Method and apparatus for the enhanced optics protection from the adverse effects of plasma under vacuum conditions

Abstract

Optics within a light source may be protected from adverse effects of plasma under low pressure operating conditions. The operation of the source involves excitation of high-temperature plasma producing radiation at a desired wavelength. The operation may include the supply of the fuel, excitation energy, supplemental substances, and extraction of produced radiation by means of refractive and / or reflective optics. The plasma generated during regular operation co-produces highly energetic charged particles and fuel vapors, which, upon interaction with the surface of the optics results in the degradation of the optics performance. To mitigate the degradation, protective gas shields and ducts may be implemented around the optics. The ducts may improve the efficiency of the protective gas shields, which may allow for the reduction of the protective gas consumption requirements.

Description

METHOD AND APPARATUS FOR THE ENHANCED OPTICS PROTECTION FROM THE ADVERSE EFFECTS OF PLASMA UNDER VACUUM CONDITIONSCROSS-REFERENCE

[0001] The present application claims the benefit under 35 U.S.C. § 119 of U.S. Provisional application 63 / 686,252, filed on August 23, 2024, titled “Method and Apparatus for the Enhanced Optics Protection from the Adverse Effects of Plasma Under Vacuum Conditions”, which is incorporated herein by reference in the entirety.TECHNICAL FIELD

[0002] The present disclosure generally relates to optical systems and, more particularly, to optical systems producing light using plasma.BACKGROUND

[0003] Plasma-based light sources, such as laser-produced plasma (LPP) sources and laser-discharge produced plasma (LDP or laser-initiated DPP) can generate soft X-ray, extreme ultraviolet (EUV), and / or vacuum ultraviolet (VUV) light for applications such as defect inspection, photolithography, or metrology. In these plasma light sources, light having the desired wavelength is emitted by plasma formed from a target material having an appropriate line-emitting or bandemitting element, the target material irradiated by a laser in a vacuum chamber to produce the plasma. One challenging aspect of the laser-produced plasma sources is to mitigate debris from the plasma generation process while minimizing the loss of light produced by the plasma. Therefore, it would be advantageous to provide a device, system, and method that cures the shortcomings described above.SUMMARY

[0004] An illumination source is described, in accordance with one or more embodiments of the present disclosure. The illumination source may include: a laser source, wherein the laser source is configured to generate a laser; a vacuum chamber; refractive optics, wherein the refractive optics are sealed to the vacuum chamber, wherein the laser is configured to refract through the refractive optics into the vacuum chamber; a target material, wherein the laser is configured to irradiate the target material at a plasma site within the vacuum chamber, wherein the laser causes the target material to produce a plasma, wherein the plasma is configured to emit illumination; a collector, wherein the collector is configured to reflect the illumination from the plasma to an intermediate focal point; first inlets; a first duct, wherein the first duct is arranged around the laser, the refractive optics, and the first inlets, wherein the first duct extends from the refractive optics towards the plasma site, wherein the first inlets are configured to inject a buffer gas into the first duct; second inlets; and a second duct, wherein the vacuum chamber houses the refractive optics, the collector, the plasma site, the first duct, and the second duct, wherein the second duct is arranged around the collector, the illumination, and the second inlets, wherein the second duct extends from the collector towards the plasma site and towards the intermediate focal point, wherein the second inlets are configured to inject the buffer gas into the second duct, wherein the buffer gas is configured to flow from the first duct and the second duct into the vacuum chamber.

[0005] An inspection system is described, in accordance with one or more embodiments of the present disclosure. The inspection system may include: an illumination source including: a laser source, wherein the laser source is configured to generate a laser; a vacuum chamber; refractive optics, wherein the refractive optics are sealed to the vacuum chamber, wherein the laser is configured to refract through the refractive optics into the vacuum chamber; a target material, wherein the laser is configured to irradiate the target material at a plasma site within the vacuum chamber, wherein the laser causes the target material to produce aplasma, wherein the plasma is configured to emit illumination; a collector, wherein the collector is configured to reflect the illumination from the plasma to an intermediate focal point; first inlets; a first duct, wherein the first duct is arranged around the laser, the refractive optics, and the first inlets, wherein the first duct extends from the refractive optics towards the plasma site, wherein the first inlets are configured to inject a buffer gas into the first duct; second inlets; and a second duct, wherein the vacuum chamber houses the refractive optics, the collector, the plasma site, the first duct, and the second duct, wherein the second duct is arranged around the collector, the illumination, and the second inlets, wherein the second duct extends from the collector towards the plasma site and towards the intermediate focal point, wherein the second inlets are configured to inject the buffer gas into the second duct, wherein the buffer gas is configured to flow from the first duct and the second duct into the vacuum chamber.

[0006] A method is described, in accordance with one or more embodiments of the present disclosure. The method may include: injecting a buffer gas into a first duct and a second duct by first inlets and second inlets, the buffer gas flowing from the first duct and the second duct into a vacuum chamber; delivering a target material to a plasma site within the vacuum chamber; generating a laser by a laser source; refracting the laser through refractive optics into the vacuum chamber, wherein the refractive optics are sealed to the vacuum chamber, wherein the first duct is arranged around the laser, the refractive optics, and the first inlets, wherein the first duct extends from the refractive optics towards the plasma site; irradiating the target material at the plasma site by the laser to produce a plasma, wherein the plasma emits illumination; and reflecting the illumination from a collector to an intermediate focal point, wherein the vacuum chamber houses the refractive optics, the collector, the plasma site, the first duct, and the second duct, wherein the second duct is arranged around the collector, the illumination, and the second inlets, wherein the second duct extends from the collector towards the plasma site and towards the intermediate focal point.

[0007] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate subject matter of the disclosure. Together, the description and drawings serve to explain the principles of the disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which:

[0009] FIG. 1A illustrates a perspective view of ducts of an illumination source, in accordance with one or more embodiments of the present disclosure.

[0010] FIG. 1B illustrates a perspective view of the ducts around a laser and illumination of the illumination source, in accordance with one or more embodiments of the present disclosure.

[0011] FIG. 1C illustrates a simplified schematic of the illumination source with the ducts, in accordance with one or more embodiments of the present disclosure.

[0012] FIG. 1D illustrates a simplified schematic of the illumination source with the inlets injecting a buffer gas into the ducts and the ducts containing the buffer gas up to the vacuum chamber, in accordance with one or more embodiments of the present disclosure.

[0013] FIG. 1E illustrates a simplified schematic of the illumination source with the ducts around the laser and the illumination, in accordance with one or more embodiments of the present disclosure.

[0014] FIG. 2 illustrates a conceptual view of an inspection system with the illumination source, in accordance with one or more embodiments of the present disclosure.

[0015] FIG. 3 illustrates a flow diagram of a method, in accordance with one or more embodiments of the present disclosure.DETAILED DESCRIPTION

[0016] The present disclosure has been particularly shown and described with respect to certain embodiments and specific features thereof. The embodiments set forth herein are taken to be illustrative rather than limiting. It should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the disclosure. Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings.

[0017] One approach to the protection of the optics is to inject the protective gas through and around the exposed optics surfaces and create localized areas of the elevated gas density. Such areas act as ion decelerating medium and avert fuel vapor flow from the regions in the vicinity of plasma. Outside of implementing the inlet arrays around and through the exposed optics, no other measures are introduced. However, the injection of the protective gas through the optics may raise manufacturing challenges. Another approach to the protection of the optics is to only inject the protective gases around the optics. When protecting optics with large surface areas, the injection of the protective gases around the optics may result in radially nonuniform protective gas density and requires the excessive amount of protective gas to be injected to meet the protection requirement across the entire surface of the optics. The protection nonuniformity described above can be mitigated by arranging the gas inlets within the optical elements, driving the cost, and reducing reliability of the optics.

[0018] Embodiments of the present disclosure are directed to a method and apparatus for the enhanced optics protection from the adverse effects of plasma under vacuum conditions. Optics within a light source may be protected from adverse effects of plasma under low pressure operating conditions. The operation of the source involves excitation of high-temperature plasma producing radiationat a desired wavelength. The operation may include the supply of the fuel, excitation energy, supplemental substances, and extraction of produced radiation by means of refractive and / or reflective optics. The plasma generated during regular operation co-produces highly energetic charged particles and fuel vapors, which, upon interaction with the surface of the optics results in the degradation of the optics performance. To mitigate the degradation, protective gas shields and ducts may be implemented around the optics. The ducts may improve the efficiency of the protective gas shields, which may allow for the reduction of the protective gas consumption requirements.

[0019] U.S. Patent Number US8963110B2, titled “Continuous generation of extreme ultraviolet light”; U.S. Patent Number US9151718B2, titled “Illumination system with time multiplexed sources for reticle inspection”; U.S. Patent Number US9151881B2, titled “Phase grating for mask inspection system”; U.S. Patent Number US9268031B2, titled “Advanced debris mitigation of EUV light source”; U.S. Patent Number US9295147B2, titled “EUV light source using cryogenic droplet targets in mask inspection”; U.S. Patent Number US9335637B2, titled “Laser-produced plasma EUV source with reduced debris generation utilizing predetermined non-thermal laser ablation”; U.S. Patent Number US9348214B2, titled “Spectral purity filter and light monitor for an EUV reticle inspection system”; U.S. Patent Number US9420678B2, titled “System and method for producing an exclusionary buffer gas flow in an EUV light source”; U.S. Patent Number US9544984B2, titled “System and method for generation of extreme ultraviolet light”; U.S. Patent Number US9918375B2, titled “Plasma based light source having a target material coated on a cylindrically-symmetric element”; U.S. Patent Number US9989758B2, titled “Debris protection system for reflective optic utilizing gas flow”; U.S. Patent Number US10021773B2, titled “Laser produced plasma light source having a target material coated on a cylindrically-symmetric element”; U.S. Patent Number US10034362B2, titled “Plasma-based light source”; U.S. Patent Number US10101664B2, titled “Apparatus and methods for optics protection from debris in plasma-based light source”; U.S. Patent Number US10217625B2, titled “Continuous-wave laser-sustained plasma illumination source”; U.S. PatentNumber US10953441 B2, titled “System and method for cleaning optical surfaces of an extreme ultraviolet optical system”; U.S. Patent Number US11112691 B2, titled “Inspection system with non-circular pupil”; U.S. Patent Number US11259394B2, titled “Laser produced plasma illuminator with liquid sheet jet target”; U.S. Patent Number US11272607B2, titled “Laser produced plasma illuminator with low atomic number cryogenic target”; U.S. Patent Number US11293880B2, titled “Method and apparatus for beam stabilization and reference correction for EUV inspection”; U.S. Patent Number US11317500B2, titled “Bright and clean x-ray source for x-ray based metrology”; U.S. Patent Number US11499924B2, titled “Determining one or more characteristics of light in an optical system”; U.S. Patent Number US11635700B2, titled “Method and apparatus for EUV mask inspection”; U.S. Patent Number US12158576B2, titled “Counterflow gas nozzle for contamination mitigation in extreme ultraviolet inspection systems”; are each incorporated herein by reference in the entirety.

