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

Abstract

A method of operating an extreme ultraviolet (EUV) radiation source includes generating a stream of droplets of target material. The method also includes illuminating a droplet of the stream using a first laser pulse to adjust a shape of the droplet into a target. The method also includes illuminating the target using a second laser pulse to transform the target into a rarefied target. A time between the first laser pulse and the second laser pulse is 0.5 to 2.5 μs. The method also includes illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation.

Description

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

[0001] This application claims priority to US Application No. 63 / 730,445, filed December 10, 2024, titled SYSTEM AND METHOD FOR OPTIMIZING TARGET EXPANSION 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 transform the target into a rarefied target. A time between the first laser pulse and the second laser pulse can be 0.5 to 2.5 ps. The method can also comprise illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation.

[0010] In some embodiments, a radiation source comprises a droplet generator, a first laser device and a second laser device. The droplet generator can generate a stream of droplets of target material. The first laser pulse 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 transform the target into a rarefied target. A time between the first laser pulse and the second laserpulse is 0.5 to 2.5 ps. 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.

[0011] 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 and a second laser device. 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 transform the target into a rarefied target. A time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps. 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 projection system can direct the beam to project an image of the pattern onto a substrate.

[0012] 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 operations can comprise setting a timing of a first laser pulse to illuminate a droplet of a stream of target material. The operations can also comprise setting a timing of a second laser pulse to illuminate the target to transform the target into a rarefied target. A time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps. The operations can also comprise setting a timing of a third laser pulse to illuminate and ionize the rarefied target to generate EUV radiation.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. 3A shows a graph as an example of variability of usable tin mass with respect to recipe setpoint(s), in accordance with embodiments of the present disclosure.

[0017] FIG. 3B shows an example of an expanded droplet target, in accordance with embodiments of the present disclosure.

[0018] FIG. 4A shows a graph as an example of variability of generated EUV power with respect to recipe setpoint(s), consistent with embodiments of the present disclosure.

[0019] FIG. 4B shows an intensity map of an EUV-generating plasma, in accordance with embodiments of the present disclosure.

[0020] FIG. 5 shows a flowchart of an example method for initializing and operating an EUV source, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0021] 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.

[0022] 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.

[0023] 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 that can 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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, unless specifically 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.

[0028] 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.

[0029] 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.

[0030] 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 radiationbeing 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.”

[0031] 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.

[0032] Illumination can be understood to be a form of radiation. Hence, the terms “radiation” and “illumination” can be used herein interchangeably.

[0033] 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.

[0034] 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.

[0035] 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 can comprise 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).

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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, laserpulse 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 Uogic Array (PUA), a Programmable Array Uogic (PAU), a Generic Array Uogic (GAU), a Complex Programmable Uogic Device (CPUD), 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. 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 UPP 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 pulse repetition 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, to generate a stable EUV-emitting plasma it is desirable to ensure 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 multi -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-pulse212, 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 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, thereby adversely impacting EUV power and stability. 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 different absorption 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 at its most optimal.

[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 main 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 the rarefaction 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 expansion process to transform droplet 204 into an optimal disc-shaped target 208, which in turn affects EUV production performance. 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 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] In one example, a given recipe for pre-pulse 212 can cause underperformance of the system, as measured by EUV conversion efficiency (as created at plasma formation region 4 (FIG. 1)) and EUV power after intermediate focus (as transmitted from radiation source SO lithographic into apparatus LA without being clipped by the aperture at the 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 givenpulse 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). For example, various recipe parameters should be carefully selected in order to create a suitable matter distribution of the plasma volume, such as usable mass of target material that participates in EUV generation, as opposed to the lost mass that does not contribute EUV photons. For simplicity of description, terms such as “tin mass” or the like can be used herein describe a target material without limiting embodiments herein to tin (embodiments can be implemented using any suitable target material).

[0054] Hence, embodiments of the present disclosure implement optimized recipe setpoints to achieve high EUV conversion efficiency at primary focus (plasma formation region 4 (FIG. 1)), higher EUV power after intermediate focus, and higher usable tin mass. The recipe setpoints disclosed herein are based on assessment of the plasma physics and confirmation via experimental or simulation results.

