Substrate for supporting reticle and related systems and methods

Abstract

Substrate devices are described for supporting, transporting, and processing a reticle, including for supporting the reticle during a step of detecting particle contamination at a surface of the reticle prior to use of the reticle in an extreme ultraviolet (EUV) lithography process. The substrate device may be designed to prevent or reduce the amount of light used in a reticle back side inspection system, such as reflected from a substrate through edge structures of the substrate and redirected to a camera of the reticle back side inspection system.

Description

Technical Field

[0001] This description relates to a substrate apparatus for supporting, transporting, and handling photomasks, including supporting the photomask during a step of detecting particulate contaminants on the surface of the photomask prior to its use in an extreme ultraviolet (EUV) lithography process. Background Technology

[0002] One of the commonly used process steps in the fabrication of integrated circuits and other microelectronic and semiconductor devices is photolithography. Broadly speaking, photolithography involves selectively exposing a specially prepared surface to a radiation source using a patterned template to create an etched surface layer. Depending on the method, the patterned template is a photomask, which is an extremely flat glass plate with a pattern to be reproduced on its surface.

[0003] Photolithography steps can be used multiple times when fabricating microelectronic devices on semiconductor wafer substrates. Useful photolithography techniques can use light of various wavelengths, including light in the ultraviolet range, deep ultraviolet range, and extreme ultraviolet range.

[0004] In some process examples, a semiconductor wafer surface can be fabricated by first depositing silicon nitride onto a surface, followed by coating with a photosensitive liquid polymer or photoresist. Next, ultraviolet (UV) light (e.g., extreme ultraviolet light, "EUV") is transmitted through or reflected from the surface of a mask or photomask to project the desired pattern onto the photoresist-covered wafer. The light-exposed portions of the photoresist are chemically modified and remain unaffected when the wafer subsequently undergoes a chemical process to remove the unexposed photoresist, thus shaping the modified photoresist on the wafer into the form of the pattern on the mask. The wafer then undergoes an etching process to remove the exposed portions of the nitride layer, resulting in a nitride pattern on the wafer precisely designed by the mask.

[0005] When performing photolithography using EUV light, patterned light is applied to the photoresist by being reflected from the patterned photomask rather than transmitted through it. To achieve maximum reflection of light from the photomask (also called a "reflective photomask") onto the photoresist, the surfaces of the photomask that receive EUV light for reflection must be as free as possible from defects, contaminants, and damage. During EUV lithography, particulate contaminants on the photomask surface can be detrimental to its performance. These contaminants can interfere with the reflected light and can also affect the shape or positioning of the photomask, either of which will alter the direction and quality of the reflected light. Summary of the Invention

[0006] To reduce or avoid particulate contaminants on the photomask surface during the lithography process, specialized equipment and systems are used to inspect the photomask surface for particulate contaminants before its use in the lithography process. These systems are sometimes referred to as “back-of-mask inspection” (RBI) devices or modules and can be included as standalone modules in larger EUV lithography systems. During inspection, the photomask is supported by the flat surface of a substrate (sometimes referred to herein as an “EUV substrate” or “substrate”). The substrate is useful for supporting, carrying, and transferring the photomask during the various steps of preparing and using it in the EUV lithography process.

[0007] The rear-side inspection system operates to detect particles present on the rear (unpatterned) surface of the photomask by illuminating the rear (unpatterned) surface of the photomask while the patterned (opposite) surface of the photomask is horizontally supported by a substrate. See also: Figure 1 And related text. Light is directed at a shallow angle to the surface behind the photomask. If the light is not interrupted by particulate contaminants at said surface, then the light will be reflected from said surface at the same shallow angle (“reflection angle”).

[0008] However, if particles are present at the surface, light guided to the surface at a shallow angle is redirected (deflected, reflected, scattered, etc.) away from the photomask surface in various directions other than the shallow reflection angle. The detector of the photomask back-side inspection system includes a camera positioned above the photomask, facing downwards toward the back surface of the photomask. The camera receives a portion of the light scattered and redirected away from the photomask surface due to particles at the surface. The camera correlates the redirected light with the presence of particulate contaminants at the surface.

[0009] In addition to light reflected from particulate contaminants, it is also possible to deflect light directed at the photomask surface from the substrate supporting the photomask during the inspection step. Light used to inspect the rear surface can pass through the transparent portion of the photomask and reach the substrate. The edge portion at the perimeter of the EUV photomask is typically transparent, and light used to detect particles on the photomask surface can pass through the transparent edge of the photomask and reach the substrate below. Normally, light can be reflected from the substrate in a direction that does not reach the camera. However, some structures present on the substrate may redirect (reflect or deflect) the light toward the camera. If light is redirected from the substrate to the camera, the camera will associate that light with the presence of particulate contaminants on the rear surface of the photomask; that is, when no particles are present, the system will generate a "false positive" reading indicating the presence of particles on the surface via the detector.

[0010] In these back-of-mask inspection systems, it is necessary to reduce or avoid false positive readings caused by light reflecting off the EUV substrate and toward the camera (which associates light with particulate contaminants).

[0011] According to this description, the substrate structure of an EUV substrate and, for example, a photomask support, can be designed to prevent or reduce the amount of light used in a photomask-back inspection system, the light being reflected from the substrate, for example, by an edge structure of the substrate and redirected to the camera of the photomask-back inspection system. The substrate may include features that reduce that amount of light, the light being deflected or reflected from the edge surface of the substrate in a direction that would cause the deflected light to reach the camera of the system and record false positive readings. These features include "low-reflectivity surfaces" applied to or formed on the edge structure.

