CN114747298A

CN114747298A - Inhibitor substances for optical systems

Info

Publication number: CN114747298A
Application number: CN202080082603.4A
Authority: CN
Inventors: S·德德亚; 马悦; A·D·拉弗格
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2019-11-27
Filing date: 2020-10-29
Publication date: 2022-07-12
Also published as: WO2021104794A1; KR20220106751A

Abstract

An Extreme Ultraviolet (EUV) light source comprising: a container configured to receive a 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 container; and an inhibitor substance. In operational use, the object accumulates debris including the target material, the radicals react with at least some of the debris to remove the debris from the object, and the inhibitor substance inhibits recombination of the radicals on the object.

Description

Inhibitor substances for optical systems

Cross Reference to Related Applications

This application claims priority from U.S. application No.62/941,518 entitled "INHIBITOR FOR AN option SYSTEM" filed on 27.11.2019, which is incorporated herein by reference in its entirety.

Technical Field

The present disclosure relates to inhibitor substances for optical systems. The optical system may be, for example, an Extreme Ultraviolet (EUV) light source.

Background

Debris may accumulate on objects in the 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 (sometimes also referred to as soft x-rays) having a wavelength of 100 nanometers (nm) or less, and light having a wavelength of, for example, 20nm or less, between 5 and 20nm, or between 13 and 14nm, may be used in a lithographic process to produce extremely small features in a substrate, such as a silicon wafer, by inducing polymerization in a resist layer. Methods of producing EUV light include, but are not necessarily limited to: a material comprising an element such as xenon, lithium or tin is converted in the plasma state into an emission line in the EUV range. In one such method, commonly referred to as lasing plasma (LPP), the desired plasma may be generated by irradiating a target material, which may take the form of droplets, plates, ribbons, streams or clusters of material, with an amplified beam which may be referred to as a drive laser, for example. For this process, plasma is typically generated in a sealed container (e.g., a vacuum chamber) and monitored using various types of metrology equipment.

Disclosure of Invention

In one general aspect, an Extreme Ultraviolet (EUV) light source includes: a container configured to receive a 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 container; and an inhibitor substance. In operational use, the object accumulates debris including the target material, the radicals react with at least some of the debris to remove the debris from the object, and the inhibitor substance inhibits recombination of the radicals on the object.

Implementations may include one or more of the following features.

The inhibitor substance may comprise a solid phase inhibitor substance. The solid phase inhibitor substance may be part of an object in the interior of the container. The solid phase inhibitor substance may be distributed throughout the object. The solid phase inhibitor substance may be on the surface of the object. Inhibitor species 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 into a bulk region of the object by no more than about 1 micrometer (μm). The object may comprise an optical element. The object may comprise reflective optical elements. The object may comprise a metal inner wall of the container. The inner wall may comprise stainless steel, molybdenum, phosphorus nickel, copper or aluminum. The radicals may include hydrogen radicals; the target material may include tin; and the inhibitor substance may comprise arsenic, antimony, bismuth, sulphur, selenium, tellurium, beryllium or cyanide. The inhibitor material may also include a gas phase inhibitor material.

In some implementations, the inhibitor species is a gas phase inhibitor species. The gas phase suppressant may comprise hydrogen sulfide or arsenic.

The delivery system may also be configured to deliver a vapor phase suppressant substance to the interior of the container. The delivery system may also be configured to deliver a vapor phase inhibitor substance to the object. The gas phase inhibitor may bind to catalytic sites on the surface of the object.

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

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 the free radicals are provided to the object to remove at least some of the debris from the object. The inhibitor substance is present while the radicals are supplied to the object, and the inhibitor substance inhibits recombination between the radicals on the object to thereby increase a reaction between the debris and the radicals.

Implementations may include one or more of the following features.

The inhibitor substance may be a solid phase inhibitor substance that is added to the object before the object is in the optical system. Adding the inhibitor substance to the object may include: doping the object with an inhibitor substance, reacting the object with the inhibitor substance, or bombarding the object with the inhibitor substance.

The inhibitor substance may be a vapor phase inhibitor substance, and the vapor phase inhibitor substance may be provided to the object. The gas phase inhibitor may also be supplied to the object together with the radicals. The inhibitor substance may also include a solid phase inhibitor substance, and the solid phase inhibitor substance may be added to the object before the object is in the optical system.

The optical system may include an Extreme Ultraviolet (EUV) light source, and the debris may include a target material that emits EUV light when in a plasma state.

In another general aspect, an apparatus for an optical system includes: a host material comprising at least one surface on which free radicals recombine; and a solid phase inhibitor substance at the at least one surface, the inhibitor substance configured to inhibit recombination between free radicals at the surface of the material.

Implementations may include one or more of the following features.

The solid phase inhibitor substance may extend into the host material.

The solid phase inhibitor substance may be distributed over at least one surface.

The optical system may comprise an Extreme Ultraviolet (EUV) light source.

At least one surface may be a coating on the host material. The coating may comprise an oxide coating or a nitride coating. The coating may also comprise titanium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, or yttrium oxide.

Implementations of any of the above techniques may include an EUV light source, an object including an inhibitor substance, a gas including an inhibitor substance, a system, a method, a process, an apparatus, or a device. 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.

Drawings

Fig. 1 is a block diagram of an Extreme Ultraviolet (EUV) light source.

