Method for operating a multiple particle beam system with detection of a ring collapse process and triggering of a material build-up process, associated computer program product, and multiple particle beam system and multi-beam particle microscope

Abstract

The invention discloses a method for operating a multiple particle beam system, in which the onset of a ring collapse process at the front facet of the cathode tip of the beam-generating apparatus of the multiple particle beam system can be detected on the basis of beam current measurements and / or beam current uniformity measurements. This detection triggers a targeted material build-up process at the front facet of the cathode tip. Specific process steps for the material build-up process are specified. Furthermore, various options for beam current measurement and / or beam current uniformity measurement are described. In particular, these measurements may be performed during the normal operation of the multiple particle beam system.

Description

[0001] Method for operating a multiple particle beam system with detection of a ring collapse process and triggering of a material build-up process, associated computer program product, and multiple particle beam system and multi-beam particle microscope

[0002] Field of the invention

[0003] In general, the invention relates to multiple particle beam systems that operate using a multiplicity of individual charged particle beams. Specifically, the invention relates to a method for operating a multiple particle beam system with detection of a ring collapse process and triggering of a material build-up process, an associated computer program product, and a multiple particle beam system and a multi-beam particle microscope.

[0004] Prior art

[0005] With the ongoing development of ever smaller and ever more complex microstructures such as semiconductor components, there is a need to further develop and optimize planar production techniques and inspection systems for producing and inspecting small dimensions of the microstructures. For instance, the development and production of the semiconductor components require monitoring of the design of test wafers, and the planar production techniques require process optimization for reliable production with high throughput. Moreover, there have been recent demands for an analysis of semiconductor wafers for reverse engineering and for a customized, individual configuration of semiconductor components. Therefore, there is a need for inspection means which can be used with high throughput to examine the microstructures on wafers with high accuracy.

[0006] Typical silicon wafers used in the production of semiconductor components have diameters of up to 300 mm. Each wafer is divided into 30 to 60 repeating regions ("dies") with a size of up to 800 mm2. A semiconductor device comprises a plurality of semiconductor structures, which are produced in layers on a surface of the wafer by planar integration techniques. Semiconductor wafers typically have a plane surface on account of the production processes. The structure size of the integrated semiconductor structures in this case extends from a few pm to the critical dimensions (CDs) of 5 nm, and the structure sizes will become even smaller in the near future; in future, structure sizes or critical dimensions (CDs) are expected to be less than 3 nm, for example 2 nm, or even less than 1 nm. In the case of the aforementioned small structure sizes, defects of the order of the critical dimensions must be identified quickly over a very large area. For multiple applications, the specification requirement regarding the accuracy of a measurement provided by an inspection device is even higher, for example by a factor of two or one order of magnitude. For instance, a width of a semiconductor feature must be measured with an accuracy better than 1 nm, for example 0.3 nm or even less, and a relative position of semiconductor structures must be determined with an overlay accuracy better than 1 nm, for example 0.3 nm or even less.

[0007] The MSEM, a multi-beam scanning electron microscope, is a relatively new development in the field of charged particle systems (“charged particle microscopes”, CPMs). For instance, a multi-beam scanning electron microscope is disclosed in US 7 244 949 B2 and in US 2019 / 0355544 A1. In the case of a multi-beam electron microscope or MSEM, a sample is irradiated simultaneously by a multiplicity of individual electron beams arranged in a field or raster. For instance, 4 to 10 000 individual electron beams may be provided as primary radiation, with each individual electron beam being separated from an adjacent individual electron beam by a pitch of 1 to 200 micrometres. For example, an MSEM has approximately 100 separate individual electron beams (“beamlets”), which are arranged for example in a hexagonal grid, with the individual electron beams being separated by a pitch of approximately 10 pm. The multiplicity of individual charged particle beams (primary beams) are focused, in each case on an individual basis, on a surface of a sample to be examined by way of common large-field optics unit including, inter alia, a common objective lens. For example, the sample can be a semiconductor wafer that is secured to a wafer holder mounted on a movable stage. When the wafer surface is illuminated by the primary individual charged particle beams, interaction products, for example secondary electrons or backscattered electrons, emanate from the surface of the wafer. Their respective start points correspond to those locations on the sample on which the plurality of primary individual particle beams are focused in each case. The amount and the 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) that are collected by the common objective lens and, following a passage through a projection imaging system of the multi-beam inspection system, are incident on a detector arranged in a detection plane. The detector comprises a plurality of detection regions, each of which comprises a plurality of detection pixels, and the detector captures an intensity distribution for each of the secondary individual particle beams. An image field of 100 pm x 100 pm, for example, is obtained in the process.

[0008] The state-of-the-art multi-beam electron microscope comprises a sequence of electrostatic and magnetic elements. At least some of the electrostatic and magnetic elements are adjustable in order to adapt the focus position and the stigmation of the multiplicity of individual charged particle beams. The state-of-the-art multi-beam system with charged particles moreover comprises at least one crossover plane of the primary or the secondary individual charged particle beams. Moreover, the state-of-the-art system comprises detection systems to facilitate the adjustment. The state-of-the-art multi-beam particle microscope comprises at least one beam deflector (deflection scanner) for collective scanning of a region of the sample surface by means of the multiplicity of primary individual particle beams in order to obtain an image field of the sample surface.

[0009] As the demands on the imaging quality increase, so do the demands on the multi-beam particle microscopes used for imaging. Stable operating parameters are very important for high-quality recordings. One of these is the beam current of the individual particle beams used to scan a sample surface.

[0010] The emission characteristic of the particle source, more precisely the uniformity of the emission characteristic over the entire utilized emission angle, is important for a uniform beam current of the individual particle beams. When using relatively large emission angles, the emission characteristic of particle sources, e.g. of thermal field emission (TFE) sources, is no longer uniform throughout. Accordingly, the irradiance at a first multi-aperture plate (known as a filter plate) in a corresponding particle beam system is no longer uniform throughout in that case either, and there are relatively large variations in the current densities in different individual beams. However, a system requirement of multi-particle inspection systems is once again that there is only a small variation, typically less than a few percent or even less than one percent, in the currents between the various individual beams such that all individual image fields of the multi-image field are scanned with an equivalent number of particles or electrons. For example, this is a precondition to obtain individual images with approximately the same brightness. The obtainable resolution of the individual images also depends on the individual beam current.

[0011] There are options for setting the beam current on an individual basis for individual particle beams. One option in this respect is disclosed by DE 10 2018 007 652 A1 , the disclosure of which is incorporated in this patent application in full by reference.

[0012] The emission characteristic of a particle source changes slowly over time; it may exhibit a drift behaviour. Measuring and correcting for this drift is known. For example, a particle beam originally emitted by the source might also change its direction. The use of particle-optical components for correcting or compensating for this slow change in direction is known. Moreover, a particle source or tip may age; for example, it may lose brightness. The brightness of the images, in turn, correlates with the brightness or luminance of the source. If the source loses brightness, then this also applies to the image brightness. One solution to this problem lies in increasing the gain of the detection system in order to compensate the reduced brightness. However, this changes the signal-to-noise ratio (SNR) at the detector, and, in the worst-case scenario, leads to a reduction in said signal-to-noise ratio, and the obtainable contrast in the images is reduced, and hence this solution is only of limited suitability.

[0013] Therefore, an adjustment of the beam-generating system or a beam-generating apparatus itself is conventional, with a voltage applied to an extractor electrode being altered according to the prior art. However, it may take several days following such a change in the extractor current for the newly set beam-generating system to once again exhibit a sufficiently constant emission characteristic; the tips must first “burn in” again.

[0014] US 2020 / 0312619 A1 discloses a correction of beam parameters in a multi-beam particle microscope on the basis of beam current measurements at a multi-aperture array. In this case, the beam-generating system is controlled by setting the extractor voltage or the acceleration voltage. A beam migration can also be avoided. However, this always relates to slow corrections of an arising drift.

[0015] In addition to the known drift compensation, high-frequency control of a beam current is known from WO 2023 / 001402 A1. For this purpose, work can be performed with a special beamgenerating apparatus that, for fast control, uses an electrostatic control lens between the anode and the extractor. The disclosure of WO 2023 / 001402 A1 is incorporated in this patent application in full by reference.

[0016] Various types of beam current measurements are known from WO 2023 / 001401 A1 and its family document DE 102021 118561 A1 ; they can be used to control the beam current or else align the charged particle beam. Among other things, there is disclosure of a sectored first multi-aperture plate or filter plate, which is part of a multi-beam particle microscope, provided with a conductive absorber layer and connected to earth. For each sector, a current of charged particles incident on the filter plate or of particles discharged from the filter plate is measured. This is implemented, in particular, in an outer region around all the apertures in the filter plate. However, sectoring may also be implemented within or between the apertures. There is also disclosure of a beam current measurement at a pre-aperture plate with a singular opening, which is arranged close to or directly in front of the filter plate in the particle-optical beam path. Charged particles emanating directly from the particle source are also detected on the pre- aperture. To this end, the pre-aperture plate is provided with an absorber layer and connected to earth, and the current of the discharged charges is once again ascertained as the measured quantity. A detection of X-ray radiation is disclosed as a further variant for determining a beam current. In this case, X-ray radiation is created during subsequent processes following the incidence of the charged particle beam on the filter plate. Measuring light in the NIR range as a result of X-ray radiation conversion can also be used for beam current measurement purposes. The disclosure of WO 2023 / 001401 A1 is incorporated in this patent application in full by reference.

[0017] A change in the beam current and a beam migration behaviour of the particle beam emitted by a particle source or tip thus can be measured and controlled according to the prior art.

[0018] However, there are further processes that influence the beam current, which have not been measured or controlled to date: These include changes in the beam current due to what is known as the ring collapse process. The so-called ring collapse process is understood to be a characteristic reshaping of a front facet of a cathode tip. In this case, some monolayers of the front facet slide laterally or in a ring-shaped fashion from the apex surface of the cathode tip. The front facet is reshaped in the process. As a result, both the total beam current and the emission characteristics of the particle source are modified, in some cases significantly and over a relatively long period of time, for example several hours or days. Inspection processes, and in particular automated inspection processes, are therefore made more difficult or are not possible during this time.

[0019] The frequency of a ring collapse process inter alia depends on the temperature of the cathode, a diffusion rate at the cathode tip and the local field at the apex of the cathode. In practice, the frequency of a ring collapse process therefore varies greatly and ranges from daily through weekly to monthly or only once during the entire service life of a cathode tip. Ring collapse processes occur relatively frequently in inspections with comparatively low beam currents and comparatively low local electric fields, for example in the case of metrology-related questions, which are investigated using a multi-beam particle microscope.

[0020] The following generally applies: A ring collapse process is a spontaneous process. Inspection processes are not reasonably feasible during the ring collapse process. This in turn leads to an unpredictable downtime of a multiple particle beam system.

[0021] US005616926A discloses a single beam system having a Schottky emission cathode, and a method for stabilizing this cathode by means of a material build-up process at the cathode. According to US005616926A, in order to trigger the material build-up process, a sample current in the sample plane or a change in a beam current density as a function of an emission angle by the cathode is measured. In US005616926A, different currents are measured for different beam deflections of the single beam in order to measure the beam current density as a function of the emission angle. These measurements therefore require interruption of the normal operation of the single beam system and the provision of special deflection elements in the single beam system.

