CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 62/941,518, filed November 27, 2019, and titled INHIBITOR SUBSTANCE FOR AN OPTICAL SYSTEM, which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to an inhibitor substance for an optical system. The optical system may be, for example, an extreme ultraviolet (EUV) light source.

BACKGROUND

[0003] Debris may accumulate on an object in an optical system. In some cases, the debris may be removed by reacting the debris with free radicals. The optical system may be an EUV light source. EUV light may be, for example, electromagnetic radiation having wavelengths of 100 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of, for example, 20 nm or less, between 5 and 20 nm, or between 13 and 14 nm, may be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers, by initiating polymerization in a resist layer. Methods to produce EUV light include, but are not necessarily limited to, converting a material that includes 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 may 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 that may be referred to as a drive laser. 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

[0004] In one general aspect, an extreme ultraviolet (EUV) light source includes: a vessel configured to receive target material that emits EUV light when in a plasma state; a delivery system configured to deliver free radicals to an interior of the vessel; an object in the interior of the vessel; and an inhibitor substance. In operational use, the object accumulates debris that includes the target material, the free radicals react with at least some of the debris to remove the debris from the object, and the inhibitor substance inhibits recombination of the free radicals on the object.

[0005] Implementations may include one or more of the following features.

[0006] The inhibitor substance may include a solid-phase inhibitor substance. The solid- phase inhibitor substance may be part of the object in the interior of the vessel. The solid- phase inhibitor substance may be distributed throughout the object. The solid-phase inhibitor substance may be on a surface of the object. The inhibitor substance may occupy catalytic sites on the surface. The solid-phase inhibitor substance may extend into a bulk region of the object. The solid-phase inhibitor substance may extend from the surface to no more than 1 micrometer (pm) into the bulk region of the object. The object may include an optical element. The object may include a reflective optical element. The object may include a metallic interior wall of the vessel. The interior wall may include stainless steel, molybdenum, phosphor nickel, copper, or aluminum. The free radicals may include hydrogen free radicals; the target material may include tin; and the inhibitor substance may include arsenic, antimony, bismuth, sulfur, selenium, tellurium, beryllium or cyanide. The inhibitor substance also may include a gas-phase inhibitor substance.

[0007] In some implementations, the inhibitor substance is a gas-phase inhibitor substance. The gas-phase inhibitor may include hydrogen sulfide or arsenic.

[0008] The delivery system also may be configured to deliver the gas-phase inhibitor substance to the interior of the vessel. The delivery system also may be configured to deliver the gas-phase inhibitor substance to the object. The gas-phase inhibitor may bind to catalytic sites on a surface of the object.

[0009] The object may include a coating on an exterior surface, and the inhibitor substance may be in the coating. The coating may include an oxide or nitride coating. The coating may include titanium nitride, zirconium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, or yttrium oxide.

[0010] In another general aspect, an object in an optical system is exposed to debris, the debris and the object each including a respective material on which free radicals recombine and/or react; and free radicals are provided to the object to remove at least some of the debris from the object. An inhibitor substance is present while the free radicals are provided to the object, and the inhibitor substance inhibits recombination between the free radicals on the object to thereby increase reaction between the debris and the free radicals.

[0011] Implementations may include one or more of the following features. [0012] The inhibitor substance may be a solid-phase inhibitor substance that is added to the object prior to the object being in the optical system. Adding the inhibitor substance to the object may include doping the object with the inhibitor substance, reacting the object with the inhibitor substance, or bombarding the object with the inhibitor substance.

[0013] The inhibitor substance may be a gas-phase inhibitor substance, and the gas-phase inhibitor substance may be provided to the object. The gas-phase inhibitor also may be provided to the object with the free radicals. The inhibitor substance also may include a solid-phase inhibitor substance, and the solid-phase inhibitor substance may be added to the object prior to the object being in the optical system.

[0014] The optical system may include an extreme ultraviolet (EUV) light source, and the debris may include target material that emits EUV light when in a plasma state.

[0015] In another general aspect, an apparatus for an optical system includes: a bulk material including at least one surface, free radicals recombine on the at least one surface; and a solid-phase inhibitor substance at the at least one surface, the inhibitor substance configured to inhibit recombination between the free radicals at the surface of the matter. [0016] Implementations may include one or more of the following features.

[0017] The solid-phase inhibitor substance may extend into the bulk material.

[0018] The solid-phase inhibitor substance may be distributed throughout the at least one surface.

[0019] The optical system may include an extreme ultraviolet (EUV) light source.

[0020] The at least one surface may be a coating on the bulk material. The coating may include an oxide or nitride coating. The coating may also include titanium nitride, zirconium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, or yttrium oxide. [0021] Implementations of any of the techniques described above may include an EUV light source, an object that includes an inhibitor substance, a gas that includes an inhibitor substance, a system, a method, a process, a device, or an apparatus. 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

[0022] FIG. 1 is a block diagram of an extreme ultraviolet (EUV) light source.

[0023] FIG. 2A is a perspective view of an exterior of an object.

[0024] FIG. 2B is a side cross-sectional view of the object of FIG. 2A.

[0025] FIGS. 3-6 are side cross-sectional views of various objects. [0026] FIG. 7 shows an example of a plot of EUV light transmission as a function of partial pressure of a gas-phase inhibitor substance.

[0027] FIG. 8 is a flow chart of a process for removing debris in an EUV light source. [0028] FIG. 9 is a block diagram of another EUV light source.

[0029] FIG. 10 is a block diagram of yet another EUV light source.

