Multi-beam charged particle microscope for inspection with increased throughput

Abstract

This invention provides a multi-beam charged particle system and a method for operating the multi-beam charged particle system to improve yield. The multi-beam charged particle system is configured to perform a series of image captures at a series of inspection locations and to determine adjustments for image captures at subsequent inspection locations based on previous image captures at previous inspection locations. This invention can be applied to high-yield wafer inspection using a multi-beam charged particle system.

Description

Technical Field

[0001] The present invention relates to a multi-beam charged particle microscope and a method of operating a multi-beam charged particle microscope with improved throughput for inspecting semiconductor features. Prior Art

[0002] WO 2005 / 024881 A2 discloses an electron microscope system that utilizes multiple electron beamlets to scan an object under examination in parallel using a bundle of electron beamlets. Primary charged particle beamlets are generated by directing primary charged particle beamlets to a multi-beam forming unit. The multi-beam forming unit comprises at least one porous plate having multiple openings. A portion of the electron beam is incident on and absorbed by the porous plate, while another portion passes through the openings of the porous plate, thereby forming electron beamlets in the beam path downstream of each opening, with a cross-section defined by the cross-section of the opening. The multiple primary charged particle beamlets are focused onto the surface of a sample by an objective lens, triggering secondary electrons or backscattered electrons to be emitted from the sample as secondary electron beamlets. The multiple secondary electron beamlets are collected and imaged onto a detector. Each secondary beamlet is incident on a separate detector element or a group of detector elements, so that the secondary electron intensity detected therefrom provides information about the sample surface at the location where the corresponding primary beamlet is incident on the sample. The primary beamlets are systematically scanned over the sample surface to produce an electron microscope image of the sample in the conventional manner of a scanning electron microscope.

[0003] To obtain high-resolution images, the surface of the object under investigation must be positioned in the optimal focal plane for each primary beamlet. The depth of focus is typically approximately 700 nm or less, for example, 500 nm, 300 nm, or less. Therefore, the axial position of the object surface and the position of the image plane forming the focal points of the multiple beamlets must be controlled with high precision. Therefore, prior to each image capture, conventional systems invoke a routine to perform an appropriate, but time-consuming, focus adjustment. For example, document DE 10 2021 200 799 B3 discloses a focus adjustment method that includes capturing a series of images at different focal positions or z-positions of a sample stage and analyzing the results to derive the optimal focus setting. Other examples, such as those disclosed in document US 10,388,487, utilize information derived from previous measurements through complex and time-consuming investigations of the object's properties and the properties of multiple particle beamlets. Therefore, a fast focus tracking routine is required for a series of inspection images, such as those performed at multiple inspection locations on a wafer. Summary of the Invention

[0004] The technical objectives indicated above are solved by the independent claims. The dependent claims relate to advantageous embodiments.

[0005] This patent application claims priority to German patent application No. 10 2023 204 715.2 filed on May 22, 2023, the contents of which are incorporated by reference in their entirety into this patent application.

[0006] The present invention provides a multi-beam charged particle system and a method of operating a multi-beam charged particle system with increased throughput for performing a series of image acquisitions at at least one inspection location on a wafer. The present invention provides a multi-beam charged particle system and a method of operating a multi-beam charged particle system with increased throughput for performing a series of image acquisitions at a series of inspection locations on a wafer. In one embodiment, one of the objects of the present invention is achieved by generating a model-based focus distribution map from a first measurement and iteratively improving subsequent image acquisitions during a series of image acquisitions on a wafer surface.

[0007] The electron microscope of the present invention utilizes a plurality of primary charged particle beamlets to irradiate a flat surface. During operation, a plurality of secondary electron beamlets are generated within the interaction volume between the plurality of primary charged particle beamlets and the wafer. The secondary electron yield within each interaction volume depends on the current and kinetic energy of the corresponding primary charged particle beamlet and the material composition within the interaction volume.

[0008] According to one embodiment, a method for wafer inspection using a multi-beam charged particle system includes determining a series of inspection locations, including at least a first inspection location and a second inspection location on a wafer surface. The method further includes capturing a first image of the first inspection location on the wafer surface using a first imaging setting of a plurality of primary charged particle beamlets, and determining a first local image contrast map LV(x, y) of the first image. The method further includes determining a measurement value MV.1 from the first local image contrast map LV(x, y) and a reference value RV.1 from a memory based on the first imaging setting. The method further includes determining at least one deviation value DV.1 between the measurement value MV.1 and the reference value RV.1. The measurement value MV.1 includes at least one of a first focus shift FM.1, a first field curvature radius RM.1, and a first image plane tilt vector TM.1; the reference value RV.1 includes at least one of a reference focus shift RF.1, a reference field curvature radius RR.1, and a reference image plane tilt vector TR.1.

[0009] The method further includes determining at least one of an adjustment of a second or subsequent inspection location and a second or subsequent imaging setting of a plurality of primary charged particle beamlets based on at least one deviation value DV.1. The method further includes performing the adjustment of the second or subsequent inspection location and the second or subsequent imaging setting. The method further includes capturing a second or subsequent image at the second or subsequent inspection location on the wafer surface using the second or subsequent imaging setting of the plurality of primary charged particle beamlets. The method relies on prior knowledge, such as an assumption of a flat wafer surface. Thus, focus information determined at a first inspection location can be used for rapid, subsequent image capture at a series of different inspection locations (e.g., different locations on a flat wafer surface). The focus information determined at the first inspection location is taken into account at additional inspection locations at different locations, and high image contrast is achieved at the additional inspection locations without having to determine a focus position at each different inspection location.

[0010] In one embodiment, the method includes calculating a difference DT.1 between a first image plane tilt vector TM.1 and a reference image plane tilt vector TR.1; and determining, based on the difference DT.1, a tilt adjustment of the wafer surface using a sample stage at a second inspection location. The tilt adjustment can be implemented using a tilt actuator of a sample stage or a wafer chuck used to hold the wafer. This allows compensation for unexpected deviations in image plane tilt during a series of image acquisitions at a series of inspection locations (e.g., at different locations on a flat wafer).

[0011] In one embodiment, the method includes calculating a difference DF.1 between a measured focus shift FM.1 and a reference focus shift FR.1; and determining, based on the difference DF.1, at least one of an axial movement of the sample stage and an adjustment of a focus shift of the multi-beam charged particle system at a second inspection location. The focus shift adjustment can be performed using an electrostatic lens of the multi-beam charged particle system or a z-stage capable of axially displacing the wafer surface. This allows compensation for an unintended focus shift before capturing an image at a subsequent inspection location. This also allows compensation for an unintended focus shift during a series of image captures, such as at a series of inspection locations at different locations on a flat wafer.

[0012] In one embodiment, the method includes calculating a difference DR.1 between a measured field curvature radius RM.1 and a reference field curvature radius RR.1; and determining, based on the difference DR.1, an adjustment for the field curvature of a multi-beam charged particle system at a second inspection location. By activating a magnetic lens of a field curvature compensator for the multi-beam charged particle system, the field curvature radius adjustment can be achieved by applying a deceleration field above the wafer surface to adjust the impact energy of the primary charged particles. This allows compensation for unintended changes in the field curvature radius before image acquisition at a subsequent inspection location. Furthermore, unintended changes in the field curvature radius can be compensated for during a series of image acquisitions at a series of inspection locations (e.g., at different locations on a flat wafer).

[0013] In one embodiment, the method is further iteratively repeated for each of a series of image acquisitions at a series of inspection locations using a series of imaging settings. The imaging settings for each image acquisition at each inspection location can be similar, identical, or different. The series of inspection locations includes at least two inspection locations located at two different positions on a wafer. Generally, all inspection locations can be different, or the series of inspection locations can include the same inspection location for a first image acquisition performed at the first inspection location followed by a second image acquisition performed at the first inspection location.

[0014] In one embodiment, the method further includes determining a second local image contrast map LV(x,y) of the second image; determining a second measurement value MV.2 from the second local image contrast map LV(x,y); determining a second reference value RV.1 from a memory based on second imaging settings; determining at least one second deviation value DV.2 between the second measurement value MV.2 and the second reference value RV.2; determining at least one of an adjustment to a third inspection location and an adjustment to a third imaging setting for the plurality of primary charged particle beamlets based on the at least one second deviation value DV.2; and capturing a third image at the third inspection location on the wafer surface using the third imaging setting for the plurality of primary charged particle beamlets. The second measurement value MV.2 includes at least one of a second focus shift FM.2, a second field curvature radius RM.2, and a second image plane tilt vector TM.2. The second reference value RV.2 includes at least one of a reference focus shift RF.2, a reference field curvature radius RR.2, and a reference image plane tilt vector RT.2. Generally speaking, in one instance, the method includes repeatedly performing the following steps: determining a local image contrast map LV(x, y); determining an i-th measurement value MV.i; determining an i-th reference value RV.i, determining at least an i-th deviation value DV.i; determining at least one of an adjustment to an (i+1)-th examination site and an adjustment to an (i+1)-th imaging setting covering a plurality of primary charged particle beamlets from at least an i-th deviation value DV.i; and obtaining a subsequent (i+1)-th image at a subsequent (i+1)-th examination site.

