Extreme ultraviolet light source obscuration bar and method

Abstract

An extreme ultraviolet (EUV) source includes: a source container that at least partially houses a volume within which, when in use, EUV light is transmitted by a collector along the optical axis from the primary focus to the intermediate focus; a shaft having a length extending from a first end to a second end of the shaft, the shaft including a passage that extends at least partially along the length of the shaft, the first end of the shaft being attached to the inner surface of the source container, and the second end being positioned inside the source container; and a head (130) connected to the second end of the shaft, the head intersecting the optical axis, the head having an exposed surface (134) exposed to the primary focus, the exposed surface having one or more apertures, the one or more apertures being in fluid communication with the passage.

Description

Technical Field

[0001] (Cross - reference to related applications)

[0001] This application claims the priority of U.S. Application No. 63 / 420,775, filed on October 31, 2022, which is hereby incorporated by reference in its entirety into this specification.

[0002]

[0002] This disclosure relates to a method and apparatus for generating extreme ultraviolet ( "EUV") radiation from a plasma created within a source vessel by irradiating a target or target material with a laser, and more particularly to an apparatus and method for controlling the flow within the source vessel of products generated by the irradiation of the target.

Background Art

[0003]

[0003] Extreme ultraviolet radiation, for example electromagnetic radiation having a wavelength of approximately 50 nm or less (sometimes referred to as soft X - rays) and including radiation having a wavelength of about 13.5 nm, can be used in photolithography processes to create very small features within or on a substrate such as a silicon wafer. Methods for generating EUV radiation involve converting a target material into a plasma state. The target material includes at least one element having one or more emission lines in the EUV portion of the electromagnetic spectrum, such as xenon, lithium, or tin. The target material can be a solid, liquid, or gas. In one such method, often called laser - produced plasma ( "LPP"), the required plasma can be generated by irradiating a target containing one or more EUV - emitting elements with one or more optical pulses using a "source" laser, typically a CO2 laser that emits infrared light with a wavelength of 10,600 nanometers (nm) or around that wavelength. The plasma is typically generated within a sealed "source vessel", which is usually a vacuum chamber.

Summary of the Invention

[0004] ]

[0004] In some common embodiments, an extreme ultraviolet (EUV) source includes: a source container that at least partially houses a volume in which, when in use, EUV light is transmitted by a collector along the optical axis from a primary focus to an intermediate focus; a shaft having a length extending from a first end to a second end of the shaft, the shaft including a passage that extends at least partially along the length of the shaft, the first end of the shaft being attached to the inner surface of the source container, and the second end being positioned inside the source container; and a head connected to the second end of the shaft, the head intersecting the optical axis, the head having a surface exposed to the primary focus, the surface having one or more apertures, the one or more apertures being in fluid communication with the passage.

[0005]

[0005] Embodiments may include one or more of the following features: One or more apertures may be oriented along one or more directions having a component that moves away from the intermediate focal point along the optical axis and a component that is perpendicular to the optical axis. One or more apertures may include a plurality of nested annular apertures. One or more apertures may include a plurality of non-overlapping holes. The head may be integrated with the shaft.

[0006]

[0006] The head may have a circular cross-section centered on the optical axis when cut perpendicular to the optical axis. The head and shaft may contain a high-melting-point material. The high-melting-point material may be a high-melting-point metal. The high-melting-point metal may be tungsten.

[0007]

[0007] The source container may include an exhaust port extending through the source container, the exhaust port being positioned between the collector and the head as measured along the optical axis. The aperture on the exposed surface of the head may be configured to create a gas curtain that extends from the exposed surface of the head and has a flow direction toward the edge closest to the intermediate focus of the exhaust port and / or toward the portion of the inner surface of the source container adjacent to the edge closest to the intermediate focus of the exhaust port when a gas flow is supplied through the passage during use. The flow direction of the gas curtain may have a component that moves away from the intermediate focus along the optical axis.

[0008]

[0008] The head does not have to have a surface that faces perpendicular to the intermediate focal point. The shaft does not have to have a surface that faces perpendicular to the intermediate focal point.

[0009]

[0009] The EUV source may include a target delivery system configured and positioned to deliver a target containing a target material to the primary focus of a collector, and a laser configured and positioned to generate a pulsed light beam having a beam waist at or near the primary focus of the collector. The target material may include any one or more of xenon, lithium, and tin. The target material may particularly include tin.

[0010]

[0010] The EUV source may include a gas supply connected to the passage, the gas may be an inert gas or hydrogen. The gas may include hydrogen in particular. The collector may include a central aperture positioned to allow the passage of a pulsed light beam toward the collector's primary and intermediate focal points along the optical axis.

[0011]

[0011] The head may be positioned such that direct light from the primary focus is not reflected by the collector to the head or is not reflected in any way. The head may shield the intermediate focus from direct light from the pulsed light beam. The head may have an anti-reflective and / or diffuse geometry facing the primary focus of the collector, so that the pulsed light beam is reflected to diffuse away from the head rather than to concentrate anywhere in the source container. The anti-reflective and / or diffuse geometry of the head may include a generally convex surface.

[0012]

[0012] The shaft does not have to have a surface that faces perpendicular to the intermediate focal point. The shaft does not have to have a surface that faces perpendicular to the primary focal point. The shaft may have an elongated cross-section when cut by a plane parallel to the optical axis and perpendicular to the length of the shaft, the longer dimension of the cross-section is in a direction generally parallel to the optical axis, and the cross-section of the passage in a plane parallel to the optical axis and perpendicular to the length of the shaft may be elongated in a direction generally parallel to the optical axis.

[0013]

[0013] The EUV source may also include a target delivery system configured and positioned to deliver a target containing the target material to the primary focus of the collector, the target delivery system including a shroud that obscures the path toward the primary focus of the collector, so that the image of the shaft is aligned with the image of the shroud in reflection from the collector surface when viewed from the primary focus of the collector. The image of the shaft may be obscured by the image of the shroud in reflection from the collector surface when viewed from the primary focus of the collector.

[0014]

[0014] The source container may include one exhaust port extending through one side of the source container, the exhaust port being positioned between the collector and the head as measured along the optical axis. The source container may include a plurality of exhaust ports extending through the source container, the exhaust ports being positioned between the collector and the head as measured along the optical axis. The aperture may be configured to create a gas curtain for each of the plurality of exhaust gas openings when a gas flow is supplied through the passage during use, the gas curtain having a flow direction from the exposed surface of the head toward the edge closest to the intermediate focal point of each of the plurality of exhaust ports and / or toward the portion of the inner surface of the source container adjacent to the edge closest to the intermediate focal point of each exhaust port. The aperture may be configured to create a radially extending gas curtain extending from the exposed surface of the head in a flow direction including a radial component perpendicular to and away from the optical axis and an axial component parallel to the optical axis and away from the intermediate focal point when a gas flow is supplied through the passage during use. The source container may include an annular exhaust port that surrounds the source container and extends through it, the annular exhaust port being positioned between the collector and the head by measurement along the optical axis.

[0015]

[0015] In other common embodiments, a method for reducing or preventing deposition inside a source container of an extreme ultraviolet (EUV) light source may include supplying a gas to a passage in an obscuration bar including a shaft and a head, the first end of the shaft being supported on the inner surface of the source container of the EUV light source, the source container surrounding the optical axis of the EUV light source, the optical axis extending from the collector through the primary focus to the intermediate focus, the head of the obscuration bar at the second end of the shaft intersects the optical axis, the head has an exposed surface exposed to the primary focus, and draining the gas through one or more apertures on the exposed surface of the head of the obscuration bar, the one or more apertures being in fluid communication with the passage.

[0016]

[0016] Embodiments may include one or more of the following features: One or more apertures may be oriented along one or more directions having a component that moves away from the intermediate focal point along the optical axis and a component that is perpendicular to the optical axis.

[0017]

[0017] The head may be integrated with the shaft of the obscuration bar. When cut perpendicular to the optical axis, the head may be circular and have a cross-section centered on the optical axis. The head does not have to have a surface that faces perpendicular to the intermediate focal point.

[0018]

[0018] The head and shaft may contain or be formed from a high-melting-point material. The high-melting-point material may be a high-melting-point metal. The high-melting-point metal may be tungsten.

[0019]

[0019] The source container may include an exhaust port extending through the source container, the exhaust port being positioned between the collector and the head as measured along the optical axis, and the method may include flowing gas from the inside of the source container through the exhaust port. The method may include generating or using a gas curtain of gas flowing out through one or more apertures on the exposed surface of the head, the gas curtain extending from the exposed surface of the head to the exhaust port and / or to the portion of the inner surface of the source container on the mid-focal side of the exhaust port. The gas curtain may extend along a direction having components that move away from the mid-focal along the optical axis. The method may include introducing a mid-focal protection gas flow that flows along the optical axis toward the collector at or near the mid-focal. Generating a gas curtain may include splitting the mid-focal protection gas flow at the head and merging the mid-focal protection gas flow with gas flowing out through one or more apertures on the exposed surface of the head to form a gas curtain.

[0020]

[0020] The method may include delivering a target containing a target material having a melting point to the main focus of a collector; irradiating the target with a light pulse at the main focus of the collector to form a plasma emitting EUV light at the main focus of the collector; and maintaining at least a portion of the source vessel at one or more temperatures below the melting point of the target material. Maintaining at least a portion of the source vessel at one or more temperatures below the melting point of the target material may include maintaining at least a portion of the source vessel at a temperature in the range of 50°C to 200°C.

