CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US Application 62/580,827, which was filed on

November 2, 2017, and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The disclosed subject matter relates to a system and method for cleaning a surface of an optic within a chamber of an extreme ultraviolet light source.

BACKGROUND

[0003] Extreme ultraviolet (EUV) light, for example, electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a material that has an element, for example, xenon, lithium, or tin, with an emission line in the EUV range in a plasma state. In one such method, often termed laser produced plasma ("LPP"), the required plasma can be produced by irradiating a target material, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment. SUMMARY

[0005] In some general aspects, a method is used to clean a surface of an optic within a chamber of an extreme ultraviolet (EUV) light source. The chamber is held at a pressure below atmospheric pressure. The method includes generating a material in a plasma state at a location adjacent to the optic surface and within the chamber. The generating includes transforming a native material that is already present within the vacuum chamber and adjacent the optic surface from a first state into the plasma state. The plasma state of the material includes free radicals of the material. The material in the plasma state is generated at least by enabling the material in the plasma state to pass over the optic surface to remove debris from the optic surface without removing the optic from the EUV light source.

[0006] Implementations can include one or more of the following features. For example, the material in the plasma state can be generated by electromagnetically inducing an electric current at the location adjacent to the optic surface within the chamber. The electric current at the location adjacent to the optic surface within the chamber can be induced by producing a time- varying magnetic field within the chamber near the optic. The time-varying magnetic field within the chamber can be produced by flowing a time-varying electric current through an electrical conductor that is placed outside a circumference of the optic surface.

[0007] The material in the plasma state can be enabled to pass over the optic surface to remove debris from the optic surface without being in the presence of oxygen.

[0008] The material in the plasma state can include at least ions, electrons, and free radicals of hydrogen.

[0009] The debris can be removed from the optic surface by chemically reacting free radicals of the material with the debris on the optic surface to form a chemical that is released from the optic surface. The method can also include removing the released chemical from the EUV chamber. The free radicals can be free radicals of hydrogen and the debris on the optic surface can include tin, such that the chemical that is released from the optic surface includes tin hydride.

[00010] The debris can be removed from the optic surface by etching the debris from the optic surface at a rate of at least 1 nanometers per min over the entire optic surface.

[00011] In other general aspects, a system includes: an extreme ultraviolet (EUV) light source and a cleaning apparatus. The EUV light source includes an EUV chamber held at a pressure below atmospheric pressure; a target delivery system that directs a target toward an interaction region in the vacuum chamber, the interaction region receiving an amplified light beam, and the target including matter that emits extreme ultraviolet light when it is converted into a plasma; and an optical collector that includes a surface that interacts with least some of the emitted extreme ultraviolet light. The cleaning apparatus is adjacent the optical collector surface and is configured to remove debris from the optical collector surface without removing the collector from the EUV chamber. The cleaning apparatus includes a plasma generator adjacent the optical collector surface and within the EUV chamber. The plasma generator generates, from native material already present within the EUV chamber and adjacent the optical collector surface in a first state, a plasma material in a plasma state at a location adjacent the optical collector surface, the plasma material including free radicals that chemically react with the debris on the optical collector surface.

[00012] Implementations can include one or more of the following features. For example, the surface of the optical collector can be a reflective surface and the interaction between the optical collector surface and the emitted extreme ultraviolet light can include a reflection of the emitted extreme ultraviolet light from the optical collector surface.

[00013] The plasma generator can include an electrical conductor placed adjacent the optical collector surface, the electrical conductor being connected to a power source that supplies a time- varying electric current through the electrical conductor to thereby produce a time-varying magnetic field adjacent the optical collector surface and induce an electric current at the location adjacent the optical collector surface. The induced electric current can be large enough to generate, from native material already present within the EUV chamber in the first state, the material in the plasma state at the location adjacent the optical collector surface. The electrical conductor can be a shape that matches a shape of the optical collector surface. The plasma generator can include a dielectric material that at least partially encloses the electrical conductor. The dielectric material can include a tubing that encloses at least a portion of the electrical conductor. The tubing can touch the electrical conductor portion.

[00014] The electrical conductor can be a shape that matches a shape of a rim at the edge of the optical collector surface.

[00015] The optical collector surface can be an ellipsoidal shape and the electrical conductor can include a circular shape that has a diameter that is larger than a circumference of the optical collector surface.

[00016] The native material already present within the chamber in the first state can include hydrogen and the material in the plasma state can include at least ions, electrons, and free radicals of hydrogen.

[00017] The chemical reaction between the free radicals and the debris on the optical collector surface can form a chemical that is released from the optical collector surface. The system can also include a removal apparatus configured to remove the released chemical from the EUV chamber. The free radicals can be free radicals of hydrogen and the debris on the optical collector surface can include tin, such that the chemical that is released from the optical collector surface includes tin hydride. The debris from the optical collector surface can be etched off by the free radicals at a rate of at least 1 nanometers per min over the entire optical collector surface.

[00018] The cleaning apparatus can include an inductively-coupled (ICP) plasma source.

[00019] In other general aspects, a method is performed for cleaning a surface of an optic within a chamber of an extreme ultraviolet (EUV) light source. The chamber is held at a pressure below atmospheric pressure. The method includes generating a material in a plasma state at a location adjacent to the surface of the optic and within the chamber. The generating includes: electromagnetically inducing an electric current at the location adjacent to the optic surface within the chamber to thereby transform a material within the vacuum chamber from a first state into the plasma state. The plasma state of the material includes free radicals of the material. The material in the plasma state is generated at least by enabling the material in the plasma state to pass over the optic surface to remove debris from the optic surface without removing the optic from the EUV light source.

[00020] Implementations can include one or more of the following features. For example, the material can be adjacent the optic surface while in the first state and prior to transformation.

[00021] The electric current can be induced at the location adjacent to the optic surface within the chamber by producing a time- varying magnetic field within the chamber near the optic. The time-varying magnetic field within the chamber can be produced by flowing a time- varying electric current through an electrical conductor that is placed outside a circumference of the optic surface.

[00022] The material in the plasma state can be enabled to pass over the optic surface to remove debris from the optic surface without being in the presence of oxygen.

[00023] The material in the plasma state can include at least ions, electrons, and free radicals of hydrogen.

