Method and apparatus for EUV mask inspection

Abstract

A method and EUV mask inspection apparatus 100 for generating a protective buffer flow in an EUV light source 110 is provided. The method includes directing light from the EUV light source 110 to a collector 120 along a light path 112. A first buffer gas 132 from a buffer gas injector 130 is injected through a plurality of through holes 122 in the collector 120. The first buffer gas 132 is directed away from a surface of the collector 120. A second buffer gas 142 is injected from a ring manifold 140 disposed around the collector 120 and disposed a first distance D1 toward the light path 112 relative to the collector 120. The second buffer gas 142 is directed away from the surface of the collector 120. The first distance D1 corresponds to a distance from the collector 120 where the first buffer gas 132 merge into a single flow.

Description

Technical Field

[0001] The present disclosure relates to an optical system, and more particularly to an optical system for semiconductor inspection.

Background Art

[0002] This application was filed on February 17, 2021, claims priority to a provisional patent application to which U.S. Patent Application No. 63 / 150,091 was issued, the disclosure of which is incorporated herein by reference. With the development of the semiconductor manufacturing industry, the requirements for yield management, particularly for measurement systems and inspection systems, are increasing. Although the critical dimensions continue to shrink, the industry needs to shorten the time to achieve high-yield and high-value production. The return on investment of semiconductor manufacturers is determined by minimizing the total time from detecting a yield problem to solving it.

[0003] Manufacturing semiconductor devices such as logic devices and memory devices typically involves processing semiconductor wafers using a number of manufacturing processes to form various features and multiple levels of the semiconductor device. For example, lithography is a semiconductor manufacturing process that includes transferring a pattern from a reticle to a photoresist disposed on a semiconductor wafer. Further examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. Multiple semiconductor devices may be manufactured while disposed on a single semiconductor wafer and may be separated into individual semiconductor devices.

[0004] The inspection process is used in various steps during semiconductor manufacturing to detect defects on the wafer, improve the yield in the manufacturing process, and thus improve the profit. Inspection has always been an important part of the manufacturing of semiconductor devices such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to successfully manufacture acceptable semiconductor devices because smaller defects can cause device failures. For example, as the dimensions of semiconductor devices decrease, even relatively small defects can cause undesirable abnormalities in the semiconductor device, so it is necessary to detect defects of decreasing size.

[0005] A specific inspection process for photomask inspection operates using extreme ultraviolet (EUV) light. The use of EUV light presents many challenges to the optical system. For example, the optical system may rely on an EUV light source that uses or can contain tin (Sn) to generate plasma. While the plasma is being generated, Sn changes from a solid or liquid state to an ionized gas. Being a metal, Sn condenses in the form of small particles or metal coatings on any component in proximity to the plasma. These particles and coatings not only reduce the transmittance and reflectivity of the optical components but also damage the optical components by increasing the surface roughness.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] Therefore, there is a need for an EUV mask inspection apparatus that protects optical components from damage by contaminants.

Means for Solving the Problem

[0008] One embodiment of the present disclosure provides a method for generating a protective buffer flow in an EUV light source. The method includes directing light along an optical path from an EUV light source to a collector, injecting a first buffer gas from a buffer gas injector through a plurality of through holes of the collector, wherein the first buffer gas is directed away from the surface of the collector, and injecting a second buffer gas from a ring manifold disposed around the collector and disposed at a first distance from the collector along the optical path, wherein the second buffer gas is directed away from the surface of the collector. The first distance can correspond to the distance from the collector where the first buffer gas converges into a single stream.

[0009] According to one embodiment of the present disclosure, the amount of the second buffer gas injected may be greater than the amount of the first buffer gas injected.

[0010] According to one embodiment of the present disclosure, the first buffer gas and the second buffer gas may be the same. The first buffer gas and the second buffer gas can each include argon, helium, hydrogen, nitrogen, neon, krypton, or xenon.

[0011] According to one embodiment of the present disclosure, the plurality of through holes of the collector may be disposed around the surface of the collector and may be spaced such that a distance of 1 mm to 50 mm exists between adjacent through holes.

