Systems and methods for lateral shear interferometry in inspection tools

Abstract

A method for detecting a wavefront in situ in an inspection system is disclosed. The method includes generating light with a light source and directing the light to a hierarchical reflective mask array structure disposed on a mask stage. The method includes directing light reflected by the stage level reflective mask grating structure to a detector level reflective mask grating structure disposed in a plane of a detector, and then collecting the light reflected by the detector level reflective mask grating structure with an optical element. The method includes forming a pupil image on the detector and laterally offsetting the stepped reflective mask with the mask steps over an array period of the stepped reflective mask array structure to provide a phase reconstruction for a lateral shear interferometer. The method includes selectively illuminating a sensor of the detector with the light reflected by the optical element.

Description

[Technical Field]

[0001] REFERENCE TO RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 63 / 177,395 (filed April 21, 2021), which is incorporated herein by reference in its entirety.

[0002] "Technical Field" The present disclosure relates generally to systems and methods for wavefront aberration detection in imaging systems, and more particularly to lateral shearing interferometry-based wavefront metrology systems and methods for use in extreme ultraviolet (EUV) mask inspection tools or deep ultraviolet wafer inspection tools. [Background technology]

[0003] In advanced defect inspection tools for photolithography masks or wafers, such as those used in semiconductor manufacturing processes, there is an emerging need for internal optical aberration control of the imaging optics due to thermally induced drift or other mechanical alignment changes during tool operation. An essential part of this control scheme is optical aberration metrology, commonly referred to as "in-situ alignment metrology" due to its permanent implementation in the tool's hardware. In-situ metrology is typically divided between two different methods: in-situ coarse alignment and in-situ fine alignment. Both in-situ coarse alignment and in-situ fine alignment are typically based on aerial imaging of test structures on a given inspection tool's CCD (charge-coupled device) image sensor. This division between the two methods is necessary because metrology with sufficient accuracy for fine alignment does not have a sufficient capture range to converge to the best alignment state from the initial alignment stage after mechanical assembly of the imaging optics.

[0004] Coarse alignment metrology methods require different structures for different aberrations, and therefore only selected aberration terms are included in the alignment strategy. This is generally supported by optical simulations, but there is a risk that higher-order aberrations will not be captured. Although coarse alignment metrology is designed to have a dynamic range sufficient to capture misalignment levels after mechanical assembly of the imaging optics, there remains a risk that individual assemblies will exceed their limits and fail to align. Similarly, the final coarse alignment state may not be well within the capture range of the fine alignment method, resulting in convergence to a false minimum. Fine alignment metrology methods suffer from very low intensity levels and therefore require extensive measurement times. Generally, like all image-based aberration metrology, the above methods cannot rigorously distinguish between amplitude and phase and are therefore susceptible to errors in the illumination system or imperfections in photometric parameters such as coating uniformity or flare. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 8,916,831 [Patent Document 2] U.S. Patent No. 11,112,691 [Patent Document 3] U.S. Patent No. 7,928,416 [Patent Document 4] U.S. Patent No. 9,709,811 [Patent Document 5] U.S. Patent No. 10,021,773 Summary of the Invention [Problem to be solved by the invention]

[0006] It is therefore desirable to provide a system and method for overcoming the shortcomings of conventional solutions. [Means for solving the problem]

[0007] An inspection system is disclosed that has in situ wavefront metrology of the light of the inspection system. In an embodiment, the system includes an imaging subsystem including a light source, a set of imaging optics, and a detector including one or more imaging sensors. In an embodiment, the system includes a stage. In an embodiment, a stage-level reflective mask grating structure is disposed in a plane of a mask fixed by the stage, and the light source is configured to illuminate the stage-level reflective mask grating structure with incident light. In an embodiment, the system includes a detector-level reflective mask grating structure disposed in a plane of the detector, and the set of imaging optics is configured to direct light reflected from the stage-level reflective mask grating structure to the detector-level reflective mask grating structure. In an embodiment, the system includes an optical element configured to collect light from the detector-level reflective mask grating structure, and the detector-level reflective mask grating structure is oriented to direct light from the detector-level reflective mask grating structure to the optical element, and the optical element is configured to form a pupil image on one or more sensors of the detector. In an embodiment, the stage is configured to provide a lateral shift motion over a grating period of the stage-level reflective mask grating structure and provide phase reconstruction for lateral shear interferometry to identify changes in the wavefront of the light of the imaging system. In an embodiment, the system includes an actuator configured to selectively impinge light reflected from the optical element onto one or more sensors of the detector.