[0015] In one example, the method includes determining a wafer bow radius BR of a wafer surface from at least three difference vectors DT.1 to DT.3, and taking wafer bow radius BR into account when determining adjustments for at least one subsequent inspection location. In another example, the method includes determining a wafer wedge angle W from at least three difference vectors DT.1 to DT.3, and taking wedge angle W into account when determining adjustments for at least one subsequent inspection location. Thus, a wafer bow and a wafer wedge are systematically determined during an inspection operation and are accounted for during a subsequent series of image acquisitions on the same wafer. Consequently, after at least three image acquisitions at three different inspection locations, a deviation of a wafer surface from an expected flat surface is determined. The deviation of a wafer surface from an expected flat surface is then accounted for at other inspection locations, and high image contrast is achieved without having to determine a focus position at each inspection location.

[0016] In one embodiment, the method further includes tracking a series of at least three difference values ​​DV.1 to DV.3 and analyzing whether the series of difference values ​​DV.1 to DV.3 converge. If the series of difference values ​​DV.1 to DV.3 do not converge, triggering calibration of the multi-beam charged particle system.

[0017] In one example, a method includes determining a circle of maximum image contrast in a local image contrast map LV(x,y); and determining a diameter DVM of the circle of maximum image contrast. The radius of field curvature is typically known and varies only slightly during a series of image acquisitions. A focus offset FM is determined from the diameter DVM and the previously determined radius of field curvature. Thus, the method enables determination of the focus offset for a single image of an arbitrary object. In one example, the method includes determining the center position of the circle of maximum image contrast and determining an image plane tilt vector TM from the center position of the circle. Thus, the method enables determination of the axial position of the image plane or wafer tilt for a single image of an arbitrary object.

[0018] In one embodiment, a method for capturing a series of images at an inspection location is disclosed. The method includes capturing a first image at an inspection location on a wafer surface using a first imaging setting for a plurality of primary charged particle beamlets. The method further includes determining a first local image contrast map LV(x, y) from the first image and determining a measurement value MV from the first local image contrast map LV(x, y), wherein the measurement value MV includes at least one of a first focus shift FM, a first radius of field curvature RM, and a first image plane tilt vector TM. The method further includes determining a reference value RV.1 from a memory based on the imaging setting, wherein the reference value RV includes at least one of a reference focus shift RF, a reference radius of field curvature RR, and a reference image plane tilt vector TR. The method further includes determining at least one deviation value DV between the measurement value MV and the reference value RV; and determining at least one of an adjustment to the inspection location and an adjustment to the imaging setting for the plurality of primary charged particle beamlets based on the at least one deviation value DV. The method further includes adjusting an inspection location or adjusting imaging settings of a plurality of primary charged particle beamlets; and capturing a second image at the inspection location on the wafer surface using the adjusted imaging settings of the plurality of primary charged particle beamlets. Adjusting the imaging settings includes at least one of adjusting a focus shift FM of the image surface and adjusting an axial position of the wafer surface. In one embodiment, the method includes image processing of a series of images, such as averaging the series of images and adjusting a lateral image shift.

[0019] In one embodiment, a multi-beam charged particle system for improving throughput is provided. The multi-beam charged particle system includes a control processor including a memory for storing executable software instructions. The control processor is configured to execute the software instructions to perform any of the aforementioned methods.

[0020] A multi-beam charged particle system includes an object irradiation unit having a multi-beamlet generator for generating a plurality of primary charged particle beamlets. The multi-beam charged particle system includes an objective lens for focusing the plurality of primary charged particle beamlets into an image plane of the object irradiation unit during use. The multi-beam charged particle system includes a first common scanning deflector for scanning the plurality of primary charged particle beamlets over a region of the image plane. The multi-beam charged particle system further includes a detection unit configured to image the plurality of secondary electron beamlets onto an image sensor. The multi-beam charged particle system further includes a beam splitter unit for directing the plurality of primary charged particle beamlets from the multi-beamlet generator to the objective lens, and for directing the plurality of secondary electron beamlets from the objective lens to the detection unit. The detection unit includes a second common scanning deflector for maintaining the focal point of the plurality of secondary electron beamlets at a constant position on an image detector.

[0021] The multi-beam charged particle system further includes a voltage supply unit connected to a wafer during use for providing an extraction voltage to the wafer to generate a deceleration field for the primary charged particles, corresponding to an acceleration or extraction field for the secondary charged particles generated in the interaction volume. The detection unit includes a plurality of adjustable electron optical elements.

[0022] It will be understood that the present invention is not limited to the embodiments and examples, but also includes combinations and variations of a plurality of embodiments and examples. Simple diagram description

[0023] Several embodiments of the present invention will now be explained in more detail with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional view of a multi-beam charged particle system according to a first embodiment; Figure 2a, b shows an example of a scanning image capture of a surface section of a wafer; Figure 3a, b illustrates an example of the effects of field curvature and image plane tilt and imaging contrast; Figure 3c illustrates the relationship between local image contrast and defocus of an ideal focus position or beam waist; Figure 4a, b shows two examples of local imaging contrast or local visibility of two different imaging settings; Figure 5 shows an example of wafer inspection work; Figure 6 shows an example of a method according to a second embodiment; Figure 7 shows an example of wafer bending; FIG8 illustrates an inspection task including wafer inspection at a plurality of inspection locations; and 9a, b illustrate an example of the method according to the second embodiment. Implementation Method

[0024] In the exemplary embodiments of the present invention described below, components with similar functions and structures are denoted by similar or identical reference numerals as much as possible.

[0025] Some array elements (e.g., a plurality of primary charged particle beamlets) are denoted by reference numerals. Depending on the application, the same reference numeral may also denote a single element in the element array. Each primary charged particle beamlet (3.1, 3.2, 3.3) is one of a plurality of primary charged particle beamlets (3). The method is described with reference to a semiconductor wafer as an object, but the method can also be applied to a mask for semiconductor wafer manufacturing, which is considered to be equivalent to a semiconductor wafer in the present invention.

[0026] FIG1 is a schematic diagram illustrating the basic features and functions of a multi-beam charged particle system 1 according to a first embodiment. It should be noted that the reference numbers used in the figure are selected to represent their relative functions. The system shown is a multi-beam scanning electron microscope (SEM) that uses a plurality of primary charged particle beamlets 3 to produce a plurality of primary charged particle beam spots 5 on a surface 25 of an object 7, such as a wafer or a photomask substrate located in the image 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. The features and functions of the multi-beam charged particle system 1 can be implemented using electrons or other types of primary charged particles, such as ions, particularly helium ions. Further details of the multi-beam charged particle system 1 are provided in PCT patent application No. PCT / EP2021 / 066255, filed on June 16, 2021, which is incorporated herein by reference in its entirety.

[0027] The multi-beam charged particle system 1 comprises an object irradiation unit 100, a detection unit 200, and a secondary electron beam splitter or beam splitter unit 400 for separating a secondary charged particle beam path 11 from a primary charged particle beam path 13. The object irradiation unit 100 comprises a charged particle multi-beam generator 300 for generating a plurality of primary charged particle beamlets 3 and adapted to focus the plurality of primary charged particle beamlets 3 on an image plane 101, wherein a surface 25 of an object or wafer 7 is positioned by a sample stage 500.

[0028] The charged particle multi-beam generator 300 generates a plurality of primary charged particle beamlet spots on an intermediate image surface 321. The charged particle multi-beam generator 300 includes at least one primary charged particle source 301, such as electrons. The at least one primary charged particle source 301 emits a diverging primary charged particle beam, which is collimated by at least one collimating lens 303 to form a collimated or parallel primary charged particle beam 309. The collimating lens 303 typically comprises one or more electrostatic or magnetic lenses, or a combination of electrostatic and magnetic lenses. The collimated primary charged particle beam 309 is incident on a multi-beam forming unit 305. The multi-beam generating unit 305 is described, for example, in US 2019 / 0259575 and US 10,741,355 B1, both of which are incorporated herein by reference in their entireties. The multi-beam forming unit 305 essentially comprises a first porous plate or filter plate 304, which is illuminated by a collimated primary charged particle beam 309. The first porous plate or filter plate 304 comprises a plurality of apertures in a grating configuration for generating a plurality of primary charged particle beamlets 3, which are generated by the collimated primary charged particle beam 309 passing through the plurality of apertures. The multi-beam forming unit 305 comprises at least one further porous plate 306, which is located downstream of the first porous plate or filter plate 304 relative to the direction of movement of electrons in the primary charged particle beam 309. For example, the porous plate 306 comprises, for example, four or eight electrostatic elements for each of the plurality of apertures, for example, to individually deflect each of the plurality of beamlets. According to some embodiments, the multi-beam forming unit 305 is equipped with an end porous plate 307. The multi-beam forming unit 305 is also equipped with an adjacent electrostatic field lens 308.1, which in some examples is integrated into the multi-beam forming unit 305. In conjunction with the field lens 308.2, the plurality of primary charged particle beamlets 3 are focused in or near the intermediate image surface 321. Each of the primary charged particle source 301 and the active porous plate 306 is controlled by a primary beam path control unit 830.