[0012] According to one embodiment of the present disclosure, the ring manifold may include a plurality of buffer gas injectors disposed circularly around the ring manifold, and the plurality of buffer gas injectors may be spaced such that a distance of 1 mm to 50 mm exists between adjacent buffer gas injectors.

[0013] According to one embodiment of the present disclosure, the first buffer gas and the second buffer gas may be directed towards the optical path upstream from the collector.

[0014] According to one embodiment of the present disclosure, the method can include the step of injecting a third buffer gas from a second ring manifold disposed around the ring manifold and disposed at a second distance from the ring manifold towards the optical path, the third buffer gas being directed away from the surface of the collector. The second distance can correspond to the distance from the ring manifold where the second buffer gas merges into a single flow.

[0015] According to one embodiment of the present disclosure, the collector may be a mirror.

[0016] One embodiment of the present disclosure provides an EUV mask inspection apparatus. The apparatus includes an EUV light source configured to emit EUV light along an optical path, a collector disposed in the optical path, a buffer gas injector configured to inject a first buffer gas through a plurality of through holes of the collector, the first buffer gas being directed away from the surface of the collector, and a ring manifold configured to inject a second buffer gas, disposed around the collector, and disposed at a first distance from the collector towards the optical path, the second buffer gas being directed away from the surface of the collector. The first distance can correspond to the distance from the collector where the first buffer gas merges into a single flow.

[0017] According to one embodiment of the present disclosure, the amount of the second buffer gas injected may be greater than the amount of the first buffer gas injected.

[0018] According to one embodiment of the present disclosure, the first buffer gas and the second buffer gas may be the same. The first buffer gas and the second buffer gas may each include argon, helium, hydrogen, nitrogen, neon, krypton, or xenon.

[0019] According to one embodiment of the present disclosure, the plurality of through holes of the collector may be arranged around the surface of the collector, and may be spaced such that a distance of 1 mm to 50 mm exists between adjacent through holes.

[0020] According to one embodiment of the present disclosure, the ring manifold may include a plurality of buffer gas injectors arranged circularly around the ring manifold, and the plurality of buffer gas injectors may be spaced such that a maximum distance of 1 mm to 50 mm exists between adjacent buffer gas injectors.

[0021] According to one embodiment of the present disclosure, the apparatus can further include a protective skirt arranged around the ring manifold that extends a second distance toward the optical path. The second distance can correspond to the distance from the ring manifold where the second buffer gas merges into a single stream.

[0022] According to one embodiment of the present disclosure, the apparatus can further include a second ring manifold configured to inject a third buffer gas, arranged around the ring manifold, and arranged at a second distance toward the optical path with respect to the ring manifold, and the third buffer gas is directed away from the surface of the collector. The second distance can correspond to the distance from the ring manifold where the second buffer gas merges into a single stream.

[0023] According to one embodiment of the present disclosure, the collector may be a mirror.

[0024] According to one embodiment of the present disclosure, the ring manifold may be sealed with respect to the collector.

[0025] According to an embodiment of the present disclosure, there may be a gap between the ring manifold and the collector. The apparatus may further include a first wall on the collector extending away from the optical path and a second wall on the ring manifold extending away from the optical path. The first wall and the second wall may be separated by a gap.

[0026] For a more complete understanding of the nature and objects of the present disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.

Brief Description of the Drawings

[0027]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6A

Figure 6B

Figure 6C

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0028] The claimed subject matter is described with respect to specific embodiments, but other embodiments including those that do not provide all of the advantages and features described herein are also within the scope of the present disclosure. Various structural, logical, process step, and electronic changes can be made without departing from the scope of the present disclosure. Accordingly, the scope of the present disclosure is defined only by reference to the appended claims.

[0029] One embodiment of the present disclosure provides an EUV mask inspection apparatus 100. As shown in FIG. 1, the apparatus 100 can include an EUV light source 110. The EUV light source 110 can be configured to emit EUV light. For example, the EUV light source 110 can utilize tin (Sn), xenon (Xe), or lithium (Li) as a plasma medium. When plasma is generated in the EUV light source 110, the plasma medium changes from a solid or liquid state to an ionized gas. The plasma can be generated by a discharge-produced plasma (DPP) or a laser-produced plasma (LPP). The EUV light can be emitted along an optical path 112. One or more optical components may be present along the optical path 112.