[0008] 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 present disclosure. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the subject matter of the present disclosure. Together, the description and the drawings serve to explain the principles of the present disclosure. [Brief explanation of the drawings]

[0009] The many advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying drawings.

[0010] [Figure 1] FIG. 1 shows a simplified block diagram of an EUV inspection system with in-situ lateral shear interferometry capability in accordance with one or more embodiments of the present disclosure. [Figure 2] FIG. 2 illustrates a top view of a stage level reflective mask grating structure in accordance with one or more embodiments of the present disclosure. [Figure 3] FIG. 3 illustrates a top view of a detector-level reflective grating structure in accordance with one or more embodiments of the present disclosure. [Figure 4] FIG. 4 shows a simplified block diagram of a DUV inspection system with in-situ lateral shear interferometry capability in accordance with one or more embodiments of the present disclosure. [Figure 5] FIG. 5 illustrates a flow diagram of a method for in situ lateral shearing interferometry functionality within a photomask inspection system in accordance with one or more embodiments of the present disclosure. [Figure 6] FIG. 6 illustrates a flow diagram of a method for in situ lateral shearing interferometry functionality within a wafer inspection system in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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 certain features thereof. The embodiments described herein are to be construed as illustrative and not restrictive. It will be readily apparent to those skilled in the art that various changes and modifications in form and detail may be made therein without departing from the spirit and scope of the present disclosure.

[0012] For initial setup and periodic maintenance of a photomask inspection system, there is a need to implement an image quality metrology tool for imaging optics permanently integrated into the photomask inspection system. A fundamental expression of the image quality of an imaging optics can be provided by wavefront aberration. Wavefront aberration, typically expressed as an expansion of Zernike polynomials, describes the deviation of a wavefront from an ideal spherical wave at a given field point and correlates with fundamental image errors such as defocus, coma, and astigmatism. Embodiments of the present disclosure are directed to an in-situ wavefront metrology system for extreme ultraviolet (EUV) mask inspection tools. The disclosed in-situ wavefront metrology system is based on lateral shearing interferometry (LSI) operating at the actinic EUV wavelength of 13.5 nm.

[0013] Actinic LSI wavefront metrology is advantageous because it combines a large initial capture range of imaging optics wavefront errors due to coarse mechanical alignment with potentially high accuracy, thereby offering the ability to replace multiple complementary aerial image-based metrology methods with a single metrology method. Additionally, the disclosed LSI wavefront metrology is faster than aerial imaging because only one structure needs to be scanned and integration times are shorter due to the non-critical optical budget. Embodiments of the present disclosure can also provide higher absolute accuracy than aerial imaging in the case of illumination defects because the LSI distinguishes between intensity and phase. In contrast to aerial imaging, all Zernike coefficients up to ~Z36 can be measured with a high capture range, thereby reducing the risk of "blind spots" in the optical setup process. When only certain Zernike coefficients are needed, phase-shifting and wavefront reconstruction algorithms can be optimized and accelerated to potentially track drifts, such as, but not limited to, heating-induced performance drift.

[0014] Additional embodiments of the present disclosure relate to wavefront metrology for wafer inspection systems with high numerical aperture in the deep ultraviolet (DUV) wavelength range of 190-270 nm.

[0015] 1 illustrates an inspection system 100 with in-situ wavefront metrology capabilities in accordance with one or more embodiments of the present disclosure. In an embodiment, the inspection system 100 is configured as a photomask inspection system operating in the EUV wavelength range (e.g., 13.5 nm). In an embodiment, the inspection system 100 includes an imaging subsystem 102. The imaging subsystem 102 may be configured to perform functions associated with inspecting a mask 114 as well as functions associated with in-situ wavefront metrology. In an embodiment, the inspection system 100 may be switched between i) a normal inspection mode and ii) a wavefront detection mode, as described further herein.

[0016] The imaging subsystem 102 may include, but is not limited to, an EUV light source 104, a set of EUV imaging optics 106, and one or more detectors 103 including one or more sensors 108 a, 108 b. It should be noted that the EUV light source 104, the set of EUV imaging optics 106, and the one or more detectors 103 may include any EUV light source 104, EUV imaging optics 106, and detector 103 known in the art of EUV-based inspection. U.S. Patent No. 8,916,831 (issued December 23, 2014) and U.S. Patent No. 11,112,691 (issued September 7, 2021) generally discuss EUV-based inspection and are incorporated herein by reference in their entireties. Light for generating EUV light sources is generally discussed in U.S. Patent 7,928,416 B2 (issued April 19, 2011); U.S. Patent 9,709,811 (issued July 18, 2017); and U.S. Patent 10,021,773 (issued July 10, 2018), which are incorporated herein by reference in their entireties.