[0022] Furthermore, different conditions apply to an emission angle of a particle source / cathode for particle beam systems that operate with a single beam than in multiple particle beam systems: The beam-current-limiting anode aperture is significantly larger in a multiple particle beam system than in a single beam system, often by more than a factor of ten, since, in a multiple particle beam system, it is necessary to initially generate as much beam current as possible as only a small fraction (e.g. 5% or less) thereof can be put to subsequent use following the formation of the multiplicity of individual particle beams. For this reason, a much larger angular range of the source cone is initially used for further beam shaping or beam generation in a multiple particle beam system. In a single beam system, the anode aperture typically cuts an angular range of only about 2.6 mrad or less out of the source cone, whereas it is often more than 30 mrad in a multiple particle beam system. This results in significantly more far-reaching requirements for the beam current and beam current uniformity in a multiple particle beam system; and the conditions in a single beam system therefore cannot be easily transferred to a multiple particle beam system.

[0023] M. S. Bronsgeest et al., ..Collapsing rings” on Schottky electron emitters, Ultramicroscopy 110 (210), pages 1243 - 1254, deals in detail with the ring collapse process in Schottky emitters for single beam systems. The geometries of Schottky emitters are scientifically examined using SEM images before, during, and after a ring collapse process. From this, conclusions are drawn as to how such a ring collapse process can be detected at an early stage and, if necessary, prevented. It is concluded that ring collapse can be prevented by setting the extractor voltage high enough. In order to detect a ring collapse process at an early stage, it is proposed to measure the front facet current and / or to measure a field enhancement factor, for example using Schottky plots. Both measurement methods require an inspection process to be interrupted. Furthermore, beam current measurements are generally considered unsuitable for the early detection of a ring collapse. Similarly, beam current instability measurements are regarded as unsuitable for the early detection of a ring collapse, since a beam current instability occurs too late. Description of the invention

[0024] The problem addressed by the present invention is therefore that of reducing the downtime of a multiple particle beam system due to a ring collapse process. The problem is solved by the independent patent claims. Advantageous embodiments of the invention are evident from the dependent patent claims.

[0025] The present patent application claims the priority of the German patent application No. 102024 127 358.5 filed on 23 September 2024, the disclosure of which in the full scope thereof is incorporated in the present patent application by reference.

[0026] Firstly, the reduction in downtime is made possible by the timely detection of the onset of a ring collapse process by means of specific beam current measurements and beam current uniformity measurements. Secondly, the ring collapse process is counteracted in a targeted manner, and the front facet of the cathode tip is rebuilt. The time required for such a material build-up at the front facet of the cathode tip is significantly shorter than the duration of a ring collapse process.

[0027] According to a first aspect of the invention, the latter relates to a method for operating a multiple particle beam system, which is described below. In this case, the multiple particle beam system comprises a beam-generating apparatus for generating a first charged particle beam. The beam-generating apparatus comprises a cathode, an extractor electrode and an anode stop, and a heating apparatus for heating the cathode. The cathode comprises a reservoir with a work-function-lowering agent and a cathode tip with a front facet. The multiple particle beam system furthermore comprises a multi-beam generator having a filter plate, wherein the filter plate has a multiplicity of apertures and wherein the filter plate is substantially traversed by the first charged particle beam, a multiplicity of individual charged particle beams being formed in the process. In that case, the method for operating the multiple particle beam system includes the following steps:

[0028] (a) defining a reference beam current and / or a reference beam current uniformity;

[0029] (b) measuring a beam current and / or measuring a beam current uniformity between the multiplicity of individual charged particle beams;

[0030] (c) comparing the measured beam current with the reference beam current and, based thereon, ascertaining a beam current deviation, and / or comparing the measured beam current uniformity with the reference beam current uniformity and, based thereon, ascertaining a beam current uniformity deviation; (d) detecting the onset of a ring collapse process at the front facet of the cathode tip on the basis of the beam current deviation and / or on the basis of the beam current uniformity deviation; and

[0031] (e) triggering a material build-up process at the front facet of the cathode tip on the basis of the detection of the onset of the ring collapse process.

[0032] The beam-generating apparatus preferably comprises a thermal field emitter, in particular a Schottky emitter. The cathode or the cathode wire may for example consist of tungsten and have a zirconium oxide reservoir. However, other materials / material combinations are also possible, such as an emitter made of molybdenum, iridium or rhenium, provided with a work- function-lowering coating made of compounds such as oxides, nitrides and carbon with zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium or lanthanum. The work-function-lowering agent such as zirconium oxide can migrate to the apex of the cathode tip and reduce the work function of electrons there in a known manner. On the front facet, which in terms of crystallography usually is a {100} surface for tungsten emitters, the work function is reduced to the greatest extent. Therefore, the emitted electron beam mainly emanates from there, or this portion of the emitted electron beam is usable for technical purposes.

[0033] According to the invention, a reference beam current and / or a reference beam current uniformity is defined. The reference beam current preferably is a total beam current, which is or would be determinable at a predefined position by means of a predefined measurement method. In this context, the reference beam current may comprise a single current value, but the reference beam current may also comprise a current interval with an upper and a lower limit. The reference beam current describes the beam current that would be expected without the occurrence of a ring collapse process. The reference beam current may be a value that is constant over time or a value interval that is constant over time, but the reference beam current may also change over time and drift processes relating to the beam current, known per se, may be included in the definition of the reference beam current.

[0034] The reference beam current uniformity is a measure of the uniformity of individual beam currents and hence of the uniformity of the beam currents of the individual charged particle beams. A uniformity of the individual particle beam currents that is as large as possible is a system requirement for multiple particle beam systems. In this case, a difference between a maximum beam current value and a minimum beam current value may only have a certain maximum value. For example, the entire range, i.e. the difference, may be no more than 10 picoamperes. It is also possible to specify a uniformity in percentages. For instance, uniformity may be defined as follows: Uniformity [%] = (maximum value - minimum value) / mean value x 0.5 x 100. Other definitions are also possible and meaningful. The specific definition of beam current uniformity will preferably have to be adapted to the specific measurement of a beam current uniformity. In this respect, the term beam current uniformity or reference beam current uniformity may refer directly to the uniformity of the individual charged particle beams generated on the filter plate. However, it is also possible to measure or define other uniformity parameters that only reflect the uniformity between the individual charged particle beams without exactly corresponding to them. Various examples of this will be given hereinafter. Then again, what applies in general terms is that the reference beam current uniformity defines a target state of beam current uniformity, provided no ring collapse process is present.

[0035] A beam current is measured and / or a beam current uniformity between the multiplicity of individual charged particle beams is measured in a further method step. The measurement of the beam current preferably is a measurement of the total beam current. This measurement can be performed directly or indirectly, i.e. the beam current can be measured directly, or the beam current can be inferred from other measured quantities. Various examples of this will be described in detail below. In an alternative to that or in addition, the measurement of beam current uniformity may be performed directly or indirectly. Various examples of this will also be described in more detail below.

[0036] In a further method step, the measured beam current is compared with the reference beam current, and, based thereon, a beam current deviation is ascertained. The beam current deviation may be represented directly as the difference between the measured beam current and the reference beam current, but, alternatively, it is also possible to ascertain whether or not the measured beam current is for example still within a reference beam current interval. In any case, the ascertained beam current deviation is the quantity that may allow detection of the onset of a ring collapse process at the front facet of the cathode tip.

[0037] In addition to that or in an alternative, the measured beam current uniformity is compared with the reference beam current uniformity, and, based thereon, a beam current uniformity deviation is ascertained. It is possible to ascertain a specific value for a beam current uniformity deviation in this case, too, but a further option lies in merely ascertaining whether or not the beam current uniformity deviation is within or outside a permitted reference interval. Once again, the onset of a ring collapse process at the front facet of the cathode tip may be detected in that case on the basis of the beam current uniformity deviation. Should a corresponding beam current deviation and / or beam current uniformity deviation be detected, the invention provides for a material build-up process at the front facet of the cathode tip to be triggered on the basis of the detection of the onset of the ring collapse process. The material build-up process is thus initiated thereby. In this context, it is preferable for the normal operation of the multiple particle beam system to be interrupted for the material build-up process. This may be implemented immediately or with a slight delay. For example, it is conceivable that a recording of a multi-image field is completed before the normal operation of the multiple particle beam system is interrupted for the material build-up process.

[0038] In order to reliably detect the onset of the ring collapse process, the beam current and / or the beam current uniformity may be measured continuously or at least regularly within certain time intervals, i.e. intermittently. In particular, it is also possible for the beam current and / or the beam current uniformity to be measured during the ongoing normal operation of a multiple particle beam system; i.e. normal operation need not be specially interrupted for such a measurement, and the beam current and / or the beam current uniformity may for example be measured simultaneously with the illumination or scanning of a sample.

[0039] According to a preferred embodiment of the invention, the beam current uniformity between the multiplicity of individual charged particle beams is measured directly or indirectly, and the material build-up process is triggered when the beam current uniformity deviation exceeds a predefined limit. In addition to that or in an alternative, the beam current, and in particular the total beam current, is measured directly or indirectly, and the material build-up process is triggered when the ascertained beam current deviation exceeds a predefined limit. In this context, this may be an upper or a lower limit for the beam current.

[0040] According to a preferred embodiment of the invention, the method moreover includes the following steps for building up material at the front facet of the cathode tip:

[0041] (f) reducing an extractor voltage applied between the cathode tip and the extractor electrode, from an initial value UEO to a reduced value UE1 . This method step is a preparatory method step and substantially serves to protect the multi-beam generator or the micro-optics unit during the subsequent method steps. For example, it is possible that the following relation may apply to the extractor voltage: UE1 / UE0 < 60%, preferably UE1 / UE0 < 55% or UE1 / UE0 < 50%. The extractor voltage is thus greatly reduced in this method step. As a result, a beam current in the sample plane or object plane of the multiple particle beam system is also greatly reduced, for example to less than 30% of its initial value. A reduction (in the absolute value) of the extractor voltage thus reduces the total beam current. (g) temporarily increasing the temperature of the cathode, in particular by increasing a heating current at the heating apparatus, from an initial temperature TO to a higher temperature T 1 . The consequence of this heating current increase is that a work-function-lowering agent located on the front facet is evaporated, and this in turn increases the work function for electrons emanating from the front facet. Phenomenologically, this means that the beam current initially rises on account of the temperature increase, since more hot electrons can leave the material due to a broadened Fermi distribution, but, as the work-function-lowering agent evaporates, said beam current subsequently reduces and typically drops to almost zero within a few minutes. The temperature of the cathode is subsequently reduced again, especially to the initial temperature TO. In this respect, the temperature of the cathode is only increased temporarily during this method step. Lowering the temperature of the cathode to the initial temperature TO may be implemented in one step, in multiple steps or even continuously. There still is no beam current present in the sample plane even when there is a return to the initial temperature TO; the work function on the cathode is too large for this (without the work- function-lowering agent present on the front facet).

[0042] The temperature of the cathode may be increased temporarily during a time interval t_temp. By preference, the following relation applies to the time interval t_temp: 1 min < t_temp < 10 min, preferably 1 min < t_temp < 8 min or most preferably 1 min < t_temp < 5 min.

