Method for operating a multi-beam particle beam microscope

Description

[0001] The invention relates to methods for operating multi-beam particle beam microscopes.

[0002] US 2015 / 0083911 A1 discloses a multi-beam particle beam microscope in which a plurality of particle beams are directed onto an object and focused there, so that a field of impact points is illuminated by the particle beams. The particle beams generate secondary electrons, which move away from the object at the impact points. A projection system is provided to collect the secondary electrons and feed them to a detection system. Secondary electrons emanating from each impact point are each formed into an electron beam, which is fed to precisely one detector element of the detection system. Detection signals from this detector element can thus be assigned to one of the electron beams and thus to one of the plurality of impact points of the particle beams on the object.To acquire a particle microscopic image, the plurality of particle beams are rasterized or scanned in parallel across the surface of the object, so that each of the particle beams illuminates a plurality of impact locations. The detected signals can thus be assigned to the plurality of impact locations of the plurality of particle beams via the respective scan position to generate the spatially resolved image data of the particle microscopic image.

[0003] WO 2018 / 172186 A1 discloses a multi-beam particle beam microscope with two detection systems for particle beams emanating from the object.

[0004] It has been shown that the particle microscopic images generated in this way show unexpected blurring and artifacts in some application situations.

[0005] Accordingly, it is an object of the present invention to propose a multi-beam particle beam microscope and a method for operating the same, with which sharper images can be produced in some application situations.

[0006] This object is achieved by providing a method for operating a multi-beam particle beam microscope having the features of the appended claim 1 and a multi-beam particle beam system having the features of the appended claim 13.

[0007] Advantageous embodiments are specified in the accompanying dependent claims 2 to 12 and 14.

[0008] According to the invention, a multi-beam particle beam microscope comprises an electron converter, an illumination system configured to illuminate a field of impact locations on an object with a plurality of particle beams, and a projection system configured to direct electron beams emanating from the impact locations onto the electron converter. The electron converter is configured to convert the energy of the electrons of the electron beams directed onto the electron converter into signals that are subsequently detected. The signals into which the energy of the electrons is converted can be signals of any directly or indirectly detectable type. For example, the electron converter can comprise a scintillator material that converts energy from the electron beams into photons that are subsequently detected.Here, the photons can again be detected indirectly: the photons generate electron-hole pairs in a semiconductor material, which in turn lead to voltage signals that are converted into digital signals and represent the detected signals. Suitable semiconductor elements for this purpose include avalanche photodiodes. Furthermore, the photons can be detected by guiding them, for example via optical fibers, to an electron multiplier, which converts the photons into voltage signals with high efficiency. The electron converter can further comprise, for example, a semiconductor material into which the electrons directed toward the electron converter penetrate and generate electron-hole pairs as direct signals, which in turn generate voltage signals.

[0009] According to the invention, the multi-beam particle beam system comprises a first detection system with a plurality of detection elements configured to detect first signals generated in the electron converter by incident electrons, and a second detection system with a plurality of detection elements configured to detect second signals generated in the electron converter by incident electrons. A detection element is an assembly of the detection system configured to detect the signals such that the corresponding detection result can be assigned to the detection element. For example, based on the detection results of two different detection elements, a decision can be made as to whether the signal triggering the detection results was detected by one or the other detection element.However, it is not possible, for example, to determine the location within a detection volume of the detection element where the signal was detected based on the detection result of a single detection element. According to exemplary embodiments, the first and / or second detection system comprises a CCD detector, and the pixels of the CCD detector are the detection elements.

[0010] According to exemplary embodiments, the first and second detection systems differ in a rate at which the respective detection elements detect the signals and / or in the number of detection elements of the respective detection system.

[0011] According to exemplary embodiments, the first and / or second detection system comprises a light detector having an array of detector elements configured to detect signals generated in the electron converter by incident electrons, wherein the signals are photons. The detection system may then further comprise an optical imaging system configured to optically image a surface of the electron converter onto the array of detection elements of the detection system.

[0012] The projection system can in particular be configured to image the surface of the object and thus the plurality of impact points of the particle beams on the object onto the surface of the electron converter.

[0013] Through the optical imaging system of the surface of the electron converter onto the field of detector elements of the light detector, an image of the impact points of the plurality of particle beams on the object is created on the field of detector elements of the light detector.

[0014] According to exemplary embodiments, the detector elements of the first detection system comprise photomultipliers. A photomultiplier consists, for example, of a photocathode and a downstream secondary electron multiplier. Photomultipliers are suitable for detecting light signals with a high detection probability and high temporal resolution. On the other hand, photomultipliers are expensive devices that take up significant space, so the number of detector elements of the first detection system and thus its spatial resolution is limited in practice if the detector elements of the first detection system are photomultipliers.

[0015] According to exemplary embodiments, the detector elements of the second detection system comprise photodiodes. Using lithographic methods, it is possible to produce arrays of photodiodes that provide a high density of detector elements and thus offer high spatial resolution. Examples of such detection systems are CCD sensors with many hundreds of thousands of detector elements or pixels. However, such detection systems have the disadvantage that they can be read out at a relatively low rate and thus comparatively slowly, which is why the detection of the light signals generated by the electron converter using a CCD sensor alone is generally not used in practice in multi-beam particle beam microscopes.

