EUV source vessel heated gas delivery apparatus and method

Abstract

A system for generating extreme ultraviolet (EUV) radiation includes a source vessel having an interior volume, a source laser configured to generate light to enter the interior volume in an entering light direction along a light axis to provide energy to a target to cause the generation of EUV within the interior volume, and a heated gas delivery apparatus (336) within the interior volume configured to deliver heated gas into the interior volume and positioned on and extending along the light axis, the heated gas delivery apparatus having outlets to dispense the heated gas into the interior volume and one or more inlet openings to receive light from the source laser to heat the heated gas delivery apparatus. A process for providing EUV radiation is also disclosed.

Description

EUV SOURCE VESSEL HEATED GAS DELIVERY APPARATUS AND METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 514,455, filed July 19, 2023, titled EUV SOURCE VESSEL HEATED GAS DELIVERY APPARATUS AND METHOD, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure relates to an extreme ultraviolet (“EUV”) source vessel having an apparatus for delivering heated gas into the source vessel, and to methods of delivering heated gas into the source vessel.BACKGROUND

[0003] Extreme ultraviolet (“EUV”) light, i.e., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), including radiation at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in or on substrates such as silicon wafers. Methods for generating EUV light include converting a target material to a plasma state. The target material includes at least one element, for example, xenon, lithium, or tin, with 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, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a “source” laser, typically a CO2 laser emitting infrared light at a wavelength at or about 10,600 nanometers (nm), to irradiate with one or more light pulses a target containing one or more EUV-line emitting elements. The plasma is typically produced in a sealed “source vessel” which is typically a vacuum chamber.

[0004] Producing the plasma from the target material inside the source vessel can generate vapor, ions, and micro particles and other debris that can deposit on and contaminate surfaces inside the source vessel, including optical surfaces. Accordingly, there is a need to remove residual target material(s) from the source vessel to reduce or eliminate surface contamination within the source vessel, a need typically met at least in part by the use of gas flows introduced into and then removed, along with the residual target material(s), from the source vessel.SUMMARY

[0005] In some general aspects, a system for generating extreme ultraviolet (EUV) radiation is provided, the system including: a source vessel having an interior volume; a source laser configured to generate light to enter the interior volume, in an entering light direction along a light axis, to provide energy to a target to cause the generation of EUV within the interior volume; and a heated gas delivery apparatus disposed within the interior volume and configured to deliver heated gas into theinterior volume, the heated gas delivery apparatus positioned on and extending along the light axis, the heated gas delivery apparatus having outlets to dispense the heated gas into the interior volume and one or more inlet openings to receive light from the source laser to heat the heated gas delivery apparatus.

[0006] Implementations can include one or more of the following features. The heated gas delivery apparatus can further include an interior surface having a reflectivity of the light of the source laser equal to or less than 0.9 to absorb at least some of the light received from the source laser through the one or more inlet openings. The heated gas delivery apparatus can further include a tapering interior surface to receive, partially reflect one or more times, and at least partially absorb the light received from the source laser through the one or more inlet openings. The tapering interior surface can be or can include a conical interior surface.

[0007] The system can further include a heated gas source connected to the heated gas delivery apparatus to deliver heated gas to the heated gas delivery apparatus. The heated gas source can be positioned outside the interior volume.

[0008] The heated gas delivery apparatus can further include a gas conduit connected to the outlets. The heated gas delivery apparatus can further include a heating element configured to heat the heated gas delivery apparatus and / or gas delivered by the heated gas delivery apparatus.

[0009] The system can further include a collector configured to reflect EUV light generated at or near a primary focus of the collector to an intermediate focus of the collector, with the collector having an optical axis essentially coincident with the light axis and a through -hole along the optical axis configured to allow the light to enter the interior volume, such that most or all EUV light reflected from the collector travels from the collector to the intermediate focus (1) inside a volume of a first approximate cone having as a base the perimeter of the collector and having as a vertex the intermediate focus, and (2) outside a volume of a second approximate cone having the through-hole as a base and the intermediate focus as a vertex, and wherein the heated gas delivery apparatus is disposed along the light axis within the second approximate cone. The heated gas delivery apparatus can have a conical exterior surface that is spaced from the surface of the second approximate cone when the heated gas delivery apparatus is at an operating temperature, such that the exterior surface does not impinge upon the second approximate cone when in operation.

[0010] The heated gas delivery apparatus can extend for a distance within a range of 90 to 110 centimeters along the light axis. The heated gas delivery apparatus can extend for a distance along the light axis within a range of 50% to 70% of a distance from the primary focus to the intermediate focus along the light axis. At least some of the outlets of the heated gas delivery apparatus can be positioned in a conical exterior surface of the heated gas delivery apparatus, with the conical exterior surface narrowing in a direction along the light axis and extending away from the entering light direction.

[0011] In some additional general aspects, a process for providing extreme ultraviolet (EUV) radiation, is provided, the process including: within the interior volume of a source vessel, irradiatinga target with light from a source laser to generate an EUV -producing plasma; producing reflected EUV light within the interior volume by reflecting EUV light from the plasma along a reflected EUV path; receiving light from the source laser into a heated gas delivery apparatus to heat the heated gas delivery apparatus; and delivering heated gas from the heated gas delivery apparatus into the interior volume of the source vessel along the reflected EUV path.

[0012] Implementations can include one or more of the following features. A side of the heated gas delivery apparatus can include outlets. The heated gas delivery apparatus can include or can have a conical shape narrowing in a direction toward an intermediate focus of the collector. The heated gas delivery apparatus can extend a distance within the range of 90 centimeters to 110 centimeters in a direction parallel to a light axis of the source laser. The heated gas delivery apparatus extends in a direction parallel to a light axis of the source laser for 50% to 70% of a distance from a primary focus to an intermediate focus of the collector.

[0013] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.DRAWING DESCRIPTION

[0014] FIG. 1A is a cross-sectional schematic diagram of aspects of an extreme ultraviolet (EUV) light source.

