Systems and methods for optimizing plasma generation in an EUV light source

Abstract

Systems and methods for initializing an extreme ultraviolet (EUV) source are discussed. The EUV radiation source may include a droplet generator configured to generate a stream of droplets of a target material, a laser source communicatively coupled to a controller. The laser source is configured to generate a first pulsed beam of light, which upon interaction with a droplet, initiates a modification of a shape of the droplet, forming a modified droplet, and generate a second pulsed beam of light configured to substantially evaporate the modified droplet, thereby generating pulses of EUV light. The EUV radiation source comprises a first actuator communicatively coupled to the controller and configured to adjust a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.

Description

SYSTEMS AND METHODS FOR OPTIMIZING PLASMA GENERATION IN AN EUV LIGHT SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 713,573, filed October 29, 2024, titled SYSTEMS AND METHODS FOR OPTIMIZING PLASMA GENERATION IN AN EUV LIGHT SOURCE, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The embodiments provided herein generally relate to extreme ultraviolet (“EUV”) light sources and their methods of operation in semiconductor device fabrication processes, and more particularly, to systems and methods for optimizing the operating conditions for generating plasma in an EUV light source.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[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 may, for example, project a pattern at 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 may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A mask inspection apparatus (e.g., actinic mask inspection apparatus) is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses reflective surfaces instead of a lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector, the multilayers may be altematingly molybdenum and silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a mass-production process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of themask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar actinic EUV wavelength (13.5 run) as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example, the 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 mask repair process so as 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.

[0006] An EUV radiation source configured for use in lithography (e.g., photolithography) apparatus or in a mask inspection apparatus may have a variety of desirable characteristics, such as high output power, high conversion efficiency, high spectral purity, among other features. The conversion efficiency, generally referred to as a fraction of the input energy to the EUV source that is converted to EUV radiation energy, may be used to determine the utility requirements, to select a target material, or to understand the limits of power scaling. In lithography and mask inspection apparatuses used for semiconductor device fabrication, maximizing EUV source output power and the source stability are of particular interest.SUMMARY

[0007] Some embodiments of the present disclosure provide systems and methods for optimizing the operating conditions for generating plasma in an EUV radiation source.

[0008] One aspect of the present disclosure is directed to an EUV radiation source comprising a droplet generator configured to generate a stream of droplets of a target material, a laser source communicatively coupled to a controller. The laser source is configured to generate a first pulsed beam of light, which upon interaction with a droplet, initiates a modification of a shape of the droplet, forming a modified droplet, and generate a second pulsed beam of light configured to substantially evaporate the modified droplet, thereby generating pulses of EUV light. The EUV radiation source comprises a first actuator communicatively coupled to the controller and configured to adjust a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.

[0009] Another aspect of the present disclosure is directed to a method for initializing an extreme ultraviolet (EUV) radiation source. The method comprises generating a stream of droplets of a target material; illuminating a droplet of the stream of droplets with a first pulsed beam of light to initiate a modification of a shape of the droplet, forming a modified droplet; illuminating the droplet with a second pulsed beam of light to substantially evaporate the modified droplet, thereby generating pulsesof EUV light, and adjusting a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.

[0010] Yet another aspect of the present disclosure is directed to a lithographic system comprising a lithographic apparatus and an EUV radiation source. The EUV radiation source configured to generate and supply EUV radiation to the lithographic apparatus. The EUV radiation source comprises a droplet generator configured to generate a stream of droplets of a target material, a laser source communicatively coupled to a controller. The laser source is configured to generate a first pulsed beam of light, which upon interaction with a droplet, initiates a modification of a shape of the droplet, forming a modified droplet, and generate a second pulsed beam of light configured to substantially evaporate the modified droplet, thereby generating pulses of EUV light. The EUV radiation source comprises a first actuator communicatively coupled to the controller and configured to adjust a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.

[0011] Yet another aspect of the present disclosure is directed to a non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of a system to cause the system to perform operations comprising activating a droplet generator to generate a stream of droplets of a target material, illuminating a droplet of the stream of droplets with a first pulsed beam of light to initiate a modification of a shape of the droplet, forming a modified droplet, illuminating the droplet with a second pulsed beam of light to substantially evaporate the modified droplet, thereby generating pulses of EUV light, and adjusting, using an actuator, a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.BRIEF DESCRIPTION OF FIGURES

[0012] The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments, taken in conjunction with the accompanying drawings.

[0013] Fig. 1 illustrates a schematic block diagram of a lithography system comprising an EUV radiation source and a lithographic apparatus, consistent with embodiments of the present disclosure .

[0014] Fig. 2 illustrates an exemplary mask inspection apparatus used in combination with an EUV radiation source, consistent with embodiments of the present disclosure.

[0015] Fig. 3 illustrates a schematic diagram of an exemplary alternative EUV radiation source, consistent with embodiments of the present disclosure.

[0016] Fig. 4 illustrates a simplified schematic of laser-target interaction in an exemplary EUV radiation source, consistent with embodiments of the present disclosure.

[0017] Fig. 5 illustrates a simplified schematic of an exemplary interaction between a pre-pulse laser beam and a target droplet, consistent with embodiments of the present disclosure.

[0018] Fig. 6 illustrates a simplified schematic of an exemplary interaction between a main pulse laser beam and a modified target droplet, consistent with embodiments of the present disclosure.

[0019] Figs. 7A-7C illustrate schematics of the laser-target interaction profiles for varying size of the main pulse beam, consistent with embodiments of the present disclosure.

[0020] Figs. 8A-8C illustrate schematics of the laser-target interaction profiles for varying offset distances in a direction parallel to the propagation of laser beam (z-direction), consistent with embodiments of the present disclosure.

[0021] Figs. 9A-9C illustrate schematics of the laser-target interaction profiles for varying offset distances in a direction parallel to the droplet stream (x-direction), consistent with embodiments of the present disclosure.

[0022] Figs. 10A-10C illustrate schematics of the laser-target interaction profiles for varying offset distances in a direction (y-direction) orthogonal to the propagation of laser beam and to the droplet stream, consistent with embodiments of the present disclosure.

[0023] Figs. 11A-11C illustrate schematics of the laser-target interaction profiles for varying energy of pre-pulse beam, consistent with embodiments of the present disclosure.

[0024] Figs. 12A-12C illustrate schematics of the laser-target interaction profiles for varying main pulse pedestal energy, consistent with embodiments of the present disclosure.

[0025] Figs. 13A-13C illustrate schematics of the laser-target interaction profiles for varying prepulse to main pulse beam time delay, consistent with embodiments of the present disclosure.

[0026] Figs. 14A and 14B illustrate schematics of exemplary transformations, via laser-interaction, of a droplet target to a modified droplet target, consistent with embodiments of the present disclosure.

[0027] Fig. 15 illustrates a process flowchart of a method for optimizing tilt angle of a target in an EUV radiation source, consistent with embodiments of the present disclosure.

[0028] Fig. 16 illustrates a process flowchart for a method for initializing an EUV radiation source, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0029] 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. 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 may include A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component may include A, B, or C, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0030] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the disclosed embodiments as recited in the appended claims.

[0031] Manufacturing semiconductor devices, typically involves processing a substrate (e.g., a semiconductor wafer) using a number of fabrication processes to form various features and multiple layers of the devices. Such layers and features are typically manufactured and processed using, e.g., deposition, photolithography, etch, chemical -mechanical polishing, and ion implantation. Multiple devices may be fabricated on a plurality of dies on a substrate and then separated into individual devices. Photolithography is a process of transferring a pattern to a radiation-sensitive material arranged on a substrate by exposing the substrate to radiation through a mask or a reticle defining the pattern. The substrate may be a silicon (Si) wafer coated with a radiation-sensitive material (e.g., a photoresist). The radiation used in a lithographic apparatus, for advanced technology nodes, may be EUV radiation having a wavelength in the range of 4-20 nm, for example, 13.5 nm. Generally, the shorter the wavelength of the radiation used to expose the photosensitive material, the better the resolution, and therefore, much smaller features can be produced on the substrate using EUV radiation.

[0032] Methods to produce the desirable 13.5 nm EUV radiation include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon (Xe), lithium (Li), or tin (Sn), with one or more emission line in the EUV range. In one such method, often termed laser- produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line -emitting element, with an optical beam, such as a laser beam.

[0033] Although specific reference may be made in this disclosure to the manufacture of ICs, it should be explicitly understood that the description herein has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin -film magnetic heads, etc. In the context of this disclosure, any use of the terms “reticle” and “wafer” should be considered as interchangeable with the more general terms “mask” and “substrate,” respectively.

[0034] Although specific reference may be made in this disclosure to a lithographic apparatus, it should be explicitly understood that the description herein may be used in other apparatuses including, but not limited to, a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non -vacuum) conditions.

[0035] Although specific reference may be made in this disclosure in the context of optical lithography, it will be appreciated that the disclosure, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0036] Where the context allows, embodiments of the disclosure may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the disclosure may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine -readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical, and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc., and in doing that may cause actuators or other devices to interact with the physical world.

[0037] Fig. 1 is a schematic block diagram of a lithography system comprising a radiation source and a lithographic apparatus, consistent with embodiments of the present disclosure. A lithography apparatus or a lithography system is an apparatus that applies a desired pattern onto a target portion of a substrate such as a silicon wafer. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W. Although not illustrated, lithography apparatus may further include a processor, a preprocessor, a microprocessor, or the like, to process obtained data, for example.

[0038] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The 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. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0039] After being thus conditioned, the EUV radiation beam B interacts with the 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 the patterned EUV radiation beam B' onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14, which are configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT.The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Fig. 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0040] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B', with a pattern previously formed on the substrate W. A relative vacuum, i.e., a small amount of gas (e.g., hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, or in the projection system PS.

[0041] The lithographic apparatus LA and radiation source SO described herein can be used in a method for performing a circuit layout patterning process. A circuit layout patterning method may comprise receiving a substrate with a disposed photoresist layer. The method may further comprise directing EUV radiation from radiation source SO to the photoresist layer to form a patterned photoresist layer. The method may further comprise developing and etching the patterned photoresist layer to form the desired circuit layout.

