Autofocus system for tracking sample surface with configurable focus offset

Abstract

An autofocus system is disclosed. The system includes an illumination source. The system includes an aperture. The system includes a projection mask. The system includes a detector assembly. The system includes a relay system configured to optically couple illumination transmitted through the projection mask to an imaging system. The relay system is also configured to project one or more patterns from the projection mask onto a sample and to transmit an image of the projection mask from the sample to the detector assembly. The system includes a controller including one or more processors configured to execute a set of program instructions. The program instructions are configured to cause the one or more processors to receive one or more images of the projection mask from the detector assembly and determine a quality of the one or more images of the projection mask.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 62 / 829,831 entitled “AUTOMATED FOCUSING SYSTEM TRACKING SPECIMEN SURFACE WITH CONFIGURABLE FOCUS OFFSET” naming Xiumei Liu, Kai Cao, Richard Wallingford, Matthew Giusti, and Brooke Bruguier as inventors, filed April 5, 2019, the entirety of which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates generally to the field of optical imaging systems, and more particularly to an automated focusing system for tracking a specimen surface with a configurable focus offset. BACKGROUND

[0004] The demand for electronic logic and memory devices with increasingly smaller footprints and features presents a wide range of manufacturing challenges beyond the desired scale. In the context of semiconductor manufacturing, accurately identifying the type and size of defects is an important step in improving throughput and yield. Furthermore, to achieve optimal imaging quality and defect detection sensitivity, the focal plane of the imaging system must be maintained.

[0005] Accordingly, it would be desirable to provide a system that overcomes one or more of the disadvantages of the previous approaches identified above. SUMMARY

[0006] An autofocus system is disclosed. In one embodiment, the system includes an illumination source. In another embodiment, the system includes an aperture. In another embodiment, the system includes a projection mask. In another embodiment, the system includes a detector assembly. In another embodiment, the system includes a relay system configured to optically couple illumination transmitted through the projection mask to an imaging system, the relay system configured to project one or more patterns from the projection mask onto a sample disposed on a stage assembly of the imaging system and to transmit an image of the projection mask from the sample to the detector assembly. In another embodiment, the system includes a controller including one or more processors configured to execute a set of program instructions stored in memory, the program instructions configured to cause the one or more processors to: receive one or more images of the projection mask from the detector assembly; and determine a quality of the one or more images of the projection mask.

[0007] A system is disclosed. In one embodiment, the system includes an imaging system. In another embodiment, the system includes an autofocus system. In another embodiment, the autofocus system includes an illumination source. In another embodiment, the autofocus system includes an aperture. In another embodiment, the autofocus system includes a projection mask. In another embodiment, the autofocus system includes a detector assembly. In another embodiment, the autofocus system includes a relay system configured to optically couple illumination transmitted through the projection mask to the imaging system, the relay system configured to project one or more patterns from the projection mask onto a sample disposed on a stage assembly of the imaging system and to transmit an image of the projection mask from the sample to the detector assembly. In another embodiment, the system includes a controller including one or more processors configured to execute a set of program instructions stored in memory, the program instructions configured to cause the one or more processors to: receive one or more images of the projection mask from the detector assembly; and determine a quality of the one or more images of the projection mask.

[0008] An autofocus system is disclosed. In one embodiment, the autofocus system includes a projection mask image quality (PMIQ) autofocus system. In another embodiment, the PMIQ autofocus system includes an illumination source. In another embodiment, the PMIQ autofocus system includes a first aperture. In another embodiment, the PMIQ autofocus system includes a first projection mask. In another embodiment, the PMIQ autofocus system includes a first PMIQ detector assembly and a second PMIQ detector assembly. In another embodiment, the system includes a normalized s-curve (NSC) autofocus system. In another embodiment, the NSC autofocus system includes an illumination source. In another embodiment, the NSC autofocus system includes a second aperture. In another embodiment, the NSC autofocus system includes a second projection mask. In another embodiment, the NSC autofocus system includes a first NSC detector assembly and a second NSC detector assembly. In another embodiment, the system includes a relay system configured to optically couple illumination from the PMIQ autofocus system and the NSC autofocus system to an imaging system, the relay system configured to project one or more patterns from the first projection mask onto a sample disposed on a stage assembly of the imaging system and to transmit an image of the first projection mask from the sample to the first PMIQ detector assembly and the second PMIQ detector assembly, the relay system configured to project one or more patterns from the second projection mask onto the sample disposed on the stage assembly of the imaging system and to transmit an image of the second projection mask from the sample to the first NSC detector assembly and the second NSC detector assembly. In another embodiment, the system includes a controller including one or more processors configured to execute a set of program instructions stored in memory, the program instructions configured to cause the one or more processors to: receive one or more signals from the first PMIQ detector assembly, the second PMIQ detector assembly, the first NSC detector assembly, and the second NSC detector assembly; and perform a dual control loop to adjust the stage assembly to maintain a focus of the imaging system based on the one or more signals from the first PMIQ detector assembly, the second PMIQ detector assembly, the first NSC detector assembly, and the second NSC detector assembly.

[0009] An autofocus system is disclosed. In one embodiment, the autofocus system includes a PMIQ autofocus system. In another embodiment, the PMIQ autofocus system includes an illumination source. In another embodiment, the PMIQ autofocus system includes a first aperture. In another embodiment, the PMIQ autofocus system includes a first projection mask. In another embodiment, the PMIQ autofocus system includes a first PMIQ detector assembly. In another embodiment, the system includes a NSC autofocus system. In another embodiment, the NSC autofocus system includes an illumination source. In another embodiment, the NSC autofocus system includes a second aperture. In another embodiment, the NSC autofocus system includes a second projection mask. In another embodiment, the NSC autofocus system includes a first NSC detector assembly. In another embodiment, the system includes a relay system configured to optically couple illumination from the PMIQ autofocus system and the NSC autofocus system to an imaging system, the relay system configured to project one or more patterns from the first projection mask onto a sample disposed on a stage assembly of the imaging system and to transmit an image of the first projection mask from the sample to the first PMIQ detector assembly, the relay system configured to project one or more patterns from the second projection mask onto the sample disposed on the stage assembly of the imaging system and to transmit an image of the second projection mask from the sample to the first NSC detector assembly. In another embodiment, the system includes a controller including one or more processors configured to execute a set of program instructions stored in memory, the program instructions configured to cause the one or more processors to: receive one or more signals from the first PMIQ detector assembly and the first NSC detector assembly; apply a digital binary return mask to the one or more signals from the first NSC detector assembly; and execute a dual control loop based on outputs from the one or more signals from the first PMIQ detector assembly, the first NSC detector assembly, and the digital binary return mask to adjust the stage assembly to maintain a focus of the imaging system.

[0010] An autofocus system is disclosed. In one embodiment, the autofocus system includes a PMIQ autofocus system. In another embodiment, the PMIQ autofocus system includes an illumination source. In another embodiment, the PMIQ autofocus system includes a first aperture. In another embodiment, the PMIQ autofocus system includes a tilted first projection mask. In another embodiment, the system includes a NSC autofocus system. In another embodiment, the NSC autofocus system includes an illumination source. In another embodiment, the NSC autofocus system includes a second aperture. In another embodiment, the NSC autofocus system includes a second projection mask. In another embodiment, the system includes a detector assembly. In another embodiment, the system includes a relay system configured to optically couple illumination from the PMIQ autofocus system and the NSC autofocus system to an imaging system, the relay system configured to project one or more patterns from the first projection mask onto a sample disposed on a stage assembly of the imaging system and to transmit an image of the first projection mask from the sample to the detector assembly, the relay system configured to project one or more patterns from the second projection mask onto the sample disposed on the stage assembly of the imaging system and to transmit an image of the second projection mask from the sample to the detector assembly. In another embodiment, the system includes a controller including one or more processors configured to execute a set of program instructions stored in memory, the program instructions configured to cause the one or more processors to: receive one or more signals from the detector assembly; apply a digital binary return mask to the one or more signals from the detector assembly; and execute a dual control loop based on the one or more signals from the detector assembly and an output of the digital binary return mask to adjust the stage assembly to maintain a focus of the imaging system.

[0011] An autofocus system is disclosed. In one embodiment, the autofocus system includes a PMIQ autofocus system. In another embodiment, the PMIQ autofocus system includes an illumination source. In another embodiment, the PMIQ autofocus system includes a first aperture. In another embodiment, the PMIQ autofocus system includes a first projection mask. In another embodiment, the PMIQ autofocus system includes one or more PMIQ detector assemblies. In another embodiment, the system includes a NSC autofocus system. In another embodiment, the NSC autofocus system includes an illumination source. In another embodiment, the NSC autofocus system includes a second aperture. In another embodiment, the NSC autofocus system includes a second projection mask. In another embodiment, the NSC autofocus system includes one or more NSC detector assemblies. In another embodiment, the system includes a relay system configured to optically couple illumination from the PMIQ autofocus system and the NSC autofocus system to an imaging system, the relay system configured to project one or more patterns from the first projection mask onto a sample disposed on a stage assembly of the imaging system and to transport an image of the first projection mask from the sample to the one or more PMIQ detector assemblies, the relay system configured to project one or more patterns from the second projection mask onto the sample disposed on the stage assembly of the imaging system and to transport an image of the second projection mask from the sample to the one or more NSC detector assemblies. In another embodiment, the system includes a controller including one or more processors configured to execute a set of program instructions stored in memory, the program instructions configured to cause the one or more processors to: receive one or more signals from the one or more PMIQ detector assemblies and the one or more NSC detector assemblies; and generate a focus error map based on the one or more signals from at least one of the one or more PMIQ detector assemblies or the one or more NSC detector assemblies.