[0015] FIG. IB is a cross-sectional schematic diagram of the EUV light source of FIG. 1A, rotated 90 degrees about the z axis and rotated such that gravity is represented downward in the plane of the page.

[0016] FIG. 2 is a block diagram of an EUV source in use with a photolithography exposure apparatus.

[0017] FIG. 3A is a perspective view of an implementation of a heated gas delivery apparatus that can be positioned in the EUV light source of FIGS. 1A or IB.

[0018] FIG. 3B is a perspective view of the heated gas delivery apparatus of FIG. 3A, rotated 90 degrees about the z axis relative to the view of FIG. 3A.

[0019] FIG. 3C is a cross-sectional view of the heated gas delivery apparatus of FIG. 3B shown as taken along the line 3C-3C of FIG. 3A.

[0020] FIG. 4 is a cross-sectional view similar to FIG. 3C of another implementation of a heated gas delivery apparatus.

[0021] FIG. 5 is a cross-sectional schematic diagram of a collector and a heated gas delivery apparatus.

[0022] FIG. 6 is a flowchart of a procedure for operating a light source including a heated gas delivery apparatus such as in FIGS. 1A, IB, 3A, 3B, 4, 5, or 6.DETAILED DESCRIPTION

[0023] FIG. 1A is a simplified schematic cross-sectional view of some components of an implementation of an EUV light source 110. As shown by the reference coordinate axis, the cross section of the EUV light source 110 in FIG. 1A is taken in an x-z plane, with x positive in the upward direction in the plane of the page, and z positive to the right in the plane of the page, the z axis aligned with a light axis A of a source laser 112 to be described below.

[0024] As shown in FIG. 1A, the EUV light source 110 includes the source laser 112 for generating a beam 113 of light pulses (for example, laser pulses) and delivering the beam 113 from the source laser 112 into the interior volume 114 of a source vessel 111 to individually irradiate targets 115 at or near an irradiation site 116. The targets 115 travel 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 vessel 111 has an interior surface 156 surrounding the interior volume 114.

[0025] As also shown in FIG. 1A, the EUV light source 110 includes the target delivery system 117a that delivers the targets 115 into the interior 114 of the source vessel 111 to the irradiation site 116. At the irradiation site 116, the targets 115 individually interact with one or more light pulses (of the beam 113) to produce a plasma 118 that produces EUV light 119 (represented in the figure by bounding rays indicated by the reference character 119). Radiation from the plasma 118, positions of the targets 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 control and operation of the EUV light source 110.

[0026] The targets 115 can be delivered along at least part of their travel through a target shroud 115s. The shroud 115s can be in the form of a tube (which can have apertures for metrology) or other shielding structure that shields or partially shields incoming targets 115 from gasses and other materials in the interior volume 114 of the source vessel 111, such that the trajectory of the targets 115 is not excessively disturbed by such gasses or other materials. Unused targets (such as those that are not converted into plasma 118) of the targets 115 can be captured in a target trap 117b.

[0027] The targets 115 are or include an EUV emitting target material such as, but not necessarily limited to, tin, lithium, xenon, or combinations thereof. The targets 115 can be in the form of liquid droplets, or alternatively can be solid particles or solid particles contained within liquid droplets. For example, the element tin can be presented as a target in the form of pure tin; a tin compound such as SnBr4, SnBr2, SnH : a tin alloy, e.g., tin-gallium alloys, tin-indium alloys; or tin -indium -gallium alloys; or a combination thereof.

[0028] The EUV light source 110 can also include a collector 120 that redirects the EUV light 119 as redirected EUV light 124. The collector 120 can be a near-normal incidence collector mirror having an optical axis coincident with the light axis A and a reflective surface 121. The reflective surface 121 can be in the form of a prolate spheroid (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 focusat a so-called intermediate focus 123, with the optical axis of the collector defined as a line extending between them along light axis A. The source vessel 111 of the EUV light source 110 thus encloses, at least in part, the volume 114 in which, when the EUV light source 110 and source vessel 111 are in use, EUV light 119 is redirected (reflected) by the collector 120 as EUV light 124 from the primary focus 122 to the intermediate focus 123 along the axis A. Reflected EUV light 124 from the collector120 can be output from the EUV light source 110 at the intermediate focus 123 and input to a device utilizing the EUV light 124, such as a lithography exposure apparatus (as shown in FIG. 2). The collector 120 is formed with an aperture in the form of a through -hole 125 through the collector 120 to allow the beam 113 of light pulses generated by the source laser 112 to pass through the through- hole 125 and reach the irradiation site 116. The through-hole 125 creates a shadow or voluminous gap 154 along the optical axis A in the reflected EUV light 124 from the collector 120.

[0029] In order to reflect the EUV light 119, the collector 120 can be in the form of a multi-layer mirror (MLM), with the reflective surface 121 having a graded multilayer coating with alternating layers of molybdenum and silicon, and in some cases, one or more high temperature diffusion barrier layers, smoothing layers, capping layers and / or etch stop layers. Other surface shapes besides the prolate spheroid can also be used for the reflective surface 121. For example, the reflective surface121 can alternatively be in the form of a parabola rotated about its major axis. In implementations, the reflective surface 121 can be configured to deliver a beam of EUV light 124 having a ring-shaped or other cross section at the intermediate focus 123. In other implementations, the reflective surface 121 can utilize coatings and layers other than or in addition to those described above.

[0030] The collector 120 can be expensive to fabricate. The efficiency and power of the light produced by the EUV light source 110 depend upon the quality of the reflective surface 121 of the collector 120. For these and other reasons, it is desirable to protect the collector 120 from damage to its reflective surface 121.

[0031] However, the collector 120 must be placed within the source vessel 111 within the volume 114 and proximate to the plasma 118 in order to collect and redirect the EUV light 119. Structures within the source vessel 111, including the collector 120, can be exposed to high energy ions and / or particles and vapor of or containing target material. The particles of target material and high energy ions and vapor, which are essentially debris or byproducts from a light-based vaporization or ablation process, can contaminate the collector’s exposed reflective surface 121. Particles of target material and energetic ions and vapor can also cause physical damage and localized heating of the reflective surface 121 of the collector 120.