[0020] FIGS. 1A-1E illustrate an illumination source 100 in accordance with one or more embodiments of the present disclosure. The illumination source 100 may provide a method and apparatus for enhanced optics protection from the adverse effects of plasma under vacuum conditions. The illumination source 100 may include one or more components, such as, but not limited to, a laser source 102, a laser 103, a vacuum chamber 104, refractive optics 106, a target material 108, a plasma 109, a collector 110, illumination 111, vacuum pumps 112, a plasma site 113, a gas supply 114, first inlets 116, second inlets 118, a first duct 120, a second duct 122, an intermediate focal point 124, a target-material generator 126, a buffer gas 128, an internal focus module 130, debris 132, a plasma-to-collector section 134, a collector-to-intermediate focus section 136, a laser-exit opening 138, a illumination-entrance opening 140, an illumination-exit opening 142, or the like.

[0021] The illumination source 100 may include the laser source 102. The laser source 102 may be a pulsed-laser source, a modulated-laser source, or the like. For example, the laser source 102 may be the pulsed-laser source. The laser source 102 may be any suitable laser source, such as a solid-state laser. The lasersource 102 may include any suitable gain medium, such as, but not limited to, a fiber-shaped, rod-shaped, or disk-shaped active media. The laser source 102 may include, but is not limited to, Nd:YAG, Er:YAG, Yb:YAG, Ti:Sapphire, Nd:Vanadate, a gas-discharge laser, an excimer laser, a MOPA configured excimer laser, an excimer laser having one or more chambers, a master oscillator / power oscillator (MOPO) arrangement, a master oscillator / power ring amplifier (MOPRA) arrangement, a power oscillator / power amplifier (POPA) arrangement, a solid state laser that seeds one or more excimer or molecular fluorine amplifier or oscillator chambers, a pulsed-gas discharge CO2 laser, or the like.

[0022] The laser source 102 may be configured to generate the laser 103. The laser 103 may also be referred to as a drive laser, a plasma-pumping laser, or the like. The laser 103 may be a pulsed laser. For example, the laser 103 may be a pulsed infrared (IR) laser. The laser 103 may include a select power and / or pulserepetition rate. For example, the laser 103 may include a relatively low power (e.g., from about 10 W to about 1 kW) and / or a relatively low pulse-repetition rate (e.g., between about 2 kHz and 50 kHz). By way of another example, the laser 103 may include a relatively high power (e.g. , 10 kW or higher) and / or a relatively high pulserepetition rate (e.g., above 50 kHz, above 100 kHz, or the like).

[0023] The illumination source 100 may include the vacuum chamber 104. The vacuum chamber 104 may be configured to maintain a vacuum pressure within the vacuum chamber 104. The vacuum pressure may refer to any pressure that is lower than atmospheric pressure. The vacuum chamber 104 may be a low-pressure container in which the plasma 109 is produced at the plasma site 113 and the illumination 111 is collected and focused. The illumination 111 may be strongly absorbed by gases, thus, reducing the pressure within the vacuum chamber 104 reduces the attenuation of the illumination 111 within the illumination source 100.

[0024] The vacuum chamber 104 may house one or more components of the illumination source 100 within the vacuum chamber 104. For example, the vacuum chamber 104 may house the refractive optics 106, the target material 108, the plasma 109, the collector 110, the plasma site 113, the first inlets 116, the second inlets 118, the first duct 120, the second duct 122, and the like.

[0025] The illumination source 100 may include the refractive optics 106. The refractive optics 106 may also be referred to as an entrance window, a vacuum window, an entrance lens, a vacuum lens, or the like. The refractive optics 106 may be a window (e.g., zero optical power), a converging lens (e.g., positive optical power), or the like. The refractive optics 106 may be sealed to the vacuum chamber 104. For example, the refractive optics 106 may be coupled to a sidewall of the vacuum chamber 104 and may maintain the vacuum pressure inside the vacuum chamber 104. In this regard, the refractive optics 106 may include a vacuum interface with the vacuum chamber 104. The laser 103 may be configured to refract through the refractive optics 106 into the vacuum chamber 104. The refractive optics 106 may provide a path for transmitting the laser 103 into the vacuum chamber 104. To maintain the low-pressure environment inside the vacuum chamber 104, the laser 103 may pass into the vacuum chamber 104 through the refractive optics 106. The refractive optics 106 may also focus the laser 103 onto the target material 108 (e.g., where the refractive optics 106 is the converging lens). The refractive optics 106 may be made of any suitable laser which is configured to refract the laser 103 and which is compatible with the vacuum environment. For example, the refractive optics 106 may made calcium fluoride (CaF2), silicon dioxide (SiO2), or the like.

[0026] The illumination source 100 may include the target material 108. The target material 108 may also be referred to as a source material, a plasmaproducing material, or the like. The target material 108 may be configured to produce the plasma 109. The laser 103 may irradiate the target material 108 at the plasma site 113 within the vacuum chamber 104. The laser 103 and / or the target material 108 may be positioned to irradiate the target material 108 in the path ofthe laser 103. The laser 103 may be focused on the target material 108. For example, the laser 103 may be focused onto the target material 108 within the vacuum chamber 104. The target material 108 may be irradiated by the laser 103 at a focal point or spot of the laser 103. The laser 103 may irradiate the target material in one or more pulses. The laser 103 may cause the target material 108 to produce the plasma 109 at the plasma site 113. The absorption cross-section of the target material 108 and / or the plasma 109 may cause the target material 108 to absorb the laser 103. For example, the wavelength of the laser 103 may match an absorption line of the target material 108. The target material 108 may include any plasma-producing target material which may produce the plasma 109 when irradiated by the laser 103, and more particularly producing high-temperature plasma which may emit the illumination 111 at the desired wavelength. For example, the target material 108 may include tin (Sn), xenon (Xe), lithium (Li), nitrogen (e.g., diatomic nitrogen (N2)), nickel (Ni), copper (Cu), beryllium (Be), nickel-carbonyl (Ni(CO)4), or combinations thereof. For instance, the target material 108 may be one of tin or xenon. The target material 108 may be in any suitable phase immediately before producing the plasma 109. For example, the target material 108 may be in the phase of a solid, a liquid, or a supercritical fluid (e.g., supercritical gas) immediately before producing the plasma 109. In embodiments, the target material 108 may be solid xenon (e.g., xenon ice) immediately before producing the plasma 109, although this is not intended to be limiting.

[0027] The laser 103 may produce the plasma 109 by initiating and / or maintaining the plasma 109. The illumination source 100 may be a laser-produced plasma (LPP) source or a laser-discharge produced plasma (LDP or laser-initiated DPP) source. The illumination source 100 may or may not include an electrode (not depicted) to assist the laser 103 in producing the plasma 109 from the target material 108. For example, the illumination source 100 may be the LPP source which may use the laser 103 to produce the plasma 109 from the target material 108 without the electrode. The absorption of the laser 103 by the target material 108 may ionize the target material 108, producing the plasma 109. For instance,the laser 103 may irradiate the target material 108 with a first pulse (pre-pulse) followed by a second pulse (main pulse) to produce the plasma 109. By way of another example, the illumination source 100 may be the LDP source may use the laser 103 in combination with the electrode to produce the plasma 109 from the target material 108. The electrodes may be coils surrounding the target material 108 which may magnetically excite the target material 108. For instance, the laser 103 may evaporate the target material 108 using the laser 103 followed by pinching the evaporation via the electrode to produce the plasma 109. By way of another instance, the electrodes may initiate the plasma 109 followed by the laser 103 maintaining the plasma 109. As depicted, the illumination source 100 may be the LPP source which may use the laser 103 to produce the plasma 109 from the target material 108 without the electrode, although this is not intended to be limiting.

[0028] The target material 108 may be delivered into the path of the laser 103 at the plasma site 113 in any suitable form, such as, but not limited to, droplets, pellets, a jet, a cylindrical-coating, or the like. The droplet may refer to a small amount of material that will be acted upon by the laser 103 and thereby converted to the plasma 109. The droplet may exist in gas, liquid, or solid phases. The pellet may be the droplet that is in a solid phase, such as by freezing upon moving into the vacuum chamber 104. The target material 108 may be supplied by other means, such as in a stream of droplets or jet of target material, which entrance point into the vacuum chamber 104 may be spaced away from the plasma site 113. In some embodiments, the target material 108 is delivered to the plasma site 113 in the form of liquid droplets, solid pellets, and / or solid particles contained within liquid droplets. The target material 108 can be in the form of a stream or jet or a series of discrete targets (i.e. liquid droplets, solid pellets, and / or solid particles contained within liquid droplets). In embodiments, the target material 108 may be droplets of solid xenon, although this is not intended to be limiting.