[0030] The apparatus 100 can further include a collector 120 disposed within the optical path 112. The collector 120 can be configured to change the direction of the EUV light from the EUV light source 110. For example, the collector 120 can be a mirror. The mirror can be in a 2D or 3D shape. For example, the mirror can be a flat ellipse (2D), a curved ellipse (3D), or an ellipse with a notch having a straight line or an arbitrary shape. The collector 120 can have various shape errors, surface roughness, and sizes depending on the distance of the collector from the EUV light source 110, the focal point where the light converges, and the amount of power of the system. The collector 120 can be curved. For example, the collector 120 can be concave or convex. The collector 120, the EUV light source 110, and the optical components along the optical path 112 can exist in a vacuum.

[0031] The device 100 can further include a buffer gas injector 130. The buffer gas injector 130 can be configured to inject a first buffer gas 132 through a plurality of through holes 122 of the collector 120. The first buffer gas 132 may be directed away from the surface of the collector 120. The first buffer gas 132 may be directed towards the optical path 112 upstream from the collector 120. The first buffer gas 132 can be supplied from a first gas source 102. The first gas source 102 may be connected to the buffer gas injector 130 by a first gas pipe 103. There may be two or more first gas sources 102. The first gas source 102 may be a gas supply source from a pressurized cylinder or a facility manifold. The plurality of through holes 122 may be arranged around the surface of the collector 120. For example, as shown in FIG. 6A, the plurality of through holes 122 may be spaced apart and cover the surface of the collector 120. The plurality of through holes 122 may be spaced apart such that a distance of 1 mm to 100 mm exists between adjacent through holes 122. When the first buffer gas 132 is injected through the plurality of through holes 122, it can be understood that the individual jets merge into a single flow at a certain distance from the collector 120, and this flow may be directed towards the optical path 112. The concept of merging can be defined at a specific position in space known as the merging line, where the angles of the flows from various sources converge to a single angle. For example, as shown in FIG. 7, the merging line can be defined at a certain distance from the collector 120 where the individual flows of the first buffer gas 132 merge into a single flow. Therefore, in the region between the flow source and the merging line, the individual flows are separated, thereby allowing contaminants to penetrate between the individual flows. However, in the region beyond the merging line, the individual flows merge into a single flow, and this flow forms a protective barrier to prevent most or all contaminants from penetrating into this flow. For example, more than 80%, more than 90%, more than 95%, or more than 99% of the contaminants can be blocked when crossing the merging line.

[0032] The device 100 can further include a ring manifold 140. The ring manifold 140 can have an annular shape. For example, the ring manifold 140 can be an annular ellipse, an annular circle, or any other annular shape. The ring manifold 140 can be configured to inject a second buffer gas 142 directed away from the surface of the collector 120. The second buffer gas 142 can be directed toward the optical path 112 upstream from the collector 120. The second buffer gas 142 can be supplied from a second gas source 104. The second gas source 104 can be a gas supply source from a pressure cylinder or a facility manifold. The second gas source 104 can be connected to the ring manifold 140 by a second gas pipe 105. There can be two or more second gas sources 104. The ring manifold 140 can be disposed around the collector 120. For example, the ring manifold 140 can be disposed radially outward from the collector 120. The ring manifold 140 can be disposed at a first distance D1 from the collector 120 in the direction of the optical path 112. The first distance D1 can correspond to the distance from the collector 120 where the first buffer gas 132 converges into a single stream. For example, the first distance D1 can be between 1 mm and 20 mm. In a particular embodiment, the first distance D1 can be 5 mm. It can be understood that when the ring manifold 140 is disposed at the first distance D1 from the collector 120 in the direction of the optical path 112, contaminants in the vicinity of the collector 120 can be less likely to reach the collector 120 by penetrating through the first buffer gas 132 due to the second buffer gas 142. The contaminants can be Sn or other materials from the plasma medium, but can also be other particles or materials such as hydrocarbons. The contaminants can be on the order of 1 nm to 50 nm in size.