DETAILED DESCRIPTION

[0030] Referring to FIG. 1, a block diagram of an extreme ultraviolet (EUV) light source

100 is shown. The light source 100 includes a vessel 109, a target supply system 140, a delivery system 130, and an inhibitor substance 155 (shown as triangles). The inhibitor substance 155 promotes the removal of debris 122 (shown as stars) from objects in an interior

101 of the vessel 109, thereby improving the overall performance of the light source 100.

The inhibitor substance 155 inhibits a reaction between free radicals 135 (shown as open circles) and substances other than the debris 122. For example, the inhibitor substance 155 prevents a reaction between the free radicals 135 and materials other than the debris 122 at a surface of the object, or decreases the rate of the reaction between the free radicals 135 and materials other than the debris 122 at the surface of the object. The presence of the inhibitor substance 155 results in a higher concentration of the free radicals 135 being available to combine with the debris 122. As a result, the free radicals 135 remove the debris 122 from the objects at a higher rate, more effectively, and/or more completely.

[0031] In operational use, the target supply system 140 delivers a stream 121 of targets to the interior 101. An interaction between a light beam 106 and target material in a target 12 lp (which is one of the targets in the stream 121) at a plasma generation site 123 produces a plasma 196 that emits EUV light 197. The target 12 lp includes target material, which is any material that has an emission line in the EUV range when in a plasma state. The target material may be, for example, tin, lithium, or xenon. Other materials may be used as the target material. For example, the element tin may 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.

[0032] The debris 122 is any substance that is capable of accumulating on exposed surfaces of objects in the interior 101. For example, the debris may be dust and/or metallic particles emitted from the plasma 196, and/or the target 12 lp. The exposed surfaces may be on any objects in the interior 101. For example, the exposed surface may be a surface 181, which is a reflective optical surface of an optical element 104 that interacts with reflections of the light beam 106 and the plasma 196 and directs the EUV light 197 to a lithography tool 199. The exposed surface may be an interior wall 103 that that is exposed to the plasma 196 and/or reflections of the light beam 106. The interior wall 103 may be a metal material such as, for example, stainless steel, molybdenum, phosphor nickel, copper, or aluminum. The exposed surface may be an orifice through which gas is provided to or removed from the interior 101. [0033] Regardless of the nature of the object, the accumulation of the debris 122 may negatively affect the performance of the object and the light source 100. For example, the presence of the debris 122 on the reflective surface 181 may reduce the amount of the EUV light 197 that the surface 181 reflects, thereby causing less EUV light 197 to be delivered to the lithography tool 199. In another example, the presence of debris 122 at an orifice hinders the ability for gas to be provided to or removed from the interior 101.

[0034] To reduce or eliminate the accumulation of debris 122, the delivery system 130 delivers free radicals 135 (shown as circles in FIG. 1) to the interior 101. The free radicals 135 are atoms, molecules, and/or ions that have an unpaired valence electron. The free radicals 135 react or combine with the debris 122 to thereby remove the debris 122 from the objects. For example, in implementations in which the target material and the debris 122 is tin (Sn) and the free radicals 135 are free radicals of hydrogen (H), the reaction between the hydrogen free radicals 135 and the tin debris 122 is:

Sn + 4 H SnH ₄

In this example, one tin (Sn) debris 122 molecule reacts with four hydrogen (H) free radicals 135 to form stannane (SntU) gas. Stannane is a gas that desorbs from the surface 181 of the objects and is exhausted from the vessel 109. The higher the concentration of the hydrogen free radicals (H), the higher the etch or removal rate of the tin debris 122. However, because the free radicals 135 are highly reactive, there is a possibility that the free radicals 135 will combine with something other than the debris 122 before having the opportunity to combine with the debris 122. For example, without the inhibitor substance 155, some of the free radicals 135 may recombine with others of the free radicals 135 on the surface 181. Once the free radicals 135 combine with another substance, the free radicals 135 are unable to combine with the debris 122 that has accumulated on the surface 181. In other words, if the free radicals 135 combine with a substance other than the debris 122, the concentration of the free radicals 135 available to combine with the debris 122 on the surface 181 decreases. To increase the concentration of the free radicals 135 that are available to combine with the debris 122, the light source 100 includes the inhibitor substance 155 in the interior 101. The inhibitor substance 155 inhibits or prevents the recombination of the free radical 135 with substances other than the debris 122, thereby increasing the concentration of free radicals 135 available to combine with the debris 122. In this way, the free radicals 135 are able to remove the debris 122 from the surface 181 more completely, at a faster rate, and with greater efficiency.

[0035] The inhibitor substance 155 is any type of matter that inhibits recombination of the free radicals 135 with substances other than the debris 122. The inhibitor substance 155 may be, for example, arsenic, antimony, bismuth, sulfur, selenium, tellurium, beryllium, or cyanide. The inhibitor substance 155 may be a compound that includes more than one type of material in combination with arsenic, antimony, bismuth, sulfur, selenium, tellurium, beryllium, or cyanide.

[0036] The inhibitor substance 155 may be solid phase and/or gas phase. In implementations in which the inhibitor substance 155 is a solid phase, the inhibitor substance 155 is on the surface 181 of the object and/or in a bulk region of the object. FIGS. 2B, 3, and 4 show examples of a solid-phase inhibitor substance. FIG. 5 shows an example of a gas- phase inhibitor substance. Moreover, in some implementations, solid-phase and gas-phase inhibitor substances are used as shown in FIGS. 1, 6, and 9.

