Extreme ultraviolet light source shielding rod and method thereof

Abstract

An extreme ultraviolet (EUV) source comprises: a source container at least partially enclosing a volume in which EUV light is transmitted by a collector along an optical axis from a primary focus to an intermediate focus when in use; a shaft having a length extending from a first end to a second end of the shaft, the shaft including a channel extending at least partially along the length of the shaft, the first end of the shaft attached to the inner surface of the source container and the second end positioned within 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 therein, the one or more apertures being in fluid communication with the channel.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to U.S. Application No. 63 / 420,775, filed Oct. 31, 2022, the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present invention relates to methods and apparatuses for generating extreme ultraviolet (“EUV”) radiation from a plasma generated in a source container by irradiating a target or target material with a laser, and particularly to apparatuses and methods for controlling the flow of products generated by target irradiation within the source container. BACKGROUND OF THE INVENTION

[0004] Extreme ultraviolet radiation (e.g., electromagnetic radiation having a wavelength of about 50 nm or shorter (sometimes also referred to as soft x-rays)) (including radiation having a wavelength of about 13.5 nm) can be used in a lithography process to create extremely small features in or on a substrate such as a silicon wafer. Methods for generating EUV radiation include converting a target material to a plasma state. The target material includes at least one element, such as xenon, lithium, or tin, having one or more emission lines in the EUV portion of the electromagnetic spectrum. The target material can be solid, liquid, or gas. In one such method (commonly referred to as laser-produced plasma (“LPP”)), the desired plasma can be generated by using a “source” laser (commonly a CO2 laser emitting infrared light having a wavelength of or about 10,600 nanometers (nm)) to irradiate a target containing one or more EUV line-emitting elements with one or more light pulses. The plasma is typically generated in a sealed “source container,” which is typically a vacuum chamber. SUMMARY OF THE INVENTION

[0005] In some general aspects, an extreme ultraviolet (EUV) source includes: a source container at least partially surrounding a volume in which, in use, EUV light is transmitted by a collector from a primary focus to an intermediate focus along an optical axis; an axis having a length extending from a first end to a second end of the axis, the axis including a channel that extends at least partially along the length of the axis, the first end of the axis being attached to an inner surface of the source container and the second end being positioned within the source container; a head connected to the second end of the axis, the head intersecting the optical axis, the head having a surface exposed to the primary focus, the surface having one or more holes that are in fluid communication with the channel.

[0006] The implementation may include one or more of the following features. One or more holes may be oriented along one or more directions having a component in a direction away from the intermediate focus along the optical axis and a component perpendicular to the optical axis. One or more holes may include a plurality of nested annular holes. One or more holes may include a plurality of non-overlapping holes. The head may be integral with the shaft.

[0007] The head may have a cross-section taken perpendicular to the optical axis that is circular and centered on the optical axis. The head and the shaft may include a refractory material. The refractory material may be a refractory metal. The refractory metal may be tungsten.

[0008] The source container may include an exhaust port extending through the source container, which is positioned between the collector and the head when measured along the optical axis. The holes in the exposed surface of the head may be configured to: when in use and supplied with an air flow through the channel, generate a gas curtain that extends from the exposed surface of the head, the gas curtain having a flow direction that is from the exposed surface of the head towards the edge of the exhaust port closest to the intermediate focus and / or towards a portion of the inner surface of the source container that is adjacent to the edge of the exhaust port closest to the intermediate focus. The flow direction of the gas curtain may have a component along the optical axis away from the intermediate focus.

[0009] The head may not have a surface perpendicular to the intermediate focus. The shaft may not have a surface perpendicular to the intermediate focus.

[0010] The EUV source may include: a target delivery system configured and positioned to deliver a target including target material to the main focus of the collector; and a laser configured and positioned to generate a pulsed beam having a beam waist at or near the main 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.

[0011] The EUV source may include a supply device for a gas connected to the channel, the gas may be an inert gas or hydrogen. The gas may specifically include hydrogen. The collector may include a central hole positioned to allow the pulsed beam to pass along the optical axis towards the main focus and the intermediate focus of the collector.

[0012] The head may be positioned such that direct light from the main focus is not or is substantially not reflected by the collector onto the head. The head may shield the pulsed beam from direct light with respect to the intermediate focus. The head may have an anti-reflection and / or diffusive geometry facing the main focus of the collector such that the pulsed beam is reflected from the head in a diffusive manner without being concentrated at any location within the source container. The anti-reflection and / or diffusive geometry of the head may include a generally convex surface.

[0013] The shaft may not have a surface perpendicular to the intermediate focus. The shaft may not have a surface perpendicular to the principal focus. When intercepted in a plane parallel to the optical axis and perpendicular to the length of the shaft, the shaft may have an elongated cross-section, the long dimension of which is in a direction substantially parallel to the optical axis, and the cross-section of the channel in a plane parallel to the optical axis and perpendicular to the length of the shaft may be elongated in a direction substantially parallel to the optical axis.

[0014] The EUV source may also include a target delivery system configured and positioned to deliver a target including target material to the principal focus of the collector, the target delivery system including a shroud that shields the path towards the principal focus of the collector such that the image of the shaft is aligned with the image of the shroud when viewed from the reflection of the principal focus of the collector from the collector surface. When viewed from the reflection of the principal focus of the collector from the collector surface, the image of the shaft may be occluded by the image of the shroud.

[0015] 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 when measured along the optical axis. The source container may include a plurality of exhaust ports extending through the source container, the exhaust ports being located between the collector and the head when measured along the optical axis. The holes may be configured to generate a respective gas curtain for each corresponding exhaust port of the plurality of exhaust ports when in use and when an air flow is supplied through the channel, the respective gas curtain having a respective flow direction from the exposed surface of the head towards the edge of the corresponding exhaust port of the plurality of exhaust ports closest to the intermediate focus and / or towards a portion of the inner surface of the source container adjacent to the edge of the corresponding exhaust port closest to the intermediate focus. The holes may be configured to generate a radially extending gas curtain when in use and when an air flow is supplied through the channel, the radially extending gas curtain extending from the exposed surface of the head, the radially extending gas curtain having a flow direction including 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 focus. The source container may include an annular exhaust port surrounding and extending through the source container, the annular exhaust port being positioned between the collector and the head when measured along the optical axis.

[0016] In other general aspects, a method of reducing or preventing deposition on the interior of a source container in an extreme ultraviolet (EUV) light source may include: supplying a gas to a channel in a shielding rod, the shielding rod including a shaft and a head, a first end of the shaft being supported on an inner surface of a source container in the EUV light source, the source container surrounding an optical axis of the EUV light source, the optical axis extending from a collector through a principal focus to an intermediate focus, the head of the shielding rod at a second end of the shaft intersecting the optical axis, the head having an exposed surface exposed to the principal focus; and causing the gas to flow out through one or more holes in the exposed surface of the head of the shielding rod, the one or more holes being in fluid communication with the channel.

[0017] The implementation may include one or more of the following features. One or more holes may be oriented along one or more directions that have a component in a direction away from the intermediate focus along the optical axis and a component perpendicular to the optical axis.

[0018] The head may be integral with the axis of the shielding rod. The head may have a cross-section taken perpendicular to the optical axis that is circular and centered on the optical axis. The head may not have a surface facing perpendicularly towards the intermediate focus.

[0019] The head and the axis may comprise or be formed of a refractory material. The refractory material may be a refractory metal. The refractory metal may be tungsten.

[0020] The source container may include an exhaust port extending through the source container, which is positioned between the collector and the head when measured along the optical axis. The method may include passing gas from inside the source container through the exhaust port. The method may include generating a gas curtain of gas flowing out through one or more of the holes in the exposed surface of the head, or using the gas flowing out through one or more of the holes in the exposed surface of the head, the gas curtain extending from the exposed surface of the head to the exhaust port and / or a portion of the inner surface of the source container on the intermediate focus side of the exhaust port. The gas curtain may extend along a direction that has a component along the optical axis away from the intermediate focus. The method may include introducing an intermediate focus protection gas flow flowing towards the collector along the optical axis at or near the intermediate focus. Generating the gas curtain may include splitting the intermediate focus protection gas flow at the head and combining the intermediate focus protection gas flow with the gas flowing out through one or more of the holes in the exposed surface of the head to form the gas curtain.

[0021] The method may include: delivering a target including a target material having a melting point to the main focus of the collector; irradiating the target with a pulsed laser at the main focus of the collector to form a plasma at the main focus of the collector, the plasma emitting EUV light; and maintaining at least a portion of the source container at one or more temperatures below the melting point of the target material. Maintaining at least a portion of the source container at one or more temperatures below the melting point of the target material may include maintaining at least a portion of the source container at a temperature within the range of 50°C to 200°C.

[0022] Causing gas to flow out through one or more holes in the exposed surface of the head of the shielding rod may include suppressing or preventing the airflow in the direction away from the collector from skimming over the exhaust port, such that the airflow in the direction away from the collector enters the exhaust port. The method may include suppressing or preventing the airflow in the direction away from the collector from skimming over the exhaust port during a time period extending from the moment of stopping irradiating the target with a light pulse in the source container for 20 milliseconds (ms) or 50 ms, or in the range from 20 ms to 50 ms. The method may include suppressing or preventing the airflow in the direction away from the collector from skimming over the exhaust port during a time period extending from the moment of starting to irradiate the target with a light pulse in the source container for 20 milliseconds or 150 ms, or in the range from 20 ms to 150 ms.