[0017] In an embodiment, the inspection system 100 includes a mask stage 110 configured to hold a mask 114. In an embodiment, the inspection system 100 includes a stage-level reflective mask grating structure 112 disposed in the plane of the mask 114 held by the mask stage 110. In an embodiment, the inspection system 100 includes a detector-level reflective mask grating structure 116 disposed in the plane of the detector 108. The light source 104 is configured to illuminate the stage-level reflective mask grating structure 112 with incident light 105. A set of imaging optics 106 is then configured to direct a light beam 107 reflected from the stage-level reflective mask grating structure 112 to the detector-level reflective mask grating structure 116.

[0018] Figure 2 shows a top view of the stage-level reflective mask grating structure 112, and Figure 3 shows a top view of the detector-level reflective mask grating structure 116. It should be noted that embodiments of the present disclosure are not limited to the specific examples provided in Figures 2 and 3, and it is envisioned that numerous grating structure arrangements may be implemented within the system 100. The detector-level reflective mask grating structure 116 includes more grating elements than the stage-level reflective mask grating structure 112.

[0019] In an embodiment, the stage-level reflective mask grating structure 112 may be illuminated with the full NA of the imaging subsystem 102. The stage-level reflective mask grating structure 112 may include, but is not limited to, a checkerboard grating design intended to generate a coherent diffracted copy (first order) of the incident incoherent light NA (numerical aperture). In order for the diffraction angle to be a reasonably small fraction (typically a few percent) of the tool NA, the grating periodicity (along the patch diagonal) may be on the order of micrometers, according to the diffraction equation with an EUV wavelength of 13.5 nm. For a 3×3 mask size in this example, this translates to a feature size (d ML), with a reflection duty cycle of approximately 55%. Note that this is a very large reflection area compared to the test structures used for aerial imaging metrology. This area can be increased by using a mask with more squares (e.g., 4x4, 5x5, NxM), so that more light can be collected for wavefront metrology.

[0020] In an embodiment, the detector-level reflective mask grating structure 116 may be positioned within a free area 117 of a sensor plane (e.g., a CCD plane) of one or more sensors 108a, 108b of one or more detectors 103. The detector-level reflective mask grating structure 116 may comprise an extended checkerboard grating having more reflective squares than the stage-level reflective mask grating structure 112. In an embodiment, the size (d DL ) is the size d of the stage level reflective mask grating structure 112 ML Such an arrangement ensures overlap with the projected image of the detector-level reflective mask grating structure 116 at multiple laterally shifted positions of up to one period in each direction. In an embodiment, the grating pitch of the detector-level reflective mask grating structure 116 may be upscaled by the magnification ratio of the imaging optics 106, resulting in aerial coverage of several millimeters wide.

[0021] Note that because only one mask level structure (e.g., a checkerboard structure) is required in system 100, the available space on the mask stage can be filled with identical redundant markers, extending the metrology contamination limit lifetime.

[0022] In an embodiment, the inspection system 100 includes a mirror 118 configured to collect the light beam 119 from the detector-level reflective mask grating structure 116. The detector-level reflective mask grating structure 116 may be oriented to direct light from the detector-level reflective mask grating structure 116 toward the mirror 118. The mirror is then configured to reflect the light beam 121 collected by the mirror 118 to one or more sensors of the detector 103 to form a pupil image on one or more sensors of the detector 108. For example, the detector-level reflective mask grating structure 116 may be slightly tilted to direct the light beam 119 from the structure 116 onto the mirror 118. In an embodiment, the mirror 118 is positioned laterally near the bottom end of the EUV imaging subsystem 102 at a distance that constitutes the track length of the interferometer.

[0023] Diffraction from the detector level reflective mask grating structure 116 can produce a 1st order diffracted pupil beam that coincides with (+ / -) the 0th order to produce a static shear interferogram (interference fringes) on a sensor (e.g., a CCD) of the detector 108. The formation of a static shear interferogram is generally described in U.S. Patent 7,333,216 B2 (issued February 19, 2008), which is incorporated herein by reference in its entirety.

