Methods for global and local optimization of imaging resolution in multibeam systems.

Abstract

A multi-beam charged particle microscope configured for improved methods of wavefront aberration determination and compensation is provided. Using a modification element, the wavefront aberration amplitude is indirectly determined and converted into normalized sensitivity units. The invention allows for compensating for wavefront aberrations using a compensation element different from the modification element. The normalized sensitivity units can be determined, for example, by an improved calibration method.

Description

[Technical field]

[0001] The present invention is applicable to multi-beam charged particle systems, and is particularly suitable for operation of a multi-beam charged particle system comprising at least an array of charged particle optical elements, and a global charged particle component. [Background technology]

[0002] With the continuous development of smaller and more dense microstructures, such as semiconductor devices, further development and optimization of planar fabrication techniques and inspection systems for small-dimension fabrication and inspection of microstructures is required. Development and fabrication of semiconductor devices requires, for example, design verification of test wafers, and planar fabrication techniques require process optimization for reliable high-throughput fabrication. In addition, in recent years, reverse engineering of semiconductor devices and analysis of semiconductor wafers for customized individual configurations are required. Therefore, high-throughput inspection tools are required for the investigation of microstructures on wafers with high accuracy.

[0003] A typical silicon wafer used in the manufacture of semiconductor devices has a diameter of up to 12 inches (300 mm). Each wafer is segmented into 30-60 repeating areas ("dies") of approximately up to 800 sq mm in size. A semiconductor device includes multiple semiconductor structures fabricated in layers on the surface of the wafer by planar integration techniques. Due to the fabrication processes involved, semiconductor wafers typically have a flat surface. Feature sizes of integrated semiconductor structures range from a few μm to 5 nm critical dimension (CD), and in the near future, feature sizes will decrease further, for example to feature sizes or critical dimensions (CD) of less than 3 nm, for example 2 nm, or even less than 1 nm. With the small feature sizes mentioned above, defects of the size of the critical dimension must be identified within very large areas within a short period of time.

[0004] The present invention therefore aims to provide a charged particle system, and a method of operation of a charged particle system, that allows high throughput interrogation of integrated semiconductor features with at least critical dimension resolution during development or during manufacturing or for reverse engineering of semiconductor devices. It is also possible to acquire high resolution images for a set of specific locations on a wafer, for example for so-called process control monitors PCM or only for critical areas.

[0005] Charged Particle MicroscopeA recent development in the field of CPM is the multi-beam charged particle microscope. For example, US Pat. No. 7,244,949 (BB) and US Pat. No. 10,896,800 (BB) disclose multi-beam charged particle microscopes. In a multi-beam charged particle microscope, such as a multi-beam scanning electron microscope, a sample is illuminated by an array of electron beamlets, including, for example, 4 to 10,000 electron beams as primary radiation, each electron beam separated from its next adjacent electron beam by a distance of 1 to 200 micrometers. For example, an MSEM has 100 separated electron beams or beamlets arranged on a hexagonal array, the electron beamlets being separated by a distance of about 10 μm. A plurality of primary charged particle beamlets are focused by an objective lens onto the surface of the sample to be studied, for example a semiconductor wafer mounted on a movable stage and fixed on a wafer chuck. During illumination of the wafer surface by the primary charged particle beamlets, interaction products, e.g., secondary electrons, emerge from multiple intersection points formed by the focal points of the primary charged particle beamlets, while the amount and energy of the interaction products depend on the material composition and topography of the wafer surface. The interaction products form multiple secondary charged particle beamlets, which are collected by an objective lens and guided onto a detector arranged at a detector plane by a projection imaging system of the multi-beam inspection system. The detector includes multiple detection areas, each of which includes multiple detection pixels, and detects the intensity distribution for each of the multiple secondary charged particle beamlets, resulting in an image patch of, e.g., 100 μm x 100 μm.

[0006] Prior art multi-beam charged particle microscopes comprise a series of electrostatic and magnetic elements. At least some of the electrostatic and magnetic elements are adjustable to adjust the focal position and astigmatism correction of the multiple secondary charged particle beams. As an example, US Pat. No. 10,535,494 proposes readjustment of the charged particle microscope when the detected intensity distribution of the focal points of the secondary charged particle beamlets deviates from a predetermined intensity distribution. Adjustment is achieved when the detected intensity distribution follows the predetermined intensity distribution. The global displacement or deformation of the intensity distribution of the secondary charged particle beamlets allows conclusions to be drawn regarding topographical effects, geometrical shape, or tilt of the sample, or charging effects of the sample. US Pat. No. 9,336,982 (BB) discloses a secondary charged particle detector with a scintillator plate for converting secondary charged particles into light.

[0007] US Patent Application Publication No. 20190355544 discloses a multi-beam charged particle microscope with an adjustable projection system for compensating for charging of the sample during scanning. The projection system is therefore configured with high-speed electrostatic elements for maintaining proper imaging of the secondary charged particle beamlets from the sample to the detector. Prior art multi-beam charged particle microscopes include a detection system to facilitate the adjustment.

[0008] It is generally desirable to change the imaging settings of a charged particle microscope. A method for changing the image acquisition settings of a multi-beam charged particle microscope from a first imaging setting to a different second imaging setting is described in U.S. Pat. No. 9,799,485 (BB).

[0009] However, in charged particle microscopes for wafer inspection, it is desired to achieve imaging resolution with high reliability and high repeatability. With a multi-beam system, the image is obtained by a number of individual charged particle beamlets, each forming an individual image segment. Even after compensation of the field curvature, each image segment imaged with an individual beamlet is obtained with a certain resolution, which depends on the beam quality of the corresponding beamlet. The resolution of each image segment may deviate from the given imaging resolution, for example they may exceed the threshold of the resolution requirement, and the resolution within each image segment may differ for different beamlets. This may be caused by field-dependent aberrations of the (global) illumination system. In addition, the resolution of a multi-beam system may change over time or depend on the inspection site of the object under investigation. In particular, the imaging resolution of a multi-beam system may be degraded by the individual aberrations of each beamlet.

[0010] It is an object of the present invention to provide a multi-beam charged particle inspection system having means for enabling reliable high accuracy and high resolution image acquisition. One object of the present invention is to provide a multi-beam charged particle inspection system having means for monitoring and controlling the resolution of each image segment for each of a plurality of primary charged particle beamlets. It is a further object of the present invention to provide a multi-beam charged particle inspection system having means for maintaining high resolution and high image contrast during reliable image acquisition of a series of image patches.

[0011] Generally, it is an object of the present invention to provide a multi-beam charged particle inspection system for wafer inspection having means for enabling reliable high-precision and high-resolution image acquisition across multiple image segments obtained by multiple charged particle beamlets. It is a further object of the present invention to monitor and compensate for individual aberrations of each beamlet of the multi-beam system and provide equally corrected aberration-free imaging conditions for each beamlet. Summary of the Invention

[0012] The present invention solves the object of the present invention by a multi-beam charged particle microscope comprising at least one global compensator and a compensator array element configured for determining and compensating the wavefront aberrations of a plurality of primary charged particle beamlets. The present invention provides a method for determining the wavefront aberrations of each of a plurality of primary charged particle beamlets. The present invention further provides a method for compensating the wavefront aberrations of each of a plurality of primary charged particle beamlets. The present invention further provides a method for monitoring the wavefront aberrations of each of a plurality of primary charged particle beamlets during a wafer inspection task. Thus, the present invention provides a method for operating a multi-beam charged particle microscope with low aberrations of each of a plurality of primary charged particle beamlets. Using the method for operating a multi-beam charged particle microscope according to the present invention, the inspection task meets the high throughput and resolution requirements of the wafer inspection task.

[0013] The invention further provides a multi-beam charged particle microscope configured to perform a method of operating a multi-beam charged particle microscope with high throughput and low aberration. The multi-beam charged particle microscope according to the invention comprises a compensator array element including a plurality of compensators for compensation of wavefront aberrations of each of a plurality of primary charged particle beamlets. The multi-beam charged particle microscope according to the invention further comprises at least one of a modification element or a global compensator. The wavefront aberrations of at least a subset of the primary charged particle beamlets are determined by modification of the beam shape of at least a subset of the primary charged particle beamlets by at least one of the modification element, the global compensator, or the compensator array element. According to an aspect of the invention, the sensitivity of the compensator array element, the global compensator, and / or the modification element is determined in normalized sensitivity units. From the wavefront aberrations in normalized sensitivity units, a control signal is calculated in normalized sensitivity units for compensation of the wavefront aberrations. The control signal is provided to the global compensator and / or the compensator array element. Determining the wavefront error and the control signal in normalized sensitivity units allows for determining the wavefront error using a first element and compensating for the wavefront error using a second element different from the first element. The first element can be any of a compensator array element, a global compensator, or a modification element. The second element can include a compensator array element, a global compensator, or both. Determining the wavefront error and the control signal in normalized sensitivity units allows for fast compensation of the wavefront aberration for the high throughput requirements of wafer inspection tasks. With improved determination of the wavefront aberration in normalized sensitivity units, the wavefront aberration is minimized with greater accuracy, achieving higher imaging performance, such as higher resolution and higher imaging contrast. Additionally, the variation of the wavefront aberration across multiple primary charged particle beamlets is minimized.

[0014] In a first embodiment, a multi-beam charged particle microscope according to the present invention is described. The multi-beam charged particle system comprises at least an array of charged particle optical elements and a global element. The global charged particle element, such as an objective lens or a beam splitter, is a first source of aberration. The array of charged particle optical elements, or the array optical element, is a second source of aberration. The multi-beam charged particle microscope according to the present invention comprises a compensator array element, including a plurality of compensators for compensation of wavefront aberrations of each of a plurality of primary charged particle beamlets, and at least one of a modification or a global compensator element. The multi-beam charged particle microscope further comprises a control unit having a memory and a processor, configured to perform a method for determining, monitoring and compensating a plurality of wavefront aberrations of a plurality of primary charged particle beamlets in a short time. The control unit is configured to derive the wavefront aberrations by modification of the beam shape of at least a subset of the primary charged particle beamlets by modification of at least one of the modification element, the global compensator or the compensator array element in normalized sensitivity units. The control unit is configured to derive a control signal for compensating for the wavefront error in normalized sensitivity units and provide the control signal to the global compensator element and / or the compensator array element.

[0015] In one example, the multi-beam charged particle microscope of the present invention further comprises a beam splitter for splitting the beam path of the multiple primary charged particle beamlets from the beam path of the multiple secondary electron beamlets, and a detection system for detecting the multiple secondary electron beamlets. The detection system includes a projection imaging system for imaging the multiple secondary electron beamlets onto a detector. The control unit is configured to vary or modify the wavefront error of at least a subset of the multiple primary charged particle beamlets using one of the modifying element, the global compensator element, and / or the compensator array element. The detector obtains multiple image series, and the control unit is configured to derive multiple wavefront errors of at least a subset of the multiple primary charged particle beamlets from the multiple image series. Thereby, the control unit is configured to convert a result from the data analysis of the multiple image series related to the multiple wavefront errors into normalized sensitivity units. The control unit is further configured to derive a control signal for compensation of all wavefront errors of the multiple primary charged particle beamlets in normalized sensitivity units. The control unit is configured to provide control signals to at least one of the global compensator elements and / or the compensator array elements, which are configured to individually or jointly compensate for wavefront aberrations of the primary charged particle beamlets. Thus, the control signals for each individual compensation element are calibrated using predefined calibration parameters such as offset and slope of a calibration transfer curve.