[0042] Reference is now made to Fig. 2, which illustrates an exemplary mask inspection apparatus, consistent with embodiments of the present disclosure. Mask inspection apparatus, also referred to herein as mask inspection system 200 may be to identify or inspect defects in a mask to be used in a lithographic process by means of lithographic apparatus LA, described in Fig. 1. The mask inspection system may comprise a radiation source 210 (e.g., an EUV radiation source SO of Fig. 1), an illumination system 220, and a detection system 230. A mask 240 may be placed on a mask stage 250 and illuminated by the illumination system 220 reflecting radiation incident from radiation source 210. The radiation coming from the illuminated mask 240 may be reflected by detection system 230 to form an image on a detector 260.

[0043] Referring back to Fig. 1, radiation source SO may be a LPP EUV radiation source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam2 into a target material, also referred to herein as a fuel material. An exemplary target material includes tin (Sn), which is provided from a target generator or a fuel generator 3. Although Sn is referred to in the following description, any suitable fuel material may be used. The target material may, for example, be in liquid form, and may, for example, be a metal or an alloy. The fuel generator3 may comprise a nozzle configured to direct the fuel, e.g., in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel droplet(s) at the plasma formation region 4. The deposition of laser energy into target material (e.g., tin droplets) creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0044] In some embodiments, laser system 1 may be spatially separated from the radiation source SO. Where this is the case, laser beam 2 may be passed from laser system 1 to 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 suitable optics. In some embodiments, laser system 1, radiation source SO, and the beam delivery system may together be considered to form a radiation system.

[0045] The EUV radiation from plasma 7 is collected and focused by a collector mirror 5. Collector mirror 5 may comprise, for example, a near-normal incidence radiation collector mirror 5, also referred to as a normal -incidence radiation collector. Collector mirror 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a wavelength of 13.5 nm). The collector mirror 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0046] Radiation that is reflected by collector mirror 5 forms EUV radiation beam B. The EUV radiation beam B may be focused at an intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. In some embodiments, radiation source SO may be arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0047] It should be appreciated that although radiation source SO is discussed herein as a LPP EUV source, any suitable source such as a free electron laser (FEL) or a discharge produced plasma (DPP) source may be used to generate EUV radiation.

[0048] Reference is now made to Fig. 3, which illustrates a schematic diagram of an exemplary alternative EUV radiation source, consistent with embodiments of the present disclosure. For generating plasma, target material 360 (e.g., Sn, Xe, or an alloy) may be provided to a rotating element 320 such as, but not limited to, rotating wheels, cylinder, or a drum, or variations thereof. In some embodiments, target material 360 may be provided via target material source 330 in liquified form to the rotating element 320, such as by means of a target material bath. Alternatively, target material may also be provided in solid or frozen form (e.g., Xe, Li, or Sn metal). In some embodiments, target material may be in a gaseous state and may be sprayed onto the rotating element 320 to replenish target material transformed to plasma. In some embodiments, rotating element 320 may be cooled to solidify target material 360.

[0049] Radiation source 300 may further comprise an excitation device 310 configured to assist in plasma formation. In some embodiments, excitation device 310 may comprise a laser source, such as a solid-state laser or a gas laser, directing a laser beam to be incident on target material 360 and form plasma at a plasma formation region 370 (analogous to plasma formation region 4 of Fig. 1). A solid- state laser, as used herein, refers to a laser source which uses a lasing medium that is a solid, for example, a neodymium -doped yttrium aluminum garnet (Nd:YAG) laser. A gas laser, as used herein,refers to a laser source in which an electric current is discharged through a gas to produce coherent light, for example, a carbon dioxide (CO2) laser, carbon monoxide (CO) laser, helium-neon (HeNe) laser, or a nitrogen (N2) laser.

[0050] Radiation source 300 may include reflective optics configured to reflect the generated EUV radiation to intermediate focus point 342. In some embodiments, the reflective optics may comprise a collector mirror 340, analogous to collector mirror 5 of Fig. 1. EUV radiation of 13.5 nm is absorbed significantly by materials, and even gases, used in a Sn-based LPP EUV radiation source. Therefore, it may be desirable to minimize EUV radiation absorption losses along the optical path in an EUV radiation source to maximize the output of EUV radiation or the conversion efficiency of EUV sources. One of several ways to mitigate absorption losses in an EUV radiation source is to maximize the reflectivity of collector mirror 340. In some embodiments, collector mirror 340 may comprise a multilayered coating of reflective and barrier materials, acting as Bragg reflectors. The multilayered coating may include a silicon-molybdenum (Si / Mo) multilayered stack, for example. Other suitable coatings may be applied as well.

[0051] In some embodiments, buffer gas flow 345 may be provided to mitigate contamination of radiation source 300 and related components from previously existing debris or debris generated in radiation source 300. Collector mirror 340 may be located in close proximity to plasma formation region 370. In addition to EUV radiation, the plasma formed at plasma formation region 370 may emit high-energy ions as well as undesirable Sn particles, which may get deposited on the reflective surface of collector mirror 340. Such debris, over a period of time, impairs the reflective coating of collector mirror 340, negatively impacting the optical characteristics, such as reflectivity, and lifetime of collector mirror 340. A buffer gas flow 345 may comprise the flow of a buffer gas, e.g., hydrogen gas, at a suitable pressure range. In some embodiments, the buffer gas flow pressure may be in a range of 50 - 150 Pascals (Pa). The flow of buffer gas may be configured to enable deceleration of atomic Sn (unionized) and Sn ions approaching the reflective surface of collector mirror 340, and further, to minimize deposition of Sn ions by enabling a chemical reaction between Sn particles and hydrogen gas to form gaseous tin hydride (SnH4). The gaseous SnH4may be removed from the EUV source vessel (not shown) using a vacuum pump 350. In some embodiments, vacuum pump 350 may be provided to create and maintain a gas pressure below atmospheric pressure inside radiation source 300.

[0052] Commonly used EUV radiation having a wavelength of 13.5 nm is obtained from plasma emissions of highly charged Sn ions. It is to be appreciated that while Li, Xe, and Sn, all of which have ions with strong resonance transitions within the desirable bandwidth, may be used to generate EUV radiation, the conversion efficiency for Sn is higher than that of Xe and Li. Near 13.5 nm wavelength, the EUV spectrum of highly charged Sn ions is dominated by intense unresolved transition arrays (UTAs) arising mainly from the resonance transitions. One of several requirements to obtain ~ 20-40 eV plasma temperatures to produce the highly charged Sn ions includes high powerdensities, among other things. This temperature requirement may be understood through the Stefan- Boltzmann law, which describes the energy emitted per second per unit surface by a black body as a function of temperature. Using this relationship, approximately 108Watts (W) may be needed for a sustained emission from a representative emitting area of ~ 1 mm2. The requirement for high power density necessitates plasma sources of a pulsed nature. Some examples of pulsed laser sources may include discharge-produced plasma (DPP) or laser produced plasma (LPP).

[0053] In an exemplary LPP EUV radiation source, a fuel droplet stream (e.g., Sn droplets) may be illuminated with a pulsed laser radiation provided by one or more laser sources. To generate EUV radiation, a series of laser pulses are injected so as to intercept each Sn droplet of the droplet stream. The Sn droplets may change shape upon interaction with the laser pulse although they may still be referred to as droplets or, alternatively, the droplet may be more generally referred to as a Sn target or a fuel target. The series of laser pulses may comprise three laser pulses. The LPP EUV radiation source may use (i) a low-energy first laser pulse, also referred to as a pre-pulse (PP), which upon being incident on the fuel droplet (e.g., Sn droplet), causes the shape of the fuel droplet to change from a substantially spherical droplet to a flatter, disk-shaped target; (ii) a low-energy second laser pulse, also referred to as a pedestal pulse or a rarefaction pulse (RP), which upon interaction, causes the disk-shaped target to expand to a disperse, rarefied target; and (iii) a high-energy third pulse, also referred to as a main pulse (MP), which upon being incident on the disperse target, causes conversion of at least a portion of the disperse target into plasma at a plasma formation region. The portion of the disperse target converted to plasma can emit radiation, including EUV radiation, during de-excitation and recombination of electrons with ions of the plasma. The fraction of the main pulse energy that is converted to EUV radiation energy may be referred to as the conversion efficiency of the radiation source. The laser pulses may include infrared (IR) radiation, for example, with a wavelength of approximately 10 pm or approximately 1 pm.

[0054] As previously discussed, the laser source providing pulsed laser radiation may comprise a 10 pm laser architecture (e.g., CO2 laser architecture) or a 1 pm laser architecture (e.g., Nd:YAG laser architecture). In a 10 pm laser architecture, the low-energy pre-pulse and the low-energy rarefaction pulse (which may otherwise be called a rarefication pulse) may be generated by a laser source configured to provide a laser beam having a wavelength in a range of 9 pm - 11 pm. On the other hand, in a 1 pm laser architecture, the low-energy pre-pulse and the low-energy rarefaction pulse may be generated by a laser source configured to provide a laser beam having a shorter wavelength of approximately 1 pm. In either laser architectures, the high-energy main pulse may be generated by a laser source configured to provide a laser beam having a wavelength in the range of around 10 pm. It is to be appreciated that the laser wavelength ranges mentioned herein are approximate and may vary within a suitable range.

[0055] With respect to excitation lasers for EUV radiation sources, currently existing systems use one or more high-power CO2 gas laser at 10.6 pm wavelength, or one or more solid-state Nd:YAGlaser at 1 pm wavelength. The conversion efficiency, and to some extent, the maximum EUV output power obtainable may depend, among other things, on the electron density of the generated plasma. To improve the conversion efficiency, it may be desirable that the electron density of the plasma formed by the fuel target irradiated by the main pulse of laser radiation is as close to, but not less than, the critical plasma density. Critical plasma density, as used herein, refers to the density of electrons in a plasma formed by irradiation of the fuel material by a laser radiation at which the plasma frequency equals the frequency of an electromagnetic electron wave in the plasma. The critical plasma density of a plasma is governed by the following equation:where e0is the permittivity of free space, meis the mass of an electron, a»Lis the angular frequency of the incident laser radiation, and e is the charge of an electron.