[0033] The ring manifold 140 can include a plurality of buffer gas injectors 144 arranged circularly around the ring manifold 140. For example, as shown in FIG. 6B, the plurality of buffer gas injectors 144 may be arranged elliptically on the upper surface of the ring manifold 140. The plurality of buffer gas injectors 144 may be spaced such that there is a distance of 1 mm to 50 mm between adjacent buffer gas injectors 144. It can be understood that when the second buffer gas 142 is injected from the plurality of buffer gas injectors 144, the individual jets merge into a single flow at a certain distance from the ring manifold 140 towards the optical path 112.

[0034] The first buffer gas 132 and the second buffer gas 142 can define a protective buffer flow surrounding the optical path 112. For example, the first buffer gas 132 can protect the center of the collector 120, and the second buffer gas 142 can protect the edge of the collector 120. In this way, the collector 120 can be protected from contaminants within the inspection system and from the plasma generated by the EUV light source 110.

[0035] The first buffer gas 132 and the second buffer gas 142 may be injected into the system in different amounts. For example, the ratio of the amount of the second buffer gas 142 to the amount of the first buffer gas 132 may be from 10:1 to 1:10. In a particular embodiment, the ratio of the amount of the second buffer gas 142 to the amount of the first buffer gas 132 may be 3:1. It can be understood that a larger flow rate may be desirable to improve the protective effect of the first buffer gas 132 and the second buffer gas 142. However, the flow rate of the first buffer gas 132 may be limited due to the size of the plurality of through holes 122 of the collector 120 as compared with the possible flow rate of the second buffer gas 142 from the ring manifold 140. In addition, the risk of contamination due to convection at the edge of the collector 120 may be greater than the risk of contamination due to diffusion at the center of the collector 120. Therefore, the flow rate of the second buffer gas 142 injected may be greater than the amount of the first buffer gas 132 injected. Thereby, the protective effect of the protective buffer flow can be improved as compared with an inspection system that injects only the first buffer gas 132.

[0036] The first buffer gas 132 and the second buffer gas 142 may be the same. For example, the first buffer gas 132 and the second buffer gas 142 may be argon, helium, hydrogen, nitrogen, neon, krypton, xenon, or a mixture thereof. Alternatively, the first buffer gas 132 and the second buffer gas 142 may be different species or mixtures. Although the first gas source 102 and the second gas source 104 are shown, if the first buffer gas 132 and the second buffer gas 142 are the same, one gas source may be used. The gas source can have a pressure of 10 Pa to 10,000 Pa. In a particular embodiment, the gas source can have a pressure of about 1000 Pa.

[0037] As shown in FIG. 2, the apparatus 100 can further include a protective skirt 150. The protective skirt 150 can have an annular shape. The protective skirt 150 can be disposed around the ring manifold 140. For example, the protective skirt 150 can be disposed on the outer surface of the ring manifold 140. The protective skirt 150 may be fixed to the ring manifold 140 using welding, fasteners, or adhesives. Alternatively, the protective skirt 150 may be an integral part of the ring manifold 140. The protective skirt 150 can extend a second distance D2 toward the optical path 112 upstream from the collector 120. The second distance D2 can correspond to the distance from the ring manifold 140 where the second buffer gas 142 merges into a single flow. For example, the second distance D2 can be between 1 mm and 20 mm. In certain embodiments, the second distance D2 can be 5 mm. It can be understood that when the protective skirt 150 extends a second distance D2 toward the optical path 112 with respect to the ring manifold 140, contaminants in the vicinity of the collector 120 can be less likely to reach the collector 120 by penetrating through the second buffer gas 142 (and the first buffer gas 132).

[0038] The plasma medium mass concentration (PMMC) on the optical component to be protected can be used as an indicator of the effectiveness of the protective buffer flow. Based on computational fluid dynamics (CFD) modeling and other methods, the improvement due to the reduction of PMMC can be calculated. In the tested examples, the PMMC was calculated on the collector 120 using various designs of the present disclosure. The test results are shown in Table 1.

Table 1

[0039] According to the test results, the protection of the collector 120 can be improved by adding the ring manifold 140 and the second buffer gas 142. The protection of the collector 120 can be further improved by adding the protective skirt 150 to the ring manifold 140.