[0029] The illumination source 100 may include the target-material generator 126. The target-material generator 126 may also be referred to as a target-materialsupply. The target-material generator 126 may deliver the target material 108 into the path of the laser 103. The target material 108 may be delivered in such a way that the target material 108 may intersect with the laser 103 as the laser 103 is focused onto the target material 108. For example, the target-material generator 126 may deliver the target material 108 at the focal point of the laser 103. The target-material generator 126 may be a droplet-type generator, a pellet-type generator, a jet-type generator, a rotating drum-type generator, or the like for delivering the target material 108 in the selected form. For example, the targetmaterial generator 126 may be configured to deliver droplets of Xenon into the vacuum chamber 104 into the focal point of the laser 103. The target-material generator 126 may pressurize and cool the Xenon, such that the Xenon liquifies. The liquid Xenon may be pumped through a nozzle as a jet. As the jet emerges from the nozzle, the Xenon may decay. As the jet decays, Xenon droplets may be formed. The droplets may be liquid or solid depending on conditions. The droplets may travel to a site in the vacuum chamber 104 where the droplets are irradiated by the laser 103. By way of another example, the target-material generator 126 may be the rotating drum-type generator. The rotating drum-type generator may include a rotating drum which may be coated with the target material 108. For example, the rotating cylinder may be coated with solid xenon, although this is not intended to be limiting. The rotating drum may provide a stable, uniform surface onto which the solid Xenon may be coated and subsequently irradiated by the laser 103. The rotating drum may also translate axially to provide additional sites from which to irradiate the target material 108 with the laser 103. After one or more pulses of the laser 103 irradiate the target material 108, the rotating drum, which may be rotating and / or axially translating, may present a new area of the target material 108 for irradiation. Each irradiation may produce a crater in the coating of the target material 108. The craters may be refilled by a replenishment system of the target-material generator 126 to provide a target material delivery system that can continuously present the target material 108 to be irradiated. The specifics of the target-material generator 126 are not intended to be limiting.

[0030] The plasma 109 may be heated to a select electron temperature. For example, the plasma 109 may be heated by the laser 103 (e.g., by pulses of the laser 103) and / or by the electrode. The electron temperature of the plasma 109 may be selected based on the wavelength of the wavelength of the illumination 111 desired for the application of the illumination source 100. For example, the plasma 109 may be high-temperature plasma. For instance, the electron temperature of the plasma 109 may be between 20 and 40 eV, or the like.

[0031] The illumination source 100 may include the illumination 111. The illumination 111 may also be referred to as exposure light. The plasma 109 may emit the illumination 111. For example, the high-temperature plasma may emit the illumination 111. The illumination 111 may be broadband. The plasma 109 may emit the illumination 111 as broadband radiation. The illumination 111 may be generated by the plasma 109 through de-excitation of excited species within the plasma 109. The plasma 109 may include various excited species, including the target material 108. The spectrum of the illumination 111 may be dependent on the composition of species within the plasma 109, energy levels of excited states of species within the plasma 109, the temperature of the plasma 109, and / or the pressure surrounding the plasma 109. In this regard, the spectrum of the illumination 111 generated by the plasma 109 may be tuned to include emission within a desired wavelength range by selecting the composition of the target material 108 to have one or more emission lines within the desired wavelength range. Often, a desired material (e.g. a desired element, a desired species, or the like) suitable for generating emission within a desired wavelength range exists in a liquid or a solid phase such that high temperatures are required to evaporate the target material 108 and maintain a desired pressure for the plasma 109. In another embodiment, the power, wavelength, and focal characteristics of the illumination source 100 are adjusted to obtain a desired conversion efficiency of absorbed energy to emission output within a desired wavelength range.

[0032] The illumination 111 may include a select wavelength. The illumination 111 may be vacuum-ultraviolet (VUV) light and / or soft X-ray light. The illuminationsource 100 may produce the illumination 111 within the vacuum chamber 104 to prevent the atmosphere from absorbing the VUV light and / or the soft X-ray light. The VUV light may have a wavelength of between 10 nm and 200 nm. The VUV light may be far ultraviolet (FUV) light and / or extreme ultraviolet (EUV) light. The FUV light may have a wavelength of between 121 and 200 nm. The EUV light may have a wavelength of between 10 nm and 124 nm. The soft X-ray light may have a wavelength of between 0.1 and 10 nm. In embodiments, the illumination 111 may be in-band EUV light having a wavelength of 13.5 nm. The in-band EUV light may also be referred to as actinic light (e.g., where the actinic light is used for inspecting a reticle or wafer at the same wavelength used for lithography). For example, the in-band EUV light may have a wavelength of 13.5 nm with 2% bandwidth. Although the illumination 111 is described as the in-band EUV light, this is not intended to be limiting. It is contemplated that the benefits provided by the illumination source 100, may be applicable to any of the VUV light and / or soft X-ray light formed by the plasma 109 operating within the vacuum chamber 104.

[0033] The illumination source 100 may include the collector 110. The collector 110 may also be referred to as a collector mirror, collector optics, reflective optics, or the like. The illumination 111 may be transmitted to the collector 110. The collector 110 may collect the illumination 111 and focus the illumination 111 to the intermediate focal point 124. The collector 110 may include two focal points. For example, the focal points of the collector 110 may include the plasma site 113 (e.g., coinciding with the final focal point of the laser 103) and the intermediate focal point 124. The collector 110 may be located off-axis from the path of the laser 103 between the refractive optics 106 and the target material 108.

[0034] The collector 110 may focus the illumination 111 to the intermediate focal point 124 by reflecting the illumination 111. For example, the collector 110 may be configured to reflect the VUV light and / or the soft X-ray light. For instance, the collector 110 may be configured to reflect the EUV light. In embodiments, the collector 110 may be configured to reflect the in-band EUV light. The collector 110 which is configured to reflect the EUV light (e.g., the in-band EUV light) may beany suitable material. For example, the material which reflects the in-band EUV light may be ruthenium (Ru), molybdenum (Mo) (e.g., Mo / Si multilayer mirrors), niobium (Nb) (e.g., niobium-carbon and silicion (NbC / Si) multilayer mirrors), engineered high density carbon films having high Sp3 content (e.g. tetrahedral (Ta-C)), or the like. The collector 110 may also be a multi-layer coating. The collector 110 may be a multi-layer mirror. The multi-layer mirror may include a graded multilayer coating with alternating layers of material. The multi-layer coating may also include high-temperature diffusion barrier layers, smoothing layers, capping layers, etch stop layers, and the like. The collector 110 may having a reflective surface in the form of a

[0035] The collector 110 may be arranged at a select incidence angle to the illumination 111. For example, the collector 110 may be a near-normal-incidence mirror, a grazing-incidence mirror, or the like. For instance, the collector 110 may be the near-normal-incidence mirror. The collector 110 may be any suitable shape to collect the illumination 111. For example, the collector 110 may be an elliptical collector, a collector with multiple surface contours, a truncated prolate spheroid (i.e. , an ellipse rotated about its major axis) or a segment thereof, or the like.

[0036] The illumination source 100 may include the internal focus module 130. The internal focus module 130 may collect and refocuses the illumination 111. The internal focus module 130 may be coupled to and disposed outside of the vacuum chamber 104. The internal focus module 130 may be a dynamic gas lock to preserve the low-pressure environment within the vacuum chamber 104. The internal focus module 130 may also protect downstream optics that interact with the illumination 111 from the debris 132.

[0037] The illumination source 100 may include the intermediate focal point 124 of the illumination 111. The collector 110 may be configured to focus the illumination 111 to the intermediate focal point 124. The intermediate focal point 124 may be inside the internal focus module 130. In this regard, the collector 110may directs the illumination 111 out of the vacuum chamber 104 and into the internal focus module 130.

[0038] The illumination source 100 may include the first duct 120. The first duct 120 may be arranged around the laser 103, the refractive optics 106, and / or the first inlets 116. The first duct 120 may extend from the refractive optics 106 and / or the first inlets 116 towards the plasma 109 at the plasma site 113.

[0039] The laser 103 may be focused from the refractive optics 106 to the plasma 109 within the first duct 120. The first duct 120 may be disposed around the path of the laser 103. The first duct 120 may be dimensioned and arranged so that the first duct 120 does not block the optical path of the laser 103 (i.e. , the first duct 120 is not a stop for the laser 103). For example, the shape of the first duct 120 may be defined to be larger than the path of the laser 103 defined by the refractive optics 106 and the beam caustic at the plasma site 113. The first duct 120 may envelop the propagation caustic of the laser 103.

[0040] The illumination source 100 may include the second duct 122. The second duct 122 may be arranged around the collector 110, the illumination 111, and / or the second inlets 118. The second duct 122 may extend from the collector 110 and / or the second inlets 118 towards the plasma site 113 and / or towards the intermediate focal point 124. The second duct 122 may envelop the propagation caustic of the laser 103.

[0041] The second duct 122 may include one or more sections. For example, the second duct 122 may include the plasma-to-collector section 134 and / or the collector-to-intermediate focus section 136. The plasma-to-collector section 134 and the collector-to-intermediate focus section 136 may be joined together around the collector 110 and / or the second inlets 118. The plasma-to-collector section 134 and the collector-to-intermediate focus section 136 may intersect to form the second duct 122. The plasma-to-collector section 134 may extend from the collector 110 and / or the second inlets 118 towards the plasma site 113. The collector-to-intermediate focus section 136 may extend from the collector 110and / or the second inlets 118 towards the intermediate focal point 124. The plasma-to-collector section 134 and the collector-to-intermediate focus section 136 may envelope respective sections of the beam caustic. The plasma-to-collector section 134 may envelope the propagation caustic of the illumination 111 from the plasma site 113 to the collector 110. The collector-to-intermediate focus section 136 may envelope the propagation caustic of the illumination 111 from the collector 110 to the intermediate focal point 124.

