Inspection system with non-circular pupil

Abstract

To provide inspection systems providing high-fidelity images and realizing efficient utilization of source illumination without imposing undue complexity and associated costs.SOLUTION: An inspection system includes an illumination source configured to generate extreme ultraviolet (EUV) light, illumination optics to direct the EUV light to a sample within a range of off-axis incidence angles corresponding to an illumination pupil distribution, collection optics to collect light from the sample in response to the incident EUV light within a range of collection angles corresponding to an imaging pupil distribution, and a detector configured to receive at least a portion of the light collected by the collection optics. Further, a center of the illumination pupil distribution corresponds to an off-axis incidence angle along a first direction on the sample, and at least one of the illumination pupil distribution and the imaging pupil distribution is non-circular such that the size along the first direction is shorter than the size along a second direction perpendicular to the first direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] (Technical field) FIELD OF THE DISCLOSURE This disclosure relates generally to inspection systems, and more particularly to extreme ultraviolet illumination inspection systems. [Background technology]

[0002] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 62 / 793,027, filed January 16, 2019, entitled "INSPECTION SYSTEM WITH NON-CIRCULAR PUPIL," inventors Damon Kvamme, Rui-fang Shi, Daniel Wack, Sseunhyeun Jo, and Xin Ye, the entire contents of which are incorporated herein by reference.

[0003] Inspection systems perform critical quality control measurements in semiconductor manufacturing and metrology and can commonly be used to inspect components such as, but not limited to, photomasks (e.g., reticles), unpatterned wafers, and patterned wafers. Typically, it is desirable for an inspection system to provide high-fidelity, low-noise images, and therefore the ability to resolve defects of interest that could impact the manufacturing process or product performance.

[0004] For example, photomask inspection systems can use illumination sources with the same or substantially similar spectra as the light sources used in lithography systems to create image-based features on those masks, enabling the resolution of defects that could affect integrated circuits fabricated using those masks. However, driven by the demand for ever-smaller feature sizes, the wavelengths used in both lithography and inspection systems have correspondingly decreased. In particular, the widespread use of extreme ultraviolet (EUV) light for lithography and inspection systems presents significant challenges in the design of both the illumination source (e.g., EUV light source) and the inspection system, typically resulting in trade-offs between illumination source brightness, system cost, and the complexity of its optical system. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 8,711,346 [Patent Document 2] U.S. Patent No. 8,785,082 [Patent Document 3] U.S. Patent No. 8,842,272 Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, it would be desirable to provide an inspection system and method that provides high fidelity images and efficient source illumination utilization without incurring excessive complexity and associated costs. [Means for solving the problem]

[0007] An inspection system is disclosed in accordance with one or more exemplary embodiments of the present disclosure. The system includes an illumination source that generates extreme ultraviolet (EUV) light. The system includes illumination optics that direct the EUV light to a specimen within a range of incidence angles, where the incidence angle range corresponds to an illumination pupil distribution at a pupil plane of the one or more illumination optics, with a center of the illumination pupil distribution corresponding to an off-axis incidence angle along a first direction on the specimen. The system also includes one or more collection optics that collect light from the specimen in response to the incident EUV light within a collection angular range, where the collection angular range corresponds to an imaging pupil distribution at a pupil plane of the one or more collection optics. In some exemplary embodiments, the center of the imaging pupil distribution corresponds to an off-axis collection angle along the first direction on the specimen. In some example embodiments, at least one of the illumination pupil distribution and the imaging pupil distribution is non-circular, and the non-circular shape has a size along a first direction that is shorter than a size along a second direction perpendicular to the first direction. In some example embodiments, the system also includes a detector that receives at least a portion of the light collected by the one or more collection optics.

[0008] A photomask inspection system is disclosed in accordance with one or more exemplary embodiments of the present disclosure. The system includes an illumination source that generates an illumination beam. The system also includes one or more illumination optics that direct the illumination beam to the photomask within a range of incidence angles, the range of incidence corresponding to an illumination pupil distribution at a pupil plane of the one or more illumination optics, with the illumination pupil distribution centered at an off-axis incidence angle along a first direction on the photomask. The system also includes one or more collection optics that collect light from the photomask in response to the incident illumination beam within a collection angular range, the collection angular range corresponding to an imaging pupil distribution at a pupil plane of the one or more collection optics. In some exemplary embodiments, the imaging pupil distribution centered at the off-axis collection angle along the first direction on the photomask. In certain illustrative embodiments, at least one of the illumination pupil distribution and the imaging pupil distribution is non-circular, and the size of the non-circular shape along a first direction is shorter than the size of the non-circular shape along a second direction perpendicular to the first direction. In certain illustrative embodiments, the system also includes a detector configured to receive at least a portion of the light collected by the one or more collection optics and generate one or more detection signals related to the received light.

[0009] An inspection method is disclosed in accordance with one or more exemplary embodiments of the present disclosure. According to certain exemplary embodiments, the method directs extreme ultraviolet (EUV) light onto a specimen within a range of incidence angles, the range of incidence corresponding to an illumination pupil distribution at a pupil plane of one or more illumination optics, with a center of the illumination pupil distribution corresponding to an off-axis incidence angle along a first direction on the specimen. According to certain exemplary embodiments, the method collects light resulting from the specimen in response to the incident EUV light within a range of collection angles, the range of collection angles corresponding to an imaging pupil distribution at a pupil plane of one or more collection optics. In certain exemplary embodiments, the center of the imaging pupil distribution corresponds to an off-axis collection angle along the first direction on the specimen. In certain exemplary embodiments, at least one of the illumination pupil distribution and the imaging pupil distribution is non-circular, and the size of the non-circular shape along the first direction is shorter than the size along a second direction perpendicular to the first direction. In accordance with certain illustrative embodiments, the method also includes generating an image of the specimen with a detector based at least in part on the light collected by the one or more collection optics.

[0010] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the general description, serve to explain the principles of the invention.

