Method of ion-beam milling a cross-section surface into a wafer with ion beam, method of operating a dual-beam system, dual beam system and wafer inspection system

Abstract

This invention provides an improved method for operating a dual-beam system, and a dual-beam system for improving and mitigating charging effects during image formation or ion beam milling. The improved method and the dual-beam system utilize at least one of generating a temporary conduction zone for charge discharge, parallel charging cessation during ion beam milling, or charging cessation using a flushing gas enclosed in a localized gas flushing volume. This improved method and dual-beam system can be used for wafer inspection with improved accuracy.

Description

Improved Method and Apparatus for Charge Compensation During 3D Tomography The present invention relates to a method for measuring patterns of semiconductor objects within a semiconductor wafer, and more particularly to a method, computer program product, and corresponding semiconductor inspection device for performing three-dimensional (3D) tomography at a wafer. With the semiconductor inspection device and the method of the present invention, charging effects during imaging are reduced and high accuracy of image formation is maintained. The method, computer program product, and semiconductor inspection device can be utilized for various inspection tasks, such as metrology of integrated circuits within a semiconductor wafer, defect detection, process monitoring, or defect review. Semiconductor structures are among the most intricate man-made structures. Semiconductor manufacturing involves precise manipulation (such as lithography or etching) of materials such as silicon or oxides at very fine scales in the nanometer (nm) range. Wafers made of silicon wafers are used as substrates for microelectronic devices (including semiconductor structures built in and on the wafer). These semiconductor structures are constructed layer by layer using repetitive processing steps involving repeated chemical, mechanical, thermal, and optical processes. The size, shape, and placement of these semiconductor structures and patterns are affected by several factors. For example, during the manufacture of 3D memory devices, the key processes are currently etching and deposition. Other involved process steps (such as lithographic exposure or implantation) can also impact the properties of these components of the integrated circuits. Therefore, the manufactured semiconductor structures are troubled by rare and different defects. Devices for metrology, defect detection, or defect review are looking for these defects. These devices are not only required during wafer manufacturing. Since this manufacturing process is very complex and highly non-linear, it is difficult to optimize production process parameters. As a remedial measure, an iterative scheme called process window qualification (PWQ) can be applied. In each iteration, test wafers are manufactured based on the current best process parameters, and different die of the wafer are exposed to different manufacturing conditions. By detecting and analyzing these test structures using devices for metrology and defect detection, the best manufacturing process parameters can be selected. In this way, the production process parameters can be fine-tuned towards optimality. Subsequently, a highly accurate quality control process and device for metrology of these semiconductor structures in the wafer are required. The manufactured semiconductor structures are manufactured by the determined process and are thus based on the knowledge background. These semiconductor structures are manufactured with a series of layers parallel to the surface of the substrate. For example, in logic patterns, metal lines travel in parallel in metal layers or high aspect ratio (HAR) structures, and metal vias travel perpendicular to these metal layers. The angle between metal lines in different layers is alternatively 0° or 90°. On the other hand, for VNAND structures, it is known that their cross-section is on average circular. Furthermore, semiconductor wafers have a diameter of 300 mm and are composed of several sites (so-called die), each containing at least one integrated circuit pattern, such as for a memory chip or for a processor chip, etc. During manufacturing, the semiconductor wafer undergoes approximately 1000 process steps, and within the semiconductor wafer, approximately 100 or more parallel layers are formed, including these transistor layers, these layers of the middle of the line, and these interconnect layers, and in memory devices, a plurality of 3D arrays of memory cells. The aspect ratio and the number of layers of the integrated circuit are continuously increasing, and these structures are increasingly growing into the third (vertical) dimension. The current height of these memory stacks is exceeding a dozen micrometers. In contrast, the minimum feature size is getting smaller and smaller. The minimum feature size or critical dimension is below 10 nm (e.g., 7 nm or 5 nm) and will be getting closer and closer to feature sizes of approximately 3 nm and below in the near future. Despite the increasing complexity and these dimensions of these semiconductor structures growing into the third dimension, the lateral dimensions of the integrated semiconductor structures are getting smaller and smaller. Therefore, it becomes challenging to measure the shape, these dimensions, and the orientation of these features and patterns in three dimensions (3D) with high precision and their overlay. The lateral measurement resolution of charged particle systems is usually limited by the sampling grating of individual image points, or the dwell time of each pixel on the sample, and the charged particle beam diameter. The sampling grating resolution can be set within the imaging system and can be adapted to the charged particle beam diameter on the sample. The general grating resolution is 2 nm or below, and the grating resolution limit can be reduced without physical limitations. The charged particle beam diameter has a finite size, which depends on these charged particle beam operating conditions and the lens. The beam resolution is limited by approximately half of the beam diameter. The lateral resolution can be below 2 nm, for example, even below 1 nm. A common way to generate 3D tomography data from a semiconductor sample on the nm scale is, for example, the so-called slicing and imaging method obtained from a dual-beam device. The slicing and imaging method is described in WO 2020 / 244795 A1. According to the method of WO 2020 / 244795 A1, 3D volume detection is obtained at a detection sample extracted from a semiconductor wafer. In another example, the slicing and imaging method is applied to the surface of a semiconductor wafer at an inclined angle, as described in WO 2021 / 180600 A1. According to this method, a 3D volume image of the detection volume is obtained by slicing and imaging a plurality of cross-sectional surfaces within the detection volume. For accurate measurement, a large number N of cross-sectional surfaces in the detection volume are generated, and the number N exceeds 100 or even more image slices. For example, in a volume with a lateral dimension of 5 μm and a slice distance of 5 nm, 1000 slices are milled and imaged. Using an exemplary sample with a plurality of HAR structures having a pitch of, for example, 70 nm, approximately 5000 HAR structures are in one field of view, and a total of more than 5 million cross-sectional HAR structures are generated. An exemplary task of semiconductor detection is to determine a set of specified parameters of a semiconductor object, such as the high aspect ratio (HAR) - the structure inside the detection volume. Such parameters are, for example, dimensions, areas, shapes, or other measurement parameters. Generally, a semiconductor contains many complex three-dimensional structures. During the manufacturing process or process development, some selected physical or geometric parameters of a representative plurality of such three-dimensional structures must be measured with high accuracy and high throughput. To monitor the manufacturing, a detection volume is defined, including the representative plurality of three-dimensional structures. Then, this detection volume is analyzed, for example, by the slicing and imaging method, resulting in a high-resolution 3D volume image of the detection volume. From these large numbers of image slices, a three-dimensional volume image is derived with high accuracy. However, even minor charging effects at the wafer or cross-sectional surfaces cause, for example, image distortion, which deteriorates the derivation of the three-dimensional volume image and reduces the accuracy of the final detection or measurement task. Moreover, even minor charging effects at the wafer or cross-sectional surfaces cause a change in local image contrast and cause misjudgment of the edge detection algorithm for the edges of features and thus for critical dimension (CD) measurement. Therefore, an object of the present invention is to provide a charge reduction method effective during imaging of a plurality of cross-sectional surfaces generated in a wafer. An object of the present invention is to provide a detection device configured to perform a charge reduction method. These objects are solved by the present invention. The present invention is defined by the claims, and details are provided by the specific embodiments and examples. In a first specific embodiment, a method for reducing the impact of charging effects by ion beam milling a cross-sectional surface into a wafer is provided. The method includes, during the ion beam milling of the cross-sectional surface, compensating for milling-induced charging by scanning a second charged particle imaging beam over at least a segment of the cross-sectional surface during the ion beam milling. The ion beam milling method is performed using an ion beam comprising an ion species selected from a group of ion species including Gallium ions, Xenon ions, Oxygen ions, Neon ions, Argon ions, and Helium ions. In an example, the method further includes adjusting a kinetic energy of the second charged particle imaging beam to reduce a surface charge generated during the ion beam milling. Using the kinetic energy, the secondary electron yield (SEY) and charging behavior of the wafer surface are adjustable. In an example, the kinetic energy is adjusted to be lower than a minimum low-energy transition energy or higher than a maximum high-energy transition energy of a material composition present at a detection site of the wafer. Using the transition energy, the kinetic energy of the charged particle beam is