System and method for lateral shearing interferometry in an inspection tool

Abstract

A method for in-situ wavefront detection within an inspection system is disclosed. The method includes generating light with a light source and directing the light to a stage-level reflective mask grating structure disposed on a mask stage. The method includes directing light reflected from the stage-level reflective structure to a detector-level mask structure disposed in the plane of a detector, and then collecting light reflected from the detector-level mask structure with an optical element. The method includes forming a pupil image on the detector, and laterally shifting the stage-level reflective mask across a grating period of the stage-level reflective mask grating structure with the mask stage to provide phase reconstruction for lateral shearing interferometry. The method includes selectively illuminating the light reflected from the optical element onto the one or more sensors of the detector.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 177,395, filed on April 21, 2021, under 35 U.S.C. §119(e), the entirety of which is incorporated herein by reference. Technical Field

[0003] The present disclosure generally relates to a system and method for wavefront aberration detection within an imaging system, and more particularly to a lateral shearing interferometry based wavefront metrology system and method for use in an extreme ultraviolet (EUV) mask inspection tool or a deep ultraviolet wafer inspection tool. Background Art

[0004] In advanced defect inspection tools for photolithography masks or wafers, such as those used in semiconductor manufacturing processes, the need for internal optical aberration control of the imaging optical system arises due to thermally induced drift or other mechanical alignment changes during tool operation. An integral part of this control scheme is optical aberration metrology, which is often referred to as 'in-situ alignment metrology' due to its permanent implementation in the tool's hardware. In-situ metrology is usually divided into two different methods, namely in-situ coarse alignment and in-situ fine alignment. Both in-situ coarse alignment and in-situ fine alignment are usually based on aerial imaging of a test structure on a CCD (charge coupled device) image sensor of a given inspection tool. This separation between the two methods is necessary because metrology with sufficiently high accuracy for fine alignment does not have sufficient capture range to converge to the optimal alignment state from the initial alignment stage after mechanical assembly of the imaging optics.

[0005] Coarse alignment metrology methods require different structures for different aberrations and therefore only include selected aberration terms in the alignment strategy. While this is usually supported by optical simulations, there is a risk that higher order aberrations are not captured. Although coarse alignment metrology is designed to have sufficient dynamic range to capture the misalignment level after mechanical assembly of the imaging optics, there is still a risk that individual assemblies may exceed the limits and fail to align. Likewise, there is a risk that the final coarse alignment state is not satisfactory within the capture range of the fine alignment method and may converge to a false minimum. The fine alignment metrology method suffers from very low intensity levels and therefore requires a lot of measurement time. In general, similar to all image-based aberration metrology, the above methods cannot strictly distinguish between amplitude and phase and are therefore subject to errors in the illumination system or defects in photometric parameters (such as coating uniformity or spot).

[0006] Therefore, it is desirable to provide a system and method to overcome the deficiencies of previous solutions. Summary of the invention

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

[0008] It should be understood that both the foregoing summary and the following detailed description are exemplary and explanatory only and do not necessarily limit the present disclosure. The accompanying drawings, which are incorporated into and constitute part of this specification, illustrate the subject matter of the present disclosure. The description and drawings together serve to explain the principles of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Many advantages of the present disclosure may be better appreciated by those skilled in the art by referring to the accompanying drawings.

[0010] Figure 1 A simplified block diagram illustrating an EUV inspection system with in-situ lateral shearing interferometry capability in accordance with one or more embodiments of the present disclosure.

[0011] Figure 2 A top level view illustrating a mask level reflective grating structure according to one or more embodiments of the present disclosure.

[0012] Figure 3 A top level view illustrating a detector stage reflective grating structure in accordance with one or more embodiments of the present disclosure.

[0013] Figure 4A simplified block diagram illustrating a DUV inspection system with in-situ lateral shearing interferometry capability in accordance with one or more embodiments of the present disclosure.

[0014] Figure 5 Flowchart illustrating a method for in-situ lateral shearing interferometry capability within a photomask inspection system in accordance with one or more embodiments of the present disclosure.

[0015] Figure 6 Flowchart illustrating a method of in-situ lateral shearing interferometry capability within a wafer inspection system in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION

[0016] Reference will now be made in detail to the disclosed subject matter illustrated in the accompanying drawings. The present disclosure has been particularly shown and described with respect to specific embodiments and specific features thereof. The embodiments set forth herein are to be regarded as illustrative and not restrictive. It should be readily apparent to one of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the present disclosure.