[0016] In a conventional multi-beam particle beam microscope, a detection system comprising photomultiplier tubes as detector elements has a number of detector elements equal to the number of particle beams directed onto the object by the illumination system. Each of these particle beams is assigned to exactly one detector element of the detection system, and each detector element of the detection system is assigned exactly one particle beam. The detection signals of any given detector element of the detection system are then assigned to the particle beam assigned to the given detector element, and the detection signals are then further assigned to the point of incidence of this particle beam on the object, onto which the particle beam was directed during scanning across the object's surface when the secondary electrons were generated that triggered the detection signals.

[0017] This conventional detection principle assumes that the projection system and the optical imaging system together are capable of ensuring that secondary electrons generated by a given particle beam impinging on the object lead to detection signals essentially of the detector element associated with the given particle beam, while the secondary electrons emanating from the point of impact of the given particle beam do not lead to detection signals of other detector elements of the detection system.

[0018] The inventors have recognized that this assumption is not always justified in practice, and in some cases, secondary electrons released from the object by a given particle beam also lead to increased detection signals from detector elements of the light detector that are different from the detector element associated with the given particle beam. This effect can be referred to as "crosstalk."

[0019] According to exemplary embodiments of the invention, detection signals from the detector elements of the second detection system are used to assign detection signals from the detector elements of the first detection system to particle beams impinging on the object. In particular, the detection signals from the detector elements of the second detection system are used to assign detection signals from the first detection system to impact locations of the particle beams on the object.

[0020] According to exemplary embodiments of the invention, a method for operating a multi-beam particle beam microscope, such as that explained above, comprises scanning a plurality of particle beams across an object to illuminate and displace an array of particle beam impingement sites on the object, and directing electron beams emanating from particle beam impingement sites on the object onto an electron converter.The method further comprises detecting first signals generated in the electron converter by incident electrons with a plurality of detection elements of a first detection system during a first period of time, detecting second signals generated in the electron converter by incident electrons with a plurality of detection elements of a second detection system during a second period of time, and assigning the signals detected during the first period of time with the detection elements of the first detection system to the impact locations based on the detection signals detected during the second period of time with the detection elements of the second detection system.

[0021] According to exemplary embodiments, the detection of the first signals with the detection elements of the first detection system occurs at a rate that is greater than a cutoff frequency, and the detection of the second signals with the detection elements of the second detection system occurs at a rate that is less than 0.5 times this cutoff frequency. This means that the detection elements of the second detection system can be read out significantly more slowly than the detection elements of the first detection system.

[0022] According to exemplary embodiments, the number of detection elements of the second detection system is more than twice as large as the number of detection elements of the first detection system. This means that the second detection system can achieve a significantly higher spatial resolution than the first detection system.

[0023] According to exemplary embodiments, the projection system is configured to image the impact locations of the particle beams on the object onto a surface of the electron converter using the electron beams emanating from the object. While the particle beams impinging on the object can be very well focused on the object and create small beam foci on the object, it is not possible in practice to image these small beam foci onto very small beam foci of the electron beams on the surface of the electron converter using the projection system. This is because the electrons forming the electron beams emerge from the object with a broad energy spectrum, so that the imaging of the electrons from the object onto the surface of the electron converter provided by the projection system is already subject to errors due to the energy range of the electrons.The electrons emanating from a single point of incidence on the object thus impinge on an extended area on the surface of the electron converter. However, it is possible to design the projection system such that electrons emanating from adjacent points of incidence of different particle beams on the object each illuminate extended areas on the surface of the electron converter, but that different extended areas do not overlap or overlap only slightly. In conventional multi-beam particle beam microscopes, it was assumed that these areas do not overlap with each other, and the optical imaging system then projects these areas onto the array of detector elements of the first detection system.

[0024] The inventors have recognized that the adjacent regions on the surface of the electron converter, which are impinged by electrons of the electron beams emanating from adjacent impingement locations on the object, can change over time and, in particular, can change during the duration of a recording of a particle microscopic image by scanning the plurality of particle beams over the object.

[0025] By evaluating the detection signals of the detector elements of the second detection system, which may have a higher spatial resolution than the first detection system, it is possible to determine the type of overlap between the areas illuminated by the electron beams on the surface of the electron converter. This detection may include an image analysis of images detected with the second detection system.

[0026] According to exemplary embodiments, detection signals detected during the first period with a given detector element of the detector elements of the first detection system are assigned to at least two different impact locations. In contrast to the conventional method, in which the light signals detected by a given detection element of the first detection system are always erroneously assigned only to the particle beam assigned to the given detector element, the method according to the embodiments described here allows for a more flexible assignment of the detection signals, which takes into account the current situation and any given current deficiencies in the image provided by the projection system.For example, based on the evaluation of the detection signals from the detector elements of the second detection system, it may be determined that, for example, 90% of the detection signals from a given detector element of the first detection system are assigned to the particle beam associated with the given detector element, while 10% of the detection signals are assigned to a specific particle beam that strikes the object adjacent to the particle beam associated with the given detector element. This, in some respects, more accurate assignment of detection signals to particle beams and thus to impact locations on the object can be used to generate particle microscopic images with comparatively better image properties.