[0012] 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 application as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the general description, serve to explain the principles of the application. BRIEF DESCRIPTION OF DRAWINGS

[0013] Those skilled in the art will better appreciate the advantages of the present application upon consideration of the detailed description of various embodiments thereof with reference to the accompanying drawings, in which:

[0014] Figure 1 A simplified schematic of a conventional autofocus (AF) system in accordance with one or more embodiments of the present application is illustrated.

[0015] Figure 2A A plot comprising a plurality of s-curves from an AF system is illustrated in accordance with one or more embodiments of the present application.

[0016] Figure 2B A normalized s-curve (NSC) plot is illustrated in accordance with one or more embodiments of the present application.

[0017] Figure 3 A comparison of cross-sectional views of a channel hole of a sample is illustrated in accordance with one or more embodiments of the present application.

[0018] Figure 4 A plot comprising a deformed s-curve and a symmetric s-curve is in accordance with one or more embodiments of the present application. Figure 1 A top view illustrating sensitivity of an AF system to pattern geometry is shown in FIG.

[0019] Figure 5 A plot comprising a deformed s-curve and a symmetric s-curve is in accordance with one or more embodiments of the present application.

[0020] Figure 6 A sample defect map acquired by an imaging system of an AF system coupled to Figure 1 A sample defect map acquired by an imaging system of an AF system is shown in FIG.

[0021] Figure 7 A plot comprising a deformed s-curve and a symmetric s-curve is in accordance with one or more embodiments of the present application.

[0022] Figure 8A A simplified schematic diagram of an AF system is illustrated in accordance with one or more embodiments of the present application.

[0023] Figure 8B An AF light pattern of an AF system is illustrated in accordance with one or more embodiments of the present application.

[0024] Figure 8C A through focus curve (TFC) of an AF system is illustrated in accordance with one or more embodiments of the present application.

[0025] Figure 8D A process flow diagram depicting a focus control loop of an AF system in accordance with one or more embodiments of the present application.

[0026] Figure 8E An additional / alternative embodiment of an AF system is illustrated in accordance with one or more embodiments of the present application.

[0027] Figure 8F An additional / alternative embodiment of an AF system is illustrated in accordance with one or more embodiments of the present application.

[0028] Figure 9AA simplified schematic of an AF system according to one or more embodiments of the application is illustrated.

[0029] Figure 9B A field of view of an AF system according to one or more embodiments of the application is illustrated.

[0030] Figure 9C An additional / alternative embodiment of an AF system according to one or more embodiments of the application is illustrated.

[0031] Figure 9D A simplified schematic of an AF system according to one or more embodiments of the application is illustrated.

[0032] Figure 9E A process flow diagram depicting a dual control loop of an AF system according to one or more embodiments of the application is illustrated.

[0033] Figure 9F A plot including an offset from a top surface of a sample according to one or more embodiments of the application is illustrated.

[0034] Figure 10 An s-curve according to one or more embodiments of the application is illustrated.

[0035] Figure 11 An additional / alternative embodiment of an AF system according to one or more embodiments of the application is illustrated.

[0036] Figure 12 A simplified schematic of an AF system according to one or more embodiments of the application is illustrated.

[0037] Figure 13 A simplified schematic of an AF system according to one or more embodiments of the application is illustrated.

[0038] Figure 14 An exemplary projection mask pattern according to one or more embodiments of the application is illustrated. DETAILED DESCRIPTION

[0039] The present application has been particularly shown and described with respect to particular embodiments and particular features thereof. The embodiments set forth herein should be viewed as illustrative rather than restrictive. Various changes and modifications can be made to the form and details of the disclosed subject matter without departing from the spirit and scope thereof.

[0040] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings.

[0041] Figure 1A simplified schematic of a conventional autofocus (AF) system 100 coupled to an imaging system 130 in accordance with one or more embodiments of the present disclosure is illustrated. In one embodiment, the AF system 100 is coupled to the imaging system 130 via a relay system 115.

[0042] In one embodiment, the AF system 100 includes an illumination source 102 configured to generate an illumination 101. The illumination source 102 can include any illumination source known in the art for generating an illumination 101, including but not limited to a broadband radiation source, a narrow frequency radiation source, or the like.

[0043] In another embodiment, the AF system 100 includes an aperture 104. For example, the AF system 100 can include a pupil aperture 104. The aperture can have any numerical aperture value known in the art. For example, the pupil aperture 104 can have a numerical aperture of 0.9 NA.

[0044] In another embodiment, the AF system 100 includes a projection mask 106 configured to project a geometric pattern 105.

[0045] The relay system 115 can include any set of optical elements known in the art for relaying an illumination. For example, the relay system 115 can include, but is not limited to, a focusing lens 114. For example, the focusing lens 114 can include a z-adjustable focusing lens 114.

[0046] In another embodiment, the AF system 100 includes one or more sets of sensors 116. For example, the AF system 100 can include a first set of sensors 116a and a second set of sensors 116b. For example, the first set of sensors 116a can be a set of focus sensors 116a and the second set of sensors 116b can be a set of normal sensors 116b.

[0047] In another embodiment, the AF system 100 includes a return mask 118.

[0048] The AF system 100 can include optical elements 108 known in the art. For example, one or more optical elements 108 can include, but are not limited to, one or more mirrors 110, one or more beam splitters 112a, 112b, and the like. Additionally, the AF system 100 can include any additional optical elements known in the art, including but not limited to one or more mirrors, one or more lenses, one or more polarizers, one or more beam splitters, one or more wave plates, and the like.

[0049] Imaging system 130 can include one or more optical elements 132, including but not limited to one or more mirrors 134, one or more objectives 136, and the like. It is noted herein that one or more optical elements 132 can include any optical element known in the art, including but not limited to one or more mirrors, one or more lenses, one or more polarizers, one or more beam splitters, one or more wave plates, and the like.

[0050] In another embodiment, imaging system 130 includes one or more detectors 138 configured to acquire illumination from sample 140 via illumination source 102 or a separate independent light source (not shown in FIG. 1). Figure 1

[0051] Sample 140 can include any sample known in the art, including but not limited to a wafer, a photomask, a reticle, and the like; a biological sample, such as but not limited to a tissue, a prosthesis, or the like; or a non-biological sample, such as but not limited to one or more curved glass plates (or sheets), one or more non-curved glass plates (or sheets), or the like. In one embodiment, sample 140 is disposed on a stage assembly 142 to facilitate movement of sample 140. In another embodiment, stage assembly 142 is an actuatable stage. For example, stage assembly 142 can include but is not limited to one or more translation stages adapted to selectively translate sample 140 along one or more linear directions (e.g., an x-direction, a y-direction, and / or a z-direction). By way of another example, stage assembly 142 can include but is not limited to one or more rotation stages adapted to selectively rotate sample 140 along a rotational direction. By way of another example, stage assembly 142 can include but is not limited to a translation stage and a rotation stage adapted to selectively translate sample 140 along a linear direction and / or rotate sample 140 along a rotational direction.

[0052] A description of an autofocus system is discussed in U.S. Patent No. 4,639,587, entitled “AUTOMATIC FOCUSING SYSTEM FOR A MICROSCOPE,” issued January 27, 1987, the entirety of which is incorporated herein by reference.

[0053] Figure 2A A plot 200 of a plurality of s-curves including from AF system 100 is illustrated in accordance with one or more embodiments of the present disclosure. Figure 2B A plot 220 of normalized s-curves of the plurality of s-curves illustrated in Figure 2A

[0054] In one embodiment, AF system 100 is configured to generate a plurality of s-curves. For example, as illustrated in FIG. 2, AF system 100 is configured to generate a plurality of s-curves 202, 204, 206, 208, 210, 212, 214, 216, 218, and 220.​​Figure 2A As shown in the plot 200, the plurality of s-curves can include a normal channel A curve 202, a normal channel B curve 204, a focused channel A curve 206, a focused channel B curve 208, and a normalized s-curve (NSC) 212.

[0055] As shown in the plot 220, the slope 210 of the curve 202 can be used to determine a linear range. Figure 2A

[0056] In the plot 220, a normalized s-curve (NSC) 222 is shown and described by:

[0057]

[0058] In Equation 1, F a represents a focus signal for channel A, F b represents a focus signal for channel B, N a represents a normal signal for channel A, and N b represents a normal signal for channel B. For example, one or more normal signals (N a , N b ) can be obtained from a normal sensor. By another example, one or more focus signals can be obtained from a focus sensor.

[0059] It is noted herein that the control system can be designed such that the z-stage is set to NSC = 0. However, a focus lens with z-axis adjustment (e.g., the focus lens 114) is needed to adjust the user-configurable focus offset of the z-plane of the sample.

[0060] Figure 3 is a comparison 300 of a desired via and a deformed via through a sample stack according to one or more embodiments of the present disclosure.

[0061] During a via etching process step, a desired via (e.g., the via 302) through a sample stack (as shown by Figure 3 ) should be cylindrical. If the sample has a uniform straight via 302 through the entire sample surface, the AF system 100 can maintain the focus at the desired focus offset. However, it is noted herein that one or more process variations of the sample can cause the AF system 100 to shift the sample surface up and / or down. As Figure 3 shown in the plot 300, this shift is caused by a tapered via 304.

[0062] ​It is noted herein that process variations can result in certain areas of the sample having tapered via holes 304 (e.g., variation in via hole size). Although the physical thickness of the sample is the same, depending on the severity of the process variation, the AF system 100 can displace the sample surface into and / or from the optimal focal plane of the high performance imaging system. As a result, loss of sensitivity in surface defect detection is caused.

[0063] Figure 4 A top view illustrating sensitivity 400 of the AF system 100 to pattern geometry of a sample in accordance with one or more embodiments of the application is shown.