[0017] For the initial setup and periodic maintenance of a photomask inspection system, it is necessary to implement an image quality metrology tool for imaging optics, i.e., permanently integrated into the photomask inspection system. A fundamental representation of the image quality of the imaging optics can be provided by the wavefront aberration. The wavefront aberration is typically expressed as an expansion of Zernike polynomials, which describe the deviation of the wavefront from an ideal spherical wave at a given field point and are related to fundamental image errors such as defocus, coma, astigmatism, and others. Embodiments of the present disclosure relate to an in-situ wavefront metrology system for an extreme ultraviolet (EUV) mask inspection tool. The in-situ wavefront metrology system of the present disclosure is based on lateral shearing interferometry (LSI) operating at an actinic EUV wavelength of 13.5 nm.

[0018] Actinic LSI wavefront metrology is advantageous because of its combination of a large initial capture range of wavefront errors due to rough mechanical alignment of imaging optics and potential high-end accuracy, thereby providing the ability to replace multiple complementary aerial image-based metrology with a single metrology method. In addition, the LSI wavefront metrology of the present disclosure is faster than aerial imaging because only one structure needs to be scanned and the integration time is shorter due to the non-critical light budget. Since LSI distinguishes between intensity and phase, embodiments of the present disclosure can also provide higher absolute accuracy than aerial imaging in the case of illumination imperfections. Compared to aerial imaging, all Zernike coefficients up to ~Z36 can be measured with a high capture range, thereby mitigating the risk of 'blind spots' during the optical setup process. If only specific Zernike coefficients are required, then the phase shift and wavefront reconstruction algorithms can be optimized and accelerated to potentially track drifts, such as but not limited to heating-induced performance drifts.

[0019] Additional embodiments of the present disclosure relate to wavefront metrology for wafer inspection systems with high numerical aperture in the deep ultraviolet (DUV) in the wavelength range of 190 to 270 nm.

[0020] Figure 1 An inspection system 100 equipped with in-situ wavefront metrology capabilities according to one or more embodiments of the present disclosure is described. In an embodiment, the inspection system 100 is configured as a photomask inspection system operating in the EUV wavelength regime (e.g., 13.5 nm). In an embodiment, the inspection system 100 includes an imaging subsystem 102. The imaging subsystem 102 can be configured to perform functions associated with inspection of a mask 114 and functions associated with in-situ wavefront metrology. In an embodiment, and as further discussed herein, a mode of the inspection system 100 can be switched between i) a normal inspection mode and ii) a wavefront detection mode.

[0021] 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 108a, 108b. Note 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 detectors 103 known in the art of EUV-based inspection. U.S. Pat. No. 8,916,831, issued on Dec. 23, 2014, and U.S. Pat. No. 11,112,691, issued on Sep. 7, 2021, generally discuss EUV-based inspection, the entire contents of which are incorporated herein by reference. Light for producing EUV sources is generally discussed in U.S. Pat. No. 7928416B2, issued on April 19, 2011, U.S. Pat. No. 9,709,811, issued on July 18, 2017, and U.S. Pat. No. 10,021,773, issued on July 10, 2018, the entire contents of which are incorporated herein by reference.

[0022] 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 when the mask 114 is 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. In turn, the set of imaging optics 106 is configured to direct a light beam 107 reflected from the stage-level reflective structure 112 to the detector-level mask structure 116.

[0023] Figure 2 A top view of the stage-level reflective mask grating structure 112 is illustrated, and Figure 3 A top view of the detector level reflective mask grating structure 116 is illustrated. Note that embodiments of the present disclosure are not limited to Figure 2 and 3 Specific examples are provided in , and it is contemplated that many grating structure arrangements may be implemented within system 100.

