Fast 3D volume inspection of semiconductor wafers with increased depth resolution

Abstract

The present invention relates to a three-dimensional circuit pattern inspection method of repetitive semiconductor structures within wafers, more particularly, to a method, computer program product and a corresponding semiconductor inspection system for determining parameters of 3D objects such as HAR structures within a semiconductor wafer with increased depth resolution. The method employs a milling and imaging of at least one cross-section surface at a slanted angle with respect to the surface of a wafer. The method, computer program product and device can be used for quantitative metrology, defect detection, process monitoring, defect review, and inspection of integrated circuits within semiconductor wafers.

Description

[0001] Fast 3D volume inspection of semiconductor wafers with increased depth resolution

[0002] Field of the invention

[0003] The present invention relates to a three-dimensional circuit pattern inspection method of repetitive semiconductor structures within wafers, more particularly, to a method, computer program product and a corresponding semiconductor inspection system for determining parameters of 3D objects such as HAR structures within a semiconductor wafer with increased depth resolution. The method employs a milling and imaging of at least one crosssection surface at a slanted angle with respect to the surface of a wafer. The method, computer program product and device can be used for quantitative metrology, defect detection, process monitoring, defect review, and inspection of integrated circuits within semiconductor wafers.

[0004] Prior art

[0005] Semiconductor structures are amongst the finest man-made structures and suffer from very few imperfections only. These rare imperfections are the signatures which defect detection or defect review or quantitative metrology devices are looking for. Fabricated semiconductor structures are based on prior knowledge, for example from design data and fabricated from a limited number of materials and processes. Furthermore, the semiconductor structures are manufactured in a sequence of layers being parallel to the surface of a silicon wafer substrate. During manufacturing, a huge number of three-dimensional semiconductor structures is generated in a wafer, wherein the fabrication process is subject to several influences. Generally, the edge shapes, areas or overlay positions of semiconductor structures may be subject to the property of involved materials, the lithography exposure, or any other involved manufacturing step, such as etching, polishing, deposition, or

[0006] implantation.A way to generate 3D data from semiconductor samples on nm scale is the so-called slice and image approach performed for example by a dual beam device. A slice and image approach is described in WO 2020 / 244795 A1. According to the method of the WO 2020 / 244795 A1, a 3D volume inspection is obtained at an inspection sample. This method has the disadvantage that a wafer has to be destroyed to obtain an inspection sample. This disadvantage has been solved by utilizing the slice and image method under a slanted angle into the surface of a semiconductor wafer, as described in WO 2021 / 180600 A1. According to this method, at least a first inspection site is determined, and 3D volume image of an inspection volume is obtained by slicing and imaging a plurality of cross-section surfaces of the inspection volume. In a first example for a precise measurement, a large number N of cross-section surfaces of the inspection volume is generated, with the number N exceeding 100 or even more. For example, in a volume with a lateral dimension of 5pm and a slicing distance of 5nm, 1000 slices are milled and imaged. However, the slice-and image method is time consuming and thus not always the preferred method. In an example, the task of the inspection is to determine a set of specific parameters of repetitive semiconductor objects such as high aspect ratio (HAR)- structures of memory devices. Cross-sections of such HAR-structures are circular on average and arranged in a regular raster perpendicular to the surface of a silicon wafer. In prior art, a method using a single cut under a shallow angle of for example 10°, has been proposed. However, with the increasing depth of the HAR-structures, exceeding for example 5pm, lateral extensions of cross sections a shallow angle increase to about 30pm or even more, which exceeds the field of view of typical scanning electron beam systems. On the other hand, with steeper milling angles of for example more than 26°, cross sections with a lateral extension of less than 10pm can be achieved. Larger milling angles, however, come along with a reduction in depth resolution, and a larger number of cross-sections is required.

[0007] WO 2023 / 117262 A1 provides further background art concerning three-dimensional circuit pattern inspection methods of an inspection volume at an inspection site of a semiconductor wafer and concerning related devices.of the invention

[0008] It is therefore an object of the invention to provide a further improvement to the slice- and image-method for deep and repetitive structures such as HAR-structures of modern 3D NAND-memory devices.

[0009] The object is solved by the independent claims. Dependent claims are directed to advantageous embodiments.

[0010] The present patent application claims the priority of German patent application No. 102025 104766.9 filed on 10 February 2025, the disclosure of which in the full scope thereof is incorporated in the present patent application by reference.