[0027] The inventors have recognized that the method described here can be particularly advantageous when the object being examined has surface charges that are inhomogeneously distributed across the surface. This is especially the case when the extent of the surface charges and the effective range of these surface charges are smaller or much smaller than the extent of the area on the object imaged by the entirety of the particle beams. Surface charges on the object can be generated, in particular, by the particle beams themselves scanning the object, which is why the configuration of the surface charges on the object can change over time.Locally present surface charges can cause one or more electron beams emanating from the particle beams' impact points on the object to be deflected in specific, quite different directions, while other electron beams are not deflected or are deflected differently due to the effects of the surface charges, which decrease with distance from the surface charges. This leads to the configuration of an area illuminated by a given electron beam on the surface of the electron converter changing over time in terms of shape and size.However, it is possible to detect this temporal change in the configuration of the illuminated areas on the electron converter, which in turn are imaged by the optical imaging system onto the detector elements of the first detection system, by evaluating the detection signals of the second detection system and to take this into account when assigning the detection signals of the detector elements of the first detection system to the particle beams and thus to the impact locations.

[0028] According to exemplary embodiments, the detection of detection signals with the detection elements of the first detection system is repeated during a plurality of first time periods, wherein the detection signals detected by a given detection element of the first detection system in one of the plurality of first time periods are at least partially assigned to a given particle beam, and wherein the detection signals detected by the given detection element of the first detection system in another of the plurality of first time periods are not partially assigned to the given impact location. This means that the assignment of detector elements and their detection signals to particle beams can change, for example, during the acquisition of a particle microscopic image.This change in assignment is based on the detection signals, which also change during this time and are detected with the detection elements of the second detection system.

[0029] The number of detection elements of the first detection system can be equal to the number of particle beams scanned across the object. However, the number of detection elements of the first detection system can also be greater than the number of particle beams scanned across the object. In particular, the number of detection elements of the first detection system can be equal to an integer multiple of the number of particle beams scanned across the object. For example, the number of detection elements of the first detection system can be equal to four or 16 times the number of particle beams.

[0030] Embodiments of the invention are explained in more detail below with reference to the figures. Herein: Figure 1 is a schematic representation of a multi-beam particle beam microscope; Figure 2 is a schematic representation of an electron detector of the Figure 1 shown multi-beam particle beam microscope; Figure 3 a schematic representation of a first detection system of the Figure 2 electron detector shown; Figure 4 a schematic representation of an array of detector elements of the Figure 3 shown first detection system; Figure 5 is a schematic representation of a plan view of an array of detector elements of a second detection system of the Figure 2 electron detector shown; Figure 6 an enlarged partial view of the array of detector elements of the Figure 4 to explain a method according to an embodiment; Figure 7 a block diagram to explain the Figure 6explained method; Figure 8 shows a representation of scan paths of the Figures 6 and 7 explained method; Figure 9 shows a further block diagram to explain the method according to Figures 6 to 8 explained method; and Figure 10 is a schematic representation of an electron detector used in the Figure 1 shown multi-beam particle beam microscope.

[0031] Figure 1is a schematic representation of a multi-beam particle microscope that uses a plurality of beams of charged particles. The multi-beam particle microscope generates a plurality of beams of charged particles that impinge on an object to be examined, generating secondary electrons that emanate from the object and are subsequently detected. The multi-beam particle microscope 1 is of the scanning electron microscope (SEM) type, which uses a plurality of primary electron beams 3 to generate a plurality of electron beam spots 5 on a surface of the object 7. The object 7 to be examined can be of any type and include, for example, a semiconductor wafer, a biological sample, an array of miniaturized elements, or the like.The surface of the object 7 is arranged in an object plane 101 of an objective lens 102 of an objective lens system 100.

[0032] The enlarged section I1 of the Figure 1 shows a plan view of the surface 101 of the object 7 with a regular rectangular field 103 of impact locations 5 of the particle beams 3 on the surface 101 of the object 7. In Figure 1 The number of impact locations is 25, arranged as a 5x5 array 103. The number of 25 particle beams 3 or impact locations is a small number chosen for the sake of simplicity. In practice, the number of beam spots can be chosen to be significantly larger, such as 20x30, 100x100, and the like.

[0033] In the illustrated embodiment, the array 103 of impact locations 5 is a substantially regular rectangular array with a constant distance P1 between adjacent impact locations 5. Exemplary values ​​of the distance P1 are 1 µm and 10 µm. However, it is also possible for the array 103 to have other symmetries, such as hexagonal symmetry.

[0034] Particle beams 3 can be very finely focused at the impact points 5. The diameters of the beam foci formed on the surface of the object can be, for example, 1 nm, 5 nm, 100 nm, and 200 nm. The focusing of the particle beams 3 to form the beam spots 5 is performed by the objective lens system 100.

[0035] The particles of the particle beams 3 striking the object generate electrons there, which emanate from the surface of the object 7. The electrons emanating from the surface of the object 7 are accelerated by electric fields provided by the objective lens 102 and formed into electron beams 9. The multi-beam particle beam microscope 1 comprises a projection system formed by the objective lens 102 and further electron lenses 205. The projection system 102, 205 provides an electron beam path 11 for feeding the plurality of electron beams 9 to an electron detector 209. The electron detector 209 comprises an electron converter 207, onto which the electron beams 9 are directed by the projection system 102, 205 and which is configured to generate photons as signals upon the impact of electrons from the electron beams 9.These photons are detected by light detectors, as described below. A material of the electron converter 207 may comprise a scintillator material, such as the phosphor material sold under the product designation R42 by El-Mul Technologies, Israel.

[0036] Section I2 in Figure 1 shows a plan view of a surface 211 of the electron converter 207, onto which the electron beams 9 impinge. Reference numeral 213 denotes locations at which centers of the impinging electron beams are arranged. In the Figure 1 In the ideal situation shown, the centers 213 are arranged in a field 217 at a regular distance P2 from each other. Example values ​​of the distance P2 are 10 µm, 100 µm, and 200 µm.