[00024] The debris can be removed from the optic surface by chemically reacting free radicals of the material with the debris on the optic surface to form a chemical that is released from the optic surface. The method can also include removing the released chemical from the EUV chamber. The free radicals can be free radicals of hydrogen and the debris on the optic surface can include tin, such that the chemical that is released from the optic surface includes tin hydride.

[00025] The material within the vacuum chamber can be native and present within the vacuum chamber.

[00026] The disclosed cleaning apparatus and method enables the ICP plasma source to be located inside a vacuum to minimize or reduce the need for modification of the EUV chamber. The design of the ICP plasma source is not placed in atmosphere during operation and thus there is no need to transport the plasma or free radicals into the EUV chamber, which reduces the complexity of the ICP plasma source described herein. In some implementations, the ICP plasma source is designed to operate in the vacuum environment of the EUV chamber by being made with segmented beads, which are made of a dielectric material such as a porcelain, ceramic, mica, polyethylene, glass, or quartz, and such a design reduces the risk of air leakage due to breakage of the ICP plasma source. The disclosed cleaning apparatus and method provides for at least 10 times faster or at least 100 times faster removal (such as etching) of debris such as tin from the optic surface when compared with prior techniques such as filament cleaning (HRG) and microwave cleaning systems.

DRAWING DESCRIPTION

[00027] Fig. 1 is a block diagram of a cleaning apparatus that removes debris from a surface of an optic within an extreme ultraviolet (EUV) light source;

[00028] Fig. 2 is a block diagram of an exemplary EUV light source in which the cleaning apparatus is designed as an inductively-coupled plasma (ICP) cleaning apparatus;

[00029] Fig. 3A is a first side perspective view of a collector mirror that can be cleaned using the cleaning apparatus of Figs. 1 or 2;

[00030] Fig. 3B is a second side perspective view of the collector mirror of Fig. 3A;

[00031] Fig. 3C is a side cross sectional view of the collector mirror of Fig. 3A;

[00032] Fig. 3D is a plan view taken along the second side of the collector mirror of Fig.

3B;

[00033] Fig. 4A is a perspective view of an ICP cleaning apparatus that can be used in the EUV light source of Figs. 1 or 2; [00034] Fig. 4B is a side cross-sectional view of the ICP cleaning apparatus of Fig. 4A taken along plane B-B;

[00035] Fig. 4C is a side cross-sectional view of section C of the ICP cleaning apparatus of Fig. 4B;

[00036] Fig. 4D is a plan view of the ICP cleaning apparatus of Figs. 4A-4C;

[00037] Fig. 5A is a perspective view of an ICP cleaning apparatus that can be used in the EUV light source of Figs. 1 or 2;

[00038] Fig. 5B is a side cross-sectional view of the ICP cleaning apparatus of Fig. 5A taken along plane B-B;

[00039] Fig. 5C is a side cross-sectional view of section C of the ICP cleaning apparatus of Fig. 5B;

[00040] Fig. 5D is a plan view of the ICP cleaning apparatus of Figs. 5A-5C;

[00041] Fig. 5E is a plan view of section E of the ICP cleaning apparatus of Fig. 5D;

[00042] Fig. 6A is a plan view of an ICP cleaning apparatus that can be used in the EUV light source of Figs. 1 or 2;

[00043] Fig. 6B is perspective view of the ICP cleaning apparatus of Fig. 6A;

[00044] Fig. 6C is a plan view of section C of the ICP cleaning apparatus of Fig. 6A;

[00045] Fig. 7 is a flow chart of a procedure for cleaning the surface of the optic using the cleaning apparatus of Figs. 1 and 2;

[00046] Fig. 8A is a schematic illustration showing steps of the procedure of Fig. 7;

[00047] Fig. 8B is a schematic illustration showing steps of the procedure of Fig. 7;

[00048] Fig. 9 is a flow chart of a procedure for transforming material in the EUV chamber from a first state into a plasma state;

[00049] Fig. 10A is a graph of removal rate versus distance from the cleaning apparatus of Figs. 1 and 2, when compared with prior cleaning techniques, where the vertical axis is a linear scale;

[00050] Fig. 10B is the graph of removal rate versus distance from the cleaning apparatus of Figs. 1 and 2, when compared with prior cleaning techniques, where the vertical axis is a nonlinear scale;

[00051] Fig. 11 is a block diagram of a lithography apparatus that receives the output of the EUV light source of Fig. 2; and [00052] Fig. 12 is a block diagram of a lithography apparatus that receives the output of the EUV light source of Fig. 2.

DESCRIPTION

[00053] Referring to Fig. 1, a cleaning apparatus 105 is configured to removing debris 107 from a surface 110 of an optic 115 within an extreme ultraviolet (EUV) light source 100. The optic 115 is contained within a cavity of an EUV chamber 125 that is held at vacuum pressures, that is, at a pressure below atmospheric pressure. The cleaning apparatus 105 includes a plasma generator 180 that enables the production or generation of a material in a plasma state (a plasma material 130) at a location that is local to or adjacent to the optic surface 110 from a material already present and native (a native material 135) within the EUV chamber 125. A material 135 is native or present within the EUV chamber 125 if it exists within the EUV chamber 125 without needing to be transported into the EUV chamber 125 from outside the EUV chamber 125. The plasma material 130 chemically reacts with the debris 107, thereby removing the debris 107 from the optic surface 110 and forming a new chemical 137 that can be removed from the EUV chamber 125. The new chemical 137 can be in a gaseous state such that when it is formed it is released from the optic surface 110 and the removal from the EUV chamber 125 involves pumping the new chemical 137 from the EUV chamber 125.

[00054] The EUV light source 100 includes a target delivery system 140 that directs a stream 145 of targets 150 toward an interaction region 155 in the EUV chamber 125. The interaction region 155 receives an amplified light beam 160. The target 150 includes matter that emits EUV light when it is in a plasma state. An interaction between the matter within the target 150 and the amplified light beam 160 at the interaction region 155 converts some of the matter in the target 150 into a plasma state, and this converted matter can be referred to as a light-emitting plasma 170. The light-emitting plasma 170 emits EUV light 165. The light-emitting plasma 170 has an element with an emission line in the EUV wavelength range. The created light-emitting plasma 170 has certain characteristics that depend on the composition of the target 150. These characteristics include the wavelength of the EUV light 165 produced by the light-emitting plasma 170.