[0012] One type of low-reflectivity surface is a surface that is shaped or angled to prevent light reflected from the low-reflectivity surface from being redirected in the direction toward the camera. Examples of this type of low-reflectivity surface include surfaces with edge structures having a low radius of curvature and surfaces with edge structures having a flat beveled ("chamfered") surface that directs any light hitting the surface in a direction away from the camera.

[0013] Alternatively, the low-reflectivity surface may be made of or coated with a material designed to have low reflectivity (e.g., low spectral reflectivity) for the wavelengths of light emitted by the illuminator. The low-reflectivity surface may be a surface treated, for example, by chemical etching, laser ablation, laser texturing, mechanical abrasion, etc., to form a roughened surface that exhibits relatively low reflectivity relative to the surface before treatment, for example, by scattering light in a direction away from the surface. The low-reflectivity surface may also be a coated material applied to the surface, wherein the coated material has low spectral reflectivity for the wavelengths of light emitted by the illuminator.

[0014] Alternatively, the substrate may include features (e.g., a “mask” or “screen”) that, when the photomask is supported by the substrate, block light emitted by the illuminator before it reaches the photomask, preventing light from passing through the transparent portion of the photomask (e.g., at the transparent edge), and preventing light from reaching the substrate at the edge structure and possibly redirecting the light from the edge structure toward the camera.

[0015] On one hand, the present invention relates to a substrate for supporting a photomask. The substrate includes: a horizontal upper substrate surface and one or more edge structures. An example of an edge structure is the edge of an aperture positioned vertically through the substrate, the aperture including an aperture edge structure formed between a vertical aperture sidewall and the upper substrate surface. Another example of an edge structure is the edge structure of a photomask support located within the aperture. The photomask support may include: a photomask support body engaging the aperture sidewall; an upper photomask support surface located above the upper substrate surface and adjusted to support a photomask above the upper substrate surface; and a photomask support edge structure formed between the photomask support body and the upper photomask support surface. The substrate including these or the edge structures may include a low-reflectivity surface at the edge structure. For example, the aperture edge structure may include a low-reflectivity surface in the form of: a bevel; a radius of curvature less than 0.3 mm; a surface having a spectral reflectivity at least 10% lower than that of the upper substrate surface; or a combination thereof. The edge structure of the photomask support may include a low-reflectivity surface in the form of: a bevel; a radius of curvature less than 0.3 mm; a surface having a spectral reflectance at least 10% lower than that of the upper substrate surface; or a combination thereof.

[0016] On the other hand, the present invention relates to a photomask backside inspection device, comprising: a vacuum chamber having an interior; a substrate according to the present description and claims, located in the interior; an illuminator capable of directing light onto the surface of a photomask supported on the upper surface of the substrate; and a camera adapted to detect light redirected by particles on the surface of the photomask.

[0017] In another aspect, the present invention relates to a method for detecting particles on the rear surface of a photomask using a photomask rear-side particle inspection system as described herein. The method includes using a photomask rear-side inspection system comprising: a vacuum chamber having an interior; a substrate according to the description and claims, located within the interior; an illuminator capable of directing light onto an upper surface of the substrate; a photomask supported on the substrate; and a camera adapted to detect light redirected from particles on the upper surface of the substrate to detect particles on the rear side of the photomask. The method further includes directing light from the illuminator onto the rear surface of the photomask and the upper surface of the substrate, and using the camera to detect light reflected from particles on the upper surface of the substrate. The method may further include using the light detected by the camera to determine the amount of particles present on the upper surface of the substrate. Attached Figure Description

[0018] Figure 1 Showing a side sectional view of the example photomask back inspection system as described.

[0019] Figure 2A and 2B This is a side perspective view of the example substrate as described.

[0020] Figure 3 This is a side perspective view of the example substrate as described.

[0021] Figure 4A , 4B 4C and 4D are side sectional views of an example substrate containing edge structures as described.

[0022] Figure 5 Showing a side sectional view of the example photomask back inspection system as described.

[0023] Figure 6 A cross-sectional view depicting another embodiment of the screen as described.

[0024] Figure 7 A cross-section of an additional embodiment as described is shown.

[0025] Figure 8 A cross-sectional view depicting another embodiment of light blocked by the aperture edge structure and the photomask support edge structure.

[0026] All figures are schematic and not drawn to scale. Detailed Implementation

[0027] Figure 1 An example of a photomask backside inspection system is shown, which can be used to detect unwanted particulate contaminants at low levels on the back surface of a photomask in the form of extremely small particles (e.g., particles with a size on the micrometer scale). Particles on the photomask surface can be dust particles, surface debris carried by different surfaces and transferred to the photomask surface by air, or surface debris generated by detachment from different surfaces and transferred to the photomask surface by air.

[0028] The particles are extremely small (e.g., having a size on the micrometer scale) and can exist in extremely low amounts on the back surface of the photomask. Even when the particle size is extremely small (based on volume, mass, or both) and if they exist in extremely low amounts on the back surface of the photomask, the systems and methods for detecting these particles are important for semiconductor manufacturing because these small particles, present at low levels on the back surface of the photomask, can have a substantial impact on the performance of the photomask during the photolithography steps.

