Multi-beam particle microscope with improved multi-beam generator for field curvature correction, and multi-beam generator

Abstract

A multi-beam generator for a charged particle multi-beam system is provided, comprising a stack of multi-aperture plates having at least a first multi-lens array for long-range focal length variation and a second multi-lens array for short-range focal length variation. The aperture diameter of the first multi-lens array is varied to encode pre-compensation for a spherically curved image field at an object plane of the multi-beam system. The aperture diameter of the second multi-lens array is varied to encode pre-compensation for residual image field errors at the object plane not pre-compensated by the first multi-lens array. A control unit of the multi-beam generator provides drive voltages U1 and U2 to the first and second lens arrays based on a current operating point of the charged particle multi-beam system.

Description

[Technical Field]

[0001] Generally, the present invention relates to a multi-beam particle microscope that operates using multiple individual particle beams. Specifically, the present invention relates to a multi-beam particle microscope having an improved multi-beam generator for field curvature correction. [Background technology]

[0002] With the continued development of increasingly smaller and more complex detailed structures, such as semiconductor components, there is a need to develop and optimize planar manufacturing techniques and inspection systems for fabricating and inspecting the small dimensions of these microstructures. For example, the development and manufacture of semiconductor components requires the monitoring of test wafer designs, and planar manufacturing techniques require process optimization for high-throughput, reliable manufacturing. Furthermore, there has recently been an increasing demand for the analysis of semiconductor wafers for reverse engineering and the individual construction of customer-specific semiconductor components. Therefore, there is a need for inspection tools that can be used at high throughput to inspect the microstructures on wafers with high precision.

[0003] Typical silicon wafers used in the manufacture of semiconductor components have a diameter of up to 300 mm. Wafers can be up to 800 mm 2A semiconductor device is divided into 30 to 60 repeating areas ("dies") of various sizes. A semiconductor device comprises multiple semiconductor structures fabricated in layers on the surface of a wafer using planar integration techniques. Due to the manufacturing process, semiconductor wafers are typically flat. The feature sizes of integrated semiconductor structures range from a few microns to a few nanometers, but in the near future, feature sizes or critical dimensions (CDs) are expected to become even smaller, corresponding to the 3-nm, 2-nm, or even smaller technology nodes of the International Technology Roadmap for Semiconductors. Such small feature sizes require rapid identification of defects of critical dimension size over very large areas. For some applications, the specification requirements for the measurement accuracy provided by inspection devices are even higher, for example, by a factor of two or an order of magnitude. For example, the width of semiconductor features must be measured with an accuracy of less than 1 nm, e.g., 0.3 nm or better, and the relative positions of semiconductor structures must be determined with an overlay accuracy of less than 1 nm, e.g., 0.3 nm or better.

[0004] Multibeam scanning electron microscopes (MSEMs) are a relatively new development in the field of charged particle systems (charged particle microscopes, CPMs). Examples of multibeam scanning electron microscopes are disclosed in U.S. Patent No. 7,244,949 (B2) and U.S. Patent Application Publication No. 2019 / 0355544. In a multibeam electron microscope, or MSEM, multiple individual electron beams arranged in a field of view, or raster, simultaneously illuminate a sample. For example, 4 to 10,000 individual electron beams can be provided as primary radiation, with each individual electron beam separated from its neighbor by a pitch of 1 to 200 micrometers. For example, an MSEM has approximately 100 separated individual electron beams ("beamlets") arranged, for example, in a hexagonal raster, with the individual electron beams separated by a distance of approximately 10 micrometers. Multiple charged individual particle beams (primary beams) are individually focused in each case onto the surface of a sample to be inspected by a common wide-field optical system, including, inter alia, a common objective lens. As an example, the sample can be a semiconductor wafer fixed on a wafer holder assembled on a movable stage. During irradiation of the wafer surface with the charged primary individual particle beams, interaction products, such as secondary electrons or backscattered electrons, are emitted from the wafer surface. Their respective starting points correspond to positions on the sample at which the multiple primary individual particle beams are focused in each case. The amount and energy of the interaction products depend, inter alia, on the material composition and the topography of the wafer surface. The interaction products form multiple secondary individual particle beams (secondary beams), which are collected by a common objective lens and, as a result of the projection imaging system of the multi-beam inspection system, are incident on a detector arranged in a detection plane. The detector comprises multiple detection areas, each of which comprises multiple detection pixels, and the detector captures the intensity distribution of each of the secondary individual particle beams. In this process, an image field of, for example, 100 μm×100 μm is acquired.

[0005] Prior art multi-beam electron microscopes include a series of electrostatic and magnetic elements, at least some of which are adjustable to adapt the focal position and astigmatism of the multiple charged individual particle beams. Prior art charged particle multi-beam systems also include at least one crossover plane for the primary or secondary charged individual particle beams. Furthermore, prior art systems include a detection system to facilitate adjustment. Prior art multi-beam particle microscopes also include at least one beam deflector ("deflection scanner") for collective scanning of an area of ​​the sample surface with the multiple primary individual beams to obtain an image field of the sample surface.

[0006] As the requirements for imaging quality increase, so do the requirements for the multi-beam particle microscope used for imaging. For example, to achieve the exceptional resolution of a multi-beam particle microscope, it is necessary to minimize the field curvature at the object plane, i.e., the focal length change. As a first measure, the electron optics or charged particle optics of the multi-beam particle microscope are optimized. However, these measures are limited by Scherzer's theorem. This limit is insufficient to meet the current needs for beam uniformity.

[0007] Therefore, it has been proposed to apply active devices for individual focal length adjustment for each beam, for example, an array of individually addressable ring electrodes as the active part of a micro-Einzel lens array. The focal length of each micro-Einzel lens depends approximately quadratically on the voltage applied to the individual lens electrodes. However, these devices for correcting field curvature for each beam are difficult and expensive to manufacture. Therefore, it is absolutely essential that every single micro-correction device functions perfectly; otherwise, this type of correction device is useless. Furthermore, it is difficult to provide every micro-Einzel lens array with voltages exceeding 50V, 100V, or even 400V, for example, which would result in serious insulation problems and a very limited lifetime for current devices. Examples of active correction devices can be found, for example, in U.S. Patent Nos. 5,834,783, 6,483,120, 6,903,353, 7,126,141, and 11,145,485.

[0008] An alternative approach proposes the use of passive devices, such as an array of monolithic Einzel lens systems, in which only one drive voltage is applied to the entire array. It is known that the focal length of an Einzel lens is approximately proportional to the diameter of the aperture in the Einzel lens's central electrode. Therefore, by appropriately varying the diameter of the apertures in a multi-aperture plate forming part of the Einzel lens array, individual focal length changes for each beam can be achieved. Therefore, by applying only one drive voltage to multi-aperture plates with different hole diameters, field curvature and image field gradient can be corrected. Examples of such passive correction devices can be found, for example, in Japanese Patent No. 60105229, U.S. Patent No. 10,504,681, U.S. Patent No. 10,784,070, and U.S. Patent No. 11,139,138. Further examples are disclosed in U.S. Patent Nos. 10,923,313 (B1) and 11,322,335 (B2).

[0009] However, the variation of aperture diameter in the described passive correction device is substantially limited: the aperture diameter cannot be larger than the beam pitch between adjacent individual particle beams and cannot be smaller than the diameter of an individual particle beam. Furthermore, the voltage applied to a monolithic multi-aperture plate with different aperture diameters cannot be arbitrarily increased without serious insulation and short-circuit problems.

[0010] WO 2007 / 028595 A2 discloses a particle-optical component having two successively arranged multi-aperture plates, the gap between which is shaped to exhibit radial dependence. When a variable potential is applied to each of the two plates, electric fields of different strengths are generated in the gap between the two multi-aperture plates, which can be used to correct field curvature. Further correction can be achieved by combining the two multi-aperture plates with a single aperture plate placed downstream.

[0011] In practice, further challenges arise: modern multi-beam particle microscopes handle ever-increasing numbers of individual particle beams, resulting in ever-increasing image field sizes. Naturally, the larger the image field, the greater the focal length variations within the image field. Additionally, multi-beam particle microscopes and systems are required to operate at different operating points, resulting in different field curvatures that must be corrected very precisely. However, according to prior art, only quasi-static field curvatures are corrected.

[0012] Therefore, the prior art solutions are inadequate, at least for large image fields, and in particular the range of field curvature correction and the variability of field curvature correction over a wide range are inadequate. Summary of the Invention

[0013] It is therefore an object of the present invention to solve one or more of the problems identified above, and in particular to provide high precision field curvature correction for multi-beam particle microscopes that operate with a large number of individual particle beams and therefore have a large image field.

[0014] A further object is to enable dynamic field curvature correction for multi-beam particle microscopes operating at different operating points.

[0015] A further object is to provide a multi-beam particle microscope that allows such a large correction of field curvature, and furthermore, that the field curvature correction should be adjustable within a wide range.

[0016] The object is achieved by the subject matter of the independent claims. Advantageous embodiments of the invention are evident from the dependent claims.

[0017] This patent application claims priority from German Patent Application No. 102023101781.0, filed on January 25, 2023, the entire disclosure content of which is incorporated herein by reference.

[0018] A very general finding of the present invention is that the above objectives can be achieved by a hybrid concept that combines multiple methods.

[0019] Another important aspect of the present invention is the analysis of errors that occur when performing field curvature correction at different operating points of a multi-beam particle microscope. Changes in operating point, such as changes in beam pitch, magnification, landing energy, working distance, etc., result in changes in the refractive power of the particle-optical lens. The change in refractive power generates different field curvatures at the object plane. The field curvature is mathematically a sphere, which means it has a radius, and is therefore also labeled an image shell.

[0020] We analyzed the errors that arise when correcting different field curvatures using a passive concept that applies Einzel lens systems with different aperture diameters. Adjusting the field curvature by changing the drive voltage applied to the central electrode of the micro-Einzel lens array results in a linear difference in the focal length changes of all individual particle beams. On the other hand, when scaling a first sphere with a first radius (corresponding to the first field curvature) to a second sphere with a second radius (corresponding to the second field curvature), mathematically, linear scaling does not exist. Therefore, perfect linear scaling by simply adjusting the applied voltage inevitably results in incomplete field curvature correction. In other words, linear scaling of spherical field curvature does not result in a larger or smaller sphere, but rather in a "compressed" or "stretched" sphere, which is no longer spherical, strictly speaking.

[0021] However, the errors introduced by compressing or expanding the image sphere have certain mathematical properties that are unique to particle-optical systems: said errors are typically third order and can be well approximated by, for example, a fourth order polynomial.

[0022] Therefore, the present invention employs a novel two-stage approach. First, a long-range focal length change is performed at a selected operating point using a micro-Einzel lens array with multiple apertures of variable diameter. Second, specific errors introduced in the long-range focal length change are specifically corrected by a subsequent short-range focal length change. This error correction can be encoded in a second micro-Einzel lens array with multiple apertures whose aperture diameters change according to the error, or by an active correction device, for example, using individually adjustable ring electrodes.

