Method of operating a multi-beam particle microscope with high-speed closed-loop beam current control, computer program product, and multi-beam particle microscope

Abstract

A method for operating a multi-beam particle microscope operating with a plurality of individual charged particle beams, comprising the steps of measuring the beam current, determining a deviation of the measured beam current from a nominal beam current, decomposing the determined deviation into a drift component and a high-frequency component, controlling the high-frequency component of the beam current using a first closed-loop beam current control means and / or compensating for the influence of the high-frequency component on the recording quality of the multi-beam particle microscope using a means different from the closed-loop beam current control means. An electrostatic control lens arranged in the beam generating system between the extractor and the anode can be used as the first closed-loop beam current control means, so that there is no need to adapt the extractor voltage of the beam generating system.

Description

Technical Field

[0001] Generally, the present invention relates to a multi-beam particle microscope that operates using a plurality of individual particle beams. Specifically, the present invention relates to a method of closed-loop beam current control in a multi-beam particle microscope, a related computer program product, and a related multi-beam particle microscope.

Background Art

[0002] With the continuous progress of miniaturized and complex microstructures, such as semiconductor components, there is a need to develop and optimize planar manufacturing technologies for manufacturing microstructures and inspection systems for inspecting micro dimensions. For example, the development and manufacture of semiconductor components require monitoring the design of test wafers, and planar manufacturing technologies require process optimization for reliable manufacturing with high throughput. There is also a recent need for the analysis of semiconductor wafers for reverse engineering and for customer-specific individual configurations of semiconductor components. Therefore, there is a need for inspection means that can be used with high throughput to examine the microstructures on the wafer with high accuracy.

[0003] Typical silicon wafers used in the manufacture of semiconductor components have a maximum diameter of 300 mm. Each wafer has a maximum of 800 mm 2It is subdivided into 30 to 60 repeating regions ("dies") of the size. The semiconductor device includes a plurality of semiconductor structures, which are manufactured by forming layers on the surface of the wafer by planar integration technology. The semiconductor wafer typically has a flat surface due to the manufacturing process. In this case, the structure size of the integrated semiconductor structure ranges from several μm to 5 nm which is the critical dimension (CD), but the structure size will be further miniaturized in the near future. In the future, it is expected that the structure size or the critical dimension (CD) will be less than 3 nm, for example 2 nm, or even less than 1 nm. In the case of the above micro-structure size, defects of the size of the critical dimension need to be quickly identified over a very large area. In some applications, the specification requirements for the measurement accuracy provided by the inspection device are, for example, two or one digit higher. For example, the width of the semiconductor feature needs to be measured with an accuracy less than 1 nm, for example 0.3 nm or even finer, and the relative position of the semiconductor structure needs to be determined with an overlay accuracy less than 1 nm, for example 0.3 nm or even finer.

[0004] The MSEM, i.e., the multi-beam scanning electron microscope, is the result of a relatively new advancement in the field of charged particle systems (charged particle microscopes (CPMs)). For example, multi-beam scanning electron microscopes are disclosed in U.S. Patent No. 7,244,949 B2 and U.S. Patent Application Publication No. 2019 / 0355544. In the case of a multi-beam electron microscope or MSEM, a plurality of individual electron beams arranged in a field of view or raster are simultaneously irradiated onto a sample. For example, 4 to 10,000 individual electron beams can be provided as a primary irradiation, and each individual electron beam is separated from an adjacent individual electron beam by a pitch of 1 to 200 micrometers. For example, an MSEM has about 100 separated individual electron beams ("beamlets"), which are arranged, for example, in a hexagonal raster, and the individual electron beams are separated by a distance of about 10 μm. A plurality of charged individual particle beams (primary beams) are individually focused onto the surface of the sample to be inspected, each time, using a common wide-field optical system, in particular a common objective lens. For example, the sample can be a semiconductor wafer fixed to a wafer holder assembled on a movable stage. During the irradiation of the wafer surface by the charged primary individual particle beams, interaction products, such as secondary electrons or backscattered electrons, are emitted from the surface of the wafer. Each of their starting points corresponds, each time, to a location on the sample where a plurality of primary individual particle beams are focused. The amount and energy of the interaction products depend, in particular, on the material composition and topography of the wafer surface. The interaction products form a plurality of secondary individual particle beams (secondary beams), which are collected by a common objective lens and, as a result of the projection imaging system of the multi-beam inspection system, are incident on a detector arranged on the detection surface. The detector includes a plurality of detection regions each including a plurality of detection pixels, and the detector captures the intensity distribution of each of the secondary individual particle beams. In this process, for example, an image field of 100 μm × 100 μm is obtained.

[0005] Conventional multi-beam electron microscopes include a series of electrostatic and magnetic elements. At least some of the electrostatic and magnetic elements are adjusted to adapt the focal positions and spherical aberrations of multiple charged particle beams. Conventional charged particle multi-beam systems further include at least one crossover plane of the primary or secondary charged particle beam. The conventional system also includes a detection system to facilitate adjustment. Conventional multi-beam particle microscopes include at least one beam deflector ("deflection scanner") for collectively scanning an area of the sample surface using multiple primary individual beams to obtain an image field of the sample surface. Further details regarding multi-beam electron microscopes and methods of operating them are described in the German patent application with application number 102020206739.2 filed on May 28, 2020, the disclosure of which is hereby incorporated by reference in its entirety into this patent application.

[0006] As the requirements for imaging quality increase, the requirements for multi-beam particle microscopes used for imaging also increase. For high-quality recording, stable operating parameters are extremely important. One of these is the beam current intensity of the individual particle beams used to scan the sample surface.

[0007] For a uniform beam current intensity of an individual particle beam, the emission characteristics of the particle beam source, more precisely the uniformity of the emission characteristics over the entire emission angle used, are important. When using a relatively large emission angle, the emission characteristics of the particle source, for example a thermionic field emission (TFE) source, become non-uniform over the whole. Consequently, the irradiance in the first multi-aperture plate in the corresponding particle beam system also becomes non-uniform over the whole, and there is a relatively large variation in the current density in different individual beams. However, in the case of a multi-particle inspection system, it is a system requirement that there is only a slight variation in the current intensity between the various individual beams, typically less than a few percent, and even less than 1 percent, so that all individual image fields of the multi-image field are scanned with an equal number of particles or electrons. For example, this is a prerequisite 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 multiple options for the individual adjustment of the beam current of an individual particle beam. One option in this regard is disclosed in German Patent Application Publication No. 102018007652A1, the disclosure of which is hereby incorporated by reference in its entirety into this patent application.

[0009] The emission characteristics of the particle source also change gradually over time and may exhibit drift behavior. For example, the particle beam originally emitted by the source may change its direction. The use of particle optical components for correcting or compensating for this slow change in direction is known.

[0010] Also, the particle source or the tip may undergo aging degradation, for example, the brightness may decrease. The brightness of the image is further correlated with the brightness or luminance of the source. If the brightness of the source decreases, this also applies to the brightness of the image. One solution to this problem is to increase the gain of the detection system to compensate for the decreased brightness. However, this changes the signal-to-noise ratio (SNR) in the detector, and in adverse situations, this causes a decrease in the above signal-to-noise ratio, resulting in a decrease in the contrast of the obtained image, so this solution has only conditional suitability.

[0011] Therefore, adjustment of the beam generating system itself has been common in the past, and according to the prior art, the voltage applied to the extractor electrode is changed. However, it may take several days after such a change in the extractor current for the newly adjusted beam generating system to again exhibit sufficiently stable emission characteristics, and it is necessary to first "burn in" the tip again.

[0012] In summary, slow changes in the emission characteristics of the source can be compensated by closed-loop control in the prior art, but fast or high-frequency changes cannot be compensated.

[0013] U.S. Patent Application Publication No. 2020 / 0312619 discloses correction of beam parameters in a multi-beam particle microscope based on beam current measurement in a multi-aperture array. In this case, the beam generating system is controlled by adjusting the extractor voltage or the acceleration voltage. Beam migration can also be avoided. However, this is always related to slow correction of the occurring drift.

[0014] "Oxygen-Processed Field Emission Source" by L.H Veneklasen et al., Journal of Applied Physics 43(1972), pp.1600 - 1604 discloses a tip with a unique shape and examines its energy spread and flicker noise.

[0015] German Patent Publication No. 102019008249B3 discloses a particle beam system including a multi-beam deflection device and a beam stop, a method for operating the particle beam system, and a related computer program product. The beam stop is arranged in the first particle optical beam path at the same height as the location where the particle beam diameter is reduced or minimized and can be used to measure the total beam current. This can be embodied as a unique cup.

[0016] German Patent No. 60034559T2 discloses a multi-electron beam lithography apparatus having mutually different beam-limiting apertures.

