Calibration of a multi-beam particle raster scanning microscope

Abstract

The application relates to a method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 4, with selecting, from the M particle beams, at least one particle beam to be calibrated, determining beam specific calibration data for a plurality of the M particle beams, and using the beam specific calibration data determined for the plurality of the M particle beams for calibrating said at least one particle beam to be calibrated.

Description

[0001] Description

[0002] Calibration of a multi-beam particle raster scanning microscope

[0003] TECHNICAL FIELD

[0004] Various examples of the disclosure generally pertain to calibrating multi-beam raster scanning charged-particle microscopes.

[0005] BACKGROUND

[0006] In scanning electron microscopes (SEMs) or other types of charged-particle raster scanning microscopes (e.g., helium ion microscopes), the imaging time is largely defined by the time it takes to scan the sample with the electron beam. To increase the signal-to-noise ratio (SNR), it is possible to increase the so-called dwell time - the time spent by the particle beam in one pixel. Larger dwell times lead to larger imaging times and improved SNR.

[0007] Recently, multi-beam SEMs (MSEMs) have been employed to provide large-scale composite images of samples. Here, multiple sequences of images are acquired contemporaneously for multiple FOVs that are arranged in a spatial pattern, to thereby define a composite FOV. The images of each sequence are combined, using frame averaging. The respective aggregate images thereby obtained are stitched to form a composite image.

[0008] In metrology applications, SEMs are used to measure critical dimensions, CD, of sample structures. By way of example widths of lines are measured and compared to their design values. For the metrology applications single beam SEMs may be used, however also multibeam SEMs are used in metrology applications to increase throughput.

[0009] When determining critical dimensions, the result should of course be constant over time and match across all beams and even across several microscope systems. However, it has been observed that the determined CD values vary across the beams, across different systems and over time.

[0010] SEMs are furthermore used to detect defects on semiconductor samples, but in Multi-beam SEMs the defect detection rate may vary from beam to beam.Accordingly a need exists to overcome the drawbacks above and reduce the variations of the results obtained with Multi-beam microscopes, such as the variation in the determination of critical dimensions in sample structures or the variation in the detected defects.

[0011] SUMMARY

[0012] This need is met by the features of the independent claims. Further aspects are described in the dependent claims.

[0013] According to a first aspect a method for calibrating a multi-beam particle beam raster scanning microscope comprising M particle beams is provided with M being larger than 2 . The method comprises selecting, from the M particle beams, at least one particle beam to be calibrated and the step of determining for a plurality of the M particle beams beam specific calibration data and to use the beam specific calibration data determined for the plurality of the M particle beams for calibrating the at least one particle beam.

[0014] In this embodiment the calibration data for other beams is used for calibrating the at least one beam to be calibrated. Accordingly, the method does not use, for calibrating said at least one beam, only data generated with this one beam to be calibrated, but data generated from the other beams of the microscope are used for calibrating the beam to be calibrated. The data used from the other beams can include different data such as a mean critical dimension determined with the other beams, a spot size determined for each of the plurality of beams, an anisotropy of the beam shape, a defect sensitivity determined for the plurality of beams.

[0015] According to a further aspect a method for calibrating a multi-beam particular raster scanning microscope comprising M particle beams is provided wherein the method comprises the step of determining with each of the M beams, a plurality of beam specific images of a calibration sample having a known dimension parameter. Furthermore, for each of the M beam specific images, a beam specific dimension parameter is determined for the calibration sample. For each of the M beams, an offset between the beam specific dimension parameter and the known dimension parameter of the calibration sample is determined. For a sample of interest, for each of the M beams, a beam specific dimension parameter is determined for the sample of interest and then the beam specific dimension parameter for the sample of interest can be corrected for each of the M beams based on the offset determined for the corresponding beam.In this embodiment, for each beam several beam specific images are generated which are then used to determine a dimension parameter of the calibration sample. As the real dimension parameter of the calibration sample is known, it is possible to determine a beam specific offset. When the dimension parameter is determined for a sample of interest, this dimension parameter can then be corrected based on the offset.

[0016] According to a further aspect a method for calibrating the multi-beam particle raster scanning microscope with M particle beams is provided wherein the method comprises the steps of determining, for each of the beams, a relation how at least one operating parameter of the multi-beam particle raster scanning microscope influences a precision how precise a dimension parameter of the calibration sample investigated with the microscope can be determined. Furthermore, a dimension parameter of a sample of interest is determined based on at least one image obtained with the microscope while monitoring the operating parameter. The determined dimension parameter of the sample of interest can then be corrected to a corrected dimension parameter based on the monitored at least one operating parameter and the determined relation.

[0017] This method is a more indirect calibration where the main factors (operating parameters of the microscope) influencing the determination of the dimension parameter are monitored and where it is known how these operating parameters influence the determined dimension parameter. When a dimension parameter such as a critical dimension is determined for a sample of interest and the operating parameter is monitored it is possible to correct the determined dimension parameter based on the monitored operating parameter and the determined relation. The operating parameters can include parameters such as a beam tilt, a spot size or a lateral position of the beam relative to an aperture plane etc..

[0018] According to a further aspect a method for calibrating the multi-beam particle raster scanning microscope with M particle beams is provided the method comprising the steps of determining, for each of M beams a relation how a charging effect of a calibration sample investigated with the multi-beam particle raster scanning microscope influences a precision how precise a dimension parameter of a calibration sample investigated with the multi-beam particle raster scanning microscope can be determined. Furthermore, a dimension parameter of a sample of interest is determined based on at least one image obtained with the multi-beam particle raster scanning microscope while determining the charging effect of the sample of interest.

[0019] Furthermore, the corresponding multi beam raster scanning microscope is provided.It is to be understood that the features mentioned above and those yet to be explained below may be used not only in the respective combinations indicated but also in other combinations or in isolation without departing from the scope of the present invention. The determined dimension parameter of the sample of interest is corrected to a corrected dimension parameter based on the determined charging and determined relation

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Fig. 1 schematically illustrates a multi-beam microscope incorporating features of the invention.

[0022] Fig. 2 shows a schematic view of how a variation of a critical dimension evolves over time using a calibration.

[0023] Fig. 3 shows an average of critical dimension in a sample obtained with multiple beams with and without calibration corresponding to the situation shown in Fig. 2.

[0024] Fig. 4 shows an improved stability of a determined critical dimension determined for the different beams with and without calibration.

[0025] Fig. 5 schematically shows how charging effects influence the determination of a measured critical dimension value.

[0026] Fig. 6 shows critical dimension values determined for different charging effects and an influence on the line width.

[0027] Fig. 7 schematically shows how an isotropy is a copy of beam shapes can influence measured critical dimension values.

[0028] Fig. 8 shows an example of an anisotropy of a beam shape.

[0029] Fig. 9 shows different designs of calibration samples including different structures provided in a calibration sample.

[0030] Fig. 10 shows a schematic representation how a beam tilt influences a determination of a critical parameter.

[0031] Fig. 11 schematically shows how a defect sensitivity including a capture rate and false positive rate depends on a contrast-to-noise ratio of the sample and the size of the defect.Fig.12 shows how a resolution of the beam influences a defect sensitivity with capture rate and false positive rate.