[0016] The multibeam charged particle microscope (1) according to the first embodiment comprises a multibeam generating unit (300) for generating a plurality of J primary charged particle beamlets (3). It further comprises an array of compensation elements (601), a global compensation element (603) and / or a modification element (605). The control unit (800) of the multibeam charged particle microscope (1) is configured to tune the multibeam charged particle microscope (1) to a set point and to modify the wavefront aberration amplitude of each or a plurality of primary charged particle beamlets (3) using the modification element (605). It is further configured to determine the wavefront aberration amplitude A(j) of each or a plurality of J primary charged particle beamlets (3) at the set point and to determine a first or global component AG1 and a second or residual component Ares(j) of the field dependence of the wavefront aberration amplitude A(j) of the plurality of J primary charged particle beamlets (3). The control unit (800) is further configured to compensate the global component AG1 by providing a control signal to the global compensation element (603) and to compensate the residual component Ares(j) by providing a plurality of control signals to the array of compensation elements (601). The global compensation element (603) can be a multi-pole element including at least a first layer of a plurality of electrostatic or magnetic poles, and the global component AG1 of the field-dependence of the wavefront aberration amplitude corresponds to the low-order field-dependence of the wavefront aberration amplitude affected by the global compensation element (603). The array of compensator components (601) includes at least a first layer having a plurality of J apertures and a plurality of electrostatic electrodes arranged within the periphery of each aperture, and the residual component Ares(j) of the field-dependence of the wavefront aberration amplitude corresponds to the residual wavefront aberration that cannot be compensated using the global compensation element (603). The control unit is further configured to convert the wavefront aberration amplitude determined by modifying the modification element (605) into normalized sensitivity units, and to determine a plurality of control signals for the array of compensation elements (601) from a residual component of the wavefront aberration amplitude in the normalized sensitivity units.The control unit (800) of the multi-beam charged particle microscope (1) may further be configured to determine a control signal for the global compensation element (603) from the global component AG1 of the wavefront aberration amplitude in normalized sensitivity units. The modification element (605) may be provided by the deflection scanner (110) or the magnetic compensation element (420) of the multi-beam charged particle microscope (1), but in another example, the modification element (605) may be identical to the global compensation element (603) of the multi-beam charged particle microscope (1). There may also be multiple, e.g., two modification elements, as well as multiple, e.g., two global compensation elements, having different low-order field dependencies corresponding to the wavefront aberration amplitudes AG1 and AG2. During use, the set points may deviate from the design set points, and due to rotation of the primary charged particle beamlets in the magnetic field, the set points may include deviations of a predetermined rotation of the raster arrangement (41) of the multiple primary charged particle beamlets (1) between the coordinate systems of the image plane (101), the array of compensator components (601), the global compensation element (603), and / or the modification element (605). Thus, the control unit is configured to compensate for rotational differences in wavefront aberrations between the compensation elements (601, 603) and / or the modification element (605).

[0017] In a second embodiment of the present invention, a method is provided for determining a plurality of wavefront aberrations of each of a plurality of primary charged particle beamlets in normalized sensitivity units. According to the method, a plurality of wavefront aberrations of at least a subset of the plurality of primary charged particle beamlets are determined. In one example, the individual wavefront aberrations of each beamlet are determined by modifying beam characteristics of each beamlet by a modifying element of a multi-beam charged particle microscope. The method includes determining a first and a second contribution to the wavefront error. The first contribution corresponds to a functional dependence of the wavefront error of the plurality of primary charged particle beamlets.

[0018] In a method for determining a number of wavefront aberration amplitudes of a multi-beam charged particle microscope (1) according to a second embodiment, first, the multi-beam charged particle microscope is adjusted or set to a set point for a first inspection task. In a second step, the wavefront aberration of a number of J primary charged particle beamlets (3) is modified by providing a series of at least SI=3 different modification control signals SV(i=1...SI) to a modification element (605) and a number of contrast values ​​C(j,i) are measured for each of the number of J primary charged particle beamlets (3) at each modification control signal SV(i=1...SI). From the number of contrast values ​​C(j,i), a number of contrast curves C(j=1...J) are determined for each of the number of J primary charged particle beamlets (3). From the number of contrast curves C(j=1...J), a number of J wavefront aberration amplitudes A(j=1...J) in normalized sensitivity units at the set point are derived. The method includes calculating a parabolic, hyperbolic, or polynomial approximation to the contrast values ​​C(j,i) for i=1...SI for each of the plurality of J primary charged particle beamlets (3). In a first example, each of the plurality of J wavefront amplitudes A(j) in normalized sensitivity units is determined from a modified control signal SV(maxC(j)) at a maximum contrast value maxC(j) divided by a normalized range RV of the modification element 605. The method may further include determining a normalized range RV by determining a maximum and minimum control signal SV required to achieve a predetermined modification of the image contrast of at least one of the plurality of primary charged particle beamlets. In a second example, each of the plurality of J wavefront amplitudes A(j) in normalized sensitivity units is determined from a parabolic coefficient KV(j) of the contrast curve C(j) in the vicinity of a local maximum or minimum of the contrast curve C(J).

[0019] The method may further include converting the wavefront aberration amplitude to a wavefront aberration amplitude vector. During use, the setpoint may deviate from the design setpoint, and due to the rotation of the primary charged particle beamlets in the magnetic field, the setpoint may include a deviation of a predetermined rotation of the raster arrangement (41) of the multiple primary charged particle beamlets (1) between the coordinate systems of the image plane (101), the array of compensator components (601), the global compensation element (603), and / or the modification element (605). In one example of the method according to the second embodiment, the multiplication of the wavefront aberration amplitude vector by a rotation matrix M may take into account a deviation of a predetermined rotation of the raster arrangement (41) of the multiple primary charged particle beamlets (1) between the coordinate systems of the image plane (101), the array of compensator components (601), the global compensation element (603), and / or the modification element (605).

[0020] In a third embodiment of the present invention, a method for compensating for a plurality of wavefront aberrations of each of a plurality of primary charged particle beamlets is provided.

[0021] The wavefront aberration of each of the multiple primary charged particle beamlets is minimized by the global compensator or the compensator array element. The method for compensating multiple wavefront aberrations of a multi-beam charged particle microscope (1) at a set point includes receiving multiple J wavefront aberration amplitudes A (j=1...J) of multiple J primary charged particle beamlets (3) in normalized sensitivity units. It further includes determining a global component AG1 of the amplitudes in normalized sensitivity units, the global component having a predetermined field dependence of the multiple J wavefront aberration amplitudes A(j) of the global compensation element (603). It further includes determining a residual component Ares(j) of the multiple residual wavefront amplitudes in normalized sensitivity units. It further includes converting the global component to a global compensation signal GCS and converting the residual component to multiple local compensation signals LCS(j). A global correction signal GCS is provided to a global compensation element (603) and a plurality of local compensation signals LCS(j) are provided to the array of compensation elements (601). In one example, the method according to the third embodiment includes determining a plurality of J wavefront aberration amplitudes A(j). The determining step may be performed according to the method of the second embodiment of the present invention.

[0022] As shown in the embodiment, different normalized sensitivity units are also possible, e.g., scaling in ranges RV or RC, or according to parabolic sensitivity constants KV and KC. Other scaling in normalized sensitivity units is also possible. With a consistent selection of normalized sensitivity units to be applied in the determination step and in the compensation step, it is possible to directly transfer measurements from the determination step to the compensation step, and to transfer compensation signals from the global compensator (603) to compensator elements of the array of compensation elements (601).

[0023] In a fourth embodiment of the present invention, a method for monitoring a plurality of wavefront aberrations of a plurality of primary charged particle beamlets is provided. According to the fourth embodiment, the method includes monitoring the wavefront aberrations of a multi-beam charged particle microscope (1) during an inspection task. The monitoring can be accomplished by a model-based control based on a predefined model of the multi-beam charged particle microscope and secondary indirect monitoring parameters. The method utilizing the model-based control can trigger the determination of a plurality of J wavefront aberration amplitudes A(j=1...J) according to the second embodiment when the image contrast predicted according to the predefined model falls below a predefined threshold. The monitoring can include monitoring the image contrast of a plurality of digital images received during the inspection task and triggering the receipt of a plurality of J wavefront aberration amplitudes A(j=1...J) when the image contrast falls below a predefined threshold.

[0024] In a fifth embodiment of the present invention, a method for determining and calibrating normalized sensitivity units of wavefront aberrations of a plurality of primary charged particle beamlets is provided.

[0025] According to a first example of the fifth embodiment, a method for calibrating a charged particle optical element of a multi-beam charged particle microscope (1) includes the steps of: (a) modifying a wavefront aberration of a plurality of J primary charged particle beamlets (3) by providing a series of at least SI=3 different control signals SC(i=1...SI) to the charged particle optical element; and (b) determining a plurality of contrast values ​​C(j=1...J, i=1...SI) for each of the plurality of J primary charged particle beamlets (3) at each different control signal SC(i=1...SI). (c) determining a plurality of contrast curves C(j=1...J,SC) for each of a plurality of J primary charged particle beamlets (3) from a plurality of contrast values ​​C(j,i), (d) determining an extremum maxC(j) and a control signal SC(maxC(j)) corresponding to each extremum maxC(j) of the contrast curve C(j,SC), and (e) determining a plurality of modifications of the wavefront amplitude A(j) affected by the charged particle optical element in normalized sensitivity units. The charged particle optical element can be any element, such as a global compensator (603), or a compensator element of the array of compensator elements (601), or a modification element (605). In one example, the multiple changes of the wavefront amplitude A(j) are obtained in normalized sensitivity units by A(j)=SC(maxC(j)) / RC, where RC is the normalized range of the charged particle optical element corresponding to the difference between the maximum and minimum control signal SC required to achieve a predetermined change of the image contrast C(j) for at least one of the multiple J primary charged particle beamlets. In another example, the multiple changes of the wavefront amplitude A(j) are obtained in normalized sensitivity units by A(j)=SIGN(j)*2KV(j)*SC(maxC(j)), using an approximate parabolic constant KC(j) for the contrast curve C(j=1...J,SC) in the vicinity of the extreme value max(C(j,SC)). Both examples of normalized sensitivity units are equivalent, but it is necessary to consistently select one normalization method according to, for example, the selection of the third embodiment of the present invention for determination and compensation. The multiple modifications of the wavefront amplitude A(j) of the charged particle optical element are then stored in a memory of the control unit (800) of the multi-beam charged particle microscope (1) in normalized sensitivity units.

[0026] According to a second exemplary fifth embodiment, a method for determining wavefront aberration in absolute units is provided. For each of a plurality of J primary charged particle beamlets, a wavefront detection pattern (61) is provided in an image plane (101) of a multi-beam charged particle microscope (1). Each wavefront detection pattern (61) comprises a plurality of repeating features (63) oriented at different rotation angles α. A plurality of measurements of the wavefront detection pattern (61) are performed in a focus sequence with NQ focus steps q=1...NQ to determine a plurality of contrast values ​​C(j,q;α). For each of the beamlets j=1...J and each rotation angle α, a plurality of contrast curves C(j;α) are fitted as a function of focus position and a maximum value maxC(j;α) is determined for each of the contrast curves C(j;α). A symmetric wavefront aberration A(j) with even order rotational symmetry is determined for each of the beamlets from the maximum difference between the two focal positions of the two maxima maxC(j;α) and maxC(j;α-90) of the two repeating features (63) oriented at 90° to each other. The symmetric wavefront aberration A(j) with even order rotational symmetry is given, for example, by astigmatism. The method may further include determining a plurality of relative image displacements dr(j;α) across the focal points of each of the plurality of repeating features (63). From the plurality of relative image displacements dr(j;α), an asymmetric wavefront aberration with odd order rotational symmetry can be derived for each of the beamlets. The asymmetric wavefront aberration with odd order rotational symmetry can be, for example, coma.

[0027] Other advantages of the embodiments of the present disclosure will become apparent from the following description in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments and examples, but includes modifications, combinations, or variations thereof. Other embodiments of the present invention will become apparent to those skilled in the art from consideration of the specification and practice of the present invention disclosed herein. For example, the use of the apparatus and methods defined in this application is not limited to the use of electron beams as charged particles. Rather, any particle beam may be used. Examples of alternative particle beams are ion beams, metal beams, and molecular beams. Furthermore, the application of the present invention is not limited to the inspection of semiconductor wafers, but is generally applicable to objects or samples involved in semiconductor manufacturing, including lithography masks. Thus, the word "wafer" is not limited to semiconductor wafers, but is intended to include lithography masks for wafer printing. However, the present invention is also applicable to other nanostructured objects, including photonic crystals and metamaterials, as well as the investigation of biological tissues and minerals. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a diagram showing a multi-beam charged particle microscope according to a first embodiment. [Diagram 2] FIG. 1 illustrates the raster configuration of a multi-beam charged particle microscope and image patches for an inspection task. [Diagram 3] FIG. 13 shows the shape of an astigmatic electron beamlet over focal length z and the corresponding contrast curve. [Figure 4] FIG. 6 shows an array 601 of compensator elements. [Diagram 5] FIG. 13 shows the field dependence of wavefront aberration amplitude in normalized units for multiple primary charged particle beamlets 3 in a hexagonal raster configuration 41. [Figure 6] FIG. 4 illustrates a method for determining the wavefront amplitude according to a second embodiment of the invention. [Figure 7] FIG. 6 shows contrast curves over a range of changes in the control parameter SV of the modification element 605. [Figure 8]FIG. 11 illustrates a compensation method according to a third embodiment of the present invention. [Figure 9] 6 shows contrast curves over variation of a control parameter SC of the array of compensation elements 601 and the global compensation element 603. FIG. [Figure 10] FIG. 13 is a diagram showing an example of a test pattern for measuring the wavefront aberration amplitude. [Figure 11] FIG. 13 illustrates a method for determining and calibrating wavefront aberration amplitude in common sensitivity units according to a fifth embodiment of the present invention. [Figure 12] FIG. 1 shows a typical contrast curve obtained by a method for determining and calibrating the wavefront aberration amplitude in common sensitivity units. [Figure 13] FIG. 1 shows a typical curve of the centroid obtained by a method for determining and calibrating the wavefront aberration amplitude in common sensitivity units. [Figure 14] FIG. 1 illustrates an example of a rotation of the coordinate system at a set point of a multi-beam charged particle microscope. [Figure 15] FIG. 13 illustrates an example of change in rotation of a coordinate system following a deviation from a predefined set point. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. Throughout the description, the same reference numerals in different drawings represent the same or similar elements, unless otherwise indicated. It should be noted that the symbols used in the figures do not represent the physical configuration of the illustrated components, but have been chosen to symbolically indicate their respective functionality.