[0056] As an example, the critical density of electrons at the desirable 20 - 40 eV temperature range for incident radiation with a wavelength of ~1 pm (e.g., generated by a Nd:YAG laser source) would be approximately 1021cm'3and the critical density of electrons in plasma generated by an incident radiation with a wavelength of -10.6 pm (e.g., generated by a CO2 laser source) would be approximately 1019cm'3. The electron density of solid or liquid atomic Sn is in the range of 1022- 1023cm'3. As the fuel target is irradiated by the main pulse of the laser radiation, and as the fuel target emits EUV radiation as a result of the irradiation, the electron density of the plasma formed from the fuel target by the radiation decreases. Such a decrease in the electron density of the plasma may cause the electron density of the plasma relevant for the absorption of laser radiation to drop below the critical plasma density, thus negatively impacting the conversion efficiency of the laser source and increasing the reflectance of the plasma.

[0057] Reference is now made to Fig. 4, which illustrates a simplified schematic of laser-target interaction in an exemplary LPP EUV radiation source, consistent with embodiments of the present disclosure. LPP EUV radiation source 400 may comprise an apparatus for generating a pulsed laser, a target-material delivery system, and reflective optics. Although not illustrated, it is to be appreciated that LPP EUV radiation source 400 may further comprise any one or more of a chamber or an enclosure vessel, a system for providing vacuum, an EUV controller, a droplet delivery control system, a droplet position detection system, EUV metrology instrument(s), among other things.

[0058] As shown in Fig. 4, EUV radiation source 400 may include laser source 410 configured to generate laser pulses and to deliver the laser pulses into a chamber (not shown). The laser pulses may travel along one or more beam paths 412 from laser source 410 into the chamber to illuminate the target material (e.g., fuel material droplets) at an irradiation region 470 (e.g., plasma formation region 4 of Fig. 1 or plasma formation region 370 of Fig. 3). The chamber may comprise an enclosuremaintained, during operation of the EUV radiation source, at a pressure below the atmospheric pressure and configured to house the reflective optics (e.g., a collector mirror).

[0059] A target-material delivery system may be configured to generate and deliver droplets 425 of a target material into the interior of the chamber to irradiation region 470 where the droplets may interact with one or more laser pulses, e.g., one or more pre-pulses and thereafter one or more main pulses, to ultimately produce a plasma and generate an EUV emission. The target-material delivery system may comprise a droplet generator 420 configured to generate either (i) one or more streams of droplets of target material exiting droplet generator 420, or (ii) one or more continuous streams of target material which exit droplet generator 420 and subsequently break into droplets due to surface tension. In some embodiments, droplet generator 420 may have a single orifice, or multiple orifices to generate a “showerhead-type” effect.

[0060] In some embodiments, droplet generator 420 may be configured to generate and deliver droplets of the target material into the chamber at a frequency within a frequency range such as 100 Hz - 500 Hz, or 200 Hz - 1 kHz, or 500 Hz - 10 kHz, or 5 kHz - 25 kHz, or 20 kHz - 100 kHz, or 30 kHz - 100 kHz, or 40 kHz - 100 kHz, or 50 kHz - 100 kHz, or 60 kHz - 100 kHz, or 70 kHz - 100 kHz, or 80 kHz - 150 kHz, or 90 kHz - 200 kHz, or other frequency ranges suitable for various applications. In some embodiments, droplet generator 420 may be configured to generate and deliver droplets of the target material at a frequency of 50 kHz. In some embodiments, the frequency of the pulsed laser radiation incident on the droplet may be substantially similar to the droplet delivery frequency. In various implementations, the pulsed laser radiation may be tightly synchronized with the droplet generation, with both having repetition rates of, for example, 100 Hz, 500 Hz, 1 kHz, 10 kHz, 20 kHz, 30 kHz, 50 kHz, 60 kHz, 100 kHz, 120 kHz, 150 kHz, 200 kHz, or other values.

[0061] The target material may include, but is not limited to, a material that includes tin (Sn), lithium (Li), xenon (Xe), or combinations thereof. The EUV emitting element, e.g., tin, lithium, or xenon may be in the form of liquid droplets, or solid particles contained within liquid droplets, or any other form which delivers the EUV emitting element to the target volume in discrete amounts. For example, Sn may be used as pure tin, as a tin compound, e.g., tin bromide (SnBr4), tin dibromide (SnB ). tin hydride (SnFL), or as a tin alloy, e.g., tin-gallium (Sn-Ga) alloys, tin-indium (Sn-In) alloys, tin- indium-gallium (Sn-In-Ga) alloys, or a combination thereof. Depending on the material used, the target material may be presented to irradiation region 570 at various temperatures, including room temperature or near room temperature (e.g., tin alloys, SnBr4), or at an elevated temperature (e.g., pure Sn), or at temperatures below room-temperature, (e.g., SnFL).

[0062] EUV radiation source 400 may further include reflective optics comprising a collector mirror 440 configured to collect and reflect the EUV radiation generated from the interaction of the main pulse with the rarefied and ionized target material. Collector mirror 440 may comprise an ellipsoidal, graded multi-layer mirror having multiple alternating layers of molybdenum and silicon, with an aperture to allow the laser pulses generated by laser source 410 to pass through to irradiation region470. Collector mirror 440 may also be referred to as a near-normal incidence radiation collector (e.g., collector mirror 5 of Fig. 1). In some embodiments, collector mirror 440 may be an ellipsoidal mirror that has a first focal point within or near irradiation region 470 and a second focal point at an intermediate focus 442 where the EUV radiation may be output from EUV light source 400 and input to a system utilizing EUV radiation, e.g., an integrated circuit lithography apparatus (as shown in Fig. 1) or a mask inspection tool (as shown in Fig. 2).

[0063] As previously described, the EUV radiation emitting LPP may be produced from liquid Sn microdroplets in two or three steps. Each individual droplet may act as a mass-limited target, thereby minimizing the load of debris. First, a low-energy pre-pulse may set in motion the deformation of a Sn droplet 425 into a target shape 425_1, which is suitable for interacting with the subsequent laser pulses. The pre-pulse may be followed by a low-energy rarefaction pulse, also referred to as a pedestal pulse, which may convert the disk-shaped Sn target 425_2 into a disperse, rarefied cloud of Sn having a lower density. The fuel target (disperse cloud of Sn atoms) may be finally illuminated by a high-energy main pulse, resulting in partial conversion of the target material to plasma. Deexcitation and recombination of electrons with ions of the Sn plasma may then generate EUV radiation, which may be collected and focused (by way of reflection) using collector mirror.

[0064] In some applications such as lithography, the output of the EUV radiation source, i.e., the EUV radiation, or pulses of EUV radiation, is directed into a scanner / stepper apparatus, which is configured to transfer patterns from a mask or a reticle to a substrate (e.g., Si wafer) coated with a radiation sensitive material (e.g., photoresist). A photolithography scanner exposes a portion of a moving wafer to a so-called “burst” of light pulses. In many cases, the pulse energy varies from pulse- to-pulse within the burst, and the accumulated energy of the burst, referred to generally as “dose,” is typically prescribed and must be controlled within a relatively small range. In addition to dose, some lithography operations also prescribe limits on the pulse-to-pulse energy variation within a burst. This is sometimes referred to as pulse-to-pulse energy stability. As the geometries of the devices shrink to smaller dimensions with advancement in technology nodes, it is desirable to maximize the output power (maximum closed-loop EUV power) and stability of the radiation signal generated by an EUV radiation source to be supplied to a scanner of a photolithography apparatus, for example.

[0065] As previously described, EUV radiation from a plasma source at least involves interaction of a pre-pulse with a droplet of a target material (e.g., Sn droplet) and interaction of a main pulse with the droplet. A software program or a machine-implemented algorithm may be used to determine setpoints for one or more variables controlling the interaction between the laser beam and the target material. A “plasma setup,” as used herein, refers to the calibration of variables relevant for controlling the interaction parameters between the laser beam and the target material. The relevant variables may include, but are not limited to, laser beam characteristics, droplet characteristics, or a combination thereof, among other things. In some cases, the software program associated with a particular EUV radiation source may be configured to determine operation setpoint(s) for multiplecontrol loops such that the operation of that EUV radiation source with the determined setpoints results in maximum closed-loop EUV output power, thereby maximizing the throughput of the system (e.g., scanner in a photolithography apparatus) configured to receive the EUV radiation.

[0066] Although the existing techniques for optimizing plasma generation in an EUV source involve implementing multiple control loops with corresponding setpoints determined by a software program, there are several drawbacks associated with the previous techniques. Some of these drawbacks include, but are not limited to, poor EUV output power reproducibility and poor EUV power signal stability. In operation, the parameters for each control loop are scanned sequentially to identify and select desirable setpoints. The setpoints may be determined based on a statistical metric such as, mean of squares of dose error, also referred to as mean squared dose error (MSDE). “Dose error,” as used herein, refers to deviation of obtained EUV energy from a desired energy dose target, and MSDE refers to the mean of the squares of dose error values. A small value of MSDE is desirable, indicating that a setpoint is producing less dose errors compared to other setpoints for a control loop, and therefore, may be selected as the operation setpoint. However, MSDE, as a qualifying metric is unreliable because it is not sensitive to all the setpoints selected during plasma setup process. As a result, some setpoints may be picked randomly based on measurement noise, causing variability in EUV output power signal, and thereby, in system performance. Therefore, it is desirable to improve the routine to select the setpoints for plasma generation based on two characteristics of the generated EUV power signal — increasing the mean of the EUV output power signal and minimizing the variation in signal, resulting in a stable, high closed-loop EUV output power.

[0067] Various implementations of the present disclosure provide systems and methods for optimizing the plasma generation setpoints for achieving maximum closed-loop EUV output power while maintaining the reproducibility and EUV power signal stability. The interaction of main pulse beam with the modified target droplet is influenced by the interaction of pre-pulse beam with the target droplet generated by the droplet generator.

[0068] Reference is now made to Fig. 5, which illustrates a simplified schematic of an exemplary interaction between a pre-pulse beam and a target droplet, consistent with embodiments of the present disclosure. An exemplary pre-pulse beam 510 having a focal point 515 may interact with a droplet 520 to initiate a modification of a shape of the droplet (leading to a modified droplet, such as a diskshaped target). The focal point 515 of pre-pulse beam 510 refers to the point where the beam’s radius is smallest, and where the beam’s power density is highest. In some embodiments, droplet 520 may be spherical or substantially spherical and a geometric center (i.e., the centroid) of droplet 520 refers to the point that is substantially equidistant from all points on the droplet’s surface. A droplet generator (e.g., droplet generator 420 of Fig. 4) may be configured to generate a stream of droplets traveling along a path, in a -x-direction (depicted in the coordinate system shown in Fig. 4). A laser source (e.g., laser source 410 of Fig. 4) may generate pre-pulse beam 510, which intersects the path ofdroplet 520. In the example shown in Fig. 4, the pre-pulse beam 510 propagates along a z-direction, orthogonal or substantially orthogonal to the direction of path of droplet 520.