[0017] European Patent Application No. 2088614A1 discloses a beam current calibration system for a single-beam particle microscope that uses either the standard detector itself for beam current measurement or a detection element additionally provided on or near the detector. Flash cleaning for removing debris from the tip is addressed, and standard compensation for current changes resulting from changing the extractor voltage or suppressor voltage is addressed. SUMMARY OF THE INVENTION

[0018] Accordingly, an object of the present invention is to provide an improved closed-loop current control method in a multi-beam particle microscope. In particular, this method will facilitate fast or high-frequency corrections. In this case, rapid readjustment of the extractor current should be avoided.

[0019] This object is achieved by the subject matter of the independent claims. Advantageous embodiments of the invention will become apparent from the dependent claims.

[0020] This patent application claims the priority of German Patent Application No. 10 2021 118 561.0, filed on July 19, 2021, the entire disclosure of which is incorporated herein by reference.

[0021] According to a first aspect of the invention, the invention is a method of operating a multi-beam particle microscope that operates using a plurality of individual charged particle beams, the method comprising the steps of measuring a beam current, determining a deviation of the measured beam current from a nominal beam current, decomposing the determined deviation into a drift component and a high-frequency component, controlling the high-frequency component of the beam current using first closed-loop beam current control means and / or compensating for the effect of the high-frequency component on the recording quality of the multi-beam particle microscope using means different from the closed-loop beam current control means. Of course, the drift component of the beam current can optionally be controlled using second closed-loop beam current control means.

[0022] The individual charged particle beams can be, for example, electrons, positrons, muons or ions, or other charged particles.

[0023] According to the present invention, a beam current is measured and a deviation of the measured beam current from a nominal beam current is determined. The nominal beam current is known or specified. In this case, this may relate to the overall nominal beam current of all individual particle beams combined, but this may also relate to the respective nominal beam current of each individual particle beam or the nominal beam current of a specific portion of a beam cone. Preferably, the nominal beam current is defined such that the overall beam current (total beam current) does not fall below a minimum value. Also preferably, each individual particle beam does not fall below at least one predetermined value of the beam current. It is also possible to define respective maximum allowable values of the nominal beam current (individual beam current and / or total beam current). The more comprehensively the beam current is measured (individual beam current, specific section of the beam spot, current on a defined area at a defined position and / or total beam current), or the more comprehensive data of the beam current becomes available, the more precisely the determined deviation of the beam current from the nominal beam current can be decomposed into a drift component and a high-frequency component. A method or algorithm essentially known from numerical calculations is used for the decomposition, for example, the relevant program code can be incorporated into the controller of a multi-beam particle microscope.

[0024] Typically, the drift component of the beam current changes continuously over time. There is usually no sudden sharp increase or change in the drift component. In this case, the drift component of the beam current only changes gradually (at a low frequency or quasi-statically) over a relatively long period, for example, over several days, weeks, or even months. For example, if a slow change is seen in the current profile measured over the entire image recording time, with optional interruptions (of course, in this case, high-speed changes may also be additionally seen), for example, during continuous operation time for recording a sample, it can be called drift. Image recording may be interrupted over a relatively long time (for example, during the operation stop of the tip at night or during maintenance work), and the beam current may have a different starting point, for example, the next day after restart, but may again have a slow trend over time.

[0025] In contrast, the high-frequency component of the beam current deviation from the nominal beam current changes relatively quickly, for example, within several minutes or hours during ongoing measurements using a multi-beam particle microscope (for example, during the measurement of a "frame" where secondary electrons for each individual beam are detected over a specific integration time within that range and used as an intensity signal, that is, during image recording). For example, high-speed high-frequency changes may occur during the image recording time for individual recordings (in which case the individual recordings consist of a complete set of multi-beam images), or during the image recording time for a region consisting of multiple individual recordings. High-frequency deviations that intermittently exceed and fall below the nominal value of the beam current often occur due to the high-frequency component. Nevertheless, during the process, the minimum value of the beam current is not reached, and it is preferable that the fluctuations are near the average value. Compared with the drift component, the high-frequency changes in the beam current occur relatively quickly. For example, the high-frequency changes are at least 500 times or 1000 times or even 10000 times faster than the low-frequency changes caused by drift.

[0026] According to the present invention, the high-frequency component of the beam current is controlled using first closed-loop beam current control means, and / or the effect that the high-frequency component has on the recording quality of the multi-beam particle microscope is compensated using means different from the closed-loop beam current control means. However, high-frequency fluctuations in the beam current can be compensated in both cases. In the first case, this can be achieved by the closed-loop beam current control means and true closed-loop control of the high-frequency component of the beam current. In the second case, this can be achieved instead by ensuring the negative effect of the high-frequency beam current fluctuations. There are several implementation options for both cases, which will be detailed below.

[0027] Optionally, the drift component of the beam current can also be controlled using second closed-loop beam current control means. In this case, both the first closed-loop beam current control means and the second closed-loop beam current control means can be provided in one part or in multiple parts. It is preferable that the first closed-loop beam current control means is different from the second closed-loop beam current control means, but the first and second closed-loop beam current control means may be the same. It should be noted in this case that high-speed closed-loop beam current control at high frequencies of the beam current usually requires different technical means from low-speed closed-loop beam current control. In principle, the means previously used in the prior art enable correction of the drift component but do not enable correction of the high-frequency component. Conversely, high-frequency correction may in principle also be performed at a lower speed such that the drift component of the beam current can likewise be corrected or controlled using specific technical means. Within the scope of this patent application, the term "closed-loop control" is used in the conventional sense of control engineering, i.e., the functional principle is the negative feedback of the measured current value to the input of a closed-loop control device, or a controller, or in this case the closed-loop beam current control means for controlling the current value.

[0028] According to a preferred embodiment of the invention, the measurement of the beam current includes the measurement of the overall beam current of the individual particle beam during the image recording procedure. The measurement of the beam current of the individual particle beam has conventionally been carried out, in the case of a multi-beam particle microscope, using, for example, a Faraday cup or an array of Faraday cups, and this measuring device is introduced not into the sample but into the object plane. However, this measurement process is carried out outside the image recording procedure and is relatively slow, often requiring more than 30 minutes for each measurement procedure. However, according to the invention, the beam current will be measured as the overall beam current during the image recording procedure. This means that it is measured during the ongoing recording. For example, during a line jump within the range of image recording, or during an image change (change from one multi-image field to the next multi-image field), the entire individual particle beam can be deflected or blanked using a multi-beam deflection device and directed towards the beam stop by means of the beam current measuring means. For example, such a beam stop in the form of a rotationally symmetric cup can be arranged in the particle optical beam path of the multi-beam particle microscope in front of the objective lens at the same height as the crossover plane. The beam current measuring means can be incorporated or connected therein. Details regarding this can be collected from German Patent No. 102019008249B3, the entire disclosure of which is incorporated herein by reference into this patent application.

[0029] According to a preferred embodiment of the present invention, the measurement of the beam current includes the measurement of the current in the multi-aperture array at a selected position during the image recording procedure. Thus, in this measurement method, it is not necessary to interrupt the image recording procedure. Instead, so to speak, the beam current can be measured in passing. The multi-aperture array is preferably arranged as a first multi-aperture array downstream of a condenser lens system in the particle optical beam path of a multi-beam particle microscope. This multi-aperture array is preferably an array that divides a first charged particle beam into a plurality of individual charged particle beams. In this case, the multi-aperture array is preferably a component of what is called a micro-optical system, which preferably consists of or includes a series of a plurality of multi-aperture plates or multi-aperture arrays. In this context, for good image quality, the first charged particle beam emitted from the particle source or tip needs to be incident on the multi-aperture array uniformly, in particular, as perpendicularly as possible, and it is also necessary to irradiate the multi-aperture array so as to be positioned as uniformly or centrally as possible. In that case, it is possible to ensure that the beam current of the individual particle beams passing through the multi-aperture array is sufficiently uniform in the individual particle beams. Uniform irradiation is achievable not only in the case of telecentric incidence of the first charged particle beam on the multi-aperture array, but also in the case of divergent incidence or convergent incidence, and in any case when the central beam axis is aligned perpendicular to the surface of the multi-aperture array. Here, the apertures in the multi-aperture array are preferably circular, but may have other shapes. Preferably, the apertures in the multi-aperture array have a regular arrangement, for example, a rectangular, square or hexagonal arrangement. In the case of a hexagonal arrangement, preferably 3n(n - 1)+1 apertures are provided, where n is any natural number.

[0030] The implementation of current measurement in a multi-aperture array requires the use of a sensor system, which can have different embodiments. In this case, individual sensors can be assigned to the apertures of the multi-aperture plate or array, but this is not essential. A very ingenious solution is to place the sensor system for measuring the beam current around the aperture of the entire multi-aperture array. By doing so, the formation of individual particle beams is not hindered, and valuable information regarding the beam current can still be obtained. According to a preferred embodiment of the present invention, the multi-aperture array has a grounded metal layer on its upper side that absorbs and discharges excess electrons. One or more individually grounded sensors for measuring the incident electron current at the position of each sensor can be placed above this metal layer. It is also possible to structure the metal layer itself and measure the beam current for each segment or with a spatial resolution based on this structuring. This measurement system can be calibrated, for example, by measuring individual particle beams using a movable stage and, for example, a Faraday cup on it. Other embodiment variations and calibration methods are also conceivable.