[00055] To be clear, the light-emitting plasma 170 of the target 150 is distinct from the plasma material 130, as follows. The light-emitting plasma 170 is produced due to the interaction between the target 150 and the amplified light beam 160. Moreover, the light-emitting plasma 170 of the target 150 is what produces the EUV light 165. By contrast, the plasma material 130 is created from the native material 135 that is found inside the chamber 125. Neither the native material 135 nor the plasma material 130 contributes to the production of the EUV light 165. Moreover, the plasma material 130 is not produced from any interaction of the native material 135 with the amplified light beam 160.

[00056] The presence of the target 150 as well as the interaction between the target 150 and the amplified light beam 160 can produce debris 107 in the form of particles, vapor residue, or pieces of matter that are present in the target 150. This debris can accumulate on surfaces of objects in the path of the light-emitting plasma 170. For example, if the target 150 includes molten metal of tin, then tin particles can accumulate on the optic surface 110. Thus, the debris 107 that forms on the optic surface 110 can include vapor residue, ions, particles, and/or clusters of matter formed from the target 150. The presence of the debris 107 can reduce the performance of the optic surface 110, and also reduce the overall efficiency of the EUV light source 100. Thus, cleaning the optic surface 110 with the cleaning apparatus 105 is beneficial to improving the performance of the EUV light source 100. The optic 115 is positioned inside of the EUV chamber 125, and removing the optic 115 results in lost time for operation of the EUV light source 100. The cleaning apparatus 105 disclosed produces the plasma material 130 locally to the optic surface 110, and in this way, the plasma material 130 does not need to be transferred from outside the EUV chamber 125 to the location that is local to or adjacent to the optic surface 110. Moreover, the debris 107 can be removed from the optic surface 110 without having to remove the optic surface 110 and the optic 115 from the EUV chamber 125. Because of its relative position, the cleaning apparatus 105 is able to remove the debris 107 at a rate that is higher than previous cleaning techniques. Additionally, the cleaning apparatus 105 is able to create the plasma material 130 in a vacuum environment and without the requiring the presence of oxygen.

[00057] The optic 115 can be an optical collector in which the surface 110 interacts with least some of the emitted EUV light 165. For example, the surface 110 of the optical collector 115 can be a reflective surface that is positioned to receive at least a portion of the EUV light 165 and to reflect this EUV light 175 for use outside the EUV light source 100. For example, the EUV light 175 can be directed toward a lithography apparatus. The reflective surface 110 can be configured to reflect light in the EUV wavelength range but absorb or diffuse or block light outside the EUV wavelength range.

[00058] The cleaning apparatus 105 includes the plasma generator 180, which is adjacent the optic surface 110 and is positioned fully within the EUV chamber 125. The cleaning apparatus 105 also includes a power source 185 that supplies power to the plasma generator 180. The native material 135 is already present within the EUV chamber 125 and at least some of this native material 135 is adjacent to the optic surface 110 and exists in a first state of matter. The plasma generator 180 generates, from the native material 135, the plasma material 130 at a location adjacent the optic surface 110. The plasma material 130 includes free radicals that chemically react with the debris 107 on the optic surface 110. These free radicals are produced from the native material 135. A free radical is an atom, molecule, or ion that has an unpaired valence electron or an open electron shell, and therefore may be seen as having a dangling covalent bond. The dangling bonds can make free radicals highly chemically reactive, that is, a free radical can react readily with other substances. Because of their reactive nature, free radicals can be used to remove a substance (such as the debris 107) from an object such as the optic surface 110. The free radicals of the plasma material 130 can remove the debris 107 by, for example, etching, reacting with, and/or combusting the debris 107.

[00059] In addition to the free radicals, the plasma material 130 can include other components that do not react with the debris 107, such as ions formed from the native material 135, electrons produced from the native material 135, and chemically neutral items. The cleaning apparatus 105 is able to remove more debris 107 as the number of free radicals present in the plasma material 130 is increased. To put this another way, the higher the density of free radicals within the plasma material 130, the higher the rate of debris removal.

[00060] In some implementations, the native material 135 is made up of hydrogen molecules in a gaseous state. In these implementations, the plasma generator 180 generates from the hydrogen molecules a plasma material that includes free radicals of hydrogen. The plasma material can also include ions of the hydrogen, electrons, and hydrogen molecules.

[00061] As mentioned above, the cavity within the EUV chamber 125 is held at vacuum, that is, at a pressure below atmospheric pressure. For example, the EUV chamber 125 can be held at a low pressure of between about 0.5 Torr (T) to about 1.5 T (for example, at 1 T), which is the pressure selected for generation of the EUV light 165. The cleaning apparatus 105 is configured to produce the plasma material 130 in the EUV chamber 125, which means it is designed to work in a vacuum (such as at 1 T). Moreover, the cleaning apparatus 105 is configured to operate on the native material 135, and in some implementations, the native material 135 available is molecular hydrogen. The cleaning apparatus 105 does not operate on the target 150. Moreover, the cleaning apparatus 105 is designed to enable its use without having to change the design or operation of the EUV chamber 125. Thus, the cleaning apparatus 105 is configured to operate in the environment in which EUV light 165 is produced most efficiently.

[00062] As mentioned above, the target delivery system 140 directs the stream 145 of targets 150 toward the interaction region 155 in the EUV chamber 125. The target delivery system 140 delivers, controls, and directs the targets 150 in the stream 145 in the form of liquid droplets, a liquid stream, solid particles, or clusters, solid particles contained within liquid droplets, or solid particles contained within a liquid stream. The target 150 can be any material that emits EUV light when in a plasma state. For example, the target 150 can include water, tin, lithium, and/or xenon. The target 150 can be a target mixture that includes a target substance and impurities such as non-target particles. The target substance is the substance that, when in a plasma state, has an emission line in the EUV range. The target substance can be, for example, a droplet of liquid or molten metal, a portion of a liquid stream, solid particles or clusters, solid particles contained within liquid droplets, a foam of target material, or solid particles contained within a portion of a liquid stream. The target substance can be, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the target substance can be the element tin, which can be used as pure tin (Sn); as a tin compound, for example, SnBr4, SnBr2, SnH4; as a tin alloy, for example, tin- gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. Moreover, in the situation in which there are no impurities, the target 150 includes only the target substance.