[0029] like Figure 1The system 100 includes a vacuum housing 110 that defines a vacuum chamber 108 internally. A photomask substrate (also referred to as an "EIP substrate" or "EUV inner box substrate") 112 is supported by a kinematic mount 114 within the vacuum chamber 108. A photomask 130 is supported by the substrate 112 at its lower (patterned) surface 134, and its upper surface (the "rear side" of the photomask) 132 faces upward toward the vacuum chamber 108 and the camera 140. An illuminator 116 generates light 122 of a wavelength that passes through a window 118. The light 122 is directed toward the surface 132 to inspect for particles that may be present on the surface 132.

[0030] Light 122 is directed toward surface 132 at a shallow angle of incidence, so that the light 122 from illuminator 116 is reflected from surface 132 at the same shallow angle (reflection angle) and will not be reflected in the direction that would allow the reflected light to be detected by camera 140. In the illustrated example, system 100 includes a camera 140 positioned above photomask 130 and at an angle perpendicular to the horizontal surface of photomask 130. Light directly reflected from surface 132 at a shallow reflection angle will not be received by camera 140. If there are no particles at surface 132 that redirect light 122 at a shallow reflection angle from surface 132, then no more than a very low and predictable amount of light will be directed toward and detected by camera 140.

[0031] However, if particulate contaminants are present at surface 132, the light 122 from illuminator 116 will be deflected or reflected by the particles and scattered as deflected light 122A within a directional range (including the direction toward camera 140) within vacuum chamber 108. The deflected light 122 received by camera 140 will be associated with the presence of particulate contaminants at surface 132. With this arrangement, camera 140 receives and detects the light 122A deflected by particulate contaminants present at surface 132, and records the deflected light as being caused by particulate contaminants at surface 132.

[0032] However, in addition to the particles present on surface 132, the structural features of substrate 112 may also enable light 122 from illuminator 116 to be reflected or deflected toward camera 140 as deflected light 122A. When this occurs, camera 140 detects the deflected light 122A and associates it with particulate contaminants on surface 132. When the signal is not generated by particulate contaminants, the result is a "false positive" reading indicating the presence of particulate contaminants on the surface.

[0033] The substrate 112 may have different structures that allow light 122 to be deflected in a certain direction into deflected light 122A so that the deflected light 122A is received by the camera 140 and produces a false positive reading. During the EUV lithography step, the substrate may be of a type useful for supporting, transferring, or otherwise supporting the photomask. Examples of useful substrates may be of the type of "photomask cassette" assembly, for example as described in U.S. Patents 9,745,119 and 9,919,863 and U.S. Patent Publication 2021 / 0057248, the entire contents of which are incorporated herein by reference. These documents describe photomask cassettes comprising a substrate supporting the photomask during transport and a cover that can be placed on the substrate and the photomask to protect the photomask during movement.

[0034] Figure 2A and 2B Examples of a substrate 112 and a photomask 130 useful in an exemplary photomask backside inspection system 100 are shown. The substrate 112 includes a position located at a top surface 150 approximately the size and shape of the photomask. The patterned surface 134 of the photomask 130 may be disposed in a "downward" direction toward the upper surface 150 of the substrate 112. The upper surface 150 may have a reflective metallic finish, such as reflective chromium, reflective nickel, or the like.

[0035] Lateral blocking pins 160 are mounted to the substrate 112 on the upper surface 150. A plurality of blocking pins 160 are securely attached to the substrate 112 at locations near the perimeter of the space used to position the photomask 130 above the surface 150, and effectively guide the photomask 130 to a desired position above the surface 150. In some embodiments, the blocking pins 160 are shaped with tapered inclined surfaces to facilitate placement of the photomask 130 between the blocking pins 160, which are arranged in a regular pattern on the surface 150 and sized and positioned so that the photomask 130 is located at a desired position above the surface 150 between the blocking pins. The blocking pins 160 may be made of metals such as steel or aluminum, or may be made of other rigid materials (including polymers).

[0036] In addition to the containment pins 160, the example substrate 112 also includes a plurality of openings through the substrate and photomask supports 162 located within those openings. Each photomask support 162 is located within an opening or "aperture" extending vertically through the substrate 112, and each photomask support 162 includes an upper surface located at a small height above an upper surface 150 of the substrate 112. The upper surfaces of each photomask support 162 are individually positioned at a small distance above surface 150 to allow the upper surface of the photomask support to contact the bottom (patterned) surface 134 of the photomask 130 and to position the bottom surface 134 at a small distance above surface 150. The upper surface of each photomask support 162 is positioned at a distance above the upper surface 150 to separate the lower surface 134 from the upper surface 150 and create a horizontally extending gap between the surfaces. The gap may have any useful size, for example, ranging from 0.001 inches to 0.010 inches.

[0037] like Figure 2A and 2B The substrate 112 includes a plurality of photomask supports 162 located diagonally on surface 150 near several pairs of blocking pins 160 at the corners of the substrate 112, to contact the surface of the photomask located near each of the four corners of the photomask. Alternatively, one or more photomask supports 162 may be positioned as desired at any other useful location on surface 150 to effectively provide support for the photomask 130 above surface 150, preferably positioned to avoid contact with patterned or otherwise sensitive areas of the patterned surface 134.

