EUV mask inspection method and equipment

Abstract

The present disclosure provides a method for generating a protective buffer flow in an EUV light source and an EUV mask inspection device. The method includes directing light from the EUV light source toward a collector along a light path. A first buffer gas from a buffer gas injector is injected through a plurality of perforations in the collector. The first buffer gas is directed away from a surface of the collector. A second buffer gas is injected from an annular manifold arranged at a periphery of the collector and at a first distance relative to the collector toward the light path. The second buffer gas is directed away from the surface of the collector. The first distance corresponds to a distance from the collector at which the first buffer gas merges into a single flow.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to provisional patent application filed on February 17, 2021 and designated as U.S. Application No. 63 / 150,091, the disclosure of which is hereby incorporated by reference. Technical Field

[0003] The present disclosure relates to optical systems, and more particularly, to optical systems for semiconductor inspection. Background Art

[0004] The evolution of the semiconductor manufacturing industry places higher demands on yield management and specifically metrology and inspection systems. Critical dimensions continue to shrink, but the industry needs to reduce the time it takes to achieve high-yield, high-value production. Minimizing the total time from detecting a yield problem to resolving said problem determines the return on investment for semiconductor manufacturers.

[0005] The manufacture of semiconductor devices such as logic and memory devices typically includes processing a semiconductor wafer using a number of manufacturing processes to form various features and multiple levels of the semiconductor device. For example, photolithography is a semiconductor manufacturing process that involves transferring a pattern from a mask to a photoresist disposed on a semiconductor wafer. Additional 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 on a single semiconductor wafer in an arrangement that separates the multiple semiconductor devices into individual semiconductor devices.

[0006] Inspection processes are used at various steps during semiconductor manufacturing to detect defects on wafers to promote higher yields and therefore higher profits in the manufacturing process. Inspection has always been an important part of manufacturing semiconductor devices such as integrated circuits (ICs). However, as the size of semiconductor devices decreases, inspection becomes more important for the successful manufacture of acceptable semiconductor devices because smaller defects may cause device failures. For example, as the size of semiconductor devices decreases, the detection of defects of reduced size has become necessary because even relatively small defects may still cause undesirable aberrations in semiconductor devices.

[0007] Some inspection processes used to inspect masks operate using extreme ultraviolet (EUV) light. The use of EUV light presents many challenges to optical systems. For example, optical systems may rely on EUV light sources that may use or contain tin (Sn) to create plasma. When the plasma is generated, Sn is converted from a solid or liquid state to an ionized gas. As a metal, Sn condenses on any component that is close to the plasma in the form of small particles or metallic coatings. These particles and coatings damage optical components by reducing the transmittance and reflectivity of optical components and increasing surface roughness.

[0008] Therefore, there is a need for an EUV mask inspection apparatus that protects optical components from being damaged by contaminants. Summary of the invention

[0009] Embodiments of the present disclosure provide a method for generating a protective buffer flow in an EUV light source. The method may include: directing light from the EUV light source toward a collector along a light path; injecting a first buffer gas from a buffer gas injector through a plurality of perforations in the collector, wherein the first buffer gas is directed away from a surface of the collector; and injecting a second buffer gas from an annular manifold arranged at a periphery of the collector and at a first distance relative to the collector toward the light path, wherein the second buffer gas is directed away from the surface of the collector. The first distance may correspond to a distance from the collector at which the first buffer gas merges into a single flow.

[0010] According to an embodiment of the present disclosure, a rate of injecting the second buffer gas may be greater than a rate of injecting the first buffer gas.

[0011] According to an 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.

[0012] According to an embodiment of the present disclosure, a plurality of perforations in a light collector may be arranged around a surface of the light collector and may be spaced such that a distance between adjacent perforations exists between 1 mm and 50 mm.

[0013] According to an embodiment of the present disclosure, the annular manifold may include a plurality of buffer gas injectors arranged in a circle around the annular manifold, and the plurality of buffer gas injectors may be spaced such that a distance between adjacent buffer gas injectors exists between 1 mm and 50 mm.

[0014] According to an embodiment of the present disclosure, the first buffer gas and the second buffer gas may be directed toward the light path upstream of the light collector.