[00063] The light-emitting plasma 170 can be considered to be a highly ionized plasma with electron temperatures of several tens of electron volts (eV). Higher energy EUV light 165 can be generated with other fuel materials (other kinds of targets 150), for example, terbium (Tb) and gadolinium (Gd). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, and then collected by the optic 115. [00064] Some of the free radicals of the plasma material 130 flow across the optic surface 110 after being formed by the plasma generator 180 by the act of diffusion. However, because the pressure in the EUV chamber 125 is relatively high (even though it is a vacuum, it is not a high vacuum), it can be challenging for the free radicals of the plasma material 130 to disperse across the optic surface 110 without additional assistance. Accordingly, the cleaning apparatus 105 can also include a gas flow mechanism 184 that is configured to push or disperse the free radicals of the plasma material 130 across the entire surface of the optic surface 110.

[00065] Referring to Fig. 2, an exemplary EUV light source 200 is shown. In the EUV light source 200, the cleaning apparatus 105 is designed as an inductively-coupled plasma (ICP) cleaning apparatus 205 that is designed to clean debris 207 from a reflective surface 210 of a collector mirror 215. The cleaning apparatus 205 includes a plasma generator 280 adjacent the reflective surface 210 and positioned fully within the EUV chamber 225. The cleaning apparatus 205 also includes a power source 285 that supplies power to the plasma generator 280.

[00066] Similar to the EUV light source 100, the EUV light source 200 includes a target delivery system 240 that supplies a stream 245 of targets 250 toward an interaction region 255 and an amplified light beam 260 that is directed toward the interaction region 255, where the interaction between the amplified light beam 260 and the target 250 converts at least some of the target 250 into a plasma state 270 that produces EUV light 265. The cleaning apparatus 205 will be discussed after the EUV light source 200 is described. The EUV light source 200 can include other components not shown in Fig. 2, such as, for example, components for monitoring aspects of the production of EUV light 265, or for controlling aspects relating to the amplified light beam 260.

[00067] The EUV light source 200 includes an optical system 261 that produces the amplified light beam 260 due to a population inversion within a gain medium or mediums. The optical system 261 can include an optical source that produces a light beam, and a beam delivery system that steers and modifies the light beam and also focuses the light beam to the interaction region 255. The optical source within the optical system 261 includes one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses that form the amplified light beam 260, and in some cases, one or more pre-pulses that form a precursor amplified light beam (not shown). Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the optical system 261 produces the amplified light beam 260 due to the population inversion in the gain media of the amplifiers even if there is no laser cavity. Moreover, the optical system 261 can produces the amplified light beam 260 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the optical system 261. The term "amplified light beam" therefore encompasses one or more of: light from the optical system 261 that is merely amplified but not necessarily a coherent laser oscillation and light from the optical system 261 that is amplified and is also a coherent laser oscillation.

[00068] The optical amplifiers used in the optical system 261 can include as a gain medium a gas that includes carbon dioxide (C0 ₂ ) and can amplify light at a wavelength of between about 9100 and 11000 nanometers (nm), and for example, at about 10600 nm, at a grain greater than or equal to 100. Suitable amplifiers and lasers for use in the optical system 261 include a pulsed laser device, for example, a pulsed gas-discharge CO2 laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, 10 kW or higher and a high pulse repetition rate, for example, 40 kHz or more.

[00069] In some implementations, as discussed above, the target 250 includes tin (Sn), and in these implementations, the debris 207 on the reflective surface 210 includes tin particles. As discussed above, the EUV chamber 225 is a controlled environment, and one of the materials that can be present and permitted within the EUV chamber 225 is molecular hydrogen (H ₂ ) 235. In this case, the cleaning apparatus 205 creates plasma material 230 from the molecular hydrogen 235. This plasma material 230 includes free radicals of hydrogen that interact with the debris 207 (which includes tin particles) on the reflective surface 210. A free radical of hydrogen is a single hydrogen element (H*). This chemical process can be represented by the following chemical formula:

H ₂ (g) <-> 2 H* (g), where g indicates that the chemical is in the gaseous state.

[00070] Specifically, the generated free radicals of hydrogen H* bond with the tin particles (Sn) on the reflective surface 210 and form a new chemical called tin hydride (SnH ₄ ) 237, which is released from the reflective surface 210. This chemical process is represented by the following chemical formula: 4 H* (g) + Sn(s) <-> SnH ₄ (g), where s indicates that the chemical is in the solid state.

[00071] The debris 207 can be etched off or removed from the reflective surface 210 at a rate of at least 1 nanometers per min over the entire reflective surface 210, and not just the regions closest to the cleaning apparatus 205. This is because the plasma material 230 is created at the location adjacent the reflective surface 210, as opposed to being created outside the EUV chamber 225 and then transported into the EUV chamber 225. This is important because the hydrogen radicals H* are short lived and will tend to recombine to reform molecular hydrogen. The design of the cleaning apparatus 205 enables the formation of the hydrogen radicals H* as close as possible to the reflective surface 210 to thereby enable more of the hydrogen radicals H* to combine with the tin particles before they have a chance to recombine with each other to reform molecular hydrogen.

[00072] Referring also to Figs. 3A-3D, the collector mirror 215 includes an aperture 216 to allow the amplified light beam 260 to pass through and reach the interaction region 255. The collector mirror 215 includes a reflective surface 210 that interacts with the EUV light 265 produced from the interaction between the target 250 and the amplified light beam 260 at the interaction region 255. The reflective surface 210 reflects the EUV light 275, which is at least a portion of the EUV light 265, to a secondary focal plane 266, where this EUV light 275 is then captured for use by a tool 290 (such as a lithography apparatus) outside the EUV light source 200. Exemplary lithography apparatuses 1190, 1290 are discussed with reference to respective Figs. 11 and 12 following a detailed description of the EUV light source 100, 200. The collector mirror 215 can be, for example, an ellipsoidal mirror that has a primary focus at the interaction region 255 and a secondary focus at the secondary focal plane 266. This means that a plane section (such as plane section C-C) is in the shape of an ellipses or a circle. Thus, the plane section C-C cuts through the reflective surface 210, and it is formed from a portion of an ellipse. A plan view of the collector mirror 215 shows that the edge 211 of the reflective surface 210 forms a circular shape.