[0011] Those skilled in the art will be able to better appreciate the many advantages of the present disclosure by reviewing the accompanying drawings, in which: [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a conceptual diagram of an inspection system with off-axis illumination and collection, according to one or more embodiments of the present disclosure. [Figure 2]FIG. 2 is a plot of circular illumination and imaging pupil distributions for inspection and lithography in accordance with one or more embodiments of the present disclosure. [Figure 3] 1 is a plot illustrating two differently shaped illumination pupil distributions for inspection in accordance with one or more embodiments of the present disclosure. [Figure 4] 1 is a plot illustrating two different shaped imaging pupil distributions for inspection in accordance with one or more embodiments of the present disclosure. [Figure 5A] FIG. 1 is a conceptual top view of a detector with square pixels. [Figure 5B] FIG. 1 is a conceptual top view of a detector with rectangular pixels. [Figure 6] FIG. 1 is a flow diagram depicting steps performed in a specimen inspection method. DETAILED DESCRIPTION OF THE INVENTION

[0013] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with reference to certain embodiments and specific features thereof. The embodiments described herein should be considered illustrative and not limiting. It will be readily apparent to those skilled in the art that various changes and modifications in form and detail can be made therein without departing from the spirit and scope of the present disclosure.

[0014] Embodiments of the present disclosure are directed to inspection systems and methods incorporating off-axis illumination and / or collection in which at least one of the illumination pupil and imaging pupil has a non-circular distribution, particularly one in which the spatial extent (e.g., size) of the non-circular distribution in the in-plane direction is smaller than that in the out-of-plane direction.

[0015] For example, an inspection system typically illuminates a specimen over a range of angles of incidence corresponding to the numerical aperture of the illumination optics and collects light from the specimen within a range of angles of collection corresponding to the numerical aperture of the collection optics, which may or may not be the same as the numerical aperture of the illumination optics. These ranges of angles of incidence and collection correspond to distributions at the illumination pupil plane and the imaging pupil plane, respectively. Thus, an off-axis illumination system can be characterized as one in which the chief ray (or approximate center) of the illumination and / or imaging pupil distributions is at an off-axis angle relative to the specimen normal (e.g., specimen normal).

[0016] Additional embodiments of the present disclosure are directed to inspection using EUV light, including, but not limited to, wavelengths less than about 120 nanometers (nm). For example, EUV light sources for lithography and / or inspection can generate light centered around wavelengths such as, but not limited to, 13.5 nm, 7 nm, etc. The present application recognizes that inspection with EUV light introduces various design concerns. For example, because EUV light is strongly absorbed by nearly all materials, EUV systems typically include only reflective components, often positioned at grazing incidence angles. Therefore, EUV imaging systems will operate in a reflective mode, i.e., a mode in which the imaging light includes light reflected and / or scattered from the specimen. Furthermore, in EUV imaging systems, the illumination pupil and imaging pupil distributions will be non-overlapping to prevent overlap between the incident angle range of the incident light and the collection range used to image the specimen, which imposes additional constraints, as described in more detail below.

[0017] It is recognized herein that inspection systems are typically designed to exhibit circular illumination and imaging pupil distributions, allowing typical optical configurations to provide consistent image quality (e.g., resolution, signal strength, noise level, etc.) regardless of orientation on the specimen. However, such a configuration in an off-axis inspection system would not utilize the full range of available numerical aperture, ultimately limiting the system's performance.

[0018] Additional embodiments of the present disclosure are directed to illuminating a specimen with a noncircular illumination pupil distribution, but with a smaller spatial extent in the plane of incidence (e.g., defined by the specimen normal and the off-axis illumination and / or collection light) than in the plane of incidence. It is noted that the size and shape of the illumination pupil distribution are typically designed to balance tradeoffs related to system performance. For example, reducing the size of the illumination pupil distribution (related to illumination etendue, i.e., illumination photon throughput) can increase the desired defect signal, while reducing the adverse effects of line edge roughness. Furthermore, in practice, because specimen reflectivity can decrease as the angle of incidence relative to the specimen normal increases, it may be desirable to limit the numerical aperture of the illumination. Illuminating a specimen with a noncircular illumination pupil allows for the numerical aperture of the illumination light to be limited along a specified direction (e.g., in the plane of incidence) while maintaining the desired illumination etendue. According to this configuration, illumination with a non-circular illumination pupil can achieve higher performance than illumination with a circular illumination pupil without making the optical system larger, more complex, or more expensive.

[0019] Additional embodiments of the present disclosure are directed to collecting light from a specimen with a non-circular imaging pupil distribution, but with a spatial extent of the non-circular imaging pupil that is smaller in directions within the plane of incidence than in directions outside the plane of incidence. For example, a non-circular aperture that extends perpendicular to the plane of incidence can capture additional scattered light from the specimen compared to collecting light within a circular imaging pupil distribution without making the optical system larger, more complex, or more costly.

[0020] This application recognizes that a non-circular imaging pupil distribution can lead to differences in resolution across different directions, since the resolution of an imaging system depends in part on the numerical aperture of the collected light. Further embodiments of the present disclosure are directed to providing uniform image resolution while maintaining a non-circular imaging pupil distribution. According to some embodiments, one or more collection optics can provide anamorphic scaling onto the detector. This configuration allows for different magnifications of the collected light along different directions (e.g., along the plane of incidence and perpendicular to the plane of incidence), thereby enabling a typical detector with square pixels to uniformly sample the imaging light. According to some embodiments, the detector can be designed with rectangular pixels. This configuration allows for differences in numerical aperture due to a non-circular imaging pupil distribution to be compensated for by varying pixel dimensions.

[0021] FIG. 1 is a conceptual diagram of an inspection system 100 with off-axis illumination and collection in accordance with one or more embodiments of the present disclosure.

[0022] The exemplary inspection system 100 includes an illumination source 102 that generates an illumination beam 104. The illumination beam 104 may include one or more specified wavelengths of light, such as, but not limited to, ultraviolet (UV), extreme ultraviolet (EUV), deep ultraviolet (DUV), and vacuum ultraviolet (VUV) radiation. For example, at least a portion of the spectrum of the illumination beam 104 may have a wavelength less than about 120 nanometers. For example, at least a portion of the spectrum of the illumination beam 104 may have a wavelength associated with lithography tools suitable for semiconductor manufacturing, such as, but not limited to, 13.5 nm, 7 nm, etc.