[0064] It is noted herein that unpatterned areas on the sample can cause the focus of the sample to be displaced relative to the patterned areas on the sample. The amount of focus displacement depends on the width of the unpatterned areas.

[0065] As Figure 4 shown in FIG. 4B, the AF system 100 is sensitive to sample pattern geometry. For example, the presence of unpatterned areas 402 in the via hole etch array 404 can cause the focus to be displaced (e.g., displaced areas 406), even though the unpatterned areas can have the same physical height as the etch array. For example, for unpatterned areas having a width between 5 pm and 30 pm, the focus can be displaced between 100 nm and 400 nm depending on the brightness and width of the pattern.

[0066] It is noted herein that the focus displacement due to unpatterned areas and the upward and / or downward displacement of the sample due to process variations result in inconsistency across the entire surface of the sample and loss of defect detection sensitivity.

[0067] Figure 5 A plot 500 including a distorted s-curve 502 and a symmetric s-curve 504 in accordance with one or more embodiments of the application is shown.

[0068] It is noted herein that one serious problem with the AF system 100 is loss of focus on the sample (e.g., 3D NAND wafer with conventional AF setup). As Figure 5 shown in FIG. 5B, this is due to the severe s-curve linear range reduction caused by the diffraction of the sample pattern when the AF light penetrates below the sample surface.

[0069] As Figure 5 shown in FIG. 6B, the 3D NAND wafer s-curve 502 is distorted when the AF light penetrates below the 3D NAND wafer surface. In contrast, a mirror surface that does not have a diffraction sample patterning exhibits a symmetric s-curve 504 when the AF light penetrates below the sample surface.

[0070] Figure 6A sample defect map 600 acquired by an imaging system 150 coupled to an AF system 100 according to one or more embodiments of the present disclosure is illustrated.

[0071] In the sample map 600, each point corresponds to one detected defect. As Figure 6 As shown in the middle, at the bottom 602 of the sample map 600 includes zero points, which means zero defects are detected at the bottom of the sample. This is due to loss of focus in the AF system 100. It is noted herein that for inspecting defects at the bottom most of a high sample stack, which can be up to about 30 pm high, the focusing lens in the AF system needs to support 80 mm of z-axis adjustment range at high resolution. This can be extremely difficult for engineering design. In addition, the s-curve will be skewed at large focus offset due to excessive spherical aberration.

[0072] Figure 7 A plot including a skewed and asymmetric s-curve 700 and a symmetric s-curve 702 according to one or more embodiments of the present disclosure is illustrated.

[0073] It is noted herein that the skewed and asymmetric s-curve is caused when the sample is moved up. As Figure 7 As shown in the middle, when the sample is moved up 10 pm along the z-axis, the skewed and asymmetric s-curve 700 is caused due to excessive spherical aberration. In contrast, when the sample focus offset is zero, the symmetric s-curve 702 is caused.

[0074] Autofocus (AF) systems have become an instrument to achieve peak defect detection sensitivity for optical-based imaging systems. These optical-based AF systems have advantages over non-optical systems. For example, optical-based AF systems have fast signal response and high sensitivity. However, optical-based AF systems, such as the AF system 100, have several disadvantages. For example, a sample can be thick and transparent (or translucent) such that light can propagate below the top surface. It is difficult to distinguish AF light returned from the top surface and the bottom surface, especially when the sample has two or more surfaces separated by a selected distance, e.g., separated by about 10 nm to 10 pm.

[0075] In addition, for optical-based AF systems, it can be very challenging to maintain the focus of the best focus plane of the imaging system on the top surface of the sample within one depth of focus (DOF). The sample itself can have different refractive indices at different locations, e.g., x-axis and y-axis, which as a trade-off modulates the AF signal strength. This becomes even more challenging when the DOF is very short. For example, for an imaging system operating at 0.9 numerical aperture (NA) with a wavelength of 200 nm, the DOF can be about 100 nm.

[0076] For fully automated imaging systems, the best focal plane must be maintained at a user-configurable focus offset (typically on the sample surface) to achieve the best image quality and thus the best detection sensitivity. An autofocus system can be integrated with a high performance imaging system for this purpose.

[0077] Based on the shortcomings of the AF system 100, embodiments of the present disclosure are directed to an autofocus (AF) system for tracking a sample surface with a configurable focus offset. In particular, embodiments of the present disclosure are directed to an AF system integrated with a high performance imaging system for achieving the best image quality and the best detection sensitivity.

[0078] Figure 8A A simplified schematic of an AF system 800 coupled to an imaging system 830 in accordance with one or more embodiments of the present disclosure is illustrated. In one embodiment, the AF system 800 is optically coupled to the imaging system 830 via a relay system 810.

[0079] In one embodiment, the AF system 800 includes an illumination source 802 configured to generate illumination 801. The illumination source 802 can include any illumination source known in the art for generating illumination 801, including but not limited to a broadband radiation source, a narrow frequency radiation source, or the like.

[0080] In another embodiment, the AF system 800 includes an aperture 804. For example, the AF system 800 can include a pupil aperture 804. The aperture can have any shape or numerical aperture value known in the art. For example, the pupil aperture 804 can have a numerical aperture of 0.9 NA.

[0081] In another embodiment, the AF system 800 includes a projection mask 806 configured to project a geometric pattern 808 onto the sample 840. For example, the projection mask 806 can include an external projection mask containing one or more selected patterns (as shown in Figure 14 and further discussed herein) that can be projected onto the plane of the sample 840 via the relay system 810 and the imaging system 830. It is noted herein that the geometric pattern 808 can be generated via any method known in the art. For example, the geometric pattern 808 can be a simple binary mask. By way of another example, the geometric pattern 808 can be generated by a spatial light modulator. By way of further example, the geometric pattern 808 can be generated by optical diffraction (or interference means).

[0082] The relay system 810 can include any set of optical elements known in the art for relaying the projection mask image 819 from the first optical system and the second optical system. For example, the relay system 810 can include, but is not limited to, a focusing lens 812. For example, the focusing lens 812 can include a z-adjustable focusing lens 812.

[0083] In another embodiment, the AF system 800 includes a detector assembly 814. For example, as shown in Figure 8A The detector assembly 814 can include, but is not limited to, one or more cameras 814. The AF system 800 can include any type of camera. For example, the AF system 800 can include, but is not limited to, a two-dimensional (2D) camera. By way of another example, the AF system 800 can include, but is not limited to, a tilted 2D camera. By way of further example, the AF system 800 can include, but is not limited to, a tilted projection mask.

[0084] In another embodiment, the detector assembly 814 is communicatively coupled to a controller 816. The controller 816 can include one or more processors 818. The one or more processors 818 are configured to execute a set of program instructions stored in a memory 820. The set of program instructions are configured to cause the one or more processors 818 to perform one or more steps of the present disclosure. In one embodiment, the one or more processors 818 are configured to receive one or more projection mask images 819 from the detector assembly 814 of the AF system 800. In another embodiment, the one or more processors 818 are configured to determine a projection mask image quality (PMIQ). For example, the one or more processors 818 can apply one or more focus metrics (which can be optimized for different purposes or applications) to determine an image quality of the projection mask images 819.

[0085] In another embodiment, the one or more processors 818 can adjust the stage assembly 842 in response to the projection mask image quality (PMIQ). For example, in response to the monitored projection mask image quality, the one or more processors 818 can dynamically adjust a vertical position (i.e., a z-position) of the stage assembly 842 such that the sample z-position is adjusted to (or maintained at) an optimal (or at least sufficient) focus position.

[0086] The AF system 800 can include any optical elements 822 known in the art for facilitating operation of the AF system and the imaging system and coupling between the AF system and the imaging system. For example, the one or more optical elements 822 can include, but are not limited to, one or more lenses 824, one or more mirrors 826, or one or more beam splitters 828a, 828b. Additionally, although not shown, the AF system 800 can include any additional optical elements known in the art, including, but not limited to, one or more polarizers, one or more beam splitters, one or more wave plates, and the like.

[0087] The imaging system 830 can include one or more optical elements 832 including, but not limited to, one or more mirrors 834, one or more objective lenses 836, and the like. In another embodiment, the imaging system 830 includes one or more detectors 838 configured to acquire illumination from the sample 840 (e.g., reflected, diffracted, or scattered). The one or more detectors 838 can be communicatively coupled to the controller 816. In this regard, the one or more processors 818 of the controller 816 can receive signals and / or image data from the one or more detectors 838 of the imaging system 830. The imaging system 830 can include any imaging system known in the art. For example, the imaging system can include an inspection system, an image-based metrology system, a machine vision system, or a biological / biomedical imaging system.

[0088] The sample 840 can include any sample known in the art including, but not limited to, a wafer, a reticle, a photomask, and the like. In one embodiment, the sample 840 is disposed on a stage assembly 842 to facilitate movement of the sample 840. In another embodiment, the stage assembly 842 is an actuatable stage. For example, the stage assembly 842 can include, but is not limited to, one or more translation stages adapted to selectively translate the sample 840 along one or more linear directions (e.g., an x-direction, a y-direction, and / or a z-direction). By way of another example, the stage assembly 842 can include, but is not limited to, one or more rotation stages adapted to selectively rotate the sample 840 along a rotational direction. By way of another example, the stage assembly 842 can include, but is not limited to, a translation stage and a rotation stage adapted to selectively translate the sample 840 along a linear direction and / or rotate the sample 840 along a rotational direction.

[0089] Figure 8B The AF system 800 is illustrated in FIG. 8. Figure 8A The AF light patterns 850, 852 of the AF system 800 illustrated in FIG. 8 in accordance with one or more embodiments of the present disclosure.