[0023] More particularly, according to a first aspect of the invention, the invention relates to a multi-beam charged particle microscope with an adjustable image shell or field curvature, the multi-beam charged particle microscope comprising: a multi-beam generator configured to generate a first field of view of a plurality of charged first individual particle beams; a first particle-optical unit having a first particle-optical beam path and configured to image the generated first individual particle beam onto an object at an object plane, wherein the first individual particle beam can impinge on the object surface at an incidence position that forms a second field of view; a detection system having a plurality of detection regions forming a third field of view; a second particle-optical unit having a second particle-optical beam path and configured to image a second individual particle beam emanating from an incidence position in the second field of view onto a third field of view of a detection region of the detection system; a magnetic and / or electrostatic objective lens through which both the first and second individual particle beams pass; a beam switch disposed in the first particle beam path between the multi-beam particle generator and the objective lens, and in the second particle beam path between the objective lens and the detection system; a sample stage for holding and / or positioning the object during object inspection; Controller and Equipped with the multi-beam charged particle microscope is adapted to operate at a plurality of operating points, and by operating the multi-beam charged particle microscope at a plurality of operating points, the first particle-optical unit generates, at an object plane, a plurality of spherically curved image fields, each of which is pre-compensated by the multi-beam generator; The multi-beam generator is a filter plate comprising a plurality of filter apertures for generating a plurality of first individual particle beams, the filter plate being connected to ground potential in use; - a stack of multi-aperture plates having at least a first multi-lens array for long-range focal length variation and a second multi-lens array for short-range focal length variation, the first multi-lens array comprising: a first multi-aperture plate comprising a plurality of first apertures and connected in use to earth potential; a second multi-aperture plate comprising a plurality of second apertures and connected in use to a first drive voltage, the diameters of the plurality of second apertures varying according to a first function of the distance of each second aperture from an optical axis (A) of the multi-beam particle microscope, the first function being adapted to pre-correct for a spherically curved image shell at the object plane; a third multi-aperture plate having a plurality of third apertures and connected to ground potential in use; Equipped with the centers of the first, second, and third plurality of openings are aligned with one another; a second multi-lens array aligned with the first multi-lens array; the second multi-lens array is configured to pre-compensate for residual image shell errors at the object plane that are not pre-compensated by the first multi-lens array; and Equipped with the controller is configured to provide a first drive voltage to the first multi-lens array based on an operating point of the multi-beam particle microscope; The controller is configured to provide a second drive voltage to the second multi-lens array based on an operating point of the multi-beam particle microscope.

[0024] It should be noted that the potential to which the first and third aperture plates are set can be any voltage, since only the potential difference relative to the particle or electron source is relevant. For practical reasons, it is useful to set this potential to ground. In the remainder of this text, "ground potential" will be used to refer to the reference potential for all other potentials mentioned.

[0025] The individual charged particle beams can be, for example, electrons, positrons, muons or ions, or other charged particles.

[0026] The apertures in the first and second multi-lens arrays are circular. Preferably, the apertures in the multi-lens arrays have a regular arrangement, such as a rectangular, square, or hexagonal arrangement. Preferably, in the case of a hexagonal arrangement, 3n(n-1)+1 apertures are provided, where n is any natural number.

[0027] The controller of the multi-beam charged particle microscope may include multiple control units. For example, each control unit of the multi-beam charged particle microscope may control a portion or function of the multi-beam charged particle microscope. For example, a separate control unit may be provided for controlling the multi-beam generator.

[0028] According to the present invention, at least a plurality of second apertures of the first multi-lens array have diameters that vary according to a first function of the distance of the respective second aperture from the optical axis of the multi-beam particle microscope. In other words, the aperture diameter is proportional to r, the distance to the optical axis Z. However, this does not prevent an offset being added to the distance r, thereby shifting the position of the apertures in the second multi-aperture plate, even if this is not explicitly mentioned in the following text.

[0029] The first plurality of apertures of a first multi-aperture plate provided upstream of a second multi-aperture plate can have a constant diameter, and the third multi-aperture plate comprising a third plurality of third apertures can also have a constant diameter, however, it is also possible for the diameters of the first plurality of apertures and / or the third plurality of apertures to vary according to a first function or another function.

[0030] The stack of multi-aperture plates comprises at least three plates of the first multi-lens array. In practice, all of these multi-aperture plates can be provided as separate plates. However, alternative design possibilities can be chosen, such as providing different layers with apertures that act as plates on a substrate.

[0031] Preferably, the filter plate is also provided as a separate plate, however, it is also possible to combine the filter plate with the first multi-aperture plate of the first multi-lens array.

[0032] The stack of multi-aperture plates can also include another multi-lens array for long-range focal length variation. The entire stack can then include, for example, five multi-aperture plates forming two consecutive Einzel lens arrays, each with an aperture diameter that varies according to a first function of the distance of the respective aperture from the optical axis. The third multi-aperture plate of the first Einzel lens array can then provide the first multi-aperture plate for another Einzel lens array. By providing two consecutive Einzel lens arrays, the voltage applied to the multi-aperture plate with variable-diameter apertures can be reduced, contributing to even greater long-range focal length variation.

[0033] In either case, the stack of multi-aperture plates, including at least a first multi-lens array, is configured for a long-range focal length change. This long-range focal length change typically provides most of the focal length change required to compensate for the field curvature at the object plane. In contrast, the second multi-lens array, designed for a short-range focal length change, achieves a relatively short focal length change. The primary function of the second multi-lens array is to precompensate for residual image shell errors at the object plane. These residual image shell errors are not precompensated by the first multi-lens array. In many cases, the residual image shell errors are not only not precompensated by the first multi-lens array, but in principle cannot be precompensated by the first multi-lens array. Precompensating for residual image shell errors corrects the shape of the image shell from a compressed or elongated image shell to a spherically curved image shell.

[0034] The first function applied to pre-compensate this spherically curved image shell in the object plane can in principle be of any type. However, in practice, this first function usually has a quadratic dependence on the distance r to the optical axis Z of the multi-beam particle microscope. A parabolic image shell in the object plane, or at least an image shell very similar to a spherical image shell, is then pre-compensated. However, it is also possible for the first function itself to already contain higher-order correction terms to generate, for at least one operating point, a pre-compensated shape of the image shell that is even closer to an ideal spherically curved image surface.

[0035] According to a preferred embodiment of the present invention, in the second multi-aperture plate, the variation of the diameter of the second aperture according to the first function is optimal for pre-compensating for a spherically curved image shell in the object plane during use at a preselected reference operating point; the controller is configured to provide a first non-zero drive voltage to the first multi-lens array at a reference operating point; The controller is configured to provide a second drive voltage, which is essentially zero, to the second multi-lens array at the reference operating point. In other words, at the reference operating point, only the first multi-lens array can be active to optimally pre-compensate for field curvature. For this correction at the reference operating point, the second multi-lens array is not required. It should be noted that the first drive voltage applied at the reference operating point is not necessarily the maximum first drive voltage. Instead, when comparing the first drive voltages provided at different operating points, the first drive voltage can be a small or moderate drive voltage.

[0036] According to another embodiment, the first function of the second multi-aperture plate is designed so that the magnitude of the residual image shell error that needs to be corrected by the second multi-lens array does not exceed a predetermined limit. In this case, although the first function may not be able to completely correct the field curvature at the reference operating point or other operating points, the residual image shell error can be well pre-compensated for all operating points, so that the entire adjustment for pre-compensating the field curvature can be most conveniently performed.

[0037] According to a preferred embodiment, the controller of the multi-beam charged particle microscope is configured to provide a non-zero first drive voltage to the first multi-lens array at the second operating point, the first drive voltage being different from the first drive voltage provided at the reference operating point; The controller is configured to provide a second non-zero drive voltage to the second multi-lens array at a second operating point, such that the change in the first drive voltage essentially reflects a long-distance focal length change necessitated by the change in operating point, and the provision of the second drive voltage essentially serves to pre-compensate for residual image shell error.

[0038] According to a preferred embodiment, the second multi-lens array comprises a fourth multi-aperture plate comprising a plurality of fourth apertures and connected to ground potential in use; a fifth multi-aperture plate comprising a plurality of fifth apertures, the fifth multi-aperture plate being connected in use to a second drive voltage U2, wherein diameters of the plurality of fifth apertures vary according to a second function of a distance r of each aperture from an optical axis Z of the multi-beam particle microscope, the second function being adapted to pre-compensate for residual image shell errors at the object plane that are not pre-compensated by the first multi-lens array; a sixth multi-aperture plate having a plurality of sixth apertures and connected to ground potential in use; Equipped with the centers of the fourth, fifth, and sixth openings are aligned with one another; The controller is configured to control the second drive voltage U2 provided to the fifth multi-aperture plate based on an operating point of the multi-beam particle microscope.

[0039] According to this embodiment, the residual shell error is essentially pre-compensated by another monolithic Einzel lens array. The necessary correction is encoded in the change in diameter of the fifth aperture of the fifth multi-aperture plate. The second function is a function of the distance r of each fifth aperture relative to the optical axis Z. However, this does not preclude adding an offset to the distance r, thereby shifting the position of the apertures in the fifth multi-aperture plate, even though this is not explicitly stated in the text below.

[0040] The encoded residual error correction can also be combined with the additional focal length variation already encoded in the second multi-aperture plate of the first multi-lens array, thereby increasing the overall focal length variation range. According to a preferred embodiment, the first multi-lens array is arranged directly adjacent to the filter plate. In other words, the first multi-lens array is the first multi-lens array after the filter plate in the direction of the particle-light beam path through the multi-beam generator. The first multi-lens array is preferably immediately followed by the second multi-lens array. However, the arrangement order of the first and second multi-lens arrays can also be reversed.

[0041] According to a preferred embodiment, the third multi-aperture plate of the first multi-lens array and the fourth multi-aperture plate of the second multi-lens array are provided as identical multi-aperture plates. Alternatively, the first multi-aperture plate of the first multi-lens array and the sixth multi-aperture plate of the second multi-lens array are provided as identical multi-aperture plates. In both cases, an overall alternation of multi-aperture plates connected to ground potential and multi-aperture plates (with variable aperture encoding correction) connected to a drive voltage is provided. The last multi-aperture plate in the sequence is again connected to ground potential.

[0042] According to a preferred embodiment, the first function f1(r) is

number

[0043] According to a preferred embodiment, the second function f2(r) is

number

[0044] According to a preferred embodiment, the second function f2(r) is not the inverse of the first function f1(r).

[0045] According to a further preferred embodiment of the present invention, the second multi-lens array comprises a multi-aperture plate with a plurality of apertures, each aperture being surrounded by a plurality of individually addressable ring electrodes; The controller is configured to provide a respective second drive voltage U2i to each of the ring electrodes based on an operating point of the multi-beam particle microscope. Therefore, in this embodiment, the second multi-lens array is realized by an active device. However, the practical problems that arise when using such an active device do not arise in this particular embodiment, and the active device is only used to correct residual shell errors, which does not require the use of high voltages. Therefore, serious insulation or short-circuit problems do not arise.

[0046] According to a preferred embodiment, for each second drive voltage U2i the following relationship holds: The relationship 0 V≦U2i≦20 V is satisfied, preferably 0 V≦U2i≦10 V. These driving voltages are approximately one order of magnitude smaller than the driving voltages required when applying ring electrodes for long-range focal length change, as attempted in the prior art.

[0047] According to a preferred embodiment, for the first drive voltage U1, the following relationship holds: U1≦100 V, preferably U1≦150 V or U1≦200 V or U1≦400 V. These relationships hold true at each operating point of the multi-beam particle microscope.

[0048] According to a preferred embodiment, the ratio of the focal length change Δz1 achieved by the first multi-lens array alone to the overall focal length change Δz achieved within the image field at the object plane is, for all operating points, Δz1 / Δz≧0.80, preferably Δz1 / Δz≧0.90, and more preferably Δz1 / Δz≧0.95. The focal length change is defined as the difference in focal length in the Z direction of one image shell. The Z direction is the propagation direction of the individual particle beams and is also parallel to the optical axis Z. The achieved overall focal length change Δz can be the sum of the focal length change Δz1 achieved with the first multi-lens array and the focal length change Δz2 achieved with the second multi-lens array. However, other multi-lens arrays or other measures may further contribute to the achieved overall focal length change Δz.

[0049] According to a preferred embodiment, the multi-beam particle microscope satisfies the following relationship: The multi-beam generator of the present invention is configured to pre-compensate for focal length changes Δz within the image field at the object plane such that Δz≧1.0 μm, preferably Δz≧3 μm, and most preferably Δz≧12 μm. Thus, the overall focal length changes Δz that can be pre-compensated for by the multi-beam generator of the present invention are large, allowing for correction of field curvature for large image fields. Exemplary sizes for large image fields are 100 μm×100 μm, 200 μm×200 μm, or 500 μm×500 μm, or even larger.