[0023] Details of one or more implementations are set forth in the accompanying drawings and the following description. Other features will be apparent from the specification, the drawings, and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1A is a cross-sectional schematic view of aspects of an extreme ultraviolet (EUV) light source.

[0025] Figure 1B is Figure 1A a cross-sectional schematic view of the EUV light source of rotated 90 degrees about the z-axis.

[0026] Figure 1C is Figure 1B a cross-sectional schematic view of the EUV light source of , wherein the EUV light source is rotated such that gravity is downward in the plane of the page.

[0027] Figure 1D is Figure 1C a cross-sectional schematic view of the EUV light source of , showing the negative impact of excessive momentum in the flow.

[0028] Figure 1E is Figure 1C a cross-sectional schematic view of the EUV light source of , showing the beneficial effect of the shielding rod flow.

[0029] Figure 1F is Figure 1E an enlarged view of an illustration of , showing details of the airflow near the shielding rod.

[0030] Figure 2 is a schematic view of an EUV source for use with a lithographic exposure apparatus.

[0031] Figure 3 is that which can be positioned at Figure 1A , Figure 1B , Figure 1C or Figure 1E a perspective view of an implementation of a shielding rod in an EUV light source of .

[0032] Figure 4 is a perspective view of another implementation of a shadow bar in an EUV light source that can be positioned at Figure 1A , Figure 1B , Figure 1C or Figure 1E .

[0033] Figure 5 is a cross-sectional view of an implementation of the axis of a shadow bar (such as the shadow bar of Figure 4 ) taken along the plane 5-5 marked on Figure 4 .

[0034] Figure 6A is a perspective view of another implementation of a shadow bar in an EUV light source that can be positioned at Figure 1A , Figure 1B , Figure 1C or Figure 1E .

[0035] Figure 6B is Figure 6A a perspective view of an implementation of the head of a shadow bar.

[0036] Figure 7 is a flowchart of a process for reducing deposition inside the source container of an EUV light source, such as at an EUV light source of Figure 1A , Figure 1B , Figure 1C or Figure 1E .

[0037] Figure 8A is a perspective view of another implementation of the shadow bar head.

[0038] Figure 8B is a schematic cross-sectional view of a part of an EUV light source having a plurality of exhaust holes and exhaust ports, showing the use of the shadow bar head of Figure 8A .

[0039] Figure 8C is a cross-sectional view of an EUV light source taken along the line and direction shown in Figure 8B of Figure 8B .

[0040] Figure 9 is a cross-sectional view of an implementation of an EUV light source and a shadow bar head similar to Figure 8C .

[0041] Figure 10 is a cross-sectional view of an implementation of an EUV light source and a shadow bar head similar to Figure 8C .

[0042] Figure 11 is a cross-sectional view of an implementation of an EUV light source and a shadow bar head similar to Figure 8C .

[0043] Figure 12A It is a schematic cross-sectional view of a part of an EUV light source having an annular exhaust port.

[0044] Figure 12B It is along Figure 12A the line and direction shown Figure 12A cross-section of the EUV light source. Detailed implementation

[0045] Figure 1A It is a simplified schematic cross-sectional view of some components of an implementation of an LPP EUV light source 110. As shown by the reference coordinate axes in the figure, Figure 1A it is shown in the x-z plane, with x being positive in the upward direction in the plane of the page and z being positive to the right in the plane of the page, and the z-axis is aligned with the optical axis A of the collector 120 described below.

[0046] As Figure 1A shown, the EUV light source 110 includes a source laser 112 that generates pulses of a light beam 113 (e.g., a laser) and delivers the pulsed light beam 113 from the source laser 112 to the interior 114 of a source container 111 to individually irradiate a target 115 within an irradiation site 116. The target 115 travels downward in the plane of the page (in the negative x direction) from a target delivery system 117a to the irradiation site 116. The source container 111 has an inner surface 156 surrounding the interior 114.

[0047] Also as Figure 1A shown, the EUV light source 110 includes a target delivery system 117a that delivers the target 115 into the interior 114 of the source container 111 to the irradiation site 116. At the irradiation site 116, the target 115 individually interacts 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 can be monitored by one or more metrology devices 150, and the information collected by the one or more metrology devices 150 can be used for the control and operation of the EUV light source 110.

[0048] The target 115 can be delivered along at least a portion of its travel through a target shield 115s. The shield 115s can be in the form of a tube (which can have holes for metrology) or other shielding structure that shields or partially shields the gas and other materials in the interior 114 of the source container 111 from the incoming target 115 so that the trajectory of the target 115 is not overly disturbed by such gas or other materials. Unused targets in the target 115 (such as targets that are not converted into the plasma 118) can be captured in a target well 117b.

[0049] The target 115 is or includes EUV-emitting target material, such as but not necessarily limited to tin, lithium, xenon, or combinations thereof. The target 115 can be in the form of droplets, or alternatively can be solid particles, or solid particles contained within droplets. For example, elemental tin can be presented as a target in the following forms: pure tin; tin compounds such as SnBr4, SnBr2, SnH4; tin alloys such as tin-gallium alloy, tin-indium alloy; or tin-indium-gallium alloy; or combinations thereof.

[0050] The EUV light source 110 can also include a collector 120. The collector 120 can be a near-normal incidence collector mirror having an optical axis A and a reflective surface 121. The reflective surface 121 can be in the form of a prolate ellipsoid (i.e., an ellipse rotated about its major axis) such that the collector 120 has a first or primary focus 122 within or near the irradiation site 116 and a second focus at a so-called intermediate focus 123, and the optical axis A is defined as the line extending between them. The source container 111 of the EUV light source 110 thus at least partially encloses the following volume: when the EUV light source 110 and the source container 111 are in use, the EUV light is transmitted by the collector 120 along the optical axis A from the primary focus 122 to the intermediate focus 123 in this volume. The reflected EUV light 124 from the collector 120 can be output from the EUV light source 110 at the intermediate focus 123 and input to a device that utilizes the EUV light 124, such as a lithographic exposure apparatus (as Figure 2 shown). The collector 120 is formed with a hole 125 to allow the beam 113 of light pulses generated by the source laser 112 to pass through the hole 125 and reach the irradiation site 116. The hole 125 creates a shadow or large volume gap 154 along the optical axis A in the reflected EUV light 124 from the collector 120.

[0051] To reflect the EUV light 119, the collector 120 can be in the form of a multilayer mirror (MLM), and the reflective surface 121 has a graded multilayer coating having alternating molybdenum layers and silicon layers, and in some cases, also has one or more high-temperature diffusion barrier layers, smoothing layers, capping layers, and / or etch stop layers. Other surface shapes than the prolate ellipsoid can also be used for the reflective surface 121. For example, the reflective surface 121 can alternatively be in the form of a parabola rotated about its major axis. In an implementation, the reflective surface 121 can be configured to deliver an EUV beam 124 having an annular or other cross-section at the intermediate focus 123. In other implementations, the reflective surface 121 can utilize other coatings and layers than the coatings and layers described above.

[0052] The manufacturing cost of collector 120 can be high. The efficiency and power of the light generated by EUV light source 110 depend on the quality of the reflective surface 121 of collector 120. For these and other reasons, it is desirable to protect collector 120 from damage to its reflective surface 121.

[0053] However, collector 120 must be placed within source container 111 and close to or near plasma 118 in order to collect and redirect EUV light 119. The structures within source container 111 (including collector 120) can be exposed to high-energy ions and / or particles and vapor that are of or contain target material. Particles of the target material, as well as high-energy ions and vapor (which are essentially debris or by-products of a light-based vaporization or ablation process), can contaminate the exposed reflective surface 121 of the collector. Particles of the target material, as well as high-energy ions and vapor, can also cause physical damage and local heating to the reflective surface 121 of collector 120.

[0054] As Figure 1A shown, EUV light source 110 can include a focusing unit 126 that includes one or more optical elements (not shown) for focusing beam 113 into a focal spot or beam waist located at or near illumination site 116.

[0055] Figure 2 is a diagram showing the implementation of EUV light source 210 (such as Figure 1A EUV light source 110 or another EUV light source) and lithographic exposure apparatus 271. Lithographic exposure apparatus 271 receives EUV light 224 generated by EUV light source 210 and reflects it in one or more illumination mirrors 272 to illuminate reflective pattern or reticle 273. The EUV light reflected from pattern or reticle 273 is further reflected and reduced by one or more reducing mirrors 274 and illuminates substrate or wafer 275 (or one or more photosensitive layers on substrate or wafer 275, not shown) to allow a patterned structure to be formed in or on substrate or wafer 275.

[0056] The optical elements and sensors within lithographic exposure apparatus 271, as well as the photosensitive layers on substrate or wafer 275, are generally sensitive to various types or even any type of radiation. Therefore, especially considering Figure 1A the high power level generated by source laser 112, it is important to prevent any portion of the light pulse beam 113 from source laser 112 (including Figure 1A beam 113a shown, which corresponds to the extension of beam 113 beyond illumination site 116) from reaching intermediate focus 123 and potentially entering a lithographic exposure apparatus, such as lithographic exposure apparatus 271.