[0055] In some embodiments, wavelength of pre-pulse 212 is set to 1 pm or less. While longer wavelengths can also be used, a shorter wavelength of 1 pm or less can be more readily absorbed by target materials such as tin. The increased absorption at wavelengths of 1 pm or less can confer more kinetic energy for target expansion with fewer photons. The energy of pre-pulse 212 can be about 2 mJ or greater. In various situations, suitable pulse energies of pre-pulse 212 can be about 2 mJ, 3 mJ, 4 mJ, 5 mJ, 7 mJ, or 10 mJ, or can be in ranges between these values (e.g., 3-5 mJ, 4-10 mJ). Expansion time TE can be set to about 0.5 to 2.5 ps (the time between the first laser pulse and the second laser pulse). In various situations, suitable values of the expansion time TE can be about 0.5 ps, 0.75 ps, 1 ps, 1.5 ps, 2 ps, or 2.5 ps, or can be in ranges between these values (e.g., 0.5-1.5 ps, 0.75-2.5 ps). Appropriate use of these values can increase the usable tin mass, as shown in FIG. 3A.

[0056] FIG. 3 A shows a graph 300 as an example of variability of usable tin mass with respect to recipe setpoint(s), in accordance with embodiments of the present disclosure. In some embodiments, the vertical axis represents the normalized usable tin mass (e.g., a value of 1 represents the original starting mass of droplet 204 (FIG. 2) prior to target expansion), as well as normalized unusable tin mass where indicated. The horizontal axis represents the expansion time TE (controllable as a recipe setpoint).

[0057] A technical significance of the setpoint parameters represented in graph 300 is a relationship between setpoint parameters and droplet behavior. Droplet 204 (FIG. 2) can have a given diameter prior to target expansion. When droplet 204 transforms into target 208 (FIG. 2), target 208 can achieve a sheet span (e.g., target disc diameter) that depends on laser energy and expansion time TE. Since each system design can have a distinct ideal target diameter, the desired target diameter can be between 200 to 800 pm (other diameter values are within the scope of embodiments of the present disclosure).

[0058] For a given wavelength (e.g., 1 pm or less) and energy (e.g., 2 to 50 mJ), the droplet can experience a corresponding expansion speed. In this example, the desired target diameter is a fixed value. For the fixed wavelength, the expansion velocity is a function of the energy of pre-pulse 212 (FIG. 2). Expansion velocity can also be represented as a function of expansion time TE. Based on the solid black data plot 305 (“sheet + center”) in graph 300, it is apparent that the fluid dynamics of droplet expansion can favor shorter expansion times (higher laser energy resulting in faster target expansion). Datapoint 302 can represent the usable tin mass of state-of-the-art systems, which is close to a factor of 0.4 of the original droplet mass. Datapoint 304 represents an increase of usable tin mass by implementing one or more embodiments described herein (e.g., about a factor of 1.5 increase). Whereas the state of the art is only capable of converting about 40% of the original tin droplet mass into usable tin, embodiments of the present disclosure allow the usable tin mass to increase (e.g., at least 60% of the mass of the original droplet). The usable tin mass is the mass that participates in EUV generation. Unusable tin mass is the mass that does not participate in EUV generation.

[0059] FIG. 3B shows an example of a target 306, consistent with embodiments of the present disclosure. In some embodiments, target 306 is an expanded droplet as described in reference to FIG. 2. Target 306 can represent a more practical (non-idealized) target 208 (FIG. 2). Target 306 can comprise a disc center 310 and a disc sheet 312. The tin mass at disc center 310 and disc sheet 312 is where the majority of the usable tin mass is located (the mass that participates in plasma EUV emission). However, due to instabilities in fluid dynamics, portions of the droplet 204 (FIG. 2) can transform into unusable tin mass, which can include a rim 314, ligaments 316, and satellite fragments 318. Rim 314 can be disposed at an edge of disc sheet 312. Ligaments 316 can be offshoots and protrusions extending from rim 314. As ligament mass detaches from rim 314, the orphaned micro - droplets can be considered satellite fragments 318. Rim 314, ligaments 316, and satellite fragments 318 perform poorly in subsequent conversion into a plasma state that produces EUV radiation.