[0021]

[0021] Allowing gas to flow out through one or more apertures on the exposed surface of the head of the obscuration bar may include suppressing or preventing gas flow away from the collector from passing through the exhaust port, thereby allowing gas flow away from the collector to enter the exhaust port. The method may include suppressing or preventing gas flow away from the collector from passing through the exhaust port for a period of 20 milliseconds (ms) or 50 ms, or within the range of 20 to 50 ms, from the moment when the irradiation of the target with light pulses in the source container is stopped. The method may include suppressing or preventing gas flow away from the collector from passing through the exhaust port for a period of 20 milliseconds or 150 ms, or within the range of 20 to 150 ms, from the moment when the irradiation of the target with light pulses in the source container is stopped.

[0022]

[0022] Details of one or more embodiments are described in the accompanying drawings and the following description. Other features will become apparent from the description and drawings, as well as from the claims. [Brief explanation of the drawing]

[0023] [Figure 1A]

[0023] This is a schematic cross-sectional view of an extreme ultraviolet (EUV) light source. [Figure 1B]

[0024] This is a schematic cross-sectional view of the EUV light source shown in Figure 1A, rotated 90 degrees around the z-axis. [Figure 1C]

[0025] A schematic cross-sectional view of the EUV light source of FIG. 1B, rotated so that gravity is represented as downward in the plane of the paper. [Figure 1D]

[0026] A schematic cross-sectional view of the EUV light source of FIG. 1C, showing the adverse effects of excessive momentum in the flow. [Figure 1E]

[0027] A schematic cross-sectional view of the EUV light source of FIG. 1C, showing the beneficial effects of the obscuration bar flow. [Figure 1F]

[0028] An enlarged view of an inset of FIG. 1E, showing details of the gas flow near the obscuration bar. [Figure 2]

[0029] A diagram of an EUV source in use with a photolithography exposure apparatus. [Figure 3]

[0030] A perspective view of one embodiment of an obscuration bar that can be positioned within the EUV light source of FIG. 1A, FIG. 1B, FIG. 1C, or FIG. 1E. [Figure 4]

[0031] A perspective view of another embodiment of an obscuration bar that can be positioned within the EUV light source of FIG. 1A, FIG. 1B, FIG. 1C, or FIG. 1E. [Figure 5]

[0032] A cross-sectional view taken along plane 5-5 marked in FIG. 4 of one embodiment of the shaft of an obscuration bar such as the obscuration bar of FIG. 4. [Figure 6A]

[0033] A perspective view of another embodiment of an obscuration bar that can be positioned within the EUV light source of FIG. 1A, FIG. 1B, FIG. 1C, or FIG. 1E. [Figure 6B]

[0034] A perspective view of one embodiment of the head of an obscuration bar of FIG. 6A. [Figure 7]

[0035] A flowchart of a procedure for reducing deposition inside the source container of an EUV light source such as the EUV light source of FIG. 1A, FIG. 1B, FIG. 1C, or FIG. 1E. [Figure 8A]

[0036] This is a perspective view of another embodiment of the head of an obscuration bar. [Figure 8B]

[0037] This is a schematic cross-sectional view of a part of an EUV light source having multiple exhaust ports and exhaust vents, showing the use of the obscuration bar head shown in Figure 8A. [Figure 8C]

[0038] This is a cross-sectional view of the EUV light source in Figure 8B, cut along the lines shown in Figure 8B and in the direction shown in Figure 8B. [Figure 9]

[0039] This is a cross-sectional view similar to that shown in Figure 8C of one embodiment of an EUV light source and an obscuration bar head. [Figure 10]

[0040] This is a cross-sectional view similar to that shown in Figure 8C of one embodiment of an EUV light source and an obscuration bar head. [Figure 11]

[0041] This is a cross-sectional view similar to that shown in Figure 8C of one embodiment of an EUV light source and an obscuration bar head. [Figure 12A]

[0042] This is a schematic cross-sectional view of a portion of an EUV light source having an annular exhaust port. [Figure 12B]

[0043] This is a cross-section of the EUV light source in Figure 12A, cut along the line shown in Figure 12A in the direction shown in Figure 12A. [Modes for carrying out the invention]

[0024]

[0044] Figure 1A is a simplified schematic cross-sectional view of some components of one embodiment of the LPP EUV light source 110. As indicated by the reference coordinate axes in the figure, Figure 1A is shown in the xz plane, where x is positive in the upward direction in the plane of the paper, z is positive in the rightward direction in the plane of the paper, and the z axis coincides with the optical axis A of the collector 120, which will be described later.

[0025]

[0045] As shown in Figure 1A, the EUV light source 110 includes a source laser 112 for generating a beam 113 of light (e.g., laser) pulses and delivering the pulsed light beam 113 from the source laser 112 into the interior 114 of the source container 111 to individually irradiate targets 115 within the irradiation area 116. The targets 115 move downward (in the negative x-direction) in the plane of the paper from the target delivery system 117a to the irradiation area 116. The source container 111 has an interior surface 156 surrounding the interior 114.

[0026]

[0046] As shown in Figure 1A, the EUV light source 110 includes a target delivery system 117a, which delivers the target 115 into the interior 114 of the source vessel 111 to the irradiation site 116. At the irradiation site 116, the target 115 interacts individually with one or more light pulses (of the light beam 113) to generate a plasma 118 that produces EUV light 119. The light from the plasma 118, the position of the target 115, and other data may be monitored by one or more metronome devices 150, and the information collected by these metronome devices 150 may be used for the control and operation of the EUV light source 110.

[0027]

[0047] The target 115 may be delivered along at least part of its trajectory through the target shroud 115s. The shroud 115s may be in the form of a tube (which may have an aperture for measurement) or other shielding structure that shields or partially shields the incoming target 115 from the gas and other materials inside the source vessel 111 114, so that the trajectory of the target 115 is not excessively disturbed by such gas or other materials. Unused targets among the target 115 (such as those not converted to plasma 118) may be captured in the target trap 117b.

[0028]

[0048] Target 115 is or includes an EUV-emitting target material, such as but not limited to tin, lithium, xenon, or a combination thereof. Target 115 may take the form of liquid droplets, or alternatively, solid particles or solid particles contained within liquid droplets. For example, elemental tin may be presented as a target in the form of pure tin, tin compounds such as SnBr4, SnBr2, SnH4, tin alloys such as tin-gallium alloys, tin-indium alloys, or tin-indium-gallium alloys, or combinations thereof.

[0029]

[0049] The EUV light source 110 may also include a collector 120. The collector 120 may be a near-perpendicular incidence collector mirror having an optical axis A and a reflective surface 121. The reflective surface 121 may be in the form of an oblate ellipsoid (i.e., an ellipse rotated around its major axis), and thus the collector 120 has a first focal point or principal focal point 122 in or near the irradiation area 116 and a second focal point, a so-called intermediate focal point 123, with the optical axis A defined as a line extending between them. Thus, the source container 111 of the EUV light source 110 at least partially accommodates the volume in which EUV light is transmitted by the collector 120 from the principal focal point 122 to the intermediate focal point 123 along the optical axis A when the EUV light source 110 and the source container 111 are in use. The reflected EUV light 124 from the collector 120 is output from the EUV light source 110 at an intermediate focal point 123 and can be input to a device that utilizes the EUV light 124, such as a lithography exposure apparatus (as shown in Figure 2). The collector 120 is formed with an aperture 125, which allows the light beam 113 of the light pulse generated by the source laser 112 to pass through the aperture 125 and reach the irradiation site 116. The aperture 125 creates a shadow or large gap 154 ​​along the optical axis A in the reflected EUV light 124 from the collector 120.

[0030]

[0050] To reflect the EUV light 119, the collector 120 may be in the form of a multilayer mirror (MLM), and the reflective surface 121 has a transitional multilayer coating comprising alternating layers of molybdenum and silicon, and optionally one or more high-temperature diffusion barrier layers, smoothing layers, capping layers, and / or etching stop layers. Other surface shapes other than an elongated ellipsoid may also be used for the reflective surface 121. For example, the reflective surface 121 may alternatively be in the form of a parabola rotated around its major axis. In some embodiments, the reflective surface 121 may be configured to deliver a beam of EUV light 124 having an annular or other cross-section at the intermediate focal point 123. In other embodiments, the reflective surface 121 may utilize coatings and layers other than or additional to those described above.

[0031]

[0051] The collector 120 can be expensive to manufacture. The efficiency and power of the light produced by the EUV light source 110 depend on the quality of the reflective surface 121 of the collector 120. For these and other reasons, it is desirable to protect the collector 120 from damage to its reflective surface 121.

[0032]

[0052] However, the collector 120 must be positioned in close proximity to or near the plasma 118 within the source vessel 111 in order to focus and redirect the EUV light 119. Structures within the source vessel 111, including the collector 120, may be exposed to high-energy ions and / or particles and vapors of the target material or containing the target material. These target material particles and high-energy ions and vapors are essentially debris or by-products from the light-based evaporation or ablation process and may contaminate the exposed reflective surface 121 of the collector. These target material particles and energetic ions and vapors may also cause physical damage and localized heating of the reflective surface 121 of the collector 120.

[0033]

[0053] As shown in Figure 1A, the EUV light source 110 may include a focusing unit 126 that includes one or more optical elements (not shown) for focusing the light beam 113 onto the irradiation area 116 or a focal point or beam waist near it.

[0034]

[0054] Figure 2 shows an embodiment of the EUV light source 210, such as the EUV light source 110 of Figure 1 or another EUV source, together with a lithography exposure apparatus 271. The lithography exposure apparatus 271 receives EUV light 224 generated by the EUV light source 210 and reflects it with one or more illumination mirrors 272 to illuminate a reflective pattern or reticle 273. The EUV light reflected from the pattern or reticle 273 is further reflected and reduced by one or more reduction mirrors 274 and irradiated onto a substrate or wafer 275 (or one or more photosensitive layers on the substrate or wafer 275, not shown) to enable the formation of a patterned structure in or on the substrate or wafer 275.