[0031] According to a preferred embodiment of the present invention, three sensors for measuring current are arranged on the upper side of the multi-aperture array, particularly in a triangular form, especially an equilateral triangular form, around the outside of the plurality of apertures. The three sensors can be exactly three sensors or at least three sensors. In the case of an ingenious arrangement or an arrangement with a geometric shape, the three sensors are sufficient to enable conclusions to be drawn regarding the beam current and the position of the irradiation beam cone incident on the multi-aperture array.

[0032] According to a preferred embodiment of the present invention, the method further includes determining the radius and / or displacement of the beam cone incident on the multi-aperture array. The optical axis of the incident beam cone corresponds to the midpoint of the multi-aperture array in the case of uniform central irradiation of the multi-aperture array, and the optical axis of the multi-beam particle microscope also usually passes through this midpoint. In this case, the beam current is usually distributed rotationally symmetrically within the beam cone, that is, in the case of a rotationally symmetric arrangement of these sensors centered on the optical axis or the midpoint of the multi-aperture array, the sensors also measure the same beam current, and there is a gradient in the radial direction. On the other hand, when the axis of the incident beam cone is displaced relative to the center of the optical axis or relative to the midpoint of the multi-aperture array, different beam current values are measured by the sensors. This displacement can be determined from the measured beam current values, precisely for both the absolute value and the direction. This displacement can be corrected using appropriate closed-loop beam current control means, and thus the beam current can also be adjusted at individual positions of the multi-aperture array. This correction or closed-loop control can also be implemented at high frequencies, which will be described in detail below.

[0033] According to a preferred embodiment of the present invention, the control of the high-frequency component of the beam current using the first closed-loop beam current control means includes adjusting the irradiation of the multi-aperture array in a high-frequency manner. In this case, the irradiation of the multi-aperture array is preferably centered and / or the beam current itself is adjusted. The beam current is adjusted by expanding or reducing the incident beam cone in a high-frequency manner. For this purpose, there are several exemplary embodiments.

[0034] According to a preferred embodiment of the present invention, the first closed-loop beam current control means includes an electrostatic double deflector in the region of the condenser lens system. In this case, the condenser lens system can include one, two, three or more condenser lenses. The condenser lens can be a magnetic lens, but can also be an electrostatic lens. The electrostatic double deflector in the region of the condenser lens system enables the beam cone generated by the condenser lens system to be offset in parallel, thus enabling adjustment of the irradiation position / center positioning of the multi-aperture array. Different from the conventionally provided magnetic deflector, the electrostatic double deflector can be driven quickly and is suitable for a high-speed feedback loop for closed-loop beam current control in a multi-beam particle microscope.

[0035] According to an additional or alternative embodiment of the present invention, the first closed-loop beam current control means includes an electrostatic condenser lens. The electrostatic condenser lens can be driven more quickly than a magnetic condenser lens and is thus similarly suitable for high-speed feedback and closed-loop beam current control in a multi-beam particle microscope.

[0036] According to a preferred embodiment of the present invention, the first closed-loop beam current control means includes an electrostatic control lens disposed between the extractor electrode of the beam generation system of the multi-beam particle microscope and the anode of the beam generation system. In principle, therefore, the beam generation system comprises further intervening high-speed electrodes. This can be a lens or a simple plate to which a voltage is applied. The electrostatic control lens is preferably biased to facilitate rapid changes in both directions (narrower or wider) of the diameter of the first beam cone incident on the multi-aperture array. The change in the diameter of the beam cone incident on the multi-aperture array caused by the electrostatic control lens is only very slight, but facilitates fine adjustment of the beam current. A further advantage of this embodiment is that there is a change in voltage in the region of the beam generation system, but this change is not carried out at the extractor electrode and it is not necessary to change the acceleration voltage itself. From a technical point of view, a space of several millimeters is obtained between the extractor electrode and the anode, in which an electrostatic control lens or a control electrode can be arranged.

[0037] Therefore, there are a plurality of embodiment variations of the first closed-loop beam current control means, and these embodiment variations can be combined in whole or in part with each other. Other embodiment variations are also possible.

[0038] In addition to, or instead of, the closed-loop beam current control of the high-frequency component of the beam current using the first closed-loop beam current control means, according to the present invention, means different from the closed-loop beam current control means can be used to compensate for the influence of the high-frequency component on the recording quality of the multi-beam particle microscope.

[0039] According to one embodiment of the present invention, a detection system of a multi-beam particle microscope is driven based on a high-frequency deviation of the beam current from the nominal beam current, and a high-frequency adjustment of the gain and / or offset of the detection system is performed based on the high-frequency deviation of the beam current from the nominal beam current. In this way, the brightness and / or contrast of the image obtained using the detection system can be adjusted. In this case, the detection system can be adjusted globally for all channels (entire detection area) or individually for individual channels (individual detection areas) of individual particle beams.

[0040] In this case, the adjustment of the gain and / or offset of the detection system causes an adjustment of the brightness and / or contrast. As a result of this adjustment option, rapid fluctuations in the beam current of all individual particle beams or a single individual particle beam in the secondary path of the multi-beam particle microscope can be compensated. The closed-loop beam current control principle according to this embodiment variation is based on the fact that the influence of beam current changes on the brightness and / or contrast of the image generated using the detection system is known in principle. Knowing this relationship facilitates the corresponding correction or compensation of beam current fluctuations. In this embodiment variation of the present invention, in addition to compensating for the influence in the high-frequency component of the beam current deviation, it is also advantageous to correct the drift of the beam current. By simultaneously controlling the drift of the beam current, it is ensured that the detection system operates within an optimal range and that the noise in the detection system does not increase excessively. Further details regarding the detector adjustment and the relationship between brightness and contrast, and the adjustment of the gain and offset of the detection system can be obtained from German Patent Application Publication No. 102018007455, the entire disclosure of which is incorporated herein by reference.

[0041] According to a further preferred embodiment of the present invention, compensating for the influence of high-frequency components on the recording quality of a multi-beam particle microscope includes adjusting the scanning speed at which a plurality of individual particle beams scan the surface of the sample. By adjusting the scanning speed, each of the first individual particle beams stays at a specific location for a longer or shorter period of time, or scans each pixel for a longer or shorter period of time. In this way, more detectable secondary particles are emitted from the sample surface. Accordingly, the beam current of the second individual particle beam incident on the detection system ultimately varies in this way. This variation is preferably uniform for all individual particle beams. For example, the scanning speed can be adjusted by changing the clock frequency of the scanning device. In this case, typical changes in the clock frequency based on the measured high-frequency deviation of the beam current from the nominal beam current, in particular based on the measured high-frequency deviation of the overall beam current, are in this case up to approximately ±10% of the nominal clock frequency, although ±15% or even ±5% are also conceivable, with the interval boundaries being included in each case. In principle, the scanning speed can be adjusted for each recorded line. However, for example, in the case of an image field change (change between mFOVs), it is also possible to adapt the settings only for a set of lines.

[0042] According to a further preferred embodiment of the present invention, the method further includes a step of logging the measured beam current values. In this case, a log is generated into which the measured beam current values at each point in time are input. The more complete the capture of the values, the more precisely both the drift correction and the high-frequency correction of the beam current can be performed, so that all available beam current values can be collected in this log quite generally.

[0043] According to a further preferred embodiment of the present invention, the method includes estimating the residual service life of the tip of the beam generation system and / or initiating the necessary replacement of the tip. The residual service life of the tip can be estimated algorithmically from the logged beam current values. This is because the investigation by the present inventors has revealed, in principle, how the beam current value progresses over the service life of the tip. The time when the tip needs to be replaced also depends on the number of operating hours or error states during operation, as well as other operating parameters such as the ambient temperature, generated vacuum, voltage fluctuations or changes during the operating time. Therefore, even in the case of the same manufacturing / specification, since the used tips are different from each other, the replacement time also differs for each tip. In the case of a relatively new tip, the beam current in the outer region of the beam cone generated by the tip is slightly larger than that in the inner region. This relationship is derived from the emission behavior of the tip having a specific tip radius, i.e., its geometry. The difference in the distribution of the inner and outer beam currents or the beam current density equalizes as the tip gets older. If an increase in this homogenization is detected, it can be inferred thereby that the replacement of the tip is imminent. Regarding the observed drift, it is also possible to readjust the voltage of the extractor electrode. In this case, in principle, it is necessary to gradually increase the extraction voltage first over the service life. However, just before a failure, the tip supplies a beam current high enough that the extraction voltage needs to be controlled downward. Therefore, this drift reversal can be regarded as a sign indicating that the tip replacement will be necessary in the near future.

[0044] The residual service life of the tip of the beam generation system revealed by the algorithm can be displayed on the multi-beam particle microscope or the residual service life can be displayed on the output unit. Also, for the purpose of tip replacement, it is possible to automatically request maintenance of the multi-beam particle microscope or to automatically order the tip.

