Systems and methods for debris mitigation in EUV light sources

Abstract

Systems and methods for debris mitigation associated with the operation of a radiation source are disclosed. The radiation source may comprise a vessel defining an interior space symmetrically around a primary optical axis, the vessel comprising a first plurality of guideway openings circumferentially arranged and angled through a wall of the vessel. The method for debris mitigation may include directing a stream of droplets of a target material into a vessel to generate a radiation upon interaction with a pulsed laser beam, and injecting a gas into an interior volume of the vessel using a first plurality of guideway openings circumferentially arranged and angled through a wall of the vessel, wherein injecting the gas through the first plurality of guideway openings enables generating a vortex within the vessel to influence a path of a contaminant in the vessel.

Description

SYSTEMS AND METHODS FOR DEBRIS MITIGATION IN EUV LIGHT SOURCESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 701,345, filed September 30, 2024, titled SYSTEMS AND METHODS FOR DEBRIS MITIGATION IN EUV LIGHT SOURCES, 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 mitigating tin (Sn) debris in EUV light sources.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 wafer inspection apparatus, a dimension measurement apparatus, 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 the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a maskcontaining the phase defects. Such phase defects may be correctly detected by using the same or similar actinic EUV wavelength (13.5 nm) 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] A metrology apparatus is an apparatus that measures critical dimension and inspect various aspects of the wafer during the semiconductor manufacturing process. A metrology apparatus can also measure and characterize physical properties of materials and components. The metrology apparatus is a precision instrument that ensures product quality and process control. In at least one embodiment, the metrology apparatus employs EUV radiation to inspect and measure dimensions of targets on the substrate.

[0007] 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

[0008] Some embodiments of the present disclosure provide systems and methods for mitigating tin debris in EUV light sources. One aspect of the disclosure is directed to a radiation source comprising a vessel defining an interior space symmetrically around a primary optical axis, the vessel comprising a first plurality of guideway openings circumferentially arranged and angled through a wall of the vessel.

[0009] Another aspect of this disclosure is directed to a method for reducing contamination associated with operation of a radiation source. The method comprises directing a stream of droplets of a target material into a vessel to generate a radiation upon interaction with a pulsed laser beam, and injecting a gas into an interior volume of the vessel using a first plurality of guideway openings circumferentially arranged and angled through a wall of the vessel, wherein injecting the gas throughthe first plurality of guideway openings enables generating a vortex within the vessel to influence a path of a contaminant in the vessel.

[0010] Yet another aspect of this disclosure is directed to a lithographic system comprising a lithographic apparatus and a radiation source configured to supply EUV radiation to the lithographic apparatus. The radiation source comprises a vessel defining an interior space symmetrically around a primary optical axis, the vessel comprising a first plurality of guideway openings circumferentially arranged and angled through a wall of the vessel.BRIEF DESCRIPTION OF FIGURES

[0011] 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.

[0012] 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 .

[0013] Fig. 2 illustrates an exemplary mask inspection apparatus used in combination with an EUV radiation source, consistent with embodiments of the present disclosure.

[0014] Fig. 3 illustrates a schematic diagram of an exemplary alternative EUV radiation source, consistent with embodiments of the present disclosure.

[0015] Fig. 4 illustrates a simplified schematic of an exemplary laser produced plasma (LPP) EUV radiation source, consistent with embodiments of the present disclosure.

[0016] Fig. 5 illustrates a simplified schematic of source of contamination in an exemplary LPP EUV radiation source.

[0017] Fig. 6A illustrates a cross-section schematic of an exemplary gas management module for a source vessel of a LPP EUV radiation source, consistent with embodiments of the present disclosure.

[0018] Fig. 6B illustrates a top planar cross-section schematic view of a source vessel along the plane A-A' shown in Fig. 6A, consistent with embodiments of the present disclosure.

[0019] Fig. 7A shows a schematic of an exemplary source vessel of a radiation source including guideway openings configured to inject gas into the vessel, consistent with embodiments of the present disclosure.

[0020] Fig. 7B illustrates a graphical representation of gas velocity as a function of distance from the edge of a frustum structure including guideway openings, consistent with embodiments of the present disclosure.

[0021] Figs. 8A, 8B, and 8C illustrate exemplary arrangements of guideway openings circumferentially arranged on a frustum structure of a source vessel, consistent with embodiments of the present disclosure.

[0022] Figs. 9A and 9B illustrate schematics of cross-section views of a frustum structure including multiple guideway openings, consistent with embodiments of the present disclosure.

[0023] Figs. 10A and 10B illustrate schematics of cross-section views of a frustum structure including multiple guideway openings, consistent with embodiments of the present disclosure.

[0024] Figs. 11A, 1 IB, and 11C illustrate schematics of cross-section views of a frustum structure including exemplary nozzle designs, consistent with embodiments of the present disclosure.

[0025] Fig. 12 illustrates a process flowchart of an exemplary method 1200 for reducing contamination associated with an operation of a fuel-based radiation source, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0026] 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.

[0027] 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 elements unless 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.

[0028] 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 theresolution, and therefore, much smaller features can be produced on the substrate using EUV radiation.

[0029] 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.

[0030] 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.

[0031] As used herein, the term “optic” and its derivatives include, but are not necessarily limited to, components which reflect, or transmit, or operate on incident light and includes, but is not limited to, lenses, windows, filters, wedges, prisms, grisms, gratings, etalons, diffusers, transmission fibers, detectors and other instrument components, apertures, stops and mirrors including multi-layer mirrors, near-normal incidence mirrors, grazing incidence mirrors, specular reflectors and diffuse reflectors. Moreover, as used herein, the term “optic” and its derivatives are not meant to be limited to components which operate solely or to advantage within one or more specific wavelength range (s) such as at the EUV output light wavelength, the irradiation laser wavelength, a wavelength suitable for metrology or some other wavelength.

[0032] 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.

[0033] 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.

[0034] 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, amachine-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.

[0035] 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.

[0036] 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 .

[0037] 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).

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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 13.5nm). 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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 EUVsources. 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.

[0049] 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 tin 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 tin (unionized) and tin ions approaching the reflective surface of collector mirror 340, and further, to minimize deposition of Sn ions by enabling a chemical reaction between tin 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.