[00073] Although the collector mirror 215 shown herein is a single curved mirror, the collector mirror 215 can take other forms. For example, the collector mirror 215 can be a Schwarzschild collector having two radiation collecting surfaces. In an implementation, the collector mirror 215 is a grazing incidence collector that includes a plurality of substantially cylindrical reflectors nested within one another.

[00074] The EUV light source 200 also includes a control apparatus 292 in communication with one or more controllable components or systems of the EUV light source 200. The control apparatus 292 is in communication with the optical system 261 and the target delivery system 240. The target delivery system 240 can be operable in response to signals from one or more modules within the control apparatus 292. For example, the control apparatus 292 can send a signal to the target delivery system 240 to modify the release point of the targets 250 to correct for errors in the targets 250 arriving at the desired interaction region 255. The optical system 261 can be operable in response to signals from one or more modules within the control apparatus 292. The control apparatus 292 can include a module for controlling the power source 285 of the cleaning apparatus 205. The various modules of the control apparatus 292 can be free-standing modules in that data between the modules is not transferred from module to module. Or, one or more of the modules within the control apparatus 292 can communicate with each other. The modules within the control apparatus 292 can be co-located or separated from each other physically. For example, the module that controls the power source 285 can be co-located with the power source 285.

[00075] The EUV system 200 also includes a removal or exhaust apparatus 295 configured to remove the released chemical 237 from the EUV chamber 225 as well as other gaseous byproducts that can form within the EUV chamber 225. The removal apparatus 295 can be a pump that removes the released chemical 237 from the EUV chamber 225. For example, once the chemical 237 is formed, it is released, and because the chemical 237 can be volatile, it is sucked to the removal apparatus 295, which removes the released chemical 237 from the EUV chamber 225.

[00076] Other components of the EUV light source 200 that are not shown include, for example, detectors for measuring parameters associated with the produced EUV light 265.

Detectors can be used to measure energy or energy distribution of the amplified light beam 260.

Detectors can be used to measure an angular distribution of the intensity of the EUV light 265.

Detectors can measure errors in the timing or focus of the pulses of the amplified light beam 260. Output from these detectors can be provided to the control apparatus 292, which can include modules that analyze the output and adjust aspects of other components of the EUV light source 200 such as the optical system 261 and the target delivery system 240.

[00077] In summary, an amplified light beam 260 is produced by the optical system 261 and directed along a beam path to irradiate the target 250 at the interaction region 255 to convert the material within the target 250 into plasma that emits light in the EUV wavelength range. The amplified light beam 260 operates at a particular wavelength (the source wavelength) that is determined based on the design and properties of the optical system 261.

[00078] Referring to Figs. 4A-4D, in some implementations, the cleaning apparatus 105 is designed as an inductively-coupled plasma (ICP) cleaning apparatus 405. The cleaning apparatus 405 includes a plasma generator 480 that receives energy or power from a power source 485. The plasma generator 480 is internal to the EUV chamber 225, although the power source 485 can be external to the EUV chamber 225. The plasma generator 480 is shaped similarly to the edge 211 of the reflective surface 210 of the collector mirror 215. Thus, because the edge 211 is circular, the plasma generator 480 is also circular.

[00079] The plasma generator 480 includes an electrical conductor 481 placed adjacent the reflective surface 210 of the collector mirror 215. The electrical conductor 481 is connected to the power source 485. The electrical conductor 481 is made of any electrically conducting material such as metal. The electrical conductor 481 is housed inside a tubing 482 and

atmospheric pressure can be maintained between the electrical conductor 481 and the tubing 482. The tubing 482 can be made of a dielectric material such as a porcelain, ceramic, mica, polyethylene, glass, or quartz. The electrical conductor 481 can be made hollow to enable it to be water cooled (by transmitting water through the inside of the conductor 481). The path of the electrical conductor 481 can conform to the shape of the edge 211 of the reflective surface 210 to enable the energy produced from the plasma generator 480 to more efficiently interact with the native material 235 adjacent the reflective surface 210 and also the edge 211. The inside diameter of the plasma generator 480 can be slightly larger than the diameter of the edge 211 of the reflective surface 210 so as not to block the amount of EUV light 275 that is reflected from the reflective surface 210 during operation of the EUV light source 200.

[00080] Any geometric configuration of the electrical conductor 481 is possible. For example, the electrical conductor 481 can have an inner diameter that is less than about several centimeters (cm) from the edge 211 of the reflective surface 210. As shown, the electrical conductor 481 is circular in shape. In other implementations in which the reflective surface 210 is a rectangular shape, then the electrical conductor 481 can be rectangular. Thus, if the reflective surface 210 is triangular in shape, then the electrical conductor 481 can also be triangular or if the reflective e surface 210 is a linear or straight form, then the electrical conductor 481 can be a linear shape.

[00081] Referring to Figs. 5A-5D, in some implementations, the cleaning apparatus 105 is designed as an inductively-coupled plasma (ICP) cleaning apparatus 505. Similar to the cleaning apparatus 405, the cleaning apparatus 505 includes a plasma generator 580 that receives energy or power from a power source 585. The plasma generator 580 is internal to the EUV chamber 225, although the power source 585 can be external to the EUV chamber 225. The plasma generator 580 is shaped similarly to the edge 211 of the reflective surface 210 of the collector mirror 215. Thus, because the edge 211 is circular, the plasma generator 580 is also circular.

[00082] The plasma generator 580 includes an electrical conductor 581 placed adjacent the reflective surface 210 of the collector mirror 215. The electrical conductor 581 is connected to the power source 585. The electrical conductor 581 is made of any electrically conducting material such as metal. The electrical conductor 581 is housed inside a tubing 582 but there is no gap between the electrical conductor 581 and the tubing 582. Thus, the tubing 582 is in contact with and touches the electrical conductor 581.

