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) TECHNICAL FIELD This disclosure relates generally to inspection systems, and more specifically 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, and entitled “INSPECTION SYSTEM WITH NON-CIRCULAR PUPIL” by Damon Kvamme, Rui-fang Shi, Daniel Wack, Sseunhyeun Jo and Xin Ye, the entirety of which is 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 thus be able to resolve defects of interest that may impact the manufacturing process or product performance.

[0004] For example, photomask inspection systems can use illumination sources with the same or substantially similar spectra to the light sources used in lithography systems to create image-based features on those masks, allowing for the resolution of defects that could affect integrated circuits created using those masks. Driven by the demand for ever smaller feature sizes, however, the wavelengths used by both lithography and inspection systems have correspondingly decreased. In particular, extreme ultraviolet (EUV) light, commonly used 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, which typically imposes a tradeoff 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 achieves 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 according to an exemplary embodiment includes an illumination source that generates extreme ultraviolet (EUV) light. The system according to an exemplary embodiment includes illumination optics that direct the EUV light to a specimen within an incidence angle range, the incidence angle range corresponding 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 according to an exemplary embodiment includes one or more collection optics that collect light from the specimen in response to the incident EUV light within a collection angle range, the collection angle range corresponding to an imaging pupil distribution at a pupil plane of the one or more collection optics. In an exemplary embodiment, the imaging pupil distribution center corresponds to an off-axis collection angle along a first direction on the specimen. In some demonstrative 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 smaller than a size along a second direction perpendicular to the first direction. In some demonstrative 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 according to an exemplary embodiment includes an illumination source that generates an illumination beam. The system according to an exemplary embodiment includes one or more illumination optics that direct the illumination beam to the photomask within an incidence angle range, the incidence angle range corresponding to an illumination pupil distribution at a pupil plane of the one or more illumination optics, and a center of the illumination pupil distribution corresponding to an off-axis incidence angle along a first direction on the photomask. The system according to an exemplary embodiment includes one or more collection optics that collect light from the photomask in response to the incident illumination beam within a collection angle range, the collection angle range corresponding to an imaging pupil distribution at a pupil plane of the one or more collection optics. In an exemplary embodiment, the imaging pupil distribution center corresponds to an off-axis collection angle along a first direction on the photomask. In some illustrative 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 less than a size along a second direction perpendicular to the first direction. In some 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] In accordance with one or more exemplary embodiments of the present disclosure, an inspection method is disclosed. In some exemplary embodiments, the method includes directing extreme ultraviolet (EUV) light to a specimen within an incidence angle range, the incidence angle range 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. In some exemplary embodiments, the method includes collecting light resulting from the specimen in response to the incident EUV light within a collection angle range, the collection angle range corresponding to an imaging pupil distribution at a pupil plane of one or more collection optics. In some exemplary embodiments, the imaging pupil distribution center corresponds to an off-axis collection angle along the first direction on the specimen. In some 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 smaller than the size of the non-circular shape along a second direction perpendicular to the first direction. In accordance with one illustrative embodiment, the method also includes generating an image of the sample with a detector based on at least a portion of 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 intended to limit 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 referring to the accompanying drawings, in which: [Brief description of the drawings]

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

[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 specifically shown and described with reference to certain embodiments and specific features thereof. The embodiments described herein should be considered as 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 an in-plane direction is smaller than in an out-of-plane direction.

[0015] For example, an inspection system may typically illuminate a specimen with a range of incidence angles corresponding to a numerical aperture of the illumination optics, and collect light from the specimen within a range of collection angles corresponding to a numerical aperture of the collection optics, which may or may not be the same as that of the illumination optics. The range of incidence angles and collection angles correspond to distributions in the illumination and imaging pupil planes, respectively. Thus, an off-axis illumination system may be characterized as one in which the chief ray (or approximate center) of the illumination and / or imaging pupil distributions presents an off-axis angle with respect to the normal of the specimen (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, an EUV light source for lithography and / or inspection can generate light centered around wavelengths of, but not limited to, 13.5 nm, 7 nm, etc. As recognized herein, 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. Thus, 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 an EUV imaging system, the illumination pupil distribution and the imaging pupil distribution will be non-overlapping so that there is no overlap between the incidence angle range of the incident light and the collection light range used to image the specimen, which imposes additional constraints, as described in more detail below.

