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

Abstract

Method for operating a multiple particle beam system (1), wherein the multiple particle beam system (1) comprises a beam generating device (300) for generating a first charged particle beam (309), and wherein the beam generating device (300) comprises a cathode (350), an extractor electrode (353) and an anode aperture (354) as well as a heating device (359) for heating the cathode (350), and wherein the cathode (350) comprises a reservoir with an output work reduction agent (361) and a cathode tip (351) with a front facet (365), wherein the multi-particle beam system (1) further comprises a multi-beam generator (305) with a filter plate (304), wherein the filter plate (304) has a plurality of apertures (304) and wherein the filter plate (304) is substantially penetrated by the first charged particle beam (309) to form a plurality of charged single-particle beams (3), and the procedure comprises the following steps: (a) Determining (S2) a reference beam current and / or a reference beam current uniformity; (b) Measuring (S3) a beam current and / or measuring a beam current uniformity between the plurality of charged single-particle beams (3); (c) Comparing (S4) the measured beam current with the reference beam current and, based on this, determining a beam current deviation and / or comparing the measured beam current uniformity with the reference beam current uniformity and, based on this, determining a beam current uniformity deviation; (d) Detecting (S5) a ring collapse process starting at the front facet (365) of the cathode tip (351) based on the beam current deviation and / or based on the beam current uniformity deviation; and (e) Triggering (S6) a material build-up process at the front facet (365) of the cathode tip (351) based on the detection of the initiating ring collapse process.

Description

Field of invention

[0001] The invention relates generally to multi-particle beam systems that operate with a multitude of charged single-particle beams. Specifically, the invention relates to a method for operating a multi-particle beam system with detection of a ring collapse process and triggering of a material build-up process, an associated computer program, a multi-particle beam system, and a multi-beam particle microscope. State of the art

[0002] With the continuous development of increasingly smaller and more complex microstructures, such as semiconductor devices, there is a need for the further development and optimization of planar fabrication techniques and inspection systems for the production and inspection of these small microstructures. The development and fabrication of semiconductor devices, for example, requires verification of test wafer designs, and planar fabrication techniques necessitate process optimization for reliable, high-throughput manufacturing. Furthermore, the analysis of semiconductor wafers for reverse engineering and the customized configuration of semiconductor devices is increasingly required. Therefore, there is a need for inspection tools that can be used with high throughput to examine microstructures on wafers with high accuracy.

[0003] Typical silicon wafers used in the production of semiconductor devices have diameters of up to 300 mm. Each wafer is divided into 30 to 60 repeating sections ("dies") with a size of up to 800 mm. 2A semiconductor device comprises multiple semiconductor structures fabricated in layers on a wafer surface using planar integration techniques. Due to the manufacturing processes, semiconductor wafers typically have a flat surface. The feature size of the integrated semiconductor structures ranges from a few micrometers to critical dimensions (CDs) of 5 nm, with even smaller feature sizes expected in the near future; future feature sizes or critical dimensions (CDs) below 3 nm, for example 2 nm, or even below 1 nm, are anticipated. At these small feature sizes, defects of critical dimension size must be identified quickly across a very large area.For several applications, the specification requirement for the accuracy of a measurement provided by an inspection instrument is even higher, for example by a factor of two or an order of magnitude. For example, the width of a semiconductor feature must be measured with sub-1 nm accuracy, such as 0.3 nm or even less, and the relative position of semiconductor structures must be determined with a sub-1 nm superposition accuracy, such as 0.3 nm or even less.

[0004] A more recent development in the field of charged particle microscopes (CPM) is the MSEM, a multi-beam scanning electron microscope. A multi-beam scanning electron microscope is disclosed, for example, in US 7,244,949 B2 and US 2019 / 0355544 A1. In a multi-beam electron microscope, or MSEM, a sample is simultaneously irradiated with a multitude of single-electron beams arranged in a field or grid. For example, 4 to 10,000 single-electron beams can be provided as primary radiation, with each single-electron beam separated from an adjacent single-electron beam by a distance of 1 to 200 micrometers. For example, an MSEM has approximately 100 separate single-electron beams (“beamlets”), which are arranged, for example, in a hexagonal grid, with the single-electron beams separated by a distance of approximately 10 µm.A multitude of charged single-particle beams (primary beams) are individually focused onto the surface of the sample under investigation by a common large-field optics system, including a common objective lens. The sample can be, for example, a semiconductor wafer mounted on a wafer holder, which is itself mounted on a movable stage. During illumination of the wafer surface with the charged primary single-particle beams, interaction products, such as secondary electrons or backscattered electrons, are emitted from the wafer surface. Their respective starting points correspond to the locations on the sample onto which the multitude of primary single-particle beams are focused. The quantity and energy of the interaction products depend, among other things, on the material composition and the topography of the wafer surface.The interaction products form several secondary single-particle beams (secondary beams) that are collected by the common objective lens and directed by a projection imaging system of the multi-beam inspection system onto a detector located in a detection plane. The detector comprises several detection areas, each containing several detection pixels, and the detector records an intensity distribution for each of the secondary single-particle beams. This results in an image field of, for example, 100 µm × 100 µm.

[0005] The prior art multi-beam electron microscope comprises a series of electrostatic and magnetic elements. At least some of the electrostatic and magnetic elements are adjustable to adapt the focus position and stigmatization of the multiple charged single-particle beams. The prior art multi-beam charged particle system also includes at least one intersection plane of the primary or secondary charged single-particle beams. Furthermore, the prior art system includes detection systems to facilitate adjustment. The prior art multi-beam particle microscope includes at least one deflection scanner for collectively scanning an area of ​​the sample surface using the multiple primary single-particle beams to obtain an image field of the sample surface.

[0006] As the demands on image quality increase, so do the demands on the multi-beam particle microscopes used for imaging. Stable operating parameters are crucial for high-quality images. One of these is the beam flux of the individual particle beams used to scan a sample surface.

[0007] For a uniform beam flux of the individual particle beams, the emission characteristic of the particle source is important, more precisely, a uniform emission characteristic across the entire emission angle used. When using larger emission angles, the emission characteristic of particle sources, e.g., thermal field emission sources (TFEs), is no longer consistently uniform. Consequently, the illuminance at a first multi-aperture plate (so-called filter plate) in a corresponding particle beam system is also no longer consistently uniform, and greater variations in the current densities occur in different individual beams.However, for multi-particle inspection systems, it is a system requirement that there be only a small variation in current intensities between the individual beams, typically less than a few percent or even less than one percent, so that all individual image fields of the multi-image field are scanned with an equivalent number of particles or electrons. This is a prerequisite, for example, for obtaining individual images with approximately the same brightness. The achievable resolution of the individual images also depends on the individual beam current.

[0008] There are ways to individually adjust the beam current for single-particle beams. One such method is disclosed in DE 10 2018 007 652 A1, the disclosure of which is incorporated in its entirety by reference into this patent application.

[0009] A particle source slowly changes its emission characteristics over time; it can exhibit drift behavior. It is known to measure and correct this drift. A particle stream originally emitted by the source can also change its direction. It is known to correct or compensate for this slow change in direction using particle-optical components.

[0010] Furthermore, a particle source or tip can age, for example, losing brightness. The brightness or luminance of the source directly correlates with the brightness of the images. If the source loses brightness, the image brightness also decreases. One approach to addressing this is to increase the gain at the detection system to compensate for the reduced brightness. However, this changes the signal-to-noise ratio (SNR) at the detector, and in the worst case, it worsens, reducing the achievable contrast in the images. Therefore, this approach is only partially suitable.

[0011] It is therefore common practice to adjust the radiation-generating system or device itself, whereby, according to the state of the art, a voltage applied to an extractor electrode is changed. However, after such a change in the extractor current, it can take several days until the newly adjusted radiation-generating system again exhibits a sufficiently constant radiation characteristic; the peaks must first "burn in" again.

[0012] US patent 2020 / 0312619 A1 discloses a correction of beam parameters in a multi-beam particle microscope based on beam current measurements at a multi-aperture array. The beam-generating system is controlled by adjusting the extractor voltage or the accelerating voltage. Beam drift can also be prevented. However, these are always slow corrections for any drift that occurs.

[0013] From WO 2023 / 001402 A1 and DE 10 2021 118 561 A1, a high-frequency control of a beam current is known – in addition to the known drift compensation. This can be achieved using a special beam generation device that employs an electrostatic control lens between the anode and extractor for fast control purposes.

[0014] The disclosure of WO 2023 / 001402 A1 is fully incorporated into the present patent application by reference.

[0015] From WO 2023 / 001401 A1, various types of beam current measurements are known that can be used for beam current control or for aligning the charged particle stream. Among other things, a sectored first multi-aperture plate or filter plate of a multi-beam particle microscope, provided with a conductive absorber layer and grounded, is disclosed. For each sector, a current of charged particles impacting the filter plate or particles being deflected from the filter plate is measured. This is done particularly in an area surrounding all apertures of the filter plate. However, the sectoring can also be performed within or between the apertures. Also disclosed is a beam current measurement on a pre-aperture plate with a single opening, which is arranged in the particle-optical beam path close to or directly in front of the filter plate.Charged particles emanating directly from the particle source are also detected on the pre-aperture. For this purpose, the pre-aperture plate is provided with an absorber layer and grounded; the current of the discharged charges is again determined as the measured quantity. As a further variant, the detection of X-rays for determining a beam current is disclosed. In this case, the X-rays are generated in subsequent processes after the charged particle beam strikes the filter plate. A measurement of light in the NIR range as a result of the conversion of X-rays can also be used for beam current measurement purposes. The disclosure of WO 2023 / 001401 A1 is incorporated in its entirety into the present patent application by reference.

[0016] A change in the beam flux and the beam migration behavior of the particle beam emitted from a particle source or tip can therefore be measured and controlled according to the state of the art.

[0017] However, there are other processes that influence the beam flux and are currently neither measured nor controlled: These include changes in the beam flux due to the so-called ring-collapsing process. Ring-collapsing refers to a characteristic reshaping of the front facet of a cathode tip. In this process, some monolayers of the front facet slide laterally or in a ring-like fashion along the apex surface of the cathode tip. This reshaping of the front facet results in changes to both the total beam flux and the emission characteristics of the particle source, sometimes significantly and over extended periods, such as several hours or days. Inspection processes, and especially automated inspection processes, are therefore hampered or even impossible during this time.