Fig. 2A is a perspective view of the exterior of an object.

Fig. 2B is a side cross-sectional view of the object of fig. 2A.

Fig. 3-6 are side cross-sectional views of various objects.

Figure 7 shows an example of a graph of EUV light transmittance as a function of partial pressure of a vapor phase inhibitor species.

FIG. 8 is a flow chart of a process for removing debris in an EUV light source.

FIG. 9 is a block diagram of another EUV light source.

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

Detailed Description

Referring to FIG. 1, a block diagram of an Extreme Ultraviolet (EUV) light source 100 is shown. The light source 100 includes a container 109, a target supply system 140, a delivery system 130, and an inhibitor substance 155 (shown as a triangle). The inhibitor substance 155 facilitates removal of debris 122 (shown as a star) from objects in the interior 101 of the container 109, thereby improving the overall performance of the light source 100. The inhibitor substance 155 inhibits the reaction between the 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 radicals 135 and a material at the surface of the object other than the debris 122, or reduces a reaction rate between the radicals 135 and a material at the surface of the object other than the debris 122. The presence of the inhibitor substance 155 makes a higher concentration of free radicals 135 available for combination with the debris 122. As a result, the radicals 135 remove the debris 122 from the object at a higher rate, more efficiently, and/or more completely.

In operational use, the target supply system 140 delivers a stream 121 of targets to the interior 101. The interaction between the beam 106 at the plasma generation site 123 and the target material in the target 121p (which is one of the targets in stream 121) produces a plasma 196 that emits EUV light 197. Target 121p includes a target material that is in a plasmaAny material having an emission line in the EUV range in state. The target material may be, for example, tin, lithium or xenon. Other materials may be used as the target material. For example, elemental tin may be used as pure tin (Sn); as tin compounds, e.g. SnBr₄、SnBr₂、SnH₄(ii) a As a tin alloy, for example a tin-gallium alloy, a tin-indium-gallium alloy or any combination of these alloys.

Debris 122 is any substance that can accumulate on the exposed surfaces of objects in interior 101. For example, the debris may be dust and/or metal particles emitted from the plasma 196 and/or the target 121 p. The exposed surface may be on any object in the interior 101. For example, the exposed surface may be surface 181, surface 181 being a reflective optical surface of optical element 104 that interacts with the reflection of beam 106 and plasma 196 and directs EUV light 197 to lithography tool 199. The exposed surface may be an inner wall 103 that is exposed to the plasma 196 and/or reflected from the light beam 106. The inner wall 103 may be a metallic material such as, for example, stainless steel, molybdenum, phosphorus nickel, copper, or aluminum. The exposed surface may be an orifice through which gas is provided to or removed from interior 101.

Regardless of the nature of the object, the accumulation of debris 122 may negatively impact the performance of the object and the light source 100. For example, the presence of debris 122 on reflective surface 181 may reduce the amount of EUV light 197 reflected by surface 181, thereby causing less EUV light 197 to be delivered to lithography tool 199. In another example, the presence of debris 122 at the apertures impedes the ability to provide gas to interior 101 or remove gas from interior 101.

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 radicals 135 are atoms, molecules, and/or ions having unpaired valence electrons. The radicals 135 react or combine with the debris 122, thereby removing the debris 122 from the object. For example, in implementations where the target material and debris 122 are tin (Sn) and the radicals 135 are hydrogen (H) radicals, the reaction between the hydrogen radicals 135 and the tin debris 122 is:

in this example, one tin (Sn) scrap 122 molecule reacts with four hydrogen (H) radicals 135 to form stannane (SnH)₄) A gas. The stannane is a gas that is desorbed from the surface 181 of the object and is exhausted from the vessel 109. The higher the concentration of hydrogen radicals (H), the higher the etch or removal rate of the tin debris 122. However, because the radicals 135 are highly reactive, there is a possibility that the radicals 135 will combine with something other than the debris 122 before having an opportunity to combine with the debris 122. For example, in the absence of the inhibitor substance 155, some of the free radicals 135 may complex with other ones of the free radicals 135 on the surface 181. Once the free radicals 135 combine with another substance, the free radicals 135 cannot combine with the debris 122 accumulated on the surface 181. In other words, if the free radicals 135 combine with a substance other than debris 122, the concentration of free radicals 135 available to combine with debris 122 on the surface 181 is reduced. To increase the concentration of free radicals 135 available for combination with debris 122, light source 100 includes an inhibitor substance 155 in interior 101. The inhibitor species 155 inhibits or prevents the recombination of the free radicals 135 with species other than the debris 122, thereby increasing the concentration of free radicals 135 available for combination with the debris 122. In this way, the radicals 135 are able to more completely remove the debris 122 from the surface 181 at a faster rate and with greater efficiency.

Inhibitor substance 155 is any type of material that inhibits free radicals 135 from complexing with substances other than debris 122. Inhibitor material 155 may be, for example, arsenic, antimony, bismuth, sulfur, selenium, tellurium, beryllium, or cyanide. Inhibitor substance 155 may be a compound including more than one type of material in combination with arsenic, antimony, bismuth, sulfur, selenium, tellurium, beryllium, or cyanide.