[0029] The multiple focal points of the primary charged particle beamlets 3 that have passed through the intermediate image surface 321 are imaged onto the image plane 101 by the field lens 103 and the objective lens 102, with the surface 25 of the object 7 located in the image plane. A voltage is applied to the object by the sample voltage supply unit 503, generating a decelerating electrostatic field between the objective lens 102 and the object surface 25. With the decelerating electrostatic field generated by the sample voltage supply unit 503, the kinetic impact energy of the primary electrons is adjusted to, for example, less than 2 keV, less than 1 keV, less than 500 eV, less than 300 eV, or even lower.

[0030] The object irradiation unit 100 further includes a collective multi-beam raster scanner 110 located near a beam intersection 108. This collective multi-beam raster scanner is capable of deflecting the plurality of primary charged particle beamlets 3 in a direction perpendicular to the propagation direction of the charged particle beamlets. In all instances, the propagation direction of the primary beamlets is the positive z-direction. The objective lens 102 and the collective multi-beam raster scanner 110 are centered around an optical axis 105 of the multi-beam charged particle system 1, which is perpendicular to the wafer surface 25. The plurality of primary charged particle beamlets 3, forming beam spots 5 in a plurality of raster configurations, are scanned synchronously across the wafer surface 25. In one example, the grating configuration of the focal spot 5 of the plurality of J primary charged particles 3 is a hexagonal grating of approximately one hundred or more primary charged particle beamlets 3, for example, J = 91, J = 100, or J = approximately 300, such as J = 331, J = 2791, or even more beamlets. The beam spots 5 are spaced approximately 6 μm to 45 μm apart and have a diameter less than 5 nm, such as 3 nm, 2 nm, or even less. In one example, the beam spot size is approximately 1.5 nm, and the distance between two adjacent beam spots is 8 μm. At each scanning position of each of the plurality of beam spots 5, a plurality of secondary electrons are generated, forming a plurality of secondary electron beamlets 9 having the same grating configuration as the beam spot 5. The intensity of the secondary charged particle beamlets 9 generated at each beam spot 5 depends on the intensity of the impinging primary charged particle beamlet 3 illuminating the corresponding beam spot 5, the material composition and layout of the wafer 7 beneath the beam spot 5, and the charging condition of the wafer 7 at the beam spot 5. The plurality of secondary charged particle beamlets 9 are accelerated by the same electrostatic field between the objective lens 102 and the object surface 25, generated by the sample voltage supply unit 503, focused by the objective lens 102, and pass through the collective multi-beam raster scanner 110 in a direction relative to the primary charged particle beamlets 3. The plurality of secondary charged particle beamlets 9 are deflected and scanned by the collective multi-beam raster scanner 110. The plurality of secondary charged particle beamlets 9 are then directed by the secondary electron beam splitter or beam splitter unit 400 to follow the secondary beam path 11 of the detection unit 200. The plurality of secondary electron beamlets 9 travel in the opposite direction to the primary charged particle beamlet 3, and the beam splitter unit 400 is typically configured to separate the secondary charged particle beam path 11 from the primary charged particle beam path 13 by a magnetic field or a combination of a magnetic field and an electrostatic field.

[0031] The detection unit 200 images the secondary electron beamlets 9 onto the image sensor 600, forming a plurality of secondary charged particle beam spots 15 on the image sensor. The detector or image sensor 600 includes a plurality of detector pixels or individual detectors. The intensity of each of the plurality of secondary charged particle beam spots 15 is individually detected, and the properties of the object surface 25 are detected with high resolution and high throughput for large image patches of the object 7. For example, for a 10×10 beamlet raster with an 8μm pitch, a single image scan using the collective multi-beam raster scanner 110 produces an image patch with a diameter of approximately 88μm×88μm, with an image resolution of, for example, 2nm or less. The image patch is sampled using half the beam spot size, resulting in 8,000 pixels per image line for each beamlet, resulting in an image patch generated by 100 beamlets containing 6.4 gigapixels. The digital image data is collected by the imaging control unit 810. Details of digital image data collection and processing using, for example, parallel processing are described in PCT Patent Application No. WO 2020151904 A2 and U.S. Patent No. US 9,536,702, which are hereby incorporated by reference herein.

[0032] The detection unit 200 further includes at least one collective raster scanner 222 connected to a scan control unit 860. The scan control unit 860 is configured to compensate for a residual difference in the positions of the plurality of secondary electron beam spots 15 of the plurality of secondary electron beamlets 9, so that the positions of the plurality of secondary electron beam spots 15 remain constant at the image sensor 600.

[0033] The detection unit 200 includes additional electrostatic or magnetic lenses 205.1 through 205.5, and a second intersection or pupil plane 21b of the plurality of secondary electron beamlets 9, in which a contrast aperture filter module 214 having a plurality of aperture filters 284a, 284b is located. The second intersection corresponds to a pupil plane 21b of the detection unit 200. In a pupil plane, a transverse coordinate system relative to the optical axis 2105 corresponds to the propagation angle of the secondary electron trajectories at the imaging surface 101. The propagation angle of the secondary electron trajectories is measured relative to the wafer surface normal, which corresponds to the optical axis 2105 of the detection unit 200.

[0034] The image sensor 600 is configured with an array of sensing areas that utilizes a pattern fused to the raster configuration of the secondary electron beamlets 9 focused onto the image sensor 600 by the projection lens 205. This enables detection of each individual secondary electron beamlet, independent of other secondary electron beamlets incident on the image sensor 600. The image sensor 600 illustrated in FIG1 may be an electron-sensitive detector array, such as a CMOS or CCD sensor. This electron-sensitive detector array may include an electron-to-photon conversion unit, such as a scintillator element or an array of scintillator elements. In another embodiment, the image sensor 600 may be configured with an electron-to-photon conversion unit or scintillator plate positioned in the focal plane of a plurality of secondary electron beam spots 15. In this embodiment, the image sensor 600 may further include a relay optical system for imaging and directing photons generated by the electron-to-photon conversion unit at the secondary charged particle beam spot 15 onto a dedicated photon detection unit, such as a plurality of photomultiplier tubes or avalanche photodiodes (not shown). Such an image sensor is disclosed in US Pat. No. 9,536,702, which is cited above and incorporated herein by reference.

[0035] While capturing an image block by scanning the plurality of primary charged particle beamlets 3, the sample stage 500 is preferably not moved. After capturing an image block, the sample stage 500 is moved to the next image block to be captured. In an alternative embodiment, the sample stage 500 is continuously moved in the second direction while the collective multi-beam raster scanner 110 scans the plurality of primary charged particle beamlets 3 in the first direction to capture an image. The sample stage movement and position are monitored and controlled by sensors known in the art, such as laser interferometers, grating interferometers, confocal microlens arrays, and the like.

[0036] During an image scan, the control unit 800 is configured to trigger the image sensor 600 to detect a plurality of time-resolved intensity signals from a plurality of secondary electron beamlets 9 at predetermined time intervals, and a digital image of an image block is accumulated and stitched together from all scanning positions of the plurality of primary charged particle beamlets 3.

[0037] The control unit 800 of the multi-beam charged particle system 1 further includes an imaging control module 810 configured to receive a data stream from the image sensor 600 and generate a digital image of the surface of the sample 7 during operation; a secondary beam path control module 820 configured to control the lens 205 and other components of the detection unit 200; a primary beam path control module 830 configured to control components of the object irradiation unit 100, which includes the charged particle multi-beam generator 300; and a sample stage control module 850. It is configured to control the position and alignment of the sample stage and includes control of the sample voltage supply unit 503; a scanning control unit 860 configured to control the scanning operation of the collective multi-beam raster scanner 110 and the scanning deflection system 222; a control operation processor unit 840 configured to perform sample inspection operations and to control the various modules described above (i.e., 810, 820, 830, 850, 860, 870); and a memory 880 for storing software, instructions, and image data. The control operation processor unit 840 is also connected to an interface (not shown) for exchanging data, instructions, software, or user interaction. A control unit 800 according to the first embodiment further includes an image processing engine 890 configured to perform image processing operations on at least one digital image.