[0024] In an embodiment, the stage-level mask grating structure 112 can be illuminated with the full NA of the imaging subsystem 102. The stage-level reflective mask grating structure 112 can include, but is not limited to, a checkerboard grating design that is intended to produce a coherent diffraction replica (1st order) of the incident incoherent light NA. In order for the diffraction angle to be a fairly small fraction of the tool NA (typically a few percent), the periodicity of the grating (along the patch diagonal) can be about one micron, according to the diffraction equation for an EUV wavelength of 13.5 nm. For the 3x3 mask size in this example, this would result in a structure size (d ML ), with a reflective duty cycle of about 55%. Note that this is a very large reflective area compared to the test structure used for aerial imaging metrology. This area can be increased by using a mask with more square shapes (e.g. 4x4, 5x5, NxM), and thus more light can be collected for wavefront metrology.

[0025] 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 the one or more detectors 103. The detector-level reflective mask grating structure 116 may include an extended checkerboard grating having more reflective squares than the mask-level grating structure 112. In an embodiment, the size (d DL ) is greater than the size d of the mask-level reflective mask structure 116 ML This arrangement ensures overlap with the projected image of the mask-level reflective mask grating structure 116 in multiple lateral offset positions of up to one period in each direction. In embodiments, the grating pitch of the detector-level reflective mask grating structure 116 can be scaled up by the magnification of the imaging optics 108, resulting in aerial coverage over several millimeters.

[0026] Note that since only one mask level structure (eg, a checkerboard structure) is required in system 100, the available space on the mask stage can be filled with identical redundant marks and the contamination-limited lifetime of the metrology is extended.

[0027] In an embodiment, the inspection system 100 includes a mirror 118 configured to collect a beam 119 from a detector-level mask structure 116. The detector-level mask structure 116 may be oriented to direct light from the detector-level mask structure 116 to the mirror 118. In turn, the mirror is configured to reflect a beam 121 collected by the mirror 118 to one or more sensors of the detector 103 to form a pupil image on the 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 beam 119 from the structure 116 onto the mirror 118. In an embodiment, the mirror 118 is positioned laterally near the lower end of the EUV imaging subsystem 102 at a distance that constitutes a track length 124 of the interferometer.

[0028] Diffraction from the detector-stage reflective mask grating structure 116 may produce + / - 1st order diffraction pupil beams that will coincide with the 0th order to produce a static shearing interferogram on a sensor (e.g., CCD) of the detector 108. Information on static shearing interferograms is generally described in U.S. Pat. No. 7,333,216 B2, issued Feb. 19, 2008, the entire contents of which are incorporated herein by reference.

[0029] In an embodiment, the mirror 118 may include a spherical mirror. For example, a spherical mirror having a focal length of the indicated track length (i.e., a radius curvature of 2x the track length) will produce a corrected pupil image via an optical 2f transform. The transform will produce a pupil image having a diameter of several hundred microns on one of the sensors (e.g., a CCD) of the detector 103, which is aligned to be projected on the sensor. This occurs regardless of the illumination field size of a few millimeters because the relationship of track length versus field size satisfies the paraxial condition of the spherical lens element. For example, given a typical pixel size of a CCD, a sufficient number of pixels will be covered by the pupil image projection to produce a digital sampling of the system wavefront with a resolution of about the shear distance, which is a small fraction of the pupil diameter. Note that the system 100 is not limited to the mirror 118 discussed herein, and it is contemplated that the system 100 may incorporate any optical element capable of performing the function of the 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 and / or directing light as described herein.

[0030] In an embodiment, the mask stage 110 is configured to provide lateral offset motion 120 across the grating period of the stage-level reflective mask grating structure 112 to provide phase reconstruction for lateral shearing interferometry to identify changes in the wavefront of light of the inspection system 100. Note that lateral offset motion around one period of the grating period provides high-fidelity phase reconstruction, which can be achieved with the mask stage 110. Phase reconstruction is generally described in D. Malacara, "Optical Shop Testing", John Wiley & Sons 1992, Chapter 1, which is incorporated herein by reference in its entirety. The stage 110 has sufficient precision in nanometers in full 3D space to provide accurate in-plane phase step motion profiles within the grating period. In an embodiment, stage motion of the grating mirror in the CCD plane of the detector 103 is not required.

[0031] 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 is configured to transmit control instructions to the mask stage 110 so as to control the movement of the mask stage 110 to perform the lateral shift described herein. In an embodiment, the controller 126 is also communicatively coupled to the detector 103 to analyze image data associated with the laterally shifted mask stage position. The controller 126 may include a processor and memory to perform these control functions.