[0043] In method step (g), according to a preferred embodiment of the invention, the following relation may apply to a temperature difference AT = T1 - TO: 250°C < AT < 400°C, preferably 300°C < AT < 350°C. In addition to that or in an alternative, the following relation may apply to the temperature TO: TO > 1700°C, preferably TO > 1750°C or most preferably TO > 1800°C.

[0044] (h) increasing the extractor voltage to an elevated value UE2, which is higher than the initial value UEO, during a time interval t_Mat. A higher local electric field is present at the cathode tip or at the front facet as a result. This allows a material build-up process to start at the front facet: As a result of the temporary absence of the work-function-lowering agent, emitter material, for example tungsten, can more easily migrate to the apex and stabilize the initial emitter shape. In parallel, a work-function-lowering agent present in the reservoir of the cathode migrates back to the front facet. According to a preferred embodiment of the invention, the following relation applies to the time interval t_Mat: t_Mat < 60 min, preferably t_Mat < 45 min and most preferably t_Mat < 30 min. According to a preferred embodiment of the invention, the following relation may apply to the value of the extractor voltage UE2: UE2 / UE0 > 150%, preferably UE2 / UE0 > 200% or most preferably UE2 / UE0 > 250%. In addition to that or in an alternative, the following relation may apply to the extractor voltage UE2: UE2 > 5000 V, preferably UE2 > 6000 V or most preferably UE2 > 6500 V.

[0045] (i) reducing the extractor voltage to the initial value UE0. In this context, the extractor voltage may be reduced gradually or in a single step or continuously to the initial value UE0.

[0046] According to a preferred embodiment of the invention, the reduction of the extractor voltage according to step (i) is performed in such a way that the beam current I does not exceed an initial value I0, which is present at the beginning of step (f). During the material build-up process, this can prevent the presence of an excessive beam current that could potentially damage the multi-beam generator.

[0047] According to a preferred embodiment of the invention, a or the beam current I is measured during the material build-up process. By preference, the beam current I is measured at short time intervals or even continuously. This allows for the best possible control of the material build-up process.

[0048] According to a preferred embodiment of the invention, the beam current uniformity between the multiplicity of individual charged particle beams is measured directly or indirectly, and the material build-up process is triggered when the following relation applies to the beam current uniformity deviation Allni: Allni > 10%, preferably Allni > 5% or Allni > 1%. In the case of multiple particle beam systems with in principle a very high beam current uniformity, it is possible to choose the beam current uniformity deviation Allni to be very small. In the case of multiple particle beam systems with slightly lower beam current uniformity of a system-related nature, the beam current uniformity deviation AUni should be chosen to be slightly larger.

[0049] According to a preferred embodiment of the invention, the beam current uniformity is measured, and the beam current uniformity is measured at the filter plate of the multi-beam generator. In this embodiment variant, the beam current uniformity is thus measured indirectly or derived from a measurement in which there is no direct measurement of a beam current of each individual particle beam generated at the filter plate. In this embodiment of the invention, the beam current uniformity can be measured during ongoing normal operation of the multiple particle beam system, the operation needs not be interrupted or specially adapted on account of the measurement. According to a preferred embodiment of the invention, at least one region, which is arranged between adjacent apertures, serves as the detection region for measuring the beam current uniformity at the filter plate of the multi-beam generator. By preference, multiple such regions are used for measuring the beam current uniformity.

[0050] According to a preferred embodiment of the invention, the filter plate of the multi-beam generator, on which the beam current uniformity is measured, comprises a conductive absorber layer, wherein the filter plate or the conductive absorber layer is divided into a multiplicity of electrically isolated sectors, each of which is connected to earth. In that case, the measurement of the beam current uniformity comprises the measurement of beam currents in each case incident on a sector of the filter plate or discharged therefrom. For example, a current meter may be provided between the connection to earth and the respective sector, for example an ammeter, in particular a picoammeter. The number of sectors on the filter plate is chosen such that a beam current uniformity of the individual particle beams can, as a minimum, be derived from the corresponding number of beam currents. However, it is not necessary to match the number of sectors to the number of individual particle beams. Instead, the number of sectors may be lower, for example there might be only six, seven or eight sectors or twelve or sixteen sectors. When sectoring the filter plate, consideration should be given to the fact that the sectoring must not adversely affect the quality of the generated individual particle beams. The provision of electrical insulation between adjacent sectors on the surface of the filter plate should therefore be avoided. This is because insulation could be electrically charged in that case. Instead, the electrical insulation is laid within the filter plate by way of an appropriate 3D design of the filter plate, or the electrical insulation is buried therein. It is also possible to improve the signal-to-noise ratio during the beam current measurement by combining regions with multiple apertures in the filter plate into one sector. Moreover, more than 90% of the particles arriving at the filter plates are usually absorbed there, and only the remaining fraction is used to generate the individual particle beams. The following additionally applies: The fewer sectors used, the larger the individual sector and the better the signal-to- noise ratio during the current measurement in that sector. Conversely, a certain minimum number of sectors is nevertheless needed in order to detect a beam current uniformity deviation at all.

[0051] According to a preferred embodiment of the invention, each of the sectors surrounds at least one of the apertures in the filter plate, and a plurality of sectors preferably surround a group of apertures in the filter plate. Furthermore, each of the apertures in the filter plate is assigned to exactly one of the sectors. In this embodiment of the invention, the sectors as detection regions for a beam current thus are arranged very close to or directly at the apertures or around the apertures. Beam current fluctuations for individual beam currents or for certain groups of individual beam currents can therefore be reproduced very well by beam current measurements in the sectors. This makes it possible to draw conclusions about beam current uniformity.

[0052] According to a further preferred embodiment of the invention, a current detection aperture plate having a singular opening is arranged between the beam-generating apparatus and the filter plate, the back side of which faces the filter plate. The back side facing the filter plate comprises a conductive absorber layer that is connected to earth. At least one beam current of secondary electrons and / or backscattered electrons incident on the back side of the current detection aperture plate can be measured in order to measure a beam current and / or a beam current uniformity. These secondary electrons and / or backscattered electrons are generated or backscattered when the first charged particle beam is incident on the filter plate. They are also a very good measure of the beam current that was originally emitted by the beam-generating apparatus.

[0053] According to a preferred embodiment of the invention, the absorber layer of the current detection aperture plate is divided into a multiplicity of electrically isolated sectors, wherein one beam current is measured per sector. As regards the sectoring of the current detection aperture plate, what has already been explained as regards sectoring the filter plate in the context of the other embodiment of the invention applies analogously. However, the sectoring of the current detection aperture plate is somewhat less critical than the structuring of the filter plate, since no individual particle beam is generated or influenced at the current detection aperture plate. However, the overall detection signal obtained at the current detection aperture plate is lower than the signal obtained directly, as it were, at the sectored filter plate.

[0054] According to a preferred embodiment of the invention, the beam current is measured, and the measurement of the beam current comprises measuring an anode current. This anode current is generated on the basis of charged particles, in particular electrons, incident on the anode stop. It turns out that measuring the anode current is a particularly good measure for modifications to the beam-generating apparatus that are caused by a ring collapse process. The anode is typically in the form of an anode stop and cuts off the outermost beam current in the beam-generating apparatus (“teeth”). This beam current is generated in the edge region of the front facet and partly generated at the sides of the front facet, and it is difficult to use from a technical point of view. At the onset of a ring collapse process, it is precisely this region of the front facet that changes first. This allows an especially timely detection of the onset of the ring collapse process.

[0055] According to a preferred embodiment of the invention, the material build-up process is triggered when the measured anode current l_A drops below a predetermined threshold value l_A_Ref. Experiments have shown that the onset of a ring collapse process is accompanied by a decrease in the anode current l_A.

[0056] According to a preferred embodiment of the invention, the following relation for the measured anode current applies to the trigger of the material build-up process: l_A / l_A_Ref < 85%, preferably l_A / l_A_Ref < 90% or most preferably l_A / l_A_Ref < 95%. By preference, the material build-up process is therefore triggered very early.

[0057] In addition to that or in an alternative, the material build-up process is triggered according to a preferred embodiment when a rate of change of the measured anode current l_A exceeds a predetermined threshold value l_Avar_Ref within a predefined change time interval. This allows normal ageing processes of the cathode tip to be taken into account accordingly when triggering the material build-up process or allows these to be distinguished from the onset of a ring collapse process. A known rate of change of the measured anode current l_A during a burn-in of the cathode tip may also be taken into account or removed by calculation.

[0058] According to a preferred embodiment of the invention, one of the aforementioned changes in the anode current may be used as a necessary criterion for triggering a material build-up process, and a change in the beam current uniformity may be used as a sufficient criterion for triggering a material build-up process.

[0059] According to a preferred embodiment of the invention, the beam current and / or the beam current uniformity is measured in a or the object plane of the multiple particle beam system. This corresponds to the sample plane d of a multi-beam particle microscope. A beam current and / or a beam current uniformity may be determined in the object plane, for example by means of one Faraday cup or by means of multiple Faraday cups (or one successively displaced Faraday cup). However, such a measurement interrupts the normal operation of the multiple particle beam system, or it should at least be meaningfully integrated into a process sequence of the multiple particle beam system in order to avoid unnecessary downtime. In principle, however, measuring the beam current and / or the beam current uniformity in the object plane is a meaningful measure for detecting the onset of the ring collapse process. According to a preferred embodiment of the invention, the beam current is measured, and the beam current is measured level with a beam crossover of the individual charged particle beams with one another. For example, this may be implemented in inspection processes during a line jump or during an image jump. In that case, it is possible to deflect all individual particle beams into a beam current measuring device, which is arranged level with the beam crossover, by means of a simple collective beam deflector. In principle, this deflection is a simple parallel offset. Such a measurement of the total beam current is therefore very simple to realize.

[0060] According to a preferred embodiment of the invention, the multiple particle beam system is a multi-beam particle microscope, which generates a multiplicity of individual particle-optical images that can be composed or are composed to form a multi-image. In addition to the beam current measurements and / or beam current uniformity measurements described above or in an alternative, the beam current uniformity can then be measured indirectly on the basis of brightness values of the generated individual images. If the brightness values of the generated individual images vary more than permitted, this may also be used to infer the onset of a ring collapse process.

[0061] It is possible for the above-described embodiment variants of the invention to be combined with one another in full or in part, provided that no technical contradictions arise as a result.

[0062] According to a second aspect of the invention, the latter relates to a computer program product having a program code for carrying out the method as described above in multiple embodiment variants. In this case, the program code can be written in any desired programming language. The program code may have a modular structure. For example, the program code may comprise a module concerning the measurement and evaluation of the beam current and / or beam current uniformity and a module concerning the material build-up process.

[0063] According to a third aspect of the invention, the latter relates to a multiple particle beam system configured to carry out the method as described above in multiple embodiment variants. In this case, for example, a program code for carrying out the method, as described above in multiple embodiment variants, can be loaded into a processor of the controller of the multiple particle beam system, and the corresponding components of the multiple particle beam system can be controlled accordingly.