[0023] Illumination source 102 can be any type of illumination source known in the art suitable for generating optical illumination beam 104. In one embodiment, illumination source 102 comprises a broadband plasma (BBP) illumination source. In this configuration, illumination beam 104 can include radiation emitted by a plasma. For example, BBP illumination source 102 can include, but is not required to include, one or more pumping light sources (e.g., one or more lasers) configured to focus into a volume of gas and cause the gas to absorb energy, thereby creating or sustaining a plasma suitable for emitting radiation. Furthermore, at least a portion of the plasma radiation can be utilized as illumination beam 104.

[0024] According to another embodiment, illumination source 102 may comprise one or more lasers capable of emitting radiation at one or more specified wavelengths.

[0025] Furthermore, the illumination beam 104 generated by the illumination source 102 can have any temporal profile. For example, the illumination source 102 can generate a continuous illumination beam 104, a pulsed illumination beam 104, or a modulated illumination beam 104.

[0026] In some embodiments, the illumination source 102 directs an illumination beam 104 to the specimen 106 via an illumination path 108. The illumination path 108 may include one or more illumination optics 110 suitable for directing, focusing, and / or shaping the illumination beam 104 onto the specimen 106. For example, the illumination optics 110 may include one or more lenses, one or more focusing elements, etc. Furthermore, the illumination optics 110 may include any combination of reflective, transmissive, or absorptive optical elements known in the art suitable for directing and / or focusing the illumination beam 104. For example, the illumination optics 110 may include reflective optics suitable for directing and / or focusing short wavelength light (e.g., EUV light), such as, but not limited to, flat mirrors, curved mirrors (e.g., ellipsoidal mirrors, parabolic mirrors, etc.).

[0027] The illumination optics 110 may further include one or more additional illumination path components suitable for shaping the illumination beam 104 and / or controlling the angular incidence (e.g., illumination pupil distribution) of the illumination beam 104 on the specimen 106. Examples of such illumination path components may include, but are not limited to, one or more apertures, one or more apodizers, one or more homogenizers, one or more diffusers, one or more polarizers, one or more filters, etc.

[0028] Also in embodiments, the specimen 106 is positioned on a specimen stage 112. The specimen stage 112 may include any suitable device for positioning and / or scanning the specimen 106 within the inspection system 100. For example, the specimen stage 112 may include any combination of a linear translation stage, a rotation stage, a tip / tilt stage, etc.

[0029] The exemplary inspection system 100 also includes a detector 114 configured to capture light emanating from the specimen 106 (e.g., collected light 116) via a collection path 118. The collection path 118 may include, but is not limited to, one or more collection optics 120 for collecting radiation from the specimen 106. For example, the collected light 116 received by the detector 114 may be light that has been reflected or scattered from the specimen 106 (e.g., by specular reflection, diffuse reflection, etc.) via the collection optics 120. For example, the collected light 116 received by the detector 114 may be light generated by the specimen 106 in response to the illumination beam 104 (e.g., luminescence due to absorption of the illumination beam 104, etc.). The collection optics 120 may include any combination of reflective, transmissive, or absorptive optical elements suitable for directing and / or focusing the collected light 116 as known in the art. For example, the focusing optical system 120 may include reflective optical systems suitable for directing and / or focusing short wavelength light (e.g., EUV light, etc.), such as, but not limited to, flat mirrors, curved mirrors (e.g., ellipsoidal mirrors, parabolic mirrors, etc.).

[0030] The detector 114 may comprise any type of detector known in the art suitable for measuring the collected light 116 received from the specimen 106. Examples of the detector 114 may include, but are not limited to, a CCD-type detector, a TDI-type detector, a photomultiplier tube (PMT), an avalanche photodiode (APD), etc. Additionally, in some embodiments, the detector 114 may comprise a spectroscopic detector suitable for identifying the wavelength of radiation emanating from the specimen 106.

[0031] The collection path 118 may further include a number of additional collection path elements, including but not limited to, one or more apertures, one or more apodizers, one or more polarizers, one or more filters, etc., that direct and / or shape the collected light 116 from the specimen 106. In some embodiments, the collection path elements define the angular range (e.g., imaging pupil distribution) within which light is collected from the specimen 106.

[0032] In some embodiments, the inspection system 100 includes a controller 122. In some embodiments, the controller 122 includes one or more processors 124 configured to execute program instructions stored on a storage medium 126 (e.g., memory). In this configuration, the one or more processors 124 included in the controller 122 may perform any of the various processing steps described elsewhere in this disclosure.

[0033] The one or more processors 124 comprising the controller 122 may include any processing element known in the art. In this sense, the one or more processors 124 may include any microprocessor-based device configured to execute algorithms and / or instructions. According to certain embodiments, the one or more processors 124 may comprise a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, or any other computer system (e.g., a networked computer) configured to execute programs, which may be configured to operate the inspection system 100 as described elsewhere in this disclosure. With further recognition, the term "processor" may be broadly defined to encompass any device having one or more processing elements that executes program instructions obtained from a non-transitory storage medium 126.

[0034] The storage medium 126 may include any storage medium known in the art suitable for storing program instructions executable by one or more associated processors 124. For example, the storage medium 126 may include a non-transitory storage medium. For example, the storage medium 126 may include, but is not limited to, read-only memory, random access memory, magnetic or optical storage devices (e.g., disks), magnetic tape, solid-state drives, etc. It is further noted that the storage medium 126 may be housed with one or more processors 124 within a common controller housing. In some embodiments, the storage medium 126 may be remote from the physical location of one or more processors 124 and the controller 122. For example, one or more processors 124 in the controller 122 may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.). Therefore, the above description should be considered as merely illustrative and not limiting on the present invention.

[0035] Controller 122 may be communicatively coupled to any component of inspection system 100 or to any additional component external to inspection system 100. In embodiments, controller 122 may be configured to receive data from components such as, but not limited to, detector 114. For example, controller 122 may receive any combination of raw data, processed data (e.g., inspection results), and / or partially processed data. In embodiments, controller 122 may perform processing steps based on the received data. For example, controller 122 may perform defect inspection steps such as, but not limited to, defect identification, classification, and sorting.

