Multi-beam inspection method with precise defect location

Abstract

A multi-beam charged particle beam system and a method for scanning image acquisition with high reliability and high repeatability is provided. The system and method enable a fast and precise transfer of image coordinates into wafer coordinates.

Description

[0001] Multi-Beam inspection method with precise defect location

[0002] Field of the invention

[0003] The present invention relates to an inspection apparatus for semiconductor objects within a semiconductor wafer, more particularly, to a multi-beam apparatus and corresponding method of operating the multi-apparatus for performing inspection of a wafer.

[0004] Prior art

[0005] Fabricated semiconductor structures are fabricated by determined processes and are based on prior knowledge. A semiconductor wafer has a diameter of 300 mm and consists of a plurality of several sites, so called dies, each comprising at least one integrated circuit pattern such as for example for a memory chip or for a processor chip. During fabrication, semiconductor wafers run through 1000 or more process steps, and within the semiconductor wafer, about 100 and more parallel layers are formed, comprising the transistor layers, the layers of the middle of the line, and the interconnect layers and, in memory devices, a plurality of memory cells. Fabricated semiconductor structures suffer from rare and different imperfections. Devices for quantitative metrology, defect-detection or defect review are looking for these imperfections. For example, locations of defect candidates are identified by optical inspection tools, and highly resolving electron microscopes are utilized for inspection of the defect candidates. Semiconductor inspection tasks require a high resolution and a high throughput. WO 2005 / 024881 A2 discloses an electron microscope system which operates with a multiplicity of electron beamlets for the parallel scanning of a sample to be inspected with a plurality of electron beamlets. The plurality of primary electron beamlets is generated by directing a primary charged particle beam onto a multi-aperture plate, which has a multiplicity of openings. One portion of the electrons of the electron beam is incident onto the multi-aperture plate and is absorbed there, and other portions of the beam transmit the plurality of openings of the multiaperture plate. Thereby, in the beam path downstream of each opening, a plurality ofprimary electron beamlets is formed. The plurality of primary electron beamlets are focused by an objective lens on a surface of a sample. Interaction volumes of the primary electrons within the sample are formed at the plurality of focus points. Secondary electrons or backscattered electrons are emitted from the interaction volumes. Thereby, a plurality of secondary electron beamlets is emitted from the sample, collected, and imaged onto a detector. Each of the secondary beamlets is incident onto separate detector elements, so that the secondary electron intensities detected therewith provide information relating to the surface of the sample at the location where the corresponding primary beamlet is incident onto the sample. The bundle of primary electron beamlets is scanned systematically over the surface of the sample and an electron microscopic image of the sample is generated. Alternatively, the sample is continuously moved by a sample stage and a scanning image is acquired.

[0006] Defects inspection and defect review with high throughput, however, require a fast and reliable determination of defect coordinates. A reliable, repeatable determination of a defect location within a scanning image of a multi-beam system with a plurality of primary charged particle beamlets may depend on system properties such as imaging settings.

[0007] It is therefore a task of the invention to provide a method of accurate determination of a defect location on a wafer using multi-beam scanning images. It is therefore a further task of the invention to provide a method of accurate positioning of a predetermined defect location at a predetermined coordinate within a multi-beam scanning image to be acquired. It is therefore a further task of the invention to provide a method of determining coordinates within a multi-beam system for scanning image acquisition with high reliability and high repeatability. It is therefore a further task of the invention to provide a faster method of determining coordinates within a multi-beam system for scanning image acquisition

[0008] DE 102021200799 B3 discloses background art.J. T. Neumann et al., Defect detection and classification on imec iN5 node BeoL test vehicle with multibeam scanning electron microscope, Journal of Micro / Nanopatterning, Materials, and Metrology 22 (2023), S. 021009 - 021009 discloses background art.

[0009] of the invention

[0010] The objects of the invention are solved by the embodiments and examples.

[0011] The present patent application claims the priority of German patent application No. 102025 101889.8 filed on 20 January 2025, the disclosure of which in the full scope thereof is incorporated in the present patent application by reference.

[0012] According to an embodiment, a method of accurate determination of a defect location with a multi-beam charged particle beam system is provided. According to the method, defect coordinates detected in multi-beam scanning images are precisely mapped or transformed to wafer coordinates. Within a multi-beam charged particle beam system, a plurality of J primary charged particle beamlets is used for parallel image acquisition. The method comprising loading a wafer with a predefined wafer coordinate system to a sample chuck of the multi-beam charged particle beam system. The method is further comprising determining a wafer loading displacement vector R3 and a wafer loading angle J3 of the wafer coordinate system with respect to the sample chuck. The method is further comprising a step of selecting an image setting of the multi-beam charged particle beam system and determining a plurality of raster positions R_MB.j of a plurality of focus spots of the plurality of J primary charged particle beamlets for the selected image setting. The method is further comprising determining a multi-beam displacement vector R1 of a line of sight (LOS) and a multi-beam rotation angle JI from the plurality of raster positions R_MB.j. The method is further comprising selecting a chuck displacement vector R4 and a chuck rotation angle J4 and positioning the sample chuck with a wafer stage at a position according to the chuck displacement vector R4 and the chuck rotation angle J4.

[0013] The method is further comprising a step of determining a data required for a coordinate transfer from multi-beam image coordinates into wafer coordinate system. The coordinatetransfer is generally comprising a coordinate transformation comprising at least one of an affine transformation or a mapping. Affine transformations comprise at least one of a translation or a rotation of a coordinate system.

[0014] The data required for a coordinate transfer is comprising at least one of a transfer matrix TMB2W, a plurality of raster positions R_MB_W.j of the plurality of focus spots of the plurality of J primary charged particle beamlets in the wafer coordinate system, and a rotation matrix TSR(JW) with a scan rotation angle JW in wafer coordinate system.

[0015] The method is further comprising acquiring a multi-beam image of a surface area of the wafer by scanning a plurality of focus spots of the plurality of primary charged particle beamlets in a scanning direction with a scan rotation angle JO. A multi-beam image of a surface area of the wafer is acquired by scanning the plurality of focus spots of the plurality of primary charged particle beamlets to a plurality of scanning positions vectors R_cs with a scan rotation angle JO. The method is further comprising determining an image position vector R_W of an image point of the multi-beam image in wafer coordinate system from a scanning positions vector R_cs and at least one of the data required for the coordinate transfer.

[0016] In an example, the chuck rotation angle J4 is selected such that the scan rotation angle JW in wafer coordinate system is JW = 0. For example, the chuck rotation angle J4 is selected in accordance with J4 = JI + JO - J3.

[0017] In an example, the transfer matrix TMB2W is determined from the wafer loading displacement vector R3, the wafer loading angle J3, the multi-beam displacement vector Rl, the multibeam rotation angle JI, the chuck displacement vector R4 and the chuck rotation angle J4. In an example, each of the plurality of raster positions R_MB_W.j in wafer coordinate system is determined by multiplication of the transfer matrix TMB2W with one of the plurality of raster positions R_MB.j of the plurality of focus spots. In an example, the image position vector R_W is determined according to R_W = R_MB_W.j + TSR (JW) * R_cs. In an example, the image position vector R_W is determined according to R_W = TMB2W * [R_MB.j + TSR(JO) * R_cs] .In an example, the method is further comprising determining an image position vector R_l of an image point within the multi-beam image by R_l = R_MB.j + TSR(JO) * R_cs, with scanning position vector R_cs and scan rotation matrix TSR(JO) according to the scan rotation angle JO. The method is further comprising transforming the image position vector R_l into a position vector R_W in wafer coordinate system by R_W = TMB2W (R1,J1; R3,J3; R4, J4) * R_l with the transfer matrix TMB2W ("multi-beam-to-wafer") depending on the wafer loading displacement vector R3, the wafer loading angle J3, the multi-beam displacement vector Rl, the multi-beam rotation angle JI, the chuck displacement vector R4 and the chuck rotation angle J4. Thereby, an accurate determination of a defect location or a location of an object of interest is determined in wafer coordinates, irrespective of a loading precision of the wafer. Thereby, the specifics of a multi-beam system, including the positions of the plurality of focus spots of the plurality of primary beamlets, and the scanning rotation angle are considered and image coordinates within a multi-beam system for scanning image acquisition are determined with high reliability and high repeatability.

[0018] In an example, the method is further comprising transforming a center position vector RD of an inspection site in wafer coordinate system into a position vector R6_R of the inspection site in a reference coordinate system and determining the chuck displacement vector R4 according to equation R4 = Rl - R6_R, with multi-beam displacement vector Rl. In an example of the method, the step of transforming the center position vector RD of the inspection site in wafer coordinate system into a position vector R6_R in reference coordinate system is comprising transforming the center position vector RD of the inspection site in wafer coordinate system into a position vector R6 in chuck coordinate system by equation R6 = TRw2c ( J 3 ) * RD + R3. The rotation matrix Tw2c ("wafer-to-chuck") is thereby depending on the wafer loading angle J3 and the wafer loading displacement vector R3. The method is further comprising the step transforming the position vector R6 in chuck coordinate system into the position vector R6_R in reference coordinate system by equation R6_R = TRRC2C (J4) * R6. The rotation matrix TRRC2C ("reference-to-chuck") is depending on the chuck rotation angle J4. Thereby, for example a fast and accurate positioning of a predetermined defect location at a predetermined coordinate with the multi-beam scanning image is enabled.In an example, the method is further comprising selecting the inspection site from a list of a plurality of inspection sites assigned to the wafer. In an example, the method is further comprising selecting the selected image setting according to at least one of the inspection sites from the list of a plurality of inspection sites.