[0024] In an embodiment, mirror 118 may include a spherical mirror. For example, a spherical mirror with a focal length of the indicated track length (i.e., a radius of curvature equal to 2× the track length) generates a corrected pupil image via an optical 2f transformation. The transformation generates a pupil image with a diameter of several hundred micrometers onto one of the sensors (e.g., a CCD) of detector 103 aligned to project it. This occurs despite an illumination field size of several millimeters because the track length-to-field size relationship satisfies the paraxial condition for spherical lens elements. For example, given a typical pixel size of a CCD, a sufficient number of pixels are covered by the pupil image projection to generate a digital sampling of the system wavefront with a resolution on the order of the shear distance, which is a small fraction of the pupil diameter. Note that system 100 is not limited to the mirror 118 discussed herein, and it is contemplated that system 100 may incorporate any optical element capable of performing the function of mirror 118. For example, the optical elements may include any combination of refractive optics (e.g., lenses, prisms), reflective optics (e.g., mirrors), or diffractive optics (e.g., diffraction gratings) suitable for collecting (focusing) and / or directing light as described herein.

[0025] In an embodiment, the mask stage 110 is configured to provide a lateral shift motion 120 over the grating period of the stage-level reflective mask grating structure 112 to provide phase reconstruction for lateral shear interferometry to identify changes in the wavefront of light in the inspection system 100. Note that a lateral shift motion per grating period provides high-fidelity phase reconstruction that can be achieved with the mask stage 110. Phase reconstruction is generally described in D. Malacara, "Optical Shop Testing," John Wiley & Sons 1992, Ch. 1, which is incorporated herein by reference in its entirety. The stage 110 has sufficient precision, on the order of nanometers, in all 3D space to provide accurate in-plane phase-stepping motion curves over the grating period. In an embodiment, stage motion of the grating mirror in the CCD plane of the detector 103 is not required.

[0026] In an embodiment, the mask stage 110 is controlled via a controller 126. In this sense, the controller 126 is communicatively coupled to the mask stage 110 and configured to send control instructions to the mask stage 110 to control movement of the mask stage 110 to perform the lateral shifts described herein. In an embodiment, the controller 126 is also communicatively coupled to the detector 103 to analyze image data relative to the laterally shifted mask stage position. The controller 126 may include a processor and memory for performing these control functions.

[0027] An estimate of the light intensity at the CCD can be made by comparing it with normal operation. The reflective portion of the five-field mask in Figure 2 has a size of several square millimeters, which corresponds to a projected image size on the CCD of several square millimeters. However, the detector-level mask grating structure 116 and the projection pupil after reflection from the spherical 2f mirror 118 back onto the CCD sensor will compress the light into a circle with an area of less than one square millimeter. Therefore, even with reflectivity losses from the two mirrors, there will still be more photons per pixel of the CCD than in normal operation. Note that detector 103 is not limited to a CCD detector and can include any type of detector known in the art that is suitable for imaging the light collected from mirror 118. For example, detector 103 can include, but is not limited to, a CCD detector, a TDI detector, a CMOS image sensor, etc.

[0028] In an embodiment, the inspection system 100 includes an actuator 122 configured to selectively cause light 121 reflected from the mirror 118 to impinge on one of the sensors 108 a, 108 b of the detector 103. As shown in FIG. 1 , the actuator 122 may include a shutter 124 optically positioned between the detector-level reflective mask grating structure 116 and the one or more sensors 108 a, 108 b of the detector 103 to i) block the reflected pattern from the beam path during a normal inspection mode, and ii) pass the light during a wavefront detection mode. In alternative and / or additional embodiments, the actuator 122 includes a tilting mechanism configured to tilt the mirror 118 so that light reflected from the mirror 118 impinges on the one or more sensors 108 a, 108 b of the detector 103 during the wavefront detection mode, and to tilt the mirror so that light reflected from the mirror does not impinge on the one or more sensors 108 a, 108 b of the detector 103 during the normal inspection mode.

[0029] In an embodiment, the actuator 122 is controlled via a controller 126. In this sense, the controller 126 is communicatively coupled to the actuator 122 and configured to send control commands to the actuator 122 to control the state of the actuator 122. For example, in the case of a shutter, the controller 126 sends a control command to the actuator 122 to cause the actuator 122 to open or close the shutter 124, thereby switching the mode of the system 100 between i) a normal operation mode and ii) a wavefront detection mode. For example, in the case of a tilt actuator, the controller 126 transmits a control command to the actuator 122 to rotate or tilt a stage that secures the mirror 118, thereby rotating the light 121 to turn the sensors 108 a, 108 b of the detector 103 off and on, thereby switching the mode of the system 100 between i) a normal operation mode and ii) a wavefront detection mode.