[0035]

[0055] The optical elements and sensors within the lithography exposure apparatus 271, as well as the photosensitive layer on the substrate or wafer 275, are typically sensitive to many types of radiation or to any type of radiation. Therefore, especially considering the high power levels generated by the source laser 112 in Figure 1A, it is important to prevent any portion of the beam 113 of the light pulse from the light source laser 112 (including the light beam 113a shown in Figure 1A, which corresponds to the portion of the light beam 113 extending beyond the irradiation site 116) from reaching the intermediate focal point 123 and, in some cases, from entering the lithography exposure apparatus such as the lithography exposure apparatus 271.

[0036]

[0056] For this purpose, beam-blocking elements such as the obscuration bar 127 of the present disclosure may be used, as shown in Figure 1A. The obscuration bar 127 may include a base 128, a shaft 129 extending from the base 128, and a head 130 supported on the shaft 129. When the obscuration bar 127 is used, the head 130 is positioned on the optical axis A of the collector 120 such that the optical axis A intersects with the head 130, as shown. The head 130 may also be positioned and sized to fit within the shadow or large gap 154 ​​in the reflected EUV light 124 from the collector 120. For example, the head 130 may have a circular cross-section that is centered on the optical axis A and fits within the shadow or large gap 154 ​​when cut perpendicular to the optical axis A. This geometry prevents the head 130 from blocking any portion or any significant portion of the EUV light 124 reflected from the collector 120 and directed toward the lithography exposure apparatus 121, while simultaneously providing good protection for the intermediate focus 123 from direct illumination by the pulsed light beams 113, 113a of the source laser 112. In other words (with respect to positioning in shadow or gap 154), the head 130 is positioned such that the direct light 119 from the primary focus 122 is reflected by the collector 120 to the head 130 with little or no effect. The head 130 may also have an anti-reflective and / or diffuse geometry facing the primary focus 122 of the collector 120, so that the light from the source laser 112 reaching the head 130 is reflected to diffuse away from the head 130 rather than being concentrated anywhere within the source vessel 111. The anti-reflective and / or diffuse geometry of the head 130 may include a generally convex surface exposed to the collector 120.

[0037]

[0057] In a source vessel using the target shroud 115s, the shaft 129 of the obscuration bar 127 can be aligned with the shroud 115s, as shown in the figure. That is, the shaft can be positioned as far as possible in the shadow created by the shroud 115s in the reflected EUV light 124. In other words, when viewed from the primary focus 122 of the collector 120, the image of the shaft 129 can be aligned with the image of the shroud 115s in the reflection from the collector surface 121. In some embodiments, when viewed from the primary focus 122 of the collector 120, the shaft 129 can be completely hidden in the shadow of the shroud 115s when the image of the shaft 129 is obscured by the image of the shroud 115s in the reflection from the collector surface 121. This arrangement reduces or eliminates the obstruction of the EUV light 124 from leaving the EUV light source 110 by the shaft 129. A gas conduit 131 is connected to the base 128 of an obscuration bar 129 and to a source of gas (not shown), such as H2 gas 132, allowing the gas to be supplied to the interior 114 of the source container 111 at or near the center or optical axis A of the source container 111 using an obscuration bar 127, as will be illustrated and described in more detail below.

[0038]

[0058] Figure 1B is a simplified schematic cross-sectional view of the EUV light source 110, rotated 90 degrees around the optical axis A to show a cross-section in the yz plane, where positive y is upward and positive z is to the right in the plane of the paper, as indicated by the reference coordinate axes. When in use, the EUV light source 110 may be in the yz plane as shown, or tilted with respect to gravity as indicated by the gravity vector G parallel to the yz plane. In this figure, the shaft 129 and base 128 of the obscuration bar 127 are behind the head 130 toward the plane of the paper. Also visible in this figure are the exhaust port 133 and the associated exhaust port 155. As shown, the exhaust port 133 is a structure that extends from the source vessel 111 and defines the exhaust port 155, which is in fluid communication with the interior 114 of the source vessel 111 and extends outward from there. Gas, along with entrained ions, vapor, and debris, can be discharged from the source vessel 111 through the exhaust port 155 of the exhaust port 133 by one or more vacuum pumps (not shown). The exhaust port 155 is positioned between the collector 120 and the head 130, measured along the optical axis A.

[0039]

[0059] As shown in Figure 1B, the head 130 of the obscuration bar 127 includes a surface or "exposed surface" 134 exposed to the primary focus 122. The exposed surface 134 may be or include an inclined surface 134s, i.e., a surface not perpendicular to axis A, and may face generally toward the exhaust port 155 of the exhaust port 133 and / or toward the portion of the internal surface 156 of the source container 111 toward the intermediate focus side of the exhaust port 155, as will be illustrated and described in more detail below.

[0040]

[0060] Figure 1C shows another cross-section of the EUV light source 110 in the yz plane, but this time the gravity G vector is oriented downward in the plane of the paper, and various gas flows that may be used within the EUV light source 110 are represented in the figure by outlined arrows.

[0041]

[0061] Referring to Figure 1C, a gas flow, such as a hydrogen (H2) gas flow at a pressure in the range of approximately 50 to 300 Pa, can be used as a buffer gas for debris and / or vapor control within the source vessel 111. Considering that a vacuum is required inside the source vessel 111 114 to avoid excessive absorption of EUV light by the gas molecules, adequately protecting the collector 120 from target material debris and vapor emitting from the irradiation site 116 would be difficult without the use of a gas flow. Hydrogen (H2) is relatively transparent to EUV radiation with a wavelength of approximately 13.5 nm and is therefore generally preferred over other candidate gases such as helium, argon, and other gases that exhibit higher absorption at approximately 13.5 nm.

[0042]

[0062] H2 gas may be introduced into the source vessel 111 to slow down and guide energetic debris (ions, atoms, and clusters) of the target material created by the irradiation of the target 115 and the irradiation site 116, and by the resulting plasma 118. The debris is slowed down by collisions with gas molecules. An H2 gas flow 136 at the central aperture 125 of the collector 120 may be used for this purpose. The flow 136, sometimes known as a "cone flow" 136, may be guided by a tube or nozzle 137, etc., from the aperture 125 at the center of the collector 120 toward the irradiation site 116 where the plasma 118 is repeatedly produced. This direction is opposite to the debris trajectory toward the collector 120 from the irradiation site 116 toward the collector 120, and thus the cone flow 136 functions to reduce damage to the collector 120 caused by the deposition, injection, and accumulation of the sputtered target material.

[0043]

[0063] When a target 115 that is or contains tin is used, using hydrogen gas with such a target 115 (as in the case of the cone flow 136) introduces another potential source of contamination into the source container 111. This is the spitting or "spitting" of molten tin from the surface inside the container that is or will be coated with molten tin, when hydrogen bubbles form and grow in or beneath the molten tin, and then burst.

[0044]

[0064] One method to prevent tin spitting is to prevent the molten target material from accumulating on the surface inside the source container 110 by keeping the surface below or well below the melting point of the target material, which is approximately 232°C in the case of tin. For example, some portions of the inner surface 156 of the source container 111 can be maintained at a temperature below 232°C, such as a temperature in the range of 50°C to 110°C. The tin deposited on such a surface will remain in a solid form, preventing or resisting spitting.

[0045]

[0065] However, deposition on low-temperature surfaces also reduces the maintenance interval of EUV sources such as EUV source 110. Deposit growth on low-temperature surfaces and liquid tin accumulation on high-temperature surfaces can be reduced by using additional gas flow.

[0046]

[0066] A gas flow, often referred to as an umbrella flow 139, may be guided along the surface of the collector 120 (from an outlet not shown). A so-called showerhead flow, in which gas flows through multiple parallel apertures generally perpendicular to the surface to be protected, may be provided in the area of ​​the source vessel 111 closest to the collector 120, such as showerhead flows S1 and S2. In additional areas, such as the area near the intermediate focus 123, protective gas flows having components of flow parallel to or guided parallel to the surface to be protected may be introduced through apertures aimed in a direction having components along or parallel to the surface to be protected. For example, gas flows such as gas flows F1, F2, F3, and F4 may be introduced to protect the internal surface 156 of the source vessel 111 in the area near the intermediate focus 123.

[0047]

[0067] A gas flow often referred to as a dynamic gas lock ("DGL") is one or more gas flows used to prevent material from leaving the EUV source 110 within the region of the intermediate focus 123. The DGL can generate a gas flow such as a DGL flow 138 directed from the area of ​​the intermediate focus 123 toward the irradiation site 116, which may also be called an "intermediate focus protection" gas flow 138.

[0048]

[0068] A stable guide flow 140 flowing away from the collector 120 may be formed primarily by a cone flow 136, in addition to an umbrella flow 139 and showerhead flows S1 and S2 (and optionally others not shown). The solid curve in Figure 1C shows an example of the guide flow 140. This guide flow 140 helps to receive and carry away vapor, ions, and materials including microparticles and nanoparticles generated from the target 115 during plasma generation from the collector 120. A counterflow 141 moving from the intermediate focus 123 toward the collector 120 may be formed primarily by a DGL flow 138, in addition to flows such as F1, F2, F3, and F4 (and optionally others not shown). The dotted curve in Figure 1C shows an example of the counterflow 141.

[0049]

[0069] Given the low pressure used within the source vessel 111, the pressure difference at the exhaust port 155 of the exhaust port 133 is not large. However, the small pressure difference at the exhaust port 155 generated by vacuuming the exhaust port 133, plus the momentum balance of the flow between the guide flow 140 and the counterflow 141 in the confluence region 142 of the two flows 140 and 141, and the fact that the confluence region 142 is close to the exhaust port 155, can create a stable guide flow of target material byproducts, accompanied and contained in the guide flow 140, into the exhaust port 155 without the target material byproducts substantially coming into contact with any inner surface of the source vessel 111.