[0019] In an example, the method is further comprising determining the multi-beam displacement vector Rl, the multi-beam rotation angle JI with respect to a reference coordinate system corresponding to a reference frame of the multi-beam charged particle beam system.

[0020] In an example, the method is further comprising determining the multi-beam displacement vector Rl, the multi-beam rotation angle JI, and the plurality of raster positions R_MB.j of the plurality of focus spots (5.j, j = 1...J) for the selected image setting from a memory of the multi-beam charged particle beam system. The memory is comprising data representing at least one predetermined beam displacement vector Rl, multi-beam rotation angle JI, and a plurality of raster position R_MB.j of the plurality of focus spots for at least one image setting. Such data can be predetermined data, for example determined during a calibration of the multi-beam charged particle beam system.

[0021] In an example, the method is further comprising determining the multi-beam displacement vector Rl, the multi-beam rotation angle JI, and the plurality of raster positions R_MB.j of the plurality of focus spots of the plurality of J primary charged particle beamlets for the selected image setting during a calibration step of the multi-beam charged particle beam system.

[0022] In an example, the method is further comprising changing an image plane of the multi-beam charged particle beam system by adjusting a focus plane of the plurality of focus spots and determining a modification of the multi-beam displacement vector Rl into Rim, a modification of multi-beam rotation angle JI into Jim, and a change of the scanning angle JO into JOm. A change of an image plane can be required for example due to a wafer curvature or wafer wedge angle. After lateral movement, a wafer surface may not be arranged within the image plane of the multi-beam charged particle beam system and a changing an imageplane of the multi-beam charged particle beam system may be required. In an example, the method is further comprising changing the scan rotation angle JOm into a scan rotation angle J0m2 such that a scanning direction is not changed within a reference coordinate system.

[0023] In an example, the method is further comprising an image processing of the multi-beam image, the image processing comprising at least one of an image stitching, contrast and brightness adjustment, noise reduction, image data compression, a feature detection, an object recognition, a feature measurement or a feature comparison. In an example, the method is further comprising a comparison of at least a segment of a multi-beam image with a reference image.

[0024] In an example, the reference image is derived from CAD data. Such a derivation may comprise an imaging simulation corresponding to the image acquisition with the multi-beam charged particle beam system.

[0025] In an example, the method is further comprising detecting at least one of a defect or an object of interest within the multi-beam image and storing a wafer coordinate R_W of the at least one detected defect or object of interest in a memory. In an example, the method is further comprising a review step of at least one defect or object of interest at the wafer coordinate R_W.

[0026] According to an embodiment, a multi-beam charged particle beam system is comprising a multi-beam charged particle beam column. The multi-beam charged particle beam column is rigidly mounted to a reference frame via a mounting flange. The reference frame is defining a reference coordinate system. The multi-beam charged particle beam system is further comprising a stage with a sample chuck for receiving a wafer and a control unit with a control operation processor and a memory. The memory is comprising software instructions, for causing when executed by the control operation processor to perform any of the method steps of the method described above.

[0027] The invention described by examples and embodiments is not limited to the embodiments and examples but can be implemented by those skilled in the art by various combinations ormodifications thereof. The present invention will be even more fully understood with reference to the following drawings:

[0028] Fig. 1 is a schematic sectional view of a multi-beam charged particle beam system 1 according to a first embodiment

[0029] Fig. 2 illustrates some details of a multi-beam charged particle beam system 1

[0030] according to the first embodiment

[0031] Fig. 3 illustrates coordinates of a plurality of focus spots in an image plane and an example of scanning coordinates

[0032] Figs.4a,b,c illustrate coordinates of an image acquisition with multi-beam charged particle beam system

[0033] Fig. 5 is a schematic sectional view of a multi-beam charged particle beam system 1 with a reference frame 1025

[0034] Figs. 6a, b illustrate examples of a method according to a third embodiment

[0035] Figs. 7a, b illustrate a wafer and a substrate holder or chuck with corresponding coordinate systems

[0036] Figs. 8a, b illustrate a transfer of image coordinates into wafer coordinates

[0037] Fig. 9 illustrates a closed loop transformation of coordinates from wafer coordinates to image coordinates and vice versa

[0038] Fig. 10 illustrates an example of an architecture of a multi-beam charged particle beam system 1

[0039] In the exemplary embodiments of the invention described below, components similar in function and structure are indicated as far as possible by similar or identical reference numerals. Some array elements, for example the plurality of primary charged particle beamlets, are identified by a reference number. Depending on the context, the same reference number may also identify a single element out or the array elements. Each primary charged particle beamlet (3.1, 3.2, 3.3) is one beamlet of the plurality of primary charged particle beamlets (3).

[0040] The schematic representation of figure 1 illustrates basic features and functions of a multibeam charged-particle system 1 according to a first embodiment. It is to be noted that thesymbols used in the figure have been chosen to symbolize their respective functionality. The type of system shown is that of a multi-beam scanning electron microscope using a plurality of primary charged particle beamlets 3 for generating a plurality of primary charged particle beam spots 5 on a surface 25 of an object or sample 7, such as a wafer or mask substrate, located with a top surface 25 in an object plane 101 of an objective lens 102. For simplicity, only three primary charged particle beamlets 3.1 to 3.3 and three primary charged particle beam spots 5.1 to 5.3 are shown, but there can be more beamlets, for example the number J of beamlets can be more than J = 60, more than J = 90, for example J = 397, J = 547, or even more, for example J = 919. The features and functions of multi-beamlet charged-particle system 1 can be implemented using electrons or other types of primary charged particles such as ions and in particular Helium ions. Further details of the microscope system 1 are provided in International Patent application WO 2022 / 262970 Al, which is hereby fully incorporated by reference.

[0041] The multi-beam charged-particle beam system 1 comprises an object irradiation unit 100 and a detection unit 200 and a secondary electron beam divider or beam splitter unit 400 for separating the secondary charged-particle beam path 13 from the primary charged-particle beam path 11. The object irradiation unit 100 comprises a charged-particle multi-beam generator 300 for generating the plurality of primary charged-particle beamlets 3 and is adapted to focus the plurality of primary charged-particle beamlets 3 in the object plane 101, in which the surface 25 of an object or wafer 7 is positioned by a sample stage 500.

[0042] The primary beam generator 300 produces a plurality of intermediate focus spots of the primary charged particle beamlets 3 in an intermediate image surface 321. The primary beamlet generator 300 comprises at least one source 301 of primary charged particles, for example electrons. The at least one primary charged particle source 301 emits a diverging primary charged particle beam 309, which is collimated by at least one collimating lens 303. The collimated primary charged particle beam 309 is incident on the multi-beam forming unit 305. A multi-beam forming unit 305 is for example explained in US 2019 / 0259575, and in US 10.741.355 Bl, both hereby incorporated by reference. The multi-beam forming unit 305 basically comprises a first multi-aperture plate or filter plate 304 illuminated by theprimary charged particle beam 309. The first multi-aperture plate or filter plate 304 comprises a plurality of apertures in a raster configuration for generation of the plurality of primary charged particle beamlets 3, which are generated by transmission of parts of the primary charged particle beam 309 through the plurality of apertures. The multi-beamlet forming unit 305 comprises at least one further multi-aperture plate 306, which is located, with respect to the direction of movement of the electrons in beam 309, downstream of the first multi-aperture or filter plate 304. For example, a second multi-aperture plate 306 comprises for example four, eight or more electrostatic elements for each of the plurality of apertures, for example to deflect each of the plurality of beamlets 3 individually. The multibeam forming unit 305 is further configured with an adjacent electrostatic field lens 331, which can be combined in the multi-beam forming unit 305. Together with a second field lens 333, each of the plurality of primary charged particle beamlets 3 is focused in or in proximity of the intermediate image surface 321. The primary charged-particle source 301 and each of the active multi-aperture plates 306 are controlled by control unit 830.

[0043] After passing the intermediate image surface 321, the plurality of primary charged particle beamlets 3 is imaged by field lens group 103 and objective lens 102 into the object plane 101.

[0044] Figure 2 illustrates further details of the multi-beam system 1. Same reference numbers are used, and reference is made also to the description of figure 1. A beam tube 151 is provided downstream of the multi-aperture arrangement 305. In the example of figure 2, the beam tube 151 is comprising several beam tube segments 151.1 to 151.5. The beam tube 151 being connected to a voltage supply with the first or tube voltage Ul. From the entrance of a beam tube 151, the plurality of primary charged particle beamlets 3 propagate along the primary beam path 11 at a constant kinetic energy ET until the exit opening 153 of the beam tube 151. The kinetic energy ET of the primary charged particle beamlets 3 during passing the beam tube 151 is for example 20keV, 30keV, 60keV, or more.

