Calibration method for a particle imaging apparatus and method for operating a calibrated particle imaging apparatus

Abstract

Several calibration methods for particle imaging systems are proposed that allow the determination of lateral focus shifts as a function of focusing changes or refocusing. Distortions in particle-optical images generated with appropriately calibrated systems can thus be reduced, and metrological investigations in these images, and especially in 3D tomography, can be performed with greater accuracy. With appropriately calibrated particle imaging systems, 3D samples, such as deep channels in semiconductor samples, can be measured more precisely. Distortion effects arising from sample geometries in electrostatic immersion fields can also be calibrated and thus corrected.

Description

Field of invention

[0001] The invention relates to particle imaging devices for generating particle-optical images. Specifically, the invention relates to a calibration method for a particle imaging device and a method for operating a calibrated particle imaging device. State of the art

[0002] Particle beam systems with particle beam columns, such as electron beam columns or ion beam columns, are known from the prior art. An example is a scanning electron microscope, in which a focused electron beam scans a region of an object to be examined, and secondary electrons or backscattered electrons generated by the incident electron beam on the object are detected as a function of the deflection of the focused particle beam in order to generate or calculate an electron microscopic image of the scanned region of the object.

[0003] Basically, the primary particle beam is generated by a beam generator with a particle source, passes through beam-shaping elements such as a condenser lens, a stigmator, or other beam-shaping devices, and is then focused onto the object under investigation by an objective lens. To achieve high resolution in the particle beam column or particle beam microscope, the particle beam must be focused as precisely as possible on the object; that is, the area illuminated by the focused particle beam on the object's surface (the "beam spot") should be as small and round as possible. For this purpose, the particle beam microscope, with its particle-optical components, is adjusted so that the image plane, onto which the particle source is imaged by the optical system, coincides with the surface of the object.This can be achieved, with the object positioned at a given distance from the objective lens, by changing the focus of the particle beam microscope until the beam spot on the object's surface is as small as possible. The focus of the particle beam microscope can be changed, for example, by altering the excitation of the objective lens and / or by changing the kinetic energy of the particles in the particle beam as they pass through the objective lens.

[0004] Furthermore, the area illuminated by the focused particle beam on the object's surface should be as round as possible. To achieve this, it is often necessary to correct imaging errors of the particle beam system using stigmatizers. Electrostatic multipole electrodes or magnetic multipoles, for example, can be used for such astigmatism correction.

[0005] After adjusting the focus of the particle beam microscope in this way, further measures are usually necessary to improve the quality of the beam focus on the object's surface. This includes adjusting the particle beam so that it passes through the objective lens essentially centrally. The underlying principle is that lens aberrations of a particle-optical lens become more pronounced the further the beam passes through the lens from its optical axis. Since the objective lens typically provides a large portion of the refractive power required to image the particle source onto the object's surface, it is advantageous, with a view to reducing the imaging errors involved in this process, to adjust the beam relative to the objective lens so that it passes through the lens as centrally as possible. For this adjustment, the particle beam system, or...The particle beam column, according to the prior art, includes, for example, one or more deflection devices for shifting the particle beam within the objective lens, which are arranged in the beam path between the particle beam source and the objective lens. By changing the excitation of this deflection device or devices, it is possible to shift the location within a principal plane of the objective lens where the center of the particle beam intersects the principal plane. This also applies analogously to objective lens systems, for example, to a system with a combination of a magnetic objective lens and an electrostatic objective lens.

[0006] Correctly adjusting the particle beam is not easy. Several methods for this already exist according to the state of the art.

[0007] It is known, for example, to use particle beam microscopes to record images at at least two different focus settings for adjustment purposes. Based on an analysis of these recorded or calculated images, the optimal excitation of the deflection device can then be set. This is based on the following consideration: When the particle beam passes centrally through the objective lens and is focused on the surface of the object, the recorded particle beam image is essentially sharp. If, starting from this setting, the focus is slightly changed and a particle beam image is recorded at this altered setting, it will be only slightly less sharp compared to the previously recorded particle beam image and otherwise essentially the same.However, if the two images, captured with different focuses, are taken using a particle beam that does not pass centrally through the objective lens, the two images will differ not only in sharpness but also in position. Changing the focus causes the second image to be shifted or offset relative to the first. Therefore, prior art employs a so-called "wobble method" to adjust the particle beam. In this method, the focus is periodically changed while images are continuously captured. A user observes the captured images, which shift back and forth if the beam is not properly adjusted, and adjusts the excitation of the deflection device(s) until all the resulting images are essentially stationary.This is a manual process that requires a certain level of experience and is therefore time-consuming. Automated adjustment methods also exist.

[0008] Particle beam systems, and in particular particle imaging devices, are also used for process control, for example in semiconductor manufacturing. With the continuous development of increasingly smaller and more complex microstructures such as semiconductor devices, there is a need for the further development and optimization of planar fabrication techniques and inspection systems for the production and inspection of small microstructures. The development and fabrication of semiconductor devices, for example, requires verification of the design of test wafers, and planar fabrication techniques require process optimization for reliable, high-throughput production. Furthermore, there is a growing demand for the analysis of semiconductor wafers for reverse engineering and for the customer-specific, individual configuration of semiconductor devices.Therefore, there is a need for inspection tools that can be used with high throughput to examine the microstructures on wafers with high accuracy.

[0009] Typical silicon wafers used in the production of semiconductor devices have diameters of, for example, 300 mm. Each wafer is divided into approximately 30 to 60 repeating regions ("dies") with a size of up to 800 mm. 2Semiconductor devices are subdivided into modules, each containing at least one integrated circuit pattern, for example, for a memory chip or a processor chip. A semiconductor device comprises several semiconductor structures fabricated in layers on the surface of a wafer using planar integration techniques. Due to the manufacturing processes, semiconductor wafers typically have a flat surface. The feature size of the integrated semiconductor structures ranges from a few micrometers to critical dimensions (CDs) of 5 nm, and in the near future, feature sizes will become even smaller; future feature sizes or critical dimensions (CDs) below 3 nm, for example, 2 nm, or even below 1 nm, are expected. At the aforementioned small feature sizes, defects of the critical dimension must be identified quickly over a very large area.For several applications, the specification requirement for the accuracy of a measurement provided by an inspection instrument is even higher, for example by a factor of two or an order of magnitude. For example, the width of a semiconductor feature must be measured with sub-1 nm accuracy, such as 0.3 nm or even less, and the relative position of semiconductor structures must be determined with a sub-1 nm superposition accuracy, such as 0.3 nm or even less.

[0010] Semiconductor structures are among the most delicate structures created by humans and are subject to various defects. Systems for quantitative 3D metrology, defect detection, or fault inspection search for these defects. Fabricated semiconductor structures rely on prior knowledge. Semiconductor structures are produced from a series of layers that run parallel to a substrate. For example, in a sample of an integrated circuit pattern, the metal traces in the metal layers run parallel to each other, or so-called HAR structures (high aspect ratio structures) and metal feedthroughs run perpendicular to the metal layers. The angle between the metal traces in different layers is either 0° or 90°. In contrast, VNAND-type structures are known to have a circular cross-section on average.

[0011] During manufacturing, semiconductor wafers undergo approximately 1000 process steps, and within the wafer, roughly 100 or more parallel layers are formed. These layers comprise the transistor layers, the conduction core layers, and the interconnect layers, as well as, in the case of memory devices, a multitude of 3D arrays of memory cells. The dimensions, shapes, and arrangements of the semiconductor structures and patterns are subject to various influences. Currently, etching and deposition are the critical processes in the fabrication of 3D memory devices. Other involved process steps, such as lithographic exposure or implantation, also influence the properties of the integrated circuit elements.

[0012] The aspect ratio and the number of layers in integrated circuits are constantly increasing, and the structures are growing into the third (vertical) dimension. The current height of memory stacks already exceeds a dozen micrometers, and this trend is continuing. In contrast, the size of the features is constantly decreasing. The minimum feature size, or critical dimension, is below 10 nm, for example, 7 nm or 5 nm, and will soon approach feature sizes below 3 nm. As the complexity and dimensions of semiconductor structures grow into the third dimension, the lateral dimensions of integrated semiconductor structures are becoming ever smaller. Therefore, it is becoming increasingly difficult to measure the shape, dimensions, and orientation of features and patterns in 3D and their superposition with high precision.

[0013] With the increasing demands on the resolution of particle imaging systems in three dimensions, the inspection and 3D analysis of integrated semiconductor circuits on wafers is becoming ever more challenging. The lateral measurement resolution of charged particle systems is typically limited by the scanning grid of the individual pixels or the residence time per pixel on the sample and the diameter of the charged particle beam. The scanning grid resolution can be set within the imaging system and adapted to the diameter of the charged particle beam on the sample or object. The typical grid resolution is 2 nm or less, but the grid resolution limit can, in principle, be reduced without physical limitations. The diameter of the charged particle beam has a limited size, which depends on the operating conditions of the charged particle beam and the lens used.The beam resolution is limited by approximately half the beam diameter or by the beam's full width at half maximum (FWHM). The resolution can be below 2 nm, e.g., even below 1 nm.

[0014] A common method for generating 3D tomography data of semiconductor samples on the nanometer scale is the so-called slice and image approach, which is implemented, for example, with a dual particle beam system. Dual particle beam systems, with a charged particle beam column for imaging and a FIB column for milling, are operated at the so-called coincidence point, meaning that the optical axis of the charged particle beam, the optical axis of the FIB, and the sample surface meet at a single point (coincide). This is advantageous because it ensures that the imaging actually occurs at the sample area affected by milling.

[0015] A sectioning and imaging method is described, for example, in WO 2020 / 244795 A1. According to the method described in WO 2020 / 244795 A1, a 3D volume inspection is performed on an inspection sample taken from a semiconductor wafer. This method has the disadvantage that a wafer must be destroyed to obtain a block-shaped inspection sample. On the other hand, the method has the advantage that the working distance between a charged particle imaging device, such as a scanning electron microscope (SEM), more precisely its column, and a sample or wafer to be imaged can be comparatively small, especially if the two columns are arranged perpendicular to each other and the optical axis of the SEM is perpendicular to the sample surface. The resolution of a particle imaging device is generally better the shorter the working distance.

[0016] An alternative method for generating 3D tomography data of semiconductor samples on the nm scale is described in WO 2021 / 180600 A1. According to the document, the disadvantage of having to destroy a wafer is overcome by a different geometric arrangement of a milling or FIB column on the one hand and an imaging column with charged particles on the other. This arrangement allows the coincidence point to be navigated across the entire wafer without destroying it. The so-called wedge cut approach utilizes the sectioning and imaging technique with an oblique cutting angle into the surface of a semiconductor wafer. However, due to geometric limitations in the arrangement of the two columns relative to each other, the working distance of the imaging column to the sample or wafer being imaged is greater compared to the approach described in WO 2020 / 244795 A1.This results in a slightly lower image resolution.

[0017] The demand for ever-increasing memory chip capacities within essentially the same chip area is leading to the design of 3D memory architectures. In many cases, these 3D architectures consist of a large number (>50) of alternating insulating and conductive layers stacked into piles 5-10 µm high or even higher. Deep channels are etched into these layers and then filled with insulating and conductive material. In some chip architectures, such stacks are fabricated and then stacked on top of each other to achieve even higher memory chip capacities on the same chip area.

[0018] A critical step in the fabrication of such structures is controlling the etching of these deep channels: Dry etching chambers are designed for unidirectional etching of the entire wafer, but the deeper the etching, the more critical even slight deviations from the desired etching direction become. In 3D memory architectures, even a slight deviation from the desired etching direction leads to undesirable channel shifts on the wafer, which must be controlled to ensure, for example, the alignment of the memory cells with the underlying chip structures. The deeper the etching, the more serious this problem becomes. Therefore, semiconductor manufacturers are interested in measuring this channel tilt across the wafer.

[0019] When measuring channel inclination using a sectioning and imaging technique, for example with a dual system comprising an electron beam column for imaging and an ion beam column for material removal (FIB-SEM system), image distortion control over a height range of 5–10 µm or even higher is required. The lateral position of the imaging particle beam must be known to within a few nanometers over the entire height range. While the position of the imaging or inspecting particle beam on the surface of a sample under inspection is known via position markers or so-called fiducials, this is not automatically the case for the lateral position of the charged particle beam when focusing into the sample.

[0020] US 2004 / 0173746A1 discloses investigations of a sample in a so-called tilting mode. Tilting a sample surface relative to a scanning electron beam can be achieved either mechanically or electronically. US 2004 / 0173746A1 proposes combining these two tilting methods to increase resolution and minimize aberrations. The phenomenon or problem of lateral focus shift during a focus change is not addressed in the publication.

[0021] US 5,894,124 A discloses an investigation of samples that are tilted relative to a scanning particle beam. Tilting the sample surface generates a lateral component of an electric field. This component is corrected by a deflecting electric field generated by deflection electrodes. The phenomenon or problem of a lateral focus shift when the focus is changed is not addressed in the publication.

[0022] US 4 728 799 A discloses a correction of height measurements in the course of calibrating a lithography system and is to be attributed to the technological background. Description of the invention

[0023] It is therefore the object of the present invention to provide a method by which inclinations, such as channel inclinations, in a semiconductor sample or in a sample of an integrated circuit can be determined more precisely. In particular, it should be possible to obtain a more accurate knowledge of the position of an imaging charged particle beam, even at depth within a sample, and this position should be controllable.

[0024] The problem is solved by the subject matter of the independent patent claims. Advantageous embodiments of the invention are described in the dependent patent claims.

[0025] The invention is based on the finding that in some particle imaging devices, a lateral beam offset can occur when the focus is changed. This offset can be so large that distortions appear in particle-optical images, and especially in 3D tomography, preventing a highly accurate determination of structural or channel inclination. The inventors have therefore investigated the causes of these distortions in more detail. These causes include, among others, a lateral beam offset when the focus changes due to minimal misalignment of the focusing particle-optical lenses, and effects occurring in an electrostatic immersion field in connection with the geometry of the sample being inspected or the wedge-shaped section.

[0026] According to the invention, several calibration methods for particle imaging devices are therefore proposed, which allow the determination of a lateral focus shift as a function of a focusing change or refocusing of a particle imaging device. Distortions in particle-optical images produced with correspondingly calibrated particle imaging devices can thus be reduced, and metrological investigations in the particle-optical images can be carried out with greater accuracy. Distortion effects occurring due to sample geometries in electrostatic immersion fields can also be calibrated and thus corrected.

[0027] According to a first aspect of the invention, it relates to a calibration method for a particle imaging apparatus. In a first method step (1a), a particle imaging apparatus is provided which operates with at least one charged particle beam. The charged particles can be, for example, electrons, positrons, muons, ions, or other charged particles. The particle imaging apparatus can be configured in various ways, for example, as a particle microscope, a SEM, a TEM, a STEM, a SEM-STEM, a dual beam system, a multi-beam particle microscope, a mask repair system, or in other ways.