[0036] Furthermore, the steps described throughout this disclosure may be performed by a single controller 122 or, alternatively, by multiple controllers. Additionally, the controller 122 may include one or more controllers housed within a common controller housing or housed within multiple housings. This allows any controller or controller combination to be individually packaged into a module suitable for integration into the inspection system 100. For example, the controller 122 may operate as a centralized processing platform. Also, for example, the controller 122 may be distributed such that portions of the controller 122 are implemented in and / or housed within any combination of the inspection tools.

[0037] Additionally, in embodiments, controller 122 may control and / or direct (e.g., via control signals) any component of inspection system 100. For example, any combination of elements in illumination path 108 and / or collection path 118 may be adjustable. In this configuration, controller 122 may modify any combination of illumination or imaging conditions, such as, but not limited to, illumination or imaging pupil distribution.

[0038] Inspection system 100 may be configured as any type of inspection system known in the art. Additionally, inspection system 100 may be, but is not required to be, an EUV inspection system 100 adapted to inspect specimen 106 with EUV light. In one embodiment, inspection system 100 is a photomask inspection system. That is, the specimen 106 inspected by the inspection system may include a photomask (e.g., a reticle). In another embodiment, inspection system 100 is configured as an EUV mask blank inspection system. EUV-based mask blank inspection is generally described in U.S. Patent No. 6,233,999 to Stokowski, issued April 29, 2014, and U.S. Patent No. 6,233,999 to Xiong et al., issued July 22, 2014, both of which are incorporated herein by reference in their entireties. In another embodiment, inspection system 100 is configured as a wafer inspection system or a reticle inspection system. EUV imaging is generally described in U.S. Patent No. 6,229,999, issued September 23, 2014, to Wack, which is incorporated herein by reference in its entirety.

[0039] Circular and non-circular pupil distributions (e.g., illumination and imaging pupil distributions) for illumination and collection light will now be described in more detail with reference to Figures 2-4. The illumination and / or imaging pupil distributions of Figures 2-4 may be implemented by an inspection system such as, but not limited to, inspection system 100 depicted in Figure 1. Furthermore, the illumination and / or imaging pupil distributions of Figures 2-4 may be suitable for EUV inspection, although this is not required.

[0040] It is recognized herein that photomask inspection and photomask lithography can be performed with different illumination and / or imaging pupil distributions, although this is not required. For example, the selection of the center point (e.g., that corresponding to the chief ray angle), size, and / or shape of the pupil distribution can vary for different applications, incorporating tradeoffs between throughput (e.g., etendue), coherence, etc. Furthermore, illumination pupil distributions and imaging pupil distributions for the same application (e.g., inspection, lithography, etc.) can have the same characteristics (e.g., pupil distribution size and / or shape), or they can differ.

[0041] Additionally, it is recognized herein that the numerical aperture (e.g., maximum angle of incident or collected light) of an inspection system and / or lithography system may typically be limited. For example, the numerical aperture of an inspection system may be limited by the physical size and focusing power of system components (e.g., illumination optics 110, collection optics 120, etc.). Also, for example, the reflectivity of specimen 106 (or a coating on specimen 106) may vary as a function of angle, making illumination and / or collection beyond the cutoff angle inefficient and / or impractical.

[0042] Additionally, it may be desirable to position the illumination pupil distribution of the inspection system inside the cutoff angle of the specimen 106 to ensure efficient use of light (e.g., illumination beam 104). However, in practice, the imaging pupil distribution can be designed with more flexibility than the illumination pupil distribution. For example, illumination use outside the cutoff angle may be inefficient. However, the imaging pupil distribution may contain both reflected light (e.g., light from the opposite side of the incident light but at the same angle) and scattered light that may emanate from the specimen 106 over a wide angular range. Therefore, expanding the imaging pupil distribution to accommodate angles beyond the angle of the reflected light can increase the captured optical signal and improve the signal-to-noise ratio of the generated image. Furthermore, the imaging pupil distribution can generally be expanded beyond the cutoff angle of the specimen 106 without degrading performance.

[0043] 2 is a plot 200 of circular illumination and imaging pupil distributions for inspection and lithography of a specimen 106 (e.g., a photomask) according to one or more embodiments of the present disclosure. In FIG. 2, the angle 0 represents the surface normal of the photomask, the distance from 0 represents the angle of incidence and / or collection angle relative to the surface normal, and θ X ,θ Y represents the azimuth angle based on the photomask. X and θ Y can correspond to the X and Y directions, respectively, in the specimen plane (e.g., the surface of specimen 106, a plane perpendicular to the surface normal, etc.). Specifically, FIG. 2 illustrates illumination pupil distribution 202 and imaging pupil distribution 204 for inspection (e.g., by inspection system 100), and illumination pupil distribution 206 and imaging pupil distribution 208 for lithography. That is, FIG. 2 illustrates the light cones associated with the incident illumination and collected light used to image a photomask in both inspection and lithography applications. Additionally, FIG. 2 illustrates that photomask coating cutoff angle 210 is approximately 14 degrees, represented by a circle centered at angle 0 (the surface normal).

[0044] 2 to 4 show the θ X 2-4 , off-axis illumination and imaging along a direction are depicted. As previously discussed, it can be useful to characterize off-axis illumination and imaging of a specimen relative to a plane of incidence. For example, the plane of incidence can be defined by a surface normal and a central ray of the incident and / or collected light. We recognize that in real systems, the incident and / or collected light is represented as a cone (e.g., associated with a numerical aperture range), and therefore the plane of incidence can be defined for each ray within that cone. However, for clarity of this disclosure, it is preferable to define the plane of incidence (e.g., interaction surface) based on the central ray of the illumination beam 104 for any application (e.g., inspection, lithography, etc.). For example, the plane of incidence in FIGS. 2-4 can be defined by a surface normal and a θ in angle space. Xdirection (or X direction on the specimen 106). Furthermore, as depicted in Figures 2-4, the inspection imaging pupil distribution 204 can be centered in the entrance plane, although this is not required.

[0045] Furthermore, it is recognized herein that the distribution of a beam (e.g., illumination beam 104) typically cannot be represented as a step function, but rather may generally have tails. Additionally, the beam may contain "hot spots" or irregularities. Therefore, the center of the illumination and / or imaging pupil distribution may be determined by any technique known in the art. For example, the center of the pupil distribution may be determined based on, but not limited to, the location of intensity peaks or a weighted intensity analysis.