[0057] For this purpose, asFigure 1A As shown, a beam blocking element such as the shadow bar 127 of the present disclosure can be used. The shadow bar 127 can include a base 128, a shaft 129 extending from the base 128, and a head 130 supported on the shaft 129. When using the shadow bar 127, as shown, the head 130 is located on the optical axis A of the collector 120 such that the optical axis A intersects the head 130. In addition, the position and size of the head 130 can be set to fit the shadow or large volume gap 154 in the reflected EUV light 124 from the collector 120. For example, the head 130 can have a cross-section taken perpendicular to the optical axis A that is circular and centered on the optical axis A and matches the shadow or large volume gap 154. This geometry prevents the head 130 from blocking any or any significant portion of the EUV light 124 reflected from the collector 120 and directed toward the lithography exposure apparatus 121, while well protecting the intermediate focus 123 from direct irradiation by the pulsed beams 113, 113a of the source laser 112. In other terms (regarding the positioning in the shadow or gap 154), the head 130 is positioned such that little or no direct light 119 from the main focus 122 is reflected by the collector 120 onto the head 130. The head 130 can also have an anti-reflective and / or diffusive geometry facing the main focus 122 of the collector 120 such that the light reaching the head 130 from the source laser 112 is thereby reflected from the head 130 in a diffusive manner rather than being concentrated at any location within the source container 111. The anti-reflective and / or diffusive geometry of the head 130 can include a generally convex surface exposed to the collector 120.

[0058] In a source container using the target shield 115s, as shown, the shaft 129 of the shadow bar 127 can be aligned with the shield 115s, that is, it can be located as much as possible within the shadow produced by the shield 115s in the reflected EUV light 124. In other words, when observed from the reflection of the main focus 122 of the collector 120 from the collector surface 121, the image of the shaft 129 can be aligned with the image of the shield 115s. In some implementations, the shaft 129 can be completely hidden within the shadow of the shield 115s, just as when observed from the reflection of the main focus 122 of the collector 120 from the collector surface 121, the image of the shaft 129 is hidden by the image of the shield 115s. This arrangement reduces or eliminates the situation where the EUV light 124 is blocked by the shaft 129 from leaving the EUV light source 110. The gas conduit 131 is connected to the base 128 of the shadow bar 129 and a source of gas (not shown) such as H2 gas 132 to allow the shadow bar 127 to be used to supply gas to the interior 114 of the source container 111 at or near the center or optical axis A of the source container 111, as will be shown and discussed in more detail below.

[0059] Figure 1Bis a simplified schematic cross-sectional view of EUV light source 110, rotated 90 degrees about the optical axis A to show a cross-section in the y-z plane, where y is positive upward in the plane of the page and z is positive to the right, as indicated by the reference coordinate axes. When in use, EUV source 110 can be tilted relative to gravity as indicated by the gravity vector G and is shown in the y-z plane or parallel to the y-z plane as shown. In this view, the axis 129 and base 128 of the shadow bar 127 enter the page behind the head 130. Also in this view, the exhaust hole 133 and associated exhaust port 155 can be seen. As shown, the exhaust hole 133 is a structure that extends from the source container 111 and defines the exhaust port 155, which is in fluid communication with and extends from the interior 114 of the source container 111. Gas, as well as entrained ions, vapor, and debris, can be exhausted from the source container 111 through the exhaust port 155 of the exhaust hole 133 by one or more vacuum pumps (not shown). When measured along the optical axis A, the exhaust port 155 is located between the collector 120 and the head 130.

[0060] As Figure 1B shown, the head 130 of the shadow bar 127 includes a surface or "exposed surface" 134 that is exposed to the main focus 122. The exposed surface 134 can be or can include an inclined surface 134s, which represents a surface that is not perpendicular to the axis A and can generally face the direction of the exhaust port 155 of the exhaust hole 133 and / or the direction of the portion 135 of the inner surface 156 of the source container 111 on the intermediate focus side of the exhaust port 155, which will be shown and discussed in more detail below.

[0061] Figure 1C is another cross-section of EUV light source 110 in the y-z plane, but the gravity G vector is now directed downward in the plane of the page and there are various gas flows that can be used for the EUV light source 110, which are represented by contoured arrows in the figure.

[0062] Referring Figure 1C , a gas flow (such as a hydrogen (H2) flow with a pressure in the range of about 50 Pa to about 300 Pa) can be used as a buffer gas for debris and / or vapor control within the source container 111. Given that the interior 114 of the source container 111 requires a vacuum to avoid excessive absorption of EUV light by gas molecules, it is difficult to adequately protect the collector 120 from target material debris and vapor emitted from the irradiation site 116 without using a gas flow. Hydrogen (H2) is relatively transparent to EUV radiation at a wavelength of about 13.5 nm and is therefore generally superior to other candidate gases such as helium, argon, and other gases that exhibit higher absorption at about 13.5 nm.

[0063] H2 gas can be introduced into the source container 111 to slow down and direct the high-energy debris (ions, atoms, and clusters) of the target material generated by the irradiation of the target 115 and the irradiation site 116 and the resulting plasma 118. The debris slows down after colliding with the gas molecules. The H2 gas flow 136 at the central hole 125 of the collector 120 can be used for this purpose. Sometimes referred to as the "conical flow" 136, the flow 136 can be directed by a tube or nozzle 137, etc., from the hole 125 at the center of the collector 120 towards the irradiation site 116 where the plasma 118 is repeatedly generated. This direction is opposite to the debris trajectory from the irradiation site 116 towards the collector 120, and thus the conical flow 136 is used to reduce the damage to the collector 120 caused by the gas-phase deposition, injection, and deposition of the sputtered target material.

[0064] When using a tin or tin-containing target 115, using hydrogen gas (such as in the conical flow 136) together with such a target 115 can result in another potential pollution source in the source container 111. This is when hydrogen bubbles form and grow in or under the molten tin and then burst, causing the molten tin to spurt out or "spit out" from the surface of the container coated with or experiencing molten tin coating.

[0065] One method for preventing tin splashing is to prevent the molten target material from accumulating on the surface of the source container 110, that is, to keep the surface below or well below the melting point of the target material. For tin, the melting point is approximately 232 °C. For example, some parts of the inner surface 156 of the source container 111 can be maintained at a temperature below 232 °C, such as in the range of 50 °C to 110 °C. Any tin deposited on such a surface remains in solid form and prevents or resists splashing.

[0066] However, deposition on cold surfaces also shortens the service interval length of EUV sources (such as EUV source 110). By using additional gas flows, the growth of deposits on cold surfaces and the accumulation of liquid tin on hot surfaces can be reduced.

[0067] A gas flow commonly referred to as the umbrella flow 139 can be directed along the surface of the collector 120 (from an outlet not shown). A so-called showerhead flow can be provided in the area of the source container 111 closest to the collector 120, where the gas flows through a plurality of parallel holes substantially perpendicular to the surface to be protected, such as showerhead flow S1 and showerhead flow S2. In additional areas such as in the vicinity of the intermediate focus 123, a protective gas flow parallel to the surface to be protected or having a flow component parallel to the surface to be protected can be introduced through holes that point in a direction having a flow component along or parallel to the surface to be protected. For example, gas flows such as gas flows Fl, F2, F3, and F4 can be introduced to protect the inner surface 156 in the area near the intermediate focus 123 of the source container 111.

[0068] An air flow, commonly referred to as a dynamic gas lock (“DGL”), is one or more air flows used to prevent any material from leaving the area of the intermediate focus 123 of the EUV source 110. The DGL can generate an air flow from the area of the intermediate focus 123 towards the illumination site 116, such as the DGL flow 138, which can also be referred to as the “intermediate focus protection” air flow 138.

[0069] The stable guiding flow 140 flowing out of the collector 120 can be mainly formed by the conical flow 136, the umbrella flow 139, and the nozzle flows SI and S2 (and optionally other flows not shown). Figure 1C The solid line curve in shows an example of the guiding flow 140. This guiding flow 140 helps to contain and carry materials away from the collector 120, including vapor, ions, and micron and nanoparticle generated by the target 115 during the generation of the plasma 118. The reverse flow 141 moving from the intermediate focus 123 towards the collector 120 can be mainly formed by the DGL flow 138 together with flows such as flows Fl, F2, F3, and F4 (and optionally other flows not shown). Figure 1C The dashed line curve in shows an example of the reverse flow 141.

[0070] Considering the low pressure used within the source container 111, the pressure difference at the exhaust port 155 of the exhaust hole 133 is not large. However, the small pressure difference generated at the exhaust port 155 by pumping the exhaust hole 133 through a vacuum pump, and the flow momentum balance between the guiding flow 140 and the reverse flow 141 at the merging region 142 of the two flows 140, 141 (where the merging region 142 is close to the exhaust port 155) can generate a stable guiding flow that entangles and contains the by-products of the target material in the guiding flow 140 into the exhaust port 155, while the by-products of the target material basically do not contact any inner surface of the source container 111.

[0071] Figure 1D Shows Figure 1C The cross-section of the EUV light source 110 of, but the air flow 140 no longer repeatedly receives and carries vapor, ions, and micron and nanoparticle generated by the target 115 used in the plasma production process. This is partially represented by the absence 118a of the plasma 118 at the illumination site 116 in the source container 111 in. Figure 1D is represented by the absence of the plasma 118 at the illumination site 116 in the source container 111.