[0028] In process step (1b), a test object is provided, wherein the test object has a test structure on its surface. This test structure has geometric dimensions that are precisely known, for example, a specific pattern of mutually orthogonal line structures. The test structures of the test object can be manufactured with high accuracy and thus serve as a reference object. The test object is essentially a 2D test object.

[0029] In process step (1c), the test object is positioned in an object plane of the particle imaging apparatus within the particle-optical beam path of the particle imaging apparatus such that a surface normal of the test object forms an angle θ1 with the particle-optical axis of the particle imaging apparatus. This angle θ1 can be 0°, but it does not have to be. With an angle θ1 of 0°, the at least one charged particle beam would strike the test structure of the test object at right angles, or telecentrically.

[0030] In a process step (1d), a first focus z1 of the particle imaging apparatus is set such that the at least one particle beam of the particle imaging apparatus is focused at least pointwise onto the surface of the test object. If the angle θ1 = 0°, the first focus z1 is set so that the focus is not only pointwise, but at several points and, in particular, at all points of the test object to be scanned later, onto the surface of the test object.

[0031] In a process step (1e), the test structure is scanned or probed with the particle beam at the first (fixed) focus z1, and a first particle-optical image of the test structure is generated. For the purposes of this application, a particle-optical image is understood to be an image generated by detecting particle radiation. A particle detector is used to detect the particle radiation, and it is possible to combine a particle detector with an optical detector. The detected particles can be, for example, electrons or other charged particles. It is possible, for instance, to detect secondary electrons, backscattered electrons, mirror electrons, or mirror ions for generating the particle-optical image. The particle-optical images are images in which the intensity of the detected particle radiation is represented as a function of an image position for a pixel.

[0032] In a procedural step (1f), a first position P1 of the test structure is determined in the first particle-optical image. This first position P1 of the test structure can denote the overall position of the test structure. However, it is also possible that the first position P1 of the test structure denotes only one of several positions of a part of the test structure. Furthermore, it is of course possible that several first positions P1 of the test structure are determined, from which, for example, the overall position P1 of the test structure can then be derived.

[0033] In a process step (1g), the test object is arranged in the particle-optical beam path of the particle imaging apparatus such that a surface normal of the test object forms a second angle θ2 with the particle-optical axis of the particle imaging apparatus, wherein this second angle θ2 is different from the first angle θ1, i.e., θ2 ≠ θ1. Preferably, the second angle θ2 is an angle other than 0°. This preferably represents a true inclination of the surface of the test object relative to the particle-optical axis of the particle imaging apparatus.

[0034] In one process step (1h), the focusing is changed and a second focusing z2 of the particle imaging apparatus is set such that a particle beam of the particle imaging apparatus is focused at least pointwise onto the surface of the test object, whereby this second focusing z2 is different from the first focusing z1, i.e. z2 ≠ z1.

[0035] In a process step (1i) the test structure is scanned with the particle beam in the second fixed focusing z2 and a second particle-optical image of the test structure is generated.

[0036] In one process step (1j), a second position P2 of the test structure is determined in the second particle-optical image. The test structure is thus imaged multiple times, at different focuses and angles. The principle for determining the second position P2 of the test structure in the second particle-optical image is the same as that for determining the first position P1 of the test structure in the first particle-optical image.

[0037] In one procedure step (1k), a positional shift d21 is determined between the second position P2 of the test structure in the second particle-optical image and the position P1 of the test structure in the first particle-optical image. Of course, it is possible that several positional shifts d21 of different substructures or characteristic points of the test structure are determined. However, at least one positional shift d21 is determined.

[0038] In a process step (1l), a focus shift fs of the particle beam in a direction orthogonal to the particle-optical axis Z is determined as a function of the focusing change Δz = z2 - z1. This determination of the focus shift fs is based on the determined lateral position shift d21 of the test structure and the angular difference Δθ = θ2 - θ1. The focus shift fs of the particle beam in a direction orthogonal to the particle-optical axis Z is therefore a lateral focus shift. This lateral focus shift is responsible for distortions that occur when the focusing changes. Since the test structure(s) of the test sample, which serves as a reference sample, are precisely known, the lateral focus shift fs can be deduced from the measured position shifts d21.According to a preferred embodiment of the invention, the focus shift fs is determined based on a discrepancy between an expected positional shift d21' of the test structure and the actual positional shift d21 of the test structure determined in step (1k). The angular difference Δθ = θ2 - θ1 is known, which allows conclusions to be drawn about the expected positional shift d21' of the test structure. Deviations from this are attributable to the lateral focus shift fs.

[0039] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, where x1 ≠ x2. Preferably, the two directions x1 and x2 are orthogonal to each other and can also be referred to as the x-direction and y-direction in a Cartesian coordinate system. In the context of this patent application, however, a y-direction, particularly in the case of a tilted test sample or specimen, denotes the direction in which the specimen is tilted. Therefore, we initially refer to two directions x1 and x2 in which the focus shift fs is determined.

[0040] According to a preferred embodiment of the invention, the first angle θ1 = 0°. This allows for a particularly simple determination of the focus shift fs.

[0041] According to a preferred embodiment of the invention, process steps (1h) to (1l) are repeated for further focusing or for several further focusings. Thus, the corresponding process steps are repeated for further focusings z3, z4, z5, and so on, and corresponding position shifts d31, d41, d51, and so on are determined. This allows for a further determination of the lateral focus shift fs as a function of different focusing changes or focusing settings. In this way, the particle imaging apparatus can be calibrated for large focusing changes, which are necessary for the inspection of 3D features such as channel structures in semiconductor samples. For larger focusing changes Δz, a lateral focus shift fs generally does not grow linearly with z, so that for several focusings z3, z4, z5, etc.,In each case, an exact determination of the lateral focus shift fs and its subsequent correction are necessary.

[0042] According to a preferred embodiment of the invention, the focus is changed incrementally by a constant value dz.

[0043] According to a preferred embodiment of the invention, the following relationship applies to the constant value dz: 1 µm ≤ dz ≤ 50 µm. Depending on the focus ranges or focus variations over which a particle imaging apparatus is to be adjustable, the incremental value dz can be selected accordingly. For example, a total focus variation range comprises approximately 100 µm, e.g., approximately + / - 50 µm around a fixed coincidence point in dual particle beam systems.

[0044] According to a preferred embodiment of the invention, the test object is arranged on a ministage with an adjustable tilt angle, and the method further comprises tilting the ministage, in particular to adjust the angle θ2. With an untilted ministage, the angle θ1 can, for example, be 0°.

[0045] According to a further preferred embodiment of the invention, the calibration method according to the invention is performed multiple times for different angles θ. This can be achieved particularly easily with the described ministage. The method can be performed, for example, for 3, 4, 5, 10 or even more angles θi. This allows the overall accuracy of the calibration method to be increased.

[0046] In the calibration method according to the first aspect of the invention, the scanning or scanning of the test structure is carried out with a fixed, i.e., constant, focus. However, there are also particle imaging devices that, when used with obliquely cut or tilted test objects, employ a variable focus adapted to the oblique or tilted position in order to achieve sharp focus at virtually any point on the surface of the test object, even with obliquely or tilted samples (adaptive focusing).

[0047] According to a second aspect of the invention, a calibration method for such a particle imaging apparatus is provided, comprising the following steps: In a first process step (2a), a particle imaging apparatus is provided which operates with at least one charged particle beam. Otherwise, the particle imaging apparatus is as described in connection with the first aspect of the invention.

[0048] In a process step (2b), a test object is provided, wherein the test object has a test structure on its surface. Regarding the details of a test object or a test structure, reference is made to what has already been stated in connection with the first aspect of the invention.

[0049] In a process step (2c) the test object is arranged in an object plane of the particle imaging apparatus in the particle-optical beam path of the particle imaging apparatus such that a surface normal of the test object forms a first angle θ1 = 0° with the particle-optical axis of the particle imaging apparatus.

[0050] In a process step (2d) a first focusing z1 of the particle imaging apparatus is set such that the particle beam of the particle imaging apparatus is focused on the surface of the test object.

[0051] In a process step (2e), the test structure is scanned with the at least one particle beam in the fixed first focusing z1, and a first particle-optical image of the test structure is acquired. Up to this point, the calibration procedure according to the second aspect of the invention can be identical to process steps (1a) to (1e) according to the first aspect of the invention, provided that the angle θ1 is 0° there.

[0052] In a process step (2f) the test object is arranged in the particle-optical beam path of the particle imaging apparatus such that a surface normal of the test object forms a second angle θ2 with the particle-optical axis of the particle imaging apparatus, which is different from the first angle θ1, where θ2 ≠ θ1.

[0053] In one process step (2g), the test structure is scanned with the particle beam using a variable focus Δz, which is adapted to the respective scanning position Rij of the particle beam, and a second particle-optical image of the test structure is acquired. In other words, the variable focus Δz is adapted to the tilt of the test object, so that (at least without lateral focus shift fs) the scanning particle beam would theoretically be focused onto the surface of the test object at every scanning position Rij.

[0054] In one process step (2h), a distortion of the test structure in the second particle-optical image relative to the first particle-optical image is determined. This determination of the distortion can be based on the identification of positional changes of the test structure or parts thereof, or on deviations from a regularity of the test structure. In principle, several methods exist for this, which are familiar to those skilled in the art.

[0055] In a process step (2i), a focus shift fs of the particle beam is determined in a direction orthogonal to the particle-optical axis Z as a function of a focusing change Δz in the direction of the particle-optical axis Z, or in the depth direction, whereby the determination of the focus shift fs is based on the determined distortion of the test structure and on the angle θ2. The focusing change Δz corresponds to the variable focusing Δz that is known for a respective raster position Rij of the particle beam or has been adaptively set for it.

[0056] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, where x1 ≠ x2. In particular, the two directions x1 and x2 can be orthogonal to each other. One of the directions can coincide with a y-direction in which the test object is tilted.

[0057] According to a preferred embodiment of the invention, the test object is arranged on a ministage with an adjustable tilting angle, and the method further comprises tilting the ministage to adjust the angle θ2.

[0058] According to a preferred embodiment of the invention, the method is carried out multiple times for different angles θ or θ2.

[0059] According to a preferred embodiment of the invention, the following relationship applies to the edge length L of an image field FOV (corresponding to the English abbreviation "Field of View") scanned with the particle beam: L ≥ 10 µm, preferably L ≥ 20 µm, and most preferably L ≥ 50 µm. In this way, a sufficiently large change in focus Δz can be represented during the scanning process. Furthermore, a sufficiently large area of ​​the test object is represented, which enables a sufficiently accurate determination of the distortion that occurs.

[0060] According to a third aspect of the invention, it relates to a calibration method for a particle imaging apparatus, comprising the following steps: In process step (3a), a particle imaging apparatus is provided. The particle imaging apparatus comprises a particle source for generating a particle beam with charged particles, the particle source being operated at an accelerating voltage EHT. Furthermore, the particle imaging apparatus includes at least two focusing lenses through which the particle beam passes. One lens is a magnetic objective lens, and the other is an electrostatic lens. The electrostatic lens is arranged downstream of the magnetic objective lens in the direction of the particle-optical beam path. The magnetic objective lens and the electrostatic lens focus the particle beam onto an object at a distance z from the magnetic objective lens.The particle imaging apparatus also includes a deflection unit designed to deflect the particle beam towards the center of the electrostatic lens. Furthermore, the apparatus includes a stage or holder designed to hold an object at a working distance WD from the magnetic objective lens. The apparatus also includes a detection unit for detecting interaction particles emanating from the object, for example, for the detection of secondary electrons. Finally, the apparatus includes a control unit for operating the particle imaging apparatus. This control unit is designed to drive the particle source to provide the accelerating voltage EHT. The control unit is also designed to drive the magnetic objective lens, for example, by varying the current in a coil of the magnetic objective lens.Furthermore, the control system is designed to actuate the deflection unit, allowing the particle beam to be deflected precisely to the center of the electrostatic lens, which serves to adjust the particle imaging apparatus. In addition, the particle imaging apparatus is adjusted in such a way that, independent of energy, a change in the accelerating voltage (EHT) has no effect on the beam position upon impact with the object. With such an adjustment, a change in the accelerating voltage (EHT) only results in a change in the beam diameter on the object; thus, the image sharpness of the recorded particle-optical image is altered, but not the beam position.

[0061] Alternatively, it is theoretically possible to reverse the order of the electrostatic lens and the magnetic objective lens. In this case, the particle beam would first be adjusted so that it passes centrally through the magnetic objective lens, and then the focus would be adjusted using only the electrostatic lens. However, in practice, this approach results in somewhat lower image quality.

[0062] Then, in a process step (3b), an object is provided at a distance z. This object can again be a test object or a test sample with regular structures, but it can also be another object with a characteristic feature, for example with a position marker on a sample surface.

[0063] In one step (3c) a reference feature of the object is located.

[0064] In one step (3d), a first particle-optical image is captured by means of focusing z. The focusing z here corresponds exactly to the working distance WD, which is why the captured particle-optical image is in principle sharply captured.

[0065] In a process step (3e) a position P1 of the reference feature is determined in the first particle-optical image.

[0066] In process step (3f), the accelerating voltage EHT is changed by an offset ΔV. It is possible to know in advance, at least approximately, which offset ΔV causes which change in the focusing Δz, e.g., from theoretical calculations or from a previous calibration. This makes it possible to choose the value of the offset ΔV appropriately.

[0067] In one process step (3g), the particle imaging apparatus is refocused by Δz by changing the control signal of the magnetic objective lens. As described, changing the accelerating voltage EHT by the offset ΔV results in the particle beam no longer being focused, but rather imaged indistinctly onto the object. However, the position of the particle beam upon impact with the object remained unchanged. Refocusing the particle imaging apparatus, or refocusing the particle beam by Δz by changing the control signal of the magnetic objective lens, can, however, lead to a change in the position of the particle beam on the object if the magnetic objective lens and the electrostatic lens are not precisely aligned with each other; the latter is practically never the case with the highest degree of accuracy.When refocusing the particle imaging apparatus by Δz by changing the control of the magnetic objective lens, the electrostatic lens is preferably controlled unchanged. Any lateral displacements or focus shifts fs are therefore solely attributable to the changed control of the magnetic objective lens, which enables calibration.

[0068] In process step (3h) a second particle-optical image is taken in the setting of the particle imaging apparatus refocused by Δz.

[0069] In process step (3i) a position P2 of the reference feature is determined in the second particle-optical image.

[0070] In process step (3j), a positional shift d21 between the second position P2 of the reference feature and the first position P1 of the reference feature is determined. It is of course also possible to determine multiple positional shifts of multiple reference features.

[0071] In process step (3k), a focus shift fs of the particle beam is determined in a direction orthogonal to the particle-optical axis Z as a function of the refocusing Δz using the magnetic objective lens, whereby the determination of the focus shift fs is based on the determined position shift d21 of the reference feature. In particular, the position shift d21 can correspond exactly to the focus shift fs.