[0045] The plurality of primary charged particle beamlets 3 are imaged and focus points 5.o and 5.i are formed in an image plane 101 by field lenses 333 and 103 (see figure 1), and by the objective lens 102. The objective lens 102 is of the type of a magnetic lens with a coil 161 and a pole shoe 163, forming a gap for the magnetic field. A current I is provided during useto the coil 161 to generate the focusing magnetic field (not shown). With the gap, the magnetic field is constrained within a volume above the surface 25 of the sample 7.

[0046] The lower pole shoe 163 is connected to a voltage supply and a second voltage U2 is provided to the pole shoe 163. Below the lower pole shoe 163, a decelerating electrode 133 is provided, connected to a voltage supply for providing a voltage U3 to the electrode. In the example shown, the decelerating electrode 133 is provided as separate electrode. Via sample voltage supple 503, a sample voltage U4 is provided by sample voltage supply 503 to a sample mounting platform for holding and contacting during use a wafer 7. According to the voltage difference between U1 and U4, an electrical field 137 is generated, which is almost parallel to the propagation direction of the primary charged particle beamlets 3 and generates a decelerating force to the primary charged particles. The sample voltage U4 is adjusted such that the third kinetic energy or landing energy EL of the primary electrons is adjusted in a range below lOkeV, below 2keV, IkeV, 800eV, below 300eV or even below lOOeV. The electrical field 137 forms a decelerating field to reduce the kinetic energy of the primary charged particle beamlets 3 before impinging on the sample surface 25 arranged in the image plane 101, such that a high resolution is achieved. The electrical field 137 forms in parallel an extraction field for extracting and accelerating secondary electrons from the wafer 7. The electrical field 137 above the surface 25 of the sample 7 is therefore also called the extraction field 137.

[0047] The object irradiation system 100 of the multi-beam charged particle beam system 1, shown in Figure 1 and 2, further comprises a collective multi-beam raster scanner 110 in proximity of a beam cross-over by which the plurality of charged particle beamlets 3 can be deflected in scanning direction perpendicular to the propagation direction of the charged particle beamlets 3. Throughout the examples, the propagation direction of the primary beamlets is in positive z-direction. Objective lens 102 and collective multi-beam raster scanner 110 are centered at an optical axis 1105 of the multi-beam charged-particle system 1. For example, the plurality of primary charged particle beamlets 3, forming the plurality of beam spots 5 arranged in a raster configuration, is scanned synchronously over the wafer surface 25. The primary beam spots 5 have a distance or pitch P of about 6pm to 45pm and a diameter of below 5nm, for example 3nm. In an example, the beam spot size is about 4nm, and thedistance between two adjacent beam spots is 8pm. At each scan position of each of the plurality of primary beam spots 5, a plurality of secondary electrons is generated, respectively, forming the plurality of secondary electron beamlets in the same raster configuration as the primary beam spots 5. The intensity of secondary charged particle beamlets, including beamlets 9.o and 9.i depends on the intensity of the impinging primary charged particle beamlet 3, the material compositions, the topography of the object 7 under the beam spot 5, and the charging condition of the sample at the beam spot 5. The plurality of secondary charged particle beamlets 9 are accelerated by the same electrostatic field 137 between objective lens 102 and object surface 25 and are collected by objective lens 102 and pass the first collective multi-beam raster scanner 110 in opposite direction to the primary beamlets 3. The plurality of secondary beamlets 9 is scanning deflected by the first collective multi-beam raster scanner 110. The plurality of secondary charged particle beamlets is then guided by the beam splitter unit 400 to follow the secondary beam path 13 to the detection unit 200. The beam divider 400 comprises a first beam divider segment 400.1 arranged in the primary beam path 11, and third beam divider segment 400.3 arranged in the secondary electron beam path 13, and a second beam divider segment 400.2 arranged in both primary and secondary electron beam path 11 and 13 and configured for dividing secondary electron beamlets 9 from the primary charged particles.

[0048] Detection unit 200 images the secondary electron beamlets 9 onto the surface 225 of an image sensor 600 to form there a plurality of secondary charged particle image spots 15. Detection unit 200 further comprises at least a second raster scanner 222, which is connected to scanning control unit 860 (see figure 1). Scanning control unit 860 is configured to compensate a difference in the scanning deflection power of the first scanning deflector 110 in the common beam path and the second scanning deflector 222, such that the positions of the plurality secondary electron focus spots 15 are kept at constant positions at image sensor 600. In the example, the detection unit 200 further comprises several beam tube segments 151.1 to 151.5. The detection unit 200 comprises first to third electron optical lenses 205.1 to 205.3, first and second electron-optical lens elements 211.1 and 211.2, first corrector or compensator 220, and at least two stigmators 264.1 and 264.2. A pupil or aperture stop 284 is arranged within a common pupil plane 258 and for example mounted on a stage for adjustment or exchange of different pupil stops 284. The pupil plane258 corresponds to a joint cross-over of the secondary electron beamlets 9. With the pupil or aperture stop 284, a momentum or energy filtering is obtained. Such a momentum or energy filtering is for example described in WO 2024 / 078739 Al, which is hereby fully incorporated by reference. The detection unit 200 further comprises the image sensor 600, which is connected to imaging control module 810, configured to receive image data during scanning operation. The first to third electron optical lenses 205.1 to 205.3, the first and second electron-optical lens elements 211.1 and 211.2, the first compensator 220, and the stigmators 264.1 and 264.2 are connected to secondary beam-path control module 840.

[0049] The detector or image sensor 600 comprises a plurality of detector pixels or individual detectors. For each of the plurality of secondary charged particle beam spots 15, the intensity is detected separately, and the property of the surface 25 of the object 7 is detected with high resolution and with high throughput. The digital image data is collected by imaging control module 810. Details of the digital image data collection and processing, using for example parallel processing, are described in international patent application WO 2020 / 151904 A2 and in US-Patent US 9.536.702, which are hereby incorporated by reference.

[0050] The image sensor 600 is configured by an array of sensing areas in a pattern compatible to the raster arrangement of the secondary electron beamlets focused by the detection unit 200 onto the image sensor 600. This enables a detection of each individual secondary electron beamlet independent from the other secondary electron beamlets. The image sensor 600 illustrated in figures 1 and 2 can be an electron sensitive detector array such as a CMOS or a CCD sensor. Such an electron sensitive detector array can comprise an electron-to-photon conversion unit, such as a scintillator element or an array of scintillator elements. The image sensor 600 can further comprise a relay optical system for imaging and guiding the photons generated by the electron-to-photon conversion unit on dedicated photon detection elements, such as a plurality of photomultipliers or avalanche photodiodes. An example of an image sensor is disclosed in US 9,536,702, which is incorporated by reference.

[0051] In an example of an acquisition of an image patch by scanning the plurality of primary charged particle beamlets 3, the stage 500 is not moved, and after the acquisition of animage patch, the stage 500 is moved to the next image patch to be acquired. In an example, the stage 500 is continuously moved in a second direction while an image is acquired by scanning of the plurality of primary charged particle beamlets 3 with the collective multibeam raster scanner 110 in a first direction. Stage movement and stage position is monitored and controlled by sensors known in the art, such as Laser interferometers, grating interferometers, confocal micro lens arrays, or similar.

[0052] During an image scan, the control unit 800 is configured to trigger the image sensor 600 to detect in predetermined time intervals a plurality of timely resolved intensity signals from the plurality of secondary electron beamlets, and the digital image of an image patch is accumulated and stitched together from all scan positions of the plurality of primary charged particle beamlets 3.

[0053] The control unit 800 of the multi-beamlet charged-particle system 1 further comprises an-imaging control module 810, configured to receive the data streams from the image sensor 600 and to generate during operation a digital image of the surface of the sample 7; a secondary beam-path control module 840, configured to control the detection unit 200; a primary beam-path control module 830, configured to control the elements of the object irradiation unit 100; a stage control module 850, configured to control the stage positioning and alignment, and including control of the sample voltage supply unit 503; a scanning operation control module 860, configured to control a scanning operation by the first collective multi-beam raster scanner 110 and the second deflection system 222; a control operation processor unit 880, configured to execute inspection tasks of samples, and configured to control the modules 810, 820, 830, 840, 850, 860 and at least one memory 890 for storing software instructions and image data. It is understood that control unit 800 comprises several different memories, which are here simplified and summarized as memory 890. The control operation processor unit 880 is further connected to an interface IX (not shown) for exchange of data, instructions, software or user interaction.