[0032] By comparison with the normal operating mode, an estimation of the light intensity at the CCD can be performed. Figure 2 The reflected portion of the 5-field mask in will have a size of several square millimeters, which corresponds to a projected image size on the CCD of several square millimeters. However, the projected pupil after reflection from the detector-level mask grating structure 116 and the spherical 2f mirror 118 back to the CCD sensor will compress the light into a circle with an area of ​​less than one square millimeter. Therefore, even with the reflectivity loss from both mirrors, the CCD will still have more photons per pixel than in normal operation. Note that the detector 103 is not limited to a CCD detector, and may include any type of detector known in the art suitable for imaging the collected light from the mirror 118. For example, the detector 103 may include, but is not limited to, a CCD detector, a TDI detector, a CMOS image sensor, or the like.

[0033] In an embodiment, the inspection system 100 includes an actuator 122 configured to selectively direct light 121 reflected from the mirror 118 onto one sensor 108a, 108b of the detector 103. Figure 1As shown, the actuator 122 may include a shutter 124 optically positioned between the detector stage reflective mask grating structure 116 and the one or more sensors 108a, 108b of the detector 103 to i) block the reflected pattern from the beam path during the normal inspection mode; and ii) pass light through during the wavefront detection mode. In alternative and / or additional embodiments, the actuator 122 includes a tilt mechanism configured to tilt the mirror 118 during the wavefront detection mode so that light reflected from the mirror 118 impinges on the one or more sensors 108a, 108b of the detector 103, and tilt the mirror during the normal inspection mode so that light reflected from the mirror cannot impinge on the one or more sensors 108a, 108b of the detector 103.

[0034] In an embodiment, the actuator 122 is controlled via the controller 126. In this sense, the controller 126 is communicatively coupled to the actuator 122 and is configured to transmit control instructions to the actuator 122 so as to control the state of the actuator 122. For example, in the case of a shutter, the controller 126 may transmit control instructions to the actuator, which causes the actuator 122 to open / close the shutter 124, thereby switching the mode of the system 100 between i) a normal operating mode and ii) a wavefront detection mode. For example, in the case of a tilt actuator, the controller 126 may transmit control instructions to the actuator, which causes the actuator 122 to rotate or tilt the stage of the fixed mirror 118, thereby rotating the light 121 off / on the sensors 108a, 108b of the detector 103, and thus switching the mode of the system 100 between i) a normal operating mode and ii) a wavefront detection mode.

[0035] Note that while much of this disclosure has focused on LSI-based wavefront detection in the context of an EUV mask inspection system, this configuration does not 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 equipped with LSI-based wavefront metrology capabilities.

[0036] Although it is allowed Figure 2 and 3 The above dimensions of the mask and grating structures depicted in vary within certain ranges depending on the precise optical parameters of the imaging system, but the metrology system cannot be simply extended to the much longer wavelengths of the DUV region (e.g., 193 nm or 266 nm) without changing the optical parameters. The wavelengths of the DUV region are 15 to 20 times larger than those of the EUV, and thus the size of the detector-level mask structure 116 would need to be increased to a size that may no longer fit in the available space between the CCDs of the detector 108.

[0037] However, the solution can be applied to DUV wafer inspection systems due to two factors that together can substantially reduce the size of the grating structure. First, the NA of a wafer inspection microscope is typically much larger than the NA of an EUV mask inspection tool, resulting in a correspondingly smaller grating pitch for a given shear distance. Second, the wafer inspection optics of a DUV tool can typically be scaled to a smaller magnification via an optical transfer element with variable magnification.

[0038] Figure 4 A DUV wafer inspection system 400 equipped with in-situ wavefront metrology capability according to one or more embodiments of the present disclosure is illustrated. Note that all embodiments, components, and steps previously described herein with respect to system 100 should be interpreted as extending to system 400 unless otherwise noted.

[0039] In an embodiment, the 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, the inspection system 400 includes an imaging subsystem 402. The imaging subsystem 402 may be configured to perform functions associated with inspection of the wafer 414 and functions associated with in-situ wavefront metrology. The 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 408a, 408b.