[0030] It should be noted that while much of this disclosure focuses on LSI-based wavefront detection in the context of an EUV mask inspection system, this configuration is not intended to limit the scope of this disclosure. Rather, the scope of this disclosure is intended to extend to any inspection system configuration. For example, in additional embodiments, system 100 may include a DUV wafer inspection tool with LSI-based wavefront metrology capabilities.

[0031] While the above dimensions of the mask and grating structure shown in Figures 2 and 3 are allowed to vary over a range, depending on the exact optical parameters of the imaging system, the metrology system cannot simply be extended to the much longer wavelengths in the DUV region (e.g., 193 nm or 266 nm) without a change in the optical parameters. Wavelengths in the DUV region are 15-20 times larger than those in EUV, and therefore the size of the detector-level reflective mask grating structure 116 would need to increase to a size where it could no longer fit into the available space between the CCDs of the detector 108.

[0032] Nevertheless, two factors, both of which can sufficiently reduce the size of the grating structures, make this solution applicable to DUV wafer inspection systems. First, the NA of the wafer inspection microscope is typically much larger than that of the EUV mask inspection tool, resulting in a correspondingly smaller grating pitch for a given shear distance. Second, the wafer inspection optics of DUV tools are typically scalable to smaller magnifications via optical transfer elements with variable magnification ratios.

[0033] 4 illustrates a DUV wafer inspection system 400 with in-situ wavefront metrology capabilities in accordance with one or more embodiments of the present disclosure. It should be noted that all embodiments, components, and steps previously described herein with respect to system 100 should be construed as extending to system 400 unless otherwise stated.

[0034] In an embodiment, inspection system 400 is configured as a wafer inspection system operating in the DUV wavelength region (e.g., 193 nm or 266 nm). In an embodiment, inspection system 400 includes an imaging subsystem 402. Imaging subsystem 402 can be configured to perform functions related to inspection of wafer 414 as well as functions related to in-situ wavefront metrology. Imaging subsystem 402 may include, but is not limited to, a DUV light source 404, a set of DUV imaging optics 406, and one or more detectors 403 including one or more sensors 408 a, 408 b.

[0035] In an embodiment, the inspection system 400 includes a wafer stage 410 configured to secure a wafer 414. In an embodiment, the inspection system 400 includes a stage-level reflective mask grating structure 412 disposed in the plane of the wafer 414 when secured by the wafer stage 410. In an embodiment, the inspection system 100 includes a detector-level reflective mask grating structure 416 disposed in the plane of the detector 403. The light source 404 is configured to illuminate the stage-level reflective mask grating structure 412 with incident light. The set of imaging optics 406 is then configured to direct the light beam reflected from the stage-level reflective mask grating structure 412 to the detector-level reflective mask grating structure 416. In an embodiment, the detector-level reflective mask grating structure 416 may be positioned within a free region 417 of a sensor plane (e.g., a CCD plane) of one or more sensors 408a, 408b of the one or more detectors 403.

[0036] In an embodiment, the inspection system 400 includes a mirror 418 configured to collect the light beam 419 from a detector-level reflective mask grating structure 416. The detector-level reflective mask grating structure 416 can be oriented to direct the light from the detector-level reflective mask grating structure 116 toward the mirror 418. The mirror 418 is then configured to reflect the light beam 421 collected by the mirror 418 to one or more sensors 408a, 408b of the detector 403 to form a pupil image on the one or more sensors of the detector 403. The detector-level reflective mask grating structure 416 can be slightly tilted to direct the light beam 419 from the structure 416 onto the mirror 418. In an embodiment, the mirror 418 can include a spherical mirror. It should be noted that the system 400 is not limited to the mirror 418 discussed herein, and it is contemplated that the system 400 can incorporate any optical element capable of performing the function of the mirror 418. For example, the optical elements may include any combination of refractive optics (e.g., lenses, prisms), reflective optics (e.g., mirrors), or diffractive optics (e.g., diffraction gratings) suitable for collecting (focusing) and / or directing light as described herein.

[0037] In an embodiment, the wafer stage 410 is configured to provide a lateral shift motion 420 over a grating period of the stage level reflective mask grating structure 412 to provide phase reconstruction for lateral shear interferometry to identify changes in the wavefront of light in the inspection system 400. In an embodiment, the wafer stage 410 is controlled via a controller 426.