[0042] The illumination 111 may be collected and focused within the second duct 122. The second duct 122 may be disposed around the path of the illumination 111. For example, the plasma-to-collector section 134 may be disposed around the path of the illumination 111 from the plasma site 113 to the collector 110. By way of another example, the collector-to-intermediate focus section 136 may be disposed around the path of the illumination 111 from the collector 110 to the intermediate focal point 124. The second duct 122 may be dimensioned and arranged so that the second duct 122 does not block the optical path of the illumination 111 (i.e., the second duct 122 is not a stop for the illumination 111). For example, the shape of the plasma-to-collector section 134 may be defined to be larger than the path of the illumination 111 defined by the beam caustic at the plasma site 113 and the collection angle of the collector 110. By way of another example, the shape of the collector-to-intermediate focus section 136 may be defined to be larger than the path of the illumination 111 defined by the collection angle of the collector 110 and the beam caustic at the intermediate focal point 124.

[0043] The second duct 122 may extend from the collector 110 through the vacuum chamber 104 into the internal focus module 130. For example, the collector-to-intermediate focus section 136 extend from the collector 110 through the vacuum chamber 104 into the internal focus module 130. For instance, the collector-to-intermediate focus section 136 extend from the collector 110 through a hole in the wall of the vacuum chamber 104 into the internal focus module 130. The second duct 122 may be continuous from the collector 110 to the internal focus module 130. For example, the collector-to-intermediate focus section 136 may becontinuous from the collector 110 to the internal focus module 130. The collector-to-intermediate focus section 136 may isolate the illumination 111 from the vacuum chamber 104. In this regard, the collector-to-intermediate focus section 136 may not be segmented into halves which do not isolate the illumination 111 ad are not continuous.

[0044] The first duct 120, the second duct 122, the plasma-to-collector section 134, and / or the collector-to-intermediate focus section 136 may include any suitable shape. The shapes may be selected to contain the buffer gas 128 and / or to prevent interfering with the laser 103 and / or the illumination 111. The shapes may also be selected to reduce the total volume of the first duct 120 and the second duct 122 while containing the buffer gas 128 and preventing interference with the laser 103 and / or the illumination 111. For example, the first duct 120, the plasma-to-collector section 134, and / or the collector-to-intermediate focus section 136 may be cone shapes. The shape of the second duct 122 may be defined by the intersecting cone shapes of the plasma-to-collector section 134 and the collector-to-intermediate focus section 136. The cone shapes of the first duct 120, the plasma-to-collector section 134, and / or the collector-to-intermediate focus section 136 may be truncated. For example, the first duct 120 may be truncated by the laser-exit opening 138. By way of another example, the plasma-to-collector section 134 may be truncated by the illumination-entrance opening 140. By way of another example, the collector-to-intermediate focus section 136 may be truncated by the illumination-exit opening 142. The cones shapes may also be right circular or oblique. For instance, the first duct 120 may be a truncated-right circular cone shape. By way of another instance, the plasma-to-collector section 134 and / or the collector-to-intermediate focus section 136 may be truncated-oblique cone shapes. The shapes of the first duct 120 and the second duct 122 may be defined by the generatrices of the propagation cones of respective of the laser 103 and the illumination 111. For example, the vertex from which the first duct 120 is truncated may be the plasma site 113 and the base of the first duct 120 may be arranged around the refractive optics 106. By way of another example, the second duct 122 may be defined by the generatrices of the incident beam originating from theplasma 109, and the reflected beam refocusing onto the intermediate focal point 124, and the common base defined by the collector 110. For instance, the vertex from which the plasma-to-collector section 134 is truncated may be the plasma site 113, the base may be arranged around the collector 110, and the oblique angle of the plasma-to-collector section 134 may be defined at the off-axis angle of the collector 110. By way of another instance, the vertex from which the collector-to-intermediate focus section 136 is truncated may be the intermediate focal point 124, the base may be arranged around the collector 110, and the oblique angle of the plasma-to-collector section 134 may be defined at the reflection of the illumination 111 from the off-axis angle of the collector 110. Thus, the shape of the second duct 122 may resemble the shape of the illumination 111 along the path from the plasma site 113 to the intermediate focal point 124. Although the shapes are described as being the cones, this is not intended as a limitation of the present disclosure. However, the cones may be beneficial for reducing the flow of the buffer gas 128 while maintaining a similar pressure as a non-conical shape.

[0045] The first duct 120 and the second duct 122 may not block respective of the illumination 111 and the laser 103 while enveloping the other of the laser 103 and the illumination 111. For example, the first duct 120 may be disposed out of the path of the illumination 111 from the plasma site 113, to the collector 110, and to the intermediate focal point 124 while enveloping the laser 103. By way of another example, the second duct 122 may be disposed out of the path of the laser 103 from the refractive optics 106 to the plasma site 113 while enveloping the illumination 111.

[0046] The illumination source 100 may include the buffer gas 128. The buffer gas 128 may also be referred to as a protective gas. The buffer gas 128 may be different than the target material 108. The buffer gas 128 may include any suitable element or composition used for the generation of laser-sustained plasmas. For example, the buffer gas 128 may include a noble gas or an inert gas (e.g., noble gas or non-noble gas) such as, but not limited to argon, hydrogen, helium, nitrogen, neon, krypton, xenon, or the like. As another example, the buffer gas 128 mayinclude a non-inert gas (e.g., mercury). As another example, the buffer gas 128 may include a mixture of a noble gas and one or more trace materials (e.g., metal halides, transition metals and the like). The buffer gas 128 may be selected to be more transmissive to the illumination 111 than the target material 108. In one instance, the buffer gas 128 is one of argon, helium, or hydrogen. In embodiments, the buffer gas 128 is argon.

[0047] The buffer gas 128 may be injected along the path of the laser 103 to protect the refractive optics 106 and / or along the path of the illumination 111 to protect the collector 110. The first inlets 116 and the second inlets 118 may inject the buffer gas 128 along the paths of respective of the laser 103 and the illumination 111. The first inlets 116 and the second inlets 118 may also be referred to as gas inlets. The buffer gas 128 may be injected within the first duct 120 and the second duct 122 by respective of the first inlets 116 and the second inlets 118. For example, the first inlets 116 may inject the buffer gas 128 around the refractive optics 106. By way of another example, the second inlets 118 may inject the buffer gas 128 around the collector 110. The buffer gas 128 may then be disposed within the first duct 120 and / or the second duct 122.

[0048] The first duct 120 and / or the second duct 122 may contain the buffer gas 128. For example, the first duct 120 and / or the second duct 122 may be a solid material which may be impermeable to the buffer gas 128 and which may be compatible with the vacuum environment of the vacuum chamber 104. The first duct 120 and / or the second duct 122 may reduce flow expansion by preventing the buffer gas 128 from expanding outwards through the sides of the first duct 120 and / or the second duct 122. The material of the first duct 120 and / or the second duct 122 may also be configured to dissipate heat and / or may not be reflective to the illumination 111. The first duct 120 may contain the buffer gas 128 up to the laser-exit opening 138. The second duct 122 may contain the buffer gas 128 up to the illumination-entrance opening 140 and / or the illumination-exit opening 142.

[0049] The buffer gas 128 may be disposed within the first duct 120. The first inlets 116 may inject the buffer gas 128 within the first duct 120. For example, the buffer gas 128 may be injected around the refractive optics 106 within the first duct 120 by the first inlets 116. The buffer gas 128 may be arranged between the plasma site 113 and the refractive optics 106 within the first duct 120 along the path of the laser 103. The first duct 120 may direct the flow of the buffer gas 128 from the first inlets 116 towards the plasma site 113. The buffer gas 128 injected from the first inlets 116 may merge into a single flow within the first duct 120.

[0050] The buffer gas 128 may be disposed within the second duct 122. The second inlets 118 may inject the buffer gas 128 within the second duct 122. For example, the buffer gas 128 may be injected around the collector 110 within the second duct 122 by the second inlets 118. The buffer gas 128 may be arranged between the plasma site 113, the collector 110, and the intermediate focal point 124 within the second duct 122 along the path of the illumination 111. For example, the buffer gas 128 disposed within the plasma-to-collector section 134 may be arranged between the plasma site 113 and the collector 110. By way of another example, the buffer gas 128 disposed within the collector-to-intermediate focus section 136 may be arranged between the collector 110 and the intermediate focal point 124. The second duct 122 may direct the flow of the buffer gas 128 from the second inlets 118 towards the plasma site 113 and / or towards the intermediate focal point 124. For example, the plasma-to-collector section 134 may direct the flow of the buffer gas 128 from the second inlets 118 towards the plasma site 113. The buffer gas 128 injected from the first inlets 116 may merge into a single flow within the plasma-to-collector section 134. By way of another example, the collector-to-intermediate focus section 136 may direct the flow of the buffer gas 128 from the second inlets 118 towards the intermediate focal point 124. The plasma-to-collector section 134 and the collector-to-intermediate focus section 136 may separate the flow of the buffer gas 128 within the second duct 122.

[0051] The production of the plasma 109 may generate the debris 132. For example, the debris 132 may be generated from various sources including, but notlimited to, the target material 108, the plasma 109, plasma-facing components, eroded surfaces in proximity of the target material 108 or the plasma 109, the target-material generator 126, or the like. The debris 132 may be undesired byproducts when producing the plasma 109. The debris 132 may include any suitable debris generated from producing the plasma 109. For example, the debris 132 may include the target material 108 (e.g., the target material 108 in gas phase), atomic vapor, free ions, neutrals, micro-particles, contaminants (e.g. hydrocarbons or organics), or the like. For instance, not all of the target material 108 that is delivered into the vacuum chamber 104 may be successfully converted to the plasma 109, such that the gas particles of the target material 108 may for the debris 132. By way of another example, the plasma 109 may emit the free ions and / or atomic vapor during the ionization and / or de-excitation. By way of another example, the debris 132 from the target material 108 in the form of fast neutrals, ions and / or as fragments may produce secondary metal debris by sputtering metal plasma materials.