[0072] Plasma production may stop for various reasons. To control the amount of radiation (“exposure dose”) received by a given exposure site on the wafer, such as the wafer 275 ( Figure 2 ), the EUV light 119, 124 generated by the EUV light source (such as the EUV light source 111) from each light pulse can be detected ( Figure 1A , Figure 1B) power and calculates in real time the total power delivered to that location. After reaching or exceeding the desired exposure dose level, further light pulses can be mis-timed immediately so that in the source container 111, as long as the exposure location is positioned for exposure, the target 115 is not struck by light pulses. This results in an abrupt halt to plasma production in the source container 111. An abrupt halt (and start) to plasma production can also occur during movement of the wafer from one exposure location to the next, or during movement from one wafer to the next, or even in lithography techniques involving sub-standard exposure time rates.

[0073] Referring again to Figure 1D , when the continuous target 115 is continuously irradiated by light pulses of the light beam 113 from the source laser 112 at the irradiation location 116 (as Figure 1C shown), vapor, micron and nano particles and other debris, and ionized plasma are repeatedly generated at and near the irradiation location 116 in the source container 111, which are thus effectively injected or deposited into the conical flow 136 at the main focus 122 of the collector 120. The net momentum of the injected material is very low or almost zero because the energy and momentum of the plasma 118 and associated materials tend to propagate and / or radiate in all directions. Thus, the injected material reduces the total momentum of the conical flow 136 and the guiding flow 140 (partially formed by the conical flow 136), which carries the injected material from the collector 120 towards the exhaust hole 133.

[0074] When plasma production stops, such as during the stepping, adjustment or other changes of the associated lithography exposure apparatus, the light pulses of the light beam 113 stop striking the target 115, and the material of the continuous target 115 simply passes through the focus of the collector 120 on its way to the target well 117b ( Figure 1A ). Thus, this unirradiated target material is not injected into or entrained in the conical flow 136 and the guiding flow 140. In the absence of the material repeatedly injected at the irradiation location 116 by plasma production, the momentum of the conical flow 136 and the flow 140 may be too large to maintain its normal balanced flow path into the exhaust hole 133 (or too large to be balanced by the reverse flow 141 as Figure 1C shown). As Figure 1DAs shown, a stream (or "burst stream") 143 can bypass the exhaust hole 133 (or in other words, the stream can pass or escape outside the normal path of the stream 140 flowing out of the exhaust hole 133). It can be considered that when plasma production stops, target-related vapor and debris are no longer contained in the stream 140, and the burst stream 143 that occurs when plasma is not generated does not cause contamination in the source container 111. However, when plasma production first stops, the stream 140 from the most recent plasma production still entrains target-derived vapor and debris, and the burst stream 143 can carry this vapor and debris out of the exhaust port 155. In addition, when plasma production has just started or restarted, the target material has just started to be entrained again in the initial high average momentum stream 140, and thus a burst stream 143 containing the target material may also occur at plasma startup.

[0075] When the burst stream 143 contains target-related material, deposition or contamination may occur on the portion 135 of the inner surface 156 of the source container 111 on the intermediate focus side of the exhaust hole 133. After passing through the exhaust port 155, the burst stream 143 or the stream 143 can also move in various other directions, which may cause an unstable flow pattern in the source container 111 and produce contamination in other regions within the interior 114 or in other regions of the inner surface 156.

[0076] As Figure 1E shown in the cross-section of the EUV light source 110 of Figure 1A , in one aspect of the present disclosure, the problem of the burst stream 143 from the stream 140 is prevented or reduced by using a shielding rod gas stream 144, which is provided into the source container 111 from the exposed surface 134 of the head 130 of the shielding rod 127 or from the exposed surface 134 in the form of the inclined surface 134s ( Figure 1E ). As shown in the illustration of Figure 1C , the shielding rod gas stream 144 flows in a direction including at least two components, a first component towards the portion 135 of the inner surface 156 of the source container 111 on the intermediate focus side of the exhaust hole 133, and a second component along the optical axis A towards the collector 120. Originating the gas stream 144 from the exposed surface 134 and flowing in a direction including these two components helps to ensure that the momentum of the gas stream 144, as well as the momentum of the reverse flow 141 mainly composed of the DGL flow 138 but potentially including other flows (such as flows F1 - F4) ( Figure 1C ) is sufficient to prevent or substantially prevent the stream 140 mainly composed of the cone flow 36 from bypassing the exhaust port 155, keeping the stream 140 within its desired mode and entering the exhaust port 155 from the collector 120 or from the irradiation site 116. When plasma is generated at the irradiation site 116 (as Figure 1DAs shown), the blocker bar airflows 144 can all remain "open", thereby eliminating or reducing any need to rapidly change or rebalance the airflow within the source container 111. The blocker bar airflows 144 can effectively form a gas curtain having a flow direction that is from the exposed surface 134 of the head 130 towards the edge of the exhaust port 155 closest to the intermediate focus 123 and / or towards the portion 135 of the inner surface 156 of the source container 111 that is adjacent to the edge of the exhaust port 133 closest to the intermediate focus 123.

[0077] Reference Figure 1F , which is Figure 1E an enlarged view of an illustration of, in some implementations, the surface 157 of the head 130 of the blocker bar 127 that faces the intermediate focus can be symmetric about the optical axis A. In Figure 1F the example of, the head 130 has a symmetric faceted pattern on its surface 157 that faces the intermediate focus (facets shown in more detail below in Figure 3 and Figure 6A ). Symmetry about the optical axis A tends to evenly divide the reverse flow 141 (or the major component DGL flow 138 of the reverse flow 141) into split flows, such as flows 141a and 141b as shown in the plane of the figure. Since the blocker bar airflow 144 is introduced through the exposed surface 134 of the head 130 rather than through the side surface 134i, the blocker bar airflow 144 does not significantly push the split flows (such as flows 141a and 141b) away from the side surface 134i of the head 130 and towards the inner surface 156 of the source container 111. The substantially even division of the reverse flow 141, and the blocker bar airflow 144 being introduced through the exposed surface 134 of the head 130 rather than through the side surface 134i, helps to maintain the stability of the reverse flow 141 and allows the divided reverse flows (such as flows 141a and 141b) to flow with the blocker bar airflow 144 to help direct the flow 140 (and flow with it) into the exhaust port 155 (see Figure 1E ). Thus, divided reverse flows such as flows 141a and 141b can help generate a gas curtain. By splitting the reverse flow 141 (or DGL flow 138 or "intermediate focus protection" flow 138) at the head 130, effectively combining the intermediate focus protection airflow 138 with the blocker bar airflow 144 flowing out through one or more holes in the exposed surface of the head 130, a gas curtain having sufficient momentum to prevent or reduce the burst flow 143 can be formed.

[0078] Figure 3 , Figure 4 and Figure 6A show various implementations of the (multiple) blocker bars 127 according to the present disclosure. Figure 3 is a perspective view of a blocker bar 327 that is Figure 1AImplementation of the shielding rod 127. As understood from the Figure 1A above description, the shielding rod 327 is used in the context of an EUV light source 110 that includes a source vessel 111 that encloses an interior 114 of the EUV source vessel 111, and when in use, EUV light 146 travels along an optical axis A from a collector 120 to an intermediate focus 123 within the interior. As Figure 3 shown, the shielding rod 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, such as when they are formed together by machining from a single block or by continuous 3D printing. The shaft 329 may include a base 328, which, if present, may also be integral with the shaft 329.

[0079] As Figure 3 indicated by the dashed line in, the shielding rod 327 also defines or includes a gas channel 347 that extends in a direction along the length L of the shaft 329. In the illustrated implementation, the channel 347 is enclosed within the shaft 329.

[0080] The head 330 and the shaft 329 may include or be formed of a refractory material, such as an oxide, nitride, or carbide ceramic, or a refractory metal, for example. Molybdenum and tungsten are two metals that may be used. Tungsten is useful due to its extremely high melting point and relatively high thermal conductivity.

[0081] As Figure 3 shown, the shaft has a length L that extends from its first end 345 to its second end 346. In use or when positioned for use, in this implementation, the first end 345 is attached to the source vessel 111 at the base 328 (as Figure 1A shown for the shielding rod 127 in). The head 330 is connected to the second end 346 of the shaft 329, and when in use or when positioned for use (as Figure 1A shown for the shielding rod 127 in), the head 330 intersects the optical axis A of the collector 120. The head 330 also has one or more holes 348 that, in this implementation, are in fluid communication (i.e., fluidly connected) with the channel 347 via a chamber 349 within the head 330.

[0082] In Figure 3 the implementation, a plurality of angled facets (facets 351a, 351b, 351c are shown therein) are symmetrically positioned on a surface 357 of the head 330 that faces the intermediate focus. These facets ensure that when the shielding rod 327 is in use or when positioned for use, the head 330 does not have a surface that faces the intermediate focus 123 perpendicularly (see, for example, Figure 1A and Figure 1E and Figure 1E(Illustration). In addition, the surface exposed to the intermediate focus is far from being perpendicular to the intermediate focus, such as being greater than 30 degrees or even greater than 45 degrees from the perpendicular to the intermediate focus. If the surface of the head 330 facing the intermediate focus is to be coated with liquid tin, this geometry reduces the likelihood of spitting in the direction of the intermediate focus 123. The symmetrical arrangement of the facets also promotes the stability of the reverse flow 141 moving around the head 330 ( Figure 1C and Figure 1F ), as described above with respect to Figure 1F stated.