[0050]

[0070] Figure 1D shows a cross-section of the EUV light source 110 in Figure 1C, but the gas stream 140 no longer repeatedly receives and carries vapor, ions, and microparticles and nanoparticles generated from the target 115 used in the plasma generation process. This is partially represented in Figure 1D by the absence of plasma 118a at the irradiation site 116 within the source vessel 111.

[0051]

[0071] Plasma generation can be stopped for a variety of reasons. To control the amount of radiation ("exposure dose") received by a given exposure site on a wafer, such as wafer 275 (Figure 2), the power of the EUV light 119, 124 (Figures 1A, 1B) generated by an EUV light source, such as EUV light source 111, can be detected from each light pulse, and the total power delivered to that site can be calculated in real time. Once a desired exposure dose level is reached or exceeded, further light pulses are immediately de-timed, and therefore, no light pulses strike the target 115 within the source vessel 111, as long as the exposure site is positioned for exposure. This results in an abrupt cessation of plasma generation in the source vessel 111. Abrupt cessation (and start) of plasma generation can occur while moving from one exposure site to the next on a wafer, or while moving from one wafer to the next, or even in lithography techniques with exposure speeds lower than standard time.

[0052]

[0072] Referring again to Figure 1D, when a series of targets 115 are continuously irradiated at the irradiation site 116 (as in Figure 1C) by light pulses of the light beam 113 from the source laser 112, vapor, microparticles and nanoparticles, as well as other debris and ionized plasma are repeatedly generated at and near the irradiation site 116 within the source vessel 111, and thus effectively injected or deposited into the cone flow 136 at the main focus 122 of the collector 120. Since the energy and momentum of the plasma 118 and associated materials tend to move and / or radiate in all directions, the net momentum of the injected material is low or almost zero. Thus, the injected material reduces the overall momentum of the cone flow 136 and the guide flow 140 (partially formed from the cone flow 136) that carries the injected material away from the collector 120 towards the exhaust port 133.

[0053]

[0073] If plasma generation stops during stepping, adjustment, or other modifications in the associated lithography exposure device, the light pulses of the light beam 113 stop striking the target 115, and the series of target material simply pass through the focal point of the collector 120 on their way to the target trap 117b (Figure 1A). Thus, this unirradiated target material is not injected into and accompanied by the cone flow 136 and guide flow 140. Without the presence of material repeatedly injected into the irradiation site 116 by plasma generation at that site, the momentum of the cone flow 136 and flow 140 may be too large to maintain their normal balanced flow path into the exhaust port 133 (or too large to be balanced by the counterflow 141 shown in Figure 1C). As shown in Figure 1D, the flow (or “breakout flow”) 143 may pass through the exhaust port 133 (or, in other words, the flow may pass beyond the exhaust port 133 or escape the normal path of the flow 140 exiting the exhaust port 133). When plasma generation stops, it might be thought that the vapor and debris related to the target are no longer contained in the flow 140, and that the breakout flow 143 that occurs when plasma is not being generated will not cause contamination in the source vessel 111. However, when plasma generation first stops, the vapor and debris led out from the target are still accompanied by the flow 140 from the most recent plasma generation, and the breakout flow 143 may carry this vapor and debris beyond the exhaust port 155. Also, when plasma generation has just started or restarted, the target material has just begun to be accompanied again by what is initially a flow 140 with a high mean momentum, and therefore a breakout flow 143 containing the target material can also occur at plasma startup.

[0054]

[0074] When the breakout flow 143 contains material related to the target, deposits or contamination may be generated on the portion 135 of the internal surface 156 of the source vessel 111 on the intermediate focal side of the exhaust port 133. The breakout flow 143 or flow 143 may also move in various other directions after passing through the exhaust port 155, potentially causing unstable flow patterns within the source vessel 111 and generating contamination in other areas within the interior 114 or other areas of the internal surface 156.

[0055]

[0075] As shown in the cross-section of the EUV light source 110 in Figure 1E, in one aspect of the present disclosure, the problem of breakout flow 143 from the flow 140 is prevented or reduced by the use of an obscuration bar gas flow 144 supplied into the source vessel 111 from the exposed surface 134 of the head 130, or from the exposed surface 134 of the head 130 in the form of an inclined surface 134s of the obscuration bar 127 (Figure 1A). As shown in the inset of Figure 1E, the obscuration bar gas flow 144 flows in a direction including at least two components, the first component toward the portion 135 of the internal surface 156 of the source vessel 111 toward the intermediate focal side of the exhaust port 133, and the second component toward the collector 120 along the optical axis A. Generating flow 144 from the exposed surface 134 and directing it in a direction that includes these two components helps ensure that the momentum of flow 144, together with the momentum of the counterflow 141 (Figure 1C), which mainly consists of the DGL flow 138 but potentially includes other flows such as flows F1-F4, is sufficient to prevent or substantially prevent flow 140, which mainly consists of the cone flow 36, from passing through the exhaust port 155, keeping flow 140 within its desired pattern, moving from the collector 120 or from the irradiation site 116 into the exhaust port 155. The obscuration bar gas flow 144 can be left "on" both when plasma is being generated at the irradiation site 116 (as shown in Figure 1C) and when plasma is not being generated (as shown in Figure 1D), eliminating or reducing the need to rapidly change or rebalance the flow in the source vessel 111. The obscuration bar gas flow 144 can effectively form a gas curtain having a flow direction from the exposed surface 134 of the head 130 toward the edge of the exhaust port 155 closest to the intermediate focal point 123 and / or toward the portion 135 of the inner surface 156 of the source container 111 adjacent to the edge of the exhaust port 133 closest to the intermediate focal point 123.

[0056]

[0076] Referring to Figure 1F, which is an enlarged view of the inset in Figure 1E, the intermediate focal opposing surface 157 of the head 130 of the obscuration bar 127 may be symmetrical about the optical axis A in some embodiments. In the example of Figure 1F, the head 130 has a symmetrical pattern of facets on its intermediate focal opposing surface 157 (the facets are shown in more detail below in Figures 3 and 6A). The symmetry about the optical axis A tends to evenly divide the opposing flow 141 (or the DGL flow 138, which is the main component of the opposing flow 141) into divided flows such as flows 141a and 141b shown in the plane of the figure. Since the obscuration bar flow 144 is introduced through the exposed surface 134 of the head 130 rather than the side 134i, the obscuration bar flow 144 does not significantly push the divided flows, such as flows 141a and 141b, toward the inner surface 156 of the source vessel 111 away from the side 134i of the head 130. The nearly uniform division of the counterflow 141, together with the obscuration bar flow 144 introduced through the exposed surface 134 of the head 130 rather than through the side 134i, helps to maintain the stability of the counterflow 141, allowing the divided counterflows, such as flows 141a and 141b, to flow together with the obscuration bar flow 144 and guide the flow 140 into the exhaust port 155 (as seen in Figure 1E) (and to flow together with the flow 140). Thus, the divided counterflows, such as flows 141a and 141b, can help generate a gas curtain. By splitting the counterflow 141 (or DGL flow 138 or “intermediate focus protection” flow 138) in the head 130 and effectively merging the intermediate focus protection gas flow 138 with the obstruction bar gas flow 144 that flows out through one or more apertures on the exposed surface of the head 130, a gas curtain can be formed with enough momentum to prevent or reduce breakout flow 143.

[0057]

[0077] Figures 3, 4, and 6A illustrate various embodiments of one or more observation bars 127 according to the present disclosure. Figure 3 is a perspective view of an observation bar 327, which is an embodiment of the observation bar 127 of Figure 1A. As can be understood from the above description of Figure 1A, the observation bar 327 is used in the context of an EUV light source 110, which includes a source container 111 housing the interior 114 of the EUV source container 111, where, during use, EUV light 146 is transmitted from a collector 120 to an intermediate focal point 123 along the optical axis A. As shown in Figure 3, the observation bar 327 includes a shaft 329 and a head 330. The head 330 may be attached to the shaft 329, or may be integral with the shaft 329, for example, by machining from a single block or when formed together by continuous 3D printing. The shaft 329 may include a base 328, and if present, this base may be integrated with the shaft 329.

[0058]

[0078] As shown by the dashed line in Figure 3, the obscuration bar 327 also defines or includes a gas passage 347 that extends along the direction of the length L of the shaft 329. In the illustrated embodiment, the passage 347 is housed inside the shaft 329.

[0059]

[0079] The head 330 and shaft 329 may contain or be formed from high-melting-point materials such as oxides, nitrides, or carbide ceramics, or high-melting-point metals. Molybdenum and tungsten are two metals that may be used. Tungsten is useful because of its very high melting point and relatively high thermal conductivity.

[0060]

[0080] As shown in Figure 3, the shaft has a length L extending from its first end 345 to its second end 346. When in use or in position for use, the first end 345 is attached to the source container 111 (as in the case of the observation bar 127 in Figure 1A) at the base 328 in this embodiment. The head 330 is connected to the second end 346 of the shaft 329, and when in use or in position for use (as in the case of the observation bar 127 in Figure 1A), the head 330 intersects the optical axis A of the collector 120. The head 330 also has one or more apertures 348, which, in this embodiment, are in fluid communication (i.e., fluidly connected) with the passage 347 via the chamber 349 inside the head 330.