[0038] Regarding this description, the photomask 130 includes a functional (reflective) patterned surface over most of the patterned surface 134, but may be unpatterned in areas adjacent to the peripheral edge of the photomask. The outer edge of the photomask (“peripheral edge”) may be unpatterned and transparent, thereby allowing light 122 to pass through the photomask at the location of the peripheral edge. Light 122 passing through the photomask 130 at the peripheral edge can reach the upper surface 150 of the substrate 112. If the location where light illuminates the upper surface 150 is reflective and includes a structure other than a horizontally flat surface (e.g., if the upper surface 150 includes rounded or cornered edges or another non-horizontal structure (referred to as an “edge structure”)), then light 122 passing through the photomask 130 at the transparent peripheral edge can illuminate the edge structure and may be reflected or deflected toward the camera 140 as deflected light 122A. The deflected light 122A can be received by the camera 140 and cause the system 100 to incorrectly associate the deflected light 122A with particulate contaminants present at the surface 132, i.e., generate a false positive reading.

[0039] As described herein, the applicant has identified structures (e.g., “low-reflectivity surfaces” on the edge structures) that reduce the occurrence of light 122 being deflected or reflected as light 122A from edge structures of a substrate (e.g., from edge structures associated with the upper surface 150, specifically at the periphery edge of the substrate 112) and redirected toward the camera 140 to produce false positive readings.

[0040] Figure 3 This is a top perspective view of an example substrate 112 with added details. Substrate 112 includes a horizontal upper surface 150 with openings or "apertures" 170 and 172. Aperture 170 is adapted to receive a photomask support 162 (not shown), and aperture 172 is each adapted to receive a retaining pin 160 (not shown). Each aperture defines a circular edge ("edge structure") between its vertical sidewall and the horizontal upper surface 150.

[0041] Figure 4A The image shows a substrate 112 in cross-section, an aperture 170 extending vertically through the substrate 112, a photomask support 162 housed within the aperture 170, a retaining pin 160, and a photomask 130. The photomask 130 is supported by the upper surface of the photomask support 162, which suspends the photomask 130 a small distance above a surface 150, wherein the patterned surface 134 of the photomask 130 is separated from the surface 150 by a gap 180. The photomask support 162 contacts the patterned surface 134 at the peripheral edge of the photomask, for example, at an unpatterned location that is transparent to light generated by an illuminator from a rear-side inspection system.

[0042] The orifice 170 includes a circular edge structure 174 at its perimeter. In the cross-section (e.g.) Figure 4A In the location shown, the edge structure 174 has a rounded surface formed between the horizontal surface 150 and the vertical sidewall of the aperture 170. The edge structure 174 has a radius of curvature specified by a circle having a radius r1.

[0043] Moreover Figure 4A At the location, the photomask support 162 includes a circular edge structure 176 at its perimeter and adjacent to its horizontal upper surface. In the cross-section (e.g., Figure 4A In the location shown, the edge structure 176 has a rounded surface formed between the horizontal upper surface of the photomask support 162 and the vertically extending outer cylindrical sidewall of the photomask support 162. The edge structure 176 has a radius of curvature specified by a circle having a radius of curvature r2.

[0044] The photomask support can have any design and can be designed to provide a support structure for the photomask, which supports the photomask at a position slightly above the upper surface of the substrate at 150 degrees, as described, while reducing the presence of edge structures on the photomask support that can reflect or deflect light from the illuminator of the photomask rear-side inspection system in the direction toward the camera of the photomask rear-side inspection system.

[0045] exist Figure 4A In this example, the photomask support 162 is a polymeric structure having a generally cylindrical upper body with a relatively flat (horizontal) upper surface 166 and edge structures 176 extending around the periphery of the upper photomask surface 166. The aperture 170 has an inner diameter slightly smaller than the outer diameter of the polymeric photomask support 162. The photomask support 162 is mounted within the aperture 170 at a position where the upper photomask surface 166 of the photomask support 162 protrudes slightly above surface 150, establishing a height of gap 180. The photomask support 162 can be held in place by fasteners (e.g., fixing screws) that allow the height of the upper surface of the photomask support to extend above surface 150 and also allow for fine adjustments to the height of the upper surface above surface 150.

[0046] According to this description, the EUV substrate and substrate structure, such as a photomask support, can be designed to prevent or reduce the amount of light used in a photomask-back inspection system, where the light is reflected from the substrate and redirected to the camera of the inspection system. The substrate may include features that reduce the amount of light, such as deflecting or reflecting light from the edge surface of the substrate in a direction that would cause deflected light to reach the camera of the system and record false positive readings; these features include "low-reflectivity surfaces" applied to or formed on the edge structure. Alternatively or additionally, the substrate may include features that reduce the amount of light that can pass through transparent portions of the photomask (e.g., at the peripheral edges of the photomask) and reach the substrate while the photomask is supported by the substrate, and that may be redirected toward the camera.

[0047] The example substrate includes a non-horizontal edge structure. With the photomask supported by the substrate, and in unpatterned areas of the photomask (e.g., at the perimeter edge), light directed to the rear side of the photomask for particle inspection can pass through the photomask and reach the substrate. The light reaching the substrate can be guided from the edge structure of the substrate and redirected to the system's camera. The example substrate described can be designed to reduce the amount of light deflected towards the camera from the edge structure of the substrate in this manner.

[0048] To reduce the amount of light deflected from the edge structure, the example substrate may include an edge structure with a low-reflectivity surface, the edge structure including, for example: an edge structure shape that reduces the amount of light reflected toward the camera; and a low-reflectivity surface on the edge structure formed by treating the edge structure surface to reflect less or absorb more light used in the back-side inspection system. Alternatively or additionally, the substrate or system may include a structure (e.g., a "screen" or "mask") located within the system (e.g., at the edge of the substrate) to selectively block light from the inspection system from reaching the edge structure of the substrate.