[0015] According to an embodiment of the present disclosure, a method may include injecting a third buffer gas from a second annular manifold arranged at a periphery of the annular manifold and at a second distance relative to the annular manifold toward the light path, wherein the third buffer gas is directed away from a surface of the light collector. The second distance may correspond to a distance from the annular manifold at which the second buffer gas merges into a single stream.

[0016] According to an embodiment of the present disclosure, the light collector may be a mirror.

[0017] Embodiments of the present disclosure provide an EUV mask inspection apparatus. The apparatus may include: an EUV light source configured to emit EUV light along a light path; a light collector arranged in the light path; a buffer gas injector configured to inject a first buffer gas through a plurality of perforations in the light collector, wherein the first buffer gas is directed away from a surface of the light collector; and an annular manifold configured to inject a second buffer gas, arranged at a periphery of the light collector and at a first distance relative to the light collector toward the light path, wherein the second buffer gas is directed away from the surface of the light collector. The first distance may correspond to a distance from the light collector at which the first buffer gas merges into a single stream.

[0018] According to an embodiment of the present disclosure, a rate of injecting the second buffer gas may be greater than a rate of injecting the first buffer gas.

[0019] According to an 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.

[0020] According to an embodiment of the present disclosure, a plurality of perforations in a light collector may be arranged around a surface of the light collector and may be spaced such that a distance between adjacent perforations exists between 1 mm and 50 mm.

[0021] According to an embodiment of the present disclosure, the annular manifold may include a plurality of buffer gas injectors arranged in a circle around the annular manifold, and the plurality of buffer gas injectors may be spaced such that there is a maximum distance between adjacent buffer gas injectors of between 1 mm and 50 mm.

[0022] According to an embodiment of the present disclosure, the apparatus may further include a protective skirt arranged at the periphery of the annular manifold, the protective skirt extending a second distance toward the light path. The second distance may correspond to a distance from the annular manifold at which the second buffer gas merges into a single flow.

[0023] According to an embodiment of the present disclosure, the apparatus may further include a second annular manifold configured to inject a third buffer gas, arranged at a periphery of the annular manifold and at a second distance relative to the annular manifold toward the light path, wherein the third buffer gas is directed away from the surface of the light collector. The second distance may correspond to a distance from the annular manifold at which the second buffer gas merges into a single stream.

[0024] According to an embodiment of the present disclosure, the light collector may be a mirror.

[0025] According to an embodiment of the present disclosure, the annular manifold may be sealed to the light collector.

[0026] According to an embodiment of the present disclosure, there may be a gap between the annular manifold and the light collector. The device may further include: a first wall on the light collector extending away from the light path; and a second wall on the annular manifold extending away from the light path. The first wall and the second wall may be separated by the gap. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] For a fuller understanding of the nature and purpose of the present disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, wherein:

[0028] Figure 1 is a schematic diagram of an EUV mask inspection device according to an embodiment of the present disclosure;

[0029] Figure 2 is a schematic diagram of an EUV mask inspection device according to another embodiment of the present disclosure;

[0030] Figure 3 is a schematic diagram of an EUV mask inspection device according to another embodiment of the present disclosure;

[0031] Figure 4 is a schematic diagram of an EUV mask inspection device according to another embodiment of the present disclosure;

[0032] Figure 5 is a schematic diagram of an EUV mask inspection device according to another embodiment of the present disclosure;

[0033] Fig. 6A is a top view of a light collector of an EUV mask inspection device according to an embodiment of the present disclosure;

[0034] Figure 6B is a top view of a ring-shaped manifold of an EUV mask inspection apparatus according to an embodiment of the present disclosure;

[0035] Figure 6C is a top view of a secondary annular manifold of an EUV mask inspection apparatus according to an embodiment of the present disclosure;

[0036] Figure 7is a flow chart of a method for generating a protection buffer flow in an EUV light source according to an embodiment of the present disclosure; and

[0037] Figure 8 is a schematic diagram illustrating the concept of individual gas jets merging into a single stream. DETAILED DESCRIPTION

[0038] Although the claimed subject matter will be described in terms of certain embodiments, other embodiments including embodiments that do not provide all of the advantages and features set forth herein are also within the scope of the present disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the present disclosure. Therefore, the scope of the present disclosure is defined solely by reference to the appended claims.

