefaction pulse (RP) from laser system 1 (FIG. 1) to rarefy target 208. The rarefaction laser pulse (e.g., second laser pulse) is represented as rarefaction pulse 214 in FIG. 2. Described differently, the expansion time TE is measured from a time instance of pre-pulse 212 to a time instance of rarefaction pulse 214. The chosen parameters of prepulse 212 can affect a choice of when rarefaction pulse 214 should be fired. It is desirable to impinge rarefaction pulse 214 on target 208 when the disc shape is optimal, with a spatial distribution that results in the highest value of conversion efficiency or EUV output energy. The identification of this optimal shape is affected by a variety of factors.

[0050] rarefaction pulse 214 can affect a next step in the process of droplet manipulation. At this step, target 208 can be rarefied into an expanded volume (e.g., going from an approximately two- dimensional disc to a three-dimensional volume), herein referred to as rarefied target 210. The time period between rarefaction pulse 214 to a next pulse can be referred to as a period of target rarefaction (or simply “rarefaction”). The rarefaction time is denoted as TR. Rarefied target 210 achieving an optimal volume and density distribution can mark the end of rarefaction time TR. At this instant, rarefied target 210 can be illuminated using a high energy main pulse (MP) from laser system 1 (FIG. 1). The main laser pulse (e.g., third laser pulse) is represented as main pulse 216 in FIG. 2. main pulse 216 can inject a large amount of energy into rarefied target 210, thereby heating rarefied target 210 to the point of ionization (plasma state). What ensues is a period of charge recombination and EUV emission. The wavelength spectrum of the emitted radiation by the plasma can depend on the material of rarefied target 210. For example, liquid tin can be ionized for producing radiation at a wavelength 13.5 nm (or one or more EUV wavelengths), useful for EUV-based lithographic processes.

[0051] In some embodiments, laser system 1 (FIG. 1) comprises three or more laser devices. A first laser device can be considered as independent from a second laser device (e.g., having distinct laser amplifier units). A third laser device (having its own independent laser amplifier) can be considered as independent from the first and second laser devices. The first laser device can generate pre-pulse 212. The second laser device can generate rarefaction pulse 214. The third laser device can generate MP pulse 216. This setup can help mitigate issues of using a common laser amplifier for all three pulses, such as undesirably reducing the energy of one pulse when attempting to increase the energy of another pulse. In other configurations, more or fewer than three lasers can be involved. For example, a setup can use two lasers, with one providing the high-energy main pulses and another providing the pre-pulses and rarefaction pulses. The laser device providing pre-pulses and therarefaction pulses can comprise a first component laser to generate pre -pulse 212 and a second component laser to generate rarefaction pulse 214.

[0052] An object of the present disclosure is to provide optimal laser parameters for the target rarefaction process to rarefy target 208 such that the EUV output is maximized. There are several quantifiable metrics of interest that are useful for gauging EUV production performance. Some metrics can be related to the plasma volume at plasma formation region 4 and others can be related to the radiation beam B as it emerges from the plasma or after it emerges from an aperture at the intermediate focus (between radiation source SO and lithographic apparatus LA), which can be quantified via detectors 16 or 17 (FIG. 1). Other metrics can be related to the EUV radiation after intermediate focus 6 (FIG. 1) that is used for lithographic patterning, which can be quantified via detector 16 (FIG. 1). Using detector 16 (FIG. 1), metrics such as EUV conversion efficiency and EUV power after intermediate focus can be quantified.

[0053] Performance metrics can include, for example, EUV power, brightness, and conversion efficiency. In one example, a given recipe for rarefaction pulse 214 and main pulse 216 can cause underperformance of a measured EUV conversion efficiency and EUV power after intermediate focus. A recipe can define a set of parameter values (setpoints) used for the execution of a process, such as initiating pre-pulse 212 (e.g., recipe can inform laser system 1 (FIG. 1) to use 1 pm wavelength, a given power level, a given pulse timing, or the like). It can be challenging and impractical to systematically map out the recipe parameter, particularly when lacking a good understanding of the physics at a given laser wavelength (e.g., 1 pm pre-pulse wavelength). An additional complexity is present in the interaction between ionization degree and volume expansion speed (rarefaction speed). Rarefaction speed can increase as a function of increasing ionization degree that occurs from the energy of rarefaction pulse 214 (FIG. 2), but not necessarily increase EUV performance at very high energies.