[0064] According to a fourth aspect of the invention, the latter relates to a multi-beam particle microscope comprising the following: a beam-generating apparatus configured to generate a first charged particle beam and comprising a cathode, an extractor electrode and an anode stop, and a heating apparatus for heating the cathode, wherein the cathode comprises a reservoir with a work-function-lowering agent, and wherein the cathode comprises a cathode tip with a front facet; a multi-beam generator having a multi-aperture array, wherein the multi-aperture array comprises a filter plate having a multiplicity of apertures that is traversed by the first charged particle beam, a multiplicity of first individual charged particle beams being formed in the process, and wherein the multi-beam generator is configured to generate a first field of a multiplicity of individual charged particle beams from the first charged particle beam; and a beam-current meter configured to measure a beam current and / or beam current uniformity; a first particle optics unit having a first particle-optical beam path and configured to direct the generated first individual particle beams at a sample such that the first particle beams impinge on the sample at incidence locations that form a second field; a detection system; a second particle optics unit having a second particle-optical beam path and configured to image second individual particle beams, which emanate from the incidence locations in the second field, onto the detection system; a particle-optical objective lens traversed by both the first and the second individual particle beams; a beam splitter arranged in the first particle-optical beam path between the multi-beam generator and the objective lens and arranged in the second particle-optical beam path between the objective lens and the detection system; and a controller configured to control the beam-generating apparatus, the particle-optical objective lens, the first particle optics unit, the second particle optics unit and the detection system, and wherein the controller is configured to detect the onset of a ring collapse process at the front facet of the cathode tip of the beam-generating apparatus on the basis of the beam current measurement and / or the beam current uniformity measurement and to control the beamgenerating apparatus for a material build-up process at the front facet of the cathode tip on the basis of the detection.

[0065] The terms and definitions used in connection with the multi-beam particle microscope are the same as in the description of the method for operating the multiple particle beam system according to the first aspect of the invention.

[0066] According to a preferred embodiment of the invention, the beam-current measuring means is configured to measure a beam current uniformity, wherein the beam-current measuring means comprises the filter plate of the multi-beam generator. The filter plate comprises a conductive absorber layer and is divided into a multiplicity of electrically isolated sectors, each of which is connected to earth. The beam currents incident on the respective sectors are determined during the operation of the multi-beam particle microscope. For example, the conductive absorber layer may be coated with gold, copper, silver, platinum or another conductive material. Moreover, what has already been stated in connection with the method according to the invention for operating a multiple particle beam system applies to this embodiment variant of the invention.

[0067] According to a preferred embodiment of the invention, each of the sectors surrounds at least one of the apertures in the filter plate, and a plurality of sectors preferably surround a group of apertures in the filter plate. Furthermore, each of the apertures in the filter plate is assigned to exactly one of the sectors. In this embodiment of the invention, the sectors as detection regions for a beam current thus are arranged very close to or directly at the apertures or around the apertures. Beam current fluctuations for individual beam currents or for certain groups of individual beam currents can therefore be reproduced very well by beam current measurements in the sectors. This makes it possible to draw conclusions about beam current uniformity.

[0068] According to a preferred embodiment of the invention, the beam-current measuring means comprises a current detection aperture plate that is arranged between the beam-generating apparatus and the filter plate. Its back side faces the filter plate and comprises a conductive absorber layer that is connected to earth. For example, the conductive absorber layer may once again comprise gold, copper, silver, platinum or another conductive material or metal. A beam current of secondary electrons and / or backscattered electrons incident on the back side of the current detection aperture plate is measured during the operation of the multi-beam particle microscope. These are created when charged particles of the first charged particle beam are incident on the filter plate or are backscattered by the latter.

[0069] According to a preferred embodiment of the invention, the absorber layer of the current detection aperture plate is divided into a multiplicity of electrically isolated sectors, and the beam-current measuring means is configured to measure one beam current per sector. For this purpose, a current meter, for example an ammeter and in particular a picoammeter, may be provided between the connection to earth and the respective sector.

[0070] According to a preferred embodiment of the invention, a conductive protective layer is arranged on the front side of the current detection aperture plate, connected to earth and electrically insulated from the conductive absorber layer. This ensures that charged particles that are incident on the front side of the current detection aperture plate do not contribute to the current measurement when measuring backscattered electrons backscattered by the filter plate and / or secondary electrons emanating from the filter plate. One disturbance variable can thus be dispensed with.

[0071] According to a further preferred embodiment of the invention, the beam-current measuring means comprises the anode stop of the beam-generating apparatus. The beam-current measuring means is configured to measure the beam current of charged particles, in particular electrons, incident on the anode stop. Moreover, all that has already been stated in connection with the method according to the invention for operating a multiple particle beam system also applies to this embodiment of the invention.

[0072] According to a fifth aspect of the invention, the latter relates to a multiple particle beam system comprising the following: a beam-generating apparatus configured to generate a first charged particle beam; a multi-beam generator having a multi-aperture array, wherein the multi-aperture array comprises a filter plate having a multiplicity of apertures that is traversed by the first charged particle beam, a multiplicity of first individual charged particle beams being formed in the process, and wherein the multi-beam generator is configured to generate a first field of a multiplicity of individual charged particle beams from the first charged particle beam; and a current detection aperture plate having a singular opening arranged between the beamgenerating apparatus and the filter plate and traversed substantially without contact by the first charged particle beam, wherein a back side of the current detection aperture plate faces the filter plate and has a conductive absorber layer that is connected to earth, and wherein a current meter is arranged between the conductive absorber layer and earth such that a beam current of secondary electrons and / or backscattered electrons incident on the conductive absorber layer is measurable during the operation of the multiple particle beam system.

[0073] In this case, as regards the features of the multiple particle beam system according to the fifth aspect of the invention, everything that has already been explained with regard to these features in connection with the first to fourth aspect of the invention applies.

[0074] According to a preferred embodiment of the invention, a conductive protective layer is arranged on a front side of the current detection aperture plate, connected to earth and electrically insulated from the conductive absorber layer. This prevents charged particles emitted directly by the beam-generating apparatus from being directly incident on the current detection aperture plate and interfering with or being overlaid on the measurement of the incident secondary electrons and / or backscattered electrons.

[0075] According to a preferred embodiment of the invention, the absorber layer of the current detection aperture plate is divided into a multiplicity of electrically isolated sectors, each of which is connected to earth. In this case, one beam-current measuring means is arranged between the absorber layer of each sector and earth, respectively, and configured to measure a beam current per sector of secondary electrons and / or backscattered electrons incident on the conductive absorber layer. The beam-current measuring means in the narrower sense can for example be an ammeter, in particular a picoammeter. The measurement can therefore be very accurate.

[0076] The various embodiments of the invention may be combined with one another in full or in part, provided that no technical contradictions arise as a result. This also applies to embodiment variants of the invention according to different aspects of the invention.

[0077] The invention will be understood even better with reference to the accompanying figures, in which:

[0078] Fig. 1 : shows a schematic illustration of a multi-beam particle microscope (MSEM);

[0079] Fig. 2: shows a schematic illustration of a beam-generating apparatus;

[0080] Fig. 3: shows a schematic illustration of details of a beam-generating apparatus;

[0081] Fig. 4: schematically illustrates a ring collapse process;

[0082] Fig. 5: schematically illustrates method steps of a method according to the invention for operating a multiple particle beam system;

[0083] Fig. 6: schematically illustrates method steps of a material build-up process;

[0084] Fig. 7: shows measurement results of beam current measurements during a ring collapse process;

[0085] Fig. 8: schematically illustrates a beam current measurement at an anode stop;

[0086] Fig. 9: schematically illustrates beam current distributions;

[0087] Fig. 10: schematically illustrates a beam current measurement at a filter plate;

[0088] Fig. 11 : schematically illustrates beam current measurements at a sectored filter plate;

[0089] Fig. 12: schematically illustrates beam current measurements at a sectored filter plate;

[0090] Fig. 13: schematically shows a current detection aperture plate; and

[0091] Fig. 14: schematically shows a sectored current detection aperture plate. Fig. 1 schematically shows a multiple particle beam system 1 in the form of a multi-beam particle microscope 1. The multi-beam particle microscope 1 comprises a beam-generating apparatus 300 having a particle source, for example an electron source. By means of the beam-generating apparatus 300, charged particles or electrons are generated for example by means of thermal field emission. The emitted charged particles form a divergent particle beam 309, and the latter is collimated by a sequence of condenser lenses 303.1 and 303.2 and incident on a multi-beam particle generator 305 having a multi-aperture arrangement. The multi-beam particle generator 305 comprises multiple multi-aperture plates 304, 306 and a field lens 307. The multi-beam particle generator 305 generates a multiplicity of individual particle beams 3 or individual electron beams 3, which are arranged in a field, which is imaged onto a further field formed by beam spots 5 in the object plane 101. The pitch between centre points of apertures in a multi-aperture plate 306 can be for example 5 pm, 100 pm and 200 pm. The diameters D of the apertures are smaller than the pitch between the centre points of the apertures; examples of the diameters are 0.2 times, 0.4 times and 0.8 times the pitches between the centre points of the apertures.

[0092] The multi-aperture arrangement 305 and the field lens 308 are configured to generate a multiplicity of focal points 323 of primary beams 3 in a grid arrangement on a surface 321 . The surface 321 needs not be a plane surface but rather can be a spherically curved surface in order to account for an image field curvature of the subsequent particle-optical system.

[0093] The multi-beam particle microscope 1 furthermore comprises a system of electromagnetic lenses 103 and an objective lens 102, which image the beam foci 323 from the intermediate image surface 321 into the object plane 101 with reduced size. In between, the first individual particle beams 3 pass through the beam splitter 400 and a collective beam deflection system 500, by means of which the multiplicity of first individual particle beams 3 are deflected during operation and the image field is scanned. The first individual particle beams 3 incident in the object plane 101 form for example a substantially regular field, wherein pitches between adjacent incidence locations 5 can be for example 1 pm, 10 pm or 40 pm. The field formed by the incidence locations 5 can have a rectangular or hexagonal symmetry, for example.

[0094] The object 7 to be examined may be of any desired type, for example a semiconductor wafer or a biological sample, and may comprise an arrangement of miniaturized elements or the like. The surface 15 of the object 7 is arranged in the object plane 101 of the objective lens 102. The objective lens 102 can comprise one or more electron-optical lenses. For example, it can be a magnetic objective lens and / or an electrostatic objective lens. The primary particles 3 incident on the object 7 generate interaction products, for example secondary electrons, backscattered electrons or primary particles, which have experienced a reversal of movement for other reasons, and these interaction products emanate from the surface of the object 7 or from the first plane 101 or object plane 101 . The interaction products emanating from the surface 15 of the object 7 are shaped by the objective lens 102 to form secondary particle beams 9. In the process, the secondary beams 9 pass through the beam splitter 400 downstream of the objective lens 102 and are supplied to a projection system 200. The projection system 200 comprises an imaging system 205 with projection lenses 206, 208 and 210, a contrast stop 214 and a multi-particle detector 207. Incidence locations 25 of the second individual particle beams 9 on detection regions of the multi-particle detector 207 are located with a regular pitch in a third field. Exemplary values are 10 pm, 100 pm and 200 pm.