[0037] The delivery system 130 includes a gas supply system 133. The gas supply system 133 includes a chamber 137 that contains a fluid of the free radicals 135 (such as a gas) that is delivered to the interior 101. The delivery system 130 delivers the free radicals 135 to the vessel 109 through a conduit 134. The conduit 134 is a pipe or other hollow structure that is able to transport the free radicals 135. For example, the conduit 134 may be a pipe that has an interior wall that is coated or lined with a material that is substantially unreactive with the free radicals 135. The conduit 134 is coupled to the vessel 109 at a port 131. The port 131 is fluidly sealed such that a vacuum environment may be maintained in the interior 101.

[0038] The gas supply system 133 may include a plurality of chambers (the chamber 137 and another chamber 138) that are not fluidly coupled to each other, but are each configured to be fluidly coupled to the conduit 134 such that the gas in either of the chambers 137 or 138, or gas in both of the chamber 137 and 138, may be delivered to the interior 101. For example, in implementations in which the inhibitor substance 155 is a gas-phase inhibitor substance, the chamber 137 includes the free radicals 135 and the chamber 138 includes the inhibitor substance 155 in a gas phase. The inhibitor substance 155 in the gas-phase may be, for example, hydrogen sulfide (FhS) gas, arsenic (As) gas, or a precursor gas such as arsine (ASH ₃ ). [0039] The gas supply system 133 also includes a gas management system 139. The gas management system 139 includes devices, components, and/or systems that are configured to direct the free radicals 135 and/or the gas-phase inhibitor 155 to the interior 101. For example, the gas management system 139 may include pumps, flow control devices (such as valves and/or fluid switches), openings through which the free radicals 135 flow, and/or nozzles.

[0040] The EUV light source 100 also includes a control system 160 that governs the operation of the delivery system 130, the gas management system 139, and/or the gas supply system 133. For example, the control system 160 may control the flow rate of the free radicals 135 or the inhibitor substance 155 (when in gas form) by controlling valves and/or pumps within the gas management system 139. The control system 160 also may be coupled to other systems and components of the EUV light source 100, such as the target supply system 140.

[0041] The control system 160 includes an electronic processing module 161, an electronic storage 162, and an I/O interface 163. The electronic processing module 161 includes one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, an electronic processor receives instructions and data from a read-only memory, a random access memory (RAM), or both. The electronic processing module 161 may be any type of suitable electronic processor.

[0042] The electronic storage 162 may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage 162 includes non-volatile and volatile portions or components. The electronic storage 162 may store data and information that is used in the operation of the control system 160. For example, the electronic storage 162 may store information about the operation of the delivery system 130. For example, in some implementations, the electronic storage 162 stores the flow rate at which the inhibitor substance 155 (when implemented in gas-phase) should flow into the interior 101.

[0043] The electronic storage 162 also stores instructions, such as one or more computer programs, that, when executed, cause the electronic processing module 161 to communicate with components in the delivery system 130. For example, the electronic storage 162 also may store instructions that cause the gas management system 139 to control the partial pressure of the inhibitor substance 155 in the interior 101 (when the inhibitor substance 155 is implemented as a gas-phase inhibitor). [0044] The I/O interface 163 is any type of interface that allows the control system 160 to receive or send information or data. For example, the I/O interface 163 may be a keyboard, mouse, or other computer peripheral device that enables an operator to operate and/or program the control system 160. The I/O interface 163 may include devices that produce a perceivable alert such as a light or a speaker. Furthermore, the I/O interface 163 may include a communications interface such as a universal serial port (USB), a network connection, or any other interface that allows communication with the control system 160.

[0045] Referring to FIG. 2A, a perspective view of an exterior of an object 280 is shown. The object 280 includes a solid-phase inhibitor substance 255 (FIG. 2B). The inhibitor substance 255 is an implementation of the inhibitor substance 155 (FIG. 1). FIG. 2B is a side cross-sectional view of the object 280 taken along the line B — B’ of FIG. 2A. The object 280 may be an optical element such as the reflective optical element 104 (as shown in FIG. 1) or any other structure in the interior 101. For example, the object 280 may be the interior wall 103 (as shown in FIG. 1).

[0046] The object 280 is a three dimensional solid structure that includes exterior surfaces 281a, 281b, 281c. The exterior surfaces 281a and 281c are substantially flat and extend generally in the X-Y plane. The exterior surface 281b is cylindrical in shape and extends in the Z direction. The object 280 may have other shapes in other implementations. For example, the surface 281a may be a curved, concave surface. In the example of FIG. 2B, the surface 281a is oriented toward a source of the debris 122. For example, the object 280 may be used as the reflective optical element 104 (FIG. 1) with the exterior surface 281a facing the plasma generation site 123. Thus, although the debris 122 may accumulate on any of the surfaces 281a, 281b, 281c, in the example discussed below, the debris 122 primarily accumulates on the surface 281a.

[0047] The surfaces 281a, 281b, 281c are adjacent to a bulk region 282. The bulk region 282 is the solid interior of the object 280. The bulk region 282 is bounded by the exterior surfaces 281a, 281b, 281c (and any other surfaces that form the exterior of the object 280). The object 280 may be made of metal material such as, for example, stainless steel, molybdenum, phosphor nickel, copper, or aluminum. The object 280 may be made of non- metal material or a material that includes materials other than metals. For example, the object 280 may include a ceramic material. The bulk region 282 and the surfaces 281a, 281b, 281c may all be the same material, or may be different materials. For example, the surface 281a may be a coating that reflects EUV light and includes the inhibitor substance 255, and the bulk region 282 may be a metallic material. The inhibitor substance 255 may be added to various coatings, such as, for example, oxide coatings and nitride coatings. To provide more specific examples, the inhibitor substance 255 may be added to any of titanium nitride, zirconium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, and yttrium oxide. The coating with the inhibitor substance 255 may be used as a coating on the bulk region 282. In these examples, the coating with the inhibitor 255 forms the surface 281a, 281b, and/or 281c.