[0054] Hence, embodiments of the present disclosure implement co-optimization of different recipe parameters (e.g., control knobs or control sliders or variables in a program executed by a processor) to arrive at optimal setpoints. The co-optimization allows higher EUV conversion efficiency at primary focus (as created at plasma formation region 4 (FIG. 1)) and higher EUV power after intermediate focus (as transmitted from radiation source SO lithographic into apparatus LA). Parameter optimization and recipe setpoints disclosed herein are based on assessment of plasma physics of the target material and confirmation via experimental or simulation results.

[0055] FIG. 3 shows a graph 300 for illustrating an example of variability of EUV performance with respect to recipe setpoint(s), consistent with embodiments of the present disclosure. In some embodiments, graph 300 is a contour plot. The description of graph 300 will reference elements shown in at least FIG. 2. The left vertical axis represents the rarefaction time TR in arbitrary time units. The rarefaction time TR is an “adjustable knob” parameter in that it is a value that can be adjusted through control programs as an input to explore the consequences on one or moreperformance outputs. Based on the resulting outputs, a value of the rarefaction time TR can be selected or controlled as a recipe setpoint. The horizontal axis represents the energy of rarefaction pulse 214 in arbitrary energy units (a second adjustable knob parameter controllable via recipe setpoint). The magnitude scale on the right represents an EUV performance metric (e.g., brightness / energy, power, intensity of an EUV pulse measured after intermediate focus using detector 16 (FIG. 1)).

[0056] To understand the measurement shown in graph 300, it is instructive to begin at the lower bound, where the RP energy and the rarefaction time both have small values. A zero-energy setting for rarefaction pulse 214 would correspond to zero rarefaction. Rarefaction is important for optimal absorption of main pulse 216, as an appropriate volumetric distribution of tin mass in rarefied target 210 can act as an index matching medium for improved absorption. As a consequence, zero rarefaction can result in poor absorption of main pulse 216, thereby yielding a very low value for the measured EUV performance metric. Similarly, a zero-time setting for rarefaction time TR can correspond to zero rarefaction, also resulting in a very low value for the measured EUV performance metric. As the energy of RP pulse 214 or rarefaction time TR are increased from zero, it is expected that the measured EUV performance metric will increase. However, the performance does not increase indefinitely with these parameters. For example, an excessive rarefaction time TR can lead to a rarefied target 210 with an inappropriate density for interaction with main pulse 216, or may lead to an overly large rarefied target 210 with an image at the intermediate focus 6 that is clipped (partially obstructed) by the aperture between radiation source SO and lithographic apparatus LA.

[0057] A technical significance of the setpoint parameters represented in graph 300 is that the increase in EUV performance is not a straightforward relationship with respect to the energy of RP pulse 214 or rarefaction time TR. The EUV performance metric can increase, as well as decrease when either of the energy of RP pulse 214 or rarefaction time TR are increased. An optimum value for the setpoints is difficult to determine and requires significant study and measurement (e.g., using detector 16 (FIG. 1)). The interplay between the energy of RP pulse 214 and rarefaction time TR can be quantified through careful analyses and experimentation, as described in embodiments herein. The quantification allows for greater confidence of selected setpoints and predicts EUV performance changes in the event that a setpoint is to be changed for any reason (e.g., a tradeoff can be quantified). In one example of parameter interplay, a first value of rarefaction time TR can be associated with maximum EUV performance at a first value for the energy of RP pulse 214. However, when the energy of RP pulse 214 is adjusted to a different second value, the first value of rarefaction time TR may no longer result in a maximum EUV performance. The black dashed line 302 tracks the shift of the optimal rarefaction time TR as a function of the RP energy.