[0045] According to a second aspect of the present invention, the present invention relates to a computer program product having program code for implementing the method described above in a plurality of embodiment variations. In this case, the program code can be subdivided into one or more sub-codes.

[0046] According to a third aspect of the present invention, the present invention is a multi-beam particle microscope, comprising a particle source, an extractor electrode, and an anode, configured to generate a first charged particle beam, and further comprising a beam generation system including an electrostatic control lens disposed between the extractor electrode and the anode, a multi-beam generator having a multi-aperture array, configured to generate a first field of a plurality of first individual charged particle beams from the first charged particle beam, first beam current measuring means in the multi-aperture array of the multi-beam generator, a first particle optical unit having a first particle optical beam path configured to direct the generated first individual particle beam toward a sample such that the first individual particle beam hits the sample at an incidence location where a second field is formed, a detection system, a second particle optical unit having a second particle optical beam path configured to image a second individual particle beam emitted from an incidence location within 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 switch disposed between the multi-beam generator and the objective lens in the first particle optical beam path and between the objective lens and the detection system in the second particle optical beam path, and a controller configured to control the beam generation system, the particle optical objective lens, the first particle optical unit, the second particle optical unit, and the detection system. The invention relates to a multi-beam particle microscope, in which a controller is configured for driving an electrostatic control lens, in particular for high-frequency driving, based on current measurement by first beam current measurement means.

[0047] The multi-beam particle microscope according to the invention is suitable for implementing the above-described method of operating a multi-beam particle microscope. In this case, the beam current can be measured using first beam current measurement means in a multi-aperture array of a multi-beam generator. In this case, the first beam current measurement means can include a plurality of sensors, in particular exactly three sensors or more than three sensors, for measuring the current above the multi-aperture array. According to a particularly preferred embodiment of the invention, exactly three sensors are arranged in a triangular form, in particular an equilateral triangular form, around the periphery of the plurality of apertures of the multi-aperture array. As described above in connection with the method, both the beam current itself and the alignment of the first charged particle beam upon incidence on the multi-aperture array can be determined using this arrangement. In this case, the high-frequency driving of the electrostatic control lens controls the beam current, and thus can correct the high-frequency deviation of the beam current from the nominal beam current. According to another embodiment variant, instead of the first beam current measurement means, second beam current measurement means for measuring the overall beam current of individual particle beams for measuring the beam current can be used, and the controller can be configured for driving an electrostatic control lens, in particular for high-frequency driving, based on current measurement using the second beam current measurement means. Examples of the second beam current control means will be described in detail below. The electrostatic condenser lens can be in the form of a simple plate having a central circular aperture to which a voltage is applied. In that case, the lens effect of this plate results from the interaction with the remaining plates or electrodes of the beam generation system. The controller is configured for appropriate feedback.

[0048] According to a preferred embodiment of the invention, the multi-beam particle microscope further In a first particle-optical beam path, a multi-beam deflection device arranged downstream of the multi-beam generator and upstream of the beam switch, and a beam stop having second beam current measuring means, the beam stop being arranged in the first particle-optical beam path upstream of the objective lens and at the same height as the crossover plane, and The controller is configured to temporarily deflect the first individual particle beam collectively using the multi-beam deflection device such that the first individual particle beam substantially impinges on the beam stop and thus does not impinge on the object plane. Such an arrangement of the beam stop and the multi-beam deflection device is already known from German Patent Publication No. 102019008249 B3 incorporated into the present application and cited above. This patent publication discloses in particular a rotationally symmetric cup having a central passage opening for defining a target into which a plurality of individual particle beams are deflected in the case of a line jump or an image jump, and the cup can include or be connected to second beam current measuring means. In this way, the beam current intensity (total beam current) can be determined during the image recording procedure using the configuration presented herein.

[0049] According to a preferred embodiment of the invention, the controller is configured for high-frequency driving of an electrostatic control lens based on current measurement using the first beam current measuring means. However, in addition to or instead of this, it is also possible to drive different particle-optical components or different components of the multi-beam particle microscope based on the beam current measured using the second beam current measuring means in order to improve the recording quality of the entire multi-beam particle microscope.

[0050] According to a preferred embodiment of the present invention, the detection system has a plurality of detection regions that form a third field of view, and a second individual particle beam emitted from the second field of view is imaged in the third field of view. Further, the controller is configured for adjusting the gain and / or offset of the detection system, particularly for high-frequency adjustment, based on beam current measurement using the second beam current measurement means. Therefore, this feedback loop aims to compensate for the influence of high-frequency components on the recording quality. It is possible to set the brightness and / or contrast of an individual image field or the entire multi-image field, that is, the detection regions of the detection system can be driven individually or globally.

[0051] The detection system can include one detector or a plurality of detectors of the same or different types. The detection system can include, for example, one or more particle detectors or be composed of them. The particle detector can further be formed in one part or many parts. However, in the detection system, it is also possible to couple one or more particle detectors and a photodetector to each other or connect them in series.

[0052] According to a preferred embodiment of the present invention, the detection system includes a particle detector and a plurality of photodetectors connected downstream thereof. Specifically, the particle detector can include a scintillator plate having a plurality of detection regions. In this case, the projection of the interaction product onto the detection regions of the particle detector is performed using an appropriate particle optical unit. In this case, the optical signal emitted by the particle detector passes in an appropriate manner to the photodetectors (detection channels) assigned to the respective detection regions of the particle detector. For example, the light emitted by the detection regions of the particle detector can be coupled into an optical fiber via a corresponding optical unit, and this fiber is further connected to an actual photodetector. The photodetector includes, for example, a photomultiplier tube, a photodiode, an avalanche photodiode, or other types of appropriate photodetectors.

[0053] According to another embodiment variant of the detection system, the detection system comprises a particle detector but not a photodetector. In this case, it is possible to detect particles directly, without the detour via photons, for example by the particles being injected into the depletion layer of a semiconductor, which can again initiate an electron avalanche. This requires correspondingly constructed semiconductor detectors comprising at least one independent conversion unit per beam.

[0054] In principle, the gain of a detection system defines the amount of output generated by the amount of input. Specifically, gain indicates the number of particles of a second particle species (outgoing) in the output relative to the number of particles of a first particle species (incoming) in the input. In the case of an avalanche photodiode, the input is formed by photons and the output is electrons. The same can be defined for all other detectors, such as DED ("direct electron detection"), PMT ("photomultiplier tube"), etc. Also, many detectors that perform conversion to an electrical signal (current or voltage drop across an output resistor) have a post-amplifier, the gain of which can be adjusted as well.

[0055] The offset also indicates how high the output signal level would be if no primary particles were to arrive. This is typically eliminated in downstream electronics using a voltage adder. Ultimately, leakage currents, etc. are therefore compensated for using these systems. Offset and gain are generally not independent of each other.

[0056] As mentioned above, when there is a high frequency readjustment of the detection system in order to compensate for effects due to high frequency deviations from the nominal beam current, it is advantageous to simultaneously correct for beam current drift, thereby ensuring that the dynamic range over which the detection system operates remains optimal and that there is no excessive increase in noise.

[0057] According to a further preferred embodiment of the present invention, the multi-beam particle microscope further includes a collective scan deflector configured to collectively deflect the first individual particle beams and scan them collectively across the sample surface. In this case, the controller is configured for driving the scan deflector and for adjusting the scan speed of the collective scan deflector based on current measurements using the first and / or second beam current measurement means. Thus, in this embodiment variant, it is not the beam current of the first individual particle beam itself that is controlled, but instead there is a correction of the overall current on the sample incident on the individual pixels. A lower beam current can be compensated by a lower scan speed, and a higher beam current can be compensated by a higher scan speed. This applies at least within a certain range. The deviation from the nominal scan speed is preferably ±12%, ±10% or ±5% or less. In this case, the scan speed is the same for all individual particle beams. In particular, a high-frequency adjustment of the scan speed can be performed during the process. For example, it is possible to adjust the scan speed substantially instantaneously for each collectively scanned multi-field of view (mFOV) or even for each line. However, it is also possible to keep the scan speed constant for the recording of multiple multi-fields of view.

[0058] According to a further preferred embodiment of the present invention, the multi-beam particle microscope further includes a condenser lens system disposed between the beam generation system and the multi-beam generator. In this case, the controller is configured for driving the condenser lens system based on current measurement using the first and / or second beam current measuring means. For example, the condenser lens system includes exactly two magnetic lenses or at least two magnetic lenses. Depending on which of these lenses is excited or how strongly each of these lenses is excited, it is possible to set the irradiation of the multi-aperture array. Also, the beam current intensity is adjusted. In the case of a condenser lens system with magnetic lenses, the driving of the condenser lens system is usually only changeable at low speed due to self-induction that occurs and opposes the applied voltage and due to the hysteresis effect that occurs. Therefore, the condenser lens system is preferably driven quasi-statically or in a low-frequency manner. However, the condenser lens system may also have one or more electrostatic lenses. The electrostatic lens can be driven more quickly, and the condenser lens system is, in principle, also suitable for high-frequency beam current adaptation or irradiation adaptation.