[0048] In the example of FIG. 2B, the inhibitor substance 255 is distributed in the bulk region 282 and on the surface 281a. The inhibitor substance 255 may be uniformly distributed throughout the object 280, or there may be higher concentrations of the inhibitor substance 255 in some portions of the object 280 than in others. For example, the object 280 may be fabricated such that there is a greater concentration of the inhibitor substance 255 at the exterior surface 281a than at any of the other exterior surfaces 281b, 281c or in the bulk region 282.

[0049] The inhibitor substance 255 may be introduced into the object 280 by, for example, doping the object 280 with the inhibitor substance 255, causing a chemical reaction between the inhibitor substance 255 and the object 280, performing chemical vapor deposition of the inhibitor substance 255 onto the object 280, or bombarding the object 280 with ions of the inhibitor substance 255. In these implementations, the process of introducing the inhibitor substance 255 into the object 280 causes the inhibitor substance 255 to become part of the object 280. For example, the object 280 may be made of a crystalline material, and a chemical doping process forms a lattice of the inhibitor substance 255 in the base crystalline material. The inhibitor substance 255 may be in a non-solid form during the introduction process. However, the inhibitor substance 255 is in solid form after being introduced into the object 280.

[0050] The free radicals 135 combine with the debris 122 to remove the debris 122 from the surface 281a of the object 280. The removal or desorbing of the debris 122 from the surface 281a is shown as a star attached to a circle with a dotted-line arrow in FIG. 2B. The inhibitor substance 255 resides at or near the surface 218a and inhibits or prevents the free radicals 135 from reacting with others of the free radicals and/or with other matter at the surface 281a. Thus, the presence of the inhibitor substance increases the ability of the free radicals 135 to recombine with the debris 122 at the surface 281a and promotes removal of the debris 122 from the surface.

[0051] Referring to FIG. 3, a side cross-sectional view of an object 380 is shown. The object includes a solid-phase inhibitor substance 355. The inhibitor substance 355 is an implementation of the inhibitor substance 155 (FIG. 1). The object 380 has substantially the same exterior shape as the object 280 (FIG. 2A). The object 380 may be an optical element such as the reflective optical element 104 (as shown in FIG. 1) or any other structure in the interior 101.

[0052] The object 380 includes exterior surfaces 381a, 381b, 381c. The exterior surfaces 381a and 381c are substantially flat and extend in the X-Y plane. The exterior surface 381b is cylindrical in shape and extends in the Z direction. In the example of FIG. 3, the surface 381a is oriented toward a source of the debris 122 such that the debris 122 primarily accumulates on the surface 381a. The surfaces 381a, 381b, 381c are adjacent to a bulk region 382.

[0053] In the example of FIG. 3, the inhibitor substance 355 is distributed in a surface region 383 and/or on the surface 381a. The surface region 383 is adjacent to the surface 381a and extends from the surface 381a into the bulk region 382 in the -Z direction. In some implementations, the surface region 383 extends no more than a few micrometers into the bulk region 382. For example, the surface region 383 may extend to about 1 or about 5 micrometers (pm) into the bulk region 382. The inhibitor substance 355 may be uniformly distributed throughout the surface region 383, or there may be higher concentrations of the inhibitor substance 355 in some portions of the surface region 383 than in others. The surface region 383 may be or may include a coating that includes the inhibitor substance 355. The inhibitor substance 355 may be added to various coatings, such as, for example, oxide coatings and nitride coatings. To provide more specific examples, the inhibitor substance 355 may be added to any of titanium nitride, zirconium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, and yttrium oxide.

[0054] As discussed above, the free radicals 135 in the surface region 383 combine with the debris 122 to remove the debris 122 from the surface 381a of the object 380. The removal or desorbing of the debris 122 from the surface 381a is shown as a star attached to a circle with a dotted-line arrow in FIG. 3. The inhibitor substance 355 occupies the surface region 383 and inhibits or prevents the free radicals 135 from reacting with others of the free radicals and/or other matter at the surface 381a. Thus, the presence of the inhibitor substance increases the ability of the free radicals 135 to recombine with the debris 122 at the surface 381a and promotes removal of the debris 122 from the surface.

[0055] Referring to FIG. 4, a side cross-sectional view of an object 480 is shown. The object 480 has substantially the same exterior shape as the object 280 (FIG. 2A). The object 480 may be an optical element such as the reflective optical element 104 (as shown in FIG. 1) or any other structure in the interior 101.

[0056] The object 480 includes exterior surfaces 481a, 481b, 481c. The exterior surface 481b is cylindrical in shape and extends in the Z direction. In the example discussed below, the surface 481a is oriented toward a source of the debris 122 such that the debris 122 primarily accumulates on the surface 481a. The surfaces 481a, 481b, 481c are adjacent to a bulk region 482. The bulk region 482 is made of a solid-phase material. The exterior surfaces 481a and 481c are substantially flat and extend in the X-Y plane.

[0057] The inhibitor substance 455 is distributed in catalytic sites 484 on the surface 481a. The catalytic sites 484 are also referred to as active sites 484. A catalytic site (or active site) is a region where substances bind and undergo a chemical reaction. In the absence of the inhibitor substance 455, the free radicals 135 recombine with others of the free radicals 135 or other matter at the catalytic sites 484. However, the object 480 includes the inhibitor substance 455 in the catalytic sites 484. By including the inhibitor substance 455 in the catalytic sites 484, the reaction of free radicals 135 with substances other than the debris 122 is reduced or eliminated. Thus, more free radicals 135 are available to react with the debris 122 and more of the debris 122 is removed.