[0038] FIG2 illustrates the scanning operation of a plurality of primary charged particle beamlets 3 during image acquisition. The scan control unit 860 is configured to provide a scan signal to the scan deflector 110 during use. Each primary charged particle beamlet 3 is thereby deflected by the collective multi-beam raster scanner 110 such that the corresponding focal spot 5.i scans across an image patch 245.i of a single beamlet ( FIG2 a ). Each image patch 245.i has a diameter AP of, for example, 8 μm to 10 μm. The scanning operation includes scanning a plurality of parallel image scan lines 241 along a scan direction 143.1 for image acquisition. At the end of each image scan line 241, each beamlet 3 moves to the starting position of the next scan line, also known as a "flyback" 243. During image acquisition along the image scan line 241, the scanning operation is controlled to achieve a dwell time of approximately 50 ns at each image point, for example, 8,000 image points per image scan line 241. The time of the flyback 243 can be very short, for example less than 20 ns in total. FIG2 b shows the parallel operation of a plurality of primary charged particle beamlets 3 to capture an image of a surface section 251 . 1 of the wafer surface, the image being composed of a plurality of smaller image tiles 245 .

[0039] Figure 3 illustrates an imaging operation using multiple primary charged particle beamlets. Figure 3a shows the through-focus distribution of five primary charged particle beamlets 3.1 to 3.5 with highly amplified field curvature. Due to the characteristics of the multi-beam charged particle system 1, the beam waists 74.1 to 74.5 of the primary charged particle beamlets 3.1 to 3.5 are formed on a curved image plane 101. The curvature radius RR and the image plane tilt vector TR of the ideal image plane 101 depend on the imaging settings of the multi-beam charged particle system 1, such as the focus setting adjusted by the objective lens 102, the selected numerical aperture, the beamlet spacing, and the landing energy adjusted by the sample voltage supply unit 503. Typically, the wafer surface 25 (not shown) is disposed in the region between the upper surface 47 having the ideal center surface 45 and the lower surface 41. This ensures that each primary beamlet is focused on the wafer surface 25 at a minimum distance, and blurring due to defocusing is kept within an acceptable range. According to a first embodiment, the operational control processor is configured to determine the resolution loss due to local defocus of each beamlet. An example is illustrated in FIG3b. A wafer surface 25 is positioned perpendicular to the z-direction so that primary charged particle beamlets 3.1 and 3.4 have minimal defocus and, therefore, minimal resolution loss due to defocus blur. Images captured during image acquisition are evaluated based on contrast measurements, such as intensity gradient or slope calculations, spectral analysis, histogram analysis, or local contrast calculation using C = (Imax-Imin) / (Imax+Imin). An image contrast measurement LV(x,y) is illustrated in FIG3b, which shows a cross-section of the contrast measurement or contrast map LV(x,y) in the y-direction, where the two maximum contrast locations correspond to the minimum defocus of the spherical focal plane deviation DZ(x,y) of the beam waists 74.1 to 74.5 relative to the y-plane.

[0040] The spherical focal plane deviation DZ(x,y) typically produces a resolution or contrast map across a plurality of circular primary beamlets, with the contrast or resolution being greatest in the annular region. Figure 3b shows a cross-section of the resolution or contrast map LV(x,y). Figure 3c plots the relationship between the local image contrast LV and the defocus z of the ideal focal position or beam waist 74 at z = 0. A focal range 51 between the lower defocus position 41 and the upper defocus position 47 can be derived from a contrast threshold 59. Typically, the local image contrast decreases linearly with increasing defocus. The local image contrast at defocus LV(z) can also exhibit asymmetries due to residual spherical image differences or dependence on the orientation of the semiconductor pattern on the wafer surface due to astigmatism. This contrast curve, which depends on the image plane deviation from the ideal focal position, can be determined during calibration of the multi-beam charged particle system 1.

[0041] During inspection, absolute contrast also depends on the material composition, the pattern to be imaged, the layout, the noise level, and charging effects on the object surface. Therefore, it is not possible to deduce the defocus or image plane position from the absolute contrast value. However, using a plurality of local contrast values ​​LV(x,y) (see FIG. 2 ) over a large image of the surface segment 251 and a model of the spherically curved and tilted image surface 101 of the multi-beam charged particle system 1, it is possible to determine the defocus or focus offset with high precision.

[0042] According to a first embodiment, a multi-beam charged particle system 1 is configured to measure a local visibility or contrast map LV(x,y) of an image of a surface segment 251 during image acquisition and overfocus at a predetermined contrast LV(z). From the contrast map LV(x,y), an offset DZ(x,y) between the object surface 25 and the ideal image plane 101 is derived. The multi-beam charged particle system 1 is configured to determine the radius RM of the actual best focus surface and the image plane tilt vector TM = [TMx, TMy] (RM and TM are denoted as the "measured" radius and the "measured" tilt vector) based on the contrast map LV(x,y).

[0043] The field curvature radius R and the image plane tilt vector T are functions of imaging settings, such as the magnification, numerical aperture, or average focal length of the plurality of primary beamlets. For each image acquisition, an imaging setting is selected. The multi-beam charged particle system 1 is configured to select different imaging settings, including the lens power of the objective lens 102, the lens power of the at least one field lens 103, and the impact energy of the primary charged particle beam 3.

[0044] FIG4 illustrates two examples of two different imaging settings. Each individual primary charged particle beam 3 is represented by a circle 17 of varying size, corresponding to the resolution of the primary charged particle beam 3. The smaller the circle 17, the greater the local image contrast obtained for the primary charged particle beam 3. The circular line 53 of equal contrast is centered about an eccentric center of symmetry 55. FIG4a illustrates the effect of a first reference field curvature radius R1 and a first reference image plane tilt vector T1 for a first imaging setting. FIG4b illustrates a second reference radius R2 and a second reference image plane tilt vector T2 for a second imaging setting, resulting in two different contrast distributions across the image field 251 of the multi-beam charged particle system 1. As shown in FIG4b, each imaging setting typically also includes different rotation angles of the plurality of primary charged particle beamlets on a wafer surface 25.

[0045] In some embodiments, a multi-beam charged particle system 1 is further configured to exchange a primary multi-beam forming unit 305 for forming a first plurality of primary charged particle beamlets with a first primary multi-beam forming unit, and to exchange a primary multi-beam forming unit 305 for forming at least a second plurality of primary charged particle beamlets with a second primary multi-beam forming unit. For each imaging setting, a reference field curvature radius RR and a reference image plane tilt vector TR can be predetermined, for example, during calibration of the multi-beam charged particle system 1. The multi-beam charged particle system 1 includes a memory configured to store a plurality of reference field curvature radii RR and a reference image plane tilt vector TR associated with a plurality of different imaging settings. The multi-beam charged particle system 1 is configured to determine, based on the selected imaging setting, the difference between a measured field curvature radius RM and a measured image plane tilt vector TM and the corresponding reference field curvature radius RR and reference image plane tilt vector TR. The multi-beam charged particle system 1 is configured to determine a focus correction signal from this difference and provide this focus correction signal to a compensation element of the multi-beam charged particle system 1. This allows a wafer surface to be maintained at an optimal focus position with minimal loss of contrast during image acquisition. The control or correction signal is determined directly from images captured during use, eliminating the need for further, time-consuming focus measurement methods. This improves throughput.

[0046] Figure 5 illustrates an example of wafer inspection using a flat wafer surface 25. The wafer surface 25 is adjusted by the stage actuator 510 of the sample stage 500 at a tilt angle g corresponding to an image plane tilt vector T. This compensates for the image plane tilt vector. The image plane tilt angle g is determined by the measured field curvature radius RM and the measured image plane tilt vector TM, where sin(g) = TM / RM. After the sample stage is adjusted to the tilt position of angle g, the measured image plane tilt vector TM ideally disappears, i.e., TM = [0, 0]. After determining the tilt position angle g for the actual captured image, the multi-beam charged particle system 1 is configured to laterally move the wafer in the tilt plane 101 to the next inspection position to capture the next inspection image. A similar principle applies to the case of wafer bowing. Further details are explained below.

[0047] FIG. 6 shows a method for improving image capture according to a second embodiment of the present invention.

[0048] In an optional step C, the multi-beam charged particle system 1 is calibrated. During the calibration step, a reference value RV of the field curvature RR, a reference focus offset DFR, and a reference image plane tilt vector TR are determined for at least one imaging setting. The reference value RV for the at least one imaging setting is stored in a memory. The reference value RV can be determined using a reference object, such as a reference wafer or any other object, such as a patterned flat wafer with semiconductor structures having a size between 3 nm and 10 nm, with a resolution requirement for an imaging task. It is not necessary to determine the reference value RV for many imaging settings during a calibration. Typically, determining the reference value RV for a limited number of imaging settings is sufficient.

[0049] In step S0, an inspection job is organized. A wafer 7 is loaded onto the sample stage 500, and a series of N inspection locations 6.1…6.N on the wafer surface 25 are received and configured. An imaging setting is selected for each of the plurality of inspection locations. For each imaging setting, a reference value RV is determined for a reference focus offset FR, a field curvature radius RR, and an image plane tilt TR. The reference focus offset FR, the reference field curvature radius RR, and the reference image plane tilt TR are typically given by differentiable functions that depend on the individual imaging settings (such as the lens excitation of the impact energy of the primary electron). Therefore, in one example, the reference value RV is determined during step S0 by interpolation, which includes using a polynomial interpolation function from predetermined reference values ​​at preselected imaging settings, which was already used during calibration of the multi-beam charged particle system 1 during step C.