[0037] The particle beams 3 are generated by an illumination system 300, which includes at least one electron source 301, at least one collimating lens 303, a multi-aperture array 305, and a field lens 307. The electron source 301 generates a divergent electron beam 309, which is collimated by the collimating lens 303 to form a beam 311 that illuminates the multi-aperture array 305.

[0038] Section I3 in Figure 1shows a top view of the multi-aperture arrangement 305. The multi-aperture arrangement 305 comprises a multi-aperture plate 313 having a plurality of openings or apertures 315 formed therein. Centers 317 of the openings 315 are arranged in a pattern 319, which corresponds to the pattern 103 formed by the impact locations 5 of the particle beams 3 on the object 7. A distance P3 between the centers 317 of the apertures 315 can have exemplary values ​​of 5 µm, 100 µm, and 200 µm. The diameters D of the apertures 315 are smaller than the distance P3 between the centers of the apertures. Exemplary values ​​of the diameters D are 0.2 x P3, 0.4 x P3, and 0.8 x P3.

[0039] Electrons of the illuminating beam 311 pass through the apertures 315 and form electron beams 3. Electrons of the illuminating beam 311 which strike the plate 313 are intercepted by the plate and do not contribute to the formation of the electron beams 3.

[0040] The multi-aperture arrangement 305 focuses the electron beams 3 such that beam foci 323 are formed in a plane 325. The section I4 in Figure 1 shows a top view of the plane 325 with the foci 323 arranged in a pattern 327. A pitch P4 of the foci 323 of the pattern 327 can be the same as or different from the pitch P3 in the pattern 319 of the multi-aperture plate 313. A diameter of the foci 323 can be, for example, 10 nm, 100 nm, and 1 µm.

[0041] The field lens 307 and the objective lens 102 provide an imaging system to image the plane 325, in which the foci 323 are formed, onto the object plane 101, so that a field 103 of impact locations 5 is formed there on the surface of the object 7.

[0042] A beam splitter 400 is provided in the beam path between the multi-aperture array 305 and the objective lens system 100. The beam splitter 400 is also part of the beam path 11 between the objective lens system 100 and the electron detector 209.

[0043] Further information on such multi-beam inspection systems and components used therein, such as particle sources, multi-aperture plates and lenses, can be obtained from the patent applications WO 2005 / 024881 A2, WO 2007 / 028595 A2, WO 2007 / 028596 A1, WO 2007 / 060017 A2, US 2015 / 0083911 A1 and WO 2016 / 124648 A1.

[0044] Figure 2shows further details of the electron detector 209. In addition to the electron converter 207, the electron detector 209 comprises a first detection system 213 with an array of detector elements 215 and a second detection system 218 with an array of detector elements 219. The first detection system 213 comprises a light detector, and the second detection system, in the illustrated embodiment, also comprises a light detector. An optical imaging system 221 is configured to light-optically image the surface 211 of the electron converter 207 onto both the array of detector elements 215 of the first detection system 213 and the array of detector elements 219 of the second detection system 218.For this purpose, the optical imaging system 221 comprises a plurality of lenses 223 and a beam splitter mirror 225, which allows part of the light incident on it to pass through to the first detection system 213 and reflects another part of the light incident on it towards the second detection system 218.

[0045] Photons generated at the electron converter 207 which pass through the beam splitter mirror 225 thus form the first signals which are detected by the first detection system 213, and photons generated at the electron converter 207 which are reflected at the beam splitter mirror 225 form the second signals which are detected by the second detection system 218.

[0046] However, it is also possible for the light striking the first detection system 213 to be reflected by the beam splitter mirror 225, while the light striking the second detection system 218 passes through the beam splitter mirror 225. In this case, the beam splitter mirror 225 can be designed such that the proportion of the light striking it that is fed to the first detection system 213 is five times, ten times, or 50 times greater than the proportion that is fed to the second detection system 218.

[0047] Details of the first detection system 213 are in Figure 3shown schematically. The first detection system 213 comprises a plurality of photomultipliers 227. The number of photomultipliers 227 can correspond to the number of particle beams 3, but it can also be greater. Each of the photomultipliers 227 is connected to one end of a light guide 229 in order to supply the light to be detected to the photomultiplier 227. The other ends of the light guides 229 are combined in a frame 231 in order to arrange the ends of the light guides 229 in a field whose geometry corresponds to the geometry of the field 217 ( Figure 1 , I2) formed by the incident electron beams 9 on the surface of the electron converter 207. The field 217 on the surface of the electron converter 207 is imaged by the optical imaging system 221 onto the field of ends of the light guides 229. Figure 4represents a plan view of the field formed by the ends of the light guides 229 held in the frame 231.

[0048] The electrons of the electron beams 9 that impinge on the electron converter 207 generate photons, a portion of which exit the electron converter 207 toward the optical imaging system 221. The optical imaging system 221 uses these photons to image the impact points 213 of the electron beams 9 onto the ends of the light guides 229. A portion of the photons enters the light guides 229 and is fed through them to the photomultipliers 227. The photomultipliers 227 convert the incoming photons into electronic signals, which are fed to a controller 235 via signal lines 233. The controller 235 is thus capable of detecting the detection signals of a given photomultiplier 227, wherein the intensity of the detected signals is substantially proportional to the intensity of one of the electron beams 9 assigned to the respective photomultiplier 227.