[0052] The debris 132 may follow the path of the laser 103 and / or the illumination 111, as both the illumination 111 and the debris 132 may be generated by the plasma 109. The debris 132 within the path of the laser 103 may contaminate the refractive optics 106. The debris 132 within the path of the illumination 111 may contaminate the collector 110 and / or attenuate the illumination 111. The contamination may degrade the performance of the optics, affect the efficiency of the generation and / or extraction of the illumination 111.

[0053] The debris 132 may contaminate the refractive optics 106 and / or the collector 110 when following respectively, the path of the laser 103 and the illumination 111. For example, high energy ions, neutrals and other micro-particles may erode / sputter the refractive optics 106 and / or the collector 110. The contamination of the refractive optics 106 and / or the collector 110 may reduce the useful lifetime and / or the efficiency and may also cause irreparable damage. Contamination of the refractive optics 106 may lead to transmission loss of the laser 103, a reduction in beam quality of the laser 103, or the like, which mayreduce the output of the illumination 111 and / or cause distortion in the shape of the illumination 111.

[0054] The illumination source 100 may be configured to protect the refractive optics 106 and / or the collector 110 from contamination of the debris 132 and / or to control the attenuation of the illumination 111 by the debris 132.

[0055] The buffer gas 128 may protect the refractive optics 106 and / or the collector 110 by slowing down and / or stopping the debris 132. For example, the buffer gas 128 may slow down and / or stop ions and neutrals of the debris 132 within the first duct 120 and / or the second duct 122 to prevent etching of the refractive optics 106 and / or the collector 110. The buffer gas 128 may establish a buffer in proximity of the refractive optics 106 and / or the collector 110, which may slow down high-energy ions and / or neutrals, thereby preventing the ions and / or neutrals from eroding the refractive optics 106 and / or the collector 110. The pressures of the buffer gas 128 within the first duct 120 and / or the second duct 122 may be sufficient to reduce ion energy (e.g. due to collisions between the ions and buffer gas 128) to a target maximum energy level, before the ions reach the refractive optics 106 and / or the collector 110. For example, the target maximum energy level may be below 100 eV, and in some cases below 30 eV.

[0056] The buffer gas 128 may also blow the debris 132 away from the first duct 120 and the second duct 122. For example, the buffer gas 128 may blow the gas of the target material 108 away from the first duct 120 and / or the second duct 122. The buffer gas 128 may blow the debris 132 transversely out of the path of the first duct 120 and / or the second duct 122. For example, the flow of the buffer gas 128 through the first duct 120 may blow the debris 132 away from the light path between the refractive optics 106 and the plasma site 113. By way of another example, the flow of the buffer gas 128 through the second duct 122 may blow the target material 108 and / or the debris 132 away from the light path between the plasma site 113 and the collector 110. The buffer gas 128 may encounter the debris 132, and by advection, creates a flow of the target material 108 and / or thedebris 132 towards the vacuum pumps 112. The buffer gas 128 may flow from the first duct 120 and / or the second duct 122 into the vacuum chamber 104. In this regard, the buffer gas 128 inside the first duct 120 and / or the second duct 122 is not static. The flow of the first duct 120 and / or the second duct 122 into the vacuum chamber 104 may prevent the diffusion of the debris 132 into the first duct 120 and / or the second duct 122.

[0057] The buffer gas 128 may or may not blow all of the target material 108 away from the second duct 122. For example, the second duct 122 may contain a mixture of the target material 108 and the buffer gas 128. The relative concentration of the target material 108 and the buffer gas 128 within the second duct 122 may be controlled by the pressure of the buffer gas 128. The concentration of the target material 108 within the second duct 122 may be relatively low due to being blown away from the second duct 122 by the buffer gas 128. The concentration of the target material 108 within the vacuum chamber 104 surrounding the outside of the first duct 120 and the second duct 122 may have higher concentration fractions of the target material 108 than inside the second duct 122 (e.g., due to the target material 108 being blown away from the ducts).

[0058] The buffer gas 128 and / or the debris 132 within the second duct 122 may attenuate the illumination 111. The illumination 111 may be strongly absorbed by most materials, including gases and atomic vapors. The debris 132, and especially the gas / atomic vapor of the debris 132, may attenuate the illumination 111. The path of the illumination 111 between the plasma site 113, the collector 110, and the intermediate focal point 124 is the area of primary concern for light attenuation.

[0059] The illumination source 100 may be configured to control the pressure of the debris 132 within the path of the illumination 111 using the buffer gas 128. The buffer gas 128 may blow away or reduce the concentration of the target material 108 within the path of the illumination 111. The buffer gas 128 may deflect the target material 108 from entering the path of the illumination 111. For example, the buffer gas 128 may deflect the target material 108 by advection. Reducing thepressure of the debris 132 along the path of the illumination 111 between the plasma site 113, the collector 110, and the intermediate focal point 124 may reduce the attenuation of the illumination 111. The buffer gas 128 may absorb less of the illumination 111 than the target material 108. The absorption cross-section of the illumination 111 may be much lower for the buffer gas 128 than for the target material 108. For example, the absorption cross-section of in-band EUV light for argon as the buffer gas 128 may be approximately 18 times smaller than the absorption cross-section of xenon as the target material 108. Therefore, the injection of the buffer gas 128 within the second duct 122 to blow away the debris 132 may be beneficial to reduce the attenuation of the illumination 111 by the debris 132, even if the buffer gas 128 does introduce some attenuation to the illumination 111. For example, the buffer gas 128 may increase the transmission of the in-band EUV light by reducing the concentration of the target material 108 on the path from the plasma site 113 to the collector 110 and then on to the intermediate focal point 124.

[0060] The flow of the buffer gas 128 within the first duct 120 and second duct 122 may establish and maintain average gas pressures within the first duct 120 and / or the second duct 122. The average gas pressures may include the pressure of the buffer gas 128 and the target material 108. The average gas pressures may be selected to provide sufficient protection from the ions and reduce the attenuation from the debris 132. The pressure of the buffer gas 128 within the second duct 122 may be selected to provide sufficient protection against the debris 132 while also not attenuating too much of the illumination 111.

[0061] The illumination source 100 may include the vacuum pumps 112. The vacuum pumps 112 may be connected to vacuum chamber 104. The vacuum pumps 112 may establish and maintain the low-pressure environment of the vacuum chamber 104. The vacuum pumps 112 may include any suitable pump. For example, the vacuum pumps 112 may be a turbo pump, turbo-molecular pump, and / or a roots pump. The vacuum pumps 112 include a dry pumping unit and / or an exhaust system. The vacuum pumps 112 may remove the target material 108,the buffer gas 128, and / or the debris 132 from the vacuum chamber 104. After the target material 108 produces the plasma 109 at the plasma site 113, the target material 108, the buffer gas 128, and / or the debris 132 may be directed towards the vacuum pumps 112 by the flow of buffer gas 128. The vacuum pumps 112 may remove the target material 108, the buffer gas 128, and / or the debris 132 from the vacuum chamber 104.

[0062] The illumination source 100 may include the gas supply 114. The gas supply 114 may supply the buffer gas 128 within the first duct 120 and the second duct 122. For example, the gas supply 114 may supply the buffer gas 128 within the first duct 120 and the second duct 122 to respective of the first inlets 116 and / or the second inlets 118. The gas supply 114 may be fluidly coupled with the first inlets 116 and / or the second inlets 118. For example, the buffer gas 128 may flow from the gas supply 114 via a manifold (not depicted) to the first inlets 116 and / or the second inlets 118. The gas supply 114 may be configured to supply the buffer gas 128 to the first inlets 116 and / or the second inlets 118, such that respective of the first duct 120 and the second duct 122 contains the buffer gas 128 at desired pressures. The gas supply 114 may adjust pressures of the buffer gas 128 within the first duct 120 and / or the second duct 122. For example, the pressures of the buffer gas 128 may be adjusted to control plasma dynamics of the plasma 109. For example, the plasma dynamics may include, but are not limited to, the rate at which the debris 132 is removed from the plasma 109, ambient pressure in the vicinity of the plasma 109, vapor pressure surrounding the plasma 109, or the composition of the plasma 109.

[0063] The gas supply 114, the first inlets 116, the second inlets 118, and / or the vacuum pumps 112 may be configured to actively control the flow the buffer gas 128. The gas supply 114, first inlets 116, the second inlets 118, and / or the vacuum pumps 112 may establish and maintain gas pressures within the first duct 120 and the second duct 122 sufficient to protect respective of the refractive optics 106 and / or the collector 110 from the debris 132. For the illumination source 100 in a steady-state regime, the pressure within the vacuum chamber 104 may bedetermined by the balance of the gas throughput and the pumping speed of the vacuum pumps 112. The pressures within the first duct 120 and the second duct 122 may be determined by the balance of the pressure in the vacuum chamber 104 and the flow from respective of the first inlets 116 and the second inlets 118.

[0064] The gas supply 114 may be configured to individually control the pressures within first duct 120 and the second duct 122. The pressure of the buffer gas 128 within the first duct 120 may be different than the pressure of the buffer gas 128 within the second duct 122 when the first inlets 116 and the second inlets 118 are injecting the buffer gas 128. The refractive optics 106 and the collector 110 may have different requirements for protection from the debris 132. The ability to individually control the pressure of the buffer gas 128 may enable meeting the requirements for protection for the optics with the higher protection requirement without supplying excess of the buffer gas 128 to the optics with the lower protection requirement. The refractive optics 106 and the collector 110 may pose different requirements to the protection and the consequences of the suboptimal protection of each of the optics may be are different. The ability to individually control may improve the protection for the optics and drive down the gas consumption requirements. The gas flow rate through the first duct 120 may be defined by the protection requirements of the refractive optics 106 and the gas flow rate through the second duct 122 may be defined by the protection requirements of the collector 110. The supply of the buffer gas 128 to the first duct 120 may be decoupled from the supply to the second duct 122, enabling meeting the protective requirements independently.