[0039] The embodiment of the present disclosure provides an EUV mask inspection device 100. Figure 1 , apparatus 100 may include an EUV light source 110. EUV light source 110 may be configured to emit EUV light. For example, EUV light source 110 may utilize tin (Sn), xenon (Xe), or lithium (Li) as a plasma medium. When plasma is generated at EUV light source 110, the plasma medium is converted from a solid or liquid state to an ionized gas. The plasma may be generated by a discharge produced plasma (DPP) or a laser produced plasma (LPP). EUV light may be emitted along an optical path 112. There may be one or more optical components along optical path 112.

[0040] The apparatus 100 may further include a light collector 120 disposed in the light path 112. The light collector 120 may be configured to redirect EUV light from the EUV light source 110. For example, the light collector 120 may be a mirror. The mirror may be a 2D or 3D shape. For example, the mirror may be an oblate ellipse (2D), a curved ellipse (3D), or an ellipse with a straight or arbitrarily shaped cutout. Depending on the distance of the light collector from the EUV light source 110, the focus to which the light is converged, and the amount of power of the system, the light collector 120 may have various form errors, surface roughness, and sizes. The light collector 120 may be curved. For example, the light collector 120 may be concave or convex. The light collector 120, the EUV light source 110, and the optical components along the light path 112 may be in a vacuum.

[0041] The apparatus 100 may further include a buffer gas injector 130. The buffer gas injector 130 may be configured to inject a first buffer gas 132 through a plurality of perforations 122 in the collector 120. The first buffer gas 132 may be directed away from a surface of the collector 120. The first buffer gas 132 may be directed toward an optical path 112 upstream of the collector 120. The first buffer gas 132 may be supplied from a first gas source 102. The first gas source 102 may be connected to the buffer gas injector 130 via a first gas conduit 103. There may be more than one first gas source 102. The first gas source 102 may be a pressurized gas cylinder or a gas supply from a facility manifold. The plurality of perforations 122 may be arranged around a surface of the collector 120. For example, Fig. 6A , the plurality of perforations 122 may be spaced so as to cover the surface of the light collector 120. The plurality of perforations 122 may be spaced such that there is a distance between 1 mm and 100 mm between adjacent perforations 122. It may be appreciated that when the first buffer gas 132 is injected through the plurality of perforations 122, the individual jets will merge into a single stream at a distance from the light collector 120 that may be oriented toward the light path 112. The concept of merging may be defined at a location in space, referred to as a merging line, where the angles of the streams from the various sources converge to a single angle. For example, as Figure 7 As shown in FIG. 1 , a merging line can be defined at a distance from the light collector 120, where the individual streams of the first buffer gas 132 merge into a single stream. Thus, in the region between the stream sources and the merging line, the individual streams are separated, which can allow contaminants to penetrate between the individual streams. However, in the region outside the merging line, the individual streams are combined into a single stream, which forms a protective barrier and prevents most or all contaminants from penetrating into the stream. For example, more than 80%, more than 90%, more than 95%, or more than 99% of the contaminants can be blocked outside the merging line.

[0042] The apparatus 100 may further include an annular manifold 140. The annular manifold 140 may have an annular shape. For example, the annular manifold 140 may be an annular ellipse and an annular circle or any other annular shape. The annular manifold 140 may be configured to inject a second buffer gas 142 directed away from the surface of the light collector 120. The second buffer gas 142 may be directed toward the light path 112 upstream of the light collector 120. The second buffer gas 142 may be supplied from a second gas source 104. The second gas source 104 may be a pressurized gas cylinder or a gas supply from a facility manifold. The second gas source 104 may be connected to the annular manifold 140 via a second gas conduit 105. There may be more than one second gas source 104. The annular manifold 140 may be arranged at the periphery of the light collector 120. For example, the annular manifold 140 may be arranged radially outward from the light collector 120. The annular manifold 140 may be arranged at a first distance D1 relative to the light collector 120 toward the light path 112. The first distance D1 may correspond to a distance from the light collector 120 at which the first buffer gas 132 merges into a single stream. For example, the first distance D1 may be between 1 mm and 20 mm. In a particular embodiment, the first distance D1 may be 5 mm. It may be appreciated that when the annular manifold 140 is arranged at the first distance D1 relative to the light collector 120 toward the light path 112, contaminants near the light collector 120 may be less likely to penetrate the first buffer gas 132 and reach the light collector 120 due to the second buffer gas 142. The contaminants may be Sn or other materials from the plasma medium, but may be other particles or materials, such as hydrocarbons. The size of the contaminants may be between 1 nm and 50 nm.