[0046] 2 and 4, the illumination pupil distribution 206 and the imaging pupil distribution 208 for lithography are shown as circular distributions (e.g., representing illumination and / or collection cones of light). Furthermore, the illumination pupil distribution 206 has a diameter of approximately 8 degrees and is centered at θ X The imaging pupil distribution 208 has a diameter of about 9 degrees and a center at θ X In this way, the illumination pupil distribution 206 and the imaging pupil distribution 208 for lithography provide a θ X 206 and 208 are both located inside the cutoff angle 210 of the photomask.

[0047] However, it should be understood that the presented characteristics of the lithography-oriented illumination pupil distribution 206 and the imaging pupil distribution 208, including, but not limited to, the size, shape, orientation, and location of the distributions, are for illustrative purposes only and should not be construed as limiting. Furthermore, the illumination pupil distribution 206 need not be the same size, shape, orientation, etc. as the lithography-oriented imaging pupil distribution 208.

[0048] 2, the illumination pupil distribution 202 and / or the imaging pupil distribution 204 for inspecting the specimen 106 may also be circular, and thus the cones of illumination light and / or collection light may be conical. In an embodiment, as depicted in FIG. 2, the center point of the illumination pupil distribution 202 (e.g., that corresponding to the chief ray angle) is θ X 2, the inspection-purpose imaging pupil distribution 204 has a diameter of approximately 13 degrees and a position of approximately 7.5 degrees along the direction, so that the inspection-purpose illumination pupil distribution 202 can be located along the edge of the cutoff angle 210 and can be entirely contained within the cutoff angle 210. Also, in the embodiment, as also illustrated in FIG. 2, the center point of the inspection-purpose imaging pupil distribution 204 (e.g., the point corresponding to the chief ray angle) is θ X The diameter is set to approximately 19 degrees at a point approximately −9 degrees along the direction. Therefore, the spread of the inspection-purpose imaging pupil distribution 204 can be made to partially exceed the cutoff angle 210. As described above in this application, by making the inspection-purpose imaging pupil distribution 204 larger in size than the inspection-purpose illumination pupil distribution 202, it becomes possible to capture scattered light from the photomask in addition to reflected light.

[0049] Next, non-circular illumination and imaging pupil distributions will be described in more detail with reference to FIGS. 3 and 4, respectively.

[0050] 3 is a plot 300 illustrating two differently shaped illumination pupil distributions for inspection in accordance with one or more embodiments of the present disclosure. For example, FIG. 3 illustrates a non-circular illumination pupil distribution 302, as well as the illumination pupil distribution for inspection 202 and the imaging pupil distribution for inspection 204 depicted in FIG. 2.

[0051] A non-circular illumination pupil distribution, such as, but not limited to, the non-circular illumination pupil distribution 302 of FIG. 3, facilitates design flexibility for the inspection system 100, thereby enabling increased sensitivity and / or throughput compared to a circular illumination pupil distribution, without increasing the size, complexity, and / or cost of the optical components. For example, the illumination optics 110 can be configured with one or more optical elements (e.g., mirrors, focusing elements, etc.) in an off-axis configuration, thereby preventing the illumination beam 104 from being incident at an orthogonal angle of incidence. That is, the illumination optics 110 can be sized to accommodate the projected size of the illumination beam 104 based on the angle of incidence of each element. Furthermore, many of the optical components are fabricated with circular dimensions due to manufacturing and / or cost considerations. Thus, in effect, by shaping portions of the illumination beam 104 along a direction perpendicular to the plane of incidence, a non-circular illumination pupil distribution 302 can be provided along a direction perpendicular to the plane of incidence, thereby making it possible to utilize an area on one or more illumination optics 110 that would otherwise go unused.

[0052] Furthermore, the extent of the non-circular illumination pupil distribution 302 may vary in directions outside the plane of incidence on the photomask (e.g., θ Y along the direction), in the plane of incidence (e.g., θ X Because the imaging pupil distribution extends farther than that in the horizontal direction (along the horizontal axis), modifications to the size, location, or shape of the imaging pupil distribution can be easily made without increasing the size, complexity, and / or cost of the optical components.

[0053] For example, as depicted in FIG. 3, a non-circular illumination pupil distribution 302 is Y direction while maintaining a good etendue or throughput of the illumination beam 104. X The numerical aperture can be reduced along the θ XNumerical aperture reduction along the direction allows the position of the inspection imaging pupil distribution 204 to be shifted toward the surface normal, as shown in Figure 3 as shifted imaging pupil distribution 304. The shifted imaging pupil distribution 304 can therefore improve performance in applications where the reflectivity of the photomask (or more generally the specimen 106) is high at angles near normal incidence. Furthermore, although not shown, increasing the size of the shifted imaging pupil distribution 304 allows for the collection of additional light (e.g., scattered light) from the specimen.

[0054] Also for example, although not shown, the size of the non-circular illumination pupil distribution, and therefore the associated etendue, may be increased without increasing the size, complexity, and / or cost of inspection system 100 .

[0055] 4 is a plot 400 illustrating two differently shaped inspection imaging pupil distributions in accordance with one or more embodiments of the present disclosure. Specifically, FIG. 4 illustrates a non-circular imaging pupil distribution 402, as well as the lithography illumination pupil distribution 206 and the inspection imaging pupil distribution 204 depicted in FIG. 2.

[0056] A non-circular imaging pupil distribution, such as, but not limited to, non-circular imaging pupil distribution 402 of FIG. 4 , facilitates design flexibility for inspection system 100, thereby enabling increased sensitivity and / or throughput compared to a circular imaging pupil distribution, without increasing the size, complexity, and / or cost of the optical components. For example, collection optics 120 can be configured with one or more optical elements (e.g., mirrors, focusing elements, etc.) in an off-axis configuration, thereby preventing collected light 116 from being incident at an orthogonal angle of incidence. That is, collection optics 120 can be sized to accommodate the projected size of collected light 116 based on the angular range of light collected from the specimen. Furthermore, many of the optical components are fabricated with circular dimensions due to manufacturing and / or cost considerations. Thus, in effect, a non-circular illumination pupil distribution 302 can be provided by shaping portions of the collected light 116 along a direction perpendicular to the plane of incidence, thereby making it possible to utilize otherwise unused area on one or more collection optics 120.