[0011] In an embodiment of the invention, a method of wafer inspection with a dual beam device is given. A dual beam device is comprising a focused ion beam column for generating and scanning a focused ion beam. The focused ion beam column arranged at a slanted angle GF to a wafer chuck. A dual beam device is further comprising a charged particle beam imaging system. The method is comprising loading a wafer on the wafer chuck and positioning an inspection site of the wafer at an intersection point of the dual beam device. The method is further comprising determining at least one periodicity vector K of a repetitive structure within the wafer in vicinity of the inspection site. A periodicity vector K of a repetitive structure is a vector pointing in a direction of periodicity of the repetitive structure.

[0012] The method is further comprising determining at least one of a scanning rotation angle GS and a wafer rotation angle GW such that a scanning or milling plane of the focused ion beam is not parallel to the at least one periodicity vector K. A vector K is not parallel to a scanning or milling plane (or vice versa) if an extension to the vector K has an intersection with the scanning or milling plane. On the other hand, a vector K is parallel to a scanning or milling plane if a vector parallel to vector K or vector K itself lies within the scanning or milling plane. For example, at least one of a scanning rotation angle GS and a wafer rotation angle GW areselected such that the intersection of a scanning or milling plane of the focused ion beam with the wafer surface is not parallel to the at least one periodicity vector K.

[0013] The method is further comprising adjusting the at least one of the scanning rotation angle GS and the wafer rotation angle GW. The method is further comprising milling, with the focused ion beam, a cross-section surface into a surface of the wafer and acquiring an image of the cross-section surface with the charged particle beam imaging system. The method is further comprising determining a plurality of measurement results within the image and compiling the measurement results of at least one parameter of the repetitive structure over depth. In an example, the angle GF between the focused ion beam and the surface of a wafer is larger than 15°, for example GF = 20°, for example GF = 26°, or even more, for example GF => 30°. In an example, a single image of the cross-section surface is acquired with the charged particle beam imaging system within a single field of view with a diameter of approximately 10pm.

[0014] By adjusting the at least one of the scanning rotation angle GS and the wafer rotation angle GW such that the at least one periodicity vector K is not parallel to scanning or milling plane of the focused ion beam, a large resolution or a small sampling rate in depth direction (perpendicular to the surface of the wafer) is achieved. Thereby, even at larger angles GF, with angles Gf exceeding 15°, for example 20°, for example 26°, or even more, for example 30°, a sufficient depth resolution of repetitive structures is achieved with a single or at least a limited number of cross section surfaces. With the larger angles GF, for example exceeding 20°, deep repetitive structures like HAR structures, exceeding depths of more than 5pm, for example 7pm, can be milled through and cross-section images can be obtained by charged particle beam imaging system within a single field of view with diameters of approximately 10pm. Therefore, no wafer movement and image stitching is required.

[0015] In an example, the method is further comprising determining a depth map of the image of the cross-section surface below the wafer surface. A depth map can be determined from structures within the wafer at known depths, such as word lines, or from simple trigonometricconsiderations based on the angles GF and GS.

[0016] In an example, the method is further comprising detecting features of the repetitive structure and performing measurements of the features within the image. In an example, the detecting of features of the repetitive structure comprises at least one of a template matching or machine learning method trained by training images comprising labelled features.

[0017] In an example, the wafer rotation angle GW is adjusted by a stage control unit connected to a wafer stage. The wafer stage is comprising a rotation stage, configured for rotating the wafer.

[0018] In an embodiment, the method is used in combination with acquiring a stack of images (“slices”) aimed at 3D volume image of an inspection volume within a wafer. During a 3D reconstruction, a plurality of cross section is formed by milling with the FIB at a slanted angle GF to the surface of the wafer, and a plurality of image slices is obtained by the charged particle imaging system. The cross-section surfaces are parallel to each other. Each of the plurality of image slices is registered with in a 3D volume and a 3D volume image is reconstructed. The registration of the image splices requires lateral alignment of each of the images. In an example of a feature-based alignment, repetitive structures are used as reference mark. If the repetitive structures have limited extension in vertical direction, their amount per slice might strongly vary from once slice to the next depending on the orientation of the periodicity vectors. This variability might substantially reduce the alignment accuracy for certain slices where the number of repetitive structures is lower. According to an embodiment, at least one of a wafer rotation angle GF or a scan rotation angle GS is selected such that the number of structures per image slice is approximately equal. Thereby, an improved accuracy of the feature-based alignment is achieved.

[0019] In an embodiment, a dual beam device for wafer inspection is given. The dual beam device is comprising a focused ion beam column for generating and scanning a focused ion beamwithin a scanning or milling plane. The focused ion beam column is arranged at an angle GF to a wafer chuck. The dual beam device is comprising a charged particle beam imaging system for acquiring an image of a cross-section surface. The dual beam device is comprising a wafer stage, comprising a rotation stage for rotating the wafer chuck. The dual beam device is comprising a control unit with a memory, the memory comprising software instructions. When executed, the software instructions are causing the dual beam device to perform a method described above.