[0049] In various example substrates, the low-reflectivity edge structure may have a shape that allows a reduced, low, or minimal amount of light from the illuminator of the photomask-back inspection system to be reflected from the edge structure toward the camera. The edge structure (e.g., 174, 176) may include a non-horizontal surface (e.g., a corner or edge) of an aperture or photomask support. To reduce the amount of light deflected from the edge structure (i.e., for the edge structure acting as a low-reflectivity edge structure), the area of ​​the non-horizontal surface of the edge structure is reduced or minimized, or it may be angled to prevent light from being redirected from the non-horizontal surface toward the camera of the photomask-back inspection system. For example, a corner with a low radius of curvature may be formed. Alternatively, the edge structure may be formed as an angled (or “beveled”) surface oriented at an angle that does not cause light to be reflected from the illuminator toward the camera, for example, causing light arriving at the beveled surface to be directed away from the camera at a shallow angle.

[0050] like Figure 4B As shown, the edge structure 174 of the aperture 170 can be fabricated to have a radius of curvature r1 of less than 0.5 mm (500 μm), for example less than 0.4, 0.3, 0.2, or 0.1 mm (100 μm). The edge structure with a low radius of curvature will present a smaller curved surface area on which light (122) can be reflected and redirected as deflected light (122A) toward the camera of the inspection system behind the photomask. The aperture 170 and edge structure 174 in the substrate 112 can be produced in a metal substrate using conventional processing techniques.

[0051] Similarly, as Figure 4B As shown, the edge structure 176 of the photomask support 162 can be fabricated to have a radius of curvature r2 of less than 0.5 mm (500 μm), for example less than 0.4, 0.3, 0.2, or 0.1 mm (100 μm). The low radius of curvature of the edge structure of the photomask support will present a smaller surface area on which light (122) can be reflected and redirected as deflected light (122A) toward the camera of the inspection system behind the photomask. The photomask support 162 and the edge structure 176 can be produced in a polymer photomask support using standard polymer molding or forming techniques.

[0052] According to alternative embodiments, such as Figure 4C As shown, the edge structure 174 of the aperture 170 can be prepared to have an angled shape relative to the direction of light received from the illuminator 116 of the photomask rear-side inspection system 100, so that any reflected light is not directed toward the camera 140. The edge structure 174 can be defined as a transition zone from the vertical sidewall of the aperture 170 to the angled surface 182. The angled surface 182 of the edge structure 174 is angled so that the light (122) from the rear-side inspection system is reflected at a shallow angle to the deflected light (122A) that is not redirected toward the camera of the photomask rear-side inspection system. Angle α1 can be any angle at which light is deflected by the angled surface 182 in a direction that does not deflect the light received by the camera 140. As indicated, examples of useful values ​​for the angle α1 between the angled surface 182 and the surface 150 can be less than 15 degrees or greater than 75 degrees. The aperture 170 and the edge structure 174 with the beveled surface 182 in the substrate 112 can be produced in the metal substrate by conventional processing techniques.

[0053] According to also Figure 4C In a similar embodiment shown, the edge structure 176 of the photomask support 162 may be prepared to have an angled shape relative to the direction of light received from the illuminator 116 of the photomask rear-side inspection system 100, so that any reflected light is not directed toward the camera 140. The angled surface 184 of the edge structure 176 is angled so that the light (122) from the rear-side inspection system is reflected at a shallow angle to the deflected light (122A) that is not redirected toward the camera of the photomask rear-side inspection system. Angle α2 may be any angle that does not direct the deflected light in a direction received by the camera 140. As indicated, examples of useful values ​​for angle α2 between the angled surface 184 and surface 150 may be less than 15 degrees or greater than 75 degrees. The photomask support 162 and the edge structure 176 may be produced in a polymer photomask support using standard polymer molding and forming techniques.

[0054] according to Figure 4D The alternative substrate shown here, the edge structure 174 of the aperture 170 can be prepared to include a low-reflectivity surface 190, based on composition or texture (roughness and smoothness) but not necessarily on shape (e.g., a low radius of curvature corner or beveled shape). The low-reflectivity surface 190 has reduced ability to reflect light 122 from the illuminator 116 of the rear-side inspection system (i.e., has low spectral reflectivity), or absorbs light 122 from the illuminator 116 of the rear-side inspection system. The shape of the edge structure can be rounded, beveled, or otherwise shaped at any radius of curvature, while the composition or texture of the edge structure surface makes the edge structure surface act as a low-reflectivity surface.

[0055] The low-reflectivity surface 190 may be any surface having a composition (e.g., by applying a coating or a treatment applied to the surface) or texture that exhibits lower reflectivity compared to the aperture 170 or other surfaces of the substrate 112, for example, lower reflectivity compared to the surface 150 of the substrate 112, the reflectivity being measured as the spectral reflectivity of light having a wavelength emitted by the illuminator of the photomask-backside inspection system. The low-reflectivity surface 190 of the edge structure 174 of the metal substrate may have a reflectivity of light emitted by the illuminator of the photomask-backside inspection system that is at least 10%, 20%, 30%, 40%, or 50% lower than the reflectivity of the surface 150 of the substrate 112, as measured as the spectral reflectivity of light emitted by the illuminator 116. The wavelength of the light may be a single wavelength or a range of wavelengths of light produced by the illuminator of the photomask-backside system and is directed toward the photomask surface to detect particulate contaminants at said surface.