[0018] The frequency of a ring collapse process depends, among other things, on the cathode temperature, the diffusion rate at the cathode tip, and the local field at the cathode apex. In practice, the frequency of a ring collapse process therefore varies considerably, ranging from daily to weekly to monthly, or only once during the entire lifetime of a cathode tip. Ring collapse processes occur relatively frequently during inspections with relatively low beam currents and relatively low local electric fields, such as in metrological investigations using a multi-beam particle microscope.

[0019] As a general rule, a ring collapse process is a spontaneous process. During a ring collapse process, inspection processes are not feasible. This, in turn, leads to an unpredictable service life for a multi-particle beam system.

[0020] US Patent 5,616,926 A discloses a single-beam system with a Schottky emission cathode and a method for stabilizing this cathode by means of a material build-up process at the cathode. According to US Patent 5,616,926 A, the material build-up process is triggered by measuring a sample current in the sample plane or a change in beam current density as a function of an emission angle from the cathode. To measure the beam current density as a function of the emission angle, US Patent 5,616,926 A measures different currents at different beam deflections of the single beam. These measurements therefore require interrupting the normal operation of the single-beam system and incorporating special deflection elements into the single-beam system.

[0021] Furthermore, regarding the emission angle of a particle source / cathode, different conditions apply to particle beam systems operating with a single beam than to multi-particle beam systems: The beam current limiting anode aperture is significantly larger in a multi-particle beam system than in a single-beam system, often by more than a factor of ten. This is because, in a multi-particle beam system, as much beam current as possible must first be generated, of which only a small fraction (e.g., 5% or less) can be used later – after the formation of the multitude of individual particle beams. Therefore, in a multi-particle beam system, a considerably larger angular range of the source cone is initially used for further beam shaping or beam generation.Typically, the anode aperture of a single-beam system cuts out an angular range of only about 2.6 mrad or less from the source cone, whereas in a multi-particle beam system it is often more than 30 mrad. This results in significantly more stringent requirements for the beam current and beam current uniformity in a multi-particle beam system; therefore, the conditions for a single-beam system cannot be directly transferred to a multi-particle beam system.

[0022] MS Bronsgeest et al., “Collapsing rings” on Schottky electron emitters, Ultramicroscopy 110 (210), pages 1243–1254, deals extensively with the ring collapse process in Schottky emitters for single-beam systems. The geometries of Schottky emitters are scientifically investigated using SEM images before, during, and after a ring collapse process. Conclusions are drawn regarding how such a ring collapse process can be detected early and, if possible, prevented. It is concluded that ring collapse can be prevented by a sufficiently high extractor voltage. To detect a ring collapse process early, the measurement of the front facet current and / or the measurement of a field gain factor, for example, using Schottky diagrams, is proposed. Both measurement methods require an interruption of the inspection process.Furthermore, beam current measurements are generally considered unsuitable for the early detection of ring collapse. Similarly, beam current instability measurements are considered unsuitable for the early detection of ring collapse, as beam current instability occurs too late. Description of the invention

[0023] The object of the present invention is therefore to reduce the service life of a multi-particle beam system by means of a ring collapse process. This object is achieved by the independent claims. Advantageous embodiments of the invention are described in the dependent claims.

[0024] The reduction in service life is made possible, firstly, by the early detection of an incipient ring collapse process using 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 material rebuildup on the front facet of the cathode tip is significantly shorter than the duration of a ring collapse process.

[0025] According to a first aspect of the invention, it relates to a method for operating a multi-particle beam system described below. The multi-particle beam system comprises a beam generating device for generating a first charged particle beam. The beam generating device includes a cathode, an extractor electrode, an anode aperture, and a heating device for heating the cathode. The cathode comprises a reservoir containing an output work reduction agent and a cathode tip with a front facet. The multi-particle beam system further comprises a multi-beam generator with a filter plate, wherein the filter plate has a plurality of apertures and wherein the filter plate is substantially penetrated by the first charged particle beam, forming a plurality of charged single-particle beams.The procedure for operating the multi-particle beam system then comprises the following steps: (a) Establishing a reference beam current and / or reference beam current uniformity; (b) Measuring a beam current and / or measuring a beam current uniformity between the multitude of charged single-particle beams; (c) Comparing the measured beam current with the reference beam current and, based on this, determining a beam current deviation and / or comparing the measured beam current uniformity with the reference beam current uniformity and, based on this, determining a beam current uniformity deviation; (d) Detecting a ring collapse process initiating at the front facet of the cathode tip based on beam current deviation and / or based on beam current uniformity deviation; and (e) Triggering a material build-up process at the front facet of the cathode tip based on the detection of the initiating ring collapse process.

[0026] The beam-generating device preferably comprises a thermal field emitter, in particular a Schottky emitter. The cathode or cathode wire can, 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-reducing coating of compounds such as oxides, nitrides, and carbon with zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium, or lanthanum. The work-function-reducing 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. The work function is most strongly reduced on the front facet, which crystallographically is usually a {100} face in tungsten emitters. Therefore, the emitted electron beam mainly originates from there.This portion of the emitted electron beam is technically usable.

[0027] According to the invention, a reference beam current and / or reference beam current uniformity is defined. The reference beam current is preferably a total beam current that can be determined at a predefined position using a predefined measurement method. The reference beam current can comprise a single current value, or it can comprise a current interval with an upper and a lower limit. The reference beam current describes the beam current that would be expected without a ring collapse process occurring. The reference beam current can be a constant value over time or a constant interval of values ​​over time, but it can also change over time, and known drift processes affecting the beam current can be included in the definition of the reference beam current.

[0028] Reference beam current uniformity is a measure of the uniformity of individual beam currents, and thus of the uniformity of the beam currents of charged individual particle beams. For multi-particle beam systems, the greatest possible uniformity of individual particle beam currents is a system requirement. The difference between a maximum and a minimum beam current value must not exceed a specific maximum value. The entire range, i.e., the difference, can, for example, be a maximum of 10 picoamperes. Uniformity can also be expressed as a percentage. For example, uniformity can be defined as follows: Uniformity [%] = (Maximum value - Minimum value) / Average value x 0.5 x 100. Other definitions are also possible and useful. The specific definition of beam current uniformity should preferably be adapted to the specific measurement of beam current uniformity.Therefore, the term beam current uniformity, or reference beam current uniformity, can refer directly to the uniformity of the charged single-particle beams generated at the filter plate. However, it is also possible to measure or define other uniformity quantities that merely reflect the uniformity between the charged single-particle beams without exactly matching them. Several examples of this will be given later. Generally speaking, the reference beam current uniformity defines a target state for beam current uniformity, provided no ring collapse process is present.

[0029] In a further process step, the beam current and / or the beam current uniformity among the multitude of charged individual particle beams is measured. Preferably, the total beam current is measured. This measurement can be performed directly or indirectly; that is, the beam current can either be measured directly or it can be deduced from other measured quantities. Several examples of this are described in detail below. Alternatively or additionally, the beam current uniformity can also be measured directly or indirectly. Several examples of this are also described in detail below.

[0030] In a further step, the measured beam current is compared with the reference beam current, and based on this comparison, a beam current deviation is determined. The beam current deviation can be expressed directly as the difference between the measured beam current and the reference beam current, or alternatively, it can be determined whether the measured beam current is still within a reference beam current interval. In any case, the determined beam current deviation is the parameter that allows for the detection of a ring collapse process beginning at the front facet of the cathode tip.

[0031] Additionally or alternatively, the measured beam current uniformity is compared with the reference beam current uniformity, and based on this, a beam current uniformity deviation is determined. Here, too, a specific value for a beam current uniformity deviation can be determined, but it is also possible to simply ascertain whether the beam current uniformity deviation lies within or outside a permissible reference interval. Based on the beam current uniformity deviation, a ring collapse process beginning at the front facet of the cathode tip can then be detected.

[0032] If a corresponding beam current deviation and / or beam current uniformity deviation is detected, a material build-up process is triggered at the front facet of the cathode tip based on the detection of the incipient ring collapse process, according to the invention. The material build-up process is thus initiated. It is preferred that the normal operation of the multi-particle beam system is interrupted for the material build-up process. This can occur immediately or with a slight time delay. For example, it is conceivable to complete the acquisition of a multi-image field before the normal operation of the multi-particle beam system is interrupted for the material build-up process.

[0033] To reliably detect the onset of the ring collapse process, the beam current and / or beam current uniformity can be measured continuously or at least intermittently at certain intervals. In particular, it is also possible to measure the beam current and / or beam current uniformity during the normal operation of a multi-particle beam system; that is, normal operation does not need to be interrupted for such a measurement, and the measurement of the beam current and / or beam current uniformity can, for example, be performed simultaneously with the illumination or scanning of a sample.

[0034] According to a preferred embodiment of the invention, the beam current uniformity among the plurality of charged single-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. Additionally or alternatively, 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 determined beam current deviation exceeds a predefined limit. This limit can be an upper or a lower limit for the beam current.

[0035] According to a preferred embodiment of the invention, the method further comprises the following steps for material build-up at the front facet of the cathode tip: (f) Reducing an extractor voltage applied between the cathode tip and the extractor electrode from an initial value UE0 to a reduced value UE1. This process step is a preparatory step and essentially serves to protect the multi-beam generator or the micro-optics in the subsequent process steps. For example, the following relationship may apply to the extractor voltage: UE1 / UE0 ≤ 60%, preferably UE1 / UE0 ≤ 55% or UE1 / UE0 ≤ 50%. The extractor voltage is thus significantly reduced in this process step. Consequently, a beam current in the sample plane or object plane of the multi-particle beam system is also significantly reduced, for example, to less than 30% of its initial value. A reduction (in magnitude) of the extractor voltage therefore reduces the total beam current. (g) Temporarily increasing the temperature of the cathode, in particular by increasing the heating current at the heating device, from an initial temperature T0 to a higher temperature T1. This increase in the heating current causes a work-work reducing agent located on the front facet to evaporate, which in turn increases the work function for electrons exiting the front facet. Phenomenologically, this means that the beam current initially increases due to the temperature increase, as a broadened Fermi distribution allows more hot electrons to leave the material. However, as the work-work reducing agent evaporates, the beam current decreases and typically drops to almost zero within a few minutes. Subsequently, the temperature of the cathode is reduced again, in particular to the initial temperature T0. Therefore, the temperature of the cathode is only temporarily increased in this process step.The reduction of the cathode temperature to the initial temperature T0 can occur in one step, in several steps, or even continuously. Even after reaching the initial temperature T0 again, there is still no beam current in the sample plane, as the work function at the cathode is too high (without a work function reduction agent on the front facet).