[0038] In an embodiment, the inspection system 400 includes an actuator 422 configured to selectively cause light 421 reflected from the mirror 418 to impinge on one of the sensors 408 a, 408 b of the detector 403. As shown in FIG. 4 , the actuator 422 may include a shutter 424 optically positioned between the detector-level reflective mask grating structure 416 and the one or more sensors 408 a, 408 b of the detector 403 to i) block the reflected pattern from the beam path during a normal inspection mode, and ii) pass the light during a wavefront detection mode. In alternative and / or additional embodiments, the actuator 422 includes a tilt mechanism configured to tilt the mirror 418 so that light reflected from the mirror 418 impinges on the one or more sensors 408 a, 408 b of the detector 403 during the wavefront detection mode, and to tilt the mirror so that light reflected from the mirror does not impinge on the one or more sensors 408 a, 408 b of the detector 403 during the normal inspection mode. In an embodiment, the actuator 422 is controlled via a controller 426.

[0039] FIG. 5 illustrates a flow diagram of a method for in-situ wavefront metrology in an EUV photomask inspection system in accordance with one or more embodiments of the present disclosure.

[0040] In step 502, light is generated. For example, EUV light may be generated by an EUV light source. In step 504, the light is directed onto a stage-level reflective mask grating disposed on a mask stage. In step 506, light reflected from the stage-level reflective mask grating structure is directed onto a detector-level reflective mask grating structure disposed at the plane of one or more sensors of a detector. In step 508, the light reflected from the detector-level mask grating structure is focused using an optical element. In step 510, a pupil image is formed on one or more sensors of the detector. In step 512, the stage-level reflective mask, together with the mask stage, is shifted laterally across the grating period of the stage-level reflective mask grating structure to provide phase reconstruction for lateral shear interferometry to identify changes in the wavefront of light in the photomask inspection system. In step 514, the light reflected from the optical element selectively impinges on one or more sensors of the detector.

[0041] FIG. 6 illustrates a flow diagram of a method for in-situ wavefront metrology in a wafer inspection system in accordance with one or more embodiments of the present disclosure.

[0042] In step 602, light is generated. For example, deep UV light may be generated by a deep UV light source. In step 604, the light is directed onto a stage-level reflective mask grating disposed on the wafer stage. In step 606, light reflected from the stage-level reflective mask grating structure is directed onto a detector-level reflective mask grating structure disposed at the plane of one or more sensors of a detector. In step 608, the light reflected from the detector-level mask grating structure is focused using an optical element. In step 610, a pupil image is formed on one or more sensors of the detector. In step 612, the stage-level reflective mask is shifted laterally with the wafer stage across the grating period of the stage-level reflective mask grating structure to provide phase reconstruction for lateral shear interferometry and identify changes in the wavefront of the light in the wafer inspection system. In step 614, the light reflected from the optical element selectively impinges on one or more sensors of the detector.

[0043] It is further contemplated that each of the above-described method embodiments may include any other step(s) of any other method(s) described herein. In addition, each of the above-described method embodiments may be performed by any of the systems described herein.

[0044] Referring again to FIG. 1 , in an embodiment, controller 126 includes one or more processors and memory. The one or more processors may include any processor or processing element known in the art. For purposes of this disclosure, the term “processor” or “processing element” may be broadly defined to encompass any device having one or more processing or logic elements (e.g., one or more microprocessor devices, one or more application-specific integrated circuit (ASIC) devices, one or more field-programmable gate arrays (FPGAs), or one or more digital signal processors (DSPs)). In this sense, one or more processors may include any device configured to execute algorithms and / or instructions (e.g., program instructions stored in memory). In an embodiment, the one or more processors may be embodied as a desktop computer, a mainframe computer system, a workstation, an image computer, a parallel processor, a network computer, or any other computer system configured to execute programs that operate or are configured to operate with system 100 as described throughout this disclosure. Additionally, the steps described throughout this disclosure may be performed by a single controller or, alternatively, by multiple controllers. Additionally, controller 126 may include one or more controllers housed within a common housing or multiple housings. In this manner, any controller or combination of controllers may be separately packaged as a module suitable for integration into system 100. Additionally, controller 126 may analyze data received from detector 103 and provide the data to additional components within or external to system 100.