[0050] According to a preferred embodiment of the present invention, the multi-beam generator includes at least one additional multi-lens array that contributes to pre-compensation of focal length changes within the image field at the object plane. This additional multi-lens array can be part of a stack of multi-aperture plates and therefore primarily contributes to long-range focal length changes. However, it can also be provided as an additional multi-lens array that contributes to pre-correction of residual image shell errors. Preferably, the additional multi-lens array can contribute overall to increasing the maximum possible focal length change. Since large focal length changes are very difficult to achieve, even small contributions to the possible focal length change can actually be of great value. Furthermore, one or more additional multi-lens arrays can pre-compensate for types of focal length changes within the image field other than field curvature, such as field gradient or tilt.

[0051] According to a preferred embodiment of the present invention, the multi-beam generator further comprises a first tilt-compensating multi-lens array, the first tilt-compensating multi-lens array comprising: a first multi-aperture plate comprising a plurality of first apertures and connected to ground potential in use; a second multi-aperture plate comprising a plurality of second apertures, the second multi-aperture plate being connected in use to a first gradient drive voltage, the diameters of the second apertures varying as a linear function f(x) of the position of each aperture in a direction x perpendicular to the optical axis Z; a third multi-aperture plate having a plurality of third apertures and connected to ground potential in use; Equipped with the centers of the first, second, and third openings are aligned with one another; The controller is configured to provide a first tilt drive voltage to the first tilt-compensated multi-lens array based, inter alia, on an operating point of the multi-beam particle microscope. The apertures in the first, second, and third multi-aperture plates are circular. The diameter of the apertures in the first and third multi-aperture plates may be constant, but may vary in the same manner as the second apertures in the second multi-aperture plate, or in a different manner.

[0052] The first tilt-compensating multi-lens array is suitable for compensating for image field gradients in the object plane. There are various reasons for image field gradients or tilts. Field gradients can be caused, for example, by aberrations or asymmetric beam splitter designs. Additionally or alternatively, an object in the object plane, such as a wafer surface, may not be perfectly perpendicular to the optical axis Z of the multi-beam particle microscope.

[0053] According to a preferred embodiment, the multi-beam generator further comprises a second tilt-compensating multi-lens array, the second tilt-compensating multi-lens array comprising: a fourth multi-aperture plate comprising a plurality of fourth apertures and connected to ground potential in use; a fifth multi-aperture plate comprising a plurality of fifth apertures, the fifth multi-aperture plate being connected in use to a second gradient drive voltage, the diameters of the fifth apertures varying as a fundamental linear function f(y) of the position of each aperture in a direction y perpendicular to the optical axis Z and which is linear independent of the direction x; a sixth multi-aperture plate having a plurality of sixth apertures and connected to ground potential in use; Equipped with the centers of the fourth, fifth, and sixth openings are aligned with one another; The controller is configured to provide a second tilt drive voltage to the second tilt-compensated multi-lens array based, inter alia, on an operating point of the multi-beam particle microscope.

[0054] Thus, the second tilt-compensated multi-lens array operates essentially in the same way as already described for the first tilt-compensated multi-lens array. Preferably, the directions x and y are perpendicular to each other.

[0055] Preferably, a third tilt-compensating multi-lens array and / or a fourth tilt-compensating multi-lens array is provided to enable tilt compensation in the −x and −y directions. A stack of four tilt-compensating multi-lens arrays with compensation properties in the x, −x, y, and −y directions can compensate for tilt in both the x and y axes, making it possible to correct beam tilt in any desired direction.

[0056] Instead of providing four different tilt-compensating multi-lens arrays, it is also possible, for example, to encode the deflected tilt compensation in the first multi-lens array. More specifically, according to a preferred embodiment, the first function representing the diameter change of the plurality of second apertures in the second multi-aperture plate of the first multi-lens array can be not only a function of the distance r from the optical axis Z, but also a linear function of the position of each aperture in the direction -x and / or the direction -y, where the directions -x and -y are perpendicular to the optical axis Z and are linear independently of each other, preferably perpendicular to each other, and the linear functions of -x and / or -y are adapted to deflect the field gradient at the object plane. The deflectable field gradient can arise, for example, due to an asymmetric beam splitter design, but can also arise for other reasons. In any case, if the overall tilt is deflected sufficiently, it is sufficient to provide two more tilt-compensating multi-lens arrays for any desired tilt compensation.

[0057] According to a preferred embodiment, the multi-beam generator further comprises an astigmatic multi-aperture plate comprising a plurality of apertures, with a plurality of individually addressable electrostatic multipole electrodes arranged around each aperture; The controller is configured to provide a set of drive voltages U4ij to each of the multipole electrodes, and each set of drive voltages U4ij is configured to provide an individual offset voltage U4ij common to all electrodes of the respective multipole electrodes based on an operating point of the multibeam particle microscope. offset It is equipped with: The multipole electrodes can be, for example, octapole or dodecapole electrodes. When the same potential is applied to segmented electrodes arranged around a particular aperture, the effect of the multipole electrodes is essentially the same as that of ring electrodes, which can accommodate focal length changes when the ring electrodes are part of an Einzel lens system. The offset voltage U4ij offset By individually adjusting , the overall range of focal length variation and the possibility of pre-compensating for field curvature (or equivalently, image field gradient) can be improved.

[0058] In general, the order of the different correctors in the multi-beam generator, e.g. the order of the multi-lens array, tilt-compensating multi-lens array and astigmatism multi-aperture plate in the multi-beam generator, can be freely chosen, however it may be advantageous to provide the astigmatism-correcting multi-aperture plate as the first corrector in the sequence of correctors, as this position may have advantages in terms of alignment / adjustment.

[0059] According to a preferred embodiment of the present invention, the multi-beam generator comprises: a termination multi-aperture plate having a plurality of termination apertures, the diameter of the termination apertures varying as a function of the distance of each aperture from the optical axis, the termination multi-aperture plate being connected to ground potential in use; an electrode aperture plate having a single aperture through which all of the first individual particle beams pass, the electrode aperture plate being connected in use to an extraction voltage; an electrode aperture plate, the centers of which are aligned with each other; In this order, The controller is configured to provide an extraction voltage to the electrode aperture plate based on the operating point of the multi-beam particle microscope, thereby varying the extraction electric field, and therefore the immersion lens effect contributes to the focal length change. The terminal multi-aperture plate is the last multi-aperture plate in the multi-beam generator when described in the direction of the particle light beam path. This terminal multi-aperture plate can be, for example, the last multi-aperture plate of a second multi-lens array for short-range focal length change, but can also be another multi-aperture plate of another multi-lens array or be provided separately.

[0060] The above effect is further enhanced if the shape of the terminating multi-aperture plate is additionally varied as a function of the distance r from the optical axis Z. This additional variation has been found to be extremely efficient and sensitive to focal length changes.

[0061] Changing the shape of the end multi-aperture plate can be achieved, for example, by changing the thickness of the end multi-aperture plate. According to a preferred embodiment, the top surface of the end multi-aperture plate is flat, and the bottom surface of the end multi-aperture plate is convexly shaped relative to the direction of the particle light beam path, or alternatively, the bottom surface of the end multi-aperture plate is concavely shaped relative to the direction of the particle light beam path. Furthermore, depending on the specific shape of the bottom surface of the end multi-aperture plate, the distance between one electrode aperture plate and the respective end aperture of the other end multi-aperture plate also changes. This contributes to a further change in the extraction field of view and contributes to a change in the focal length.

[0062] Further investigation by the inventors revealed that the focal length change can be further increased by optimizing the plate dimensions, especially those of the first multi-lens array that are applied to long-distance focal length changes. The maximum field curvature correction potential can be achieved by using the shortest electrode length of the second multi-aperture plate, the shortest gap not only between the first and second multi-aperture plates but also between the second and third multi-aperture plates of the first microlens array, and the counter electrode of the Einzel lens system with the smallest thickness. More specifically, the following geometric conditions are recommended:

[0063] According to a preferred embodiment, the following relationship holds for the thickness L2 of the second multi-aperture plate of the first multi-lens array: L2≦50 μm, preferably L2≦30 μm.

[0064] According to a preferred embodiment, the following relationship holds for the thickness L1 of the first multi-aperture plate of the first multi-lens array: L1≧80 μm, preferably L1≧100 μm, and / or the following relationship holds for the thickness L3 of the third multi-aperture plate of the first multi-lens array: L3≧80 μm, preferably L3≧100 μm.

[0065] According to a preferred embodiment, the following relationship holds for the gap G1 between the first multi-aperture plate and the second multi-aperture plate of the first multi-lens array: G1≦10 μm, preferably G1≦5 μm; and / or Regarding the gap G2 between the second multi-aperture plate and the third multi-aperture plate of the first multi-lens array, the following relationship holds: G2≦10 μm, preferably G2≦5 μm.

[0066] In principle, the above findings regarding the dimensions of Einzel lens arrays are transferable to other multi-lens arrays as well.

[0067] According to a further preferred embodiment, the multi-beam generator further comprises means for generating a voltage gradient on the second multi-aperture plate of the first multi-lens array, and the controller is configured to provide a gradient drive voltage to the second multi-aperture plate to pre-compensate for a field gradient at the object plane. Since the focal length change depends on the applied voltage, providing a voltage gradient can further contribute to the focal length change. Providing a voltage gradient provides another possibility to adapt a linear change in focal length.

[0068] According to a preferred embodiment, the second multi-aperture plate has a high-resistivity coating on its surface that contacts the opposite side of the second multi-aperture plate to generate an adjustable voltage gradient. For example, there may be two opposing contacts, particularly elongated contacts, but there may also be a further pair of opposing contacts. Additionally or alternatively, another pair of opposing contacts may be provided, arranged to provide an additional gradient in a further direction, preferably orthogonal to the gradient direction generated by the first pair of opposing contacts.

[0069] According to further embodiments of the present invention, the multi-beam particle microscope further comprises means for in-situ plasma cleaning of the monolithic multi-aperture plate and / or means for providing a small partial pressure of hydrogen gas during use of the multi-beam particle microscope for cleaning purposes. Hydrogen can be provided continuously during operation of the multi-beam particle microscope, or in a pulsed or intermittent manner between different recordings or imaging of different frames, or generally during pauses in the imaging process. Additionally or alternatively, means can be provided for continuously heating the monolithic multi-aperture plate. Heating contributes to cleaning the monolithic multi-aperture plate. Cleaning is particularly suitable when only passive correction devices, such as the monolithic multi-aperture plate, are applied. Furthermore, cleaning is very important because impurities or debris in apertures with precisely varying diameters can reduce the accuracy of the high-precision corrections required to pre-compensate for field curvature or image field tilt.

[0070] The embodiments of the first aspect of the present invention can be combined with each other fully or partially unless a technical contradiction arises.

[0071] According to a second aspect of the invention, the invention relates to a multi-beam generator for a charged particle multi-beam system, the multi-beam generator comprising: a filter plate comprising a plurality of filter apertures for generating a plurality of first individual particle beams, the filter plate being connected to ground potential during use; A stack of multi-aperture plates having at least a first multi-lens array for long-range focal length change and a second multi-lens array for short-range focal length change, the first multi-lens array comprising: a first multi-aperture plate comprising a plurality of first apertures and connected to ground potential in use; a second multi-aperture plate comprising a plurality of second apertures, the second multi-aperture plate being connected in use to a first drive voltage, wherein diameters of the plurality of second apertures vary according to a first function of distance of each second aperture from an optical axis (A) of the multi-beam system, the first function adapted to pre-compensate for a spherically curved image field at an object plane of the multi-beam system; a third multi-aperture plate having a plurality of third apertures and connected to ground potential in use; Equipped with the centers of the first, second, and third openings are aligned with one another; a second multi-lens array aligned with the first multi-lens array; a second multi-lens array configured to pre-compensate for residual image field errors at the object plane that are not pre-compensated by the first multi-lens array; and A control unit, configured to provide a first drive voltage to the first multi-lens array based on an operating point of the multi-beam system; a control unit configured to provide a second drive voltage to the second multi-lens array based on an operating point of the multi-beam system; and It is equipped with:

[0072] The multi-beam generator may be provided with some or all of the features already described with respect to the first aspect of the invention relating to a multi-beam charged particle microscope comprising a multi-beam generator. Of course, the multi-beam generator is also applicable to other charged particle multi-beam systems. Explicit reference is again made to the first aspect of the invention.