[0032] As also shown in FIG. 1A, the EUV light source 110 can include a focusing unit 126 that includes one or more optical elements (not shown) for focusing the light beam 113 to a focal spot or beam waist at or near the irradiation site 116.

[0033] FIG. 2 is a diagram showing an implementation of an EUV light source 210 such as EUV light source 110 of FIG. 1 A or another EUV source, with a lithography exposure apparatus 271 . Thelithography exposure apparatus 271 receives EUV light 224 produced by the EUV light source 210 and reflects it in one or more illumination mirrors 272 so as to illuminate a reflective pattern or reticle 273. EUV light reflected from the pattern or reticle 273 is further reflected and reduced by one or more reducing mirrors 274 and irradiated on a substrate or wafer 275 (or on one or more photosensitive layers on the substrate or wafer 275, not shown) to allow the formation of patterned structures in or on the substrate or wafer 275.

[0034] With reference to FIGS. 1A and 2, optical elements and sensors within the lithography exposure apparatus 271, as well as the photosensitive layers on the substrate or wafer 275, are typically sensitive to many types or even to any type of radiation. It is therefore important, especially given high power levels produced by the source laser 112 of FIG. 1A, to prevent any part of the beam 113 of light pulses from source laser 112 (including the beam 113e shown in FIG. 1A that corresponds to the portion of the beam 113 that extends beyond the irradiation site 116) from reaching the intermediate focus 123 and potentially entering a lithography exposure apparatus such as lithography exposure apparatus 271.

[0035] To this end, as shown in FIG. 1A, a beam blocking element such as an obscuration bar 127 of the present disclosure can be used. The obscuration bar 127 can include a base 128, a shaft 129 extending from the base 128, and a head 130 supported on the shaft 129. In use of the obscuration bar 127, the head 130 is positioned on axis A, as shown, such that the optical axis (and light axis) A intersects the head 130. Moreover, the head 130 can be positioned and sized to fit within the shadow or voluminous 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, which is circular and centered on the optical axis A and matched to the shadow or voluminous gap 154. This geometry prevents the head 130 from blocking any, or any significant part, of the EUV light 124 that is reflected from the collector 120 and directed toward the lithography exposure apparatus 121, while simultaneously wellprotecting the intermediate focus 123 from direct illumination by the beam 113 of pulses of the source laser 112. Expressed in other terms (regarding positioning in the shadow or gap 154), the head 130 is positioned such that little or no direct EUV light 119 from the primary focus 122 is reflected by the collector 120 as EUV light 124 to the head 130.

[0036] In source vessels in which a target shroud 115s is used, as shown, the shaft 129 of the obscuration bar 127 can be aligned with the shroud 115s, that is, it can be positioned as much as possible within a shadow created by the shroud 115s in the reflected EUV light 124. Expressed in other terms, an image of the shaft 129 can be aligned with an image of the shroud 115s, when viewed from the primary focus 122 of the collector 120 in reflection from the collector surface 121. In some implementations, the shaft 129 can be completely hidden in the shadow of the shroud 115s, as when the image of the shaft 129 is hidden by the image of the shroud 115s when viewed from the primary focus 122 of the collector 120 in reflection from the collector surface 121. This arrangement reducesor eliminates reflected EUV light 124 being prevented from exiting the EUV light source 110 by the shaft 129.

[0037] A heated gas delivery apparatus 136 can be attached to (or, as shown and discussed below, formed as a single piece with) the head 130 of the obscuration bar 127. As will be shown below, the head 130 of the obscuration bar 127 can be attached to or be formed as a single piece with the shaft 129. A gas supply passage 131 is connected to the base 128 and shaft 129 of the obscuration bar 127 and to a source 13 la of gas, such as hydrogen (H2) gas 132, allowing the obscuration bar 127 to be used to supply heated gas to the interior volume 114 of the source vessel through the heated gas supply apparatus 136 represented by the arrows HG. The heated gas represented by the arrows HG is heated by light received into the heated gas delivery apparatus 136 as will be shown below. Some additional heating will occur from source laser light and EUV light received on exterior surfaces of the heated gas delivery apparatus 136. Optionally, additional heating can be provided, such as by a heater 13 Ih at or near the gas source 13 la and / or the gas supply passage 131, as shown, or by resistive heating in the obscuration bar 127 and / or in the obscuration bar head 130 or heated gas delivery apparatus 136, described further below.

[0038] FIG. IB is a simplified schematic cross-sectional view of the EUV light source 110 of FIG. 1, rotated 90 degrees around the z axis to show a cross section in a y-z plane (as indicated by the reference coordinate axis) and rotated such that gravity is downward in the plane of the page. When in use, the EUV source 110 can be inclined with respect to gravity as shown, though other orientations can also be used. In this view, the shaft 129 and the base 128 of the obscuration bar 127 are behind the head 130 into the page. Also in this view, an exhaust port 133 and an associated exhaust opening 155 are visible. As shown, the exhaust port 133 is a structure that extends from the source vessel 111 and defines the exhaust opening 155 that is in fluid communication with, and extends out from, the interior 114 of the source vessel 111. Gases and entrained ions, vapor, and debris can be evacuated from the source vessel 111 by one or more vacuum pumps (not shown) through the exhaust opening 155 of the exhaust port 133. The exhaust opening 155 is positioned, measured along the optical axis A, between the collector 120 and the head 130.

[0039] As also shown in the implementation of FIG. IB, the heated gas delivery apparatus 136 is formed as a unit with the head 130 of the obscuration bar 127. The heated gas delivery apparatus 136 in this implementation thus includes a surface or “exposed surface” 134 exposed to the primary focus 122. The exposed surface 134 can be or can include a slanted surface 134s, meaning a surface that is not perpendicular to the axis A, and can be facing generally in the direction of the exhaust opening 155 of the exhaust port 133 and / or in the direction of a portion 135 of the interior surface 156 of the source vessel 111 on the intermediate focus side of the exhaust opening 155.