[0065] Since no absorption requirements are posed by the gas density for the refractive optics 106, a proper adjustment of the length of the first duct 120 may allow for maintaining high inside pressure, and corresponding protection for such the refractive optics 106, thus eliminating the requirement for protective optical elements and improving the efficiency of the plasma excitation. For example, the pressure of the buffer gas 128 within the first duct 120 may be higher than the pressure of the buffer gas 128 within the second duct 122 when the first inlets 116and the second inlets 118 are injecting the buffer gas 128. The buffer gas 128 may be transparent to the laser 103. In this regard, the concentration of the buffer gas 128 within the first duct 120 may be increased above the pressure of the buffer gas 128 within the second duct 122 without impacting the ability of the laser 103 to produce the plasma 109.

[0066] The first duct 120 may include the laser-exit opening 138. The first duct 120 may be hollow from the refractive optics 106 to the laser-exit opening 138. The laser-exit opening 138 may be the closest portion of the first duct 120 to the plasma site 113. The laser 103 and / or the buffer gas 128 may exit the first duct 120 into the vacuum chamber 104 via the laser-exit opening 138. The laser-exit opening 138 may or may not be aligned on-axis to the laser 103. For example, the laserexit opening 138 is depicted as on-axis to the laser 103, although this is not intended to be limiting.

[0067] The second duct 122 may include the illumination-entrance opening 140 and the illumination-exit opening 142. The second duct 122 may be hollow from the collector 110 to the illumination-entrance opening 140 and / or to the illumination-exit opening 142. The illumination-entrance opening 140 may be the closest portion of the second duct 122 to the plasma site 113. The illumination-exit opening 142 may be the closest portion of the second duct 122 to the intermediate focal point 124. The illumination 111 may enter the second duct 122 from the vacuum chamber 104 via the illumination-entrance opening 140. The illumination 111 may exit the second duct 122 (e.g., into the internal focus module 130) via the illumination-exit opening 142. The buffer gas 128 may exit the second duct 122 via the illumination-entrance opening 140 and / or the illumination-exit opening 142. The plasma-to-collector section 134 may define the illumination-entrance opening 140. The collector-to-intermediate focus section 136 may define the illuminationexit opening 142.

[0068] The second duct 122 may produce a transversely directed (e.g., transverse to the optical axis of the illumination 111) flow of the buffer gas 128between the collector 110 and the plasma site 113 to push the debris 132 out of the path of the illumination 111. For example, the plasma-to-collector section 134 may produce the transversely directed flow. For instance, the illumination-entrance opening 140 produce the transversely directed flow. The illumination-entrance opening 140 may be angled off-axis relative to the optical axis of the illumination 111 between the plasma site 113 and the collector 110. The angle of the illumination-entrance opening 140 off-axis to the illumination 111 may be beneficial to deflect the debris 132 using the buffer gas 128 without the flow of the buffer gas 128 being head on to the debris 132 (e.g., potentially reducing flow rate).

[0069] The illumination-entrance opening 140 may be angled away from the laser-exit opening 138. The illumination-entrance opening 140 being angled away from the laser-exit opening 138 may decouple the interaction of the buffer gas 128 inside the first duct 120 exiting the laser-exit opening 138 from the buffer gas 128 inside the second duct 122 exiting the illumination-entrance opening 140. For example, the debris 132 may deflect from the buffer gas 128 exiting the laser-exit opening 138 and may go to a side of the vacuum chamber 104 opposite to the second duct 122. By way of another example, the debris 132 may deflect from the buffer gas 128 exiting the illumination-entrance opening 140 and may go to a side of the vacuum chamber 104 opposite to the first duct 120.

[0070] The illumination-exit opening 142 may be smaller than the illuminationentrance opening 140. The illumination-exit opening 142 being smaller than the illumination-entrance opening 140 may cause the buffer gas 128 to stagnate at the illumination-exit opening 142 and may cause most of the buffer gas 128 to form counterflow to the debris 132 and exit toward the plasma site 113 through the illumination-entrance opening 140.

[0071] The length of the first duct 120 and / or the second duct 122 may or may not extend up to the plasma site 113. For example, the length of the first duct 120 and / or the second duct 122 may be offset from the plasma site 113. In one example, the second duct 122 may be closer to the plasma site 113 than the firstduct 120. For example, the illumination-entrance opening 140 may be closer to the plasma site 113 than the laser-exit opening 138.

[0072] The pressure inside of the vacuum chamber 104 (e.g., outside of the first duct 120 and / or the second duct 122 but within the vacuum chamber 104) may be lower than the pressures of the buffer gas 128 inside the first duct 120 and / or the second duct 122 when the first inlets 116 and the second inlets 118 are injecting the buffer gas 128. The first duct 120 and / or the second duct 122 may prevent filling the vacuum chamber 104 with excess of the buffer gas 128 outside of the paths of the laser 103 and / or the illumination 111 within the vacuum chamber 104. As one example, the pressure inside the vacuum chamber 104 may be about 10 times less than the pressure inside the first duct 120 and / or about 6 times less than the pressure inside the second duct 122.

[0073] The illumination source 100 may be used in any suitable application. The illumination 111 may be collected for use in a semiconductor process. For example, the illumination source 100 may be used within an inspection system 200, a lithography system (not depicted), or the like. The illumination 111 may be the in-band EUV light which may be particularly suitable for use in metrology and / or mask inspection activities (e.g., actinic mask inspection and including blank or patterned mask inspection) using the inspection system 200.

[0074] FIG. 2 illustrates an inspection system 200, in accordance with one or more embodiments of the present disclosure. The inspection system 200 may be an EUV reticle inspection tool, an EUV inspection system, a EUV mask projection system, or the like. For example, the inspection system 200 may be an actinic inspection system by using the in-band EUV light that may represent what will be realized using EUV light during lithography. The inspection system 200 may operate in a vacuum to prevent the atmosphere from absorbing the illumination 111 and / or the collected light 209.

[0075] The inspection system 200 may include illumination source 100. The inspection system 200 may be configured to inspect the sample 203. Theinspection system 200 may be configured to inspect the sample 203 by illuminating the sample 203 with illumination 111, collect collected light 209 from the sample 203, and detecting field images 236 based on the collected light 209. The collected light 209 may include a field plane 232, the field images 236,

[0076] The illumination 111 and / or the collected light 209 may be extreme ultraviolet (EUV) light. The EUV light may have a wavelength of between 10 nm and 121 nm. For example, the EUV light may have a wavelength between 5 nm and 30 nm. For instance, the EUV light may have a wavelength between 5 and 15 nm. In embodiments, the illumination 111 may be in-band EUV light having a wavelength of 13.5 nm. For example, the in-band EUV light may have a wavelength of 13.5 nm with 2% bandwidth. Although the illumination 111 and / or the collected light 209 is described as the in-band EUV light, EUV light at other wavelength ranges may also be used. The illumination 111 and / or the collected light 209 may be continuous, pulsed, modulated, or the like. For example, the illumination 111 and / or the collected light 209 may be pulsed.

[0077] The inspection system 200 may direct the illumination 111 to the sample 203. For example, the inspection system 200 may be configured to direct the illumination 111 to the sample 203 along the illumination path 205. The illumination path 205 may be an optical path for providing the illumination 111 to the sample 203 being inspected. The illumination path 205 may include the illumination optics 204 which direct the illumination 111 to the sample 203. The illumination optics 204 may include one or more optical components (not depicted). The illumination optics 204 may include and / or be downstream of the internal focus module 130 in the illumination path 205. The optical components may be optical mirrors (e.g., due to the wavelength of the illumination 111). The illumination optics 204 may reflect the illumination 111 such that the illumination 111 illuminates the sample 203. The illumination optics 204 may include a series of condensing mirrors configured to condense the illumination 111 into a narrow beam directed to the sample 203. The illumination optics 204 may also include a multiplexing mirror to multiplex the illumination 111 from one or more of the illumination source 100. The opticalcomponents may also process and / or shape the illumination 111 prior to directing onto the sample 203. For example, the illumination optics 204 may include collector optics, homogenizers, spectral purity filters, relays, condensers, and the like. The collector optics may collect the illumination 111 from the illumination source 100 and direct the illumination 111 to the sample 203. The homogenizer may change the illumination 111 from a gaussian beam to a flat-top beam. The flat-top beam may also be referred to as a top-hat beam. The spectral purity filter may filter wavelengths (e.g., drive laser wavelengths of the illumination source 100) from the illumination 111. The relays may relay the illumination 111 between any of the various optical components of the illumination optics 204. The condenser may condense the illumination 111 into a converging beam on the sample 203. The illumination path 205 may also include an additional aperture stop (not depicted), which may be referred to as an illumination-aperture stop.

[0078] The sample 203 may include a mask blank, a photomask, a wafer, a die, or the like. The photomask may also be referred to as a reticle. For example, the sample 203 may be a photomask used in extreme ultraviolet (EUV) lithography.

[0079] The stage 210 may support the sample 203. The stage 210 may be an actuatable stage. The illumination 111 and / or the collected light 209 may be scanned in a scanning direction over the sample 203. The stage 210 may scan the illumination 111 and the collected light 209 in a scanning direction over the sample 203. The sample 203 may be scanned under the illumination 111 and / or the collected light 209 by actuating the stage 210. The stage 210 may include any device suitable for positioning and / or scanning the sample 203 within the inspection system 200. For example, the stage 210 may include any combination of linear translation stages, rotational stages, tip / tilt stages, or the like. For example, the stage 210 may include, but is not limited to, one or more translational stages suitable for translating the sample 203 along one or more linear directions (e.g., x-direction, y-direction, and / or z-direction). By way of another example, the stage 210 may include, but is not limited to, one or more rotational stages suitable for rotating the sample 203 along a rotational direction. By way of another example,the stage 210 may include, but is not limited to, a rotational stage and a translational stage suitable for translating the sample 203 along a linear direction and / or rotating the sample 203 along a rotational direction.