[0050] Commonly used EUV radiation having a wavelength of 13.5 nm is obtained from plasma emissions of highly charged tin ions. It is to be appreciated that while Li, Xe, and tin, all of which have ions with strong resonance transitions within the desirable bandwidth, may be used to generate EUV radiation, the conversion efficiency for tin is higher than that of Xe and Li. Near 13.5 nm wavelength, the EUV spectrum of highly charged tin 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 tin ions includes high power densities, 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).

[0051] In an exemplary LPP EUV radiation source, a fuel droplet stream (e.g., tin 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 tin droplet of the droplet stream.The tin 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 tin 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., tin 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.

[0052] 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.

[0053] 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:YAG laser 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.

[0054] 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 tin 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.

[0055] Reference is now made to Fig. 4, which illustrates a simplified schematic of an exemplary LPP EUV radiation source, consistent with embodiments of the present disclosure. LPP EUV radiation source 400 comprises a laser source 410 for generating a pulsed laser and delivering laser pulses into a chamber 405. The laser pulses may travel along one or more paths from laser source 410 into chamber 405 to illuminate one or more target material at an irradiation region 462.

[0056] LPP EUV radiation source 400 further includes an EUV controller 420 configured to control triggering one or more lamps or laser devices in laser source 410 to thereby generate light pulses for delivery into chamber 405. LPP EUV radiation source 400 further includes a droplet position detection system 470, which may include one or more droplet imagers configured to provide an output indicative of the position of one or more droplets of the target material, e.g., relative to irradiation region 462. The imager(s) may provide this output to a droplet position detection feedback system 430, which can, e.g., compute a droplet position and trajectory, from which a droplet position error can be computed, e.g., on a droplet-by-droplet basis. The droplet position error may then be provided as an input to controller 420, which can, for example, provide a position, direction or timing correction signal to laser source 410 to control a source timing circuit or to control a beam position and shaping system, e.g., to change the location or focal power of the light pulses being delivered to irradiation region 462 in chamber 405.

[0057] EUV controller 420 is electronically connected to LPP EUV radiation source 400 and is electronically connected to other components as well. EUV controller 420 may be a computer configured to execute various controls of droplet detection system 470, laser source 410, a droplet delivery control system 440, or a droplet delivery mechanism 450. EUV controller 420 also includesprocessing circuitry configured to execute various signal and image processing functions. In some embodiments, EUV controller 420 includes one or more processors (not shown). A processor may be a generic or specific electronic device capable of manipulating or processing information. For example, the processor may include any combination of any number of a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), an optical processor, a programmable logic controllers, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Uogic Array (PUA), a Programmable Array Uogic (PAU), a Generic Array Uogic (GAU), a Complex Programmable Uogic Device (CPUD), a Field-Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), and any type circuit capable of data processing. The processor may also be a virtual processor that includes one or more processors distributed across multiple machines or devices coupled via a network. EUV controller 420 may further include one or more memories (not shown). A memory may be a generic or specific electronic device capable of storing codes and data accessible by the processor (e.g., via a bus). For example, the memory may include any combination of any number of a random-access memory (RAM), a read-only memory (ROM), an optical disc, a magnetic disk, a hard drive, a solid- state drive, a flash drive, a security digital (SD) card, a memory stick, a compact flash (CF) card, or any type of storage device. The codes may include an operating system (OS) and one or more application programs (or “apps”) for specific tasks. The memory may also be a virtual memory that includes one or more memories distributed across multiple machines or devices coupled via a network.

[0058] LPP EUV radiation source 400 further includes a droplet delivery control system 440 operable in response to a signal (which in some implementations may include the droplet error described above, or some quantity derived therefrom) from EUV controller 420, to e.g., modify the release point of the target material from droplet delivery mechanism 450 to correct for errors in the droplets arriving at the desired irradiation region 462. For LPP EUV radiation source 400, droplet delivery mechanism 450 may include, for example, a droplet generator creating either 1) one or more streams of droplets or 2) one or more continuous streams that exit the generator and subsequently break into droplets due to surface tension. In either case, droplets may be generated and delivered to irradiation region 462 such that one or more droplets may simultaneously reside in irradiation region 462, allowing one or more droplets to be simultaneously irradiated by an initial pulse, e.g., prepulse to form an expanded target suitable for exposure to one or more subsequent laser pulse(s), e.g., main pulse(s), to generate an EUV emission. In some embodiments, a multi -orifice dispenser may be used to create a “showerhead-type” effect. In general, LPP EUV radiation source 400, the droplet generator may be modulating or non-modulating and may include one or several orifice(s) through which target material is passed to create one or more droplet streams.

[0059] 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 maybe 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, tin may be used as pure tin, as a tin compound, e.g., tin bromide (SnBr4). tin dibromide (SnB ). tin hydride (SnH4). 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 462 at various temperatures, including room temperature or near room temperature (e.g., tin alloys, SnBr4), or at an elevated temperature (e.g., pure tin), or at temperatures below room-temperature, (e.g., SnH4).

[0060] LPP EUV radiation source 400 further includes a droplet catcher 455, also referred to herein as a tin catcher, or a tin bucket, configured to catch or collect excessive target material. In some embodiments, the droplets of target material may intentionally or be accidentally missed by the laser beam, and collected by droplet catcher 455. In some embodiments, droplet catcher 455 is aligned with droplet delivery mechanism 450 along direction D2 such that droplet catcher 455 and droplet delivery mechanism 450 are placed on diametrically opposite ends of a reflective optics 460.

[0061] Continuing with Fig. 4, LPP EUV radiation source 400 also includes reflective optics 460, e.g., a collector mirror in the form of a truncated ellipsoid having, e.g., a graded multi-layer coating with alternating layers of molybdenum and silicon. In some embodiments, reflective optics 460 is formed with an aperture 461 to allow the light pulses generated by laser source 410 to pass through and reach irradiation region 462. Aperture 461 is also configured to introduce one or multiple buffer gas flows. As shown, the reflective optics 460 may be, e.g., an ellipsoidal mirror that has a first focus within or near irradiation region 462 and a second focus at an intermediate region 464, also referred to herein as an intermediate focus, where the EUV light may be output from LPP EUV radiation source 400 and input to a device utilizing EUV light, e.g., an exposure tool 490 (e.g., a lithography tool or a metrology tool). In some embodiments, reflective optics 460 may be positioned such that the closest operable point on the reflective optics 460 is located at a distance, “ ” from irradiation region 462. It is to be appreciated that other optics may be used in place of the ellipsoidal mirror for collecting and directing light to an intermediate location for subsequent delivery to a device utilizing EUV light, for example reflective optics 460 may be parabolic or may be configured to deliver a beam having a ring-shaped cross-section to an intermediate location.