[0040] In an embodiment, the inspection system 400 includes a wafer stage 410 configured to hold 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 the wafer 414 is held by the wafer stage 410. In an embodiment, the inspection system 400 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. In turn, the set of imaging optics 406 is configured to direct a light beam reflected from the stage-level reflective structure 412 to the detector-level mask structure 416. In an embodiment, the detector-level reflective mask grating structure 416 may be positioned within a free area 417 of a sensor plane (e.g., a CCD plane) of one or more sensors 408a, 408b of the one or more detectors 403.

[0041] In an embodiment, the inspection system 400 includes a mirror 418 configured to collect a beam 419 from a detector-level mask structure 416. The detector-level mask structure 416 may be oriented to direct light from the detector-level mask structure 416 to the mirror 418. In turn, the mirror 418 is configured to reflect a 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 may be slightly tilted to direct the beam 419 from the structure 416 onto the mirror 418. In an embodiment, the mirror 418 may include a spherical mirror. Note that the system 400 is not limited to the mirror 418 discussed herein, and it is contemplated that the system 400 may 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 and / or directing light as described herein.

[0042] In an embodiment, wafer stage 410 is configured to provide lateral offset motion 420 across the grating period of stage-level reflective mask grating structure 412 to provide phase reconstruction for lateral shearing interferometry to identify changes in the wavefront of light for inspection system 400. In an embodiment, wafer stage 410 is controlled via controller 426.

[0043] In an embodiment, the inspection system 400 includes an actuator 422 configured to selectively direct light 421 reflected from the mirror 418 onto one sensor 408a, 408b of the detector 403. Figure 4 As shown, the actuator 422 may include a shutter 424 optically positioned between the detector stage reflective mask grating structure 416 and the one or more sensors 408a, 408b of the detector 403 to i) block the reflected pattern from the beam path during the normal inspection mode; and ii) pass light through during the wavefront detection mode. In alternative and / or additional embodiments, the actuator 422 includes a tilt mechanism configured to tilt the mirror 418 during the wavefront detection mode so that light reflected from the mirror 418 impinges on the one or more sensors 408a, 408b of the detector 403, and tilt the mirror during the normal inspection mode so that light reflected from the mirror cannot impinge on the one or more sensors 408a, 408b of the detector 403. In an embodiment, the actuator 422 is controlled via a controller 426.

[0044] Figure 5 Flowchart 500 illustrates a method of in-situ wavefront metrology in a photomask inspection system in accordance with one or more embodiments of the present disclosure.

[0045] 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 to a stage-level reflective mask grating disposed on a mask stage. In step 506, the light reflected from the stage-level reflective structure is directed to a detector-level mask structure disposed in the plane of one or more sensors of a detector. In step 508, the light reflected from the detector-level mask structure is collected with an optical element. In step 510, a pupil image is formed on one or more sensors of the detector. In step 512, a stage-level reflective mask is laterally shifted across a grating period of the stage-level reflective mask grating structure with a mask stage to provide phase reconstruction for lateral shearing interferometry to identify changes in the wavefront of light for a photomask inspection system. In step 514, the light reflected from the optical element is selectively illuminated onto one or more sensors of the detector.

[0046] Figure 6 Flowchart 600 illustrates a method of in-situ wavefront metrology in a wafer inspection system according to one or more embodiments of the present disclosure.

[0047] In step 602, light is generated. For example, the DUV light may be generated by a DUV light source. In step 604, the light is directed to a stage-level reflective mask grating disposed on a wafer stage. In step 606, the light reflected from the stage-level reflective structure is directed to a detector-level mask structure disposed in the plane of one or more sensors of a detector. In step 608, the light reflected from the detector-level mask structure is collected with 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 laterally shifted across a grating period of the stage-level reflective mask grating structure with a wafer stage to provide phase reconstruction for lateral shearing interferometry to identify changes in the wavefront of light for a wafer inspection system. In step 614, the light reflected from the optical element is selectively illuminated onto one or more sensors of the detector.

[0048] It is further contemplated that each of the embodiments of the method described above may include any other steps of any other method(s) described herein. In addition, each of the embodiments of the method described above may be performed by any of the systems described herein.