[0057] For example, if the spread of the non-circular imaging pupil distribution 402 is Y Along the direction, θ X 4. The farther along the direction, the more light (e.g., scattered light) can be captured from the specimen 106 without increasing the size or complexity (and associated cost) of the inspection imaging pupil distribution 204. For example, scattered light can be collected (e.g., as collected light 116) in the region 404 depicted in FIG. 4. According to another embodiment, not shown, the angle θ can be adjusted to not exceed the cutoff angle 210. X The size of the non-circular imaging pupil distribution along the direction can be set.

[0058] Furthermore, as previously described herein, a non-circular imaging pupil distribution can be combined with a non-circular illumination pupil distribution (e.g., non-circular illumination pupil distribution 302). This configuration allows combining the advantages of a non-circular distribution for both illumination and imaging. For example, the non-circular illumination pupil distribution 302 of FIG. 3 can be combined with the non-circular imaging pupil distribution 402 of FIG. 4. Furthermore, although not shown, θX By shifting the non-circular imaging pupil distribution 402 along the axial direction, more of the collected light 116 that is near the surface normal and emanates from the specimen 106 can be collected.

[0059] The non-circular pupil distribution may generally have any non-circular shape. For example, the non-circular illumination pupil distribution 302 and the non-circular imaging pupil distribution 402 in FIGS. 3 and 4 each have a major axis that is perpendicular to the plane of incidence (e.g., perpendicular to the surface normal and the ray at the center of the illumination pupil distribution) θ Y In some embodiments, the non-circular illumination pupil distribution 302 and / or the non-circular imaging pupil distribution 402 may be any non-circular shape.

[0060] Additionally, a non-circular pupil distribution may be formed using any combination of components of inspection system 100, including, but not limited to, one or more apertures. For example, a non-circular illumination pupil distribution (e.g., non-circular illumination pupil distribution 302 in FIG. 4) may be formed by one or more apertures on illumination path 108. Also, for example, a non-circular imaging pupil distribution (e.g., non-circular imaging pupil distribution 402 in FIG. 3) may be formed by one or more apertures on collection path 118.

[0061] It is recognized herein that the resolution of an imaging system depends in part on the numerical aperture of collected light 116. In that regard, a non-circular imaging pupil distribution (e.g., non-circular illumination pupil distribution 302 in FIG. 3) will have a resolution of θ Y θ compared to that along the direction X This can lead to differences in resolution along a direction. For example, the resolution (e.g., sampling resolution) along a particular direction of an image generated by inspection system 100 can depend on factors such as, but not limited to, the numerical aperture of collected light 116 along that direction, the magnification along that direction, and the pixel size of detector 114 along that direction.

[0062] In some embodiments, the inspection system 100 provides at least one of a non-circular illumination pupil distribution and a non-circular imaging pupil distribution, and further provides multiple directions (e.g., θ X Direction and θY The resolution is constant (or nearly constant) across the image (or direction).

[0063] In one embodiment, the detector 114 of the inspection system 100 may include rectangular pixels with different lengths along two orthogonal directions to provide uniform image resolution along the two orthogonal directions. This configuration allows the difference in pixel length along the two orthogonal directions to compensate for differences in numerical aperture and / or magnification associated with a non-circular imaging pupil.

[0064] 5A and 5B are conceptual top views of detector 114 having square and rectangular pixels, respectively. For example, as depicted in FIG. 5A, detector 114 can have square pixels 502 with uniform lengths 504 along two orthogonal directions. For example, as depicted in FIG. 5B, detector 114 can have rectangular pixels 502 with short side lengths 506 and long side lengths 508.

[0065] In one embodiment, the inspection system 100 has a non-circular imaging pupil (e.g., the non-circular imaging pupil distribution 402 depicted in FIG. 4) that has a larger extent in directions outside the plane of incidence than in directions within the plane of incidence, and a detector 114 with rectangular pixels 502 as depicted in FIG. 5B. For example, the plane of incidence depicted in FIGS. 2-4 may be defined as a plane of incidence that is perpendicular to the surface normal (angle 0) and θ X Furthermore, the orientation of the pixel 502 of the detector 114 can be defined by the short side length 506 being θ Y Along the direction, the long side length is θ X In addition, the size of the non-circular imaging pupil distribution 402 and the size of the pixel 502 (e.g., the short side length 506 and / or the long side length 508) can be co-optimized to adjust the θ X direction, θ Y For example, the θ X Size along the direction and θ YBy controlling the size along the direction based on known pixel dimensions (e.g., short side length 506 and / or long side length 508), the θ X direction, θ Y This allows for consistent sampling of the collected light 116 along both directions.

[0066] According to one embodiment, anamorphic magnification can be performed in the collection path 118 of the inspection system 100. For example, θ X Magnification along the direction is θ Y According to this configuration, the magnification along the θ X and / or θ Y By scaling the magnification along the direction, the θ X direction, θ Y This allows for a consistent image resolution (eg, sampling resolution) for the collected light 116 along both directions.

[0067] Additionally, any type of detector 114 having pixels of any size or shape may be used to perform anamorphic magnification in inspection system 100. In one embodiment, inspection system 100 has a non-circular imaging pupil that extends more outside the plane of incidence than within the plane of incidence (e.g., non-circular imaging pupil distribution 402 depicted in FIG. 4), a detector 114 with square pixels 502 as depicted in FIG. 5A, and collection optics 120 that performs anamorphic magnification.