[0073] The present invention may be better understood with reference to the accompanying drawings. [Brief explanation of the drawings]

[0074] [Figure 1] Schematic diagram of a multi-beam particle microscope (MSEM). [Figure 2] FIG. 1 illustrates an example of a multi-beam generator. [Figure 3] FIG. 10 is a diagram illustrating the effect of field curvature and image field gradient on the resolution of a multi-beam particle system. [Figure 4] FIG. 10 is a diagram illustrating a schematic diagram of the effect of pre-compensation for image curvature and image field gradient on the resolution of a multi-beam particle system. [Figure 5] 1A and 1B are diagrams illustrating schematically important aspects of field curvature correction according to the present invention; [Figure 6] 1A and 1B illustrate a schematic diagram of a multi-beam generator according to the present invention; [Figure 7] FIG. 10 is a diagram typically showing the top surface of a multi-aperture plate with different aperture diameters. [Figure 8] 1 is a graphical representation of afterimage shell error. [Figure 9] FIG. 10 is a diagram illustrating a multi-aperture plate having different aperture diameters. [Figure 10] FIG. 10 is a schematic diagram illustrating a multi-aperture plate of a tilt-compensated multi-lens array. [Figure 11] FIG. 10 is a diagram illustrating another multi-aperture plate of a multi-beam generator. [Figure 12] 10A and 10B show schematic diagrams of another multi-beam generator according to the present invention; [Figure 13] 10A-10C are diagrams showing schematic diagrams of end multi-aperture plates with different shapes. [Figure 14] 10A and 10B are diagrams illustrating another end multi-aperture plate having a different shape. [Figure 15] 10A and 10B show schematic diagrams of another multi-beam generator according to the present invention; [Figure 16] 10A and 10B show schematic diagrams of another multi-beam generator according to the present invention; [Figure 17] 10A and 10B show schematic diagrams of another multi-beam generator according to the present invention; [Figure 18] FIG. 2 is a diagram illustrating the dimensions of a multi-lens array. DETAILED DESCRIPTION OF THE INVENTION

[0075] FIG. 1 schematically illustrates a multi-beam particle microscope 1. The multi-beam particle microscope 1 includes a beam generator 300 with a particle source 301, e.g., an electron source. A diverging particle beam 309 is collimated by a series of condenser lenses 303.1 and 303.2 and impinges on a multi-aperture arrangement 305. The multi-aperture arrangement 305 includes a plurality of multi-aperture plates 306 and a field lens 307. A plurality of individual particle beams 3, e.g., individual electron beams 3, are generated by the multi-aperture arrangement 305. The center points of the apertures in the multi-aperture plate arrangement 305 are positioned within a field of view that is imaged onto another field of view formed by a beam spot 5 at the object plane 101. The center-to-center distance between the apertures in the multi-aperture plate 306 can be, for example, 5 μm, 100 μm, and 200 μm. The diameter D of the aperture is smaller than the center-to-center distance of the apertures; examples of the diameter are 0.2, 0.4, and 0.8 times the center-to-center distance of the apertures.

[0076] The multi-aperture arrangement 305 and the field lens system 308 are configured to generate multiple focal points 323 of the first individual particle beam 3 in a raster arrangement within a plane 321. The plane 321 does not need to be a flat surface, but can be a spherically curved surface to pre-compensate for the field curvature of the subsequent particle-optical system. Alternatively, the beam focal points 323 may be virtual. The diameters of the beam focal points 323 are, for example, 10 nm, 100 nm, 1 μm, etc.

[0077] The multi-beam particle microscope 1 further includes a system of electromagnetic lenses 103 and objective lenses 102 that demagnify and image the beam focus 323 from the intermediate image plane 321 onto the object plane 101. The first individual particle beams 3 pass through a beam splitter 400 and a collective beam deflection system 500, which are used to deflect the multiple first individual particle beams 3 during operation to scan the image field. The first individual particle beams 3 incident on the object plane 101 form, for example, a substantially regular field or array, and the distance between adjacent incident positions or beam spots 5 can be, for example, 1 μm, 10 μm, or 40 μm. The field formed by the positions of the incident spots or beam spots 5 can have, for example, rectangular or hexagonal symmetry. The diameter of the beam spots 5 formed on the object plane 101 can be small. Exemplary values ​​of the diameter are 1 nm, 5 nm, 10 nm, 100 nm, and 200 nm. The focusing of the individual particle beam 3 to form the beam spot 5 is performed by an objective lens system 102 .

[0078] The object to be inspected may be of any type, for example a semiconductor wafer or a biological sample, and may comprise an arrangement of miniaturized elements, etc. The surface 15 of the object 7 is arranged in the objective plane 101 of the objective lens 102. The objective lens 102 may comprise one or more electron-optical lenses, for example a magnetic objective lens and / or an electrostatic objective lens, etc.

[0079] Primary particles 3 impinging on an object 7 generate interaction products, such as secondary electrons, backscattered electrons, or primary particles that have undergone a reversal of motion for other reasons, which emanate from the surface or object plane 101 of the object 7. The interaction products emanating from the surface 15 of the object 7 are shaped by an objective lens 102 to form a secondary particle beam 9 or second individual particle beam 9. The secondary particle beam 9 then passes through a beam splitter 400 positioned after the objective lens 102 and is directed along the particle-light beam path to a projection system 200. The projection system 200 includes an imaging system 205 having multiple projection lenses or projection lens systems 208, 209, and 210, a contrast aperture 214, and a multi-particle detector 207. The incidence positions 25 of the secondary individual particle beam 9 in the detection region of the multi-beam particle detector 207 are within a third field of view with a fixed spacing between the incidence positions. Exemplary values ​​are 10 μm, 100 μm, and 200 μm.

[0080] The multi-beam particle microscope 1 further comprises a computer system or control unit 10, which may be of single or multi-component design, designed to control the individual particle optical components of the multi-beam particle microscope 1 and to evaluate and analyze the signals acquired by means of the multi-detector 207 or detection unit 207.

[0081] Further information on such multi-beam particle beam systems or multi-beam particle microscopes and components such as particle sources, multi-aperture plates and lenses used therein can be taken from WO2005 / 024881A2, WO2007 / 028595A2, WO2007 / 028596A1, WO2011 / 124352A1 and WO2007 / 060017A2, as well as DE102013016113A1 and DE102013014976A1, the full disclosures of which are incorporated herein by reference.

[0082] FIG. 2 illustrates an example of a multi-beam generator 305. According to FIG. 2, the multi-beam generator 305 includes a series of five multi-aperture plates 304 and 306.2-306.5 arranged in the z-direction of propagating electrons, and a global lens 307. Each multi-aperture plate 304, 306.2-306.5 includes a plurality of apertures 85.1-85.5 spaced apart at the same lateral distance P1, and each plate is aligned to generate and shape a plurality of primary charged particle beamlets 3. The plurality of multi-aperture plates 304 and 306.2-306.5 and the global lens electrode 307 are spaced apart by spacers 83.1-83.4 and spacer 86. The multi-beam generating unit 305 is illustrated in cross section (x,z). Only four apertures 85.1-85.5 are illustrated in each multi-aperture plate, along with the inner membrane zone 335 and support zone 333. A portion of the collimated incident electron beam 309 passes through aperture 85.1, forming multiple individual charged particle beams 3. The filter plate 304 includes a metal layer 99 on the beam entrance side to stop and absorb the electron beam 309 impinging around the multiple apertures 85.1. The bulk material of the filter plate 304 is made of a conductive material, such as doped silicon, and is connected to ground. The second multi-aperture or ground electrode plate 306.2 is made of a conductive material, such as doped silicon, and is connected to ground (0 V). The third multi-aperture plate 306.3 is a two-layer lenslet plate. The first layer 306.3a includes multiple ring electrodes 79 for multiple apertures, each configured to individually change the focal position of a corresponding primary charged particle beamlet, e.g., charged particle beamlets 3.1-3.4. The second layer 306.3b downstream of the first layer 306.3a is isolated from the first layer and is made of a conductive material, such as doped silicon. The second layer 306.3b is connected to ground (0 V).During use, the ground electrode plate 306.2, the first layer 306.3a, and the second layer 306.3b form a plurality of individually adjustable Einzel lenses for the plurality of primary charged particle beamlets 3. The multi-beam generating unit 305 further includes a fourth multi-aperture or multi-astigmatism plate 306.4. The multi-astigmatism plate includes four or more electrodes 81, e.g., eight electrodes, for each of the plurality of apertures 85.4 (not labeled in FIG. 2). During use, different voltages ranging from -20V to +20V can be applied to each of the electrodes, thereby individually affecting each beamlet 3.1-3.4. For example, an asymmetric voltage difference can deflect each beamlet 3.1-3.4 by up to several microns in each direction to pre-compensate for distortion aberrations of the illumination unit 100. For example, the astigmatism of each beamlet 3.1-3.4 can be compensated for. Using an offset voltage, each multipole element can further be implemented as an Einzel lens. Each multipole element, together with the second layer 306.3b and the hybrid lens plate 306.5, both connected to ground (0 V or a suitable reference potential), can form an offset circular lens field. This further extends the focal length DF. The fifth multi-aperture plate or hybrid lens plate 306.5 is fabricated from doped silicon and forms an additional electrode connected to ground. In the example of FIG. 2, the first condenser lens 307 is connected to the multi-beam generator 305. The global lens 307 includes a ring electrode 82 and can be supplied with a high voltage of 3 kV to 20 kV, e.g., 12 kV to 17 kV. The lens 307, on the other hand, forms a global electrostatic lens field for globally focusing multiple primary charged particle beamlets 3, including beamlets 3.1 to 3.4. The electrostatic lens field penetrates each of the apertures, eg aperture 85.5, of hybrid lens plate 306.5, creating an additional electrostatic lens field with focusing power at each aperture of hybrid lens plate 306.5.However, the electrostatic lens fields of hybrid lens plate 306.5 cannot be individually adjusted and do not allow for variable field tilt or compensation for variable amounts of field curvature. Using optional additional field lens 308, each of multiple primary charged particle beamlets 3, including beamlets 3.1-3.4, is focused, during use, onto a curved and tilted intermediate image plane 321 to form an astigmatism-corrected focal spot.

[0083] FIG. 3 schematically illustrates the effect of field curvature and image field gradient on the resolution of a multi-beam particle system. FIG. 3 illustrates the geometric characteristics at the focal plane in a cross-sectional view. Five beam cones 62.1-62.5 of individual primary beams 3 near the focal plane are exemplarily illustrated. Each beam cone 62.1-62.5 has a beam waist with a minimum spot area 74.1-74.5. The beam waists 74.1-74.5 are spherical and lie on a curved surface with radius R, or can be approximated by a sphere with radius R. Furthermore, because the center 43 of the spherical image field is not located on the optical axis Z, the image curvature is not symmetrical on both sides of the optical axis Z. This is due to the current field gradient or image field tilt, indicated by line 45 in FIG. 3a.

[0084] As a result of the field curvature and field tilt, there is no sharp focal plane and there is a gap 51 between the lower focal plane 47 and the upper focal plane 49. In addition, Figure 3a illustrates the y-axis passing through the position of the axial primary beam, designated by reference numeral 41 in the figure.