[0069] Fig. 5 further illustrates offsets dx, dy, and dz, associated with the interaction of pre-pulse beam 510 with droplet 520. Setpoints associated with the interaction of pre-pulse beam 510 and droplet 520 include measurements of offset distances between the focal point 515 of pre-pulse beam 510 and a geometric center of droplet 520. Fig. 5 includes an inset that illustrates reference directions for purposes of this discussion. In some embodiments, offset dz, also referred to herein as laser-to- droplet in z-direction (L2DZ), refers to the offset distance (typically in microns (pm)) between the focal point 515 of pre-pulse beam 510 and the geometric center of droplet 520, in a direction parallel to the propagation of pre-pulse beam 510, i.e., the z-direction, as shown. Offset dz(L2DZ) may determine the droplet z-direction offset distance relative to the waist of pre-pulse beam 510, which substantially coincides with focal plane of pre-pulse beam 510. The focal plane of pre-pulse beam 510 refers to the plane extending perpendicular to the z-direction and including the focal point 515, as shown. In some embodiments, EUV radiation source (e.g., EUV radiation source 400) may include a controller (e.g., processor, a microprocessor, a CPU, etc.) configured to initiate, via a software program, an actuator to adjust offset dz. In some embodiments, adjusting offset dzmay include adjusting an input signal, such as an electrical signal, to the actuator to adjust a position of pre-pulse beam 510 in z-direction to adjust the pre-pulse fluence on droplet 520, thereby optimizing or otherwise adjusting the target size for interaction with main pulse. The pre-pulse fluence, as used herein, refers to a ratio of the pre-pulse beam energy to the pre-pulse beam size incident on a target droplet. The pre-pulse fluence is highest along the focal plane (or the waist) of the pre-pulse beam. As used herein, an actuator refers to a hardware component of a machine or a device that produces force, torque, or displacement in response to an input signal. In some embodiments, one or more actuators may be used to enable movement, or displacement, or adjustment of a characteristic of an optical beam. An actuator may be a pneumatic actuator (input signal is air pressure), or a hydraulic actuator (input signal is fluid pressure), or an electrical actuator (input signal is an electrical signal). Initiating an actuator may include activating the actuator, calibrating the actuator, applying or adjusting an input signal to the actuator to adjust the output signal, for example, displacement of optical mirrors associated with the actuator. The impact of adjusting offset L2DZ on the characteristics of the modified droplet target and its interaction with the main pulse are discussed in detail with reference to Figs. 8A-8C.

[0070] In some embodiments, offset dx, also referred to herein as offset of laser-to-droplet in x- direction (L2DX), refers to the offset distance (typically in microns (pm)) between focal point 515 of pre-pulse beam 510 and a geometric center of droplet 520, in a direction parallel to the path of droplet 520 or the path of the stream of droplets, i.e., the x-direction, as shown. In some embodiments, EUV radiation source (e.g., EUV radiation source 400) may include a controller (e.g., processor, a microprocessor, a CPU, etc.) configured to initiate, via a software program, an actuator to adjustoffset dx. In some embodiments, the actuator to adjust offset dxmay be different from the actuator configured to adjust offset dz. In some embodiments, the actuator to adjust offset dxmay include a timing control for firing the pre-pulse beam 510. In some embodiments, a single actuator may be configured to adjust one or more offsets. The impact of adjusting offset L2DX on the characteristics of the modified droplet target and its interaction with the main pulse are discussed in detail with reference to Figs. 9A-9C.

[0071] Offset dy, also referred to herein as offset of laser-to-droplet in y-direction (L2DY), refers to the offset distance (typically in microns (pm)) between focal point 515 of pre-pulse beam 510 and a geometric center of droplet 520, in a direction orthogonal to the path of droplet 520 or the path of the stream of droplets and orthogonal to the propagation direction of pre-pulse beam 510, i.e., the y- direction, as shown. In some embodiments, EUV radiation source (e.g., EUV radiation source 400) may include a controller (e.g., processor, a microprocessor, a CPU, etc.) configured to initiate, via a software program, an actuator to adjust offset dy. The impact of adjusting offset L2DY on the characteristics of the modified droplet target and its interaction with the main pulse are discussed in detail with reference to Figs. 10A-10C.

[0072] Reference is now made to Fig. 6, which illustrates a simplified schematic of an exemplary interaction between a main pulse beam and a modified droplet target, consistent with embodiments of the present disclosure. An exemplary main pulse beam 610 having a main pulse beam size MPWilluminates a disk-shaped modified droplet target 625. As used herein, the main pulse beam size refers to the diameter of the main pulse beam corresponding to the pre-pulse z-focus plane. In some embodiments, main pulse beam size may be referred to as the main pulse beam width as well. Fig. 6 further illustrates an offset 630 between an optical axis 601 of main pulse beam 610 and a geometric center of modified droplet target 625 in a direction orthogonal to optical axis 601 of main pulse beam 610. In some embodiments, offset 630 represents a cumulative offset between main pulse beam 610 and modified droplet target 625 in the x- direction and in the y-direction. In some embodiments, EUV radiation source (e.g., EUV radiation source 400) may include or communicate with a controller (e.g., processor, a microprocessor, a CPU, etc.) configured to initiate, via a software program, an actuator to adjust main pulse beam size without impacting a position of the pre-pulse beam in the z-direction.

[0073] Illumination of modified droplet target 625 with main pulse beam 610 may be accompanied by back-reflection of a portion of the beam signal (or beam power) in a direction 650 (negative z direction) opposite to the propagation direction of main pulse beam 610. The back -reflected power, also referred to as reverse power, propagates in the reverse direction with respect to the propagation of main pulse beam 610. The back-reflected power may be useful, in various situations, as a source of diagnostic information regarding the interaction between the main pulse beam 610 and the modified droplet target 625. Conversely, the back -reflected may be undesirable because it represents energy that is not absorbed by the modified droplet target 625 and thus is not converted into EUV radiation. In addition, the reverse power or the back-reflected power may travel back to the seed laser table,negatively impacting the operation and performance of the laser source. To mitigate the back- reflected power, the disk-shaped modified droplet target 625 may be formed so that it is not flat or perpendicular to the direction of the main pulse beam 610. As shown, Ryrepresents a tilt angle of the disk-shaped modified droplet target 625 around the y-direction. Similarly, a quantity Rx(not shown) can represent a tilt angle of the disk-shaped modified droplet target 625 around the x-direction. In some embodiments, the desired tilt angle Rxor Rymay be based on one or more of the target size, overlap between the target and the main pulse beam size, a comparison of the main pulse beam size and the target size, among other features.

[0074] Some conditions of plasma generation to achieve the desirable high stability and high openloop output power EUV radiation from a EUV radiation source include, but are not limited to, producing a large target for interaction with main pulse beam, comparably matched main pulse beam size and target size, maximum overlap between main pulse beam and target, among other conditions. It is to be appreciated that the term “target” refers to the modified droplet target (e.g., modified droplet target 625). One or more offsets such as L2DX, L2DY, L2DZ, or pre-pulse energy, main pulse pedestal energy (i.e., rarefaction pulse energy), pre-pulse to main pulse timing delay, dose margin, among other parameters, may be individually or collectively adjusted, using software- adjustable actuators, to have setpoint values that optimize the plasma generation process such that the EUV radiation generated is highly stable, reproducible, and has maximum EUV power. The impact of adjustment of main pulse beam size on the overlap between the modified target and the main pulse beam is discussed in detail with reference to Figs. 7A-7C.

[0075] Plasma optimization techniques employed in previous EUV radiation sources failed to recognize various features that could improve the generation of EUV power. For example, past designs did not adequately support control of the main pulse beam size. In previous systems, actuators or control techniques coupled the main pulse beam size with the offset dz(z-direction offset) of a prepulse beam. The main pulse beam size could only be indirectly adjusted by adjusting the control loop setpoints for L2DZ (or the offset in the z-direction, discussed with reference to Figs. 8A-8C).

[0076] Embodiments of the present disclosure provide systems and methods to optimize plasma control loop setpoints including a control loop setpoint for individually controlling main pulse beam size. Reference is now made to Figs. 7A-7C, which illustrate schematics of the laser-target interaction profdes for varying main pulse beam size, consistent with embodiments of the present disclosure. In some embodiments, an EUV radiation source (e.g., EUV radiation source 400) may include a controller (e.g., processor, a microprocessor, a CPU, etc.) configured to initiate, via a software program, an actuator to individually adjust main pulse beam size. The main pulse beam size may be independently adjusted without affecting a position of the pre-pulse with respect to the target droplet in the z-direction. Adjustment of main pulse beam size may influence the extent or degree of overlap between the modified droplet target with the main pulse beam.

[0077] In some embodiments, the main pulse beam size (MPW) may be adjusted by adjusting the pressure in a pressurized pre-pulse box (not shown). The pre-pulse box pressure may serve to function as an actuator for controlling main pulse beam size. Other actuators for adjusting MPWare also envisioned, including selectable or adjustable delay lines and selectable or adjustable focusing optics. Figs. 7A, 7B, and 7C illustrate a pre-pulse beam 710, a droplet 720 illuminated by pre-pulse beam 710 and having an offset in the z-direction with respect to the focal plane 705 of pre-pulse beam 710, a main pulse beam 730, and a modified droplet target 725. Focal plane of pre-pulse beam 710 refers to the plane perpendicular to the z-direction along which the focal point (e.g., focal point 515 of Fig. 5) of pre-pulse beam 710 lies. In some embodiments, focal plane 705 may substantially coincide with the waist of pre-pulse beam or the plane along which the pre-pulse fluence is the highest.