meant under which (for a specified material composition) no secondary electrons are generated. During the ion beam milling, for example, a positive surface charge is generated. As the kinetic energy is lower than the low-energy transition energy or higher than the high-energy transition energy, a negative surface charge is generated by the second charged particle imaging beam, and thereby, the positive charge due to the ion beam milling is compensated. In an example, the method further includes generating a temporary conduction region between the cross-sectional surface and a capacitance by scanning the second charged particle imaging beam between the cross-sectional surface and the capacitance to effect a flow of charge from the cross-sectional surface to the capacitance. Thereby, the charging due to the ion beam milling is drained to the capacitance, and the effect of surface charging is avoided or compensated. Generating the temporary conduction region can be performed during the ion beam milling. Before generating the temporary conduction region between the cross-sectional surface and the capacitance, the kinetic energy of the charged particle imaging beam can be adjusted. In an example, the method further includes at least one step of image formation by scanning the second charged particle imaging beam over the cross-sectional surface of the wafer to form a two-dimensional image of the cross-sectional surface. The method can include several repetitions of the steps of image formation, ion beam milling, and compensating for milling-induced charging to acquire a plurality of two-dimensional images of a plurality of cross-sectional surfaces to reduce the impact of charging during the ion beam milling or image formation. According to a second specific embodiment, an image acquisition method includes at least one step of image formation to form a two-dimensional image of a segment of a surface of a wafer, such as a cross-sectional surface generated by ion beam milling. The step of image formation is achieved by scanning a charged particle imaging beam over the segment of the surface. The charged particle imaging beam is, for example, an electron beam or an ion beam, such as a helium ion beam. The image acquisition method further includes a step of generating a temporary conduction region between the segment of the surface and a capacitance. The temporary conduction region is generated by scanning the charged particle imaging beam between the segment of the surface and the capacitance. Thereby, the flow of charge is realized and the charged current flows from the segment of the surface to the capacitance. Thereby, the local surface charging of the segment of the surface caused by scanning the charged particle imaging beam over the segment is reduced, and the charging effect is alleviated. In an example, the image acquisition method further includes at least one step of ion beam milling a segment of the surface of a wafer using an ion beam. Thereby, a cross-sectional surface is formed as a segment of the surface. In an example, the optical axis of the ion beam is set at an angle GFE between 30° and 60° with the optical axis of the charged particle imaging beam. Thereby, the cross-section is formed at an inclined angle into the wafer. In an example, the step of generating a temporary conduction region by scanning the charged particle imaging beam is performed during the ion beam milling step. The step of generating a temporary conduction region can be performed during image formation and ion beam milling. In an example, the kinetic energy of the charged particle imaging beam is adjusted before generating the temporary conduction region. Thereby, the secondary electron yield and the penetration depth of the charged particles are adjusted, and the conductivity of the temporary conduction region is increased. In an example, the step of generating a temporary conduction region is repeated several times during image formation and during ion beam milling, and the surface charge during image formation or ion beam milling can flow to the capacitance. Thereby, the surface charge is alleviated during image formation and ion beam milling. In an example, the method further includes a step of compensating for milling-induced charging during ion beam milling by scanning the charged particle imaging beam over a segment of the cross-sectional surface during ion beam milling, for example, in parallel with the ion beam milling process. In an example, before compensating for the milling-induced charging by scanning the charged particle imaging beam, the method includes adjusting the kinetic energy of the charged particle imaging beam. Thereby, the secondary electron yield during the charged particle beam scanning is adjusted and configured to reduce the milling-induced charging. In an example, the steps of image formation, ion beam milling, and generating a temporary conduction region are repeated to acquire a plurality of two-dimensional images of a plurality of cross-sectional surfaces and reduce the impact of charging during ion beam milling or image formation. Thereby, a plurality of two-dimensional images are acquired and a three-dimensional volume image of the detection volume of the wafer at the detection site is generated. In an example, several different temporary conduction regions are generated to the same or different capacitances. In an example, the method further includes the step of determining the capacitance adjacent to the segment of the surface of the wafer. The step of determining the capacitance may include forming the capacitance adjacent to the segment of the surface by deposition of a metal volume onto the surface of the wafer. In another example, a large capacitance such as a conduction power or ground line may be used in the wafer design adjacent to the detection site, which may be used as the capacitance for discharging surface charges via the temporary conduction region. In an example, the method further includes a step of physically connecting a probe to the capacitance to remove the accumulated charge from the capacitance. The probe may be a needle of pipe capable of conducting charge, which may be moved and positioned by a high-precision actuator. Such probes are known in the art. In an example, the method further includes the step of setting a rinsing head at the detection site above the surface of the wafer, and providing a rinsing gas to the rinsing head via a gas supply to form a local gas rinsing volume between the dual-beam device and the surface of the wafer. The rinsing gas is selected and provided for neutralizing surface charges generated during image formation or ion beam milling. Suitable rinsing gases are, for example, selected from the group of gases including Hydrogen, Nitrogen, Xenon, Argon, Neon, or Helium. In an example, the method further includes performing an image processing on the at least one two-dimensional image including image processing operations (selected from the group of operations including image registration, depth map determination, distortion compensation, magnification adjustment, noise removal, contrast enhancement, image normalization, and thresholding, three-dimensional volume image generation, feature detection, feature extraction, template matching, or machine learning object detector). In an example, the method further includes adjusting a first detection site of a wafer at the optical axis of the charged particle imaging system. In a third specific embodiment, a method for operating a dual-beam device to reduce charging effects is provided. The method of operating the dual-beam system includes providing a gas flush head at a detection site adjacent to a surface of a wafer, and providing a flush gas to the gas flush head via a gas supply to form a local gas flush volume. The method further includes guiding a charged particle imaging beam of the dual-beam device through a first beam passage opening of the gas flush head, and guiding an ion beam of the dual-beam device through a second beam passage opening of the gas flush head. By providing the gas flush head with the first and second beam passage openings, a confined local gas flush volume is achieved, and the flush gas is provided in a confined area above the wafer surface at the detection side. The method further includes at least one step of ion beam milling at least one cross-sectional surface at the detection site into the surface of the wafer using the ion beam, and at least one step of forming at least one two-dimensional image of the at least one cross-sectional surface by scanning the charged particle imaging beam over the at least one cross-sectional surface. The method further includes selecting and providing the flush gas, which is configured to neutralize surface charges generated during ion beam milling or charged particle imaging. Suitable flush gases can be selected, for example, from a group of gases including hydrogen, nitrogen, xenon, argon, neon, or helium. In an example, the method further includes adjusting the detection site of the wafer at the optical axis of the charged particle imaging system. In yet another specific embodiment, a dual-beam system configured to mitigate charging effects during ion beam milling or imaging using a charged particle imaging beam is provided. The dual-beam system includes a charged particle imaging system, an ion beam system, a sample stage having a wafer support stage or chuck, and a control unit. The control unit includes a memory. The memory is configured to store a set of instructions. The control unit includes a processing engine configured to execute the set of instructions to cause the dual-beam system to perform any of the methods according to the first to third specific embodiments. In an example, the ion beam system is configured to generate a focused ion beam (FIB) including ions selected from a group including gallium ions, xenon ions, oxygen ions, neon ions, argon ions, and helium ions. In an example, the optical axis of the ion beam system is set at an angle GFE with the optical axis of the charged particle imaging system, and the angle GFE is between 30° and 80°. In another example, the angle GFE = 90°. The dual-beam system can be part of a wafer detection system. According to an example, the dual-beam system further includes a gas purge head disposed between a surface of the charged particle imaging system, the ion beam system, and a wafer or a wafer support stage. The gas purge head includes a first beam passage opening for guiding a charged particle imaging beam through the gas purge head; and a second beam passage opening for guiding an ion beam through the gas purge head. The gas purge head is connected to a gas supply configured to supply a purge gas to the gas purge head