[0058] Hence, in some embodiments, the output of an EUV radiation source is optimized by measuring an EUV performance metric as a function of both a first adjustable parameter and a second adjustable parameter of the EUV radiation source, the first adjustable parameter being a time delay between the second and third laser pulses (e.g., rarefaction time TR) and the second adjustableparameter is an energy of the second laser pulse (e.g., RP pulse 214). The measurements of the EUV performance metric can be analyzed (e.g., using control system 15 (FIG. 1)) to determine values of the first and second adjustable parameters associated with a maximum value of the EUV performance metric. Based on the analysis, a time setpoint for the time delay between the second and third laser pulses and an energy setpoint for the energy of the second laser pulse can be selected, thereby co optimizing the two adjustable parameters for optimal EUV output.

[0059] Based on the co-optimization testing, EUV output can be at an optimum when using a rarefaction time TR of about 20 ns, 30 ns, 50 ns, 70 ns, 90 ns, 100 ns, 120 ns, 150 ns, 175 ns, or 200 ns, or in ranges therebetween, or in the range of 20 to 200 ns, 30 to 180 ns, or 50 to 150 ns, or the like, along with a RP pulse energy of about 2 mJ, 3 mJ, 4 mJ, 5 mJ, 7.5 mJ, 10 mJ, or in ranges therebetween or in the range of 2.0 to 10 mJ, 2.0 to 4.5 mJ, 2.5 to 4.0 mJ, or 3.0 to 3.5 mJ, or the like. RP pulse energy can also depend on RP beam size and shape, which is described further below.

[0060] In some embodiments, there are several adjustable knob parameters that can affect EUV output. Each knob parameter can be considered a degree of freedom. An object of the present disclosure is to optimize each adjustable knob parameter (that is, to identify values for the knob parameters that results in a maximum EUV output). Examples of adjustable knob parameters can be as follows. The first and second knob parameters can be the two discussed above with respect to graph 300. They can be treated as coupled parameters that are optimized via two-dimensional (2D) co-optimization. A third knob parameter can be the aim of rarefaction pulse 214 with respect to an x- position of target 208 (the x-direction is the direction of travel of the droplet stream, perpendicular to a z-direction defined by the propagation direction of rarefaction pulse 214). Alternatively, or in addition, the third knob parameter can be a timing of rarefaction pulse 214 relative to the timing of arrival of a droplet at the plasma formation region 4. A fourth knob parameter can be the aim of rarefaction pulse 214 with respect to a y-position of target 208 (the y-direction is perpendicular to x- direction and the z-direction). The third and fourth knobs can be treated as coupled parameters that are optimized via two-dimensional (2D) co-optimization. A fifth knob parameter can be the duration of rarefaction pulse 214. A sixth knob parameter can be the spot size of rarefaction pulse 214.

[0061] FIG. 4 shows a graph 400 for illustrating an example of variability of EUV performance with respect to the third and fourth knob parameters for the incident position of rarefaction pulse 214 on a target 402, consistent with embodiments of the present disclosure. The description of graph 400 will reference elements shown in at least FIG. 2. In some embodiments, target 402 represents a different view of target 208. The direction of rarefaction pulse 214 is perpendicular to the page (perpendicular to the x and y coordinate axes drawn on target 402). Target 402 can have a disc shape with a center 404 (e.g., the origin of the x and y coordinate axes).

[0062] Graph 400 is a contour plot. The left vertical axis of graph 400 represents a y-position of rarefaction pulse 214 as incident on target 402. The horizontal axis of graph 400 represents an x- position of rarefaction pulse 214 as incident on target 402. The laser x and y-positions are adjustablevia setpoints. The magnitude scale of graph 400 represents an EUV performance metric (e.g., measured after intermediate focus using detector 16 (FIG. 1)). Similar to the co -optimization described in reference to FIG. 3, EUV performance as a function of the third and fourth knobs can be quantified with the aid of detector 16 (FIG. 1)). In graph 400, region 406 represents the highest EUV performance. Region 406 is approximately coincident with center 404, the zeroth x-position and y- position on the disc. When setting up a new EUV source, or when setting up an EUV source after a refurbishment or other downtime, a two-dimensional scan such as depicted in graph 400 can be used to identify the operating parameters (e.g., mechanical tunings, electrical signals, software variables, or other control parameters) that can result in optimal initial conditions for operating the source.