[0095] The multi-beam particle microscope 1 furthermore comprises a computer system or control unit or controller 10, which in turn can be embodied integrally or in multipartite fashion and which is designed both to control the individual particle-optical components of the multi-beam particle microscope 1 and to evaluate and analyse the signals obtained by the multi-detector 207 or detection unit.

[0096] Further information relating to such multi-beam particle beam systems or multi-beam particle microscopes 1 and component parts used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881 A2, WO 2007 / 028595 A2, WO 2007 / 028596 A1 , WO 2011 / 124352 A1 and WO 2007 / 060017 A2 and the German patent applications DE 102013016 113 A1 and DE 102013 014 976 A1 , the disclosure of each of which is incorporated in this patent application in full by reference.

[0097] Figure 2 schematically shows a beam-generating apparatus 300. The beam-generating apparatus 300 comprises a cathode 350 having a cathode tip 351 for emitting charged particles, for example electrons. The cathode can be heated, as indicated schematically in Figure 2 by the depicted heating wire 359. Furthermore, the beam-generating apparatus 300 comprises an extractor electrode 353 and optionally a suppressor electrode 356. Cathode 350, suppressor electrode 356 and extractor electrode 353 together form what is known as the beam head 349. This is indicated by the dotted quadrilateral in Figure 2. The suppressor electrode 356 and the extractor electrode 353 each have a cylindrical shape with a cylindrical lateral surface and a flat front region: In the example shown, the flat front region 359 of the suppressor electrode 356 comprises an opening 360, through which the cathode tip 351 projects. The extractor electrode 353 has a planar front region 357, which in turn has an opening 358. The emitted particle beam 352 passes through this opening. The particle beam 352 is then incident on an anode electrode 354 or anode stop 354, which, in the example shown, is formed in a planar fashion and has an opening 355. By means of this opening 355, the charged particle beam 352 is trimmed and acquires the shape of the divergent particle beam 309 that is also illustrated schematically in Figure 1.

[0098] The anode stop 354 can be displaced relative to the beam head 349, specifically both in the z-direction and in a plane orthogonal to the particle-optical axis Z.

[0099] The constituent parts of the beam-generating apparatus 300 can be controlled by means of the controller 10. The applied acceleration voltage, the extraction voltage, the suppressor voltage and a heating current for the cathode 350 may be controlled by means of the controller in this way.

[0100] During normal operation of the beam-generating apparatus 300 or during operation of the associated multiple particle beam system 1 , an extraction voltage of for example a few kV, for example approximately + / -2 kV, + / -3 kV, + / -4 kV, + / -5 kV, + / -6 kV to + / -10 kV, is present between the cathode tip 351 and the extractor electrode 353.

[0101] Between the cathode tip 351 and the anode electrode 354, an acceleration voltage of a few 10 kV is present during operation, for example + / -10 kV, + / -25 kV, +1-27 kV, + / - 30 kV, + / - 35 kV, + / -40 kV or significantly more, for example up to + / -300 kV. It is preferably the case here that the anode electrode 354 is at earth potential or only a low voltage is applied to it, while the actual high voltage is present at the cathode 350.

[0102] Between the optionally provided suppressor electrode 356 and the cathode 350, during operation of the beam-generating apparatus 300 a suppressor voltage is present which can be for example a few 100 volts, e.g. + / -200 V, + / -300 V, + / -400 V, + / -500 V or + / -600 V.

[0103] During a material build-up process at the front facet 365 of the cathode 350, the extraction voltage and the heating current at the cathode 350, in particular, may be changed.

[0104] Figure 2 schematically illustrates the described voltages between the electrodes for the case where the cathode 350 emits electrons. The conditions would be reversed if positively charged particles were emitted. The example illustrated in Figure 2 should be understood as nonlimiting in this respect. This also applies to subsequent Figure 8. Fig. 3 schematically shows details of a beam-generating apparatus 300. Specifically, Fig. 3a schematically shows details of the beam head 349 and, in particular, the cathode 350. The cathode tip 351 protrudes from the opening 360 in the suppressor electrode 356. The emitted particle beam 352 thereupon passes through the opening 358 in the extractor electrode 353. The cathode 350 itself comprises a reservoir 361 with a work-function-lowering agent and may be heated by means of a heating wire 359. The cathode may be made of tungsten, for example, and zirconium oxide, for example, may be used as work-function-lowering agent. However, other materials / material combinations are also possible, such as an emitter made of molybdenum, iridium or rhenium, provided with a work-function-lowering coating made of compounds such as oxides, nitrides and carbon with zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium or lanthanum.

[0105] Fig. 3b, left, is an enlarged illustration of the cathode 350. It is possible to identify the cathode wire 364, with the reservoir 361 also being arranged thereon. The cathode wire 364 usually consists of monocrystalline tungsten. The heating wire 359 usually consists of polycrystalline tungsten. However, other materials are also possible. The front region 362 of the cathode 350 is normally formed by an etched shaft 363. The cathode tip 351 in the narrower sense is situated at the tapered end of this etched shaft 363. This is shown at the far right of Fig. 3b in greatly enlarged fashion. The cathode 350 tapers to a point in rounded off fashion above the tapered shaft 367, the rounded-off region being referred to as the apex 366. The front facet 365 of the cathode tip 351 is situated at the tip of the apex 366. The front facet 365 has a fixed crystallographic orientation, usually a {100} surface. The charged particles, in particular electrons, emanate from this surface during the operation of the beam-generating apparatus 300. The orientation of the crystallographic surface has an influence on the work function of the electrons. In this respect, the operation of the cathode 350 is a complex interaction between an electric field present at the cathode 350, the geometry of the cathode tip 351 , the nature of the front facet 365 and, in particular, the crystallographic orientation of this surface, the material of the cathode 350 and the work-function-lowering agent used. In summary, the emission characteristic of a cathode 350 is dependent on all parameters that describe the electric field at the cathode tip 351 and on those parameters that describe the chemical potential for the (curved) metal surface in the presence of the electric field. The chemical potential in turn depends on the strength of the electric field.

[0106] In thermodynamics, a stability criterion (dynamic equilibrium) for the geometric shape of a cathode tip 351 can be derived only under the assumption of a uniform chemical potential p. In practice, however, this assumption does not hold true since there is no value for the axial electric field that would lead to a homogeneous chemical potential on the surface of the apex 366 and of the shaft 367 of the cathode tip 351. Therefore, a gradient or fluctuation of the chemical potential always leads to a mass transport by means of surface diffusion, whereby migrating emitter material can modify the shape of the apex and the manifestation of the crystallographic facets.

[0107] The processes at the cathode tip 351 , as outlined briefly above, are therefore complex. In addition to a continuous ageing process of a cathode tip 351 , which leads to a drift process in the beam current emitted by the cathode 350, there are further processes that may adversely affect the beam current and in particular the beam current uniformity of the emitted charged particles. These include what is known as the ring collapse process, which is illustrated schematically in Fig. 4. What is known as the ring collapse process refers to a characteristic reshaping of the front facet 365 of a cathode tip 351. Some layers of a structural combination of an emitter material / work-function-lowering agent unit, such as, for example, a ZrOW unit of the front facet 365, slide off the apex surface 366 of the cathode tip 351 laterally or in ringshaped fashion. The front facet 365 is thus transformed. The sliding process is illustrated by arrows in Fig. 4: In Fig. 4a, the extent of the front facet 365 is greater than in Fig. 4b, where a layer of a structural combination of an emitter material / work-function-lowering agent unit such as, for example, a ZrOW unit of the front facet 365 has already slid off. As a result of sliding, both the total beam current and the emission characteristic of the cathode 350 are modified, typically significantly and over a relatively long period of time, for example several hours or days. Inspection processes, and automated inspection processes in particular, are therefore more difficult or not meaningfully feasible during the ring collapse process. The frequency of a ring collapse process varies greatly and inter alia depends on the temperature of the cathode 350, on a diffusion rate at the cathode tip 351 and on the local field at the apex 366 of the cathode. Weak local electric fields promote the frequency with which ring collapse processes occur. However, it is not possible to predict a ring collapse process; a ring collapse process remains a spontaneously occurring event. However, the onset of the ring collapse process can be measured, and appropriate countermeasures may be taken.

[0108] Fig. 5 schematically illustrates method steps of the method according to the invention for operating a multiple particle beam system, in particular a multi-beam particle microscope 1. The multiple particle beam system is provided in an initial method step S1. The multiple particle beam system 1 comprises a beam-generating apparatus 300 for generating a first charged particle beam 309. Furthermore, the beam-generating apparatus 300 comprises a cathode 350, an extractor electrode 353 and an anode stop 354 and also a heating apparatus 359 for heating the cathode 350. In this case, the cathode 350 comprises a reservoir 361 with a work- function-lowering agent and a cathode tip 351 with a front facet 365. Moreover, the multiple particle beam system 1 comprises a multi-beam generator 305 having a filter plate 304, wherein the filter plate 304 has a multiplicity of apertures 304a and wherein the filter plate 304 is substantially traversed by the first charged particle beam 309, 311 , a multiplicity of (first) individual charged particle beams 3 being formed in the process.

[0109] A reference beam current and / or a reference beam current uniformity is defined in method step S2. These may be individual numerical values or else range specifications. The reference beam current preferably is a total beam current, which is / would be determinable at a predefined position by means of a predefined measurement method. In this context, the reference beam current may comprise a single current value, but the reference beam current may also comprise a current interval with an upper and a lower limit. The reference beam current describes the beam current that would be expected without the occurrence of a ring collapse process. The reference beam current may be a value that is constant over time or a value interval that is constant over time, but the reference beam current may also change over time and drift processes relating to the beam current, known per se, may be included in the definition of the reference beam current. The reference beam current uniformity is a measure of the uniformity of individual beam currents and hence of the uniformity of the beam currents of the individual charged particle beams. A uniformity of the individual particle beam currents that is as large as possible is a system requirement for a multiple particle beam system. In this case, a difference between a maximum beam current value and a minimum beam current value may only have a certain maximum value. For example, the entire range, i.e. the difference, may be no more than 10 picoamperes. It is also possible to specify a uniformity in percentages. For instance, uniformity may be defined as follows: Uniformity [%] = (maximum value - minimum value) / mean value x 0.5 x 100. Other definitions are also possible and meaningful. The specific definition of beam current uniformity will preferably have to be adapted to the specific measurement of a beam current uniformity. In this respect, the term beam current uniformity or reference beam current uniformity may refer directly to the uniformity of the individual charged particle beams generated on the filter plate. However, it is also possible to measure or define other uniformity parameters that only reflect the uniformity between the individual charged particle beams without exactly corresponding to them. Various examples of this will be given hereinafter. Then again, what applies in general terms is that the reference beam current uniformity defines a target state of beam current uniformity, provided no ring collapse process is present.

[0110] In a further method step S3, a beam current and / or a beam current uniformity are / is measured between the multiplicity of the individual charged particle beams 3, in particular during the normal operation of the multiple particle beam system 1. The beam current and / or beam current uniformity may be measured in different ways in this context, as will be explained in more detail hereinafter. In principle, the beam current and / or beam current uniformity can be measured directly and immediately or else indirectly. In any case, conclusions regarding the beam current and / or beam current uniformity can be drawn on the basis of a measurement process.