[0062] In some embodiments, irradiation of the target material at irradiation region 462 produces a plasma and generates an EUV emission. In addition, as a by-product of this process, debris may be generated that exit the plasma, typically, in all directions. Generally, the debris’ initial energy exiting the plasma will vary over a range, with the range being affected by a number of factors including, but not limited to, the wavelength, energy, intensity and pulse-shape of the irradiating light, and the composition, size, shape and form of the target material. Also indicated above, these debris, if unabated, may degrade nearby optics, such as mirrors, laser input windows, metrology windows, filters, etc. To mitigate some issues associated with degradation due to unutilized debris, LPP EUVradiation source 400 may include a gas management system including a regulated gas source 475 for introducing one or more gases into chamber 405 and an exhaust mechanism 480 for removing gas from chamber 405. In some embodiments, regulated gas source 475 may be configured to introduce in chamber 405 between irradiation region 462 and reflective optic 460, the gas establishing a gas number density n, (i.e. number of molecules / volume) sufficient to operate over the distance d to reduce ion energy to a target maximum energy level before the ions reach the optic. For example, a gas number density sufficient to reduce ion energy to a target maximum energy level between about 10-200 eV, and in some cases below 30 eV may be provided.

[0063] Suitable gases may include hydrogen (H2), e.g., greater than 50% hydrogen (protium and / or deuterium isotopes), helium, or combinations thereof. For example, for a plasma generating ions having a maximum initial ion energy and distance d ~15 cm from the plasma, a suitable gas for reducing ion energy below about 30 eV may be hydrogen gas at a pressure of about 500 mTorr at room temperature may be suitable. It is to be appreciated that gas introduced into the chamber may react with chamber conditions, debris, or the plasma to dissociate or create ions, e.g. atomic hydrogen, hydrogen ions which may be effective for cleaning, etching, or ion slowing.

[0064] In some embodiments, gas source 475 may introduce several gases, for example, H2, He, Ar, or HBr, either separately and independently, or the gas may be introduced as a mixture. Moreover, although Fig. 4 illustrates the gas being introduced at one location, it is to be appreciated that the gas may be introduced at multiple locations, may be removed at multiple locations, or may be evacuated at multiple locations. The gas may be supplied via a tank or may be generated locally. As illustrated in Fig. 4, gas management system may further include a second gas source 478 providing a stream of gas that flows along a direction from intermediate focus 464 toward irradiation region 462.

[0065] During operation of a LPP EUV radiation source, tin debris may be formed in various phase states (e.g., ions, particles, atomic, liquid), sizes, or locations within the EUV source vessel placed inside a chamber (e.g., chamber 405), and may originate from various sources. Tin debris may deposit on the surface of collector mirror or travel towards and through the intermediate focus into tool(s) connected with the EUV radiation source. Tin in vapor form may be evacuated from the vessel or the chamber using one or more vacuum pumps, however, tin particles in solid or liquid state may be harder to remove. One of several sources of particulate contamination in an exposure tool connected to an EUV radiation source is tin particles in solid or liquid state generated by “spitting.” Tin “spitting” refers to the phenomenon of tin splatter that occurs when hydrogen radicals interact with molten tin, creating hydrogen bubbles, which upon bursting result in ejection of tin particles over a large angle range in the source vessel. To mitigate this issue, in some systems, the walls of the source vessel are maintained at a lower temperature to minimize debris traveling towards intermediate focus (intermediate region) and eventually to tools connected through the intermediate focus. Alternatively, or additionally, tin debris traveling towards the intermediate focus may be suppressed using physicalbarriers. However, the use of physical barriers may limit the EUV transmission due to absorption of EUV radiation.

[0066] Fig. 5 illustrates a simplified schematic of sources of tin debris in a LPP EUV radiation source. Fig. 5 shows a droplet generator 510, a droplet catcher 520 placed directly opposite droplet generator 510 along the path of the droplets, a source vessel 540 including an exhaust port 530, an opening 564 in source vessel 540 substantially coincident with an intermediate focus, and an EUV radiation receiving apparatus 590.

[0067] As shown in Fig. 5, one of several sources of tin debris may be portions of tin droplet at primary focus or irradiation region due to an inadequate clearance between pulses. In some cases, tin debris may be generated at or near the primary focus due to an incomplete target droplet bum by a pre-pulse beam or the main pulse beam. The tin particles generated at primary focus may deposit on inner liners of the source vessel, or may escape into apparatus 590 through intermediate focus. Another potential source of tin debris may include particles generated from droplet deflected towards intermediate focus via liquid tin particles back-splashing from droplet catcher 520 and pushed through towards intermediate focus. Yet another potential source of tin debris may be tin particles generated due to tin spitting from heated or non -heated modules. In some cases, exhaust port 530 may be heated to temperatures above the melting point of tin to enable vaporization. While the heated exhaust port may facilitate removal of tin vapors through a pumping mechanism (e.g., a vacuum pump), heating the exhaust port, in presence of hydrogen radicals, may cause tin spitting.

[0068] Embodiments of the present disclosure provide systems and methods for mitigating tin debris in EUV light sources using flow-based solutions. Reference is now made to Fig. 6A, which illustrates a cross-section schematic of an exemplary gas management module for a source vessel of a LPP EUV radiation source, consistent with embodiments of the present disclosure. LPP EUV radiation source 600, also referred to as a radiation source 600, may include a source vessel 610 having a primary optical axis 601, a collector mirror 670 having a primary focus 660 and an intermediate focus 650, a laser source 680 generating a pulsed laser beam 685, an exhaust port 630, and a gas management system comprising guideway openings 640-A1-640-A6 and 640-B1-640-B6. Although not illustrated, it is to be appreciated that radiation source 600 may include a second gas source (e.g., gas source 478 of Fig. 4) providing a stream or a jet of gas that flows upstream (i.e., negative z direction) along primary optical axis 601 from intermediate focus 650 towards primary focus 660 (e.g., irradiation region 462 of Fig. 4). In this context, “upstream” refers to an overall direction of flow of gas, particle, or debris, opposite to the direction of propagation of the pulsed laser beam (e.g., pulsed laser beam 685) along primary optical axis 601 (from intermediate focus 650 towards primary focus 660), and “downstream” refers to an overall direction of flow of gas, particle, or debris, along the direction of propagation of the pulsed laser beam along primary optical axis 601 (from primary focus 660 towards intermediate focus 650). It is to be further appreciated that intermediate focus 650 and primary focus 660 may be aligned with primary optical axis 601 . In someembodiments, geometric center of opening of collector mirror 670 and the path of laser beam 685 may be aligned with primary optical axis 601 as well.