[0054] The axial position of the image plane 101 is typically adjusted by focusing means within the object irradiation unit 100, for example an adjustment of at least one of the objective lens 102, a field lens 103, and the decelerating field 137. For example, if a desired landing energyLE is selected, decelerating field 137 is adjusted and the axial position of the image plane 101 is changed. Within the image plane 101, a plurality of beam spots 5 is formed in a raster arrangement 251. Figure 3a illustrates a typical example of a raster arrangement 251 with a plurality of focus spots 5 in the image plane 101. Within the multi-beam coordinate system 1115 with coordinates [x_MB, y_MB], a plurality of position coordinate vectors R_MB.j with coordinates (x_MB.j, yJVIB.j) of the plurality of primary beam spots 5.j is given. The position coordinate vectors R_MB.j of each beam spot 5.j with j = 1....J are for example determined during a calibration of the plurality of beam spots 5 at a qualified reference object.

[0055] The origin of the multi-beam coordinate system 1115 is defined at the line-of-sight (LOS) of the multi-beam system 1. The line-of-sight of the multi-beam system 1 is for example defined by the center of gravity of all primary beamlets 3. For example, the line-of-sight is given by the central beam spot 5.1 at [x_MB.l = 0, y_MB.l = 0], In an example of an image acquisition, each primary beamlet 3 is synchronously scanned over a small segment on the surface 25 of the sample 7. Scanning is for example performed by common scanning deflector 110, formed for example by a multipole element. Each beam spot, for example beam spot 5.j, is scanned in scanning coordinate system (SCS) 1119 with scanning coordinates [xs,ys], and for each beam spot 5.j, a scanning field 241.j is acquired during image scanning. An example of a scanning field 241. j is illustrated in figure 3b. Within each scanning field 241.j, image point coordinates are defined in the scanning coordinate system 1119, centered at an unscanned beam focus position R_MB.j of a single beam spot 5.j. For example, an image point 1006 is determined by the position of the beam spot 5.j within the multi-beam coordinate system MB plus the local scanning coordinate vector R_cs = (x_sc, y_sc) in scanning coordinate system 1119.

[0056] Objective lens 102 generates a magnetic field for focusing the primary beamlets 3. Typically, also the field lenses 103 and 333 are formed by magnetic field lenses. Each imaging by magnetic fields may result in a rotation of the plurality of beamlets 3 around the optical axis 1105. A part of the magnetic field of the objective lens 102 is downstream of the common scanning deflector 110, thereby also influencing the scanning coordinate system 1119.

[0057] Therefore, each scanning coordinate system 1119 with coordinates [xs, ys] of each scanning field 241.j is typically rotated with respect to the multi-beam coordinate system 1115 byscan rotation angle JO. Figure 4a illustrates an example of a scan rotation angle JO between scanning coordinate system 1119 with respect to the multi-beam coordinate system 1115. Some exemplary beam spots 5.31, 5.41 and 5.51 are illustrated with their corresponding scanning fields 241.31, 241.41, 241.51, each formed by rotated rectangular scanning field 241.31, 241.41, 241.51 within the multi-beam coordinate system 1115. The scanning coordinate system 1119 with coordinates [xs, ys] is illustrated at the center beam spot 5.1. A scan rotation angle JO for each specific image setting (for example, for different landing energies LE) can for example be predetermined at a reference or calibration object. Scan rotation angle JO can generally be adjusted, for example according to predetermined needs of an inspection task. For example, the scan rotation angle JO can be adjusted by a mathematical rotation of the scanning signals provided to for example an octupole scanner 110. Such a scan rotation is for example described in WO 2021156198 Al, which is hereby incorporated by reference.

[0058] The control unit 800, for example the image control module 810, comprises software instructions and hardware, which is configured for determining image coordinates R_l of each image point acquired during scanning image acquisition. An example of a determination of an image coordinate R_l is illustrated in figure 4b. The image coordinate R_l of an image position 1006 is determined within the scanning field 241.j of an individual beamlet 5.j with beamlet number j at scanning coordinates R_cs. In the multi-beam coordinate system 1115, coordinates R_l = (x_mb.j,y_mb.j) of an image point (or pixel) 1006 are given by

[0059] (1) R_l = R_MB.j + TSR(JO) * R_cs

[0060] with the scanning position vector R_cs of the image point 1006. A scanning position vector R_cs in scanning coordinate system 1119 with scanning coordinates [xs, ys] is transformed into multi-beam coordinate system 1115 by scan rotation matrix TSR, representing the scan rotation by angle JO, and by adding to the coordinate of the corresponding unscanned beam position R_MB.j of beamlet 5.j within multi-beam coordinate system 1115.

[0061] The line-of-sight (LOS) of a multi-beam charged particle beam system 1 is typically determined within a reference coordinate system (RCS) 1111 with coordinates [RCS_x, RCS_y], Ideally, the Line-of-sight (LOS) is not altered during a change of an image setting of amulti-beam charged particle beam system 1. The line-of-sight (LOS) of a multi-beam charged particle beam system 1, however, may depend on a variety of parameters such as for example a magnification, a numerical aperture of primary beamlets, or a landing energy LE of an imaging task to be performed with the multi-beam system 1. Figure 4c illustrates an example of a displacement vector R1 = [xl, yl] of the line-of-sight (LOS) within the reference coordinate system 1111. Within the reference coordinate system 1111, the multi-beam coordinate system 1115 is displaced by multi-beam displacement vector R1 and rotated by rotation angle JI. The control unit 800, for example the image control module 810, further comprises software instructions and hardware, which is configured for determining image coordinates R_RC of each image point 1006 in the reference coordinate system 1111. For example, during image acquisition, the image position vector R_l according to eq. (1) is transformed into image position vector R_RC in reference coordinate system 1111 according to

[0062] (2) R_RC = TRMB2RC (JI) * R_l + R1

[0063] with the rotation matrix TMB2Rc("multi-beam-to-reference") describing the multi-beam rotation angle JI and the multi-beam displacement vector Rl.

[0064] The reference coordinate system 1111 is for example determined according to a mechanical reference of a multi-beam charged particle beam system 1. Figure 5 illustrates an example of a multi-beam charged particle beam system 1 comprising a reference frame 1025. The multibeam charged particle beam system 1 is comprising a multi-beam charged particle beam column 1001, comprising an object irradiation unit 100 and a detection unit 200, and reference is made to figures 1 and 2 and the description above. The multi-beam charged particle beam column 1001 is attached via a mounting flange 1031 to the reference frame 1025. Reference frame 1025 is construed and formed with high stiffness and low thermal expansion. The reference frame 1025 also serves as mechanical reference for encoders 21, which are in control of the position and movement of stage 500. The stage 500 is comprising a substrate holder or chuck 551 configured for receiving and holding for example a wafer 7. The stage 500 further comprises several movement and rotation axes, including rotation axis 555.t, z-stage 555. z, and x-y-stage 555. xy. Stage position or stage movement are controlled by encoders 21. Various types of encoders 21 are well known in the field, for example Laser interferometers with Laser beams 27, grating interferometers, confocal sensors, and the like.An absolute position of the substrate holder 551 relative the reference frame 1025 can be obtained with absolute position sensors 1021. For example, encoders 21 may include absolute position sensors 1021, for example encoders may comprise a homing positing, from which on an absolute distance is measured. As an alternative, or in addition, at least one absolute position sensor 1021 is attached to reference frame 1025. Such sensors comprise for example capacitive sensors or confocal sensors and are for example capable of determining an absolute position of substrate holder 551 at a predetermined position relative to the reference frame 1025.

[0065] The multi-beam charged particle beam system 1 further comprises a control unit 800.

[0066] Control unit 800 comprises a user interface 805 with a user interface display 1400 and user command devices 1401. Operation control unit 800 further comprises at least one memory 890 for storing instructions and at least one processing engine 880 for executing the instructions. Instructions comprise for example image processing instructions. After images acquisition is executed, imaging control module 810 is triggered to receive via detector 600 image information of a plurality of secondary electron beamlets9. Imaging control module 810 is synchronously operating with scanning control unit 860.

[0067] For example, a memory 890 is storing instructions for executing a calibration method of determining a multi-beam coordinate system 1115 with coordinates [MB_x, MB_y] with respect to the reference coordinate system 1111. According to a second embodiment, a method of determining a multi-beam coordinate system 1115 with respect to the reference coordinate system 1111 comprises the steps of

[0068] a) Selecting at least one of a plurality of image settings corresponding to an inspection task and setting a multi-beam charged particle beam column 1001 into a selected image setting. An image setting is including system settings comprising at least one of a landing energy (LE), a magnification (i.e. beam pitch), a numerical aperture, a beam energy, a scanning pattern and a scan rotation angle JO.

[0069] b) Selecting a calibration object, the calibration object comprising test patterns which enable a precise determination of the positions of the plurality of beam spots 5 with respect to the calibration object. Background information concerning calibration objects with test patterns for multi-beam charged particle beam systems aredisclosed in WO 2013 / 032949 Al, DE 102018124903 B3, and US 9,991,089 B2, which are incorporated by reference.

[0070] c) Placing the calibration object in the image plane 101 of the multi-beam charged particle beam system 1 via stage 500. For example, a calibration object is attached to the substrate chuck 551 at a predetermined position C and the substrate chuck 551 is moved by stage 500 such that the position C and rotation angle JC of the calibration object is known within the reference coordinate system 1111.