[0111] In a method step S4, the measured beam current is compared with the reference beam current, and, based thereon, a beam current deviation is ascertained. In addition to that or in an alternative, the measured beam current uniformity is compared with the reference beam current uniformity, and, based thereon, a beam current uniformity deviation is ascertained. The ascertained beam current deviation and / or beam current uniformity deviation is evaluated below: Whether or not a ring collapse process has set in at the front facet 365 is detected or ascertained in method step S5. In this case, the onset of a ring collapse process at the front facet of the cathode tip is detected on the basis of the beam current deviation and / or on the basis of the beam current uniformity deviation. Should the beam current deviation and / or the beam current uniformity deviation be correspondingly large, step S6 provides for a material build-up process at the front facet 365 of the cathode tip 351 to be triggered on the basis of the detection of the onset of the ring collapse process. Otherwise, the beam current and / or beam current uniformity between the multiplicity of individual charged particle beams 3 is measured again in step S3. In this way, the onset of a ring collapse process can be ascertained in a timely manner, and countermeasures can be taken by means of the material build-up process.

[0112] According to a preferred embodiment of the invention, the material build-up process is triggered when the beam current uniformity between the multiplicity of individual charged particle beams 3 is measured and when the beam current uniformity deviation exceeds a predefined limit. In addition to that or in an alternative, the beam current can be measured, and the material build-up process can be triggered when the beam current deviation exceeds a predefined limit.

[0113] For example, it is possible that the material build-up process is triggered when the following relation applies to the beam current uniformity deviation Allni: Allni > 10%, preferably Allni > 5% or Allni > 1%. In the case of multiple particle beam systems with in principle a very high beam current uniformity, it is possible to choose the beam current uniformity deviation AUni to be very small. In the case of multiple particle beam systems with slightly lower beam current uniformity of a system-related nature, the beam current uniformity deviation Allni should be chosen to be slightly larger.

[0114] Fig. 6 schematically illustrates method steps of a material build-up process. In method step S7, an extractor voltage applied between the cathode tip 351 and the extractor electrode 353 is reduced, from an initial value UEO to a reduced value UE1. As a result of significantly reducing the extractor voltage, a beam current (in the sample plane) is significantly reduced, for example to less than 30% of its initial value. At the same time, the total current emitted by the cathode 350 is reduced. A reduction in the total current protects the sensitive micro-optics unit of a multiple particle beam system 1 during the subsequent method steps. For example, the extractor voltage UEO may be lowered to the reduced value UE1 in such a way that the following relation applies: UE1 / UE0 < 60%, preferably UE1 / UE0 < 55% or UE1 / UE0 < 50%. However, the reduced value UE1 may also be chosen to be even smaller.

[0115] In method step S8, the temperature of the cathode 350 is temporarily increased, in particular by increasing a heating current at the heating apparatus 359, from an initial temperature TO to a higher temperature T1. The temporary increase in the temperature of the cathode 350 initially leads to an increase in the beam current on account of the increased temperature, but then also leads to the evaporation of the work-function-lowering agent from the front facet 365 of the cathode tip 351. With increasing evaporation, the beam current drops again, until ultimately practically no beam current is emitted from the cathode 350 anymore because the work function required for this has become too large. According to an example, the temperature of the cathode 350 can be increased temporarily during a time interval t_temp, wherein the following relation may apply to the time interval t_temp: 1 min < t_temp < 10 min, preferably 1 min < t_temp < 8 min or 1 min < t_temp < 5 min. The time interval t_temp is therefore relatively short.

[0116] In addition to that or in an alternative, a temperature difference AT = T1 - TO during method step S8 may fulfil the following relation: 250°C < AT < 400°C, preferably 300°C < AT < 350°C. In addition to that or in an alternative, the following relation may apply to the temperature TO itself: TO > 1700°C, in particular TO > 1750°C or TO > 1800°C.

[0117] In method step S9, the extractor voltage is increased again, to be precise to an elevated value UE2 that is higher than the initial value UEO, to be precise during a time interval t_Mat. On account of the now higher local electric field, the actual material build-up starts. Emitter material increasingly migrates to the apex of the cathode and is accumulated on side and front facets, stabilizing the latter. The front facet 365 itself is reshaped and rebuilt. The following relation may apply to the elevated value UE2 of the extractor voltage UE2: UE2 / UE0 > 150%, preferably UE2 / UE0 > 200% or UE2 / UE0 > 250%. In addition to that or in an alternative, the following relation may apply to the extractor voltage UE2: UE2 > 5000 V, preferably UE2 > 6000 V or UE2 > 6500 V. Even higher values of the extractor voltage UE2 are eventually possible, e.g. up to 8 kV or 10 kV.

[0118] Increasing the extractor voltage in method step S9 is temporary and implemented during a time interval t_Mat. The following relation may apply to the time interval t_Mat: t_Mat < 60 min, preferably t_Mat < 45 min or t_Mat < 30 min. The duration of method step S9 is therefore also relatively short in comparison with the significantly longer duration of a ring collapse process.

[0119] In a further method step S10, the extractor voltage is reduced to the initial value UE0. The point of this is to lower the extractor voltage substantially back to the initial value UE0. According to an example, the reduction of the extractor voltage is performed in such a way in method step S10 that the beam current I does not exceed an initial value I0, which is present at the beginning of step S7. Thus, an overshooting beam current is prevented, in turn protecting the multi-beam generator of the multiple particle beam system 1.

[0120] In optional method step S11 , the beam current and / or the beam current distribution can be monitored following the material build-up process. This method step serves to improve process reliability. As a rule, however, both the beam current and the beam current distribution will adopt satisfactory values after the material build-up process has been performed.

[0121] Fig. 7 shows measurement results of beam current measurements during a ring collapse process. Accordingly, the beam current I is plotted on the y-axis, which is normalized to 1 before the ring collapse process. The time t is plotted on the x-axis, to be precise in days in the measurement example shown. The entire ring collapse process in Fig. 7 lasted about two weeks.

[0122] Measured points that show the beam current at a sample or at an object or in the object plane of a multiple particle beam system (here: multi-beam particle microscope 1) are plotted in the diagram as circles. During the first four days of the ring collapse process, the beam current in the region of the sample initially remains almost constant, before the beam current then suddenly increases quite significantly and drops quite significantly shortly thereafter (on days 7 and 8 respectively). Then, the beam current in the region of the sample slowly recovers and approximately reaches its initial value after about two weeks. In the case of a beam current measurement in the region of the object plane 101 , the change in the beam current that occurs during a ring collapse process is thus clearly recognizable but only appears relatively late.

[0123] The situation is different with regard to a beam current that is detected at the anode stop: According to Fig. 8, the anode stop 354 is equipped with a conductive absorber layer 368. This conductive absorber layer 368 is connected to earth, wherein a current measuring instrument, for example an ammeter, in particular a picoammeter, is provided between earth and the conductive absorber layer 368. The ascertained values are transmitted to the controller 10. The conductive absorber layer 368 is electrically insulated from the anode stop 354, to which a voltage is applied. As illustrated in Fig. 8, the anode stop 354 curtails the emitted particle beam 352 at the edges. This curtailed region of the beam current corresponds to the edges of the front facet 365 of the cathode tip 351 . The onset of a ring collapse process becomes noticeable at a particularly early stage in this edge region as a result of monolayers of the front facet 349 slipping onto the apex 366. Accordingly, it is also evident from Fig. 7 that the beam current measured at the anode (small triangular measured points in Fig. 7) indeed already changes at the onset of the ring collapse process, starting to sink in this case. From day three of the ring collapse process at the latest, the change in the anode beam current can be significantly detected. This may also be the case earlier. This naturally depends on the system stability of the multiple particle beam system as a whole and on the accuracy with which the beam currents can be measured.

[0124] According to an example, it is possible that the material build-up process is triggered when the measured anode current l_A drops below a predetermined threshold value l_A_Ref. For example, a quotient of the anode current l_A and the predetermined threshold value l_A_Ref may fulfil the following relation: l_A / l_A_Ref < 85%, preferably l_A / l_A_Ref < 90% or l_A / l_A_Ref < 95%. It is possible to already detect the onset of the ring collapse process in the case of a relatively small drop in the measured anode current.

[0125] In addition to that or in an alternative, it is also possible to remove other effects by calculation when the anode current is measured. For example, it is possible to remove by calculation a drift of the anode current on account of a normal ageing process of a cathode tip 351. Therefore, it may also be meaningful to trigger the material build-up process at the front facet 349 when a rate of change of the measured anode current l_A exceeds a predetermined threshold value l_Avar_Ref within a predefined change time interval. Changes on account of normal drift are usually much slower than changes in the anode current that are caused by the onset of a ring collapse process. Fig. 7 also plots a dotted region, in which a beam current uniformity deviation Allni was observed during the ring collapse process. The beam current uniformity deviation was observable approximately from day 6 through day 10 of the ring collapse process. An observed beam current uniformity deviation is thus also a good measure for the detection of a ring collapse process.

[0126] Fig. 9 schematically illustrates different types of beam current distributions. The values of individual beam currents are illustrated in Fig. 9 using different greyscale values or pattern fillings. In this context, a light pattern filling means a low beam current, while a dark pattern filling means a high beam current. In Fig. 9A, nine individual measurements of beam currents are shown by way of example. These have the same value; the beam current distribution in Fig. 9a is therefore uniform. In this case, the individual measured values may correspond to an individual particle beam 3 formed in the filter plate 304, but it is also possible that the individual measured values arise on account of a combination of multiple individual particle beams to form a measurement sector or measurement region. This depends on the type of measurement of the beam current uniformity that was performed and will be described in more detail below.

[0127] In Fig. 9b, the beam current distribution is no longer uniform in all directions: The beam current in Fig. 9b increases steadily from column C1 to column C5. By contrast, the beam current is constant within the respective column C1 to C5. The beam current distribution shown in Fig. 9b can be observed during a ring collapse process, while the distribution in Fig. 9a represents the initial state before a ring collapse process and after a completed ring collapse process.

[0128] Fig. 9c in turn shows an insufficiently homogeneous beam current distribution: In the example shown, a beam current increases radially from the inside to the outside. The beam current is lowest in the central region RO. The beam current is high in the ring-shaped region R1 , which directly adjoins the central region RO, and the beam current is even higher in the more outward ring R2 of the individual beam current measurements. This non-uniform beam current distribution can in turn be observed during the ring collapse process.

[0129] Other deviations not explicitly shown here are also possible in addition to the deviations from the beam current uniformity shown in Fig. 9.