[0080] The illumination 111 may reflect from the sample 203 as collected light 209. The collected light 209 may reflect via specular reflection, scattering, diffusion, or the like. The illumination path 205 and the imaging path 207 may be spatially separated. The illumination 111 and the collected light 209 may be off-axis when being directed to and reflected from, respectively, the sample 203. The collected light 209 may reflect from the sample 203 off-axis to the illumination 111. The collected light 209 may be patterned light. For example, the collected light 209 may be patterned according to the mask, the wafer, and / or the die of the sample 203. The pattern may also indicate defects associated with the sample 203. The illumination source 100 may also illuminate the sample 203 via critical illumination. The collected light 209 may be the EUV light (e.g., the in-band EUV light).

[0081] The imaging path 207 may be the optical path for collected light 209 from the sample 203 being inspected to the detector 208. The imaging optics 206 may be configured to direct the collected light 209 from the sample 203 to the detector 208 along the imaging path 207. The imaging path 207 may include the imaging optics 206 which direct the collected light 209 to the detector 208. The imaging optics 206 may be between the field plane 232 and the detector 208.

[0082] The imaging optics 206 may output a projection of the collected light 209 onto the detector 208. The imaging optics 206 may collect the collected light 209 and form images at the detector 208. For example, the imaging optics 206 may be configured to image the field images 236 on the detector 208. The field images 236 may be a conjugate of the field plane 232.

[0083] The field plane 232 may also be referred to as an object plane, a reticle plane, a mask plane, or the like. The field plane 232 may be the reflection of the illumination 111 on the sample 203. The field plane 232 may be represented by one or more field points. The collected light 209 reflected, diffracted, or scatteredfrom different locations on the sample 203 may be detected in different locations in the field plane 232, regardless of the collection angle.

[0084] The field images 236 may be conjugates to the field plane 232. For example, the collected light 209 emanating from a particular point of the field plane 232 at any angle may be imaged to a corresponding to a particular point in the field images 236. The field images 236 may be a final conjugate plane of the field plane 232.

[0085] The detector 208 may be configured to detect the field images 236 from the collected light 209. The detector 208 may be a time-delay-integration detector array.

[0086] The detector 208 may be configured to provide the field images 236 to the controller 212. The controller 212 may receive the field images 236 from the detector 208. The controller 212 may use the field images 236 to detect one or more defects on the sample 203, or the like.

[0087] FIG. 3 illustrates a flow diagram of a method 300, in accordance with one or more embodiments of the present disclosure. The embodiments and the enabling technologies described previously herein in the context of the illumination source 100 should be interpreted to extend to the method 300. It is further noted, however, that the method 300 is not limited to the architecture of the illumination source 100.

[0088] In a step 310, first inlets and second inlets may inject a buffer gas into a first duct and a second duct. For example, the first inlets 116 and the second inlets 118 may inject the buffer gas 128 into the first duct 120 and the second duct 122. The buffer gas 128 may to flow from the first duct 120 and the second duct 122 into the vacuum chamber 104.

[0089] In a step 320, a target material may be delivered to a plasma site within a vacuum chamber. For example, the target material 108 may be delivered to the plasma site 113 within the vacuum chamber 104.

[0090] In a step 330, a laser source may generate a laser. For example, the laser source 102 may generate the laser 103.

[0091] In a step 340, the laser may refract through refractive optics into the vacuum chamber. For example, the laser 103 may refract through the refractive optics 106 into the vacuum chamber 104. The first duct 120 may be arranged around the laser 103, the refractive optics 106, and the first inlets 116. The first duct 120 may extend from the refractive optics 106 towards the plasma site 113.

[0092] In a step 350, irradiating the target material at the plasma site by the laser to produce a plasma, the plasma emitting illumination. For example, the target material 108 may be irradiated at the plasma site 113 by the laser 103 to produce the plasma 109, the plasma 109 emitting the illumination 111.

[0093] In a step 360, reflecting the illumination from a collector to an intermediate focal point. For example, the collector 110 may reflect the illumination 111 to the intermediate focal point 124. The vacuum chamber 104 may house the refractive optics 106, the collector 110, the plasma site 113, the first duct 120, and the second duct 122. The second duct 122 may be arranged around the collector 110, the illumination 111, and the second inlets 118. The second duct 122 may extend from the collector 110 towards the plasma site 113 and towards the intermediate focal point 124.

[0094] Referring generally again to the figures. Reducing the consumption requirement of the buffer gas 128 by the first duct 120 and / or the second duct 122 may provide several benefits, such as, but not limited to, reducing the cost of the source, reducing the size of the source, reducing the complexity of the source, reducing the ease-of-repair of the source, and the like.

[0095] The depiction of the buffer gas 128 in FIG. 1D separately from the laser 103 and the illumination 111 in FIG. 1 E is not intended to be limiting, and is merely in the interest of clarity (e.g., to depict the rays of the laser 103 and the illumination 111 insides the ducts and to depict the buffer gas 128 merging together inside theducts). The buffer gas 128, the laser 103, and the illumination 111 may each be disposed within the vacuum chamber 104 at the same time.

[0096] The volume that needs to be filled with the buffer gas 128 may be the first duct 120 and the second duct 122, rather than that of the vacuum chamber 104, which results in the reduced protective gas consumption. Computational Fluid Dynamics modelling indicates dramatic reduction of the flow of the buffer gas 128 gas flow through the second duct 122 compared to the traditional open flow architecture to maintain the same optics protection. The flow of the buffer gas 128 through the vacuum chamber 104 is projected to be reduced significantly, correspondingly lowering the requirements for the pumping capacity. The concentration of the fuel vapors reaching the collector 110 is expected to be reduced by as an order of magnitude. Due to the mechanical isolation of the flow of the buffer gas 128 inside the second duct 122 and the protective gas counterflow from the opening, transmission of the illumination 111 between the plasma 109 and the collector 110 is expected to substantially improve.

[0097] The reduction in the excess pressure of the buffer gas 128 outside of the paths of the laser 103 and / or the illumination 111 may enable reducing the supply requirements for the gas supply 114 and / or the pumping requirements for the vacuum pumps 112. For example, the illumination source 100 may use fewer of the vacuum pumps 112 which may reduce the size, complexity, and / or cost of the illumination source 100. The lower pressure of the buffer gas 128 within the vacuum chamber 104 may or may not increase a risk of metal sputtering from the walls of the vacuum chamber 104 caused by the debris 132. However, the first duct 120 and the second duct 122 may protect respective of the refractive optics 106 and the collector 110 from the metal sputtering.

[0098] The laser 103 may be directed from the laser source 102 to the refractive optics 106 via one or more additional components (not depicted). The additional components may include laser combiners, focusing optics, or the like. The laser combiners may combine the laser 103 from multiple of the sources. The numberof the sources which are combined using the laser combiner may be selected based on the power output of the sources, the desired power / brightness of the illumination 111 , the efficiency at which the illumination 111 is generated, and the like. The illumination source 100 may also include multiple of the refractive optics 106 for combining multiple of the laser 103 into the same focal spot on the target material 108. The focusing optics may adjust the position and / or focus of the laser 103 on the target material 108.

[0099] The first inlets 116 and the second inlets 118 may include a plurality of holes (not depicted) arranged around the respective of the refractive optics 106 and the collector 110. For instance, the plurality of holes may be arranged circularly around the refractive optics 106 and the collector 110. When the buffer gas 128 is injected through the plurality of holes, the individual jets of the buffer gas 128 may merge into a single flow at some distance from the holes, which may be toward the plasma site 113. The merging concept may be defined at a certain location in space, known as a merging line, where the angles of the flow from various sources converge to a single angle. In the area beyond the merging line, the individual flows combine to a single flow, which forms a protective barrier and prevents most or all contaminants from penetrating the flow of the buffer gas 128. The second duct 122 may enable injecting the buffer gas 128 around the collector 110 with sufficient pressure to protect the collector 110 from the debris 132 without injecting the buffer gas 128 through holes inside the collector 110. For example, due to the reduced slow speed of the buffer gas 128 in the vicinity of the collector 110, the gas inlets through the collector 110 may no longer be necessary, thus the cost of the collector 110 may be reduced and the reliability may be improved.

[0100] The illumination source 100 may or may not inject the buffer gas 128 by inlets which are additional to the first inlets 116 and / or the second inlets 118. For example, the first inlets 116 and the second inlets 118 may be the only inlets by which the buffer gas 128 is injected into the vacuum chamber 104, although this is not intended to be limiting. It is further contemplated that the illumination source100 may inject the buffer gas 128 into the collector-to-intermediate focus section 136 adjacent to the internal focus module 130.

[0101] The illumination-entrance opening 140 may be a removable and / or adjustable piece of the plasma-to-collector section 134, allowing for the fine tuning of the counterflow direction to optimize source performance.

[0102] A controller may include one or more controllers housed in a common housing or within multiple housings. In this way, any controller or combination of controllers may be separately packaged as a module suitable for integration into a system. Further, the controllers may analyze data received from detectors and feed the data to additional components within the system or external to the system.

[0103] A controller may include one or more processors and / or memory. The memory may maintain program instructions which may be executable by the processors, causing the controller to perform any of the various functions of the controller.

[0104] The one or more processors may include any processor or processing element known in the art. For the purposes of the present disclosure, the term “processor” or “processing element” may be broadly defined to encompass any device having one or more processing or logic elements (e.g., one or more microprocessor devices, one or more application specific integrated circuit (ASIC) devices, one or more field programmable gate arrays (FPGAs), or one or more digital signal processors (DSPs)). In this sense, the one or more processors may include any device configured to execute algorithms and / or instructions (e.g., program instructions stored in memory). In one embodiment, the one or more processors may be embodied as a desktop computer, mainframe computer system, workstation, image computer, parallel processor, networked computer, or any other computer system configured to execute a program. Moreover, different subsystems of the system may include a processor or logic elements suitable for carrying out at least a portion of the steps described in the present disclosure. Therefore, the above description should not be interpreted as a limitation on theembodiments of the present disclosure but merely as an illustration. Further, the steps described throughout the present disclosure may be carried out by a single controller or, alternatively, multiple controllers.