[0040] Various gas flows that can be used in the EUV light source 110 are represented in the figure by outline-style arrows. Gas flows such as flows of hydrogen (H2) gas at pressures in the range of about 50 to about 300 Pascals (Pa) can be used within the source vessel 111 as a buffer gas for debrisand / or vapor control. Given that a vacuum is needed in the interior 114 the source vessel 111 to avoid gas molecules excessively absorbing the EUV light 119, 124, it would be difficult to protect the collector 120 adequately from target material debris and vapor emanating from the irradiation site 116 without the use of such gas flows. Hydrogen (H2) is relatively transparent to EUV light having a wavelength of about 13.5 nm, and so is generally preferred over other candidate gases such as helium, argon, and other gases that exhibit a higher absorption at about 13.5 nm.

[0041] H2 gas can be introduced into the source vessel 111 to slow down and guide energetic debris (ions, atoms, and clusters) of target material created by irradiation of targets 115 and irradiation site 116 and by the resulting plasma 118. The debris is slowed down by collisions with the gas molecules. A flow CF of H2 gas at the center aperture 125 of the collector 120 can be used for this purpose. Sometimes known as a “cone flow” CF, the flow CF can be guided by a tube or nozzle 137 or the like from the aperture 125 at the center of the collector 120 toward the irradiation site 116 at which the plasma 118 is repeatedly created. This direction is counter to a debris trajectory from the irradiation site 116 toward the collector 120, and the cone flow CF thus serves to reduce damage to the collector 120 caused by vapor deposition, implantation, and deposition of sputtered target material.

[0042] A gas flow that is often referred to as an umbrella flow 139 can be directed along the surface of the collector 120 (from outlets not shown). So-called showerhead flows, in which gas flows through multiple parallel apertures generally perpendicular to the surface to be protected, such as showerhead flow S 1 and showerhead flow S2, can be provided in areas of the source vessel 111 nearest the collector 120. In additional regions such as regions near the intermediate focus 123, protective gas flows parallel to, or having a component of flow directed parallel to, the surface to be protected can be introduced through apertures aimed in directions having a 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 interior surface 156 of the source vessel 111 in regions near the intermediate focus 123.

[0043] A gas flow often referred to as a dynamic gas lock (“DGL”) is one or more gas flows used to prevent any material leaving the EUV source 110 in the region of the intermediate focus 123. A DGL can produce a gas flow such as DGL flow 138 from the area of the intermediate focus 123 toward the irradiation site 116, which flow 138 can also be termed an “intermediate-focus-protecting” gas flow 138.

[0044] A stable guided debris-containing flow 140 flowing away from the collector 120 can be formed and maintained by the cone flow CF, together with the umbrella flow 139 and the showerhead flows SI and S2 (and optionally others not shown). The solid curved lines in FIG. 1C illustrate example boundaries of the debris-containing guided flow 140. This guided flow 140 helps contain and carry away from the collector 120 materials, including vapor, ions, and micro and nanoparticles, generated from the targets 115 during production of plasma 118.

[0045] An opposing flow 141 moving from the intermediate focus 123 toward the collector 120 can be formed mainly by the DGL flow 138 together with the heated gas from the heated gas delivery apparatus 136 (arrows HG) and flows such as flows (“curtain” flows) Fl, F2, F3, and F4 (and optionally others not shown). The curved boundary line 141b, together with the interior surfaces 156 of the vessel 111 in the upper parts of the figure, represent example boundaries of the opposing flow 141. Heated gas (arrows 143) can also be flowed from the exposed surface 134 or exposed slanted surface 134s.

[0046] Given the low pressures used within the source vessel 111, pressure differentials at the exhaust opening 155 of the exhaust port 133 are not large. But a small pressure differential at the exhaust opening 155 produced by vacuum pumping the exhaust port 133, together with flow momentum balances between the guided flow 140 and the opposing flow 141 at a merging region 142 of the two flows 140, 141, with the merging region 142 being near the exhaust opening 155, can create a stable guided flow of target material byproducts entrained and contained in the guided flow 140 into the exhaust opening 155 without the target-material byproducts substantially contacting any inner surfaces of the source vessel 111.

[0047] When targets 115 that are tin or tin-containing are used, the use of hydrogen gas (such as in the cone flow CF) with such targets 115 results in another potential source of contamination in the source vessel 111. This is the ejection or “spitting” of molten tin, from surfaces in the vessel coated or subject to coating with molten tin, when hydrogen bubbles form and grow in or under the molten tin and then burst.

[0048] One way to prevent tin spitting is to prevent molten target material from accumulating on a surface in the source vessel 110 is by keeping the surface below or well below the melting point of the target material, which fortin is about 232 °C. For example, some portions of the interior surface 156 of the source vessel 111 can be maintained at a temperature below 232 °C, such as a temperature in the range of 50 °C to 110 °C or 50 °C to 100 °C. Any tin which deposits on such a surface is kept in solid form and prevents or resists spitting.

[0049] Use of “cold” surfaces, while reducing spitting, can also decrease the temperature and increase the density of the gas in the interior volume 114 of the vessel 111. Higher gas density inside the interior volume 114 increases absorption of the EUV light, particularly for reflected EUV light 124 (FIG. 1A) reflected from the collector 120 passing through the typically cooler “upper” areas of the interior volume 114 (toward the top of the page of FIG. IB).

[0050] Delivery of heated gas represented by arrows HG from the into the interior volume 114 of the vessel 111 from the side(s) of the heated gas delivery apparatus 136, and optionally from the exposed surface 134, 134s, helps provide a region 144 of lower density gas (illustrated by the small -dash boundaries) through which a significant portion of the EUV light 124 traveling from the collector 120 to the intermediate focus 123 can pass. In practice, there are usually not sharp boundaries betweenheated and unheated gas in the interior volume 114; rather, there can be a temperature gradient with higher temperatures nearer the heated gas delivery apparatus and the flows represented by arrows HG. Accordingly, the small-dash boundary in FIG. IB represents an example region of relatively higher temperature and lower density gas. The lower density gas in the region 144 thereby reduces absorption of the EUV light 124 and this allows increased brightness of the EUV light produced (the EUV light 124 that is output) by the EUV source 110, increasing the performance and economy of operation of the EUV source 110 and of downstream devices such as the lithography apparatus 217 (FIG. 2). The heated gas delivery apparatus 136 can also be used within a vessel having “hot” interior surfaces if desired, to obtain similar benefits from density gas in the region around the heated gas delivery apparatus 136. The heated gas delivery apparatus 136 can include or be formed from high temperature metals such as molybdenum, tungsten, rhenium, and their alloys. Ceramic materials having relatively good conductivity and formability can also be used, such as aluminum nitride, for example.