[0056] A typical surface 150 of substrate 112 is a metallic (e.g., chromium, nickel, or similar) surface that highly reflects light in the visible wavelength range. Typically, edge surfaces 174 and apertures 170 comprise the same reflective metallic surface, for example, a coating applied to a metallic (e.g., aluminum) substrate structure. The metallic surface of the edge structure can reflect light received from the illuminator of the photomask substrate inspection system and can redirect that light toward the system's camera, thereby causing false positive readings. The metallic (e.g., chromium) surface of the substrate may have a spectral reflectivity that reflects more than 60% of the wavelengths emitted by the illuminator of the photomask-back inspection system. For example, a metallic surface (150) or edge structure (174) made of chromium may have a spectral reflectivity greater than 60% (e.g., approximately 63%) for light with wavelengths emitted by the illuminator.

[0057] Examples of low-reflectivity surfaces 190 may have a spectral reflectance of less than 63%, less than 60%, or less than 55%, 50%, 45%, 40%, 30%, or 20% for light of the same wavelength emitted by an illuminator. Examples of low-reflectivity surfaces may be made of oxides (e.g., chromium oxide or nickel oxide) on the metal surface of the edge structure.

[0058] According to the example substrate, the highly reflective metallic surface of the edge structure 174 of the aperture 170 can be modified to reduce the reflectivity of the surface (measured as spectral reflectivity) and produce a low-reflectivity surface 190. For example, the highly reflective metallic coating on the surface of the edge structure (e.g., 174) can be treated by chemical etching, laser texturing, laser ablation, or mechanical abrasion, or a combination thereof, to reduce the reflectivity of the surface and produce a low-reflectivity surface 190.

[0059] The treatment may affect the chemical composition of the metal, the texture of the metal surface, or both. The low-reflectivity surface 190 resulting from the treatment may have a reflectivity (measured as spectral reflectivity) lower than, or at least 10%, 20%, 30%, 40%, or 50% lower than, the reflectivity of the surface 150 of the substrate 112, or the reflectivity of the untreated surface of the edge structure 174, compared to, or both. For example, the low-reflectivity surface may have a spectral reflectivity lower than 63%, lower than 60%, or lower than 55%, 50%, 45%, 40%, 30%, or 20% for light emitted at wavelengths from the illuminator.

[0060] Alternatively or additionally, the reflective metallic coating on the surface of the edge structure 174 of the metal substrate may be coated with a material different from that of surface 150 and having a lower reflectivity for the light 122 emitted by illuminator 116, or a material that absorbs the light 122 emitted by illuminator 116 to produce a low-reflectivity surface 190. Compared to the reflectivity of surface 150 of substrate 112 and compared to the reflectivity of the untreated (uncoated) surface of edge structure 174, the low-reflectivity surface 190 produced by coating (optionally in combination with chemical treatment, laser treatment, mechanical abrasion, etc.) may be at least 10%, 20%, 30%, 40%, or 50% lower in reflectivity (measured as spectral reflectivity) of the light emitted by the illuminator of the photomask-back inspection system.

[0061] The coating that can be useful for producing a low-reflectivity surface 190 can be any coating that can be applied to the surface of the edge structure, and the coating has a reflectivity, as measured by spectral reflectivity, that is lower than that of the surface 150 of the substrate 112 or lower than that of the untreated surface of the edge structure 174 or lower than both; for example, a spectral reflectivity that is lower than 63%, lower than 60%, or lower than 55%, 50%, 45%, 40%, 30%, or 20% for light of a wavelength emitted by an illuminator.

[0062] Also there Figure 4D As shown in the illustration, the edge structure 176 of the photomask support 162 can be prepared to have a low-reflectivity surface 192. The ability of the low-reflectivity surface 192 to reflect the light 122 of the illuminator 116 of the rear inspection system is reduced or absorbed to prevent the light 122 from becoming reflected light from the edge structure surface toward the camera 140.

[0063] The low-reflectivity surface 192 may be any surface with a chemical composition or texture that results in a desired low reflectivity but optionally does not necessarily include a lower reflectivity compared to other surfaces of the photomask support 162, the reflectivity being measured by the spectral reflectivity of light emitted at a wavelength by the illuminator of the photomask rear-side inspection system.

[0064] In the example substrate described herein, the edge structure 176 of the photomask support 162 may include a texture that exhibits the desired low reflectivity of the edge structure surface and produces a low-reflectivity surface 192. For example, the polymeric surface of the edge structure (e.g., 176) may be treated to roughen the surface by chemical etching, laser texturing, laser ablation, or mechanical abrasion, or a combination thereof, to reduce the reflectivity of the surface and produce a low-reflectivity surface 192. During the molding process of forming the photomask support 162, the surface may also be formed to have a non-smooth (e.g., roughened) low-reflectivity surface. The low-reflectivity surface 192 formed by treating the surface to form a low-reflectivity surface (textured, roughened surface) polymeric surface by chemical etching, laser texturing, laser ablation, or mechanical abrasion may have a surface with a reflectivity (measured by spectral reflectivity) lower than that of the polymeric surface before treatment by at least 10%, 20%, 30%, 40%, or 50% lower than that of the polymeric surface before treatment, in terms of the reflectivity of light emitted by the illuminator of the photomask back-side inspection system compared to the reflectivity of the polymeric surface before treatment.