[0072] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis, wherein x1 ≠ x2. In particular, the two directions x1 and x2 can again be orthogonal to each other.

[0073] According to a preferred embodiment of the invention, process steps (3f) to (3k) are repeated for different offsets ΔV of the accelerating voltage EHT. This enables more precise calibration as well as calibration over the largest possible range Δz.

[0074] According to a preferred embodiment of the invention, the offset ΔV is changed incrementally by a constant value dV. This allows the calibration process to be carried out particularly easily and automatically.

[0075] According to a preferred embodiment of the invention, the electrostatic lens is an electrostatic objective lens. In particular, it is possible for this electrostatic lens to be provided by supplying a potential at an end region of a beam tube within the magnetic objective lens (so-called "Gemini lens"). The pole pieces of the magnetic objective lens are each at ground potential, and preferably the object under investigation is also at ground potential. Thus, an electrostatic immersion field is not used.

[0076] According to an alternative embodiment of the invention, the electrostatic lens achieves its lensing effect by means of a potential difference between the object on the one hand and the magnetic objective lens on the other. This can be achieved, for example, by placing the object at a non-zero potential while the magnetic objective lens is at ground potential. For example, a sample voltage of -4 kV can be applied to a wafer if the charged particles of the particle beam are electrons.

[0077] However, it is also possible that the electrostatic lens is designed differently according to the third aspect of the invention. It is also possible to combine the two embodiments of the electrostatic lens described above.

[0078] According to a fourth aspect of the invention, it relates to a calibration method for a particle imaging apparatus, comprising the following steps: In a first process step (4a), a particle imaging apparatus is provided, which operates with at least one charged particle beam and can operate in a first and a second operating mode. In the first operating mode, the particle imaging apparatus operates with a first depth of field TS1 and a first resolution A1, and in the second operating mode, it operates with a second depth of field TS2 and a second resolution A2. The first depth of field TS1 is greater than the second depth of field TS2. Furthermore, the first resolution A1 is lower than the second resolution A2. The depth of field TS is defined in each case as the defocus df at which the diameter of a beam waist (starting from the smallest, best possible beam diameter in the focal plane) increases by 10%.Generally, there is an optimal aperture angle at which the beam diameter is minimal and the resolution is optimal. The beam diameter increases for both larger aperture angles (deterioration due to lens aberrations) and smaller aperture angles (deterioration due to diffraction effects). Within the scope of the invention, a smaller aperture angle is chosen, at least temporarily, to achieve a better depth of field, resulting in an increased beam diameter and thus a less than optimal resolution for image acquisition.

[0079] In process step (4b), a wedge-shaped test object with 3D structures is provided, the section being defined by an angle GF to the surface of the test object. The 3D structures can be, for example, channel structures extending into the depth of the test object over a large distance, for example, more than 10 µm, more than 50 µm, or more than 100 µm. The wedge-shaped test object can be generated before process step (4b) or during process step (4b). According to a preferred embodiment of the invention, providing the wedge-shaped test object with the 3D structures includes ablating the test object with the 3D structures in a wedge shape using a focused ion beam (FIB).

[0080] In process step (4c), the test object is arranged relative to the particle imaging apparatus such that the particle-optical axis Z of the particle imaging apparatus and the uncut surface of the test object are orthogonal to each other, with the 3D structures at different depths z of the test object exposed in the direction of the particle-optical axis Z of the particle imaging apparatus. Ideally, the direction of the 3D structures coincides with, or substantially coincides with, the particle-optical axis Z.

[0081] In process step (4d), the wedge-shaped test object is scanned in the first operating mode with a fixed focus setting z1 = z1ij, generating a first particle-optical image of the test object with the 3D structures. The imaging is thus performed with a large depth of field TS1, whereby the different arrangement of the 3D structures with respect to height or depth z is relatively insignificant for the imaging. The imaging of the 3D structures in the particle-optical image shows no distortion due to any focus changes; it only varies slightly with regard to the sharpness of the depicted 3D structures.

[0082] In process step (4e) the positions P1 of the 3D structures are determined in the first particle-optical image.

[0083] In process step (4f), the wedge-shaped test object is scanned in the second operating mode with adaptive focusing z2ij, which is adapted to the respective raster position Rij of the particle beam, and a second particle-optical image of the test object with the 3D structures is generated. The adaptive focusing z2ij describes a focusing process in which the beam is focused sharply, i.e., with minimal beam waist, on specific raster positions of the test object. The scanning and generation of the second particle-optical image thus takes place with a shallower depth of field TS2, but with a better resolution A2, or in other words, with a larger numerical aperture NA2.

[0084] In the process step (4g) the positions P2 of the 3D structures are determined in the second particle-optical image.

[0085] In process step (4h), the positions P1 and P2 are compared with each other, and the respective displacements d21 = P2 - P1 of the 3D structures are determined. These displacements d21 are the result of a lateral focus shift fs of the particle beam during the respective focusing change Δz = z2ij - z1.

[0086] In process step (4i) a lateral focus shift fs of the particle beam is determined in a direction orthogonal to the particle-optical axis Z as a function of a focusing change Δz = z2ij - z1, wherein the determination of the focus shift fs is based on the determined displacements d21 of the 3D structures.

[0087] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, wherein x1 ≠ x2.

[0088] According to a preferred embodiment of the invention, process steps (4b) to (4i) are repeated, in particular multiple times, for different angles GF. The process is then repeated for several differently wedge-shaped test objects with 3D structures. It is particularly possible to ablate a first wedge-shaped section again, so that a different wedge-shaped test object with a different angle GF is produced. For this purpose, for example, a focused ion beam (FIB), particularly according to the "sectioning and imaging" method, can be used.

[0089] According to a preferred embodiment of the invention, the following relationship applies to a ratio of first depth of field TS1 and second depth of field TS2: 5 ≤ TS1 / TS2 ≤ 30. Additionally or alternatively, the following relationship applies to a ratio of first resolution A1 and second resolution A2: 2 ≤ A1 / A2 ≤ 6.

[0090] According to a fifth aspect of the invention, this relates to a calibration method for a particle imaging apparatus. In a first process step (5a), a particle imaging apparatus is provided which operates with at least one charged particle beam and which can operate in a first operating mode and in a second operating mode. In the first operating mode, the object to be imaged is not arranged in an electrostatic immersion field, while in the second operating mode, the object to be imaged is arranged in an electrostatic immersion field. This can be achieved, for example, by configuring a control system for the particle imaging apparatus to apply, or switch on, or switch off an electrical potential on the object to be imaged or on a sample stage holding the object.

[0091] In process step (5b) a first wedge-shaped test object with 3D structures is provided, the section being defined by a first angle GF1 to the surface of the test object.

[0092] In process step (5c) the test object is arranged relative to the particle imaging apparatus such that the particle-optical axis Z of the particle imaging apparatus and the uncut surface of the test object are, in particular, essentially orthogonal to each other, wherein the 3D structures are exposed at different depths Z of the test object in the direction of the particle-optical axis of the particle imaging apparatus.

[0093] In process step (5d) the first wedge-shaped test object is scanned with a calibrated particle beam in the first operating mode without an electrostatic immersion field and a first particle-optical image of the test object with the 3D structures is generated.

[0094] In process step (5e) the positions P1 of the 3D structures are determined in the first particle-optical image.

[0095] In process step (5f) the first wedge-shaped test object is scanned with a calibrated particle beam in the second operating mode with a first electrostatic immersion field and a second particle-optical image of the test object with the 3D structures is generated.

[0096] In the process step (5g) the positions P2 of the 3D structures are determined in the second particle-optical image.

[0097] In the process step (5h) the positions P1 and P2 are compared with each other and the respective displacements d21 = P2 - P1 of the 3D structures are determined.

[0098] In the procedure step (5i) a lateral focus shift fs in a direction orthogonal to the particle-optical axis Z is determined as a function of the strength of the electrostatic immersion field, whereby the determination of the focus shift fs is based on the determined displacements d21 of the 3D structures.

[0099] In this embodiment of the invention, the lateral focus shift fs is based on small bends of equipotential lines of the electrostatic field at the wedge-shaped surface of the test object, or results from the geometry of the wedge-shaped test object. The lateral focus shift is solely due to this effect. A lateral focus shift resulting from an imprecise alignment of successive particle-optical lenses has already been corrected or calibrated beforehand. Therefore, the scanning in steps (5d) and (5f) is performed with a previously calibrated particle beam or a particle imaging apparatus calibrated in this respect.

[0100] According to a preferred embodiment of the invention, the lateral focus shift fs is determined for a plurality of grid positions Rij. This is advantageous because the shape of the wedge-shaped test object is defined along the respective grid positions. The grid spacing can be selected with varying degrees of fineness. It must be fine enough so that the geometric distortions and potential lines on the surface of the wedge-shaped test sample can be accurately reproduced during the screening process.

[0101] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, wherein again x1 ≠ x2.

[0102] According to a preferred embodiment of the invention, process steps (5f) to (5i) are carried out repeatedly, in particular multiple times, each time with a different strength of the electrostatic immersion field. It is possible for the strength of the electrostatic immersion field to be changed incrementally, for example over a range from about 500 V / mm to about 4000 V / mm.

[0103] According to a preferred embodiment of the invention, the process steps (5b) to (5i) are repeated, in particular several times, each time for wedge-shaped test objects of different geometries. The determination of the focus shift fs in a direction orthogonal to the particle-optical axis Z is then carried out as a function of the geometry of the wedge-shaped test object.

[0104] According to a preferred embodiment of the invention, the angle GF is varied for each wedge-shaped test object of different geometries. Additionally or alternatively, the maximum cutting depth T is varied for each wedge-shaped test object of different geometries. Additionally or alternatively, the maximum cutting width B is varied for each wedge-shaped test object of different geometries. In this way, it is possible to determine the separate influence of different geometric parameters on the focus shift fs. Calibration can thus be performed very precisely. According to an advantageous embodiment of the invention, the different parameters are varied separately and sequentially, and their influence on the focus shift fs is determined separately.

[0105] The following applies to all calibration methods of the first to fifth aspects of the invention: According to a preferred embodiment of the invention, the particle imaging apparatus is one of the following: a particle microscope, a SEM, a TEM, a STEM, a SEM-STEM, a dual particle beam system, a multi-beam particle microscope, or a mask repair system. This list is not exhaustive. The described lateral focus shift can occur in all of the aforementioned particle imaging apparatuses, either due to a less than 100% accurate alignment of successive particle-optical lenses or due to electrostatic or magnetic immersion fields on an object or sample surface.

[0106] According to a preferred embodiment of the invention, the particle imaging apparatus comprises a magnetic objective lens and an electrostatic objective lens. This objective lens system can, in particular, operate according to the so-called "Gemini principle".

[0107] In particular, an end region of a beam tube that protrudes into the magnetic objective lens can be at an electrical potential, for example at a potential of several kV.

[0108] According to a preferred embodiment of the invention, the electrostatic objective lens is arranged or formed within the pole shoes of the magnetic objective lens.

[0109] According to a sixth aspect of the invention, it relates to a method for operating a particle imaging apparatus. The particle imaging apparatus can be one of the particle imaging apparatus described above in several embodiments.

[0110] In a first process step (6a) a particle imaging apparatus is provided which operates with at least one charged particle beam.

[0111] In a process step (6b) the particle imaging apparatus is calibrated with respect to a focus shift fs of the particle beam in a direction orthogonal to the particle-optical axis Z as a function of a focusing change Δz or refocusing Δz of the particle imaging apparatus, in particular the calibration is carried out according to one of the calibration methods as described above in accordance with the first to fourth aspects of the invention.Additionally or alternatively, the particle imaging apparatus is calibrated with respect to a focus shift fs of the particle beam based on the strength of an electrostatic immersion field at an object to be imaged and / or based on a geometry of an object to be imaged in an electrostatic immersion field, wherein this calibration can, but does not have to, be carried out in particular according to a calibration method that has been described in connection with the fifth aspect of the invention.

[0112] In one process step (6c), at least one particle-optical image of an object is generated using the particle imaging apparatus. This prevents distortion in the particle-optical image due to lateral focus shift, which offers particular advantages in metrological applications.

[0113] According to a preferred embodiment of the invention, a plurality of mutually parallel oriented cross-sectional images through a 3D structure are generated with the calibrated particle imaging apparatus, and the method further comprises the following steps: (6d) Generating a 3D volume image from the cross-sectional images; and (6e) Determining a tilt of the 3D structure based on the 3D volume image. The calibrated particle imaging apparatus may preferably be a dual particle beam system, which operates, for example, using a system consisting of a particle microscope and a focused ion beam (FIB), for example, using the sectioning and imaging method.

[0114] According to a preferred embodiment of the invention, the 3D structure comprises a NAND structure.

[0115] It is possible to combine different embodiments of the invention, including different aspects of the invention, provided that this does not create any technical contradictions.

[0116] The invention will be better understood with reference to the accompanying figures. These show: Fig. Figure 1: schematically shows a particle imaging apparatus using a particle microscope as an example; Fig. Figure 2: schematically shows a section of a particle imaging apparatus with an objective lens system comprising a magnetic objective lens and an electrostatic objective lens; Fig. Figure 3: schematically shows a section of a particle imaging apparatus with an electromagnetic immersion field on the object; Fig. 4: schematically illustrates a focus shift caused by particle-optical lenses that are not precisely aligned with each other; Fig. Figure 5: schematically illustrates the occurrence of a lateral focus shift when the focus is changed in the z-direction; Fig. 6: schematically illustrates a focus shift caused by an electrostatic immersion field on a wedge-shaped sample; Fig. 7: schematically illustrates problems in determining the inclination of a 3D structure due to an existing focus shift; Fig. Figure 8: schematically shows different arrangements of a rehearsal table and a ministage; Fig. Figure 9: schematically shows a scanning of a sample with different sample arrangements; Fig. 10: schematically shows a flowchart for a calibration method according to the invention; Fig. Figure 11: schematically shows a scanning of a sample with different sample arrangements and tilt-adapted focusing; Fig. 12: schematically shows a flowchart for a calibration method according to the invention; Fig. Figure 13: schematically shows a focus shift during refocusing; Fig. 14: schematically shows a flowchart for a calibration method according to the invention; Fig. 15: schematically shows a dual particle beam system and the 3D volume images produced with it; Fig. 16: schematically shows a dual particle beam system that operates using the sectioning and imaging method with a wedge-shaped section; Fig. 17: schematically illustrates different depths of field for an imaging particle beam; Fig. 18: schematically shows a flowchart for a calibration method according to the invention; Fig. 19: schematically illustrates positional shifts of obliquely cropped 3D structures in two different recording modes in a perspective view and in a top view; Fig. 20: schematically shows a flowchart for a calibration method according to the invention; Fig. 21: schematically shows various wedge-shaped test objects; and Fig. Figure 22: schematically shows a flowchart for a procedure for operating the particle imaging apparatus.