[0130] Fig. 10 schematically shows a beam current measurement at the filter plate 304 of a multibeam generator 305. In the example shown, the filter plate 304 is illuminated telecentrically by the illuminating particle beam 311. In the process, the openings 304a in the filter plate 304 are traversed by some charged particles in the form of individual charged particle beams 3. However, the majority of the charged particles incident on the filter plate 304 are absorbed in a conductive absorber layer 341 of the filter plate 304. The conductive absorber layer 341 is connected to earth. A current meter 370 is now arranged between this earth and the conductive absorber layer 341 in order to measure the beam current incident on the filter plate 304 or on the absorber layer 341 and in order to transmit the corresponding measured value to the controller 10. The beam current incident on the filter plate 304 may represent a measure of the total beam current. According to a preferred embodiment variant of the invention, however, the beam current detected at the filter plate 304 may also be used for the detection of a beam current distribution and hence for the determination of the beam current uniformity, specifically during the normal operation of the multiple particle beam system:

[0131] Fig. 11 schematically illustrates beam current measurements at a sectored filter plate 304. Fig. 11 shows a top view of the filter plate 304, and so it is possible to identify the conductive absorber layer 341 with openings 304a arranged therein. The filter plate 304 or the conductive absorber layer 341 is divided into a multiplicity of electrically isolated sectors, these are the sectors B1 to B6 in the example shown. In the example shown, the sectors B1 to B6 are arranged centrically around the central aperture 304a in the filter plate 304. Now the beam currents that are incident on a sector or region B1 to B6 of the filter plate 304 in each case are measured. The measurement principle in this context is the same as already illustrated in Fig. 10. However, measured values 11 to I6 are now obtained, and these are ascertained by means of current meters 370.1 to 370.6 and transmitted to the controller 10. The respective detected beam currents 11 to I6 may be normalized, and the size of the regions B1 to B6 may be taken into account accordingly in this normalization. In that case, it is possible that identical and normalized measured values 11 to I6 reflect an existing beam current uniformity. However, if the measured values or the correspondingly normalized measured values 11 to I6 differ and deviate from one another, it is possible from this to draw a conclusion regarding a deviation from the required beam current uniformity. Measurement and evaluation of beam currents or beam current uniformities may be automated. According to the example shown, each of the sectors B1 to B6 surrounds at least one of the apertures 304a in the filter plate 304 and, apart from the central aperture in the filter plate arranged in the sector B1 , the sectors B2 to B6 each surround a group of apertures 304a in the filter plate 304. Furthermore, each of the apertures 304a in the filter plate 304 is assigned to exactly one of the sectors B1 to B6. In this embodiment of the invention, the sectors B1 to B6 as detection regions for a beam current thus are arranged very close to or directly at the apertures 304a or around the apertures 304a. Beam current fluctuations for individual beam currents or for certain groups of individual beam currents can therefore be reproduced very well by beam current measurements in the sectors B1 to B6. This makes it possible to draw conclusions about beam current uniformity.

[0132] Fig. 12 schematically illustrates beam current measurements at a further sectored filter plate 304. Unlike in Fig. 11 , the sectoring in Fig. 12 has not been carried out in shells, but a further six regions B2 to B7, the footprint of each of which is essentially triangular, are provided around the central opening in the filter plate with a central region B1. Such sectoring also allows the measurement of beam current uniformity deviations.

[0133] Naturally, it is also possible to sector the filter plate 304 in another way. In this context, one sector may also be provided for each aperture 304a. However, this is not mandatory and may even be detrimental: The absorber surface 341 of a filter plate 304 is significantly larger than the area of apertures 304a of all particle beams taken together. The larger the area in which incident charged particles are used for the current measurement, the better the signal-to-noise ratio of the measured beam current. Sectoring that is not too fine thus possibly allows for a better and more reliable detection of beam current uniformity deviations.

[0134] In practice, the sectoring of the filter plate 304 will be implemented such that no insulators are exposed on the surface of the filter plate 304 and able to be charged by incident charged particles. Instead, insulators will be hidden or buried inside the filter plate 304.

[0135] Fig. 13 schematically shows a further option allowing the measurement of a beam current. Fig. 13 illustrates sections of a beam tube 390, in which the charged particle beam 311 that was emitted by the cathode 350 is guided. The beam tube 390 opens into a vacuum chamber 392 in the region of the multi-beam generator 305. The opening 391 and the vacuum chamber 392 are also only indicated schematically in Fig. 13. The beam tube and the vacuum chamber or their walls are connected to earth in the example shown. A current detection aperture plate 380 is shown schematically in Fig. 13. In this case, the current detection aperture plate 380 is arranged between the beam-generating apparatus 300 and the filter plate 304 of the multibeam generator 305. Its back side 384 faces the filter plate 304 and comprises a conductive absorber layer 382 that is connected to earth. Now, a beam current of secondary electrons and / or backscattered electrons incident on the back side 384 of the current detection aperture plate 380 is measured: Said electrons arise when the illuminating particle beam 311 is incident on the filter plate 304. Thus, this beam current measurement is an indirect beam current measurement, which nevertheless allows very reliable conclusions to be drawn about the total beam current emitted by the cathode 350. In the example shown, a conductive protective layer 383, which is electrically insulated from the conductive absorber layer 382, is arranged on the top side 385 of the current detection aperture plate 380. As a result, charged particles that might be incident on the front side 385 of the current detection aperture plate 380 do not contribute to the falsification of the measured beam current. Instead, only secondary particles or secondary electrons and / or backscattered electrons are measured.

[0136] It is possible to sector the current detection aperture plate 380 in Fig. 13 in order to measure not only a total beam current but also a beam current uniformity in this way. Fig. 14 shows a corresponding example of a sectored current detection aperture plate 380. In Fig. 14, the current detection aperture plate 380 is shown in its view from below. Thus, it is possible to identify the conductive absorber layer 382, which is arranged on the underside 384 of the current detection aperture plate 380. The central opening 381 , through which the particle- optical axis Z extends centrally, is found in the middle. In Fig. 14, the conductive absorber layer 382 of the current detection aperture plate 380 is divided into a multiplicity of electrically insulated sectors B1 to B4, in this case into four electrically insulated sectors. However, a division into fewer or more sectors could also be undertaken. In this context, Fig. 14 only shows the basic principle. A respective beam current is measured for each sector B1 to B4. To this end, the conductive absorber layer 382 of each sector B1 to B4 is connected to earth, wherein a respective current meter 370.1 to 370.4 is provided between earth and the sector B1 to B4. The measured values from each current meter 370.1 to 370.4 may in turn be transmitted to the controller 10. In the example shown, the sectors B1 to B4 are the same size. The measured current values 11 to I4 are therefore also identical should beam current uniformity be present or in the event of an ideal beam current uniformity. Should said measured current values deviate from one another, ideal beam current uniformity no longer exists either. In this way, in turn, the onset of a ring collapse process may be ascertained by means of a beam current measurement or a beam current uniformity measurement, and a material build-up process may be triggered.

[0137] Multiple options for measuring the beam current and for measuring the beam current uniformity were described in the above-described exemplary embodiments of the invention. In this context, these measurement methods should not be construed as exhaustive. For example, the beam current and / or the beam current uniformity may also be measured level with a beam crossover of individual charged particle beams 3 with each other (cf. beam crossover 108 in Fig. 1). In addition, it is naturally possible to measure a total beam current or else individual beam currents in an object plane 101 (cf., yet again, Fig. 1 of the present patent application). It is possible to combine the different beam current measurements and / or beam current uniformity measurements in part or in full.

[0138] Moreover, it is possible to (optionally additionally) infer a modified beam current or a modified beam current uniformity on the basis of other measured quantities: According to a further example, the multiple particle beam system is a multi-beam particle microscope, which generates a multiplicity of individual particle-optical images that can be composed or are composed to form a multi-image. The beam current uniformity can be measured indirectly in the process, to be precise on the basis of brightness values of the individual images generated.

[0139] The invention discloses a method for operating a multiple particle beam system, in which the onset of a ring collapse process at the front facet of the cathode tip of the beam-generating apparatus of the multiple particle beam system can be detected on the basis of beam current measurements and / or beam current uniformity measurements. This detection triggers a targeted material build-up process at the front facet of the cathode tip. Specific process steps for the material build-up process are specified. Furthermore, various options for beam current measurement and / or beam current uniformity measurement are described. In particular, these measurements may be performed during the normal operation of the multiple particle beam system.

[0140] List of reference signs

[0141] 1 Multiple particle beam system, multi-beam particle microscope

[0142] 3 Primary particle beams, first individual particle beams

[0143] 5 Beam spots, incidence locations

[0144] 7 Object, sample, wafer

[0145] 9 Secondary particle beams, second individual particle beams

[0146] 10 Computer system, controller

[0147] 15 Sample surface, wafer surface

[0148] 25 Image point of a second individual particle beam

[0149] 101 Object plane

[0150] 102 Objective lens

[0151] 103 Field lens

[0152] 105 Axis

[0153] 108 Beam crossover

[0154] 200 Detector system

[0155] 205 Projection lens system

[0156] 206 Projection lens 207 Multi-particle detector

[0157] 208 Projection lens

[0158] 210 Projection lens

[0159] 212 Beam crossover

[0160] 214 Aperture filter, contrast stop

[0161] 222 Collective anti-deflection system

[0162] 300 Beam-generating apparatus

[0163] 301 Particle source, beam-generating system

[0164] 303 Collimation lens system

[0165] 304 Multi-aperture array, filter plate

[0166] 304 Aperture in the filter plate

[0167] 305 Micro-optics unit, multi-aperture arrangement, multi-beam particle generator, multibeam generator

[0168] 306 Multi-aperture plate, multi-aperture array

[0169] 307 Field lens, aperture plate

[0170] 308 Field lens

[0171] 309 Particle beam

[0172] 311 Illuminating particle beam

[0173] 321 Intermediate image plane

[0174] 323 Beam foci

[0175] 341 Conductive absorber layer

[0176] 350 Cathode

[0177] 351 Cathode tip

[0178] 352 Emitted particle beam

[0179] 353 Extractor electrode

[0180] 354 Anode stop

[0181] 355 Opening in the anode stop

[0182] 356 Suppressor electrode

[0183] 357 Front region of the extractor electrode

[0184] 358 Opening in the extractor electrode

[0185] 359 Heating apparatus, heating wire

[0186] 360 Opening in the suppressor electrode

[0187] 361 Reservoir with work-function-lowering agent

[0188] 362 Front region of the cathode

[0189] 363 Etched shaft

[0190] 364 Cathode wire

[0191] 365 Front facet 366 Apex

[0192] 367 Shaft

[0193] 370 Current meter

[0194] 380 Current detection aperture plate

[0195] 381 Opening

[0196] 382 Conductive absorber layer

[0197] 383 Conductive protective layer

[0198] 384 Back side

[0199] 385 Front side

[0200] 390 Beam tube

[0201] 391 Opening

[0202] 392 Vacuum chamber

[0203] 400 Beam splitter, magnet arrangement

[0204] 500 Scan deflector

[0205] 600 Displacement stage or positioning device x Direction y Direction z Direction

[0206] Z Particle-optical axis

[0207] B Sector

[0208] R0... R2 Ring, shell

[0209] C1...C6 Column

Claims

38Claims1. Method for operating a multiple particle beam system, wherein the multiple particle beam system comprises a beam-generating apparatus for generating a first charged particle beam, and wherein the beam-generating apparatus comprises a cathode, an extractor electrode and an anode stop, and a heating apparatus for heating the cathode, and wherein the cathode comprises a reservoir with a work-function-lowering agent and a cathode tip with a front facet, wherein the multiple particle beam system furthermore comprises a multi-beam generator having a filter plate, wherein the filter plate has a multiplicity of apertures and wherein the filter plate is substantially traversed by the first charged particle beam, a multiplicity of individual charged particle beams being formed in the process, and wherein the method includes the following steps:(f) defining a reference beam current and / or a reference beam current uniformity;(g) measuring a beam current and / or measuring a beam current uniformity between the multiplicity of individual charged particle beams;(h) comparing the measured beam current with the reference beam current and, based thereon, ascertaining a beam current deviation, and / or comparing the measured beam current uniformity with the reference beam current uniformity and, based thereon, ascertaining a beam current uniformity deviation;(i) detecting the onset of a ring collapse process at the front facet of the cathode tip on the basis of the beam current deviation and / or on the basis of the beam current uniformity deviation; and(j) triggering a material build-up process at the front facet of the cathode tip on the basis of the detection of the onset of the ring collapse process.