[0051] FIG. 3 A is a perspective view of an implementation of a heated gas delivery apparatus 336 that can be positioned in the EUV light source 110 of FIGS. 1A or IB, and used as heated gas delivery apparatus 136. The heated gas delivery apparatus 336 (or obscuration bar head 330) is shown in a view corresponding to that of heated gas delivery apparatus 136 (or obscuration bar head 130) of FIG. IB, but with the z-axis to the right in the plane of the page of FIG. 3 A. FIG. 3B is a perspective view of the heated gas delivery apparatus 336 and the obscuration bar head 330 rotated 90 degrees about the z axis relative to FIG. 3A, such that the obscuration bar 327 as a whole is visible, including a shaft 329 and a base 328 in addition to the head 330. The shaft 329 includes facets 352a and 352c (and corresponding facets on the side not visible) such that the shaft 329 tapers or narrows toward the positive and toward the negative z-axis direction. FIG. 3C is a cross-sectional view of the heated gas delivery apparatus 336 and the obscuration bar 327 of FIGS. 3A and 3B, taken along the line 3C-3C shown in FIG. 3A.

[0052] As seen in FIGS. 3A, 3B, and 3C, the heated gas delivery apparatus 336 (which also serves as the head 330 of the obscuration bar 327) has a tapered shape, tapering in the positive-z direction, or (in FIGS. 1A and IB) in a direction along the light axis A toward the intermediate focus 123, to a tip 336t. Outlets 336o in a side surface 336s (a tapering surface) can be used to deliver heated gas into the interior volume of a light source vessel, such as the interior volume 114 of the vessel 111 of the light source 110 of FIGS. 1A and IB. Outlets 336o can be in the form of circular or approximately circular holes, as shown, but can take other forms such as slits and other shapes. The tip 336t can be closed, that is, it can have no outlets. A surface or “exposed surface” 334 is positioned on the negative-z-axis extreme of the heated gas delivery apparatus 336 (or on a side exposed to the primary focus 122 in FIGS 1A and IB). The exposed surface 334 can be or can include a slanted surface 334s meaning a surface that is not perpendicular to the axis A (of FIGS. 1 A and IB).

[0053] As further seen in FIGS. 3B and 3C, the exposed surface 334 can include one or more inlet openings 336io. The openings 336io can, when in use, be radiated by beam 113e (FIG. 1A and FIG. 3C) from the source laser 112 (FIGS. 1A, IB) and can admit a portion of the light of the beam 113e into an interior 336i of the heated gas delivery apparatus 336 in the form of interior radiation RADi (FIG. 3C). The interior radiation RADi irradiates an interior surface 336is of the heated gas delivery apparatus 336 to heat the heated gas delivery apparatus 336 and thereby heat gas delivered by or through the heated gas delivery apparatus 336, such as hydrogen gas Fb, to produce heated gas HG. The interior surface 336is a tapering interior surface to receive, absorb, and partially reflect one or more times the radiation RADi within the interior 336i . In this implementation, the inlet openings 336io in the exposed surface 334 also serve as gas outlets (with outflow not shown in FIGS. 3A-3C), and, in this implementation, can take the form of concentric ring outlets 348c and holes 348h (FIG. 3B). In this implementation, the interior surface 336is illuminated by the interior radiation RADi is also the obscuration-bar-head portion of an interior surface of a gas conduit 347 extending along and inside the shaft 329 and along and inside the heated gas delivery apparatus 336 or head 330 (FIG. 3C).

[0054] As seen in FIG. 3C, the shaft 329 of the obscuration bar 330 can optionally include a resistive heating element RHE positioned along the gas conduit 347 to add heat to the gas that passes through. A restive heating element can also or alternatively be included in the heated gas delivery apparatus or head 336, 330. A coating or other surface modification 336isc can be included on the interior surface 336is or a portion thereof to lower the reflectivity of the interior surface such that the interior radiation RADi is reflected less than it would be by an unmodified portion of the interior surface 336is . In the case wherein the reflectivity of the unmodified or uncoated internal surface is greater than 0.9, the coating or other surface modification 336isc can be useful to make the reflectivity equal to or below 0.9, for example. Both the interior reflectivity of equal to or less than 0.9 and the tapering shape in the positive z-axis direction of the interior 336i of the heated gas delivery apparatus 336, a shape which helps produce multiple reflections within in the interior 336i, can assist in capturing energy from the interior radiation RADi to heat the heated gas delivery apparatus 336.

[0055] FIG. 4 is a cross-sectional view similar to FIG. 3C, showing another implementation of a heated gas delivery apparatus 436 included as part of an obscuration bar 427, the obscuration bar having a shaft 429, a base 428, and a head 430 which is also the as heated gas delivery apparatus 436. Similarly to FIG. 3C and as seen in FIG. 4, the heated gas delivery apparatus 436 has a tapered shape, tapering in the positive-z direction, or (in FIGS. 1A and IB) in a direction along the light axis A toward the intermediate focus 123, to a tip 436t (FIG. 4). Outlets 436o in a side surface 436s (a tapering surface) can be used to deliver heated gas into the interior volume of a light source vessel, such as the interior volume 114 of the vessel 111 of the light source 110 of FIGS. 1 A and IB. The tip 436t can be closed, that is, it can have no outlets. A surface or “exposed surface” 434, is positioned on the negative-z-axis extreme of the heated gas delivery apparatus 436 (or on a side exposed to theprimary focus 122 in FIGS 1A and IB). The exposed surface 434 can be or can include a slanted surface 434s, meaning a surface that is not perpendicular to the axis A (of FIGS. 1A and IB).