[0085] Figure 3b shows a cut along the y-axis of Figure 3a, with the x and y planes corresponding to the paper plane in Figure 3b. The resolution for imaging an object located in the x and y plane is shown. The resolution is optimal for the smallest beam waists 74.1-74.5, but only a portion of the beam waists 74.1-74.5 is precisely located in the object plane or x and y plane. The spot size shown in Figure 3b corresponds to the diameter of the beam cone in the x and y plane or object plane 101. Figure 3b also shows a resolution change with a rather concentric structure for spot 55, but spot 55 is not the beam waist of central beam cone 62.3, but the adjacent beam cone 62.2. Point 53.6 shows an approximately concentric ring or hexagon with a focal point having a similar resolution. On the right side, reference numerals 57.1, 57.2, and 57.3 indicate beam cone diameters that are too large to meet the resolution criteria of a multibeam charged particle microscope or any other multibeam system. Therefore, a suitable correction of the image field at the object plane 101 is required. To achieve uniform resolution of the multiple individual particle beams 3, not only the field curvature but also the field gradient needs to be pre-compensated.

[0086] Figure 4 illustrates schematically the effect of correct pre-compensation for image curvature and image field gradient on the resolution of a multi-beam particle system. In contrast to the situation shown in Figure 3a, in Figure 4a all beam waists 74.1-74.5 are located exactly on the y-axis. There is no image field curvature or field gradient.

[0087] As a result, the resolution in the object plane or x,y plane is very uniform and the spot size, which corresponds to the size of the beam cone in the object plane 101, is small for each of the first individual particle beams 3, as shown in FIG. 4b.

[0088] FIG. 5 schematically illustrates an important aspect of field curvature correction according to the present invention. FIG. 5a schematically shows a spherical image shell 50a having a radius R around a center M. The spherical image shell 50a corresponds to the field curvature at a first operating point. A change in the operating point, such as the beam pitch, magnification, landing energy, or working distance, results in a change in the refractive power of the particle-optical lens of the multi-beam charged particle microscope or system. The change in refractive power generates a different field curvature at the object plane, thereby generating a different sphere with a different radius R'. Therefore, to adapt the field curvature pre-compensation to the change in operating point, it is necessary to find an appropriate transformation in the pre-compensator or multi-beam generator used.

[0089] However, applying the passive concept using Einzel lens systems with different aperture diameters presents a drawback: when adjusting the image curvature by varying the drive voltage applied to the central electrode of the micro-Einzel lens array, the difference in focal length change is linear for all individual particle beams. On the other hand, when attempting to scale a first sphere with a first radius (corresponding to the first image field curvature) to a second sphere with a second radius (corresponding to the second image field curvature), mathematically, linear scaling does not exist. Therefore, simply scaling a spherical field curvature by simply adjusting the applied voltage inevitably results in incomplete correction of the field curvature. In other words, linearly scaling a spherical field curvature using a passive device does not result in a larger or smaller sphere, but rather in a "compressed" or "stretched" sphere, which is no longer spherical, strictly speaking. This is exactly what is shown in Figure 5b, where the right side shows a "compressed sphere" 50b, which is an ellipsoid.

[0090] For ease of illustration, Figure 5 shows the entire image shells 50a and 50b. However, in practice, only a portion of the sphere 50a or ellipsoid 50b is important and corresponds to the actual image field. This region is shown schematically by boxes 550 on either side of Figure 5. While the error introduced by approximating the ellipsoid 50b as a sphere is small, it makes a difference when applying large image field sizes when dealing with a large number of individual particle beams, especially when performing imaging processes that require high resolution.

[0091] Therefore, the present invention employs a novel approach to performing two-stage correction of field curvature. First, a long-range focal length change is performed at a selected operating point using a micro-Einzel lens array with multiple apertures of varying diameters. Second, the specific error introduced by the long-range focal length change at the selected operating point is specifically corrected by a subsequent short-range focal length change. This error correction can be encoded in a second micro-Einzel lens array with multiple apertures with error-specific changes in aperture diameter, or it can be encoded by an active correction device, for example, using individually adjustable ring electrodes. Returning to exemplary FIG. 5, the long-range focal length change ignores the error introduced when changing the operating point and treats the ellipsoid 50b as if it were a sphere 50a. However, the short-range focal length change accurately corrects the error.

[0092] 6 schematically illustrates a multi-beam generator 305 according to the present invention. In the illustrated embodiment, the multi-beam particle generator 305 comprises a first multi-lens array 350 and a second multi-lens array 360. Furthermore, the multi-beam generator 305 may further comprise other components not shown in FIG. 6, such as a filter plate with a plurality of filter openings for generating a plurality of first individual particle beams.

[0093] The first multi-lens array 350 is provided for long-range focal length variation and includes a first multi-aperture plate 351 with a plurality of first apertures, which is connected to ground potential during use. It also includes a second multi-aperture plate 352 with a plurality of second apertures, which is connected to a first drive voltage U1 during use. The second apertures have diameters d1, d2, d3, d4, and d5, which vary according to a first function f1 of the distance r of each second aperture from the optical axis Z of the multi-beam particle microscope. In the illustrated example, a separate first particle beam 3.3 is located on the optical axis Z. On the optical axis Z, the diameter d3 of the second multi-aperture plate 352 is smallest. The diameters d2 and d4 of two adjacent apertures in the second multi-aperture plate 352 are larger, with the diameter d1=d4, through which particle beams 3.1 and 3.5 pass, respectively, having the largest diameter. The first function f1, which represents the diameter of the apertures of the second multi-aperture plate 352, is adapted to pre-compensate for the spherically curved image shell at the object plane. Thus, the focal points of the first individual particle beams 3.1-3.5 are formed at the intermediate image plane 321, which has a spherically curved surface. The focal length change Δz is defined as the change in focal length in the z-direction. The pre-compensation achieved by the first multi-lens array 350 can be optimized for the first image shell. The diameters d1-d5 can be selected accordingly. In particular, the first function f1(r) is a quadratic polynomial, or more generally, an nth-order polynomial, with n∈N and n≧2.

[0094] The second multi-lens array 360 is provided for short-distance focal length variation. In the illustrated example, the second multi-lens array 360 includes a fourth multi-aperture plate 361 having a plurality of fourth apertures, which is connected to ground potential during use. A fifth multi-aperture plate 362 is provided having a plurality of fifth apertures with diameters d6-d10. This fifth multi-aperture plate 362 is connected to a second drive voltage U2 during use. The plurality of fifth apertures have diameters d6-d10 that vary according to a second function f2 of the distance r from the optical axis Z of the multi-beam particle microscope 1 to each aperture. This second function is adapted to pre-compensate for residual image shell errors in the object plane 101 that are not pre-compensated by the first multi-lens array 350. Additionally, a sixth multi-aperture plate 363 is provided having a plurality of six apertures, which is connected to ground potential during use. The centers of all apertures of all multi-aperture plates 351, 352, 353, 361, 362, 363 are aligned with one another and in each case the apertures are circular. The controller 10 or a separate control unit of the controller 10 is configured to control a second drive voltage U2 provided to the fifth multi-aperture plate 362 based on an operating point of the multi-beam particle microscope 1.

[0095] Typically, diameters d6-d10 are not the same as diameters d1-d5. Instead, diameters d6-d10 encode a second function f2(r). According to a preferred embodiment, function f2(r) is an n-th order polynomial, n∈N and n≧4. Thus, the second function encodes error correction of the image shell of third order or higher. Typically, second function f2(r) is also not the inverse of first function f1(r). The diameters of the holes in plates 351, 353, 361, or 363 need not be the same as the diameters of the corresponding holes in plates 352 or 362. They may be scaled from these diameters or may be the same within each plate.

[0096] Instead of providing the second multi-lens array 306 as a micro-Einzel lens array with apertures of different diameters, a plurality of individually addressable ring electrodes arranged around each aperture can be used. The controller 10 or a respective controller unit can provide a separate second drive voltage U2i to each of the ring electrodes at the operating point of the multi-beam particle microscope.

[0097] The voltage U1 or U2 that can be applied to the monolithic multi-aperture plate 352 or 362 can be relatively high, for example, U1≦100 V, preferably U1≦150 V, or U1≦200 V, or U1≦400 V. U2 can also be of the same order of magnitude, but is usually lower because U2 typically applies only to short focal length changes. However, technically, U2 can be as high as U1.

[0098] In contrast, the voltage provided to the individually addressable ring electrodes as second drive voltage U2i is preferably much more limited to avoid insulation or short circuit problems. According to an embodiment, the second drive voltage U2i has the relationship 0V≦U2i≦20V, preferably 0V≦U2i≦10V.

[0099] The ratio of the focal length change Δz1 achieved by the first multi-lens array 350 alone to the overall focal length change Δz achieved within the image field at object plane 101 is maintained at all operating points such that Δz1:Δz≧0.90, preferably Δz1:Δz≧0.95. It is noted that this ratio is the same whether determined relative to the intermediate image plane 321 or the object plane 101. In practice, the total focal length change Δz is Δz≧3 μm, but larger values, e.g., Δz>12 μm, are preferred.

[0100] FIG. 7 exemplarily illustrates top views of multi-aperture plates with different aperture diameters. As an example, a possible implementation of the second multi-aperture plate 352 of the first multi-lens array 350 for long-distance focal length variation is shown. The second multi-aperture plate 352 is connected to a first drive voltage U1 during use. The diameters of the apertures in the second multi-aperture plate 352 illustrate a radial dependency. In other words, the aperture diameters vary as a first function of the distance r from the optical axis Z, which, in the illustrated example, extends through the center C into the page, to the respective second apertures. The diameter of the aperture located at the center C is the smallest diameter aperture. Therefore, the overall arrangement is centered.

[0101] FIG. 7b illustrates a first function f1 for varying the aperture diameter. In principle, different functions and dependences of r on the aperture diameter d can be selected. The aperture diameter can be, for example, linear, quadratic, cubic, or hyperbolic. In principle, it is also possible to use different sets of aperture plates 352i with different dependences of r. This allows the required field curvature correction to be approximated by changing the plate voltage Ui accordingly. A Taylor expansion of the entire plate can be applied. However, according to the present invention, such a large set is not necessary; in principle, two micro-einzel lens arrays 350, 360 have already proven sufficient, with the first micro-einzel lens array 350 being used for long-distance focal length changes and the second multi-lens array 360 being used for short-distance focal length changes. Therefore, in practice, the second multi-aperture plate 352 of the first multi-lens array 350 typically has a quadratic dependence of r on diameter changes.

[0102] FIG. 8 is a graphical representation of the residual image shell error that can be corrected for short-distance focal length changes using the second multi-lens array 360 in accordance with the present invention. More specifically, FIG. 8a shows a first image shell 501 corresponding to a first operating point. The image shell 501 is spherical and has a first radius corresponding to a first field curvature FC1. A second image shell 502 is also shown, which is also spherical and has a second radius corresponding to a second field curvature FC2. The image shells 501 and 502 are assumed to be ideal and are pre-compensated for in accordance with the present invention by using the multi-beam generator 305. Essentially, the first multi-lens array 350 for long-distance focal length changes is applied to change the first image shell 501 into the second image shell 502. More specifically, a first drive voltage U1 is applied to a second multi-aperture plate 352 having a plurality of second apertures with varying aperture diameters, e.g., quadratic. However, linear scaling of the first driving voltage U1 does not accurately transform the first image shell 501 into the second image shell 502 because all focal lengths change linearly in the z direction. In other words, there is no linear scaling in the z direction that can transform the first image shell 501 into the second image shell 502. The residual image shell error produced by linear scaling of the voltage is shown as graph 503 in FIG. 8a. Note that the residual image shell error shown is amplified by a factor of 1000. The mathematical nature of this residual image shell error is a fourth-order polynomial, with not only fourth-order terms but also second-order terms. This characteristic is observed for all possible changes in the operating point in a specially simulated multibeam particle microscope. Therefore, for a multi-aperture plate 362 with apertures of various diameters, it is possible to encode the residual image shell error correction as a polynomial according to graph 503. In other words, graph 503 corresponds to a second function f2(r) that corrects the residual image shell error.