[0065] Alternatively or additionally, the polymeric surface of edge structure 176 may be coated with a material different from the polymeric material of edge structure 176 and having a desired low reflectivity for light 122 emitted by illuminator 116, or a material that absorbs light 122 emitted by illuminator 116 to produce low-reflectivity surface 192. Compared to the reflectivity of the uncoated polymeric surface of edge structure 176 or compared to the reflectivity of surface 150 of substrate 112, the low-reflectivity surface 192, through coating (optionally combined with chemical treatment, laser treatment, mechanical abrasion, etc.), can be at least 10%, 20%, 30%, 40%, or 50% lower in reflectivity (measured by spectral reflectivity) of light emitted by the illuminator of the photomask-back side inspection system.

[0066] The coating that can be useful for producing a low-reflectivity surface 192 can be any coating that can be applied to the polymeric surface of the edge structure 176, and the coating has a reflectivity that is at least lower than that of the uncoated surface of the edge structure 176 as measured by spectral reflectivity.

[0067] In other example systems, the substrate used in the photomask back-side inspection system may include, for example, a structure of a light screen (“screen” or “mask”) that reduces the amount of light or prevents light from being directed toward the photomask at transparent areas (e.g., at the periphery edge of the photomask), thereby preventing light from passing through the transparent areas of the photomask, preventing light from reaching the substrate supporting the photomask, and preventing light from reaching edge structures on the substrate that may redirect light toward the camera of the inspection system.

[0068] Figure 5 The display includes Figure 1An example of a system 100 is a photomask backside inspection system. This system includes a vacuum housing 110, a vacuum chamber 108, a photomask substrate 112, a kinematic device 114, and a photomask 130. The photomask 130 has a lower (patterned) surface 134 supported by the substrate 112 and an upper surface (the "back" side" of the photomask) 132 facing upwards towards the vacuum chamber 108 and the camera 140. An illuminator 116 generates light 122 of a wavelength that passes through a window 118. The light 122 is directed toward the surface 132 to inspect particles that may be present at the surface 132.

[0069] The substrate 112 is described as including an edge structure that, if light passes through the photomask 130 and reaches the edge structure (which may be a structure 174, 176 or both of the aperture 170 and the photomask support 162 as described), then the edge structure is able to deflect the light from the illuminator 116 in the direction toward the camera 140.

[0070] Figure 5 The system 100 further includes a light screen 186 supported by a substrate 112. The light screen (or “screen”) 186 can be any structure that can be placed at the edge of the substrate 112, i.e., positioned between the illuminator 116 and the photomask 130, specifically between the illuminator 116 and the edge structures of the substrate 112 (such as those exemplified by structures 174, 176). In this embodiment, the light screen 186 is vertically positioned. The light screen 186 does not transmit light 122 and prevents light 122 from reaching the photomask 130, or prevents light 122 from reaching the edge structures 174, 176 that can redirect light 122 toward the camera 140.

[0071] The screen 186 can be placed at any useful location on the substrate to prevent light from reaching the photomask 130 or the edge structure of the base (e.g., 174, 176). Figure 2A and 2B The useful location of screen 186 and associated apertures (not shown) are illustrated. Screen 186 is positioned near the peripheral edge of substrate 112 and near the edge structure of photomask support 162. Additionally, Figure 6 Another embodiment is depicted in which the substrate 112 acts as a light screen for the photomask support body 162 and prevents light from impacting the edge 176 of the photomask support. In this embodiment, the edge of the photomask support is positioned below the upper surface of the substrate 112. The edge structure of the photomask 130 is a transition area from the blocking pin 160 to the beveled edge structure. Additionally, Figure 8 Another embodiment in which the edge structure of the photomask support blocks light from the edge structure of the aperture is depicted.

[0072] The photomask support positioned on the substrate can also reflect light due to its surface or the fasteners used to hold it to the substrate. Occasional light reflections can also be problematic and produce false positive readings. In cases such as... Figure 7 In another embodiment depicted, a substrate 112 for supporting a photomask (not shown) includes a horizontal substrate surface 115 and a photomask support 162 located on the horizontal substrate surface 115. The photomask support 162 includes a base 194 and at least one photomask contact feature 196 extending from the base 194. The at least one photomask contact feature is located at a distal end of the support structure 162. The base 194 may include one or more fasteners 198 to retain the base on the horizontal substrate surface. The one or more fasteners 198 are recessed into countersunk holes 200 to shield edges from shallow-angle light. In another embodiment, the fastener 198 or edge structure 202 includes a low-reflectivity surface formed by: (i) a bevel, (ii) a radius of curvature less than 0.3 mm, (iii) a surface having a spectral reflectance at least 10% lower than that of the upper substrate surface, or (iv) a combination thereof.

Claims

1. A substrate for supporting a photomask, the substrate comprising: a horizontal substrate surface, an aperture positioned vertically through the plate, the aperture comprising an aperture edge structure formed between a vertical aperture sidewall and the substrate surface, a photomask support within the aperture, the photomask support comprising: a photomask support body that engages the aperture sidewall, an upper photomask support surface that is positioned above the horizontal plate surface and adapted to support a photomask above the substrate surface, and a photomask support edge structure formed between the photomask support body and the photomask support surface, wherein: the aperture edge structure comprises a low-reflective surface comprising: a bevel, a radius of curvature less than 0.3 millimeters, a surface having a spectral reflectivity that is at least 10% lower than a spectral reflectivity of the upper substrate surface, or a combination of these; the photomask support edge structure comprises a low-reflective surface comprising: a bevel, a radius of curvature less than 0.3 millimeters, a surface having a spectral reflectivity that is at least 10% lower than a spectral reflectivity of the upper substrate surface, or a combination of these; or at least one light screen positioned on the substrate surface, wherein the at least one light screen is non-transmissive to light and prevents light from reaching the aperture edge structure or the photomask support edge structure.