[0049] Figure 5shows a top view of the detector elements 219 of the second detection system 218. The detector elements 219 are formed by the photodiodes of a CCD sensor 237, in which the photodiodes 219 are arranged in a rectangular array. The number of detector elements 219 of the second detection system 218 can be, for example, 128x128, 1024x1024, or other values. The detector elements 219 of the CCD sensor 237 are read line by line from the array of detector elements 219 via lines 239 and transmitted to the controller 235.

[0050] Figure 6 is an enlarged partial representation of the Figure 4 shown field of the ends of the light guides 229. Here, only the four light guides are shown, which are in Figure 4 are arranged at the top left of the field. These ends of the light guides 229 can be designated by field indices (1,1); (2,1); (1,2); and (2,2).

[0051] A circle represented by a broken line 241 encloses an area within which 90% of the photons would strike the end of the optical fiber 229 with the field index (1,1) in the situation described above as ideal, in which each particle beam is assigned to exactly one detector element of the first detection system and each detector element of the first detection system is assigned exactly one particle beam 3. In particular, in this situation, the particle beam which is in the field 101 (compare I1, Figure 1 ) illuminates the impact point 5 located at the top left and generates secondary electrons, which are formed into an electron beam 9, which is in the field 211 (compare I2, Figure 1) on the surface 211 of the electron converter 207 illuminates the top left segment and generates photons there, which are imaged by the imaging optics 221 onto the end of the light guide (1,1). If this is the case, all detection signals detected by the photomultiplier 227 connected to the light guide (1,1) can be assigned to the particle beam located in the top left of the field 101.

[0052] Deviating from this ideal situation, situations occur in practice in which the photons triggered by this one particle beam do not land 90% in the circle 241, but in a circle arranged offset from it and in Figure 6circle 243 shown by a solid line. This displacement can be due, for example, to the presence of electrical charges on the surface of the object 7, which deflect the beam 9 of secondary electrons from its ideal trajectory, so that the photons triggered by this beam at the electron converter 207 mostly land within the circle 243. This means that some of these photons enter the fiber end (2,1) and another part enters the fiber end (1,2). Figure 6 In the situation shown, it could be assumed that the detection signals attributable to the particle beam 3 described above result from a sum of detection signals detected by different photomultipliers 227. For the Figure 6 In the situation shown, the intensity I of detection signals attributable to said particle beam 3 could, for example, be as follows: I = 0,8 × I 1 1 + 0 , 15 × I 2 1 + 0 , 05 × I 1 2

[0053] The factors 0.8, 0.15, and 0.05 result from a geometric consideration of the overlap of circle 243 with the surfaces of the ends of fibers 229. If necessary, this consideration can also take into account the inhomogeneous distribution of photon intensities within the circle. The distribution of these intensities typically follows a Gaussian function with a high central maximum and tails extending outside of circle 243.

[0054] The position of circle 243 cannot be determined using the first detection system alone. However, the position of circle 243 can be detected using the second detection system 218, which receives the same distribution of light intensities as the first detection system, but offers better spatial resolution due to its larger number of detector elements. By analyzing images detected by the second detection system 218, it is thus possible to determine a circle 243 associated with each of the particle beams 3 on the surface of the ends of the optical fibers 229. Based on the overlap of these circles 243 with the ends of the optical fibers 229, it is then possible to determine which portions of the detection signals from the individual detector elements of the first detection system are to be assigned to which particle beams 3.

[0055] The factors chosen above as examples were created under the assumption that the detector elements (1,1), (2,1), and (1,2) do not receive any signals attributable to particle beams other than the one represented by circle 243. Typically, a system of equations must be solved to obtain the correct assignments of detection signals to particle beams. The solution of such a system of equations is simplified if the number of detection elements of the first detection system is greater than the number of particle beams.

[0056] The method explained here thus makes it possible to assign detection signals detected by multiple detector elements 215 of the first detection system 213 to each of the particle beams 3. This is done based on an image analysis of images acquired by the second detection system 218. This method results in images with higher contrast and fewer artifacts, since not all detection signals detected by a given detector element 215 of the first detection system 213 are always assigned to exactly one of the particle beams 3.

[0057] This procedure is explained again below using the block diagram of the Figure 7explained. There, reference numeral 251 denotes a vector of signal intensities I1, I2, ... I25, which are detected by the 25 detector elements 215 of the first detection system 213 within a first time period. During a second time period, which can be before the first time period, after the first time period, shorter or longer than the first time period, and within the first time period or containing the first time period, an image is recorded with the second detection system 218, which image is subjected to an image analysis 253. Based on this image analysis 253, the proportions to which the detection signals detected by the detector elements 215 of the first detection system 213 are to be assigned to individual particle beams 3 are determined.These components can, for example, be represented as a matrix with which vector 251 is multiplied to obtain a vector 255 whose elements I'1, I'2, ... I'25 represent the detected intensities assigned to the individual beams. As mentioned above, a solution to a system of equations or another complex calculation, such as an iterative calculation, may be necessary to obtain this matrix. In the case where the number of detector elements of the first detection system is greater than the number of particle beams, vector 251 has a number of components equal to the number of detector elements of the first detection system, while the number of components of vector 255 is equal to the number of particle beams. If a matrix is ​​used to obtain vector 255 from vector 251, it would accordingly be non-square.

[0058] Arrows 257 in Figure 7represent the fact that detection signals detected by a detector element are assigned to multiple beams. For example, intensities detected by the first detector element I1 are assigned to beams 1, 2, and 6, but not to the other beams.