[0105] The memory medium may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors. For example, the memory medium may include a non-transitory memory medium. By way of another example, the memory medium may include, but is not limited to, a read-only memory (ROM), a random-access memory (RAM), a magnetic or optical memory device (e.g., disk), a magnetic tape, a solid-state drive, and the like. The memory medium may include flash memory cells, or other type memory, discrete EPROM or EEPROM, or the like. It is further noted that memory medium may be housed in a common controller housing with the one or more processors. In one embodiment, the memory medium may be located remotely with respect to the physical location of the one or more processors and controller. For instance, the one or more processors of controller may access a remote memory (e.g., server), accessible through a network (e.g., internet, intranet, and the like).

[0106] It is further contemplated that each of the embodiments of the methods described above may include any other step(s) of any other method(s) described herein. In addition, each of the embodiments of the method described above may be performed by any of the systems described herein.

[0107] One skilled in the art will recognize that the herein described components operations, devices, objects, and the discussion accompanying them are used as examples for the sake of conceptual clarity and that various configuration modifications are contemplated. Consequently, as used herein, the specific exemplars set forth and the accompanying discussion are intended to be representative of their more general classes. In general, use of any specific exemplar is intended to be representative of its class, and the non-inclusion ofspecific components, operations, devices, and objects should not be taken as limiting.

[0108] As used herein, directional terms such as “top,” “bottom,” “over,” “under,” “upper,” “upward,” “lower,” “down,” and “downward” are intended to provide relative positions for purposes of description, and are not intended to designate an absolute frame of reference. Various modifications to the described embodiments will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments.

[0109] With respect to the use of substantially any plural and / or singular terms herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. The various singular / plural permutations are not expressly set forth herein for sake of clarity.

[0110] The herein described subject matter sometimes illustrates different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "connected," or "coupled," to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being "couplable," to each other to achieve the desired functionality. Specific examples of couplable include but are not limited to physically mixable and / or physically interacting components and / or wirelessly interactable and / or wirelessly interacting components and / or logically interacting and / or logically interactable components.

[0111] Furthermore, it is to be understood that the invention is defined by the appended claims. It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” and the like). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., “a” and / or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, and the like” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “ a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, and the like). In those instances where a convention analogous to “at least one of A, B, or C, and the like” is used, in general such a construction is intended in the sense one having skill in the art wouldunderstand the convention (e.g., “ a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, and the like). It will be further understood by those within the art that virtually any disjunctive word and / or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

[0112] It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes. Furthermore, it is to be understood that the invention is defined by the appended claims.

Claims

CLAIMSWhat is claimed:

1. An illumination source comprising:a laser source, wherein the laser source is configured to generate a laser; a vacuum chamber;refractive optics, wherein the refractive optics are sealed to the vacuum chamber, wherein the laser is configured to refract through the refractive optics into the vacuum chamber;a target material, wherein the laser is configured to irradiate the target material at a plasma site within the vacuum chamber, wherein the laser causes the target material to produce a plasma, wherein the plasma is configured to emit illumination;a collector, wherein the collector is configured to reflect the illumination from the plasma to an intermediate focal point;first inlets;a first duct, wherein the first duct is arranged around the laser, the refractive optics, and the first inlets, wherein the first duct extends from the refractive optics towards the plasma site, wherein the first inlets are configured to inject a buffer gas into the first duct;second inlets; anda second duct, wherein the vacuum chamber houses the refractive optics, the collector, the plasma site, the first duct, and the second duct, wherein the second duct is arranged around the collector, the illumination, and the second inlets, wherein the second duct extends from the collector towards the plasma site and towards the intermediate focal point, wherein the second inlets are configured to inject the buffer gas into the second duct, wherein the buffer gas is configured to flow from the first duct and the second duct into the vacuum chamber.

2. The illumination source of claim 1 , wherein the target material comprises at least one of tin or xenon, wherein the buffer gas comprises at least one of argon, helium, or hydrogen.

3. The illumination source of claim 1, wherein the illumination comprises at least one of a vacuum-ultraviolet light or a soft X-ray light.

4. The illumination source of claim 3, wherein the illumination comprises the vacuum-ultraviolet light, wherein the vacuum-ultraviolet light comprises one of far ultraviolet light or extreme ultraviolet light.

5. The illumination source of claim 4, wherein the illumination comprises the extreme ultraviolet light.

6. The illumination source of claim 1, wherein the collector is located off-axis from a path of the laser between the refractive optics and the target material.

7. The illumination source of claim 1, wherein the intermediate focal point is disposed outside of the vacuum chamber.

8. The illumination source of claim 7, comprising an internal focus module, wherein the internal focus module is coupled to and disposed outside of the vacuum chamber, wherein the intermediate focal point is disposed within the internal focus module.

9. The illumination source of claim 8, wherein the second duct extends from the collector through the vacuum chamber into the internal focus module.

10. The illumination source of claim 1, wherein the second duct comprises a plasma-to-collector section and a collector-to-intermediate focus section, wherein the plasma-to-collector section and the collector-to-intermediate focus section are joined around the collector, wherein the plasma-to-collector section extends from the collector towards the plasma site, wherein the collector-to-intermediate focus section extends from the collector to the intermediate focal point.

11. The illumination source of claim 10, wherein the plasma-to-collector section and the collector-to-intermediate focus section comprises cone shapes.

12. The illumination source of claim 11 , wherein the plasma-to-collector section and the collector-to-intermediate focus section comprise truncated-oblique cone shapes, wherein a base of the plasma-to-collector section and the collector-to-intermediate focus section are arranged around the collector.

13. The illumination source of claim 1 , wherein the first inlets are configured to inject the buffer gas around the refractive optics, wherein the second inlets are configured to inject the buffer gas around the collector.

14. The illumination source of claim 1, comprising a gas supply, wherein the gas supply is configured to supply the buffer gas to the first inlets and the second inlets.

15. The illumination source of claim 14, wherein the gas supply is configured to individually control pressures within the first duct and the second duct, wherein a pressure of the buffer gas within the first duct is different than a pressure of the buffer gas within the second duct when the first inlets and the second inlets are injecting the buffer gas.

16. The illumination source of claim 15, wherein the pressure of the buffer gas within the first duct is higher than the pressure of the buffer gas within the second duct when the first inlets and the second inlets are injecting the buffer gas.

17. The illumination source of claim 1 , wherein the first duct comprises a laserexit opening, wherein the laser is configured to exit the first duct into the vacuum chamber via the laser-exit opening; wherein the first duct is configured to contain the buffer gas up to the laser-exit opening;wherein the second duct comprises an illumination-entrance opening and an illumination-exit opening, wherein the illumination is configured to enter the second duct from the vacuum chamber via the illumination-entrance opening, wherein the illumination is configured to exit the second duct via the illuminationexit opening, wherein the second duct is configured to contain the buffer gas up to the illumination-entrance opening and the illumination-exit opening.

18. The illumination source of claim 17, wherein the illumination-entrance opening is angled off-axis relative to an optical axis of the illumination between the plasma site and the collector.

19. The illumination source of claim 18, wherein the illumination-entrance opening is angled away from the laser-exit opening.

20. The illumination source of claim 17, wherein the illumination-exit opening is smaller than the illumination-entrance opening.

21. The illumination source of claim 17, wherein the illumination-entrance opening is closer to the plasma site than the laser-exit opening.

22. The illumination source of claim 1, wherein a pressure inside the vacuum chamber is lower than pressures inside the first duct and the second duct when the first inlets and the second inlets are injecting the buffer gas.

23. An inspection system comprising:an illumination source comprising:a laser source, wherein the laser source is configured to generate a laser;a vacuum chamber;refractive optics, wherein the refractive optics are sealed to the vacuum chamber, wherein the laser is configured to refract through the refractive optics into the vacuum chamber;a target material, wherein the laser is configured to irradiate the target material at a plasma site within the vacuum chamber, wherein the laser causes the target material to produce a plasma, wherein the plasma is configured to emit illumination;a collector, wherein the collector is configured to reflect the illumination from the plasma to an intermediate focal point;first inlets;a first duct, wherein the first duct is arranged around the laser, the refractive optics, and the first inlets, wherein the first duct extends from the refractive optics towards the plasma site, wherein the first inlets are configured to inject a buffer gas into the first duct;second inlets; anda second duct, wherein the vacuum chamber houses the refractive optics, the collector, the plasma site, the first duct, and the second duct, wherein the second duct is arranged around the collector, the illumination, and the second inlets, wherein the second duct extends from the collector towards the plasma site and towards the intermediate focal point, wherein the second inlets are configured to inject the buffer gas into the second duct,wherein the buffer gas is configured to flow from the first duct and the second duct into the vacuum chamber.

24. A method comprising:injecting a buffer gas into a first duct and a second duct by first inlets and second inlets, the buffer gas flowing from the first duct and the second duct into a vacuum chamber;delivering a target material to a plasma site within the vacuum chamber; generating a laser by a laser source;refracting the laser through refractive optics into the vacuum chamber, wherein the refractive optics are sealed to the vacuum chamber, wherein the first duct is arranged around the laser, the refractive optics, and the first inlets, wherein the first duct extends from the refractive optics towards the plasma site;irradiating the target material at the plasma site by the laser to produce a plasma, wherein the plasma emits illumination; andreflecting the illumination from a collector to an intermediate focal point, wherein the vacuum chamber houses the refractive optics, the collector, the plasma site, the first duct, and the second duct, wherein the second duct is arranged around the collector, the illumination, and the second inlets, wherein the second duct extends from the collector towards the plasma site and towards the intermediate focal point.

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