[0103] 8b illustrates a second example for the transformation of a first image shell 501 into a second image shell 502. Here, a slightly different design of the multi-beam generator was used as the basis for the simulation. Again, the graph 504 showing the residual image shell error (again magnified 1000 times) is a fourth-order polynomial. In this case, it is a fourth-order polynomial with only fourth-order terms.

[0104] It is noted that Figures 8a and 8b simply illustrate the change in diameter for a constant offset diameter, all units being in μm.

[0105] 9 shows a schematic representation of multi-aperture plates 352, 362 with different aperture diameters. More specifically, multi-aperture plate 352 is the second multi-aperture plate of the first multi-lens array 350 adapted for long-range focal length variation. The diameter variation is adapted to pre-compensate for a spherically curved image shell at the object plane. In the illustrated example, the aperture diameter of multi-aperture plate 352 is measured at r with respect to the center, denoted as C. 2 Therefore, the second multi-aperture plate 352 encodes a parabolic term for field curvature correction as a monolithic electrode.

[0106] In contrast, the second multi-aperture plate 362 of the second multi-lens array 360 encodes a third-order residual shell error correction, and the diameter variation in the plate 362 corresponds to a fourth-order polynomial, and thus r 4 and in the illustrated example, r 2 , and also includes a term. This dependence of r can be understood, for example, when analyzing the change in diameter along line 510. Along said line 510, the diameter increases and decreases, not simply increases. The embodiment shown in FIG. 9 illustrates that field curvature correction up to third order can be achieved relatively easily using only two micro-Einzel lens arrays that specifically encode the change in diameter. It is not necessary to provide a full expansion, e.g., a full Taylor expansion, to achieve third-order correction of field curvature.

[0107] FIG. 10 schematically illustrates multi-aperture plates 370 and 371 of a tilt-compensated multi-lens array. The tilt-compensated multi-lens array is suitable for compensating for image field gradients in the object plane, which can occur for several reasons, such as an asymmetric beam splitter design or simply a slightly tilted wafer surface. The tilt-compensated multi-lens array can comprise a micro-Einzel lens array. Essentially, there are three multi-aperture plates, with the first and third multi-aperture plates connected to ground potential during use, and the second multi-aperture plate in between is connected to a specific drive voltage, e.g., a first tilt drive voltage, during use. An example of such a second multi-aperture plate 370 with multiple second apertures is shown in FIG. 10a. The diameter of the second apertures varies as a linear function f(x) of the position of each aperture in a direction x perpendicular to the optical axis Z. In other words, the diameter of the second apertures of the multi-aperture plate 370 is relatively large on the left side and small on the right side, and this variation is represented by a linear function. In use, a first gradient drive voltage is connected to the second multi-aperture plate 370, and the controller 10 or a respective control unit of the controller 10 is configured to provide the first gradient drive voltage to the first tilt-compensated multi-lens array, in particular based on the operating point of the multi-beam particle microscope 1. It is possible for the field gradient to vary according to the operating point of the multi-beam particle microscope. However, it is also possible for a pre-compensated tilt to be provided to compensate for a tilt position of the probe or object, such as a wafer surface that is not 100% orthogonal to the optical axis Z of the system at the object plane. This tilt remains the same even if the operating point of the multi-beam particle microscope changes.

[0108] FIG. 10b schematically illustrates a second multi-aperture plate 371 of a second tilt-compensated multi-lens array. In use, a second tilt drive voltage is provided to the second multi-aperture plate 371. Plate 370 allows tilt compensation in the x direction, while multi-aperture plate 371 allows tilt compensation in the y direction. Again, the diameter of the second apertures in plate 371 varies as a linear function in the y direction, but also according to a linear function of the position of each aperture. Preferably, direction x and direction y are perpendicular to each other and both perpendicular to optical axis Z. However, in principle, direction x and direction y could also be chosen to be linear independently of each other.

[0109] Regarding tilt compensation, it should be noted that tilt compensation using only one multi-aperture plate encoded in linear Cartesian coordinates is only suitable for compensating for tilt in one direction, such as tilt to the right. However, tilt can also be provided in the opposite direction for the same coordinates. Therefore, to be able to compensate for field gradients or tilts in any direction, a third and fourth tilt-compensating multi-lens array are also required, enabling tilt compensation not only in the (-x) direction but also in the (-y) direction. The overall tilt compensation unit can be constructed, for example, with a sequence of four micro-Einzel lens arrays, including the first, second, third, and fourth tilt-compensating multi-lens arrays described above. In a multi-beam particle microscope, if it is already known from the beginning that a field gradient will occur, for example due to an asymmetric beam splitter design, it is also possible to pre-compensate for the tilt of the respective image plane by already encoding the pre-compensation in the first multi-lens array, which forms the basis of the long-range focal length change. FIG. 11 schematically illustrates respective examples, in which the second multi-aperture plate 352 of the first multi-lens array 350 comprises a plurality of second apertures, the diameters of which vary according to a first function of the distance of each second aperture from the optical axis Z of the multi-beam particle microscope, but with an additional shift ΔS. As illustrated in the example of FIG. 11a, when a parabolic shape for pre-compensating for image curvature is encoded in the multi-aperture plate 352, the vertex S is shifted from the center C to a position S. For this zenith position S, the radius dependence is as described above with respect to FIG. 7a. Reference is again made to FIG. 3 of the present patent application. If the spherical image shell is further tilted, the center 43 will be positioned off-axis from the particle-optical axis Z of the system 1.

[0110] FIG. 12 shows a schematic diagram of another multi-beam generator 305 according to the present invention. Essentially, the multi-beam generator 305 includes a first multi-lens array 350 for long-range focal length variation and a second multi-lens array 360 for short-range focal length variation. A first drive voltage U1 is applied to the second multi-aperture plate 352, and a second drive voltage U2 is applied to the fifth multi-aperture plate 362. The other multi-aperture plates 351, 353, and 363 (corresponding to 361) are connected to ground potential during use. The second multi-aperture plate 352 includes apertures of different diameters, and as previously described, the fifth multi-aperture plate 362 also includes multiple apertures of different diameters. So, essentially, to reiterate, the first micro-einzel lens array 350 encodes pre-correction of the spherically curved image shell at the object plane 101, and the second micro-einzel lens array 360 encodes correction of residual image shell error at the object plane.

[0111] However, according to the embodiment shown in FIG. 12, the sixth multi-aperture plate 363, which is connected to ground potential during use, also functions as the terminal multi-aperture plate 390. The terminal multi-aperture plate 390 is defined as the last multi-aperture plate in the sequence of multi-aperture plates in the multi-beam generator 305. The terminal multi-aperture plate 390 is followed by an electrode aperture plate 380 with a single aperture through which all first individual particle beams 3 pass. This electrode aperture plate 380 is connected to an extraction voltage U3 during use. The controller 10 or its respective control unit is configured to apply the extraction voltage U3 to the electrode aperture plate 380 based on the operating point of the multi-beam particle microscope 1, thereby varying the extraction field and, therefore, the immersion lens effect, contributing to the focal length change. The electric field of the immersion lens comprising the ground electrode 390 and the electrode aperture plate 380 is different for each individual particle beam 3.1-3.5. This is because the electric field leaks or penetrates more or less into the apertures of the terminal multi-aperture plate 390 depending on the diameter of each aperture. Therefore, by encoding a specific diameter pattern into the terminating multi-aperture plate 390 and by appropriately controlling the extraction voltage U3 of the electrode aperture plate 380, further possibilities for focal length variation can be provided.

[0112] It is noted that the multi-beam generator 305 shown in FIG. 12 may include additional elements or features not specifically illustrated in FIG.

[0113] FIG. 13 shows a schematic representation of a terminal multi-aperture plate 390 with a different shape. This terminal multi-aperture plate 390 can be the sixth multi-aperture plate 363 of the second multi-lens array 360, but in principle it could be another multi-aperture plate located at the terminal position, and thus as the last multi-aperture plate inside the multi-beam generator 305. Again, the terminal multi-aperture plate 390, in combination with the electrode aperture plate 380 with a single central aperture, forms an immersion lens. According to the illustrated embodiment, the terminal multi-aperture plate 390 comprises a number of terminal apertures with different diameters d91-d95. The diameters d91-d95 vary as a function of the distance r of each aperture from the optical axis Z. Furthermore, the terminal multi-aperture plate 390 has a top surface 391 that is planar and a bottom surface 392 that is convexly shaped relative to the direction of the particle light beam path. Therefore, the terminal multi-aperture plate 390 has a maximum thickness h in the vicinity of the central aperture with diameter d93 through which the third individual particle beam 3.3 passes. max The thickness of the plate 390 further decreases away from the particle-optical axis Z, and the heights h1 and h2 are max It turns out that varying the shape of the terminal multi-aperture plate 390, more specifically its thickness h, can further contribute to the variation of the immersion lens effect for each individual particle beam 3.1-3.5. Therefore, it is possible to encode another possibility into the terminal multi-aperture plate 390 for individual focal length variation for each beam.

[0114] Figure 14 shows a schematic representation of another end multi-aperture plate 390 with a different shape. Compared to the embodiment shown in Figure 13, the shape of the bottom surface 392 is different, the bottom surface 392 being concave with respect to the direction of the particle light beam path. The thickness of the end multi-aperture plate 390 is therefore at its minimum at the central aperture through which the particle beam 3.3 passes in the example shown. The minimum thickness is h min As the distance from the particle optical axis Z increases, the thickness increases, for example, to h3 and h4. The change in diameter of the aperture in FIG. 14 is the same as in FIG.

[0115] Depending on the shape of the bottom surface 392, the sign of the extraction voltage must be adapted. In combination with a convex shape of the bottom surface 392 of the terminal multi-aperture plate 390, the extraction voltage is negative (for electrons or other negatively charged particles providing a discrete particle beam). In combination with a concave bottom surface 392, the extraction voltage U3 provided at the electrode aperture plate 380 is positive (for electrons or other negatively charged particles providing a discrete particle beam). It should be noted that in principle, other shapes of the terminal multi-aperture plate 390, and in particular the bottom surface 392, are also possible, for example, stepped or partly linearly varying shapes.

[0116] Figures 15-17 schematically illustrate other embodiments of a multi-beam generator 305 in accordance with the present invention. Figures 15-17 are not to scale and do not explicitly illustrate the variations in diameter of particular multi-aperture plates, as explained in more detail above. Instead, the primary aspect illustrated in Figures 15-17 is the combination of modules or multi-lens arrays that form the multi-beam generator 305.

[0117] 15 includes the following elements or units in the following order: first, a filter plate 304 is provided with a plurality of filter openings for generating a plurality of first individual particle beams, and the filter plate 304 is connected to ground potential during use;

[0118] After the filter plate 304 is a first multi-lens array 350 for long-range focal length variation. It comprises a stack of multi-aperture plates forming a micro-Einzel lens array. In use, the central multi-aperture plate is connected to a first drive voltage U1, and a diameter change in the first multi-lens array encodes pre-compensation of the spherically curved image shell at the object plane.

[0119] Thereafter, a second multi-lens array 360 for short-range focal length variation is provided. In the embodiment shown in Figure 15, the second multi-lens array 360 is realized as another micro-Einzel lens array with a central multi-aperture plate with a plurality of fifth apertures whose diameters vary according to a second function of the distance of each aperture from the optical axis Z, this second function being adapted to pre-compensate for residual image shell errors in the object plane that are not pre-compensated by the first multi-lens array 350.