[0050] The number N of different examination locations can be any number between N=2 and greater, for example, N=10, N=100, or N=1000 or greater. At each examination location, an image corresponding to the image field size of the plurality of primary charged particle beams 3 is captured (see the image of surface segment 251 in FIG2 ). An image field size can be, for example, approximately 100 μm x 100 μm or greater. A series of examination locations 6.1, 6.2, and further examples are shown in FIG7 and 8 and described below.

[0051] In one embodiment, the inspection process involves repeated imaging at the same location on the wafer surface, meaning that a subsequent inspection location is identical to a previously inspected location. For example, a series of images is acquired from a surface segment 251, where each individual image has a fast scan time and high noise levels. This reduces the charge on the wafer surface 25. With a longer time delay between image captures of the same inspection location, the charge accumulated on the wafer surface 25 decays, preventing subsequent image capture from being degraded by the charge from the previous image capture. By averaging the series of images to obtain a final image, the noise level is reduced.

[0052] In loop IAI (1…N), each of at least N = 2 different inspection locations 6.i (where i = 1…N) at different positions is subsequently adjusted by the sample stage within the image field of the multi-beam charged particle system 1, and at least one first image at the first inspection location 6.1 and one second image at the second inspection location 6.2 are captured.

[0053] Each loop consists of the following steps: Step AF.i is used to set the i-th imaging setting for the i-th image capture and to adjust the i-th inspection part 6.i on the wafer surface 25 to be in or near the image surface 101 of the i-th imaging setting. Step Ii, obtain the i-th inspection image at the i-th inspection part 6.i. In step LVD.i, the local image contrast map LV(x,y) of the i-th image is determined. In step MV.i, a measurement value MV.i of at least one of a focus shift FM.i, a field curvature radius RM.i, and an image plane tilt vector TM.i is determined from the local image contrast map LV(x,y) of the i-th image. In step CP, the measured value MV.i is compared with the reference value RV.i, where the reference value RV.i is determined based on the reference value RV stored for the i-th image setting. A control step is used to determine whether the required number N of inspections has been completed (by determining whether i+1≤N or whether the maximum number N of inspection locations has been reached).

[0054] During each LVD step, a local image contrast map LV(x,y), also called local visibility, of an image of the surface segment 251 is determined using a fast procedure selected from a group of procedures, the procedures comprising: an intensity gradient determination, a local histogram analysis of the grayscale levels, a local visibility determination according to V = (Imax-Imin) / Imax+Imin), a local calculation of the normalized image logarithmic slope (NILS), a local spectrum analysis, and an analysis of the convolution result of an edge filter.

[0055] During each step MV.i, the actual value of at least one of the defocus FM.i, the field curvature radius RM.i, and the image plane tilt TM.i is determined. To determine this, in one example, a defocus contrast model with a model radius and a model tilt is applied, and the measured value MV.i is determined by optimization. In one example, a predetermined defocus contrast model is applied to the local visibility LV.i and the focus shift FM, and the field curvature radius RM is determined by a best-fit approximation. For illustration, please refer to Figures 3b and 3c and Figure 9. For larger focus shifts or defocus DZ(x,y), as a good approximation, the local visibility LV decreases linearly with the absolute value of the defocus DZ(x,y). It is also known that the field curvature radius RM is positive in the coordinate system of Figure 3, where the z-axis points upward relative to the propagation direction of the primary charged particles. In other words, the center of the field curvature is located above the wafer surface 25. Therefore, the ambiguity in the sign of the defocus curve DZ(x,z) is resolved by conventional knowledge.

[0056] The beamlet 3 with the smallest distance from the beam waist 74 to a wafer surface 25 produces the greatest possible image contrast, where DZ(x,y) = 0. Due to spherical field curvature, the maximum image contrast LV within the image field is achieved on a circular ring within the image field (see FIG4 , where the circle 57 of minimum beam spot diameter and maximum image contrast is highlighted by the bold line). In one embodiment, the focus offset FM is determined by the diameter DVM of the circle of maximum image contrast 57 and the field curvature radius RM. In one embodiment, the image plane tilt vector TM is determined by the center of the circle of maximum image contrast 57.

[0057] In step CP, the measured value MV.i is compared with the reference value RV.i, which is determined based on the imaging settings of the i-th image. The deviation DV.i between the measured value MV.i and the reference value RV.i is determined using the following formulas: DF.i = FM.i – RF.i, DR.i = RM.i - RR.i, and DT.i = TM.i – TR.i.

[0058] In one embodiment, step MV includes determining the rotation angle of the plurality of primary charged particle beamlets 3 (see FIG4 b ). Typically, a position change of the image surface 101 of the magnetomotive objective lens 102 can also cause a change in the configuration rotation angle of the plurality of primary charged particle beamlets 3 .

[0059] The differences or deviations DV.i are provided to step AF.(j) for use in the next or any subsequent image acquisition (where j > i). Thus, for the next adjustment in step AF.(j), the reference value RV.(j) can be adjusted and corrected using the deviations DF.i, DR.i, and DM.i determined in the previous step MV.i of the previous image acquisition step Ii.

[0060] In one embodiment, the deviations DF.i, DR.i, and DT.i are further tracked in tracking step IR. During tracking step IR, the deviations DF, DR, and DT are analyzed. In a first embodiment, the adaptive focus and image plane tilt adjustment according to step AF converges during a series of image acquisitions, and the deviations DF, DR, and DT decrease. In a second embodiment, the adaptive focus and image plane tilt adjustment according to step AF does not converge. This lack of convergence may be due to at least two reasons: A) The multi-beam charged particle system 1 needs to recalibrate the reference value RV. This may occur due to drift or degradation of the system performance of the multi-beam charged particle system 1. B) The wafer surface 25 is not flat.

[0061] Typically, wafer surface 25 is flat, but wafer 7 may be subject to wafer bow or may be non-parallel or wedge-shaped, meaning the upper and lower wafer surfaces are not completely parallel. The range of deviations DF, DR, and DT generated by wafer bow and / or a wedge-shaped wafer is systematic. For example, a wafer's wedge angle W can be determined based on a constant component of a range of tilt angles DT. For example, a small wafer bow with a bend radius BR can be determined based on at least three tilt angles at three different inspection locations with different local surface tilts BT.1 to BT.3. Figure 7 illustrates the local surface tilts BT.1 to BT.3 based on the wafer bend radius BR. Figure 7 illustrates an example of a convex wafer bow with a convex wafer surface 25. The three inspection locations 6.1 to 6.3 are depicted using three different surface normal vectors BT.1 to BT.3, from which the center of curvature CBR of the wafer bow is derived. Surface normal vectors BT.1 to BT.3 are determined by the difference vector DT between the measured image plane tilt vector TM and the expected reference tilt vector TR. With a known bend radius BR, local wafer tilt can be determined at any location on surface 25 of wafer 7. An example will be described for the next subsequent inspection location 6.4. At the xy coordinates of inspection location 6.4, the local surface normal vector AT of wafer surface 25 is calculated from the center of curvature CBR of the wafer bend. Based on the local surface normal vector AT at wafer inspection location 6.4, the wafer can be adjusted in step AF.4. For better illustration, the effects of wafer bow are highly exaggerated in FIG7 . In the presence of wafer bow, wafer 75 is moved using the method according to the second embodiment via sample stage 500, not in the tilt plane as shown in FIG5 , but rather only on a virtual sphere based on the curvature of wafer 7, so that wafer surface 25 remains close to image surface 101.

[0062] In step SP, for example, a series of at least tilt angles DT are monitored, and it is determined whether systematic effects due to a wedge-shaped wafer or a wafer bow are the most likely source of the non-convergence of the deviations DF, DR, and DT. A wafer wedge W and a wafer bow radius BR can then be numerically derived and taken into account during the determination of the measurement values ​​in a subsequent step MV. The local tilt of a wafer surface is thereby determined at each subsequent inspection location, and during step AF, the wafer surface 25 is adjusted in the image surface 101 of the multi-beam charged particle system 1 at the subsequent inspection location with the optimal focus shift FR, including compensating for the local tilt of the wafer surface 25 at each subsequent inspection location. Compensation for the local tilt can be achieved, for example, by using a sample stage actuator 510 (see FIG. 5 ), which is configured to adjust the tilt angle of a wafer holding device or wafer chuck of the sample stage 500.

[0063] In one embodiment, if a series of difference values ​​DF, DR, and TM do not converge even after taking into account the wafer wedge angle W or the wafer bend radius BR, calibration of the multi-beam charged particle system 1 according to step C is initiated, and at least one new set of reference values ​​RV for at least one imaging setting is determined. The new reference values ​​are stored in a memory for use during a series of image acquisitions at a plurality of inspection locations according to an inspection task.