[0059] According to a preferred embodiment of the present invention, the multi-beam particle microscope further includes, in particular, an electrostatic double deflector within the region of the condenser lens system. In this case, the controller is configured, in particular, for high-frequency driving of the double deflector based on current measurement using the first and / or second beam current measurement means. Using the double deflector, it is possible to perform a parallel offset of the beam cone of the first charged particle beam in such a way that the first charged particle is directed towards the multi-aperture array of the multi-beam generator centered. In particular, beam current measurement using the first beam current measurement means in the multi-aperture array makes it possible to determine whether the beam cone of the first charged particle beam is offset relative to the central axis of the multi-aperture array or relative to the optical axis of the entire system. This offset can be corrected. This correction is instantaneous and thus possible at high frequencies, but in principle, it is also possible to drive the double deflector quasi-statically in order to obtain a sufficiently accurate adjustment of the condenser lens system or the entire system, or to perform drift correction.

[0060] According to a further preferred embodiment of the present invention, the controller is configured for low-frequency driving of the extractor electrode based on current measurement using the first and / or second beam current measurement means. Therefore, this is not related to high-speed control but is related to drift correction. It has been found from experience that after the driving of the extractor electrode changes, it takes a certain amount of time, for example, about 2 to 3 days, until the particle source or the tip of the beam generation system adapts to the new situation and "burns in".

[0061] The above-described embodiment variations according to the first to third aspects of the present invention can be combined with each other in whole or in part as long as no technical contradiction results.

[0062] In this context, the present invention will be better understood with reference to the accompanying drawings.

Brief Description of the Drawings

[0063]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0064] FIG. 1 is a schematic view of a particle beam system 1 in the form of a multi-beam particle microscope 1 that uses a plurality of particle beams. The particle beam system 1 generates a plurality of particle beams that strike an object to be inspected in order to generate interaction products, such as secondary electrons, that are emitted from the object and then detected. The particle beam system 1 is of a scanning electron microscope (SEM) type that uses a plurality of primary particle beams 3 that are incident on the surface of an object 7 at a plurality of positions 5 and generate a plurality of electron beam spots, or spots, that are spatially separated from each other there. The object 7 to be inspected can be of any desired type, such as a semiconductor wafer or a biological sample, and can include an arrangement such as microelements. The surface of the object 7 is disposed on a first surface 101 (objective surface) of an objective lens 102 of an objective lens system 100.

[0065] An enlarged excerpt I1 of FIG. 1 shows a plan view of the objective surface 101 having a square field of view 103 of the incident position 5 formed on the first surface 101. In FIG. 1, the number of incident positions is 25 that form a 5×5 field of view 103. The number 25 of incident positions is a number selected for a simplified figure. In practice, the number of beams, and thus the number of incident positions, can be selected to be a much larger number, such as 20×30, 100×100, etc.

[0066] In the illustrated embodiment, the field of view 103 of the incident position 5 is substantially square with a constant pitch P1 between adjacent incident positions. Examples of values of the pitch P1 are 1 micrometer, 10 micrometers, and 40 micrometers. However, the field of view 103 can also have other symmetric shapes, such as hexagonal symmetry.

[0067] The diameter of the beam spot formed within the first surface 101 can be made small. Examples of values of this diameter are 1 nanometer, 5 nanometers, 10 nanometers, 100 nanometers, and 200 nanometers. The focusing of the particle beam 3 to form the beam spot 5 is performed by the objective lens system 100.

[0068] The primary particles hitting the object generate interaction products, such as secondary electrons, backscattered electrons, or primary particles whose movement is reversed for other reasons, from the surface of the object 7 or from the first surface 101. The interaction products emitted from the surface of the object 7 are shaped by the objective lens 102 to form a secondary particle beam 9. The particle beam system 1 provides a particle beam path 11 for guiding a plurality of secondary particle beams 9 to the detector system 200. The detector system 200 includes a particle optical unit having a projection lens 205 for directing the secondary particle beam 9 towards the particle multi-detector 209.

[0069] In the extract view I2 of FIG. 1, a plan view of the plane 211 of the individual detection regions of the particle multi-detector 209 where the secondary particle beam 9 is incident at the position 213 is shown. The incident position 213 is within the field of view 217 having a regular pitch P2 from each other. Examples of the value of the pitch P2 are 10 micrometers, 100 micrometers, and 200 micrometers.

[0070] The primary particle beam 3 is generated by a beam generator 300 including at least one particle source 301 (e.g., an electron source), at least one collimation lens 303, a multi-aperture array 305, and a field lens 307. The particle source 301 generates a divergent particle beam 309, which is collimated or at least substantially collimated by the collimation lens 303 to form a beam 311 that irradiates the multi-aperture array 305.

[0071] FIG. 1 shows an extract view I3 of a multi-aperture array 305. The multi-aperture array 305 includes a multi-aperture plate 313 having a plurality of openings or apertures 315 formed therein. The midpoints 317 of the openings 315 are disposed within a field of view 319 that is imaged onto the field of view 103 formed by the beam spot 5 on the object plane 101. The pitch P3 between the midpoints 318 of the apertures 315 can have examples of values of 5 micrometers, 100 micrometers, and 200 micrometers. The diameter D of the aperture 315 is smaller than the pitch P3 between the midpoints of the apertures. Examples of values of the diameter D are 0.2×P3, 0.4×P3, and 0.8×P3.

[0072] Particles of the irradiation particle beam 311 pass through the apertures 315 to form the particle beam 3. Particles of the irradiation beam 311 that hit the plate 313 are absorbed by the plate 313 and do not contribute to the formation of the particle beam 3.

[0073] Due to the applied electrostatic field, the multi-aperture array 305 focuses each of the particle beams 3 so that a beam focus 323 is formed within the plane 325. Alternatively, the beam focus 323 can be virtual. The diameter of the beam focus 323 can be, for example, 10 nanometers, 100 nanometers, and 1 micrometer.

[0074] The field lens 307 and the objective lens 102 provide a first imaging particle optical unit for imaging the plane 325 where the beam focus 323 is formed onto the first plane 101 such that the incident position 5 or the field of view 103 of the beam spot occurs there. When the surface of the object 7 is disposed on the first plane, the beam spot is correspondingly formed on the objective surface.

[0075] The objective lens 102 and the projection lens array 205 provide a second imaging particle optical unit for imaging the first surface 101 onto the detection surface 211. Thus, the objective lens 102 is a lens that is part of both the first and second particle optical units, while the field lens 307 belongs only to the first particle optical unit and the projection lens 205 belongs only to the second particle optical unit.

[0076] A beam switch 400 is arranged between the multi-aperture array 305 and the objective lens system 100 in the beam path of the first particle optical unit. The beam switch 400 is also part of a second optical unit in the beam path between the objective lens system 100 and the detector system 200.

[0077] Further information regarding such multi-beam particle beam systems and components used therein, such as particle sources, multi-aperture plates and lenses, can be obtained from international patent applications WO2005 / 024881A2, WO2007 / 028595A2, WO2007 / 028596A1, WO2011 / 124352A1 as well as WO2007 / 060017A2, and German patent application publications No. 102013016113 and No. 102013014976, the disclosures of which are incorporated herein by reference in their entireties.

[0078] FIG. 2 shows a schematic view of the adjustment of the beam cone of the irradiation beam 311 when incident on the multi-aperture array 313. By adjusting the beam cone, the beam current for each individual particle beam 3 can be adjusted. First, a particle or a divergent particle beam 309 is emitted by a source 301. The divergent particle beam 309 passes through a collimation lens system or a condenser lens system 303 including two condenser lenses 303.1 and 303.2 in this embodiment. FIG. 2 shows two different settings of the condenser lens system 303 here. In the first setting, the condenser lens 303.1 is activated and the condenser lens 303.2 is deactivated. As a result, the particles of the divergent particle beam 309 are collimated at the condenser lens 303.1 and hit the multi-aperture array 313 as an irradiation particle beam 311.1 having a diameter d1. In the second case, the condenser lens 303.1 is deactivated and the condenser lens 303.2 is activated. Thus, the divergent particle beam 309 is further spread and collimated only at the second condenser lens 303.1 so that an irradiation particle beam 311.2 having a diameter d2 is incident on the multi-aperture plate 313. The number of particles incident on the multi-aperture array 313 is the same in both cases, but the density is different. Therefore, when the multi-aperture array 313 having an aperture 315 (not shown) is traversed, individual particle beams 3 having different beam current intensities depending on the diameter of the irradiation spot are formed.

[0079] In the illustrated embodiment, the condenser lenses 303.1 and 303.2 are magnetic lenses in each case. However, it is also possible to replace one or both of the magnetic lenses with electrostatic condenser lenses. Further, it is possible to change the number of condenser lenses in the entire condenser lens system 303, that is, to provide only one lens or to provide three or more lenses. Also, one or more deflectors can be provided for adjusting the irradiation beam 311. These adjustment means and the type of condenser lens affect how quickly the irradiation spot can be adjusted. This will be described in detail below within the scope of this patent application. First, what is illustrated here is how different beam currents of the individual particle beams occur when different irradiation spots are used.