[0071] d) Determining the plurality of spot positions of the plurality of beam spots 5. j with j = 1....J relative to the calibration object.

[0072] e) Determining the multi-beam coordinate system 1115 from the plurality of spot positions of the plurality of beam spots 5. For example, the step of determining the multi-beam coordinate system 1115 comprises determining a center of gravity of the plurality of spot positions and thereby determining the line-of-sight (LOS). The step of determining the multi-beam coordinate system 1115 further comprises determining the symmetry-axes of the plurality of spot positions. The origin of the multi-beam coordinate system 1115 is determined to coincide with the center of gravity or line- of-sight (LOS), and at least one coordinate axis, for example axis x_MB (see figure 4), is selected as one axis of symmetry. After determining the multi-beam coordinate system 1115, the positions R_MB.j of each beam beam spot 5.j is determined. f) Determining the position M and rotation angle JM of the multi-beam coordinate system 1115 relative to the calibration object.

[0073] g) Determining, from the position C and rotation angle JC of the calibration object and the position M and rotation angle JM, the relative position R1 = C + M and the rotation angle Jl= JC+JM of the multi-beam coordinate system 1115 relative to the reference coordinate system 1111 (see Figure 4c).

[0074] h) Storing the relative position R1 as the multi-beam displacement vector R1 and the multi-beam rotation angle JI of the multi-beam coordinate system 1115 relative to the reference coordinate system 1111 assigned to the selected image setting.

[0075] i) During calibration, further a scan rotation angle JO of an imaging setting can be determined and stored in a memory. Furthermore, during calibration, image scaling of the scanning pattern can be determined, including anamorphic scaling.According to a third embodiment, a method of performing an inspection task comprises detecting a defect during imaging of the surface of a wafer 7 and deriving the defect location in wafer coordinates. An example is illustrated in figure 6a. During a wafer fabrication process, a defect inspection step DI comprises a detection of defects within predefined areas on a wafer. A wafer fabrication process may comprise steps i = 1...1000 or more process steps WF(i). For example, after wafer fabrication step WF(i), a wafer is subject to a defect inspection step DI, wherein it is searched for defects within preselected areas on a wafer. Such predefined areas can comprise process control monitors PCM distributed over certain positions on a wafer. Such predefined areas can comprise critical areas CA for the performance of the fabrication process until step WF(i). A defect inspection task DI requires high throughput with a large area tool. In prior art, such tools for defect inspection have been optical inspection tools using light with large spot size and low resolution. Optical inspection tools are unable to resolve defects on wafers with critical dimensions of semiconductor structures down to few nm. According to the third embodiment, a multibeam charged particle beam system 1 is used as a defect inspection tool for a defect inspection step DI. For the defect inspection step DI, a list of areas AL comprising areas to be imaged by a defect inspection tool is provided. The area list AL can comprise the areas comprising process control monitors PCM or critical areas CA on each wafer. Process control monitors PCM or critical areas CA are configured and selected during the design phase of a wafer fabrication process.

[0076] During defect inspection step DI, defects are detected, and locations of the defects on a wafer surface are determined with high precision. Defect locations are stored in review file RF. More details of defect inspection step DI according to the third embodiment are illustrated in figure 6b and described below. In a subsequent review step DR, at least for some defects, a defect review is executed. A review apparatus may be configured for high-resolution imaging, including advanced imaging modes using for example angular-resolved backscattered electron imaging, secondary or backscattered electron spectroscopy, or x-ray spectroscopy. An example of such a high-resolution electron-optical imaging system is disclosed in WO 2023072919 A2, which is incorporated here within by reference. A review apparatus typically has a field of view of less than 10pm, for example less than 6pm. A required time interval for an image acquisition with a review apparatus, however, isproportional to the area to be imaged. Therefore, review step DR is preferably executed over small areas with dimensions of less than 1pm, 500nm or even less, for example lOOnm. Thereby, a high throughput of a review step DR is achieved. Executing the review step DR on small areas requires determining the locations of defects on a wafer surface with high precision, at least with a higher precision of less than 1pm, 500nm, or even higher precision, for example with about lOOnm or less.

[0077] An example of the defect inspection step DI according to the third embodiment is illustrated in figure 6b. In a first step SI, a sample 7 such as a wafer 7 or a mask 7 is loaded to the sample holder or chuck 551. Figure 7a illustrates an example of a wafer 7 with a wafer coordinate system 1121 with coordinates [W_x, W_y], The wafer 7 is placed by a handling robot on the surface of sample holder or chuck 551 with a limited accuracy. Figure 7b illustrates a wafer with wafer coordinate system 1121, placed onto a sample holder 551 with chuck coordinate system 1125 with coordinates [C_x, C_y] . Same reference numbers are used as within the foregoing figures and reference is made to the description thereof. Due to the limited loading accuracy, wafer coordinate system 1121 is displaced with respect to chuck coordinate system 1125 by displacement vector R3 and rotated by wafer loading angle J3. Typically, displacement vector R3 can be about 50pm or more, for examplelOOpm.

[0078] Loading angle J3 can be about lOmrad or more, for example 20mrad.

[0079] In a second step S2, the position of the wafer coordinate system 1121 is determined within chuck coordinate system 1125, and wafer loading displacement vector R3 and wafer loading angle J3 are determined. Method steps for such a wafer registration are well known in the art. For example, a reference marker or calibration mask 1551 is attached to the stage adjacent to the chuck 551. By comparison of a distance and an orientation angle between the reference marker or calibration mask 1551 and a reference marker on a wafer, an absolute position of the wafer 7 and a wafer loading displacement vector R3 and wafer loading angle J3 can be determined. As illustrated in figure 7b, the position of chuck coordinate system 1125 is determined with reference to the reference frame 1025 by encoders 21, including for example two encoders 21.x and 21.y for orthogonal directions. Thereby, a position displacement vector R4 of the chuck coordinate system 1125 of the sample chuck 551 with respect to the reference coordinate system 1111 can be determined.Wafer loading displacement vector R3 and wafer loading angle J3 are stored temporarily in memory 890 for repeated use during inspection tasks on the same wafer 7.

[0080] In a third step S3, an image acquisition of an inspection task is initialized. Step S3 comprises selection of at least one inspection site 6.i (with i = 1,2,...) and placement of the at least one inspection site 6.i for example at the line-of-sight (LOS). Step S3 further comprises determination of data for a coordinate transfer from multi-beam image coordinates into wafer coordinates and storing the data in memory 890 for later use.

[0081] In step S3.1, at least one inspection site 6.i for inspection of a surface area on the wafer 7 is selected and placed for example at the line-of-sight (LOS). The center position RD of an inspection site 6.i can be predetermined, for example according to design positions, or selected by user selection via user input 1401. For example, an inspection site 6.i (with i = 1,2,...) is selected according to process control monitors integrated during wafer fabrication within each die. For example, at an inspection site 6.i, a defect is suspected. For example, an inspection site 6.i is determined according to an arrangement of critical features. For example, an inspection site 6.i is selected according to a routine inspection. In an example, an inspection site 6.i comprises a large surface area of a die, for example of about 80pm or more in diameter. For example, an inspection site 6.i and corresponding center position RD is received from an area list AL. An area list can comprise a plurality of areas or inspection sites and corresponding center positions RD to be inspected.

[0082] The center position RD of the inspection site 6.i is given in wafer coordinate system 1121 and transferred into chuck coordinate system 1125 via

[0083] (3) R6 = TRW2C (J3) * RD + R3

[0084] with the rotation matrix Twzc ("wafer-to-chuck") comprising wafer loading angle J3 and wafer loading displacement vector R3, both determined in step S2 and stored in memory 890 (see Figure 6b and 7b).

[0085] Step S3.1 further comprises selecting an image setting. For the selected image setting, a relative position R1 = (xl, yl) and the rotation angle JI of the multi-beam coordinate system 1115 relative to the reference coordinate system 1111 is determined. The determination cancomprise receiving the multi-beam displacement vector R1 and the multi-beam rotation angle JI from a memory 890. An example is illustrated in figure 8a. Same reference numbers are used as within the foregoing figures and reference is made to the description thereof. For example, each of multi-beam displacement vectors and the rotation angles of multibeam coordinate systems 1115 relative to the reference coordinate system 1111 can be determined according to the calibration method of the second embodiment. A plurality of multi-beam displacement vectors and multi-beam rotation angles for a plurality of image settings can for example be stored in a memory 890 of the multi-beam charged particle beam system 1. A multi-beam displacement vector R1 and multi-beam rotation angle JI at a specific imaging setting can be determined according to an interpolation between previously determined multi-beam displacement vectors and multi-beam rotation angles.