[0043] The annular manifold 140 may include a plurality of buffer gas injectors 144 arranged circularly around the annular manifold 140. For example, Figure 6B , the plurality of buffer gas injectors 144 may be arranged in an ellipse at the top surface of the annular manifold 140. The plurality of buffer gas injectors 144 may be spaced apart such that there is a distance between 1 mm and 50 mm between adjacent buffer gas injectors 144. It may be appreciated that when the second buffer gas 142 is injected from the plurality of buffer gas injectors 144, the individual jets will merge into a single stream at a certain distance from the annular manifold 140 toward the light path 112.

[0044] The first buffer gas 132 and the second buffer gas 142 can define a protective buffer flow around the light path 112. For example, the first buffer gas 132 can protect the center of the light collector 120, and the second buffer gas 142 can protect the edge of the light collector 120. In this way, the light collector 120 can be protected from contaminants in the inspection system and plasma generated by the EUV light source 110.

[0045] The first buffer gas 132 and the second buffer gas 142 may be injected into the system at different rates. For example, the ratio between the rates of the second buffer gas 142 and the first buffer gas 132 may be between 10:1 and 1:10. In a particular embodiment, the ratio between the rates of the second buffer gas 142 and the first buffer gas 132 may be 3:1. It can be appreciated that a larger flow rate may be desired 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 perforations 122 in the light collector 120 compared to the possible flow rate of the second buffer gas 142 from the annular manifold 140. In addition, the contamination risk from convection at the edge of the light collector 120 may be greater than the contamination risk from diffusion at the center of the light collector 120. Therefore, the flow rate of the second buffer gas 142 injected may be greater than the rate of the first buffer gas 132 injected. This may improve the protective effect of the protective buffer flow compared to an inspection system that injects only the first buffer gas 132.

[0046] 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 described, one gas source may be used in the case where the first buffer gas 132 and the second buffer gas 142 are the same. The gas source may have a pressure from 10 Pa to 10,000 Pa. In a specific embodiment, the gas source may have a pressure of about 1,000 Pa.

[0047] like Figure 2 , the apparatus 100 may further include a protective skirt 150. The protective skirt 150 may have an annular shape. The protective skirt 150 may be arranged at the periphery of the annular manifold 140. For example, the protective skirt 150 may be disposed on the outer surface of the annular manifold 140. The protective skirt 150 may be fixed to the annular manifold 140 using welding, fasteners, or adhesives. Alternatively, the protective skirt 150 may be an integral part of the annular manifold 140. The protective skirt 150 may extend a second distance D2 toward the light path 112 upstream of the light collector 120. The second distance D2 may correspond to a distance from the annular manifold 140 at which the second buffer gas 142 merges into a single stream. For example, the second distance D2 may be between 1 mm and 20 mm. In a specific embodiment, the second distance D2 may be 5 mm. It can be appreciated that when the protective skirt 150 extends the second distance D2 relative to the annular manifold 140 toward the light path 112 , contaminants near the light collector 120 may be less likely to penetrate the second buffer gas 142 (and the first buffer gas 132 ) and reach the light collector 120 .

[0048] The plasma medium mass concentration (PMMC) on the optical component to be protected can be used as an indication of the effectiveness of the protection buffer flow. Based on computational fluid dynamics (CFD) modeling and other methods, the improvement through reducing PMMC can be calculated. In the test example, PMMC was calculated on the light collector 120 using various designs of the present disclosure. The test results are shown in Table 1.