The inhibitor material 155 can be in a solid phase and/or a gas phase. In implementations where the inhibitor substance 155 is a solid phase, the inhibitor substance 155 is on a surface 181 of the object and/or in a bulk region of the object. Fig. 2B, 3 and 4 show examples of solid phase inhibitor substances. Fig. 5 shows an example of a vapor phase inhibitor substance. Further, in some implementations, a solid phase inhibitor substance and a gas phase inhibitor substance are used as shown in fig. 1, 6, and 9.

The delivery system 130 includes a gas supply system 133. The gas supply system 133 includes a chamber 137 that contains a fluid (such as a gas) of radicals 135 that is delivered to the interior 101. The delivery system 130 delivers the radicals 135 to the vessel 109 via a conduit 134. The conduit 134 is a tube or other hollow structure capable of transporting the radicals 135. For example, conduit 134 may be a tube having an inner wall that is coated or lined with a material that is substantially non-reactive with radicals 135. The conduit 134 is coupled to the container 109 at port 131. The port 131 is fluidly sealed such that a vacuum environment may be maintained in the interior 101.

The gas supply system 133 may include a plurality of chambers (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 gas in either

chamber

137 or 138, or gas in both

chambers

137 and 138, may be delivered to the interior 101. For example, in implementations where the inhibitor substance 155 is a vapor phase inhibitor substance, the chamber 137 includes the radicals 135 and the chamber 138 includes the inhibitor substance 155 in a vapor phase. The inhibitor substance 155 in the gas phase may be, for example, hydrogen sulfide (H)₂S) gas, arsenic (As) gas or precursor gas (such As arsine (AsH)₃))。

The gas supply system 133 also includes a gas management system 139. The gas management system 139 includes devices, components, and/or systems configured to direct the radicals 135 and/or the vapor phase suppressant 155 to the interior 101. For example, the gas management system 139 may include a pump, a flow control device (such as a valve and/or a fluid switch), an opening through which the radicals 135 flow, and/or a nozzle.

The EUV light source 100 also includes a control system 160, the control system 160 governing 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 radicals 135 or the inhibitor species 155 (when in gaseous form) by controlling valves and/or pumps within the gas management system 139. The control system 160 may also be coupled to other systems and components of the EUV light source 100, such as the target supply system 140.

Control system 160 includes an electronic processing module 161, an electronic storage device 162, and an I/O interface 163. The electronic processing module 161 comprises one or more processors (such as a general purpose or special purpose microprocessor) adapted for executing a computer program, as well as 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.

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

Electronic storage 162 also stores instructions, such as one or more computer programs, that when executed, cause electronic processing module 161 to communicate with components in delivery system 130. For example, electronic storage device 162 may also store instructions that cause gas management system 139 to control the partial pressure of inhibitor substance 155 in interior 101 (when inhibitor substance 155 is implemented as a gas phase inhibitor).

I/O interface 163 is any type of interface that allows control system 160 to receive or transmit information or data. For example, the I/O interface 163 may be a keyboard, mouse, or other computer peripheral that enables an operator to operate and/or program the control system 160. The I/O interface 163 may include a device that produces a perceptible alarm, such as a light or a speaker. Further, the I/O interfaces 163 may include communication interfaces, such as a universal serial port (USB), a network connection, or any other interface that allows communication with the control system 160.

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

Object 280 is a three-dimensional solid structure including

outer surfaces

281a, 281b, 281 c. The

outer surfaces

281a and 281c are substantially flat and generally extend in the X-Y plane. The outer surface 281b is cylindrical in shape and extends in the Z direction. In other implementations, the object 280 may have other shapes. For example, the surface 281a may be a curved concave surface. In the example of fig. 2B, the surface 281a is oriented towards the source of debris 122. For example, the object 280 may be used as a reflective optical element 104 (fig. 1) having an outer surface 281a facing the plasma generation site 123. Thus, although debris 122 may accumulate on any of

surfaces

281a, 281b, 281c, in the examples discussed below, debris 122 accumulates primarily on surface 281 a.

Surfaces

281a, 281b, 281c are adjacent to body region 282. The body region 282 is a solid interior of the object 280. The body region 282 is bounded by the

outer surfaces

281a, 281b, 281c (and any other surfaces that form the exterior of the object 280). The object 280 may be made of a metallic material such as, for example, stainless steel, molybdenum, phosphorus nickel, copper, or aluminum. The object 280 may be made of a non-metallic material or a material including a material other than metal. For example, the object 280 may comprise a ceramic material. The body region 282 and

surfaces

281a, 281b, 281c may be the same material, or may be different materials. For example, surface 281a may be a coating that reflects EUV light and includes inhibitor substance 255, while body region 282 may be a metallic material. Inhibitor substance 255 may be added to various coatings (such as, for example, oxide coatings and nitride coatings). To provide a more specific example, the inhibitor substance 255 may be added to any of titanium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, and yttrium oxide. A coating with inhibitor substance 255 may be used as a coating on the body region 282. In these examples, the coating with inhibitor 255

forms surfaces

281a, 281b, and/or 281 c.

In the example of fig. 2B, inhibitor substance 255 is distributed in body region 282 and on surface 281 a. The inhibitor substance 255 may be uniformly distributed throughout the object 280, or the concentration of the inhibitor substance 255 may be higher in some portions of the object 280 than in other portions. For example, object 280 may be fabricated such that the concentration of inhibitor substance 255 at outer surface 281a is greater than the concentration of inhibitor substance 255 at any of the other

outer surfaces

281b, 281c or in body region 282.