[0020] According to the embodiments of the invention, the acquisition time of a 3D volume image of a semiconductor object of interest by the slice- and image-method is reduced, and an image accuracy is increased. Thereby, the throughput of an inspection task is increased and an inspection system for 3D inspection of a semiconductor object of interest is provided with increased throughput and increased accuracy. The invention provides a device and a method for 3D inspection of an inspection volume in a wafer and for the determination of a set of parameters of semiconductor features inside of the inspection volume with high accuracy and higher throughput. The methods and system configured for performing a method can be utilized for quantitative metrology, defect detection, process monitoring, defect review, and inspection of integrated circuits within semiconductor wafers.

[0021] The invention described by examples and embodiments is not limited to the embodiments and examples but can be implemented by those skilled in the art by various combinations or modifications. The present invention will be even more fully understood with reference to the following drawings:

[0022] Figure 1 shows an illustration of an inspection system for 3D volume inspection with a dual beam device.

[0023] Figure 2 is an illustration of a method of volume inspection in a wafer with a slanted cross-section milling and imaging by the dual beam device.

[0024] Figure 3 illustrates an example of an image sliceFigure 4 shows an illustration of an image slice according to the first embodiment Figure 5 shows an illustration of a milling according to the second embodiment Figure 6 shows an example of a dual beam device capable of performing the embodiments

[0025] Figure 7 illustrates the method according to the embodiments

[0026] Throughout the figures and the description, same reference numbers are used to describe same or similar features or components. The coordinate system is selected that the wafer surface 55 coincides with the XY-plane.

[0027] Recently, for the investigation of 3D inspection volumes in semiconductor wafers, a slice and imaging method has been proposed, which is applicable to inspection volumes inside a wafer. Thereby, a 3D volume image is generated at an inspection volume inside a wafer in the so called “wedge-cut” approach or wedge-cut geometry, without the need of a removal of a sample from the wafer. The slice- and image method is applied to an inspection volume with dimensions of few pm, for example with a lateral extension of 5pm to 10pm or up to 50pm in wafers with diameters of 200mm or 300mm. A V-shaped groove or wedge is milled in the top surface of a semiconductor wafer to make accessible a cross-section surface at a slanted angle to the top surface. 3D volume images of inspection volumes are acquired at a limited number of measurement sites, for example representative sites of dies, for example at process control monitors (PCM), or at sites identified by other inspection tools. The slice and image method will destroy the wafer only locally, and other dies may still be used, or the wafer may still be used for further processing. The methods and inspection systems according to the 3D Volume image generation are described in WO 2021 / 180600 A1, which is fully incorporated herein by reference. The current invention is an improvement and extension to the methods and inspection systems according to the 3D Volume image generation, where a higher throughput is required.The invention is applicable for semiconductor devices consisting of semiconductor-elements with high aspect ratio and / or located in multiple layers inside the device. Manufacturing of such devices strongly relies on the ability to characterize the semiconductor-elements in 3D. An inspection system of the invention is illustrated in figure 1. According to the first embodiment, an improved wafer inspection system 1000 for 3D volume inspection is given. The improved wafer inspection system 1000 for high-throughput 3D volume inspection is configured for a slice- and imaging method under wedge cut geometry with a dual beam device 1. For a wafer 8, several measurement sites, comprising measurement sites 6.1 and 6.2, are defined in a location map or inspection list generated from an inspection tool or from design information. The wafer 8 is placed on a wafer support table 15. The wafer support table 15 is mounted on a stage 155 with actuators and position control 21. Actuators and means for precision control 21 for a wafer stage 155 such as Laser interferometers are known in the art. A stage control unit 16 receives information about the actual position of the wafer stage 155 and is configured to control the wafer stage 155 and to adjust a measurement site 6.1 of the wafer 8 at the intersection point 43 of the dual-beam device 1. The dual beam device 1 is comprising a FIB column 50 with a FIB optical axis 48 and a charged particle beam (CPB) imaging system 40 with optical axis 42. At the intersection point 43 of both optical axes of FIB and CPB imaging system, the wafer surface 55 is arranged at a slant angle GF to the FIB axis 48. FIB axis 48 and CPB imaging system axis 42 include an angle GFE. In the coordinate system of figure 1, the normal to the wafer surface 55 is given by the z-axis. The focused ion beam (FIB) 51 is generated by the FIB-column 50 and is impinging under angle GF on the surface 55 of the wafer 8. Slanted cross-section surfaces are milled into the wafer by ion beam milling at the inspection site 6.1 under approximately the slant angle GF at a predetermined y-position, which is controlled by the stage 155 and position control 21. In the example of figure 1, the slant angle GF is approximately 30°. The actual slant angle of the slanted cross-section surface can deviate from the slant angle GF by up to 1° to 4° due to the beam divergency of the focused ion beam, for example a Gallium-lon beam, or due to variable material properties with respect to milling along thecross-section surface. The Fib column 50 comprises a scanning deflector 53, which is controlled by scan control unit 23. By scanning deflector 53, the FIB beam is deflected, for example in x-direction, thereby generating a scanning plane by the optical axis 48 of the FIB column 50 and the x-direction. Thereby, an extended cross section in x-direction is generated into the wafer surface 55. In an embodiment, the scan controller 23 is configured to introduce a scan rotation of the scanning plane. Such scan rotation is described below in more detail. With the charged particle beam imaging system 40, images of the milled surfaces are acquired. In the example of Figure 1, the charged particle beam imaging system 40 is arranged with its charged particle beam 44 perpendicular to the wafer surface 55 and parallel to the z-axis. In other configurations, the optical axis 42 of the charged particle beam imaging system 40 is arranged at an angle to the z-axis.