[0036] The temporary increase in cathode temperature can occur during a time interval t_temp. Preferably, the following relationship 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.

[0037] In process step (g), according to a preferred embodiment of the invention, the following relationship can apply for a temperature difference ΔT = T1 - T0: 250°C ≤ ΔT ≤ 400°C, preferably 300°C ≤ ΔT ≤ 350°C. Additionally or alternatively, the following relationship can apply for the temperature T0: T0 ≥ 1700°C, preferably T0 ≥ 1750°C, or most preferably T0 ≥ 1800°C.

[0038] (h) Increasing the extractor voltage to an elevated value UE2, which is higher than the initial value UE0, for a time interval t_Mat. This results in a higher local electric field at the cathode tip or front facet. This allows a material build-up process to begin at the front facet: Due to the temporary absence of the work-reducing agent, emitter material, for example tungsten, can migrate more easily to the apex and stabilize the initial emitter shape. In parallel, any work-reducing agent present in the cathode reservoir migrates back to the front facet. According to a preferred embodiment of the invention, the following relationship applies to the time interval t_Mat: t_Mat ≤ 60 min, preferably t_Mat ≤ 45 min, and most preferably t_Mat ≤ 30 min.

[0039] According to a preferred embodiment of the invention, the following relationship can apply to the value of the extractor voltage UE2: UE2 / UE0 ≥ 150%, preferably UE2 / UE0 ≥ 200%, or most preferably UE2 / UE0 ≥ 250%. Additionally or alternatively, the following relationship can apply to the extractor voltage UE2: UE2 ≥ 5000 V, preferably UE2 ≥ 6000 V, or most preferably UE2 ≥ 6500 V. (i) Reducing the extractor voltage to the initial value UE0. It is possible that the extractor voltage is reduced to the initial value UE0 stepwise, in a single step, or continuously.

[0040] According to a preferred embodiment of the invention, the reduction of the extractor voltage according to step (i) is carried out such that the beam current I does not exceed an initial value I0 present at the beginning of step (f). This prevents an excessive beam current during the material build-up process, which could potentially damage the multi-beam generator.

[0041] According to a preferred embodiment of the invention, a beam current, or beam current I, is measured during the material build-up process. Preferably, the beam current I is measured at short intervals or even continuously. This allows for the best possible control of the material build-up process.

[0042] According to a preferred embodiment of the invention, the beam current uniformity between the plurality of charged single-particle beams is measured directly or indirectly, and the material build-up process is triggered when the following relationship applies to the beam current uniformity deviation ΔUni: ΔUni ≥ 10%, preferably ΔUni ≥ 5% or ΔUni ≥ 1%. For plurality-particle beam systems with inherently very high beam current uniformity, it is possible to select a very small beam current uniformity deviation ΔUni. For plurality-particle beam systems with somewhat lower beam current uniformity due to system-related factors, the beam current uniformity deviation ΔUni should be selected to be somewhat larger.

[0043] According to a preferred embodiment of the invention, the beam flow uniformity is measured, and this measurement is performed at the filter plate of the multi-beam generator. In this embodiment, the beam flow uniformity is thus measured indirectly, or derived from a measurement in which the beam flow of each individual particle beam generated at the filter plate is not directly measured. In this embodiment of the invention, the measurement of the beam flow uniformity can be performed during the normal operation of the multi-particle beam system; the operation does not need to be interrupted or specially adapted for the measurement.

[0044] According to a preferred embodiment of the invention, at least one area arranged between adjacent apertures serves as a detection area for measuring the beam current uniformity at the filter plate of the multi-beam generator. Preferably, several such areas are used for measuring the beam current uniformity.

[0045] According to a preferred embodiment of the invention, the filter plate of the multi-beam generator, on which the beam current uniformity is measured, has a conductive absorber layer, wherein the filter plate or the conductive absorber layer is divided into a plurality of electrically isolated sectors, each of which is grounded. Measuring the beam current uniformity then comprises measuring beam currents that impinge on or are derived from a sector of the filter plate. For example, a current measuring device, such as an ammeter, in particular a picoammeter, can be provided between the grounding and the respective sector. The number of sectors on the filter plate is selected such that a beam current uniformity of the individual particle beams can at least be derived from the corresponding number of beam currents.However, it is not necessary to adjust the number of sectors to the number of individual particle beams. The number of sectors can instead be lower, for example, only six, seven, or eight sectors, or twelve or sixteen sectors. When sectorizing the filter plate, it is important to consider that the sectorization must not negatively affect the quality of the generated individual particle beams. Therefore, it is essential to avoid placing electrical insulation between adjacent sectors on the surface of the filter plate, as this could cause the insulation to become electrically charged. Instead, the electrical insulation should be integrated into the filter plate through appropriate 3D design, or buried within it.Furthermore, combining areas with multiple apertures on the filter plate into a single sector improves the signal-to-noise ratio during beam current measurement. Additionally, filter plates typically absorb more than 90% of the incoming particles, with only the remaining fraction used to generate the single-particle beams. Moreover, the fewer sectors used, the larger each individual sector and the better the signal-to-noise ratio during current measurement at that sector. Conversely, a certain minimum number of sectors is still required to detect any deviation in beam current uniformity.

[0046] According to a preferred embodiment of the invention, each sector surrounds at least one of the apertures of the filter plate, and preferably a plurality of sectors surround a group of apertures of the filter plate. Furthermore, each aperture of the filter plate is assigned to exactly one of the sectors. In this embodiment of the invention, the sectors are thus arranged as detection areas for a beam stream very close to or directly around the apertures. Beam stream fluctuations in individual beam streams or in specific groups of individual beam streams can therefore be very well represented by measuring beam streams in the sectors. This allows conclusions to be drawn about the beam stream uniformity.

[0047] According to a further preferred embodiment of the invention, a current detection aperture plate with a singular opening is arranged between the beam generating device and the filter plate, the rear side of which faces the filter plate. The rear side facing the filter plate has a conductive absorber layer which is grounded. To measure a beam current and / or beam current uniformity, at least one beam current of secondary electrons and / or backscattered electrons striking the rear side of the current detection aperture plate can be measured. These secondary electrons and / or backscattered electrons are generated or backscattered when the first charged particle beam strikes the filter plate. They are also a very good measure of the beam current that was originally emitted by the beam generating device.

[0048] According to a preferred embodiment of the invention, the absorber layer of the current detection aperture plate is divided into a plurality of electrically isolated sectors, with a beam current being measured in each sector. The same principles apply to the sectoring of the current detection aperture plate as have already been described for the sectoring of the filter plate in connection with the other embodiment of the invention. However, the sectoring of the current detection aperture plate is somewhat less critical than the structuring of the filter plate, since no individual particle beams are generated or influenced at the current detection aperture plate. Nevertheless, the overall detection signal obtained at the current detection aperture plate is lower than the signal obtained directly at the sectored filter plate.

[0049] According to a preferred embodiment of the invention, the beam current is measured, and this measurement includes measuring an anode current. This anode current is generated based on charged particles, particularly electrons, striking the anode aperture. It has been found that measuring the anode current provides a particularly good measure of changes in the beam-generating device caused by a ring collapse process. The anode is typically designed as an anode aperture and, in the beam-generating device, cuts off the outermost beam current (the "teeth"). This beam current is generated at the edge of the front facet and partly at its sides, and is technically difficult to utilize. When a ring collapse process begins, precisely this area of ​​the front facet changes first. This allows for particularly early detection of the onset of the ring collapse process.

[0050] According to a preferred embodiment of the invention, the material build-up process is triggered when the measured anode current I_A falls below a predetermined threshold value I_A_Ref. Experiments have shown that a decrease in the anode current I_A is associated with the onset of a ring collapse process.

[0051] According to a preferred embodiment of the invention, the following relationship applies to the measured anode current for triggering the material build-up process: I_A / I_A_Ref ≤ 85%, preferably I_A / I_A_Ref ≤ 90%, or most preferably I_A / I_A_Ref ≤ 95%. Thus, the material build-up process is preferably triggered very early.

[0052] Additionally or alternatively, according to a preferred embodiment, the material build-up process is triggered when the rate of change of the measured anode current I_A exceeds a predetermined threshold I_Avar_Ref within a predefined change time interval. In this way, normal aging processes of the cathode tip can be appropriately considered when triggering the material build-up process, and a distinction can be made between this and an incipient ring collapse process. A known rate of change of the measured anode current I_A during the burn-in of the cathode tip can also be taken into account or excluded.

[0053] According to a preferred embodiment of the invention, one of the above-mentioned changes in the anode current can be used as a necessary criterion for triggering a material build-up process, and a change in the beam current uniformity can be used as a sufficient criterion for triggering a material build-up process.

[0054] According to a preferred embodiment of the invention, the beam current and / or beam current uniformity are measured in the object plane of the multi-beam particle beam system. This corresponds to the sample plane d of a multi-beam particle microscope. Beam current and / or beam current uniformity can be determined in the object plane, for example, using a Faraday cup or several Faraday cups (or a successively shifted Faraday cup). However, such a measurement interrupts the normal operation of the multi-beam particle beam system or should at least be meaningfully integrated into a process sequence of the multi-beam particle beam system to avoid unnecessary downtime. In principle, however, measuring the beam current and / or beam current uniformity in the object plane is a useful measure for detecting the onset of the ring collapse process.

[0055] According to a preferred embodiment of the invention, the beam current is measured at the point where the charged individual particle beams cross over each other. This can occur, for example, during inspection processes during an interlacing or frame shift. It is then possible to deflect all individual particle beams into a beam current measuring device located at the point of beam crossing using a simple collective beam deflector. This deflection is essentially a simple parallel offset. Therefore, such a measurement of the total beam current is very easy to implement.

[0056] According to a preferred embodiment of the invention, the multi-particle beam system is a multi-beam particle microscope that generates a multitude of particle-optical images which can be combined or are combined to form a multi-image. In addition to or as an alternative to the beam current measurements and / or beam current uniformity measurements described above, the beam current uniformity can then be measured indirectly based on the brightness values ​​of the generated individual images. If the brightness values ​​of the generated individual images vary more than permissible, this can also indicate the onset of a ring collapse process.

[0057] It is possible to combine the above-described embodiments of the invention in whole or in part, provided that no technical contradictions result.