[0083] Figure 4 The shielding rod 427 is shown, which is Figure 1A Another implementation of the shielding rod 127 of. Figure 4 Shown is a perspective view rotated 180 degrees from the Figure 3 perspective view, such that the exposed surface 434 of the head 430 is visible in the form of the inclined surface 434s of the head 430 (e.g., see Figure 1E and Figure 1E Illustration, showing the exposed surface 134 in the form of the inclined surface 134s of the head 130 of the shielding rod 127). A plurality of holes 448 exist in the exposed inclined surfaces 434, 434s in the form of a plurality of non-overlapping holes 448a, and do not exist in the side surface 434i.

[0084] In Figure 4 implementation, the surface of the head 430 facing the intermediate focus has a conical surface 453. Similar to the facets 351a, 351b, 351c in the Figure 3 implementation, this conical surface 453 ensures that the head 430 does not have a surface perpendicular to the intermediate focus 123. In addition, the conical surface 453 can be exposed to the intermediate focus at an angle away from the perpendicular to the intermediate focus, such as being greater than 30 degrees or even greater than 45 degrees from the perpendicular to the intermediate focus. In addition, in Figure 4 , the shaft 429 has facets 452a and 452b (not shown) on the surface of the shaft 429 facing the intermediate focus, and has facets 452c and 452d (not shown) on the surface of the shaft 429 facing the collector. Therefore, in this implementation, the shaft 429 lacks a surface perpendicular to the intermediate focus 123 and lacks a surface perpendicular to the collector 120 or the main focus 122 near the collector 120.

[0085] From Figure 4It will be appreciated that the holes 448 are generally substantially perpendicular to the exposed inclined surfaces 434, 434s. Specifically, when the shielding rod 427 is in use or is installed for use, the holes 448 are oriented along one or more directions that have a component in the direction away from the intermediate focus 123 along the optical axis A and a component perpendicular to the optical axis A. The exposed inclined surfaces 434, 434s may be convex, which helps to diffuse any light reflections from the source laser 112. The convex exposed inclined surfaces 434, 434s may also allow for a wider airflow region to be generated from the holes 448, which are in the form of holes that are generally perpendicular to the exposed inclined surfaces 434, 434s. It should be noted that other implementations are possible, such as an implementation that omits the inclined surface 434s to facilitate the alignment of the inclined holes in the exposed surface 434 in substantially the same direction as the holes 448.

[0086] Figure 5 is a cross-sectional view of an implementation of the shaft 529 that is similar to the shaft 429, such as taken along Figure 4 the line 5-5 as shown. The shaft 529 includes facets 552a, 552b, 552c, and 552d, which are similar to the Figure 4 facets. In addition, the shaft 529 defines an internal channel 547, similar to the Figure 3 internal channel 347. As shown in the Figure 5 implementation, when taken in a plane parallel to the optical axis A (i.e., parallel to the "z" direction) and perpendicular to the length of the shaft 529, the shaft 529 has an elongated cross-section, and the long dimension of the cross-section is in a direction substantially parallel to the optical axis (i.e., substantially parallel to the "z" direction). This shape of the shaft 529 that is elongated in the "z" direction allows the shaft 529 to be thinned in the x-y plane so as to be better hidden within the shadow of the target shroud 115s (if any), while still allowing for sufficient airflow in the channel 547, which is also elongated in the "z" direction.

[0087] Figure 6A is a perspective view of the shielding rod 627, which is another implementation of the Figures 1A - 1E shielding rod 127 (and head 130). As can be seen from the reference coordinates, Figure 6A the view in Figure 6AAs shown, when viewed along the optical axis A, the head 630 has a circular cross-section. In this implementation, the hole 648 in the exposed surface 634 (rather than in the side surface 634i) is in the form of an oval nested annular hole 648b. The exposed surface 634 with the hole 648 is also the inclined surface 634s such that the flow from the annular hole 648b has components in both the negative "z" direction and the negative "y" direction, as indicated by the arrows below the head 630. The shaft 629 and the base 628 are similar to some of the other implementations described above. Additionally, the shaft 629 includes facets 652c and 652d on its surface, and such facets face the negative "z" direction, or face Figures 1A - 1E the collector 120. As with other implementations, the exposed inclined surfaces 634, 634s can have a convex shape overall. As Figure 6B shown, in the perspective view of the head 630 of the shielding rod 627 in Figure 6A , when viewed along the positive x-axis as shown by the reference coordinates in the figure, this optional overall convexity of the exposed inclined surfaces 634, 634s is shown in Figure 6B . As Figure 6B shown, a part 634a of the exposed inclined surfaces 634, 634s close to the inner annular hole in the annular hole protrudes more than a part 634b of the exposed inclined surfaces 634, 634s close to the outer annular hole in the annular hole 648b, thus making the exposed inclined surfaces 634, 634s present an overall convex shape. As Figure 3 shown, there are a plurality of facets on the surface 657 of the head 630 facing the intermediate focus, where 651a, 651b, and 651C are visible.

[0088] Referring to Figure 7 , the process P100 is performed to prevent unnecessary deposition in the source container 111 of the EUV light source 110. In step S10, the gas 132 ( Figure 1A) is supplied to a channel 347 within the shadow bars 127, 327, 427, 627 that include shafts 129, 329, 429, 629 and heads 130, 330, 430, 630. A first end 345 of the shafts 129, 329, 429, 629 is supported on an inner surface 156 of a source container 111 within the EUV light source 110. The source container 111 surrounds an optical axis A of the EUV light source 110, and the optical axis A extends between a condenser 120 and an intermediate focus 123 of the EUV light source 110. The shadow bars 127, 327, 427, 627 include heads 130, 330, 430, 630 located at a second end 346 of the shafts 129, 329, 429, 629, the heads intersecting the optical axis A and including exposed surfaces 134, 434, 634 that are exposed to a main focus 122. Next, in step S20, a gas 132 flows through the channel 347 of the shadow bar and exits from the head, and enters an interior 114 of the source container 111 through the exposed surfaces 134, 434, 634 of the heads 130, 330, 430, 630 and / or one or more holes 348, 448, 648 in the shafts 129, 329, 429, 529, 629 of the shadow bars 127, 327, 427, 627. The holes 348, 448, 648 can be oriented along one or more directions that have a component away from the intermediate focus 123 and a component perpendicular to the optical axis A.

[0089] In the implementation of process P100, the heads 130, 330, 430, 630 can be integral with the shafts 129, 329, 429, 529, 629 of the shadow bars 127, 327, 427, 627. The heads 130, 330, 430, 630 can have a cross-section taken perpendicular to the optical axis A that is circular and centered on the optical axis A. The heads 130, 330, 430, 630 can not have a surface that faces the intermediate focus 123 perpendicularly. The heads 130, 330, 430, 630 and the shafts 129, 329, 429, 529, 629 can include a refractory material or be formed of a refractory material. The refractory material can be, for example, an oxide, nitride, or carbide ceramic, or a refractory metal such as, for example. The metal can be molybdenum or tungsten. The refractory metal can be tungsten.

[0090] In the implementation of process P100, the source container 111 may include an exhaust port 155 defined by an exhaust hole 133, the exhaust port 155 extending through the source container 111, wherein the exhaust port 155 is positioned between the collector 120 and the head 130 when measured along the optical axis A. Process P100 may also include causing gas to flow out of the source container 111 through the exhaust port 155. Process P100 may include generating a gas curtain at least in part from or together with the outflow of gas 132, the gas flowing out through one or more holes 348, 438, 638 in the exposed surfaces 134, 434, 634 of the head, and extending to the exhaust port 155 and / or a portion 135 of the inner surface 156 of the source container 111 on the intermediate focus side of the exhaust port 155. The gas curtain may extend in a direction having a component along the optical axis A away from the intermediate focus 123. Process P100 may include introducing an intermediate focus protection gas flow in the form of DGL flow 138 at or near the intermediate focus 123 and flowing along the optical axis A towards the collector 120. Generating the gas curtain may include splitting the intermediate focus protection gas flow 138 at the head 130, 330, 430, 630 and combining the intermediate focus protection gas flow 138 with the gas flowing out through one or more holes 348, 448, 648 in the exposed surfaces 134, 434, 634 of the head 130, 330, 430, 630 to form a gas curtain.

[0091] The implementation of the method may include delivering a target 115 including a target material to the main focus 122 of the collector 120, the target material having a melting point, and irradiating the target 115 with a light (e.g., laser) pulse at the main focus 122 of the collector 120 to form a plasma 118 at the main focus 122 of the collector 120, the plasma 120 emitting EUV light 119, and maintaining at least a portion of the source container 111 at one or more temperatures below the melting point of the target material. At least a portion of the source container 111 may be maintained at a temperature below 232 °C or below 200 °C, such as in the range of 50 °C to 200 °C, 50 °C to 150 °C, or even 50 °C to 110 °C.