[0080] FIG. 3 shows a schematic view of an irradiation spot having current density fluctuations upon incidence on the multi-aperture array 313. First, FIG. 3a shows the current density in the cross-section of the irradiation particle beam 311. The beam current density values are slightly different within this cross-section. This variation is substantially caused by the geometry of the particle source 301 or the tip 340 of the particle source 301. In this case, the outer region 351 of the beam 311 in the illustrated embodiment has a higher current density than the more inwardly located regions 352, 353, and 354. The current density is lowest in the central region 354. The different currents or current densities are indicated by different patterns / shadings in FIG. 3a, with darker shadings indicating more current. Of course, the transition from the outside to the inside can be made continuous, and the distinct concentric rings in FIG. 3a are used only for clearly showing the principle.

[0081] Figure 3b shows the beam 311 upon incidence on the multi-aperture plate 313. In this embodiment, the multi-aperture plate 313 has 19 openings 315 arranged in a hexagonal shape in the illustrated embodiment. This hexagonal arrangement is further clarified by a hexagon 316 drawn using dashed lines. Thus, the particles of the particle beam 311 pass through the various openings 315, and individual particle beams 3 are formed. The remaining particles of the particle beam 311 are incident on the plate 313. The plate 313 is usually grounded so that the incident charge is discharged. This can also be utilized for measuring current (see below).

[0082] From Figure 3b, it is clear that some of the openings 315 allow more charged particles or a higher current to pass through than the other openings. The opening 315.1 is located in the outer region, and thus relatively more charged particles pass through, and the individual particle beam formed has a slightly higher current density. The opening 315.2 is arranged slightly closer to the center of the beam cone 311, and the particle current passing through it is slightly lower than in the case of the opening 315.1. The opening 315.3 is located at the center, and a beam current lower than all other beam currents passes through it.

[0083] Figure 4 shows a schematic diagram of the beam current intensities of a plurality of individual particle beams 3 arranged in a hexagonal shape. A field is assigned to each of the 19 individual particle beams 3 within the beam current intensity diagram 360, and the fields are numbered in the example of the figure. In this case, the figure in Figure 4 does not show the image field of view, but only shows the diagram of the measured beam current intensities. These current intensities can be measured by conventional means, such as by a Faraday cup, arranged on a movable sample stage (stage). Such conventional measurements take a relatively long time, about 30 minutes for each individual measurement. The various gray scale values of the pattern filling in Figure 4 also indicate various beam current intensities in this case, and darker fillings represent higher beam current intensities than lighter fillings.

[0084] As a rule, the beam current intensity of each individual particle beam 3 is well known or can be measured very accurately. For recording with a multi-beam particle microscope, it is necessary to ensure that each individual particle beam 3 supplies a sufficient beam current, that is, the beam current must not fall below a certain limit. For example, for each individual particle beam, it is possible to require an individual beam current of at least 500 pA or 600 pA, for example at least 560 pA, at least 570 pA or at least 580 pA. It is also possible to define the nominal beam current as an allowable range. As a rule, this requires a degree of uniformity of the individual beam currents in addition to the minimum beam current or in addition to the average beam current. In this case, the difference between the maximum beam current value and the minimum beam current value may have only a certain maximum value. For example, over the entire range, that is, the difference, can be 10 pA or less. The degree of uniformity can also be specified in the form of a percentage value. For example, the degree of uniformity can be defined as degree of uniformity [%]=(maximum value - minimum value) / average value×0.5×100. Other definitions are possible and appropriate.

[0085] It is also possible to measure the total beam current. For this purpose, it is not necessary to measure the individual particle beams 3 individually. For example, when all individual particle beams are blanked at the same position / on the same detector, it is also possible to determine the total beam current for all individual particle beams simultaneously (see the description regarding FIG. 9).

[0086] According to a preferred embodiment of the present invention, the multi-aperture array 313 has, on its upper side, a grounded metal layer that absorbs and discharges excess electrons. One or more individually grounded sensors 370 that measure the incident electron current at the position of each sensor can be arranged above this metal layer. It is also possible to structure the metal layer itself and measure the beam current for each section or with a spatial resolution based on this structuring. This measurement system can be calibrated based on, for example, an individual particle beam 3 measured using a movable stage and, for example, a Faraday cup thereon. Other embodiment variations and calibration methods are also conceivable.

[0087] FIG. 5 shows a schematic diagram of the first beam current measuring means 370 above the multi-aperture array 313, and the multi-aperture array 313 is irradiated by beam cones 311 having different diameters. In the illustrated embodiment, the multi-aperture array 313 has a total of 91 holes for generating 91 individual particle beams 3. Also in this case, the arrangement of the apertures 315 is hexagonal. The first beam current measuring means 370 is arranged around the outer periphery of the hexagonal arrangement of the apertures 315, and the first beam current measuring means 370 has a figure consisting of three parts in the example of the figure. The first beam current measuring means 370 includes three sensors 370.1, 370.2, and 370.3 above the multi-aperture array 313. The three sensors 370.1, 370.2, and 370.3 are arranged in the form of an equilateral triangle around the outer periphery of the plurality of apertures. This geometric arrangement makes it possible to obtain a lot of information about the spread and position of the irradiated particle beam 311 incident on the multi-aperture array 313 while using a small number of sensors. In the example shown in FIG. 5, the beam cone of the irradiated particle beam 311 hits the multi-aperture plate 313, and the midpoint M of the irradiation spot is the same as the midpoint P of the multi-aperture array 313 where the aperture 1 is located. In the case of incidence positioned at the center of the irradiated particle beam 311 on the multi-aperture plate 313, if it is an appropriately symmetric and equally spaced arrangement, the same intensity of current hits all the sensors 370.1, 370.2, and 370.3. For example, if the characteristics or gradients of the current intensity are known throughout the beam cone as a result of calibration, the radius r1 or diameter d1 of the irradiation spot of the irradiated particle beam 311 can be determined from the measured current intensity. FIGS. 5a and 5b have different irradiation spot sizes, and the irradiation spot is larger in FIG. 5b and has a radius r2 instead of just the radius r1 as in FIG. 5a. The current intensities measured by the sensors 370.1, 370.2, and 370.3 according to FIG. 5b are the same for each of the sensors, but the absolute value is lower than in the case of FIG. 5a.

[0088] FIG. 6b shows an incidence that, unlike FIG. 5, is not positioned at the center of the irradiation particle beam 311 to the multi-aperture array 313. FIG. 6b rather shows an incidence deviated from the center or a positional deviation V at the time of incidence. In FIG. 6a, the midpoint M of the beam spot and the midpoint P of the multi-aperture array 313 spatially coincide. In FIG. 6b, the midpoint M of the beam spot, where the radius r1 remains unchanged compared to FIG. 6a, is displaced relative to the midpoint P of the multi-aperture array 313. This positional deviation V is also depicted in FIG. 6b in the same manner. Therefore, the sensors 370.1, 370.2, and 370.3 will measure different current intensities. In this case, the respective deviations from each other of the measured values form a characteristic pattern that enables an estimation regarding the positional deviation based on the previously performed calibration.

[0089] As described above regarding how different beam currents are generated and regarding different types of beam measurements, the focus will now shift to the correction of beam current fluctuations. In this context, FIG. 7 shows a beam generation system 301 having additional electrostatic elements for correction. In the illustrated embodiment, the beam generation system 301 is constructed as follows. The beam generation system 301 includes a tip portion 340 surrounded laterally in a cylindrical surface shape by a suppressor electrode 341, and the suppressor electrode 341 serves to suppress the emission of electrons laterally from the tip portion. For example, the tip portion 340 can be a thermionic emitter operated at a heating current intensity of several amperes. A voltage of several hundred volts with respect to the tip portion 340 is applied to the suppressor 341. A voltage of several kilovolts with respect to the tip portion 340 is applied to an extractor electrode 342 disposed at a distance from the tip portion 340. In this case, the distance between the tip portion 340 and the extractor 342 is typically several hundred micrometers, for example 200 μm or 400 μm. The anode 343 is typically disposed 1 centimeter directly below the tip portion 340. The acceleration potential between the tip portion 340 and the anode 343 is several tens of kilovolts, for example 25 kV, 30 kV or 35 kV. So far, the beam generation system 301 corresponds to a known beam generation system. However, the arrangement of the electrostatic control electrode 344 between the extractor 342 and the anode 343 is novel. There is a space of several millimeters, for example 6 mm, 8 mm or 10 mm between these two elements, which is sufficient to spatially arrange an additional electrode between the extractor 342 and the anode 343. In the simplest case, the electrostatic control lens 344 can be provided as a simple plate to which an appropriate voltage is applied. Then, a lens effect appears in conjunction with the adjacent electrostatic field.