[0058] Referring to FIG. 5, a side cross-sectional view of an object 580 is shown. In the example of FIG. 5, a gas-phase inhibitor substance 555 is used to inhibit reactions between the free radicals 135 and substances other than the debris 122. The gas-phase inhibitor substance 555 is an implementation of the inhibitor substance 155 (FIG. 1). The object 580 has substantially the same exterior shape as the object 280 (FIG. 2A). The object 580 may be an optical element such as the reflective optical element 104 (as shown in FIG. 1) or any other structure in the interior 101. The object 580 includes exterior surfaces 581a, 581b,

581c. The exterior surfaces 581a and 581c are substantially flat and extend in the X-Y plane. The exterior surface 581b is cylindrical in shape and extends in the Z direction. The surfaces 581a, 581b, 581c are adjacent to a bulk region 582.

[0059] In the example of FIG. 5, the debris 122 accumulates on the surface 581a. The debris is removed from the surface 581a using the inhibitor substance 555 (which is in the gas phase). The surface 581a is oriented toward a source of the debris 122 such that the debris 122 primarily accumulates on the surface 581a.

[0060] The inhibitor substance 555 may be directed towards the object 580 by the delivery system 130 (FIG. 1). In some implementations, the gas management system 139 controls the flow of the inhibitor substance 555 into the interior 101 such that the inhibitor substance 555 has a uniform flow pattern (for example, velocity, temperature, and direction) across the surface 581a. In some implementations, the gas management system 139 controls the flow of the inhibitor substance 555 such that the flow pattern is non-uniform across the surface 581a. [0061] The free radicals 135 combine with the debris 122 to remove the debris 122 from the surface 581a of the object 580. The removal or desorbing of the debris 122 from the surface 581a is shown as a star attached to a circle with a dotted-line arrow in FIG. 5. The recombination of the free radicals 135 with substances other than the debris 122 is inhibited by the gas-phase inhibitor substance 555 and the concentration of the free radicals 135 available to recombine with the debris 122 is increased. As a result, a higher portion of the debris 122 on the surface 581a is removed.

[0062] Referring to FIG. 6, a side cross-sectional view of an object 680 is shown. The object 680 includes a solid-phase inhibitor substance 655s (shown as right-side-up triangles in FIG. 6) and is exposed to a gas-phase inhibitor substance 655g (shown as upside-down triangles in FIG. 6). In other words, FIG. 6 shows an example in which both a gas-phase inhibitor substance and a solid-phase inhibitor substance are used.

[0063] The object 680 has substantially the same exterior shape as the object 280 (FIG.

2A). The object 680 may be an optical element such as the reflective optical element 104 (as shown in FIG. 1) or any other structure in the interior 101. The object 680 includes exterior surfaces 681a, 681b, 681c. The exterior surfaces 681a and 681c are substantially flat and extend in the X-Y plane. The exterior surface 681b is cylindrical in shape and extends in the Z direction. In the example of FIG. 6, the surface 681a is oriented toward a source of the debris 122 such that the debris 122 primarily accumulates on the surface 681a. The surfaces 681a, 681b, 681c are adjacent to a bulk region 682.

[0064] The inhibitor substance 655s, which is a solid-phase inhibitor substance, is distributed in the bulk region 682 and on the surface 681a. The solid-phase inhibitor substance 655s may be uniformly distributed throughout the object 680, or there may be higher concentrations of the solid-phase inhibitor substance 655s in some portions of the object 680 than in others. The gas-phase inhibitor substance 655g is directed towards the surface 681a of the object 680 by the delivery system 130 (FIG. 1).

[0065] The free radicals 135 combine with the debris 122 to remove the debris 122 from the surface 681a of the object 680. The removal or desorbing of the debris 122 from the surface 681a is shown as a star attached to a circle with a dotted-line arrow in FIG. 6. The recombination of the free radicals 135 with substances other than the debris 122 is inhibited by the gas-phase and solid-phase inhibitor substance 655, and the concentration of the free radicals 135 available to recombine with the debris 122 is increased. As a result, a higher portion of the debris 122 on the surface 681a is removed.

[0066] In implementations in which the inhibitor substance is in the gas-phase, the inhibitor substance is selected or conditioned such that the impact on the transmission of EUV light is minimal. FIG. 7 shows a plot 700 of the percentage of EUV light that is transmitted as a function of partial pressure of a gas-phase inhibitor substance that is inside a vacuum chamber of an EUV light source. The data shown in FIG. 7 is simulated data. In the example shown in FIG. 7, the inhibitor substance was a gas of hydrogen sulfide (H2S). The pressure of the hydrogen was 1.8 millibars (mbar) and the temperature of the H2S was 200 °C. The percentage transmission shown in the plot 700 is the percentage transmission over a 1 meter path length. If all of the EUV light is transmitted, then the percentage transmission would be 100%. If none of the EUV light is transmitted, then the percentage transmission would be 0%.

[0067] Prior to introducing the inhibitor substance into the vacuum chamber, the partial pressure of the inhibitor substance is zero. In the example represented by the plot 700, the percentage transmission of the EUV light was approximately 87% prior to the introduction of the inhibitor substance. The percentage transmission of the EUV light was approximately 84% when the gas-phase inhibitor substance was present in the vacuum chamber at a partial pressure of 0.02 mbar. The percentage transmission of the EUV light was approximately 74% when the gas-phase inhibitor substance was present in the vacuum chamber at a partial pressure of 0.1 mbar. For pressures less than about 0.06 mbar, the percentage transmission of EUV light drops by about 6% or less. Thus, a H2S gas-phase inhibitor substance may be added in trace amounts without significantly impacting the transmission of the EUV light.