[0040] As shown in FIG. 3, the apparatus 100 can further include a second ring manifold 160. The second ring manifold 160 can have an annular shape. For example, the second ring manifold 160 can be an annular ellipse, an annular circle, or any other annular shape. The second ring manifold 160 can be configured to inject a third buffer gas 162 directed away from the surface of the collector. The third buffer gas 162 can be directed toward the optical path 112 upstream of the collector 120. The third buffer gas 162 can be supplied from a third gas source 106. The third gas source 106 can be a pressurized cylinder or a gas supply source from a facility manifold. The third gas source 106 can be connected to the second ring manifold 160 by a third gas pipe 107. There can be two or more third gas sources 106. The second ring manifold 160 can be disposed around the ring manifold 140. For example, the second ring manifold 160 can be disposed radially outward from the ring manifold 140. The second ring manifold 160 can be disposed at a second distance D2 from the ring manifold 140 toward the optical path 112. The second distance D2 can correspond to the distance from the ring manifold 140 where the second buffer gas 142 merges into a single stream. For example, the second distance D2 can be from 1 mm to 20 mm. In a particular embodiment, the second distance D2 can be 5 mm. When the second ring manifold 160 is disposed at the second distance D2 from the ring manifold 140 toward the optical path 112, it can be understood that contaminants in the vicinity of the collector 120 are less likely to reach the collector 120 by penetrating through the second buffer gas 142 (and the first buffer gas 132) due to the third buffer gas 162.

[0041] The second buffer gas 142 and the third buffer gas 162 may be injected into the system in the same amount. Alternatively, the second buffer gas 142 and the third buffer gas 162 may be injected into the system in different amounts. The third buffer gas 162 may be argon, helium, hydrogen, nitrogen, neon, krypton, xenon, or a mixture thereof.

[0042] The second ring manifold 160 may include a plurality of second buffer gas injectors 164 arranged circularly around the second ring manifold 160. For example, as shown in FIG. 6C, the plurality of second buffer gas injectors 164 may be arranged elliptically on the upper surface of the second ring manifold 160. The plurality of second buffer gas injectors 164 may be spaced such that a maximum distance of 1 mm to 50 mm exists between adjacent second buffer gas injectors 164. It can be understood that when the third buffer gas 162 is injected from the plurality of second buffer gas injectors 164, the individual jets merge into a single stream at a distance from the second ring manifold 160 toward the optical path 112.

[0043] According to an embodiment of the present disclosure, as shown in FIG. 4, the ring manifold 140 may be sealed with respect to the collector 120. For example, the ring manifold 140 may be arranged radially outward from the collector 120 so that there is no gap between the ring manifold 140 and the collector 120. To ensure sealing between the ring manifold 140 and the collector 120, an O-ring, gasket, or any other sealing device may be provided between the ring manifold 140 and the collector 120. The sealing device may be arranged on the ring manifold 140, on the collector 120, or on any other component of the system. The ring manifold 140 may also extend away from the optical path 112 so that there is no gap between the ring manifold 140 and the collector 120. Thus, the back surface of the collector 120 can be protected from contaminants.

[0044] According to one embodiment of the present disclosure, as shown in FIG. 5, there may be a gap 170 between the ring manifold 140 and the collector 120. For example, the ring manifold 140 may be disposed radially outward from the collector 120 such that there is a gap 170 between the ring manifold 140 and the collector 120. With the gap 170, the back surface of the collector 120 cannot be protected from contaminants. However, the apparatus 100 can further include a first wall 172 and a second wall 174. The first wall 172 may be disposed on the collector 120 and extend away from the optical path 112. The second wall 174 may be disposed on the ring manifold 140 and extend away from the optical path 112. The first wall 172 and the second wall 174 may be separated by the gap 170. The first wall 172 and the second wall 174 may extend away from the collector 120 and the ring manifold 140 by a distance L. It can be understood that contaminants entering the gap 170 can collide with the first wall 172 and the second wall 174, protecting the back surface of the collector 120. The length L of the gap 170 between the first wall 172 and the second wall 174 can be determined based on the width of the gap 170 and the pressure of the system.

[0045] Using the EUV mask inspection apparatus 100, based on the protective buffer flow defined by the first buffer gas 132 and the second buffer gas 142, not only the light intensity on the reticle but also the lifetime of the optical collector 120 close to the EUV light source 110 can be increased, thereby optimizing the inspection performance.