2. Method according to Claim 1, wherein the beam current uniformity between the multiplicity of individual charged particle beams is measured, and the material build-up process is triggered when the beam current uniformity deviation exceeds a predefined limit; and / or wherein the beam current is measured, and the material build-up process is triggered when the beam current deviation exceeds a predefined limit.

3. Method according to either of the preceding claims, wherein the material build-up process comprises the following steps:39(f) reducing an extractor voltage applied between the cathode tip and the extractor electrode, from an initial value UEO to a reduced value UE1 ;(g) temporarily increasing the temperature of the cathode, in particular by increasing a heating current at the heating apparatus, from an initial temperature TO to a higher temperature T 1 ;(h) increasing the extractor voltage to an elevated value UE2, which is higher than the initial value UEO, during a time interval t_Mat;(i) reducing the extractor voltage to the initial value UEO.

4. Method according to the preceding claim, wherein the reduction of the extractor voltage according to step (i) is performed in such a way that the beam current I does not exceed an initial value IO, which is present at the beginning of step (f).

5. Method according to either of Claims 3 and 4, wherein the following relation applies to the extractor voltage UE1 : UE1 / UE0 < 60%, in particular UE1 / UE0 < 55% or UE1 / UE0 < 50%.

6. Method according to any of Claims 3 to 5, wherein the following relation applies to a temperature difference AT= T1 - TO: 250°C < AT < 400°C, in particular 300°C < AT < 350°C; and / or wherein the following relation applies to the temperature TO: TO > 1700°C, in particular TO > 1750°C or TO > 1800°C.

7. Method according to any of Claims 3 to 6, wherein the temporary increase in the temperature of the cathode is implemented during a time interval t_temp, and wherein the following relation applies to the time interval t_temp: 1 min < t_temp < 10 min, in particular 1 min < t_temp < 8 min or 1 min < t_temp < 5 min.

8. Method according to any of Claims 3 to 7, wherein the following relation applies to the extractor voltage UE2: UE2 / UE0 > 150%, in particular UE2 / UE0 > 200% or UE2 / UE0 > 250%; and / or wherein the following relation applies to the extractor voltage UE2: UE2 > 5000 V, in particular UE2 > 6000 V or UE2 > 6500 V.

9. Method according to any of Claims 3 to 8,wherein the following relation applies to the time interval t_Mat: t_Mat < 60 min, in particular t_Mat < 45 min or t_Mat < 30 min.

10. Method according to any of the preceding claims, wherein a or the beam current I is measured during the material build-up process.

11. Method according to any of the preceding claims, wherein the beam current uniformity between the multiplicity of individual charged particle beams is measured, and the material build-up process is triggered when the following relation applies to the beam current uniformity deviation AUni: Allni > 10%, in particular Allni > 5% or Allni > 1%.

12. Method according to any of the preceding claims, wherein the beam current and / or beam current uniformity is measured during the ongoing normal operation of the multiple particle beam system.

13. Method according to any of the preceding claims, wherein the beam current uniformity is measured, and wherein the beam current uniformity is measured at the filter plate of the multi-beam generator.

14. Method according to the preceding claim, wherein at least one region, which is arranged between adjacent apertures, serves as the detection region for measuring the beam current uniformity at the filter plate of the multibeam generator.

15. Method according to either of the two preceding claims, wherein the filter plate comprises a conductive absorber layer, and wherein the filter plate is divided into a multiplicity of electrically insulated sectors, each of which is connected to earth; and wherein the measurement of the beam current uniformity comprises the measurement of beam currents in each case incident on a sector of the filter plate.

16. Method according to the preceding claim, wherein each of the sectors surrounds at least one of the apertures in the filter plate, in particular wherein a plurality of sectors surround a group of apertures in the filter plate; and wherein each of the apertures in the filter plate is assigned to exactly one of the sectors.

17. Method according to any of the preceding claims, wherein a current detection aperture plate is arranged between the beam-generating apparatus and the filter plate, the back side of said current detection aperture plate facing the filter plate and the back side of said current detection aperture plate comprising a conductive absorber layer that is connected to earth, and wherein the beam current and / or a beam current uniformity are measured, and wherein at least one beam current of secondary electrons and / or backscattered electrons incident on the back side of the current detection aperture plate is measured.

18. Method according to the preceding claim, wherein the absorber layer of the current detection aperture plate is divided into a multiplicity of electrically insulated sectors, and wherein one beam current is measured per sector.

19. Method according to any of the preceding claims, wherein the beam current is measured, and wherein the measurement of the beam current comprises the measurement of an anode current that is generated on the basis of charged particles, in particular electrons, incident on the anode stop.

20. Method according to the preceding claim, wherein the material build-up process is triggered when the measured anode current l_A drops below a predetermined threshold value l_A_ref.

21. Method according to the preceding claim, wherein the following relation applies to the trigger of the material build-up process: l_A / l_A_ref < 85%, in particular l_A / l_A_ref < 90% or l_A / l_A_ref < 95%.

22. Method according to either of Claims 20 and 21 , wherein the material build-up process is triggered when a rate of change of the measured anode current l_A exceeds a predetermined threshold value l_Avar_ref within a predefined change time interval.

23. Method according to any of the preceding claims, wherein the beam current and / or beam current uniformity is measured in an object plane.

24. Method according to any of the preceding claims, wherein the beam current is measured, and wherein the beam current is measured level with a beam crossover of the individual charged particle beams with one another.

25. Method according to any of the preceding claims, wherein the multiple particle beam system is a multi-beam particle microscope, which generates a multiplicity of individual particle-optical images that can be composed or are composed to form a multi-image, wherein the beam current uniformity is measured indirectly on the basis of brightness values of the individual images generated.

26. Computer program product comprising a program code for carrying out the method according to any of the preceding claims.

27. Multiple particle beam system configured to carry out the method according to any of Claims 1 to 25.

28. Multi-beam particle microscope, comprising the following: a beam-generating apparatus configured to generate a first charged particle beam and comprising a cathode, an extractor electrode and an anode stop, and a heating apparatus for heating the cathode, wherein the cathode comprises a reservoir with a work-function-lowering agent, and wherein the cathode comprises a cathode tip with a front facet; a multi-beam generator having a multi-aperture array, wherein the multi-aperture array comprises a filter plate having a multiplicity of apertures that is traversed by the first charged particle beam, a multiplicity of first individual charged particle beams being formed in the process, and wherein the multi-beam generator is configured to generate a first field of a multiplicity of individual charged particle beams from the first charged particle beam; and a beam-current measuring means configured to measure a beam current and / or beam current uniformity; a first particle optics unit having a first particle-optical beam path and configured to direct the generated first individual particle beams at a sample such that the first particle beams impinge on the sample at incidence locations that form a second field; a detection system; a second particle optics unit having a second particle-optical beam path and configured to image second individual particle beams, which emanate from the incidence locations in the second field, onto the detection system;a particle-optical objective lens traversed by both the first and the second individual particle beams; a beam splitter arranged in the first particle-optical beam path between the multi-beam generator and the objective lens and arranged in the second particle-optical beam path between the objective lens and the detection system; and a controller configured to control the beam-generating apparatus, the particle-optical objective lens, the first particle optics unit, the second particle optics unit and the detection system, and wherein the controller is configured to detect the onset of a ring collapse process at the front facet of the cathode tip of the beam-generating apparatus on the basis of the beam current measurement and / or the beam current uniformity measurement and to control the beam-generating apparatus for a material build-up process at the front facet of the cathode tip on the basis of the detection.

29. Multi-beam particle microscope according to Claim 28, wherein the beam-current measuring means is configured to measure a beam current uniformity, and wherein the beam-current measuring means comprises the filter plate, wherein the filter plate comprises a conductive absorber layer, and wherein the filter plate is divided into a multiplicity of electrically insulated sectors, each of which is connected to earth, and wherein the beam currents incident on the respective sectors are determined during the operation of the multi-beam particle microscope.

30. Multi-beam particle microscope according to the preceding claim, wherein each of the sectors surrounds at least one of the apertures in the filter plate, in particular wherein a plurality of sectors surround a group of apertures in the filter plate; and wherein each of the apertures in the filter plate is assigned to exactly one of the sectors.

31. Multi-beam particle microscope according to any of Claims 28 to 30, wherein the beam-current measuring means comprises a current detection aperture plate arranged between the beam-generating apparatus and the filter plate, the back side of said current detection aperture plate facing the filter plate and the back side of said current detection aperture plate comprising a conductive absorber layer that is connected to earth, and wherein a beam current of secondary electrons and / or backscattered electrons incident on the back side of the current detection aperture plate is measured during operation.

32. Multi-beam particle microscope according to the preceding claim, wherein the absorber layer of the current detection aperture plate is divided into a multiplicity of electrically insulated sectors, and wherein the beam-current measuring means is configured to measure one beam current per sector, respectively.

33. Multi-beam particle microscope according to any of Claims 31 to 32, wherein a conductive protective layer is arranged on a front side of the current detection aperture plate, connected to earth and electrically insulated from the conductive absorber layer.

34. Multi-beam particle microscope according to any of Claims 28 to 33, wherein the beam-current measuring means comprises the anode stop and is configured to measure the beam current of charged particles, in particular electrons, incident on the anode stop.

35. Multiple particle beam system, comprising the following: a beam-generating apparatus configured to generate a first charged particle beam; a multi-beam generator having a multi-aperture array, wherein the multi-aperture array comprises a filter plate having a multiplicity of apertures that is traversed by the first charged particle beam, a multiplicity of first individual charged particle beams being formed in the process, and wherein the multi-beam generator is configured to generate a first field of a multiplicity of individual charged particle beams from the first charged particle beam; and a current detection aperture plate having a singular opening arranged between the beam-generating apparatus and the filter plate and traversed substantially without contact by the first charged particle beam, wherein a back side of the current detection aperture plate faces the filter plate and has a conductive absorber layer that is connected to earth, and wherein a current meter is arranged between the conductive absorber layer and earth such that a beam current of secondary electrons and / or backscattered electrons incident on the conductive absorber layer is measurable during the operation of the multiple particle beam system.

36. Multiple particle beam system according to the preceding claim, wherein a conductive protective layer is arranged on a front side of the current detection aperture plate, connected to earth and electrically insulated from the conductive absorber layer.

37. Multiple particle beam system according to either of Claims 35 and 36, wherein the absorber layer of the current detection aperture plate is divided into a multiplicity of electrically insulated sectors, each of which is connected to earth; wherein one beam-current measuring means is arranged between the absorber layer of each sector and earth, respectively, and configured to measure a beam current per sector of secondary electrons and / or backscattered electrons incident on the conductive absorber layer.

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