[0059] The method further includes scanning the particle beams 3 across the surface of the object. For this purpose, the controller 235 can, for example, comprise a scan generator that provides a suitable scan signal 259. Based on the scan signal 259, a beam deflector is supplied with time-varying voltages. This beam deflector is arranged in the region of the beam path 13, for example, in or near the objective lens 102, in order to collectively deflect the bundle of particle beams 3 based on the scan signal, so that the impact points 5 of the particle beams 3 are shifted on the surface of the object 7.

[0060] For example, the impact locations 5 of the particle beams 3 on the surface of the object 7 can be gradually shifted to new impact locations 5 after a predetermined period of time, which can be equal to the first period, so that a very large number of impact locations 5 are successively illuminated with the particle beams 3. The detected detection signals are then assigned to these impact locations to form the particle microscopic image. Figure 7a vector 261 is shown, which represents detected signal intensities I" that are assigned to impact locations (1i,1i), (2i, 2i), ... in such a time step during a first time interval i. The variable i represents an index of the successively performed time steps. The assignment of the detection signals of the vector 255 assigned to the individual particle beams 3 to the detection signals of the vector 261 assigned to individual impact locations is carried out based on the scan signal 259 of the scan generator.

[0061] An example of the operation of the scan generator and the generation of the scan signal 259 is shown below using the Figure 8This shows a top view of the surface of the object 7 and impact locations 5, which are successively illuminated during the scanning within the time steps performed. The 255 impact locations, which are successively illuminated by the first particle beam 3, are labeled (11,11), (12,12), ... (1225,1225). These lie within a square area 263 of the surface of the object, which contains 225 impact locations. The impact locations generated successively by the scanning are shown in Figure 8connected by a line representing the scan path of the particle beam. It can be seen that the scan path has a meandering shape. In particular, the region 263 of the surface of the object 7 contains nine regions 265, each containing 25 impact locations that are consecutively contained in the scan path. The regions 265 of the surface of the object are significantly smaller than the regions 263 of the surface of the object. In order to correctly assess the ratios of the sizes of the regions 263 and 265, the convex hulls of the impact locations contained in the regions 263 and 265, respectively, are considered. The convex hull of a set of impact locations is the smallest possible area with a convex edge that contains all of these impact locations.

[0062] The maximum lateral extent of the convex hull of the impact locations lying in the surface area 265 is Figure 8denoted by l1, the minimum lateral extent of these impact locations is denoted by l2, and the maximum lateral extent of the convex hull of the impact locations contained in the surface area 263 is Figure 8 denoted by L1, while the minimum lateral extent of these impact locations is denoted by L2. It can be seen that: 2 × l 1 < L 1 and 2 × l 2 < L 2

[0063] This means that the surface areas 265 are significantly smaller in terms of their lateral extent than the surface areas 263.

[0064] The scanning process is described below using the Figure 9explained again. In summary, the scanning process proceeds as follows: The scan signals 259 generated by the scan generator are not changed during a first period of time. During the first period of time, the individual particle beams 3 thus illuminate unchanged impact locations on the surface of the object. A vector 251 of detection signals detected by the photomultipliers 227 during the first period of time is recorded. This measure, which takes place during the first period of time, is described in Figure 9 represented by a block 281. The scan signal is then modified to shift the points of incidence of the particle beams on the object along their scan paths by one position. There, the measure of block 281 is repeated. After 25 such repetitions, which are Figure 9represented by a block 282, all 25 impact locations are scanned that lie in one of the surface areas 265. This measure of scanning the 25 impact locations within one of the surface areas 265 is in Figure 9 represented by a block 283. The execution of the measures of block 283 takes a second time period, which is approximately 25 times longer than the first time period.

[0065] During the second time period, an image is also recorded with the detector elements 219 of the second detection system 218. It is assumed that the number of detector elements 219 of the second detection system is 4096. The recording of an image is in Figure 9 represented by block 284. The action of capturing an image with the second detection system is repeated once within the second period, as represented by block 285 in Figure 9The execution of the one-time repetition of the recording of this image, which takes place simultaneously with the measures marked 283, is in Figure 9 represented by a block 286. The measures 283 and 286 can be carried out in parallel or simultaneously, since the first detection system 213 and the second detection system 218 can simultaneously detect corresponding signals due to the beam splitter 225. The simultaneous execution of the measures 283 and 286 is in Figure 9represented by a block 287 and includes, on the one hand, the detection of detection signals which are assigned to the impact locations arranged within one of the regions 265 and the detection of an image with the second detection system 218. Based on the image detected by the second detection system 218, ie based on the image analysis 253 carried out on the basis of this image, all 25 vectors 251 obtained within the second period are converted into 25 vectors 255 which each represent the detection signals assigned to the individual particle beams.

[0066] This means that the detection signals obtained in 25 successively performed measures 281 are assigned to the individual particle beams according to the same assignment, this assignment being based on a single image acquired in measure 284, also during the second period, with the second detection system.

[0067] In general, it would be desirable to perform measure 284 once during each first time period, thus capturing as many images with the second detection system as measures 281 are performed. However, in the example shown, capturing and reading images with the CCD detector at the frequency corresponding to the first time period is not possible, which is why measure 284 is only performed once, while measure 281 is performed 25 times. However, the CCD detector has a sufficiently high spatial resolution to determine, through image analysis, which of the detector elements 215 of the first detection system 213 are receiving detection signals that can be assigned to individual particle beams.