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 makes the inhibitor substance 255 part of the object 280. For example, object 280 may be made of a crystalline material and the chemical doping process forms a lattice of inhibitor substance 255 in the base crystalline material. Inhibitor substance 255 may be in a non-solid form during the introduction process. However, the inhibitor substance 255 is in a solid form after being introduced into the object 280.

The radicals 135 combine with the debris 122 to remove the debris 122 from the surface 281a of the object 280. In fig. 2B, the removal or desorption of debris 122 from surface 281a is shown as a star with dashed arrows attached to a circle. Inhibitor species 255 are present at or near surface 218a and inhibit or prevent free radicals 135 from reacting with other ones of the free radicals and/or with other materials at surface 281 a. Thus, the presence of the inhibitor species increases the ability of the free radicals 135 to complex with the debris 122 at the surface 281a and facilitates removal of the debris 122 from the surface.

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

Object 380 includes

outer surfaces

381a, 381b, 381 c.

Outer surfaces

381a and 381c are substantially flat and extend in the X-Y plane. The outer surface 381b is cylindrical in shape and extends in the Z direction. In the example of FIG. 3, surface 381a is oriented toward the source of debris 122 such that debris 122 accumulates primarily on surface 381 a.

Surfaces

381a, 381b, 381c are adjacent to body region 382.

In the example of fig. 3, inhibitor species 355 are distributed in surface region 383 and/or on surface 381 a. Surface region 383 is adjacent to surface 381a and extends in the-Z direction from surface 381a into body region 382. In some implementations, the surface region 383 extends into the body region 382 no more than a few microns. For example, the surface region 383 may extend about 1 or about 5 micrometers (μm) into the body region 382. Inhibitor substance 355 may be uniformly distributed over surface region 383, or inhibitor substance 355 may be at a higher concentration in some portions of surface region 383 than in other portions. The surface region 383 may be or may include a coating that includes an inhibitor substance 355. Inhibitor species 355 can be added to various coatings, such as, for example, oxide coatings and nitride coatings. To provide a more specific example, inhibitor substance 355 may be added to any of titanium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, and yttrium oxide.

As described above, the 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 desorption of debris 122 from surface 381a is shown in FIG. 3 as a star with dashed arrows attached to a circle. The inhibitor species 355 occupies the surface region 383 and inhibits or prevents the radicals 135 from reacting with other ones of the radicals and/or other materials at the surface 381 a. Thus, the presence of inhibitor species increases the ability of the free radicals 135 to complex with the debris 122 at the surface 381a, and facilitates the removal of the debris 122 from the surface.

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

The object 480 includes

outer surfaces

481a, 481b, 481 c. The outer surface 481b is cylindrical in shape and extends in the Z-direction. In the example discussed below, the surface 481a is oriented toward the source of debris 122 such that debris 122 is accumulated primarily on the surface 481 a. The

surfaces

481a, 481b, 481c are adjacent to the body region 482. Body region 482 is made of a solid phase material. The

outer surfaces

481a and 481c are substantially flat and extend in the X-Y plane.

Inhibitor species 455 are distributed among the catalytic sites 484 on the surface 481 a. The catalytic sites 484 are also referred to as active sites 484. A catalytic site (or active site) is a region where a substance binds and undergoes a chemical reaction. In the absence of inhibitor species 455, free radicals 135 complex with other free radicals or other materials in free radicals 135 at catalytic site 484. However, object 480 includes inhibitor substance 455 in catalytic sites 484. By including inhibitor species 455 in catalytic sites 484, the reaction of free radicals 135 with species other than 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.

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

outer surfaces

581a, 581b, 581 c. The

outer surfaces

581a and 581c are substantially flat and extend in the X-Y plane. The outer surface 581b is cylindrical in shape and extends in the Z-direction. The

surfaces

581a, 581b, 581c are adjacent to the body region 582.

In the example of fig. 5, debris 122 accumulates on surface 581 a. Debris is removed from surface 581a using inhibitor substance 555 (in the vapor phase). The surface 581a is oriented toward the source of debris 122 such that debris 122 accumulates primarily on the surface 581 a.

Inhibitor substance 555 may be directed toward object 580 via delivery system 130 (fig. 1). In some implementations, gas management system 139 controls the flow of inhibitor substance 555 into interior 101 such that inhibitor substance 555 has a uniform flow pattern (e.g., velocity, temperature, and direction) across surface 581 a. 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 581 a.

The radicals 135 combine with the debris 122 to remove the debris 122 from the surface 581a of the object 580. The removal or desorption of debris 122 from surface 581a is shown in FIG. 5 as a star shape with dashed arrows attached to a circle. The recombination of the radicals 135 with species other than the debris 122 is inhibited by the vapor phase inhibitor species 555 and the concentration of radicals 135 available for recombination with the debris 122 increases. Accordingly, the upper portion of the debris 122 on the surface 581a is removed.

Referring to FIG. 6, a side cross-sectional view of an object 680 is shown. The object 680 includes solid phase inhibitor substance 655s (shown as a right triangle in fig. 6) and is exposed to gas phase inhibitor substance 655g (shown as an inverted triangle 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.