[0086] Step S3 further comprises step S3.2. During step S3.2, a chuck displacement vector R4 and a chuck rotation angle J4 is determined and a movement of the sample chuck 551 is executed. In step S3.2, stage control unit 850 is triggered to move the sample chuck 551 to the position corresponding to a chuck displacement vector R4 and to apply chuck rotation angle J4 within the reference coordinate system 1111. The position and rotation of the sample holder or chuck 551 is monitored by encoders 21 and controlled by stage control unit 850. A setpoint or zero-position of the wafer or sample chuck 551 with respect to the reference coordinate system 1125 is stored in the memory 890 of the multi-beam charged particle beam system 1 and for example determined during a calibration of the multi-beam charged particle beam system 1. For example, setpoint or zero-position of the wafer chuck 551 with respect to the reference coordinate system 1111 is determined by absolute position sensor 1021 of the multi-beam charged particle beam system 1 (see figure 5). For example, a calibration object 1551 is attached adjacent to the chuck 551. As illustrated in figure 8b, the position of chuck coordinate system 1125 is determined within the reference coordinate system 1111 by encoders 21, including for example two encoders 21.x and 21. y for orthogonal directions. Thereby, a position displacement vector R4 of the chuck coordinate system 1125 of the sample chuck 551 with respect to the reference coordinate system 1111 can be determined. Further encoders (not shown) are configured for determining and monitoring a chuck rotation angle J4 of a rotation stage 555. t may be integrated for example as grating interferometers within rotation stage 555.t.In an example, the chuck displacement vector R4 is determined such that center position RD of an inspection site 6.i is positioned at the line-of-sight LOS. The center position vector R6 of the inspection site 6.1 in chuck coordinate system 1125 is transformed into reference coordinates (1111) via

[0087] (4) R6_R = TR RC2C (J4) * R6

[0088] with the rotation matrix TRRC2c("reference-to-chuck") with chuck rotation angle J4.

[0089] With the relative position vector R1 of the selected imaging setting, the displacement vector R4 of the substrate holder or chuck 551 within the reference coordinate system 1111 is determined according to

[0090] (5) R4 = R1 - R6_R

[0091] Vectors R4, Rl, R6 and R6_R are illustrated in figure 8b. It should be noted that identical vectors R6 and R6_R are measured in different coordinate systems.

[0092] Step S3.2 further comprises selection and determination of a chuck rotation angle J4. Chuck rotation angle J4 is for example determined according to a rotation angle JI of the multibeam coordinate system 1115 within the reference coordinate system 1111. In an example, chuck rotation angle J4 is selected according to a scanning orientation according to the scan rotation angle JO. For example, a chuck rotation angle J4 is selected according to J4 = J 1 + JO -J3, such that a scanning direction xs is parallel to a wafer coordinate W_x.

[0093] In a third step S3.3 of step S3, data for a coordinate transfer from multi-beam image coordinates into wafer coordinates are determined and stored in memory 890.

[0094] In an example, the data comprises a transfer matrix TMB2W ("multi-beam-to-wafer"). The transfer matrix TMB2W is depending on the wafer loading displacement vector R3 and the wafer loading angle J3 according to step S2. The transfer matrix TMB2W is further depending on the multi-beam displacement vector Rl, the multi-beam rotation angle JI. The multibeam displacement vector Rl and corresponding multi-beam rotation angle JI are determined according to the selected image setting in step S3.1. The transfer matrix TMB2W is further depending on the chuck displacement vector R4 and the chuck rotation angle J4according to step S3.2. An example of the transfer matrix TMB2W is described and illustrated below at the description of step S4 in more detail.

[0095] In an example, the data comprises the unscanned beam or raster positions R_MB_W.j of focus spots 5.j of beamlets 3.j within the wafer coordinate system 1121. The unscanned beam positions R_MB_W.j of beamlets 5.j are determined according to a multiplication of the positions R_MB.j of each beam spot 5.j with the transfer matrix TMB2W according to equation (6):

[0096] (6) R_MB_W.j = TMB2W (R1,J1; R3,J3; R4,J4) * R_MB.j

[0097] In an example, the data comprises the rotation matrix TSR(JW) for consideration of the scanning rotation with the scan rotation angle JW in wafer coordinate system 1121. The scan rotation angle JW in wafer coordinates is determined according to JW = (J3 + J4) - (JI + JO), with the scan rotation angle JO in multi-beam coordinates, the multi-beam rotation angle JI, the wafer loading angle J3 and the chuck rotation angle J4.

[0098] In a fourth step S4, a multi-beam scanning image SI is acquired and transferred to wafer coordinates. The multi-beam scanning image SI is stored in a memory.

[0099] In a first example, the multi-beam scanning image SI is acquired by scanning deflection of the plurality of primary charged particle beamlets 3 with the common scanning deflector 110. During step S4, for at least one image position, a corresponding coordinate on the surface 25 of the wafer 7 in the wafer coordinate system 1121 is determined. For example, for each image position within the multi-beam scanning image, a corresponding coordinate on the surface 25 of the wafer 7 in the wafer coordinate system 1121 is determined.

[0100] In an example of step S4, an image position vector R_l of an image point or pixel 1006 (see figure 4b) within a multi-beam image is determined according to eq. (1). The image position vector R_l is transformed into image position vector R_RC in reference coordinate system 1111 according to eq. (2) with the transfer matrix TMB2RC ("multi-beam-to-reference") comprising relative position vector R1 and the rotation angle JI (see Figure 4c and 8a).The image position vector R_RC is transformed into chuck coordinates R_C in chuck coordinate system 1125 by

[0101] (7) R_C = TRC2C (J4) * [R_RC - R4]

[0102] with the transfer matrix TRC2C (J4) comprising the chuck rotation angle J4 and chuck displacement vector R4. With wafer displacement vector R3 and loading angle J3 determined in step S2, the corresponding position vector R_W in wafer coordinates is obtained according to

[0103] (8) R_W = TC2W (J 3) * [R_C - R3]

[0104] with the transfer matrix TRc2w ("chuck-to-wafer").

[0105] In an example, the sequence of coordinate transformations is summarized in one transfer matrix TMB2W ("multi-beam-to-wafer") and the image position vector R_W in wafer coordinates is determined according to

[0106] (9) R_W = TMB2W (R1,J1; R3,J3; R4,J4) * R_l

[0107] In an example, transfer matrix TMB2W is given by matric multiplication, including for example multiplication of transfer matrix TMB2Rc("Multi-beam-to-reference"), rotation matrix TRRC2C ("reference-to-chuck"), transfer matrix TRc2w ("chuck-to-wafer") described above, including consideration of shift or displacement operations described above. Generally, the coordinate transfer by transfer matrix TMB2W ("Multi-Beam-to-Wafer") is comprising a coordinate transformation, comprising at least one of an affine transformation or a mapping. An affine transformation comprises at least one of a translation or a rotation of a coordinate system. In further examples, an affine transformation further includes a scaling, a hyperbolic rotation, a shear mapping, and compositions thereof in any combination and sequence.

[0108] In an example, transfer matrix TMB2W is computed during step S3.3 and stored in the memory 890. In an example of step S4, each image coordinate R_l is transformed into wafer coordinates with the transfer matrix TMB2W received from memory 890.

[0109] With R_l given according to equation (1), image points R_W in wafer coordinates are determined according to

[0110] (10) R_W = TMB2W (R1,J1; R3,J3; R4,J4) * [R_MB.j + TSR(JO) * R_cs]

[0111] In an example, transfer matrix TMB2W is computed during step S3.3 and stored in the memory 890. In an example of step S4, an image point at scanning coordinate R_cs is transformedT1

[0112] into wafer coordinates with the transfer matrix TMB2W, the position R_MB.j of a beam spot 5.j and the scan rotation matrix TSR(JO) received from memory 890.

[0113] In an example, image position vector R_W in wafer coordinates is determined according to (11) R_W = R_MB_W.j + TSR (JW) * R_cs

[0114] with the unscanned beam position R_MB_W.j of beamlets 5.j within wafer coordinate system 1121. Unscanned beam position R_MB_W.j and rotation matrix TSR(JW) are determined in step S3.3 and stored in the memory 890. In step S4, image points R_W in wafer coordinates are determined according to equation (11) by using scanning coordinates R_cs and the data received from memory 890.

[0115] In an example, chuck rotation angle J4 is selected according to J4 = JI + JO - J3, and JW = 0 and equation (11) further simplifies to R_W = R_MB_W.j + R_cs.

[0116] A closed loop of vectors R_l, Rl, R3, R4 and R_W is illustrated in figure 9. Same reference numbers are used as within the foregoing figures and reference is made to the description thereof. Multi-beam displacement vector Rl, corresponding multi-beam rotation angle JI, and raster positions R_MB.j of beamlets for a selected image setting are for example predetermined during a calibration Cl and stored in a memory 890. The scan rotation angle JO is calibrated during the same calibration step Cl or adjusted by a scan rotation described above. The Wafer loading displacement vector R3 and corresponding wafer loading rotation angle J3 are determined in step S2 during a wafer registration. The chuck displacement vector R4 and corresponding chuck rotation angle J4 are controlled by stage control unit 850 and monitored by encoders 21.

[0117] In the examples described above, a predetermined center position RD of an inspection site (6.1, 6.2, 6.i) in wafer coordinates is placed at the line-of-sight LOS and chuck displacement vector R4 is determined according to eq. (5).