[0068] The measures of block 287 are repeated nine times, as indicated by block 289, in order to successively scan with each of the particle beams one of the nine surface areas 265 contained in the surface area 263. The totality of these measures is shown in Figure 9 designated by a block 290 and comprises the acquisition of particle microscopic image data by 25 particle beams, each of which is directed to 255 impact locations, so that an intensity measurement is carried out at each of 6,375 impact locations.

[0069] Within block 290, each particle beam illuminates 225 impact locations arranged in nine different object areas 265, which are scanned sequentially. During the scanning of each of the surface areas 265, an image is acquired with the second detection system, which serves as a basis for determining an assignment of detection signals detected by the individual detection elements of the first detection system to the individual particle beams.

[0070] Thus, nine different assignments are used for 225 different successively acquired vectors 251 of detection signals. The scan path is selected such that, taking into account the lateral extent of the convex hull of the impact locations contained in the surface region 263, the convex hull of the impact locations to which the same assignment is applied is as small as possible. This is made possible in particular by the choice of the meandering scan path. This choice of scan path is based on the consideration that changes in the assignment of detection signals to particle beams are caused by surface charges present locally on the surface. These surface charges do not act uniformly on all particle beams, but only on particle beams whose impact locations on the object are close to the surface charge. Furthermore, it is assumed that these surface charges change slowly during scanning.This consideration justifies using the same assignment of detection signals to particle beams for several impact locations arranged one after the other in the scan path.

[0071] The scanning method was previously explained based on simplified values ​​for the number 25 of particle beams used, the number nine of surface areas 265 contained in the surface area 263 scanned by a particle beam, and the number 25 for the number of impact locations contained in a surface area 265. In practice, the individual numerical values ​​can be chosen to be significantly larger.

[0072] In the presentation of the Figure 7The assignment of detection signals detected by the individual detector elements of the first detection system to the individual particle beams is performed based on the image analysis 253 before the assignment of the detection signals assigned to the individual particle beams to the individual impact locations based on the scan signal 259 of the scan generator. This order can be reversed, and the assignment based on the image analysis 253 to individual particle beams can be performed after the assignment of the individual particle beams to the impact locations based on the scan signal 259.

[0073] Figure 10 shows a further variant of an electron detector 209, which is used in the multi-beam particle beam microscope of the Figure 1 The electron detector 209 has an electron converter 207, on the surface 211 of which the electron beams 9 are arranged in the representation of the Figure 10from the left. The energy of the electrons of the electron beams 9 is converted by the electron converter 207 into two different types of signals, which are detected by two different detection systems.

[0074] A first detection system 213 of the two detection systems comprises detector elements 215, which are semiconductor elements, such as silicon drift detectors and PIN diodes. These are also part of the electron converter in that the electrons of the electron beams 9 penetrating them are converted into first signals, namely electron-hole pairs, in the semiconductor elements. These signals trigger electrical signals in the semiconductor elements, which, after suitable amplification, are output to a controller 235 of the multi-beam particle beam microscope 1. The semiconductor elements of the detector elements 215 are electron detectors that can be read at high rates, such as 40 MHz to 400 MHz, and convert impinging electrons into electrical signals. The number of detector elements 215 of the first detection system 213 can be equal to or greater than the number of electron beams 9 impinging on the electron converter 207.

[0075] A second detection system 218 of the electron detector 209 includes a light detector 237, such as a CCD sensor, having an array of detector elements or pixels 219. The number of detector elements 219 of the second detection system 218 is substantially greater than the number of detector elements 215 of the first detection system 213.

[0076] Between the electron converter 207 and the light detector 237, an optical imaging system 221 is provided, which optically images the surface 211 of the electron converter 207 onto the field of detector elements 219 of the second detection system 218.

[0077] Due to its configuration comprising the semiconductor elements 215, the electron converter 207 is designed to convert a portion of the electrons of the electron beams 9 striking it into the first signals, namely the electron-hole pairs, which are detected by the first detection system 213. A further portion of the electrons of the electron beams 9 striking the electron converter 207 is converted into second signals, namely photons, which emerge from the surface 211 of the electron converter 207, onto which the electron beams 9 also strike. In the illustration of the Figure 10 These photons exit the electron converter 207 to the left.

[0078] A portion of these photons emerging from the surface 211 of the electron converter 207 is imaged by the optical imaging system 221 onto the light detector 237 and detected by its detection elements 219. The images detected by the light detector 237 are transmitted to the controller 235.

[0079] The signals detected by the second detection system 218 can be photons of various types. Firstly, the electron beams 9 impinging on the surface 211 of the electron converter 207 cause local heating there. This local heating generates photons that are detectable by the light detector 237 in the infrared range of the radiation spectrum. These photons have photon energies of, for example, 1 meV to 500 meV.

[0080] To detect these photons, the light detector 237 and the optical imaging system 221 are advantageously designed as an infrared camera. The infrared camera can thus generate a thermal image of the surface 211 of the electron converter 207 and then evaluate it. The thermal image represents the distribution of the electron intensities impinging on the surface 211 of the electron converter 207. The readout rate of the light detector 237 is significantly slower than the readout rate of the detection elements 215 of the first detection system 213, but the number of detection elements 219 of the second detection system 218 is significantly greater than the number of detection elements 215 of the first detection system 213.As already explained above, based on an image analysis of the images detected by the light detector 237, an improved assignment of the signals detected by the detection elements 215 of the first detection system 213 to the electron beams 9 impinging on the electron converter 207 and finally to the impact locations of the particle beams 3 on the object 7 can be carried out.