[0030] The schematic diagram of Figure 1 illustrates the basic features and functionality of a multi-beamlet charged particle microscope 1 according to a first embodiment of the present invention. The type of system shown is that of a scanning electron microscope (SEM) that uses multiple primary electron beamlets 3 to generate multiple primary charged particle beam spots 5 on the surface of an object 7, such as a wafer placed in the object plane 101 of an objective lens 102. For simplicity, only five primary charged particle beamlets 3 and five primary charged particle beam spots 5 are shown.

[0031] The microscope system 1 comprises an object illumination unit 100, a detection unit 200 and a beam splitter unit 400 for separating a secondary charged particle beam path 11 from a primary charged particle beam path 13. The object illumination unit 100 comprises a charged particle multi-beamlet generator 300 for generating a plurality of primary charged particle beamlets 3 and is adapted to focus the primary charged particle beamlets 3 in an object plane 101, where a surface 25 of a wafer 7 is positioned by a sample stage 500. The sample stage 500 comprises a stage motion controller, which comprises a plurality of motors configured to be independently controlled by control signals. The stage motion controller is connected to a control unit 800.

[0032] The primary beamlet generator 300 generates a number of primary charged particle beamlet spots 311 in an intermediate image plane 321, which is typically a spherically curved surface to compensate for the field curvature of the object illumination unit 100. The primary beamlet generator 300 includes a source 301 of primary charged particles, e.g., electrons. The primary charged particle source 301 emits a diverging primary charged particle beam 309, which is collimated, for example, by collimating lenses 303.1 and 303.2 to form a collimated beam. The collimating lenses 303.1 and 303.2 typically consist of one or more electrostatic or magnetic lenses, or a combination of electrostatic and magnetic lenses. The collimated primary charged particle beam enters the primary multi-beamlet forming unit 305. The multi-beamlet forming unit 305 essentially comprises a first multi-aperture plate 306.1 illuminated by a primary charged particle beam 309. The first multi-aperture plate 306.1 comprises a number of apertures in a raster arrangement for the generation of a number of primary charged particle beamlets 3, which are generated by transmission of a collimated primary charged particle beam 309 through the number of apertures. The multi-beamlet forming unit 305 comprises at least a further multi-aperture plate 306.2 arranged downstream of the first multi-aperture plate 306.1 with respect to the direction of movement of the electrons in the beam 309. For example, the second multi-aperture plate 306.2 has the function of a microlens array and is preferably set to defined potentials such that the focal positions of the number of primary beamlets 3 in the intermediate image plane 321 are adjusted. The multi-beamlet forming unit 305 further includes a compensator array element 601 including an individual electrostatic element for each of a plurality of apertures for individually influencing each of the plurality of beamlets 3. The compensator array element 601 consists of one or more multi-aperture plates with electrostatic elements such as multi-pole electrodes, or a series of multi-pole electrodes, to form an astigmatism corrector array configured to individually compensate for the wavefront aberration of each of the primary charged particle beamlets.The multi-beamlet forming unit 305 is configured with a first immersion field lens 307 forming an immersion field within a plurality of apertures of an exit multi-aperture plate, in this example a second multi-aperture plate 306.2 of the multi-beamlet forming unit 305. By means of the first immersion field lens 307, the second field lens 308 and the second multi-aperture plate 306.2 the plurality of primary charged particle beamlets 3 are focused in or near the intermediate image plane 321.

[0033] Located in or near the intermediate image plane 321 is a beam steering multi-aperture plate 390 having multiple apertures with electrostatic deflectors capable of individually deflecting each of the multiple charged particle beamlets 3. The apertures of the beam steering multi-aperture plate 390 are configured to have larger diameters to allow the passage of multiple primary charged particle beamlets 3 even if the focal spots of the primary charged particle beamlets 3 deviate from their design positions. The primary charged particle source 301, and the active multi-aperture plate mechanisms 306.1...306.3, and the beam steering multi-aperture plate 390 are controlled by a primary beamlet control module 830, which is connected to the control unit 800.

[0034] The multiple focal points of the primary charged particle beamlets 3 passing through the intermediate image plane 321 are imaged by the field lenses 103.1 and 103.2 and the objective lens 102 into the image plane 101, where the surface of the wafer 7 is positioned by the sample stage 500. The object illumination system 100 includes a deflection system 110 in the vicinity of a first beam crossover 108, where the multiple charged particle beamlets 3 can be deflected in a direction perpendicular to the beam propagation direction (here the z-direction). The deflection system 110 is connected to a control unit 800. The objective lens 102 and the deflection system 110 are centered on the optical axis 105 of the multi-beamlet charged particle microscope system 1, which is perpendicular to the wafer surface 25. The wafer surface 25, arranged in the image plane 101, is then raster scanned by means of the deflection system 110. Thereby, the multiple primary charged particle beamlets 3, forming multiple beam spots 5 arranged in a raster configuration, are synchronously scanned over the surface 25. In one example, the raster arrangement of the focal spots 5 of the plurality of primary charged particle beamlets 3 is a hexagonal raster of about 100 or more primary charged particle beamlets 3. The primary beam spots 5 have a distance of about 6 μm to 15 μm and a diameter of less than 5 nm, for example, 3 nm, 2 nm, or even less. In one example, the beam spot size is about 1.5 nm and the distance between two adjacent beam spots is 8 μm. The object illumination unit 100 according to the first example of the first embodiment of the present invention further includes a global compensation element 603 in the vicinity of the deflection scanner 110. The global compensation element 603 may be configured as a magnetic element having six, eight, or more individually addressable poles for generating a magnetic field to compensate or affect the wavefront aberration of the plurality of primary charged particle beamlets. The global compensation element 603 may be configured by a single element or by at least two multipole elements arranged in sequence. The global compensation element 603 is controlled by a primary beamlet control module according to the method of the present invention, which will be described below. In a second example of the first embodiment, the object illumination unit 100 includes a modification element 605 configured to modify the wavefront aberration of at least a subset of the plurality of primary charged particle beamlets 3.1, but may be realized by, for example, magnetic correction element 420, deflection unit 110, or global compensation element 603. In the example of FIG. 605, modification element 605 is located near magnetic correction element 420, but other locations are also possible, for example, near beam deflection unit 110, inside a pole element of magnetic objective lens 102, or near global compensation element 603.

[0035] At each scanning position of each of the multiple primary beam spots 5, multiple secondary electrons are generated respectively, forming multiple secondary electron beamlets 9 in the same raster configuration as the primary beam spots 5. The plurality or intensity of the secondary charged particles generated at each beam spot 5 depends on the intensity of the impinging primary charged particle beamlets 3 illuminating the corresponding spot 5, the material composition and the topography of the object under the beam spot. The secondary charged particle beamlets 9 are accelerated by the electrostatic field generated by the sample charging unit 503, collected by the objective lens 102, and directed by the beam splitter 400 to the detection unit 200. The detection unit 200 images the secondary electron beamlets 9 onto the image sensor 207, forming multiple secondary charged particle image spots 15 thereon. The detector includes multiple detector pixels or individual detectors. For each of the multiple secondary charged particle beam spots 15, the intensity is detected separately, and the material composition of the wafer surface is detected with high resolution for large image patches with high throughput. For example, for a raster of 10 x 10 beamlets with a pitch of 8 μm, an image patch of approximately 88 μm x 88 μm is generated with one image scan by the deflection system 110, with an image resolution of, for example, 2 nm. The image patch is sampled with, for example, half the beam spot size of 2 nm, and therefore with a pixel count of 8000 pixels per image line per beamlet, so that an image patch generated by 100 beamlets contains 6.4 gigapixels. Image data is collected by the control unit 800. Details of the collection and processing of image data, for example using parallel processing, are described in German Patent Application No. 102019000470.1 and U.S. Patent No. 9,536,702 (BB), which are incorporated herein by reference.

[0036] The secondary electron beamlets 9 pass through the first deflection system 110, are scan-deflected by the first scanning system 110 and are guided by the beam splitter unit 400 to follow the secondary beam path 11 of the detection unit 200. The secondary electron beamlets 9 travel in a direction opposite to the primary charged particle beamlets 3, and the beam splitter unit 400 is configured to separate the secondary beam path 11 from the primary beam path 13, typically using a magnetic field or a combination of magnetic and electrostatic fields. A magnetic correction element 420 is present in the primary beam path 13 and, optionally, also in the secondary beam path 11. The projection system 205 of the detection unit 200 further includes at least a second deflection system 222, connected to the projection system control unit 820. The control unit 800 is configured to compensate for residual errors in the positions of the multiple focal points 15 of the multiple secondary electron beamlets 9 so that the positions of the multiple focused secondary electron spots 15 remain constant at the image sensor 207.

[0037] The projection system 205 of the detection unit 200 includes at least a second crossover 212 of the plurality of secondary electron beamlets 9, in which an aperture 214 is arranged. In one example, the aperture 214 further includes a detector (not shown), connected to a projection system control unit 820. The projection system control unit 820 is further connected to at least one electrostatic lens 206 of the projection system 205, which includes further electrostatic or magnetic lenses 208, 209, 210. The projection system 205 further includes at least a first multi-aperture corrector 220, connected to the projection system control unit 820, having an aperture and an electrode for individually influencing each of the plurality of secondary electron beamlets 9.

[0038] The image sensor 207 is configured with an array of sensitive areas in a pattern that matches the raster arrangement of the secondary electron beamlets 9 focused on the image sensor 207 by the projection lens 205. This allows detection of each individual secondary electron beamlet 9 independent of other secondary electron beamlets 9 incident on the image sensor 207. A number of electrical signals are generated, converted into digital image data and processed to the control unit 800. During an image scan, the control unit 800 is configured to trigger the image sensor 207 to detect a number of time resolved intensity signals from the multiple secondary electron beamlets 9 within a predefined time interval, and a digital image of an image patch is accumulated and stitched together from all the scanning positions of the multiple primary charged particle beamlets 3.

[0039] The image sensor 207 shown in FIG. 1 may be an electron-sensitive detector array, such as a CMOS or CCD sensor. Such an electron-sensitive detector array may include an electron-photon conversion unit, such as a scintillator element, or an array of scintillator elements. In another embodiment, the image sensor 207 may be configured as an electron-photon conversion unit or scintillator plate located in the focal plane of the multiple secondary electron particle image spots 15. In this embodiment, the image sensor 207 may further include relay optics for imaging and guiding the photons generated by the electron-photon conversion unit at the secondary charged particle image spots 15 onto dedicated photon detection elements, such as multiple photomultiplier tubes or avalanche photodiodes (not shown). Such an image sensor is disclosed in U.S. Pat. No. 9,536,702 (BB), which is incorporated herein by reference. In one example, the relay optics further includes a beam splitter for splitting and guiding the light to a first slow photodetector and a second fast photodetector. The second high-speed photodetector is constituted by an array of photodiodes, such as avalanche photodiodes, that are fast enough to resolve the image signals of the multiple secondary electron beamlets according to the scanning speed of the multiple primary charged particle beamlets. The first low-speed photodetector is preferably a CMOS or CCD sensor that provides high-resolution sensor data signals for monitoring the focal spot 15 or the multiple secondary electron beamlets 9, as well as for controlling the operation of the multi-beam charged particle microscope as described in more detail below.

[0040] During acquisition of an image patch by scanning the multiple primary charged particle beamlets 3, the stage 500 is preferably not moved, and after acquisition of an image patch, the stage 500 is moved to the next image patch to be acquired. The stage movements and the stage position are monitored and controlled by sensors known in the art, such as laser interferometers, grating interferometers, confocal microlens arrays, or the like. For example, a position sensing system determines the lateral and vertical displacements and rotations of the stage using laser interferometers, capacitive sensors, confocal sensor arrays, grating interferometers, or any combination thereof.