[0060] In some embodiments, the dotted gray data plot 303 in the graph represents the usable tin mass of the disc center 310. The solid gray data plot 301 can represent the usable tin mass of the disc sheet 312. The solid black data plot 305 can represent the total sum of the usable mass from both disc center 310 and disc sheet 312. The dash-dotted black data plot 307 represents unusable tin mass from rim 314. The dashed black data plot 309 represents unusable tin mass from satellite fragments 318.

[0061] With the timing of rarefaction pulse 214 (FIG. 2) being defined with respect to the timing of pre-pulse 212 (FIG. 2) (expansion time TE), the data in graph 300 shows that a decrease in the setpoint for the expansion time corresponds to an increase in usable tin mass (shown as a left-arrow along the total usable tin mass). To reduce a setpoint for the expansion time, higher energies can be used (faster expansion). Another observation is that longer expansion times can undesirably allow more of the tin mass to be shifted toward rim 314, ligaments 316, and satellite fragments 318 (unusable). Hence, reducing the setpoint for expansion time can desirably bias the tin mass more toward usable and away from unusable. In some embodiments, a time between the first laser pulse (pre-pulse 212 (FIG. 2))and the second laser pulse (rarefaction pulse 214 (FIG. 2)) is set to about 0.5 to 2.5 ps, 1.0 to 2.5 ps, 1.5 to 2.5 ps, 1.5 to 2.0 ps, 1.0 to 2.0 ps, 1.0 to 1.5 ps, or the like.

[0062] It is desirable to finish target expansion early, which can be accomplished by instigating faster target expansion to reach the desired target lateral size. In addition to, or as an alternative, shorter wavelengths can also be used to increase target expansion speeds. Comparing to a baseline of a 1 pm wavelength for pre-pulse 212 (FIG. 2), a wavelength setpoint of 532 nm can achieve a same speed of target expansion for less energy (e.g., about factor of 2 reduction compared to the 1 pm baseline). A wavelength setpoint of 266 nm can achieve a same speed of target expansion for even less energy (e.g., about a factor 4 for reduction compared to the 1 pm baseline). Hence, the wavelength of the first laser pulse can be about 1 pm or less, 800 nm or less, 600 nm or less, 400 nm or less, 300 nm or less, about 532 nm, about 266 nm, or the like.

[0063] How the laser energy is distributed in time (temporal shape) can also help to achieve a more optimal usable tin outcome. Hence, the duration (e.g., full width at half maximum (FWHM) of uptime) of the first laser pulse can be about 1 to 20 ns, 5 to 20 ns, 10 to 20 ns, 15 to 20 ns, 10 to 15 ns, or the like. Furthermore, a beam spot of the second laser pulse, at target 208 (FIG. 2), can have a span (e.g., diameter) of about 400 pm or greater so as to illuminate the entire disc. A wavelength of rarefaction pulse 214 can be about 2 pm or less, 1.55 pm, 1 pm, or the like.

[0064] In some embodiments, the irradiance (energy divided by duration and illumination area) of rarefaction pulse 214 (FIG. 2) exerts the most control of the ionization degree or extent of rarefied target 210 (FIG. 2). The ionization degree is directly related to plasma brightness (EUV brightness) (the greater the amount of ionized tin mass, the greater the amount of EUV photons emitted). Furthermore, there can be an optimal ionization degree for EUV production. One of the goals of an optimized recipe is to achieve optimal ionization for EUV production. The rarefaction time TR can exert the most control over the density distribution of the volumetric target (the greater the value of rarefaction time TR, the greater the volume of rarefied target 210). Rarefaction speed can increase as a function of increasing ionization degree that occurs from the energy rarefaction pulse 214 (FIG. 2) (partial or pre-ionization). To take advantage of this behavior, the setpoint for the energy of the second pulse laser (rarefaction pulse 214 (FIG. 2)) can be set such that it is sufficient to create a partially ionized rarefied target 210 (FIG. 2). In other words, an energy of the second laser pulse can be greater than a threshold for partial ionization of rarefied target 210 (FIG. 2).