[0120] Thereafter, an astigmatic multi-aperture plate 388 is provided, comprising a plurality of apertures, around each of which a plurality of individually addressable electrostatic multipole electrodes are arranged. In use, a set of drive voltages U4ij is provided to each of the multipole electrodes adapted for astigmatic purposes. In addition, each set of drive voltages U4ij is provided with an individual offset voltage U4ij common to all electrodes of the respective multipole electrodes, based on the operating point of the multibeam particle microscope. offset This individual offset voltage U4ij can be offset contributes to a focal length change that can be adapted individually for each individual particle beam 3 . Thereafter, an immersion lens array 395 is provided comprising a terminating multi-aperture plate 390 which in use is connected to ground potential, and an electrode aperture plate 380 having a single aperture which in use is connected to an extraction voltage. The immersion lens array 395 may be designed, for example, as described with respect to Figures 12 to 14.

[0121] In general, the order of the different correctors in the multi-beam generator 305 can be freely chosen, for example the order of the multi-lens arrays 350, 360, the tilt-compensating multi-lens arrays 370, 371 and the astigmatic multi-aperture plate 388 in the multi-beam generator 305. However, it may be advantageous to provide the astigmatic multi-aperture plate 388 as the first corrector in the sequence of correctors, as this position may have advantages from an alignment / adjustment point of view.

[0122] Figure 16 shows another multi-beam generator 305 that differs from the multi-beam generator 305 shown in Figure 15 in that the second multi-lens array 360 is not geometry encoded but comprises a multi-aperture plate with a plurality of apertures, with a plurality of individually addressable ring electrodes disposed around each aperture. The controller 10 of the multi-beam generator 305 is configured to provide a respective second drive voltage U2i to each of the ring electrodes based on an operating point of the multi-beam particle microscope.

[0123] FIG. 17 shows another order of elements of the multi-beam generator 305, in which the filter plate 304 is provided first. This is followed by a first multi-lens array 350 for long-distance focal length variation. In this embodiment, the diameter variation encoded in this micro-lens array 350 is adapted to pre-compensate for a spherically curved image shell in the object plane and also to offset the tilt of the image plane, for example, due to an asymmetric beam splitter design. This means that the first function representing the diameter variation of the plurality of second apertures in the second multi-aperture plate 352 of the first multi-lens array 350 is not only a function of the distance r from the optical axis Z, but also a linear function of the position of each aperture in the directions -x and / or -y, which are perpendicular to the optical axis Z and are independently linear and preferably orthogonal to each other.

[0124] Then, a first tilt-compensating multi-lens array 370 is provided to compensate for the first tilt in the x direction, followed by a second tilt-compensating multi-lens array 371 to compensate for the tilt of the image plane in the y direction. This means that linear functions f(x), f(y) of the position of each aperture in the x or y direction are encoded in the tilt-compensating multi-lens arrays 370, 371.

[0125] The tilt-compensating multi-lens arrays 370, 371 are followed by an astigmatic multi-aperture plate 388, as already explained with reference to Figures 15 and 16. This is followed by a terminating multi-aperture plate 390, which is set to ground potential. Of course, the multi-beam generator 305 can also be provided with further elements, such as, for example, an immersion lens array 395.

[0126] Optionally, a shielding layer 396 may be provided between subsequent multi-lens arrays.

[0127] FIG. 18 illustrates the dimensions of a multi-lens array. FIG. 18 is not drawn to scale. Further investigation by the inventors showed that the focal length change can be further increased by optimizing the plate dimensions, particularly those of the first multi-lens array 350 that are applicable to long-distance focal length changes. The maximum field curvature correction potential can be achieved with the shortest electrode length L2 of the second multi-aperture plate 352, the shortest gaps G1 and G2 between the first multi-aperture plate 351 and the second multi-aperture plate 352 as well as between the second multi-aperture plate 352 and the third multi-aperture plate 353 of the first microlens array 350, and by using counter electrodes 351 and 353 of the Einzel lens system with minimum thicknesses L1 and L3.

[0128] According to a preferred embodiment, the following relationship holds for the thickness L2 of the second multi-aperture plate 352 of the first multi-lens array 352: L2≦50 μm, preferably L2≦30 μm.

[0129] According to a preferred embodiment, the following relationship holds for the thickness L1 of the first multi-aperture plate of the first multi-lens array: L1≧80 μm, preferably L1≧100 μm. Additionally or alternatively, the following relationship holds for the thickness L3 of the third multi-aperture plate 353 of the first multi-lens array 350: L3≧80 μm, preferably L3≧100 μm.

[0130] According to a preferred embodiment, the following relationship holds for the gap G1 between the first multi-aperture plate 351 and the second multi-aperture plate 352 of the first multi-lens array 350: G1≦10 pm, preferably G1≦5 pm. Additionally or alternatively, the following relationship holds for the gap G2 between the second multi-aperture plate 352 and the third multi-aperture plate 353 of the first multi-lens array 350: G2≦10 μm, preferably G2≦5 μm.

[0131] In principle, the above observations regarding the dimensions of the Einzel lens array 350 are applicable to other multi-lens arrays as well.

[0132] According to an embodiment, a multi-beam generator 305 for a charged particle multi-beam system 1 is provided, comprising a stack of multi-aperture plates having at least a first multi-lens array 350 for long-range focal length variation and a second multi-lens array 360 for short-range focal length variation. The aperture diameter of the first multi-lens array 350 is varied to encode pre-compensation for a spherically curved image field in an object plane of the multi-beam system 1. The aperture diameter of the second multi-lens array 360 is varied to encode pre-compensation for residual image field errors in an object plane not pre-compensated by the first multi-lens array 350. A control unit 10 of the multi-beam generator 305 provides drive voltages U1 and U2 to the first lens array 350 and the second lens array 360 based on a current operating point of the charged particle multi-beam system 1.

[0133] The above-described embodiments are not intended to limit the present invention, instead they represent exemplary embodiments only.

[0134] It is noted that U.S. Pat. No. 11,322,335 (B2) proposes a quadratic dependence of the aperture diameter in a multi-aperture array for field curvature compensation. In particular, a purely quadratic dependence of the aperture diameter once fixed would require a separate aperture plate with a variable aperture diameter whose spatial dependence is fourth or higher order. This need is illustrated in FIG. 8b of this patent application. However, with larger image fields, e.g., containing more than 90, 300, or even more individual particle beams, the image shell error becomes larger, and the quadratic limitation of the multi-aperture plate is no longer sufficient for correction. For correction at a selected operating point or setpoint of the multi-beam particle microscope, the radial dependence of the aperture diameter in the multi-aperture plate should instead be designed according to the following criteria:

[0135] (a) The spatial path of the focal spot must follow an image shell. The portion of the image shell can be determined by mathematically simple techniques, for example, as a root or cosine function of the angle relative to the axis of symmetry. This spherical portion of the image shell is deformed by the spatially invariant focusing effect of the deceleration field. Overall, this is a power series f(r)~fO+Σ of the focal length f as a function of the distance r from the axis of symmetry, with coefficients Ai. i Air i where in addition to the first quadratic order, at least one higher quartic order is considered. For larger image fields with even more individual particle beams, e.g., 90 or more, 300 or more, even higher orders can be considered, such as the 6th order and even higher.

[0136] (b) The achieved focal length change behaves approximately linearly with the change in aperture diameter of the multi-aperture plate. The example aperture diameter design described here takes this nonlinearity into account.

[0137] However, even with an ideal design of the multi-aperture plate for a preferred operating point or setting of the multi-beam particle microscope, the problem of image shell changes with changes in the operating point or setting remains.

[0138] Example 1: a first multi-aperture plate with variable diameter apertures designed for field curvature or image shell correction at an operating point or set point selected taking into account criteria (a) and (b); a second aperture plate with a variable diameter aperture designed for field curvature or image shell correction at operating points or set points deviating from a selected operating point or set point; Equipped with a multi-beam system. [Explanation of symbols]

[0139] 1. Multibeam particle microscope 3 Primary particle beam (individual particle beam) 5 Beam spot, incident position 7 Object 9 Secondary particle beam 10 Computer system, controller 15 Sample surface, wafer surface 25 Secondary charged particle image spot 41 axial y-axis passing through the position of the primary beam 43 center 45 tilted image plane 47 Lower focal plane 49 Upper focal plane 50 Statue Shell 51 z range or interval 53 Nearly concentric rings or hexagons with foci of similar resolution 55 spots 57 Rejected primary beams that do not meet resolution criteria 60 Optical axis 62 Beam cone of the primary beam near the focal plane 74 Minimum Spot Range 79 Ring Electrode 81 Multipole Electrode 82 Ring electrode 83 Spacer 85 Aperture 86 Spacer 99 Absorption and Conduction Layers 101 Object plane 102 Objective Lens 105 Optical axis of multi-beam charged particle microscope 200 Detection System 205 Projection Lens System 206 Electrostatic Lens 207 Particle Multi-Detector 208 Projection Lens 209 Projection Lens 210 Projection Lens 212 Crossover 214 Aperture Filter 218 Deflector System 220 multi-aperture collector, individual deflector array 222 Collective Deflection System, Anti-Scan 300 Beam Generator 301 Particle Source, Beam Generation System 303 Collimation Lens System 304 Multi-aperture array, filter plate 305 Multi-aperture arrangement, multi-beam generator 306 Micro-Optical Systems 307 Field of view lens 308 Field of View Lens 309 Particle Beam 321 Intermediate image plane 323 Beam Focus 333 Support Zone 335 Membrane Zone 350 The first multi-lens array for long-range focal length variation 351 First multi-aperture plate 352 Second Multi-Aperture Plate 353 Third Multi-Aperture Plate 360° Second multi-lens array for short-range focal length change 361 First multi-aperture plate 362 Second Multi-Aperture Plate 363 Third Multi-Aperture Plate 370 Multi-aperture plate for tilt-compensated multi-lens array 371 Multi-aperture plate for tilt-compensated multi-lens array 380 Electrode Aperture Plate 381 Single Aperture Plate 388 Astigmatism Multi-aperture Plate, Multi-astigmatist 390 End Multi-Aperture Plate 391 Top surface 392 bottom 395 Immersion Lens Array 396 Shield Layer 400 Beam Switch 500 collective scanning deflector 501 Elephant Shell 502 Elephant Shell 503 Afterimage Shell Error 504 Afterimage Shell Error 510 line 550 Box 600 sample stage d diameter of opening h Height / thickness of multi-aperture plate R radius M midpoint or center C center S vertex ΔS shift Z optical axis Δz focal length change G1 Gap G2 Gap L1 Thickness L2 thickness L3 Thickness

Claims

1. 1. A multi-beam charged particle microscope with an adjustable image shell, comprising: a multi-beam generator configured to generate a first field of view of a plurality of charged first individual particle beams; a first particle-optical unit having a first particle-light beam path and configured to image the generated first individual particle beam onto an object at an object plane, wherein the first individual particle beam can impinge on the object surface at an incidence position that forms a second field of view; a detection system having a plurality of detection regions forming a third field of view; a second particle-optical unit having a second particle-optical beam path and configured to image a second individual particle beam emanating from the entrance location in the second field of view onto the third field of view of the detection area of ​​the detection system; a magnetic and / or electrostatic objective lens through which both the first individual particle beam and the second individual particle beam pass; a beam switch disposed in the first particle beam path between the multi-beam particle generator and the objective lens, and in the second particle beam path between the objective lens and the detection system; a sample stage for holding and / or positioning the object during inspection of the object; Controller and Equipped with the multi-beam charged particle microscope is adapted to operate at a plurality of operating points, and by operating the multi-beam charged particle microscope at the plurality of operating points, the first particle-optical unit generates, at the object plane, a plurality of spherically curved image fields, which are pre-compensated by the multi-beam generator, respectively; The multi-beam generator includes: a filter plate comprising a plurality of filter apertures for generating the plurality of first individual particle beams, the filter plate being connected to ground potential during use; a stack of multi-aperture plates having at least a first multi-lens array for long-range focal length change and a second multi-lens array for short-range focal length change; wherein the first multi-lens array comprises: a first multi-aperture plate comprising a plurality of first apertures and connected to ground potential in use; a second multi-aperture plate comprising a plurality of second apertures, the second multi-aperture plate being connected in use to a first drive voltage, the diameters of the plurality of second apertures varying according to a first function of a distance of each second aperture from an optical axis (A) of the multi-beam particle microscope, the first function being adapted to pre-correct for a spherically curved image shell at the object plane; a third multi-aperture plate having a plurality of third apertures and connected to ground potential in use; Equipped with the centers of the first, second, and third openings are aligned with one another; the second multi-lens array is aligned with the first multi-lens array, and the second multi-lens array is configured to pre-compensate for residual image shell errors at the object plane that are not pre-compensated by the first multi-lens array; the controller is configured to provide a first drive voltage to the first multi-lens array based on the operating point of the multi-beam particle microscope; The multi-beam charged particle microscope, wherein the controller is configured to provide a second drive voltage to the second multi-lens array based on the operating point of the multi-beam particle microscope.