[0028] During imaging, a beam of charged particles 44 is scanned by a scanning unit of the charged particle beam imaging system 40 along a scan path over a cross-section surface of the wafer at measurement site 6.1, and secondary particles as well as backscattered particles are generated. Particle detector 17.1 and optional internal particle detector 17.2 collect at least some of the secondary particles and / or backscattered particles and communicate the particle count with a control unit 19. Other detectors for other kinds of interaction products such as x-rays or photons may be present as well. Control unit 19 is in control of the charged particle beam imaging column 40 and of the FIB column 50 and connected to a control unit 16 to control the position of the wafer mounted on the wafer support table 15 via the wafer stage 155. Operation control unit 2 communicates with control unit 19, which triggers placement and alignment for example of measurement site 6.1 of the wafer 8 at the intersection point 43 via wafer stage movement and triggers repeatedly operations of FIB milling, image acquisition and stage movements. Control unit 19 and Operation control unit 2 comprises a memory for storing instructions in form of software code and at least one processer to execute during operation the instructions, for example to execute the methods described in the embodiments. A memory is further provided to store digital image data. Operation controlunit 2 may further comprise a user interface or an interface to other communication interfaces to receive instructions, prior information and to transfer inspection results.

[0029] Each cross-section surface is milled by the FIB beam 51, and imaged by the charged particle imaging beam 44, which is for example scanning electron beam of a SEM or a Helium-lon-beam of a Helium ion microscope (HIM).

[0030] The operation control unit 2 is configured to perform a 3D inspection inside an inspection volume 160 in a wafer 8. The operation control unit 2 is further configured to reconstruct the properties of semiconductor structures of interest from the 3D volume image. In an example, features and 3D positions of the semiconductor structures of interest, for example the positions of the HAR structures, are detected by the image processing methods, for example from HAR centroids. A 3D volume image generation including image processing methods and feature based alignment is further described in WO 2020 / 244795 A1, which is hereby incorporated by reference.

[0031] Figure 2 illustrates further details of the slice and imaging method in the wedge cut geometry. By executing the slicing and imaging method in wedge-cut geometry, a crosssection 52 generated. Figure 2 illustrates the wedge cut geometry at the example of a 3D-memory stack. The cross-section surface 52 is milled with a FIB beam 51 at an angle GF of approximately 30° to the wafer surface 55, but other angles GF, for example between GF = 20° and GF = 60° are possible as well. With scanning deflector, the fib beam 51 is scanning deflected in x-direction. The cross-section surface 52 is generated by scanning the FIB-beam 51 in a scanning or milling plane 54. Scanning or milling plane 54 is comprising the optical axis 48 (or a line parallel to the optical axis 48) of the FIB column 50 and a line parallel to the x-axis.