[0032] Fig. 13 shows how a defect sensitivity can be controlled over time.

[0033] Fig. 14 shows a variation of the capture rate between different beams.

[0034] Fig. 15 shows how a variation between different beams of the capture rate can be kept within a certain threshold.

[0035] Fig. 16 schematically shows how an aperture plane of the multi-beam microscope can be imaged.

[0036] Fig. 17 to 19 schematically show how a charging of a sample can be determined based on images generated from the aperture.

[0037] DETAILED DESCRIPTION

[0038] In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only.

[0039] The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection.

[0040] Functional blocks may be implemented in hardware, firmware, software, or a combination thereof.

[0041] Hereinafter, techniques of operating and controlling a multi-beam charged particle raster scanning microscope will be disclosed. Specifically, it will be discussed in more detail how multi beam microscopes can be calibrated effectively and how variations in parameters determined by the microscope by different beams can be reduced. The parametersdetermined by the microscope can include parameters such as a critical dimension, CD, a defect sensitivity including capture rates or false positive rates.

[0042] FIG. 1 is a schematic illustration of a multi-beam particle raster scanning microscope, here based on electrons, so that a multi-beam Scanning electron microscope, MSEM 1, is provided. Further information relating to such MSEMs and components used therein, such as, for instance, particle sources, multi-aperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881, WO 2007 / 028595, WO 2007 / 028596, WO 2011 / 124352 and WO 2007 / 060017 and the German patent applications having the publication numbers DE 102013016113 A1 and DE 102013014 976 A1, the disclosure of which in the full scope thereof is incorporated by reference in the present application.

[0043] The MSEM 1 uses a plurality of charged particle beams (also referred to as beamlet) for imaging a sample 7. The MSEM 1 generates a plurality of M primary beamlets 3.1, 3.2, 3.3 which strike the sample 7 to generate interaction products, e.g., secondary electrons, which emanate from the sample 7, form secondary beamlets 9.1, 9.2, 9.3, and are subsequently detected.

[0044] Each one of the primary and secondary beamlets 3.1 , 3.2, 3.3, 9.1 , 9.2, 9.3 is formed and guided by a respective imaging subsystem of the MSEM 1. Each imaging subsystem is associated with a respective FOV. Images acquired by a respective imaging subsystem depict the respective FOV. The multiple FOVs are arranged in a spatial pattern to thereby define a composite FOV.

[0045] The primary beamlets 3.1, 3.2, 3.3 are formed by electrons which are incident on a surface of the sample 7 at a plurality of locations and generate a plurality of primary electron beam focus spots 5.1 5.2, 5.3 that are spatially separated from one another. The sample 7 to be examined can be of any desired type, e.g., a semiconductor wafer or a semiconductor mask, and can comprise an arrangement of miniaturized elements. The surface of the sample 7 is arranged in a sample plane 101 of an objective lens system 102 of a first particle optical unit 100 (also referred to as illumination system). A diameter of the minimal beam spots or focus spots 5.1 5.2, 5.3 shaped in the sample plane 101 can be small. Exemplary values of this diameter are below four nanometers, for example three nm or less. The focusing of the primary beamlets 3.1 , 3.2, 3.3 for shaping the focus spots 5.1 5.2, 5.3 is carried out by the objective lens system 102. In this case, the objective lens system 102 can comprise a magnetic immersion lens. Further examples of focusing means are described in the German patent DE 102020125534 B3, the entire content of which is herewith incorporated in the disclosure.The number M of primary beamlets 3.1, 3.2 and 3.3 (i.e., the number of FOVs) may be five, 25, 90 to 100, or more (for sake of simplicity, only three primary beamlets 3.1, 3.2 and 3.3 with corresponding focus points 5.1, 5.2 and 5.3 are shown in FIG 1).

[0046] In practice, the number of beamlets M, and hence the number of FOVs, can be chosen to be significantly greater, such as, for example, M = 10x 10, M = 20 x 30 or M = 100 x 100. Exemplary values of the pitch between the incidence locations and FOVs are 1 micrometer, 10 micrometers, or more, for example 40 micrometers.

[0047] The number of primary and secondary beamlets M defines the number of FOVs. Each imaging subsystem has a respective FOV. The respective FOV is defined by scanning the respective pair of primary and secondary beamlets (e.g., beamlets 3.1 and 9.1) over the sample 7 in the respective FOV.

[0048] The primary beamlets 3.1, 3.2, 3.3 striking the sample 7 generate interaction products, e.g., secondary electrons, back-scattered electrons, which emanate from the surface of the sample 7, or primary particles that have experienced a reversal of movement for other reasons. The interaction products emanating from the surface of the sample 7 are shaped by the objective lens system 102 to form the secondary beamlets 9.1, 9.2, 9.3. Secondary electrons included in the secondary beamlets 9.1, 9.2, 9.3 are used for imaging.

[0049] The MSEM 1 provides a detection beam path for guiding the plurality of secondary beamlets 9.1, 9.2, 9.3 to a secondary electron imaging system 200. The secondary electron imaging system 200 includes several electron-optical lenses 205.1 to 205.5 for directing the secondary beamlets 9.1, 9.2, 9.3 towards a spatially resolving detector system 600.

[0050] The imaging with the secondary electron imaging system 200 is strongly magnifying such that both the pattern of the primary beamlets on the wafer surface and the size and shape of focal points of the primary beamlets are imaged in much magnified fashion. By way of example, a scale factor I magnification is between 100x and 300x such that one nm on the wafer surface is imaged enlarged to between 100 nm and 300 nm. In an example, an image field of a multi-beam device with for example 100 pm diameter is enlarged to approximately 30 mm.

[0051] The primary beamlets 3.1, 3.2, 3.3 are generated in a beam generation apparatus 300 comprising at least one particle source 301 (e.g., an electron source), at least one collimation lens 303, a multi-aperture arrangement 305 and a first field lens 331 and a second field lens 333. The particle source 301 generates at least one diverging particle beam 309, which is at least substantially collimated by the at least one collimation lens 303, and which illuminates the multi-aperture arrangement 305. The multi-aperture arrangement 305 includes an aperture plate 304 (also referred to as filter plate or multihole aperture plate), which has a plurality of M openings formed therein in a first rasterarrangement. Particles of the illuminating particle beam 309 pass through the M apertures or openings of the first aperture plate 304 and form the plurality M of primary beamlets 3.1, 3.2, 3.3. Particles of the illuminating particle beam 309 which strike the first aperture plate 304 are absorbed by the latter and do not contribute to the formation of the primary beamlets 3.1, 3.2, 3.3. A multi-aperture arrangement 305 usually has at least a further multi-aperture plate 306, for example a lens array, a stigmator array, or an array of deflection elements.

[0052] Together with the field lens 331 and a second field lens 333, the multi-aperture arrangement 305 focuses each of the primary beamlets 3.1 , 3.2, 3.3 in such a way that focal points are formed in an intermediate image surface 321. Alternatively, the beam foci and the intermediate image surface 321 can be virtual. The intermediate image surface 321 can be curved to pre-compensate a field curvature of the imaging system arranged downstream of the intermediate image surface 321.