[0092] In the implementation of this method, causing the gas 132 to flow out through one or more of the holes 348, 448, 648 in the exposed surfaces 134, 434, 634 of the heads 130, 330, 430, 630 of the shielding rods 127, 327, 427, 627 may include inhibiting or preventing the gas 140 from passing through the exhaust port 155 in a direction away from the collector 120, resulting in the gas 140 entering the exhaust port 155 in a direction away from the collector 120. For example, during a time period extending from the moment when the irradiation of the target 115 with light pulses in the source container 111 is stopped for 20 milliseconds (ms) or 50 ms, or during a time period within the range of 20 ms to 50 ms, the air flow 140 in a direction away from the collector 120 can be inhibited or prevented from bypassing the exhaust port 155. For example, during a time period extending from the moment when the irradiation of the target with light pulses in the source container starts for 20 ms or 150 ms, or during a time period within the range of 20 to 150 ms, the air flow 140 in a direction away from the collector 120 can also be inhibited or prevented from bypassing the exhaust port 155.

[0093] Figure 8A is a perspective view of another implementation of the head 830 of the shielding rod, which can be positioned Figure 8B inside the EUV source container 811 shown in partial cross-section. Figure 8C is taken along Figure 8B the cross-section line and direction shown in Figure 8B a cross-sectional view of the source container 811. The head 830 of the shielding rod can be supported on the shaft 829, as Figure 8C shown. The shaft 829 is attached to the inner surface 856 of the source container 811 and may include a base 828 that facilitates the attachment.

[0094] Referring to Figures 8A - 8C , the hole 848 in the head 830 of this implementation is positioned in the exposed surface 834 in the form of a conical surface 834c. Similar to the case of the head 330 in Figure 3 , there are a plurality of facets on the surface 857 of the head 830 facing the intermediate focus, where 851a, 85lb, and 851c are visible. The hole 848 in the conical surface 834c extends more or less perpendicular to the conical surface 834c, such that the hole 848 is configured to generate a radially extending gas curtain when supplied with an air flow through a channel ( Figures 8A - 8C the channel is not shown in Figure 3 but see, for example, the channel 347 in Figures 1A - 1E ) in the shaft 829 of the shielding rod 827 during use. The flow direction of the radially extending gas curtain includes a radial component perpendicular to and away from the optical axis A, and an axial component parallel to the optical axis A and away from the intermediate focus (the intermediate focus is not shown but see, for example, Figure 8C andFigure 8B as indicated by the arrow in Figure 8B . The resulting gas curtain is substantially in the form of a conical fan, extending outwardly from the exposed surface 834 of the head 830 towards the inner surface 856 of the source container 811. As described in the above implementation, the holes are not located on or in the side surface 834i of the head 830.

[0095] Figures 8A - 8C The head 830 shown can be advantageously used in a source container including a plurality of exhaust ports extending through the source container, such as Figure 8B and Figure 8C the source container 811 of Figure 8C , where there are two exhaust holes 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 shielding rod can support the head 830, as Figure 8C shown.

[0096] Figure 9 is a cross-section taken from the same viewing angle as Figure 8C Figure 8C , showing an implementation of a source container 911 having four exhaust holes 933a - 933d and four corresponding exhaust ports 955a - 955d. (For a clearer view of the features shown, Figure 9 the base and shaft of the shielding rod are omitted in Figure 9 .) The head 930 also has an exposed surface 934 in the form of a conical surface 934c, and when in use, it generates a gas curtain substantially in the form of a conical fan (indicated by the arrows in Figure 9 Figure 9 ), which extends from the exposed surface of the head 930 outwardly to the inner surface 956 of the source container 911. The exposed surfaces 834, 934 in the form of conical surfaces (such as conical surfaces 834c, 934c) help to diffuse and / or widely distribute (i.e., avoid concentration) the power from the source laser (such as Figure 1A the source laser 112 of Figure 1A ) reaching the head 930. Other surface shapes can be used, and the holes, such as hole 848, can still be placed along a selected direction even within the exposed surfaces 834, 934 other than the conical surfaces to generate a radially extending gas curtain, such as Figure 8C and Figure 9 as indicated by the arrows in Figure 9 .

[0097] Figure 10 and Figure 11 Other implementations are shown in the cross-sections of Figure 11 , with views similar to those of Figure 8C and Figure 9 Figure 9 .

[0098] In Figure 10In the implementation shown, the source container 1011 with an inner surface 1056 includes a plurality of exhaust holes, in this case two exhaust holes 1033a, 1033b, which have corresponding plurality of exhaust ports 1055a, 1055b that extend through the source container 1011, similar to the source container 811 of FIG. 8. The head 1030 of the shielding rod (supported on a shaft not shown for the sake of observing the features shown in the figure) has two inclined facets 1034e, 1034d on an exposed surface 1034 in the form of an inclined surface (or double-inclined surface) 1034s of the head 1030. Each facet has a hole (not shown) that is configured to create a corresponding gas curtain for each respective exhaust port among the plurality of exhaust ports 1055a, 1055b in the source container 1011 when in use and when an air flow is supplied from a channel (not shown but see reference Figure 3 and the related description above). In the case of the implementation shown, two gas curtains are created, as indicated by the two sets of arrows.

[0099] In Figure 11 the implementation shown, the source container 1111 with an inner surface 1156 includes a plurality of exhaust holes, in this case two exhaust holes 1133a, 1133b, which have corresponding plurality of exhaust ports 1155a, 1155b that extend through the source container 1111, similar to the source container 811 of FIG. 8, but the exhaust holes 1133a, 1133b and the exhaust ports 1155a, 1155b are not symmetrically arranged within the source container 1111. In this implementation, the head 1130 of the shielding rod (supported on a shaft not shown in the figure) has two facets 1134f, 1134g on an exposed surface 1134 of the head 1130. The facets 1134f, 1134g are each positioned to at least partially face in the direction of a respective one of the plurality of exhaust ports 1155a, 1155b. Each facet has a hole (not shown) that is configured to generate a corresponding gas curtain when in use and when an air flow is supplied from a channel (not shown) in or on the support shaft, the corresponding gas curtain extending from the exposed surface 1134 and pointing towards each respective exhaust port among the plurality of exhaust ports 1155a, 1155b in the source container 1011. In the case of the implementation shown, two gas curtains are created, as indicated by the two sets of arrows.

[0100] Figure 12A A partial cross-section of another implementation of the source container 1211 of an EUV source is shown, the source container 1211 having an inner surface 1256. Figure 12B It is a cross-section taken along Figure 12A the line 12B shown in Figure 12A . Refer to Figure 12A and Figure 12B, the source container 1211 includes an annular exhaust hole 1233r, which has an associated annular exhaust port 1255r that surrounds the source container 1211 and extends through the source container, where the annular exhaust port is positioned between a collector (not shown, for example, see Figures 1A - 1E ) and the head 1230 when measured along the optical axis A. The exhaust port 1233r includes an annular scrubber 1260. As Figure 12B shown, the exhaust gas received in the annular exhaust port 1255r through the annular scrubber 1260 is removed through one or more vacuum ports (two in this implementation) 1262a, 1262b and the annular scrubber 1260, and the vacuum ports are connected to one or more vacuum pumps (not shown), as indicated by the arrows within the annular exhaust port 1255r and within the vacuum ports 1262a, 1262b.

[0101] Figure 12A and Figure 12B The shielding rod 1227 (including the head 1230 supported on the shaft 1229, which may include a base 1228) in Figures 8A - 8C can be implemented in the same or a similar manner as in Figures 8A - 8C , and the exposed surface 1234 on the head 1230 is in the form of a conical surface 1234c. The holes (not shown) in the conical surface 1234c can be configured to generate a gas curtain when in use and when supplied with an air flow through a channel (not shown) in the shaft 1229. The gas curtain extends radially from the exposed surface 1234, and its flow direction includes 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 focus (as indicated by the arrows in Figure 12A ) and close to Figure 12B the center. As with the other implementations described above, the holes are not located on the side surface 1234i of the head 1230.

[0102] These aspects and implementations can be further described using the following clauses:

[0103] 1. An extreme ultraviolet (EUV) source, comprising:

[0104] A source container that at least partially encloses the following volume: when in use, EUV light is transmitted from a main focus to an intermediate focus by a collector along an optical axis within the volume;

[0105] A shaft having a length extending from a first end of the shaft to a second end, the shaft including a channel that at least partially extends along the length of the shaft, the first end of the shaft being attached to an inner surface of the source container, and the second end being positioned within the source container;

[0106] A head, connected to the second end of the shaft, the head intersecting the optical axis, the head having an exposed surface exposed to the main focus, the exposed surface having one or more holes, the one or more holes being in fluid communication with the channel.

[0107] 2. The EUV source according to clause 1, wherein the exposed surface is an inclined surface.

[0108] 3. The EUV source according to clause 1, wherein the one or more holes are oriented in one or more directions having a component in a direction away from the intermediate focus along the optical axis and a component perpendicular to the optical axis.

[0109] 4. The EUV source according to clause 1, wherein the one or more holes include a plurality of nested annular holes.

[0110] 5. The EUV source according to clause 1, wherein the one or more holes include a plurality of non-overlapping holes.