[0065] Optimization of rarefaction time TR can increase EUV power output. The optimal setpoint for rarefaction time can desirably distribute the tin mass of rarefied target 210 (FIG. 2) for increased more stable and higher ionization. In some embodiments, a time between the second laser pulse (rarefaction pulse 214 (FIG. 2)) and the third laser pulse (main pulse 216 (FIG. 2)) is set to about 200 ns or less. In various situations, suitable values of the rarefaction time TR can be about 25 ns, 50 ns, 75 ns, 100 ns, 125 ns, 150 ns, 175 ns, 200 ns, or 250 ns, or can be in ranges between these values (e.g., 50-100 ns, 75-175 ns). In various situations, suitable spot sizes of the second laser pulse, where it reaches thetarget, can be about 500 pm, 550 pm, 600 pm, 650 pm, 700 pm, 800 pm, 900 pm, or 1000 pm, or can be in ranges between these values (e.g., 600-800 pm, 700-1000 pm). How the laser energy is distributed in time (pulse shape) can also help to achieve a more optimal EUV production, which is explained below with respect to FIG. 4A.

[0066] FIG. 4A shows a graph 400 as an example of variability of generated EUV power with respect to recipe setpoint(s), in accordance with embodiments of the present disclosure. In some embodiments, the vertical axis of graph 400 represents EUV power after intermediate focus (e.g., measured using detector 16 (FIG. 1)). The power is normalized to arbitrary units. The horizontal axis represents the duration (e.g., FWHM of uptime) of rarefaction pulse 214 (FIG. 2) (controllable as a recipe setpoint).

[0067] A technical significance of the setpoint parameters represented in graph 400 is a relationship between setpoint parameters and the properties of rarefied target 210 (FIG. 2) that relate to EUV performance. The total EUV emission can depend on the plasma temperature, density, and charge state occurring after rarefied target 210 is fully ionized by main pulse 216 (FIG. 2). In turn, the optimal combination of temperature, density, and charge state of the plasma ionized by main pulse 216 can depend on the ionization degree and tin density distribution of rarefied target 210 at primary focus, which is controlled by both the duration and rarefaction time TR of rarefaction pulse 214 (FIG. 2). The setpoint for rarefication time TR can be set to about 10 ns or less. The setpoint for pulse duration can be 10 ns or less (explained in more detail below). As shown in graph 400, reducing the pulse duration for rarefaction pulse 214 (e.g., by using laser energies 2 mJ or greater) can improve EUV output after intermediate focus (e.g., measured using detector 16 (FIG. 1)).

[0068] To confirm the correspondence between plasma dynamics and EUV output, detector 17 (FIG. 1) can be used to quantify the plasma activity at primary focus (e.g., plasma formation region 4 (FIG. 1)). An example measurement using detector 17 (FIG. 1) is illustrated in FIG. 4B.

[0069] FIG. 4B shows an example intensity map 402 of an EUV-generating plasma, consistent with embodiments of the present disclosure. In some embodiments, detector 17 is a camera that captures an image of plasma 7 at primary focus (FIG. 1). The vertical axis represents a position along a “z” direction (e.g., the z direction can be parallel to the travel direction of main pulse 216 (FIG. 2). The horizontal axis can represent a position along an “x” direction (e.g., the x direction can be parallel to the travel direction of tin droplets). The intensity scale (energy) goes from low energy (dark) to high energy (bright), which can correspond to detector counts. Intensity map 402 is meant to illustrate the volume that contains the majority of the EUV-p reducing plasma.