[0017] This application recognizes that inspection systems are typically designed to exhibit circular illumination and imaging pupil distributions such that a typical optical configuration can provide consistent image quality (e.g., resolution, signal strength, noise level, etc.) regardless of orientation on the specimen. However, such a configuration for an off-axis inspection system would not utilize the full range of available numerical aperture, ultimately limiting the system 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 of the noncircular illumination pupil in a direction within the plane of incidence (e.g., defined by the specimen normal and the off-axis illumination light and / or collection light) than in a direction outside 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 the illumination etendue, i.e., throughput of illumination photons) can increase the desired defect signal, at the expense of the adverse effects of line edge roughness. Furthermore, in practice, since the reflectivity of the specimen may decrease as the angle of incidence relative to the specimen normal increases, it may be desirable to limit the numerical aperture of the illumination. By illuminating a specimen with a noncircular illumination pupil, the numerical aperture of the illumination light can be limited along a specified direction (e.g., in the plane of incidence) while maintaining the desired illumination etendue. With this configuration, illumination with a non-circular illumination pupil can provide improved performance compared to illumination with a circular illumination pupil, without making the optical system larger, more complex, or more costly.

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

[0020] The present application recognizes that a non-circular imaging pupil distribution can lead to different resolutions in different directions since the resolution of the 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 with 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), allowing a typical detector with square pixels to sample the imaging light uniformly. 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 differences in pixel size.

[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] An embodiment of the 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, including, 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 a lithography tool suitable for semiconductor manufacturing, including, but not limited to, 13.5 nm, 7 nm, and the like.

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

[0024] According to another embodiment, illumination source 102 may include 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 may have any temporal profile. For example, the illumination source 102 may generate a continuous illumination beam 104, a pulsed illumination beam 104, or a modulated illumination beam 104.

[0026] In an embodiment, the illumination source 102 also directs an illumination beam 104 to the sample 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 sample 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 suitable for directing and / or focusing the illumination beam 104 as known in the art. For example, the illumination optics 110 may include reflective optics 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.).

[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 incidence angular range (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 an embodiment, the specimen 106 is disposed 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 embodiment of the 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 (e.g., specularly, diffusely, etc.) from the specimen 106 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] Detector 114 may comprise any type of detector known in the art suitable for measuring collected light 116 received from specimen 106. Examples of 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, detector 114 may comprise a spectroscopic detector suitable for identifying wavelengths of radiation emanating from 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 one embodiment, the inspection system 100 also includes a controller 122. In one embodiment, the controller 122 also includes one or more processors 124 configured to execute program instructions stored on a storage medium 126 (e.g., a memory). In this configuration, the one or more processors 124 included in the controller 122 may perform any of the various process steps described elsewhere in this disclosure.

[0033] The one or more processors 124 that comprise 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. In some 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 that 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 execute 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 the associated processor(s) 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, a read-only memory, a random access memory, a magnetic or optical storage device (e.g., disk), a magnetic tape, a solid-state drive, and the like. It is further noted that the storage medium 126 may be housed with the processor(s) 124 in a common controller housing. According to some embodiments, the storage medium 126 may be remote to the physical location of the processor(s) 124 and the controller 122. For example, the processor(s) 124 of the controller 122 may access a remote memory (e.g., a server) accessible over a network (e.g., the Internet, an intranet, etc.). Thus, the above description should be taken as merely illustrative and not as a limitation on the present invention.

[0035] Controller 122 may be communicatively coupled to any component of inspection system 100 or to any additional components external to inspection system 100. In an embodiment, 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 an embodiment, 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, sorting, etc.

[0036] Further, 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 multiple controllers housed in a common controller housing or housed in multiple housings. In this manner, any controller or combination of controllers may be packaged separately into modules 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 and / or housed in any combination of the inspection tools.

[0037] Additionally, in embodiments, controller 122 may control (e.g., via control signals) and / or direct 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. 6,399,433 issued to Stokowski on April 29, 2014, and U.S. Patent No. 6,399,433 issued to Xiong et al. on 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. 6,399,323 issued to Wack on September 23, 2014, the entirety of which is incorporated herein by reference.

[0039] Circular and non-circular pupil distributions (e.g., illumination and imaging pupil distributions) for illumination and collection light are now described in detail with reference to Figures 2-4. The illumination and / or imaging pupil distributions of Figures 2-4 may be realized by an inspection system such as, but not limited to, inspection system 100 depicted in Figure 1. Additionally, 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 inspection of a photomask and lithography with a photomask can be performed with separate 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 distributions can be different for different applications to accommodate 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 incidence or collection 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 a cutoff angle inefficient and / or impractical.