[0069] Source vessel 610 of radiation source 600 may comprise a substantially conical vessel. In some embodiments, source vessel 610 may comprise a single conical structure placed inside a chamber (e.g., chamber 405 of Fig. 4). The chamber may comprise a pressure chamber maintained at a negative pressure, with respect to atmospheric pressure, using a pumping mechanism. In some embodiments, source vessel 610 may comprise a conical structure 604 (containing the apex) and one or more frustum structures 608. In some embodiments, conical structure 604 is a hollow conical structure having a tapered body that converges to a narrow opening at its apex, and frustum structure 608 is a hollow frustum having two opening bases defined by its side wall. For example, frustum structure 608 includes a first component 608A and a second component 608B. In some embodiments, first component 608A and second component 608B are integrated into a single unit. In some embodiments, first component 608A and second component 608B are separate components but coupled to each other. In such a configuration, frustum structure 608 is connected to conical structure 604. As illustrated in Fig. 6A, a vertex of the conical structure 604 may substantially coincide with intermediate focus 650 and along primary optical axis 601. Source vessel 610 comprises an opening at the apex of the conical structure 604, the opening configured to allow passage of EUV radiation into a processing tool or apparatus (e.g., a photolithography apparatus or a metrology apparatus). The opening at the apex of conical structure 604 may further be configured to receive and direct flow of a buffer gas (e.g., hydrogen gas) from intermediate focus 650 towards primary focus 660.

[0070] In some embodiments, conical structure 604 and frustum structure 608 may be connected to each other such that frustum structure 608 and conical structure 604 are aligned along primary optical axis 601. As illustrated in Fig. 6A, a first base of frustum structure 608 may be connected to a second of conical structure 604 and the height of frustum structure 608 is defined by a distance along primary optical axis 601 between its first and second bases. In some embodiments, the height of frustum structure 608 extends to include exhaust port 630. In some embodiments, the height of frustum structure 608 extends to a component including exhaust port 630. A right conical structure refers to a cone where its axis, a line connecting its vertex to a center of its base, is orthogonal to its base. An oblique conical structure refers to a cone where its axis is not orthogonal to its base. A right frustum structure, in this context, refers to the portion of a cone that lies between two parallel planes cutting the cone. An oblique frustum structure refers to the portion of a cone that lies between two nonparallel planes cutting the cone. The height of the frustum structure is the average distance (along a height axis of the cone) between the two bases of the frustum structure. In some embodiments, conical structure 604 has a right conical configuration, while conical structure 604 comprises an oblique conical component and an oblique frustum component. Depending on various requirements, such as a design for apparatus maintenance of LPP EUV radiation source, in some embodiments, at least one of conical structure 604, first component 608A of frustum structure 608, and second component 608B offrustum structure 608 has an oblique configuration. For example, a combination of conical structure 604 and frustum structure 608 has a right conical configuration, while conical structure 604 has an oblique conical configuration and frustum structure 608 has an oblique frustum configuration. In another example, conical structure 604 has a right conical configuration and frustum structure 608 has a right frustum configuration, while each of first component 608A and second component 608B of frustum structure 608 has an oblique frustum configuration. It is to be appreciated that the heights of the conical structure and frustum structure as shown in Fig. 6A are exemplary, virtual, and nonlimiting.

[0071] An effective gas flow-based approach to suppress, divert, or block tin particles or debris traveling towards intermediate focus 650 includes introducing high-momentum jets of gas in an area between intermediate focus 650 and exhaust port 630 to create a strong, tangential, swirling flow along the walls of source vessel 610 and angled towards exhaust port 630 such that the gas is traveling away from intermediate focus 650 in a direction towards exhaust port 630 or towards primary focus 660.

[0072] In some embodiments, frustum structure 608 of source vessel 610 may include a first plurality of guideway openings 640-Al - 640-A6 configured to direct a buffer gas flow into source vessel 610. For example, first guideway openings 640-Al - 640-A6 is arranged in first component 608A of frustum structure 608. In some embodiments, one or more guideway openings 640-Al - 640-A6 may correspond to the passage of a gas originating from second gas source 478 of Fig. 4. A guideway opening, in this context, may also be referred to as a passage, a slot, a pathway, or a channel, or a through-hole in the wall of the source vessel serving the function of transporting gas from an external source into an interior volume of the source vessel. It is to be appreciated that although the cross- sectional view of an exemplary first component 608A of frustum structure 608 shows only two guideway openings (640-Al and 640-A4), first component 608A may include any suitable number of guideway openings based on space constraints or other practical implementation issues. In some embodiments, the number of guideway openings ranges from about 5 to about 35 per row. If the number is greater than 35, in some instances, each guideway opening is too narrow, resulting in higher pressure loss. If the number is smaller than 5, in some instances, each guideway opening is spaced farther apart, resulting in less coverage.

[0073] In some embodiments, one or more guideway openings are circumferentially arranged and angled through a wall of source vessel 610. In some embodiments, guideway openings 640-Al and 640-A4 are circumferentially arranged along an inner wall of frustum structure 608. In some embodiments, an exemplary guideway opening 640-Al (as illustrated in Fig. 6A) has a rectangular cross-section and is arranged along the inner wall of frustum structure 608. In some embodiments, exemplary guideway opening 640-Al has a rectangular shape and a rectangular cross-section, and may be circumferentially arranged along the walls of frustum structure 608 such that it is oriented substantially tangential to the circumference of the wall of frustum structure 608 or source vessel 610.As used herein, “substantially tangential” refers to the direction having an angle in the range of about 30 to about 75 degrees with the tangent to the circumference of the wall at a given location along the circular edge of frustum structure 608. In some embodiments, gas flowing through exemplary guideway opening 640-Al forms a gas jet initially having a rectangular cross-section.