[0070] The plasma state as measured in intensity map 402 corresponds to data point 403 in graph 400, which is associated with 7.3 ns pulse duration for rarefaction pulse 214 (FIG. 2). The measurement of intensity map 402 can be repeated for each data point in graph 400, which can show that the EUV-producing volume is relatively unchanged with respect to the different pulse durations tested in graph 400. A technically significant feature is that the EUV performance can be increasedwithout a substantial change to the plasma volume. Shortening the pulse duration of rarefaction pulse 214 (FIG. 2) can increase EUV power, brightness, and conversion efficiency. Hence, in various situations, the setpoint for pulse duration for rarefaction pulse 214 (FIG. 2) can be 10 ns or less (e.g., 4, 5, 6, 7.3, 8.6, or 10 ns FWHM or in ranges therebetween). In various situations, suitable pulse energies of rarefaction pulse 214 can be about 3 mJ, 4 mJ, 5 mJ, 7 mJ, 10 mJ, 15 mJ, 20 mJ, or can be in ranges between these values (e.g., 3-5 mJ, 4-10 mJ). In various situations, suitable wavelengths of rarefaction pulse 214 can be about 1 pm, 1.2 pm, 1.4 pm, 1.5 pm (e.g., 1.55) pm, 1.6 pm, 1.8 pm, 2.0 pm, or 2.2 pm, or can be in ranges between these values (e.g., 1-2 pm, 1.5-1.6 pm).

[0071] In some embodiments, to generate rarefaction 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).

[0072] FIG. 5 shows an example method 500 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. 1-4B, such as control system 15 of FIG. 1.

[0073] In some embodiments, at step 502, a stream of droplets of target material is generated using fuel generator 3.

[0074] At step 504, a droplet of the stream (e.g., droplet 204 (FIG. 2)) can be illuminated using first laser pulse (e.g., pre-pulse 212 (FIG. 2). The first laser pulse can adjust a shape of the droplet into a target (e.g., targets 208 or 306 (FIGS. 2 and 3 B)) . The shape can be a thin disc sheet.

[0075] At step 506, the target can be illuminated using a second pulse laser (e.g., rarefaction pulse 214 (FIG. 2)). The second laser pulse can transform the target into a rarefied target (e.g., rarefied target 210 (FIG. 2)). A time between the first laser pulse and the second laser pulse can be about 0.5 to 2.5 ps.

[0076] At step 508, the rarefied target can be illuminated using a third laser pulse (e.g., main pulse 216 (FIG. 2)). The rarefied target can be ionized to generate EUV radiation.

[0077] Embodiments described herein can comprise other operations based on the devices and functions described in reference to FIGS. 1-4B.

[0078] 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.

[0079] A non-transitory computer-readable medium can 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, consistent with embodiments in the present disclosure. For example, the instructions stored in the non-transitory computer-readable medium can be executed by the circuitry of the controller for performing method 500 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), anyother 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.