during use to form a local gas purge volume. The purge gas is selected from a group of gases including hydrogen, nitrogen, xenon, argon, neon, or helium. In an example, the first beam passage opening and the second beam passage opening are spatially separated and are disposed at an angle corresponding to an angle GFE between the ion beam system and the charged particle imaging system. In an example, the first beam passage opening and the second beam passage opening are at least partially overlapped. The gas purge head is retractably mounted on an actuating mount. Although these examples and specific embodiments are illustrated with these examples of semiconductor wafers, it is understood that the present invention is not limited to semiconductor wafers, but may also be applied, for example, to reticles or masks used in semiconductor manufacturing. Throughout these illustrations and descriptions, the same reference numbers are used to illustrate the same features or components. The coordinate system is selected such that the wafer surface 55 coincides with the XY plane. To investigate the 3D inspection volume in a semiconductor wafer, slicing and imaging methods applicable to inspecting the volume inside the wafer have been proposed. In an example, a 3D volume image is generated from the inspection volume inside the wafer by the so-called "wedge-cut" method or wedge-cut geometry without removing a sample piece from the wafer. The slicing and imaging method is applicable to inspection volumes with dimensions of a few micrometers (μm), such as having a lateral extension of 5 μm to 10 μm in a wafer with a diameter of 200 mm or 300 mm. The lateral extension can also be larger and can be up to 30 or 50 micrometers at most. V-grooves or edges are milled in the top surface of the integrated semiconductor wafer so that a cross-sectional surface at an angle to the top surface can be accessed. 3D volume images of the inspection volume are captured at a finite number of inspection sites, such as representative sites of the die, such as at process control monitors (PCM), or at sites identified by other inspection tools. The slicing and imaging method will only locally damage the wafer, and other dies are still likely to be used, or the wafer is still likely to be used for further processing. The methods and inspection systems based on the 3D volume image are described in WO 2021 / 180600 A1, which is hereby incorporated by reference in its entirety. An example of a wafer inspection system 1000 for 3D volume inspection is illustrated in FIG. 1. The wafer inspection system 1000 is configured for a slicing and imaging method in a wedge-cut geometry with a dual-beam device 1. For the wafer 8, several inspection sites (including inspection sites 6.1 and 6.2) are defined in a position map or inspection list generated from inspection tools or from design information. The wafer 8 is placed on a wafer support stage 15. The wafer support stage 15 is mounted on a stage 155 with an actuator and positioning control. Actuators and means (such as a laser interferometer) for precision control of the wafer stage are known in the art. The control unit 16 is configured to control the wafer stage 155 and adjust the inspection site 6.1 of the wafer 8 at the intersection point 43 of the dual-beam device 1. The dual-beam device 1 includes an FIB column 50 having an FIB optical axis 48; and a charged particle beam (CPB) imaging system 40 having an optical axis 42. At the intersection point 43 of the two optical axes of the FIB and CPB imaging systems, the wafer surface 55 is set at an inclined angle GF with respect to the FIB axis 48. The FIB axis 48 and the CPB imaging system axis 42 include an angle GFE, and the CPB imaging system axis forms an angle GE with the normal of the wafer surface 55. In the coordinate system of FIG. 1, the normal of 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 irradiated on the surface 55 of the wafer 8 at the angle GF. The inclined cross-sectional surface is milled into the wafer at the inspection site 6.1 at approximately the inclined angle GF by the ion beam.In the example of FIG. 1, the inclined surface angle GF is approximately 30°. Due to the beam divergence of the focused ion beam (such as a gallium ion beam), the actual inclined surface angle of the inclined surface cross-section surface may deviate from the inclined surface angle GF by up to 1° to 4°. Generally, the FIB column 50 can be, for example, a gallium FIB with or without a Wien filter or a similar mechanism that allows an alloy-based source (such as silicon, gold, etc.), or a gas field ion source (GFIS), a plasma source, or a duo-plasmatron FIB that uses other types of ion species such as xenon, oxygen, or argon ions, or related technologies (such as "cluster" or "low temperature" ion sources). Generally, the FIB column 50 is used to generate a focused ion beam, if necessary, at different charge states of the ions. An imaging system 40 for charged particle beams tilted at an angle GE with respect to the wafer normal is used to capture images of the milled surfaces. In the example of FIG. 1, the angle GE is approximately 15°. However, other settings are also possible, such as using GE = GF, so that the CPB imaging system axis 42 is perpendicular to the FIB axis 48 (and GFE = 90°), or GE = 0°, so that the CPB imaging system axis 42 is perpendicular to the wafer surface 55. During imaging, the charged particle beam 44 is scanned by the scanning unit of the charged particle beam imaging system 40 along a scanning path on the cross-section surface of the wafer 8 at the detection site 6.1, and secondary particles and scattered particles are generated. For example, the secondary electron particle detector 17.1 collects at least some of the secondary particles and scattered particles and communicates the particle count to the control unit 19. Other detectors for other interaction products may also be present, such as the in-lens detector 17.2 for collecting backscattered charged particles. The control unit 19 controls the charged particle beam imaging column 40 of the FIB column 50 and is connected to the stage control unit 16 to control the positioning of the wafer 8 mounted on the wafer support stage 15 via the wafer stage 155. The control unit 19 communicates with the operation control unit 2, which triggers, via the wafer stage movement, the placement and alignment, for example, of the detection site 6.1 of the wafer 8 at the intersection point 43 and triggers the repeated operations of FIB milling, image capture, and stage movement. Each new intersection surface is milled by the FIB beam 51 and imaged by a charged particle imaging beam 44, such as a scanning electron beam or a helium ion beam of a helium ion microscope (HIM). In an example, the dual beam system includes a first focused ion beam system 50 disposed at a first angle GF1; and a second focused ion column disposed at the second angle GF2, and the wafer is rotated between milling at the first angle GF1 and the second angle GF2 while imaging is performed by an imaging charged particle beam column 40 disposed, for example, perpendicular to the wafer surface 55. The dual beam system 1 further includes a gas injection system (GIS) 79 having a gas nozzle connected via a valve (not shown) to at least one gas reservoir (not shown). Thereby, a controlled amount of precursor gas can be provided during milling or imaging, and for example, a metal coating can be produced. For example, alignment marks or fiducials can be produced. For example, a tungsten metal coating is produced by providing tungsten hexacarbonyl. The metal coating can be shaped by ion beam milling, and alignment marks or fiducials are formed close to the detection site. Thereby, accurate registration and image alignment of the plurality of cross-sectional images are achieved. By using a dedicated precursor gas, the milling operation by the FIB 51 can be enhanced. For example, the homogeneity of the milling operation in a composition of different materials can be improved, and curtaining can be reduced. The composition of materials in a semiconductor wafer can include silicon, silicon dioxide, silicon nitride, copper, aluminum, or other materials. The preferred precursor gas system includes at least one of ammonia, ammonium hydroxide, ammonium carbamate, bromine, chlorine, hydrazine, hydrogen peroxide, hadacidin, iodine, diiodoethane, isopropanol, methyl difluoroacetate, nitroethane, nitroethanol, nitrogen, nitrogen tetroxide, nitrogen trifluoride, nitromethane, nitropropane, nitrobutane, oxygen, ozone, PMCPS, tungsten hexacarbonyl, water, or xenon difluoride. However, it may also be other gases, such as methoxy acetylchloride, methyl acetate, methyl nitroacetate, ethyl acetate, ethyl nitroacetate, propyl acetate, propyl nitroacetate, nitro ethyl acetate, methyl methoxyacetate, and acetic acid or thioacetic acid, hexafluoroacetylacetone, silazane, trifluoroacetamide, dicobalt octacarbonyl, molybdenum hexacarbonyl, and combinations thereof. Furthermore, the dual-beam system 1 further includes a contact pin 81. The contact pin 81 is connected to a manipulator (not shown) for precisely moving the contact pin 81, such as under the control of the charged particle beam 44 during image capture. Thus, the structures present on the wafer surface can be contacted and electrically connected to the control device 19. Figure 2 illustrates the wedge cut geometry for the 3D memory stack. Figure 2 illustrates the situation when the surface 52 is the new cross-sectional surface finally milled by the FIB 51. The cross-sectional surface 52 is scanned, for example, by a scanning electron microscope (SEM) beam 44 that is set at a normal incidence to the wafer surface 55 in the example of Figure 2, and high-resolution cross-sectional image slices are generated. The cross-sectional surfaces 53.1…53.N are subsequently milled with the FIB beam 51 at an angle GF of approximately 30° with respect to the wafer surface 55, but other angles GF (e.g., between GF = 20° and GF = 60°) are also possible. The cross-sectional image slices contain first cross-sectional image features formed by the intersection with high aspect ratio (HAR) structures or vias (e.g., the first cross-sectional image features of the HAR structures 4.1, 4.2, and 4.3); and second cross-sectional image features formed by the intersection with the layers L.1…L.M that contain, for example, SiO 2 , SiN, or tungsten lines. Some of these lines are also referred to as "word-lines". The maximum number M of layers typically exceeds 50, such as more than 100 or even more than 200. The HAR structures and the layers extend through most of the volume in the wafer, but may contain gaps. The HAR structures typically have a diameter of less