[0111] 6. The EUV source according to clause 1, wherein the head and the shaft are integral.

[0112] 7. The EUV source according to clause 1, wherein the head has a cross-section taken perpendicular to the optical axis, the cross-section being circular and centered on the optical axis.

[0113] 8. The EUV source according to clause 1, wherein the head and the shaft comprise a refractory material.

[0114] 9. The EUV source according to clause 8, wherein the refractory material is a refractory metal.

[0115] 10. The EUV source according to clause 9, wherein the refractory metal includes tungsten.

[0116] 11. The EUV source according to clause 1, wherein the source container includes an exhaust port extending through the source container, the exhaust port being positioned between the collector and the head when measured along the optical axis.

[0117] 12. The EUV source according to clause 11, wherein the exposed surface is an inclined surface, the inclined surface being substantially oriented in the direction of the exhaust port and / or in the direction of a portion of the inner surface of the source container on the intermediate focus side of the exhaust port.

[0118] 13. The EUV source according to clause 11, wherein the hole is configured to: when in use and supplied with an air flow through the channel, generate a gas curtain having a flow direction that is from the exposed surface of the head towards the edge of the exhaust port closest to the intermediate focus and / or towards a portion of the inner surface of the source container, the portion of the inner surface being adjacent to the edge of the exhaust port closest to the intermediate focus.

[0119] 14. The EUV source according to clause 11, wherein the flow direction of the gas curtain has a component along the optical axis away from the intermediate focus.

[0120] 15. The EUV source according to clause 1, wherein the head does not have a surface that faces the intermediate focus perpendicularly.

[0121] 16. The EUV source according to clause 1, wherein the shaft does not have a surface that faces the intermediate focus perpendicularly.

[0122] 17. The EUV source according to clause 1, further comprising:

[0123] a target delivery system configured and positioned to deliver a target to the main focus of the collector, the target comprising a target material; and

[0124] a laser configured and positioned to generate a pulsed beam having a beam waist at or near the main focus of the collector.

[0125] 18. The EUV source according to clause 17, wherein the target material comprises xenon, lithium, or tin.

[0126] 19. The EUV source according to clause 18, wherein the target material comprises tin.

[0127] 20. The EUV source according to clause 18, further comprising a gas supply device connected to the channel, the gas comprising an inert gas or hydrogen.

[0128] 21. The EUV source according to clause 20, wherein the gas comprises hydrogen.

[0129] 22. The EUV source according to clause 17, wherein the collector comprises a central hole positioned to allow the pulsed beam to pass along the optical axis towards the main focus and the intermediate focus of the collector.

[0130] 23. The EUV source according to clause 22, wherein the head is positioned such that direct light from the main focus is not reflected by the collector onto the head.

[0131] 24. The EUV source according to clause 23, wherein the head directly shields the pulsed beam for the intermediate focus.

[0132] 25. The EUV source according to clause 17, wherein the head has an anti-reflective and / or diffusive geometry facing the main focus of the collector, such that the pulsed beam is reflected from the head in a diffusive manner without being concentrated at any location within the source container.

[0133] 26. The EUV source according to clause 25, wherein the anti-reflective and / or diffusive geometry of the head includes a substantially convex surface.

[0134] 27. The EUV source according to clause 17, wherein the shaft does not have a surface that is perpendicular to the intermediate focus.

[0135] 28. The EUV source according to clause 1, wherein the shaft does not have a surface that is perpendicular to the main focus.

[0136] 29. The EUV source according to clause 1, wherein when intercepted in a plane parallel to the optical axis and perpendicular to the length of the shaft, the shaft has an elongated cross-section, the long dimension of the cross-section being in a direction substantially parallel to the optical axis, and wherein the cross-section of the channel in a plane parallel to the optical axis and perpendicular to the length of the shaft extends in a direction substantially parallel to the optical axis.

[0137] 30. The EUV source according to clause 1, further comprising:

[0138] A target delivery system configured and positioned to deliver a target to the main focus of the collector, the target comprising a target material, the target delivery system including a shield that shields the path towards the main focus of the collector,

[0139] wherein when observed from the reflection of the main focus of the collector from the collector surface, the image of the shaft is aligned with the image of the shield.

[0140] 31. The EUV source according to clause 30, wherein when observed from the reflection of the main focus of the collector from the collector surface, the image of the shaft is blocked by the image of the shield.

[0141] 32. The EUV source according to clause 31, wherein when intercepted in a plane parallel to the optical axis and perpendicular to the length of the shaft, the shaft has an elongated cross-section, the long dimension of the cross-section being in a direction substantially parallel to the optical axis.

[0142] 33. The EUV source according to clause 1, wherein the source container includes an exhaust port that extends through one side of the source container, and the exhaust port is positioned between the collector and the head when measured along the optical axis.

[0143] 34. The EUV source according to clause 1, wherein the source container includes a plurality of exhaust ports that extend through the source container, and the exhaust ports are positioned between the collector and the head when measured along the optical axis.

[0144] 35. The EUV source according to clause 34, wherein the holes are configured to, when in use and supplied with an air flow through the channel, generate a respective gas curtain for each respective exhaust port of the plurality of exhaust ports, the respective gas curtain having a respective flow direction that is from the exposed surface of the head towards the edge of the respective exhaust port of the plurality of exhaust ports that is closest to the intermediate focus and / or towards a portion of the inner surface of the source container, the portion of the inner surface being adjacent to the edge of the respective exhaust port that is closest to the intermediate focus.

[0145] 36. The EUV source according to clause 34, wherein the holes are configured to, when in use and supplied with an air flow through the channel, generate a radially extending gas curtain that extends from the exposed surface of the head, the radially extending gas curtain having a flow direction that includes 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 focus.

[0146] 37. The EUV source according to clause 1, wherein the source container includes an annular exhaust port that surrounds and extends through the source container, and the annular exhaust port is positioned between the collector and the head when measured along the optical axis.

[0147] 38. The EUV source according to clause 37, wherein the holes are configured to, when in use and supplied with an air flow through the channel, generate a radially extending gas curtain that extends from the exposed surface of the head, the radially extending gas curtain having a flow direction that includes 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 focus.

[0148] 39. A method for reducing or preventing deposition on the interior of a source container in an extreme ultraviolet (EUV) light source, the method comprising:

[0149] Gas is supplied to a channel in a shadow bar, the shadow bar including a shaft and a head, a first end of the shaft being supported on an inner surface of a source container in an EUV light source, the source container surrounding an optical axis of the EUV light source, the optical axis extending from a collector through a main focus to an intermediate focus of the EUV light source, the head of the shadow bar located at a second end of the shaft intersecting the optical axis, the head having an exposed surface exposed to the main focus; and

[0150] The gas is caused to flow out through one or more holes in the exposed surface of the head of the shadow bar, the one or more holes being in fluid communication with the channel.

[0151] 40. The method according to clause 39, wherein the exposed surface is an inclined surface.

[0152] 41. The method according to clause 39, wherein the one or more holes are oriented in one or more directions having a component in a direction away from the intermediate focus along the optical axis and a component perpendicular to the optical axis.

[0153] 42. The method according to clause 39, wherein the head is integral with the shaft of the shadow bar.

[0154] 43. The method according to clause 39, wherein the head has a cross-section taken perpendicular to the optical axis, the cross-section being circular and centered on the optical axis.

[0155] 44. The method according to clause 39, wherein the head does not have a surface perpendicular to and facing the intermediate focus.

[0156] 45. The method according to clause 39, wherein the head and the shaft comprise a refractory material.

[0157] 46. The method according to clause 39, wherein the refractory material is a refractory metal.

[0158] 47. The method according to clause 43, wherein the refractory metal is tungsten.

[0159] 48. The method according to clause 39, wherein the source container includes an exhaust port that extends through the source container, the exhaust port being positioned between the collector and the head when measured along the optical axis, the method further including causing gas to flow from inside the source container through the exhaust port.

[0160] 49. The method according to clause 48, wherein the exposed surface is an inclined surface that generally faces in the direction of the exhaust port and / or in the direction of a portion of the inner surface of the source container on the intermediate focus side of the exhaust port.

[0161] 50. The method according to clause 48 further includes: generating a gas curtain that includes the gas flowing out through one or more holes in the exposed surface of the head, the gas curtain extending from the exposed surface of the head to the exhaust port and / or a portion of the inner surface of the source container on the intermediate focus side of the exhaust port.

[0162] 51. The method according to clause 50, wherein the gas curtain extends in a direction that has a component away from the intermediate focus along the optical axis.

[0163] 52. The method according to clause 50 further includes: introducing, at or near the intermediate focus, an intermediate focus protection gas flow that flows along the optical axis toward the collector.

[0164] 53. The method according to clause 52, wherein generating the gas curtain includes: splitting the intermediate focus protection gas flow at the head and combining the intermediate focus protection gas flow with the gas flowing out through one or more holes in the exposed surface of the head to form the gas curtain.

[0165] 54. The method according to clause 50 further includes:

[0166] delivering a target to the main focus of the collector, the target including a target material that has a melting point;

[0167] irradiating the target with a light pulse at the main focus of the collector to form a plasma at the main focus of the collector, the plasma emitting EUV light; and maintaining at least a portion of the source container at one or more temperatures below the melting point of the target material.