[0049] Reference again Figure 1In an embodiment, the 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 the purpose of this disclosure, the term "processor" or "processing element" may be broadly defined to include 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, the one or more processors may include any device configured to execute algorithms and / or instructions (e.g., program instructions stored in a memory). In an embodiment, the one or more processors may be embodied as a desktop computer, a main computer system, a workstation, an image computer, a parallel processor, a networked computer, or any other computer system configured to execute a program configured to operate the system 100 or operate in conjunction with the system 100, as described throughout this disclosure. In addition, the steps described throughout this disclosure may be performed by a single controller or alternatively by multiple controllers. In addition, the controller 126 may include one or more controllers housed in a common housing or in multiple housings. In this manner, any controller or combination of controllers may be individually packaged as a module suitable for integration into system 100. Furthermore, controller 126 may analyze data received from detector 103 and feed the data to additional components within system 100 or external to system 100.

[0050] The memory medium may include any storage medium known in the art suitable for storing program instructions that can be executed by the associated one or more processors. For example, the memory medium may include a non-transitory memory medium. By way of another example, the memory medium may include, but is not limited to, a read-only memory (ROM), a random access memory (RAM), a magnetic or optical memory device (e.g., a disk), a tape, a solid-state drive, and the like. It should be further noted that the memory medium may be housed in a common controller housing with the one or more processors. In one embodiment, the memory medium may be remotely located relative to the physical location of the one or more processors. For example, the one or more processors may access a remote memory (e.g., a server) that may be accessed via a network (e.g., the Internet, an intranet, and the like).

[0051] Those skilled in the art will recognize that the components, operations, devices, objects, and accompanying discussions described herein are used as examples for conceptual clarity, and various configuration modifications are contemplated. Therefore, as used herein, the specific examples and accompanying discussions set forth are intended to represent their more general classes. In general, the use of any specific example is intended to represent its class, and the failure to include specific components, operations, devices, and objects should not be considered limiting.

[0052] With respect to the use of substantially any plural and / or singular terms herein, those skilled in the art may translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. For clarity, the various singular / plural permutations are not explicitly set forth herein.

[0053] The subject matter described herein sometimes illustrates different components contained in or connected to other components. It should be understood that such depicted architectures are only exemplary, and in fact many other architectures that realize the same functionality can be implemented. In a conceptual sense, any arrangement of components to realize the same functionality is effectively "associated" so that the desired functionality is realized. Therefore, any two components combined to realize a specific functionality in this article can be considered to be "associated" with each other so that the desired functionality is realized, 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 realize the desired functionality, and any two components that can be so associated can also be considered to be "coupleable" to each other to realize the desired functionality. Specific examples that can be coupled include but are not limited to components that can be physically matched and / or physically interacted and / or components that can wirelessly interact and / or wirelessly interact and / or components that can logically interact and / or components that can logically interact.

[0054] In addition, it should be understood that the present disclosure is defined by the appended claims. Those skilled in the art will understand that, in general, the terms used herein and especially in the appended claims (e.g., the body of the appended claims) are generally intended to be "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 "including" should be interpreted as "including but not limited to", and the like). Those skilled in the art will further understand that if a specific number of introductory claim statements is intended, such intent will be explicitly stated in the claim, and in the absence of such a statement, such intent is absent. For example, to aid understanding, the appended claims may contain the use of the introductory phrases "at least one" and "one or more" to introduce claim statements. However, the use of these phrases should not be construed as implying that the introduction of a claim recitation by the indefinite article "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim contains the introductory phrases "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should generally be construed to mean "at least one" or "one or more"); the same is true of the use of definite articles to introduce claim recitations. In addition, even if a specific number of introductory claim recitations is explicitly recited, those skilled in the art will recognize that such recitation should generally be construed to mean at least the recited number (e.g., the mere recitation "two recitations" without other modifiers generally means at least two recitations or two or more recitations). Furthermore, in those instances where a convention similar to “at least one of A, B, and C, and the like” is used, generally speaking, such construction is contemplated in the sense that a person skilled in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include, but is not limited to, systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or both A, B, and C, and the like). In those instances where a convention similar to “at least one of A, B, or C, and the like” is used, generally speaking, such construction is contemplated in the sense that a person skilled in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include, but is not limited to, systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or both A, B, and C, and the like). Those skilled in the art will further understand that any transitional words and / or phrases (whether in the specification, claims or drawings) that present two or more alternative items should be understood to contemplate the possibility of including one, either or both of the items.For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

[0055] It is believed that the present disclosure and its many attendant advantages will be understood from the foregoing description, and it will be appreciated that various changes may be made to the form, construction, and arrangement of components without departing from the disclosed subject matter or sacrificing all of its significant advantages. The described form is illustrative only, and the appended claims are intended to cover and encompass such changes. Furthermore, it should be understood that the present disclosure is defined by the appended claims.