than 100 nm, such as about 80 nm, or for example 40 nm. Thus, the cross-sectional image slices contain the first cross-sectional image features as the intersections or cross-sections of the HAR structures at different depths (Z) at the respective XY positions. In the case of vertically cylindrical memory HAR structures, the resulting first cross-sectional image features are circular or elliptical structures at various depths determined by the positions of the structures on the inclined cross-sectional surface 52. The memory stack extends in the Z direction perpendicular to the wafer surface 55. The thickness d or the minimum distance d between two adjacent cross-sectional image slices is adjusted to a value of typically an order of magnitude of several nm, such as 30 nm, 20 nm, 10 nm, 5 nm, 4 nm, or even smaller. Once a material layer of a predetermined thickness d is removed using the FIB, the next cross-sectional surface 53.i…53.J is exposed and can be accessed for imaging using the charged particle imaging beam 44. During repeated milling and imaging, a plurality of cross-sections are formed and a plurality of cross-sectional images are obtained such that a detection volume of size LX x LY x LZ is properly sampled and, for example, a 3D volume image can be generated. Thereby, the damage to the wafer is limited to the detection volume 160 plus the damaged volume in the y direction of length LYO. With a detection depth LZ of about 10 µm, the additional extension of the damaged volume in the y direction is typically limited to less than 20 µm. Figure 3 shows an example of a cross-sectional image slice 311 corresponding to a cross-section surface 52 generated by an imaging charged particle beam 44. The cross-sectional image slice 311 includes an edge line 315 between the bevel cross-section of the wafer and the surface 55 at the edge coordinate y1. Up to the edge, the image slice 311 shows several cross-sections 307.1…307.S of the HAR structures intersected by the cross-section surface 52. In addition, the image slice 311 includes cross-sections of several word lines 313.1 to 313.3 at different depths or z positions. Using these word lines 313.1 to 313.3, a depth map Z of the bevel cross-section surface 52 1 (x, y) can be generated. Also, after segmenting and annotating the cross-sectional image of the semiconductor object to be observed, the HAR channel cross-sections are identified and the properties of the HAR channel cross-sections are determined by machine learning methods. Examples are described in WO 2022 / 223229A1 and PCT / EP2022 / 082590, which are hereby incorporated by reference. Figure 4 illustrates an example of charging effects during milling and imaging. The same reference numerals as in Figures 2 and 3 are used, and reference is made to the description of Figures 2 and 3. Due to the exposure to the charged particle beam, local surface charges may be generated. The wafer 8 is typically made of a semiconductor material such as silicon and may include local isolation capacitances that can accumulate charges. Local charges may be generated on the surface or inside an insulator such as SiO 2 (silicon dioxide), and these local charges may adhere to the surface of the insulator or remain inside for a relatively long time. Local charges may also accumulate in the isolation conductors, which form the ability to collect charges. Locally trapped charges typically decay only slowly, for example due to thermal diffusion. These local charges may deflect or degrade the charged particle imaging beam 44 and, for example, cause local distortion or local focus shift, or may have an impact on the secondary or backscattered electron yield. Local charges may also deflect or degrade the ion beam 51 and cause the ion beam 51 to deflect away from the cross-section surface 53 during milling. As a result, the cross-section surface 53 may have a curved shape. Local charges can be generated by the charged particle imaging beam 44 as well as by using the ion beam 51. In both cases, positive and negative charging may occur, depending on the kinetic energy and material composition of the sample. FIG. 4a illustrates a milling step using an ion beam 51 with positive charged ion species (such as gallium, argon, or xenon ions). During milling, these positive ions may rip off electrons from the newly formed cross-sectional surface 53 and form a positive charge 91 at and below the next cross-sectional surface 53. FIG. 4b illustrates an imaging step using an electron beam 44. Depending on the landing kinetic energy of these primary electrons, fewer secondary electrons may be extracted from the cross-sectional surface 53 as the primary electrons are irradiating, thereby forming a local negative charge 93 near the surface 53. FIG. 5 illustrates the charging behavior of an electron beam imaging system. Generally, the charging of a surface or a capacitance depends on the material composition and the landing kinetic energy of these primary electrons. FIG. 5 shows a secondary electron yield (SEY) curve 61 for a first material composition and an SEY curve 62 for a second material composition. As the landing kinetic energy between the equilibrium points ELT and EHT, a positive charge is formed. As the landing kinetic energy between EHT2 and EHT1, for example, according to the SEY curves of these two material compositions, parts of the cross-sectional surface 53 may accumulate positive charges during the scanning and acquisition of a digital image of the cross-sectional surface 53, while other parts will accumulate negative charges. However, within a semiconductor wafer 8, only a limited number of material compositions exist, such as a first material composition with a secondary electron yield (SEY) curve 61 (with a maximum value at the high energy transition point EHT1), and a second material composition with a secondary electron yield (SEY) curve 62 (with a minimum value at the low energy transition point ELT2). According to a first embodiment of the disclosed content, a method for milling and imaging a plurality of cross-sectional surfaces to reduce the impact of charging effects is provided. According to the method, a series of milling and imaging of a plurality of cross-sectional surfaces includes the following steps: - milling (into a detection volume of a wafer 8) a cross-sectional surface 53 by an ion beam, including generating a surface charge (91, see FIG. 4a) using an ion beam 51, - scanning (by an electron beam 44) at least each segment of the cross-sectional surface 53 with primary electrons having a landing kinetic energy lower than a minimum low energy transition energy ELT2 or higher than a maximum high energy transition energy EHT1 of the material compositions present in a sample (see FIG. 5), thereby compensating for the surface charge generated during ion beam milling by forming a compensating negative surface charge (93, see FIG. 4b). In an example, ion beam milling and a first electron beam scan are performed in parallel. The electron beam current, scan frequency, and landing energy of the first electron beam scan can be adjusted to reduce the surface charge generated during ion beam milling below a predetermined threshold, such that the deflection angle of the ion beam 51 caused by the surface charge during milling is 70 milliradians (mrad) or less, for example 30 mrad or less, 10 mrad or less, or even 1 mrad or less. In an example, ion beam milling and electron beam scanning are performed sequentially. During a first electron beam scan, a positive surface charge 91 is reduced. Image capture is performed during a second electron beam scan. Thus, the electron beam 44 is not affected by any surface charge 91 generated during milling. The first and second electron beam scans can be performed using first and second parameters of the electron beam scanning operation, where the parameters include at least one of a landing energy, a scan frequency, and an electron beam current. Using the method according to the first embodiment, the surface bending of the cross-sectional surface 53 is minimized. Using the method according to the first embodiment, a high-quality electron beam image can be obtained while reducing the impact of the surface charge 91 generated during ion beam milling. According to a second specific embodiment of the disclosed content, a method for image capture to reduce the impact of charging effects is provided. An example of the method according to the second specific embodiment is illustrated in FIG. 6. FIG. 6 shows detection site 6.1 on the surface 55 of wafer 8. The cross-sectional surface 53 is milled by ion beam 51 and imaged by electron beam 44. The cross-sectional surface 53 and the wafer surface 55 form an edge 315. Generally, the wafer surface 55 and the cross-sectional surface are insulators or composed of insulating material compositions with large areas, such as pure silicon or silicon dioxide. According to the method, a large capacitance 71 is generated adjacent to the detection site 6.1. The capacitance 71 can be generated, for example, by the deposition of a coating of a metal layer. The capacitance 71 may also exist as a large capacitance within the structured wafer, such as formed by large metal interconnects or the like. During milling or image capture, the charged particle beam 44 is swept from the cross-sectional surface 53 to the capacitance 71. With the exposure using the charged particle imaging beam 44, the wafer surface 55 becomes conductive and the charges generated or accumulated on or below the surface 53 are guided to the capacitance 71. With the exposure using the charged particle imaging beam 44, a temporary guiding connection 73 is formed. Thus, the surface charges 91 or 93 flow through the temporary guiding connection or conduction region 73 to the capacitance of the charge sink 71. Thus, the surface charges 91 or 93 are reduced. An example of the method is further illustrated in FIG. 7. During the exposure using the charged particle beam 44, a local conduction region or guiding connection 73 is formed. The underlying physical principle is the excitation of valence electrons in the semiconductor, which is thus an inter-bandgap insulator. As the kinetic energy of the charged particles of the charged particle beam 44 exceeds the inter-bandgap of the order of 1 eV to 10 eV, electrons are kicked out of the valence band and thus hole conduction becomes possible. Thus, by sweeping the charged particle beam 44 over the wafer surface 55 with a landing kinetic energy higher than the inter-bandgap energy, a temporary guiding connection 73 is formed between the cross-sectional surface 53 and the charge sink 71. Thus, the local charges 91 or 93 flow to the charge sink 71. The charge sink 71 may be formed by a metal structure above