[0090] In the embodiment shown in FIG. 7, only the condenser lens 303.1 of the condenser lens system 303 is excited, and the second condenser lens 203.2 is stopped, but this may be different. In the illustrated embodiment, the irradiation particle beam 311 hits the multi-aperture array 313 telecentrically. This enables high-speed or high-frequency driving of the electrostatic control lens 344, and as a result, there is a slight variation in the diameter of the beam spot when it enters the multi-aperture array 313. FIG. 7 shows two different beam paths of the divergent particle beam 309 or the irradiation particle beam 311. In the former case, the irradiation spot formed when entering the multi-aperture array 313 has a diameter d1, while in the latter case, it has a diameter d2 and is thus expanded over a slightly larger range.

[0091] In this case, the principle of changing the diameter of the irradiation spot is the same as that already detailed in the context of FIGS. 2 to 6, but the means used to change the size of the irradiation spot are different. In this case, an electrostatic high-speed control lens 344 is used instead of the conventional means. In this case, the electrostatic control lens 344 is preferably biased positively or negatively in order to achieve a particularly high-speed voltage change. For example, it can be the potential between the potential of the anode 343 and the potential of the extractor 342, including the upper and lower limits of the range. A specific potential can be selected based on the geometry of the electrodes.

[0092] For example, when measurement is performed above the multi-aperture array 313 using, for example, a first beam current measuring means such as the sensor system according to FIGS. 5 and 6 of the beam current here, in order to excite the electrostatic control lens 344 for closed-loop beam current control, that is, to apply a specific voltage to the electrostatic control lens 344, an appropriate control signal can be generated using the controller 10 of the multi-beam particle microscope 1 based on this measurement. This feedback loop is extremely fast, and in this way, constant irradiation of the micro-optical system or the multi-aperture array 313 can be obtained during the operation of the multi-beam particle microscope 1.

[0093] In particular, the high-frequency component of the beam current deviation during the image recording procedure using the multi-beam particle microscope 1 can be controlled using the electrostatic control lens 344. In this case, high-frequency closed-loop control is implemented in the beam generation system 301, but it is not necessary to adapt the entire acceleration voltage or change the extractor voltage. As a result, unlike the case of changing the extractor voltage, a substantially constant operation of the tip 340 is ensured without the need for re-burn-in.

[0094] FIG. 8 shows a further design option for the closed-loop beam current control means. FIG. 8 shows the radiation of the divergent particle beam 309 that moves along the optical axis 105 and is generated using the beam generation system 301. This radiation passes through the condenser lens system 33 having the first condenser lens 303.1 and the second condenser lens 303.2. In the illustrated embodiment, each condenser lens is a magnetic lens. An electrostatic double deflector having components 345 and 346 is disposed in the region of the condenser lens system 303. In the illustrated embodiment, with respect to the particle optical beam path, component 345 is downstream of the first condenser lens 303.1 and component 346 is downstream of the second condenser lens 303.2. However, other arrangements of the double deflector in the region of the condenser lens system 303 are possible. For example, both components 345 and 346 can be disposed downstream of the second condenser lens 303.2 with respect to the particle optical beam path.

[0095] The beam 311 can be offset in parallel using the double deflector. When incident on the multi-aperture array 313, the beam 311 is offset relative to the optical axis 105 by the vector V. In this case, the electrostatic double deflectors 345 and 346 can be driven rapidly and are suitable for high-frequency correction of the offset when the multi-aperture array 313 is irradiated. Further, the double deflectors 345 and 346 can be driven based on the current value measured using the first beam current measuring means, for example, measured using the sensor 370 on the surface of the multi-aperture plate 313. This feedback loop can also be used for high-speed closed-loop current control during the image recording procedure.

[0096] Also, one of the condenser lenses 303 can be formed as an electrostatic condenser lens 303. This electrostatic condenser lens 303 can also be driven rapidly and quasi-instantaneously in order to vary the diameter d of the irradiation spot upon incidence on the multi-aperture plate 313 as a result. Also in this case, the drive can be implemented in the form of a feedback loop based on current measurement, which is determined, for example, using the sensor 370 above the multi-aperture array 313.

[0097] FIG. 9 schematically shows a multi-beam particle microscope 1 having a closed-loop beam current control means and a compensator driven using a controller 10. In this case, the controller 10 can be formed in one part or in a number of parts, and the entire multi-beam particle microscope 1 can in principle be controlled using the controller 10. In particular, the controller 10 controls the beam generation system 301, the components of the first particle optical unit, the second particle optical unit and the detection system 200, and further components of the multi-beam particle microscope 1 which may or may not be explicitly shown. In the schematic view of FIG. 9, only the most important control elements and aspects in the context of the present invention are represented by connection lines to the selected particle optical components. In particular, the multi-beam particle microscope 1 is suitable for implementing the method according to the present invention for operating the multi-beam particle microscope 1 in the described embodiment variations.

[0098] First, the beam current is measured using various beam current measuring means, and the measured value is transmitted to the controller 10. In the illustrated embodiment, the first beam current measuring means is arranged above the micro-optical system, particularly above the multi-aperture array 313. In this case, this can be one or more sensors 370, for example, as shown in FIGS. 5 and 6. Further, in the illustrated embodiment, the total beam current is measured using a sensor arranged on or assigned to the beam stop 111. In this case, the individual particle beam 3 is steered in the first particle optical beam path upstream of the objective lens 102 to the beam stop 111 arranged at the same height as the crossover plane using the multi-beam deflector 390. In particular, the controller 10 can be configured to direct the first individual particle beam 3 into the beam stop 111 during a line jump or an image jump when scanning the sample surface. Thus, the total beam current can be measured during the image recording procedure. The measured beam current value is further transmitted to the controller 10. From the measured beam current value, the deviation from a predefined nominal beam current is determined. This deviation is decomposed into a drift component and a high-frequency component using an algorithm implemented in the controller 10. Thus, both the drift component of the beam current and the high-frequency component of the beam current can be controlled or compensated using the closed-loop beam current control means.

[0099] The components of the multi-beam particle microscope 1 are driven in a manner known per se for static setting of the beam current or for drift correction. This includes adjustment of the extractor voltage in the beam generation system 301 and driving of the condenser lens system 303. The deflector 304, additionally illustrated in FIG. 9, serves to statically adjust the irradiation beam 311 upon incidence on the micro-optical system 306. However, the multi-beam particle microscope 1 includes further components and control elements for high-frequency driving for the purpose of controlling the beam current.

[0100] An electrostatic control electrode 344 (not shown in FIG. 9) driven by the controller 10 using a feedback loop is provided as a component of the beam generation system 301. In addition to or instead of this, the condenser lens of the condenser lens system 303 can be designed as a high-speed electrostatic condenser lens and can be driven equally rapidly. As a result, the diameter of the beam incident on the micro-optical system 306 can be corrected rapidly.

[0101] For the high-speed correction of the lateral offset of the irradiation spot, one or more electrostatic deflectors, in particular an electrostatic double deflector as shown, for example, in FIG. 8, may be provided additionally or alternatively in the condenser lens system 303. These deflectors can likewise be driven using a feedback signal based on the measured current value.

[0102] In addition to or instead of the high-speed closed-loop beam current control as described above, a further control loop is implemented in the multi-beam particle microscope 1 shown in FIG. 9. For example, the scanning speed of the scanning deflector 110 can be adapted based on the measured beam current. For example, it is possible to deviate from the nominal scanning speed by up to approximately 10% or up to approximately 5% or up to approximately 1% based on the beam current value measured at that time. When the scanning speed is increased, the beam current incident on a specific region / pixel on the sample decreases, while when the scanning speed is decreased, the beam current incident thereon increases.

[0103] In addition to or instead of the above, it is also possible to drive the detection system 200 based on the measured beam current, and the controller 10 is used for the high-frequency adjustment of the gain and / or offset of the detection system 200. In this case, the detection system 200 can be adjusted globally for all detection regions or channels, or individually for the individual channels / detection regions of the individual secondary particle beams 9. For further details, reference is made to the description (summary part and drawing part) shown above in connection with the description of the present invention.

[0104] FIG. 10 schematically shows a flowchart of a method according to the present invention for operating the multi-beam particle microscope 1 also in this case. In method step S1, the beam current is measured. In this case, preferably as the beam current, for example, it can be determined for a predetermined region or an individual particle beam 3 on the surface of the multi-aperture array 313, for example, using a common beam stop 111 including a sensor system, and / or it can be determined collectively for all individual particle beams 3, which is the beam current at that time.

[0105] In a further method step S2, the deviation of the measured beam current from the nominal beam current is determined. For example, the nominal beam current can be the minimum beam current, but an interval of allowable beam current may also be provided, and it is also possible that various beam current uniformities are required when defining the nominal beam current.