[0061]

[0081] In the embodiment shown in Figure 3, multiple angled facets (of which facets 351a, 351b, and 351c are illustrated) are symmetrically positioned on the intermediate focal opposing surface 357 of the head 330. These facets ensure that the head 330 does not have a surface perpendicular to the intermediate focal 123 when the obscuration bar 327 is in use or positioned for use (see, for example, Figures 1A and 1E and the inset in Figure 1E). Also, any surface exposed to the intermediate focal is at an angle far from perpendicular to the intermediate focal, such as greater than 30 degrees or greater than 45 degrees from perpendicular to the intermediate focal. This geometry reduces the possibility of spitting in the direction of the intermediate focal 123 when the intermediate focal opposing surface of the head 330 is coated with liquid tin. The symmetrical arrangement of the facets also promotes the stability of the counterflow 141 moving around the head 330 (Figures 1C and 1F), as described above with respect to Figure 1F.

[0062]

[0082] Figure 4 shows an obscuration bar 427, which is another embodiment of the obscuration bar 127 of Figure 1A. Illustrated in Figure 4 is a perspective view rotated 180 degrees from the perspective view of Figure 3, thereby allowing the exposed surface 434 of the head 430 to be seen in the form of the inclined surface 434s of the head 430 (see, for example, Figure 1E and the inset in Figure 1E, which show the exposed surface 134 in the form of the inclined surface 134s of the head 130 of the obscuration bar 127). Multiple apertures 448 are present in the form of multiple non-overlapping holes 448a on the exposed inclined surfaces 434, 434s but not on the side surfaces 434i.

[0063]

[0083] In the embodiment shown in Figure 4, the intermediate focal-facing surface of the head 430 has a conical surface 453. Similar to the facets 351a, 351b, and 351c in the embodiment shown in Figure 3, this conical surface 453 ensures that the head 430 does not have a surface that faces perpendicular to the intermediate focal 123. Furthermore, the conical surface 453 may be exposed to the intermediate focal at an angle far from being perpendicular to the intermediate focal, such as greater than 30 degrees or greater than 45 degrees. In addition, in Figure 4, the shaft 429 has facets 452a and 452b (not shown) on the intermediate focal-facing surface of the shaft 429, and facets 452c and 452d (not shown) on the collector-facing surface of the shaft 429. Therefore, in this embodiment, the shaft 429 lacks a surface perpendicular to the intermediate focal point 123, and also lacks a surface perpendicular to the collector 120 or the primary focal point 122 near the collector 120.

[0064]

[0084] As can be seen from Figure 4, the aperture 448 is typically essentially perpendicular to the exposed inclined surfaces 434, 434s. Specifically, when the obscuration bar 427 is in use or mounted for use, the aperture 448 is oriented along one or more directions having a component that moves away from the intermediate focal point 123 along the optical axis A and a component that is perpendicular to the optical axis A. The exposed inclined surfaces 434, 434s may also be convex, which can help diffuse any reflection of light from the source laser 112. Convex exposed inclined surfaces 434, 434s can also enable the generation of a wider area of ​​gas flow from the aperture 448, which takes the form of a hole that is roughly perpendicular to the exposed inclined surfaces 434, 434s. Note that other embodiments are possible, such as embodiments in which an inclined aperture on the exposed surface 434, which is oriented in roughly the same direction as the aperture 448, is preferred over the inclined surface 434s.

[0065]

[0085] Figure 5 is a cross-sectional view of one embodiment of shaft 529 similar to shaft 429, such as when cut along line 5-5 shown in Figure 4. Shaft 529 includes facets 552a, 552b, 552c, and 552d similar to the facets in Figure 4. Shaft 529 also defines an internal passage 547 similar to the internal passage 347 in Figure 3. As shown in the embodiment of Figure 5, shaft 529 has an elongated cross-section when cut in a plane parallel to the optical axis A (i.e., parallel to the "z" direction) and perpendicular to the length of shaft 529, with the longer dimension of the cross-section being in a direction approximately parallel to the optical axis (i.e., approximately parallel to the "z" direction). This shape of shaft 529, which is elongated in the "z" direction, allows shaft 529 to become thinner in the xy plane and be better concealed in the shadow of the target shroud 115s when it is present, while still allowing sufficient gas flow in the elongated passage 547, which is also elongated in the "z" direction.

[0066]

[0086] Figure 6A is a perspective view of an obstruction bar 627, which is another embodiment of the obstruction bar 127 (and head 130) from Figures 1A to 1E. As can be seen from the reference coordinates, Figure 6A is observed upward along the "z" axis, or in other words, upward along the optical axis A. As seen in Figure 6A, the head 630 has a circular cross-section when viewed along the optical axis A. In this embodiment, the aperture 648 takes the form of an elliptical, nested annular aperture 648b on the exposed surface 634, rather than on the side surface 634i. The exposed surface 634 having the aperture 648 is here again an inclined surface 634s, and as a result, the flow from the annular aperture 648b has components in both the negative "z" direction and the negative "y" direction, as indicated by the arrow below the head 630. The shaft 629 and base 628 are similar to some of the other embodiments described above. Furthermore, the shaft 629 includes facets 652c and 652d on its surface, such facets facing in the negative "z" direction or toward the collector 120 in Figures 1A to 1E. As in other embodiments, the exposed inclined surfaces 634, 634s may have a convex shape overall. This optional convex shape overall of the exposed inclined surfaces 634, 634s is shown in Figure 6B, in a perspective view of the head 630 of the obscuration bar 627 in Figure 6A, viewed along the positive x-axis as indicated by the reference coordinates in the figure. As seen in Figure 6B, the portion 634a of the exposed inclined surfaces 634, 634s closer to the inside of the ring aperture protrudes more than the portion 634b of the exposed inclined surfaces 634, 634s closer to the outside of the ring aperture 648b, giving the exposed inclined surfaces 634, 634s a convex shape overall. As shown in Figure 3, multiple facets exist on the intermediate focal point opposing surface 657 of the head 630, of which 651a, 651b, and 651C are visible.

[0067]

[0087] Referring to Figure 7, procedure P100 is performed to prevent undesirable deposition in the source vessel 111 of the EUV light source 110. In step S10, gas 132 (Figure 1A) is supplied to passage 347 in the obscuration bars 127, 327, 427, 627, which include shafts 129, 329, 429, 629 and heads 130, 330, 430, 630, in the EUV light source 110. The first ends 345 of the shafts 129, 329, 429, 629 are supported on the inner surface 156 of the source vessel 111 of the EUV light source 110. The source vessel 111 surrounds the optical axis A of the EUV light source 110, which extends between the collector 120 and the intermediate focal point 123 of the EUV light source 110. The obscuration bars 127, 327, 427, 627 include heads 130, 330, 430, 630 at the second ends 346 of the shafts 129, 329, 429, 629, the heads intersect the optical axis A and include exposed surfaces 134, 434, 634 exposed at the primary focus 122. Next, in step S20, gas 132 flows through the passages 347 of the obscuration bars and out of the heads, and flows into the interior 114 of the source container 111 through the exposed surfaces 134, 434, 634 of the heads 130, 330, 430, 630 and / or through one or more apertures 348, 448, 648 of the shafts 129, 327, 429, 529, 629 of the obscuration bars 127, 327, 427, 627. Apertures 348, 448, and 648 can be oriented along one or more directions having a component that moves away from the intermediate focal point 123 and a component that is perpendicular to the optical axis A.

[0068]

[0088] In the embodiment of procedure P100, the heads 130, 330, 430, and 630 may be integrated with the shafts 129, 329, 429, 529, and 629 of the obscuration bars 127, 327, 427, and 627. When cut perpendicular to the optical axis A, the heads 130, 330, 430, and 630 may have a circular cross-section centered on the optical axis A. The heads 130, 330, 430, and 630 may not have a surface perpendicular to the intermediate focal point 123. The heads 130, 330, 430, and 630 and the shafts 129, 329, 429, 529, and 629 may contain or be formed from a high-melting-point material. The high-melting-point material may be, for example, an oxide, nitride, or carbide ceramic, or a high-melting-point metal. The metal may be molybdenum or tungsten. Tungsten can be a high-melting-point metal.

[0069]

[0089] In an embodiment of procedure P100, the source container 111 may include an exhaust port 155 defined by an exhaust port 133, the exhaust port 155 extending through the source container 111, and the exhaust port 155 is positioned between the collector 120 and the head 130 as measured along the optical axis A. Procedure P100 may further include allowing gas to flow out from the inside of the source container 111 through the exhaust port 155. Procedure P100 may include generating a gas curtain from or by gas flowing out through one or more apertures 348, 438, 638 of the exposed surfaces 134, 434, 634 of the head, which at least partially extend to the exhaust port 155 and / or to the portion 135 of the internal surface 156 of the source container 111 on the intermediate focal side of the exhaust port 155. The gas curtain may extend along a direction having components away from the intermediate focal 123 along the optical axis A. Procedure P100 may include introducing an intermediate focus protection gas flow in the form of a DGL flow 138 flowing along the optical axis A toward the collector 120 at or near the intermediate focus 123. Generating a gas curtain may include splitting the intermediate focus protection gas flow 138 at heads 130, 330, 430, 630 and merging the intermediate focus protection gas flow 138 with gas flowing out through one or more apertures 348, 448, 648 of the exposed surfaces 134, 434, 634 of heads 130, 330, 430, 630 to form a gas curtain.

[0070]

[0090] Embodiments of the method may include delivering a target 115 containing a target material having a melting point to a primary focus 122 of a collector 120, irradiating the target 115 with light (e.g., laser) pulses at the primary focus 122 of the collector 120 to form a plasma 118 at the primary focus 122 of the collector 120, wherein the plasma 120 emits EUV light 119, and maintaining at least a portion of a source vessel 111 at one or more temperatures below the melting point of the target material. At least a portion of the source vessel 111 may be maintained at a temperature below 232°C, or below 200°C, for example, in the range of 50°C to 200°C, or in the range of 50°C to 150°C, or even in the range of 50°C to 110°C.