For example, when the partial pressure of the hydrogen sulfide (H2S) inhibitor substance 155 is less than about 6 mbar, the percentage transmission of EUV light is reduced by 6% or less. [0068] Referring to FIG. 8, a flow chart of a procedure 800 is shown. The procedure 800 may be used to remove debris from an optical element in an EUV light source.

[0069] An object in an optical system is exposed to debris (810). The debris 122 accumulates on the object. In the example discussed below, the optical system is the EUV light source 100 (FIG. 1). The object may be an optical element such as the reflective optical element 104 (as shown in FIGS. 1 and 9), any other structure in the interior 101 of the vessel 109, the object 280 (as shown in FIGS. 2A and 2B), the object 380 (as shown in FIG. 3), the object 480 (as shown in FIG. 4), the object 580 (as shown in FIG. 5), or the object 680 (as shown in FIG. 6). [0070] The free radicals 135 are directed toward the object (820). The object is made of an object material. The debris 122 is made of a debris material. The object material may be, for example, a metal material such as, for example, stainless steel, molybdenum, phosphor nickel, copper, or aluminum. The object material may be a non-metal material, such as, for example, a dielectric coating. The debris material may include, for example, tin, dust, or particles of any type of target material. The free radicals 135 are able to react with or recombine with the object material and the debris material. In other words, it is possible for the free radicals 135 to combine with the object material or the debris material. However, the debris 122 is removed when the free radicals 135 combine with the debris material. As discussed above, the inhibitor substance 155 reduces or eliminates reactions of the free radicals 135 with materials other than the debris 122. To promote removal of the debris 122, the free radicals 135 are directed toward the object while the inhibitor substance 155 is present.

[0071] In some implementations, the inhibitor substance 155 is a solid-phase inhibitor substance that is added to the object prior to the object being in the EUV light source 100.

For example, the object may be the object 280 (FIGS. 2A and 2B). As discussed above, the inhibitor substance 255 is introduced into the object 280 by, for example, doping the object 280 with the inhibitor substance 255, causing a chemical reaction between the inhibitor substance 255 and the object 280, performing chemical vapor deposition of the inhibitor substance 255 onto the object 280, or by bombarding the object 280 with ions of the inhibitor substance 155. The addition of the solid-phase inhibitor substance occurs prior to installing the object 280 in the vessel 109.

[0072] In other implementations, the inhibitor substance 155 is a gas-phase inhibitor substance that is provided to the object with the free radicals 135. For example, and referring also to FIG. 5, the inhibitor substance 555 may be directed toward the object 580 by the delivery system 130 while the free radicals 135 are also directed toward the object 580 by the delivery system 130 so that the inhibitor substance 555 and the free radicals 135 are present at the object 580 at the same time.

[0073] In further implementations, the inhibitor substance 155 includes both a solid-phase inhibitor substance that is added to the object prior to the object being installed in the EUV light source 100 and a gas-phase inhibitor substance that is provided to the object while the object is being used in the EUV light source 100. For example, the object may be the object 680 as shown in FIG. 6. [0074] Referring to FIG. 9, a block diagram of another EUV light source 900 is shown.

The EUV light source 900 is the same as the EUV light source 100 (FIG. 1), except the EUV light source includes a gas-phase inhibitor delivery system 950 that is separate from a free- radical delivery system 930. In the EUV light source 900, the control system 160 is coupled to the gas-phase inhibitor delivery system 950 and the free-radical delivery system 930. In the light source 900, the inhibitor substance 955 includes both a solid phase inhibitor substance 955s (shown as right-side-up triangles in FIG. 9) and a gas phase inhibitor substance 955g (shown as upside-down triangles in FIG. 9).

[0075] The free radical delivery system 930 includes a free radical gas supply system 933. The free radical gas supply system 933 includes a chamber 937 that contains a fluid of the free radicals 135 (such as a gas). The free radical delivery system 930 delivers the free radicals 135 to the interior 101 via a fluid conduit 934. The fluid conduit 934 is coupled to the vessel 109 at a port 931. The port 931 is fluidly sealed such that a vacuum environment may be maintained in the interior 101. The free radical gas supply system 933 also includes a free radical gas management system 939. The free radical gas management system 939 includes flow control devices such as, for example, pumps and valves.

[0076] The inhibitor delivery system 950 includes an inhibitor gas supply system 953. The inhibitor gas supply system 953 includes a chamber 957 that contains the gas-phase inhibitor substance 955g. The inhibitor delivery system 950 delivers the gas-phase inhibitor substance 955g to the interior 101 via a fluid conduit 954. The fluid conduit 954 is coupled to the vessel 109 at a port 951. The port 951 is fluidly sealed such that a vacuum environment may be maintained in the interior 101. The inhibitor gas supply system 953 also includes an inhibitor gas management system 959. The free radical gas management system 959 includes flow control devices such as, for example, pumps and valves.