[0090] In one embodiment, the AF light reflected from the surface of the sample forms the AF light pattern 850. In another embodiment, the AF light penetrating into the sample forms the light AF light pattern 852. It is noted herein that the AF light pattern 850 illustrates that the AF system 800 has optimal PMIQ when the tightly focused spot is reflected from the sample surface. Further, it is noted herein that the AF light pattern 852 has a focus that is spread out along the XYZ axes. The AF light pattern 852 illustrates that the tightly focused spot can be spread out in the XYZ directions, which implies a degraded point spread function due to the penetration of light into the sample.

[0091] It is noted herein that if the projection mask (PM) image projected from the projection mask (PM) to the sample has a high numerical aperture and diffraction is limiting the imaging quality, then the PM image reflected by the sample has only the best image quality when it is reflected from the top surface. As Figure 8B As explained in the '622 Application, for a highly focused point that can be considered a point spread function (PSF) of the optical system, when the light from the PM penetrates the sample and is reflected from one or more points on the bottom surface, the sample thickness and material refraction will cause the point size to spread in the lateral directions and along the optical axis. This causes aberrations of the nearly perfect PSF. Thus, the PM imaging quality degrades.

[0092] Figure 8C A through focus curve (TFC) 860 of the AF system 800 according to one or more embodiments of the present application is illustrated.

[0093] In one embodiment, the one or more processors 818 of the AF system 800 are configured to determine an image quality of the one or more images 819 of the projection mask 806 based on applying one or more focus metrics to the one or more images 819 of the projection mask 806. For example, the one or more processors 818 can be configured to apply a through focus curve (e.g., the TFC 860) to the one or more images 819 of the projection mask 806. It is noted herein that the PM IQ can be quantitatively measured using one or more focus metrics that can be adjusted for different application purposes as the sample 840 is moved up and down. Figure 8C The TFC 860 shown in the '622 Application includes five repetitions measured on a sample 840 (e.g., a mirror sample). The peak-to-peak variation of the five repetitions is about 30 nm.

[0094] Figure 8D A process flow diagram depicting a focus control loop 870 of the AF system 800 according to one or more embodiments of the present application is illustrated. It is noted that the descriptions of the various embodiments, components, and operations described herein previously with respect to the PM IQ are to be interpreted as extending to the dual control loop 870 unless otherwise noted. It is further noted that the various steps 872-892 are not to be interpreted as limited to a particular order depicted in the '622 Application or described herein. Rather, it is noted that the control loop 870 can begin at any number of positions in the control loop 870 and bypass and / or repeat any number of steps. Figure 8D

[0095] In step 872, the focus control loop 870 generates a focus target.

[0096] In step 874, the focus control loop 870 adjusts the focus target.

[0097] In step 876, the focus control loop 870 generates a height of the sample.

[0098] ​In step 878, the focus control loop 870 applies a control algorithm (e.g., control algorithm 1).

[0099] In step 880, the focus control loop 870 adjusts the stage assembly based on the output of the control algorithm (e.g., control algorithm 1). For example, the system 800 can be configured to adjust the stage assembly 842 in the z-direction.

[0100] In step 882, the focus control loop 870 acquires one or more measurements using one or more PMIQ optics.

[0101] In step 884, the focus control loop 870 acquires a PMIQ TFC via a detector assembly. For example, the detector assembly 814 of the AF system 800 can be configured to acquire a PMIQ TFC.

[0102] In step 886, the focus control loop 870 communicates data based on the PMIQ TFC. For example, the second loop 962 can be configured to communicate selected data to selected destinations via data paths for processing.

[0103] In step 888, the focus control loop 870 processes one or more PM images to calculate a focus error and a sign.

[0104] In step 890, the focus control loop 870 communicates data to a stage assembly. For example, the system 800 can be configured to communicate data to the stage assembly 842.

[0105] In step 892, the focus control loop 870 generates one or more focus errors. For example, the focus control loop 870 can calculate a focus error expressed as a distance (e.g., in nanometers).

[0106] It is noted herein that the control system can be designed such that the sample z position is dynamically adjusted to maintain the sample z position at the peak position of the TFC.

[0107] Figure 8EAn AF system 800 according to one or more additional / alternative embodiments of the present disclosure is illustrated. In this embodiment, the detector assembly 814 of the AF system 800 includes one or more tilted 2D cameras 896 configured to tilt into and out of the focal plane. For example, the one or more tilted 2D cameras 896 can tilt into and out of the focal plane in at least one of an rX direction (rotation about the x-axis) or an rY direction (rotation about the y-axis). It is noted herein that the optical axis can be defined as the z-axis. In this regard, a complete TFC can be obtained using the one or more tilted 2D cameras 896 as the sample 840 is moved in the XY plane for automated imaging. Further, each point of the TFC can be mapped from one or more images of the projection mask to each XY position on the one or more tilted 2D cameras. Further, it is noted herein that the one or more tilted 2D cameras 896 can be configured to obtain the TFC without continuously moving the sample 840 in the z-direction.

[0108] Figure 8F An AF system 800 according to one or more additional / alternative embodiments of the present disclosure is illustrated. In this embodiment, the detector assembly 814 of the AF system 800 includes one or more 2D cameras 814 and one or more transparent plates 898. For example, the one or more transparent plates 898 can be disposed in front of the one or more 2D cameras 814 and have a thickness that varies across the one or more transparent plates 898. It is noted herein that the one or more transparent plates 898 can be formed of any transparent material known in the art, including but not limited to glass, quartz, or the like.

[0109] Figure 9A A simplified schematic of an AF system 900 coupled to an imaging system 930 according to one or more embodiments of the present disclosure is illustrated. In particular, Figure 9A An AF system 900 configured for dual mode simultaneous operation is illustrated. The AF system 900 can include a PMIQ projection system 903 (or PMIQ module) and a NSC projection system 905 (or NSC module). In another embodiment, the AF system 900 is coupled to an imaging system 930 via a relay system 910. In this regard, the relay system 910 is configured to optically couple illumination from the PMIQ autofocus system and the NSC autofocus system to the imaging system 930.

[0110] In this embodiment, each of the PMIQ projection system 903 and the NSC projection system 905 can include its own illumination source, aperture, projection mask, illumination level control, and NA setting. For example, the PMIQ projection system 903 can include, but is not limited to, a first illumination source 902a, a first aperture 904a, and a first projection mask 906a. The NSC projection system 905 can include, but is not limited to, a second illumination source 902b, a second aperture 904b, and a second projection mask 906b.

[0111] In one embodiment, the illumination source 902a of the PMIQ autofocus projection system 903 is configured to operate in a continuously-on state mode. In another embodiment, the illumination source 902b of the NSC autofocus projection system 905 includes a first illumination channel (Channel A) and a second illumination channel (Channel B). The output of the illumination source 902b of the NSC autofocus projection system 905 can be time-multiplexed to mitigate cross-talk between the first illumination channel (Channel A) and the second illumination channel (Channel B).

[0112] In another embodiment, the first projection mask 906a and the second projection mask 906b are positioned such that the first projection mask 906a projects in a first half portion of the field of view and the second projection mask 906b projects in a second half portion of the field of view. In this regard, as shown in FIG. 9B, the system 900 can be configured such that the PMIQ projection system 903 uses a first half portion (e.g., left half portion) of the field of view and the NSC projection system 905 uses a second half portion (e.g., right half portion) of the field of view. Such an arrangement helps mitigate optical cross-talk between the PMIQ autofocus projection system 903 and the NSC autofocus projection system 905. For example, as shown in FIG. 9B, the projection from the PMIQ projection system 903 can occupy a left side 913a of the field of view (FOV) while the projection from the NSC projection system 905 can occupy a right side 913b of the FOV. Figure 9B Figure 9B

[0113] In another embodiment, the first projection mask 906a and the second projection mask 906b can have one or more different characteristics. For example, the first projection mask 906a and the second projection mask 906b can have different grid mask patterns, grid mask pitches, or grid mask orientations. The projection masks 906a, 906b can include one or more external projection masks containing one or more selected patterns (as shown in FIG. 9C and further discussed herein) that can be projected onto the plane of the sample 940 via the relay system 910 and the imaging system 930. Figure 14

[0114] It is noted herein that the PMIQ projection system 905 can operate using a 0.9 NA setting for both the illumination path and the collection path 907, 909, respectively. Further, the NSC projection system 905 can operate using a reduced NA in both the illumination path and the collection path 907, 909, respectively. For example, the NSC projection system 905 can operate using a NA that is less than 0.9 NA. For example, the NSC projection system 905 can operate using a NA that is between 0.4 NA and 0.6 NA. Further, the NSC projection system can operate using a NA of 0.5 NA. However, it is noted herein that the setting of the NA can be optimized based on the application. ​​​

[0115] In another embodiment, the AF system 900 includes one or more PMIQ detector assemblies. For example, as shown in FIG. 9B, the PMIQ projection system 903 can include a first PMIQ detector assembly 914a and a second PMIQ detector assembly 914b. For example, the first PMIQ detector assembly 914a and the second PMIQ detector assembly 914b can include, but are not limited to, a first 2D camera 914a and a second 2D camera 914b, respectively. It is noted herein that the one or more 2D cameras 914 can have a predetermined z-offset to obtain several discrete points on the TFC curve. Figure 9A

[0116] In another embodiment, the AF system 900 includes one or more NSC detector assemblies. For example, as shown in FIG. 9C, the NSC projection system 903 can include a first detector assembly 920a and a second detector assembly 920b. For example, the NSC projection system 903 can include a first sensor 920a and a second sensor 920b. The first sensor 920a can include one or more focus sensors configured to receive one or more focus signals (e.g., F a , F b ) from one or more illumination channels (e.g., channel A and channel B) of the NSC projection system 905. The second sensor 920b can include one or more normal sensors 920b configured to receive one or more normal signals (e.g., N a , N b) ) from one or more illumination channels (e.g., channel A and channel B) of the NSC projection system 905. Figure 9A

[0117] In another embodiment, the AF system 900 includes one or more collection pupil aperture stops 918. For example, the AF system 900 can include a first collection pupil aperture stop 918a and a second collection pupil aperture stop 918b associated with the first sensor 920a and the second sensor 920b of the NSC projection system 905, respectively. For example, the first collection pupil aperture stop 918a can have a first numerical aperture (e.g., 0.5 NA). In another example, the second collection pupil aperture stop 918b can have a second numerical aperture (e.g., 0.5 NA). It is noted herein that the one or more collection pupil aperture stops 918a, 918b can have any numerical aperture value.