[0074] In some embodiments, frustum structure 608 of source vessel 610 may include a plurality of rows of guideway openings. A row of guideway openings may include a plurality of guideway openings circumferentially arranged along an inner wall of frustum structure 608. For example, a second row of guideway openings 640-B1 - 640-B6 is arranged in second component 608B of frustum structure 608. In some embodiments, each guideway openings 640-B1 - 640-B6 is aligning with each guideway openings 640-Al - 640-A6 when viewing along primary optical axis 601. In some embodiments, second row of guideway openings 640-B1 - 640-B6 and first row of guideway openings 640-Al - 640-A6 are arranged circumferentially offset when viewing along primary optical axis 601. Second row of guideway openings 640-B1 and 640-B6 is arranged between first row of guideway openings and exhaust port 630. In some embodiments, each of guideway openings 640-Al - 640-A6 is oriented at an angle 01 relative to an interior wall of frustum structure 608, within a range of approximately 43 to 88 degrees, and each of guideway openings 640-B1 - 640-B6 is oriented at an angle 02 relative to the interior wall of frustum structure 608, within a range of approximately 43 to 88 degrees. Such selected ranges balance between a flow component along the xz-plane to create a centrifugal force field and a flow component along the -y-direction to slow the debris. It is to be appreciated that although Fig. 6A illustrates only two rows, any number of suitable rows of guideway openings may be provided. In this context, an upper row of guideway openings refers to the plurality of guide way openings arranged circumferentially along the walls of the portion of frustum structure 608 closer to conical structure 604, and a lower row of guideway openings refers to the plurality of guideway openings arranged circumferentially along the walls of the portion of frustum structure 608 closer to exhaust port 630.

[0075] In some embodiments, first row of guideway openings (e.g., guideway openings 640-Al - 640-A6 when there are 6 guideway openings) and second row of guideway openings (e.g., guideway openings 640-B1 - 640-B6 when there are 6 guideway openings) comprises equal number of guideway openings placed axisymmetrically with respect to z-axis to maintain symmetry of gas flow in frustum structure 608 of source vessel 610. In some embodiments, the first row and the second row may have different number of guideway openings. For example, considering the volume and flow coverage of various components of source vessel 610, the first row has fewer guideway openings than the second row. In some embodiments, frustum structure 608 may include two or more rows of guideway openings, and each row may have an equal or an unequal number of guideway openings.

[0076] Reference is now made to Fig. 6B, which illustrates a top planar cross-section view of the first row of guideway openings in first component 608A of frustum structure 608 along the A -A' plane shown in Fig. 6A, consistent with embodiments of the present disclosure. As previouslydiscussed, the first row of guideway openings may include a plurality of guideway openings 640-Al, 640-A2, 640-A3, 640-A4, 640-A5, and 640-A6 circumferentially arranged along the inner wall of frustum structure 608. In some embodiments, guideway openings 640-Al - 640-A6 are equally or unequally spaced along the circumference of frustum structure 608. As shown, each guideway opening may have a rectangular cross-section, thereby producing gas jets having a rectangular crosssection.

[0077] In some embodiments, at least one guideway opening may be oriented at an angle in a range from about 30 to about 75 degrees with respect to a circumference of the wall of source vessel 610. In some embodiments, as shown, at least one guideway opening may be oriented substantially tangential to the circumference of the wall of source vessel 610. In this disclosure, substantially tangential refers to the orientation of a guideway opening at an angle 03 from about 30 to about 75 degrees with respect to a tangent to the wall of source vessel 610 at the location where the guideway opening intersects with the wall of source vessel 610 or the frustum structure 608.

[0078] In some embodiments, one or more guideway openings (e.g., guideway openings 640-Al - 640-A6) may be configured to inject a gas (e.g., a hydrogen buffer gas) jet tangentially or substantially tangentially along the circumference of frustum structure 608, forming a swirling flow inside a portion of frustum structure 608 (as illustrated in and discussed with reference to Fig. 7). The swirling flow may create a centrifugal force field radiating outward from primary optical axis 601, thereby deflecting particles (e.g., tin particles or debris) towards the walls of source vessel 610 (or frustum structure 608). In some embodiments, the velocity of gas jets injected into source vessel 610 through one or more guideway openings 640-Al - 640-A6 may be substantially similar to maintain uniformity in centrifugal force circumferentially along the walls of source vessel 610. In some embodiments, while the velocity of gas jets passing through multiple guideway openings of a row of guideway openings may be similar, the velocity of gas jets passing through guideway openings of different rows may be similar or dissimilar.

[0079] Referring back to Fig. 6A, frustum structure 608 may include one or more guideway openings that are angled to direct gas away from intermediate focus 650 and towards exhaust port 630. In addition to one or more guideway openings (e.g., guideway openings 640-Al - 640-A6) being rectangular and circumferentially arranged along the inner wall of frustum structure 608, one or more guideway openings may be angled in a manner that the gas flowing through a rectangular guideway opening in a direction parallel to its minor axis is angled away from intermediate focus 650 and towards exhaust port 630. In some embodiments, one or more guideway openings (e.g., 640-Al - 640-A6 and 640-B1 - 640-B6) have a circular, triangular, or elliptical shape. The angled flow of buffer gas flowing through angled guideway openings may have a large downward velocity component, i.e., in the negative z-direction. In some embodiments, a combination of: (i) the swirling flow caused by the substantially tangential placement of guideway openings along the circumference of source vessel 610, which imparts a centrifugal force to prevent debris (e.g., tin particles) travelingtowards intermediate focus 650, and (ii) angled flow, caused by angled guideway openings, directed away from intermediate focus 650 (i.e., towards primary focus 660 or exhaust port 630), creates a strong, spiraling flow of buffer gas, also referred to herein as a vortex, which may suppress, block, or divert the downstream tin particles to move towards inner walls of source vessel 610, thereby minimizing the migration of tin particles towards intermediate focus 650. The vortex created by the hybrid flow pattern (tangential and downward) of buffer gas may minimize particle residence time in the flow, thereby further suppressing tin particles from traveling towards intermediate focus. The term “vortex,” in this context refers to a region in which the flow of the gas (or a fluid) revolves around an axis line, which may be straight or curved. As used herein, particle residence time refers to the length of time a particle is floating in the source vessel before impinging a wall surface of source vessel, and therefore demobilized, or before removal from source vessel through evacuation via exhaust port.