[0071]

[0091] In embodiments of the method, allowing gas 132 to flow out through one or more apertures 348, 448, 648 on the exposed surfaces 134, 434, 634 of the heads 130, 330, 430, 630 of the obscuration bars 127, 327, 427, 627, may include suppressing or preventing gas flow 140 away from the collector 120 from passing through the exhaust port 155, thereby allowing gas flow 140 away from the collector 120 to enter the exhaust port 155. Suppressing or preventing gas flow 140 away from the collector 120 from passing through the exhaust port 155 may occur, for example, during a period of 20 milliseconds (ms) or 50 ms, or within the range of 20 to 50 ms, from the moment the irradiation of the target 115 in the source vessel 111 is stopped. Suppression or prevention of the gas flow 140 moving away from the collector 120 passing through the exhaust port 155 can occur, for example, during a period of 20 ms or 150 ms from the moment the light pulse is started to irradiate the target in the source container, or during a period within the range of 20 to 150 ms.

[0072]

[0092] Figure 8A is a perspective view of another embodiment of the head 830 of the obscuration bar, which may be positioned within the EUV source vessel 811 shown in a partial cross section in Figure 8B. Figure 8C is a cross section of the source vessel 811 of Figure 8B, cut along the cross section lines and directions shown in Figure 8B. The head 830 of the obscuration bar may be supported by a shaft 829 as shown in Figure 8C. The shaft 829 is attached to the inner surface 856 of the source vessel 811 and may include a base 828 to facilitate installation.

[0073]

[0093] Referring to Figures 8A to 8C, the aperture 848 of the head 830 in this embodiment is positioned on the exposed surface 834, which has the form of a conical surface 834c. As in the case of the head 330 in Figure 3, there are multiple facets on the intermediate focal opposing surface 857 of the head 830, of which 851a, 851b, and 851c are visible. The aperture 848 on the conical surface 834c extends more or less perpendicular to the conical surface 834c, and therefore, when a gas flow is supplied through the passage in the shaft 829 of the obscuration bar 827 during use (this passage is not shown in Figures 8A to 8C, but see, for example, passage 347 in Figure 3), the aperture 848 is configured to create a gas curtain that extends radially in the flow direction, as indicated by the arrows in Figures 8C and 8B, and includes a radial component perpendicular to the optical axis A and away from the optical axis A, and an axial component parallel to the optical axis A and away from the intermediate focal point of the source vessel 811 (the intermediate focal point is not shown, but see Figures 1A to 1E). Thus, the resulting gas curtain essentially takes the form of a conical fan extending outward from the exposed surface 834 of the head 830 toward the internal surface 856 of the source vessel 811. Similar to the embodiments described above, the aperture is not positioned above or within the side 834i of the head 830.

[0074]

[0094] The head 830 shown in Figures 8A to 8C may be usefully used in a source container that includes multiple exhaust ports extending through the source container, such as the source container 811 in Figures 8B and 8C, which has two exhaust ports 833a, 833b and two corresponding exhaust ports 855a, 855b on opposite sides of the source container 811. The shaft 829 and base 828 of the obscuration bar may support the head 830 as shown in Figure 8C.

[0075]

[0095] Figure 9 is a cross-section from the same viewpoint as Figure 8C, showing an embodiment of a source vessel 911 having four exhaust ports 933a-933d and four corresponding exhaust vents 955a-955d. (The base and shaft of the obscuration bar are omitted from Figure 9 for clear observation of the illustrated features.) The head 930 also has an exposed surface 934 in the form of a conical surface 934c, which, when in use, generates a gas curtain in the form of an essentially conical fan (represented by an arrow in Figure 9) extending outward from the exposed surface of the head 930 toward the internal surface 956 of the source vessel 911. The exposed surface 834, 934 in the form of a conical surface, such as the conical surface 834c, 934c, helps to diffuse and / or disperse (i.e., avoid concentration) the power from the source laser (such as source laser 112 in Figure 1A) that reaches the head 930. Other surface shapes may be used, and apertures such as aperture 848 may still be positioned along a direction selected to generate a radially extending gas curtain, as indicated by the arrows in Figures 8C and 9, even if they extend into exposed surfaces 834, 934 other than the conical surface.

[0076]

[0096] Further embodiments are shown in the cross-sections of Figures 10 and 11, which are similar to the figures in Figures 8C and 9.

[0077]

[0097] In the embodiment shown in Figure 10, a source container 1011 having an internal surface 1056 has a plurality of exhaust ports, two in this case 1033a and 1033b, and a plurality of corresponding exhaust openings 1055a, 1055b that extend through the source container 1011, similar to the source container 811 in Figure 8. The head 1030 of the obscuration bar (supported on a shaft not shown in the figure for the sake of easier observation of the illustrated features) has two inclined facets 1034e, 1034d on the exposed surface 1034 in the form of an inclined surface (or double inclined surface) 1034s of the head 1030. Each facet has an aperture (not shown), which is configured to create a gas curtain for each of the plurality of exhaust gas openings 1055a, 1055b of the source container 1011 when a gas flow is supplied from a passage in or over the support shaft (not shown, see Figure 3 and the relevant description above) during use. In the illustrated embodiment, two gas curtains are created, as indicated by the two sets of arrows.

[0078]

[0098] In the embodiment shown in Figure 11, a source container 1111 having an internal surface 1156 has a plurality of exhaust ports, in this case two 1133a and 1133b, and a plurality of corresponding exhaust ports 1155a, 1155b that extend through the source container 1111, similar to the source container 811 in Figure 8. However, the exhaust ports 1133a, 1133b and the exhaust ports 1155a, 1155b are not symmetrically arranged within the source container 1111. In this embodiment, the head 1130 of the obscuration bar (supported by a shaft not shown in the figure) has two facets 1134f, 1134g on the exposed surface 1134 of the head 1130. Each of the facets 1134f, 1134g is positioned to at least partially face the respective directions of the plurality of exhaust ports 1155a, 1155b. Each facet has an aperture (not shown), which, when a gas flow is supplied from a passage in or above the support shaft (not shown) during use, creates a gas curtain that extends from the exposed surface 1134 and is directed toward each of the multiple exhaust gas openings 1155a, 1155b in the source container 1011. In the illustrated embodiment, two gas curtains are created, as indicated by two sets of arrows.

[0079]

[0099] Figure 12A shows a partial cross-section of another embodiment of the source vessel 1211 of the EUV source, which has an internal surface 1256. Figure 12B is a cross-section of Figure 12A cut along line 12B shown in Figure 12A. Referring to Figures 12A and 12B, the source vessel 1211 includes an annular exhaust port 1233r, with an associated annular exhaust port 1255r encircling the source vessel 1211 and extending through the source vessel, the annular exhaust port being positioned between the collector (not shown; see, for example, Figures 1A to 1E) and the head 1230, measured along the optical axis A. The exhaust port 1233r includes an annular scrubber 1260. As shown in Figure 12B, the exhaust gas received into the annular exhaust port 1255r through the annular scrubber 1260 is removed through one or more vacuum ports (two in this embodiment) 1262a, 1262b connected to one or more vacuum pumps (not shown), as indicated by the arrows in the annular exhaust port 1255r and in the vacuum ports 1262a, 1262b and through the annular scrubber 1260.

[0080]

[0100] The obstruction bar 1227 in Figures 12A and 12B, including a head 1230 supported on a shaft 1229 which may include a base 1228, may be implemented in the same or similar manner as in Figures 8A to 8C, with the exposed surface 1234 on the head 1230 taking the form of a conical surface 1234c. The aperture (not shown) of the conical surface 1234c may be configured to create a gas curtain extending radially from the exposed surface 1234 in a flow direction including a radial component perpendicular to and away from the optical axis A, and an axial component parallel to the optical axis and away from the intermediate focal point, as indicated by the arrows near the center of Figures 12A and 12B, when a gas flow is supplied through a passage (not shown) in the shaft 1229 during use. As in the other embodiments described above, the aperture is not positioned on the side surface 1234i of the head 1230.

[0081]