[0045] The memory medium may include any storage medium known in the art suitable for storing program instructions executable by the associated one or more processors. For example, the storage medium may include a non-transitory storage medium. As another example, the memory medium may include, but is not limited to, read-only memory (ROM), random access memory (RAM), magnetic or optical memory devices (e.g., disks), magnetic tape, solid-state drives, etc. Furthermore, it should be noted that the memory medium may be housed within a common controller housing along with one or more processors. In one embodiment, the memory medium may be located remotely relative to the physical location of one or more processors. For example, one or more processors may access a remote memory (e.g., a server) accessible via a network (e.g., the Internet, an intranet, etc.).

[0046] Those skilled in the art will recognize that the component operations, devices, objects, and accompanying discussion described herein are used as examples for conceptual clarity, and that various configuration modifications are contemplated. Thus, as used herein, the specific examples described and accompanying discussion are intended to be representative of their more general classes. In general, the use of any specific example is intended to represent that class, and the non-inclusion of specific components, operations, devices, and objects should not be construed as limiting.

[0047] With respect to the use of virtually any plural and / or singular term herein, those skilled in the art will be able to convert from the plural to the singular and / or from the singular to the plural as appropriate to the context and / or application. The various singular / plural permutations are not expressly set forth herein for ease of understanding.

[0048] The subject matter described herein illustrates different components that are, in some cases, included within or connected to other components. It should be understood that such depicted architectures are merely exemplary, and that in fact many other architectures that achieve the same functionality may be implemented. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Thus, any two components herein that combine to achieve a particular function can be considered to be “associated” with each other such that the desired functionality is achieved, regardless of the architecture or intermediate components. Similarly, any two components so associated can also be considered to be “connected” or “coupled” to each other to achieve the desired functionality, and any two components that can be so associated can also be considered to be “couplable” with each other to achieve the desired functionality. Specific examples of what can be coupled include, but are not limited to, physically coupleable and / or physically interacting components and / or wirelessly interacting and / or wirelessly interacting components and / or logically interacting and / or logically interacting components.

[0049] It should further be understood that the present invention is defined by the appended claims. In general, those skilled in the art will understand that the terms used in this specification, and particularly in the appended claims (e.g., the body of the appended claims), are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but not limited to," etc.). Those skilled in the art will further understand that if a specific number of introduced claim recitations is intended, such intention will be expressly recited in the claim; in the absence of such recitation, no such intention exists. For example, as an aid to understanding, the following appended claims may include the use of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be interpreted as meaning that the introduction of a claim recitation with the indefinite article "a" or "an" limits any particular claim containing such an introduced claim recitation to an invention containing only one such recitation. The same applies to the use of express articles used to introduce claim recitations, even when the same claim includes the introductory phrase "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should typically be interpreted to mean "at least one" or "one or more"). Those skilled in the art will also recognize that even when a specific number of introduced claim recitations is explicitly recited, such a recitation should typically be interpreted to mean at least the recited number (e.g., a bare recitation of "two recitations" without other modifiers typically means at least two recitations, or two or more recitations).Furthermore, in instances where a conventional expression similar to "such as at least one of A, B, and C" is used, generally such a configuration is intended in the sense that one skilled in the art would understand the conventional expression (e.g., "a system having at least one of A, B, and C" includes, but is not limited to, systems having A only, B only, C only, A and B together, A and C together, B and C together, and / or A, B, and C together). In instances where a conventional expression similar to "such as at least one of A, B, or C" is used, generally such a configuration is intended in the sense that one skilled in the art would understand the conventional expression (e.g., "a system having at least one of A, B, or C" includes, but is not limited to, systems having A only, B only, C only, A and B together, A and C together, B and C together, and / or A, B, and C together). Those skilled in the art will further appreciate that virtually any disjunctive word and / or phrase presenting two or more alternative terms, wherever it appears in the description, claims, or drawings, should be understood to contemplate the possibility of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

[0050] It is believed that 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 elements without departing from the disclosed subject matter or sacrificing all of its material advantages. The described forms are merely illustrative, and it is the intent of the following claims to embrace and include such modifications. It is further understood that the invention is defined by the appended claims.

Claims

1. 1. An inspection system for performing in situ wavefront metrology of light of the inspection system, comprising: an imaging subsystem comprising a light source, a set of imaging optics, and a detector including one or more sensors for imaging; The stage and a stage-level reflective mask grating structure disposed in the plane of a mask fixed by the stage, the light source configured to illuminate the stage-level reflective mask grating structure with incident light; a detector-level reflective mask grating structure disposed in the plane of the detector, the imaging optics set configured to direct light reflected from the stage-level reflective mask grating structure to the detector-level reflective mask grating structure; an optical element configured to collect light from the detector-level reflective mask grating structure, the detector-level reflective mask grating structure oriented to direct light from the detector-level reflective mask grating structure towards the optical element, the optical element configured to form a pupil image on one or more sensors of the detector; an actuator configured to selectively cause light reflected from the optical element to impinge on the one or more sensors of the detector; Including, the stage is configured to provide a lateral shifting motion over a grating period of the stage level reflective mask grating structure to provide phase reconstruction for lateral shear interferometry to identify changes in a wavefront of light in an imaging system. system.