[0077] The control system 160 governs the operation of the free radical delivery system 930, the inhibitor delivery system 950, the free radical gas management system 939, the inhibitor gas management system 959, the free radical gas supply system 933, and/or the inhibitor gas supply system 953. For example, the control system 160 may control the flow rate of the free radicals 135 and the gas-phase inhibitor substance 955g by controlling valves and/or pumps in the free radical gas management system 939 and the inhibitor gas management system 959, respectively. The control system 160 also may be coupled to other systems and components of the EUV light source 900, such as the target supply system 140. [0078] Referring to FIG. 10, an implementation of an LPP EUV light source 1000 is shown. The LPP EUV light source 1000 is an implementation of the EUV light source 100 (FIG. 1). An interior 1007 of a vacuum chamber 1030 of the LPP EUV light source 1000 includes one or more of the inhibitor substances described above in either a gas phase, a solid phase, or both. As described above, the inhibitor substance (either in a gas phase, a solid phase, or both) in the interior 1007 prevents a reaction between the free radicals 135 and materials other than the debris 122 at a surface of an object, resulting in a higher concentration of the free radicals 135 being available to combine with the debris 122 and remove the debris 122 from the objects more effectively.

[0079] The LPP EUV light source 1000 is formed by irradiating a target mixture 1014 at a plasma formation region 1005 with an amplified light beam 1010 that travels along a beam path toward the target mixture 1014. The target material in the targets of the stream 121 discussed with respect to FIG. 1 may be or include the target mixture 1014. The plasma formation region 1005 is within the interior 1007 of a vacuum chamber 1030. When the amplified light beam 1010 strikes the target mixture 1014, a target material within the target mixture 1014 is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 1014. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

[0080] The light source 1000 includes a drive laser system 1015 that produces the amplified light beam 1010 due to a population inversion within the gain medium or mediums of the laser system 1015. The light source 1000 includes a beam delivery system between the laser system 1015 and the plasma formation region 1005, the beam delivery system including a beam transport system 1020 and a focus assembly 1022. The beam transport system 1020 receives the amplified light beam 1010 from the laser system 1015, and steers and modifies the amplified light beam 1010 as needed and outputs the amplified light beam 1010 to the focus assembly 1022. The focus assembly 1022 receives the amplified light beam 1010 and focuses the beam 1010 to the plasma formation region 1005.

[0081] In some implementations, the laser system 1015 may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. 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 laser system 1015 produces an amplified light beam 1010 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 1015 may produce an amplified light beam 1010 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 1015. The term “amplified light beam” encompasses one or more of: light from the laser system 1015 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 1015 that is amplified and is also a coherent laser oscillation.

[0082] The optical amplifiers in the laser system 1015 may include as a gain medium a filling gas that includes C02 and may amplify light at a wavelength of between about 9100 nm and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 900 times. Suitable amplifiers and lasers for use in the laser system 1015 may include a pulsed laser device, for example, a pulsed, gas-discharge C02 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, lOkW or higher and high pulse repetition rate, for example, 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers in the laser system 1015 may also include a cooling system such as water that may be used when operating the laser system 1015 at higher powers.

[0083] The light source 1000 includes a collector mirror 1035 having an aperture 1040 to allow the amplified light beam 1010 to pass through and reach the plasma formation region 1005. The collector mirror 1035 may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation region 1005 and a secondary focus at an intermediate location 1045 (also called an intermediate focus) where the EUV light may be output from the light source 1000 and may be input to, for example, an integrated circuit lithography tool (not shown). The light source 1000 may also include an open-ended, hollow conical shroud 1050 (for example, a gas cone) that tapers toward the plasma formation region 1005 from the collector mirror 1035 to reduce the amount of plasma-generated debris that enters the focus assembly 1022 and/or the beam transport system 1020 while allowing the amplified light beam 1010 to reach the plasma formation region 1005. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation region 1005.

[0084] The light source 1000 may also include a master controller 1055 that is connected to a droplet position detection feedback system 1056, a laser control system 1057, and a beam control system 1058. The light source 1000 may include one or more target or droplet imagers 1060 that provide an output indicative of the position of a droplet, for example, relative to the plasma formation region 1005 and provide this output to the droplet position detection feedback system 1056, which may, for example, compute a droplet position and trajectory from which a droplet position error may be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 1056 thus provides the droplet position error as an input to the master controller 1055. The master controller 1055 may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 1057 that may be used, for example, to control the laser timing circuit and/or to the beam control system 1058 to control an amplified light beam position and shaping of the beam transport system 1020 to change the location and/or focal power of the beam focal spot within the chamber 1030.

[0085] The supply system 1025 includes a target material delivery control system 1026 that is operable, in response to a signal from the master controller 1055, for example, to modify the release point of the droplets as released by a target material supply apparatus 1027 to correct for errors in the droplets arriving at the desired plasma formation region 1005.

[0086] Additionally, the light source 1000 may include light source detectors 1065 and 1070 that measures one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector 1065 generates a feedback signal for use by the master controller 1055. The feedback signal may be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

[0087] The light source 1000 may also include a guide laser 1075 that may be used to align various sections of the light source 1000 or to assist in steering the amplified light beam 1010 to the plasma formation region 1005. In connection with the guide laser 1075, the light source 1000 includes a metrology system 1024 that is placed within the focus assembly 1022 to sample a portion of light from the guide laser 1075 and the amplified light beam 1010. In other implementations, the metrology system 1024 is placed within the beam transport system 1020. The metrology system 1024 may include an optical element that samples or re directs a subset of the light, such optical element being made out of any material that may withstand the powers of the guide laser beam and the amplified light beam 1010. A beam analysis system is formed from the metrology system 1024 and the master controller 1055 since the master controller 1055 analyzes the sampled light from the guide laser 1075 and uses this information to adjust components within the focus assembly 1022 through the beam control system 1058. [0088] Thus, in summary, the light source 1000 produces an amplified light beam 1010 that is directed along the beam path to irradiate the target mixture 1014 at the plasma formation region 1005 to convert the target material within the mixture 1014 into plasma that emits light in the EUV range. The amplified light beam 1010 operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system 1015. Additionally, the amplified light beam 1010 may be a laser beam when the target material provides enough feedback back into the laser system 1015 to produce coherent laser light or if the drive laser system 1015 includes suitable optical feedback to form a laser cavity.