[0032] A high-resolution cross-section image slice of the cross-section surface 52 is acquired for example by SEM beam 44. The cross-section image slice comprises first cross-section image features, formed by intersections with high aspect ratio (HAR) structures or vias (for example first cross-section image features of HAR-structures 4.1, 4.2, and 4.3) and secondcross-section image features formed by intersections with layers L.1 ... L.M, which comprise for example SiO2, SiN- or Tungsten-planes. Some of the lines are also called “word-lines”. The maximum number M of layers is typically more than 50, for example more than 100 or even more than 200. The HAR-structures and layers extend throughout most of the inspection volume in the wafer but may comprise gaps. The HAR structures typically have diameters about or below 100nm, for example about 80nm, or for example 40nm. The HAR structures are arranged in a regular, for example hexagonal raster with a pitch of about below 300nm, for example even below 200nm. The cross-section image slices contain therefore first cross-section image features as intersections or cross-sections of the HAR structures at different depth (Z) at the respective XY-location. In case of vertical memory HAR structures of a cylindrical shape, the obtained first cross-sections image features are circular or elliptical structures at various depths determined by the locations of the structures on the sloped cross-section surface 52. The memory stack extends in the Z-direction perpendicular to the wafer surface 55. The thickness d or minimum distances d between two adjacent cross-section averaged image slices is adjusted to values typically in the order of few nm, for example 30nm, 20nm, 10nm, 5nm, 4nm or even less.

[0033] The inspection volume 160 (see figure 2) typically has a lateral extension of LX = LY = 5pm to 15pm in x-y plane, and a depth LZ of 2pm to 15pm below the wafer surface 55. However, the extensions can also be larger and reach for example 50pm.

[0034] Figure 3a shows an image slice 311 generated by the imaging charged particle beam 44 and corresponding to cross-section surface 52. The image slice 311 comprises an edge line 315 between the slanted cross-section surface 52 and the surface 55 of the wafer. Wafercoordinates xw,yw are illustrated. Right to the edge 315, the image slice 311 shows several cross-sections 307.1...307. S through HAR structures which are intersected by the crosssection surface 311. In addition, the image slice 311 comprises cross-sections 313.1 to 313. n of several word lines at different depths or z-positions. Each image pixel is assigned to a depth z below the wafer surface 55. For example, a depth map 317 of the cross-section image 311 is computed. Various methods for determining the depth map 317 are known fromprior art; see for example US 7,005,640 B2 or US 2007 / 0141732 A1 or US 10,026,590 B2 for depth computation according to simple trigonometry. For example, a depth map 317 is computed from cross sections 313.1 to 313. n of several word lines at different known depths or z-positions. The depth of the word-lines 313.1 to 313. n is typically known or the word-lines 313.1 to 313. n are used as reference or scale for the determination of the depth map 317. The method using word lines is for example described in US 2022 / 0392793 A1, which is hereby included by reference. An intersection through the depth map 317 along line AA’ is shown in Figure 3b. Cross-sections of HAR structures are identified, such that along intersection AA’, and the z-positions of cross-sections of HAR structures 307 are determined. Thereby, with a single cross-section surface 52, a certain depth or z-sampling of HAR structures 307 is achieved. In the example of figure 3, the HAR structures are measured along intersection AA’ at five different depth positions. With the HAR structures arranged in hexagonal pattern with periodicity in certain directions with periodicity vectors KY, K60, K120 and KX, only a limited number of different z-positions or depths are sampled by each image 311. In the example of figure 3, for example the average diameter of the HAR structures can be determined at ten different depths or z-positions. Generally, any measurement M can be derived over depth z with a limited resolution (illustrated on the right side of figure 3b).

[0035] A periodicity vector K of a repetitive structure is a vector pointing in a direction of periodicity of the repetitive structure. Periodicity vectors may also be understood as frequency vectors in reciprocal or spatial frequency space. Periodicity vectors to the lowest spatial frequencies of the periodic structure are thus pointing from one structure of the periodic structure in the direction of next neighboring structure of a periodic structure. For example, in the hexagonal arrangement of HAR structures shown in Figure 3a, each individual HAR structure has six next neighbors. A first periodicity vector is pointing from a single HAR cross-section to the next neighbor in y-direction (vector KY), a second periodicity vector is pointing from a single HAR cross-section to the next neighbor in direction at an angle of 60° to the y-direction (vector K60), and a third periodicity vector is pointing from a single HAR cross-section to the next neighbor in direction at an angle of 120° to the y-direction (vector K120). There arefurther periodicity vectors like periodicity vector KX, pointing in x-direction to the next neighbor in x-direction. In the example of HAR structures illustrated in figure 3, the periodicity vectors KY = GO, KX = K90, K60 and K120 are all parallel to the surface 55 of a wafer 8. In another example of periodic structure arranged in a rectangular pattern periodicity vectors KO, K45, or K90. For simplification, the numbers in the name of the periodicity vectors K are describing the angle with the y-axis, so for example KX can also be written as K90.