[0041] In Fig. 2 an embodiment of a method of wafer inspection by image patch acquisition is explained in more detail. The wafer is positioned such that its wafer surface 25 is in the focal plane of the multiple primary charged particle beamlets 3 with the center 21.1 of the first image patch 17.1. The predefined positions of the image patches 17.1...k correspond to the inspection sites of the wafer for the inspection of semiconductor features. The predefined positions of the first inspection site 33 and the second inspection site 35 are loaded from an inspection file according to a standard file format. The predefined first inspection site 33 is divided into several image patches, e.g. the first image patch 17.1 and the second image patch 17.2, and the first center position 21.1 of the first image patch 17.1 is aligned under the optical axis of the multi-beam charged particle microscope system for the first image acquisition step of the inspection task. Methods for aligning the wafer are well known in the art so that the wafer surface 25 is registered and a coordinate system of wafer coordinates is generated.

[0042] The plurality of primary beamlets are distributed in a regular raster configuration 41 within each image patch and are scanned by a scanning mechanism to generate a digital image of the image patch. In this example, the plurality of primary charged particle beamlets 3 are arranged in a rectangular raster configuration 41 with M lines with n primary beam spots 5.11, 5.12 to 5.1N and beam spots 5.11 to 5.MN in a first line with N beam spots. For simplicity, only M=5 by N=5 beam spots are shown, but the number of beam spots, M by N, may be larger and the plurality of beam spots 5.11 to 5.MN may have different raster configurations 41, such as a hexagonal or circular raster.

[0043] Each of the primary charged particle beamlets is scanned across the wafer surface 25, as shown in the example of a primary charged particle beamlet with beam spots 5.11-5.MN with scan paths 27.11-27.MN of the multiple primary charged particle beamlets. The scanning of each of the multiple primary charged particle beamlets 3 is performed according to a preselected scanning program, for example by back and forth deflection by the scanning deflector 110 in the x-direction and an associated interlaced deflection by the scanning deflector 110 in the y-direction. For image acquisition, multiple secondary electrons are emitted at the scanning positions of the focal points 5.11-5.MN, generating multiple secondary electron beamlets 9. The multiple secondary electron beamlets 9 are collected by the objective lens 102, pass through the first deflection system 110, are guided to the detection unit 200 and are detected by the image sensor 207. The sequential stream of data of each of the multiple secondary electron beamlets 9 is converted into multiple 2D data sets synchronously with the scan paths 27.11...27.MN to form digital image data of each image subfield 29.11-29.MN. The multiple digital images of the multiple subfields 29.11-29.MN are finally stitched together by an image stitching unit to form a digital image of the first image patch 17.1. The operation is repeated for the second image patch 17.2 in the first inspection site 33, and after acquisition of the image data in the first inspection site, the wafer 7 is moved to the next inspection site 35 having image patch 17.k.

[0044] Next, the requirements or specifications of the wafer inspection task are given. For high throughput wafer inspection, the image acquisition of the image patches 17.1...k and the stage movement between the image patches 17.1...k must be fast. On the other hand, strict specifications of the image quality, such as image resolution, image accuracy and repeatability, must be maintained. For example, the requirement for image resolution is typically 2 nm or less. For example, the edge position of a feature, typically the absolute position accuracy of the feature, must be determined with high absolute accuracy. Typically, the requirement for position accuracy is about 50% or less of the resolution requirement. The image contrast and dynamic range must be sufficient to obtain an accurate representation of the semiconductor features and material composition of the wafer under inspection. Typically, the dynamic range must be better than 6 or 8 bits and the image contrast must be better than 80%. The dominant aberration limiting the resolution and contrast of each image data of the image subfield 29.mn corresponding to the focal point 5.mn of the primary charged particle beamlet 3.mn is given by the primary astigmatism. Other aberrations may also be present, such as trefoil or higher order astigmatism. First order astigmatism has two components AST0=A0r 2 cos 2φ AST45=A45r 2 sin 2φ where r is the distance of the particle trajectory from the beamlet central trajectory and φ is the polar angle. In other words, the radius r and the polar angle φ describe the pupil coordinates of the charged particle trajectory in the primary beamlet. In FIG. 3, the effect of wavefront aberration of the primary charged particle beamlet 3 is explained with the example of AST0. FIG. 3a shows a perfectly aligned beamlet 3 propagating along the z-axis, which is perpendicular to the wafer surface 25 (not shown). In the image plane 101, the astigmatic beamlet 3 forms a circle of least confusion spot 74. The diameter of the circle of least confusion spot 74 depends on the amplitude of the wavefront aberration (here AST0) and increases with increasing amplitude. Thus, the image contrast or resolution in the image plane 101 decreases with increasing wavefront aberration amplitude. In a first line focal plane 72 with a positive z-position at a distance AD / 2 from the image plane 101 a first line focus 76.1 is formed, and in a second line focal plane 78 with a negative distance AD / 2 a second line focus 76.2 is formed at a relative angle of 90° to the first line focus 76.1. Outside the plane the shape of the beamlets is elliptical. The astigmatic difference AD ​​(reference 73) is proportional to the amplitude A0 of AST0.

[0045] In FIG. 3b, two exemplary contrast curves are shown across the focus in z-distance. The first contrast curve 81 shows the contrast across the focus in a typical semiconductor probe with horizontally and vertically aligned structures (HV structures). In the two planes 73 and 78, the line focus is either parallel or perpendicular to the HV structures, and the maximum image contrast of the image of the wafer surface segment is therefore shown to maximum. In the image plane 101, the contrast curve 81 shows a local minimum, where the contrast curve at the minimum has an approximately parabolic shape. The second contrast curve 83 shows an example of an arbitrary probe with arbitrarily oriented structures. Here, the contrast curve has a maximum in the image plane 101, where the contrast curve at the maximum has an approximately parabolic shape. In both cases, the contrast curve shows a local extremum of image contrast in the image plane 101, with an approximately parabolic shape around the local extremum (either a maximum or a minimum).

[0046] FIG. 4 shows an array 601 of compensating elements. Such arrays of compensators are known in the art, for example, as known in U.S. Pat. No. 10,147,582 (BB), which is incorporated herein by reference. The multi-aperture array 601 includes a plurality of apertures arranged in a raster arrangement 41 of a plurality of primary charged particle beamlets 3, in this example a hexagonal raster arrangement. Two of the apertures are indicated with reference numbers 685.1 and 685.2. Within the periphery of each of the plurality of apertures, a plurality of electrodes 681.1-681.8 are arranged, in this example the number of electrodes is eight, although other numbers such as four, six or more are possible. The electrodes are electrically insulated from each other and from the support of the array 601 of compensators. Each of the plurality of electrodes 681 is connected to a control module by one of conductive lines 607 or wire connections (not all shown). By application of individual and predetermined voltages to each of the electrodes 681, different effects can be achieved for each of the multiple primary charged particle beamlets passing through each of the apertures 685. In one example, multiple, e.g., two or three, such multi-aperture plates 601 are arranged in series. A method of operation of the multi-beam charged particle microscope 1 with the array of compensators 601 is described in the following embodiments.

[0047] In a multi-beam system, the wavefront aberrations usually have two different contributions. The first contribution is from a combination of global elements of the multi-beam system 1, such as the deflector 110, the objective lens 102, or the beam splitter unit 400. The second contribution is from the primary multi-beamlet forming unit 305. FIG. 5a shows a typical field dependence of the primary AST0 of a plurality of primary charged particle beamlets 3 according to a hexagonal raster configuration 41. The circles identify the positive amplitudes A0 and the squares identify the negative A0 of the primary charged particle beamlets 3, the diameter of the symbols being proportional to the amplitude A0 of the wavefront aberration AST0. The distribution of the wavefront aberration amplitudes has a third contribution with an orderly field dependence. As is visible in FIG. 5a, the amplitude A0 of the AST0 shows a dominant linear dependence over the field coordinates in x and y. In addition, the distribution of the wavefront aberration amplitude shows a fourth contribution, which is illustrated in Fig. 5a in some examples of beamlets identified with reference 3d. Here, the wavefront aberration AST0 shows a local deviation from the linear dependence. Fig. 5b shows the residual wavefront aberration after subtraction of the ordered field dependence of the wavefront aberration (here, linear dependence in x and y). The diameter corresponds to the residual fourth contribution of the wavefront aberration AST0 on an enlarged scale compared to Fig. 5a.

[0048] According to a second embodiment of the present invention, a method for determining the wavefront aberration amplitude of each of the primary charged particle beamlets is provided. Usually, it is not possible to measure the wavefront aberration directly. Instead, and for general use, a number J of wavefront aberrations A (j=1...J) are indirectly determined by a series of contrast measurements at different control parameters of the modification element 605. From the series of measurements, a control parameter value for maximum image contrast is derived. The control parameter value SV(maxC(j)) at maximum or minimum image contrast corresponds to the wavefront aberration AV in units of the control parameter of the modification element 605 and is therefore unique to the modification element 605. The wavefront aberration A in normalized sensitivity units is then derived, for example in relative units, and in the following the symbol A is used for the amplitude of the wavefront error in normalized sensitivity units. As described in the third embodiment below, the derivation of the normalized wavefront amplitude A(j) in normalized sensitivity units allows the determination of the wavefront aberration amplitude using a first element or modification element 605, and the compensation of the wavefront aberration using a second element or compensation element 601 or 603 that is different from the first element or modification element 605. The modification element 605 can be any element that affects the wavefront aberration of interest, such as the magnetic correction element 420, or the scanning deflector 110, or other components. In an alternative example, a global compensation element 603 is used as the modification element 605 to change the wavefront aberration of interest. The control parameter can be either the voltage of an electrostatic element or the current of a magnetic field element. By utilizing the global modification element 605, the method of determining the wavefront aberration amplitude A(j) can be performed with improved speed and reduced complexity, since it is only necessary to change the control parameter SV of a single element. In the first example, the wavefront aberration amplitude AV is then converted to the wavefront aberration amplitude AC in units of the maximum range of the compensation element. In a second example, the wavefront aberration amplitude AV is converted to an absolute wavefront aberration amplitude A in normalized sensitivity units.

[0049] In Fig. 6 an example of the method according to the second embodiment is described. The method is configured to determine the wavefront aberration amplitude including the first and second contributions. The method is applicable to any object 7 as long as the object 7 generates image contrast. The method is described on the example of AST0, but it can be applied for any wavefront aberration such as AST45, coma, higher order astigmatism, or trefoil aberration.

[0050] In an initial step SR, the surface 25 of the object 7 is adjusted into the image plane 101 of the multi-beamlet charged particle microscope 1. Such a method is described in German patent application no. 102021200799.6, filed on 29 January 2021, which is incorporated herein by reference. An initial setting of the multi-beamlet charged particle microscope 1 is determined, for example by a focus sequence, and an optimal focal plane is determined as a set point. The control unit 800 then triggers the determination of AST0.

[0051] In step SD, a sequence of contrast measurements i=1 to i=SI is repeated. For each contrast measurement, a control parameter SV of the modification element 605 is varied over a sequence of control parameters SV(i=1 to SI). The number of contrast measurements SI is typically selected such that SI>=3, and preferably SI is 5 or more. For each control parameter SV(i) of the modification element, an image contrast C(j,i) for each of a plurality of J charged particle beamlets at a feature present on the object surface 25 is determined. The plurality of image contrast values ​​C(j=1...J,I=1...SI) are temporarily stored in a memory.

[0052] In step SF, the contrast curves C(j,i) for each of the plurality of J primary charged particle beamlets are analyzed and for each beamlet j, an optimal control parameter SV(maxC(j)) for a maximum contrast value maxC(j) is numerically determined. The determination can be performed, for example, by polynomial fitting, for example by fitting a parabola to the measurement points. An example is shown in FIG. 7. For a primary charged particle beamlet j, the image contrast is measured at five different control parameter values ​​SV(i=1) to SV(i=5). The measurement value for i=3 corresponds to the set point 49, and the control parameter value SV(3) in this example is set to 0 (SV(3)=0). However, this does not necessarily have to be the case. According to the determination of the set points during step SR, the control parameter value of the modification element 605 can have an offset control parameter value deviating from 0.

[0053] A parabolic fit C(j;i), identified with reference numeral 53, is fitted to the measurement points C(j,i=1) to C(j,i=5) and a maximum max(C(j)) (see reference numeral 55) is determined. An optimal control parameter value SV(maxC(j)) corresponding to the maximum contrast value 55 is determined. The maximum contrast value max(C(j)) may be different for each of the plurality of J primary charged particle beamlets, for example due to other wavefront aberrations or certain other imperfections of this beamlet. Furthermore, the parabolic shape of each contrast curve C(j) may be different for each j=1...J beamlet depending on the sensitivity of the wavefront aberrations of this beamlet to parameter changes of the modifier.