[0053] The at least one field lens 103 and the objective lens system 102 provide a first imaging particle optical unit for imaging the surface 321 , in which the beam foci are formed, onto the sample plane 101 such that a second pattern of focus spots 5.1, 5.2, 5.3 of the primary beamlets 3.1, 3.2, 3.3 is formed there. Typically, the surface 25 of the sample 7 is arranged in the sample plane 101, and the focal spots 5,1 5.2, 5.3 are correspondingly formed on the object surface 25. The plurality of primary beamlets 3.1, 3.2, 3.3 form a crossover point 108, in the vicinity of which a first deflection scanner 110 is arranged. The first deflection scanner 110 is used to deflect the plurality of primary beamlets 3.1 , 3.2, 3.3 collectively and synchronously such that the plurality of focus spots 5.1, 5.2, 5.3 are moved contemporaneously over the surface 25 of the sample 7. Raster scanning is implemented, thereby imaging the sample 7. The first deflection scanner 110 is driven by a scanning control unit 860 such that in an inspection mode of operation, a plurality of two-dimensional image data of the surface is acquired. Additionally, the MSEM 1 can include further static deflectors configured to adjust the position of the plurality of the primary beamlets 3.1, 3.2, 3.3.

[0054] The objective lens system 102 and the projection lenses 205 provide a secondary electron imaging system 200 for imaging the sample plane 101 onto an imaging plane 225. The objective lens system 102 is thus a lens or a lens system that is part of both the first and the second particle optical unit, while the field lenses 103, 331 and 333 belong only to the first particle optical unit 100, and the projection lenses 205 belongs only to the secondary electron imaging system 200.

[0055] A beam divider 400 is arranged in the beam path of the first particle optical unit 100 between the field lens 103 and the objective lens system 102. The beam divider 400 isalso part of the second optical unit in the beam path between the objective lens system 102 and the projection lenses 205.

[0056] The first deflection scanner 110 is arranged in a primary electron beam path or in a joint electron beam path. In the example shown in FIG. 1, the secondary beamlets 9.1, 9.2, 9.3 transmit during use the first deflection scanner 110 in opposite direction and the scanning movement of the secondary beamlets 9.1, 9.2, 9.3 is partially compensated. The secondary electrons have typically a different kinetic energy compared to the primary electrons. Therefore, the scanning movement of the moving irradiation positions is only partially compensated. To fully compensate the scanning movement of the secondary beamlets 9.1, 9.2, 9.3, the collective beam deflector 222 is arranged in the secondary electron beam path.

[0057] The secondary electron imaging system 200 includes the second, collective beam deflector 222 which is arranged in the vicinity of a crossover point of the secondary beamlets 9.1, 9.2, 9.3. The second, collective beam deflector 222 is operated synchronously with the first deflection scanner 110 and compensates during use a beam deflection of the secondary beamlets 9.1, 9.2, 9.3 such that centers 15 of the beamlets 9 remain at constant position on the imaging plane 225. Thereby, each secondary beamlet 9 is kept within the area of a set of detection elements, which is assigned to the individual secondary beamlet 9.

[0058] The secondary electron imaging system 200 includes electron-optical lenses 205.1 to 205.5 to adjust a focus plane of the secondary beamlets 9.1 , 9.2, 9.3. A defocus can be applied. The electron-optical lenses 205.1 to 205.5 can thus implement corrective elements to correct the focus plane. The electron-optical lenses 205.1 to 205.5 are shown as magneto-optical elements but are not limited to magneto-optical elements and can comprise also electro-static lens elements or stigmators. With the electron-optical lenses 205.1 to 205.5, the secondary beamlets 9.1, 9.2, 9.3 can be focused into the imaging plane 225 of the secondary electron imaging system 200.

[0059] The secondary electron imaging system 200 can include a plurality of further corrective elements, for example at least one of a multi-aperture array element, a deflector or an exchangeable aperture stop. Together with the objective lens system 102, the lenses serve to focus the secondary beamlets 9.1, 9.2, 9.3 on the spatially resolving detector system 600 and, in the process, allow to correct or compensate the magnification and rotation of the pattern of the secondary beamlets 9.1, 9.2, 9.3 in the imaging plane 225. Thereby, the pattern of the plurality of secondary beamlets 9.1, 9.2, 9.3 can stabilized. For example, a first and second magnetic lenses 205.4 and 205.5 (as further examples of corrective elements) are designed in reversed order to one another and have oppositely directed magnetic fields. A Larmor rotation of the secondary beamlets 9.1, 9.2, 9.3 can becompensated by suitably applying control signals to (driving) the magnetic lenses 205.4 and 205.5. The secondary electron imaging system 200 - in the illustrated example -includes further corrective elements, specifically a multi-aperture plate 216.

[0060] The MSEM 1 furthermore is associated with a processing device 800 configured both for controlling the individual particle optical components of the multiple particle beam system and for evaluating and analyzing the signals obtained by the detector system 600. The processing device 800 can be separated from the MSEM 1 or can be part of the MSEM 1. For example, the processing device 800 can be configured to acquire pairs of test images and then evaluate the test images to determine values of one or more imaging parameters. In this case, the control or processing device 800 can be constructed from a plurality of individual electronic computers or electronic components. By way of example, the processing device 800 includes a control processor 880, a control module 840 for the control of the electro-optical elements of the secondary electron imaging system 200, and a control module 830 for the control of the electro-optical elements of the primary beamlet generation unit. The processing device 800 is further connected to a control module 503 for supplying a voltage to the sample 7, said voltage also being referred to as extraction voltage. Thereby, during use, an extraction field is generated between the objective lens system 102 and the surface 25 of the sample 7. During use, the extraction field decelerates the primary charged particles of the primary beamlets 3.1, 3.2, 3.3 before the object surface 25 is reached and generates an additional focusing effect on the plurality of primary beamlets 3.1, 3.2, 3.3. At the same time, the extraction field serves during use to accelerate the secondary particles out of the surface 25 of the sample 7.

[0061] Further, the processing device 800 includes the scanning control unit 860 for the raster scanning.

[0062] The detector system 600 includes a plurality of sets of detection elements with one set of detection elements for each secondary beamlet 9. During use, each set of detection elements is configured to record the intensity signal of the assigned secondary beamlet 9. The plurality of intensity signals for the plurality of secondary beamlets 9.1 , 9.2, 9.3 is transferred to the image data acquisition unit 810, where the image data is processed and stored in memory 890. Accordingly, multiple images are acquired, one for each imaging subsystem. These multiple images (or an aggregated image determined based on images of respective sequences) can be combined to a composite image having a composite FOV.