[0049] Table 1

[0050]

[0051]

[0052] According to the test results, the protection of the light collector 120 can be improved by adding the annular manifold 140 and the second buffer gas 142. The protection of the light collector 120 can be further improved by adding the protective skirt 150 to the annular manifold 140.

[0053] like Figure 3 , the apparatus 100 may further include a secondary annular manifold 160. The secondary annular manifold 160 may have an annular shape. For example, the secondary annular manifold 160 may be an annular ellipse and annular circle or any other annular shape. The secondary annular manifold 160 may be configured to inject a third buffer gas 162 directed away from the surface of the light collector. The third buffer gas 162 may be directed toward the light path 112 upstream of the light collector 120. The third buffer gas 162 may be supplied from a third gas source 106. The third gas source 106 may be a pressurized gas cylinder or a gas supply from a facility manifold. The third gas source 106 may be connected to the secondary annular manifold 160 by a third gas conduit 107. There may be more than one third gas source 106. The secondary annular manifold 160 may be arranged at the periphery of the annular manifold 140. For example, the secondary annular manifold 160 may be arranged radially outward from the annular manifold 140. The secondary annular manifold 160 may be arranged at a second distance D2 relative to the annular manifold 140 toward the light path 112. The second distance D2 may correspond to a distance from the annular manifold 140 at which the second buffer gas 142 merges into a single stream. For example, the second distance D2 may be between 1 mm and 20 mm. In a particular embodiment, the second distance D2 may be 5 mm. It may be appreciated that when the secondary annular manifold 160 is arranged at the second distance D2 relative to the annular manifold 140 toward the light path 112, contaminants near the light collector 120 may be less likely to penetrate the second buffer gas 142 (and the first buffer gas 132) and reach the light collector 120 due to the third buffer gas 162.

[0054] The second buffer gas 142 and the third buffer gas 162 may be injected into the system at the same rate. Alternatively, the second buffer gas 142 and the third buffer gas 162 may be injected into the system at different rates. The third buffer gas 162 may be argon, helium, hydrogen, nitrogen, neon, krypton, xenon, or a mixture thereof.

[0055] The secondary annular manifold 160 may include a plurality of secondary buffer gas injectors 164 arranged circularly around the second annular manifold 160. For example, Figure 6C , the plurality of second buffer gas injectors 164 may be arranged in an ellipse at the top surface of the second annular manifold 160. The plurality of secondary buffer gas injectors 164 may be spaced apart such that there is a maximum distance between 1 mm and 50 mm between adjacent secondary buffer gas injectors 164. It may be appreciated that when the third buffer gas 162 is injected from the plurality of secondary buffer gas injectors 164, the individual jets will merge into a single stream at a certain distance from the secondary annular manifold 160 toward the light path 112.

[0056] According to the embodiments of the present disclosure, Figure 4 , the annular manifold 140 can be sealed to the light collector 120. For example, the annular manifold 140 can be arranged radially outward from the light collector 120 so that there is no gap between the annular manifold 140 and the light collector 120. An o-ring, a gasket, or any other sealing device can be provided between the annular manifold 140 and the light collector 120 to ensure the seal between the annular manifold 140 and the light collector 120. The sealing device can be disposed on the annular manifold 140, the light collector 120, or any other component of the system. The annular manifold 140 can also extend away from the light path 112 so that there is no gap between the annular manifold 140 and the light collector 120. Therefore, the back of the light collector 120 can be protected from contamination.

[0057] According to the embodiments of the present disclosure, Figure 51 , there may be a gap 170 between the annular manifold 140 and the light collector 120. For example, the annular manifold 140 may be arranged radially outward from the light collector 120 so that there is a gap 170 between the annular manifold 140 and the light collector 120. In the presence of the gap 170, the back of the light collector 120 may not be protected from contamination. However, the apparatus 100 may further include a first wall 172 and a second wall 174. The first wall 172 may be disposed on the light collector 120 and extend away from the light path 112. The second wall 174 may be disposed on the annular manifold 140 and extend away from the light 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 a distance L away from the light collector 120 and the annular manifold 140. It can be appreciated that contaminants entering the gap 170 may collide with the first wall 172 and the second wall 174 and may protect the back of the light collector 120. The length L of the gap 170 between the first wall 172 and the second wall 174 may be determined based on the width of the gap 170 and the pressure of the system.