[00083] The tubing 582 can be made of a dielectric material such as a porcelain, ceramic, mica, polyethylene, glass, or quartz. The electrical conductor 581 can be made hollow to enable it to be water cooled (by transmitting water through the inside of the conductor 581). The path of the electrical conductor 581 conforms to the shape of the edge 211 of the reflective surface 210 to enable the energy produced from the plasma generator 580 to more efficiently interact with the native material 235 adjacent the reflective surface 210 and also the edge 211. The inside diameter of the plasma generator 580 can be slightly larger than the diameter of the edge 211 of the reflective surface 210 so as not to block the amount of EUV light 275 that is reflected from the reflective surface 210 during operation of the EUV light source 200.

[00084] In some implementations, as shown in Fig. 5E, the tubing 582 is replaced with a plurality of segmented beads 583, each of which is made up of a dielectric material. The beads 583 contact each other and also contact the electrical conductor 581. Moreover, there is no air inside the beads 583. Thus, there is no chance for air leakage even if the plasma generator 580 breaks due to external shock.

[00085] One advantage to using the solid (filled) tubing 582 or the beads 583 is that there is no air gap (such as found in the plasma generator 480) between the conductor 581 and the dielectric material (the tubing 582 or beads 583). This gap is effectively a gap filled with air (mostly oxygen). For example, there is a risk that the tubing 482 could break due to an external shock, and if this happens, then the air in the region between the tubing 482 and the conductor 481 would be released into the EUV chamber 125. The design shown in Figs. 5A-5E reduces this risk of exposing the EUV chamber 125 to air and oxygen.

[00086] An exemplary plasma generator 680 is shown in Figs. 6A and 6B, and a close-up view of a segment of the plasma generator 680 is shown in Fig. 6C. The plasma generator 680 includes an electrical conductor 681 placed adjacent the reflective surface 210 of the collector mirror 215. The electrical conductor 681 is connected to the power source 685 and is made of any electrically conducting material such as metal. The electrical conductor 681 is housed inside a sheath 682 made up of a plurality of solid segments 683 that resemble beads. The solid segments 683 are arranged to lock into each other to form a continuous shape around the electrical conductor 681. As discussed above, there is no substantial air gap between the sheath 682 and the electrical conductor 681. The solid segments 683 of the sheath 682 can be made of a dielectric material such as a porcelain, ceramic, mica, polyethylene, glass, or quartz.

[00087] Referring to Fig. 7, a procedure 700 is performed for cleaning the surface 110 of the optic 115, which is within the EUV chamber 125 that is held at a pressure below atmospheric pressure. Reference is made to Figs. 8A and 8B, which schematically shows the steps of the procedure 700. As shown, a material in a plasma state (the plasma material 130) is generated at the location adjacent to the optic surface 110, where the generation occurs within the EUV chamber 125 (705). As discussed above, the plasma material 130 includes at least ions, electrons, and free radicals of the material. The generation of the plasma material 130 includes

transforming a material, which is already within the EUV chamber 125 and adjacent the optic surface 110 from a first state (the native material 135) into the plasma state (the plasma material 130) (710). The plasma state of the material (the plasma material 130) includes free radicals of the material. For example, if the native material 135 includes molecular hydrogen, then step 710 involves transforming the molecular hydrogen into a plasma state that includes hydrogen free radicals H*. The material in the plasma state is enabled to pass over the optic surface 110 to remove debris 107 from the optic surface 110 without removing the optic 115 from the EUV chamber 125 (715).

[00088] As shown in Figs. 8A and 8B, the plasma material 130 can chemically react with the debris 107 to form the new chemical 137, which is in a gaseous state and is released from the optic surface 110. For example, if the native material 135 includes molecular hydrogen and the debris 107 includes tin particles, then step 615 involves a reaction between the hydrogen free radicals H* and the tin Sn to produce as the new chemical tin hydride 237.

[00089] Referring to Fig. 9, in some implementations in which the cleaning apparatus 105 is an ICP cleaning apparatus (such as the cleaning apparatus 205, 405, or 505), a procedure 910 is performed for transforming the native material 135 into the plasma material 130. In the ICP process, a time-varying electric current is flowed (from the power source 285, 485, 585) through the electrical conductor 481, 581 (which is placed outside the circumference of the optic surface 110) (911) The flow of the time-varying electric current produces a time- varying magnetic field within the EUV chamber 225 and around the electric current (912). And, the produced time- varying magnetic field around the electric current induces an electric field or current at the location adjacent to the optic surface 210 and within the EUV chamber 225 (913). The induced electric current (913) is large enough to generate, from the native material 135 within the EUV chamber 125, the plasma material 130 at the location adjacent the reflective surface 210.

Specifically, the changing or time- varying magnetic field (912) induces an electric current in the region around the conductor 481,581. Additionally, this induced electric current produces its own magnetic field that opposes the time-varying magnetic field that is produced at 913, and this opposing magnetic field generates its own current, or induced electric field, which is carried by the native material 135 near the plasma generator. The energy produced from this induced field converts the native material 135 into the plasma material 130 (it induces the plasma material 130 in the EUV chamber 125).

[00090] Moreover, the size of the induced electric field in the area around the conductor 481, 581 is proportional to size of the conductor 481, 581. Thus, in order to obtain a larger area or volume of plasma material 130, the size of the conductor 481, 581 should be increased. [00091] The procedures 700 and 910 are performed without the presence of oxygen as a catalyst or an element to a reaction. The procedure 700 can also include the step of removing the released chemical from the EUV chamber 125 for example, using the exhaust apparatus 295.

[00092] Referring to Figs. 10A and 10B, graphs 1000 and 1050, respectively, show results of tests that were performed to determine the rate of removal of the debris 107 from the optic surface 110 using the procedures 700 and 910 and the ICP cleaning apparatus 205, 405, 505 (ICP) when compared to two prior techniques (labeled as HRG and MW in the graphs). The horizontal axis of the graphs 1000 and 1050 corresponds to a distance from the ICP cleaning apparatus 205, 405, 505. The vertical axis of the graphs 1000 and 1050 corresponds to the removal rate (in arbitrary units). From this data there is a noticeable improvement in the removal rate using the ICP cleaning apparatus 205, 405, 505 when compared with prior techniques. For example, at a value of 5 arbitrary units from the ICP cleaning apparatus 205, 405, 505, the ICP cleaning apparatus 205, 405, 505 removed debris 107 at a rate that is one hundred times greater than the removal rate of the HRG and MW techniques. Additionally, the removal rate using the ICP cleaning apparatus 205, 405, 505 is steady and does not drop off appreciably as the distance from the ICP cleaning apparatus 205, 405, 505 is increased. In some implementations, it is possible to reach a rate of removal of greater than 100 nm per minute from 1-12 centimeters (cm) from the ICP cleaning apparatus 205, 405, 505. The outer diameter of the optic surface 210 of a typical collector mirror for use in an EUV light source is about 60 cm.