[0117] Fig. Figure 1 schematically shows a particle imaging apparatus 100 using the example of a particle microscope in the form of a scanning electron microscope. The scanning electron microscope 100 has a particle source 1 that generates an electron beam 122, wherein the electron beam 122 passes through a condenser lens 4, an xy stigmator 24, a beam deflector 25, and an objective lens 11, so that the electron beam 122 is focused onto the surface of an object 8 or a sample 8, the sample or object 8 being held by a sample stage 15. The working distance WD between the lower end of the objective lens 11 and the surface of the sample 8 is also shown.

[0118] In the example shown, the condenser lens 4 is a magnetic condenser lens with a pole shoe 21 and a coil 23, which is excited by means of the control 20.

[0119] The xy-stigmator 24 is an electrostatic stigmator with a plurality of, for example, eight electrodes arranged around the electron beam 122, the electrodes being controlled or excited by the controller 20. In the example shown, four of these electrodes form an x-stigmator, and four other electrodes of these eight form a y-stigmator. Both the x-stigmator and the y-stigmator can generate a quadrupole field by means of their respective four electrodes. Alternatively, the xy-stigmator can have eight coils for generating a magnetic field, each of which is controlled by the controller 20 to generate a quadrupole field for the x-stigmator or the y-stigmator, respectively. The xy-stigmator 24 thus provides, in a single component, the combined functions of both an x-stigmator and a y-stigmator to influence, adjust, or correct the astigmatism of the particle beam 122.

[0120] The beam deflector 25 can be a magnetic or an electrostatic beam deflector, which in turn is controlled by the controller 20 to scan the point of impact of the particle beam 122 across the surface of the object 8. An electron detector 17 is provided in the example shown to detect secondary electrons and backscattered electrons emanating from the point of impact of the electron beam 122 on the object surface 8. The controller 20 is configured to correlate measured electron intensities, obtained by the electron detector 17, with the points of impact of the particle beam 122 on the object surface 8 according to the state of the beam deflector 25, in order to acquire a particle-optical image of the object 8 or its surface.

[0121] The objective lens 11 comprises a pole shoe 29 and a coil 31, which can be controlled or excited by means of the control unit 20. Additionally, the particle beam column in the example shown includes an electrostatic objective lens, the lensing effect of which is activated by applying a voltage to an end region of the beam tube 10 (the beam tube is in Fig. (Figure 1, only partially shown) is generated inside the magnetic objective lens 11. The applied voltage is controllable by the controller 20. The pole pieces 29 of the objective lens 11 are preferably at a different potential, in particular at ground potential. The condenser lens 4 and the objective lenses 10, 11 focus the particle beam 122 such that the minimum beam diameter, and thus the beam focus, is imaged in focus at the working distance WD from the objective lens 11, provided that the x-stigmator and the y-stigmator are adjusted so that the beam cross-section has a circular cross-section in the region of the beam focus. A sharp particle-optical image can be recorded precisely when the surface of the object 13 coincides with the beam focus, i.e., when the object 13 is at the correct distance from the objective lens 11.

[0122] The sample stage 14 includes an actuator (not shown) which can also be controlled by the controller 20. The actuator is configured to adjust the position of the object 13 or its surface in the z-direction or in the direction of the particle beam 122. Therefore, the controller 20 can either vary the excitation of the condenser lens 4 and the objective lens 11 or the objective lenses 10, 11, or adjust the position of the object 8 in the z-position by means of the actuator in order to create the beam focus precisely on the object surface.

[0123] Fig. Figure 2 schematically shows a section of a particle imaging apparatus 100 with an objective lens system comprising a magnetic objective lens 11 and an electrostatic objective lens 10. Furthermore, in Fig. Figure 2 shows the optical analogue of the electrostatic objective lens with reference numeral 10' and the optical analogue of the magnetic objective lens with reference numeral 11'. The effect of the magnetic lens can be controlled by the control unit 20 by appropriately exciting the coil 31. At the opening of the grounded pole shoes 29, the magnetic field emerges from the pole shoes and exerts its particle-optical effect, indicated by 11'. The electrostatic lens is formed by a high potential applied to the end region of the beam tube 10, while the sample or object 8, for example a wafer, is at ground potential, i.e., U = 0 V, in the example shown. In the example shown, the voltage applied to the end region of the beam tube 10 is 8 kV, but it can also be more or less; typically, however, it is in the kV range.

[0124] In the example shown, the magnetic field of the magnetic objective lens 11 and the electrostatic field of the electrostatic objective lens 10 are each located within the objective lens, which is a physical object. During operation, a charged particle beam 122 passes sequentially through first the magnetic objective lens 11' and then the electrostatic objective lens 10'. Due to their design, these two particle-optical lenses are never perfectly aligned with each other. Slight misalignments of the particle-optical lenses relative to each other can contribute to a lateral focus shift fs when the focus of the magnetic objective lens 11 is changed. For adjustment purposes, the control of the electrostatic objective lens 10' is periodically varied.When a particle beam passes centrally through the electrostatic lens 10', this periodic change in the electrostatic field only alters the sharpness of the image, but not the position of the particle beam on the object 8. During this adjustment process, the magnetic field strength of the magnetic objective lens 11, 11' is normally not changed. However, the strength of the magnetic field of the magnetic objective lens 11 is changed at a later time when a different working distance WD or a different focus z is to be set with the particle imaging apparatus 100. In this context, if the particle-optical lenses 10', 11' of the objective lens system are not adjusted or aligned with 100% precision, a lateral focus shift fs can occur, which can cause problems, especially in metrological applications and particularly when examining 3D objects.This can cause distortions in particle-optical images or in a 3D volume image. In principle, the arrangement of electrostatic objective lens 10' and magnetic objective lens 11' can also be reversed. That is, the particle beam would be adjusted so that it passes centrally through the magnetic objective lens 11'. However, this is in . Fig. The arrangement shown in Figure 2 is somewhat better, since image errors tend to depend more on the electrostatic objective lens 10' than on the magnetic objective lens 11', so that the particle beam is optimized for the electrostatic objective lens.

[0125] Fig. Figure 3 schematically shows a section of a particle imaging apparatus 100 with an electromagnetic immersion field at the object 8. In the illustrated embodiment, the magnetic objective lens 11 is the only objective lens. The pole pieces 29a, 29b of the magnetic objective lens 11 are grounded in the example shown, i.e., at ground potential, U = 0 V. However, a negative voltage is applied to the sample 8 in the example shown, e.g., U S = -4 kV. An electrostatic lens is thus formed between the lower pole piece 29b and the sample surface 8, the optical analogue of which is in Fig. 3 is shown with the reference numeral 10'. In this embodiment with an electrostatic immersion field, two particle-optical lenses are also successively provided: first the magnetic objective lens 11 or its optical analogue 11', and then the electrostatic lens or its optical analogue 10'. Here, too, it is possible that the two particle-optical lenses 11', 10' are not aligned with absolute precision, meaning their lens centers or axes do not coincide exactly. It is also possible to use the embodiments of Fig. 2 and Fig. to combine 3 together. At the end of beam tube 10, for example, 8 kV would then be present, and at the sample, for example, a voltage of U would be applied. S = -4kV applied.

[0126] Fig. 4 shows for the in the Fig. 2 and Fig. The three illustrated examples show the focus shift fs that can occur in each case: First, the axis A1 is shown, which runs centrally through the magnetic objective lens 11 or its optical analogue 11'. Also shown is the corresponding axis of the electrostatic lens or its optical analogue 10', labeled A2. These two axes are shifted parallel to each other in the example shown. The axis A2 also corresponds to the particle-optical axis Z of the particle imaging apparatus 100. A particle beam 122 moving along the axis A2 does not pass centrally through either lens, which is why it is deflected in each case. This displacement manifests itself after passing through the lens system 11', 10' as the lateral focus shift fs.

[0127] Fig. Figure 5 schematically illustrates the occurrence of the lateral focus shift fs when the focus is changed in the z-direction: In Fig. Figure 5a shows the optimal case, in which no focus shift fs occurs when the focus z is changed by + / - dz: Instead, the foci f1, f2, and f3 simply shift along the z-axis when the focus is changed. In contrast, the real case, which is shown in Fig. As shown in Figure 5b: When the focus z is changed by + / - dz, the foci f1, f2, and f3 shift not only along the z-direction but also laterally in a y-direction. The position in the y-direction depends on the respective focus z. It should be noted that the lateral focus shift fs occurs within a plane orthogonal to the particle-optical axis Z; therefore, the total focus shift fs only coincides with the y-direction randomly or by definition. Fig. 5b together.

[0128] Fig. Figure 6 schematically illustrates a focus shift fs caused by an electrostatic immersion field on a wedge-shaped section of sample 8. The wedge-shaped section can be created, for example, with a focused ion beam (FIB). The section angle GF relative to the planar surface of sample 8 is also shown in the example. The total section depth is T, and the width of the section B is also shown. In the example shown, a voltage of -1000 V is applied to sample 8. The equipotential lines 123.1, 123.2, 123.3, and 123.4 are shown above the sample surface. These are slightly drawn into the wedge-shaped section or do not run parallel to the otherwise planar outer surface of object 8. Accordingly, charged particle beams 122 are deflected laterally as they approach the angled sample surface. During the example shown, the particle beam 122...Since particle 1 strikes sample 8 practically perpendicularly, the charged particle beam 122.2 is deflected to the left in the example shown; its point of impact is shifted slightly laterally. This lateral focus shift fs2 is shown in . Fig. Figure 6 is also shown. The third single-particle beam 122.3, however, is deflected in the opposite direction shortly before reaching the sample surface, resulting in a lateral focus shift fs3 to the right in the example shown. The depicted lateral focus shift fs is therefore solely the result of the electrostatic immersion field applied to the sample. The effect itself depends on the geometry of the section of sample 8 and the voltage applied to the sample.

[0129] Fig. Figure 7 schematically illustrates problems in determining the inclination of a 3D structure due to an existing lateral focus shift fs. Fig. Figure 7a shows a first 3D structure 60 extending in the depth direction, or z-direction. A first particle beam 122.1 experiences a lateral focus shift fs when its focus changes into the depth. Without considering this lateral focus shift fs, one would assume, in corresponding particle-optical images or a volume image of the 3D structure 60, that the 3D structure 60 is tilted. In reality, however, it is not tilted. The opposite case is shown in Figure 7a. Fig. 7b: Here too, for the exemplary charged particle beam 122.2, a lateral focus shift fs occurs when the depth focusing z is changed. Although the 3D structure 61 is actually inclined, without knowledge of the lateral focus shift fs, one would conclude that the 3D structure 61 is straight or not inclined. This exemplifies the importance of calibrating a lateral focus shift fs as a function of a focusing or focusing change dz.

[0130] Fig. Figure 8 schematically shows the arrangement of a sample 8, 28. In Fig. 8a A wafer 28 is arranged on a sample stage 15. It lies flat on the sample stage 15. For smaller samples 8, a ministage 18 is additionally provided, which, in addition to height adjustment, also has a tilting adjustment for arranging a sample 8 in a sample chamber. This is shown in Fig. 8b and Fig. 8c illustrated. In Fig. Figure 8b shows a sample 8 on a ministage 18, which is not tilted. The sample surface therefore extends in the y-direction, the z-direction corresponds to the sample normal. Fig. In 8c, sample 8 is tilted on the ministage 18 by the angle θ. Thus, the height z in the sample surface changes along the y-direction. These various adjustment options can be utilized in a calibration procedure according to the invention:

[0131] Fig. Figure 9 schematically shows a scanning of a sample 8 in different arrangements of the sample 8. A precisely known test structure, whose dimensions are exactly known, is present on the sample surface 8a. Two-dimensional test samples of this type already exist. Therefore, these types of test samples can be used for calibration purposes.

[0132] In Fig. In 9a, sample 8 is static, meaning the normal to the sample surface 8a) corresponds to the z-direction. Furthermore, the height of the sample surface does not change in the y-direction. The exemplary single-particle beams 121.1, 121.2, and 121.3 each strike the sample surface 8a at the same height z, focused into position. Fig. In 9b, however, sample 8 with its sample surface 8a is tilted by the angle θ. If one now scans along the direction y, the particle beam only focuses on the sample surface 8a at the y-position y3. Structures of the sample that are recorded at positions y1 and y2 become blurred. Nevertheless, if the tilt angle θ and the structure of the surface 8a are known, a position of the test structure can generally be determined. Now, in the example of Fig. 9b If the depth focus in the z-direction is changed, but the angle θ remains unchanged, then any displacements of the test structure observed after scanning are due to a lateral focus shift fs. This can be repeated for different depth focuses z, if necessary, e.g., for z1, z2, and z3.

[0133] Fig. Figure 10 schematically shows a flowchart for a calibration method according to a first aspect of the invention.

[0134] In a first process step S1, a particle imaging apparatus 100 is provided, which operates with at least one charged particle beam 122. The charged particles can be, for example, electrons, positrons, muons, ions, or other charged particles. Furthermore, the particle imaging apparatus 100 can be configured in various ways, for example as a particle microscope, a SEM, a TEM, a STEM, a SEM-STEM, a dual beam system, a multi-beam particle microscope, a mask repair system, or in other ways.

[0135] In process step S2, a test object 8 is provided, wherein the test object has a test structure on its surface 8a. This test structure has geometric dimensions that are precisely known, for example, a specific pattern of mutually orthogonal line structures. The test structures of the test object 8 can be manufactured with high accuracy and thus serve as a reference object.

[0136] In process step S3, the test object 8 is positioned in an object plane of the particle imaging apparatus 100 within the particle-optical beam path of the particle imaging apparatus 100 such that a surface normal of the test object 8 forms an angle θ1 with the particle-optical axis Z of the particle imaging apparatus 100. This angle θ1 can be 0°, but it does not have to be 0°. With an angle θ1 of 0°, the at least one charged particle beam 122 would strike the test structure of the test object 8 at right angles, or telecentrically.

[0137] In process step S4, a first focusing z1 of the particle imaging apparatus 100 is adjusted such that the at least one particle beam 122 of the particle imaging apparatus 100 is focused at least pointwise onto the surface of the test object 8. If the angle θ1 = 0°, the first focusing z1 is adjusted so that the focus is not only pointwise, but at several points and, in particular, at all points of the test object 8 to be scanned later, onto the surface 8a of the test object 8.

[0138] In process step S5, the test structure is scanned or probed with the particle beam 122 at the first (fixed) focus z1, and a first particle-optical image of the test structure is generated. For the purposes of this application, a particle-optical image is understood to be an image generated by detecting particle radiation. A particle detector is used to detect the particle radiation, and it is possible to combine a particle detector with an optical detector. The detected particles can be, for example, electrons or other charged particles. It is possible, for instance, to detect secondary electrons, backscattered electrons, mirror electrons, or mirror ions for generating the particle-optical image. The particle-optical images are images in which the intensity of the detected particle radiation is represented as a function of an image position for a pixel.