[0064] After capturing the last Nth inspection image of a wafer inspection run on the wafer, the inspection image of the inspection location, the deviations DF, DR, and TR, the wafer wedge angle W, and the wafer bend radius BR are stored in memory for later use. The later use can include, for example, defect detection, feature size determination, distribution analysis of specific defects or feature sizes on the wafer surface 25, or repeated inspection of the same wafer taking into account the determined wedge angle W or bend radius BR of the wafer obtained during the previous inspection run.

[0065] The determination of the local contrast LV, the determination of the measured value MV, and the comparison with the reference value can be performed during and in parallel with a series of image acquisitions. Adjustments to an imaging setting determined from this comparison can be applied to subsequent image acquisitions at the same or different examination sites. This method is not limited to application to a direct series of two pairs of directly consecutive image acquisitions but can be applied to any series of image acquisitions within a series. From any first image acquisition or series of image acquisitions, adjustments to an imaging setting or sample stage can be derived for any subsequent image acquisition or series of image acquisitions.

[0066] 8 illustrates a simplified example of wafer inspection work in one instance of wafer 7 with a complex number of wafer bare crystals 71 on surface 25 . In this instance, the first three inspection sites 6.1 to 6.3 are located at the periphery of wafer 7 . Thereby, the wafer bending radius BR or wedge angle W of one wafer can be determined with high precision. Next, inspection sites 6.4 to 6.7 are selected at the process control monitors (PCM) to perform the measurements, for example, which are sensitive to exposure or etching processes. Next, a cluster of inspection sites 66 , which includes a complex number of adjacent image fields of small bundles of primary charged particles, is used to image bare crystals 71 of a larger region or at least one wafer. Thereby, the distribution or defect density of defect 67 is determined. For each image retrieval, the wafer surface 25 with the best focus offset RF is adjusted to the image surface 101 of the multi-beam charged particle system 1 using the contrast information of the previously retrieved images. Utilizing the multi-beam charged particle system 1 according to the first embodiment and the method of operating the multi-beam system according to the second embodiment, it is capable of examining a large area of ​​the one wafer surface 25 in a shorter time and the characteristics of semiconductor features such as fins, interconnects, HAR channels, transistor gates, doping regions, process control monitors, etc. are able to be measured in a short time. Furthermore, it is able to detect defects, such as those caused by contamination during the wafer process, or mask defects. The results enable process improvements, such as adjustment of process parameters for individual process steps during mask repair operations, cleaning operations, or semiconductor wafer fabrication.

[0067] 9 illustrates one instance of the method according to the second embodiment. Figure 9a shows a first measurement of a localized image contrast LV1 ( x , y ) at a first examination site 6.1. Due to a larger focal offset FM1, the circle 57.1 of maximum image contrast is located at a peripheral position. From the local image contrast LV1 ( x , y ), a first deviation DZ 1 ( x , y ) from the wafer surface 25 from a complex number of primary bundles is calculated. The first deviation DZ1 ( x , y ) shows a field curvature with radius R1 and a first large defocus or focus offset FM1. The focal offset FM1 has an effect on the local imaging contrast LV1 ( x , y ), and with the focal offset FM1 of the first image retrieval, the main part of the image for (given through the contrast critical value 59) a specific contrast or resolution requirement is out of focus.

[0068] FIG9 b shows the result of the method according to the second embodiment. FIG9 b shows an ideal local image contrast LVR(x,y) with a balanced focus offset FR of the wafer surface 25 relative to the image surface 101, with the beamlets at an ideal reference position DZR(x,y) and an image contrast LVR(x,y) above a contrast threshold 59 for a maximum number of primary charged particle beamlets 3 (not shown).

[0069] In this example, the field curvature radius R1 and the image plane tilt vector do not differ from the reference values. However, the difference between the measured local contrast curve LV1(x,y) and the reference local contrast curve LVR(x,y) is due to the effect of the focal length difference DF = FM1-FR. Figures 3a and 3b illustrate an example with an image plane tilt vector. An image plane tilt vector TM is determined based on the displacement of the center of symmetry of the local contrast curve LV(x,y). In one example, the field curvature radius RM is determined, for example, from the curvature of the local image contrast map LV(x,y) or the difference between a maximum contrast value and a minimum contrast value of the local image contrast map LV(x,y). For example, as the field curvature radius increases, the focal length on the image field decreases, and the contrast difference between a minimum local contrast value and a maximum local contrast value of the local image contrast map LV(x,y) decreases.

[0070] According to the method of the second embodiment, during step MV.1, focus shift FM1 is determined based on the local image contrast LV1(x,y). For example, focus shift FM1 is calculated by applying a defocus contrast model to LV1(x,y) or based on the diameter of circle 57.1 of maximum image contrast. Based on predetermined knowledge of field curvature radius R1 (ultimately verified during step MV by calculating RM.1), focus shift FM1 is determined during a first measurement from a single image of surface 25 of a structured semiconductor wafer 7. In step CP, focus shift FM1 is compared with a reference focus shift FR based on the imaging settings used at the first examination location 6.1. During step AF.2, a difference DF is determined and used to adjust the second imaging settings for a second measurement at the second examination location.

[0071] According to another example, the method of the second embodiment is also suitable for tracking drift or degradation of a multi-beam charged particle system, thereby, for example, performing frame averaging or multi-mode image acquisition.

[0072] According to a first embodiment of frame averaging, a series of images is captured at a first inspection location (6.1) with a short dwell time, thereby reducing the charging of charged objects. However, each individual image exhibits an increased noise level. To compensate for the increased noise level, an average image is calculated from a series of at least two, three, five, or more images. However, during the acquisition of the multiple images, drift may occur as the position of the inspection location 6.1 and the position of the surface 25 of the wafer 7 change. Furthermore, charge may slowly accumulate on the surface of the wafer and affect the image surface 101 (caused by charging). Due to the charging, the axial position or focus shift of the plurality of primary charged particle beamlets 3 and the shape of the image surface 101 change during the series of images. By determining the focus shift during or after each image acquisition, focus shift can be continuously compensated during the acquisition of the series of images. Focus shift reduces image contrast and is therefore not possible to compensate for through digital post-processing, such as by laterally shifting the images. By using frame averaging and on-the-fly focus correction according to the method of the second embodiment, high-resolution images can be obtained.

[0073] However, adjustment of image settings is not limited to correction of focus shift FM or image plane tilt TM. In a second example of multimodal imaging, a series of images is acquired using, for example, different contrast modes for secondary electron imaging or different kinetic energies of the primary beamlet. Since the secondary electron yield depends on the kinetic energy of the primary electrons, different charging characteristics of the semiconductor sample are achieved using a first image setting and a second image setting using first and second kinetic energies of the primary electrons. According to the second embodiment, drift in focus shift FM or image plane tilt TM is determined and compensated for during the acquisition of the series of images acquired using different kinetic impact energies of the primary electrons. Focus shift FM.1 or image plane tilt TM.1 is determined based on a first measurement using the first image setting and transferred to a second measurement using the second image setting. Another example of multimodal imaging is a series of images using different aperture filters 284.a and 284.b in the secondary electron beam path. This allows for measurement of different charging contrasts or topographic contrasts, and image fusion of a series of at least two different images can provide an image of the wafer surface with higher precision.

[0074] Therefore, according to the second embodiment, a method for wafer inspection using a multi-beam charged particle system comprises the following steps: capturing a first image at a first inspection location (6.1) on a wafer surface (25) using a first imaging setting of a plurality of primary charged particle beamlets (3); and capturing at least one second image at a second inspection location (6.2) on the wafer surface (25) using a second imaging setting of a plurality of primary charged particle beamlets (3). In one embodiment, the second inspection location (6.2) is the same as the first inspection location (6.1), and a series of at least two repeated image captures are performed at the first inspection location (6.1), wherein between each image capture, a measurement value MV.i consisting of at least one of a focus shift FM.i, a field curvature radius RM.i, and an image plane tilt vector TM.i is determined. From the measurement value MV.i, at least one deviation value DV.i is determined, and subsequent imaging settings of the plurality of primary charged particle beamlets (3) are adjusted according to the at least one deviation value DV.i. The second image acquisition at the first examination site (6.1) can be obtained as a repeated image with a second imaging setting similar to the first imaging setting, wherein only the image plane or the image plane tilt is adjusted. The second image acquisition at the first examination site (6.1) can be obtained as a repeated image with a second imaging setting different from the first imaging setting, wherein the second imaging setting differs from the first imaging setting in that at least one of the imaging settings consists of a kinetic impact energy of a primary electron, a scanning direction or scanning pattern, an aperture filter 284, and an aperture filter placement.