[0078] In some embodiments, the pressure in pressurized pre-pulse box may be adjusted to adjust the main pulse beam size, thereby adjusting the overlap between main pulse beam 730 and modified droplet target 725. As shown in Fig. 7A, main pulse beam size MPw.i, measured at the horizontal plane corresponding to the pre-pulse z-plane focus, is substantially large compared to the target size. The pressure in pressurized pre-pulse box may be adjusted to a different pressure value, using a software -adjustable actuator, to produce main pulse beam 730 having a smaller main pulse beam size MPW.2 (illustrated in Fig. 7B), which is more matched with respect to modified droplet target size. The software -adjustable actuator may be further adjusted to adjust the pressure in the pre-pulse pressure box to produce main pulse beam 730 having a still smaller main pulse beam size MPW.3 (illustrated in Fig. 7C), which is small compared to modified droplet target size. It is to be appreciated that there may be a negligible but finite degree of crosstalk between the pressure in pre-pulse pressure box and the separation in x- and y-directions of pre-pulse beams to main pulse beams.

[0079] Figs. 8A-8C illustrate schematics of the laser-target interaction profiles for varying offset distances in a direction parallel to the propagation of pre-pulse laser beam (z-direction), consistent with embodiments of the present disclosure. Figs. 8A-8C show a schematic of a pre-pulse beam 810 and a main pulse beam 830. The offset dzindicates the z-direction offset of the position of droplet with respect to the focal point of pre-pulse beam or the pre-pulse beam waist. The offset dzmay determine the pre-pulse fluence on the target droplet, thereby influencing the size of the modified droplet target generated by the interaction of pre-pulse beam and the droplet.

[0080] Fig. 8A illustrates a position of droplet 820-1 interacting with pre-pulse beam 810 such that the offset in z-direction is a positive value, +dz, indicating that the z-position of droplet 820-1 is lower than the reference z-position, or the focal point of pre-pulse beam 810. Modified droplet target 825-1 corresponds to the target formed after interaction of pre-pulse beam 810 with droplet 820-1 with a +dzoffset. As shown in Fig. 8B, droplet 820-2, having a zero offset (dz= 0) is illuminated with the maximum pre-pulse fluence of pre-pulse beam 810, thereby generating a larger modified droplet target 825-2 compared to modified droplet target 825-1 or modified droplet target 825-3. Fig. 8C shows droplet 820-3, having a -dzoffset with a negative value, forming a smaller modified droplettarget 825-3 compared to modified droplet target 825-2 because the pre-pulse fluence incident on droplet 820-3 is lower than the pre-pulse fluence incident on droplet 820-2. In operation, however, the measured dzmay not accurately represent the actual dzdue to errors in metrology, crosstalk, among other factors. For example, an L2DZ setpoint may be a negative offset (-dz) in the range of 100 pm - 200 pm, or 110 pm - 200 pm, or 120 pm - 200 pm, or 130 pm - 200 pm, or 140 pm - 200 pm, or 150 pm - 200 pm. In some embodiments, the L2DZ offset may be -dz= 175 pm.

[0081] In some embodiments, offset dzmay be adjusted using a software-adjustable actuator. In some embodiments, adjusting an actuator to adjust offset dzmay include adjusting a position of an optical mirror associated with the laser source and configured to influence a position of a laser beam with respect to the droplet based on the modified droplet target size.

[0082] Reference is now made to Figs. 9A-9C, which illustrate schematics of the laser-target interaction profiles for varying offset distances in a direction parallel to the droplet stream (x- direction), consistent with embodiments of the present disclosure. Figs. 9A, 9B, and 9C illustrate a pre-pulse beam 910 and a main pulse beam 930. Interaction between target droplets 920-1, 920-2, and 920-3 and pre-pulse beam 910 may result in formation of corresponding modified droplet targets 925- 1, 925-2, and 925-3, as shown in Figs. 9A, 9B, and 9C, respectively.

[0083] In some embodiments, offset dxmay be adjusted using a software -adjustable actuator. In some embodiments, adjusting an actuator to adjust offset dxmay include adjusting a timing of firing a pre-pulse beam (e.g., pre-pulse beam 910) as the droplet travels in the (-x) direction, away from the droplet generator (e.g., droplet generator 420 of Fig. 4). Offset dx, or L2DX, may be used as a setpoint for control loops and an optimum offset dxmay be determined based on one or more of the desired values for target size, target tilt (Ry), or target velocity (Vx) in the x-direction, or overlap between target and main pulse, using the determined setpoint. In some embodiments, a setpoint (e.g., offset dxvalue) that produces the highest open-loop EUV power, indicating a significant overlap between main pulse beam 930 and a modified droplet target in the x-direction, may be used.

[0084] Fig. 9A shows an example of early firing of pre-pulse beam 910 interacting with droplet 920- 1 such that the main pulse 930 and modified droplet target 925-1 are not substantially overlapped in the x-direction. In some embodiments, a tilt of modified droplet target around the y-direction may be adjusted based on the timing of firing. For example, an early firing may result in a positive target tilt angle +Ryaround the y-direction, a late firing may result in a negative target tilt angle -Ryaround the y-direction. As previously discussed, it may be desirable to produce modified droplet target having a tilt at least to minimize back -reflected or reverse power, among other reasons. The mitigation of reverse power may protect optical equipment from damage and may enhance EUV power and stability. Fig. 9B shows an example of timing of firing of pre-pulse beam 910 interacting with droplet 920-2 such that the main pulse 930 and modified droplet target 925-2 are substantially overlapped in the x-direction and the modified droplet target 925-2 has a negligible tilt (Ry~ 0) around the y- direction. Fig. 9C shows an example of a late timing of firing of pre-pulse beam 910 interacting withdroplet 920-3 such that the main pulse 930 and modified droplet target 925-3 are not substantially overlapped in the x-direction and the modified droplet target 925-3 has a negative tilt (-Ry) around the y-direction. It is to be appreciated that the actuator may be configured to adjust the timing of firing of pre-pulse beam, which further determines the offset dx, thereby influencing one or more of the target size, the target tilt, the overlap between the target and main pulse beam in the x-direction, or the target velocity in the x-direction.

[0085] Reference is now made to Figs. 10A-10C, which illustrate schematics of the laser-target interaction profiles for varying offset distances in a direction (y-direction) orthogonal to the propagation of laser beam (pre-pulse beam and main pulse beam) and orthogonal to the droplet stream, consistent with embodiments of the present disclosure. Figs. 10A, 10B, and 10C illustrate a pre-pulse beam 1010 and a main pulse beam 1030. Interaction between target droplets 1020-1, 1020- 2, and 1020-3 and pre-pulse beam 1010 may result in formation of corresponding modified droplet targets 1025-1, 1025-2, and 1025-3, as shown in Figs. 10A, 10B, and 10C, respectively.

[0086] In some embodiments, offset dymay be adjusted using a software -adjustable actuator. Offset dy, or L2DY, may be used as a setpoint for control loops and an optimum offset dymay be determined based on desired values for one or more of the target size, target tilt around the x-direction (Rx), or a target velocity (Vy) in the y-direction, or an overlap between target and the main pulse in the y- direction, using the determined setpoint. In some embodiments, a setpoint (e.g., offset dyvalue) that produces a minimum number of pulses at maximum driver laser gain command (DLGC) (or other limiting parameter of the main pulse beam or other limit of an EUV system), resulting in a more stable EUV output and generates a L2DY setpoint desirable for dose performance, may be used for optimizing plasma generation.

[0087] Fig. 10A illustrates a position of droplet 1020-1 interacting with pre-pulse beam 1010 such that the offset in y-direction is +dy, indicating that the y-position of droplet 820-1 is lower than the reference position, or the focal point of pre-pulse beam 1010. Modified droplet target 1025-1 corresponds to the target formed after interaction of pre-pulse beam 1010 with droplet 1020-1 with a positive +dyoffset. A corresponding view in the y-z plane is shown for reference. As shown in Fig. 10B, droplet 1020-2, having a zero offset (dy= 0) in the y-direction is illuminated with a higher fluence of the pre-pulse beam 1010, thereby generating a larger modified droplet target 1025-2 compared to modified droplet target 1025-1 or modified droplet target 1025-3. Fig. 10C shows droplet 1020-3, having a negative -dyoffset, forming a smaller modified droplet target 1025-3 compared to modified droplet target 1025-2 because the pre-pulse fluence incident on droplet 1020-3 is lower than the pre-pulse fluence incident on droplet 1020-2. In operation, the measured offset dymay not accurately represent the actual dydue to errors in metrology, crosstalk, among other factors.

[0088] In some embodiments, the L2DY offset, used as a setpoint for control loops to optimize plasma generation, may be optimized by an in-line plasma tune-up (IPT) DLGC railing algorithmoperating on production wafers in real-time. An IPT DLGC railing algorithm may enable automatic adjustment of the offset to dynamically update the L2DY offset or the L2DY setpoint.

[0089] In some embodiments, in addition to optimizing the L2DY offset, the IPT DLGC railing algorithm may be used to optimize the L2DX offset as well. In some embodiments, the IPT DLGC railing algorithm may optimize the L2DX and L2DY offsets sequentially or simultaneously.

[0090] Reference is now made to Figs. 11A-11C, which illustrate schematics of the laser-target interaction profdes for varying energy of pre-pulse beam, consistent with embodiments of the present disclosure. In some embodiments, an EUV radiation source (e.g., EUV radiation source 400) may include a controller (e.g., processor, a microprocessor, a CPU, etc.) configured to initiate, via a software program, an actuator to individually adjust pre-pulse beam energy. As used herein, the “prepulse beam energy” refers to an integrated energy of the pre-pulse beam incident on a target droplet. Adjustment of the pre-pulse beam energy may influence the size of the modified droplet target generated.

[0091] In some embodiments, the pre-pulse beam energy may be controlled via a pre-pulse energy controller. The pre-pulse energy controller may use a variety of detectors for measuring the fluence, power, or energy, of a pre-pulse beam. These measurements may be made with the pre-pulse beam illuminating droplets or alternatively, timed so that they do not illuminate droplets. In operation, the pre-pulse energy setpoint may differ from the setpoints determined during plasma optimization process due to reverse power gain stripping, or differing drive laser gain command (DLGC) or other systematic factors introduced by the measurement process.. Figs. 11A, 1 IB, and 11C illustrate a prepulse beam 1110, a droplet 1120 illuminated by pre-pulse beam 1110 and having an offset in the z- direction (-dz) with respect to the focal plane 1105 of pre-pulse beam 1110, a main pulse beam 1130, and modified droplet targets 1125-1, 1125-2, and 1125-3, respectively.