[0080] Some embodiments can 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 transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation.2. The method of clause 1, wherein: an energy of the first laser pulse is 2 mJ or greater; and a wavelength of the first laser pulse is 1 pm or less.3. The method of any one of clauses 1 or 2, wherein an energy of the second laser pulse is greater than a threshold for partial ionization of the rarefied target.4. The method of any one of clauses 1 to 3, wherein a time between the second laser pulse and the third laser pulse is 200 ns or less.5. The method of any one of clauses 1 to 4, wherein: an energy of the second laser pulse is 2 mJ or greater; a wavelength of the second laser pulse is 1 to 2 pm; a duration of the second laser pulse is 10 ns or less; and a beam spot of the second laser pulse, at the target, has a span of 400 pm or greater.6. The method of any one of clauses 1 to 5, wherein: a laser system of the radiation source comprises first, second, and third laser amplifiers; and the method further comprises: generating the first laser pulse via the first laser amplifier; generating the second laser pulse via the second laser amplifier; and generating the third laser pulse via the third laser amplifier.7. The method of any one of clauses 1 to 6, further comprising ionizing a mass of the rarefied target that is at least 60% of the mass of the droplet.8. 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 transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and a second laser device configured to illuminate the rarefied target using a third laser pulse generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target using the third laser pulse.9. The radiation source of clause 8, wherein: a first component laser configured to generate the first laser pulse; and a second component laser configured to generate the second laser pulse.10. The radiation source of clause 8, wherein: an energy of the first laser pulse is 2 mJ or greater; and a wavelength of the first laser pulse is 1 pm or less.11. The radiation source of any one of clauses 9 or 10, wherein an energy of the second laser pulse is greater than a threshold for partial ionization of the rarefied target.12. The radiation source of any one of clauses 9 to 11, wherein a time between the second laser pulse and the third laser pulse is 200 ns or less.13. The radiation source of any one of clauses 9 to 12, wherein: an energy of the second laser pulse is 2 mJ or greater; a wavelength of the second laser pulse is 1 to 2 pm; a duration of the second laser pulse is 10 ns or less; and a beam spot of the second laser pulse, at the target, has a span of 400 pm or greater.14. The radiation source of any one of clauses 9 to 13, wherein the first laser device is configured to generate an ionizable mass from the droplet that is at least 60% of the mass of the droplet.15. The radiation source of any one of clauses 9 to 14, further comprising a control system configured to control one or more properties of the first, second, and third laser pulses.16. 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 transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and a second laser device configured to illuminate the rarefied target using a third laser pulse generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target using the third laser pulse; anda projection system configured to direct the beam to project an image of the pattern onto a substrate.17. The lithographic apparatus of clause 16, wherein: a first component laser configured to generate the first laser pulse; and a second component laser configured to generate the second laser pulse.18. The lithographic apparatus of clause 16, wherein: an energy of the first laser pulse is 2 mJ or greater; and a wavelength of the first laser pulse is 1 pm or less.19. The lithographic apparatus of any one of clauses 16 or 17, wherein an energy of the second laser pulse is greater than a threshold for partial ionization of the rarefied target.20. The lithographic apparatus of any one of clauses 16 to 18, wherein a time between the second laser pulse and the third laser pulse is 200 ns or less.21. The lithographic apparatus of any one of clauses 16 to 19, wherein: an energy of the second laser pulse is 2 mJ or greater; a wavelength of the second laser pulse is 1 to 2 pm; a duration of the second laser pulse is 10 ns or less; and a beam spot of the second laser pulse, at the target, has a span of 400 pm or greater.22. The lithographic apparatus of any one of clauses 16 to 20, wherein the first laser device is configured to generate an ionizable mass from the droplet that is at least 60% of the mass of the droplet.23. The lithographic apparatus of any one of clauses 16 to 21, wherein the radiation source further comprises a control system configured to control one or more properties of the first, second, and third laser pulses.24. 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 to cause the EUV radiation source to perform operations, the operations comprising: setting a timing of a first laser pulse to illuminate a droplet of a stream of target material; setting a timing of a second laser pulse to illuminate the target to transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and setting a timing of a third laser pulse to illuminate and ionize the rarefied target to generate EUV radiation.25. The non-transitory computer-readable medium of clause 24, wherein: an energy of the first laser pulse is 2 mJ or greater; and a wavelength of the first laser pulse is 1 pm or less.26. The non-transitory computer-readable medium of any one of clauses 24 or 25, wherein an energy of the second laser pulse is greater than a threshold for partial ionization of the rarefied target.27. The non-transitory computer-readable medium of any one of clauses 24 to 26, wherein a time between the second laser pulse and the third laser pulse is 200 ns or less.28. The non-transitory computer-readable medium of any one of clauses 4 to 27, wherein: an energy of the second laser pulse is 2 mJ or greater; a wavelength of the second laser pulse is 1 to 2 pm; a duration of the second laser pulse is 10 ns or less; and a beam spot of the second laser pulse, at the target, has a span of 400 pm or greater.29. The non-transitory computer-readable medium of any one of clauses 24 to 28, wherein: a laser system of the radiation source comprises first, second, and third laser amplifiers; and the operations further comprise: generating the first laser pulse via the first laser amplifier; generating the second laser pulse via the second laser amplifier; and generating the third laser pulse via the third laser amplifier.30. The non-transitory computer-readable medium of any one of clauses 24 to 29, wherein the operations further comprise ionizing a mass of the rarefied target that is at least 60% of the mass of the droplet.31. 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; a second laser device configured to illuminate the target using a second laser pulse to transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and a third laser device configured to illuminate the rarefied target using a third laser pulse generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target using the third laser pulse.

[0081] 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 transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and illuminating the rarefied target using a third laser pulse to ionize the rarefied target to generate EUV radiation.

2. The method of claim 1, wherein: an energy of the first laser pulse is 2 mJ or greater; and a wavelength of the first laser pulse is 1 pm or less.