[00093] The cleaning apparatus 105, 205, 405, 505 provides a much higher removal debris removal rate than prior techniques in part because the density of the free radicals in the plasma material 130 at the optic surface 110, 210 is much higher than prior techniques. And, this is due in part to the fact that the free radicals of the plasma material 130 are efficiently created locally to the optic surface 110, 210 (instead of being transported in from outside the EUV chamber 125) and also due to the design of the ICP cleaning apparatus 205, 405, 505, which is able to operate within a vacuum environment and without the use of oxygen or water, or any other additional material that is not natively found in the EUV chamber 125.

[00094] Referring to Fig. 11, in some implementations, the cleaning apparatus 105 (or 205, 405, 505) is implemented within an EUV light source 1100 that supplies EUV light 1175 to a lithography apparatus 1190. The lithography apparatus 1190 includes an illumination system (illuminator) IL configured to condition a radiation beam B (for example, EUV light 1175); a support structure (for example, a mask table) MT constructed to support a patterning device (for example, a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (for example, a wafer table) WT constructed to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (for example, a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (for example, including one or more dies) of the substrate W.

[00095] The illumination system IL can include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure MT can be a frame or a table, for example, which can be fixed or movable as required. The support structure MT can ensure that the patterning device is at a desired position, for example, with respect to the projection system PS.

[00096] The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam can correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. The patterning device can be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase- shift, and attenuated phase- shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[00097] The projection system PS, like the illumination system IL, can include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It can be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[00098] As here depicted, the apparatus is of a reflective type (for example, employing a reflective mask).

[00099] The lithographic apparatus can be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such "multi-stage" machines, the additional tables can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other tables are being used for exposure.

[000100] The illuminator IL receives an extreme ultraviolet radiation beam (the EUV light 1175) from the EUV light source 1000. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, for example, xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line- emitting element, with a laser beam. The EUV light source 1100 can be designed like the EUV light source 100 or 200. As discussed above, the resulting plasma emits output radiation, for example, EUV radiation, which is collected using the optic 115, 215 (or a radiation collector).

[000101] The radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (for example, an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, for example, so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (for example mask) MA with respect to the path of the radiation beam B. Patterning device (for example mask) MA and substrate W can be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2.

[000102] The depicted apparatus could be used in at least one of the following modes:

1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (that is, a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (that is, a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT may be determined by the (de )magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (for example, mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[000103] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[000104] Fig. 12 shows an implementation of the lithographic apparatus 1290 in more detail, including the EUV light source 1200, the illumination system IL, and the projection system PS. The EUV light source 1200 is constructed and arranged as discussed above when describing EUV light sources 100, 200.

[000105] The systems IL and PS are likewise contained within vacuum environments of their own. The intermediate focus (IF) of the EUV light source 1200 is arranged such that it is located at or near an aperture in an enclosing structure. The virtual source point IF is an image of the radiation emitting plasma (for example, the EUV light 165).

[000106] From the aperture at the intermediate focus IF, the radiation beam traverses the illumination system IL, which in this example includes a facetted field mirror device 1222 and a facetted pupil mirror device 1224. These devices form a so-called "fly's eye" illuminator, which is arranged to provide a desired angular distribution of the radiation beam 1221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA (as shown by reference 1260). Upon reflection of the beam 1221 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 1226 is formed and the patterned beam 1226 is imaged by the projection system PS via reflective elements 1228, 1230 onto a substrate W held by the substrate table WT. To expose a target portion C on substrate W, pulses of radiation are generated while substrate table WT and patterning device table MT perform synchronized movements to scan the pattern on patterning device MA through the slit of illumination.

[000107] Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures similar to EUV chamber 125. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there can be more mirrors present than those shown. For example, there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in Fig. 12.

[000108] Referring again to Fig. 1, the target delivery system 140 can include a droplet generator arranged within the EUV chamber 125, and arranged to fire a high frequency stream 145 of droplets toward the interaction region 155. In operation, the amplified light beam 160 is delivered in a synchronization with the operation of droplet generator, to deliver pulses of radiation to turn each droplet (each target 150) into the light-emitting plasma 170. The frequency of delivery of the droplets can be several kilohertz, for example 50 kHz.

[000109] In some implementations, the energy from the amplified light beam 160 is delivered in at least two pulses: namely, a pre pulse with limited energy is delivered to the droplet before it reaches the interaction region 155, in order to vaporize the fuel material into a small cloud, and then a main pulse of energy is delivered to the cloud at the interaction region 155, to generate the light-emitting plasma 170. A trap (which can be, for example, a receptacle) is provided on the opposite side of the EUV chamber 125, to capture fuel (that is, the target 150) that is not, for whatever reason, turned into plasma.

[000110] The droplet generator in the target delivery system 140 includes a reservoir that contains the fuel liquid (for example, molten tin) and a filter and a nozzle. The nozzle is configured to eject droplets of the fuel liquid toward the interaction region 155. The droplets of fuel liquid can be ejected from the nozzle by a combination of pressure within the reservoir and a vibration applied to the nozzle by a piezoelectric actuator (not shown).

[000111] Other aspects of the invention are set out in the following numbered clauses.

1. A method of cleaning a surface of an optic within a chamber of an extreme ultraviolet (EUV) light source, the chamber being held at a pressure below atmospheric pressure, the method comprising:

generating a material in a plasma state at a location adjacent to the optic surface and within the chamber, the generating comprising transforming a native material that is already present within the vacuum chamber and adjacent the optic surface from a first state into the plasma state;

wherein the plasma state of the material includes free radicals of the material;

wherein generating the material in the plasma state comprises enabling the material in the plasma state to pass over the optic surface to remove debris from the optic surface without removing the optic from the EUV light source.