[0046] One embodiment of the present disclosure can provide a method 200 for generating a protective buffer flow in an EUV light source. As shown in FIG. 8, the method 200 can include the following steps.

[0047] In step 210, the light from the EUV light source is directed towards the collector. The EUV light source can be configured to emit EUV light. For example, the EUV light source can utilize Sn, Xe, or Li as a plasma medium or to generate a plasma medium. When plasma is generated in the EUV light source, the plasma medium changes from a solid or liquid state to an ionized gas. The plasma can be generated by DPP or LPP. The EUV light may be emitted along the optical path.

[0048] In step 220, the first buffer gas is injected from the buffer gas injector through a plurality of through holes of the collector. The collector may be disposed in the optical path. The collector can be configured to change the direction of the EUV light from the EUV light source. For example, the collector may be a mirror. The mirror may be in a 2D or 3D shape. For example, the mirror may be a flat ellipse (2D), a curved ellipse (3D), or an ellipse having a notch with a straight line or any shape. The collector may be curved. For example, the collector may be concave or convex. The first buffer gas may be directed away from the surface of the collector. The first buffer gas may be directed towards the optical path upstream from the collector.

[0049] The plurality of through holes may be disposed around the surface of the collector. For example, as shown in FIG. 6A, the plurality of through holes may be spaced apart and can cover the surface of the collector. The plurality of through holes may be spaced such that a distance of 1 mm to 100 mm exists between adjacent through holes. It can be understood that when the first buffer gas is injected through the plurality of through holes, the individual jets merge into a single flow at a certain distance from the collector towards the optical path.

[0050] In step 230, a second buffer gas is ejected from a ring manifold disposed around the collector and disposed at a first distance from the collector in the optical path direction with respect to the collector. The second buffer gas may be directed away from the surface of the collector. The second buffer gas may be directed into the optical path upstream from the collector. The first distance can correspond to the distance from the collector where the first buffer gas merges into a single stream. For example, the first distance may be from 1 mm to 20 mm. In a particular embodiment, the first distance may be 5 mm. It can be understood that when the ring manifold is disposed at the first distance from the collector in the optical path direction, contaminants in the vicinity of the collector are less likely to reach the collector by penetrating through the first buffer gas due to the second buffer gas. The contaminants may be Sn or other materials from the plasma medium, but may also be other particles or materials such as hydrocarbons. The contaminants may be on the order of 1 nm to 50 nm in size.

[0051] The first buffer gas and the second buffer gas may be directed away from the surface of the collector. The shape of the surface of the collector can affect the direction of the first buffer gas and the second buffer gas, but the through holes in the collector can also affect the direction of the first buffer gas and the second buffer gas. The first buffer gas and the second buffer gas may be angled with respect to each other for a converging or diverging flow. For example, the angle between the first buffer gas and the second buffer gas may be from -45 degrees to 45 degrees. In a particular embodiment, the angle between the first buffer gas and the second buffer gas may be 0 degrees (i.e., the first buffer gas and the second buffer gas are parallel).

[0052] The ring manifold can comprise a plurality of buffer gas injectors arranged circularly around the ring manifold. For example, as shown in FIG. 6B, the plurality of buffer gas injectors may be arranged elliptically on the upper surface of the ring manifold. The plurality of buffer gas injectors may be spaced such that a maximum distance of 1 mm to 50 mm exists between adjacent buffer gas injectors. When the second buffer gas is injected from the plurality of buffer gas injectors, it can be understood that the individual jets merge into a single flow at a distance from the ring manifold towards the optical path.

[0053] The first buffer gas and the second buffer gas can define a protective buffer flow surrounding the optical path. For example, the first buffer gas can protect the center of the collector, and the second buffer gas can protect the edge of the collector. In this way, the collector can be protected from contaminants within the inspection system and from the plasma generated by the EUV light source.