[0056] As further seen in FIG. 4, the exposed surface 434 can include one or more inlet openings 436io, with one shown in the cross section of FIG. 4. The opening 436io can, when in use, be radiated by beam 113e (FIG. 1A and FIG. 4) from the source laser 112 (FIGS. 1A, IB) and can admit a portion of the light of the beam 113e into an interior 436i of the heated gas delivery apparatus 436 in the form of interior radiation RADi (FIG. 4). The interior radiation RADi irradiates an interior surface 436is of the heated gas delivery apparatus 436 to heat the heated gas delivery apparatus 436 and thereby heat gas delivered by or through the heated gas delivery apparatus 436 to produce heated gas HG. In this implementation, the inlet opening(s) 436io in the exposed surface 434 do not also serve as gas outlets. This implementation can be useful if no gas flow is desired from the exposed surface 434 (an alternative represented by closed end CE of a gas sub-conduit 447o at the lower end of the exposed surface 434 of FIG. 4), or if gas flow is desired from the exposed surface 434 but baffles BF are used that significantly limit or prevent radiation from the beam 113e from reaching a gas conduit or sub- conduit 447p (an alternative represented at the upper end of the exposed surface 434 of FIG. 4).

[0057] In this implementation, one or more interior regions such as interior region 436i are accessible to the beam 113e through one or more associated inlet openings 436io that admit some of the light of beam 113e into the interior region(s) 436i, resulting in interior radiation RADi. As noted, the interior region(s) 436i are not part of a gas conduit within the heated gas delivery apparatus or heat 436, 430. Instead, one or more gas sub-conduits 447o, 447p are fed from a gas conduit 447 positioned within and along the shaft 427. Gas sub-conduits 447o, 447p can be opposite sides of a single ring conduit running along inside the outer surface 436s, or may be fluidically connected by one or more ring or half-ring feed channels, such as by a single ring or half-ring feed channel positioned within the heated gas delivery apparatus or head 436, 430 (not shown) at or near the conduit 447.

[0058] The interior radiation RADi illuminates an interior surface of the heated gas delivery apparatus 436 in the form of the interior surface 436is of the interior region 436i such that the heated gas delivery apparatus 436 is heated, thereby heating gas delivered by or through the heated gas delivery apparatus 436 to produce heated gas HG. A coating or other surface modification 436isc can be included on the interior surface 436is or a portion thereof to lower the reflectivity of the interior surface such that the interior radiation RADi is reflected less than it would be by an unmodified portion of the interior surface 436is. In the case wherein the reflectivity of the unmodified or uncoated internal surface is greater than 0.9, the coating or other surface modification 436isc can be useful to make the reflectivity equal to or below 0.9, for example. Both the interior reflectivity of equal to or less than 0.9 and the tapering shape in the positive z-axis direction of the interior region 436i of the heated gas delivery apparatus 436, a shape which helps produce multiple reflections within in the interior region 436i, can assist in capturing energy from the interior radiation RADi to heat the heated gas delivery apparatus 436.

[0059] FIG. 5 is a cross-sectional diagram including a collector 520, such as the collector 120 of FIGS. 1A and IB, and a heated gas delivery apparatus 536, such as the heated gas delivery apparatus 136 of FIGS. 1A and IB, 336 of FIGS. 3A-3C, and 436 of FIG. 4. Some beneficial aspects of the gas heated gas delivery apparatuses and processes disclosed herein can be explained in part with respect to FIG. 5.

[0060] The outer and inner perimeters of the of the collector 520 or the collector surface 521, define the outer edges or edge rays of reflected EUV light 524 reflected in the collector surface 521 from a plasma 518 at or near a primary focus 522 of the collector 520. The EUV light 524 reflected from the collector 520 is essentially all confined within a first cone defined by the outer edge of the collector520 or the collector reflective surface 521 as its base and the intermediate focus 523 as its vertex. This first cone may be referred to as a first approximate cone, or may be considered as a first slightly truncated cone, as the intermediate focus effectively covers some area and is thus not a single point. In FIG. 5, the first approximate cone is represented in cross section by the triangle having vertices Al, B, C. Similarly, the EUV light 524 reflected from the collector 520 is essentially all confined outside of a second cone or second approximate cone or second truncated cone. The second approximate cone is defined by the perimeter of the through hole 525 or of the inner edge of the collector reflective surface521 as its base and the intermediate focus 523 as its vertex. In FIG. 5, the second approximate cone is represented in cross section by the triangle having vertices Al, D, E. The second approximate cone corresponds to the “shadow or voluminous gap 154” along the optical axis A of FIG. 1A.

[0061] While essentially all confined there, the intensity of the reflected EUV light 524 from the collector 520 is not evenly distributed within the volume between the first and second cones. To illustrate this point, as represented in FIG. 5, angles given by BFG and by DFG are equal and thus essentially equally divide the EUV light 524 leaving the primary focus 522 toward the left side of the collector 520 in the plane of FIG. 5. However, as shown, radiation traveling within angle BFG spreads across more of the surface 521 of the collector 520 than radiation traveling within the angle DFG. Accordingly, after reflection in the collector 520, nominally about one-half of the radiation (EUV light 524) from the collector 520 travels within the grey-highlighted region in FIG. 5, and one-half travels outside the grey-highlighted region. Accordingly, the grey-highlighted region is a concentrated region CR in which EUV light 524 is more concentrated than EUV light 524 radially outside of concentrated region CR.