[0075] In one example, focus shift correction during acquisition of a series of images for frame averaging or multimodal imaging can be combined with lateral drift correction during post-processing of the acquired images.

[0076] This method enables a fast focus tracking function in a multi-beam charged particle system with multiple primary charged particle beamlets during an inspection job, without requiring specialized focus adjustment methods or algorithms using a series of images at multiple focal planes and reference test objects. The method relies on predetermined knowledge or information about the field curvature radius and image plane tilt vector for a given imaging setup. Based on this predetermined information, deviations from the ideal image plane and wafer surface position can be determined from the inspection image based on model or analytical derivation, and the deviations can be adjusted for subsequent image acquisition. The method is also configured to detect and account for wedge-shaped wafers or wafer bow.

[0077] The present invention is described by the following items:

[0078] Item 1: A method for inspecting a wafer using a multi-beam charged particle system (1), comprising: Determining at least one first inspection portion (6.1) and a second inspection portion (6.2) on the wafer surface (25); Using a first imaging setting of the plurality of primary charged particle beamlets (3), a first image is captured at the first inspection portion (6.1) on the wafer surface (25); Determining a first local image contrast map LV(x,y) of the first image; determining a measurement value MV.1 from the first local image contrast map LV(x, y), wherein the measurement value MV.1 comprises at least one of a first focus shift FM.1, a first field curvature radius RM.1, and a first image plane tilt vector TM.1; Determining a reference value RV.1 from a memory according to the first imaging setting, wherein the reference value RV.1 includes at least one of a reference focus offset RF.1, a reference field curvature radius RR.1, and a reference image plane tilt vector TR.1; Determine at least one deviation DV.1 between the measured value MV.1 and the reference value RV.1; Determining at least one of an adjustment of the second examination site (6.2) and an adjustment of a second imaging setting of the plurality of primary charged particle beamlets (3) from the at least one deviation value DV.1; and A second image is captured at the second inspection position (6.2) on the wafer surface (25) using the second imaging setting of the plurality of primary charged particle beamlets (3).

[0079] Item 2: The method according to Item 1, comprising: Calculating the difference DT.1 between the first image plane tilt vector TM.1 and the reference image plane tilt vector TR.1; From the difference DT.1, a tilt angle adjustment of the wafer surface (25) using a sample stage (500) at the second inspection location (6.2) is determined.

[0080] Item 3: The method according to item 1 or 2, comprising: Calculating the difference DF.1 between the measured focus shift FM.1 and the reference focus shift FR.1; From the difference DF.1, at least one of an axial movement of the sample stage (500) and an adjustment of a focus offset of the multi-beam charged particle system (1) at the second inspection position (6.2) is determined.

[0081] Item 4: The method according to any one of items 1 to 3, comprising: Calculate the difference DR.1 between the measured field curvature radius RM.1 and the reference field curvature radius RR.1; From the difference DR.1, an adjustment of the field curvature of the multi-beam charged particle system (1) at the second examination site (6.2) is determined.

[0082] Item 5: The method according to any one of items 1 to 4, further comprising: Determining a second local image contrast map LV(x,y) of the second image; determining a second measurement value MV.2 from the second local image contrast map LV(x, y), wherein the second measurement value MV.2 comprises at least one of a second focus shift FM.2, a second field curvature radius RM.2, and a second image plane tilt vector TM.2; Determining a second reference value RV.1 from a memory according to the second imaging setting, wherein the second reference value RV.2 includes at least one of a reference focus offset RF.2, a reference field curvature radius RR.2, and a reference image plane tilt vector RT.2; Determining at least a second deviation value DV.2 between the second measurement value MV.2 and the second reference value RV.2; Determining at least one of an adjustment of a third examination site (6.3) and an adjustment of a third imaging setting of the plurality of primary charged particle beamlets (3) based on the at least one second deviation value DV.2; A third image is captured at the third inspection position (6.3) on the wafer surface (25) using the third imaging setting of the plurality of primary charged particle beamlets (3).

[0083] Item 6: The method as described in Item 5 further comprises repeating the following steps: determining a local image contrast map LV(x, y), determining an i-th measurement value MV.i, determining an i-th reference value RV.i, determining at least an i-th deviation value DV.i, determining an adjustment of a subsequent j-th inspection part (6.j) from the at least i-th deviation value DV.i, where j>i, and at least one of the adjustments of a j-th imaging setting of the plurality of primary charged particle beamlets (3); and capturing a j-th image at the j-th inspection part (6.j).

[0084] Item 7: The method according to Item 6, comprising: Determining a wafer bending radius BR of a surface of a wafer (7) from at least three difference vectors DT.1 to DT.3; and During the determination of the adjustment of a subsequent inspection location (6.j), the wafer bending radius BR is taken into account.

[0085] Item 8: The method according to Item 6 or 7, comprising: Determining a wedge angle W of a wafer (7) from at least three difference vectors DT.1 to DT.3; and This wedge angle W is taken into account during the determination of the adjustment of a subsequent examination location (6.j).

[0086] Item 9: The method according to any one of items 6 to 8, comprising: Track a series of at least three difference values ​​DV.1 to DV.3; Analyze whether the series of difference values ​​DV.1 to DV.3 converge; and When the series of difference values ​​DV.1 to DV.3 do not converge, calibration of the multi-beam charged particle system is triggered.

[0087] Item 10: The method according to any one of items 1 to 9, comprising: In the local image contrast map LV(x,y), a circle (57) of maximum image contrast is determined; Determining the diameter DVM of the circle (57) of maximum image contrast; and From the diameter DVM of the circle (57), a focus offset FM is determined.

[0088] Item 11: The method according to Item 10, comprising: Determining a center position of the circle (57) of maximum image contrast; From the center position of the circle, an image plane tilt vector TM (57) is determined.

[0089] Item 12: The method as described in any one of items 1 to 11, further comprising repeated image acquisition at an examination site.

[0090] Item 13: The method as described in any one of items 1 to 12, wherein the second image capture is sequentially followed by the first image capture.

[0091] Item 14: A multi-beam charged particle system comprising a control processor (800), the control processor including a memory (880) for storing executable software instructions, the control processor (800) being configured to execute the software instructions to perform the method described in any one of items 1 to 13.

[0092] Item 15: A method for acquiring a series of images of an examination site (6.1), comprising: Using a first imaging setting of a plurality of primary charged particle beamlets (3), a first image is captured at the inspection portion (6.1) on a wafer surface (25); Determining a first local image contrast map LV(x,y) of the first image; determining a measurement value MV.1 from the first local image contrast map LV(x, y), wherein the measurement value MV.1 comprises at least one of a first focus shift FM.1, a first field curvature radius RM.1, and a first image plane tilt vector TM.1; Determining a reference value RV.1 from a memory according to the imaging setting, wherein the reference value RV.1 includes at least one of a reference focus offset RF.1, a reference field curvature radius RR.1, and a reference image plane tilt vector TR.1; Determine at least one deviation DV.1 between the measured value MV.1 and the reference value RV.1; Determining at least one of an adjustment of the examination site (6.1) and an adjustment of the imaging settings of the plurality of primary charged particle beamlets (3) from the at least one deviation value DV.1; and A second image is captured at the inspection location (6.1) on the wafer surface (25) using the adjusted imaging settings of the plurality of primary charged particle beamlets (3).

[0093] Item 16: The method of Item 15, wherein the adjustment of the imaging setting comprises at least one of an adjustment of a focus shift FM of the image surface (101) and an adjustment of an axis position of the wafer surface (25).

[0094] Item 17: The method according to Item 15 or 16, further comprising image processing of the series of images.

[0095] Item 18: The method as described in Item 17, wherein the image processing includes adjusting the averaging and lateral image shift of the series of images.

[0096] Item 19: A method for determining a focal plane deviation in a multi-beam charged particle system (1) based on a single image capture, comprising: An image is captured at a first inspection location (6.1) on a wafer surface (25) using a first imaging setting of a plurality of primary charged particle beamlets (3); Determine a local image contrast map LV(x,y) of the image; and At least one of a focus shift FM, a field curvature radius RM, and an image plane tilt vector TM is determined from the local image contrast map LV(x,y).

[0097] Item 20: The method of Item 19, further comprising determining an image circle (57) of maximum contrast, and determining a focus shift FM from the circle (57) of maximum contrast.

[0098] Item 21: The method as described in Item 19 or 20, further comprising determining the image plane tilt vector TM based on the displacement of a symmetry center of the local image contrast map LV(x,y).

[0099] Item 22: The method according to any one of items 19 to 21, further comprising determining the field curvature radius RM from the curvature of the local image contrast map LV(x, y) or the difference between a maximum contrast value and a minimum contrast value of the local image contrast map LV(x, y).

[0100] Item 23: The method of any one of items 19 to 22, further comprising receiving at least one of a reference focus offset RF, a reference field curvature radius RR, and a reference image plane tilt vector TR; and At least one deviation value DV between at least one of a focus shift FM, a field curvature radius RM and an image plane tilt vector TM is determined from a reference focus shift RF, a reference field curvature radius RR or a reference image plane tilt vector TR.