[0118] ​​In another embodiment, the AF system 900 includes a return mask 916. For example, the AF system 900 can include a return mask 916 having the same pattern as the projected mask. By way of another example, the AF system 900 can include a return mask having a different pattern than the projected mask. It is noted herein that the return mask can be used in the focus channel to generate a focus signal for the NSC projection system 905. The return mask 916 acts as an optical valve against reflecting the projected mask image. When the sample is in focus, the focus sensor channels A and B receive the same amount of light. When the sample is out of focus, one channel receives more light than the other channel and vice versa. The direction of the out of focus can be determined by which channel receives more light.

[0119] In another embodiment, the one or more PMIQ detector assemblies 914 and the one or more NSC sensors 920 are communicatively coupled to a controller 921. The controller 921 can include one or more processors 925. The one or more processors 925 are configured to execute a set of program instructions stored in memory 927. The set of program instructions are configured to cause the one or more processors 925 to perform one or more steps of the present disclosure. In one embodiment, the one or more processors 925 are configured to receive one or more signals from the first PMIQ detector assembly 914a, the second PMIQ detector assembly 914b, the first NSC detector assembly 920a, and the second NSC detector assembly 920b. In another embodiment, the one or more processors 925 are configured to perform a dual control loop based on the one or more signals from the first PMIQ detector assembly, the second PMIQ detector assembly, the first NSC detector assembly, and the second NSC detector assembly to adjust the stage assembly 942 (e.g., z position) to maintain (or establish) a focus of the imaging system 930.

[0120] The relay system 910 can include any set of optical elements known in the art for relaying illumination from the first optical system and the second optical system. For example, the relay system 910 can include, but is not limited to, a focusing lens 912. For example, the focusing lens 912 can include a z-adjustable focusing lens 912.

[0121] The imaging system 930 can include any imaging system known in the art and the description of the imaging system 830 provided previously herein should be interpreted as extending to the imaging system 930. The imaging system 930 can include one or more optical elements 932 including, but not limited to, one or more mirrors 934, one or more objective lenses 936, and the like. In another embodiment, the imaging system 930 includes one or more detectors configured to acquire illumination from the sample 940 (e.g., reflected, diffracted, or scattered). The one or more detectors can be communicatively coupled to the controller 921. In this regard, the one or more processors of the controller can receive signals and / or image data from the one or more detectors of the imaging system 930. The imaging system 930 can include any imaging system known in the art. For example, the imaging system can include an inspection system, an image-based metrology system, a machine vision system, or a biological / biomedical imaging system.

[0122] The sample 940 and stage assembly 942 can include any sample and stage assembly known in the art and the description of the sample 840 and stage assembly 842 provided previously herein should be interpreted as extending to the sample 940 and stage assembly 942.

[0123] The AF system 900 can include any optical elements 922 known in the art for facilitating operation of the AF system and the imaging system and coupling between the AF system and the imaging system. For example, the one or more optical elements 922 can include, but are not limited to, one or more prisms 924 (e.g., top surface reflection or internal reflection), one or more lenses 926a, 926b, and one or more beam splitters 928a, 928b, and / or 928c. Additionally, although not shown, the AF system 900 can include any additional optical elements known in the art including, but not limited to, one or more polarizers, one or more beam splitters, one or more mirrors, one or more wave plates, and the like.

[0124] Figure 9C Additional / alternative embodiments of the AF system 900 in accordance with one or more embodiments of the present disclosure are described. It should be noted that the description of the systems 800 and 900 should extend to the embodiments depicted in FIGS. 8A-8B, unless otherwise mentioned herein. Figure 9B

[0125] In this embodiment, the AF system 900 includes a PMIQ camera 950a and a NSC camera 950b. For example, the system 900 can include a 2D camera 950a configured as a PMIQ camera and a 2D camera 950b configured as a NSC AF camera. In another embodiment, the AF system 900 can apply a digital binary return mask 952 during image processing to calculate the total integral energy transmitted.

[0126] ​In this embodiment, the PMIQ camera 950a and the NSC camera 950b are communicably coupled to the controller 921 of the AF system 900. In this regard, the NSC camera 950b replaces the normal channel and the focus channel of Figure 9A the PMIQ camera 950a. Further, the one or more processors 925 of the controller 921 can be configured to generate the NSC using data from the NSC camera 950b.

[0127] The digital binary return mask 952 can be configured to calculate the total integrated energy within the full field of view (FOV) of the NSC AF camera 950b as F a , F b , respectively. In this regard, a single camera (e.g., the NSC camera 950b) can be used to generate the NSC signal in a computational manner. It is noted herein that this embodiment can reduce the development cost of high precision optics and reduce focus detection artifacts from optical defects.

[0128] It is noted herein that the second illumination source 902b of the NSC projection system 905, which includes channels A and B, is configured to be turned on in a temporal sequence. Further, for the purposes of the present disclosure, the term “N a signal” or “N b signal” refers to the total integrated energy within the FOV of the AF camera 950a when the channel A and / or B light is turned on.

[0129] In another embodiment, the NSC can be calculated using the total integrated energy within the FOV of the NSC camera 950b. For example, when the sample is moved in the z-direction, the lateral shift of a single edge of the projected mask image can be detected when one illumination channel is turned on. For example, if channel A (or B) illuminates the projected mask from the right (or left) side of the pupil, the lateral motion of the right (or left) edge of the projected mask can be analyzed. The direction of motion for the two channels should be opposite. After subtracting the motion of the two channels, the detection sensitivity can be doubled. The NSC signal obtained using edge motion detection can reduce or even avoid energy from sub-surface reflections, which in turn causes a bias in the total energy in F a , F b , N a , N b . Conceptually, the image edge can be easily detected by taking the derivative of the original image.

[0130] Figure 9D Additional / alternative embodiments of the AF system 900 according to one or more embodiments of the present disclosure are described.

[0131] In one embodiment, one or more projection masks 906 of the AF system 900 include one or more tilted projection masks 954. For example, one or more tilted projection masks 954 may be tilted to obtain a complete TFC curve without moving one or more components of the AF system 900. In this additional / alternative embodiment, the geometry of the tilted projection mask 954 may be designed to match the geometry of the second projection mask 906b (e.g., ...). Figure 9B (The content shown in the text is different.)

[0132] In another embodiment, the detector assembly 914 of the AF system 900 includes a camera 914. For example, the camera 914 may be configured to obtain the PMIQ from the left side 913a of the FOV 911 and the NSC from the right side 913b of the FOV 911, as... Figure 9B As shown in the diagram. In this embodiment, camera 914 is communicatively coupled to controller 921. One or more processors 925 of controller 921 can be configured to measure PMIQ and generate NSC signals.

[0133] In this embodiment, the left PMIQ image and the right NSC image can be read simultaneously. For example, the imaging processing algorithm can split the FOV into two halves. For example, a separate image processing algorithm can be used to process the PMIQ and NSC images to obtain the corresponding focus signals for PMIQ and NSC. The two focus signals can be combined through a dual control loop, as described herein. Figure 9E Further description.

[0134] It should be noted in this article that the second illumination source 902b of the NSC projection system 905 is configured for time multiplexing for A / B channel differentiation.

[0135] Figure 9E A process flow diagram illustrating a dual control loop 960 according to one or more embodiments of the present invention is provided.

[0136] The dual control loop 960 may include a first control loop 961 and a second control loop 962 to maintain or establish focus for the imaging system. In this embodiment, the first control loop 961 implements the NSC autofocus routine and the second control loop 962 implements the PMIQ autofocus routine, which are consistent with the NSC and PMIQ embodiments previously described herein. Therefore, the various embodiments, components, and operations described with respect to the NSC and / or PMIQ embodiments should be interpreted as extending to the dual control loop 960, unless otherwise stated.

[0137] The second loop 962 can be configured to correct one or more process variations caused by the defocusing of the first loop 961. For example, the second loop 962 can be configured to detect the peak position TFC, so that the second loop 962 can find the optimal focal plane from the sample surface.

[0138] In one embodiment, as a starting point, the first control loop 961 can adjust the stage assembly based on the NSC optics and control feedback. Then, in the case where there is a process variation (e.g., caused by a de-focusing of the first control loop 961), the second control loop 962 can detect a focus error signal. The focus error signal can be computed from a set of defocused images acquired through the PMIQ optics and one or more detector assemblies. The control algorithm of the second loop 962 can compute one or more focus metrics of the PMIQ images at each focus offset to obtain a few discrete points on a defocus curve or FC. The measured TFC data points can be used to compute the offset of the best focus plane of the PMIQ with respect to the sample surface at the current position. This offset corresponds to the focus error. Then, this quantity is converted to the NSC signal and can be fed back into the first control loop 961. After detecting the focus error signal, the first control loop 961 can then move the stage assembly to a new z position such that the focus error is completely corrected.