[0063] In the following different calibration options for the microscope 1 discussed in figure 1 are explained in more detail. Especially when the microscope is used in metrology applications, the determined information such as the critical dimension, CD, should be constant overtime and match across all beams and even across several SEMs (i.e. tools). In the followingdifferent methods are discussed to reduce the variation of parameters obtained with different beams. The effect can be reduced with calibration, numerical postprocessing of the determined critical dimensions and / or by postprocessing the obtained images. For the improvement of the beam-to-beam matching and the tool-to-tool matching and for a better long-term stability and repeatability of the obtained values it is possible to install a calibration sample next to a sample of interest wherein the calibration sample can be installed on wafer stage as close as possible to the sample of interest. The calibration sample preferably comprises patterns such as patterns discussed in more detail in connection with Fig. 10 below and is made of material that is not prone to charging or contamination effects. Furthermore, it should be made sure that no tilt is provided relative to the sample of interest and that the lines have a known width, meaning known CD values such as 16 nm or 10 nm.

[0064] The workflow for the direct calibration can now include the following steps:

[0065] -In the following it is assumed that the microscope has M beams and for each of the M beams P images of the calibration sample are captured and the CD values for the calibration sample are determined for the different beams. The CD values CDcalib are determined as follows:

[0066] ■ P images of calibration sample are captured for all M beams, measured CD-values CDcalibJJ with i=1 , ... ,M, j=1 ,... ,P are determined. For each beam the offset to the design CD value is determined (e.g. via CDoffsetJ = mean_j(CDcalib_i,j) - CDdesign or CDoffsetJ = medianJ(CDcalibJJ) - CDdesign) for i=1,... ,M)

[0067] ■ Now for the following N images captured on the sample of interest, the determined offset can be used to correct the measured CD values CDmeasuredJ.k (i=1 ,... ,M, k=1.... ,N) by this offset: CDmeasuredCorrectedJ.k = CDmeasuredJ.k - CDoffsetJ

[0068] ■ Repeat the calibration procedure

[0069] The best values of P and N depend on the stability / noise of the SEM: increasing P helps to obtain a higher confidence on the calibration values. How high N can be chosen without the need for recalibration depends on the stability / noise of the tool: If the tool is prone to drifts (e.g., of the beams / of the stage), recalibration has to be performed more often, i.e., N has to be chosen accordingly small / the time interval between recalibration has to be chosen accordingly small.A rough estimate on the expected beam-to-beam (B2B)-Matching of the corrected measured CD values can be estimated given the standard deviation of repeated CD measurements oCD(for 91 beams): B2BMatching « 5 • oCD■

[0070]

[0071] Calibration can also be performed on several calibration structures, e.g., on lines & spaces with varying width or sidewall angle as discussed in connection with Fig. 10. This way, one obtains several calibration offsets CDoffset_i,l for each beam i=1,... ,M.

[0072] Depending on the measured structure on the sample of interest (i.e., depending on its design width / sidewall angle / ...), an according CDoffset-value can be chosen (e.g., by interpolation of the CDoffset_i,l).

[0073] Fig. 2 shows the difference of the beam-to-beam matching where graph 21 shows the noncalibrated difference of the CD value in nanometers wherein graph 22 shows the calibrated value using the above discussed calibration. In the example shown a calibration sample with a 60 nm line width was acquired and from the images of the different beams the line width was determined 5 times a day and without calibration the determined values differed by 2 nm or more whereas in the calibrated graph the difference could be reduced to 0.5 nm for the determined line width.

[0074] Fig. 3 shows the average CD value as determined by the different beams in the image wherein the values cover the range from 61-63 nm. Each tile shown in the figure corresponds to one image beam. The right side of Fig. 3 shows that the average CD values vary within a range of 0.5 nm.

[0075] Fig. 4 shows how the CD stability improved after calibration. In Fig. 4 the same calibration sample was sampled over 7 days and for each beam the range of CD values is indicated. As shown, over the different beams, without the calibration the CD range is lower than 1.5 nm but also higher than 1.20 nm for some of the beams, wherein this range is reduced to below 0.5 nm for most of the beams after the calibration. The situation discussed above describes a direct calibration approach of the CD values.

[0076] In the following a more indirect calibration of the CD values is discussed. Here the calibration could be performed as follows: It is possible to determine operating parameters of the microscope 1 which influence the precision how precise the dimensional parameters such as the CD values can be determined. These operating parameters can include parameters such as a beam tilt relative to the sample, a spot size of the beam, a focus position of the beam, a lateral position or a shape of the beam in a contrast aperture plane.These values can be determined for the microscope in regular intervals such as every N images on dedicated calibration samples where these parameters can be determined or directly from the acquired images of the sample. Furthermore, the sensitivity of the CD values for a sample of interest on these operating parameters can be determined for each imaging condition of the SEM. By way of example the change of the measured CD value per degree of beam tilt can be determined for each landing energy. This can be done either by measurement or by simulations using 3D models of the sample and simulation of the electron sample interaction via Monte Carlo methods. The sensitivity of the CD values is determined for the calibration sample so that the relation can be determined how a degree of beam tilt influences the precision of the determined CD value or how the spot size or the focus position influences the determined CD value. Then it is possible to determine a measurement such as a CD measurement in a sample of interest, and the operating parameters such as beam tilt etc. discussed above are monitored and as the influence of these parameters on the CD value has been determined in advance, it is possible to consider this operating parameter induced change of the CD measurement in the structure or sample of interest. During the CD measurement of the sample of interest one can look up the values of the key parameters of the microscope. Multiplication of these values with the sensitivities determined in advance yields the necessary CD offsets by which the measured CD values have to be corrected. By way of example, it is possible to determine in advance a correlation between beam tilt and CD error (such as 1 degree of beam tilt leads to an CD error of 0.1 nm), and from the monitored beam tilt one can deduce the CD error.

[0077] In the following a method for calibrating the beams is discussed in which images obtained with the different beams are postprocessed to improve the comparability of the different CD values determined by the different beams. In regular intervals it is possible to determine the spot size w_i for each beam, with i=1 , ... ,M either on a dedicated calibration sample or by evaluating the image sharpness of the captured images. When the spot size for each of the M beams is known it is possible to select a target spot size such as the maximum of the spot sizes as w_max and this maximum can be either the maximum value of all spot sizes or can be any other value constant over time to improve the CD stability. The images obtained by the different beams can now be postprocessed such that each image ends up having an image sharpness corresponding to this spot size w_max. This postprocessing can include the application of a Gaussian filter bearing the image

[0078] As an example, assuming a Gaussian spot size w_i of each beam, to each image l_i, i=1 , ... ,M a Gaussian blur of width dSigmaJ = sqrt(w_maxA2 - w_iA2) has to applied. Theresulting images IblurredJ, i=1 ,M all have an image sharpness (Gaussian blurriness) of wBlurredJ = sqrt(w_iA2 + dSigmaJA2) = sqrt(w_iA2 + w_maxA2 - w_iA2) = w_max.

[0079] Now the CD values shall be determined on the post-processed images I blurredj.

[0080] As a result, the impact of a varying spot size (e.g., through beam-to-beam variation or due to defoci / drifts over time) on the determined CD values can corrected.

[0081] If the spot profile wj for each beam i=1 ,M is known (e.g., by extracting it from a SEM image on a dedicated test sample), each image l_i may also be post-processed by a deconvolution with the spot profile wj, resulting in deblurred images IdeblurredJ, i=1,... ,M, with the same improved image sharpness.