2. The substrate of claim 1, wherein the aperture edge structure comprises a low- reflective coating having a spectral reflectivity that is at least 50% lower than the spectral reflectivity of the upper substrate surface.

3. The substrate of claim 1, wherein the aperture edge structure comprises a textured surface formed by a process selected from: chemical etching, laser texturing, laser ablation, or mechanical abrasion.

4. The substrate of claim 1, wherein the photomask support edge structure comprises a low-reflective coating having a spectral reflectivity that is at least 50% lower than the spectral reflectivity of the upper substrate surface.

5. The substrate of claim 1, wherein the photomask support edge structure comprises a textured surface formed by a process selected from: chemical etching, laser texturing, laser ablation, or mechanical abrasion.

6. The substrate of claim 1, further comprising a photomask supported at the substrate upper surface.

7. The substrate of claim 1, wherein the bevel of the aperture edge structure is angled outside of a range of 15 and 75 degrees.

8. The substrate of claim 1, wherein the bevel of the photomask edge structure is angled outside of a range of 15 and 75 degrees.

9. The substrate of claim 1, wherein a lower edge of the bevel of the aperture edge structure is positioned in a lower position relative to the photomask support edge structure such that the photomask support edge structure shields the lower edge of the bevel of the aperture edge structure from light.

10. The substrate of claim 1, wherein a lower edge of the bevel of the reticle support edge structure is in a lower position relative to the substrate such that the substrate shields the lower edge of the bevel of the reticle support edge structure from light.

11. A reticle backside inspection apparatus comprising: a vacuum chamber comprising an interior, a substrate of claim 1 at the interior, an illuminator capable of directing light toward the substrate, and a camera adapted to detect light redirected by a particle on a backside surface of a reticle supported on an upper surface of a substrate.

12. A method of using a reticle backside inspection apparatus of claim 11, the method comprising: supporting a reticle at an upper surface of the substrate; directing light from the illuminator toward the reticle; using the camera to detect light reflected by a particle at a surface of the reticle.

13. The method of claim 12, comprising using the light detected by the camera to determine an amount of particles at the surface of the reticle.

14. A reticle backside inspection apparatus comprising: a vacuum chamber comprising an interior, a substrate having an upper surface and an edge structure, an illuminator capable of directing light onto the substrate upper surface, and a camera, wherein the substrate comprises: a horizontal upper substrate surface, an aperture positioned vertically through the plate, the aperture comprising an aperture edge structure formed between a vertical aperture sidewall and the upper substrate surface, a reticle support within the aperture, the reticle support comprising: a reticle support body engaging the aperture sidewall, an upper reticle support surface above the horizontal plate upper surface and adapted to support a reticle above the upper substrate surface, and a reticle support edge structure formed between the reticle support body and the upper reticle support surface, and wherein: the aperture edge structure comprises a low reflectance surface comprising: a bevel, a radius of curvature less than 0.3 millimeters, a surface having a spectral reflectance less than 60% at wavelengths of light emitted by the illuminator, or a combination of these; the reticle support edge structure comprises a low reflectance surface comprising: a bevel, a radius of curvature less than 0.3 millimeters, a surface having a spectral reflectance less than 60% at wavelengths of light emitted by the illuminator, or a combination of these; or at least one light screen positioned on the substrate surface, wherein the at least one light screen does not transmit light and prevents light from reaching the aperture edge structure or the reticle support edge structure.

15. The inspection apparatus of claim 14, wherein the aperture edge structure comprises a low reflectance coating having a spectral reflectance less than 50% at wavelengths of light emitted by the illuminator.

16. The inspection apparatus of claim 14, wherein the aperture edge structure comprises a textured surface formed by a process selected from: chemical etching, laser texturing, laser ablation, or mechanical abrasion.

17. The inspection apparatus of claim 14, wherein the reticle support edge structure comprises a low-reflectance coating having a spectral reflectivity of less than 50% at wavelengths of light emitted by the illuminator.

18. The inspection apparatus of claim 14, wherein the reticle support edge structure comprises a textured surface formed by a process selected from chemical etching, laser texturing, laser ablation, or mechanical abrasion.

19. A substrate for supporting a reticle, the substrate comprising: (a) a horizontal substrate surface, (b) a reticle support on the horizontal substrate surface, the reticle support having: (i) a base, and (ii) a support structure extending from the base, the support structure having an edge structure and at least one reticle contact feature on a distal end of the reticle support structure, wherein the base includes (1) a fastener to hold the base on the horizontal substrate surface, and wherein the fastener is recessed into a counterbore, thereby concealing the edge from light at shallow angles, or (2) the fastener or the edge structure comprises a low-reflectance surface comprising: a bevel, a radius of curvature less than 0.3 millimeters, a surface having a spectral reflectivity at least 10% lower than a spectral reflectivity of the upper substrate surface, or a combination thereof.

20. The substrate of claim 19, wherein the edge structure comprises a textured surface formed by a process selected from chemical etching, laser texturing, laser ablation, or mechanical abrasion.

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