[0054] In a first example, a wavefront amplitude A(j) in normalized sensitivity units is determined for each of a plurality of primary beamlets 3 as A(j)=SV(maxC(j)) / RV for a predefined control parameter range RV of the modification element. The determination is repeated or performed in parallel for each beamlet. The control parameter range RV of the modification element 605 is determined, for example, in a previous calibration step and adjusted such that the same range of wavefront aberration amplitudes can be achieved with the control parameter range RV or modification element 605 as with the global compensation element 603, or with the control parameter range of each element of the array of compensation elements 601. The corresponding range or modification and compensation elements are then adjusted so that the compensator elements can be controlled with the determined wavefront amplitude A in normalized sensitivity units.

[0055] In a second equivalent example, the wavefront amplitude A(j) in normalized sensitivity units is derived from the curvature of the contrast curves 53. For each of the J contrast curves with parabolic coefficients KV describing the parabolic portion, the parameters of the parabolic fit are extracted: C(j;SV)=maxC(j)-KV(j)(SV-SV(maxC(j))) 2

[0056] Next, the wavefront aberration amplitude A(j) in normalized sensitivity units at a set point 49 in the image plane 101 is determined according to the derivation of the parabolic contrast curve at SV=0: A(j)=SIGN(j)*2KV(j)*SV(maxC(j)).

[0057] The absolute amplitude sign(j) is determined according to the definition of the coordinate system. In the second example, the parabolic coefficient KV(j) describes the sensitivity of the wavefront aberration to the change of the control parameter and is therefore also called the parabolic sensitivity parameter. In Fig. 5a, the result of the determination of the absolute wavefront amplitude A(j) in normalized sensitivity units is shown.

[0058] For greater accuracy, a higher order approximation or fitting to the measured contrast values ​​can be performed, for example a hyperbolic fit or a higher order polynomial fit. From the approximate fit curve to the contrast values ​​at SV=0, the wavefront aberration amplitude in normalized sensitivity units is derived.

[0059] For the determination method, it is not necessary to acquire a digital image from the entire image sub-field as shown in FIG. 2. The contrast measurement for each beamlet can also be performed in a smaller image sub-field with a small number of scan positions and scan lines, for example 128 x 128 pixels or 256 x 256 pixels. For objects with similar surface structures, for example as structured semiconductor wafers, it is also possible to perform the contrast measurement in different, smaller image sub-fields within the scanning range of the deflection scanner 110. This minimizes the charging effect on the contrast measurement for different parameters of the modifier. In one example, the determination method is not applied to all beamlets of the plurality of J beamlets.

[0060] Thus, a method for determining a plurality of wavefront aberration amplitudes A(j) includes the steps of: (a) setting the multibeam microscope (1) to a set point for an inspection task; (b) modifying wavefront aberrations of a plurality of J primary charged particle beamlets (3); (c) determining a plurality of contrast curves C(j=1...J) for each of the plurality of J primary charged particle beamlets (3) from a plurality of contrast values ​​C(j,i); and (d) determining a plurality of J wavefront aberration amplitudes A(j=1...J) in normalized sensitivity units at the set point from the plurality of contrast curves C(j=1...J). The wavefront aberration is modified by providing a series of at least SI=3 different modification control signals SV(i=1...SI) to the modification element (605) and measuring a plurality of contrast values ​​C(j,i) at each modification control signal SV(i=1...SI) for each of the plurality of J primary charged particle beamlets (3). Each contrast curve C(j) can be obtained by calculation of a parabolic, hyperbolic or polynomial approximation to the contrast values ​​C(j,i) for i=1...SI for each of the plurality of J primary charged particle beamlets (3). In a first example, the wavefront amplitude A(j) in normalized sensitivity units is determined from the modified control signal SV(maxC(j)) at the maximum contrast value maxC(j) divided by the normalized range RV of the modification element 605. The normalized range RV can be determined from the difference between the maximum and minimum control signals SV required to achieve a predetermined modification of the image contrast of at least one beamlet of the plurality of primary charged particle beamlets. In a second example, the wavefront amplitude A(j) in normalized sensitivity units is determined from the parabolic coefficient KV(j) of the contrast curve C(j).

[0061] During use, the set points may deviate from the design set points, and due to the rotation of the primary charged particle beamlets in the magnetic field, the set points may include deviations of a predetermined rotation of the raster arrangement (41) of the multiple primary charged particle beamlets (1) between the coordinate systems of the image plane (101), the array of compensator components (601), the global compensation element (603), and / or the modification element (605). In one example of the method according to the second embodiment, the wavefront aberration amplitude is converted into a wavefront aberration amplitude vector, for example, the vector [AST0,AST45]. The multiplication of the wavefront aberration amplitude vector [AST0,AST45] by a rotation matrix M can take into account deviations of a predetermined rotation of the raster arrangement (41) of the multiple primary charged particle beamlets (1) between the coordinate systems of the image plane (101), the array of compensator components (601), the global compensation element (603), and / or the modification element (605).

[0062] For example, after the wavefront aberration amplitudes A(j) present in the multi-beam charged particle system 1 have been determined according to the method of the second embodiment, the wavefront aberration amplitudes A(j) can be minimized by at least a compensator element. According to a third embodiment of the present invention, a method for compensating for the wavefront aberration amplitudes A(j) of a plurality of J primary charged particle beamlets is described. In Fig. 8, an example of a method for compensating for the wavefront aberration amplitudes A(j) in a multi-beam charged particle microscope 1 is described.

[0063] In a first compensation trigger step CTS, the wavefront aberration amplitudes A(j) of a plurality of J primary charged particle beamlets 3 of the multi-beam charged particle microscope 1 are received and analyzed. Whenever the wavefront aberration amplitudes A(j) of a primary charged particle beamlet exceed a predefined maximum threshold value of wavefront aberration amplitudes, compensation is triggered by a subsequent step of the method.

[0064] The wavefront aberration amplitude A(j) may be received, for example, from a method of determining the wavefront aberration amplitude A(j) according to the first embodiment. Other inputs may also be received, for example, from other measurement results or from monitoring and model-based control. The model-based control predicts a first expected behavior, for example, a change in the wavefront aberration amplitude, based on a given model assumption and a second indirect monitoring parameter, such as the operating time of the multi-beam charged particle microscope or the temperature of a component. A model-based control algorithm is further disclosed in International Application EP2021 / 061216, filed May 28, 2020, which is incorporated herein by reference. In one example, the determination of the wavefront aberration amplitude A(j) at the set point 49 is triggered by a monitoring method. In one example, during use of the monitoring method, a change in image contrast of a digital image generated during an inspection or metrology task is monitored. A wavefront aberration measurement is triggered when the image contrast changes, for example, below a predetermined threshold of a minimum contrast, for at least one of the plurality of primary charged particle beamlets. The monitoring method will be explained in more detail in the fourth embodiment.

[0065] In a compensation determination step CDS, the amplitude A(j) is analyzed. In a compensation analysis step CAS, at least a first component AG1 of the amplitude is determined, which has a field dependency of at least a first global compensation element, by a best fit of the field dependency to the amplitude A(j). Typically, the global compensation element exhibits a low-order field dependency on the wavefront aberration, such as a constant field dependency, a linear field dependency or a quadratic field dependency. In one example, the field dependency of the wavefront aberration is expanded in the form of a so-called double Zernike expansion, in which the wavefront aberration and the field dependency are expanded in the form of Zernike polynomials. For the normalized calculation of the amplitude AG1 of the field dependency, the maximum field radius is set to 1.

[0066] The global compensation element can be the global compensation element 603, the deflection scanner 110, the magnetic field component 420, or any other electronic optical element in the primary beam path that has a large effect on the wavefront aberration of interest but has a small effect on other properties of the multi-beam charged particle microscope 1. It is also possible to use several global compensation elements, for example the first global compensation element 603 and the deflection scanner 110 as the second global compensation element. The deflection scanner 110 can be realized, for example, as an electrostatic or magnetic octopole element, and a compensation signal for compensating for the wavefront aberration can be provided as an offset signal to some of the eight poles. This can, for example, compensate for an approximately constant offset of the astigmatism. In an alternative example, an element including six or a multiple of six poles can be provided with the ability to compensate for the trefoil aberration.

[0067] The first component AG1, which can be compensated by at least one global compensation element, is subtracted from the amplitude A(j) to obtain a second component Ares(j) of the residual of the residual wavefront amplitude, which second component Ares(j) cannot be corrected using a global compensation element.

[0068] From the first component AG1 a global correction signal GCS is derived for at least one global compensation element, the global correction signal GCS being derived from the field-dependent amplitude AG1 and from a predetermined sensitivity of the global compensation element.

[0069] As a result of the determination of the predetermined sensitivity of the global compensation element, similar contrast curves as shown in Fig. 7 are obtained for at least the representative beamlet and for the change of the control signal of the global compensation element, but with a difference in scaling. The contrast over the change of the compensator again shows a parabolic shape with a parabolic sensitivity parameter KC. The predetermined parabolic parameter KC is stored in the memory of the control unit 800. Further details of the determination of the calibration parameters are described in the fifth embodiment of the invention.

[0070] The global correction signal GCS is given by the following equation: GCS=SIGN*AG1 / (2*KC).

[0071] From the second component, a number J of local compensation signals LCS(j) are calculated for each of the J primary charged particle beamlets. For each of the compensator elements of the array element 601, a number of predetermined individual parabolic parameters KLC(j) are used to calculate the number of local compensation signals LCS(j) by: LCS(j)=SIGN(j)*Ares(j) / (2*KLC(j)).

[0072] The signs are predetermined according to the definition of the coordinate system, for example defined during the step of determining the wavefront amplitude so as to coincide with the definition of the signs.

[0073] In one example, the compensation determination step CDS is performed by a control unit 800. The control unit 800 thus comprises a memory for storing the field dependence of the global compensator element 603 and the predetermined parabolic sensitivity parameter KC of the global compensator element 603. In the memory, furthermore, a plurality of predetermined parabolic sensitivity parameters KLC(j) for j=1...J are stored. In another example, the predetermined parabolic sensitivity parameter KC of the global compensator element is stored in a motion control unit of the global compensator element, and the conversion to a global correction signal GCS is performed in the motion control unit of the global compensator element. Similarly, the calculation of a plurality of local compensation signals LCS(j) can be performed in a motion control unit of the array 601 of global compensation elements.

[0074] The compensation determination step CDS is performed here using the wavefront aberration amplitude A(j) in normalized sensitivity units, described with the example of parabolic sensitivity parameters KV, KC, and KLC(j). Other examples of scaling of the wavefront amplitude are also possible, for example from the modification element 605 to the two compensation elements 603 and 601. One example is to utilize a scaling of the wavefront amplitude with different maximum parameter ranges of the compensation elements 601 and 605, similar to the scaling A(j)=SV(maxC(j)) / RV in normalized sensitivity units of the modification elements as described above in the first example of the second embodiment.

[0075] In a compensation execution step CES, the global compensation signal GCS is provided to a global compensation element, e.g. element 603, which achieves, in step GCE, global compensation of the first component of the wavefront aberration. The multiple local compensation signals LCS(j) are provided to an array of compensation elements 601, which achieves, in step LCE, local compensation of the residual wavefront aberration.

[0076] In an optional verification step CVS, the compensated wavefront aberration is measured again, for example using the method described in the first embodiment. To achieve even better compensation of the wavefront aberration, the method steps CDS, CES and CVS can also be repeated iteratively until the compensated wavefront aberration falls below a predefined threshold.

[0077] Thus, a method according to a third embodiment for compensating for multiple wavefront aberrations of a multi-beam charged particle microscope (1) at a set point includes the steps of: (a) receiving a plurality of J wavefront aberration amplitudes A (j=1...J) of a plurality of J primary charged particle beamlets (3) in normalized sensitivity units; (b) determining a global component AG1 of the amplitudes in normalized sensitivity units, the global component having a predetermined field dependence of the plurality of J wavefront aberration amplitudes A(j) of a global compensation element (603); (c) determining a residual component Ares(j) of a plurality of residual wavefront amplitudes in normalized sensitivity units; (d) converting the global component into a global compensation signal GCS; (e) converting the residual component into a plurality of local compensation signals LCS(j); (f) providing the global compensation signal GCS to the global compensation element (603); and (g) providing the plurality of local compensation signals LCS(j) to the array of compensation elements (601). The first step (a) may include a method for determining a plurality of J wavefront aberration amplitudes A(j) according to the second embodiment. According to the method of the third embodiment, the wavefront aberration amplitudes A(j) are converted into normalized sensitivity units, and the sensitivities of the compensation elements are described in the same normalized sensitivity units. In a first example, a normalized range RC of the global compensation element (603) and a normalized range RL of the array of compensation elements (601) are predetermined and stored in the memory of the control unit (809). In a first example, the wavefront aberration amplitudes and the control signals for the compensator elements are normalized using the normalized ranges RV, RC, and RL. In a second example, parabolic sensitivity constants KV, KC, or KLC(j) are used instead, and consistent scaling of the wavefront aberration amplitudes and the control signals for driving the compensators is achieved.