Object 680 has substantially the same external shape as object 280 (fig. 2A). The object 680 may be an optical element, such as a reflective optical element 104 (shown in FIG. 1) or any other structure in the interior 101. The object 680 includes

outer surfaces

681a, 681b, 681 c. The

outer surfaces

681a and 681c are substantially flat and extend in the X-Y plane. The outer surface 681b is cylindrical in shape and extends in the Z-direction. In the example of FIG. 6, the surface 681a is oriented toward the source of debris 122 such that debris 122 primarily accumulates on the surface 681 a. The

surfaces

681a, 681b, 681c are adjacent to the body region 682.

Inhibitor substance 655s, which is a solid phase inhibitor substance, is distributed in the bulk region 682 and on the surface 681 a. The solid phase inhibitor substance 655s may be uniformly distributed throughout the object 680, or the concentration of the solid phase inhibitor substance 655s may be higher in some portions of the object 680 than in other portions of the solid phase inhibitor substance 655 s. The vapor phase inhibitor substance 655g is directed by the delivery system 130 (figure 1) toward the surface 681a of the object 680.

The radicals 135 combine with the debris 122 to remove the debris 122 from the surface 681a of the object 680. The removal or desorption of debris 122 from the surface 681a is shown in FIG. 6 as a star with dashed arrows attached to a circle. The recombination of the free radicals 135 with species other than the debris 122 is inhibited by the gas and solid phase inhibitor species 655, and the concentration of free radicals 135 available for recombination with the debris 122 increases. Thus, the higher portion of the debris 122 on the surface 681a is removed.

In implementations where the inhibitor substance is in the gas phase, the inhibitor substance is selected or tuned so as to minimize the effect on transmission of EUV light. Figure 7 shows a graph 700 of the percentage of EUV light transmitted as a function of the partial pressure of the vapor phase inhibitor species inside the vacuum chamber of the EUV light source. The data shown in fig. 7 are analog data. In the example shown in fig. 7, the inhibitor substance is hydrogen sulfide (H2S) gas. The pressure of hydrogen was 1.8 millibar (mbar) and the temperature of H2S was 200 ℃. The percent transmission shown in graph 700 is the percent transmission over a path length of 1 meter. If all EUV light is transmitted, the transmission percentage is 100%. If no EUV light is transmitted, the transmission percentage is 0%.

The partial pressure of the inhibitor substance is zero prior to introduction of the inhibitor substance into the vacuum chamber. In the example represented by graph 700, the percent transmission of EUV light prior to introduction of the inhibitor substance is about 87%. The percentage transmission of EUV light is about 84% when the vapour phase inhibitor species is present in the vacuum chamber at a partial pressure of 0.02 mbar. The percentage transmission of EUV light is about 74% when the vapour phase inhibitor species is present in the vacuum chamber at a partial pressure of 0.1 mbar. For pressures less than about 0.06mbar, the percent transmission of EUV light drops by about 6% or less. Therefore, the H2S vapor phase inhibitor substance can be added in trace amounts without significantly affecting transmission of EUV light. For example, when the partial pressure of the hydrogen sulfide (H2S) inhibitor species 155 is less than about 6mbar, the percentage transmission of EUV light is reduced by 6% or less.

Referring to fig. 8, a flow chart of a process 800 is shown. The process 800 may be used to remove debris from an optical element in an EUV light source.

An object in the optical system is exposed to debris (810). Debris 122 accumulates on the object. In the examples discussed below, the optical system is an EUV light source 100 (fig. 1). The object may be an optical element, such as a reflective optical element 104 (as shown in fig. 1 and 9), any other structure in the interior 101 of the container 109, an object 280 (as shown in fig. 2A and 2B), an object 380 (as shown in fig. 3), an object 480 (as shown in fig. 4), an object 580 (as shown in fig. 5), or an object 680 (as shown in fig. 6).

The 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 metallic material such as, for example, stainless steel, molybdenum, phosphorus nickel, copper or aluminum. The object material may be a non-metallic 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 capable of reacting or complexing with the object material and the debris material. In other words, the radicals 135 may combine with the object material or the debris material. However, when the radicals 135 combine with the debris material, the debris 122 is removed. As described above, the inhibitor substance 155 reduces or eliminates the reaction of the radicals 135 with materials other than the debris 122. To facilitate removal of the debris 122, the radicals 135 are directed toward the object while the inhibitor substance 155 is present.

In some implementations, the inhibitor substance 155 is a solid phase inhibitor substance that is added to the object before the object is in the EUV light source 100. For example, the object may be object 280 (fig. 2A and 2B). As described 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 the installation of the object 280 in the container 109.

In other implementations, the inhibitor substance 155 is a gas phase inhibitor substance that is provided to the object having the radicals 135. For example, and referring also to fig. 5, the inhibitor substance 555 can be directed by the delivery system 130 to the object 580 while the radicals 135 are also directed by the delivery system 130 to the object 580, such that the inhibitor substance 555 and the radicals 135 are present at the object 580 at the same time.

In further implementations, the inhibitor substance 155 includes a solid phase inhibitor substance that is added to the object before the object is installed in the EUV light source 100 and a vapor phase inhibitor substance that is provided to the object while the object is used in the EUV light source 100. For example, the object may be an object 680 as shown in FIG. 6.