2. in the second multi-aperture plate, the variation of the diameter of the second aperture according to the first function is optimal for pre-compensating for a spherically curved image shell at the object plane at a preselected reference operating point during use; the controller is configured to provide a first non-zero drive voltage to the first multi-lens array at the reference operating point; The multi-beam particle microscope of claim 1 , wherein the controller is configured to provide a second drive voltage to the second multi-lens array at the reference operating point, the second drive voltage being zero.

3. the controller is configured to provide a non-zero first drive voltage to the first multi-lens array at a second operating point, the first drive voltage being different from the first drive voltage provided at the reference operating point; The multi-beam particle microscope of claim 2 , wherein the controller is configured to provide a second non-zero drive voltage to the second multi-lens array at the second operating point.

4. The second multi-lens array includes: a fourth multi-aperture plate comprising a plurality of fourth apertures and connected to ground potential in use; a fifth multi-aperture plate comprising a plurality of fifth apertures and connected in use to a second drive voltage U2, wherein diameters of the plurality of fifth apertures vary according to a second function of a distance of the respective aperture from the optical axis (A) of the multi-beam particle microscope, the second function being adapted to pre-compensate for the residual image shell errors at the object plane that are not pre-compensated by the first multi-lens array; a sixth multi-aperture plate comprising a plurality of sixth apertures and connected to ground potential in use; Equipped with the centers of the fourth, fifth, and sixth openings are aligned with one another; 4. The multi-beam particle microscope of claim 1, wherein the controller is configured to control the second drive voltage U2 provided to the fifth multi-aperture plate based on the operating point of the multi-beam particle microscope.

5. The third multi-aperture plate of the first multi-lens array and the fourth multi-aperture plate of the second multi-lens array are provided as the same multi-aperture plate, or 5. The multi-beam particle microscope according to claim 4, wherein the first multi-aperture plate of the first multi-lens array and the sixth multi-aperture plate of the second multi-lens array are provided as the same multi-aperture plate.

6. The first function f1(r) is [Equation 1] 6. The multi-beam particle microscope according to claim 1, wherein n is an n-th order polynomial where n≧2.

7. The second function f2(r) is [Equation 2] 7. The multi-beam particle microscope according to claim 4, wherein n is an n-th order polynomial where n≧4.

8. 8. The multi-beam particle microscope according to claim 4, wherein the second function f2(r) is not an inverse function of the first function f1(r).

9. the second multi-lens array comprises a multi-aperture plate having a plurality of apertures, each aperture being surrounded by a plurality of individually addressable ring electrodes; 4. The multi-beam particle microscope of claim 1, wherein the controller is configured to provide a respective second drive voltage U2i to each of the ring electrodes based on the operating point of the multi-beam particle microscope.

10. For each of the second drive voltages U2i, the following relationship is satisfied:

10. The multi-beam particle microscope according to claim 9, wherein 0V≦U2i≦20V, in particular 0V≦U2i≦10V, holds true.

11. For the first drive voltage U1, the following relationship exists: U1≦100V, in particular U1≦150V or U1≦200V or U1≦400V The multi-beam particle microscope according to any one of claims 1 to 10, wherein the following holds true:

12. The ratio of the focal length change Δz1 achieved by the first multi-lens array alone to the overall focal length change Δz achieved within the image field at the object plane satisfies the following relationship for all operating points: Δz1 / Δz≧0.80, particularly Δz1 / Δz≧0.90 or Δz1 / Δz≧0.95 The multi-beam particle microscope according to any one of claims 1 to 11, wherein the following holds true:

13. The multi-beam particle microscope has the following relationship:

13. The multi-beam particle microscope according to claim 1, configured to pre-compensate for focal length changes Δz within the image field in the object plane, where Δz≧1.0 m, in particular Δz≧3 μm or Δz≧12 μm.

14. 14. The multi-beam particle microscope according to any one of claims 1 to 13, wherein the multi-beam generator comprises at least one further multi-lens array that contributes to pre-compensation of the focal length variations within the image field in the object plane.

15. The multi-beam generator includes: a first multi-aperture plate comprising a plurality of first apertures and connected to ground potential in use; a second multi-aperture plate comprising a plurality of second apertures, the second multi-aperture plate being connected in use to a first gradient drive voltage, the diameters of the second apertures varying as an elementary linear function f(x) of the position of each aperture in a direction x perpendicular to the optical axis Z; a third multi-aperture plate having a plurality of third apertures and connected to ground potential in use; a first tilt-compensated multi-lens array comprising: the centers of the first, second, and third openings are aligned with one another; 15. The multi-beam particle microscope of claim 14, wherein the controller is configured to provide the first tilt drive voltage to the first tilt-compensated multi-lens array, particularly based on the operating point of the multi-beam particle microscope.

16. The multi-beam generator includes: a first multi-aperture plate comprising a plurality of first apertures and connected to ground potential in use; a second multi-aperture plate comprising a plurality of second apertures, the second multi-aperture plate being connected in use to a second gradient drive voltage, the diameters of the second apertures varying as a fundamental linear function f(y) of the position of each aperture in a direction y perpendicular to the optical axis Z and which is linear independent of the direction x; a third multi-aperture plate having a plurality of third apertures and connected to ground potential in use; a second tilt-compensated multi-lens array comprising: the centers of the first, second, and third openings are aligned with one another; 16. The multi-beam particle microscope of claim 15, wherein the controller is configured to provide the second tilt drive voltage to the second tilt-compensated multi-lens array, particularly based on the operating point of the multi-beam particle microscope.

17. 17. The multi-beam charged particle microscope according to claim 1, wherein the first function representing the diameter variation of the plurality of second apertures in the second multi-aperture plate of the first multi-lens array is not only a function of the distance r from the optical axis Z but also a linear function of the position of each aperture in a direction −x and / or a direction −y, the directions −x, −y being perpendicular to the optical axis Z and linear independently of each other, and the linear functions of −x and / or −y being adapted to deflect a field gradient in the object plane.

18. the multi-beam generator further comprises an astigmatic multi-aperture plate comprising a plurality of apertures, with a plurality of individually addressable electrostatic multipole electrodes disposed around each aperture; The controller is configured to provide a set of drive voltages U4ij to each of the electrostatic multipole electrodes, each set of drive voltages U4ij being based on the operating point of the multibeam particle microscope and configured to provide an individual offset voltage U4ij common to all electrodes of the respective multipole electrodes. offset The multi-beam particle microscope according to any one of claims 1 to 17, comprising:

19. The multi-beam generator includes: a termination multi-aperture plate having a plurality of termination apertures, the diameters of the termination apertures varying as a function of the distance of each of the apertures from the optical axis, the termination multi-aperture plate being connected to ground potential in use; an electrode aperture plate having a single aperture through which all of the first individual particle beams pass and connected in use to an extraction voltage, the centers of the terminal multi-aperture plate and the electrode aperture plate being aligned with each other; In this order, 19. The multi-beam particle microscope of any one of claims 1 to 18, wherein the controller is configured to provide the extraction voltage to the electrode aperture plate based on the operating point of the multi-beam particle microscope, thereby varying the extraction field and therefore the immersion lens effect contributes to the focal length change.

20. 21. A multi-beam particle microscope according to claim 19 or 20, wherein the shape of the terminating multi-aperture plate varies as a function of distance from the optical axis.

21. the top surface of the end multi-aperture plate is planar and the bottom surface of the end multi-aperture plate is convexly shaped with respect to the direction of the particle-light beam path; or 21. The multi-beam particle microscope of claim 20, wherein a top surface of the end multi-aperture plate is planar and a bottom surface of the end multi-aperture plate is concavely shaped relative to the direction of the particle light beam path.

22. 22. The multi-beam particle microscope according to claim 1, wherein the following relationship holds for a thickness L2 of the second multi-aperture plate of the first multi-lens array: L2≦50 μm, in particular L2≦30 μm.

23. and / or with respect to the thickness L1 of the first multi-aperture plate of the first multi-lens array, the following relationship holds: L1≧80 μm, in particular L1≧100 μm; 23. The multi-beam particle microscope according to any one of claims 1 to 22, wherein the following relationship holds for a thickness L3 of the third multi-aperture plate of the first multi-lens array: L3≧80 μm, in particular L3≧100 μm.

24. Regarding the gap G1 between the first multi-aperture plate and the second multi-aperture plate of the first multi-lens array, the following relationship is satisfied: G1≦10 μm, particularly G1≦5 μm and / or Regarding the gap G2 between the second multi-aperture plate and the third multi-aperture plate of the first multi-lens array, the following relationship is satisfied: G2≦10 μm, particularly G2≦5 μm The multi-beam particle microscope according to any one of claims 1 to 23, wherein the following holds true:

25. the multi-beam generator further comprising means for generating a voltage gradient on the second multi-aperture plate of the first multi-lens array; 25. The multi-beam particle microscope of claim 1, wherein the controller is configured to provide a gradient drive voltage to the second multi-aperture plate to pre-compensate for a field gradient at the object plane.

26. 26. The multi-beam particle microscope of claim 25, wherein the second multi-aperture plate comprises a high-resistivity coating on a surface thereof that contacts an opposite side of the second multi-aperture plate to create an adjustable voltage gradient.

27. said multi-beam particle microscope further comprising means for in-situ plasma cleaning of the monolithic multi-aperture plate; and / or 27. The multi-beam particle microscope according to any one of claims 1 to 26, wherein the multi-beam particle microscope comprises means for providing a small partial pressure of hydrogen gas during use of the multi-beam particle microscope for cleaning purposes.

28. A multi-beam particle microscope according to any preceding claim, further comprising means for continuously heating the monolithic multi-aperture plate.

29. 1. A multi-beam generator for a charged particle multi-beam system, comprising: a filter plate comprising a plurality of filter apertures for generating a plurality of first individual particle beams, the filter plate being connected to ground potential in use; a stack of multi-aperture plates having at least a first multi-lens array for long-range focal length change and a second multi-lens array for short-range focal length change, The first multi-lens array comprises: a first multi-aperture plate comprising a plurality of first apertures and connected to ground potential in use; a second multi-aperture plate comprising a plurality of second apertures, the second multi-aperture plate being connected in use to a first drive voltage, wherein diameters of the plurality of second apertures vary according to a first function of a distance of each second aperture from an optical axis (A) of the charged particle multi-beam system, the first function being adapted to pre-compensate for a spherically curved image field in an object plane of the charged particle multi-beam system; a third multi-aperture plate having a plurality of third apertures and connected to ground potential in use; Equipped with the centers of the first, second, and third openings are aligned with one another; the second multi-lens array is aligned with the first multi-lens array; the second multi-lens array is configured to pre-compensate for residual image field errors at the object plane that are not pre-compensated by the first multi-lens array; and a control unit configured to provide a first driving voltage to the first multi-lens array based on an operating point of the charged particle multi-beam system, and to provide a second driving voltage to the second multi-lens array based on the operating point of the charged particle multi-beam system; A multi-beam generator comprising:

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