[0082] Now the CD values shall be determined on the post-processed images IdeblurredJ. As a result, the impact of a varying spot size (e.g., through beam-to-beam variation or due to defoci / drifts over time) on the determined CD values can corrected.

[0083] In connection with Figs. 5 and 6 a further correction of determined CD values is discussed which occur by charging or contamination effects. When the electron beam hits the sample of interest, or the calibration sample a positive or negative charging of the sample can occur in view of the fact that the electrons either accumulate and stay in this sample or the electrons induce the emission of electrons and in Fig. 5 curve 31 shows how the CD value increases over the different measurements which are carried out at the same location on a sample. Due to charging effects the measured value increases due to contamination or charging. Fig. 6 shows CD values determined from simulated SEM images with a varying voltage on the left side and how the CD value of 33 nm evolves in dependence on the voltage resulting from the charging effect.

[0084] The charging effects could be determined, by way of example by analyzing the secondary electron signal as described in more detail in DE 102021 124099 A1 and as discussed in more detail in connection with Fig. 16 to 19 below. The use of pre-calibrated offset values and the measured CD values could be corrected to obtain the true structure size without charging effects.

[0085] The calibration can furthermore be improved by using the anisotropy of the beam shapes in the multi-beam SEM. In a single beam SEM a correction of the beam profile is possible more easily than in a multi-beam SEM. Even with a multi-stigmator which allows to correct astigmatism for each beam individually, beam profile asymmetries can occur, by way of example due to coma. As the CD values are impacted by the extent of the electron beamin the direction vertical to the inspected structure, the anisotropy of the beam profile needs to be taken into account when calibrating CD values. The above discussed direct calibration method discussed above in connection with Fig. 2 and 3 could be enhanced by determining the offset values for structures of different orientation and using the according offset values when correcting the measured CD values. Referring to Fig. 9 different structures are shown which can be used in a calibration sample such as the structures 40, 42, 46 and 48 which describe different lines having a defined width and a defined orientation. The structure 44 represents a pyramid or structure 50 as a resolution sample could be used or a structure such as structure 52.

[0086] For these different structures a CD calibration offset can be determined depending on the individual beam spot shape structure size etc. The pyramid structure as shown by 44 could be used to monitor the relative changes in the individual beam tilt and the resolution structure 50 could be used to the determine the individual beam directional sharpness. All these structures shown in Fig. 9 could be provided on a calibration sample which is preferably of conducting material in order to avoid any charging effects. Each of the beams can be used to determine CD values for all of the structures 40-52 shown in Fig. 9 and it is then possible to determine the influence of the geometry on the determined CD values as the CD values are known. When the sample of interest is examined where the general geometry is known as being similar to one of the geometries shown in Fig. 9 it is possible to determine the corresponding influence determined with this calibration sample. By way of example the calibration discussed in connection with Figs. 2 and 3 can be enhanced by determining the offset values for the structures of different orientation and when the structure in the sample of interest is known the corresponding offset values can be determined and used when correcting the measured CD values in the sample of interest. In a similar way for the indirect calibration discussed above the spot size in each direction, the directional sharpness or the spot size in the direction vertical to the inspected structure can be used as a parameter.

[0087] Fig. 7 shows different examples for the determined beam shapes and Fig. 8 shows the value CG (contrast gradient) which is a measure for the spot size. The ellipse in Fig. 7 depicts the shape of a spot which is not round. CG_x_projected is the size of the spot when projected onto the horizontal axis (x) and CG_y_projected is the size of the spot when projected onto the vertical axis (y). When imaging a line, the spot size perpendicular to the direction of the line is what influences the measured CD value. I.e., when imaging a vertical line, the spot size in horizontal direction (CG_x_projected) is what determines the apparent line width (CD value).Fig. 8 shows (depending on the beam number from 1 to 91) the overall CG value, the CG_x_projected value and the CG_y_projected value. One sees that in this case, there is a difference between CG_x_projected and CG_y_projected of approximately 1 nm for almost all beams, i.e. , the spots are not round.

[0088] In addition to the mean structure 60 one other beam shape 62 is shown which corresponds to one of the shapes shown in Fig. 7. Thus, as indicated above the spot size in each direction, the directional sharpness, in the horizontal plane, the plane vertical to the beam and the numerical aperture can be used as a parameter. Furthermore, the anisotropy of the spot size can be used in the embodiment discussed above where the images were postprocessed based on the target spot size. In general the extraction of the contours from the multi-beam SEM images can be improved with the knowledge of the directional sharpness of each beam instead of just the usual CG value which is an average in all directions. Summarizing the exact three-dimensional orientation of the beam shapes is used for each beam and can be considered when determining a CD value based on an image obtained by a certain beam.

[0089] Fig. 10 shows how a beam tilt cannot be only determined based on the pyramid structures shown in Fig. 9 but also based on a structure 63 as shown. The structure 63 comprises a region 66 which is normally perpendicular to an ideal beam 72 and a region 64 which is parallel to the ideal beam 72 whereas the actual beam 74 has a certain inclination angle, a beam tilt, relative to the ideal beam. Based on the beam tilt a line profile is obtained as shown in the right part of Fig. 10 and the difference of the left edge width to the right edge width and / or intensity, the edge width asymmetry, helps to determine the beam tilt as this difference between the edges is proportional to the beam tilt. Accordingly, the patterns shown in Fig. 9 or 10 of different orientations such as vertical and horizontal and the beam tilts relative to the sample can be measured and the beam tilts can be adjusted such that the beam tilt is minimal. A monitoring of the edge width asymmetry may also be used as a criterion to ensure the stability of the beam tilt relative to the sample. In combination with a monitoring of the lateral crossover position in the contrast aperture plane, also tilts of the sample may be distinguished from tilts of the illuminating beams.

[0090] In the following a workflow is discussed using the geometry of the calibration sample having the different geometries as shown in Fig. 9. Furthermore, it is discussed how the information can be used to improve the determination of the critical dimension determined in a sample of interest. Using the calibration sample shown in Fig. 9, P images are captured for all the M beams with all of the orientations of the patterns shown in Fig. 9. Accordingly, the calibration of the CD offset values for the different expected structure sizes and structure orientations are known. Now for the following N images captured on the sample of interestthe determined CD offset values are used to correct the measured CD values. For each image and structure of interest, extract the structure size and orientation from the image or look it up in a database if the design specification is known and correct the measured CD values by the calibrated CD offset values using a lookup table, interpolation, and / or extrapolation of the calibrated CD offset values. This can be either done in real-time or by postprocessing if the calibration values and images are stored.

[0091] Furthermore, it is possible in addition that images obtained with the pattern shown in Fig.