[0139] It should be noted that the various steps 964-980 should not be interpreted as being limited to Figure 9E the particular order depicted in FIG. 9 or described herein. Rather, it should be noted that the dual control loop 960 can start at any number of positions in the control loop 960 and bypass and / or repeat any number of steps.

[0140] In step 964, the first loop 961 acquires one or more NSC signals with one or more NSC optics. For example, the controller 921 of the AF system 900 can be configured to acquire one or more NSC signals from one or more NSC optics.

[0141] In step 966, the first loop 961 applies a first control algorithm (e.g., control algorithm 1) to the one or more NSC signals acquired in step 964.

[0142] In step 968, the first loop 961 adjusts the stage assembly based on the output of the first control algorithm (e.g., control algorithm 1). For example, the controller 921 of the AF system 900 can be configured to adjust the stage assembly 942. For example, the stage assembly 942 can be adjusted in the z direction.

[0143] It should be noted herein that the first loop 961 of the dual control loop 960 can be configured as a feedback loop. The bandwidth of the first loop 961 can be adjusted based on the application of the control loop and / or one or more hardware choices. Further, it should be noted herein that when there is a focus error, it is an indication of a process variation. The magnitude of the focus error correlates to the severity of the process variation.

[0144] In step 970, the second loop 962 acquires one or more measurements using one or more PMIQ optics.

[0145] In step 972, the second loop 962 acquires a PMIQ through focus curve (TFC) via the detector assembly. For example, one or more detectors 914a, 914b of the AF system 900 can acquire a PMIQ TFC. For example, the TFC or several discrete points on the TFC obtained via the PMIQ subsystem of step 970 can be used to generate a PMIQ TFC.

[0146] In step 974, the second loop 962 communicates data based on the PMIQ TFC. For example, the second loop 962 can be configured to communicate selected data to selected destinations via data paths for processing.

[0147] In step 976, the second loop 962 calculates a focus error and sign based on the communicated data. For example, the second loop 962 can calculate a focus error expressed as a distance (e.g., in nanometers). It should be noted that focus error is typically measured in nanometers, while the NSC count represents an electronic digital signal that has a linear relationship to focus error in a linear region of data ( Figure 2A ) with a slope corresponding to the S-curve slope.

[0148] In step 978, the second loop 962 passes the data in the NSC count to the first loop 961. In step 980, the second loop 962 applies a second control algorithm (e.g., control algorithm 2) to the data passed to the first loop 961. In this regard, the second loop 962 can convert the focus error to an NSC signal. Then, the second loop 962 can feed the NSC signal of the focus error into the first loop 961 of the dual control loop 960. Then, the first control loop 961 can use the focus error to adjust the z-position of the stage assembly to fully correct the focus error.

[0149] The NSC autofocus routine and the PMIQ autofocus routine can operate independently for 2D wafer inspection and 3D NAND wafer inspection non-array regions. Further, the PMIQ autofocus routine can operate independently where process variation is small and the required linear range is less than about 500 nm.

[0150] Figure 9F A plot 990 including an offset from a top surface of a sample is illustrated in accordance with one or more embodiments of the present disclosure.

[0151] It is noted herein that for inspecting defects deep in the sample stack, large focus offsets are required. In one embodiment, the AF system 900 is configured to set the sample at a user-configurable focus offset that conforms to a sample top surface focus trajectory that can be recorded during sample surface inspection, as indicated by the curve 992 in FIG. 9B. Figure 9F

[0152] In another embodiment, the AF system 900 can be configured to collect one or more focus error signals that are passed to a first loop 962 of the control loop 960. When there is a focus error, it is an indication of a process variation. The magnitude of the focus error correlates to the severity of the process variation. When the focus error map (FEM) is used in conjunction with a defect distribution map (e.g., FIG. 9A), valuable information can be provided to the user to find the root cause of the yield limiting factor through correlation of the two maps or with other process control parameters. Figure 5

[0153] In another embodiment, a focus error map (FEM) can be generated using the NSC projection system 905. Due to the focus sensitivity of the NSC AF principle to process variations, the defect distribution map is not very reliable. However, the focus error map can still contain valuable information for the user to identify the yield limiting factor by correlating it with other process parameters.

[0154] It is noted herein that the NSC projection system 905 can operate as a standalone AF system that provides the desired functionality and excellent focus tracking performance for 2D wafer inspection and other non-array areas of 3D NAND wafers where PMIQ can be challenging but the NSC AF principle works well. It is noted herein that the only difference with NSC AF is that it has a reduced numerical aperture relative to the PMIQ projection system 903 in both the illumination and collection paths. This can increase the focus tracking random noise. It can be mitigated by increasing the ratio of the focus signal (F a , F b ) to the normal signal (N a , N b ) which is 1 :2 in the NSC projection system 905. For example, the second beam splitter 928b can utilize a different split ratio than 50 / 50. For example, the second beam splitter 928b can transmit 66% to the focus channel and reflect 33% to the normal channel. By another example, a neutral density filter can be used to reduce the light in the normal channel.

[0155] Figure 10 An s-curve 1000 is illustrated in accordance with one or more embodiments of the present disclosure.

[0156] As Figure 10 ​​The linear range 1002 of the s-curve 1000 can be extended in both the illumination and collection paths of the NSC projection system 905 using a reduced NA setting relative to the PMIQ projection system 903, as shown. For example, the linear range 1002 of the s-curve can be extended in both the illumination and collection paths of the NSC projection system 905 using a 0.5 NA setting. The NSC projection system 905 can have a numerical aperture that is less than 0.9 NA. For example, the NSC projection system 905 can have a numerical aperture between 0.4 NA and 0.6 NA. For example, the NSC projection system 905 can have a numerical aperture of 0.5 NA.

[0157] Figure 11 Additional / alternative embodiments of the AF system 900 according to one or more embodiments of the disclosure are described. In particular, the PMIQ projection system 903 of the AF system 900 can be implemented using a different magnification than the NSC projection system 905. In this regard, the PMIQ projection system 903 and the NSC projection system 905 can share the same illumination (e.g., as shown in FIG. 1) with FOV splitting. Figure 9B In addition, the PMIQ projection system 903 can have a lead / lag relative to the NSC projection system 905 for left-to-right and right-to-left scans, as shown.

[0158] Additionally, if the intrinsic pattern of the sample can be resolved using a 2D camera, the grid mask of the PMIQ projection system 903 field of view can be eliminated. In this regard, instead of detecting the external projected pattern image quality, the imaging quality of the sample pattern is directly detected and analyzed. Similar control and focus metric algorithms can be applied in the systems previously described herein. It is noted herein that the focus metric is not limited to edge slope. For example, robust contrast, cumulative density function (CDF), high frequency energy, or the like can be applied individually or in combination to determine the best focus plane.

[0159] Figure 12 Additional / alternative embodiments of the AF system 900 according to one or more embodiments of the disclosure are described. In particular, the low NA setting of the NSC projection system 905 can be configured for 3D NAND wafer surface tracking, which requires insensitivity to process variations. It is noted herein that this allows for different magnifications, independent aberrations, and focus control of 2D and 3D inspection modules.

[0160] In one embodiment, the AF system 900 includes a detector 1200. For example, the AF system 900 can include a camera 1200. In another embodiment, the AF system 900 includes a stepped focus delay 1202.

[0161] In another embodiment, the AF system 900 includes a plurality of focus sensors 920a', 920a". In another embodiment, the AF system 900 includes a plurality of normal sensors 920b', 920b".

[0162] Figure 13 Additional / alternative embodiments of the AF system 900 in accordance with one or more embodiments of the present disclosure are described. In particular, the PMIQ projection system 903 and the NSC projection system 905 of the AF system 900 can be configured to operate using similar NA settings. For example, the AF system 900 can be configured to have the same NA by transmitting illumination through one or more beam splitters.

[0163] In one embodiment, the AF system 900 includes a tilted camera 1300. In another embodiment, the AF system 900 includes a transparent plate 1302 disposed in front of the detector assembly 914. The transparent plate 1302 can be formed of any transparent material known in the art, including but not limited to glass.

[0164] In this embodiment, the illumination source 902 can include a light emitting diode (LED) illumination source 902.

[0165] Figure 14 An exemplary projection mask pattern 1400 in accordance with one or more embodiments of the present disclosure is described.

[0166] In one embodiment, the projection mask pattern 1400 includes a column pitch pattern 1402. In another embodiment, the projection mask pattern 1400 includes a square (or rectangular) box pattern 1404. In another embodiment, the projection mask pattern 1400 includes a star pattern 1406. It is noted herein that the projection mask pattern 1400 can include any specially designed pattern, and thus the above discussion should not be construed as limiting the scope of the present disclosure.

[0167] In another embodiment, the projection mask pattern 1400 can include a grid mask pattern configured to enhance focus detection sensitivity. For example, a series of binary square boxes oriented at different angles with respect to the sample x-axis can allow for more information about different aberration types to be used to detect imaging quality. It is noted herein that the imaging contrast of a grid mask can be enhanced by applying a special coating or altering the design of the transmission material and preventing the properties of the light and dark portions of the mask.

[0168] It is noted herein that the AF systems 800, 900, 1100, 1200, 1300 can have several advantages over the AF system 100. For example, the AF systems can track sample surfaces with process variations and unpatterned areas. For example, the sample surface planes determined using the AF system 800 are insensitive to process variations and unpatterned areas in the array. In this regard, surface defect detection sensitivity can enable full authorization of high performance imaging systems, such as the imaging system 830. By way of another example, the AF systems have increased ability to detect defocus errors. For example, when using the AF system at 0.9 NA AF test on a 3D NAND wafer, defocus errors were detected at 40 nm, which is fully within one depth of focus.