[0042] Additionally, it may be desirable to have the illumination pupil distribution of the inspection system located 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 may be more flexible in design than the illumination pupil distribution. For example, illumination use outside the cutoff angle may be inefficient. However, the imaging pupil distribution may include both reflected light (e.g., light on 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 range of angles. Thus, by expanding the imaging pupil distribution to accommodate angles beyond the angle of the reflected light, more optical signals may be captured and the signal-to-noise ratio of the generated image may be increased. Furthermore, the imaging pupil distribution may generally be expanded beyond the cutoff angle of the specimen 106 without degrading performance.

[0043] 2 is a plot 200 of a circular illumination pupil distribution and an imaging pupil distribution 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 relative to the surface normal, and θ X ,θ Y represents the azimuth angle based on the photomask. X and θ Y can be mapped 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 the inspection and lithography tools. Additionally, FIG. 2 illustrates the photomask coverage cutoff angle 210 to be approximately 14 degrees, represented by a circle centered at an angle of 0 (the surface normal).

[0044] In general, Figs. 2 to 4 show the θ X 2-4, it may be useful to characterize the off-axis illumination and imaging of a specimen with respect to a plane of incidence. For example, the plane of incidence may be defined by a surface normal and a central ray of the incident and / or collected light. It is recognized that in real systems, the incident and / or collected light is represented as a cone (e.g., associated with a range of numerical apertures), and the plane of incidence may be defined for each ray in the cone. However, for purposes of clarity of this disclosure, it is better 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 may be defined by a surface normal and a θ in angle space. X2-4, the inspection imaging pupil distribution 204 can be centered in the entrance plane, although this is not required.

[0045] Further, the present application recognizes that the distribution of a beam (e.g., illumination beam 104) cannot typically be represented as a step function, but rather may generally have tailed distributions. Additionally, the beam may have "hot spots" or irregularities. Therefore, the center of the illumination and / or imaging pupil distribution may be identified by any technique known in the art. For example, the center of the pupil distribution may be identified 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 for lithography and the imaging pupil distribution 208 are shown as circular distributions (e.g., representing illumination and / or collection cones of light). Additionally, the illumination pupil distribution 206 has a diameter of approximately 8 degrees and is centered at θ X direction, the diameter of the imaging pupil distribution 208 is about 9 degrees and the center is θ X In this way, the illumination pupil distribution 206 and the imaging pupil distribution 208 for lithography are X 2, the illumination pupil distribution 206 for lithography and the imaging pupil distribution 208 for imaging are both inside the cutoff angle 210 of the photomask.

[0047] However, it will 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 position of those distributions, are for illustrative purposes only and should not be taken 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 cone 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-oriented imaging pupil distribution 204 has a center point (e.g., that corresponding to the chief ray angle) of θ X The diameter is set to about 19 degrees at a position about −9 degrees along the direction. Therefore, the spread of the imaging pupil distribution for inspection 204 can be made to partially exceed the cutoff angle 210. As described above in the present application, by making the imaging pupil distribution for inspection 204 larger in size than the illumination pupil distribution for inspection 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 Figures 3 and 4, respectively.

[0050] 3 is a plot 300 illustrating two different inspection illumination pupil distributions having different shapes 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 inspection illumination pupil distribution 202 and the inspection imaging pupil distribution 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, can facilitate design flexibility for the inspection system 100 and thus increase 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 such that the illumination beam 104 is not 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. Additionally, many of the optical components are fabricated with circular dimensions due to manufacturing and / or cost concerns. Thus, in effect, by shaping portions of the illumination beam 104 along a direction perpendicular to the plane of incidence, it is possible to provide a non-circular illumination pupil distribution 302 along a direction perpendicular to the plane of incidence, thereby making it possible to utilize 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 that are 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 x-direction (along the x-direction), 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 has a θ 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 thus improve performance in applications where the reflectivity of the photomask (or more generally the specimen 106) is high at angles near normal incidence. Additionally, although not shown, increasing the size of the shifted imaging pupil distribution 304 allows additional light (e.g., scattered light) to be collected from the specimen.

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

[0055] 4 is a plot 400 illustrating two different inspection imaging pupil distributions having different shapes 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, the non-circular imaging pupil distribution 402 of FIG. 4, can facilitate flexible design of the inspection system 100 and thus increase 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, the collection optics 120 can be configured in an off-axis configuration with one or more optical elements (e.g., mirrors, focusing elements, etc.) such that the collected light 116 is not incident at an orthogonal incidence angle. That is, the collection optics 120 can be sized to accommodate the projected size of the collected light 116 based on the angular range over which light is collected from the specimen. Additionally, many of the optical components are fabricated with circular dimensions due to manufacturing and / or cost concerns. 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 area on one or more collection optics 120 that would otherwise go unused.