[0058] Using the EUV mask inspection apparatus 100 , inspection performance may be optimized by increasing light intensity on the reticle and the lifetime of the optical concentrator 120 close to the EUV light source 110 based on the protection buffer flow defined by the first buffer gas 132 and the second buffer gas 142 .

[0059] Embodiments of the present disclosure may provide a method 200 for generating a protective buffer flow in an EUV light source. Figure 8 As shown in FIG. 2 , method 200 may include the following steps.

[0060] At step 210, light from an EUV light source is directed toward a light collector. The EUV light source may be configured to emit EUV light. For example, the EUV light source may utilize Sn, Xe, or Li as a plasma medium or generate a plasma medium. When plasma is generated at the EUV light source, the plasma medium is converted from a solid or liquid state to an ionized gas. The plasma may be generated by a DPP or LPP. EUV light may be emitted along a light path.

[0061] In step 220, a first buffer gas is injected from a buffer gas injector through a plurality of perforations in a light collector. The light collector may be arranged in the light path. The light collector may be configured to redirect EUV light from an EUV light source. For example, the light collector may be a mirror. The mirror may be a 2D or 3D shape. For example, the mirror may be an oblate ellipse (2D), a curved ellipse (3D), or an ellipse with a straight or arbitrarily shaped cutout. The light collector may be curved. For example, the light collector may be concave or convex. The first buffer gas may be directed away from a surface of the light collector. The first buffer gas may be directed toward the light path upstream of the light collector.

[0062] A plurality of perforations may be arranged around the surface of the light collector. Fig. 6A As shown in FIG. 1 , the plurality of perforations may be spaced so as to cover the surface of the light collector. The plurality of perforations may be spaced such that there is a distance between adjacent perforations of between 1 mm and 100 mm. It can be appreciated that when the first buffer gas is injected through the plurality of perforations, the individual jets will merge into a single stream at a distance from the light collector toward the light path.

[0063] In step 230, a second buffer gas is injected from an annular manifold arranged at a first distance relative to the collector toward the light path. The second buffer gas may be directed away from the surface of the collector. The second buffer gas may be directed toward the light path upstream of the collector. The first distance may correspond to a distance from the collector at which the first buffer gas merges into a single stream. For example, the first distance may be between 1 mm and 20 mm. In a particular embodiment, the first distance may be 5 mm. It may be appreciated that when the annular manifold is arranged at a first distance relative to the collector toward the light path, contaminants near the collector may be less likely to penetrate the first buffer gas and reach the collector due to the second buffer gas. The contaminants may be Sn or other materials from the plasma medium, but may be other particles or materials, such as hydrocarbons. The size of the contaminants may be between 1 nm and 50 nm.

[0064] The first buffer gas and the second buffer gas may be directed away from the surface of the light collector. The shape of the surface of the light collector may affect the direction of the first buffer gas and the second buffer gas, but the perforations in the light collector may 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 relative to each other for convergent or divergent flows. For example, the angle between the first buffer gas and the second buffer gas may be between -45 degrees and 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).

[0065] The annular manifold may include a plurality of buffer gas injectors arranged in a circle around the annular manifold. Figure 6B , multiple buffer gas injectors may be arranged in an ellipse at the top surface of the annular manifold. The multiple buffer gas injectors may be spaced such that there is a maximum distance between adjacent buffer gas injectors of between 1 mm and 50 mm. It may be appreciated that when injecting the second buffer gas from the multiple buffer gas injectors, the individual jets will merge into a single stream at a distance from the annular manifold toward the light path.

[0066] The first buffer gas and the second buffer gas can define a protective buffer flow around the light path. For example, the first buffer gas can protect the center of the light collector and the second buffer gas can protect the edge of the light collector. In this way, the light collector can be protected from contaminants in the inspection system and plasma generated by the EUV light source.