[0063] FIG. 5 shows a graph 500 for illustrating an example of variability of EUV performance with respect to the fifth knob parameter for the duration of rarefaction pulse 214 (FIG. 2), consistent with embodiments of the present disclosure. In some embodiments, how the rarefaction pulse energy is distributed in time (temporal shape) can affect the EUV performance. The description of graph 500 will reference elements shown in at least FIG. 2. The vertical axis of graph 500 represents an EUV performance metric (e.g., measured after intermediate focus using detector 16 (FIG. 1)). The horizontal axis of graph 500 represents the energy of rarefaction pulse 214 in arbitrary energy units (adjustable knob parameter controllable via recipe setpoint). The plot lines shown in graph 500 were measured at various durations of rarefaction pulse 214. The duration can correspond to a full width at half maximum (FWHM) of pulse uptime. The solid plot corresponds to a pulse duration of 7.30 ns. The dashed plot corresponds to a pulse duration of 9.45 ns. The dotted plot corresponds to a pulse duration of 13.10 ns. The dash-dot-dot plot corresponds to a pulse duration of 17.45 ns.

[0064] In some embodiments, ionization of rarefied target 210 is a non-linear behavior with an ionization threshold. As the energy of rarefaction pulse 214 is increased, EUV performance experiences a sudden increase when the ionization threshold is met. Graph 500 shows that the ionization threshold is not fixed, but is a function of pulse duration. Shorter pulse duration can shift the ionization threshold to lower energy values.

[0065] Graph 500 also shows that over-ionization is also present and undesirable. As the pulse energies are increased into the regime of over-ionization (regimes that shift as a function of pulse duration), the EUV output is reduced rather than increased. Shorter RP pulses are technically significant, as EUV performance can be increased, as well as reducing the energy consumption by the second laser device of laser system 1 (FIG. 1). Hence, the setpoint for pulse duration of rarefaction pulse 214 can be about 1 ns, 2 ns, 3 ns, 5 ns, 6 ns, 7 ns, 10 ns, 12 ns, 15 ns, 17 ns, or 20 ns, or in ranges therebetween or in the range of 1 to 20 ns, 1 to 10 ns, 5 to 10 ns, or 7 to 10 ns, or the like.

[0066] In some embodiments, the size of beam spot, of rarefaction pulse 214 at target 208, can be optimized. This is the sixth adjustable knob parameter. The EUV performance metric can be measured after intermediate focus using detector 16 (FIG. 1)) for various setpoints of spot size (e.g. diameter of cross-section of the beam spot). Based on the measurements, it was found that beam spotsizes of about 400 un, 500 pun, 600 pun, 750 pun, or 1000 pun, or in ranges therebetween, or in the range of 400 to 1000 pun, 500 to 1000 pun, or 500 to 750 pun, or the like, produced similar peak EUV performance. The selected beam spot size can influence the setpoint choice for RP pulse energy. For example, an energy up to 10m J can be used when using a large beam spot size.

[0067] In some embodiments, to generate RP pulse 214 (FIG. 2), the second laser device of laser system 1 (FIG. 1) can be an ultrafast laser (e.g., a picosecond laser, a femtosecond laser, or the like).

[0068] FIG. 6 shows an example method 600 for initializing and operating an EUV source, consistent with embodiments of the present disclosure. The method can be executed using devices and functions described in reference to FIGS. 1-5, such as control system 15 of FIG. 1.

[0069] In some embodiments, at operation 602, a stream of droplets of target material is generated using droplet generator 3 (FIG. 1). The droplets are aimed at a primary focus or plasma formation region 4 (FIG. 1) for subsequent irradiation by multiple lasers.