[0054] The first buffer gas and the second buffer gas may be injected into the system in different amounts. For example, the ratio of the amount of the second buffer gas to the amount of the first buffer gas may be from 10:1 to 1:10. In a particular embodiment, the ratio of the amount of the second buffer gas to the amount of the first buffer gas may be 3:1. It can be understood that a larger flow rate may be desirable in order to improve the protective effect of the first buffer gas and the second buffer gas. However, the flow rate of the first buffer gas may be limited due to the size of the plurality of through-holes of the collector compared to the possible flow rate of the second buffer gas from the ring manifold. In addition, the risk of contamination due to convection at the edge of the collector may be greater than the risk of contamination due to diffusion at the center of the collector. Therefore, the flow rate of the second buffer gas injected may be greater than the amount of the first buffer gas injected. This can improve the protective effect of the protective buffer flow compared to an inspection system that injects only the first buffer gas.

[0055] The first buffer gas and the second buffer gas may be the same. For example, the first buffer gas and the second buffer gas may be argon, helium, hydrogen, nitrogen, neon, krypton, xenon, or a mixture thereof. Alternatively, the first buffer gas and the second buffer gas may be different species or mixtures.

[0056] Method 200 can further include providing a protective skirt disposed around the ring manifold that extends a second distance toward the optical path upstream from the collector. The protective skirt may be disposed on the outer surface of the ring manifold. The protective skirt may be fixed to the ring manifold using welding, fasteners, or an adhesive. Alternatively, the protective skirt may be an integral part of the ring manifold. The second distance can correspond to the distance from the ring manifold where the second buffer gas merges into a single flow. For example, the second distance may be from 1 mm to 20 mm. In a particular embodiment, the second distance may be 5 mm. It can be understood that when the protective skirt extends a second distance toward the optical path with respect to the ring manifold, contaminants in the vicinity of the collector may be less likely to reach the collector by penetrating through the second buffer gas (and the first buffer gas).

[0057] Method 200 can further include the step of injecting a third buffer gas from a second ring manifold disposed around the ring manifold and disposed at a second distance from the ring manifold in the optical path direction. The third buffer gas may be directed away from the surface of the collector. The third buffer gas may be directed into the optical path upstream from the collector. The second distance can correspond to the distance from the ring manifold where the second buffer gas merges into a single stream. For example, the second distance may be from 1 mm to 20 mm. In certain embodiments, the second distance may be 5 mm. It can be understood that when the second ring manifold is disposed at a second distance from the ring manifold in the optical path direction, contaminants near the collector are less likely to reach the collector by penetrating through the second buffer gas (and the first buffer gas) due to the third buffer gas.

[0058] The second buffer gas and the third buffer gas may be injected into the system in the same amount. Alternatively, the second buffer gas and the third buffer gas may be injected into the system in different amounts. The third buffer gas may be argon, helium, hydrogen, nitrogen, neon, krypton, xenon, or a mixture thereof.

[0059] According to one embodiment of the present disclosure, the ring manifold may be sealed with respect to the collector. For example, the ring manifold may be disposed radially outward from the collector such that there is no gap between the ring manifold and the collector. The ring manifold may also extend away from the optical path such that there is no gap between the ring manifold and the collector. Thus, method 200 can protect the back surface of the collector from contaminants.

[0060] According to one embodiment of the present disclosure, there may be a gap between the ring manifold and the collector. For example, the ring manifold may be arranged radially outward from the collector such that there is a gap between the ring manifold and the collector. If there is a gap, the back surface of the collector cannot be protected from contaminants. However, method 200 can further include providing a first wall extending away from the optical path on the collector and providing a second wall extending away from the optical path on the ring manifold. The first wall and the second wall may be separated by the gap.

[0061] The first wall and the second wall may extend away from the collector and the ring manifold by a certain distance. It can be understood that contaminants entering the gap can collide with the first wall and the second wall, protecting the back surface of the collector. The length of the gap between the first wall and the second wall may be determined based on the width of the gap and the pressure of the system.

[0062] Using method 200 for generating a protection buffer flow in an EUV light source, not only the light intensity on the reticle but also the lifetime of the optical collector close to the EUV light source can be increased based on the protection buffer flow defined by the first buffer gas and the second buffer gas, thereby optimizing the inspection performance.

[0063] Although the present disclosure has been described with respect to one or more specific embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the scope of the present disclosure. Accordingly, the present disclosure is considered to be limited only by the appended claims and their reasonable interpretations.