[0057] For example, the spread of the non-circular imaging pupil distribution 402 is Y Along the direction, θ X 4. Since the aperture extends further along the θ direction, it is possible to capture additional light (e.g., scattered light) 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 aperture may be adjusted to a value of θ 2 so as not to exceed the cutoff angle 210. X The size of the non-circular imaging pupil distribution along the direction can be set.

[0058] Additionally, as 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 the advantages of a non-circular distribution to be combined 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. Additionally, although not shown, a non-circular illumination pupil distribution (e.g., as described for imaging pupil distribution 304) can be combined with a non-circular illumination pupil distribution (e.g., as described for imaging pupil distribution 304) forX 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 emerges into the specimen 106 can be obtained.

[0059] The non-circular pupil distribution may generally be any non-circular shape. For example, the non-circular illumination pupil distribution 302 and the non-circular imaging pupil distribution 402 in FIG. 3 and FIG. 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] As recognized herein, 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., a non-circular illumination pupil distribution 302 in FIG. 3) may be affected by θ Y θ compared to that along the direction X For example, the resolution (e.g., sampling resolution) along a particular direction of an image generated by inspection system 100 may depend on factors such as, but not limited to, the numerical aperture of collected beam 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) over the range of the scanning direction.

[0063] In one embodiment, the detector 114 of the inspection system 100 may include rectangular pixels with different lengths along two mutually orthogonal directions to provide uniform image resolution along the two mutually orthogonal directions. In this configuration, the difference between the pixel lengths along the two mutually orthogonal directions may 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 length 504 along two orthogonal directions. For example, as depicted in FIG 5B, detector 114 can have rectangular pixels 502 exhibiting short side length 506 and long side length 508.

[0065] In one embodiment, the inspection system 100 has a non-circular imaging pupil that is larger in extent in directions outside the incidence plane (e.g., the non-circular imaging pupil distribution 402 depicted in FIG. 4) than in directions within the incidence plane, and a detector 114 with rectangular pixels 502 as depicted in FIG. 5B. For example, the incidence plane in FIGS. 2-4 may be viewed as an angle θ X Furthermore, the orientation of the pixels 502 of the detector 114 can be defined as a pixel having a short side length 506 of 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, it is possible to obtain uniform sampling along both the θ direction and the θ direction of a non-circular imaging pupil distribution 402. X Size along the direction and θ YThe size along the direction is controlled based on known pixel dimensions (e.g., short side length 506 and / or long side length 508) to X direction, θ Y This allows for consistent sampling of 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 Scaling the magnification along the direction allows for a θ X direction, θ Y It is possible to provide a constant 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 provide anamorphic magnification in the inspection system 100. In one embodiment, the inspection system 100 has a non-circular imaging pupil that is larger in extent in directions outside the plane of incidence (e.g., the non-circular imaging pupil distribution 402 depicted in FIG. 4), a detector 114 having square pixels 502 as depicted in FIG. 5A, and collection optics 120 that provides anamorphic magnification.

[0068] In the inspection system 100 (e.g., the condenser optical system 120, the detector 114, etc.), the condensed light 116 may be sampled at any resolution. In the inspection system 100 according to one embodiment, a specified sampling resolution is provided based on the point spread function (PSF) of the condenser optical system 120. For example, by co-optimizing the size of the pixel 502 on the detector 114 and the magnification provided by the condenser optical system 120, it is possible to provide a specified number of pixels based on the size of the PSF. According to the understanding in the present application, the sampling resolution based on the PSF can represent the balance between design trade-offs in the inspection system. According to this configuration, a specified balance is brought about by the specified sampling resolution, thereby enabling critical sampling. For example, by increasing the sampling resolution, generally, the image quality (e.g., image fidelity) can be improved, but on the other hand, the sampling time can be lengthened, thereby increasing the operation throughput. Furthermore, increasing the sampling resolution may require increasing the magnification, thereby increasing the complexity and cost of the condenser optical system 120, and / or aberration may occur. The inspection system 100 according to one embodiment provides approximately 4.88 pixels 502 across the PSF of the condenser optical system 120, that is, what can be regarded as critical sampling. However, as can be understood, the value of 4.88 is presented solely for illustrative purposes and should not be construed as a limitation. Roughly, the inspection system 100 may provide any sampling resolution related to any specified performance level.

[0069] Furthermore, in reality, the PSF may not be circular and may exhibit different sizes along different directions (e.g., the directions corresponding to the θ X direction and the θ Y direction, the X direction and the Y direction on the specimen, etc.). In such a configuration, the specified performance level can be achieved by individually specifying the range of the numerical aperture, the magnification, and the sizes along the various directions of the pixel 502.