2. The method of clause 1, wherein generating the material in the plasma state comprises electromagnetically inducing an electric current at the location adjacent to the optic surface within the chamber.

3. The method of clause 2, wherein inducing the electric current at the location adjacent to the optic surface within the chamber comprises producing a time- varying magnetic field within the chamber near the optic.

4. The method of clause 3, wherein producing the time-varying magnetic field within the chamber comprises flowing a time-varying electric current through an electrical conductor that is placed outside a circumference of the optic surface.

5. The method of clause 1, wherein enabling the material in the plasma state to pass over the optic surface to remove debris from the optic surface is done without the presence of oxygen. 6. The method of clause 1, wherein the material in the plasma state includes at least ions, electrons, and free radicals of hydrogen.

7. The method of clause 1, wherein removing debris from the optic surface comprises chemically reacting free radicals of the material with the debris on the optic surface to form a chemical that is released from the optic surface.

8. The method of clause 7, further comprising removing the released chemical from the EUV chamber.

9. The method of clause 7, wherein the free radicals are free radicals of hydrogen and the debris on the optic surface includes tin, such that the chemical that is released from the optic surface includes tin hydride.

10. The method of clause 1, wherein removing debris from the optic surface comprises etching the debris from the optic surface at a rate of at least 1 nanometers per min over the entire optic surface.

11. A system comprising:

an extreme ultraviolet (EUV) light source comprising:

an EUV chamber held at a pressure below atmospheric pressure;

a target delivery system that directs a target toward an interaction region in the vacuum chamber, the interaction region receiving an amplified light beam, and the target comprising matter that emits extreme ultraviolet light when it is converted into a plasma; and

an optical collector that includes a surface that interacts with least some of the emitted extreme ultraviolet light; and

a cleaning apparatus adjacent the optical collector surface and configured to remove debris from the optical collector surface without removing the collector from the EUV chamber, the cleaning apparatus comprising a plasma generator adjacent the optical collector surface and within the EUV chamber, wherein the plasma generator generates, from native material already present within the EUV chamber and adjacent the optical collector surface in a first state, a plasma material in a plasma state at a location adjacent the optical collector surface, the plasma material including free radicals that chemically react with the debris on the optical collector surface. 12. The system of clause 11, wherein the surface of the optical collector is a reflective surface and the interaction between the optical collector surface and the emitted extreme ultraviolet light includes a reflection of the emitted extreme ultraviolet light from the optical collector surface.

13. The system of clause 11, wherein:

the plasma generator comprises an electrical conductor placed adjacent the optical collector surface, the electrical conductor being connected to a power source that supplies a time- varying electric current through the electrical conductor to thereby produce a time- varying magnetic field adjacent the optical collector surface and induce an electric current at the location adjacent the optical collector surface, and

the induced electric current is large enough to generate, from native material already present within the EUV chamber in the first state, the material in the plasma state at the location adjacent the optical collector surface.

14. The system of clause 13, wherein the electrical conductor is a shape that matches a shape of the optical collector surface.

15. The system of clause 13, wherein the plasma generator comprises a dielectric material that at least partially encloses the electrical conductor.

16. The system of clause 15, wherein the dielectric material comprises a tubing that encloses at least a portion of the electrical conductor.

17. The system of clause 16, wherein the tubing touches the electrical conductor portion.

18. The system of clause 14, wherein the electrical conductor is a shape that matches a shape of a rim at the edge of the optical collector surface.

19. The system of clause 13, wherein the optical collector surface is an ellipsoidal shape and the electrical conductor includes a circular shape that has a diameter that is larger than a circumference of the optical collector surface.

20. The system of clause 11, wherein the native material already present within the chamber in the first state includes hydrogen and the material in the plasma state includes at least ions, electrons, and free radicals of hydrogen.

21. The system of clause 11, wherein the chemical reaction between the free radicals and the debris on the optical collector surface form a chemical that is released from the optical collector surface. 22. The system of clause 21, further comprising a removal apparatus configured to remove the released chemical from the EUV chamber.

23. The system of clause 21, wherein the free radicals are free radicals of hydrogen and the debris on the optical collector surface includes tin, such that the chemical that is released from the optical collector surface includes tin hydride.

24. The system of clause 21, wherein the debris from the optical collector surface is etched off by the free radicals at a rate of at least 1 nanometers per min over the entire optical collector surface.

25. The system of clause 11, wherein the cleaning apparatus includes an inductively- coupled plasma source.

26. A method of cleaning a surface of an optic within a chamber of an extreme ultraviolet (EUV) light source, the chamber being held at a pressure below atmospheric pressure, the method comprising:

generating a material in a plasma state at a location adjacent to the surface of the optic and within the chamber, the generating comprising:

electromagnetically inducing an electric current at the location adjacent to the optic surface within the chamber to thereby transform a material within the vacuum chamber from a first state into the plasma state;

wherein the plasma state of the material includes free radicals of the material;

wherein generating the material in the plasma state comprises enabling the material in the plasma state to pass over the optic surface to remove debris from the optic surface without removing the optic from the EUV light source.

27. The method of clause 26, wherein the material is adjacent the optic surface while in the first state and prior to transformation.

28. The method of clause 26, wherein inducing the electric current at the location adjacent to the optic surface within the chamber comprises producing a time-varying magnetic field within the chamber near the optic.

29. The method of clause 28, wherein producing the time-varying magnetic field within the chamber comprises flowing a time- varying electric current through an electrical conductor that is placed outside a circumference of the optic surface. 30. The method of clause 26, wherein enabling the material in the plasma state to pass over the optic surface to remove debris from the optic surface is done without the presence of oxygen.

31. The method of clause 26, wherein the material in the plasma state includes at least ions, electrons, and free radicals of hydrogen.

32. The method of clause 26, wherein removing debris from the optic surface comprises chemically reacting free radicals of the material with the debris on the optic surface to form a chemical that is released from the optic surface, the method further comprising removing the released chemical from the EUV chamber.

33. The method of clause 32, wherein the free radicals are free radicals of hydrogen and the debris on the optic surface includes tin, such that the chemical that is released from the optic surface includes tin hydride.

34. The method of clause 26, wherein the material within the vacuum chamber is native and present within the vacuum chamber.

[000112] Other implementations are within the scope of the following claims.