Referring to fig. 9, a block diagram of another EUV light source 900 is shown. EUV light source 900 is the same as EUV light source 100 (fig. 1) except that EUV light source includes a vapor phase suppressant delivery system 950 that is separate from radical delivery system 930. In the EUV light source 900, the control system 160 is coupled to a vapor suppressant delivery system 950 and a radical delivery system 930. In light source 900, inhibitor species 955 includes solid phase inhibitor species 955s (shown as a right triangle in fig. 9) and gaseous inhibitor species 955g (shown as an inverted triangle in fig. 9).

The radical delivery system 930 includes a radical gas supply system 933. The radical gas supply system 933 includes a chamber 937 of fluid (such as gas) containing radicals 135. The radical delivery system 930 delivers the radicals 135 to the interior 101 via the fluid conduit 934. Fluid conduit 934 is coupled to container 109 at port 931. Port 931 is fluidly sealed so that a vacuum environment can be maintained in interior 101. The radical gas supply system 933 also includes a radical gas management system 939. The radical gas management system 939 includes flow control devices, such as pumps and valves.

The suppressant delivery system 950 includes a suppressant gas supply system 953. The suppressor gas supply system 953 includes a chamber 957 containing gas phase suppressor substance 955 g. The suppressant delivery system 950 delivers the vapor phase suppressant species 955g to the interior 101 via a fluid conduit 954. The fluid conduit 954 is coupled to the container 109 at a port 951. Port 951 is fluidly sealed so that a vacuum environment can be maintained in interior 101. The suppressor gas supply system 953 further comprises a suppressor gas management system 959. The radical gas management system 959 includes flow control devices such as, for example, pumps and valves.

Control system 160 manages the operation of radical delivery system 930, inhibitor delivery system 950, radical gas management system 939, inhibitor gas management system 959, radical gas supply system 933, and/or inhibitor gas supply system 953. For example, the control system 160 may control the flow rate of the radicals 135 and the vapor phase inhibitor species 955g by controlling valves and/or pumps in the radical gas management system 939 and the inhibitor gas management system 959, respectively. The control system 160 may also be coupled to other systems and components of the EUV light source 900, such as the target supply system 140.

Referring to fig. 10, an implementation of LPPEUV light source 1000 is shown. LPPEUV light source 1000 is an implementation of EUV light source 100 (fig. 1). The interior 1007 of the vacuum chamber 1030 of the LPPEUV light source 1000 includes one or more of the above-described inhibitor substances in a gas phase, a solid phase, or both. As described above, the inhibitor species (in the gas phase, solid phase, or both) in the interior 1007 prevents reactions between the radicals 135 and materials at the surface of the object other than the debris 122, resulting in a higher concentration of radicals 135 that can be used to combine with the debris 122 and more effectively remove the debris 122 from the object.

LPPEUV light source 1000 is formed by irradiating target mixture 1014 with an amplified light beam 1010 traveling along a beam path toward target mixture 1014 at plasma formation region 1005. The target material in the target of stream 121 discussed with respect to fig. 1 can be or include a target mixture 1014. The plasma formation region 1005 is within the interior 1007 of the vacuum chamber 1030. When the amplified light beam 1010 strikes the target mixture 1014, the target material within the target mixture 1014 is converted to a plasma state having elements with emission lines in the EUV range. The plasma generated has certain characteristics that depend on the composition of the target material within the target mixture 1014. These characteristics may include the wavelength of EUV light generated by the plasma and the type and amount of debris released from the plasma

The light source 1000 includes a driving laser system 1015, the driving laser system 1015 generating an amplified light beam 1010 due to a population inversion within one or more gain media 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 that includes a beam transport system 1020 and a focusing assembly 1022. The beam transport system 1020 receives the amplified light beam 1010 from the laser system 1015, and manipulates 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 light beam 1010 to the plasma formation region 1005.

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 a desired wavelength with high gain, an excitation source, and internal optics. The optical amplifier may or may not have a laser mirror or other feedback device that forms the laser cavity. Thus, even without a laser cavity, laser system 1015 produces an amplified light beam 1010 due to population inversion in the gain medium of the laser amplifier. Further, if there is a laser cavity to provide sufficient feedback to the laser system 1015, the laser system 1015 may generate the amplified light beam 1010 as a coherent laser beam. The term "amplified light beam" encompasses one or more of the following: light from the laser system 1015 that is only amplified but not necessarily coherent laser oscillation; light from the laser system 1015 that is amplified and also coherently laser oscillated.

The optical amplifier in the laser system 1015 may include a fill gas as a gain medium, the fill gas including CO2, and may amplify light having a wavelength between about 9100nm and about 11000nm, and particularly about 10600nm, with a gain greater than or equal to 900 times. Suitable amplifiers and lasers for use in laser system 1015 may include pulsed laser devices, e.g., pulsed gas discharge CO2 laser devices that produce radiation at about 9300nm or about 10600nm, e.g., with DC or RF excitation, operating at relatively high power (e.g., 10kW or more) and high pulse repetition rates (e.g., 40kHz or more). The pulse repetition rate may be, for example, 50 kHz. The optical amplifier in the laser system 1015 may also include a cooling system, such as water, which may be used when operating the laser system 1015 at higher powers.