[0067] The first buffer gas and the second buffer gas may be injected into the system at different rates. For example, the ratio between the rate of the second buffer gas and the rate of the first buffer gas may be between 10:1 and 1:10. In a particular embodiment, the ratio between the rate of the second buffer gas and the rate of the first buffer gas may be 3:1. It can be appreciated that a larger flow rate may be desired 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 multiple perforations in the collector compared to the possible flow rate of the second buffer gas from the annular manifold. In addition, the risk of contamination from convection at the edge of the collector may be greater than the risk of contamination from diffusion at the center of the collector. Therefore, the flow rate of the second buffer gas may be greater than the rate of the first buffer gas. This may improve the protective effect of the protective buffer flow compared to an inspection system that only injects the first buffer gas.

[0068] 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.

[0069] The method 200 may further include providing a protective skirt arranged at the periphery of the annular manifold, the protective skirt extending a second distance toward the light path upstream of the collector. The protective skirt may be disposed on the outer surface of the annular manifold. The protective skirt may be fixed to the annular manifold using welding, fasteners, or adhesives. Alternatively, the protective skirt may be an integral part of the annular manifold. The second distance may correspond to a distance from the annular manifold at which the second buffer gas merges into a single stream. For example, the second distance may be between 1 mm and 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 light path relative to the annular manifold, contaminants near the collector may be less likely to penetrate the second buffer gas (and the first buffer gas) and reach the collector.

[0070] The method 200 may further include: injecting a third buffer gas from a second annular manifold arranged at a periphery of the annular manifold and arranged at a second distance relative to the annular manifold toward the light path. The third buffer gas may be directed away from the surface of the collector. The third buffer gas may be directed toward the light path upstream of the collector. The second distance may correspond to a distance from the annular manifold at which the second buffer gas merges into a single stream. For example, the second distance may be between 1 mm and 20 mm. In a particular embodiment, the second distance may be 5 mm. It can be understood that when the second annular manifold is arranged at a second distance relative to the annular manifold toward the light path, due to the third buffer gas, contaminants near the collector may be less likely to penetrate the second buffer gas (and the first buffer gas) and reach the collector.

[0071] The second buffer gas and the third buffer gas may be injected into the system at the same rate. Alternatively, the second buffer gas and the third buffer gas may be injected into the system at different rates. The third buffer gas may be argon, helium, hydrogen, nitrogen, neon, krypton, xenon, or a mixture thereof.

[0072] According to an embodiment of the present disclosure, the annular manifold can be sealed to the light collector. For example, the annular manifold can be arranged radially outward from the light collector so that there is no gap between the annular manifold and the light collector. The annular manifold can also extend away from the light path so that there is no gap between the annular manifold and the light collector. Therefore, the method 200 can protect the back of the light collector from contamination.

[0073] According to an embodiment of the present disclosure, there may be a gap between the annular manifold and the light collector. For example, the annular manifold may be arranged radially outward from the light collector so that there is a gap between the annular manifold and the light collector. In the case of the presence of the gap, the back of the light collector may not be protected from contamination. However, the method 200 may further include: providing a first wall on the light collector extending away from the light path; and providing a second wall on the annular manifold extending away from the light path. The first wall and the second wall may be separated by the gap.

[0074] The first wall and the second wall may extend a distance away from the light collector and the annular manifold. It can be appreciated that contaminants entering the gap may collide with the first wall and the second wall and may protect the back of the light 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.

[0075] Using the method 200 for generating a protection buffer flow in an EUV light source, inspection performance may be optimized by increasing light intensity on a reticle and lifetime of optical collectors close to the EUV light source based on the protection buffer flow defined by the first buffer gas and the second buffer gas.

[0076] Although the present disclosure has been described with respect to one or more specific embodiments, it will be appreciated that other embodiments of the present disclosure may be made without departing from the scope of the present disclosure. Therefore, the present disclosure is deemed to be limited only by the appended claims and reasonable interpretation thereof.