[0078] The determination of the wavefront amplitude A(j) according to the method of the second embodiment is a fast and reliable method for determining wavefront aberrations. The method can be applied to any object that generates some image contrast. In particular, there is no need to replace the inspection site of the object with a dedicated measurement object. The method is further applicable for monitoring the wavefront aberrations throughout the inspection or measurement task. According to a fourth embodiment of the present invention, monitoring of the multi-beam charged particle microscope 1 is performed by regular application of the determination method according to the second embodiment. In one example, the monitoring is performed by extracting contrast parameters of images obtained from image sub-fields at different, consecutive inspection sites. Typically, the average image contrast at different inspection sites should not change during wafer inspection. If the contrast change exhibits a certain field dependence of the wavefront aberrations, such as, for example, a linear field dependence at x any of AST0 as shown in FIG. 5a, the determination step of the wavefront amplitude according to the second embodiment is triggered, followed by the compensation method according to the third embodiment. In a fourth embodiment, the control unit 800 is configured to monitor a contrast change of the plurality of charged particle beamlets during a wafer inspection task. The control unit 800 is further configured to analyze the contrast change and to determine at least a first field dependency of a first wavefront aberration, e.g., AST0 or AST45. If at least a first field dependency is detected by the control unit 800, a wavefront amplitude of the corresponding wavefront aberration AST45 is determined. The control unit 800 is further configured to determine a further field dependency, e.g., a second field dependency of a second wavefront aberration, e.g., AST0. If a second field dependency is detected by the control unit 800, a wavefront amplitude of the corresponding wavefront aberration AST0 is determined.

[0079] In a further example of a method for monitoring a multi-beam charged particle microscope 1 according to the fourth embodiment, the throughput is further increased by monitoring only selected primary charged particle beamlets. From the plurality of primary charged particle beamlets 3, a small number of two or more beamlets are selected that are representative of the low-order field dependence of the wavefront aberration. In one example, at least two selected beamlets with a large spatial distance relative to each other are used to monitor the linear field dependence of the wavefront aberration of interest, for example AST0 or AST45. In one example of a monitoring method, the wavefront aberration of three selected beamlets with a large distance relative to each other is changed using corresponding compensators of the array of compensation elements 601 while leaving the plurality of other primary beamlets unchanged. The control unit triggers the change in the wavefront aberration of only these three selected beamlets during monitoring. From the change in the image contrast generated by the selected beamlets, the wavefront aberration amplitude of the selected beamlets is determined, and the first component or the low-order field dependence of the wavefront aberration is determined. The first component or linear field dependence of the wavefront aberration is then compensated by a suitable global compensator 603 with the linear field dependence of the wavefront aberration. The residual wavefront error amplitude Ares is compared with a threshold. If Ares exceeds the threshold, the full determination and compensation according to the second and third embodiments of the invention is triggered. According to a third example, the wavefront aberration of any selected beamlet can be changed with a corresponding compensator of the array of compensation elements 601. During the monitoring, the wavefront aberration of different selected beamlets can be changed, which allows, for example, to determine different low-order field dependences of AST0, AST45, coma or trefoil aberrations.

[0080] According to a further example, a model-based control based on a predefined model of the multi-beam charged particle microscope (1) and secondary indirect monitoring parameters is utilized in the monitoring method, whereby the image contrast of a plurality of digital images received during an inspection task is predicted according to a predefined model, and a wavefront determination and / or compensation step is triggered when the predicted image contrast falls below a predefined threshold.

[0081] A requirement or specification of a wafer inspection task is the throughput. The throughput depends on several parameters, for example the measurement area per acquisition time itself. The measurement area per acquisition time is determined by the dwell time, the resolution, and the number of beamlets. A typical example of the dwell time is 20ns to 80ns. Thus, the pixel rate in the high-speed image sensor 207 is in the range of 12Mhz to 50MHz, and about 20 image patches or frames can be acquired per minute. Between the acquisition of two image patches, the wafer is moved laterally by the wafer stage to the next point of interest. For 100 beamlets, a typical example of the throughput in high resolution mode with a pixel size of 0.5nm is about 0.045sqmm / min (square millimeters per minute), and for a larger number of beamlets and a lower resolution, for example 10000 beamlets and a dwell time of 25ns, a throughput of more than 7sqmm / min is possible. The stage movement, including the acceleration and deceleration of the stage, is one of the limiting factors for the multi-beam inspection system throughput. Faster acceleration and deceleration of the stage in a short time usually requires a complex and expensive stage or induces dynamic vibrations in a multi-beam charged particle system. Because no additional stage movement is required, the second to fourth embodiments of the present invention allow determination and compensation of wavefront aberrations such as astigmatism with a high throughput of wafer inspection tasks, while keeping image performance specifications well within the requirements for resolution and repeatability. For example, monitoring according to the third example with only three beamlets allows for the performance of monitoring in parallel with wafer or mask inspection, where multiple primary beamlets are used for wafer or mask inspection, except for a few selected beamlets for monitoring.

[0082] In the fifth embodiment, a method for determining the parabolic sensitivity parameters KC and KLC(j) is shown. In a first example, the parabolic sensitivity parameters KC and KLC(j) are determined in a similar manner to the determination method according to the second embodiment. However, unlike the determination method according to the second embodiment, the parabolic sensitivity parameters KC or KLC(j) are determined by changing the compensation element 601 or 603 and by obtaining a contrast curve over the corresponding element change. FIG. 9 shows the contrast curves in two different examples for the same beamlet j. The contrast curve 53 of the representative beamlet of the global compensation element 603 is obtained by changing the control parameter SC of the global compensation element 603 with, for example, SI=5 different control parameter values ​​SC (SI=1...I). A parabolic fit to I obtained contrast values ​​C1 (i=1...I) is performed. The maximum contrast value 55 at the maximum contrast position SC(maxC1(j)) and the corresponding control parameter value are determined. The range RC of the global compensation element 603 is adjusted to compensate for a range of wavefront aberrations, e.g., the same as the range RV of the modification of the wavefront aberration amplitude by the modification element 605. The parabolic constant KC is determined and stored in the memory of the control unit 800. A representative beamlet is selected according to the field dependence of the global compensation element 603. Similar operations are performed for the compensators of the array elements 601 for each beamlet j=1...J. A second contrast value C2(i=1...SI) is calculated for each beamlet j by the array elements 601. The parabolic sensitivity coefficient KLC(j) is determined by changing the control parameters of the compensators of the compensators of the element 601. FIG. 9 shows the on contrast curve 51 with a maximum value maxC2 (with index 56). Typically, the compensators of the array of compensators 601 have a larger effect or sensitivity to wavefront aberrations and are driven over a smaller range RL. The method is repeated for each of the J beamlets j=1...J. The parabolic constant KLC(j) is typically slightly different and larger than the parabolic constant KC of the global compensator. Determination of the parabolic constants KC and KLC(j) is possible even when wavefront aberrations are present.The situation in FIG. 9 shows a situation when wavefront aberrations are present, since the maximum contrasts maxC1 and maxC2 are at parameter values ​​SC that deviate from 0. The calibration method according to the first example of the fifth embodiment can be performed on a wafer surface having structures that generate image contrast. Thus, the calibration method according to the first example can be performed during a wafer inspection task, for example, when the compensation method for wavefront errors according to the third embodiment does not converge after several iterations. In one example, individual compensators of the array of compensators 601 may undergo drift or contamination, which requires a recalibration of the compensators. In such a case, the calibration method according to the first example can also be performed only for individual compensators of the array of compensators 601 that exhibit a deviating behavior during the compensation method.

[0083] A first example of a method for calibrating a charged particle optical element, such as a global compensator (603), or a compensator element of an array of compensator elements (601), or a modification element (605), comprises the steps of: (a) modifying a wavefront aberration of a plurality of J primary charged particle beamlets (3) by providing a series of at least SI=3 different control signals SC(i=1...SI) to the charged particle optical element; (b) determining a plurality of contrast values ​​C(j=1...J, (c) determining a plurality of contrast curves C(j=1...J,SC) for each of the plurality of J primary charged particle beamlets (3) from the plurality of contrast values ​​C(j,i), (e) determining an extreme value maxC(j) and a control signal SC(maxC(j)) corresponding to each extreme value maxC(j) of the contrast curve C(j,SC), and (f) determining a plurality of changes in wavefront amplitude A(j) affected by the charged particle optical element in normalized sensitivity units. The normalized sensitivity units in a first example are given by A(j)=SC(maxC(j)) / RC, where RC is the normalized range of the charged particle optical element corresponding to the difference between the maximum and minimum control signal SC required to achieve a predetermined change in image contrast C(j) for at least one of the plurality of J primary charged particle beamlets. The normalized sensitivity units according to the second example are given by A(j)=SIGN(j)*2KV(j)*SC(maxC(j)), with an approximate parabolic constant KC(j) for the contrast curve C(j=1...J,SC) in the vicinity of the extremum max(C(j,SC)). The multiple modifications of the wavefront amplitude A(j) of the charged particle optical element in normalized sensitivity units are stored in a memory of the control unit (800) of the multi-beam charged particle microscope (1).

[0084] In a further example of the calibration method according to the fifth embodiment, an absolute calibration of the sensitivity of the modification element 605 or compensator element 601 or 603 to wavefront aberrations is described. The absolute calibration is performed on a specific test pattern for the determination of the wavefront aberrations. In FIG. 10, an example of a test pattern or wavefront detection pattern 61 is shown. The wavefront detection pattern 61 comprises several repeating features 63 arranged at different rotation angles. The wavefront detection pattern 61 of FIG. 10 comprises eight repeating features 63 arranged at equidistant rotation steps of 22.5°. Each feature 63 is labeled with a label 69, where the rotation angles (0, 23, 45, 67, 90, 113, 135 and 158) are used as labels. Each feature 63 comprises a first grating pattern 65 for the determination of the wavefront aberrations in the form of astigmatism. Each feature 63 comprises a second line pattern 67 for the determination of the wavefront aberrations in the form of coma. A test wafer is provided with a plurality of identical wavefront detection patterns 61, at least one detection pattern 61 being arranged in a raster arrangement 41 of a plurality of primary charged particle beamlets 3. In figure 11 the calibration method is further illustrated.

[0085] In a first step M1, a test wafer is aligned in the image plane of the multi-beam charged particle microscope 1 such that a test pattern 61 is located within each of the image sub-fields 29 corresponding to each of the multiple primary charged particle beamlets 3.

[0086] In an iterative step M2, the compensator element to be calibrated is set to a first control parameter value SC(1) of a series of predetermined control parameter values ​​SC(i=1...SI). The compensator element can be any compensator of the array elements 601 or the global compensator 603. Calibration of the modification element 605 can also be performed.

[0087] In an iterative step M3, a focus sequence over a predetermined focus interval with focus steps dz(q), q=1...NQ, is performed. At each focus position, a digital image is acquired and a number of contrast values ​​C(j,q;α) for the regular grid pattern 65 with angle α are determined. In addition, the relative displacement drα of the center of gravity of the line pattern 67 with angle α is determined.

[0088] In step M4, the contrast curves across the focal points are evaluated for the control parameter values ​​SC(i). First, a parabolic contrast curve is fitted to the contrast measurements across the focal stack. An example is shown in FIG. 12. FIG. 12 shows six contrast curves C0, C23, C67, C90, C113, and C158 for a first grating pattern 65 at angles 0°, 22.5°, and 67.5°, and a second grating pattern 65 at angles 90°, 112.5°, and 157.5° perpendicular to the first pattern. The two contrast curves C23 and C113 show strong image contrast with maxima maxC23 and maxC23, which correspond to an astigmatism oriented at 22.5° relative to the x-axis. For other orientations of the pattern 63, the two line foci 76.1 and 76.2 of the astigmatic beamlet as shown in Fig. 3 are rotated with respect to the line grating 65.2 or 65.7 and are oriented neither perpendicularly nor parallel to the line grating 65.2 or 65.7. The distance between the two focus positions of the two maximum contrast values ​​then determines the absolute astigmatism value AST oriented at 22.5°, which corresponds to the control parameter value SC(i). This value is also called the astigmatic focus difference of the HV structure (HV stands for horizontal-vertical, meaning two grating patterns oriented perpendicular to each other).

[0089] The method steps M2 to M4 may be repeated until a predetermined sequence of control parameter values ​​SC(i), I=i...SI, is achieved. For each control parameter value SC(i=1...SI), an astigmatism focus difference value is determined.