[0101] The embodiments and examples may be further described using the following clauses. 1. A source container that, when in use, at least partially contains a volume through which EUV light is transmitted by a collector along the optical axis from the primary focus to the intermediate focus, A shaft having a length extending from a first end to a second end, the shaft including a passage, the passage extending at least partially along the length of the shaft, the first end of the shaft being attached to the inner surface of the sauce container, and the second end being positioned inside the sauce container, An extreme ultraviolet (EUV) source comprising a head connected to a second end of a shaft, the head intersects the optical axis, the head has an exposed surface exposed at the principal focal point, the exposed surface has one or more apertures, the one or more apertures are in fluid communication with a passage. 2. The exposed surface is a sloped surface, and the EUV source is as specified in Clause 1. 3. An EUV source according to Clause 1, wherein one or more apertures are oriented along one or more directions, having components that are oriented away from the intermediate focal point along the optical axis and components that are oriented perpendicular to the optical axis. 4. One or more apertures comprise multiple nested annular apertures in the EUV source of Clause 1. 5. One or more apertures comprising multiple non-overlapping holes, as per Clause 1, in the EUV source. 6. The head is integrated with the shaft, according to the EUV source in Clause 1. 7. The head is an EUV source according to clause 1, having a circular cross-section centered on the optical axis when cut perpendicular to the optical axis. 8. The head and shaft are made of high-melting-point material, according to the EUV source of Clause 1. 9. High melting point materials are high melting point metals, as specified in clause 8 of the EUV source. 10. The high melting point metal is tungsten, according to clause 9 of the EUV source. 11. The source vessel is provided with an exhaust port extending through the source vessel, the exhaust port being positioned between the collector and the head as measured along the optical axis, according to the EUV source of Clause 1. 12. The exposed surface is an inclined surface facing generally toward the exhaust port and / or toward the portion of the inner surface of the source vessel toward the intermediate focal side of the exhaust port, according to Clause 11. 13. An EUV source according to Clause 11, wherein the aperture is configured to create a gas curtain having a flow direction from the exposed surface of the head toward the edge closest to the midpoint of the exhaust port and / or toward the portion of the inner surface of the source vessel adjacent to the edge closest to the midpoint of the exhaust port when a gas flow is supplied through the passage during use. 14. The gas curtain flow direction has components that move away from the intermediate focal point along the optical axis, according to the EUV source of clause 11. 15. The head is an EUV source according to clause 1, wherein the head does not have a surface that faces perpendicular to the intermediate focal point. 16. The shaft of the EUV source according to clause 1 has no surface facing perpendicular to the intermediate focal point. 17. A target delivery system configured and positioned to deliver a target containing a target material to the primary focus of a collector, A laser configured and positioned to generate a pulsed light beam having a beam waist at or near the primary focus of the collector, An EUV source as specified in Clause 1, further comprising the features described above. 18. The target material is an EUV source according to Clause 17, comprising xenon, lithium, or tin. 19. The target material is tin, and the EUV source is according to clause 18. 20. An EUV source according to clause 18, further comprising a gas supply section connected to a passage, wherein the gas is an inert gas or hydrogen. 21. The gas is hydrogen, and the EUV source is specified in clause 20. 22. An EUV source according to Clause 17, wherein the collector has a central aperture positioned to allow the passage of a pulsed light beam toward the primary and intermediate focal points of the collector along the optical axis. 23. The head is positioned such that direct light from the principal focus is not reflected back to the head by the collector, according to the EUV source of clause 22. 24. The head is an EUV source according to clause 23, which shields the intermediate focal point from direct light from the pulsed light beam. 25. The head has an anti-reflective and / or diffuse geometry facing the principal focus of the collector, so that the pulsed light beam is reflected to diffuse away from the head rather than to concentrate at any point within the source vessel, according to Clause 17 of the EUV source. 26. The anti-reflective and / or diffuse geometry of the head is provided for the EUV source of Clause 25, with a generally convex surface. 27. The shaft of the EUV source according to clause 17 has no surface facing perpendicular to the intermediate focal point. 28. An EUV source according to Clause 1, wherein the shaft does not have a surface perpendicular to the primary focal point. 29. The EUV source of Clause 1, wherein the shaft has an elongated cross-section when cut by a plane parallel to the optical axis and perpendicular to the length of the shaft, the longer dimension of the cross-section is in a direction generally parallel to the optical axis, and the cross-section of the passage in a plane parallel to the optical axis and perpendicular to the length of the shaft is elongated in a direction generally parallel to the optical axis. 30. A target delivery system configured and positioned to deliver a target comprising a target material to the primary focus of a collector, further comprising a target delivery system comprising a shroud that shields the path toward the primary focus of the collector, The image of the shaft is aligned with the image of the shroud in reflection from the collector surface when viewed from the collector's primary focus, according to the EUV source of Clause 1. 31. The EUV source of clause 30, in which the image of the shaft is obscured by the image of the shroud in reflection from the collector surface when viewed from the primary focus of the collector. 32. The EUV source according to Clause 31, wherein the shaft has an elongated cross-section when cut by a plane parallel to the optical axis and perpendicular to the length of the shaft, and the longer dimension of the cross-section is in a direction approximately parallel to the optical axis. 33. The source vessel is provided with one exhaust port extending through one side of the source vessel, the exhaust port being positioned between the collector and the head as measured along the optical axis, according to the EUV source of Clause 1. 34. The source vessel is provided with multiple exhaust ports extending through the source vessel, the exhaust ports being positioned between the collector and the head as measured along the optical axis, according to the EUV source of Clause 1. 35. The aperture is configured to create a gas curtain for each of the multiple exhaust gas openings when a gas flow is supplied through the passage during use, and each gas curtain has a flow direction from the exposed surface of the head toward the edge closest to the intermediate focus of each of the multiple exhaust ports and / or toward the portion of the inner surface of the source vessel adjacent to the edge closest to the intermediate focus of each exhaust port, according to Clause 34. 36. An EUV source of Clause 34, wherein the aperture is configured to create a radially extending gas curtain extending from the exposed surface of the head in the flow direction, comprising a radial component perpendicular to the optical axis and away from the optical axis, and an axial component parallel to the optical axis and away from the intermediate focal point, when a gas flow is supplied through the passage during use. 37. The source vessel is provided with an annular exhaust port that surrounds the source vessel and extends through the source vessel, the annular exhaust port being positioned between the collector and the head as measured along the optical axis, according to the EUV source of Clause 1. 38. An EUV source of Clause 37, wherein the aperture is configured to create a radially extending gas curtain extending from the exposed surface of the head in the flow direction, comprising a radial component perpendicular to the optical axis and away from the optical axis, and an axial component parallel to the optical axis and away from the intermediate focal point, when a gas flow is supplied through the passage during use. 39. A method for reducing or preventing deposition inside a source container of an extreme ultraviolet (EUV) light source, The gas is supplied to a passage within an obscuration bar comprising a shaft and a head, wherein the first end of the shaft is supported on the inner surface of the source container of an EUV light source, the source container surrounds the optical axis of the EUV light source, the optical axis extends from the collector through the principal focus to the intermediate focus of the EUV light source, and the head of the obscuration bar at the second end of the shaft intersects the optical axis, and the head has an exposed surface exposed to the principal focus. The method involves venting gas through one or more apertures on the exposed surface of the head of an obscuration bar, wherein the one or more apertures are in fluid communication with a passage, and the venting is performed. A method for providing this. 40. The exposed surface is an inclined surface, according to the method of Clause 39. 41. The method of Clause 39, wherein one or more apertures are oriented along one or more directions having a component in the direction away from the intermediate focal point along the optical axis and a component perpendicular to the optical axis. 42. The head is integrated with the shaft of the obscuration bar, in the manner of Clause 39. 43. The method of Clause 39, wherein the head, when cut perpendicular to the optical axis, is circular and has a cross-section centered on the optical axis. 44. The method of Clause 39, wherein the head does not have a surface that faces perpendicular to the intermediate focal point. 45. The head and shaft are made of a high melting point material, according to the method of Clause 39. 46. ​​A high melting point material is a high melting point metal, according to the method of Clause 39. 47. The high melting point metal is tungsten, according to the method of clause 43. 48. The method of Clause 39, wherein the source container is provided with an exhaust port extending through the source container, the exhaust port being positioned between the collector and the head as measured along the optical axis, and the method further comprises flowing gas from the inside of the source container through the exhaust port. 49. The method of Clause 48, wherein the exposed surface is an inclined surface facing generally in the direction of the exhaust port and / or the portion of the inner surface of the source container facing the intermediate focal side of the exhaust port. 50. The method of Clause 48, further comprising generating a gas curtain having gas flowing out through one or more apertures on the exposed surface of the head, wherein the gas curtain extends from the exposed surface of the head to the exhaust port and / or to the portion of the inner surface of the source vessel on the intermediate focal side of the exhaust port. 51. The gas curtain extends along a direction having components that move away from the intermediate focal point along the optical axis, in the manner of Clause 50. 52. The method of clause 50, further comprising introducing an intermediate-focus protection gas flow that flows toward the collector along the optical axis at or near the intermediate focal point. 53. The method of Clause 52, wherein generating a gas curtain comprises splitting an intermediate focus protective gas flow at the head and merging the intermediate focus protective gas flow with gas flowing out through one or more apertures on the exposed surface of the head to form a gas curtain. 54. Delivering a target having a melting point to the main focus of the collector, The process involves irradiating the target with a light pulse at the collector's main focus to form a plasma emitting EUV light at the collector's main focus, and maintaining at least a portion of the source vessel at one or more temperatures below the melting point of the target material. The method of Clause 50 further includes the following. 55. The method of Clause 54, comprising maintaining at least a portion of the source container at one or more temperatures below the melting point of the target material, or maintaining at least a portion of the source container at a temperature in the range of 50°C to 200°C. 56. The method of Clause 39, comprising allowing gas to flow out through one or more apertures on the exposed surface of the head of the obscuration bar, thereby inhibiting or preventing gas flow away from the collector from passing through the exhaust port, thereby allowing gas flow away from the collector to enter the exhaust port. 57. The method of Clause 51, comprising suppressing or preventing a gas flow away from the collector from passing through the exhaust port during a period of up to 20 milliseconds after stopping the irradiation of the target with light pulses in the source container. 58. The method of Clause 51, comprising suppressing or preventing a gas flow away from the collector from passing through the exhaust port during a period of 20 milliseconds from the start of irradiating a target with a light pulse in the source container.

[0082]

[0102] The above-described embodiments and other embodiments are within the scope of the following claims.

Claims

1. A source container that, during use, at least partially contains a volume through which EUV light is transmitted by a collector along the optical axis from the primary focus to the intermediate focus, A shaft having a length extending from a first end to a second end, the shaft including a passage, the passage extending at least partially along the length of the shaft, the first end of the shaft being attached to the inner surface of the sauce container, and the second end being positioned inside the sauce container, An extreme ultraviolet (EUV) source comprising: a head connected to the second end of the shaft, the head intersects the optical axis, the head has an exposed surface exposed to the principal focal point, the exposed surface has one or more apertures, the one or more apertures are in fluid communication with the passage.

2. The EUV source according to claim 1, wherein the exposed surface is an inclined surface.

3. The EUV source according to claim 1, wherein the one or more apertures are oriented along one or more directions having a component in a direction away from the intermediate focal point along the optical axis and a component perpendicular to the optical axis.