[0058] According to a second aspect of the invention, it relates to a computer program product comprising program code for executing the method as described above in several embodiments. The program code can be written in any programming language. The program code can be modular in structure. For example, the program code can include a module for measuring and evaluating the beam current and / or beam current uniformity, as well as a module for the material build-up process.

[0059] According to a third aspect of the invention, it relates to a multiple particle beam system configured to execute the method as described above in several embodiments. For example, program code for executing the method, as described above in several embodiments, can be loaded into a processor of the multiple particle beam system's control system, and the corresponding components of the multiple particle beam system can be controlled accordingly.

[0060] According to a fourth aspect of the invention, it relates to a multi-beam particle microscope comprising the following: A beam generating device configured to generate a first charged particle beam and comprising a cathode, an extractor electrode and an anode aperture, and a heating device for heating the cathode, wherein the cathode comprises a reservoir with an output work reduction agent and wherein the cathode has a cathode tip with a front facet; a multi-beam generator with a multi-aperture array, wherein the multi-aperture array has a filter plate with a plurality of apertures, which is penetrated by the first charged particle beam to form a plurality of charged first single-particle beams, and wherein the multi-beam generator is configured to generate from the first charged particle beam a first field of a plurality of charged single-particle beams; a beam current measuring instrument configured to measure beam current and / or beam current uniformity; a first particle optics with a first particle-optical beam path configured to direct the generated first single-particle beams onto a sample, such that the first particle beams hit the sample at points of impact that form a second field; a detection system; a second particle optic with a second particle-optical beam path configured to image second single-particle beams emanating from the points of impact in the second field onto the detection system; a particle-optical objective lens through which both the first and second individual particle beams pass; 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 generation device, the particle optical objective lens, the first particle optics, the second particle optics, and the detection system, and wherein the control is configured to detect a ring collapse process initiating at the front facet of the cathode tip of the beam generating device based on beam current measurement and / or beam current uniformity measurement, and to control the beam generating device for a material build-up process at the front facet of the cathode tip based on the detection.

[0061] 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 multi-beam particle beam system according to the first aspect of the invention.

[0062] According to a preferred embodiment of the invention, the beam current measuring device is configured to measure beam current uniformity, and the beam current measuring device comprises the filter plate of the multi-beam generator. The filter plate has a conductive absorber layer and is divided into a plurality of electrically isolated sectors, each of which is grounded. During operation of the multi-beam particle microscope, the beam currents incident on the respective sectors are determined. The conductive absorber layer can be coated, for example, with gold, copper, silver, platinum, or another conductive material. Otherwise, what has already been stated in connection with the inventive method for operating a multi-beam particle beam system applies to this embodiment of the invention.

[0063] According to a preferred embodiment of the invention, each sector surrounds at least one of the apertures of the filter plate, and preferably a plurality of sectors surround a group of apertures of the filter plate. Furthermore, each aperture of the filter plate is assigned to exactly one of the sectors. In this embodiment of the invention, the sectors are thus arranged as detection areas for a beam stream very close to or directly around the apertures. Beam stream fluctuations in individual beam streams or in specific groups of individual beam streams can therefore be very well represented by measuring beam streams in the sectors. This allows conclusions to be drawn about the beam stream uniformity.

[0064] According to a preferred embodiment of the invention, the beam current measuring device comprises a current detection aperture plate arranged between the beam generating device and the filter plate. Its rear side faces the filter plate and has a conductive absorber layer that is grounded. The conductive absorber layer can, in turn, comprise, for example, gold, copper, silver, platinum, or another conductive material or metal. During operation of the multi-beam particle microscope, a beam current of secondary electrons and / or backscattered electrons incident on the rear side of the current detection aperture plate is measured. These are generated when charged particles of the first charged particle beam strike the filter plate or are backscattered by it.

[0065] According to a preferred embodiment of the invention, the absorber layer of the current detection aperture plate is divided into a plurality of electrically isolated sectors, and the beam current measuring device is configured to measure one beam current per sector. For this purpose, a current measuring device, for example an ammeter and in particular a picoammeter, can be provided between the ground and the respective sector.

[0066] According to a preferred embodiment of the invention, a conductive protective layer, which is grounded and electrically insulated from the conductive absorber layer, is arranged on the front face of the current detection aperture plate. This ensures that charged particles striking the front face of the current detection aperture plate do not contribute to the current measurement when measuring backscattered electrons and / or secondary electrons emitted from the filter plate. A source of interference is thus eliminated.

[0067] According to a further preferred embodiment of the invention, the beam current measuring device comprises the anode aperture of the beam generating device. The beam current measuring device is configured to measure the beam current of charged particles, in particular electrons, striking the anode aperture. Furthermore, everything that has already been stated in connection with the inventive method for operating a multi-particle beam system also applies to this embodiment of the invention.

[0068] According to one example, this refers to a multi-particle beam system that has the following characteristics: A beam-generating device configured to produce a first charged particle beam; a multi-beam generator with a multi-aperture array, wherein the multi-aperture array has a filter plate with a plurality of apertures, which is penetrated by the first charged particle beam to form a plurality of charged first single-particle beams, and wherein the multi-beam generator is configured to generate from the first charged particle beam a first field of a plurality of charged single-particle beams; and a current detection aperture plate with a singular opening arranged between the beam generating device and the filter plate, through which the charged first particle beam is penetrated essentially without contact, wherein a rear side of the current detection aperture plate faces the filter plate and has a conductive absorber layer connected to an earth, and wherein a current measuring device is arranged between the conductive absorber layer and the grounding, so that a beam current of secondary electrons and / or backscattered electrons striking the conductive absorber layer can be measured during operation of the multi-particle beam system.

[0069] For the features of the multiple particle beam system according to the example, everything that has already been stated with regard to these features in connection with the first to fourth aspects of the invention applies.

[0070] In one example, a conductive protective layer is arranged on the front face of the current detection aperture plate. This layer is grounded and electrically insulated from the conductive absorber layer. This prevents charged particles emitted directly from the beam-generating device from striking the current detection aperture plate and interfering with or superimposing the measurement of the incident secondary electrons and / or backscattered electrons.

[0071] In one example, the absorber layer of the current detection aperture plate is divided into a multitude of electrically isolated sectors, each connected to a ground. Between the absorber layer of each sector and the ground, a beam current measuring device is positioned, configured to measure the beam current of secondary electrons and / or backscattered electrons incident on the conductive absorber layer in each sector. The beam current measuring device, in the narrower sense, could be, for example, an ammeter, particularly a picoammeter. This allows for very precise measurement.

[0072] The various embodiments of the invention can be combined wholly or partially, provided that this does not result in any technical contradictions. This also applies to embodiments of the invention according to different aspects of the invention.

[0073] The invention will be better understood with reference to the accompanying figures. These show: Fig. 1: shows a schematic representation of a multi-beam particle microscope (MSEM); Fig. Figure 2 shows a schematic representation of a beam generating device; Fig. 3: shows details of a beam generating device in a schematic representation; Fig. Figure 4: schematically illustrates a ring collapse process; Fig. 5: schematically illustrates process steps of a method according to the invention for operating a multiple particle beam system; Fig. 6: schematically illustrates the process steps of a material build-up process; Fig. Figure 7 shows measurement results from beam current measurements during a ring collapse process; Fig. 8: schematically illustrates a beam current measurement at an anode aperture; Fig. 9: schematically illustrates beam current distributions; Fig. Figure 10: schematically illustrates a beam current measurement on a filter plate; Fig. 11: schematically illustrates beam current measurements on a sectored filter plate; Fig. 12: schematically illustrates beam current measurements on a sectored filter plate; Fig. Figure 13: schematically shows a current detection aperture plate; and Fig. Figure 14 shows a schematic representation of a sectorized current detection aperture plate.

[0074] Fig. Figure 1 schematically shows a multi-beam particle beam system 1 in the form of a multi-beam particle microscope 1. The multi-beam particle microscope 1 has a beam generation device 300 with a particle source, for example, an electron source. Charged particles or electrons are generated by the beam generation device 300, for example, by thermal field emission. The emitted charged particles form a diverging particle beam 309, which is collimated by a sequence of condenser lenses 303.1 and 303.2 and strikes a multi-beam particle generator 305 with a multi-aperture arrangement. The multi-beam particle generator 305 comprises several multi-aperture plates 304, 306 and a field lens 307. A multitude of single-particle beams 3 or 3 are emitted by the multi-beam particle generator 305.Single-electron beams 3 are generated, arranged in a field which is mapped onto another field formed by beam spots 5 in the object plane 101. The distance between the centers of apertures of a multi-aperture plate 306 can be, for example, 5 µm, 100 µm, and 200 µm. The diameters D of the apertures are smaller than the distance between the centers of the apertures; examples of the diameters are 0.2 times, 0.4 times, and 0.8 times the distances between the centers of the apertures.

[0075] The multi-aperture arrangement 305 and the field lens 308 are configured to generate a multitude of focal points 323 of primary beams 3 in a grid arrangement on a surface 321. The surface 321 need not be a flat surface, but can be a spherically curved surface to accommodate field curvature of the subsequent particle optical system.

[0076] The multi-beam particle microscope 1 further comprises a system of electromagnetic lenses 103 and an objective lens 102, which reduce the size of the beam foci 323 from the intermediate image plane 321 onto the object plane 101. The first individual particle beams 3 pass through the beam splitter 400 and a collective beam deflection system 500, which deflects the multitude of the first individual particle beams 3 during operation and scans the image field. The first individual particle beams 3 incident on the object plane 101 form, for example, a substantially regular field, with distances between adjacent point locations 5 being, for example, 1 µm, 10 µm, or 40 µm. The field formed by the point locations 5 can, for example, have a rectangular or hexagonal symmetry.

[0077] The object 7 to be examined can be of any type, for example a semiconductor wafer or a biological sample, and it can comprise an array of miniaturized elements or the like. The surface 15 of the object 7 is located in the object plane 101 of the objective lens 102. The objective lens 102 can comprise one or more electron-optical lenses. It can be, for example, a magnetic objective lens and / or an electrostatic objective lens.

[0078] The primary particles 3 striking object 7 generate interaction products such as secondary electrons, backscattered electrons, or primary particles that have undergone a reversal of motion for other reasons. These products originate from the surface of object 7 or from the first plane 101 or object plane 101. The interaction products emanating from the surface 15 of object 7 are shaped into secondary particle beams 9 by the objective lens 102. After passing through the objective lens 102, the secondary beams 9 pass through the beam splitter 400 and are directed to a projection system 200. The projection system 200 has an imaging system 205 with projection lenses 206, 208 and 210, a contrast aperture 214 and a multi-particle detector 207. The impact points 25 of the second single-particle beams 9 on the detection areas of the multi-particle detector 207 are located in a third field at a regular distance from each other.Examples of values ​​are 10 µm, 100 µm and 200 µm.