[0070] At operation 604, a droplet of the stream of droplets can be illuminated using a first laser pulse (pre-pulse 212 (FIG. 2)). The first laser pulse can be used to adjust a shape of droplet 204 into a disc-shaped target 208 (FIG. 2)).

[0071] At operation 606, the target can be illuminated using a second laser pulse (e.g., rarefaction pulse 214 (FIG. 2)). The second laser pulse can be used to rarefy the target to produce a rarefied target (e.g., rarefied target 210 (FIG. 2)).

[0072] At operation 608, the rarefied target can be illuminated using a third laser pulse (e.g., main pulse 216 (FIG. 2)). The third laser pulse can be used to ionize the rarefied target to generate EUV radiation.

[0073] At operation 610, an EUV performance metric can be measured as a function of a time delay between the second and third laser pulses (e.g., rarefaction time TR (FIG. 2)) and as a function of an energy of the second laser pulse.

[0074] At operation 612, a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse are determined based on a maximum value of the EUV performance metric determined from the measuring. This determination can involve a repetition of operations 602, 604, 606, 608, and 610 with varying values of the time between the second and third laser pulses and the energy of the second laser pulse.

[0075] Embodiments described herein can comprise other operations based on the devices and functions described in reference to FIGS. 1-5. For example, the method can further comprise selecting the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the EUV radiation source.

[0076] Embodiments of the present disclosure exploit low temperature plasma transport physics to engineer the ionization degree and volumetric tin distribution of rarefied target 210 (FIG. 2) to enhance absorption of main pulse 216 (FIG. 2) and control EUV emission volume.

[0077] A non-transitory computer-readable medium may be provided that stores instructions for a processor of a controller (e.g., control system 15 in FIG. 1) for controlling various elements of a lithographic apparatus for initializing and operating an EUV source (e.g. provided in FIG. 6), consistent with embodiments in the present disclosure. For example, the instructions stored in the non-transitory computer-readable medium may be executed by the circuitry of the controller for performing method 600 in part or entirely. Common forms of non-transitory media include, for example, a floppy disk, a flexible disk, hard disk, solid-state drive, magnetic tape, or any other magnetic data storage medium, a Compact Disc Read-Only Memory (CD-ROM), any other optical data storage medium, any physical medium with patterns of holes, a Random Access Memory (RAM), a Programmable Read-Only Memory (PROM), and Erasable Programmable Read-Only Memory (EPROM), a FLASH-EPROM or any other flash memory, Non-Volatile Random Access Memory (NVRAM), a cache, a register, any other memory chip or cartridge, and networked versions of the same.