the wafer surface 55 or by metal deposition in a trench. After the capacitance limit of the charge sink 71 is reached, the charge sink 71 can be connected to a large capacitance, such as an external capacitance, such as a ground level, via a contact pin 81. In the example of FIG. 7, the charge sink 71 is generated by the deposition of a certain volume of conductive material (such as metal). The volume can be milled into the wafer surface 55 in advance. The volume can also be provided as an additional layer on the wafer surface 55. In some examples, a large capacitance may already exist near the detection site 160, and this large capacitance can be used as the charge sink 71. Such large capacitances are formed, for example, by ground paths or voltage supply lines within the semiconductor wafer 8. FIG. 8 illustrates an example of the method according to the second specific embodiment. In step I, the detection site 6.i is adjusted by the wafer stage 155 at the intersection point 43 of the optical axis 42 of the charged particle imaging system (40) or the dual beam device 1, and the processes for slicing and imaging are determined and initialized. The process may include a partial registration of the coordinates at the detection site and the generation of alignment reference points. During step S0, the capacitance 71 is determined to be adjacent to the detection volume 160. If no capacitance 71 is present, the capacitance 71 is formed by the deposition of a metal volume, for example by the deposition of a tungsten layer. In an example, the capacitance 71 is formed in a groove first milled into the surface 55 of the wafer 8 by the ion beam 51. In step S1, a cross-sectional surface 53 is formed by ion beam milling into the surface 55 of the wafer. In step S2, the electron beam 44 is scanned between the milled area of the cross-sectional surface 53 and the capacitance 71 (see FIG. 7). Thereby, a local and temporary conduction region 73 is generated, which enables the flow of charge between the cross-sectional surface 53 and the capacitance 71. The flowing charge may alternatively be a positively charged "hole" or a negatively charged electron. Thereby, the surface charge generated during the ion beam milling is reduced. In step S3, an image of the cross-sectional surface 53 is formed by the imaging electron beam 44. During this imaging operation, the electron beam 44 is scanned over the cross-sectional surface 53. At each scan position, secondary electrons are emitted and collected by the detector 17.1 or 17.2 (see FIG. 1). According to each scan position, the signal corresponding to the collected secondary electrons depends on, for example, the material composition or surface topography at the surface 55 of the wafer 8. Generally, during step S3, a digital image of a segment of the surface (55) of the wafer 8 is formed. In an example, the cross-sectional surface 53 is equivalent to a segment of the surface 55 of the wafer 8. In step S4, the electron beam 44 is scanned between the area of the cross-sectional surface 53 and the capacitance 71 (see FIG. 7). Thereby, a local and temporary conduction region 73 is generated, which enables the flow of charge between the cross-sectional surface 53 and the capacitance 71. The flowing charge may alternatively be a positively charged "hole" or a negatively charged electron. Thereby, the surface charge generated during the imaging using the electron beam 44 is reduced. Step S4 may alternatively be performed after the image acquisition of the cross-sectional surface 53 is completed, or may be repeated several times during the image acquisition. For example, after each predefined sequence of, for example, 10 or 20 scan lines, the discharge operation according to step S4 may be interleaved between consecutive sequences of scan lines. In an optionally performed step S5, the probe 81 is connected to the capacitance 71 by an actuator (not shown), and a physical contact is formed. Thereby, the charge accumulated in the capacitance 71 is connected to the ground potential and removed from the capacitance 71. In step S6, depending on the detection task, it is evaluated whether the cross-sections of the required quantity N are milled and imaged. The milling step S1 and the imaging step S3 of the required quantity N may include N = 1, N = 100, or N = 1000, or more iterations. In step S7, the plurality of two-dimensional images of the plurality of N cross-sections are processed, and a detection result is determined. Image data processing may include at least one member of the group of data processing methods including: - Image registration, - Depth map determination, - Distortion compensation, including magnification adjustment, - Image processing methods, such as noise removal, contrast enhancement, image normalization, and threshold setting, - Three-dimensional volume image generation, - Feature detection and feature extraction, such as including template matching or machine learning algorithms, such as object detectors. Examples of step S7 are explained in more detail in WO2021 / 083581 A1, WO2021 / 083551 A1, WO2022 / 223229 A1, WO2021 / 180600 A1, which are incorporated by reference. As a final result, for example, the localization of target features of a plurality of target features, the relative localization of at least two target features, the size, shape, or area of the target features, the deviation, error, and statistical properties from the target features are determined and provided to the user. Figure 9 shows the result of step S7. In Figure 9a, the trajectory of the center coordinates of the HAR channel is shown. Each horizontal line corresponds to one contour 387 of the feature measured at the depth z inside the detection volume of the wafer. Thus, the HAR channel can be analyzed and, for example, the average inclination angle g of the average channel trajectory 363 is determined. Figure 9b illustrates the distribution of the measured radius r2 of a plurality of wafer samples. The radius r2 shows a significant drift on the wafer sample, which can be an indicator of process drift during the manufacturing process of the wafer. A wafer detection system configured to execute the method according to the specific embodiments is further illustrated in Figure 10. The wafer detection system 1000 includes a dual-beam system 1. The dual-beam system is illustrated in more detail in Figure 1 and reference is made to the description content of Figure 1. The basic features of the dual-beam system 1 are a first charged particle or FIB column 50 for milling and a second charged particle beam imaging system 40 for high-resolution imaging of the cross-section surface. The dual-beam system 1 includes at least one detector 17 for detecting secondary particles that can be electrons or photons. The dual-beam system 1 further includes a wafer support stage 15 configured to hold a wafer 8 during use. The wafer support stage 15 is position-controlled by a stage control unit 16 connected to the control unit 19 of the dual-beam system 1. The control unit 19 is configured with a memory and logic to control the operation of the dual-beam system 1. The wafer detection system 1000 further includes an operation control unit 2. The operation control unit 2 includes at least one processing engine 201, which can be formed by a plurality of parallel processors including a graphics processing unit (GPU) processor and a shared unified memory. The operation control unit 2 further includes a solid state drive (SSD) memory or a magnetic disk memory or a storage 203, which is used to store data, such as training data, a trained machine learning algorithm, and a plurality of cross-sectional images. The operation control unit 2 further includes a user interface 205, including a user interface display 400 and a user command device 401, which are configured to receive inputs from a user and display quotes or results to a user. The operation control unit 2 further includes a memory or a storage 219, which is used to store the process information of the image generation process of the dual-beam device 1 and is used to store software instructions that can be executed by the processing engine 201. The process information of the image generation process using the dual-beam device 1 can include, for example, a library of the equivalent effects during the image generation and a list of predetermined material contrasts. The software instructions include software for performing a method according to the first or second specific embodiment. The operation control unit 2 is further connected to an interface unit 231, which is configured to receive more commands or data from external devices or networks, such as computer-aided design (CAD) data. The interface unit 231 is further configured to exchange information, such as receiving instructions from external devices or providing measurement results to external devices, or storing a set of training data, a trained machine learning algorithm, or a plurality of cross-sectional images in an external storage. The detection system 1000 is configured to receive user information for performing the methods according to the specific embodiments, such as CAD information of the semiconductor object to be observed, the location of the detection site, or the detection result. The processing engine 201 is further configured to execute the method illustrated in FIG. 8 and described above. The processing engine 201 is configured to display information via the user display 400 and receive user inputs via the user interface 401. FIG. 11 illustrates a dual-beam device 1 of a wafer inspection system 1000 according to a third specific embodiment. FIG. 11a illustrates a cross-section of various parts of a scanning electron beam device 40 configured to capture an image of a cross-sectional surface 53 inside a detection volume 160. The two charged particle beams 44 and 51 of the dual-beam system 1 are arranged in the y-z plane. The cross-sectional surface 53 is generated by ion beam milling using an ion beam 51 that forms an angle GFE between 55° and 90° with the electron beam 44 in the y-z plane (see FIG. 1). The scanning electron beam device 40 includes a magnetic lens 82 having a coil 83 and a magnetic yoke 85, which forms a magnetic lens field in the region indicated by reference numeral 89. In this example, the magnetic lens is a radial gap lens, where the gap of the magnetic yoke 85 is parallel to the electron beam axis (parallel to the electron beam 44). Inside the magnetic lens 82, at least two multipole elements 87 are arranged and configured to scan and deflect the electron beam 44. For example, the at least two multipole elements 87 are connected to a control unit (not shown), which is further configured to supply a voltage or current