[0092] 10 are used to monitor the beam tilts and directional sharpness by capturing images of the pyramid structures 44 or the resolution structure 50 on the calibration sample. The precalibrated sensitivity values of the CD values can be used on the beam tilt changes to correct the CD values. The pre-calibration of the sensitivities can either be done in simulations or by using the calibration sample. It is possible to capture images of the calibration sample having the structures shown in Fig. 9 with different deflector settings and using a beam tilt. For each deflector setting also an image of the pyramid structures is captured in order to monitor the change in beam tilt. Similarly, the stigmation or focus series where the beam is deliberately defocussed to various extends and images of both the lines and structures 40-48 and of the resolution structure 50 may be used to correlate directional sharpness with CD offsets.

[0093] As discussed above, the microscope can be used to determine critical dimensions in metrology applications. Another use case is defect inspection where the microscopes are used to detect defects on semiconductor samples. Here also multi-beam microscopes may be used to detect defects to increase the throughput. The capture rates of defect detection algorithms applied to the images depend on the beam resolution and noise levels of the images in relation to the defect size and the brightness or visibility of the defects. A defect visibility may be defined as a difference in scanning electron yield at the corresponding pixels between a sample with and without defect. In principle, the capture rates should be as high as possible. In reality however finite beam resolution and noise levels limits the achievable capture rates as a trade-off to throughput. When multiple beams are used, a variation of the capture rates from beam-to-beam or from microscope to microscope or also from time to time can play a role. The variation of capture rates may be important when performing statistical analysis on the number of defects found when comparing different manufacturing processes or when detecting drifts over time in the manufacturing process error rates.

[0094] The defect sensitivity and hence also the capture rates or false positive rates depend on the contrast to noise ratio, CNR, of the sample and the resolution of the beams. Fig. 11shows depending on the difference of the CNR to a target CNR how much the false positive rate (FP) differs from the target FP of 5 % (lines 85) and how much the capture rate (CR) differs from the target CR of 90 % (lines 81). Each line corresponds to a different defect size (ranging from 2 to 7 nm).

[0095] In a similar way Fig. 12 shows graphs for the false positive rate and capture rate depending on the difference of the beam resolution (CG) to a target beam resolution.

[0096] A monitoring of the contrast-to-noise ratio and the contrast gradient values during image acquisition allows to estimate changes in defect sensitivity or allows determining defect sensitivity variations from beam-to-beam, from microscope to microscope (tool-to-tool) or time-to-time even without knowing the ground truth, the true number of defects on the sample. If a large contrast-to-noise ratio / contrast gradient variation across the beams is detected, a realignment of the column may be performed to reduce it and ensure matching defect sensitivity. Such a realignment can include a fast autofocus or auto-stigmation or also a realignment of the projection path. If this fast realignment does not provide the necessary CNR / CG homogeneity more complete realignments of the column may be performed.

[0097] A possible workflow to reduce the time-to-time drifts of defect sensitivities may include the steps to acquire images for all beams (full field of view, i.e., mFoV) and evaluating the contrast to noise ratio and the contrast gradient for each of the individual field of views (sFoV) of the acquired images.

[0098] By way of example a defect detection may be carried out and if the contrast to noise ratio or the contrast gradient has drifted too much and reaches some threshold value such as the threshold values 91 and 92 shown in Fig. 13, a realignment of the column may be performed to optimize the contrast to noise ratio / contrast gradient. Furthermore, the acquisition may be repeated at the current position or the realignment may be used at the next position. Then the system may move to the next sample position and the steps above are repeated.

[0099] When scanning large areas of the sample for defects, on average the same number of defects is to be expected to be found in each of the individual field of views. The monitoring of the total defect count for each beam individually may thus be used to detect the possible B2B variations in the defect sensitivity. As an example for each beam specific FOV, the total number of defects detected per beam can be evaluated. If the variation across the beams is larger than a certain threshold, for example a real alignment of the column may be performed to ensure better beam to beam matching or it is possible to adjust the spotgeometry or the beam current as discussed in further detail below. Certain parts of the sample such as regions near the edges of a die may be excluded from this statistical evaluation as outer beams may systematically see different structures than the inner ones. If the design data of the sample of interest is known, it may be used to identify the regions where the same number of defects per SFOV are to be expected excluding the aforementioned special cases. Such a realignment can in most cases include a fast autofocus or auto-stigmation or also a realignment of the projection path. Furthermore, complete realignments of the column can be used.

[0100] In order to obtain a sufficient beam-to-beam matching of the defect sensitivity or a matching between different tools one of the following steps may be carried out:

[0101] The first option includes an adjustment of the spot shape or size and / or of the noise levels to homogenize the contrast gradient or contrast to noise ratio values across all beams or tools. In the case of a correction of the beam-to-beam matching this could be obtained with appropriate micro-optics which allow to manipulate the electron trajectories on a per beam basis using one multipole per beam in order to stigmate or reshape the beam or one lens per beam is used to defocus the beam. In case of a correction of a tool-to-tool matching global electron optical elements such as conventional multipoles or lenses may be used to achieve this effect. Alternatively, the images may be manipulated in postprocessing by convolution or deconvolution of the images with appropriate spot shapes. Noise levels across the images may also be homogenized by artificially adding noise. In the case of a correction of beam-to-beam matching this leads to all beams having the defect sensitivity of the worst beam. However, the beam-to-beam variation is improved which in some applications be more important.

[0102] Furthermore, the beam current may be adjusted in order to ensure the beam-to-beam matching to homogenize the contrast-to-noise ratio and thus the defect sensitivity. If the tool-to-tool matching is to be improved this may be done by adjusting the beam current of all beams at once through adjustment to the condenser lenses or the electron source, the heating current or the extraction voltage. If the beam-to-beam matching is to be improved and the beam current can be adjusted on a per beam basis, the contrast-to-noise ratio values and hence the defect sensitivity may be adjusted on a per beam basis as well.

[0103] As a further alternative to the postprocessing of the images for a homogenization of the monitored values and the defect sensitivity, the monitored values may simply be saved in addition to the acquired images. When performing a statistical analysis of the defect counts, these monitored values may be converted to possible decreases or increases of the capture rates. Hence, appropriate correction factors may be applied to the defect counts extractedfrom the images themselves. These correction values may either be extracted from simulation or from experimental work where the ground truth is known and the imaging parameters such as contrast gradient, contrast-to-noise ratio, beam current may be controlled in calibration. As an example, such a calibration may be performed in the following way:

[0104] For varying spot shapes, CG values, CNR or beam current the capture rates can be determined on a calibration sample containing defects of a varying size or type. Later, when determining defect counts on a sample of interest the measured contrast gradient or contrast-to-noise ratio values on the sample of interest may be used to determine the expected capture rates for each beam. The defect count extracted from the acquired image may be rescaled according to the ratio of the expected capture rate for each beam to the target capture rate.

[0105] Figs. 14 and 15 show how a control of the contrast-to-noise ratio (or contrast gradient values) may lead to an improved B2B matching of CNR (or CG) and hence influence the variation of the capture rates. Fig. 14 shows how CNR values may vary from beam to beam if no control mechanism is used: The values spread far across predetermined upper and lower target thresholds (dashed lines). Fig. 15 shows how this spread of the CNR values across the beams may be reduced to lie in between the threshold values (dashed lines) for all beams if a control mechanism is used.