[0080] Fig. 7A illustrates gas flow patterns in an exemplary radiation source, consistent with embodiments of the present disclosure. Image 700A illustrates the gas flow fields in a conical structure 704 and a frustum structure 708 of source vessel 710. In some embodiments, frustum structure 708 includes multiple components, such as a first component 708A and a second component 708B. In some embodiments, first component 708A and second component 708B are integrated into a single unit. In some embodiments, first component 708A and second component 708B are separate components but coupled to each other. Frustum structure 708 (analogous to frustum structure 608 of Fig. 6A) includes two rows of six guideway openings. Although not explicitly illustrated, source vessel 710 includes two rows of six guideway openings placed axisymmetrically. The guideway openings within each row may be equidistant from each other. Images 700B and 700C illustrate a cross-section view of frustum structure of source vessel 710 along plane B-B' and C-C', respectively. Images 700B and 700C illustrate the injection of angled gas jets to cause a clockwise, swirling flow of the gas at horizontal planes B-B' and C-C', respectively. In some embodiments, guideway openings 740-1 - 740-6 may be oriented to cause a counterclockwise, swirling flow of gases traveling towards exhaust port 730.

[0081] As illustrated in images 700B, and 700C, the high-momentum (high-velocity) gases injected through guideway openings (e.g., guideway openings 740-1 - 740-6) create a strong, swirling, tangential flow in frustum structure 708 of source vessel 710. In some embodiments, one or more guideway openings have a rectangular, circular, triangular, or elliptical shape. The swirling flow creates a centrifugal force to push the debris or tin particles floating and traveling towards an intermediate focus 750, towards an inner wall of source vessel 710, thus preventing tin particles from escaping source vessel and entering an exposure tool, for example. In some embodiments, a central jet 705 of buffer gas, in addition to gas injected through guideway openings (e.g., guideway openings 740-1 - 740-6), is introduced through an opening located at or close to intermediate focus 750 in source vessel 710. Central jet 705 may be configured to push tin particles traveling towardsintermediate focus 750 away from the exposure tool and towards an exhaust port 730. Central jet 705 may travel along a primary optical axis 701 of source vessel 710.

[0082] Fig. 7B illustrates a graphical representation of gas velocity as a function of distance from the edge of a frustum structure including guideway openings. Starting from the circumferential edge of source vessel 710 in the x-direction along a horizontal plane (e.g., plane B-B' or C-C'), the gas velocity at the inner wall of source vessel 710 may be zero or negligibly small. The gas velocity may be the highest in the central region around primary optical axis 701 and in the region of swirling flow caused by the high-velocity gases injected through guideway openings. It is to be appreciated that although Fig. 7A illustrates only two rows, any number of suitable rows of guideway openings, ranging from one to four, may be provided to accommodate diverse design requirement.

[0083] In some embodiments, gases injected through guideway openings (e.g., guideway openings 740-1 - 740-6), stabilize central jet 705 along primary optical axis 701. Stabilization of central jet 705 may include reduction of tilt or displacement away from primary optical axis 701. In some embodiments, gases injected through guideway openings may also maintain uniformity in gas velocity across the cross-section of central jet 705.

[0084] Reference is now made to Figs. 8A, 8B, and 8C, which illustrate exemplary arrangements 800A, 800B, and 800C, respectively, of guideway openings circumferentially arranged on a com structure and / or a frustum structure of a source vessel, consistent with embodiments of the present disclosure. In some embodiments, one or more guideway openings have a rectangular, circular, triangular, or elliptical shape. Arrangement 800A of Fig. 8A illustrates a source vessel 810 analogous to source vessel 610 of Fig. 6A, including a conical structure and a frustum structure (not explicitly labeled). Arrangement 800A includes one row of guideway openings 840 configured to inject buffer gas 845 into source vessel 810. As previously described, guideway opening 840 may be circumferentially arranged on inner wall of source vessel 810 and oriented substantially tangential to the inner wall of frustum structure or to the wall of source vessel 810. In addition, guideway opening 840 may be angled downward (away from intermediate focus) toward an exhaust. Buffer gas 845 may be injected in a direction parallel to the orientation of guideway opening 840 with respect to the primary optical axis. Buffer gas 845 injected through guideway opening 840 may form a downward spiraling gas flow 850, also referred to herein as a vortex, because of the angled and substantially tangential orientation of guideway opening 840. A central jet 805, analogous to central jet 705 of Fig. 7A, may be injected through an opening at the intermediate focus to further suppress, block, or divert tin particles traveling towards the intermediate focus, thereby preventing debris from escaping source vessel 810 and entering an exposure tool.

[0085] Arrangements 800B and 800C include two and three rows of guideway openings 840, respectively. In some embodiments, source vessel may include any suitable number of rows of guideway openings, based on physical and spatial constraints, requirement, application, among otherfactors. In some embodiments, spacing between adjacent rows of guideway openings may be uniform or non-uniform, or substantially uniform.

[0086] Figs. 9A and 9B illustrate schematics of cross-section views of a frustum structure including multiple guideway openings, consistent with embodiments of the present disclosure. In some embodiments, the orientation of the swirling flow generated by introducing buffer gas through guideway openings may be adjusted. A cross-section view 900A of frustum structure in Fig. 9A illustrates a counter-clockwise orientation of the swirl in source vessel 910A by synchronizing the introduction of buffer gas in the counter-clockwise direction 945 CC. A cross-section view 900B of frustum structure in Fig. 9B illustrates a clockwise orientation of the swirl in source vessel 910B by synchronizing the introduction of buffer gas in the clockwise direction 945 C. It is to be appreciated that although source vessels 910A and 91 OB are illustrated as having ten guideway openings configured to generate a total of ten jets, the number of guideway openings is not limited and may be adjusted, as appropriate.

[0087] Figs. 10A and 10B illustrate schematics of cross-section views of a frustum structure including multiple rows of guide way openings, consistent with embodiments of the present disclosure. In some embodiments, source vessel 1010A may include multiple rows of guideway openings. A first row and a second row of guideway openings, shown in Figs. 10A and 10B, respectively, may include a different number of guideway openings. It is to be appreciated that although the orientation of the swirl generated by the buffer gas may be the same, the velocity of the gases in the swirl (or the centrifugal force) may be different.