The light source 1000 includes a collector mirror 1035 with a stop 1040 to allow the amplified light beam 1010 to pass through to the plasma formation region 1005. Collector mirror 1035 may be, for example, an ellipsoidal mirror having a primary focus at plasma formation region 1005 and a secondary focus (also referred to as an intermediate focus) at intermediate position 1045, where EUV light may be output from 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 shaped enclosure 1050 (e.g., a cone of gas), the hollow conical shaped enclosure 1050 tapering from the collector mirror 1035 toward the plasma formation region 1005 to reduce the amount of plasma-generating debris entering the focusing assembly 1022 and/or the beam transport system 1020, while allowing the amplified light beam 1010 to reach the plasma formation region 1005. To this end, a gas flow may be provided in a hood oriented towards the plasma formation region 1005.

Light source 1000 may also include a master controller 1055 connected to droplet position detection feedback system 1056, laser control system 1057, and beam control system 1058. The light source 1000 may include one or more target or droplet imagers 1060, the target or droplet imagers 1060 providing an output indicative of, for example, the position of a droplet relative to the plasma formation region 1005 and providing the output to a droplet position detection feedback system 1056, which droplet position detection feedback system 1056 may, for example, calculate the droplet position and trajectory from which droplet position errors may be calculated on a droplet-by-droplet or average basis. Thus, droplet position detection feedback system 1056 provides the droplet position error as an input to master controller 1055. Thus, the master controller 1055 may, for example, provide laser position, orientation, and timing correction signals to the laser control system 1057, which laser control system 1057 may be used, for example, to control laser timing circuitry, and/or to provide laser position, orientation, and timing correction signals to the beam control system 1058 to control the shaping and amplified beam position of the beam transport system 1020 to change the position and/or power of the beam focus within the chamber 1030.

The supply system 1025 comprises a target material delivery control system 1026, the target material delivery control system 1026 being operable, in response to a signal from the main controller 1055, to, for example, modify the release point of a droplet released by the target material supply device 1027 to correct for errors in the droplet reaching the desired plasma formation region 1005.

In addition, the light source 1000 may include light source detectors 1065 and 1070 that measure one or more EUV light parameters including, but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular wavelength band, energy outside a particular wavelength band, and angular distribution of EUV intensity and/or average power. Light source detector 1065 generates a feedback signal that is used by master controller 1055. The feedback signal may, for example, indicate errors in parameters such as timing and focusing of the laser pulses to properly intercept the droplet at the correct position and time for efficient and effective EUV light generation.

The light source 1000 may also include a guiding laser 1075, which guiding laser 1075 may be used to align various segments of the light source 1000 or to facilitate steering the amplified light beam 1010 to the plasma formation region 1005. In conjunction with the guiding laser 1075, the light source 1000 includes a metrology system 1024 positioned within the focus assembly 1022 to sample the amplified light beam 1010 and portions of the light from the guiding laser 1075. In other implementations, the metrology system 1024 is placed within the beam transport system 1020. The metrology system 1024 may include optical elements that sample or redirect a subset of the light, made of any material that can withstand the power of the directed laser beam and the amplified light beam 1010. The beam analysis system is formed by metrology system 1024 and master controller 1055, because master controller 1055 analyzes the sampled light from pilot laser 1075 and uses this information to adjust the components within focusing assembly 1022 through beam control system 1058.

Thus, in summary, the light source 1000 produces an amplified light beam 1010, the amplified light beam 1010 being directed along a beam path to irradiate the target mixture 1014 at the plasma formation region 1005 to convert target material within the mixture 1014 into plasma that emits light in the EUV range. The amplified light beam 1010 operates at a specific wavelength (also referred to as the drive laser wavelength) 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 sufficient feedback to the laser system 1015 to generate a coherent laser, or if the driving laser system 1015 includes appropriate optical feedback to form a laser cavity.

Other aspects of the invention are set forth in the following numbered clauses.

1. An Extreme Ultraviolet (EUV) light source comprising:

a container configured to receive a 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 container; and

an inhibitor substance, wherein, in operational use, the object accumulates debris, the debris 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 according to clause 1, wherein the inhibitor substance comprises a solid-phase inhibitor substance.

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

4. An EUV light source according to clause 3, wherein the solid-phase inhibitor substance is distributed throughout the object.

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

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

7. The EUV light source according to 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 into a bulk region of the object by no more than about 1 micrometer (μm).

9. The EUV light source according to clause 2, wherein the object comprises an optical element.

10. The EUV light source according to clause 9, wherein the object comprises a reflective optical element.

11. The EUV light source according to clause 2, wherein the object comprises a metal inner wall of the container.

12. The EUV light source according to clause 11, wherein the inner wall comprises stainless steel, molybdenum, nickel-phosphorus, copper or aluminum.

13. The EUV light source according to clause 2, wherein the radicals comprise hydrogen radicals; the target material comprises tin; inhibitor substances include arsenic, antimony, bismuth, sulfur, selenium, tellurium, beryllium or cyanide.

14. The EUV light source according to clause 2, wherein the inhibitor substance further comprises a vapor phase inhibitor substance.

15. The EUV light source according to clause 1, wherein the inhibitor substance comprises a vapor phase inhibitor substance.

16. The EUV light source according to clause 1, wherein the delivery system is further configured to deliver a vapor phase inhibitor substance to the interior of the container.

17. The EUV light source of clause 16, wherein the delivery system is configured to deliver a vapor phase suppressant substance to the object.

18. The EUV light source according to clause 17, wherein the vapor phase suppressant binds to a catalytic site on the surface of the object.