[0139] In process step S6, a first position P1 of the test structure is determined in the first particle-optical image. This first position P1 of the test structure can denote the overall position of the test structure. However, it is also possible that the first position P1 of the test structure denotes only one of several positions of a part of the test structure. Furthermore, it is of course possible that several first positions P1 of the test structure are determined, from which, for example, the overall position P1 of the test structure can then be derived.

[0140] In process step S7, the test object 8 is arranged in the particle-optical beam path of the particle imaging apparatus 100 such that a surface normal of the test object 8 forms a second angle θ2 with the particle-optical axis Z of the particle imaging apparatus 100, wherein this second angle θ2 is different from the first angle θ1, i.e., θ2 ≠ θ1. Preferably, the second angle θ2 is an angle other than 0°. This preferably represents a true inclination of the surface of the test object 8 relative to the particle-optical axis Z of the particle imaging apparatus 100.

[0141] In a process step S8, the focusing is changed and a second focusing z2 of the particle imaging apparatus 100 is set such that a particle beam 122 of the particle imaging apparatus 100 is focused at least pointwise onto the surface of the test object, whereby this second focusing z2 is different from the first focusing z1, i.e. z2 ≠ z1.

[0142] In process step S9, the test structure is scanned with the particle beam 122 in the second fixed focusing z2 and a second particle-optical image of the test structure is generated.

[0143] In process step S10, a second position P2 of the test structure is determined in the second particle-optical image. The test structure is thus imaged multiple times, at different focuses and angles. The principle for determining the second position P2 of the test structure in the second particle-optical image is the same as that for determining the first position P1 of the test structure in the first particle-optical image.

[0144] In procedure step S11, a positional shift d21 is determined between the second position P2 of the test structure in the second particle-optical image and the position P1 of the test structure in the first particle-optical image. Of course, it is possible that several positional shifts d21 of different substructures or characteristic points of the test structure are determined. However, at least one positional shift d21 is determined.

[0145] In process step S12, a focus shift fs of the particle beam 122 is determined in a direction orthogonal to the particle-optical axis Z as a function of the focusing change Δz = z2 - z1. This determination of the focus shift fs is based on the determined lateral position shift d21 of the test structure and the angular difference Δθ = θ2 - θ1. The focus shift fs of the particle beam in a direction orthogonal to the particle-optical axis Z is therefore a lateral focus shift. This lateral focus shift is responsible for distortions that occur when the focusing changes. Since the test structure(s) of the test sample 8, which represents a reference sample, are precisely known, the lateral focus shift fs can be deduced from the measured position shifts d21.According to a preferred embodiment of the invention, the focus shift fs is determined based on a discrepancy between an expected positional shift d21' of the test structure and the actual positional shift d21 of the test structure determined in step (1k). The angular difference Δθ = θ2 - θ1 is known, which allows conclusions to be drawn about the expected positional shift d21' of the test structure. Deviations from this are attributable to the lateral focus shift fs.

[0146] The calibration procedure ends in step S13.

[0147] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, where x1 ≠ x2. Preferably, the two directions x1 and x2 are orthogonal to each other and can also be referred to as the x-direction and y-direction in a Cartesian coordinate system. In the context of this patent application, however, a y-direction, particularly in the case of a tilted test sample 8 or sample 8, denotes the direction in which the sample 8 is tilted. Therefore, we initially refer to two directions x1 and x2 in which the focus shift fs is determined.

[0148] According to a preferred embodiment of the invention, the first angle θ1 = 0°. This allows for a particularly simple determination of the focus shift fs.

[0149] According to a preferred embodiment of the invention, process steps S8 to S12 are repeated for further focusing or for several further focusings. Thus, the corresponding process steps are repeated for further focusings z3, z4, z5, and so on, and the corresponding positional shifts d31, d41, d51, and so on are determined. This allows for a further determination of the lateral focus shift fs as a function of different focusing changes or focusing settings. In this way, the particle imaging apparatus 100 can be calibrated for large focusing changes Δz, which are necessary for the inspection of 3D features, for example, of channel structures 60, 61 in semiconductor samples.

[0150] According to a preferred embodiment of the invention, the focus is changed incrementally by a constant value dz.

[0151] According to a preferred embodiment of the invention, the following relationship applies to the constant value dz: 1 µm ≤ dz ≤ 50 µm. Depending on the focus ranges or focus variations over which a particle imaging apparatus 100 is to be adjustable, the incremental value dz can be selected accordingly.

[0152] According to a preferred embodiment of the invention, the test object 8 is arranged on a ministage 18 with an adjustable tilt angle θ, and the method further comprises tilting the ministage 18, in particular to adjust the angle θ2. With an untilted ministage, the angle θ1 can, for example, be 0°.

[0153] According to a further preferred embodiment of the invention, the calibration method according to the invention is performed multiple times for different angles θ. This can be achieved particularly easily with the described Ministage 18. The method can be performed, for example, for 3, 4, 5, 10 or even more angles θi. This allows the overall accuracy of the calibration method to be increased.

[0154] Fig. Figure 11 schematically shows a scanning of a sample 8 with different arrangements of the sample 8 using tilt-adjusted focusing z or adaptive focusing. The focusing z is – unlike in Fig. 9 - in the y-direction in Fig. 11b) variable and adapted to the inclination θ. If the inclination θ is known, the required focusing z as a function of a raster position Rij is also known in principle. In this respect, a sample surface 8a) with a known test structure can be scanned with focus. However, with this otherwise regular scanning, distortions may occur due to a lateral focus shift fs, which can be observed in the corresponding particle-optical image.

[0155] Fig. Figure 12 schematically shows a flowchart for a calibration method according to a second aspect of the invention, wherein tilt-adapted focusing is used: In a first process step S20, a particle imaging apparatus 100 is provided, which operates with at least one charged particle beam 122. Otherwise, the particle imaging apparatus 100 is subject to the same characteristics as already described in connection with the first aspect of the invention.

[0156] In process step S21, a test object 8 is provided, wherein the test object 8 has a test structure on its surface 8a. Regarding the details of a test object 8 or a test structure, reference is made to what has already been stated in connection with the first aspect of the invention.

[0157] In a process step S22, the test object 8 is arranged in an object plane of the particle imaging apparatus 100 in the particle-optical beam path of the particle imaging apparatus 100 such that a surface normal of the test object 8 forms a first angle θ1 = 0° with the particle-optical axis Z of the particle imaging apparatus 100.

[0158] In a process step S23, a first focusing z1 of the particle imaging apparatus 100 is set such that the particle beam 121 of the particle imaging apparatus 100 is focused onto the surface of the test object 8.

[0159] In process step S24, the test structure is scanned with the at least one particle beam 122 at the fixed first focusing z1, and a first particle-optical image of the test structure is acquired. Up to this point, the calibration procedure according to the second aspect of the invention can be identical to process steps S1 to S5 according to the first aspect of the invention, provided that the angle θ1 is 0° there.

[0160] In a process step S25, the test object 8 is arranged in the particle-optical beam path of the particle imaging apparatus 100 such that a surface normal of the test object 8 forms a second angle θ2 with the particle-optical axis Z of the particle imaging apparatus 100, which is different from the first angle θ1, where θ2 ≠ θ1.

[0161] In process step S26, the test structure is scanned with the particle beam 122 with a variable focus Δz, which is adapted to the respective scanning position Rij of the particle beam 122, and a second particle-optical image of the test structure is acquired. In other words, the variable focus Δz is adapted to the tilt of the test object 8, so that (at least without lateral focus shift fs) the scanning particle beam would theoretically be focused onto the surface 8a of the test object 8 at every scanning position Rij.

[0162] In process step S27, a distortion of the test structure in the second particle-optical image relative to the first particle-optical image is determined. This determination of the distortion can, in turn, be based on the determination of positional changes of the test structure or parts thereof, or on deviations from a regularity of the test structure. In principle, several possibilities exist for this, which are familiar to those skilled in the art.

[0163] In process step S28, a focus shift fs of the particle beam 122 is determined in a direction orthogonal to the particle-optical axis Z, or in the depth direction T, as a function of a focusing change Δz in the direction of the particle-optical axis Z. The determination of the focus shift fs is based on the determined distortion of the test structure and on the angle θ2. The focusing change Δz corresponds to the variable focusing Δz, which is known for a given raster position Rij of the particle beam or has been adaptively set for that position.

[0164] The procedure ends at S29 on the average.

[0165] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, where x1 ≠ x2. In particular, the two directions x1 and x2 can be orthogonal to each other. One of the directions can coincide with a y-direction in which the test object 8 is tilted.

[0166] According to a preferred embodiment of the invention, the test object 8 is arranged on a ministage 18 with an adjustable tilting angle, and the method further comprises tilting the ministage 18 to adjust the angle θ2.

[0167] According to a preferred embodiment of the invention, the method is carried out multiple times for different angles θ or θ2.

[0168] According to a preferred embodiment of the invention, the following relationship applies to the edge length L of an image field FOV (corresponding to the English abbreviation "Field of View") scanned with the particle beam 122: L ≥ 10 µm. In this way, a sufficiently large change in focus Δz can be represented during the scanning process. In addition, a sufficiently large area of ​​the test object 8 is represented, which enables a sufficiently accurate determination of the distortion that occurs.

[0169] Fig. Figure 13 schematically shows a focus shift during refocusing. This effect can also be used for calibration purposes: In Fig. In 13a, the particle beam 122 is focused onto the sample surface 8a at a precisely known position. A characteristic feature or a position marker can be used for this purpose. Fig. In section 13b, the focusing depth is now changed by the value Δz, without any lateral focus shift fs occurring. With a particle imaging apparatus 100 described above, this can be achieved, for example, by changing the accelerating voltage EHT at the particle source. It is not necessary to change the magnetic lens 11 or the electrostatic lens component 10, if present. The adjustment by the combined objective lens system 11, 10 remains unchanged when the accelerating voltage EHT is changed. The change in the accelerating voltage EHT is described in Fig. 13 indicated by the specification ΔV ≠ 0V.

[0170] In Fig. 13c now refocusing takes place, exclusively by changing the control of the magnetic objective lens 11: This results in a lateral focus shift fs. The magnitude of this lateral focus shift fs depends on the refocusing Δz.

[0171] Fig. Figure 14 schematically shows a flowchart for a calibration process according to the invention in accordance with the fourth aspect of the invention, as this is generally implemented in Fig. 13 has already been outlined: In process step S30, a particle imaging apparatus 100 is provided. The particle imaging apparatus 100 comprises a particle source 1 for generating a particle beam 122 with charged particles, wherein an accelerating voltage EHT is applied to the particle source 1 during operation. Furthermore, the particle imaging apparatus 100 includes at least two focusing lenses through which the particle beam 122 passes. One lens is a magnetic objective lens 11, 11', and the other is an electrostatic lens 10, 10'. The electrostatic lens 10' is arranged downstream of the magnetic objective lens 11' in the direction of the particle-optical beam path. The magnetic objective lens 11, 11' and the electrostatic lens 10, 10' focus the particle beam 122 onto an object 8 at a distance z from the magnetic objective lens.The particle imaging apparatus 100 further comprises a deflection unit 24, which is configured to deflect the particle beam 122 towards the center of the electrostatic lens 10, 10'. The particle imaging apparatus 100 also includes a stage 15 or object holder, which is configured to hold an object 8 at a working distance WD from the magnetic objective lens 11. Furthermore, the particle imaging apparatus 100 includes a detection unit 17 for detecting interaction particles emanating from the object 8, for example, for the detection of secondary electrons. Finally, the particle imaging apparatus 100 includes a control unit 20 for controlling the particle imaging apparatus 100. The control unit 20 is configured to drive the particle source 1 to provide the accelerating voltage EHT.The control unit 20 is further configured to control the magnetic objective lens 11, or, for example, to vary the current in a coil 31 of the magnetic objective lens 11. Furthermore, the control unit 20 is configured to control the deflection unit 24, thereby deflecting the particle beam 122 precisely to the center of the electrostatic lens 10, 10', which serves to adjust the particle imaging apparatus. Moreover, the particle imaging apparatus 100 is adjusted in such a way that an energy-independent adjustment is such that a change in the accelerating voltage EHT has no effect on the beam position upon impact with the object 8. With such an adjustment, a change in the accelerating voltage EHT only results in a change in the beam diameter on the object 8; thus, the image sharpness of the recorded particle-optical image is changed, but not the beam position.

[0172] Then, in process step S31, an object 8 is provided at a distance z. This object 8 can again be a test object 8 or a test sample 8 with regular structures, but it can also be another object with a characteristic feature, for example with a position marker on a sample surface.

[0173] In step S32, a reference feature of object 8 is located.

[0174] In step S33, a first particle-optical image is captured by means of focusing z. The focusing z here corresponds exactly to the working distance WD, which is why the captured particle-optical image is, in principle, sharply focused.

[0175] In process step S34, a position P1 of the reference feature is determined in the first particle-optical image.

[0176] In process step S35, the accelerating voltage EHT is changed by an offset ΔV.

[0177] In process step S36, the particle imaging apparatus 100 is refocused by Δz by changing the control signal of the magnetic objective lens 11. As described, changing the accelerating voltage EHT by the offset ΔV results in the particle beam 122 no longer being focused, but rather imaged indistinctly onto the object 8. However, the position of the particle beam 122 upon impact with the object 8 remained unchanged. Refocusing the particle imaging apparatus, or refocusing the particle beam 122 by Δz by changing the control signal of the magnetic objective lens 11, can, however, lead to a change in the position of the particle beam 122 on the object 8 if the magnetic objective lens 11, 11' and the electrostatic lens 10, 10' are not precisely aligned with each other; the latter is practically never the case with the highest degree of accuracy.When refocusing the particle imaging apparatus 100 by Δz by changing the control of the magnetic objective lens 11, the electrostatic lens 10 is preferably controlled unchanged. Any lateral displacements or focus shifts fs are therefore solely attributable to the changed control of the magnetic objective lens 11, which enables calibration.

[0178] In process step S37, a second particle-optical image is taken in the setting of the particle imaging apparatus 100, refocused by Δz.

[0179] In process step S38, a position P2 of the reference feature is determined in the second particle-optical image.

[0180] In process step S39, a positional shift d21 is determined between the second position P2 of the reference feature and the first position P1 of the reference feature. It is, of course, also possible to determine multiple positional shifts of multiple reference features.

[0181] In process step S40, a focus shift fs of the particle beam 122 is determined in a direction orthogonal to the particle-optical axis Z as a function of the refocusing Δz using the magnetic objective lens 11, wherein the determination of the focus shift fs is based on the determined position shift d21 of the reference feature. In particular, the position shift d21 can correspond exactly to the focus shift fs.

[0182] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, where x1 ≠ x2. In particular, the two directions x1 and x2 can again be orthogonal to each other.

[0183] According to a preferred embodiment of the invention, process steps S35 to S40 are repeated for different offsets ΔV of the accelerating voltage EHT. This enables more precise calibration as well as calibration over the largest possible range.

[0184] According to a preferred embodiment of the invention, the offset ΔV is changed incrementally by a constant value dV. This allows the calibration process to be automated particularly easily.