[0036] The increasing depth of HAR structures of more than 5pm and the limited lateral field of view of a scanning electron microscope requires large milling angles GF of for example more than GF > 26°, for example GF = 30°. Thereby, however, the depth resolution or z- sampling-rate of measurement parameter M is limited, and in some examples, not sufficient. For example, with a pitch of 160nm between HAR structures, a depth resolution is limited to about dz ~ 80nm and an angle GF ~ 26°. In an example it has been proposed to repeat the milling step and image acquisition step. Thereby, depth resolution dz or z-sampling of measurements M of HAR structures can be increased, for example to dz < 50nm, however, at the expense of throughput.

[0037] According to a first embodiment, a depth resolution or z-sampling of measurements of HAR structures is increased by a wafer rotation. The effect of a wafer rotation is illustrated in Figure 4. As within the figures 1 to 3, the FIB optical axis 48 of Fib Column 50 is within an y-z-plane and the scanning direction of FIB beam 51 is the x-direction, such that edge 315 between cross-section surface 52 and wafer surface 55 is again parallel to the x-direction. However, wafer 8 is rotated by angle GW, such that none of the periodicity vectors KY, K60, K120 and KX is parallel to the x-direction.

[0038] In Figure 4, only periodicity vector KY is shown, deviating from y-direction by wafer rotation angle GW. By wafer rotation GW, the z-sampling of measurements M of HAR-structures is very much increased, in the example of figure 4 by a factor of three, but the factor can be much larger. Thereby, the need for further milling and imaging operations is reduced and throughput of a measurement task is increased. The effect is illustrated win figure 4b,showing a much denser z-sampling and higher depth resolution of a measurement task of measurement parameter M of HAR-structures.

[0039] According to a second embodiment, a depth resolution or z-sampling of measurements of HAR structures is increased by a scan rotation. Figure 5a illustrates the example according to figures 1 to 4 with scanning direction S1 of fib-beam 51 in x-direction, generating an edge 315a between cross-section surface 52a and wafer surface 55 which is parallel to the x-direction. The cross-section plane 52a is within a scanning or milling plane 54a which comprises the optical axis 48 of FIB column 50 and a line parallel to the x-axis. Figure 5b illustrates the example with scanning direction S2 of fib-beam 51 after a scan rotation by angle GS. After scan rotation by scan rotation angle GS, scanning direction S2 is no longer parallel to x-direction, but generates a rotated scanning or milling plane 54b which comprises the optical axis 48 of FIB column 50 and the scanning direction S2. Rotated scanning or milling plane 54b is not parallel to x-direction. Therefore, edge 315b between cross-section surface 52b and wafer surface 55 is not parallel to the x-direction anymore and the milling plane 54b is not parallel to any periodicity vectors KY, K60, K120 or KX anymore. Thereby, a z-sampling is increased and a higher depth resolution of a measurement task of HAR-structures is achieved. Thereby, the need for further milling and imaging operations is reduced and throughput of a measurement task is increased.

[0040] In both embodiments, a z-sampling is increased and a higher depth resolution of a measurement task of periodic structures such as HAR-structures is achieved. The increased z-sampling is achieved by an arrangement of a scanning or milling plane 54 by FIB beam 51 such that the scanning or milling plane 54 is not parallel to any of the periodicity vectors K (for example KY, K60, K120 or KX) of the periodic structures. Periodic structures such as HAR structures may be arranged for example in hexagonal structure or any other type of repetitive structure.

[0041] Figure 6 shows an example of a dual beam device 1 with a wafer stage 155 comprising a stage component 155. t for wafer rotation. Same reference numbers as within figure 1 areused, and reference is also made to the description of figure 1. The system further comprises a wafer chuck 151 for holding during use a wafer 8. The position of the stage 155 is controlled by position encoders such as laser interferometer 21. Generally, encoders are known in the art. The Fib column 50 and the imaging charged particle beam column 40 are both mounted to a rigid frame 25, which also serves as a mechanical reference for the position encoder 21. The arrangement of the scanning or milling plane 54 such that none of the periodicity vectors K of a periodic feature is parallel to the scanning or milling plane 54 is achieved by at least one of a wafer rotation by rotation angle GW or a scan rotation GS. Wafer rotation by angle GW is controlled by stage control unit 16, and scan rotation is achieved by scanning deflector 53 of the Fib column 50, controlled by scan controller 23. Figure 7 illustrates a method according to the first or second embodiment.

[0042] In a step A1 , an inspection site 6 within a wafer 8 is positioned in vicinity of an intersection point 43 of the dual beam device 1. In a step A2, at least one periodicity vector K of a repetitive structure at the inspection site 6 is determined. The at least one periodicity vector K is for example determined from CAD data, from a surface image of the wafer surface (55), or from an image of a previously milled cross-section surface into the wafer surface (55). In an example, the wafer surface 55 is achieved after a delayering of certain layers covering the repetitive structure.