[0092] In some embodiments, a setpoint for pre-pulse beam energy may be determined after a setpoint for offset in the z-direction is determined to enable formation of a larger modified droplet target produced by interaction of pre-pulse beam. Fig. 11A illustrates a modified droplet target 1125-1 produced by a smaller dose of pre-pulse beam energy in comparison to modified droplet targets 1125 - 2 and 1125-3 of Figs. 1 IB and 11C, respectively, which are formed by a larger dose of pre-pulse beam energy. In some embodiments, a desired target size may be used to determine the setpoint for pre-pulse beam energy. In some embodiments, the desired target size may be in the range of 200 pm - 600 pm, or 250 pm - 600 pm, or 300 pm - 600 pm, or 350 pm - 600 pm, or 400 pm - 600 pm, or 400 pm - 500 pm, or 425 pm - 475 pm, or any desirable range. In some embodiments, the desired target size may be 450 pm.

[0093] Reference is now made to Figs. 12A-12C, which illustrate schematics of the laser-target interaction profiles for varying main pulse pedestal energy, consistent with embodiments of the present disclosure. In some embodiments, an EUV radiation source (e.g., EUV radiation source 400) may include or communicate with a controller (e.g., processor, a microprocessor, a CPU, etc.)configured to initiate, via a software program, an actuator to adjust main pulse pedestal energy. As used herein, the “main pulse pedestal energy” refers to an energy of a leakage pulse or a pedestal pulse applied to rarefy the modified droplet before illuminating it with the main pulse. Adjustment of the main pulse pedestal energy may influence the extent of rarefaction of target droplet prior to illumination with the main pulse beam, which may impact one or more of the target size, the overlap between the target and main pulse beam, or debris mitigation, among other things.

[0094] In some embodiments, a setpoint for main pulse pedestal energy may be determined after the setpoint for offset in the z-direction is determined to enable formation of a larger modified droplet target. Fig. 12A illustrates an example of an under-rarefied target 1225-1 formed due to illumination of droplet 1220 by an inadequate main pulse pedestal energy. In some embodiments, an underrarefied target may generate more debris compared to a substantially rarefied target, and therefore, it may be desirable to adequately rarefy a target before illuminating with the main pulse beam.

[0095] Fig. 12B illustrates an example of an optimally rarefied target 1225-2 formed due to illumination of droplet 1220 by an appropriate main pulse pedestal energy. Fig. 12C illustrates an example of an over-rarefied target 1225-3 formed due to illumination of droplet 1220 by an excessive main pulse pedestal energy, resulting in a poor conversion efficiency and thereby lower EUV output power. In some embodiments, the setpoint for main pulse pedestal energy may be determined based on the maximum open-loop EUV power achievable.

[0096] Reference is now made to Figs. 13A-13C, which illustrate schematics of the laser-target interaction profiles for varying pre-pulse to main pulse time delay, consistent with embodiments of the present disclosure. The “pre-pulse to main pulse time delay” refers to the time elapsed between the pre-pulse and main pulse triggers. The lower limit of pre-pulse to main pulse time delay may be constrained by the target expansion time required to make a large target and the upper limit of prepulse to main pulse time delay may be constrained by the integration window of the scanner energy sensors or other limits. In some embodiments, an EUV radiation source (e.g., EUV radiation source 400) may include a controller (e.g., processor, a microprocessor, a CPU, etc.) configured to initiate, via a software program, an actuator to adjust the pre-pulse to main pulse time delay. Adjustment of the pre-pulse to main pulse time delay may influence one or more of an overlap between main pulse and the target, or the target size, among other things.

[0097] Fig. 13A illustrates an example of a short delay between pre -pulse beam 1310 and main pulse beam 1330. The short time delay limits the overall expansion of the target to full width and also limits the overlap between modified droplet target 1325-1 and main pulse beam 1330, thereby affecting the ability of the system to generate high open-loop EUV power. In some embodiments, the pre-pulse to main pulse time delay may be adjusted, using a software -adjustable actuator (or a timing circuit of a controller), to adjust the overall target size, target tilt, or the overlap between the target and the main pulse beam. In some embodiments, one or more upper limits of the adjustment of pre-pulse to main pulse delay may be determined based on its impact on target size, on the shape of the plasma, or onthe position of the plasma with respect to the intermediate focus. In some embodiments, an upper limit of the pre-pulse to main pulse delay may be imposed to avoid compromising the shape of the plasma or the position of the plasma with respect to the intermediate focus.

[0098] Fig. 13B illustrates an example of a desirable pre-pulse to main pulse time delay resulting in maximum overlap between modified droplet target 1325-2 and main pulse beam 1330. Fig. 13C illustrates an example of a long delay between pre-pulse beam 1310 and main pulse beam 1330, resulting in inadequate overlap between modified droplet target 1325-3 and main pulse beam 1330.

[0099] In some embodiments, the pre-pulse to main pulse time delay may be used in combination with adjustment of offset dxand offset dyto achieve a desired target tilt Ryand a desired target overlap with the main pulse beam. In some embodiments, the desired target tilt Ry, around the y-direction, may be in the range of 10° - 30°, or 12° - 30°, or 14° - 30°, or 16° - 30°, or 18° - 30°, or 10° - 28°, or 10° - 26°, or 10° - 24°, or 10° - 22°, or 10° - 20°, or any suitable range. In some embodiments, the desired target tilt Rymay be 19° ± 3°.

[0100] Figs. 14A and 14B illustrate schematics of an exemplary technique to optimize the target tilt Ryby adjusting a combination of setpoints, consistent with embodiments of the present disclosure. Fig. 14A illustrates a time-sequence of events representing the transformation of a target droplet 1420 into a modified target droplet 1425-2 with an undesired tilt angle or tilt direction. A pre-pulse beam 1410 having an initial pre-pulse beam energy, offset dx(L2DX), and offset dy(L2DY), may illuminate droplet 1420 to modify the shape of droplet 1420, forming modified droplet target 1425-1, which, due to its momentum and due to expansion, moves and evolves overtime into target 1425-2. Main pulse beam 1430, having an initial pre-pulse to main pulse time delay, may illuminate modified droplet target 1425-2. While there may be a significant overlap between modified droplet target 1425-2 and main pulse beam 1430, the negative target tilt is undesirable, indicating that the initially selected prepulse to main pulse time delay is not optimized. To overcome the issue of undesired target tilt, the pre-pulse to main pulse time delay may be increased, allowing modified droplet target 1425-2 to travel further outside main pulse beam 1430, forming 1425-3. The adjustment in pre-pulse to main pulse time delay causes a de -optimization of the overlap between modified droplet target 1425-3 and main pulse beam 1430. At this point, L2DX and L2DY may be adjusted for the increased pre-pulse to main pulse time delay, as illustrated in Fig. 14B. Adjusting offset dxinfluences the timing and location of interaction between pre-pulse beam 1410 and droplet 1420-2. As previously discussed with reference to Fig. 9A, an early firing of a pre -pulse beam (e.g., pre-pulse beam 1410) on a droplet target (e.g., droplet 1420-2) with a more positive dxresults in a more positive target tilt +Ry, and lower target velocity Vx. The slower moving modified droplet target 1425-4, with the desired positive target tilt +Ry, may be better synchronized with main pulse beam 1430 due to the longer pre-pulse to main pulse time delay. In addition, the lower velocity may allow for increased expansion of modified droplet target 1425-4 before being illuminated by main pulse beam 1430. This plasma setpoint optimization routine produces a plasma generation recipe with optimized setpoints for L2DX, L2DY,and pre-pulse to main pulse time delay to form modified droplet target 1425-5, having a desired target tilt and a desired overlap with main pulse beam 1430.

[0101] As previously discussed, embodiments of the present disclosure provide systems and methods to independently and directly control the main pulse beam size using an actuator configured to perform the adjustment of main pulse beam size. This ability provides various combinations of setpoints to optimize the plasma generation to enhance the stability and output power of the EUV pulses, thereby providing an improved plasma optimization routine compared to previous optimization techniques. In addition, the proposed plasma optimization routine, implemented through embodiments of this disclosure, utilizes different metrics for selection of each existing setpoint to create large Sn targets centered in sufficiently large main pulse beam. Sn targets, which are centered in a sufficiently large main pulse beam, may produce higher open loop EUV signal as well as more stable (less variability) EUV signal on a shot-to-shot basis, leading to lower dose margin. Energy per pulse at maximum driver laser (DL) gain is referred to as the maximum open loop EUV power. Dose margin, as used herein, refers to a percentage of the maximum open loop capability that is deliberately under-used in order to run in a closed loop EUV scenario with sufficient overhead to deliver EUV with no dose errors (or with an acceptably low incidence of dose errors). In plasma setup processes, it is desirable to seek operating setpoints that provide high EUV power output, but with high stability— which enables low dose margins. With a setup that enables low dose margins, the usable power (closed loop EUV power) is increased, which enables operation with increased power and thus increased overall die throughput.

[0102] Reference is now made to Fig. 15, which illustrates a process flowchart of an exemplary method 1500 for optimizing tilt angle of a target in an EUV radiation source, consistent with embodiments of the present disclosure. In some embodiments, it may be desirable to optimize the target tilt Ryto produce stable and high closed-loop EUV output power. The exemplary method 1500 to optimize the target tilt Rymay be altered to modify the order of steps and to include additional or fewer steps.

[0103] Step 1510 includes checking whether an initially measured target tilt Ryis within a desired range of tilt angles. In some embodiments, the desired target tilt Ry, with respect to the x-direction, may be in the range of 10° - 30°, or 12° - 30°, or 14° - 30°, or 16° - 30°, or 18° - 30°, or 10° - 28°, or 10° - 26°, or 10° - 24°, or 10° - 22°, or 10° - 20°, or any suitable range.

[0104] A target tilt below the minimum desired tilt (target tilt A of method 1500), may increase the back-reflected power or the reverse power, increasing the risk of equipment damage (e.g., laser seed) and limiting the forward power gain of the laser beams to generate high EUV output power. A target tilt above the maximum desired tilt (target tilt B of method 1500) may not adequately overlap with the main pulse beam, thereby reducing the conversion efficiency and the achievable closed-loop EUV output power. Therefore, it is desirable to maintain the target tilt within a desirable range of tilt angles.