[0088] Reference is now made to Figs. 11A, 1 IB, and 11C, which illustrate schematics of exemplary arrangements 1100A, 1100B, and 1100C, respectively, of guideway openings circumferentially arranged on a conical structure and / or a frustum structure of a source vessel, consistent with embodiments of the present disclosure. In some embodiments, one or more guideway openings have a rectangular, circular, triangular, or elliptical shape. In some embodiments, the velocity of gas jets injected through guideway openings may be adjusted based on a characteristic of a nozzle of the guideway opening. A characteristic of the nozzle may include, but is not limited to, a length, a contour, a cross-section, a size, an angle with respect to the direction of gas flow through the nozzle, among other things. In some embodiments, guideway opening 1140A of Fig. 11A may comprise a converging nozzle configured to produce high-speed subsonic jets of gas. Subsonic gas jets flowing through guideway opening 1140A may have a velocity in the range of 50 m / s - 300 m / s. In some embodiments, subsonic gas jets flowing through guideway opening 1140A may have a velocity in the range of 100 m / s to 250 m / s, or any suitable subsonic range.

[0089] In some embodiments, guideway opening 1140B of Fig. 1 IB may comprise a diverging nozzle configured to produce low-speed gas jets. In some embodiments, the nozzle of guideway opening 1140C may comprise a converging-diverging (CD) nozzle configured to produce supersonicgas jets injected into source vessel of a radiation source. A supersonic gas jet may have gas velocity in the range of 270 m / s - 700 m / s, or any suitable supersonic range.

[0090] In some embodiments, though not illustrated, one or more guideway openings may have a circular, elliptical, triangular, or a non-rectangular cross-section. In some embodiments, the aspect ratio of a rectangular guideway opening may be varied to tailor the implementation based on a requirement while achieving optimum performance. In some embodiments, the downward tilt angle of the rectangular guideway openings with respect to primary optical axis may be varied.

[0091] Reference is now made to Fig. 12, which illustrates a process flowchart of an exemplary method 1200 for reducing contamination associated with an operation of a fuel-based radiation source, consistent with embodiments of the present disclosure. Method 1200 may include steps performed to mitigate contamination (e.g., tin debris) in radiation sources. In some embodiments, the illustrated method 1200 may be altered to modify the order of steps and to include additional or fewer steps.

[0092] Step 1210 includes directing a stream of droplets of a target material into a conical vessel (e.g., source vessel 610 of Fig. 6A) to generate a radiation (e.g., EUV radiation) upon interaction with a laser beam (e.g., pulsed laser beam 685 of Fig. 6). 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 (SnH4). 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.

[0093] The droplets are generated using a droplet generator (e.g., droplet delivery mechanism 450 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 that 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.

[0094] Step 1220 includes injecting a gas into an interior volume of the conical vessel using a first plurality of guide way openings circumferentially arranged and angled through a wall of the conical vessel, wherein injecting the gas through the first plurality of guideway openings enable generating a vortex within the conical vessel to influence a path of a contaminant in the vessel.

[0095] As stated above, it is appreciated that a second plurality of guideway openings (or more) can also be used to enable the generation of the vortex and that the configuration of the plurality of guideway openings throughout the conical vessel can be set according to design goals.

[0096] 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, frequencymodulation, activating one or more actuators for laser beam control, simulation by executing algorithms, computing, and at least some steps of method 1200. 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.

[0097] 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.

[0098] The embodiments of the present disclosure may further be described using the following clauses:1. A radiation source, comprising: a vessel defining an interior space symmetrically around a primary optical axis, the vessel comprising a first plurality of guide way openings circumferentially arranged and angled through a wall of the vessel.2. The radiation source of clause 1, further comprising a pressure chamber enclosing the vessel and an optic, the optic configured to reflect radiation generated from a target material upon interaction with a pulsed laser beam.3. The radiation source of clause 2, wherein the optic comprises a collector mirror having a primary focus and an intermediate focus aligned with the primary optical axis.4. The radiation source of any one of clauses 1-3, wherein at least one guideway opening of the first plurality of guideway openings is oriented at an angle in a range between 30 and 75 degrees with respect to a circumference of the wall of the vessel.5. The radiation source of clause 4, wherein the at least one guideway opening of the first plurality of guideway openings is oriented substantially tangential with respect to the circumference of the wall of the vessel.6. The radiation source of any one of clauses 1-5, wherein the first plurality of guideway openings is configured to direct a gas flow to an interior volume of the vessel at a non-orthogonal angle with respect to the primary optical axis.7. The radiation source of clause 6, wherein the first plurality of guideway openings is configured to direct the gas flow at an angle in a range between 30 and 75 degrees with respect to the circumference of the wall of the vessel.8. The radiation source of any one of clauses 6 and 7, wherein the first plurality of guide way openings enables the gas flow to generate a swirling flow within the vessel, the swirling flow exerting a centrifugal force on the wall of the vessel.9. The radiation source of clause 8, wherein the swirling flow has a clockwise or a counterclockwise orientation.10. The radiation source of any one of clauses 6-9, wherein the first plurality of guideway openings enables the gas flow to generate a vortex in the interior volume of the vessel, the vortex configured to influence a path of a contaminant in the vessel.11. The radiation source of clause 10, wherein the first plurality of guideway openings enables the gas flow to divert the contaminant towards the wall of the vessel.12. The radiation source of any one of clauses 10 and 11, further comprising an exhaust port associated with the vessel, and wherein the first plurality of guideway openings enables the gas flow to direct the contaminant toward the exhaust port.13. The radiation source of any one of clauses 10-12, wherein the contaminant comprises liquid tin, solid tin, ionized tin, or unionized tin.14. The radiation source of any one of clauses 1-13, wherein a cross-section of the at least one guideway opening of the first plurality of guide way openings is rectangular, circular, triangular, or elliptical.15. The radiation source of clause 14, wherein a cross-section of the gas flow from the at least one rectangular guideway opening is rectangular, circular, triangular, or elliptical.16. The radiation source of any one of clauses 1-15, wherein the at least one guideway opening of the first plurality of guideway openings comprises a converging nozzle.17. The radiation source of any one of clauses 1-15, wherein the at least one guideway opening of the first plurality of guideway openings comprises a diverging nozzle.18. The radiation source of any one of clauses 1-15, wherein the at least one guideway opening of the first plurality of guideway openings comprises a converging -diverging nozzle.19. The radiation source of any one of clauses 1-18, wherein the guideway openings of the first plurality of guideway openings are uniformly or non -uniformly spaced along the wall of the vessel.20. The radiation source of clause 1, wherein the vessel comprises a second plurality of guideway openings circumferentially arranged and angled through the wall of the vessel at a different location from the first plurality of guideway openings.21. The radiation source of clause 20, wherein at least one guide way opening of the second plurality of guideway openings is oriented at an angle in a range between 30 and 75 degrees with respect to a circumference of the wall of the vessel.22. The radiation source of any one of clauses 20 and 21, wherein the first and the second plurality of guideway openings comprise an equal number of guideway openings.23. The radiation source of any one of clauses 20 and 21, wherein the first and the second plurality of guideway openings comprise an unequal number of guideway openings.24. The radiation source of any one of clauses 21-23, wherein a cross-section of the at least one guideway of the first plurality of guide way openings and of the at least one guideway of the second plurality of guideway openings is rectangular, circular, triangular, or elliptical.25. The radiation source of any one of clauses 2-24, wherein the vessel further comprises an opening configured to allow passage of radiation generated from the target material to an output system connected to the pressure chamber.26. The radiation source of any one of clauses 1-25, wherein the vessel comprises a conical structure and a frustum structure connected to the conical structure, and wherein the first plurality of guideway openings is through the wall of the frustum structure.27. The radiation source of clause 26, wherein the frustum structure has an oblique frustum configuration.28. A method for reducing contamination associated with operation of a radiation source, the method comprising: directing a stream of droplets of a target material into a vessel to generate a radiation upon interaction with a pulsed laser beam; injecting a gas into an interior volume of the vessel using a first plurality of guideway openings circumferentially arranged and angled through a wall of the vessel, wherein injecting the gas through the first plurality of guideway openings enables generating a vortex within the vessel to influence a path of a contaminant in the vessel.29. A lithographic system, comprising: a lithographic apparatus; and a radiation source of clause 1, configured to supply extreme ultraviolet (EUV) radiation to the lithographic apparatus.30. The lithographic system of clause 29, wherein the radiation source is coupled to the lithographic apparatus, and the radiation source includes a first plurality of guideway openings and a second plurality of guideway openings circumferentially arranged and angled through a wall of a vessel of the radiation source.31. The lithographic system of clause 30, wherein the radiation source includes an exhaust port coupled to the vessel, wherein the second plurality of guide way openings is arranged between the exhaust port and the first plurality of guideway openings.