[0079] The multi-beam particle microscope 1 further comprises a computer system or a control unit or controller 10, which in turn may be designed as a single unit or as a multi-part unit, and which is designed both for controlling the individual particle-optical components of the multi-beam particle microscope 1 and for evaluating and analyzing the signals obtained with the multi-detector 207 or the detection unit.

[0080] Further information on such multi-beam particle beam systems or multi-beam particle microscopes 1 and components used therein, such as particle sources, multi-aperture plates 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 10 2013 016 113 A1 and DE 10 2013 014 976 A1, the disclosures of which are fully incorporated into the present application by reference.

[0081] Fig. Figure 2 schematically shows a beam generating device 300. The beam generating device 300 comprises a cathode 350 with a cathode tip 351 for emitting charged particles, for example, electrons. The cathode can be heated, which in Fig. 2 schematically indicated by the heating wire 359 shown. Furthermore, the beam generation device 300 comprises an extractor electrode 353 and optionally a suppressor electrode 356. The cathode 350, suppressor electrode 356 and extractor electrode 353 together form the so-called beam head 349. This is shown in Fig. 2 indicated by the dotted square. The suppressor electrode 356 and the extractor electrode 353 each have a cylindrical shape with a cylindrical shell and a flat front region: In the example shown, the flat front region 359 of the suppressor electrode 356 includes an opening 360 through which the cathode tip 351 protrudes. The extractor electrode 353 has a flat front region 357, which in turn has an opening 358. This opening is penetrated by the emitted particle beam 352. The particle beam 352 then strikes an anode electrode 354 or anode aperture 354, which in the example shown is flat and has an opening 355. By means of this opening 355, the charged particle beam 352 is clipped and takes the form of the diverging particle beam 309, which is also in Fig. 1 is shown schematically.

[0082] The anode aperture 354 can be moved relative to the beam head 349, both in the z-direction and in a plane orthogonal to the particle-optical axis Z.

[0083] The components of the beam generation device 300 can be controlled by means of the control unit 10. The applied acceleration voltage, the extraction voltage, the suppressor voltage, and a heating current for the cathode 350 can be regulated by the control unit.

[0084] In normal operation of the beam generating device 300 or in operation of the associated multi-particle beam system 1, an extraction voltage of, for example, a few kV is applied between the cathode tip 351 and the extractor electrode 353, for example, approximately + / - 2kV, + / - 3kV, + / - 4 kV, + / - 5 kV, + / - 6 kV up to + / - 10 kV.

[0085] Between the cathode tip 351 and the anode electrode 354, an accelerating voltage of several tens of kV is applied during operation, for example, + / - 10 kV, + / - 25 kV, + / - 27 kV, + / - 30 kV, + / - 35 kV, + / - 40 kV or significantly higher, for example up to + / - 300 kV. Preferably, the anode electrode 354 is at ground potential or is only subjected to a low voltage, while the actual high voltage is applied to the cathode 350.

[0086] During operation of the beam generating device 300, a suppressor voltage is applied between the optionally provided suppressor electrode 356 and the cathode 350, which can be, for example, several hundred volts, e.g. + / - 200V, + / - 300V, + / - 400V, + / - 500V or + / - 600V.

[0087] During a material build-up process at the front facet 365 of the cathode 350, in particular the extraction voltage and the heating current at the cathode 350 can be changed.

[0088] In Fig. Figure 2 shows the described voltages between the electrodes schematically for the case where the cathode emits 350 electrons. With the emission of positively charged particles, the relationships would be reversed. The in Fig. The second example shown should not be understood as restrictive. The same applies to the following one. Fig. 8.

[0089] Fig. Figure 3 schematically shows details of a beam generating device 300. Specifically, it shows Fig. Figure 3a schematically shows details of the beam head 349 and, in particular, the cathode 350. The cathode tip 351 protrudes from the opening 360 of the suppressor electrode 356. The emitted particle beam 352 then passes through the opening 358 of the extractor electrode 353. The cathode 350 itself comprises a reservoir 361 containing a work-work reducing agent and can be heated by means of a heating wire 359. The cathode can, for example, be made of tungsten, and zirconium oxide can be used as the work-work reducing agent. However, other materials / material combinations are also possible, such as an emitter made of molybdenum, iridium, or rhenium with a work-work reducing coating of compounds such as oxides, nitrides, and carbon with zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium, or lanthanum.

[0090] Fig. Figure 3b on the left is an enlarged view of the cathode 350. The cathode wire 364, to which the reservoir 361 is also attached, is visible. The cathode wire 364 is usually made of monocrystalline tungsten. The heating wire 359 is usually made 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. At the tapered end of this etched shaft 363 is the cathode tip 351 in the narrower sense. This is shown in Fig. Figure 3b, shown on the far right, is greatly enlarged. Above the tapered shaft 367, the cathode 350 tapers to a rounded point; this rounded area is called the apex 366. At the tip of the apex 366 is the front facet 365 of the cathode tip 351. The front facet 365 has a fixed crystallographic orientation, typically a {100} face. During operation of the beam-generating device 300, charged particles, particularly electrons, emanate from this facet. The orientation of the crystallographic facet influences the work function of the electrons. Therefore, the operation of the cathode 350 is a complex interplay between an electric field applied to the cathode 350, the geometry of the cathode tip 351, the properties of the front facet 365, and especially the crystallographic orientation of this facet, the material of the cathode 350, and the work function reduction agent used.In summary, the radiation characteristic of a cathode 350 depends on all parameters that describe the electric field at the cathode tip 351, as well as 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.

[0091] In thermodynamics, a stability criterion (dynamic equilibrium) for the geometric shape of a cathode tip 351 can only be derived under the assumption of a uniform chemical potential µ. In practice, however, this assumption cannot be maintained, 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 the shaft 367 of the cathode tip 351. Therefore, a gradient or fluctuation of the chemical potential always leads to mass transport via surface diffusion, whereby migrating emitter material can change the shape of the apex and the crystallographic facets.

[0092] The processes at the cathode tip 351, briefly outlined above, are therefore complex. Besides a continuous aging process of the cathode tip 351, which leads to a drift process in the beam current emitted from the cathode 350, there are other processes that can negatively affect the beam current and, in particular, the beam current uniformity of the emitted charged particles. These include the so-called ring collapse process, which is schematically represented in Fig. Figure 4 illustrates this. The so-called ring collapse process refers to a characteristic deformation of the front facet 365 of a cathode tip 351. In this process, several layers of a structural combination consisting of an emitter material-work function reduction unit, such as a ZrOW unit, slide laterally or in a ring-like fashion from the front facet 365 onto the apex surface 366 of the cathode tip 351. This deformation of the front facet 365 is described in Figure 4. The sliding process is described in Figure 4. Fig. 4 represented by the arrows: In Fig. 4a the extent of the front facet 365 is larger than in Fig. 4b, where a layer of a structural combination of an emitter material-output work reduction unit, such as a ZrOW unit of the front facet 365, has already slipped away. As a consequence of the slippage, both the total beam current and the radiation characteristics of the cathode 350 change, typically significantly and over a longer period, for example, several hours or days. Inspection processes, and especially automated inspection processes, are therefore difficult or not practically possible during the ring collapse process. The frequency of a ring collapse process varies considerably and depends, among other things, on the temperature of the cathode 350, a diffusion rate at the cathode tip 351, and the local field at the apex 366 of the cathode. Low local electric fields favor the frequency of ring collapse processes.However, predicting a ring collapse process is not possible; a ring collapse process remains a spontaneously occurring event. Nevertheless, the onset of the ring collapse process can be measured, and appropriate countermeasures can be taken.

[0093] Fig. Figure 5 schematically illustrates the process steps of the inventive method for operating a multi-beam particle beam system, in particular a multi-beam particle microscope 1. In an initial process step S1, the multi-beam particle beam system is provided. The multi-beam particle beam system 1 comprises a beam generating device 300 for generating a first charged particle beam 309. Furthermore, the beam generating device 300 comprises a cathode 350, an extractor electrode 353, an anode diaphragm 354, and a heating device 359 for heating the cathode 350. The cathode 350 includes a reservoir 361 with an output work reduction agent and a cathode tip 351 with a front facet 365.

[0094] Furthermore, the multi-particle beam system 1 has a multi-beam generator 305 with a filter plate 304, wherein the filter plate 304 has a multiplicity of apertures 304a and wherein the filter plate 304 is substantially penetrated by the first charged particle beam 309, 311 forming a multiplicity of charged (first) single-particle beams 3.

[0095] In process step S2, a reference beam current and / or a reference beam current uniformity is defined. This can involve single numerical values ​​or range specifications. The reference beam current is preferably a total beam current that can be determined at a predefined position using a predefined measurement method. The reference beam current can comprise a single current value, or it can encompass a current interval with upper and lower limits. The reference beam current describes the beam current that would be expected without a ring collapse process occurring.The reference beam current can be a constant value or a constant interval of values ​​over time. It can also change over time, and known drift processes affecting the beam current can be incorporated into its definition. Reference beam current uniformity is a measure of the uniformity of individual beam currents, and thus of the uniformity of the beam currents of the charged individual particle beams. For multi-particle beam systems, the greatest possible uniformity of the individual particle beam currents is a system requirement. The difference between a maximum and a minimum beam current value must not exceed a certain maximum value. For example, the entire range, i.e., the difference, can be a maximum of 10 picoamperes. Uniformity can also be expressed as a percentage.Uniformity can be defined, for example, as follows: Uniformity [%] = (Maximum value - Minimum value) / Mean value x 0.5 x 100. Other definitions are also possible and useful. The specific definition of beam current uniformity should preferably be adapted to the specific measurement of beam current uniformity. In this respect, the term beam current uniformity or reference beam current uniformity can refer directly to the uniformity of the charged single-particle beams generated at the filter plate. However, it is also possible to measure or define other uniformity quantities that only reflect the uniformity between the charged single-particle beams without exactly corresponding to them. Several examples of this will be given later. In general, the reference beam current uniformity defines a target state for beam current uniformity, provided that no ring collapse process is present.