[0062] This concentration effect tends to be increased by the properties of the surface 521 of the collector 520. EUV light 524 reflected at lower angles of incidence (closer to perpendicular to the surface 521, as within the concentrated region CR) tends to be reflected more completely (with higher reflectance) than EUV light 524 reflected at higher angles of incidence (as outside the concentrated region CR). Thus more than one-half of the light traveling from the collector 520 to the intermediate focus 523 generally travels within the relatively radially narrow concentrated region CR. By introducing heated gas HG directly into the concentrated region CR (and from there into surroundingregions) and thus reducing gas absorption losses particularly strongly in the concentrated region CR, the brightness of a given EUV light source 110 can be increased, such as by 1%, 2%, 3%, or 4% or more. To deliver the heated gas directly to the concentrated region CR without impinging on the region CR and thus interfering with the transmission of EUV light 524, the exterior surface of the heated gas delivery apparatus 536 can be conical and can be spaced from the surface of the second approximate cone when the heated gas delivery apparatus 536 is at an operating temperature, suchthat the exterior surface of the heated gas delivery apparatus 536 does not encroach upon the second approximate cone when in operation. The operating temperature of the heated gas delivery apparatus 536 (and of heated gas delivery apparatuses 336 and 436) can be in the range of 400° C to 2000° C, or 400° C to 1200° C, or 400° C to 800° C, for example. The heated gas delivery apparatus can extend for a distance within the range of 90 centimeters (cm) to 110 cm, or 95 cm to 105 cm, along the axis A. Alternatively or in addition, the heated gas delivery apparatus can extend in the range of from 50% to 70%, or 55% to 65%, of a distance from the primary focus 522 to the intermediate focus 523 along the axis A. The tip of the heated gas delivery apparatus 536 can extend to within 10 cm to 25 cm, or 10 cm to 20 cm, or 10 cm to 15 cm, from the intermediate focus 523.

[0063] FIG. 6 is a flow diagram of a process P600 for operating an EUV light source (such as the EUV light source 110) according to the present disclosure. As seen in FIG. 6, the process P600 includes irradiating, within an interior volume 114 (FIGS. 1A, IB) of an EUV light source 110 (FIGS. 1A, IB), a target (such as target 115 of FIG. 1A) with light (such as beam 113 of FIG. 1A) from a source laser (112 of FIG. 1A) to produce an EUV -emitting plasma (118 of FIGS. 1A, IB; 518 of FIG. 5) (S10); reflecting, from a collector (120 of FIGS. 1A, IB; 520 of FIG. 5) along a path of reflected EUV light within the interior volume (114 of FIGS. 1A, IB), EUV light (124 of FIGS. 1A, IB; 524 of FIG. 5) from the plasma (118 of FIGS. 1A, IB; 518 of FIG. 5) (S20); receiving light (such as beam 113 of FIG. 1A) from the source laser (112 of FIG. 1A) into a heated gas delivery apparatus (136 of FIGS. 1A, IB; 336 of FIGS. 3A-3C; 436 of FIG. 4; 536 of FIG. 5) to heat the heated gas delivery apparatus (136 of FIGS. 1A, IB; 336 of FIGS. 3A-3C; 436 of FIG. 4; 536 of FIG. 5) (S30); and delivering heated gas (HG of FIGS. 1A, IB, 3C, 4, and 5) from the heated gas delivery apparatus (136 of FIGS. 1A, IB; 336 of FIGS. 3A-3C; 436 of FIG. 4; 536 of FIG. 5) into the interior volume (114 of FIGS. 1A, IB) along the path of reflected EUV light (124 of FIGS. 1A, IB; 524 of FIG. 5).

[0064] The implementations can be further described in the following numbered clauses:1. A system for generating extreme ultraviolet (EUV) radiation, the system including: a source vessel having an interior volume; a source laser configured to generate light to enter the interior volume, in an entering light direction along a light axis, to provide energy to a target to cause the generation of EUV within the interior volume; and a heated gas delivery apparatus disposed within the interior volume and configured to deliver heated gas into the interior volume, the heated gas delivery apparatus positioned on and extending along the light axis, the heated gas delivery apparatus havingoutlets to dispense the heated gas into the interior volume and one or more inlet openings to receive light from the source laser to heat the heated gas delivery apparatus.2. The system of clause 1, wherein the heated gas delivery apparatus further includes an interior surface having a reflectivity of the light equal to or less than 0.9 to absorb at least some of the light received from the source laser through the one or more inlet openings.3. The system of clause 1, wherein the heated gas delivery apparatus further includes a tapering interior surface to receive, partially reflect one or more times, and at least partially absorb the light received from the source laser through the one or more inlet openings.4. The system of clause 3, wherein the tapering interior surface includes a conical interior surface.5. The system of clause 1, further including a heated gas source connected to the heated gas delivery apparatus to deliver heated gas to the heated gas delivery apparatus.6. The system of clause 5, wherein the heated gas source is positioned outside the interior volume.7. The system of clause 1, wherein the heated gas delivery apparatus further includes a gas conduit connected to the outlets.8. The system of clause 1, wherein the heated gas delivery apparatus further includes a heating element configured to heat the heated gas delivery apparatus and / or gas delivered by the heated gas delivery apparatus.9. The system of clause 1, further including a collector configured to reflect EUV light generated at or near a primary focus of the collector to an intermediate focus of the collector, the collector having an optical axis essentially coincident with the light axis and a through-hole along the optical axis configured to allow the light to enter the interior volume, such that most or all EUV light reflected from the collector travels from the collector to the intermediate focus inside a volume of a first approximate cone having as a base the perimeter of the collector and having as a vertex the intermediate focus, and outside a volume of a second approximate cone having the through-hole as a base and the intermediate focus as a vertex, and wherein the heated gas delivery apparatus is disposed along the light axis within the second approximate cone.10. The system of clause 9, wherein the heated gas delivery apparatus has a conical exterior surface that is spaced from the surface of the second approximate cone when the heated gas delivery apparatus is at an operating temperature, such that the exterior surface does not impinge upon the second approximate cone when in operation.11. The system of clause 9, wherein the heated gas delivery apparatus extends for a distance within a range of 90 to 110 centimeters along the light axis.12. The system of clause 9, wherein the heated gas delivery apparatus extends for a distance along the light axis within a range of 50% to 70% of a distance from the primary focus to the intermediate focus along the light axis.13. The system of clause 1, wherein at least some of the outlets of the heated gas delivery apparatus are positioned in a conical exterior surface of the heated gas delivery apparatus, the conical exteriorsurface narrowing in a direction along the light axis and extending away from the entering light direction.14. The system of clause 13, wherein the heated gas delivery apparatus extends a distance within a range of 90 to 110 centimeters along the light axis.15. The system of clause 13, wherein the heated gas delivery apparatus extends a distance along the light axis within a range of 50% to 70% of a distance from the primary focus to the intermediate focus along the light axis.16. The system of clause 13, wherein a tip of the heated gas delivery apparatus extends to within 10 centimeters to 15 centimeters from an intermediate focus of a collector.17. A process for providing extreme ultraviolet (EUV) radiation, the process including: within the interior volume of a source vessel, irradiating a target with light from a source laser to generate an EUV-producing plasma; producing reflected EUV light within the interior volume by reflecting EUV light from the plasma along a reflected EUV path; receiving light from the source laser into a heated gas delivery apparatus to heat the heated gas delivery apparatus; and delivering heated gas from the heated gas delivery apparatus into the interior volume of the source vessel along the reflected EUV path.18. The process of clause 17, wherein a side of the heated gas delivery apparatus includes outlets.19. The process of clause 18, wherein the heated gas delivery apparatus includes a conical shape narrowing in a direction toward an intermediate focus of the collector.20. The process of clause 19, wherein the heated gas delivery apparatus extends a distance within the range of 90 centimeters to 110 centimeters in a direction parallel to a light axis of the source laser.21. The process of clause 19, wherein the heated gas delivery apparatus extends in a direction parallel to a light axis of the source laser for 50% to 70% of a distance from a primary focus to an intermediate focus of the collector.