4. The EUV source according to claim 1, wherein the one or more apertures comprise a plurality of nested annular apertures.

5. The EUV source according to claim 1, wherein the one or more apertures comprise a plurality of non-overlapping holes.

6. The EUV source according to claim 1, wherein the head is integrated with the shaft.

7. The EUV source according to claim 1, wherein the head, when cut perpendicular to the optical axis, is circular and has a cross-section centered on the optical axis.

8. The EUV source according to claim 1, wherein the head and the shaft are made of a high melting point material.

9. The EUV source according to claim 8, wherein the high melting point material is a high melting point metal.

10. The EUV source according to claim 9, wherein the high melting point metal includes tungsten.

11. The EUV source according to claim 1, wherein the source container is provided with an exhaust port extending through the source container, and the exhaust port is positioned between the collector and the head by measurement along the optical axis.

12. The EUV source according to claim 11, wherein the exposed surface is an inclined surface that generally faces in the direction of the exhaust port and / or the direction of the portion of the inner surface of the source container that faces the intermediate focal point side of the exhaust port.

13. The EUV source of claim 11, wherein the aperture is configured to create a gas curtain having a flow direction from the exposed surface of the head toward the edge of the exhaust port closest to the intermediate focus and / or toward the portion of the inner surface of the source container adjacent to the edge of the exhaust port closest to the intermediate focus when a gas flow is supplied through the passage during use.

14. The EUV source according to claim 11, wherein the flow direction of the gas curtain has a component that moves away from the intermediate focal point along the optical axis.

15. The EUV source according to claim 1, wherein the head does not have a surface that faces perpendicular to the intermediate focal point.

16. The EUV source according to claim 1, wherein the shaft does not have a surface perpendicular to the intermediate focal point.

17. A target delivery system configured and positioned to deliver a target containing a target material to the primary focus of the collector, A laser configured and positioned to generate a pulsed light beam having a beam waist at or near the primary focus of the collector, The EUV source according to claim 1, further comprising the following:

18. The EUV source according to claim 17, wherein the target material comprises xenon, lithium, or tin.

19. The EUV source according to claim 18, wherein the target material comprises tin.

20. The EUV source according to claim 18, further comprising a gas supply unit connected to the passage, wherein the gas is an inert gas or hydrogen.

21. The EUV source according to claim 20, wherein the gas comprises hydrogen.

22. The EUV source according to claim 17, wherein the collector comprises a central aperture positioned to allow the pulsed light beam toward the primary focus and intermediate focus of the collector along the optical axis.

23. The EUV source according to claim 22, wherein the head is positioned such that direct light from the principal focus is not reflected back to the head by the collector.

24. The EUV source according to claim 23, wherein the head shields the intermediate focal point from direct light from the pulsed light beam.

25. The EUV source of claim 17, wherein the head has an anti-reflective and / or diffuse geometry facing the principal focus of the collector, so that the pulsed light beam is reflected to diffuse away from the head rather than to concentrate at any location within the source container.

26. The EUV source according to claim 25, wherein the anti-reflective and / or diffuse geometry of the head comprises a generally convex surface.

27. The EUV source according to claim 17, wherein the shaft does not have a surface perpendicular to the intermediate focal point.

28. The EUV source according to claim 1, wherein the shaft does not have a surface perpendicular to the main focal point.

29. The EUV source according to claim 1, wherein the shaft has an elongated cross-section when cut by a plane parallel to the optical axis and perpendicular to the length of the shaft, the longer dimension of the cross-section is in a direction generally parallel to the optical axis, and the cross-section of the passage in the plane parallel to the optical axis and perpendicular to the length of the shaft is elongated in a direction generally parallel to the optical axis.

30. A target delivery system configured and positioned to deliver a target comprising a target material to the primary focus of a collector, further comprising a target delivery system comprising a shroud that obstructs the path toward the primary focus of the collector, The EUV source according to claim 1, wherein the image of the shaft is aligned with the image of the shroud in reflection from the collector surface when viewed from the primary focus of the collector.

31. The EUV source of claim 30, wherein the image of the shaft is obscured by the image of the shroud in reflection from the collector surface when viewed from the primary focus of the collector.

32. The EUV source according to claim 31, wherein the shaft has an elongated cross-section when cut by a plane parallel to the optical axis and perpendicular to the length of the shaft, and the longer dimension of the cross-section is in a direction generally parallel to the optical axis.

33. The EUV source according to claim 1, wherein the source container comprises a single exhaust port extending through one side of the source container, the exhaust port being positioned between the collector and the head by measurement along the optical axis.

34. The EUV source according to claim 1, wherein the source container comprises a plurality of exhaust ports extending through the source container, and the exhaust ports are positioned between the collector and the head by measurement along the optical axis.

35. The aperture is configured to create a gas curtain for each of the plurality of exhaust gas openings when a gas flow is supplied through the passage during use, and each gas curtain has a flow direction from the exposed surface of the head toward the edge of each of the plurality of exhaust ports closest to the intermediate focus and / or toward the portion of the inner surface of the source container adjacent to the edge of each exhaust port closest to the intermediate focus, according to claim 34.

36. The EUV source of claim 34, wherein the aperture is configured to produce a radially extending gas curtain extending from the exposed surface of the head in a flow direction, comprising a radial component perpendicular to the optical axis and away from the optical axis, and an axial component parallel to the optical axis and away from the intermediate focal point, when a gas flow is supplied through the passage during use.

37. The EUV source according to claim 1, wherein the source container comprises an annular exhaust port that surrounds the source container and extends through the source container, and the annular exhaust port is positioned between the collector and the head by measurement along the optical axis.

38. The EUV source of claim 37, wherein the aperture is configured to produce a radially extending gas curtain extending from the exposed surface of the head in a flow direction, comprising a radial component perpendicular to the optical axis and away from the optical axis, and an axial component parallel to the optical axis and away from the intermediate focal point, when a gas flow is supplied through the passage during use.

39. A method for reducing or preventing deposition inside a source container of an extreme ultraviolet (EUV) light source, The gas is supplied to a passage within an obscuration bar comprising a shaft and a head, wherein the first end of the shaft is supported on the inner surface of a source container of an EUV light source, the source container surrounds the optical axis of the EUV light source, the optical axis extends from the collector through the principal focus to the intermediate focus of the collector, the head of the obscuration bar at the second end of the shaft intersects the optical axis, and the head has an exposed surface exposed to the principal focus, The gas is discharged through one or more apertures on the exposed surface of the head of the obscuration bar, wherein the one or more apertures are in fluid communication with the passage, and the gas is discharged. A method for providing this.

40. The method of claim 39, wherein the exposed surface is an inclined surface.

41. The method of claim 39, wherein the one or more apertures are oriented along one or more directions having a component in a direction away from the intermediate focal point along the optical axis and a component perpendicular to the optical axis.

42. The method of claim 39, wherein the head is integrated with the shaft of the obscuration bar.

43. The method of claim 39, wherein the head, when cut perpendicular to the optical axis, is circular and has a cross-section centered on the optical axis.

44. The method of claim 39, wherein the head does not have a surface that faces perpendicular to the intermediate focal point.

45. The method of claim 39, wherein the head and the shaft are made of a high melting point material.

46. The method of claim 39, wherein the high melting point material is a high melting point metal.

47. The method of claim 46, wherein the high melting point metal is tungsten.

48. The method of claim 39, wherein the source container is provided with an exhaust port extending through the source container, the exhaust port is positioned between the collector and the head as measured along the optical axis, and the method further comprises flowing gas from the inside of the source container through the exhaust port.

49. The method of claim 48, wherein the exposed surface is an inclined surface that generally extends in the direction of the exhaust port and / or faces the portion of the inner surface of the source container that is on the intermediate focal side of the exhaust port.

50. The method of claim 48, further comprising generating a gas curtain comprising the gas flowing out through one or more apertures of the exposed surface of the head, wherein the gas curtain extends from the exposed surface of the head to the exhaust port and / or to the portion of the inner surface of the source container on the intermediate focal side of the exhaust port.

51. The method of claim 50, wherein the gas curtain extends in a direction having components that move away from the intermediate focal point along the optical axis.

52. The method of claim 50, further comprising introducing an intermediate-focus protection gas flow that flows toward the collector along the optical axis at or near the intermediate focus.

53. The method of claim 52, wherein generating the gas curtain comprises splitting the intermediate focus protection gas flow at the head and merging the intermediate focus protection gas flow with the gas flowing out through one or more apertures on the exposed surface of the head to form the gas curtain.

54. To deliver a target comprising a target material having a melting point to the main focus of the collector, Irradiating the target with a light pulse at the main focus of the collector to form a plasma emitting EUV light at the main focus of the collector, and maintaining at least a portion of the source container at one or more temperatures below the melting point of the target material. The method of claim 50, further comprising the above.

55. The method of claim 54, wherein maintaining at least a portion of the source container at one or more temperatures below the melting point of the target material comprises maintaining at least a portion of the source container at a temperature in the range of 50°C to 200°C.

56. The method of claim 39, comprising allowing the gas to flow out through one or more apertures on the exposed surface of the head of the obscuration bar, thereby suppressing or preventing the gas flow away from the collector from passing through the exhaust port, and allowing the gas flow away from the collector to enter the exhaust port.

57. The method of claim 51, further comprising suppressing or preventing the gas flow away from the collector from passing through the exhaust port during a period of 20 milliseconds after stopping the irradiation of the target with light pulses in the source container.

58. The method of claim 51, further comprising suppressing or preventing the gas flow away from the collector from passing through the exhaust port during a period of 20 milliseconds from the start of irradiating the target with light pulses in the source container.

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