[0096] In a further process step S3, the beam current and / or beam current uniformity is measured among the multitude of charged individual particle beams 3, particularly during the normal operation of the multiple particle beam system 1. The beam current and / or beam current uniformity can be measured in various ways, as will be explained in detail later. In principle, the measurement of the beam current and / or beam current uniformity can be direct or indirect. In any case, conclusions about the beam current and / or beam current uniformity can be drawn from the measurement process.

[0097] In process step S4, the measured beam current is compared with the reference beam current, and a beam current deviation is determined based on this comparison. Additionally or alternatively, the measured beam current uniformity is compared with the reference beam current uniformity, and a beam current uniformity deviation is determined based on this comparison. The determined beam current deviation and / or beam current uniformity deviation is then evaluated. In process step S5, it is detected whether or not a ring collapse process is beginning at front facet 365. This detection of a ring collapse process beginning at the front facet of the cathode tip is based on the beam current deviation and / or the beam current uniformity deviation.If the beam current deviation and / or the beam current uniformity deviation is sufficiently large, a material build-up process is triggered in step S6 at the front facet 365 of the cathode tip 351 based on the detection of the incipient ring collapse process. Otherwise, in step S3, the beam current and / or the beam current uniformity among the multitude of charged single-particle beams 3 is measured again. In this way, the onset of a ring collapse process can be determined promptly, and countermeasures can be taken using the material build-up process.

[0098] According to a preferred embodiment of the invention, the material build-up process is triggered when the beam current uniformity between the plurality of charged single-particle beams 3 is measured and when the beam current uniformity deviation exceeds a predefined limit. Additionally or alternatively, the beam current can be measured and the material build-up process triggered when the beam current deviation exceeds a predefined limit.

[0099] For example, the material build-up process may be triggered if the following relationship applies to the beam current uniformity deviation ΔUni: ΔUni ≥ 10%, preferably ΔUni ≥ 5% or ΔUni ≥ 1%. For multi-particle beam systems with inherently very high beam current uniformity, it is possible to choose a very small beam current uniformity deviation ΔUni. For multi-particle beam systems with somewhat lower beam current uniformity due to system-related factors, the beam current uniformity deviation ΔUni should be chosen to be somewhat larger.

[0100] Fig. Figure 6 schematically illustrates the process steps of a material build-up process. In process step S7, an extractor voltage applied between the cathode tip 351 and the extractor electrode 353 is reduced from an initial value UE0 to a reduced value UE1. This significant reduction in the extractor voltage drastically reduces the beam current (in the sample plane), for example, to less than 30% of its initial value. Simultaneously, the total current emitted by the cathode 350 is reduced. This reduction in the total current protects the sensitive micro-optics of a multi-particle beam system 1 during subsequent process steps. The reduction of the extractor voltage UE0 to the reduced value UE1 can be achieved, for example, by the following relationship: UE1 / UE0 ≤ 60%, preferably UE1 / UE0 ≤ 55% or UE1 / UE0 ≤ 50%. The reduced value UE1 can also be chosen to be even smaller.

[0101] In process step S8, the temperature of the cathode 350 is temporarily increased, in particular by increasing the heating current at the heating device 359, from an initial temperature T0 to a higher temperature T1. This temporary increase in the temperature of the cathode 350 initially leads to an increase in the jet current due to the elevated temperature, but then also to the evaporation of the work-reducing agent from the front facet 365 of the cathode tip 351. As the evaporation progresses, the jet current decreases again until finally practically no jet current is emitted from the cathode 350 because the required work function has become too high. The temporary increase in the temperature of the cathode 350 can be carried out according to an example during a time interval t_temp, where the following relation can 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.

[0102] Additionally or alternatively, a temperature difference ΔT = T1 - T0 during process step S8 can satisfy the following relationship: 250°C ≤ ΔT ≤ 400°C, preferably 300°C ≤ ΔT ≤ 350°C. Additionally or alternatively, the following relationship can apply to the temperature T0 itself: T0 ≥ 1700°C, in particular T0 ≥ 1750°C or T0 ≥ 1800°C.

[0103] In process step S9, the extractor voltage is increased again to a higher value UE2 than the initial value UE0 for a time interval t_Mat. Due to the now higher local electric field, the actual material buildup begins. Emitter material increasingly migrates towards the apex of the cathode and enriches the side and front facets, stabilizing the latter. The front facet 365 itself is reshaped and rebuilt. The following relationship can apply to the increased value UE2 of the extractor voltage UE2: UE2 / UE0 ≥ 150%, preferably UE2 / UE0 ≥ 200% or UE2 / UE0 ≥ 250%. Additionally or alternatively, the following relationship can apply to the extractor voltage UE2: UE2 ≥ 5000 V, preferably UE2 ≥ 6000 V or UE2 ≥ 6500 V. Even higher values ​​for the extractor voltage UE2 are possible, e.g. up to 8 kV or 10 kV.

[0104] The increase in extractor voltage in process step S9 is temporary and occurs during a time interval t_Mat. The following relationship can apply to the time interval t_Mat: t_Mat ≤ 60 min, preferably t_Mat ≤ 45 min or t_Mat ≤ 30 min. The duration of process step S9 is therefore relatively short compared to the significantly longer duration of a ring collapse process.

[0105] In a further process step S10, the extractor voltage is reduced to the initial value UE0. The aim here is to lower the extractor voltage to the initial value UE0. For example, the reduction of the extractor voltage in process step S10 is carried out in such a way that the beam current I does not exceed the initial value I0 present at the beginning of step S7. This prevents an excessive beam current, which in turn protects the multi-beam generator of the multi-particle beam system 1.

[0106] In the optional process step S11, the beam current and / or beam current distribution can be checked after the material build-up process. This process step serves to improve process reliability. However, as a rule, both the beam current and the beam current distribution will assume satisfactory values ​​after the material build-up process has been carried out.

[0107] Fig. Figure 7 shows measurement results from beam current measurements during a ring collapse process. The beam current I is plotted on the y-axis, normalized to 1 before the ring collapse process. Time t is plotted on the x-axis, in the illustrated measurement example in the form of days. The entire ring collapse process in Fig. 7 took about two weeks.

[0108] Circular measurement points are plotted in the diagram, showing the beam flux at a sample, an object, or in the object plane of a multi-particle beam system (here: multi-beam particle microscope 1). During the first four days of the ring collapse process, the beam flux in the sample area remains approximately constant before suddenly increasing very sharply and then dropping sharply shortly thereafter (around days 7 and 8, respectively). Afterward, the beam flux in the sample area slowly recovers and reaches approximately its initial value after about two weeks. Therefore, the beam flux change occurring during a ring collapse process is clearly visible in a beam flux measurement in the object plane 101, although it only becomes apparent relatively late in the process.

[0109] The situation is different with regard to a beam current that is detected at the anode aperture: According to Fig. In section 8, the anode aperture 354 is equipped with a conductive absorber layer 368. This conductive absorber layer 368 is grounded, with a current measuring instrument, for example an ammeter, in particular a picoammeter, being provided between the grounding and the conductive absorber layer 368. The measured values ​​are transmitted to the control unit 10. The conductive absorber layer 368 is electrically insulated from the anode aperture 354, which is energized. As shown in Fig. As shown in Figure 8, the anode aperture 354 cuts off the emitted particle beam 352 at its edges. This cut-off region of the beam stream corresponds to the edges of the front facet 365 of the cathode tip 351. An incipient ring collapse process becomes particularly noticeable in this edge region through the slippage of monolayers of the front facet 349 onto the apex 366. Accordingly, one can see in Fig. 7 also that the beam current measured at the anode (small triangular measuring points in Fig. 7) In fact, the anode beam current changes at the very beginning of the ring collapse process, in this case beginning to decrease. The change in the anode beam current is detectable significantly from day three of the ring collapse process at the latest. This may also be the case earlier. This naturally depends on the overall system stability of the multi-particle beam system and on the accuracy with which the beam currents can be measured.

[0110] For example, the material buildup process can be triggered when the measured anode current I_A falls below a predetermined threshold I_A_Ref. A ratio of the anode current I_A to the predetermined threshold I_A_Ref could, for instance, satisfy the following relationship: I_A / I_A_Ref ≤ 85%, preferably I_A / I_A_Ref ≤ 90% or I_A / I_A_Ref ≤ 95%. It is possible to detect the beginning of the ring collapse process even with a relatively small drop in the measured anode current.

[0111] Additionally or alternatively, it is also possible to factor out other effects when measuring the anode current. For example, it is possible to factor out anode current drift due to the normal aging process of a cathode tip 351. Therefore, it can also be useful to trigger the material buildup process at the front facet 349 when the rate of change of the measured anode current I_A exceeds a predetermined threshold I_Avar_Ref within a predefined change time interval. Changes due to normal drift are usually significantly slower than the changes in anode current resulting from the onset of a ring collapse process.

[0112] In Fig. Figure 7 shows a dotted area where a beam flow uniformity deviation ΔUni was observed during the ring collapse process. This deviation was observable from approximately day 6 to day 10 of the ring collapse process. Therefore, an observed beam flow uniformity deviation is also a good indicator for detecting a ring collapse process.

[0113] Fig. Figure 9 schematically illustrates different types of beam current distributions. The values ​​of individual beam currents are shown in Fig. Figure 9 illustrates this through different shades of gray or pattern fills. A light pattern fill indicates low beam current, a dark pattern fill indicates high beam current. Fig. Figure 9A shows nine exemplary individual measurements of beam currents. These have the same value, representing the beam current distribution in Fig. 9a is therefore uniform. The individual measured values ​​can correspond to a single-particle beam 3 formed in the filter plate 304, but it is also possible that the individual measured values ​​result from the aggregation of several single-particle beams into a measuring sector or measuring area. This depends on the type of beam flow uniformity measurement performed and will be described in more detail below.

[0114] 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. Within each column C1 to C5, however, the beam flux is constant. The in Fig. The beam current distribution shown in 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.

[0115] Fig. Figure 9c again shows an insufficiently homogeneous beam flux distribution: In the example shown, the beam flux increases radially from the inside out. The beam flux is lowest in the central region R0. In the annular region R1, which directly borders the central region R0, the beam flux is increased, and in the outer ring R2, where individual beam flux measurements are taken, the beam flux is even further increased. This non-uniform beam flux distribution can again be observed during the ring collapse process.

[0116] In addition to the in Fig. In addition to the nine deviations from beam stream uniformity shown, other deviations are also possible, which are not explicitly shown here.