[0185] According to a preferred embodiment of the invention, the electrostatic lens is an electrostatic objective lens 10'. In particular, it is possible that this electrostatic lens is provided by supplying a potential at an end region 10 of a beam tube within the magnetic objective lens 11 (so-called "Gemini lens"). The pole pieces 29 of the magnetic objective lens 11 are each at ground potential, and preferably the object 8 to be examined is also at ground potential. Thus, an electrostatic immersion field is not used.

[0186] According to an alternative embodiment of the invention, the electrostatic lens 10' exerts its lensing effect by means of a potential difference between the object 8 on the one hand and the magnetic objective lens 11 on the other. This can be achieved, for example, by placing the object 8 at a potential other than zero, while the magnetic objective lens is at ground potential. For example, a sample voltage of up to -4 kV can be applied to a wafer 8 if the charged particles of the particle beam are electrons.

[0187] However, it is also possible that the electrostatic lens 10' is designed differently according to the third aspect of the invention. Furthermore, it is possible to combine the two above embodiments of electrostatic lenses 10'.

[0188] Fig. Figure 15 shows a schematic representation of the usual cross-sectional imaging method for obtaining a 3D volume image of an integrated semiconductor sample 8. The cross-sectional method, also known as the "slice and image" method, achieves three-dimensional (3D) volume imaging using a step-and-repeat approach. First, the integrated semiconductor sample 8 is prepared for the usual cross-sectional imaging method using known techniques. A small block or slice is taken from a wafer and then analyzed. Hereinafter, the terms "cross-sectional image" and "slice" are used synonymously. In one step, a thin surface layer of the material is removed. This material can be removed in various ways known in the industry, including the use of a focused ion beam for milling or polishing at an angle through a focused ion beam (FIB) column 50.For example, the focused ion beam 51 propagates almost parallel to the z-axis and is scanned in the y-direction to mill the upper surface 55 of the sample 8 (which may, for example, be part of a wafer) and expose a new cross-sectional area 52 in a yz-plane. This makes the newly exposed cross-sectional area 52 accessible for imaging. In a further step, the cross-sectional area 52 is scanned with a charged particle beam (CPB) imaging system 40, e.g., a scanning electron microscope (SEM) or a second FIB, to obtain a cross-sectional image 1000.1. The optical axis 42 of the charged particle beam imaging system 40 can be arranged parallel to the x-direction or inclined at an angle to the x-direction.Both secondary and backscattered electrons are captured by a detector (not shown) to reveal material contrast within the integrated semiconductor sample and are visible as different shades of gray in cross-sectional image 1000.1. Metal structures produce lighter measurement results. Surface layer removal by milling and the generation of a cross-sectional image are repeated for cross-sectional areas 53 and 54 and other cross-sectional areas at equal intervals d. A sequence of 2D cross-sectional images 1000, consisting, for example, of N cross-sectional image sections 1000.2, 1000.3, ... 1000.N at different depths, is generated to create a three-dimensional 3D dataset. The representative cross-sectional image section 1000.1 is obtained by measuring a commercially available Intel processor-integrated semiconductor chip with 14 nm technology.

[0189] In this process, at least first and second cross-sectional images are generated by successively milling cross-sectional areas into the integrated semiconductor sample with a focused ion beam to expose or make accessible a sequence of cross-sectional areas for imaging. Each cross-sectional area of ​​the integrated semiconductor sample is then imaged using a charged particle beam imaging system. A 3D image of the integrated semiconductor structure is reconstructed from the sequence of N 2D cross-sectional image slices 1000. The spacing d between the cross-sectional images 1000.1, 1000.2, 1000.3 can be controlled by the FIB milling or polishing process and can be, for example, between 1 nm and 30 nm.

[0190] In the example above, the cross-sectional image planes are oriented perpendicular to the upper surface 55 of the integrated semiconductor wafer 8, with the normal to the upper surface 55 of the wafer being aligned parallel to the z-direction, as shown in Fig. This results in 2D cross-sectional images that are aligned parallel to the yz-plane, or in other words, the cross-sectional image planes enclose the z-axis or wafer normal axis, and the x-direction of imaging is parallel to the wafer surface. The conventional cross-sectional imaging method in this conventional geometry is therefore only applicable to samples taken from a wafer.

[0191] It is also possible, of course, to interchange the positions of the FIB column 50 and the CPB imaging column 40. Then the optical axis of the CPB can be aligned perpendicular to the upper surface 55 of the sample 8. This arrangement is advantageous for imaging HAR structures that extend into the depth of the sample 8.

[0192] In both configurations described above, the working distance of the FIB columns and the CPB imaging columns can be individually adjusted. It is therefore possible to optimize both working distances. Generally speaking, the shorter the working distance, the better the image resolution. Since a short working distance can be set without geometric limitations, high-resolution imaging is possible with the CPB imaging columns 40.

[0193] However, one general problem remains the same: A measurement point on a wafer must be located near an edge of the wafer to enable a measurement with the illustrated geometric arrangement of columns 40, 50. Otherwise, the wafer must be destroyed and a sample 8 or a piece of the wafer removed to artificially create an edge and thus a situation suitable for further analysis.

[0194] Fig. Figure 16 shows a method for generating a 3D volume image using the sectioning and imaging technique, which is applied to a test volume within a wafer in the so-called "wedge-section" approach or wedge-section geometry, without the need to remove a sample 8 from the wafer. The sectioning and imaging technique is applied to a test volume with dimensions of a few µm, for example, 5 µm to 10 µm lateral extent in 200 mm or 300 mm wafers, without the need to remove samples from the wafer 8. A groove or edge is milled into the top surface 55 of an integrated semiconductor wafer 8 to expose a cross-sectional area at an angle to the top surface 55. 3D volume images of test volumes are acquired at a limited number of measurement points, e.g., at representative locations of dies 6.1, 6.2, etc. B. at process control monitors (PCM) or at locations identified by other testing tools.In the cutting and imaging process, wafer 8 is only locally destroyed, and other dies can still be used, or wafer 8 can still be used for further processing.

[0195] Fig. Figure 16 shows the wafer inspection system 500, configured for a sectioning and imaging process under wedge-section geometry with a dual-beam device 1. For a wafer 8, several measuring points, including measuring points 6.1 and 6.2, are defined in a layout plan or inspection list generated by 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 table 155 with actuators and position control. Actuators and means for the precision control of a wafer table, such as laser interferometers, are known in the art. A control unit 16 is configured to control the wafer table 155 and to position a measuring point 6.1 of the wafer 8 at the intersection 43 of the dual-beam device 100.The dual-beam device 100 comprises an FIB column 50 with an optical FIB axis 48 and a charged particle beam (CPB) imaging system 40 with an optical axis 42. At the intersection 43 of the two optical axes of the FIB and CPB imaging systems, the wafer surface 55 is arranged at an oblique angle GF to the FIB axis 48. The FIB axis 48 and the axis of the CPB imaging system 42 form an angle GFE, and the optical axis of the CPB imaging system 42 forms an angle GE with the perpendicular to the wafer surface 55. In the coordinate system of . Fig. The normal to the wafer surface 55 is defined by the z-axis. The focused ion beam (FIB) 51 is generated by the FIB column 50 and strikes the surface 55 of the wafer 8 at an angle GF. Inclined cross-sectional areas are milled into the wafer at the test point 6.1 by ion beam milling at approximately an angle GF. In the example of Fig. The inclined angle GF is approximately 30°. Due to the beam divergence of the focused ion beam, e.g., a gallium ion beam, the actual inclination angle of the inclined cross-sectional area can deviate from the inclined angle GF by up to 1° to 4°. Images of the milled surfaces are acquired using the imaging system 40, inclined at an angle GE to the wafer normal, with a charged particle beam. In the example of Fig. The angle GE is approximately 15°. However, other arrangements are also possible, e.g., with GE = GF, so that the axis of the CPB imaging system 42 is perpendicular to the FIB axis 48, or GE = 0°, so that the axis of the CPB imaging system 42 is perpendicular to the wafer surface 55.

[0196] During imaging, a beam of charged particles 44 is scanned by a scanning unit of the charged particle beam imaging system 40 along a scanning path across a cross-sectional area of ​​the wafer 8 at the measurement point 6.1, generating secondary and scattered particles. The particle detector 17 collects at least some of the secondary and scattered particles and transmits the particle count to a control unit 19. Other detectors for other types of interaction products may also be present. The control unit 19 controls the imaging column 40 and the FIB column 50 and is connected to a control unit 16 to control the position of the wafer 8 mounted on the wafer support table 15 by means of the wafer table 155. The control unit 19 communicates with the operational control unit 20, which controls the placement and orientation of, for example, measurement point 6.1 of wafer 8 at intersection 43 triggers the movement of the wafer table and repeatedly triggers FIB milling, image acquisition and table movements.

[0197] Each new cut surface is milled by the FIB beam 51 and imaged by the charged particle imaging beam 44, which is, for example, a scanning electron beam or a helium ion beam of a helium ion microscope (HIM).

[0198] However, since both the FIB column 50 and the CPB imaging column 40 are positioned above the wafer surface 55, geometric constraints exist for their arrangement. It is no longer possible to optimize both working distances individually, as columns 40 and 50 already obstruct each other at short working distances. As a general practical rule, the position of the FIB column 50 limits and thus "defines" the working distance of the CPB imaging column 40. The working distance of the CPB imaging column 40 cannot normally be chosen to be shorter than 4 to 5 mm. In contrast, the working distance of the CPB imaging column 40, as shown in Fig. 16 shown, can be shortened to approximately 2 mm. Therefore, in the Fig. In the wedge-shaped arrangement shown in 16, the image resolution is typically an order of magnitude lower than in the one shown in Fig. 15 shown edge arrangement.

[0199] It is also possible to perform a calibration procedure for a particle imaging apparatus 100 and a lateral focus shift fs occurring during a focus change based on different depths of field. Fig. Figure 17 schematically illustrates different depths of field for an imaging particle beam 122. Fig. Figure 17 on the left shows a first particle beam 122.1, whose beam cone is focused on the focal plane F. The beam cone of particle beam 122.1 has an opening angle φ1 to the particle-optical axis Z. This opening angle φ1 is relatively small. The size of the beam spot in the focal plane F therefore changes only slowly along the z-direction. Thus, particle beam 122.1 has a relatively large depth of field.

[0200] In Fig. Figure 17 on the right shows another charged particle beam 122.2, which is also focused on the focal plane F. Its beam cone has a relatively large opening angle φ2 to the particle-optical axis Z. The diameter of a beam spot originating from the focal plane therefore changes more rapidly in the z-direction than in the example shown on the left. The depth of field of the particle beam 122.2 is thus relatively small. The depth of field TS is mathematically defined as the defocus, i.e., the distance in the z-direction from the focal plane F, at which the diameter of the beam spot increases by 10%. Typically, the diameter of a beam spot in the focal plane is about 1 nm to 2 nm. Depending on the opening angle φ, the depth of field is then typically between 200 nm and 500 nm.

[0201] In a shot with a large depth of field (left in Fig. 17) Due to the associated smaller numerical aperture, only a lower resolution can be achieved. The situation is different with the one in Fig. In the example shown on the right (17), the numerical aperture is relatively large, allowing for high resolution.

[0202] Fig. Figure 18 now schematically shows a flowchart for a calibration method according to the fourth aspect of the invention, taking into account the relationships with the depth of field TS described above and optionally also the described sectioning and imaging method: In a first process step S50, a particle imaging apparatus 100 is provided, which operates with at least one charged particle beam 122 and can operate in a first operating mode and a second operating mode. In the first operating mode, the particle imaging apparatus 100 operates with a first depth of field TS1 and a first resolution A1, while in the second operating mode, it operates with a second depth of field TS2 and a second resolution A2. The first depth of field TS1 is greater than the second depth of field TS2. Furthermore, the first resolution A1 is lower than the second resolution A2. The depth of field TS is defined in each case as the defocus df at which the diameter of a beam waist (for the smallest, best possible beam diameter) increases by 10%. Generally, there is an optimal aperture angle at which the beam diameter is minimal and the resolution is optimal.The beam diameter increases for both larger opening angles (deterioration due to lens aberrations) and smaller opening angles (deterioration due to diffraction effects). Within the scope of the invention, a smaller opening angle is selected, at least temporarily, to achieve a better depth of field, resulting in a larger beam width and thus a less than optimal resolution for image acquisition.

[0203] In process step S51, a wedge-shaped test object 8 with 3D structures is provided, the section being defined by an angle GF to the surface of the test object 8. The 3D structures can be, for example, channel structures 60, 61 extending into the depth of the test object over a considerable distance, for example, more than 10 µm, more than 50 µm, or more than 100 µm. The wedge-shaped test object can be generated before process step (S51) or during process step (S51). According to a preferred embodiment of the invention, providing the wedge-shaped test object 8 with the 3D structures includes ablating the test object with the 3D structures in a wedge shape using a focused ion beam (FIB).

[0204] In process step S52, the test object 8 is positioned relative to the particle imaging apparatus 100 such that the particle-optical axis Z of the particle imaging apparatus 100 and the uncut surface of the test object 8 are orthogonal to each other, with the 3D structures at different depths z of the test object exposed in the direction of the particle-optical axis Z of the particle imaging apparatus 100. Ideally, the direction of the 3D structures coincides with, or substantially coincides with, the particle-optical axis Z.

[0205] In process step S53, the wedge-shaped test object 8 is scanned in the first operating mode with a fixed focus setting z1 = z1ij, generating a first particle-optical image of the test object with the 3D structures. The imaging is thus performed with a large depth of field TS1, whereby the different arrangement of the 3D structures with respect to height or depth z is relatively insignificant for the imaging. The imaging of the 3D structures in the particle-optical image shows no distortion; it only varies slightly with regard to the sharpness of the depicted 3D structures.

[0206] In process step S54, the positions P1 of the 3D structures are determined in the first particle-optical image.

[0207] In process step S55, the wedge-shaped test object 8 is scanned in the second operating mode with adaptive focusing z2ij, which is adapted to the respective raster position Rij of the particle beam. A second particle-optical image of the test object with the 3D structures is then generated. The adaptive focusing z2ij describes a focusing process in which the beam is focused sharply, i.e., with minimal beam waist, on specific raster positions of the test object 8. The scanning and generation of the second particle-optical image thus occur with a shallower depth of field TS2, but with a higher resolution A2, or in other words, a larger numerical aperture NA2.

[0208] In process step S56, the positions P2 of the 3D structures are determined in the second particle-optical image.

[0209] In process step S57, the positions P1 and P2 are compared with each other, and the respective displacements d21 = P2 - P1 of the 3D structures are determined. These displacements d21 are the result of a lateral focus shift fs of the particle beam122 during the respective focusing change Δz = z2ij - z1.

[0210] In process step S58, a lateral focus shift fs of the particle beam 122 is determined in a direction orthogonal to the particle-optical axis Z as a function of a focusing change Δz = z2ij - z1, whereby the determination of the focus shift fs is based on the determined shifts d21 of the 3D structures.