[0101] Item 24: The method as described in any one of items 19 to 23, further comprising determining at least one of an adjustment of a second inspection portion (6.2) and an adjustment of a second imaging setting of the plurality of primary charged particle beamlets (3) based on the at least one deviation value DV.

[0102] Item 25: The method as described in Item 24, wherein the second inspection site (6.2) is the same as the first inspection site (6.1).

[0103] 1: Multi-beam charged particle system 3: A small beam of charged particles 5: Primary charged particle beam spot / beam spot 6: Check the area 7: Objects / Wafer 9: Secondary electron beam / secondary charged particle beam 11: Secondary charged particle beam path 13: Primary charged particle beam path 15: Secondary charged particle beam spot / secondary electron beam spot 17: Circle 21: Pupil plane 25: Object surface / wafer surface / surface 41: Lower surface / lower defocus position 45: Ideal center surface 47: Upper surface / upper defocus position 51:Focus range 53: Circular lines of equal contrast 55: Eccentric symmetry center 57: Circle of maximum image contrast 59: Contrast threshold 66: Check the area 67: Defect 71: Bare crystal 74: Waist 100: Object irradiation unit 101: Image Surface 102: Objective lens 103: Field lens 105: Optical axis 108: beam intersection 110: Collective multi-beam raster scanner / scanning deflector 200: Detection Unit 205: Lens element 211: Intermediate Image Plane 214: Contrast aperture filter module 222: Collective raster scanner / scanning deflection system 245: Image tile 251: Surface segment / image field 284: Aperture filter 300: Charged particle multi-beam generator 301: Primary charged particle source 303: Collimating lens 304: filter plate 305:Multi-beam forming unit 306: porous plate / active porous plate 307: End-end porous plate 308: Electrostatic field lens / field lens 309: Primary charged particle beam 321: Intermediate image surface 400: beam splitter unit 500: Sample stage 503: Sample voltage supply unit 510: Sample stage actuator 600: Image sensor 800: Control unit 810: Imaging control module 820: Secondary beam path control module 830: Primary beam path control module 840: Control operation processor unit 850: Sample stage control module 860: Scanning control unit 880: Memory 890: Image processing engine 2105: Optical Axis

Claims

1. A method for wafer inspection using a multi-beam charged particle system (1), comprising: determining at least one first inspection location (6.1) and a second inspection location (6.2) different from the first inspection location (6.1) on a wafer surface (25); capturing a first image at the first inspection location (6.1) on the wafer surface (25) using a first imaging setting of a plurality of primary charged particle beams (3); determining a first local image contrast map LV(x,y) of the first image; determining a measurement value MV.1 from the first local image contrast map LV(x,y), wherein the measurement value MV.1 includes at least one of a first focus offset FM.1, a first field curvature radius RM.1 and a first image plane tilt vector TM.1; determining a reference value RV.1 from a memory according to the first imaging setting, wherein the reference value RV.1 includes at least one of a reference focus offset RF.1, a reference field curvature radius RR.1 and a reference image plane tilt vector TR.1; Determine at least one deviation value DV.1 between the measured value MV.1 and the reference value RV.1; From the at least one deviation value DV.1, determine at least one of the adjustment of the second inspection site (6.2) and the adjustment of a second imaging setting of the plurality of primary charged particle beams (3); and use the second imaging setting of the plurality of primary charged particle beams (3) to capture a second image at the second inspection site (6.2) on the wafer surface (25).

2. The method as described in claim 1, comprising: calculating the difference DT.1 between the first image plane tilt vector TM.1 and the reference image plane tilt vector TR.1; and determining, from the difference DT.1, the tilt angle adjustment of the wafer surface (25) of a sample stage (500) at the second inspection location (6.2).

3. The method as described in claim 1 or 2, comprising: calculating the difference DF.1 between the measured focal offset FM.1 and the reference focal offset FR.1; and determining, from the difference DF.1, at least one of an axial movement of the sample stage (500) and an adjustment of a focal offset of the multi-beam charged particle system (1) at the second inspection location (6.2).

4. The method as described in claim 1, comprising: calculating the difference DR.1 between the measured field curvature radius RM.1 and the reference field curvature radius RR.1; and determining, from the difference DR.1, an adjustment of the field curvature of the multi-beam charged particle system (1) at the second inspection location (6.2).

5. The method as described in claim 1, further comprising: determining a second local image contrast map LV(x,y) of the second image; determining a second measurement value MV.2 from the second local image contrast map LV(x,y), wherein the second measurement value MV.2 includes at least one of a second focus offset FM.2, a second field curvature radius RM.2, and a second image plane tilt vector TM.2; determining a second reference value RV.1 from a memory, based on the second imaging settings, wherein the second reference value RV.2 includes at least one of a reference focus offset RF.2, a reference field curvature radius RR.2, and a reference image plane tilt vector RT.2; and determining at least one second deviation value DV.2 between the second measurement value MV.2 and the second reference value RV.

2. From the at least one second deviation value DV.2, determine at least one of the adjustment of a third inspection location (6.3) and the adjustment of a third imaging setting of the plurality of primary charged particle beams (3); and use the third imaging setting of the plurality of primary charged particle beams (3) to capture a third image at the third inspection location (6.3) on the wafer surface (25).

6. The method as described in claim 5 further comprises repeatedly the following steps: determining a local image contrast map LV(x,y); determining an i-th measurement value MV.i; determining an i-th reference value RV.i; determining at least one i-th deviation value DV.i; determining, from the at least one i-th deviation value DV.i, an adjustment for a subsequent j-th inspection site (6.j), wherein j>i, and at least one of the adjustments for a j-th imaging setting of the plurality of primary charged particle beams (3); and capturing a j-th image.

7. The method as described in claim 6, comprising: determining the wafer bending radius BR of the surface of a wafer (7) from at least three difference vectors DT.1 to DT.3; and taking into account the wafer bending radius BR during the adjustment of determining a subsequent inspection location (6.j).

8. The method as described in claim 6 or 7, comprising: determining a wedge angle W of a wafer (7) from at least three difference vectors DT.1 to DT.3; and taking the wedge angle W into account during the adjustment of determining a subsequent inspection location (6.j).

9. The method as described in claim 6 or 7, comprising: tracking a series of at least three difference values ​​DV.1 to DV.3; analyzing whether the series of difference values ​​DV.1 to DV.3 converges; and triggering calibration of the multi-beam charged particle system when the series of difference values ​​DV.1 to DV.3 does not converge.

10. The method as described in claim 1 or 2, comprising: determining a circle (57) of maximum image contrast in the local image contrast map LV(x,y); determining a diameter DVM of the circle (57) of maximum image contrast; and determining a focus offset FM from the diameter DVM of the circle (57).

11. The method as described in claim 10, comprising: determining a center position of the circle (57) with maximum image contrast; and determining an image plane tilt vector TM (57) from the center position of the circle.

12. The method as described in claim 1 or 2, further comprising capturing a repeating image at an examination site (6.1, 6.2, 6.3, 6.4).

13. The method as described in any one of claims 1 to 2 and 4, wherein the second image capture is sequentially followed by the first image capture.

14. A multi-beam charged particle system comprising a control processor (800) including a memory (880) for storing executable software instructions, the control processor (800) being configured to execute the software instructions to perform the method as described in any one of claims 1 to 2 and 4.

15. A method for acquiring a series of images at an inspection site (6.1), comprising: acquiring a first image at the inspection site (6.1) on a wafer surface (25) using a first imaging setting of a plurality of primary charged particle beams (3); determining a first local image contrast map LV(x,y) of the first image; determining a measurement value MV.1 from the first local image contrast map LV(x,y), wherein the measurement value MV.1 includes at least one of a first focus offset FM.1, a first field curvature radius RM.1, and a first image plane tilt vector TM.1; determining a reference value RV.1 from a memory, based on the imaging setting, wherein the reference value RV.1 includes at least one of a reference focus offset RF.1, a reference field curvature radius RR.1, and a reference image plane tilt vector TR.1; and determining at least one deviation value DV.1 between the measurement value MV.1 and the reference value RV.

1. From the at least one deviation value DV.1, determine at least one of the adjustment of the inspection site (6.1) and the adjustment of the imaging settings of the plurality of primary charged particle beams (3); and use the adjusted imaging settings of the plurality of primary charged particle beams (3) to capture a second image at the inspection site (6.1) on the wafer surface (25).

16. The method as described in claim 15, wherein the adjustment of the imaging settings includes at least one of adjusting a focus offset FM of the image surface (101) and adjusting an axial position of the surface (25) of the wafer (7).

17. The method as described in claim 15 or 16, further comprising image processing of the series of images.

18. The method as described in claim 17, wherein the image processing includes adjusting the series of images by averaging and lateral image displacement.

Images