[0078] Some embodiments may further be described using the following clauses:1. A method of operating an extreme ultraviolet (EUV) radiation source, comprising: generating a stream of droplets of target material; illuminating a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target; illuminating the target using a second laser pulse to rarefy the target; illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation; measuring an EUV performance metric as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse; and determining a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measuring.2. The method of clause 1, further comprising selecting the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the EUV radiation source.3. The method of clause 2, wherein selecting the value for the time between the second and third laser pulses sets the time between the second and third laser pulses to between 50 and 150 ns.4. The method of any one of clauses 2 or 3, wherein selecting the value for the energy of the second laser sets the energy of the second laser pulse to between 2.5 and 10 mJ.5. The method of any one of clauses 1 to 4, wherein a beam spot of the second laser pulse, at the target, has a diameter between 400 and 1000 pm.6. The method of any one of clauses 1 to 5, wherein a beam spot of the second laser pulse, at the target, has a diameter between 500 and 750 pm.7. The method of any one of clauses 1 to 6, wherein a duration of the second laser pulse is between 1 and 20 ns.8. A non-transitory computer-readable medium that stores a set of instructions that is executable by at least one processor of an extreme ultraviolet (EUV) radiation source configured to use a stream of droplets of target material to cause the EUV radiation source to perform operations, the operations comprising: illuminating a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target; illuminating the target using a second laser pulse to rarefy the target; illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation; and measuring an EUV performance metric as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse; and determining a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measuring.9. The non-transitory computer-readable medium of clause 8, wherein the operations further comprise selecting the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the EUV radiation source.10. The non-transitory computer-readable medium of clause 9, wherein selecting the value for the time between the second and third laser pulses sets the time between the second and third laser pulses to between 50 and 150 ns.11. The non-transitory computer-readable medium of any one of clauses 9 or 10, wherein selecting the value for the energy of the second laser sets the energy of the second laser pulse to between 2.5 to 10 mJ.12. The non-transitory computer-readable medium of any one of clauses 8 to 11, wherein a beam spot of the second laser pulse, at the target, has a diameter between 400 and 1000 pm.13. The non-transitory computer-readable medium of any one of clauses 8 to 12, wherein a beam spot of the second laser pulse, at the target, has a diameter between 500 and 750 pm.14. The non-transitory computer-readable medium of any one of clauses 8 to 13, wherein a duration of the second laser pulse is between 1 to 20 ns.15. A radiation source, comprising: a droplet generator configured to generate a stream of droplets of target material; a first laser device configured to illuminate a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target and to illuminate the target using a second laser pulse to rarefy the target; anda second laser device configured to illuminate the rarefied target using a third laser pulse to generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target; and a control system having one or more processors and configured to: analyze measurements of an EUV performance metric of the radiation source as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse; and determine a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measurements.16. The radiation source of clause 15, wherein the first laser device comprises: a first component laser configured to generate the first laser pulse; and a second component laser configured to generate the second laser pulse.17. The radiation source of clause 15 or 16, wherein the controller is further configured to select the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the radiation source.18. The radiation source of clause 17, wherein the controller is further configured to set the time between the second and third laser pulses to between 50 and 150 ns.19. The radiation source of any one of clauses 17 or 18, wherein the controller is further configured to set the energy of the second laser pulse to between 2.5 and 10 mJ.20. The radiation source of any one of clauses 16 to 19, wherein a beam spot of the second laser pulse, at the target, has a diameter between 400 to 1000 pm.21. The radiation source of any one of clauses 16 to 20, wherein a beam spot of the second laser pulse, at the target, has a diameter between 500 and 750 pm.22. The radiation source of any one of clauses 16 to 21, wherein a duration of the second laser pulse is between 1 and 20 ns.23. A lithographic apparatus, comprising: a radiation source configured to generate a beam of extreme ultraviolet (EUV) radiation to illuminate a pattern of a patterning device, the radiation source comprising: a droplet generator configured to generate a stream of droplets of target material; a first laser device configured to illuminate a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target and to illuminate the target using a second laser pulse to rarefy the target; a second laser device configured to illuminate the rarefied target using a third laser pulse to generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target; and a control system having one or more processors and configured to:analyze measurements of an EUV performance metric of the radiation source as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse; and determine a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measurements; and a projection system configured to direct the beam to project an image of the pattern onto a substrate.24. The lithographic apparatus of clause 23, wherein the first laser device comprises: a first component laser configured to generate the first laser pulse; and a second component laser configured to generate the second laser pulse.25. The lithographic apparatus of clause 23 or 24, wherein the controller is further configured to select the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the radiation source.26. The lithographic apparatus of clause 25, wherein the controller is further configured to set the time between the second and third laser pulses to between 50 and 150 ns.27. The lithographic apparatus of any one of clauses 25 or 26, wherein the controller is further configured to set the energy of the second laser pulse to between 2.5 and 10 mJ.28. The lithographic apparatus of any one of clauses 23 to 27, wherein a beam spot of the second laser pulse, at the target, has a diameter between 400 to 1000 pm.29. The lithographic apparatus of any one of clauses 23 to 28, wherein a beam spot of the second laser pulse, at the target, has a diameter between 500 and 750 pm.30. The lithographic apparatus of any one of clauses 23 to 29, wherein a duration of the second laser pulse is between 1 and 20ns.

[0079] It will be appreciated that the embodiments of the present disclosure are not limited to the exact construction that has been described above and illustrated in the accompanying drawings and that various modifications and changes may be made without departing from the scope thereof.