[0099] 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 of the 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. A radiation source, comprising: a vessel defining an interior space symmetrically around a primary optical axis, the vessel comprising a first plurality of guideway openings circumferentially arranged and angled through a wall of the vessel.

2. The radiation source of claim 1, further comprising a pressure chamber enclosing the vessel and an optic, the optic configured to reflect radiation generated from a target material upon interaction with a pulsed laser beam, wherein the optic comprises a collector mirror having a primary focus and an intermediate focus aligned with the primary optical axis.

3. The radiation source of claim 1, wherein at least one guideway opening of the first plurality of guideway openings is oriented at an angle in a range between 30 and 75 degrees with respect to a circumference of the wall of the vessel.

4. The radiation source of claim 1, wherein at least one guide way opening of the first plurality of guideway openings is oriented substantially tangential with respect to a circumference of the wall of the vessel.

5. The radiation source of claim 1, wherein the first plurality of guideway openings is configured to direct a gas flow to an interior volume of the vessel at a non-orthogonal angle with respect to the primary optical axis.

6. The radiation source of claim 5, wherein the first plurality of guideway openings is configured to direct the gas flow at an angle in a range between 30 and 75 degrees with respect to the circumference of the wall of the vessel.

7. The radiation source of claim 6, wherein the first plurality of guideway openings enables the gas flow to generate a swirling flow within the vessel, the swirling flow exerting a centrifugal force on the wall of the vessel.

8. The radiation source of claim 6, wherein the first plurality of guideway openings enables the gas flow to generate a vortex in the interior volume of the vessel, the vortex configured to influence a path of a contaminant in the vessel.

9. The radiation source of claim 8, wherein the first plurality of guideway openings enables the gas flow to divert the contaminant towards the wall of the vessel.

10. The radiation source of claim 1, further comprising an exhaust port associated with the vessel, and wherein the first plurality of guideway openings enables a gas flow to direct a contaminant toward the exhaust port.

11. The radiation source of claim 1, wherein a cross-section of at least one guideway opening of the first plurality of guideway openings is rectangular, circular, triangular, or elliptical.

12. The radiation source of claim 11, wherein a cross-section of the gas flow from the at least one rectangular guideway opening is rectangular, circular, triangular, or elliptical.

13. The radiation source of claim 1, wherein at least one guide way opening of the first plurality of guideway openings comprises at least one of a converging nozzle, a diverging nozzle ,and a converging -diverging nozzle.

14. The radiation source of claim 1, wherein the vessel comprises a second plurality of guideway openings circumferentially arranged and angled through the wall of the vessel at a different location from the first plurality of guideway openings.

15. The radiation source of claim 14, wherein at least one guideway opening of the second plurality of guideway openings is oriented at an angle in a range between 30 and 75 degrees with respect to a circumference of the wall of the vessel.

16. The radiation source of claim 1, wherein the vessel comprises a conical structure and a frustum structure connected to the conical structure, and wherein the first plurality of guideway openings is through the wall of the frustum structure.

17. The radiation source of claim 16, wherein the frustum structure has an oblique frustum configuration.

18. A method for reducing contamination associated with operation of a radiation source, the method comprising: directing a stream of droplets of a target material into a vessel to generate a radiation upon interaction with a pulsed laser beam;injecting a gas into an interior volume of the vessel using a first plurality of guideway openings circumferentially arranged and angled through a wall of the vessel, wherein injecting the gas through the first plurality of guideway openings enables generating a vortex within the vessel to influence a path of a contaminant in the vessel.

19. A lithographic system, comprising: a lithographic apparatus; and a radiation source coupled to the lithographic apparatus, wherein the radiation source includes a first plurality of guide way openings and a second plurality of guide way openings circumferentially arranged and angled through a wall of a vessel of the radiation source.

20. The lithographic system of claim 19, wherein the radiation source includes an exhaust port coupled to the vessel, wherein the second plurality of guide way openings is arranged between the exhaust port and the first plurality of guideway openings.

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