Claims

1. A method for generating a protective buffer flow in an EUV light source, comprising: directing light from the EUV light source along a light path toward a light collector; injecting a first buffer gas from a buffer gas injector through a plurality of perforations in the light collector, wherein the first buffer gas is directed away from a surface of the light collector; and injecting a second buffer gas from a manifold buffer gas injector included in an annular manifold, the annular manifold being arranged at a periphery of the light collector such that there is a gap between the annular manifold and the light collector and the annular manifold is arranged at a first distance relative to the light collector toward the light path, wherein the manifold buffer gas injector is away from the surface of the light collector, wherein the second buffer gas is directed away from the surface of the light collector and is directed away from the surface of the light collector further than the first distance, wherein the light collector has a first wall extending away from the light path and the annular manifold has a second wall extending away from the light path, and wherein the first wall and the second wall are separated by the gap; The first distance corresponds to a distance from the light collector at which the first buffer gases merge into a single stream. 2 . The method of claim 1 , wherein a rate of injecting the second buffer gas is greater than a rate of injecting the first buffer gas. The method of claim 1 , wherein the first buffer gas and the second buffer gas are the same. 4 . The method of claim 3 , wherein the first buffer gas and the second buffer gas each comprise argon, helium, hydrogen, nitrogen, neon, krypton, or xenon.

5. The method of claim 1, wherein the plurality of perforations in the light collector are arranged around the surface of the light collector and are spaced such that there is a distance between adjacent perforations of between 1 mm and 50 mm.

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

7. The method of claim 1, wherein the first buffer gas and the second buffer gas are directed toward the light path upstream of the light collector.

8. The method according to claim 1, further comprising: injecting a third buffer gas from a second annular manifold disposed at a periphery of the annular manifold and disposed at a second distance relative to the annular manifold toward the light path, wherein the third buffer gas is directed away from the surface of the light collector; Wherein the second distance corresponds to a distance from the annular manifold at which the second buffer gas merges into a single stream.

9. The method of claim 1, wherein the light collector is a mirror.

10. An EUV mask inspection device, comprising: an EUV light source configured to emit EUV light along an optical path; a light collector arranged in the light path; a buffer gas injector configured to inject a first buffer gas through a plurality of perforations in the light collector, wherein the first buffer gas is directed away from a surface of the light collector; an annular manifold including a manifold buffer gas injector configured to inject a second buffer gas, the annular manifold being arranged at a periphery of the light collector such that there is a gap between the annular manifold and the light collector and the annular manifold being arranged at a first distance relative to the light collector toward the light path, wherein the manifold buffer gas injector is away from the surface of the light collector, and wherein the second buffer gas is directed away from the surface of the light collector and is directed farther away from the surface of the light collector than the first distance; a first wall of the light collector extending away from the light path; and a second wall on the annular manifold extending away from the light path, wherein the first wall and the second wall are separated by the gap; The first distance corresponds to a distance from the light collector at which the first buffer gases merge into a single stream.

11. The apparatus of claim 10, wherein a rate of injecting the second buffer gas is greater than a rate of injecting the first buffer gas.

12. The apparatus of claim 10, wherein the first buffer gas and the second buffer gas are the same.

13. The apparatus of claim 12, wherein the first buffer gas and the second buffer gas each comprise argon, helium, hydrogen, nitrogen, neon, krypton, or xenon.

14. The apparatus of claim 10, wherein the plurality of perforations in the light collector are arranged around the surface of the light collector and are spaced such that there is a distance between 1 mm and 50 mm between adjacent perforations.

15. The apparatus of claim 10, wherein the annular manifold comprises a plurality of manifold buffer gas injectors arranged around the annular manifold, and the plurality of manifold buffer gas injectors are spaced such that a maximum distance between adjacent manifold buffer gas injectors is between 1 mm and 50 mm.

16. The apparatus of claim 10, further comprising: a protective skirt disposed at the periphery of the annular manifold, the protective skirt extending toward the light path by a second distance; Wherein the second distance corresponds to a distance from the annular manifold at which the second buffer gas merges into a single stream.

17. The apparatus of claim 10, further comprising: a second annular manifold configured to inject a third buffer gas disposed at a periphery of the annular manifold and at a second distance relative to the annular manifold toward the light path, wherein the third buffer gas is directed away from the surface of the light collector; Wherein the second distance corresponds to a distance from the annular manifold at which the second buffer gas merges into a single stream.

18. The apparatus of claim 10, wherein the light collector is a mirror.

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