[0089] Other aspects of the invention are set out in the following numbered clauses.

1. An extreme ultraviolet (EUV) light source comprising: a vessel configured to receive target material that emits EUV light when in a plasma state; a delivery system configured to deliver free radicals to an interior of the vessel; an object in the interior of the vessel; and an inhibitor substance, wherein, in operational use, the object accumulates debris that includes the target material, the free radicals react with at least some of the debris to remove the debris from the object, and the inhibitor substance inhibits recombination of the free radicals on the object.

2. The EUV light source of clause 1, wherein the inhibitor substance comprises a solid- phase inhibitor substance.

3. The EUV light source of clause 2, wherein the solid-phase inhibitor substance is part of the object in the interior of the vessel.

4. The EUV light source of clause 3, wherein the solid-phase inhibitor substance is distributed throughout the object.

5. The EUV light source of clause 2, wherein the solid-phase inhibitor substance is on a surface of the object.

6. The EUV light source of clause 5, wherein the inhibitor substance occupies catalytic sites on the surface.

7. The EUV light source of clause 5, wherein the solid-phase inhibitor substance extends into a bulk region of the object.

8. The EUV light source of clause 7, wherein the solid-phase inhibitor substance extends from the surface to no more than about 1 micrometer (pm) into the bulk region of the object.

9. The EUV light source of clause 2, wherein the object comprises an optical element. 10. The EUV light source of clause 9, wherein the object comprises a reflective optical element.

11. The EUV light source of clause 2, wherein the object comprises a metallic interior wall of the vessel.

12. The EUV light source of clause 11, wherein the interior wall comprises stainless steel, molybdenum, phosphor nickel, copper, or aluminum.

13. The EUV light source of clause 2, wherein the free radicals comprise hydrogen free radicals; the target material comprises tin; and the inhibitor substance comprises arsenic, antimony, bismuth, sulfur, selenium, tellurium, beryllium or cyanide.

14. The EUV light source of clause 2, wherein the inhibitor substance further comprises a gas-phase inhibitor substance.

15. The EUV light source of clause 1, wherein the inhibitor substance comprises a gas- phase inhibitor substance.

16. The EUV light source of clause 1, wherein the delivery system is further configured to deliver the gas-phase inhibitor substance to the interior of the vessel.

17. The EUV light source of clause 16, wherein the delivery system is configured to deliver the gas-phase inhibitor substance to the object.

18. The EUV light source of clause 17, wherein the gas-phase inhibitor binds to catalytic sites on a surface of the object.

19. The EUV light source of clause 15, wherein the gas-phase inhibitor comprises hydrogen sulfide or arsenic.

20. The EUV light source of clause 1, wherein the object comprises a coating on an exterior surface, and the inhibitor substance is in the coating.

21. The EUV light source of clause 20, wherein the coating comprises an oxide or nitride coating.

22. The EUV light source of clause 21, wherein the coating comprises titanium nitride, zirconium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, or yttrium oxide.

23. A method comprising: exposing an object in an optical system to debris, the debris and the object each comprising a respective material on which free radicals recombine and/or react; and providing free radicals to the object to remove at least some of the debris from the object, wherein an inhibitor substance is present while the free radicals are provided to the object, and the inhibitor substance inhibits recombination between the free radicals on the object to thereby increase reaction between the debris and the free radicals.

24. The method of clause 23, wherein the inhibitor substance is a solid-phase inhibitor substance that is added to the object prior to the object being in the optical system.

25. The method of clause 24, wherein adding the inhibitor substance to the object comprises doping the object with the inhibitor substance, reacting the object with the inhibitor substance, or bombarding the object with the inhibitor substance.

26. The method of clause 23, wherein the inhibitor substance is a gas-phase inhibitor substance, and the method further comprises providing the gas-phase inhibitor substance to the object.

27. The method of clause 26, wherein the gas-phase inhibitor is provided to the object with the free radicals.

28. The method of clause 27, wherein the inhibitor substance further comprises a solid- phase inhibitor substance, and the solid-phase inhibitor substance is added to the object prior to the object being in the optical system.

29. The method of clause 23, wherein the optical system comprises an extreme ultraviolet (EUV) light source, and the debris comprises target material that emits EUV light when in a plasma state.

30. An apparatus for an optical system, the apparatus comprising: a bulk material comprising at least one surface ,the at least one surface being a surface where free radicals recombine; and a solid-phase inhibitor substance at the at least one surface, the inhibitor substance configured to inhibit recombination between the free radicals at the surface of the matter.

31. The apparatus of clause 30, wherein the solid-phase inhibitor substance extends into the bulk material.

32. The apparatus of clause 30, wherein the solid-phase inhibitor substance is distributed throughout the at least one surface.

34. The apparatus of clause 30, wherein the optical system comprises an extreme ultraviolet (EUV) light source.

35. The apparatus of clause 30, wherein the at least one surface is a coating on the bulk material.

36. The apparatus of clause 35, wherein the coating comprises an oxide or nitride coating.

37. The apparatus of clause 36, wherein the coating comprises titanium nitride, zirconium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, or yttrium oxide. [0090] Other implementations are within the scope of the claims.