[0070] It is recognized herein that an inspection system 100 having a non-circular imaging pupil and constant resolution across multiple directions (e.g., obtained with some combination of anamorphic scaling and square or rectangular pixels 502) can operate in the same or nearly 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 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 using the directions defined in Figures 2 to 4). θY / NA θX ) while increasing the θ of the inspection system 100. 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 held constant). Y In the structure along the X-axis of the inspection system 100, an inspection sensitivity increase of about 19% was realized at a numerical aperture ratio of 1.25, an inspection sensitivity increase of about 32% was realized at a numerical aperture ratio of 1.5, and an inspection sensitivity increase of about 48% was realized at a numerical aperture ratio of 2. X In the structure according to the present invention, an approximately 12% increase in inspection sensitivity was achieved with a numerical aperture ratio of 1.25, 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 developed using a circular illumination pupil with a NA ratio of approximately 1.37, along with an approximately elliptical illumination pupil distribution. θY / NA θX In addition, a common system in which only the illumination pupil distribution is modified (e.g., the imaging pupil distribution is kept constant) was used to generate each model. 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 3.6% was realized with a numerical aperture ratio of 1.37. X For structures based on the numerical aperture of 1.37, an increase in inspection sensitivity of about 0.4% was realized. For round defects, an increase in inspection sensitivity of about 1.4% was realized at a numerical aperture of 1.37. Furthermore, an increase in average multilayer reflectance of about 10% was realized at a numerical aperture of 1.37.

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

[0075] In accordance with an embodiment, a method 600 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, which is depicted in FIGS. 2-4 as θ X In this case, the illumination pupil distribution can be associated with a range of angles of incidence of the incident light.

[0076] The light directed to the specimen can have any specified spectrum, including any specified wavelength or range of wavelengths. In one embodiment, the light directed to the specimen includes at least a portion of EUV light. Thus, if EUV light is directed to 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 a 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 an 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, and further, the spread of the non-circular pupil distribution may be shorter along a first direction relative to 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 can facilitate tuning of the numerical aperture of the incident and / or collected light along a first direction (e.g., in a plane of incidence defined by the surface normal of the specimen and the center of the illumination pupil distribution). In some embodiments, the extent of the numerical aperture of the incident light (e.g., the spread 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] For example, a non-circular imaging pupil distribution can facilitate increasing the amount of scattered light that is 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 the associated optical components.

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

[0083] The subject matter described herein is at times depicted as various components embedded within or connected to other components. It is to be understood that the depicted architectures are merely exemplary, and in fact many other architectures can be implemented to achieve the same functionality. Conceptually, if any arrangement of components achieves the same functionality, then the arrangement of components is substantially "associated" with each other to achieve the desired functionality. Thus, any two components in this application that are combined to achieve a particular functionality can be considered to be "associated" with each other 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 each other to achieve the desired functionality, and any two components that can be so associated can also be viewed as "combinable" with each other to achieve the desired functionality. Examples of coupleable 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 in the form, construction and arrangement of the parts may be made without departing from the disclosed subject matter or diminishing all of its essential advantages. The form described is merely illustrative, and it is the intent of the following claims to encompass and embrace all such modifications. Moreover, it will 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 EUV light to the specimen within a range of incidence angles corresponding to an illumination pupil distribution at a pupil plane of the one or more illumination optics, the illumination pupil distribution centered corresponding to an off-axis angle of incidence along a first direction on the specimen; one or more anamorphically magnified focusing optical systems configured to collect light resulting from the sample in response to the incident EUV light within a collection angle range corresponding to an imaging pupil distribution at a pupil plane of the one or more focusing optical systems, the imaging pupil distribution having a center corresponding to an off-axis collection angle along a first direction on the sample, the imaging pupil distribution being non-circular with a size along the first direction smaller than a size along a second direction perpendicular to the first direction, and the imaging pupil distribution being smaller in a direction within the incident plane than in a direction outside the incident plane; a detector configured to receive at least a portion of the collected light by the one or more collection optics, the detector having a plurality of rectangular pixels having a short side length and a long side length, the long side length responsible for sampling the collected light along the first direction and the short side length responsible for sampling the collected light along the second direction; An inspection system comprising:

2. 2. The inspection system of claim 1, wherein the imaging pupil distribution is elliptical, with a major axis aligned along the second direction.

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

4. 4. The inspection system of claim 3, 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 size of the rectangular pixels of the detector and the anamorphic magnification provided by the collection optics are co-optimized to provide a specified number of pixels based on the size of a point spread function (PSF) of the collection optics.

Images