[0105] Step 1520 includes determining whether target tilt Ryis within a desired range between angles A and B. If target tilt Ryis outside the desired range, the pre-pulse to main pulse time delay may be adjusted to adjust the target tilt Ry. If target tilt Ryis below the minimum desired tilt, determined in step 1530, the pre-pulse to main pulse time delay may be increased (step 1550), and if target tilt Ryis above the maximum desired tilt (determined in step 1540), the pre-pulse to main pulse time delay may be reduced (step 1560). The pre-pulse to main pulse time delay may be adjusted in time intervals in the range of 50 nanoseconds to 300 nanoseconds (ns). In some embodiments, the pre-pulse to main pulse time delay may be iteratively increased or reduced by 150 ns.

[0106] As an example, if the target tilt Ryis undesirably low, the pre-pulse to main pulse time delay may be increased, allowing the modified droplet target to travel further in x-direction beyond the main pulse beam, causing a de-optimization of the overlap between modified droplet target and the main pulse beam. For the increased pre-pulse to main pulse time delay, offset dx(L2DX) and offset dy(L2DY) may be adjusted (steps 1570 and 1580, respectively). Adjusting offset dxinfluences the timing and location of the interaction between the pre-pulse beam and droplet. As previously discussed with reference to Fig. 9A, an early firing of a pre-pulse beam on a droplet target with a positive dxresults in a positive target tilt +Ry, and lower target velocity Vx. The slower moving modified droplet target, with the desired positive target tilt +Ry, may be better synchronized with main pulse beam due to the longer pre-pulse to main pulse time delay. In addition, the lower velocity may allow for maximum expansion of modified droplet target before being illuminated by main pulse beam.

[0107] Method 1500 further includes re-checking whether the adjusted target tilt Ryis within a desirable range. One or more of the above steps may be repeated to optimize the target tilt to form plasma, from which high stability, high closed-loop EUV output power may be generated.

[0108] Reference is now made to Fig. 16, which illustrates a process flowchart of an exemplary method 1600 for initializing an EUV radiation source, consistent with embodiments of the present disclosure. Method 1600 may include steps performed to optimize plasma generation by optimizing the main pulse beam size to minimize a variation of energy of the EUV light pulses. In some embodiments, the illustrated method 1600 may be altered to modify the order of steps and to include additional or fewer steps.

[0109] Step 1610 includes generating a stream of droplets of atarget material (e.g., droplets 425 of Fig. 4). The target material may include, but is not limited to, a material that includes Sn, Li, Xe, or combinations thereof. The EUV emitting element, e.g., tin, lithium, or xenon may be in the form of liquid droplets, or solid particles contained within liquid droplets, or any other form which delivers the EUV emitting element to the target volume in discrete amounts. For example, Sn may be used as pure tin, as a tin compound, e.g., tin bromide (SnBr4), tin dibromide (SnBr2), tin hydride (SnH ). or as a tin alloy, e.g., tin-gallium (Sn-Ga) alloys, tin-indium (Sn-In) alloys, tin-indium -gallium (Sn-In-Ga) alloys, or a combination thereof.

[0110] The droplets are generated using a droplet generator (e.g., droplet generator 420 of Fig. 4). The droplet generator is configured to generate either (i) one or more streams of droplets of target material or (ii) one or more continuous streams of target material which exit the droplet generator and subsequently break into droplets due to surface tension. In some embodiments, the droplet generator may have a single orifice, or multiple orifices. The droplet generator is configured to generate and deliver the plurality of droplets at a frequency between 50 kHz and 200 kHz.

[0111] Step 1620 includes illuminating a droplet with a first pulse of light to initiate a modification of a shape of the droplet, forming a modified droplet. A light pulse, in this context, refers to a pulsed laser radiation generated from a laser source (e.g., radiation source SO of Fig. 1, radiation source 210 of Fig. 2, or excitation device 310 of Fig. 3, or laser source 410 of Fig. 4). The laser source may be a part of an EUV radiation source such as a laser produced plasma (LPP) EUV source. In some embodiments, a fuel droplet stream (e.g., Sn droplets) may be illuminated with a pulsed laser radiation provided by one or more laser sources. To generate EUV radiation, a series of laser pulses are injected so as to intercept each Sn droplet of the droplet stream. The Sn droplets may change shape upon interaction with the laser pulse although they may still be referred to as droplets or, alternatively, the droplet may be more generally referred to as a Sn target or a fuel target. The series of laser pulses may comprise two or more laser pulses for each droplet. The EUV radiation source may use a low -energy first laser pulse, also referred to as a pre-pulse, which upon being incident on the fuel droplet causes the shape of the fuel droplet to change from a substantially spherical droplet to a flatter, disk-shaped target.

[0112] Step 1630 includes illuminating the droplet with a second pulse of light to substantially evaporate and highly ionize the modified droplet, thereby generating pulses of EUV light. The laser source may provide a high-energy pulse, also referred to as a main pulse, which upon being incident on the target, causes conversion of at least a portion of the target into plasma at a plasma formation region (e.g., plasma formation region 4 of Fig. 1, plasma formation region 370 of Fig. 3, or plasma formation region 470 of Fig. 4). The portion of the target converted to plasma can emit radiation, including EUV radiation, during de-excitation and recombination of electrons with ions of the plasma. Although not explicitly mentioned, step 1630 may further include illuminating the modified droplet with a third pulse of light to disperse and / or reduce a density of the modified droplet prior to being substantially evaporated, the third pulse of light having a pedestal energy configured to be adjustable based on a desired reduction in the density of the modified droplet.

[0113] Step 1640 includes adjusting a width of a beam that provides the second pulse to minimize a variation of energy of the pulses of EUV light. In some embodiments, an EUV radiation source (e.g., EUV radiation source 400 of Fig. 4) may include or communicate with a controller configured to initiate, via a software program, an actuator to individually adjust a size of a beam that provides the main pulse. The width of the main pulse beam (or the main pulse beam size) may be independently adjusted without affecting a position of the pre-pulse with respect to the target droplet in the z-direction. In some embodiments, adjusting main pulse beam size may influence the extent or degree of overlap between the modified droplet target with the main pulse beam. The main pulse beam size (MPW) may be adjusted by adjusting the pressure in a pressurized pre-pulse box (not shown). The prepulse box pressure may serve to function as an actuator for controlling main pulse beam size. The pressure in pressurized pre-pulse box may be adjusted to adjust the main pulse beam size, thereby adjusting the overlap between main pulse beam and modified droplet target (e.g., modified droplet target 725 of Fig. 7A).

[0114] A non-transitory computer readable medium may be provided that stores instructions for one or more processors of a controller to directing components to carry out, among other things, activating laser sources, generating and delivering droplets into a chamber, power amplification, frequency modulation, activating one or more actuators for laser beam control, simulation by executing algorithms, computing, and at least some steps of methods 1500 and 1600. 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.

[0115] Block diagrams in the figures may illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer hardware or software products according to various exemplary embodiments of the present disclosure. In this regard, each block in a schematic diagram may represent certain arithmetical or logical operation processing that may be implemented using hardware such as an electronic circuit. Blocks may also represent a module, segment, or portion of code that comprises one or more executable instructions for implementing the specified logical functions. It should be understood that in some alternative implementations, functions indicated in a block may occur out of the order noted in the figures. For example, two blocks shown in succession may be executed or implemented substantially concurrently, or two blocks may sometimes be executed in reverse order, depending upon the functionality involved. Some blocks may also be omitted. It should also be understood that each block of the block diagrams, and combination of the blocks, may be implemented by special purpose hardware-based systems that perform the specified functions or acts, or by combinations of special purpose hardware and computer instructions.