[0065] The above-described implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A system for generating radiation, the system comprising: a source vessel having an interior volume; a source laser configured to generate light to enter the interior volume, in an entering light direction along a light axis, to provide energy to a target to cause the generation of EUV within the interior volume; and a heated gas delivery apparatus disposed within the interior volume and configured to deliver heated gas into the interior volume, the heated gas delivery apparatus positioned on and extending along the light axis, the heated gas delivery apparatus having outlets to dispense the heated gas into the interior volume and one or more inlet openings to receive light from the source laser to heat the heated gas delivery apparatus.

2. The system of claim 1, wherein the heated gas delivery apparatus further comprises an interior surface having a reflectivity of the light equal to or less than 0.9 to absorb at least some of the light received from the source laser through the one or more inlet openings.

3. The system of claim 1, wherein the heated gas delivery apparatus further comprises a tapering interior surface to receive, partially reflect one or more times, and at least partially absorb the light received from the source laser through the one or more inlet openings.

4. The system of claim 3, wherein the tapering interior surface comprises a conical interior surface.

5. The system of claim 1, further comprising a heated gas source connected to the heated gas delivery apparatus to deliver heated gas to the heated gas delivery apparatus.

6. The system of claim 5, wherein the heated gas source is positioned outside the interior volume.

7. The system of claim 1, wherein the heated gas delivery apparatus further comprises a gas conduit connected to the outlets.

8. The system of claim 1, wherein the heated gas delivery apparatus further comprises a heating element configured to heat the heated gas delivery apparatus and / or gas delivered by the heated gas delivery apparatus.

9. The system of claim 1, further comprising a collector configured to reflect EUV light generated at or near a primary focus of the collector to an intermediate focus of the collector, the collector havingan optical axis essentially coincident with the light axis and a through -hole along the optical axis configured to allow the light to enter the interior volume, such that most or all EUV light reflected from the collector travels from the collector to the intermediate focus inside a volume of a first approximate cone having as a base the perimeter of the collector and having as a vertex the intermediate focus, and outside a volume of a second approximate cone having the through-hole as a base and the intermediate focus as a vertex, and wherein the heated gas delivery apparatus is disposed along the light axis within the second approximate cone.

10. The system of claim 9, wherein the heated gas delivery apparatus has a conical exterior surface that is spaced from the surface of the second approximate cone when the heated gas delivery apparatus is at an operating temperature, such that the exterior surface does not impinge upon the second approximate cone when in operation.

11. The system of claim 9, wherein the heated gas delivery apparatus extends for a distance within a range of 90 to 110 centimeters along the light axis.

12. The system of claim 9, wherein the heated gas delivery apparatus extends for a distance along the light axis within a range of 50% to 70% of a distance from the primary focus to the intermediate focus along the light axis.

13. The system of claim 1, wherein at least some of the outlets of the heated gas delivery apparatus are positioned in a conical exterior surface of the heated gas delivery apparatus, the conical exterior surface narrowing in a direction along the light axis and extending away from the entering light direction.

14. The system of claim 13, wherein the heated gas delivery apparatus extends a distance within a range of 90 to 110 centimeters along the light axis.

15. The system of claim 13, wherein the heated gas delivery apparatus extends a distance along the light axis within a range of 50% to 70% of a distance from the primary focus to the intermediate focus along the light axis.

16. The system of claim 13, wherein a tip of the heated gas delivery apparatus extends to within 10 centimeters to 15 centimeters from an intermediate focus of a collector.

17. A process for providing extreme ultraviolet (EUV) radiation, the process comprising:within the interior volume of a source vessel, irradiating a target with light from a source laser to generate an EUV-producing plasma; producing reflected EUV light within the interior volume by reflecting EUV light from the plasma along a reflected EUV path; receiving light from the source laser into a heated gas delivery apparatus to heat the heated gas delivery apparatus; and delivering heated gas from the heated gas delivery apparatus into the interior volume of the source vessel along the reflected EUV path.

18. The process of claim 17, wherein a side of the heated gas delivery apparatus comprises outlets, and the heated gas delivery apparatus comprises a conical shape narrowing in a direction toward an intermediate focus of the collector.

19. The process of claim 18, wherein the heated gas delivery apparatus extends a distance within the range of 90 centimeters to 110 centimeters in a direction parallel to a light axis of the source laser.

20. The process of claim 18, wherein the heated gas delivery apparatus extends in a direction parallel to a light axis of the source laser for 50% to 70% of a distance from a primary focus to an intermediate focus of the collector.

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