[0106] In connection with Figs. 16-19 it is discussed in more detail how an imaging of the aperture plane of the microscope 1 could be used to determine a charging of the sample. Fig. 16 shows in the upper part a usual imaging method including the sample 7 the aperture 175 and the detector 178. The aperture 175 is arranged in the vicinity of a crossover point of the secondary beamlets 9.1, 9.2, 9.3. The beam as shown in Fig. 16 is one beam of the multiple beams and there is one beam per detector channel. In the usual imaging mode, beam 171 emitted from the center is imaged to the center on the detector and beam 172 not originating from the center is not imaged to the center at the detector 178. As shown in the lower part of Fig. 16, the strength of lens 179 is readjusted such as to image the aperture plane onto the detector plane. This means that beam 173 originating from the center is not imaged to the center of the detector whereas beam 174 not originating from the center is imaged onto the detector center by readjusting lens 179. The imaging of the aperture plane is further discussed in DE1020211240199A1.

[0107] Accordingly in the usual imaging node shown in the upper part of Fig. 16 each pixel of each detector channel contains information on the secondary electron signal at a specificscan position of each beam. In the aperture imaging mode, only one beam is used and the others are disabled. Each pixel of a certain detector channel contains the information on a secondary electron signal at a specific scan position passing through a certain region in the aperture plane. As the position in the aperture plane depends on the secondary electron momentum at the sample, each detector channel basically leads to an SEM image of the sample resulting from the secondary electrons within a certain start direction and start energy range. It was found that the charging of the sample leads to a certain signature in the resulting SEM images in the aperture imaging mode shown in the lower part of Fig. 16. This allows a determination which sample regions charge up and by how much providing the sign and the amplitude of the local sample potential.

[0108] Fig. 17 shows in the upper row the usual images 181-183 of one beam at different voltages occurring at the samples. The lower part shows the corresponding aperture image. Accordingly, image 185 is the aperture image corresponding to 0V charge in the sample and the lower image 185 shows a homogeneous distribution of the beams within each of the fields. If there is a positive charge of +10V in the sample, the electrons from the corresponding region are either decelerated or deflected and the aperture image 186 shows that an electron starting on the left side from the middle is deflected towards the right side by the positive voltage so that the aperture image 186 in each field of view is not a homogeneous image. Furthermore the aperture image 187 is shown for a voltage of -10V at the sample where a deflection in the other direction can be deduced in the aperture image 187. Image 184 is a comparison image where no charging occurs in sample but where there is a circle of different material in the middle and the aperture image 188 clearly shows that this contrast is not a result of a charging but a material contrast.

[0109] Fig. 18 shows a further example where a line of potential zero is provided with the surrounding area being grounded wherein the line and the substrate are of the same material. The first row again shows the normal sample images 191, 193, 195 of one beam whereas the second row, images 192, 194 and 196 show the corresponding aperture images. Without the applied voltage the line is not detected in the normal image 191 and the aperture image is also homogeneous. With a line at +10V as shown in the sample image 193, the corresponding aperture image 194 shows that the region to the left of the line is bright for the channels to the right and vice versa. For the line of -10V the sample image 195 of the secondary electrons shows a collection on the left and on the right side of the channel whereas the corresponding aperture image indicates that the region to the left of the line is bright for channels to the left and vice versa.Referring to Fig. 19 aperture images 197-199 are shown for different voltages from -5 to -20V. The images 197 to 199 show a line of potential II at -5. -20 and -20 V with the surrounding area being grounded, wherein the line and the substrate are of the same material. The higher the voltage is, the more the bright signal in the aperture image is deflected to the right side. By looking at the individual detector channels the potential of the charged area can be determined, and the position of the vertical bright region depends on the voltage. Accordingly, the position and distribution of the secondary electrons in the aperture image can be used to determine a charging distribution of the sample of interest. As noted in the description of Figs. 6 and 7 above, this information may for example be used to correct measured CD values on charging samples.

[0110] The application relates to the following clauses:

[0111] 1. A method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 2, the method comprising:

[0112] - selecting, from the M particle beams, at least one particle beam to be calibrated, - determining beam specific calibration data for a plurality of the M particle beams, - using the beam specific calibration data determined for the plurality of the M particle beams for calibrating said at least one particle beam to be calibrated.

[0113] 2. The method of clause 1, wherein determining the beam specific calibration data comprises

[0114] - determining a first calibration value for each of the plurality of the M particle beams, - determining a mean calibration value based on the first calibration values of the plurality of the M particle beams,

[0115] - using the mean calibration value for calibrating said at least one particle beam to be calibrated.

[0116] 3. The method of clause 2 wherein determining the beam specific calibration data comprises

[0117] - determining a beam specific image of a sample structure of sample of interest with each of the plurality of the M particle beams,

[0118] - determining the first calibration value for each of the beam specific images.

[0119] 4. The method of clause 3, wherein the first calibration value comprises a critical dimension value of the sample structure, wherein the mean calibration value is a mean critical dimension value.5. The method of clause 2 wherein determining the beam specific calibration data comprises

[0120] - determining a spot size for each of the plurality of the M particle beams in a beam specific image of a sample structure,

[0121] - determining a target spot size based on the spot sizes of the plurality of the M particle beams,

[0122] - postprocessing the beam specific images with the target spot size.

[0123] 6. The method of clause 5, wherein the target spot size is a largest spot size determined for the plurality of the M beams.

[0124] 7. The method of any preceding clause, determining the beam specific calibration data comprises

[0125] - generating a beam specific image of a sample of interest with each of the plurality of the M particle beams,

[0126] - determining a defect sensitivity in each of the plurality of beam specific images,

[0127] - determining a defect sensitivity variation from the determined defect sensitivities, - using the defect sensitivity variation for calibrating the M particle beams.

[0128] In clause 7 determining defect sensitivity can for exampleinvolve determining the CNR and CG values and estimating the defect sensitivity from these values.

[0129] 8. The method of clause 7, wherein using the defect sensitivity variation comprises determining whether the defect sensitivity is within a defined threshold defect sensitivity, and if this is not the case, adapting a setup of the multi-beam particle raster scanning microscope.

[0130] 9. The method of clause 7 or 8, determining the defect sensitivity from time to time and if the defect sensitivity is approaching the defined threshold defect sensitivity, the setup of the multi-beam particle raster scanning microscope is initiated.

[0131] In clauses 8 and 9, correcting defect sensitivity can include using autofocus, autostigmation, projection path realignment, adding noise and other measures to achieve similar CNR and CG values for each beam.10. The method of any preceding clause, wherein selecting the at least one particle beam comprises selecting the M particle beams, wherein the beam specific calibration data are used for calibrating the M particle beams.

[0132] 11. A method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 4, the method comprising:

[0133] - determining, with each of the M particle beams, a plurality of beam specific images of a calibration sample having a known dimension parameter,

[0134] - determining, for each of the plurality of beam specific images, a beam specific dimension parameter for the calibration sample,

[0135] - determining, for each of the M particle beams, an offset between the beam specific dimension parameter and the known dimension parameter of the calibration sample, - determining, for each of the particle beams, for a sample of interest, a beam specific dimension parameter for the sample of interest,

[0136] - correcting, for each of the particle beams, the beam specific dimension parameter for the sample of interest based on the offset determined for the corresponding beam.