[0081] As an alternative to the design of the light detector 237 and the optical imaging system 221 as an infrared camera, a camera operating in the visible range of the light spectrum or other ranges of the light spectrum can also be used if a Figure 10A layer 271 of scintillator material, shown by dashed lines, is provided. The scintillator material converts the energy of a portion of the electrons of the electron beams 9 into photons with energies of, for example, 1 eV to 4 eV, which are thus greater than the energies of the thermal radiation generated by heating the surface 211 of the electron converter 207 with photon energies of, for example, 1 meV to 500 meV.

[0082] The imaging of the photons generated by the layer 271 of scintillator material onto the light detector 237 generates an image there, which represents the distribution of the electron intensities impinging on the surface 211 of the electron converter 207. These images can be read out again to the controller 235 and used to assign the signals detected by the detection elements 215 of the first detection system 213 to the individual electron beams 9 and ultimately to the impact locations of the particle beams 3 on the object 7.

Claims

1. Method for operating a multi-beam particle beam microscope (1), wherein the method comprises: scanning a multiplicity of particle beams (3) over an object (7); directing electron beams (9) emanating from impingement locations (5) of the particle beams at the object onto an electron converter (207); detecting first signals generated by impinging electrons in the electron converter by means of a plurality of detection elements (215) of a first detection system (213) during a first time period; and detecting second signals generated by impinging electrons in the electron converter by means of a plurality of detection elements (219) of a second detection system (218) during a second time period, wherein a number of the detection elements of the second detection system is more than double the magnitude of a number of the detection elements of the first detection system; characterized by assigning to the impingement locations the signals which were detected by means of the detection elements of the first detection system during the first time period, specifically on the basis of the detection signals which were detected by means of the detection elements of the second detection system during the second time period.

2. Method according to Claim 1, wherein detecting the first signals generated by the impinging electrons in the electron converter by means of the plurality of detection elements of the first detection system is carried out at a rate that is greater than a limit frequency, and wherein detecting the second signals generated by the impinging electrons in the electron converter by means of the plurality of detection elements of the second detection system is carried out at a rate that is less than 0.5 times the limit frequency.

3. Method according to either of Claims 1 and 2, wherein the detection signals which were detected by means of a single detection element of the detection elements of the first detection system during the first time period are assigned to at least two different impingement locations.

4. Method according to any of Claims 1 to 3, furthermore comprising: assigning the detection signals which were recorded by means of each detection element of the detection elements of the first detection system during the first time period to one or a plurality of particle beams of the multiplicity of particle beams, wherein the detection signals which are assigned to an identical one of the multiplicity of particle beams are assigned to an identical impingement location.

5. Method according to any of Claims 1 to 4, wherein scanning the multiplicity of particle beams comprises deflecting the particle beams on the basis of a deflection signal that changes over time; and wherein assigning to the impingement locations the detection signals which were detected by means of the detection elements of the first detection system during the first time period is furthermore carried out on the basis of the deflection signal.

6. Method according to any of Claims 1 to 5, wherein the number of the detection elements of the first detection system is greater than a number of the multiplicity of particle beams, and / or wherein the number of the detection elements of the first detection system is equal to an integral multiple of the number of the multiplicity of particle beams, and / or wherein the number of the detection elements of the first detection system is equal to the number of the multiplicity of particle beams.

7. Method according to any of Claims 1 to 6, wherein the first signals generated in the electron converter are photons.

8. Method according to any of Claims 1 to 7, wherein the detection elements of the first detection system comprise photomultipliers and / or avalanche photodiodes; and / or wherein the first signals generated in the electron converter are electrical signals.

9. Method according to any of Claims 1 to 8, wherein the second signals generated in the electron converter are photons, wherein the detection elements of the second detection system are arranged next to one another in an array, and wherein the method furthermore comprises optically imaging a surface of the electron converter onto the array of detection elements of the second detection system.

10. Method according to Claim 9, furthermore comprising using photons for the optical imaging which emerge from the electron converter at a side on which the electron beams impinge on the electron converter.

11. Method according to Claim 10, wherein the photons detected by the detection elements of the second detection system have an energy of 1 meV to 500 meV.

12. Method according to any of Claims 9 to 11, wherein the detection elements of the second detection system comprise photodiodes, wherein the second detection system comprises a CCD, in particular.

13. Multi-beam particle beam system, comprising: an electron converter (207); an illumination system (300) configured to illuminate an array of impingement locations (5) at an object (7) with a multiplicity of particle beams (3); a projection system (102, 205) configured to direct electron beams (9) emanating from the impingement locations onto the electron converter; a first detection system (213) having a plurality of detection elements (215) configured to detect first signals generated by impinging electrons in the electron converter; a second detection system (218) having a plurality of detection elements (219) arranged next to one another in an array and configured to detect second signals generated by impinging electrons in the electron converter, wherein the second signals are photons; and an optical imaging system (221) configured to image a surface of the electron converter optically onto the array of detection elements of the second detection system; characterized in that such photons which emerge from that surface of the electron converter onto which the electron beams are also directed onto the electron converter are directed onto the detection elements of the second detection system by the optical imaging system.

14. Multi-beam particle beam system according to Claim 13, wherein the electron converter comprises a scintillator material which converts energy of the electron beams into photons.