[0117] Fig. Figure 10 schematically shows a beam current measurement at the filter plate 304 of a multi-beam generator 305. In the example shown, the filter plate 304 is illuminated telecentrically by the illuminating particle beam 311. Some charged particles pass through the openings 304a of the filter plate 304 in the form of charged single-particle beams 3. However, the majority of the charged particles striking the filter plate 304 are absorbed in a conductive absorber layer 341 of the filter plate 304. The conductive absorber layer 341 is grounded. A current measuring device 370 is arranged between this grounding and the conductive absorber layer 341 to measure the beam current striking the filter plate 304 or the absorber layer 341 and to transmit the corresponding measured value to the controller 10. The beam current striking the filter plate 304 can represent a measure of the total beam current.According to a preferred embodiment of the invention, the beam current detected at the filter plate 304 can also be used for the detection of a beam current distribution and thus for the determination of the beam current uniformity, during the normal operation of the multi-particle beam system: . Fig. Figure 11 schematically illustrates beam current measurements on a sectored filter plate 304. Fig. Figure 11 shows a top view of the filter plate 304, revealing the conductive absorber layer 341 with openings 304a arranged within it. The filter plate 304, or rather the conductive absorber layer 341, is divided into a multitude of electrically isolated sectors; in the example shown, these are sectors B1 to B6. In this example, sectors B1 to B6 are arranged concentrically around the central aperture 304a of the filter plate 304. The beam currents that impinge on each sector, or region B1 to B6, of the filter plate 304 are now measured. The measurement principle is the same as already described in [reference missing]. Fig. Figure 10 is shown. However, measured values ​​I1 to I6 are now obtained, which are determined using current meters 370.1 to 370.6 and transmitted to the controller 10. The detected beam current I1 to I6 can be normalized, and the size of the ranges B1 to B6 can be taken into account during this normalization. Identical and normalized measured values ​​I1 to I6 can then reflect existing beam current uniformity. However, if the measured values, or the corresponding normalized measured values ​​I1 to I6, are different and deviate from each other, this can indicate a deviation from the required beam current uniformity. The measurement and evaluation of beam currents and beam current uniformity can be automated.According to the example shown, each of the sectors B1 to B6 surrounds at least one of the apertures 304a of the filter plate 304. Apart from the central aperture of the filter plate, which is located in sector B1, sectors B2 to B6 each surround a group of apertures 304a of the filter plate 304. Furthermore, each of the apertures 304a of the filter plate 304 is assigned to exactly one of the sectors B1 to B6. In this embodiment of the invention, sectors B1 to B6 are thus arranged as detection areas for a beam stream very close to or directly around the apertures 304a. Beam stream fluctuations in individual beam streams or in specific groups of individual beam streams can therefore be very effectively visualized by measuring beam streams in sectors B1 to B6. This allows conclusions to be drawn about the beam stream uniformity.

[0118] Fig. Figure 12 schematically illustrates beam current measurements on another sectorized filter plate 304. Unlike in Fig. 11 is the sectorization in Fig. The measurements were not taken in stages, but rather six further areas, B2 to B7, are provided around the central opening in the filter plate with a central area B1, the base of which is essentially triangular. Such sectoring also enables the measurement of beam flow uniformity deviations.

[0119] Of course, it is also possible to sector the filter plate 304 in other ways. One sector can be provided for each aperture 304a. However, this is not strictly necessary and may even be disadvantageous: The absorber area 341 of a filter plate 304 is significantly larger than the combined area of ​​apertures 304a of all particle beams. The larger the area on which incident charged particles are used for current measurement, the better the signal-to-noise ratio of the measured beam current. Therefore, a less fine sectoring may allow for better and more reliable detection of beam current uniformity deviations.

[0120] In practice, the sectorization of the filter plate 304 will be carried out in such a way that no insulators are exposed on the surface of the filter plate 304 and can be charged by charged particles striking them. Instead, insulators will be concealed or buried inside the filter plate 304.

[0121] Fig. Figure 13 schematically shows another way in which a beam current can be measured. A section of this is shown in Figure 13. Fig. 13 a beam tube 390 in which the charged particle beam 311, emitted from the cathode 350, is guided. In the area of ​​the multi-beam generator 305, the beam tube 390 opens into a vacuum chamber 392. The opening 391 and the vacuum chamber 392 are in Fig. Figure 13 is also only shown schematically. The jet tube and the vacuum chamber, or rather their walls, are grounded in the example shown. This is shown in Fig. Figure 13 schematically shows a current detection aperture plate 380. The current detection aperture plate 380 is arranged between the beam generation device 300 and the filter plate 304 of the multi-beam generator 305. Its rear side 384 faces the filter plate 304 and has a conductive absorber layer 382, ​​which is grounded. The beam current of secondary electrons and / or backscattered electrons striking the rear side 384 of the current detection aperture plate 380 is measured. These electrons are generated when the illuminating particle beam 311 strikes the filter plate 304. This beam current measurement is therefore an indirect beam current measurement, which nevertheless allows very reliable conclusions to be drawn about the total beam current emitted by the cathode 350.

[0122] In the example shown, a conductive protective layer 383, which is electrically insulated from the conductive absorber layer 382, ​​is arranged on the upper surface 385 of the current detection aperture plate 380. As a result, charged particles that might strike the front surface 385 of the current detection aperture plate 380 do not contribute to distorting the measured beam current. Instead, only secondary particles or secondary electrons and / or backscattered electrons are measured.

[0123] It is possible to use the current detection aperture plate 380 in Fig. 13 to sectorize in order to measure not only a total beam flux, but also a beam flux uniformity. Fig. Figure 14 shows a corresponding example of a sectorized current detection aperture plate 380. Fig. Figure 14 shows the current detection aperture plate 380 in its bottom view. The conductive absorber layer 382, ​​located on the underside 384 of the current detection aperture plate 380, is visible. The central opening 381, through which the particle-optical axis Z runs, is located in the middle. Fig. 14 The conductive absorber layer 382 of the current detection aperture plate 380 is divided into a plurality of electrically isolated sectors B1 to B4, in this case into four electrically isolated sectors. However, a division into fewer or more sectors could also be made. Fig. Figure 14 illustrates only the basic principle. A beam current is measured for each sector B1 to B4. For this purpose, the conductive absorber layer 382 of each sector B1 to B4 is grounded, with a current measuring device 370.1 to 370.4 being provided between the grounding point and each sector B1 to B4. The measured values ​​of each current measuring device 370.1 to 370.4 can then be transmitted to the controller 10. In the example shown, sectors B1 to B4 are each of equal size. The measured current values ​​I1 to I4 are therefore also identical if beam current uniformity is present or ideal. If they deviate from each other, ideal beam current uniformity is no longer present. In this way, the start of a ring collapse process can be determined by means of a beam current measurement or a beam current uniformity measurement, and a material build-up process can be triggered.

[0124] In the exemplary embodiments of the invention described above, several possibilities for measuring the beam current and beam current uniformity have been described. These measurement methods are not to be understood as exhaustive. For example, the measurement of the beam current and / or the beam current uniformity can also be carried out at the level of a beam crossing of charged single-particle beams 3 (see beam crossing 108 in [reference]). Fig. 1) Furthermore, it is of course possible to measure a total beam flux or individual beam fluxes in an object plane 101 (see again Fig. 1 of the present patent application). It is possible to combine the various beam current measurements and / or beam current uniformity measurements completely or partially.

[0125] Furthermore, it is possible to infer a change in beam current or beam current uniformity from other measured parameters (possibly in addition to the main ones): According to another example, the multi-particle beam system is a multi-beam particle microscope that generates a multitude of individual particle-optical images that can be combined or are combined to form a multi-image. The beam current uniformity can then be measured indirectly, based on the brightness values ​​of the generated individual images.

[0126] A method for operating a multi-particle beam system is disclosed, in which an incipient ring collapse process at the front facet of the cathode tip of the beam generation device of the multi-particle beam system can be detected based on 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 described. Furthermore, various methods for beam current measurement and / or beam current uniformity measurement are described. These measurements can be performed, in particular, during the normal operation of the multi-particle beam system. Reference symbol list 1. Multi-beam particle system, multi-beam particle microscope 3 primary particle beams, first single-particle beams 5 beam spots, points of impact 7. Object, sample, wafer 9 secondary particle beams, second single-particle beams 10 Computer system, control 15 Sample surface, wafer surface 25 pixels of a second single-particle beam 101 Object level 102 lens 103 Field lens 105 axle 108 Beam crossing, Cross-over 200 detector system 205 Projection lens system 206 Projection lens 207 Multi-particle detector 208 Projection lens 210 Projection lens 212 Beam crossing, Cross-over 214 aperture filter, contrast diaphragm 222 Collective Anti-Deflection System 300 beam generating device 301 Particle source, radiation-generating system 303 Collimation lens system 304 multi-aperture array, filter plate 304 Aperture in the filter plate 305 Micro-optics, multi-aperture arrangement, multi-beam particle generator, multi-beam generator 306 multi-aperture plate, multi-aperture array 307 Field lens, aperture plate 308 Field lens 309 Particle beam 311 illuminating particle beam 321 Intermediate image plane 323 beam foci 341 conductive absorber layer 350 cathode 351 Cathode tip 352 emitted particle beam 353 Extractor electrode 354 Anode aperture 355 Opening of the anode aperture 356 Suppressor electrode 357 Front area of ​​the extractor electrode 358 Opening of the extractor electrode 359 Heating device, heating wire 360° opening of the suppressor electrode 361 Reservoir with discharge work reduction agent 362 front area of ​​the cathode 363 etched shaft 364 cathode wire 365 Front facet 366 Apex 367 shaft 370 current meter 380 Current detection aperture plate 381 Opening 382 conductive absorber layer 383 conductive protective layer 384 reverse 385 Front 390 jet nozzle 391 Opening 392 Vacuum chamber 400 beam switch, magnetic arrangement 500 scan deflectors 600 Moving table or positioning device x direction y direction z direction Z particle optical axis Sector B R0...R2 Ring, bowl Columns C1...C6