[0118] In a second example, a multi-beam scanning image SI is acquired by continuously moving the substrate holder or chuck 551 by stage 500 in a first direction and scanning the plurality of primary charged particle beamlets 3 in a direction perpendicular to the movement directionof the substrate holder or chuck 551. In the example of a scanning stage, chuck displacement vector R4 is a function over time t with R4 = f(t). In an example, the movement direction of the substrate holder or chuck 551 with respect to the multi-beam coordinate system 1115 is approximately perpendicular to the scanning deflection according to the scan rotation angle JO, and equations (1) to (11) can be applied in analogy to the first example with chuck displacement vector-function R4 (ts) synchronized with time intervals ts corresponding to the scanning frequency 1 / ts.

[0119] In a fifth step S5, at least one specific object of interest, for example a defect is detected in the multi-beam image and a wafer coordinate R_W of the detected defect is stored in memory. For example, software instructions are executed to cause the processing engine 880 to perform image processing, for example comprising at least one of an image stitching, contrast and brightness adjustment, noise reduction, image data compression. For example, image processing is further comprising at least one of a feature detection, object recognition, feature measurement or feature comparison step. The determination of a defect or any other specific object of interest may comprise various methods, such as user selection via user interface 805, or automated pattern detection methods including pattern matching, a comparison to design data, a die-to-die comparison, or machine learning methods of feature or pattern detection and classification. For example, a comparison to design data can be performed with the precisely known image data in wafer coordinates determined during step S4. For example, a predefined feature is detected in an image and assigned to a position coordinate in wafer coordinate system 1121. Processed image data is stored in memory 890 for further use.

[0120] In the fifth step S5, the location R_W of the at least one specific object of interest is determined in wafer coordinates 1121 and stored for example in an inspection file assigned to the specific wafer. For example, a list of wafer coordinates R_W of a plurality of detected defects is determined and stored in a review file RF. Step S5 must not necessarily be executed after completion of step S4 but can be performed at least partially in parallel to step S4.In an example, the method further comprises a step S6. Step S6 is executed during image acquisition step S4. In an example, a wafer surface 25 deviates from the ideal object plane 101 of the multi-beam charged particle beam system 1 and for example an adjustment of the focus plane of the plurality of beam spots 5 is required. Such a change of an image plane 101 is for example required if the stage 500 does not comprise a motion axis 555. z for adjustment of the wafer 7 in RCS z-direction. The adjustment or change of the focus plane or object plane 101 can be achieved by adjustment or change of an excitation of an electron-optical element within the object irradiation unit 100 or by a change of the extraction field by sample voltage supply 503. For example, a focus position or object plane 101 is achieved by changing the power of magnetic objective lens 102. An adjustment of the focus plane of the plurality of beam spots 5 typically comes along with at least one of an effect selected from the group of effects comprising a change of the displacement vector R1 of the line-of-sight (LOS), a change of the rotation angle JI of the multi-beam coordinate system 1115 within the reference coordinate system 1111, and a change of the scanning angle JO. During step S6, a focus deviation between the image plane 101 and the surface of the wafer 7 is detected and a change of the image plane 101 is triggered. During step S6, the at least one effect of the change of the image plane 101 is determined, comprising at least a modification of multi-beam displacement vector R1 into Rim, a modification of multi-beam rotation angle JI into Jim, and a change of the scanning angle JO into JOm. For example, during step S6, the transfer matrix transfer matrix TMBZW is modified into TMBZW (Rim, Jim; R3,J3; R4,J4). For example, according to the modified scan rotation angle JOm, a modified scan rotation angle JWm in wafer coordinates is obtained.

[0121] In an example, step S6 comprises a change of a modified scan rotation angle JOm. For example, the modified scan rotation JOm is changed back to scanning angle JO according to the selected image setting, such that an orientation of a scanning direction (e.g. xs) is not changed relative to the multi-beam coordinate system 1115. For example, a modified scan rotation JOm is changed to a scan rotation angle J0m2 such that a rotation of the modified scanning angle JOm by modified multi-beam rotation angle Jim is compensated and a scanning direction is not changed within the reference coordinate system 1111. Such changes of the modified scan rotation angle JOm can be achieved by a scan rotation of the scanning commands described above, for example by a digital rotation of the scanningcommands provided for example to a multi-pole scanning deflector 110. Thereby, for example a scanning rotation JW = 0 can be maintained during image acquisition.

[0122] The method according to the third embodiment is for example implemented with software code for execution within a control unit 800 of a multi-beam charged particle beam system 1. When executed, wafer loading and registration, image setting of the multi-beam charged particle beam system 1, stage movement and image acquisition are performed in an efficient manner to quickly transform image coordinates into wafer coordinates with high precision. Thereby, image coordinates of for example defects are transferred to wafer coordinates for later use for example in subsequent review tasks. According to a fourth embodiment, a method of a defect review is given. For a wafer 7, a plurality of inspection sites 6.i, for example comprising inspection sites 6.1 and 6.2 is received from for example user interface 805 or via a file such as a KLARF-file or RF-file described above, comprising a list of inspection sites 6.1, 6.2 and more. The task of a defect review is to navigate a plurality of inspections sites 6 (including 6.1 and 6.2) into the field of view of the multi-beam charged particle system 1 and to determine whether a suspected defect is present at the corresponding inspection site. After loading the wafer 7 to the sample holder or chuck 551, and registration of the wafer 7 to the sample holder or chuck 551, the coordinates of inspection sites 6.1 or 6.2 are not known within chuck coordinate system 1125 (Steps SI and S2). For each inspection site 6, the corresponding chuck position vectors R4 and chuck rotation angles J4 are determined in step S3. Images are acquired in Step S4 and transformed into wafer coordinates. The images within wafer coordinates are analyzed by image processing methods comprising at least one method of a feature extraction, feature classification, and dimension or area measurement. The methods may comprise template matching, image comparison, or machine learning methods. As a result, at image coordinates corresponding to an inspections site 6, for example a defect or a nuisance is determined. Optionally, a defect is classified into predetermined defect classes.

[0123] Figure 10 illustrates a further example of a multi-beam charged particle beam system 1 according to the first embodiment. Same reference numbers are used as in figures 1, 2 and 5, and reference is made to the description above. In the example of figure 10, the reference frame 1025 is further mounted via active dampers 513 on a mounting platform 507. Themotion axes 555 of the stage 500 is attached to a stage bearing 509, which is rigidly attached to the mounting platform 507. With the active dampers 513, dynamic vibrations from stage 500 are not transferred to the frame 25. Further, mounting platform 507 is rigidly connected by mounting pedestals 531 to a fab floor 511 of high mass. Therefore, dynamic vibrations from stage 500 are absorbed within fab floor 511 without causing any deterioration to the multi-beam charged particle beam column 1001 mounted to the frame 1025. Further shown is vacuum enclosure 523, which either encloses multi-beam charged particle beam column 1001 or is dynamically isolated with respect to multi-beam charged particle beam column 1001 by bellows 521. From encoder signals from encoder 21, the position and rotation of the sample holder or chuck 551 is determined, for example the actual position of the coordinate system of wafer chuck 551 with respect to reference frame 1025 is given by chuck displacement vector R4 and chuck rotation angle J4.

[0124] With the apparatus and the method according to the examples and embodiments, image information obtained during image acquisition with a multi-beam charged particle beam system 1 and unambiguously transformed into wafer coordinates. Further, sites for inspection, such as suspected defect positions on a wafer in wafer coordinates can be transferred into image coordinates and movement commands for a wafer stage of the multibeam system, such that suspected defect positions can be reviewed unambiguously. The apparatus and the method according to the examples and embodiments consider a rotation of the raster of the plurality of primary charged particle beamlets as well as a rotation angle of a scanning direction, executed either by a common scanning beam deflector or a continuously moving stage.