to the multipole elements 87, configured to correct the aberration of the electron beam 44, and adjust the focal plane of the electron beam 44, for example, according to the inclination angle GFE between the cross-sectional planes 53 generated by the ion beam 51 at an angle GFE between 55° and 90° with respect to the electron beam. The dual-beam system 1 of the third specific embodiment further includes a gas flush head 171 having a local gas flush volume 181, which can be positioned above the wafer surface 55 at the location of the detection site 160. The gas flush head 171 can be connected, for example, to an actuator for positioning adjustment and for forming a retraction system (not shown). The gas flush head 171 is connected to a gas supply 177 having a valve 179. The gas flush head 171 further includes a first opening 173 for passing the scanning electron beam 44 during use; and a second opening 175 for passing the ion beam 51 during use. During use, the local gas flush volume 181 is enclosed by the gas flush head 171 and the wafer surface 55. During use, the flush gas is supplied by the gas supply 177 controlled by a control device connected to the valve 179. Thus, a high gas pressure of the flush gas is generated in the local gas flush volume 181. By including, for example, an ionized gas or a combination of ionized gases supplied by the gas supply 177, the local surface charge generated at the cross-sectional surface 53 during milling or imaging is neutralized by the ions of the ionized gas. The flush gas is preferably an inert gas, such as nitrogen (N 2), xenon (Xe), argon (Ar), neon (Ne), or helium (He). Thereby, deterioration such as deflection, local distortion, or local focus shift during ion beam milling or charged particle beam imaging is reduced. More details are disclosed in US 8,552,406 B2, which is incorporated herein by reference. Examples of the purge gas include gases from the gas group provided above, including ammonia, ammonium hydroxide, ammonium carbamate, bromine, chlorine, hydrazine, hydrogen peroxide, sodium amide, iodine, diiodoethane, isopropyl alcohol, methyl difluoroacetate, nitroethane, nitroethanol, nitrogen, dinitrogen tetroxide, nitrogen trifluoride, nitromethane, nitropane, nitrobutane, oxygen, ozone, PMCPS, tungsten hexacarbonyl, water, or xenon difluoride. However, other gases are also possible, such as methoxyacetyl chloride, methyl acetate, methyl nitroacetate, ethyl acetate, ethyl nitroacetate, propyl acetate, propyl nitroacetate, ethyl nitroacetate, methyl methoxyacetate, and acetic acid or thioacetic acid, hexafluoroacetylacetone, silazane, trifluoroacetamide, cobalt octacarbonyl, molybdenum hexacarbonyl, and combinations thereof. Using these first and second beams through openings 173 and 175, both of the two particle beams 44 and 51 of the dual beam system 1 are capable of entering and transmitting through the local gas purge volume 181. A cross-section in the x-y plane is shown in FIG. 11b. FIG. 12 illustrates another example according to the third embodiment. The same reference numerals are used as in FIG. 10, and reference is made to the description of FIG. 11. In the example of FIG. 12, the first beam through opening 173 and the second beam through opening 175 overlap and form a single beam passing through the opening in an elongated shape in the y direction and having a narrower dimension in the x direction. Using these two examples, the opening sizes of the beam through openings 173, 175 for the two charged particle beams are reduced, and the high gas pressure of the purge gas for charge neutralization can be maintained within the local gas purge volume 181 during use. The general scanning area of the charged particle beams 44 and 51 is about 10 µm. The general opening sizes of the beam through openings 173, 175 can be provided to be below 100 µm, such as below 80 µm, such as below 50 µm. Thereby, it is possible to scan the charged particle beam over the general scanning area, and the high purge gas pressure inside the local gas purge volume 181 is maintained. Using the method, the dual beam system, and the wafer inspection system 1000 configured to perform the method according to the disclosed content, deterioration such as deflection, local distortion, or local focus shift during ion beam milling or charged particle beam imaging is reduced. The method and the wafer inspection system 1000 can be used for quantitative metrology, but can also be used for defect detection, process monitoring, defect re-detection, and inspection of integrated circuits in semiconductor wafers. The present invention can be illustrated by the following examples: Example 1: An image acquisition method, comprising: - Image formation by scanning a charged particle imaging beam (44) over a segment of one surface (53, 55) of a wafer (8) to form a two-dimensional image of the segment of the surface (53, 55); - Scanning the charged particle imaging beam (44) between the segment of the surface (53, 55) and a capacitance (71) to effect a flow of charge from the segment of the surface (53, 55) to the capacitance (71), thereby generating a transient conduction region (73) between the segment of the surface (53, 55) and the capacitance (71). Example 2: The method of Example 1, comprising: - Ion beam milling a segment of one surface (53, 55) of a wafer (8) using an ion beam (51), wherein the ion beam (51) is disposed at an angle GFE with respect to the charged particle imaging beam (44). Example 3: The method of Example 2, wherein the step of generating a transient conduction region (73) by scanning the charged particle imaging beam (44) is performed during the ion beam milling step. Example 4: The method of any one of Examples 1 to 3, wherein the step of generating a transient conduction region (73) by scanning the charged particle imaging beam (44) is performed during the image formation step. Example 5: The method of any one of Examples 1 to 4, further comprising, before generating a transient conduction region (73) by scanning the charged particle imaging beam (44) between the segment of the surface (53, 55) and the capacitance (71), adjusting a kinetic energy of the charged particle imaging beam (44). Example 6: The method of any one of Examples 2 to 5, comprising, during the milling step, a step of compensating for milling-induced charging by scanning the charged particle imaging beam (44) over a segment of the cross-sectional surface (53) during ion beam milling. Example 7: The method of Example 6, further comprising, before compensating for milling-induced charging by scanning the charged particle imaging beam (44), adjusting a kinetic energy of the charged particle imaging beam (44). Example 8: The method of any one of Examples 2 to 7, comprising repeating the steps of image formation, generating a transient conduction region (73), ion beam milling, and compensating for milling-induced charging to acquire a plurality of two-dimensional images of a plurality of cross-sectional surfaces (53) and reduce the impact of charging during ion beam milling or image formation. Example 9: The method of any one of Examples 1 to 8, comprising: - Determining the capacitance (71) adjacent to the segment of the surface (53, 55) of the wafer (8). Example 10: The method of Example 9, wherein the step of determining the capacitance (71) comprises forming the capacitance (71) adjacent to the segment of the surface (53, 55) by deposition of a metal volume onto the surface (53, 55).Example 11: The method according to any one of Examples 1 to 10 further includes: - physically connecting a probe (81) to the capacitance (71) to remove the accumulated charge from the capacitance (71). Example 12: The method according to any one of Examples 1 to 11 further includes - providing a rinse head (171) above the surface (55) of the wafer (8), - providing a rinse gas from a gas supply (177) to the rinse head (171) to form a local gas rinse volume (181). Example 13: The method according to Example 12 includes selecting and providing the rinse gas configured to neutralize surface charges generated during image formation or ion beam milling. Example 14: The method according to Example 13, wherein the rinse gas is selected from the group including nitrogen (N. 2), a gas group of xenon (Xe), argon (Ar), neon (Ne), or helium (He). Example 15: The method according to any one of Examples 1 to 14, further comprising performing an image processing on one or more two-dimensional images including an image processing operation (selected from the operation group including image registration, depth map determination, distortion compensation, magnification adjustment, noise removal, contrast enhancement, image normalization, and threshold setting, three-dimensional volume image generation, feature detection, feature extraction, template matching, or machine learning object detector). Example 16: The method according to any one of Examples 1 to 15, further comprising adjusting a first detection site (6.1) of a wafer (8) at the optical axis (42) of the charged particle imaging system (40). Example 17: A method of ion beam milling a cross-sectional surface (53) into a wafer (8) using an ion beam (51), comprising, during the ion beam milling, compensating for milling-induced charging by scanning a charged particle imaging beam (44) over a segment of the cross-sectional surface (53) during the ion beam milling. Example 18: The method according to Example 17, further comprising adjusting a kinetic energy of the charged particle imaging beam (44) to reduce a surface charge generated during the ion beam milling. Example 19: The method according to Example 18, comprising adjusting the kinetic energy to be lower than a minimum low energy transition energy ELT2 or higher than a maximum high energy transition energy EHT1 of a material composition present at a detection site (6.i) of the wafer (8). Example 20: The method according to any one of Examples 17 to 19, further comprising - generating a temporary conduction region (73) between the cross-sectional surface (53, 55) and a capacitance (71) by scanning the charged particle imaging beam (44) between the cross-sectional surface (53) and the capacitance (71) to effect a flow of charge from the cross-sectional surface (53) to the capacitance (71). Example 21: The method according to Example 20, wherein the step of generating a temporary conduction region (73) by scanning the charged particle imaging beam (44) is performed during the ion beam milling. Example 22: The method according to Example 20 or Example 21, further comprising adjusting a kinetic energy of the charged particle imaging beam (44) before generating a temporary conduction region (73) between the cross-sectional surface (53) and the capacitance (71) by scanning the