[0116] The embodiments of the present disclosure may further be described using the following clauses:1. An extreme ultraviolet (EUV) radiation source, comprising: a droplet generator configured to generate a stream of droplets of a target material;a laser source communicatively coupled to a controller and configured to: generate a first pulsed beam of light, which upon interaction with a droplet, initiates a modification of a shape of the droplet, forming a modified droplet, and generate a second pulsed beam of light configured to substantially evaporate the modified droplet, thereby generating pulses of EUV light; and a first actuator communicatively coupled to the controller and configured to adjust a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.2. The radiation source of clause 1, wherein the first actuator is further configured to independently adjust the width of the second pulsed beam without influencing a position of the first pulsed beam in a direction parallel to a propagation of the first pulsed beam.3. The radiation source of clause 2, further comprising a second actuator communicatively coupled to the controller and configured to adjust an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in the direction parallel to the propagation of the first pulsed beam.4. The radiation source of clause 3, wherein adjustment of the offset distance enables maximization of a size of the modified droplet for interaction with the second pulsed beam.5. The radiation source of any one of clauses 3 and 4, wherein the second actuator is further configured to adjust the offset distance to a predetermined value, and wherein the predetermined value of the offset distance is in a range from 150 microns (pm) to 200 microns (pm) with respect to the focal point of the first pulsed beam.6. The radiation source of clause 5, wherein the offset distance is 175 pm with respect to the focal point of the first pulsed beam.7. The radiation source of any one of clauses 1-6, further comprising a third actuator communicatively coupled to the controller and configured to adjust an energy of the first pulsed beam of light generated by the laser source.8. The radiation source of clause 7, wherein an adjustment of the energy of the first pulsed beam of light causes an adjustment of a size of the modified droplet to a desired size.9. The radiation source of clause 8, wherein the desired size of the modified droplet is in a range of 400 pm - 500 pm.10. The radiation source of clause 9, wherein the desired size of the modified droplet is 450 pm.11. The radiation source of any one of clauses 1-10, further comprising a fourth actuator communicatively coupled to the controller and configured to adjust an energy of the second pulsed beam of light generated by the laser source.12. The radiation source of clause 11, wherein an adjustment of the energy of the second pulsed beam of light causes an adjustment of the energy of the generated pulses of EUV light.13. The radiation source of any one of clauses 1-12, further comprising a fifth actuator communicatively coupled to the controller and configured to adjust an offset distance between a focalpoint of the first pulsed beam and a geometric center of the droplet in a direction parallel to the stream of droplets.14. The radiation source of any one of clauses 1-13, further comprising a sixth actuator communicatively coupled to the controller and configured to adjust an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in a direction orthogonal to the stream of droplets and to the first pulsed beam.15. The radiation source of any one of clauses 1-14, wherein the laser source is configured to generate a third pulsed beam of light to interact with the modified droplet to reduce a density of the modified droplet prior to being substantially evaporated, the third pulsed beam of light having a pedestal energy.16. The radiation source of clause 15, further comprising a seventh actuator communicatively coupled to the controller and configured to adjust the pedestal energy to adjust a reduction in the density of the modified droplet.17. The radiation source of any one of clauses 1-16, wherein the droplet generator is configured to generate the stream of droplets at a frequency in a range of 50 kHz - 100 kHz.18. The radiation source of any one of clauses 1-17, wherein the target material comprises a metal or an alloy.19. A method for initializing an extreme ultraviolet (EUV) radiation source, the method comprising: generating a stream of droplets of a target material; illuminating a droplet of the stream of droplets with a first pulsed beam of light to initiate a modification of a shape of the droplet, forming a modified droplet; illuminating the droplet with a second pulsed beam of light to substantially evaporate the modified droplet, thereby generating pulses of EUV light; and adjusting a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.20. The method of clause 19, further comprising independently adjusting the width of the second pulsed beam without influencing a position of the first pulsed beam in a direction parallel to a propagation of the first pulsed beam.21. The method of clause 20, further comprising adjusting an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in the direction parallel to the propagation of the first pulsed beam.22. The method of clause 21, wherein adjusting the offset distance enables maximizing a size of the modified droplet for interaction with the second pulsed beam.23. The method of any one of clauses 21 and 22, further comprising adjusting the offset distance to a predetermined value, and wherein the predetermined value of the offset distance is in a rangefrom -150 microns (pm) to -200 microns (pm) with respect to the focal point of the first pulsed beam.24. The method of any one of clauses 19-23, further comprising adjusting an energy of the first pulsed beam of light to adjust a size of the modified droplet to a desired size in a range of 400 pm - 500 pm.25. The method of any one of clauses 19-24, further comprising adjusting an energy of the second pulsed beam of light to adjust the energy of the pulses of EUV light.26. The method of any one of clauses 19-25, further comprising adjusting an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in a direction parallel to the stream of droplets.27. The method of any one of clauses 19-26, further comprising adjusting an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in a direction orthogonal to the stream of droplets and to the first pulsed beam.28. The method of any one of clauses 19-27, further comprising illuminating the modified droplet with a third pulsed beam of light to reduce a density of the modified droplet prior to being substantially evaporated, the third pulsed beam of light having a pedestal energy configured to be adjustable based on a desired reduction in the density of the modified droplet.29. The method of any one of clauses 19-28, wherein the target material comprises a metal or an alloy.30. The method of any one of clauses 19-29, further comprising generating the stream of droplets at a frequency in a range of 50 kHz - 100 kHz.31. A lithographic system, comprising: a lithographic apparatus; and an extreme ultraviolet (EUV) radiation source of clause 1, configured to generate and supply EUV radiation to the lithographic apparatus.32. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of a system to cause the system to perform operations comprising: activating a droplet generator to generate a stream of droplets of a target material; illuminating a droplet of the stream of droplets with a first pulsed beam of light to initiate a modification of a shape of the droplet, forming a modified droplet; illuminating the droplet with a second pulsed beam of light to substantially evaporate the modified droplet, thereby generating pulses of EUV light; and adjusting, using an actuator, a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.33. The non-transitory computer readable medium of clause 32, wherein the operations further comprise independently adjusting the width of the second pulsed beam without influencing a position of the first pulsed beam in a direction parallel to a propagation of the first pulsed beam.34. The non-transitory computer readable medium of clause 33, wherein the operations further comprise adjusting an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in the direction parallel to the propagation of the first pulsed beam.35. The non-transitory computer readable medium of clause 33, wherein adjusting the offset distance enables maximizing a size of the modified droplet for interaction with the second pulsed beam.36. The non-transitory computer readable medium of any one of clauses 34 and 35, wherein the operations further comprise adjusting the offset distance to a predetermined value, and wherein the predetermined value of the offset distance is in a range from -150 microns (pm) to -200 microns (pm) with respect to the focal point of the first pulsed beam.37. The non-transitory computer readable medium of any one of clauses 32-36, wherein the operations further comprise adjusting an energy of the first pulsed beam of light to adjust a size of the modified droplet to a desired size in a range of 400 pm - 500 pm.38. The non-transitory computer readable medium of any one of clauses 32-37, wherein the operations further comprise adjusting an energy of the second pulsed beam of light to adjust the energy of the pulses of EUV light.39. The non-transitory computer readable medium of any one of clauses 32-38, wherein the operations further comprise adjusting an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in a direction parallel to the stream of droplets.40. The non-transitory computer readable medium of any one of clauses 32-39, wherein the operations further comprise adjusting an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in a direction orthogonal to the stream of droplets and to the first pulsed beam.41. The non-transitory computer readable medium of any one of clauses 32-40, wherein the operations further comprise illuminating the modified droplet with a third pulsed beam of light to reduce a density of the modified droplet prior to being substantially evaporated, the third pulsed beam of light having a pedestal energy configured to be adjustable based on a desired reduction in the density of the modified droplet.42. The non-transitory computer readable medium of any one of clauses 32-41, wherein the operations further comprise generating the stream of droplets at a frequency in a range of 50 kHz - 100 kHz.43. The non-transitory computer readable medium of any one of clauses 32-42, wherein the target material comprises a metal or an alloy.

[0117] 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. The present disclosure has been described in connection with various embodiments, other embodiments ofthe invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

CLAIMS1. An extreme ultraviolet (EUV) radiation source, comprising: a droplet generator configured to generate a stream of droplets of a target material; a laser source communicatively coupled to a controller and configured to: generate a first pulsed beam of light, which upon interaction with a droplet, initiates a modification of a shape of the droplet, forming a modified droplet, and generate a second pulsed beam of light configured to substantially evaporate the modified droplet, thereby generating pulses of EUV light; and a first actuator communicatively coupled to the controller and configured to adjust a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.

2. The radiation source of claim 1, wherein the first actuator is further configured to independently adjust the width of the second pulsed beam without influencing a position of the first pulsed beam in a direction parallel to a propagation of the first pulsed beam.

3. The radiation source of claim 2, further comprising a second actuator communicatively coupled to the controller and configured to adjust an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in the direction parallel to the propagation of the first pulsed beam.

4. The radiation source of claim 3, wherein adjustment of the offset distance enables maximization of a size of the modified droplet for interaction with the second pulsed beam.

5. The radiation source of any one of claims 3 and 4, wherein the second actuator is further configured to adjust the offset distance to a predetermined value, and wherein the predetermined value of the offset distance is in a range from 150 microns (pm) to 200 microns (pm) with respect to the focal point of the first pulsed beam.

6. The radiation source of claim 5, wherein the offset distance is 175 pm with respect to the focal point of the first pulsed beam.

7. The radiation source of any one of claims 1-6, further comprising a third actuator communicatively coupled to the controller and configured to adjust an energy of the first pulsed beam of light generated by the laser source.

8. The radiation source of claim 7, wherein an adjustment of the energy of the first pulsed beam of light causes an adjustment of a size of the modified droplet to a desired size.

9. The radiation source of claim 8, wherein the desired size of the modified droplet is in a range of 400 pm - 500 pm.

10. The radiation source of claim 9, wherein the desired size of the modified droplet is 450 pm.

11. The radiation source of any one of claims 1-10, further comprising a fourth actuator communicatively coupled to the controller and configured to adjust an energy of the second pulsed beam of light generated by the laser source.

12. The radiation source of claim 11, wherein an adjustment of the energy of the second pulsed beam of light causes an adjustment of the energy of the generated pulses of EUV light.

13. The radiation source of any one of claims 1-12, further comprising a fifth actuator communicatively coupled to the controller and configured to adjust an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in a direction parallel to the stream of droplets.

14. The radiation source of any one of claims 1-13, further comprising a sixth actuator communicatively coupled to the controller and configured to adjust an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in a direction orthogonal to the stream of droplets and orthogonal to the first pulsed beam.

15. The radiation source of any one of claims 1-14, wherein the laser source is configured to generate a third pulsed beam of light to interact with the modified droplet to reduce a density of the modified droplet prior to being substantially evaporated, the third pulsed beam of light having a pedestal energy.

16. The radiation source of claim 15, further comprising a seventh actuator communicatively coupled to the controller and configured to adjust the pedestal energy to adjust a reduction in the density of the modified droplet.

17. The radiation source of any one of claims 1-16, wherein the droplet generator is configured to generate the stream of droplets at a frequency in a range of 50 kHz - 100 kHz.

18. The radiation source of any one of claims 1-17, wherein the target material comprises a metal or an alloy.

19. A method for initializing an extreme ultraviolet (EUV) radiation source, the method comprising: generating a stream of droplets of a target material; illuminating a droplet of the stream of droplets with a first pulsed beam of light to initiate a modification of a shape of the droplet, forming a modified droplet; illuminating the droplet with a second pulsed beam of light to substantially evaporate the modified droplet, thereby generating pulses of EUV light; and adjusting a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.

20. The method of claim 19, further comprising independently adjusting the width of the second pulsed beam without influencing a position of the first pulsed beam in a direction parallel to a propagation of the first pulsed beam.

21. The method of claim 20, further comprising adjusting an offset distance between a focal point of the first pulsed beam and a geometric center of the droplet in the direction parallel to the propagation of the first pulsed beam.

22. The method of claim 19, further comprising adjusting an energy of the second pulsed beam of light to adjust the energy of the pulses of EUV light.

23. A lithographic system, comprising: a lithographic apparatus; and an extreme ultraviolet (EUV) radiation source of claim 1, configured to generate and supply EUV radiation to the lithographic apparatus.

24. A non-transitory computer readable medium that stores a set of instructions that is executable by one or more processors of a system to cause the system to perform operations comprising: activating a droplet generator to generate a stream of droplets of a target material; illuminating a droplet of the stream of droplets with a first pulsed beam of light to initiate a modification of a shape of the droplet, forming a modified droplet; illuminating the droplet with a second pulsed beam of light to substantially evaporate the modified droplet, thereby generating pulses of EUV light; andadjusting, using an actuator, a width of the second pulsed beam to minimize a variation of energy of the pulses of EUV light.

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