3. The method of claim 1, wherein an energy of the second laser pulse is greater than a threshold for partial ionization of the rarefied target.

4. The method of claim 1, wherein a time between the second laser pulse and the third laser pulse is 200 ns or less.

5. The method of claim 1, wherein: an energy of the second laser pulse is 2 mJ or greater; a wavelength of the second laser pulse is 1 to 2 pm; a duration of the second laser pulse is 10 ns or less; and a beam spot of the second laser pulse, at the target, has a span of 400 pm or greater.

6. The method of claim 1, wherein: a laser system of the radiation source comprises first, second, and third laser amplifiers; and the method further comprises: generating the first laser pulse via the first laser amplifier; generating the second laser pulse via the second laser amplifier; and generating the third laser pulse via the third laser amplifier.

7. The method of claim 1, further comprising ionizing a mass of the rarefied target that is at least8. 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 transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and a second laser device configured to illuminate the rarefied target using a third laser pulse generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target using the third laser pulse.

9. The radiation source of claim 8, 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.

10. The radiation source of claim 8, wherein: an energy of the first laser pulse is 2 mJ or greater; and a wavelength of the first laser pulse is 1 pm or less.

11. The radiation source of claim 8, wherein an energy of the second laser pulse is greater than a threshold for partial ionization of the rarefied target.

12. The radiation source of claim 8, wherein a time between the second laser pulse and the third laser pulse is 200 ns or less.

13. The radiation source of claim 8, wherein: an energy of the second laser pulse is 2 mJ or greater; a wavelength of the second laser pulse is 1 to 2 pm; a duration of the second laser pulse is 10 ns or less; and a beam spot of the second laser pulse, at the target, has a span of 400 pm or greater.

14. The radiation source of claim 8, wherein the first laser device is configured to generate an ionizable mass from the droplet that is at least 60% of the mass of the droplet.

15. The radiation source of claim 8, further comprising a control system configured to control one or more properties of the first, second, and third laser pulses.

16. 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 transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and a second laser device configured to illuminate the rarefied target using a third laser pulse generate EUV radiation based on ionization of the rarefied target using the third laser pulse; and a projection system configured to direct the beam to project an image of the pattern onto a substrate.

17. 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 to cause the EUV radiation source to perform operations, the operations comprising: setting a timing of a first laser pulse to illuminate a droplet of a stream of target material; setting a timing of a second laser pulse to illuminate the target to transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and setting a timing of a third laser pulse to illuminate and ionize the rarefied target to generate EUV radiation.

18. The non-transitory computer-readable medium of claim 17, wherein: an energy of the first laser pulse is 2 mJ or greater; and a wavelength of the first laser pulse is 1 pm or less.

19. The non-transitory computer-readable medium of claim 17, wherein an energy of the second laser pulse is greater than a threshold for partial ionization of the rarefied target.

20. The non-transitory computer-readable medium of claim 17, wherein a time between the second laser pulse and the third laser pulse is 200 ns or less.

21. The non-transitory computer-readable medium of claim 17, wherein: an energy of the second laser pulse is 2 mJ or greater; a wavelength of the second laser pulse is 1 to 2 pm; a duration of the second laser pulse is 10 ns or less; anda beam spot of the second laser pulse, at the target, has a span of 400 pm or greater.

22. The non-transitory computer-readable medium of claim 17, wherein: a laser system of the radiation source comprises first, second, and third laser amplifiers; and the operations further comprise: generating the first laser pulse via the first laser amplifier; generating the second laser pulse via the second laser amplifier; and generating the third laser pulse via the third laser amplifier.

23. The non-transitory computer-readable medium of claim 17, wherein the operations further comprise ionizing a mass of the rarefied target that is at least 60% of the mass of the droplet.

24. 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; a second laser device configured to illuminate the target using a second laser pulse to transform the target into a rarefied target, wherein a time between the first laser pulse and the second laser pulse is 0.5 to 2.5 ps; and a third laser device configured to illuminate the rarefied target using a third laser pulse generate extreme ultraviolet (EUV) radiation based on ionization of the rarefied target using the third laser pulse.

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