2. The system of claim 1 , wherein the light source is configured to produce extreme ultraviolet (EUV) light.

3. 3. The system of claim 2, comprising an EUV photomask inspection system.

4. The system of claim 3 , wherein the stage comprises a mask stage.

5. The system of claim 1 , wherein the light source is configured to produce deep ultraviolet (DUV) light.

6. 6. The system of claim 5, comprising a DUV wafer inspection system.

7. The system of claim 6 , wherein the stage comprises a wafer stage.

8. 10. The system of claim 1, wherein the light source is configured to illuminate the stage level reflective mask grating structure with incident light having a full numerical aperture (NA) of an imaging subsystem.

9. 10. The system of claim 1, wherein the stage level reflective mask grating structure is configured to generate coherent first order diffracted light of incident incoherent light.

10. 10. The system of claim 1, wherein the detector-level reflective mask grating structure includes more grating elements than the stage-level reflective mask grating structure.

11. 10. The system of claim 1, wherein the grating pitch of the detector-level reflective mask grating structure is magnified by a magnification factor of the imaging optics.

12. 2. The system of claim 1, wherein the detector level reflective mask grating structure is positioned in a plane of the detector within an area where the detector level reflective mask grating structure can be freely positioned.

13. 10. The system of claim 1, wherein the one or more sensors include one or more charge-coupled devices (CCDs).

14. 14. The system of claim 13, wherein the projection of the pupil image onto the one or more charge-coupled devices extends across two or more pixels of the one or more charge-coupled devices.

15. The system of claim 1 , wherein the optical element comprises a mirror.

16. The system of claim 15 , wherein the mirror comprises a spherical mirror.

17. 17. The system of claim 16, wherein the spherical mirror has a focal length corresponding to a track length of an imaging subsystem.

18. 2. The system of claim 1, wherein the actuator comprises a shutter positioned between the detector level reflective mask grating structure and one or more sensors of the detector, for blocking a reflective pattern from the beam path during a normal inspection mode and for allowing light to pass during a wavefront detection mode.

19. 2. The system of claim 1, wherein the actuator comprises a tilt actuator configured to tilt the mirror so that light reflected from the mirror impinges on the one or more sensors of the detector during a wavefront detection mode, and to tilt the mirror so that light reflected from the mirror does not impinge on the one or more sensors of the detector during a normal inspection mode.

20. 1. A method for in-situ wavefront metrology of extreme ultraviolet (EUV) light in a photomask inspection system, comprising: generating EUV light; directing the EUV light onto a stage-level reflective mask grating structure disposed on a mask stage; directing EUV light reflected from the stage-level reflective mask grating structure through an optical imaging system onto a detector-level reflective mask grating structure positioned in the plane of one or more sensors of a detector; using an optical element to collect EUV light reflected from the detector level reflective mask grating structure and form a pupil image on one or more sensors of the detector; laterally shifting the stage level reflective mask grating structure along with a mask stage over a grating period of the stage level reflective mask grating structure to provide phase reconstruction for lateral shear interferometry to identify changes in a wavefront of EUV light in a photomask inspection system; selectively impinging EUV light reflected from the optical element onto one or more sensors of the detector; A method comprising:

21. 1. A method for in-situ wavefront metrology of light for deep ultraviolet (DUV) wafer inspection, comprising: generating DUV light; directing DUV light onto a stage-level reflective mask grating structure disposed on a mask stage; directing DUV light reflected from the stage-level reflective mask grating structure through an optical imaging system onto a detector-level reflective mask grating structure positioned in the plane of one or more sensors of a detector; using optical elements to collect DUV light reflected from the detector-level reflective mask grating structure and form a pupil image on one or more sensors of the detector; laterally shifting the stage level reflective mask grating structure along with a mask stage over a grating period of the stage level reflective mask grating structure to provide phase reconstruction for lateral shear interferometry to identify changes in a wavefront of DUV light in a photomask inspection system; selectively impinging DUV light reflected from the optical element onto one or more sensors of the detector; A method comprising:

Images