Claims

[1] Method for operating a multiple particle beam system (1), wherein the multiple particle beam system (1) comprises a beam generating device (300) for generating a first charged particle beam (309), and wherein the beam generating device (300) comprises a cathode (350), an extractor electrode (353) and an anode aperture (354) as well as a heating device (359) for heating the cathode (350), and wherein the cathode (350) comprises a reservoir with an output work reduction agent (361) and a cathode tip (351) with a front facet (365), wherein the multi-particle beam system (1) further comprises a multi-beam generator (305) with a filter plate (304), wherein the filter plate (304) has a plurality of apertures (304) and wherein the filter plate (304) is substantially penetrated by the first charged particle beam (309) to form a plurality of charged single-particle beams (3), and the procedure comprises the following steps: (a) Determining (S2) a reference beam current and / or a reference beam current uniformity; (b) Measuring (S3) a beam current and / or measuring a beam current uniformity between the plurality of charged single-particle beams (3); (c) Comparing (S4) the measured beam current with the reference beam current and, based on this, determining a beam current deviation and / or comparing the measured beam current uniformity with the reference beam current uniformity and, based on this, determining a beam current uniformity deviation; (d) Detecting (S5) a ring collapse process starting at the front facet (365) of the cathode tip (351) based on the beam current deviation and / or based on the beam current uniformity deviation; and (e) Triggering (S6) a material build-up process at the front facet (365) of the cathode tip (351) based on the detection of the initiating ring collapse process. [2] Method according to claim 1, wherein the beam current uniformity between the plurality of charged single-particle beams (3) is measured and the material build-up process is triggered when the beam current uniformity deviation exceeds a predefined limit; and / or where 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 any of the preceding claims, the material build-up process includes the following steps: (f) Reducing (S7) an extractor voltage applied between the cathode tip (351) and the extractor electrode (353) from an initial value UE0 to a reduced value UE1; (g) Temporarily increasing (S8) the temperature of the cathode (350), in particular by increasing a heating current at the heating device (359), from an initial temperature T0 to a higher temperature T1; (h) Increase (S9) the extractor voltage to an increased value UE2, which is higher than the initial value UE0, during a time interval t_Mat; (i) Reduce (S10) the extractor voltage to the initial value UE0. [4] Method according to the preceding claim, wherein the reduction of the extractor voltage according to step (i) is carried out such that the beam current I does not exceed an initial value I0 which is present at the beginning of step (f). [5] Method according to one of claims 3 to 4, wherein the following relationship applies to the extractor voltage UE1: UE1 / UE0 ≤ 60%, in particular UE1 / UE0 ≤ 55% or UE1 / UE0 ≤ 50%. [6] Method according to any one of claims 3 to 5, where, for a temperature difference ΔT = T1 - T0, the following relationship applies: 250°C ≤ ΔT ≤ 400°C, in particular 300°C ≤ ΔT ≤ 350°C; and / or where the following relation applies to the temperature T0: T0 ≥ 1700°C, in particular T0 ≥ 1750°C or T0 ≥ 1800°C. [7] Method according to any one of claims 3 to 6, wherein the temporary increase of the temperature of the cathode (350) takes place 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 one of claims 3 to 7, where the following relationship applies to the extractor voltage UE2: UE2 / UE0 ≥ 150%, in particular UE2 / UE0 ≥ 200% or UE2 / UE0 ≥ 250%; and / or where the following relation applies to the extractor voltage UE2: UE2 ≥ 5000V, in particular UE2 ≥ 6000V or UE2 ≥ 6500 V. [9] Method according to any one 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 one of the preceding claims, wherein the beam current I is measured during the material build-up process. [11] Method according to one of the preceding claims, wherein the beam current uniformity between the plurality of charged single-particle beams (3) is measured and the material build-up process is triggered when the following relation applies to the beam current uniformity deviation ΔUni: ΔUni ≥ 10%, in particular ΔUni ≥ 5% or ΔUni ≥ 1%. [12] Method according to one of the preceding claims, wherein the measurement of the beam current and / or the beam current uniformity is carried out during the ongoing normal operation of the multiple particle beam system (1). [13] Method according to one of the preceding claims, wherein the beam stream uniformity is measured and wherein the measurement of the beam stream uniformity is carried out on the filter plate (304) of the multi-beam generator (305). [14] Method according to the preceding claim, wherein for measuring the beam stream uniformity on the filter plate (304) of the multi-beam generator (305) at least one area arranged between adjacent apertures (304) serves as the detection area. [15] Method according to one of the two preceding claims, wherein the filter plate (304) has a conductive absorber layer (341) and wherein the filter plate (304) is divided into a plurality of electrically isolated sectors (B), each of which is grounded; and wherein measuring the beam current uniformity includes measuring beam currents that each strike a sector (B) of the filter plate (304). [16] Method according to the preceding claim, wherein each of the sectors (B) surrounds at least one of the apertures (304) of the filter plate (304), in particular wherein a plurality of sectors (B) surrounds a group of apertures (304) of the filter plate (304); and wherein each of the apertures (304) of the filter plate (304) is assigned to exactly one of the sectors (B). [17] Method according to any of the preceding claims, wherein a current detection aperture plate (380) is arranged between the beam generating device (300) and the filter plate (304), the rear side (384) of which faces the filter plate (304) and the rear side (384) of which has a conductive absorber layer (382) which is grounded, and where the beam flux and / or beam flux uniformity are measured, and wherein at least one beam current of secondary electrons and / or backscattered electrons incident on the back (384) of the current detection aperture plate (380) is measured. [18] Method according to the preceding claim, wherein the absorber layer (382) of the current detection aperture plate (380) is divided into a plurality of electrically isolated sectors (B) and wherein a beam current is measured in each sector (B). [19] Method according to one of the preceding claims, wherein the beam current is measured and wherein the measurement of the beam current comprises measuring an anode current generated based on charged particles, in particular electrons, striking the anode aperture (354). [20] Method according to the preceding claim, wherein the material build-up process is triggered when the measured anode current I_A falls below a predetermined threshold I_A_ref. [21] Method according to the preceding claim, wherein the following relation applies for triggering the material build-up process: I_A / I_A_ref ≤ 85%, in particular I_A / I_A_ref ≤ 90% or I_A / I_A_ref ≤ 95%. [22] Method according to one of claims 20 to 21, wherein the material build-up process is triggered when a rate of change of the measured anode current I_A exceeds a predetermined threshold I_Avar_ref within a predefined change time interval. [23] Method according to one of the preceding claims, wherein the measurement of the beam current and / or the beam current uniformity is carried out in an object plane (101). [24] Method according to one of the preceding claims, wherein the beam current is measured and wherein the measurement of the beam current is carried out at the level of a beam crossing (108) of the charged single-particle beams (3) with each other. [25] Method according to any one of the preceding claims, wherein the multi-particle beam system (1) is a multi-beam particle microscope (1) that produces a multiplicity of particle-optical single images which can be combined or are combined to form a multi-image, where the beam stream uniformity is measured indirectly based on brightness values ​​of the generated individual images. [26] Computer program product comprising program code for executing the method according to any of the preceding claims. [27] A multi-particle beam system (1) configured to perform the method according to any one of claims 1 to 25. [28] Multi-beam particle microscope (1) comprising the following: A beam generating device (300) configured to generate a first charged particle beam (309) and comprising a cathode (350), an extractor electrode (353) and an anode aperture (354) as well as a heating device (359) for heating the cathode (350), wherein the cathode (350) comprises a reservoir with an output work reduction agent (361) and wherein the cathode (350) has a cathode tip (351) with a front facet (365); a multi-beam generator (305) with a multi-aperture array (306), wherein the multi-aperture array (306) has a filter plate (304) with a plurality of apertures (304) which is penetrated by the first charged particle beam (309) to form a plurality of charged first single-particle beams (3), and wherein the multi-beam generator (305) is configured to generate from the first charged particle beam (309) a first field of a plurality of charged single-particle beams (3); a beam current measuring instrument configured to measure beam current and / or beam current uniformity; a first particle optics with a first particle-optical beam path configured to direct the generated first single-particle beams (3) onto a sample (7) such that the first particle beams (3) hit the sample (7) at impact locations (5) forming a second field; a detection system (200); a second particle optics with a second particle-optical beam path, configured to image second single-particle beams (9) emanating from the impact points (5) in the second field onto the detection system (200); a particle-optical objective lens (102) through which both the first (3) and the second single-particle beams (9) pass; a beam splitter (400) arranged in the first particle-optical beam path between the multi-beam generator (305) and the objective lens (102), and arranged in the second particle-optical beam path between the objective lens (102) and the detection system (200); and a controller (10) configured to control the beam generating device (300), the particle optical objective lens (102), the first particle optics, the second particle optics, and the detection system (200), and wherein the controller (10) is configured to detect a ring collapse process starting at the front facet (365) of the cathode tip (351) of the beam generating device (300) based on the beam current measurement and / or the beam current uniformity measurement, and to control the beam generating device (300) for a material build-up process at the front facet (365) of the cathode tip (351) based on the detection. [29] Multibeam particle microscope (1) according to claim 28, wherein the beam current measuring device is configured to measure a beam current uniformity and wherein the beam current measuring device comprises the filter plate (304), wherein the filter plate (304) has a conductive absorber layer (341) and wherein the filter plate (304) is divided into a plurality of electrically isolated sectors (B) which are each grounded, and wherein in the operation of the multi-beam particle microscope (1) the beam currents striking the respective sectors (B) are determined. [30] Multi-beam particle microscope (1) according to the preceding claim, wherein each of the sectors (B) surrounds at least one of the apertures (304) of the filter plate (304), in particular wherein a plurality of sectors (B) surrounds a group of apertures (304) of the filter plate (304); and wherein each of the apertures (304) of the filter plate (304) is assigned to exactly one of the sectors (B). [31] Multi-beam particle microscope (1) according to any one of claims 28 to 30, wherein the beam current measuring means comprises a current detection aperture plate (380) arranged between the beam generating device (300) and the filter plate (304), the rear side (384) of which faces the filter plate (304) and the rear side (384) of which has a conductive absorber layer (382) which is grounded, and wherein in operation a beam current of secondary electrons and / or backscattered electrons striking the back (384) of the current detection aperture plate (380) is measured. [32] Multi-beam particle microscope (1) according to the preceding claim, wherein the absorber layer (382) of the current detection aperture plate (380) is divided into a plurality of electrically isolated sectors (B) and wherein the beam current measuring device is configured to measure one beam current per sector (B). [33] Multi-beam particle microscope (1) according to one of claims 31 to 32, wherein a conductive protective layer (383) is arranged on a front side (385) of the current detection aperture plate (380), which is grounded and is electrically insulated from the conductive absorber layer (382). [34] Multi-beam particle microscope (1) according to any one of claims 28 to 33, wherein the beam current measuring means comprises the anode aperture (354) and is configured to measure the beam current of charged particles, in particular electrons, striking the anode aperture (354).

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