[0043] In a third step A3, at least one of a scanning rotation angle GS of the scanning or milling plane 54 of the focused ion beam 51 or a wafer rotation angle GW is determined such that the scanning or milling plane 54 of the focused ion beam 51 is not parallel to the at least one periodicity vector K. In step A4, at least one of a scan rotation or a wafer rotation is affected by at least one of a scan control unit 23 or a stage control unit 16. In step A5, a cross section is milled into the surface 55 of the wafer under FIB angle GF, and an image of the crosssection surface is acquired. In step A6, the image of the cross-section surface is processed any analyzed. Image processing and analyzing may comprise at least one of a

[0044] - determination of a depth map for example from the word lines or from simple trigonometricconstruction; for example, the depth map is determined relative to a surface 55 of the wafer 8;

[0045] - a number or image processing operations such as noise reduction, thresholding, contrast enhancement;

[0046] - a detection or extraction of features, for example including machine learning methods, template matching; machine learning method can be trained by training images comprising labelled features;

[0047] - performing measurements of parameters of the detected features, such as distances, diameters, areas or shapes, thereby assigning each measurement result M to a depth according to the depth map;

[0048] - compiling the measurement results M of at least one parameter of the repetitive structure over depth, thereby generating a statistical assembly of measurement results M over depth of the repetitive structure (see figure 4b, right side).

[0049] Measurement parameters of interest can be a diameter, an area, a shape parameter (e.g. an ellipticity or eccentricity), a position deviation relative to a periodic raster, or the like.

[0050] The method allows a large resolution dz of small sampling rate in depth direction (perpendicular to the surface 55 of the wafer) even at larger angles GF, with angles Gf exceeding 15°, for example 20°, for example 26°, or even more, for example 30°. Thereby, deep repetitive structures like HAR structures, exceeding depth of more than 5pm, for example 7pm, can be milled through and cross-section images can be obtained by charged particle beam imaging system 40 within a single field of view with diameters of approximately 10pm.

[0051] For example, as illustrated in figure 5b, the cross-section surface 52b is comprised within the milling plane 54b, which has to the wafer surface 55 a first angle GF and a second angle GS. The first angle GF is according to the angle of the focused ion beam column (50) with respect to the wafer surface 55 (or the wafer chuck 151 , respectively). The second angle GS selected and adjusted by scan controller 23 and effected by scanning deflector 53 of thefocused ion beam column (50). The intersection 315b of the milling plane 54b with the wafer surface 55 is not parallel to any periodicity vector K of the repetitive structure within the wafer 8, in this example an HAR structure of a NAND-device.

[0052] The invention described by examples is however not limited to the examples or claims but can be implemented by those skilled in the art by various combinations or modifications thereof.

[0053] For example, the method is not limited to the increase of a depth sampling or z-resolution of a determination of a measurement M of a periodic structure. For example, in an embodiment, a method step comprising at least one of a wafer rotation GW of scanning rotation GS is used in combination with acquiring a stack of images (“slices”) aimed at 3D volume image of an inspection volume within a wafer. During a 3D reconstruction, a plurality of cross section is formed by milling with the FIB at a slanted angle GF to the surface of the wafer, and a plurality of image slices is obtained by the charged particle imaging system. The crosssection surfaces are parallel to each other. Each of the plurality of image slices is registered with in a 3D volume and a 3D volume image is reconstructed. The registration of the image splices requires lateral alignment of each of the images. In an example of a feature-based alignment, repetitive structures are used as reference mark. If the repetitive structures have limited extension in vertical direction, their amount per slice might strongly vary from one slice to the next, depending on the orientation of the periodicity vectors. This variability substantially reduces the alignment accuracy for certain slices where the number of repetitive structures is lower. According to an embodiment, at least one of a wafer rotation angle GF or a scan rotation angle GS is selected such that the number of structures per image slice is approximately equal. Thereby, an improved accuracy of the feature-based alignment is achieved.