[0211] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, wherein x1 ≠ x2.

[0212] According to a preferred embodiment of the invention, process steps S51 to S58 are repeated, in particular multiple times, for different angles GF. The process is thus repeated for several differently wedge-shaped test objects 8 with 3D structures. It is also possible to ablate a first wedge-shaped section again, so that a different wedge-shaped test object with a different angle GF is produced. For this purpose, for example, a focused ion beam, in particular according to the "sectioning and imaging" method, can be used.

[0213] According to a preferred embodiment of the invention, the following relationship applies to a ratio of first depth of field TS1 and second depth of field TS2: 5 ≤ TS1 / TS2 ≤ 30. Additionally or alternatively, the following relationship applies to a ratio of first resolution A1 and second resolution A2: 2 ≤ A1 / A2 ≤ 6.

[0214] The procedure ends in step S59 in the example shown.

[0215] Fig. Figure 19 schematically illustrates an inclined sample surface 8a with cutaway 3D structures 60 and an associated particle-optical image 1000.1.

[0216] Fig. Figure 19 schematically illustrates positional shifts of obliquely cropped 3D structures 60 in particle-optical images generated using two different imaging modes. Fig. Figure 19a shows a perspective view, in Fig. 19b the corresponding top view. Shown is a superposition of two particle-optical images in a superposition image 1000.1. The first particle-optical image was created in the first mode according to the method of Fig. 18 recorded, the second particle-optical image was taken in the second mode according to the procedure of Fig. 18 recorded. In the first mode with large depth of field, the position of the cropped 3D structures 60.1 is very regular, or in the example shown, is provided at positions that are in Fig. 19 are represented by dark hatched circles. In this image of the 3D structure, there was no lateral focus shift fs. This differs from the image taken in the second operating mode with shallow depth of field but adjusted focus: In this case, the position of the 3D structures 60 is represented by the light dotted circles. Fig. Figure 19 shows that the position of structures 60.1 is identical in both images (therefore, no dark hatching is visible). As the focus changes in depth or in the z-direction, the positions of the 3D structures in the two images increasingly diverge. In the bottom row, with the most significant change in focus in the z-direction, a lateral position shift fs is most clearly visible. This is indicated in Figure 19. Fig. 19b in the 3D structure 60.2 in an exemplary manner: The lateral focus shift fs is composed of a shift dx in the x-direction and a shift dy in the y-direction, where the y-direction coincides with the direction in which the sample 8 is tilted or obliquely cut and in which the z-focus was simultaneously adjusted during image acquisition in the second mode. In the example shown, the lateral focus shift fs, which depends on the focus change Δz, can be determined by calculating the lateral shifts dx and dy line by line.

[0217] Fig. Figure 20 schematically shows a flowchart for another calibration method according to the invention, namely for a calibration method according to the fifth aspect of the invention. This calibration method serves to determine a lateral focus shift fs in a direction orthogonal to the particle-optical axis Z as a function of the strength of an electrostatic immersion field for wedge-shaped test objects 8. Moreover, calibration can also be performed depending on the geometry of the wedge-shaped section.

[0218] In a first process step S70, a particle imaging apparatus is provided that operates with at least one charged particle beam and can operate in a first and a second operating mode. In the first operating mode, the object to be imaged is not located in an electrostatic immersion field, while in the second operating mode, the object to be imaged is located in an electrostatic immersion field. This can be achieved, for example, by configuring the particle imaging apparatus to apply an electrical potential to the object to be imaged or to a sample stage holding the object, or to switch it on or off.

[0219] In process step S71, a first wedge-shaped test object with 3D structures is provided, whereby the section is defined by a first angle GF1 to the surface of the test object.

[0220] In process step S72, the test object is arranged relative to the particle imaging apparatus in such a way that the particle-optical axis Z of the particle imaging apparatus and the uncut surface of the test object are, in particular, orthogonal to each other, with the 3D structures being exposed at different depths Z of the test object in the direction of the particle-optical axis of the particle imaging apparatus.

[0221] In process step S73, the first wedge-shaped test object 8 is scanned with a calibrated particle beam 122 in the first operating mode without an electrostatic immersion field, and a first particle-optical image of the test object 8 with the 3D structures is generated.

[0222] In process step S74, the positions P1 of the 3D structures are determined in the first particle-optical image.

[0223] In process step S75, the first wedge-shaped test object is scanned with an adjusted particle beam 122 in the second operating mode with a first electrostatically calibrated immersion field, and a second particle-optical image of the test object 8 with the 3D structures is generated.

[0224] In process step S76, the positions P2 of the 3D structures are determined in the second particle-optical image.

[0225] In process step S77, the positions P1 and P2 are compared with each other and the respective displacements d21 = P2 - P1 of the 3D structures are determined.

[0226] In process step S78, a lateral focus shift fs is determined in a direction orthogonal to the particle-optical axis Z as a function of the strength of the electrostatic immersion field, whereby the determination of the focus shift fs is based on the determined displacements d21 of the 3D structures.

[0227] In this embodiment of the invention, the lateral focus shift fs is based on small bends of equipotential lines of the electrostatic field at the wedge-shaped surface of the test object, or results from the geometry of the wedge-shaped test object. The lateral focus shift is solely due to this effect. A lateral focus shift due to an imprecise alignment of successive particle-optical lenses 10', 11' has already been corrected or calibrated beforehand. Therefore, scanning in steps S73 and S75 is performed with a previously calibrated particle beam 122 and a particle imaging apparatus 100, respectively.

[0228] According to a preferred embodiment of the invention, the lateral focus shift fs is determined for a plurality of grid positions Rij. This is advantageous because the shape of the wedge-shaped test object is defined by the respective grid positions. The grid can be selected with varying degrees of fineness. It must be fine enough so that the geometric distortions of the potential lines on the surface of the wedge-shaped test sample can be accurately reproduced during the screening process.

[0229] According to a preferred embodiment of the invention, the focus shift fs is determined in a first direction x1 orthogonal to the particle-optical axis Z and in a second direction x2 orthogonal to the particle-optical axis Z, wherein again x1 ≠ x2.

[0230] According to a preferred embodiment of the invention, process steps S75 to S78 are carried out repeatedly, in particular multiple times, each time with a different strength of the electrostatic immersion field. It is possible for the strength of the electrostatic immersion field to be changed incrementally, for example over a range from approximately 500 V / mm to approximately 4000 V / mm.

[0231] According to a preferred embodiment of the invention, process steps S75 to S78 are repeated, in particular repeated several times, each time for wedge-shaped test objects of different geometries. The determination of the focus shift fs in a direction orthogonal to the particle-optical axis Z is then carried out as a function of the geometry of the wedge-shaped test object.

[0232] According to a preferred embodiment of the invention, the angle GF is varied for each of the 8 wedge-shaped test objects of different geometries. Additionally or alternatively, the maximum cutting depth T is varied for the 8 wedge-shaped test objects of different geometries. Additionally or alternatively, the maximum cutting width B is varied for the 8 wedge-shaped test objects of different geometries. In this way, it is possible to determine the separate influence of different geometric parameters on the focus shift fs. The calibration can thus be very precise. According to an advantageous embodiment of the invention, the different parameters are varied separately and sequentially, and their influence on the focus shift fs is determined separately.

[0233] In the example shown, the procedure ends at step S79.

[0234] The following applies to all calibration methods of the first to fifth aspects of the invention: According to a preferred embodiment of the invention, the particle imaging apparatus 100 is an apparatus from the following list of particle imaging apparatuses 100: a particle microscope, a SEM, a TEM, a STEM, a SEM-STEM, a dual particle beam system, a multi-beam particle microscope, a mask repair system. This list is not exhaustive. The described lateral focus shift can occur in all of the aforementioned particle imaging apparatuses 100, either due to a less than 100% accurate alignment of successive particle-optical lenses or due to electrostatic or magnetic immersion fields on an object or sample surface.

[0235] According to a preferred embodiment of the invention, the particle imaging apparatus comprises a magnetic objective lens and an electrostatic objective lens. This objective lens system can, in particular, operate according to the so-called "Gemini principle".

[0236] In particular, an end region of a beam tube that protrudes into the magnetic objective lens can be at an electrical potential, for example at a potential of several kV or keV.

[0237] According to a preferred embodiment of the invention, the electrostatic objective lens is arranged or formed within the pole shoes of the magnetic objective lens.

[0238] Fig. Figure 21 schematically shows various wedge-shaped test objects: Fig. Figure 21a shows a first wedge-shaped test object with a width B1 and a maximum depth T1. The cutting angle GF1 is also shown.

[0239] In Fig. Figure 21b shows a second test object with a different geometry: The section angle GF2 is larger than the section angle GF1. Fig. 21a. The width B2 corresponds to the width B1 in Fig. 21a. The total depth of the partial section T2 is greater than the depth T1 in Fig. 21a.

[0240] In Fig. 21c shows a third obliquely cut test sample 8. Its total width B3 is larger than in the previous examples, the depth T3 corresponds to the depth T2 from . Fig. 21b. The section angle GF3 is smaller than in the other examples. These variations of geometric parameters can be continued. Furthermore, it is of course possible to use geometries designed differently instead of the wedge-shaped section. However, the wedge-shaped sections are particularly practical, as they can be generated very effectively using a focused ion beam (FIB).

[0241] Fig. Figure 22 shows an exemplary flowchart for a method for operating a particle imaging apparatus 100. According to a sixth aspect of the invention, this relates to a method for operating a particle imaging apparatus 100. The particle imaging apparatus 100 can be a particle imaging apparatus as described above in several embodiments.

[0242] In a first process step S80, a particle imaging apparatus 100 is provided, which operates with at least one charged particle beam 122.

[0243] In a process step S81, the particle imaging apparatus 100 is calibrated with respect to a focus shift fs of the particle beam 122 in a direction orthogonal to the particle-optical axis Z as a function of a focusing change Δz or refocusing Δz of the particle imaging apparatus 100, in particular the calibration is carried out according to one of the calibration methods as described above in accordance with the first to fourth aspects of the invention.Additionally or alternatively, the particle imaging apparatus 100 is calibrated with respect to a focus shift fs of the particle beam 122 based on the strength of an electrostatic immersion field at an object 8 to be imaged and / or based on a geometry of an object 8 to be imaged in an electrostatic immersion field, wherein this calibration can, but does not have to, be carried out in particular according to a calibration method that has been described in connection with the fifth aspect of the invention.

[0244] In process step S82, at least one particle-optical image of an object is generated using the particle imaging apparatus. This prevents distortion in the particle-optical image due to lateral focus shift, which offers particular advantages in metrological applications.

[0245] According to a preferred embodiment of the invention, a plurality of mutually parallel sectional images through a 3D structure are generated with the calibrated particle imaging apparatus, and the following further process steps are carried out: In step S83, a volume image is generated from the cross-sectional images, and in step S84, a tilt of the 3D structure is determined based on the cross-sectional images.

[0246] The calibrated particle imaging apparatus 100 can preferably be a dual particle beam system, which operates, for example, by means of a system consisting of a particle microscope and a focused ion beam, for example using a sectioning and imaging technique. According to a preferred embodiment of the invention, the 3D structure comprises a NAND structure.

[0247] The procedure described as an example ends in step S85.

[0248] The various embodiments and aspects of the invention can be combined in whole or in part, provided that no technical contradictions result. Furthermore, it is explicitly pointed out that the described embodiments of the invention are not to be interpreted as limiting the invention.

[0249] Several calibration methods for particle imaging devices 100 are proposed, which allow the determination of a lateral focus shift fs as a function of a focusing change Δz or a refocusing Δz of a particle imaging device 100. Distortions in particle-optical images generated with appropriately calibrated particle imaging devices 100 can thus be reduced, and metrological investigations in the particle-optical images, and especially in 3D tomographies, can be carried out with greater accuracy. With appropriately calibrated particle imaging devices 100, 3D samples such as deep channels in semiconductor samples can be measured more accurately. Distortion effects occurring due to sample geometries in electrostatic immersion fields can also be calibrated and thus corrected. Reference symbol list 1 particle source 4 Condenser lens 6 Measuring point, measuring range, investigation area 8 Sample, object, wafer 8a Object surface with structure, test sample 10 End section beam tube at potential, electrostatic objective lens 10' optical analogue of the electrostatic objective lens 11 magnetic lens 11' Optical analogue to the magnetic objective lens 15 Sample table 16 Sample table control unit 17 Detection system 18 Ministage 19 Control unit 20 Control 21 Pole shoe 23 coil 24 x,y-Stigmator 25 beam deflectors, grid device 29 Polschuh 31 coil 40 imaging particle beam column 42 Optical axis of the imaging system 43 Intersection 44 imaging particle beam 46 scanning path, grid path 48 optical axis of the FIB 50 FIB column 51 focused ion beam 52 Cut surface, surface 53 Cut surface, surface 54 Cut surface, surface 55 Surface area of ​​the object, surface area of ​​the sample, surface area of ​​the wafer 60 3D structure, channel 61 3D structure, channel 122 particle beam 100 particle beam system, particle imaging apparatus, scanning electron microscope, SEM 155 Wafer stage 500 wafer inspection system 1000 3D images, volumetric images 1000.1 Cross-sectional image 1000.2 Cross-sectional image 1000.3 Cross-sectional image WD working distance GF angle, cutting angle GE angle, angle during imaging process GFE angle, arrangement angle between the columns U S Potential at the object, tension at the object Potential U at the end of the beam tube / inside the lens, voltage at the end of the beam tube / inside the lens x direction y direction (inclination direction) z direction Z particle optical axis