19. The EUV light source according to clause 15, wherein the vapor phase inhibitor comprises hydrogen sulfide or arsenic.

20. The EUV light source according to clause 1, wherein the object comprises a coating on the outer surface and the inhibitor substance is in the coating.

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

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

23. A method, comprising:

exposing an object in the 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 species is present while the free radicals are provided to the object, and the inhibitor species inhibits recombination between the free radicals on the object to thereby increase a reaction between the debris and the free radicals.

24. The method according to clause 23, wherein the inhibitor substance is a solid phase inhibitor substance added to the object before the object is in the optical system.

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

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

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

28. The method according to clause 27, wherein the inhibitor substance further comprises a solid phase inhibitor substance, and the solid phase inhibitor substance is added to the object before the object is 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 a target material that emits EUV light when in a plasma state.

30. An apparatus for an optical system, the apparatus comprising:

a host material comprising at least one surface, the at least one surface being a surface at which radicals recombine; and

a solid phase inhibitor substance, at the at least one surface, the inhibitor substance configured to inhibit recombination between free radicals at the surface of the material.

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

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

33. The apparatus of clause 30, wherein the optical system comprises an Extreme Ultraviolet (EUV) light source.

34. The apparatus according to clause 30, wherein the at least one surface is a coating on the host material.

35. The apparatus of clause 34, wherein the coating comprises an oxide coating or a nitride coating.

36. The apparatus of clause 35, wherein the coating comprises titanium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, or yttrium oxide.

Other implementations are within the scope of the following claims.

Claims

1. An Extreme Ultraviolet (EUV) light source comprising:

an object in the interior of the container; and

an inhibitor substance, wherein, in operational use, the object accumulates debris, the debris comprises the target material, the radicals react with at least some of the debris to remove the debris from the object, and the inhibitor substance inhibits recombination of the radicals on the object.

2. An EUV light source according to claim 1, wherein the inhibitor substance comprises a solid phase inhibitor substance.

3. An EUV light source according to claim 2, wherein the solid-phase inhibitor substance is part of the object in the interior of the container.

4. An EUV light source according to claim 3 wherein the solid phase inhibitor substance is distributed throughout the object.

5. An EUV light source according to claim 2, wherein the solid phase inhibitor substance is on the surface of the object.

6. An EUV light source according to claim 5 wherein the inhibitor substance occupies catalytic sites on the surface.

7. An EUV light source according to claim 5 wherein the solid phase inhibitor substance extends into a bulk region of the object.

8. An EUV light source according to claim 7 wherein the solid phase inhibitor substance extends from the surface into the bulk region of the object no more than about 1 micrometer (μm).

9. An EUV light source according to claim 2, wherein the object comprises an optical element.

10. An EUV light source according to claim 9 wherein the object comprises a reflective optical element.

11. An EUV light source as claimed in claim 2, wherein the object comprises a metal inner wall of the container.

12. The EUV light source of claim 11, wherein the inner wall comprises stainless steel, molybdenum, phosphorus nickel, copper, or aluminum.

13. An EUV light source according to claim 2, wherein the radicals comprise hydrogen radicals; the target material comprises tin; and the inhibitor substance comprises arsenic, antimony, bismuth, sulfur, selenium, tellurium, beryllium or cyanide.

14. An EUV light source as claimed in claim 2 wherein the inhibitor species further comprises a vapour phase inhibitor species.

15. An EUV light source as claimed in claim 1 wherein the inhibitor species comprises a vapour phase inhibitor species.

16. An EUV light source as claimed in claim 1 wherein the delivery system is further configured to deliver the vapour phase inhibitor substance to the interior of the container.

17. An EUV light source according to claim 16, wherein the delivery system is configured to deliver the vapour phase inhibitor substance to the object.

18. An EUV light source according to claim 17 wherein the vapour phase inhibitor binds to catalytic sites on the surface of the object.

19. EUV light source according to claim 15, wherein the vapor inhibitor comprises hydrogen sulfide or arsenic.

20. An EUV light source as claimed in claim 1, wherein the object comprises a coating on an outer surface and the inhibitor substance is in the coating.

21. An EUV light source according to claim 20, wherein the coating comprises an oxide coating or a nitride coating.

22. An EUV light source according to claim 21, wherein the coating comprises titanium nitride, zirconium oxide, aluminium 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 a reaction between the debris and the free radicals.

24. The method of claim 23, wherein the inhibitor substance is a solid phase inhibitor substance added to the object before the object is in the optical system.

25. The method of claim 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 claim 23, wherein the inhibitor species is a vapor phase inhibitor species, and the method further comprises: providing the gas-phase inhibitor substance to the object.

27. The method of claim 26, wherein the vapor phase suppressant is provided to the object with the free radicals.

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

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

30. An apparatus for an optical system, the apparatus comprising:

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 material.

31. The apparatus of claim 30, wherein the solid phase inhibitor substance extends into the host material.

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

33. The apparatus of claim 30, wherein the optical system comprises an Extreme Ultraviolet (EUV) light source.

34. The apparatus of claim 30, wherein the at least one surface is a coating on the host material.

35. The apparatus of claim 34, wherein the coating comprises an oxide coating or a nitride coating.

36. The apparatus of claim 35, wherein the coating comprises titanium nitride, zirconium oxide, aluminum oxide, titanium oxide, hafnium oxide, or yttrium oxide.