charged particle imaging beam (44). Example 23: The method according to any one of Examples 17 to 22, comprising: - determining the capacitance (71) adjacent to the segment of the cross-sectional surface (53). Example 24: The method according to Example 23, wherein the step of determining the capacitance (71) comprises forming the capacitance (71) adjacent to the cross-sectional surface (53) by depositing a metal volume onto the surface (55) of the wafer (8) and adjacent to the cross-sectional surface (53).Example 25: The method as in any one of Examples 17 to 24 further comprises: - physically connecting a probe (81) to the capacitance (71) to remove the accumulated charge from the capacitance (71). Example 26: The method as in any one of Examples 17 to 25 further comprises - providing a rinse head (171) above the surface (55) of the wafer (8), - supplying a rinse gas from a gas supply (177) to the rinse head (171) to form a local gas rinse volume (181). Example 27: The method as in Example 26 comprises selecting and providing the rinse gas configured to neutralize the surface charge generated during ion beam milling. Example 28: The method as in Example 27, wherein the rinse gas is selected from the group including nitrogen (N. 2), a gas group of xenon (Xe), argon (Ar), neon (Ne), or helium (He). Example 29: The method according to any one of Examples 17 to 28, further comprising - forming an image by scanning the charged particle imaging beam (44) over the cross-sectional surface (53) of a wafer (8) to form a two-dimensional image of the cross-sectional surface (53). Example 30: The method according to Example 29, wherein the step of generating a temporary conduction region (73) by scanning the charged particle imaging beam (44) is performed during the step of image formation. Example 31: The method according to any one of Examples 17 to 30, comprising repeating the steps of image formation, generating a temporary conduction region (73), ion beam milling, and compensating for milling-induced charging to capture a plurality of two-dimensional images of a plurality of cross-sectional surfaces (53) and reduce the impact of charging during ion beam milling or image formation. Example 32: The method according to Example 31, further comprising performing an image processing on the one or more two-dimensional images comprising image processing operations (selected from the operation group including image registration, depth map determination, distortion compensation, magnification adjustment, noise removal, contrast enhancement, image normalization, and setting a threshold, three-dimensional volume image generation, feature detection, feature extraction, template matching, or machine learning object detector). Example 33: The method according to any one of Examples 17 to 32, further comprising adjusting a first detection site (6.1) of a wafer (8) at the optical axis (42) of the charged particle imaging system (40). Example 34: A method of operating a dual-beam system (1), comprising: - providing a gas flushing head (171) at a detection site (6) adjacent to a surface (55) of a wafer (8), - supplying a flushing gas to the gas flushing head (171) via a gas supply (177) to form a local gas flushing volume (181), - guiding a charged particle imaging beam (44) through a first beam passage opening (173) of the gas flushing head (171), - guiding an ion beam (51) through a second beam passage opening (175) of the gas flushing head (171), - performing ion beam milling on a plurality of cross-sectional surfaces (53) at the detection site (6) into the surface (55) of the wafer (8) using the ion beam (51), - forming an image of a plurality of two-dimensional images of the cross-sectional surfaces (53) by scanning the charged particle imaging beam (44) over each cross-sectional surface (53). Example 35: The method according to Example 34, comprising selecting and providing the flushing gas configured to neutralize surface charges generated during ion beam milling or charged particle image formation. Example 36: The method according to Example 35, wherein the flushing gas is selected from including nitrogen (N 2), a gas group of xenon (Xe), argon (Ar), neon (Ne), or helium (He). Example 37: The method according to any one of Examples 34 to 36 further includes adjusting the detection site (6) of the wafer (8) at the optical axis (42) of the charged particle imaging system (40). Example 38: A dual-beam system (1) includes - a charged particle imaging system (40); - an ion beam system (50); - a sample stage (155) having a wafer support (15); - a control unit (2, 19) including a memory (219) that stores a set of instructions; and a processing engine (201) configured to execute the set of instructions to cause the dual-beam system (1) to perform any of the methods of Examples 1 to 37. Example 39: The dual-beam system (1) according to Example 38, wherein the ion beam system (50) generates a focused ion beam (51) including ions selected from a group including gallium ions, xenon ions, oxygen ions, neon ions, argon ions, and helium ions. Example 40: The dual-beam system (1) according to Example 38 or Example 39, wherein an optical axis (48) of the ion beam system (50) is set at an angle GFE with respect to the optical axis (42) of the charged particle imaging system (40), and wherein the angle GFE is between 30° and 80°. Example 41: A wafer detection system (1000) includes a dual-beam system according to any one of Examples 38 to 40. Example 42: A dual-beam system (1) includes - a charged particle imaging system (40), - an ion beam system (50), - a sample stage (155) having a wafer support (15), - a gas flushing head (171) disposed between the charged particle imaging system (40), the ion beam system (50), and a surface (55) of a wafer (8), wherein the gas flushing head (171) includes - a first beam passage opening (173) for guiding a charged particle imaging beam (44) through the gas flushing head (171), - a second beam passage opening (175) for guiding an ion beam (51) through the gas flushing head (171), and wherein the gas flushing head (171) is connected to a gas supply (177) for providing a flushing gas to the gas flushing head (171) during use to form a local gas flushing volume (181). Example 43: The dual-beam system (1) according to Example 42, wherein the gas supply (177) is configured to provide a gas selected from including nitrogen (N 2), a purge gas of a gas group of xenon (Xe), argon (Ar), neon (Ne), or helium (He). Example 44: A dual-beam system (1) as in Example 42 or Example 43, wherein the first beam passes through the opening (173) and the second beam passes through the opening (175) are spatially separated and are arranged at an angle corresponding to an angle GFE between the ion beam system (50) and the charged particle imaging system (40). Example 45: A dual-beam system (1) as in any one of Examples 42 or 44, wherein the gas purge head (171) is retractably mounted on an actuated mounting. Example 46: A dual-beam system (1) as in any one of Examples 42 or 45, wherein the ion beam system (50) generates a focused ion beam (51) comprising ions selected from a group comprising gallium ions, xenon ions, oxygen ions, neon ions, argon ions, and helium ions. Example 47: A dual-beam system (1) as in any one of Examples 42 or 46, further comprising a control unit (2, 19) comprising a memory (219) that stores a set of instructions; and a processing engine (201) configured to execute the set of instructions to cause the dual-beam system (1) to perform any one of the methods of Examples 1 to 37. However, the invention illustrated by examples and specific embodiments is not limited to the examples, but can be implemented by those skilled in the art by various combinations or modifications. 1: Dual-beam system 2: Operation control unit 4: Cross-sectional image feature 6: Detection site 8: Wafer 15: Wafer support stage 16: Stage control unit 17: Electron detector 19: Control unit 40: Charged particle beam (CPB) imaging system 42: Optical axis of the imaging system 43: Intersection point 44: Imaging charged particle beam 48: FIB optical axis 50: FIB column 51: Focused ion beam 52: Cross-sectional surface 53: Cross-sectional surface 55: Wafer top surface 61: First SEY exceeding kinetic energy 62: Second SEY exceeding kinetic energy 63: Low-energy equilibrium point 65: High-energy equilibrium point 71: Charge pile-up 73: Temporary guiding connection 75: Gap 79: Gas injection nozzle 81: Contact pin 82: Magnetic lens 83: Coil 85: Yoke 87: Multipole element 89: Magnetic lens field 91: Positive charge 93: Negative charge 155: Wafer stage 160: Detection volume 171: Flushing head 173: First beam passage opening 175: Second beam passage opening 177: Gas supply 179: Valve 181: Local gas flushing volume 201: Processing engine 203: Memory 205: User interface 219: Memory 231: Interface unit 307: Measured cross-sectional image of the HAR structure 309: Image segment 311: Cross-sectional image slice 313: Word line 315: Edge with the surface 317: Ring region of the HAR structure 318: Noise 320: Partially merged outer contour 321: Center positioning 325: Defect or deviation 327: Pixel-level annotation ring 363: Average HAR channel track 379: Contour gap 381: Initial contour proposal 383: Contour line of the first feature 385: Second contour proposal 387: Contour of the feature 391: Scaling vector 400: User interface display 401: User command device 1000: Wafer detection system I: Steps S0 - S7: Steps The present invention, illustrated by examples and specific embodiments, is not limited to these specific embodiments and examples, but can be implemented by those skilled in the art through various combinations or modifications of these specific embodiments and examples. The present invention will even be more fully understood with reference to the following drawings: FIG. 1 shows an exemplary diagram of a wafer detection or metrology system for 3D volume detection using a dual-beam device. FIG. 2 is an exemplary diagram of the slicing and imaging method for volume detection in a wafer. FIG. 3 illustrates an example of a cross-sectional image obtained by the slicing and imaging method FIG. 4 illustrates an example of charging effects during milling and imaging of a sample FIG. 5 illustrates charging effects related to the landing kinetic energy of primary electrons FIG. 6 illustrates a first example of a charge alleviation method FIG. 7 illustrates further details of the charge alleviation method FIG. 8 illustrates the charge alleviation method FIG. 9 shows the result of the detection FIG. 10 shows a wafer detection system according to an embodiment FIG. 11 illustrates a first example of a third embodiment FIG. 12 illustrates a second example of the third embodiment I: Step S0 - S7: Steps