[0068] The inspection system 100 (e.g., the collection optics 120, the detector 114, etc.) may sample the collected light 116 at any desired resolution. In one embodiment, the inspection system 100 provides a specified sampling resolution relative to the point spread function (PSF) of the collection optics 120. For example, the size of the pixels 502 on the detector 114 and the magnification provided by the collection optics 120 may be co-optimized to provide a specified number of pixels relative to the size of the PSF. It is recognized herein that the sampling resolution relative to the PSF can represent a balance between design tradeoffs in the inspection system. This configuration provides a specified balance, thereby enabling critical sampling. For example, increasing the sampling resolution generally improves image quality (e.g., image fidelity), while also increasing the sampling time and, therefore, operational throughput. Furthermore, increasing the sampling resolution may require increasing the magnification, which may increase the complexity and cost of the collection optics 120 and / or introduce aberrations. In one embodiment, inspection system 100 provides approximately 4.88 pixels 502 across the PSF of collection optics 120, which may be considered critical sampling. However, it should be understood that the value of 4.88 is provided for illustrative purposes only and should not be considered limiting. In general, inspection system 100 may provide any sampling resolution for any specified performance level.

[0069] Furthermore, in reality, the PSF may not be circular and may be oriented in different directions (e.g., θ X Direction and θ Y The pixel 502 may have different sizes along different directions (such as along the x- and y-axis directions on the specimen) and different sizes along different directions. In such configurations, the range of the numerical aperture, the magnification, and the size of the pixel 502 along different directions can be specified separately to achieve a specified level of performance.

[0070] It is recognized herein that an inspection system 100 having a non-circular imaging pupil and constant resolution across multiple directions (e.g., that obtained with some combination of anamorphic scaling and square or rectangular pixels 502) can operate in the same or substantially the same manner as an inspection system 100 having a circular imaging pupil. For example, image analysis and / or processing of the resulting images can be performed using the same or substantially the same techniques with little or no modification. Thus, the advantages of the non-circular imaging pupil described herein can be obtained without a significant increase in size, complexity, or cost.

[0071] Additionally, the present application recognizes that a non-circular pupil for illumination and / or imaging can be suitable for inspection of a wide range of defect types regardless of their orientation on the specimen 106.

[0072] As an example, we can model the defect sensitivity for various defect types as the ratio of the out-of-plane numerical aperture to the in-plane numerical aperture (e.g., NA if we use the directions defined in Figures 2 to 4). θY / NA θX ) and using a nearly elliptical imaging pupil distribution. Additionally, a common system was used to generate each model, where only the imaging pupil distribution was modified (e.g., the illumination pupil distribution was kept constant). Y In the structure along the X-axis (θ of the inspection system 100) on the specimen 106, an increase in inspection sensitivity of about 19% was realized at a numerical aperture ratio of 1.25, an increase in inspection sensitivity of about 32% at a numerical aperture ratio of 1.5, and an increase in inspection sensitivity of about 48% at a numerical aperture ratio of 2. X For structures based on the numerical aperture ratio (NRA) of 1.25, an approximately 12% increase in inspection sensitivity was achieved with a numerical aperture ratio of 1.5, an approximately 20% increase in inspection sensitivity with a numerical aperture ratio of 1.5, and an approximately 27% increase in inspection sensitivity with a numerical aperture ratio of 2. For round defects, an approximately 19% increase in inspection sensitivity was achieved with a numerical aperture ratio of 1.25, an approximately 28% increase in inspection sensitivity with a numerical aperture ratio of 1.5, and an approximately 48% increase in inspection sensitivity with a numerical aperture ratio of 2.

[0073] As another example, a model of defect sensitivity for various defect types may be created using a circular illumination pupil with a NA ratio of approximately 1.37, along with an approximately elliptical illumination pupil distribution. θY / NA θX Furthermore, a common system was used to generate each model, where only the illumination pupil distribution was modified (e.g., the imaging pupil distribution was kept constant). Y In the structure along the X direction (θ of the inspection system 100) on the specimen 106, an increase in inspection sensitivity of about 3.6% was realized with a numerical aperture ratio of 1.37. X For structures based on the numerical aperture ratio (NRA) of 1.37, an increase in inspection sensitivity of approximately 0.4% was achieved. For round defects, an increase in inspection sensitivity of approximately 1.4% was achieved at a numerical aperture ratio of 1.37. Furthermore, an increase in average multilayer reflectivity of approximately 10% was achieved at a numerical aperture ratio of 1.37.

[0074] 6 is a flow diagram depicting steps performed in a specimen inspection method 600. Applicant notes that previously described embodiments and implementing technologies within the context of inspection system 100 are understood to apply to method 600. However, it is further noted that method 600 is not limited to the architecture of inspection system 100.

[0075] The method 600 according to an embodiment includes directing 602 light to a specimen based on an illumination pupil distribution, where the illumination pupil distribution is centered in a first direction on the specimen (e.g., the X direction on the specimen, depicted in FIGS. 2-4 as θ 1 ). X In this case, the illumination pupil distribution can be mapped to the incidence angle range of the incident light.

[0076] The light directed at the specimen can have any specified spectrum, including any specified wavelength or wavelength range. In one embodiment, the light directed at the specimen includes at least a portion of EUV light. Therefore, if EUV light is directed at the specimen, optical elements suitable for EUV light (e.g., mirrors, focusing elements, etc.) can be incorporated.

[0077] In accordance with an embodiment, method 600 also includes collecting 604 light from the specimen corresponding to an imaging pupil distribution, where the center of the imaging pupil distribution corresponds to an off-axis collection angle along a first direction on the specimen, and where at least one of the illumination pupil distribution and the imaging pupil distribution is non-circular.

[0078] The method 600 according to the embodiment also includes generating 606 an image of the specimen based on the collected light.

[0079] Any combination of illumination pupil distribution and imaging pupil distribution may be non-circular. Furthermore, the extent of the non-circular pupil distribution may be smaller along a first direction than in another direction (e.g., a second direction on the specimen that is perpendicular to the first direction).

[0080] For example, a non-circular illumination pupil distribution facilitates tuning of the numerical aperture of the incident and / or collected light along a first direction (e.g., in the plane of incidence defined by the surface normal to the specimen and the center of the illumination pupil distribution). According to certain embodiments, the extent of the numerical aperture of the incident light (e.g., the extent of the illumination pupil distribution along the first direction) can be reduced along the first direction and increased in other directions. Thus, the angle of the incident and / or collected light along the first direction can be selected to be near the surface normal without sacrificing the etendue of the incident light. This configuration can provide improved performance compared to a circular illumination pupil distribution without increasing the size, complexity, and / or cost of associated optical components.