[0169] By way of another example, the NSC projection system 905 has an extended s-curve linearity range by using a reduced numerical aperture in both the illumination path and the collection path. This extended linearity range can avoid losing focus, which is typical when previous methods set the NA at 0.9 NA for both the illumination aperture and the collection aperture. As shown in FIG. 6A, the nominal symmetric s-curve becomes distorted in the lower half. The s-curve linearity range can be drastically reduced and thus prone to losing focus. Figure 10 By way of another example, the extended s-curve linearity range also enables feeding the focus error based on the NSC signal to the

[0170] By way of another example, the extended s-curve linearity range also enables feeding the focus error based on the NSC signal to the Figure 9E By way of another example, the split field of view can avoid cross talk between one or more components of the AF system. By way of another example, the control loops of one or more components can operate independently for non-array areas of 2D wafer inspection and 3D NAND wafer inspection. For example, the PMIQ control loop can operate independently.

[0171] By way of another example, the through focus curve (TFC) can alternatively be obtained using a tilted 2D camera without moving the sample z-stage or other moving parts in the optical system, which can otherwise introduce vibrations, air sway, and / or acoustic noise. By way of further example, the through focus curve (TFC) can alternatively be obtained using a set of glass plates with different thicknesses without moving the wafer z-stage or other moving parts in the optical system, which can otherwise introduce vibrations, air sway, and / or acoustic noise.

[0172] By way of further example, the AF field of view on the sample plane can be digitally truncated when a smaller size is needed. In addition, unwanted sample features that can interfere with the PMIQ detection can also be digitally masked. Furthermore, an energy-based method or an edge motion-based method can be used to computationally generate the NSC curve of the AF system 100.

[0173] By another example, for bottom defect detection, a sample can be set at a user configurable focus offset relative to the sample surface topography, which is recorded during surface defect inspection. It should be noted that the PMIQ method and / or the PMIQ+NSC method works very well to track the wafer surface at the array region for 3D NAND inspection. However, when inspecting defects at the bottom of the wafer stack or at large focus offsets, a different strategy can be used to mitigate the excessive travel range requirement of the focusing lens in the NSC method. In this alternative method, the z stage z0(x,y) can be recorded during inspection at the wafer surface. For wafer bottom or large focus offset inspection, a constant user configurable focus offset is used, which can be added such that for inspection at large focus offsets, the autofocus tracks to a virtual plane with a constant offset from the top surface, as shown in Figure 9F

[0174] By a further example, a focus error map (FEM) can be collected during inspection. In addition, a focus error map can be collected prior to inspection. The FEM can be valuable for users to find out the root cause of process variation and / or yield limiting factors.

[0175] It is noted herein that one or more components of the systems 800, 900 can be communicatively coupled to various other components of the systems 800, 900 in any manner known in the art. For example, the one or more processors 818, 925 can be communicatively coupled to each other and other components via wired (e.g., copper wire, fiber optic cable, and the like) or wireless connections (e.g., RF coupling, IR coupling, WiMax, Bluetooth, 3G, 4G, 4G LTE, 5G, and the like). By another example, the controller 816, 921 can be communicatively coupled to one or more components of the systems 800, 900 via any wired or wireless connection known in the art.

[0176] ​In one embodiment, the one or more processors 818, 925 can include any one or more processing components known in the art. In this regard, the one or more processors 818, 925 can include any microprocessor-type device configured to execute software algorithms and / or instructions. In one embodiment, the one or more processors 818, 925 can be comprised of a desktop computer, mainframe computer system, workstation, image computer, parallel processors, or other computer systems (e.g., networked linked computers) configured to execute programs configured to operate the system 800, 900, as described throughout the present disclosure. It should be appreciated that the steps described throughout the present disclosure can be carried out by a single computer system or, alternatively, multiple computer systems. Further, it should be appreciated that the steps described throughout the present disclosure can be carried out by any one or more of the one or more processors 818, 925. Generally, the term "processor" can be broadly defined to encompass any device having one or more processing components that execute program instructions from memory 820, 927. Moreover, different subsystems of the system 800, 900 can include processors or logic elements suitable to carry out at least a portion of the steps described throughout the present disclosure. Accordingly, the above description should not be interpreted as a limitation on the present disclosure and is for illustration only.

[0177] The memory 820, 927 can include any storage media suitable for storing program instructions and data received from the system 800, 900 that can be executed by the associated one or more processors 818, 925. For example, the memory 820, 927 can include a non-transitory memory medium. For example, the memory 820, 927 can include, but is not limited to, read-only memory (ROM), random access memory (RAM), magnetic or optical memory devices (e.g., optical disk), magnetic tape, solid state drives, and the like. It should be further noted that the memory 820, 927 can be housed in a common controller housing with the one or more processors 818, 925. In alternative embodiments, the memory 820, 927 can be remotely located with respect to the physical location of the processors 818, 925, controllers 816, 921, and the like. In another embodiment, the memory 820, 927 maintains program instructions for causing the one or more processors 818, 925 to carry out the various steps described by the present disclosure.

[0178] In one embodiment, the user interface is communicatively coupled to the controller 816, 921. In one embodiment, the user interface can include, but is not limited to, one or more desktop computers, tablet computers, smart phones, smart watches, or the like. In another embodiment, the user interface includes a display for displaying data of the system 800, 900 to a user. The display of the user interface can include any display known in the art. For example, the display can include, but is not limited to, a liquid crystal display (LCD), an organic light emitting diode (OLED) based display, or a CRT display. Those skilled in the art will recognize that any display device capable of integration with the user interface is suitable for implementation in the present disclosure. In another embodiment, a user can input selections and / or instructions via the use of the user interface input devices in response to data displayed to the user.

[0179] Those skilled in the art will recognize that the components, e.g., operations, apparatuses, objects, and the like described herein are used as example and are intended to be representative of that which is conceptually similar, such modifications varying from the illustrations as can be desired. Thus, as is apparent from this disclosure, the embodiments set forth are intended to be illustrative rather than limiting in all respects. In general, the scope of the disclosure encompasses all modifications that can fall within the scope of the claims along with their equivalents.

[0180] Those skilled in the art will appreciate that there are various tools (e.g., hardware, software, and / or firmware) by which a process and / or system and / or other technology (described herein) can be implemented, and that the preferred vehicle will vary with the context of usage. For example, if speed and accuracy is very important, as embodied herein, then a predominantly hardware and / or firmware vehicle can be selected; alternatively, if flexibility is paramount, then a predominantly software embodiment can be preferred; or yet again, some combination of hardware, software, and / or firmware can be selected. Accordingly, there are several feasible vehicles by which the processes and / or apparatuses and / or other technology described herein can be implemented, none of which is inherently superior to the others, as any vehicle to be utilized is a selection dependent upon the context in which the vehicle is to be utilized and the specific concerns (e.g., speed, flexibility, or predictability) of the implementer, any of which can vary.

[0181] The foregoing description is presented to enable one of ordinary skill in the art to make and use the application as provided in the context of a particular application and its requirements. As used herein, directional terms such as "top," "bottom," "upper," "lower," "up," "down," "downward," "upward," and "downwards" are intended to provide relative positions for purposes of description and are not intended to designate absolute reference frames. Those skilled in the art will appreciate modifications to the described embodiments, and the generic principles defined herein can be applied to other embodiments. Thus, the present application is not intended to be limited to the particular embodiments disclosed, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0182] With respect to substantially any plural and / or singular terms herein, those having skill in the art can adapt the terminology to describe particular embodiments, and / or claims, where applicable. Various singular / plural permutations can not be explicitly addressed herein for clarity.

[0183] All of the methods described herein can include storing results of one or more steps of the method embodiments in a memory. The results can include any of the results described herein and can be stored in any manner known in the art. The memory can include any of the memories described herein or any other suitable storage medium known in the art. After the results have been stored, the results can be accessed in the memory and used by any of the method or system embodiments described herein, formatted for display to a user, used by another software module, method, or system, etc. Further, the results can be stored "permanently," "semi-permanently," temporarily, or for a period of time. For example, the memory can be random access memory (RAM) and the results can not necessarily remain in the memory indefinitely.

[0184] It is further contemplated that each of the embodiments of the methods described above can include any other step(s) of any other method(s) described herein. Additionally, each of the embodiments of the methods described above can be performed by any of the systems described herein.

[0185] The subject matter described herein is sometimes illustrated using different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermediate components. Likewise, any two components so associated can also be viewed as being "connected" or "coupled" to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being "couplable" to each other to achieve the desired functionality. Specific examples of couplable include but are not limited to physically mateable and / or physically interacting components and / or wirelessly interactable and / or wirelessly interacting components and / or logically interacting and / or logically interactable components.