Claims

1. A method for milling a cross-sectional surface (53) into a wafer (8) using an ion beam (51), comprising, during the ion beam milling, compensating for a milling-induced surface charge by scanning a charged particle imaging beam (44) over a segment of the cross-sectional surface (53) during the ion beam milling; and further comprising, adjusting a kinetic energy of the charged particle imaging beam (44) to reduce the milling-induced surface charge generated during the ion beam milling.

2. The method as described in claim 1, comprising adjusting the kinetic energy below a minimum low-energy transition energy ELT2 of the material composition present at a detection site (6) of the wafer (8) or above a maximum high-energy transition energy EHT1.

3. The method as described in claim 1 or claim 2 further comprises - achieving charge flow from one of the cross-sectional surfaces (53) to the capacity (71) by scanning the charged particle imaging beam (44) between the cross-sectional surfaces (53, 55) and a capacity (71), thereby creating a temporary conduction region (73) between the cross-sectional surfaces (53) and the capacity (71).

4. The method as described in claim 3, wherein the step of generating the temporary conductive region (73) by scanning the charged particle imaging beam (44) is performed during the ion beam milling.

5. The method as described in claim 1, comprising: - determining the capacity (71) adjacent to the cross-sectional surface (53).

6. The method as described in claim 5, wherein the step of determining the capacity (71) comprises forming the capacity (71) by depositing a metal volume onto the surface (55) of the wafer (8) and adjacent to the cross-sectional surface (53).

7. The method as described in claim 1 further comprises: - connecting a probe (81) to the capacity (71) entity to remove accumulated charge from the capacity (71).

8. The method as described in claim 1 further comprises - disposing a flushing head (171) above the surface (55) of the wafer (8), - supplying a flushing gas to the flushing head (171) by a gas supply (177) to form a local gas flushing volume (181).

9. The method as described in claim 8, comprising selecting and providing the flushing gas configured to neutralize the surface charge generated during ion beam milling.

10. The method as described in claim 9, wherein the flushing gas system is selected from a group of gases including hydrogen, nitrogen, xenon, argon, neon, or helium.

11. The method as described in claim 1 further comprises - image formation by scanning the charged particle imaging beam (44) on the cross-sectional surface (53) of the wafer (8) to form a two-dimensional image of the cross-sectional surface (53).

12. The method as described in claim 11, wherein the step of generating a temporary conduction region (73) by scanning the charged particle imaging beam (44) is performed during the image formation step.

13. The method as described in claim 1, comprising the steps of repeated image formation, generating a temporary conductive region (73), ion beam milling, and compensating for surface charge caused by the milling, to acquire multiple two-dimensional images of multiple cross-sectional surfaces (53) to reduce the impact of a charge during ion beam milling or image formation.

14. The method as described in claim 13 further comprises performing an image processing on one or more two-dimensional images containing image processing operations (selected from the group of operations including image registration, depth map determination, distortion compensation, magnification adjustment, noise removal, contrast enhancement, image normalization, and setting a threshold, three-dimensional volumetric image generation, feature detection, feature extraction, template matching, or machine learning object detector).

15. The method as described in claim 1 further includes adjusting one of the detection sites (6) of the wafer (8) at the optical axis (42) of the charged particle imaging system (40).

16. A method of operating a dual-beam system (1), comprising: - disposing a gas flushing head (171) at a detection site (6) adjacent to a surface (55) of a wafer (8); - supplying a flushing gas to the gas flushing head (171) via a gas supply (177) to form a local gas flushing volume (181); - guiding a charged particle imaging beam (44) through a first beam passage opening (173) of the gas flushing head (171); - guiding an ion beam (51) through a second beam passage opening (175) of the gas flushing head (171); - using the ion beam (51) to perform ion beam milling on a plurality of cross-sectional surfaces (53) at the detection site (6) into the surface (55) of the wafer (8); - forming a plurality of two-dimensional images of the cross-sectional surfaces (53) by scanning the charged particle imaging beam (44) on each of the plurality of cross-sectional surfaces (53).

17. The method as described in claim 16, comprising selecting and providing the flushing gas configured to neutralize surface charges generated during ion beam milling or charged particle imaging.

18. The method as described in claim 17, wherein the flushing gas system is selected from a group of gases including hydrogen, nitrogen, xenon, argon, neon, or helium.

19. The method as described in any one of claims 16 to 18 further comprises adjusting the detection site (6) of the wafer (8) at the optical axis (42) of the charged particle imaging system (40).

20. A dual-beam system (1) comprising: - a charged particle imaging system (40); - an ion beam system (50); - a sample stage (155) having a wafer support stage (15); - a control unit (2, 19) including a memory (219) storing a set of instructions; and a processing engine (201) configured to execute the set of instructions to cause the dual-beam system (1) to perform any of the methods of request 1 to request 19.

21. The dual-beam system (1) as described in claim 20, wherein the ion beam system (50) generates a focused ion beam (51) containing ions selected from a group including gallium ions, xenon ions, oxygen ions, neon ions, argon ions, and helium ions.

22. The dual-beam system (1) as described in claim 20 or claim 21, wherein one optical axis (48) of the ion beam system (50) is set at an angle GFE with the optical axis (42) of the charged particle imaging system (40), and wherein the angle GFE is between 30° and 80°.

23. A wafer inspection system (1000) comprising a dual-beam system as described in any one of claims 20 to 22.

24. A dual-beam system (1) comprising - a charged particle imaging system (40), - an ion beam system (50), - a sample stage (155) having a wafer support stage (15), - a gas flushing head (171) disposed between the charged particle imaging system (40), the ion beam system (50), and a surface (55) of a wafer (8), wherein the gas flushing head (171) comprises - a first beam through opening (173) for guiding a charged particle imaging beam (44) through the gas flushing head (171), - a second beam through opening (175) for guiding an ion beam (51) through the gas flushing head (171), and wherein the gas flushing head (171) is connected to a gas supply (177) for supplying a flushing gas to the gas flushing head (171) during use to form a local gas flushing volume (181).

25. The dual-beam system (1) as claimed in claim 24, wherein the gas supply (177) is configured to provide a flushing gas selected from a group of gases including hydrogen, nitrogen, xenon, argon, neon, or helium.

26. The dual-beam system (1) as described in claim 24 or claim 25, wherein the first beam through opening (173) and the second beam through opening (175) are spatially separated and are angularly set corresponding to an angle GFE between the ion beam system (50) and the charged particle imaging system (40).

27. The dual-beam system (1) as described in claim 24, wherein the gas flushing head (171) is retractable and mounted on a moving mount.

28. The dual-beam system (1) as described in claim 24, wherein the ion beam system (50) generates a focused ion beam (51) containing ions selected from a group including gallium ions, xenon ions, oxygen ions, neon ions, argon ions, and helium ions.

29. The dual-beam system (1) as described in claim 24 further includes a control unit (2, 19) including a memory (219) storing a set of instructions; and a processing engine (201) configured to execute the set of instructions to cause the dual-beam system (1) to perform any of the methods described in claims 1 to 19.

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