[0090] Then, in step M4, the central position dr of the line pattern 67 across the focal point can also be analyzed. Figure 13 is an illustration of the result of the analysis. In this example, the wavefront aberration of interest is not astigmatism, but coma. Due to wavefront aberration in the form of coma, the maximum position dr of the line image drifts across the focal point and shows a curve. Figure 13 shows three examples of curves with examples of relative displacement drα of the line image across focal position z for angles α=0°, α=23°, and α=45°. In this example, there is coma aberration oriented below 45°.

[0091] The change in the focal position according to step M3 may be performed by a stage 500 having an actuator for displacing the object surface 25 in the z-direction, or by any other means for changing the focal position of the multi-beam charged particle microscope 1.

[0092] 11 can be performed for the calibration of absolute sensitivity parameters of the modification element 605 or the compensation element 601 or 603. The method can also be generally applied for the absolute measurement of wavefront aberrations of a multi-beam charged particle microscope 1.

[0093] Accordingly, a method for determining a wavefront aberration for each beamlet of a plurality of J primary charged particle beamlets of a multi-beam charged particle microscope (1) comprises the steps of: (a) providing a wavefront detection pattern (61) in an image plane (101) of the multi-beam charged particle microscope (1) for each beamlet of the plurality of J primary charged particle beamlets, each wavefront detection pattern (61) including a plurality of repeating features (63) oriented at different rotation angles α; (b) performing a plurality of measurements of the wavefront detection pattern (61) in a focus sequence having NQ focus steps q=1...NQ to determine a plurality of contrast values ​​C(j,q;α). (c) for each beamlet j=1...J and each rotation angle α, approximating a plurality of contrast curves C(j;α) as a function of focus position; (d) deriving a maximum value maxC(j;α) for each contrast curve C(j;α); and (e) determining a symmetric wavefront aberration A(j) having even-order rotational symmetry, such as stigmatic AST0 or AST45, for each beamlet from the maximum difference between two focus positions of two maxima maxC(j;α) and maxC(j;α-90) of two repeating features (63) oriented at 90° with respect to each other.

[0094] The method may further include determining a plurality of relative image displacements dr(j;α) across the focal points of each of the plurality of repeating features (63), and determining an asymmetric wavefront aberration having odd-order rotational symmetry, e.g., COMA0 or COMA90, for each of the beamlets from the maximum relative image displacement dr across the focal points.

[0095] In an example of an embodiment of the present invention, a rotation of a raster arrangement 41 of a number of primary beamlets 3 is considered. Such an example is shown in FIG. 14. The rotation of the raster arrangement 41 can be the result of magneto-optical lens elements, such as, for example, field lenses as lenses 103.1 and 103.2, objective lens 102, or other magneto-optical elements. Sometimes such a rotation of a number of J primary charged particle beamlets 3 is called Larmor rotation. Due to Larmor rotation, the coordinate systems (x,y) of different multipole elements can have different orientations. In FIG. 14, an example of a first set point or reference set point is shown. The coordinate system is chosen to have the z coordinate in the propagation direction of the primary charged particle beamlets 3, as in FIG. 1. The rotation between the multi-array elements of the primary multi-beamlet forming unit 305, including the compensator array element 601, is adjusted so that focal points or dwell points 5 (see dwell points 5.0, 5.1 and 5.3 in FIG. 14c) are formed in a raster configuration 41 in the xy coordinate system of the image plane 101. The dwell points 5 are formed on the surface 25 of a sample, e.g. a wafer, arranged in the image plane 101. FIG. 14a shows the corresponding array of compensator elements 601 in a coordinate system (x1, y1), rotated with respect to the sample coordinate system (x, y). FIG. 14b) shows the rotated coordinate system (x5, y5) of the global modification element 605. The rotation is now explained with the example of compensation of the wavefront aberration AST0 of the primary charged particle beamlet 3.3. In the case of AST0, the line focus is elongated along the x and y directions of the image plane 101, which is shown with great emphasis at the elongated dwell point 5.3, which corresponds to the beamlet 3.3. According to an example, the wavefront aberration AST0 is modified by the modification element 605, in this case by providing a first, positive voltage to the electrodes 615.1 and 615.5 and a second, negative voltage to the electrodes 615.3 and 615.7 of the modification element 605 (FIG. 14b). A number of primary charged particle beamlets pass through the modification element 605 in common in the intersection area 189. The modification element 605 is arranged in a coordinate system (x5, y5), rotated according to the set point of the multi-beam charged particle microscope 1.The electrodes 615.1-615.8 are oriented in a coordinate system (x5,y5), appropriately rotated to modify the wavefront aberration AST0 of the plurality of primary charged particle beamlets. Then, compensation control signals are determined and provided to the electrodes 681 of the corresponding compensator elements 683.3. For example, a third positive voltage is provided to the electrodes 681.1 and 681.5, and a fourth negative voltage is provided to the electrodes 681.3 and 681.7 of the compensator elements 683.3 of the array of compensation elements 601 (FIG. 14a). The method of determining the control signals, for example the third and fourth voltages, has been described above in the compensation determination step CDS and the compensation execution step CES. The electrodes 681.1-681.8 are oriented in a coordinate system (x1,y1), appropriately rotated to compensate the AST0 of the primary beamlet 3.3. The global compensation element 603 (not shown) may be arranged in a coordinate system (x3, y3) rotated according to a reference set point of the multi-beam charged particle microscope 1. The electrodes of the global compensation element 603 (not shown) are oriented in the coordinate system (x3, y3) rotated appropriately to compensate for the first component AG1 of the amplitude having the field dependence of the global compensation element of the wavefront aberration AST0 of the multiple primary charged particle beamlets.

[0096] The relative rotation angle between the image plane 101 and the coordinate system of the array element 601 of the compensator, the at least one global compensation element 603 and / or the modification element 605 is usually adjusted during manufacturing and calibration of the multi-beam primary charged particle microscope 1 for a predetermined reference first set point. However, the relative rotation angle can change for different set points, for example for a second set point with a different magnification, a third set point with a different numerical aperture of the beamlets, or a fourth set point with a different focal length to the image plane 101. An example is shown in FIG. 15. According to the different set points, the raster arrangement 41 in the image plane 101 is rotated by an angle φ, as shown in FIG. 15c. However, the physical implementation of the modification element 605 and the array of raster elements 601 remains unchanged. The modification of the wavefront aberration by the element 605 now depends on the relative rotation angle γ5' between the (x,y) coordinate system of the image plane 101. The modification of AST0 in the image plane 101 can be accomplished in two ways.

[0097] In a first manner, the modification signals provided to the modification elements are adjusted to cause a rotation of the electrostatic modification field within the intersection volume 189. This can be achieved by considering all eight poles of the modification element 605. For example, a positive voltage is provided to the four electrodes 615.1, 615.8, 615.5, and 615.4, and a negative voltage is provided to the electrodes 615.2, 615.3, 615.6, and 615.7. By appropriate adjustment of the voltage levels provided to the electrodes, field rotation can be achieved. Rotation is further improved by a large number of poles of the multi-pole element, for example 12 poles or more.

[0098] In the second scheme, AST0 is transformed into a vector [AST0(0),AST45(0)]. The vector is rotated by a rotation matrix M(γ5') to [AST0(γ5'),AST45(γ5')]=M(γ5')*[AST0(0),AST45(0)]. The rotation matrix M is usually specific for each wavefront aberration. In a similar approach, the compensation value GCS or LCS for the compensation element is calculated by considering the rotation matrix by M(γ) using the Larmor rotation angle between the coordinate systems of the modification element and the compensation element. Thus, the relative rotation between the image plane 101 and the coordinate systems of the array element 601 of the compensator, the at least one global compensation element 603, and / or the modification element 605 is considered by the wavefront aberration as a vector function, for example, with [AST0,AST45] or [COMA0,COMA90], according to the order of rotational symmetry of the wavefront aberration. The corresponding rotation angles may be determined during a calibration step and stored in the memory of the control unit 800.

[0099] The methods of the second to fifth embodiments may be implemented in a multi-beam charged particle microscope (1) for automatic application of any of the methods or triggered by user input. Thus, the multi-beam charged particle microscope (1) is configured to have a control unit (800) including a processor and memory with software code and programmed hardware such as an FPGA, configured to perform any of the methods of the second to fifth embodiments. The multi-beam charged particle microscope (1) of the first embodiment further comprises a multi-beam generating unit (300) for generating a plurality of primary charged particle beamlets (3) during use. The multi-beam generating unit (300) further comprises an array of compensation elements (601). The multi-beam charged particle microscope (1) of the first embodiment further comprises a global compensation element (603) and / or a modification element (605). The control unit (800) is configured to tune the multi-beam charged particle microscope (1) to a set point during use and to determine the wavefront aberration amplitude A(j) of each or a plurality of primary charged particle beamlets (3) at the set point. The control unit (800) is configured to determine a field-dependent global component AG1 and a residual component Ares(j) of the wavefront aberration amplitude A(j) of the plurality of J primary charged particle beamlets (3) during use. The control unit (800) is further configured to compensate the global component AG1 by the global compensation element (603) and the residual component Ares(j) by the array of compensation elements (601) during use. During the determination of the wavefront aberration amplitude A(j), the control unit (800) is configured to modify the wavefront aberration amplitude of each of the plurality of primary charged particle beamlets (3) using the modification element (605). The global compensation element (603) can be a multi-pole element including at least a first layer of multiple electrostatic or magnetic poles, and the global component AG1 of the field dependence of the wavefront aberration amplitude corresponds to the low-order field dependence of the wavefront aberration amplitude affected by the global compensation element (603).The array of compensator components (601) includes at least a first layer having a plurality of J apertures and a plurality of electrostatic electrodes arranged within the periphery of each aperture, and a field-dependent residual component Ares(j) of the wavefront aberration amplitude corresponds to a residual wavefront aberration that cannot be compensated for using the global compensation element (603). According to a first embodiment, the control unit is further configured to convert the wavefront aberration amplitude determined by the modification of the modification element (605) into normalized sensitivity units during use and to determine a plurality of control signals for the array of compensation elements (601) from the residual component of the wavefront aberration amplitude in the normalized sensitivity units. The control unit (800) is further configured to determine a control signal for the global compensation element (603) from the global component AG1 of the wavefront aberration amplitude in the normalized sensitivity units. In one example, the modification element (605) is provided by the deflection scanner (110) or the magnetic compensation element (420) of the multi-beam charged particle microscope (1), or is identical to the global compensation element (603) of the multi-beam charged particle microscope (1).

[0100] Fast control and recognition of wavefront aberrations are important not only for high resolution and high image contrast, but also for high image repeatability. Under high image repeatability, first and second repeated digital images are generated under repeated image acquisition of the same area, and it is understood that the difference between the first and second repeated digital images is less than a predetermined threshold. For example, the difference in image contrast between the first and second repeated digital images should be less than 10%, preferably less than 5%. In this way, similar image results are obtained even if the imaging operation is repeated. This is important, for example, for image acquisition and comparison of similar semiconductor structures in different wafer dies, or for comparison of the acquired images with representative or reference images obtained from image simulations, or from CAD data, or from a database. [Explanation of symbols]

[0101] 1. Multi-beamlet charged particle microscope 3 Primary electron beamlet 5 Primary charged particle beam spot 7 Samples or objects 9 Secondary electron beamlet 11 Secondary charged particle beam path 13 Primary charged particle beam path 15 Secondary charged particle image spot 17 Elephant Patch 19 Image patch overlap 21 Center of the statue patch 25 Surface of object 7, wafer surface 27 Scanning Path 29 Image partial field of view 33 First inspection site 35 Second Test Site 41 Raster Configuration 49 Set Points 51 Second contrast curve fitted to contrast measurement 53 First contrast curve fitted to contrast measurement 55 Maximum of the first contrast curve 57 Maximum of the second contrast curve 61 Wavefront Detection Pattern 63 Repeating features of wavefront detection pattern 65 Regular Grid Pattern 67 Line Pattern 69 Orientation Index 72 First Line Focal Plane 74 Minimum Circle of Confusion Spot 76 Linear focus 78 Second Line Focal Plane 81 First Contrast Curve 83 Second Contrast Curve 100 Object Irradiation Unit 101 Object plane 102 Objective Lens 103 Field Lens 105 Optical axis 108 Beam Crossover 110 Common deflection system 189 Intersection Area 200 detection units 205 Projection System 207 Image Sensor 208 Electrostatic or magnetic lenses 209 Electrostatic or magnetic lenses 210 Electrostatic or magnetic lenses 212 The Second Crossover 220 Secondary electron multi-aperture corrector 222 Second common deflection system 300 Multi-Beamlet Generator 301 Source 303 Collimating Lens 305 Primary Multi-Beamlet Formation Unit 306.1 First multi-aperture plate 306.2 Second and further multi-aperture plates 307 First Electrostatic Field Lens 308 Second Field Lens 309 Diverging Primary Charged Particle Beam 311 Primary charged particle beamlet spot 321 Intermediate image plane 390 Beam Steering Multi-Aperture Plate 400 Beam Splitter 420 Magnetic Compensation Element 500 sample units 503 Sample Charging Unit 601 Array of Compensating Elements 603 Global Compensation Element 605 Changes 615 poles 681 Electrode 683 Multipole Elements 685 Aperture 687 Wiring Connection 800 Control Unit 820 Projection System Control Unit 830 Primary Beamlet Control Module

Claims

1. A multi-beam charged particle microscope (1), comprising: a multi-beam generating unit (300) for generating a plurality of primary charged particle beamlets (3), comprising an array of compensation elements (601); a global compensation element (603), - modification elements (605), a control unit (800), said control unit (800) comprising: - adjusting said multi-beam charged particle microscope (1) to a set point; - modifying the wavefront aberration amplitude of each of said plurality of primary charged particle beamlets (3) using said modification element (605); determining the wavefront aberration amplitude A(j) of each of the plurality of primary charged particle beamlets (3) at the set point; determining a global component AG1 and a residual component Ares(j) of the field dependence of the wavefront aberration amplitude A(j) of the plurality J of primary charged particle beamlets (3), - compensating said global component AG1 by said global compensation element (603), - compensating said residual components Ares(j) by said array of compensation elements (601); A multi-beam charged particle microscope (1) configured to perform

2. 2. The multi-beam charged particle microscope of claim 1, wherein the global compensation element (603) is a multi-pole element including at least a first layer of a plurality of electrostatic or magnetic poles, and the global component AG1 of the field dependence of the wavefront aberration amplitude corresponds to a low-order field dependence of the wavefront aberration amplitude affected by the global compensation element (603).