[0081] Additionally, for example, a non-circular imaging pupil distribution can facilitate increasing the amount of scattered light collected to generate an image of the specimen, thereby increasing signal strength and improving image quality compared to a circular illumination pupil without increasing the size, complexity, and / or cost of associated optical components.

[0082] According to some embodiments, the pixel size of the detector for generating the specimen image and / or the magnification of the optics collecting light from the specimen (e.g., in step 604) and generating the image of the specimen (e.g., in step 606) can be adjusted to achieve uniform image resolution for the specimen. For example, a non-circular imaging pupil distribution may in practice result in different resolutions along different directions. According to some embodiments, imaging the specimen with anamorphic scaling and square pixels in steps 604 and 606 achieves uniform image resolution along two orthogonal directions. According to some embodiments, imaging the specimen with uniform magnification and rectangular pixels of appropriate size in steps 604 and 606 achieves uniform image resolution along two orthogonal directions. According to some embodiments, imaging the specimen with anamorphic scaling and rectangular pixels of appropriate size in steps 604 and 606 achieves uniform image resolution along two orthogonal directions. According to some embodiments, imaging the specimen with anamorphic scaling and rectangular pixels of appropriate size in steps 604 and 606 achieves uniform image resolution along two orthogonal directions.

[0083] The subject matter described herein is sometimes depicted as various components embedded within, or connected or coupled to, other components. It should be understood that the depicted architectures are merely exemplary, and that in fact, many other architectures are possible to implement the same functionality. Conceptually, if any arrangement of components achieves the same functionality, then that arrangement is substantially "associated" with one another to achieve the desired functionality. Thus, any two components herein that are combined to achieve a particular function can be considered to be "associated" with one another to achieve the desired functionality, regardless of the architecture or intervening components. Similarly, any two components so associated can also be viewed as "connected" or "coupled" to one another to achieve the desired functionality, and any two components that can be so associated can also be viewed as "combinable" with one another to achieve the desired functionality. Examples of "couplable" include, but are not limited to, physically interactable and / or physically interacting components, and / or wirelessly interactable and / or wirelessly interacting components, and / or logically interactable and / or logically interacting components.

[0084] The present disclosure and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes can be made in the form, construction and arrangement of the parts without departing from the disclosed subject matter or diminishing all of its essential advantages. The described form is merely illustrative, and it is the intent of the following claims to encompass and embrace all such modifications. It will further be understood that it is the appended claims which define the invention.

Claims

1. an illumination source configured to generate extreme ultraviolet (EUV) light; one or more illumination optics configured to direct the extreme ultraviolet light toward a specimen; one or more collection optics configured to collect extreme ultraviolet light from the specimen and to perform anamorphic magnification on the collected extreme ultraviolet light such that a magnification of the collected extreme ultraviolet light along a first direction is different from a magnification of the collected extreme ultraviolet light along a second direction; a detector including a plurality of rectangular pixels configured to receive at least a portion of the collected extreme ultraviolet light from the one or more collection optics having anamorphic magnification; wherein dimensions of the rectangular pixels are configured to compensate for numerical aperture differences associated with the anamorphic magnification of the extreme ultraviolet (EUV) light.

2. 10. The inspection system of claim 1, wherein the imaging pupil distribution is non-circular.

3. The inspection system of claim 2 , wherein the imaging pupil distribution is elliptical with a major axis along the second direction.

4. 2. The inspection system of claim 1, wherein a sampling resolution of the collected light on the detector is uniform along the first direction and the second direction.

5. 2. The inspection system of claim 1, wherein the rectangular pixels have a short side length and a long side length, the long side length responsible for sampling along a first direction of the collected light and the short side length responsible for sampling along a second direction of the collected light.

6. 2. The inspection system of claim 1, wherein the detector comprises at least one of a CCD type or a CCD-TDI type detector.

7. 10. The inspection system of claim 1, wherein the illumination source comprises a broadband plasma illumination source.

8. 10. The inspection system of claim 1, comprising a photomask inspection system.

9. 10. The inspection system of claim 1, comprising a wafer inspection system.

10. 1. An inspection system comprising: a detector including a plurality of rectangular pixels configured to receive anamorphically scaled extreme ultraviolet light from one or more collection optics, a magnification of the anamorphically scaled extreme ultraviolet light along a first direction differs from a magnification of the anamorphically scaled extreme ultraviolet light along a second direction, and dimensions of the rectangular pixels are configured to compensate for differences in numerical aperture associated with the anamorphically scaled extreme ultraviolet light.

11. 11. The inspection system of claim 10, wherein the imaging pupil distribution of the extreme ultraviolet light is non-circular.

12. The inspection system of claim 11 , wherein the imaging pupil distribution is elliptical with a major axis along the second direction.

13. 11. The inspection system of claim 10, wherein a sampling resolution of the collected light on the detector is uniform along the first direction and the second direction.

14. 11. The inspection system of claim 10, wherein the rectangular pixels have a short side length and a long side length, the long side length responsible for sampling along a first direction of the collected light and the short side length responsible for sampling along a second direction of the collected light.

15. 11. The inspection system of claim 10, wherein the extreme ultraviolet light is generated by a broadband plasma illumination source.

16. 11. The inspection system of claim 10, wherein the detector includes at least one of a CCD type or a CCD-TDI type detector.

17. 11. The inspection system of claim 10, comprising a photomask inspection system.

18. 11. The inspection system of claim 10, comprising a wafer inspection system.

19. generating extreme ultraviolet light; directing the extreme ultraviolet light toward a specimen through one or more illumination optics; collecting the extreme ultraviolet light from the specimen with one or more collection optics; anamorphically scaling the collected extreme ultraviolet light such that the collected extreme ultraviolet light has a magnification along a first direction and a magnification along a second direction that is different from the magnification along the first direction; detecting the anamorphically scaled collected extreme ultraviolet light with a detector including a plurality of rectangular pixels; wherein dimensions of the rectangular pixels are configured to compensate for numerical aperture differences associated with the anamorphic magnification of the extreme ultraviolet (EUV) light.

20. 20. The method of claim 19, wherein the imaging pupil distribution is non-circular.

21. 20. The method of claim 19, wherein the sampling resolution of the collected light on the detector is uniform along the first direction and the second direction.

Images