[0168] 55. The method according to clause 54, wherein maintaining at least a portion of the source container at one or more temperatures below the melting point of the target material includes: maintaining at least a portion of the source container at a temperature in the range of 50°C to 200°C.

[0169] 56. The method according to clause 39, wherein causing the gas to flow out through one or more holes in the exposed surface of the head of the shielding rod includes: suppressing or preventing the gas flow in the direction away from the collector from skimming over the exhaust port such that the gas flow in the direction away from the collector enters the exhaust port.

[0170] 57. The method according to clause 51 includes suppressing or preventing an air flow in a direction away from the collector from skimming over the exhaust port during a time period extending 20 milliseconds from when light pulse irradiation of the target in the source container is stopped.

[0171] 58. The method according to clause 51 includes suppressing or preventing an air flow in a direction away from the

[0172] collector from skimming over the exhaust port during a time period extending 20 milliseconds from when light pulse irradiation of the target in the source container is started.

[0173] The above implementation and other implementations are within the scope of the appended claims.

Claims

1. An extreme ultraviolet (EUV) source, comprising: A source container that at least partially encloses a volume in which, during use, EUV light is transmitted by a collector along an optical axis from a primary focus to an intermediate focus; An axis having a length extending from a first end of the axis to a second end, the axis including a channel that extends at least partially along the length of the axis, the first end of the axis being attached to an inner surface of the source container, and the second end being positioned within the source container; A head connected to the second end of the axis, the head intersecting the optical axis, the head having an exposed surface that is exposed to the primary focus, the exposed surface having one or more holes that are in fluid communication with the channel.

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 holes are oriented along one or more directions having a component in a direction away from the intermediate focus 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 holes include a plurality of nested annular holes.

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

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

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

8. The EUV source according to claim 1, wherein the head and the shaft comprise a refractory material.

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

10. The EUV source according to claim 9, wherein the refractory metal includes tungsten.

11. The EUV source according to claim 1, wherein the source container includes an exhaust port extending through the source container, the exhaust port being positioned between the collector and the head when measured along the optical axis.

12. The EUV source according to claim 11, wherein the exposed surface is an inclined surface that is substantially oriented in the direction of the exhaust port and / or in the direction of a part of the inner surface of the source container, the part of the inner surface being on the intermediate - focus side of the exhaust port.

13. The EUV source according to claim 11, wherein the holes are configured to produce a gas curtain when in use and supplied with an air flow through the channel, the gas curtain having a flow direction from the exposed surface of the head towards the edge of the exhaust port closest to the intermediate focus and / or towards a part of the inner surface of the source container, the part of the inner surface being adjacent to the edge of the exhaust port closest to the intermediate focus.

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

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

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

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

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 device connected to the channel, the gas comprising 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 hole positioned to allow the pulsed beam to pass along the optical axis towards the main focus and the intermediate focus of the collector.

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

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

25. The EUV source according to claim 17, wherein the head has an anti-reflective and / or diffusive geometry facing the main focus of the collector such that the pulsed beam is reflected from the head in a diffusive manner and not concentrated at any location within the source container.

26. The EUV source according to claim 25, wherein the anti-reflective and / or diffusive 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 focus.

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

29. The EUV source according to claim 1, wherein when intercepted in a plane parallel to the optical axis and perpendicular to the length of the axis, the axis has an elongated cross-section, the long dimension of the cross-section being in a direction substantially parallel to the optical axis, and wherein the cross-section of the channel in a plane parallel to the optical axis and perpendicular to the length of the axis is elongated in a direction substantially parallel to the optical axis.

30. The EUV source according to claim 1, further comprising: A target delivery system configured and positioned to deliver a target to the primary focus of the collector, the target including a target material, the target delivery system including a shield that shields a path toward the primary focus of the collector, Wherein an image of the axis is aligned with an image of the shield when viewed from the primary focus of the collector in reflection from the surface of the collector.

31. The EUV source according to claim 30, wherein when viewed from the principal focus of the collector in reflection from the collector surface, the image of the axis is occluded by the image of the shroud.

32. The EUV source according to claim 31, wherein when intercepted in a plane parallel to the optical axis and perpendicular to the length of the axis, the axis has an elongated cross-section, the long dimension of the cross-section being in a direction substantially parallel to the optical axis.

33. The EUV source according to claim 1, wherein the source vessel includes an exhaust port extending through one side of the source vessel, the exhaust port being positioned between the collector and the head when measured along the optical axis.

34. The EUV source according to claim 1, wherein the source vessel includes a plurality of exhaust ports extending through the source vessel, the exhaust ports being positioned between the collector and the head when measured along the optical axis.

35. The EUV source according to claim 34, wherein the holes are configured to, when in use and supplied with an air flow through the channel, produce a respective gas curtain for each respective exhaust port of the plurality of exhaust ports, the respective gas curtain having a respective flow direction, the respective flow direction being from the exposed surface of the head towards the edge of the respective exhaust port of the plurality of exhaust ports closest to the intermediate focus and / or towards a portion of the inner surface of the source vessel, the portion of the inner surface being adjacent to the edge of the respective exhaust port closest to the intermediate focus.

36. The EUV source according to claim 34, wherein the holes are configured to, when in use and supplied with an air flow through the channel, produce a radially extending gas curtain extending from the exposed surface of the head, the radially extending gas curtain having a flow direction including 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 focus.

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

38. The EUV source according to claim 37, wherein the hole is configured to generate a radially extending gas curtain when in use and when an air flow is supplied through the channel, the radially extending gas curtain extending from the exposed surface of the head, the radially extending gas curtain having a flow direction that includes 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 focus.

39. A method of reducing or preventing deposition on an interior of a source container in an extreme ultraviolet EUV light source, the method comprising: Supply gas to a channel in a shielding rod that includes an axis and a head, the first end of the axis being supported on an inner surface of a source container in an EUV light source, the source container surrounding an optical axis of the EUV light source that extends from a collector through a primary focus to an intermediate focus of the collector, the head of the shielding rod at the second end of the axis intersecting the optical axis, the head having an exposed surface that is exposed to the primary focus; And Cause the gas to flow out through one or more holes in the exposed surface of the head of the shielding rod, the one or more holes being in fluid communication with the channel.

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

41. The method according to claim 39, wherein the one or more holes are oriented along one or more directions that have a component in a direction away from the intermediate focus along the optical axis and a component perpendicular to the optical axis.

42. The method according to claim 39, wherein the head and the shaft of the shielding rod are integral.

43. The method according to claim 39, wherein the head has a cross-section taken perpendicular to the optical axis, the cross-section being circular and centered on the optical axis.

44. The method according to claim 39, wherein the head does not have a surface that faces the intermediate focus perpendicularly.

45. The method according to claim 39, wherein the head and the shaft comprise a refractory material.

46. The method according to claim 39, wherein the refractory material is a refractory metal.

47. The method according to claim 46, wherein the refractory metal is tungsten.

48. The method according to claim 39, wherein the source container includes an exhaust port that extends through the source container, the exhaust port being positioned between the collector and the head when measured along the optical axis, the method further comprising: Cause gas to flow through the exhaust port from inside the source container.

49. The method according to claim 48, wherein the exposed surface is an inclined surface, the inclined surface being substantially oriented in the direction of the exhaust port and / or in the direction of a part of the inner surface of the source container, the part of the inner surface being on the intermediate focus side of the exhaust port.

50. The method according to claim 48, further comprising: Generate a gas curtain that includes the gas flowing out through one or more holes in the exposed surface of the head, the gas curtain extending from the exposed surface of the head to the exhaust port and / or a portion of the inner surface of the source container on the intermediate focus side of the exhaust port.

51. The method according to claim 50, wherein the gas curtain extends in a direction having a component along the optical axis away from the intermediate focus.

52. The method according to claim 50, further comprising: At or near the intermediate focus, introduce an intermediate focus protection gas flow that flows toward the collector along the optical axis.

53. The method according to claim 52, wherein generating the gas curtain comprises: Split the intermediate focus protection gas flow at the head and combine the intermediate focus protection gas flow with the gas flowing out through one or more holes in the exposed surface of the head to form the gas curtain.

54. The method according to claim 50, further comprising: Deliver a target to the primary focus of the collector, the target including a target material having a melting point; Irradiate the target with a light pulse at the primary focus of the collector to form a plasma at the primary focus of the collector, the plasma emitting EUV light; and maintaining at least a portion of the source container at one or more temperatures below the melting point of the target material.

55. The method according to claim 54, wherein maintaining at least a part 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 within the range of 50°C to 200°C.

56. The method according to claim 39, wherein causing the gas to flow out through one or more holes in the exposed surface of the head of the shielding rod comprises: Inhibiting or preventing an air flow in a direction away from the collector from passing through the exhaust port, such that the air flow in the direction away from the collector enters the exhaust port.

57. The method according to claim 51, further comprising: During a time period extending 20 milliseconds from when light pulse irradiation of the target in the source container is stopped, inhibiting or preventing the air flow in the direction away from the collector from passing through the exhaust port.

58. The method according to claim 51, further comprising: During a time period extending 20 milliseconds from when light pulse irradiation of the target in the source container is started, inhibiting or preventing the air flow in the direction away from the collector from passing through the exhaust port.

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