Claims

CLAIMS1. A method of operating an extreme ultraviolet (EUV) radiation source, comprising: generating a stream of droplets of target material; illuminating a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target; illuminating the target using a second laser pulse to rarefy the target; illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation; measuring an EUV performance metric as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse; and determining a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measuring.

2. The method of claim 1, further comprising selecting the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the EUV radiation source.

3. The method of claim 2, wherein selecting the value for the time between the second and third laser pulses sets the time between the second and third laser pulses to between 50 and 150 ns.

4. The method of claim 2, wherein selecting the value for the energy of the second laser sets the energy of the second laser pulse to between 2.5 and 10 mJ.

5. The method of claim 1, wherein a beam spot of the second laser pulse, at the target, has a diameter between 400 and 1000 pm.

6. A non-transitory computer-readable medium that stores a set of instructions that is executable by at least one processor of an extreme ultraviolet (EUV) radiation source configured to use a stream of droplets of target material to cause the EUV radiation source to perform operations, the operations comprising: illuminating a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target; illuminating the target using a second laser pulse to rarefy the target; illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation; andmeasuring an EUV performance metric as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse; and determining a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measuring.

7. The non-transitory computer-readable medium of claim 6, wherein the operations further comprise selecting the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the EUV radiation source.

8. The non-transitory computer-readable medium of claim 7, wherein selecting the value for the time between the second and third laser pulses sets the time between the second and third laser pulses to between 50 and 150 ns.

9. The non-transitory computer-readable medium of claim 6, wherein a beam spot of the second laser pulse, at the target, has a diameter between 500 and 750 pm.

10. The non-transitory computer-readable medium of claim 6, wherein a duration of the second laser pulse is between 1 to 20 ns.

11. A radiation source, comprising: a droplet generator configured to generate a stream of droplets of target material; a first laser device configured to illuminate a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target and to illuminate the target using a second laser pulse to rarefy the target; and a second laser device configured to illuminate the rarefied target using a third laser pulse to generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target; and a control system having one or more processors and configured to: analyze measurements of an EUV performance metric of the radiation source as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse; and determine a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measurements.

12. The radiation source of claim 11, wherein the first laser device comprises: a first component laser configured to generate the first laser pulse; anda second component laser configured to generate the second laser pulse.

13. The radiation source of claim 11, wherein the controller is further configured to select the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the radiation source.

14. The radiation source of claim 13, wherein the controller is further configured to set the time between the second and third laser pulses to between 50 and 150 ns.

15. The radiation source of claim 13, wherein the controller is further configured to set the energy of the second laser pulse to between 2.5 and 10 mJ.

16. The radiation source of claim 11, wherein a beam spot of the second laser pulse, at the target, has a diameter between 400 to 1000 pm.

17. The radiation source of claim 11, wherein a beam spot of the second laser pulse, at the target, has a diameter between 500 and 750 pm.

18. The radiation source of claim 11, wherein a duration of the second laser pulse is between 1 and 20 ns.

19. A lithographic apparatus, comprising: a radiation source configured to generate a beam of extreme ultraviolet (EUV) radiation to illuminate a pattern of a patterning device, the radiation source comprising: a droplet generator configured to generate a stream of droplets of target material; a first laser device configured to illuminate a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target and to illuminate the target using a second laser pulse to rarefy the target; a second laser device configured to illuminate the rarefied target using a third laser pulse to generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target; and a control system having one or more processors and configured to: analyze measurements of an EUV performance metric of the radiation source as a function of a time between the second and third laser pulses and as a function of an energy of the second laser pulse; anddetermine a value for the time between the second and third laser pulses and a value for the energy of the second laser pulse based on a maximum value of the EUV performance metric determined from the measurements; and a projection system configured to direct the beam to project an image of the pattern onto a substrate.

20. The lithographic apparatus of claim 19, wherein the controller is further configured to select the value for the time between the second and third laser pulses and the value for the energy of the second laser as setpoints to operate the radiation source.

21. The lithographic apparatus of claim 20, wherein the controller is further configured to set the energy of the second laser pulse to between 2.5 and 10 mJ.

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