[0054] A list of reference numbers is provided:

[0055] 1 Dual Beam Device

[0056] 2 Operation Control Unit4 cross section image features

[0057] 6 measurement sites

[0058] 8 wafer

[0059] 15 wafer support table

[0060] 16 stage control unit

[0061] 17 Secondary or backscattered Electron detector 19 Control Unit

[0062] 21 Position sensor

[0063] 23 scan controller

[0064] 40 charged particle beam (CPB) imaging system 42 Optical Axis of imaging system

[0065] 43 Intersection point

[0066] 44 Imaging charged particle beam

[0067] 48 Fib Optical Axis

[0068] 50 FIB column

[0069] 51 focused ion beam

[0070] 52 cross section surface

[0071] 53 scanning deflector

[0072] 54 scanning or milling plane

[0073] 151 wafer chuck

[0074] 155 wafer stage

[0075] 160 inspection volume307 measured cross section image of HAR structure 311 cross section image slice

[0076] 313 cross sections through word lines

[0077] 315 edge with surface

[0078] 317 depth map

[0079] 1000 wafer inspection system

Claims

Claims1. A method of wafer inspection with a dual beam device (1), the dual beam device (1) comprising a focused ion beam column (50) for generating and scanning a focused ion beam (51) arranged at a slanted angle GF to wafer chuck (151), and a charged particle beam imaging system (40), the method comprising:- loading a wafer (8) on the wafer chuck (151) and positioning an inspection site (6) of the wafer (8) at an intersection point (43) of the dual beam device (1),- determining at least one periodicity vector K of a repetitive structure within the wafer (8) in vicinity of the inspection site (6),- determining at least one of a scanning rotation angle GS and a wafer rotation angle GW such that a scanning or milling plane (54) of the focused ion beam (51) is not parallel to the at least one periodicity vector K,- adjusting the at least one of the scanning rotation angle GS and the wafer rotation angle GW,- milling, with the focused ion beam (51), a cross-section surface (52) into a surface (55) of the wafer (8),- acquiring an image of the cross-section surface (52) with the charged particle beam imaging system (40),- determining a plurality of measurement results within the image and- compiling the measurement results of at least one parameter of the repetitive structure over depth.

2. The method of claim 1, further comprising determining a depth map of the image of the cross-section surface (52) below the wafer surface (55).

3. The method of claim 1 or 2, further comprising detecting features of the repetitive structure and performing measurements of the features within the image.

4. The method of any of the claims 1 to 3, wherein the angle GF between the focused ion beam (51) and the wafer chuck (151) is larger than 15°, for example 20°, for example 26°, or even more, for example 30°.

5. The method of any of the claims 1 to 4, wherein acquiring the image of the crosssection surface (52) is obtained as a single image with the charged particle beam imaging system (40) within a single field of view with a diameter of approximately 10pm.

6. The method of any of the claims 1 to 5, wherein the wafer rotation angle GW is adjusted by a stage control unit (16) connected to a wafer stage (155) configured for rotating the wafer (8).

7. The method of any of the claims 1 to 6, wherein scan rotation angle GS is adjusted by scan control unit (23), connected a scanning deflector (53) of the focused ion beam column (50), configured for scanning deflection the focused ion beam (51) within the scanning or milling plane (54).

8. The method of any of the claims 1 to 7, further comprising detecting features of the repetitive structure and performing measurements of the features within the image, wherein detecting features of the repetitive structure comprises at least one of a template matching or machine learning method trained by training images comprising labelled features.

9. A method of wafer inspection with a dual beam device (1), the dual beam device (1) comprising a focused ion beam column (50) for generating and scanning a focused ion beam (51) arranged at a slanted angle GF to wafer chuck (151), and a charged particle beam imaging system (40), the method comprising:- loading a wafer (8) on the wafer chuck (151) and positioning an inspection site (6) of the wafer (8) at an intersection point (43) of the dual beam device (1),- determining at least one periodicity vector K of a repetitive structure within the wafer (8) in vicinity of the inspection site (6),- determining at least one of a scanning rotation angle GS and a wafer rotation angle GW such that the intersection of a scanning or milling plane (54) of the focused ion beam (51) with the wafer surface is not parallel to the at least one periodicity vector K,- adjusting the at least one of the scanning rotation angle GS and the wafer rotation angle GW,- milling, with the focused ion beam (51), a cross-section surface (52) into a surface (55) of the wafer (8),- acquiring an image of the cross-section surface (52) with the charged particle beam imaging system (40),- determining a plurality of measurement results within the image and- compiling the measurement results of at least one parameter of the repetitive structure over depth.

10. A dual beam device (1) for wafer inspection, comprising:- a focused ion beam column (50) for generating and scanning a focused ion beam (51) within a scanning or milling plane (54), the focused ion beam column (50) arranged at an angle GF to a wafer chuck (151),- a charged particle beam imaging system (40) for acquiring an image of a cross section surface (52),- a wafer stage (155) comprising a rotation stage 155. t for rotating the wafer chuck (151), and- a control unit (19) with a memory, the memory comprising software instructions when executed, causing the dual beam device (1) to perform a method of any of the claims 1 to 9.

Images