[0125] A list of reference numbers is provided:

[0126] I multi-beam charged-particle beam system

[0127] 3 primary charged particle beamlet or plurality of primary charged particle beamlets 5 primary charged particle beam spot

[0128] 7 object or sample

[0129] 9 secondary electron beamlet, forming the plurality of secondary electron beamlets II primary beam path

[0130] 13 secondary electron beam path15 focus spot of secondary electron beam 21 encoder

[0131] 25 surface of object or sample

[0132] 27 Laser beam

[0133] 100 object irradiation unit

[0134] 101 image plane

[0135] 102 objective lens

[0136] 103 field lens

[0137] 110 collective multi-beam raster scanner 133 decelerating electrode

[0138] 137 equipotential lines of electric field

[0139] 151 beam tube

[0140] 153 Beam exit opening

[0141] 161 coil

[0142] 163 pole shoe

[0143] 165 lower pole shoe segment

[0144] 200 detection unit

[0145] 205 magneto-dynamic lens

[0146] 211 electron-optical lens

[0147] 220 multi-pole corrector

[0148] 222 second raster scanner

[0149] 225 secondary electron image plane

[0150] 241 scanning field

[0151] 251 raster arrangement

[0152] 258 cross-over or pupil position

[0153] 264 multi-pole corrector

[0154] 284 Aperture stop

[0155] 300 charged-particle multi-beamlet generator 301 charged particle source

[0156] 303 collimating lenses

[0157] 304 filter plate

[0158] 305 multi-beam forming unit306 multi-aperture plates

[0159] 309 primary electron beam

[0160] 321 intermediate image surface

[0161] 331 first field lens

[0162] 333 second field lens

[0163] 400 beam splitter or divider unit

[0164] 500 sample stage

[0165] 503 Sample voltage supply

[0166] 507 mounting platform

[0167] 509 stage bearing

[0168] 511 fab floor

[0169] 513 active damper

[0170] 521 bellow

[0171] 523 vacuum enclosure

[0172] 531 mounting pedestals

[0173] 551 substrate holder or chuck

[0174] 555 movement or rotation axis

[0175] 600 image sensor

[0176] 800 control unit

[0177] 805 User Interface

[0178] 810 imaging control module

[0179] 820 sensor module

[0180] 830 primary beam-path control module

[0181] 840 secondary beam-path control module 850 stage control module

[0182] 860 scanning control unit

[0183] 880 control operation processor

[0184] 890 memory

[0185] 1001 multi-beam charged particle beam column 1006 image point

[0186] 1021 absolute position sensor

[0187] 1025 reference frame1031 mounting flange

[0188] 1105 optical axis

[0189] 1111 reference coordinate system 1115 multi-beam coordinate system 1119 scanning coordinate system 1121 wafer coordinate system 1125 chuck coordinate system 1127 stage coordinate system 1551 calibration object

Claims

35Patent claims1. A method of defect inspection with a multi-beam charged particle beam system (1), using a plurality of J primary charged particle beamlets (3.j, j = 1..J), the method comprising:- loading a wafer (7) with a predefined wafer coordinate system (1121) to a sample chuck (551) of a multi-beam charged particle beam system (1);- determining a wafer loading displacement vector R3 and a wafer loading angle J3 of the wafer coordinate system (1121) with respect to the sample chuck (551);- selecting an image setting;- determining a multi-beam displacement vector R1 of a line of sight (LOS) and a multi-beam rotation angle JI, and a plurality of raster positions R_MB.j of a plurality of focus spots (5.j, j = 1..J) of the plurality of J primary charged particle beamlets (3.j, j = 1..J) for the selected image setting;- selecting and positioning the sample chuck (551) with a wafer stage (500) at a position according to a chuck displacement vector R4 and a chuck rotation angle J4, - determining a data for a coordinate transfer from multi-beam image coordinates into wafer coordinate system (1121), the data comprising at least one of a transfer matrix TMB2W, a plurality of raster positions R_MB_W.j of the plurality of focus spots (5.j, j = 1..J) of the plurality of J primary charged particle beamlets (3.j, j = 1..J) in the wafer coordinate system (1121), and a rotation matrix TSR(JW) with a scan rotation angle JW in wafer coordinate system (1121),- acquiring a multi-beam image of a surface area of the wafer (7) by scanning a plurality of focus spots (5) of the plurality of primary charged particle beamlets (3) to a plurality of scanning position vectors R_cs with a scan rotation angle JO;- determining an image position vector R_W of an image point (1006) of the multibeam image in wafer coordinate system (1121) from a scanning position vector R_cs and at least one of the data for a coordinate transfer.

2. The method of claim 1, wherein the chuck rotation angle J4 is selected such that the scan rotation angle JW in wafer coordinate system is JW = 0.

363. The method of claim 1, wherein the chuck rotation angle J4 is selected in accordance with J4 = JI + JO - J3.

4. The method of any of the claims 1 to 3, wherein the transfer matrix TMB2W is determined from the wafer loading displacement vector R3, the wafer loading angle J3, the multi-beam displacement vector Rl, the multi-beam rotation angle JI, the chuck displacement vector R4 and the chuck rotation angle J4.

5. The method of claim 4, wherein each of the plurality of raster positions R_MB_W.j in wafer coordinate system (1121) is determined by multiplication of the transfer matrix TMB2W with one of the plurality of raster positions R_MB.j of the plurality of focus spots (5.j, j = 1...J).

6. The method of any of the claims 1 to 5, wherein the image position vector R_W is determined according toR_W = R_MB_W.j + TSR (JW) * R_cswith the scanning position vector R_cs and the scan rotation matrix TSR(JW).

7. The method of any of the claims 1 to 5, wherein the image position vector R_W is determined according toR_W = TMB2W * [RJVIB.j + TSR (JO) * R_cs]with the scanning position vector R_cs and the scan rotation matrix TSR(JW).

8. The method of any of the claims 1 to 5, further comprising determining an image position vector R_l of an image point (1006) by R_l = R_MB.j + TSR(JO) * R_cs, and wherein the image position vector R_W is determined by multiplying the transfer matrix TMB2W with the image position vector R_L9. The method of any of the claims 1 to 8, further comprisingselecting an inspection site (6.1, 6.2) from a list of a plurality of inspection sites (6.1, 6.2) assigned to the wafer (7).

10. The method of claim 9, further comprising- transforming a center position vector RD of the inspection site (6.1, 6.2) in wafer coordinate system (1121) into a position vector R6_R of the inspection site (6.1, 6.2) in a reference coordinate system (1111);- selecting the chuck displacement vector R4 according R4 = R1 - R6_R, with the multibeam displacement vector Rl.

11. The method of claim 10, wherein the step of transforming the center position vector RD of the inspection site (6.1, 6.2) in wafer coordinate system (1121) into a position vector R6_R in reference coordinate system (1111) is comprising- transforming the center position vector RD of the inspection site (6.1, 6.2) in wafer coordinate system (1121) into a position vector R6 in chuck coordinate system (1125) byR6 = TRW2C ( J3) * RD + R3with the rotation matrix Tw2c("wafer-to-chuck") depending on the wafer loading angle J3 and the wafer loading displacement vector R3, and- transforming the position vector R6 in chuck coordinate system (1125) into the position vector R6_R in reference coordinate system (1111) byR6_R = TR RC2C (J4) * R6with the rotation matrix TRRC2c("reference-to-chuck") depending on the chuck rotation angle J4.

12. The method of any of the claims 1 to 11, further comprising selecting the selected image setting according to at least one of inspection site (6.1, 6.2) from a list of a plurality of inspection sites (6.1, 6.2).

13. The method of any of the claims 1 to 12, further comprising determining the multibeam displacement vector Rl, the multi-beam rotation angle JI with respect to a reference coordinate system (1111) corresponding to a reference frame (1025) of the multi-beam charged particle beam system (1).

14. The method of any of the claims 1 to 13, further comprising determining the multibeam displacement vector Rl, the multi-beam rotation angle JI, and the plurality of raster positions R_MB.j of the plurality of focus spots (5.j, j = 1.. J) for the selected image setting from a memory of the multi-beam charged particle beam system (1), the memory comprising data representing at least one predetermined beam displacement vector Rl, multi-beam rotation angle JI, and a plurality of raster position R_MB.j of the plurality of focus spots (5.j, j = 1.. J) for at least one image setting.

15. The method of any of the claims 1 to 14, further comprising determining the multibeam displacement vector Rl, the multi-beam rotation angle JI, and the plurality of raster positions R_MB.j of the plurality of focus spots (5.j, j = 1...J) of the plurality of J primary charged particle beamlets (3.j, j = 1...J) for the selected image setting during a calibration step of the multi-beam charged particle beam system.

16. The method of any of the claims 1 to 15, further comprising changing an object plane (101) of the multi-beam charged particle beam system (1) by adjusting a focus position of the plurality of focus spots (5.j, j = 1...J), and determining a modification of the multi-beam displacement vector Rl into Rim, a modification of multi-beam rotation angle JI into Jim, and a change of the scanning angle JO into JOm.

17. The method of claim 16, further comprising changing the scan rotation angle JOm into a scan rotation angle J0m2 such that a scanning direction is not changed within a reference coordinate system (1111).

18. The method of any of the claims 1 to 17, further comprising an image processing of the multi-beam image, the image processing comprising at least one of an image stitching, contrast and brightness adjustment, noise reduction, image data compression, a feature detection, an object recognition, a feature measurement or a feature comparison.3919. The method of any of the claims 1 to 18, further comprising a comparison of at least a segment of a multi-beam image with a reference image.

20. The method of claim 19, wherein the reference image is derived from CAD data.

21. The method of any of the claims 1 to 20, further comprising detecting at least one of a defect or an object of interest within the multi-beam image and storing a wafer coordinate R_W of the at least one detected defect or object of interest in a memory.

22. The method of claim 21, further comprising a review step of at least one defect or object of interest at the wafer coordinate R_W.

23. A multi-beam charged particle beam system (1), comprising- a multi-beam charged particle beam column (1001), mounted to a reference frame (1025) via a mounting flange (1031), the reference frame defining a reference coordinate system (1111),- a stage (500) with a sample chuck (551) for receiving a wafer (7),- a control unit (800) with a control operation processor (880) and a memory (890), the memory (890) comprising software instructions, for causing when executed by the control operation processor (880) to perform any of the methods according to claim 1 to 22.

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