[0186] Furthermore, it should be understood that the invention is defined by the appended claims. Those skilled in the art will understand that, generally, the terms used herein and especially in the appended claims (e.g., the body of the appended claims) are intended to be “open-ended” terms (e.g., the term “comprising” should be interpreted as “comprising but not limited to,” the term “having” should be interpreted as “at least having,” the term “comprising” should be interpreted as “comprising but not limited to,” and the like). Those skilled in the art will further understand that if a particular number of claims are intended to be introduced, then this intention is explicitly stated in the claims, and if such a statement is not present, this intention does not exist. For example, to aid understanding, the following appended claims may include the introductory phrases “at least one” and “one or more” to introduce the claims. However, the use of these phrases should not be construed as implying that the introduction of a claim statement by the indefinite article “a” or “an” limits any particular claim containing this introductory claim statement to the invention containing only that statement, even if the same claim contains the introductory phrases “a or more” or “at least one” and indefinite articles (e.g., “a” or “an”) (e.g., “a” and / or “an” should generally be interpreted as meaning “at least one” or “a or more”); the same applies to the use of definite articles for introducing a claim statement. Furthermore, even if a specific number of introductory claim statements is explicitly stated, those skilled in the art will recognize that this statement should generally be interpreted as meaning at least the number stated (e.g., a bare statement of “two statements” without other modifiers generally means at least two statements or two or more statements). Furthermore, in the examples where a convention similar to "at least one of A, B, and C and the like" is used, this construction is generally desired 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, A and B, A and C, B and C, and / or systems having A, B, C, and the like). In the examples where a convention similar to "at least one of A, B, or C and the like" is used, this construction is generally desired 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, A and B, A and C, B and C, and / or systems having A, B, C, and the like). Those skilled in the art will further understand that transitional words and / or phrases (whether in the description, claims, or figures) that present two or more alternative terms should be understood as potentially including one, any, or both of the terms. For example, the phrase “A or B” would be understood as potentially including “A” or “B” or “A and B”.

[0187] It is believed that the present application and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes can be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its materials advantages. The form described is merely exemplary and it is intended to encompass and provide for alterations of its parts so that they come within the scope of the claims of the present application. It will be understood by those skilled in the art that the present application is to cover the modifications and variations of this application included within the scope of the appended claims and their equivalents.

Claims

1. An autofocus system comprising: a projection mask image quality (PMIQ) autofocus system comprising: an illumination source that emits light; an optical element comprising a first lens and a second lens, wherein the light emitted from the illumination source first passes through the first lens; a first aperture, wherein the light that passes through the first lens sequentially passes through the first aperture and the second lens; a first projection mask that receives the light that passes through the second lens; and a first PMIQ detector assembly and a second PMIQ detector assembly; a normalized s-curve (NSC) autofocus system comprising: an illumination source; a second aperture; a second projection mask; and a first NSC detector assembly and a second NSC detector assembly; a relay system, wherein the relay system is configured to project one or more patterns from the first projection mask onto a sample disposed on a stage assembly of an imaging system, wherein an image of the first projection mask is then transmitted from the sample to at least one or more detectors of the imaging system and the relay system, respectively, and the relay system further transmits the image of the first projection mask from the sample to the first PMIQ detector assembly and the second PMIQ detector assembly, wherein the relay system is configured to project one or more patterns from the second projection mask onto the sample disposed on the stage assembly of the imaging system, wherein an image of the second projection mask is then transmitted from the sample to the at least one or more detectors of the imaging system and the relay system, respectively, and the relay system further transmits the image of the second projection mask from the sample to the first NSC detector assembly and the second NSC detector assembly; and a controller including one or more processors, wherein the one or more processors are configured to execute a set of program instructions stored in memory, wherein the program instructions are configured to cause the one or more processors to: receive signal and / or image data from the one or more detectors of the imaging system; receive one or more signals from the first PMIQ detector assembly, the second PMIQ detector assembly, the first NSC detector assembly, and the second NSC detector assembly; and perform a dual control loop based on the one or more signals from the first PMIQ detector assembly, the second PMIQ detector assembly, the first NSC detector assembly, and the second NSC detector assembly to adjust the stage assembly to maintain a focus point of the imaging system.

2. The system of claim 1, wherein the first projection mask and the second projection mask are positioned such that the first projection mask projects in a first half portion of a field of view and the second projection mask projects in a second half portion of the field of view to mitigate optical cross-talk between the PMIQ autofocus system and the NSC autofocus system.

3. The system of claim 2, wherein at least one of a grating mask pattern, a grating mask pitch, or a grating mask orientation of the first projection mask is different from the second projection mask.

4. The system of claim 1, wherein the illumination source of the PMIQ autofocus system is configured to operate in a continuously on state.

5. The system of claim 1, wherein the illumination source of the NSC autofocus system includes a first illumination channel and a second illumination channel, wherein an output of the illumination source of the NSC autofocus system is time multiplexed to mitigate cross talk between the first illumination channel and the second illumination channel.

6. The system of claim 1, wherein the NSC autofocus system has a reduced numerical aperture relative to the PMIQ autofocus system in at least one of an illumination path or a collection path.

7. The system of claim 6, wherein the NSC autofocus system has an extended s-curve linear range.

8. The system of claim 6, wherein the NSC autofocus system has a numerical aperture less than 0.9 NA.

9. The system of claim 8, wherein the NSC autofocus system has a numerical aperture between 0.4 NA and 0.6 NA.

10. An autofocus system comprising: a projection mask image quality (PMIQ) autofocus system comprising: an illumination source; a first lens; a first aperture; a second lens; a first projection mask; and a first PMIQ detector assembly; a normalized s-curve (NSC) autofocus system comprising: an illumination source; a second aperture; a second projection mask; and a first NSC detector assembly; a relay system; wherein the illumination source emits light, the light emitted from the illumination source sequentially passes through the first lens, the first aperture, and the second lens, and is received by the first projection mask; wherein the relay system is configured to project one or more patterns from the first projection mask onto a sample disposed on a stage assembly of an imaging system, wherein an image of the first projection mask is then transmitted from the sample to at least one or more detectors of the imaging system and the relay system, respectively, and the relay system further transmits the image of the first projection mask from the sample to the first PMIQ detector assembly, wherein the relay system is configured to project one or more patterns from the second projection mask onto the sample disposed on the stage assembly of the imaging system, wherein an image of the second projection mask is then transmitted from the sample to the one or more detectors of the imaging system and the relay system, respectively, and the relay system further transmits the image of the second projection mask from the sample to the first NSC detector assembly; and a controller including one or more processors, wherein the one or more processors are configured to execute a set of program instructions stored in memory, wherein the program instructions are configured to cause the one or more processors to: receive signals and / or image data from the one or more detectors of the imaging system; receive one or more signals from the first PMIQ detector assembly and the first NSC detector assembly; apply a digital binary return mask to the one or more signals from the first NSC detector assembly; and execute a dual control loop based on the one or more signals from the first PMIQ detector assembly, the first NSC detector assembly, and an output of the digital binary return mask to adjust the stage assembly to maintain a focus of the imaging system.

11. An autofocus system comprising: a projection mask image quality (PMIQ) autofocus system comprising: an illumination source; a first lens; a first aperture; a second lens; a tilted first projection mask; a normalized s-curve (NSC) autofocus system comprising: an illumination source; a second aperture; a second projection mask; a detector assembly; a relay system; wherein the illumination source emits light, the light emitted from the illumination source passes sequentially through the first lens, the first aperture, and the second lens, and is received by the tilted first projection mask; wherein the relay system is configured to project one or more patterns from the tilted first projection mask onto a sample disposed on a stage assembly of an imaging system, wherein an image of the tilted first projection mask is then transmitted from the sample to one or more detectors of the imaging system and the relay system, respectively, and the relay system further transmits the image of the tilted first projection mask from the sample to the detector assembly, wherein the relay system is configured to project one or more patterns from the second projection mask onto the sample disposed on the stage assembly of the imaging system, wherein an image of the second projection mask is then transmitted from the sample to the one or more detectors of the imaging system and the relay system, respectively, and the relay system further transmits the image of the second projection mask from the sample to the detector assembly; and a controller including one or more processors, wherein the one or more processors are configured to execute a set of program instructions stored in memory, wherein the program instructions are configured to cause the one or more processors to: receive signals and / or image data from the one or more detectors of the imaging system; receive one or more signals from the detector assembly; apply a digital binary return mask to the one or more signals from the detector assembly; and execute a dual control loop based on the one or more signals from the detector assembly and an output of the digital binary return mask to adjust the stage assembly to maintain a focus of the imaging system.

12. The system of claim 11, wherein the tilted first projection mask is configured to provide a through focus curve (TFC).

13. The system of claim 11, wherein at least one of a grid mask pattern, a grid mask pitch, or a grid mask orientation of the tilted first projection mask is different than the second projection mask.

14. An autofocus system comprising: a projection mask image quality (PMIQ) autofocus system comprising: an illumination source; a first lens; a first aperture; a second lens; a first projection mask; and one or more PMIQ detector assemblies; a normalized s-curve (NSC) autofocus system comprising: an illumination source; a second aperture; a second projection mask; and one or more NSC detector assemblies; a relay system; wherein the illumination source emits light, the light emitted from the illumination source sequentially passes through the first lens, the first aperture, and the second lens, and is received by the first projection mask; wherein the relay system is configured to project one or more patterns from the first projection mask onto a sample disposed on a stage assembly of an imaging system, wherein an image of the first projection mask is then transmitted from the sample to at least one or more detectors of the imaging system and the relay system, respectively, the relay system further transmitting the image of the first projection mask from the sample to the one or more PMIQ detector assemblies, wherein the relay system is configured to project one or more patterns from the second projection mask onto the sample disposed on the stage assembly of the imaging system, wherein an image of the second projection mask is then transmitted from the sample to the at least one or more detectors of the imaging system and the relay system, respectively, the relay system further transmitting the image of the second projection mask from the sample to the one NSC detector assembly; and a controller including one or more processors, wherein the one or more processors are configured to execute a set of program instructions stored in memory, wherein the program instructions are configured to cause the one or more processors to: receive signals and / or image data from the one or more detectors of the imaging system; receive one or more signals from the one or more PMIQ detector assemblies and the one or more NSC detector assemblies; and generate a focus error map based on the one or more signals from at least one of the one or more PMIQ detector assemblies or the one or more NSC detector assemblies.

15. The system of claim 14, wherein at least one of a grid mask pattern, a grid mask pitch, or a grid mask orientation of the first projection mask is different than the second projection mask.

Images