Claims

[1] Calibration procedure for a particle imaging apparatus (100) comprising the following steps: (1a) Providing a particle imaging apparatus (100) operating with at least one charged particle beam (122); (1b) Providing a test object (8) wherein the test object (8) has a test structure on its surface (8a); (1c) Arrange the test object (8) in an object plane of the particle imaging apparatus (100) in the particle-optical beam path of the particle imaging apparatus (100) such that a surface normal of the test object (8) forms an angle θ1 with the particle-optical axis of the particle imaging apparatus (100); (1d) Setting a first focus z1 of the particle imaging apparatus (100) such that the particle beam (122) of the particle imaging apparatus is focused at least pointwise onto the surface (8a) of the test object (8); (1e) Scanning the test structure with the particle beam (122) in the first focusing z1 and generating a first particle-optical image of the test structure; (1f) Determining a first position P1 of the test structure in the first particle-optical image; (1g) Arrange the test object (8) in the particle-optical beam path of the particle imaging apparatus (100) such that a surface normal of the test object (8) forms an angle θ2 with the particle-optical axis (Z) of the particle imaging apparatus (100), where θ2 ≠ θ1; (1h) Changing the focus and setting a second focus z2 of the particle imaging apparatus (100) such that a particle beam (122) of the particle imaging apparatus (100) is focused at least pointwise onto the surface of the test object (8), where z2 ≠ z1; (1i) Scanning the test structure with the particle beam (122) in the second focusing z2 and generating a second particle-optical image of the test structure; (1j) Determining a second position P2 of the test structure in the second particle optical image; (1k) Determining a positional shift d21 between the second position P2 of the test structure in the second particle-optical image and the position P1 of the test structure in the first particle-optical image; (1l) Determining a focus shift fs of the particle beam (122) in a direction orthogonal to the particle-optical axis Z as a function of the focusing change Δz = z2 - z1, wherein the determination of the focus shift fs is based on the determined lateral position shift d21 of the test structure and based on the angular difference Δθ = θ2-θ1. [2] Calibration method according to claim 1, wherein the determination of the focus shift fs is based on a discrepancy between an expected position shift d21' of the test structure and the actual position shift d21 of the test structure determined in step (1k). [3] Calibration method according to one of the preceding claims, wherein the focus shift fs is determined in a first direction x1 orthogonal to the particle optical axis Z and in a second direction x2 orthogonal to the particle optical axis Z, wherein x1 ≠ x2. [4] Calibration method according to one of the preceding claims, wherein the first angle θ1 = 0°. [5] Calibration method according to one of the preceding claims, wherein the method steps (1h) to (1l) are repeated for further focusing or for several further focusing. [6] Calibration method according to the preceding claim, wherein the focus is changed incrementally by a constant value dz. [7] Calibration method according to the preceding claim, wherein the following relation applies to the constant value dz: 1µm ≤ dz ≤ 50µm. [8] Calibration method according to any one of the preceding claims, wherein the test object (8) is arranged on a ministage (18) with an adjustable tilt angle, and the method further comprises tilting the ministage (18), in particular to adjust the angle θ2. [9] Calibration method according to one of the preceding claims, wherein the method is carried out multiple times for different angles θ. [10] Calibration procedure for a particle imaging apparatus (100) comprising the following steps: (2a) Providing a particle imaging apparatus (100) operating with at least one charged particle beam (122); (2b) Providing a test object (8) wherein the test object (8) has a test structure on its surface (8a); (2c) Arrange the test object (8) in an object plane of the particle imaging apparatus (100) in the particle-optical beam path of the particle imaging apparatus (100) such that a surface normal of the test object (8) forms an angle θ1=0° with the particle-optical axis (Z) of the particle imaging apparatus (100); (2d) Setting a first focus z1 of the particle imaging apparatus (100) such that the particle beam (122) of the particle imaging apparatus (100) is focused onto the surface (8a) of the test object; (2e) Scanning the test structure with the particle beam (122) in the fixed first focusing z1 and taking a first particle-optical image of the test structure; (2f) Arrange the test object (8) in the particle-optical beam path of the particle imaging apparatus (100) such that a surface normal of the test object (8) forms an angle θ2 with the particle-optical axis (Z) of the particle imaging apparatus (100), where θ2 ≠ θ1; (2g) Scanning the test structure with the particle beam (122) with a variable focusing Δz adapted to the respective scanning position Rij of the particle beam (122), and taking a second particle-optical image of the test structure; (2h) Determine a distortion of the test structure in the second particle optical image relative to the first particle optical image; and (2i) Determining a focus shift fs of the particle beam (122) in a direction orthogonal to the particle-optical axis Z as a function of a focusing change Δz in the direction of the particle-optical axis Z, wherein the determination of the focus shift fs is based on the determined distortion of the test structure and on the angle θ2. [11] Calibration method according to the preceding claim, wherein the focus shift fs is determined in a first direction x1 orthogonal to the particle optical axis Z and in a second direction x2 orthogonal to the particle optical axis Z, wherein x1 ≠ x2. [12] Calibration method according to any one of claims 10 to 11, wherein the test object (8) is arranged on a ministage (18) with an adjustable tilt angle, and the procedure further includes tilting the ministage (18) to adjust the angle θ2. [13] Calibration method according to one of claims 10 to 12, wherein the method is carried out multiple times for different angles θ. [14] Calibration method according to one of claims 10 to 13, wherein the following relation applies to the edge length L of an image field FOV scanned with the particle beam (122): L ≥ 10µm, in particular L ≥ 20 µm or L ≥ 50µm. [15] Calibration procedure for a particle imaging apparatus (100) comprising the following steps: (3a) Providing a particle imaging apparatus (100) comprising the following: a particle source (1) for generating a particle beam (122) with charged particles, wherein an accelerating voltage EHT is applied to the particle source (1) during operation; at least two focusing lenses (10, 11) through which the particle beam (122) passes and which comprise, on the one hand, a magnetic objective lens (11) and, on the other hand, an electrostatic lens (10), wherein the electrostatic lens (10) is arranged in the direction of the particle-optical beam path downstream of the magnetic objective lens (11), wherein the magnetic objective lens (11) and the electrostatic lens (10) focus the particle beam (122) at a distance z from the magnetic objective lens (11) onto an object (8), a deflection unit (25) which is configured to deflect the particle beam (122) towards the center of the electrostatic lens (10), an object stage or object holder (15) which is set up to hold an object (8) at a working distance WD to the magnetic objective lens (11); a detection unit (17) for detecting interaction particles emanating from the object (8); and a control (20) for controlling the particle imaging apparatus (100), wherein the control (20) is set up to control the particle source (1) to provide the accelerating voltage EHT, wherein the control (20) is set up to control the magnetic objective lens (11), and wherein the control unit (20) is configured to control the deflector unit (25), and wherein the particle imaging apparatus (100) is adjusted in such a way that an energy-independent change in the accelerating voltage EHT has no effect on the beam position when it hits the object (8); (3b) Providing an object (8) at a distance z; (3c) Locating a reference feature of the object (8); (3d) Acquiring a first particle-optical image by means of focusing; (3e) Determining a position P1 of the reference feature in the first particle-optical image; (3f) Changing the accelerating voltage EHT by an offset ΔV; (3g) Refocusing the particle imaging apparatus (100) by Δz by changing the control of the magnetic objective lens (11); (3h) Taking a second particle-optical image in the setting of the particle imaging apparatus refocused by Δz (100); (3i) Determining a position P2 of the reference feature in the second particle-optical image; (3j) Determine a positional shift d21 between the second position P2 of the reference feature and the first position of the reference feature; and (3k) Determining a focus shift fs of the particle beam (122) in a direction orthogonal to the particle-optical axis Z as a function of the refocusing Δz using the magnetic objective lens (11), wherein the determination of the focus shift fs is based on the determined position shift d21 of the reference feature. [16] Calibration method according to the preceding claim, wherein the focus shift fs is determined in a first direction x1 orthogonal to the particle optical axis Z and in a second direction x2 orthogonal to the particle optical axis Z, wherein x1 ≠ x2. [17] Calibration method according to one of claims 15 to 16, wherein the method steps (3f) to (3k) are repeated for different offsets ΔV of the accelerating voltage EHT. [18] Calibration method according to the preceding claim, wherein the offset ΔV is changed incrementally by a constant value dV. [19] Calibration method according to any one of the preceding claims 15 to 18, wherein the electrostatic lens is an electrostatic objective lens. [20] Calibration method according to any one of the preceding claims 15 to 18, wherein the electrostatic lens (10) exerts its lensing effect by means of a potential difference between the object (8) and pole shoes (29) of the magnetic objective lens (11), in particular where the object (8) lies on a non-zero potential. [21] Calibration procedure for a particle imaging apparatus (100) comprising the following steps: (4a) Providing a particle imaging apparatus (100) which operates with at least one charged particle beam (122) and which can operate in a first operating mode and in a second operating mode, wherein the particle imaging apparatus (100) operates in the first operating mode with a first depth of field TS1 and a first resolution A1 and wherein the particle imaging apparatus (100) operates in the second operating mode with a second depth of field TS2 and a second resolution A2, where the first depth of field TS1 is greater than the second depth of field TS2, and where the first resolution A1 is less than the second resolution A2; (4b) Providing a wedge-shaped test object (8) with 3D structures, wherein the section is defined by an angle GF to the surface of the test object (8); (4c) Arrange the test object (8) relative to the particle imaging apparatus (100) such that the particle-optical axis Z of the particle imaging apparatus (100) and the uncut surface of the test object (8) are orthogonal to each other, wherein the 3D structures at different depths z of the test object (8) are exposed in the direction of the particle-optical axis (Z) of the particle imaging apparatus (100); (4d) Scanning the wedge-shaped test object (8) in the first operating mode and with a fixed setting z1=z1ij of the focusing and generating a first particle-optical image of the test object (8) with the 3D structures; (4e) Determining positions P1 of the 3D structures in the first particle-optical image; (4f) Scanning the wedge-shaped test object (8) in the second operating mode and with adaptive focusing z2ij, which is adapted to the respective raster position Rij of the particle beam (122), and generating a second particle-optical image of the test object (8) with the 3D structures; (4g) Determining positions P2 of the 3D structures in the second particle-optical image; (4h) Comparing positions P1 and P2 with each other and determining the respective displacements d21=P2-P1 of the 3D structures; and (4i) Determining a focus shift fs of the particle beam (122) in a direction orthogonal to the particle-optical axis Z as a function of a focusing change Δz=z2ij-z1, wherein the determination of the focus shift fs is based on the determined displacements d21 of the 3D structures. [22] Calibration method according to claim 21, wherein providing the wedge-shaped test object (8) with the 3D structures comprises a wedge-shaped ablation of the test object (8) with the 3D structures by means of a focused ion beam (51). [23] Calibration method according to the preceding claim, wherein the focus shift fs is determined in a first direction x1 orthogonal to the particle optical axis Z and in a second direction x2 orthogonal to the particle optical axis Z, wherein x1 ≠ x2. [24] Calibration method according to one of claims 21 to 23, wherein the method steps (4b) to (4i) are carried out repeatedly, in particular repeatedly, for different angles GF. [25] Calibration method according to any one of claims 21 to 24, where the following relationship applies to the ratio of first depth of field TS1 to second depth of field TS2: 5 ≤ TS1 / TS2 ≤ 30; and / or where the following relation holds for a ratio between first resolution A1 and second resolution A2: 2 ≤ A1 / A2 ≤ 6; [26] Calibration procedure for a particle imaging apparatus (100) comprising the following steps: (5a) Providing a particle imaging apparatus (100) which operates with at least one charged particle beam (122) and which can operate in a first operating mode and in a second operating mode, wherein in the first operating mode an object (8) to be imaged is not arranged in an electrostatic immersion field and wherein in the second operating mode the object to be imaged is arranged in an electrostatic immersion field; (5b) Providing a first wedge-shaped test object (8) with 3D structures (60), wherein the section is defined by a first angle GF1 to the surface of the test object; (5c) Arrange the test object (8) relative to the particle imaging apparatus (100) such that the particle-optical axis Z of the particle imaging apparatus (100) and the uncut surface of the test object (8) are in particular orthogonal to each other, wherein the 3D structures (60) are exposed at different depths z of the test object (8) in the direction of the particle-optical axis (Z) of the particle imaging apparatus (100); (5d) Scanning the first wedge-shaped test object (8) with a calibrated particle beam (122) in the first operating mode without electrostatic immersion field and generating a first particle-optical image of the test object (8) with the 3D structures (60); (5e) Determining positions P1 of the 3D structures (60) in the first particle-optical image; (5f) Scanning the first wedge-shaped test object (8) with an adjusted particle beam (122) in the second operating mode with a first electrostatic immersion field and generating a second particle-optical image of the test object (8) with the 3D structures (60); (5g) Determining positions P2 of the 3D structures (60) in the second particle-optical image; (5h) Comparing positions P1 and P2 with each other and determining the respective displacements d21=P2-P1 of the 3D structures (60); and (5i) Determining a focus shift fs in a direction orthogonal to the particle-optical axis Z as a function of the strength of the electrostatic immersion field, wherein the determination of the focus shift fs is based on the determined displacements d21 of the 3D structures (60). [27] Calibration method according to the preceding claim, wherein the focus shift fs is determined for a plurality of grid positions Rij. [28] Calibration method according to one of claims 26 to 27, wherein the focus shift fs is determined in a first direction x1 orthogonal to the particle optical axis Z and in a second direction x2 orthogonal to the particle optical axis Z, wherein x1 ≠ x2. [29] Calibration method according to one of claims 26 to 28, wherein the method steps (5f) to (5i) are carried out repeatedly, in particular multiple times, each time with a different strength of the electrostatic immersion field. [30] Calibration method according to any one of claims 26 to 29, wherein the process steps (5b) to (5i) are carried out repeatedly, in particular multiple times, each time for wedge-shaped test objects (8) of different geometries; and wherein the determination of the focus shift fs in a direction orthogonal to the particle-optical axis Z is carried out as a function of the geometry of the wedge-shaped test object (8). [31] Calibration method according to the preceding claim, where the angle GF varies for the wedge-shaped test objects (8) of different geometries; and / or where the maximum cutting depth (T) varies for the wedge-shaped test objects (8) of different geometries; and / or where the maximum cutting width (B) varies for the wedge-shaped test objects (8) of different geometries. [32] Calibration method according to any one of claims 1 to 31, wherein the particle imaging apparatus (100) is an apparatus from the following list of particle imaging apparatus (100): a particle microscope, a SEM, a TEM, a STEM, a SEM-STEM, a dual particle beam system, a multi-beam particle microscope, a mask repair system. [33] Calibration method according to any one of claims 1 to 32, wherein the particle imaging apparatus (100) comprises a magnetic objective lens (11) and an electrostatic objective lens (10). [34] Calibration method according to the preceding claim, wherein the electrostatic objective lens (10) is arranged or formed within the pole shoes (29) of the magnetic objective lens (11). [35] Method for operating a particle imaging apparatus (100) comprising the following steps: (6a) Provision of a particle imaging apparatus 100), which operates with at least one charged particle beam (122); (6b) Calibrating the particle imaging apparatus (100) with respect to a focus shift fs of the particle beam (122) in a direction orthogonal to the particle-optical axis Z as a function of a focusing change Δz or refocusing Δz of the particle imaging apparatus (100), in particular calibrating according to any one of claims 1 to 25, and / or Calibrating the particle imaging apparatus (100) with respect to a focus shift fs of the particle beam (122) based on the strength of an electrostatic immersion field at an object to be imaged (8) and / or based on a geometry of an object to be imaged (8) in an electrostatic immersion field, in particular calibrating according to one of claims 26 to 31; (6c) Generating at least one particle-optical image of an object (8) using the particle imaging apparatus (100). [36] Method for operating a particle imaging apparatus (100) according to the preceding claim, wherein a plurality of mutually parallel oriented cross-sectional images through a 3D structure (60) are generated with the calibrated particle imaging apparatus (100) and wherein the method further comprises the following steps: (6d) Creating a volumetric image from the cross-sectional images; (6e) Determining a tilt of the 3D structure (60) based on the volume image. [37] Method for operating a particle imaging apparatus (100) according to the preceding claim, wherein the 3D structure (60) comprises a NAND structure.

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