3. 3. The multi-beam charged particle microscope of claim 1, wherein the array of compensator components (601) comprises at least a first layer having a plurality of J apertures and a plurality of electrostatic electrodes arranged within the periphery of each aperture, and the residual component Ares(j) of the field dependence of the wavefront aberration amplitude corresponds to a residual wavefront aberration that cannot be compensated for using the global compensation element (603).

4. 3. The multi-beam charged particle microscope (1) of claim 1 or 2, wherein the control unit is further configured to convert the wavefront aberration amplitude determined by the modification of the modification element (605) into normalized sensitivity units and to determine a plurality of control signals for the array of compensation elements (601) from the residual component of the wavefront aberration amplitude in normalized sensitivity units.

5. 5. The multi-beam charged particle microscope (1) of claim 4, wherein the control unit is configured to determine a control signal for the global compensation element (603) from the global component AG1 of the wavefront aberration amplitude in normalized sensitivity units.

6. 3. The multi-beam charged particle microscope (1) of claim 1 or 2, wherein the modifying element (605) is provided by a deflection scanner (110) or a magnetic correction element (420) of the multi-beam charged particle microscope (1).

7. 3. The multi-beam charged particle microscope (1) according to claim 1 or 2, wherein the modification element (605) is identical to the global compensation element (603) of the multi-beam charged particle microscope (1).

8. 3. The multi-beam charged particle microscope (1) of claim 1 or 2, wherein the set point comprises a predetermined rotational deviation of the raster arrangement (41) of the plurality of primary charged particle beamlets (1) between the coordinate systems of the image plane (101), the array of compensator components (601), the global compensation element (603), and / or the modification element (605), and the control unit is configured to compensate for the rotational difference of the wavefront aberration between the compensation elements (601, 603) and / or the modification element (605).

9. A method for determining a plurality of wavefront aberration amplitudes of a multi-beam charged particle microscope (1), comprising: a) setting the multi-beam microscope (1) to a set point for an inspection task; b) modifying the wavefront aberration of a plurality of J primary charged particle beamlets (3) by providing a series of at least SI=3 modification control signals SV(i=1...SI) to a modification element (605) and measuring a plurality of contrast values ​​C(j,i) at each modification control signal SV(i=1...SI) for each of the plurality of J primary charged particle beamlets (3); c) determining a plurality of contrast curves C(j=1...J) for each of the plurality of J primary charged particle beamlets (3) from the plurality of contrast values ​​C(j,i); d) determining a plurality of J wavefront aberration amplitudes A(j=1...J) in normalized sensitivity units at the set point from the plurality of contrast curves C(j=1...J); A method comprising:

10. 10. The method of claim 9, wherein step c comprises the calculation of a parabolic, hyperbolic or polynomial approximation to the contrast values ​​C(j,i) for each of the plurality of J primary charged particle beamlets (3), i=1...SI.

11. 11. The method of claim 9 or 10, wherein each of a plurality of J wavefront amplitudes A(j) in normalized sensitivity units is determined from at least one of a modified control signal SV(maxC(j)) at a maximum contrast value maxC(j) divided by a normalized range RV of the modification element 605 and / or a parabolic coefficient KV(j) of the contrast curve C(j).

12. 12. The method of claim 11, further comprising determining the normalization range RV by determining maximum and minimum control signals SV required to achieve a predetermined change in the image contrast of at least one of the plurality of primary charged particle beamlets.

13. - converting said wavefront aberration amplitude into a wavefront aberration amplitude vector; - multiplying said wavefront aberration amplitude vector by a rotation matrix M to take into account a given rotational deviation of said raster arrangement (41) of said plurality of primary charged particle beamlets (1) between the coordinate systems of the image plane (101), the array of compensator elements (601), the global compensation element (603) and / or the modification element (605); The method of claim 9 or 10, further comprising:

14. A method for compensating for a plurality of wavefront aberrations of a multi-beam charged particle microscope (1) at a set point, comprising: a) receiving a number of J wavefront aberration amplitudes A, j=1...J, of a number of J primary charged particle beamlets (3) in normalized sensitivity units; b) determining a global component AG1 of amplitude in normalized sensitivity units, said global component having a predetermined field dependence of said plurality of J wavefront aberration amplitudes A(j) of the global compensation element (603); c) determining a plurality of residual wavefront amplitudes in normalized sensitivity units, residual components Ares(j); d) converting said global components into a global correction signal GCS; e) converting the residual components into a plurality of local compensation signals LCS(j); f) providing said global correction signal GCS to a global compensation element (603); g) providing said plurality of local compensation signals LCS(j) to an array of compensation elements (601); A method comprising:

15. The method of claim 14 , wherein step a) comprises determining the plurality of J wavefront aberration amplitudes A(j).

16. 4. The method of claim 3, wherein the step of determining the plurality of J wavefront aberration amplitudes A(j) comprises: modifying said wavefront aberration of said plurality of J primary charged particle beamlets (3) by providing a series of at least SI=3 modification control signals SV, i=1...SI, to a modification element (605); - measuring a number of contrast values ​​C(j,i) for each of said plurality of J primary charged particle beamlets (3) at each modified control signal SV(i=1...SI); - determining from said plurality of contrast values ​​C(j,i) a plurality of contrast curves C(j=1...J) for each of said plurality of J primary charged particle beamlets (3); - determining a number J of wavefront aberration amplitudes A(j=1...J) in normalized sensitivity units from said number of contrast curves C(j=1...J); 16. The method of claim 15, comprising:

17. 17. The method of claim 16, further comprising calculating a parabolic, hyperbolic or polynomial approximation to the contrast values ​​C(j,i) for each of the plurality of J primary charged particle beamlets (3), i=1...SI.

18. 18. The method of claim 17, wherein each of a plurality of J wavefront amplitudes A(j) in normalized sensitivity units is determined from at least one of a modified control signal SV(maxC(j)) at a maximum contrast value maxC(j) divided by a normalized range RV of the modification element 605 and / or a parabolic coefficient KV(j) of the contrast curve C(j).

19. - converting said wavefront aberration amplitudes A(j) into wavefront aberration amplitude vectors; - taking into account a given rotational deviation of the raster arrangement (41) of the plurality of primary charged particle beamlets (1) at the set point between the array of compensator components (601), the global compensation element (603) and / or the modification element (605) by multiplication of the wavefront aberration amplitude vector by a rotation matrix M; 16. The method of claim 14 or 15, further comprising:

20. 16. The method according to claim 14 or 15, wherein in step d) the global correction signal GCS is obtained from the amplitude AG1 in normalized sensitivity units by multiplication of a predetermined normalization range RC of the global compensation element (603) or from a predetermined parabolic sensitivity parameter KC of the global compensation element (603).

21. 16. The method according to claim 14 or 15, wherein in step e) the plurality of local compensation signals LCS(j) are obtained from the plurality of residual wavefront aberration amplitudes Ares(j) in normalized sensitivity units by multiplication of a predetermined normalization range RL of each of the compensator elements of the array of compensator elements (601) or from a plurality of predetermined parabolic sensitivity parameters KLC(j) of the array of compensation elements (601).

22. The method according to claim 14 or 15, wherein step a) further comprises monitoring the wavefront aberrations of the multi-beam charged particle microscope (1) during an inspection task.

23. 23. The method of claim 22, wherein the monitoring comprises model-based control based on a predetermined model of the multi-beam charged particle microscope and a secondary indirect monitoring parameter.

24. 23. The method of claim 22, further comprising: monitoring an image contrast of a plurality of digital images received during an inspection task; and triggering the receiving of the plurality of J wavefront aberration amplitudes A, j=1...J, when an image contrast falls below a predetermined threshold.

25. A method for calibrating a charged particle optical element of a multi-beam charged particle microscope (1), such as a global compensator (603), or a compensator element of an array of compensator elements (601), or a modification element (605), comprising the steps of: modifying the wavefront aberration of a plurality of J primary charged particle beamlets (3) by providing a series of at least SI=3 different control signals SC, with i=1...SI, to said charged particle optical element; - measuring a number of contrast values ​​C(j=1...J, i=1...SI) for each of said plurality of J primary charged particle beamlets (3) at different control signals SC(i=1...SI); - determining from said plurality of contrast values ​​C(j,i) a plurality of contrast curves C(j=1...J,SC) for each of said plurality of J primary charged particle beamlets (3); determining an extreme value maxC(j) and said control signal SC(maxC(j)) which corresponds to said extreme value maxC(j) of each of said contrast curves C(j,SC); - determining a number of modifications of the wavefront amplitude A(j) affected by the charged particle optical element in normalized sensitivity units; A method comprising:

26. 26. The method of claim 25, wherein the multiple changes in wavefront amplitude A(j) in normalized sensitivity units are given by A(j)=SC(maxC(j)) / RC, where RC is a normalized range of the charged particle optical element corresponding to the difference between maximum and minimum control signals SC required to achieve a predetermined change in the image contrast C(j) for at least one of the multiple J primary charged particle beamlets.

27. 26. The method of claim 25, wherein the multiple modifications of the wavefront amplitude A(j) in normalized sensitivity units are given by A(j)=SIGN(j)*2KV(j)*SC(maxC(j)), with KC(j) being an approximate parabolic constant for the contrast curve C(j=1...J,SC) in the vicinity of the extremum max(C(j,SC)).

28. 27. The method according to claim 25 or 26, further comprising the step of storing the plurality of modifications of the wavefront amplitudes A(j) of the charged particle optical elements in normalized sensitivity units in a memory of a control unit (800) of the multi-beam charged particle microscope (1).

29. 1. A method for determining a wavefront aberration of each beamlet of a plurality of J primary charged particle beamlets of a multi-beam charged particle microscope (1), comprising: - providing a wavefront detection pattern (61) in an image plane (101) of the multi-beam charged particle microscope (1) for each of said beamlets of said plurality of J primary charged particle beamlets, each wavefront detection pattern (61) comprising a plurality of repeating features (63) oriented at different rotation angles α; performing a plurality of measurements of said wavefront detection pattern (61) in a focus sequence having NQ focus steps q=1...NQ and determining a plurality of contrast values ​​C(j,q;α); - approximating, for each of said beamlets j=1...J and each of said rotation angles α, a number of contrast curves C(j;α) as a function of focus position; deriving said maximum value maxC(j;α) for each of said contrast curves C(j;α); - determining for each of said beamlets a symmetric wavefront aberration A(j) with even order of rotational symmetry from the maximum difference between the two focal positions of two maxima maxC(j;α) and maxC(j;α-90) of two repeating features (63) oriented at 90° to each other; A method comprising:

30. 30. The method of claim 29, further comprising determining a plurality of relative image displacements dr(j;α) across the focal points of each of the plurality of repeating features (63), and determining an asymmetric wavefront aberration having odd-order rotational symmetry for each of the beamlets from the maximum relative image displacement dr(j;α) across the focal points.

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