Claims

1. A method for generating a protective buffer flow in an EUV light source, comprising: directing light along an optical path from the EUV light source to a collector; injecting a first buffer gas from a buffer gas injector through a plurality of through-holes of the collector, wherein the first buffer gas is directed away from the surface of the collector; injecting a second buffer gas from a ring manifold disposed around the collector and disposed at a first distance from the collector along the optical path, wherein the second buffer gas is directed away from the surface of the collector; wherein the first distance is the distance from the collector at which the first buffer gas merges into a single flow.

2. The method according to claim 1, wherein the amount of the second buffer gas injected is greater than the amount of the first buffer gas injected.

3. The method according to claim 1, wherein the first buffer gas and the second buffer gas are the same.

4. The method according to claim 3, wherein the first buffer gas and the second buffer gas each contain argon, helium, hydrogen, nitrogen, neon, krypton, or xenon.

5. The method according to claim 1, wherein the plurality of through-holes of the collector are arranged around the surface of the collector and spaced such that a distance of 1 mm to 50 mm exists between adjacent through-holes.

6. The method according to claim 1, wherein the ring manifold comprises a plurality of buffer gas injectors arranged around the ring manifold and spaced such that a distance of 1 mm to 50 mm exists between adjacent buffer gas injectors.

7. The method according to claim 1, wherein the first buffer gas and the second buffer gas are directed along the optical path upstream of the collector.

8. The method according to claim 1, wherein Injecting a third buffer gas from a second ring manifold disposed around the ring manifold and disposed at a second distance from the ring manifold toward the optical path, wherein the third buffer gas is directed away from the surface of the collector further comprising wherein the second distance is the distance from the ring manifold where the second buffer gas merges into a single stream A method characterized by this. **Claim 9** The method according to claim 1, characterized in that the collector is a mirror. **Claim 10** An EUV mask inspection apparatus, comprising an EUV light source configured to emit EUV light along an optical path; a collector disposed in the optical path; a buffer gas injector configured to inject a first buffer gas through a plurality of through holes of the collector, wherein the first buffer gas is directed away from the surface of the collector; a ring manifold configured to inject a second buffer gas, disposed around the collector, and disposed at a first distance from the collector toward the optical path, wherein the second buffer gas is directed away from the surface of the collector; comprising The EUV mask inspection apparatus is characterized in that the first distance is the distance from the collector where the first buffer gas merges into a single stream. **Claim 11** The apparatus according to claim 10, characterized in that the amount of the second buffer gas injected is greater than the amount of the first buffer gas injected. **Claim 12** The apparatus according to claim 10, characterized in that the first buffer gas and the second buffer gas are the same. **Claim 13** The apparatus according to claim 12, characterized in that the first buffer gas and the second buffer gas each contain argon, helium, hydrogen, nitrogen, neon, krypton, or xenon. **Claim 14** The apparatus according to claim 10, characterized in that the plurality of through holes of the collector are disposed around the surface of the collector and are spaced such that a distance of 1 mm to 50 mm exists between adjacent through holes. **Claim 15** The apparatus according to claim 10, wherein the ring manifold comprises a plurality of buffer gas injectors arranged around the ring manifold, and the plurality of buffer gas injectors are spaced such that a maximum distance of 1 mm to 50 mm exists between adjacent buffer gas injectors.

16. The apparatus according to claim 10, a protective skirt disposed around the ring manifold and extending a second distance toward the optical path further comprising wherein the second distance is the distance from the ring manifold at which the second buffer gas merges into a single stream characterized in that

17. The apparatus according to claim 10, a second ring manifold configured to inject a third buffer gas, disposed around the ring manifold, and disposed at a second distance from the ring manifold toward the optical path, wherein the third buffer gas is directed away from the surface of the collector further comprising wherein the second distance is the distance from the ring manifold at which the second buffer gas merges into a single stream characterized in that

18. The apparatus according to claim 10, wherein the collector is a mirror.

19. The apparatus according to claim 10, wherein the ring manifold is sealed with respect to the collector.

20. The apparatus according to claim 10, wherein there is a gap between the ring manifold and the collector, a first wall on the collector extending away from the optical path, a second wall on the ring manifold extending away from the optical path, further comprising wherein the first wall and the second wall are separated by the gap characterized in that

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