[0137] 12. The method of clause 11, wherein the beam specific dimension parameter is determined as structure specific value for several different calibration structures provided on the calibration sample, the method further comprising:

[0138] - determining a structure provided in the sample of interest,

[0139] - determining a structure specific calibration value corresponding to the structure provided in the sample of interest,

[0140] - correcting the beam specific dimension parameter with the structure specific calibration value.

[0141] 13. A method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 4, the method comprising:

[0142] - determining, for each of the M beams, a relation how at least one operating parameter of the multi-beam particle raster scanning microscope influences a precision how precise a dimension parameter of a calibration sample investigated with the multi-beam particle raster scanning microscope can be determined,

[0143] - determining a dimension parameter of a sample of interest based on at least one image obtained with the multi-beam particle raster scanning microscope while monitoring the operating parameter,- correcting the determined dimension parameter of the sample of interest to a corrected dimension parameter based on the monitored at least one operating parameter and the determined relation.

[0144] 14. The method of clause 13, wherein the operating parameter comprises at least one of the following:

[0145] - a beam tilt of the corresponding beam relative to the sample of interest,

[0146] - a spot size of the beam on the sample of interest,

[0147] - a lateral position of the beam relative to an aperture plane.

[0148] 15. A method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 4, the method comprising:

[0149] - determining, for each of M beams a relation how a charging effect of a calibration sample investigated with the multi-beam particle raster scanning microscope influences a precision how precise a dimension parameter of a calibration sample investigated with the multi-beam particle raster scanning microscope can be determined,

[0150] - determining a dimension parameter of a sample of interest based on at least one image obtained with the multi-beam particle raster scanning microscope while determining the charging effect of the sample of interest,

[0151] - correcting the determined dimension parameter of the sample of interest to a corrected dimension parameter based on the determined charging and determined relation.

[0152] 16. The method of clause 15, wherein the charging is determined based on an aperture image determined in an aperture imaging mode in which an aperture-plane present in the multi-beam particle raster scanning microscope is imaged.

[0153] 17. A multi-beam particle raster scanning microscope configured to carry out a method as mentioned in any of clauses 1 to 16.

Claims

Claims1. A method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 2, the method comprising:- selecting, from the M particle beams, at least one particle beam to be calibrated, - determining beam specific calibration data fora plurality of the M particle beams, - using the beam specific calibration data determined for the plurality of the M particle beams for calibrating said at least one particle beam to be calibrated.

2. The method of claim 1, wherein determining the beam specific calibration data comprises- determining a first calibration value for each of the plurality of the M particle beams, - determining a mean calibration value based on the first calibration values of the plurality of the M particle beams,- using the mean calibration value for calibrating said at least one particle beam to be calibrated.

3. The method of claim 2 wherein determining the beam specific calibration data comprises- determining a beam specific image of a sample structure of sample of interest with each of the plurality of the M particle beams,- determining the first calibration value for each of the beam specific images.

4. The method of claim 3, wherein the first calibration value comprises a critical dimension value of the sample structure, wherein the mean calibration value is a mean critical dimension value.

5. The method of claim 2 wherein determining the beam specific calibration data comprises- determining a spot size for each of the plurality of the M particle beams in a beam specific image of a sample structure,- determining a target spot size based on the spot sizes of the plurality of the M particle beams,- postprocessing the beam specific images with the target spot size.- 26 -6. The method of claim 5, wherein the target spot size is a largest spot size determined for the plurality of the M beams.

7. The method of any preceding claim, determining the beam specific calibration data comprises- generating a beam specific image of a sample of interest with each of the plurality of the M particle beams,- determining a defect sensitivity in each of the plurality of beam specific images,- determining a defect sensitivity variation from the determined defect sensitivities, - using the defect sensitivity variation for calibrating the M particle beams.

8. The method of claim 7, wherein using the defect sensitivity variation comprises determining whether the defect sensitivity is within a defined threshold defect sensitivity, and if this is not the case, adapting a setup of the multi-beam particle raster scanning microscope.

9. The method of claim 7 or 8, determining the defect sensitivity from time to time and if the defect sensitivity is approaching the defined threshold defect sensitivity, the setup of the multi-beam particle raster scanning microscope is initiated.

10. The method of any preceding claim, wherein selecting the at least one particle beam comprises selecting the M particle beams, wherein the beam specific calibration data are used for calibrating the M particle beams.

11. A method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 4, the method comprising:- determining, with each of the M particle beams, a plurality of beam specific images of a calibration sample having a known dimension parameter,- determining, for each of the plurality of beam specific images, a beam specific dimension parameter for the calibration sample,- determining, for each of the M particle beams, an offset between the beam specific dimension parameter and the known dimension parameter of the calibration sample, - determining, for each of the particle beams, for a sample of interest, a beam specific dimension parameter for the sample of interest,- correcting, for each of the particle beams, the beam specific dimension parameter for the sample of interest based on the offset determined for the corresponding beam.

12. The method of claim 11, wherein the beam specific dimension parameter is determined as structure specific value for several different calibration structures provided on the calibration sample, the method further comprising:- determining a structure provided in the sample of interest,- determining a structure specific calibration value corresponding to the structure provided in the sample of interest,- correcting the beam specific dimension parameter with the structure specific calibration value.

13. A method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 4, the method comprising:- determining, for each of the M beams, a relation how at least one operating parameter of the multi-beam particle raster scanning microscope influences a precision how precise a dimension parameter of a calibration sample investigated with the multi-beam particle raster scanning microscope can be determined,- determining a dimension parameter of a sample of interest based on at least one image obtained with the multi-beam particle raster scanning microscope while monitoring the operating parameter,- correcting the determined dimension parameter of the sample of interest to a corrected dimension parameter based on the monitored at least one operating parameter and the determined relation.

14. The method of claim 13, wherein the operating parameter comprises at least one of the following:- a beam tilt of the corresponding beam relative to the sample of interest,- a spot size of the beam on the sample of interest,- a lateral position of the beam relative to an aperture plane.

15. A method for calibrating a multi-beam particle raster scanning microscope comprising M particle beams, with M > 4, the method comprising:- determining, for each of M beams a relation how a charging effect of a calibration sample investigated with the multi-beam particle raster scanning microscope influences a precision how precise a dimension parameter of a calibration sample investigated with the multi-beam particle raster scanning microscope can be determined,- determining a dimension parameter of a sample of interest based on at least one image obtained with the multi-beam particle raster scanning microscope while determining the charging effect of the sample of interest,- correcting the determined dimension parameter of the sample of interest to a corrected dimension parameter based on the determined charging and determined relation.

16. The method of claim 15, wherein the charging is determined based on an aperture image determined in an aperture imaging mode in which an aperture-plane present in the multi-beam particle raster scanning microscope is imaged.

17. A multi-beam particle raster scanning microscope configured to carry out a method as mentioned in any of claims 1 to 16.

Images