Claims

1. An inspection system having in-situ wavefront metrology of light of said inspection system, said system comprising: an imaging subsystem including a light source, a set of imaging optics, and a detector including one or more imaging sensors; Carrier; a stage-level reflective mask grating structure disposed in the plane of the mask held by the stage, wherein the light source is configured to illuminate the stage-level reflective mask grating structure with incident light; a detector-level reflective mask grating structure disposed in a plane of the detector, wherein 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; an optical element configured to collect light from the detector-level reflective mask grating structure, wherein the detector-level reflective mask grating structure is oriented to direct light from the detector-level reflective mask grating structure to the optical element, wherein the optical element is configured to form a pupil image on the one or more sensors of the detector, wherein the stage is configured to provide lateral offset motion across a grating period of the stage-level reflective mask grating structure to provide phase reconstruction for lateral shearing interferometry to identify changes in a wavefront of light for the imaging subsystem; and An actuator is configured to selectively illuminate light reflected from the optical element onto the one or more sensors of the detector.

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

3. The system of claim 2, wherein the inspection system comprises an EUV photomask inspection system. 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 generate deep ultraviolet (DUV) light.

6. The system of claim 5, wherein the inspection system comprises a DUV wafer inspection system.

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

8. 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 NA of the imaging subsystem.

9. The system of claim 1, wherein the stage-level reflective mask grating structure is configured to produce a set of coherent diffraction replicas having an incoherent light NA.

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

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

12. The system of claim 1, wherein the detector-level reflective mask grating structure is disposed within a free area of ​​the detector plane.

13. The system of claim 1, wherein the one or more sensors comprise one or more charge coupled devices.

14. The system of claim 13, wherein a 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. The system of claim 16, wherein the spherical mirror has a focal length corresponding to a track length of the imaging subsystem.

18. A system according to claim 1, wherein the actuator includes a shutter positioned between the detector stage reflective mask grating structure and the one or more sensors of the detector to block the reflection pattern from the beam path during normal inspection mode and to allow light to pass during wavefront detection mode.

19. A system according to claim 1, wherein the actuator includes a tilt actuator, which is configured to tilt the mirror during a wavefront detection mode so that light reflected from the mirror is irradiated onto the one or more sensors of the detector and to tilt the mirror during a normal inspection mode so that light reflected from the mirror cannot be irradiated onto the one or more sensors of the detector.

20. A method for in-situ wavefront metrology of extreme ultraviolet (EUV) light for a photomask inspection system, comprising: Producing EUV light; directing the EUV light to a stage-level reflective mask grating structure disposed on a mask stage; directing EUV light reflected from the stage-level reflective mask grating structure to a detector-level reflective mask grating structure in a plane of one or more sensors disposed in a detector by an optical imaging system; collecting EUV light reflected from the detector-stage reflective mask grating structure with an optical element, forming a pupil image on the one or more sensors of the detector; using the mask stage to laterally shift the stage-level reflective mask grating structure across a grating period of the stage-level reflective mask grating structure to provide phase reconstruction for lateral shearing interferometry to identify changes in the wavefront of the EUV light for the photomask inspection system; and EUV light reflected from the optical element is selectively irradiated onto the one or more sensors of the detector.

21. A method for in-situ wavefront metrology of light for deep ultraviolet (DUV) wafer inspection, comprising: Producing DUV light; directing the DUV light to a stage-level reflective mask grating structure disposed on a wafer stage; directing, by an optical imaging system, DUV light reflected from the stage-level reflective mask grating structure to a detector-level reflective mask grating structure in a plane of one or more sensors disposed in a detector; collecting the DUV light reflected from the detector stage reflective mask grating structure with an optical element, forming a pupil image on the one or more sensors of the detector; using the wafer stage to laterally shift the stage-level reflective mask grating structure across a grating period of the stage-level reflective mask grating structure to provide phase reconstruction for lateral shearing interferometry to identify changes in the wavefront of the DUV light of a wafer inspection system; and The DUV light reflected from the optical element is selectively irradiated onto the one or more sensors of the detector.

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