[0106] In a further method step S3, the determined deviation is decomposed into a drift component and a high-frequency component. The drift component of the beam current usually changes continuously over time, and thus over a relatively long period, for example, over several days, weeks or even months. In contrast, the high-frequency component changes relatively rapidly within seconds, minutes or hours, for example, during ongoing measurements / image recordings using the multi-beam particle microscope 1. The high-frequency changes in the beam current occur relatively rapidly compared to the drift component, for example, at least 500 times or 1000 times or even 10000 times faster than the low-frequency changes caused by drift.

[0107] In a further method step S4, the high-frequency component of the beam current is controlled using the first closed-loop beam current control means. A plurality of embodiments of such high-frequency closed-loop control have already been described in connection with FIG. 9. For example, the electrostatic control lens 344 of the beam generation system 301 can be used as a closed-loop beam current control element. High-speed driving of the electrostatic condenser lens 303 is also possible. Using a high-speed electrostatic deflector in the condenser lens system 303, especially an electrostatic double deflector, the lateral offset can be compensated almost instantaneously.

[0108] Also, in method step S7, optionally, the drift component of the beam current may be controlled using second closed-loop beam current control means. The second closed-loop beam current control means can be the same as the first closed-loop beam current control means, but it is preferably not the same as the first closed-loop beam current control means. For example, the magnetic condenser lens can be driven quasi-statically to compensate for the drift.

[0109] In addition to, or instead of, the closed-loop beam current control, and in particular in addition to, or instead of, the high-frequency beam current correction, further means can be taken to correct the influence of the high-frequency beam current change on the recording quality of the multi-beam particle microscope 1.

[0110] According to method step S5, the detection system 200 of the multi-beam particle microscope 1 is driven based on the deviation of the beam current from the nominal beam current, and the high-frequency adjustment of the gain and / or offset of the detection system 200 is performed based on the high-frequency deviation of the beam current from the nominal beam current. In this case, the detection system 205 can be adjusted globally for all detection channels or individually for the individual detection channels of the secondary individual particle beam 9.

[0111] In addition to, or instead of, the above, in method step S6, the scanning speed at which the plurality of individual particle beams 3 scan the surface of the sample 7 can be adjusted. The beam current per pixel on the sample 7 can be decreased in the feedback loop by increasing the scanning speed, while the beam current can be increased by decreasing the scanning speed.

[0112] The method described in FIG. 10 is preferably a computer-implemented method and can thus be incorporated into the computer or controller 10 of the multi-beam particle microscope 1.

Description of the reference numerals

[0113] 1 Multi-beam particle microscope 3 Primary particle beam (individual particle beam) 5 Beam spot, incident position 7 Object 9 Secondary particle beam 10 Computer system, controller 11 Secondary particle beam path 13 Primary particle beam path 25 Sample surface, wafer surface 100 Objective lens system 101 Object plane 102 Objective lens 103 Field of view 105 Optical axis of multi-beam particle microscope 108 Crossover 110 Batch scanning deflector 111 Beam stop with second current measuring means 200 Detector system 205 Projection lens 207 Detection region 208 Deflector for adjustment 209 Particle multi-detector 211 Detection plane 212 Crossover 213 Incident position 214 Aperture filter 215 Detection region 216 Active element 217 Field of view 218 Deflector system 220 Multi-aperture corrector, individual deflector array 222 Batch deflection system, anti-scan 300 Beam generator 301 Particle source, beam generation system 303 Collimation lens system 304 Deflector 305 Multi-aperture array 306 Micro-optical system 307 Field lens 308 Field lens 309 Diverging particle beam 311 Irradiating particle beam 313 Multi-aperture plate, multi-aperture array 315 Aperture of multi-aperture plate 316 Hexagon 317 Midpoint of aperture 319 Field of view 323 Beam focus 325 Intermediate image plane 326 Field-of-view lens system 340 Tip 341 Suppressor 342 Extractor electrode 343 Anode 344 Electrostatic control electrode 345 Deflector 346 Deflector 351 Region 352 Region 353 Region 354 Region 360 Beam current intensity diagram 370 First beam current measuring means 390 Multi-beam deflector 400 Beam switch 420 Magnetic element 500 Sample stage 503 Voltage source for sample d1 Beam cone diameter d2 Beam cone diameter r Beam cone radius M Beam spot midpoint P Midpoint of multi-aperture plate, midpoint of multi-aperture array V Displacement between beam cone midpoint and midpoint of multi-aperture array S1 Measure beam current S2 Determine deviation of beam current S3 Decompose into drift component and high-frequency component S4 Control high-frequency component S5 Adjust detector S6 Adjust scanning speed Control the S7 drift component

Claims

1. A multi-beam particle microscope, comprising: a beam generation system including a particle source, an extractor electrode, and an anode, configured to generate a first charged particle beam, and further including an electrostatic control lens disposed between the extractor electrode and the anode; a multi-beam generator having a multi-aperture array and configured to generate a first field of view of a plurality of first individual charged particle beams from the first charged particle beam; first beam current measuring means in the multi-aperture array of the multi-beam generator; a first particle optical unit having a first particle optical beam path configured to direct the first individual particle beam toward a sample such that the generated first individual particle beam hits the sample at an incident position forming a second field of view; a detection system; a second particle optical unit having a second particle optical beam path configured to image a second individual particle beam emitted from the incident position within the second field of view onto the detection system; a particle optical objective lens through which both the first and second individual particle beams pass; a beam switch disposed in the first particle optical beam path between the multi-beam generator and the objective lens and in the second particle optical beam path between the objective lens and the detection system; a controller configured to control the beam generation system, the particle optical objective lens, the first particle optical unit, the second particle optical unit, and the detection system; wherein the controller is configured for driving the electrostatic control lens, in particular for high-frequency driving, based on current measurement by the first beam current measuring means; a multi-beam particle microscope.

2. a multi-beam deflection device disposed downstream of the multi-beam generator and upstream of the beam switch in the first particle optical beam path; a beam stop having second beam current measuring means, the beam stop being disposed upstream of the objective lens in the first particle optical beam path at the same height as the crossover plane. The controller is configured to collectively and temporarily deflect the first individual particle beam using the multi-beam deflection device so that the first individual particle beam substantially impinges on the beam stop and thus does not impinge on the object plane. The multi-beam particle microscope according to claim 1.

3. The controller is configured for driving the electrostatic control lens, particularly high-frequency driving, based on the current measurement by the first beam current measuring means. The multi-beam particle microscope according to claim 2.

4. A multi-beam particle microscope, A beam generation system including a particle source, an extractor electrode, and an anode and configured to generate a first charged particle beam, further including an electrostatic control lens disposed between the extractor electrode and the anode. A multi-beam generator having a multi-aperture array and configured to generate a first field of view of a plurality of first individual charged particle beams from the first charged particle beam. Second beam current measuring means configured to measure the total beam current of the individual particle beams. A first particle optical unit having a first particle optical beam path configured to direct the first individual particle beam toward the sample such that the generated first individual particle beam hits the sample at an incident position forming a second field of view. A detection system, A second particle optical unit having a second particle optical beam path configured to image a second individual particle beam emitted from the incident position within the second field of view onto the detection system. A particle optical objective lens through which both the first and second individual particle beams pass. A beam switch disposed in the first particle optical beam path between the multi-beam generator and the objective lens and disposed in the second particle optical beam path between the objective lens and the detection system. Including a controller configured to control the beam generation system, the particle optical objective lens, the first particle optical unit, the second particle optical unit, and the detection system. The controller is configured for driving the electrostatic control lens, particularly high-frequency driving, based on the current measurement by the second beam current measuring means. A multi-beam particle microscope.

5. The detection system has a plurality of detection regions that form a third field of view, and the second individual particle beam emitted from the second field of view is imaged on the third field of view. The controller is configured for adjusting the gain and / or offset of the detection system, particularly for high-frequency adjustment, based on current measurement using the first and / or second beam current measurement means, of the multi-beam particle microscope according to claim 1 or 4. **Claim 6** The multi-beam particle microscope according to claim 5, wherein the detection regions of the detection system are individually driven. **Claim 7** The multi-beam particle microscope according to claim 5, wherein the detection regions of the detection system are driven globally. **Claim 8** Further comprising a batch scanning deflector configured to collectively deflect the first individual particle beams and collectively scan them on the sample surface. The controller is configured to drive the scanning deflector and to adjust the scanning speed of the batch scanning deflector based on current measurement using the first and / or second beam current measurement means, of the multi-beam particle microscope according to claim 1 or 4. **Claim 9** Further comprising a condenser lens system disposed between the beam generation system and the multi-beam generator. The controller is configured to drive the condenser lens system based on current measurement by the first and / or second beam current measurement means, of the multi-beam particle microscope according to claim 1 or 4. **Claim 10** Particularly, further comprising an electrostatic double deflector in the region of the condenser lens system. The controller is configured to drive the double deflector, particularly for high-frequency driving, based on beam current measurement by the first and / or second beam current measurement means, of the multi-beam particle microscope according to claim 9. **Claim 11** The controller is configured to perform low-frequency driving of the extractor electrode based on current measurement by the first and / or second beam current measurement means, of the multi-beam particle microscope according to claim 1 or 4.

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